DISCLOSURE OF THE INVENTION

[0039] The first present invention provides an optical switch including the following elements. At least a plurality of optical transmission lines are provided for transmissions of optical signals. Each of the at least plurality of optical transmission lines have at least an impurity doped fiber. At least an excitation light source is provided for emitting an excitation light. At least an excitation light switch is provided which is connected to the excitation light source and also connected to the at least plurality of optical transmission lines for individual switching operations to supply the excitation light to the at least plurality of optical transmission lines to feed the excitation light to the impurity doped fiber on the at least plurality of optical transmission lines, thereby causing an excitation of the impurity doped fiber on selected one of the at least plurality of optical transmission lines so as to permit a transmission of the optical signal through the excited impurity doped fiber, whilst unselected one of the impurity doped fibers is unexcited whereby the optical signals are absorbed into the unselected one of the impurity doped fibers thereby to discontinue transmission of the optical signal by the unselected one of the impurity doped fibers.

[0040] It is preferable that the optical switch further includes: a single input side optical transmission line; and a single input side optical coupler connected to the single input side optical transmission line, and wherein the at least plurality of optical transmission lines comprise first and second optical transmission lines which are connected through the single input side optical coupler to the single input side optical transmission line, and the first and second optical transmission lines have first and second impurity doped fibers, and wherein the at least excitation light source comprises a single excitation light source, and the at least excitation light switch comprises a single excitation light switch which has first and second output terminals for selecting any one of the first and second output terminals, and the first output terminal is connected through a first optical coupler to the first impurity doped first to feed the excitation light to the first impurity doped fiber only when the first output terminal is selected by the single excitation light switch, and the second output terminals is connected through a second optical coupler to the second impurity doped fiber to feed the excitation light to the second impurity doped fiber only when the second output terminal is selected by the single excitation light switch.

[0041] It is preferable further comprise first and second optical filers. The first optical filter is provided on the first optical transmission line and positioned between the first optical coupler and an output terminal of the first optical transmission line so as to remove a noise from the first optical signal when the first impurity doped fiber is excited. The second optical filter is provided on the second optical transmission line and positioned between the second optical coupler and an output terminal of the second optical transmission line so as to remove a noise from the second optical signal when the second impurity doped fiber is excited.

[0042] It is preferable further comprise the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A first optical isolator is provided between the input side optical coupler and the first optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A second optical isolator is provided between the input side optical coupler and the second optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the second optical transmission line.

[0043] It is preferable further comprise the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A circulator is provided as the input side optical coupler and an optical isolator provided between the input side optical transmission line and the first and second optical transmission lines.

[0044] It is preferable that the first optical coupler is inserted between the first impurity doped fiber and an output terminal of the first optical transmission line so as to feed the excitation light to the first impurity doped fiber in an opposite direction to a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and an output terminal of the second optical transmission line so as to feed the excitation light to the second impurity doped fiber in an opposite direction to a transmission of the second optical signal through the second impurity doped fiber excited.

[0045] It is preferable that the first optical coupler is inserted between the first impurity doped fiber and the input side optical coupler so as to feed the excitation light to the first impurity doped fiber in the same direction as a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and the input side optical coupler so as to feed the excitation light to the second impurity doped fiber in the same direction as a transmission of the second optical signal through the second impurity doped fiber excited.

[0046] It is preferable that the optical switch has two inputs and two outputs and comprises a pair of first and second optical switches connected to each other through at least an interconnecting optical transmission line, and wherein each of the first and second optical switches further comprises the following elements. A single input side optical coupler is provided which is connected to the single input side optical transmission line. First and second optical transmission lines are connected through the single input side optical coupler to the single input side optical transmission line. The first and second optical transmission lines have first and second impurity doped fibers. A single excitation light source is provided. A single excitation light switch is provided which has first and second output terminals for selecting any one of the first and second output terminals. The first output terminal is connected through a first optical coupler to the first impurity doped fiber to feed the excitation light to the first impurity doped fiber only when the first output terminal is selected by the single excitation light switch. The second output terminal is connected through a second optical coupler to the second impurity doped fiber to feed the excitation light to the second impurity doped fiber only when the second output terminal is selected by the single excitation light switch.

[0047] It is preferable that each of the first and second optical switches further comprises first and second optical filters. The first optical filter is provided on the first optical transmission line and positioned between the first optical coupler and an output terminal of the first optical transmission line so as to remove a noise from the first optical signal when the first impurity doped fiber is excited. The second optical filter is provided on the second optical transmission line and positioned between the second optical coupler and an output terminal of the second optical transmission line so as to remove a noise from the second optical signal when the second impurity doped fiber is excited.

[0048] It is preferable that each of the first and second optical switches further comprises the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A first optical isolator is provided between the input side optical coupler and the first optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A second optical isolator is provided between the input side optical coupler and the second optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the second optical transmission line.

[0049] It is preferable that each of the first and second optical switches further comprises the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A circulator is provided as the input side optical coupler and an optical isolator provided between the input side optical transmission line and the first and second optical transmission lines.

[0050] It is preferable that, for each of the first and second optical switches, the first optical coupler is inserted between the first impurity doped fiber and an output terminal of the first optical transmission line so as to feed the excitation light to the first impurity doped fiber in an opposite direction to a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and an output terminal of the second optical transmission line so as to feed the excitation light to the second impurity doped fiber in an opposite direction to a transmission of the second optical signal through the second impurity doped fiber excited.

[0051] It is preferable that, for each of the first and second optical switches, the first optical coupler is inserted between the first impurity doped fiber and the input side optical coupler so as to feed the excitation light to the first impurity doped fiber in the same direction as a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and the input side optical coupler so as to feed the excitation light to the second impurity doped fiber in the same direction as a transmission of the second optical signal through the second impurity doped fiber excited.

[0052] It is preferable that the optical switch has two inputs and two outputs and comprises a pair of first and second optical switches connected to each other through at least an interconnecting optical transmission line, and a common excitation light source connected to the first and second optical switches, and wherein each of the first and second optical switches further comprises the following elements. A single input side optical coupler is provided which is connected to the single input side optical transmission line. First and second optical transmission lines are provided which are connected through the single input side optical coupler to the single input side optical transmission line. The first and second optical transmission lines have first and second impurity doped fibers. A single excitation light switch is provided which is connected to the common excitation light source, the single excitation light switch having first and second output terminals for selecting any one of the first and second output terminals, and the first output terminal being connected through a first optical coupler to the first impurity doped fiber to feed the excitation light to the first impurity doped fiber only when the first output terminal is selected by the single excitation light switch, and the second output terminal being connected through a second optical coupler to the second impurity doped fiber to feed the excitation light to the second impurity doped fiber only when the second output terminal is selected by the single excitation light switch.

[0053] It is preferable that the at least plurality of optical transmission lines are separated from each other for separate transmission of different optical signals on the plurality of separated optical transmission lines. Each of the separated optical transmission lines has a single impurity doped fiber. The at least excitation light source comprises a single excitation light source. The at least excitation light switch comprises a single excitation light switch for separate switching operations to the at least plurality of optical transmission lines to separately control individual excitations of the impurity doped fibers on the least plurality of optical transmission lines.

[0054] It is preferable that the at least plurality of optical transmission lines are separated from each other for separate transmission of different optical signals on the plurality of separated optical transmission lines, and each of the separated optical transmission lines has a single impurity doped fiber, and the at least excitation light source comprises two excitation light source, and further at least excitation light switch comprises a single optical cross connector for separate switching operations to the at least plurality of optical transmission lines to separately control individual excitations of the impurity doped fibers on the at least plurality of optical transmission lines.

[0055] The second present invention provides an optical switch comprising the following elements. A first optical transmission line is provided for transmitting a first optical signal. An optical reflectivity variable mirror is provided which is capable of varying a reflectivity in a range of 0% to 100% for reflecting the first optical signal. The optical reflectivity variable mirror is connected with the first optical transmission line. A second optical transmission line is provided which is connected through the optical reflectivity variable mirror to the first optical transmission line. An optical transmitter is provided which is connected through the second optical transmission line to the optical reflectivity variable mirror for transmitting a second optical signal. If the optical reflectivity variable mirror sets the reflectivity at less than 100%, then the first optical signal is reflected by the optical reflectivity variable mirror so that the first optical signal is outputted from the first optical transmission line, if the optical reflectivity variable mirror sets the reflectivity at 100%, then the first optical signal is transmitted through the optical reflectivity variable mirror, whilst the second optical signal transmitted from the optical transmitter is also transmitted through the optical reflectivity variable mirror to be outputted from the first optical transmission line.

[0056] The third present invention provides an optical add-drop multiplexer comprising at least a single set of the following elements. A first optical transmission line is provided for transmitting a first optical signal. An optical coupler is provided on the first optical transmission line for dividing the first optical signal into first and second divided optical signals. A fourth optical transmission line is provided which is connected with the optical coupler for transmitting the first divided optical signal. An optical receiver is provided which is connected through the fourth optical transmission line to the optical coupler for receiving the first divided optical signal. An optical reflectivity variable mirror is provided which is capable of varying a reflectivity in a range of 0% to 100%. The optical reflectivity variable mirror is connected with first optical transmission line for reflecting the second divided optical signal. A second optical transmission line is provided which is connected through the optical reflectivity variable mirror to the first optical transmission line. An optical transmitter is provided which is connected through the second optical transmission line to the optical reflectivity variable mirror for transmitting a second optical signal. If the optical reflectivity variable mirror sets the reflectivity at less than 100%, then the first optical signal is reflected by the optical reflectivity variable mirror so that the first optical signal is outputted from the first optical transmission line. If the optical reflectivity variable mirror sets the reflectivity at 100%, then the first optical signal is transmitted through the optical reflectivity variable mirror, whilst the second optical signal transmitted from the optical transmitter is also transmitted through the optical reflectivity variable mirror to be outputted from the first optical transmission line.

[0057] It is preferable that the optical add-drop multiplexer comprises a plurality of the optical add-drop multiplexers, and further comprising an optical device having at least any one of multiplexing function and demultiplexing function so that the optical add-drop multiplexers are operable to different wavelength optical signals.

[0058] The fourth present invention provides an optical add-drop multiplexer comprising at least a single set of the following elements. An input side optical transmission line is provided for transmitting a first optical signal. An input side optical coupler is provided on the first optical transmission line for dividing the first optical signal into first and second divided optical signals. First and second optical transmission lines are provided which are connected with the input side optical coupler for transmissions of the first and second divided optical signals respectively. The first and second optical transmission lines have first and second impurity doped fibers. An optical receiver is provided which is connected through the first optical transmission line to the first impurity doped fiber for receiving the first divided optical signal only when the first impurity doped fiber is excited. An optical transmitter is provided which is connected through the second optical transmission line to the second impurity doped fiber for transmitting a second optical signal through the second impurity doped fiber to the input side optical transmission line for output of the second optical signal only when the second impurity doped fiber is excited. At least an excitation light source is provided for emitting an excitation light. An excitation light switch is provided which is connected to the excitation light source and also connected to the first and second optical transmission lines for selective switching operations to supply the excitation light to any one of the first and second optical transmission lines to feed the excitation light to selected one of the first and second impurity doped fibers, thereby causing an excitation of the selected one of the first and second impurity doped fibers, whilst unselected one of the first and second impurity doped fibers is unexcited.

[0059] It is preferable that the optical add-drop multiplexer comprises a plurality of the optical add-drop multiplexers, and further comprising an optical device having at least any one of multiplexing function and demultiplexing function so that the optical add-drop multiplexers are operable to different wavelength optical signals.

[0060] The fifth present invention provides an optical gate switch comprising the following elements. A first optical transmission line is provided for transmitting an input optical input signal. A second optical transmission line is provided for transmitting an optical output signal. A fourth optional transmission line is connected through an optical coupler to both the first and second optional transmission lines. The fourth optional transmission line has at least a impurity doped fiber and a wavelength band selective optical reflecting mirror capable of selecting a wavelength band of a light to be reflected. The impurity doped fiber is positioned between the wavelength band selective optical reflecting mirror. An excitation light source is provided which is connected to the wavelength band selective optical reflecting mirror for controlling an emission of an excitation light so that if the excitation light source emits the excitation light to feed the excitation light to the impurity doped fiber so as to excite the impurity doped fiber, whereby the optical input signal is transmitted through the excited impurity doped fiber and amplified by the excited impurity doped fiber and subsequently the amplified optical signal is reflected by the wavelength band selective optical reflecting mirror before the reflected optical signal is then transmitted through the excited impurity doped fiber and further amplified by the excited impurity doped fiber for subsequent output of the further amplified optical signal through the output signal optical transmission line.

[0061] The sixth present invention provides an optical add-drop multiplexer comprising at least a single set of the following elements. A first optional transmission line is provided for transmitting an input optical input signal. A second optical transmission line is provided for transmitting an optical output signal. A fourth optional transmission line is provided which is connected through an optical coupler to both the first and second optional transmission lines. The fourth optional transmission line has at least a impurity doped fiber and a wavelength band selective optical reflecting mirror capable of selecting a wavelength band of a light to be reflected. The impurity doped fiber is positioned between the wavelength band selective optical reflecting mirror. An optical receiver is provided which is connected through a second optical coupler to the fourth optical transmission line so that the second optical coupler is positioned between the first optical coupler and the impurity doped fiber for allowing the optical receiver receives a part of the optical input signal. An optical transmitter is provided which is connected through a fourth optical coupler to the output signal transmission line for transmitting a second optical signal as a substitute output signal only when no output signal is supplied from the impurity doped fiber. An excitation light source is provided which is connected to the wavelength band selective optical reflecting mirror for controlling an emission of an excitation light so that if the excitation light source emits the excitation light to feed the excitation light to the impurity doped fiber so as to excite the impurity doped fiber, whereby the optical input signal is transmitted through the excited impurity doped fiber and amplified by the excited impurity doped fiber and subsequently the amplified optical signal is reflected by the wavelength band selective optical reflecting mirror before the reflected optical signal is then transmitted through the excited impurity doped fiber and further amplified by the excited impurity doped fiber for subsequent output of the further amplified optical signal through the output signal optical transmission line.

[0062] It is preferable that the optical add-drop multiplexer comprises a plurality of the optical add-drop multiplexers, and further comprising an optical device having at least any one of multiplexing function and demultiplexing function so that the optical add-drop multiplexers are operable to different wavelength optical signals.

[0063] The seventh present invention provides an optical transmission line junction structure comprising at least three optical transmission lines for transmuting optical signals and an optical device having at least any one of wavelength multiplexing and demultiplexing functions connected to the at least three optical transmission lines, so that the optical device having at least any one of multiplexing and demultiplexing functions serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

[0064] It is preferable that the optical device comprises an optical multiplexer/demultiplexer.

[0065] It is preferable that the optical device comprises an optical multiplexer.

[0066] It is preferable that the optical device comprises an optical demultiplexer.

[0067] The eighth present invention provides an optical transmission line junction structure comprising at least three optical transmission lines for transmuting optical signals and an optical circulator connected to the at least three optical transmission lines, so that the optical circulator serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

[0068] The ninth present invention provides an optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of the plurality of optical transmission line junctions has an optical device having at least any one of wavelength multiplexing and demultiplexing functions, which is connected to the at least three optical transmission lines, so that the optical device having at least any one of multiplexing and demultiplexing functions serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

[0069] It is preferable that all of the plurality of optical transmission line junctions have the optical devices.

[0070] It is preferable that at least one of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

[0071] It is preferable that the at least one of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

[0072] It is preferable that at least one of the plurality of looped optical transmission lines has at least single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

[0073] It is preferable that at least two of the plurality of looped optical transmission lines are connected to an optical multiplexer/demultiplexer, whilst a single looped optical transmission line is separated by the at least two of the plurality of looped optical transmission lines from the optical multiplexer/demultiplexer, so that optical signals are individually transmitted along the plurality of looped optical transmission lines, and wherein all of the plurality of optical transmission line junctions have the optical devices.

[0074] It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

[0075] It is preferable that each of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

[0076] It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

[0077] It is preferable that the optical device comprises an optical multiplexer/demultiplexer.

[0078] It is preferable that the optical device comprises an optical multiplexer.

[0079] It is preferable that the optical device comprises an optical demultiplexer.

[0080] The tenth present invention provides an optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of the plurality of optical transmission line junctions has an optical circulator, which is connected to the at least three optical transmission lines, so that the optical circulator serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

[0081] It is preferable that all of the plurality of optical transmission line junctions have the optical circulators.

[0082] It is preferable that at least one of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

[0083] It is preferable that at least one of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

[0084] It is preferable that at least one of the plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

[0085] It is preferable that at least two of the plurality of looped optical transmission lines are connected to an optical multiplexer/demultiplexer, whilst a single looped optical transmission line is separated by the at least two of the plurality of looped optical transmission lines from the optical multiplexer/demultiplexer, so that optical signals are individually transmitted along the plurality of looped optical transmission lines, and wherein all of the plurality of optical transmission line junctions have the optical circulators.

[0086] It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

[0087] It is preferable that each of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

[0088] It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

[0089] The eleventh present invention provides an optical gate switch comprising the following elements. A main optical transmission line is provided. First and second optical multiplexer/demultiplexers are also provided on the main optical transmission line so that the first and second optical multiplexer/demultiplexers are separated from each other. The first and second optical multiplexer/demultiplexers are connected with first and second subordinate optical transmission lines respectively. An impurity doped fiber is provided on the main optical transmission line and positioned between the first and second optical multiplexer/demultiplexers. An excitation light source is provided which is connected through the first subordinate optical transmission line to the first optical multiplexer/demultiplexer so that the excitation light source emits an excitation light which is transmitted through the first subordinate optical transmission line and the first optical multiplexer/demultiplexer to the impurity doped fiber. The second optical multiplexer/demultiplexer transmits the optical signal onto the main optical transmission line and also transmits a leaked part of the excitation light onto the second subordinate optical transmission line.

[0090] It is preferable to further comprise an optical reflecting mirror provided on the second subordinate optical transmission line for reflecting the leaked part of the excitation light to the impurity doped fiber.

[0091] It is preferable to further comprise a secondary excitation light source on the second subordinate optical transmission line.

[0092] Preferred Embodiments

[0093] First Embodiment

[0094] A first embodiment according to the present invention will be described in detail with reference to FIG. 1 which is a diagram illustrative of a first novel optical switch having a single input and two outputs. The optical switch has an input side coupler 21 which is connected to a first optical transmission line 110 on which an optical input signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical input signal is divided by the input side coupler 21 into two parts. The optical switch has second and third optical transmission lines 120 and 121 which are connected to the input side coupler 21 . The two divided optical signals are then transmitted through the second and third optical transmission lines 120 and 121 for output thereof. The second optical transmission line 120 is connected to a first output side coupler 22 . The third optical transmission line 121 is connected to a second output side optical coupler 23 . A first erbium doped fiber EDF 11 is provided on the second optical transmission line 120 between the input side coupler 21 and the first output side coupler 22 . A second erbium doped fiber EDF 12 is provided on the third optical transmission line 120 between the input side coupler 21 and the second output side coupler 23 . The first and second erbium doped fibers EDF 11 and EDF 12 have a length of 50 meters. The first and second erbium doped fibers EDF 11 and EDF 12 may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF 11 and EDF 12 respectively. The optical switch further has an excitation light switch 41 which is connected through a first excitation light transmission line 111 to the first output side coupler 22 as well as which is connected through a second excitation light transmission line 112 to the second output side coupler 23 . The optical switch further has an excitation light source 31 which is connected to the excitation light switch 41 . The excitation light source 31 emits an excitation light with a wavelength of 1480 nanometers The excitation light switch 41 is operated to switch the excitation light to any one of the first and second excitation light transmission lines 111 and 112 to supply any one of the first and second erbium doped fibers EDF 11 and EDF 12 .

[0095] If the excitation light switch 41 is operated to switch to supply the excitation light to the first erbium doped fiber EDF 11 , then the first erbium doped first EDF 11 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF 11 without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the second optical transmission line 120 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the excitation light switch 41 to be fed through the first excitation light transmission line 111 and the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the excitation light switch 41 whereby a leaked excitation light is then fed through the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF 11 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF 11 . As a result, an optical output signal from the third optical transmission line 121 has an intensity of −60 dBm or less. The excitation light switch 41 causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0096] As a modification to the above first embodiment, the above excitation light switch 41 may be replaced by a polymer optical switch.

[0097] If the polymer optical switch 41 is operated to switch to supply the excitation light to the first erbium doped fiber EDF 11 , then the first erbium doped fiber EDF 11 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF 11 without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the second optical transmission line 120 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the polymer optical switch 41 to be fed through the first excitation light transmission line 111 and the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the polymer optical switch 41 whereby a leaked excitation light is then fed through the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF 12 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF 12 . As a result, an optical output signal from the third optical transmission line 121 has intensity of −60 dBm or less. The polymer optical switch 41 causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0098] If the polymer optical switch 41 is operated to switch to supply the excitation light to the second erbium doped fiber EDF 12 , then the second erbium doped fiber EDF 12 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF 12 without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the third optical transmission line 121 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the polymer optical switch 41 to be fed through the second excitation light transmission line 112 and the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the polymer optical switch 41 whereby a leaked excitation light is then fed through the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF 11 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF 11 . As a result, an optical output signal from the third optical transmission line 121 has an intensity of −60 dBm or less. The polymer optical switch 41 causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0099] As a further modification to the above first embodiment, the excitation light has a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

[0100] In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

[0101] It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

[0102] It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

[0103] The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

[0104] It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

[0105] It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

[0106] It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

[0107] It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

[0108] It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

[0109] The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO ₃ switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

[0110] Second Embodiment

[0111] A second embodiment according to the present invention will be described in detail with reference to FIG. 2 which is a diagram illustrative of a second novel optical switch having a single input and two outputs. A structural difference of the second novel optical switch from the first novel optical switch is only in further providing first and second optical filters on two output sides in order to eliminate or remove amplified noises from the optical output signals.

[0112] The optical switch has an input side coupler 21 which is connected to a first optical transmission line 110 on which an optical input signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical input signal is divided by the input side coupler 21 into two parts. The optical switch has second and third optical transmission lines 120 and 121 which are connected to the input side coupler 21 . The two divided optical signals are then transmitted through the second and third optical transmission lines 120 and 121 for output thereof. The second optical transmission line 120 is connected to a first output side coupler 22 . The third optical transmission line 121 is connected to a second output side optical coupler 23 . A first erbium doped fiber EDF 11 is provided on the second optical transmission line 120 between the input side coupler 21 and the fist output side coupler 22 . A second erbium doped fiber EDF 12 is provided on the third optical transmission line 120 between the input side coupler 21 and the second output side coupler 23 . The first and second erbium doped fibers EDF 11 and EDF 12 have a length of 50 meters. The first and second erbium doped fibers EDF 11 and EDF 12 may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF 11 and EDF 12 respectively. The optical switch further has an excitation light switch 41 which is connected through a first excitation light transmission line 111 to the first output side coupler 22 as well as which is connected through a second excitation light transmission line 112 to the second output side coupler 23 . The optical switch further has an excitation light source 31 which is connected to the excitation light switch 41 . The excitation light source 31 emits an excitation light with a wavelength of 1480 nanometers. The excitation light switch 41 is operated to switch the excitation light to any one of the first and second excitation light transmission lines 111 and 112 to supply any one of the first and second erbium doped fibers EDF 11 and EDF 12 .

[0113] Further, in this second embodiment, a first optical filter 51 is provided on the second optical transmission line 120 and positioned closer to the output side than the first output side optical coupler 22 . If the first erbium doped fiber EDF 11 is excited, then this first erbium doped fiber EDF 11 also serves as an optical power amplifier which, however, amplifies not only the divided optical signal from the first optical transmission line 110 but also noises induced in the optical signals, for which reason it is preferable to remove or eliminate the noises from the optical output signal by the first optical filter 51 in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch 41 . Similarly, a second optical filter 52 is provided on the third optical transmission line 121 and positioned closer to the output side than the second output side optical coupler 23 . If the second erbium doped fiber EDF 12 is excited, then this second erbium doped fiber EDF 12 also serves as an optical power amplifier which, however, amplifies not only the divided optical signal from the first optical transmission line 110 but also noises included in the optical signals, for which reason it is preferable to remove or eliminate the noises from the optical output signal by the second optical filter 52 in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch 41 .

[0114] If the excitation light switch 41 is operated to switch to supply the excitation light to the first erbium doped fiber EDF 11 , then the first erbium doped fiber EDF 11 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF 11 without any optical absorption. The optical output signal is then fed to the first optical filter 51 to remove or eliminate the noises from the optical output signal by the first optical filter 51 in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch 41 . Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line 120 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the excitation light switch 41 to be fed through the first excitation light transmission line 111 and the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the excitation light switch 41 whereby a leaked excitation light is then fed through the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF 12 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF 12 . As a result, an optical output signal from the third optical transmission line 121 is free of any substantive noise and has an intensity of −60 dBm or less. The excitation light switch 41 causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0115] If the excitation light switch 41 is operated to switch to supply the excitation light to the second erbium doped fiber EDF 12 , then the second erbium doped fiber EDF 12 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF 12 without any optical absorption. The optical output signal is then fed to the second optical filter 52 to remove or eliminate the noises from the optical output signal by the second optical filter 52 in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch 41 . Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line 120 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the excitation light switch 41 to be fed through the second excitation light transmission line 112 and the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the excitation light switch 41 whereby a leaked excitation light is then fed through the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF 11 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF 11 . As a result, an optical output signal from the third optical transmission line 121 is free of any substantive noise and has an intensity of −60 dBm or less. The excitation light switch 41 causes an insertion loss of 2 dBm and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0116] As a modification to the above second embodiment, the above excitation light switch 41 may be replaced by a polymer optical switch similarly to the first embodiment.

[0117] If the polymer optical switch 41 is operated to switch to supply the excitation light to the first erbium doped fiber EDF 11 , then the first erbium doped fiber EDF 11 is excited whereby the divided optical signal with the wavelength of 1550 nanoseconds is transmitted through the first erbium doped fiber EDF 11 without any optical absorption. The optical output signal is then fed to the first optical filter 51 to remove or eliminate the noises from the optical output signal by the first optical filter 51 in order to avoid deterioration in signal-to-noise ratio due to provision of the polymer optical switch 41 . Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line 120 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the polymer optical switch 41 to be fed through the first excitation light transmission line 111 and the first output side optical coupler 22 to the first erbium doped EDF 11 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the polymer optical switch 41 whereby a leaked excitation light is then fed through the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF 12 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF 12 . As a result, an optical output signal from the third optical transmission line 121 is free of any substantive noise and has an intensity of −60 dBm or less. The polymer optical switch 41 causes an insertion loss of 20 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0118] If the polymer optical switch 41 is operated to switch to supply the excitation light to the second erbium doped fiber EDF 12 , then the second erbium doped fiber EDF 12 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF 12 without any optical absorption. The optical output signal is then fed to the second optical filter 52 to remove or eliminate the noises from the optical output signal by the second optical filter 52 in order to avoid deterioration in signal-to-noise ratio due to provision of the polymer optical switch 41 . Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line 120 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the polymer optical switch 41 to be fed through the second excitation light transmission line 112 and the second output side coupler 23 to the second erbium doped fiber EDF 12 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the polymer optical switch 41 whereby a leaked excitation light is then fed through the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF 11 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF 11 . As a result, an optical output signal from the third optical transmission line 121 is free of any substantive noise and has no intensity of −60 dBm or less. The polymer optical switch 41 causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0119] As a further modification to the above second embodiment, the excitation light has a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

[0120] In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

[0121] It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

[0122] It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

[0123] The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

[0124] It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

[0125] It is furthermore possible to replace the erbium doped fiber by rear earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

[0126] It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

[0127] It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separated from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

[0128] It is yet more possible to set freely a ratio of optical devices at the optical coupler in accordance with the various design choices.

[0129] It is still further possible to freely set the transmission-band width in accordance with the number of the optical signals to be transmitted through the optical switch.

[0130] It is yet further possible to provide optical filters and optical isolators since the excitation light and returned light provide no influence to input and output sides of the optical switch.

[0131] The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO ₃ switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

[0132] Third Embodiment

[0133] A third embodiment according to the present invention will be described in detail with reference to FIG. 3 which is a diagram illustrative of a third novel optical switch having a single input and two outputs. A structural difference of the third novel optical switch from the first novel optical switch is in further providing first and second optical isolators as well as first and second optical mirrors in order to increase an efficiency of excitation of the erbium doped fiber with allowance of a sufficient optical absorption.

[0134] The optical switch has an input side coupler 21 which is connected to a first optical transmission line 110 on which an optical switch signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical input signal is divided by the input side coupler 21 into two parts. The optical switch has second and third optical transmission lines 120 and 121 which are connected to the input side coupler 21 . The two divided optical signals are then transmitted through the second and third optical transmission lines 120 and 121 for output thereof. The second optical transmission line 120 is connected to a first output side coupler 22 . The third optical transmission line 121 is connected to a second output side optical coupler 23 . A first erbium doped fiber EDF 11 is provided on the second optical transmission line 120 between the input side coupler 21 and the first output side coupler 22 . A second erbium doped fiber EDF 12 is provided on the third optical transmission line 120 between the input side coupler 21 and the second output side coupler 23 . The first and second erbium doped fibers EDF 11 and EDF 12 have a length of 50 meters. The first and second erbium doped fibers EDF 11 and EDF 12 may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF 11 and EDF 12 respectively. The optical switch further has an excitation light switch 41 which is connected through a first excitation light transmission line 111 to the first output side coupler 22 as well as which is connected through a second excitation light transmission line 112 to the second output side coupler 23 . The optical switch further has an excitation light source 31 which is connected to the excitation light switch 41 . The excitation light source 31 emits an excitation light with a wavelength of 1480 nanometers. The excitation light switch 41 is operated to switch the excitation light to any one of the first and second excitation light transmission line 111 and 112 to supply any one of the first and second erbium doped fibers EDF 11 and EDF 12 .

[0135] In addition, a first optical isolator 61 is provided on the second optical transmission line 120 and positioned between the input side optical coupler 21 and the first erbium doped fiber EDF 11 . The first optical isolator 61 permits only a unidirectional transmission of the optical signal from the input side optical coupler 21 to the first erbium doped fiber EDF 11 , however, preventing an opposite direction transmission of the optical signal from the first erbium doped fiber EDF 11 to the input side optical coupler 21 . A second optical isolator 62 is provided on the third optical transmission line 121 and positioned between the input side optical coupler 21 and the second erbium doped fiber EDF 12 . The second optical isolator 62 permits only a unidirectional transmission of the optical signal from the input side optical coupler 21 to the second erbium doped fiber EDF 12 , however, preventing an opposite direction transmission of the optical signal from the second erbium doped fiber EDF 12 to the input side optical coupler 21 . Moreover, a first optical reflective mirror 71 is provided on a first terminal of the second optical transmission line 120 so that the divided optical signal having passed through the first erbium doped fiber EDF 11 is reflected by the first optical reflective mirror 71 toward the first erbium doped fiber EDF 11 , whereby the divided optical signal passes through the first erbium doped fiber EDF 11 two times. If the first erbium doped fiber EDF 11 is excited, then this first erbium doped fiber EDF 11 serves as an amplifier. This two times transmissions of the divided optical signal by the first optical reflective mirror 71 increases the efficiency of the excitation of the first erbium doped fiber EDF 11 even if the power of the excitation light emitted from the excitation light source 31 is not so high. The reflected optical signal is thus transmitted through the first erbium doped fiber EDF 11 and divided into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fourth optical transmission line 122 whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the first optical isolator 61 so that no light is transmitted back to the first optical transmission line 110 . Furthermore, a second optical reflective mirror 72 is provided on a second terminal of the third optical transmission line 121 so that the divided optical signal having passed through the second erbium doped fiber EDF 12 is reflected by the second optical reflective mirror 72 toward the second erbium doped fiber EDF 12 , whereby the divided optical signal passes through the second erbium doped fiber EDF 12 two times. If the second erbium doped fiber EDF 12 is excited, then this second erbium doped fiber EDF 12 serves as an amplifier. This two times transmissions of the divided optical signal by the second optical reflective mirror 72 increases the efficiency of the excitation of the second erbium doped fiber EDF 12 even if the power of the excitation light emitted from the excitation light source 31 is not so high. The reflected optical signal is thus transmitted through the second erbium doped fiber EDF 12 and divided into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fifth optical transmission line 123 whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the second optical isolator 62 so that no light is transmitted back to the first optical transmission line 110 .

[0136] If the excitation light switch 41 is operated to switch to supply the excitation light to the first erbium doped fiber EDF 11 , then the first erbium doped fiber EDF 11 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF 11 without any optical absorption and then the optical signal is reflected by the first optical reflective mirror 71 for subsequent returning to the first erbium doped fiber EDF 11 . This two times transmissions of the divided optical signal by the first optical reflective mirror 71 increases the efficiency of the excitation of the first erbium doped fiber EDF 11 , even if the power of the excitation light emitted from the excitation light source 31 is not so high. The reflected optical signal is thus transmitted through the first erbium doped fiber EDF 11 and divided by an optical coupler into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fourth optical transmission line 122 whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the first optical isolator 61 so that no light is transmitted back to the first optical transmission line 110 . The optical signal with an intensity of 0 dBm is outputted from the output terminal of the fourth optical transmission line 122 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the excitation light switch 41 to be fed through the first excitation light transmission line 111 and the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the excitation light switch 41 whereby a leaked excitation light is then fed through the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF 12 , for which reason the divided optical signal with the wavelength of 1550 nanometers in absorbed into the second erbium doped fiber EDF 12 . A leaked divided optical signal is also reflected by the second optical reflective mirror 72 and the reflected leaked optical signal is again transmitted through the second erbium doped fiber EDF 12 . As a result, an optical output signal from the fifth optical transmission line 123 has an intensity of −80 dBm or less. The excitation light switch 41 causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

[0137] If the excitation light switch 41 is operated to switch to supply the excitation light to the second erbium doped fiber EDF 12 , then the second erbium doped fiber EDF 12 is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF 12 without any optical absorption and then the optical signal is reflected by the second optical reflective mirror 72 for subsequent returning to the second erbium doped fiber EDF 12 . This two times transmissions of the divided optical signal by the second optical reflective mirror 72 increases the efficiency of the excitation of the second erbium doped fiber EDF 12 even if the power of the excitation light emitted from the excitation light source 31 is not so high. The reflected optical signal is thus transmitted through the second erbium doped fiber EDF 12 and divided by an optical coupler into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fifth optical transmission line 123 whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the second optical isolator 62 so that no light is transmitted back to the first optical transmission line 110 . The optical signal with an intensity of 0 dBm is outputted from the output terminal of the fourth optical transmission line 122 . Accurately, the majority part of the excitation light emitted from the excitation light source 31 is switched by the excitation light switch 41 to be fed through the second excitation light transmission line 112 and the second output side optical coupler 23 to the second erbium doped fiber EDF 12 . On the other hand, the minority part of the excitation light emitted from the excitation light source 31 might be leaked through the excitation light switch 41 whereby a leaked excitation light is then fed through the first output side optical coupler 22 to the first erbium doped fiber EDF 11 . However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF 11 , for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF 11 . A leaked divided optical signal is also reflected by the first optical reflective mirror 71 and the reflected leaked optical signal is again transmitted through the first erbium doped fiber EDF 11 . As a result, an optical output signal from the fourth optical transmission line 122 has an intensity of −80 dBm or less. The excitation light switch 41 causes an insertion loss of 2 dBm and a crosstalk of 20 dB which allow the optical switch to be free from any