Note: Descriptions are shown in the official language in which they were submitted.
 IO152O25CA 02264421 1999-03-05OPTICAL FIBER AMPLIFIER AND OPTICAL AMPLIFICATION DEVICE FORBIDIRECTIONAL OPTICAL TRANSMISSION USING THE OPTICAL FIBERAMPLIFIERBACKGROUND OF THE INVENTIONl. Field of the InventionThe present invention relates to an optical fiberamplifier and an optical amplification device for use inbidirectional optical transmission using the optical fiberamplifier and to the configuration of the optical fiberamplifier which amplifies signal light directly in an opticaltransmission device. More particularly, the presentinvention relates to an optical fiber amplifier used in abidirectional light transmission system.2. Description of the Related ArtIn a longâdistance optical fiber communication system,optical repeaters are used to amplify signal light attenuatedin transmission optical fibers. In many cases, optical fiberamplifiers, which use rareâearth doped optical fibers toamplify light, are used as optical repeaters.In the optical fiber amplifier, pump light and signallight must be in a rare-earth doped optical fiber. The pumplight is used to pump the optical fiber to amplify the signallight. Therefore, the optical fiber amplifier requires awavelength division multiplexer (WDM) to multiplex the signallight and the pump light. .n....\â........»...........,.i . . . . .... .._  10152025CA 02264421 1999-03-052The conventional optical fiber amplifier has a pumplight source which outputs pump light, a rareâearth dopedoptical fiber which is an optical amplification medium, and awavelength division multiplexer which multiplexes the pumplight and the signal light and sends the multiplexed light tothe rareâearth doped optical fiber. A typical wavelengthdivision multiplexer is an optical fiber fusionâsplicedcoupler or an optical filter composed of multiâlayerdielectric films.On the other hand, the conventional optical fiberamplifier used in a oneâdirectional light transmission deviceis not so complicated, while the conventional optical fiberamplifier used in a bidirectional light transmission deviceis complicated and therefore requires many optical parts.The biâdirectional light transmission device using theconventional optical fiber amplifier must have two opticalfiber amplifiers to amplify up and down signal light. Eachoptical fiber amplifier has its own pump light source, rare-earth (for example, erbium) doped optical fiber, andwavelength division multiplexer. The bidirectional lighttransmission device sends and receives both the up and downsignal light via a single optical fiber. Therefore, thedevice must separate these two types of signal light, up anddown, sent or received Via the erbium doped optical fibersused by the up and down signal optical fiber amplifiers. Todo so, the device has two optical circulators before andafter the erbium doped optical fibers.......,..,. .M.............._......, . . .. U .10152025CA 02264421 1999-03-053However, the conventional optical fiber amplifier hastwo components between the pump light source and the erbiumdoped optical fiber; one is an optical coupler coupling thepump light source and the erbium doped optical fiber and theother is the wavelength division multiplexer. They attenuatethe output from the pump light source and reduce the power ofthe pump light sent to the rare-earth doped optical fiber,thereby reducing the gain of the signal light achieved by theerbium doped optical fiber.An attempt to configure the optical amplification devicefor bidirectional light transmission using the above opticalfiber amplifiers requires two optical circulators and twowavelength division multiplexers. This results in moreoptical parts in the optical amplification device, making thedevice larger.SUMMARY OF THE INVENTIONIt is an object of the present invention to provide anoptical fiber amplifier and an optical amplification devicefor use in biâdirectional light transmission using theoptical fiber amplifier. In particular, it is an object ofthe present invention to provide a simplyâstructured opticalfiber amplifier and, especially, an optical amplificationdevice for use in biâdirectional light transmission.The optical fiber amplifier according to the presentinvention comprises a pump light source outputting pump.......M..........â...w............ .. A .â...._..m..\...».......,,,.... .â10152025CA 02264421 1999-03-05light; and an optical amplification medium receiving the pumplight, amplifying received signal light, and outputtingamplified signal light. The signal light is input from, andthe amplified signal light is output from, one end of theoptical amplification medium. Between the other end of theoptical amplification medium and the pump light source isprovided an optical filter selectively transmitting the pumplight and selectively reflecting light with a wavelengthbandwidth equal to that of the signal light.The optical fiber amplifier according to the presentinvention comprises a pump light source outputting pump lightwith a first wavelength; an optical amplification mediumreceiving the pump light, amplifying signal light with asecond wavelength, and outputting amplified signal light; andan optical filter selectively transmitting light with thefirst wavelength and selectively reflecting the light withthe second wavelength, the optical filer being providedbetween the pump light source and the optical amplificationmedium. In addition, the optical fiber amplifier has opticalcoupling means for outputting the signal light received froma first optical I/O terminal to a second optical I/O terminaland for outputting the amplified signal light received fromthe second optical I/O terminal to a third optical I/Oterminal, the second optical I/O terminal being connected toone end of the optical amplification medium.The optical filter may be a fiber bragg grating or mayinclude a multiâlayer dielectric filter. The optical coupler ...-.....,.............,...,_...,.., . . ,..,_.,u___,,_,.,. ._,...,_.. .. . , , .,.,.,,,,â.,,,,_w,_, âN1O152025CA 02264421 1999-03-055may be an optical circulator or a directional opticalcoupler. The optical amplification medium is a rareâearthdoped optical fiber such as a erbium doped optical fiber or apraseodymium doped optical fiber.The optical fiber amplifier according to the presentinvention further comprises a pump light cut filterselectively removing the pump light, the pump light cutfilter being provided after the third optical I/O terminal orbetween the optical amplification medium and the secondoptical I/O terminal.In the conventional optical fiber amplifier, awavelength division multiplexer is provided before the erbiumdoped optical fiber to multiplex the pump light and thesignal light. On the other hand, in the optical fiberamplifier according to the present invention, the signallight is input into, and the amplified signal light is outputfrom, one end of the optical amplification medium. The pumplight source is at the other end. The optical filter,provided between the pump light source and the erbium dopedoptical fiber, transmits only the pump light and reflects thesignal light. This configuration eliminates the need for thewavelength division multiplexer which was required in theconventional configuration.An optical amplification device for biâdirectional lighttransmission using the optical fiber amplifier according tothe present invention basically uses the aboveâdescribedoptical fiber amplifier. To amplify the up signal light, the10152025CA 02264421 1999-03-056device has a first optical fiber amplifier comprising a firstpump light source outputting first pump light with a firstwavelength; a first optical amplification medium receivingthe first pump light, amplifying up signal light with asecond wavelength, and outputting amplified up signal light;and a first optical filter selectively transmitting lightwith the first wavelength and selectively reflecting thelight with the second wavelength, the first optical filerbeing provided between the first pump light source and thefirst optical amplification medium. Similarly, a secondoptical fiber amplifier comprises a second pump light sourceoutputting second pump light with a third wavelength; asecond optical amplification medium receiving the second pumplight, amplifying down signal light with a fourth wavelength,and outputting amplified down signal light; and a secondoptical filter selectively transmitting the light with thethird wavelength and selectively reflecting the light withthe fourth wavelength, the second optical filer beingprovided between the second pump light source and the secondoptical amplification medium.The first optical fiber amplifier and the second opticalfiber amplifier are connected by an optical coupler. Theoptical coupling means comprises a first optical I/O terminalreceiving the up signal light and outputting the amplifieddown signal light; a second optical I/O terminal outputtingthe up signal light and receiving the amplified up signallight, the second optical I/O terminal being connected to one10152025CA 02264421 1999-03-05end of the first optical amplification medium; a thirdoptical I/O terminal outputting the amplified up signal lightreceived from the second optical I/O terminal and receivingdown signal light; and a fourth optical I/O terminaloutputting the down signal light and receiving the amplifieddown signal light, the fourth optical I/O terminal beingconnected to one end of the second optical amplificationmedium.The first wavelength equals the third wavelength. Thatis, the wavelength of the up signal light may be equal tothat of the down signal light. In this case, instead ofusing the first pump light source and the second pump lightsource, the optical amplification device for bi-directionallight transmission using the optical fiber amplifieraccording to the present invention may use one pump lightsource outputting the pump light with a first wavelength.The device may use an optical branching circuit to branchthis pump light into a first pump light and a second pumplight.In the above configuration, the optical filter may be afiber grating. The optical coupler may be an opticalcirculator. The optical amplification medium is a rareâearthdoped optical fiber such as an erbium doped optical fiber.The optical amplification device for biâdirectionallight transmission using the optical fiber amplifieraccording to the present invention may further comprises afirst pump light cut filter selectively removing the first10152025CA 02264421 1999-03-05pump light, the first pump light cut filter being connectedto the third optical I/O terminal; and a second pump lightcut filter selectively removing the second pump light, thesecond pump light cut filter being connected to the firstoptical I/O terminal.The positions of the first pump light cut filter and thesecond pump light cut filter described above may be changed.That is, the first pump light cut filter may be providedbetween the first optical amplification medium and the secondoptical I/O terminal to selectively remove the first pumplight. And, the second pump light cut filter may beprovided between the second optical amplification medium andthe fourth optical I/O terminal to selectively remove thesecond pump light.The optical amplification device for bidirectional lighttransmission using the optical fiber amplifier according tothe present invention is designed basically around theoptical fiber amplifier described above. In addition, thedevice may use a fullyâcirculating type 4âterminal opticalcirculator. This configuration requires only one opticalcirculator instead of two. Combining this configuration withthe elimination of the wavelength division multiplexer makesthe optical amplification device for bidirectional lighttransmission smaller than the conventional one.BRIEF DESCRIPTION OF THE DRAWINGS,. ..s.....~................âi â. I , .. ,. .,.,...,. .. .10152025CA 02264421 1999-03-05The above and other objects, features and advantages ofthe present invention will be become more apparent from thefollowing detailed description when taken in conjunction withthe accompanying drawings wherein:Fig. 1 is a diagram showing an example of theconfiguration of a conventional optical fiber amplifier.Fig. 2 is a diagram showing the configuration of abidirectional light transmission device having theconventional optical fiber amplifier.Fig. 3 is a diagram showing the configuration of thefirst embodiment of the optical fiber amplifier according tothe present invention.Fig. 4 is a diagram showing the configuration of thesecond embodiment of the optical fiber amplifier according tothe present invention.Fig. 5 is a diagram showing the configuration of thethird embodiment of the optical fiber amplifier according tothe present invention.Fig. 6 is a diagram showing the configuration of thefirst embodiment of the bidirectional light transmissiondevice using the optical fiber amplifier according to thepresent invention.Fig. 7 is a diagram showing the configuration of thesecond embodiment of the bidirectional light transmissiondevice using the optical fiber amplifier according to thepresent invention. 10152025CA 02264421 1999-03-0510Fig. 8 is a diagram showing the configuration of thethird embodiment of the bidirectional light transmissiondevice using the optical fiber amplifier according to thepresent invention.Fig. 9 is a diagram showing the configuration of thefourth embodiment of the bidirectional light transmissiondevice using the optical fiber amplifier according to thepresent invention.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSBefore describing an optical amplification device forbidirectional light transmission according to the presentinvention, a conventional optical amplification device isdescribed to help understand the present invention.Fig. l is a diagram showing an example of theconfiguration of the conventional optical fiber amplifier.In a longâdistance optical fiber communication system,optical repeaters are used to amplify signal light attenuatedin transmission optical fibers. In many cases, optical fiberamplifiers, which use rare-earth doped optical fibers toamplify light, are used as optical repeaters.In the optical fiber amplifier, pump light and signallight must be in the rareâearth doped optical fiber. Thepump light is used to pump the optical fiber to amplify thesignal light.Therefore, the optical fiber amplifierrequires a wavelength division multiplexer (optical ._. ..........._....._..»...........__....... _..,-... ,. .. ..................................w.... . .10152025CA 02264421 1999-03-0511multiplexer) to multiplex the signal light and the pumplight.As shown in Fig. l, the conventional optical fiberamplifier has a pump light source 407 which outputs pumplight, a rareâearth doped optical fiber 405 which is anoptical amplification medium, and a wavelength divisionmultiplexer 404 which multiplexes the pump light and thesignal light and sends the multiplexed light to the rare-earth doped optical fiber. A typical wavelength divisionmultiplexer is an optical fiber fusion-spliced coupler or anoptical filter composed of multiâlayer dielectric films.The conventional optical fiber amplifier used in a one-directional light transmission device is not so complicated,while the conventional optical fiber amplifier used in abidirectional light transmission device is complicated andtherefore requires many optical parts.Fig. 2 is a diagram showing the configuration of abidirectional light transmission device having theconventional optical fiber amplifiers. The optical fiberamplifier amplifying the up signal light comprises the pumplight source 407, erbium doped optical fiber 405, andwavelength division multiplexer 403. Likewise, the opticalfiber amplifier amplifying the down signal light comprisesthe pump light source 408, erbium doped optical fiber 406,and wavelength division multiplexer 404. An optical filter410 and an optical filter 409 are provided for the up signaland the down signal, respectively, to cut the pump light.. .._--..m............................... .....,,... .n............._... . ,. 10152O25CA 02264421 1999-03-0512The bidirectional light transmission device sends andreceives both the up and down signal light via a singleoptical fiber. Therefore, the device must separate these twotypes of signal light, up and down, sent or received via theerbium doped optical fibers used by the up and down signaloptical fiber amplifiers. To do so, the device has twooptical circulators, 401 and 402, before and after the erbiumdoped optical fibers 405 and 406.The conventional optical fiber amplifier has twocomponents between the pump light source and the erbium dopedoptical fiber; one is an optical coupler coupling the pumplight source and the erbium doped optical fiber and the otheris the wavelength division multiplexer. They attenuate theoutput from the pump light source and reduce the power of thepump light sent to the rare-earth doped optical fiber,thereby reducing the gain of the signal light achieved by theerbium doped optical fiber.An attempt to configure the optical amplification devicefor bidirectional light transmission using the above opticalfiber amplifiers requires two optical circulators and twowavelength division multiplexers. This results in moreoptical parts in the optical amplification device, making thedevice larger.Next, the optical fiber amplifier and the opticalamplification device using the optical fiber amplifieraccording to the present invention will be described more indetail...,..w..M..._.............. . . u...........a...........w.......... ...\ ., .. â,....p.__,..10152025CA 02264421 1999-03-0513Fig. 3 is a diagram showing the configuration of thefirst embodiment of the optical fiber amplifier according tothe present invention. The optical fiber amplifier accordingto the present invention comprises a pump light source 107which outputs pump light, a fiber bragg grating 105 whichfunctions as an optical fiber selectively transmitting thepump light and selectively reflecting the signal light, anerbium doped optical fiber 103 which is an opticalamplification medium, and a 3-terminal optical circulator101.The signal light enters the optical fiber amplifierthrough an optical I/O terminal 1 of the 3-terminal opticalcirculator 101. The signal light is then output from anoptical I/O terminal 2 of the 3-terminal optical circulator101 for transmission to the erbium doped optical fiber 103.On the other hand, the pump light output from the pumplight source 107 is sent to the erbium doped optical fiber103 through the fiber bragg grating 105. This pump lightamplifies the signal light in the erbium doped optical fiber103. The signal light, reflected in the fiber bragg grating105, is sent back through to an optical I/O terminal 2 of the3-terminal optical circulator 101. The amplified signallight entered from the optical I/O terminal 2 is output fromthe optical I/O terminal 3.The optical fiber amplifier according to the presentinvention does not have the wavelength division multiplexerfor multiplexing the signal light and the pump light, as with....l.M...........u..,... . , 0 ,.,.....~\..l.....-.â......,.,... âV . . .. V 10152025CA 02264421 1999-03-0514the conventional optical fiber amplifier; instead, it hasthe optical circulator and the optical filter, such as thefiber bragg grating, which reflect and transmit only theselected wavelength light. This configuration allows thesignal light to be entered, and the amplified signal light tobe output, from the same end of the erbium doped opticalfiber.In this embodiment, the fiber bragg grating is used toselectively transmit the pump light and selectively reflectthe signal light. Instead of the fiber brag grating, anoptical filter such as the one using a multiâlayer dielectricfiler may be used. The erbium doped optical fiber is alsoused in this embodiment as a rare-earth doped optical fiberwhich functions as the light amplification medium. Insteadof the erbium doped optical fiber, some other rareâearthdoped optical fiber such as a praseodymium doped opticalfiber may be used. In addition, instead of the opticalcirculator, an optical directional coupler may be used.Figs. 4 and 5 are diagrams showing the configuration ofthe second and the third embodiments of the optical fiberamplifier according to the present invention. The opticalfiber amplifiers shown in those figures differ from thatshown in Fig. 3 in that a pump light cut filter 110 isinstalled after the erbium doped optical fiber lO3 to cut thepump light included in the amplified signal light.In the second embodiment shown in Fig. 4, the pump lightcut filter 110 is connected to the optical I/O terminal 3 of1O152025CA 02264421 1999-03-0515the 3âterminal optical circulator 101. In the thirdembodiment shown in Fig. 5, the pump light cut filter 110 isprovided between the optical I/O terminal 2 of the 3âterminaloptical circulator 101 and the erbium doped optical fiber103. Providing the pump light cut filter 110 after theerbium doped optical fiber 103 reduces noises included in theamplified signal light.Next, the bidirectional light transmission device usingthe optical fiber amplifier according to the presentinvention will be described.Fig. 6 is a diagram showing the configuration of thefirst embodiment of the bidirectional light transmissiondevice using the optical fiber amplifier according to thepresent invention. The optical fiber amplifier for the upsignal light comprises the pump light source 107, fiber bragggrating 105, and erbium doped optical fiber 103. Likewise,the optical fiber amplifier for the down signal lightcomprises a pump light source 108, fiber bragg grating 106,and erbium doped optical fiber 104.The aboveâdescribed optical fiber amplifiers use afullyâcirculating type 4âterminal optical circulator 102.The optical circulator used in the bidirectional lighttransmission device using the optical fiber amplifieraccording to the present invention is a fullyâcirculatingtype 4âterminal optical circulator. It has optical I/Oterminals 1 to 4.Light entered through one terminal isoutput to the next terminal. That is, light entered through 10152025CA 02264421 1999-03-0516optical I/O terminal 1 is output to the optical I/O terminal2, light entered through optical I/O terminal 2 is output tothe optical I/O terminal 3, light entered through optical I/Oterminal 3 is output to the optical I/O terminal 4, and lightentered through optical I/O terminal 4 is output to theoptical I/O terminal 1.The erbium doped optical fiber lO3 of the optical fiberamplifier for the up signal is connected to the optical I/Oterminal 2 of the fullyâcirculating type 4âterminal opticalcirculator lO2. The erbium doped optical fiber 104 of theoptical fiber amplifier for the down signal is connected tothe optical I/O terminal 4 of the fully-circulating type 4-terminal optical circulator 102. The up signal light enteredthrough the optical I/O terminal l is amplified as in theoneâdirectional operation shown in Fig. 3 and is output fromthe optical I/O terminal 3. On the other hand, the downsignal light entered through the optical I/O terminal 3 isamplified similarly and is output from the optical I/Oterminal 4.The optical fiber amplifier according to the presentinvention allows the signal light to be entered, and theamplified signal light to be output, from the same end ofthe erbium doped optical fiber. This enables one opticalcirculator to connect both optical fiber amplifiers, thusallowing one optical circulator to build a bidirectionallight transmission device. As compared with the conventionaloptical fiber amplifier requiring two optical circulators, ......,.........4M..........._... . ., . . . .. ,...m...«.......u.......m._.. . .. , . . , . ........... >\âlV ............ . ., .10152025CA 02264421 1999-03-0517the optical fiber amplifier according to the presentinvention is simpler in configuration.Fig. 7 is a diagram showing the configuration of thesecond embodiment of the bidirectional light transmissiondevice using the optical fiber amplifier according to thepresent invention.Basically, the bidirectional light transmission deviceof the second embodiment shown in Fig. 7 is similar to theone in the first embodiment shown in Fig. 6 except that onlyone pump light source lO7 is provided. This pump lightsource is used for both the up signal light and the downsignal light. The pump light output from the pump lightsource lO7 is branched into two by an optical branchingcircuit 109 and is sent to each erbium doped optical fiberthrough the fiber bragg grating 105 and the fiber bragggrating 106. This configuration is possible when thewavelength bandwidth of the up signal light is the same asthat of the down signal light and when the pump light has thesame wavelength.Next, other embodiments of the bidirectional lighttransmission device using the optical fiber amplifieraccording to the present invention will be described.Figs. 8 and 9 are diagrams showing the configuration ofthe third and fourth embodiments of the bidirectional lighttransmission device using the optical fiber amplifieraccording to the present invention.As with the opticalfiber amplifier according to the present invention shown in l0152025CA 02264421 1999-03-0518Fig. 4 or Fig. 5, a pump light cut filter 111 or 112 isprovided after the erbium doped optical fiber.In the third embodiment shown in Fig. 8, the pump lightcut filters 111 and 112 are connected to the optical I/Oterminal 3 and the optical I/O terminal 1 of the opticalcirculator, respectively. On the other hand, in the fourthembodiment shown in Fig. 9, the pump light cut filters 111and 112 are provided between the optical I/O terminal 2 ofthe optical circulator and the erbium doped optical fiber 103and between the optical I/O terminal 4 of the opticalcirculator and the erbium doped optical fiber 104,respectively. These pump light cut filters may be replacedby band pass filters provided that they cut light withdifferent wavelengths and transmit only light with awavelength approximately equal to that of the signal light.That is, they are required to cut not only the pump light butalso spontaneous emission light introduced by the lightamplification medium during light amplification. As aresult, they reduce noises in the output signal light.Referring now to Fig. 6 again, the following describeshow light is transmitted in the first embodiment of thebidirectional light transmission device using the opticalfiber amplifier according to the present invention.In this embodiment, light with a wavelength of 1550nm isused for the up signal light and light with a wavelength ofl560nm for the down signal light. The fiber bragg grating105 reflects 99% or more of light with a wavelength of.â...,,..¢ . .. ,. , . . ...w...r.......................-. ....,..10152025CA 02264421 1999-03-0519l550i2nm and transmits 99% or more of light with a wavelengthof 1470:20nm. The fiber bragg grating 106 reflects 99% ormore of light with a wavelength of 1560:2nm and transmits 99%or more of light with a wavelength of 1470i20nm. Light withthe center wavelength of 1470nm and with the output of 80mWis used for the pump light sources 107 and 108. The fully-circulating type 4âterminal optical circulator 102 has theinsertion loss of 1dB and the isolation of 40dB.Under the above condition, when the light output of theup signal light entered through the optical I/O terminal 1 ofthe fullyâcirculating type 4âterminal optical circulator 102was âl0dBm, the amplified signal light of +15dBm was outputfrom the optical I/O terminal 3 of the fullyâcirculating type4âterminal optical circulator 102. The same result wasobtained for the down signal light.As described above, the optical fiber amplifieraccording to the present invention has means for receivingthe signal light and outputting the amplified signal light atone end and, at the other end, the pump light source. Theoptical filter transmits only the pump light and reflects thesignal light back to the optical amplification medium. Thisconfiguration eliminates the need for the wavelength divisionmultiplexer required for the conventional configuration.The optical amplification device for bidirectional lighttransmission designed around the optical fiber amplifieraccording to the present invention, with the fully-circulating type 4âterminal optical circulator, requires only.»................u...... H. V .. . . ...,.,,,.1015CA 02264421 1999-03-0520one optical circulator instead of two. Combining thisconfiguration with the elimination of the wavelength divisionmultiplexer makes the optical amplification device forbidirectional light transmission smaller than theconventional one.While this invention has been described in connectionwith certain preferred embodiments, it is to be understoodthat the subject matter encompassed by way of this inventionis not to be limited to those specific embodiments. On thecontrary, it is intended for the subject matter of theinvention to include all alternative, modification, andequivalents as can be included within the spirit and scope ofthe following claims.
