Method and apparatus for making wavelength adjustments in a wavelength division multiplex system

Abstract: Method and apparatus for making wavelength and power adjustments to a laser in a wavelength division multiplex system includes an extender card containing fiber optic transmission devices and path attenuators is inserted in the system in place of a card containing the laser to be adjusted. The extender card includes switches to put the laser under adjustment in known states for making power adjustments and wavelength adjustments without affecting system operation. After adjustments have been made, the extender card is removed and the laser card is reinstalled in the system.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to information communication systems, and more particularly to information communication systems employing laser transmitters and receivers in a wavelength division multiplex system. 2. PRIOR ART The wavelength has to be adjusted for a good performance of a wavelength division multiplex/demultiplex fiber optic communications systems. Disadvantageously only the optical input signal power of each laser transmitter can be adjusted in the system disclosed in US-patent application with the number US-A-5 225 922. There a controller controls the power of the transmitter signal in dependence on the quality of the signal at a receiver by amplifying or attenuating the signal directly behind each transmitter. In a wavelength division multiplex/demultiplex fiber optic communications link, the wavelength of each laser must be adjusted to correspond with that of one of a number of multiplexor grating peaks without interrupting the multiplexor functioning of other lasers that are properly tuned, each to its own grating peak. In the past, wavelength adjustment of lasers in a wavelength division multiplex system was mostly accomplished by measuring the power output at the transmitting output of the link, disabling all but the laser to be adjusted and tuning it for maximum power output, thus disrupting the entire link. One example of wavelength division multiplex system with wavelength adjustment of lasers is disclosed in UK patent application with the number GB-A-2 170 370. There the bias of a transmitter, which wavelength will be adjusted, is modulated with a frequency of an additional oscillator. Thus, the output signal of the transmitter is modulated with this frequency for being identifiable by a detector at the output signal of a multiplexor. A controlled controls the wavelength of the transmitter in dependence on the signal of the detector by regulating the temperature of the transmitter with a Peltier device. But this system needs disadvantageously a lot of devices e.g. an additional oscillator, an electric switch, a detector, a controller and many Peltier devices, which are all fixed means of the system. It is important for efficient and nondisruptive operation of the WDM system that the wavelength of any laser in the system be adjusted without disrupting the entire communications link. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to present a better method and system for adjusting wavelength and power output of one of a number of lasers in a wavelength division multiplex (WDM) communications link without disrupting operation of other laser transmitters in the WDM system. Accordingly, method and apparatus for making wavelength and power adjustments to a laser in a wavelength division multiplex system includes an extender card containing fiber optic transmission devices and path attenuators is inserted in the system in place of a card containing the laser to be adjusted. The extender card includes switches to put the laser under adjustment in known states for making power adjustments and wavelength adjustments without affecting system operation. After adjustments have been made, the extender card is removed and the laser card is reinstalled in the system. It is an advantage of the present invention that wavelength and power of individual lasers, each mounted on a different card in a wavelength division multiplex system can be readily adjusted without system disruption. It is another advantage of the present invention that power and wavelength of a laser can be readily adjusted while the system continues in operation. The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. BRIEF DESCRIPTION OF THE DRAWING For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a block diagram of a wavelength division multiplex system in accordance with the present invention. FIGURE 2 is a schematic diagram of a card containing a laser connected to a multiplexer in accordance with the present invention. FIGURE 3 is a schematic diagram of the card of FIGURE 2 mounted on an extender card for adjusting power and wavelength of the laser contained on the card in accordance with the present invention. FIGURE 4 is a schematic diagram of the extender card of FIGURE 3 in accordance with the present invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In a wavelength division multiplex system, it may be required to make adjustments to one component of the system while the remaining system components continue to operate without interruption. As an example, laser wavelength and power output of one laser in the system may be adjusted while other lasers in the system continue to transmit data. Adjustments of power output and wavelength of a laser must be made without disconnecting the common fiber link interconnecting elements of the system. Referring now to FIGURE 1, a wavelength division multiplex system (WDM) for transmitting information across a fiber optic link will be described. Referring now to FIGURE 1, WDM system 100 includes a number of laser transmitters 102, 104, 106 and 108, each connected by a separate fiber optic link to optical multiplexor 110 which multiplexes each of the transmitted optical signals to a single fiber optic link 112 using wavelength division multiplex techniques as are well-known in the art. Optical multiplexor 110 may be implemented by any one of several well-known WDM devices. At the receiving end, the common fiber link 112 is connected to demultiplexer 114 which employs a wavelength division demultiplex on the signals present on common fiber link 112 and provides separate signals to optical receivers 116, 118, 120 and 122, respectively, depending on the wavelength of the received signals. For efficient operation, and for ease of adjustment, each of the laser transmitters 102, 104, 106 and 108 are mounted on a separate card 132, 134, 136 and 138, respectively, installed in system 100. Referring now to FIGURE 2, the normal installation of card 132 in system 100 will be described. Card 132 carrying laser transmitter 102 is installed in back plane 140 of system 100 and connected to multiplexor 110 during normal operation. Referring now to FIGURE 3, a preferred embodiment of the present invention will be described in greater detail. Card 132 on which laser transmitter 102 is mounted is removed from back plane 140 of system 100 and is mated with extender card 300 which is inserted in back plane 140 at the position from which card 132 was removed. Laser transmitter 102 then provides an output through connectors 308 on cards 300 to mutliplexor 110 as before. However, with extender card 300 in place in the system, the power and wavelength of laser transmitter 102 can be adjusted without interruption of other laser transmitters 104, 106 and 108 in the information communication system 100. Referring now to FIGURE 4, extender card 300 will be described in greater detail. A primary component of extender card 300 is control circuit 302 which includes mode select switch 320 for controlling the state of operation of laser transmitter 102 between an operational state and a test state. Connectors 308 are used for inserting an optical attenuator which provides an adjustable attenuation in the optical path between laser transmitter 102 and multiplexor 110. Laser transmitter card 132 is mated with extender card along edge 306 which includes electrical connectors 312, electrical and optical connectors 310, and fiber optic connectors 308. Extender card 300 is connected to back plane 140 along edge 304. Edge 304 includes connectors 314 which carries both electrical and optical signals and connector 316. A second set of optical connectors (not shown) may also be inserted in the path of fiber link 327 for test purposes. The optical output from laser transmitter 102 is connected through connector 310 on extender card 300 to fiber link 322 to connector 308 which connects to an external variable attenuator. The output of the variable attenuator is connected on fiber link 324 to connector 314 which transmits the attenuated optical signal to multiplexor 110. Mode select logic 302 allows a laser transmitter 102 to be controlled by coax line 325 in response to control signals received by mode select logic 302 from backplane 140 on coax lines 326. Alternatively, the laser transmitter 102 can be controlled by logic 1 or 0 levels generated by mode select logic 302. The mode selection as between normal and test operation is controlled by mode select switch 320 on extender card 300. Mode select switch 320 also includes a second switch which controls the binary levels of mode select logic 302 when in the test mode. Using feedback from the open fiber detection circuitry of the optical link, the wavelength of laser transmitter 102 is adjusted to optimum. The optical attenuator connected to connector 308 is adjusted until overall optical path loss is just sufficient for the optical receiver at the other end of the optical link to signal by a reverse channel that it has detected the light transmitted. The center of the range over which the returned signal indicates adequate received light is a proper adjustment for the laser transmitter 102. After power and wavelength adjustments have been made for laser transmitter 102, extender card 300 is removed from back plane 140 and laser transmitter card 132 is reinstalled in back plane 140 as in FIGURE 2 and normal operation is continued without disruption of other elements of the system.

Claims: A wavelength division multiplex optical information transmission system (100), comprising: a plurality of transmitters (102, 104, 106, 108), each operating at a different predetermined wavelength; a multiplexor (100) connected to said plurality of transmitters (102, 104, 106, 108) for multiplexing outputs of each of said transmitters to a common link employing a predetermined multiplexing technique; a transmission link (112) connecting said multiplexor (110) to a demultiplexor (114) for transmitting information; a demultiplexor (114) connected to said transmission link (112) for demultiplexing signals transmitted over said transmission link (112) employing a predetermined demultiplexing technique; a plurality of detectors (116, 118, 120, 122), each having one of a plurality of predetermined detection characteristics for recovering information transmitted from said multiplexor (110) to said demultiplexor(114); and characterized by comprising an adjustment means (300), which is temporarily interposable between one of said transmitters (102, 104, 106, 108) and said multiplexor (110) for adjusting characteristics of said transmitter (102) without disconnecting said transmission link (112), whereby said adjustment means (300) is removable, after said characteristics of said one of said transmitters (102, 104, 106, 108) have been adjusted. System (100) according to Claim 1 wherein each of said plurality of transmitters (102, 104, 106, 108) is a laser transmitter, and each of said plurality of detectors (116, 118, 120, 122) is an optical detector and said transmission link (112) is a fiber optic transmission link. System (100) according to Claim 1 wherein said multiplexor (110) is an optical multiplexor and said predetermined multiplexing technique is a wavelength division multiplex technique. System (100) according to Claim 3 wherein said predetermined demultiplexing technique is a wavelength division demultiplexing technique. System (100) according to Claim 1 wherein said adjustment means (300) further comprises: means (302, 320) for controlling a mode of operation of one of said plurality of transmitters (102, 104,106, 108); and means (302, 308, 314, 324) for controlling path attenuation between said one transmitter (102) and said demultiplexor (114) for facilitating power output and wavelength output of said one transmitter (102). System (100) according to Claim 5 wherein said adjustment means (300) is mounted on an extender device interposed between said one transmitter (102) and said multiplexor (110). A test device (300) for facilitating power and wavelength output adjustment of a transmitter (102, 104, 106, 108) in a wavelength division multiplex system (100) having a plurality of transmitters (102, 104, 106, 108) multiplexed onto a common transmission medium (112), comprising: a first connector (312) for connecting said test device (300) to one or more control inputs of said transmitter (102); a second connector (310) for connecting said control device (302) to one or more outputs of said transmitter (102); a variable attenuation device connected to said second connector (310) and to a test output connector (314) connected to an input of said multiplexor (110); and control means (302, 320) for controlling a mode of operation of said one transmitter (102); and characterized by said test device (300) being arranged to be temporarily interposable between one of said transmitters (102, 104, 106, 108) and said multiplexor (110) for adjusting characteristics of said transmitter (102), without disconnecting said transmission link (112) in said wavelength division multiplex system (100), and removable, after said characteristics of said one of said transmitters (102, 104, 106, 108) have been adjusted. A method for adjusting one or more output characteristics of one of a plurality of transmitters (102, 104, 106, 108) operating in a wavelength division multiplexed system (100) without disrupting operation of other transmitters (104, 106, 108) in said system (100), comprising the steps of: a) controlling the mode of operation of said one transmitter (102) to a test mode; b) inserting attenuation in a path between an output of said one transmitter (102) and said multiplexor (110) to provide a minimum detectable signal at a receiver (116, 118, 120, 122); c) adjusting a first operational characteristic of said transmitter (102) to an optimum value based on system feedback; and characterized by the steps of: d) interposing (300) a test device between said one transmitter (102) and a multiplexor (110) before step a) for adjusting characteristics of said transmitters (102) in said system (100); and e) removing said test device (300) from the system (100) to allow normal operation of said one transmitter (102) after step c). A method according to Claim 8 where each of said plurality of transmitters (102, 104, 106, 108) are laser transmitters, each operating at a different predetermined wavelength. A method according to Claim 8 wherein said multiplexor (110) is an optical multiplexor employing a wavelength division multiplex technique to multiplex a plurality of laser output signals on a common transmission medium (112). A method according to Claim 8 wherein said common transmission medium (112) is a fiber optic link connected between said multiplexor (110) and a demultiplexor (114) at a receiving end of said system (100). A method according to Claim 9 further comprising the step of controlling a variable attenuator on said test device (300).

Inventors: JANNIELLO FRANK JAMES, NEUNER RICHARD ANDREW, JANNIELLO, FRANK JAMES, NEUNER, RICHARD ANDREW
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1996
Main classification: H04B10
IPC codes: H04J14, H04B10