Abstract:
A key requirement of a fiber optic communication system is its ability to transmit data from one location to another relatively free of errors in the data stream. The data stream error rate is a function of the error rate of the laser module utilized to transmit the data. A fast and efficient method of testing a laser module, in order to estimate its bit error rate, is to measure side mode suppression ratios of the laser module output while operating the laser module at each of a first and second bias setting, and to generate a test result for the laser module in accordance with the difference between the first and second side mode suppression ratio measurements. Furthermore, a system is provided for performing this laser module testing method.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a system and method for testing a laser module by measuring its side mode suppression ratio. 
     BACKGROUND OF THE INVENTION 
     A key requirement of a fiber optic communication system is its ability to transmit data from one location to another relatively free of errors in the data stream. The data stream error rate is a function of the error rate of the laser module utilized to transmit the data. For fiber optic communication systems where the goal is to transmit data over long distances at high bit rates, the typical acceptable error rate is on the order of one error in one trillion bits (or 1×10 −12 ). In the future, even lower error rates may be required. 
     The prior art has provided a method for measuring the actual error rate of a laser module by utilizing a bit error rate tester to compare data transmitted through the laser module with its output over a length of time. This method is time-consuming and requires dedication of expensive capital equipment for a long period of time for each laser module tested. In addition, the method must be repeated at several different temperatures in order to simulate actual use conditions. The testing process causes wear and tear to the laser module, which has a finite useful life. 
     Accordingly, there remains a need in the art for a rapid, efficient and easily-automated method for testing a laser module in order to determine whether its bit error rate falls within acceptable limits for the intended application of the laser module, and a system capable of performing this method. 
     SUMMARY OF THE INVENTION 
     The present invention addresses, inter alia, the foregoing needs by providing a relatively fast and efficient method of testing a laser module by measuring a first side mode suppression ratio of the laser module output while operating the laser module at a first bias setting, measuring a second side mode suppression ratio of the laser module output while operating the laser module at a second bias setting, and generating a test result for the laser module in accordance with the difference between the first and second side mode suppression ratio measurements. The present invention further provides a system for performing the method of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a preferred embodiment of the apparatus according to the present invention; 
     FIG. 2 is a flow chart depicting a method of the present invention; 
     FIG. 3 is a graph of the relationship between power and current supplied to a laser module; 
     FIG. 4 is a graph of the relationship between power and wavelength for an exemplary typical laser module; and 
     FIG. 5 is a graph of the relationship between bit error rates and ΔSMSR. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The structure and function of the preferred embodiments can best be understood by reference to the drawings. Where the same reference numerals appear in multiple figures, the numerals refer to the same or corresponding structure in those figures. 
     As shown in FIG. 1, in a laser module testing system  100 , a laser module  120  is capable of being mounted to and operated by evaluation board  110 . Laser module  120  may include a distributed feedback (DFB) laser with a threshold current level. Evaluation board  110  supplies power to laser module  120 , including AC and DC current. Data source  115  supplies a modulated electrical data stream signal to evaluation board  110 . Data source  115  can be, for example, a commercially available bit error rate transmitter (BERT) or a low cost circuit (typically mounted on a printed circuit board) specifically designed for such purpose. Data source  115  should be capable of delivering data stream signals that are similar to the data streams that will be encountered by laser module  120  in actual field use. Evaluation board  110  transmits the electrical data stream signal from data source  115  to laser module  120 . Laser module  120  converts the electrical signal into an optical signal output. Output from the laser module is directed to a testing device  140  capable of measuring optical output of the laser module with respect to wavelength. In the preferred embodiment, testing device  140  is an optical spectrum analyzer (OSA). Output from laser module  120  is directed to testing device  140  through a length of optical fiber  130 . Evaluation board  110  and testing device  140  are coupled to CPU  150 , user interface  160 , and memory  171 . Memory  171  may include high speed random access memory and non-volatile memory such as disk storage. Memory  171  stores, inter alia, operating system  172 , file system  174 , test result data  176 , test control program  180 , evaluation board control module  182 , OSA control module  184  and test data evaluation module  188 . More generally, memory  171  stores control software for execution by the CPU  150  of computer  170 , where the control software is capable of evaluating the laser module  120  by controlling function of the laser module  120 , testing device  140  and evaluation board  110  in order to perform all steps of the method of the present invention. 
     FIG. 2 shows a method  200  of the present invention. During step  202 , laser module  120  is set at a first bias setting, including associated AC and DC current level settings. Further, data is provided to and transmitted by the laser module during at least the measurement steps  204 ,  208  of the method. Preferably, the data transmission rate of the laser module during the testing is similar to the data rate to be used during normal operation of the laser diode, e.g., a data rate in the range of 1 Gb/s to 10 Gb/s. 
     During step  204 , a first side mode suppression ratio is measured from output from laser module  120  while the laser module is operated at the first bias setting. During step  206 , laser module  120  is set at a second bias setting, including associated AC and DC current level settings. One may predetermine which of the first or second bias settings will comprise a lower DC current level voltage than the other bias setting. In the preferred embodiment, the first bias setting includes the DC and AC currents suitable for field operation of laser module  120 , and the second bias setting includes a DC current level that is approximately 5 mA lower than the first bias setting. More generally, the difference between the DC current of the first and second settings is less than 15 mA, and preferably is 5 mA or less. During step  208 , a second side mode suppression ratio is measured from output from laser module  120  while the laser module is operated at the second bias setting. During step  210 , a difference between the first and second side mode suppression ratio measurements is determined, and then in step  212  a test result is generated for laser module  120  in accordance with that difference. In accordance with the test result, one may evaluate the suitability of laser module  120  for such uses as long haul data communication through optical fiber. 
     In a preferred embodiment, the first and second side mode suppression ratios are computed using logarithmic (e.g., decibel) units, for example: 
     
       
           SMSR =10 Log 10 (Peak1/Peak2) 
       
     
     (where Peak1 and Peak2 represent the highest and second highest optical power peaks as a function of wavelength). As a result, the difference between the first and second side mode suppression ratios, ΔSMSR, represents a percentage change in the side mode suppression ratio between the first and second bias settings, as opposed to an absolute change in the side mode suppression ratio. In other embodiments, absolute measurement units, or other suitable units may be used to represent the first and second side mode suppression ratios and the difference therebetween. 
     In the preferred embodiment, evaluation board  110  supplies both a data stream received from data source  115  and AC and DC current to laser module  120  in accordance with instructions from control software executed by computer  170 . Further, in the preferred embodiment, testing device  140  measures the side mode suppression ratios at each of the first and second bias settings and generates the test result in accordance with the difference between the first and second side mode suppression ratio measurements. 
     In alternate embodiments of the present invention, the division of tasks performed during method  200  may be different from that described above. For example, some of the tasks performed by the testing device  140  in the preferred embodiment may be performed by the computer  170  of the evaluation board  110 , and some or all of the tasks performed by the computer  170  in the preferred embodiment may be performed by a processor on the evaluation board  110  or by another device. In one particular alternate embodiment of the present invention, computer  170  is coupled to evaluation board  110  and device  140 , and utilizes control software to determine the first and second side mode suppression ratios and to evaluate the laser module. It does this by sending first control signals to evaluation board  110  to operate laser module  120  at a first bias setting, receiving first information from testing device  140  associated with operation of laser module  120  at the first bias setting and using the first information to determine a first side mode suppression ratio of the laser module output while operating the laser module at the first bias setting. The computer  170  continues evaluating the laser module by sending second control signals to evaluation board  110  to operate laser module  120  at a second bias setting, receiving second information from testing device  140  associated with operation of the laser module at the second bias setting and using the second information to determine a second side mode suppression ratio of the laser module output while operating the laser module at the second bias setting. 
     In the alternate embodiment being described, the computer  170  generates a test result for laser module  120  in accordance with a difference between the first and second side mode suppression ratio measurements. In this alternate embodiment, the testing device  140  generates the first information and second information in the form of optical power output measurements at multiple wavelengths. The computer  170  uses these measurements to determine highest and second highest peaks of the output optical power as a function of wavelength. From the highest and second highest peaks in the first information the computer computes the first side mode suppression ratio, and from the highest and second highest peaks in the second information the computer computes the second side mode suppression ratio. 
     Referring to FIG. 3, graph  300  shows a relationship between power and current supplied to laser module  120  during steps  202 ,  204 ,  206  and  208 , which can be used to depict the key features of the levels of AC and DC current supplied to laser module  120  in conjunction with first and second bias settings. Levels of AC and DC current supplied to laser module  120  at a first bias setting can be determined by increasing a DC current supplied to laser module  120  until a desired optical power output is achieved (as represented by DC operating point  302 ), and then varying an AC current supplied to the laser module until a desired extinction ratio is achieved. At the first bias setting, the laser module has an operating range represented by operating points  302  and  304  in FIG. 3. A second bias setting can be determined by increasing or decreasing a DC current supplied to laser module  120  ( 306 ) while maintaining an AC current to the laser module that is substantially equal to the AC current level of the first bias setting. At the second bias setting, the laser module has an operating range represented by operating points  306  and  308 . The first or second bias setting having the lower DC current level causes laser module  120  to operate at a range of current levels that is at least partially below the laser module&#39;s threshold current level. This “below threshold” mode of operation is represented by point  308  in FIG.  3 . 
     Referring to FIG. 4, graph  400  shows a relationship between wavelength and optical output power for laser module  120  while the laser module is operated at either a first or second bias setting. Wavelength peaks generated by plotting wavelength vs. power are used to measure a side mode suppression ratio of laser module  120  at each of the first and second bias settings. In the preferred embodiment, the data contained in graph  400  is generated by testing device  140  based on output from laser module  120  received through optical fiber  130 . A side mode suppression ratio measurement for the laser module operating at a given bias setting is measured by computing the ratio of the power values at the highest wavelength peak ( 402 ) and second highest wavelength peak ( 404 ). If the power values are measured in decibel units, as is the case in the preferred embodiment, this ratio is computed by subtracting the second highest power value from the highest power value. In the preferred embodiment, testing device  140  performs this calculation. In alternate embodiments, a side mode suppression ratio can be calculated by computer  170  or manually by a user, using power vs. wavelength data generated by testing device  140 . 
     Referring to FIG. 5, graph  500  shows the relationship between actual bit error rates measured for several laser modules  120  and ΔSMSR (the difference between first and second side mode suppression ratio measurements) for those laser modules in accordance with the method of the present invention. Graph  500  demonstrates that a ΔSMSR measurement below approximately ten decibels (10 db) is indicative of an actual bit error rate on the order of 1×10 −13  for the laser module. In the preferred embodiment, a ΔSMSR measurement of less than five decibels (5 db) is considered sufficiently reliable to deem a laser module suitable for such uses as long haul data communication through optical fiber at the full data rate for which the laser module has been designed, for which the typical acceptable error rate is on the order of one error in one trillion bits (or 1×10 −12 ). Laser modules whose ΔSMSR is above 5 db may not, in some circumstances, be considered suitable for such uses, and are labeled for use at lower data transmission rates (e.g., 1 Gb/s instead of 2.5 Gb/s, or 2.5 Gb/s instead of 5 Gb/s) than the laser modules whose ΔSMSR is below 5 db, and/or for shorter transmission lengths than the laser modules whose ΔSMSR is below 5 db. In alternate embodiments, a ΔSMSR other than 5 db may be used as the threshold to determine the laser&#39;s suitability for a particular use. 
     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.