Abstract:
A testing method of a semiconductor laser emitting a wavelength under a test different from a reference wavelength in a given wavelength range includes: a first step of obtaining a length of an optical fiber under the test satisfying a reference dispersion condition at the wavelength under the test, based on the reference dispersion condition for the test and a unit dispersion amount of the optical fiber; and a second step of inputting a modulation signal that is a modulated laser light of the semiconductor laser having a wavelength as the wavelength under the test into an optical fiber having substantially the same length as the length obtained in the first step and evaluating an output of the optical fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-064863, filed on Mar. 17, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to a testing method of a semiconductor laser and a laser testing device. 
         [0004]    (ii) Related Art 
         [0005]      FIG. 1  illustrates a block diagram of a laser testing device for measuring a bit error rate that is one of transmission property tests of a modulation signal. As illustrated in  FIG. 1 , a laser device  101  is coupled to a light-receiving device  103  through an optical fiber  102  and a variable attenuator  107 . 
         [0006]    A controller  104  includes a pattern generator  105  and an evaluation unit  106 . The pattern generator  105  inputs a given test data pattern into the laser device  101 . Then, the laser device  101  outputs an optical signal according to the test data pattern. The optical signal gets to the light-receiving device  103  through an optical fiber  102  having a given length. The evaluation unit  106  evaluates data of the optical signal obtained through a through line in synchronization with the output data of the pattern generator  105  with use of an electrical signal from the light-receiving device  103 . And, the controller  104  controls attenuation amount of the variable attenuator  107 . 
         [0007]    The laser testing device evaluates degradation (bit error rate) of an optical signal getting to the light-receiving device  103  with respect to the changing of the attenuation amount of the variable attenuator  107 , and determines the laser device  101  as defective or non-defective. For example, Japanese Patent Application Publication No. 1-204491 discloses the above-mentioned laser testing device. 
       SUMMARY 
       [0008]    In general, the optical fiber  102  having a given transmission dispersion amount is provided when a bit error rate or an eye pattern of a laid optical fiber is tested. A variety of laser light having different wavelength around a reference wavelength is used in a wavelength multiplexing communication such as WDM (Wavelength Division Multiplexing). 
         [0009]    However, the same optical fiber  102  is used with respect to any type of laser device in the bit error rate test or in the eye pattern test. Therefore, the dispersion amount of the optical fiber is not considered. 
         [0010]    For example, dispersion amount of laser light having passed through the optical fiber  102  is enlarged when the laser device outputs a wavelength longer than a reference wavelength of a dispersion amount of the optical fiber  102 . Therefore, the bit error rate or the eye pattern is evaluated with use of a standard harder than a predetermined transmission dispersion amount when the laser device outputs a wavelength longer than the reference wavelength of the dispersion amount of the optical fiber  102 . Therefore, a non-defective product may be determined as “defective”. 
         [0011]    The dispersion amount of laser light having passed through the optical fiber  102  is reduced when the laser device outputs a wavelength shorter than a reference wavelength of the dispersion amount of the optical fiber  102 . Therefore, the bit error rate or the eye pattern is evaluated with use of a standard lighter than a predetermined transmission dispersion amount when the laser device outputs a wavelength shorter than the reference wavelength of the dispersion amount of the optical fiber  102 . Therefore, a defective product may be determined as “non-defective”. 
         [0012]    It is an object of the present invention to provide a laser testing method and a laser testing device capable of evaluating data transmission property accurately. 
         [0013]    According to an aspect of the present invention, there is provided a testing method of a semiconductor laser emitting a wavelength under a test different from a reference wavelength in a given wavelength range, including: a first step of obtaining a length of an optical fiber under the test satisfying a reference dispersion condition at the wavelength under the test, based on the reference dispersion condition for the test and a unit dispersion amount of the optical fiber; and a second step of inputting a modulation signal that is a modulated laser light of the semiconductor laser having a wavelength as the wavelength under the test into an optical fiber having substantially the same length as the length obtained in the first step and evaluating an output of the optical fiber. 
         [0014]    According to an aspect of the present invention, there is provided a laser testing device including: an optical fiber unit that selects a length of an optical fiber based on information relating to a wavelength of laser light under a test; and an evaluation unit that receives modulated laser light and evaluates an optical signal from the optical fiber with respect to the optical fiber having the length selected by the optical fiber unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates a block diagram of a laser testing device for measuring a bit error rate that is one of transmission property tests of a modulation signal; 
           [0016]      FIG. 2  illustrates a block diagram of a laser testing device in accordance with a first embodiment; 
           [0017]      FIG. 3  illustrates an example of a flow chart executed by the controller; 
           [0018]      FIGS. 4A and 4B  illustrate a changing of dispersion amount according to a wavelength; and 
           [0019]      FIG. 5  illustrates another example of an optical switch and a sub optical fiber. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0020]      FIG. 2  illustrates a block diagram of a laser testing device in accordance with a first embodiment. As illustrated in  FIG. 2 , the laser testing device includes a laser device  1 , a light-receiving device  3 , a controller  4 , a variable attenuator  7  and an optical fiber unit  100 . 
         [0021]    The laser device  1  is a modulable laser device. The laser device  1  is coupled to the light-receiving device  3  through optical switches  2   a  and  2   b , a through line  10   a , the optical fiber unit  100  and the variable attenuator  7 . 
         [0022]    The optical fiber unit  100  includes a main optical fiber  10 , optical switches  8   a  and  8   b , and a plurality of sub optical fibers  11   a  to  11   e . The main optical fiber  10  has, for example, shortest fiber length according to a desirable supposed dispersion, and is coupled between the laser device  1  and the optical switch  8   a . The sub optical fibers  11   a  to  11   e  are coupled between the optical switch  8   a  and the optical switch  8   b . The optical switch  8   b  is coupled to the variable attenuator  7 . The optical switches  8   a  and  8   b  select one of the sub optical fibers  11   a  to  11   e . Thus, an optical fiber length from the laser device  1  to the variable attenuator  7  is selected. The sub optical fiber  11   e  is a through line and has a short fiber length. In  FIG. 2 , five kinds of sub optical fibers are provided. However, more than one sub optical fiber has only to be provided. 
         [0023]    The variable attenuator  7  is an attenuator adjusting attenuation amount of a light from the optical fiber unit  100 . The light-receiving device  3  includes a photo diode and so on, and receives a light having passed through the variable attenuator  7 . A wavelength detector  9  detects a wavelength output from the laser device  1 . The controller  4  includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory) and so on. The CPU executes a given program and generates a pattern generator  5  and an evaluation unit  6  in the controller  4 . 
         [0024]    Next, a description will be given of an operation of the laser testing device in accordance with the embodiment.  FIG. 3  illustrates an example of a flow chart executed by the controller  4 . A description will be given of the operation of the laser testing device with reference to  FIG. 2  and  FIG. 3 . As illustrated in  FIG. 3 , the pattern generator  5  inputs a given testing data pattern into the laser device  1 . Thus, the laser device  101  outputs an optical signal according to the testing data pattern. At the moment, the optical switch  2   a  couples an optical path to the wavelength detector  9 . An output wavelength of the laser device  1  is controlled so that a desirable wavelength (a wavelength under test) is obtained (Step S 1 ). 
         [0025]    Here, information relating to the wavelength of the laser device  1  is not limited. For example, the information may be a measured value of the output wavelength of the laser device  1  controlled in Step S 1 , or information relating to an output wavelength of the laser device  1  obtained in advance. 
         [0026]    Next, the controller  4  controls the optical switches  2   a  and  2   b  so that the light-receiving device  3  receives the output of the laser device  1  through the through line  10   a  and the variable attenuator  7 . The testing data pattern of Step S 1  is continuously fed into the laser device  1 . An electrical signal generated in the light-receiving device  3  is fed into the evaluation unit  6 . The evaluation unit  6  evaluates the electrical signal from the light-receiving device  3  in synchronization with the output data of the pattern generator  5 . Every attenuation amount given by the variable attenuator  7  is evaluated. Thus, error information including degradation of the optical signal getting to the light-receiving device  3  with each attenuation amount is obtained (Step S 2 ). 
         [0027]    Next, the controller  4  selects a length of an optical fiber under test, based on the information relating to the output wavelength of the laser device  1  in Step S 1  (Step S 3 ). Here, a dispersion amount condition of an optical fiber affecting the laser light is provided in an evaluation of transmission property of a modulation signal. It is preferable that the dispersion condition is satisfied in order to restrain an influence on a transmission waveform. And so, the controller  4  selects the length of the optical fiber so that the dispersion condition of the optical fiber is satisfied in Step S 3 . 
         [0028]    A description will be given of details of selection of the optical fiber length in a C-band range (Conventional-band: 1530 nm to 1565 nm). The controller  4  determines a coefficient of the following Equation (1) based on a difference between a reference wavelength 1550 nm of a unit dispersion amount of an optical fiber and a wavelength of the laser light under test when selecting the optical fiber length. 
         [0000]      Optical fiber length (km)=dispersion capacity of an optical fiber (ps/nm)/unit dispersion amount with respect to an optical fiber length (ps/(nm·km)  [Equation 1]
 
         [0029]    Generally, an optical fiber to which zero dispersion amount is set has a unit dispersion amount D(λ) of optical fiber at a wavelength λ. Therefore, the optical fiber under transmission test of a modulation signal has only to have the optical fiber length obtained with Equation (1). 
         [0030]    The dispersion amount increases at a longer wavelength compared to a reference wavelength and decreases at a shorter wavelength compared to the reference wavelength in the optical fiber to which the zero dispersion amount is set. In this case, as illustrated in  FIGS. 4A and 4B , the transmission waveform is changed. And so, the controller  4  shortens the optical fiber length compared to a set length at longer wavelength compared to the reference wavelength so that the dispersion amount condition of the optical fiber in the transmission test of the modulation signal. 
         [0031]    For example, in accordance with Equation (1), the unit dispersion amount with respect to a length of an optical fiber is 16.7 ps/(nm-km) and the length of the optical fiber is 95 km when a wavelength of laser light under test is 1560 nm longer than the reference wavelength, in a case where dispersion tolerance of the optical fiber is 1600 ps/nm (International Standard regarding optical communication: ITU-G (ITU: International Telecommunication Union)). 
         [0032]    The length of the optical fiber is long compared to a set length, if the wavelength of the laser light is shorter than the reference wavelength. For example, the length of the optical fiber is set to be 110 km, if the wavelength of the laser light under test is 1520 nm shorter than the reference wavelength. It is possible to select the length of the optical fiber in S-band range (Short-hand: 1460 nm to 1530 nm) or L-band range (Long-band: 1565 nm to 1625 nm) as well as the above-mentioned C-band range. 
         [0033]    The calculation method of a length of an optical fiber is changed according to a relationship between an optical fiber condition under modulation transmission test and wavelength of laser light under test. For example, a description will be given of a case where modulation transmission of laser light having wavelength around 1300 nm in an optical fiber having zero dispersion amount at 1300 nm wavelength is tested. In this case, it is necessary to consider that the dispersion amount is zero at 1300 nm wavelength, if the modulation transmission of the laser light around 1300 nm is tested in the optical fiber in which zero dispersion amount is set at 1300 nm. In this case, the dispersion amount increases when the wavelength gets away from 1300 nm toward both longer side and shorter side. Therefore, a coefficient for shortening the length of the optical fiber is selected in the selection even if the wavelength gets away from the center wavelength (1300 nm) toward both longer side and shorter side. The unit dispersion amount when the reference wavelength is 1550 nm is 16.7 ps/(nm·km) in the optical fiber in which zero dispersion amount is set at 1300 nm. 
         [0034]    Next, the controller  4  controls the optical switches  2   a  and  2   b  so as to select the length of the optical fiber selected in Step S 3  (Step S 4 ). Next, the controller  4  controls the optical switches  2   a  and  2   b  so that the light-receiving device  3  receives the output of the laser device  1  through the optical fiber unit  100  and the variable attenuator  7 . The laser device  1  continuously receives the testing data pattern of Step S 1 . The evaluation unit  6  receives an electrical signal generated in the light-receiving device  3 . The evaluation unit  6  obtains error information in which degradation (bit error) of the optical signal fed into the light-receiving device  3  is collected per attenuation amount (Step S 5 ). 
         [0035]    Next, the evaluation unit  6  compares the error information obtained in Step S 2  (error having passed through the through line  10   a ) and the error information obtained in Step S 5  (error information having passed through the optical fiber unit  100 ). And the evaluation unit  6  determines the laser device  1  as non-defective or defective (Step S 6 ). After that, the controller  4  completes the flowchart. 
         [0036]    In accordance with the embodiment, a length of an optical fiber having adequate dispersion amount is selected according to a wavelength of laser light under test. This allows adequate evaluation of a transmission waveform. Therefore, data transmission property is evaluated adequately. The evaluation of the data transmission property may be what is called eye pattern evaluation as well as the bit error evaluation of the embodiment. 
         [0037]    In the optical fiber unit  100  illustrated in  FIG. 2 , the optical switches  8   a  and  8   b  select one of the sub optical fibers  11   a  to  11   d  as a pathway. However, the selection of the pathway is not limited. For example, the optical switches  8   a  and  8   b  may select the pathway by combining a plurality of the sub optical fibers.  FIG. 5  illustrates another example of an optical switch and a sub optical fiber. 
         [0038]    As illustrated in  FIG. 5 , each of the sub optical fibers  11  is an optical fiber having different length. The optical switches  8   a  and  8   b  are capable of selecting an optical fiber having desirable length by combining at least one of the sub optical fibers  11 . A return fiber  14  connects an input/output ports of the switched  8   a  and  8   b . The return fiber  14  and the optical switches  8   a  and  8   b  may couple a plurality of the sub optical fibers  11  each other. Thus, an optical fiber having arbitrary length can be selected. The optical switches  8   a  and  8   b  may let an optical amplifier  12  in the optical fiber in which a plurality of the sub optical fibers  11  are combined, when the optical fiber has a length larger than a predetermined length. The optical amplifier  12  amplifies attenuated optical power caused long distance transmission. The optical switches  8   a  and  8   b  may let a wavelength filter  13  in the optical fiber. The wavelength filter  13  removes noise-level light. 
         [0039]    The present invention is not limited to the specifically disclosed embodiments and variations but may include other embodiments and variations without departing from the scope of the present invention.