JP2009224639A - Test method and test device of semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a test method of mode synchronous semiconductor laser, for easily evaluating whether a mode synchronous operation is normal or abnormal and also for facilitating automation. <P>SOLUTION: A value of a photocurrent generated in a saturable absorption region is measured with an ampere meter 113 while a value of drive current supplied to a gain region is changed by a drive circuit 111, for acquiring a relationship between a drive current and a photocurrent. Then, a drive current value at an abrupt photocurrent increase is detected. Further, if a drive current value actually used is larger than the drive current value corresponding to kink, the semiconductor laser is determined to perform a mode synchronous operation. Otherwise, it is determined not to perform the mode synchronous operation. Since soundness/defect of the semiconductor laser is determined using kink, its determining operation is simple and automation is easy. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a test method and apparatus for a mode-locked laser diode (MLLD) that generates an optical pulse train by modulating light excited in a gain region in a saturable absorption region. The present invention can be applied to a test of a mode-locked semiconductor laser for high-speed optical communication, for example.

  In a relay device or the like of an optical communication network, received signal light is converted into an electric signal, signal processing is performed by an electric circuit, and the processed electric signal is converted into an optical signal again and transmitted. In such an apparatus, the signal processing speed is limited to the processing speed of the electric circuit, and is about 40 Gbps at the maximum. For this reason, in order to realize further high-speed processing, a technique for performing signal processing with an optical signal is desired. In order to realize optical signal processing, it is necessary to use an optical pulse train having a very high repetition frequency. A mode-locked semiconductor laser is known as means for generating a high-frequency optical pulse train.

  The mode-synchronized semiconductor laser performs a CW (Continuous Wave) operation or a mode-synchronized operation depending on driving conditions. The CW operation is an operation that generates light having a constant peak intensity, that is, CW light. On the other hand, the mode synchronization operation is an operation for generating an optical pulse train by changing the light intensity of CW light at a predetermined time period. In the present application, 'mode-locked semiconductor laser' refers to a semiconductor laser obtained by adding a mechanism for changing the light intensity of the CW light at a predetermined time period to a mechanism for generating CW light.

  FIG. 18 is a schematic diagram conceptually showing one structural example of a resonator provided in a mode-locked semiconductor laser.

  In the resonator 100 of FIG. 18, the gain region 101 performs optical excitation when a DC forward current Iop is injected. The light generated by this light excitation is reflected back and forth within the resonator 100 by being reflected by a diffraction grating (not shown) in the reflection surface 105 and the distributed Bragg reflection region 103. The excitation light is amplified every time it passes through the gain region 101. However, the gain region 101 does not perform amplification when the gain is saturated.

  Saturable absorption region 102 absorbs part of the generated light when application of DC reverse bias voltage Vre is started. The absorption coefficient at this time decreases as the incident light intensity increases. The light absorbed in the saturable absorption region 102 becomes the photocurrent Imod.

  The distributed Bragg reflection region 103 extracts a desired frequency band component from the generated light using a diffraction grating (not shown). By changing the applied voltage Vb to change the refractive index of the distributed Bragg reflection region 103, the optical distance between the diffraction gratings can be changed, and thereby the extracted frequency band can be adjusted.

  The phase adjustment region 104 is a region for adjusting the total optical distance between both end faces 105 and 106 of the resonator 100 so as to be an integral multiple of the generated light wavelength. By changing the applied voltage Vp, the refractive index (and hence the optical distance) of the phase adjustment region 104 can be adjusted. When the optical distance of the resonator is an integral multiple of the desired wavelength, only the light of the desired wavelength is amplified and the light of other wavelengths is attenuated as the generated light reciprocates in the resonator.

  In such a configuration, when the generated light repeats reciprocation in the resonator 100, the amplification factor by the gain region 101 and the absorption factor by the saturable absorption region 102 periodically vary. Since these fluctuation phases do not coincide with each other, the generated light has a higher amplification factor than the absorptance and a higher absorption factor than the amplification factor, and forms a pulse waveform. When the light intensity of the pulsed light exceeds a predetermined value, it is output from both end faces 105 and 106.

  Conventionally, an aging test has been performed to evaluate the reliability and the like of a mode-locked semiconductor laser. An aging test is a test in which a mode-locked semiconductor laser is operated continuously for a long period to evaluate the occurrence of a failure. The aging test is performed in a process of selecting an element that stably operates from mass-produced mode-locked semiconductor laser elements, and a process of confirming whether the selected element actually operates stably. The time required for the aging test is, for example, 5000 hours per test. In the aging test, it is evaluated whether or not sufficient laser light intensity can be maintained, and whether or not normal mode-locking operation can be maintained.

  As a method for evaluating the laser light intensity, for example, an APC (Auto Power Control) method and an ACC (Auto Current Control) method are known.

  The APC method is a method in which the mode-locked semiconductor laser is continuously operated while automatically controlling the forward current Iop (see FIG. 18) in the gain region 101 so that the laser light intensity becomes constant. In this APC method, it is determined that a failure has occurred when the value of the forward current Iop exceeds a predetermined value. An aging test method using the APC method is disclosed in Patent Document 1 below, for example. In the test method of Patent Document 1, the laser light intensity is determined by measuring the photocurrent Imod (see FIG. 18) (see, for example, paragraph 0012 of Patent Document 1). According to this method, it is possible to perform more accurate detection than when the intensity of the laser output light is directly detected by the photodetector, and therefore it is possible to automatically control the intensity of the laser light with high accuracy.

  The ACC method is a method of measuring the laser light intensity by continuously operating the mode-locked semiconductor laser with the forward current Iop being a fixed value. In the ACC method, it is determined that a failure has occurred when the laser light intensity falls below a predetermined value.

  On the other hand, as a method for evaluating normality / abnormality of the mode synchronization operation, a method of observing an autocorrelation waveform, an optical spectrum, and an RF (Radio Frequency) spectrum of an output optical pulse train is known.

  FIG. 19A is a waveform diagram showing an example of an autocorrelation waveform of an optical pulse train, where the horizontal axis represents time [picoseconds] and the vertical axis represents the intensity of the autocorrelation function [arbitrary unit]. As shown in FIG. 19A, since the relationship between the time and intensity of laser light output can be observed from the autocorrelation waveform, the presence or absence of pulse oscillation can be directly evaluated.

  FIG. 19B is a waveform diagram illustrating an example of an optical spectrum of an optical pulse train, where the horizontal axis represents wavelength [nm] and the vertical axis represents light intensity [arbitrary unit]. As shown in FIG. 19B, when the mode synchronization operation is performed, each wavelength component has a certain phase relationship. The envelope α as shown in the figure is obtained from each wavelength component. On the other hand, in the CW operation, the phase relationship between the wavelength components is not constant. The optical spectrum in FIG. 19B is also called a longitudinal mode spectrum. The longitudinal mode spectrum corresponds to a Fourier transform of the time waveform of the optical pulse train (that is, the autocorrelation waveform in FIG. 19A). Accordingly, the half width W2 of the envelope α shown in FIG. 19B varies depending on the reciprocal of the half width W1 of the optical pulse (see FIG. 19A). Therefore, the product of the half widths W1 × W2 is a constant. This product W1 × W2 is referred to as a time zone width product. If the time waveform matches a Gaussian function, the time zone width product is 0.44. Therefore, by obtaining this time zone width product, a deviation from the theoretical value of each optical pulse constituting the optical pulse train (see FIG. 19A) can be evaluated.

FIG. 19C is a waveform diagram showing an example of an RF spectrum of an optical pulse train, where the horizontal axis represents frequency [GHz] and the vertical axis represents intensity (dB) [arbitrary unit]. As shown in FIG. 19C, when a normal mode synchronization operation is performed, a peak β is generated at every predetermined frequency f (here, f = 40 GHz) under a certain condition. The frequency f of the peak β is obtained by f = c / 2nL (n is the refractive index and L is the resonator length). On the other hand, when normal mode cycle operation is not performed, a peak does not occur at the frequency f under the above conditions, and a number of peaks may occur at every predetermined pulsation frequency. Such an abnormal peak is called pulsation. When pulsation occurs, there is a disadvantage that amplitude modulation occurs in the optical pulse train and a stationary optical pulse train cannot be obtained.
JP-A-10-253700

  As described above, regarding the laser light intensity, highly accurate evaluation can be automatically performed by using the method of Patent Document 1.

  On the other hand, in the test for evaluating the normality / abnormality of the optical pulse train, an operator needs to observe the autocorrelation waveform, the optical spectrum, and the RF spectrum each time.

  As described above, the aging test is continuously performed for a long time of, for example, about 5000 hours. For example, the evaluation process is performed 20 times at 15-minute intervals, then 20 times at 1-hour intervals, and further 248 times at 20-hour intervals. The reason for the short evaluation interval at the beginning of the test is to detect an initial failure. The higher the frequency of evaluation, the greater the workload for performing this evaluation.

  In the normal aging test, a number of mode-locked semiconductor laser elements are tested simultaneously for statistical reasons. The number of elements simultaneously used in the actual aging test is, for example, 25. The greater the number of elements to be tested simultaneously, the greater the workload for performing this evaluation.

  For the reasons described above, a test for evaluating normal / abnormal mode-synchronous operation has required a great human burden.

  An object of the present invention is to provide a test method and a test apparatus that can easily evaluate whether a mode-locked semiconductor laser is good or bad and that can be easily automated.

  (1) In the first invention, a gain region for generating and amplifying light when a drive current is supplied, and a light intensity output from the gain region when a reverse bias voltage is applied can be modulated. The present invention relates to a method for testing a semiconductor laser having a resonator with a saturated absorption region.

  Then, a measurement step for obtaining the relationship between the drive current and the photocurrent by measuring the value of the photocurrent generated in the saturable absorption region while changing the value of the drive current, and a kink of the relationship obtained in the measurement step And a pass / fail determination step for determining whether or not the resonator performs mode-locking operation within a predetermined drive current value range based on the kink detection result.

  (2) The second invention is capable of modulating the intensity of light output and generated from the gain region when a reverse bias voltage is applied, and a gain region that generates and amplifies light when a drive current is supplied. The present invention relates to a test apparatus for testing a semiconductor laser having a resonator having a saturated absorption region.

  Then, a drive circuit that changes the value of the drive current, an ammeter that measures the value of the photocurrent generated in the saturable absorption region, and an ammeter that measures the photocurrent while changing the value of the drive current in the drive circuit Thus, the relationship between the drive current and the photocurrent is acquired, the kink of the relationship is detected, and whether or not the resonator performs the mode synchronization operation within a predetermined drive current value range based on the kink detection result. A control circuit for determination.

  In the present invention, since switching between the CW optical oscillation operation and the mode synchronization operation is detected by a kink generated in the relationship between the drive current and the photocurrent, normality / abnormality of the mode synchronization operation can be easily evaluated. Labor saving and automation of the test become easy.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the size, shape, and arrangement relationship of each component are shown only schematically to the extent that the present invention can be understood, and the numerical conditions described below are merely examples. .

<First Embodiment>
A method for testing a mode-locked semiconductor laser according to the first embodiment of the present invention will be described with reference to FIGS.

  First, the principle that is the basis of the test method according to this embodiment will be described.

  FIG. 1 is a conceptual diagram showing the configuration of the measuring apparatus used in this embodiment. In FIG. 1, the components given the same reference numerals as those in FIG. 18 are the same as those in FIG.

  As shown in FIG. 1, the measuring apparatus 110 includes a drive circuit 111, a reverse bias power source 112, an ammeter 113, a photodiode 114, and a control circuit 115.

  The drive circuit 111 supplies a drive current Iop to the gain region 101 and supplies control voltages Vb and Vp to the distributed Bragg reflection region 103 and the phase adjustment region 104. As described above, the drive current Iop is a DC forward current. The voltages Vb and Vp are fixed voltages determined according to the set wavelength of the mode-locked semiconductor laser.

  The reverse bias power source 112 applies a DC reverse bias voltage Vre to the saturable absorption region 102. The reverse bias voltage Vre is a fixed voltage.

  The ammeter 113 measures the photocurrent Imod. The value of the photocurrent Imod is sent to the control circuit 115.

The photodiode 114 receives the laser beam output from the resonator end face 106 of the distributed Bragg reflection region 103 and outputs a current _IL having a value corresponding to the laser beam intensity. The value of the current I _L is sent to the control circuit 115.

  The control circuit 115 acquires the measurement results Imod and IL of the ammeter 113 and the photodiode 114 while changing the drive current Iop supplied from the drive circuit 111 to the gain region 101.

  Next, the measurement result of the measuring apparatus 110 will be described with reference to FIGS.

  FIG. 2 is a graph showing the results of measurement by the measurement device 110. In FIG. 2, the horizontal axis represents the drive current Iop [mA], and the vertical axis represents the light intensity [mW], the photocurrent [mA], and the drive voltage [V]. In FIG. 2, a curve a1 indicates the relationship between the drive current Iop and the drive voltage Vop. A curve b1 shows the relationship between the drive current Iop and the output current IL of the photodiode 114. A curve c1 indicates the relationship between the drive current Iop and the photocurrent Imod.

  As can be seen from FIG. 2, a kink occurs in the curve c1 indicating the relationship between the drive current Iop and the photocurrent Imod near the drive current Iop = 75 mA. On the other hand, no singular point is generated in the curves a1 and b1. In this embodiment, the kink indicates a portion where the change in the photocurrent Imod rapidly increases with respect to the change in the drive current Iop.

  3 to 6 show the characteristics of the output laser beam of the mode-locked semiconductor laser used in this embodiment. (A) is an autocorrelation waveform, (B) is an optical spectrum, and (C) is RF spectrum. 3 shows a case where the drive current Iop is 50 mA, FIG. 4 shows a case where the drive current Iop is 75 mA, FIG. 5 shows a case where the drive current Iop is 80 mA, and FIG. 6 shows a case where the drive current Iop is 100 mA.

  As can be seen from the autocorrelation waveforms in FIGS. 3A and 4A, when the drive current Iop is 50 mA and 75 mA (that is, when the drive current Iop is smaller than the kink occurrence point), the laser output intensity is time. Are substantially constant and do not form pulses. Further, as can be seen from the optical spectra of FIGS. 3B and 4B, the half-value widths W3 and W4 are very wide as compared to the mode-locking operation (see FIG. 19B). Furthermore, as can be seen from FIG. 3C and FIG. 4C, pulsation occurs at every fixed frequency. In this manner, it was confirmed that the mode-locked semiconductor laser was performing the CW operation when the drive current Iop was smaller than the kink generation current.

  On the other hand, as can be seen from the autocorrelation waveforms of FIGS. 5A and 6A, when the drive current Iop is 80 mA or 100 mA (that is, when the drive current Iop is larger than the kink occurrence point), The laser output intensity changes with time to form a pulse. Further, as can be seen from the optical spectra of FIGS. 5B and 6B, the half-value width of the output light is very narrow. Further, as can be seen from FIG. 5C and FIG. 6C, an intensity peak exists only at 40 GHz, and no pulsation occurs. Thus, it was confirmed that the mode-locked semiconductor laser was performing the mode-locking operation when the drive current Iop was larger than the kink generation current.

  As can be seen from the above description, whether or not the mode-locked semiconductor laser is performing the mode-locking operation can be determined from the kink of the curve indicating the relationship between the drive current Iop and the photocurrent Imod without observing the autocorrelation waveform or the like. Can be judged. This is presumed to be due to the following reasons.

  As can be seen from FIGS. 5A and 6A, when the mode-locking operation is performed, the semiconductor laser periodically repeats the output and blocking of the laser beam. Here, the laser beam is output from the end faces 105 and 106 when the intensity is equal to or higher than a predetermined value, and is blocked when the intensity is lower than the predetermined value. Therefore, when the laser beam is interrupted, the light absorption rate by the saturable absorption region 102 is very high, and the photocurrent Imod increases accordingly. On the other hand, when the laser beam is output, the photocurrent Imod is small because the light absorption rate by the saturable absorption region 102 is low.

  Here, during the CW operation, the laser beam is always maintained in the output state. Therefore, the average value of the photocurrent Imod is considered to be small.

  On the other hand, at the time of the mode synchronization operation, the laser beam is repeatedly output / cut off. Therefore, it is considered that the average value of the photocurrent Imod increases as the photocurrent Imod periodically increases.

  As a result, when the laser output operation is switched from the CW operation to the mode synchronization operation, it is considered that the average value of the photocurrent Imod is rapidly increased and a kink is generated.

A method for testing a mode-locked semiconductor laser using this principle will be described below. The test according to this embodiment can be performed using, for example, an apparatus as shown in FIG. However, the photocurrent I _L and the driving voltage measurement Vop is not required and therefore it is not necessary to provide a photodiode 114.

  First, as described above, the control circuit 115 obtains the relationship between the currents Iop and Imod by measuring the value of the photocurrent Imod while changing the value of the drive current Iop (see FIG. 2).

  Next, the control circuit 115 detects the kink of the relationship obtained in the measurement step. In this embodiment, as shown in FIG. 2, it is determined that a portion where the photocurrent Imod increases rapidly with respect to the increase in the drive current Iop is a kink. In order to determine whether or not it is a sudden increase, for example, the increase rate of the photocurrent Imod with respect to Iop may be compared with a predetermined threshold value. This threshold value may be obtained experimentally according to the type of the mode-locked semiconductor laser.

  Subsequently, based on the kink detection result, the control circuit 115 determines whether or not the resonator performs a mode-locking operation with a driving current value when the mode-locking semiconductor laser is actually driven. For this purpose, a setting range of the drive current value in actual use may be determined, and it may be determined whether or not the set range is larger than the drive current value corresponding to the kink. That is, when the entire setting range of the drive current value actually used is included on the current value side larger than the kink, it is determined that the mode-locking operation is normally performed, and a current value in which part or all of the setting range is smaller than the kink If it is included, it may be determined that the mode synchronization is not normally performed.

  As described above, according to this embodiment, it is possible to determine whether or not the semiconductor laser is operating in mode synchronization without observing the autocorrelation waveform, the optical spectrum, and the RF spectrum. Therefore, according to this embodiment, the normal / abnormal evaluation of the mode synchronization operation can be easily performed, and further automation is facilitated.

<Second Embodiment>
Next, a method for testing a mode-locked semiconductor laser according to the second embodiment of the present invention will be described with reference to FIGS.

  First, the principle that is the basis of the test method according to this embodiment will be described.

FIG. 7A and FIG. 8A are graphs showing the results of measurement by the measurement device 110. Note that the measurements as shown in FIGS. 7A and 8A can be performed using the same test apparatus (see FIG. 1) as in the first embodiment. 7A and 8A, the horizontal axis represents the drive current Iop [mA], and the vertical axis represents the light intensity [mW], the photocurrent Imod [mA], and the drive voltage Vop [V]. In FIGS. 7A and 8A, curves a2 and a3 indicate the relationship between the drive current Iop and the drive voltage Vop. Curve b2, b3 includes a driving current Iop, it shows the relationship between the output current I _L of the photodiode 114. Curves c2 and c3 show the relationship between the drive current Iop and the photocurrent Imod.

  FIGS. 7B and 8B show the relationship between the drive current Iop and a value ΔI−Imod obtained by differentiating the photocurrent Imod with the drive current Iop. 7B and 8B, the horizontal axis represents the drive current value Iop [mA], and the vertical axis represents the value [no unit] obtained by differentiating the photocurrent Imod with the drive current Iop.

In the first embodiment described above, it is determined that the semiconductor laser has switched from the CW operation to the mode-synchronized operation when the photocurrent Imod increases rapidly. On the other hand, in the examples of FIGS. 7A and 8A, a sudden increase in the photocurrent Imod is not detected. Instead, the rapid decrease in the photocurrent Imod and the photocurrent I _L A sudden increase was detected. At this time, no singular point is generated in the change of the drive voltage (see curves a1 and a2). A sudden increase in the photocurrent I _L is considered to be due to mode hopping. Mode hopping is a phenomenon in which the output light of a semiconductor laser is suddenly shifted from one longitudinal mode to another mode. According to the results of the study by the present inventor, when mode hopping occurs, a sudden increase in photocurrent Imod is not detected despite the switching from CW operation to mode synchronization operation. There is a case where the current Imod suddenly decreases (see FIG. 7A). Furthermore, even when the switching from the CW operation to the mode synchronization operation does not occur, the photocurrent Imod may decrease rapidly with the occurrence of mode hopping (see FIG. 8A). Whether or not mode hopping occurs and whether or not switching at the time of CW operation / mode synchronization is performed at the same time as mode hopping is considered to be caused by differences in element structures, manufacturing variations, and the like.

  9 and 10 show the output laser light characteristics of the mode-locked semiconductor laser device used in the measurement of FIG. 7. FIG. 9 shows the case where the drive current Iop is 115 mA, and FIG. 10 shows the case where the drive current Iop is 117 mA. It is. 9 and 10, (A) is an autocorrelation waveform, (B) is an optical spectrum, and (C) is an RF spectrum.

As can be seen from FIG. 9, in the mode-locked semiconductor laser device used in the measurement of FIG. 7, when the driving current Iop is 115 mA, the autocorrelation waveform and the optical spectrum are close to the mode-locking operation, but the RF spectrum Then pulsation was observed. Note that the mode synchronization frequency is 40 GHz as in the first embodiment. On the other hand, as can be seen from FIG. 10, when the driving current Iop was 117 mA, the autocorrelation waveform, the optical spectrum, and the RF spectrum showed normal mode-locking operation. Therefore, in the mode-locked semiconductor laser device corresponding to FIG. 7, while the driving current Iop of 117mA from 115mA, i.e. near the occurrence of a rapid increase in rapid decline and photocurrent I _L photocurrent Imod, CW It can be seen that the transition from the operation to the mode synchronization operation is performed.

  FIGS. 11 to 14 show output laser light characteristics of the mode-locked semiconductor laser device used in the measurement of FIG. 8, FIG. 11 shows the case where the drive current Iop is 50 mA, and FIG. 12 shows the case where the drive current Iop is 60 mA. 13 shows a case where the drive current Iop is 65 mA, and FIG. 14 shows a case where the drive current Iop is 100 mA. 11 to 14, (A) is an autocorrelation waveform, (B) is an optical spectrum, and (C) is an RF spectrum.

As can be seen from FIGS. 11 to 14, in the mode-locked semiconductor laser device corresponding to FIG. 8, the autocorrelation waveform, the optical spectrum, and the RF spectrum have the same characteristics as those in FIG. 19 in the entire driving current Iop of 50 to 100 mA. Therefore, it showed normal mode synchronization operation. From this, it can be seen that the mode-locked semiconductor laser device corresponding to FIG. 8 operates in the mode-locked manner in the entire drive current Iop. That is, in the example of FIG. 8, in the vicinity of the occurrence of a rapid increase in rapid decline and photocurrent I _L photocurrent Imod, the transition from CW operation to mode-locking operation is not performed. However, the center frequency of the optical spectrum is shifted to the high frequency side while the drive current Iop is shifted from 60 mA to 65 mA. This shift is likely due to mode hopping.

  As described above, since mode hopping has occurred, the transition from the CW operation to the mode synchronization operation may not be detected by the method that only detects the kink of the photocurrent Imod (see the first embodiment). In contrast, in this embodiment, the CW operation / mode synchronization operation is determined using the relationship between the drive current Iop and the differential value ΔI-Imod (see FIGS. 7B and 8B). . As described above, the differential value ΔI−Imod is a value obtained by differentiating the photocurrent Imod with the drive current Iop.

  As can be seen from FIG. 7B, the variation range of the differential value ΔI-Imod is very large during the CW operation, but the variation range is small during the mode synchronization operation. Further, in the example of FIG. 8B, that is, when the mode synchronization operation is always performed, the fluctuation range of the differential value ΔI-Imod is always a small value. Therefore, by comparing the fluctuation range of the differential value ΔI-Imod with a predetermined width, it is possible to determine whether the semiconductor laser element is operating in CW mode or mode-locking. The predetermined width that serves as a criterion for discrimination differs depending on the design conditions of the mode-locked semiconductor laser.

A method for testing a mode-locked semiconductor laser using this principle will be described below. The test according to this embodiment can be performed using, for example, an apparatus as shown in FIG. However, the photocurrent I _L and the driving voltage measurement Vop is not required and therefore it is not necessary to provide a photodiode 114. In addition, the control circuit 115 is provided with an arithmetic function for obtaining a differential value ΔI−Imod.

  First, as described above, the control circuit 115 acquires the relationship between the currents Iop and Imod by measuring the value of the photocurrent Imod while changing the value of the drive current Iop (FIG. 7A). 8 (A)).

  Next, the control circuit 115 detects a kink in the relationship between the currents Iop and Imod. In this embodiment, as shown in FIG. 7A and FIG. 8A, when the photocurrent Imod rapidly decreases with respect to the increase in the drive current Iop, it is determined that it is a kink. In order to detect a sudden decrease, the increase rate of the photocurrent Imod with respect to Iop may be compared with a predetermined threshold value. This threshold value may be obtained experimentally according to the type and design value of the mode-locked semiconductor laser.

  When the kink is detected, the control circuit 115 determines whether or not the resonator performs the mode synchronization operation within the range of the drive current value allowed when the mode synchronization semiconductor laser is actually driven. When the kink is 'abrupt decrease', the control circuit 115 divides the driving current Iop into a region where the driving current Iop is smaller than that of the kink and a region where the driving current Iop is larger than the kink, It is determined whether or not the semiconductor laser is mode-locked in the region. Then, the control circuit 115 determines whether or not the actually used drive current value corresponds to the mode synchronous operation region. Note that the determination method when the kink is ‘abrupt increase’ may be the same method as in this embodiment or the same method as in the first embodiment described above.

  As described above, according to this embodiment, it is possible to determine whether or not the semiconductor laser is operating in mode synchronization without observing the autocorrelation waveform, the optical spectrum, and the RF spectrum. Therefore, according to this embodiment, the normal / abnormal evaluation of the mode synchronization operation can be easily performed, and further automation is facilitated.

<Third Embodiment>
Next, a method for testing a mode-locked semiconductor laser according to the third embodiment of the present invention will be described with reference to FIGS.

  In this embodiment, a case will be described in which an aging test relating to mode-locking operation is performed in parallel with an aging test of laser light intensity and oscillation threshold.

  The test according to this embodiment can be performed using, for example, an apparatus as shown in FIG. However, the control circuit 115 has a function for performing an aging test of the laser light intensity and the oscillation threshold, a calculation function for obtaining the differential value ΔI-Imod and the secondary differential value Δ (ΔI-Imod), and the like. Provided.

  As described above, the APC method and the ACC method are known as methods for evaluating the laser beam intensity. In this embodiment, the ACC method, that is, a method of measuring the laser light intensity while continuously operating the mode-locked semiconductor laser with the driving current Iop fixed. This is because in the case of a mode-locked semiconductor laser, the repetition period of laser output changes when the driving conditions are changed, and it seems difficult to perform an evaluation test close to actual use. As the fixed value of the drive current Iop, for example, a current value when the semiconductor laser is actually used is employed.

  15 to 17 are diagrams for explaining the test method according to this embodiment. FIG. 15 is a flowchart, and FIGS. 16A to 16C and FIGS. 17A to 17C are graphs. is there.

In FIGS. 16A and 17A, the horizontal axis represents the drive current Iop [mA], and the vertical axis represents the laser light intensity P [mW] and the photocurrent Imod [mA]. In FIG. 16B and FIG. 17B, the horizontal axis is the drive current Iop [mA], and the vertical axis is the differential value ΔI−Imod [no unit] of the photocurrent Imod. In FIG. 16C and FIG. 17C, the horizontal axis represents the drive current Iop [mA], and the vertical axis represents the secondary differential value Δ (ΔI−Imod) [mA ⁻¹ ] of the photocurrent Imod.

  First, the semiconductor laser for evaluation is carried into a constant temperature bath, and the temperature inside the bath is raised (see step S1).

When the inside of the tank reaches the set temperature, the control circuit 115 starts supplying the drive current Iop to the semiconductor laser. Further, voltages Vre, Vb, and Vp are applied to the saturable absorption region 102, the distributed Bragg reflection region 103, and the phase adjustment region 104. Thereby, the laser output of the semiconductor laser is started. Then, the control circuit 115, the drive current Iop in a state set to a fixed value described above, by measuring the output current I _L of the photodiode 114, and obtains the initial value P0 of the laser beam intensity. At the same time, the control circuit 115 acquires the initial value Imod0 of the photocurrent. Subsequently, as in the second embodiment, the control circuit 115 scans the drive current Iop and measures the relationship between the drive current Iop and the laser beam intensity P (FIG. 7A and FIG. 8A). ) (See curves b2 and b3), and obtain an initial value Ith0 of the oscillation threshold current (that is, the minimum value of the drive current Iop necessary for the semiconductor laser to emit light) from this measured value (see step S2). .

  Thereafter, the control circuit 115 repeatedly performs the following evaluation process (see steps S3 to S11).

  In step S3, the control circuit 115 stands by until the next evaluation process is performed while performing laser output with the drive current Iop set to the above-described fixed value until the next evaluation process is performed. The time schedule of the evaluation process may be the same as the conventional one, for example, 20 times at 15 minute intervals first, then 20 times at 1 hour intervals, and then 248 times at 20 hour intervals.

When it is time to evaluate processing, the control circuit 115, first, the obtaining a laser beam intensity P from the current I _L of the photodiode 114, for measuring the photocurrent Imod (see step S4).

  Next, the control circuit 115 calculates a change amount ΔP (= P0−P) of the laser light intensity P, and further obtains a change rate (that is, 100 × ΔP / P0 [%]) with respect to the initial value P0. If the rate of change is outside the predetermined range, the control circuit 115 determines that the semiconductor laser has failed and ends the test (see step S5). The allowable range of change rate is, for example, ± 10%, but may be arbitrarily determined according to the type of semiconductor laser.

On the other hand, when the rate of change of the laser beam intensity P is within the predetermined range, the control circuit 115 subsequently scans the drive current Iop, and the relationship between the drive current Iop and the laser beam intensity P (FIG. 16A, Measure the curves b4 and b5 in FIG. As described above, the laser beam intensity P is the current I _L, it is possible to know. At the same time, the control circuit 115 also measures the relationship between the drive current Iop and the photocurrent Imod (see curves c4 and c5 in FIGS. 16A and 17A) (see step S6).

  Subsequently, the control circuit 115 determines the threshold current Ith of the semiconductor laser from the relationship between the drive current Iop and the laser beam intensity P. Next, the control circuit 115 calculates a change amount ΔIth (= Ith0−Ith) of the threshold current, and further obtains a change rate (that is, 100 × ΔIth / Ith0 [%]) with respect to the initial value Ith0. If the rate of change is outside the predetermined range, the control circuit 115 determines that the semiconductor laser has failed and ends the test (see step S7). The allowable range of change rate is, for example, ± 10%, but may be arbitrarily determined according to the type of semiconductor laser.

  On the other hand, when the rate of change of the threshold current Ith is within the predetermined range, the control circuit 115 then obtains the change amount ΔImod (= Imod0−Imod) using the photocurrent Imod measured in step S4. Then, the control circuit 115 obtains a change rate (that is, 100 × ΔImod / Imod0 [%]) with respect to the initial value Imod0.

  If the rate of change of the photocurrent Imod is within the predetermined range, the control circuit 115 finishes the evaluation process for that time, returns to step S3, and waits for the next evaluation process (see step S8). The allowable range of the change rate is, for example, ± 10%, but may be arbitrarily determined according to the type of semiconductor laser.

  On the other hand, if the rate of change is greater than + 10%, the control circuit 115 determines whether or not the semiconductor laser is operating in a mode-locked manner as follows (see steps S9 to S11). In this determination process, the relationship between the drive current Iop and the photocurrent Imod measured in step S6 described above is used.

  First, the measurement result indicating the relationship between the drive current Iop and the photocurrent Imod is differentiated by the drive current Iop, and the relationship between the drive current Iop and the differential result ΔI-Imod (FIG. 16B, FIG. 17B). Curve d4, d5). Then, the control circuit 115 detects the drive current Iop2 when the kink occurs (see step S9).

  Then, the control circuit 115 detects the fluctuation range of the differential result ΔI-Imod in the sections R1 and R2 shown in FIGS. 16B and 17B (see step S10). In this embodiment, in consideration of measurement errors and the like, the section R1 is set to a section of −5 to −35 mA with respect to the value of the drive current value Iop2, and the section R2 is set to be based on the value of the drive current value Iop2. And a section of +5 to +35 mA. Furthermore, in this embodiment, in order to detect the fluctuation range of the differentiation result ΔI-Imod, the relationship between the driving current Iop and the differentiation result ΔImod is further differentiated by the driving current Iop. Then, the relationship between the drive current Iop and the second derivative result Δ (ΔI−Imod) is obtained. As shown in FIGS. 16C and 17C, the secondary differential result Δ (ΔI−Imod) fluctuates around the zero coordinate. The control circuit 115 calculates the absolute value of each local maximum value and each local minimum value of this curve, and further calculates the average value of these absolute values divided into sections R1 and R2. When the average value is equal to or smaller than a predetermined value (here, “0.02”), it is determined that the corresponding section is a mode synchronization section.

  In the example of FIG. 16C, the average value in the section R1 is 0.049, and the average value in the section R2 is 0.015. For this reason, it is determined that the section R1 is a CW operation section and the section R2 is a mode synchronization section.

  On the other hand, in the example of FIG. 17C, the average values in the sections R1 and R2 are both 0.012. For this reason, it is determined that both the sections R1 and R2 are mode synchronization sections.

  Next, the control circuit 115 determines whether or not the operation at the drive current Iop when actually used is a mode-synchronized operation based on the determination results of the sections R1 and R2 (see step S11). If the drive current Iop belongs to the CW operation section, it is determined as a failure and the test is terminated. If the drive current Iop belongs to the section R2, the test returns to step S3 and the test is continued.

  As described above, according to this embodiment, the normal / failure of the semiconductor laser can be determined without observing the autocorrelation waveform, the optical spectrum, and the RF spectrum in the aging test. Therefore, according to this embodiment, the aging test can be easily performed, and further automation is easy.

  In this embodiment, the CW operation / mode synchronization operation is determined using the relationship between the drive current Iop and the differential value ΔI-Imod, as in the second embodiment. However, the same method as that of the first embodiment, that is, the method of determining that the mode-locking operation is performed only in the region where the drive current Iop is larger than the kink occurrence point can be adopted in the aging test of this embodiment. .

  By the aging test of this embodiment, it is also possible to determine a driving condition (driving current Iop, reverse bias voltage Vre, etc.) that stabilizes the operation of the semiconductor laser. For example, under a driving condition in which the kink of the photocurrent Imod does not occur in the vicinity of the driving current Iop when actually used, or in a driving condition in which the fluctuation range of ΔImod is small even if the kink occurs, the semiconductor laser It is considered that the operation is stable. Therefore, by performing the aging test according to this embodiment under various driving conditions, it becomes possible to find a driving condition that can maintain the operation stability over a long period of time by using the kink of the photocurrent Imod. .

  In each of the above embodiments, the case where the present invention is applied to a semiconductor laser element has been described as an example. However, if an ammeter for photocurrent Imod or a drive control circuit is incorporated in a module, the present invention is applied to a laser. It can also be applied to module testing.

  In each of the above-described embodiments, the case where the mode-locked semiconductor laser having the same structure as that of FIG. 18 is tested has been described as an example. This invention can be applied even if it is.

  In each of the above-described embodiments, for example, the case where the evaluation test is automated using the test apparatus of FIG. 1 has been described. However, the experimenter directly observes the characteristic graph shown in FIG. Even in such a case, the work load can be reduced as compared with the conventional method of observing the autocorrelation waveform, optical spectrum, and RF spectrum, and the effects of the present invention can be obtained.

It is a conceptual diagram which shows the structure of the test apparatus used by the 1st-3rd embodiment. It is a graph which shows the characteristic of the mode synchronous semiconductor laser which concerns on 1st Embodiment. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 1st Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is an RF spectrum. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 1st Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is an RF spectrum. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 1st Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is an RF spectrum. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 1st Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is an RF spectrum. It is a graph which shows the characteristic of the mode synchronous semiconductor laser which concerns on 2nd Embodiment. It is a graph which shows the characteristic of the mode synchronous semiconductor laser which concerns on 2nd Embodiment. It is a graph which shows the characteristic of the mode-locking semiconductor laser which concerns on 2nd Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is RF spectrum. It is a graph which shows the characteristic of the mode-locking semiconductor laser which concerns on 2nd Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is RF spectrum. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 2nd Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is RF spectrum. It is a graph which shows the characteristic of the mode-locking semiconductor laser which concerns on 2nd Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is RF spectrum. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 2nd Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is RF spectrum. It is a graph which shows the characteristic of the mode locked semiconductor laser which concerns on 2nd Embodiment, (A) is an autocorrelation waveform, (B) is an optical spectrum, (C) is RF spectrum. It is a flowchart for demonstrating the test method which concerns on 3rd Embodiment. It is a graph which shows the characteristic of the mode synchronous semiconductor laser which concerns on 3rd Embodiment, (A) is the relationship between a drive current and a laser beam intensity, and the relationship between a drive current and a photocurrent, (B) is a drive current and a photocurrent. A first derivative result of the relationship with the current, and (C) a second derivative result of the relationship between the drive current and the photocurrent. It is a graph which shows the characteristic of the mode synchronous semiconductor laser which concerns on 3rd Embodiment, (A) is the relationship between a drive current and a laser beam intensity, and the relationship between a drive current and a photocurrent, (B) is a drive current and a photocurrent. A first derivative result of the relationship with the current, and (C) a second derivative result of the relationship between the drive current and the photocurrent. It is a schematic diagram which shows notionally one structural example of the resonator provided in a mode synchronous semiconductor laser. It is a figure which shows the prior art example of the method of evaluating normal / abnormality of mode synchronous operation | movement, (A) is a wave form diagram which shows an example of the autocorrelation waveform of an optical pulse train, (B) shows an example of the optical spectrum of an optical pulse train. Waveform diagram (C) is a waveform diagram showing an example of an RF spectrum of an optical pulse train.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Resonator 101 Gain area | region 102 Saturable absorption area | region 103 Distributed Bragg reflection area | region 104 Phase adjustment area | region 110 Measuring apparatus 111 Drive circuit 112 Power supply for reverse bias 113 Ammeter 114 Photodiode 115 Control circuit

Claims (8)

A resonance comprising a gain region that generates and amplifies light when a drive current is supplied, and a saturable absorption region that modulates the intensity of light output from the gain region when a reverse bias voltage is applied A test method for a semiconductor laser having a detector, comprising:
A measurement step of obtaining a relationship between the drive current and the photocurrent by measuring a value of the photocurrent generated in the saturable absorption region while changing the value of the drive current;
A kink detection step for detecting a kink of the relationship obtained in the measurement step;
A pass / fail determination step for determining whether or not the resonator performs mode-locking operation at a predetermined drive current value based on the kink detection result;
A method for testing a semiconductor laser, comprising: The kink detecting step is a first step of determining that the portion where the photocurrent increases by a first threshold value or more with respect to the increase of the driving current is the kink, and
The pass / fail determination step determines that the mode synchronization operation is performed when the predetermined drive current value is larger than the drive current value corresponding to the kink, and otherwise determines that the mode synchronization operation is not performed. Is the second step
The method for testing a semiconductor laser according to claim 1. The kink detecting step is a step of determining that the portion where the photocurrent decreases by a second threshold value or more with respect to the increase of the driving current is the kink, and
The pass / fail judgment step divides the variation range of the differentiation result into a third step for differentiating the relationship acquired by the measurement step with the drive current, and a region where the drive current is smaller and larger than the kink. In the fourth step for comparing with a predetermined fluctuation range, and in a region where the fluctuation range of the differential result is larger than the predetermined fluctuation range, the resonator performs a CW oscillation operation, and the fluctuation range of the differential result is the predetermined fluctuation range. A fifth step for determining that the resonator is in the mode-synchronized operation in a smaller region, and whether the predetermined drive current value corresponds to the mode-synchronized operation based on the determination result of the fifth step. A sixth step of determining whether or not
The method for testing a semiconductor laser according to claim 1.   5. The semiconductor laser testing method according to claim 4, wherein an evaluation process including the measurement step, the kink detection step, the pass / fail determination step, and the pass / fail determination step is executed a plurality of times with a predetermined time schedule. A resonance comprising a gain region that generates and amplifies light when a drive current is supplied, and a saturable absorption region that modulates the intensity of light output from the gain region when a reverse bias voltage is applied A semiconductor laser test apparatus for testing a semiconductor laser having a container,
A drive circuit for changing the value of the drive current;
An ammeter for measuring the value of photocurrent generated in the saturable absorption region;
Obtaining the relationship between the drive current and the photocurrent by causing the ammeter to measure the photocurrent while changing the value of the drive current in the drive circuit, detecting a kink of the relationship, and A control circuit for determining whether or not the resonator performs mode-locking operation at a predetermined drive current value based on a kink detection result;
A semiconductor laser testing apparatus comprising: The control circuit comprises:
A portion where the photocurrent increases by a first threshold value or more with respect to the increase in the driving current is determined to be the kink; and
If the predetermined drive current value is larger than the drive current value corresponding to the kink, it is determined to perform the mode synchronization operation; otherwise, it is determined not to perform the mode synchronization operation.
6. The semiconductor laser testing apparatus according to claim 5, wherein: The control circuit comprises:
A portion where the photocurrent decreases by a second threshold value or more with respect to an increase in the drive current is determined to be the kink; and
The relationship obtained by the control circuit is differentiated by the driving current, the fluctuation width of the differentiation result is compared with a predetermined fluctuation width divided into a region where the driving current is smaller and a region larger than the kink, and the differentiation result The resonator performs the CW oscillation operation in a region where the fluctuation width of the differential is larger than the predetermined fluctuation width, and the resonator performs the mode-locking operation in a region where the fluctuation width of the differential result is smaller than the predetermined fluctuation width. And based on the determination result, determine whether or not the predetermined drive current value corresponds to the mode synchronization operation.
6. The semiconductor laser testing apparatus according to claim 5, wherein:   Whether the control circuit acquires the relationship between the drive current and the photocurrent, detects the kink, and whether the resonator performs mode-locking operation at a predetermined drive current value based on the detection result of the kink. 6. The semiconductor laser testing apparatus according to claim 5, wherein the process of determining whether or not is executed a plurality of times according to a predetermined time schedule.

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