JPH0645700A - Testing method for laser diode - Google Patents

Abstract

PURPOSE:To provide a method for testing a laser diode in which the number of testing steps of a code error ratio is reduced and an efficiency of testing is enhanced in the method for testing the rate of the diode. CONSTITUTION:A test of a laser diode before a code error ratio is measured obtains the following formulae by using a measured value of a threshold current (Ith), a bias current (Ib) and a pulse current (Ip). f=ln((alpha-k)/(alpha-1)). alpha=(Ib+Ip)Ith. k=Ib/Ith (k<1). Corresponding relation between the f and a code error ratio (BER) obtained separately by measurement is previously obtained, and a range of the f is preset based on the corresponding relation to obtain a desired code error ratio, the Ith of the diode is measured, the f is obtained by the formulae, and the diode having the f of a set range is sorted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode test method, and more particularly to a laser diode code error rate test method.

[0002]

2. Description of the Related Art A main application of laser diodes is high bit rate digital optical communication. The laser diode used for this converts the current pulse into an optical signal so that the waveform of the input current pulse is not collapsed as much as possible,
Required to send. FIG. 5A is a block diagram of a digital optical communication device. A pulse current as a transmission electric signal is applied to a laser diode to generate laser light, and the laser light is sent to a light receiving element (APD) through an optical fiber. Then, the transmitted laser light is photo-electrically converted by the light receiving element. By the way, the received electric signal which is converted from light to electricity usually has a distorted waveform as shown in FIG. The waveform distortion of the received electric signal occurs mainly due to the distortion of the waveform in the laser diode of the light emitting unit and the like, the optical fiber and the light receiving element of the light receiving unit, and the superposition of noise generated in the light emitting unit and the light receiving unit. This causes a bit error. For example, A in FIG.
The original transmission signal 0 is erroneously detected as the identification code 1 in the section.

Among the causes of the above waveform distortion, the waveform distortion caused by the laser diode is caused by the increase of injected carriers,
This occurs because there is a time delay until the stimulated emission occurs and a time delay until the injected carriers disappear after the pulse is stopped. The higher the communication speed and the higher the density, the more likely it is that the communication will occur. Regarding the waveform distortion due to the above laser diode,
The code error rate is a test item for evaluating the pulse transmission fidelity of the laser diode. That is, the code error rate refers to the ratio of signals that are erroneously detected among the received signals.

Conventionally, a trial of the code error rate of a laser diode is performed.
The test is conducted as follows. FIG. 3A shows a conventional example.
It is a block diagram of the measuring device of a code error rate. In Figure 3 (a)
And 1 is the pulse current (I_P) Laser diode (L
D) A pulse pattern generator supplied to 3 and 2 are
Bias current (I_b) Supply
In the ias circuit, the threshold current (I
_th) When biased below and I_thBias above
There are cases. 3 is a laser diode to be measured, 4
Is I_POf the laser light generated by the application of
Back reflector for adjusting the amount of reflected light, 5 is a back reflector
An optical fiber that conveys the laser light that has passed through the reflector 4 to the receiver.
Eva, 6 is an attenuator that attenuates the amount of incident light before it enters the receiver.
With a physical attenuator, for example, attenuate the amount of incident light
The error bit is output to the optical receiver 8
It is used, for example, to find out the limit point. 7 is Optica
Optical-electrical conversion of laser light that has passed through the attenuator 6.
For example, APD (avalanche photodiode) etc.
Of the light receiving element, and 8 is photoelectrically converted by the light receiving element 7.
Optical that generates pulses in response to electrical signals (photocurrent)
Optical receiver, 9 measures the BER for the received signal
Error detector.

When the BER is measured by using the above measuring device, each device is carefully adjusted so that an unnecessary error does not occur, and all the laser diodes are measured. Then, the quality is determined according to a preset BER quality criterion.

[0006]

According to the above test method, the higher the bit rate is required, the more elaborate the adjustment of the equipment is required, and the longer measurement time is required. Therefore, when measuring a large amount, in order to ensure mass productivity,
Requires a large number of measurement systems. Therefore, there has been a problem that much time and cost are required.

The present invention was created in view of the problems of the conventional example, and an object thereof is to provide a laser diode test method capable of reducing the number of code error rate test steps and increasing the test efficiency. .

[0008]

The above-mentioned problem is a test of a laser diode performed prior to the measurement of the code error rate, and the measured value of the threshold current (I _th ) which is the starting current of laser oscillation and the test condition Using the bias current (I _b ) and the pulse current (I _P ), the following equation f = ln ((α−k) / (α−1)) where f = td / τn td: oscillation delay time τn: Injected carrier lifetime α = I _OP / I _th I _OP = I _b + I _p k = I _b / I _th (k <1) and the bit error rate (BER) separately obtained by actual measurement. And the range of f is preset based on the correspondence so that a desired code error rate can be obtained, and the threshold current (I _th ) of the laser diode to be tested is measured. Then, by obtaining f by the above formula, the setting range of f It is achieved by a laser diode test method characterized by screening the incoming laser diode.

[0009]

[Operation] BER is related to the distortion of the waveform due to the laser diode. The distortion of the waveform due to the laser diode is the time delay (td) until the injection carrier increases and stimulated emission occurs, and the injection carrier after the pulse stops. Occurs because there is a time delay until it disappears. Also, td and I
The relationship between _th , I _b and I _P is expressed by the following equation. f = ln ((α−k) / (α−1)) where f = td / τn τn: Lifetime of injected carrier α = I _OP / I _th I _OP = I _b + I _P k = I _b / I _th (k <1) FIG. 2 is a diagram showing the correspondence between α and f when k = 0.8.

As described above, the inventor of the present application has confirmed that the threshold current (I _th ), the bias current (I _b ), and the pulse current (I _P ) which are the starting currents of the laser oscillation are transmitted through f through the code error rate ( BER), I _th , I
_The following studies were conducted in order to make a preliminary evaluation of BER by _b and I _P. That is, FIG.
It is a figure which shows the corresponding relationship between the measurement result of BER, and f.

According to FIG. 3B, the BE that has been set in advance is set.
Samples having f of 0.15 or more were BER defective without exception, and samples having f of 0.15 or less were BER good products with respect to the quality criterion of R. Therefore, in the case of the experiment, it suffices to select those having α such that f falls within the range of 0.15 or less based on the correspondence relationship in FIG. By doing this, the BE is actually measured.
Since it is possible to preliminarily select a group having a high possibility of becoming R non-defective products, it is possible to reduce the number of samples for which BER should be measured and to improve the efficiency of measurement, as compared with the conventional case.

As a result, it is possible to reduce the number of steps for measuring the BER and effectively use the measuring device.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A laser diode testing method according to an embodiment of the present invention will be described below with reference to the drawings.

(1) Investigation for Applying to Test Method of Example of the Present Invention The inventor of the present application has found that the threshold current (I
_th), paying attention to the fact the bias current (I _b) and the pulse current to be actually applied to the optical communication apparatus (I _P) is involved in bit error rate (BER) via f (= td / τn) Then, an investigation was conducted to make a preliminary evaluation of BER by I _th , I _b, and I _P.

(A) Correspondence between α and f FIG. 4 (a) is a diagram showing the relationship between light output and current, FIG.
FIG. 4B shows a pulse waveform of the injection current (I), FIG.
(C) is a diagram showing changes over time of the injected carrier density (n),
FIG. 4D is a diagram showing the change over time in the photon density (n _ph ) generated in the laser diode due to the injection current.

As shown in FIG. 4A, the relationship between the light output of the laser diode and the current is divided into a spontaneous emission region and a laser oscillation region, and the current value at the boundary is the threshold current (I _th ). Become. At this boundary, in the laser oscillation region, the light output increases in proportion to the current with a larger proportional coefficient than in the spontaneous emission region.

Therefore, when performing digital optical communication, a pulse current is applied so that the laser diode operates in the laser oscillation region. In this case, as one of the methods for biasing the laser diode, there is a method in which a bias current (I _b ) slightly smaller than I _th is supplied to the laser diode in order to accelerate the response. This allows the laser diode to respond quickly when a pulse current is applied.

However, even if this is done,
There is a time delay (td) until the injected carriers increase and stimulated emission occurs, and a time delay until the injected carriers disappear after the pulse is stopped (FIGS. 4B to 4D). Therefore, the waveform of the transmission current pulse is distorted by the laser diode, and the waveform distortion by the laser diode causes a code error on the receiving side.

By the way, as expressed by the following equation, td is related to I _th , I _b and I _P. f = ln ((α−k) / (α−1)) where f = td / τn τn: Lifetime of injected carrier α = I _OP / I _th I _OP = I _b + I _P k = I _b / I _th (k <1) FIG. 2 is a diagram showing the correspondence between α and f when k = 0.8. The vertical axis of FIG. 2 represents f on a logarithmic scale, and the horizontal axis represents α on a normal scale.

According to FIG. 2, f rapidly decreases as α increases, that is, as I _th decreases. Thus, I _th , I _b, and I _P are expected to be related to BER via f, and I _th , I _b, and I _P
It may be possible to have a preliminary assessment of the ER.

(B) Correspondence between BER actual measurement result and f FIG. 3 (a) is a block diagram of the BER measuring device, 11 is I _P
A pulse pattern generator for supplying the laser diode (LD) 13 with I _b
In the bias circuit for supplying, and a case where biasing the I _th or more when the bias below I _th. In the case of the embodiment, it is biased to I _b = 0.8 × I _th . 13 is a laser diode (LD) as an object to be measured, 14 is I _P
Back reflector for adjusting the amount of laser light generated at the end of the fiber generated by the application of the laser beam, 15 is an optical fiber for carrying the laser light passing through the back reflector 14 to the receiver, and 16 is the amount of light incident before entering the receiver. Is an optical attenuator that attenuates the incident light amount, and is used, for example, to attenuate the amount of incident light and search for a limit point at which an error bit appears in the output of the optical receiver 18. , 17 is a light receiving element such as an APD for photoelectrically converting the laser light passing through the optical attenuator, and 18 is a light receiving element 1
An optical receiver that generates a pulse in response to an electric signal (photocurrent) photoelectrically converted by 7 and an error detector 19 that measures the BER of the received signal.

FIG. 3 (b) is a diagram showing the correspondence between f and the actual measurement result of BER measured by the above measuring device. f was obtained by the relational expression between f and α. According to FIG. 3 (b), samples having f of 0.15 or more were inevitably defective with respect to the measured BER, and samples having f of 0.15 or less, with respect to preset BER quality criteria. Was good for the measured BER without exception.

From the above, based on the correspondence relationship of FIG.
By selecting those having α such that f falls within the set range (0.15 or less in the case of experiment), the measured B
It is possible to preliminarily select a sample that has a very high possibility of becoming a non-defective ER.

(2) Test Method of the Embodiment of the Present Invention FIG. 1 is a flow chart for explaining the test method of the laser diode according to the embodiment of the present invention.

First, I _th of all the laser diodes to be tested is measured. Then, I _b = k × I _th (k = 0.8 in the case of the embodiment), I _b and I _P are substituted into the following formula α = (I _b + I _P ) / I _th , and α And f is further calculated from the relational expression between f and α.

Next, in order to obtain the desired BER, the range of f (0.15 or less in the case of the embodiment) set based on the correspondence between BER and f separately obtained by actual measurement is set. It is determined whether or not.

Next, the samples not falling within the range of f are B
The BER is actually measured by the measuring device shown in FIG. 3A for the sample which is excluded as the ER defect and falls within the range of f. At this time, f falls within the set range (0.15 or less in the case of the embodiment), and only the sample having a very high possibility of being a good BER by actual measurement is measured.
Very few have ER defects.

After that, the BER is removed except for the ones with bad BER.
Perform the following test steps on non-defective products. As described above, according to the embodiment of the present invention, by selecting those having α such that f falls within the setting range based on the correspondence relationship of FIG. Very high samples can be pre-sorted. As a result, it is possible to reduce the number of samples for which BER should be measured and to improve the efficiency of measurement, as compared with the conventional case. Therefore, B
It is possible to reduce the number of steps for measuring the ER and effectively use the measuring device.

[0029]

As described above, according to the laser diode test method of the present invention, a desired code error rate is obtained due to the correspondence between BER and f (= td / τn) obtained by actual measurement. By measuring I _th for f set so as to be obtained, the following expression f = ln ((α−k) / (α−1)) where f = td / τn td: oscillation delay time τn: Life time of injected carrier α = I _OP / I _th I _OP = I _b + I _p k = I _b / I _th (k <1) Based on the selection, a laser diode is selected so as to fall within the setting range of f. There is.

Therefore, it is possible to preliminarily select a sample which is highly likely to be a good BER by actual measurement, reduce the number of samples for which the BER should be measured, and improve the efficiency of the measurement, as compared with the conventional case. be able to.

This makes it possible to reduce the number of steps for measuring the BER and effectively use the measuring device.

[Brief description of drawings]

FIG. 1 is a flowchart illustrating a laser diode testing method according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between f and α according to the embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a code error rate measuring device for a laser diode according to an embodiment of the present invention and a diagram showing measurement results of the code error rate.

FIG. 4 is a diagram illustrating an optical output current and a pulse response of a laser diode.

FIG. 5 is a configuration diagram of an optical communication device of a conventional example and an explanatory diagram of signal waveforms of respective parts.

[Explanation of symbols]

 11 pulse pattern generator, 12 bias circuit, 13 laser diode (LD), 14 back reflector, 15 optical fiber, 16 optical attenuator, 17 light receiving element (APD), 18 optical receiver, 19 error detector.

Claims (1)

[Claims] 1. A laser diode test carried out prior to measurement of a code error rate, comprising a measured value of a threshold current (I _th ) which is a starting current of laser oscillation, a bias current (I _b ) and a pulse of a test condition. Using the current (I _P ), the following equation f = ln ((α−k) / (α−1)) where f = td / τn td: oscillation delay time τn: lifetime of injected carrier α = I _OP / I _th I _OP = I _b + I _p k = I _b / I _th (k <1) and the correlation between the bit error rate (BER) separately obtained by actual measurement are obtained in advance, In addition, the range of f is preset based on the correspondence so that a desired code error rate is obtained, the threshold current (I _th ) of the laser diode to be tested is measured, and f is obtained by the above formula. By selecting the laser diode within the setting range of f. The method of testing a laser diode, characterized by.