JPH0461182A - Method and device for measuring semiconductor laser current-optical output characteristic - Google Patents

Abstract

PURPOSE:To grasp easily the current-optical output characteristic of a semiconductor laser equivalent to that upon actual use of the same by providing to the semiconductor laser a current of a shorter pulse width than a thermal time constant of the semiconductor laser and detecting optical output intensity of the semiconductor laser in synchronism with the current pulse. CONSTITUTION:A changeover switch 32 is thrown to the side of a low frequency oscillator 33 to measure a thermal time constant of a semiconductor laser 31. Then, the changeover switch 32 is thrown to the side of a triangular wave generator 34 to measure a current pulse in the triangular current pulse in the triangular wave generator 34 is set to be a shorter pulse width than the thermal time constant based upon a control signal from a control circuit 39, and output light from the semiconductor laser 31 based upon the triangular current pulse is divided into a plurality of time series optical pulses by controlling a shutter speed with the control circuit 39 and passing a high speed shutter 35 through the optical path between the semiconductor laser 31 and a photodiode 37. Those light pulses are thus entered into the photodiode 37.

Description

[Detailed Description of the Invention] Summary 7. Method for measuring current-optical output characteristics of a semiconductor laser and device 1. Easily obtains the above characteristics equivalent to the current-optical fit force characteristics of a semiconductor laser in actual use. 1-1 The purpose is to understand and obtain the semiconductor L/- method.
- The optical output intensity of the above-mentioned conductor laser is detected in synchronization with the thermal time constant of the laser, a current pulse/L pulse with a pulse width shorter than J2, and the current pulse. Yonizuru.

INDUSTRIAL APPLICATION FIELD The present invention relates to a method and apparatus for measuring current-light output characteristics of a semiconductor light source.

In the general optical communication system currently in practical use, a semiconductor laser is driven by a series of current pulses corresponding to the transmitted information to form optical pulses that blink at a predetermined timing2. Light pulses are transmitted through optical fibers. Direct modulation of a semiconductor laser is possible because the carved conductor laser has a linear optical output characteristic (4) with respect to the drive current (5) below a predetermined threshold current. When constructing an optical communication system using this type of direct modulation method, it is important to understand the current-optical output characteristics of the semiconductor laser in order to obtain good system characteristics. It is..

2. Description of the Related Art Conventionally, the current-optical output characteristics (IL characteristics) of a semiconductor laser have generally been measured by driving the semiconductor laser at C%V (continuous current driving).

Problems to be Solved by the Invention FIG. 9 shows an example of current-light output characteristics of a semiconductor laser. This figure is a graph comparing the characteristics during CW driving (solid line) at low temperature, room temperature, and high temperature with the characteristics during actual use (broken line) when driven by high-speed current pulses. It is clear that the characteristics differ between CW driving and actual use, and this tendency becomes more pronounced as the temperature rises. Therefore, if only one current-optical output characteristic is known during CW driving, the driving conditions of the semiconductor laser should be determined when constructing a system 7'1 that applies the direct modulation method of the semiconductor laser. is difficult to optimize.

If the drive line (1) of the semiconductor laser is not optimized, waveform deterioration is likely to occur especially when operating the semiconductor laser at high speed.

Therefore, the object of the present invention is to make it possible to easily grasp the above-mentioned characteristics equivalent to the current-light output characteristics of a semiconductor laser in actual use.

In order to solve the problems, the present invention will be described in detail with reference to FIG. Figure 1 (a
, ) When a high-speed drive current pulse (corresponding to the transmission speed of the system) as shown in 2 is applied to a semiconductor laser having the current-light output characteristics shown in 1, the light received by the current-light output characteristics measuring device is If the instrument were of a typical low-bandwidth type, it would not be able to respond to these fast pulses and would simply measure the average light intensity; It is not possible to grasp the current-light output characteristics during use. Furthermore, even when an incident angle wave current pulse 3 having the same transmission speed is applied, the characteristics cannot be similarly determined. In this way, when trying to understand the current-light output characteristics of a semiconductor laser using current pulses during actual use, it is necessary to use a measurement device equipped with a light receiver that responds to optical pulses based on current pulses during actual use. equipment is required. However, this type of measuring device lacks versatility and requires two hours of measurement time.

Therefore, in the present invention, a current pulse having a pulse width shorter than the thermal time constant of the semiconductor laser is applied to the semiconductor laser, and the optical output intensity of the semiconductor laser is detected in synchronization with the current pulse. In this method, a current pulse with a pulse width shorter than the thermal time constant of the semiconductor laser is applied, making it possible to obtain characteristics almost equivalent to those in actual use without being affected by heat. -4. Note that even with such a short pulse width current pulse, 1. A light pulse based on the pulse can be sufficiently received by a light receiver of a normal band. The thermal time constant 71D of a semiconductor laser can be determined by measuring the time taken to reach a thermal equilibrium state when a low frequency pulse is applied to the semiconductor laser, as shown in FIG. 1(b). . τ
1o is usually a time equivalent to 10 to 1.00 KH2.

Therefore, the thermal time constant τ1. By measuring ゎ in advance and using a current pulse having a pulse width shorter than this, characteristics equivalent to those in actual use can be obtained. Top-
As the current distribution pulse, for example, a triangular wave current pulse (pulse width 1; 1<rLD) as shown in FIG. You can find out by examining the following relationship. The thermal time constant of the semiconductor laser is τ□. , transmission speed (
f) When the number of bits sent in one cycle is B, rLD<f
If the relationship /B holds true, it can be said that there is almost no effect of heat during pulse driving, so as stated above, τ, 0
It will be better to use a triangular current pulse with a shorter pulse width. For example, f=-1001, 1llz, B1
Assuming 0 bit (ryp), τL11 is normally 1 (
Although the time is approximately equivalent to 1.00 KHz, it is possible to measure characteristics equivalent to those during actual pulse driving (for example, 1.00 MHz) at a speed of approximately 1 MHz to 1.00 KHz.

FIG. 2 is a diagram showing a first configuration of a measuring device used to implement the method of the present invention. This device includes a triangular wave current pulse generating means 12 which gives a triangular wave current pulse to a semiconductor laser 11 with a pulse width shorter than the thermal time constant of the semiconductor laser 11.
and 1. [--a high-speed shutter 13 that divides the output light of the semiconductor laser 11 based on the triangular wave current pulse into a plurality of time-series optical pulses, and a light that performs optical-to-electrical conversion of the optical pulses and detects the intensity of the optical pulses. pulse intensity detection means 14; and comparison means 1 for comparing the intensity of the detected optical pulse with the instantaneous value of the triangular wave current pulse synchronized with the optical pulse;
5.

In the method of the present invention using this device, the semiconductor laser 1
A triangular current pulse having a pulse width shorter than the thermal time constant of the semiconductor laser 11 is applied to the semiconductor laser 11, and the output light of the semiconductor laser 11 based on the triangular current pulse is divided into a plurality of time-series optical pulses by a high-speed shutter 13. , the intensity of the optical pulse is detected in correspondence with the instantaneous value of the diagonal current pulse.

FIG. 3 is a diagram showing a second configuration of the measuring device used to implement the method of the present invention. This device includes a current sweeper 21 that generates a current that increases or decreases over time;
a switching means 22 for applying the current from the current sweeper 21 to the semiconductor laser 11 intermittently at a cycle shorter than the thermal time constant of the semiconductor laser 11 as a current pulse; A light pulse intensity detection means 23 converts the light pulse into electricity and detects the intensity of the light pulse, and compares the intensity of the detected light pulse with the level of the current pulse synchronized with the light pulse. The comparison means 24 is also included.

In the method of the present invention using this device, a current that increases or decreases over time is applied to the semiconductor laser 11 intermittently as current pulses at a cycle shorter than the thermal time constant of the semiconductor laser 11, and the current pulse is Based on this, the intensity of the optical pulse output from the semiconductor laser 11 is detected in correspondence with the current pulse.

FIG. 4 is a diagram showing an example of a waveform diagram at each point in FIG. 2. That is, Fig. 1(a) shows the current waveform at the output point ■ of the triangular wave current pulse generating means 12, and Fig. 1 Q)) shows the light intensity waveform at the output point ■ of the semiconductor laser (■, L)) 1.1. (C) is a light intensity waveform at the output point (3) of the high-speed shutter 13. By using a high-speed/low-speed sensor in this manner, the intensity of each optical pulse can be detected in synchronization with the instantaneous time of the hexagonal wave current pulse.

FIG. 5 is a diagram showing an example of a waveform diagram at each point in FIG.
The current waveform at 0)) in the same figure is the switching means 22
The current waveform at the output point (1) is the current waveform at the output point (1), and (C) in the same figure is the light intensity waveform at the output point of the semiconductor laser 11. In this example, we apply a current of 1' to the conductive laser that increases as time passes, but we apply a current that decreases by 1'X' as time passes. In the example of Lll, -, the current is changed by the switching means stage.
, L, , ``'r Semiconductor as a current pulse - the i:', ', ', -'f

Since it is, I used high-speed gloss 4),)-na1. lol
According to the method or apparatus of the present invention, the intensity of the output light pulse can be detected synchronously with the current pulse (1'). Since the current pulse is applied to the semiconductor laser with a short pulse width, it is possible to obtain current-optical characteristics equivalent to 22 in actual use without being affected by heat. There is no need to use a high-speed drive current pulse corresponding to the speed j,2), so the optical receiver of the measuring device is not required to have special band characteristics. Therefore, it becomes possible to easily grasp the characteristics equivalent to the characteristics of the semiconductor laser when it is actually used. .

Example The present invention will be explained in detail below.

FIG. 6 is a block diagram of a V measuring device showing an embodiment of the first configuration. Reference numeral 31 denotes a semiconductor laser whose current and optical output characteristics are to be measured, and this semiconductor laser 3j is connected to a low frequency oscillator 33 and a triangular wave generator 34 via a changeover switch 32. 35 is a high-speed shutter that divides the output light of the semiconductor laser 31 into a plurality of time-series optical pulses, and 36 is a drive circuit for the high-speed shutter 35. The light pulse that has passed through the high-speed shutter 35 is sent to the photodiode 3
7 into a photocurrent pulse, this photocurrent pulse is amplified by an amplifier circuit 38 and input to a control circuit 39.

The control circuit 39 is also connected to a high-speed shutter drive circuit 36, a triangular wave generator 34, and a low frequency oscillator 33.

The operation of this device will be explained. First, changeover switch 3
2 on the low frequency oscillator 33 side, and the thermal time constant of the semiconductor laser 31 is measured. When measuring the thermal time constant, a low frequency current pulse of 1 is applied from the low frequency oscillator 33 to the conductor 1/-the 3j by a signal from the control circuit 39, and a response signal at the photodimate; 37 is sent to the control circuit 39. The thermal time constant is calculated based on the input. This output Pi 1st speed Nyanota: 35 is open. Next, the switch 32 is switched to the triangular wave generator 34 side, and the current-light output characteristics are measured. At this time, the pulse width of the triangular wave current pulse in the triangular wave generator 34 is set to be shorter than the thermal time constant based on the control signal from the control circuit 39. The output light of the semiconductor laser 31 based on this triangular wave current pulse is controlled by a control circuit 3 to control the speed of the laser beam.
The light passes through a high-speed shutter 35 controlled by 9, and is divided into a plurality of time-series light pulses, which are then manually applied to a photodimate 37. Since the current generated in the photodiode 37 is proportional to the intensity of the optical pulse, the current-light output characteristics can be determined by processing the intensity of the time-series optical pulses in correspondence with the instantaneous values of the triangular current pulses. Obtainable.

FIG. 7 shows a measuring device 1077 showing an embodiment of the second configuration.
It is a diagram. 1A, the conductor laser 41 is the changeover switch 4
12 to a low frequency oscillator 43 and switching circuit 4
Connected to 4. Reference numeral 45 is a current sweeper connected to the switch/lung circuit 44.The optical pulse output from the semiconductor laser 41 is directly converted into an electric signal by the photodynode 46, and this signal is amplified by the amplifier 47 and sent to the control circuit. 148 manpower. Control circuit 48 is also connected to low frequency oscillator 43 and switching circuit 44 .

Operation 4 of this device will be explained. As in the embodiment of the first configuration, the low frequency oscillator 13 and the control circuit 48 function to measure the thermal time constant of the semiconductor laser 41. Then, a current pulse having a pulse width shorter than the thermal time constant is generated. The switching circuit 44 is controlled so that the current sweeper 21
The current from the semiconductor laser 4 is transmitted intermittently as current pulses.
1 is man-powered. , and the light pulse from the semiconductor laser 41 based on this current pulse is transmitted to the photodimate 4.
6 converts the light pulse into a photocurrent pulse proportional to the intensity of the light pulse and inputs it to the control circuit 48.

The intensity of the optical pulse detected in this way is compared with the level of a current pulse from the switching circuit 44 synchronized with the optical pulse to obtain the current/light output characteristics.

In the previous embodiments, as shown in FIG. 8(a), the pulse width of the optical pulse is constant at Δt. For example, even if the angular wave current pulse is repeatedly applied to the semiconductor laser a plurality of times, there is a limit to the measurement accuracy.

That is, in order to improve measurement accuracy, it is necessary to make Δt smaller, which requires a high-bandwidth photodiode, which is not practical. Therefore, the measurement accuracy is improved as follows without using a high-bandwidth photodiode. That is, the pulse width of the optical pulse is randomly determined, and the intensity of the optical pulse is repeatedly detected a plurality of times corresponding to the optical pulse, and the results are averaged.

In this case, if "1" continues in a random pattern, the temperature of the semiconductor laser will rise undesirably, so it is preferable to create a pattern of "ro" before and after "-1". For example, a rung like "rlooloololol..."
, patterns can be used. In FIG. 8j, the waveform of the first configuration is shown, but the waveform of the first configuration? The same effect can be obtained by randomizing the current switching in the same way.

As described in detail, the present invention has the effect that characteristics equivalent to the current-light output characteristics of a semiconductor laser in actual use can be easily determined.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the principle of the present invention, FIG. 2 is a diagram showing the first configuration of the measuring device of the present invention, FIG. 3 is a diagram showing the second configuration of the measuring device of the present invention, and FIG. 2
FIG. 5 is a diagram showing an example of a waveform diagram at each point in FIG. 3; FIG. 5 is a diagram showing an example of a waveform diagram at each point in FIG. 3; FIG. Figure 7 is a block diagram of a measuring device showing an embodiment of the second configuration of the present invention, Figure 8 is an explanatory diagram of another embodiment, and Figure 9 is an example of current-light output characteristics. FIG. 1.31.41... Semiconductor laser 2... Triangular wave current pulse generation means, 3.35... High speed shutter, 4.23... Optical pulse intensity detection means, 5.24... Comparison means, 1 .45...Current sweeper, 2...Switching means.

Claims (1)

[Claims] 1. A current pulse having a pulse width shorter than the thermal time constant of the semiconductor laser (11) is applied to the semiconductor laser (11), and the light of the semiconductor laser (11) is applied in synchronization with the current pulse. Current of semiconductor laser characterized by detecting output intensity -
How to measure optical output characteristics. 2. A triangular wave current pulse having a pulse width shorter than the thermal time constant of the semiconductor laser (11) is applied to the semiconductor laser (11), and the semiconductor laser (11) based on the triangular wave current pulse
) is divided into a plurality of time-series light pulses by a high-speed shutter (13), and the intensity of the light pulses is detected in correspondence with the instantaneous value of the triangular wave current pulse. Measurement method described. 3. Triangular wave current pulse generating means (12) for providing a triangular wave current pulse having a pulse width shorter than the thermal time constant of the semiconductor laser (11) to the semiconductor laser (11); ) that divides the output light into multiple time-series optical pulses (13
), an optical pulse intensity detection means (14) for detecting the intensity of the optical pulse by photo-electrically converting the optical pulse, and the triangular wave current pulse synchronizing the intensity of the detected optical pulse with the optical pulse. A current-light output characteristic measuring device for a semiconductor laser, characterized in that it comprises a comparing means (15) for comparing with an instantaneous value of. 4. Apply a current that increases or decreases over time to the semiconductor laser (11) intermittently as a current pulse at a cycle shorter than the thermal time constant of the semiconductor laser (11), and based on the current pulse, the semiconductor laser (11) 2. The measuring method according to claim 1, wherein the intensity of the optical pulse outputted from the current pulse is detected in correspondence with the current pulse. 5. A current sweeper (21) that generates a current that increases or decreases over time, and a semiconductor laser (1) that directs the current from the current sweeper (21) to a semiconductor laser (1).
a switching means (22) which supplies the semiconductor laser (11) with a current pulse intermittently at a period shorter than the thermal time constant of 1); A light pulse intensity detection means (23) for detecting the intensity of the light pulse through optical-to-electrical conversion; and a comparison means (23) for comparing the intensity of the detected light pulse with the level of the current pulse synchronized with the light pulse. 24) A semiconductor laser current-light output measuring device characterized by comprising: 6. The method or device according to any one of claims 2 to 5, wherein the pulse width of the light pulse is random, and the detection of the intensity of the light pulse is repeatedly performed in correspondence with the light pulse. .