JPH0222529A - Method and device for measuring characteristics of semiconductor laser - Google Patents

Abstract

PURPOSE:To measure oscillation wavelength characteristics depending upon an injected current under the specific temperature condition of a semiconductor laser(LD) element by detecting the sudden phase discontinuation point of the waveform of an intensity signal obtained from a photodetector. CONSTITUTION:A temperature control circuit(ATM) 2 controls the temperature of an LD element 3 to a constant value, which is determined by a computer 7. The driving current of the element 3 is controlled by an LD driving circuit 1 and varied to adjust the oscillation wavelength of the element 3. Further, the element 3 varies in the refractive index of its waveguide according to the variation of the injected current, so that the oscillation wavelength varies. Then the divergent light emitted by the element 3 is collimated by a collimator lens 4 into parallel light, which is transmitted through an optical fiber 5, so that the photodetecting element 6 detects the intensity of the light. The output of the element 6 is inputted to the computer 7 and the output of the element 6 is inputted while the driving of the element 3 is controlled to perform signal processing for detecting the mode hopping characteristics of the element 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method and apparatus for measuring characteristics of a semiconductor laser, and more particularly to a method and apparatus for measuring characteristics of a semiconductor laser for measuring oscillation wavelength characteristics of a semiconductor laser element.

[Prior Art] Semiconductor lasers (hereinafter referred to as LDs) are simpler, cheaper, smaller, and lighter in device configuration than gas lasers, etc., and are widely used as light sources for optical communication, audio, and video optical discs, etc. . In addition, applications to light sources for optical interferometers have recently been actively researched.

[Problem B to be solved by the invention] In an LD element, the oscillation wavelength changes depending on the current injected into the element, but the relationship between the two is not linear, and the oscillation wavelength becomes unstable at a certain injection current value. It has a so-called mode hopping characteristic that changes continuously.

Interferometers perform measurements by observing interference fringes formed by wavelength scanning, but when mode hopping occurs, the oscillation wavelength changes, so it is necessary to avoid mode hopping points when using the interferometer. Moreover, mode hopping does not always occur at a constant injection current value, and the mode hopping point varies depending on temperature conditions, etc., so to avoid mode hopping, consider the injection current and temperature conditions and the characteristics of the oscillation wavelength in advance. must be measured.

For this purpose, it is necessary to measure the oscillation wavelength of the LD element using a spectrometer using a diffraction grating, an interferometer, or the like.

However, methods that use a spectroscope or the like have the following problems.

1) It is necessary to scan the diffraction grating and the reflecting mirror of the interferometer with a laser beam, which takes time for measurement and is troublesome to scan.

2) A large-scale device is required, and in particular, if a structure is adopted in which a spectrometer or the like is introduced into the system to be measured in order to evaluate the LD element mounted in a device that uses it as a light source, the entire device becomes complicated and large.

Furthermore, this is not a practical method if it is necessary to calibrate each device one by one before shipping.

An object of the present invention is to solve the above-mentioned problems and to make it possible to easily and inexpensively measure the characteristics of an LD element.

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a semiconductor laser characteristic measurement method and apparatus for measuring the oscillation wavelength characteristics of a semiconductor laser element. Laser light from a semiconductor laser device whose conditions can be controlled as desired is passed through an optical filter having spectral characteristics such that light transmittance or light reflectance monotonically increases or decreases with respect to wavelength in a band near the oscillation wavelength of the semiconductor laser device. The temperature of the semiconductor laser element is controlled to a predetermined value by inputting the current into the light receiving element, and then changing the injection current of the semiconductor laser element and detecting a sharp phase discontinuity point in the intensity signal waveform obtained from the light receiving element. A configuration was adopted to measure the oscillation wavelength characteristics that depend on the injection current of a semiconductor laser device under a predetermined temperature condition.

[Function] According to the above configuration, a change in the wavelength of the semiconductor laser is converted to a change in intensity via the optical filter. It is possible to measure the oscillation wavelength characteristics that depend on the injection current under a predetermined temperature condition of the laser element.

[Example] Hereinafter, the present invention will be described in detail based on the example shown in the drawings.

FIG. 1 shows the configuration of an apparatus for measuring characteristics of a semiconductor laser device employing the present invention.

In FIG. 1, the laser light source is a single mode oscillation LD element 3, which is temperature controlled by an ATV (temperature control circuit) 2. ATV2 controls the temperature of LD element 3 to a desired constant value. The control temperature value is determined by the computer 7.

Further, the drive current of the LD element 3 is controlled by the LD drive circuit 1, and the oscillation wavelength of the LD element 3 is adjusted by changing this drive current. In the LD element 3, the refractive index of the waveguide changes due to a change in the injection current, and the oscillation wavelength changes.

The diverging light emitted from the LD element 3 is passed through the collimating lens 4.
is converted into parallel light by The collimated laser beam passes through an optical filter 5 having spectral characteristics, and the light intensity is detected by a light receiving element 6.

The optical filter 5 has spectral characteristics such that light transmittance (or light reflectance) monotonically increases or decreases with respect to wavelength in a band near the oscillation wavelength of the LD element 3.

The output of the light receiving element 6 consisting of a photodiode or the like is input to the computer 7 via an input/output port and other interface circuits. The computer 7 is composed of a microprocessor, a memory, etc., and drives the LD element 3 via the LD drive circuit 1 according to a program to be described later.
The output of the light receiving element 6 is input while controlling the driving of the LD element 3, and signal processing for detecting the mode hopping characteristic of the LD element 3 is performed.

Next, the operation of the above configuration will be explained.

The object of measurement in the present invention is an LD element with single mode oscillation and variable wavelength. Here, FIG. 2 shows an example of the injection current-oscillation wavelength characteristic of a single mode oscillation LD element.

As shown in the figure, the injection current and the oscillation wavelength have a linear relationship in a certain section, but the oscillation wavelength jumps significantly due to mode hops in several places. The point at which mode hopping occurs mainly depends on the temperature of the LD element 3 and the injection current. When measuring mode hopping for this purpose, either the temperature or the injection current must be kept constant.

As mentioned above, the optical filter 5 used has spectral characteristics such that the light transmittance (or light reflectance) monotonically increases or decreases in the vicinity band including the oscillation wavelength of the LD element 3 whose characteristics are to be measured. do. FIG. 3 shows an example of the transmission characteristics of the optical filter 5. In FIG. 3, the horizontal axis is the wavelength of the incident light, λ. indicates a certain oscillation wavelength of the LD element 3.

Here, the LD element 3 has a mode hopping characteristic as shown in FIG. 4 under a certain temperature condition, that is, the injection current 1
Between 0 and ll, the wavelength of the laser beam is λ. The wavelength increases monotonically from λ1 to λ, jumps from λ1 to λ2 at 11, and then increases monotonically with the injection current from λ2.The injection current is monotonically increased with time by the LD drive circuit 1. Let's consider the case.

On the other hand, the change in the output light intensity of the LD element 3 continues to increase monotonically for the same injection current change, as shown in FIG. The intensity is so small that it is usually difficult to distinguish it from the signal waveform of intensity changes.

However, when this laser beam passes through the optical filter 5 having the spectral characteristics shown in FIG. 3, the light intensity changes after passing through as shown in FIG. 6. This is because the light transmittance of the optical filter 5 depends on the wavelength, so when the oscillation wavelength jumps due to mode hopping of the LD element 3, the transmittance changes significantly before and after that, and there is a clear difference in the intensity change. Continuous points appear.

By detecting this discontinuous point, it is possible to know the mode hopping characteristics of the LD element 3 under a certain temperature condition. For detecting discontinuous points, an optical filter 5 obtained from the light receiving element 6 is used.
One possible method is to take the first derivative of the light intensity signal waveform after transmission.

The injection current i of the LD element 3 is controlled to increase monotonically with respect to time, and the intensity of the transmitted light of the optical filter 5 is adjusted to the light receiving element 6.
When detected, a detected waveform as shown in FIG. 6 is obtained. When this waveform is first differentiated, a waveform as shown in FIG. 7 is obtained.

As shown in Figure 7, where there are discontinuous points -
Since the order differential value changes greatly, mode hopping can be detected by comparing it with an appropriate threshold level.

Next, a procedure for measuring the mode hopping characteristic of the LD element 3 according to the present invention will be explained with reference to FIG. The procedure shown in FIG. 14 is executed by the computer 7.

First, in step S1 of FIG. 14, the temperature of the LD element 3 is made constant at an arbitrary value by the ATM 2.

Subsequently, in step S2, the oscillation wavelength of the LD element 3 is changed. FIG. 8 shows a waveform of one cycle (T) of the injection current injected into the LD element 3. That is, the injection current is changed at a constant rate with respect to time, and wavelength scanning is performed at a constant rate.

Subsequently, in step S3, the light from the LD element 3 is transmitted through the optical filter 5 and received by the light receiving element 6 to obtain a light intensity change signal, and the waveform of the light intensity change signal is input into the computer 7. FIG. 9 shows an example of the waveform of the light intensity change signal. 9th
In the figure, mode hopping occurs at the time point M. When inputting the intensity signal waveform to the computer 7,
Actually, a quantized data string is captured at every predetermined sampling time.

Next, in step S4, the computer 7 performs a first-order differential calculation of the intensity signal waveform. FIG. 10 shows a waveform obtained by firstly differentiating the waveform of FIG. 9. Actually, this waveform is expressed inside the computer 7 in the form of a data string similar to the above.

Next, in step S5, a detection level is set, and mode hopping is detected by detecting a location where the −th differential waveform has an extremely large value (a location where the rate of change is large). Specifically, mode hopping points are detected by sequentially comparing the −th order differential data string with a predetermined threshold value.

FIG. 11 shows the mode hopping points detected from FIG. 10. Here, the value of the injection current at which mode hopping occurred is calculated in correspondence with FIG. 8, and the mode hopping detected when the wavelength of the LD element 3 is increased is plotted on the horizontal axis. The white circles indicate the mode hopping detected when the value is decreased, and the black circles indicate mode hopping.

Subsequently, in step S6, measurements are repeated under the same conditions, and the average value of the injection current values at which mode hopping occurs is calculated.

Next, in step S7, the LD element 3 is
Shift the temperature at regular intervals and repeat the above process to detect mode hopping at each temperature.

FIG. 12 is a plot of mode hopping points under different temperature conditions measured by the above process, with temperature on the vertical axis and injection current on the horizontal axis. Here, as in Figure 11, the mode hopping when the wavelength increases is indicated by a white circle.
The mode hopping when the wavelength decreases is shown by a black circle.

As is clear from FIG. 12, the value of the injection current at which mode hopping occurs varies depending on the temperature.

According to the above embodiment, the injection current to the LD element 3 is changed over time, and a sudden phase jump is detected from the first differential value of the intensity signal change signal obtained through the optical filter having spectral characteristics. Hopping points can be detected.

The configuration of the device is very simple, and the characteristics of the LD element can be measured at low cost. In particular, since it does not use a complicated optical system that requires mechanical control, measurements can be made in a short time.
Furthermore, mode hopping characteristics can be continuously detected regarding the injection current across the entire oscillation region of the LD element.

Furthermore, even when the above configuration is incorporated into a device that uses an LD element as a light source, it can be implemented more easily and inexpensively than a configuration in which a conventional spectrometer is incorporated. In that case, for example, the 12th
As shown in the figure, mode hopping points can be measured according to temperature conditions, so when operating the device, ATM2 is used to prevent mode hopping from occurring within the desired injection current range.
By controlling the temperature of the element 3, the device can be operated only in a region where the injection current/oscillation wavelength of the LD element 3 linearly corresponds.

Furthermore, it is also possible to carry out the above-mentioned mode hopping measurement at a timing such as when the apparatus is started up, and to perform an automatic calibration operation in which various constants during operation of the apparatus are changed based on the mode hopping conditions obtained thereby.

Note that the first-order differentiation of the intensity signal waveform shown in FIG. 7 can also be performed by an analog differentiation circuit.

In addition, if the graph in Figure 12 is folded in the middle as shown in Figure 13, the temperature at which mode hopping does not occur, and the injection current This is useful for finding conditions.

Such processing can be easily executed if the data string corresponding to FIG. 12 is stored in the memory of the computer 7.

It goes without saying that an interference filter, a dichroic filter, etc. can be used as the optical filter 5.

Moreover, although the configuration of a light transmission type optical system has been shown above, it goes without saying that similar measurements can be performed with a light reflection type optical system.

[Effects of the Invention] As is clear from the above, according to the present invention, in a semiconductor laser characteristic measurement method and apparatus for measuring the oscillation wavelength characteristics of a semiconductor laser element, injection current and temperature conditions during driving can be controlled as desired. A possible semiconductor laser device receives the laser light from the semiconductor laser device through an optical filter that has spectral characteristics such that the light transmittance or light reflectance monotonically increases or decreases with respect to wavelength in a band near the oscillation wavelength of the semiconductor laser device. The temperature of the semiconductor laser element is controlled to a predetermined value, and the injected current of the semiconductor laser element is changed to detect an abrupt phase discontinuity point in the intensity signal waveform obtained from the light receiving element. Since we have adopted a configuration that measures the oscillation wavelength characteristics that depend on the injected current at a given temperature condition of the laser element, changes in the wavelength of the semiconductor laser are converted into changes in intensity via an optical filter, which can be obtained from the light receiving element. By detecting the sharp phase discontinuity point of the intensity signal, it is possible to measure the oscillation wavelength characteristic that depends on the injection current under a predetermined temperature condition of the semiconductor laser device. The measurement system does not require mechanical control, can be constructed very simply and inexpensively, and can perform measurements in a short time. In addition, even when the measuring device is incorporated into a device that uses a semiconductor laser element as a light source, it is only necessary to add hardware such as an optical filter and a light receiving element, and the entire device can be configured easily, inexpensively, compactly, and lightweight. There is.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the structure of an interferometer adopting the present invention, Fig. 2 is a diagram showing mode hopping of the LD element in the device shown in Fig. 1, and Fig. 3 is a diagram showing the optical filter shown in Fig. 1. Figure 4 is a diagram showing the wavelength characteristics with respect to the injection current of the LD element when there is mode hopping, and Figure 5 is a diagram showing the output light intensity characteristics of the LD element. , Fig. 6 is a diagram showing the light intensity characteristics after passing through the optical filter, Fig. 7 is a diagram showing the first-order differential waveform of the waveform in Fig. 6, and Fig. 8 is a diagram showing the injection when measuring the characteristics of the LD element. Figure 9 is a diagram showing the current, Figure 9 is a diagram of the intensity signal obtained by the drive characteristics shown in Figure 8, Figure 10 is a diagram showing the first differential waveform of the waveform in Figure 9, and Figure 11 is a diagram showing the linear differential waveform of the waveform in Figure 9. An explanatory diagram showing the mode hopping points detected from Figure 10, Figure 12 is an explanatory diagram showing the mode hopping points obtained by changing the temperature conditions, and Figure 13 is an explanatory diagram showing the mode hopping points detected by changing the temperature conditions. Fig. 14 is a flowchart showing the measurement control procedure in the present invention. 1... LD drive circuit 2... ATM3... LD
i-column 4... Collimating lens 5... Optical filter 6... Light-receiving element 7... Computer patent applicant Kowa Co., Ltd. Figure 2: λLD
Diagram 11 Fortress Nishi'Ifgi "Hand +ety) Flowch Volume-"
Figure 14-Door door

Claims (1)

[Claims] 1) In a semiconductor laser characteristic measurement method for measuring the oscillation wavelength characteristics of a semiconductor laser device, a laser beam of a semiconductor laser device whose injection current and temperature conditions during driving can be controlled as desired is applied to the semiconductor laser device. The temperature of the semiconductor laser element was controlled to a predetermined value by inputting it to the light receiving element through an optical filter having spectral characteristics such that the light transmittance or light reflectance monotonically increases or decreases with respect to wavelength in a band near the oscillation wavelength of By changing the injection current of the upper semiconductor laser element and detecting a sharp phase discontinuity point in the intensity signal waveform obtained from the light receiving element, the oscillation wavelength characteristic that depends on the injection current at a predetermined temperature condition of the semiconductor laser element is determined. 1. A method for measuring characteristics of a semiconductor laser. 2) A method for measuring characteristics of a semiconductor laser according to claim 1, characterized in that the temperature conditions at the time of measuring the oscillation wavelength characteristics are changed, and the oscillation wavelength characteristics of the semiconductor laser element are measured under a plurality of temperature conditions. . 3) A semiconductor laser characteristic measuring device for measuring the oscillation wavelength characteristics of a semiconductor laser device, which includes means for controlling the injection current to a desired value when driving the semiconductor laser device, and controlling the temperature of the semiconductor laser device to a desired value. and a light receiving means for receiving laser light from a semiconductor laser element through an optical filter having spectral characteristics such that light transmittance or light reflectance monotonically increases or decreases with respect to wavelength in a band near the oscillation wavelength of the semiconductor laser element. the temperature of the semiconductor laser element is controlled to a predetermined value by the temperature control means, the injection current of the semiconductor laser element is changed by the injection current control means, and an abrupt phase shift of the intensity signal waveform measured by the light receiving element is caused. 1. An apparatus for measuring characteristics of a semiconductor laser, comprising measurement control means for measuring an oscillation wavelength characteristic depending on an injection current of the semiconductor laser element under a predetermined temperature condition by detecting continuous points. 4) The semiconductor according to claim 3, wherein the temperature control means changes the temperature conditions when measuring the oscillation wavelength characteristics, and measures the oscillation wavelength characteristics of the semiconductor laser element under a plurality of temperature conditions. Laser characteristic measurement device.