JP2006322917A - Laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser device capable of executing highly reliable and accurate distance measurement utilizing a self-coupling effect of a laser element regardless of the change of a wavelength changing rate characteristic of the laser element. <P>SOLUTION: Feedback control of a driving condition of the laser element is performed so that a modulated laser light component is kept constant, based on information of the modulated laser light component generated by a self-coupling effect between a wavelength modulated laser light output from the laser element and reflected light from a reference point, set beforehand, of the laser light. For example, a driving current of the laser element is corrected based on a correction coefficient calculated theoretically, or feedback control of an amplitude of the driving current or its period increased and decreased periodically in order to perform wavelength modulation of the laser light is performed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laser device that detects a state of a measurement object, for example, a distance to the measurement object, vibration / displacement of the measurement object, and the like using a self-coupling effect in a laser element.

  One optical distance measurement technique uses a self-coupling effect (also referred to as a self-mixing effect) of a semiconductor laser element (see, for example, Patent Documents 1 and 2). In this method, for example, as shown in FIG. 1, laser light (output light) output from a laser element (LD) 1 driven using a predetermined modulation signal is irradiated toward the detection target 2 and the detection target is also detected. An output light in which an interference signal generated by a self-coupling effect between the laser light (return light) reflected by the object 2 and returned to the laser element 1 and the output light is superimposed is received by a light receiver (PD) 3, The output is subjected to frequency analysis or the like to measure the distance (L), speed, vibration and the like to the detection object 2.

  That is, when the oscillation wavelength of the output light of the laser element 1 is continuously changed, the return light reflected by the detection object 2 interferes with the output light of the laser element 1 to satisfy the resonance condition (strengthening). At the wavelength, the amplification efficiency of the laser device 1 is slightly increased, and at the wavelength satisfying the attenuation condition (weakening), the amplification efficiency is slightly decreased. As a result, the output of the light receiver 3 is repeatedly increased and decreased. For example, when a triangular waveform drive current is applied to a laser element whose output light oscillation wavelength changes according to the applied current value, the current value is continuously proportional to the passage of time in one period of the triangular wave. Increases after reaching the peak. In response to this, the wavelength of the output light emitted from the laser element continuously increases, and after reaching the peak, the wavelength of the output light continuously decreases.

Thus, while the wavelength of the output light continuously increases and decreases, the resonance condition and the attenuation condition between the output light and the return light are alternately satisfied many times. As a result, the light receiver 3 obtains a waveform in which minute interference components (resonance component and attenuation component) are superimposed on the triangular wave as shown in FIG. This interference component includes information such as the distance L between the laser element 1 and the detection target 2. Therefore, if this waveform is analyzed, it is possible to obtain the distance, speed, vibration, and other states from the frequency of the resonance component to the detection target 2. For example, the state of the detection target 2 can be obtained by differentiating the modulated light, extracting a signal component superimposed on a triangular wave, and counting the signal component.
JP 10-246782 A JP-A-11-287859

  Incidentally, the intensity (light quantity) P of the output light from the laser element 1 varies with the drive current I, and the oscillation wavelength λ also varies with the drive current I. That is, a slight thermal expansion / contraction occurs in the laser element 1 due to the increase / decrease in the drive current I, and the oscillation wavelength λ of the laser light slightly increases / decreases (drifts) accordingly. Moreover, the intensity (light quantity) P of the output light from the laser element 1 and the oscillation wavelength λ also change depending on the temperature T of the laser element 1. Then, the wavelength modulation band of the laser light output from the laser element 1 changes, and accordingly, the frequency of the interference component generated by the self-coupling action in the laser element 1 also changes. As a result, there arises a problem that the distance to the measurement object 2 detected by analyzing the interference component of the laser beam is shifted.

  The present invention has been made in consideration of such circumstances, and its purpose is to correct a measurement error caused by a temperature drift of the laser element, and to perform distance measurement and vibration using the self-coupling effect of the laser element. It is an object of the present invention to provide a laser device that can perform displacement detection and the like with high reliability and high accuracy.

In order to achieve the above-described object, a laser apparatus according to the present invention irradiates a measurement target with wavelength-modulated laser light and introduces the laser light reflected by the measurement target. A laser element is provided, and the modulated laser beam generated by the self-coupling effect of the laser beam in the laser element is analyzed to measure the state of the measurement object, such as the distance to the measurement object and the vibration / displacement of the measurement object. To do,
Based on the information of the modulated laser light component generated by the self-coupling effect of the laser element by the reflected light from the preset reference point, the driving condition of the laser element is fed back so as to keep the modulated laser light component constant. It is characterized by having means for controlling.

  Incidentally, the preset reference point is from the background surface of the measurement target area when no detection target is present in the measurement target area, or the incident / exit plane of the laser beam output from the laser element with respect to the measurement target area. Become. The feedback control of the driving conditions of the laser element is performed, for example, by obtaining a correction coefficient at which the frequency of the modulated laser light component becomes a preset frequency and correcting the driving condition of the laser element according to the correction coefficient. Alternatively, when the wavelength modulation of the laser light is performed by periodically increasing / decreasing the driving current of the laser element, feedback control of the driving condition of the laser element is performed using the amplitude of the driving current or the driving current. The increase / decrease period may be changed.

  In the laser apparatus having the above-described configuration, information on the modulated laser light component generated by the self-coupling effect of the laser element due to the reflected light from a preset reference point, specifically, the interference component between the output light and the reflected light Since the drive condition of the laser element is feedback controlled so as to keep the beat number and frequency constant, the modulated laser light component itself can be kept constant even if a temperature drift occurs in the laser element. Therefore, for example, it is possible to accurately obtain the distance to the measurement object obtained by analyzing the modulated laser beam component. That is, according to the laser device of the present invention, it is possible to measure the distance to the measurement object with high accuracy and to detect the vibration / displacement of the measurement object without being affected by the temperature drift of the laser element. Can be performed. In addition, the measurement accuracy can be guaranteed only with simple control.

A laser apparatus according to an embodiment of the present invention will be described below with reference to the drawings.
FIG. 3 is a schematic configuration diagram of a main part of the laser apparatus according to this embodiment. This laser device is basically configured in the same manner as the laser device shown in FIG. 1, and in particular, the laser element 1 is housed in a sealed case 8 together with the light receiver 3 and is modularized (packaged). The laser light output from the laser element 1 is irradiated onto the measurement object 2 through the transparent cover 7 such as glass provided on the sealed case 8 so as to face the front surface of the laser element 1, and the measurement object 2. The reflected light from the light returns to the laser element 1 through the transparent cover 7.

  The frequency analysis unit 5 that detects an interference component generated by the self-coupling effect of the laser element 1 from the output of the light receiver 3 receives the laser beam (output light) output from the laser element 1 and the measurement object 2. In addition to the interference component with the reflected light, the interference component between the laser beam (output beam) and the laser beam (return beam) reflected by the transparent cover 7 is also detected. In particular, the frequency analysis unit 5 feeds back information on interference components between the reflected light and output light from the transparent cover 7 to the wavelength modulation unit 4 so as to control the modulation conditions of the laser element 1 as will be described later. It is configured. In the figure, reference numeral 6 denotes a distance detection unit that obtains the distance to the measurement object 2 from the interference component of the laser beam subjected to frequency analysis.

  Specifically, as shown in FIG. 4, a schematic configuration of a main part thereof, the window portion is formed on the front surface of the laser element 1 which is housed in a sealed case 8 together with the light receiver 3 and modularized, and the laser A transparent cover (transparent body) 7 for protecting the element 1 is provided. In particular, an antireflection film 9 is provided on the inner side surface of the transparent cover 7 positioned on the laser element 1 side to prevent reflection of laser light emitted from the laser element 1, and measurement of the transparent cover 7 is performed. The outer surface (externally exposed surface of the laser length measuring device) positioned on the object 2 side is set so that slight reflection occurs. The outer side surface (externally exposed surface of the laser length measuring device) that has been processed so that slight reflection of the transparent cover 7 occurs is used as a measurement reference point in the laser length measuring device, as will be described later.

  That is, when laser light is input / output through a transparent body (transparent cover 7) such as glass, the laser light is slightly reflected at the interface between the transparent body and air. In order to prevent such reflection, an antireflection film is formed exclusively by coating the surface of the transparent body with a filler in which a low refractive index material is dispersed. Also in the laser length measuring device according to the present invention, an antireflection film (AR coating) 9 is provided on the surface of the transparent cover 7 in order to suppress unnecessary reflection on the transparent cover 7 which is the laser light incident / exit surface. At this time, the antireflection film 9 is provided only on the inner surface of the transparent cover 7 so that the laser beam is reflected on the outer surface of the transparent cover 7. A part of the laser beam output from the laser element 1 is reflected by the outer surface of the transparent cover 7 and returns to the laser element 1.

  The outer surface of the transparent cover 7 is of course not subjected to antireflection processing, but is intentionally subjected to processing necessary to obtain reflected light having an intensity that causes a self-coupling effect in the laser element. You may do it. Specifically, the outer surface of the transparent cover 4 can be mirror-polished, or an optical film having a certain reflectivity can be formed by coating. However, the main role of the laser device is to irradiate the measurement object 2 with laser light and return the reflected light to the laser element 1 to measure the distance to the measurement object 2. It is desirable to suppress reflections at as much as possible.

  According to the laser device configured as described above, the laser light output from the laser element 1 and irradiated toward the measurement object 2 via the transparent cover 7 is reflected by the measurement object 2, and the transparent cover The laser beam reflected by the outer surface of the transparent cover 7 is also returned to the laser element 1. As a result, in the laser element 1, interference due to the self-coupling effect between the output light and the reflected light from the measurement object 2 occurs, and interference due to the self-coupling effect between the output light and the reflected light from the transparent cover 7 occurs. .

  Then, the interference components of the laser beam corresponding to the reflected light (return light) respectively generated by the measurement object 2 and the transparent cover 7 are generated as frequency components fo and f1 which are different from each other as shown in FIG. The frequencies fo and f1 of these interference components correspond to the distance Lo from the laser element 1 to the transparent cover 7 and the distance L from the measurement object 2, respectively. Accordingly, if the interference components frequencies fo and f1 generated by the self-coupling effect of the laser element 1 are detected and distance information Lo and L corresponding to these frequency frequencies fo and f1 are obtained, the above-described transparent cover 7 is obtained. As a measurement reference point, the distance Ls from the measurement reference point (transparent cover 7) to the measurement object 2 can be measured with high accuracy as [= L−Lo].

In addition, if laser light is reflected on the outer surface of the transparent cover 7 in this way, there is an individual difference (variation) in the positional relationship between the laser element 1 and the transparent cover 7 in the laser length measuring device. In addition, since the distance L _S to the measurement object 2 can be measured with high accuracy using the outer surface of the transparent cover 7 as a measurement reference point, reliability can be achieved regardless of individual differences in mass-produced laser length measuring instruments. It is possible to perform a high distance measurement. In particular, when this type of laser length measuring device is installed and used at a site such as a factory, the distance (position) of the measurement object 2 from the preset measurement origin is often the measurement object. In this case, it is only necessary to determine (adjust) the mounting position of the laser length measuring device by abutting the outer surface of the transparent cover 7 described above serving as the laser light incident / exit end surface of the laser length measuring device with the measurement origin. The handling can be facilitated.

  Incidentally, as described above, the intensity (light quantity) P of the output light from the laser element 1 and the oscillation wavelength λ also change depending on the temperature T of the laser element 1. Then, the wavelength modulation band of the laser light output from the laser element 1 changes, and accordingly, the frequency of the interference component generated by the self-coupling action in the laser element 1 also changes. As a result, there arises a problem that the distance to the measurement object 2 detected by analyzing the interference component of the laser beam is shifted.

  Therefore, in the laser length measuring device according to the present invention, paying attention to the interference component due to the self-coupling effect between the laser light output from the laser element 1 and the reflected light from the outer surface of the transparent cover 7 used as the measurement reference point described above, The drive condition of the laser element 1 is feedback-controlled based on the interference component due to the reflected light from the measurement reference point. Specifically, information such as the beat number and frequency of the interference component is controlled by feedback control of the drive current itself of the laser element 1 and the amplitude or period of the drive current that is periodically increased or decreased to modulate the wavelength of the laser beam. Is kept constant.

  That is, in a laser apparatus that measures the distance L to the measurement object using the self-coupling effect of the semiconductor laser element 1, the oscillation wavelength λ of the laser element 1 is the wavelength of the laser element 1 according to the drive current. When the change rate changes with the change rate characteristic, the interference signal generated by the self-coupling effect also changes. Specifically, for example, as shown in FIG. 6A, a drive current composed of a periodic triangular wave is applied to the laser element 10 to cause laser oscillation, and an interference signal generated by the self-coupling effect between the output light and the reflected light is generated. When the superimposed laser beam is received by the light receiver 13, the amount of light received (electromotive current) by the light receiver 13 is as shown in FIG. However, if the wavelength change rate characteristic of the laser element 1 changes as shown in FIG. 6C due to a temperature change or the like, the interference condition changes along with the change of the oscillation wavelength. The quantity (electromotive current) changes as shown in FIG. That is, the interference that occurred at the frequency f (0) due to the oscillation wavelength shift occurs at the frequency f (1). This shift in the wavelength of the interference component causes a measurement error.

  Therefore, in the laser length measuring device according to the present invention, the frequency of the interference component described above is detected within the range of change in the oscillation wavelength of the laser element 10 as described above. Then, for example, the correction coefficient K for correcting the deviation of the interference frequency as [K · f (1) → f (0)] is calculated, the drive current of the laser element 10 is corrected based on the correction coefficient K, and the oscillation is performed. The wavelength λ itself is corrected.

  Based on the detected frequency of the interference component, for example, as shown in FIG. 5 (e), the period of the drive current (triangular wave) applied to the laser element 10 is adjusted, so that the frequency as shown in FIG. 5 (f) is obtained. As shown in FIG. 5 (h), interference occurs at f (0), or the amplitude of the drive current (triangular wave) applied to the laser element 10 is adjusted as shown in FIG. 5 (g). In addition, interference may occur at the frequency f (0). That is, in the measurement system using the self-coupling effect of the laser element 1, the frequency component of the interference signal generated by the self-coupling effect indicates the distance information to the reflection point of the laser beam. In order to correct the wavelength itself, the drive current may be corrected according to a correction coefficient, or the resonance condition may be corrected by adjusting the period and / or amplitude of the drive current that is continuously increased or decreased.

  Thus, as described above, in the laser apparatus according to the present invention, the drive condition of the laser element 1 is feedback controlled so as to keep the frequency of the interference component generated by the reflected light from the measurement reference point constant. Therefore, even if the wavelength change rate characteristic of the laser element 1 changes, measurement conditions such as distance measurement to the measurement object 2 based on the interference component due to the self-coupling effect of the laser element 1 can be kept constant. As a result, it is possible to improve the measurement reliability and always perform highly accurate distance measurement.

  In addition, as described above, the drive current of the laser device 1 is corrected based on the correction coefficient calculated theoretically, or the amplitude of the drive current that is periodically increased or decreased to modulate the wavelength of the laser beam or its period is fed back. Since only the control is required, the reliability of the distance measurement and the measurement accuracy can be sufficiently increased under a simple control system. Therefore, its practical advantage is great.

  In the above-described embodiment, the outer surface of the transparent cover 7 fitted into the window of the sealed case 8 in the laser length measuring device is used as a measurement reference point. For example, only the laser element 1 is enclosed in a can package. When the laser element is used alone, it goes without saying that the transparent cover 7 provided in the window portion of the can package may be used as a measurement reference point. Further, when an optical fiber is provided facing the laser element 1 and laser light is input / output from the distal end portion toward the measurement object 2 through the optical fiber, the optical fiber is opposed to the measurement object 2 of the optical fiber. It is also possible to use a laser light incident / exit surface (fiber end surface) provided as a measurement reference point.

  Further, in the case where the measurement object 2 is provided in a predetermined space (measurement object area) and the measurement object 2 does not exist in the measurement object area, the laser beam output from the laser element 1 Is reflected on the background surface of the measurement object. The background surface (background) of the measurement object is usually set to be fixed (invariant) in many cases. Therefore, as shown in FIG. 4, the background surface (background) 11 of the measurement target region is used as a reference point, the interference component due to the reflected light from the background surface 11 is detected, and the drive current of the laser element 1 is calculated. Of course, feedback control is also possible. In short, the present invention can be implemented with various modifications without departing from the spirit of the present invention.

The figure which shows the basic composition of the laser apparatus using the self-coupling effect of a laser element. The figure which shows the example of the intensity | strength of the wavelength-modulated laser beam output from a laser element, and its interference component. The figure which shows schematic structure of the laser apparatus which concerns on one Embodiment of this invention. The figure which shows typically the mode of reflection in the background surface (background) of the transparent cover and measurement object area | region of the laser beam output from the laser element. The figure which shows the difference in the frequency of the interference component produced by the return light from a several reflective point. The figure which shows the form of correction | amendment by feedback control of the change of the frequency of the interference component by the reflected light from the reference point in the laser apparatus which concerns on this invention, and the drive condition of a laser element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser element 2 Measurement object 3 Light receiver 7 Transparent cover (transparent body)
10 Optical fiber 11 Background of measurement target area

Claims (4)

A self-coupled laser element that irradiates the measurement target with wavelength-modulated laser light and introduces the laser light reflected by the measurement target is provided. The self-coupling effect of the laser light in this laser element A laser device that detects the state of the measurement object by analyzing the modulated laser beam generated by
Based on the information of the modulated laser light component generated by the self-coupling effect of the laser element by the reflected light from the preset reference point, the driving condition of the laser element is fed back so as to keep the modulated laser light component constant. A laser device characterized by controlling.   The preset reference point is a background surface of the measurement target region when a detection target does not exist in the measurement target region, or an entrance / exit surface of the laser light output from the laser element with respect to the measurement target region. The laser device according to claim 1.   The feedback control of the driving condition of the laser element is performed by obtaining a correction coefficient at which the frequency of the modulated laser light component becomes a preset frequency and correcting the driving condition of the laser element according to the correction coefficient. Item 2. The laser device according to Item 1.   The wavelength modulation of the laser beam is performed by periodically increasing / decreasing the driving current of the laser element, and feedback control of the driving condition of the laser element is performed by changing the amplitude of the driving current or the increasing / decreasing of the driving current. The laser apparatus according to claim 1, wherein the laser apparatus is performed while changing a cycle.

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