JPH10197619A - Laser radar - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the S/N of detected signals by further reducing stray light by using a semiconductor laser using a wavelength having a temperature coefficient smaller than that of the wavelength used by a Fabry-Perot semiconductor laser and an optical interference filter. SOLUTION: A semiconductor laser 1 is constituted so that the temperature coefficient of the oscillation wavelength of the laser 1 may become shorter than that of the oscillation wavelength of a Fabry-Perot semiconductor laser. The reflected light of the light emitted from the laser 1 from an object to be irradiated is detected by means of a light receiving means 5. When the means 5 detects the reflected light, the means 5 uses an optical interference filter having a narrow half-value width of wavelength of, for example, 20nm. A microcomputer outputs a light emission command of a short-time pulse to the laser 1 by controlling the gate timer 11 of a distance calculating means 9 and a light emission driving means 2 and, at the same time, starts the counting up of a distance counter 12. When a photodiode 7 receives the reflected light from the object, the microcomputer stops the counting up and a signal converter 13 outputs the counted distance as a distance signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser radar, and more particularly to a laser radar for measuring an inter-vehicle distance used in an apparatus for preventing collision of a vehicle, and in particular, to reduce the background, that is, stray light, and to reduce the S / N ratio of a detection signal. The present invention relates to an improved laser radar.

[0002]

2. Description of the Related Art Conventional distance measurement methods using a laser radar include a phase difference method and a pulse method. In a rear-end collision prevention laser radar used in a vehicle collision prevention apparatus, a pulse method that requires a small average optical power is mainly used because it is necessary to detect a long distance at a relatively wide angle. In this pulse method, a short-time pulse light is emitted from a semiconductor laser as a light source, and a distance is calculated by measuring a time until the pulse-shaped laser light is reflected from an object to be measured in front and returns. It is.

[0003] Since the amount of laser light reflected back from the object to be measured and returned is very weak, as shown in FIG.
An avalanche photodiode 100 that can obtain a large amplification sensitivity for weak light is used as a photodetector. When this avalanche photodiode is used, it is necessary to reduce the background, that is, the incidence of stray light, in order to secure a large S / N ratio. Therefore, an optical bandpass filter 101 such as an optical interference filter is used on the entire surface of the avalanche photodiode. In FIG.
Is a light receiving amplifier, 103 is a distance calculating means for calculating a distance by measuring a time from when the pulsed laser light is emitted and reflected back to the object to be measured and returns, 104 is a power supply unit, and 105 is a power supply unit. A semiconductor laser emission driving unit 106 is a semiconductor laser serving as a light source. 7, reference numeral 107 denotes a light emitting unit;
108 is a light receiving means, 109 is a collimating lens of laser, 110 is a condensing lens, 11 is a power input terminal, 112
Is a distance signal output terminal.

[0004]

In principle, the background light can be reduced and the S / N ratio of the detection signal can be secured as far as the line width of the semiconductor laser 106 or the half width of the optical interference filter is narrower. . However, since it is a laser radar for vehicles, the ambient temperature environment may be about -30 to + 80 ° C. Conventionally, a Fabry-Perot type semiconductor laser is used as a light source in a laser radar, and the temperature coefficient of wavelength is about 0.5 nm / ° C. Therefore, the wavelength variation due to temperature is estimated to be 55 nm. For this reason, it is necessary to set the half width of the optical bandpass filter to 50 to 100 nm,
There was a risk that stray light could not be removed. For this reason, in order to obtain a desired S / N ratio of the detection signal, it is sufficient to increase the output of the semiconductor laser as a light source, but it has been pointed out that power consumption increases. Therefore, the present invention uses a semiconductor laser having a smaller temperature coefficient of wavelength than a Fabry-Perot semiconductor laser, thereby using a light interference filter having a wavelength half width narrower than that of a conventional light interference filter to further reduce stray light and detect the light. It is an object of the present invention to provide a laser radar capable of improving an S / N ratio of a signal.

[0005]

According to the present invention, there is provided a laser radar having a first cladding layer, an active layer, a second cladding layer, a contact layer, and a current stop layer having a stripe groove for forming a current path on a semiconductor substrate. And a plurality of electrode layers are sequentially laminated, and a plurality of concave portions having a rectangular cross section from the electrode layer to the second cladding layer are arranged at regular intervals in the stripe groove direction as a light source. The light receiving means for detecting the light reflected from the irradiated object and returning the light has an optical interference filter having a narrow wavelength half width.

[0006] In such a semiconductor laser, as a phenomenon surface, when an optical gain curve represented by a graph in which the horizontal axis represents the wavelength and the vertical axis represents the intensity of the oscillated light is seen, the optical gain is increased due to an increase in the ambient temperature. Even if the curve changes, the oscillation wavelength is at the same position, so that the temperature coefficient of the oscillation wavelength is small. On the other hand, in the case of a Fabry-Perot type semiconductor laser, the temperature coefficient of the oscillation wavelength is as described above because the peak of the optical gain curve shifts to the longer wavelength side due to an increase in the ambient temperature, in the same graph. Big. It is preferable that the temperature coefficient of the oscillation wavelength is 0.06 to 0.08 nm / ° C., since even if the ambient temperature changes by about −30 to + 80 ° C., the wavelength variation due to the temperature becomes 9 nm or less.
If a semiconductor laser having such a temperature coefficient of oscillation wavelength of 0.08 nm / ° C. is used, the above-described ambient environmental temperature in a Fabry-Perot semiconductor laser having a temperature coefficient of oscillation wavelength of 0.5 nm / ° C. used in a conventional laser radar is used. Is about 1/6 or less as compared with the wave fluctuation of 55 nm / .degree.

The use of such a semiconductor laser makes it possible to use an optical interference filter having a wavelength half width narrower than that of the conventional laser radar. Therefore, when stray light is reduced and the output of the light source is the same as that of the conventional laser radar, the S / N ratio of the detection signal can be improved. When the S / N signal is set to the same level as that of the conventional laser radar, the output of the laser light source can be reduced, and the power consumption can be reduced.

The first and second cladding layers have conductivity and are made of a semiconductor such as InP. If one of the first and second cladding layers is a p-type semiconductor layer, the other is an n-type semiconductor layer. The active layer is a medium that causes a laser operation, and is made of, for example, InGaAsP.

The contact layer is for reducing the contact resistance between the electrode layer and the second cladding layer, which is a semiconductor layer, and is made of, for example, a semiconductor such as InGaAs. This contact layer is formed by the semiconductor n
It becomes n-type or p-type corresponding to the type and p-type.

[0010] The current stop layer has a function of flowing a current applied to the electrode layer made of Au or the like to the contact layer through the stripe groove, and is made of, for example, SiO2. Each of the plurality of recesses has a maximum depth of 1 μm. When the length of the resonator is approximately 250 to 400 μm and the pitch (the interval from this concave portion to the adjacent concave portion including a certain concave portion) is 0.2 to 0.3 μm, the number of these concave portions is 830 to 830.
It will be in the range of 2,000. These juxtaposed concave portions may be on both sides of the stripe groove, or may be on one side of the stripe groove. Preferably on both sides.

The laser radar according to the present invention comprises a laser emission driving means, a semiconductor laser as a light source driven by the emission driving means, a light receiving means, and a return light from the semiconductor laser when the light is emitted. Means for calculating the distance from the semiconductor laser to the irradiation object from the time taken until the detection of the object.

Further, in the laser radar of the present invention, a semiconductor laser may be used in which a plurality of the above-mentioned stripe grooves are arranged in a non-parallel manner and a plurality of resonators are formed. In such a laser, the operating ambient temperature environment is -3.
Even when the temperature changes from 0 to + 80 ° C., the interval between the plurality of concave portions of each resonator, that is, the grating period is changed by the difference in directionality due to the non-parallel arrangement of the stripe grooves, and the vicinity of the peak wavelength of the optical gain curve at each temperature. If it is set to oscillate, the resonator suitable for the predetermined temperature can be switched and used, the deviation between the oscillation wavelength and the peak wavelength of the optical gain curve is reduced, the threshold current value is increased, and the optical output-injection is performed. It is possible to suppress a decrease in the slope of the current characteristic slope efficiency (electric-light conversion efficiency).

[0013] In the case of a laser radar using a laser having a plurality of resonators, a resonator determination is performed by selecting the plurality of resonators according to a change in measured temperature and outputting a resonator selection signal to the laser emission driving means. Use means.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. In FIG. 1 showing one embodiment, 1 is a semiconductor laser, 2 is a light emission driving means of the semiconductor laser 1, 3
Denotes a power supply unit of the light emission driving unit. The semiconductor laser 1, the light emission driving unit 2 and the power supply unit 3 form a main part of the light emission unit 4.

In FIG. 1, reference numeral 5 denotes a light receiving means. The light receiving means 5 is a means for detecting light which is emitted from the semiconductor laser 1 and is reflected by an unillustrated object to be irradiated and returns. It includes an optical interference filter 6, an avalanche photodiode 7, and a light receiving amplifier 8. Optical interference filter 6
For example, those having a narrow wavelength half width can be used, for example, those having a wavelength half width of 20 nm can be used. 9 is a distance calculating means usually used for the laser radar,
It is connected to the light emission driving means 2 and the light receiving amplifier 8. The distance calculation means 9 calculates the distance from the semiconductor laser 1 to the irradiation target from the time from the time when the light is emitted from the semiconductor laser 1 to the time when the light returns to the light receiving means 5.

The distance calculating means 9 will be described in detail below. This means 9 includes an oscillator 10, a gate timer 11, which is connected to the light receiving amplifier 8 and the laser emission driving means 2, and which is connected to the light receiving amplifier 8, the light receiving amplifier 8, and the gate timer 11. And a distance counter 12 for outputting a distance signal and a signal converter 13. Usually, these gate timers 1
1. The distance counter 12 and the signal converter 13 constitute a part of a microcomputer and are controlled by a computer.
The microcomputer not shown is a gate timer 1
1 and the light emission driving means 2 to issue a short-time pulse light emission command to the semiconductor laser 1,
Start counting up. The pulsed laser light is reflected by the object to be irradiated ahead and returns, and the photodiode 7
, The count-up of the distance counter 12 is stopped. Then, the counted distance is used as the signal converter 13.
To output a distance signal. The frequency of the oscillator for counting up is about 100 to 200 MHz. In FIG. 1, reference numeral 14 denotes a laser collimating lens,
Reference numeral 15 denotes a condenser lens, 16 denotes a power input terminal, and 17 denotes a distance signal output terminal.

As shown in FIGS. 2 and 3, the semiconductor laser 1 has a rectangular plate-like n-type InP semiconductor substrate 20 having a lower surface on which a common electrode 21 made of Au or the like is formed. The first cladding layer 22 of InP
An active layer 23 made of GaAsP, a second clad layer 24 made of p-type InP, and a contact layer 25 made of p-type InGaAs are sequentially laminated. Contact layer 2
5 is a stripe groove 2 extending in the longitudinal direction of the substrate 20.
A current stop layer 28 made of SiO2 is formed so as to form 6, 27. Highly conductive electrode layers 30, 31 made of Au or the like are formed on the current stop layer 28 and the stripe grooves 26, 27. Also,
A groove 32 extending in the longitudinal direction is formed between the stripe grooves 26 and 27 so as to bisect the short direction of the semiconductor laser 1.

The groove 32 has two electrode layers 30 and 31
Are provided so that they can be independently driven, and the depth of the groove 32 is such that the bottom surface of the groove 32 is located near the interface between the second cladding layer 24 and the active layer 23 in the second cladding layer 24. What is necessary is just to have. Stripe grooves 26, 27
Are formed on both sides of each of the plurality of recesses 3 having a rectangular cross section extending from the surface of the electrode layers 30 and 31 toward the active layer 23.
3 are formed in the direction of the stripe grooves 26 and 27 at regular intervals.
Are formed. As shown in FIG. 3, these recesses 33 are formed to such a depth that the bottom surface of each recess does not reach the active layer 23, and the maximum depth thereof is 1 μm. These recesses 3
Although the number of 3 is only 12 in FIG. 2, the length of the resonator is approximately 250 to 400 μm, and the pitch (the interval between the concave portion 33 including the concave portion 33 and the adjacent concave portion 33). ) Is 0.2 to 0.3 μm, 8
The range is 30 to 2,000.

In such a semiconductor laser 1, near the active layer 23 serving as an optical waveguide, as shown in FIG.
A plurality of diffraction gratings 34 are formed along the lines. When a drive current is injected from the electrode layers 30 and 31 through the stripe grooves 26 and 27, light is generated in the active layer 23 and vibrates and couples. Only light having a wavelength determined by the period of the diffraction grating 34 becomes a laser beam and oscillates in a feedback manner. That is, as shown in FIG. 2, a first resonator that emits light by a drive current injected from the electrode layer 30 through the stripe groove 26 is formed in a region of the active layer 23 located below the stripe groove 26, In the region of the active layer 23 located below the stripe groove 27, the electrode layer 3
A second resonator that emits light by the drive current injected from No. 1 is formed. Stripe groove 27 corresponding to second resonator
Are formed at an angle θ, for example, 5 degrees with respect to the vertical direction with respect to the short side of the semiconductor laser 1, and the stripe groove 26 corresponding to the first resonator is perpendicular to the short side of the semiconductor laser 1. Extending in the direction.

As described above, if the stripe grooves 26 and 27 are formed non-parallel, that is, the resonance directions of the first resonator and the second resonator are non-parallel, the stripe grooves of the first resonator and the second resonator are respectively formed. Diffraction grating 34 intersecting 26, 27
The pitch between the plurality of concave portions 33 constituting the group is different. Therefore, the wavelength of light emitted from the first resonator is different from the wavelength of light emitted from the second resonator.

For example, the combination of the active layer material / cladding layer material having an oscillation wavelength of 1550 nm is InGaAsP.
In the / InP-based semiconductor material, when the inclination angle of the resonator (stripe groove) is set to 5 degrees as described above, the oscillation wavelength changes by 6 nm. As described above, in the ambient temperature change, the first resonator, which is one resonator, is used on the low temperature side, and when the ambient temperature increases, the first resonator having a resonance direction that is non-parallel to the resonance direction of the first resonator is used. The laser light is output from the second resonator. As a result, the difference between the oscillation wavelength and the peak wavelength of the optical gain curve can be reduced, and an increase in the threshold current value and a decrease in the slope efficiency (electric-light exchange efficiency) can be suppressed.

The switching between the two resonators is performed by a resonator selector 35 as shown in FIG. The resonator selecting unit 35 includes a temperature sensor 36 and a resonator determining unit 37, and a current having a magnitude corresponding to a use environment temperature of the laser measurement according to the present embodiment is supplied from the temperature sensor 36 to the resonator determining unit 37. Then, a selection signal corresponding to the resonator to be selected in accordance with the magnitude of the current supplied from the resonator determination unit 37 is output to the laser emission driving unit 2. The laser emission driving unit 2 drives one of the first resonator 38 and the second resonator 39 selected according to the selection signal.

The light interference filter 6 in the light emitting means 5 is
FIG. 5 shows the relationship between the wavelength (nm) on the horizontal axis and the light transmittance on the vertical axis.
And a center wavelength of 1,300 nm or 1,5 as shown in FIG.
The half width at 50 nm is 20 nm.

Next, the measurement of the distance between the object to be irradiated and the laser radar according to the above embodiment will be described. In FIG. 1, a semiconductor laser 1 driven by a light emission drive unit 2 in a light emission unit 4 emits light toward an irradiation target (not shown). This light is reflected by the irradiation object, and the reflected light is received by the light receiving means 5. That is, the reflected light first passes through the optical interference filter 6 and reduces stray light that becomes noise, and reaches the avalanche photodiode 7. The electric signal photoelectrically converted by the diode reaches the light receiving amplifier 8. Then, as described above, the distance calculating unit 9 calculates the distance between the semiconductor laser 1 and the irradiation target, and outputs a distance signal from the distance signal output terminal 17.

Further, the first and second resonators 38 and 39 of the semiconductor laser 1 are changed by the ambient temperature change. A signal for driving any one of 39 is output to the laser emission driving means 2. Then, the driving unit 2 drives one of the resonators 38 and 39 to emit laser light.

[0026]

According to the laser radar of the present invention,
A semiconductor laser in which a plurality of recesses having a rectangular cross section from the electrode layer to the second cladding layer are arranged at regular intervals in the stripe groove direction is used, and accordingly, the half-width of the wavelength is changed to that of the conventional optical interference filter. Since a narrower optical interference filter can be used, the background, that is, stray light is significantly reduced as compared with a conventional laser radar using a Fabry-Perot type semiconductor laser as a light source.
The S / N ratio of the detection signal can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing an embodiment of a laser radar according to the present invention.

FIG. 2 is a perspective view showing an example of a semiconductor laser used for the laser radar of the present invention.

FIG. 3 is a sectional view taken along the line II of FIG. 2;

FIG. 4 is a circuit block diagram showing another embodiment of the laser radar of the present invention when the semiconductor laser of FIG. 2 is used.

FIG. 5 is a graph showing the characteristics of an example of an optical interference filter used in the laser radar according to the present invention, with the horizontal axis representing the wavelength and the vertical axis representing the transmittance.

FIG. 6 is a graph showing the characteristics of another example of the optical interference filter used in the laser radar according to the present invention, in which the horizontal axis represents the wavelength and the vertical axis represents the transmittance.

FIG. 7 is a circuit block diagram showing a conventional laser radar.

DESCRIPTION OF SYMBOLS 1 semiconductor laser 2 laser emission drive means 3 power supply unit 4 light emission means 5 light reception means 6 optical interference filter 7 avalanche photodiode 8 light reception amplifier 9 distance calculation means 20 substrate 22 first cladding layer 23 active layer 24 first 2 cladding layer 25 contact layer 26, 27 stripe groove 28 current stop layer 30, 31 electrode layer 33 recess 35 resonator selecting means 38 first resonator 39 second resonator

Claims (3)

[Claims] A first cladding layer, an active layer, a second cladding layer, a contact layer, a current stop layer having a stripe groove forming a current path, and an electrode layer are sequentially laminated on a semiconductor substrate. A light source is a semiconductor laser in which a plurality of recesses each having a rectangular cross section extending over the second cladding layer are arranged at regular intervals in the stripe groove direction, and light emitted from the semiconductor laser is reflected from an irradiation object. A laser radar, wherein the light receiving means for detecting the returning light has an optical interference filter having a narrow wavelength half width. And calculating a distance from the semiconductor laser to the object to be irradiated from a light emission driving unit of the semiconductor laser and a time from when the semiconductor laser emits light to when the return light is detected by the light receiving unit. 2. The laser radar according to claim 1, further comprising a distance calculating unit. 3. A semiconductor laser comprising a plurality of stripe grooves arranged in a non-parallel manner to form a corresponding plurality of resonators as a light source, and the plurality of resonators are used in accordance with a change in measured temperature. 2. The laser radar according to claim 1, further comprising a resonator determination unit that selectively outputs a resonator selection signal to the laser emission driving unit.