JP3510569B2 - Optical frequency modulation distance meter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distance measuring method or a multiple position measuring method (FMCW method: Frequency Mower) based on a measuring principle using a light source capable of continuous optical frequency sweep.
dulated Continuous Wave. OFDR method: Optical Frequ
ency Domain Reflectometory. Hereinafter, the optical frequency modulation type range finder using the "optical frequency modulation type measuring method" will be described.

[0002]

2. Description of the Related Art In the field of optical measurement, an optical frequency modulation type measuring method is applied to a range finder, an optical fiber sensor, an optical fiber flaw detector, and the like. This optical frequency modulation method is a method of measuring a distance or multiple positions using a light source capable of continuously sweeping an optical frequency. Specifically, the light wave from this light source is frequency-modulated (FM: Frequency Modulati
on), the frequency-modulated light wave is emitted from the measurement location toward the object to be measured, the reflected light reflected from the object to be measured is received again, and the reflected light is reflected by the reference light on the measurement side by interference. A frequency difference corresponding to an optical path delay time (optical path difference, distance) with light, that is, a beat (beat) frequency is measured. This is a method of obtaining a desired measurement amount (in this case, distance, but other than this, temperature and strain are also possible).

That is, in this measuring method, an optical frequency modulated wave repeatedly output from a light source capable of linear sweeping in time is heterodyne detected by an interference optical system, and a desired measurement amount is obtained from frequency information of a beat signal at that time. This is a measuring method, and its beat frequency has a characteristic that it changes depending on the optical path difference time (distance) between the reference light and the object to be measured. Here, "linearly sweeping in time" means changing the frequency at regular intervals with respect to the time axis.

A conventional frequency shift feedback laser (FSF:
Frequency Shifted Feedback (hereinafter referred to as FSF laser) is shown in FIG.
Will be explained. FIG. 7 shows a case of an FSF laser light source 90 using a solid-state laser as a specific example. FSF
The laser light source 90 includes an excitation light source 1, a coupling lens 2, a resonator mirror 3, a gain medium 4, and an acousto-optic device (AOM: Acoust).
ic Optical Modulator) 5 and a resonator mirror 6. That is, an acousto-optic element 5 in which an ultrasonic wave of a modulation frequency f ₀ is propagated is inserted as a frequency shift element inside the laser resonator, and the first-order diffracted light 8 subjected to Doppler frequency shift is reflected by the resonator mirror 6. , Is configured to return to the gain medium 4 again.

A solid crystal is usually used as the gain medium 4, and a laser resonator is constituted by the pumping light source 1 and the resonator mirrors 3 and 6 that match the absorption wavelength of the crystal. Then, the FSF laser output light 7 is extracted from the 0th-order diffracted (non-diffracted) light separated from the acoustooptic device 5. The process of generating the optical frequency modulation signal is 2 because the first-order diffracted light 8 passes through the acousto-optic element 5 twice every time it makes one revolution in the resonator.
A Doppler shift of f ₀ is received, and it makes multiple turns to cause laser oscillation. As a result, its laser output undergoes continuous frequency chirping, which causes the FSF laser source 9
The optical frequency modulation signal is 0.

Then, the output light 7 of the FSF laser light source 90 is guided to the interference optical system 100, and is separated into a reference light 11 and a measurement light 12 by a light distributor / combiner 9, and reference mirrors which are respective reflection mirrors. 10, reflected again by the mirror 13 to be measured and multiplexed by the light distributor / multiplexer 9, and the respective reflected lights of the reference light 11 and the measurement light 12 are combined to form an interference light 14. This interference light 14 is square-law detected (heterodyne detection) and photoelectrically converted by a photodetector 15 to detect a beat signal. Then, the reference beam 11 is detected by the frequency analyzer 16 that identifies the frequency component of the beat signal.
The frequency spectrum (beat frequency value) corresponding to the optical path difference between the measurement light 12 and the measurement light 12 is obtained, and the optical path difference (distance) is finally obtained from the frequency value by the electronic calculator 17.

[0007]

The FSF laser light source 90 has been conventionally required to be low in cost and improved in measurement sensitivity by reducing the number of parts. However, achievement of the object is hindered by the following reasons. ing.

First, the reason why the cost reduction of the FSF laser light source 90 is prevented will be described. The conventional FSF laser light source 90 includes the solid-state laser type described above and the fiber laser type using an optical fiber amplifier. In the former solid-state laser, the pumping light source and the gain medium can be separated, and any wavelength and output can be set arbitrarily by each gain medium crystal, but the device configuration becomes complicated and the pumping light source also has a relatively large output laser. Therefore, the cost is generally high. In addition, the fiber laser has the advantage of being alignment-free because it is an all-fiber optical system. However, as with the solid-state laser, a light source for exciting the optical fiber must be provided separately, and the polarization in the fiber is further increased. For the wavefront control, there are many components such as a polarizing element and insertion of an attenuator for preventing the influence of the nonlinear effect of the fiber due to the high output light. As a result, any of the above types (solid, fiber) must be expensive.

Next, the reason why the sensitivity of the optical frequency modulation type rangefinder using the FSF laser is hindered from increasing will be described. This measurement method is highly sensitive in principle because of the detection principle based on heterodyne detection, but it is premised that the laser oscillation mode is a single mode. In the conventional FSF laser light source 90 described above, the resonator configuration is a normal Fabry-Perot resonator (by the resonator mirrors 3 and 6 in FIG. 7), and at the same time, the resonator length is usually about 20 cm.
This results in a condition (multimode oscillation) in which a plurality of oscillation modes can exist within the gain profile width (wavelength width of gain that allows laser oscillation) specific to the gain medium. As a result, the phase components of the respective modes allow relatively random behavior, and the influence thereof becomes self-noise, which causes a decrease in sensitivity at the time of detection. For this reason, the advantage of high sensitivity in heterodyne detection in principle is not applicable to the actual device configuration for the reasons described above, and sensitivity reduction remains a problem.

The present invention has been made to solve the above problems, and is an optical frequency modulation method capable of reducing the cost by simplifying the structure of the FSF light source and preventing the deterioration of sensitivity due to the conventional multimode oscillation. The purpose is to provide a rangefinder.

[0011]

An optical frequency modulation type rangefinder according to a first aspect of the present invention includes a frequency shift feedback resonator type semiconductor laser which outputs an optical frequency modulation signal whose optical frequency changes with time in a required period. , An optical frequency modulation signal output from this semiconductor laser is divided into reference light and measurement light by an optical distributor, of which the measurement light is irradiated and reflected by the object to be measured, and an optical system that interferes with the reference light again. A photodetector that detects the beat signal specific to the optical path difference between the reference light and the measurement light caused by the interference of this optical system and converts it into an electrical signal, and the frequency that measures the frequency component of the beat signal detected by this photodetector. and analyzer, includes a computer for calculating the final distance from the frequency value determined with the measurement parameters by the frequency analyzer, the frequency shift attributable
Light that circulates back in the laser cavity of a return-cavity semiconductor laser
In addition, by the acousto-optic element inserted in the resonator,
A laser cavity configuration providing a Doppler frequency shift.
The external mirror on one side is set to the frequency of the light circulating in the resonator.
On the other hand, it also serves as a spectral characteristic or a mode selector element,
The optical frequency modulation signal always maintains single mode oscillation.
It is an external mirror for the purpose of disability .

[0012]

A second invention is the optical frequency modulation rangefinder according to the first invention, wherein the laser resonator has a front end face of the semiconductor laser coated with antireflection and a rear end face of the one end face of the laser resonator. In addition, the external mirror also serving as the mode selector element is configured as the other end surface.

[0014]

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) An optical frequency modulation rangefinder according to a first embodiment of the present invention will be described with reference to FIGS. 1, 2 and 3. FIG. 1 is a block diagram of an optical frequency modulation type rangefinder according to the present invention, and FIGS. 2 and 3 show the operation of the present invention and the difference in principle from the prior art.

The optical frequency modulation range finder according to the present invention comprises:
It is composed of an FSF laser light source 110 and an interference optical system 100. The FSF laser light source 110 includes a semiconductor laser (gain medium) 20, a coupling lens 26, an acousto-optic element 23,
It is composed of a mode selector element 27. Above FS
The F laser light source 110 obtains an oscillation output from the semiconductor laser 20 and its driving power supply 21. The semiconductor laser 2
A rear end face of 0 (external mirror M ₁ ) 28 and a mode selector element (external mirror M ₃ ) 27 constitute a laser resonator. In this case, the front end face (external mirror M ₂ ) 29 of the semiconductor laser 20 is usually end face treated with a non-reflection coating (reflectance <0.2%) in order to obtain the FSF resonator effect. In some cases, the above-mentioned end face treatment may not be necessary due to the difference in the individual semiconductor laser elements.

First, the light output from the semiconductor laser 20 itself is guided to the acousto-optic device (AOM) 23 via the lens 26 to be parallel light. Further, the ultrasonic wave 30 is excited in the acoustooptic device 23 by the application of the signal of the modulation frequency f ₀ from the AOM modulation signal source 22, and propagates in the device in the direction of the arrow. At this time, in the acousto-optic element 23, a Bragg diffraction phenomenon occurs due to the interaction between the light and the acoustic wave (photoacoustic effect), and is separated into 0th-order light (non-diffracted light) 25 and 1st-order diffracted light (returned light) 24. To do. Among them, the first-order diffracted light 24 has an optical frequency f ₀ due to the Doppler frequency shift.
It is output after being deviated. The first-order diffracted light 24 has a Fabry-Perot resonator condition in which only one arbitrary oscillation mode component is reflected by the mode selector element (external mirror M ₃ ) 27, and such reflection causes the first-order diffracted light 24. The signal 24 is again input to the acoustooptic device 23, where it is again subjected to the Doppler frequency shift f ₀ and is returned to the semiconductor laser 20 via the lens 26.

As described above, the resonator of this embodiment is a semiconductor
Rear end face of body laser 20 (external mirror M ₁28) and Modeset
Rector element (external mirror M₃) 27, a resonator
Frequency 2f for each round of the inside₀Doppler frequency
Receive a shift. It makes multiple revolutions (reflections)
Laser oscillation, the optical frequency modulation of which is arbitrary single mode
Shows the characteristic of linear frequency chirp.

Here, as an example of the mode selector element 27 which is one of the features of the present invention, a Fabry-Perot resonator (hereinafter referred to as an etalon) structure with two pairs of arbitrary reflectance mirrors is shown, and a single mode based on it is shown. The difference between the oscillation and the conventional FSF laser oscillation will be described with reference to FIGS. 2 and 3.

First, the conventional FSF laser oscillation will be described with reference to FIG. The upper diagram of FIG. 2A shows the relationship between gain and loss, and the lower diagram shows the actual oscillation mode distribution in that relationship. As is clear from this figure, in the conventional FSF laser oscillation, the loss difference Δα between the resonator modes is
Is small over a wide gain wavelength width G _osc (λ), which results in a condition in which a plurality of modes can oscillate, that is, multimode oscillation. The principle of the present invention is shown in FIG.
By making the loss curve L (λ) extremely steep optical frequency dependency as shown in (b), the loss difference Δα is small with respect to only one arbitrary mode, and the loss difference Δα with respect to other modes.
α can be increased. As a result, oscillations in other modes are suppressed, resulting in arbitrary single-mode oscillation. The etalon shown as an example of the mode selector element 27 controls the optical frequency dependence of the loss curve L (λ). This etalon is a wavelength-selective optical element, and in the present embodiment, it is applied as a single-end reflecting mirror of an FSF resonator for the purpose of increasing the sensitivity of an optical frequency modulation type rangefinder, and promotes single-mode oscillation. It is used as one of the means to continuously generate frequency chirps.

Next, referring to FIG. 3, referring to a comparison of the time change between the conventional and the present invention, the optical path difference (delay time τ) between the reference light and the measurement light of the interference optical system 100. Optical frequency difference (solid line and broken line),
That is, the generation process of the beat frequency will be described. First, a common feature of the optical frequency modulation of the FSF laser is that the oscillation is always maintained by the repeated chirping of each mode due to the chirp of each oscillation mode. That optical frequency [nu ₂ by q-th mode born in optical frequency [nu ₂ in FIG. 3 is temporally frequency chirp
Until it disappears, and the next mode is newly reproduced again from the optical frequency ν ₁ . Laser oscillation is generated by repeating the optical frequency modulation operation.

The conventional FSF laser shown in FIG. 3 (a) oscillates in a multi-mode, that is, the modes simultaneously perform optical frequency chirping at the same time. Specifically, the arbitrary time region ΔT
In this case, in this case, four modes (q = 0 to 3) simultaneously exist. The beat frequency component Δ corresponding to the optical path time difference τ between the reference light and the measurement light for each mode
Four ν _b are generated, and the time integration of their superposition becomes a beat frequency spectrum. The state of the spectrum will be described with reference to FIG. When phase noise is generated in each mode, the more the number of modes, the more it contributes to noise as a beat signal, that is, the measurement sensitivity deteriorates. In principle, the measurement method using optical frequency modulation does not necessarily have to be multimode, and single mode is sufficient, which improves the intensity ratio (S / N ratio) of the beat signal to noise. That is, high sensitivity, which is an advantage of the frequency modulation method, can be realized.

The present invention has been made on the basis of the above points. Hereinafter, the same description as the above will be described with reference to FIG. 3B in the case of the single mode optical frequency modulation according to the present invention. . In the same time range ΔT as in FIG. 3A, the present invention is characterized in that the number of modes is always only one. Therefore, the combination with the measuring light shown by the broken line is always one set, and the influence of the phase noise generated for each mode can be minimized. As a result, in the S / N comparison of the beat frequency spectra in the multi-mode oscillation and the single-mode oscillation of FIG. 4, the noise floor (noise level) can be lowered even if the signal strength is the same, and as a result, the measurement sensitivity is improved due to the improvement of the S / N. It can contribute to improvement.

The FSF laser light source 110 in the first embodiment has been described above. Next, returning to the configuration of FIG. 1 again, until the distance is obtained by the output light 25 from the FSF laser light source 110 and the interference optical system 100. Explain the flow. In this embodiment, the 0th order light 25 of the output of the FSF laser is used.
However, depending on the case, the mode selector element 27
The same effect can be obtained by using a part of the first-order diffracted light 24 that passes through.

Output light 25 from the FSF laser light source 110
Is split into a reference beam 11 and a measurement beam 12 by a light distributor / combiner 9 of the interference optical system 100, which are reflected back to the light distributor / combiner 9 by a reference mirror 10 and a measurement target mirror 13, respectively. Then, they are overlapped to form the interference light 14. The interference light 14 is square-law detected and photoelectrically converted by the photodetector 15, and a beat signal peculiar to the optical path difference (distance) from the optical distributor / multiplexer 9 to the mirrors 10 and 13 is generated. The frequency component of this beat signal is used to obtain a beat frequency value by a frequency analysis device 16 such as an FFT analyzer, an RF spectrum analyzer, and a frequency counter, and the optical path is calculated from the relational expression between the measured value and the optical frequency modulation measurement parameter using an electronic computer 17. Find the difference distance.

(Second Embodiment) Next, an optical frequency modulation type rangefinder according to a second embodiment of the present invention will be described with reference to FIG. The difference between the first embodiment and this embodiment is the configuration of the mode selector element 50, and the interference optical system 100
Other configurations including is the same as the first embodiment.

The mode selector element 50, which is a feature of this embodiment, has a fiber Bragg reflection diffraction grating (external mirror M ₃ ) 41 formed on the rear end side of the fiber 40.
They form a laser resonator. Acousto-optic element 2
The first-order diffracted light 24 output from the mode selector element 5 is output as fiber propagation light 43 through the coupling lens 42.
The light propagates through 0 and is reflected by the fiber Bragg reflection diffraction grating 41, which is the rear end surface, and is returned to the semiconductor laser 20 via the lens 42 and the acoustooptic device 23. As a result, laser oscillation of single mode optical frequency modulation, which is a feature of the present invention, can be obtained. The oscillation principle and operation are the same as those in the first embodiment, and thus the description of this part is omitted.

Due to the features of the mode selector element 50 in the present embodiment, since its structure is a fiber component, it can be duplicated more easily than a bulk element, suitable for mass production, and low cost can be realized. In addition, by wrapping the fiber in any laser resonator length, the same storage space can be secured without being restricted by the resonator length, making it easy to downsize the device and standardize the housing, and portability. It can also contribute to cost reduction.

(Third Embodiment) Next, an optical frequency modulation rangefinder according to a third embodiment of the present invention will be described with reference to FIG. The difference between this embodiment and the first and second embodiments is that the mode selector element is not used on the laser resonator side, only the simple external mirror (M ₃ ) 60 is used, and the same role as the mode selector element is achieved. Is included as a single single mode oscillation light source having a diffraction grating structure (active layer structure) 61 in the gain medium of the semiconductor laser 20. Usually, the internal structure of such a semiconductor laser is called a distributed feedback type (DFB: Distributed FeedBack) or a Bragg reflector type (DBR: Distributed Bragg Reflector). Further, the FSF laser light source 11 having this characteristic
The configuration of the interference optical system 100 other than 0 is the same as that of the first and second embodiments.

Also in the configuration of this embodiment, the principle of generation of single-mode optical frequency modulation, which is a feature of the present invention, is the same as that in FIGS. 2 and 3, and the description thereof is omitted here.

Due to the features of the semiconductor laser of this embodiment,
Mode selector element (etalon) 2 referred to in the first embodiment
7. Even if the mode selector element (fiber Bragg reflection diffraction grating) 41 referred to in the second embodiment is not provided outside the semiconductor laser 20, an external mirror whose sole purpose is to return first-order diffraction to the semiconductor laser 20. It can be dealt with by replacing with 60.

In this embodiment, the optical frequency modulation oscillation, which is a feature of the present invention, can be performed with an inexpensive external mirror without using the mode selector element, which is an optical component common to the first and second embodiments. Therefore, according to the present embodiment, the FSF
By reducing the cost of the entire laser light source 110 and simplifying the laser resonator structure, it is possible to contribute to a reduction in alignment (optical adjustment) operation (improvement in ease of use).

[0034]

As described in detail above, according to the present invention, the front end face of a semiconductor laser is subjected to antireflection coating and generated by a laser resonator composed of external mirrors (M ₁ , M ₂ , M ₃ ). By directly coupling the first-order diffracted light to the semiconductor laser through the coupling lens, the semiconductor laser itself can serve as both the pumping light source and the gain medium. The resonator can be simplified by configuring the resonator so that one of the laser resonator mirrors is the rear end face of the semiconductor laser. As a result, the number of parts can be reduced as compared with the conventional solid-state FSF laser, and an inexpensive semiconductor laser having a relatively low output and an arbitrary wavelength can be used, and the cost of the FSF laser light source can be reduced as a whole.

Further, by using one of the reflection mirrors of the FSF laser resonator as an external mirror (M ₃ ) which also serves as a mode selector element, it is possible to realize an optical frequency modulation method from multimode oscillation to arbitrary single mode oscillation. It is possible to prevent deterioration of sensitivity. Therefore, according to the present invention, the structure of the frequency shift feedback laser, which is the light source thereof, can be simplified due to the features of the device configuration and the optical frequency modulation method related thereto, and thus the system cost can be reduced and the optical adjustment can be easily performed. In addition, the measurement sensitivity can be improved due to the characteristics of the optical frequency modulation that oscillates.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical frequency modulation rangefinder according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the principle feature of oscillation of an FSF laser according to the present invention in comparison with the principle of a conventional FSF laser.

FIG. 3 is a diagram for explaining the characteristics of the change over time in the optical frequency modulation of the oscillation of the FSF laser of the present invention in comparison with the case of the conventional FSF laser.

FIG. 4 shows a relationship (S / N) between signal strength and noise strength of a beat frequency spectrum obtained by using an FSF laser light source and an interference optical system according to the present invention, and shows a conventional FSF.
The figure compared with the case of a laser and explaining.

FIG. 5 is a diagram showing a configuration of an optical frequency modulation type rangefinder according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of an optical frequency modulation rangefinder according to a third embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of a conventional optical frequency modulation type rangefinder using an FSF laser.

[Explanation of symbols]

9 ... Optical distribution / multiplexer 10 ... Reference mirror 11 ... Reference light 12 ... Measurement light 13 ... Measurement target mirror 14 ... Interference light 15 ... Photodetector 16 ... Frequency analysis device 17 ... Computer 20 ... Semiconductor laser 21 ... Semiconductor Laser driving power source 22 ... AOM modulation signal source 23 ... Acousto-optic element 24 ... First-order diffracted light 25 ... Output light 26 ... Lens 27 ... Mode selector element (external mirror M ₃ ) 28 ... Rear end surface of semiconductor laser (external mirror M) ₁ ) 29 ... Front end face of semiconductor laser (external mirror M ₂ ) 30 ... Ultrasonic wave 40 ... Fiber 41 ... Fiber Bragg reflection diffraction grating (external mirror M ₃ ) 42 ... Lens 43 ... Fiber propagation light 50 ... Mode selector element 60 ... External mirror (M ₃ ) 61 ... Diffraction grating structure (active layer structure) 100 ... Interference optical system 110 ... FSF laser light source

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 10-82858 (JP, A) JP 8-37342 (JP, A) JP 8-262137 (JP, A) JP 5- 203412 (JP, A) JP-A-9-172215 (JP, A) JP-A-2-308583 (JP, A) JP-A-8-75433 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01B 9/02 G01B 11/00 G01S 17/32 H01S 3/10

Claims (2)

(57) [Claims] 1. A frequency-shifted feedback resonator type semiconductor laser that outputs an optical frequency modulation signal whose optical frequency changes with time at a required period, and an optical distributor refers to the optical frequency modulation signal output from this semiconductor laser. The light is divided into light and measurement light, of which the measurement light is radiated and reflected by the object to be measured and interferes with the reference light again, and the optical path difference between the reference light and the measurement light caused by the interference of this optical system A photodetector that detects the beat signal and converts it to an electrical signal, a frequency analyzer that measures the frequency component of the beat signal detected by this photodetector, and a final value based on the frequency values and measurement parameters obtained by this frequency analyzer. And a computer for calculating a distance, and a laser resonance of the frequency shift feedback resonator semiconductor laser.
Acousto-optics inserted in the resonator for the light that goes back around the resonator
Depending on the element, a laser that gives a Doppler frequency shift for each revolution
An external mirror on one side of the laser resonator configuration
Spectral characteristics or mode selection for the frequency of the rotating light
The optical frequency modulation signal that doubles as a
Optical frequency modulation range finder, which is an external mirror for the purpose of ensuring the oscillation of the Le mode. 2. A optical frequency modulation scheme rangefinder according to claim 1 Symbol placement, mode with the laser resonator, and antireflection coating on the front end face of the semiconductor laser, the rear end surface and one end surface of the laser resonator An optical frequency modulation rangefinder, characterized in that an external mirror that also serves as a selector element is configured as the other end face.