JP3049849B2 - Laser doppler velocimeter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser Doppler velocimeter capable of obtaining a measurement result with high accuracy by modulating an optical modulator with a sinusoidal modulation signal.

[0002]

2. Description of the Related Art Conventionally, there is a laser Doppler velocimeter which measures, for example, a moving speed of a moving body, a vibration speed of a vibrating body, a flow velocity of a fluid, and the like as objects to be measured. Report OQE85-160, page 25-
30 pages). In other words, when the object to be measured is irradiated with a light beam obtained from a laser light source, a reflected light beam having a frequency shifted according to the speed of the object to be measured can be obtained.
By measuring the frequency shift amount of the reflected light beam with respect to the light beam irradiated on the measurement target, the speed of the measurement target can be detected.

In a laser Doppler velocimeter, a reference light beam whose frequency is shifted by a predetermined amount with respect to a light beam obtained from a laser light source is formed based on such a measurement principle, and the reference light beam is used as a reference. The velocity of the object to be measured is measured by measuring the frequency shift amount of the reflected light beam.

FIG. 2 shows an example of such a laser Doppler velocimeter.
Is a substrate having an electro-optical effect (eg, LiNbO ₃ ),
On this substrate 2, a waveguide 3b branched at a branch portion 3a,
An optical waveguide 3 formed substantially in a Y-shape by 3c and 3d is provided. An optical modulator 4 including electrodes 4a and 4b on both sides of the waveguide 3b is provided in the waveguide 3b.

[0005] 5 is located at the end of the substrate 2 is a semiconductor laser, the light beam L _F outputted from the front surface of the semiconductor laser 5, towards the object of interest to be measured S (eg vibrating surface of the speaker) The reflected light L _F ′ is radiated to the waveguide 3
c from the end 3c ₁ and a light beam L output from the back of the semiconductor laser 5.
_R is adapted to be incident on the end portion 3b ₁ of the waveguide 3b.

[0006] The light beam L _R that is output incident on the waveguide 3b from the back of the semiconductor laser 5 is phase-modulated by the optical modulator 4, it will be the reference light. That is, the electrode 4a in the optical modulator 4, by incidence of light beam L _R waveguide 3b since the refractive index of the waveguide is changed in accordance with the voltage applied to 4b, the applied voltage (modulation signal) Thus, a light beam having a phase changed in proportion to the above, that is, a reference light can be obtained.

Here, as a modulation signal applied to the electrodes 4a and 4b, a sine wave signal of a predetermined frequency (angular frequency ω _m ) output from a sine wave generation circuit 6 is supplied via a variable gain amplifier 7. Have been. Therefore, the variable gain amplifier 7
Is supplied to the electrodes 4a and 4b as a modulation signal with a sine wave signal of a predetermined voltage level, whereby an arbitrary degree of phase modulation can be set.

The electric field E _{ref of the} reference light obtained by the optical modulator 4 is

(Equation 1) Is represented by Here, ω ₀ is the angular frequency of the laser beam, m
₁ is a phase modulation index, φ ₁ is an initial phase, and J _q is a q-order Bessel function.

The phase modulation index m ₁ is

(Equation 2) Is represented by Here, α is the modulation efficiency (0 <α <1),
_ns is the refractive index of the substrate 2; γ is the electro-optic constant of the substrate 2;
₁ is a voltage of the modulation signal applied to the electrodes 4a, 4b, l ₁ electrode 4a, 4b length of, d ₁ is the electrode 4a, a 4b interval.

On the other hand, the electric field E _sig of the reflected light L _F ′ irradiated from the front side of the semiconductor laser 5 and reflected on the object S to be measured and guided from the end 3c ₁ to the waveguide 3c is:

(Equation 3) Is represented as Here, k ₀ is the wavelength of the laser beam, φ
₂ is the initial phase.

[0011] reference light and the reflected light are multiplexed is guided to the waveguide 3d from the branch portion 3a, will be received by the photodetector 8 located at the end 3d ₁ part of the waveguide 3d. Here, the output current I _DET (t) of the photodetector 8 is calculated from the reference light and the reflected light as in the following (Equation 4).

(Equation 4) Here, E ₁ = E ^* ₁ , E ₂ = E ^* ₂ , and ξ ₁ are the sensitivities of the photodetector 8. Note that the component including 2ω ₀ cannot be detected by the photodetector 8 and is ignored.

The output current I _DET (t) of the photodetector 8 is ω
_m and 2ω _m , and are supplied to a band-pass filter 9 having a center frequency to extract an AC component. That is, the output i _DET (t) of the bandpass filter 9 is

(Equation 5) Indicated by Here, it is assumed that ω _m >> 2k ₀ v (t). The output of the band pass filter 9 is input to the frequency mixing circuit 10.

The sine wave signal (angular frequency ω _m ) output from the sine wave generation circuit 6 is also supplied to a 分 frequency dividing circuit 11 and a phase shifter 12,

(Equation 6) Are generated, and this reference signal is supplied to the frequency mixing circuit 10. Therefore, the frequency mixing circuit 10 multiplies the output of the bandpass filter 9 of (Expression 5) by the reference signal of (Expression 6).

The output of the frequency mixing circuit 10 is supplied to a band-pass filter 13 having a center frequency of (3/2) ω _m
Only the signal component near (3/2) ω _m is extracted. The output of the bandpass filter 13 is shown by (Equation 7).

(Equation 7)

The output of the band-pass filter 13 is supplied to a demodulation circuit 14 and demodulated, whereby the vibration frequency of the measured object S is obtained, and the velocity v of the object S can be obtained from the vibration frequency of the object S. but not, the output of the bandpass filter 13 shown by the above equation (7) can not be demodulated unconditionally, phase modulation index m ₁ is required to be adjusted.

Here, if the first-order and second-order components of the Bessel function are equal at a certain phase modulation index, the above equation (7) is obtained.
Can be organized. Now, from the Bessel function of FIG. 3, when the phase modulation index m ₁ = 2.6, the matching condition J
It can be seen that ₁ (m ₁ ) = J ₂ (m ₁ ) is satisfied. At this time, the above (Equation 7) is arranged as the following (Equation 8).

(Equation 8)

The first term on the right side of (Equation 8) is the in-phase signal (in
-phase signal), and the second term on the right side is a phase quadrature signal (out-
of-phase signal). Here, if, for example, φ ₃ = 0, π, 2π,... Is adjusted by the phase shifter 12, the in-phase signal component is obtained from (Equation 8).

(Equation 9) Only the demodulation circuit 1
4, the vibration frequency f _D of the measured object S is

(Equation 10) Will be obtained as Here, λ ₀ is the wavelength of the laser light. After all, if λ ₀ is known, it is understood that the velocity v of the object S can be obtained from the vibration frequency f _D of the object S.

[0018]

However, such a laser Doppler velocimeter requires a frequency mixing circuit 10, a phase shifter 12, and the like, so that the signal processing circuit becomes complicated and the electric circuit cannot be miniaturized. There is a problem that it is a major obstacle to downsizing the laser Doppler speedometer.

Further, as described above, the matching condition J ₁ (m ₁ ) =
In order to satisfy J ₂ (m ₁ ), it is necessary to set the phase modulation index m ₁ high (m ₁ = 2.6). Then, as can be seen from the above (Equation 2), it is necessary to set the length l ₁ of the electrodes 4a and 4b to be somewhat longer and increase the voltage V _{1 of the} modulation signal. However the voltage V ₁ of the modulated signal is not preferable that too high a restriction in the electrical design. Therefore, it is necessary to make the length l ₁ of the electrodes 4a and 4b longer. Therefore, it is necessary to lengthen the waveguide 3b, which leads to an increase in the size of the substrate 2, which also hinders the promotion of downsizing of the laser Doppler velocimeter.

Further, although the frequency mixing circuit 10 is used, unnecessary signals are likely to be generated due to the nonlinearity of the circuit. For this reason, there is also a problem that the signal-to-noise ratio decreases and the speed detection sensitivity becomes insufficient.

[0021]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and irradiates an object to be measured with a light beam emitted from a laser light source to obtain reflected light thereof. The light beams emitted from the first and second phase modulators, in which sinusoidal signals having different frequencies are applied as modulation signals, to obtain first and second reference lights, The first and second combined lights obtained by combining the reflected light with the first and second reference lights, respectively, are converted into light detection signals by first and second light detection means. An object of the present invention is to provide a laser Doppler velocimeter that measures the velocity of an object to be measured based on the sum of optical detection signals output from a second optical detector.

[0022]

The first and second laser Doppler velocimeters are used.
The first and second reference lights are obtained by applying two sinusoidal signals having different frequencies to the phase modulator as modulation signals, and are combined with the reflected lights, respectively, and detected by the first and second photodetectors. Thus, a signal corresponding to the speed of the object to be measured can be extracted as the sum of the outputs of the first and second photodetectors only by optimizing the respective phase modulation indices.
Further, the degree of freedom in selecting the phase modulation index increases, and the phase modulation index can be set to a small value.

[0023]

FIG. 1 is a block diagram showing an embodiment of a laser Doppler velocimeter according to the present invention. Reference numeral 20 denotes an entire laser Doppler velocimeter. 21 is a substrate having an electro-optic effect (for example,
LiNbO ₃ ), and on the substrate 21, branch portions 22a, 2a
Waveguides 22e, 22 branched by 2b, 22c, 22d
f, 22g, 22h, 22i, 22j, 22k, 22m
An optical waveguide 22 having a substantially X-shape is provided.

The electrode 2 on both sides of the waveguide 22g
An optical modulator 23 including 3a and 23b is provided.
The electrodes 24a, 24b on both sides of the waveguide 22f are also provided.
An optical modulator 24 consisting of 24b is provided.

[0025] 25 is located at the end of the substrate 21 is a semiconductor laser, the light beam L _F output from the front surface of the semiconductor laser 25 is, toward the object S plane of vibration or the like of the object to be measured, for example, a speaker And the reflected light L _F ′
There has been made to be incident from the end portion 22h ₁ to waveguide 22h. Then, the reflected light L _F ′ incident on the waveguide 22h is branched at the branch portion 22b, and the waveguides 22i and 22f
led to j.

On the other hand, the light beam L _R output from the back of the semiconductor laser 25 is adapted to be incident from an end portion 22e ₁ to waveguide 22e, the light incident on the waveguide 22e beam L _R is bifurcation The waveguide 22f is branched at 22a.
And 22g.

[0027] The light beam L _R that is output guided to the waveguide 22f and 22g from the back of the semiconductor laser 5 are respectively phase-modulated by the light modulator 23 and 24, will be the reference light. That is, the light modulators 23 and 24 are
Since the refractive index of the waveguide changes in accordance with the voltage applied to the waveguides 22a, 23b and 24a, 24b, the waveguide 22f
And a light beam L _R passing through 22 g, in proportion to the applied voltage (modulation signal) can be changed the phase, and the reference light it.

[0028] 26 is a sine wave generating circuit for generating a sine wave of angular frequency omega _m, sine wave signal output from the sine wave generating circuit 26, the electrode 23a of the optical modulator 23 as a modulation signal, the 23b Has been applied. Further, the sine wave signal output from the sine wave generation circuit 26 is also supplied to a 回路 frequency divider circuit 27, where the sine wave signal is divided by the 分 frequency divider circuit 27, The obtained signal is further supplied to the variable gain amplifier 28. Then, signals given predetermined gains by the variable gain amplifiers 22 and 24 are applied to the electrodes 24a and 24b of the phase modulator 24 as modulation signals.

At this time, the light beam L _R emitted from the back side of the semiconductor laser 5 and guided to the waveguides 22f and 22g.
Are phase-modulated by the phase modulators 23 and 24, the electric fields E _ref1 and E _ref2 of the reference light are

[Equation 11] as well as,

(Equation 12) Is represented by Where ω ₀ is the angular frequency of the laser beam, η
₁ is a reference light branching ratio, m ₁ and m ₂ are phase modulation indices, φ
₀ and φ ₁ are the initial phase, J _p is the p-order Bessel function, J
_q is a q-order Bessel function.

Here, the phase modulation index m _i (i = 1,2) is

(Equation 13) Can be represented by Here, α is the modulation efficiency (0 <α
<1), _ns is the refractive index of the substrate 21 in the Y direction,
Electro-optic constant of, V _i is the electrode 23a, 23b or the electrode 2
The voltage of the modulation signal applied to 4a, 24b, l _i
3a, 23b or the electrodes 24a, 24b the length of, the d _i electrodes 23a, 23b or the electrodes 24a, a distance 24b.

On the other hand, the laser light L _F emitted from the front side of the semiconductor laser 5 and applied to the object S moving at the speed v is reflected by the object S to become reflected light L _F ′ and guided. After being incident on the wave path 22h, it is branched as described above and introduced into the waveguides 22i and 22j. The two reflected electric fields E _sig1 and E _sig2 are

[Equation 14] as well as,

(Equation 15) Is represented as Where η ₂ is the reflected light branching ratio, φ
₂ is the initial phase.

Reference light obtained by passing through waveguide 22g
Is applied to the reflected light passing through the waveguide 22i and the branch portion 22c.
And the combined light is coupled to the end 22m of the waveguide 22m.
₁ Irradiation is performed on the photodetector 29 disposed in the portion.
You. The reference light obtained by passing through the waveguide 22f is:
In the reflected light passing through the waveguide 22j and the branch portion 22d
The combined light is combined with the end 22k of the waveguide 22k. ₁ Department
The light is irradiated on the photodetector 30 disposed in the minute.

For the photodetectors 29 and 30, for example, a photodiode array having a common cathode is used. Therefore, the sum I _DET (t) of the output currents of the two photodetectors 29, 30
Is

(Equation 16) Becomes Where E ₁ = E ^* ₁ , E ₂ = E ^* ₂ , ξ ₁ and ξ
₂ is the sensitivity of the photodetectors 29 and 30, respectively. In addition,
The component containing 2ω ₀ cannot be detected by the photodetector 8 and is ignored.

The sum I of the output currents of the photodetectors 29 and 30
_DET (t) is a bandpass filter 3 centered on ω _m
1 and an alternating current is taken out. That is, the output of the bandpass filter 31 is

[Equation 17] Indicated by Here, it is assumed that ω _m >> 2k ₀ v (t).

Here, the matching condition is

(Equation 18) By setting each value so that is satisfied,
The output of the band-pass filter 31 represented by (Equation 17) is further organized,

[Equation 19] Becomes

The first term on the right side in (Equation 19) is an in-phase signal (in-phase signal) component, and the second term on the right side is a phase quadrature signal (out-of-phase signal) component. Here, a predetermined level of DC offset voltage is applied to the electrodes 24a and 24b of the optical modulator 24 by the circuit system represented by the capacitor C and the coil L, and φ ₁ −φ ₀ = 0, π, 2π,.・
If it is adjusted so that

(Equation 20) Only can be taken out.

By inputting the output of the band-pass filter 31 shown in (Equation 20) to the demodulation circuit 32, the vibration frequency f _D of the object S to be measured is changed.

(Equation 21) Will be obtained as Here, λ ₀ is the wavelength of the laser light. That is, if λ ₀ is known, the speed v of the object S is obtained from the vibration frequency f _D of the object S.

As described above, the laser Doppler velocimeter of the present embodiment is provided with two phase modulators 23 and 24 to which sinusoidal signals having different frequencies are applied, obtains two reference lights, and combines them with the reflected light. Then, by entering the photodetectors 29 and 30, a signal that can be demodulated in the demodulation circuit 32 can be easily extracted from the sum of the outputs of the photodetectors 29 and 30. That is, no frequency mixing circuit or phase shifter is required, and the number of bandpass filters is reduced. Therefore, the signal processing circuit system can be greatly simplified. Furthermore, since no unnecessary noise is generated because no frequency mixing circuit is used, the speed detection sensitivity is improved.

[0039] Furthermore, the matching conditions shown in the above equation (18), but can be regarded as the optical detector sensitivity ξ ₁ = ξ ₂ by using a photodiode array to the photodetector 29 and 30, characteristics of this case the diode Need not necessarily be balanced. The values of the branch ratio η ₁ of the reference light and the branch ratio η ₂ of the reflected light depend on the optical waveguide 22 on the substrate 21.
It can be set arbitrarily depending on the design of. Therefore, the setting of the phase modulation indices m ₁ and m ₂ greatly increases the degree of freedom.

That is, the values of the phase modulation indices m ₁ and m ₂ are no longer limited to values in which the first-order and second-order components of the Bessel function become equal as in the conventional case. Therefore, the phase modulation indices m ₁ and m ₂ can be set to smaller values. For example, when the phase modulation index m ₁ = m ₂ = 0.5, the electrodes 23a, 23b, 24
The lengths of a and 24b are one of the electrode lengths of the conventional optical modulator.
/ 5 or less, which can greatly contribute to downsizing of the substrate 21.

The configuration of the laser Doppler velocimeter of the present invention is not limited to the above-described embodiments, and various types can be considered.
The set value of the phase modulation index is the numerical value (m ₁
= M ₂ = 0.5). Needless to say, it should be set based on specific circuit design and board design conditions.

[0042]

As described above, the laser Doppler velocimeter according to the present invention generates a modulation signal to be applied to the electrodes of the phase modulator by two sine-wave signals having different frequencies. The system can be greatly simplified, and the degree of freedom in selecting a phase modulation index increases. By setting the phase modulation index to be small, the length of the electrode of the optical modulator can be significantly reduced. . Therefore, there is an effect that it greatly contributes to promotion of downsizing of the laser Doppler speedometer. Further, unnecessary signals generated in the course of signal processing can be eliminated in principle. As a result, the signal-to-noise ratio is improved, and the sensitivity of speed detection of an object can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional laser Doppler velocimeter.

FIG. 3 is an explanatory diagram of a Bessel function.

[Explanation of symbols]

 REFERENCE SIGNS LIST 20 laser Doppler velocimeter 21 substrate 22 optical waveguide 23, 24 optical modulator 25 semiconductor laser 26 sine wave generating circuit 27 1/2 frequency dividing circuit 28 variable gain amplifier 29, 30 photodetector 31 bandpass filter 32 demodulation circuit

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-2075 (JP, A) Patent 2829966 (JP, B2) Patent 2779038 (JP, B2) Patent 2689503 (JP, B2) JP 6-10687 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01S 17/00-17/95 G01P 3/36 G01P 5/00

Claims (1)

(57) [Claims] 1. An object to be measured is irradiated with a light beam emitted from a laser light source to obtain reflected light, and a sine wave signal having a different frequency is applied as a modulation signal to the light beam emitted from the laser light source. To the first and second phase modulators to obtain first and second reference lights, and the first and second reference lights are combined with the reflected light to obtain the first and second reference lights, respectively. The first and second combined lights are converted into light detection signals by first and second light detection means, and the light is detected based on the sum of the light detection signals output from the first and second light detectors. A laser Doppler velocimeter configured to measure the velocity of a measurement object.