JPS61235719A - Fiber interferometer - Google Patents

Abstract

PURPOSE:To make output variation small and to take a measurement with high precision by suppressing variation of resonance condition due to return light to a laser by adding an AC component to a current supplied to the laser and then widening the spectral width of laser projection light. CONSTITUTION:The projection light from the laser light source 1 is incident on a branch 21 of a fiber coupler and sent equally to branches 22 and 23. Light entering the branch 22 is further distributed equally to branches 32 and 33 to enter a fiber loop 4 and an optical phase modulator 5. Light propagated in the loop 4 returns to the fibers coupler 3 while having an optical phase difference. This light enters the coupler 2 and is incident on a photodetector 7. The intensity of the detected light is inputted to a signal processing circuit 12, which performs synchronous detection with a driving signal from a driving signal generator 13 and outputs the result. This constitution can not deal with variation in the intensity of the incident light by itself, so a photodetector 6 is provided at the branch 23 and its photodetection output is compared with the output voltage of a reference voltage source 8 to control a laser supply current by a control circuit 10 with the comparison output, thereby controlling the light intensity of the light source 1.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention uses a fiber as an optical path, and a physical change that occurs in the optical path does not cause a phase difference in propagating light, and this is measured as a change in the intensity of interference light.・Gyro, temperature, pressure,
This invention relates to an interferometric sensor that measures flow rate, underwater acoustics, etc.

[Background of the invention]

A fiber sensor that constitutes an interferometer using a fiber as an optical path and detects changes in physical quantities applied to the fiber by converting them into interference intensity is disclosed in, for example, Japanese Patent Laid-Open No. 55-93010. These sensors can have various optical system configurations,
There are various detection targets.

In both of these methods, the incident light of the laser is divided into two optical paths using a half mirror, etc., and the phase difference that occurs when the lights are brought together again is calculated by the different changes in physical quantities that the light that propagates through these two optical paths undergoes. It is configured to be detected. At this time, the difference in phase difference is observed as a change in the interference light intensity, so if the laser light intensity incident on the fiber changes, the detector indication will change even if there is no change in the physical quantity to be measured. , there was a problem of loss of accuracy.

The causes of such changes in laser light intensity are (1)
(2) The optical coupling efficiency between the laser and the fiber changes due to changes in temperature, etc. (2) The laser, which is an optical resonator, receives reflections from the optical system after laser emission, causing the light to re-laser. By returning to , the resonance state is modulated by so-called external conditions, resulting in a change in the resonance state and, therefore, a fluctuation in the output.

etc. can be mentioned.

[Purpose of the invention]

An object of the present invention is to provide an optical fiber interferometer that eliminates changes in the incident light from the laser light source described above, has small output fluctuations, and is capable of highly accurate measurement.

[Summary of the invention]

The present invention is one of the causes of the above-mentioned incident light intensity fluctuation.
By adding an alternating current component to the current supplied to the laser, the spectrum width of the laser emitted light is widened, and even slight fluctuations in the resonance conditions are suppressed. By detecting a portion of the light that actually enters the fiber and passes through the fiber coupler, the change in intensity can be determined, and this can be used to control and stabilize the entire laser co-feed current. .

[Embodiments of the invention]

Hereinafter, embodiments of the present invention will be described using a fiber gyro configuration as a representative example of a fiber interferometer, with reference to FIG.

In FIG. 2, the emitted light from the laser light source 1 enters a fiber 21, and 21 is one branch of the fiber coupler 2. The laser light source has the configuration shown in FIG. 3, and is configured so that the light emitted from the semiconductor laser diode 42 is focused through a lens 44 on the end of a fiber connected to a stainless steel book 46. The fiber coupler 2 has the configuration shown in FIG. 4, for example. That is, the cladding layer of a single mode fiber 52, 54 is thinned by etching, melting, or research, so that light leaks from the core. Two light beams are prepared, and mutual optical coupling is achieved by the leaked light. Therefore, the incident light from 21 is equally divided into branch branches 22 and 23 and output. The light emitted to 22 is transferred to another fiber.
The light enters one branch 31 of the coupler 3, is equally distributed to branches 32 and 33, and enters both ends of the serial optical path of the fiber loop 4 and optical phase modulator 5. In the configuration of the optical fiber gyro, the light propagating through the fiber loop 4 from both directions is affected by changes in rotational angular velocity, and the light propagates through the fiber loop 4 again.
It returns to coupler 3 with an optical phase difference. These lights interfere due to the phase difference and one intensity is modulated. This light enters the fiber coupler 2, branches into a branch 24, and enters the photodetector 7. The optical phase modulator connected in series with the fiber loop 4 continuously modulates the interference light intensity,
Facilitates detection of weak forces. The detected light intensity is modulated and input to the signal processing circuit 12 as an alternating current electric signal, which is synchronously detected with the drive signal of the drive signal generator 13 of the one-phase modulator 5 and output processed.

The above configuration alone cannot cope with changes in the intensity of the incident light of the laser light source 7. Therefore, another photodetector 6 is provided in the one branch 23 of the fiber coupler 2, and its photodetecting power is used as a reference. The output voltage of the voltage reducer 8 is compared with the comparator circuit 9. The laser supply current is controlled by the control circuit 10 based on the output, and the light intensity of the laser light source is controlled to be constant. Although this keeps the average intensity of the laser beam constant, it cannot suppress the instability of laser oscillation due to the reflected return light of the laser beam and the accompanying instantaneous fluctuations in the output intensity (mode hopping). Therefore, an AC voltage modulator 11 was provided in series with 10 to superimpose an AC component on the current supplied to the laser. When an alternating current is superimposed on a laser, the resonance state is modulated and the spectrum of the laser emitted light becomes multimode. FIG. 5 shows the change in the spectrum when AC is superimposed. When the spectrum is broadened in this way, the influence of external resonance conditions is alleviated, and the mode hopping described above can be eliminated.

[Embodiments of the invention]

In the second embodiment, the light from the laser light source 1 is split by a beam splitter 3, and then transferred to an optical fiber loop 4.
The beams rotate in the clockwise and counterclockwise directions, merge again through the beam splitter 3, and reach the photodetector circuit 7. During this time, both the clockwise and counterclockwise light waves undergo phase modulation by the phase modulator 30.

The electric field strength of the light incident on the photodetector circuit 7 is E, = A cos ((11t + Φ (1) + ψay )
−(1) Eo, =Acos ((11t+Φ(t
−v ) tPcaw) −(2). Here, E6Wy Fi:6 (Ill are the electric fields of clockwise and counterclockwise light, respectively. Φ.

is the amount of phase shift caused by the phase modulator.

Φ, = φm811 (i>jy φ, two-phase modulation degree ω,: modulation angular frequency ... (3)
Also, τ is the propagation delay time in the optical fiber loop, L τ=□ ...(4)n
: Fiber refractive index L: Fiber loop length C: Speed of light.

E in the photodetection circuit 7. The power of the interference light by w* Eaaw is detected, and the output P as an interferometer is P = (E a
-+ E caw) ”= A” [1+-zos 2
((11t+Φ(t) +'faw)+-cos2(
ωt+Φ(−τ)+ψ. CI+)+cos(2ωt+
Φ(1) + Φ(-τ) + F aw + 'f' O
aw) + cos (Φ(1)-Φ(-τ)+'l"c+w
-To. w) ] = (DC component) + (2ω component) + (harmonic component of ω and ω1)... The 0 rotation angular velocity information that becomes (5) is the harmonic of (ω and ω) in equation (5). wave component), which is processed by the signal processing circuit 60 to obtain an angular velocity signal.

In equation (5), (2ω component) is an optical frequency and is substantially detected as a DC component. In this way, the interferometer output signal contains a considerable DC component, and the S/
It can be seen that when the light intensity of the light source is increased to improve the N ratio, this DC component also increases.

The above fact is an obstacle when photoelectrically converting the interferometer output signal. That is, in the photodetection circuit 7, a photodiode detects an optical signal as a photocurrent, but a calculator circuit is required to convert this from current to voltage. At this time, if the photocurrent from the photodiode is directly converted into current-to-voltage, if the DC component is predominant, the electronic circuit for current-to-voltage conversion may exceed its dynamic range and become saturated.

Therefore, in this embodiment, as shown in FIG. 1, the photoelectric conversion circuit is provided with frequency selection characteristics, particularly characteristics that suppress DC gain. The circuit shown in FIG. 1 will be explained in detail below.

The photodiode 100 is biased by a bias voltage vIl, and when an optical signal is incident thereon, it generates a photocurrent corresponding to the optical signal. This photocurrent signal is input to the inverting input terminal of the operational amplifier 200, and is subjected to current-to-voltage conversion by a so-called transimpedance circuit.

Here, the capacitors 301 and 305 are set to have a sufficiently small value with respect to the phase modulation angular frequency ω,
Therefore, the phase modulation angular frequency component is mainly fed back through the resistor 302, and the DC signal is fed back through the resistors 303 and 304.

Therefore, by setting the feedback resistor 302 for the modulated signal sufficiently large and the feedback resistors 303 and 304 for the DC signal sufficiently small, the operating point can be set near Ov even if the optical input signal has many DC components. , it is possible to perform current-to-voltage conversion of the modulated signal over a wide dynamic range with sufficient gain.

Resistors 306 and 307 increase the current-voltage conversion gain for the modulated signal by voltage-dividing feedback. In other words, the value of the ratio of the resistors 306 and 307 is multiplied by the resistor 302 as a multiplier to increase the effective value of the resistor 302.

FIG. 6 shows another embodiment, in which a photodiode 100
The photocurrent signal is first passed through a resistor 310 for current-to-voltage conversion.Then, an operational amplifier 200 and an impedance 4
It is amplified by an inverting amplifier circuit according to 01,402. Here, the impedances 401 and 402 have filter characteristics that suppress the DC gain and emphasize the modulated signal.

According to this embodiment, it is possible to detect an information signal from the modulated output signal of the interferometric fiber sensor while preventing saturation due to the DC component, thereby increasing the dynamic range of the sensor and improving the S/N ratio. effective.

〔Effect of the invention〕

According to the present invention, it is possible to suppress the intensity of the laser beam incident on the fiber and its instantaneous fluctuations, and it is possible to significantly improve the accuracy and stability of the fiber interferometer. In the experimental example, the output drift could be reduced to 1/100 or less compared to the case where the feedback by the detector 6 and the AC superimposing circuit 11 were not provided.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram showing a detection circuit of an optical fiber interferometer, Fig. 2 is an embodiment of the present invention, Fig. 3 is a laser light source used in the present invention, and Fig. 4 is a circuit diagram showing a detection circuit of an optical fiber interferometer.・Each configuration of the coupler, FIG. 5 is a spectral waveform diagram of the laser light source, and No. Va shows another embodiment of the detection circuit. 1... Laser/light source, 2... Fiber coupler (
21, 22, 23, 24 same branching technique), 3... Fiber coupler (31, 32, 33, 34 same branching technique), 4...
...Fiber loop, 5...Optical phase modulator, 6.
... Photodetector, 7... Photodetector, 8... Reference voltage source, 9... Differential amplifier, 10... Laser current source, 11
... AC current modulator, 12 ... Synchronous detection circuit, 13
...Second drink 5 mouths

Claims (1)

[Claims] 1. In a fiber interferometer that includes a laser light source, an optical fiber that guides the light source, and a fiber coupler that has a structure that allows the light propagating in the fiber to leak through the cladding of the fiber, a fiber branch that connects to the laser input end. A photodetector is provided at the output end of the branch of the fiber coupler in which the light incident from the branch branches and propagates through the cladding part of the fiber, and the supply current of the laser light source is controlled by the output of the detector. A fiber interferometer characterized by: