JP2641891B2 - Phase modulation measurement device - Google Patents

Description

The present invention relates to an apparatus for measuring phase modulation in an optical fiber.

The invention relates in particular to a device for measuring the rotational speed as in a gyroscope.

Fiber optic gyroscopes and devices that measure phase modulation use coils made of optical fibers,
Coherent radiation is supplied from one end of the coil. As the coil rotates about its axis, the speed of the radiation propagating along the optical fiber changes significantly, causing a phase shift in the radiation emerging from the other end of the coil. By comparing the phase of the emitted radiation with the phase of radiation emitted from the same light source that does not pass through the coil or propagates in the opposite direction through the coil, rotation speed or other phase modulation can be measured. .

There are various problems when measuring phase modulation using the above apparatus. First, if the intensities of the radiated light passing through the two optical paths change separately, the measurement accuracy will be degraded. In addition, errors occur when the optical fiber is discontinuous or when the temperature changes.

Therefore, an object of the present invention is to provide a phase modulation measuring method and a measuring apparatus which can solve the above-mentioned disadvantages.

A phase modulation measuring device according to the present invention is a device for measuring phase modulation in an optical fiber, wherein the first device is a frequency-modulated coherent radiation light, the first device including an optical fiber.
A light source device that emits the measurement radiation light propagating through the first optical path and the reference radiation light that propagates through the second optical path that is different from the first optical path; and a heterodyne mixture of the radiation lights emitted from the first and second optical paths. A detector that combines and detects various harmonics of the modulation frequency in the homodyne signal, and compares the amplitude of one harmonic in the homodyne signal with the amplitude of the higher harmonic adjacent to this harmonic. And a comparator for extracting an indication of phase modulation between the first optical path and the second optical path.

 The fibers can be arranged in a coil.

The light source device emits frequency-modulated radiated light to opposite ends of the coil, and the detector combines the radiated light emitted from each end of the coil with each reference radiated light by homodyne mixing to form the two light beams. A combined signal including harmonics to the reference radiation light related to the total phase difference between the optical paths is generated, and various harmonics of the modulation frequency in each homodyne signal are detected, and the comparator detects each harmonic in each homodyne signal. Comparing the amplitude of one harmonic of the above with the amplitude of the adjacent higher-order harmonic,
It can be configured to extract readings of phase modulation along the coil in each direction.

Alternatively, the measuring device of the present invention includes two coils arranged coaxially with each other, and emits frequency-modulated radiation from the light source device to both coils, and the radiation is transmitted along the coils. The detector propagates in the opposite direction, and the detector combines the emitted light emitted from each coil with each reference emitted light by homodyne mixing, and detects various harmonics of the modulation frequency in the combined signal. Can compare the amplitude of a certain harmonic in each homodyne signal with the amplitude of an adjacent higher-order harmonic, and take out an indication of the phase modulation of each coil.

The measurement device of the present invention may include another comparator for comparing the output of the comparator for each phase modulation, and may be configured to retrieve an averaged signal representing the phase modulation. The measuring device of the present invention can extract the reading of the rotation speed about the axis of each coil. Desirably, the length of each coil is approximately one-half the wavelength of the radiation propagating along the coil. The comparator can compare the amplitude of the third harmonic with the amplitude of the fourth harmonic. The apparatus of the present invention includes a reflector that reflects, as reference radiation, radiation emitted to one end of the coil or each coil, and combines the radiation emitted from the coil or each coil.
The apparatus of the present invention includes one or more amplifiers connected to the detector for normalizing the relative amplitude of adjacent harmonics.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

In FIG. 1, the fiber optic gyroscope device includes a light source 1 which emits coherent radiation, the radiation having a frequency modulated by a modulation unit 2 to a low frequency of 420 kHz. The light source 1 may be a diode-pumped solid-state ring laser manufactured by "Lightwave Electronics Co., Ltd." or U.S. Pat.
Preferably, the laser described in the above item is used. The modulated radiation is split equally by a beam splitter 3 into two separate beams. The light emitted from the detection optical path is made incident on one end 5 of an optical fiber 10 wound as a coil 11 via a half mirror 4 arranged at an angle. The optical fiber is a single mode fiber having a length of 500 m. Radiation light from another detection optical path is made incident on the other end 7 of the coiled optical fiber 10 via the second inclined half mirror 6.

The half mirror 4 is arranged such that a part of the radiated light supplied to one end 5 of the coil 11 is incident on the first radiation detector 21 as a reference signal via the second half mirror 6. The half mirror 4 reflects the light emitted from one end 5 of the coil 11 toward the second radiation detector 31. Second
Is arranged such that a part of the light energy supplied to the other end 7 of the coil 11 via the first half mirror 4 is incident on the second detector 31 as reference light. The second half mirror 6 reflects the radiation emitted from the other end 7 of the coil 11 toward the first detector 21.

Therefore, the first detector 21 receives the combined radiated light of the radiated light passing through the coil 11 along the clockwise direction and the reference radiated light before being incident on the coil, while the second detector 31 controls the coil 11 The combined radiated light of the radiated light passing along the counterclockwise direction and the reference radiated light before entering the coil is received.

The detectors 21 and 31 function as nonlinear heterodyne detectors that generate a harmonic spectrum of the modulation frequency Wm shown in FIG. Signals representing the amplitudes of the third and fourth harmonics are supplied to comparators 24 and 34 via amplifiers 22, 23 and 32, 33, respectively. Comparators 24 and 34 compare the relative amplitudes, as described below, and output an output signal on lines 25 and 35 representing the phase shift induced in the two radiation paths when coil 11 rotates about its axis. Each occurs. These phase shifts are approximately equal in magnitude and opposite in sign and produce corresponding changes in the relative amplitudes of the third and fourth harmonics. As the coil attempts to accelerate or decelerate, a phase modulation transition occurs for the coil, which occurs in the Fourier spectrum as side bands equally spaced around each harmonic frequency. Therefore, the carrier itself is suppressed, that is, the composite signal is generated as a double sideband of the suppressed carrier signal, and the carrier is reproduced before the amplitude detection is performed. The signals from the comparators 24 and 34 are supplied to a third comparator 40. The third comparator 40 generates an averaged signal representing the phase shift induced by the phase modulation measuring device and supplies this signal to the utilization means 50. The utilization means 50 extracts a signal representing the rotational speed from the phase shift, displays or stores the value using the extracted signal, or uses the value in a steering system or a control system.

The method of operating the gyroscope will be described with reference to FIG.

The amplitude of the harmonic with respect to the reference radiation generated by the detectors 21 and 31 is determined by the difference between the radiation propagating in the coil at the wavelength of the light source 1 in the opposite direction and the reference radiation reflected by the half mirror 4 or 6 and directly received. Related to the total phase difference between them. This phase difference is the arc tangent of the ratio of the amplitude of the even harmonic signal to the adjacent odd harmonic signal. This ratio is a function of the Bessel function of the specific harmonic order, as well as a function of the propagation time in the optical fiber.
When 11 is stationary and the modulation depth of the light source 1 is constant, all the Bensel functions are constant.

The amplitude e _{t of the} transmission signal, that is, the amplitude of the signal before entering the coil is given by the following equation.

In _{e t = E t sin (w} t + m f sin w n t) ... (1) where, E _t is the maximum signal amplitude; W is the light source 1 angular velocity; W _n modulation of the angular velocity; m _f is f / modulation rate given by f _m; Delta] f is the frequency variation; f _n is a modulation frequency; similarly, the amplitude e _r of the received signal, i.e. the amplitude of the signal after passing through the coil is given by the following equation.

e _r = E _r sin [w (tr) + m _f sin w _n (t r) + φ] (2) where r is a time delay induced by the coil; φ is a phase induced by the coil Transition; Heterodyne or Fourier spectrum is given by:

Here, Jn (X) is an nth-order Bessel function of the argument X, and the argument X is Since the phase angle φ is measured by using the ratio of adjacent Bessel functions, an output that does not depend on the signal level of the absolute value is generated.

The frequency change Δf and the modulation frequency are made the best based on the argument of the Bessel function (formula (4)), and the maximum sensitivity at a specific harmonic can be formed ignoring the attenuation effect. When the maximum value of J (x) at the argument x = 4.2 is used in the third harmonic, the following equation is derived from the equation (4).

When f _m = 200 kHz and sinλ / 4 = 1 Δf = 420 kHz The zero-order and fifth-order Bessel functions J ₀ (x) and J ₅ (x) are shown in FIG. 3 together with the sine function. The length of the coil 11 is set to be half the wavelength (180 °) of the radiation propagating through the coil.
That is, sin θ is mathematically equal to 1 and the maximum value is x = 4.2
Is selected so as to correspond to the first maximum of the Bessel function J ₃ (x) of the _third harmonic in, that is, to maximize these products. Since the peaks of adjacent higher harmonics corresponding to J (x) are lower than the peaks of the third harmonic, amplifiers 22 and 32 are arranged to generate higher amplification than amplifiers 23 and 32. And the relative amplitudes for the third and fourth harmonics are normalized. Alternatively, the argument that J ₃ (x) = J ₄ , that is, x =
An argument of 4.88 can also be used. From equation (4), Here, for a Bessel function larger than Δf = 488 kHz J ₁ (x) where f _m = 200 kHz and sinλ / 4 = 1, a smaller amplitude exists at a small argument x that is about 1 or less. That is, it has an effect distinguishable from Rayleigh noise. By using a reference signal having a small time delay, a Bessel function having a large argument can be used. In this case, two adjacent higher order harmonics than the first harmonic
The amplitudes of the harmonics, for example the amplitudes of the second and third harmonics,
Alternatively, it is useful to compare the amplitudes of the fourth and fifth harmonics.

Any nonlinear optical power or Kerr effect is suppressed since the entire spectrum of each beam is affected equally.

By rotating at high speed around the axis of coil 11,
An equivalent Doppler shift occurs. For example, when the rotation speed is 1000 ° / sec, a Doppler shift of 2.8 Hz occurs. In some applications it is useful to detect this Doppler shift, while in other applications the Doppler shift occurs via low frequency phase modulation of the reference beam.

Instead of using a single coil, two coils 61 and 61 made of optical fibers wound as shown in FIG. 4 and arranged coaxially are used, and a single wavelength light is incident on each coil. Can be done. In this measuring device, a directional coupler can be used instead of the beam splitter. Since the entrance and exit points of both coils 61 and 62 are separated, a balanced mixer can be used. Therefore, four detectors 71 to 74 are used. Detector 71
And 72 both receive a part of the radiation supplied to the entrance of the coil 62 in the counterclockwise direction and a part of the radiation emitted from this coil. One detector 71 is a half mirror on the incident side.
The radiated light reflected at 75 and transmitted through the exit-side mirror is received. The other detector 72 receives the incident-side radiation reflected by the half mirror 75 and reflected by the half mirror 76. Similarly, one detector 71 is a coil reflected by the half mirror 76.
The detector 72 receives the radiation emitted from the coil 62 and receives the radiation emitted from the coil 62 through the half mirror 76. The other detectors 73 and 74 are likewise clockwise coils.
Half mirrors located on the entrance and exit sides of 61
Positioned relative to 77 and 78. On the other hand, detector 73
Receives the emission light on the incident side reflected by the half mirrors 77 and 78 and the emission light on the emission side transmitted through the half mirror 78 on the emission side. The other detector 74 receives the reference radiation light reflected by the incident half mirror 77 and transmitted through the emission half mirror 78 and the detection radiation light reflected by the emission half mirror 78.

By using a balanced mixer, noise and errors due to harmonic amplitude modulation distortion in the light source 1 due to frequency modulation are reduced, and it is particularly effective to use a detector of opposite polarity. The reproduction of the harmonic carrier described above
Distinguished from harmonic amplitude modulation distortion.

The present invention is not limited to the embodiment described above, and various modifications are possible. For example, instead of frequency modulated radiation from a light source, other angle modulated radiation can also be used, especially for phase or wavelength modulation. For phase modulation, a phase modulator is included in the device as shown in units 1 'and 2' of FIG.

The present invention is not limited to the measurement of the rotational speed, but can be applied to other phase modulation measuring devices using coils. For example, as shown in FIG. 5, phase modulation can occur due to changes in pressure and temperature experienced by the detector coil 51 and its former 52. The modulation frequency is also related to the length of the coil 51. Reference radiation is 5A
As shown in the figure, the light is separated from the detection radiation approaching the detection coil 51 by an element such as a half mirror or a prism 54.

It is also possible to use a directional coupler 56 as shown in FIG. 5B or a single transmission / reception fiber 55 having a half mirror 57 (or equivalent to a half mirror) as shown in FIG. 5C.

Further, as shown in FIGS. 6A and 6B, a composite of measurement fibers 51A to 51D driven by the same light source can be used.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a gyroscope, FIG. 2 is a graph showing a frequency spectrum of emitted light in the gyroscope, FIG. 3 is a graph showing Bessel functions of various orders, and FIG. FIG. 5 is a diagram showing a configuration of a modification, FIG. 5 is a diagram showing a configuration of a device for measuring another physical value, and FIGS. 5A to 5C are diagrams of a configuration of a modification of the device shown in FIG. FIG. 6A and FIG. 6B are diagrams showing a configuration of another modification. DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Modulation unit 3 ... Beam splitter 4, 6 ... Half mirror 10, 61, 62, 51 ... Optical fiber 11, 51, 52, 61 ... Coil 21, 31 ... Detector 22 , 23,32,33 …… Amplifier 24,34,40 …… Comparator

Claims (10)

(57) [Claims] An apparatus for measuring phase modulation in an optical fiber, comprising: a frequency-modulated coherent radiated light that propagates along a first optical path including an optical fiber (11, 61, 62, 51). A second different from the emitted light and the first optical path
Light source device that emits reference radiation propagating in the optical path of
2) combining the emitted light from the first and second optical paths by heterodyne mixing to generate a combined signal including harmonics to the reference emitted light related to the total phase difference between the two optical paths. Together with a detector (21, 31, 71 to 74) for detecting various harmonics of the modulation frequency in the combined signal;
A comparator for comparing the amplitude of a harmonic in the combined signal with the amplitude of an adjacent higher harmonic, the phase between the first optical path and the second optical path. A phase modulation measuring device configured to extract a modulation reading. 2. The optical fiber is a coil (11, 61, 62, 51).
2. The phase modulation measurement device according to claim 1, wherein the phase modulation measurement device is arranged as: 3. A frequency-modulated radiated light from said light source device (1, 2) is emitted to opposite ends of said coil (11), and said detectors (31, 21) are provided from each end of said coil. The emitted radiation is combined with each reference radiation by homodyne mixing, and various harmonics of the modulation frequency in the combined signal are detected, and the comparator (24, 34) detects a certain high frequency in the combined signal. 3. The method according to claim 2, wherein the amplitude is compared with the amplitude of an adjacent higher-order harmonic, and an indication of phase modulation in the coil is taken out.
Item 8. The phase modulation measuring device according to Item 1. 4. A light source device comprising two coils (61 and 62) arranged coaxially with each other, and the frequency-modulated radiation from the light source device is projected on both coils, and these radiations travel along the coils. And propagate in the opposite direction, and the detectors (71-74)
The emitted light emitted from each coil (61 and 62) is combined with each reference emitted light by homodyne mixing, and various harmonics of the modulation frequency in each combined signal are detected. The amplitude of a certain harmonic in the obtained combined signal is compared with the amplitude of a higher harmonic adjacent to the higher-order side, and an indication of phase modulation of each coil is taken out. 3. The phase modulation measurement device according to claim 2. 5. The comparator for each phase modulation.
5. A phase modulation system as claimed in claim 3, further comprising a further comparator for comparing the outputs of the phase modulation means and adapted to extract an averaged signal representative of the phase modulation. measuring device. 6. The coil or each coil (11, 61, 62, 51)
The phase modulation measurement device according to any one of claims 2 to 5, characterized in that an indication of a rotation speed around is taken out. 7. The coil or each coil (11, 61, 62, 51)
7. The phase modulation measuring apparatus according to claim 2, wherein a length of the phase modulation measurement is substantially equal to a half wavelength of radiation at the time of propagation through the coil. 8. The apparatus according to claim 1, wherein said comparator compares the amplitude of a third harmonic with the amplitude of a fourth harmonic. The phase modulation measurement device according to any one of the above. 9. The coil or each coil (11, 61, 62, 51)
A reflector (4,6,75-78,53) for reflecting the radiated light supplied to one end of the as a reference radiated light, and configured to couple to the radiated light emitted from the coil or each coil. The phase modulation measurement device according to any one of claims 2 to 8, characterized in that: 10. An apparatus comprising one or more amplifiers (22, 23, 32, 33) connected to said detector and configured to normalize relative amplitudes of adjacent harmonics. The phase modulation measurement device according to any one of claims 1 to 9, wherein: