JP2000205863A - Optical gyro - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ring laser gyro wherein detection in rotational direction is possible with no such mechanical structure as dither (minute vibration) while rock-in is hard to occur and no optical detector required. SOLUTION: A ring laser provided with one or more tapered parts at a part of optical waveguide, an optical filter 3 which selectively reflects the laser light emitted from the ring laser, and an electric terminal 14 which detects a voltage, current, or impedance in the optical waveguide, are provided. Here, the tapered part makes the reflection loss of laser light circulating in one direction less than that of laser light circulating in the other direction so that the laser light circulating in one direction oscillates, and the optical filter 3 changes the intensity of reflection light according to the oscillation frequency of laser light so that the rotational speed and direction of the optical waveguide is detected based on the voltage, current, or impedance detected from the electric terminal 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical gyro, and more particularly, to a ring laser type optical gyro integrated on a semiconductor substrate.

[0002]

2. Description of the Related Art Conventionally, as a gyro for detecting an angular velocity of a moving object, a mechanical gyro having a rotor and a vibrator and an optical gyro are known. In particular, optical gyros are being revolutionized in the gyro technical field because they can be activated instantaneously and have a wide dynamic range. The optical gyro includes a ring laser gyro, an optical fiber gyro, a passive ring resonator gyro, and the like. Of these, the first to be developed is a ring laser type gyro using a gas laser, which has already been put to practical use in aircraft and the like. Recently, a semiconductor laser gyro integrated on a semiconductor substrate has been proposed as a small, high-precision ring laser gyro. As this known document, there is, for example, JP-A-5-288556 ("semiconductor laser gyro").

According to the above publication, as shown in FIG.
A ring-shaped gain waveguide 1100 is formed on a semiconductor substrate 1000 having a pn junction.
A carrier is injected from the electrode 2200 into the inside to generate laser oscillation. Then, a part of each of the laser light propagating clockwise and counterclockwise in the gain waveguide 1100 is extracted and caused to interfere in the light absorption region 1700, and the interference light intensity is extracted from the electrode 2300 as a photocurrent. I have.

[0004]

However, in the conventional ring laser type gyro, the rotation direction cannot be detected. For this reason, dither (micro vibration) is applied, and the rotation direction is determined from the correlation between the dither (micro vibration) direction and the signal.

In the ring laser type gyro, the oscillation frequency is separated into two with rotation. However, when the rotational speed is small, the difference between the oscillation frequencies becomes small. In this case, due to the nonlinearity of the medium, a lock-in phenomenon in which the oscillation frequency is pulled into one mode has occurred. In order to cancel this lock-in phenomenon, the above-mentioned dither (small vibration) is applied to the ring laser type gyro.

Accordingly, the present invention provides a ring laser gyro that can detect the rotation direction without a mechanical mechanism such as dither (micro vibration), hardly causes lock-in, and does not require a photodetector. The task is to provide.

[0007]

According to the present invention, there is provided a ring laser for generating, as a main mode, a laser beam circulating in an optical waveguide in one direction, and a ring laser emitted from the ring laser. An optical filter for filtering laser light to an intensity corresponding to the wavelength and inputting the filtered light to the ring laser again, and means for detecting a change in voltage, current, or impedance of the ring laser are provided.

Further, the present invention provides a ring laser having a tapered portion at one or more locations of an optical waveguide, and a laser beam emitted from the ring laser which is filtered to an intensity corresponding to the wavelength and is again filtered. An optical filter for inputting to the ring laser, and an electric terminal for detecting a change in voltage, current, or impedance of the ring laser are provided.

Further, the present invention provides a ring laser in which optical elements are provided at one or more locations of an optical waveguide, and a laser beam emitted from the ring laser is filtered to an intensity corresponding to the wavelength, and is again filtered. Optical filter to be input to the ring laser, the voltage of the ring laser,
Current, or an electrical gyro having an electrical terminal for detecting a change in impedance, wherein the optical element is
The transmission loss for the laser light circulating in one direction in the optical waveguide is made smaller than the transmission loss for the laser light circulating in the other direction.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the embodiments of the present invention, first, the operation principle of the optical gyro of the present invention will be described with reference to mathematical expressions.

In the present invention, a ring laser that generates laser light circulating in one direction in an optical waveguide as a main mode is a ring laser that generates only laser light circulating in an optical waveguide in one direction, or A ring laser in which the intensity of laser light circulating in one direction in an optical waveguide is much greater than the intensity of laser light circulating in another direction. The SN ratio of a signal detected by the optical gyro of the present invention is affected by the intensity ratio between the main mode laser light circulating in one direction and the sub mode laser light circulating in the other direction. Therefore, in the present invention, it is preferable to generate only the main mode laser light. However, as long as the S / N ratio of the signal detected by the optical gyro can be secured to a sufficient value, laser light in the sub-mode may be generated.

Suppose that the laser light in the ring laser circulates clockwise, and its wavelength is λ ₁ . When the ring laser is rotated clockwise, the oscillation frequency f ₁ of the clockwise laser light is larger than the oscillation frequency f ₁₀ at the time of non-rotation, as shown in Expression (1).

[0013]

(Equation 1) Only decrease. Here, S ₁ is a closed area surrounded by the optical path of the laser light, L ₁ is the optical path length of the laser light, and Ω is the angular velocity of rotation. On the other hand, when the ring laser is rotated counterclockwise, the oscillation frequency f ₁ of the clockwise laser light becomes
Compared to the oscillation frequency f ₁₀ during non-rotation, as shown in equation (2),

[0014]

(Equation 2) Only increase.

When the laser light emitted from the ring laser enters the optical filter, the intensity of the reflected light changes according to the oscillation frequency of the laser light. At this time, if the reflected light from the optical filter is incident on the ring laser, the laser light originally present in the ring laser and the laser light reflected from the optical filter interfere with each other.

As a result, the carrier concentration in the ring laser fluctuates. This variation in carrier concentration is related to the intensity of the laser light reflected from the optical filter,
The electrical conductivity of the medium constituting the laser is proportional to the carrier concentration. Therefore, the amount of fluctuation of the carrier of the ring laser is a constant current drive,
It can be detected as a change in terminal voltage. On the other hand, in the case of constant voltage driving, it is observed as a change in the injection current.
Further, a change in impedance between the anode and the cathode may be directly measured using an impedance meter.

The relationship between the oscillation frequency during non-rotation and the reflection wavelength of the optical filter is determined in advance, so that the increase and decrease of the oscillation frequency and the increase and decrease of the intensity of the laser light incident on the ring laser correspond to one to one. By doing so, not only the rotation speed but also the rotation direction can be detected.

Since the change in oscillation frequency due to rotation is small, an optical filter having a sharp spectrum is required. For example, there is a method in which a gas is sealed in a cell to use absorption lines of atoms and molecules, and a method of using a laser oscillation spectrum line width. When using the absorption lines of atoms and molecules, for example, by using the shift of the spectrum line due to the application of an electric field, such as the Stark effect used in laser spectroscopy, the oscillation frequency of the ring laser and the resonance frequency of the optical filter can be reduced. Relationship can be controlled.

Now, in order to oscillate only the laser light that propagates in one rotation direction in the ring laser, it is necessary to apply a loss only to the light that propagates in the opposite direction. For example, by providing a tapered portion in a part of the optical waveguide, for light incident on the tapered portion,
Total reflection condition shifts.

In the present invention, for example, the tapered portion is formed as a portion in which the width of the optical waveguide gradually increases along one direction of the propagation direction of the laser light. in this case,
The shape of the tapered portion in a plane parallel to the plane including the propagation path of the circulating laser light has a sawtooth shape.

A mirror loss occurs for light incident on such a tapered portion. Since the angle of incidence on the tapered portion varies depending on the direction in which the light circulates, the loss is so large that laser light circulating in a certain direction becomes impossible to oscillate, and the loss is small for light circulating in the opposite direction. be able to. As a result, it is possible to oscillate only the laser light that propagates in a circular manner in one rotation direction.

Instead of providing a tapered portion in a part of the optical waveguide, an optical element such as an optical isolator may be inserted in the optical path. An optical isolator transmits light having a certain polarization direction in only one direction. Therefore, by inserting an optical isolator, if the polarization of the light circling in the opposite direction is the same as in a normal laser,
A ring laser can be made in which only light propagating in one direction is present. Of course, as long as the optical element has a function of transmitting light only in one direction, not limited to the optical isolator, other optical elements such as an optical circulator may be used.

When the first laser light emitted from the first ring laser is incident on the second laser, the first laser light and the second The second laser light existing in the second laser interferes with the second laser light. By setting the oscillation frequencies of the first laser light and the second laser light at the time of non-rotation to be different from each other, beat light corresponding to the difference between the oscillation frequencies is generated at the time of non-rotation. In this state, when the first ring laser and the second ring laser are simultaneously rotated, the oscillation frequency of the first laser light changes according to Expressions (1) and (2). As a result, the beat frequency changes in the second laser. At this time, if the second laser has an electric terminal for detecting voltage, current or impedance, a beat signal can be detected from this electric terminal. This beat signal can be detected as a change in the terminal voltage if it is driven at a constant current. On the other hand, if it is a constant voltage drive,
It is observed as a change in injection current. Further, a change in impedance between the anode and the cathode may be directly measured using an impedance meter.

The rotation direction can be detected by observing the increase / decrease of the beat frequency from the non-rotation time. However, the rotation direction can be detected only when the oscillation frequency of the first laser beam is f ₁ and the oscillation frequency of the second laser beam is f ₂
(> F ₁ ) when the difference between the oscillation frequencies satisfies the following condition. That is, as shown in equation (3),

[0025]

(Equation 3) If the first laser light and the second laser light have the same non-rotating oscillation wavelength, as shown in equation (4),

[0026]

(Equation 4) And the beat frequency f ₂ −f ₁ takes a positive or negative value. If the absolute values of the beat frequencies are equal, the same signal is output from the terminal, and in this case, the rotation direction cannot be detected. On the other hand, if the sign of the beat frequency is always the same (however, the sign is positive in the description) and only the absolute value changes depending on the rotation direction, the rotation direction can be detected.

The operating principle of the optical gyro according to the present invention has been described above.

In the first embodiment of the present invention, a ring laser including a tapered optical waveguide in at least a part of the optical waveguide so that only a laser beam propagating in one direction is present; The ring laser includes an optical filter that selectively reflects laser light emitted from the ring laser, and the ring laser includes an electric terminal that detects voltage, current, or impedance.

In the second embodiment, a ring laser including at least a part of an optical waveguide including an optical element in which only a laser beam that propagates in a circular direction in one direction, and a ring laser emitted from the ring laser An optical filter for selectively reflecting laser light is provided, and the ring laser is provided with an electric terminal for detecting voltage, current or impedance.

In the third embodiment, a total reflection surface is provided on the optical waveguide of the ring laser. In this configuration, if the ring laser is provided with a total reflection surface and a ring resonator is constituted only by this total reflection surface, mirror loss is eliminated and the laser oscillation threshold value is reduced.

In a fourth embodiment, the ring laser and the optical filter are optically coupled by an optical waveguide.

[0032]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a drawing that best illustrates the features of the present invention, in which 3 is an optical filter, 10 is a quartz tube, 11 is an asymmetric tapered portion of an optical waveguide, and 12 is a mirror. , 13 is an anode, 14 is an electric terminal,
15 is a cathode, 100 is a counterclockwise laser beam, 1
01 is a laser beam emitted from the mirror, and 102 is a laser beam reflected by the optical filter.

In the above configuration, a quartz block was cut out using a drill to form a quartz tube 10. afterwards,
The mirror 12 was attached to the quartz tube 10. Further, an anode 13 and a cathode 15 were attached to the quartz tube 10. Next, a helium gas and a neon gas are put into the quartz tube 10, and a voltage is applied between the anode 13 and the cathode 15, so that a discharge starts and a current flows. Then, the loss to the clockwise laser light is large due to the tapered portion 11 of the optical waveguide. As a result, only the counterclockwise laser light 100 oscillates in the quartz tube 10. Therefore, no pull-in, ie, lock-in, occurs between the laser beams propagating in a circular shape in opposite directions to each other.

As the optical filter 3, a gas cell containing helium gas and neon gas in a quartz cell was used. Further, an electrode was inserted into the gas cell, and an electric field was applied to the gas. However, the conditions were such that no discharge occurred. At this time, the position of the absorption spectrum changes due to the Stark effect. Thus, the positional relationship between the oscillation frequency of the laser beam 100 and the absorption spectrum can be controlled. Here, a gas cell was used as the optical filter,
A laser may be used as the optical filter. When light near the oscillation frequency of the laser light enters the laser, it can be selectively amplified. Moreover, since the oscillation spectrum width of the laser light is narrow, the laser can be used as an optical filter having steep selectivity. However, it is important to suppress the light intensity of the incident light so that injection locking does not occur.

When the quartz tube 10 is stationary, the oscillation frequency of the laser beam 100 is 4.73 × 10 ¹⁵ Hz and the oscillation wavelength λ is 632.8 nm. Laser light 101 is emitted from a part of a mirror constituting the optical resonator and inserted into the optical filter 3. When the laser beam 102 reflected by the optical filter 3 returns to the quartz tube 10, the laser beam 10
2 reduces the population inversion. Therefore, the potential of the electric terminal 14 increases by 10 mV as compared with when the reflected light 102 does not exist.

When the quartz tube 10 is rotated clockwise at a rate of 1 degree per second and the length of one side of the resonator is 10 cm, the oscillation frequency f ₁ of the counterclockwise laser light 100 is 248. Increase by 3 kHz. Since the reflectance of the optical filter 3 is higher than this oscillation frequency, the light intensity of the laser light 102 increases. As a result, the potential of the electric terminal 14 increases by 5 mV compared to when the quartz tube 10 does not rotate. On the other hand, when the quartz tube 10 rotates counterclockwise at a speed of 1 degree per second, the oscillation frequency f ₁ of the laser light 100 becomes 2
Decrease by 48.3 kHz. In this case, since the reflectance of the optical filter 3 is small, the light intensity of the laser beam 102 decreases. As a result, the potential of the electric terminal 14 is reduced by 5 mV compared to when the quartz tube 10 does not rotate.

As described above, the direction of rotation can be detected by increasing or decreasing the potential of the electric terminal 14. In addition, since the absolute value of the amount of change in potential corresponds to the rotation speed on a one-to-one basis, the rotation speed can also be measured. That is, according to the present invention, the rotation speed and the rotation direction can be simultaneously detected.

Here, helium gas and neon gas are used, but any gas that oscillates laser may be used. Also, the shape of the optical waveguide surrounded by the optical path is shown in FIG.
And any shape such as a hexagon, a triangle, or a circle.

(Second Embodiment) FIG. 2 is a drawing that best illustrates the features of the present invention. In FIG. 2, reference numeral 16 denotes an optical isolator.

In the above configuration, the difference from FIG. 1 is that an optical isolator 16 is used instead of the tapered optical waveguide 11.
That is, was used. An optical isolator transmits light having a certain polarization direction in only one direction. Therefore, by inserting an optical isolator, if the polarization of the light circling in the opposite direction is the same as in a normal laser,
A ring laser can be made in which only light propagating in one direction is present. Of course, as long as the optical element has a function of transmitting light only in one direction, not limited to the optical isolator, other optical elements such as an optical circulator may be used.

(Third Embodiment) FIG. 3 is a drawing that best illustrates the features of the third embodiment. In FIG. 3, reference numeral 20 denotes a ring resonator type semiconductor laser, 21 denotes an asymmetric tapered portion of an optical waveguide, and 23 denotes a semiconductor laser. Is an anode, 24 is an electric terminal, 25 is a cap layer, 26 is a cladding layer, 27 is a light guide layer, 28
Is an active layer, 29 is a light guide layer, 34 is a semiconductor substrate, 35
Is a cathode, 200 is a counterclockwise laser beam, 201
Denotes a laser beam emitted, and 202 denotes a laser beam reflected by the optical filter 3. The upper figure is a top view, and the lower figure is a cross-sectional view cut along AA 'of the upper figure.

The manufacturing method in the above configuration will be described. First, an n-InP substrate 34 is formed using a metal organic chemical vapor deposition method.
(Thickness: 350 μm) and 1.3 μm undoped InGa constituting the ring resonator type semiconductor laser 20.
AsP light guide layer 29 (0.15 μm thickness), 1.55
Undoped InGaAsP active layer 28 of 0.1 μm composition (thickness 0.1 μm), undoped InGaAs of 1.3 μm composition
sP light guide layer 27 (0.15 μm thickness), p-InP
Cladding layer 26 (2 μm thick), p-
The InGaAsP cap layer 25 (thickness 0.3 μm) is grown. After crystal growth, p-I was applied using a spin coater.
Photoresist AZ-1350 on nP cap layer
(Made by Hoechst) is applied so that the film thickness becomes 1 μm.
After pre-baking at 80 ° C. for 30 minutes, the wafer was exposed with a mask. The width of the optical waveguide after development and rinsing is 5 μm, and the maximum value of the width of the optical waveguide in the tapered portion 21 is 8 μm and the minimum value is 5 μm. The length of one circumference of the optical waveguide is 600 μm. After that, the wafer is introduced into the reactive ion etching equipment,
Etching was performed using chlorine gas to a depth of 3 μm. Finally, Cr / Au was formed as an anode 23 on the p-InP cap layer 25 by vapor deposition. In addition, AuGe /
Ni / Au was deposited. After that, annealing was performed in a hydrogen atmosphere to make ohmic contact.

In the above configuration, since the refractive index of the semiconductor is different from that of air, reflection occurs at the interface. Assuming that the refractive index of the semiconductor is 3.5, total reflection occurs when the angle between the normal to the interface and the laser beam is 16.6 degrees or more. In the mode that receives total reflection, the oscillation threshold value is smaller by the mirror loss than the other modes, so that oscillation starts at a low injection current level. Moreover, since the gain is concentrated in this oscillation mode, oscillation in other modes is suppressed. In FIG. 3, the angle between the laser beam and the normal to the interface is 45 degrees, which satisfies the condition of total reflection. However, the angle between the normal to the interface and the laser beam was set to 16.5 degrees so that only the interface from which the laser beam was extracted deviated from the total reflection condition. The oscillation threshold at room temperature is 3 mA. The driving current of the ring resonator type laser 20 is 4.5 mA, and when this laser is stationary, the laser beam 2
The oscillation wavelength λ of 00 is 1.55 μm. When the laser beam 202 reflected by the optical filter 3 returns to the ring resonator type laser 20, the population inversion is reduced by the laser beam 202. Therefore, the potential of the electric terminal 24 increases by 10 mV as compared with when the reflected light 202 does not exist.

When the ring resonator type laser 20 is rotated clockwise at a rate of 30 degrees per second, such as camera shake or vehicle vibration, the counterclockwise laser beam 2 is emitted.
Oscillation frequency f ₃ of 00 is increased by 130Hz. Since the reflectance of the optical filter 3 is higher than this oscillation frequency, the light intensity of the laser beam 202 increases. As a result, the potential of the electric terminal 24 increases by 5 mV compared to when the ring resonator type laser 20 does not rotate. On the other hand, when the ring resonator type laser 20 is rotated counterclockwise per second 30 degrees of speed, the oscillation frequency f ₃ of the laser beam 200 is decreased by 130 Hz. In this case, since the reflectance of the optical filter 3 is small, the light intensity of the laser beam 202 decreases. As a result, the potential of the electric terminal 24 is reduced by 5 mV as compared with the case where the ring resonator type laser 20 does not rotate.

As described above, the rotation direction can be detected by increasing or decreasing the potential of the electric terminal 24. In addition, since the absolute value of the amount of change in potential corresponds to the rotation speed on a one-to-one basis, the rotation speed can also be measured. That is, according to the present invention, the rotation speed and the rotation direction can be simultaneously detected.

Here, InG is used as the semiconductor material.
aAsP type was used, but GaAs type, ZnSe type
Any material system, such as a system, an InGaN system, and an AlGaN system, may be used. Also, the shape of the optical waveguide that is surrounded by the optical path is not limited to a square as shown in FIG. 5, but may be any shape such as a hexagon, a triangle, or a circle.

(Fourth Embodiment) FIG. 4 is a drawing that best illustrates the features of the fourth embodiment. In FIG. 4, reference numeral 5 denotes an optical waveguide, 210 denotes a laser beam propagating through the optical waveguide 5, 211
Is a laser beam emitted from the optical waveguide 5, 212 is a laser beam reflected by the optical filter 3, and 220 is an optical waveguide 5
Is a laser beam that propagates through

In the above configuration, since the ring resonator type laser 20 and the optical filter 3 are optically coupled by the optical waveguide 5, the coupling efficiency is improved. Therefore, under the same driving conditions as in the fifth embodiment, the amplitude of the signal voltage obtained from the electric terminal 24 was 7 mV.

(Fifth Embodiment) FIG. 5 is a drawing that best illustrates the features of the fifth embodiment. In FIG. 5, reference numeral 22 denotes an optical waveguide.

In the above configuration, since the optical waveguide of the ring resonator type laser 20 and the optical waveguide 22 are connected, the coupling efficiency is improved. Therefore, under the same driving conditions as in the fifth and seventh embodiments, the amplitude of the signal voltage obtained from the electric terminal 24 became 10 mV.

[0052]

According to the first and second embodiments of the present invention described above, the terminal voltage at the time of rotation is observed, and the increase or decrease from the non-rotation is obtained, whereby the rotation direction and the rotation are obtained. Speed can be detected. In addition, since there is only the laser light that propagates in a circular direction in one direction, an optical gyro without lock-in can be realized.

Further, according to the third embodiment, since there is no mirror loss, the laser oscillation threshold value is reduced. As a result, an optical gyro with low power consumption can be realized.

Further, according to the fourth embodiment, an optical gyro having small optical loss and low power consumption can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an optical gyro according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an optical gyro according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating an optical gyro according to a third embodiment of the present invention.

FIG. 4 is a diagram illustrating an optical gyro according to a fourth embodiment of the present invention.

FIG. 5 is a diagram illustrating an optical gyro according to a fifth embodiment of the present invention.

FIG. 6 is a plan view of a conventional semiconductor laser gyro.

[Explanation of symbols]

 Reference Signs List 3 optical filter 4 laser 5 optical waveguide 10 quartz tube 11 asymmetric tapered portion of optical waveguide 12 mirror 13 anode 14 electrical terminal 15 cathode 16 optical isolator 20 ring resonator type semiconductor laser 21 optical waveguide tapered portion 22 optical waveguide 23 anode 24 electricity Terminal 25 Cap layer 26 Cladding layer 27 Light guide layer 28 Active layer 29 Light guide layer 34 Semiconductor substrate 35 Cathode 41 Anode 42 Cathode 43 Electrical terminal 100 Counterclockwise laser beam 101 Laser beam 102 Laser beam 200 Counterclockwise laser beam 201 laser light 202 laser light 210 laser light 211 laser light 212 laser light 220 laser light

Claims (5)

[Claims] 1. A ring laser that generates a laser beam circulating in one direction in an optical waveguide as a main mode, and a laser beam emitted from the ring laser is filtered to an intensity corresponding to the wavelength, and the ring laser is re-filtered. An optical gyro, comprising: an optical filter to be input to a laser; and means for detecting a change in voltage, current, or impedance of the ring laser. 2. A ring laser having a tapered portion at one or more locations in an optical waveguide, and a laser beam emitted from the ring laser filtered to an intensity corresponding to the wavelength and input to the ring laser again. An optical gyro comprising: an optical filter to be operated; and an electric terminal for detecting a change in voltage, current, or impedance of the ring laser. 3. A ring laser provided with an optical element at one or more locations of an optical waveguide, and a laser beam emitted from the ring laser is filtered to an intensity corresponding to the wavelength and input to the ring laser again. An optical gyro including an optical filter to be operated and an electric terminal for detecting a change in voltage, current, or impedance of the ring laser, wherein the optical element circulates in the optical waveguide in one direction. An optical gyro, wherein transmission loss for laser light is made smaller than transmission loss for laser light circulating in another direction. 4. The optical gyro according to claim 1, wherein the optical waveguide has a total reflection surface. 5. The optical gyro according to claim 1, further comprising a coupling optical waveguide that optically couples the ring laser and the optical filter.