JPH0260127B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a gyro used as an angular velocity sensor of an inertial navigation device for determining the self-position, speed, attitude, etc. of a moving body such as an aircraft in an inertial space. This invention relates to an optical fiber laser gyroscope using a laser light source and an optical fiber.

<Prior Art> Figure 1 shows the basic configuration of an optical fiber laser gyroscope. 1 is a semiconductor laser that generates a coherent laser beam, L ₁ is a lens that focuses the emitted light from 1 into a parallel beam b, 2 is a reflected beam b ₁ of beam b, and a transmitted beam
b This is a semi-transmissive mirror (beam splitter) that splits the beam into _two . The two-split beams b ₁ and b ₂ enter the single mode optical fiber 3 wound in a loop through condensing lenses L ₂ and L ₃ as counterclockwise light and clockwise light from both ends of the single mode optical fiber 3 . The emitted light beams b ₂ ′ and b ₁ ′ from the optical fibers are transmitted and reflected by the semi-transparent mirror 2 again via the condensing lenses L ₂ and L ₃ , and are reflected as interference light b ₃ via the condensing lens L ₄ . The light enters a light receiving element 4 such as a photodiode.

Here, when the entire optical system rotates at an angular velocity Ω with respect to the inertial space in a plane parallel to the plane of the paper, the counterclockwise light b ₁
Due to the SAGNAC effect, a phase difference of Δθ=4πlr/cλ・Ω (1) occurs between the clockwise light b 2 and the clockwise light b ₂ . However, l and r are the fiber length and loop radius, and c and λ are the light speed and wavelength of the laser beam in vacuum.

Therefore, when b ₁ ′=Acosωt and b ₂ ′=Acos(ωt+Δθ), the intensity I of the interference light b ₃ observed by the photodetector 4 is I=|b ₁ ′+b ₂ ′| ² =Acosωt +Acos(ωt+Δθ)| ² =A|cos ² ωt+cos ² (ωt+Δθ) +2cosωtcos(ωt+θ)| =A(1+cosΔθ) =A{1+cos(4πlr/Cλ・Ω)} (2)

As is clear from equation (2), I
Since the change in is proportional to the cosine of Ω, the rate of change is extremely small when Ω is extremely small, which has the disadvantage of low sensitivity during minute rotations.

In order to solve this problem, the configuration of the optical system should be devised so that equation (2) is proportional to the sine, that is, the phase is delayed by 90°. More than 10 methods have been proposed, including the optical heterodyne method and the optical heterodyne method, but all of them significantly reduce power utilization efficiency, and it is difficult to realize an optical fiber that transmits two polarization states with the same characteristics. It also has the disadvantage of requiring a large number of additional elements, making the structure complicated and expensive.
Volume No. 10 (October 1981), pages 937 to 946).

<Configuration of the present invention> The present invention provides an optical fiber laser gyroscope that can create a 90° phase delay with respect to interference waves without requiring any complicated optical system configuration or additional elements. , its configuration features are as follows: (1) A linearly polarized light source is used as the laser light source.

(2) A polarization-maintaining optical fiber is used as the optical fiber.

(3) By using a metal thin film that has a phase delay characteristic due to reflection, such as an Inconel half mirror, as a semi-transmissive mirror and setting the incident angle to a predetermined value, it has a phase delay of 90° for the twice reflected light. let

It is a combination of Commercially available products can be used for all of the elements (1), (2), and (3).

The specific structure of the present invention will be explained below with reference to Examples. In FIG. 2, 1 is a semiconductor laser that generates a linearly polarized laser beam, 5 is a semi-transparent mirror made of a metal thin film such as an Inconel half mirror, and 6 is an optical fiber having polarization preserving characteristics. P indicates the polarization plane of the laser beam, which is parallel to the plane of the paper. φ indicates the angle of incidence and reflection angle of each beam on the semi-transmissive mirror 5, and is selected to a specific value. Condenser lenses L ₁ to _{L 4} ,
The light receiving element 4 has the same configuration as that shown in FIG.
The basic configuration for receiving b ₃ and determining the angular velocity Ω is also the same as that in Fig. 1.

Next, the characteristics of the metal thin film semi-transmissive mirror 5, which is a feature of the present invention, will be explained. Figure 3 shows the relationship between the phase delay α during two reflections and the incident angle φ for metals.
This is a comparison between Au, Ni and Inconel half mirrors. Inconel half mirror is made by depositing an alloy of Ni, Cr, and Fe on a glass substrate.
It has properties intermediate between glass and metal, and has a characteristic that at a specific incident angle of approximately 75°, the phase delay of two reflections is 90°. On the other hand, there is no phase delay for transmitted light.

Therefore, if φ≒75° in Figure 2,
Since b ₁ and b ₁ ′ are reflected twice by the semi-transmissive mirror 5, and beams b ₂ and b ₂ ′ are transmitted twice, these two
A 90° phase difference is created between the two beams, and a 90° phase delay is generated in the interference light b _3. Equation (2) is: I=A {1+cos (4πlr/Cλ・Ω−90°)} = A {1+sin(4πlr/CλΩ)} (3) As shown in FIG. 4, the maximum sensitivity is achieved when the rotational angular velocity Ω is zero, making it possible to improve the measurement limit of minute angular velocity changes.

That is, according to the present invention, a metal thin film having a phase delay characteristic is used as a semi-transmissive mirror that splits the laser beam from the light source into two, and a polarization preserving type is used as the optical fiber, which are easily available elements. A phase difference of 90° can be created with an extremely simple configuration.

FIG. 5 shows an example of the application of the present invention when Ω or l and r become large and the phase difference becomes one cycle or more of the wavelength. This is the same as the embodiment described above, and the output of equation (3) is obtained by the light receiving element 4. 7 is beam b from the light source
A second semi-transmissive mirror inserted in the middle of the mirror reflects the beam 5 that forms the first semi-transmissive mirror once, and separates the interference light b ₄ between the beams that passes through it once and sends it to the condenser lens L ₅ . The light is made incident on the second light receiving element 8 through the light receiving element 8. In this case, the two light beams forming the interference light b ₄ are each reflected once and transmitted through the metal thin film semi-transmissive mirror 5, so the phase lag is equal and the phase difference is 0, and the interference light b ₄ has a phase lag. does not occur. Therefore, the output generated at the light receiving element 8 is (2)
As shown in the formula, it is proportional to the cosine, which is the same as in the conventional example shown in FIG.

9 and 10 occur in light receiving elements 4 and 8 (3)
This is an amplifier that amplifies the output expressed by equation (2) and removes the bias component, and adjusts the gain to make the amplitude uniform. Therefore, the outputs x ₁ and x ₂ are respectively x ₁ =Asin4πlr/C·Ω x ₂ =Acos4πlr/C·Ω. In FIG. 6, the solid line indicates x ₁ and the dotted line indicates x ₂ . x ₁ and x ₂ are root-mean-squared through square calculators 11, 12, adder 13, and square root circuit 14 to calculate amplitude A, and this signal and inverter 15
The inverted signal -A obtained through the analog-to-digital converter 16 is supplied as a reference signal.
Reference numeral 17 denotes a changeover switch driven by a command from the microprocessor 18, which selects the signal with higher sensitivity from x ₁ or x ₂ and supplies it to the input terminal of the analog-to-digital converter 16. The selected signal x ₁ or x ₂ is digitally converted and read into the microprocessor 18. On the other hand, x ₁ and x ₂ are pulse shaping
Guided by the signal processing circuit 19, as shown in FIG. 7, the timing at which the rate of change of x ₁ and x ₂ becomes maximum,
That is, a pulse P is generated every 90° (P ₁ , P ₂ , P ₃
...) is supplied to the counter 20. counter 20
The integrated pulse number is calculated by the microprocessor 18.
The microprocessor executes a calculation to interpolatively add the number of pulses from the counter 20 and the digital value from the analog-to-digital converter;
The current angular velocity Ω is displayed on the display device 21.

In Fig. 7, if the current angular velocity is Ω ₀ , four pulses P ₁ to P ₄ are generated every 90° from 19, and from P ₄ to the next 90°, x ₁ Or interpolate using analog variation of _x2 . That is A/2
The value x ₀ of x ₁ corresponding to Ω ₀ is used as a reference. The microprocessor 18 determines whether to use x ₁ or x ₂ as the interpolation signal and controls the changeover switch 17 . The judgment criterion is to select the one with higher detection sensitivity, but by checking both x ₁ or x ₂ ,
The signal closer to A/2 is treated as a signal with higher sensitivity.

In such a configuration, since the pulse P is digital and the amplitude A is used as a reference signal in digital conversion of the analog signals x ₁ and x ₂ , highly accurate digital conversion is performed in which fluctuations in the amplitude A are compensated for. is possible. Furthermore, since the analog signal with higher sensitivity is automatically selected, it is possible to realize an optical fiber laser gyroscope with high precision and high sensitivity over an angular velocity higher than minute angular velocity as a whole.

In the above embodiments, a polarization-maintaining optical fiber is used as the optical fiber, but when a normal single mode optical fiber is used, the beam b reflected and transmitted through the semi-transmissive mirror 5 as shown in FIG. A polarizing plate or polarizing prism 2 is placed between ₁ and b ₂ and the input end of the optical fiber.
2 and 23 are installed, and of the beams b ₁ ′ and b ₂ ′ that pass through the optical fibers and are emitted, only the polarized light component P parallel to the plane of the paper is extracted as interference light b ₃ .

In addition, an Inconel half mirror is exemplified as a metal thin film semi-transparent mirror that is easily available, but
It is also possible to create semi-transmissive mirrors using various metal vapor depositions, in which case the phase delay characteristics due to double reflection will be different from those of Inconel half mirrors.
Therefore, the specific angle of incidence when applied to the present invention is
Needless to say, the angle will be other than 75°.

<Effects> The effects of the optical fiber laser gyroscope of the present invention explained above are summarized as follows: (1) Since the characteristics of a semi-transmissive mirror made of a thin metal film are used as a means to create a phase delay, it is not possible to use a complex structure like the conventional technology. No additional elements are required, and an optical fiber laser gyro with high sensitivity to minute angular velocities can be realized with an extremely simple configuration.

(2) There is no need to create a difference between the clockwise and counterclockwise optical paths to create a phase delay, and the difference between the two optical paths is zero, so errors due to optical path difference fluctuations due to optical system fluctuations and spectral fluctuations of the laser light source are eliminated. Does not occur.

(3) Since clockwise light and counterclockwise light have the same polarization plane, their transfer functions within the optical fiber are exactly the same, and the effects of transfer function fluctuations due to temperature fluctuations (which is a problem with orthogonal polarization systems) can be eliminated. I don't accept it.

(4) As shown in Figure 5, the sine proportional signal x ₁ and the cosine proportional signal x ₂ can be easily detected, so when Ω becomes large or l and r become large, select and use the signal with high sensitivity. Furthermore, since the amplitude can be calculated from the root mean square of both signals and analog changes can be detected based on this, highly accurate measurement that is not affected by amplitude fluctuations is possible.

(5) Since a sine proportional signal and a cosine proportional signal can be obtained at the same time, if a method is adopted in which a pulse is generated when the rate of change in each signal is maximum as shown in Figure 7, depending on the increasing and decreasing direction of Ω, Since the order of pulse generation is reversed, the direction of change in Ω can be easily seen, and furthermore, the direction of rotation can be easily identified.

(6) All optical elements can be constructed from passive elements, so the construction is simple and errors are small.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of a conventional optical fiber laser gyroscope, Fig. 2 is a configuration diagram showing an embodiment of the laser gyroscope of the present invention, Fig. 3 is a characteristic diagram of an Inconel half mirror, and Fig. 4 is a diagram of the laser gyroscope of the present invention. FIG. 5 is a block diagram showing an application example of the present invention, FIGS. 6 and 7 are output characteristic diagrams thereof, and FIG. 8 is a block diagram showing the main parts of another embodiment of the present invention. It is. 1... Semiconductor laser, 4... Light receiving element, 5...
Metal thin film semi-transmissive mirror, 6... polarization preserving optical fiber, 9, 10... amplifier, 16... analog-digital converter, 17... changeover switch, 18...
...Microprocessor, 20...Counter, 21
...Indicator, _L1 to _L5 ...Condensing lens.

Claims (1)

[Claims] 1. A coherent light source of linearly polarized light, a semi-transparent mirror made of a metal thin film that enters the emitted light of the light source at a predetermined angle and divides it into two directions by reflection and transmission, and It has an optical fiber that inputs the split light beam into both end faces of the optical fiber, and a light receiving element that receives the interference light that is mixed and interfered by the light emitted from the optical fiber being reflected and transmitted through the semi-transparent mirror again. An optical fiber laser gyroscope characterized in that a phase difference of 90° is given between the two mixed and interfering light beams by setting the angle of incidence of the mirror to a specific value. 2. The optical fiber laser gyroscope according to claim 1, characterized in that an Inconel half mirror is used as the semi-transmissive mirror, and the angle of incidence on the semi-transmissive mirror is approximately 75°. 3. An optical fiber laser gyroscope according to claim 1, characterized in that a polarization maintaining optical fiber is used as the optical fiber. 4. The optical fiber laser gyroscope according to claim 1, characterized in that a single mode optical fiber is used as the optical fiber, and polarizing plates or polarizing prisms are installed on both end faces of the optical fiber. 5 A coherent light source of linearly polarized light, a first semi-transmissive mirror made of a metal thin film that enters the output light of this light source at a predetermined angle and divides it into two directions by reflection and transmission, and a light beam divided into two directions. an optical fiber that enters the light into its both end faces, a first light receiving element that receives interference light that is mixed and interfered by the light emitted from the optical fiber being reflected and transmitted through the semi-transmissive mirror again, and transmitted through the semi-transmissive mirror. and a second semi-transparent mirror that separates the interference light that is reflected and returns to the light source from the optical path of the light source;
a second light-receiving element that receives the interference light from the semi-transmissive mirror; a circuit that calculates the square root of the sum of squares of the outputs of the first and second light-receiving elements; An optical fiber laser gyroscope characterized by extracting the output of two light receiving elements. 6 In claim 5, the above first and second
An optical fiber laser gyroscope characterized in that a selection means is provided for switching the one of the light receiving elements that is more sensitive to the angular velocity to be measured.