A coupling device applied to a brillouin fiber optic gyroscope

Abstract

The application discloses a coupling device applied to a Brillouin fiber gyroscope, comprising four 1550nm wavelength fiber collimators A, B, C and D and a prism, wherein the prism is coated with a 1550nm wavelength antireflection film, a semi-transmission semi-reflection film and a high-reflection film on specific surfaces respectively. The coupling device is extremely small in size and high in stability, can realize the purpose of low-threshold excitation of stimulated Brillouin laser without the condition of double-frequency light beams satisfying the resonance condition at the same time, simplifies the overall scheme and improves the stability of the gyroscope system.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic gyroscopes, specifically relating to a coupling device for Brillouin fiber optic gyroscopes. It can be used as an interferometer with a constant optical path difference, or as an optical filter based on a Mach-Zehnder interferometer. Background Technology

[0002] Fiber optic gyroscopes possess outstanding advantages in principle, including high precision, all-solid-state operation, and high reliability, playing a crucial role in inertial navigation systems and showing a continuous upward trend in development. After more than 40 years of development, fiber optic gyroscopes have become mainstream inertial instruments, widely used in attitude control, positioning and navigation, oil well inclination measurement, and other military and civilian fields. To date, fiber optic gyroscopes have mainly gone through the development stages of interferometric, resonant, and Brillouin fiber optic gyroscopes. Interferometric fiber optic gyroscopes typically use broadband light sources, require no frequency stabilization, and have a simple structure; however, to ensure gyroscope accuracy, the fiber optic cable needs to be thousands of kilometers long and precisely looped, making integration difficult. The core component of resonant fiber optic gyroscopes is the fiber optic resonant cavity, requiring only tens of meters of fiber or millimeter-level on-chip waveguides to ensure high gyroscope accuracy. However, limited by factors such as light source linewidth, stability, various optical noises, and system complexity, resonant fiber optic gyroscopes remain in the laboratory research stage. With the advancement of related technologies, the development of ultra-narrow linewidth lasers, photonic crystal fibers, and various devices has brought opportunities to Brillouin fiber optic gyroscopes.

[0003] Brillouin fiber optic gyroscopes are based on the stimulated Brillouin scattering (SBS) and Sagnac effects, and their structures are as follows: Figure 1 As shown, the pump light is split into two beams of equal intensity after passing through coupler 1, and then enters the fiber ring resonator after passing through coupler 2. When the pump light intensity in the resonator reaches a threshold, SBS light with a frequency difference equal to the Brillouin frequency shift is generated in the ring cavity, moving in the opposite direction to the pump light. Due to the optical Sagnac effect, the frequencies of the two clockwise and counterclockwise SBS light beams change with the rotational speed of the ring resonator. By performing beat frequency processing on the SBS light, the rotational angular velocity of the system can be obtained. As a third-generation fiber optic gyroscope, it has advantages such as linear signal output, large dynamic range, and high sensitivity, and has application potential in realizing gyroscope miniaturization and high sensitivity.

[0004] To reduce the overall size of the gyroscope, the fiber length of the ring resonator needs to be shortened as much as possible. However, the excitation threshold of stimulated Brillouin scattering increases as the fiber length decreases. Therefore, traditional Brillouin fiber gyroscope schemes employ a dual-resonance method to reduce the threshold power of the pump light. This involves setting the initial length of the fiber resonator and combining it with real-time PZT control of the fiber ring length or real-time tuning of the laser to ensure that the pump light and the Brillouin laser simultaneously satisfy the resonance condition of the fiber ring resonator. However, this method places extremely strict requirements on the initial fiber length of the resonator, and the control system for simultaneously stabilizing the dual-frequency beams is very complex and difficult to implement. Consequently, the stability of the output laser is poor, directly affecting the detection accuracy of the inertial navigation system.

[0005] This invention overcomes the problems of using traditional coupling devices in Brillouin fiber optic gyroscopes. The proposed coupling device is extremely small in size and has high stability. It can achieve the purpose of low-threshold excitation of stimulated Brillouin laser without the need for two frequency beams to simultaneously meet the resonance condition, thus simplifying the overall scheme and improving the stability of the inertial navigation system. Summary of the Invention

[0006] This invention proposes a coupling device applicable to Brillouin fiber optic gyroscopes. The invention is characterized by miniaturization, no melting point, high reliability, and suitability for mass production. Brillouin fiber optic gyroscopes using this coupling device are easier to control and have lower threshold voltages.

[0007] The coupling device proposed in this invention has frequency-dependent transmission characteristics that allow the pump light to pass through the fiber ring resonator in a single pass without resonance, while the Brillouin laser signal can resonate in the fiber ring resonator, thereby achieving stable pumping operation. At the same time, based on the relative positions of the longitudinal mode and the center of the Brillouin gain curve, stable single-frequency beam stabilization can be achieved by combining a simple feedback frequency stabilization circuit.

[0008] Existing technologies use dual-frequency beams to simultaneously satisfy resonance conditions to reduce the pump laser threshold and lower the power of the pump laser in Brillouin fiber gyroscopes. To simplify the frequency stabilization of the gyroscope system, frequency stabilization of only a single-frequency beam can achieve the effect of reducing the excitation threshold of the Brillouin laser. It is expected that the coupling device in the gyroscope can achieve the following: (1) a large proportion of the pump light enters the fiber resonant cavity, passes through only once, and does not resonate in the cavity; (2) the SBS laser must circulate through the fiber ring resonant cavity multiple times, and the loss in the cavity is minimal.

[0009] For the coupling device proposed in this invention, the refractive index and size of the prism satisfy Δl=c / 2nf B When, where c is the speed of light, n is the refractive index of the prism, Δl is the maximum vertical distance from the top surface of the prism to the bottom, f B Brillouin frequency shift (f B =2nυ af p / c,υ a Let f be the speed of sound inside the prism. p (The frequency of the input pump laser) The transmission spectrum of this device is a periodic function of the beam frequency, and its spectral shape conforms to the law of sine and cosine functions, so it can be used as a periodic filter. This coupling device can meet the requirement of reducing the difficulty of Brillouin gyroscope frequency stabilization if and only if the frequency of the pump laser and the frequency of the Brillouin laser are located at the valley and peak of the transmission spectrum, respectively, that is, the value of the stimulated Brillouin divergence frequency difference matches the frequency corresponding to the abscissa of the peak value of the output spectrum.

[0010] The present invention adopts the following technical solution:

[0011] The present invention provides a coupling device for Brillouin fiber optic gyroscopes, comprising four 1550nm wavelength fiber collimators A, B, C, and D, and a prism with a specific surface coated with anti-reflection, semi-transparent and semi-reflective, and high-reflection films.

[0012] The prism is gem-shaped, and its outer surface includes a left side, a top side, a right side, a lower right side, and a lower left side connected clockwise. The top side is parallel to the horizontal plane, the left and right sides are parallel to each other and perpendicular to the top side, and the lower left and lower right sides form a right-angled isosceles triangle. The left and right sides have equal heights, each being half the length of the top side. The prism is cut along the junction of the top and left sides, parallel to the lower left side, and along the junction of the top and right sides, parallel to the lower right side, forming two intersecting internal planes, denoted as the left inner side and the right inner side, respectively. The left, top, and right sides are coated with an anti-reflective coating with a wavelength of 1550nm, the left and right inner sides are coated with a semi-transparent and semi-reflective coating with a wavelength of 1550nm, and the lower left and lower right sides are coated with a high-reflective coating with a wavelength of 1550nm.

[0013] 1550nm wavelength fiber collimator A is located on the left side of the prism, 1550nm wavelength fiber collimators B and C are located on the upper side of the prism, and 1550nm wavelength fiber collimator D is located on the right side of the prism.

[0014] Furthermore, the pump laser is input through fiber collimator A. After entering the prism, the beam is split into two beams by a 1550nm wavelength semi-transparent and semi-reflective membrane on the left inner side. One beam is reflected to a 1550nm wavelength high-reflective membrane on the lower left side. After two reflections, it is split into two beams at a 1550nm wavelength semi-transparent and semi-reflective membrane on the right inner side. The beams are reflected to fiber collimator C and transmitted to fiber collimator D, respectively. The other beam is transmitted to the 1550nm wavelength semi-transparent and semi-reflective membrane on the right inner side. The light reflected there reaches fiber collimator D, and the transmitted light reaches fiber collimator C.

[0015] The pump laser is input through fiber collimator B. After entering the prism, the beam is split into two beams by a semi-transparent and semi-reflective membrane with a wavelength of 1550nm on the left inner side. One beam is reflected to a semi-transparent and semi-reflective membrane with a wavelength of 1550nm on the right inner side. The reflected light reaches fiber collimator D, and the transmitted light reaches fiber collimator C. The other beam is transmitted to a highly reflective membrane with a wavelength of 1550nm on the lower left side. After two reflections, it is split into two beams at a semi-transparent and semi-reflective membrane with a wavelength of 1550nm on the right inner side. The two beams are reflected to fiber collimator C and transmitted to fiber collimator D, respectively.

[0016] Furthermore, the maximum vertical distance Δl from the upper side surface of the prism to the bottom satisfies:

[0017]

[0018] Where c is the speed of light and n is the refractive index of the prism. Let υ be the Brillouin frequency shift. a Let f be the speed of sound inside the prism. p The frequency of the input pump laser.

[0019] Furthermore, the optimal working distance for the fiber collimators A, B, C, and D is 1.5nΔl.

[0020] The present invention also provides a Brillouin fiber optic gyroscope, including the above-mentioned coupling device, and further including a 1550nm pump laser source, a 50 / 50 fiber coupler, a filter, a detector, and a fiber ring resonator. The 1550nm pump laser source emits pump laser, which is split into two beams by the 50 / 50 fiber coupler and enters fiber collimator A and fiber collimator B respectively, and enters the fiber ring resonator at the ports of fiber collimator C and fiber collimator D respectively. The Brillouin laser-sensitive angular velocity excited by the pump laser in the fiber ring resonator returns to the 50 / 50 fiber coupler through fiber collimator A and fiber collimator B, and enters the detector after the pump light and Rayleigh scattered light are removed by the filter, thereby sensing the angular velocity.

[0021] The beneficial effects of this invention compared to the prior art are as follows:

[0022] 1. The Brillouin fiber optic gyroscope based on the present invention does not require matching the pump light frequency with the resonant point of the fiber optic ring resonator, thus simplifying the frequency stabilization system.

[0023] 2. Compared to the existing technology of fusion splicing two fiber couplers, the beam splitter film is directly attached to the surface of the prism, resulting in a stable structure without fusion points, which increases the optical path reliability of the Brillouin fiber optic gyroscope.

[0024] 3. The present invention has a small size, which is beneficial for the miniaturization design of fiber optic gyroscopes, and has strong feasibility and practicality.

[0025] 4. The coupling device proposed in this invention has a simple structure, can be mass-produced, and is conducive to the industrialization of the equipment. Attached Figure Description

[0026] Figure 1 A schematic diagram of a Brillouin fiber optic gyroscope;

[0027] Figure 2 This is a schematic diagram of a coupling device used in Brillouin fiber optic gyroscopes.

[0028] Figure 3 This is an optical path diagram inside the coupling device;

[0029] Figure 4 When a tuned laser is input into fiber collimator A, the normalized light intensity of the output at ports C and D of the fiber collimator is as follows:

[0030] Figure 5 This is a schematic diagram of a Brillouin fiber optic gyroscope structure based on this coupling device;

[0031] Figure 2-5 In the middle: 1-1550nm wavelength fiber collimator A, 2-1550nm wavelength fiber collimator B, 3-1550nm wavelength fiber collimator C, 4-1550nm wavelength fiber collimator D, 5-1550nm wavelength antireflection coating, 6-1550nm wavelength semi-transparent and semi-reflective coating, 7-1550nm wavelength high reflectivity coating, 8- fused silica or K9 glass prism, 9-1550nm pump laser source, 10-1550nm wavelength 50 / 50 fiber coupler, 11-1550nm wavelength ultra-narrow fiber filter, 12- detector, 13- fiber ring resonator. Detailed Implementation

[0032] The invention will now be described in detail with reference to the accompanying drawings.

[0033] This invention provides a coupling device for use in Brillouin fiber optic gyroscopes, such as... Figure 2 As shown. The scheme includes four 1550nm wavelength fiber collimators and one specially shaped prism made of fused silica, K9 glass or other light-transmitting material, on which a 1550nm wavelength antireflection coating, a semi-transparent and semi-reflective coating and a high-reflection coating are respectively deposited on specific surfaces.

[0034] Specifically, the prism is gem-shaped, and its outer surface includes a left side, a top side, a right side, a lower right side, and a lower left side connected clockwise. The top side is parallel to the horizontal plane, the left and right sides are parallel to each other and perpendicular to the top side, and the lower left and lower right sides form a right-angled isosceles triangle. The left and right sides have equal heights, each being half the length of the top side. The prism is cut along the junction of the top and left sides, parallel to the lower left side, and along the junction of the top and right sides, parallel to the lower right side, forming two intersecting internal planes, denoted as the left inner side and the right inner side, respectively. The left, top, and right sides are coated with an antireflective coating 5 with a wavelength of 1550nm, the left and right inner sides are coated with a semi-transparent and semi-reflective coating 6 with a wavelength of 1550nm, and the lower left and lower right sides are coated with a high-reflective coating with a wavelength of 1550nm.

[0035] 1550nm wavelength fiber collimator A is located on the left side of the prism, 1550nm wavelength fiber collimators B and C are located on the upper side of the prism, and 1550nm wavelength fiber collimator D is located on the right side of the prism.

[0036] The implementation of the coupling device used in the Brillouin fiber optic gyroscope specifically includes the following steps:

[0037] Step 1: Select a suitable light-transmitting medium as the material for making the prism, such as fused silica or K9 glass;

[0038] Step 2: Calculate the prism size based on the refractive index of the medium, so that the frequency corresponding to the peak-to-peak value of the device's output end matches the frequency difference between the pump light and the Brillouin laser.

[0039] Specifically, the normalized light intensity at the output of the coupling device is To satisfy the frequency difference corresponding to the peak-to-peak value (i.e., the frequency change of the horizontal coordinate corresponding to adjacent peaks and troughs), Figure 4 f0) and Brillouin frequency shift f B For them to be equal, they must be such that f p -f0 is the Brillouin frequency shift f B That is, the maximum vertical distance from the top side of the prism to the bottom.

[0040] Step 3: Fabricate the prism and coat it with anti-reflective coating, high-reflective coating, and semi-transparent / semi-reflective coating at the corresponding positions;

[0041] Step 4: Calculate the required collimator working distance based on the optical path of the beam inside the prism;

[0042] Specifically, taking the light input from fiber optic collimator A to the outputs of fiber optic collimators C and D as an example, calculate the optical path length and the collimator working distance (the same applies to input from fiber optic collimator B and outputs from fiber optic collimators C and D). From Figure 3 It can be seen that light entering the prism from fiber collimator A, undergoing two transmissions, and exiting from fiber collimator C has an optical path length of nΔl; light entering the prism from fiber collimator A, undergoing four reflections, and exiting from fiber collimator C has an optical path length of 2nΔl. Light entering the prism from fiber collimator A, undergoing one transmission and one reflection, and exiting from fiber collimator D has an optical path length of nΔl; light entering the prism from fiber collimator A, undergoing three reflections and one transmission, and exiting from fiber collimator D has an optical path length of 2nΔl. Therefore, the optimal working distance for fiber collimator A is the midpoint of the two optical path lengths, 1.5nΔl (the same applies to fiber collimators B, C, and D).

[0043] Step 5: Adjust the three-dimensional position and pitch angle of the collimator to minimize losses and optimize its polarization keeping characteristics;

[0044] Step 6: Securely assemble the components.

[0045] The internal optical path diagram of the present invention is as follows: Figure 3 As shown. The pump laser is input through fiber collimator A and fiber collimator B. Taking the laser input through fiber collimator A as an example, after entering the prism, the beam is split into two beams by the 1550nm wavelength semi-transparent and semi-reflective membrane 6 on the left inner side. One beam is reflected to the 1550nm wavelength high-reflective membrane 7 on the lower left side. After two reflections, it is split into two beams at the 1550nm wavelength semi-transparent and semi-reflective membrane 6 on the right inner side. The beams are reflected to fiber collimator C and transmitted to fiber collimator D, respectively. The other beam is transmitted to the 1550nm wavelength semi-transparent and semi-reflective membrane 6 on the right inner side. The light reflected at this point reaches fiber collimator D, and the transmitted light reaches fiber collimator C. Taking a laser beam input through fiber collimator B as an example, after entering the prism, the beam is split into two beams by a semi-transparent and semi-reflective membrane 6 with a wavelength of 1550nm on the left inner side. One beam is reflected to the semi-transparent and semi-reflective membrane 6 with a wavelength of 1550nm on the right inner side, where the reflected light reaches fiber collimator D and the transmitted light reaches fiber collimator C. The other beam is transmitted to a high-reflective membrane 7 with a wavelength of 1550nm on the lower left side. After two reflections, it is split into two beams at the semi-transparent and semi-reflective membrane 6 with a wavelength of 1550nm on the right inner side, and is reflected to fiber collimator C and transmitted to fiber collimator D, respectively.

[0046] The structural dimensions of this invention affect the coupling characteristics. Figure 4The diagram shows the normalized output light intensities at ports C (dashed line) and D (solid line) of the fiber collimator when a tuned laser is input to collimator A under ideal conditions. It also represents the ideal frequency positions of the pump light and the Brillouin laser. When the designed structural dimensions are met, all the pump light enters the resonant cavity, enabling Brillouin laser excitation at the lowest threshold condition.

[0047] The coupling device provided by this invention can be applied to Brillouin fiber optic gyroscopes, and its simplest structural diagram is shown below. Figure 5 As shown, a 1550nm pump laser source 9 emits a pump laser, which is split into two beams by a 50 / 50 fiber coupler 10 and enters fiber collimators A and B respectively. The beams then enter the fiber ring resonator 13 at ports C and D respectively. The Brillouin laser-sensitive angular velocity excited by the pump light within the resonator returns to the 50 / 50 fiber coupler 10 through fiber collimators A and B. After the pump light and Rayleigh scattering light are removed by filter 11, the light enters detector 12, where it is further sensitive to the angular velocity. The above design comprehensively considers issues such as the pump light threshold and frequency stabilization difficulty in Brillouin fiber gyroscopes. When the structural dimensions of the coupling device meet the requirements... Figure 4 In an ideal scenario, low-intensity excitation of Brillouin lasers can be achieved without real-time control of the pump light frequency to meet the resonant point of the resonant cavity, and the Brillouin laser can circulate and propagate within the resonant cavity to achieve the purpose of sensitive angular velocity.

[0048] This invention addresses the problems of high pump threshold and complex frequency stabilization systems in existing Brillouin fiber optic gyroscopes by proposing a coupling device for use in Brillouin fiber optic gyroscopes. First, a structural schematic and internal optical path diagram of the coupling device are presented, and a dimensional reference formula for prism design is listed. Finally, a simplified structure of a Brillouin fiber optic gyroscope based on this coupling device is proposed. The advantages of this invention include a significant reduction in the pump threshold of the Brillouin fiber optic gyroscope, miniaturization, no melting point, high reliability, and practicality. It is also suitable for mass production and has broad application prospects.

Claims

1. A coupling device for use in Brillouin fiber optic gyroscopes, characterized in that, It includes four 1550nm wavelength fiber collimators A, B, C, and D, as well as a prism with anti-reflection, semi-transparent and semi-reflective, and high-reflection coatings on a specific surface; The prism is gem-shaped, and its outer surface includes a left side, a top side, a right side, a lower right side, and a lower left side connected clockwise. The top side is parallel to the horizontal plane, the left and right sides are parallel to each other and perpendicular to the top side, and the lower left and lower right sides form a right-angled isosceles triangle. The left and right sides have equal heights, each being half the length of the top side. The prism is cut along the junction of the top and left sides, parallel to the lower left side, and along the junction of the top and right sides, parallel to the lower right side, forming two intersecting internal planes, denoted as the left inner side and the right inner side, respectively. The left, top, and right sides are coated with an anti-reflective coating with a wavelength of 1550nm, the left and right inner sides are coated with a semi-transparent and semi-reflective coating with a wavelength of 1550nm, and the lower left and lower right sides are coated with a high-reflective coating with a wavelength of 1550nm. 1550nm wavelength fiber collimator A is located on the left side of the prism, 1550nm wavelength fiber collimators B and C are located on the upper side of the prism, and 1550nm wavelength fiber collimator D is located on the right side of the prism.

2. The coupling device for a Brillouin fiber optic gyroscope according to claim 1, characterized in that, The pump laser is input through fiber collimator A. After entering the prism, the beam is split into two beams by a 1550nm wavelength semi-transparent and semi-reflective membrane on the left inner side. One beam is reflected to a 1550nm wavelength high-reflective membrane on the lower left side. After two reflections, it is split into two beams at a 1550nm wavelength semi-transparent and semi-reflective membrane on the right inner side. The beams are reflected to fiber collimator C and transmitted to fiber collimator D, respectively. The other beam is transmitted to a 1550nm wavelength semi-transparent and semi-reflective membrane on the right inner side. The light reflected from the semi-transparent and semi-reflective membrane reaches fiber collimator D, and the transmitted light reaches fiber collimator C. The pump laser is input through fiber collimator B. After entering the prism, the beam is split into two beams by a 1550nm wavelength semi-transparent and semi-reflective membrane on the left inner side. One beam is reflected to a 1550nm wavelength semi-transparent and semi-reflective membrane on the right inner side. The light reflected at the semi-transparent and semi-reflective membrane reaches fiber collimator D, and the transmitted light reaches fiber collimator C. The other beam is transmitted to a 1550nm wavelength high-reflective membrane on the lower left side. After two reflections, it is split into two beams at a 1550nm wavelength semi-transparent and semi-reflective membrane on the right inner side, and is reflected to fiber collimator C and transmitted to fiber collimator D, respectively.

3. The coupling device for a Brillouin fiber optic gyroscope according to claim 1, characterized in that, The maximum vertical distance from the upper side surface to the bottom of the prism satisfy: in, c At the speed of light, n The refractive index of the prism, For Brillouin frequency shift, The speed of sound inside the prism. The frequency of the input pump laser.

4. The coupling device for a Brillouin fiber optic gyroscope according to claim 1, characterized in that, The optimal working distance for the fiber optic collimators A, B, C, and D is: .

5. A Brillouin fiber optic gyroscope, characterized in that, The coupling device, including any one of claims 1-4, further includes a 1550nm pump laser source, a 50 / 50 fiber coupler, a filter, a detector, and a fiber ring resonator. The 1550nm pump laser source emits pump laser light, which is split into two beams by the 50 / 50 fiber coupler and enters fiber collimators A and B respectively. The beams then enter the fiber ring resonator at the ports of fiber collimators C and D respectively. The Brillouin laser-sensitive angular velocity excited by the pump laser in the fiber ring resonator returns to the 50 / 50 fiber coupler through fiber collimators A and B. After the pump light and Rayleigh scattering light are removed by the filter, the beam enters the detector and is then sensitive to the angular velocity.

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