CN1162684C - Semiconductor side-pumped solid-state laser gyroscope and its electro-optic modulation method - Google Patents

Abstract

The invention relates to a semiconductor side-pumped solid-state laser gyroscope and an electro-optic modulation method thereof, relating to the structural design of a semiconductor-pumped solid-state laser gyroscope. It is mainly composed of annular cavity mirror group, gain medium, pumping device, electro-optic crystal, beam splitter, optical delay device and readout circuit. It is characterized in that the pumping device adopts side pumping, and the side pumping It consists of focusing coupling lens, semiconductor laser array and semiconductor laser temperature control device. The invention can better overcome the problem of thermal stress, and at the same time, the annular cavity is a triangular cavity without negative area, and its structure is similar to that of a gas laser gyroscope, and its ready-made technology can be fully utilized. Electro-optic dithering uses two electro-optic crystals with the same parameters, and uses sine wave signals with a phase difference of 180° to modulate them respectively, which can significantly reduce the gyroscope lock-in area. It has the advantages of small size, stable operation and long life, and can be widely used in inertial technologies such as inertial navigation, inertial guidance and inertial measurement.

Description

Semiconductor side-pumped solid-state laser gyroscope and its electro-optic modulation method

technical field

The invention belongs to the field of semiconductor-pumped solid-state laser technology and sensor technology, and in particular relates to the structural design of a semiconductor-pumped solid-state laser gyroscope.

Background technique

As an inertial measurement device, the gyroscope is the core of inertial navigation and guidance technology. Over the past ten years, with the development of optoelectronic technology, optical gyroscopes have become a strong competitor of traditional mechanical gyroscopes and electromechanical gyroscopes. Laser gyroscopes have gained recognition in aerospace, aviation, navigation, and ground combat vehicles due to their many characteristics. Wide application of weapons and equipment. Laser gyro has become one of the important development directions.

In principle, optical gyroscopes can be divided into two categories: resonant type and interference type. The representative of the former is the laser gyro with He-Ne gas ring laser as the core, which is also a navigation-grade optical gyro widely used at present. The representative of the latter is a fiber optic gyroscope with a fiber optic interferometer composed of multi-turn fibers as the core (Zhang Yanshen et al. Laser Gyro Technical Report and Papers, Department of Precision Instruments, Tsinghua University, October 1999). Although the gas laser gyroscope is the most accurate among the current optical gyroscopes, it has its inherent disadvantages: ①The gain medium of the gas laser gyroscope is He-Ne mixed gas. In order to ensure the constant gas composition and concentration, it needs to be replaced frequently. Inflatable gas, so the sealing of the cavity is very high; ②The gain is a mixed gas, in order to ensure the oscillation condition, the gas gain must have a certain limit optical path, that is, the cavity is difficult to miniaturize, and the gas discharge requires high voltage; ③He-Ne The working laser wavelength of the gas laser gyroscope is relatively short (633nm), so the coating process of the reflector of the ring resonator is very demanding, which greatly increases the cost; ④ In order to eliminate the inherent lock-up, the gas laser gyroscope introduces a mechanical The additional components of the jitter not only increase the complexity of the gyro system, but also bring additional noise to the system; ⑤ The total efficiency is very low, less than <0.01%. Due to the existence of many problems, it is difficult to further improve the accuracy of the gas laser gyroscope.

Since the sensing signal of the optical gyroscope is laser, there is inevitably a blocking phenomenon, that is, when the rotational speed of the measured object is lower than a certain threshold, the output of the gyroscope is nonlinear or has no output. Traditionally, there are three ways to reduce lockup: one is to add a fixed rotational speed to the entire gyroscope (Raytheon Co., USA); ); The third is to use the magneto-optic effect.

With the development of solid-state laser technology, semiconductor-pumped solid-state laser (DPSSL) has shown significant advantages over lamp-pumped solid-state laser (LDSSL). DPSSL has higher overall efficiency than LDSSL, simple structure, and stable laser output. In particular, the progress of monolithic solid-state laser technology, narrow linewidth, high-power laser output, and frequency stabilization technology have promoted the research of solid-state laser gyro technology. Compared with other current optical gyroscopes, the semiconductor-pumped solid-state laser gyroscope has the following advantages: ①It is an all-solid-state solution, which ensures that the product is suitable for long-term storage, miniaturization, low production cost, and weak temperature effect; ②Solid gain medium is an application technology The mature rare earth metal crystal material has a long lasing wavelength (1.064mm), which reduces the requirements for the coating process of the mirror, which can greatly reduce the production cost of the gyroscope; ③ and the current He-Ne gas laser gyroscope Similarly, the solid-state laser gyro pumped by the semiconductor laser is also an active resonant cavity laser gyro, so the advantages of the traditional gas laser gyro are also reflected in this type of gyro system, so it will be a high-precision laser gyro ④In the semiconductor laser pumped solid-state laser gyroscope system, the signal acquisition and processing is to use the two-way light to generate the beat frequency after combining the light at the output end, which maintains a linear relationship with the system angular velocity, so the system signal detection technology can be greatly reduced. difficulty. Therefore, semiconductor-pumped solid-state laser gyroscopes have attracted people's attention. In 1999, Halldorsson et al. (United States Patent, No.5,960,022) proposed a 3-DOF semiconductor-pumped solid-state laser gyroscope, using multiple semiconductor lasers as the pumping light source, the ring cavity is a rectangular cavity of a single piece of glass, and the gain medium uses Trivalent rare earth metal crystal (doped with Nd ³⁺ ), the laser works in continuous wave mode, and the acousto-optic frequency shift technology is used to reduce the measurement lock-in area. The advantage is that the solid gain is used instead of the gas gain, and the optical method is used instead of the mechanical method. Reduce lockups. Its disadvantage is that it uses acousto-optic frequency shifting technology to generate Doppler effect to reduce blocking, and the structure is complicated. In the document "Diels, Jean-Claude et al. Progress toward a compact solid stateactive laser gyroscope Proc. SPIE Vol.3616, p.136-142.", a semiconductor-pumped pulse mode-locked solid-state laser gyroscope is proposed, as shown in the figure As shown in 4, the ultrashort pulse laser technology is applied to the laser gyro technology, the semiconductor laser end pumping method is adopted, the mirror is used to form an 8-shaped ring cavity, and the multi-quantum well saturable absorber (MQW) is used as a mode-locking device. Since the laser works in the pulse mode, after a reasonable design of the resonator, the two optical pulses propagating in the cavity will not meet on the optical components that constitute the resonator, thus eliminating the lock-up intrinsically, instead of using a gas laser gyroscope The mechanical method of the instrument, the magneto-optic effect or the acousto-optic effect. However, due to the inevitable backscattering of pulsed light on the MQW and energy coupling, the laser gyroscope uses a square wave modulation signal for electro-optic modulation to obtain a frequency bias effect and reduce latch-up. The disadvantage is that due to the semiconductor end-pumping method, it is difficult to further miniaturize the annular cavity, and because the end-pumping inevitably uses an 8-shaped cavity, there is a negative area, and the calibration factor of the gyroscope will be reduced. At the same time, the end-pumping method will Uneven thermal stress that causes crystal gain reduces gyroscope life and requires more optical components and complex structures.

Contents of the invention

The purpose of the present invention is in order to overcome the weak point of above-mentioned optical gyroscope, propose a kind of semiconductor side pump solid gyroscope and its electro-optic modulation method, make it further simplify the structure, overcome thermal stress problem preferably, improve measurement accuracy, reduce lock zone.

The purpose of the present invention is achieved through the following technical solutions:

A semiconductor side-pumped solid-state laser gyroscope, mainly composed of an annular cavity mirror group, a gain medium, a pumping device, an electro-optic crystal and its electro-optic modulation circuit, a beam splitter, an optical delayer and a readout circuit, its characteristics In that: the annular cavity is an isosceles triangular cavity composed of two curved mirrors and a plane mirror, and two electro-optic crystals with the same parameters are symmetrically placed on the waist of the isosceles triangle; the pumping device adopts a side pump The side pump consists of a focusing coupling lens, a semiconductor laser array and a semiconductor laser temperature control device.

The semiconductor laser temperature control device is composed of a semiconductor cooling chip, a heat sink, a thermistor and a temperature control circuit.

The plane mirror and the curved mirror are the plane mirror and the curved mirror coated with a high reflection film with a reflectivity of not less than 99.9% for the wavelength of 1064nm.

The electro-optic crystal adopts a lithium niobate electro-optic crystal whose end surface is coated with a high-transmittance film of 1064nm and whose reflectivity is not higher than 0.2%.

The gain medium is yttrium vanadate (Nd:YVO4) doped with 0.5% neodymium, and the geometric shape is a slab. The input and output ends are coated with a high-transparency film with a reflectivity of no higher than 0.1% for a wavelength of 1064nm, and the side away from the cylindrical lens is coated with a wavelength of 1064nm. A high-reflection film with a reflectivity of not less than 98%, and a two-color film with a reflectivity of not less than 1% for a wavelength of 808nm and a reflectivity of not less than 98% for a wavelength of 1064nm are coated on the side next to the cylindrical lens.

The focusing coupling lens adopts a cylindrical lens coated with a high-transmittance film with a reflectivity not higher than 1% for a wavelength of 808nm.

The present invention also provides an electro-optic modulation method for the semiconductor side-pumped solid-state laser gyroscope, the method utilizes an electro-optic modulation circuit composed of a signal generator and a power amplifier driving a lithium niobate crystal (LiNbO ₃ ) to modulate the electro-optic crystal , to generate frequency bias and reduce locked area, characterized in that two symmetrically placed electro-optic crystals with the same parameters are respectively modulated by a sine wave modulation signal with a phase difference of 180 degrees.

The advantage of the present invention is that due to the use of semiconductor laser array side pumping device, semiconductor laser array and focusing cylindrical lens to pump the crystal gain medium, the energy of the light-receiving surface of the gain medium is relatively uniform, and the problem of thermal stress can be better overcome. At the same time, the annular cavity is The triangular cavity has no negative area, and the semiconductor laser array is installed inside the ring cavity or at a position perpendicular to the cavity plane, which facilitates the miniaturization of the gyroscope; at the same time, the structure is similar to that of the gas laser gyroscope, making full use of the gas laser gyroscope Ready-made technology; electro-optic crystals are used for electro-optic modulation, which has a similar effect to mechanical jitter, but the jitter frequency can be much higher than that of mechanical jitter; in order to reduce loss, the electro-optic crystal end face is coated with high-transparency film; the modulation signal uses a sine wave with a phase difference of 180 degrees The modulation signals are modulated separately, which can significantly reduce the locked area of the gyroscope; the readout circuit adopts a subdivision circuit, which can greatly improve the measurement accuracy of the gyroscope.

Description of drawings

Fig. 1 is a schematic structural diagram of a semiconductor side-pumped solid-state laser gyroscope proposed by the present invention.

Fig. 2 is a schematic diagram of the side pump device of the present invention.

Fig. 3 is a schematic diagram of the electro-optic modulation structure of the present invention.

Fig. 4 is a schematic diagram of the structure of the diode-pumped pulse mode-locked solid-state laser gyroscope proposed by Diels.

Detailed ways

Concrete structure, operating principle and embodiment of the present invention are described in detail below in conjunction with accompanying drawing:

Fig. 1 is the embodiment structure schematic diagram of semiconductor side-pumped solid-state laser gyroscope that the present invention proposes, and it mainly consists of mirror group 1,6,8, gain medium 2, focusing coupling lens 3, semiconductor laser array 4, semiconductor laser temperature control Device 5, electro-optic crystals 7, 13, beam splitter 9, readout circuit 10, optical delayer 11, modulation circuit 12 and piezoelectric ceramics (PZT) 14. The semiconductor laser 4 is used as a pumping light source, and the gain medium 2 is pumped after being coupled by the focusing coupling lens 3; the mirror group 1, 6, and 8 form a ring resonator; the electro-optic crystals 7, 13 are used for electro-optic modulation to generate a biasing effect; bidirectional The light 15, 17 is output through the reflector 8, and the beat frequency is generated after the light is combined by the optical delay device 11 and the beam splitter plate 9, and the readout circuit 10 reads out the beat frequency signal, which can be measured by the The speed of the measured object relative to the inertial space. The annular cavity is an isosceles triangle cavity composed of two curved mirrors 1, 6 and a plane mirror 8, and two electro-optic crystals 7, 13 with the same parameters are symmetrically placed on the waist of the isosceles triangle.

Fig. 2 is a schematic diagram of the side pump device of the present invention. The present invention adopts the side-pumped (side-pumped) mode, and the side-pumping device is composed of the focusing coupling lens 3, the semiconductor laser array 4 and the semiconductor laser temperature control device 5: the semiconductor cooling plate 21 is installed on the heat sink 18, and heat-conducting silica gel is used Paste the semiconductor cooling sheet 21 on the semiconductor laser array 4 to dissipate heat, and place a thermistor 19 inside the substrate of the semiconductor laser array 4 to measure the temperature of the semiconductor laser array; the thermistor 19 feeds back the temperature to the temperature control circuit 20. The temperature control circuit 20 drives the semiconductor cooling chip 21, so that the semiconductor laser array 4 works at a constant temperature and prevents the drift of the pumping wavelength.

Fig. 3 is a schematic diagram of the electro-optic modulation structure of the present invention, which is composed of an electro-optic modulation circuit and an electro-optic crystal. When there is no external voltage, the electro-optic crystal has a refractive index of n. Place two electro-optic crystals 7 and 13 with the same parameters symmetrically on the waist of the triangular cavity, first add a constant u ₀ to the crystals 7 and 13, and the refractive index becomes n ₀ ; the voltage modulation voltage signal uses a triangular wave or a sawtooth The shape wave u(t) is also added to the electro-optic modulation crystals 7 and 13, wherein u(t) is the electro-optic modulation crystal 7 and -u(t) is the electro-optic modulation crystal 13, which are modulation voltages with opposite phases. When the light beam 15 reaches the electro-optic crystal 7, the modulation voltage is applied to the electro-optic crystal 7, and due to the electro-optic effect, the refractive index of the electro-optic crystal will change to n ₀ +Δn, where

$\mathrm{<span\; class="notranslate">\; \&Delta;\; n</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 13</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

The optical path of the light beam 15 will also change l·Δn, a part of the light beam 15 is output through the plane mirror 8, and a part continues to advance through the electro-optic crystal 13. Due to the extremely short time, the modulation voltage on the electro-optic crystal 13 is -u(t) .The refractive index of the electro-optic crystal is n ₀ -Δn, and the optical path change amount of the light beam 15 -l·Δn returns to the initial phase; similarly, the light beam 17 also changes in the same way, and the difference is that the light beam 17 passes through the flat mirror 8 to output the light The path change amount is -l·Δn, and the optical path change amount of the beam 15 is l·Δn, which is equivalent to the ring cavity having an initial rotational speed Ω ₀ as

${\mathrm{<span\; class="notranslate">\; \&Omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="13"\; label="r"\; filenames="C0211667700081.png"\; state="\{\{state\}\}">r</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 13</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Among them: l is the crystal thickness; r ₁₃ is the electro-optic coefficient; c is the speed of light in vacuum.

This creates an electro-optical dithering effect. In order to reduce the loss, the end face of the electro-optic crystal is plated with lithium niobate whose reflectivity is not higher than 0.2% for the wavelength of 1064nm.

Figure 4 is a schematic diagram of the structure of a semiconductor-pumped pulsed mode-locked solid-state laser gyroscope proposed by Diels, which consists of a semiconductor laser, a collimator lens, a shaping prism, a curved mirror, a gain crystal, a ring cavity plane mirror, a lens, a saturable absorber, and an electro-optic modulation crystal. A figure-eight cavity is used, and multiple quantum wells (MQW) are used as saturable absorbers to generate mode-locked pulsed laser light. The semiconductor laser is a GaAlAs/GaAs laser with a lasing wavelength of 808nm, the crystal is Nd:YVO ₄ , and the electro-optical conversion efficiency is greater than 50%. Since the backscattering of the MQW causes latch-up, in order to reduce the latch-up, the electro-optic crystal _LiNbO3 is used for modulation to generate a bias frequency. Two-way light is output from the reflector, and one path of light is delayed through the optical path, and then combined.

The working principle of this side pump gyroscope is as follows: as shown in Figure 1, the pumping light λ ₁ =808nm sent by the semiconductor laser array 4 passes through the focusing coupling lens (using a plated 808nm reflectance not higher than 1%) Cylindrical lens with highly transparent film), collimated and focused to crystal gain 2 [Yttrium vanadate (Nd:YVO4) doped with 0.5% neodymium], the geometric shape is slab, and the reflectivity of the input and output ends is not higher than 0.1 for 1064nm wavelength % high-transmittance film, the side away from the cylindrical lens is coated with a high-reflection film with a reflectance of no less than 98% for a wavelength of 1064nm, and the side close to the cylindrical lens is coated with a reflectance of no higher than 1% for a wavelength of 808nm and no higher than 1% for a wavelength of 1064nm lower than 98% of the dichroic film); the crystal gain absorbs λ ₁ and emits fluorescence at a series of wavelengths; the annular cavity formed by the mirrors 1, 6, and 12 constitutes a traveling wave cavity, and the wavelength is 1.06 μm due to mode competition and resonant cavity mode selection The light wave will form a two-way oscillation in the cavity; output through the output mirror 8; the two-way light passes through the beam splitter 9, the optical delay device 11, and generates resonance on the readout circuit 10, so that the beat frequency signal of the two-way light wave can be obtained, along the Moving the optical delayer 11 in the direction 16 can adjust the phase difference of the two-way light waves; the electro-optic modulation crystals 7 and 13 are used to generate electro-optic jitter and reduce the locking effect of the gyroscope.

Embodiment: The structure diagram of this system is shown in Figure 1. The detailed steps are as follows: the semiconductor laser array 4 used is a 6W GaAlAs/GaAs double heterojunction laser array with a peak wavelength of 810±5nm, fixed on a semiconductor refrigerator, and controlled by a temperature control system. Gain medium 2 uses a crystal whose absorption wavelength matches the peak wavelength of the semiconductor laser. We choose very mature Nd:YVO ₄ instead of Nd:YAG. The doping concentration of Nd:YVO ₄ crystal is ≈0.5%, the geometric shape is lath, the size is 10×5×5mm, and the input and output end faces (that is, two 5×5 mm faces) are plated with a reflectance of 1064nm wavelength not higher than 0.1%. High-transmittance film, the side away from the cylindrical lens is coated with a high-reflection film with a reflectance of no less than 98% for a wavelength of 1064nm, and the side close to the cylindrical lens is coated with a reflectance of no higher than 1% for a wavelength of 808nm and not lower than that for a wavelength of 1064nm In 98% dichroic film, the crystal is wrapped with indium foil (to achieve good thermal contact) and placed in a copper block, which is cooled and temperature-controlled by a peltier cooler. The cylindrical lens has a focal length of 500um and a length of 13mm. The annular cavity is an isosceles triangular cavity composed of two curved mirrors and a plane mirror. The waist length is 150mm, and the base is 200mm. The curved surface mirror of the film system with an incident angle of 24°, the flat mirror 8 is a film system with a diameter of 20mm and coated with a film system of R>99.9%@1064nm (incidence angle of 42°), forming an annular cavity. The optical delay device 11 is coated with a film system with high reflection to the wavelength of 1.06 μm, which is used for optical delay and light combination. The beam splitter plate 9 is plated with λ=1.06 μm, T=50%, and is also used for combining light. The readout circuit 10 is used to read out signals. The electro-optic crystals used for electro-optic modulation are two electro-optic crystals with the same parameters. The end face is coated with a lithium niobate electro-optic crystal with a reflectivity of no higher than 0.2% for a wavelength of 1064nm. The geometric size is 3×3×5mm (with the travel direction of the traveling wave) The coincident length on the y optical axis is 5mm), symmetrically placed on the waist of the isosceles triangle. This system measures the rotational speed of the object relative to the inertial space and can significantly reduce the lock zone.

Claims (7)

1. semiconductor side pumped solid laser gyroscope, mainly by the ring cavity reflector group, gain media, pump arrangement, electro-optic crystal and electrooptical modulating circuit thereof, beam-splitting board, optical time delay device and sensing circuit are formed, it is characterized in that: described ring cavity is that the electro-optic crystal of two equal parameters is positioned on the waist of isosceles triangle symmetrically by two curved mirrors and the isosceles triangle chamber that level crossing is formed; Described pump arrangement adopts profile pump, and profile pump is by focusing on coupled lens, and semiconductor laser array and semiconductor laser attemperating unit are formed.

2. semiconductor side pumped solid laser gyroscope according to claim 1 is characterized in that: described semiconductor laser attemperating unit is made up of semiconductor chilling plate, heat radiator, thermistor, temperature control circuit.

3. semiconductor side pumped solid laser gyroscope according to claim 1 is characterized in that: described level crossing and curved mirror adopt plating wavelength 1064nm reflectivity to be not less than the level crossing and the curved mirror of 99.9% high-reflecting film.

4. according to claim 1, the described semiconductor side pumped solid laser gyroscope of 2 or 3 arbitrary claims, it is characterized in that: described electro-optic crystal adopts the end face plating wavelength 1064nm reflectivity not to be higher than the lithium niobate electro-optic crystal of 0.2% high transmittance film.

5. according to the semiconductor side pumped solid laser gyroscope of right 4, it is characterized in that gain media is the vanadic acid yttrium (Nd: YVO4) of neodymium-doped 0.5%, geometric configuration is a lath, the plating of input and output end face is not higher than 0.1 high transmittance film to 1064nm wavelength reflectivity, away from the plating of the one side of post lens wavelength 1064nm reflectivity is not less than 98% high-reflecting film, the one side plating of adjacent post lens is not higher than 1% and wavelength 1064nm reflectivity is not less than 98% Double-color film to wavelength 808nm reflectivity.

6. semiconductor side pumped solid laser gyroscope according to claim 1 is characterized in that: described focusing coupled lens adopt plating to wavelength 808nm reflectivity for not being higher than the post lens of 1% high transmittance film.

7. electrooptical modulation method that adopts semiconductor side pumped solid laser gyroscope as claimed in claim 1 is by signal generator, drive lithium columbate crystal (LiNbO ₃) the electrooptical modulating circuit modulation electric luminescent crystal formed of power amplifier, produce the offset frequency effect, reduce the lock district, it is characterized in that: use the sinewave modulation signal of phase differential 180 degree to modulate the electro-optic crystal of two equal parameters that symmetry places respectively.

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