Double longitudinal mode laser offset frequency locking method and device based on thermoelectric cooler
Technical Field The invention belongs to the technical field of laser application, in particular to a thermoelectric refrigerator based on double-longitudinal mode laser offset locking method and its device. Background Art In recent years, in order to mask aligner and numerical control machine tool to represent ultra-precise measurement and processing technology in a large-scale, high-precision, more space-degree-of-freedom synchronous measuring direction, the laser interference measuring system to the requirements of the new, shown in:on the one hand, the requirement of reducing interference in the processing line width of the measuring system from the measurement uncertainty 10-7 to 10-8, and then for the relative frequency stability of the laser source to achieve 10-8; on the other hand, large measuring scale and more space-degree-of-freedom synchronous measuring lead to interference in measuring system dramatically increased the total laser power consumption, far more than single frequency stabilization of the laser output laser power, at the same time there is a need to adopt the plurality of combined measuring laser frequency. However, different relative stability of frequency in the frequency stabilized laser, laser wavelength, wavelength differences drift direction, and the like, this will bring about laser interference measuring system the measurement precision of the different spatial degrees of freedom, wavelength reference and space coordinate the problem of inconsistent, thereby affecting the entire multi-dimensional laser interference measuring system integrated measurement accuracy. In order to guarantee the laser interference in measuring accuracy of the measuring system, using the plurality of the frequency of the laser frequency to achieve consistency 10-8, the frequency of the frequency stabilized laser has become a consistency between ultra-precise measurement and processing technology development is one of the key issues need to be resolved. The reference frequency is different in accordance with the frequency, frequency stabilized laser can be divided into two major categories: a laser gain curve to the center frequency of the reference frequency as the control frequency, such as lamb sunkenly frequency stabilized laser, frequency stabilized laser of double longitudinal mode, semaan frequency stabilized laser; to another kind of atom or molecule absorption line of the reference frequency as the control frequency, if the iodine is full and absorption frequency stabilized laser. Lamb sunkenly frequency stabilized laser to the center frequency of the gain curve of the reference frequency, utilizing the piezoelectric ceramic adjusting resonant cavity to length and laser oscillation frequency, when the laser operating frequency is located in the position of the center frequency of the gain curve, because the hole burning effect, laser output light power with an extremely small value. Lamb sunkenly frequency stabilized laser frequency stability and lamb recess and its symmetry related to the width of, the work air pressure of the a lower narrowed the, relative to the stability of the laser center frequency can reach 10-9, but at the same time will also be greatly reduce the output optical power of the laser. Because the center frequency of the laser gain curve along with the working gas pressure and discharge conditions and change, and multiple frequency stabilized laser on the physical parameter the same height can not be done, so its frequency stabilization control of the differences between the reference frequency, thus leading to a plurality of frequency stabilized laser output frequency of the laser the consistency is low, can reach only 10-6 -10-7. Furthermore, lamb sunkenly frequency stabilized laser output laser light intensity and frequency are provided with a certain modulation, modulation depth is about 10MHz. And lamb sunkenly frequency stabilized laser structure of the piezo-electric crystal and ectromechanic, the fabrication cost is expensive, poor anti-vibration performance, long preheating time 2-3 hours. Frequency stabilized laser of double longitudinal mode and fills the graceful steady frequency laser output laser includes two kinds of different polarization state and frequency of the laser, is generally used in the optical power of the two kinds of optical difference frequency of the feedback signal as a control, the two kinds of optical frequency stability of the laser gain curve on the symmetrical position of the center frequency. Frequency stabilized laser of double longitudinal mode and fills the graceful steady frequency laser ectromechanic on the structure and cavity structure can be adopted. Ectromechanic adjust the cavity length of the laser frequency of the device is piezoelectric ceramic device implementation, the structure of the frequency stabilized laser relative frequency stability than 10-9, long preheating time, however, poor anti-vibration performance. Adjusting the cavity length of the laser cavity frequency implementation of device has a electric heating device and thermoelectric refrigerator, the use of two kinds of implementation relative frequency stabilized laser cavity of the device is capable of achieving the stability of frequency 10-8, and the preheating time is less than 20 minutes. However, the frequency of adjusting the length of the electric heater element, preheating target temperature is generally higher than the natural preheating the equilibrium temperature of the laser tube, so the different ambient temperatures with greater difference preheating time, at the same time the high preheating temperature of the photoelectric conversion device and other devices the drift of the performance parameters, the control circuit lead to instability of the system frequency, higher working temperature to also reduce the service life of the laser tube. In order to solve the electric heater member adjusting the length of the frequency stabilization method of the above-mentioned shortcoming, Harbin industrial University based on thermoelectric refrigerator a double-longitudinal mode laser frequency stabilization method (Chinese Patent CN100382398: based on thermoelectric refrigerator double-longitudinal mode laser frequency stabilization method and device). The method of the thermoelectric refrigerator plus reverse current to the laser tube is pre-heated to its natural operation thermal balance temperature, through controlling the thermoelectric refrigerator change magnitude and direction of the current laser resonant the cavity cavity is long to control the laser output double-longitudinal mode optical power difference is zero, to achieve the purpose of the frequency. Avoiding the existing frequency stabilization device with the different temperatures of the environment is lengthened preheating time, the preheating effect is not ideal, is easy to be influenced by the external environmental temperature, defects of the effect of changes in the speed of the air. However, in the design of the heat transfer structure, the thermoelectric refrigerator is mounted on the same side of the laser tube, the laser tube wall by non-uniform heating or cooling, temperature gradient exists, so as to cause a radial distortion of the laser tube, the stability of the laser frequency affect the output. In addition such frequency stabilized laser of double longitudinal mode is still in the center frequency of the laser gain curve of the frequency of the frequency-control as a reference point, and this center frequency susceptible to temperature, the influence of factors such as the atmospheric pressure changes, it is difficult to exceed the accuracy of the frequency of the laser relative to 10-8. In addition the above-mentioned two kinds of structure frequency stabilized laser of double longitudinal mode and fills the graceful steady frequency laser, its frequency-stabilizing control the reference frequency of the control actuator has nothing to do with the resonant cavity length, essentially is the center frequency of the laser gain curve, because of the working pressure, difference and other parameters of the discharge current, a plurality of frequency stabilized laser output laser frequency still shortcomings of low consistency, is generally 10-6 -10-7. The absorption frequency stabilized laser the iodine is full ectromechanic and the structure, its resonant cavity is placed with high purity low pressure iodine vapor absorption chamber, and to utilize the piezoelectric ceramic adjusting the length of the cavity, the operating frequency of the laser is locked in the127 I2 molecules on the hyperfine absorption lines. Because only the air pressure of the iodine absorption chamber 1-10Pa, mainly depends on the width of the peak absorption of the natural line width of the absorption substance, its value is only dozens of KHz, lamb is far less than the width of the dent, therefore the iodine is full and absorb relative to the center frequency of a frequency stabilized laser can reach stability of frequency 10-11 -10-12. On the other hand, due to the adoption of the external reference frequency, the frequency is not influenced by laser working substance pressure and the impact of the discharge current and other parameters, so in the case of a plurality of frequency stabilized laser, its frequency stabilization control of the frequency reference is highly consistent, frequency can be as high as consistency 10-11 -10-12. However, cavity iodine steady frequency laser output light modulation of the frequency-modulated laser, the modulation depth of the wave frequency to several MHz, therefore, on the whole, the accuracy of the frequency of the laser relative to 10-8. Furthermore, frequency stabilized laser output the iodine is full and the absorption of only several tens of μW laser power, due to the use of piezoelectric ceramic adjusting element, process the structure is complicated, the price is expensive, the piezoelectric material creep and use period is short, a long preheating time, poor anti-vibration performance. In order to overcome the output of he-Ne laser iodine steady frequency with modulation of the laser frequency, the disadvantages of small luminous power, the United States Lawrence Livemore laboratory research R.R.Donaldson offset locking of the 633 nm helium-neon laser (R.R.Donaldson, S.R.Paterson.Design and Construction of a Large, Vertical-axis Diamond Turning Machine.Proc.Of SPIE. 1983, (433): 62-67). The laser has the characteristics that a free operation of the high-precision laser tracking another iodine steady frequency laser, laser iodine steady frequency and deviate from a fixed frequency value, thus not only maintains the laser center frequency iodine steady frequency the advantages of relatively high accuracy, but also can be of the non-modulated output frequency of the high-power laser, the relative accuracy of the frequency reaches 10-9, output power reaches 15mW. However, this kind of laser employs a resonant cavity structure ectromechanic and piezoelectric ceramic adjusting element, long preheating time is removed, outside of the anti-vibration characteristics, very large volume of the entire laser device. At present, this kind of laser used only for individual special large ultra-precision machining equipment, and additional anti-vibration measures need to be taken. To sum up, lamb sunkenly frequency stabilized laser to achieve frequency stability 10-9, poor anti-vibration performance, however, a modulation frequency, can not be applied to the industrial measuring field, and the frequency of the plurality of frequency stabilized laser only reaches the consistency 10-6 -10-7; ectromechanic frequency stabilized laser of double longitudinal mode and semaan frequency stabilized laser can reach the relative frequency stability 10-9, long preheating time, however, bad resistance to shock, can not be applied to the industrial measurement field; inner cavity type double-longitudinal mode laser with the Zeeman laser to achieve the relative frequency stability 10-8, strong adaptive capacity working environment, however, the control frequency of the reference frequency is essentially the center frequency of the laser gain curve, because of the working pressure, the difference of the discharge current and other parameters, there are still multiple frequency stabilized laser frequency is low in the consistency, cannot meet the ultra-precision measuring and processing technology the development of frequency stabilized laser frequency consistency 10-8 requirements; the iodine is full and absorption frequency stabilized laser is frequency stability and consistency 10-11, , but the output power is small, the requirement to the work environment is high, generally only used for measuring the calibrating field; the iodine is full based on piezoelectric ceramic and to and absorption frequency stabilized laser as a reference the offset locking laser, its frequency stability and consistency reach 10-9, but the structure is complicated, anti-vibration ability, suitable occasions are strictly limited. From this it can be seen, the existing frequency stabilized laser technology will be difficult to meet a new generation of ultra-precision machining the requirements of the development of the measuring technology. Content of the invention In response to the deficiency of the existing laser frequency stabilization technique, the invention provides a thermoelectric refrigerator based on double-longitudinal mode laser offset locking method, its purpose is to solve the current multi-dimensional laser interference measuring system because the frequency of the frequency stabilized laser poor consistency between the integrated measurement due to the problem of precision on the low side, and the ultra-precise processing the rapid development of the measuring technology provides a frequency good consistency, long service life and can be directly applied to the industrial field model laser light source. The invention also provides a thermoelectric refrigerator based on double-longitudinal mode laser offset locking device. The purpose of this invention is realized through the following technical scheme: Based on thermoelectric refrigerator a double longitudinal mode laser offset locking method, the method comprises the following steps: (1) opening frequency stabilized laser of double longitudinal mode A power supply, after the course through the preheating and stabilization, laser A internal laser tube output polarization direction of the two mutually-orthogonal longitudinal mode light , by the polarization beam splitter taken out as one of the longitudinal mode A of the output light of the laser light, the output light is separated into a fiber beam splitter n ≥ 1 path, denoted as light beam X1, X2, ... , Xn, wave frequency the same as the vr, as a double longitudinal mode laser B1, B2, ... , Bn locking the reference frequency of the frequency offset; (2) open at the same time double-longitudinal mode laser B1, B2, ... , Bn power supply, measuring the current environment temperature T0 and to the determination of the preheating target temperature value Tset, and T0 <Tset by the thermoelectric cooler to the double-longitudinal mode laser B1, B2, ... , Bn to preheat the laser tube, and according to the current temperature Treal and preheating target temperature Tset thermoelectric refrigerator constantly adjust the difference of the size of the reverse current, the temperature of the make the laser tube gradually tends to pre-set temperature value Tset, and eventually reach a thermal equilibrium state, output of the laser tube of the laser includes the polarization direction of the two mutually-orthogonal longitudinal mode light , separated by the polarization light splitting device one of the longitudinal mode light is used as a double-longitudinal mode laser B1, B2, ... , Bn the output light, as light beam Y1, Y2, ... , Yn, corresponding entry of the wave frequency v1, v2, ... , vn; (3) double-longitudinal mode laser B1, B2, ... , Bn after the end of its course of preheating the entering the frequency locking control process, the light beam X1, X2, ... , Xn is respectively connected with the beam Y1, Y2, ... , Yn optical mixer and form of the beat frequency optical signal path n, using high-frequency photoelectric detector converts the n way beat frequency optical signal into the n path signal, by the signal conditioning the rear, the frequency value is measured by the frequency measuring module, as the Δ v1, Δv2, ... , Δvn, wherein Δ vi = | vi-vr | (i=1, 2, ... , N); (4) frequency stabilized laser of double longitudinal mode B1, B2, ... , Bn difference in their respective wave frequency Δ v1, Δv2, ... , Δvn value change section of the same a laser frequency locking, and all the laser pre-set offset reference value Δ vset the same, will be obtained by measuring the frequency difference of the light wave Δ v1, Δv2, ... , Δvn as the feedback signal of the closed-loop control of the locked, the offset reference value with the pre-set Δ vset seeking difference, according to the frequency difference of light wave Δ v1, Δv2, ... , Δvn and the offset reference value Δ vset the resulting difference is the positive, negative sum size adjustment thermoelectric refrigerator of the forward current is applied, reverse and size, thus to control the laser tube refrigeration and heating, so as to change the temperature of the laser tube, the cavity length and laser longitudinal mode frequency, the Δ v1, Δv2, ... , Δvn tends to Δ vset; (5) when the Δ v1 =Δv2 =... =Δvn =Δvset time, double-longitudinal mode laser B1, B2, ... , Bn frequency locking control process is finished, the polarization light splitting device by separating the single longitudinal mode laser Y1, Y2, ... , Yn in the same lock the frequency of a frequency value, in other words v1 = v2 =... = vn = vr +Δvset (or v1 = v2 =... = vn = vr-Δvset); (6) the pre-adjusted to a reference value of offset Δv 'set, repeating step (4), (5), double-longitudinal mode laser B1, B2, ... , Bn output laser Y1, Y2, ... , Yn the lock the frequency of the frequency value of the re-set vr +Δv 'set (or vr-Δv'set) the upper, thus the output of the frequency of the laser the adjustment. Based on thermoelectric refrigerator a double longitudinal mode laser offset locking device, comprises a double longitudinal mode stable frequency laser power supply, frequency stabilized laser of double longitudinal mode, frequency stabilization state indication lamp, fiber-optical beam splitter, device n ≥ 1 is also included in the same structure, a double-longitudinal mode laser parallel relations, wherein each double-longitudinal mode laser is the assembly structure of the: double-longitudinal mode laser power supply and the laser tube is connected with, a main polarizing beam splitter is placed before the output end of the laser supervisor , vice-polarization beam splitter is placed in the laser tube pair with the output end of the optical fiber between an input end of the combiner, the optical fiber bundle with another input terminal of is connected to one of the output end of the optic fiber beam splitter, the polarization analyzer is placed in the output end of the optical fibre beam combiner between the high-speed photodetector, a high-speed photodetector, high-speed frequency divider, pre-amplifier, a post-amplifier, high-speed comparator, frequency measuring module, a microprocessor, D/A converter, thermoelectric cooler driver, thermoelectric refrigerator, is connected with the heat transfer structure, wherein the heat transfer structure from the laser tube from the inside to the outside of the internal heat-conducting layer are sequentially assembled, copper pipe heat conducting layer, the intermediate heat-conducting glue layer, thermoelectric refrigerator, external heat conduction adhesive layer, a radiator, a heat-insulating layer, and the thermoelectric cooler and the radiator are respectively provided with two, symmetrical to the laser tube is two sides, the electric current of the thermoelectric refrigerator through a thermoelectric refrigerator drive control end, is connected with the microprocessor D/A converter, laser tube temperature sensor in the internal heat-conducting glue layer, and its output end is connected with the microprocessor, the ambient temperature sensor is placed in the double-longitudinal mode laser external, its output end is connected with the microprocessor, the frequency lock state indicating lamp is connected to the microprocessor. The high-speed photoelectric detector bandwidth greater than 500MHz. The invention has the following characteristics and good results: (1) the present invention adopts a laser offset locking technology to a plurality of double-longitudinal mode laser frequency locking in parallel, all offset locking frequency stabilized laser of double longitudinal mode output laser is provided with a uniform frequency value, its relative frequency stability reaches 10-8, frequency to achieve consistency 10-8, overcome in the traditional frequency stabilized laser due to frequency reference is not consistent between to the frequency of the frequency stabilized laser can only achieve consistency 10-6 -10-7 deficiency, this is different from the prior art one of the innovative point. (2) the present invention adopts a frequency stabilized laser of double longitudinal mode in the cavity structure, and to thermoelectric refrigerator as the cavity length adjustment execution element, at the same time using thermoelectric refrigerator design symmetrical heat transfer structure, elimination of the laser tube of the laser due to uneven heating of the radial distortion influence to the stability of the output frequency; compared with the external cavity frequency stabilized laser preheating time is short, strong shock-resistant kinetic energy, can be directly used for industrial site; and the electric heating device as a cavity resonator compared with length-adjustable actuating element, the laser tube is reduced the temperature of the work frequency, enhancing the environmental adaptability, reduce the preheating time, reduced the photoelectric conversion device and the temperature drift on the other device performance parameters to the impact of the frequency stabilization effect, improve the service life of the laser tube, this is different from the prior art, bis the innovative point. (3) by changing the preset offset reference value Δ vset, of offset can be adjusted in this invention locked frequency stabilized laser of double longitudinal mode output value of the frequency of the laser, thus can more conveniently generate wavelength of the laser can be tuned continuously, this is different from the existing technical innovation of three points. Description of drawings Figure 1 is a schematic diagram of the principle of the device of the invention Figure 2 (a) to the device of the invention in a structure diagram of the double-longitudinal mode laser Figure 2 (b) to fig. 2 (a) A-Aa cross-sectional view in, that is, heat transfer a structure diagram Figure 3 is a schematic diagram of a closed-loop control system of the device of the invention in the course of preheating double-longitudinal mode laser Figure 4 is the device of the invention in double-longitudinal mode laser preheating the equilibrium temperature curve and the environment temperature Figure 5 is a schematic diagram of a closed-loop control system in the device of the present invention double-longitudinal mode laser frequency locking process Figure 6 is a schematic diagram of the relative position of this invention double-longitudinal mode laser frequency locking position and the reference frequency Figure 7 is a schematic diagram of the mutual relations between the thermoelectric refrigerator in this invention the heat exchange the direction of the current direction Figure 8 is graph of the preheating temperature of the embodiment of the invention in different initial environment Figure 9 is the invention ordinary power balance type frequency stabilized laser of double longitudinal mode output laser frequency drift map Figure 10 as in the present invention double-longitudinal mode laser output laser frequency drift map In the Figure, 1 frequency stabilized laser power supply, 2 frequency stabilized laser of double longitudinal mode, B1, B2, ... , Bn double-longitudinal mode laser, 3 frequency-state indicator, 4 optical fiber beam splitter, 5 laser power supply, 6 a microprocessor, 7 the ambient temperature sensor, 8 laser tube temperature sensor, 9 laser tube, 10D/A converter, 11 thermoelectric cooler driver, 12 thermoelectric refrigerator, 13 pair of polarization light splitter, 14 a main polarizing beam splitter, 15 optical fiber combiner, 16 an analyzer, 17 a high-speed photodetector, 18 high-speed frequency divider, 19 pre-amplifier, 20 a post-amplifier, 21 high-speed comparator, 22 frequency measuring module, 23 locked state indicator, 24 internal heat conducting adhesive layer, 25 copper pipe heat conducting layer, 26 intermediate conduction rubber layer, 27 external heat conduction adhesive layer, 28 radiator, 29 thermal insulation layer. Mode of execution The following illustrated in the Figure the implementation of this invention a detailed description of embodiment. Based on thermoelectric refrigerator in this invention the double-longitudinal mode laser offset locking device comprises a frequency stabilized laser power supply 1, frequency stabilized laser of double longitudinal mode 2, frequency stabilization state indicating lamp 3 and the optical fiber beam splitter 4, device n ≥ 1 is also included in the same structure, are connected in parallel relationship between a double-longitudinal mode laser B1, B2, ... , Bn, wherein each double-longitudinal mode laser B1, B2, ... , Bn is the assembly structure of the: double-longitudinal mode laser power supply 5 and the laser tube 9 is connected with the, main polarizing beam splitter 14 is placed in the laser tube 9 before the main output, vice-polarization beam splitter 13 is placed in the laser tube 9 pair with the optical fiber output end of the combiner 15 between one of the input end of the, optical fiber combiner 15 with the other input end of optical fiber beam splitter 4 is connected to one of the output end of the, an analyzer 16 is placed in the optical fibre beam combiner 15 and the output end of the high-speed photoelectric detector 17 between, high-speed photoelectric detector 17, high-speed frequency divider 18, pre-amplifier 19, a post-amplifier 20, high-speed comparator 21, frequency measuring module 22, the microprocessor 6, D/A converter 10, thermoelectric cooler driver 11, thermoelectric refrigerator 12, is connected with the heat transfer structure, wherein the heat transfer structure from the laser tube 9 from inside to outside in sequence to the internal heat-conducting glue layer assembly 24, copper tube heat conducting layer 25, an intermediate heat-conducting adhesive layer 26, thermoelectric refrigerator 12, external heat conduction glue layer 27, radiator 28, insulating layer 29 composed, and the thermoelectric cooler 12 and the radiator 28 each have two, symmetrical to the laser tube 9 is of the two sides of, thermoelectric cooler 12 the control end of the current through the thermoelectric cooler driver 11, D/A converter 10 with the microprocessor 6 is connected, laser tube temperature sensor 8 is in the internal heat conducting adhesive layer 24 in, its output end is connected with the micro-processor 6, the ambient temperature sensor 7 is placed in the double-longitudinal mode laser B1, B2, ... , Bn external, its output end is connected with the micro-processor 6, locked state indicator light 23 the microprocessor 6. The high-speed photo-detector 17 a detection bandwidth greater than 500MHz. In view of the device includes a plurality of offset locking frequency stabilized laser of double longitudinal mode B1, B2, ... , Bn, and laser B1, B2, ... , Bn preheating and frequency control of locking is completely consistent process, the following only the frequency stabilized laser of double longitudinal mode B1 described the process, the description is equally applicable to apparatus in any one of the other offset locking frequency stabilized laser of double longitudinal mode. At the start of operation, the opening frequency stabilized laser of double longitudinal mode power supply 1, frequency stabilized laser of double longitudinal mode 2 the process enters the preheating and stabilization, when the above-mentioned process is finished, the state indicator light frequency 3, said frequency stabilized laser of double longitudinal mode 2 has entered the frequency stabilized state, the output of the beam splitter is coupled into the fiber laser by 4, are separated into n way the frequency of reference light beam, denoted as light beam X1, X2, ... , Xn, frequency recorded as vr. In frequency-state indicator light 3-enabled time, open double-longitudinal mode laser power supply 5, double-longitudinal mode laser B1 enters the preheating process, Figure 3 is a double-longitudinal mode laser environment temperature and preheating heat balance temperature curve chart, different ambient temperatures different thermal equilibrium temperature preheating the same, but each of the thermal balance of the laser tube under the state of the external environmental temperature is the same as heat exchange energy, and the heat exchange energy and the temperature difference the relevant, in other words the temperature and the environmental temperature have fixed temperature difference. According to the preheating temperature curve determination of the preheating heat balance temperature Tset. Figure 4 is a schematic diagram of a closed-loop control system for double-longitudinal mode laser preheating process. The microprocessor 6 according to the environmental temperature sensor 7 measuring ambient temperature of the thermal equilibrium temperature of setting the preheating Tset, and will Tset preheating closed-loop control system as the reference input quantity, at the same time in order to laser tube temperature sensor 8 measured by laser tube 9 temperature Treal as a feedback signal, the microprocessor 6 to calculate the difference of the two, and according to the MPC control algorithm, outputting the digital control signal, by D/A converter 10 digital-to-analog converted to an analog signal, the analog signal through the thermoelectric cooler driver 11 is amplified, is used for controlling the thermoelectric cooler 12 the working current, to the laser tube 9 for preheating. The laser tube 9 reach thermal equilibrium temperature of preheating Tset the rear, the microprocessor 6 switching double-longitudinal mode laser B1 enter the frequency locking control process. Figure 5 is the device of the invention in double-longitudinal mode laser frequency locking process a schematic diagram of the closed-loop control system. Laser tube 9 the main, auxiliary output end of the mutually orthogonal directions of polarization are outputting two longitudinal mode light , by the polarization beam splitter 13 and 14 are respectively separate the master, the auxiliary output terminal of the two longitudinal mode light , horizontal polarization of the auxiliary output longitudinal mode light used for frequency locking control, denoted as light beam Y1, its frequency as the v1, main output end of the longitudinal horizontal polarization of light is used as a frequency stabilized laser of double longitudinal mode B1 the output light. Beam Y1 beam combiner is coupled into the fiber 15, and the reference beam X1 are combined into one beam, through the analyzer 16 reference beam after X1 and light beam Y1 polarization in the same direction, to form the beat frequency optical signal, and the high-speed photoelectric detector 17 are converted to the voltage signal, the voltage signal passes through high-speed frequency divider 18, pre-amplifier 19, a post-amplifier 20, high-speed comparator 21, a square-wave signal, is sent into the frequency measuring module 22 for frequency measurement, to obtain light beam X1 and light beam Y1 optical frequency difference Δ v1 = | v1-vr |. Frequency measuring module 22 the frequency of the measured Δ v1 to beam X1 center frequency vr and light beam Y1 frequency v1 the absolute value of the difference, beam Y1 frequency value v1 = vr-Δv1 or v1 = vr +Δv1, in other words v1 may be located on vr the left side or the right side, as shown in Figure 6. In order to make the frequency stabilized laser of double longitudinal mode B1, B2, ... , Bn output laser is provided with a uniform frequency value, all need to be offset locking frequency stabilized laser of double longitudinal mode unified locking to vr the same side, there is a need to judge v1 and vr of the relative position. The microprocessor 6 controls the thermoelectric cooler 12 to the laser tube 9 is heated, its temperature rises to the Tset +ΔT, in the process at the same time measuring the Δ v1 value change. Under v1 and the relationship between the length of the laser tube In the formula, c is lightspeed, q to longitudinal mode ordinals, η is the refractive index of the resonant cavity, the cavity length of the laser tube l. When the temperature rises, increased l cavity length of the laser tube, v1 reduced, if Δ v1 increase, then v1 located in vr the left side of the; reverse, v1 located in vr the right side. The embodiment would v1 locking to vr the right side, if the heat balance temperature Tset lower v1 actually located vr the left side, it is aimed at laser B1, adjusting the thermal balance temperature is T 'set, the v1 transferred to vr the right side. The frequency measuring module 22 measured Δ v1 frequency locking value as the feedback signal of the closed-loop control system, at the same time, the pre-set offset reference value Δ vset (this embodiment heating Δ vset = 65MHz) as a reference input of the control system, the microprocessor 6 to calculate the difference of the two, according to the two difference value of the positive and negative and the size passes through MPC control algorithm processing obtain the output a digital control signal, the converter D/A 10 converted to an analog voltage, thermoelectric cooler driver 11 for the power amplification, final regulating thermoelectric cooler 12 the direction and the magnitude of the working current, to change the degree of heating or refrigerating in order to change the laser tube 9 temperature, and then adjust the laser tube 9 of laser resonance cavity length and the frequency of the output light, the Δ v1 tends to Δ vset. When the Δ v1 ≈Δvset time, double-longitudinal mode laser B1 frequency locking process is finished, the locked state indicator light 23, said double-longitudinal mode laser B1 enter working condition of stability, at this moment v1 = vr +Δvset. The offset reference value will be adjusted to preset Δv 'set, repeat the above-mentioned frequency locking process, the double-longitudinal mode laser B1 for adjusting the frequency of the output laser vr +Δv'set. Figure 7 shows that embodiment thermoelectric cooler 12 the direction of the current relationship with each other in the direction of heat energy. Embodiment thermoelectric cooler 12 which is applied to the semiconductor material of the Peltier effect and other relevant thermoelectric effect semiconductor component manufacturing design, appropriated small volume, long service life, vibration and noise-free without any pollution, and the like, principle is that: when a block N-type semiconductor material and a P-type semiconductor material to form a thermocouple to the time, in this circuit are connected to the direct current, can be the transfer of energy occurs in:N-shaped element by the electric current flows to the P-type element, joint to absorb heat, become cold end; current flows from the P-type component N-type element, joint release heat. As the hot end. The direction of heat energy by the decision of the current direction, absorbs heat and emit heat by the current size of the size of the decision. When the embodiment thermoelectric cooler 12 when the positive extreme input current, heat energy from the laser tube 9 output, sequentially through the internal heat-conducting adhesive layer 24, copper tube heat conducting layer 25, an intermediate heat-conducting adhesive layer 26, thermoelectric refrigerator 12, external heat conduction glue layer 27, reaches the radiator 28, the heat sink 28 with a larger area, so the heat easily through the air convection and radiation into the air in the form of; when the embodiment thermoelectric refrigerator 12 when a negative extreme input current, radiator 28 through the air convection and radiation in the form of absorbing heat energy from air, sequentially through the external heat conduction glue layer 27, thermoelectric refrigerator 12, intermediate conduction rubber layer 26, the heat conducting layer of copper tube 25, internal heat conducting adhesive layer 24, to reach the laser tube 9. Fig. 8 provides the device of the invention the embodiment of different initial environment curve the preheating temperature, changes from the curve trend in different can be obtained under the initial temperature environment, changes in laser preheating curve is basically identical, in 15 minutes, the target temperature to the temperature rise from the 0.1 within [...] , and the temperature change rate is very small, basic to reach a thermal equilibrium. Description device in different industrial field, after time substantially uniform preheating, heat balance can be obtained, providing a good frequency stabilized conditions. Figure 9 is an ordinary power balance type frequency stabilized laser of double longitudinal mode output laser frequency drift Figure, diagram provides two power balance type frequency stabilized laser of double longitudinal mode at 15 months in the laser sampling measurement of the variation of the frequency curve, the frequency of the two laser the iodine is full and absorbed by the helium-neon laser beat frequency to frequency. As can be seen from the Figure, two laser output of the laser is about frequency difference 210MHz, and exist long-term, slow drift, the change of the frequency drift of different lasers is not the same tendency, therefore, to the ordinary power balance type frequency stabilized laser of double longitudinal mode, its laser frequency generally only achieve consistency 10-6 -10-7. Figure 10 is double-longitudinal mode laser output laser frequency map long-term drift of the invention offset locking. The example calibration n=2, double-longitudinal mode laser B1 and B2the iodine is full and the frequency of the absorbed by the helium-neon laser beat frequency obtained by frequency, as can be seen from the chart, in for a period of 15 months in a sample measuring time, double-longitudinal mode laser B1 and B2 the frequency of outputting the laser is that long-term, slow drift, this is because the presence of the feedback control of the long term drift of the reference frequency. However, laser B1 with the laser B2 the control frequency of the same of the reference frequency, the frequency drift of the trend is consistent in general, its frequency to achieve consistency 10-8. A double longitudinal mode laser offset frequency locking method and a device based on a thermoelectric cooler belong to the technical field of laser application. In the invention, the frequency of the output laser of a power balance type double longitudinal mode frequency stabilized laser A is taken as the reference frequency, and simultaneously the thermoelectric cooler is used for controlling the temperature to adjust the cavity length so that a fixed difference value is maintained between the frequency of the output laser of n>=1 double longitudinal mode lasers B1, B2, ..., Bn and the reference frequency, thus ensuring the output laser of the double longitudinal mode lasers B1, B2, ..., Bn to have uniform frequency value and the relative frequency stability and the frequency invariance to reach 10 and overcoming the shortcoming that the frequency invariance between the frequency stabilized lasers only reaches 10-10 because of inconsistency of the reference frequency in the traditional frequency stabilized lasers. 1, based on thermoelectric refrigerator a double longitudinal mode laser offset locking method, which is characterized in that the method comprises the following steps: (1) opening frequency stabilized laser of double longitudinal mode A power supply, after the course through the preheating and stabilization, laser A internal laser tube output polarization direction of the two mutually-orthogonal longitudinal mode light , by the polarization beam splitter taken out as one of the longitudinal mode A of the output light of the laser light, the output light is separated into a fiber beam splitter n ≥ 1 path, denoted as light beam X1, X2, ... , Xn, wave frequency the same as the vr, as a double longitudinal mode laser B1, B2, ... , Bn locking the reference frequency of the frequency offset; (2) open at the same time double-longitudinal mode laser B1, B2, ... , Bn power supply, measuring the current environment temperature T0 and to the determination of the preheating target temperature value Tset, and T0 <Tset, by the thermoelectric cooler to the double-longitudinal mode laser B1, B2, ... , Bn to preheat the laser tube, and according to the current temperature Treal and preheating target temperature Tset thermoelectric refrigerator constantly adjust the difference of the size of the reverse current, the temperature of the make the laser tube gradually tends to pre-set temperature value Tset, and eventually reach a thermal equilibrium state, output of the laser tube of the laser includes the polarization direction of the two mutually-orthogonal longitudinal mode light , separated by the polarization light splitting device one of the longitudinal mode light is used as a double-longitudinal mode laser B1, B2, ... , Bn the output light, as light beam Y1, Y2, ... , Yn, corresponding entry of the wave frequency v1, v2, ... , vn; (3) double-longitudinal mode laser B1, B2, ... , Bn after the end of its course of preheating the entering the frequency locking control process, the light beam X1, X2, ... , Xn is respectively connected with the beam Y1, Y2, ... , Yn optical mixer and form of the beat frequency optical signal path n, using high-frequency photoelectric detector converts the n way beat frequency optical signal into the n path signal, by the signal conditioning the rear, the frequency value is measured by the frequency measuring module, as the Δ v1, Δv2, ... , Δvn, wherein Δ vi = | vi-vr | (i=1, 2, ... , N); (4) frequency stabilized laser of double longitudinal mode B1, B2, ... , Bn difference in their respective wave frequency Δ v1, Δv2, ... , Δvn value change section of the same a laser frequency locking, and all the laser pre-set offset reference value Δ vset the same, will be obtained by measuring the frequency difference of the light wave Δ v1, Δv2, ... , Δvn as the feedback signal of the closed-loop control of the locked, the offset reference value with the pre-set Δ vset seeking difference, according to the frequency difference of light wave Δ v1, Δv2, ... , Δvn and the offset reference value Δ vset the resulting difference is the positive, negative sum size adjustment thermoelectric refrigerator of the forward current is applied, reverse and size, thus to control the laser tube refrigeration and heating, so as to change the temperature of the laser tube, the cavity length and laser longitudinal mode frequency, the Δ v1, Δv2, ... , Δvn tends to Δ vset; (5) when the Δ v1 =Δv2 =... =Δvn =Δvset time, double-longitudinal mode laser B1, B2, ... , Bn frequency locking control process is finished, the polarization light splitting device by separating the single longitudinal mode laser Y1, Y2, ... , Yn in the same lock the frequency of a frequency value, in other words v1 = v2 =... = vn = vr +Δvset (or v1 = v2 =... = vn = vr-Δvset); (6) the pre-adjusted to a reference value of offset Δv 'set, repeating step (4), (5), double-longitudinal mode laser B1, B2, ... , Bn output laser Y1, Y2, ... , Yn the lock the frequency of the frequency value of the re-set vr +Δv 'set (or vr-Δv'set) the upper, thus the output of the frequency of the laser the adjustment. 2, based on thermoelectric refrigerator a double longitudinal mode laser offset locking device, comprises a double longitudinal mode stable frequency laser power supply (1), frequency stabilized laser of double longitudinal mode (2), the status indicator lamp frequency (3), the optic fiber beam splitter (4), characterized in that the device also includes n ≥ 1 the same structure, are connected in parallel relationship between a double-longitudinal mode laser (B1, B2, ... , Bn), wherein each double-longitudinal mode laser (B1, B2, ... , Bn) is the assembly structure of the: double-longitudinal mode laser power supply (5) and the laser tube (9) is connected, a main polarizing beam splitter (14) is placed in the laser tube (9) before the main output, vice-polarization light splitter (13) is placed in the laser tube (9) the output end of the optical fiber bundle pair (15) between an input end of the, optical fiber combiner (15) and the other input end of optical fiber beam splitter (4) is connected to one of the output end of the, an analyzer (16) is placed in the optical beam combiner (15) and the output end of the high-speed photoelectric detector (17) between, a high-speed photodetector (17), high-speed frequency divider (18), the pre-amplifier (19), a post-amplifier (20), high-speed comparator (21), the frequency measuring module (22), a microprocessor (6), D/A converter (10), thermoelectric cooler driver (11), thermoelectric cooler (12), is connected with the heat transfer structure, wherein the heat transfer structure from the laser tube (9) in turn from the inside to the outside of the internal heat-conducting glue layer (24), the heat conducting layer of copper tube (25), an intermediate heat-conducting glue layer (26), thermoelectric cooler (12), external heat conduction adhesive layer (27), a radiator (28), heat-insulating layer (29) form, and the thermoelectric cooler (12) and the radiator (28) each have two, symmetrical to the laser tube (9) is two sides, thermoelectric cooler (12) the control end of the current through the thermoelectric cooler driver (11), D/A converter (10) and the microprocessor (6) is connected, laser tube temperature sensor (8) is in the internal heat-conducting glue layer (24) in, its output end is connected with the micro-processor (6), the ambient temperature sensor (7) is placed in the double-longitudinal mode laser (B1, B2, ... , Bn) external, its output end is connected with the micro-processor (6), locked state indicator (23) with the microprocessor (6). 3, based on thermoelectric refrigerator offset locking device for double-longitudinal mode laser according to Claim 2, characterized in that a high-speed optical detector (17) is larger than the detection bandwidth 500MHz.