CN1138323C - Dual-refraction dual-frequency Zeeman laser device with stable frequency difference and its method for stabilizing frequency difference - Google Patents

Abstract

The present invention discloses a method for stabilizing frequency difference of a zeeman dual-refraction dual-frequency laser with stable frequency difference, which relates to a structure of a helium-neon zeeman dual-refraction dual-frequency laser and a method for stabilizing frequency difference thereof. The present invention is characterized in that on the basis that reliable and suitable frequency difference is obtained by applying a two-dimensional thrust augmentation ring, computer control is utilized for stabilizing the frequency difference by stabilizing the frequency of a laser. In the present invention, a two-dimensional force application mode replaces the original one-dimensional force application mode, so materials of the thrust augmentation ring are optimized and selected, and meanwhile, a frequency-stabilizing device for the laser is added; the drift of frequency difference of the laser is further reduced, the reliability of the device and the stability of the frequency are improved, and the laser enters a period of practical application.

Description

Frequency Difference Stabilized Zeeman Birefringent Dual-Frequency Laser and Its Frequency Difference Stabilization Method

technical field

The invention relates to a laser, in particular to a structure of a helium-neon Zeeman-birefringence dual-frequency laser and a frequency stabilization method for the laser.

Background technique

A "Birefringent Dual-Frequency Laser Without Frequency Difference Blocking and Its Frequency Difference Accuracy Control Method" proposed in the current prior art (Chinese Patent: Application No.: 99103513.5), the dual-frequency laser consists of a laser gain tube, a one-dimensional The force device is composed of an external transverse magnetic field. This technology solves the problem that the dual-frequency laser cannot output a frequency difference greater than 3 MHz and less than 40 MHz, thus breaking through the limitation of the existing dual-frequency laser interferometer on the measurement speed, which is of great significance in the history of the invention of dual-frequency lasers . Although this type of laser can output a suitable frequency difference suitable for dual-frequency laser interferometer applications, its frequency difference stability cannot meet the specific requirements of industrial applications. The frequency difference of the Zeeman-birefringence dual-frequency laser still drifts. There are three reasons for this: first, when the external temperature changes, the afterburner ring and the photoelastic element (the half-cavity is an anti-reflection window, and the whole cavity is the output window) will expand with heat and contract with cold, but if the materials of the booster ring and the photoelastic element are different and the coefficients of linear expansion are different, the magnitudes of thermal expansion and cold contraction will also be different. For example, the reinforcing ring is Invar (coefficient of linear expansion is about 5×10 ^-7 ), and the window material is K9 glass (coefficient of linear expansion is about 4×10 ^-5 ), the diameter of the window is larger than that of Invar when the temperature rises. The faster the expansion, the greater the pressure of the afterburner ring on the window, causing the internal stress of the window to become larger, the birefringence effect is strengthened, and the frequency difference becomes larger, and vice versa. The variance introduced by this is on the order of a few MHz. The second is that the device for applying force to the photoelastic element in the existing Zeeman-birefringence dual-frequency laser is a one-dimensional force application method. Since the frequency difference of this laser is proportional to the stress generated in the photoelastic element, it is necessary to generate When the frequency difference is small, the stress applied is also small, which makes the force device easy to loose relative to the photoelastic element, and the reliability is poor. Third, as the temperature of the laser tube heats up, the length of the laser cavity becomes longer, the two frequencies of the laser will drift, and the gain line must be swept. Due to the frequency pulling effect, the frequency difference will change by several KHz.

Contents of the invention

The purpose of the present invention is on the basis of above-mentioned patent technology, proposes a kind of frequency difference stable Zeeman birefringence dual-frequency laser and its frequency difference stabilization method, to overcome the existing Zeeman birefringence dual-frequency laser frequency difference drift problem, The purpose of simultaneously stabilizing the frequency difference and the frequency is achieved, the reliability of the device is improved, and the laser enters the stage of practical application.

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

A frequency difference stabilization method for Zeeman-birefringence dual-frequency lasers with frequency difference stabilization, the method is characterized in that on the basis of obtaining a reliable and suitable frequency difference by applying a two-dimensional afterburning ring, and then using computer control to stabilize the frequency of the laser To stabilize the frequency difference, the steps of the method are as follows:

(1). A pair of mutually orthogonal diameter directions of the photoelastic element in the laser resonator applies force perpendicular to the beam diameter, that is, adopts a two-dimensional force application method to obtain a reliable and suitable frequency difference;

(2). The material of the force applying device should be selected so that it and the material of the photoelastic element should meet the relationship of matching and sealing, so that its linear expansion coefficient and the linear expansion coefficient of the photoelastic element material differ by 0% to 10%;

(3). A frequency difference stabilizing device controlled by a computer is installed on the above-mentioned laser. The device first detects the two-way light intensity of O light and E light separated by a Wollaston prism through a photodetector, and the photoelectric receiving module will Optical signals are converted into electrical signals;

(4). The above-mentioned electrical signal enters the computer through AD conversion for real-time acquisition, and outputs the control signal of the equal light intensity method or the extreme value locking method after the computer program operation;

(5). The above-mentioned control signal is converted to the piezoelectric amplification module through DA, and the frequency-stabilizing actuator is driven through voltage amplification, and finally the frequency-stabilizing actuator is used to complete the frequency-stabilizing servo control.

A Zeeman-birefringence dual-frequency laser with stable frequency difference for realizing the above method, comprising a helium-neon (He-Ne) laser gain tube, a photoelastic element in a laser resonator and a force applying device and a reflector on it The lens, the quartz package shell and the external transverse magnetic field formed by the magnetic strips arranged outside the quartz package shell and arranged up and down are characterized in that the force ring is a two-dimensional force ring with force screws; in order to further reduce To reduce the frequency difference drift of this kind of laser and improve the frequency stability, the above-mentioned laser also adds a frequency stabilization device, which is composed of a Wollaston prism, a photodetector, a photoelectric receiving module, a piezoelectric amplifier module, and a frequency stabilization actuator. and a computer equipped with AD/DA data acquisition card and control software; the output end of the photodetector is connected with the input end of the photoelectric receiving module, and the output end of the photoelectric receiving module is connected with the AD end of the data acquisition card, and the data The DA terminal of the acquisition card is connected with the input terminal of the piezoelectric amplifier module, and the output terminal of the piezoelectric amplifier module is connected with the frequency-stabilizing actuator.

The technical feature of the present invention is that the two-dimensional force ring is made of a valve whose linear expansion coefficient is consistent with that of the photoelastic element.

The above-mentioned two-dimensional force ring can adopt circular, square or other shaped force rings that can apply a pair of mutually orthogonal radial force to the photoelastic element.

When the laser resonator of the above laser is a half cavity, the frequency stabilizing actuator adopts piezoelectric ceramics; when the laser resonator is a full cavity, the frequency stabilizing actuator adopts a heating wire.

In the present invention, the original one-dimensional force application method is replaced by a two-dimensional force application method, and the material of the force ring is optimized, and at the same time, a laser frequency stabilization device is added, thereby further reducing the frequency difference of the laser. Drift, and improve the reliability and frequency stability of the device, making the laser enter the stage of practical application.

Description of drawings:

Fig. 1 is a schematic diagram of Zeeman-birefringence dual-frequency laser light intensity with cavity length tuning curve of the present invention

Fig. 2 is a schematic structural view of an embodiment of the present invention, that is, when the force ring is circular.

Fig. 3 is another embodiment of the present invention, that is, a schematic structural view of the force ring adopting a square shape.

Fig. 4 is a schematic cross-sectional view of the installation structure of the booster ring.

Fig. 5 is a schematic structural diagram of a Zeeman-birefringence dual-frequency laser with a frequency stabilization device.

Figure 6 is a circuit diagram of the photoelectric receiving module

Fig. 7 is a circuit diagram of the voltage amplifying module.

Figure 8 is a flow chart of the control software program.

Detailed ways

The principle, structure, specific implementation and preferred mode of the present invention will be described in detail below in conjunction with the accompanying drawings:

The force application method in the present invention is a two-dimensional force application method, that is, on a pair of mutually orthogonal diameter directions of the photoelastic element, the force is applied perpendicular to the beam diameter, and we call this force application device a two-dimensional force application ring. , circular (as shown in Figure 2) or square (as shown in Figure 3) and other shapes of force rings that can apply a pair of mutually orthogonal diametrical forces to the photoelastic element can be used. There are four screw holes at the two mutually perpendicular diameter positions of the circular booster ring and the center positions of the four sides of the square booster ring, and each screw hole is equipped with a flat top screw located in the ring. Component afterburner. The laser beam passes through the photoelastic element, and the frequency difference generated by the laser is proportional to the stress difference between the two perpendicular directions of force, which can be expressed as, $\mathrm{\Δv}\∝({\mathrm{\Δn}}_1-\mathrm{\Δ}{\mathrm{no}}_2)=\frac{1}{2}{\mathrm{no}}_0^3(p_{12}-p_{11})({\mathrm{the\; s}}_1-{\mathrm{the\; s}}_2)---\left(1\right)$ where Δv is the output frequency difference of the laser, Δn ₁ and Δn ₂ are the principal refractive indices of the two principal stress directions, n ₀ is the refractive index of the photoelastic element, p ₁₂ and p ₁₁ are the photoelastic coefficient tensors, s ₁ , s ₂ is the main strain tensor. This new type of two-dimensional force applying device obtains a small frequency difference by providing a small stress difference, which allows us to apply a large enough force when obtaining a small frequency difference to prevent the force ring from loosening and ensure reliability.

In order to overcome the drift of the frequency difference during the elongation and shortening of the resonator, the present invention uses a two-dimensional force ring to obtain a reliable and suitable frequency difference, and uses computer control to stabilize the frequency of the laser to stabilize the frequency difference. The invention also includes performing frequency stabilization on the Zeeman-birefringence dual-frequency laser to stabilize the frequency difference. The existing dual-frequency laser sources widely used in dual-frequency laser interferometers are mostly longitudinal Zeeman dual-frequency lasers, and most of them use the equal-intensity method to thermally stabilize the frequency. Since the left-handed light and right-handed light gain lines are symmetrical about the center frequency in the longitudinal Zeeman effect, the equal light intensity point is located at the center frequency and is unique, so the equal light intensity method is simple and easy to implement. Because the laser in the present invention is under the transverse Zeeman effect, the gain lines of the light (O light and E light) of two frequencies are as shown in Figure 1, respectively by the π component 1 and the σ component 2 in the transverse Zeeman effect Provides buffs. For such a gain curve, no matter the Zeeman-birefringence dual-frequency laser with full cavity or half cavity, the present invention has the problem of selecting the frequency stabilization point in its frequency stabilization method, and two methods can be adopted, namely Equal light intensity method and light intensity extreme value locking method, wherein the light intensity extreme value locking method is also divided into π light extreme value locking method and σ light depression locking method. Both of the frequency stabilization methods are methods of stabilizing the frequency difference by stabilizing the light intensity and thus stabilizing the frequency. The equal light intensity method is to lock the frequency at one of the two positions where the light intensity of the two modes is equal (3 points or 4 points); the π light extreme value locking method is to lock the frequency at the maximum value of the π gain line ( 5 points); the σ optical notch locking method is to lock the frequency at the minimum value of the σ gain line (6 points). The frequency of the constant light intensity method is not at the center frequency, and the frequency of the extreme locking method is at the center frequency.

Example 1:

Example 1:

The two-dimensional force ring is illustrated by taking a circular 7 (as shown in FIG. 2 ) or a square 10 (as shown in FIG. 3 ) as an example. The force ring 7 is fastened on the output window 13 of the full-cavity laser or on the anti-reflection window 9 of the half-cavity laser through four flat top screws 8 . Get the required frequency difference by adjusting the tightness of the top wire and fix the force ring on the window. Then use epoxy resin to stick the top wire and the window, the top wire and the force ring respectively, so as to ensure its reliability. The materials of the energizing ring and the top wire are valves whose linear expansion coefficient is consistent with that of the anti-reflection window. The relationship between the metal and the photoelastic element material k4 glass should meet the matching and sealing relationship, that is, the difference between the linear expansion coefficients of the two materials should be less than 10%, which can ensure the consistency of their expansion and contraction. The structure of the afterburning device after bonding is shown in Figure 4, in which 11 is a laser gain tube.

Example 2

Fig. 5 is a schematic structural diagram of a half-cavity Zeeman-birefringence dual-frequency laser system equipped with a frequency difference stabilization device. It mainly includes a laser gain tube 11, an anti-reflection window 9, a reflector 16, a two-dimensional force ring 7, a packaged quartz shell 12, and an external lateral direction formed by a magnetic strip 14 arranged outside the quartz packaged shell and arranged up and down. A semi-cavity Zeeman-birefringence dual-frequency laser and a frequency stabilization device composed of a magnetic field, the frequency stabilization device consists of a Wollaston prism 18, a photodetector 19, a photoelectric receiving module 21, an AD/DA data acquisition card 24, and a A computer 25 with control software, a piezoelectric amplifier module 22 and piezoelectric ceramics 17 are formed. In practical application, the photoelectric receiving module 21 and the piezoelectric amplifying module 22 are integrated and assembled in the control electrical box 23; the AD/DA data acquisition card 24 and the control software are installed in the computer 25. The photoelectric detector adopts silicon photocell, the data acquisition card is 12-bit precision (PC6333 from Zhongtai Company), and the voltage amplification range can reach 0-300V. When the frequency is stable, through the computer control, the photodetector detects the light intensity of the O light and the E light separated by the Wollaston prism and sends it to the PHOTOINPUT terminal of the photoelectric receiving module. The signal is amplified and filtered by the photoelectric receiving module. The OUTPUT terminal outputs to the data acquisition card, enters the computer for real-time acquisition through AD conversion, and outputs the control signal of the equal light intensity method or the extreme value locking method after computer program operation, and then outputs to the INPUT terminal of the piezoelectric amplifier module through DA conversion. The voltage amplification is output from the OUTPUT terminal and drives the piezoelectric ceramics, and finally the piezoelectric ceramics completes the frequency stabilization servo control. Thus, the frequency difference is stabilized by stabilizing the laser frequency. The frequency stability is better than 1×10 ^-7 for industrial applications, and the frequency difference stability can reach more than 1×10 ^-4 .

In the frequency stabilization system of this embodiment, the selection of frequency stabilization methods, the judgment of frequency stabilization points and the optimization of control signals are all realized by software control, which has strong versatility. The software judges the frequency stabilization point by the shape of the gain curve. At the beginning of frequency stabilization, the preset voltage drives the piezoelectric ceramic to scan the cavity length of the laser, collects and records two adjacent average values of the two light intensities in real time, and calculates the slopes of the two gain curves at this point. Through the logical judgment of the software, the equal light intensity method stabilizes the frequency at the point where the two light intensities are equal, and distinguishes points 3 and 4 through the slope. The extreme value locking method stabilizes the frequency at the extreme value of the light intensity and the slope is zero, that is 5 o'clock or 6 o'clock. The control software includes a digital PID program link to optimize the output control signal.

In addition, for the frequency stabilization of the full-cavity Zeeman-birefringence dual-frequency laser, the system can be used as long as the actuator is replaced, that is, the piezoelectric ceramic is replaced by a heating wire.

Claims (5)

1. the difference on the frequency antihunt means of the stable Zeeman-birefringence double-frequency laser of a difference on the frequency, the method is characterized in that by using two-dimensional force-adding ring and obtain on the basis of reliable suitable frequency difference, utilize computer control to come stabilized frequency poor by the stable laser frequency again, the step of its method is as follows:

(1). perpendicular to the reinforcing of light beam diameter, promptly adopt two-dimentional force mechanism on a pair of mutually orthogonal diametric(al) of the photoelastic components in laserresonator, it is poor to obtain reliable suitable frequency;

(2). the material of preferred force application apparatus, make the relation that should satisfy matched seal between itself and the photoelastic components material, make the coefficient of linear expansion of its coefficient of linear expansion and photoelastic components material differ 0～10%;

(3). be provided with one by computer-controlled frequency regulator on above-mentioned laser, this device is at first surveyed 0 light and the E light two-way light intensity of separating through wollaston prism by photodetector, accepts module by photoelectricity and changes light signal into the signal of telecommunication;

(4). above-said current signal enters computer by the AD conversion and gathers in real time, as calculated the control signal of output isocandela method or extreme value lock method behind the machine sequential operation;

(5). above-mentioned control signal is changed to the piezoelectricity amplification module by DA, driven the frequency stabilization executive component, finish the frequency stabilization SERVO CONTROL by the frequency stabilization executive component at last through voltage amplification.

2. adopt the stable Zeeman-birefringence double-frequency laser of a kind of difference on the frequency of method according to claim 1, comprise a He-Ne Lasers gain tube (11), and laserresonator in photoelastic components (9) and on force application apparatus, mirror lens (16), quartz packaged shell (12) and outside and magnetic stripe (14) that arrange up and down forms adds transverse magnetic field by being arranged on the quartz packaged shell, it is characterized in that: described augmentor is the two-dimensional force-adding ring that has afterburning screw; This laser has also increased a frequency regulator, this device is by wollaston prism (18), photodetector (19), photoelectricity receiver module (21), piezoelectricity amplification module (22), frequency stabilization executive component (17) and the computer (25) that AD/DA data collecting card (24) and PID Control Software be housed are formed, the output of described photodetector links to each other with the input of photoelectricity receiver module, the output of photoelectricity receiver module links to each other with the AD of data collecting card end, the DA end of data collecting card links to each other with the input of piezoelectricity amplification module, and the output of piezoelectricity amplification module links to each other with the frequency stabilization executive component.

3. according to the described two-frequency laser of claim 2, it is characterized in that: but described two-dimensional force-adding ring is to be made by the corresponding to valve of the coefficient of linear expansion of its coefficient of linear expansion and photoelastic components.

4. according to claim 2 or 3 described a kind of two-frequency lasers, it is characterized in that: described two-dimensional force-adding ring is circular, the square reinforcing ring that maybe can apply other shape of a pair of mutually orthogonal diametric(al) diameter reinforcing to photoelastic components.

5. according to the described a kind of two-frequency laser of claim 2, it is characterized in that: when laserresonator was half chamber, described frequency stabilization executive component adopted piezoelectric ceramic, and when laserresonator was full chamber, the frequency stabilization executive component adopted heater strip.

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