JPH04106989A - Method and apparatus for stabilizing two frequency gas laser - Google Patents

Abstract

PURPOSE:To stabilize an accurate frequency without respect to a laser output light intensity of insufficient control accuracy by stabilizing an oscillation frequency based on a control beam frequency generated by the beat of a longitudinal mode beat frequency and a reference frequency. CONSTITUTION:A control beat formed of a DBM 18 is input to an F/V converter 24 through an amplifier 16B, a comparator 20 and a frequency divider 22, and a control beam frequency is converted to a corresponding control voltage by the converter 24. The control voltage is compared with a reference voltage Vs, its differential voltage is input to a controller 26 of next heater, and the heater 12 of a laser 10 generates heat based on the differential voltage. A phase comparator (PC) 30 compares the reference frequency f1 from a crystal oscillator 28 with the phase of a regression frequency f2, and generates an erroneous voltage proportional to the frequency difference of the two signals. This becomes the control voltage of a VCO 34, which is supplied in a direction for reducing the frequency difference.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method and apparatus for stabilizing a dual-frequency gas laser, which is suitable for use in precision distance measurement and the like.

[Conventional technology]

A three-frequency laser that utilizes the beat between the longitudinal modes of a laser that oscillates in two modes can be used to measure distance, etc. For example, see 'Optronics', pages 116 to 122. V
ol, 7, No. 8.1988, 633n11
A three-frequency gas laser as a rangefinder that utilizes the intermode beat of a He-Ne laser is disclosed. In the case of a He-Ne laser, it oscillates within a gain profile with a width of approximately 1.5 GHz, and the frequency interval between modes, that is, the beat frequency of 7 days, is the length of the laser cavity and the speed of light is C. It can be expressed by equation (1). f day=C/(2・L)...(
1) When applying the beat frequency 7 days generated by the inter-mode beat to distance measurement, it is required to obtain a stable beat frequency 7 days in order to achieve highly accurate measurement. However, as is clear from the above equation (1), when the length of the resonator changes, the beat frequency also changes. Therefore, in order to stabilize the beat frequency 7 days, it is important to keep the resonator length constant. Therefore, in the above-mentioned rangefinder, which consists of an internal mirror laser that oscillates in two modes, the polarization directions of each mode are orthogonal to each other, and each mode is separated by a beam splitter, and the intensity of each mode is The resonator lengths are controlled so that they are equal, and the oscillation frequency is stabilized. in particular,
The laser tube is heated by a heater disposed around the laser tube, and the length of the resonator is controlled by adjusting the amount of heat generated at that time.

[Problem to be solved by the invention]

However, when controlling the beat frequency based on variations in the intensity of the laser output light, such as variations in the surface light intensity of the oscillation mode, the changes in the intensity and the changes in the beat frequency do not necessarily match, so the beat frequency There was a problem in that it was difficult to control the 7-day period in a stable state. The present invention was made to solve the above-mentioned conventional problems, and does not depend on the laser output light intensity with insufficient control accuracy.
An object of the present invention is to provide a method and device for stabilizing a three-frequency gas laser that enables highly accurate frequency stabilization.

[Means to achieve the task]

The present invention provides a method for stabilizing a three-frequency gas laser in which a laser tube is provided with an internal mirror for reflecting and resonating laser light, in which the beat between the longitudinal mode beat frequency oscillated from the laser tube and the reference frequency is determined. The above-mentioned problem has been achieved by generating a control beat using the control beat frequency and stabilizing the oscillation frequency based on the control beat frequency. The present invention provides a three-frequency gas laser device in which a laser tube is provided with an internal mirror for reflecting and resonating laser light, and includes photoelectric conversion means for converting light oscillated from the laser tube into an electrical signal, and outputting a reference frequency signal. a reference frequency generating means for
The above-mentioned problem is similarly achieved by providing a control beat generation means for creating and outputting a control beat signal from the beat frequency signal from the photoelectric conversion means and the reference frequency signal.

[Action and effect]

In the method of the present invention, the laser oscillation frequency is stabilized by taking a beat between the longitudinal mode beat frequency oscillated from the laser tube and a stable reference frequency and using it as the control beat frequency. In this way, by controlling the length of the laser tube based on the frequency, for example by thermal expansion, it is possible to control the length of the laser tube with much higher precision than when controlling the length of the laser tube based on fluctuations in the intensity of the laser output light. As a result, the oscillation frequency of the laser can be stabilized with high precision. Further, in the device of the present invention, the longitudinal mode beat frequency is converted into an electric signal by the photoelectric conversion means, and the beat frequency signal and the reference frequency signal from the reference frequency generation means are
By inputting the input to the control beat generation means, the beat generation means can take the beat between the longitudinal mode beat frequency and the reference frequency and easily generate the control beat frequency, so that the laser oscillation frequency can be reliably adjusted. can be stabilized with high precision.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an outline of the configuration of a three-frequency gas laser device that is an embodiment of the present invention, and FIG. 2 is for explaining the operation of a PLL that is a reference frequency generating means applied to this embodiment. FIG. The three-frequency gas laser device of this embodiment includes an internal bell-shaped three-frequency gas laser 10 disposed inside a reflecting mirror (not shown) for resonating laser light.
A film heater 12 is disposed around the laser tube made of Pyrex glass. Behind the laser 10 is an avalanche photo diode (optical conversion means, hereinafter referred to as APD) 14 that receives bimodally oscillated laser light and converts it into an electrical signal.
A double balanced mixer (control beat generating means, hereinafter referred to as DBM) 18 is connected to the APD 14 via two amplifiers 16A, and the D B
The amplified electrical signal can be input to M18. Further, the DBM 18 has a PLL (Phase Locked Lo) for outputting a reference frequency signal.
(op) 20 is connected, and in the DBM 18, it is possible to create a heat for control between the beat frequency of the longitudinal mode beat input to the above-mentioned via the PCl3 and the above-mentioned reference frequency. Note that the configuration and functions of the PLL 20 will be described later. The frequency of the control heat created by the DBM 18 is input to the FV converter 24 via the amplifier 16B, comparator 20 and frequency divider 22, and the control beat frequency is input to the FV converter 24. Converted to control voltage. The control JjJ voltage is compared with a predetermined reference voltage V,3, and the difference voltage is input to the control circuit 26 of the next heater, and the laser 10 is controlled based on this difference voltage.
The heater 12 is designed to generate heat. The PLL 20 includes a crystal oscillator 2 that oscillates a reference frequency.
For the phase comparator (PC) 30 connected to A programmable counter (shown in FIG. 2) 40 is connected in sequence, and the counter 40 is connected to the PC 30 to form a closed loop. The PLL 20 is a phase-locked loop used to oscillate a stable frequency that is a reference for controlling the laser 10. Its basic functions will be explained based on FIG. A phase comparator (PC) compares the phase and frequency of the reference frequency f1 and the regression frequency f2 from the crystal oscillator 28, and generates an error voltage proportional to the frequency difference between these two signals. This becomes the control gIJ voltage of the VCO 34 and is supplied in a direction to reduce the frequency difference. In this way, when fl becomes equal to f2, the PLL
20 becomes locked. -If the degree is locked, fl and
As long as there is no difference in f2, the state of fl=f2 continues,
Even if there is a difference between fl and f2 in this locked state, the PC 30 detects this difference and controls the VCO 34 so that fl = f2 to maintain the locked state. In this way, the PLL 20 oscillates at a stable frequency. It becomes possible to do this. To explain in detail the basic elements constituting the PLL 20, the loop filter 32 is composed of a low-pass filter (LPF), and usually only one type of low-pass filter or two types of low-pass filters are used. Use a continuous connection. If there is a difference error when the PLL 20 is in a steady state,
The phase comparator 30 always outputs an output signal as an elongated pulse at the period of the reference signal. Therefore, this pulse is applied to the loop filter 32 and integrated into a direct current, which controls the frequency and phase of the VC034. Further, the VCO 34 is an oscillator whose oscillation frequency changes linearly with respect to the input voltage (control voltage), and the output is generally expressed by the following equation (2). Vo (t) = v2Ao −C
05(ωot+Koft vd(t)d↑) ,,,
(2) Here, VoN) is the output of the VCO, vd(t) is the control f31II voltage, ω0 is the free-running oscillation frequency of the VCO, and K
o is the gain coefficient of the VCO (rad , z v ,'s
), F'AO is the output amplitude of the VCO.
This is a pre-frequency divider that pre-divides a high frequency that cannot be directly divided by the programmable counter 40 before the zero, and makes it possible to divide the frequency by the programmable counter. As a result, even if the reference frequency remains the same, the lock frequency increases by the frequency division number of the grease scaler. Therefore, if this is written as an equation, it becomes the following equation (3). fv = M-N-f2 = M-N-fl (3) where fv is the output frequency of the VCO 34, M is the frequency division ratio of the grease scaler, and N is the frequency division ratio of the programmable counter. Next, the operation of this embodiment will be explained. First, the PLL 20 is activated, and a steady state is established with the control I- so that the reference frequency f1 from the crystal oscillator 28 and the local frequency f2 match. Make the frequency fs output. On the other hand, a longitudinal mode beat laser beam is irradiated onto the APD 14 from behind the laser 10, and the laser beam is photoelectrically converted to generate an electric signal corresponding to the beat frequency fB, and the electric signal is sent to the amplifier 16A. amplified by the DB
Output to M18. And in the above DBM18,
The above beat frequency f input as an electric signal, respectively.
A beat is taken between the snake and the above reference frequency fs, and f8
A control beat frequency F day having a frequency of fs is oscillated. Control beat frequency FB output from the above DBM18
is input to the FV converter 24 via the amplifier 16B, the comparator 20, and the frequency divider 22, where it is converted into the control voltage Vc corresponding to the beat frequency F8. This control voltage Vc is compared with a preset reference voltage Vs, and the difference voltage (Vc Vs) is applied to the heater force control circuit 2.
6 is output. Then, the amount of heat generated by the heater 12 of the laser 10 is adjusted based on the voltage difference, and the amount of thermal expansion of the laser tube is adjusted, thereby controlling the length of the laser tube, that is, the resonator length. At that time, for example, by controlling the length of the laser tube so that the voltage difference becomes zero, it becomes possible to lock the beat frequency to a certain one day and cause the laser 10 to oscillate at that frequency. If the beat frequency, which is the difference between the absolute frequencies of the two fir modes, is determined in this way, the absolute frequency can be determined conversely. Furthermore, since the frequency is directly detected, it has the advantage of high reproducibility. In addition, since the beat frequency 1 day corresponds to the absolute frequency, it also has wavelength selectivity.Furthermore, by increasing the stability of the reference frequency fS, the beat frequency 1 day can also be stabilized, making it less susceptible to the effects of gain profile changes. do not have. When controlling the temperature of the laser tube as described above, keeping the length of the laser tube constant, and locking the beat frequency 1B, 632
Bimodal oscillation in nn wavelength band (cavity length 15-30a)
The heating frequency of the He-Ne laser changes periodically with an interval of λ7/2 (λ: wavelength) depending on the length of the laser tube (for example, "Optronics JP116~
P122, VOl, 7, No. 8.1988). Therefore, the beat frequency of any Saga on the beat frequency curve can be locked at one day. In addition, in this case, if it is desired to lock at the center of the concavity of the curve, the lock can be achieved accurately by utilizing the fact that the first-order differential of the curve is zero. Furthermore, by selecting the lock position, it is also possible to select the polarization direction, beat frequency, or absolute frequency of the output light of the laser. Next, the results of evaluating the three-frequency gas laser device of this example using the system shown in FIG. 3 will be described. The above system is the same as shown in FIGS. 1 and 2 above, except that a pen recorder 40 is connected to the FV converter 24 and the laser beam output from the front of the laser 10 is measured as described below. It is the same as a three-frequency gas laser device. For measurements performed in front of the laser, the light emitted from the front of the laser 10 is passed through a beam splitter 42 to another stabilizing laser 44.
After superimposing the light from the
Divide the frequency by /64 and measure the frequency. At that time, the frequency is GP-IB (GeneraPurpose
(Interface Bus) Built-in counter 5
o, and at the same time create a data file on the personal computer 52. In addition, the following were used as specific components of the above system. As a laser, a bimodal internal mirror type He-Ne laser UNL-21OR manufactured by Radio Corporation was used. The specifications are as follows. Beam diameter (1, / e2m > 0.6 Beam divergence angle (mrad full width) 13 Longitudinal modulus spacing (Cy'2 LMth) 676 (1
日) Torikami pressure (kV) 10 Tube voltage (V) 1250 (=596) Outer diameter dimensions Total length (u), -230 Tube diameter (In)
30 In addition, a film heater (360Ω) manufactured by MINCO was adhered to the above-mentioned tube with double-sided tape. In addition, the PLL uses a one-chip IC (MC14 manufactured by Motorola) that has a phase comparator and a programmable counter.
5152-1) was used. This IC divides the reference signal input by 64 or 128, and the frequency from vCO is set from 3 to 1023 by a programmable counter.
It is possible to divide the frequency up to As a crystal oscillator, we used a small and highly stable Toshiba TS511B29 that can output a 1Mf (z reference signal).
Do not use. A lag lead filter shown in the typical circuit of FIG. 4 was used as the loop filter (LPF). This contributes to expanding the frequency pulling range by compensating for phase lag and increasing the gain in direct current. This circuit outputs a stable frequency, and it is also possible to change the output frequency by adjusting the gain. In addition, as vCO, Cl-10I manufactured by Niss EC Co., Ltd.
Using V(W>), PD-1 manufactured by R& was used as a divider to distribute one of the output frequencies from the vCO to the prescaler and the other to the DBM. , M5447 manufactured by Mitsubishi Electric
5 Don't use P. This is a frequency divider with an ECL circuit configuration, and the maximum IGl (2
The frequency at t can be divided. The PLL composed of the above elements outputs a reference frequency fs of 640M, and the beat frequency input from the laser 10 in D B M2S is 7 days < 676.
MH2>, and control beat frequency F
B was output, and the following temperature control was performed. Frequency stabilization by the film heater 12 uses a differential amplifier, inputs the voltage Vc of the beat frequency after FV conversion and the reference voltage vs to the other, and applies current to the heater so as to eliminate this voltage difference. This was done by controlling the length of the tube. For temperature control, a temperature control device equipped with a non-contact relay using a photocoupler was used. In addition, the counter used to measure the frequency of the laser beam output from the front of the laser 10 has a built-in GP-IB 11.
- A universal counter with a measuring range of ~120ME (z), with a counting time of 4 between 1 (ls and 10 St).
We selected from among the types and used those that could be measured. This counter can also be operated from a computer via GP-I B, and the results can easily be stored in a computer data file. In addition, the computer mentioned above is PC-9 manufactured by NEC.
801UVl::GP, -IB board (PC9801-
29N) was expanded and used. The measurement is performed using a frequency counter 50 with a built-in GP-I B interface, and the data is analyzed by a computer. Using that data, calculate the Allan variance and evaluate the stability. This makes it possible to intuitively understand stability from short-term to long-term. Here, the stability is the ratio of the original amount to the changed amount, and for example, if a 1 m length changes by 1 μf, the stability is 10 −6 . The above Allan dispersion will be explained using FIG. Assume that N values y(t) that vary over time are continuously measured every 1S time. At this time, if the average value of the measured values of y (↑) for only time τ from the second is y (k), then the next average value is expressed as y (k + τ/ls), and in this case, the Allan variance σ2 ( 2, τ) is defined by the following equation (4). σ2 (2, τ) = [(y(k+τ/j3) y(k)12/2]・
・・・・・・・・・(4) Here, [] represents an infinite time average. Next, the results of laser frequency stabilization using the above system will be explained. Since stabilizing the oscillation frequency also leads to stabilizing the laser intensity, measurements were performed on both frequency and intensity. The laser intensity is measured using a 1-v converter < APD as described above.
) to receive the light from the front of the laser. First, FIG. 6 shows the results of measuring the heat frequency f without any control. From this figure, we can see that the frequency fluctuates periodically, and we can also see that the speed of the fluctuation is gradually slowing down. When the results shown in FIG. 6 are analyzed and the relationships between the elongation of the laser tube and temperature and time are graphed, the results are shown in FIGS. 7 and 8, respectively. Next, the temperature inside the box (not shown) containing the laser 10 is controlled to be constant, and at the same time, a voltage of 12V is applied to the film heater 12, and when the tube is sufficiently extended, control for stabilization is performed. I put it on. The results are shown in FIGS. 9(A) and (B). Note that FIG. 9(B) shows the measurement results after FIG. 9(A), and the control for stabilization was started when the beat frequency fluctuation started and was locked in that state. Frequency stabilization is controlled by heating the laser tube with a heater, so it is only possible to stretch the original tube from a contracted state. There is a need. That is, the laser must be in a state where it contracts when the heater is turned off. FIG. 10 is a diagram showing enlarged variations in frequency and output intensity in the control state. Moreover, FIG. 11 and FIG. 12 are diagrams showing measured values and Allan dispersion, respectively, when the beat frequency of the laser is measured by a frequency counter. In both figures (A)
is the measured value, and this is just the value measured by the counter, so the actual beat frequency is 64@ on the vertical axis. (B) is the calculation result of Allan variance,
The vertical axis represents relative stability, and the horizontal axis represents calculation interval in time. Note that Figure 11 shows the results when the laser is elongated, and the measured values are only shown up to 60 seconds, but this is because the measurement data is expanded to see the movement of the measured data, and the actual measurement is 20 seconds. I went for a minute. The above measurement results show the following. From Figure 6, the fluctuation range of the beat frequency without control is 4.93 MH2 (2,91 to 7.84 MHz), and in Figure 9, the fluctuation range before stabilization is 416 ME (z (
It was 0.80-4.96M [(2). However, as shown in Fig. 9, after stabilization (start of control), the above fluctuation range becomes 0.05M) [2 (3.61 to 3.56M soil), and even in Fig. 10, which is shown enlarged, the fluctuation range is 0.05M) [2 (3.61 to 3.56M). 04M7 (
1.84 to 1.88MH2), where,
The beat frequency is the output voltage from the FV converter from 5V to 8M.
It is converted as an output. In addition, the fluctuation range of the laser light intensity is 0 in Figure 6 before stabilization.
．． 19V (2,89~3.08V), but after stabilization, it is below O, OIV<2.95~2.95V) in Figure 9, and 0.025V (5,174~5V) in Figure 10. .199V
). Note that the laser output has no absolute unit, so the measured values only represent relative fluctuations. In addition, the frequency control voltage is 2.25V (3.6M
The center voltage for output stabilization was 2.95 V in FIG. 9 and 5.184 ■ in FIG. 10. The relative stability of the output after stabilization was calculated from the following equation (5) and was found to be: 024%. Since the number of times when it is not stabilized is only a few percent, it means that the stability has improved by one stroke. Stability = ((main power peak-to-peak value) / 2) ÷ (center voltage for output stabilization) ... (wo) From Figure 12, even under control, it fluctuates slowly in waves. However, this is probably because the measurements were taken in a room with temperature fluctuations. In other words, the temperature of the laser tube itself is controlled, but since the mirror is exposed to the outside, the angle of the mirror may have moved slightly due to the influence of external temperature changes, or it may have warped. Conceivable. Further, the stability of the laser was determined from FIG. 9(B). Specifically, as shown in FIG. 13, which is an enlarged view of the - part of FIG. 9(B) at the time of control start, a triangle is created, X is determined, and the value is substituted into the following equation (6). λ is the wavelength (633 nl), y is the fluctuation width of the heating frequency in the controlled state, and ΔL is the extension of the laser tube. ×/〈λ/2)=(V/2)/ΔL... (6
)Furthermore, the frequency stability is determined by the following equation (7). ΔL/L=Δf/f ・・・・・・・・・
(7) Calculations using the values obtained in FIG. 13 above yield the following results. ΔL=0.387 (nn) L=22.2 (CIll),°, ΔL/L=1.74×10−9 Therefore, the relative stability is 1°74×10−9. Let's compare this with the stability due to Allan dispersion. From the Allan variance shown in FIG. 12, it becomes as follows. Stability seen in the short term 2゜lX10-" Stability seen in the long term 6゜0 Compared to the short-term stability, this is an order of magnitude different. Even in the long term, it is 1/3 of what was calculated in the diagram. This is because both the laser intensity measurement and the beat frequency measurement use forward light and cannot be measured at the same time, so these two data do not record the same state, and the stable state may be better depending on the difference in the position where the control is applied. Because there are good and bad cases, the difference appears in our opinion. Also, the vertical axis of these Allan dispersions is not the calculation result itself, but the calculation result is the center frequency of the He-Ne laser 4,
Relative stability is calculated by dividing by 74 x 10-1+. As detailed above, according to this example, the laser frequency fluctuates by 10-6 in an uncontrolled state, but it was found that stability of i o -10 was obtained from the results of Allan dispersion. . This shows that high stability can be achieved by control using longitudinal mode beats. Although the present invention has been specifically described above, it goes without saying that the present invention is not limited to what has been shown in the above embodiments, and can be modified in various ways without departing from the gist thereof. For example, in the above embodiment, the laser was stabilized based on thermal expansion due to heating of the laser tube, but the invention is not limited to this.
For example, as in the case of Japanese Patent Application No. 61-179743, the oscillation frequency may be controlled by applying a bending load to the end of the laser provided with an internal mirror for resonance. Furthermore, the PLL is not limited to the configuration shown in the embodiments described above, and for example, the loop filter can use other low-pass filters in addition to the lag-lead filter.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically showing a three-frequency gas laser device that is an embodiment of the present invention, and FIG. 2 is a PL applied to the three-frequency gas laser device.
3 is a block diagram showing the system applied to the performance evaluation of a three-frequency gas laser device; FIG. 4 is a circuit diagram of a loop filter that constitutes the PLL; The figure shows the line section used to explain Allan dispersion.
A diagram showing the measurement results when the laser is not stabilized. Figure 7 is a diagram showing the relationship between the elongation of the laser tube and temperature. Figure 8 is a diagram showing the relationship between the elongation of the laser tube and time. Figure 9 is a diagram showing the measurement results when the laser is stabilized midway, Figure 10 is a diagram showing an enlarged measurement result of the laser in a stabilized state, Figure 11 is a graph showing the measured value of the beat frequency and its Raman dispersion when the laser is not stabilized, Figure 12 is a graph corresponding to Figure 11 when the laser is stabilized, Figure 13 1 is a diagram for explaining a method of calculating laser stability; FIG. O... Laser, 2... Heater, 4... Avalanche photo diode, 8...
Double balanced mixer, 0...PLL, 4...FV converter, 0...phase comparator.

Claims (4)

[Claims] (1) In a method for stabilizing a dual-frequency gas laser in which the laser tube is equipped with an internal mirror for reflecting and resonating laser light, the beat between the longitudinal mode beat frequency oscillated from the laser tube and the reference frequency is determined. A method for stabilizing a dual-frequency gas laser, comprising generating a control beat and stabilizing an oscillation frequency based on the control beat frequency. (2) A method for stabilizing a dual-frequency gas laser according to claim 1, characterized in that the oscillation frequency is stabilized by controlling the length of the laser tube based on the control beat frequency. (3) In a dual-frequency gas laser device in which the laser tube is equipped with an internal mirror for reflecting and resonating laser light, a photoelectric conversion means converts the light oscillated from the laser tube into an electrical signal, and outputs a reference frequency signal. A dual-frequency gas laser comprising: a reference frequency generating means; and a control beat generating means for creating and outputting a control beat signal from the beat frequency signal from the photoelectric conversion means and the reference frequency signal. Device. (4) In the above-mentioned claim 3, further comprising: FV conversion means for converting the control beat frequency from the control beat generation means into voltage; and heating means disposed around the laser tube, the FV
A dual-frequency gas laser device, characterized in that a control voltage is applied from the conversion means to the heating means, and the length of the laser tube is controlled by thermal expansion to stabilize the oscillation frequency.