JPH0722681A - Multimode modulation laser system - Google Patents

Abstract

(57) [Summary] (Modified) [Objective] To provide a multimode modulation laser system in which the 0th-order light and the 1st-order diffracted light are coaxially combined and the polarization planes of both are aligned. [Structure] In this multimode modulation laser system, laser light oscillated from a single mode laser device 1 is incident on an AOM 3 of an acoustooptic system 5 through a polarization beam splitter 2. The 0th-order light and the 1st-order diffracted light are emitted from the AOM 3. The emitted 0th-order light and 1st-order diffracted light are reflected by the optical mirrors 8 and 9 of the reflection optical system 10, and enter the AOM 3 again through the λ / 4 wavelength plates 6 and 7. This allows
The 0th-order light and the 1st-order diffracted light are coaxially combined, and their polarization planes are aligned. The 0th-order light and the 1st-order diffracted light are refracted at right angles by the polarization beam splitter 2 and extracted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multimode modulation laser system, and more particularly to a multimode modulation laser system capable of aligning the optical axis and polarization plane of laser light of each longitudinal mode.

[0002]

2. Description of the Related Art Generally, a laser beam for separation having a specific wavelength, which is absorbed only by a specific atom or molecule, is oscillated, and the atom or molecule is selectively excited by this laser beam to be ionized.
Separation and recovery by an electric field or a chemical reaction is widely performed.

By the way, a laser device used for separating atoms and molecules includes a laser oscillator device that oscillates a tunable laser beam whose wavelength can be selectively tuned, and a laser system that excites and amplifies the oscillated laser beam. And are used to select specific atoms or molecules for multi-stage excitation. Further, some atoms have a hyperfine structure, and in order to efficiently excite atoms having a hyperfine structure, laser light having several frequencies suitable for the transition structure level is output. A laser device or a plurality of single mode laser devices is required.

Therefore, conventionally, as a method of forming two or more frequencies coaxially using these laser devices, a method of assembling a laser resonator for multimode oscillation or combining single mode laser light with a dichroic mirror or the like is used. The method of doing is proposed.

Conventionally, distance measurement using a laser beam has been widely performed. As a typical method thereof, the phase difference of the intensity-modulated wave between the return light from the target and the emitted light is measured. Apply several types of interference fringes to the measured object (target) that have slightly different scale distances from the measured distance
Interference measurement using laser light interference, such as a matching method that measures the distance from the scale line to measure the distance, or a synthetic wavelength method that measures the distance from the interference intensity signal of the light by simultaneously inserting two wavelengths of laser light into the interferometer. Distance is known. In the case of using light modulation, since modulated light must be formed coaxially, in many cases, an electro-optic modulator (EO) is used.
M: Electro Optic Modulator) is used.

[0006]

In order to selectively and efficiently excite a specific atom or molecule using a laser device, a laser beam whose wavelength is adjusted with high precision so as to correspond to each transition structure is output. However, it is almost impossible to control the intensity of the laser light of each frequency formed coaxially and the frequency interval of each with the laser light of multi-mode oscillation. On the other hand, when using a plurality of single-mode laser devices, it is possible to control each frequency and intensity before coaxial combining, but when the wavelengths of each single-mode laser light are close, dichroic mirrors are used. , Laser light 10
Coaxial synthesis near 0% is not possible without the use of polarized light. That is, if the reflectance of the wavelength (frequency) of one of the laser beams is increased, the laser beam of the other wavelength (frequency) that has been transmitted is reflected because the wavelengths are close to each other. If one laser beam is vertically polarized and the other laser beam is horizontally polarized by using the polarization of the laser beam, nearly 100% coaxial synthesis is possible, but the polarization plane of the synthesized laser beam is There is a problem that they are not available.

Further, regarding the distance measuring technique, in the case of measurement using light in the atmosphere, a major correction factor is a change in the refractive index of air. Therefore, there is a problem that the weather conditions such as temperature and humidity must be accurately measured and corrected by an empirical formula or the like. When considering distance measurement by an interferometry method using a polychromatic laser beam capable of coping with such an environmental change, wavelength stability of each of the laser beams of a plurality of wavelengths (polychromatic) becomes a problem.

The present invention has been made in consideration of the above-mentioned circumstances, and it is possible to arbitrarily control the longitudinal mode interval of laser light and the intensity of each mode and to form a laser beam having a uniform plane of polarization. It is an object to provide a multimode modulation laser system.

[0009]

In order to solve the above-mentioned problems, a multi-mode modulation laser system according to the present invention is installed on the optical path of a single mode laser beam, and the longitudinal mode interval of the laser beam and the mode of each mode. An acousto-optic system whose intensity is adjustable, a reflection optical system including a wave plate and an optical mirror, and reflecting a laser beam of each mode emitted from the acousto-optic system to re-enter the acousto-optic system. It is a thing.

[0010]

In the multi-mode modulation laser system according to the present invention, an acousto-optic modulator (AOM: as an acousto-optic system).
When a single mode laser beam is incident on the Acoust Optic Modulator), the intensity and direction of the diffracted light of the laser beam
It changes depending on the strength and frequency of the ultrasonic waves. Therefore, the longitudinal mode interval of laser light and the intensity of each mode are
It can be controlled arbitrarily.

By the way, when the longitudinal mode interval of the laser light and the intensity of each mode are controlled, the polarization planes of each mode are not coaxially aligned, and therefore the reflection optical system returns the light to the AOM of the acousto-optical system again. As a result, a laser beam having a uniform plane of polarization is formed coaxially.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 shows a basic two-mode laser system as an example of a multimode modulation laser system according to the present invention. In the figure, reference numeral 1 indicates a stable wavelength (mode) and control. It is a laser device that oscillates possible single-mode laser light, and an acousto-optic modulator (AOM) driven by the polarization beam splitter 2 and the driver 4 is provided on the optical path of the laser light oscillated from the single-mode laser device 1. ) 3 are installed respectively. This A
The OM 3 constitutes the acousto-optic system 5. The AOM 3 generates ultrasonic waves in a medium such as a single crystal or glass to cause a periodical refractive index change, and thereby constitutes a phase type diffraction grating, and a laser beam is incident on the AOM 3. The intensity and direction of the diffracted light are changed according to the intensity and frequency of the ultrasonic wave.

As shown in FIG. 1, a reflecting optical system 10 composed of λ / 4 wave plates 6 and 7 and optical mirrors 8 and 9 is installed on the downstream side of the AOM 3, and the reflecting optical system 10 is provided. Reference numeral 10 is adapted to return the 0th-order light (the frequency of the incident laser light) and the 1st-order diffracted light (the light shifted by the frequency of the ultrasonic wave) to the AOM 3 which is the acoustooptic system 5 again. Then, the reflected 0th-order light and 1st-order diffracted laser light are bent at a right angle to the incident laser light from the single-mode laser device 1 by the polarization beam splitter 2, and the two-mode laser light is coaxial. In addition, it is now possible to obtain it with a uniform plane of polarization.

Next, the operation of this embodiment will be described.

When a laser beam is incident on the AOM 3 of the acoustooptic system 5, the intensity and direction of the diffracted laser beam change depending on the intensity and frequency of the ultrasonic wave. Due to this operation principle, the frequency of the incident laser light is shifted by an amount corresponding to the frequency of the ultrasonic wave. The frequency shift amount can be arbitrarily adjusted, and the intensity of each of the incident laser lights can be changed by adjusting the output of the ultrasonic wave to change the ratio between the 0th-order light and the 1st-order diffracted light.

That is, the incident laser light entering the AOM 3 undergoes Bragg diffraction due to the incident wavelength λ and the frequency δ of the ultrasonic wave, and the first-order diffracted light is generated at an angle 2θ with respect to the laser light having the incident angle θ. Θ at this time is

[Equation 1] Indicated by. In this state where the light passes through the AOM 3 once, two modes can be created at an angle of 2θ between the 0th-order light and the 1st-order diffracted light, but they are not coaxial and are coaxial. Need to be synthesized.

Conventionally, since this combination is performed by using a deflected light beam splitter and the deflected light of a laser, there is a problem that the planes of polarization of the combined beams are not aligned. However, in order to solve this problem. In this embodiment, a method of returning the modulated first-order diffracted light to the AOM 3 again by using the optical mirror 9 which is the reflection optical system 10. At this time, A
A λ / 4 wave plate 7 is inserted between the OM 3 and the optical mirror 9 to change the polarization of the light entering the AOM 3 again from vertical polarization to horizontal polarization or from horizontal polarization to vertical polarization. Similarly, for the 0th-order light, the optical mirror 8 returns the light to the AOM 3 and the λ / 4 wavelength plate 6 changes the polarization direction.

When the first-order diffracted light is incident on the AOM 3 of the acousto-optic system 5 again, the modulation frequency can be changed twice. For example, 1 GHz modulation with one AOM3
If it can be performed with, it becomes possible to modulate at 2 GHz by passing it through twice. At this time, the Bragg angle θ is so small that the modulation frequency is negligible with respect to the wavelength λ (several tens MHz to several GH).
z), the angle θ upon re-incident is almost unchanged.

The 0th-order light and the 1st-order diffracted light that have entered the AOM 3 again are bent at a right angle to the incident laser light by the polarization beam splitter 2 installed in front of the AOM 3, and the 0th-order light of the wavelength λ and the 1 of the wavelength λ + 2δ. Two modes of the second-order diffracted light are obtained on the same axis and in which the planes of polarization are aligned.

The optical loss here is the amount of transmission of the AOM 3, and usually 95% or more is ensured by one transmission, so that the loss is 10% or less even if it is passed twice.
Therefore, an efficient method is to create the two modes, and the intensity ratio of the two modes can be freely adjusted by the driver 4 of the AOM 3.

FIG. 2 shows a case where a plurality of the systems shown in FIG. 1 are combined to form a multimode system.

That is, in the system shown in FIG. 2, a polarization beam splitter 2, an AOM 3 and a driver 4 which are acousto-optic systems 5, and a λ / 4 wave plate 6 which is a reflection optical system 10.
7 and optical mirrors 8 and 9, as well as polarization beam splitters 2a and 2b, AOMs 3a and 3b, drivers 4a and 4
b, λ / 4 wave plates 5a5b, 6a, 6b and optical mirrors 7a, 7b, 8a, 8b are additionally provided, and the multimode laser light extracted by the polarization beam splitter 2b is emitted from the excitation laser device 11. The laser light is incident on the laser amplifier 12 together with the laser light and the laser output can be increased.

FIG. 3 shows a multi-longitudinal mode modulation laser system capable of switching two modes at high speed in the laser system shown in FIG. 1, and shows the laser beam at the center of the AOM 3 of the acoustooptic system 5. A convex lens 13 is installed between the polarization beam splitter 2 and the AOM 3 so as to have a beam weight, and concave optical mirrors 14 and 15 are used instead of the optical mirrors 8 and 9 to form a reflection optical system 16.
Are configured. The acousto-optic system 5 and the reflection optics system 16 can switch each mode at high speed like a flip-flop circuit to increase the rising and falling speeds.

Excitation of sodium atom (Na) having a hyperfine structure at the D1 level using the basic two-longitudinal mode modulation laser system shown in FIGS. 1 and 3 will be described with reference to FIG. .

FIG. 4 shows the excitation using a certain polarized light laser beam in Na-D1, in which the symbol mF represents a magnetic quantum number and the symbols F and F'represent energy levels. There is.

When the wavelength excited from F = 1 to F ′ = 1 is λ, the level difference between F = 1 and F = 2 is 1772 MHz,
Since the level difference between F ′ = 1 and F ′ = 2 is 192 MHz, the excitation wavelength from each energy level is

[Equation 2] And the maximum difference is 1772MHz + 192MHz =
It becomes 1964 MHz = 1.964 GHz.

That is, by using the two-mode modulation laser system of the present invention, the mode interval is about 2 GHz,
The frequency of the ultrasonic wave of the AOM 3 of the acousto-optic system 5 is about 1 GHz.
Therefore, the Na atom can be efficiently excited.

It should be noted that the details of each excitation wavelength are about 2
Although there is a difference of about 00 MHz, there is a Doppler effect, power broadening, etc. between the vapor of Na and the laser beam, and the difference of about this degree is effectively influenced without being affected. You can

The same multi-longitudinal mode laser system is used to selectively excite a specific isotope atom having a hyperfine structure, for example, U-235 isotope, also in uranium (U) atom. can do. Also, FIG.
In the case of the longitudinal mode system shown in Fig. 2, the wavelength of the laser light is such that a specific uranium isotope is excited at a wavelength λ for a certain time and is excited by shifting the wavelength of the laser light by λ + δ at the next time. An exciting method of jumping is also possible. In this case, there is a feature that the peak intensity of one mode (wavelength) can be increased, and this is an advantageous method when specific uranium isotope excitation is sufficiently performed in a short time.

Next, a basic multimode modulation laser system applied to precision distance measurement will be described with reference to FIGS. These systems are shown in FIGS.
The two-mode modulation laser system shown in Fig. 2 was developed for distance measurement. The basic principle of distance measurement is that the Michelson interferometer is used to perform interferometric measurement in two modes, that is, two wavelengths. It is designed to correct the fluctuation and the refractive index of.

That is, the multi-longitudinal mode laser system shown in FIG. 5 basically uses the laser system shown in FIG. 1, and between the single mode laser device 1 and the polarization beam splitter 2 there is a rotary type. A λ / 2 wavelength plate 21 is installed, and in the polarization beam splitter 2, a part of the horizontal polarization component is reflected at a right angle and is incident on an optical mirror 22 forming a reflection optical system 21. The rate of incidence on the optical mirror 22 can be adjusted by the angle of the λ / 2 wave plate 20.

Between the optical mirror 22 and the polarization beam splitter 2, as shown in FIG. 5, is a λ / 4 wave plate 23.
The λ / 4 wavelength plate 23 polarizes the return light from the optical mirror 22 from horizontal to vertical,
It is adapted to pass through the polarization beam splitter 2.
Then, this transmitted light, together with the return laser light from the AOM 3 of the acousto-optic system 5, the photodetector 2 to which the phase meter 25 is connected.
4 and the photodetector 24 measures the interference.

Since the laser light from the optical mirror 22 is vertically polarized, the return laser light from the AOM 3 is horizontally polarized, and both laser lights are linearly polarized and their polarization planes are orthogonal to each other. , These light direct photo detector 2
Even when the light beam is incident on No. 4, a beat signal (a signal having a strong or weak interference) is not detected.

Therefore, in the multiple longitudinal mode laser system shown in FIG. 5, the deflected light beam splitter 2 and the photodetector 24 are used.
A λ / 4 wavelength plate 26 is installed between the two, and both polarizations pass through the λ / 4 wavelength plate 26 with an axis having a polarization plane inclined by 45 degrees. Then, the same amount of the same component is extracted from both polarizations, and the beat signal can be detected by the photodetector 24.

On the other hand, the distance measuring portion is set on the downstream side of the AOM 3, as shown in FIG. That is, A
On the downstream side of OM3, instead of the optical mirrors 8 and 9 of the reflective optical system 10 in the system shown in FIG.
A reflecting mirror 27 for setting an optical axis parallel to the next light and a reflector 28 as an object to be measured are installed as a reflection optical system 29. Each of the 0th-order light and the 1st-order light reflected by the reflector 28 is installed. The return light is re-incident on the AOM 3 to be coaxially combined, further reflected by the polarization beam splitter 2, and incident on the photodetector 24. In the multi-longitudinal mode laser system shown in FIG. 5, the position of the reflector 28 is adjusted to facilitate the alignment of the point where the reflected light from the reflective optical system 29 re-enters the AOM 3.

The multi-longitudinal mode laser system shown in FIG. 6 basically uses the laser system shown in FIG. 3, and between the single mode laser device 1 and the polarization beam splitter 2 there is a rotary type. A λ / 2 wavelength plate 20 is installed, and in the polarization beam splitter 2, a part of the horizontal polarization component is reflected at a right angle and is incident on the photodetector 24.

On the other hand, the distance measuring portion is set on the upstream side of the AOM 3 which is the acousto-optic system 5, as shown in FIG. That is, the corner cube prism 38 as the object to be measured is installed on the downstream side of the λ / 4 wavelength plate 26 so that the return laser light after being combined into two wavelengths by the AOM 3 is used for distance measurement. Has become.

FIG. 7 shows a system in which a plurality of AOM basic systems of FIG. 6 are provided and a large number of wavelengths (modes) can be used. The AOM basic system 40 is composed of, for example, the acousto-optic system 5 and the catoptric system 16.

That is, in this multi-longitudinal mode laser system, as shown in FIG. 7, three AOM basic systems 40 are used for one single mode laser device 1, and light from each of these AOM basic systems 40 is used. In addition to the polarization beam splitter 2, polarization beam splitters 42a and 42b are additionally provided for coaxially synthesizing.

In this multi-longitudinal mode laser system, however, the single mode laser device 1 is one, and the stability of the mode interval depends on the frequency of the ultrasonic wave. Therefore, the frequency stabilization of the single mode laser device 1 itself is performed. The feature is that it does not affect so much, and by using multiple wavelengths, not only is it easy to deal with air fluctuations and changes in the refractive index, but each mode is switched by the method described in the system shown in FIG. Then, pulse distance measurement becomes possible.

That is, the distance L to the object to be measured is τ, which is the time required for the pulsed laser light to travel back and forth to the object to be measured.
Then,

## EQU00003 ## L = C.multidot..tau.2 where C is the speed of light.

As described above, the system of FIG. 7 can check not only one distance measuring method but also a plurality of distance measuring techniques in time, so that the obtained information amount (measurement amount) can be confirmed by different methods. In addition, it can be corrected, and more precise distance measurement becomes possible.

[0044]

As described above, according to the present invention,
It is possible to arbitrarily control the interval between the longitudinal modes of laser light and the intensity of each mode, and it is possible to form a laser beam having a uniform plane of polarization. Therefore, when applied to a laser system for isotope separation, the laser light is set so as to correspond to the optimum resonance absorption line for atoms and molecules due to the frequency modulation action of the acousto-optic system. It becomes possible to effectively excite in the transition structure. Therefore, unlike a conventional laser system in which the ratio of laser light that does not match each resonance absorption line and does not contribute to pumping is large, in the multimode modulation laser system according to the present invention, the conversion efficiency of pumping laser light with respect to laser output is high. It is extremely high and the energy loss can be greatly reduced. In addition, by appropriately changing the distribution ratio of each laser light mode, it is possible to adjust the intensity to an optimum level corresponding to each transition structure. Therefore, it is easy to optimize the utilization efficiency with respect to the total output of the laser light, and it is possible to greatly improve the operational management and economical efficiency of the laser device.

Further, when it is applied to a distance measuring technique, it is a big problem that the fluctuation of the atmosphere, that is, a change in the refractive index of air, is affected by a two-wavelength or multi-wavelength type interference technique, a phase detection method by laser frequency modulation and a pulse By using distance measurement comprehensively, the accuracy of refractive index correction can be improved and the resolution can be increased.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a basic example of a multi-mode modulation laser system according to the present invention.

FIG. 2 is a configuration diagram showing an example of a case where a plurality of multimode modulation laser systems of FIG. 1 are combined to form a multimode modulation laser system.

3 is a configuration diagram showing a multimode modulation laser system capable of switching two modes at high speed in the laser system of FIG.

FIG. 4 is an explanatory diagram showing an example of excitation in a D1 level hyperfine structure of a sodium atom.

5 is a configuration diagram showing a multi-mode modulation laser system in which the laser system of FIG. 1 is adapted for distance measurement.

6 is a configuration diagram showing a multi-mode modulation laser system in which the laser system of FIG. 3 is adapted for distance measurement.

7 is a configuration diagram showing an example of a multi-mode modulation laser system when a plurality of laser systems in FIG. 6 are combined for multi-color interference distance measurement.

[Explanation of symbols]

1 Single Mode Laser Device 2, 2a, 2b, 42a, 42b Polarization Beam Splitter 3, 3a, 3b Acousto-optic Modulator (AOM) 4, 4a, 4b Driver 5 Acousto-optic System 6, 6a, 6b, 7a, 7b, 23 , 26 λ / 4 wave plate 8, 8a, 8b, 9, 9a, 9b, 22 Optical mirror 10, 10a, 10b, 16, 29 Reflective optical system 11 Convex lens 12, 13 Concave mirror 20 λ / 2 wave plate 24 Photodetector 27 Reflector 28 Reflector 38 Corner Cube Prism 40 AOM Basic System

Claims (1)

[Claims] 1. An acousto-optic system installed on the optical path of a single-mode laser beam and capable of adjusting the longitudinal mode interval of the laser beam and the intensity of each mode, a wave plate and an optical mirror, A multi-mode modulation laser system, comprising: a reflection optical system that reflects laser light of each mode emitted from the system and re-enters the acousto-optic system.