JPH06342955A - Frequency stabilizing method for laser and frequency stabilized laser - Google Patents

Abstract

PURPOSE:To control a laser to stabilize its oscillation in a wide range without impairing a quality of a laser light. CONSTITUTION:A metal reflecting mirror 4 is disposed near a totally reflecting surface 6 of a laser medium 1, a distance between the mirror 4 and the surface 6 is regulated by a piezoelectric element 5 to vary a phase of a laser light to be totally reflected thereby to control a substantial optical path length of the medium 1, thereby stabilizing an oscillating frequency of the laser.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency stabilizing laser which actively stabilizes the oscillation frequency of laser light emitted from a laser resonator.

[0002]

2. Description of the Related Art Laser oscillation frequency control is particularly important in the field of optical measurement using a laser, particularly in the field of length measurement and distance measurement utilizing interference of laser light. The reason for utilizing the interference of light is that the wavelength of the light, that is, the frequency is used as a measuring rule. Therefore, many attempts have been made to actively stabilize the oscillation frequency of the laser, and some of them have already been put to practical use.

There are the following two factors that determine the oscillation frequency of the laser. (1) Gain region of laser medium (2) Effective optical path length (effective length) L of laser resonator L (1) and (2) are both important factors. The gain region is mainly determined by the laser medium, and it is difficult for it to change due to external disturbances, and since the gain region is originally much wider than the target value for frequency stability, the stability of the oscillation frequency is improved. Does not have much effect. As an example, even a gas laser, which is said to have a relatively narrow gain width, has a gain region of about 1 GHz and is often used for optical measurement of Nd: Y.
A solid-state laser such as AG has a gain region of about 100 GHz or more. On the other hand, 1 MH in the fields of length measurement and distance measurement
Stability of z or less, or 1 kHz or less in some cases is required. Also, the effective length L of the laser resonator shown in (2)
Is the oscillation frequency ν

## EQU1 ## ν = nC / 2L (1) (where C is the speed of light in a vacuum, and n is an integer), so that it directly affects the stability of the oscillation frequency. Therefore, in order to stabilize the oscillation frequency,
It is necessary to control the effective optical path length L of the laser resonator.

The following techniques have been proposed as techniques for controlling the effective length L of the laser resonator, for example. 1. 1. Move the reflector that constitutes the laser resonator in the optical axis direction. 2. A glass plate is placed inside the laser resonator, and the optical path in the glass plate is changed by the inclination. 3. Place a birefringent crystal in the laser cavity and rotate it to change the refractive index. The pressure is applied to the laser medium to distort it and change the refractive index. 5. An electro-optical element whose refractive index changes with an applied voltage is placed in the laser resonator.

[0005]

However, the above-mentioned conventional technique for stabilizing the laser oscillation frequency has the following problems. First, in the techniques shown in 1 to 3, since the relatively heavy elements such as the reflecting mirror, the glass plate, and the birefringent crystal are moved to control the oscillation frequency, the controllable oscillation frequency band is narrow. There's a problem. That is, when the element is mechanically moved by a drive mechanism or the like, a large phase delay occurs with respect to the control signal when the element is moved at a frequency higher than the oscillation frequency determined by the weight of the element. The oscillation frequency of the device is generally inversely proportional to the square root of the weight, and becomes lower as the weight becomes heavier.
Therefore, in order to drive and control the element with good responsiveness, it is necessary to drive and control the element at a frequency lower than the oscillation frequency, and the band is narrowed in a heavy element.

In the technique shown in FIG. 4, pressure is applied to the laser medium to change the refractive index of the laser medium. At this time, the element for applying pressure to the laser medium is enlarged because of the above-mentioned control band. Can not do it. Therefore, pressure is applied to the laser medium by the small element, but the small element does not uniformly apply pressure to the laser medium, so that not only does the laser medium change the cavity length in the optical axis direction, but also the optical axis. Distortion in the direction orthogonal to is also generated, and the refractive index in the laser medium can be distributed. As a result, there is a problem that the profile of the laser light passing through the laser medium (the quality of light in the cross-sectional direction) is affected.

Further, the techniques shown in 2, 3, and 5 have a problem that a space for inserting an element in the laser resonator is required, and the resonator as a whole becomes large, and an instability factor increases. As an example of the instability factor, the effect of thermal expansion due to temperature fluctuations may increase due to the size increase of the resonator. As is clear from the above formula (1), the effective length of the resonator changes due to thermal expansion, which becomes a factor that hinders stabilization of the oscillation frequency. Another problem is that low-frequency vibrations that are difficult to remove with a vibration eliminator or the like are likely to occur. Further, there is a problem that an air flow is generated in the space around the element, and the air flow makes the laser oscillation unstable.

An object of the present invention is to provide a frequency-stabilized laser capable of obtaining a wide control band and a stable oscillation frequency without deteriorating the quality of laser light.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Explaining in association with FIG. 1 showing an embodiment, the invention of claim 1 of the present invention is as follows.
It is applied to a laser frequency stabilization method for actively stabilizing the oscillation frequency of laser light emitted from the laser resonator 1 in which the total reflection surface 6 is interposed in the optical path OP. The above-mentioned object is achieved by controlling the optical path length of the laser resonator 1 by reflecting the laser light leaking from the total reflection surface 6 toward the total reflection surface 6. Further, the invention of claim 2 is applied to a frequency-stabilized laser that actively stabilizes the oscillation frequency of the laser light emitted from the laser resonator 1 in which the total reflection surface 6 is interposed in the optical path OP. The above-described object is to reflect the laser light leaking from the total reflection surface 6 toward the total reflection surface 6 by the reflector 4 arranged in the vicinity of the laser light reflection portion of the total reflection surface 6 This is achieved by controlling the optical path length of the resonator 1. Here, the effective optical path length of the laser resonator 1 is changed by changing the distance between the reflector 4 and the total reflection surface 6 so that the oscillation frequency of the laser light emitted from the laser resonator 1 becomes a predetermined frequency. You may provide the control means 7-9 which controls. Further, the piezoelectric element 5 may drive and control the reflector 4 to change the distance between the reflector 4 and the total reflection surface 6.

[0010]

[Function] Total reflection means that when the incident angle exceeds a certain angle (critical angle) when the light travels from a medium having a high refractive index to a medium having a low refractive index, the medium has a lower refractive index. Is a phenomenon in which all light is lost and all light is reflected at the interface between media. However, at this time, when viewed microscopically, not all light is reflected at the interface,
Light is leaking from the interface to a distance of about the wavelength of light. This light is called evanescent light, and it exists only near the interface and decays exponentially when it leaves the interface. Therefore, it is known that the actual total reflection shifts the reflection position by about the wavelength of light as compared with the geometrical reflection at the interface. This phenomenon is called the Goos-Hanchen shift.

When an object is brought close to the total reflection surface,
It has been known that when the object is a transparent body or a light absorber, light is absorbed by the object, and the reflectance of the surface that is totally reflected is lowered. However, what happens when the object is a reflector, and what happens to the phase of the light that is totally reflected when the reflector is brought close to the total reflection surface, and such total reflection It has not been well known how the effective length of an optical resonator including a plane changes.

The present inventor brings the reflector close to the total reflection surface so that the phase of the light that is totally reflected is almost exponential depending on the approach distance while the reflector is in the vicinity of the total reflection surface. It was found that the change of the laser beam was changed to, and it was applied to the effective length control of the laser cavity. That is, the laser light leaked from the total reflection surface 6 by the reflector 4 is reflected toward the total reflection surface 6 and the phase of the laser light totally reflected is changed, whereby the effective optical path length of the laser resonator 1 is changed. Was controlled to stabilize the frequency of the laser light emitted from the laser resonator 1.

Incidentally, in the section of means and action for solving the above problems for explaining the constitution of the present invention, the drawings of the embodiments are used for making the present invention easy to understand. It is not limited to.

[0014]

FIG. 1 is a schematic diagram showing a ring type YAG laser to which a frequency stabilized laser according to the present invention is applied. In FIG. 1, reference numeral 1 denotes a laser medium made of Nd: YAG (oscillation frequency 1064 nm), which is formed in a substantially hemispherical shape. Surface of this laser medium 1 (spherical surface)
90% for light with a wavelength of 1064 nm.
The partial reflection coating 2 is formed so as to be 99% reflective and transmissive for light of 780 nm. Also,
The plane (the lower surface in the figure) of the laser medium 1 is a total reflection surface 6 without any reflection coating or the like. Three
Is a semiconductor laser for excitation having a wavelength of 780 nm, and light from this semiconductor laser is incident on the laser medium 1 through one of the partially reflective coatings 2. This incident light follows the clockwise optical path OP shown in FIG.
Laser oscillation of 064 nm occurs. Therefore, the size and shape of the laser medium 1 and the formation position of the partial reflection coating 2 are determined so that the effective optical path length L of this optical path satisfies the above-mentioned expression (1).

On the other hand, on the total reflection surface 6 of the laser medium 1, a metal reflecting mirror 4 is arranged in proximity. The metal reflecting mirror 4 is arranged so as to face the reflecting portion of the above-mentioned laser oscillation optical path OP, and the distance from the total reflection surface 6 can be adjusted by the piezoelectric element actuator 5 arranged on the back surface. Has been done. Here, if the laser beam diameter is made to be the smallest at the total reflection surface 6, the size of the metal reflecting mirror 4 may be about the beam diameter at that time, and several tens of μm is sufficient.

The metal reflecting mirror 4 changes the phase of the light reflected by the total reflection surface 6 according to the distance from the total reflection surface 6, and consequently changes the substantial optical path length L of the optical path OP. Let FIG. 2 is a diagram showing the relationship between the distance between the metal reflecting mirror 4 and the total reflection surface 6 and the phase change of light according to the experimental results of the inventor. Here, the wavelength is 1064 nm, the incident angle of the light beam on the total reflection surface 6 is 85 °, and the refractive index of the laser medium 1 is 1.5. The metal reflecting mirror 4 may change the phase of light on the total reflection surface 6 by 0 to 2π, and therefore,
In the case of FIG. 2, the metal reflecting mirror 4 may be moved by 0 to 80 nm. The moving distance of the metal reflecting mirror 4 by the piezoelectric element actuator 5 is set so that the phase of light on the total reflection surface 6 can be changed by 0 to 2π by the metal reflecting mirror 4, and the incident angle is the critical angle. Is approximately twice the wavelength of the laser light (2 × 1064 nm≈2
μm), which is the maximum distance traveled.

The laser light generated by the laser oscillation is emitted from the other partial reflection coating 2 of the laser medium 1, this laser light is branched by the partial reflection mirror 7, and one of them is guided to the optical difference frequency measuring means 8. . The configuration of the optical difference frequency measuring means 8 is arbitrary, but in the present embodiment, the optical difference frequency measuring means 8 as shown in FIG. 3 is used.

In FIG. 3, the laser light from the partial reflecting mirror 7 is guided to the non-linear optical element 10, and the non-linear optical element 10 generates the second harmonic (wavelength: 512 nm) for the 1064 nm laser light. The second harmonic wave is subjected to phase modulation by the optical phase modulator 11, is branched into two directions by the partial reflection mirror 12, and the two branched second harmonic waves are reflected by the reflection optical system 13 having a plurality of reflection mirrors. It is guided to the iodine cell 14 filled with iodine gas, and substantially opposes in the iodine cell 14. The iodine atom has a saturation absorption line of 512 nm which is equal to the second harmonic of the laser light,
It has an accuracy of several Hz or less.

The transmitted light intensity of one of the second harmonic beams that has passed through the iodine cell 14 is detected by the photodetector 15 and input to the IN port of the lock-in amplifier 16. The lock-in amplifier 16 detects the phase difference between the sine wave input from the oscillator 17 to the REF port and the transmitted light intensity of the second harmonic beam detected by the photodetector 15, and depending on this phase difference and the intensity. Output signal from the OUT port. The sine wave from the oscillator 17 is the voltage-frequency converter 1
It is input to the optical phase modulator 11 via 8.

When the frequency of the laser beam from the partial reflecting mirror 7 is a predetermined frequency, the intensity of the transmitted light from the iodine cell 14 is maximized and the output from the lock-in amplifier 16 is also maximized. On the other hand, when the frequency of the laser beam from the partial reflecting mirror 7 shifts to the high frequency or low frequency side, the intensity of the transmitted light from the iodine cell 14 decreases, but the saturated absorption line of the iodine atom is substantially bilaterally symmetric with 512 nm at the center. Since it has a shape, it cannot be detected only from the transmitted light intensity whether it is shifted to the high frequency side or the low frequency side. Therefore, the second
The harmonics are phase-modulated, and when they are shifted to the high frequency side and the low frequency side, the phase difference is set to 180 °, and which side is shifted is detected. This allows
The second harmonic is stabilized in the center of the saturated absorption line of iodine atom.

If the offset value is added to the output of the lock-in amplifier 16 by the offset adder 19, then 5
It is also possible to stabilize at a frequency deviating from 12 nm by a predetermined frequency.

The difference signal from the reference frequency output from the optical difference frequency measuring means 8 is appropriately amplified by the piezoelectric element driving power source 9 and applied to the piezoelectric element 5. Thereby, the distance between the metal reflecting mirror 4 and the total reflection surface 6 is adjusted and controlled so that the oscillation frequency of the laser light emitted from the laser medium 1 becomes a predetermined frequency. At this time, as shown in FIG.
For the distance between the metal reflector 4 and the total reflection surface 6, the optical path OP
Since the effective change in the optical path length of 1 appears almost exponentially, it is desirable to insert a logarithmic amplifier (not shown) between the optical difference frequency measuring means 8 and the piezoelectric element driving power supply 9. As a result, the effective optical path length L of the optical path OP in the laser medium 1 is kept constant, and the laser oscillation frequency can be stabilized.

Therefore, according to the present embodiment, the metal reflecting mirror 4 is arranged close to the total reflection surface 6 of the laser medium 1,
By adjusting the distance between the metal reflecting mirror 4 and the total reflection surface 6, the substantial optical path length of the optical path OP is changed to stabilize the oscillation frequency. The element (metal reflecting mirror 4) to be drive-controlled can be made sufficiently smaller than that of a mirror, a glass plate, or a crystal, and as a result, the metal reflecting mirror 4 can be made lighter and a wide control band can be secured. it can. Further, unlike the above-mentioned conventional example, there is no need to apply pressure or the like to the laser medium 1, the quality of laser light is not impaired, and in addition, it is necessary to insert an element into the laser medium 1 (laser resonator). Since it does not exist, highly stable oscillation frequency control can be performed.

The frequency-stabilized laser of the present invention is
The details are not limited to the one embodiment described above, and various modifications are possible. As an example, in one embodiment, the Nd: YAG laser medium is used and the semiconductor laser is used as the excitation source, but the known medium and the excitation source are not limited to this and can be used.

[0025]

As described in detail above, according to the present invention, the substantial optical path length of the laser resonator is changed by reflecting the laser light leaking from the total reflection surface of the laser resonator by the reflector. In order to stabilize the oscillation frequency, the control element (reflecting mirror,
The element to be drive-controlled can be made sufficiently smaller than that of a glass plate or crystal, and a wide control band can be secured. Further, unlike the above-mentioned conventional example, there is no need to apply pressure or the like to the laser resonator, the quality of the laser light is not impaired, and in addition, there is no need to insert an element in the laser resonator, so High oscillation frequency control can be performed.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a ring type YAG laser which is an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a relationship between a distance between a metal reflecting mirror and a total reflection surface and a phase change.

FIG. 3 is a schematic configuration diagram showing details of optical difference frequency measuring means in one embodiment.

[Explanation of symbols]

 1 YAG laser medium 2 Partial reflection coating 3 Excitation semiconductor laser 4 Metal reflector 5 Piezoelectric element 6 Total reflection surface 7 Partial transmission mirror 8 Optical difference frequency measuring means 9 Piezoelectric element driving power supply

Claims (4)

[Claims] 1. A laser frequency stabilizing method for actively stabilizing the oscillation frequency of laser light emitted from a laser resonator having a total reflection surface interposed in the optical path, wherein the laser light leaks from the total reflection surface. A method for stabilizing a frequency of a laser, wherein the optical path length of the laser resonator is controlled by reflecting light toward the total reflection surface. 2. A frequency-stabilized laser that actively stabilizes the oscillation frequency of laser light emitted from a laser resonator having a total reflection surface interposed in the optical path, in the vicinity of the laser light reflection portion of the total reflection surface. The frequency-stabilized laser is characterized in that the optical path length of the laser resonator is controlled by reflecting the laser light leaking from the total reflection surface toward the total reflection surface by the reflectors arranged in this manner. 3. The frequency stabilized laser according to claim 2, wherein the distance between the reflector and the total reflection surface is such that the oscillation frequency of the laser light emitted from the laser resonator has a predetermined frequency. And a control means for controlling the effective optical path length of the laser resonator by changing the frequency. 4. The frequency-stabilized laser according to claim 3, wherein the control unit drives and controls the reflector by a piezoelectric element to change the distance between the reflector and the total reflection surface. A frequency-stabilized laser.