JPH11233868A - Wavelength-selecting type laser oscillator and wavelength selecting method in laser oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid the adverse effect of the high-frequency drift and the instability of diffraction efficiency. SOLUTION: In the interior of the resonator constituted of a total reflection mirror 10 and an output mirror 20 having the specified transmittance, a laser medium 30 with the excitation light source and an acoustooptic variable wavelength filter 50 are arranged on a resonating optical path at the suitable position relationship. Furthermore, an rf-driver 70 has the control computer 60, which selectively varies the frequency of a driving voltage to be applied on the acoustoptic variable wavelength filter 50. By selecting the frequency through this control computer 60, the incident light having a plurality of oscillating lines supplied from the laser medium 30 is separated into diffracted lights λ2 and λ3, which are the oscillating lines in correspondence with that frequency and non-diffracted light λ1, which is the specified oscillating line. Only this non-diffracted light λ1 is resonated, and the laser light having the specified wavelength is outputted. On the resonating optical path in the interior of the resonator, a nonlinear output crystal 40 for generating the second higher harmonic is inserted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength-selective laser oscillation device using an acousto-optic tunable filter as a wavelength selection element, and a wavelength selection method in laser oscillation.

[0002]

2. Description of the Related Art In a wavelength-selective laser oscillator using an acousto-optic wavelength tunable filter, wavelength selection is made faster and more accurate than a wavelength-selection method that drives a mechanical element such as rotating a prism. And the adverse effects of disturbance such as vibration and temperature changes and changes with time are small. Therefore, a relatively stable oscillation wavelength and laser output can be obtained.

FIG. 1 shows the principle of such a wavelength-selective laser oscillation device. Inside a resonator constituted by the total reflection mirror 10 and the output mirror 20, a laser medium 30 having an excitation light source and a nonlinear optical crystal 4 for generating a second harmonic are provided.
0 and an acousto-optic wavelength tunable filter (acousto-optic element in the figure) 50 are arranged, and have an appropriate arrangement relationship with each other. The acousto-optic wavelength tunable filter 50 includes:
A predetermined high-frequency voltage corresponding to a command signal from the control computer 60 is selectively applied from the rf driver 70.
In such a laser oscillation device, an excitation energy exceeding an oscillation threshold is supplied from the excitation light source to the laser medium 30.
, The laser oscillation starts and the second harmonic laser light transmitted through the emission mirror 20 is obtained.

At this time, in order to obtain a laser beam having a desired wavelength, an oscillation line having a corresponding wavelength component among a plurality of oscillation lines included in input light supplied from the laser medium 30 is first converted to an acousto-optic wavelength. Diffracted light λ by variable filter
Take out as 1. The total reflection mirror is accurately positioned with respect to the exit direction of the diffracted light to be extracted, and the reflection angle is precisely adjusted. This causes the diffracted light to resonate, and the second harmonic is extracted as laser light. This is the principle of wavelength selection by the acousto-optic tunable filter.

The operation of the acousto-optic tunable filter will be briefly described. As is well known, an ultrasonic wave having a wavelength corresponding to a high-frequency voltage applied from an rf driver is generated inside an acousto-optic tunable filter. The excitation light is separated into diffracted light λ1 and undiffracted light (zero-order light) λ2 and λ3 according to the wavelength of the ultrasonic wave, and is emitted from the acousto-optic tunable filter. The obtained first-order diffracted light becomes a target of resonance.

[0006]

According to the wavelength selection principle of the acousto-optic tunable filter as described above, an attempt is made to extract a desired wavelength component as diffracted light. This has caused some problems. First, there is the problem of high frequency drift. Each time the diffracted light passes by the high frequency voltage applied to the acousto-optic tunable filter, that is, the frequency of the ultrasonic wave, the Doppler shift occurs. For this reason, the frequency band of the obtained laser light is widened and the band cannot be narrowed. Further, since the diffraction efficiency does not reach 100%, a loss due to diffraction occurs, and the threshold value of laser oscillation increases.

Next, there is a problem of instability of diffraction efficiency. When a high-frequency voltage is applied to the acousto-optic tunable filter, heat is generated and the temperature increases with the energization time. The diffraction efficiency is degraded by the rise in the temperature. Conversely, when the temperature decreases, the diffraction efficiency recovers. That is, the diffraction efficiency fluctuates due to the temperature drift. In addition, due to the adverse effect of the temperature drift, the refractive index in the acousto-optic tunable filter becomes non-uniform, which causes not only a change in diffraction efficiency but also deformation and displacement of laser light. Due to the above problems, the oscillation wavelength and the laser output become unstable.

The present invention has been made in view of such circumstances, and has as its object to provide a wavelength-selective laser oscillation device and an oscillation wavelength selection method that can avoid the adverse effects of high-frequency drift and instability of diffraction efficiency. To provide.

[0009]

In order to achieve the above object, in a laser oscillation apparatus according to the present invention, an excitation light source is provided inside a resonator constituted by a total reflection mirror and an output mirror having a predetermined transmittance. The laser medium and the acousto-optic element are arranged on the resonance optical path with an appropriate positional relationship, and the drive medium further comprises drive voltage control means for selectively varying the frequency of the drive voltage applied to the acousto-optic element. By selecting the frequency by means, the input light having a plurality of oscillation lines supplied from the laser medium is converted by the acousto-optic element into the diffracted light as the oscillation line corresponding to the frequency and the predetermined oscillation line. A laser oscillation device that separates the light into a diffracted light and resonates the separated light to selectively output a laser light having a predetermined wavelength. Only it is arranged to reflect toward the output mirror as a resonant object. Here, the acousto-optic wavelength tunable filter is an AOTF (acoustooo).
Optical Tunable Filter, also called “acousto-optic wavelength tuning filter” or “acousto-optic element”.

[0010] Preferably, the acousto-optic device is an acousto-optic wavelength tunable filter. More preferably, in the laser oscillation device, a nonlinear optical crystal for generating a second harmonic is inserted on a resonance optical path inside the resonator.

In the laser oscillator, it is preferable that the drive voltage control means changes the frequency of the drive voltage at a high speed, thereby switching the predetermined wavelength of the laser light at a high speed. Further, when a crystal having a neodymium (Nd) ion as a light emission center is used as a laser medium, the input light including a 900 nm band, a 1000 nm band, and a 1300 nm band can be obtained therefrom.

Further, the laser medium is GYAG
(Gd _x Y _3-x Al ₅ O ₁₂ ) as a host, a 900 nm band, a 1000 nm band, and a 130 nm band can be used.
A gain equal to or higher than a certain level required for output can be obtained in all the oscillation lines that are the basis of the three primary colors such as the 0 nm band, and can be practically used as a laser device of three primary colors. Especially 900n
In the m band, gains closer to the 1000 nm band and the 1300 nm band can be obtained. That is, the problem that the oscillation intensity in the 900 nm band is too small like a conventional simple YAG laser and cannot be practically used as a laser device of three primary color oscillations can be solved.

In addition, if the acousto-optic tunable filter is constituted by an acousto-optic device for a Q switch, the acousto-optic device for a Q switch also has the function of a tunable filter in a laser device of Q-switch oscillation. Therefore, a wavelength-selective laser oscillation device can be realized with the same number of components as before without newly adding a component for wavelength selection.

In order to achieve the above object, in the wavelength selecting method according to the present invention, there is provided a method for selecting an arbitrary frequency from among a plurality of predetermined frequencies in the driving voltage applied to the acousto-optical element in the laser oscillation device. When the input light having a plurality of oscillation lines supplied from the laser medium is separated into the diffracted light and the undiffracted light by the acousto-optic element, the oscillation according to the selected frequency is performed. A desired oscillation line is extracted as undiffracted light by removing the line as the diffracted light, and the undiffracted light is resonated to selectively output laser light having a predetermined wavelength.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS ===== Outline ===== A fundamental difference of the present invention from the above-mentioned prior art lies in the principle of wavelength selection by an acousto-optic element. That is, which of the diffracted light and the undiffracted light (zero-order transmitted light) obtained from the acousto-optic element is to be subjected to resonance is different. Conventionally, diffracted light has been the object of resonance. In contrast, in the present invention, the object to be resonated is undiffracted light. That is, as shown in FIG. 2, unnecessary frequency components of the excitation light were removed as diffracted lights λ2 and λ3, and a total reflection mirror was provided for the remaining undiffracted light λ1. Except for this point, it is basically common to the conventional technology. Therefore, the following description will focus on the difference between the object of resonance and the non-diffracted light.

===== Configuration Example of Laser Device ===== FIG. 2 shows a configuration of a wavelength-selective solid-state laser oscillation device according to the present invention. This is because "the semiconductor laser pumped second harmonic N
d: YAG laser ". As described above, the total reflection mirror is installed so as to reflect the undiffracted light obtained from the acousto-optic tunable filter as the acousto-optic element. In this configuration example, the total reflection mirror and the emission mirror are positioned on a straight line that is the optical axis of the laser light that is emitted at resonance, and the mirrors are completely opposed to each other for axis alignment. Of course, a so-called Z-shaped resonator in which two or more total reflection mirrors are combined can also be configured.

===== Example of Specific Laser Oscillation ==
=== << Selection of Wavelength >>>> As specific oscillation wavelengths (oscillation lines) selected by the acousto-optic wavelength tunable filter, for example, 900 nm band, 1000 nm band, and 13
One or two of the three wavelength bands of the 00 nm band are selected. The specific wavelength in each band is 947n
m, 1064 nm, and 1318 nm. Each second harmonic corresponding to these three wavelengths is blue in the visible region,
It corresponds to each emission color of green and red. If, for example, any two wavelengths are simultaneously selected from among them, all the emission colors in the visible region can be freely extracted. In this case, the oscillation wavelength can be varied at high speed by switching the frequency of the drive voltage applied to the acousto-optic wavelength tunable filter under computer control at high speed. This enables not only industrial applications such as material processing and measurement and academic research but also practical use as a scanning light source for a display as a display device.

<<<< Setting of Each Constituent Component >>>> As the acousto-optic wavelength tunable filter, a conventional well-known and commonly used one can be used. In addition, the total reflection mirror and the output mirror are 947 nm and 106 nm in accordance with the three wavelength options described above.
The reflectance is 99% for each wavelength of 4 nm and 1318 nm. The mirror surface of the output mirror is provided with an antireflection film for each of the wavelengths of 473 nm, 532 nm, and 659 nm as the second harmonic. One nonlinear optical crystal is sequentially arranged on the optical axis to generate the second harmonic of each wavelength of 947 nm, 1064 nm, and 1318 nm. As the laser medium, a conventional Nd: YAG laser crystal that can oscillate with a commercial power supply is used. An appropriate semiconductor laser that oscillates in a continuous mode is attached to the laser medium as an excitation light source. 947n
In diffracting and removing each of the wavelengths of m, 1064 nm, and 1318 nm, the optimum frequencies of the driving voltages applied to the acousto-optic wavelength tunable filter are each 114 M.
Hz, 92 MHz, and 83 MHz.

<<< Selection of One Wavelength >>>> Two driving voltages of 83 MHz and 114 MHz were simultaneously applied to the acousto-optic tunable filter. Then, in the acousto-optic tunable filter, 1318 nm and 9
The 47 nm wavelength is diffracted together while the 1064 nm
Only the wavelengths of. As a result, green laser light of 532 nm oscillated. Similarly, the frequency of the driving voltage is set to 83M
Hz and 92 MHz, 473 nm blue laser light, or 92 MHz and 114 MHz, 65
9 nm red laser light can be obtained. As described above, by simultaneously applying drive voltages of arbitrary two frequencies, it is possible to select monochromatic laser light of each second harmonic corresponding to the remaining one frequency.

<<<< Simultaneous Selection of Two Wavelengths >>>> The frequency of the drive voltage applied to the acousto-optic wavelength tunable filter is set to 114M.
Hz only. Then, only the wavelength of 947 nm is diffracted by the acousto-optic tunable filter, while 10
Wavelengths of 64 nm and 1318 nm were transmitted. As a result, green laser light of 532 nm and red laser light of 659 nm oscillated simultaneously. Similarly, if the driving voltage frequency is 83 MHz, a combination of a 473 nm blue laser beam and a 532 nm green laser beam, or 92
MHz, blue laser light of 473 nm and 659
A combination of nm red laser light can be obtained.
In this way, by applying a drive voltage of any one frequency, laser beams of each second harmonic corresponding to the remaining two frequencies can be obtained at the same time.

===== Others ===== (a) Various design changes can be made without departing from the gist of the present invention. For example, as is well known, the second harmonic generation crystal may be provided outside the resonator.
Also, without providing a nonlinear optical crystal for second harmonic generation,
A type in which a resonated fundamental wave is extracted as laser light is also included in the present invention. (B) When installing the total reflection mirror, it is only necessary to position the laser beam on the linear optical axis of the laser beam emitted in resonance, and to precisely adjust it to the subtle diffraction angle of the diffracted light as in the past. Work burden is reduced. Further, conventionally, all the diffracted lights of different wavelengths are reflected by the total reflection mirror which is adjusted to a fixed angle. If it were to reflect diffracted light with slightly different diffraction angles for each wavelength,
Very accurate reflections cannot be expected and the acceptable wavelength range is limited. On the other hand, in the present invention, since the undiffracted light transmitted straight through the acousto-optic wavelength tunable filter is always set as the resonance target, extremely accurate reflection by the reflecting mirror can be realized and the allowable wavelength range is widened.

[0022]

The non-diffracted light obtained from the acousto-optic tunable filter is subjected to resonance. Therefore, various problems in the related art due to the fact that the object of resonance is diffracted light can be avoided. First, it became unnecessary to consider the adverse effects of high-frequency drift. That is, the undiffracted light does not cause the Doppler shift caused by the acousto-optic wavelength tunable filter. Accordingly, the frequency band of the obtained laser beam does not expand, and the band can be narrowed. The loss for undiffracted light is only the surface reflection of the crystal used for the acousto-optic device. In addition, the surface reflection can be substantially reduced to zero because a crystal to which an anti-reflection coat is applied is usually used. Therefore, the threshold value of laser oscillation can be lowered. Therefore, power consumption can be reduced.

Next, it is not necessary to consider the adverse effect of the instability of the diffraction efficiency. That is, even if the diffraction efficiency fluctuates due to the temperature drift, the present invention in which the undiffracted light is subjected to resonance does not have a significant adverse effect. Further, it is not necessary to prepare an expensive acousto-optic tunable filter to increase the diffraction efficiency. Rather, the diffraction efficiency may be low because the present invention utilizes the undiffracted light that has been lost when diffracted light is subjected to resonance. In this regard, a normal AO-Q switch having a diffraction effect with respect to a plurality of oscillation wavelengths (for example, three types serving as the basis of three primary color oscillations) can be used as a device having a wavelength selection function.

In the past, in order to obtain diffracted light of sufficient intensity, it was necessary to strongly excite ultrasonic waves in the acousto-optic tunable filter, and a relatively large input power was required. However, since the diffracted light in the present invention includes only unnecessary wavelengths to be removed, the intensity of the diffracted light may be small, and the input power can be reduced. As described above, since the adverse effects of the high-frequency drift and the instability of the diffraction efficiency can be avoided, the oscillation wavelength and the laser output can be stabilized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a wavelength selection principle of a conventional wavelength selection type laser oscillation device.

FIG. 2 is a schematic configuration diagram showing a wavelength selection principle of the wavelength selection type laser oscillation device of the present invention.

[Explanation of symbols]

 Reference Signs List 10 total reflection mirror 20 output mirror 30 laser medium 40 nonlinear optical crystal 50 acousto-optic wavelength tunable filter 60 control computer (drive voltage control means) 70 rf driver (drive voltage control means)

Claims (8)

[Claims] 1. A laser medium and an acousto-optic element attached to an excitation light source are arranged on a resonance optical path in a resonator constituted by a total reflection mirror and an output mirror having a predetermined transmittance. And a drive voltage control means for selectively varying a frequency of a drive voltage applied to the acousto-optic element. A plurality of oscillations supplied from the laser medium are selected by selecting the frequency by the drive voltage control means. The input light having a line is separated by the acousto-optic element into diffracted light as an oscillating line corresponding to the frequency and undiffracted light as a predetermined oscillating line, and the separated light is resonated to a predetermined wavelength. A laser oscillation device that selectively outputs laser light, wherein the total reflection mirror is arranged to reflect only the undiffracted light toward the output mirror as a resonance target. Features. 2. The wavelength-selective laser oscillation device according to claim 1, wherein said acousto-optic device is an acousto-optic tunable filter. 3. The wavelength-selective laser oscillation device according to claim 1, wherein a nonlinear optical crystal for generating a second harmonic is inserted on the resonance optical path inside the resonator. . 4. The apparatus according to claim 1, wherein the drive voltage control means changes the frequency of the drive voltage at a high speed, thereby switching the predetermined wavelength of the laser light at a high speed. Wavelength selective laser oscillator. 5. The wavelength-selective laser oscillation device according to claim 1, wherein said laser medium is formed of a crystal having neodymium ions as emission centers. 6. The wavelength-selective laser oscillation device according to claim 5, wherein said laser medium is composed of a crystal using GYAG as a host. 7. The wavelength-selective laser oscillation device according to claim 2, wherein said acousto-optic tunable filter is constituted by an acousto-optic element for a Q switch. 8. A wavelength selection method used in the laser oscillation device according to claim 1, wherein the wavelength is arbitrary among a plurality of predetermined frequencies in the drive voltage applied to the acousto-optic element. By selecting the frequency of, when the input light having a plurality of oscillation lines supplied from the laser medium is separated into the diffracted light and the undiffracted light by the acousto-optic element, according to the selected frequency By removing the oscillation line as the diffracted light, the desired oscillation line is extracted as undiffracted light, and the undiffracted light is resonated to selectively output laser light having a predetermined wavelength.