JPH1065254A - Laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of the noise for an internal resonator type laser, which generates the second harmonic wave for a laser beam, and to obtain the stable output of a laser. SOLUTION: A laser is provided with an Nd:YAG crystal 5, which comes incident with an excitation light 20, is excited by the energy of the light 20 and generates a laser beam, mirrors 1, 2, 3 and 4, which constitute a laser oscillator for forming the optical path of the laser beam which passes through the crystal 5, and a KTP crystal 6, which is arranged in the laser oscillator and generates the second harmonic wave of the laser beam, and is constituted into such a structure that a laser beam having a plurality of longitudinal modes is generated. In this case, the wavelength and the mode which constitute the laser beam generated in the crystal 5 in the longitudinal modes, are adjusted on the condition that a second high frequency and a sum frequency are generated, including the second high frequency and the sum frequency, which are set into the same wavelength to each other by a non-linear optical member, having a phase mutually weaken, or the sum frequencies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal resonator type laser device for generating a second harmonic.

[0002]

2. Description of the Related Art In a laser device that generates a second harmonic with high efficiency by inserting a nonlinear optical crystal into a laser resonator, when the laser device has a plurality of longitudinal modes,
At the same time as the second harmonic of each of the plurality of longitudinal modes of the laser light, a sum frequency between the longitudinal modes is generated. Due to the generation of the sum frequency, intense mode competition noise generally called a green problem may occur. Currently, solutions are awaited in various applications, and several solutions have been proposed.

FIG. 9 shows an example of a conventional low-noise laser device, which generates a low-noise second harmonic light described in Optics Letters, vol. 13, page 805, published in 1988. FIG. 2 is a configuration diagram of an internal cavity laser device. FIG. 9 shows a Nd: YAG crystal 31, which is a laser medium, and a nonlinear optical crystal that satisfies the type II phase matching condition and generates the second harmonic of a laser beam having a wavelength of 1064 nm emitted from the Nd: YAG crystal 31. KTP crystal 32, which is arranged at an azimuth of 45 ° with respect to the c-axis of KTP crystal 32, Nd:
A quarter-wave plate 33 for a laser beam having a wavelength of 1064 nm emitted from the YAG crystal 31, a laser resonator, and an output mirror 34 for outputting the second harmonic generated by the KTP crystal 32 are shown.

In this laser device, since the quarter-wave plate 33 is arranged at an azimuth of 45 ° with respect to the c-axis of the KTP crystal 32, the eigenpolarization mode of the laser light in the resonator is controlled. , The non-linear polarization which generates the sum frequency is canceled in the KTP crystal 32, and the sum frequency is not generated, so that the noise due to the green problem is not generated. FIG. 10 shows a conventional example of a low-noise laser device using a different method.
Journal of Quantum Electronics (I
EEE, Journal of Quantum El
Electronics, vol. 28, p. 1164.

Here, an Nd: YAG crystal 41, a laser diode 36 emitting an excitation light 37, and an excitation light 37 emitted from the laser diode 36 are condensed to form an Nd: YAG crystal.
Lens 38 to be incident on crystal 41, Nd: YAG crystal 4
K, which is a nonlinear optical crystal having a type II phase matching condition for generating a second harmonic of the laser light emitted in step 1.
4 with respect to the c-axis of TP crystal 42 and KTP crystal 42
A Brewster plate 43 arranged to obtain the highest transmittance for linearly polarized light polarized in the 5 ° azimuth is shown.

In this laser device, a generally well-known birefringent filter is formed due to the birefringence of the KTP crystal 42 and the polarization direction dependence of the transmittance of the Brewster plate 43, and the laser beam is controlled for each longitudinal mode. Are given different resonator losses, so that only one longitudinal mode with the least loss oscillates selectively, so that no sum frequency is generated and the generation of noise is suppressed. As a method of oscillating a laser in a single longitudinal mode, a method using a ring resonator is well known.

FIG. 11 shows a conventional example of a low-noise laser device using a different method, which is a technical digest of advanced solid-state lasers (Technical Dig) published in 1996.
est of Advanced Solid Sta
te Lasers) Post Dead Line PD4.

In this conventional example, an Nd: YVO ₄ crystal 45 which is a laser medium having a wide oscillation wavelength width is used.
The mirrors 51 to 57 constitute a laser resonator. Also, here, an excitation optical system including an optical fiber end 46, a collimating lens 48, and a condenser lens 49 is shown, and each excitation light emitted from each excitation optical system is
The light passes through the mirrors 52 and 53 and enters the Nd: YVO ₄ crystal.

Further, in the laser resonator, Nb: YV
An LBO crystal 40, which is a nonlinear optical crystal that generates a second harmonic of the laser light generated by the O ₄ crystal 45, and a lens 41 for adjusting a laser mode in the LBO crystal 40 are arranged. In this conventional example, an Nd: YVO ₄ crystal 45, which is a laser medium having a wide oscillation wavelength width, is used, and the optical path is turned back by mirrors 54, 55, and 56 to form a laser resonator.
By increasing the length to about m, the number of longitudinal modes of the laser oscillation light is increased to about 100. As a result, fluctuations in the output of each longitudinal mode due to the green problem of the second harmonic light generated in the LBO crystal 40 are mitigated by canceling the fluctuations of the outputs of the many longitudinal modes, and the overall output obtained is obtained. Is reduced.

[0010]

However, in the method of inserting a quarter-wave plate shown in FIG. 9, it is necessary to oscillate two orthogonal polarization modes in one longitudinal mode each. In order to keep the tolerance of the reflectance of the crystal coating and the alignment angle of the mirror narrow and to maintain a noise-free state for a long time, it is necessary to precisely control the temperature of each component and the entire resonator.

Similarly, in the method of arranging Brewster plates shown in FIG. 10, in order to stably maintain one longitudinal mode, the temperature of the KTP crystal and the length of the resonator are precisely adjusted. You need to control. Also, when a high output exceeding 1 W is taken out, the gain in the laser medium is increased and the longitudinal mode of the laser light is likely to be plural, and when the number becomes plural, noise is generated due to a green problem and the stability is lacking. There is fear.

Further, in the conventional example shown in FIG. 11, since the length of the resonator is increased and the number of oscillating longitudinal modes is set to 100 or more, the oscillating wavelength band is widened and the wavelength of phase matching is increased. Since the allowable width is limited, the efficiency of wavelength conversion may decrease. In addition, the distribution of the longitudinal mode is easily influenced by the reflection of the optical member or the like inserted inside the resonator, and there is a possibility that the longitudinal mode of a distant frequency may oscillate, in which case noise due to a green problem may occur. is there. Further, since it is necessary to increase the length of the resonator to 1 m or more, miniaturization is difficult, and furthermore, the configuration of the resonator tends to be complicated, and mechanical stability may be reduced. When actually assembling the laser device, it is extremely important that the device is easily adjusted and has a high degree of design freedom.

The present invention has been made in view of the above circumstances, and has as its object to provide a laser device capable of preventing generation of noise and obtaining a stable output.

[0014]

According to the present invention, there is provided a laser apparatus for generating a laser beam having a plurality of longitudinal modes by receiving excitation light and being excited by the energy of the excitation light. Two mirrors forming a laser resonator forming an optical path of laser light passing through a medium, and a plurality of longitudinal modes arranged in the laser resonator and forming a laser beam generated by the laser medium; The laser medium includes a nonlinear optical member that generates a second harmonic caused by one longitudinal mode and a sum frequency caused by two longitudinal modes of a plurality of longitudinal modes constituting the laser light. The wavelength and the phase of the longitudinal mode constituting the laser light generated in the second high frequency and the sum frequency having the same wavelength and the mutually destructing phases are mutually the same by the nonlinear optical member. Rui includes the sum frequency to each other, the second
It is characterized in that it is adjusted to conditions for generating a high frequency and a sum frequency.

A green problem is generated when a laser beam generated in a laser medium is converted into a second high frequency or a sum frequency.
It is caused by scrambling for the energy of the original laser light by the second high frequency and the sum frequency,
Although a detailed description will be given later, when the second high frequency and the sum frequency or the sum frequencies have the same wavelength and mutually destructive phases, such competition for energy is suppressed, and the generation of a green problem is suppressed.

Here, in the laser apparatus of the present invention, one of the two mirrors directs the second harmonic and the sum frequency emitted from the nonlinear optical member to the nonlinear optical member. It is preferable that the reflectivity of the one mirror for the second harmonic and the sum frequency be adjusted to a reflectivity satisfying the above condition.

If the reflectivity of the mirror is too low, the above condition will not be met, and the reflectivity of the mirror also greatly contributes to the suppression of the green problem. Further, in the laser device of the present invention, the laser medium may be positioned along an optical path of the laser light sandwiched between two mirrors constituting a laser resonator, and the two mirrors may be arranged along the optical path. In any case, it is preferable that the laser beam is disposed at a position separated by at least 1/3 of the entire length of the optical path of the laser beam sandwiched between the two mirrors.

When the laser medium is placed near the center of the laser resonator, the green problem is suppressed and stable oscillation occurs. This is because, in such an arrangement condition, the energy of the longitudinal mode is distributed so that the energy of the central longitudinal mode is large and the energy on both sides thereof is about half that of the central mode. In this case, the energy stored in the laser medium is Is efficiently extracted, the phases of the longitudinal modes are stabilized, and it is considered that this contributes to the suppression of the green problem.

Further, in the laser device of the present invention,
It is preferable that an optical member for limiting the number of longitudinal modes of the laser beam is provided in the laser resonator. By doing so, skipping of the vertical mode interval can be avoided, and the possibility of occurrence of a green problem is reduced. Further, in the laser device of the present invention, the difference between the frequencies of the two longitudinal modes whose frequencies are farthest from each other among the plurality of longitudinal modes constituting the laser light generated in the laser medium is within 1.5 GHz. It is preferable that the temperature be adjusted to

As shown in an embodiment described later, the frequency between two longitudinal modes having the farthest frequencies is 1.5 GHz.
If the above conditions are satisfied, the generation of the green problem is suppressed, but if the frequency of the two longitudinal modes having the most distant frequencies is sandwiched within 1.5 GHz, the laser light , Almost all of the longitudinal modes satisfy the above conditions, and the generation of a green problem is extremely stably suppressed.

[0021]

Embodiments of the present invention will be described below. A laser device described below as a first embodiment includes a laser medium that receives excitation light and is excited by the energy of the excitation light to generate laser light composed of a plurality of longitudinal modes. A first mirror that reflects the laser light emitted in a direction along the first optical path passing through the first optical path in a direction in which the laser light returns to the laser medium along the first optical path; It is arranged at a position sandwiching the laser medium with respect to one mirror,
The excitation light is transmitted, the excitation light is made incident on the laser medium, and the laser light emitted from the laser medium is reflected in a direction along a second optical path extending in a direction different from the direction in which the first optical path extends. A second mirror disposed on the second optical path and extending the laser light reflected by the second mirror in a direction along the second optical path in a direction different from the direction in which the second optical path extends A third mirror that reflects in the direction along the third optical path, and is disposed on the third optical path, and the laser light reflected in the direction along the third optical path by the third mirror is used as the laser light. Third along the third optical path
A fourth mirror reflecting in a direction returning to the third mirror, and a plurality of laser light beams generated by the laser medium, which are arranged on a third optical path between the third mirror and the fourth mirror. Second due to one of the vertical modes
A nonlinear optical member for generating a high frequency and a sum frequency resulting from two longitudinal modes of the plurality of longitudinal modes constituting the laser light, wherein the third reflecting mirror is configured to emit the laser light generated by the laser medium; And the second mirror transmits the second high frequency and the sum frequency generated by the non-linear optical member, and the fourth mirror transmits the laser light generated by the laser medium to the second high frequency and the sum frequency generated by the non-linear optical member. And a laser device that reflects both.

Further, in the laser device of the second embodiment, which will be described after the description of the first embodiment, the laser medium is disposed on the second optical path instead of being disposed on the first optical path. It is a laser device arranged. According to the first and second embodiments of the present invention, when adjusting the laser mode in the resonator, the first mirror can be moved in the direction along the first optical path, and the fourth mirror can be moved. The mirror can be moved in the direction along the third optical path, so that the degree of freedom of adjustment is high. When the third mirror is a concave mirror, the curvature of the concave mirror may be gentle,
The distance between the third mirror and the fourth mirror can be widened, and the rate of change in the characteristics with respect to the change in the position of the fourth mirror is small. Therefore, adjustment for suppressing the occurrence of the green program is easy. And has excellent long-term stability. As a result, the degree of freedom in design is greatly improved.

According to the first and second embodiments of the present invention, the second harmonic generated by the non-linear optical member can be extracted outside using the third mirror as an output mirror. Here, in the first and second embodiments, it is preferable that the third mirror is a concave mirror.

Compensating for the thermal lens effect of the laser medium, and narrowing the laser mode in the nonlinear optical member disposed between the third mirror and the fourth mirror,
This is for generating the second harmonic with high efficiency. Further, in the first and second embodiments, the first and second
Preferably, an optical member for limiting the number of longitudinal modes of the laser light is provided on any one of the third optical path and the third optical path.

When the number of longitudinal modes generated in the laser medium is limited, the effect of the wavelength dependence of the phase matching condition in the nonlinear optical member is reduced, and the efficiency of wavelength conversion to the second harmonic is improved. Further, as described above, the reduction in the number of the vertical modes greatly contributes to the suppression of the green problem. Further, in the first embodiment, the excitation light is focused in the laser medium, and the excitation light is focused in the laser medium, which is disposed at a position sandwiching the second mirror with respect to the laser medium. It is preferable to provide a converging optical member that can be adjusted in the direction along the optical path.
In the embodiment, the excitation light is condensed in the laser medium, which is disposed at a position between the laser medium and the second mirror, and the condensing position of the excitation light is along the second optical path. It is preferable to provide a focusing optical member that can be adjusted in the direction.

By making the focusing position of the excitation light adjustable, the overlap between the excitation mode and the laser mode in the laser medium can be adjusted optimally. By adjusting the focus position accordingly, a laser output corresponding to the power of the excitation light can be obtained. FIG. 1 is a view showing the structure of a laser device according to a first embodiment of the present invention.

In the laser device shown in FIG. 1, one of the laser media, Nd: YA having a diameter of 5 mm and a length of 5 mm, is used.
A G crystal 5 is arranged, and the Nd: YAG crystal 5 is irradiated with excitation light 20. The excitation light 20 is guided to an optical fiber 21 and exits from an end face 21 a thereof, is collimated by a collimating lens 22, and is further collimated by a focusing lens 2.
3, the light passes through the plane mirror 2 which is an example of the above-mentioned second mirror, and is condensed in the Nd: YAG crystal 5. Here, the condenser lens 23 is fixed on a movable stage 24 that is movable in the direction of the arrow AB shown in the figure, and the position of the condenser lens 23 is adjusted.

The plane mirror 2 is an example of the above-mentioned second mirror. In this case, the Nd: YAG crystal 5 has a high reflectance of 99.9% or more with respect to the oscillation laser light having a wavelength of 1064 nm. , And has a transmittance of 95% for light having a wavelength of 809 mm of the excitation light. Also Nd: YAG
Oscillating laser light (wavelength 1) is applied to both end faces 5a and 5b of crystal 5.
064 nm) and a dielectric film having a reflectance of 0.5% or less with respect to excitation light (wavelength 809 nm).

In the Nd: YAG crystal 5, laser oscillation occurs due to the energy of the excitation light, and the above-mentioned wavelength 1
064 nm laser light is generated, and the generated laser light is applied to the Nd: YAG crystal 5 in a direction along the first optical path 11.
Emitted from On the first optical path 11, a plane mirror 1 (an example of the above-described first mirror) having a reflectivity of 99.9% with respect to laser light (wavelength: 1064 nm) is arranged. In the present embodiment, the distance between the plane mirror 1 and the plane mirror 2 is 30 cm, and the Nd: YAG crystal 5 is arranged at a position close to the plane mirror 2.

In the first embodiment shown in FIG. 1, a Brewster plate 8 is further arranged on the first optical path 11. The Brewster plate 8 has a KTP in which the direction of the polarized light having the highest transmittance is the second high frequency or the sum frequency.
The crystal 6 (the KTP crystal 6 will be described later) is fixed at 45 ° to the direction of the c-axis, which is one of the crystal axes. Under this condition, the generation efficiency of the second harmonic in the KTP crystal 6 is the highest, and at the same time, the number of longitudinal modes of the oscillation spectrum is greatly reduced in order to perform the wavelength selection action most effectively as a birefringent filter. The influence of the wavelength dependence of the matching condition is reduced, and the conversion efficiency to the second harmonic is further improved. Further, since the number of longitudinal modes of the oscillation spectrum is significantly reduced, the green problem is effectively suppressed as described later.

However, the purpose of arranging the Brewster plate 8 is not to limit the longitudinal mode of the laser beam to one, but to set the longitudinal mode of laser oscillation at a distant position that is not equally spaced on the frequency axis. This is to suppress the occurrence.
The laser light emitted from the Nd: YAG crystal 5 toward the plane mirror 2 is reflected in a direction along a second optical path 12 extending in a direction different from the direction in which the first optical path 11 extends. This second
The light path 12 has a wavelength of 1064 nm.
It has a high reflectance of 9.9% or more, and has a transmittance of 95% with respect to the light of wavelength 532 nm, which is the second high frequency of the laser light of wavelength 1064 nm, generated by the KPT crystal 6, and has a curvature radius of 70 mm. A concave mirror 3 (an example of the above-mentioned third mirror) is arranged, and the concave mirror 3 functions as an output mirror for the second harmonic. The laser light reflected by the plane mirror 2 and traveling along the second optical path 12 to reach the concave mirror 3 is
Thus, the light is reflected in a direction along a third optical path 13 extending in a direction different from the direction in which the second optical path 12 extends. In the present embodiment, the distance between the plane mirror 2 and the concave mirror 3 is about 25 cm. On the third optical path 13, a wavelength 106
It has a high reflectivity of 99.9% or more for light of 4 nm, and also has a high reflectivity of 99.5% or more for light of wavelength 532 nm which is the second harmonic of laser light of wavelength 1064 nm. The plane mirror 4 (an example of the above-described fourth mirror) is disposed, and the laser light reflected by the concave mirror 3 and traveling along the third optical path 13 is transmitted by the plane mirror 4 to the third optical path 13. Is reflected in the direction of returning to the concave mirror 3 along. In the present embodiment, the distance between the concave mirror 3 and the plane mirror 4 is about 4 cm.

Third between the concave mirror 3 and the plane mirror 4
The KTP crystal 6 which is a non-linear optical crystal having a type II phase matching crystal orientation that generates a second harmonic of wavelength 532 when light having a wavelength of 1064 nm is incident on the optical path 13 of FIG.
Is arranged. The wavelength 1 is applied to both end faces of the KTP crystal 6.
The reflectance is 0.2% or less for the light of 064 nm and the wavelength is 53
A dielectric film having a reflectance of 0.4% or less for light of 2 nm is formed. The KTP crystal 6 is arranged so that its C-axis direction is at 45 ° with respect to the direction of the polarized light having the highest transmittance in the Brewster plate 8,
It is designed to maximize the conversion efficiency to the second harmonic. Wavelength 532 emitted bidirectionally from KTP crystal 6
Of the second harmonics of nm, the light directed to the plane mirror 4 is reflected by the plane mirror 4, is collected in the same direction, and is extracted from the concave mirror 3 to the outside of the resonator. Lens 7 is 5
This is a lens for collimating 32 nm light.

In the case of the laser device having the structure shown in FIG. 1, the distance between the plane mirror 1 and the plane mirror 2 is adjusted by moving the plane mirror 1 in the direction in which the first optical path 11 extends. The distance between the concave mirror 3 and the plane mirror 4 can be adjusted by moving the plane mirror 4 in the direction in which the third optical path 13 extends, and the Nd: YAG crystal 5 can be adjusted by these adjustments. The laser cavity can be kept stable by compensating for the thermal lens effect inside the Nd: YAG crystal 5, and the overlap between the excitation mode and the laser mode inside the Nd: YAG crystal 5 can be adjusted optimally to increase the efficiency of laser oscillation. . In addition, the degree of freedom in designing the laser mode in the resonator is high, and even if the radius of curvature of the concave mirror 3 is increased to 70 mm, the K disposed in the third optical path 13 is reduced.
Since the laser mode in the TP crystal 6 can be reduced to a small value, the distance between the concave mirror 3 and the plane mirror 4 is set to 4 cm.
The nonlinear optical crystal having a long length can be arranged with a margin, and the generation efficiency of the second harmonic can be improved. In addition, the allowable range of the change in the distance between the concave mirror 3 and the plane mirror 4 is wide, the adjustment is easy, and the influence of the output fluctuation due to disturbance is small. Therefore, by adopting the structure shown in FIG. 1, it is possible to obtain a laser output with the highest efficiency in any excited state in any excited state, and it is not necessary to dispose a pinhole for adjusting the laser mode. A diffraction limited beam can be obtained.

As described above, the condenser lens 23
Is movable in the direction of the arrow AB, and the position of the condenser lens 23 is adjusted so that the power of the laser beam is maximized. By adjusting the position of the condenser lens 23, Nd: Y
The focal length and the excitation mode of the thermal lens inside the AG crystal 5 are adjusted, so that the output characteristics are adjusted to be optimal according to the laser resonator.

As described above, according to the embodiment shown in FIG. 1, since the adjustment is easy and the laser resonator can be stably maintained, it is easy to achieve the condition in which the green program is stably suppressed. Can be adjusted. still,
In the embodiment shown in FIG. 1, N is an example of a laser medium.
Although a d: YAG crystal is used, the laser medium used here does not need to be an Nd: YAG crystal.
It may be an O ₄ crystal, a Nd: YLF crystal, or any other laser medium. As a pumping light source used for pumping, for example, a semiconductor laser diode, a pumping light source that generates pumping light having a suitable wavelength corresponding to a laser medium to be used is selected.

In the third embodiment, the KTP crystal is used as the nonlinear optical material for generating the second harmonic. However, it is not always necessary to use the KTP crystal.
It may be a BO crystal or an LBO crystal,
Other non-linear optical materials may be used. FIG. 2 is a view showing the structure of a second embodiment of the laser device of the present invention.

The difference from the embodiment shown in FIG. 1 is that Nd: Y
The point is that the AG crystal 5 is arranged in the second optical path 12. Thus, the laser medium is the first medium as shown in FIG.
May be arranged on the second optical path as shown in FIG.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of experiments using a laser device having the structure shown in FIG. 1 will be described below. Here, in the laser device shown in FIG. 1, while changing the length of the laser resonator and the reflectivity of a part of the mirror for a specific wavelength, the longitudinal mode of output laser light and the time variation of power were measured.

FIG. 3 shows the green output in the configuration of FIG. 1 (here, since the second harmonic and the sum frequency are green light, the second harmonic and the sum frequency are collectively referred to as “green light”). , Collectively refer to those outputs as "green output"
FIG. 4 is a diagram showing a longitudinal mode distribution of laser oscillation light having a wavelength of 1064 nm slightly leaked from the laser resonator, and FIG. 5 is a Fabry diagram showing a longitudinal mode distribution of emitted green light. It is a figure showing the result of having measured using a bellows interferometer. Both the free spectral intervals of the interferometer are 3 GHz, and the two spectral distributions shown in FIGS. 4 and 5 are the same spectra observed on the measurement principle. Hereinafter, one of the distributions will be described.

Here, as shown in FIG. 4, a plurality of longitudinal modes of laser oscillation light are observed in the laser resonator, and in the KTP crystal arranged in the laser resonator, as shown in FIG. The generation of not only the second harmonic corresponding to each longitudinal mode but also the generation of a sum frequency between those modes is observed,
Although the number of vertical modes has increased, a stable green output was obtained as shown in FIG. 3 even in the state where the sum frequency is generated as described above.

Here, by comparing the longitudinal mode distributions of FIG. 4 and FIG. 5, the cause of no noise can be explained as follows. In the longitudinal mode distribution as shown in FIG. 4, as shown in FIG. 6A, typical three longitudinal modes, that is, the intensities of the longitudinal modes of frequencies # 1, # 2, and # 3 are I1, I2, and I3, respectively, and the intensity ratio is 1: Assuming that the ratio is 2: 1, the longitudinal mode distribution of the second harmonic corresponding to each longitudinal mode is theoretically proportional to the square of each intensity, and is given as shown in FIG. 6 (B). The sum frequency between the longitudinal modes is proportional to twice the intensity product of each mode and is given as shown in FIG. Therefore, when the phase relationship between these modes is random, the longitudinal mode distribution of the obtained green light generally includes the intensity for each frequency shown in FIG. 6B and the frequency distribution shown in FIG. It is given as a simple sum with each intensity. Now, in the case where the frequencies υ1, υ2, and υ3 are the frequencies of the adjacent longitudinal modes at equal intervals,
The second harmonic 2υ2 of υ2 and the sum frequency υ1 of υ1 and υ3
+ Υ3 has almost the same frequency. Therefore, when the longitudinal mode of the laser oscillation is given as shown in FIG. 4, the mutual intensity distribution of the green light is, as shown in FIG.
υ2 should be the strongest. However, FIG. 5 showing the distribution of the green light from which the measurement result was obtained is completely different from the above, and in particular, the green light at 2υ2, which should be the strongest, is weak on the contrary. This phenomenon can be explained as follows. If the phase of the second harmonic 2υ2 in FIG. 6 (B) and the sum frequency υ1 + υ3 in FIG. 6 (C) are always out of phase by about 180 degrees, the nonlinear polarizations generated in the KTP cancel each other out and are extracted 2
It is led that the green light in # 2 becomes weak as shown in FIG. Since a green problem has a cause due to a change in conversion efficiency of energy from an original laser beam to a second harmonic or a sum frequency or a competition for energy between the second harmonic and a sum frequency, as described above, When the phase between the longitudinal modes acts so that the second harmonic and the sum frequency cancel each other and weaken each other, the generation of the green problem is suppressed.

In the above experiments, each longitudinal mode is extremely stable, and the competition between the longitudinal modes and the so-called green problem can be achieved without particularly precise alignment of the temperature, position, angle and resonator of the KTP crystal. No noise was generated. FIG. 7 shows a case where the distance between the plane mirror 1 and the plane mirror 2 is reduced to 10 cm in the configuration of FIG. 1 and the other distances are the same as those described with reference to FIG. Longitudinal mode distribution (Fig. 7
(A)) and the result of measuring the longitudinal mode distribution of the emitted green light (FIG. 7 (B)) by the Fabry-Bellowe interference method. As the distance between the plane mirror 1 and the plane mirror 2 is reduced, the distance between the longitudinal modes of the laser oscillation light is increased, the number of the longitudinal modes is increased to five, and the relative intensity distribution is also changed. This is because the laser resonator length is shortened and the longitudinal mode interval of laser oscillation is widened, and the relative position of the Nd: YAG crystal is closer to the end of the laser resonator, so that the energy extraction efficiency is improved. That is, the Nd: YAG crystal is
d: Reflects a change in efficiency when each longitudinal mode of the laser beam extracts energy accumulated in the YAG crystal. Also in this configuration, as can be understood from FIG. 7B, the mode of the sum frequency of the center and the longitudinal modes adjacent to the center, which should be the strongest, is weakened, and the phase between the longitudinal modes is always constant. It can be seen that it is stable and a so-called synchronous state is realized. Even in this state, no green problem occurs.

FIG. 8 shows that, in the configuration of FIG. 1, the plane mirror 4 which reflects green light in the resonator is replaced with a plane mirror having a transmittance of 95% for green light, and the plane mirror 1 is replaced with a plane mirror. FIG. 7 is a diagram illustrating a temporal change of a green output when a distance between the flat mirror 2 and the plane mirror 2 is reduced to 10 cm.
Under this condition, the output fluctuates periodically, indicating that a green problem has occurred. From this, it was found that the internal reflection of the green light to the KTP crystal acts as a major factor for realizing the mode locking described above. That is, when green light generated inside the KTP crystal is reflected outside the KTP crystal and passes through the KTP crystal again, reverse wavelength conversion by reversible optical parametric occurs, and the green light is converted to 1064 nm.
It is considered that the converted 1064 nm light has exactly the same wavelength as the laser oscillation light, so that it easily causes interference and synchronizes the phase of each longitudinal mode or promotes it.

Even when the above-mentioned plane mirror 4 is replaced with a plane mirror having a high transmittance for green light, if the distance between the plane mirror 1 and the plane mirror 2 is from 20 cm to 40 cm, the noise due to the green problem is generated. Did not occur. This relates to the relative position of the laser medium Nd: YAG in the laser resonator, and the position of the laser medium in the laser resonator is
And the plane mirror 4 have a laser resonator length of 1
In the case of being separated by / 3 or more, the intensity of the longitudinal mode is distributed such that the intensity of the longitudinal mode at the center frequency is the highest, and the intensity of the longitudinal mode at the frequencies at both ends is about half the intensity. This is spatially in such a longitudinal mode distribution,
Because the spatial mode energy distribution in the laser medium where the longitudinal mode at the center frequency could not be completely extracted, the energy was extracted so that the longitudinal modes at the adjacent frequencies divide into two, maximizing the extraction efficiency. In terms of time, considering the time integration of energy extraction, it is more efficient to extract the energy stored in the laser medium when each longitudinal mode has a specific phase difference than in a random phase state. This is because you can do it. Therefore, in such a positional relationship of the laser medium, it is considered that the phase of the oscillating longitudinal mode is easily synchronized and the occurrence of the green problem can be suppressed.

When considering a mode-locked state in which no noise is generated, the polarization components of the KTP crystal for generating the second harmonic and the sum frequency need to interfere with each other so as to weaken each other. In order to cause interference, it is necessary that the longitudinal modes of laser oscillation have the same interval on the frequency axis. In a general laser device, the longitudinal modes of actual oscillated light often do not have equal intervals due to a so-called etalon effect or a spatial hole burning effect due to residual reflection of constituent optical components and the like. If the oscillating vertical modes are not equidistant,
Even if the phases of the longitudinal modes are synchronized, they do not interfere with each other, and a longitudinal mode in which only the second harmonic or only the sum frequency exists alone may occur. At that time, a green problem may occur. Therefore, it is desirable that an optical member for limiting the number of longitudinal modes of the laser beam is disposed in the laser resonator, and that the interval between the longitudinal modes due to an etalon effect, a spatial hole burning effect, or the like is avoided. In the configuration of FIG.
A so-called birefringent filter is formed by the P crystal and the Brewster plate, and the number of oscillating longitudinal modes is limited to about 3 to 5 arranged at regular intervals.

Further, in order for mode harmonics to occur and their second harmonics and the sum frequency to strongly interfere with each other, the most distant longitudinal mode must be set so that all of the longitudinal modes simultaneously satisfy the phase matching condition. It is required that the distance between them is as narrow as possible. In this experiment, the interval between the most distant longitudinal modes of the laser oscillation light having a wavelength of 1064 nm in the configuration of FIG. 1 is as narrow as 1.5 GHz or less, and the number thereof is small.

[0047]

As described above, according to the present invention,
A laser device capable of preventing generation of noise and obtaining a stable output is realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of a laser device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a structure of a second embodiment of the laser device of the present invention.

FIG. 3 is a diagram showing a time change of a green output when no noise occurs.

FIG. 4 shows laser oscillation light 106 when noise does not occur.
It is the figure which showed the measurement result of the longitudinal mode distribution of 4 nm.

FIG. 5 shows a green output 532 when no noise occurs.
FIG. 9 is a diagram showing a measurement result of a longitudinal mode distribution of nm.

FIG. 6 is a diagram showing a longitudinal mode distribution of a green output which is expected when the longitudinal modes of the laser oscillation light are not phase-synchronized or synchronized.

FIG. 7 shows the distance between the plane mirror 1 and the plane mirror 2 being 10c.
FIG. 13 is a diagram showing a measurement result of a longitudinal mode distribution of laser oscillation light of 1064 nm and green output of 532 nm when the length is reduced to m.

FIG. 8 shows the distance between the plane mirror 1 and the plane mirror 2 being 10c.
m is a diagram showing the result of measuring the time change of the green output observed when the plane mirror 4 is replaced with a mirror having a high transmittance for green light.

FIG. 9 is a diagram showing an example of a conventional low-noise laser device.

FIG. 10 is a diagram showing an example of a conventional low-noise laser device using a different method.

FIG. 11 is a diagram showing an example of a conventional low-noise laser device using a different method.

[Explanation of symbols]

 1, 2, 4 Planar mirror 3 Concave mirror 5 Nd: YAG crystal 6 KTP crystal 8 Brewster plate 11 First optical path 12 Second optical path 13 Third optical path 20 Excitation light

Claims (5)

[Claims] 1. A laser medium that receives excitation light and is excited by the energy of the excitation light to generate laser light having a plurality of longitudinal modes, and a laser resonator that forms an optical path of the laser light passing through the laser medium. Two mirrors constituting the second harmonic, which are arranged in the laser resonator and are caused by one longitudinal mode of a plurality of longitudinal modes constituting a laser beam generated in the laser medium; A nonlinear optical member that generates a sum frequency due to two longitudinal modes of the plurality of longitudinal modes constituting the laser light, wherein the wavelength and phase of the longitudinal mode constituting the laser light generated in the laser medium are A second phase having the same wavelength and mutually depleting phases by the nonlinear optical member.
A laser device characterized by being adjusted to a condition for generating a second high frequency and a sum frequency including a high frequency and a sum frequency or a sum frequency. 2. One of the two mirrors reflects a second harmonic and a sum frequency emitted from the nonlinear optical member toward the nonlinear optical member. 2. The laser device according to claim 1, wherein the reflectance of one mirror with respect to the second harmonic and the sum frequency is adjusted to a reflectance that satisfies the condition. 3. A laser medium according to claim 1, wherein said laser medium is located on an optical path of said laser light sandwiched between two mirrors constituting said laser resonator, and is located along said optical path from any of said two mirrors. 2. The laser device according to claim 1, wherein the laser device is disposed at a position separated by at least one-third of an entire optical path of the laser light sandwiched between the two mirrors. 4. The laser device according to claim 1, wherein an optical member for limiting the number of longitudinal modes of the laser light is provided in the laser resonator. 5. A plurality of longitudinal modes constituting a laser beam generated by the laser medium are adjusted so that the difference between the frequencies of two longitudinal modes whose frequencies are farthest from each other is within 1.5 GHz. The laser device according to claim 1, wherein: