JPH11163451A - Laser device - Google Patents

Abstract

(57) Abstract: The present invention relates to an internal resonator type laser device that generates a second harmonic, and prevents generation of noise to obtain a stable output. Kind Code: A1 An excitation light is incident and excited by the energy of the excitation light to generate a laser beam.
d: YAG crystal 5, mirrors 1, 2, 3, and 4 forming laser resonators forming an optical path of laser light passing through the Nd: YAG crystal 5, and the laser light arranged in the laser resonator And a KTP crystal 10 that generates a second harmonic of the above, and a laser device that generates a laser beam having a plurality of longitudinal modes. The second high-frequency and the sum frequency including the second high-frequency and the sum frequency or the sum frequency having the same wavelength and the mutually destructive phases by the nonlinear optical member;
Has been adjusted. Further, the laser oscillation is intermittently performed by the acousto-optic element 9.

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 longitudinal mode of the laser light, a sum frequency between the longitudinal modes is generated. The generation of this sum frequency is
Each of the longitudinal modes causes a common loss, which causes the modes to be energetically coupled to each other, and may cause severe mode competition noise generally called a green problem. This green problem becomes extremely harmful noise when applied to a laser device, and several noise control methods have been proposed so far.

FIG. 2 shows an example of a conventionally proposed low-noise laser apparatus. The low-noise laser apparatus described in Optics Letters, vol. 13, page 805, published in 1988. FIG. 2 is a configuration diagram of an internal resonance type laser device that generates two harmonic light. FIG. 2 shows an Nd: YAG crystal 31, which is a laser medium,
KTP crystal 32 which is a nonlinear optical crystal that satisfies the type II phase matching condition and generates a second harmonic of a laser beam having a wavelength of 1064 nm emitted from Nd: YAG crystal 31, and has a 45 ° relative to the c-axis of KTP crystal 32. Wavelength 10 emitted from Nd: YAG crystal 31 arranged in ° orientation
An output mirror 34 that constitutes a quarter-wave plate 33 for 64 nm laser light, a laser resonator, and outputs the second harmonic generated by the KTP crystal 32 is shown.

In this laser device, since the quarter-wave plate 33 is arranged at an angle 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, Non-linear polarization which generates a sum frequency is canceled in the KTP crystal 32, and no sum frequency is generated. Therefore, noise due to the green problem is not generated. FIG. 3 shows a conventional example of a low-noise laser device using a different method, which is published in 1992 by I Triple E and Journal of Quantum Electronics (IE).
EE, Journal of Quantum Ele
tronics) 28, 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 the TP crystal 42 and the 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 between the longitudinal modes of the laser. Different resonator losses are provided, so that only one longitudinal mode with the least loss oscillates selectively, so that no sum frequency is generated and noise generation is suppressed. As a method of oscillating a laser in a single longitudinal mode, a method using a ring resonator is well known.

FIG. 4 shows a conventional example of a low-noise laser apparatus using a different method, which is a technical digest of advanced solid-state lasers (Technical D) published in 1996.
igest of Advanced Solid S
state Lasers) post deadline PD4.

In this conventional example, an Nd: YVO ₄ crystal 45 which is a laser medium having a wide oscillation wavelength width is used.
A laser resonator is formed by the two mirrors 51 and 57, and the optical path is folded by five mirrors 52 to 56 in the middle. Here, an excitation optical system including an optical fiber end 46, a collimator lens 48, and a condenser lens 49 is shown, and each excitation light emitted from each excitation optical system passes through each mirror 52, 53. And Nd: YVO
_Four incident on the 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 the quarter-wave plate shown in FIG. 2, it is necessary to oscillate two orthogonal polarization modes in each one longitudinal mode. 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 inserting a Brewster plate shown in FIG. 3, 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.

In the two conventional examples described above, it is necessary to always limit the longitudinal mode of the laser beam to one or two. However, an optical member is provided in the resonator to intermittently prevent oscillation. If the laser oscillation is performed intermittently, the number of longitudinal modes may further increase due to temperature fluctuations generated in the laser crystal and fluctuations in the refractive index caused by the fluctuations.

When the length of the resonator is increased and the number of oscillating longitudinal modes is set to 100 or more, as in the prior art shown in FIG. 4, 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 laser device is easily adjusted and has a high degree of design freedom. In addition, this method uses Nd: Y
It cannot be applied to many other laser devices that do not have a wide oscillation wavelength band, such as VO ₄ crystal.

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a laser device capable of preventing generation of noise and obtaining a stable output.

[0015]

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 constituting a laser resonator forming an optical path of laser light passing through a medium, a plurality of mirrors being arranged on an optical path of laser light in the laser resonator and constituting laser light generated in the laser medium; The second high frequency caused by one of the longitudinal modes and two of the plurality of longitudinal modes constituting the laser beam.
A nonlinear optical member that generates a sum frequency due to the longitudinal mode of the book, and a laser light that is disposed on the optical path of the laser light in the laser resonator and intermittently interrupts laser oscillation on the optical path. A switching optical member for reducing the wavelength, and a wavelength and a phase of a longitudinal mode constituting the laser light generated in the laser medium are wavelengths at least partially overlapping each other by the nonlinear optical member, and weakened by each other. The second harmonic and the sum frequency including the second harmonic and the sum frequency having the same phase are adjusted to conditions for generating the second harmonic and the sum frequency.

The green problem is caused by scrambling for the energy of the original laser light by the second high frequency and the sum frequency when the laser light generated in the laser medium is converted into the second high frequency and the sum frequency. Are
When the second high frequency and the sum frequency or the sum frequencies have the same wavelength and mutually destructive phases, such a scramble for energy is suppressed, and the generation of a green problem is suppressed.

Here, in the laser device of the present invention, one of the two mirrors constituting the laser resonator is configured to convert the second harmonic and the sum frequency emitted from the non-linear optical member. It is preferable that the reflectivity of the one mirror to the second harmonic and the sum frequency is 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 １ ／ or more of the entire optical path of the laser beam sandwiched between the two mirrors.

When the laser medium is placed at 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 one longitudinal mode at the center is large, and the energy of the longitudinal mode is distributed so that the energy on both sides is about half of that. In this case, the energy stored in the laser medium is stored. It is considered that the obtained energy is efficiently extracted, and at the same time, the phase relationship between the longitudinal modes is stabilized, which 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.

[0021]

Embodiments of the present invention will be described below. FIG. 1 is a view showing the structure of a laser device according to a first embodiment of the present invention. The laser device shown in FIG. 1 has a diameter of 5 mm and a length of 5 mm, which is one of the laser media.
Nd: YAG crystal 5 is arranged, and this Nd: YAG crystal 5
The excitation light 20 is applied to the AG crystal 5. Excitation light 20
Is guided from the optical fiber 21, exits from the end face 21 a, is collimated by the collimating lens 22, further passes through the condenser lens 23, passes through the plane mirror 2, and becomes N
d: Light is condensed in the YAG crystal 5. Here, the condenser lens 2
Numeral 3 is fixed on a movable stage 24 that is movable in the directions of arrows AB shown in the figure, and the position of the condenser lens 23 is adjusted.

The flat mirror 2 is Nd: Y here.
It has a high reflectivity of 99.9% or more with respect to the laser beam having a wavelength of 1064 nm of the oscillation laser beam in the AG crystal 5, and has a transmittance of 95% with respect to the light having a wavelength of 809 mm of the excitation light. In addition, both end surfaces 5a and 5b of the Nd: YAG crystal 5 have
A reflectance of 0.1 for the oscillation laser light (wavelength 1064 nm).
A dielectric film having a reflectance of 1% or less and a reflectance of 0.5% or less with respect to excitation light (wavelength 809 nm) is formed.

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, the plane mirror 1 having a reflectivity of 99.9% with respect to a laser beam (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 N
d: The 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 and an acousto-optic element 9 are further arranged on a first optical path 11. The Brewster plate 8 is set so that the direction of the polarized light having the highest transmittance is 45 ° with respect to the direction of the c-axis which is one of the crystal axes of the KTP crystal 6 that generates the second high frequency and the sum frequency. Fixed. 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 oscillation spectra is greatly reduced in order to perform the wavelength selection action most effectively as a birefringent filter. Has less influence on the wavelength, and the conversion efficiency to the second harmonic is further improved.

However, the purpose of arranging the Brewster plate 8 is not to limit the number of longitudinal modes of the laser beam to one, but to place the longitudinal modes of the laser at distant positions that are not equally spaced on the frequency axis. This is to suppress the occurrence of. The acousto-optic element 9 transmits laser oscillation light along the optical path 11 when no voltage is applied, but when a high-frequency voltage is applied, a periodic distribution of the refractive index due to the photoelastic effect is formed in the element, The periodic distribution of the refractive index Nd: the light propagating along the optical path 11 from the YAG crystal 5 is diffracted and deviated from the optical path 11, so that the laser oscillation stops or is significantly attenuated.

The Nd: YAG crystal is continuously irradiated with the excitation light 20 even while the laser oscillation is hindered, and recombination in the Nd: YAG crystal 5 due to stimulated emission occurs due to the stoppage of the oscillation. And the upper level of the laser
Much more electrons are stored than in a state of continuous oscillation. Then, immediately after the application of the voltage to the acousto-optic element 9 is stopped, laser oscillation is started again. At this time, a large amount of stimulated emission light is generated at once from the laser upper level where electrons are accumulated, and continuous An extremely strong pulsed laser oscillation light that cannot be obtained in a typical oscillation state can be obtained. This strong laser oscillation light can generate the second harmonic with high efficiency in the nonlinear optical crystal. This is because the efficiency of wavelength conversion in the nonlinear optical crystal is proportional to the intensity of the incident laser light.

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 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 is disposed, and the concave mirror 3 functions as an output mirror of the second harmonic. The laser light which is reflected by the plane mirror 2 and travels along the second optical path 12 to reach the concave mirror 3 passes through the concave mirror 3 to a third optical path 13 extending in a direction different from the direction in which the second optical path 12 extends. It is reflected in the direction along. In the present embodiment, the distance between the plane mirror 2 and the concave mirror 3 is about 25 cm. The third optical path 13 has a high reflectivity of 99.9% or more with respect to the light having a wavelength of 1064 nm, and also has a reflectivity of 992 nm with respect to the light having a wavelength of 532 nm which is the second harmonic of the laser light having a wavelength of 1064 nm. A flat mirror 4 having a high reflectivity of 0.5% or more is disposed, and is reflected by the concave mirror 3 and is reflected by the third optical path 13.
The laser beam that has traveled along the
Is reflected in the direction returning to the concave mirror 3 along the optical path 13 of FIG. In the present embodiment, the distance between the concave mirror 3 and the plane mirror 4 is about 4 cm.

The third between the concave mirror 3 and the plane mirror 4
In the optical path 13, a KTP crystal 6, which is a nonlinear optical crystal having a type II phase matching condition and generating a second harmonic having a wavelength of 532 when light having a wavelength of 1064 nm is incident, is disposed. Both ends of the KTP crystal 6 have a wavelength of 106
Reflectance of 0.2% or less for light of 4 nm, wavelength of 532n
A dielectric film having a reflectance of 0.4% or less with respect to light of m is formed. The KTP crystal 6 has its C-axis direction set to 4 ° with respect to the polarization
They are arranged so as to be at 5 °, whereby the conversion efficiency to the second harmonic is devised so as to be maximum.
Of the second harmonics having a wavelength of 532 nm emitted bidirectionally from the KTP crystal 6, the light traveling toward the plane mirror 4 is reflected by the plane mirror 4, collected in the same direction, and taken out of the cavity from the concave mirror 3. It is. Lens 7 is 532n
This is a lens for collimating m light.

In the embodiment shown in FIG. 1, the Nd: YAG crystal 5, which is a laser medium, has an optical path of laser light in the laser resonator viewed from two mirrors 1 and 4 constituting the laser resonator. It is located almost in the center. By arranging the laser medium at the center in this way, for the oscillation wavelength of one longitudinal mode, the longitudinal mode immediately adjacent to that longitudinal mode uses the energy stored in the laser medium as the laser beam most efficiently. Since it is selected as a combination to be extracted, irregularities do not occur in the longitudinal modes, the relative phase relationship between the longitudinal modes is stabilized, and the second harmonic and the sum frequency effectively interfere in the nonlinear optical crystal. , The green problem is suppressed, and the generation of noise is suppressed.

[0030] In the embodiment shown in FIG. 1, it is an example of the laser medium Nd: Although YAG crystal is used, the laser medium used here is Nd: not necessarily a YAG crystal, the Nd: YVO ₄ crystal Nd: YL
It may be an F crystal, a composite crystal containing a laser element, or any other laser medium (crystal base material, laser light emitting element to be added). In this embodiment, a fiber output type semiconductor laser diode is used as the excitation light source.
Excitation may be performed directly by a semiconductor laser diode, or a lamp, a gas laser, or the like may be used. An excitation light source that generates excitation light having a suitable wavelength corresponding to the laser medium to be used is selected.

In this 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 nonlinear optical crystals may be used. As the nonlinear optical material for generating the second harmonic, a suitable nonlinear optical material having high generation efficiency of the second harmonic is selected according to the laser medium and the laser oscillation wavelength to be used. Further, in this embodiment, the type II phase matching condition is used for the nonlinear optical crystal. However, the type I phase matching condition may be used. When a crystal having the type I phase matching condition is used, the polarization direction of the oscillating light in the laser resonator is incident parallel to the crystal axis at which the highest conversion efficiency of the crystal can be obtained. Alternatively, an optical material that has a periodic domain inversion distribution and can perform quasi-phase matching with the laser oscillation wavelength may be used.

In this embodiment, as a method of limiting the number of longitudinal modes of laser oscillation light, a birefringent filter is formed by a Brewster plate inserted in a resonator and KTP to limit the number of longitudinal modes. However, it is not always necessary to adopt this combination, and another birefringent filter element may be inserted into the resonator. Further, it is not necessary to use a birefringent filter to limit the number of filters, and an etalon or a narrow band filter may be used, or a combination of these may be used.

Further, in this embodiment, an acousto-optical element is used to cause intermittent oscillation of laser light. However, it is not always necessary to use an acousto-optical element, and an electro-optical element may be used. A photoelastic element may be used. In that case, a combination with a polarizer, a wave plate, or the like is selected as necessary for efficient intermittent oscillation. At the same time as the selection of the combination, the orientation of those elements in the laser resonator is also selected so as to be the orientation at which efficient conversion is performed in the adopted nonlinear optical crystal. Although the embodiment of the present invention is shown in FIG. 1 as an embodiment of the present invention, the configuration of the laser device for realizing claim 1 of the present invention includes the number of mirrors for changing the laser optical path, the curvature of the mirror, Various other configurations can be realized by using conventionally known technologies and optical components such as a lens and a pinhole for adjusting the laser oscillation mode in the resonator. However, the principle configuration for suppressing noise is summarized in the configuration described in claim 1 and is included in the scope of the present invention as long as the configuration in claim 1 is satisfied.

[0034]

As described above, according to the present invention,
Generation of noise is prevented, and a stable output can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of an embodiment of a laser device of the present invention.

FIG. 2 is a diagram showing an example of a conventional low-noise laser device.

FIG. 3 is a diagram showing an example of a conventional low-noise laser device using a different method.

FIG. 4 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 9 Acousto-optic element 11 First optical path 12 Second optical path 13 Third optical path 20 Excitation light

Claims (4)

[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, which are arranged on an optical path of laser light in the laser resonator, and are second mirrors generated by one of a plurality of longitudinal modes constituting laser light generated in the laser medium. A nonlinear optical member that generates a high frequency and a sum frequency resulting from two longitudinal modes of the plurality of longitudinal modes that constitute the laser light, and a nonlinear optical member that is disposed on an optical path of the laser light in the laser resonator; The laser light on the optical path, intermittently, comprising a switching optical member for lowering the level below the level that hinders laser oscillation, and the wavelength of the longitudinal mode constituting the laser light generated in the laser medium and Conditions under which the second harmonic and the sum frequency are generated by the nonlinear optical member, the second harmonic and the sum frequency including at least partially overlapping wavelengths and the second harmonic and the sum frequency having mutually destructive phases. To
A laser device characterized by being adjusted. 2. One of two mirrors constituting the laser resonator reflects a second harmonic and a sum frequency emitted from the non-linear optical member toward the non-linear optical member. 2. The laser device according to claim 1, wherein the reflectance of the one mirror with respect to the second harmonic and the sum frequency is adjusted to a reflectance that satisfies the condition. . 3. The laser medium according to claim 1, wherein the laser medium is positioned on an optical path of the laser light sandwiched between two mirrors constituting the laser resonator, and the laser medium is disposed along any one of the two mirrors along the optical path. 2. The laser device according to claim 1, wherein the laser device is disposed at a position separated by at least one-third of the 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.