JPH0621554A - Laser light generating equipment - Google Patents

Abstract

PURPOSE:To effectively restrain mode competition noise, by detecting retardation of optical parts used in a resonator generating second harmonic laser light, with a detecting means for polarization surface rotation amount, and negative- feedback-controlling the phase retardation amount in the laser resonator, according to the detected output. CONSTITUTION:A detected output from a photo detector 23 of a detection part 20 of polarization surface rotation amount is sent to a TE element 23 via an amplifier 31, as the constitution for controlling the retardation in a laser resonator 13. The TE element 32 controls the temperature of each of the optical parts of the resonator 13, in particular, a nonlinear optical element 17 such as quarter-wavelength plate 15 and a KTP, and controls the retardation of the optical parts. Retardation change is always measured, and negative feedback control is applied, thereby remarkably stabilizing the retardation. Hence the mode competition noise caused by the retardation change can be effectively restrained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light generator, and more particularly to a laser light generator having a laser light source for generating high-order harmonic laser light using a nonlinear optical crystal element.

[0002]

2. Description of the Related Art It has hitherto been proposed to efficiently perform wavelength conversion by utilizing a high power density inside a resonator, for example, an external resonance type SHG (second harmonic generation).
Alternatively, SHG and the like using a nonlinear optical element inside the laser resonator have been tried.

As an example of the second harmonic generation type in the laser resonator, there is known one in which a laser medium and a nonlinear optical crystal element are arranged between at least one pair of reflecting mirrors constituting the resonator. In the case of this type of laser light generator, the nonlinear optical crystal element in the resonator is arranged so that the second harmonic laser light is phase-matched with the fundamental laser light, so that the second harmonic laser light is efficiently generated. Can be taken out.

As a method of realizing the above phase matching, a type I or type II phase matching condition is established between the fundamental laser light and the second harmonic laser light. In other words, the principle of type I phase matching is to use the ordinary ray of the fundamental laser light to generate a phenomenon in which two photons polarized in the same direction produce one photon having a frequency doubled. To do. On the other hand, type II phase matching is to make two fundamental polarizations that are orthogonal to each other enter a nonlinear optical crystal element so that the phase matching conditions can be established for each of the two polarizations. The fundamental wave laser light is divided into an ordinary ray and an extraordinary ray inside the nonlinear optical crystal element to cause phase matching with the extraordinary ray of the second harmonic laser light.

However, when the second harmonic laser light is to be generated using the type II phase matching condition, the intrinsic polarization of the fundamental laser light is changed every time the fundamental laser light repeatedly passes through the nonlinear optical crystal element. Since the phase changes, the generation of the second harmonic laser light may not be stably continued.

That is, each time the fundamental laser light generated in the laser medium repeatedly passes through the nonlinear optical crystal element due to the resonance operation, the phases of orthogonal natural vibrations (that is, p-wave component and s-wave component) are shifted. Then, in each part of the resonator, it becomes impossible to obtain a steady state in which the fundamental wave laser lights efficiently reinforce each other, so that a strong resonance state (strong standing wave) cannot be formed.
As a result, the conversion efficiency of the fundamental wave laser light into the second harmonic laser light may be deteriorated, and noise may be generated in the second harmonic laser light.

Therefore, the applicant of the present application filed Japanese Patent Application Laid-Open No. 1-220.
In Japanese Patent Publication No. 879, in a laser light source configured to generate a second harmonic laser beam by a non-linear optical crystal element, a birefringent element such as a quarter-wave plate is inserted in the resonance optical path of a fundamental wave laser beam. By doing so, a laser light source is proposed which stabilizes the second harmonic laser light emitted as the output laser light.

FIG. 6 shows an example of a laser light source, that is, a laser light generator disclosed in the above-mentioned Japanese Patent Laid-Open No. 1-220879. The laser light generator shown in FIG. 6 is a laser medium (laser rod) using Nd: YAG.
The resonator 101 includes a reflection surface (dichroic mirror) 103 formed on the incident surface of 102 and a reflection surface (dichroic mirror) inside the output concave mirror 104, and the resonator 101 has Nd. A YAG laser medium 102, a nonlinear optical crystal element 106 made of KTP (KTiOPO ₄ ), and a birefringent element 107 which is a quarter-wave plate made of, for example, a quartz plate are arranged.

Laser medium 102 in this resonator 101
The excitation laser light emitted from the excitation semiconductor laser 111 is incident on the incident surface 103 of the collimator lens 11.
2. By being incident through the objective lens 113,
The fundamental wave laser light LA (ω) is generated. The fundamental wave laser light LA (ω) passes through the nonlinear optical crystal element 106 and the birefringent element 107 and is reflected by the reflecting surface of the concave mirror 104,
The incident surface (reflection surface) 10 is passed through the birefringent element 107, the nonlinear optical crystal element 106, and the laser medium 102 again in this order.
It is reflected at 3. Therefore, the fundamental laser light LA (ω)
Will resonate so as to reciprocate between the reflecting surface 103 on the incident surface of the laser medium 102 of the resonator 101 and the reflecting surface on the inner side of the output concave mirror 104.

Birefringent element 10 such as the quarter-wave plate described above.
7, in the plane perpendicular to the light propagation direction, as shown in FIG. 7, the direction of the extraordinary light direction refractive index n _{e (7)} is the extraordinary light direction refractive index n _{e (6 The} optical axis position is tilted by a predetermined azimuth angle θ, for example, θ = 45 °.

In the above configuration, the fundamental wave laser light LA
(Ω) generates the second harmonic laser light LA (2ω) when passing through the nonlinear optical crystal element 106 through the resonance optical path, and the second harmonic laser light LA (2ω) is generated by the concave mirror 10.
It is transmitted as an output laser beam after passing through 4.

In this state, the fundamental laser light LA
Each ray forming (ω) is a nonlinear optical crystal element 106.
By passing through the birefringent element (1/4 wavelength plate of the fundamental wave) 107 set in the azimuth angle θ = 45 ° with respect to the laser beam, the power of the laser light in each part of the resonator 101 is at a predetermined level Stabilized to. This is the laser medium 1
The fundamental laser light LA (ω) generated in No. 02 is resonated so as to pass through the nonlinear optical crystal element 106
When the second harmonic laser light LA (2ω) of II is generated, the birefringent element 107 suppresses the coupling due to the sum frequency generation between the two intrinsic polarization modes of the fundamental laser light LA (ω) orthogonal to each other. By doing so, the oscillation is stabilized.

By thus providing the quarter-wave plate (the birefringent element 107) in the resonator 101, the KTP
It is effective in suppressing longitudinal mode conflict noise between orthogonal polarizations regardless of the residual retardation (phase delay amount) of (nonlinear optical crystal element 106) or the phase delay amount. Especially, when the phase delay amount of KTP is 90 °, Nd: YAG
Since the intrinsic polarization mode in the (laser medium 102) is circularly polarized light, there is an advantage that the saturation of the gain is uniform and longitudinal mode conflict noise between the same polarized lights is less likely to occur.

[0014]

The KTP phase delay control type SHG laser of the KTP, quarter-wave plate is obtained by analyzing the eigenpolarization mode in the Nd: YAG laser resonator as described above. It has been found that the retardation error causes the eigenpolarization plane to rotate around the optical axis. Rotation of the eigenpolarization plane about the optical axis causes longitudinal mode competition noise between orthogonal polarizations. Since the retardation error of the KTP and the quarter-wave plate is also caused by the temperature change of these, the SHG laser of this type requires precise temperature control.

Therefore, the intracavity SHG generation type Nd: Y
In the AG laser, the KTP placed in the resonator,
In order to reduce or stabilize the retardation variation of optical components such as a quarter-wave plate, the temperature of these optical components is precisely controlled to a predetermined constant temperature.

However, no matter how precise the temperature control is,
When the optical path of the laser fluctuates, a retardation error occurs in the KTP and the quarter-wave plate, and the optimum point of temperature control is deviated, which cannot be dealt with, and retardation fluctuation due to aging is inevitable. is the current situation.

The present invention has been made in view of such circumstances, and in an intracavity SHG (subject 2 harmonic generation) type laser light generator, a nonlinear optical crystal element (KTP) generated by temperature change or the like is generated. Etc.), it is possible to stabilize the variation in retardation of the birefringent element (1/4 wavelength plate or the like), and to provide a laser light generator capable of suppressing mode competition noise caused by the variation in retardation. To do.

[0018]

A laser light generator according to the present invention comprises an excitation light source element, a laser medium excited by the excitation light from the excitation light source element, and the laser device arranged before and after the laser medium. A pair of reflecting means for forming a laser resonator by reflecting a fundamental wave laser beam generated from a medium, a nonlinear optical crystal element arranged in the laser resonator, and a pair of reflecting surfaces of the laser resonator. A detection means for detecting the rotation amount of the polarization plane of the fundamental wave laser light output from one of the two, and a control means for controlling the phase delay amount in the laser resonator according to the output from the detection means. By doing so, the above-mentioned problems are solved.

Here, the control means may control the phase delay amount of these optical components by controlling the temperature of the optical components in the laser resonator. Further, an electro-optical crystal element may be provided in the laser resonator, and the electro-optical crystal element may be controlled by the control means to control the phase delay amount in the laser resonator.

Here, the laser medium is Nd:
YAG, Nd: YVO ₄ , LNP, Nd: BEL or the like is used, and as the above-mentioned nonlinear optical crystal element, KTP, L
N, BBO, LBO, etc. are used.

As the laser resonator serving as the second harmonic generation (SHG) laser light source, a non-linear optical crystal element in which the fundamental wave laser light generated in the laser medium excited by the excitation light is provided inside the resonator is used. Resonance operation is performed so that the type II second harmonic laser light is generated, and the coupling due to the sum frequency generation between the two polarization modes of the fundamental laser light is suppressed to stabilize in two modes. It is possible to use an optical means provided in the resonator path for performing such oscillation. A birefringent element such as a quarter-wave plate can be used for this optical means.

[0022]

The change in retardation (amount of phase delay) of the optical component in the resonator can be detected by the means for detecting the rotation amount of the polarization plane. Therefore, the temperature control or the electro-optic crystal element control is performed according to the detection signal. Thus, the retardation in the resonator can be controlled to stabilize the retardation fluctuation caused by the fluctuation of the laser optical path or the like, and the mode competition noise due to the retardation fluctuation can be effectively suppressed.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing a schematic structure of an embodiment of a laser beam generator according to the present invention. In FIG. 1, laser light as excitation light is emitted from a laser diode 11 which is a semiconductor laser element as an excitation light source element. This excitation laser light is condensed by the lens 12,
The light is incident on the laser medium 16 using, for example, Nd: YAG via the reflecting mirror 14 and the quarter-wave plate 15 of the laser resonator 13. As the reflecting mirror 14, a concave mirror whose inside is concave is used. The laser medium 16 generates a fundamental wave laser light LA (ω) in response to the excitation light, and the fundamental wave laser light LA (ω) is, for example, KTP (KTiO).
It is reflected by the reflecting mirror 18 through the nonlinear optical crystal element 17 using PO ₄ ). A plane mirror or the like is used as the reflecting mirror 18. Conversely, a flat mirror may be used as the reflecting mirror 14 and a concave mirror may be used as the reflecting mirror 18.

Nonlinear optical crystal element 17 such as KTP
Is a type II phase-matched laser,
The second harmonic laser light LA (2
ω) is generated. For example, if the wavelength λ of the fundamental wave laser light LA (ω) is 1064 nm, the second harmonic laser light LA
The wavelength of (2ω) is 532 nm which is λ / 2. Reflector 1
The reflection surface of No. 4 transmits the excitation light (for example, a wavelength of 810 nm), and the fundamental wave laser light LA generated by the laser medium 16 is generated.
The reflection surface of the reflecting mirror 18 has a characteristic of reflecting (ω), and a characteristic of reflecting the fundamental laser light LA (ω) and transmitting the second harmonic laser light LA (2ω). Have These reflecting mirrors 14 and 18 can be formed by so-called dichroic mirrors. Therefore, the laser medium 16
The fundamental-wave laser light LA (ω) generated in 1 reciprocates between the reflecting mirrors 14 and 18 of the laser resonator 13 to oscillate the laser light. The quarter-wave plate 15 is set in an azimuth angle θ = 45 ° with respect to the nonlinear optical crystal element 17, and functions to stabilize the power of the laser light in each part of the resonator 13. have.

That is, the applicant of the present invention has previously described this quarter-wave plate 15 in the specification and drawings of Japanese Patent Application Laid-Open No. 1-220879 and Japanese Patent Application No. 2-125854, and Japanese Patent Application No. 3-1.
A quarter-wave plate which is a birefringent element used based on the technology disclosed in the specification and drawings of No. 7068, and which is set in an azimuth angle θ = 45 ° with respect to the nonlinear optical crystal element 17. By passing through 15, the resonator 1
This is for stabilizing the power of the laser light in each part of 3.

By inserting the quarter-wave plate 15, (i) non-linear coupling between polarization modes due to sum frequency generation is eliminated, and mode competition between polarization modes can be prevented. (Ii) Since the spatial phase difference between the two polarization modes is 90 °, the spatial hole burning effect can be suppressed by oscillation of the two polarization modes, and stable oscillation of two vertical modes (two polarization modes) can be obtained. To be That is, the action and effect can be obtained.

Next, the light emitted from the laser resonator 13 of the second harmonic generation type in the laser resonator is an SHG laser light having a wavelength of 532 nm, and the above-mentioned wavelength of 1064 nm is a leakage component. The fundamental wave laser light is included. The dichroic mirror 19 has this wavelength 1
It is for separating the leaked light component of 064 nm from the SHG light of wavelength 532 nm, and is arranged outside the reflecting mirror 18 of the laser resonator 13. The leaked light component separated by the dichroic mirror 19 is sent to a detection unit 20 for detecting the rotation amount of the polarization plane.

The polarization plane rotation amount detection unit 20 is sent to an etalon (Fabry-Perot plate) 21 to separate only one polarization utilizing the frequency difference between two orthogonal polarizations, and this is separated by an analyzer 22. The amount of light is sent to the detector (photodetector) 23 and detected. Here, when the plane of polarization of the two orthogonally polarized light is rotated by, for example, Δθ, the amount of light passing through the analyzer 22 changes as shown in FIG. 2, so that the rotation of the plane of polarization can be detected.

Here, FIG. 3 shows a quarter-wave plate 15 and a nonlinear optical crystal element (KTP, etc.) 1 used in the resonator 13.
Retardation of optical components such as 7 (amount of phase delay)
Shows that the inherent polarization plane of the SHG green laser light is rotated by the fluctuation. In FIG. 3, the retardation of KTP, which is the nonlinear optical crystal element 17, is represented by δ ₁ , and the retardation of the quarter-wave plate 15 is represented by δ ₂ . The quarter wave plate 15 has a normal value when the phase delay amount δ ₂ is 90 °. θ represents the azimuth angle of the intrinsic polarization. Here, when the phase delay amount δ _{1 of} KTP is 90 °, the azimuth angle θ of the intrinsic polarization is proportional to the phase delay amount δ ₂ of the quarter wavelength plate 15 as shown by the curve plotted with the triangle mark in FIG. And you can see that it changes. At this time, 1/4 wave plate 1
The c-axis of 5 is 45 with respect to the c-axis of KTP as described above.
It is in the direction of °.

When the phase delay amount δ _{1 of} KTP is 0 °, 1
The azimuth angle of the intrinsically polarized light is 45 ° regardless of the retardation δ ₂ of the / 4 wavelength plate 15, which is preferable, but in this case, the azimuth angle of the c axis of the ¼ wavelength plate 15 is 45 °. If they are deviated, the azimuth of the intrinsic polarized light rotates in proportion to this, so there are advantages and disadvantages, and the advantages and disadvantages cannot be generally stated. Generally, the 90 ° phase delay amount type in which spatial hole burning is uniform is more advantageous, and therefore a method of detecting the retardation variation will be described below.

From the above analysis, it has been found that in the 90 ° phase delay type, when the retardation varies, the eigen polarization plane rotates by an angle of 1/2 of the retardation variation. The polarization plane is rotated by the laser medium (YAG, etc.)
The leak light of the fundamental wave may be detected by a photodetector through an analyzer, and specifically, for example, the detection unit 20 of FIG. 1 may be used. As in the detection unit 20, only the leaked light having a wavelength of 1064 nm is separated and taken out by the etalon 21, and the amount of light passing through the analyzer 22 is detected by the photodetector 23. As shown in FIG. Detect the rotation of the plane of polarization.

The photodetector 23 of the polarization plane rotation amount detecting section 20.
The detection output from is as a configuration for controlling the retardation (phase delay amount) in the laser resonator 13,
It is sent to a so-called TE element (thermo-electric element, TE cooler) 32 which is a temperature control element via an amplifier 31. The amplifier 31 amplifies the difference between the output from the photodetector 23 and the reference voltage V _ref and sends it to the TE element 32.
The TE element 32 is an optical component of the resonator 13, especially 1
Quarter-wave plate 15 and nonlinear optical element 17 such as KTP
The temperature of the optical components is controlled to control the retardation of these optical components. The control direction at this time is set to 1 in the resonator 13 when the detected retardation becomes large.
Negative feedback control that reduces the retardation of the / 4 wavelength plate 15, the nonlinear optical element 17, and the like, and controls the rotation angle of the intrinsic polarization plane to a constant value.

Next, FIG. 4 shows a modified example of the polarization plane rotation amount detection unit 20 of FIG. 1, in which the leaked light component of the wavelength 1064 nm extracted by the etalon 21 is x by the polarization beam splitter 25. The y-axis polarization component is divided into the y-axis polarization component, the x-axis polarization component is the photodetector 26, and the y-axis polarization component is the photodetector 27.
Are to be detected respectively. These photo detectors 2
By taking the differential of the detected light amount from 6 and 27,
It is possible to detect the polarization plane rotation amount with higher accuracy. The detected light amounts shown in FIG. 5 are Δx and Δy, respectively, depending on the rotation of the polarization plane.
Only changes (opposite directions). In the system of FIG. 4, the polarization beam splitter 25 functions as the analyzer.

The other parts of FIG. 4 may be constructed in the same manner as in FIG. The outputs from the photodetectors 26 and 27 may be differentially amplified by, for example, a differential amplifier. In addition, the sum of the amounts of light detected from the photodetectors 26 and 27 is used to detect the photodetectors 26 and 2
If the difference in the amount of detected light from 7 is standardized (normalized), it is possible to avoid the influence of the fluctuation of the laser output on the detection signal.

As described above, the retardation can be extremely stabilized by constantly measuring the retardation variation and performing the negative feedback control so as to reduce the variation. Therefore, it is possible to effectively suppress the mode competition noise caused by the change in retardation.

The present invention is not limited to the above-described embodiment, and for example, an electro-optical (electro-optical conversion) crystal element in which a phase delay amount changes in a laser resonator according to an electronic signal. Is inserted and arranged, and the detection unit 20 of FIG.
The electro-optical crystal element is driven according to the amount of rotation of the polarization plane detected by the configuration of FIG. 4 or the retardation in the resonator detected by another method to reduce the fluctuation of the retardation. Alternatively, the negative feedback control may be performed. Further, the laser medium and the nonlinear optical crystal element are not limited to Nd: YAG and KTP, as a matter of course.

[0037]

As is apparent from the above description, according to the laser light generating apparatus of the present invention, the optical components used in the resonator for generating the type II second harmonic laser light, in particular, Retardation (phase delay amount) of a nonlinear optical crystal element such as a quarter-wave plate or KTP is detected by a polarization plane rotation amount detection means, and the phase delay amount in the laser resonator is detected according to the detection output. Since the negative feedback control is performed, it is possible to stabilize the retardation variation caused by the laser optical path variation, and effectively suppress the mode competition noise caused by the retardation variation.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a schematic configuration of a basic embodiment of a laser light generator according to the present invention.

FIG. 2 is a diagram used for explaining rotation amount detection of a polarization plane of the embodiment.

FIG. 3 is a diagram showing a relationship between a retardation of a quarter-wave plate in a laser resonator and a rotation angle of a polarization plane.

FIG. 4 is a schematic configuration diagram showing a modification of the polarization plane rotation amount detection unit 20 of FIG.

FIG. 5 is a diagram used for explaining rotation amount detection of a polarization plane in the modification example of FIG. 4;

FIG. 6 is a configuration diagram showing a schematic configuration of a conventional example of a laser light generator.

7 is an explanatory diagram of an azimuth angle of a birefringent element used in the conventional example of FIG.

[Explanation of symbols]

 11 ... Laser diode 13 ... Laser resonator 14 ... Reflecting mirror (concave mirror) 15 ... Quarter wave plate 16 ... Laser medium 17 ...・ ・ ・ Nonlinear optical crystal element 18 ・ ・ ・ Reflecting mirror (planar mirror) 20 ・ ・ ・ Detecting unit of polarization plane rotation amount 21 ・ ・ ・ Etalon 22 ・ ・ ・ ・ ・ Analyzer 23, 26, 27 ... Photodetector 25 ... Polarization beam splitter 31 ... Amplifier 32 ... TE (thermo-electric) element

Claims (3)

[Claims] 1. A pumping light source element, a laser medium pumped by pumping light from the pumping light source element, and a laser which is arranged in front of and behind the laser medium and reflects a fundamental wave laser beam generated from the laser medium. A pair of reflecting means forming a resonator, a nonlinear optical crystal element disposed in the laser resonator, and a polarization plane of the fundamental laser light output from one of the pair of reflecting surfaces of the laser resonator. And a control means for controlling the amount of phase delay in the laser resonator according to the output from the detecting means. 2. The laser light generator according to claim 1, wherein the control means controls the temperature of the optical components in the laser resonator to control the amount of phase delay of these optical components. . 3. An electro-optic crystal element is provided in the laser resonator, and the electro-optic crystal element is controlled by the control means to control the amount of phase delay in the laser resonator. 1. The laser light generator according to 1.