KR20090004685A - Laser light second harmonic generator - Google Patents

Abstract

A generation apparatus for a second harmonic wave of laser light is provided to generate laser beam in different wavelength to the wave direction. A laser beam light source(1) generates laser beam of the fundamental component. The non-linear optical crystal(6) has the first section and the second end face. The laser light of the fundamental component is condensed from the concentration optic system. The laser light is incident in the non-linear optical crystal. The reflector systems(8) returns the second harmonic wave component generated in the nonlinear crystal and the beam mixing the fundamental component through the second end face within the nonlinear crystal. A beam splitter(4) takes out the laser beam of the second harmonic wave component from which is outputted from the first section second harmonic wave component which is not wavelength-converted fundamental component at least.

Description

Laser light second harmonic generator {GENERATION APPARATUS FOR THE SECOND HARMONIC WAVE OF LASER LIGHT}

The present invention injects a laser beam output from a laser resonator or its amplifier in the periodic direction of a periodic polarization structure of a nonlinear crystal for converting wavelengths having a stoichiometric composition forming a periodic polarization structure, and a laser having a wavelength different from the fundamental wave in the propagation direction. Provided are a high efficiency, high stability harmonic generator and a converter for generating light.

In the electronic industry, the miniaturization of DRAM, SRAM, and the like has progressed year by year, and the optical integration of internal circuits has been achieved accordingly. Moreover, high density recording in the field of information recording is preferable. In response to such high integration, high density, and miniaturization, in laser application apparatuses such as laser processing and surface inspection, continuous output and short wavelength of the laser beam are preferable, and long-term stability is also required. M ² to indicate the degree of condensing quality of the beam The parameter is not limited to 1, which is the value of a single transverse mode, but near quality is required. Non-linear optical crystals are used for oscillation of short-wavelength lasers, and non-linear single crystals are installed in the laser optical path to obtain harmonic components.Also, by applying an electric field to the non-linear optical crystals, a polarization of a spatial periodic structure is provided, thereby providing periodic polarization. It has been reported that harmonic generators form crystals, form waveguides of light in the periodic direction of the periodic structure, introduce a laser beam from the outside into the waveguide, and convert the incident beam into harmonics as the waveguide propagates.

The wavelength conversion efficiency of such a device is high in peak laser power, so nonlinear phenomena are likely to occur, and high conversion efficiency is realized. However, the wavelength conversion efficiency from the continuous oscillation output remained low. For this reason, conventionally, a resonator is formed in the continuous laser output optical path, the length of the resonator is controlled by a piezo element or the like so as to resonate with the incident beam of the fundamental wave, and the wavelength is converted into a nonlinear optical element provided in the resonator. 2 Harmonic generators or optical parametric oscillators are designed. However, this configuration entails the complexity of the control system for controlling the fundamental wave beam in the phase resonance state of the propagation beam in the resonator, thus making it difficult to ensure stability.

  <Patent Document 1> US Patent No. 6,654,392

Patent Document 2: US Patent No. 5,800,767

<Patent Document 3> US Patent No. 5,838,486

<Patent Document 4> Japanese Patent Laid-Open No. 1999-212127

<Patent Document 5> Japanese Patent Application Laid-Open No. 2004-191963

<Patent Document 6> Japanese Patent Application Laid-Open No. 2003-15176

<Patent Document 7> Japanese Patent Laid-Open No. 2005-150252

The problem to be solved in the present invention is to achieve high conversion efficiency, structure simplification, and output stabilization in a harmonic generation device or a wavelength conversion device of a laser device.

The problem to be solved in the present invention is to achieve high conversion efficiency, structure simplification, and output stabilization in a harmonic generation device or a wavelength conversion device of a laser device.

(Means to solve the task)

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, this invention is a laser light 2nd harmonic generator, The laser light source which produces the laser light of a fundamental wave component, and the nonlinear optical crystal which converts the laser light of the said fundamental wave component into 2nd harmonics. It has a periodic polarization structure having a first cross section and a second cross section facing each other and having a periodic direction in the axial direction perpendicular to the first cross section and the second cross section, the polarization direction in a direction perpendicular to this axial direction. A nonlinear optical crystal having a periodic polarization structure, a condensing optical system that condenses the laser light of the fundamental wave component, and enters the nonlinear optical crystal, a second harmonic component generated in the nonlinear crystal radiated from a second cross section; A reflection optical system for returning the beam mixed with the fundamental wave components into the nonlinear crystal through a second cross section, and the nonlinear crystal And a beam splitter for extracting a laser beam of at least the second harmonic component from the second harmonic component outputted from the first end face and the fundamental wave component not wavelength-converted after passing through the reciprocating optical path therein. .

By passing through the reciprocating optical path in the nonlinear crystal, the heat generation by the laser light is uniform, and the temperature distribution is uniform in the nonlinear optical crystal, so that the wavelength conversion matching conditions are maintained under the same conditions in the longitudinal direction. Therefore, there is an effect that the efficiency of conversion to harmonics can be improved and stabilized, and the device can be miniaturized.

An optical isolator is provided between the laser light source and the nonlinear optical crystal. By providing a light isolator, the oscillating beam which has passed becomes the light reflected and returned from the optical system provided thereafter, and has an effect that it does not enter a laser light source and cause disturbance to an oscillating beam.

Moreover, the temperature control means was provided for the temperature stabilization of the said nonlinear optical crystal. The reciprocating optical path in the nonlinear optical crystal is coaxial. In addition, the nonlinear optical crystals doped with MgO or ZnO having a periodic polarization structure (PPLT (periodically poled LiTaO ₃ : periodically polarized lithium tantalite), PPSLT (periodically poled stoichiometric LiTaO ₃₎ : Periodically polarized stoichiometric lithium tantalite), PPLN (periodically poled LiNbO ₃₎ : Periodically polarized lithium niobate) or PPSLN (periodically poled stoichiometric LiNbO ₃₎ : Stoichiometrically polarized lithium niobate).

The laser light source may be a laser light source for generating continuous oscillation light. Since the laser light source is a continuous wave, it is possible to generate second harmonics with high efficiency and stabilization even in the continuous wave.

In addition, the laser light source is characterized in that a fiber laser oscillator or a fiber amplifier for outputting the oscillating light of a single wavelength spectrum (spectrum). In addition, the wavelength of the continuous oscillation light is characterized by a single wavelength of 700nm or more and 1100nm or less.

Moreover, the said nonlinear optical crystal is characterized by having a waveguide structure. By providing a waveguide structure, the fundamental wave beams confined in the waveguide can maintain high intensity and propagate in the nonlinear optical crystal to lengthen the interaction length, thereby improving the efficiency and stabilization of the conversion efficiency to harmonics. We can plan.

On the other hand, the present invention is a device for converting the laser light of the fundamental wave component generated from the laser light source to the second harmonic, the first cross section and the first cross-section facing each other to convert the laser light of the fundamental wave component to the second harmonic A nonlinear optical crystal having a periodic polarization structure having two cross sections, having a periodic direction in an axial direction perpendicular to the first and second cross sections, and having a polarization direction in a direction perpendicular to the axial direction; And a condensing optical system for condensing laser light of the fundamental wave component and incident the nonlinear optical crystal, and a beam in which the second harmonic component and the fundamental wave component generated in the nonlinear crystal radiated from a second cross section are mixed. After passing through the reflecting optical system for returning into the nonlinear crystal through the cross section and the reciprocating optical path in the nonlinear crystal, And a beam splitter configured to extract a laser beam of at least the second harmonic component from the second harmonic component outputted from the fundamental component and the fundamental wave component not wavelength-converted. By passing through the reciprocating optical path in the nonlinear crystal, the heat generation by the laser light is uniform, and the temperature distribution is uniform in the nonlinear optical crystal, so that the wavelength conversion matching conditions are maintained under the same conditions in the longitudinal direction. Therefore, there is an effect that the efficiency of conversion to harmonics can be improved and stabilized, and the device can be miniaturized.

In addition, the nonlinear optical crystal is a PPLT (periodically poled LiTaO ₃ doped with MgO or ZnO having a periodic polarization structure). : Periodically polarized lithium tantalite), PPSLT (periodically poled stoichiometric LiTaO ₃₎ : Periodically polarized stoichiometric lithium tantalite), PPLN (periodically poled LiNbO ₃₎ : Periodically polarized lithium niobate) or PPSLN (periodically poled stoichiometric LiNbO ₃₎ : Stoichiometrically polarized lithium niobate).

The laser light second harmonic generator or laser light wavelength conversion device of the present invention has the effect that the efficiency of the conversion to harmonics can be improved and stabilized, and the device can be miniaturized.

1 shows an embodiment of the present invention. It is very suitable for the laser light source 1 to be a continuous oscillation laser. The laser light having a fundamental wave wavelength for converting into harmonics is output from the laser light source 1.

When making the laser light source 1 continuous oscillation, the structure as shown in the Example of FIG. 2 can be used. In FIG. 2A, the oscillation output from the distributed feedback laser (DFB) laser 30 having controlled oscillation wavelength is incident on the amplifying fiber 32. The amplification fiber 32 is coupled with excitation power from the laser diode 31 for excitation. The output power 33 amplified by this amplified fiber can be used as a fundamental wave laser light source subjected to wavelength conversion.

In order to polarize the output of a continuous oscillation fiber laser in a clear direction, it can be realized by giving the fiber itself which emits a laser output the function of polarizing output and polarization preservation, The structure using it can use the structure similar to FIG. 2B. One or more Bragg gratings are formed at at least one position in the vicinity of both ends 35 and 36 of the amplifying fiber 38 as reflection elements for wavelength selection, and excitation from the laser diode 37 for excitation. Power is coupled to the amplifying fiber 38. The output power 39 is oscillated continuously at the oscillation wavelength determined by Bragg grating. If necessary, power amplification can be achieved by separately using an amplification fiber (not shown). Yb-added fibers are used for such amplified fiber 32 and amplified fiber 38. The wavelength can be selected from the range of 978 nm-1100 nm.

For the generation of the fundamental wave beam, it is possible to use a laser light source and an amplifier whose gain medium is a titanium doped sapphire crystal having a wavelength variable characteristic. It is known that a high beam quality is preferable for realizing a high efficiency wavelength conversion by a nonlinear optical crystal, and the maximum conversion efficiency is acquired when it has a single frequency spectrum. The oscillation wavelength is variable from 700 nm to 1000 nm. It goes without saying that even when such a light source is operated in a pulse mode, high wavelength conversion efficiency can be obtained by cyclic polarization inversion nonlinear crystal.

The output beam 9 from the laser light source 1 enters the light isolator 2. Although the optical isolator 2 may not be provided, it is preferable to provide so that the oscillation beam which has passed becomes the light reflected and returned from the optical system provided afterwards, so that the oscillation beam may not enter the laser light source 1 and cause disturbance to the oscillation beam. Do. The oscillation beam 10 which has passed through the optical isolator 2 controls the linearly polarized direction of the oscillation beam 10 by the 1/2 wave plate 3, and sets it as the beam 11. The half-wave plate 3 is provided to align the reciprocating optical path formed in the periodically polarized nonlinear optical crystal 6 provided thereafter with a predetermined polarization direction. In the periodically polarized nonlinear optical crystal 6, when the polarization direction is formed in the direction perpendicular to the ground, the polarization direction is vertical in the direction perpendicular to the ground as shown by a double circle in order to produce a nonlinear optical action with maximum efficiency. That is, the polarization direction and the polarization direction need to be defined in parallel. 3 shows this relationship. The traveling direction 51 of the beam of the nonlinear optical element 6 is parallel to the periodic direction 52 of polarization, and the polarization direction 61 is parallel to the polarization direction 62. The fundamental wave beam passes through the beam splitter 4 inclined at 45 ° with respect to the optical path, and is also focused on the condenser lens 5 to form a fine condensing spot.

In the case where the waveguide is formed in the nonlinear optical crystal 6, a condensing spot is formed on the first end face 6-1 of the nonlinear optical crystal 6, and the fundamental wave beam confined in the waveguide is high. The intensity is maintained to propagate within the nonlinear optical crystal 6. While the waveguide propagates the waveguide, a part of the fundamental wave component is converted into the second harmonic and directed to the second end face 6-2 of the nonlinear optical crystal 6. From the second end face 6-2, the fundamental wave component and the second harmonic component are emitted into the space as the divergent beam. The divergent beam is reflected by a reflecting optical system 8 such as a concave reflector, and returned to the second end face 6-2 of the nonlinear optical crystal 6. The reflection characteristics of the reflection optical system 8 have high reflection characteristics with respect to two wavelengths of the fundamental wave and the second harmonic wave. In the case where the waveguide is formed and the concave reflecting mirror is used as the reflecting optical system 8, the radius of curvature is defined as the emission point of the divergent beam in the second end face 6-2 of the nonlinear optical crystal 6 The distance from the reflecting surface of the concave reflecting mirror is selected so as to efficiently reflect back to the waveguide in the nonlinear optical crystal 6. In the case of using a nonlinear optical crystal without a waveguide, the condenser lens 5 and the reflective optical system 8 are disposed so that the fundamental wave has a condensing point near the center of the crystal.

The nonlinear optical crystal is a polymerically poled LiTaO ₃ doped with MgO or ZnO having a periodic polarization structure. : Periodically polarized lithium tantalite), PPSLT (periodically poled stoichiometric LiTaO ₃₎ : Periodically polarized stoichiometric lithium tantalite), PPLN (periodically poled LiNbO ₃₎ : Periodically polarized lithium niobate) or PPSLN (periodically poled stoichiometric LiNbO ₃₎ : Periodically polarized stoichiometric lithium niobate) is very suitable.

When the nonlinear optical crystal is MgO: PPSLT or MgO: PPSLN, many of them have the effect of absorbing the power of the fundamental wave through the light when the wavelength of the second harmonic is shorter than 532 nm (green light) or the like. Indicates. Therefore, when the optical arrangement is constituted by a single path as in the prior art, the absorption power in the beam at the incident end face side and the exit end face side of the nonlinear crystal is different, so that the temperature distribution occurs in the crystal axis direction. This causes the deviation from the temperature phase matching condition, and the wavelength conversion efficiency is lowered. The temperature in the nonlinear crystal can be controlled to a temperature range of about +/- 0.1 DEG C using well-known electronic cooling (ETC) technology or a heater.

The beam 16, coming out of the output end 6-1 of the first section via the reciprocating optical path and including the second harmonic component and the fundamental wave component not wavelength-converted, passes through the condensing lens 5 again to form a parallel beam ( 17), and is selectively separated and reflected from the fundamental wave component by the beam splitter 4 reflecting at least the second harmonic component with a high reflectance, and taken out to the outside as the harmonic beam 18, which is provided for the purpose of use. . The fundamental wave component passes through the beam splitter 4, and when the isolator 2 is provided, it is excluded as a component having a different polarization direction from the inside so that the isolator 2 does not return to the laser light source 1. do.

In this improved reciprocating beam passing method, the calorific value associated with light absorption at each location along each longitudinal direction of the nonlinear optical crystal 6 is averaged because of the reciprocating path, so that a uniform temperature distribution is formed. That is, in the vicinity of the first end face 6-1, the incident fundamental wave beam is strong and the transform beam is weak, whereas in the return beam, the fundamental wave beam is weak and the transform beam is strong, while the second end face 6-2 is provided. The calorific value in the vicinity of is reduced in the fundamental wave component with the increase of the second harmonic component in the incidence direction, and the same in the feedback light. The heat generated by absorption generated by each wavelength component by the place is equalized in the entire waveguide region. Therefore, since the temperature distribution in the periodically polarized nonlinear optical crystal is uniform, wavelength conversion matching conditions are maintained under the same conditions in the longitudinal direction, thereby enabling highly efficient wavelength conversion over the entire length of the periodically polarized nonlinear optical crystal. It is clear that the uniformity of the temperature distribution can be realized much more stably and easily than the conventional method of passing only one direction, and the economic effect of high conversion efficiency, output stabilization, cost effect resulting from the ease of device construction, etc. Compared with the structure of the conventional single path, many effects are obtained. By making the reciprocating path into the coaxial position, the temperature distribution also becomes the distribution of the concentric shape, which is effective in stabilizing the beam.

In addition, in the example of FIG. 1, although the structure which reflects to the 2nd end surface 6-2 of the nonlinear optical element 6 by using a concave mirror to the reflecting optical system 8 is employ | adopted, instead of a convex lens and a plane mirror It is also possible to configure. In this case, a convex lens for parallelizing the beams is disposed so as to coincide with the second end face 6-2 of the focal position thereof (waveguide), and a flat mirror for reflecting the parallel beam with the convex lens is provided. do. Moreover, even if the laser light source 1 is applied to a pulse laser and a wavelength tunable laser instead of a continuous oscillation laser, an effect is acquired.

     <Example>

MgO: PPSLT was employed for the nonlinear optical crystal, and the length of the crystal (the length between the first cross section and the second cross section) was 40 mm. A second harmonic generator according to the present invention for injecting a laser light from a laser light source having a single frequency of 1064 nm and a wavelength of 1064 nm as a single-mode fiber laser and its amplifier configuration into a reciprocating optical path. Using the configuration, 3.7 W was obtained as a single frequency, single mode, laser output of wavelength 532 nm. The conversion efficiency to the second harmonic was 39% in optical power conversion efficiency. Therefore, a high value was obtained in the wavelength conversion efficiency of the continuous oscillation output.

The beam quality of the second harmonic in the above configuration is actually M ² Meter using M ² If the measured value, the direction of x M ² = 1.12, y direction of M ² = 1.13 were obtained. On the condition that the conversion power when only one optical path passed was low, the value was M ² = 1.23 in the x direction and M ² = 1.19 in the y direction. Therefore, in the present invention in which both M ² in the x direction and the y direction use a reciprocating optical path, second harmonics of high quality are obtained and high conversion efficiency is realized.

According to the present invention, a laser beam emitted from a fiber laser forms a reciprocating optical path in a periodically polarized nonlinear optical crystal such as PPLT, PPSLT, PPLN or PPSLN doped with MgO or ZnO, and a fundamental wave laser is formed from an incident cross section. The laser wavelength conversion which introduces a beam and makes the wavelength-converted harmonic the same cross section as the output cross section improves the efficiency and stabilization of the conversion efficiency of a laser beam, miniaturizes the apparatus, and obtains a practical harmonic continuous oscillation beam. Provide the device.

In the wavelength conversion device of the present invention, a reciprocating optical path is formed in a waveguide of a periodically polarized nonlinear optical crystal, and a wavelength conversion continuous oscillation output can be obtained with high efficiency for the laser beam of continuous oscillation. Compared with the short wavelength laser oscillator which conventionally required a large power drive using krypton or argon ion gas laser to obtain a short wavelength continuous laser, the power efficiency can be greatly improved, and the device can be miniaturized and the short wavelength It can be widely used for laser applications such as laser micromachining using a continuous oscillation output, fine pattern exposure, high precision fine inspection, high density information recording, and the like.

Some embodiments of the present invention have been described above. It is clear that changes can be made to these without departing from the technical idea of the invention described in the claims.

As an application of the present invention, the fundamental wave output of continuous oscillation can be obtained with a short wavelength of the second harmonic and with high efficiency, so that the circuit element of the conductive link of the redundancy circuit of the laser micromachining apparatus, the silicon wafer of the semiconductor memory, Removal and cutting in layers of multi-layered electronic devices, correction of defects in liquid crystal display panels and plasma display devices, trimming of capacitors, resistances, and induction coefficients formed on substrate surfaces, trimming of circuit board functions, and the like. It can be applied to laser precision machining of semiconductor substrates, DVD mastering, defect inspection of silicon wafers, reticle defect inspection for lithography and the like. This makes it possible to realize high-efficiency wavelength conversion by continuous oscillation, and to perform operations that have been performed at the conventional pulse output at the continuous output, thereby improving work efficiency and simplifying the detection of very small defects. Since it became feasible, the contribution of the conventional improvement is large.

1 is an explanatory diagram of a harmonic generating device according to the present invention.

2 is a diagram showing an example of a fiber laser and a fiber amplifier device for generating a high quality beam used as a laser light source in the present invention.

3 is a diagram illustrating a relationship between a periodic direction, a polarization direction, a polarization direction, and a beam propagation direction in a nonlinear optical crystal having periodic polarization.

  <Explanation of symbols for the main parts of the drawings>

1 laser light source 2 light isolator

3 1/2 wave plate

4 beam splitter 5 condensing lens

6 Nonlinear Optical Crystals with Periodic Polarization

6-1 First cross section (of nonlinear optical crystal)

6-2 2nd cross section

8 reflection optics

9 output beam 10 oscillation beam

11 beam

16 Beam with second harmonic component and fundamental wave component not wavelength converted

17 parallel beams

18 harmonic beams

30 DFB lasers

31, 37 laser diode for excitation

32, 38 amplified fiber

33, 39 output power

35, 36 Bragg Grating

51 Beam direction

52 cycle direction (polarized)

61 Polarization Direction 62 Polarization Direction

Claims (11)

A laser light source for generating laser light of a fundamental wave component, A nonlinear optical crystal for converting laser light of the fundamental wave component into second harmonics, having a first cross section and a second cross section facing each other, and having a periodic direction in an axial direction perpendicular to the first cross section and the second cross section. A nonlinear optical crystal having a periodic polarization structure and having a periodic polarization structure having a polarization direction in a direction perpendicular to the axial direction, A condensing optical system for condensing the laser light of the fundamental wave component and making it enter the nonlinear optical crystal; A reflection optical system for returning a beam in which the second harmonic component and the fundamental wave component generated in the nonlinear crystal radiated from the second cross section into the nonlinear crystal through a second cross section, A laser having a beam splitter for extracting a laser beam of at least said second harmonic component from said second harmonic component and said fundamental wave component not wavelength converted after passing through a reciprocating optical path in said nonlinear crystal; Optical second harmonic generator. The method of claim 1, The laser light second harmonic generator which provided the optical isolator between the said laser light source and the said nonlinear optical crystal. The method according to claim 1 or 2, A laser light second harmonic generator provided with a temperature control means for temperature stabilization of the nonlinear optical crystal. The method according to any one of claims 1 to 3, A laser light second harmonic generator in which the reciprocating optical path in the nonlinear optical crystal is coaxial. The method according to any one of claims 1 to 4, The nonlinear optical crystals are doped with MgO or ZnO having a periodic polarization structure (PPLT (periodically poled LiTaO ₃ : periodically polarized lithium tantalite), PPSLT (periodically poled stoichiometric LiTaO ₃₎ : Periodically polarized stoichiometric lithium tantalite), PPLN (periodically poled LiNbO ₃₎ : Periodically polarized lithium niobate) or PPSLN (periodically poled stoichiometric LiNbO ₃₎ : Periodically polarized stoichiometric lithium niobate). The method according to any one of claims 1 to 5, The laser light second harmonic generating device wherein the laser light source is a laser light source for generating continuous oscillation light. The method of claim 6, And a fiber laser oscillator or fiber amplifier, wherein said laser light source outputs oscillating light of a single wavelength spectrum. The method according to claim 6 or 7, A laser light second harmonic generating device having a wavelength of 700 nm or more and a single wavelength of 1100 nm or less. The method according to any one of claims 1 to 8, The second harmonic generator of laser light, wherein the nonlinear optical crystal has a waveguide structure. An apparatus for converting laser light of a fundamental wave component generated from a laser light source into second harmonics, Converting the laser light of the fundamental wave component into second harmonics, and having a first cross section and a second cross section facing each other, and having a periodic polarization structure in a axial direction perpendicular to the first cross section and the second cross section; A nonlinear optical crystal having a periodic polarization structure having a polarization direction in a direction perpendicular to the axial direction, A condensing optical system for condensing the laser light of the fundamental wave component and making it enter the nonlinear optical crystal; A reflection optical system for returning a beam in which the second harmonic component and the fundamental wave component generated in the nonlinear crystal radiated from the second cross section into the nonlinear crystal through a second cross section, A laser having a beam splitter for extracting a laser beam of at least said second harmonic component from said second harmonic component and said fundamental wave component not wavelength converted after passing through a reciprocating optical path in said nonlinear crystal; Optical wavelength conversion device. The method of claim 10, The nonlinear optical crystals are doped with MgO or ZnO having a periodic polarization structure (PPLT (periodically poled LiTaO ₃ : periodically polarized lithium tantalite), PPSLT (periodically poled stoichiometric LiTaO ₃₎ : Periodically polarized stoichiometric lithium tantalite), PPLN (periodically poled LiNbO ₃₎ : Periodically polarized lithium niobate) or PPSLN (periodically poled stoichiometric LiNbO ₃₎ : Periodically polarized stoichiometric lithium niobate).

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