KR100284759B1 - Second harmonic generator - Google Patents

Abstract

The present invention describes a second harmonic generating device.

The second harmonic generator according to the present invention provides an optical resonance section of a predetermined distance, means for providing an incident path through which pumped light is incident, and an emission path through which harmonics generated in the resonance section are emitted, and installed in the resonance section. A gain medium generating a fundamental wave by the incident pumping light, two adjacent nonlinear single crystal element elements disposed in the resonance section and generating harmonics from the fundamental wave, between the gain medium and the nonlinear single crystal element. It is provided with the polarization control means provided in. In this case, the two adjacent single crystal elements (Extraordinary axis) The axis is configured to be 90 ° to each other to reduce the temperature change of the nonlinear single crystal device due to the phase-delay, thereby improving the physical and optical stability of the SHG.

Description

Second harmonic generator

1 is a schematic configuration diagram of a conventional second harmonic generator.

FIG. 2 is a view showing an arrangement structure of traveling light for a nonlinear single crystal device of the conventional second harmonic generator shown in FIG.

3 is a schematic structural diagram of a first embodiment of the present invention.

4 is a schematic structural diagram of a second embodiment of the present invention.

5 is a schematic structural diagram of a third embodiment of the present invention.

6 is a schematic structural diagram of a fourth embodiment of the present invention.

7 is a view showing a first example of an arrangement structure of a nonlinear single crystal device of a second harmonic generator according to the present invention.

8 is a view showing a second example of the arrangement structure of the nonlinear single crystal element of the second harmonic generator according to the invention.

The present invention relates to a second harmonic generator, and relates to a second harmonic generator capable of stable temperature control and improved operation stability.

Intracavity second harmonic generation (ISHG) is a method of oscillating blue and green visible light by doubling the optical frequency of low intensity continuous wave laser light using a nonlinear optical effect. This method can obtain green light sources from solid state lasers such as Nd (Neodymium) lasers, which are not capable of direct oscillation of visible light, and are particularly useful for small solid state lasers using diode pumping.

The ISHG has a structure in which a laser gain medium for oscillating a laser light of fundamental frequency and a nonlinear single crystal element for oscillating a second harmonic are installed inside a resonator.

In general, an ISHG resonator has two mirrors on a light propagation path that maintains a predetermined distance, and the mirror has a high reflection (HR) coating film that is transparent to pumping laser light and highly reflective to fundamental waves, among which One mirror (on the harmonic output side) has an AR (Anti Reflection) coating film, which is transparent to harmonics.

The second harmonic output obtained from the ISHG as described above has an incidental phenomenon caused by the nonlinear effect. This unwanted phenomenon is caused by short-term noise generated by SFG (Sum-frequency Generation) between the FLM (Fundamental Longitudinal Mode) oscillating simultaneously in the resonator and SFG between Polarization Mode. That is it.

The short term noise generally appears in the 10-100 KHz band, which poses a very disadvantageous problem for applications requiring stable light output. This is called the “Green Problem”, and the root of this problem is found in articles such as the Journal of Optical Society of America, B Vol 3, 1175-1180 (1986) and Physical Review A V4, 2778-2790 (1990). It became.

To address this “Green Problem,” several structures of ISHGs have been developed: Optics Letters V13 805-807 (1988), Optics Letters VIS 1141-1143 (1990), Journal of Quantum Electronics V28, 1164-1168 (1992) and Suggested by US Pat. No. 4,910,740 and the like.

Figure 1 schematically shows the ISHG of the proposed structure through the Journal of Quantum Electronics V28. Referring to this, a resonator is constructed by arranging two first and second mirrors 2 and 7 which are HR coated with respect to a fundamental wave, and a gain medium (Gain Medium, 3) The Brewster Plate 4 and the nonlinear single crystal element 5 are provided. A beam splitter 8 for separating a part of the harmonics outputted is provided at the rear of the second mirror, and the color filter 9 and the photo detector 17 are provided on the traveling path of the light separated by the beam splitter 8. And a temperature control device 6 is provided in the single crystal element 5, and an electric signal obtained from the photo detector 10 is provided as a reference value between the temperature control device and the photo detector 10. And a temperature control circuit 11 for controlling the temperature control device 6 according to the result of the comparison. In addition, a pumping laser light source 1 is provided in front of the first mirror 2 to supply pumping laser light into the resonator.

In general, the function of the Brewster plate installed inside the resonator is a polarizing element, whereas in the case of ISHG shown in FIG. 1, the Brewster plate has a function of mode selection in addition to the function of a simple polarizer. Have

According to the mode selection, only one of a plurality of cavity modes simultaneously oscillated in the laser oscillation band is selected to change to a single mode, thereby avoiding the green problem.

In order for only one specific cavity mode to resonate with a constant gain, the polarization of light passing through the Brewster plate 4 must be a p-polarization in which the polarization direction is parallel to the Incident plane of the Brewster plate, In other cases, light loss occurs when passing through the Brewster plate 4, and in the case of the conventional ISHG, when the loss exceeds the gain, the oscillation is stopped.

Looking at the problem of the conventional ISHG in detail as follows.

FIG. 2 shows the relationship between the optical axis direction of the nonlinear single crystal element used in the conventional ISHG shown in FIG. 1 and the incident surface (parallel to the p-polarization direction) of the Brewster plate.

In the case of Type II phase-matching, the second harmonic conversion efficiency is that the polarization direction of the fundamental beam is (Extraordinary axis) axis It is incident on the (Ordinary axis) in the 45 ° direction and maximized when the fundamental wave is perpendicular to the machined plane at the phase-matching angle (23.5 °). When incident on the single crystal element as described above, the polarization of the fundamental wave is Wow Separated by an axis. Axis Since the axes support different refractive indices, the components separated by the two axes are relatively phase-delayed as they pass through the single crystal device. The length of the single crystal element is ℓ, Axis After the first round-trip through the single crystal element, the index of refraction of the axis is n _e and n _o , and the wavelength λ of the fundamental wave. Wow The phase-delay (δφ) generated between the components separated by the axis is defined by the following equation.

In addition, when the linear p-polarized beam passing through the Brewster plate passes through the single crystal element 5 and passes through the Brewster plate again after one loud-trip, (Condition, δφ) As follows.

δφ = mπ, m = 0,1,2,... …

For example, δφ is 160π when the n _o , n _e measurements of the KTP single crystal already published in the paper, λ = 1 μm, and the single crystal length (l) are applied to 5 mm. When δ φ is not an integer multiple of π, circular or elliptical polarization results in a large optical loss when passing through the Brewster plate, resulting in an unstable SHG output.

Therefore, in order to make δφ to be a multiple of an integer, the value of (n _e -n _o ) must be adjusted by finely controlling the temperature of the single crystal element. In general, since the value of (n _e -n _o ) varies with a minute width according to the temperature change of the surrounding environment, a temperature control method is required as shown in FIG. 1 to obtain a stable SHG output.

The temperature control method required for this is to convert the SHG light obtained from the beam splitter 8 through the color filter 9 to convert the electrical signal into the photo detector 10 and then compare it with the reference level in the temperature control circuit 11. By driving the appropriate temperature control device 6, the single crystal element temperature is kept within a predetermined range so as to maintain a stable output. However, the temperature control of the single crystal device is accompanied by a difficulty that must be made within the range of 0.01 ℃.

An object of the present invention is to provide a second harmonic generating device capable of stable operation by enabling stable temperature control.

In order to achieve the above object, the second harmonic generator of the present invention provides an optical resonance section of a predetermined distance, and provides an incident path through which pumped light is incident and an emission path through which harmonics generated within the resonance section are emitted. Means, a gain medium provided in the resonance section for generating a fundamental wave by incident pumping light, two adjacent nonlinear single crystal elements in the resonance section for generating harmonics from the fundamental wave, and the gain medium And a polarization control means provided between the nonlinear single crystal element.

In the above-described ISHG of the present invention, the polarization control means may be a general Brewster plate, which is provided on the light transmission surface of the gain medium adjacent to the nonlinear single crystal element as another type. Can be acquired by the photo exit surface.

In the latter type in which the polarization control means is provided in the gain medium, the optical axis of the gain medium and the optical axis of the nonlinear single crystal element maintain a predetermined angle.

And in all of the types of the present invention SHG, the means for providing the optical resonance section can be obtained by two mirrors maintaining a predetermined distance, and as another type, at least one mirror is the gain medium or the It may be provided on one side of the nonlinear single crystal device. Mirrors provided in the gain medium and the nonlinear single crystal element are provided at the side where the pumping light is incident and at the side where the harmonic is emitted, that is, at both ends of the resonance section. The gain medium may be a single crystal device such as Nd: YAG, and the nonlinear single crystal device may be KTP.

In addition to the above structure, a means for temperature control of the nonlinear single crystal element may be provided, and the temperature control means separates a part of the harmonic output as in the conventional SHG package, converts it into an electrical signal, and obtains The result of comparing the current with the reference value may be fed back to the temperature control circuit to control the temperature of the nonlinear single crystal device.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

3 is a schematic configuration diagram of a first embodiment of the present invention, FIG. 4 is a schematic configuration diagram of a second embodiment of the present invention, FIG. 5 is a schematic configuration diagram of a third embodiment of the present invention, and FIG. A schematic diagram of a fourth embodiment of the invention, FIG. 7 shows a first example of the arrangement of a nonlinear single crystal element of a second harmonic generator according to the present invention, and FIG. 8 shows a second harmonic generator according to the invention. 2 is a view showing a second example of an arrangement structure of a nonlinear single crystal element.

[First Embodiment]

First, referring to FIG. 3, a resonator is configured by arranging two first and second mirrors 22 and 27 coated with HR on a fundamental wave to maintain a predetermined interval. Between the first mirror 22 and the second mirror 27, that is, inside the resonator, a gain medium such as Nd: YAG, Gain Medium 23, a Brewster plate 24 as a polarizing element, and KTP, etc. The same nonlinear single crystal element 25 is provided. The single crystal element 25 is composed of first and second single crystal element elements 25a and 25b of the same length and made of the same material, which characterizes the present invention, and the first single crystal element adjacent to the Brewster plate 24. AR coating surfaces 251a and 251b are formed on both sides of the elements 25a and 25b for fundamental and second harmonics.

A beam splitter 28 for separating a part of the harmonics output is provided at the rear of the second mirror, and the color filter 29 and the photo detector 210 are provided on the path of the light separated by the beam splitter 28. And a temperature control device 26 is provided in the single crystal elements 25a and 25b, and is obtained from the photo detector 210 between the temperature control device 26 and the photo detector 210. A temperature control circuit 211 is provided for comparing the electrical signal with a reference value and controlling the temperature control device 26 according to the result of the comparison. In addition, a pumping laser light source 21 such as a laser diode is provided in front of the first mirror 22 to supply pumping laser light into the resonator.

In the present invention SHG having the above structure, the present invention of the single crystal element 25 is characterized by two single crystal element elements 25a and 25b.

Second Embodiment

Referring to FIG. 4, a gain medium (Gain Medium, 33) such as Nd: YAG or the like, and a Brewster plate (34) as a polarizing element is provided inside the resonator to which the laser light from the pumping laser light source 31 is supplied. And nonlinear single crystal elements 35 such as KTP are provided on the optical axis of one resonator.

The single crystal element 35 is characterized by the present invention and is composed of first and second single crystal element elements 35a and 35b of the same length and made of the same material, and the first single crystal element adjacent to the Brewster plate 34. AR coated surfaces 351a and 351b are formed on both sides of the element 35a and on the front surface of the second single crystal element element 35b for fundamental and second harmonics. Then, one surface of the gain medium facing the pumping laser source 31 is processed into a curved surface and the opposite surface is processed into a plane perpendicular to the optical axis, wherein the curved surface of the gain medium is subjected to HR coating for fundamental waves. The first mirror 32 is provided, and the second mirror 37 which is HR coated is provided on the harmonic emission surface of the second single crystal element element 35b.

And, behind the second mirror 37, a beam splitter 38 for separating a part of the harmonics outputted is provided, and the color filter 39 and the photo detector on the traveling path of the light separated by the beam splitter 38. 310 is provided, and the nonlinear single crystal element 35 is provided with a temperature control device 36, and between the temperature control device 36 and the photo detector 310 from the photo detector 310. A temperature control circuit 311 is provided for comparing the obtained electrical signal with a reference value and controlling the temperature control device 36 according to the result of the comparison.

Third Embodiment

Referring to FIG. 5, two first and second mirrors 42 and 47 which are HR coated with respect to the fundamental wave are arranged to maintain a predetermined interval to supply laser light from the laser pumping light source 41. The receiving resonator is configured. Between the first mirror 41 and the second mirror 47, i.e., inside the resonator, a gain medium (Gain Medium, 43) such as Nd: YAG, a nonlinear single crystal element 45 such as KTP, It is provided to maintain a predetermined angle. On one surface of the gain medium 23 opposite to the nonlinear single crystal element 25, an inclined surface for maintaining a so-called Brewster angle, that is, an inclined surface 44 as a polarizing element is provided.

An AR coating surface 43a is formed with respect to the fundamental wave on one surface (perpendicular to the optical axis) of the gain medium opposite the first mirror 42. In addition, AR coating surfaces 451a and 451b are formed on both sides of the single crystal element elements 45a and 45b adjacent to the gain medium 43 for fundamental and second harmonics.

And, behind the second mirror 47, a beam splitter, a color filter, a photo detector, a temperature control circuit and a temperature control device of the same type as in the first and second embodiments are provided.

Fourth Embodiment

Referring to FIG. 6, inside the resonator to which the laser light from the pumping laser light source 51 is supplied, a nonlinear single crystal element 55 such as gain medium (Gain Medium, 53) such as Nd: YAG, KTP, etc. It is provided on the optical axis inclined by predetermined angle. An inclined surface 54 for maintaining a so-called Brewster angle is provided on one surface of the gain medium 53 opposite to the nonlinear single crystal element 55.

The nonlinear single crystal element 55 is divided into first and second nonlinear single crystal element elements 55a and 55b as characterizing the present invention. Then, one surface of the gain medium facing the pumping laser light source 51 is processed into a curved surface, and the first mirror 52 is provided by HR coating treatment for the fundamental wave, and the second single crystal element element is provided. A second mirror 57 which is HR coated on the fundamental wave and AR coated on the harmonic is provided on the harmonic emission surface of 55b. An AR coating surface 551a is formed on the incident surface of the second single crystal element 55b with respect to the fundamental wave and the second harmonic wave.

And, behind the second mirror 57, beam splitters, color filters, photo detectors, temperature control circuits, and temperature control devices of the same type as in the first and second embodiments are provided.

The embodiments of the present invention described above have an array structure of nonlinear single crystal elements as shown in FIG. 7 and FIG.

Referring first to FIGS. 7 and 8, the single crystal element elements 25a, 35a, 45a, 55a (25b, 35b, 45b, 55b) are (Extraordinary axis) axis Place the (Ordinary axis) axis and fundamental wave polarization at a 45 ° angle, and align the face of each element perpendicular to the incident beam. And the single crystal element elements 25a, 35a, 45a, 55a. Of the axes and the single crystal element elements 25b, 35b, 45b, 55b. Arrange 90 degrees between the axes.

In the case of the single crystal element elements 25a, 35a, 45a, 55a (25b, 35b, 45b, 55b) in the arrangement shown in FIG. 7, the beams of the fundamental wave are first single crystal element elements 25a, 35a, 45a, 55a. Accumulated through (Extraordinary axis) axis The phase-delay δφ between the (Ordinary axis) axis components is canceled when passing through the second single crystal element elements 25b, 35b, 45b, 55b, and the second single crystal element element 25b even in the case of a beam traveling from the opposite side. The phase-delays generated when passing through 35b, 45b, and 55b are canceled by the first single crystal element elements 25a, 35a, 45a, and 55a. Therefore, when the beam of the fundamental wave passes through the single crystal element and completes one round-trip, the total round-trip delay becomes zero (零), resulting in no change in the polarization state.

The same phenomenon described above also occurs in the case of the single crystal element elements 25a, 35a, 45a, 55a (25b, 35b, 45b, 55b) in the arrangement shown in FIG. In the case of type II phase-matching, in order to enable such phase-delay cancellation, the two single crystal element elements must be the same length and compositional composition as described above. When these conditions are met, changes in ambient temperature will not be a source of SHG output instability.

Claims (7)

Means for providing an optical resonant section of a predetermined distance and providing an incidence path through which pumped light is incident, an outgoing path through which harmonics are generated within the resonant section, and a pumping light installed in the resonant section A second harmonic generating device having a gain medium generating a wave, a nonlinear single crystal element disposed in the resonance section for generating harmonics from the fundamental wave, and polarization control means provided between the gain medium and the nonlinear single crystal element. The nonlinear single crystal device of claim 1, wherein the nonlinear single crystal device comprises a first nonlinear single crystal device element and a second nonlinear single crystal device element, wherein the first nonlinear single crystal device element and the second nonlinear single crystal device element (Extraordinary axis) The second harmonic generator, characterized in that the angle between the axes to maintain 90 °. 2. The method of claim 1, wherein the angles of the nonlinear single crystal element elements Axis (Ordinary axis) The second harmonic generator, characterized in that the axis is maintained 45 ° with respect to the polarization direction of the fundamental wave. The second harmonic generating device according to claim 2, wherein the nonlinear single crystal element has the same composition and the same length. The second harmonic generating device according to any one of claims 1 to 3, wherein the means for providing the light resonance section is provided by a coating layer formed on the gain medium and the nonlinear single crystal element. The method of claim 1, wherein the means for providing the optical resonance section is formed on one curved surface of the medium facing the pumping laser light source, the coating layer having a high reflectivity with respect to the fundamental wave and the non-linear single crystal element A second harmonic generator, which is formed on the emission surface of the second harmonic, and has a coating layer having a high reflection rate for the fundamental wave and a low reflection rate for the harmonic wave. The second harmonic generator according to claim 1, wherein the polarization control means is a Brewster plate interposed between the gain medium and the nonlinear single crystal element. 2. The polarization control means according to claim 1, wherein the polarization control means forms a Brewster angle on the gain medium and is provided by an inclined surface that is inclined at a predetermined angle to the optical axis of the gain medium and the optical axis of the nonlinear single crystal element. Harmonic Generator.