JP6508084B2 - Wavelength conversion optical device and laser device - Google Patents

Description

  The present invention relates to a wavelength conversion optical device and a laser device using a quasi phase matching (QPM) element that converts a fundamental wave into a third wave or a fourth wave with a single element.

  The QPM element is composed of a wavelength conversion element in which a periodically poled structure is formed. There is known a method of converting a fundamental wave into a third harmonic (third harmonic) in this QPM element. This method wavelength-converts the fundamental wave into a second harmonic (second harmonic) by a second harmonic converter (SHG unit) formed of a first nonlinear crystal, and converts the second harmonic by the second harmonic converter. And the fundamental wave not converted to the second harmonic wave by the second harmonic wave converter are generated as a sum frequency by the third harmonic wave converter (THG unit) composed of the second nonlinear crystal.

  However, in the case where the second harmonic conversion unit and the third harmonic conversion unit are included in series in one element, and the temperature of the entire element is uniformly adjusted, the second harmonic conversion unit and the third harmonic conversion are caused by the dispersion of fundamental wavelength. If the units do not simultaneously phase match, measures are required.

  In such a case, the refractive index is changed using parameters other than the temperature of the entire element and the temperature, and both the second harmonic conversion unit and the third harmonic conversion unit are phase-matched together at a given wavelength. A method can be considered. As a method for converting the refractive index, voltage application is known (Patent Document 1 to Patent Document 5).

  Patent Document 6 discloses a QPM waveguide in which a proton exchange optical waveguide 105 is formed on the surface as shown in FIGS. 7 and 8. In FIG. 7, a voltage is uniformly applied to the core portion by the electrode 110 regardless of the polarization inversion direction. In FIG. 8, the electrode 110 is formed such that the direction of voltage application changes in accordance with the direction of the polarization direction of the polarization inversion unit 104. In any of the examples, when a voltage is applied to the crystal of the polarization inversion unit 104, a change in refractive index occurs to adjust the phase matching condition.

  Further, in Non-Patent Document 1, as shown in FIG. 9, the third harmonic wave conversion part THG is formed in a fanned out polarization inversion pattern, and the element is shifted in the vertical direction to change the polarization inversion period, The second harmonic wave conversion unit SHG and the third harmonic wave conversion unit THG can be simultaneously phase-matched in any part.

  Further, in Patent Document 7, as shown in FIG. 10, phase adjustment conditions are satisfied by adjusting, for example, the pressure and / or temperature that uniformly acts on two joined nonlinear optical crystals 20A and 20B. A wavelength converter is described having pressure adjusting means 32 for adjusting.

Japanese Patent Application Laid-Open No. 3-283686 Unexamined-Japanese-Patent No. 5-100266 Unexamined-Japanese-Patent No. 5-142608 Japanese Patent Laid-Open No. 5-289135 JP 6-273816 A JP 7-270632 A JP, 2010-271633, A

Junji Hirohashi et al .: PPMgSLT device for cascaded 355 nm generation

  In Patent Documents 1 to 6, when a voltage is applied to the crystal, the refractive index changes and the phase matching condition is adjusted. However, when a voltage is uniformly applied to the periodical polarization inversion portion, the change in refractive index changes depending on the polarization direction, so that reflection occurs at the interface where the polarization changes and a loss occurs. In order to eliminate this loss, it is necessary to reverse the voltage application direction for each polarization, but fine processing is necessary, and a short circuit between electrodes is likely to occur.

  In Non-Patent Document 1, in order to obtain high output, it is often necessary to increase the diameter of the beam and to increase the working length. In this case, since the polarization inversion period changes continuously in the direction perpendicular to the optical axis in the element, the efficiency of the fundamental wave component that does not contribute to phase matching decreases as the beam diameter increases. In addition, although the allowable range of the phase matching temperature characteristic becomes narrower as the working length is longer, the efficiency is lowered because one element in the device in which phase matching is strictly performed is one point in the direction perpendicular to the optical axis.

  The object of the present invention is to provide a wavelength conversion optical device and a laser device which have no loss due to reflection at an interface, and can avoid the decrease in efficiency with a large beam diameter and a long working length.

  In order to solve the above-mentioned subject, the present invention has a first periodical polarization inversion part which consists of a parallel polarization inversion pattern having a first polarization inversion period, and a second harmonic conversion which converts a fundamental wave to a second harmonic wave. And a second periodical polarization inversion unit comprising a parallel polarization inversion pattern arranged in series in the traveling direction of light and the second harmonic wave conversion unit and having a second polarization inversion period shorter than the first polarization inversion period And arranged on one surface of the pseudo phase matching element, and disposed in series, the pseudo phase matching element including a third harmonic conversion unit that generates a third harmonic based on the fundamental wave and the second harmonic. Temperature adjustment element for adjusting the temperature of the second harmonic wave converter and the third harmonic wave converter, the second harmonic wave converter disposed in series on the other surface of the pseudo phase matching element, and A pressure application unit for applying a pressure to the third harmonic wave conversion unit; Characterized in that to simultaneously phase matching the second harmonic conversion unit and the third harmonic conversion unit by changing the pressure of the temperature and the pressure applying part of the settling device.

  The present invention also provides a double wave conversion unit having a first periodical polarization inversion unit consisting of parallel polarization inversion patterns having a first polarization inversion period, and converting a fundamental wave into a second harmonic wave; Wave conversion part, and a third periodical polarization inversion part comprising a parallel polarization inversion pattern arranged in series in the traveling direction of light and having a third polarization inversion period shorter than the first polarization inversion period; A quasi phase matching element including a fourth harmonic conversion unit that generates a fourth harmonic based on waves, and the second harmonic conversion unit disposed on one surface of the quasi phase matching device and disposed in series A temperature adjustment element for adjusting the temperature of the fourth harmonic conversion unit, and pressure applied to the second harmonic conversion unit and the fourth harmonic conversion unit disposed on the other surface of the quasi phase matching element and disposed in series The temperature adjustment element and the pressure application unit. Characterized in that to simultaneously phase matching the second harmonic conversion unit and the fourth harmonic conversion unit by changing the force.

  According to the present invention, the pressure application unit applies pressure to the first periodic polarization inversion unit and the second periodic polarization inversion unit to change the pressure, and the temperature control element is tripled with the second harmonic conversion unit. By changing the temperature of the wave conversion unit, the second harmonic conversion unit and the third harmonic conversion unit are simultaneously phase-matched. In this case, since the refractive index changes due to pressure are the same regardless of the direction of polarization of the periodically poled portion, no reflection occurs at the interface where the polarization changes, and no loss occurs.

  In addition, since the polarization inversion pattern is a parallel thin line pattern inside each of the second harmonic wave conversion unit and the third harmonic wave conversion unit, it is possible to avoid the efficiency drop with a large beam diameter and a long working length.

It is sectional drawing along the X-axis (the advancing direction of light) of the coordinate axis (X, Y, Z axis) of the wavelength conversion optical apparatus of Example 1 of this invention. It is sectional drawing between ZZ of the wavelength conversion optical apparatus of Example 1 of this invention. It is sectional drawing between XX of the wavelength conversion optical apparatus of Example 1 of this invention. Design time required to simultaneously phase match the second harmonic conversion unit and the third harmonic conversion unit when the fundamental wavelength of the wavelength conversion optical device according to the first embodiment of the present invention deviates from the designed value at design time It is a figure which shows the difference of the assumption temperature and the difference of design time assumption value pressure. It is sectional drawing of the wavelength conversion optical apparatus of Example 3 of this invention. It is a figure which shows the laser apparatus containing the wavelength conversion optical apparatus of Example 4 of this invention. It is a figure which shows an example of the wavelength conversion element described in patent document 6. FIG. It is a figure which shows another example of the wavelength conversion element described in patent document 6. FIG. It is a figure which shows an example of the wavelength conversion element described in the nonpatent literature 1. FIG. It is a figure which shows an example of the wavelength conversion element described in patent document 7. FIG.

  Hereinafter, embodiments of a wavelength conversion optical device and a laser device of the present invention will be described in detail based on the drawings.

  In Patent Document 7, the QPM element is not used, and the junction surface of the two nonlinear optical crystals 20A and 20B is parallel to the optical axis, whereas the wavelength conversion optical device of the present invention is the front half. A second harmonic wave conversion unit and a third harmonic wave conversion unit in the second half are arranged in series in the optical axis direction.

  Further, according to the present invention, the phase matching conditions of the second harmonic wave conversion unit and the third harmonic wave conversion unit, which are different portions arranged in series, are simultaneously satisfied by adjusting the pressure (in combination with the temperature change). It features.

  FIG. 1 is a cross-sectional view along the X-axis (the traveling direction of light) of the coordinate axes (X, Y, Z axes) of the wavelength conversion optical device of Example 1 of the present invention. FIG. 2 is a cross-sectional view of the wavelength conversion optical device of the first embodiment of the present invention taken along the line Z-Z. FIG. 3 is a cross-sectional view of the wavelength conversion optical device of Example 1 of the present invention taken along line X-X.

  The wavelength conversion optical device shown in FIG. 1 includes a second harmonic wave conversion unit 1, a third harmonic wave conversion unit 2, a temperature control element (corresponding to the temperature control element of the present invention) 3, and a pressure application unit 4. The second harmonic conversion unit 1 and the third harmonic conversion unit 2 are formed of an integrated QPM element.

  The second harmonic conversion unit 1 and the third harmonic conversion unit 2 are made of non-linear crystals, and Mg doped LiNbO3, that is, MgLN is used as this crystal. Alternatively, MgSLT may be used as a crystal of the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2.

  The optical axis is parallel to the plane formed by the X crystal axis and the Y crystal axis of the crystal, the fundamental wave wavelength λ of the light is around 1550 nm, and the polarization direction coincides with the Z axis crystal.

  As shown in FIG. 3, the second harmonic wave conversion unit 1 has a first periodical polarization inversion unit consisting of parallel polarization inversion patterns 1a having a first polarization inversion period, and doubles the incident fundamental wave. Convert to waves.

  The third harmonic wave conversion unit 2 is a second harmonic wave conversion unit 1 and a second polarization inversion pattern 2a parallel to the light traveling direction and having a second polarization inversion cycle shorter than the first polarization inversion cycle. The third harmonic which is a sum frequency is generated from the fundamental wave which has a periodical polarization inversion part of the following and is not wavelength-converted by the second harmonic wave converter 1 and the second harmonic wave wavelength-converted by the second harmonic wave converter 1. Do.

The temperature control element 3 is formed of a Peltier element or the like, and is disposed on the lower surface (Z ₋ side of the crystal) of the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 disposed in series. The temperature of the third harmonic wave conversion unit 2 is adjusted.

The pressure application unit 4 includes a hydraulic mechanism, a piezo element, etc., and is disposed on the upper surface (Z ₊ side of the crystal) of the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 disposed in series. A pressure is applied to the unit 1 and the third harmonic wave conversion unit 2.

  The second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 which are crystals, the temperature control element 3 and the pressure application unit 4 are adhered by an adhesive or the like so that they can be pulled in the Z-axis direction.

  For example, when the pressure p0 is applied downward from the Z + direction (the downward direction is the positive direction of the pressure), certain conditions such as the fundamental wave wavelength 1550 nm and the element temperature 35 ° C. The third harmonic wave conversion unit 2 is designed to be phase-matched together.

For example, wavelength λ ₀ = 1550 nm, temperature T ₀ = 35 ° C., polarization inversion period Λ _{S of} the second harmonic conversion unit 1 = 20.87 μm, polarization inversion period Λ _{T of} the third harmonic conversion unit 2 = 7.09 μm, pressure Phase matching is performed when p ₀ = 0 MPa.

  FIG. 4 shows the difference between the estimated temperature at the design time required for simultaneously phase matching the second harmonic conversion unit 1 and the third harmonic conversion unit 2 when the fundamental wave wavelength deviates from the design estimated value. The difference from the design-time assumed pressure is shown.

  When the phase matching temperature is ΔT and the applied pressure is changed by Δp as shown in FIG. 4 with respect to the dispersion Δλ (nm) of the wavelength of the fundamental wave, the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 simultaneously Phase match.

  When using a fiber laser as the fundamental wave light source, the variation Δλ of the fundamental wavelength is typically | Δλ | <0.1 nm. The variation Δλ in this range can be accommodated by | ΔT | <0.01 K and | Δp | <100 MPa. K is Kelvin. T is an absolute temperature.

  As described above, by changing the temperature of the temperature adjustment element 3 and the pressure of the pressure application unit 4 with respect to the variation Δλ of the wavelength of the fundamental wave, the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 are simultaneously phased. It can be matched.

Next, how to determine ΔT and Δp will be described. First, the temperature-applied dispersion curve in a state where no pressure is applied to the crystal is n (λ, T). _Let the polarization inversion period of the second harmonic wave conversion unit 1 be Λ _S and let the polarization inversion period of the third harmonic wave conversion unit 2 be Λ _T. In the design, it is assumed that the second harmonic conversion unit 1 and the third harmonic conversion unit 2 are simultaneously phase-matched at the wavelength λ ₀ , the temperature T ₀ , and the pressure p ₀ .

The function ni (λ, T) i = 1, 2, 3... Is defined by equation (1).
ni (Δλ, ΔT, Δp) ≡ n ((λ ₀ + Δλ) / i, T ₀ + ΔT, p ₀ + Δp)
... (1)
Using the above notation, when ΔT = 0 and Δp = 0, 2 _S = λ ₀ / {2 n ₂ (0,0,0) − in the _second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2, respectively. 2n ₁ (0, 0, 0)} ... (2)
Λ _T = λ ₀ / {3 n ₃ (0, 0, 0)-n ₁ (0, 0)-2 n ₂ (0, 0, 0)}
... (3)
It has become. From the state of equations (2) and (3), the phase matching wavelength is λ ₀ → λ ₀ + Δλ for both the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 as T ₀ → T ₀ + ΔT and p ₀ → p ₀ + Δp. That is, the following equations (4) and (5) are simultaneously established.
Λ _S = (λ ₀ + Δλ) / {2 n ₂ (Δλ, ΔT, Δp) -2 n ₁ (Δλ, ΔT, Δp)}
... (4)
Λ _T = (λ ₀ + Δλ) / {3 n ₃ (Δλ, ΔT, Δp) -n ₁ (Δλ, ΔT, Δp) -2 n ₂ (Δλ, ΔT, Δp)} (5)
In the equations (4) and (5), assuming that the wavelength shift Δλ is a given amount, the two equations (4) and (5) are satisfied by adjusting the two variables ΔT and Δp. become. Since there are two variables and two equations, generally, two variables ΔT and Δp can be obtained.

Next, n (λ, T, p) used for calculation in the first embodiment will be described. If crystals are MgLN, principal stress σ _11, σ _22, with respect to sigma _33, the coefficient [pi _31, delta using a ^{_{π 33 (1 / n e 2}} ) = π 31 · σ 11 + π 31 · σ 22 + π ₃₃ · σ ₃₃ (6)
It becomes.

Equation (6) can be transformed into equation (7) because the change in refractive index is minute.
^{_{Δn e = -n e 3 (π}} 31 · σ 11 + π 31 · σ 22 + π 33 · σ 33) / 2 ... (7)
When pressure p is applied in the Z direction using a MgLN Z plate, it is assumed that σ ₃₃ = −p, σ ₁₁ = σ ₂₂ = 0. Then,
^{_{Δn e = n e 3 · π}} 33 · p / 2 ... (8)
The Sellmeier equations extraordinary refractive index _{n e} and _{n e (λ, T),} n (λ, T, p) and n (λ, T, p) ≡n e + Δn e = n e + (n e 3 · It was set as (pi) ₃₃ * p) / 2 = _ne ((lambda, T) ³ * (pi) ₃₃ * p / 2. (pi) ₃₃ was 0.32 * 10 <^-12> [Pa <^-1> ].

  As described above, according to the wavelength conversion optical device of the first embodiment, the pressure application unit 4 applies pressure to the first periodic polarization inversion unit and the second periodic polarization inversion unit, and the temperature control element 3 doubles. By adjusting the temperatures of the wave conversion unit 1 and the third harmonic conversion unit 2, the second harmonic conversion unit 1 and the third harmonic conversion unit 2 are simultaneously phase-matched. For this reason, the refractive index changes due to pressure are the same regardless of which direction the polarization direction of the periodically poled portion turns, so no reflection occurs and no loss occurs at the interface where the polarization changes.

  In addition, since the polarization inversion pattern is a parallel thin line pattern in each of the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2, it is possible to avoid the efficiency decrease at a large beam diameter and a long working length. .

Next, a wavelength conversion optical apparatus according to a second embodiment of the present invention will be described. In the wavelength conversion optical device according to the first embodiment, p≡p ₀ + Δp <0 may be necessary in order to cope with the wavelength variation of | Δλ | <0.1 nm. In this case, since the crystal is pulled to the pressure application unit 4 side, the interface between the temperature adjustment element 3 and the second harmonic conversion unit 1 and the third harmonic conversion unit 2, the second harmonic conversion unit 1 and the third harmonic conversion It is necessary to consider the adhesive strength of the interface between the part 2 and the pressure application part 4.

On the other hand, the wavelength conversion optical device of Example 2 has a wavelength λ ₀ = 1550 nm, a temperature T ₀ = 35 ° C., a pressure p ₀ = 100 MPa, Λ _S = 20.87 μm, and Λ _T = 7.09 μm. Make it phase-matched.

  Since the relationship of Δλ-ΔT-Δp is the same as that shown in FIG. 4, similar to the wavelength conversion optical device of the first embodiment, the typical wavelength variation of the fiber laser | Δλ | <0.1 nm, | It is possible to cope with ΔT | <0.01 K and | Δp | <100 MPa.

However, in this range, the pressure p≡p ₀ + Δp applied to the second harmonic wave conversion unit 1 and the third harmonic wave conversion unit 2 is positive, so that pressure is always applied from the pressure application unit 4 side to the crystal side Ru. Thereby, the adhesion strength of the interface between the temperature adjustment element 3 and the second harmonic conversion unit 1 and the third harmonic conversion unit 2 and the interface between the second harmonic conversion unit 1 and the third harmonic conversion unit 2 and the pressure application unit 4 is It doesn't matter.

  FIG. 5 is a cross-sectional view of a wavelength conversion optical apparatus according to a third embodiment of the present invention. The wavelength conversion optical device of the third embodiment shown in FIG. 5 is the same as the wavelength conversion optical device of the first embodiment shown in FIG. 1 except that the third harmonic conversion unit 2 is replaced with the third harmonic conversion unit 2. A fourth harmonic wave conversion unit 5 is disposed.

  The fourth harmonic conversion unit 5 includes a third period including a parallel polarization inversion pattern having a second polarization inversion pattern and a third polarization inversion period arranged in series in the traveling direction of light and having a third polarization inversion period shorter than the first polarization inversion period. It generates a fourth harmonic wave based on the second harmonic wave from the second harmonic wave conversion unit 1 having a target polarization inversion unit.

  According to the wavelength conversion optical device of Example 3 configured as described above, the pressure application unit 4 applies pressure to the first periodic polarization inversion unit and the third periodic polarization inversion unit, and By adjusting the temperatures of the second harmonic wave conversion unit 1 and the fourth harmonic wave conversion unit 5, the second harmonic wave conversion unit 1 and the fourth harmonic wave conversion unit 5 are simultaneously phase-matched.

  In addition, the second harmonic conversion unit 1 and the fourth harmonic conversion unit 5 are simultaneously phase-matched by changing the temperature of the temperature control element 3 and the pressure of the pressure application unit 4 with respect to the dispersion of the fundamental wave wavelength. Can.

  FIG. 6 is a view showing a laser apparatus including a wavelength conversion optical apparatus according to a fourth embodiment of the present invention. The laser device shown in FIG. 6 includes a laser light source 11 having a semiconductor laser, lenses 12a and 12b, a QPM element 10, a lens 13, and an optical fiber 14.

  The semiconductor laser included in the laser light source 11 oscillates and outputs the fundamental wave of laser light, and is excited by carrier injection consisting of electrons and holes injected by current drive, and carrier pair annihilation of injected electrons and holes is excited Output laser light generated by stimulated emission generated at the time of The lenses 12 a and 12 b guide the fundamental wave of the laser light from the laser light source 11 to the QPM element 10.

  The QPM element 10 is a wavelength conversion optical device according to any of the first to third embodiments, and receives the fundamental wave of the laser light from the lens 12b, converts the fundamental wave into a harmonic, and emits the harmonic to the lens 13. The lens 13 guides, to the optical fiber 14, the harmonic laser light whose wavelength has been converted by the QPM element 10.

  Thus, since the QPM element 10 which is the wavelength conversion optical device of the first to third embodiments can be applied to a laser device, the wavelength conversion of the first to third embodiments is also performed in the laser device of the fourth embodiment. The effect of the optical device can be obtained.

  The present invention is applicable to a semiconductor laser device.

1 Double wave converter
DESCRIPTION OF SYMBOLS 1a, 2a Polarization inversion pattern 2 3rd harmonic wave conversion part 3 temperature control element 4 pressure application part 5 4th harmonic wave conversion part 11 laser light source 12a, 12b lens 10 QPM element 13 lens 14 optical fiber

Claims (4)

A second harmonic conversion unit having a first periodical polarization inversion unit formed of parallel polarization inversion patterns having a first polarization inversion period, and converting a fundamental wave into a second harmonic; The fundamental wave and the second harmonic are provided with a second periodical polarization inversion portion formed of parallel polarization inversion patterns arranged in series in the traveling direction and having a second polarization inversion period shorter than the first polarization inversion period. And a quasi phase matching element including a third harmonic conversion unit that generates a third harmonic based on
A temperature control element for adjusting the temperature of the second harmonic conversion unit and the third harmonic conversion unit disposed on one side of the quasi phase matching device and disposed in series;
And a pressure application unit that applies a pressure to the second harmonic conversion unit and the third harmonic conversion unit that are disposed on the other surface of the quasi phase matching element and are disposed in series.
A wavelength conversion optical device characterized in that the second harmonic conversion unit and the third harmonic conversion unit are simultaneously phase-matched by changing the temperature of the temperature adjustment element and the pressure of the pressure application unit. A second harmonic conversion unit having a first periodical polarization inversion unit formed of parallel polarization inversion patterns having a first polarization inversion period, and converting a fundamental wave into a second harmonic; 4 times based on the second harmonic, having a third periodical polarization inversion portion consisting of parallel polarization inversion patterns arranged in series in the traveling direction and having a third polarization inversion period shorter than the first polarization inversion period A quasi phase matching element including a fourth harmonic wave conversion unit that generates a wave;
A temperature control element for adjusting the temperature of the second harmonic conversion unit and the fourth harmonic conversion unit disposed on one side of the quasi phase matching element and disposed in series;
And a pressure application unit that applies a pressure to the second harmonic conversion unit and the fourth harmonic conversion unit that are disposed on the other surface of the pseudo phase matching element and are disposed in series.
A wavelength conversion optical device characterized in that the second harmonic conversion unit and the fourth harmonic conversion unit are simultaneously phase-matched by changing the temperature of the temperature adjustment element and the pressure of the pressure application unit.   3. The waveguide type wavelength conversion optical device according to claim 1, wherein the quasi phase matching element is made of MgSLT or MgLN. A semiconductor laser for oscillating the fundamental wave;
The wavelength conversion optical device according to any one of claims 1 to 3, wherein the fundamental wave from the semiconductor laser is incident thereon;
A laser apparatus comprising:

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