JP2006059964A - Quasi phase-matching wavelength conversion element for wavelength converted laser, and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve generation efficiency of laser in a wavelength converted laser device that converts basic wave light into harmonic beams, using a quasi phase-matching wavelength converting element. <P>SOLUTION: A thin film whose material is a mixture of silicon oxide and silicon nitride is formed at end faces 4a and 4b, to reduce the reflectivity of both end faces 4a and 4b of a quasi phase-matching wavelength conversion (QPM) element 4 arranged in an optical resonator 7. The mixing ratio of the mixture is so controlled as to be a specified refractive index, which is acquired from the refractive index of base body of QPM element at a used wavelength and that of air which is an incident (or outgoing) medium. So, the reflectivity of the end faces 4a and 4b can be suppressed, down to 0.01% or smaller, amounting to improvement by one digit or more as compared to before. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is inserted in the optical path of an optical resonator that resonates and amplifies fundamental wave light emitted from a laser medium pumped by pumping light, and performs quasi-phase matching for generating harmonic light from the fundamental wave light. The present invention relates to a wavelength conversion element used and a manufacturing method thereof.

  In recent years, short-wavelength lasers such as green and blue have been attracting attention in a wide range of fields such as interferometers, optical pickups for optical disks, and printing devices. It is being advanced. As one of such short wavelength laser devices, there is known a wavelength conversion laser device in which a wavelength conversion element is inserted in an optical path of a fundamental laser beam to generate harmonic light, and the harmonic light is extracted outside. ing.

In this type of conventional wavelength conversion laser apparatus, KN (KNbO ₃ ) or KTP (KTiO ₄ ), which is a nonlinear optical crystal, is used as a wavelength conversion element. However, such a crystal has a temperature dependency of wavelength. There is a problem that crystals that can be used with respect to wavelength are limited. On the other hand, in recent years, a wavelength conversion element using a quasi phase matching (QPM) action has attracted attention. Such a quasi-phase-matching wavelength conversion element (hereinafter referred to as “QPM element”) is a laser having a predetermined wavelength by forming a periodic polarization inversion layer inside a bulk crystal such as LiNbO ₃ or LiTaO _3. It is intended to achieve pseudo phase matching with light, and it is possible to cope with almost any wavelength by changing the period of the polarization inversion layer.

  FIG. 1 is a schematic view showing a general configuration of a wavelength conversion laser device using a QPM element (see, for example, Patent Document 1). The laser device includes a semiconductor laser 1 that generates excitation light, a lens 2 that collects the excitation light from the semiconductor laser 1, and a laser that stimulates and emits laser light including fundamental light by being excited by the excitation light. Medium 3, QPM element 4 that generates second harmonic light from stimulated fundamental wave light, etalon 5 that selectively transmits light of a specific wavelength, and output that transmits part of the light while reflecting light And mirror 6.

The operation of this laser apparatus will be schematically described. The excitation light emitted from the semiconductor laser 1 is condensed by the lens 2 and irradiated to the laser medium 3. In the laser medium 3, a reflection layer 3 a that efficiently transmits the excitation light and reflects the fundamental wave light and the harmonic light with high reflectivity is formed on the incident surface of the excitation light. An optical resonator 7 is constituted by the output mirror 6. Thereby, the fundamental wave light stimulated and emitted from the laser medium 3 by the excitation light is oscillated and amplified in the optical resonator 7. As the laser medium 3, a substance corresponding to oscillation having a wavelength twice the laser wavelength to be extracted is used. For example, in order to extract a blue emission wavelength of 473 nm, Nd: YAG (946 nm) and a green emission wavelength of 523 nm are extracted. For this purpose, Nd: YVO ₄ (1064 nm) or the like is used.

  The QPM element 4 inserted in the optical resonator 7 generates light having a wavelength half that of the fundamental wave light, that is, double harmonic light, by the nonlinear optical effect. Therefore, the fundamental wave light and the harmonic light are mixed inside the optical resonator 7. The fundamental light is reflected by the output mirror 6, but the harmonic light is transmitted through the output mirror 6. As a result, as shown in FIG. 1, only the harmonic light is selectively emitted from the output mirror 6 to the right. The etalon 5 is not an essential component in resonance, but here has a function of selecting one mode among a plurality of modes generated in the process of oscillation.

  In the wavelength conversion laser device having such a configuration, it is important to stabilize the quasi phase matching condition in the QPM element 4 in order to efficiently extract harmonic light. For this purpose, the laser oscillation wavelength is made to coincide with the fundamental wavelength that gives the maximum conversion efficiency at the time of wavelength conversion in the QPM element 4, and the reflectance at the both end faces 4 a and 4 b of the QPM element 4 is suppressed to reduce the optical resonator 7. It is necessary to keep the Fresnel reflection loss low and raise the oscillation wavelength feedback efficiency to a sufficiently high value.

JP 2003-304019 A

  The reason why the reflectance at the end faces 4a and 4b of the QPM element 4 is suppressed is that an ideal standing wave is not obtained in the optical resonator 7 when the reflected wave at the end face increases. However, conventionally, the end face of a QPM element used in a wavelength conversion laser device generally has a reflectivity of about 1%, and a reflectivity of about 0.1% or more is particularly intended for low reflection. Show. The lower the reflectance, the better. However, practically, if the reflectance is about 0.01% or less, it is a range that can be regarded as almost non-reflecting, and this can be one target. There is no established method that can achieve such low reflection.

  The present invention has been made in view of these problems, and in the QPM element for wavelength conversion laser, the generation efficiency of the harmonic laser is reduced by suppressing the reflection at the end face as the light incident surface and the light exit surface extremely small. The main purpose is to increase.

The first invention made to solve the above problems is inserted in an optical path of an optical resonator that resonates and amplifies fundamental light emitted from a laser medium pumped by pumping light. In a quasi phase matching wavelength conversion element for a wavelength conversion laser for generating harmonic light,
A thin film made of a mixture of an oxide and a nitride of a predetermined material is formed on both end surfaces of the light incident surface and the light emission surface of a substrate that generates harmonics, and the mixing ratio of the oxide and nitride Is adjusted so that the refractive index of the thin film at the laser wavelength to be used becomes a predetermined value calculated based on the refractive index of the incident side substance and the refractive index of the outgoing side substance sandwiching the thin film. It is characterized by that.

Further, the second invention made to solve the above-mentioned problem is inserted in an optical path of an optical resonator that resonates and amplifies fundamental light emitted from a laser medium excited by excitation light, and the fundamental light A method of manufacturing a quasi phase matching type wavelength conversion element for wavelength conversion laser for generating harmonic light from
By performing a film forming process using a predetermined substance as a target in an atmosphere gas in which oxygen gas and nitrogen gas are mixed in a predetermined ratio on both end surfaces which are a light incident surface and a light output surface of a substrate that generates harmonics. And forming a thin film made of a mixture of the oxide and nitride of the predetermined substance, and adjusting the mixing ratio of oxygen gas and nitrogen gas during the film forming process, so that the thin film at the laser wavelength to be used is adjusted. The refractive index is set to a predetermined value calculated based on the refractive index of the incident-side material and the refractive index of the output-side material sandwiching the thin film.

  Specifically, as the predetermined substance, various substances can be used as long as they form oxides and nitrides, but considering the stability of the generated thin film and the validity of the film formation time. For example, silicon, titanium, or aluminum is preferable.

For example, when the predetermined substance is silicon (Si), the thin films formed on both end faces of the base body are a mixture of silicon oxide (SiO ₂ ) and silicon nitride (Si ₃ N ₄ ). Although depending on the wavelength, the refractive index of silicon oxide is 1.44, the refractive index of silicon nitride is 2.02, and the refractive index of the mixture varies depending on the mixing ratio of the two. On the other hand, the reflectivity of this thin film depends on the refractive index of both materials sandwiching the thin film and the refractive index of this thin film itself, and now the refraction of one material (typically air) sandwiching the thin film. When the refractive index is n0 and the refractive index of the other substance (typically the substrate) is ns, the condition that the reflectance at the thin film is minimized is that the refractive index n1 of the thin film satisfies the following equation.
n1 = (n0 · ns) ^1/2 (1)

  As described above, since the refractive index depends on the wavelength, the refractive index n1 of the target thin film is calculated from the above equation (1) corresponding to the wavelength of the laser light to be actually used, and the mixture is set to the value. The mixing ratio may be controlled. Specifically, since it is possible to change the ratio of the mixture by adjusting the mixing ratio of oxygen gas and nitrogen gas during the film forming process, the oxygen gas flow rate and nitrogen under predetermined film forming conditions in advance. The relationship between the ratio of the gas flow rate and the refractive index of the thin film is experimentally investigated, and the ratio of the oxygen gas flow rate and the nitrogen gas flow rate for obtaining the target refractive index is obtained. One is enough.

  Thus, according to the quasi phase matching wavelength conversion element for wavelength conversion laser and the method for manufacturing the same according to the present invention, when light is incident on the element by the thin films formed on both end faces of the base of the QPM element. In addition, reflection at the end face can be suppressed to an extremely low level when the light is emitted. Specifically, the reflectance can be suppressed to a level of about 0.01% or less, which is one digit or more smaller than the conventional one. Thereby, the loss in the resonator is reduced, the wavelength conversion efficiency in the QPM element is improved, and the output of the harmonic laser extracted to the outside can be increased.

  Hereinafter, the QPM element for wavelength conversion laser which is one Example of this invention is demonstrated concretely.

Generally, when a thin film made of a material having a refractive index n1 smaller than the refractive index ns is formed on a substrate having a refractive index ns, the thin film contributes to suppressing the reflectance. Assuming that the geometric thickness of the thin film is d1, the optical film thickness is n1 · d1, and the reflectance is minimal at a wavelength λ0 that satisfies the following equation (2).
n1 · d1 = (1/4) · λ0 + (m / 2) · λ0 (2)
However, m = 0, 1, 2,...
The reflectance Rmin at this time is given by the following equation (3).
Rmin = {(n1 ² −n0 · ns ) / (N1 ² + n0 · ns ) } ² … (3)
Here, n0 is the refractive index of the incident medium.
From equation (3), the condition for the reflectance Rmin to be minimal (theoretically zero) is
n1 ² −n0 · ns = 0
From this, the above equation (1) is obtained.

Consider a case where MgO: LiTaO ₃ , which is a second harmonic generation element, is used as the base of the QPM element. The refractive index ns of this MgO: LiTaO ₃ is 2.143 (at λ = 946 nm). On the other hand, in the laser apparatus configured as shown in FIG. 1, since the incident medium is air, the refractive index n0 is about 1. Accordingly, the refractive index n1 of the thin film having a minimum reflectance is obtained as 1.46 from the equation (1). FIG. 2 is a conceptual diagram showing the relationship between the mixing ratio of silicon oxide (SiO ₂ ) and silicon nitride (Si ₃ N ₄ ) and the refractive index. The refractive index of silicon oxide is 1.44, and the refractive index of silicon nitride is 2.02. The refractive index can be arbitrarily controlled between 1.44 and 2.02 by the mixing ratio of both. Here, it can be seen that in order to obtain the target refractive index of 1.46, a thin film in which most of the silicon oxide is mixed with a small amount of silicon nitride may be formed. Note that although a straight line is drawn in the graph of FIG. 2, this is not necessarily a straight line, and a preliminary experiment is actually performed under predetermined film formation conditions to determine the mixing ratio and refractive index of the substance. It is necessary to seek the relationship.

In general, vapor deposition or sputtering is used as a method for forming a thin film on a substrate. However, it is also possible to form a film by these conventional methods. Specifically, in order to form a thin film of a mixture of silicon oxide and silicon nitride as described above, for example, vapor deposition may be performed in an atmosphere of a mixed gas of oxygen gas and nitrogen gas using silicon as a target. At that time, by adjusting the flow rate of oxygen gas and nitrogen gas supplied as the atmospheric gas, the mixing ratio of oxide and nitride, that is, the composition of the thin film laminated on the surface of the substrate (SiO ₂ ) _x (Si ₃ N ₄ ) _1-x can be controlled.

FIG. 3 is an example of an actual measurement result of the relationship between the film forming conditions of (SiO ₂ ) _x (Si ₃ N ₄ ) _1-x , the refractive index, and the deposition rate. The film forming conditions are as follows: nitrogen gas flow rate: 20 sccm / min, temperature: 130 ° C., high frequency power: 500 W, and oxygen gas flow rate is changed within a range of 3-20 sccm / min. As the oxygen gas flow rate increases, the mixing ratio of silicon oxide having a small refractive index increases, so that the refractive index of the film decreases. Here, the base has a refractive index of 3.5, and the refractive index giving a minimum reflectance is 1.87. Therefore, as shown in FIG. 3, the flow rate of the oxygen gas may be determined so that the refractive index is 1.87.

  However, the refractive index depends on the wavelength. FIG. 4 shows the result of calculating the wavelength dependence of the reflectance when a thin film is formed so that the reflectance is minimized at a wavelength of 946 nm. As can be seen in this figure, since the refractive index changes according to the wavelength, even if the reflectance is determined to be minimal at a certain wavelength, the reflectance does not become minimal at other wavelengths, but becomes very large. End up. Therefore, it is important to calculate the refractive index at the target wavelength when determining the target value of the refractive index based on the equation (1).

  As described above, by forming thin films having appropriate refractive indexes on both end faces 4a and 4b of the QPM element 4 shown in FIG. 1, the reflectance at the end faces is very small, specifically 0. It is possible to suppress it to 0.01% or less. The thickness of the thin film can be uniquely derived based on the formula (2) if the refractive index n1 is determined.

  In addition, it is obvious that any appropriate modification or modification within the scope of the present invention is included in the scope of the claims of the present invention.

The schematic block diagram of the general wavelength conversion laser apparatus using a QPM element. The conceptual diagram which shows the relationship between the mixing ratio of silicon oxide and silicon nitride, and a refractive index. _{(SiO 2) x (Si 3} N 4) shows an example of a measurement result of the relationship between the film formation conditions of the _1-x and the refractive index and deposition rate. The figure which shows the result of having calculated the wavelength dependence of the reflectance at the time of forming a thin film so that a reflectance may become the minimum at a wavelength of 946 nm.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser 2 ... Lens 3 ... Laser medium 3a ... Reflective layer 4 ... QPM element 4a, 4b ... End face 5 ... Etalon 6 ... Output mirror 7 ... Optical resonator

Claims (3)

Quasi-phase matching for wavelength conversion laser for generating harmonic light from the fundamental wave light inserted in the optical path of the optical resonator that resonates and amplifies the fundamental wave light emitted from the laser medium pumped by the pumping light Type wavelength conversion element,
A thin film made of a mixture of an oxide and a nitride of a predetermined material is formed on both end surfaces of the light incident surface and the light emission surface of a substrate that generates harmonics, and the mixing ratio of the oxide and nitride Is adjusted so that the refractive index of the thin film at the laser wavelength to be used becomes a predetermined value calculated based on the refractive index of the incident side substance and the refractive index of the outgoing side substance sandwiching the thin film. A quasi phase matching wavelength conversion element for a wavelength conversion laser device. Pseudo phase matching type for wavelength conversion laser for generating harmonic light from the fundamental wave light inserted in the optical path of an optical resonator that resonates and amplifies the fundamental wave light emitted from the laser medium pumped by the pumping light A method for manufacturing a wavelength conversion element, comprising:
By performing a film forming process using a predetermined substance as a target in an atmosphere gas in which oxygen gas and nitrogen gas are mixed in a predetermined ratio on both end surfaces which are a light incident surface and a light output surface of a substrate that generates harmonics. And forming a thin film made of a mixture of the oxide and nitride of the predetermined substance, and adjusting the mixing ratio of oxygen gas and nitrogen gas during the film forming process, so that the thin film at the laser wavelength to be used is adjusted. A quasi phase matching wavelength for wavelength conversion laser, characterized in that the refractive index is a predetermined value calculated based on the refractive index of the incident side material and the refractive index of the output side material sandwiching the thin film A method for manufacturing a conversion element.   The said predetermined substance is either silicon, titanium, or aluminum, The quasi phase matching type | mold wavelength conversion element for wavelength conversion lasers of Claim 1, or the pseudo | simulation for wavelength conversion lasers of Claim 2 characterized by the above-mentioned. A manufacturing method of a phase matching type wavelength conversion element.

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