JP2007158020A - Optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a method of reducing the FSR and utilizing the refractive index change of PLZT at maximum, and to provide an electrode with a little optical loss and a device with a little wave front deterioration, if PLZT cavity optical switches are arrayed. <P>SOLUTION: A plurality of rectangular parallelepiped cavities 12 are arranged with specified equal pitches in parallel to each other on the upside of a board 11. Electrodes 13, 14 are formed on both lengthwise sides impermeable to light, constituting an optical switch array 10. A plurality of cylindrical lenses 22 are arranged in parallel with the same arranging pitch on a transparent board 21, and the arrays are aligned and laminated together. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a compact solid-state laser light source that can be used as a light source for projectors, printers, and the like.

For solid-state laser light sources, a method for reducing the size by joining elements to each other has been studied (see, for example, Patent Document 1). However, this small solid-state laser can only modulate with the LD. In LD modulation, since the emission life of the laser crystal is long, it is difficult to modulate high-speed response.
For this reason, a method of performing high-speed response using a device having an acousto-optic element outside is generally used, but when it is OFF, it is necessary to throw light out and there is a loss of energy. In addition, the optical system becomes large as external modulation and is not suitable for miniaturization. In order to solve this, a light source having a Q switch function is required. The Q switch function is a function that allows a switch unit to be placed in a solid-state laser cavity, emits light to the outside when necessary, and accumulates light in the cavity when unnecessary. There is an example in which an optical system using an electro-optic element is studied as the Q switch function (see, for example, Patent Document 2). In this case, the polarization of light is controlled by the electro-optic element, and only necessary light is converted into polarized light that is emitted to the outside. However, since the optical system becomes complicated and many optical elements are included, the apparatus has a large energy loss.

On the other hand, a bandpass filter in which an electro-optic material is sandwiched between dielectric multilayer films has been studied (for example, see Patent Document 3). The dielectric multilayer film is obtained by periodically laminating dielectrics having different refractive indexes, and a pair having a large refractive index difference such as SiO ₂ and TiO ₂ is used. By controlling the thickness of the dielectric, it can function as a reflective film that reflects light of an arbitrary wavelength. By inserting a layer that disturbs periodicity as a defect layer between the dielectric multilayer films, a cavity can be formed and a function as an optical filter that transmits only an arbitrary wavelength can be provided. This is used in the communication field as a bandpass filter or the like. Research has been conducted on using an electro-optic material as an internal material of the cavity and controlling the optical thickness by changing the refractive index by applying a voltage. By changing the cavity length, it also functions as an optical switch for controlling the transmittance for a single wavelength light source. In addition, since it is a free space planar switch that does not require an optical system such as a waveguide, it has the potential to develop a spatial modulator. The electro-optic material has a very fast response speed, and the Mach-Zehnder type modulator has a response speed of 40 GHz level.

The incident light to the optical switch has a half width of the spectrum and is about 1 nm. In order to realize the OFF state in which the light transmittance is 0, the FSR (free spectrum range) indicating the peak-to-peak distance of the transmission spectrum needs to be 2 nm or more. When the FSR is 2 nm or less, even if the spectrum is moved, the spectrum of the incident light is always applied to the peak having a high transmittance, and the transmittance cannot be reduced to zero.
PLZT, which is a commonly used electro-optic material, is a ceramic and a polycrystalline body. PLZT generally has a large change in refractive index in the direction in which a voltage is applied, and a small change in refractive index in the vertical direction. It is necessary to consider a method that can make maximum use of the refractive index variation.

The electrode portion does not function as an optical switch. The light reflected by the optical switch is subjected to multiple reflection in the cavity, and at this time, the metal electrode portion is irradiated with light. At this time, if the electrode portion is transparent or light is absorbed, light is attenuated there, and light cannot be accumulated in the cavity, and light is wasted. There is a need for a method that does not waste light.
From the electro-optic constant of PLZT, if the distance between the electrodes is 1 nm as the applied voltage, about 1 kV is required. In recent years, there are transistors corresponding to a large voltage, but applying a few kV at a high speed is very expensive. On the other hand, if the distance between the electrodes is reduced, the aperture through which light passes is reduced. As a method of reducing the distance between the electrodes and increasing the opening, a method of using a PLZT cavity type optical switch as an array is conceivable. However, when the array is formed, the wavefront of light deteriorates.

JP 2004-281932 A JP 2002-208749 A JP 2003-207753 A

  The present invention seeks to solve the above-described problems, seeks a method that can reduce the FSR and maximize the change in the refractive index of the PLZT, provides an electrode with low optical loss, and provides a PLZT cavity-type light. Provide a device with little wavefront degradation even if switches are arrayed.

According to the first aspect of the present invention, in the optical element in which the wavelength conversion material and the optical filter are integrated on the incident side and the emission side of the wavelength conversion material, the optical filter on the emission side may optionally have transmittance. It is a filter which can control.
According to a second aspect of the present invention, in the optical element of the first aspect, the optical filter on the emission side is a cavity type optical switch using an electro-optical material.
According to a third aspect of the present invention, in the optical element according to the second aspect, when the cavity of the electro-optic material has a spectral half-value width of light oscillated from the wavelength conversion material as λm, λm satisfies the following condition: It is characterized by satisfying.
λm <FSR
However,
FSR = λ ² / (2 × n × d)
λ: spectral center wavelength of light oscillated from wavelength conversion material n: refractive index of electro-optic material d: thickness of electro-optic material

According to a fourth aspect of the present invention, in the optical element according to the second or third aspect, the electro-optical material is PLZT and has a thickness of 10 μm or less.
According to a fifth aspect of the present invention, in the optical element according to any one of the second to fourth aspects, incident light incident on the wavelength converting material is linearly polarized light, the polarization direction thereof, and the electro-optical material. The application direction of the voltage to be applied is parallel.
According to a sixth aspect of the present invention, in the optical element according to any one of the third to fifth aspects, the electrode of the cavity type optical switch serves as a reflecting surface.
According to a seventh aspect of the present invention, in the optical element according to any one of the second to fifth aspects, the cavity-type optical switch is arrayed, and a microlens is arranged in a shape corresponding to the array. Features.

According to the present invention,
Since a filter that can arbitrarily control the transmittance is provided on the light output side of the wavelength conversion material, it is not necessary to throw away the light even when it is OFF, so that the light can be used effectively, and the light usage efficiency as an optical switch Will improve.
Since the optical filter on the emission side is a cavity type optical switch using an electro-optic material, the transmittance can be controlled, and the light that does not pass becomes reflected light, and the reflected light undergoes regular reflection and returns to the incident direction. Moreover, since it is an electro-optic material, it has a high-speed response and can be switched at an arbitrary timing.
Since the spectral half-value width λm of the light oscillated from the wavelength conversion material is set to be smaller than the FSR, the transmission spectrum is not affected by the spectrum of the incident light, and the transmittance when OFF can be reduced. .
Other effects will be described in the detailed description.

FIG. 1 is a diagram showing an outline of the configuration of the present invention.
In the figure, reference numeral 1 denotes a laser crystal, 2 denotes an incident side optical filter, 3 denotes a nonlinear material, 4 denotes an emission side optical filter, 5 denotes PLZT, 6 denotes an electrode, and 7 and 8 denote dielectric multilayer films.
As the incident light to the optical switch, it is advantageous to use the output of a wavelength conversion material. Here, the wavelength variable material is defined as a material that converts the wavelength of incident light and emits outgoing light having a different wavelength. For example, it refers to a material that absorbs one wavelength and emits another wavelength, such as a laser crystal. Examples of such a material include a general direct transition type semiconductor, which absorbs high energy light, and emits light with a band gap of a low energy level or an impurity level such as a phosphor. Further, a material that emits the second harmonic and uses a material that converts light of a low energy level into a high energy level may be used.
With such a material, by controlling the wavelength of the optical filter for incident light and the optical filter on the emission side, the incident light can be transmitted and a cavity can be formed only for the wavelength of the emission light. Here, the cavity is a structure sandwiched between two mirror surfaces and confining light inside by repeating multiple reflections. The intensity of the light confined inside can be kept higher than the intensity of the incident light.
If the transmittance of the optical filter on the emission side can be controlled in a state where light is confined in this cavity, the emitted light can be extracted arbitrarily for a certain amount of time. This transmittance control is in other words ON / OFF control. At the time of OFF, since the light is returned to the inside of the cavity, it is not necessary to throw away the light. As a result, light can be used effectively, and the light utilization efficiency as an optical switch is improved. In order to facilitate understanding, the configuration and operation will be described with examples of numerical values.

The laser crystal (Nd: YAG) 1 is irradiated through the incident side optical filter 2 using 808 nm light from an LD light source (not shown) as incident light. The incident side optical filter 2 is designed to transmit 808 nm and reflect 532 nm and 1064 nm. The laser crystal emits light of 1064 nm, and this light enters the nonlinear material (KTP) 3 and emits a second harmonic of 532 nm. An exit side optical filter 4 is also formed on the exit side of the non-linear material 3, and the entrance side is a passive optical filter, but the exit side is an active optical filter. The emission side optical filter 4 is a cavity type optical switch. The cavity type optical switch is composed of a microfabricated PLZT 5, an electrode 6, and dielectric multilayer films 7 and 8. Dielectric multilayer films 7 and 8 are formed on both surfaces of the PLZT, respectively, and cavities are assembled there. Switching occurs by changing the resonance wavelength of the optical length of the cavity by applying a voltage. Since the principle of the cavity type optical switch is well known, the explanation is omitted here. The cavity-type optical switch can arbitrarily control the transmittance, whereby light of 532 nm is emitted from the wavelength conversion material.
As described above, the cavity type optical switch has a configuration in which a material capable of controlling a change in refractive index is sandwiched between the incident side and the output side by the mirror surface. The transmittance can be controlled by arbitrarily selecting the wavelength that causes multiple reflection on the mirror surface. The cavity type optical switch can control the transmittance, and light that does not pass through becomes reflected light. The reflected light undergoes regular reflection and returns to the incident direction. Moreover, since it is an electro-optic material, it has a high-speed response and can be switched at an arbitrary timing.

In order to reduce the transmittance at OFF, it is necessary that the spectrum of the incident light and the peak position of the transmittance do not overlap on the spectrum. For that purpose, it is necessary to make the transmission spectrum FSR (free spectrum range) wider than the spectrum of incident light.
This is formulated as follows.
FSR = λ ² / (2 × n × d)
λ: spectral center wavelength of the light oscillated from the wavelength converting material n: refractive index of the electro-optic material d: thickness of the electro-optic material Here, when the spectral half width of the light oscillated from the wavelength converting material is λm,
λm <FSR
The value of FSR is set so that
If this condition is satisfied, the transmission spectrum will not be applied to the spectrum of the incident light, and the transmittance at OFF can be reduced.

  When the wavelength of the incident light is about 500 nm, the electro-optic material is PLZT having a refractive index n = 2.5, and the thickness is made 10 μm or less, so that the FSR is about 5 nm when applied to the above formula. . Since the half width of the spectrum of the incident light is about 1 nm, the transmittance at OFF can be sufficiently reduced. Further, when the wavelength is around 500 nm, the wavelength is green. Currently, solid-state lasers are used because semiconductor lasers do not have a light source with a green wavelength. In the present invention, a small high-speed modulator is provided for a solid-state laser having a visible light wavelength.

FIG. 2 is a diagram showing an arrayed cavity optical switch and a microlens array. 2A is a microlens array, FIG. 2B is an optical switch, and FIG. 2C is a partially enlarged cross-sectional view.
In the figure, reference numeral 10 is an optical switch array, 11 is a substrate, 12 is a cavity, 13 and 14 are electrodes, 20 is a microlens array, 21 is a substrate, and 22 is a microlens.
In FIG. 2A, a plurality of cylindrical lenses 22 are arranged on a transparent substrate 21 with the generatrix parallel to each other at a predetermined equal interval pitch to form a microlens array 20.

In FIG. 4B, the light transmission direction is the thickness direction of the substrate 11 and the direction from the bottom to the top of the figure. On the upper surface of the substrate 11, a plurality of rectangular parallelepiped cavities 12 are arranged in parallel at the same arrangement pitch as the cylindrical lens 22, and electrodes 13 and 14 are disposed on both side surfaces that do not transmit light in the longitudinal direction. Is formed. The electrodes 13 and 14 extend in a comb shape from both end portions of the substrate 11 facing each other in the depth direction in the figure and alternately enter the gaps of the cavities 12 to constitute the optical switch array 10. The upper and lower multilayer films are not shown.
FIG. 4C is a diagram for explaining the relationship between each cavity and the electrode. An electrode (for example, the electrode 13) that enters a gap between two rectangular parallelepiped cavities having multilayer films formed on the upper and lower surfaces rises to the side surfaces of the cavities on both sides to constitute one electrode for the cavity. On the other side surface of each cavity, the other electrode (in this example, electrode 14) rises similarly to the side surfaces of two adjacent cavities to form the other electrode.
In order to configure the wavelength conversion device, the microlens array 20 is brought into close contact with the upper surface of the optical switch array 10 and aligned so that the centerline of each microlens coincides with the centerline of the rectangular parallelepiped cavity. Fix it.
In this device, cylindrical microlenses were arranged in accordance with the PLZT electrode direction. As a result, light having a shape close to a spherical wave emitted from between the PLZT electrodes can be converted into substantially parallel light by the lens. Even if a plurality of lights are mixed in an array, interference does not occur if each is a parallel light. Thereby, it is possible to avoid a phenomenon such as diffraction that occurs when there is an array of pinholes.

FIG. 3 is a schematic view of the operation of the array type wavelength conversion device.
An incident light beam Li enters from the bottom of the figure, and an outgoing light beam whose wavelength has been converted by the laser crystal 1 and the nonlinear material 3 enters a cavity-type optical switch made of PLZT. Lo is controlled.
PLZT cavity-type optical switches are arrayed, and microlenses are arranged corresponding to the cavities. The microlens array 20 converts light emitted from the narrow opening of the PLZT into parallel light without being diffracted.

FIG. 4 is a diagram showing a part of the embodiment of the present invention.
In the figure, reference numerals X, Y, and Z denote coordinate axes, and Px denotes the polarization direction of incoming and outgoing light beams.
In the present embodiment, Nd: YAG is used as the laser crystal 1 and potassium phosphotitanate (KTiOPO4: hereinafter referred to as KTP) is used as the nonlinear optical element 3, but the cavity may be formed of other materials. Nd: YAG and KTP are polished to a thickness of about 200 μm, and the two are directly joined. An incident side optical filter 2 is formed on a surface corresponding to the incident surface. The incident side optical filter 2 is designed to transmit 808 nm and completely reflect 1064 nm and 532 nm. The structure uses a transparent material such as TiO ₂ or SiO ₂ , and the layer structure is designed to be an optimum structure by an optical simulator.
The incident light beam is aligned in advance with the linearly polarized light Px oriented in the coordinate x direction in FIG.

A cavity-type optical switch 10 is disposed on the opposite surface of the incident side optical filter 2 as the output side optical filter 4. Continuous light (CW light) Li from the LD from below is emitted upward as light Lo modulated by this optical switch. The modulation is performed by voltage, and the electrodes 9 are left and right. When the electric signal enters the electrode 9, the optical switch 10 converts the electric signal into an optical signal. The optical switch 10 includes an electro-optic material and side electrodes 6 and 6 sandwiched between mirror surfaces. This time, dielectric multilayer films 7 and 8 were used as mirror surfaces. SiO ₂ and TiO ₂ were alternately laminated on the PLZT as a dielectric multilayer film (not shown). The dielectric multilayer film was composed of 6 pairs of SiO ₂ and TiO ₂ . By changing the number of pairs, it is possible to change the half width of the transmission spectrum. If you want a narrower half-value width, you can increase the number of pairs. By using 6 pairs, the half width of the transmission spectrum is about 1 nm.
As the electro-optic material, PLZT having a large electro-optic constant was used. The film thickness of the electro-optic material is set so that the optical path length is an even multiple 2N of 1/4 of the incident wavelength. As a result, the wavelength of the incident light matches the peak position of the transmittance spectrum. The film thickness was about 10 μm. PLZT is finely processed so that the distance between the electrodes is about 200 μm, and Al is deposited on the side surface.
According to this configuration, since the voltage application direction and the polarization direction of light coincide (be parallel), the amount of change in the refractive index of PLZT can be utilized to the maximum.

The KTP substrate and the Nd: YAG substrate are each polished to about 200 μm and bonded. A dielectric multilayer film is formed as an incident side optical filter on the Nd: YAG surface side.
PLZT is produced by sintering with a composition of 9:35:65 and a thickness of 500 μm. Optical polishing is performed on both sides of the PLZT substrate. The substrate is placed on a polishing jig, and first, one surface is polished to improve flatness. If this flatness is not increased, the PLZT substrate will be warped when the back surface is polished and thinned. A dielectric multilayer film is deposited on the surface after the next polishing. This surface is turned over and attached to a jig, and the back surface is polished. At this time, the polishing is gradually advanced while the measurement is performed so that the PLZT substrate left after polishing becomes about 10 μm.

The polished PLZT substrate having a thickness of 10 μm is attached to the KTP substrate. At this time, a UV curable resin having a refractive index substantially equal to that of the KTP substrate is used so that the thickness of the resin becomes uniform. Next, a resist is applied from this state. The resist is spin-coated to a thickness of about 2 μm and baked at 80 ° C. for about 1 hour. After applying this resist, a groove is cut by dicing. For dicing, a resin blade was used, and the blade width was about 100 μm and the depth was about 20 μm. As a result, the resist is peeled off only at the diced portion, and the resist is left at the other portions. In this state, a metal thin film to be an electrode is formed. The film forming apparatus is devised so that film formation on the side surface proceeds by using DC sputtering and setting the voltage as small as possible. The metal to be deposited may be Al, Ag, etc. that are generally used. When worrying about oxidation from PLZT, Pt, Au, or the like can also be used. This time, Al was used and the film thickness was about 500 nm. Under these conditions, it can be confirmed with an optical microscope or the like that Al is also formed on the side surfaces of the diced grooves.
If a highly reflective material is used for the electrode, light leakage from the upper surface of the KTP can be prevented. That is, light reaching the electrode portion on the upper surface from the inside of the KPT is reflected there and returns to the cavity direction again, so that there is no loss of light at the electrode portion. As a result, an optical switch with high light utilization efficiency is obtained, and the light utilization efficiency as an optical element is increased.

  Lift-off is performed in a state where Al is deposited on the resist. In lift-off, the sample is immersed in an organic solvent such as acetone, and the resist is dissolved by applying ultrasonic waves. In the portion where the resist is melted, Al deposited thereon is peeled off. Al wiring remains only in the dicing groove portion where there is no resist. As can be seen in the cross-sectional view, the Al electrode is peeled off by lift-off on the upper surface of PLZT to form an opening. A dielectric multilayer film is formed in the opening. Since the opening becomes an insulating region isolated from the left and right electrodes, a lateral electric field can be applied to the opening by applying a voltage to the electrodes on both sides. Pads are formed so that they can be connected to the dicing grooves, and wiring is performed on the pads by wire bonding. By connecting a ground and a signal to each of these electrodes, an electric field can be arbitrarily applied to PLZT. Continuous light that is linearly polarized is incident from below. When the polarization direction is perpendicular to the electric field, the electro-optic effect is received.

It is a figure which shows the outline | summary of a structure of this invention. It is a figure which shows the cavity type | mold optical switch and micro lens array which were arrayed. It is an effect | action schematic diagram of an array type wavelength conversion device. It is a figure which shows a part of embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser crystal 2 Incident side optical filter 3 Non-linear material 4 Outgoing side optical filter 5 PLZT
6 Electrode 7, 8 Dielectric multilayer 10 Optical switch array 20 Micro lens array Li Incident light beam Lo Outgoing light beam

Claims (7)

  In an optical element in which a wavelength conversion material and an optical filter are integrated on the incident side and the emission side of the wavelength conversion material, the optical filter on the emission side is a filter that can arbitrarily control the transmittance. An optical element.   2. The optical element according to claim 1, wherein the optical filter on the emission side is a cavity type optical switch using an electro-optic material. 3. The optical element according to claim 2, wherein the cavity of the electro-optic material satisfies the following condition when λm is a spectral half width of light oscillated from the wavelength conversion material. .
λm <FSR
However,
FSR = λ ² / (2 × n × d)
λ: spectral center wavelength of light oscillated from wavelength conversion material n: refractive index of electro-optic material d: thickness of electro-optic material   4. The optical element according to claim 2, wherein the electro-optic material is PLZT and has a thickness of 10 [mu] m or less.   5. The optical element according to claim 2, wherein incident light incident on the wavelength conversion material is linearly polarized light, and a polarization direction of the incident light is parallel to an application direction of a voltage applied to the electro-optical material. An optical element characterized by the above.   6. The optical element according to claim 3, wherein an electrode of the cavity type optical switch serves as a reflecting surface. 6. The optical element according to claim 2, wherein the cavity-type optical switch is arrayed, and a microlens is arranged in accordance with the array.

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