JP2008124287A - Wavelength conversion device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-type wavelength conversion device having high conversion efficiency. <P>SOLUTION: The wavelength conversion device comprises a substrate 2, a reflection layer 3, a wavelength conversion layer 4, and a surface-luminescent laser portion 6 having an active layer 61, a p-side reflection mirror 62, and an n-side reflection mirror 63. The reflection layer 3 formed on the substrate 2 and the n-side reflection mirror 63 of the surface-luminescent laser portion 6 are disposed to face each other with the wavelength conversion layer 4 interposed therebetween. The p-side reflection mirror 62 for reflecting the light emitted from the surface-luminescent laser portion 6 and the reflection layer 3 form a resonator. The wavelength conversion layer 4 is configured to have a thickness equal to or less than coherent length, thus confining light into the wavelength conversion layer 4, extending an effective conversion light path length, and preventing a reduction in efficiency due to inversion of the phase of wavelength-converted light. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength conversion element that emits light in a plane type, and particularly to improvement of conversion efficiency.

  In various fields of optoelectronics, the role of a laser diode (hereinafter referred to as LD) as a light source is significant. However, since the wavelength of the LD is determined by the constituent material composition, there is a blank wavelength region in which direct light emission from the LD has not been put into practical use. In particular, in the visible light, the green wavelength region near the wavelength of 550 nm is not put into practical use by the LD alone, and is a blank wavelength region. A laser light source that gives a wavelength in this blank wavelength region can be realized by a wavelength conversion technique using an excitation laser and a wavelength conversion element. This is a technique for generating an arbitrary wavelength from the sum frequency and the difference frequency of the frequency by irradiating the wavelength conversion material with excitation laser light.

  If RGB three-color LD light sources are prepared in this way, they can be used as light sources for image forming apparatuses such as projectors and televisions. In the case of a full-color laser light source, higher performance as a light source can be expected as compared with conventional lamps and LEDs that have recently attracted attention in terms of color reproducibility, power consumption, and light directivity. Further, not only visible light but also optical communication using infrared wavelength division multiplexing technology has great expectations for wavelength conversion elements.

  The green laser currently in practical use emits laser light at around 1064 nm by exciting a solid laser material such as neodymium-doped vanadate crystals using an LD that emits light at a near infrared wavelength for photoexcitation. A wavelength conversion method in second harmonic generation using a nonlinear optical crystal such as lithium oxide or KTP (potassium phosphotitanate) is used. As described above, in a wavelength band where a direct transition material system has not been developed, since a solid laser medium or a second harmonic crystal is used, there is a problem that the light emission efficiency is lower and the cost is higher than that of a single LD.

  As a method for increasing the luminous efficiency, there are a method of processing a crystal giving a second harmonic into an optical waveguide, or using a polarization inversion technique. However, since the optical waveguide structure requires highly accurate alignment of the input light, the manufacturing cost increases. Further, the wavelength tolerance for quasi-phase matching by the polarization inversion technique is very strict, 1 nm or less, and it is basically necessary to provide a temperature control mechanism in order to stabilize the output. Furthermore, in order to obtain sufficient conversion efficiency by these methods, there is a problem that the size of the element is about centimeters and the cost cannot be reduced.

  There is a great expectation for a light source that emits uniform light with a visible light source as a light source of an image generation apparatus. One of devices capable of generating laser light from a plane is a vertical cavity surface emitting laser. Although the surface emitting laser has a small output, a high output can be expected as a whole because it can be arranged in a two-dimensional array. In addition, since the threshold current is also low, high output drive is possible with low power consumption.

  As a method for converting the optical output of the surface emitting laser to extract visible laser light, the surface emitting second harmonic generation element disclosed in Patent Document 1 is an active layer and a multilayer film of the surface emitting laser. Focusing on the fact that III-V semiconductors, which are materials that form reflectors, are both laser materials and materials that exhibit nonlinear effects, the wavelength conversion element is made of the same material as the surface emitting laser. ing.

Patent Document 2 discloses a light source device using a surface emitting laser and a wavelength conversion element. This light source device has a structure in which a wavelength conversion structure generally used for a surface emitting laser is simply applied to a vertical cavity surface emitting laser.
Japanese Patent No. 3244529 JP-A-2005-99160

  There is a problem that the nonlinear effect of the semiconductor material to be wavelength-converted by the surface-emitting second harmonic generation element disclosed in Patent Document 1 is small, and crystal growth by manipulating such a crystal plane is difficult. Furthermore, it is a problem in selecting a material that a material having a large nonlinear effect other than a semiconductor cannot be used.

  In addition, regarding the wavelength conversion structure for the surface emitting laser shown in Patent Document 2, the configuration used in the conventional wavelength conversion element is applied as it is, and there is almost no detailed description for the surface emitting laser. In addition, because the reflectivity of the output mirror of the surface emitting laser is high, the output light is basically small, and the power of the fundamental laser cannot be increased, so the conversion efficiency may not be increased. There is.

  An object of the present invention is to solve such problems and to provide a surface-type wavelength conversion element having high conversion efficiency.

  The wavelength conversion element according to the present invention includes a substrate, a reflective layer, a wavelength conversion layer, and a surface emitting laser unit, and the surface emitting laser unit is configured to reflect a light output side reflecting mirror, an active layer that emits light, and emitted light. A wavelength conversion element having a reflective mirror for reflecting, wherein a reflective layer is formed on the substrate, and the reflective layer and the reflective mirror on the light output side of the surface-emitting laser unit are disposed opposite to each other with the wavelength conversion layer interposed therebetween, The reflecting mirror for reflecting the light emitted from the surface emitting laser part and the reflecting layer form a resonator, and the thickness of the wavelength conversion layer is less than or equal to the coherent length.

  A polarization control layer may be provided between the active layer of the surface emitting laser part and the wavelength conversion layer.

  Moreover, it is desirable to provide a light confinement layer between the active layer of the surface emitting laser part and the wavelength conversion layer. This optical confinement layer is preferably formed by a photonic crystal slab structure. The photonic crystal arrangement in the photonic crystal slab structure of the light confinement layer may be non-uniform or may have a defect.

  Moreover, it is desirable to have a condensing layer between the active layer of the surface emitting laser part and the wavelength conversion layer. This condensing layer is preferably formed of a photonic crystal.

  Furthermore, a light diffusion layer may be provided on the light output side of the substrate on the output side.

  The wavelength conversion layer may be composed of a plurality of layers.

  Further, it is desirable that the surface emitting laser portion is arranged in an array to constitute a wavelength conversion element.

  The present invention comprises a substrate, a reflection layer, a wavelength conversion layer, a reflection mirror on the light output side, an active layer that emits light, and a surface-emitting laser section that has a reflection mirror that reflects the emitted light, and is formed on the substrate. The reflecting layer and the reflecting mirror on the light output side of the surface emitting laser part are disposed opposite to each other with the wavelength conversion layer interposed therebetween, and the reflecting mirror formed on the substrate reflects the light emitted from the surface emitting laser part and the resonator. The wavelength conversion layer thickness is less than the coherent length, the light is confined in the wavelength conversion layer, the effective conversion optical path length is extended, and the conversion efficiency by reversing the phase of the wavelength converted light is improved. Increase.

  In addition, a highly efficient wavelength conversion element can be obtained by providing a polarization control layer and a light confinement layer between the active layer and the wavelength conversion layer of the surface emitting laser section.

  Furthermore, by forming the optical confinement layer with a photonic crystal slab structure, the group velocity of light can be slowed down, the interaction between light and the wavelength conversion element can be strengthened, and a highly efficient wavelength conversion element can be obtained. Obtainable.

  In addition, by providing a condensing layer between the active layer and the wavelength conversion layer of the surface-emitting laser unit, it is possible to perform wavelength conversion with high efficiency by preventing a reduction in the amount of feedback of light to the active layer. . By forming this condensing layer with a photonic crystal, light can be condensed with the photonic crystal regardless of the position of the light emitting point of the surface emitting laser part, and a highly efficient wavelength conversion element can be obtained. .

  Furthermore, by providing a light diffusion layer on the output side on the light output side, the coherency of the laser light can be relaxed.

  Moreover, a highly efficient wavelength conversion element can be obtained by comprising a wavelength conversion layer in multiple layers.

  In addition, by arranging the surface emitting laser portions in an array to constitute a wavelength conversion element, even if the oscillation wavelength varies, wavelength conversion is performed for that wavelength, so that a reduction in efficiency can be suppressed, and high efficiency is achieved. A wavelength conversion element can be obtained.

In describing the wavelength conversion element of the present invention, first, the conversion efficiency of the wavelength conversion element will be described. Here, wavelength conversion by second harmonic generation (SHG) will be described as a typical example of wavelength conversion. Since the second harmonic is proportional to the square of the power density of the laser beam that gives the fundamental wave, if the output power of the laser beam that gives the fundamental wave is P ₀ and the magnitude of the beam of the fundamental wave is w, the second harmonic wave The wave output P ₁ can be expressed as P ₁ = α · (P ₀ / w) ² . Here, α is a constant. In order to increase the output of the second harmonic wave, the output power P ₀ of the fundamental laser beam may be increased and the beam size w of the fundamental wave may be reduced. Therefore, in the present invention, in order to increase the output power P ₀ of the fundamental laser beam, the output of the normal surface emitting laser, that is, the output from the reflecting mirror having a high reflectance is not used, but the surface emitting laser is used. We focused on utilizing the property that the light intensity generated inside the resonator of this is sufficiently larger than the light output from the reflector. Thus, it is possible to increase the light output P ₀ of the fundamental wave, it is possible to increase the conversion efficiency.

Specifically, the reflectance on the wavelength conversion layer side is adjusted as a reflecting mirror for forming a surface emitting laser, and a reflection layer having a high reflectance is formed on the light output side, so that the wavelength conversion layer is placed in the surface emitting laser. The light generated inside the surface emitting laser directly interacts with the wavelength conversion layer.
In addition, in order to reduce the beam size w of the fundamental wave, a reflection layer is introduced to localize the light and increase the conversion efficiency. Furthermore, by using a wavelength conversion layer that is thinner than the coherent length, a configuration is provided in which phase inversion is prevented and an increase in efficiency is simultaneously satisfied by acting as a resonator.

  FIG. 1 is a side sectional view showing the configuration of the wavelength conversion element of the present invention. The wavelength conversion element 1 includes a substrate 2, a reflection layer 3, a wavelength conversion layer 4, a spacer 5, and a surface emitting laser unit 6. The substrate 2 may be any material that is transparent to output light, such as glass, optical crystal, or semiconductor, but is preferably a material that has a thermal expansion coefficient, a refractive index, and the like that are substantially the same as those of other layers. For example, in order to output green light of 550 nm, glass, quartz, or an optical crystal is desirable.

  The reflective layer 3 is formed of a multilayer mirror so that a high reflectance can be obtained with low loss. By making the reflective layer 3 have a multilayer structure, the reflectivity can give a high reflectivity to the fundamental wave and can give a low reflectivity to the wavelength-converted output light. Here, the fundamental wave is light before wavelength conversion, and is laser light emitted from the surface emitting laser unit 6 by current injection. The output light refers to laser light obtained by converting the fundamental wave.

As an example of this multilayer reflector, the fundamental wave is 1100 nm, the output light is a second harmonic, a wavelength conversion element having a second harmonic of 550 nm, the fundamental wave has a high reflectivity, and the output wave A simulation was performed for a structure that provides low reflectivity. In the simulation, the reflectance with respect to the wavelength was calculated using the interference matrix method for normal incidence. The multilayer film is assumed to be formed of SiO ₂ and TiO ₂ that are transparent to the fundamental wave and the output wave, and the respective refractive indexes are set to 1.45 and 2.3. The refractive index has chromatic dispersion, but in the case of this configuration, there is almost no large change, and since the characteristics are not greatly affected, the chromatic dispersion is ignored in the simulation. Further, although the exact value of the refractive index varies depending on the film forming conditions, desired reflection characteristics can be obtained by adjusting the respective film thicknesses. FIG. 2 shows simulation results when the film thicknesses of SiO ₂ and TiO ₂ are 120 nm and 190 nm, respectively. As shown in FIG. 2, a multilayer film having a reflectance of 99.9% or more at a wavelength of 1100 nm and a reflectance of 4% or less at a wavelength of 550 nm can be formed. Since the film thickness of this multilayer film reflecting mirror can be controlled in the order of nm, it is a film forming condition that is sufficiently possible with the current technology. By using this multilayer mirror for the reflective layer 3, it is possible to confine the fundamental wave inside the element and output only the second harmonic.

An optical crystal having a large nonlinear constant is used for the wavelength conversion layer 4. Generally, KTP, lithium niobate, lithium tantalate, or the like is used. Depending on the material of the wavelength conversion layer 4, a large conversion efficiency can be obtained when the direction of a specific crystal axis and the vibration direction of light coincide with each other. Therefore, it is necessary to select an appropriate crystal axis. With respect to lithium niobate, the c-axis, which is the optical axis of the crystal, is set in the wavelength conversion plane. Moreover, the thickness of the wavelength conversion layer 4 is made thinner than the coherent length, and has a structure sandwiched between the reflective layer and the reflective mirror. Since the standing wave is formed between the reflecting layer and the reflecting mirror of the surface emitting laser when the laser oscillates, the oscillation state of the light in the wavelength conversion layer is constant. Therefore, if the thickness of the wavelength conversion layer 4 is made thinner than the coherent length, the second harmonic generated at an arbitrary position of the wavelength conversion layer 4 can be superimposed without canceling the phase, and the output light A reduction in efficiency can be prevented. Here, the coherent length Lc is a phase inversion distance due to a phase shift caused by refractive index dispersion between the fundamental wave and the second harmonic, where the wavelength of the fundamental wave is λ _F , the refractive index is n _F , When the refractive index of the harmonic is n _SH ,
Lc = λ _F / 4 (n _SH −n _F )
It is a value represented by
If the second harmonic is around 550 nm, the coherent length for lithium tantalate and lithium niobate is 3 to 4 μm. Therefore, when this material is used, the thickness of the wavelength conversion layer 4 is desirably 3 μm or less.

  The spacer 5 connects the wavelength conversion layer 4 and the surface emitting laser unit 6 and can take various forms depending on the application. In the case of simple connection, a connection with an adhesive or a flat dielectric film is formed on the spacer 5 and then connected to the surface emitting laser section 6 by bonding. The spacer 5 can also have various functions. For example, the spacer 5 can be connected by forming a non-reflective structure having almost no reflectivity with respect to the fundamental wave. This non-reflective structure can be formed by a multilayer film or a nano structure.

In addition, a reflection layer 7 can be formed on the wavelength conversion layer 4 as shown in FIG. The reflection layer 7 is formed of a multilayer film so as to have a low reflectance with respect to the fundamental wave and a high reflectance with respect to the output wave. FIG. 3 shows a result of a simulation performed on a structure that exhibits a low reflectance for the fundamental wave and a high reflectance for the output wave. It was assumed that the fundamental wave was 1100 nm, the output light was 550 nm, which is the second harmonic, and the multilayer film was formed of SiO ₂ and TiO ₂ . The respective refractive indexes were 1.45 and 2.3, and the reflectance with respect to the wavelength was calculated using an interference matrix method for normal incidence when the film thicknesses of SiO ₂ and TiO ₂ were 60 nm and 95 nm, respectively. It shows that a multilayer film having a reflectance of 99.9% or more at a wavelength of 550 nm and a reflectance of 7% or less at a wavelength of 1100 nm can be formed. By forming the multilayer film having such a configuration, the output wave can be output efficiently.

  The surface emitting laser section 6 is formed to face the wavelength conversion layer 4 with the spacer 5 interposed therebetween. Since the basic configuration of the surface emitting laser unit 6 does not change regardless of the surface emitting laser, a GaInAs / GaAs surface emitting laser will be described below as a representative example.

  The surface emitting laser section 6 having a GaInAs / GaAs material composition is a material system having an oscillation wavelength of around 1 μm and stable characteristics. In addition, since the second harmonic is in the visible light wavelength band, various applications can be expected. However, it is known that the oscillation wavelength of GaInNAs / GaAs material system in which nitrogen (N) is mixed with GaInAs also oscillates in the same wavelength band, and shows excellent performance with little deterioration of threshold current and temperature characteristics. Therefore, it is necessary to use them properly for the purpose and technological progress.

  The surface emitting laser unit 6 includes an active layer 61, a p-side reflecting mirror 62, an n-side reflecting mirror 63, a p-side electrode 64, an n-side electrode 65, a support base 66, a current confinement portion 67, and a buried portion 68.

  In the active layer 61, a pair of injected electrons and holes is combined to generate light. The active layer 61 is formed of a GaInAs / GaAs multiple quantum well having a thickness of about 10 nm, and the quantum well is adjusted to a thickness that forms a resonator for one wavelength by being sandwiched between AlGaAs layers. The p-side reflecting mirror 62 is formed of a reflecting mirror having a pair of p-doped GaAs / AlAs. The multilayer film preferably has a high reflectance, and is preferably an element design having a reflectance of 99.9% or more. In order to realize this reflectivity, it is necessary to increase the number of layers of the multilayer film, but it is necessary to have a multilayer film configuration in consideration of an increase in electric resistance. The n-side reflecting mirror 63 is formed of a reflecting mirror having a pair of n-doped GaAs / AlAs. The reflectance of the n-side reflecting mirror 63 does not need to be large, and is adjusted to the reflectance immediately before laser oscillation occurs. Without the n-side reflecting mirror 63, the laser characteristics are unstable due to the influence of the return light. However, if the reflectivity is sufficient to cause laser oscillation, sufficient light power is not input to the wavelength conversion layer 4. Since the conversion efficiency is lowered, the reflectance is adjusted. Since the p-side electrode 64 is hole injection, it needs to be located close to the quantum well active layer 61. On the other hand, the n-side electrode 65 is a position where the output light is not suppressed since it is an electron injection with a high mobility. In the figure, the support pedestal 66 is provided on the wavelength conversion layer 61 side, but may be disposed beside the n-side reflecting mirror 63. Beside the n-side reflecting mirror 63 has an advantage that the connection between the spacer 5 and the support base 66 is facilitated. The active layer 61, the p-side reflecting mirror 62, and the n-side reflecting mirror 63 are processed into a mesa shape by etching, and a current confinement portion 67 is formed by selectively oxidizing AlAs. In addition to being electrically insulated by this selective oxidation portion, it is possible to confine light by the low refractive index layer, so that the laser characteristics are improved. This is one of the factors that stabilize the characteristics of the surface emitting laser portion 6 of this material system. In the portion removed by etching, a buried portion 68 is formed of an organic material such as polyimide or BCB, and an electrode and electric wiring for injecting current are formed thereon. The support pedestal 66 serves as a pedestal for forming the surface emitting laser portion 6, but it needs to be sufficiently thin in order to minimize the influence of loss due to light diffraction.

  The light generated in the active layer 61 of the surface emitting laser unit 6 forms a standing wave inside the resonator formed by the reflecting layer 3 and the p-side reflecting mirror 62, thereby being strongly confined and causing laser oscillation. Wake up. By causing this laser oscillation, the optical power is dramatically accumulated inside the resonator, so that the wavelength conversion efficiency in the wavelength conversion layer 4 is increased, the second harmonic generation is promoted, and the output light is extracted as the optical output. Can do. Since the light in the active layer 61 causes recombination of hole electrons by stimulated emission, the light emission efficiency is remarkably higher than that of light emitted from a normal LED.

  Next, the second wavelength conversion element 1a will be described. As shown in the side sectional view of FIG. 4, the second wavelength conversion element 1 a includes a polarization control layer 8 provided between the active layer 61 and the wavelength conversion layer 4 of the surface emitting laser section 6 of the wavelength conversion element 1. Have The polarization control layer 8 is formed of a multilayer film or an artificial nanostructure such as a subwavelength grating or a photonic crystal.

  The surface emitting laser section 6 basically has a property that the linearly polarized light is generated but the polarization direction is not stable because of the symmetry of the crystal forming the active layer 61. Therefore, attempts have been made to stabilize the polarization plane by a method of introducing asymmetry to the mesa shape of the surface emitting laser section 6 or a method of providing an optical anisotropy by crystal growth of an active layer on an inclined substrate. It was. However, these methods have many problems in light output and efficiency as compared with the normal surface emitting laser unit 6. On the other hand, the polarization control layer 8 is provided between the active layer 61 of the surface emitting laser section 6 and the wavelength conversion layer 4 to control the reflectance of the polarization of light generated in the active layer 61. Furthermore, by arranging it close to the wavelength conversion layer 4, it is also possible to adjust so as to increase the electric field vibration in the same direction as the crystal axis of the wavelength conversion layer 4. That is, when the wavelength conversion layer 4 is formed of an optical crystal, the wavelength is converted most efficiently if the vibration of the electric field is appropriate in the direction in which the nonlinear constant becomes the largest. Therefore, it is necessary to fix the polarization direction with respect to the crystal axis of the optical crystal, and it is desirable to provide the polarization control layer 8 close to the wavelength conversion layer 4. In addition, by controlling the reflectance of polarized light, it is possible to select the polarization of the light that oscillates the laser, and the oscillation efficiency of the laser light can be increased, so that a large light output can be obtained with low power consumption. It becomes.

  Next, the third wavelength conversion element 1b will be described. As shown in the side sectional view of FIG. 5, the third wavelength conversion element 1 b includes a light confinement layer 9 between the active layer 61 of the surface emitting laser section 6 and the wavelength conversion layer 4. The optical confinement layer 9 is formed by a subwavelength grating or a photonic crystal made of a nanostructure having a size of about the wavelength. That is, after forming the material on the wavelength conversion layer 4 side, the structure is formed by lithography and dry etching.

  As described above, the wavelength conversion efficiency increases in proportion to the power density of incident light. The power density of light is determined by the power of the fundamental wave and the spot shape. The power of the fundamental wave is increased by adjusting the reflectance of the light output side reflecting mirror of the surface emitting laser unit 6. Further, the light confinement layer 9 is used for controlling the spot shape. That is, the light emitted from the surface emitting laser section 6 has a wavefront spread due to the effect of diffraction, and the beam spot shape when entering the wavelength conversion layer 4 becomes large. Moreover, since the oblique component is included, there is a problem that the light reflected by the reflective layer 3 propagates to the left and right of the wavelength conversion layer 4. In order to solve this problem, by providing the optical confinement layer 9 made of nanostructures, the direction of the wave vector of incident light is changed by the diffraction effect of the structure, and the effect of suppressing the light spread to the left and right is suppressed. The fundamental wave power density can be increased while keeping the beam spot shape small.

  Even if the light confinement layer 9 is a one-dimensional diffraction grating, it has an effect of suppressing light propagation in the lateral direction, but has no effect of suppressing the direction in which the diffraction grating is not formed. Therefore, a two-dimensional photonic crystal slab structure 91 as shown in the plan view of FIG. 6A and the front view of FIG. The photonic crystal slab structure 91 is a structure in which a two-dimensional photonic crystal is formed in a thin film surface having a thickness of about a wavelength. FIG. 6A shows the case where the circular holes 92 forming the photonic crystal are formed in a square lattice shape, but light that requires various arrangements such as a triangular lattice as shown in the plan view of FIG. Can be selected according to the characteristics. Further, here, a circular hole structure having a small refractive index is formed in the dielectric slab structure, but a cylindrical structure having a large refractive index may be formed in the dielectric slab structure. For these refractive index distributions, it is necessary to select an optimum arrangement according to the characteristics of light. Furthermore, the structure for forming the photonic crystal is not necessarily a circle, and may be a polygon, an ellipse, or the like.

  Since this photonic crystal also has the effect of localizing light, it is possible to reduce the group velocity of light. By forming a photonic crystal structure that uses a portion where the slope of the wave number-frequency curve of the photonic band is small, it is possible to give a group velocity delay effect that reduces the speed of light to 1/100 or less. The effect of strengthening the interaction with the conversion element can be given at the same time.

  6 shows a configuration in which the optical confinement layer 9 and the wavelength conversion layer 5 of the two-dimensional photonic crystal slab structure 91 are formed of different materials. As shown in FIGS. 7A and 7B, the wavelength conversion is performed. The light confinement layer 9 may be formed on the layer 4 itself. FIG. 7A shows a case where the photonic crystal slab structure 91 is formed partway through the wavelength conversion layer 4, and FIG. 7B shows that the photonic crystal slab structure 91 has the same thickness as the wavelength conversion layer 4. The case where it is formed is shown. The case shown in (c) is used when the light and confinement of the photonic crystal and the anomalous dispersion effect need to be expressed more strongly than in the case shown in (b).

  In order to form such a photonic crystal slab structure 91 in an optical crystal, a semiconductor process technique is used. FIG. 8 shows an electron micrograph when a triangular lattice photonic crystal array having a diameter of 400 nm and a period of 600 nm is formed on lithium niobate, which is an optical crystal. This shows that the patterning of the photonic crystal array in which the roughness of the side surface and the bottom surface is suppressed to 10 nm or less has been successful, and it is possible to form the photonic crystal in the wavelength conversion layer 4.

  In the above description, the case where the photonic crystal array of the photonic crystal slab structure 91 is uniformly arranged has been described. However, it is not necessary to arrange the photonic crystal array uniformly, and as shown in FIG. A defect structure in which a missing portion 93 is provided in a part of a circular hole 92 forming a crystal may be used. By providing the missing portion 93 in this manner, light can be localized at the defective portion 93 and the beam spot can be reduced. Further, since the size of the defect portion 93 is several μm, light can be confined in a very small portion, and the optical power can be increased. Further, as shown in FIG. 9B, the beam spot shape can be adjusted by providing a portion 94 having a changed size in a part of the circular hole 92 forming the photonic crystal. The characteristics of the photonic crystal array can be adjusted by the shape and period of the structure that gives the difference in refractive index. As shown in FIG. It is possible to adjust the characteristics by giving a difference or by changing the period according to the direction as shown in FIG.

  The photonic crystal slab structure 91 is created by applying semiconductor process processing such as lithography and etching. Therefore, since the shape can be set flexibly with respect to the patterning shape, the shape as shown in FIG. 9 can be manufactured by the same manufacturing method as that for forming the periodic structure.

  Next, the fourth wavelength conversion element 1c will be described. As shown in the side sectional view of FIG. 10, the fourth wavelength conversion element 1 c includes the microlens layer 10 between the support base 66 of the surface emitting laser unit 6 and the wavelength conversion layer 4. The light emitted from the surface emitting laser section 6 has a wavefront spread due to the effect of diffraction, so that the fundamental wave does not enter the reflecting layer 3 or the wavelength converting layer 4 vertically, and the amount of light feedback to the active layer 61 decreases. End up. As a method for preventing this diffraction of light, there is a method of feeding back before diffraction occurs by bringing the reflective layer 3 and the active layer 61 close to each other. In this case, it is necessary to make the support base 66 thin. If the support pedestal 66 is made thin, it becomes difficult to handle the surface emitting laser section 6 and cannot be made sufficiently thin. Therefore, the microlens layer 10 is provided between the support pedestal 66 and the wavelength conversion layer 4 to prevent light from being diffracted so that the light enters the reflection layer 3 perpendicularly. The microlens layer 10 is formed of glass or plastic and is connected to the support base 66 by an adhesive or bonding. The diameter of the microlens 101 of the microlens layer 10 may be about the same as or larger than the size of the light emitting portion of the surface emitting laser section 6. Further, in order to prevent characteristic deterioration due to positional deviation between the light emitting portion of the surface emitting laser section 6 and the microlens 101, it is desirable that the size of the microlens 101 has a margin. For example, if the diameter of the light emitting portion is 10 μm, the diameter of the microlens 101 may be 10 μm or more. Furthermore, the diameter may be 20 μm to 50 μm in consideration of positional deviation and diffraction. Further, when the surface emitting laser unit 6 is arranged in an array, a microlens array may be formed with respect to the arrangement of the surface emitting laser unit 6.

  Further, when the microlens array is formed, the efficiency greatly changes with respect to the positional deviation between the light emitting unit and the microlens 101. Therefore, as shown in FIG. 11, a three-dimensional photonic crystal layer 11 is provided between the wavelength conversion layer 4 and the surface emitting laser unit 6 so as to collect light using the super lens effect of the photonic crystal. good. The super lens effect of the three-dimensional photonic crystal layer 11 provided in the fifth wavelength conversion element 1d is characterized in that there is no deviation of the condensing point depending on the image formation position, unlike the image formation by a normal lens. That is, if the photonic crystal layer 11 is formed, light is collected by the photonic crystal regardless of the position of the light emitting point of the surface emitting laser section 6. Thus, the three-dimensional photonic crystal exhibiting the super lens effect can be formed using dielectric fine particles. For example, a self-organized three-dimensional photonic crystal can be obtained by immersing the substrate in a solution in which fine particles of polystyrene are dispersed and pulling it up from the solution. The characteristics can be adjusted by adjusting the diameter of the polystyrene fine particles. Further, a three-dimensional photonic crystal may be formed by an autocloning method. Although this method is slightly complicated, the difference in refractive index can be selected relatively easily, the degree of freedom of structure is high, and a method of directly forming a film on the wavelength conversion layer 4 or connecting a formed film is connected. Can be formed.

  Next, the fifth wavelength conversion element 1e will be described. As shown in the side sectional view of FIG. 12, the fifth wavelength conversion element 1e has a light diffusion layer 12 on the emission side of the wavelength-converted light. Since the light diffusion layer 12 diffuses the output light to change the phase and the emission direction, the coherency of the laser light can be reduced. In this way, by reducing the coherency of the laser light, speckle noise of the image generation apparatus can be reduced. The light diffusion layer 12 can be realized by forming irregularities of the order of nm on the surface or forming a film containing a scattering material. The material to be formed may be transparent to the output light, and fine processing may be directly performed on the substrate 2 without performing special film formation.

  Further, as shown in the wavelength conversion element 1 f of FIG. 13, a multi-wavelength conversion layer 13 having a multilayer structure in which the wavelength conversion layer 131 is sandwiched between the buffer layers 132 may be provided as the wavelength conversion layer 4. The thickness of the wavelength conversion layer 131 of the multi-wavelength conversion layer 13 is smaller than the coherent length, and the thickness of the buffer layer 132 is adjusted so as to cause pseudo phase matching. As a manufacturing method of such a multi-wavelength conversion layer 13, an optical crystal is bonded to a substrate on which a multilayer film is formed, and the crystal is thinned by a method such as polishing. Multi-wavelength conversion is performed by depositing a material having a refractive index and thermal expansion coefficient comparable to that of the wavelength conversion layer 131 on the thin film layer by vapor deposition or sputtering, and then flattening and repeating optical crystal bonding and thinning. Layer 13 can be formed. Further, although it is difficult to manufacture, an optical crystal in which the c-axis direction is reversed may be used for the buffer layer.

  Next, FIG. 14 shows a wavelength conversion element 1g in which the surface emitting laser sections 6 are arranged in an array. The wavelength conversion layer 4 and the reflection layer 3 of the wavelength conversion element 1f may be formed uniformly, and separation by patterning or the like is not necessary. Here, FIG. 14 shows the case where the surface emitting laser units 6 are provided for three elements, but the number of the surface emitting laser units 6 is not limited.

  Surface emitting lasers arranged in an array form have variations in oscillation wavelength due to a difference in temperature rise distribution caused by current injection, resulting in a difference in wavelength conversion efficiency due to polarization inversion elements. As the number of light emitting points increases, this tendency becomes stronger, so that temperature control becomes more severe as the number of arrays increases. On the other hand, in the structure like the wavelength conversion element 1f, even if the oscillation wavelength varies, wavelength conversion is performed with respect to the wavelength, so that a decrease in efficiency can be suppressed. Further, the slight variation in the oscillation wavelength has the effect of reducing the coherency, and therefore, when used in an image generation apparatus, it is possible to suppress the occurrence of defects due to speckle.

  For example, when the surface emitting laser section 6 of 20 μm square is arranged at an element interval of 100 μm, about 100 elements can be arranged within 1 mm square, and the light output from the wavelength conversion element formed by one surface emitting laser section 6 is 10 mW. If it is, 1 W of optical output can be obtained with 100 elements. High output from such a high-density region is difficult with LEDs, and can exhibit the characteristics of a laser element. As an effective light source for forming a very small image display device by using this element. Available.

  Next, for example, a typical manufacturing method of the wavelength conversion element 1 in which the wavelength of the fundamental wave is 1064 nm and the second harmonic wave of 532 nm is output by wavelength conversion will be described. Although the design values are slightly different for other wavelength bands, the manufacturing method is almost the same.

First, the lithium niobate substrate 2 having the c-axis in the plane is prepared, and the reflective layer 3 is formed on the substrate ₂ with 10 pairs or more of SiO ₂ and TiO ₂ . The reflective layer 3 is adjusted to have a high reflectance with respect to the fundamental wave wavelength and a low reflectance with respect to the output wave wavelength. A substrate transparent to the output wave wavelength is connected to the reflective layer 3 formed on the substrate 2 by an adhesive. At this time, as a transparent substrate, a lithium niobate substrate was used in order to match the thermal expansion coefficient in consideration of the post-process. The connected substrate is thinned to 3 μm by polishing to form the wavelength conversion layer 4 having a thickness equal to or less than the coherent length. It is also possible to use a method for improving the mass productivity by ion slicing for the thinning. Thereafter, as a part of the spacer 5 portion, a multilayer reflective layer 7 having a high reflectance with respect to the output wave wavelength and a low reflectance with respect to the fundamental wave wavelength is formed on the wavelength conversion layer 4. The reflective layer 7 is also formed of 10 pairs or more of SiO ₂ and TiO ₂ , but the reflection wavelength band is changed by changing the film thickness. On the other hand, an n-side reflector 63 made of an n-GaAs / AlAs multilayer film, an active layer 61 of a GaInAs / GaAs multiple quantum well, and a p-side reflector made of a p-GaAs / AlAs multilayer film are formed on a GaAs substrate by a crystal growth apparatus. A surface emitting laser substrate on which 62 is formed is prepared. The n-side reflecting mirror 63 is adjusted to reflectivity so as to increase the basic power while alleviating instability due to the return light. The p-side reflecting mirror 62 is adjusted to a number of pairs that does not cause a problem due to an increase in electrical resistance due to an increase in the number of pairs in the multilayer film, while increasing the reflectance to 99.9% or more. A p-side electrode 64 and an n-side electrode 65 based on gold are formed on the substrate by vapor deposition. The n-side electrode 64 is formed on the back surface, and the light output portion is masked in advance before vapor deposition, and the masking portion is peeled off by a lift-off method to form a light output portion. A mesa shape is formed on the surface by lithography and etching, and a current confinement portion 67 is formed by selective oxidation. Thereafter, thermal annealing is performed to make ohmic contact between the electrode and the semiconductor. Thereafter, the mesa is embedded with polyimide, the electrode portion is cueed, and the final electrode wiring is patterned. The wavelength conversion element 1 can be formed by connecting the substrate with the wavelength conversion layer 4 manufactured by the above process and the surface emitting laser unit 6 with an organic adhesive. At this time, in addition to the adhesive, the optical contact, low-temperature bonding, and direct bonding by normal temperature bonding may be performed for connection.

It is side surface sectional drawing which shows the structure of the wavelength conversion element of this invention. It is a change characteristic figure of the reflectance to the wavelength of the reflective layer which consists of a multilayer-film reflective mirror. It is a change characteristic figure of the reflectance to the wavelength of the reflective layer provided in the light incidence side of a wavelength conversion layer. It is side surface sectional drawing which shows the structure of a 2nd wavelength conversion element. It is side surface sectional drawing which shows the structure of a 3rd wavelength conversion element. It is a block diagram of the photonic crystal slab structure which comprises an optical confinement layer. It is side surface sectional drawing which shows the structure of the other photonic crystal slab structure which comprises an optical confinement layer. It is a schematic diagram which shows the arrangement | sequence of the photonic crystal slab structure which comprises an optical confinement layer. It is a top view which shows the arrangement | sequence of the photonic crystal which comprises a light confinement layer. It is side surface sectional drawing which shows the structure of a 4th wavelength conversion element. It is side surface sectional drawing which shows the structure of a 5th wavelength conversion element. It is side surface sectional drawing which shows the structure of a 6th wavelength conversion element. It is side surface sectional drawing which shows the structure of the wavelength conversion element which has a multi wavelength conversion layer. It is side surface sectional drawing which shows the structure of the wavelength conversion element which has a surface emitting laser part arranged on the array.

Explanation of symbols

1; wavelength conversion element, 2; substrate, 3; reflective layer, 4; wavelength conversion layer, 5; spacer,
6; surface emitting laser part, 7; reflective layer, 8; polarization control layer, 9; light confinement layer,
10; microlens layer, 11; three-dimensional photonic crystal layer, 12; light diffusion layer,
13; multi-wavelength conversion layer, 61; active layer, 62; p-side reflecting mirror, 63; n-side reflecting mirror,
64; p-side electrode; 65; n-side electrode; 67; support pedestal; 67;
68; buried portion, 91; photonic crystal slab structure.

Claims (9)

A wavelength conversion unit including a substrate, a reflection layer, a wavelength conversion layer, and a surface emitting laser unit, the surface emitting laser unit including a reflecting mirror on the light output side, an active layer that emits light, and a reflecting mirror that reflects the emitted light; An element,
A reflective layer is formed on the substrate;
The reflecting layer and the reflecting mirror on the light output side of the surface emitting laser unit are disposed opposite to each other with the wavelength conversion layer interposed therebetween, and the reflecting mirror that reflects the light emitted from the surface emitting laser unit and the reflecting layer are resonators. Form the
The wavelength conversion element, wherein the wavelength conversion layer has a thickness equal to or less than a coherent length.   The wavelength conversion element of Claim 1 which has a polarization control layer between the active layer of the said surface emitting laser part, and the said wavelength conversion layer.   The wavelength conversion element according to claim 1, further comprising an optical confinement layer between the active layer of the surface-emitting laser section and the wavelength conversion layer.   The wavelength conversion element according to claim 3, wherein the optical confinement layer is formed of a photonic crystal slab structure.   The wavelength conversion element according to claim 1, further comprising a condensing layer between the active layer of the surface-emitting laser unit and the wavelength conversion layer.   The wavelength conversion element according to claim 5, wherein the condensing layer is formed of a photonic crystal.   The wavelength conversion element according to claim 1, wherein a light diffusion layer is provided on the output side of the substrate on the light output side.   The wavelength conversion element according to claim 1, wherein the wavelength conversion layer includes a plurality of layers.   The wavelength conversion element according to claim 1, wherein the surface emitting laser units are arranged in an array.

Images