JPH11174506A - Wavelength conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength conversion element with which high efficiency is assured under pseudo phase matching by a single period without providing the element with the spread of spectra of space frequencies. SOLUTION: An optical waveguide 21 is formed by using a semiconductor material which is high transparent to a basic wave and second harmonic wave. This optical waveguide 21 is provided with a current source 22 for periodically implanting carriers in the light transmission direction (y) in the optical waveguide, by which a diffraction grating periodically lowered in refractive index by a band filling effect with respect to a conversion wavelength existing near the basic absorption end in a semiconductor is formed. The wave number between the basic wave and the conversion wave (second harmonic wave) is matched by this diffraction grating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a wavelength conversion element, and more particularly to an optical waveguide type wavelength conversion element.

[0002]

2. Description of the Related Art As a method for obtaining a second harmonic, there is a method utilizing nonlinear polarization of a medium. Electrons are changed by the electric field of the light wave in the medium to cause polarization. When the optical electric field is weak, the polarization is proportional to the electric field. However, when the light is strong, the nonlinear component cannot be ignored.

When a sine wave having an angular frequency ω is incident on this medium, a light wave distorted from the sine wave due to nonlinear polarization is generated. This wave contains the component of the angular frequency 2ω which is the second harmonic in addition to the component of the angular frequency ω. Therefore, the fundamental wave can be converted into the second harmonic by extracting the component of the angular frequency 2ω.

However, in order to increase the conversion efficiency,
It is necessary that the phase of the component already converted and propagated matches the phase of the component newly converted with the propagation of the incident wave.

[0005] A wavelength conversion element is extremely important in industry as a device for obtaining coherent short-wavelength light. If a wavelength conversion element that can efficiently convert the wavelength even with the output of the semiconductor laser can be realized, the effect is remarkable.

In recent years, the spatial power density of the fundamental wave has been increased by forming the wavelength conversion element into an optical waveguide type,
The possibility of realizing a relatively high conversion efficiency even with a fundamental wave input light intensity of about several tens mW has begun to be demonstrated.

However, in the waveguide type wavelength conversion element which has been realized or proposed, the phase matching condition is constantly maintained even when the refractive index changes due to the temperature change or the incident wavelength changes. There has been no proposal that achieves both high efficiency and stability such that stable high wavelength conversion can be obtained.

[0008] Every nonlinear optical material basically has a refractive index dispersion, and phase matching, which is a requirement for obtaining high conversion, that is, matching the refractive index of the fundamental wave with the refractive index of the second harmonic. The condition cannot be realized without any auxiliary means. For this reason, various methods such as a matching method between an ordinary ray fundamental wave and an extraordinary ray second harmonic using birefringence of a crystal are used. The method is adopted,
In the case of the waveguide type, a quasi-phase matching method using a periodic array of the refractive index and a periodic array of the second-order nonlinear effect (reversal period of spontaneous polarization) is often adopted.

[0009] In order to increase the wavelength and the temperature range, it is often practiced to spatially disperse the spectrum of the spatial frequency of the periodic array.

An example of a wavelength conversion element using this quasi-phase matching method is disclosed in Japanese Patent Laid-Open Publication No.
This is disclosed in Japanese Patent Application Laid-Open No. 6-273816 (hereinafter referred to as Prior Art 2).

The prior art 1 is an optical waveguide device having a waveguide layer made of an insulating organic nonlinear material having a second-order nonlinear optical effect, which is characterized by forming a periodic polarization structure in the waveguide layer. A variable voltage is applied to the electrodes to prevent the polarization components formed from causing orientation relaxation due to light absorption and the like, and to prevent the phase matching condition between the fundamental wave and the second harmonic from shifting and the wavelength conversion from becoming unstable. Means for applying,
In addition, an organic nonlinear material having a short relaxation time so as to respond to a variable voltage is used.

On the other hand, Prior Art 2 discloses an optical waveguide type 2 using an insulating inorganic nonlinear crystal having a second-order nonlinear optical effect.
The second harmonic generation element is characterized by applying a voltage from the side to an optical waveguide that is configured to give a periodic change in the equivalent refractive index of the optical waveguide mode in the transmission direction. The electrode and a DC power supply connected thereto are provided so as to change the original refractive index of the crystal in the waveguide through the electrode. The purpose is to relax the allowable electrode setting range that satisfies the matching condition.

[0013]

However, if the spectrum of the spatial frequency is widened, there is a drawback that the efficiency decreases because the effective nonlinear constant per unit frequency (wavelength) decreases.

Accordingly, an object of the present invention is to secure high efficiency under quasi-phase matching by a single period without providing a spectrum spread of a spatial frequency, and to achieve redundancy (wide wavelength and wide temperature range). To provide a wavelength conversion element.

[0015]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides an optical waveguide formed of a polar substance having a high transmittance with respect to a fundamental wave and a second harmonic, and a light transmitting through the optical waveguide. And a carrier injection means for periodically injecting carriers in the light transmission direction.

According to the present invention, the refractive index in the light transmission direction can be periodically changed by periodically injecting carriers into the optical waveguide. Thereby, the phases of the fundamental wave and the second harmonic can be matched.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A feature of the present invention is that a current source for injecting periodic carriers into a waveguide is provided, and a diffraction grating that acts only on a second harmonic generated by a periodic carrier distribution is basically used. To achieve phase matching between the wave and the second harmonic,
It is another object of the present invention to provide a highly efficient and stable wavelength conversion element in which the fluctuation of the refractive index and the fundamental wavelength is compensated by changing the size of the injected carrier.

Both the prior arts 1 and 2 and the present invention share a common idea of matching the phase between the fundamental wave and the second harmonic through the periodic structure of the refractive index or the periodic structure of the polarization. Forming a periodic structure using a method, that is, utilizing a decrease in the refractive index of an injection site by carrier injection in a semiconductor material is a unique feature of the present invention.
The refractive index change is much larger than the electro-optic effect and the like used in the prior art 2, and is not limited to the correction of the period setting allowable error described in the effect of the prior art 2, but also the dispersion and fluctuation of the fundamental wavelength. It can also absorb large changes such as movement.

Here, the periodic structure of the refractive index change will be briefly described. The fact that the spatial period of the refractive index change diffracts light is well known in the example of light diffraction by sound waves in a solid or liquid.

When the wavelength of the sound wave is short and the interaction length with the light is sufficiently long, the incident light wave is all diffracted by the sound wave into first-order diffracted light. This is called a Bragg diffraction region.

Now, the periodic structure of the refractive index change of the amplitude ΔN formed with the period Λ fixed in the x direction in the medium includes a forward sound wave ΔN · exp (−jKx) and a backward sound wave ΔN ·
exp (+ jKx) can be interpreted as interfering with the formation of a standing wave. Here, Kx represents the wave number in the x direction, and Kx = 2π / Λ.

In the present invention, the periodic structure of the change of the refractive index indicates the structure of the period of the decrease in the refractive index caused by the band filling effect due to the carrier injection.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is an external perspective view of a first embodiment of the wavelength conversion element according to the present invention.

The wavelength conversion element is an optical waveguide 2 made of a polar substance having high transmittance with respect to the fundamental wave and the second harmonic.
1, for example, an optical waveguide 21 using an optical semiconductor material such as GaN or AlN crystal that is transparent to short-wavelength blue light and has a second-order nonlinear effect, and a current for applying a current i to the optical waveguide 21 Source 22.

Further, the optical waveguide 21 is a light transmitting n
A mold guide layer 1, a p-type clad layer 3 and an n-type clad layer 2 sandwiching the n-type guide layer 1 from above and below,
A p-type contact layer 5 and an n-type contact layer 4 sandwiching the mold guide layer 1 and the two cladding layers 2 and 3 from above and below,
It comprises a p-type electrode 6 provided on the p-type contact layer 5 and formed in a ladder shape, and an n-type electrode 7 provided on the n-type contact layer 4 and formed in a planar shape.

Then, a current i is applied between the p-type electrode 6 and the n-type electrode 7 by the current source 22.

In the optical waveguide 21, a structure satisfying optical and electronic conditions is formed by a crystal growth method such as a vapor phase or a liquid phase so that carriers in the crystal can be increased or decreased by current injection.

The p-type electrode 6 provided for injecting the carriers is formed in a ladder shape so as to have a periodic structure with a period Λ. On the other hand, the n-type electrode 7 is uniformly formed in a planar shape.

For the sake of convenience, the direction in which light passes through the optical waveguide 21 is denoted by y, and the direction perpendicular to this direction (the horizontal direction of the optical waveguide 21).
Is represented by x, y, and x at right angles (the vertical direction of the optical waveguide 21).

Next, the operation will be described in detail. FIG.
Is a wave number versus frequency characteristic diagram of a light wave, and FIGS. 3 and 4 are refractive index change characteristic diagrams of the optical waveguide 21 in the light traveling direction y.

Referring to FIG. 1, a fundamental wave 9 is applied to n-type guide layer 1 from one end face of optical waveguide 21 with current i flowing between p-type electrode 6 and n-type electrode 7 by current source 22. Is injected, a second harmonic 10 having a frequency twice that of the crystal constituting the optical waveguide 21 is generated from the other end face of the optical waveguide 21 due to a second-order nonlinear effect.

The generation of the second harmonic requires that the phase matching condition be satisfied.

FIG. 2 shows the relationship between the frequency and the wave number of the guided light. As shown in the figure, the dispersion curve (solid line) 11 of the guided mode guided by the n-type guide layer 1 is represented by the bulk dispersion curve (dashed line) 12 of the crystal layer constituting the n-type guide layer 1 and n Between the bulk dispersion curve (broken line) 13 of the crystal layers constituting the p-type and p-type cladding layers 2 and 3, and includes a fundamental wave (wave number k (ω)) and a second harmonic (wave number k (2ω)). ) Needs to satisfy k (ω) + k (ω) = k (2ω) (1), but this condition cannot be obtained with ordinary optical materials due to the refractive index dispersion. .

Therefore, as shown in FIG. 3, a current i is applied between the p-type electrode 6 and the n-type electrode 7 to
Frequency near the fundamental absorption edge of the n-type and n-type cladding layers 2 and 3 (meaning a region where light does not pass through a region where light passes)
The refractive index is reduced at a period of に よ っ て due to the band-filling effect of only the light. That is, the refractive index in the light transmission direction y of the optical waveguide 21 is increased or decreased by the period Λ.

The wave number reduced by the decrease in the refractive index is represented by K
Assuming that (疑), pseudo phase matching is performed via this spatial grating (a portion where the refractive index of the optical waveguide 21 increases and decreases with the period Λ), and k (ω) + k (ω) = k (2ω) −K (Λ (2) is satisfied, and the wavelength conversion is performed efficiently.

That is, referring to FIG. 2, dispersion curve (solid line)
The curve of the dispersion curve (solid line) 11 changes so that the point F of the frequency 2ω corresponding to the 11 wave number k (2ω) moves horizontally leftward by the wave number K (Λ) and is located at the point G.

Next, a second embodiment will be described. In the case of the first embodiment, even if the equation (2) holds, if the ambient temperature changes or the incident wavelength further changes, the condition of the equation (2) does not hold. This solution is a second embodiment.

FIG. 4 shows a refractive index change characteristic of the optical waveguide 21 in the light traveling direction y in the second embodiment.

In the second embodiment, the current i flowing between the p-type electrode 6 and the n-type electrode 7 by the current source 22, that is, the amount of injected carriers is changed.

Referring to FIG. 4, if the amount of the current to be changed is Δi, the period of the refractive index 率 does not change and the average refractive index ΔNave changes.

FIG. 5 is a wavelength-refractive index characteristic diagram with current i as a parameter. Referring to the figure, the injection current value is i
By setting + Δi, only the refractive index can be changed without changing the period Λ of the refractive index. Thereby, the pseudo phase matching condition can be satisfied again.

Thus, even if there is a change in the ambient temperature or a change in the incident wavelength, it is possible to adjust the injection current (2).
The quasi-phase matching of the equation is maintained, and high wavelength conversion is maintained.

Next, a third embodiment will be described. In the third embodiment, the optical waveguide 21 has a different frequency ω.
Two waves 1 and ω2 are injected to generate a sum frequency or difference frequency ω3.

For example, a light wave having a wavelength of 1.3 μm and a wavelength of 0.3 μm.
By simultaneously injecting a light wave of 8 μm into the optical waveguide 21 and adjusting the applied current i, a wavelength of 0.51
μm green light can be generated.

Similarly, by injecting light waves of two different wavelengths into the optical waveguide 21 at the same time and adjusting the applied current i, it is possible to generate light having the sum of these wavelengths.

On the other hand, in the case where a light wave of one wavelength is injected into the optical waveguide 21 (the first and second embodiments), a light wave of 1.3 μm is injected into the optical waveguide 21 and the second method is adopted. If the current i is adjusted to generate the second harmonic, red light of 0.65 μm is obtained, and the wavelength is 0.83 μm.
By injecting a light wave of m into the optical waveguide 21 and adjusting the current i so as to generate the second harmonic, blue light of 0.415 μm can be obtained.

Therefore, the three primary colors of red, green, and blue light can be sequentially generated by selecting the injection lightwave and setting the injection current, and can be used for a laser display or a laser printer.

In the present invention, the wavelength conversion element is made of GaN or A.
By using an optical semiconductor material such as 1N crystal, the light receiving device and the electronic circuit can be monolithically integrated with the wavelength conversion element, and the control circuit configuration for stabilizing the converted wave intensity and the high converted wave intensity can be obtained. The speed modulation circuit can also be integrated integrally.

[0049]

According to the present invention, an optical waveguide formed of a polar substance having a high transmittance with respect to a fundamental wave and a second harmonic, and a carrier periodically arranged in a transmission direction of light transmitted through the optical waveguide. And carrier injecting means for injecting the laser beam, thereby ensuring high efficiency under a quasi-phase matching by a single period without providing a spread of a spatial frequency spectrum, and providing redundancy (widening of wavelength and widening of temperature range). ) Can be given.

[Brief description of the drawings]

FIG. 1 is an external perspective view of a first embodiment of a wavelength conversion element according to the present invention.

FIG. 2 is a wave number versus frequency characteristic diagram of a light wave of the wavelength conversion element.

FIG. 3 is a refractive index change characteristic diagram in the light traveling direction of an optical waveguide of the wavelength conversion element.

FIG. 4 is a graph showing a refractive index change characteristic of an optical waveguide of the wavelength conversion element in a light traveling direction.

FIG. 5 is a wavelength-refractive index characteristic diagram with the current of the wavelength conversion element as a parameter.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 n-type guide layer 2 n-type cladding layer 3 p-type cladding layer 4 n-type contact layer 5 p-type contact layer 6 p-type electrode 7 n-type electrode 21 optical waveguide 22 current source

Claims (7)

[Claims] 1. An optical waveguide formed of a polar substance having a high transmittance with respect to a fundamental wave and a second harmonic, and a carrier injection means for periodically injecting a carrier in a transmission direction of light transmitted through the optical waveguide. And a wavelength conversion element. 2. The optical waveguide according to claim 1, wherein the optical waveguide has a guide layer that transmits the light, two clad layers having different polarities sandwiching the guide layer from above and below, and polarities sandwiching the guide layer and the two clad layers from above and below. Are different from each other, and the carrier injection means is provided on one of the two contact layers and is provided on a first electrode formed in a ladder shape and on the other of the two contact layers. 2. The wavelength conversion element according to claim 1, comprising a second electrode formed in a planar shape, and a current source applied between the first and second electrodes. 3. The wavelength conversion element according to claim 2, wherein the first electrodes are formed at regular intervals in correspondence with a period of a decrease in the refractive index of the optical waveguide caused by the carrier injection. 4. The wavelength conversion element according to claim 2, wherein said current source is configured to be variable in current. 5. The wavelength conversion element according to claim 1, wherein one fundamental wave is input to the optical waveguide, and a second harmonic of the fundamental wave is obtained as an output. 6. The wavelength according to claim 1, wherein a fundamental wave of two different wavelengths is input to the optical waveguide, and a sum or a difference frequency of the wavelengths is obtained as an output. Conversion element. 7. The wavelength conversion element according to claim 1, wherein the polar substance forming the optical waveguide is an optical semiconductor material.