JP2738155B2 - Waveguide type wavelength conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion device for a semiconductor laser, which is capable of realizing a small coherent short wavelength light source.

[0002]

2. Description of the Related Art A wavelength conversion element, especially a second harmonic generation (SHG) element, is extremely important in industry as a device for obtaining coherent short-wavelength light which is difficult to obtain with an excimer laser or the like.

[0003] Semiconductor lasers are used in various optical communication devices and optical information devices as light sources that emit small, high-output coherent light. Currently, the wavelength of light obtained from this semiconductor laser is a wavelength in the near infrared region of 0.78 μm to 1.55 μm. In order to apply this semiconductor laser to devices such as displays more widely, red, green, blue, etc.
Shorter wavelength light is required, but it is difficult to realize this type of semiconductor laser with the current technology.
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, semiconductor laser manufacturing technology has been developed,
Higher output characteristics than ever have been obtained. Therefore, if an optical waveguide type SHG element is configured,
A decrease in energy density due to light diffraction can be avoided, and there is a possibility that a wavelength conversion element can be realized at a relatively high conversion rate even at a light intensity of about a semiconductor laser. As such an example,
An optical waveguide is formed in a lithium niobate crystal, near infrared light is transmitted through the optical waveguide, and a second harmonic wave radiated (Cherenkov radiation) into the crystal substrate is obtained from the SH.
There is an invention of a G element (JP-A-60-14222, JP-A-61-94031). The SHG element of this method has a feature that precise temperature control is not required because the phase matching condition between the fundamental wave and the SHG wave is automatically set. However, since the electromagnetic field distribution of the fundamental wave, which is the guided light, and the SHG light, which is the emitted light, are significantly different, the SHG
The conversion efficiency into light is low, and the output level of the semiconductor laser (up to about 100 mW) is about 0.2%, which is not practical.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the disadvantage that the conventional waveguide-type SHG element has a low conversion efficiency and to provide a waveguide having a structure in which the phase matching condition fluctuates. To provide a wavelength conversion element.

[0006]

Means for reflecting a fundamental wave at both ends of a linear waveguide, means for converting polarization by 90 degrees before and after reflection in the means, and means for guiding the fundamental wave to the linear waveguide Thus, a highly efficient and stable waveguide type wavelength conversion element can be obtained.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings based on embodiments.

FIG. 1 is a view showing the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention. Reference numeral 1 denotes a lithium tantalate (LiTaO ₃ ) crystal plate, and the substrate orientation is a z-plate (that is, the normal to the substrate is the z-axis). On the surface of this crystal, a linear main waveguide 2 and an input waveguide 3 for guiding output light of the semiconductor laser 10 connected to the crystal end face to the main waveguide are formed. Reflecting means for reflecting the fundamental wave (dielectric multilayer films 4 and 5 on the crystal end faces in this embodiment) are provided at both ends of the linear main waveguide 1. With respect to the fundamental wave reciprocating in the main waveguide by this reflection means, the outgoing path 8 and the returning path 9 have polarizations orthogonal to each other.
Means (polarization rotators) 6 and 7 for rotating polarized light composed of interdigital electrodes are set near the reflecting surfaces 4 and 5, respectively. The fundamental wave in the 0.8 μm band that outputs the semiconductor laser 10 polarized in the z direction of the substrate 1 is incident on the incident waveguide 3.
, And guided to the main waveguide 2 toward the reflector 4.
The guided light reflected by the reflector 4 becomes a polarized light 9 parallel to the substrate orthogonal to the polarized light 8 on the outward path and travels toward the reflector 5 located at the other end of the main waveguide. The guided light reflected by the reflector 5 is
This time, the polarized light 8 is perpendicular to the substrate and travels toward the reflector 4 facing the substrate. That is, the reflectors 4 and 5 and the main waveguide 2 form a resonator.

The second-order nonlinear optical constant of LiTaO ₃ is d
₃₃ big, other constants (for example, _{d 13, d 24, d 15} ) below the smaller 1/10 of d _33. Therefore, when the fundamental wave guided light is parallel to the z-axis, the SHG light is strongly emitted as the substrate emitted light. In other words, the light is converted into SHG light much stronger when the forward-polarized light 8 is used than when the backward-polarized light 9 is used.
As described above, the resonator formed by the reflectors 4 and 5 and the main waveguide 2 is different from a normal resonator, and has different polarizations in the reciprocating direction. However, as shown in FIG. 1, strong SHG light 11 is generated in one direction.

The polarization rotator, which is in close proximity to the reflectors 4 and 5, serves exactly as a 1/4 wavelength plate as described above. This effect is achieved by causing the polarization rotation effect by electro-optical constants e _24. FIG. 2 shows a cross section of the main waveguide 2 of FIG. 1 cut at the center in the axial direction. The electric field in the y-direction to make the interdigital electrodes provided on the waveguide causes a coupling between the electric field component E _x and E _z of the fundamental wave through the e _24. It is well known that if the period 大 き く of the electrode is set equal to the reciprocal of the difference between the equivalent refractive index in the forward polarization mode and the equivalent refractive index in the backward polarization mode, this coupling is increased. If the electric field applied to the interdigital electrode is appropriate, | E
_x | = | E _z | (circular polarization state in a normal light beam), and reciprocating under the polarization rotator 6 converts the polarization into orthogonal polarization. The above period Λ is such that the anisotropy of the refractive index is 10 in LiTaO ₃ which is the substrate in the present embodiment.
^Since it is as small as ^-3, it is very easy to make it as large as several hundred μm.

However, if the above period is single, if the thickness of the waveguide or the crystal refractive index fluctuates or changes in temperature, the equivalent refractive index of the waveguide changes, and the above condition is not satisfied. In addition, the orthogonal transformation of polarized light between the forward and backward paths is not efficiently performed, and the features of the present invention are lost. In this case, as is well known, redundancy can be provided by providing a chirped electrode period (a structure in which the period gradually changes in the direction in which light travels).

Here, the case where the polarization of the light guided to the incident waveguide is parallel to the z-axis perpendicular to the substrate has been described, but the polarization parallel to the substrate may be input. In this case, the SHG light is FIG.
The light is emitted from the side opposite to the end face shown in FIG.

The present invention is not limited to the materials described in the above embodiments, but can be applied to many other nonlinear optical crystal materials such as lithium niobate and KTP by changing the electrode period of the polarization rotator. Can be done.

Although a case has been described in which a dielectric multilayer film is provided on the crystal end face as means for reflecting a fundamental wave, a grating well-known as a means for reflecting guided light can be used.

[0015]

As described above, according to the present invention, a highly efficient and stable waveguide type wavelength conversion element can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention.

FIG. 2 is a sectional view illustrating a sectional structure of a polarization rotator.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 LiTaO ₃ substrate 2 main waveguide 3 input waveguide 4, 5 reflector (dielectric multilayer film) 6, 7 deflection rotator 10 semiconductor laser

Claims (1)

(57) [Claims] 1. A Cherenkov-type wavelength conversion element
And the nonlinearity involved in the TE-polarized fundamental wave and the TM-polarized fundamental wave
Nonlinear optical crystal plates are arranged so that the shape optical constants differ
Means for reflecting a fundamental wave at both ends of a linear waveguide provided on the substrate, means for converting polarization by 90 degrees before and after reflection in the means, and means for guiding the fundamental wave to the linear waveguide. 1. A waveguide type wavelength conversion element comprising: