JPH05297428A - Wavelength conversion method - Google Patents

Abstract

PURPOSE:To improve conversion efficiency and to facilitate the conversion of a higher harmonic wave to parallel beams by selecting the thickness and width of a proton exchange optical waveguide in such a manner that the propagation constant of a basic wave and the propagation constant of the higher harmonic wave are equaled to each other. CONSTITUTION:The wavelength element for converting the basic wave of >=1.5mum and <=1.6mum wavelength to the higher harmonic wave is formed of the proton exchange optical waveguide 2 having a nonlinear optical effect formed on the Z-face surface of an LiNbO3 crystal. The thickness and width of the proton exchange optical waveguide 2 are so determined that the propagation constant of the basic wave P1 and the propagation constant of the higher harmonic wave P2 are equaled to each other at this time. The basic wave P1 propagates in the optical waveguide 2 from an incident part 10. The basic wave P1 propagating in the optical waveguide 2 is converted to the higher harmonic wave P2 of the half of the wavelength by the nonlinear optical effect of the optical waveguide 2. The higher harmonic wave P2 remains within the waveguide 2 and propagates at the same speed as the speed of the basic wave P1 if the effective refractive indices of the basic wave P1 and the higher harmonic wave P2 are equaled to each other. The efficient wavelength conversion is thus executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion method used in the fields of optical information processing using coherent light, optical applied measurement, and optical communication.

[0002]

2. Description of the Related Art In the fields of optical information processing, optical applied measurement, and optical communication, there is a need for a standard laser light source with a highly stable wavelength in the 1.5 μm wavelength band with less fiber loss. To realize this standard laser light source, a system using wavelength conversion is used.

FIG. 9 shows a wavelength conversion method for the 1.5 μm band.
The light wavelength conversion element as shown in FIG. The conventional wavelength conversion element will be described below. The fundamental wave P1 is incident on the incident portion 10 of the optical waveguide 2 formed on the substrate 1 made of LiNbO ₃ . In order to generate harmonics efficiently, it is necessary to make the speeds of the fundamental wave and harmonics equal. Generally, in a dielectric material, the refractive index increases as the wavelength of light decreases, resulting in a difference in the speed of the fundamental wave and the harmonic wave.
Therefore, conventionally, the thickness of the optical waveguide 2 is controlled so that the harmonic P2 is obliquely radiated from the optical waveguide 2 into the substrate 1 to effectively match the speed.

FIG. 10 shows a conventional wavelength conversion element for converting a fundamental wave propagating through an optical waveguide into a harmonic wave propagating through the same optical waveguide. Reference numeral 1 is a LiNbO ₃ substrate, and 2 is an optical waveguide formed by diffusing titanium. The fundamental wave P1 (wavelength 1.05 μm) incident from the incident portion 10 is converted into a harmonic wave P2 propagating in the optical waveguide 2 by the nonlinear optical effect of the optical waveguide 2 and emitted from the emitting portion 11.

This wavelength conversion element has a fundamental wave P1 and a harmonic wave P1.
By making the two polarization directions perpendicular to each other, the birefringence of the substrate is utilized to match the speed. In this element, since the higher harmonic wave P2 propagates through the optical waveguide, highly efficient conversion is possible. Further, since the light is emitted from the emitting unit 11 to the outside as a point light source, it can be easily converted into parallel light by a convex lens.

[0006]

However, when SHG is applied to the standard laser light source, the output of P2 must be 1 μW or more in order to obtain sufficient characteristics. However, the conventional wavelength conversion method shown in FIG. In the area, it was not enough, and high output was a problem. In addition, the harmonics need to be converted into parallel rays for practical purposes, but the harmonics converted by the conventional wavelength conversion element require a complicated optical lens, and making parallel rays with a simple lens was also an issue. ..

Although the above-mentioned problem does not exist in the wavelength conversion element using the wavelength conversion element shown in FIG. 10, wavelength conversion of the fundamental wave having a wavelength of 1.5 μm to 1.6 μm cannot be performed due to wavelength dispersion.

[0008]

[Means for Solving the Problems] Wavelength of 1.5 μm or more
A wavelength conversion element for converting a fundamental wave of 6 μm or less into a harmonic is manufactured by a proton exchange optical waveguide having a nonlinear optical effect formed on the Z-plane surface of a LiNbO ₃ crystal. At this time, the thickness and width of the proton exchange optical waveguide are selected so that the propagation constant of the fundamental wave and the propagation constant of the harmonic wave become equal.

[0009]

[Function] Since the propagation constant of the fundamental wave is equal to that of the harmonic wave, the fundamental wave can be efficiently converted into the harmonic wave in the optical waveguide. Further, since the harmonic wave propagates inside the optical waveguide, it can be extracted in a form that can be easily converted into parallel rays.

[0010]

1 is a perspective view of a wavelength conversion device according to a first embodiment of the first invention. The wavelength conversion device of the present invention will be described below with reference to this drawing.

A substrate 1 made of LiNbO ₃ has a plane (Z plane) perpendicular to the Z axis as its surface. Reference numeral 2 is an optical waveguide comprising a substrate 1 and a proton exchange layer formed of pyrophosphoric acid. Although the nonlinear optical effect of the proton exchange layer is smaller than that of the substrate, the light propagating inside also exists in the substrate at the same time, and as a result, the optical waveguide has the nonlinear optical effect. It should be noted that at the end of the optical waveguide 2, an entrance portion 10 and an exit portion 1 are provided.
1 is provided.

The wavelength conversion method of the first embodiment will be described below. The fundamental wave P1 (wavelength 1.56 μm) is incident on the incident portion 1
Propagate from 0 to the optical waveguide 2. The fundamental wave P1 propagating through the optical waveguide 2 is converted into a harmonic P2 (wavelength 0.78 μm) that is half the wavelength due to the nonlinear optical effect of the optical waveguide 2. In the conventional wavelength conversion device, since the effective refractive index of the fundamental wave P1 and the effective refractive index of the higher harmonic wave P2 in the optical waveguide 2 are different, the higher harmonic wave P2 is emitted obliquely from the optical waveguide 2 to the substrate 1.
However, in the present invention, the width and thickness of the optical waveguide are determined so that the effective refractive indices of the fundamental wave P1 and the harmonic wave P2 are equal.

FIG. 2 shows LiNbO.₃And proton exchange layer
Represents the wavelength dispersion of. n_s ^wIs the refractive index of the fundamental wave of the substrate
N_f ^wRepresents the refractive index of the fundamental wave of the proton exchange layer
It n _s ^2wIs the refractive index of the harmonics of the substrate n_f ^2wIs proton exchange
The refractive index of the higher harmonic wave of the replacement layer is shown. Generally optical waveguide 2
The effective refractive index of light propagating in
It changes between the refractive index of the substrate 1 and the refractive index of the optical waveguide 2.
Therefore, from the figure, the target wavelength of 1.5 μm to 1.6
In the range of μm, the refractive indices of the fundamental wave and the harmonics become equal
Understand that Therefore, the thickness of the optical waveguide 2 is optimized.
This is achieved by doing so.

FIG. 3 is a diagram showing the relationship between the thickness of the optical waveguide having the wavelength of the fundamental wave P1 of 1.56 μm and the effective refractive index. n
1 is the effective refractive index of the 0th mode for the fundamental wave P1,
2 is the effective refractive index of the secondary mode of the harmonic P2. Figure 3
When the thickness is 1.3 μm, the effective refractive indices of the fundamental wave P1 and the higher harmonic wave P2 become equal.

When the effective refractive indices of the fundamental wave P1 and the higher harmonic wave P2 are equalized in this way, the higher harmonic wave P2 stays in the waveguide 2 and propagates at the same speed as P1 for efficient wavelength conversion. The harmonic wave P2 that stays and propagates in the optical waveguide 2 is emitted from the emission unit 11 to the outside of the wavelength conversion element. Here, P
The 0th-order mode and the 1st-order mode of 2 were not used because there was no point where their effective refractive indices were equal to the 0th-order mode of the fundamental wave P1.

The harmonic wave P2 emitted from the emitting portion 11 of the optical waveguide 2 becomes light that diverges around the emitting portion 11. The light diverging from such one point can be easily converted into parallel light with a simple convex lens.

Next, a method of manufacturing this optical wavelength conversion element will be described with reference to FIG. First, a mask 20 for producing the optical waveguide 2 is produced on the LiNbO ₃ substrate 1 (FIG. 4).
(A)). The mask 20 is made of a Ta thin film (300 A) and has a stripe window (width 1 μm) in the portion where the optical waveguide is formed.
m) is formed. A Ta film is formed by sputtering, a photo process, and a dry etching process are used for the formation.

After this mask was prepared, it was placed in pyrophosphoric acid for 2 minutes.
Heat treatment is performed at 30 ° C. for 50 minutes. At this time, the stripe window portion of the substrate 1 is proton-exchanged, and the width is 3 μm and the thickness is 1.3 μm.
m optical waveguides 2 are formed (FIG. 4B).

Finally, after removing the mask 20 (FIG. 4C), the end face of the substrate is polished to form the entrance portion 10 and the exit portion 11 (FIG. 4D).

In this way, the optical wavelength conversion device shown in FIG. 1 can be manufactured. The length of this element is 6 mm. In FIG. 1, a semiconductor laser beam (wavelength: 1.56 μ)
m) was guided from the incident section 3, the wavelength was 0.78 μm.
A harmonic wave P2 of m was taken out from the emitting portion 11. A harmonic of 2.4 μW was obtained with an input of 10 mW of fundamental wave. The near-field image of the higher harmonic wave is of the secondary mode as shown in FIG. 5 and indicates that it is a point light source. This harmonic could be easily converted into parallel light by a convex lens.

The wavelength of the fundamental wave is 1.5 μm to 1.6 μm
It is necessary to optimize the thickness of the optical waveguide in order to realize an arbitrary wavelength conversion element in the range. 6 shows the fundamental wave P1
It shows the optimum optical waveguide thickness for the wavelength of. In order to convert the wavelength range of 1.5 μm to 1.6 μm, the depth of the optical waveguide must be in the range of 1.2 μm to 1.4 μm.

The output of the harmonic P2 depends on the width of the optical waveguide, and the width of the optical waveguide is important for obtaining a desired output.
FIG. 7 shows the relationship between the width of the optical waveguide and the harmonic wave P2. The thickness of the optical waveguide is 1.3 μm, which is the optimum value of FIG. 6, and the fundamental wave P1.
When the wavelength is 1.56 μm. Fundamental wave P1
Is 10 mW, in order to obtain a harmonic wave P2 of 1 μW or more, the optical waveguide width needs to be in the range of 2 μm or more and 7 μm or less.

Next, a second embodiment of this wavelength conversion method will be described with reference to FIG. 21 is a temperature adjusting cooler
Built-in semiconductor lasers 22, 23, 24 are lenses, 25
Is the SHG element used in the first embodiment, 26 is a cell filled with vapor of rubidium atom or cesium atom, and 27 is S
i is a photodetector, 28 is an LD control circuit, 29 is a synchronous detector, and 30 is a reference signal oscillator. Semiconductor laser 2
The oscillation wavelength of 1 is 1.56 μm at 25 degrees Celsius. A small modulation is applied to the injection current of the semiconductor laser 21 by the LD control circuit 13 to slightly change the oscillation wavelength of the semiconductor laser 21. At that time, the semiconductor laser 21 is controlled with a temperature accuracy of 0.1 degree.

The basic light P1 from the semiconductor laser 1 is made incident on the SHG element 25 by the lenses 22 and 23. At this time, the fundamental light P1 is wavelength-converted by the SHG element 25 and converted into a harmonic wave P2 having a wavelength of 780 nm. The converted harmonics are converted into parallel rays by the lens 24,
The light passes through the cell 26 and is detected by the photodetector.

The signal transmitted through the cell 26 and received by the photodetector 27 is detected by the synchronous detector 29 and differentiated. The intensity of the transmitted light becomes the minimum when the differential component becomes zero. Since the sign changes before and after that, it becomes possible to stabilize the wavelength of the harmonic P2 at the center of the absorption line by using this as a control signal. Since the wavelength of the harmonic P2 is obtained by converting the wavelength of the fundamental wave P1 into a half wavelength by the SHG element 25,
Stabilization of the harmonic P2 is equivalent to stabilization of the fundamental wave P1. As described above, by using the present invention, a stabilized light source having a wavelength of 1.56 μm can be realized.

[0026]

As described above, according to the present invention, it is possible to realize a wavelength conversion method with high conversion efficiency and easy parallelization of harmonics, which is effective for applications such as wavelength reference lasers.

[Brief description of drawings]

FIG. 1 is a perspective view of a wavelength conversion element according to a first embodiment.

FIG. 2 is a diagram showing wavelength dispersion of LiNbO ₃ and a proton exchange layer.

FIG. 3 is a diagram showing the relationship between the thickness of an optical waveguide and the effective refractive index.

FIG. 4 is a diagram illustrating a method of manufacturing a wavelength conversion element.

FIG. 5 is a diagram showing a near-field image of a harmonic wave.

FIG. 6 is a diagram showing an optimum optical waveguide thickness with respect to a wavelength of a fundamental wave P1.

FIG. 7 is a diagram showing a relationship between an optical waveguide width and a harmonic wave P2.

FIG. 8 is a configuration diagram of a wavelength stabilized light source that represents a second embodiment of the present invention.

FIG. 9 is a diagram showing a conventional wavelength conversion method.

FIG. 10 is a perspective view of a conventional waveguide-waveguide type wavelength conversion element.

[Explanation of symbols]

1 LiNbO ₃ Substrate 2 Optical Waveguide 10 Incident Part 11 Emitting Part 20 Mask P1 Fundamental Wave P2 Harmonic 21 Semiconductor Laser 22 Lens 23 Lens 24 Lens 25 SHG Element 26 Cell 27 Photo Detector 28 LD control circuit 29 Synchronous detector 30 Reference signal oscillator

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tetsuo Taniuchi 1006 Kadoma, Kadoma-shi, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A wavelength conversion method for converting a fundamental wave having a wavelength of 1.5 μm or more and 1.6 μm or less into a second harmonic by using a wavelength conversion element, wherein the wavelength conversion element has a surface perpendicular to the Z axis. A LiNbO ₃ crystal and a proton exchange optical waveguide formed on a plane perpendicular to the Z axis of the LiNbO ₃ crystal,
The wavelength conversion method, wherein the thickness and the width of the proton exchange optical waveguide are selected so that the propagation constant of the fundamental wave and the propagation constant of the second harmonic wave are equal to each other. 2. The fundamental wave has a wavelength of 1.5 μm or more and 1.6 μm.
And the thickness of the proton exchange optical waveguide is 1.2 or less.
The wavelength conversion method according to claim 1, wherein the wavelength conversion method is μm or more and 1.4 μm or less and the width is 2 μm or more and 7 μm or less.