JPH03188417A - Waveguide type optical modulation element - Google Patents

Abstract

PURPOSE:To improve light propagation loss in an entire element and to perform high-efficiency optical modulation to guided light by Brag diffraction by arranging two optical modulation parts in series with respect to a light advancing direction and constituting the light modulation part so that selective and variable modulation may be performed from identical propagated light. CONSTITUTION:A 1st thin film type lens 6 for converting light which is made incident from a linear type optical waveguide 2 on an input side and naturally dispersed in a fan shape into collimated guided light and a 2nd thin film type lens 7 for condensing the collimated guided light are provided inside a plane type optical waveguide 3. Then, 1st and 2nd interdigital type electrodes 10 and 11 which generate 1st and 2nd surface acoustic waves 8 and 9 having a wave surface and a propagating direction to respectively satisfy the condition of the Bragg diffraction for the collimated guided light are provided on a piezoelectric substrate 1. Thus, the attenuation of propagated light quantity is drastically improve and modulation selectively is obtained in the Bragg diffraction of the light by the surface acoustic waves, then the optical modulation is performed with high efficiency.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention utilizes the acousto-optic effect to realize frequency transition of light (
It relates to an optical modulation element that realizes a frequency shift (hereinafter referred to as a frequency shift), and in particular, by combining a thin film guiding waveguide and an acousto-optical modulation element, it emits two light beams each modulated at two different frequencies from one incident light. The present invention relates to a waveguide type optical modulator.

[Conventional technology]

One of the subfringe interferometry methods for measuring position and distance with an accuracy less than the wavelength of light is a method that uses optical heterogain interference.

In an optical heterodyne interferometer, the frequencies of the two interfering lights are slightly different, so after the reference light and the reflected light from the test object are interfered with, photoelectric conversion is performed, and the electrical signal is observed as a beat signal of the difference frequency. In this case, the position information of the test object can be detected as the difference between the phase of the beat signal and the phase of the reference signal.

On the other hand, with a normal interferometer that makes two lights of the same frequency interfere, the positional information of the object under test is detected as the brightness and darkness of interference fringes, so the measurement accuracy that can be expected is at most 1/2 wavelength. However, since the phase of an electrical signal obtained by an optical heterodyne interferometer can be measured relatively easily with an accuracy of about one-tenth of 2π regardless of changes in signal amplitude, optical phase information can also be measured with high accuracy. For example, it has been reported that optical heterodyne interferometry was used to measure surface roughness and a height resolution of O, lnm was obtained.

(References G, E, Sovaeaargren A
ppl, Opt, 20p610 1981) An important technique in optical heterogain interferometry is the frequency shift technique, which involves the use of a light source that can obtain beat signals with different frequencies and frequencies that can be detected by existing photodetectors. is necessary. There are roughly three types of methods for obtaining such a light source. 1st
The second method uses two frequency-stabilized lasers with frequency offset locked. In conclusion, these two methods are too large-scale and have many difficult problems to apply to optical heterogain interferometry.The third method is currently the most commonly used method and is based on a single laser. This is a method of dividing light into two parts and using an optical phase modulation element in one or both parts to shift the frequency.

In the early days, rotating diffraction gratings, rotating polarizing elements, and the like were used as optical phase modulation elements, but today, acousto-optical modulation elements that utilize Bragg diffraction are often used.

Acousto-optic modulators propagate ultrasonic waves through optical materials such as high-density flint glass or lead molybate to form a phase grating, and utilize the phenomenon of Bragg diffraction caused by the interaction of light and ultrasonic waves to determine the frequency. It is a shift. There are two ways to use this for interference: one uses the 0th-order and 1st-order diffracted light obtained by one modulation element, and the other uses the 1st-order diffracted light from each of two modulation elements with different drive frequencies. There is a way to do this. In the latter method, a frequency shift can be given to each orthogonal component of the polarization state, and the two
Highly useful as a frequency light source.

Acousto-optic modulators have the advantage of having no mechanically moving parts, being compact, and allowing for a high shift frequency. In addition, when configured as a dual-frequency light source, there are many component parts such as beam splinters, reflective mirrors, and wavelength plates, which require adjustment, and the overall size becomes complex and large.
It still has some drawbacks, such as being vulnerable to mechanical disturbances.

[Problem to be solved by the invention]

Claims (1)

[Scope of Claims] A piezoelectric substrate (1) having optical transparency; and an input side provided on the surface layer of the piezoelectric substrate (1) for guiding light incident from an end surface of the piezoelectric substrate. a linear optical waveguide (2); a planar optical waveguide (3) connected to the output end of the input linear optical waveguide (2); and a first optical waveguide connected to the planar optical waveguide (3), respectively. and second output-side linear optical waveguides (4), (5), which are provided inside the planar optical waveguide (3) and naturally fan-shaped from the output end of the input-side linear optical waveguide (2). The first thin film lens (6
), and the planar optical waveguide (3) in order to focus the parallel light after passing through the first thin film lens (6) into the first and second linear optical waveguides (4) and (5). ) and a second thin film lens (7) provided inside the planar optical waveguide (3) having different frequencies from each other.
First and second surface acoustic waves (8) and (9) having wavefronts and propagation directions that satisfy the conditions of Bragg diffraction for parallel light guided within the piezoelectric substrate (1) are The first and second interdigital electrodes (10) and (11)
) A waveguide type optical modulation element.