JPH03100530A - Two wavelength light source element - Google Patents

Abstract

PURPOSE:To prevent the modulating action which is not required for a waveguide light in an optical modulation element by providing a groove and a sound absorbing material for interrupting and absorbing an ultrasonic wave between two optical modulation elements so that propagated surface acoustic waves do not mutually exert influence on the other optical modulation element. CONSTITUTION:On the surface layer of a piezoelectric substrate 1 having light transmissivity, a branch type optical waveguide 2, plane type optical waveguides 3, 4 connected to one end of the branch type optical waveguide 2, and linear type optical waveguides 5, 6 connected to the plane type optical waveguides 3, 4 are provided. In this state, an incident light is modulated by an optical modulating part provided on the inside, and in that case, surface acoustic waves 15, 16 which pass through the optical modulating part are almost absorbed by a groove 13 provided between two optical modulating parts and a sound absorbing material 14, and does not mutually exert influence. In such a manner, to the waveguide light, only the optical modulation of high efficiency by the Bragg diffraction can be executed, and the external disturbance is eliminated by unnecessary surface acoustic waves 15, 16.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention utilizes the acousto-optic effect to shift the frequency of light (
Hereinafter, this will be referred to as frequency shift. ), and particularly relates to a two-wavelength light source element that emits two beams modulated at two different frequencies from one incident light by combining a thin film optical waveguide and an acousto-optic modulator. .

(Prior Art) As one of the subfringe interferometry methods for measuring positions and distances with an accuracy equal to or less than the wavelength of light, there is a method using an optical heterodyne interferometer.

In an optical heterogain interferometer, since the frequencies of the two interfering lights are slightly different, when the reference light and the reflected light from the test object are interfered with and then photoelectrically converted, the electrical signal can be 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 to be measured is detected as the brightness and darkness of the 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 heterogain 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 an optical heterodyne interferometer was used to measure surface roughness and a height resolution of 0.1 nm was obtained.

(References: G, E, Sommargren
Appl, Opt, 20p610 1.981).

An important technique in the measurement method using an optical heterodyne interferometer is the frequency shift technique, which requires a light source that can obtain beat signals with different frequencies and frequencies that can be detected by existing photodetectors. Such light sources are considered to be roughly divided into three types.

The first method is to simultaneously oscillate one laser light source in different frequency modes, and the second method is to use two frequency-stabilized lasers with frequency offset locked.

In conclusion, these two methods are too large-scale and have many problems that are difficult to apply to measurement methods using optical heterogain interferometers.

The third method is currently the most commonly used method.
This is a method in which the laser beam from the stand is divided into two parts and an optical phase modulation element is used 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 create a phase grating by propagating ultrasonic waves through an optical material such as high-density flint glass or lead molybate, and use the Bragg diffraction phenomenon caused by the interaction of light and ultrasonic waves to determine the frequency. It is a shift. The methods used for interference include a method that uses 0th-order and 1st-order diffracted light obtained by one modulation element, and a method that uses 1st-order diffracted light from each of two modulation elements with different driving frequencies. There is.

In the latter method, a frequency shift can be given to each orthogonal component of the polarization state, and it is highly useful as a two-frequency light source of orthogonally polarized light.

Acousto-optic modulators have the advantage of having no mechanically moving parts, being compact, and allowing a high shift frequency.
On the other hand, it is not suitable for mass production and is expensive, requires high-precision optical adjustment to satisfy Bragg diffraction conditions, and when configured as a dual-frequency light source, there are many components such as beam splitters, reflection mirrors, and wave plates. As a result, it has become complicated and large in size and is vulnerable to mechanical disturbances.

(Problems to be Solved by the Invention) The problem with the dual-frequency light source using the acousto-optic modulator described above is the fatal flaw of the so-called three-dimensional optical system using a bulk type optical system. . As a measure to completely improve this problem, a method of optically integrating two acousto-optic modulators by combining them with a thin film optical waveguide can be considered. -For example, an invention by the same applicant/inventor.
"Two-frequency light generation module" (Patent application 1986-229146)
Please refer to No. However, optical integration creates new problems. In this case, the most serious problem is the unnecessary interference effect on the light of the acoustic waves emitted from the two acousto-optic modulators.

That is, the acoustic waves generated on a small optical substrate are difficult to extinguish easily, and when modulated, they exert a modulating effect even on poorly guided light.

In thin-film waveguide optical integrated circuits, light is guided in a thin layer on the surface of the substrate, and acoustic waves are also concentrated on the surface of the substrate as surface acoustic waves, so the interaction between the two is strong, and there are no effective countermeasures to deal with this. is necessary.

(Means for Solving the Problems) As a method for reducing the influence of the above-mentioned unnecessary surface acoustic waves on the light in the thin film guided waveguide, the present invention provides a A structure with grooves was adopted.

That is, the energy of the surface acoustic waves is concentrated in the surface layer of the substrate, and the depth thereof is approximately the wavelength of the surface acoustic waves. Therefore, if a groove is provided at a depth equal to or greater than the wavelength of the surface acoustic wave, the surface acoustic waves generated from the acousto-optic modulator and propagating in opposite directions will be blocked by the groove, and the surface acoustic waves within the other acousto-optic modulator will be blocked by the groove. The influence of can be removed. However, in the above structure, the problem of reflection of surface acoustic waves from the inner wall of the groove remains.

Therefore, in the present invention, a sound absorbing material is applied to the groove portion to fill it up. By doing this, (1) the surface acoustic waves generated from one acousto-optic modulator can be prevented from propagating into another acousto-optic modulator, and (2) the surface acoustic waves generated from one acousto-optic modulator can be prevented from propagating into another acousto-optic modulator. It is also possible to prevent the elastic waves from being reflected by the grooves and propagating into the acousto-optic modulator again, causing unnecessary interference between the surface acoustic waves. Eliminates the effects of unnecessary modulation on light.

(Function) By this method, the two-wavelength light source element of the present invention modulates the incident light in the light modulation section provided inside the device, and at this time, the surface acoustic wave that has passed through the light modulation section is transmitted to the two light modulation sections. Most of the sound is absorbed by the grooves and sound-absorbing material placed between them, and they no longer affect each other. This makes it possible to perform only highly efficient optical modulation of guided light using Bragg diffraction, and eliminates unnecessary disturbances caused by surface acoustic waves.

(Example) FIG. 1 shows an example of the configuration of a two-wavelength light source element according to the present invention.

In this embodiment, a branched optical waveguide 2, a first planar optical waveguide 3 connected to one end of the branched optical waveguide 2, and a first planar optical waveguide 3 are provided on the surface layer of a piezoelectric substrate 1 having optical transparency. A first linear optical waveguide 5 connected to the planar optical waveguide 3 is provided. Inside the first planar optical waveguide 3, there is a first thin film lens 7 for converting naturally dispersed light incident from one end of the branched optical waveguide 2 into parallel guided light, and a first thin film lens 7 for converting the light that is naturally dispersed after entering from one end of the branched optical waveguide 2 into parallel guided light. A second thin film lens 8 is provided to condense the light. Further, a first interdigital electrode 11 is provided on the piezoelectric substrate 1 to generate a surface acoustic wave at an angle (Brang angle) that causes a Brang diffraction phenomenon with respect to the parallel guided light. It is. At the other end of the branched optical waveguide 2,
A thin film type optical system having the same configuration as that described above is connected, so a detailed explanation will be omitted.

Between the first planar optical waveguide 3 and the second planar optical waveguide 4, first and second surfaces generated and progressed at the first and second interdigital electrodes 11 and 12, respectively, are disposed between the first and second planar optical waveguides 3 and 4. elastic wave 15
．． A groove 13 for blocking the waves 16 and a sound absorbing material 14 for absorbing the blocked surface acoustic waves are provided. Further, the end surfaces 17 and 18 of the piezoelectric substrate 1 are mirror-polished so that light can enter and exit efficiently.

The light of frequency f0 that is incident on the light incidence point 19 of the piezoelectric substrate 1 is guided through the branched optical waveguide 2 and split into two.
The light enters the planar optical waveguide 3 and is converted by the first thin film lens 7 into guided light having a large beam width. The first surface acoustic wave 15 generated by applying a sine wave signal with an excitation frequency f to the first interdigital electrode 11 enters the parallel guided light portion at a Bragg angle and efficiently increases the light frequency. Perform modulation. The light frequency-modulated in this manner is focused by the second thin film lens 8, enters the first linear optical waveguide 5, and is guided to the first light output point 2o of the piezoelectric substrate 1. be waved. The frequency of the light emitted from the first light emitting point 2o is fo+L.

With the same effect as above, the light frequency modulated within the second planar optical waveguide 4 enters the second linear optical waveguide 6 and reaches the second light output point 21 of the piezoelectric substrate l. Emits more light.

The frequency of the light emitted from the second light emitting point 21 is shifted by the frequency ro of the sine wave signal applied to the second interdigital electrode 12, and becomes fo+f.

As the piezoelectric substrate constituting the two-wavelength light source element of the present invention, a substrate such as a lithium niobate crystal or a lithium tantalate crystal can be used. The titanium diffusion method, which is known as a wave path manufacturing method, and the proton exchange method can be used to form thin film lenses. Interdigital electrodes that generate surface acoustic waves can be realized using microfabrication technology using photolithography.

Furthermore, the first and second surface acoustic waves 15.16 generated by the interdigital electrodes 11.12 are excited and propagated not only in the direction shown in FIG. 1 but also in the opposite direction.
By applying a sound absorbing material to the side end surfaces 22 and 23 of the piezoelectric substrate at the positions where the surface acoustic waves propagating in the opposite direction collide, the effect of removing unnecessary ultrasonic waves can be enhanced.

In addition to mechanically processing the substrate surface as described above, unnecessary ultrasonic waves can be removed by changing the physical properties of the substrate surface by impurity diffusion, ion implantation, etc. A method of diffusing or penetrating the energy into the inside of the substrate is also considered.

(Effects of the Invention) In this invention, in order to modulate light at two different frequencies, the guided light is divided into two by two branched optical waveguides, and the guided light propagating through each leading waveguide is modulated with different frequencies. The structure uses surface acoustic waves for modulation.

In addition, grooves and sound absorbing materials are provided between the two light modulation elements to block and absorb ultrasonic waves, so that the propagating surface acoustic waves do not affect each other and the light modulation elements. This prevents unnecessary modulation effects from occurring on the light waveguided within the element.

Therefore, it has become possible to realize a two-wavelength light source element that utilizes Bragg diffraction of light due to surface acoustic waves.

[Brief explanation of drawings]

FIG. 1 shows an embodiment of the structure of a two-wavelength light source element according to the present invention. In the figure, 1 is a piezoelectric substrate, 2 is a branched optical waveguide, and 3 is a piezoelectric substrate.
and 4 are first and second planar optical waveguides, 5 and 6 are first and second linear optical waveguides, 7 to 10 are first to fourth thin film lenses, and 11 and 12 are first and second linear optical waveguides. 2 interdigital electrodes, 1
3 is a groove, 14 is a sound absorbing material, 15 and 16 are first and second surface acoustic waves, 17 and 18 are end faces of a piezoelectric substrate, 19 is a light incident point, 20 and 21 are first and second light beams Emission points, 22 and 2
3 indicates the side end surfaces of the piezoelectric substrate 1, respectively.

Claims (1)

[Scope of Claims] A piezoelectric substrate (1) having optical transparency; a branched optical waveguide (2); first and second planar optical waveguides (3) and (4) respectively connected to two output ends of the branched optical waveguide (2); first and second linear optical waveguides (5) connected to second planar optical waveguides (3) and (4), respectively;
(6) and; provided inside each of the first and second planar optical waveguides (3) and (4), and naturally dispersed in a fan shape from the two output ends of the branched optical waveguide (2). first and third thin film lenses (7) and (9) for converting thin film guided light into parallel light; passing through the first and third thin film lenses (7) and (9); In order to narrow down the parallel light beams into the first and second linear optical waveguides (5) and (6), a plurality of linear optical waveguides (3) and (4) are provided inside each of the planar optical waveguides (3) and (4). The second and fourth thin film lenses (8) and (10) have different frequencies, and the first and second planar optical waveguides (
3), (4) The first waveform has a wavefront and a propagation direction to satisfy Bragg's diffraction conditions for parallel light guided within
and first and second interdigital electrodes (11) and (12) for generating second surface acoustic waves (15) and (16) on the piezoelectric substrate (1); Groove (13) for blocking the progress of surface acoustic waves (15) and (16) of No. 2
and; a sound absorbing material (14) applied to the groove portion for absorbing the first and second surface acoustic waves (15) and (16).