JPH0711649B2 - Cross-polarized dual frequency light source - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is configured by an optical integrated circuit that frequency-modulates light by using a crystal substrate having optical transparency and an electro-optical effect, an optical waveguide, and the like. Single linearly polarized light (either TE mode or TM mode) and two orthogonal linearly polarized lights (TE mode, TM mode) output with a frequency difference The present invention relates to an orthogonal polarization dual frequency generating light source.

[Prior Art] A high-quality coherent light source has become easily available, and an interferometric measurement technique utilizing the phase difference of the light wavefront has made rapid progress.

Conventionally used interferometers with a single-frequency light source have detected changes in the phase plane, such as the interval between interference fringes and the blinking period of the interference pattern, as changes in the amount of light, and have applied it to minute displacement measurements. That is, the optical path difference in the interferometer is
If the interference pattern due to the output light is a lattice fringe, the lattice spacing is changed by changing the refractive index and the uneven shape of the DUT.
If the interference pattern due to the output light is a blinking signal, the blinking interval is directly converted to the light source wavelength and measured as the refractive index value of the measured object or the unevenness of the surface.

Since this measurement method detects the interference output light quantity as a DC level signal, it has reached its limit in measurement accuracy and noise performance. Recently, therefore, an optical heterodyne interferometry method for interfering light waves of two different frequencies has attracted attention. Has begun to be.
In this optical heterodyne interferometry method, a beat signal due to a difference in frequency between two light sources is used as a carrier of output light.

Usually, the frequency of the beat signal is kept constant with high accuracy, and therefore AC signal processing using the carrier as a center frequency signal becomes possible, resulting in a dramatic improvement in accuracy and noise performance. An important technique in the optical heterodyne interferometry is a frequency shift technique, which requires a dual-frequency light source having different frequencies and capable of obtaining a beat signal that can be detected by an existing photodetector.

There are three general methods for generating such a light source.
Types are being considered.

(1) The first method is a method of simultaneously oscillating one laser light source in modes with different frequencies, and (2) the second method is a method of using two frequency-stabilized lasers with frequency offset locking. Is.

In conclusion, the above methods (1) and (2) are too large in scale and have many problems that are difficult to apply to the optical heterodyne interferometry.

(3) The third method is the most used method at present, and is a method of dividing the light of one laser into two and performing frequency shift by using an optical phase modulation element for one or both of them. .

As the optical phase modulation element, a rotating diffraction grating, a rotating polarization element, or the like was used in the early days, but nowadays, a bulk type acoustic light modulating element using Bragg diffraction is often used. This bulk-type acousto-optic modulator is one in which ultrasonic waves are propagated in an optical material such as tellurium glass to form a phase grating, and the Doppler frequency shift due to Bragg diffraction is used. As a method of using this in an interferometer, a method of using diffracted light of 0th order and 1st order obtained by one optical phase modulation element and one of two optical phase modulation elements of different driving frequencies are used. There is a method of using the second-order diffracted light. In the latter method, frequency shifts can be applied to the orthogonal components of the polarization state, so that it is highly useful as an orthogonal polarization dual-frequency generation light source.

[Problems to be Solved by the Invention]

However, the bulk acousto-optic modulator has the advantages that it has no mechanically movable parts, is small, and can shift at a high frequency, but it is not suitable for mass production and is expensive. High-precision optical adjustment that satisfies Bragg diffraction conditions. Is necessary, it is difficult to match the orthogonal components of the polarization state on the same optical axis,
Further, when configured as a dual-frequency light source, many components such as a beam splitter, a reflection mirror, and a wave plate are required, and the overall size becomes complicated and large, which leaves drawbacks such as weakness against mechanical disturbance.

[Means for Solving the Problems]

As described above, the problem with the dual-frequency light source that uses the bulk-type acousto-optic modulator is a fundamental defect of a so-called stereoscopic optical system that uses bulk-type optical components.

Therefore, in the orthogonal polarization dual-frequency generation light source according to the present invention, an optical waveguide formed on a crystal substrate having optical transparency and an electro-optical effect and an electro-optical effect of the crystal substrate are used to provide an optical frequency shifter. Optical integration was performed to overcome the drawbacks of bulk type optical system.

Further, when the light propagation direction, which is a physical characteristic of the crystal substrate having the light transmittance and the electro-optical effect, is the z-axis direction and the applied electric field direction is the x-axis direction, the phenomenon that the principal axis of the crystal is converted is used. Then, a method is adopted in which orthogonal components of the polarization state on the same optical axis are given different frequency shifts and output.

〔Example〕

FIG. 1 shows a configuration diagram of an embodiment of a dual-polarized orthogonal polarization light source according to the present invention.

1 has a light transmitting property and an electro-optical effect 1
3 shows a crystal substrate of axial crystals.

In this uniaxial crystal, two of the main refractive indices are usually the same, and the other one is different, so the operating characteristics of the device to be manufactured differ depending on the substrate in which cutting direction of the crystal is used, and the characteristics of the device are determined. The crystal orientation is a very important factor in the process.

In this embodiment, for example, a substrate made of lithium niobate rather than Y-CUT may be used.

Reference numeral 2 denotes an optical waveguide type Mach-Zehnder interferometer formed by an optical waveguide on the crystal substrate 1 by a Ti (titanium) thermal diffusion method. However, this optical waveguide has an initial Ti film thickness of 350
Å, initial Ti film width 4μm, diffusion temperature 995 ℃, diffusion time 6h,
The one prepared under the conditions of the primary diffusion atmosphere Ar and the secondary diffusion atmosphere O ₂ is used, and the optical waveguide in which the light propagation mode is the single mode at this time is used.

This optical waveguide type Mach-Zehnder interferometer 2 is composed of a Y-branch type optical waveguide 3, a first linear type optical waveguide 4a, a second linear type optical waveguide 4b and a multiplexing interference type optical waveguide 5. The first and second linear optical waveguides 4a and 4b are mounted such that the traveling direction of light is the z-axis direction of the uniaxial crystal substrate 1.

With this shape, one input light is emitted from the Y-branch optical waveguide 3
Then, it is divided into two with equal phase and equal amount of light, and after propagating through the first linear optical waveguide 4a and the second linear optical waveguide 4b respectively, one is again generated by the multiplexing interference optical waveguide 5. Will be synthesized.

Reference numerals 6 to 8 respectively denote the first to third electrodes that constitute the optical phase compensating means.

The first electrode 6, the second electrode 7, and the third electrode 8 are integrated to form a three-electrode type optical phase compensator 23, and further, modulation is performed without main axis conversion. Therefore, it is installed so that the applied electric field direction is applied in the y-axis direction on the first and second linear optical waveguides 4a and 4b.

9-12 show the 4th-7th electrodes, respectively.

The fourth electrode 9 and the fifth electrode 10 and the sixth electrode 11 and the seventh electrode 12 form a pair, respectively, and the first electrode 6 and
The second electrode 7 and the third electrode 8 are located after the first electrode 7,
And the second linear optical waveguides 4a and 4b are sandwiched so that the applied electric field direction is the x-axis direction, and the electrodes of the first and second optical phase modulators 24 and 25 are formed.

13 and 14 efficiently take light into the optical waveguide,
A light incident port obtained by optically polishing the end surface of the crystal substrate 1 and a light emitting port for outputting the light are shown.

15 is a DC power supply for supplying a constant voltage to the optical phase compensator 23 composed of the first electrode 6, the second electrode 7 and the third electrode 8, and 16 is the first and the first electrode. An AC power source for applying an AC voltage to each of the two optical phase modulators 24 and 25, 17 denotes the phase of the electric signal generated from the AC power source 16 with respect to the first optical phase modulator 24 and the second optical phase modulator 24. A π / 2 phase shifter that changes π / 2 [rad] with the phase modulator 25 is shown.

The operation of the orthogonal polarization dual-frequency generation light source according to the present invention will be described below with reference to FIGS. 1, 2, and 3.

In FIG. 1 and FIG. 2, the laser light incident on the orthogonally polarized wave dual frequency generation light source according to the present invention is a single linearly polarized light (either TE mode or TM mode) and has a single spectrum. Use the one that has.

The laser light incident from the light incident port 13 is split into the same phase and the same quantity of light in the Y-branching optical waveguide 3 while maintaining the polarization state when input.

Here, as shown in FIG. 2, after being split, the light guided on the optical path (I) side is the first guided light 21, and the light guided on the optical path (II) side is the second guided light. I'll call it 22.

Next, the split first guided light 21 and second split guided light 22 are
It simultaneously passes through the optical phase compensator 23 constituted by the first electrode 6, the second electrode 7 and the third electrode 8.

However, a potential difference is provided by the DC power supply 15 between the first electrode 6, the second electrode 7, and the third electrode 8 forming the optical phase compensator 23, so that the electro-optic The refractive index changes due to the effect, and the phase velocities of light change immediately below the first electrode 6 and the third electrode 8 of the first and second linear optical waveguides 4a and 4b.

The relative phase difference between the first guided light 21 and the second guided light 22 due to the change in the phase velocity changes depending on the voltage applied to the DC power supply 15. Here, the relative phase difference is adjusted to be π / 2 [rad].

In the optical phase compensator 23, the traveling direction of light is the z-axis direction,
Since the electric field direction is the y-axis direction, the principal axis conversion does not occur, and the polarization state is the same as when inputting.

The first guided light 21 and the second guided light 22 having a relative phase difference of π / 2 [rad] are respectively the fourth electrode 9 and the fifth electrode 10.
And a first optical phase modulator 24 composed of
And the seventh electrode 12 are input to the second optical phase modulator 25. At this time, the first optical phase modulator 24 continuously phase-modulates the first guided light 21 with the external signal sin Ωt, and the second optical phase modulator 25 receives the second signal with the external signal cos Ωt.
The phase of the guided light 22 is continuously modulated. However, Ω is the angular frequency of the modulation signal. Therefore, the frequencies of the first guided light 21 and the second guided light 22 cause frequency shift at an integral multiple of the angular frequency of the external modulation signal as represented by the Bessel function. Furthermore, the main axis of the crystal rotates because the light traveling direction is the z-axis direction and the applied electric field direction is the x-axis direction. As a result, the polarization state changes and both TE and TM modes are generated. It

In FIGS. 3 (a) and 3 (b), the first optical phase modulator 24 is shown.
TE mode, TM output from the second optical phase modulator 25
The spectrum for each mode is displayed by the rotation spectrum. The TE mode spectrum output from the first optical phase modulator 24 is on the left side of FIG. 3 (a), and the TM mode spectrum output from the first optical phase modulator 24 is on the right side of FIG. 3 (a). The left side of FIG. 3 (b) is the TE mode spectrum output from the second optical phase modulator 25, and the right side of FIG. 3 (b) is the TM mode spectrum output from the second optical phase modulator 25. is there. However, this display is based on the assumption that the magnitude of the amplitude in each spectrum is ignored and that the phase of the second guided light 22 is delayed by π / 2 [rad] with respect to the first guided light 21. Is.

As described above, the optical frequency after passing through the optical phase modulator is represented by a Bessel function because both modulation signals are essentially sinusoidal waves although their phases are different. Therefore, a signal shifted by an integer multiple of the angular frequency Ω of the external modulation signal is generated, and its amplitude changes according to the modulation index. For example, adjusting the intensity of the electric field to the optical phase modulator so that the modulation index is "1", components of the TE mode for components _{(ω 0 + Ω) (ω} 0 + 5Ω) is reduced 32dB Can TM
The component of (ω ₀ + 2Ω) and the component of (ω ₀ + 4Ω) can be similarly reduced with respect to the component of ω ₀ of the mode.
Here, ω ₀ is the angular frequency of incident light. When deciding the modulation index, the amount that is canceled when combined in the subsequent stage is also taken into consideration.

By receiving the phase modulation and adjusting the modulation index,
For example, (ω ₀ + Ω), the first guided light 21 and the second guided light 22 in which unnecessary frequency components other than ω ₀ are reduced are combined by the combining interference type optical waveguide 5, and TE mode Substantially two kinds of light having different frequencies of orthogonal polarization in the TM mode are emitted from the light emission port 14.

The comprehensive output is shown in FIG. 3 (c). The left side of FIG. 3C is the TE mode spectrum, and the right side is the TM mode spectrum. Similar to FIGS. 3A and 3B, the magnitude of the amplitude in each spectrum is ignored.

[Modification of Example]

In the examples so far, the uniaxial crystal substrate represented by LiNbO ₃ has been described.

The substrate material to which the technical idea of the present invention can be applied may be any material that exhibits an electro-optical effect.

For example, GaAs is an isotropic crystal because it belongs to the cubic crystal system, but is equivalent to a uniaxial crystal when the body diagonal line is taken as the axis. As described above, a substrate having an electro-optical effect even if it is an isotropic crystal can be used as the substrate of the present invention.

In addition, the optical phase compensator 23 described in the above embodiment
Is the first guided light 21 guided in the optical path (I) and the optical path (II)
The relative phase difference with the second guided light 22 that guides
d) This is one example of the optical phase compensating means provided for making the optical phase compensating means.

(1) As a first example, the Mach-Zehnder interferometer 2
Asymmetrical and changing the optical path lengths of the optical path (I) and the optical path (II) to make the relative phase difference between the first guided light 21 and the second guided light 22 π / 2 [rad]. Phase compensating means (2) As a second example, a material that changes the equivalent refractive index of the optical waveguide is selected and attached on either of the first and second linear optical waveguides 4a and 4b. And the first guided light
It is an optical phase compensating means for making the relative phase difference between 21 and the second guided light 22 π / 2 [rad].

〔The invention's effect〕

As described above, the optical layer including the Y-branch optical waveguide, the first and second linear optical waveguides, and the multiplexing interference optical waveguide on the surface layer of the crystal substrate having the optical transparency and the electro-optical effect as described above. The wave type Mach-Zehnder interferometer is manufactured so that the light propagating direction is the z-axis direction.
Optical phase compensating means for providing a difference in the propagation constants of the linear optical waveguides to generate a phase difference of π / 2 [rad] between the lights propagating through the two optical waveguides. Two phase modulations, which are placed before or after the optical phase compensator with the linear optical waveguide sandwiched therebetween, are composed of a fourth electrode, a fifth electrode, a sixth electrode and a seventh electrode. Since the device is configured such that the direction of the applied electric field is the x-axis direction, it is possible to output two types of light that are orthogonally polarized on the same optical axis and have different frequencies. In addition, this eliminates the need for large components such as beam splitters and reflecting mirrors. Therefore, the semiconductor process technology has enabled mass production and also eliminates the need for highly accurate optical adjustment, and has solved the problems that cannot be solved by the conventional technology.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an orthogonally polarized wave dual frequency generation light source according to the present invention, and FIG. 2 is an orthogonally polarized wave 2 according to the present invention.
Figure 3 is a simplified block diagram of the frequency generation light source. Figure 3 shows the spectrum of the TE mode and TM mode of the output light as a rotation spectrum, ignoring the amplitude, and (a) is the first optical phase modulation. The output light from the device 24, the left side is the TE mode spectrum, the right side is the TM mode spectrum, (b) is the output light from the second optical phase modulator 25, the left side is the TE mode spectrum, and the right side is TM. Mode spectrum, (c) is the combined output light, the left side is the TE mode spectrum, the right side is
The TM mode spectra are shown respectively. In the figure, 1 is a crystal substrate, 2 is an optical waveguide type Mach-Zehnder interferometer, 3 is a Y-branch optical waveguide, 4a and 4b are first and second linear optical waveguides, 5 is a multiplexing interference optical waveguide, 6 ~
12 is the first to seventh electrodes, 13 is a light entrance, 14 is a light exit,
15 is a DC power supply, 16 is an AC power supply, 17 is a π / 2 phase shifter, 21 and 22
Is the first and second guided light, 23 is the optical phase compensator as the optical phase compensating means, and 24 and 25 are the first and second optical phase modulators, respectively.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koichiro Miyagi 5-1027 Minami-Azabu, Minato-ku, Tokyo Anritsu Co., Ltd. (72) Inventor Akito Otani 5-1027 Minami-Azabu, Minato-ku, Tokyo Anritsu Incorporated (56) References Japanese Patent Laid-Open No. Sho 61-13231 (JP, A)

Claims (1)

[Claims] 1. A crystal substrate (1) having a light-transmitting property and an electro-optical effect, and a Y-branch type optical waveguide (3) provided on a surface layer of the crystal substrate, the first and second optical waveguides. An optical waveguide type Mach-Zehnder interferometer (2) comprising a linear optical waveguide (4a, 4b) and a multiplexing interference optical waveguide (5), and propagation constants of the first and second linear optical waveguides. Optical phase compensating means (23) that causes a phase difference of π / 2 between the lights propagating through both optical waveguides, and the above-mentioned first so that the electric field application is in the direction of converting the principal axis of the crystal. The fourth and fifth electrodes (9, 10) and the sixth and seventh electrodes are placed in front of or behind the optical phase compensating means with the first and second linear optical waveguides sandwiched therebetween. (11,12)
And first and second optical phase modulators (24, 25) that are driven by sinusoidal modulation signals that are out of phase with each other by π / 2
An orthogonal polarization dual-frequency generation light source comprising: and orthogonally polarized light on the same optical axis and outputting substantially two types of light having different frequencies.