JPH0695119B2 - Optical integrated frequency analyzer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical integrated frequency analyzer that analyzes the frequency of a high-frequency electric signal by utilizing the interaction between surface acoustic waves and light.

[Conventional technology]

Figure 5 is a paper by Kanazawa and Atsumi et al. “Force International Conference on Integrated Optics and Optical Fiber Communication, Tokyo, Technical Digest, pp. 258-259,”.
1983 "(4th Int.Conf.on Integrated Optics and Opti
cal Fiber Communication, Tokyo, Technical Digest, pp.
258-259, 1983) is a block diagram of an integrated optical frequency analysis device reported. In the figure, 1 is a piezoelectric substrate such as LiNbO ₃ and 2 is a two-dimensional optical waveguide prepared by vapor-depositing a metal such as Ti or Ni on the surface of the piezoelectric substrate 1 and then thermally diffusing it.
3 is a semiconductor laser attached to the end face of the optical waveguide 2, 4
And 5 are first and second geodesic lenses manufactured on the optical waveguide 2, and 6 and 7 are transducers created on the optical waveguide 2 and between the first geodesic lens 4 and the second geodesic lens 5. And a damper, 8 is a photodetector attached to the end face of the optical waveguide 2 facing the semiconductor laser 3, 9 is divergent light emitted from the semiconductor laser 3, 10 is parallel light, 11 is non-diffracted light, and 12 is Diffracted light, 13 is a surface acoustic wave excited by the transducer 6.

Next, the operation will be described. The divergent light 9 emitted from the semiconductor laser 3 and guided to the optical waveguide 2 is converted into parallel light 10 by the first geodesic lens 4, enters the second geodesic lens 5, and is further converted into converged light. The light is focused on the photodetector 8. When a high frequency electric signal is applied to the transducer 6, the transducer 6 excites a surface acoustic wave 13 having a period Λ corresponding to the frequency of the high frequency electric signal in the optical waveguide 2. The period Λ
Is the velocity of the surface acoustic wave 13 propagating in the optical waveguide 2 is Vs,
When the frequency of the high frequency electric signal is f _r , it is given by the first equation.

The surface acoustic wave 13 traverses the parallel light 10 and is then absorbed by a damper. When the surface acoustic wave 13 crosses the parallel light 10, the surface acoustic wave 13 acts on the parallel light 10 as a diffraction grating having a period Λ, and the parallel light 10 and the surface acoustic wave 13 are crossed so as to satisfy the Bragg condition. Therefore, a part of the parallel light 10 is diffracted at the angle θ ₁ given by the second equation.

Here, λ is the wavelength of the light emitted from the semiconductor laser 3, and neff is the effective refractive index for the light guided to the optical waveguide 2. That is, the parallel light 10 is divided into the non-diffracted light 11 and the diffracted light 12, which are respectively converged by the second geodesic lens 5 and condensed at the points A and B on the photodetector 8. The distance 1 between the light condensing points A and B is given by the third equation, where f ₂ is the focal length of the second geodesic lens 5.

l = f ₂ · θ ₁ (3) Here, θ ₁ is the angle shown in the second equation. By knowing l from the third equation, the frequency f _r of the high frequency electric signal applied to the transducer 6 can be obtained.

By the way, the first geodesic lens 4 and the second geodesic lens 5 are formed by forming a hemispherical recess on the surface of the piezoelectric substrate 1 and forming a metal such as Ti or Ni on the surface of the recess like the optical waveguide 2. Is produced by thermally diffusing the above, and the above optical waveguide layer has the same refractive index distribution as that of the above optical waveguide 2. When guided light enters the first and second geodesic lenses 4 and 5 from the optical waveguide 2, the guided light travels along the recess. At this time, the guided light travels along the shortest optical path according to the Fermat principle, so that the guided light is bent by the above-mentioned dent, and the dent has a function of a lens.

[Problems to be Solved by the Invention]

Since the conventional integrated optical frequency analyzer is configured as described above, the condensing point B of the diffracted light 12 cannot be detected by the photodetector 8 with a high signal-to-noise ratio (S / N). It is necessary that the condensed spot of B has no side lobe and that the background light is not superimposed.

In order to eliminate the side ropes, it is necessary to make the first and second geodesic lenses 4 and 5 free of aberrations. For this purpose, the shapes of the first and second geodesic lenses 4 and 5 should be designed values. On the other hand, extremely high precision processing must be performed so that the error is 1 μm or less. The above processing is performed by first cutting the piezoelectric substrate 1 with a diamond bite and then optically polishing the cut surface. However, it is necessary to process each lens one by one, and it requires a long processing time. Therefore, there is a problem that productivity is poor and expensive processing is performed. Further, the above processing has a problem that the reproducibility is poor and the yield of the lens is low.

Since the background light is generated by the scattering of the guided light, the optical waveguide 2
And the loss of the first and second geodesic lenses 4 and 5 can be reduced. However, the loss of the optical waveguide 2 is relatively small when LiNbO ₃ is thermally diffused with Ti, and is about 0.5 dB / cm. However, in the conventional optical integrated frequency analysis device, the second method is required to increase the frequency resolution. Since it is necessary to increase the focal length f ₂ of the geodesic lens 5, the piezoelectric substrate 1 also becomes relatively long, about 7 cm. Therefore, even in the optical waveguide 2 in which Ti is thermally diffused, the guided light suffers a loss of 3.5 dB, and a part of the lost guided light becomes background light. Further, it has been reported that the loss of the geodesic lens is as small as about 3 dB, but a loss of 6 dB occurs because the first and second geodesic lenses 4 and 5 are required, and is lost here. A part of the guided light becomes background light. Total guided light as above
The above S / N is at most 30d due to the loss of 9.5 dB and the fact that part of the lost guided light becomes background light.
There was a problem that B was the limit.

The present invention has been made in order to solve the above problems, and the focused spot of the diffracted light 12 at the point B has a high S /
The purpose is to obtain an inexpensive device that can be detected by N and has good productivity.

[Means for Solving the Problems]

The integrated optical frequency analyzer according to the present invention has a first focal point in the XZ plane when the emitted light of the semiconductor laser emitted in the Z direction is defined as two X and Y directions perpendicular to the Z direction and orthogonal to each other. It has a distance and is the first one in the YZ plane.
Condensing with a lens having a second focal length shorter than the focal length of to obtain the first converging point in the XZ plane and the second converging point in the YZ plane, and the optical waveguide and the transducer. The input end face of the piezoelectric substrate having the surface is installed at the second converging point to input the emitted light into the optical waveguide, and the input end face of the slab waveguide is provided on the output end face of the piezoelectric substrate. By connecting them, the first converging point is obtained at the output end face of the slab waveguide.

[Action]

In the present invention, a convergent light for converging emitted light diverging from the semiconductor laser at different first and second converging points by using lenses having different focal lengths in the XZ plane and the YZ plane. The piezoelectric substrate is positioned at the second converging point of the input end face to input the converged light into the optical waveguide, and the slab waveguide connects the input end face to the output end face of the piezoelectric substrate. Since the guided light is received from the optical waveguide and guided to the output end face located at the first focusing point to obtain the focused spot of the emitted light at the output end face, the focused spot of the diffracted light is high. It can be detected by the / N ratio.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. First
In the figure, 1 is a piezoelectric substrate, 2 is an optical waveguide, 3 is a semiconductor laser, 6 is a transducer, 7 is a damper, 8 is a photodetector, 11 is non-diffracted light, 12 is diffracted light, 13 is a surface acoustic wave, and 14 is a surface acoustic wave.
Is a lens and 15 is a slab waveguide.

Next, the operation will be described. The divergent light emitted from the semiconductor laser 3 in the Z direction has a focal length f ₃ in the XZ plane, and is shorter than f ₃ in the YZ plane, where the two directions perpendicular to the Z direction and orthogonal to each other are the X and Y directions. By the lens having the focal length f ₄ , it is condensed on the input end face D of the piezoelectric substrate 1 having the optical waveguide 2, the transducer 6 and the damper 7 on the surface in the YZ plane, and the output end face of the slab waveguide in the XZ plane. Focused on E. The light flux incident on the input end surface D of the piezoelectric substrate 1 has a linear intensity distribution along the X direction,
The light is input to the optical waveguide 2 in the XZ plane and becomes guided light. The guided light propagates through the optical waveguide 2 by a distance l _{1 and then} propagates through the optical waveguide 2.
The light is incident on the slab waveguide 15 connected to the output end face of and is condensed on the output end face E of the slab waveguide 15. When a high frequency electric signal is applied to the transducer 6, the surface acoustic wave 13 is excited in the optical waveguide 2. The guided light is partially diffracted by the surface acoustic wave 13 and is separated into diffracted light and non-diffracted light.
The non-diffracted light 11 and the diffracted light 12 are collected at points A and B on the output end face of the slab waveguide 15, respectively. A photodetector 8 is coupled to the output end face of the slab waveguide 15 to detect the above-mentioned condensing points A and B. The method of obtaining the frequency of the high-frequency electric signal applied to the transducer 6 from the condensing points A and B is the same as in the conventional example.

By the way, in the slab waveguide 15, as shown in FIG. 2, a guide layer 16 is sandwiched by a clad layer 17 having a lower refractive index than the guide layer 16, and the guided light is incident on the guide layer 16 by the optical waveguide 2. It propagates while repeating total reflection at the boundary between the guide layer 16 and the cladding layer 17. The guide layer 16 and the clad layer 17 are made of optical glass. If an optical glass transparent to the wavelength of the emitted light of the semiconductor laser 3 is selected, the propagation loss of the guided light in the guide layer 16 is 0.001 dB / cm or less and almost ignored. Therefore, the scattering of guided light, which causes background light, can be almost ignored. Further, l ₁ in FIG. 1 may be about ₁ cm. Therefore, the loss that the guided light receives is about 0.5 dB in the optical waveguide 2. The loss in the lens 14 can be made almost negligible by providing the lens 14 with an antireflection film. Further, since the lens 14 is a lens for spatially propagating light, unlike a waveguide type lens such as a geodesic lens, an aplanatic lens can be obtained because the setting / manufacturing technology has been established, and the spots of the condensing points A and B can be obtained. Since the light emitted from the semiconductor laser 3 is a Gaussian beam, the side lobe of the focused spot can be made negligibly small by setting the aperture of the lens 14 to a size such that the Gaussian beam is not eclipsed.

As described above, in the present embodiment, the loss of the light emitted from the semiconductor laser 3 before reaching the photodetector 8 is about 10 dB when input to the optical waveguide 2, and the propagation loss in the optical waveguide 2 is 0.5 dB. In the conventional example, the loss when inputting to the optical waveguide 2 is about 10 dB, the propagation loss in the optical waveguide 2 is about 3.5 dB, and the first and second geodesic lenses 6 and 7 In contrast to the total loss of 6 dB, which is 19.5 dB, the intensity of the guided light reaching the photodetector 8 is increased by 9 dB. Here, considering the case where the focused spot of the diffracted light 12 is detected by the photodetector 8 at the point B, FIG. 1 shows that the guided light intersecting with the surface acoustic wave 13 is convergent light as compared with the conventional example. However, the diffraction efficiency is reduced, but it can be suppressed to 2 dB or less by setting the convergence angle of the converged light to 1 ° or less. Therefore, in the device of this embodiment, 7d
Since a focused spot of the diffracted light 12 with an intensity higher than B can be obtained, the S / N when detecting this focused spot is 7 dB.
It can be improved above. Further, since the lens 14 can be easily designed and manufactured, a long-time processing and a low yield do not require an expensive geodesic lens, and an inexpensive device can be configured.

FIG. 3 is a diagram showing the configuration of an optical integrated frequency analyzer according to another embodiment of the present invention. In this embodiment, the lens 14 of the embodiment of FIG. 1 is replaced by a bulk lens 18 and a cylinder lens.
It is composed of 19. The bulk lens 18 collects the divergent light from the semiconductor laser 3 on the output end face E of the slab waveguide 15, and the cylinder lens 19 collects the convergent light from the bulk lens 18 only in the YZ plane on the input end face of the piezoelectric substrate 1. It is condensed on D and the converged light is input to the optical waveguide 2.

FIG. 4 is a block diagram showing still another embodiment of the present invention. The apparatus of this embodiment is the cylinder lens 19 of the apparatus shown in FIG.
The slab lens is replaced by 20. The slab lens 20 has a constant refractive index n ₀ in the X direction as shown in FIG. 5 (b) and a fourth refractive index n ₀ in the Y direction as shown in FIG. 5 (c).
It has a refractive index distribution n (Y) given by the formula.

_{n (Y) Y = n 0} (1-g 2 Y 2) 1/2 ... (4) where g is a constant representing the refractive index gradient. Such a slab lens 20 is sold, for example, by HOYA Corporation. The function of the slab lens 20 is the same as that of the cylinder lens 19 in the apparatus of FIG. 3, but by adjusting the length l ₂ shown in FIG.
The light flux from 14 can be condensed in the YZ plane to form a linear intensity distribution on the end face G along the X direction. By bonding the end face G and the input end face D of the piezoelectric substrate 1
A light beam having a linear intensity distribution can be input to the optical waveguide 2 in the XZ plane. To align and fix the cylinder lens 19 of FIG. 3 with the piezoelectric substrate 1, first fix the cylinder lens 19 to the holder, and then the holder and the piezoelectric substrate 1 are fixed.
Although a method of fixing and is adopted, in FIG. 4, the end surface G of the slab lens 20 and the input end surface D of the piezoelectric substrate 1 are fixed.
Can be easily fixed by simply surface-bonding and.

In the above embodiments, the piezoelectric substrate 1 is LiNbO ₃ and the optical waveguide 2 is LiNbO _{3 in} which Ti is thermally diffused. However, any piezoelectric substrate 1 having an acousto-optical effect may be used. The waveguide 2 may be manufactured by a proton exchange method or the like. Further, although the bulk lens 18 collects the emitted light of the semiconductor laser 3 both in the XZ plane and in the YZ plane in the above-described embodiment, a lens that condenses only in the XZ plane may be used.

〔The invention's effect〕

As described above, according to the present invention, the guided light is excited and the guided light is condensed by using the lenses having different focal lengths in the XZ plane and the YZ plane, and the guided light is guided by the slab waveguide. Since it is configured to lead to the detector, a focused spot of guided light without side lobes and background light can be obtained, so the focused spot can be detected with a high S / N, and an expensive waveguide type lens is required. Therefore, there is an effect that an inexpensive device can be obtained.

[Brief description of drawings]

1 is a block diagram of an optical integrated frequency analyzer according to an embodiment of the present invention, FIG. 2 is a block diagram of a slab waveguide, and FIGS. 3 and 4 are optical integrated frequencies according to other embodiments of the present invention. FIG. 5 is a configuration diagram of an analyzer, FIG. 5 is a diagram showing a shape and a refractive index distribution of a slab lens, FIG. 6 is a diagram showing how a light beam is condensed in the slab lens, and FIG. 7 is a conventional optical integrated frequency analysis. It is a block diagram of an apparatus. 1 is a piezoelectric substrate, 2 is an optical waveguide, 3 is a semiconductor laser, 4
Is a first geodesic lens, 5 is a second geodesic lens, 6 is a transducer, 7 is a damper, 8 is a photodetector, 9 is divergent light, 10 is parallel light, 11 is non-diffracted light, 12 is diffracted light, 13 is a surface acoustic wave, 14 is a lens, 15 is a slab waveguide, 16 is a guide layer, 17 is a clad layer, 18 is a bulk lens, 19 is a cylinder lens, and 20 is a slab lens. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (1)

[Claims] 1. When a semiconductor laser that emits emitted light in the Z direction and two directions perpendicular to the Z direction and orthogonal to each other are the X and Y directions, the first focal length is in the XZ plane,
It has a second focal length that is shorter than the first focal length in the YZ plane, and emits the emitted light of the semiconductor laser at the first focal point in the XZ plane and the second focal point in the YZ plane. A lens for converging on a light point, an optical waveguide existing in the XZ plane where the input end face is installed at the second converging point and into which the emitted light is input, and a waveguide light propagating in the optical waveguide are inclined. A piezoelectric substrate having a transducer on its surface that excites part of the guided light to separate the guided light into diffracted light and undiffracted light, and a transducer for exciting a surface acoustic wave. A slab waveguide connected to the output end face, the output end face being installed at the first condensing point; and a photodetector installed at the output end face of the slab waveguide for detecting the condensing position of the diffracted light. Providing and analyzing the frequency of the electrical signal applied to the transducer Optical integrated frequency analyzer characterized.