JP2758464B2 - Frequency analyzer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency analyzer that analyzes the frequency of an electric signal by utilizing the interaction between surface acoustic waves and light.

[Prior Art] FIG. 5 is a report by Kanazawa and Atsumi et al., “Force International Conference on Integrated Optics and Optical Fiber Communication, Tokyo, Technical Digest, 258-2”
59, 1983 ”(4th Int. Conf.on Integrated Optics an
d optical Fiber Communication, Tokyo, Technical Dige
FIG. 1 is a configuration diagram of a frequency analyzer reported in St., pp. 258-259, 1983). In the figure, 1 is a piezoelectric substrate such as LiNbO ₃ , 2 is a two-dimensional optical waveguide manufactured by depositing a metal such as Ti or Ni on the surface of the piezoelectric substrate 1 and then thermally diffusing the same.
3 is a semiconductor laser attached to the end face of the optical waveguide 2,
4 and 5 are the first and second fabricated on the optical waveguide 2.
, 6 and 7 are a transducer and a damper manufactured on the optical waveguide 2 and between the first and second geodesic lenses 4 and 5, and 8 is opposed to the semiconductor laser 3 of the optical waveguide 2. A photodetector attached to the end face, 9 is a divergent light emitted from the semiconductor laser 3, 10 is a parallel light, 11 is an undiffracted light, 12 is a diffracted light, and 13 is a surface acoustic wave excited by the transducer 6. It is.

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 convergent light,
The light is condensed on the photodetector 8. When an electric signal is applied to the transducer 6, the surface acoustic wave having a period Λ corresponding to the frequency of the electric signal is generated by the transducer 6.
13 is excited in the optical waveguide 2. The period Λ is given by the first equation, where V _{s is} the velocity of the surface acoustic wave 13 propagating in the optical waveguide 2 and fr is the frequency of the electric signal.

After traversing the parallel light 10, the surface acoustic wave 13 is absorbed by the damper. When the surface acoustic wave 13 crosses the parallel light 10, the surface acoustic wave 13 acts as a diffraction grating having a period of Λ with respect to the parallel light 10, and the parallel light 10 and the surface acoustic wave 13 intersect so as to satisfy the Bragg condition. and for that, a part of the parallel light 10 is diffracted at an angle theta ₁ given by the second equation.

Here, λ is the wavelength of the light emitted from the semiconductor laser 3, and n _eff is the effective refractive index for the light guided through the optical waveguide 2.
That is, the parallel light 10 is split into the non-diffracted light 11 and the diffracted light 12, each of which is converged by the second geodesic lens 5 and condensed on points A and B on the photodetector 8. The distance 1 between the converging points A and B is given by the third equation, where fz is the focal length of the second geodesic lens 5.

1 = fz · θ ₁ (3) Here, θ ₁ is an angle represented by the second equation. By knowing l from the third equation, the frequency fr of the electric signal applied to the transducer 6 can be obtained.

Incidentally, the first and second geodesic lenses 4 and 5 are formed by forming a hemispherical recess on the surface of the piezoelectric substrate 1, and are formed on the surface of the recess in the same manner as in the manufacture of the optical waveguide 2.
The optical waveguide is manufactured by thermally diffusing a metal such as Ti or Ni. The optical waveguide has the same refractive index distribution as the 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 depression. At this time, the guided light travels on the shortest optical path according to the Fermat's principle, and is bent by the above-described dent, and the dent acts as a lens.

[Problems to be solved by the invention]

Since the conventional frequency analysis device is configured as described above, the focus point B of the diffracted light 12 is detected using the photodetector 8 with a high signal-to-noise ratio (S / N). It is necessary that the focused spot of B has no side lobe and no background light.

By the way, in order to eliminate this side lobe,
In addition, the second geodesic lenses 4 and 5 must have no aberration. For this purpose, the shape must be processed with high precision so that an error with respect to a design value is 1 micron or less. The above processing is performed by first cutting the piezoelectric substrate 1 with a diamond cutting tool, and then optically polishing the cut surface. There is a problem in that the processing is expensive and the yield is low because of extremely poor reproducibility.

On the other hand, to eliminate the background light, the background light is mainly
Since the light is generated by the substrate propagating light 20 and the first and second scattered lights 21 and 22 shown in FIG. 7 and FIG. 6 and 7 are views of FIG. 5 as viewed from above and from the side, and FIG. 7 is a cross-sectional view passing through the center of the geodesic lenses 4 and 5 in FIGS. 1 and 2.

The substrate propagating light 20 of the light emitted from the semiconductor laser 3 is not guided by the optical waveguide 2 but penetrates the piezoelectric substrate 1 and propagates. The first scattered light 21 is generated when the guided light passes through the first and second geodesic lenses 4 and 5, and the second scattered light 22 is generated when the guided light propagates through the optical waveguide 2. What happens.

And it is difficult to reduce the substrate propagation light 20,
There is a problem that background light generated by this is inevitable. Further, in order to eliminate the first scattered light 21, it is necessary to eliminate the loss of the first and second geodesic lenses 4 and 5. However, at present, even a small loss is about 3 dB. Therefore, the first and second two geodesic lenses have a loss of as much as 6 dB, and a part of the lost light becomes a background light. Further, in order to eliminate the second scattered light 22, it is necessary to eliminate the loss of the optical waveguide 2. However, even if the optical waveguide 2 is manufactured by diffusing Ti, which is said to have a small loss, the loss is about 0.5 dB / cm. There is a current situation. Since the optical waveguide 2 from the semiconductor laser 3 to the photodetector 8 has a length of about 7 cm, the guided light suffers a loss of 3.5 dB and a part of the lost light becomes a background light.

As described above, the loss of the optical waveguide 2, the first and second geodesic lenses 4 and 5, not only generates the background light but also attenuates the guided light by 9.5 dB in total, so that the S / N becomes 30 dB at most. There was a problem that it did not lead to conversion.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to detect a condensed spot of the diffracted light 12 at the point B with high S / N, and at the same time, to provide a high productivity and an inexpensive frequency analyzer. The purpose is to gain.

[Means for solving the problem]

According to the frequency analysis apparatus of the present invention, when a semiconductor laser that emits outgoing light in the Z direction and two directions perpendicular to the Z direction that are orthogonal to each other are the X direction and the Y direction, the first focal point is located in the XZ plane. And has a second focal length different from the first focal length in the YZ plane, and emits the light emitted from the semiconductor laser to a first focal point in the XZ plane, And a lens that converges on a second converging point, an optical waveguide that exists in the XZ plane, an input end face is installed on the second converging point, and the outgoing light is input into the optical waveguide and A piezoelectric substrate provided with a transducer that excites a surface acoustic wave that obliquely intersects the guided light propagating through and diffracts a part of the guided light and separates the guided light into diffracted light and undiffracted light; The piezoelectric substrate is connected to an output end face, and the output end face is the first end face. A slab waveguide installed at a converging point, a photodetector installed at an output end face of the slab waveguide to detect a condensing position of the diffracted light, and an output end face of the piezoelectric substrate or the slab waveguide And a reflection film or an absorption film formed on a portion of the input end face which does not hinder the guided light, and analyzes the frequency of the electric signal applied to the transducer.

[Action]

In the present invention, the lens has different focal lengths in the XZ plane and the YZ plane, and converts outgoing light diverging from the semiconductor laser into convergent light at different first and second focal points. The convergence light is input to the optical waveguide with the end face located at the second condensing point, and the slab waveguide receives the guided light from the optical waveguide by connecting the input end face to the output end face of the piezoelectric substrate. The light is guided to the output end face located at the first condensing point to obtain a condensed spot of the emitted light at the output end face, and the reflection film or the absorption film is formed on the output end face of the piezoelectric substrate or the input end face of the slab waveguide. Since it is configured so as to block the substrate propagation wave in the piezoelectric substrate from entering the slab waveguide by being located at a portion that does not hinder the guided light, it is possible to detect the spot of the diffracted light with a high S / N. it can.

〔Example〕

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a frequency analyzer according to one embodiment of the present invention, in which 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 undiffracted light, 12 is diffracted light, 13 is a surface acoustic wave, 14 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 in the XY plane and a focal length shorter than f in the YZ plane, assuming that two directions perpendicular to the Z direction and orthogonal to each other are the X and Y directions. f '
In the YZ plane, the light is focused on the input end face D of the piezoelectric substrate 1 having the optical waveguide 2, the transducer 6, and the damper 7 in the YZ plane, and the slab waveguide 15 in the XZ plane. Is focused on the output end face E of The light beam incident on the input end face D of the piezoelectric substrate 1 has a linear intensity distribution along the X direction, and is input to the optical waveguide 2 in the XZ plane to become guided light.

The guided light propagates through the optical waveguide 2 for a distance l ₁ , then enters the slab waveguide 15 connected to the output end face of the piezoelectric substrate 1, and is focused on the output end face E of the slab waveguide 15. Here, when an 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 waves 13 and separated into a diffracted light 12 and a non-diffracted light 11. The diffracted light 12 and the undiffracted light 11 converge at points B and A 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 light converging points A and B. The method of obtaining the frequency of the electric signal applied to the transducer 6 from the light converging points A and B is the same as in the conventional example.

By the way, the slab waveguide 15 is such that the guide layer 16 is sandwiched between first and second cladding layers 17 and 18 having a lower refractive index than the guide layer 16 as shown in FIG. 2 and enters the guide layer 16 and propagates while repeating total reflection at the boundary between the guide layer 16 and the first and second cladding layers 17 and 18. Guide layer 16 and first and second layers
The cladding layers 17 and 18 are made of optical glass with little absorption of light, the propagation loss of guided light is less than 0.001 dB / cm and can be almost ignored, and therefore, the scattering of guided light which causes background light can be almost ignored. . Also, l 1 in FIG. ₁ is 1 cm
Therefore, the loss in the optical waveguide 2 is only about 0.5 dB. Further, the loss in the lens 14 can be made almost negligible by providing the antireflection film. further,
Since the lens 14 is for spatially propagating light, unlike a waveguide type lens such as a geodesic lens, an almost aberration-free lens can be obtained because the design and manufacturing technology has been established, and the spots at the focal points A and B can be obtained. Is almost diffraction-limited, and since the emitted light of the semiconductor laser 3 is a Gaussian beam, the side lobe of the focused spot can be ignored by setting the aperture of the lens 14 to a size such that the Gaunsian beam is not vignetted. Can be smaller. further,
A reflection film or an absorption film is formed on the output end face of the piezoelectric substrate 1 or the input end face of the slab waveguide 15. FIG. 3 shows an example in which the reflection film 19 is formed. FIG. 3 (a) shows an output end face of the piezoelectric substrate 1, and a reflection film indicated by oblique lines.
Reference numeral 19 is formed in a portion other than the optical waveguide 2. FIG. 3 (b) shows an input end face of the slab waveguide 15, and a reflection film 19 shown by oblique lines is formed on the first cladding layer 17.

As shown in FIG. 4, the reflection film 19 reflects the light that has become the substrate propagation light 20 in the piezoelectric substrate 1 instead of the guided light 23 when the light is made incident on the optical waveguide 2. 20 cannot enter the slab waveguide 15. For this reason, the substrate propagation light 20 does not enter the photodetector 8 and becomes the background light.

The reflection film 19 is made of, for example, Al (aluminum) or Au.
It can be easily formed by evaporating a metal such as (gold). Instead of the reflection film 19, an absorption film that absorbs the substrate propagation light 20 may be used. The absorption film can be easily formed by evaporating a metal such as C (carbon).

As described above, in the present embodiment, the loss of the light emitted from the semiconductor laser 3 to reach the photodetector 8 is the loss of 10 dB when input to the optical waveguide 2 and the propagation loss of 0.5 dB in the optical waveguide 2. In the conventional example, the loss at the time of input to the optical waveguide 2 is 10 dB, the propagation loss in the optical waveguide 2 is 3.5 dB, the first and second geodesic lenses 4
5 and the loss of 6 dB, which is a total of 19.5 dB, while the intensity of the guided light reaching the photodetector 8 can be increased by 9 dB.

Here, considering the case where the condensed 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 a convergent light as compared with the conventional example. The diffraction efficiency is reduced only by this, but by setting the convergence angle of the convergent light to 1 ° or less, it is possible to suppress the decrease to 2 dB or less. Therefore, in this embodiment, a condensed spot of the diffracted light 12 having an intensity greater than that of the conventional example by 7 dB or more can be obtained, and since there is almost no background light, the S / N can be greatly improved.

In addition, since the lens 14 can be easily designed and manufactured, an inexpensive device can be configured as compared with a conventional example using a geodesic lens which is an expensive lens due to long processing and low yield.

[Effects of the Invention] As described above, according to the frequency analyzer according to the present invention, the semiconductor laser that emits emitted light in the Z direction, and two directions perpendicular to the Z direction that are orthogonal to each other are defined as the X direction and the Y direction. The first focal length in the XZ plane and YZ
It has a second focal length different from the first focal length in the plane, and emits the light of the semiconductor laser to a first focal point in the XZ plane and a second focal point in the YZ plane. A lens for condensing light at a light condensing point, and an optical waveguide that exists in the XZ plane, the input end face is located at the second light condensing point, and the optical waveguide to which the outgoing light is input and the light propagating through the optical waveguide. And a piezoelectric substrate having a transducer that excites a surface acoustic wave that diffracts part of the guided light and separates the guided light into diffracted light and undiffracted light, and an input end face of the piezoelectric substrate. A slab waveguide connected to the output end face, the output end face being provided at the first converging point, and a photodetector being provided at the output end face of the slab waveguide and detecting a condensing position of the diffracted light; The output end face of the piezoelectric substrate or the input end face of the slab waveguide. Since a reflection film or an absorption film formed on a portion that does not hinder the guided light is provided, and the frequency of the electric signal applied to the transducer is analyzed, a focused spot without side lobes and background light can be obtained. Therefore, the above focused spot can be detected with a high S / N, and an inexpensive device can be obtained because an expensive waveguide type lens is not required.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a frequency analyzer according to an embodiment of the present invention, FIG. 2 is a configuration diagram of a slab waveguide of the device of FIG. 1,
FIG. 3 is a view showing a portion where a reflection film is formed in the apparatus shown in FIG. 1, FIG. 4 is a view showing a state of blocking light transmitted through the substrate, FIG. 5 is a configuration diagram of a conventional frequency analyzer, and FIG. Is a top view of a conventional frequency analyzer, and FIG. 7 is a side view of the conventional frequency analyzer. In the figure, 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 undiffracted light, 12 is diffracted light, 13 is surface acoustic wave, 14 is a lens, 15 is a slab waveguide, 16 is a guide layer, 17 is a first cladding layer,
18 is the second cladding layer, 19 is the reflection film, 20 is the substrate propagation light,
21 is the first scattered light, 22 is the second scattered light, and 23 is the guided light. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. A semiconductor laser that emits outgoing light in a Z direction, and two directions perpendicular to the Z direction that are orthogonal to each other are an X direction and a Y direction.
In the case where the direction is the direction, it has a first focal length in the XZ plane, and has a second focal length different from the first focal length in the YZ plane.
The first focal point in the plane and the second focal point in the YZ plane
A lens that converges at the converging point, and an optical waveguide that exists in the XZ plane, the input end face is located at the second converging point, and the optical waveguide to which the outgoing light is input and the optical waveguide that propagates through the optical waveguide. A piezoelectric substrate having a transducer for exciting a surface acoustic wave that obliquely intersects the wave light and diffracts a part of the guided light to separate the guided light into diffracted light and undiffracted light; A slab waveguide connected to the output end face of the slab waveguide, the output end face being provided at the first converging point; and a photodetector provided at the output end face of the slab waveguide, detecting the condensing position of the diffracted light. And a reflection film or an absorption film formed on a portion of the output end face of the piezoelectric substrate or the input end face of the slab waveguide that does not hinder the guided light, and analyzes a frequency of an electric signal applied to the transducer. Lap characterized by that Wave number analyzer.