JP3708892B2 - Optical frequency comb generator and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical frequency comb generator, and is used in a field that requires a multi-wavelength, high-coherence standard light source such as optical communication, optical CT, or optical frequency standard, or a light source that can also utilize coherence between wavelengths. Applied.
[0002]
[Prior art]
In the case of measuring the optical frequency with high accuracy, heterodyne detection is performed in which the light to be measured is made to interfere with other light and an electric signal having an optical beat frequency generated is detected. The band of the laser beam that can be measured in this heterodyne detection is limited to the band of the light receiving element used in the detection system, and is about several tens of GHz.
[0003]
On the other hand, with the recent development of optoelectronics, it is necessary to further expand the measurable band of light in order to perform laser light control for frequency division multiplexing and frequency measurement of absorption lines distributed over a wide range.
[0004]
In order to meet such a demand for expanding the measurable bandwidth, a wideband heterodyne detection system using an optical frequency comb generator as shown in, for example, Japanese Patent Laid-Open No. 7-58386 has been proposed. . This optical frequency comb generator generates comb-shaped sidebands arranged at equal intervals on the frequency axis over a wide band, and the frequency stability of the generated sideband is equal to the frequency stability of incident light. It is almost equivalent. By heterodyne detection of the generated sideband and light to be measured, a wideband heterodyne detection system over several THz can be constructed.
[0005]
FIG. 3 is a configuration diagram of a conventional waveguide-type optical frequency comb generator 20. The waveguide type optical frequency comb generator 20 includes a waveguide type optical modulator 200. The waveguide type optical modulator 200 includes a substrate 201, a waveguide 202, an electrode 203, an incident side reflection film 204, an emission side reflection film 205, and an oscillator 206.
[0006]
The substrate 201 is a wafer such that, for example, a large crystal such as LiNbO ₃ or GaAs having a diameter of 3 to 4 inches grown by a pulling method is perpendicular to the x axis (when the crystal growth direction is the z axis). Or cut out so as to be perpendicular to the y-axis (hereinafter referred to as y-cut).
[0007]
The waveguide 202 is formed, for example, by diffusing Ti, and the refractive index of the layer constituting the waveguide 202 is set higher than that of other layers such as the substrate 201 in order to propagate light. The light incident on the waveguide 202 propagates while being totally reflected at the interface between the waveguide 202 and the substrate 201.
[0008]
The electrode 203 is made of a metal material such as Ti, Pt, or Au, for example, and drives and inputs an electric signal having a frequency fm supplied from the outside to the waveguide 202. As shown in FIG. 3, by providing the electrodes 203 so as to face each other on both sides of the waveguide 202, the angle formed by the light propagation direction in the waveguide 202 and the traveling direction of the modulated electric field becomes a right angle.
[0009]
The incident-side reflection film 204 and the emission-side reflection film 205 are provided to resonate light incident on the waveguide 202 and resonate by reciprocally reflecting light passing through the waveguide 202.
[0010]
The incident-side reflection film 204 is disposed on the light incident side of the waveguide type optical modulator 200, and light having a frequency ν ₁ is incident from a light source (not shown). Further, the incident-side reflection film 204 reflects light reflected by the emission-side reflection film 205 and having passed through the waveguide 202.
[0011]
The exit-side reflection film 205 is disposed on the light exit side of the waveguide type optical modulator 200 and reflects light that has passed through the waveguide 202. The exit-side reflection film 205 emits light that has passed through the waveguide 202 to the outside at a constant rate.
[0012]
The oscillator 206 is connected to the electrode 203 and supplies an electric signal having a frequency fm.
[0013]
In the waveguide-type optical frequency comb generator 20 having the above-described configuration, an electric signal synchronized with the time when the light reciprocates in the waveguide 202 is used as a drive input from the electrode 203 to the waveguide-type optical modulator 200. Compared with the case where the optical phase modulator 111 is passed only once, deep phase modulation several tens of times or more can be applied. Thereby, like the bulk type optical frequency comb generator 10, an optical frequency comb having a wide sideband can be generated, and the frequency interval between adjacent sidebands is the frequency fm of the inputted electric signal. Become equivalent.
[0014]
[Problems to be solved by the invention]
By the way, when manufacturing this waveguide type optical frequency comb generator 20, before laminating the incident side reflection film 204 and the emission side reflection film 205, both end surfaces are mirror-polished and finished with high accuracy. Incidentally, the reason for polishing is to prevent light scattering and to make the light propagation direction perpendicular to the incident side reflection film 204 and the emission side reflection film 205. At this time, in order to prevent damage such as scratches and notches at the edge portion of the substrate, the same waveguide type optical frequency comb generator 20 is generally polished in a state of being overlapped and fixed at the same time. is there.
[0015]
However, since the upper surface of the waveguide type optical frequency comb generator 20 has a level difference corresponding to the film thickness of the electrode 203, there are regions where damage is likely to occur and regions where damage is unlikely to occur. For example, since the upper surface of the electrode 203 is completely in close contact with the overlapping waveguide type optical frequency comb generator 20, it can be mirror-polished with high accuracy without causing damage at the edge portion of the substrate. In many cases, the edge portion is damaged during polishing because it is not in close contact with it. Incidentally, if an edge portion where the waveguide 202 is formed is damaged, light leaks from the waveguide 202 through the incident side reflection film 204, which is a great obstacle to improving finesse.
[0016]
Therefore, the present invention has been devised in view of the above-described problems, and reduces the loss of light propagating in the waveguide by reducing damage to the edge portion where the waveguide is formed during polishing. An object of the present invention is to provide a highly efficient optical frequency comb generator that can be reduced and a method for manufacturing the same.
[0017]
[Means for Solving the Problems]
An optical frequency comb generator according to the present invention comprises an oscillating means for oscillating a modulation signal of a predetermined frequency, and an incident side reflection film and an emission side reflection film, which are parallel to each other, to The resonating means for resonating light incident through the side reflecting film and an electro-optic crystal whose refractive index changes when an electric field is applied are formed in an x-cut or y-cut substrate, and the incident side The reflection film and the emission-side reflection film are arranged at the light incident end and the light emission end, respectively, and modulate the phase of the light resonated in the resonance means according to the modulation signal supplied from the oscillation means, An optical waveguide that generates sidebands centered on the frequency of incident light at intervals of the frequency of the modulation signal, and an electric field applied to the optical waveguide on the substrate based on the modulation signal oscillated from the oscillation means. An electrode for applying the light is provided, and the electrodes are arranged on the substrate so as to be parallel to each other on both sides of the optical waveguide, cover the entire surface except for the optical waveguide and the vicinity thereof, and light incident on the waveguide The width is changed as the end or the light emitting end is approached, and the upper portion of the light incident end and the upper portion of the light emitting end of the waveguide are formed to cover.
[0018]
According to another aspect of the present invention, there is provided an optical frequency comb generator manufacturing method for generating an optical frequency comb by modulating the phase of light resonated in an optical waveguide according to a supplied modulation signal. An optical waveguide forming step of forming an optical waveguide in the substrate; and on the substrate, the optical waveguide is disposed on both sides of the optical waveguide so as to be parallel to each other, covers the entire surface except for the optical waveguide and the vicinity thereof; An electrode forming step of forming an electrode covering the upper portion of the light incident end and the upper portion of the light emitting end in the waveguide by changing the width as approaching the light incident end or the light emitting end, and the metal layer is formed. A polishing step of laminating a plurality of substrates and simultaneously polishing the light incident end and the light emitting end, and an incident-side reflecting film and an emitting-side reflecting film parallel to each other on the light incident end and the light emitting end of the optical waveguide. husband And having a reflective film laminating step of laminating.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0023]
FIG. 1 is a diagram showing a configuration of a waveguide type optical frequency comb generator 1 to which the present invention is applied. The waveguide type optical frequency comb generator 1 includes a substrate 11, a waveguide 12, an electrode 13, a reflective film 14, a metal layer 15, and an oscillator 16.
[0024]
The substrate 11 is made of, for example, a large crystal such as LiNbO ₃ or LiTiO ₃ having a diameter of 3 to 4 inches grown by a pulling method (when the growth direction of the crystal is taken as the z axis) so as to be perpendicular to the x axis. Cut out into a wafer shape (hereinafter referred to as x-cut) or cut out so as to be perpendicular to the y-axis (hereinafter referred to as y-cut).
[0025]
The waveguide 12 is formed, for example, by diffusing Ti, Zn, H ⁺ or the like, and the refractive index of the layer constituting the waveguide 12 is larger than that of the other layers such as the substrate 11 in order to propagate light. It is set high. The light incident on the waveguide 12 propagates while being totally reflected at the boundary surface between the waveguide 12 and the substrate 11.
[0026]
The electrode 13 is made of a metal material such as Ti, Pt, Au, or Cu, for example, and drives and inputs an electric signal having a frequency fm supplied from the oscillator 16 to the waveguide 12. As shown in FIG. 1, by providing the electrodes 13 so as to face each other on both sides of the waveguide 12, the angle formed by the propagation direction of the light in the waveguide 12 and the traveling direction of the modulation electric field becomes a right angle. The electrode 13 is formed with a length in the longitudinal direction (light propagation direction) slightly shorter than that of the waveguide 12. In other words, it is formed so as to leave a region where the electrode 13 is not formed at both ends of the substrate 11.
[0027]
The reflective film 14 is provided to resonate the light incident on the waveguide 12 and is disposed on the incident side end face and the emission side end face, respectively. The reflection film 14 resonates by reciprocally reflecting light passing through the waveguide 12.
[0028]
The reflection film 14 disposed on the incident side end face receives light having a frequency ν _{1 from} a light source (not shown). Further, the reflection film 14 disposed on the incident side end face reflects the light reflected by the opposing reflection film 14 and passed through the waveguide 12.
[0029]
The reflective film 14 disposed on the emission side end surface reflects the light that has passed through the waveguide 12. Further, the reflection film 14 disposed on the emission side end face emits the resonated light to the outside at a constant rate by setting the reflectance to be slightly low.
[0030]
The metal layer 15 is made of a metal material such as Au or Cu, and is formed on the waveguide 12. The metal layer 15 is disposed on each of the incident side and the exit side of the waveguide 12, and is formed so as to increase in width as it approaches the light incident end or the light emitting end, for example, as shown in FIG. . Incidentally, the metal layer 15 may be a triangle whose width becomes wider as it approaches the light incident end or the light emitting end, or may be a trapezoid. The film thickness of the metal layer 15 is required to be equal to or greater than the film thickness of the electrode 13 as shown in the side view shown in FIG. It is also an essential condition that the metal layer 15 is disposed so as not to be in electrical contact with the electrode 13.
[0031]
By forming the metal layer 15 when the waveguide-type optical frequency comb generator 1 is manufactured, damage during polishing can be reduced, and finesse can be improved. Details of this manufacturing process will be described later. Further, since the metal layer 15 is formed in the vicinity of the incident side end face and in the vicinity of the emission side end face, the metal layer 15 hardly overlaps the position of the electrode 13 shorter than the waveguide 12. For this reason, when the high frequency wave transmitted through the oscillator 16 is applied to the electrode 13, the influence of the metal layer 15 can be suppressed. In other words, the formation of the metal layer 15 does not affect the generated optical frequency comb even when an electric signal is applied through the electrode 13. This is because the metal layer 15 has a shape that increases in width as it approaches the light incident end or light output end, such as a triangle or a trapezoid, so that the proportion of the metal layer 15 in the region near the electrode 13 is small. Therefore, further effects can be expected.
[0032]
In addition, since it becomes possible to suppress an electrostatic capacitance by making the area of the metal layer 15 small and to suppress the influence on a high frequency characteristic, it is desirable that the surface area of the metal layer 15 is as small as possible.
[0033]
If the presence of the metal layer greatly affects the modulation electric field, it is necessary to cut off the metal layer after the waveguide-type optical frequency comb generator is manufactured. However, when the metal layer needs to be laminated on the upper portion of the waveguide 12, there is a possibility of causing damage to the waveguide 12 when the metal layer is cut, which causes a deterioration of finesse. In addition, it introduces a process of cutting the metal layer after fabrication, which increases the burden of labor. In particular, an optical device such as the present invention can be manufactured in large quantities from an optical communication system or the like by following as simple a production process as possible. Can meet demand.
[0034]
In this respect, the metal layer 15 described above has almost no influence on the modulation electric field applied to the waveguide 12 even if it is laminated as it is when generating a frequency comb. It is no longer necessary to cut out such a metal layer 15 after producing the vessel 1. For this reason, the waveguide type optical frequency comb generator 1 to which the present invention is applied does not damage the waveguide 12 by cutting off the metal layer 15, so that the finesse is not deteriorated and the production process is simplified. Therefore, the burden of labor can be reduced, and as a result, production efficiency can be improved.
[0035]
The present invention is not limited to the embodiment shown in FIG. For example, as shown in FIG. 2, as an alternative to the metal layer 15, a waveguide-type optical frequency comb generator 2 in which an electrode 13 covers the upper part of the incident end face and the upper part of the exit end face in the waveguide 12 is applied. Also good. Hereinafter, the waveguide type optical frequency comb generator 2 will be described with reference to FIG. In addition, the same code | symbol is attached | subjected to the component same as the waveguide type optical frequency comb generator 1, and description is abbreviate | omitted.
[0036]
The waveguide type optical frequency comb generator 2 includes a substrate 11, a waveguide 12, an electrode 23, a reflective film 14, a metal layer 15, and an oscillator 16.
[0037]
The electrode 23 is made of a metal material such as Ti, Pt, Au, or Cu, for example, and drives and inputs an electric signal having a frequency fm supplied from the outside to the waveguide 12. As shown in FIG. 2, by providing the electrodes 23 so as to face each other on both sides of the waveguide 12, the angle formed by the propagation direction of light in the waveguide 12 and the traveling direction of the modulation electric field becomes a right angle. The electrode 23 is formed so that the length in the longitudinal direction (light propagation direction) is substantially equal to that of the waveguide 12. In other words, it is formed so as to leave a region where the electrode 12 is not formed at both ends of the substrate 11.
[0038]
Further, as shown in FIG. 2, the electrode 23 may cover the upper portion of the waveguide 12 so as to increase in width as it approaches the light incident end or the light emitting end. Thereby, the role played by the metal layer 15 can be replaced by the electrode 23.
[0039]
Next, a manufacturing method of the waveguide type optical frequency comb generator 1 to which the present invention is applied will be described.
[0040]
First, a large crystal such as LiNbO ₃ or GaAs having a diameter of 3 to 4 inches grown by a pulling method or the like is x-cut or y-cut to form a substrate 11, and, for example, Ti is diffused in the substrate to obtain a waveguide. 12 is formed.
[0041]
Next, the electrode 13 made of the above-described metal material is formed on the surface of the substrate 11 by performing processes such as patterning, vapor deposition, and lift-off, for example.
[0042]
Next, the metal layer 15 is formed through processes such as patterning, vapor deposition, and lift-off, for example. By forming the metal layer 15, the step at the edge portion of the waveguide type optical frequency comb generator can be eliminated.
[0043]
Next, the incident side end face and the emission side end face are polished. Incidentally, the reason for polishing is to prevent light scattering and to make the light propagation direction perpendicular to the reflection film 14.
[0044]
When the end face is actually polished, the same waveguide type optical frequency comb generator is polished at the same time in a state where it is fixed by overlapping several stages. By laminating the metal layer 15 corresponding to the film thickness of the electrode 13, a step corresponding to the film thickness of the electrode 13 can be eliminated. For this reason, edge portions on both end faces of the waveguide are not exposed to the surface during polishing, and damage such as scratches and cutouts is hardly caused. Further, since the upper surface of the electrode 13 is in close contact with the overlapping waveguide type optical frequency comb generator, it can be mirror-polished with high accuracy without causing damage at the edge portion described above. Since the metal layer 15 having the same thickness as the electrode 13 is in close contact with each other, uniform stress is applied to any surface during polishing. As a result, there are no areas where damage is likely to occur and areas where damage is unlikely to occur at the edge portion of the substrate, and polishing can be performed with high accuracy. Further, since damage at the edge portion of the waveguide 202 can be prevented during polishing, light leakage can be prevented and finesse can be expected to be improved.
[0045]
Finally, the reflection film 14 is laminated on the light incident end and the light exit end of the optical waveguide, and the oscillator 16 and the electrode 13 are wired.
[0046]
Incidentally, when the waveguide type optical frequency comb generator 2 is manufactured, in the process of forming the electrode 23, processing such as patterning is performed so as to have the shape of FIG.
[0047]
That is, in the method of manufacturing a waveguide-type optical frequency comb generator to which the present invention is applied, the film thickness is greater than or equal to the film thickness of the electrode at the upper part of the light incident end and the upper part of the light output end of the optical waveguide By introducing a step of forming a metal layer so that the width becomes wider as it approaches the incident end or the light emitting end, the influence on the modulation electric field supplied for generating the optical frequency comb is reduced, and the light is reduced. Finesse can be improved by reducing damage during polishing of the incident end and the light emitting end.
[0048]
【The invention's effect】
As described above in detail, the optical frequency comb generator and the manufacturing method thereof according to the present invention are arranged on the substrate so as to be parallel to both sides of the optical waveguide, and the entire surface excluding the optical waveguide and its vicinity. Introducing an electrode forming step of forming an electrode covering the upper part of the light incident end and the upper part of the light emitting end of the waveguide, the width of which is changed as the light incident end or the light emitting end of the waveguide is approached. To do. Thereby, finesse can be improved by reducing damage at the time of polishing of the light incident end and the light emitting end while reducing the influence on the modulation electric field supplied when generating the optical frequency comb.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a specific configuration example of a waveguide-type optical frequency comb generator to which the present invention is applied.
FIG. 2 is a diagram for explaining a configuration in which a role of a metal layer is replaced by an electrode in a waveguide type optical frequency comb generator to which the present invention is applied.
FIG. 3 is a diagram showing a configuration of a conventional waveguide-type optical frequency comb generator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Waveguide type optical frequency comb generator, 11 Substrate, 12 Waveguide, 13 Electrode, 14 Reflective film, 15 Metal layer, 16 Oscillator

Claims (2)

Oscillating means for oscillating a modulation signal of a predetermined frequency;
A resonance means configured to resonate light incident through the incident-side reflection film, the incident-side reflection film and the emission-side reflection film being parallel to each other;
An electro-optic crystal whose refractive index changes when an electric field is applied, is formed in an x-cut or y-cut substrate, and the incident-side reflection film and the emission-side reflection film are formed at the light incident end and the light emission end. The phase of the light resonated in the resonance means is modulated in accordance with the modulation signal supplied from the oscillation means, and the sideband centered on the frequency of the incident light is used as the modulation signal. Optical waveguides generated at frequency intervals;
On the substrate, an electrode for applying an electric field to the optical waveguide on the basis of a modulation signal oscillated from the oscillating means,
The electrodes are arranged on the substrate so as to be parallel to both sides of the optical waveguide, cover the entire surface except the optical waveguide and the vicinity thereof, and approach the light incident end or light emitting end of the waveguide. An optical frequency comb generator having a width changed so as to cover an upper part of the light incident end and an upper part of the light outgoing end of the waveguide . In a method of manufacturing an optical frequency comb generator for generating an optical frequency comb by modulating the phase of light resonated in an optical waveguide according to a supplied modulation signal,
an optical waveguide forming step of forming an optical waveguide in an x-cut or y-cut substrate; On the substrate, the optical waveguide is disposed so as to be parallel to both sides of the optical waveguide, covers the entire surface except for the optical waveguide and the vicinity thereof, and the width is changed as it approaches the light incident end or light emitting end of the waveguide. Forming an electrode covering the upper part of the light incident end and the upper part of the light emitting end of the waveguide;
A polishing step of laminating a plurality of substrates on which the metal layer is formed, and polishing the light incident end and the light emitting end simultaneously,
A reflection film laminating step of laminating an incident side reflection film and an emission side reflection film parallel to each other on the light incident end and the light exit end of the optical waveguide
A method of manufacturing an optical frequency comb generator, comprising:

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