JP3848883B2 - Optical resonator and optical frequency comb generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical resonator and an optical frequency comb generator, and requires a standard light source having high coherence at multiple wavelengths, such as optical communication, optical CT, and optical frequency standard, or a light source capable of utilizing coherence between wavelengths. It is applied to the field.
[0002]
[Prior art]
The optical resonator is applied to various optical application technologies such as optical frequency measurement and minute signal detection in optical communication. In particular, with the recent development of optoelectronics, a waveguide-type optical resonance that resonates the light confined in the waveguide in order to meet the demands of laser light control for frequency division multiplexing and frequency measurement of absorption lines distributed over a wide range. The device is used a lot.
[0003]
A conventional configuration example of such a waveguide type optical resonator will be described in detail.
[0004]
FIG. 17 shows a conventional waveguide type optical resonator 7. The waveguide type optical resonator 7 includes a substrate 71, a waveguide 72, an incident side reflection film 73, and an emission side reflection film 74.
[0005]
The substrate 71 is, for example, a 3-4 inch diameter LiNbO grown by a pulling method. ₃ A large crystal such as GaAs or GaAs is cut into a wafer. On the cut-out substrate 71, a process such as mechanical polishing or chemical polishing may be performed in order to grow the waveguide 72 by proton exchange, titanium diffusion, or the like.
[0006]
The waveguide 72 is set to have a higher refractive index than the other layers including the substrate 71 in order to propagate the light incident through the incident-side reflection film 73. For this reason, the light incident on the waveguide 72 propagates while repeating meandering due to the refractive index gradient, and exits to the outside via the exit-side reflection film 74. Note that the waveguide 72 preferably has a single mode of propagation mode.
[0007]
The incident-side reflection film 73 and the emission-side reflection film 74 are so-called reflection mirrors formed on the end face of the waveguide, and resonate by reciprocally reflecting light propagating through the waveguide 72. Incidentally, the incident-side reflection film 73 and the emission-side reflection film 74 are dielectric multilayer films having a reflectance close to 100%, for example, and are obtained by alternately stacking thin films having different refractive indexes. In order to take out the resonated light to the outside, the reflectance of the exit side reflection film 74 is set slightly lower than 100%.
[0008]
An incident side optical fiber 8 and / or an output side optical fiber 9 may be arranged outside the optical resonator.
[0009]
The incident side optical fiber 8 emits light through the core layer 82 formed inside the clad layer 81. The emitted light propagates through the waveguide 72 in the waveguide type optical resonator 7 through the incident side reflection film 73.
[0010]
Light is incident on the output side optical fiber 9 from the waveguide type optical resonator 7 through the output side reflection film 74. The emission side optical fiber 9 propagates the incident light through the core layer 92 formed inside the cladding layer 91.
[0011]
Incidentally, the optical coupling between the incident side optical fiber 8, the outgoing side optical fiber 9, and the waveguide 72 is not limited to the case where light enters and exits through a gap as shown in FIG. The light-emitting side optical fiber 9 and the waveguide 72 may be configured to contact each other. In addition, when light is incident / exited through an aspheric lens (not shown), a front spherical lens (not shown) is provided at the end of each optical fiber. In some cases, the spot size of the light is adjusted.
[0012]
An application example of the waveguide type optical resonator 7 using this optical waveguide is an optical frequency comb generator or the like. This optical frequency comb generator generates comb-shaped sidebands arranged at equal intervals on the frequency axis over a wide band. Then, by performing heterodyne detection on the generated sideband and the light to be measured, it is possible to construct a wideband heterodyne detection system over several THz.
[0013]
FIG. 18 shows the basic structure of this conventional optical frequency comb generator 30.
[0014]
The optical frequency comb generator 30 uses an optical resonator 310 including an optical phase modulator 311 and reflecting mirrors 312 and 313 disposed so as to face each other via the optical phase modulator 311.
[0015]
The optical resonator 310 resonates light Lin incident at a slight transmittance through the reflecting mirror 312 between the reflecting mirrors 312 and 313, and part of the light Lout is emitted through the reflecting mirror 313. The optical phase modulator 311 is made of an electro-optic crystal for optical phase modulation whose refractive index changes when an electric field is applied, and the frequency applied to the electrode 316 with respect to the light passing through the optical resonator 310. Phase modulation is applied according to the electric signal of fm.
[0016]
In this optical frequency comb generator 30, an electric signal synchronized with the time when light reciprocates in the optical resonator 311 is used as a drive input from the electrode 316 to the optical phase modulator 311, whereby the optical phase modulator 311 is turned on once. It is possible to apply deep phase modulation several tens of times or more compared to the case of passing only through. As a result, hundreds of higher-order sidebands can be generated, and the frequency interval fm between adjacent sidebands is all equal to the frequency fm of the input electrical signal.
[0017]
[Problems to be solved by the invention]
Incidentally, in order to reduce the loss and increase the efficiency of the waveguide type optical resonator 7, it is necessary to improve the finesse of the entire resonator by reducing the optical loss in the waveguide 72. The optical loss in the waveguide mold 72 is caused by damage to the waveguide 72 itself, reflectivity or inclination of the incident side reflection film 73 and the emission side reflection film 74, and the like.
[0018]
However, since the above-described waveguide 72 is formed in the vicinity of the surface of the waveguide-type optical resonator 7, damage is likely to occur structurally. FIG. 19 shows the size of each layer of the waveguide type optical resonator 7, FIG. 19 (a) is a front view, and FIG. 19 (b) is a side view. As shown in FIG. 19B, the waveguide type optical modulator 7 has a LiNbO with a thickness of 500 μm. ₃ A waveguide 72 having a thickness of about 3 to 5 μm is formed on the substrate, and a 1 μm thick SiO 2 is further formed thereon. ₂ A film is formed. For this reason, as shown in FIG. 19 (a), if there is a slight damage to the corners of the surface, the incident side reflection film or SiO ₂ There is a problem that light leaks from the waveguide 72 through the film and finesse cannot be improved.
[0019]
Further, in the above-described waveguide-type optical resonator 7, internal stress based on the thermal expansion coefficient of each material or the like may be generated on the end face when the incident-side reflection film 73 and the emission-side reflection film 74 are deposited. Accordingly, as shown in FIGS. 20 (a) and 20 (b), for example, a corner portion is distorted in the incident-side reflecting film 73, so that an error occurs in the reflectance characteristics of the incident-side reflecting film 73 and the emitting-side reflecting film 74. To cause light loss.
[0020]
Further, in the optical frequency comb generator 30 described above, it is necessary to use a high-reflectance reflecting mirror in order to suppress loss of light to be resonated. This high-reflecting reflecting mirror is supplied from an external light source. It also reflects light, and there is a problem that light loss at the time of incidence of light increases.
[0021]
Therefore, the present invention has been devised in view of the above-described problems, and an object thereof is to provide a highly efficient optical resonator with reduced loss of light propagating in a waveguide and enhanced finesse. . It is another object of the present invention to provide a high-efficiency optical frequency comb generator with reduced optical loss at the time of incidence and increased finesse.
[0022]
[Means for Solving the Problems]
The present invention An incident-side optical fiber that emits light through an incident-side end surface on which a reflective film is formed, an optical waveguide that propagates light emitted from the incident-side optical fiber, and an output-side end surface on which a reflective film is formed are provided. And an output-side optical fiber disposed so as to face the incident-side optical fiber through the waveguide so that light propagating through the optical waveguide can be reciprocally reflected from the incident-side end surface by the output-side end surface. And an oscillation means for oscillating a modulation signal of a predetermined frequency The incident side end surface and the output side end surface resonate light propagating through the optical waveguide. The optical waveguide is made of an electro-optic crystal whose refractive index changes when an electric field is applied, and modulates the phase of light resonated in the optical waveguide according to the modulation signal supplied from the oscillation means. A side band centered on the frequency of the light emitted from the incident side optical fiber is generated at the frequency interval of the modulation signal. Optical resonator Then, the output side end face has a transmittance set for each frequency in accordance with the light intensity of the sideband so as to flatten the generated sideband. It is characterized by that.
[0023]
This optical resonator is configured to transmit light propagating inside the optical waveguide to an incident side end face formed at the tip of the incident side optical fiber and an output side formed at the tip of the output side optical fiber. Resonate with the end face.
[0024]
An optical frequency comb generator according to the present invention includes an oscillating unit that oscillates a modulated signal having a predetermined frequency, and an incident-side reflecting mirror and an emitting-side reflecting mirror that are parallel to each other, and is incident through the incident-side reflecting mirror. Resonating means for resonating light and an electro-optic crystal whose refractive index changes when an electric field is applied, and arranged between the incident-side reflecting mirror and the emitting-side reflecting mirror and supplied from the oscillating means Optical modulation means for modulating the phase of the light resonated in the resonance means based on a signal and generating a sideband centered on the frequency of the incident light at intervals of the frequency of the modulation signal, The output side end face has a transmittance set for each frequency according to the light intensity of the sideband so as to flatten the generated sideband. It is characterized by that.
[0025]
The optical frequency comb generator according to the present invention includes an oscillation unit that oscillates a modulation signal having a predetermined frequency, and an incident-side reflection film and an emission-side reflection film that are parallel to each other. A resonating means for resonating the emitted light and an electro-optic crystal whose refractive index changes when an electric field is applied, and is arranged between the incident-side reflecting film and the emitting-side reflecting film and supplied from the oscillating means Optical modulation means for modulating the phase of the light resonated in the resonance means in accordance with the modulation signal, and generating sidebands centered on the frequency of the incident light at intervals of the frequency of the modulation signal. The incident light is emitted from an optical fiber having a high reflection film formed on the end face, and is resonated between the high reflection film and the incident side reflection film. The incident-side reflecting mirror has a maximum transmittance at the frequency of the incident light, and the exit-side end surface is in accordance with the sideband light intensity so as to flatten the generated sideband. And set the transmittance for each frequency It is characterized by that.
[0026]
These optical frequency comb generators reduce the loss of light resonating inside the electro-optic crystal by controlling the reflectance of the incident-side reflecting mirror so that only the light supplied from the light source can be transmitted.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0028]
FIG. 1 is a diagram showing a configuration of a waveguide type optical resonator 1 to which the present invention is applied. The waveguide-type optical resonator 1 includes a substrate 11 on which a waveguide 12 is laminated, and an incident-side optical fiber 19 and an emission-side optical fiber 30 installed so as to oppose each other via the substrate 11.
[0029]
The substrate 11 is a 3-4 inch diameter LiNbO grown by, for example, a pulling method. ₃ A large crystal such as is cut into a wafer. On this cut-out substrate 11, a process such as mechanical polishing or chemical polishing may be performed in order to grow the waveguide 12.
[0030]
The manufacturing process of the waveguide 12 is as follows. First, LiNbO used as the substrate 11 is used. ₃ And LiTaO ₃ Then, patterning is performed on the crystal such as, and after Ti is deposited, lift-off is performed. Then, heat treatment is performed at 1000 ° C. for about 10 hours in an oxygen wet atmosphere to thermally diffuse the deposited Ti. Incidentally, the produced high refractive index waveguide 12 is made of, for example, SiO. ₂ By covering with a clad layer 13 of a low refractive index material such as, light incident from the outside can be propagated through the waveguide 12.
[0031]
In the manufactured waveguide 12, an end face on which light is incident (hereinafter referred to as end face A) and an end face on which light is emitted (hereinafter referred to as end face B) are guided by the waveguide type optical resonator 1. After cutting out from the wafer, for example, mechanical polishing, chemical polishing, or the like is performed to reduce the surface roughness (desirably, approximately λ / 20 when the wavelength used is λ). Further, the end faces A and B are adjusted so as to be perpendicular to the waveguide 12 (desirably, the error is ± 0.1 ° or less).
[0032]
The incident side optical fiber 19 propagates light through the core 22 formed inside the clad 21. The incident-side optical fiber 19 emits light to the waveguide 12 through the dielectric multilayer film 23 formed on the end face. The surface of the dielectric multilayer film 23 is polished to such an extent that scattering does not occur when it strikes the end face A (for example, approximately λ / 20 with respect to the used wavelength λ), and the surface roughness is reduced. Reduce.
[0033]
The exit-side optical fiber 30 receives light from the waveguide 12 through the dielectric multilayer film 33 formed on the end face. The emission side optical fiber 30 propagates the light incident from the waveguide 12 through the core 32 formed inside the clad 31. Incidentally, the surface of the dielectric multilayer film 33 is also polished to the same extent as the dielectric multilayer film 23.
[0034]
The incident side optical fiber 19 and the outgoing side optical fiber 30 may be finished by polishing the surfaces of the dielectric multilayer films 23 and 33 in a convex shape, as shown in FIG. Thereby, butting to the end surfaces A and B of the incident side optical fiber 19 and the emission side optical fiber 30 becomes easy. It is also possible to provide this convex surface on the waveguide.
[0035]
Incidentally, the incident side optical fiber 19 and the output side optical fiber 30 are fixed so that the surfaces of the dielectric multilayer films 23 and 33 completely abut against the end faces A and B. That is, the optical coupling system between the optical fibers 20 and 30 and the waveguide 12 is configured to directly input and output light without using an aspheric lens.
[0036]
The thicknesses of the layers constituting the dielectric multilayer films 23 and 33 are about λ / 4 with respect to the operating wavelength λ, and are formed by alternately stacking thin films having different refractive indexes. The dielectric multilayer films 23 and 33 can be controlled to have a desired reflectance by alternately laminating materials having different refractive indexes according to the wavelength of light to be reflected. For this reason, it becomes possible to make the dielectric multilayer films 23 and 33 into mirrors that are almost totally reflected with respect to the wavelength of light that resonates inside the waveguide 12 by the material to be laminated. In other words, a pair of parallel reflecting mirrors can be arranged to face each other via the waveguide 12. Thereby, the light confined in the waveguide 12 can be Fabry-Perot resonated.
[0037]
Incidentally, in the present invention, since it is assumed that the dielectric multilayer films 23 and 33 are abutted against the end faces A and B, in order to obtain a desired reflectivity, it depends on the refractive index of the material used for the waveguide 12. (For example, LiNbO ₃ And LiTaO ₃ The material is selected (for the refractive index). Is made.
[0038]
The reflectivity of the dielectric multilayer film 23 is set to be slightly lower than 100%. Thereby, the light propagating through the core 22 can be incident and coupled to the waveguide 12 with a small transmittance. The incident-coupled light propagates in the waveguide 12 and is reflected by the dielectric multilayer film 33, and propagates in the reverse direction in the waveguide 12. The light propagating in the opposite direction is reflected by the dielectric multilayer film 23 again. By repeating this, light can be confined and resonated inside the waveguide 12.
[0039]
Further, depending on the setting of the reflectance of the dielectric multilayer film 33, the light resonating inside the waveguide 12 can be slightly transmitted to the outside and coupled to the output side optical fiber 30. It is also possible to use for measurement or the like by taking out the light emitted and coupled to the emission side optical fiber 30 through the core 32.
[0040]
Next, the shapes of the tips of the incident side optical fiber 19 and the emission side optical fiber 30 will be described.
[0041]
FIG. 3 shows the shape of the tip of the incident side optical fiber 19 on which the dielectric multilayer film 23 is formed. In the example shown in FIG. 3, the diameter of the clad 21 is about 125 μm, and the diameter of the core 22 is about 3 to 5 μm. The beam diameter of light propagating around the core 22 is about 10 μm.
[0042]
That is, since the periphery of the core 22 in the dielectric multilayer film 23 is encased in the thick clad 21, it is designed not to be damaged from the outside. In particular, distortion due to scratches, damage, and internal stress tends to occur at the corners of the dielectric multilayer film 23 as shown in FIG. However, the dielectric multilayer film 23 that covers the core layer with the thick clad 21 is hardly affected by the periphery of the core 22 even if peeling of several tens of μm occurs at this corner portion.
[0043]
As described above, the dielectric multilayer films 23 and 33 that are hardly affected by the damage around the core 22 are abutted from both ends of the waveguide 12, so that light does not leak from the waveguide 12, thereby reducing light loss. In addition, the finesse of the entire waveguide type optical resonator 1 can be improved.
[0044]
In the waveguide type optical resonator 1 to which the present invention is applied, the light propagating in the waveguide 12 is divided into the dielectric multilayer film 23 of the incident side optical fiber 19 and the emission side fiber 30 respectively disposed at both ends of the waveguide 12. Resonance is caused by the dielectric multilayer film 33 formed at the tip of the substrate. As a result, the waveguide type optical resonator 1 to which the present invention is applied can reciprocate and reflect light by the dielectric multilayer films 23 and 33 which are hardly affected by damage, and therefore finesse can be achieved without leaking light. Can be improved.
[0045]
Next, an optical frequency comb generator with reduced optical loss at the time of incidence and increased finesse will be described.
[0046]
FIG. 5 is a diagram showing a configuration of a bulk type optical frequency comb generator 10 to which the present invention is applied. The bulk type optical frequency comb generator 10 includes an optical phase modulator 111 and an optical resonance composed of an entrance-side reflecting mirror 112 and an exit-side reflecting mirror 113 disposed so as to face each other via the optical phase modulator 111. 110 and an oscillator 117.
[0047]
The optical resonator 110 resonates light Lin incident at a slight transmittance through the incident-side reflecting mirror 112 between the incident-side reflecting mirror 112 and the emitting-side reflecting mirror 113, and transmits a part of the light Lout on the emitting side. The light is emitted through the reflecting mirror 113.
[0048]
The optical phase modulator 111 is, for example, lithium niobate (LiNbO ₃ ) And the like, and is an optical device that phase-modulates light that passes based on a supplied electrical signal. This optical phase modulator 111 uses a physical phenomenon such as the Pockels effect in which the refractive index changes in proportion to the electric field and the Kerr effect in which the refractive index changes in proportion to the square of the electric field, and modulates the light passing therethrough. Do.
[0049]
The incident side reflection mirror 112 and the emission side reflection mirror 113 are provided to resonate the light incident on the optical resonator 110, and resonate by reciprocally reflecting the light passing through the optical phase modulator 111.
[0050]
The incident-side reflecting mirror 112 is disposed on the light incident side of the optical phase modulator 111, and has a frequency ν from a light source (not shown). ₁ Light Lin is incident. The incident-side reflecting mirror 112 reflects light that has been reflected by the emitting-side reflecting mirror 113 and passed through the optical phase modulator 111.
[0051]
The exit-side reflecting mirror 113 is disposed on the light exit side of the optical phase modulator 111 and reflects light that has passed through the optical phase modulator 111. The exit-side reflecting mirror 113 emits the light that has passed through the optical phase modulator to the outside at a constant rate.
[0052]
The incident-side reflecting mirror 112 and the emitting-side reflecting mirror 113 are not only disposed outside the optical phase modulator 111 as shown in FIG. 5, but also the incident-side end face and the emitting side of the optical phase modulator 111. A multi-layer film end face mirror may be mounted on the end face.
[0053]
The electrodes 116 are formed on the top and bottom surfaces of the optical phase modulator 111 so that the direction of the modulation electric field is perpendicular to the light propagation direction. The electrode 116 drives and inputs the electric signal supplied from the oscillator 117 to the optical phase modulator 111. The oscillator 117 is connected to the electrode 116 and supplies an electric signal having a frequency fm (for example, about 10 GHz).
[0054]
In the bulk-type optical frequency comb generator 10 having the above-described configuration, an optical signal synchronized with the time when light reciprocates in the optical resonator 110 is used as a drive input from the electrode 116 to the optical phase modulator 111, so that the optical phase Compared with the case of passing through the modulator 111 only once, deep phase modulation of several tens of times or more can be performed. Thereby, hundreds of sidebands can be generated over a wide band centering on the frequency of incident light. Further, the frequency interval between adjacent sidebands is equal to the frequency fm of the input electric signal.
[0055]
The frequency of the light to be measured can be determined based on a large number of optical frequency combs generated by the bulk optical frequency comb generator 10. For example, as shown in FIG. ₁ Is modulated at the frequency fm by the optical phase modulator 111 to generate an optical frequency comb composed of sidebands having a frequency interval fm as shown in FIG. This optical frequency comb has a frequency ν ₂ The measured light beams are superposed and incident on the photodetector 11. Frequency of light to be measured ν ₂ 6 (b), if the beat frequency Δν between the Nth sideband generated as an optical frequency comb is measured by the frequency counting device 12, | ν ₁ −ν ₂ | Can be determined, and accordingly, the frequency ν of the light to be measured ₂ It is also possible to determine.
[0056]
Next, the reflectance of the incident-side reflecting mirror 112 and the emitting-side reflecting mirror 113 constituting the optical resonator 110 will be described.
[0057]
As shown in FIG. 7A, the reflectance of the incident side reflecting mirror 112 is determined by the frequency ν of incident light. ₁ Is set to be minimum. In addition, the reflectance of the incident-side reflecting mirror 112 has a frequency ν ₁ In bands other than ν ₁ Is set to be higher than the reflectance at. The reflectance distribution curve is ₁ However, the slope of the curve may be not only steep but also gradual. In other words, incident light can be transmitted through the incident-side reflecting mirror with a certain bandwidth. Furthermore, ν ₁ Also, the magnitude of the reflectance in is not limited to asymptotically approaching 0%, but may be approximately 100%.
[0058]
Further, since the transmittance of the incident-side reflecting mirror 112 shows a contrast property with the reflectance, the frequency ν of the incident light ₁ At the maximum. The frequency ν ₁ In other bands, the transmittance is low. That is, when the transmittance is t% and the reflectance is r%,
t [%] + r [%] 100 [%]
The relationship is established.
[0059]
The exit-side reflecting mirror 113 maintains a constant high reflectivity as shown in FIG. 8 in order to reduce the loss of resonant light. On the other hand, when the generated optical frequency comb is made to interfere with the light to be measured, it is necessary to emit the light to the outside at a certain rate, and the emission-side reflecting mirror 113 cannot be set to total reflection. Therefore, as shown in FIG. 8, the output-side reflecting mirror 113 has a reflectance slightly lower than 100%, for example, about 97%.
[0060]
The exit-side reflecting mirror 113 emits all the generated optical frequency combs over a wide band by controlling the reflectance distribution curve to a flat shape.
[0061]
The bulk type optical frequency comb generator 10 to which the present invention is applied has a frequency ν supplied from a light source by controlling the reflectance of the incident side reflecting mirror 112 and the emitting side reflecting mirror 113 as described above. ₁ Can easily enter the optical phase modulator 111 via the incident-side reflecting mirror 112. In addition, when a modulation signal is driven and input to the light that resonates inside the optical phase modulator 111, a large number of sidebands are generated over a wide band. ₁ As shown in FIG. 7 (b), most of the sidebands of the generated optical frequency comb are not transmitted to the outside via the incident side reflecting mirror 112, and the optical phase is not set. The round trip reflection is repeated inside the modulator 111.
[0062]
Further, the bulk type optical frequency comb generator 10 to which the present invention is applied gradually reduces the bandwidth (BW) through which incident light can be transmitted via the incident-side reflecting mirror 112, thereby reducing light loss during resonance. It can be gradually reduced. Thereby, as shown in FIG.7 (c), the light quantity of each side band will increase gradually as BW is narrowed.
[0063]
On the other hand, the present invention is not applied at all even when the bandwidth through which incident light can be transmitted is, for example, BW is 80 GHz and about 8 times the frequency fm as shown in FIG. Compared to the case, it is possible to reduce the optical loss at the time of resonance. That is, according to the present invention, the incident-side reflecting mirror 112 has a frequency ν of incident light. ₁ Even if the frequency band other than can be transmitted, the frequency ν ₁ It is sufficient that the reflectance is minimum. Further, the bulk type optical frequency comb generator 10 to which the present invention is applied has a frequency ν of incident light. ₁ Allows only transmission and the frequency ν ₁ Reflectance at 0%, frequency ν ₁ When the reflectance in a band other than 100% is 100%, that is, in an ideal state, as shown in FIG. 7 (c), it is possible to suppress the loss of light at the time of resonance to the minimum, and the sideband light intensity is maximized. Become.
[0064]
This bulk type optical frequency comb generator 10 has a frequency ν of incident light. ₁ It is possible to prevent the transmission of the sidebands other than the band outside. Thereby, optical loss can be reduced and an optical frequency comb can be generated efficiently. In addition, the incident light frequency ν ₁ In FIG. 2, the transmittance is maximized, and the optical loss at the time of incidence can be reduced. Therefore, the bulk type optical frequency comb generator 10 can be further improved in efficiency. Even when a light source with a small output is used, the bulk optical frequency comb generator 10 can increase the resonant light output.
[0065]
The optical frequency comb generator according to the present invention is not limited to the configuration of the bulk optical frequency comb generator 10 described above. For example, as shown in FIG. 9, the present invention can also be applied to a waveguide-type optical frequency comb generator 20 using a waveguide-type optical modulator.
[0066]
The waveguide type optical frequency comb 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.
[0067]
The substrate 201 is a 3-4 inch diameter LiNbO grown by, for example, a pulling method. ₃ A large crystal such as GaAs or GaAs is cut into a wafer. In order to epitaxially grow the waveguide 202 layer on the cut substrate 201, processing such as mechanical polishing or chemical polishing is usually performed.
[0068]
The waveguide 202 is arranged for propagating light, and the refractive index of the layer constituting the waveguide 202 is set higher than that of other layers such as a substrate. The light incident on the waveguide 202 propagates while being totally reflected at the boundary surface of the waveguide 202.
[0069]
The electrode 203 is made of a metal material such as Al, Cu, Pt, or Au, for example, and drives and inputs an electric signal having a frequency fm supplied from the outside to the waveguide 202. Also, by providing this electrode 203, the propagation direction of light in the waveguide and the traveling direction of the modulation electric field become the same. In addition, electrodes other than the electrode 203 are required to be grounded.
[0070]
As shown in FIG. 10, this waveguide type optical modulator 200 can also be provided with electrodes 303 so as to face each other on both sides of the waveguide. In FIG. 10, the light propagation direction in the waveguide is perpendicular to the modulation electric field direction.
[0071]
That is, the waveguide type optical modulator 200 can control the light propagation mode according to the type of electrode provided.
[0072]
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. The oscillator 206 is connected to the electrode 203 and supplies an electric signal having a frequency fm.
[0073]
The incident-side reflection film 204 is disposed on the light incident side of the waveguide type optical modulator 200 and has a frequency ν from a light source (not shown). ₁ Light is incident. Further, the incident-side reflection film 204 reflects light reflected by the emission-side reflection film 205 and having passed through the waveguide 202.
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.
[0074]
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.
[0075]
In addition, since the waveguide type optical frequency comb generator 20 can modulate light by pushing light into a narrow optical waveguide as compared with the bulk type optical frequency comb generator 10, the modulation index β is increased. Can do. FIG. 11 shows the relationship between the frequency and the optical frequency comb generated by the bulk type optical frequency comb generator 10 or the waveguide type optical frequency comb generator 20. The waveguide-type optical frequency comb generator 20 having a modulation index β of about 3 rad has a larger number of sidebands and a larger amount of sideband than the bulk-type optical frequency comb generator 10 having a modulation index of 1 rad or less. That is, the frequency ν for the total amount of light in the generated sideband ₁ The ratio of the amount of light of the waveguide type optical frequency comb generator 20 is lower than that of the bulk type optical frequency comb generator.
[0076]
The reflectances of the incident-side reflecting mirror 112 and the incident-side reflecting film 204 have a frequency ν ₁ At the frequency ν ₁ Is emitted to the outside via the incident-side reflecting mirror 112 or the incident-side reflecting mirror 204. That is, the frequency ν ₁ The waveguide-type optical frequency comb generator 20 having a low ratio of the amount of light can suppress the amount of light emitted to the outside through the incident-side reflection film as compared with the bulk-type optical frequency comb generator 10. Thereby, the waveguide type optical frequency comb generator 20 can further reduce the optical loss at the time of modulation as compared with the bulk type optical frequency comb generator 10.
[0077]
Further, the waveguide type optical frequency comb generator 20 can be reduced in size as compared with the bulk type optical frequency comb generator 10 using a bulk crystal, and parasitic capacitance and parasitic inductance can be suppressed. Become. As a result, the applied voltage can be reduced, so that the speed of the device can be increased, and integration with other ultrafast optical devices is also possible.
[0078]
Next, the reflectance of the incident side reflection film 204 and the emission side reflection film 205 constituting the waveguide type optical frequency comb generator 20 will be described.
[0079]
The reflectance of the incident-side reflecting film 204 is the same as the reflectance of the incident-side reflecting mirror 112 in the bulk type optical frequency comb 10, and as shown in FIG. ₁ Is controlled to be minimum. The frequency ν ₁ In other bands, control is performed so that the reflectance is high.
[0080]
Similarly, the transmittance of the incident-side reflection film 204 exhibits a contrasting property with the reflectance, and therefore the frequency ν of the incident light. ₁ At the maximum.
[0081]
The exit-side reflection film 205 maintains a constant high reflectivity similarly to the exit-side reflector 113 in the bulk optical frequency comb 10 in order to reduce the loss of resonance light.
[0082]
Further, the waveguide type optical frequency comb generator to which the present invention is applied can further adopt the configuration described below.
[0083]
FIG. 12 shows a side view of the incident-side coupling system 4 of the waveguide-type optical frequency comb generator 50 into which the Fabry-Perot resonant light is incident. The incident side coupling system 4 includes an optical fiber 60 that emits light from an optical fiber core 602 and a waveguide-type optical frequency comb generator 50. In the incident side coupling system 4, the light emitted from the optical fiber core 602 is incident on the incident side reflection film 501 in the waveguide type optical frequency comb generator 50 and the fiber reflection film 601 applied to the end face of the optical fiber 60. Fabry-Perot resonance. That is, only light that satisfies the resonance condition obtained from the length of the gap between the incident-side reflecting film 501 and the fiber reflecting film 601 and the light frequency is transmitted through the incident-side reflecting film 501 and is incident on the waveguide 202 with high efficiency. Can be combined.
[0084]
13A and 13B show the frequency ν with respect to the gap between the incident side reflection film 501 and the fiber reflection film 601. ₁ The relationship between the reflectance and the transmittance of the incident light on the incident-side reflective film 501 is shown. By changing this gap, the frequency ν ₁ If the resonance condition in is satisfied, the reflectance becomes low and the transmittance becomes high. That is, if the resonance is satisfied and the gap is controlled to a length corresponding to the group velocity of light, the frequency ν ₁ Only the incident light can be efficiently transmitted through the incident-side reflective film 501.
[0085]
The gap length is desirably as short as possible, and the optical path length of the gap is desirably controlled to about 10 times the wavelength including the case where the gap is filled with an adhesive or the like.
[0086]
14A and 14B show the relationship between the reflectance and transmittance for each frequency in the incident-side reflection film 501 when the length of the gap between the incident-side reflection film and the fiber reflection film is a. ing. Frequency ν ₁ In other bands, the transmittance is low and the reflectance is high, so that it is possible to prevent external transmission of the generated sideband. Thereby, light can be efficiently confined in the waveguide type optical frequency comb generator 50, and light loss can be reduced.
[0087]
The reflectance set in the incident side reflection film 501 is the frequency ν of incident light. ₁ It is possible to obtain the effects of the present invention not only when the frequency is set to be minimum, but also when set freely in all frequency bands. In the case where the reflectance asymptotically approaches 100% in all frequency bands, ν ₁ Sidebands generated in other bands can be reflected most efficiently and confined inside the waveguide type optical frequency comb generator 50.
[0088]
The waveguide-type optical frequency comb generator 20 to which the present invention is applied has a frequency ν supplied from a light source by controlling the reflectance of the incident-side reflecting film 204 and the emitting-side reflecting film 205 as described above. ₁ Can easily enter the waveguide-type optical modulator via the incident-side reflection film 204. In addition, when a modulation signal is driven and input to light that resonates inside the waveguide 202, a large number of sidebands are generated over a wide band, but the transmittance of the incident-side reflection film 204 has a frequency ν. ₁ In other bands, it is set low. As a result, most of the sidebands of the generated optical frequency comb are not transmitted to the outside via the incident-side reflection film 204, and are repeatedly reciprocated within the waveguide 202.
[0089]
That is, the waveguide-type optical frequency comb generator 20 including the incident-side reflection film 204 can increase the speed of the device, and, like the bulk-type optical frequency comb 10, can reduce optical loss and improve finesse. Therefore, an optical frequency comb can be generated efficiently. Further, the waveguide type optical frequency comb generator 20 has a frequency ν of incident light. ₁ Since the transmittance is maximum at, light loss can be reduced at the time of incidence, and further improvement in efficiency can be achieved. In addition, even when a light source with a small output is used, the waveguide-type optical frequency comb generator 20 can increase the resonant light output.
[0090]
The waveguide-type optical frequency comb generator 20 and the waveguide-type optical resonator 1 according to the present invention can be further applied to a waveguide-type optical frequency comb generator 5 described below.
[0091]
As shown in FIG. 15, the waveguide type optical frequency comb generator 5 includes a waveguide type optical modulator 6, an incident side optical fiber 19, and an output side optical fiber 30.
[0092]
The waveguide type optical modulator 6 includes a substrate 11, a waveguide 12, a cladding layer 13, and an electrode 51. The same components and members as those of the above-described waveguide type optical resonator 1 are referred to the description of the waveguide type optical resonator 1.
[0093]
The electrode 51 is provided on the cladding layer 13 and is made of, for example, a metal material such as Al, Cu, Pt, or Au, and drives and inputs an electric signal having a frequency fm supplied from an oscillator to the waveguide 12.
[0094]
The incident-side optical fiber 19 and the emission-side optical fiber 30 are fixed so that the dielectric multilayer films 23 and 33 formed at the tips completely abut against the end faces A and B. Thereby, similarly to the waveguide type optical resonator 1, the light confined in the waveguide 12 can be reflected back and forth through the dielectric multilayer films 23 and 33.
[0095]
In the waveguide-type optical frequency comb generator 5 having the above-described configuration, an electric signal synchronized with the time when light reciprocates in the waveguide 12 is used as a drive input from the electrode 51 to the waveguide-type optical modulator 6. Compared with the case of passing through the inside of the waveguide 12 only once, deep phase modulation of several tens of times or more can be applied. Thereby, hundreds of sidebands can be generated over a wide band centering on the frequency of incident light. Further, the frequency interval between adjacent sidebands is equal to the frequency fm of the input electric signal. Based on a number of sidebands generated by the waveguide type optical frequency comb generator 5, the frequency of the light to be measured can be determined.
[0096]
Also in this waveguide type optical frequency comb generator 5, the dielectric multilayer films 23 and 33, which are hardly damaged around the core 22, are abutted from both ends of the waveguide 12, so that light does not leak from the waveguide 12. Light loss can be reduced, and sidebands can be generated efficiently.
[0097]
Further, in this waveguide type optical frequency comb generator 5, by controlling the laminated material of the dielectric multilayer film 23, the reflectance of the dielectric multilayer film 23 is changed to the incident side as shown in FIG. Frequency ν of light emitted from the optical fiber ₁ It is also possible to set so as to be the minimum.
[0098]
This allows the frequency ν ₁ Can easily enter the waveguide 12 through the dielectric multilayer film 23. In addition, when a modulation signal is driven and input to light that resonates inside the waveguide 12, a large number of sidebands are generated over a wide band, but the transmittance of the dielectric multilayer film 23 has a frequency ν. ₁ As shown in FIG. 7B, most sidebands of the generated optical frequency comb are not transmitted to the outside through the dielectric multilayer film 23, as shown in FIG. The round-trip reflection is repeated inside the multilayer film 33. For this reason, the optical loss at the time of resonance can be further suppressed, and the amount of light in the sideband can be increased.
[0099]
The waveguide type optical frequency comb generator 5 has a frequency ν. ₁ It is also possible to flatten the light intensity distribution of the light emitted from the filter by changing the transmittance exponentially by paying attention to the light intensity that changes exponentially according to the frequency difference Δf.
[0100]
For example, the intensity Pout of the light emitted from the dielectric multilayer film 33 can be approximately expressed by Expression (11) within a range not affected by the group refractive index dispersion.
Pout = TinTout × exp {− | Δf | Los / (βfm)} Pin (11)
In this equation (11), Tin is the transmittance of the dielectric multilayer film 23, Tout is the transmittance of the dielectric multilayer film 33, β is a modulation index during reciprocation in the waveguide 12, Los is a loss rate of light reciprocating in the waveguide 12, and is represented by a constant in the equation (11).
[0101]
The generated sideband is flattened by controlling the transmittance of the dielectric multilayer film 33 based on the equation (11).
[0102]
The dielectric multilayer film 33 sets the transmittance for each frequency in accordance with the light intensity of the generated sideband. In other words, the transmittance of the dielectric multilayer film 33 is set by paying attention to the physical property of the sideband in which the light intensity increases or decreases according to the frequency.
[0103]
The light intensity P of the sideband inside the waveguide 12 with respect to Δf _inside Can be represented by the following equation.
dP _inside / DΔf = −Los / (βfm) P _inside When Δf> 0 (12)
dP _inside / DΔf = Los / (βfm) P _inside When Δf <0 (13)
This equation (12) is obtained when Δf> 0, that is, the frequency ν. ₁ Represents the rate of change of light intensity in the higher band, and equation (13) is obtained when Δf <0, that is, the frequency ν. ₁ It represents the rate of change of light intensity in the lower band.
[0104]
P calculated by the equations (12) to (13) _inside Thus, the light intensity Pout of the emitted light can be calculated based on the equation (14).
Pout = Tout × P _inside (14)
The light intensity P of the sideband in the waveguide 12 that can be expressed by the above formula. _inside Is flattened for each spectrum and emitted to the outside. In other words, by setting the transmittance Tout for each frequency band in the dielectric multilayer film 33, the light intensity emitted to the outside is controlled.
[0105]
The condition of the transmittance Tout of the dielectric multilayer film 33 is calculated by substituting into the formulas (12) to (14) as dPout / dΔf = 0, and the following formulas (41) and (42). Can be represented.
dTout / dΔf = Los / (βfm) Tout (41)
dTout / dΔf = −Los / (βfm) Tout (42)
Based on the equations (41) and (42), the transmittance Tout of the dielectric multilayer film 22 can be determined, for example, as shown in FIG.
[0106]
That is, by controlling the transmittance of the dielectric multilayer film 33 as described above, the generated sideband can be flattened while improving the finesse of the optical frequency comb generator 5.
[0107]
【The invention's effect】
As described above in detail, the optical resonator to which the present invention is applied is configured so that light propagating inside the optical waveguide is incident on the incident-side end face formed at the distal end of the incident-side optical fiber and is emitted at both ends of the optical waveguide. Resonance is caused by the exit side end face formed at the tip of the side optical fiber. The output side end face has a transmittance set for each frequency according to the light intensity of the sideband so as to flatten the generated sideband. Therefore, the optical resonator to which the present invention is applied reflects light back and forth by the incident end face and the exit end face that are hardly affected by damage. Can Improve finesse without leaking light While flattening the generated sideband be able to.
[0108]
As described in detail above, the optical frequency comb generator according to the present invention Then, the emission side end face sets the transmittance for each frequency according to the light intensity of the sideband so as to flatten the generated sideband. To improve finesse without leaking light While flattening the generated sideband be able to.
[0109]
Therefore, the optical frequency comb generator according to the present invention can reduce the optical loss at the time of incidence while suppressing the loss of the light to be resonated.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a configuration example of a waveguide type optical resonator.
FIG. 2 is a diagram for explaining a case where the surface of a dielectric multilayer film is polished and finished in a convex shape.
FIG. 3 is a diagram for explaining a tip shape of an incident-side optical fiber on which a dielectric multilayer film is formed.
FIGS. 4A and 4B are diagrams for explaining a case where scratches and damage occur in corner portions of a dielectric multilayer film. FIGS.
FIG. 5 is a diagram for explaining a specific configuration example of a bulk-type optical frequency comb generator.
FIG. 6 is a diagram for explaining the principle of frequency measurement of light to be measured by an optical frequency comb.
FIG. 7 is a diagram for explaining the reflectance of an incident-side reflecting mirror.
FIG. 8 is a diagram for explaining the reflectance of the exit-side reflecting mirror.
FIG. 9 is a diagram for explaining a specific configuration example of a waveguide-type optical frequency comb generator.
FIG. 10 is a diagram for explaining a waveguide-type optical frequency comb generator in which electrodes are provided so as to face each other on both sides of the waveguide.
FIG. 11 is a diagram showing a relationship between an optical frequency comb generated by a bulk type optical frequency comb generator or a waveguide type optical frequency comb generator, and the frequency.
FIG. 12 is a side view of an incident side coupling system of a waveguide type optical frequency comb generator into which light having Fabry-Perot resonance is incident.
FIG. 13 shows a frequency ν with respect to the gap between the incident side reflection film and the fiber reflection film. ₁ It is the figure which showed the relationship between the reflectance in the incident side reflection film of the incident light of, and the transmittance | permeability.
FIG. 14 is a diagram showing the relationship between reflectance and transmittance for each frequency in the incident-side reflecting film when the length of the gap between the incident-side reflecting film and the fiber reflecting film is a.
FIG. 15 is a diagram for explaining an application example of a waveguide type optical resonator and a waveguide type optical frequency comb generator;
FIG. 16 is a diagram for explaining the reflectance of a dielectric multilayer film.
FIG. 17 is a diagram for explaining a configuration example of a conventional waveguide type optical resonator.
FIG. 18 is a diagram for explaining a specific configuration example of a conventional optical frequency comb generator.
FIG. 19 is a diagram for explaining the size of each layer of a conventional waveguide-type optical resonator.
FIG. 20 is a diagram for explaining a case where distortion occurs in an incident side reflection film in a conventional waveguide type optical resonator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Waveguide type optical resonators 1, 11 Substrate, 12 Waveguide, 13 Clad layer, 19 Incident side optical fiber, 21, 31 Clad layer, 22, 32 Core layer, 23, 33 Dielectric multilayer film, 30 Emission side light Fiber, 10 bulk type optical frequency comb generator,
5,20 Waveguide type optical frequency comb generator

Claims (8)

An incident-side optical fiber that emits light through an incident-side end surface on which a reflective film is formed, an optical waveguide that propagates light emitted from the incident-side optical fiber, and an output-side end surface on which a reflective film is formed are provided. An output-side optical fiber disposed so as to face the incident-side optical fiber via the waveguide so that the light propagating through the optical waveguide can be reciprocally reflected from the incident-side end surface by the output-side end surface ; Oscillating means for oscillating a modulated signal of a predetermined frequency, the incident side end face and the exit side end face resonate light propagating through the optical waveguide, and the optical waveguide is applied with an electric field to provide a refractive index. The frequency of the light emitted from the incident side optical fiber is modulated by modulating the phase of the light resonated in the optical waveguide according to the modulation signal supplied from the oscillation means. The center and the sideband an optical resonator to generate at intervals of the frequency of the modulation signal,
The optical resonator according to claim 1, wherein a transmittance is set for each frequency according to the light intensity of the side band so that the emission side end face is flattened .   2. The optical resonator according to claim 1, wherein the incident-side end surface and / or the emitting-side end surface are configured to contact an end surface of the optical waveguide.   The reflective film formed on the incident side end face and / or the reflective film formed on the output side end face is composed of a dielectric multilayer film in which materials having different refractive indexes are alternately laminated. Item 5. The optical resonator according to Item 1. 2. The optical resonator according to claim 1 , wherein the incident side end face has a maximum transmittance at a frequency of light emitted from the incident side optical fiber .   Oscillating means for oscillating a modulation signal of a predetermined frequency;
  Resonating means configured to resonate light incident through the incident-side reflecting mirror, which includes an incident-side reflecting mirror and an exit-side reflecting mirror that are parallel to each other;
  An electro-optic crystal whose refractive index changes when an electric field is applied, and is arranged between the incident-side reflecting mirror and the emitting-side reflecting mirror. In the resonance means according to the modulation signal supplied from the oscillation means Optical modulation means for modulating the phase of the resonated light and generating sidebands centered on the frequency of the incident light at intervals of the frequency of the modulation signal;
  The optical frequency comb generator according to claim 1, wherein the emission side end face has a transmittance set for each frequency in accordance with the light intensity of the sideband so as to flatten the generated sideband.   6. The optical frequency comb generator according to claim 5, wherein the incident-side reflecting mirror and the emitting-side reflecting mirror are reflecting films formed on the incident-side end face and / or the exit-side end face of the light modulation means. 6. The optical frequency comb generator according to claim 5, wherein the incident side end face has a maximum transmittance at a frequency of light emitted from the incident side optical fiber.   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, and is arranged between the incident-side reflection film and the emission-side reflection film. In the resonance means according to the modulation signal supplied from the oscillation means Optical modulation means for modulating the phase of the resonated light and generating sidebands centered on the frequency of the incident light at intervals of the frequency of the modulation signal;
  The incident light is emitted from an optical fiber having a high reflection film formed on an end face, and is resonated between the high reflection film and the incident side reflection film.
  The incident side reflecting mirror has a maximum transmittance at the frequency of the incident light,
  The optical frequency comb generator according to claim 1, wherein the emission side end face has a transmittance set for each frequency in accordance with the light intensity of the sideband so as to flatten the generated sideband.

Images