JP3910834B2 - Optical frequency comb generator - 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 a generated optical beat frequency 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 respond to the demand for expanding the measurable bandwidth, a wideband heterodyne detection system using an optical frequency comb generator has been constructed. This optical frequency comb generator generates comb-like sidebands arranged at equal intervals on the frequency axis over a wide band, and the frequency stability of this sideband is almost equal to the frequency stability of incident light. It is. 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. 13 shows the basic structure of this conventional optical frequency comb generator 30.
[0006]
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.
[0007]
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 is changed by applying an electric field, and the modulation applied to the electrode 316 with respect to the light passing through the optical resonator 310. Phase modulation is applied according to the electrical signal of frequency fm.
[0008]
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 modulation frequency frequency fm of the input electric signal.
[0009]
The intensity P of the light emitted from the reflecting mirror 313_outCan be expressed by the following expression (1) within a range not affected by the group refractive index dispersion.
P_out= T_inT_outexp {− | Δf | Los / (βfm)} P_in    (1)
In this equation (1), T_inIs the transmittance of the reflecting mirror 312, T_outIs the transmittance of the reflecting mirror 313, and P_inIs the light intensity of the incident light, β is the modulation index during reciprocation in the optical resonator 310, Los is the loss rate of light reciprocating in the optical resonator 310, and is expressed as a constant in the above equation (1). . If the loss factor of light in the resonator 310 is only transmission to the outside through the reflecting mirrors 312, 313, Los becomes T_inAnd T_outThe sum of
[0010]
FIG. 14 shows P in each band._outThe ratio of the incident light to the light intensity is shown. In FIG. 14, the vertical axis represents the ratio of the light intensity of the outgoing light to the light intensity of the incident light (P_out/ P_inThe horizontal axis represents the difference Δf between the frequency of each generated sideband and the frequency of the incident light. Further, in FIG. 14, the transmittance of the reflecting mirrors 312 and 313 is 0.005 in all frequency bands, and the phase modulation is performed with the modulation frequency fm being 5 GHz.
[0011]
In addition, as shown in equation (1), P_outIs exponentially attenuated with respect to the absolute value of Δf, and as shown in FIG. 14, when Δf = 0, in other words, it is represented by a curve that maximizes the light intensity at the frequency of the incident light.
[0012]
[Problems to be solved by the invention]
By the way, when determining the frequency of the light to be measured based on the many optical frequency combs generated by the optical frequency comb generator 30 described above, for example, the frequency ν₁Is modulated at a frequency fm by an optical phase modulator 311 to generate an optical frequency comb composed of sidebands having a frequency interval fm. And this optical frequency comb is changed to frequency ν₂| Ν by measuring the beat frequency Δν with the Nth sideband generated as an optical frequency comb by superposing the measured light of₁−ν₂｜ and finally the frequency ν of the light to be measured₂To decide.
[0013]
By the way, by flattening the light intensity distribution of the generated sideband, the sensitivity of the optical frequency comb can be made constant in all bands, and the frequency of the light to be measured can be accurately measured. As a result, it is possible to reduce the design burden in the subsequent circuit for detecting the generated sideband.
[0014]
However, in the conventional optical frequency comb generator 30, as shown in FIG. 14, the sideband light intensity decreases as the absolute value of Δf increases, in other words, away from the frequency of the incident light. Especially in a band that has a considerable frequency difference from the incident light frequency, the sideband light intensity decreases exponentially, so the sideband light intensity distribution varies and the frequency of the light under measurement is measured with high accuracy. There was a problem that it was difficult to do.
[0015]
Therefore, the present invention has been proposed in view of the above-described circumstances, and an optical frequency comb capable of measuring the frequency of the light to be measured with high accuracy even in a band having a considerable frequency difference from the frequency of the incident light. An object of the present invention is to provide an optical frequency comb generator that generates
[0016]
[Means for Solving the Problems]
  An optical frequency comb generator according to the present invention includes an oscillating unit that oscillates a modulated signal having a predetermined frequency, an incident-side reflecting mirror, and an emitting-side reflecting mirror that are parallel to each other, in order to solve the above-described problems Resonating means for resonating light incident through the side reflecting mirror, and an electro-optic crystal whose refractive index is changed by applying an electric field, arranged between the incident side reflecting mirror and the emitting side reflecting mirror, The phase of the light resonated in the resonance means is modulated in accordance with the modulation signal supplied from the oscillation means, and a side band centered on the frequency of the incident light is generated at the frequency interval of the modulation signal. Light emitting means, and the exit-side reflecting mirror comprises:In order to flatten the light intensity characteristics near the frequency of the incident light,Depending on the light intensity of the generated sideband, the transmittance for each frequencyButIs set,Filter directly on the sideband generated inside the resonance means composed of the incident side reflection mirror and the emission side reflection mirror.It is characterized by that.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0019]
FIG. 1 shows an optical frequency comb generator 10 capable of controlling the light intensity of emitted light as a first embodiment. The optical frequency comb generator 10 includes an optical resonator 110 that includes an optical phase modulator 111 and an incident-side reflecting mirror 112 and an emitting-side reflecting mirror 113 that are installed to face each other via the optical phase modulator 111. And a filter 114 and an oscillator 117.
[0020]
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.
[0021]
The optical phase modulator 111 is, for example, lithium niobate (LiNbO₃) 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.
[0022]
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.
[0023]
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).₁Light 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.
[0024]
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. In particular, the optical frequency comb generator 10 needs to emit light to the outside at a constant rate with respect to the generated optical frequency comb, so the transmittance of the exit-side reflecting mirror 113 cannot be set to zero. For this reason, the exit-side reflecting mirror 113 sets the transmittance to, for example, 0.005, and emits the light that has passed through the optical phase modulator to the filter 114 at a constant rate.
[0025]
Figure 2 shows the light intensity P of the outgoing light in each band._outLight intensity of incident light (= P_in)). By the way, the light intensity P of this emitted light_outIs calculated based on a detailed simulation including light dispersion, and the generated sideband spectral distribution is approximated by an envelope as shown in FIG. The vertical axis represents the ratio of the light intensity of the outgoing light to the light intensity of the incident light (P_out/ P_inThe horizontal axis represents the frequency of each sideband generated and the frequency ν of the incident light.₁Difference Δf.
[0026]
As shown in FIG. 2, the intensity P of the light emitted from the exit-side reflecting mirror 113_outWhen Δf = 0, in other words, it becomes a maximum at the frequency of the incident light and becomes a curve that changes exponentially according to the frequency difference Δf.
[0027]
Note that the incident-side reflecting mirror 112 and the emitting-side reflecting mirror 113 are not only disposed outside the optical phase modulator 111, but also on the incident-side end face and the emitting-side end face of the optical phase modulator 111. It may be mounted as a mirror.
[0028]
The filter 114 emits the light emitted from the emission-side reflecting mirror 113 to the outside with a certain transmittance. Details of the transmittance of the filter 114 will be described later.
[0029]
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).
[0030]
In the optical frequency comb generator 10 having the above-described configuration, an electric signal synchronized with the time when the light reciprocates in the optical resonator 110 is used as a drive input from the electrode 116 to the optical phase modulator 111, whereby the optical phase modulator It is possible to apply deep phase modulation several tens of times or more compared to the case of passing through 111 once. 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. The frequency of the light to be measured can be determined by measuring the beat frequency based on a large number of optical frequency combs generated by the optical frequency comb generator 10.
[0031]
Next, the transmittance of the filter 114 will be described.
[0032]
As shown in FIG. 3, the transmittance of the filter 114 indicates the frequency ν of incident light.₁Is set to be minimum. Further, the transmittance of the filter 114 has a frequency ν.₁In bands other than ν₁In some cases, the transmittance is set to be higher than the transmittance in FIG. The transmittance distribution curve is represented by ν as shown in FIG.₁However, the slope of the curve may be not only steep but also gradual.
[0033]
Further, as shown in FIG. 4, the transmittance distribution curve pays attention to the light intensity that changes exponentially according to the frequency difference Δf, and the transmittance of the light emitted from the filter changes exponentially. It is also possible to flatten the light intensity distribution.
[0034]
For example, the intensity P of the light emitted from the reflecting mirror 113_outCan be approximately expressed by equation (2) within a range not affected by the group refractive index dispersion.
P_out= T_inT_outexp {− | Δf | Los / (βfm)} P_in    (2)
In this equation (2), T_inIs the transmittance of the incident side reflecting mirror 112, T_outIs the transmittance of the exit-side reflecting mirror 113, β is a modulation index during reciprocation within the optical resonator 310, Los is a loss rate of light reciprocating within the optical resonator 110, and in this equation (2) Expressed as a constant. If the loss factor of light in the optical resonator 110 is only transmission to the outside through the incident-side reflecting mirror 112 and the emitting-side reflecting mirror 113, Los is T_inAnd T_outThe sum of
[0035]
Based on this equation (2), the transmittance is set so that the light intensity distribution of the light emitted from the filter 114 becomes flat. That is, the light output P of the emitted light that changes exponentially according to the wavelength_outIn order to flatten the filter, the transmittance of the filter 114 is also changed exponentially according to the wavelength.
[0036]
In this first embodiment, the optical frequency comb generator 10 to which the present invention is applied flattens the light intensity distribution of the light emitted from the filter by controlling the transmittance characteristics of the filter 114 as described above. Can be made. Thereby, in the first embodiment of the present invention, it is possible to generate an optical frequency comb that can measure the frequency of the light to be measured with higher accuracy even in a band having a substantial frequency difference from the frequency of the incident light. it can.
[0037]
Next, as a second embodiment of the present invention, an optical frequency comb generator 20 that directly filters light in an optical resonator will be described in detail with reference to FIG. Note that the same circuit components and members as those of the signal detection device 1 according to the first embodiment are referred to the description of the first embodiment, and the description thereof is omitted.
[0038]
The optical frequency comb generator 20 includes an optical resonator 110 including an optical phase modulator 111 and an incident-side reflecting mirror 112 and an emitting-side reflecting mirror 115 which are disposed so as to face each other via the optical phase modulator 111. And an oscillator 117. The optical frequency comb generator 20 is different from the first embodiment in that the generated sideband is flattened by controlling the transmittance of the exit-side reflecting mirror 115 without providing a filter.
[0039]
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 115, and a part of the light Lout is emitted from the emitting side. The light is emitted through the reflecting mirror 115.
[0040]
The incident-side reflecting mirror 112 and the emitting-side reflecting mirror 115 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.
[0041]
The exit-side reflecting mirror 115 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 115 emits the light that has passed through the optical phase modulator 111 to the outside with a certain transmittance. Details of the transmittance of the exit-side reflecting mirror 115 will be described later.
[0042]
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).
[0043]
Next, the transmittance of the output side reflecting mirror 115 will be described.
[0044]
The output side reflecting mirror 115 sets the transmittance for each frequency in accordance with the light intensity of the generated sideband. In other words, paying attention to the physical property of the sideband in which the light intensity increases or decreases according to the frequency, the transmittance of the exit-side reflecting mirror 115 is set. Therefore, the sideband light intensity P inside the resonator 110_insideIt is necessary to consider about.
[0045]
Expressions (3.1) to (3.2) are obtained by calculating the light intensity P of the sideband inside the resonator 110 with respect to the above-described Δf estimated from the expression (2)._insideShows the rate of change.
dP_inside/ dΔf = −Los / (βfm) P_inside      When Δf> 0
(3.1)
dP_inside/ dΔf = Los / (βfm) P_inside      When Δf <0
(3.2)
That is, the equations (3.1) to (3.2) can be expressed by differential equations relating to Δf, and the sideband light intensity P_insideCan be obtained as a function of Δf.
[0046]
This equation (3.1) represents the rate of change of light intensity when Δf> 0, that is, in a band higher than the frequency of incident light, and equation (3.2) is the case when Δf <0, ie, incident It represents the rate of change of light intensity in a band lower than the frequency of light. Further, in the case of Δf = 0, that is, in the case of the frequency of the incident light, it can be expressed by the light intensity of the light transmitted through the incident side reflecting mirror 112. For example, as shown in the equation (3.3), the incident light Light intensity P_inAnd transmittance T of the incident side reflecting mirror 112_inIt can be expressed by the product of
P_inside= T_in× P_in                (3.3)
P calculated by the equations (3.1) to (3.3)_insideFrom the expression (3.4), the light intensity P of the emitted light_outCan be calculated.
P_out= T_out× P_inside              (3.4)
T_outAnd T_inIt is also possible to derive Expression (2) by rearranging Expressions (3.1) to (3.4), where is a constant.
[0047]
The light intensity P of the sideband in the resonator 110 that can be expressed by the above formula._insideIs flattened for each spectrum and emitted to the outside. In other words, the transmittance T for each frequency band in the output-side reflecting mirror 113._outBy setting, the intensity of light emitted to the outside is controlled.
[0048]
The transmittance T of the exit-side reflecting mirror 113_outThe condition of dP_outBy substituting into the formulas (3.1) to (3.4) as / dΔf = 0, the following formulas (4.1) and (4.2) can be expressed.
dT_out/ dΔf = Los / (βfm) T_out                    (4.1)
dT_out/ dΔf = −Los / (βfm) T_out                  (4.2)
Based on the equations (4.1) and (4.2), the transmittance T of the exit-side reflecting mirror 113 is calculated._outCan be determined.
[0049]
Depending on the setting of the transmittance in the case of Δf = 0 (hereinafter, this transmittance is referred to as an initial value), many solutions can be obtained when calculating the above equations (4.1) and (4.2). May be. Also, 0 <T_outSince there is a physical restriction of <1, it may not hold for all Δf.
[0050]
FIG. 6 shows the transmittance T of the exit-side reflecting mirror 113 based on the equations (4.1) and (4.2)._outThe result of having been obtained is shown. Los is the total loss during the round trip through the resonator 110, and Los = T_in+ T_outAnd T_in= 0.005. In FIG. 6, curve B and curve C have different initial values.
[0051]
FIG. 7 shows the light intensity distribution in each band of outgoing light when the outgoing-side reflecting mirror 113 is used. In FIG. 7, a curve indicated by A ′ is obtained by superimposing the light intensity distributions shown in FIG. A curve B ′ indicates the transmittance T shown by the curve B in FIG._outOutput light P when set to_outRepresents the light intensity distribution. Further, a curve C ′ indicates that the exit-side reflecting mirror 113 has a transmittance T shown by the curve C in FIG._outOutput light P when set to_outRepresents the light intensity distribution. As shown in FIG. 7, the curves B ′ and C ′ have flattened light intensity characteristics in the vicinity of the frequency of the incident light.
[0052]
In FIG. 7, the curve B ′ has a light intensity higher than that of the curve A ′ in the band except for the vicinity of the frequency of the incident light, and in the curve C ′, and the first embodiment of the present invention. Even if compared with this form, an optical frequency comb can be generated with high efficiency. Since this can directly filter the sideband generated inside the optical resonator, the optical loss is reduced as compared with the first embodiment in which the light emitted from the exit-side reflecting mirror 113 is further filtered. This is because it can be reduced.
[0053]
Further, by controlling the initial value, it is possible to flatten the light intensity characteristic in a wide band as shown by the curve B ′, or to generate an optical frequency comb having a high light intensity in a narrow band as shown by the curve C ′. That is, in the second embodiment of the present invention, the practitioner can arbitrarily select whether to give priority to the bandwidth or the light intensity by controlling the initial value.
[0054]
Furthermore, by controlling this initial value, it is possible not only to achieve flattening of the sideband in the entire band, but also to flatten the intensity distribution of the sideband in some bands. In such a case, the transmittance T_outAs shown in FIG. 6, there is a case where Δf = 0 does not become the minimum and, for example, a right-upward curve is obtained.
[0055]
Figure 8 shows the transmittance T_outAnd the absolute value of the normalized frequency derivative of transmittance. In other words, this FIG.₀= Los-T_outThe relationship between the optimal transmittance and the rate of change in transmittance is shown with respect to the value of.
[0056]
When the optical frequency comb generator according to the second embodiment is actually designed, a mirror used as the output-side reflecting mirror 113 is partially selected from the mirrors having reflection characteristics that can be designed. A mirror showing the characteristics of
[0057]
FIG. 9 shows Lorentzian reflection characteristics and T_out= Slope of curve dT when = 0.01_outThe transmittance characteristic of the exit-side reflecting mirror 113, where / dΔf is −13 dB / THz, is shown. When a mirror having the transmittance characteristics shown in FIG. 9 and satisfying the conditions of FIG. 8 is used as the exit-side reflecting mirror 113, the generated optical frequency comb is flattened as shown in FIG. It becomes possible.
[0058]
In other words, in the second embodiment, the optical frequency comb generator 20 to which the present invention is applied reduces the light intensity of the outgoing light by controlling the transmittance characteristic of the outgoing-side reflecting mirror 115 as described above. It is possible to flatten the generated sideband while preventing the above-mentioned problem. The optical frequency comb generator 20 can also be applied to a communication light source that exhibits stable light intensity in each band, for example. Furthermore, when this optical frequency comb generator 20 is applied to, for example, an optical CT, it is possible to increase the resolution.
[0059]
The present invention is not limited to the first embodiment and the second embodiment described above. For example, the present invention can be applied to a waveguide type optical frequency comb generator 30 as shown in FIG.
[0060]
The waveguide type optical frequency comb 21 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.
[0061]
The substrate 201 is a 3-4 inch diameter LiNbO grown by, for example, a pulling method.₃A large crystal such as GaAs or GaAs is cut into a wafer. In order to epitaxially grow the waveguide 202 layer on the cut-out substrate 201 or to diffuse Ti on the heated substrate, processing such as mechanical polishing or chemical polishing is usually performed.
[0062]
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.
[0063]
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. 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.
[0064]
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.
[0065]
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).₁Light 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.
[0066]
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.
[0067]
Next, since the transmittance of the exit-side reflecting film 205 constituting the waveguide type optical frequency comb generator 30 is the same as the transmittance of the exit-side reflecting mirror 115 described in the second embodiment, the exit-side reflection is performed. The description of the transmittance of the mirror is cited, and the description is omitted.
[0068]
FIG. 12B shows the light intensity distribution of each band of the emitted light when the emitting side reflection film 205 is set as D, E, F shown in FIG. Even when the present invention is applied to, for example, the waveguide type optical frequency comb generator 30, the light intensity distribution of the emitted light can be arbitrarily controlled by similarly controlling the initial value.
[0069]
In addition, when the present invention is applied to the waveguide-type optical frequency comb generator 30, the size can be reduced as compared with the optical frequency comb generators 10 and 20 using a bulk crystal. Parasitic inductance can be suppressed. 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.
[0070]
【The invention's effect】
  As described in detail above, the optical frequency comb generator to which the present invention is appliedIn the output side reflector, the transmittance is set for each frequency in accordance with the light intensity of the generated sideband so that the light intensity characteristic is flattened in the vicinity of the frequency of the incident light. Filtering is directly performed on the sideband generated inside the resonance means composed of the reflecting mirror and the exit-side reflecting mirror.
[0073]
As a result, the present invention makes it possible to flatten the generated sideband while preventing a decrease in the light intensity of the emitted light, and with higher accuracy even in a band that has a considerable frequency difference from the frequency of the incident light. An optical frequency comb that can measure the frequency of the light to be measured can be generated.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a first embodiment of an optical frequency comb generator to which the present invention is applied;
FIG. 2 shows the light intensity of outgoing light in each band (= P_out) Incident light intensity (= P_inIt is a figure for demonstrating ratio with respect to.
FIG. 3 is a diagram for explaining the transmittance of each frequency of the filter.
FIG. 4 is a diagram showing a light intensity distribution for each frequency of light emitted from a filter.
FIG. 5 is a diagram for explaining a second embodiment of an optical frequency comb generator to which the present invention is applied;
FIG. 6 is a diagram showing an example of setting the transmittance of the exit-side reflecting mirror in the second embodiment.
FIG. 7 is a diagram showing a light intensity distribution for each frequency of emitted light in the second embodiment.
FIG. 8 is a diagram showing the relationship between transmittance and normalized frequency differentiation of transmittance.
[Figure 9] T_outFIG. 9 is a diagram showing the transmittance characteristics of the exit-side reflecting mirror with respect to the frequency difference Δf when a mirror having the characteristics of FIG. 8 is selected as the exit-side reflecting mirror.
10 is a diagram showing the light intensity characteristics of outgoing light when a mirror having the characteristics shown in FIG. 9 is selected as the outgoing-side reflecting mirror. FIG.
FIG. 11 is a diagram for explaining a specific configuration example of a waveguide-type optical frequency comb generator;
FIGS. 12A and 12B are diagrams showing an example of setting the transmittance of the exit-side reflecting film and the light intensity distribution of each band of the emitted light when the exit-side reflecting film is selected.
FIG. 13 is a diagram for explaining a specific configuration example of a conventional optical frequency comb generator.
FIG. 14 is a diagram showing a light intensity distribution of outgoing light for each band in a conventional optical frequency comb generator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Optical frequency comb generator, 11 Optical detector, 12 Frequency counter, 20 Waveguide type optical frequency comb generator, 110 Optical resonator, 111 Optical phase modulator, 112 Incident side reflecting mirror, 113,115 Outgoing side reflection Mirror, 114 filter, 116 electrodes, 117 oscillator

Claims (3)

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;
It said emission side reflecting mirror, so as to flatten the light intensity characteristics in a frequency neighborhood of the incident light, according to the light intensity of the generated sidebands, the transmittance is set for each frequency, the incident side reflection An optical frequency comb generator characterized by directly filtering a sideband generated inside a resonance means composed of a mirror and an exit side reflecting mirror . The exit-side reflecting mirror has a change in light intensity with respect to a frequency difference between the frequency of each sideband of the sideband generated in the resonance means composed of the entrance-side reflecting mirror and the exit-side reflecting mirror and the frequency of the incident light. based on the rate, so as to flatten the light intensity characteristics in a frequency neighborhood of the incident light, the optical frequency comb generator according to claim 1, wherein setting the transmittance for each frequency. 3. The optical frequency comb generator according to claim 2, 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 modulator.

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