JP3891977B2 - Optical frequency comb generator and optical modulator - Google Patents

Description

  The present invention relates to an optical frequency comb generator and an optical modulator, and requires a multi-wavelength, high-coherence standard light source such as optical communication, optical CT, and optical frequency standard, or a light source that can also use coherence between wavelengths. It is applied to the field.

  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 light 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.

  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 optical control for frequency division multiplexing and frequency measurement of absorption lines distributed over a wide range.

  In order to meet the demand for an increase in the measurable bandwidth, a wideband heterodyne detection system using an optical frequency comb generator (see, for example, Patent Document 1) has been proposed. 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.

  FIG. 6 shows the basic structure of this conventional optical frequency comb generator 3.

  The optical frequency comb generator 3 uses an optical resonator 100 including an optical phase modulator 31 and reflecting mirrors 32 and 33 installed so as to face each other with the optical phase modulator 31 interposed therebetween.

  The optical resonator 100 resonates light Lin incident through the reflecting mirror 32 with a small transmittance between the reflecting mirrors 32 and 33, and emits a part of the light Lout through the reflecting mirror 33. The optical phase modulator 31 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 36 with respect to the light passing through the optical resonator 100. Phase modulation is applied according to the electric signal of fm.

  In this optical frequency comb generator 3, an electric signal synchronized with the time when the light reciprocates in the optical resonator 100 is driven and input from the electrode 36 to the optical phase modulator 31, so that the optical phase modulator 31 is only once. Compared with the case of passing, it becomes possible to apply deep phase modulation of several tens of times or more. 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.

  Further, the conventional optical frequency comb generator is not limited to the above-described bulk type. For example, as shown in FIG. 7, the present invention is also applicable to a waveguide type optical frequency comb generator 200 using a waveguide.

  The waveguide type optical frequency comb generator 20 includes a waveguide type optical modulator 200. The waveguide type optical modulator 200 includes a substrate 201, a waveguide 202, an electrode 203, an incident side reflection film 204, an emission side reflection film 205, and an oscillator 206.

The substrate 201 is obtained by cutting a large crystal such as LiNbO ₃ or GaAs having a diameter of 3 to 4 inches grown by a pulling method into a wafer shape. 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.

  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.

  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. Further, the propagation direction of light in the waveguide and the traveling direction of the modulation electric field are the same.

  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.

The incident-side reflection film 204 is disposed on the light incident side of the waveguide type optical modulator 200, and light having a frequency ν ₁ is incident from a light source (not shown). Further, the incident-side reflection film 204 reflects light reflected by the emission-side reflection film 205 and having passed through the waveguide 202.
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.

  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.

JP2003-202609A.

  However, in the conventional waveguide type optical frequency comb generator 20 described above, only the light propagating in the forward direction shown in FIG. For this reason, there has been a problem that the modulation efficiency is remarkably lowered as compared with the case where phase modulation is applied to light propagating in the backward direction in addition to light propagating in the forward direction (hereinafter referred to as reciprocal modulation).

  Also, in order to obtain the same degree of modulation as in the reciprocal modulation, the intensity of the electric signal that is driven and input from the electrode 203 to the waveguide 202 must be increased, and a large applied voltage is required during phase modulation. If the required power increases, the burden on the drive circuit including the electrode 203 increases, leading to an increase in the size of the entire heterodyne detection system described above and an increase in cost based thereon. Furthermore, this increase in the applied voltage may cause a failure of the waveguide type optical frequency comb generator 20 itself.

  Therefore, the present invention has been devised in view of the above-described problems, and the object of the present invention is to perform phase modulation by performing reciprocal modulation with a simple configuration for light resonating in the optical resonator. An object of the present invention is to provide an optical frequency comb generator and an optical modulator that can improve modulation efficiency without increasing the required power.

An optical frequency comb generator to which the present invention is applied includes an oscillating means 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. A resonance means for resonating the light incident through the incident-side reflecting mirror by propagating it in the forward or backward direction, and the oscillation means disposed between the incident-side reflecting mirror and the emitting-side reflecting mirror. Optical modulation that modulates the phase of the light resonated in the resonance means in accordance with the modulation signal supplied from, and generates a side band centered on the frequency of the incident light at the frequency interval of the modulation signal The optical modulation means includes at least an optical waveguide formed of a substrate having an electro-optic effect, and a modulation signal formed on the optical waveguide and oscillated from the oscillation means in a forward direction or Consists electrode for propagating the road direction, reflection means for reflecting the modulated signal supplied to one end of said electrode from said oscillating means is provided only on the other side of the electrode, supplied from the oscillation means Without resonating the modulated signal, the phase of light propagating in the forward direction is modulated by the modulation signal propagating in the forward direction, and the phase of light propagating in the backward direction is propagated in the backward direction. Modulate with modulation signal.

Further, in order to solve the above-described problems, an optical modulator to which the present invention is applied has an oscillating unit that oscillates a modulation signal of a predetermined frequency and light incident through any one end face in the forward direction. Or an optical propagation means for propagating in the backward direction, and an optical modulation means arranged between the end faces and modulating the phase of the propagating light according to the modulation signal supplied from the oscillation means, and The means comprises at least an optical waveguide formed on a substrate having an electro-optic effect, and an electrode for propagating a modulation signal formed on the optical waveguide and oscillated from the oscillation means in the forward direction or the backward direction, reflecting means for reflecting the modulated signal supplied to one end of said electrode from said oscillating means is provided only on the other side of the electrode, without resonating the modulated signal supplied from the oscillation means, on The phase of the light propagating in the outward direction modulated by a modulation signal propagating into the forward direction, modulated by a modulation signal propagating through the phase of the light propagating to the backward direction to the backward direction.
Further, in order to solve the above-described problems, the optical modulator according to the present invention is configured to oscillate a modulation signal having a predetermined frequency and light incident through any one end face in the forward direction or A light propagation means for propagating in the return path direction, at least one reflecting mirror on the optical path including the light propagation means, a light reflection means for returning light propagating in the optical path direction from the incident end to the incident side, and an incident end; And an optical modulation means that modulates the phase of the propagating light according to the modulation signal supplied from the oscillation means, and the optical modulation means has at least an electro-optic effect. And an electrode for propagating a modulated signal formed on the optical waveguide and oscillated from the oscillating means in the forward direction or the backward direction, and from the oscillating means to one end of the electrode. On the side Reflecting means for reflecting the paper modulated signal is provided only on the other side of the electrode, without resonating the modulated signal supplied from the oscillation means, the phase of the light propagating to the forward direction Modulation is performed by the modulation signal propagating in the forward direction, and the phase of light propagating in the backward direction is modulated by the modulation signal propagating in the backward direction.

  In the optical frequency comb generator and the optical modulator to which the present invention is applied, not only the light propagating in the waveguide direction but also the light propagating in the backward direction can be phase-modulated, so that the modulation efficiency is increased. be able to.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of an optical frequency comb generator 1 to which the present invention is applied. The optical frequency comb generator 1 includes a waveguide type optical modulator 2. The waveguide type optical modulator 2 includes a substrate 11, a waveguide 12 that is formed on the substrate 11 and modulates the phase of propagating light, and the direction of the modulation electric field is substantially perpendicular to the light propagation direction. In this manner, the electrode 13 provided on the upper surface of the waveguide 12, the incident-side reflection film 14 and the emission-side reflection film 15 disposed so as to face each other via the waveguide 12, and one end side of the electrode 13 are disposed. And an oscillator 16 that oscillates a modulation signal having a frequency fm, and a phase shifter 18 and a reflector 19 disposed on the other end side of the electrode 13.

The substrate 11 is obtained by cutting a large crystal such as LiNbO ₃ or GaAs having a diameter of 3 to 4 inches grown by a pulling method into a wafer shape. In order to epitaxially grow the waveguide 12 layer on the cut substrate 11, a process such as mechanical polishing or chemical polishing is usually performed.

  The waveguide 12 is arranged for propagating light, and the refractive index of the layer constituting the waveguide 12 is set higher than that of other layers such as a substrate. The light incident on the waveguide 12 propagates while being totally reflected at the boundary surface of the waveguide 12. The waveguide 12 modulates light passing through physical phenomena 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.

  The electrode 13 is made of a metal material such as Ti, Pt, or Au, for example, and drives and inputs a modulation signal having a frequency fm supplied from the outside to the waveguide 12. With respect to the electrode 13, phase modulation is applied to light propagating in the waveguide 12 by a modulation signal having a frequency fm supplied from the oscillator 16.

  The incident side reflection film 14 and the emission side reflection film 15 resonate by propagating the light incident on the waveguide 12 in the forward direction or the backward direction shown in FIG. The incident-side reflection film 14 is disposed on the light incident side of the waveguide 12, and light supplied from the outside enters through the incident-side reflection film 14. The exit-side reflection film 15 is disposed on the light exit side of the waveguide 12 and emits a part of the light propagating through the waveguide 12 to the outside.

  The reflector 19 reflects the modulation signal oscillated from the oscillator 16. The phase shifter 18 adjusts the phase of the reflected modulation signal.

  Next, the configuration of the electrode 13 in the optical frequency comb generator 1 to which the present invention is applied will be described in more detail.

  FIG. 2 is a top view of the electrode 13. As shown in FIG. 2, the electrode 13 is configured in a U-shape from one end side where the oscillator 16 is provided to the other end side where the phase shifter 18 and the reflector 19 are provided. Yes. When this electrode 13 is considered as a so-called transmission path for a high-frequency modulation signal, if there are so-called right-angled bent portions in the corner portions 13a and 13b, the transmission characteristics are significantly deteriorated due to the wavelength dependence of the modulation signal. For this reason, you may make it round corner part 13a, b so that a right-angled bending part may not arise.

  When a modulation signal having a frequency fm oscillated from the oscillator 16 is supplied to the electrode 13 having such a shape, the modulation signal propagates through the electrode 13 in the forward direction, and travels through the waveguide 12 in the forward direction. The phase of the light propagating to can be modulated. The modulated signal that has propagated on the electrode 13 in the forward direction is reflected by the reflector 19 as it is, and after being phase-adjusted by the phase shifter 18, is then propagated through the electrode 13 in the backward direction. As a result, the phase of the light propagating in the backward direction in the waveguide 12 can be modulated. Incidentally, the phase adjusted by the phase shifter 18 is set so that the phase modulation applied to the light propagating in the backward direction in the waveguide 12 is the same as the phase shift modulation for the light propagating in the forward direction. Also good.

  That is, in the optical frequency comb generator 1 to which the present invention is applied, not only the light propagating through the waveguide 12 in the forward direction but also the light propagating in the backward direction can be phase-modulated, so that the modulation efficiency is increased. be able to.

  Further, in the optical frequency comb generator 1 having the above-described configuration, when an electric signal synchronized with the time when the light reciprocates in the waveguide 12 is used as a driving input from the electrode 13, the optical frequency comb generator 1 passes through the waveguide 12 only once. Compared to the above, it becomes possible to apply deep phase modulation several tens of times or more. As a result, an optical frequency comb having sidebands over a wide band can be generated, and the frequency intervals between adjacent sidebands are all equal to the frequency fm of the input electrical signal.

  In addition, since the optical frequency comb generator 1 can modulate light by pushing light into a narrow waveguide 12, the modulation index can be increased, and the optical frequency comb generator is generated in comparison with a bulk type optical frequency comb generator. The number of sidebands to be performed and the amount of sideband light can be increased.

  In the phase shifter 18 in the optical frequency comb generator 1 to which the present invention is applied, the phase of the modulation signal reflected from the reflector 19 may be adjusted as described below.

As shown in FIG. 2, when the length from the corner portion 13a of the electrode 13 to the exit-side reflection film 15 is Δl and the group refractive index in the waveguide 12 is _ng , light is transmitted from the corner portion 13a to the waveguide. A time t required to propagate along 12 and reflect the reflection film 15 on the emission side and return to the corner portion 13a is expressed by the following equation (1).
t = 2n _g Δl / c (c: speed of light)...
Here, when the modulation frequency is ω _m ,
ω _m + θ = 2mπ (m = 0, 1, 2,...) Equation 2
When θ is changed by adjusting the phase shifter 18 so as to satisfy the condition, the phase of the modulation signal propagating in the backward direction matches the phase of light propagating in the waveguide 12 in the backward direction. . That is, the phase shifter 18 adjusts the phase of the modulation signal so as to satisfy the following Expression 3.
θ = 2mπ-2n _g k ₀ Δl (where k ₀ = ω _m / c)
Thus, by adjusting the phase of the modulation signal adjusted by the phase shifter 18 according to the shape of the electrode 13, the frequency fm of the modulation signal, and the group refractive index _ng of the waveguide 12, The phase can be adjusted with high accuracy.

  For this reason, in the optical frequency comb generator 1, not only the light propagating through the waveguide 12 in the forward direction but also the light propagating in the backward direction can be phase-modulated with high efficiency, so that the modulation efficiency is doubled at maximum. It can be increased to near. Further, since the modulation efficiency can be effectively improved without increasing the voltage applied to the electrode 13, the power consumption can be reduced, and the heterodyne detection system itself in which the optical frequency comb generator 1 is disposed is made slim. And cost can be greatly reduced.

  The optical frequency comb generator 1 to which the present invention is applied is not limited to the embodiment described above, and is applied to the optical frequency comb generator 7 having an L-shaped electrode shown in FIG. May be. In this optical frequency comb generator 7, the same components as those of the optical frequency comb generator 1 are denoted by the same reference numerals and description thereof is omitted.

  As shown in the top view of FIG. 3, the optical frequency comb generator 7 includes an electrode 73 provided on the top surface of the waveguide 12 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction, An incident-side reflection film 14 and an emission-side reflection film 15 installed so as to face each other via the waveguide 12, and an oscillator 16 that is disposed on one end side of the electrode 73 and oscillates a modulation signal having a frequency fm. ing.

  The electrode 73 is provided with a cutting point 73a for reflecting the modulation signal supplied from the other end.

  When a modulation signal having a frequency fm oscillated from the oscillator 16 is supplied to the electrode 73 having such a shape, the modulation signal propagates on the electrode 73 in the forward direction, and travels in the waveguide 12. The phase of the light propagating in the direction can be modulated. The modulated signal that has propagated on the electrode 73 in the forward direction is reflected by the cut point 73a and is then propagated in the backward direction. As a result, the phase of light propagating in the waveguide 12 in the backward direction can be modulated, and the modulation efficiency can be improved in the same manner as the optical frequency comb generator 1. In particular, this optical frequency comb generator 7 is advantageous in that it is not necessary to dispose a phase shifter or a reflector 19, so that loss of the modulation signal can be suppressed.

  Incidentally, the position of the cutting point 73a may be adjusted as described below.

As shown in FIG. 3, when the length from the cut point 73a of the electrode 73 to the exit side reflection film 15 is Δl ₂ and the group index of refraction in the waveguide 12 is _ng , light is guided from the cut point 73a. A time t ₂ from propagation along the waveguide 12 to reflection on the exit-side reflection film 15 and return to the corner portion 73a is expressed by the following Expression 4.
t ₂ = 2ng _g Δl ₂ / c (c: speed of light) (4)
Here, when the modulation frequency is ω _m ,
ω _m t ₂ = 2mπ (m = 0, 1, 2,...) Equation 5
When the condition is satisfied, the light propagating in the backward direction is modulated with the same phase as the forward direction. Here, when the modulation frequency ω _m = 2πfm, the length Δl ₂ is adjusted so as to satisfy Equation 6 below.
Δl = mπ / n _g k ₀ (where k ₀ = ω _m / c)
Thus, by adjusting the cut point 73 a in the electrode 73 according to the frequency of the modulation signal and the group refractive index of the waveguide 12, highly efficient phase modulation can be performed.

  The present invention may be applied to the optical modulator shown in FIG. The optical modulator 8 may receive light through any of the end faces 84 and 85, and each incident light propagates in the waveguide 12 in the forward or backward direction while being modulated as described above. Then, the light is emitted to the outside through the opposed end surfaces 85 and 84. For this reason, the optical modulator 8 can efficiently modulate not only the phase of the propagating light but also the intensity, the polarization, etc., regardless of the end face on which the light is incident.

  The optical modulator 8 is not limited to the above-described embodiment, and the configuration of the optical frequency comb generator 7 having an L-shaped electrode may be applied as it is as an optical modulator.

  Incidentally, in this embodiment, the optical frequency comb generator 400 that generates an optical comb has been described as an example. However, the present invention is not limited to such a case, and only modulation of light propagating in the waveguide 412 is performed. Of course, it may be applied as the waveguide type optical modulator 402 to be performed.

It is a figure which shows the structure of the optical frequency comb generator to which this invention is applied. It is a figure for demonstrating about the structure of the electrode in the optical frequency comb generator to which this invention is applied. It is a figure for demonstrating per structure of the optical frequency comb generator which has an L-shaped electrode. It is a figure for demonstrating about the optical frequency comb generator which can modulate the phase of the light to propagate efficiently irrespective of the end surface into which light injects. It is a figure which shows the fundamental structure of the conventional optical frequency comb generator. It is a figure which shows the fundamental structure of the conventional waveguide type optical frequency comb generator.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical frequency comb generator, 2 Waveguide type optical modulator, 11 Substrate, 12 Waveguide, 13 Electrode, 14 Incident side reflective film, 15 Outgoing side reflective film, 16 Oscillator, 18 Phase shifter, 19 Reflector

Claims (8)

Oscillating means for oscillating a modulation signal of a predetermined frequency;
Resonating means that is composed of an entrance-side reflecting mirror and an exit-side reflecting mirror that are parallel to each other, and resonates by propagating light incident through the entrance-side reflecting mirror in the forward direction or the backward direction;
The incident light is arranged between the incident-side reflecting mirror and the emitting-side reflecting mirror and modulates the phase of the light resonated in the resonance means in accordance with the modulation signal supplied from the oscillation means. Optical modulation means for generating a sideband centered at the frequency of the modulation signal at intervals of the frequency of the modulation signal,
The light modulating means includes an optical waveguide formed on a substrate having at least an electro-optic effect, and an electrode formed on the optical waveguide for propagating a modulation signal oscillated from the oscillating means in a forward direction or a backward direction. The reflection means for reflecting the modulation signal supplied from the oscillation means to the one end side of the electrode is provided only on the other end side of the electrode, and resonates the modulation signal supplied from the oscillation means. The phase of light propagating in the forward direction is modulated by the modulation signal propagating in the forward direction, and the phase of light propagating in the backward direction is modulated by the modulation signal propagating in the backward direction. An optical frequency comb generator. 2. The optical frequency comb generator according to claim 1, 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. . As the reflection means, one end of the electrode is provided with a reflector for reflecting the modulation signal supplied from the other end, and a phase shifter for adjusting the phase of the reflected modulation signal. The optical frequency comb generator according to claim 1, wherein   4. The light according to claim 3, wherein the phase shifter adjusts the phase of the reflected modulated signal in accordance with the shape of the electrode, the frequency of the modulated signal, and the group refractive index of the waveguide. Frequency comb generator. 2. The optical frequency comb generator according to claim 1 , wherein the reflection means is provided with a cutting point for reflecting a modulation signal supplied from the other end at one end of the electrode.   6. The optical frequency comb generator according to claim 5, wherein a cut point in the electrode is adjusted according to a frequency of the modulation signal and a group refractive index of the waveguide. Oscillating means for oscillating a modulation signal of a predetermined frequency;
A light propagation means for propagating the light incident through any one end face in the forward direction or the backward direction;
An optical modulation unit that is arranged between the end faces and modulates the phase of the propagating light according to the modulation signal supplied from the oscillation unit;
The light modulating means includes an optical waveguide formed on a substrate having at least an electro-optic effect, and an electrode formed on the optical waveguide for propagating a modulation signal oscillated from the oscillating means in a forward direction or a backward direction. The reflection means for reflecting the modulation signal supplied from the oscillation means to the one end side of the electrode is provided only on the other end side of the electrode, and resonates the modulation signal supplied from the oscillation means. The phase of light propagating in the forward direction is modulated by the modulation signal propagating in the forward direction, and the phase of light propagating in the backward direction is modulated by the modulation signal propagating in the backward direction. An optical modulator. Oscillating means for oscillating a modulation signal of a predetermined frequency;
A light propagation means for propagating the light incident through any one end face in the forward direction or the backward direction;
A light reflecting means comprising at least one reflecting mirror on an optical path including the light propagating means, and returning light propagating in the optical path direction from the incident end side to the incident side;
An optical modulation unit that is arranged between the incident end and the reflecting mirror and modulates the phase of the propagating light according to the modulation signal supplied from the oscillation unit;
The light modulating means includes an optical waveguide formed on a substrate having at least an electro-optic effect, and an electrode formed on the optical waveguide for propagating a modulation signal oscillated from the oscillating means in a forward direction or a backward direction. The reflection means for reflecting the modulation signal supplied from the oscillation means to the one end side of the electrode is provided only on the other end side of the electrode, and resonates the modulation signal supplied from the oscillation means. The phase of light propagating in the forward direction is modulated by the modulation signal propagating in the forward direction, and the phase of light propagating in the backward direction is modulated by the modulation signal propagating in the backward direction. An optical modulator.

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