JP3891976B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator, and is applied to a field requiring 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. The

  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. 8 shows the basic structure of the 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. 9, 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, the modulation efficiency of the conventional waveguide type optical frequency comb generator 20 strongly depends on the polarization direction of light propagating in the waveguide 202. For this reason, there is a problem that only light in a specific polarization direction can be modulated.

  In addition, it is necessary to dispose a polarization preserving fiber in order to adjust the polarization direction of the light propagating in the waveguide 202, but there is a problem that labor and cost are excessive.

  Accordingly, the present invention has been devised in view of the above-described problems, and its object is to provide light that can improve modulation efficiency without being governed by the polarization direction of incident light. It is to provide a modulator.

In order to solve the above-described problems, an optical modulator to which the present invention is applied controls separation means for separating incident light according to the polarization direction, and controls the polarization direction of each of the separated lights in the same direction. A polarization control means for oscillating, an oscillating means for oscillating a modulation signal of a predetermined frequency, a light propagation means for propagating light incident through any one end face in the forward or backward direction, and an end face arranged between the end faces And an optical modulation unit that modulates the phase of the propagating light in accordance with the modulation signal supplied from the oscillation unit, and the optical modulation unit is an optical waveguide formed by a substrate having at least an electro-optic effect. A waveguide and an electrode formed on the optical waveguide for propagating the modulated signal oscillated from the oscillating means in the forward or backward direction, and the modulated signal supplied from the oscillating means to one end of the electrode Reflect Reflecting means is provided only on the other side of the electrode, without resonating the modulated signal supplied from the oscillation means, the modulated signal propagating through the phase of the light propagating to the forward direction to the forward direction Modulates and modulates the phase of the light propagating in the return path direction by the modulation signal propagating in the return path direction.

In order to solve the above-described problems, an optical modulator to which the present invention is applied controls separation means for separating incident light according to the polarization direction, and controls the polarization direction of each of the separated lights in the same direction. Polarization control means for oscillating, oscillation means for oscillating a modulated signal of a predetermined frequency, and reflecting mirrors parallel to each other, and light incident from the polarization control means through any one of the reflecting mirrors at different angles Resonance means that resonates by propagating in the forward or backward direction, and light modulation means that modulates the phase of light resonated in the resonance means in accordance with the modulation signal supplied from the oscillation means, The optical modulation means transmits an optical waveguide formed on a substrate having at least an electro-optic effect, and a modulation signal formed on the optical waveguide and oscillated from the oscillation means in the forward direction or the backward direction. Consists electrodes for, 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, supplied modulated signal from said oscillation means The phase of the light propagating in the forward direction is modulated by the modulation signal propagating in the forward direction, and the phase of the light propagating in the backward direction is modulated by the modulation signal propagating in the backward direction. To do.

In the optical modulator to which the present invention is applied, if the refractive index and the modulation efficiency of the material constituting the light propagation means are strongly dependent on a specific polarization direction, the light is separated by the light separation means according to the polarization direction. The polarization direction of each light can be controlled in the same direction. In addition, not only the light propagating through the waveguide in the forward direction but also the light propagating in the backward direction can be phase-modulated, so that the modulation efficiency can be increased. Thereby, even if the supplied light has any polarization component, highly efficient phase modulation can be realized without depending on this.

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

  The present invention is applied to a polarization-independent modulation system 9 shown below.

  As shown in FIG. 1, the modulation system 9 includes an optical modulator 8, a first polarization splitting / combining coupler 91 that separates light supplied to the optical modulator 8 into respective polarization components, A first polarization controller 93 that rotationally controls the polarization direction for one polarization component separated by the polarization separating / combining coupler 91, and a first polarization controller that rotationally controls the polarization direction for one polarization component emitted from the optical modulator 8. 2 polarization controllers 94, a second polarization controller 94 and a second polarization separation / combination coupler 92 that combines the light of each polarization component emitted from the optical modulator 8 and outputs the combined light to the outside. ing.

  The first polarization separation / combination coupler 91 separates the linearly polarized light in the horizontal direction and the linearly polarized light in the vertical direction from the light supplied from the outside. The separated light including the linearly polarized component in the horizontal direction is supplied to the first polarization controller 93, and the light including the linearly polarized component in the vertical direction is supplied to the optical modulator 8 as it is.

  The first polarization controller 93 rotates the polarization component of the light from the horizontal direction to the vertical direction, and supplies this to the optical modulator 8. Thereby, the polarization direction of the propagating light can be made the same. For this reason, if the refractive index of the material constituting the waveguide 12 and the modulation efficiency are strongly dependent on a specific polarization direction, the polarization direction of each light can be controlled in the same direction according to the polarization direction. it can.

  The second polarization controller 94 rotates the polarization component of the light emitted from the optical modulator 8 from the vertical direction to the horizontal direction, and supplies this to the second polarization separation / synthesis coupler 92. In the second polarization separation / combination coupler 92, the light including the horizontal polarization component is combined with the light including the vertical polarization component emitted from the other end of the optical modulator 8 and emitted to the outside. Will be.

  As shown in FIGS. 1 and 2, the optical modulator 8 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 in the light propagation direction. An electrode 83 provided on the upper surface of the waveguide 12 so as to be substantially perpendicular to the waveguide, a first end surface 84 and a second end surface 85 disposed so as to face each other via the waveguide 12, and an electrode 83 Is provided with an oscillator 16 that oscillates a modulation signal having a frequency fm, a phase shifter 18 and a reflector 19 that are provided on the other end side of the electrode 83.

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 83 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 83, 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 first end face 84 and the second end face 85 resonate by propagating the light incident on the waveguide 12 in the forward direction or the backward direction shown in FIG.

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

  The light incident through the first end face 84 propagates through the waveguide 12 in the backward direction and is emitted through the second end face 85. Further, the light incident through the second end face 84 propagates through the waveguide 12 in the forward direction and is emitted through the second end face 85. In other words, the light modulator 8 may receive light through any of the end faces 84 and 85, and each incident light is modulated in the waveguide 12 while being modulated as described above. And exits to the outside through the opposing end faces 84 and 85. For this reason, the light modulator 8 can efficiently modulate the propagating light without being controlled by the end face on which the light is incident.

  Incidentally, in the above-described example, the case where the phase of light is modulated has been described as an example. However, the present invention is not limited to such a case, and the light intensity, polarization, etc. are modulated using any electro-optic effect. You may make it do. The present invention can also be applied to a Mach-Zehnder type optical modulator or the like.

  The optical modulator 8 can also be applied to a ring-type electro-optic modulator described below.

  As shown in FIG. 3, the ring-type electro-optic modulator 80 includes an optical phase modulator 111, and a first mirror 112 and a second mirror that are disposed so as to face each other via the optical phase modulator 111. 113, an optical resonator 110 formed on the top and bottom surfaces of the optical phase modulator 111 such that the direction of the modulation electric field is substantially perpendicular to the light propagation direction, and each of the above-described components. And a temperature compensation housing 116 to be incorporated. Further, the first optical path 121 and the second optical path 122 made of optical fibers for propagating light to enter and exit the first mirror 112, respectively, are disposed at the end of the first optical path 121. The first collimator 131, the second collimator 132 disposed at the end of the second optical path 122, and the polarization splitting / combining coupler coupled to the first optical path 121 and the second optical path 122, respectively. 141.

  The optical phase modulator 111 is an optical device that phase-modulates light that passes through based on a supplied electrical signal. The optical phase modulator 111 may be made of the same material as that of the waveguide 12. Incidentally, the reflectance of each side surface of the optical phase modulator 111 is controlled so as to totally reflect the incident light. For this reason, as shown in FIG. 3, the light incident from the oblique direction through the first mirror 112 is totally reflected by the side surface of the optical phase modulator 111 and reflected by the second mirror 113. Therefore, a path as if to draw a ring in the optical phase modulator 111 is taken.

  The first mirror 112 and the second mirror 113 are provided to resonate light incident on the optical resonator 110, and resonate by reciprocally reflecting light passing through the optical phase modulator 111.

  The first mirror 112 is disposed on the light incident side of the optical phase modulator 111, and light is incident from the first collimator 131 or the second collimator 132. In addition, a part of the light reflected by the second mirror 113 and passed through the optical phase modulator 111 is reflected, and a part of the light is emitted to the outside.

  The electrode 115 is connected to an antenna 4 for transmitting and receiving radio signals. Phase modulation is applied to the light propagating in the optical phase modulator 111 with respect to the electrode 115 by a radio signal having a frequency fm supplied from the outside.

  The polarization separation / combination coupler 141 separates or combines the horizontal linearly polarized light and the vertical linearly polarized light from the light transmitted from the optical fiber communication network 440. The separated light including the linearly polarized light component in the horizontal direction propagates through the first optical path 121, and the light including the linearly polarized light component in the vertical direction propagates through the second optical path 122.

  The first optical path 121 rotates the optical fiber and rotates the polarization component of light from the horizontal direction to the vertical direction. Thereby, the polarization direction of the light propagating through the first optical path 121 and the light propagating through the second optical path 122 can be made the same. Note that the second optical path 122 may also have a function of rotating the polarization direction.

  The first collimator 131 and the second collimator 132 convert the light propagating through the first optical path 121 and the second optical path 122 into parallel light, respectively, to the first mirror 112 constituting the optical resonator 110. Exit.

  In such a ring type electro-optic modulator 80, light propagating from the optical fiber communication network 440 is first incident on the polarization splitting / combining coupler 141. Even if the incident light includes various polarization components, the incident light is separated according to the direction of each linearly polarized light and propagates through the first optical path 121 and the second optical path 122. The separated light is emitted as parallel light through the first collimator 131 and the second collimator 132 after the polarization direction is controlled to rotate. Each emitted light passes through the first mirror 112 from the A direction and the B direction, and propagates while being totally reflected by the side surface of the optical phase modulator 111. Then, each light reflected by the second mirror 113 returns to the first mirror 112 again, a part of the light is reflected, and a part of the light is emitted to the outside through the first mirror 112.

  That is, the light incident from the A direction propagates in the optical phase modulator 111 so as to draw a ring counterclockwise as shown in FIG. 3, and is emitted in the B direction through the first mirror 112. Then, the light emitted from the B direction propagates through the second optical path via the second collimator 132 and returns to the polarization beam splitting / combining coupler 141. Similarly, light incident from the B direction propagates in a clockwise direction so as to draw a ring, and is emitted through the first mirror 112 in the A direction. Then, the light emitted from the A direction propagates through the second optical path via the second collimator 132 and returns to the polarization beam splitting / combining coupler 141. Incidentally, the lights returned to the polarization splitting / combining coupler 141 are combined with each other and transmitted to the optical fiber communication network 440 again.

  In such a ring type electro-optic modulator 80, if the refractive index and the modulation efficiency of the material constituting the optical phase modulator 111 are strongly dependent on a specific polarization direction, the polarization is changed according to the polarization direction. The polarization direction of each light separated in the separation / combination coupler 141 can be controlled to the same direction. Thereby, even if the supplied light has any polarization component, highly efficient phase modulation can be realized without depending on this.

  Further, even if the optical fiber disposed in the optical fiber communication network 440 is not a polarization preserving fiber, highly efficient phase modulation can be performed while controlling the polarization direction. For this reason, by using this ring type electro-optic modulator 80 as the optical modulator 81 mounted on each base station 12, the general versatility of the entire system can be enhanced.

  Note that the material and structure of the temperature compensation housing 116 of the ring type electro-optic modulator 80 may compensate for changes in the thermal expansion coefficient and thermal refractive index of the crystal constituting the optical phase modulator 111. As a result, it is possible to realize high-precision modulation that eliminates the temperature environment in which the base station 12 is installed.

  The optical modulator 8 can also be applied to a ring-type electro-optic modulator 89 described below.

  As shown in FIG. 4, the ring type electro-optic modulator 89 includes an optical phase modulator 111, a first mirror 112 and a second mirror disposed so as to face each other via the optical phase modulator 111. An optical resonator 110 comprising a mirror 113, electrodes 115 formed on the top and bottom surfaces of the optical phase modulator 111 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction, and the above-described components And a temperature compensation housing 116 in which is incorporated. The ring-type electro-optic modulator 89 includes a first optical path 121 and a second optical path 122 made of optical fibers for propagating light incident on the first mirror 112, and a first optical path 121. Are connected to the first collimator 131 disposed at the end of the second optical path, the second collimator 132 disposed at the end of the second optical path 122, the first optical path 121, and the second optical path 122, respectively. And a polarization beam splitting / combining coupler 141. Further, the ring type electro-optic modulator 89 has a third optical path 123 and a fourth optical path 124 made of optical fibers for propagating light emitted from the second mirror 113, and at the end of the third optical path 123. The third collimator 133 provided, the fourth collimator 134 provided at the end of the fourth optical path 124, the third optical path 123, and the polarization connected to the fourth optical path 124, respectively. And a wave separation / synthesis coupler 142.

  In the ring type electro-optic modulator 89 shown in FIG. 4, the same components as those of the ring type electro-optic modulator 80 are denoted by the same reference numerals, and description thereof is omitted.

  The polarization beam splitting / combining coupler 142 combines light propagating through the third optical path 123 and the fourth optical path 124.

  The third optical path 123 rotates the optical fiber and rotates the polarization component of light from the horizontal direction to the vertical direction. Thereby, the polarization directions of the light propagating through the third optical path 123 and the light propagating through the fourth optical path 124 can be orthogonalized. Note that the fourth optical path 124 may have a function of rotating the polarization direction. Furthermore, an isolator may be inserted in the first optical path 121 and the second optical path 122 as necessary.

  The third collimator 133 and the fourth collimator 134 couple the light emitted from the second mirror 113 to the optical fibers constituting the third optical path 123 and the fourth optical path 124, respectively.

  FIG. 5 is a perspective view of the ring type electro-optic modulator 89. As shown in FIG. 5, the first mirror 112 and the second mirror 113 may not only be directly attached to the end face of the optical phase modulator 111 but also use external mirrors. Further, the crystal side surface constituting the optical phase modulator 111 is a total reflection surface, and modulates the light circulating around by the electrodes 115 provided above and below. Note that if one of the first collimator 131 and the third collimator 133 or the second collimator 132 and the fourth collimator 134 is removed, it operates as a normal optical frequency comb generator.

  Incidentally, in the ring type electro-optic modulator 89, when the side surfaces on which the electrodes 115 provided on the upper and lower sides of the optical phase modulator 111 are reflected as shown in FIG. 3 and FIG. 4 are drawn like a ring. However, the present invention is not limited thereto, and as shown in FIG. 5, light incident at different angles from the horizontal direction may be reflected so as to draw a ring on the lateral side surface. When the ring-type electro-optic modulator 89 is applied as an actual communication device, the configuration shown in FIG. 5 is more general and can be said to be superior in terms of practicality.

  In such a ring type electro-optic modulator 89, light propagating from the optical fiber communication network 440 is first incident on the polarization splitting / combining coupler 141. Even if the incident light includes various polarization components, the incident light is separated according to the direction of each linearly polarized light and propagates through the first optical path 121 and the second optical path 122. The separated light is emitted as parallel light through the first collimator 131 and the second collimator 132 after the polarization direction is controlled to rotate. Each emitted light passes through the first mirror 112 from the A direction and the B direction, and propagates while being totally reflected by the side surface of the optical phase modulator 111. Then, the light emitted from the second mirror 113 is emitted to the third optical path 123 and the fourth optical path 124 through the third collimator 133 and the fourth collimator 134, respectively. The light propagating through the third optical path 123 is adjusted for the polarization direction and then reaches the polarization splitting / combining coupler 142. Incidentally, the lights returned to the polarization splitting / combining coupler 141 are combined with each other and transmitted to the optical fiber communication network 440 again.

  Incidentally, the ring-type electro-optic modulator 80 to which the present invention is applied is not limited to the above-described embodiment. For example, the ring-type electro-optic modulator 80 shown in FIG. You may apply to the optical modulator 90. FIG. In the ring-type electro-optic modulator 90 shown in FIG. 6, the same components as those of the ring-type electro-optic modulator 80 are denoted by the same reference numerals and description thereof is omitted.

  The ring-type electro-optic modulator 90 includes an optical phase modulator 111 and an optical resonator including a first mirror 212 and a second mirror 213 that are disposed so as to face each other with the optical phase modulator 111 interposed therebetween. 210, and electrodes 115 formed on the top and bottom surfaces of the optical phase modulator 111 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction. The ring-type electro-optic modulator 90 includes a collimator lens 231 that converts the light from the optical fiber communication network 440 into parallel light, and a birefringence element 241 that separates the light emitted from the collimator lens 231 according to each polarization component. The half-wave plate 232 disposed on the optical path of light having one polarization component separated in the birefringent element 241 (hereinafter referred to as the first optical path 221), the first optical path 221 and And a plano-convex lens 233 provided on an optical path of light having another polarization component separated in the birefringent element 241 (hereinafter referred to as a second optical path 222).

  The first mirror 212 and the second mirror 213 are provided to resonate the light incident on the optical resonator 210, and resonate by reciprocally reflecting the light passing through the optical phase modulator 111.

  The first mirror 212 is disposed on the light incident side of the optical phase modulator 111, and light is incident from the plano-convex lens 233. The second mirror 213 is configured as a so-called concave mirror so that the light focused by the plano-convex lens 233 becomes parallel light again.

  The birefringent element 241 separates horizontal linearly polarized light and vertical linearly polarized light from the light that has been converted into parallel light by the collimator lens 231 by using birefringence whose refractive index changes depending on the polarization direction. The separated light including the linearly polarized component in the horizontal direction propagates through the first optical path 221, and the light including the linearly polarized component in the vertical direction propagates through the second optical path 222.

  The half-wave plate 232 is disposed on the first optical path 221 so that the high speed axis is inclined by 45 ° from the polarization direction of the light. Thereby, the polarization component of the light can be rotated from the horizontal direction to the vertical direction, and the polarization direction of the light propagating through the first optical path 221 and the light propagating through the second optical path 222 can be made the same. . Note that the second optical path 222 may also have a function of rotating the polarization direction.

  In such a ring type electro-optic modulator 90, light propagated from the optical fiber communication network 440 is incident on the birefringent element 241 through the collimator lens 231. The incident light is separated into the first optical path 221 and the second optical path 222 according to the direction of each linearly polarized light, even if it includes various polarization components. Among these, the light separated into the first optical path 221 passes through the half-wave plate 232, the rotation direction of the polarization is controlled, and the light propagating through the second optical path 222 is converged by the plano-convex lens 233. Thus, the light is incident on the optical phase modulator 111 from different directions (A direction and B direction).

  The light incident from the A direction propagates in the optical phase modulator 111 so as to draw a ring clockwise as shown in FIG. 6, is emitted in the B direction through the first mirror 212, and is a plano-convex lens 233. And then returns to the birefringent element 241. Similarly, the light incident from the B direction propagates in a counterclockwise manner so as to draw a ring, is emitted in the A direction via the first mirror 212, and is further birefringent element 241 via the half-wave plate 232. Return to.

  Even in such a ring-type electro-optic modulator 90, even if the light propagating through the optical fiber communication network 440 has any polarization component, high-efficiency phase modulation can be realized without depending on this. Can do.

  In the ring-type electro-optic modulator 90, each component may be incorporated in the temperature compensation casing 116 as described above.

  In the ring type electro-optic modulator 80 to which the present invention is applied, the optical phase modulator 111 may be further arranged in the configuration shown in FIG.

  The configuration shown in FIG. 7 is a so-called loop configuration in which the first optical path 121 is connected to the second optical path 122 through the optical resonator 110 and the optical phase modulator 111. As a result, the first mirror 112 is disposed near the terminal end of the first optical path 121, and the second mirror 113 is disposed near the terminal end of the second optical path 122.

  After the light including the linearly polarized component in the horizontal direction separated by the polarization beam splitter / combiner 141 propagates through the first optical path 121 and is incident on the optical phase modulator 111 via the first mirror 112 and modulated. Then, the light returns to the polarization preserving coupler 141 via the second optical path 122. The light including the linearly polarized light component in the vertical direction separated by the polarization beam splitting / combining coupler 141 propagates through the second optical path 122, enters the optical phase modulator 111 via the second mirror 113, and is modulated. Then, the light returns to the polarization preserving coupler 141 via the first optical path 121. By the way, the distance between the first path 121 and the second path 122 is the same in order to multiplex the lights returning from the first optical path 121 and the second optical path 122 to the polarization preserving coupler 141. To do.

  That is, in the configuration shown in FIG. 7, the input and output of light to the optical phase modulator 111 can be realized via different optical paths 121 and 122. Further, in this configuration, even if the input / output of light is switched between the optical paths 121 and 122, the same phase modulation can be performed.

  Further, the ring type electro-optic modulator 80 to which the present invention is applied is not limited to the above-described embodiment. For example, a ring type electro-optic modulation in which a Faraday rotator is provided inside the resonator as shown in FIG. The vessel 10 may be applied. In the ring electro-optic modulator 10, the same components and members as those of the ring electro-optic modulator 80 described above are denoted by the same reference numerals and description thereof is omitted.

  The ring-type electro-optic modulator 10 includes an optical phase modulator 111 and an optical resonator composed of a first mirror 112 and a second mirror 113 that are disposed so as to face each other via the optical phase modulator 111. 110 and electrodes (not shown) formed on the top and bottom surfaces of the optical phase modulator 111 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction. The ring-type electro-optic modulator 10 includes a first Faraday rotator 321 having a 45 ° polarization rotation disposed between the first mirror 112 and the optical phase modulator 111, a second mirror 113, and an optical phase modulation. A second Faraday rotator 322 is provided between the container 111 and the container 111.

  Arbitrary polarization components are incident on the first mirror 112. The first mirror 112 reflects a part of the incident light and transmits a part thereof.

  The first Faraday rotator 321 rotates the polarization direction of the light passing through the first mirror 112 by −45 ° and emits the light to the optical phase modulator 111. Further, the first Faraday rotator 321 further rotates the polarization direction of the light from the optical phase modulator 111 by −45 ° and emits the light to the first mirror 112.

  The second Faraday rotator 322 rotates the polarization direction of the light that has passed through the optical phase modulator 111 by −45 ° and emits the light to the second mirror 113. The second Faraday rotator 322 rotates the polarization direction of the light from the second mirror 113 by −45 ° and emits the light to the optical phase modulator 111.

  That is, in the ring type electro-optic modulator 10, when light having a polarization component (hereinafter referred to as 45 ° polarization) shifted by 45 ° from the vertical direction is supplied as shown in FIG. The first Faraday rotator 321 is rotated by −45 ° to make the polarization direction vertical. The light composed of the polarization component in the vertical direction passes through the optical phase modulator 111 as it is, and then is rotated by −45 ° with respect to the polarization direction by the second Faraday rotator 322 and is shifted by −45 ° from the vertical direction. Component (hereinafter referred to as -45 ° polarized light). The −45 ° polarized light is reflected by the second mirror 113 and then rotated again by −45 ° with respect to the polarization direction by the second Faraday rotator 322 to become a horizontal polarization component. The light composed of the polarization component in the horizontal direction passes through the optical phase modulator 111 as it is, and is rotated again by −45 ° with respect to the polarization direction by the first Faraday rotator 321 to become 45 ° polarization. It will be emitted through 112.

  Similarly, in the ring type electro-optic modulator 10, when -45 ° polarized light is supplied as shown in FIG. 8B, it is rotated by -45 ° by the first Faraday rotator 321. The polarization direction is set to the horizontal direction. The light composed of the polarization component in the horizontal direction passes through the optical phase modulator 111 as it is, and then is rotated by −45 ° in the polarization direction by the second Faraday rotator 322 to become 45 ° polarization. This 45 ° -polarized light is reflected from the second mirror 113 and then rotated again by −45 ° with respect to the polarization direction by the second Faraday rotator 322 to become a vertically polarized component. The light composed of the polarization component in the vertical direction passes through the optical phase modulator 111 as it is, and is rotated again by −45 ° with respect to the polarization direction by the first Faraday rotator 321 to become −45 ° polarization. The light is emitted through the mirror 112.

  As described above, in the ring type electro-optic modulator 10, by disposing the Faraday rotators 321 and 322 before and after the optical phase modulator 111, only two orthogonal polarization components are propagated in the optical phase modulator 111. Can be made. The polarization directions of the two orthogonal polarization components differ from each other depending on the propagation direction in the optical phase modulator 111, but have the same propagation path and optical distance, respectively. For this reason, the optical resonator 110 itself is degenerated for each light having two orthogonal polarization components. In such a case, high-efficiency phase modulation is possible without being governed by the polarization direction of incident light.

  Further, the present invention is not limited to the ring type electro-optic modulator 10 described above, but a ring type electro-optic modulator 300 in which a quarter wavelength plate is installed inside the optical resonator 110 as shown in FIG. You may make it apply to.

  This ring type electro-optic modulator 300 includes an optical phase modulator 111 and an optical resonator 110, and a quarter-wave plate 331 disposed between the first mirror 312 and the optical phase modulator 111. And a quarter-wave plate 332 disposed between the second mirror 113 and the optical phase modulator 111.

  The quarter-wave plates 331 and 332 give a phase difference of π / 2 to the light passing therethrough when they have vertical and horizontal polarization components.

  In other words, in the ring type electro-optic modulator 300, when clockwise circularly polarized light (hereinafter referred to as clockwise circularly polarized light) is supplied, it is converted by the quarter wavelength plate 331 into vertical linearly polarized light. And The light composed of the polarization component in the vertical direction passes through the optical phase modulator 111 as it is, and is converted into counterclockwise circularly polarized light (hereinafter referred to as counterclockwise circularly polarized light) by the quarter wavelength plate 332. The counterclockwise circularly polarized light is reflected from the second mirror 113 and then converted into linearly polarized light in the horizontal direction from the quarter-wave plate 322 and passes through the optical phase modulator 111 as it is. The light is converted to right-handed circularly polarized light again by the quarter-wave plate and is emitted through the first mirror 112.

  Further, in the ring type electro-optic modulator 300, when a counterclockwise circularly polarized light is supplied, it is converted into a linearly polarized light in the horizontal direction by the quarter wavelength plate 331. The light composed of the polarization component in the horizontal direction passes through the optical phase modulator 111 as it is, and is converted into clockwise circular polarization by the quarter wavelength plate 332. Then, the clockwise circularly polarized light is reflected from the second mirror 113 and then converted into linearly polarized light in the vertical direction from the quarter-wave plate 322 and passes through the optical phase modulator 111 as it is. The light is converted to counterclockwise circularly polarized light again by the ¼ wavelength plate and is emitted through the first mirror 112.

  As described above, in the ring-type electro-optic modulator 300, by arranging the quarter wavelength plates 331 and 332 before and after the optical phase modulator 111, two polarization components orthogonal to each other in the optical phase modulator 111. Can only propagate. The polarization directions of the two orthogonal polarization components differ from each other depending on the propagation direction in the optical phase modulator 111, but have the same propagation path and optical distance, respectively. For this reason, the optical resonator 110 itself is degenerated for each light having two orthogonal polarization components. In such a case, high-efficiency phase modulation is possible without being governed by the polarization direction of incident light.

  In particular, the ring-type electro-optic modulators 10 and 300 are selectively used according to the polarization component of the supplied light, so that the inside of the optical phase modulator 111 is independent of the polarization direction of linearly polarized light and the direction of circularly polarized light. Only two polarization components orthogonal to can be propagated, and high efficiency of modulation can be promoted.

It is a figure for demonstrating about a polarization-independent modulation system. It is a figure for demonstrating the detail of an optical modulator. It is a figure shown about the structure of a ring type | mold electro-optic modulator. It is a figure shown about the other structure of a ring type | mold electro-optic modulator. It is a perspective view of a ring type electro-optic modulator. It is a figure shown about the other structure of a ring type | mold electro-optic modulator. It is a figure which shows about the structure of a ring type | mold electro-optic modulator which implement | achieves the input and output of a laser beam via a different optical path. It is a figure shown about the ring type | mold electro-optic modulator which provided the birefringent element inside the resonator. It is a figure shown about the structure of the ring-type electro-optic modulator which installed the quarter wavelength plate inside the optical resonator. 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

  8 optical modulator, 91 first polarization splitting / combining coupler, 92 second polarization splitting / combining coupler, 93 first polarization controller, 94 second polarization controller, 11 substrate, 12 waveguide, 83 electrodes 84 First end face 85 Second end face 16 Oscillator 18 Phase shifter 19 Reflector

Claims (4)

Separating means for separating the light to be incident according to the polarization direction;
Polarization control means for controlling the polarization direction of each of the separated lights in the same direction;
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.   2. The optical modulator according to claim 1, wherein the light propagation means propagates the incident light while totally reflecting the inside of the crystal. Separating means for separating the light to be incident according to the polarization direction;
Polarization control means for controlling the polarization direction of each of the separated lights in the same direction;
Oscillating means for oscillating a modulation signal of a predetermined frequency;
Resonating means configured by reflecting mirrors parallel to each other, and resonating by propagating light incident through any one of the reflecting mirrors at different angles from the polarization control means in the forward direction or the backward direction;
Optical modulation means for modulating the phase of the light resonated in the resonance means in accordance with the modulation signal supplied from the oscillation means,
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.   4. The optical modulator according to claim 3, wherein the separating means in the optical modulator is made of a birefringent material.

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