JP2007335952A - Optical microwave mixer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical microwave mixer capable of attaining a higher S/N ratio than that of prior arts. <P>SOLUTION: The optical microwave mixer includes: a PBS 2 for splitting input light into two polarization components orthogonal to each other; a PBS 11 for again splitting the two input lights split by the PBS 2 and received respectively via first and second optical paths into two orthogonal polarization components; an electrooptic modulation element 3 for applying intensity modulation in response to RF modulation to the transmitted light through the application of an applied voltage with the radio frequency; an electrooptic modulation element 10 for applying intensity modulation in response to RF modulation to the transmitted light through the application of an applied voltage with the radio frequency; first bias adjustment means 5, 6 for performing bias adjustment to maximize the modulation factor by the electrooptic modulation element 3; second bias adjustment means 7, 8 for executing bias adjustment to maximize the modulation factor by the electrooptic modulation element 10; and optical receiving means 12 to 15 for applying photoelectric conversion to the two orthogonal polarization components split by the PBS 11 and received to provide an output. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical microwave mixer that applies microwave modulation to light.

  Although this is not an optical microwave mixer, there is a polarization control circuit shown in FIG. 9 as a conventional technology of this type (see, for example, Patent Document 1). This polarization control circuit will be described with reference to FIG. FIG. 9 is a diagram showing a configuration of a conventional polarization control circuit.

  In FIG. 9, the conventional polarization control circuit includes a polarization splitting beam splitter (PBS) 101, a mirror 102, a light intensity detection means 103, a processing circuit 108, a mirror 109, and a polarization plane rotation element 110. A Mach-Zehnder optical interferometer 120 is provided.

  The light intensity detection means 103 is provided with light splitters 104 and 105 and light intensity detectors 106 and 107.

  The Mach-Zehnder optical interferometer 120 includes a first input port 121, a second input port 122, a beam splitter (BS) 123, a phase shifter 124, a mirror 125, a mirror 126, and a beam splitter (BS) 127. In addition, a first output port 128 and a second output port 129 are provided.

  Further, in FIG. 9, PBS 101 and optical splitter 104, PBS 101 and optical splitter 105, optical splitter 104 and optical intensity detector 106, optical splitter 105 and optical intensity detector 107, optical splitter 104 and BS 123, optical The branching device 105 and BS123, the BS123 and the phase shifter 124, the phase shifter 124 and BS127, and the BS123 and BS127 are optical paths in space, respectively. Further, the light intensity detector 106 and the processing circuit 108, the light intensity detector 107 and the processing circuit 108, and the processing circuit 108 and the phase shifter 124 are connected by an electric circuit. Connected by.

  Next, the operation of the conventional polarization control circuit will be described. First, a light wave in an arbitrary polarization state input from the outside is separated into A component and B component which are linearly polarized components orthogonal to each other by the PBS 101.

  Thereafter, each light is detected by light intensity detecting means 103 including optical branching units 104 and 105 for extracting a part of the A component and B component, and light intensity detectors 106 and 107 for monitoring the respective light intensities. The intensity is detected, and the processing circuit 108 compares the light intensities of the A component and the B component to determine the strong component, and outputs the result. In addition, when the intensity | strength of both components is equal, it determines beforehand so that either component may be selected.

  Thereafter, the polarization direction of the B component is rotated by 90 degrees by the polarization plane rotation element 110 so as to be the same as the polarization direction of the A component.

  The Mach-Zehnder optical interferometer 120 has first and second input ports 121 and 122, and first and second output ports 128 and 129, which are input from the first input port 121 and transmitted through the BS 123. A phase shifter 124 is provided for providing a phase shift to the component.

  Here, when the phase shift of the phase shifter 124 is 0, the input from the first input port 121 is output to the second output port 129, and the input from the second input port 122 is output to the first output port 128. Make adjustments in advance. When the phase shift of the phase shifter 124 is π, the input from the first input port 121 is output to the first output port 128, and the input from the second input port 122 is output to the second output port 129. Make adjustments in advance.

  Further, the A component and the B component are input to the first input port 121 and the second input port 122, respectively, and the following control is performed based on the output result from the processing circuit 108. That is, if the intensity of the A component is stronger than that of the B component, the phase shift is set to 0 and the A component is output from the second output port 129. If the intensity of the B component is stronger than that of the A component, the phase shift is set to π. Thus, control is performed so that the B component is output from the second output port 129. Then, the output light from the second output port 129 is used as external output light.

Japanese Patent Laid-Open No. 9-281537

  However, in the conventional polarization control circuit, only the light of the second output port 129 after being separated by the BS 127 is used as external output light, and the light of the first output port 128 is not used as external output light. Therefore, if an optical microwave mixer is configured by adding an electro-optic modulation element or the like to the above configuration, when the modulation signal is a sine wave, only one of the two output lights is used, so that the light intensity utilization efficiency is 3 dB. There was a problem of causing the above loss.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain an optical microwave mixer capable of obtaining a higher S / N ratio than the conventional one.

  An optical microwave mixer according to the present invention includes a first polarization separation beam splitter that separates input light into two mutually orthogonal polarization components, and a first polarization separation beam splitter that is separated by the first polarization separation beam splitter. A second polarization splitting beam splitter that splits two input lights input through the second optical path into two mutually orthogonal polarization components, and an application with an RF frequency disposed in the first optical path A first electro-optic modulation element that applies a voltage to modulate the intensity of the transmitted light according to the frequency modulation of the RF, and is disposed in the second optical path, and applies an applied voltage with an RF frequency to A second electro-optic modulation element that performs intensity modulation in accordance with frequency modulation; a first bias adjustment means that performs bias adjustment so that the degree of modulation by the first electro-optic modulation element is maximized; The second bias adjusting means for adjusting the bias so that the degree of modulation by the electro-optic modulation element is maximized and the second polarization separation type beam splitter, and are inputted through the third and fourth optical paths. And two optical receiving means for photoelectrically converting and outputting two orthogonal polarization components.

  The optical microwave mixer according to the present invention has an effect that an S / N ratio higher than the conventional one can be obtained.

Embodiment 1 FIG.
An optical microwave mixer according to Embodiment 1 of the present invention will be described with reference to FIGS. 1 is a diagram showing a configuration of an optical microwave mixer according to Embodiment 1 of the present invention. FIG. 3 is a diagram showing a configuration for eliminating a phase difference between two signal lights by the optical microwave mixer according to Embodiment 1 of the present invention. FIG. 4 is a diagram showing another configuration of the optical microwave mixer according to Embodiment 1 of the present invention. FIG. 5 is a diagram showing a configuration when the input light is linearly polarized light and a polarization maintaining optical fiber is used in the optical microwave mixer according to Embodiment 1 of the present invention. In the following, in each figure, the same reference numerals indicate the same or corresponding parts.

  1, the optical microwave mixer according to the first embodiment includes a fiber collimator 1, a polarization splitting beam splitter (PBS) 2, an electro-optic modulation element 3, a mirror 4, and a λ / 2 wavelength plate. 5, a λ / 4 wavelength plate 6, a λ / 2 wavelength plate 7, a λ / 4 wavelength plate 8, a mirror 9, an electro-optic modulation element 10, a polarization splitting beam splitter (PBS) 11, Fiber collimators 12 and 13, an extra optical fiber 14, and an optical receiver 15 are provided.

  The term fiber collimator is composed of an optical fiber and a collimating lens system, and has a function of outputting light input from the optical fiber to space as collimated light, or coupling collimated input light to the optical fiber. Refers to parts.

  In FIG. 1, the fiber collimator 12 and the optical receiver 15, the fiber collimator 13 and the surplus optical fiber 14, and the surplus optical fiber 14 and the optical receiver 15 are connected by an optical circuit, for example, connected by an optical fiber. Has been.

  The fiber collimator 1 has a function of outputting input light to space as collimated light. The PBS 2 has a function of separating input light into two mutually orthogonal polarization components. The electro-optic modulation elements 3 and 10 have a function of applying intensity modulation according to RF frequency modulation to transmitted light that has passed through the element by applying an applied voltage with an RF frequency. The mirrors 4 and 9 have a function of bending the input light by 90 degrees. The λ / 2 wavelength plates 5 and 7 and the λ / 4 wavelength plates 6 and 8 have a function of performing bias adjustment so that the degree of modulation is maximized. The PBS 11 has a function of separating two input lights into polarized components orthogonal to each other again. The fiber collimators 12 and 13 have a function of coupling collimated input light to an optical fiber. The surplus optical fiber 14 has a function of causing a delay in the time until the passing light reaches the optical receiver 15. The optical receiver 15 has a function of photoelectrically converting two input lights and outputting a sum signal thereof.

  Next, the operation of the optical microwave mixer according to the first embodiment will be described with reference to the drawings. FIG. 2 is a diagram showing the operation of the optical microwave mixer according to Embodiment 1 of the present invention.

  First, laser light is input from the optical fiber end of the fiber collimator 1.

Next, after the laser beam emitted from the fiber collimator 1 is separated into two mutually orthogonal polarization components by the PBS 2, an electro-optic modulation element such as an LN (LiNbO ₃ ) crystal is used for each laser beam. 3 and 10 are inserted and passed. At that time, by applying an applied voltage with a sinusoidal RF frequency to the electro-optic modulation elements 3 and 10, intensity modulation corresponding to the frequency modulation of the RF is applied to the two transmitted lights that have passed through the element.

  Further, as shown in FIG. 1, the λ / 2 wave plates 5 and 7 and the λ / 4 wave plates 6 and 8 are inserted, the wave plates are rotated, and the bias adjustment is performed so that the modulation degree is maximized. Here, the λ / 2 wave plates 5 and 7 and the λ / 4 wave plates 6 and 8 may be arranged anywhere between the two optical paths between the PBS 2 and the PBS 11 and may be placed in one optical path. It is only necessary to include one / 2 wavelength plate and one λ / 4 wavelength plate.

  Further, as shown in FIG. 1, mirrors 4 and 9 are arranged between PBS2 and PBS11, and after bending each laser beam by 90 degrees, the two laser beams are again separated into mutually orthogonal polarized components by PBS11. .

  Here, the two laser beams separated by the PBS 11 are both coupled to the optical fiber via the fiber collimators 12 and 13.

  Further, by propagating to one optical path after being coupled by the two fiber collimators 12 and 13, a delay time that is an odd multiple of half the reciprocal of the modulation frequency of the voltage applied to the electro-optic modulation elements 3 and 10 is generated. An extra optical fiber 14 is added, and after photoelectric conversion, it is received by an optical receiver 15 that can obtain two sum signals.

  Next, the effect of the optical microwave mixer according to the first embodiment will be described. In FIG. 1, the modulated signal light, which is two modulated laser lights obtained after being combined by the fiber collimators 12 and 13, is, for example, as shown in FIG. Then, the other component is a component obtained by subtracting the sine wave component from the DC component that is the input optical power in terms of energy, so that the sine wave is shifted in phase by half a wavelength from the signal light as shown in FIG. It becomes signal light.

  Therefore, as described in the above operation, by propagating to one optical path after being coupled by the two fiber collimators 12 and 13, half of the reciprocal of the modulation frequency of the voltage applied to the electro-optic modulation elements 3 and 10 is obtained. By adding an extra optical fiber 14 that causes an odd multiple of the delay time and receiving it by an optical receiver 15 that can obtain two sum signals after photoelectric conversion, the signal light power can be increased by a maximum of 3 dB. As a result, the power, that is, the S / N ratio can be increased by a maximum of 6 dB.

  Note that the signal light power is increased by 3 dB and the power, that is, the S / N ratio, is 6 dB only when the propagation delay time due to the surplus optical fiber 14 completely coincides with an odd multiple of half of the reciprocal of the modulation frequency. It is.

  Next, the necessity of the λ / 2 wave plates 5 and 7 will be described below. The λ / 2 wave plates 5 and 7 have a phase difference of π, and generally rotate polarized light by 2φ when arranged at a declination angle φ, so that linearly polarized light has a plane of polarization while maintaining linearly polarized light. Rotate 2φ. On the other hand, the λ / 4 wavelength plates 6 and 8 have a phase difference of π / 2, and convert linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light. Here, when the transmittance after transmission through the electro-optic modulation elements 3 and 10 is T, and the phase difference in the entire optical path including the elements is Γ, the relationship between T and Γ is expressed by the following equation (1). The

Here, the voltage when the phase difference π is generated by applying an applied voltage to the electro-optic modulation elements 3 and 10 including the λ / 2 wavelength plates 5 and 7 and the λ / 4 wavelength plates 6 and 8 in the optical path is expressed as V _π. Then, Γ when the modulation voltage V _m sinω _m t is applied is expressed by the following equation (2).

Substituting equation (2) into equation (1), the following equation (3) is obtained as V _m << V _π .

  Next, Γ when only the λ / 4 wave plates 6 and 8 are included in the optical path is expressed by the following equation (4).

When Expression (4) is substituted into Expression (1), the following Expression (5) is obtained as V _m << V _π .

  Comparing equation (3) and equation (5), the difference due to the presence / absence of the λ / 2 wave plates 5 and 7 is only that the sign of transmittance modulation is reversed. However, in actuality, there is a possibility that the direction of polarized light may be slightly changed by transmitting through the electro-optic modulation elements 3 and 10, so in FIG. 1, the λ / 2 wavelength plates 5 and 7 are added to freely change the direction of polarized light. I was able to adjust to.

In the above description, the λ / 2 wavelength plates 5 and 7 and the λ / 4 wavelength plates 6 and 8 are used as bias adjusting means. However, in order to satisfy the formula (3) or the formula (5), the transmittance is 1 For example, a means of applying an offset voltage of V _π / 2 to the electro-optic modulation elements 3 and 10 may be used as long as it is biased to be / 2.

Further, after being coupled by the fiber collimators 12 and 13 and propagating to one optical path, a surplus that causes a delay time that is an odd multiple of half the reciprocal of the modulation frequency of the voltage applied to the electro-optic modulation elements 3 and 10. Although the optical fiber 14 is added as described above, even if there is a difference between the actually added optical fiber length and the calculated optical fiber length, the delay time caused by the difference can be sufficiently ignored with respect to the reciprocal of the modulation frequency. It doesn't matter as long as it is. For example, if the RF modulation frequency is 200 MHz, the surplus optical fiber length L ₁ that causes a half-wavelength delay of the modulation frequency is assumed that the refractive index of the optical fiber is 1.5 and the speed of light is 3 × 10 ⁸ m / s. Is calculated by the following equation (6).

  Therefore, for example, if the optical fiber length actually added is 0.505 m, the difference from the calculated optical fiber length is 0.5 cm, and the delay time caused by this difference is sufficiently negligible with respect to the reciprocal of the modulation frequency. Adjustment of the fiber length may be unnecessary.

  Further, in FIG. 1, after being separated by the PBS 2, each polarization component is modulated and combined by the PBS 11 again to obtain the signal light. Therefore, if there is a phase difference between the two signal lights, The degree of modulation will deteriorate.

Therefore, as a solution to the above problem, for example, as shown in FIG. 3, the light passing through the electro-optic modulation element 3 in the optical path from PBS2 to PBS11 is passed through the electro-optic modulation element 10 in the optical path A, The optical path B is the optical path B, the optical path A is the distance L _A , the optical path B is the distance L _B , the distance from the RF signal output unit 51 through which the RF signal propagates to the electro-optic modulation element 3 is L _A ′, When the distance from the output unit 51 to the electro-optic modulation element 10 is L _B ′, L _A ′ and L _B ′ may be adjusted so that the following expression (7) is satisfied.

  In addition, the input light in the first embodiment is an unmodulated laser beam and is intended to be subjected to RF modulation. However, as another method of use, for example, a laser beam including RF modulation in advance is down-converted. It is also possible to change the modulation frequency applied to the laser light to a desired frequency, such as converting to a baseband frequency.

In FIG. 1, one mirror 4 and one mirror 9 are installed in each optical path. For example, as shown in FIG. 4, two mirrors are installed in one optical path, and only one of the optical paths is bent 180 degrees. It doesn't matter. However, as described above, if there is a phase difference between the two signal lights separated by the PBS 2, the degree of modulation after multiplexing will deteriorate. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  The type of optical fiber used in the first embodiment may be any of a single mode optical fiber, a multimode optical fiber, and a polarization maintaining optical fiber.

  However, when the input light is linearly polarized light and a polarization-maintaining optical fiber is used, the above-described effects can be sufficiently obtained with one electro-optic modulation element even if the configuration is not as shown in FIG. For example, as shown in FIG. 5, the configuration may be such that the PBS 2, the mirrors 4, 9, the λ / 2 wavelength plate 7, the λ / 4 wavelength plate 8, and the electro-optic modulation element 10 are removed.

  Further, the fiber collimators 1, 12, and 13 are not a fiber collimator, but are lens optical systems (fiber coupling means) that output optical fibers and collimated light to space or couple collimated light to optical fibers. It does not matter.

  In FIG. 1, the input / output light is propagated through the optical fiber using the fiber collimators 1, 12, and 13. However, this is changed to spatial propagation, and the surplus optical fiber 14 is replaced with an extra spatial optical path (extra optical path). For example, a spatially coupled optical receiver 15 may be used, and one of the output lights may be coupled to the optical receiver 15 by making two output lights parallel by using a mirror or the like.

  However, with respect to the above spatial propagation, propagation by an optical fiber enables miniaturization, high reliability, easy handling, and high placement freedom.

Embodiment 2. FIG.
An optical microwave mixer according to Embodiment 2 of the present invention will be described with reference to FIG. FIG. 6 is a diagram showing a configuration of an optical microwave mixer according to Embodiment 2 of the present invention.

  In FIG. 6, the same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 6, a balanced receiver 16 is provided instead of the optical receiver 15.

  In FIG. 6, the fiber collimators 12 and 13 and the balanced receiver 16 are connected by an optical circuit, for example, by an optical fiber.

  The balanced receiver 16 has a function of photoelectrically converting two input lights and outputting a difference signal thereof.

  Next, differences in the operation of the optical microwave mixer (FIG. 6) according to the second embodiment from the optical microwave mixer (FIG. 1) of the first embodiment will be described. Until the signal light is multiplexed again by the PBS 11, it is the same as in the first embodiment. Thereafter, the signal light coupled to the optical fiber by the fiber collimators 12 and 13 is subjected to photoelectric conversion of the two input lights, and Reception is performed using a balanced receiver 16 that can obtain the difference signal.

  Further, differences from FIG. 1 due to the effect of the optical microwave mixer of FIG. 6 will be described. As described above, the two signal lights obtained after being combined by the fiber collimator 12 and the fiber collimator 13 become sine wave signal lights whose signs are reversed, so that the signal is the same as in the optical microwave mixer of the first embodiment. The optical power can be increased up to 3 dB, and the power, that is, the S / N ratio can be increased up to 6 dB.

In FIG. 1, the surplus optical fiber 14 generates a delay that is an odd multiple of half of the reciprocal of the modulation frequency of the voltage applied to the electro-optic modulation elements 3 and 10, but in FIG. 6, a balanced receiver 16 is used. Thus, since the difference signal can be obtained, the extra optical fiber 14 is further unnecessary as compared with FIG. For example, when the modulation frequency is a low frequency such as 100 kHz, the surplus optical fiber length L ₂ that causes a half-wavelength delay of the modulation frequency has an optical fiber refractive index of 1.5 and a light speed of 3 ×. If it is 10 ⁸ m / s, it is calculated as the following formula (8).

  Therefore, when the modulation frequency is low, it can be said that the use of the method of the first embodiment requires a huge optical fiber length, and the method of the second embodiment is particularly effective.

  The signal light power is increased by 3 dB and the power, that is, the S / N ratio is increased by 6 dB because the length of the two optical fibers from the fiber collimators 12 and 13 to the balanced receiver 16 is completely The delay time due to the difference between the lengths of the two optical fibers when they are equal or until they reach the balanced receiver 16 from the fiber collimators 12 and 13 is the reciprocal of the modulation frequency of the voltage applied to the electro-optic modulation elements 3 and 10. Only when it is exactly equal to an odd multiple of half.

  Similarly to the first embodiment, the difference depending on the presence / absence of the λ / 2 wavelength plates 5 and 7 is that the positive / negative of the modulation of the transmittance is reversed, but actually the electro-optic modulation elements 3 and 10 are transmitted. As a result, the direction of polarized light may change slightly. In FIG. 6, λ / 2 wavelength plates 5 and 7 are added so that the direction of polarized light can be freely adjusted.

In the above description, the λ / 2 wavelength plates 5 and 7 and the λ / 4 wavelength plates 6 and 8 are used as bias adjusting means. However, in order to satisfy the formula (3) or the formula (5), the transmittance is 1 For example, a means of applying an offset voltage of V _π / 2 to the electro-optic modulation elements 3 and 10 may be used as long as it is biased to be / 2.

  In addition, as described above, the lengths of the two optical fibers until they reach the balanced receiver 16 after being combined by the fiber collimators 12 and 13 are the same as described above. As discussed in the equation (1), the delay time caused by the difference is not necessarily equal as long as it is sufficiently negligible with respect to the reciprocal of the modulation frequency.

Further, as described above, in FIG. 6, if there is a phase difference between the two signal lights separated by PBS 2, the degree of modulation after multiplexing will deteriorate. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  In addition, the input light in the second embodiment is an unmodulated laser beam and is intended to be subjected to RF modulation. However, as another method of use, for example, a laser beam including RF modulation in advance is down-converted. It is also possible to change the modulation frequency applied to the laser light to a desired frequency, such as converting to a baseband frequency.

In FIG. 6, one mirror 4 and 9 is installed in each optical path. However, as described above, two mirrors are installed in one optical path and only one optical path is bent 180 degrees. I do not care. However, as described above, if there is a phase difference between the two signal lights separated by the PBS 2, the degree of modulation after multiplexing will deteriorate. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  The type of optical fiber used in the second embodiment may be any of a single mode optical fiber, a multimode optical fiber, and a polarization maintaining optical fiber.

  However, when the input light is linearly polarized light and a polarization-maintaining optical fiber is used, as described above, even if the configuration is not as shown in FIG. Since it can be obtained, the configuration may be such that the PBS 2, the mirrors 4, 9, the λ / 2 wavelength plate 7, the λ / 4 wavelength plate 8, and the electro-optic modulation element 10 are removed.

  The fiber collimators 1, 12, and 13 may not be separately fiber collimators, but may be lens optical systems that output optical fibers and collimated light to space or couple collimated light to optical fibers. .

  Moreover, in FIG. 6, although the input / output light is propagated by the optical fiber using the fiber collimators 1, 12, and 13, this is changed to the spatial propagation, and for example, a balanced receiver 16 is used for the balanced receiver 16, One of the output lights may be coupled to the balanced receiver 16 by making the two output lights parallel by using a mirror or the like.

  However, with respect to the above spatial propagation, propagation by an optical fiber enables miniaturization, high reliability, easy handling, and high placement freedom.

Embodiment 3 FIG.
An optical microwave mixer according to Embodiment 3 of the present invention will be described with reference to FIG. FIG. 7 is a diagram showing a configuration of an optical microwave mixer according to Embodiment 3 of the present invention.

  In FIG. 7, the same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 7, photodiodes (PD) 17 and 18, A / D converters 19 and 20, an inverter 21, and an adder 22 are provided instead of the optical receiver 15.

  PD17 and PD18 are connected to fiber collimator 12 and fiber collimator 13, respectively, A / D converter 19 and A / D converter 20 are connected to PD17 and PD18, respectively, and inverter 21 is an A / D converter. The adder 22 is connected to the A / D converter 19 and the inverter 21.

  The fiber collimators 12 and 13 and the PDs 17 and 18 are connected by an optical circuit, for example, by an optical fiber. Further, between the PDs 17 and 18 and the A / D converters 19 and 20, between the A / D converter 20 and the inverter 21, and between the A / D converter 19, the inverter 21 and the adder 22. Are connected by an electric circuit, for example, by a cable such as a coaxial cable.

  The PDs 17 and 18 have a function of performing photoelectric conversion. The A / D converters 19 and 20 have a function of converting an analog signal into a digital signal. The inverter 21 has a function of inverting the sign of the signal and outputting it. The adder 22 has a function of adding and outputting two input signals.

  Next, differences in the operation of the optical microwave mixer (FIG. 7) according to the third embodiment from the optical microwave mixer (FIG. 1) of the first embodiment will be described. As described above, the two signal lights obtained after being combined by the fiber collimator 12 and the fiber collimator 13 become sine wave signal lights whose signs are reversed. Therefore, as shown in FIG. 7, after photoelectric conversion by the PDs 17 and 18, A / D conversion is performed by the A / D converters 19 and 20, and one signal is inverted by the inverter 21, and then the two signals are converted. Are added by the adder 22.

  Further, differences from FIG. 1 due to the effect of the optical microwave mixer of FIG. 7 will be described. In the optical microwave mixer of FIG. 7, as in FIG. 1, the signal light power can be increased up to 3 dB, and the power, that is, the S / N ratio can be increased up to 6 dB. However, as described above, the configuration of FIG. When the frequency becomes low, the delay time becomes very long, so a huge amount of excess optical fiber 14 is required. On the other hand, in the configuration of FIG. 7, adjustment is performed only by signal processing after conversion into an electric signal, and an effect that the excess optical fiber 14 is unnecessary is generated.

  As in the first embodiment, the difference depending on the presence / absence of the λ / 2 wave plates 5 and 7 is that the positive / negative of the modulation of the transmissivity is reversed, but actually the electro-optic modulation elements 3 and 10 are transmitted. As a result, the direction of polarization may change slightly. Therefore, in FIG. 7, λ / 2 wavelength plates 5 and 7 are added so that the direction of polarization can be freely adjusted.

In the above description, the λ / 2 wavelength plates 5 and 7 and the λ / 4 wavelength plates 6 and 8 are used as bias adjusting means. However, in order to satisfy the formula (3) or the formula (5), the transmittance is 1 For example, a means of applying an offset voltage of V _π / 2 to the electro-optic modulation elements 3 and 10 may be used as long as it is biased to be / 2.

  Similarly to the second embodiment, also in this third embodiment, the lengths of the two optical fibers after reaching the PDs 17 and 18 after being coupled by the fiber collimators 12 and 13 should be made equal. Although it is preferable, even if there is a difference in optical fiber length, the delay time caused by the difference may be of a level that can be sufficiently ignored with respect to the reciprocal of the modulation frequency.

Further, as described above, in FIG. 7, if there is a phase difference between the two signal lights separated by PBS 2, the modulation degree after multiplexing deteriorates. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  In addition, the input light in the third embodiment is an unmodulated laser beam and is intended to be subjected to RF modulation. However, as another method of use, for example, a laser beam including RF modulation in advance is down-converted. It is also possible to change the modulation frequency applied to the laser light to a desired frequency, such as converting to a baseband frequency.

In FIG. 7, one mirror 4 and one mirror 9 are installed in each optical path. However, as described above, two mirrors are installed in one optical path, and only one of the optical paths is bent 180 degrees. I do not care. However, as described above, if there is a phase difference between the two signal lights separated by the PBS 2, the degree of modulation after multiplexing will deteriorate. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  In addition, the type of optical fiber used in the third embodiment may be any of a single mode optical fiber, a multimode optical fiber, and a polarization maintaining optical fiber.

  However, when the input light is linearly polarized light and a polarization-maintaining optical fiber is used, as described above, even if the configuration is not as shown in FIG. Since it can be obtained, the configuration may be such that the PBS 2, the mirrors 4, 9, the λ / 2 wavelength plate 7, the λ / 4 wavelength plate 8, and the electro-optic modulation element 10 are removed.

  Similarly to the first embodiment, the fiber collimators 1, 12, and 13 are not separately provided with a fiber collimator, but output to the space as an optical fiber and collimated light, or couple the collimated light to the optical fiber. It may be a lens optical system.

  In FIG. 7, the input / output light is propagated by the optical fiber using the fiber collimators 1, 12, and 13, but this is changed to the spatial propagation, and for example, the PDs 17 and 18 are spatially coupled. It may be combined.

  However, with respect to the above spatial propagation, propagation by an optical fiber enables miniaturization, high reliability, easy handling, and high placement freedom.

Embodiment 4 FIG.
An optical microwave mixer according to Embodiment 4 of the present invention will be described with reference to FIG. FIG. 8 is a diagram showing a configuration of an optical microwave mixer according to Embodiment 4 of the present invention.

  In FIG. 8, the same parts as those in FIG. 7 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 8, a phase shifter 23 and a multiplexer 24 are provided instead of the A / D converters 19 and 20, the inverter 21, and the adder 22.

  The phase shifter 23 is connected to the PD 18, and the multiplexer 24 is connected to the PD 17 and the phase shifter 23.

  Further, the PD 18 and the phase shifter 23 and the PD 17 and the phase shifter 23 and the multiplexer 24 are connected by an electric circuit, for example, by an electric cable such as a coaxial cable.

  The phase shifter 23 has a function of freely adjusting the phase of the analog signal. The multiplexer 24 has a function of combining and outputting analog signals.

  Next, differences in the operation of the optical microwave mixer (FIG. 8) according to the fourth embodiment from the optical microwave mixer (FIG. 7) of the third embodiment will be described. As described above, the two signal lights obtained after being combined by the fiber collimator 12 and the fiber collimator 13 become sine wave signal lights whose signs are reversed. Therefore, as shown in FIG. 8, after photoelectric conversion by the PDs 17 and 18, one signal is phase-shifted by a half wavelength using the phase shifter 23, and then the two signals are multiplexed by the multiplexer 24.

  The effect of the optical microwave mixer of FIG. 8 will be described. In FIG. 7, the adjustment for shifting the phase by a half wavelength is performed with a digital signal, whereas in FIG. 8, the same adjustment is performed with an analog signal, and FIG. 8 has the same effect as FIG. Then, since the phase shifter 23 is used for adjusting the phase, the phase can be adjusted freely, and the adjustment of the optical fiber length is completely unnecessary as compared with the first to third embodiments. have.

  As in the first embodiment, the difference depending on the presence / absence of the λ / 2 wave plates 5 and 7 is that the positive / negative of the modulation of the transmissivity is reversed, but actually the electro-optic modulation elements 3 and 10 are transmitted. As a result, the direction of polarized light may change slightly. In FIG. 8, λ / 2 wavelength plates 5 and 7 are added so that the direction of polarized light can be freely adjusted.

In the above description, the λ / 2 wavelength plates 5 and 7 and the λ / 4 wavelength plates 6 and 8 are used as bias adjusting means. However, in order to satisfy the formula (3) or the formula (5), the transmittance is 1 For example, a means of applying an offset voltage of V _π / 2 to the electro-optic modulation elements 3 and 10 may be used as long as it is biased to be / 2.

Further, as described above, in FIG. 8, if there is a phase difference between the two signal lights separated by the PBS 2, the degree of modulation after multiplexing will deteriorate. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  In addition, the input light in the fourth embodiment is an unmodulated laser light and is intended for RF modulation. However, as another method of use, for example, laser light including RF modulation in advance is down-converted. It is also possible to change the modulation frequency applied to the laser light to a desired frequency, such as converting to a baseband frequency.

In FIG. 8, one mirror 4 and one mirror 9 are installed in each optical path. However, as described above, two mirrors are installed in one optical path and only one optical path is bent 180 degrees. I do not care. However, as described above, if there is a phase difference between the two signal lights separated by the PBS 2, the degree of modulation after multiplexing will deteriorate. Therefore, as a solution to the above problem, for example, in FIG. 3, the distance L _A ′ from the RF signal output unit 51 to the electro-optic modulation element 3 or the electro-optics from the RF signal output unit 51 so that Expression (7) is satisfied. The distance L _B ′ to the modulation element 10 may be adjusted.

  The type of optical fiber used in the fourth embodiment may be any of a single mode optical fiber, a multimode optical fiber, and a polarization maintaining optical fiber.

  However, when the input light is linearly polarized light and a polarization-maintaining optical fiber is used, as described above, even if the configuration is not as shown in FIG. Since it can be obtained, the configuration may be such that the PBS 2, the mirrors 4, 9, the λ / 2 wavelength plate 7, the λ / 4 wavelength plate 8, and the electro-optic modulation element 10 are removed.

  Similarly to the first embodiment, the fiber collimators 1, 12, and 13 are not separately provided with a fiber collimator, but output to the space as an optical fiber and collimated light, or couple the collimated light to the optical fiber. It may be a lens optical system.

  Further, in FIG. 8, the input / output light is propagated by the optical fiber using the fiber collimators 1, 12, and 13, but this is changed to the spatial propagation, and for example, the PDs 17 and 18 are spatially coupled. It may be combined.

  However, with respect to the above spatial propagation, propagation by an optical fiber enables miniaturization, high reliability, easy handling, and high placement freedom.

It is a figure which shows the structure of the optical microwave mixer which concerns on Embodiment 1 of this invention. It is a figure which shows operation | movement of the optical microwave mixer which concerns on Embodiment 1 of this invention. It is a figure which shows the structure for eliminating the phase difference of two signal light with the optical microwave mixer which concerns on Embodiment 1 of this invention. It is a figure which shows another structure of the optical microwave mixer which concerns on Embodiment 1 of this invention. It is a figure which shows the structure in case the input light is a linearly polarized light and uses a polarization-maintaining optical fiber with the optical microwave mixer which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the optical microwave mixer which concerns on Embodiment 2 of this invention. It is a figure which shows the structure of the optical microwave mixer which concerns on Embodiment 3 of this invention. It is a figure which shows the structure of the optical microwave mixer which concerns on Embodiment 4 of this invention. It is a figure which shows the structure of the conventional polarization control circuit.

Explanation of symbols

  1 Fiber Collimator, 2 Polarization Separating Beam Splitter (PBS), 3 Electro-optic Modulator, 4 Mirror, 5 λ / 2 Wave Plate, 6 λ / 4 Wave Plate, 7 λ / 2 Wave Plate, 8 λ / 4 Wave Plate, 9 mirror, 10 electro-optic modulation element, 11 polarization splitting beam splitter (PBS), 12 fiber collimator, 13 fiber collimator, 14 extra optical fiber, 15 optical receiver, 16 balanced receiver, 17 photodiode (PD) ), 18 photodiode (PD), 19 A / D converter, 20 A / D converter, 21 inverter, 22 adder, 23 phase shifter, 24 multiplexer, 51 RF signal output section.

Claims (14)

A first polarization separation type beam splitter that separates input light into two mutually orthogonal polarization components;
A second polarization separation type beam splitter that separates two input lights separated by the first polarization separation type beam splitter and inputted through the first and second optical paths into two mutually orthogonal polarization components. When,
A first electro-optic modulation element that is disposed in the first optical path and applies intensity modulation according to RF frequency modulation to transmitted light by applying an applied voltage with an RF frequency;
A second electro-optic modulation element that is disposed in the second optical path and applies intensity modulation according to RF frequency modulation to transmitted light by applying an applied voltage with an RF frequency;
First bias adjusting means for performing bias adjustment so that a modulation degree by the first electro-optic modulation element is maximized;
Second bias adjusting means for performing bias adjustment so that the degree of modulation by the second electro-optic modulation element is maximized;
Optical receiving means for photoelectrically converting and outputting two orthogonal polarization components separated by the second polarization splitting beam splitter and inputted via the third and fourth optical paths, An optical microwave mixer. The optical receiving means includes
A delay time that is an odd multiple of half the reciprocal of the modulation frequency of the voltage applied to the first and second electro-optic modulation elements arranged in the third optical path or the fourth optical path is caused by light propagation. Extra light path,
The optical microwave mixer according to claim 1, further comprising: an optical receiver that photoelectrically converts two input lights from the second polarization splitting beam splitter and the extra optical path and outputs a sum signal. . The optical receiving means includes
A balanced receiver that photoelectrically converts two mutually orthogonal polarization components input via the third and fourth optical paths and outputs a difference signal;
The optical microwave mixer according to claim 1, wherein the third and fourth optical paths from the second polarization separation type beam splitter to the balanced receiver have the same length. The optical receiving means includes
A first photodiode disposed in the third optical path and photoelectrically converting a polarization component from the second polarization splitting beam splitter;
A second photodiode disposed in the fourth optical path and photoelectrically converting a polarization component from the second polarization splitting beam splitter;
A first A / D converter for converting an analog signal from the first photodiode into a digital signal;
A second A / D converter for converting an analog signal from the second photodiode into a digital signal;
An inverter that inverts and outputs the sign of the digital signal from the first A / D converter or the second A / D converter;
An adder that adds and outputs two signals from the first A / D converter or the second A / D converter and the inverter;
The length of the third optical path from the second polarization splitting beam splitter to the first photodiode and the length of the fourth optical path from the second polarization splitting beam splitter to the second photodiode The optical microwave mixer according to claim 1, wherein the two are equal. The optical receiving means includes
A first photodiode disposed in the third optical path and photoelectrically converting a polarization component from the second polarization splitting beam splitter;
A second photodiode disposed in the fourth optical path and photoelectrically converting a polarization component from the second polarization splitting beam splitter;
A phase shifter that freely adjusts the phase of an analog signal from the first photodiode or the second photodiode;
The optical microwave mixer according to claim 1, further comprising: a multiplexer that multiplexes and outputs two signals from the first photodiode or the second photodiode and the phase shifter. . An RF signal output unit for outputting an RF modulation signal to be applied to the first and second electro-optic modulation elements;
The sum of the first distance of the first optical path between the first and second polarization splitting beam splitters and the third distance from the RF signal output unit to the first electro-optic modulation element is: Equal to the sum of the second distance of the second optical path between the first and second polarization splitting beam splitters and the fourth distance from the RF signal output section to the second electro-optic modulation element The optical microwave mixer according to any one of claims 1 to 5, wherein the optical microwave mixer is provided. An electro-optic modulation element that applies an applied voltage with an RF frequency to modulate intensity of the transmitted light according to the frequency modulation of the RF;
Bias adjusting means for adjusting the bias so that the degree of modulation by the electro-optic modulation element is maximized;
A polarization splitting beam splitter that splits input light input from the bias adjusting means into two mutually orthogonal polarization components;
And an optical receiving means for photoelectrically converting and outputting two mutually orthogonal polarization components separated by the polarization splitting beam splitter and inputted via the third and fourth optical paths. Wave mixer. The optical receiving means includes
An extra optical path disposed in the third optical path or the fourth optical path, such that a delay time that is an odd multiple of half the reciprocal of the modulation frequency of the voltage applied to the electro-optic modulation element is caused by light propagation;
The optical microwave mixer according to claim 7, further comprising: an optical receiver that photoelectrically converts two input lights from the polarization-separated beam splitter and the surplus optical path and outputs a sum signal. The optical receiving means includes
A balanced receiver that photoelectrically converts two mutually orthogonal polarization components input via the third and fourth optical paths and outputs a difference signal;
The optical microwave mixer according to claim 7, wherein the third and fourth optical paths from the polarization splitting beam splitter to the balanced receiver have the same length. The optical receiving means includes
A first photodiode that is disposed in the third optical path and photoelectrically converts a polarization component from the polarization splitting beam splitter;
A second photodiode disposed in the fourth optical path and photoelectrically converting a polarization component from the polarization splitting beam splitter;
A first A / D converter for converting an analog signal from the first photodiode into a digital signal;
A second A / D converter for converting an analog signal from the second photodiode into a digital signal;
An inverter that inverts and outputs the sign of the digital signal from the first A / D converter or the second A / D converter;
An adder that adds and outputs two signals from the first A / D converter or the second A / D converter and the inverter;
The lengths of the third optical path from the polarization splitting beam splitter to the first photodiode and the fourth optical path from the polarization splitting beam splitter to the second photodiode are equal. The optical microwave mixer according to claim 7. The optical receiving means includes
A first photodiode that is disposed in the third optical path and photoelectrically converts a polarization component from the polarization splitting beam splitter;
A second photodiode disposed in the fourth optical path and photoelectrically converting a polarization component from the polarization splitting beam splitter;
A phase shifter that freely adjusts the phase of an analog signal from the first photodiode or the second photodiode;
The optical microwave mixer according to claim 7, further comprising: a multiplexer that multiplexes and outputs two signals from the first photodiode or the second photodiode and the phase shifter. . The optical receiving means includes
Between the second polarization splitting beam splitter or the polarization splitting beam splitter and the optical receiver, and between the second polarization splitting beam splitter or the polarization splitting beam splitter and the extra optical path. Further comprising first and second fiber coupling means for coupling spatially propagated light to the optical fiber,
The surplus optical path is an optical fiber;
The first fiber coupling means and the optical receiver, the second fiber coupling means and the surplus optical path, and the surplus optical path and the optical receiver are connected by an optical fiber. The optical microwave mixer according to claim 2 or 8. The optical receiving means includes
And further comprising first and second fiber coupling means for coupling spatially propagated light to an optical fiber between the second polarization splitting beam splitter or the polarization splitting beam splitter and the balanced receiver,
10. The first fiber coupling means and the balanced receiver, and the second fiber coupling means and the balanced receiver are connected by an optical fiber. Optical microwave mixer. The optical receiving means includes
The second polarization separation type beam splitter or the polarization separation type beam splitter and the first photodiode, and the second polarization separation type beam splitter or the polarization separation type beam splitter and the second A first and a second fiber coupling means for coupling spatially propagated light to the optical fiber between the photodiodes;
The first fiber coupling means and the first photodiode, and the second fiber coupling means and the second photodiode are connected by an optical fiber. The optical microwave mixer according to 4, 5, 10, or 11.

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