JP2012034182A - Optical fiber microwave transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical fiber microwave transmission device capable of compensating for fluctuations in an optical path length.SOLUTION: An optical fiber microwave transmission device includes: an E/O converter 2 for outputting a modulated light modulated by a microwave; an optical demultiplexer 4 for demultiplexing an optical frequency comb-shape signal into two optical frequency comb-shape signals S1, S2; an optical multiplexer 5 for multiplexing the modulated light and the comb-shape signal S1; a variable delay line 7 for varying an optical path length according to a control signal C1; an optical frequency shifter 13 for shifting the frequency of the comb-shape signal S1 reflected by a partial reflector 9; WDMs 14, 15 for demultiplexing light having wavelengths of λm and λfrom the comb-shape signals S1, S2 according to a control signal C2; a phase comparator 20 for multiplexing the light of λm and λand obtaining a phase difference of the microwave converted photoelectrically; and a control device 21 for controlling the variable delay line 7 and the WDMs 14, 15 to obtain the smaller optical path length difference between the two optical paths based on the phase difference, and a larger frequency difference when the optical path length difference becomes smaller than a predetermined threshold value.

Description

  The present invention relates to an optical fiber microwave transmission device that compensates for fluctuations in the optical path length of an optical fiber in a system that transmits modulated light modulated by a microwave signal through an optical fiber.

  In general, when a light wave propagates through an optical fiber, if there is a temperature change around the optical fiber or vibration of the optical fiber, the optical path length varies due to the expansion and contraction of the optical fiber. Accordingly, even in RoF (Radio On Fiber) transmission in which a microwave signal is superimposed on a light wave and transmitted through an optical fiber, the phase of the microwave signal demodulated after transmission through the optical fiber due to fluctuations in the optical path length of the optical fiber. Variation and delay time variation occur. For this reason, in order to improve the phase stability of the demodulated microwave signal, it is necessary to compensate for variations in the optical path length of the optical fiber that is the transmission path.

  Conventionally, in this type of optical fiber transmission system, a method has been proposed in which fluctuations in the optical path length of an optical fiber are measured and the fluctuations are compensated based on the measurement results (see, for example, FIG. 1 of Non-Patent Document 1). In the following description, the names in parentheses are the names described in FIG.

  In the conventional optical path length variation compensation method, the laser light output from the laser (ECLD) is branched into two by a first optical coupler (10 dB coupler), and one of the branched laser lights is the first optical circulator, light An optical fiber (delay line) is transmitted through an optical fiber stretcher that changes the length of the fiber, a second optical circulator, and a second optical coupler (3 dB coupler). ), And the optical fiber is reciprocated by the second optical circulator through the acousto-optic light modulator (AOM). The frequency of the reciprocating laser light is shifted by an acousto-optic light modulator. The reciprocating laser light has its optical path switched by the first optical circulator, and is combined with the other laser light branched by the first optical coupler in a third optical coupler (3 dB coupler).

  This combined light is converted into an electrical signal by a photoelectric converter (PD). Here, the frequency of the electrical signal is the frequency of the microwave signal frequency-shifted by the acousto-optic light modulator. When the optical path length of the optical fiber varies due to a temperature change or the like, the phase of the beat signal of the electrical signal subjected to photoelectric conversion varies according to the variation of the optical path length.

  The phase detector (PSD or DPFD) compares the phase of the reference signal from the reference signal source (synth. 55 MHz) with the beat signal, and the optical phase shifter matches the phase of the signal light with the desired phase. For example, a control signal such as a voltage is output. This control signal is input to the optical phase shifter via the loop filter, and the phase of the optical signal is shifted by an amount corresponding to the control signal. Here, a feedback circuit is configured by repeating an operation in which a control signal obtained from a beat signal from a photoelectric converter (PD) is input to an optical phase shifter, and this feedback circuit configures an optical path due to disturbance. Control to compensate for the variation in length.

  Therefore, the RF-modulated signal light combined by the optical multiplexer passes through the optical phase shifter and the optical circulator, is demultiplexed by the optical demultiplexer, and is electrically detected by photoelectric detection by the photoelectric converter. Although converted into a signal, this converted RF signal has high phase stability.

Musha, et. Al., “Rubust and precise length correction of 25-km fiber for distribution of local oscillator,” 2005 Digest of the LEOS Summer Topical Meeting, TuB4.4, p.123, 2005. (Figure 1)

  The conventional optical path length variation compensation method splits the laser light into two, performs heterodyne detection, and compares the phase of the beat signal obtained by this detection with the reference signal for frequency shift used to obtain the beat signal Thus, the phase of the optical signal is detected, and the fluctuation of the optical path length is obtained with high accuracy. However, such a configuration has a problem in that it cannot cope with fluctuations in the optical path length that is longer (about 1 μm) than the phase of the light wave.

  The present invention has been made to solve the above-described problems. The measurable range of the optical path length is made variable, and when the measurable range is narrowed, it is highly accurate, and when the measurable range is widened, it is rough. An object of the present invention is to obtain an optical fiber microwave transmission device that can measure fluctuations in the optical path length and can compensate for fluctuations in the optical path length.

  An optical fiber microwave transmission device according to the present invention includes an electrical / optical conversion unit that outputs modulated light that is intensity-modulated by an input first microwave signal, and a plurality of optical signals that are arranged at predetermined frequency intervals. Optical frequency comb signal generating means for generating an optical frequency comb signal, and optical demultiplexing for demultiplexing the optical frequency comb signal generated by the optical frequency comb signal generating means into first and second demultiplexed optical frequency comb signals A first optical multiplexing unit for combining the modulated light and the first demultiplexed optical frequency comb signal to output a first combined signal; and an output side of the first optical combining unit An optical transmission means for transmitting the first combined signal input from the input port to the input / output port and transmitting the first demultiplexed optical frequency comb signal input from the input / output port to the output port And the light transmission hand One end of which is connected to the input / output port of the optical path, and based on the first control signal, the optical path length varying means for varying the length of the optical path, and the first multiplexing output from the other end of the optical path length varying means An optical fiber for transmitting a signal, and a partial reflection means for transmitting the modulated light and reflecting the first demultiplexed optical frequency comb signal among the first multiplexed signal transmitted by the optical fiber; First optical / electrical conversion means for demodulating the modulated light transmitted through the partial reflection means and outputting the first microwave signal, a reference microwave signal source for outputting a reference microwave signal, and the reference In response to a microwave signal, the frequency of the first demultiplexed optical frequency comb signal reflected by the partial reflection means and output from the output port of the optical transmission means via the optical fiber and the variable delay means is obtained. An optical signal having first and second wavelengths out of the second demultiplexed optical frequency comb signal demultiplexed by the optical demultiplexing means based on the optical frequency shifting means for shifting and the second control signal; Based on the first optical wavelength demultiplexing means for demultiplexing and the second control signal, the third and second of the first demultiplexed optical frequency comb signals frequency-shifted by the optical frequency shift means A second optical wavelength demultiplexing unit for demultiplexing an optical signal having a wavelength of 4, a first wavelength optical signal output from the first wavelength demultiplexing unit, and an output from the second wavelength demultiplexing unit Second optical multiplexing means for combining the optical signals having the third wavelength and outputting a second combined signal; and an optical signal having the second wavelength output from the first wavelength demultiplexing means And a third optical multiplexing unit that combines the optical signals of the fourth wavelength output from the second wavelength demultiplexing unit and outputs a third combined signal. Means, a first photoelectric conversion means for photoelectrically converting the second combined signal and outputting a second microwave signal, and a third microwave signal by photoelectrically converting the third combined signal A second photoelectric conversion unit that outputs a phase difference, a phase comparison unit that compares the phase of the second and third microwave signals to obtain a phase difference, and outputs the phase difference, and the phase difference and the first and second components From the optical demultiplexing means to the second optical multiplexing means or from the optical demultiplexing means to the third based on a frequency difference that is a wavelength difference between two wavelengths respectively selected from the wave optical frequency comb signals. An optical path length difference between the two optical paths through which the first and second demultiplexed optical frequency comb signals to the optical multiplexing means pass is calculated, and the first control signal is set so that the optical path length difference becomes small. Output to the optical path length variable means, and the calculated optical path length difference is a predetermined value. When the value is smaller than the value, the two wavelengths of the optical signal selected from the first and second demultiplexed optical frequency comb signals are switched, and the two wavelengths are selected so as to increase the frequency difference. And a control means for outputting the second control signal instructing to the first and second optical wavelength demultiplexing means, respectively.

  According to the optical fiber microwave transmission device of the present invention, the measurable range of the optical path length is variable, and when the measurable range is narrowed, the accuracy is high, and when the measurable range is widened, the optical path length varies. It is possible to measure and to compensate for variations in the optical path length.

It is a block diagram which shows the structure of the optical fiber microwave transmission apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the optical wavelength demultiplexer of the optical fiber microwave transmission apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows operation | movement of the optical frequency shifter of the optical fiber microwave transmission apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the partial reflector of the optical fiber microwave transmission apparatus which concerns on Embodiment 2 of this invention.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of an optical fiber microwave transmission device of the invention will be described with reference to the drawings.

Embodiment 1 FIG.
An optical fiber microwave transmission device according to Embodiment 1 of the present invention will be described with reference to FIGS. 1 is a block diagram showing a configuration of an optical fiber microwave transmission device according to Embodiment 1 of the present invention. In FIG. 1, the optical fiber is drawn with a solid line, and the electric wire is drawn with a broken line. In the following, the same reference numerals in the drawings indicate the same or corresponding parts.

  1, an optical fiber microwave transmission device according to Embodiment 1 of the present invention includes a microwave input unit 1 that inputs a microwave and a microwave that outputs modulated light that is intensity-modulated by the input microwave signal. An optical frequency comb signal generator (optical frequency comb signal) that generates a wave / optical converter (E / O) (electrical / optical conversion means) 2 and an optical frequency comb signal in which a large number of optical signals are arranged at predetermined frequency intervals Generating means) 3 and an optical demultiplexer (optical demultiplexer) for demultiplexing (branching) the optical frequency comb signal generated by the optical frequency comb signal generator 3 into two of the first and second demultiplexed optical frequency comb signals. (Wave means) 4, an optical multiplexer (first filter) for multiplexing the modulated light output from the microwave / optical converter 2 and the first demultiplexed optical frequency comb signal demultiplexed by the optical demultiplexer 4. Optical multiplexing means) 5 is provided.

  As an intensity modulation method of the microwave / optical converter 2, for example, a direct modulation method in which a semiconductor laser is driven with a predetermined bias current and modulated with an input microwave signal based on the bias level, There is an external modulation method using an optical modulator in which continuous wave (CW) driven laser light is input and the transmittance of the CW light is varied by an input microwave signal. Although the intensity modulation method will be described here, it goes without saying that other modulations such as phase modulation can be applied.

  The optical frequency comb signal generator 3 generates a light wave (optical frequency comb signal) in which a large number of optical signals are arranged at a predetermined frequency interval. The optical frequency comb signal generator 3 uses a mode-locked laser, a LiNbO3 optical modulation, or the like. It can be realized by various methods such as a method of generating using a vessel.

  The optical fiber microwave transmission device according to the first embodiment of the present invention is based on the optical circulator (light transmission means) 6 and the drive signal (first control signal), and the length of the optical path (the combined signal). Of the variable delay line (optical path length varying means) 7 for varying the delay time, the optical fiber 8, and the multiplexed signal transmitted by the optical fiber 8, and transmits the modulated light and the first demultiplexed optical frequency. A partial reflector (partial reflection means) 9 that reflects the comb signal, and an optical / microwave converter (O / E) (first light) that demodulates the modulated light transmitted through the partial reflector 9 into a microwave signal / Electric conversion means) 10 and a microwave output unit 11 for outputting a demodulated microwave signal.

  Note that the input / output relationship of the optical circulator 6 is that the light wave input from the left input port in the figure is output to the right input / output port, and the light wave input from the right input / output port is output to the lower output port. And The variable delay line 7 can be realized by, for example, an optical fiber stretcher that changes the length of the optical fiber. The partial reflector 9 can be realized by a band pass filter or the like that transmits only the wavelength band of the light wave output from the microwave / optical converter 2 and reflects the other wavelength band. It can be realized by various means such as a means using a band multilayer filter and a means using a fiber Bragg grating. The optical / microwave converter 10 mainly includes a photodiode (PD) and, if necessary, a matching circuit, a microwave amplifier, and a bias voltage circuit.

Further, the optical fiber microwave transmission device according to the first embodiment of the present invention includes a reference microwave signal source 12 that outputs a reference microwave signal, and a first component reflected in accordance with the reference microwave signal. An optical frequency shifter (optical frequency shift means) 13 for shifting the frequency of the wave optical frequency comb signal and a second demultiplexed optical frequency comb signal demultiplexed by the optical demultiplexer 4 will be described later. An optical wavelength demultiplexer (WDM) (first optical output) that demultiplexes optical signals of two wavelengths (corresponding to frequencies) selected by the second control signal from the control device, that is, outputs from different output ports, respectively. Light of the same two wavelengths (corresponding to frequencies) selected by the second control signal from the first demultiplexed optical frequency comb signal shifted in frequency by the optical wavelength demultiplexing means 14 and the optical frequency shifter 13. Demultiplex signal That is, the light wavelength demultiplexer output from different output ports and (WDM) (second optical wavelength demultiplexing means) 15, the same wavelengths output from the optical wavelength demultiplexer 14, 15 (lambda _m) light between An optical coupler (second optical multiplexing means) 16 that combines signals and an optical coupler (third optical multiplexing) that combines optical signals of the same wavelength (λ _n ) output from the optical wavelength demultiplexers 14 and 15. (Wave means) 17, an optical / microwave converter (O / E) (first photoelectric conversion means) 18 that photoelectrically converts the combined signal output from the optical coupler 16 and outputs a microwave signal, and light An optical / microwave converter (O / E) (second photoelectric conversion means) 19 that photoelectrically converts the combined signal output from the coupler 17 and outputs a microwave signal is provided.

The optical frequency shifter 13 is realized by, for example, an AO modulator (AOM: Acoust Optic Modulator) that shifts the frequency of input light by applying ultrasonic waves to an acoustooptic device, a Mach-Zehnder type optical modulator using LiNbO3, or the like. can do. The wavelengths of the two optical signals respectively output from the optical wavelength demultiplexers 14 and 15 are λ _m and λ _n .

Furthermore, the optical fiber microwave transmission device according to the first embodiment of the present invention compares the phases of the microwave signals output from the optical / microwave converters 18 and 19 and obtains and outputs the phase difference between them. A wavelength difference between two wavelengths λ _m and λ _n respectively selected from the comparator (phase comparison means) 20 and the phase difference output from the phase comparator 20 and the first and second demultiplexed optical frequency comb signals. Based on (frequency difference Δf), the optical path length difference ΔL between the two optical paths is calculated, and the variable delay line 7 is driven so that the optical path length difference ΔL becomes small. A first function for obtaining a control signal and outputting it to the driver 22 and, if the calculated optical path length difference ΔL is smaller than a predetermined threshold, select from the first and second demultiplexed optical frequency comb signals, respectively. Two waves to do And a second function of outputting a second control signal for instructing selection of two wavelengths to the optical wavelength demultiplexers 14 and 15, respectively, so as to increase the wavelength difference, that is, the frequency difference Δf. (Control means) 21 and a driver 22 for outputting a drive signal for driving the variable delay line 7 based on a first control signal from the control device 21 are provided. The phase comparator 20, the control device 21, and the driver 22 can be integrally configured by a microcomputer or the like.

  FIG. 2 is a diagram showing the configuration of the optical wavelength demultiplexer of the optical fiber microwave transmission device according to the first embodiment of the present invention.

  In FIG. 2, the optical wavelength demultiplexer 14 is configured by combining an arrayed waveguide grating (AWG) (optical signal wavelength separation means) 141 and an optical switch 142. The same applies to the optical wavelength demultiplexer 15.

  Next, the operation of the optical fiber microwave transmission device according to the first embodiment will be described with reference to the drawings.

  FIG. 3 is a diagram illustrating the operation of the optical frequency shifter of the optical fiber microwave transmission device according to the first embodiment of the present invention. 2A shows the spectrum of the optical frequency comb signal generated by the optical frequency comb signal generator 3, and FIG. 2B shows the spectrum of the optical frequency comb signal whose frequency is shifted by the optical frequency shifter 13. Show.

The microwave / optical converter 2 outputs modulated light that is intensity-modulated by a microwave signal to be transmitted, which is input from the microwave input unit 1. On the other hand, the optical frequency comb signal generator 3, the optical frequency comb signal aligned optical signals of a plurality of wavelengths at a predetermined frequency interval (λ1, λ2, λ3, ... , λ m, ..., λ n, ...) the occurrence To do. Next, the optical demultiplexer 4 demultiplexes the optical frequency comb signal generated by the optical frequency comb signal generator 3 into two of the first and second demultiplexed optical frequency comb signals. The demultiplexing ratio (branch ratio) of the optical demultiplexer 4 is set according to the loss of each optical path after demultiplexing. Next, the optical multiplexer 5 combines the modulated light output from the microwave / optical converter 2 and the first demultiplexed optical frequency comb signal demultiplexed by the optical demultiplexer 4.

  The modulated light output from the microwave / optical converter 2 on which the microwave signal to be transmitted is superimposed proceeds to the variable delay line 7 via the optical multiplexer 5 and the optical circulator 6 and is transmitted via the optical fiber 8. Then, the light is transmitted through the partial reflector 9 and demodulated into a microwave signal by the light / microwave converter 10, and a microwave signal to be transmitted is output from the microwave output unit 11. The modulated light input to the optical / microwave converter 10 is directly detected by a photodiode in the optical / microwave converter 10, and the microwave signal input to the optical / microwave converter 10 is demodulated.

  Here, the variable delay line 7 varies the optical path length in accordance with the drive signal output from the driver 22. Further, since the transmittance of the optical fiber 8 is very small, for example, 0.2 dB / km or less in the wavelength 1.5 micron band, the microwave / optical converter 2 and the optical / microwave converter 10 are installed separately. However, since the propagation loss of the transmission path is small, it is effective for transmitting a microwave signal to a distant place.

  On the other hand, as described in the background art, when the transmission path becomes longer, the variation in the optical path length due to, for example, a temperature change of the optical fiber 8 becomes larger. The phase (delay time) of the output microwave signal varies.

  The partial reflector 9 reflects the first demultiplexed optical frequency comb signal out of the combined signal of the modulated light passing through the variable delay line 7 and the optical fiber 8 and the first demultiplexed optical frequency comb signal. The modulated light transmitted without being reflected by the partial reflector 9 is demodulated into a microwave signal by the light / microwave converter 10 and output from the microwave output unit 11 as described above.

  The first demultiplexed optical frequency comb signal reflected by the partial reflector 9 travels in the reverse direction through the optical fiber 8 and the variable delay line 7 and is input to the input / output port of the optical circulator 6. Output from the output port.

  The reference microwave signal source 12 outputs a reference microwave signal, and the optical frequency shifter 13 converts the first demultiplexed optical frequency comb signal output from the optical circulator 6 into the reference microwave from the reference microwave signal source 12. The frequency is shifted according to the signal and output. Here, since the first demultiplexed optical frequency comb signal includes a large number of optical signals, each frequency of the large number of optical signals corresponds to the frequency of the reference microwave signal as shown in FIG. 3B. shift.

  As described above, the optical frequency comb signal generated by the optical frequency comb signal generator 3 is demultiplexed (branched) into two of the first and second demultiplexed optical frequency comb signals by the optical demultiplexer 4. . The second demultiplexed optical frequency comb signal demultiplexed downward in FIG. 1 by the optical demultiplexer 4 is input to the optical wavelength demultiplexer 14.

The optical wavelength demultiplexer 14 demultiplexes the two components of wavelengths λ _m and λ _n selected by the second control signal from the control device 21 from the second demultiplexed optical frequency comb signal. Output from a different output port.

On the other hand, the optical wavelength demultiplexer 15 selects the wavelength λ _m and the wavelength λ _m selected by the second control signal from the control device 21 from the first demultiplexed optical frequency comb signal frequency-shifted by the optical frequency shifter 13. each output from different output ports and demultiplex two components of lambda _n. The notation of the wavelength of the optical signal output from the optical wavelength demultiplexer 15 is precisely frequency-shifted, so it should be expressed as λm ′ and λn ′ different from λm and λn, for example, In order to simplify the description, the wavelengths λ _m and λ _n are described.

  The bandwidth of each wavelength output from the optical wavelength demultiplexers 14 and 15 is narrower than the frequency comb interval, but wider than the frequency of the reference microwave signal.

Here, the detailed operation of the optical wavelength demultiplexers 14 and 15 will be described. As shown in FIG. 2, in the arrayed waveguide grating (AWG) 141 in the optical wavelength demultiplexer 14, the second demultiplexed optical frequency comb signals (λ1, λ2, λ3,. λ _m ,..., λ _n ,. Next, the optical switch 142 inputs optical signals of respective wavelengths (λ1, λ2, λ3,..., Λ _m ,..., Λ _n ,...) From the arrayed waveguide grating 141 from a plurality of input ports. In accordance with the second control signal from the control device 21, the optical signals (λ _m , λ _n ) of the wavelengths input from the two selected input ports are output from separate output ports. The same applies to the optical wavelength demultiplexer 15.

The optical coupler 16 includes an optical signal having a wavelength λ _m in the second demultiplexed optical frequency comb signal output from the optical wavelength demultiplexer 14, and the first frequency shifted from the optical wavelength demultiplexer 15. and an optical signal of wavelength lambda _m in demultiplexed optical frequency comb signal multiplexing.

Similarly, the optical coupler 17 outputs the optical signal of the wavelength λ _n in the second demultiplexed optical frequency comb signal output from the optical wavelength demultiplexer 14 and the frequency shifted from the optical wavelength demultiplexer 15. multiplexes the optical signal of wavelength lambda _n in the first demultiplexing optical frequency comb signal.

  The optical / microwave converter 18 photoelectrically converts the combined signal output from the optical coupler 16 and outputs a microwave signal. Similarly, the optical / microwave converter 19 photoelectrically converts the combined signal output from the optical coupler 17 and outputs a microwave signal.

Each of the optical / microwave converters 18 and 19 outputs a microwave signal having the same frequency as that of the reference microwave signal. The phase difference between the microwave signals output from the optical / microwave converters 18 and 19 is the phase difference between the two optical paths branched by the optical demultiplexer 4. Since the wavelength of the combined signal to the microwave converters 18 and 19 is different from λ _m and λ _n , the phase difference of the photoelectrically converted microwave signal is also different depending on these wavelengths.

Optical signal of the wavelength lambda _m input to the optical / micro wave transducer 18, an optical signal whose frequency is a frequency corresponding offset of the reference microwave signal, since an optical signal which has not been frequency corresponding offset, optical / micro The wave converter 18 outputs a beat signal of two optical signals, that is, a microwave signal having a beat frequency corresponding to the frequency of the reference microwave signal.

Similarly, the optical signal of wavelength lambda _n is input to the optical / micro wave transducer 19, the optical signal of wavelength lambda _n in the second demultiplexing optical frequency comb signal outputted from the optical wavelength demultiplexer 14 and Since the optical signal having the wavelength λ _n in the first wavelength-demultiplexed optical frequency comb signal output from the optical wavelength demultiplexer 15 and shifted in frequency, the optical / microwave converter 19 outputs two optical signals. Beat signal, that is, a microwave signal having a beat frequency corresponding to the frequency of the reference microwave signal is output.

Here, the phase of the optical signal having the wavelength λ _m output from the optical / microwave converter 18 passes from the optical demultiplexer 4 to the optical multiplexer 5, the optical circulator 6, the variable delay line 7, and the optical fiber 8. The optical path is switched by the optical circulator 6 and the optical path to the optical coupler 16 via the optical frequency shifter 13 and the optical wavelength demultiplexer 15. The optical path length difference from the optical path from the optical demultiplexer 4 to the optical coupler 16 via the optical wavelength demultiplexer 14 (denoted as ΔL) changes with the wavelength λ _m of the optical signal.

Similarly, the optical / micro wave transducer 19 of the optical signal of the wavelength lambda _n output from the phase varies at a wavelength of the optical signal lambda _n.

  Therefore, the phase difference Δφ between the microwave signals output from the optical / microwave converter 18 and the optical / microwave converter 19 can be expressed by the following equation (1).

  Δφ / (2π) = ΔL · N / c · Δf (1)

Here, ΔL is the optical path length difference, N is the refractive index of the optical fiber 8, c is the speed of light, Δf is the frequency of the optical signal with wavelength λ _m (c / λ _m ) and the frequency of the light wave with wavelength λ _n (c / λ _n ).

  Therefore, the optical path length difference ΔL can be expressed by the following equation (2).

  ΔL = Δφ / (2π) · c / N · 1 / Δf (2)

  Here, if the frequency difference Δf is set to a predetermined value (value understood in advance), the optical path length difference ΔL can be obtained from the measured value of the phase difference Δφ.

  If the phase difference Δφ is 2π or less, the maximum measurable optical path length difference ΔLmax can be expressed by the following equation (3).

  ΔLmax <c / N · 1 / Δf (3)

Therefore, when the frequency difference Δf between the wavelengths λ _m and λ _n is small, the optical path length difference ΔL is long, and when the frequency difference Δf is large, the optical path length difference ΔL is short. This ΔL is the optical path length difference with respect to the measured value of the phase difference Δφ.

  The phase comparator 20 compares the phases of the microwave signals output from the optical / microwave converter 18 and the optical / microwave converter 19 and obtains and outputs the phase difference between them. That is, the phase difference Δφ between the two microwave signals is obtained based on the above equation (1).

The control device 21 compares the phase difference Δφ obtained by the phase comparator 20 and the wavelength difference (frequency difference Δf) between the two wavelengths λ _m and λ _n selected from the first and second demultiplexed optical frequency comb signals. Thus, the optical path length difference ΔL between the two optical paths can be calculated. That is, the optical path length difference ΔL is obtained based on the above equation (2). A first control signal to the driver 22 that drives the variable delay line 7 is obtained and output to the driver 22 so that the optical path length difference ΔL is reduced. Based on the first control signal from the control device 21, the driver 22 outputs a drive signal for driving the variable delay line 7 so that the optical path length difference ΔL becomes small.

  Next, when the calculated optical path length difference ΔL becomes smaller than a predetermined threshold, the control device 21 switches between two wavelengths respectively selected from the first and second demultiplexed optical frequency comb signals, A second control signal for instructing selection of two wavelengths is output to the optical wavelength demultiplexers 14 and 15 so as to increase the difference, that is, the frequency difference Δf.

Thereby, when the wavelength difference between the two wavelengths λ _m and λ _n , that is, the frequency difference Δf is small (initial state), the measurement range ΔLmax can be widened although the measurement resolution of the optical path length difference ΔL is low. By increasing the wavelength difference between the two wavelengths λ _m and λ _n while sequentially switching them and decreasing the optical path length difference ΔL by the variable delay line 7, the measurement resolution of the optical path length difference can be increased.

As described above, in the initial state, the control device 21 selects two input ports of the optical switch 142 of the optical wavelength demultiplexers 14 and 15 having a small wavelength difference between the two wavelengths λ _m and λ _n , that is, the frequency difference. To do. Then, the second control signal is output to the optical wavelength demultiplexers 14 and 15 so as to select two input ports. Thereafter, the two input ports of the optical switch 142 are selected so as to gradually increase the frequency difference Δf.

  For example, in the expression “ΔL = c / N · 1 / Δf” obtained by modifying the expression (3), assuming that the frequency difference Δf = 100 GHz and the refractive index N = 1.45 of the optical fiber 8, the optical path length difference ΔL = 2. 1 mm is obtained. At this time, within the range where the length of the optical fiber 8 is 2.1 mm, the phase difference Δφ and the length of the optical fiber 8 are in a one-to-one correspondence. The absolute value of the length can be known, and the length of the desired optical fiber 8 can be controlled.

  On the other hand, in the conventional configuration, it is possible to detect and control the phase in the order of the optical wavelength, but it is difficult to control the length of the optical fiber 8 beyond that. However, by using the configuration of the first embodiment shown in FIG. 1, it becomes possible to control the length of the optical fiber 8 in accordance with the frequency difference Δf between the two optical signals. The effect of increasing the length of the fiber 8 is obtained.

  As described above, the optical fiber microwave transmission device according to the first embodiment uses the optical wavelength demultiplexers 14 and 15 to select an arbitrary number of optical frequency signals output from the optical frequency comb signal generator 3. These two optical signals can be selected. That is, since the frequency difference Δf between the two optical signals can be selected, the measurable optical path length difference ΔL can be changed in a wide range. Thereby, the variation | change_quantity of the optical path length by the optical fiber 8 etc. can be measured in a wide range.

  Furthermore, by converting the measurement result (phase difference Δφ) into a drive signal for the variable delay line 7, it becomes possible to compensate for variations in the optical path length.

Embodiment 2. FIG.
An optical fiber microwave transmission device according to Embodiment 2 of the present invention will be described with reference to FIG. FIG. 4 is a diagram showing a configuration of a partial reflector of an optical fiber microwave transmission device according to Embodiment 2 of the present invention. The other configuration of the optical fiber microwave transmission device according to the second embodiment of the present invention is the same as that of the first embodiment.

  In FIG. 4, the partial reflector 9 includes an optical wavelength demultiplexing filter (WDM) 91 and a Faraday rotator mirror (FR) (polarization rotating means) 92.

  The partial reflector 9 can also be realized by the configuration shown in FIG. In this case, only the wavelength of the modulated light output from the microwave / optical converter 2 is transmitted, and the other is reflected by the optical wavelength demultiplexing filter 91, which converts the first demultiplexed optical frequency comb signal to microwave / optical conversion. The light is split to a different optical path from the device 2. The demultiplexed first demultiplexed optical frequency comb signal is reflected by a Faraday rotator mirror 92 that rotates the polarization direction by 90 degrees from the time of incidence, and is input to the optical wavelength demultiplexing filter 91 from the opposite side.

  At this time, the polarization direction of the first demultiplexed optical frequency comb signal (right to left in FIG. 1) returning to the optical fiber 8 is inclined by 90 degrees with respect to the traveling time (left to right in FIG. 1). Therefore, instead of the optical circulator 6, a polarization beam splitter (PBS) whose beam output direction differs depending on the polarization direction may be used. Thereby, even if the polarization direction of the light fluctuates in the optical fiber 8, it is orthogonal in the reciprocating process, so that the fluctuation of the polarization can be canceled.

  1 Microwave input part, 2 Microwave / optical converter, 3 Optical frequency comb signal generator, 4 Optical demultiplexer, 5 Optical multiplexer, 6 Optical circulator, 7 Variable delay line, 8 Optical fiber, 9 Partial reflector 10 optical / microwave converter, 11 microwave output unit, 12 reference microwave signal source, 13 optical frequency shifter, 14 optical wavelength demultiplexer, 15 optical wavelength demultiplexer, 16 optical coupler, 17 optical coupler, 18 Optical / microwave converter, 19 optical / microwave converter, 20 phase comparator, 21 controller, 22 driver, 91 optical wavelength demultiplexing filter, 92 Faraday rotator mirror, 141 arrayed waveguide grating, 142 optical switch.

Claims (3)

Electrical / optical conversion means for outputting modulated light intensity-modulated by the input first microwave signal;
An optical frequency comb signal generating means for generating an optical frequency comb signal in which a plurality of optical signals are arranged at predetermined frequency intervals;
Optical demultiplexing means for demultiplexing the optical frequency comb signal generated by the optical frequency comb signal generating means into first and second demultiplexed optical frequency comb signals;
First optical multiplexing means for combining the modulated light and the first demultiplexed optical frequency comb signal to output a first combined signal;
The first multiplexed signal input from the input port connected to the output side of the first optical multiplexing means is transmitted to the input / output port, and the first demultiplexed light input from the input / output port An optical transmission means for transmitting a frequency comb signal to the output port;
One end connected to the input / output port of the light transmission means, and an optical path length varying means for varying the length of the optical path based on a first control signal;
An optical fiber for transmitting the first combined signal output from the other end of the optical path length varying means;
Of the first combined signal transmitted by the optical fiber, partially reflecting means that transmits the modulated light and reflects the first demultiplexed optical frequency comb signal;
First optical / electrical conversion means for demodulating the modulated light transmitted through the partial reflection means and outputting the first microwave signal;
A reference microwave signal source for outputting a reference microwave signal;
The frequency of the first demultiplexed optical frequency comb signal reflected by the partial reflection means according to the reference microwave signal and output from the output port of the optical transmission means via the optical fiber and the variable delay means Optical frequency shifting means for shifting
Based on a second control signal, a first optical wavelength component for demultiplexing optical signals of the first and second wavelengths from the second demultiplexed optical frequency comb signal demultiplexed by the optical demultiplexing means. Wave means,
Based on the second control signal, a second optical signal having a third wavelength and a fourth wavelength are demultiplexed from the first demultiplexed optical frequency comb signals frequency-shifted by the optical frequency shifting means. Optical wavelength demultiplexing means;
A second multiplexed signal is obtained by multiplexing the first wavelength optical signal output from the first wavelength demultiplexing means and the third wavelength optical signal output from the second wavelength demultiplexing means. Second optical multiplexing means for outputting
A third multiplexed signal is obtained by multiplexing the second wavelength optical signal output from the first wavelength demultiplexing means and the fourth wavelength optical signal output from the second wavelength demultiplexing means. A third optical multiplexing means for outputting
First photoelectric conversion means for photoelectrically converting the second combined signal and outputting a second microwave signal;
Second photoelectric conversion means for photoelectrically converting the third combined signal to output a third microwave signal;
Phase comparison means for comparing the phases of the second and third microwave signals to obtain and output a phase difference;
Based on the phase difference and a frequency difference which is a wavelength difference between two wavelengths respectively selected from the first and second demultiplexed optical frequency comb signals, the optical demultiplexing unit to the second optical multiplexing unit Or the optical path length difference between the two optical paths through which the first and second demultiplexed optical frequency comb signals pass from the optical demultiplexing means to the third optical multiplexing means, and the optical path length difference is calculated as The first control signal is output to the optical path length varying means so as to decrease, and when the calculated optical path length difference becomes smaller than a predetermined threshold, the first and second demultiplexed optical frequency combs The second control signal for instructing the selection of two wavelengths is switched to two wavelengths of the first and second optical wavelengths so that the two wavelengths of the optical signal selected from the signals are switched and the frequency difference is increased. Control means for outputting to the wave means respectively An optical fiber microwave transmission device comprising: The first and second wavelength demultiplexing means include:
Optical signal wavelength separation means for inputting the first or second demultiplexed optical frequency comb signal and separating and outputting from different output ports for each wavelength;
Optical signals of respective wavelengths separated by the optical signal wavelength separation means are input from a plurality of input ports, and optical signals of wavelengths input from two input ports selected according to the second control signal from among the input ports. The optical fiber microwave transmission device according to claim 1, wherein the optical fiber microwave transmission device includes optical switches that output from different output ports. The partial reflection means includes
Of the first combined signal transmitted by the optical fiber, light that transmits only the wavelength of the modulated light and demultiplexes the first demultiplexed optical frequency comb signal to an optical path different from the optical fiber. A wavelength demultiplexing filter;
The polarization direction of the demultiplexed first demultiplexed optical frequency comb signal is rotated by 90 degrees and reflected, and the first demultiplexed optical frequency comb signal is sent from the side opposite to the optical fiber to the optical wavelength demultiplexing means. And a polarization rotation means for returning,
The optical fiber microwave transmission device according to claim 1 or 2, wherein a polarization beam splitter is used instead of the light transmission means.

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