JP2014216804A - Optical fiber microwave transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber microwave transmitter capable of stably transmitting even a modulation signal such as a pulse modulation signal or a frequency chirp signal with a high phase.SOLUTION: An optical fiber microwave transmitter includes: a first optical transmission path for transmitting first modulation light intensity-modulated by an IF signal; a second optical transmission path arranged side by side with the first optical transmission path for transmitting second modulation light intensity-modulated by a second high frequency signal output by a voltage control oscillator; optical portion reflection means for reflecting a part of the second modulation light, and for allowing the other portion to pass; first frequency conversion means for outputting a sum frequency signal of the high frequency signal obtained by demodulating the first modulation light and the high frequency signal obtained by demodulating the second modulation light; and phase comparing means for comparing the high frequency signal generated from an electric signal obtained by demodulating the second modulation light reflected by the optical portion reflection means with a reference signal, and for outputting an error signal having a voltage corresponding to the difference frequency components, and for controlling the frequency of the second high frequency signal by inputting the error signal to the voltage control oscillator.

Description

  The present invention relates to an optical fiber microwave transmission device that compensates for variations in the optical path length of an optical fiber transmission line due to a temperature change or the like in an apparatus that transmits modulated light modulated by a microwave signal to a remote location via an optical fiber. .

In general, when a light wave propagates through an optical fiber, if there is a change in temperature around the optical fiber or vibration with respect to the optical fiber, the optical path length fluctuates due to expansion and contraction of the optical fiber. Therefore, 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 varies due to the variation in the optical path length of the optical fiber. And delay time fluctuations 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.

As a conventional optical path length stabilization means for solving such problems, phase fluctuations (instantaneous frequency fluctuations) caused by optical path length fluctuations due to disturbance are monitored, and the frequency of the microwave signal source is reversed in the direction opposite to the fluctuation direction. There is one that compensates for phase fluctuations at the transmission destination (see, for example, FIG. 1 of Patent Document 1).
In the conventional optical path length variation compensation system disclosed in Patent Document 1, first, the laser light output from the laser is branched into two by the first optical distributor. One of the branched laser beams is shifted in frequency by the frequency of a microwave signal output from a VCO (Voltage Controlled Oscillator) by an optical frequency shifter. This laser beam and the other branched laser beam are combined by an optical multiplexer and transmitted to a distant place through an optical circulator and an optical fiber.

At the transmission destination, a part of the transmission light is incident on the first photoelectric conversion means via the partial reflection mirror, and an electric signal corresponding to the frequency of the microwave signal is output.
Here, when the optical path length fluctuates due to an actual length, a refractive index change, or the like accompanying a temperature change of the optical fiber that is the transmission path, the phase of the microwave signal output from the photoelectric conversion means fluctuates.

On the other hand, the transmission light reflected by the partial reflection mirror travels back and forth through the optical fiber and is input to the second photoelectric conversion means via the optical circulator, and a microwave signal is output.
Here, the phase of the microwave signal and the reference microwave signal are compared, and an error signal is input to the VCO via the loop filter, so that the reciprocated modulated light (corresponding to the microwave signal) and the reference signal are A phase synchronization circuit is configured between the two.
In this way, in the conventional optical path length stabilization device, the feedback circuit that corrects the phase fluctuation of the microwave signal reciprocating through the optical fiber that is the transmission path is formed, so that the optical path length of the optical fiber that is the transmission path is formed. Fluctuation compensation control is performed to obtain high phase stability at the transmission destination.

JP2012-142841A

As described above, in the conventional optical path length stabilization device as disclosed in Patent Document 1, the phase difference between the modulated light reflected and reciprocated at the transmission destination and the reference signal source of the transmission source is detected, The variation in the phase difference was compensated by frequency control by the VCO.
However, since a normal VCO can output only CW (Continuous Wave), it cannot be applied to various modulation signals such as a pulse modulation signal and a frequency chirp signal.
As described above, the conventional device disclosed in Patent Document 1 has a problem that a signal that can be transmitted is limited to CW.

  The present invention has been made to solve the above problems, and provides a device capable of stably transmitting even a modulated signal such as a pulse modulated signal or a frequency chirp signal with respect to a conventional device. The purpose is to do.

An optical fiber microwave transmission device according to the present invention,
An IF signal source that outputs a first high-frequency signal that is an IF signal;
First electro-optic conversion means for outputting first modulated light intensity-modulated with the first high-frequency signal;
A first optical transmission line for transmitting the first modulated light;
First photoelectric conversion means for demodulating the first modulated light transmitted through the first optical transmission path into a high-frequency signal;
A voltage controlled oscillator that outputs a second high-frequency signal having a predetermined frequency according to the input voltage;
Second electro-optic conversion means for outputting second modulated light intensity-modulated with the second high-frequency signal;
A second optical transmission line that is arranged in parallel with the first optical transmission line and transmits the second modulated light;
A partial light reflecting means for reflecting a part of the second modulated light transmitted through the second optical transmission line and passing the other part;
Second photoelectric conversion means for demodulating the second modulated light that has passed through the light partial reflection means into a high-frequency signal;
A first high-frequency signal output from the first photoelectric conversion means and a high-frequency signal output from the second photoelectric conversion means are input, and a first frequency signal or a difference frequency signal of these high-frequency signals is output. Frequency conversion means;
The second modulated light output from the second electro-optic conversion means is output to the second optical transmission line, and is reflected by the light partial reflection means and reciprocates through the second optical transmission line. An optical circulator that outputs the modulated light of 2 by switching the optical path in a direction different from that of the second electro-optic conversion means;
Third photoelectric conversion means for demodulating the second modulated light whose optical path is switched by the optical circulator into a third high-frequency signal;
Second frequency conversion means for inputting the third high-frequency signal and the second high-frequency signal output from the voltage-controlled oscillator and outputting a sum frequency signal of these high-frequency signals;
A reference signal source for outputting a reference signal having a reference frequency;
Third frequency converting means for converting the frequency of the sum frequency signal output from the second frequency converting means into a high frequency signal in the same frequency band as the reference frequency;
The high-frequency signal frequency-converted by the third frequency conversion means and the reference signal are compared, and an error signal having a voltage corresponding to the difference frequency component is output, and the error signal is input to the voltage-controlled oscillator. Phase comparison means for controlling the frequency of the second high-frequency signal;
It is characterized by comprising.

  According to the present invention, there is an effect that it is possible to obtain an optical fiber microwave transmission device that can stably transmit even a modulated signal such as a pulse modulated signal or a frequency chirp signal.

1 is a block diagram showing an optical fiber microwave transmission device according to Embodiment 1 of the present invention. It is a block diagram which shows the optical fiber microwave transmission apparatus by Embodiment 2 of this invention. It is a block diagram which shows the optical fiber microwave transmission apparatus by Embodiment 3 of this invention. It is a figure explaining operation | movement of the optical fiber microwave transmission apparatus by Embodiment 3 of this invention. It is a block diagram which shows the optical fiber microwave transmission apparatus by Embodiment 4 of this invention. It is a figure explaining operation | movement of the optical fiber microwave transmission apparatus by Embodiment 4 of this invention. It is a block diagram which shows the optical fiber microwave transmission apparatus by Embodiment 5 of this invention. It is a figure explaining operation | movement of the optical fiber microwave transmission apparatus by Embodiment 5 of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected and demonstrated about the part which is the same or it corresponds. Moreover, in each figure, an electric wire is shown as a continuous line and an optical fiber is shown with a broken line.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing the overall configuration of an optical fiber microwave transmission device (hereinafter abbreviated as “RoF (Radio over Fiber) device”) according to Embodiment 1 of the present invention.
In FIG. 1, 1 is an IF signal source, 2 is a first E / O conversion means which is a first electro-optic conversion means, 3 is a reference signal source, 4 is a phase comparison means, 5 is a loop filter, and 6 is voltage control. VCO which is an oscillator, 7 is a second E / O conversion means which is a second electro-optical conversion means, 8 is an optical circulator, 9 is a third O / E conversion means which is a third photoelectric conversion means, Microwave frequency mixer as second frequency conversion means, 11 is frequency dividing means as third frequency conversion means, 12 is an optical fiber as optical transmission line, and 13 is first photoelectric conversion means. O / E conversion means, 14 a partial light reflection means, 15 a second O / E conversion means which is a second photoelectric conversion means, and 16 a microwave frequency mixer which is a first frequency conversion means.
The RoF device transmits modulated light modulated by a microwave signal input to a transmission source to a remote transmission destination via an optical fiber and outputs the transmission light.

As shown in FIG. 1, the RoF apparatus includes an IF (Intermediate Frequency) signal source 1, a first E / O (electric / optical) conversion means 2, and a reference signal source on the transmission side. 3, phase comparison means 4, loop filter 5, VCO (Voltage Controlled Oscillator) 6, second E / O conversion means 7, optical circulator 8, third O / E (light / electricity) conversion means 9, a microwave frequency mixer 10 and a frequency dividing means 11.
The transmission side includes a first O / E conversion unit 13, a partial light reflection unit 14, a second O / E conversion unit 15, and a microwave frequency mixer 16.
Each of the transmission source and the transmission destination is connected by an optical fiber 12.
In the figure, a solid line indicates an electric signal transmission path, and a broken line indicates an optical signal transmission path (such as an optical fiber).

The operation will be described below.
The electrical signal output from the IF signal source 1 is output as modulated light (for example, intensity-modulated light) modulated by the input electrical signal by the first E / O conversion means 2, and this modulated optical signal passes through the optical fiber 12. And is demodulated into an electrical signal (IF signal) by the first O / E conversion means 13.

On the other hand, the microwave signal output from the VCO 6 (hereinafter referred to as LO (local oscillation) signal) is modulated by the second E / O conversion means 7 to become a modulated optical signal. This modulated optical signal is transmitted to the transmission destination via the optical circulator 8 and the optical fiber 12.
At the transmission destination, a part of the modulated optical signal is reflected by the optical partial reflection unit 14, and the rest is demodulated into an LO signal by the second O / E conversion unit 15. As the partial light reflection means 14, there are various realization means such as a half mirror.
At the transmission destination, the IF signal is frequency-converted (up-converted) by the LO signal in the microwave frequency mixer 16 and output as an RF (Radio Frequency) signal.

Here, if the frequency of the IF signal is f _IF and the frequency of the VCO output signal is f _LO , the output signal from the microwave mixer 16 is an _RF signal having a frequency f _RF = f _IF + f _LO . The microwave mixer 16 also outputs a signal of | f _LO −f _IF | which is a difference frequency, but is omitted here.

  In general, pulse modulation, chirp modulation, and the like are performed in an IF frequency band, which is a relatively low frequency, and are often frequency-converted (up-converted) to an RF frequency band by an LO signal. In the configuration of FIG. 1, the modulated IF signal and the CW LO signal (VCO output signal) are superimposed on the optical signal and transmitted to the transmission destination. Then, after photoelectric conversion at each transmission destination, frequency conversion is performed by the microwave frequency mixer 16 to obtain a modulated signal that is pulse-modulated or chirp-modulated in the RF frequency band.

Here, the IF signal output from the IF signal source 1 is set to sin (2πf _IF · t), and the LO signal output from the VCO 6 is set to sin (2πf _LO · t). The initial phase is 0. Also, let L be the optical length of each optical fiber that transmits modulated light based on IF signals and modulated light based on LO signals, and let c be the speed of light. Then, the IF signal demodulated by the first O / E conversion means 13 is sin (2πf _IF · t + 2πf _IF · L / c), and the LO signal demodulated by the second O / E conversion means 15 is sin ( 2πf _LO · t + 2πf _LO · L / c), and the output signal from the microwave frequency mixer 16 becomes sin {2π (f _IF + f _LO ) · t + 2π (f _IF + f _LO ) · L / c}.

Here, when the optical length of each fiber varies by ΔL, the output RF signal is
sin {2π (f _IF + f _LO ) · t + 2π (f _IF + f _LO ) · (L + ΔL) / c}.
Here, _assuming that f _LO >> f _IF , the component due to f _LO is dominant in the phase fluctuation accompanying the optical path length fluctuation ΔL.
Therefore, the phase fluctuation of the transmission destination can be suppressed by compensating the phase fluctuation due to the optical path on which the LO signal is superimposed.

For example, when the frequency f _{LO of the} LO signal is 10 GHz, the wavelength is 30 mm. Therefore, when the optical path length variation of 1 mm occurs, the phase variation becomes a large value of 12 degrees.
On the other hand, when the frequency f _{IF of the} IF signal is 100 MHz, the wavelength is 3 m. Therefore, even if the optical path length variation of 1 mm occurs, the phase variation is 0.12 degrees, which is an order that can be ignored in many applications.
Thus, the phase fluctuation of the LO signal becomes dominant, so that it is possible to suppress the phase error of the RF signal even if only the phase fluctuation of the LO signal is compensated.

Next, operations related to LO signal phase fluctuation compensation will be described.
As described above, a part of the modulated light transmitted to the transmission destination is reflected by the optical partial reflection means 14, travels back and forth through the optical fiber 12, and passes through the optical circulator 8 to the LO by the third O / E conversion means 9. Demodulated to a signal.
The LO signal output from the third O / E conversion means 9 and the LO signal output from the VCO 6 are combined by the microwave mixer 10, and a microwave signal having a frequency twice that of the LO signal is output.

The frequency of the microwave signal is halved by the frequency dividing means 11 and becomes the same frequency as the LO signal.
A signal having the same frequency as the LO signal is subjected to phase fluctuations caused by fluctuations in the optical path length of the optical fiber 12 serving as a transmission path. For this reason, the phase comparison means 4 compares the phase difference with the signal from the reference signal source 3 (in this case, the same frequency signal as the LO signal) and outputs an electrical signal corresponding to the error signal from the phase comparison means 4. The band is limited by the loop filter 5 and applied to the VCO 6 as a control voltage. Thus, a PLL (Phase Locked Loop) circuit from the transmission source to the transmission destination is configured for the LO signal.

The following is a brief description using equations. The LO signal output from the VCO 6 is set as sin {2πf _LO · t + φ (t)}. A frequency (phase) change component due to the VCO 6 is added as a phase term φ (t) to the above equation. The LO signal output from the VCO 6, converted into modulated light, reciprocated from the transmission destination, and photoelectrically converted by the third O / E conversion means 9,
sin {2πf _LO · t + φ (t) + 2πf _LO · 2 (L + ΔL) / c}
You can. As described above, L and ΔL are the optical path length L of the optical fiber and the variation ΔL. The reason why (L + ΔL) is doubled in the equation is that light reciprocates through the optical fiber 12.

The second microwave mixer 10 outputs the following electrical signal that is the sum component of the photoelectrically converted LO signal and the VCO output signal.
sin {2 (2πf _LO ) · t + 2φ (t) + 2πf _LO · 2 (L + ΔL) / c}
This frequency is halved by the frequency dividing means 11,
sin {2πf _LO · t + φ (t) + 2πf _LO · (L + ΔL) / c}
Is output as

Output signal of reference signal source 3
sin (2πf _LO · t)
Compare the above two formulas,
φ (t) + 2πf _LO · (L + ΔL) / c = 0
Φ (t) = − 2πf _LO · (L + ΔL) / c
So,
The LO signal output from the second O / E conversion means 15 at the delivery destination is
sin {2πf _LO · t + φ (t) + 2πf _LO · (L + ΔL) / c} = sin (2πf _LO · t)
Thus, the signal is the same as the signal from the reference signal source 3 including the phase.
As a result, the influence of fluctuations in the optical transmission path can be reduced and the signal to be transmitted can be stabilized.
Although transmission phase components due to electrical components and optical components are omitted in the transmission path, the above description will not be affected even if these components are omitted.

  As described above, in the optical fiber microwave transmission device, the IF signal that is various modulation signals such as the pulse modulation signal and the frequency chirp signal and the LO signal that is the CW wave are separately transmitted, and the RF signal is transmitted from these at the transmission destination. In addition, the LO signal, which is a CW wave, is controlled by the PLL, so that the transmission including the RF signal can be stabilized and the phase error of the RF signal can be suppressed.

  As described above, according to the first embodiment, in a RoF device that transmits a modulation signal to a distant place, when an IF signal and an LO signal are combined at a distribution destination and an RF signal is output, a round trip is made from the transmission destination. By monitoring the phase fluctuation of the LO signal at the distribution source and controlling the phase (frequency) of the VCO output signal at the transmission direction in the direction to cancel the phase fluctuation at the transmission destination, the LO signal and the A device capable of transmitting an RF signal can be provided.

  In the present embodiment, the configuration for obtaining the sum frequency signal of the LO signal and the IF signal from the microwave mixer 16 has been described. However, the present invention is not limited to this, and the LO signal and the IF signal from the microwave mixer 16 are not limited. It is good also as a structure which obtains a difference frequency signal. In this case, the frequency of the RF signal is the difference frequency between the LO signal and the IF signal, but it is obvious that the effect of the present invention can be obtained in this case as well.

Embodiment 2. FIG.
FIG. 2 is a block diagram showing the configuration of the second RoF apparatus according to Embodiment 2 of the present invention. In the second embodiment shown in FIG. 2, the source and destination fiber transmission line sections in the first embodiment shown in FIG. 1 are shared by optical wavelength multiplexing. Other configurations are the same, and the same reference numerals are given and description thereof is omitted.
In FIG. 2, reference numeral 17 denotes first optical wavelength multiplexing / demultiplexing means, and 18 denotes second optical wavelength multiplexing / demultiplexing means.

  In the second embodiment, the wavelength band of light output from the first E / O conversion unit 2 and the wavelength band of light output from the second E / O conversion unit 7 are set to different bands. For example, the wavelength band of light output from the first E / O conversion means 2 is set to 1.3 μm band, and the wavelength band of light output from the second E / O conversion means 7 is set to 1.5 μm band. .

These lights are combined by the first optical wavelength multiplexing / demultiplexing means 17, transmitted to the far side through the optical fiber 12, and separated according to the wavelength band by the second optical wavelength multiplexing / demultiplexing means 18.
Here, the light output from the first E / O conversion means 2 is transmitted to the first O / E conversion means 13 by the second optical wavelength multiplexing / demultiplexing means 18, and the second E / O conversion is performed. The light output from the means 7 is transmitted to the light partial reflection means 14 and the second O / E conversion means 15 by the second optical wavelength multiplexing / demultiplexing means 18.

A part of the light input to the optical partial reflection unit 14 via the second optical wavelength multiplexing / demultiplexing unit 18 is reflected, and the second optical wavelength multiplexing / demultiplexing unit 18 and the optical fiber 12 are reflected from the opposite direction. The signal is reciprocated to the first optical wavelength multiplexing / demultiplexing means 17 and input to the optical circulator 8.
Other operations are the same as those shown in the first embodiment.

  As described above, according to the second embodiment, the effects shown in the first embodiment can be obtained, and the optical fiber 12 serving as a transmission path can be shared by the IF signal and the LO signal. The number can be reduced.

Embodiment 3 FIG.
FIG. 3 is a block diagram showing the configuration of the RoF device according to Embodiment 3 of the present invention, and FIG. 4 is a schematic diagram showing the frequency arrangement of electrical signals or optical signals at main positions in FIG. In FIG. 3, the same components as those in FIGS. 1 to 2 are denoted by the same reference numerals and description thereof is omitted.
In FIG. 3, 19 is a microwave synthesizing means which is a high frequency synthesizing means, 20 is a second microwave band BPF which is a second microwave bandpass filter, and 21 is a first microwave bandpass filter. The microwave band BPF.

  As shown in FIG. 3, the RoF apparatus includes an IF (intermediate frequency) signal source 1, a reference signal source 3, a phase comparison unit 4, a loop filter 5 on the transmission side, as in FIGS. , A VCO (voltage controlled oscillator) 6, an optical circulator 8, a third O / E converter 9, a microwave frequency mixer 10, a frequency divider 11, and a microwave synthesizer 19, a first E / O conversion means 2 and a second microwave band BPF (band pass filter) 20.

The transmission side is composed of a light partial reflection means 14, a first O / E conversion means 13, and a first microwave band BPF 21.
The transmission source and the transmission destination are connected by an optical fiber 12.

The operation will be described below with reference to the configuration diagram of FIG. 3 and the spectrum (frequency) arrangement diagram of FIG.
Each spectral arrangement in FIG. 4 is an arrangement at the location of the symbol in parentheses in FIG.
When the electric signal (frequency f _IF ) output from the IF signal source 1 and the LO signal (frequency f _LO ) output from the VCO 6 are combined by the microwave combining unit 19, an electric signal including each component is output ( (Refer FIG.4 (c)).

When this is intensity-modulated by the first E / O conversion means 2, sidebands offset by the frequency of each electrical signal on the upper and lower sides of the optical carrier frequency (f _c ) as shown in FIG. _{f c -f LO, f c -f} IF, f c + f IF, f c + f LO) is generated.
These are transmitted by the optical fiber 12, a part is reflected by the light partial reflection means 14 of the transmission destination, and the other is photoelectrically converted by the first O / E conversion means 13.

In the first O / E conversion means 13, a difference frequency component of each input optical signal is generated. Therefore, as shown in FIG. 4 (e), the frequency, f _IF , 2f _IF , f _LO −f _IF , f _LO , f Each component of _LO + f _IF and 2f _LO is output. Here, f _LO + f _IF is the desired RF signal f _RF .
Each electrical signal is selectively output by the first microwave band BPF 21 for the f _RF component (see FIG. 4F).

The first microwave band BPF 21 may selectively output the f _LO -f _IF component instead of selectively outputting the f _LO + f _IF component. In this case, the f _LO -f _IF component becomes a desired RF signal (f _RF ).

On the other hand, the optical signal reflected by the optical partial reflection means 14 travels back and forth through the transmission line optical fiber 12 and is demodulated into an electrical signal by the third O / E conversion means 9 via the optical circulator 8 (FIG. 4 (g )reference).
Since the third O / E conversion means 9 outputs a large number of electrical signals as described above, the f _LO component is selectively output by the second microwave band BPF 20 (see FIG. 4 (h)).
Hereinafter, by adopting the same configuration as in the first and second embodiments, a PLL can be constructed for the LO signal.

  As described above, after combining the IF signal and LO signal, E / O conversion is performed in a lump, and O / E conversion is performed after transmission through the optical fiber, so microwave frequency conversion (mixing) means is not required. Thus, the configuration of the transmission destination can be simplified.

  In the third embodiment, the second microwave band BPF 20 is provided between the third O / E conversion means 9 and the second microwave band frequency mixer 10, but the frequency extracted by the BPF is changed. Thus, the BPF 20 may be installed at the subsequent stage of the second microwave frequency mixer 10 or the subsequent stage of the frequency dividing means 11.

Embodiment 4 FIG.
FIG. 5 is a block diagram showing the configuration of the RoF apparatus according to Embodiment 4 of the present invention, and FIG. 6 is a schematic diagram showing the frequency arrangement of electrical signals or optical signals at main positions in FIG. In FIG. 5, the same components as those in FIG. 3 are denoted by the same reference numerals, and the description thereof is omitted.
In FIG. 5, 22 is an optical filter which is an optical band rejection filter.

In the fourth embodiment, the light output from the first E / O conversion means 13 between the light partial reflection means 14 and the first O / E conversion means 13 is different from the third embodiment. An optical filter 22 for removing the carrier frequency (f _c ) component is provided.

Thereby, the spectrum after passing through the optical filter 22 is as shown in FIG.
When this is photoelectrically converted by the first O / E conversion means 13, the difference frequency component of each spectrum is output as an electric signal, and as shown in FIG. 6 (e), 2f _IF , f _LO -f _IF , f Each component of _LO + f _IF (= f _RF ) and 2f _LO is output.
As in the third embodiment, f _RF (= f _LO + f _IF ) is extracted from these spectra (FIG. 6 (f)) and output.

In the fourth embodiment, since it is suppressed optical carrier frequency f _c component by the optical filter 22, it is possible to reduce unnecessary electrical signal after photoelectric conversion. Therefore, the performance of the first microwave band BPF 21 such as unnecessary wave suppression ratio and bandwidth can be relaxed.

Embodiment 5. FIG.
FIG. 7 is a block diagram showing the configuration of the RoF apparatus according to Embodiment 5 of the present invention, and FIG. 8 is a schematic diagram showing the frequency arrangement of electrical signals or optical signals at main positions in FIG. In FIG. 7, the same components as those in FIG. 3 or FIG.
In FIG. 7, reference numeral 23 denotes a frequency multiplier.

  In the fifth embodiment, a Mach-Zehnder (hereinafter abbreviated as MZ) type optical modulator is used as the first E / O conversion means 2 in comparison with the third embodiment, and the VCO 6 and the second microwave frequency are used. A frequency multiplier 23 is provided between the mixers 10.

  Normally, when an optical signal is modulated by a microwave or the like using an MZ modulator to obtain modulated light, the level of the output spectrum of the modulated light can be changed by a bias voltage applied to the MZ modulator. When performing general intensity modulation, the bias voltage is set at a point where the transmittance of the MZ modulator is 50% of the maximum value. In that case, the frequency spectrum of the light output from the MZ modulator has a distribution similar to that shown in FIG.

In the fifth embodiment, the bias voltage of the LN modulator is set at a point where the transmittance is minimized.
Such modulation at the bias point is generally known as CS-DSB (Carrier Suppression-Double Side Band) modulation.

In the fifth embodiment, the bias point of the MZ modulator is set to the minimum transmittance point.
At this time, the spectrum of the optical signal output MZ modulator as shown in FIG. 8 (d), the becomes that f _c component is suppressed.
At this time, the electrical signals output from the first O / E conversion unit 13 and the third O / E conversion unit 9 are as shown in FIGS. 8E and 8G, and FIG. In contrast to (g), unnecessary signals (for example, f _IF , f _LO, etc.) are suppressed.
Therefore, it is possible to relax the requirements for the first microwave BPF21 for retrieving the desired RF signal _{(f RF = f LO + f} IF).

The modulated light reflected at the transmission destination and reciprocated through the optical fiber 12 is converted into an electrical signal by the third O / E conversion means 9 via the optical circulator 8 (see FIG. 8G).
The electrical signal output from the third O / E conversion means 9 is extracted by the second microwave band BPF 20 at a frequency of 2f _LO .
On the other hand, the frequency of the LO signal output from the VCO 6 is doubled by the multiplier 23. After that, as in the first to fourth embodiments, the second microwave frequency mixer 10 converts the frequency of the electrical signal output from the second microwave band BPF 20 by the LO signal multiplied by two.

  The frequency-converted electric signal is converted by the frequency dividing means 11 into the same frequency as the reference signal frequency. If the reference signal frequency from the reference signal source 3 is the same as the LO signal, the frequency dividing means 11 may divide the frequency of the electric signal by four. The phase comparison means 4 compares the phase (frequency) of the reference signal from the reference signal source 3 with the frequency-divided electrical signal, and returns an error signal to the VCO 6 via the loop filter 5. Thereby, a phase synchronization circuit is configured.

Here, the electrical signal (frequency 2f _LO ) output from the second microwave band BPF 20 is
sin {2π (2f _LO ) · t + 2φ (t) + 2π (2f _LO ) · 2 (L + ΔL) / c}
You can. As described above, L is the optical path length of the optical fiber, and ΔL is the amount of variation.
Since the frequency of this signal is twice that of _fLO , φ (t) is also doubled.
Next, the LO signal sin {2πf _LO · t + φ (t)} output from the VCO 6 is converted to sin {2π (2f _LO ) · t + 2φ (t)} which is a signal whose frequency is doubled by the frequency multiplying means 23. Converted.

When these are frequency-converted by the second microwave mixer 10, an electric signal having a sum component represented by the following equation is output.
sin {2π (4f _LO ) · t + 4φ (t) + 2π (2f _LO ) · 2 (L + ΔL) / c}
By making this frequency 1/4 times by the frequency dividing means,
sin {2πf _LO · t + φ (t) + 2πf _LO · (L + ΔL) / c}
As in the first to fourth embodiments, it can be compared with the reference signal from the reference signal source 3, and a PLL circuit can be constructed.

Although the signal having the frequency 2f _LO is extracted by the second microwave band BPF 20 here, it goes without saying that the signal may be extracted by using an HPF (High Pass Filter). Even HPF can be regarded as a broad bandpass filter.

  In addition, the frequency multiplier 23 in FIG. 7 may not be provided, and a frequency 2 frequency divider may be provided between the second microwave band BPF 20 and the second microwave mixer 10 instead. In this case, the frequency dividing means 11 may be a frequency divider instead of a frequency divider.

  As described above, in the fifth embodiment, a Mach-Zehnder type optical modulator is used for the first E / O conversion means 2 and carrier suppression modulation is performed with the bias point of the MZ modulator as the bottom point. Since the component in the vicinity of the desired signal frequency is suppressed during photoelectric conversion, the performance with respect to the microwave filter can be relaxed.

Embodiment 6 FIG.
In the above second to fifth embodiments, the configuration for stabilizing the phase of the LO signal has been shown. However, the optical path length variation is converted from the output signal from the phase comparison means 4, and the offset phase of the IF signal is controlled. May be.

  In the second to fifth embodiments, since the optical fiber 12 in the transmission path is shared by the IF signal and the LO signal, the optical path length variation is the same for the IF signal. Therefore, based on the frequency difference (frequency ratio), an error signal of the LO signal is used to estimate the amount of phase fluctuation with respect to the IF signal, and using this, the phase of the IF signal is also compensated for higher phase stability. It is possible to transmit an RF signal at a degree.

  In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

1 IF signal source, 2 1st E / O conversion means, 3 reference signal source, 4 phase comparison means, 5 loop filter, 6 VCO, 7 2nd E / O conversion means, 8 optical circulator, 9 3rd O / E conversion means, 10 microwave frequency mixer, 11 frequency dividing means, 12 optical fiber, 13 first O / E conversion means, 14 optical partial reflection means, 15 second O / E conversion means, 16 microwave Frequency mixer, 17 First optical wavelength multiplexing / demultiplexing means, 18 Second optical wavelength multiplexing / demultiplexing means, 19 Microwave synthesis means, 20 Second microwave band BPF, 21 First microwave band BPF, 22 Optical filter, 23 frequency multiplier

Claims (4)

An IF signal source that outputs a first high-frequency signal that is an IF signal;
First electro-optic conversion means for outputting first modulated light intensity-modulated with the first high-frequency signal;
A first optical transmission line for transmitting the first modulated light;
First photoelectric conversion means for demodulating the first modulated light transmitted through the first optical transmission path into a high-frequency signal;
A voltage controlled oscillator that outputs a second high-frequency signal having a predetermined frequency according to the input voltage;
Second electro-optic conversion means for outputting second modulated light intensity-modulated with the second high-frequency signal;
A second optical transmission line that is arranged in parallel with the first optical transmission line and transmits the second modulated light;
A partial light reflecting means for reflecting a part of the second modulated light transmitted through the second optical transmission line and passing the other part;
Second photoelectric conversion means for demodulating the second modulated light that has passed through the light partial reflection means into a high-frequency signal;
A first high-frequency signal output from the first photoelectric conversion means and a high-frequency signal output from the second photoelectric conversion means are input, and a first frequency signal or a difference frequency signal of these high-frequency signals is output. Frequency conversion means;
The second modulated light output from the second electro-optic conversion means is output to the second optical transmission line, and is reflected by the light partial reflection means and reciprocates through the second optical transmission line. An optical circulator that outputs the modulated light of 2 by switching the optical path in a direction different from that of the second electro-optic conversion means;
Third photoelectric conversion means for demodulating the second modulated light whose optical path is switched by the optical circulator into a third high-frequency signal;
Second frequency conversion means for inputting the third high-frequency signal and the second high-frequency signal output from the voltage-controlled oscillator and outputting a sum frequency signal of these high-frequency signals;
A reference signal source for outputting a reference signal having a reference frequency;
Third frequency converting means for converting the frequency of the sum frequency signal output from the second frequency converting means into a high frequency signal in the same frequency band as the reference frequency;
The high-frequency signal frequency-converted by the third frequency conversion means and the reference signal are compared, and an error signal having a voltage corresponding to the difference frequency component is output, and the error signal is input to the voltage-controlled oscillator. Phase comparison means for controlling the frequency of the second high-frequency signal;
An optical fiber microwave transmission device comprising: An IF signal source that outputs a first high-frequency signal that is an IF signal;
First electro-optic conversion means for outputting first modulated light intensity-modulated with the first high-frequency signal;
A voltage controlled oscillator that outputs a second high-frequency signal having a predetermined frequency according to the input voltage;
Second electro-optic conversion means for outputting a second modulated light that is intensity-modulated by the second high-frequency signal and has a wavelength band different from that of the first modulated light;
First optical wavelength multiplexing / demultiplexing means for multiplexing and demultiplexing the first modulated light and the second modulated light according to these wavelength bands;
An optical transmission line for transmitting the first modulated light and the second modulated light multiplexed by the first optical wavelength multiplexing / demultiplexing means;
Second optical wavelength multiplexing / demultiplexing means for demultiplexing and multiplexing the first modulated light and the second modulated light transmitted through the optical transmission line according to these wavelength bands;
First photoelectric conversion means for demodulating the first modulated light demultiplexed by the second optical wavelength multiplexing / demultiplexing means into a high-frequency signal;
A light partial reflecting means for reflecting a part of the second modulated light demultiplexed by the second light wavelength multiplexing / demultiplexing means and allowing the other part to pass;
Second photoelectric conversion means for demodulating the second modulated light that has passed through the light partial reflection means into a high-frequency signal;
A first high-frequency signal output from the first photoelectric conversion means and a high-frequency signal output from the second photoelectric conversion means are input, and a first frequency signal or a difference frequency signal of these high-frequency signals is output. Frequency conversion means;
The second modulated light output from the second electro-optic conversion means is output to the first optical wavelength multiplexing / demultiplexing means, and is reflected by the light partial reflection means to reciprocate the optical transmission path. An optical circulator that outputs the modulated light by switching the optical path in a direction different from that of the second electro-optic conversion means,
Third photoelectric conversion means for demodulating the second modulated light whose optical path is switched by the optical circulator into a third high-frequency signal;
Second frequency conversion means for inputting the third high-frequency signal and the second high-frequency signal output from the voltage-controlled oscillator and outputting a sum frequency signal of these high-frequency signals;
A reference signal source for outputting a reference signal having a reference frequency;
Third frequency converting means for converting the frequency of the sum frequency signal output from the second frequency converting means into a high frequency signal in the same frequency band as the reference frequency;
The high-frequency signal frequency-converted by the third frequency conversion means and the reference signal are compared, and an error signal having a voltage corresponding to the difference frequency component is output, and the error signal is input to the voltage-controlled oscillator. Phase comparison means for controlling the frequency of the second high-frequency signal;
An optical fiber microwave transmission device comprising: An IF signal source that outputs a first high-frequency signal that is an IF signal;
A voltage controlled oscillator that outputs a second high-frequency signal having a predetermined frequency according to the input voltage;
High-frequency synthesis means for synthesizing the first high-frequency signal and the second high-frequency signal;
First electro-optic conversion means for outputting modulated light intensity-modulated with a high-frequency signal synthesized by the high-frequency synthesis means;
An optical transmission line for transmitting the modulated light output by the first electro-optic conversion means;
A partial light reflecting means for reflecting a part of the modulated light transmitted through the optical transmission path and passing the other part;
First photoelectric conversion means for demodulating the modulated light that has passed through the light partial reflection means into a high-frequency signal;
A first microwave band-pass filter that extracts a high-frequency signal in a desired frequency band from the high-frequency signal output from the first photoelectric conversion means;
The modulated light output from the first electro-optic conversion means is output to the optical transmission path, and the modulated light reflected by the optical partial reflection means and reciprocated through the optical transmission path is converted into the first electro-optic conversion. An optical circulator for switching and outputting the optical path in a direction different from the means;
Third photoelectric conversion means for demodulating the modulated light whose optical path is switched by the optical circulator into a high-frequency signal;
A second microwave band pass filter for extracting a high frequency signal of a desired frequency band from the high frequency signal demodulated by the third photoelectric conversion means;
Second frequency converting means for inputting the high-frequency signal output from the second microwave bandpass filter and the second high-frequency signal output from the voltage-controlled oscillator, and outputting a sum frequency signal of these high-frequency signals. When,
A reference signal source for outputting a reference signal having a reference frequency;
Third frequency converting means for converting the frequency of the sum frequency signal output from the second frequency converting means into a high frequency signal in the same frequency band as the reference frequency;
The high-frequency signal frequency-converted by the third frequency conversion means and the reference signal are compared, and an error signal having a voltage corresponding to the difference frequency component is output, and the error signal is input to the voltage-controlled oscillator. Phase comparison means for controlling the frequency of the second high-frequency signal;
An optical fiber microwave transmission device comprising:   4. The optical fiber microwave transmission device according to claim 3, further comprising an optical band rejection filter provided in a stage preceding the first photoelectric conversion means.

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