JPH10215220A - Radio wave reception method and radio wave reception system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate fading in a television radio wave reception system, especially resulting from fluctuation in a characteristic of a transmission optical fiber. SOLUTION: In the radio wave reception system, a reception end provided with an antenna 2 that receives a radio wave and an optical intensity modulator 3 to which a voltage corresponding to a strength of a radio wave received by the antenna 2 is applied, and a transmission end provided with a light source and a photodetector 7 are interconnected by a single mode optical fiber. In this case, the transmission end is provided with a polarized light scrambler 12 and with a control means 17 that detects a polarized light state and controls the light polarized state depending on the detected light polarized state so that the light polarized state of the light source is changed periodically. Thus, a light source without polarization of light is configured at a low cost independently of the environmental temperature and the stable and inexpensive television radio wave reception system is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television signal receiving system, and more particularly to an improvement for eliminating a fading phenomenon caused by fluctuations of a transmission optical fiber. The present invention relates to an improvement in a radio wave receiving system using an optical fiber in the field of mobile communication, not limited to a television radio wave receiving system.

[0002]

2. Description of the Related Art In a television relay broadcasting station where a transmitting station and a receiving station are separated from each other, a transmission system using an optical cable has been proposed instead of a conventional transmission system using a coaxial cable.

FIG. 11 is a block diagram showing the basic configuration of such a transmission system. Reference numeral 1 denotes a television signal, which is received by the antenna 2 and outputs a voltage corresponding to the intensity of the television signal. Reference numeral 3 denotes a light intensity modulator, and an antenna 2
Is applied, and the intensity of light is changed in proportion to the applied voltage. The receiving station 4 including the antenna 2 and the light intensity modulator 3 and the transmitting station 5 including the light sources 6a and 6b, the photodetector 7, and the amplifier 8 are located at a distance of several kilometers, and optical fibers 9 and 10 are located between them. Connected by In the light intensity modulator 3,
The television wave converted into a change in light is converted back to an electric signal by the photodetector 7 and output 11 as a television signal via the amplifier 8.

In such a system, the light intensity modulator 3
A ferroelectric light intensity modulator using the electro-optic effect is used. Specifically, an optical intensity modulator having a Mach-Zehnder optical waveguide formed on a lithium niobate substrate is used. The modulator adopts a polarization input type configuration using an r33 coefficient from the viewpoint of modulation efficiency, and the degree of modulation of light output changes according to the polarization state of incident light.

On the other hand, a light source 6 for outputting linearly polarized light and the modulator 3 are connected by an optical fiber 9. If a polarization plane maintaining type optical fiber (hereinafter, referred to as “PMF”) is used as the optical fiber 9, the polarized light incident on the light intensity modulator 3 is:
Although it can be kept almost constant, PMF is very expensive and has poor cost performance when used over several kilometers. On the other hand, when a single mode optical fiber (hereinafter, referred to as “SMF”) is used, it is extremely advantageous in terms of cost, but due to mechanical and thermal fluctuations of the optical fiber, the polarization of the optical fiber output is reduced. The state fluctuates, causing a so-called fading phenomenon in which the television signal fluctuates.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to prevent the influence of optical fiber fluctuation when the SMF is used.
In the conventional example, a so-called non-polarized light source is used, in which two light sources 6a and 6b are used as light sources and polarizations of respective outputs are orthogonal to each other. In this case, there are problems that it is necessary to maintain the orthogonality of the polarization directions of both light sources, that the light outputs of both light sources need to be matched, that the utilization efficiency is half of the total light output, and that heat is generated from the light sources. In addition, there are drawbacks such as the fact that the light source is very expensive and the cost performance is poor.

[0007]

According to the present invention, there is provided a receiving end including an antenna for receiving a radio wave, a light intensity modulator to which a voltage corresponding to the intensity of the radio wave received by the antenna is applied, a light source and a light source. In a radio wave receiving method in which a transmitter and a detector having a detector are connected by a single mode optical fiber, at the transmitter, the polarization of the light source is scrambled so that the polarization state of the light source light periodically changes. The polarization state is detected, and the polarization state is controlled according to the detected polarization state. The present invention has a receiving end including an antenna for receiving a radio wave and a light intensity modulator to which a voltage corresponding to the intensity of the radio wave received by the antenna is applied.
In a radio wave reception system in which a light source and a transmission end including a photodetector are connected by a single-mode optical fiber, a transmission end, a polarization scrambler, and a polarization state of the light source light are periodically changed. And a control means for detecting the polarization state and controlling the polarization state in accordance with the detected polarization state.

According to the present invention, in a radio wave receiving system, a polarization scrambler and a light source having a predetermined polarization direction are incident on the polarization scrambler at a transmitting end, and the frequency of the received radio wave is twice or more. Polarization driving means for driving the polarization scrambler at a frequency f1. The present invention provides, in a radio wave receiving system, a branching unit that transmits an optical output of the polarization scrambler to a receiving end and branches a part of the optical output to a transmitting end, and a branched light branched by the branching unit. And polarization control means for controlling the polarization driving means in accordance with the polarization state of the branched light detected by the polarization monitoring means. According to the present invention, in the radio wave receiving system, the polarization scrambler is an optical waveguide type modulator in which an optical waveguide is formed on a lithium niobate substrate. According to the present invention, in the radio wave receiving system, the polarization monitoring means includes a driving frequency f1 of the polarization scrambler.
A piezoelectric element driving means for driving a piezoelectric element with an oscillator having a frequency f2 lower than that of a single mode optical fiber for changing a polarization state of light branched from the branching means in accordance with a driving frequency of the piezoelectric element; A polarizer that transmits only one-way polarized light output from the optical fiber, a second photodetector that photoelectrically converts the light transmitted from the polarizer, and an output from the second photodetector. Filter means for passing only the frequency f2 component. According to the present invention, in the radio wave receiving system, the polarization control means receives a signal of the frequency f2 from the polarization monitoring means and detects a signal amplitude of the frequency f2, and a DC output from the detection means and a predetermined value. And comparing means for comparing the reference value with the reference value and outputting a DC voltage corresponding to the difference. According to the present invention, in the radio wave receiving system, the polarization driving means includes a frequency f
1 oscillator and a variable gain amplifier that amplifies the output voltage of the oscillator and receives the comparison output from the polarization control means to vary the amplification.
The present invention is characterized in that the radio wave receiving system comprises an averaging means for averaging the output from the photodetector. The present invention provides, in a radio wave receiving system, an optical waveguide branch formed by an optical waveguide and a branched optical waveguide provided on a lithium niobate substrate on which the polarization scrambler is formed; A polarization controller driven at a frequency f2.

According to the present invention, as described above, only one light source is required, no fine adjustment is required unlike the prior art, the influence of heat generation is reduced, and the cost can be reduced. Was.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, portions having the same function are denoted by the same reference numerals, the dotted line portion is constituted by an optical fiber, and the solid line portion is constituted by a conductor.

FIG. 1 is a block diagram showing an embodiment of the present invention. The light source 6 is an LD-pumped YAG laser, a linearly polarized laser having a relative noise intensity RIN value of −170 dBm / Hz and a low noise wavelength of 1.3 μm. The light from the light source 6 is polarized by the PMF 13 and is polarized by the polarization scrambler 1.
2 is input with a predetermined polarization. Polarization scrambler 12
Is an optical waveguide type modulator made of lithium niobate, and the polarization state of the optical output changes according to the driving voltage 38 from the polarization driving unit 18.

FIG. 9 is an explanatory view of a polarization scrambler. More specifically, FIG. 9 (a) shows the time change of the polarization state of the scrambler and its output light, and FIG.
(B) is a figure which shows the time change of the polarization on the Boincare sphere 20. In the polarization scrambler 12, four different electro-optic coefficients r13 and r33 are set for each direction.
When light is incident in the polarization direction of 5 ° and an AC voltage having a frequency f1 is applied in the z-axis direction, the polarization state of the output changes as shown by a locus of a certain closed curve 21 on the Boincare sphere 20 at the frequency f1. I do. In FIG. 9, the closed curve 21 corresponds to the state P,
A locus that changes to L, Q, and R and returns to P is shown. Which closed curve is taken is determined by the applied AC voltage value. The average degree of polarization during the period 1 / f1 is evaluated as a DOP value.

The DOP value is defined by the following equation (1). DOP = (<S1> ² + <S2> ² + <S3> ² ) ^1/2 / S0 (1) where <S1> is a time average value of the horizontal polarization component, and <S
2> is the time average of the 45 ° polarization component, <S3> is the time average of the counterclockwise circular polarization component, and S0 is the incident light intensity. The smaller the DOP value, the light is more unpolarized.

FIG. 8 is a diagram showing the applied voltage dependence of the DOP value. Applied voltage, DOP values according deviates from the optimum value (V _S) increases.

The characteristics of the polarization scrambler 12 have a temperature dependency, and it is necessary to adjust the applied voltage depending on the ambient temperature. The polarization monitor 16 and the polarization controller 1 of FIG.
7. The light branching unit 15 and the polarization driving unit 18 are feedback control units for automatically applying the optimum temperature-dependent voltage, and branch a part of the output light from the polarization scrambler 12, The polarization state is detected, a control voltage corresponding to the polarization state is output, and the applied voltage 38 to the polarization scrambler 12 is adjusted so that the polarization state is always kept in the non-polarization state.

The other light from the light splitting means 15 is SMF
9 and is sent to the light intensity modulator 3 of the receiving station 4. In the light intensity modulator 3, the television wave 1 received by the antenna 2 is added as a change in light intensity as an optical signal, and SM
The light is returned to the photodetector 7 provided at the transmitting station 5 via F10. The output from the photodetector 7 passes the TV radio signal,
Low-pass filter 19 that does not pass higher frequency components
And only the television signal is output as the output 11.

The effect of the polarization scrambler will be described with reference to FIG.
explain in detail. For convenience of explanation, the television radio signal is a digital signal (FIG. 7A) having a period T3 (T3 (= 1 / f3) >> T1 (= 1 / f1), and 100% light intensity modulation is obtained. 7 (b), (c) and (d) show
The DOP when the DOP value is optimum and the optical fiber 9 fluctuates (generally f4 << f3) when the DOP value is optimum and the fiber 9 has no fluctuation
The output from the photodetector 7 when the value deviates from the optimum value and the optical fiber 9 fluctuates is shown. The vertical line indicates the scramble frequency f1, and the envelope 24 indicates the peak-to-peak (pp) transmission. When the DOP value deviates from the optimum value as shown in FIG. 7D, the symmetry of the signal envelope is lost, and as a result, the average value fluctuates. When these are passed through a low-pass filter 19 that cuts the f1 component, the waveforms are as shown in FIGS. 7 (b '), (c') and (d '). FIGS. 7 (b ') and (c') show that the applied signal 22
Waveforms 25 and 26, which indicate that the influence of the fluctuation of the optical fiber 9 has been eliminated. But D
If the OP value deviates from the optimum value, a fluctuation component is superimposed on the television signal as shown in FIG. 7 (d '). As described above, since only a single polarization component is extracted from the polarization component scrambled at the frequency f1, the extracted light (that is, the output of the light intensity modulator 3) includes the intensity modulation component at the frequency f1. The average value of the f1 component becomes the signal component. That is, in this system, the light utilization rate is 5
0% (-3 dB).

Next, a method of controlling the polarization scrambler in the embodiment of the present invention will be described in detail. FIG.
FIG. 2 is a block diagram illustrating a portion related to polarization scrambler control of FIG. 1 in detail. A part (about 1%) of the output from the polarization scrambler 12 is split by the optical splitter 15 and enters the polarization monitor 16. The polarization monitoring means 16 includes a piezoelectric element 27 for adding a polarization fluctuation corresponding to the optical fiber fluctuation of the transmission path, and the piezoelectric element 27 has an SMF3 on its outer periphery.
0 is wound, and distortion occurs in the radial direction in accordance with the applied voltage of the low frequency f2, thereby changing the refractive index in the radial direction to the SMF 30 and changing the polarization state of the output light from the SMF 30. is there. An oscillator 28 having a frequency f2 lower than that of a television signal and an amplifier 2 for driving the piezoelectric element 27;
9, a polarizer 31 that transmits only polarized light in a certain direction out of the output light from the SMF 30 and sends it to the second photodetector, a second photodetector 32, and a second photodetector 32. Low-pass filter 33 that transmits only the f2 component of the output
And

FIG. 4 shows how the output of the filter 33 changes in accordance with the voltage applied to the polarization scrambler 12. The f2 component is minimized at the optimum applied voltage (Vs), and the f2 component increases as the voltage deviates from Vs.
The fluctuation added to the SMF 30 via the piezoelectric element 27 is
This is equivalent to the fluctuation of the transmission path 9, and a small frequency f2 component means that the polarization scrambler is functioning effectively. That is, the voltage 38 applied to the polarization scrambler 12 may be controlled so that the output of the filter 33 is minimized.

Further, in FIG. 2, in FIG.
Reference numeral 8 denotes an oscillator 34 having a frequency f1, a thermostat 35 for stabilizing the output of the oscillator 34, and a variable gain in which the output of the oscillator 34 is amplified and the gain is changed according to the size of the control means 37. And an amplifier 36. Frequency f1
Of which the amplitude is changed according to the control means 37
Is output and applied to the polarization scrambler 12.

The voltage of the control means 37 is
The polarization control unit 17 receives the output from the filter 33, performs a square detection, a detector 39, rectifies the detection output, a reference value 42, a reference value 42, And a comparison circuit 41 that compares the peak hold value with the peak hold value and outputs a DC voltage corresponding to the difference as a control signal 37. The reference value 42 means a DOP value allowed by the system, and a DOP value equal to or less than the reference value.
The polarization scrambler 12 is driven so that a value is always obtained.

The filter 33, the detection circuit 39, the peak hold circuit 40, and the like are ordinary circuits, and may be, for example, an integrator, a synchronous detection circuit, and an averaging circuit, respectively, which have the same functions. Of course.

As described above, according to the present invention, the polarized light from the light source is polarization-scrambled by the polarization scrambler 12, and the light source can be depolarized. Further, the polarization state is monitored by using a part of the output light from the polarization scrambler 12, and feedback control is performed so that the DOP value is always equal to or less than the reference value, so that the non-polarization state is stable regardless of the operating temperature. A light source can be realized.

FIG. 3 shows another example of a stress applying method for monitoring fluctuation. This is a so-called squeezer-type polarization modulator that presses the SMF 30 against the metal 31 by the piezoelectric element 27 ′, and can be used as a substitute for the piezoelectric element 27 in FIG.

The system according to the present invention has great advantages, such as being extremely advantageous in terms of cost and requiring no fine adjustment as in the conventional example.

FIG. 5 shows another embodiment of the present invention. This is a method of superimposing the polarization modulation of the frequency f2 for monitoring the polarization state on the polarization scrambler 12 as a pilot signal. The superimposition circuit 42 adds the signals of the frequencies f1 and f2.

The split light is directly transmitted to the polarizer 31.
, And enters the photodetector 32, and the filter 33 extracts only the frequency f2 component. The gain of the variable gain amplifier 36 is controlled via the polarization controller 17 so as to minimize the f2 component. In this case, the return light from the light intensity modulator 3 also includes a component of the frequency f2,
Here, as the averaging circuit 19, a bandpass filter 43 that transmits only the frequency components of the television wave without passing the frequencies f1 and f2 components is used.

FIG. 6 is a diagram showing a third embodiment which is a modification of the polarization monitoring means. On a lithium niobate substrate 44, a first polarization scrambler 45, an optical waveguide branch 46, and a second polarization scrambler 47 in the branched optical waveguide are provided. The first polarization scrambler is driven at frequency f1, and the second polarization scrambler is driven at frequency f2. As above, the frequency f2
The driving voltage of the frequency f1 is feedback-controlled so that the component is minimized.

FIG. 10 is a diagram showing a fourth embodiment.
In this embodiment, the light intensity modulator 48 is a reflection type modulator, and the transmitting station 5 and the receiving station 4 are connected to a single SMF.
Connected at 49. The optical signal on which the television signal from the optical intensity modulator 48 is superimposed propagates through the SMF 49 in the reverse direction and is separated in the optical circulator 50. The treatment after the separation, the polarization scrambler function and the control are the same as those of the embodiment shown in FIGS. With this configuration, the same effect as that of the embodiment can be obtained.

[0030]

The effect of depolarizing a light source using a polarization scrambler will be described with reference to FIGS. FIG. 12 is a diagram showing a detected waveform of the television signal output 11 in the embodiment of the present invention using the polarization scrambler. 100 denotes a carrier, 101 and 102 denote envelopes of the carrier 100 which is a video signal, and 103 denotes a carrier.
Average level, height 1 proportional to the amount of light to detector 7
04. Reference numeral 105 denotes a video amplitude. As shown in the figure, the average level is 1 due to the effect of the polarization scrambler.
03 maintains a constant level. FIG. 13 shows FIGS.
In ten embodiments of the present invention, the polarization scrambler 12
FIG. 7 is a diagram showing a waveform of a television signal output 11 when the function of FIG. Average level 1 due to fiber fluctuation
06 varies between a maximum point 107 and a minimum point 109.
The height 108 at the maximum point 107 is twice the height 104 in FIG. As shown in the figure, the video amplitude 105 changes according to the height of the average level 106, and the amplitude becomes 0 at the minimum point 109. That is, a fading phenomenon occurs in which the image amplitude changes due to the fluctuation of the fiber, which is a temporally slow fluctuation. However, if the light source is depolarized by using a polarization scrambler as described in FIG. 12, the signal amplitude is reduced to １ ／, but a constant average level 103 which is not affected by the fiber fluctuation is obtained. Can be

As described above in detail, according to the present invention, a non-polarized light source that does not depend on the environmental temperature can be configured at a low cost, and a stable and inexpensive TV radio wave receiving system can be realized.

In each of the embodiments described above, the television radio wave receiving system has been described. However, the same configuration can be applied to the mobile communication field, and the same effect can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of a radio wave receiving system according to the present invention.

FIG. 2 is a block diagram of a feedback unit of the first embodiment of the radio wave receiving system according to the present invention.

FIG. 3 is a squeezer-type polarization modulator.

FIG. 4 is a diagram showing dependence of an output of a filter 33 on an applied voltage 38.

FIG. 5 is a diagram showing a second embodiment.

FIG. 6 is a diagram showing a third embodiment.

FIG. 7 is a diagram illustrating a function of a polarization scrambler.

FIG. 8 is a diagram showing a relationship between a DOP value and an applied voltage.

FIG. 9 is a diagram showing a state of output polarization of a polarization scrambler.

FIG. 10 is a diagram showing a fourth embodiment.

FIG. 11 is a diagram showing a principle configuration of a conventional radio wave receiving system.

FIG. 12 is a television signal output waveform based on the system of the present invention.

FIG. 13 is a television signal output waveform of a system without a polarization scrambler.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Television wave 2 Antenna 3,48 Light intensity modulator 9,10,49 SMF 6,6 'YAG laser 7,32 Photodetector 12,44 Polarization scrambler 15 Optical branching 16 Polarization monitoring means 17 Polarization control means 18 Polarization drive Means 27, 27 'Piezoelectric element 33, 43 Filter

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masahisa Raw Rock 2-2-1 Jinnan, Shibuya-ku, Tokyo Inside the Japan Broadcasting Corporation Broadcasting Center (72) Inventor Hiroki Fujio 2-2-2 Jinnan, Shibuya-ku, Tokyo No. 1 Inside the Japan Broadcasting Corporation Broadcasting Center (72) Inventor Tadashi Ishikawa 2-2-1 Jinnan, Shibuya-ku, Tokyo Inside the Japan Broadcasting Corporation Broadcasting Center

Claims (10)

[Claims] 1. A receiving end comprising an antenna for receiving a radio wave, a light intensity modulator to which a voltage corresponding to the intensity of the radio wave received by the antenna is applied, and a transmitting end comprising a light source and a photodetector. And a single-mode optical fiber, wherein at the transmitting end, the polarization of the light source is scrambled, and the polarization state of the light source light is periodically changed so that the polarization state is detected. A radio wave receiving method comprising: controlling a polarization state according to the polarization state. 2. A receiving end including an antenna for receiving a radio wave, a light intensity modulator to which a voltage corresponding to the intensity of the radio wave received by the antenna is applied, and a transmitting end including a light source and a photodetector. And a single-mode optical fiber, in a transmission end, a polarization scrambler, and a polarization state is detected so that the polarization state of the light source light changes periodically. And a control means for controlling a polarization state according to the following. 3. A receiving end including an antenna for receiving a radio wave, a light intensity modulator to which a voltage corresponding to the intensity of the radio wave received by the antenna is applied, and a transmitting end including a light source and a photodetector. In a radio wave receiving system connected by a single mode optical fiber, at a transmission end, a polarization scrambler, the light source light is set to a predetermined polarization direction, and is incident on the polarization scrambler, and the frequency of the received radio wave is A polarization driving unit for driving the polarization scrambler at a frequency f1 that is twice or more the frequency f1. 4. A branching unit that transmits an optical output of the polarization scrambler to a receiving end and branches a part of the optical output to a transmitting end; and determines a polarization state of the branched light branched by the branching unit. 4. The radio wave according to claim 3, further comprising: a polarization monitoring unit for detecting, and a polarization control unit for controlling the polarization driving unit in accordance with a polarization state of the split light detected by the polarization monitoring unit. Receiving system. 5. The radio wave receiving system according to claim 2, wherein the polarization scrambler is an optical waveguide type modulator having an optical waveguide formed on a lithium niobate substrate. . 6. The polarization monitoring means comprises: an oscillator having a frequency f2 lower than a driving frequency f1 of the polarization scrambler; a piezoelectric element driving means for driving a piezoelectric element; and a polarization state of the branched light from the branching means. A single-mode optical fiber that changes according to the driving frequency of the piezoelectric element, a polarizer that transmits only one-directional polarization of output light from the single-mode optical fiber, and a second that photoelectrically converts light transmitted from the polarizer. 6. The radio wave receiving system according to claim 4, further comprising: two photodetectors; and filter means for passing only a frequency f2 component of an output from said second photodetector. 7. The polarization control means receives the signal of the frequency f2 from the polarization monitoring means, detects the signal amplitude of the frequency f2, and detects a DC output from the detection means and a predetermined reference value. 7. A radio wave receiving system according to claim 4, further comprising: comparing means for comparing DC power and outputting a DC voltage corresponding to the difference. 8. The polarization driving means includes: an oscillator having a frequency f1; and a variable gain amplifier that amplifies an output voltage of the oscillator and changes the degree of amplification by receiving a comparison output from the polarization control means. The radio wave receiving system according to any one of claims 3 to 7, wherein the radio wave receiving system is provided. 9. The radio wave receiving system according to claim 3, further comprising averaging means for averaging an output from said photodetector. 10. An optical waveguide branch portion formed of an optical waveguide and a branched optical waveguide portion provided on a lithium niobate substrate on which the polarization scrambler is formed, and a frequency f2 is provided to the branch optical waveguide portion. The radio wave receiving system according to any one of claims 3 to 9, further comprising polarization control means driven by: