JP3845047B2 - High frequency signal transmission system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-frequency signal transmission system for transmitting a high-frequency signal via an optical transmission line.
[0002]
[Prior art]
When transmitting a high-frequency signal such as a millimeter wave of about 1 km or more, all the techniques using a transmission path such as a wireless transmission path, a coaxial cable, and a waveguide have a large problem that they are lossy and cannot be bent. Therefore, research on Radio on Fiber (ROF) has been conducted as a technique for transmitting a high-frequency signal over a long distance using a low-loss and broadband optical fiber cable.
[0003]
[Problems to be solved by the invention]
In a general ROF, an optical signal is intensity-modulated in accordance with a millimeter wave signal to be transmitted, transmitted over a long distance using an optical fiber cable, and then subjected to photoelectric conversion with a high-speed photodiode to extract the original millimeter wave signal. When considering increasing the intensity of the output millimeter wave signal from the photodiode as one of the performance improvements of such a system, the intensity modulation is almost equivalent to amplitude modulation (AM). Conceivable.
(1) Increasing the intensity of light as a carrier wave.
(2) Increasing the intensity of the input millimeter wave signal to increase the modulation index.
However, the method (1) is limited by the saturation input light intensity of the photodiode, and the method (2) is limited by the nonlinear distortion of the modulator.
[0004]
For example, the prior art document “E. Ackerman et al.,“ Maximum Dynamic Range Operation of a Microwave External Modulation Fiber-Optic Link ”, IEEE Transaction on Microwave Theory Technique, Vol. 41, No. 8, pp.1299-1306. , August 1993 ", a detailed analysis is performed on ROF using a Mach-Zehnder type light intensity modulator (hereinafter referred to as an MZ type light intensity modulator), which is limited by third-order intermodulation distortion and spurious. A bias voltage for maximizing the dynamic range (SFDR) in the absence of noise is revealed. However, no means for solving the above-described problems is disclosed.
[0005]
The object of the present invention is to solve the above-mentioned problems and to transmit a high-frequency signal such as a millimeter wave with a low loss and a signal-to-noise power ratio that is greatly improved as compared with the prior art. It is to provide a signal transmission system.
[0006]
[Means for Solving the Problems]
A high-frequency signal transmission system according to the present invention is a high-frequency signal transmission system in which an optical signal transmitter and an optical signal receiver are connected via an optical transmission line,
The optical signal transmitter is
An optical carrier that has first and second optical waveguides branched from one optical waveguide that transmits an input optical carrier and uses a predetermined bias voltage for the optical carrier that transmits the second optical waveguide according to the input high-frequency signal A Mach-Zehnder optical intensity modulator that outputs an optical signal that is modulated in accordance with the high-frequency signal by synthesizing optical signals from the first and second optical waveguides;
Optical filtering means for band-filtering an optical carrier and one of the first sidebands of the intensity-modulated optical signal output from the Mach-Zehnder type optical intensity modulator and outputting it to the optical transmission line. Prepared,
The optical signal receiver is
An optical amplifying means for amplifying an optical signal received through the optical transmission line with a predetermined positive amplification and outputting it;
Photoelectric conversion means for photoelectrically converting the optical signal output from the optical amplification means into a high-frequency signal and outputting it,
The bias voltage of the Mach-Zehnder type optical intensity modulator is adjusted so that the power ratio between the optical carrier wave output from the optical amplification means and the first sideband is substantially 1.
[0007]
In the high-frequency signal transmission system, preferably, the high-frequency signal input to the Mach-Zehnder optical intensity modulator is modulated according to the input data signal.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the present invention will be described below with reference to the drawings.
[0009]
FIG. 1 is a block diagram showing a configuration of a high-frequency signal transmission system according to an embodiment of the present invention. In the high-frequency signal transmission system according to the present embodiment, the optical signal transmitter 100 and the optical signal receiver 200 are connected via an optical fiber cable 300 that is an optical transmission path. The optical signal transmitter 10 includes the optical bandpass filter 5, the optical signal receiver 200 includes the optical amplifier 31, and the bias voltage of the MZ type optical intensity modulator 10 is compared with the optical carrier wave output from the optical amplifier 31 and the first The sideband power ratio is adjusted so as to be substantially 1. That is, in the prior art document 1, a high-frequency signal is used under the condition that the optical bandpass filter 5 and the optical amplifier 31 are used to make only two optical signals, or the optical power input to the photodiode 32 is limited. Although a method for maximizing the output has not been studied, in this embodiment, the signal power of the high-frequency signal output from the photodiode 32 can be maximized by using the configuration of FIG. Thus, it is possible to transmit a high-frequency signal such as a low-loss signal with a significantly improved signal-to-noise power ratio as compared with the prior art.
[0010]
By the way, in the case of the conventional ROF, the photodiode is used as an envelope detector that outputs a high-frequency current proportional to the light intensity, but in principle, proportional to (light intensity) = (square of the optical electric field). It is a square detector that outputs the high frequency current. Therefore, once we move away from the intensity modulation method and consider a modulation method that maximizes the output high-frequency current when the input light intensity to the photodiode is limited, the input millimeter-wave signal whose intensity is equal and the difference frequency is to be transmitted. This means that only the two optical signals are transmitted. In addition, this modulation method is not affected by fading due to dispersion of the optical fiber cable compared to the case of transmitting more than three optical signals, and because the occupied bandwidth is narrow, the number of multiplexing at the time of wavelength multiplexing transmission can be increased. There are also advantages.
[0011]
In this embodiment, by using the above-described modulation method focused by the inventors, the input light intensity of the photodiode and the input signal intensity to the modulator are limited as compared with the conventional intensity modulation method. However, the present invention provides a configuration of a high-frequency signal transmission system using ROF that can obtain a larger output high-frequency current.
[0012]
Next, the configuration of the high-frequency signal transmission system according to the present embodiment will be described with reference to FIG.
[0013]
In FIG. 1, a laser light source 1 oscillates in a stable single mode to supply an optical carrier, and uses a temperature-controlled, eg, DFB type laser light source. The optical carrier wave generated by the laser light source 1 is incident on the optical waveguide 14 of the MZ type light intensity modulator 10.
[0014]
In the MZ type light intensity modulator 10, optical waveguides 11, 12, 14, 15, an optical distributor 16 and an optical combiner 17 are formed on the LiNbO ₃ substrate 20, and an electric field is applied to the optical waveguide 11. A pair of electrode lines 13 a are formed in the vicinity of the optical waveguide 11 so that the electric wave can be applied, and a pair of electrode lines 13 b is formed on the optical waveguide 12 so that an electric field can be applied to the optical waveguide 12. Closely formed. The optical carrier from the laser light source 1 enters the optical distributor 16 via the optical waveguide 14 and is divided into two by the optical distributor 16, and one optical carrier is output to the optical combiner 17 via the optical waveguide 11. The other optical carrier wave is output to the optical combiner 17 via the optical waveguide 12. The optical combiner 17 combines the two input optical carriers and outputs them to the optical bandpass filter 5 via the optical waveguide 15.
[0015]
Here, the variable bias voltage source 4 is connected to the electrode line 13a. The modulator 3 is connected to one end of the electrode line 13b via the coaxial cable 6, and the terminating resistor 7 is connected to the other end. One of the electrode lines 13a and 13b is grounded. The high-frequency signal from the high-frequency signal generator 2 is subjected to predetermined digital modulation such as QPSK according to the data signal input from the modulator 3, and the modulated high-frequency signal is output to the electrode line 13b.
[0016]
Therefore, the optical carrier wave propagating through the optical waveguide 11 is subjected to a certain phase change proportional to the bias voltage by the variable bias voltage source 4 due to the electro-optic effect of the MZ type light intensity modulator 10, and then output to the optical combiner 17. On the other hand, the optical carrier wave propagating through the optical waveguide 12 is phase-modulated with a phase change proportional to the voltage of the high-frequency signal output from the modulator 3, and then output to the optical combiner 17. By combining these two optical carriers, the light intensity is modulated.
[0017]
FIG. 2 is a graph showing an example of input / output characteristics of the MZ light intensity modulator 10 of FIG. As shown in FIG. 2, the MZ light intensity modulator 10 has input / output characteristics such that the output light intensity changes in a sine wave form with respect to the input bias voltage. The MZ type optical intensity modulator 10 needs to have a frequency characteristic that can be modulated with a high-frequency signal to be transmitted. For example, the impedance of the electrode line 13b matches that of an external circuit, and the modulation signal propagating therethrough A traveling wave type in which the velocity and the velocity of the light wave propagating in the optical waveguide 12 are matched is used. A high-frequency signal to be transmitted and a bias voltage are given as an input signal to the MZ type optical intensity modulator 10, and the bias voltage has a maximum output light intensity change proportional to the input signal voltage generally used ( Instead of V0) in FIG. 2, a value as described below is set.
[0018]
As shown in FIG. 3, in the optical waveguide 12, a large number of modulation sidebands (51a-53a, 51b-53b, etc.) are formed above and below the optical carrier 50 of the optical carrier frequency fc for phase modulation by an input high-frequency signal. ) Occurs. In the optical spectrum diagrams of FIGS. 3 to 7, the frequency f in the optical frequency region is taken on the horizontal axis, and the amplitude of each spectrum is expressed by the length of the I axis and the Q axis orthogonal to each other in the IQ plane. The relative phase between the spectra is represented by the angle from the axis.
[0019]
On the other hand, as shown in FIG. 4, in the optical waveguide 11, the optical carrier 60 is delayed by a predetermined delay phase φ by the bias voltage, and the delayed optical carrier 60a is output without being modulated. When these two optical signals are combined, the optical carrier wave having the same wavelength interferes with the phase difference determined by the bias voltage and changes its intensity, but the modulation sideband does not change.
[0020]
Therefore, in the present embodiment, as shown in FIG. 5, the bias voltage is adjusted at the output end of the optical combiner 17 so that the intensity of the optical carrier wave 70 is the same as the intensity of the first sideband 71a. In the MZ type optical intensity modulator 10, a large number of modulation sidebands (71a-73a, 71b-73b, etc.) are generated, but as shown in FIG. Only the band 71a (or only the first lower sideband 71b) may be passed, and the other modulation sidebands are removed.
[0021]
Next, after the optical signal from the optical bandpass filter 5 is transmitted over a long distance by the optical fiber cable 300, as shown in FIG. 7, the optical amplifier 31 in the optical signal receiver 200 is used in the vicinity of the saturated light input of the photodiode 32. Amplify with a predetermined positive amplification degree. Further, the amplified optical signal enters the photodiode 32 and is photoelectrically converted. The high-frequency signal after the photoelectric conversion is input to the demodulator 23, and the data signal is demodulated and output. The photodiode 32 is a current proportional to the square of the input optical electric field, and is a product of the direct current and the optical carrier wave selected by the optical bandpass filter 31 and the first upper side band, that is, a current proportional to the input high frequency signal. Output.
[0022]
【Example】
FIG. 8 is an experimental result of the high-frequency signal transmission system of FIG. 1, wherein the power ratio of the first upper sideband to the optical carrier wave at the output terminal of the optical amplifier 31 with respect to the bias voltage of the MZ type optical intensity modulator 10, and 3 is a graph showing high-frequency output power at an output terminal of a photodiode 32.
[0023]
In the experiment of FIG. 8, while maintaining the input light intensity to the photodiode 32 at a constant value of −6 dBm and changing the bias voltage of the MZ type light intensity modulator 10, The output high-frequency signal intensity from the photodiode 32 was measured. The input high frequency signal is 35 GHz. As apparent from FIG. 8, when the intensity ratio is 0 dB, that is, 1, the output high frequency intensity is maximum, and the normal bias point at which the fundamental wave component is maximum at the output end of the MZ type optical intensity modulator 10. It can be seen that an improvement amount of 17 dB is obtained compared to the case of using. This is because the optical carrier intensity is strong at the normal bias point (V0), and the gain of the optical amplifier cannot be set high due to saturation of the photodiode 32.
[0024]
As described above, in the high-frequency signal transmission system according to this embodiment, the bias voltage of the MZ-type optical intensity modulator 10 is substantially equal to the power ratio between the optical carrier wave output from the optical amplifier 31 and the first sideband. Therefore, the signal level of the high-frequency signal output from the photodiode 32 can be substantially maximized. As a result, a high-frequency signal such as a millimeter wave can be transmitted with low loss and a signal-to-noise power ratio that is significantly improved as compared with the prior art.
[0025]
In the above embodiment, the variable bias voltage source 4 is connected to the electrode line 13a. However, the present invention is not limited to this and may be connected to the electrode line 13b. In this case, the electrode line 13a can be omitted, and the device configuration is simplified.
[0026]
【The invention's effect】
As described above in detail, the high-frequency signal transmission system according to the present invention is a high-frequency signal transmission system in which an optical signal transmitter and an optical signal receiver are connected via an optical transmission line, The transmitter includes first and second optical waveguides branched from one optical waveguide that transmits an input optical carrier wave, and the optical carrier wave that transmits the second optical waveguide according to the input high frequency signal A Mach-Zehnder type optical intensity modulator that modulates using a bias voltage and outputs an optical signal intensity-modulated in accordance with the high-frequency signal by synthesizing optical signals from the first and second optical waveguides; An optical filtering means for band-filtering the optical carrier wave and one first sideband of the intensity-modulated optical signal output from the Zehnder-type optical intensity modulator and outputting it to the optical transmission line; The optical signal receiver is Optical amplifying means for amplifying an optical signal received through the optical transmission line with a predetermined positive amplification and outputting it, and optically converting the optical signal output from the optical amplifying means into a high-frequency signal for output The bias voltage of the Mach-Zehnder optical intensity modulator is adjusted so that the power ratio between the optical carrier wave output from the optical amplification means and the first sideband is substantially 1 To do. Therefore, it is possible to transmit a high-frequency signal such as a millimeter wave with a low loss and a signal-to-noise power ratio that is significantly improved as compared with the prior art.
[0027]
In the high-frequency signal transmission system, preferably, the high-frequency signal input to the Mach-Zehnder optical intensity modulator is modulated according to the input data signal. Therefore, a high-frequency signal such as a millimeter wave modulated according to the data signal can be transmitted with a low loss and a signal-to-noise power ratio greatly improved as compared with the conventional technique. Further, the distance over which the data signal can be transmitted without error can be increased.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a high-frequency signal transmission system according to an embodiment of the present invention.
2 is a graph showing an example of input / output characteristics of the Mach-Zehnder light intensity modulator (MZ light intensity modulator) of FIG.
3 is a spectrum diagram showing the spectrum of an optical signal output from the optical waveguide 12 of FIG. 1 on an IQ plane. FIG.
4 is a spectrum diagram showing the spectrum of an optical signal output from the optical waveguide 11 of FIG. 1 on an IQ plane. FIG.
FIG. 5 is a spectrum diagram showing the spectrum of an optical signal output from the optical combiner 17 in FIG. 1 on an IQ plane.
6 is a spectrum diagram showing the spectrum of an optical signal output from the optical bandpass filter 5 of FIG. 1 on an IQ plane. FIG.
7 is a spectrum diagram showing the spectrum of an optical signal output from the optical amplifier 31 of FIG. 1 on an IQ plane. FIG.
8 is an experimental result of the high-frequency signal transmission system of FIG. 1, wherein the ratio of the power of the first upper sideband to the optical carrier wave at the output terminal of the optical amplifier 31 with respect to the bias voltage of the MZ type optical intensity modulator 10; 3 is a graph showing high-frequency output power at an output terminal of a photodiode 32.
[Explanation of symbols]
1 ... Laser light source,
2. High frequency signal generator,
3 ... Modulator,
4 ... Variable bias voltage source,
5 ... Optical bandpass filter,
6 ... Coaxial cable,
7: Resistance for termination,
10: Mach-Zehnder light intensity modulator (MZ light intensity modulator),
11, 12, 14, 15 ... optical waveguide,
13a, 13b ... a pair of electrode lines,
16: Optical distributor,
17 ... photosynthesizer,
20 ... LiNbO ₃ substrate 31 ... Optical amplifier,
32 ... Photodiode,
33 ... demodulator,
100: optical signal transmitter,
200: optical signal receiver,
300: Optical fiber cable.

Claims (2)

A high-frequency signal transmission system in which an optical signal transmitter and an optical signal receiver are connected via an optical transmission line,
The optical signal transmitter is
An optical carrier that has first and second optical waveguides branched from one optical waveguide that transmits an input optical carrier and uses a predetermined bias voltage for the optical carrier that transmits the second optical waveguide according to the input high-frequency signal A Mach-Zehnder optical intensity modulator that outputs an optical signal that is modulated in accordance with the high-frequency signal by synthesizing optical signals from the first and second optical waveguides;
Optical filtering means for band-filtering an optical carrier and one of the first sidebands of the intensity-modulated optical signal output from the Mach-Zehnder type optical intensity modulator and outputting it to the optical transmission line. Prepared,
The optical signal receiver is
An optical amplifying means for amplifying an optical signal received through the optical transmission line with a predetermined positive amplification and outputting it;
Photoelectric conversion means for photoelectrically converting the optical signal output from the optical amplification means into a high-frequency signal and outputting it,
By adjusting the bias voltage of the Mach-Zehnder optical intensity modulator so that the power ratio between the optical carrier wave output from the optical amplification means and the first sideband is substantially 1 , the optical signal Even under conditions where the input signal strength to the photoelectric conversion means of the receiver is limited, the loss of the high-frequency signal is reduced and the signal is lower than when the power ratio is not substantially 1. A high-frequency signal transmission system characterized by increasing a noise-to-noise ratio .   2. The high-frequency signal transmission system according to claim 1, wherein the high-frequency signal input to the Mach-Zehnder optical intensity modulator is modulated in accordance with the input data signal.

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