JPH10261994A - Subcarrier transmission system - Google Patents

Abstract

(57) [Problem] To provide a subcarrier transmission system capable of transmitting an optical signal over a longer distance with higher efficiency than the conventional technology. SOLUTION: A subcarrier transmission system is 1.3 μm.
A light source 1 for generating an optical signal of a wavelength other than the m-band, an optical modulator 100, and a single-mode optical fiber cable 6 for a 1.3-μm wavelength band connected to an output end thereof are provided. The optical modulator 100 performs optical phase modulation in accordance with an optical demultiplexer 11 that distributes and outputs an optical signal output from the light source 1 and an optical signal that receives one optical signal from the optical demultiplexer 11. And an optical multiplexer 12 for multiplexing the other optical signal from the optical demultiplexer 11 and the optical signal output from the optical phase modulator 7 and outputting to the cable 6. Prepare. Here, the length of the cable 6 is set so that the power of the photoelectric conversion signal of the output optical signal is larger than when no insertion is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a subcarrier transmission system for transmitting an RF signal using a subcarrier signal by modulating an optical signal with a radio frequency signal (hereinafter, referred to as an RF signal).

[0002]

2. Description of the Related Art Optical communication systems have been used for baseband transmission of digital signals whose transmission band is from DC to a predetermined frequency. Conventional optical communication systems have been designed using optical signals in the 1.3 μm band, but with the advent of optical amplifiers, the wavelength of light has shifted from 1.3 μm to 1.55 μm, which can be used in multimode. Was.

On the other hand, in recent years, attention has been focused on a subcarrier transmission system for transmitting an RF signal having a relatively narrow transmission band near a predetermined frequency using an optical fiber.
In particular, at high frequencies such as millimeter waves, the transmission loss of ordinary coaxial cables is large, and expectations for a subcarrier transmission system using a low-loss optical fiber are increasing. FIGS. 2 and 3 show the spectral distributions of the signals with respect to the wavelengths of the baseband transmission system and the subcarrier transmission system, respectively.
Shown in

The wavelength of an optical signal is from 1.3 μm band to 1.55
Although the shift to the μm band has been made, the usual single mode optical fiber cable for 1.3 μm is designed so that the chromatic dispersion value, which is the wavelength dependence of the transmission speed, becomes 0 at the wavelength of 1.3 μm. At 1.55 μm, the wavelength dispersion value is 0.
Not. In conventional intensity modulation of light, there are sidebands on both sides of the carrier in the frequency range of light, so that the presence of chromatic dispersion significantly degrades the signal strength,
It has been thought that high-speed data transmission is not possible. Also,
Similarly, in a subcarrier transmission system, transmission over a certain distance has been considered difficult. For example, if there is no phase variation of light by modulating as Mach-Zehnder type optical modulator, a length of the optical fiber cable is L, the modulation frequency of the modulation signal and f _m, the chromatic dispersion value Assuming that D is the wavelength of the light source and l is the wavelength of the light source, the output is output from the photoelectric converter connected downstream of the optical fiber cable in the case of π / 2 or -π / 2, which is the conventional bias phase condition. The power P _out of the photoelectric conversion signal can be expressed by the following equation.

[0005]

[Number 1] _{P out = K [cos {π} (λ 2 D / c) Lf m 2}] 2 , where the conversion coefficient K can be expressed by the following equation.

K = ｛(J ₁ (m) / 2) · _Poptin · r _PD ｝ ² R _in

Further, J _n (x) is an nth-order Bessel function, m is a phase modulation index of the Mach-Zehnder type optical modulator, _Poptin is an average light intensity to the photoelectric converter,
r _PD is the conversion efficiency of the photoelectric converter, and is a photovoltaic current value generated by the light input intensity. Further, R _in is the impedance when the photoelectric converter is viewed at the input end of the photoelectric converter, and c is the speed of light (= 3 × 10 ⁸ m / sec).

Here, the chromatic dispersion value D is a first-order coefficient when the group velocity of the optical fiber cable is expanded with a polynomial expression on the wavelength axis. Thus, the modulation frequency f _m is 1.55μm wavelength at 30 GHz, the wavelength dispersion value at that time 17
In the case of ps / (nm · km), the signal intensity of the optical signal becomes 0 at about 4 km (for example, see U.Gliese
et al., “Chromatic Dispersion in Fiber-Optic Micr
owave and Millimeter-Wave Links ", IEEE Transactions
on Microwave Theory Thechnology, Vol. 44, No. 10, pp. 1
716-1724, 1996. " ). In particular, in a millimeter wave having a high frequency, a receivable distance is extremely short, which is several kilometers (for example, see the related art document 2 “R. Hofstetter et al.,“ Disper
sion Effects in Optical Millimeter-Wave Systems Us
ing Self-Heterodyne Method for Transport and Gener
ation ", IEEE Transactions on Microwave Theory Techn
ology, Vol. 43, No. 9, pp. 2263, September 1995 ". ).

In order to solve this problem, a method of devising the light source to make the sideband only on one side (for example, see D. Wake et al., “Optical Generation of Millim
eter-Wave Signals for Fiber-Radio Systems Using a
Dual-MOde DFB Semiconductor Laser ", IEEE Transactio
ns on Microwave Theory Technology, Vol. 43, No. 9, pp. 2
270-2276, 1995 "and prior art document 4"JJO'Reilly et
al., “Optical generation of very narrow linewidth
millimeter wave signals ", Electronics Letters, Vol.
28, No. 25, pp. 2309-2311, 1992 ". ) And a method using a chromatic dispersion shifted fiber having a chromatic dispersion value of 0 at 1.55 μm has been devised.

[0009]

However, the method of devising the light source is still under development, and there are many problems that need to be solved before practical use, and often leads to an increase in cost. The method using the chromatic dispersion shifted fiber is firstly that the chromatic dispersion shifted fiber is slightly more expensive because it has a more complicated structure than a normal single mode optical fiber cable for 1.3 μm wavelength, and that the chromatic dispersion shifted fiber is already used for 1.3 μm. Many single mode optical fiber cables are laid, and it is considered that enormous cost is required to replace them. The concept of the transmission distance is basically based on the study of digital baseband transmission, and there is no idea of using the characteristics of a subcarrier transmission system having a relatively narrow band.

An object of the present invention is to solve the above-mentioned problems and to provide a subcarrier transmission system capable of transmitting an optical signal over a longer distance with higher efficiency as compared with the prior art.

[0011]

According to a first aspect of the present invention, there is provided a subcarrier transmission system, comprising: a light source for generating and outputting an optical signal having a wavelength other than the 1.3 μm band;
A subcarrier transmission system comprising: a 1.3 μm band single mode optical fiber cable connected to an output end of the optical modulator, wherein the optical modulator converts an optical signal output from the light source to a predetermined signal. Optical demultiplexing means for distributing and outputting the power at the power demultiplexing ratio, and optical phase modulation of one of the optical signals output from the optical demultiplexing means with a predetermined bias phase in accordance with the input electric signal and output. An optical phase modulating means, and an optical multiplexing means for multiplexing the other optical signal output from the optical demultiplexing means and the optical signal output from the optical phase modulating means and outputting the multiplexed optical signal to the optical fiber cable. The length of the optical fiber cable is set so that the power of the photoelectric conversion signal of the optical signal output from the optical fiber cable is larger than when the optical fiber cable is not inserted. Characterized in that was.

According to a second aspect of the present invention, there is provided the subcarrier transmission system according to the first aspect, wherein the bias phase is such that the power of the photoelectric conversion signal of the optical signal output from the optical fiber cable is: The optical fiber cable is set to be larger than when the optical fiber cable is not inserted.

Further, the subcarrier transmission system according to claim 3 is the subcarrier transmission system according to claim 2, wherein the value of the power division ratio is substantially 1, and the bias phase is substantially 0. Is set.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a subcarrier transmission system according to an embodiment of the present invention. The subcarrier transmission system according to this embodiment has the configuration shown in FIG.
As shown in FIG. 1, a conventional Mach-Zehnder optical modulator 10 having one optical phase modulator 7 which is not a push-pull type
0 is connected to a light source 1 that generates an unmodulated optical signal having a wavelength of 1.55 μm, and a light source 1 that operates as a chromatic dispersion element 50 having a predetermined chromatic dispersion value is provided after the optical modulator 100. A single-mode optical fiber cable 6 for 3 .mu.m is connected.

First, a condition where the power of the photoelectric conversion signal output from the photoelectric converter 3 connected downstream of the optical fiber cable 6 is maximized will be considered. As can be seen from Equation 1, the signal strength becomes 0 at a certain distance, but when the distance is exceeded, the signal strength increases again. In a digital signal baseband transmission system, since the relative bandwidth is large, even if the signal strength increases in a specific transmission band, the signal strength in other transmission bands decreases, so that the optical signal cannot be transmitted. In a small subcarrier transmission system, an optical signal can be transmitted under the condition that the signal strength becomes high again. This can be achieved by adjusting the chromatic dispersion value of the entire optical fiber cable 6 to be transmitted so as to satisfy the following equation.

[0017]

Equation 3] ^{π (λ 2 D / c)} Lf m 2 = nπ

One way to satisfy the above equation (3) is to change the entire length L of the optical fiber cable 6. The fiber length L in that case can be expressed by the following equation.

[0019]

[Number 4] L = (nc) / (λ 2 Df m 2), n = 1,2,3, ...

In particular, at a high frequency such as 94 GHz, the period at which the intensity changes is as short as about 800 μm (see FIG. 5). If the transmission distance does not become the predetermined length, transmission is possible by connecting a single mode fiber 6 having a predetermined length shorter than the transmission distance and a chromatic dispersion shift fiber to a fractional part. Also, instead of chromatic dispersion shifted fiber,
It is also possible to adjust with a high dispersion fiber or a wavelength dispersion element using a fiber grating. Further, even when the fiber length L is not a predetermined length, the Mach-Zehnder optical modulator 10 shown in FIG.
Signals can be transmitted by combining the single mode optical fiber cable 6 with the zero. When the Mach-Zehnder optical modulator 100 shown in FIG. 1 is used instead of the push-pull type, the optical modulator 100 itself has a chirp characteristic in which the wavelength changes according to the modulation signal. 6, the power P _{out of the} photoelectric conversion signal output from the photoelectric converter 3 connected to the output end of the optical fiber cable 6
Can be expressed by the following equation.

[0021]

P _out = K [sin Bias + π (λ ² D / c) f
^{_{m 2} + J 0 (m}} ) sin {π (λ 2 D / c) f m 2}] 2

Here, the bias phase Bias can be expressed by the following equation.

[Equation 6] Bias = π · V _π / V _bias

Here, V _π is a half-wave voltage (see FIG. 4).
And V _bias is a bias voltage. Further, in a range where the phase modulation index m is small (m <0.3), the function value J ₀ (m) ベ ッ 1 of the Bessel function is obtained, and therefore the above equation 4 is expressed by the following equation.

[0024]

P _out = K [sin７Bias + π (λ ² D / c) f
^{_{m 2} + sin {π (}} λ 2 D / c) f m 2}] 2

Here, when the bias phase Bias = 0,

P _out = K [ ² sin ＝ π (λ ² D / c)
f _m ² ｝] ²

Equation 9] ^{(λ 2 D / c) f} m 2 = 1/2 + n, n is by selecting the condition to be an integer, the photoelectric conversion signal power P
_out becomes the maximum, and the power P _out of the photoelectric conversion signal is smaller than that at the conventional bias point in the baseband transmission system (that is, when there is no chromatic dispersion element 50 and the bias phase is ± π / 2). It can be seen that it is improved by 6 dB. Even when the bias phase, which is the conventional bias point, is ± π / 2, the power P _out of the photoelectric conversion signal can be increased by 3 dB by selecting a condition that satisfies Equation 9.

Next, a subcarrier transmission system according to the present embodiment will be described with reference to FIG. Here, the laser diode 1, the modulation signal generator 2, and the optical modulator 100
Is provided on the transmission side, while the photoelectric converter 3 is provided on the reception side via the optical fiber cable 6. In FIG. 1, on a LiNbO ₃ substrate 10 having an electro-optic effect, optical waveguides 21, 22, 23, 24, an optical demultiplexer 11 and an optical multiplexer 12 configured using the optical waveguide, and an optical phase modulator The container 7 is formed. Here, the optical waveguides 21 and
The formation of 2, 23, 24 is performed by thermally diffusing Ti onto the LiNbO ₃ substrate 10 as is known in the art.
This is performed by forming a thin film optical waveguide having an elliptical cross section made of O ₃ . After the modulation signal generator 2 which generates a RF signal modulated with e.g. digital data signals, as well as applied to the optical phase modulator 7 via the coupling capacitor C _c, variable DC voltage source 8 is a DC bias voltage V _bias the via an inductor L _b for high frequency removal is applied to the optical phase modulator 7. In response, the optical phase modulator 7 modulates the phase of the input optical signal according to the phase shift proportional to the applied signal. Here, by changing the DC bias voltage V _bias of the variable DC voltage source 8, the bias phase Bias
Can be changed.

In FIG. 1, a laser diode 1 generates an unmodulated optical signal having a wavelength of, for example, 1.55 μm other than the 1.3 μm band, and inputs the same to an optical splitter 11 via an optical fiber cable 5 and an optical waveguide 21. Is done. Next, the optical demultiplexer 1
1 splits an input optical signal into two at a power splitting ratio of 1: 1 and outputs one optical signal to an optical multiplexer 12 via an optical waveguide 22 and an optical phase modulator, while outputting the other optical signal. The signal is output to the optical multiplexer 12 via the optical waveguide 23. Further, the optical multiplexer 12 multiplexes the input two optical signals, and uses the multiplexed optical signal as the optical waveguide 24 and the single mode optical fiber cable 6 for 1.3 μm wavelength that operates as the wavelength dispersion element 50. Then, the signal is output to a photoelectric converter 3 having, for example, a photodiode and having, for example, a square detection characteristic.
The photoelectric converter 3 photoelectrically converts an input optical signal and outputs a photoelectric conversion signal that is an electric signal.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventor formed an optical modulator having the above-described configuration, and performed computer simulations and experiments. These simulations and experiments yielded similar results and are as follows.

First, FIG. 6 shows the dependence of the power of the photoelectric conversion signal on the length of the optical fiber cable 6 when the bias phase Bias is changed, and shows the power of the photoelectric conversion signal when the bias phase Bias is changed. frequency-dependent (here, the frequency is the modulation frequency f _m of the modulation signal.) shows the Figure 7. Here, the chromatic dispersion value is set to 17 ps / (nm · km) at the wavelength of the optical signal of 1.55 μm, the frequency is set to 30 GHz in the length dependency of the optical fiber cable 6, and the light is set to the optical frequency in the frequency dependency. Fiber cable 6
Was set to 5 km.

As is clear from FIGS. 6 and 7, if the fluctuation of the power P _out of the photoelectric conversion signal is allowed by about 6 dB by adjusting the bias phase Bias, the length of the arbitrary optical fiber cable 6 and the modulation signal transmission can be performed with the modulation frequency f _m. In addition, at the point where the bias phase Bias = 0, the power P _{out of the} photoelectric conversion signal is lower than that of the conventional bias point where the bias phase Bias is set to + π / 2 or −π / 2.
Is improved by 6 dB.

As described above, according to the present embodiment, the single mode optical fiber cable 6 for the wavelength of 1.3 μm is provided between the optical modulator 100 as the transmitting unit and the photoelectric converter 3 as the receiving unit. By adjusting the length of the optical fiber cable 6 and adjusting the chromatic dispersion value, it is possible to prevent signal deterioration due to chromatic dispersion. Carrier transmission becomes possible. An optical modulator 100 having a chirp;
By combining with the optical fiber cable 6, the power of the photoelectric conversion signal can be improved by 6 dB as compared with the conventional technology.

<Modification> FIG. 8 is a block diagram showing a configuration of a subcarrier transmission system of a modification according to the present invention. In this modification, two optical phase modulators 7 are compared with the above-described embodiment having one optical phase modulator 7.
a, 7b, and a phase-inverter 9 is provided. Here, the optical phase modulator 7a performs phase modulation on the input optical signal according to an electric signal that is a superimposed voltage of the modulation signal from the modulation signal generator 2 and the DC bias voltage from the variable DC voltage source 8. On the other hand, the optical phase modulator 7b modulates the phase of the input optical signal in accordance with the electric signal inverted from the electric signal by the phase inverter 9, and outputs the resulting signal. Further, a single-mode cable 6 for a wavelength of 1.3 μm and a photoelectric converter 3 provided on the receiving side are connected to a stage subsequent to the optical modulator 200 provided on the transmitting side.

[0033] Figure 5 shows the length dependence of the optical fiber cable 6 of the power of the photoelectric conversion signal in the case of changing the modulation frequency f _m of the modulation signal. As is apparent from FIG. 5, the length of the optical fiber cable 6 can be set to a length at which a sufficient photoelectric conversion signal power for transmitting an optical signal can be obtained, or the optical fiber cable 6 is inserted. The length can be set to a value at which substantially the same photoelectric conversion signal power can be obtained as when no power is supplied.

In the modification configured as described above, a single-mode optical fiber cable for a wavelength of 1.3 μm is provided between the push-pull optical modulator 200 as a transmitting unit and the photoelectric converter 3 as a receiving unit. 6, it is possible to prevent signal deterioration due to chromatic dispersion, and it is possible to perform subcarrier transmission over a distance longer than the conventionally considered limit.

[0035]

As described in detail above, in the subcarrier transmission system according to the first aspect of the present invention, 1.3.
a light source that generates and outputs an optical signal having a wavelength other than the μm band;
What is claimed is: 1. A subcarrier transmission system comprising: an optical modulator; and a single-mode optical fiber cable for a 1.3 μm band connected to an output end of the optical modulator, wherein the optical modulator outputs from the light source. Optical demultiplexing means for distributing an optical signal at a predetermined power demultiplexing ratio and outputting the same, and optical phase modulation of one of the optical signals output from the optical demultiplexing means at a predetermined bias phase in accordance with an input electric signal. And an optical phase modulator for combining the other optical signal output from the optical demultiplexer and the optical signal output from the optical phase modulator and outputting the resultant signal to the optical fiber cable. Wave means, the length of the optical fiber cable, the power of the photoelectric conversion signal of the optical signal output from the optical fiber cable is larger than when the optical fiber cable is not inserted It is sea urchin set. Therefore, by connecting the optical modulator serving as the transmitting unit and the photoelectric converter serving as the receiving unit using a single mode optical fiber cable for 1.3 μm wavelength and adjusting the length of the optical fiber cable. By adjusting the chromatic dispersion value, it is possible to prevent signal deterioration due to chromatic dispersion, and it is possible to perform subcarrier transmission over a distance longer than the conventionally considered limit. Further, by combining an optical modulator having a chirp and an optical fiber cable, it is possible to improve the power of the photoelectric conversion signal by 6 dB as compared with the related art.

[0036] In the subcarrier transmission system according to the second aspect, in the subcarrier transmission system according to the first aspect, the bias phase is determined by the power of the photoelectric conversion signal of the optical signal output from the optical fiber cable. Are set to be larger than when the optical fiber cable is not inserted. Therefore, by further adjusting the bias phase, the prior art and claim 1 can be obtained.
Compared to the system described, the power of the photoelectric conversion signal of the optical signal can be increased, and the transmission distance in the subcarrier transmission system can be increased.

Further, in the subcarrier transmission system according to claim 3, in the subcarrier transmission system according to claim 2, the value of the power splitting ratio is substantially 1, and the bias phase is substantially. Set to 0. Therefore, by further setting the bias phase to 0,
The power of the photoelectric conversion signal of the optical signal can be increased as compared with the prior art and the system according to claims 1 and 2,
The transmission distance in the subcarrier transmission system can be lengthened.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a subcarrier transmission system according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a spectrum of a baseband transmission system.

FIG. 3 is a diagram illustrating a spectrum of a subcarrier transmission system.

FIG. 4 is a graph showing a characteristic of an output light intensity of the optical modulator with respect to an input voltage to the optical modulator of FIG. 1;

FIG. 5 is a diagram illustrating the subcarrier transmission system of FIG.
5 is a graph showing the dependence of the photoelectric conversion signal power on the length of an optical fiber cable.

FIG. 6 is a diagram illustrating the subcarrier transmission system of FIG.
5 is a graph showing the dependence of the photoelectric conversion signal power on the length of the optical fiber cable when the bias phase is changed.

FIG. 7 is a diagram illustrating the subcarrier transmission system of FIG.
5 is a graph showing the frequency dependence of the photoelectric conversion signal power when the bias phase is changed.

FIG. 8 is a block diagram illustrating a configuration of a subcarrier transmission system according to a modification of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Laser diode, 2 ... Modulation signal generator, 3 ... Optical converter, 5 ... Optical fiber cable, 6 ... Optical fiber cable for 1.3 μm wavelength, 7, 7a, 7b ... Optical phase modulator, 8 ... Variable direct current Voltage source, 9: phase inverter, 10: LiNbO ₃ substrate, 11: optical demultiplexer, 12: optical multiplexer, 21, 22, 23, 24: optical waveguide, 50: wavelength dispersion element, 100: optical modulator , 200: a push-pull optical modulator, _Cc : a coupling capacitor, _Lb : a high-frequency removing inductor.

Claims (3)

[Claims] 1. A light source for generating and outputting an optical signal having a wavelength other than the 1.3 μm band, an optical modulator, and a single mode optical fiber for a 1.3 μm band connected to an output end of the optical modulator. A subcarrier transmission system including a cable, wherein the optical modulator distributes an optical signal output from the light source at a predetermined power splitting ratio and outputs the split signal, and the optical splitter Optical phase modulation means for performing optical phase modulation with a predetermined bias phase in accordance with an input electric signal and outputting one optical signal output from the means, and the other optical signal output from the optical demultiplexing means and Optical multiplexing means for multiplexing an optical signal output from the optical phase modulation means and outputting the multiplexed optical signal to the optical fiber cable, wherein the length of the optical fiber cable is equal to the length of the optical signal output from the optical fiber cable. Photoelectric conversion signal Characterized in that the power is set to be higher than when the optical fiber cable is not inserted. 2. The power of a photoelectric conversion signal of an optical signal output from the optical fiber cable,
2. The apparatus according to claim 1, wherein the optical fiber cable is set to be larger than when the optical fiber cable is not inserted.
A subcarrier transmission system as described. 3. The subcarrier transmission system according to claim 2, wherein the value of the power splitting ratio is substantially 1, and the bias phase is set to substantially 0.