JP2955658B1 - Subcarrier optical transmission system - Google Patents

Abstract

A subcarrier optical transmission system is provided. An angular frequency ωm modulated with information to be transmitted.
The optical DSB signal obtained by intensity-modulating the optical wave 1 having the frequency ω ₀ , which is the main carrier, with the sub-carrier signal 6 is transmitted through the single-mode optical fiber 4. At the input end or output end of the optical DSB signal, the sum of the group delay time differences between the two sidebands with respect to the main carrier in the optical DSB signal generated by the chromatic dispersion of the optical fiber 4 in the optical fiber transmission is equalized using the dispersion equalizer 3. After that, the optical DSB signal is square-detected at the receiving end using the light receiving element 5, and the original subcarrier signal is reproduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a subcarrier optical transmission system used for an optical millimeter wave access network or the like.

[0002]

2. Description of the Related Art Conventionally, when an optical DSB signal having an optical spectrum as shown in FIG. 6 is transmitted through an optical fiber, deterioration occurs due to the influence of fiber dispersion. FIG. 7 is a theoretical calculation result showing the relationship between the signal strength and the optical fiber length. At this time, fm = 60GHz
And periodic fading occurs, for example, 1 km
The signal disappears completely.

[0003] In order to avoid the influence of such fiber dispersion, conventionally, one of the two sidebands is removed and the optical SSB is removed.
A method of transmitting a signal has been used. As shown by the theoretical values in FIG. 7, in the case of the optical SSB signal, there is no change in the signal strength with respect to the distance. In order to convert an optical DSB signal into an optical SSB signal (FIG. 6), a method of removing one sideband by an optical filter after the optical modulator as shown in FIG. 8 has conventionally been adopted. FIG. 8 shows a conventional example, in which 81 is a continuous wave laser light source, 82 is an optical modulator, 83 is an optical filter, 84 is an optical fiber transmission line, 85 is a photodetector, 86 is a millimeter wave signal, and 87 is a reproduced millimeter. Represents a wave signal.

[0004]

In this method, since the amount of light passing through the optical filter changes sharply with the change in the frequency of the light source, there is a danger that the signal intensity fluctuates greatly. I could not say it.

[0005] In addition to this method, by using an optical modulator having a special structure, the laser light is modulated and the direct light SSB is modulated.
Although it is possible to generate signals, it is difficult to increase the frequency of the optical modulator.
And the configuration and operation are complicated, which is not a practical method.

[0006]

SUMMARY OF THE INVENTION The present invention has been proposed in view of the above-mentioned conventional drawbacks, and has been proposed as a method of transmitting a subcarrier having an angular frequency ωm (frequency is fm) modulated with information to be transmitted through an optical fiber. An optical DSB signal obtained by intensity-modulating an optical wave having a frequency ω ₀ (frequency is f ₀ ), which is a main carrier, with the sub-carrier signal is transmitted through a single-mode optical fiber. In the optical fiber transmission, the sum of the group delay time difference between the two sidebands with respect to the main carrier in the optical DSB signal generated by the chromatic dispersion of the optical fiber is equalized using a dispersion equalizer, and then, at the receiving end, the optical DS
An object of the present invention is to provide a subcarrier optical transmission system for performing square-law detection of a B signal using a light receiving element and reproducing an original subcarrier signal.

According to the present invention, as a method of dispersion equalization, when the propagation constant of the optical fiber is β (ω), the group delay time difference formula 1 with respect to the main carrier of both sidebands of the optical DSB signal with respect to the fiber length L and An object of the present invention is to provide a subcarrier optical transmission system for equalizing the average value of Expression 2, that is, Expression 3, by passing an optical DSB signal through a dispersion equalizer having dispersion characteristics of opposite signs.

Further, the present invention provides a subcarrier optical transmission system which uses a dispersion equalizing optical fiber having a dispersion value of D 'and a length of d as a dispersion equalizer and which is set to satisfy Expression 4. To provide.

Further, the present invention uses a so-called optical fiber grating as the dispersion equalizer, and expresses a sideband of a frequency at the incident end thereof and a sideband of the frequency at the other end thereof. A sub-carrier is provided at each of the input and output ends, wherein the length of the optical fiber grating is set to Equation 6 so that the optical DSB signal corresponds to the delay time difference between the two sidebands generated during propagation of the optical fiber. An optical transmission system is provided.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a conceptual diagram of an optical millimeter wave wireless access network. CS (Central Station) and BS (Base Station) and wireless terminals WT scattered around it
(Wireless Terminals).

[0011] CS and BS are connected by an optical fiber. In the CS → BS downlink, an optical signal on which information is superimposed in the CS is distributed to each BS via an optical fiber, and a radio signal obtained by converting a received optical signal in the BS is transmitted to each WT from an antenna. On the other hand, in the uplink from BS to CS, the radio signal from each WT is
It is converted to an optical signal within the optical fiber and collected to CS through an optical fiber. Here, a 60 GHz band millimeter wave is used as a wireless carrier, and a radius of 100
It is assumed to be applied to access networks centered on pico cells of about m.

Optical millimeter-wave wireless communication can provide a broadband wireless service in a narrow range in an access system, and development of a millimeter-wave band is also effective in solving the problem of frequency resources in a microwave band. In the near future, introduction is being considered. That is, the present invention is expected to be applied to a technology for transmitting a millimeter-wave wireless signal without distortion in an optical millimeter-wave wireless communication by connecting a control station and a wireless base station with an optical fiber.

First, the principle of the present invention will be described. Optical DSB obtained by modulation with millimeter wave signal by optical modulator
The electric field of the signal is expressed by the following equation.

[0014]

(Equation 7)

The electric field after transmission of the optical fiber at the distance L is expressed by the following equation.

[0016]

(Equation 8)

Where β (ω) is the propagation constant of the optical fiber,
α is the chirp parameter of the optical DSB signal, c is the speed of light in vacuum, l = c / f ₀ , and D is the dispersion of the optical fiber. The photocurrent obtained by square-detecting this with a photodetector is given by the following equation.

[0018]

(Equation 9)

The above equation shows that a millimeter wave having a frequency ωm is reproduced. However, if the amplitude is

[0020]

(Equation 10)

It can be seen that when the condition is satisfied, a phenomenon called so-called fading occurs in which the signal intensity becomes zero.
This shows a periodic behavior with respect to the optical fiber length L (FIG. 7) and the modulation frequency ωm. Here, if it is assumed that the light has passed through another dispersion medium having a dispersion value D 'and another length d before detection, the obtained photocurrent is given by the following equation.

[0022]

[Equation 11]

From the above equation, the variance value D ′ is calculated by the following equations (12) and (4)
It is understood that fading can be removed when the condition is satisfied.

[0024]

(Equation 12)

That is, if the chirp parameter α is ignored, the dispersion value D ′ is the opposite sign to the dispersion value D of the optical fiber,
The condition is that the total variance is equal (DL + D'd = 0). Then 2
Two specific embodiments are shown. 2A and 2B show a first embodiment of the present invention, in which 1 is a continuous wave laser light source, 2 is an optical modulator, 3 is an optical fiber for equalization,
Represents an optical fiber transmission line, 5 represents a photodetector, 6 represents a millimeter-wave signal, and 7 represents a reproduced millimeter-wave signal. The optical fiber for equalization 3 is disposed before or after the optical fiber transmission line 4.

In an ordinary optical fiber, as shown in FIG.
While there is a zero-dispersion wavelength near nm, the wavelength l of the laser light source is usually 1550n, at which the loss of the optical fiber is the lowest.
Set to m. Therefore, since the variance D takes a positive sign,
The dispersion value D 'of the equalizing optical fiber needs to be a negative sign. This can be realized by shifting the zero-dispersion wavelength to a longer wavelength side than 1550 nm by changing the fiber core diameter and the refractive index difference. In fact, this type of fiber has already been commercialized as a dispersion compensating fiber.

The chirp parameter α takes a value of -1 <α <1 in an electroabsorption modulator which is generally widely used, and varies depending on a DC bias voltage. Therefore, in an actual system, the equalization optical fiber 3 that roughly satisfies DL + D′ d = 0 is used, and fine adjustment is performed using the chirp parameter α, so that the equalization conditions of Expressions 4 and 12 are completely obtained. Is the most realistic way. FIG. 7 shows a second embodiment of the present invention, in which 1 is a continuous wave laser light source,
2 is an optical modulator, 9 is an optical fiber grating, 10 is an optical circulator, 3 is an optical fiber transmission line, 5 is a photodetector, and 11 is a reproduced millimeter wave signal.

The optical fiber grating 9 has a periodic refractive index change in the longitudinal direction of the fiber. As shown in FIG. 5, an optical fiber grating having a length d in which two gratings having different periods are arranged at both ends is used. Here, the period of the grating is set so as to satisfy the Bragg condition of the following equation so that both sidebands of the optical DSB signal are reflected by each grating (diffraction of 180 °).

[0029]

(Equation 13)

Further, the interval between the two gratings is set to the difference between the optical path lengths of the reciprocation of the band waves on both sides.

[0031]

[Equation 14]

Here, as shown in FIG. 5, a grating for reflecting the lower band wave having a large group delay time is arranged at the incident end, and a grating for the upper band wave having a small group delay time is arranged at the other end. The effect of dispersion generated in the fiber transmission line can be equalized, and both sidebands are temporally aligned at the incident point of the photodetector. The present fiber grating has already been put into practical use, and this embodiment can be realized at present.

Since the chirp parameter α is variable with respect to the bias voltage of the optical modulator and the wavelength of the light source, there is an advantage that by finely adjusting the chirp parameter α, complete dispersion equalization can be realized.

As described above, the present invention has been described based on the embodiments described in the drawings. However, the present invention is not limited to the above-described embodiments, but may be implemented in any manner unless the structure described in the claims is changed. can do.

[0035]

In summary, according to the present invention, according to the present invention, the chirp parameter α takes a value of -1 <α <1 in a widely used electroabsorption modulator, and varies depending on the DC bias voltage. Therefore, in an actual system, an equalizing optical fiber that roughly satisfies DL + D'd = 0 is used, and fine adjustment is performed with the chirp parameter α to obtain perfect equalization conditions, thereby achieving perfect dispersion. There are great effects such as an advantage that equalization can be realized.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a configuration of an optical millimeter wave wireless access network of the present invention.

FIGS. 2A and 2B are configuration diagrams of a subcarrier optical transmission system according to the first embodiment of the present invention.

FIG. 3 is a conceptual diagram showing the wavelength of a laser diode and the optical characteristics of an optical fiber.

FIG. 4 is a configuration diagram of a subcarrier optical transmission system according to a second embodiment of the present invention.

FIG. 5 is a conceptual diagram showing the principle of an optical fiber grating.

FIG. 6 is a characteristic diagram illustrating an optical spectrum when an optical DSB signal is transmitted through an optical fiber.

FIG. 7 is a characteristic diagram showing theoretical calculation results of optical signal strength and optical fiber length.

FIG. 8 is a configuration diagram showing a configuration of a conventional optical communication.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 continuous wave laser light source 2 optical modulator 3 equalizing optical fiber 3 ′ optical filter 4 optical fiber transmission line 5 photodetector 6, 8 millimeter wave signal 7, 11 reproduced millimeter wave signal 9 optical fiber grating 10 circulator

Claims (4)

(57) [Claims] 1. An angular frequency ω modulated with information to be transmitted.
m (frequency fm) as a method of optical fiber transmission subcarriers of a primary carrier frequency omega ₀ (frequency f ₀₎ light both sideband obtained by intensity modulating the light waves in the sub-carrier signal
(Optical DSB: Double sideband) signal is transmitted through a single mode optical fiber, and at the input end or output end of the optical fiber,
In the optical fiber transmission, the sum of the group delay time differences of both sidebands with respect to the main carrier in the optical DSB signal generated by the chromatic dispersion of the optical fiber is canceled using a dispersion equalizer (hereinafter, referred to as a dispersion equalizer).
A subcarrier optical transmission system characterized in that after receiving the signal, the receiving end detects the optical DSB signal by square detection using a light receiving element and reproduces the original subcarrier signal. 2. As a method of dispersion equalization, when the propagation constant of the optical fiber is β (ω), the light D with respect to the fiber length L
Group delay time difference of main band of both sidebands of SB signal And The average value of Wherein the optical DSB signal is passed through a dispersion equalizer having a dispersion characteristic of an opposite sign to equalize. Where c is the speed of light in vacuum, l = c /
f ₀ and D represent the dispersion of the optical fiber, and α represents the chirp parameter of the optical DSB signal. 3. A variance equalizer having a variance value of D ′ and a length of
Using an optical fiber for dispersion equalization of d, A subcarrier optical transmission system, which is set so as to satisfy Equation 4. 4. The dispersion equalizer has an effective refractive index of n
_e , for selectively reflecting the sideband of the frequency at the input end and the sideband of the frequency at the other end, using a so-called optical fiber grating. A grating having a period represented by Equation 5 is provided at each of the input and output ends, and the length of the optical fiber grating is set so that the optical DSB signal corresponds to the delay time difference between both sidebands generated during propagation of the optical fiber. Subcarrier optical transmission system, characterized in that: