JP2000056279A - Light transmitter for transmitting high frequency fsk signal through optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an FSK signal after receiving light by performing double side band modulation with which the carrier wave is suppressed according to the FSK signal having specified center frequency. SOLUTION: A light source 1 generates the light having signal frequency ν(single wave) 2, and a Mach-zehnder type optical modulator 3 makes a light phase difference at a synthesis point 4 πand performs the double side band modulation, where the carrier wave is suppressed according to the FSK signal whose center frequency is f/2, to the signal wave 2 of the light generated by the light source 1. The FSK modulated wave obtained by modulating a digital signal 5 by an FSK modulator 6 is applied to the modulator 3, DSB-SC optical modulation is executed by the modulator 3 and transmission through a single mode optical fiber 9 is performed. The spectrum 12 of the FSK signal whose center frequency is (f) is obtained as output from the light transmitter 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber transmission technique for a high-frequency signal which is hardly affected by the dispersion of an optical fiber.
The present invention relates to an optical transmitter for transmitting an optical fiber, which transmits a K signal.

[0002]

2. Description of the Related Art Optical fibers have different light propagation speeds depending on the wavelength used (dispersion of optical fibers). Modulating the light intensity with a high frequency sine wave results in a wide Double Side Band (hereinafter referred to as DSB) modulated wave that can be observed in the wavelength order. Since the arrival times of the upper optical wave, the optical carrier wave, and the lower optical wave are different, there is a problem that a signal is lost at a periodic transmission distance. As an optical modulation method capable of reducing the influence of dispersion, there are single side band (hereinafter, referred to as SSB) optical modulation in which an optical upper wave or an optical lower side wave is removed, and a suppressed carrier double-sideband in which an optical carrier is removed. (Double Si
de Band Suppressed Carrier (hereinafter referred to as DSB-SC) Two methods of optical modulation are known.

A Mach-Zehnder type optical modulator is generally used to generate an optical sideband. Among digital modulation methods for high-frequency signals, ASK modulation (Amplitude Shift K)
Eying) and PSK modulation (Phase Shift Keying) are reported to be able to be transmitted by the above-described optical modulation method. For example, in the SSB modulation, an upper wave, an optical carrier, and a lower wave obtained by intensity-modulating with a sine wave having a frequency f are optically demultiplexed, and intensity modulation or phase modulation is performed on any of the side waves with a baseband digital signal. Is transmitted and multiplexed with the carrier wave, the ASK signal of frequency f or PSK
A signal can be obtained.

[0004] In DSB-SC modulation, two optical side waves obtained by modulating a sine wave having a frequency of f / 2 are demultiplexed, and one of them is subjected to intensity modulation or phase modulation by a baseband digital signal, and the other is subjected to another modulation. If the light is multiplexed with the optical side wave and transmitted, an ASK signal or a PSK signal having a frequency f can be obtained after light reception. However, since it is difficult to greatly change the frequency of light with an external optical modulator, FSK modulation (FSK
No conventional technology can be found for requency shift keying).

[0005] A method similar to the optical fiber transmission of a high-frequency FSK signal according to the present invention (Kobayashi, Ogura, Izutsu: "Phase modulation of microwave using optical frequency modulation and chromatic dispersion of optical fiber", Optical Microwave Interaction Working Group, OMI
96-19, 1997. ). In this method, a digital baseband signal is superimposed on an injection current of a light source, and optical FSK modulation is performed. Subsequently, in a Mach-Zehnder optical modulator having a photosynthesis phase difference of π, a DSB-SC modulation is further performed using a sine wave having a frequency f. Then, an optical fiber transmission is performed, and after receiving the light, a PSK signal having a center frequency of 2f is received. This method uses optical FSK by utilizing the dispersion of an optical fiber.
The signal is converted into an electric PSK signal. When the signal is distributed to reception points at different transmission distances, the reception characteristic is deteriorated due to dispersion. On the other hand, according to the present invention, even when distribution is performed to reception points at different transmission distances, the FS
This is a system that can receive a K-modulated signal, and has a different principle.

[0006]

In a Mach-Zehnder type optical modulator, the phase or intensity of light can be changed, but the frequency cannot be changed, so that an FSK signal cannot be obtained after receiving light.

Therefore, an object of the present invention is to provide an optical fiber transmission of a high-frequency FSK signal which can obtain an FSK signal after light reception after optical fiber transmission even if a Mach-Zehnder type optical modulator is used without using an optical demultiplexer. An optical transmitter is provided.

[0008]

In order to achieve the above object, an object of the present invention is to provide a light source for generating a single-wave light and a Mach-Zehnder type optical modulator having a photosynthesis phase difference of π. A Mach-Zehnder type optical modulator for performing double-sideband modulation on a single wave of light generated by the light source with a carrier suppressed by an FSK signal having a center frequency of f / 2. In an optical receiver for receiving light modulated and transmitted by an optical fiber, an FSK signal having a center frequency f is obtained after receiving light by the optical receiver.

[0009]

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

FIG. 1 shows an embodiment of the present invention and a configuration example of an optical transmission system using the present invention. In FIG.
Reference numeral 1 denotes a light source that generates light of a single frequency ν (single wave) as indicated by reference numeral 2, reference numeral 2 denotes a spectrum (characteristic) of the light source 1,
Is a Mach-Zehnder type optical modulator, and an optical phase difference is selected to be π at a combining point of 4. 5 is a digital signal to be transmitted. 6 is an FSK modulator having a center frequency of f / 2, and 13 is an optical transmitter of the present invention. 7 is a spectrum at the output of the FSK modulator 6.

Reference numeral 8 denotes a DSB-SC optical modulation spectrum at the combining point 4, reference numeral 9 denotes an optical fiber, and reference numeral 10 denotes an optical distributor. Reference numeral 11 denotes an optical receiver, and reference numeral 12 denotes a spectrum of an FSK signal having a center frequency f at the output of the optical receiver 11.

In order to confirm the effect of using such a system, 100 Mbp of a center frequency of 6.8 GHz obtained by modulating the digital signal 5 by the FSK modulator 6 is used.
An FSK modulated wave with a similar random code of s was added to the Mach-Zehnder optical modulator 3. DSB-SC optical modulation is performed by the Mach-Zehnder type optical modulator 3 and transmitted by the single mode optical fiber 9.
The CN ratio of the 3.6 GHz FSK signal and the error rate after demodulation were measured.

For comparison, an FSK signal having a center frequency of 13.6 GHz created by the FSK modulator 6 is applied to the same Mach-Zehnder type optical modulator 3, and the optical phase difference at the combining point 4 is set to a value other than π, for example, π / 2. The optical fiber 9 was subjected to intensity modulation (DSB modulation having a carrier wave) and transmitted.

As a result, when the transmission distance is changed in the range of 0 to 20 km under the condition that the received light power is constant, the change in the CN ratio is within 1.5 dB in the DSB-SC optical modulation system.
In the intensity modulation method having a carrier, the signal was lost at 15 km. In the DSB-SC optical modulation method, the transmission distance is 0 to 0.
10 at -26 dBm received power in the range of 20 km
An error rate of ^-9 or less was obtained.

[0015]

As described above, since the optical transmission system of the present invention operates as a doubler, the frequency shift of the FSK signal to be transmitted is only half the frequency shift of the FSK signal to be received. Good FSK modulation characteristics with respect to input voltage versus frequency characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a system configuration and a spectrum characteristic of light according to an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 light source 2 spectrum of light source 3 Mach-Zehnder type optical modulator 4 synthesis point 5 digital signal 6 FSK modulator 7 spectrum of FSK modulator output 8 optical modulation spectrum of DSB-SC light 9 optical fiber 10 optical distributor 11 optical receiver 12 Spectrum of FSK signal in optical receiver 11 13 Optical transmitter of the present invention

Continued on the front page (72) Inventor Kimiyuki Oyamada 1-10-11 Kinuta, Setagaya-ku, Tokyo Japan Broadcasting Corporation Broadcasting Research Institute F-term (reference) 2H079 BA05 CA04 GA06 HA12 KA20 5K002 AA01 AA03 CA01 CA15 DA05 FA01

Claims (1)

[Claims] 1. A light source for generating a single-wave light, and a Mach-Zehnder optical modulator having a photosynthesis phase difference of π, wherein a single wave of light generated by the light source is
A Mach-Zehnder optical modulator for performing double-sideband modulation in which a carrier is suppressed by an FSK signal having a center frequency of f / 2; An optical transmitter for transmitting a high-frequency FSK signal to an optical fiber, wherein an FSK signal having a center frequency f is obtained after receiving light by the optical receiver.