KR100198948B1 - Optical transmission device having improved s/n - Google Patents

Abstract

1. Technical field to which the invention described in the claims belongs

An optical transmission apparatus with improved signal to noise ratio.

2. Technical Challenges to be Solved by the Invention

To improve the signal-to-noise ratio (SNR) of the optical fiber amplified light source at the receiving end while maintaining a narrow spectral width at the transmitting end.

3. The point of the solution of the invention

A light source for modulating and amplifying the light and a light source for amplifying the original optical signal to be reflected by the generating means by enlarging the oscillation line width of the high output optical fiber amplifying light source input from the generating means, And a light receiving means for extracting new data and a clock signal from the optical signal input from the light source line width increasing means.

4. Important Uses of the Invention

Used in optical communication systems.

Description

Optical transmitters with improved signal to noise ratio

The present invention relates to an optical transmission apparatus with improved signal-to-noise ratio and, in particular, by broadening a bandwidth of a signal arriving at a light receiving end using a four wave mixing (FWM) phenomenon in a nonlinear loop mirror and a dispersion- To an improved optical transmission apparatus.

Recently, an optical communication system based on a WDM (Wavelength Division Multiplexing) scheme has been actively researched as an Erbium-Doped Fiber Amplifier (EDFA) using an erbium-doped optical fiber as a gain medium is commercialized. In the optical frequency division multiplexing (WDM) system, various wavelengths are used for transmission, so that the frequency of each channel must be stably arranged to form a transmission light source.

The conventional transmission light source adopts a method of measuring the output wavelength of the semiconductor laser and selecting a light source of a required wavelength. The semiconductor laser has a disadvantage in that it is difficult to accurately control the oscillation wavelength, and the oscillation wavelength fluctuates irregularly due to aging. Therefore, we are looking for an alternative that can form a transmission light source stably and economically.

Recently, a wide spectrum of ASE (Amplified Spontaneous Emission) in an erbium-doped fiber amplifier (EDFA) has been proposed using optical filters to narrow down the spectrum and use it as a light source for multiple channels.

The advantage of this method is that it is possible to replace a semiconductor laser, which is a light source for multiple optical frequency division multiplexing (WDM), by only one optical fiber amplifier, and that the center frequency of each channel can also be easily set to the standard using an optical filter .

Unlike conventional semiconductor lasers, however, the incoherent light source exhibits a large intensity noise (Excess Intensity Noise) in proportion to the optical output of the optical receiver.

In general, the signal-to-noise ratio (SNR) in optical receivers is known to be proportional to the bandwidth of the spectrally reduced optical signal and inversely proportional to the electrical signal bandwidth of the receiver linear channel. To improve the signal-to-noise ratio (SNR) at a given transmission rate, the bandwidth of the optical signal has to be increased, but this increases the transmission penalty due to the chromatic dispersion of the optical fiber, which is an obstacle to the long-distance transmission.

Therefore, in a new optical communication system using such a light source, it is required to improve the signal-to-noise ratio (SNR) of the optical fiber amplified light source while maintaining a narrow spectral width at the transmitting end.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, an object of the present invention is to provide a method and apparatus for narrowly amplifying a wide optical spectrum amplified by an amplified erbium-doped fiber amplifier (EDFA) In the non-linear loop mirror and the dispersion-shifted optical fiber, the FWM phenomenon is used to broaden the bandwidth of the signal arriving at the optical receiver, so that all optical signal processing (all-optical signal processing) And to improve the signal-to-noise ratio of the optical signal.

1 is a block diagram of an optical transmission apparatus according to the present invention;

2 is a graph showing an output spectrum of a spectrum-amplified optical fiber amplification light source,

3 shows a pattern of an optical signal modulated at 2.5 Gb / s,

4 is a graph showing reception sensitivity of an optical transmission apparatus according to the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

11,14,16 Erbium doped fiber amplifier (EDFA)

12: Optical filter

13: External modulator

15,113: Dispersion-shifted optical fiber

17: Nonlinear loop mirror

18: Optical receiver

111: optical coupler

114: polarization controller

116: Avalanche photodiode (APD)

117: Amplifier

118: Retiming circuit

According to an aspect of the present invention, there is provided an optical signal processing apparatus including: means for generating an optical signal; Optical filtering means for spectrally dividing the optical signal inputted from the optical signal generating means to generate an optical fiber amplified light source; Optical modulating means for intensity-modulating the optical fiber amplified light source incident from the optical filtering means to an NRZ (Non Return to Zero) signal; First optical amplifying means for amplifying an intensity-modulated signal level of the optical modulating means and transmitting the amplified signal through a dispersion-shifted optical fiber; Second optical amplifying means for amplifying an optical signal transmitted from the first optical amplifying means through the dispersion-shifted optical fiber; A light source line width enlarging means for enlarging an oscillation line width of the high output optical fiber amplifying light source input from the second optical amplifying means to cause the original optical signal to be reflected by the second optical amplifying means, And light receiving means for extracting new data and a clock signal from the optical signal input from the light source line width increasing means.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of an optical transmission apparatus according to the present invention. In FIG. 1, reference numerals 11, 14 and 16 denote erbium-doped optical fiber amplifiers (EDFA), reference numeral 12 denotes an optical filter, reference numeral 13 denotes an external modulator, reference numerals 15 and 113 denote dispersion- A nonlinear loop mirror, 18 an optical receiver, 111 an optical coupler, 114 a polarization controller, 116 an aberrant photodiode (APD), 117 an amplifier, and 118 a retiming circuit, respectively.

An input signal is not applied to the first erbium-doped optical fiber amplifier (EDFA1) 11, and a broad light source is generated that is spontaneously amplified (ASE). The optical signal generated in the first erbium-doped optical fiber amplifier (EDFA1) 11 is input to the optical filter 12, and the optical filter 12 spectrally demodulates it to output the optical fiber amplification light source 119. [

The LiNbO3 external modulator 13 intensity-modulates (intensity modulates) the optical fiber amplified light source incident from the optical filter 12 into a NRZ (Non-Return to Zero) signal and outputs it to a second erbium-doped optical fiber amplifier (EDFA2) .

A second erbium doped fiber amplifier (EDFA2) 14 amplifies the intensity modulated signal level of the external modulator 13 and the dispersion shifted optical fiber 15 is coupled to a second erbium doped fiber amplifier 14, And transmits the amplified optical signal.

The third erbium-doped optical fiber amplifier (EDFA3) 16 amplifies the optical signal transmitted through the dispersion-shifted optical fiber 15 and has an input end having low noise and high gain characteristics and an output end having an optical output characteristic of +16 dBm or more Is a two-stage optical fiber amplifier.

The nonlinear loop mirror 17 includes an optical coupler 111, a dispersion-shifted optical fiber 113 and a polarization controller 114 to expand the oscillation line width of the high-output optical fiber amplification light source input from the third erbium- .

That is, the 3 dB optical coupler 111 serves to supply the output signal of the third erbium-doped optical fiber amplifier 16 equally to both input ends of a nonlinear loop mirror (NLM) 17, After passing through the mirror, it performs a function of connecting an enlarged signal to the optical receiver 18.

The zero dispersion wavelength (Dispersion Zero Wavelength) of the dispersion shifted optical fiber 113 inside the nonlinear loop mirror coincides with the oscillation center wavelength of the input optical fiber amplification light source. The input high power optical fiber amplification light source causes a four wave mixing (FWM) phenomenon as a nonlinear optical phenomenon in the dispersion shifted optical fiber 113, and the oscillation line width is enlarged. The original signal light incident on the nonlinear loop mirror 17 is mostly reflected toward the input end 112 through the optical coupler 111 and the output end 113 connected to the optical receiver 18 is reflected by the wide wavelength The signal components of the band mainly appear.

The polarization controller 114 adjusts the phase of the two signal lights traveling in the nonlinear loop mirror so as to maximize the line width of the output signal. Therefore, since the signal light incident on the optical receiver 18 has a wide line width, the signal-to-noise ratio significantly increases compared with the original signal light.

The optical receiver 18 is composed of an avalanche photodiode (APD) 116, a preamplifier and a main amplifier 117, and a retiming circuit 118, And extracts new data and clock signals.

Fig. 2 shows the output spectrum of the optical fiber amplification light source before and after passing through the nonlinear loop mirror.

The line width of the spectrally amplified optical fiber amplification light source 21 with the optical filter 12 was 0.92 nm, but after passing through the nonlinear loop mirror 22, it was enlarged to 1.62 nm. This means that the signal-to-noise ratio of the optical fiber amplified light source is improved by about 1.76 times.

FIG. 3 is an eye pattern in which an optical signal modulated with an NRZ signal of 2.5 Gbit / s is measured with an optical receiver having a bandwidth of 1.7 GHz.

It can be seen that the signal-to-noise ratio is improved since the excess intensity noise is reduced in the eye pattern 32 after passing through the nonlinear loop mirror as compared with the eye pattern 31 of the spectrally reduced optical fiber amplification light source by the optical filter 12.

FIG. 4 shows a bit error ratio (BER) characteristic curve of the optical transmission apparatus according to the present invention.

The optical fiber amplification light source 41 having a 0.92 nm line width that does not pass through the nonlinear loop mirror has a BER 3x The error floor phenomenon is shown in Fig. However, when the line width was enlarged to 1.62 nm, the receiver sensitivity was almost the same (42) when the dispersion-shifted optical fiber was not used and after the transmission (43), and the error-free transmission characteristic was confirmed at -28 dBm. In addition, in the case of using a semiconductor laser light source, the difference in receiving sensitivity was about 2.5 dB compared with that in (44), and the bit error rate characteristic curve showed a gentle slope because the beat noise was increased in proportion to the optical power to be.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. It is not.

The present invention as described above makes it possible to reduce the line width of a transmission light source by using a four-wave mixing phenomenon in a nonlinear loop mirror and an optical fiber when narrowing down a wide optical spectrum of high output generated from an erbium- It is possible to improve the signal-to-noise ratio of the optical fiber amplified light source and to enlarge the line width of the spectrally reduced channel by the optical signal processing method. Therefore, the present invention can be applied regardless of the modulation speed of the transmission light source.

In addition, when the present invention is applied to a light source for an optical frequency division multiplexing system, when the naturally amplified optical spectrum is narrowly narrowed by using a Fabry-Perot filter and used as a light source of a plurality of channels, Therefore, it is possible to secure more channels with only one optical fiber amplifier.

Claims (4)

Means for generating an optical signal; Optical filtering means for spectrally dividing the optical signal inputted from the optical signal generating means to generate an optical fiber amplified light source; Optical modulating means for intensity-modulating the optical fiber amplified light source incident from the optical filtering means to an NRZ (Non Return to Zero) signal; First optical amplifying means for amplifying an intensity-modulated signal level of the optical modulating means and transmitting the amplified signal through a dispersion-shifted optical fiber; Second optical amplifying means for amplifying an optical signal transmitted from the first optical amplifying means through the dispersion-shifted optical fiber; A light source line width enlarging means for enlarging an oscillation line width of the high output optical fiber amplifying light source input from the second optical amplifying means to cause the original optical signal to be reflected by the second optical amplifying means, And And light receiving means for extracting new data and a clock signal from the optical signal inputted from the light source line width increasing means. The method according to claim 1, Wherein the light source line width increasing means comprises: The output signal of the second optical amplifying means is divided into two signals to be incident on a loop and a signal of which the line width is enlarged among the signals received through the loop is outputted to the light receiving means, Optical coupling means for reflecting light; Means for causing a four-wave mixing phenomenon as a nonlinear optical phenomenon to an optical signal incident from the optical coupling means; And And a polarization adjusting means for adjusting the phase of the optical signal causing the four-wave mixing phenomenon and the signal light entering from the optical coupling means to maximize the line width of the output signal. 3. The method of claim 2, Wherein the optical coupling means is a 3-dB optical coupler. 3. The method of claim 2, Wherein the means for causing the four-wave mixing phenomenon includes a dispersion-shifted optical fiber coupled to an optical fiber loop.