KR100326151B1 - Raman fiber amplifier with enhanced signal-to-noise ratio and method for the use thereof - Google Patents

Abstract

Disclosed are a Raman fiber amplifier and a method of using the same, which induces Raman scattering of an input optical signal to improve signal-to-noise ratio performance. The basic feature of the present invention is to reduce the signal-to-noise ratio (SNR) by converting the noise output of the signal input to the optical fiber amplifier to other long wavelengths by induced Raman scattering. According to the present invention, the SNR value of the signal amplified by the Raman optical fiber amplifier can be significantly improved. Therefore, it is possible to smoothly receive data having low signal strength even in long distance optical transmission.

Description

Raman fiber amplifier with enhanced signal-to-noise ratio and method for the use

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier and a method of using the same, and more particularly, to a Raman fiber amplifier and a method of using the same, which induces Raman scattering on an input optical signal to improve signal-to-noise ratio performance.

Throughout the 1970s and 1980s, high-power Raman fiber lasers and amplifiers have been actively studied by several research groups, but with the advent of Er-Doped Fiber Amplifiers (EDFAs), the study has almost ceased. It became. However, recent studies have reported dual-clad fiber lasers with outputs ranging from several watts to several tens of watts using high power laser diode pumping, which is difficult to achieve with conventional single mode fiber lasers. Corresponding. With the advent of these high power fiber lasers and the commercialization of high-density wavelength-division multiplexing (WDM) fiber optic communications, Raman fiber amplifiers are the It has been in the spotlight recently as a candidate. The physical basis for a Raman fiber amplifier is Stimulated Raman Scattering (SRS), which occurs in optical fibers, which was first published in 1972 after Raman fiber lasers and amplifiers, all-optical signal processing, It is widely applied to nonlinear optical modulators for high power fiber lasers. Recently, EDFA and Raman fiber amplifiers are used together, and they are also applied to hybrid optical amplifiers having a gain flatness of 3 dB and a wide gain flat band of 75 nm.

Meanwhile, in order to realize high quality data transmission and low bit error rate in high speed optical communication, a signal-to-noise ratio (SNR) must be large. However, in long distance optical transmission, SNR and signal levels are lowered due to Rayleigh scattering and other nonlinear effects. In order to smoothly receive data with low signal strength, optical amplification of a signal is required, and the gain required is about 30 dB. In this amplification process, noise is primarily amplified by amplified spontaneous emission (ASE) and other nonlinear interactions. In particular, the performance of long-range optical transmission systems employing many channels, such as DWDM (Dense WDM), is often limited by optical amplifier noise. Thus, to reduce noise levels, some optical amplifiers use intermediate breakers, attenuators, or fiber Bragg gratings. However, they are only effective in a given spectral region.

In addition, a three-level laser configuration such as EDFA cannot eliminate the ASE completely, resulting in a two-fold reduction in SNR compared to the input SNR. This means that the noise figure characteristic parameter is more than 3 dB.

Accordingly, an object of the present invention is to provide a Raman optical fiber amplifier that can significantly improve the SNR value of the signal to be amplified.

Another technical problem of the present invention is to provide a method of effectively using the Raman fiber amplifier.

1A-1E are graphs for explaining the concept of the present invention;

2 is a block diagram of a germanium doped Raman optical fiber amplifier according to an embodiment of the present invention;

3A to 3D are graphs showing the results of observing signal changes in the optical fiber longitudinal direction after injecting signals into the amplifier of FIG. 2;

4A and 4B are graphs illustrating how the primary Stokes signal is amplified for various secondary Stokes control pulse signals when using a 50m long Raman optical fiber;

5 is a graph showing a change in the signal-to-noise ratio in the optical fiber longitudinal direction.

Raman optical fiber amplifier of the present invention for solving the above technical problem, the optical fiber of a predetermined length doped with an excitable element to amplify the optical signal passing through the inside thereof; Means for introducing a primary Stokes signal to be amplified in said optical fiber in a conventional Raman optical fiber amplifier having means for optically pumping said doped element; Means for introducing a secondary stokes control signal into said optical fiber for causing amplification by induced Raman scattering, together with a pumping light propagating along said optical fiber and a primary stokes signal; And means for separating the primary stokes signal amplified in the optical fiber. At this time, it is preferable that the core portion of the optical fiber is doped with germanium, and more preferably, GeO2 is contained in the core portion at 10 mol% to 30 mol%.

According to another aspect of the present invention, there is provided a Raman optical fiber amplifier, wherein a pulse signal is used as the primary stalk signal and a primary stalk signal is used as the secondary stalk control signal. And a pulse signal having a shape inverted relative to one another, a signal group consisting of a pulse signal having an inverted and extended pulse width and a continuous wave signal, respectively.

The present invention proposes a method of lowering the noise level of a germanium-doped Raman fiber amplifier (GDRA) in the temporal domain, pumping light in the longitudinal direction of the optical fiber, and primary stokes. And how the secondary Stokes signal changes over time. In optical communications, the main noise is amplified during the time interval between signal pulses. Non-Return to Zero (NRZ) and Return to Zero (RZ) signals have a time interval between signal pulses. If the signal is present at a constant intensity here, the ASE is not sufficiently amplified and remains fairly small. If the signal does not exist here at a certain intensity, then the ASE increases considerably. In the present invention, a photodiode is used as the detector which serves to integrate the total intensity between both ends of the signal pulse in the time domain in accordance with the trend of the signal intensity. This optical spectrum analyzer allows observation of the WDM spectrum along with a wide range of ASEs. Therefore, the noise can be reduced by reducing the noise output between signal pulses or switching to another long wavelength. In other words, the point of the present technology is to convert the noise output between the signal pulses of the primary Stokes wavelength into the output of the secondary Stokes wavelength. Therefore, a device having such a function may be referred to as a Raman Optical Noise Filter (RONF). The concept for this is illustrated in FIGS. 1A-1E. As a means of pumping a Raman amplifier, the output light I _p of a 1064 nm wavelength niodimium (Nd) doped double clad high power fiber laser was used. 1A is a graph showing this output light I _p over time. A 1120 nm primary Stokes signal I _S1 with digital transmission data was then incident for amplification. 1B is a graph showing the primary stokes signal I _S1 over time. Next, a second Stokes pulse I _S2 of 1180 nm wavelength shown in FIG. 1C was used to lower the noise level. In theory, the wavelengths of the pumping light I _p , the signal light I _S1 , and the control pulse light I _S2 may be selected as long as they satisfy the Raman amplification condition. For example, when 13 GHz and 5 GHz are used as Raman shift and gain bandwidths, respectively, 1420 nm, 1507 ± 20 nm, and 1615 ± 22 nm, or 1453 nm, 1555 nm, and 1665 nm wavelength bands, respectively, It can be used as the wavelength of primary and secondary Stokes signal light. Without control pulses, the primary Stokes signal is amplified at both high and low signal levels, 'on' and 'off', respectively. While the 'on' pulse of the control signal is applied during the 'off' pulse in the signal, the output of the amplified 'off' signal pulse is in the 'on' pulse of the control pulse in accordance with the interaction with the pumped light by Raman amplification. Switch to one. While the 'on' pulse of the control pulse is applied while the 'on' pulse in the signal, the output of the amplified 'on' signal pulse does not change much over the length of a given fiber. Accordingly, the noise level caused by the first stokes 'off' pulse is reduced, and FIGS. 1D and 1E show the first stokes signal I _S1 amplified differently depending on whether the second stokes pulse I _S2 is applied or not. Respectively. Referring to FIGS. 1D and 1E, it can be seen that a high SNR value is shown when the second Stokes pulse I _S2 is applied.

2 is a block diagram of a GDRA according to an embodiment of the present invention. Referring to FIG. 2, the pumping light I _p for the germanium-doped Raman optical fiber 230 is output from the optical fiber laser 220 pumped by the laser diode 210. In this embodiment, the output of the laser diode 210 was 40W and the wavelength of the output light was 810 nm. Further, as the optical fiber laser 220, the wavelength of output light (I _p) which 1064㎚, the output of 10W Nd doped double clad CW (Continuous Wave) laser optical fibers have been used. On the other hand, since the Raman conversion efficiency is low, the germanium-doped Raman optical fiber 230 of sufficient length was used in preparation for the case where the pumping output is low. The pumping light I _p is directly incident on the germanium doped Raman optical fiber 230, while the signal I _s1 and the control pulse I _s2 are coupled and incident by the first WDM combiner 240. The combined pumping light I _p , the signal I s ₁ and the control pulse I s ₂ propagate along the germanium doped Raman optical fiber 230 to interact for Raman amplification. Then, the amplified signal I s _′ is separated by the second WDM combiner 250. In this example, pulse lengths in the nanoseconds range were used to ignore the walk-off effects of pulses and other nonlinear phenomena. For example, at 1064 nm pumping wavelength the walk-off parameter is approximately 2 μs / nm, so dividing the pulse duration of 7 μs by this parameter results in a walk-off length of approximately 35 km. However, in the present embodiment, since the germanium doped Raman optical fiber 230 having a length much shorter than 35 km is used, the work-off effect can be ignored.

The operation of the apparatus of the present embodiment described above will be described by the following equation. The interaction of the pumping light I _p , the primary stokes signal I _s1 and the secondary stokes control pulses I _s2 in the germanium-doped Raman optical fiber 230 is differentiated with respect to the optical fiber longitudinal direction z. As an equation, it can be expressed as the following equations (1) to (3).

Where g _R is the Raman gain coefficient at the pumping light wavelength λ _P of 1064 nm, and α _P , α _S1 and α _S2 are the optical fiber loss coefficients for the pump light, the primary stokes signal, and the secondary stokes signal. Represent each. Further, ω _P , ω _S1 and ω _S2 represent the frequencies of the pump light, the primary stokes signal, and the secondary stokes signal, respectively.

On the other hand, the Raman gain coefficient of a typical non-polarization maintaining silica single mode optical fiber has a value of approximately 4.43 × 10 ^-12 cm / W at a wavelength of 1064 nm, which is equal to ( It is known to increase in proportion to 1 + 80 Δ). Δ represents the difference of the relative refractive index. As a medium for Raman amplification, a high concentration germanium-doped single-mode fiber (20 Mitsubishi Cable Industries; Model No. DX-C617-0A) was used as a medium for Raman amplification in the apparatus of the present embodiment, wherein Δ is a value of 0.0181. The Raman gain factor was 1.084 × 10 ⁻¹¹ cm / W. The core diameter was 2.6 µm, the cutoff wavelength was 980 nm, and the mode field diameter was 3.17 µm for the 1064 nm wavelength. On the other hand, with respect to the optical fiber loss coefficient parameter, Rayleigh scattering having a property inversely proportional to the square of the wavelength acts as a main loss factor. Since the optical fiber loss coefficient α _P of the germanium doped optical fiber used in this embodiment is 2.68 ㏈ / km at a wavelength of 1064 nm, the optical fiber loss coefficients at the first and second stokes wavelengths are α _S1 and α _S2. ) Is also easy to calculate.

On the other hand, in order to analyze the performance of the apparatus of the present embodiment, the power of the pumping light, the "on" signal of the primary stokes and the "on" signal of the secondary stokes was set to 10 W, 1 kW and 1 W, respectively. In addition, pulse signals that are set such that contrast ratios of the powers of the 'on' signal and the 'off' signal are both 100 are used as the first and second stokes signals. In addition, in order to improve the contrast ratio, the power levels of the 'on' signal and the 'off' signal for the primary and secondary stokes signals are inverted with respect to each other. In some cases, the secondary stokes signal may be inverted with respect to the primary stokes signal and its pulse width may be extended, or a continuous wave signal may be used at all. The signal in which the 'on' / 'off' is inverted can be obtained by generating an inverted electric signal at the same time when generating the data to be transmitted through the optical fiber and transmitting the modulated optical signal therefrom. . The inverted signal with the extended pulse width can be obtained by inverting the inverted and also generating the transmitting data and transmitting the modulated optical signal therefrom after generating the electric signal with the extended pulse width. The continuous wave signal can be easily obtained by directly oscillating the laser diode.

3A to 3D show the results of observing the signal change in the optical fiber longitudinal direction after injecting each signal as described above. For the sake of clarity, the output power is shown, not the output strength. In addition, the length of the germanium doped Raman optical fiber used in the present embodiment is about several tens of meters, but 200m long was used to better observe the power change trend. 3A to 3D, the powers of the pumping light, the primary stokes signal and the secondary stokes signal are denoted by P _p , P _S1 and P _S2 , respectively. Each of the Stokes signal is "on" the power in each P _S1 (ON) and P _S2 (ON) when the respective Stokes signal, respectively P _S1 (OFF) the power when the "off" and P _S2 ( OFF). 3A and 3B, when both P _S2 (ON) and P _S2 (OFF) are zero, the 'on' and 'off' powers of the primary stokes signal after propagating 63 m and 83 m, respectively It can be seen that at least 90% of the power P _{p of the} pumped light is converted to the primary Stokes signal power. In addition, it can be seen that the power of the amplified primary Stokes signal is maintained up to 200 m except for attenuation due to fiber loss.

On the other hand, when P _S2 (ON) is 1 W and P _S2 (OFF) is 10 Hz, the power of the primary stokes signal is amplified, but as shown in FIGS. 3C and 3D, the main power is the secondary stokes signal. Is converted into power. The power of the amplified primary Stokes signal reaches its maximum during propagation along the fiber and then decreases back to zero. In this case, it can be seen that after propagating 113m and 103m respectively, more than 90% of the power of the pumped light is converted to the secondary Stokes signal power for the 'on' and 'off' powers of the secondary Stokes signal. In the process of propagation through the optical fiber increases faster than P _S1 (ON) the P _S1 (OFF) and P _S2 (OFF) is further increased more quickly because of suffering a large gain that P _S2 (OFF) than P _S2 (ON) Done. In a given fiber length, as compared with the case without the P _{S1 S2} is P (ON) reduced to less if there is a P _S2 and reduce a lot of _S1 P (OFF). Thus, the SNR in the case that P _S2 is greater than the SNR in the absence of P _S2.

4A and 4B show graphs illustrating how the primary Stokes signal is amplified with respect to whether the secondary Stokes control pulse signal is applied when a 50m long Raman optical fiber is used. In the whole case, the input SNR was 100 and P _S1 (ON) was 1.0 Hz.

When there is no P _S2 as shown in Fig. 4A, P _S1 (OFF) and P _S1 (ON) are 0.222 W and 6.432 W, respectively, and the SNR value is 28.97. Meanwhile, FIG. 4B illustrates a case where a second stokes control pulse signal having an inverted shape is applied to the first stokes signal. In the case of P _S2 as shown in FIG. 4B, P _S1 (OFF) and P _S1 (ON) are 0.078W and 6.328W, respectively, and the SNR value is 81.13, which is 2.8 times higher than the value without P _S2. It is big.

5 is a graph showing a change in the signal-to-noise ratio in the optical fiber longitudinal direction. It is shown in comparison with the case where the second stokes control pulse signal I _S2 is applied or not. Referring to FIG. 5, it can be seen that the applied case has a larger SNR value than the case where it is not. Further, when the second stokes control pulse signal I _S2 is not applied, the SNR value simply decreases as the length of the optical fiber increases, while the second stokes control pulse signal I _S2 is applied. In the optical fiber length of 35m, the SNR value gradually decreases after reaching the maximum value. This maximum value is 186.1, which is 2.2 times the SNR value when the secondary Stokes control pulse signal I _S2 is not applied. At this length, P _S1 (ON), the power of the primary Stokes signal, gained 30 에 with respect to the power of the input signal and showed a value of 1.02 W. When this result was obtained, noise due to ASE and double Rayleigh scattering in the GDRA was not taken into account because the germanium doped optical fiber used in the examples was about tens of meters long and had a large pumping power. However, these should be considered in practical optical communications in the 1550 nm wavelength band where the available pumping power is limited to 1 W or less and the required fiber length is several tens of kilometers.

According to the present invention, the SNR value of the signal amplified by the Raman optical fiber amplifier can be significantly improved. Therefore, it is possible to smoothly receive data having low signal strength even in long distance optical transmission.

Claims (5)

A length of optical fiber doped with an excitable element to amplify the optical signal passing through the inside thereof; Means for light pumping the doped element; Means for introducing a primary stalk signal to be amplified in the optical fiber; Means for introducing a secondary stokes control signal into said optical fiber for causing amplification by induced Raman scattering, together with a pumping light propagating along said optical fiber and a primary stokes signal; And a means for separating the primary stokes signal amplified within the optical fiber. The Raman optical fiber amplifier of claim 1, wherein the core portion of the optical fiber is doped with germanium. The Raman optical fiber amplifier according to claim 2, wherein GeO2 is contained in the core portion at 10 mol% to 30 mol%. Preparing a length of optical fiber doped with an excitable element to amplify the optical signal passing through the inside thereof; Injecting a pumping light for optically pumping the doping element into the optical fiber; Introducing a primary stalk signal to be amplified in the optical fiber; Introducing a secondary stokes control signal into said optical fiber for causing amplification by induced Raman scattering, together with a pumping light propagating along said optical fiber and a primary stokes signal; And separating the amplified primary stokes signal in the optical fiber. The pulse signal of claim 4, wherein the primary stokes signal is a pulse signal, and the secondary stokes control signal has an inverted shape with respect to the primary stokes signal, and has an inverted and extended pulse width. Method of using a Raman optical fiber amplifier, characterized in that any one selected from the group of signals consisting of a pulse signal and a continuous wave signal.