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The gain and noise characteristics of distributed Raman amplification (DRA) based on random fiber laser (RFL) (including forward and backward random laser pumping) have been experimentally investigated through comparison with conventional bi-directional 1st-order and 2nd-order pumping. The results show that, the forward random laser pumping exhibits larger averaged gain and gain fluctuation while the backward random laser pumping has lower averaged gain and nonlinear impairment under the same signal input power and on-off gain. The effective noise figure (ENF) of the forward random laser pumping is lower than that of the bi-directional 1st-order pumping by ~2.3dB, and lower than that of bi-directional 2nd-order pumping by ~1.3dB at transparency transmission, respectively. The results also show that the spectra and power of RFL are uniquely insensitive to environmental temperature variation, unlike all the other lasers. Therefore, random-lasing-based distributed fiber-optic amplification could offer low-noise and stable DRA for long-distance transmission.
©2013 Optical Society of America
1. Introduction
The Erbium-doped fiber amplifier (EDFA) and distributed Raman amplification (DRA) are two major signal amplification technologies in modern optical fiber communication and sensing systems [1–3]. For EDFA and DRA, one of the basic challenges for further performance improvement arises from the trade-off between the amplified spontaneous (ASE) noise accumulation and nonlinear impairment. It is well known that conventional EDFA and DRA indicates pronounced gain fluctuation along the transmission fiber, giving rise to the larger nonlinear impairment for the regime with higher gain, and the deteriorated optical signal-to-noise ratio (OSNR) for the region with lower gain. It has been rigorously proved that, in case of transparency propagation, the best balance is attained as the spatial distribution of the gain keeps constant [4]. For this purpose, higher-order DRA using multiple pumps has been proposed [5], and 2nd-order amplification scheme based on an ultra-long cavity laser has been proposed and demonstrated [6–8]. An experimental demonstration of 8 × 40Gb/s unrepeated transmission over 320km has been reported very recently [9].
The random fiber laser (RFL) [10–19], designated as a milestone in laser physics and nonlinear optics, has been suggested for use in optical fiber communication and sensing firstly by S. K. Turitsyn et al. [10]. The real application of the RFL to distributed fiber-optic sensing has been demonstrated recently by the authors [18]. The relative intensity noise (RIN) transfer of the RFL was also analyzed very recently [19]. In this paper, the gain and noise characteristics of fully DRA based on the RFL have been experimentally investigated and compared with conventional bi-directional Raman amplification, for the first time. The temperature response of the RFL was also measured to confirm its stability for use in long-distance fiber-optic transmission.
2. Experimental setup
The arrangement of the experimental system is shown in Fig. 1(a). Four amplification schemes were studied over 93km standard single mode fiber (SMF). The 1550nm signal from the distributed feedback (DFB) laser was injected into the left side of the SMF via a wavelength-division-multiplexer (WDM). For conventional bi-directional 1st-order pumping, a 1480 nm pump was injected to the two sides of the SMF via a 50:50 coupler. For the bi-directional 2nd-order pumping, a pair of 1454nm fiber Bragg gratings (FBGs) with 95% reflectivity were added to the two sides of the SMF, and a 1366 nm fiber Raman laser was used as the 2nd-order primary pump [6–8]. This structure was similar to the conventional 2nd-order pumping where no FBGs were used [8].
Fig. 1 (a) Experimental arrangement to measure the gain and noise figure for various pumping configurations. (b) Experimental setup for testing the temperature response of RFL.
For the explored forward (backward) random laser pumping, the scheme was similar to that of the bi-directional 2nd-order pumping, except that the right (left) FBG was removed to avoid the facet-end feedback. Note that only one FBG was reserved in order to decrease the lasing threshold [16]. The 1454nm ‘modeless’ random laser is generated through the purely random Rayleigh distributed feedback and Raman amplification by 1366nm primary pump along the transmission span (due to the lack of closed facet-end feedback, the random lasing cavity is ‘mirrorless’ [10–19]). The 1550nm signal is further amplified by the 1454nm-band fully-distributed random laser pumping.
In the experimental setup shown in Fig. 1(a), a power meter (PM) and an optical time-domain reflectometry (OTDR) were used to measure the input-output gain and gain distribution, respectively. An optical spectrum analyzer (OSA) was used to measure the noise figure (NF) according to [3]:
where SNRin and SNRout are the input and output signal-to-noise ratios, respectively, G is the net gain, hν is the photon energy, PASE is the power of ASE in the resolution bandwidth Δν.The effective noise figure (ENF) was used to compare the noise performance directly with EDFA, defined as [3]:
where α is the fiber loss coefficient for signal, L is the span length.For the transmission span with the same signal input power and identical dispersion map, the accumulated nonlinear effect induced by self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave-mixing (FWM) can be quantified through the ratio RNL of averaged power [3]:
where Pave and Pref are the averaged signal powers when pump is on and off, respectively, G(z) is the gain distribution. Leff is the effective length [3].The experimental setup for testing the temperature response of RFL over 100km SMF is shown in Fig. 1(b). The fiber was placed in a temperature controlled chamber, and the injected power of 1366nm pump was ~1.5W. The output power and optical spectrum were recorded every 20 minutes from −40 to + 40°C, by a 10°C step.
3. Results and discussions
3.1 Gain characteristics
Figure 2(a) shows the measured on-off gain as function of input power of the primary pump for various pumping configurations. In the measurement procedure, the 1550nm signal power coupled into the fiber was attenuated to ~-35dBm to avoid the gain saturation. The primary pump power was adjusted to obtain the given on-off gain. It is found that, for the same on-off gain (>2dB), the required primary pump power for random laser pumping is larger than that of the conventional bi-directional 2nd-order pumping by ~2-2.5dB. The conventional bi-directional 1st-order pumping shows the lowest pump power requirement.
Fig. 2 (a) Measured on-off gain as function of input power of primary pump for various pumping configurations. (b)Measured gain distribution for various pumping configurations at transparency transmission point (18.6dB on-off gain). (c) Ratio of averaged power when pump is on and off as a function of on-off gain, where the signal input power is the same.
Figure 2(b) gives the measured gain distribution for various pumping configurations at transparency transmission point (18.6dB on-off gain). It is shown that, the forward random laser pumping exhibits larger averaged gain and gain variation (~5.5dB); the backward random laser pumping shows smaller averaged gain and gain variation (~3.8dB); the bi-directional 2nd-order exhibits the lowest gain variation (~2.5dB).
The ratio of averaged signal power as function of on-off gain, when pump is on and off, is shown in Fig. 2(c). The ratio has no obvious difference for <10dB on-off gain. For >10dB on-off gain, the forward random laser pumping exhibits larger ratio by ~0-3dB with increased on-off gain. At transparency transmission point, the backward random laser pumping shows the smallest ratio (~4.4dB) and thus the lowest nonlinear impairment under the conditions of the same signal input power and on-off gain.
3.2 Effective noise figure
Figure 3(a) shows the measured ENF as function of the on-off gain. It is observed that, for <20dB on-off gain, the ENF for conventional bi-directional 1st-order pumping is larger than that of the bi-directional 2nd-order pumping by an amount of ~1.1dB. Another feature is that, for <10.5dB on-off gain, the distinction for forward and backward random lasing pumping is slight, i.e. the difference in ENF is <0.5dB.
Fig. 3 (a) Measured effective noise figure as function of on-off gain for various pumping configurations. (b) Effective noise figure versus ratio of averaged power for various pumping configurations. The signal input power is the same.
It is very interested to note that, if the on-off gain is >10.5dB, the forward random laser pumping shows the lowest ENF. The improvement is enhanced gradually with the increased on-off gain. For an instance, to the transparency transmission, the ENF for the forward random laser pumping is lower than that of the bi-directional 2nd-order pumping and the bi-directional 1st-order pumping by ~1.3dB and ~2.3dB, respectively. This result can be explained by the following formula derived from Refs [3, 4]:
where nsp is the spontaneous emission factor, defined aswhere kb is the Boltzmann constant, Ω is the Raman shift, and T is the temperature. From Eq. (4), for the same net gain G(L) and span length L, the ENF is determined by the gain distribution G(z). From the measured gain distribution shown in Fig. 2(b), the forward random laser pumping shows larger gain value G(z) (see red curve), according to Eq. (4), the corresponding ENF is also smaller. This result is further verified by the comparison of the experimental and theoretical values at the transparency transmission point shown in Table 1. There is a good agreement between the experimental results and the theoretical values given by Eq. (4). Additionally, the ENF of the backward random laser pumping is worse than that of both the bi-directional 2nd-order scheme and the forward random laser pumping from moderate gains onwards.
Table 1. Experimental and Theoretical Comparison for Effective Noise Figure at Transparency Transmission Point (18.6 dB On-Off Gain)
The above discussions are focused on the nonlinear impairment and ASE noise individually. In practice, both of them have influence on transmission performance. For smaller signal input power or gain, the nonlinear effect could be neglected, and the major impairment factor arises from noise accumulation. In this case, the forward random laser pumping is more preferable due to lower ENF according to Fig. 3(a) and Table 1. If the signal input power or gain are further increased, the trade-off between nonlinear and ASE noise impairments should be considered. Figure 3(b) shows the ENF versus ratio of averaged power, here the signal input power is the same. From this figure, for the same averaged power or nonlinear impairment, the ENF of 2nd-order bi-directional pumping is the lowest, which implies a better balance. This fact is attributed to its more uniform gain distribution [4]. In both cases, the problem associated with pump-signal RIN transfer could be overcome by using the primary pump with lower RIN [5].
3.3 Temperature insensitive characteristic
According to the experimental setup shown in Fig. 1(b), the temperature response of the RFL was investigated to confirm the stability of the RFL-based fully DRA. The optical spectrum, output power, and central wavelength as function of temperature variation are shown in Figs. 4(a) and (b), it is observed that the spectral shape, output power and central wavelength of random lasing are fairly stable, with the increased fiber temperature ranging from −40 to + 40°C. This result would ensure the excellent environmental stability for the optical amplification based on random lasing.
Fig. 4 (a) Output optical spectrum of random lasing at −40°C and + 40°C.(b) Central wavelength and output power of random lasing with increased fiber temperature every 20 minutes.
Although our experiment was demonstrated for single channel transmission, the scheme can be easily used for multi-channel wavelength-division-multiplexing (WDM) systems. In this case, the wide-band and flattened Raman gain profile could be realized by using multiple FBGs with optimized wavelength spacing and reflectivity to generate multi-wavelength random laser pumping. The relatively narrow spectrum of the RFL shown in Fig. 4(a) could be further broadened by using a wide-band FBG (for example, chirped FBG) to suppress the nonlinearity of the random laser along fiber due to the stimulated Brillouin scattering (SBS).
4. Conclusions
The gain and noise characteristics of RFL-based fully-distributed amplification have been investigated experimentally and compared with conventional bi-directional 1st-order and 2nd-order pumping. The experimental results show that, the forward random laser pumping exhibits larger averaged gain and gain fluctuation; the backward random laser pumping shows smaller averaged gain and nonlinear impairment for the same signal input power and on-off gain. The noise figure of the forward random laser pumping is lower than that of the bi-directional 1st-order pumping and the bi-directional 2nd-order pumping by ~2.3dB and ~1.3dB at transparency transmission, respectively. The RFL is uniquely insensitive to environmental temperature variation, leading to its excellent stability for long-distance optical transmission. Hence, low-noise and stable long-distance fiber-optic transmission can be achieved by using random-lasing-based fully-distributed amplification demonstrated in this work.
Acknowledgments
The authors would like to thank Dr. X. F. Chen and Prof. L. Zhang in Aston University for providing the 1454nm FBGs. We also thank the reviewers for their helpful comments. This work is supported by the National Nature Science Foundation of China (NSFC) under grants No. 61290312, 61205079, 61205048, and 61106045, the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT),the Construction Plan for Scientific Research Innovation Teams of Universities in Sichuan Province under grant No. 12TD008, and the 251 Talents Program of Sichuan Normal University.
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