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A 1.3 µm Raman fiber amplifier pumped by 1.22 µm semiconductor disk laser in co-propagation geometry is demonstrated. Measured relative intensity noise of −148 dB/Hz over frequency range up to 3.5 GHz was measured at 900 mW of pump power. 9 dB gain was achieved with co-propagating pumping geometry with less than 2 dB additional noise induced by amplifier to the signal. Nearly shot-noise-limited operation of semiconductor disk laser combined with the diffraction-limited beam allows for efficient core-pumping of the single-mode fiber Raman amplifiers and represents a highly practical approach which takes full advantage of co-propagating pumping.
©2011 Optical Society of America
1. Introduction
The requirement for relatively high pump power in a single-mode fiber needed for Raman amplifiers provides a serious challenge for communication technology [1–3]. Development of high pump power sources resulted in broad deployment of Raman amplifiers in fiber-optic transmission systems making them one of the first widely commercialized nonlinear optical devices in telecommunications [4,5]. Broadband nonresonant gain determined by pump wavelength selection can be further extended by using multiple-wavelength pumping and improve the gain flatness [5]. Distributed Raman amplifiers demonstrate improvement of the noise figure and reduced nonlinear penalty of fiber systems, allowing for longer amplifier spans, higher bit rates, closer channel spacing, and operation near the zero-dispersion wavelength [3,6].
Raman lasers and amplifiers are essentially core-pumped devices since cladding pumping scheme offers low gain efficiency [7]. Consequently, a relatively large pump power launched into a single-mode fiber core is required to achieve noticeable gain [8,9]. Available commercial laser diodes, however, produce single-mode fiber coupled power only up to 1 W and at very few wavelengths [10]. Alternative pumping with powerful fiber lasers comes at high cost and high power consumption. It typically implements a high-power cladding-pumped fiber laser followed by a Raman convertor/laser shifting the pump wavelength to the required value [11].
Fast response time of Raman gain could cause additional noise due to transfer of pump fluctuations to the signal [12]. Pump-signal interaction in a long fiber line exhibits an averaging effect of noise transfer which is dependent on pumping direction. When co-propagating pumping scheme is used, the averaging effect is low compared with counter-propagating technique due to the small walk-off between pump and signal and consequently tighter requirements on the noise level of pump lasers should be applied [3]. Co-propagating pumping is, however, advantageous over the technique using only counter-propagating scheme because the signal can be maintained at low level throughout each span of transmission line [13]. It is expected that co-propagating Raman pumping could improve system performance and significantly increase the amplifier spacing under condition that the pump source has low-noise characteristics. It was shown that a relative intensity noise (RIN) of the pump source should not exceed −120 dB/Hz for the co-propagating scheme [5]. The availability of low-noise pump sources is a critical matter in further improvement of the links using Raman amplification. Currently, due to shortage of efficient low-noise pump sources, the counter-propagating pumping scheme for Raman amplifiers is of preferential use.
A promising pumping approach for Raman fiber amplifiers could utilize semiconductor disk lasers (SDL) which have demonstrated to offer low-noise, high power with diffraction-limited beam [14]. It has been shown that RIN of semiconductor lasers can reach extremely low level close to shot noise limit, provided that the laser operates in the so-called class-A regime [15,16]. This regime is attained when the photon lifetime in the laser cavity becomes much longer than the carrier lifetime in the active medium. The laser operating under this condition exhibits a relaxation-oscillation free flat spectral noise density. The emergence of low-noise high-power disk lasers operating in a wavelength range of 1.2-1.6 µm could radically change the conventional technology of Raman fiber amplifiers and lasers [17,18].
In this study we demonstrate a 1.3 µm Raman fiber amplifier pumped by a 1.22 µm low-noise semiconductor disk laser with output power up to 1.6 W launched into single-mode fiber. RIN of −143 dB/Hz close to shot-noise limit has been demonstrated over a wide spectral bandwidth for co-propagating pumping amplifier configuration with small-signal gain of 8 dB.
2. Noise performance of 1.22 µm semiconductor disk laser
The 1.22 µm SDL used as a pump source for Raman amplifier is described in detail elsewhere [14]. Up to 1.8 W of linearly polarized optical power could be launched into single-mode fiber. The intensity noise was measured using a low-noise 3.5 GHz bandwidth photodiode [19].To ensure linear response of photodiode, SDL output power was kept below 1 mW using optical attenuator. The shot noise level is estimated to be −156 dB/Hz. RIN spectrum of the 1.22 μm pump SDL measured within frequency range from 1 MHz to 3 GHz at output power of 800 mW is shown in Fig. 1 .
At frequencies of several MHz it can be seen that RIN is white and lies close to the estimated shot noise level. Intensity peak at the cavity fundamental frequency of 820 MHz is beat note between the laser signal and amplified spontaneous emission (ASE) [15]. Measured RIN level of −148 dB/Hz at relatively high output power confirms class-A regime of SDL operation.
3. 1.3 µm Raman fiber amplifier pumped by semiconductor disk laser
1.3 µm Raman fiber amplifier pumped in co-propagation direction by the SDL is shown schematically in Fig. 2(a) :
The 900 m long Raman fiber used in the experiment has elliptic core with 25 mol% of GeO2 resulting in the core/cladding index difference of ∆n = 0.03, numerical aperture of 0.25 and Raman gain of g0 = 21 dB/(km × W). Mode field area of the Raman fiber is 9 μm2 at 1.3 μm. Accurate dehydration during preform fabrication ensures low loss of 2.2 dB/km. Two polarization controllers were implemented to optimize the performance of the polarization-dependent Raman amplifier.
Similar low-noise 1.298 µm disk laser with fundamental frequency of the cavity of 21.5 GHz was used as a signal source in the gain measurements. Signal disk laser revealed the operation with noise floor level of −151 dB/Hz at 30 mW of output power. Signal was appropriately attenuated to ensure small signal gain condition.
Optical spectrum at the amplifier output for 800 mW of pump power is shown in Fig. 2(b). Variation of amplifier gain with pump power in the 900 m-long Raman amplifier is shown in Fig. 3 .
9 dB gain was obtained for the highest output power of 1.6 W. Specifically, at pump power of 1.3 W, 2 mW of signal was boosted to 11 mW with 130 mW of unabsorbed pump at the end of the amplifier.
RIN measurements of the signal laser before and after 6 dB amplification was performed for 1 W of pump power. The signal at the photodiode was always kept below 600 μW to maintain the shot noise at the level of −157 dB/Hz. Results of RIN measurements are plotted in Fig. 4 .
The increased noise level of the Raman amplifier at low frequency range can be explained by pump to signal noise transfer which has a strong influence in co-propagating configurations [3,5]. Measured RIN of −90 dB/Hz for frequencies below 30 MHz can be suppressed by noise control systems, for example by modulating the pump drive current out of phase with detected variations of laser intensity by specially designed RF circuits [20,21]. For bandwidth from 100 MHz to 3 GHz meaningful for optical communications, the noise increase after amplification was below 1.8-2.3 dB for the whole spectral range. These values are comparable with excessive noise obtained in counter-propagating discrete Raman amplifiers [22,23].
4. Conclusion
In conclusion, a 1.3 µm Raman fiber amplifier pumped by a 1.22 µm semiconductor disk laser is demonstrated. Amplification with relative intensity noise of −148 dB/Hz and less than 2 dB noise penalties is achieved over the frequency range from 100 MHz to 3 GHz. Using low-noise high-power semiconductor disk lasers for pumping Raman amplifiers takes full advantage of co-propagating scheme and establishes novel environment for optical communication.
Acknowledgments
The authors express their gratitude to Y. Chamorovskiy from Kotel’nikov Institute of Radio-Engineering and Electronics, Russian Academy of Sciences for providing Ge-doped nonlinear fiber.
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