1. Introduction

The modern world has observed strong growth in communications data traffic capacity. To accommodate this demand, optical networks should exploit the whole low-loss transmission window of silica fibers [1]. Development of broadband systems has been progressing by keeping the pace with the technology development to expand the bandwidth of optical amplifiers. The band broadening technology for 1.53-1.6 μm C- and L-bands is based on flattened and low population inversion erbium doped fiber amplifiers (EDFA) [2]. The spectral range around 1.3 μm, where single-mode fibers have low dispersion, is unattainable with EDFAs [3,4]. One solution to this problem is an implementation of Raman amplification which enables gain at an arbitrary wavelength by selecting suitable pump source [5]. Based on this concept, broadband amplifier systems have been developed, in which Raman amplification and EDFA are adopted for the 1.4 µm and the 1.5 µm bands, respectively.

Development of pump sources providing a power sufficient for obtaining practical Raman gain, made Raman amplifiers one of the most widely commercialized nonlinear optical devices in telecommunications [6–8]. Distributed Raman fiber amplifiers exhibit improved noise figure and reduced nonlinear penalty of fiber systems compared to EDFAs, allowing for longer amplifier spans, higher transmission bit rates and closer channel spacing [7–9].

Raman amplifiers require relatively high pump powers to achieve noticeable gain and they are essentially core-pumped devices since different cladding pump schemes offer low efficiency [10–12]. State-of-the-art laser diodes available commercially offer up to 1 W of single-mode fiber coupled power and can be used in at a few wavelengths [13]. Alternative approach utilizing pumping with powerful fiber lasers comes at high cost and low efficiency. It typically implements high-power cladding-pumped fiber laser followed by the Raman convertor/laser shifting the pump wavelength to required value [14].

Semiconductor disk laser (SDL) is a promising pump source for Raman fiber amplifiers. SDLs offer low-noise, high output power with diffraction-limited beam characteristics [15]. It has been demonstrated that relative intensity noise (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 [16,17]. 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.

Low-noise performance and high pump are crucial prerequisites for implementation of co-propagating Raman fiber amplifier schemes [7,8,11]. When co-propagating pumping is applied, the signal can be maintained at low level throughout each span of transmission line compared to other pumping methods [8,18]. It is expected that co-propagating pumping of Raman fiber amplifier would improve system performance and significantly increase the amplifier spacing. However, co-propagating pump configuration implies tighter requirements on pump source noise parameters [11,19]. It was demonstrated that RIN of the pumping source should not exceed −120 dB/Hz for co-propagating scheme [8]. Availability of low-noise high-power pumping sources is a critical matter in further improvement of the links using Raman amplification.

Raman amplifiers typically have significant amount of unabsorbed pump power decreasing the overall amplifier efficiency [20,21]. Erbium doped and thulium doped fiber amplifiers integrated with Raman amplifier have been successfully implemented for 1.46-1.6 μm and were shown to exhibit higher pump conversion efficiency and improved control of spectral gain profile [22,23]. Since Raman gain is virtually available at any wavelength, similar approach is also viable for O-band centered around 1.3 µm. However, the implementation of hybrid amplifier scheme was thus far impractical at this spectral range since neodymium-doped and praseodymium-doped amplifiers both show low gain and complicated handling [3,4]. With the invention of bismuth-doped silica fiber demonstrating considerable gain, the development of hybrid O-band fiber amplifier becomes feasible [1,24–26]. Since bismuth-doped fiber amplifier demonstrate moderate pump conversion efficiency and broadband spectra, using hybrid scheme combining Raman and bismuth gain media appears to be well motivated [25].

In this study we demonstrate 1.3 µm hybrid Raman-bismuth fiber amplifier pumped by 1.22 µm low-noise semiconductor disk laser with output power up to 1.6 W launched into single-mode fiber. The unique feature of this double-gain system is that both amplifiers require the same pump source because pump-Stokes spectral shift in Raman amplifier and pump-gain bandwidth separation in bismuth fiber have the same value.

Small signal gain of 18 dB was obtained in hybrid dual-gain amplifier with RIN of −139 dB/Hz over a wide spectral bandwidth. Signal gain was enhanced by 9 dB and unabsorbed pump level decreased by 8 dB compared to individual Raman amplifier. Emerging of low-noise high-power disk lasers operating in a wavelength range 1.2-1.6 µm combined with novel types of active fibers could radically change the conventional technology of fiber amplifiers and lasers [25–28]. Combination of Raman and bismuth-doped fiber amplifiers pumped with a single source takes advantage of efficient hybrid amplifier operating in telecom O-band.

2. 1.22 µm semiconductor disk laser as a low-noise pump source

1.22 µm SDL used as pump source for hybrid amplifier produces the power coupled to single-mode fiber up to 1.8 W [15]. RIN of pump laser and fiber amplifier was tested with low-noise 3.5 GHz bandwidth photodiode equipped with an optical attenuator to ensure linear response of detector [2,29,30]. Power at the receiver was kept below 1 mW. Estimated shot noise level of −156 dB/Hz is derived from RIN spectrum of 1.22 μm SDL measured for a frequency range from 1 MHz to 3 GHz at output power of 800 mW, as seen from Fig. 1 .

 

Fig. 1 RIN spectrum of a 1.22 µm pump laser at the output power of 800 mW.

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At frequencies higher than 50 MHz it can be seen that laser RIN behaves like white-noise and lies close to estimated shot noise level. Intensity peak at the cavity fundamental frequency of 820 MHz is beat note between laser signal and amplified spontaneous emission (ASE) [16]. Measured RIN of −148 dB/Hz at relatively high output power validates class-A operation regime of SDL.

3. 1.3 µm hybrid amplifier in co-propagating configuration

Scheme of 1.3 µm Raman-bismuth hybrid fiber amplifier pumped in co-propagation direction by the SDL is shown on Fig. 2 .

 

Fig. 2 Hybrid fiber amplifier setup.

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900 m-long Raman fiber was used in experiment. Fiber has elliptical core with 25 mol% of GeO₂ resulting in the core/cladding index difference of ∆n = 0.03, numerical aperture of 0.25. Estimated Raman gain of the fiber is g₀ = 21 dB/(km × W). Mode field area of the Raman fiber is 9 μm² at 1.3 μm. Accurate dehydration during perform fabrication ensured loss of 2.2 dB/km in O-band wavelength range.

Bismuth-doped fiber was drawn from perform synthesized by surface-plasma chemical vapor deposition (SPCVD) method [31]. Phosphorous served as an additive to shape refractive index profile resulting in core/cladding index difference is 5 × 10⁻³. 52 m-long fiber was used to provide optimal amplification and absorption of residual pump emerged at the output of Raman fiber. Polarization controller was implemented to optimize the performance of polarization-dependent Raman gain.

Low-noise 1.3 µm SDL with fundamental frequency of the cavity of 21.5 GHz used as a signal source exhibited RIN of −151 dB/Hz at 30 mW of output power. Output power 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. 3(a) . The amplifier gain as a function of pump power is plotted Fig. 3(b) for Raman amplifier with and without bismuth fiber spliced to its output.

 

Fig. 3 (a) Optical spectrum taken from amplifier output and (b) amplifier gain versus 1.22 μm SDL pump power.

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In isolated Raman amplifier the gain of 9 dB was obtained with 1.6 W of launched pump and 340 mW of detected unabsorbed pump after 900 m long Raman fiber. By splicing bismuth fiber with the output of Raman amplifier, the signal gain was increased up to 18 dB at the same pump power with residual pump after amplifier dropped down to 55 mW. Measured on-off gain value of under these conditions was 32 dB.

RIN measurements of signal laser performed at the input of Raman-bismuth amplifier and after 15 dB amplification have been accomplished for 1 W of pump power. The signal at the photodiode input was always kept below 600 μW to maintain the shot noise at the level of −157 dB/Hz. Results of RIN measurements are plotted at Fig. 4 .

 

Fig. 4 RIN characteristics of Raman amplifier and signal source.

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Increased noise level of hybrid amplifier at low frequency range typical for co-propagating scheme is due to pump-to-signal transfer [7,8]. With noise control system for frequencies below 30 MHz, RIN can be suppressed down to −95 dB/Hz to comply with the requirements of communication networks [20,21,32]. For bandwidth over the 100 MHz to 3 GHz meaningful for optical communications, the noise increase after amplification was below 8 dB for whole spectral range. Estimated noise figure for given signal is 6.5 dB. This result is comparable with excessive noise observed in counter-propagating discrete Raman amplifiers and in to-date reported bismuth amplifiers [11,25,33].

4. Conclusion

We have demonstrated 1.3 µm hybrid Raman-bismuth fiber amplifier co-pumped by a single 1.22 µm semiconductor disk laser in co-propagating geometry. By integrating the non-linear Raman fiber with bismuth doped fiber, the amplifier gain was boosted from 9 dB for isolated Raman amplifier to 18 dB for the same pump power which corresponds to on-off gain of 32 dB. RIN below −140 dB/Hz measured for hybrid system in a broad frequency range represents the superior performance compared with isolated bismuth amplifiers reported to date and is similar to the noise figure of counter-pumping Raman amplifiers. The hybrid bismuth-Raman amplifier system exhibits an elevated overall efficiency owing to more complete pump consumption. Using low-noise high-power semiconductor disk lasers for pumping Raman amplifier combined with active fibers would take advantage of co-propagating pumping scheme and opens new opportunities for broadband optical communication networks.