Theoretical and experimental study of multifunction C+L band hybrid fiber amplifiers
Introduction
The rapid growth of data traffic in optical communication networks push the extensive study of wideband optical amplifiers, as the roles they play have become more and more important. Conventional erbium-doped fiber amplifiers (EDFAs) operating in the C-band wavelength-division-multiplexing (WDM) system is quite mature nowadays. However, because the entire conventional band (C band) of 1530–1565 nm is fully utilized, the demands of wider bandwidth for optical amplification are studied extensively. For the L-band amplification, Raman fiber amplifier (RFA) is known to have a lower noise figure (NF) than that of the longer band (L-band) of 1570–1605 nm EDFA [1]. Consequently, a hybrid amplifier is highly promising for multi-terabit dense WDM (DWDM) systems. The hybrid Raman/Erbium-doped fiber amplifier was designed for maximizing the span length and/or minimizing the impairments of fiber nonlinearities. It was also used to enlarge the EDFA gain-bandwidth [2]. In previous work, a wide-band and gain-flattened hybrid RFA/EDFA using several pump lasers was demonstrated [3]. However, the employment of multi-pump lasers is unavoidable for such hybrid EDFA/RFA configuration. In [4], a C-band RFA was employed to control and flatten the entire gain profile of an in-line EDFA. However, the operating spectral range was restricted to the C band. The device proposed in [5] was a dispersion-compensating hybrid amplifier with two pump lasers at wavelengths of 1545 and 1565 nm. About 10 years ago, Nicholson et al. proposed the usage of fiber Bragg gratings (FBGs) to reflect the residual pump power back into the DCF as a secondary pump source. They experimentally demonstrated the enhancement of the whole Raman gain profile [6]. Another theoretical study was carried on the cascaded EDFA/RFA scheme, which is pumped bidirectional using a 1495 nm pump laser [7]. However, the gain-flattening and dispersion management for C+L band channels in that hybrid amplifier scheme have not yet been achieved simultaneously. In [8], a 1411 nm pump laser was used to convert a 1490 nm Raman pump for serial type of C/L EDFA/RFA. However, the loop mirror introduced a 3 dB extra loss which is a drawback.
In previous work [9], we demonstrated gain equalized C+L hybrid amplifier by adjusting the pump power ratio properly. But the residual dispersion is a problem and precise gain equalization was not yet achieved. We also proposed a serial type hybrid C+L band hybrid amplifier [10] and a by-pass EDF type [11] hybrid C+L band hybrid amplifier. Although these device are also based on the pump sharing concept for the hybrid C+L band EDFA/RFA, the C-band amplification was affected by the Rayleigh scattering and reflectivity of the C band pump reflector (PR)of so high as 99% is just an ideal case. Here, we simulate and demonstrate an improved bridge-type, hybrid C+L band EDFA/RFA using a single-pump laser diode (LD). Optimum dispersion compensation and power equalization for all C+L band channels are realized. It may gain-flatten and dispersion manage the WDM channels by using the proposed hybrid amplifier.
Section snippets
Experimental setup and theory
Fig. 1 depicts the proposed C+L band hybrid amplifier scheme using single-wavelength pump source. All C+L band channels reach the 3-port optical circulator (OC) after 50 km standard single mode fiber (SMF, Corning SMF-28) transmission and are then divided into two groups using a C/L WDM coupler. While the core diameter and attenuation are 9 μm and 0.2 dB/km, respectively. For the 15 WDM-channel signals of 1530-, 1535-, 1540-, 1545-, 1550-, 1555-, 1560- and 1565 nm for the C band; 1570-, 1575-,
Results and discussion
To describe the simulation process, we use the dispersion map to explain the length of each DCF segment. Fig. 3(a) presents the chromatic dispersion (ps/nm) curves both for SMF and DCF in the C+L band region. At 1550 nm, the dispersion and dispersion slope of SMF are 17 ps/nm/km and 0.058 ps/km/nm2; and those of DCF are −95 ps/nm/km and −0.62 ps/km/nm2, respectively. The fiber loss for DCF is set at 0.4 dB/km with FBGs located at different positions in the DCF, the total dispersion and dispersion
Conclusions
A loop-back scheme hybrid fiber amplifier was theoretical and experimental studied. The fiber amplifier has characteristics of double-passed dispersion compensator design. Power equalization is realized by adjusting the pump ratio and optimizing the FBG reflectivity for the corresponding channels; Chromatic dispersion for all C+L band channels could be optimally compensated. A PR is used to recycle the residual pumping power. Both the hybrid fiber amplifier and BER performance characteristics
Acknowledgments
The authors are supported in part by the NSC of Taiwan, under contracts NSC 101-2622-E-011-018-CC2 and NSC 100-2221-E-011-092-MY2.
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