Abstract
We present an experimental demonstration of polarization-independent performance in a forward and backward-pumped 2nd-order ultralong cavity Raman laser amplifiers with highly polarized pumps. Our findings show that the depolarization of the Stokes component due to gain saturation leads to polarization-insensitive performance in terms of output gain and relative intensity noise in the signal. These results pave the way for the use of individual highly polarized low-RIN semiconductor laser diodes in Raman-amplified optical communications.
Highlights
Polarization dependent gain (PDG) [1,2] in Raman amplifiers is a potential source of amplitude noise in optical fiber communications that is generally considered to impose the need for depolarized pump lasers
System designers typically rely on two different kinds of pump lasers depending on the specific on-off gain requirements of the system: intense pumping such as the one required for distributed amplification [3] is provided by inherently noisy but depolarized fiber lasers, whereas lower-powered orthogonally polarized pairs of low-noise semiconductor lasers can be combined to produce circularly polarized pumps for discrete amplification purposes [4,5]
Regardless of the propagation direction, the degree of polarization (DOP) of the Stokes component at 1455 nm is below 20% at all considered gain levels when the dispersion compensating fiber (DCF) is used, whereas it is slightly higher in the single mode fiber (SMF)-based cavity, starting at 60% for 5 dB on-off gain and gradually reducing for higher on-off gains
Summary
Polarization dependent gain (PDG) [1,2] in Raman amplifiers is a potential source of amplitude noise in optical fiber communications that is generally considered to impose the need for depolarized pump lasers. Random distributed feedback laser amplifiers [12,13] and the use of broadband pumps [14] have proven useful for extending the communication distance in long-haul transmission systems Such interesting results have been achieved at the expense of a strong degradation of efficiency and higher architecture complexity, which translates into increased cost. Laser (URFL) amplifiers [17], which allows the use of highly polarized primary pumps with no negative impact on the PDG, gain efficiency or RIN transfer We study both FW and BW pumping configurations, demonstrating that the use of the URFL architecture effectively removes the need for orthogonally polarized sources and polarization beam combiners to achieve circular polarization
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