Towards wide-bandwidth ultra-flat FOPAs

Abstract

Fibre optical parametric amplifiers (FOPAs) offer the potential for high gain and >100 nm bandwidth at arbitrary wavelengths for increased transmission capacity. We will cover the main principles of the FOPA and discuss our approach to obtaining broad flat gain and performance improvement via simultaneous Raman amplification. Keywords: optical parametric amplifiers, broadband amplifiers, Raman scattering. INTRODUCTIONModern optical transmission systems are approaching the nonlinear Shannon limit, and techniques are being explored to extend performance beyond it [1]. An alternative way of enhancing capacity is to increase the transmission bandwidth (BW). Some fibres, such as OFS AllWave ® allow low-loss (<0.32 dB/km ) transmission over a wide range (1310 -1625 nm). This presents an available BW of 44 THz, with only ~9 THz covered by C and L-band Erbium-doped fibre amplifiers (EDFAs). The development of amplifiers capable of providing one-stage amplification over the full transmission band would offer a fivefold transmission capacity increase.The most promising amplifiers capable of operating in that window are the fibre Raman amplifier (FRA) and the FOPA. However, the gain BW of FRA has so far been limited to ~100 nm, and requires multiple (>5) pumps [2]. Conversely, a single-pump FOPA with a total gain BW >30 THz has been demonstrated [3]. Additionally, the FOPA has a higher gain coefficient than FRA, and the FOPA can operate in a phase-sensitive regime enabling noise figures below 3 dB [4]. However, the FOPA still has numerous challenges -for example, the unidirectional nature of the gain prohibits backward pumping and means pump noise can transfer to signals. Secondly, the FOPA suffers from four-wave mixing between signals leading to crosstalk. Thirdly, pump power is limited by stimulated Brillouin scattering (SBS), and techniques used to mitigate it (such as pump dithering) cause signal degradation [5]. Finally, it is difficult to obtain polarization insensitive gain, although recent progress has been made [6]. This paper reviews the FOPA principle and recent achievements of wide gain BW operation suitable for signal amplification. We will explore how Raman gain from the parametric pump can be used to extend FOPA gain BW.

Looped Polarization-Insensitive Fiber Optical Parametric Amplifiers for Broadband High Gain Applications

“…Looped PI-FOPAs have enabled dual-polarization WDM amplification with net gain up to 20 dB across bandwidth up to 18 nm [21] and the first-ever employment of FOPA in a recirculating loop for multi-span transmission [22]. Looped PI-FOPAs essentially consist of two or more single-polarization FOPAs so they have a great potential to inherit achievements of polarization-sensitive FOPAs both degenerate and nondegenerate (i. e. single-and dual-pump) and thus to deliver ultra-wide bandwidth high gain polarization-insensitive parametric amplification [6], [23].…”