Optical phase-sensitive amplifiers (PSAs) leverage nonlinear interactions to achieve noise figures below the conventional quantum limit. Dual-pump phase-insensitive amplification has been studied in wavelength conversion, phase conjugation and spectral inversion, but phase-sensitive amplification remains challenging due to stimulated Brillouin scattering (SBS) in optical fibers. While pump-phase modulation helps mitigate SBS, it broadens the idler spectrum, complicating practical implementation. We demonstrate a counter-phase pump modulation approach to suppress idler spectral broadening using optical coherent combining and delayed self-heterodyne techniques to achieve excellent noise figure (NF) performance in dual-pump PSAs. Our methods achieve over 30 dB suppression of residual phase modulation tones and significantly improve the idler spectrum. We obtained a 20 dB gain with an NF of 1.45 dB and a minimal NF penalty of 0.2 dB due to pump-phase modulation. This achievement enables PSAs to support optical communication, wavelength conversion, and deep-space communication applications.
Optical phase-sensitive amplifiers (PSAs) leverage nonlinear interactions to achieve noise figures below the conventional quantum limit. Dual-pump phase-insensitive amplification has been studied in wavelength conversion, phase conjugation and spectral inversion, but phase-sensitive amplification remains challenging due to stimulated Brillouin scattering (SBS) in optical fibers. While pump-phase modulation helps mitigate SBS, it broadens the idler spectrum, complicating practical implementation. We demonstrate a counter-phase pump modulation approach to suppress idler spectral broadening using optical coherent combining and delayed self-heterodyne techniques to achieve excellent noise figure (NF) performance in dual-pump PSAs. Our methods achieve over 30 dB suppression of residual phase modulation tones and significantly improve the idler spectrum. We obtained a 20 dB gain with an NF of 1.45 dB and a minimal NF penalty of 0.2 dB due to pump-phase modulation. This achievement enables PSAs to support optical communication, wavelength conversion, and deep-space communication applications.
Phase-sensitive amplification (PSA) is a unique optical amplification scheme, featuring a low-noise property (i.e., quantum limited noise figure of 0 dB) and a phase clamping effect available for phase noise suppression [1]. It is based on optical parametric interaction between signal and pump lights. It is known that the interaction efficiency and then the signal gain depend on the phase matching between those interacting lights. There are typically two frequency arrangements for PSA in an optical fiber: (i) one pump (frequency: fp) with signal (frequency: fs) and conjugated idler (frequency: fi) satisfying fs+ fi= 2fp[Fig. 1(a)], and (ii) two pumps (frequencies: fp1, fp2) with signal (frequency: fs) satisfying fp1+ fp2= 2fs[Fig. 1(b)]. The phase matching condition for the former is well known, which is equivalent to that of phase-insensitive amplification where the signal light is amplified without an idler incident, such that the phase matching is satisfied when the center wavelength of interacting lights is in the normal dispersion region, i.e., a wavelength longer than the zero-dispersion wavelength of a fiber, as indicated in Fig. 1(a) [2]. On the other hand, the phase-matching condition for the latter has not been explicitly investigated, to the author's knowledge. Although several experimental demonstrations of dual-pump fiber PSA have been reported, many of them employed the wavelength allocation of the center wavelength being in the normal dispersion region, without detailed considerations on the phase matching [3-6]. Thereupon, this paper studies the phase matching condition of dual-pump PSA in an optical fiber.
Flat and wide-bandwidth gain spectrum, along with phase-sensitive gain with no need to generate amplifier input idlers, can be achieved with a coupled dual-core fiber optical parametric amplifier. In this paper, we analyze the noise properties of such an amplifier. We achieve a 3 dB noise figure in the phase-insensitive case and a minimum of -6 dB noise figure in the phase-sensitive configuration. An alternative phase-sensitive configuration is also studied, that avoids the use of idlers at the input of the amplifier, leading to a 3 dB flat spectrum combined noise figure for the output signals. Pump transfer noise in phase-insensitive and phase-sensitive configurations is also studied along with the noise figure variation against the length of the amplifier.
Frequency nondegenerate phase-sensitive amplifiers (PSAs) have the potential to realize broadband and noiseless amplification. However, the rigorous requirement of phase- and wavelength-locking of the input waves has significantly hampered their progress. In this paper, we review recent research on this type of optical amplifier. This work is based on a copier-PSA scheme, which consists of a parametric phase-insensitive copier followed by one or more PSAs. Broadband gain and noise performance of a fiber-based nondegenerate PSA has been characterized, both theoretically and experimentally. A record-low 1.1 dB noise figure was measured at >;26 dB gain, and a clear phase dependent gain was observed. Moreover, potential applications including phase noise squeezing and ultralow noise, multichannel and modulation-format-transparent linear amplification with up to 6 dB link noise figure advantage over conventional EDFA-amplified links have been experimentally demonstrated. The prospects and practical challenges of this intriguing amplification technology are also discussed.
Summary form only given. When a signal propagates through a transmission line, it is deteriorated by amplitude and phase noise, which reduces its signal-to-noise ratio (SNR) [1]. According to Shannon's theory, the SNR determines the maximum spectral efficiency. It is therefore necessary to limit the noise [3]. For this purpose, one can use phase sensitive amplifiers (PSA) instead of phase insensitive amplifiers (PIA) like Erbium-doped fiber amplifiers. PSAs amplify only one phase quadrature of the signal while deamplifying the other, thanks to a required phase relationship between the pump and the signal. This unique property, known as phase-squeezing, allows to obtain noise figures (NF) as low as 0 dB whereas PIAs, based on stimulated emission, cannot be go below the 3 dB quantum limit [2]. Moreover, the phase-squeezing property of PSAs can be used for quantum communication or even in metrology [1]. PSA can be realized with χ(2)material through degenerate parametric amplification [2]. The large spectral separation between the pump and the signal makes it very easy to filter out the pump after amplification. A ± 11 dB amplification with a sub-quantum NF and a regeneration of a DPSK signal was experimentally performed in LiNbO3 [3]. A fiber based approach would however be preferable for telecommunication purposes. PSA's based on four wave-mixing have been demonstrated, but the long length and the difficulty to separate pump from the signal [1] hinders its practical implementation. Here we present the first demonstration of a PSA based on periodically poled silica fiber (PPSF). This design has the advantage of all-fiber devices as well as simple pump filtering. The experimental set-up is depicted in Figure 1. A 20 mW CW fiber laser at 1550 nm is sent through a 99:1 coupler to be divided into a signal and a pump. The pump is modulated into 1 ns wide pulses with a repetition rate of 50 MHz. It is then amplified and frequency doubled in a PPLN crystal. The peak power of the 775 nm pump pulses reaches 10W. The pump and CW signal are then combined in a WDM and sent through a 30 cm long PPSF [4] where the signal undergoes parametric amplification. A second WDM is used to separate the pump from the amplified signal. The wave plates and the polarization controller in the pump path, allow to simultaneously control the polarization of the pump and the power in the fundamental mode of the PPSF (at 775 nm the fibre is slightly multimode). The relative phase between the pump and signal is controlled by fine tuning the frequency of the laser via a built-in piezo.
Phase-sensitive amplification (PSA) based on nonlinear parametric interaction provides low-noise amplification such that the noise figure can be 0 dB in principle. However, when an optical fiber is used as a nonlinear medium for PSA, multiple four-wave-mixing (FWM) processes other than the PSA process are induced, which can degrade the PSA noise performance, especially in dual-pump PSA with a narrow signal-pump wavelength separation. This paper investigates the influence of multiple FWM processes on quantum noise of dual-pump PSA in an optical fiber. The noise figure is calculated using quantum-mechanical operators, considering multiple FWM processes with the PSA process. Calculation examples are also presented, which indicate that the noise figure is degraded by 0.2–0.4 dB from the ideal 0 dB, depending on the signal-pump wavelength separation and fiber parameters.
Phase sensitive amplification (PSA) is a remarkably powerful tool if implemented correctly - it allows amplification with a noise figure lower than quantum mechanics normally dictates, and also allows the development of systems that coherently process the phase of their optical inputs. PSA at infrared wavelengths can be achieved using second or third order optical nonlinearities - indeed the first demonstrations of PSA utilised degenerate three photon mixing in bulk crystals, allowing the observation of effects such as quantum quadrature squeezing. This PhD research project was aimed at translating the fascinating science of PSAs into applications, based on Kerr nonlinearity in optical fibres, capable of being deployed in modern core optical networks runnning at 40 Gbit/s and higher. This objective was an integral part of the European Union FP7 project PHASORS. Studies, both theoretical and experimental, were carried out on wideband non-degenerate PSAs. The inline cascaded fiber optic parametric amplifier (FOPA), in which a first phase insensitive FOPA is used to generate phase locked signal-idler pairs, followed by a second FOPA in which PSA takes place, was used to experimentally demonstrate PSA gain characteristics in linear and saturated modes of operation, with PSA obtained over 20 nm in the telecom C-band. Focus was then re-directed towards applying a dual pump PSA to DPSK regeneration. The amplitude and phase limiting characteristics of these devices were experimentally studied, in particular revealing that amplitude saturation arises due to an interplay between input phase and nonlinear phase matching along the nonlinear fibre. This feature was used to identify a regime of operation for DPSK regeneration combining simultaneous phase and amplitude regeneration. A practical DPSK regenerator based on a degenerate dual pump phase sensitive FOPA was built. The device advanced the state of the art by incorporating a pump synthesis stage that allowed black-box operation. Detailed measurements using noise sources with varying frequency distributions in both amplitude and in phase, are presented, and the ability of the system to improve the phase and amplitude characteristics of signals at its input was verified. Also presented are results showing the regenerator installed in the middle of an 800 km dark fiber link. Finally, a novel scheme was proposed and demonstrated that utilised parametric mixing to perform arbitrary phase quantization. This relied on the coherent addition of an M-level PSK signal with a conjugated (M-1)th phase harmonic, scaled by a coefficient m, to achieve M-level phase quantization. The concept was successfully demonstrated with QPSK data signals at 56 Gbaud, as well as used to quantize lower bandwidth test signals from 2 to 6 phase levels.