Optical parametric amplifiers (PAs) utilize highly efficient nonlinear effects in an optical fiber and they have the ability to operate in phase-sensitive mode. The inclusion of PAs in optical systems can give an ultimate limit for lumped amplified links in terms of achievable signal to noise ratio. The drawback is the complexity stemming from the requirement of phase matching a number of waves at each amplifier, phase lock, and the extra spectrum consumed by idler waves. In this chapter, the theories of a parametric amplifier, including the quantum-optical equations, are given. The properties of the gain saturation and the noise figure are presented. Practical amplifier systems and their applications in phase-sensitive amplification, all-optical signal regeneration, and squeeze state generation are outlined.
Optical parametric amplifiers (PAs) utilize highly efficient nonlinear effects in an optical fiber and they have the ability to operate in phase-sensitive mode. The inclusion of PAs in optical systems can give an ultimate limit for lumped amplified links in terms of achievable signal to noise ratio. The drawback is the complexity stemming from the requirement of phase matching a number of waves at each amplifier, phase lock, and the extra spectrum consumed by idler waves. In this chapter, the theories of a parametric amplifier, including the quantum-optical equations, are given. The properties of the gain saturation and the noise figure are presented. Practical amplifier systems and their applications in phase-sensitive amplification, all-optical signal regeneration, and squeeze state generation are outlined.
Optical parametric amplifiers rely on second-order susceptibility (three-wave mixing) or third-order susceptibility (four-wave mixing) in a nonlinear process where the energy of incoming photons is not changed (elastic scattering). In the latter case, two pump photons are converted to a signal and to an idler photon. Under certain conditions, related to the phase evolution of the waves involved, this conversion can be very efficient, resulting in large amplification of an input signal. As the nonlinear process can be very fast, all-optical applications aside from pure amplification are also possible. If the amplifier is implemented in an optical input-phase-sensitive manner, it is possible to amplify a signal wave without excess noise, i.e., with a noise figure of 0 dB. In this paper, we will provide the fundamental concepts and theory of such amplifiers, with a focus on their implementation in highly nonlinear optical fibers relying on four-wave mixing. We will discuss the distinctions between phase-insensitive and phase-sensitive operation and include several experimental results to illustrate their capability. Different applications of parametric amplifiers are also discussed, including their use in optical communication links.
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.
Optical communication links are usually made with erbium-doped fiber amplifiers, which amplify the signal waves in a phase-insensitive (PI) manner. They can also be made with parametric fiber amplifiers, in which the signal waves interact with idler waves. If information is transmitted using only the signals, parametric amplifiers are PI and their noise figures are comparable to those of erbium amplifiers. However, transmitting correlated information in the signals and idlers, or copying the signals prior to transmission, allows parametric amplifiers to be phase-sensitive (PS), which lowers their noise figures. The information capacities of two-mode PS links exceed those of the corresponding PI links by 2 b/s-Hz.
We experimentally demonstrate a single-pumped, non-degenerate phase-sensitive parametric amplifier with a precise control of phase and amplitude of the in-going waves and investigate in detail its gain, attenuation and saturation properties in comparison with operation in phase insensitive amplifier (PIA) mode. We experimentally observe the variation of the gain and attenuation as a function of the relative phase, pump power and the signal-idler power ratio. The phase sensitive gain spectrum is studied over a 24 nm symmetrical bandwidth and we achieve a maximum phase sensitive amplification (PSA) gain of 33 dB. A departure from the theoretical maximum attenuation as the gain increases is observed and explained.
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.