Applications of four-wave mixing and cross phase modulation in highly nonlinear fibers
Seyedeh Bentolhoda Nazemosadat Arsanjani
Date of Issue2012
School of Electrical and Electronic Engineering
Network Technology Research Centre
Wavelength Division Multiplexing (WDM) is one of the most common techniques used in fiber-optic communication systems in which multiple optical signals at various wavelengths are combined and transmitted through a single fiber. One of the key components in WDM systems are optical amplifiers. The most widely used amplifier is the Erbium-doped fiber amplifier (EDFA). However, its operating wavelength doesn’t cover all the low loss wavelength range (1450-1650 nm) of optical fibers. Hence, to overcome the bandwidth limitation of EDFAs, alternative optical amplifiers have been investigated. One of the candidates is the fiber optical parametric amplifier (FOPA). FOPAs operate based on a fiber nonlinearity known as four-wave mixing (FWM). FWM arises from the third order nonlinear susceptibility in a fiber and occurs when at least two waves with different frequencies co-propagate in the fiber. When a signal beam at angular frequency ω_s is launched into a fiber along with a strong optical pump beam at ω_p, the signal is amplified through a parametric process and another wave, called the idler, is generated at 〖2ω〗_p-ω_s. For a modulated signal, the modulation format will be transferred to the idler; this feature enables the FOPA to be used for shifting the optical frequency of a signal in addition to amplifying it. In order to use any optical amplifier in WDM systems, it should have a flat gain spectrum over its operating range, and FOPAs aren’t an exception. A proposed method for achieving a FOPA with a flat gain spectrum is to set up the amplifier by cascading several fiber segments which have different parameters. A part of this research is allocated to optimizing the parameters of each of these fiber segments using a genetic algorithm such that the resulting FOPA has a flat gain spectrum. It is shown that by optimizing the fourth order dispersion of each fiber section a broader flat gain spectrum can be achieved. Fluctuations of the zero dispersion wavelength (ZDW) have been considered for an amplifier which has optimum fourth order dispersion; the results show that as long as the average ZDW of each fiber segment is maintained close to the optimum value, ZDW fluctuations won’t have a large effect on the flatness of the gain spectrum. The noise properties and the operation of a multi-section FOPA in the depleted pump regime have also been studied. The four wave mixing behavior in a novel fiber loop mirror, composed of two dispersion shifted fibers along with a single mode fiber in between them, is theoretically investigated. Due to the phase shift which the SMF introduces this configuration provides higher conversion efficiency for signals in the vicinity of the pump, compared to previously reported fiber loop mirrors. Furthermore, a new design for de-multiplexing and de-modulating a differential phase shift keying (DPSK) optical time-domain multiplexed signal is proposed and investigated. The scheme consists of a nonlinear medium and a dispersive fiber connected through a 3-dB coupler to form a fiber loop mirror. Most optical signal processing operations are based on optical-electrical-optical (O/E/O) methods in which the operation bit rate is often limited by the electrical response time. In addition, utilizing ultrahigh speed photodiodes, data modulators and electronic devices are costly. Therefore, all-optical signal processing is highly desired. In this research work applications of FWM and cross phase modulation (XPM) in all-optical signal processing are discussed. A multiple format conversion module based on FWM for converting non return-to-zero (NRZ) or return-to-zero (RZ) on-off keying (OOK) modulation format to carrier-suppressed return-to-zero (CSRZ)-OOK and NRZ/RZ-DPSK signals to CSRZ-DPSK is presented. Moreover, a scheme for performing all-optical logic exclusive-OR (XOR) between phase shift keying (PSK) data and OOK signals is described. This logic gate is based on XPM in a highly nonlinear fiber (HNLF). All-optical generation of 4 levels and 8 levels RZ-amplitude and phase shift keying (APSK) modulation format based on FWM and XPM in a HNLF is presented.
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics