Broad wavelength generation and conversion with multi modal Four Wave Mixing in silicon waveguides

Four-wave mixing (FWM) in silicon waveguides is considered to be a promising effect to realize the wavelength conversion function for wavelength-division-multiplexing optical communication systems. Compared to the degenerate FWM with a single pump, the nondegenerate FWM with two pumps shows more flexibility in phase-matching condition and has more opportunities to acquire broader conversion bandwidth. The bandwidth enhancement is theoretically analyzed for the two-pump FWM and an enhancement of 25% is experimentally demonstrated. Also, an ultra-broadband wavelength conversion is presented based on two-pump FWM by fixing one pump near the signal and scanning the other pumps.

In this invited talk, we will review our recent development of an all-optical wavelength converter which satisfies the requirements for applications in practical all-optical networks. While providing a wideband conversion for almost all the C-band, excellent performance with dual-polarization quadrature phase shift keying (DP-QPSK) signals show a true modulation format independence with phase preserving and polarization insensitive operations. The converter features a counter-dithering pump scheme for cascaded dual-stage degenerate four wave mixing (FWM) processes to suppress the stimulated Brillouin scattering (SBS) in highly nonlinear fiber, and thus significantly improve the conversion efficiency, making it a low-loss and low-noise conversion. As a result, the total noise figure of the wavelength converter is reduced to 6.2 dB which is comparable to an EDFA. The cascaded operation of the wavelength converter is also investigated in a wavelength translating loop, showing negligible impacts of the phase noise transferred from pump signals on the idlers, or residual phase noise induced by the counter-dithering scheme. The obtained high cascadability as that of an EDFA indicates the true practicality of the developed wavelength converter for real system use.

One of many fiber nonlinear effects is a four-wave mixing (FWM) phenomenon that is resonant when the phase matching condition is satisfied. It only occurs for particular combinations of fiber dispersion and signal frequencies. In particular, phase matching is achieved for signals with very similar frequencies propagating near the zero dispersion wavelength of the fiber. The FWM is a very unpleasant transmission phenomenon in a transparent optical network, but it could be used for building all-optical devices such as wavelength converters, parametric amplifiers, optical demultiplexers, chromatic dispersion compensators, as well as signal to noise regenerators. At the process of device optimization or network planning it is important to know the fiber nonlinear coefficient (NLC). This measurement can be efficiently taken by the observation of FWM in a particular optical fiber. Besides fiber NLC measurement, FWM also enables the measurement of some other important fiber parameters, such as zero dispersion wavelength, chromatic dispersion and polarisation mode dispersion.

Layered 2D graphene oxide (GO) films are integrated with micro-ring resonators (MRRs) to experimentally demonstrate enhanced nonlinear optics. Both uniformly coated (1-5 layers) and patterned (10-50 layers) GO films are integrated on complementary-metal-oxide-semiconductor (CMOS)-compatible doped silica MRRs using a large-area, transfer-free, layer-by-layer GO coating method with precise control of the film thickness. The patterned devices further employ photolithography and lift-off processes to enable precise control of the film placement and coating length. Four-wave-mixing (FWM) measurements for different pump powers and resonant wavelengths show a significant improvement in efficiency of ≈7.6 dB for a uniformly coated device with 1 GO layer and ≈10.3 dB for a patterned device with 50 GO layers. The measurements agree well with theory, with the enhancement in FWM efficiency resulting from the high Kerr nonlinearity and low loss of the GO films combined with the strong light-matter interaction within the MRRs. The dependence of GO's third-order nonlinearity on layer number and pump power is also extracted from the FWM measurements, revealing interesting physical insights about the evolution of the GO films from 2D monolayers to quasi bulk-like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.

Spontaneous four–wave mixing (SFWM) in a microring resonator is considered theoretically. The joint spectral amplitude and Schmidt number of generated biphoton field are calculated numerically for different pump pulses. Optimal pulse shapes that provide maximum efficiency of SFWM and minimum frequency correlations between signal and idler photons are found.

A detailed analysis of the efficiency of four-wave mixing (FWM) is performed for a closely spaced energy state quantum dash semiconductor optical amplifier (SOA). The analysis takes into account the effect of all discrete energy states, the gain dispersion of the quantum dash, and the effect of doping on the FWM characteristics. Our analysis reveals that large FWM efficiency can be obtained when the energy of the pump signal is equal to the first excited state. We find that at low-applied current doping the dashes by p -type concentration enhances the FWM efficiency. While at high applied current, we find that doping the SOA with p -type concentration slightly enhances the FWM efficiency of the ground state and the first excited state and lowers the FWM efficiency of the higher excited states. Also we find that doping the dashes by n -type concentration slightly modifies the FWM efficiency spectrum.

In this chapter, Four Wave Mixing (FWM) and its types are elaborated in detail. It also describes the mathematical equations which provide basis for mathematical modeling and subsequent realization on various platforms. In order to achieve optical frequency shifting by FWM in silicon-on-insulator (SOI) waveguide, a nonlinear phenomenon has been presented. The FWM process in SOI waveguides is also discussed with an emphasis on the effects of two-photon absorption and the consequent free-carrier effects.

Optical vortices (OVs) are singular optical beams with a spiral phase dislocation in their wavefront. When two pump beams at the frequencies1 and2 interact in a Kerr-type nonlinear medium, the degenerate four-wave mixing (FWM) process results in the generation of new distinct sum and difference frequency components at the output. Importantly, the FWM process is expected to preserve the topological charge of the OVs, and thus it can be employed for the generation of white-light vortices, in contrast to the vortex propagation in a Raman nonlinear medium [1]. However, as the FWM process is accompanied by noticeable nonlinear instabilities due to self- and cross-phase modulation, the phase information in the newly generated frequency components may be destroyed [2]. Therefore, it is important to reveal the possible regimes where the FWM process will dominate nonlinear instabilities, leading to the stable generation of white-light optical vortices.

In this paper, an optically controlled tunable delay scheme has been proposed using four-wave mixing (FWM) wavelength conversion in a 35-cm highly nonlinear bismuth-oxide fiber (Bi-NLF) together with group velocity dispersion (GVD) in a chirped fiber Bragg grating (CFBG). The Bi-NLF offers a very large nonlinearity and gives rise to significant FWM over a short fiber segment. With the use of a CFBG, a delay range over 185 ps has been experimentally demonstrated. To investigate the performance of the tunable delay, we have applied the scheme for variable delays of 10-Gb/s amplitude-shift keying (ASK) and differential phase-shift keying (DPSK) data signals. The bit error rate (BER) measurements show a power penalty of less than 3.5 dB for both amplitude- and phase-modulated data formats. To further increase the delay time, the CFBG has been replaced by a dispersion compensated fiber (DCF) that provides a wider bandwidth of operation. A variable delay up to 840 ps has been obtained using dual-pump FWM that offers a conversion bandwidth of about 40 nm. The large conversion range helps to minimize GVD-induced pulse distortion as a shorter DCF can be used for a given delay. The Bi-NLF provides an enhanced stimulated Brillouin scattering (SBS) threshold, a reduced latency, and an increased compactness of the approach for practical applications.

Four -Wave Mixing and Phase Conjugation in Plasmas Using Ionization NonlinearitiesJohn F. Federici' and Ernie J. ValeoPrinceton Plasma Physics Laboratory. Princeton UniversityP.O. Box 451, Princeton. New Jersey 08544ABSTRACTPlasmas have recently received considerable attention as potentially efficient long wavelength (infrared to mi-crowave) phase conjugators using degenerate four -wave mixing (DFWM). In such processes, two electromagnetic(EM) pump waves (`forward' and `backward') plus a `probe' electromagnetic wave interact through the nonlinear lowfrequency susceptibility of the medium to produce a fourth `signal' or `conjugate' electromagnetic wave. The processmay also be viewed as an Bragg scattering of an EM wave from a low frequency density grating. The density gratingis generated by the coupling of one pump wave to the probe wave via pondermotive or thermal pressure forces. Inthis paper, the possibility of using ionization nonlinearities in weakly ionized plasmas for wave mixing applicationsis discussed. By locally heating the weakly ionized plasma, one may selectively ionized portions of the plasma toproduce a low frequency density grating. The applicable wavelength regimes, plasma parameters, and response timesfor ionization nonlinearities will be discussed.I. INTRODUCTIONPhase conjugation has many applications for signal processing, beam combining, tracking and pointing, laserbeam clean up, phase distortion compensation, etc..1,2 It has long been recognized that plasmas are efficient longwavelength (infrared through microwaves to radio waves) nonlinear optical mediums for phase conjugation by four -wave mixing (FWM).3 -6 Recent experiments have demonstrated nonlinear thermal forces' as well as pondermotiveforces8 for FWM in plasmas. This paper focuses on a new nonlinearity for plasmas based on laser- induced ionizationimbalances within the plasma.In collisionless plasmas, the pondermotive force dominates the thermal force. When the electron collisional mean -free -path is short compared to the beat -wave wavelength, thermal pressure forces become important. If a plasmawere weakly ionized, there would be another nonlinear force in plasmas - the ionization 'force' - which may beexploited for wave mixing and other nonlinear optical effects. Like the thermal pressure force, the ionization forceresults from the heating of the plasma by the beat -wave. However, as the weakly ionized plasma is heated, moreneutrals are ionized; instead of pushing the electrons into a grating structure, the electron grating is created directlyby ionization.The nonlinear mechanism in the plasma which generates the electron density grating for FWM is similar tothe mechanism for ionization instability driven electron -density waves. These waves are also called `striations' or`ionization waves'. A common type of striations occur naturally in discharge plasmas.''Semiconductors exhibit an analogous `ionization nonlinearity' for FWM.12 In semiconductors, the coupled pumpand probe waves generate a large concentration of free carriers (electron -hole plasma) by optically induced transitionsof electrons between the valance and conduction bands. For laser wavelengths near a semiconductor bandgap, the

Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) has several important features, such as, high speed and high FWM conversion efficiency as well as optical demultiplexing (DEMUX) (Mecozzi et al., 1995; Mecozzi & Mrk, 1997; Das et al., 2000). The are several applications of FWM in SOAs for all-optical devices, such as, wavelength converters (Vahala et al., 1996), optical samplers (Inoue & Kawaguchi, 1998), optical phase conjugators (Kikuchi & Matsumura, 1998) and optical multiplexers/demultiplexers (Kawanishi et al., 1997; Kawanishi et al., 1994; Morioka et al., 1996; Uchiyama et al., 1998; Tomkos et al., 1999; Kirita et al., 1998; Buxens et al., 2000) have been demonstrated for optical communication systems. When a pulse of a time-multiplexed signal train (for example, a probe pulse) and a pump pulse are injected simultaneously into an SOA, gain and refractive index in the SOA are modulated and an FWM signal is generated by the modulations. Thus, we can obtain a demultiplexed signal as an FWM signal at the output of SOA. All-optical demultiplexing has been experimentally demonstrated up to 200 Gbit/s (Morioka et al., 1996). Tomkos et al., (Tomkos et al., 1999) suggested a number of ways to improve the performance of the dual-pump demultiplexer at 40 Gbit/s as follows; adjustment of the input wavelengths at the peak gain wavelength of the SOA under saturation conditions, the use of higher pump power at the input of the device, or/and the use of pulsed pumps with short pulsewidths. For the higher bit-rate, the overlap of the input to the FWM signal pulses may appear both in the time and spectral domain. The pattern effect may also appear in the FWM signal due to the slow components of the optical nonlinearities in SOAs (Saleh & Habbab, 1990). These effects degrade the usefulness of the FWM in SOAs as a practical DEMUX device in optical network/ communication systems. Therefore, it is very important to analysis the optical DEMUX characteristics based on FWM in SOAs for the ultrafast multi-bit input optical pulses.

We propose an optical parametric diffuser (OPD), a novel optical wavelength conversion device for wide wavelength band optical sampling systems. The OPD is made by using a multiple quantum well (MQW) structure with different band gap materials. Our aim is to achieve wavelength conversion of wider than 30 THz using the OPD and we simulated the wavelength conversion by four-wave mixing (FWM) for pico-second order optical pulses into the OPD. Experimental results indicated a conversion efficiency of -10 dB at a wavelength conversion frequency of 10 THz. To confirm this simulation result, we fabricated a basic OPD and measured the characteristics of wavelength conversion. These measurements gave a conversion efficiency of -22 dB (approx. -16 dB for the basic OPD itself) at a wavelength conversion frequency of 5 THz. Comparison of the simulation and experimental results suggested the possibility of high wavelength conversion efficiency even above 20 THz conversion frequency by optimizing the OPD structure.

Future optical networks may require both wavelength conversion and bit shape regeneration in an all-optical domain. The possibility of pulse reshaping while providing wavelength conversion may support new demands over medium and large distances links (Kelly, 2001). Indeed, during propagation the optical data signal suffers deterioration due to the amplified spontaneous emission (ASE) from optical amplifiers, pulse distortion from intrinsic dispersion, crosstalk, and attenuation. All-optical regenerators may be important components for the restoration of these signals, providing complexity and cost reductions with the avoidance of optoelectronic conversions. The regeneration could be 2R (reamplification and reshaping) or 3R, which also provide retiming to solve jitter (Simon et al., 2008). Several 3R regenerators using the semiconductor optical amplifier (SOA) have been proposed, such as cascaded SOAs setups (Funabashi et al., 2006) or SOA based MachZehnder interferometers (MZI) (Fischer et al., 1999). However for small and medium distances systems, where the signal amplitude noise and distortions form the main problem and where jitter has fewer magnitudes, the simpler 2R processes can be adequate to keep signal quality (Simon et al., 1998). In addition, the SOA is a helpful device for both 2R-regeneration (Ohman et al., 2003) and wavelength conversion (Durhuus et al., 1996). Several techniques for 2R-regeneration based on SOAs have been proposed and tested, for example by using four-wave mixing (FWM) (Simos et al., 2004), cross-gain modulation (XGM) (Contestabile et al., 2005), integration within MZI (Wang et al., 2007), multimode interferometric SOA (Merlier et al., 2001), cross-phase modulation (XPM) with filtering (Chayet et al., 2004), and feed forward technique (Conforti et al., 1999). However, these techniques require complex designs and involve critical operation points, even the simplest ones based on XGM features. In addition, most of these techniques are not capable of wavelength conversion and regeneration simultaneously. Recently a regenerator based on cross-gain modulation was proposed using one SOA for wavelength conversion (in a counter-propagating mode) and another deeply saturated SOA (synchronized by an optical delay line) to achieve cross-gain compression (Contestabile 2005). This efficient approach has similarities with the all-optical feed-forward techniques. In addition, this regenerator could not done wavelength conversion if the wavelength of the

Future high capacity wavelength division multiplexed (WDM) optical networks will rely on transmitting signals with information encoded on both the amplitude and phase of multiple optical carriers. To optimise the efficiency of these networks, they are likely to employ some level of wavelength switching [1], and a key function required by wavelength switching is wavelength conversion [1]. Wavelength conversion of phase modulated signals in wavelength switched optical networks will need to be undertaken using a coherent nonlinear process, such as four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) [2] employing a fast wavelength tuneable pump laser to ensure an optical channel can be dynamically routed to different wavelengths. A crucial issue that is likely to have a major impact on the wavelength conversion of such high capacity signals using FWM is the phase noise transfer to the converted signal. A detailed study of FWM wavelength converted continuous-wave signals in [3] showed that the phase noise transfer from the pump to the idler, increased the linewidth of the idler by a factor of 4 relative to the pump. Such a large line broadening is highly detrimental to the detection of phase-encoded signals, especially since the tuneable lasers that exhibit small wavelength switching times (<;50 ns) and would be employed in wavelength switched networks tend to have linewidths of the order of a few MHz [4]. Therefore estimating the linewidth limits that enable FWM wavelength conversion using this scheme is vital to select a tuneable pump laser with suitable linewidth.

: The focus of this research project is to investigate new approaches to switching multiple wavelengths within small volumes and low power consumption that is independent of data rates We also have basic experimental results that show a path toward implementation of this type of switching component. The potential impact on future DoD and military needs will be where the movement of large amounts of data is needed in space and power sensitive applications (e.g. fighter aircraft, ship based networks, rapidly deployed networks, harsh environments).