Nonlinear photonics with applications in lightwave communications

This doctoral dissertation investigates the use of nonlinear photonics in targeted Lightwave communication applications. Different highly nonlinear optical materials have been considered for the investigation of Lightwave communications data carriers, with a focus on the optical carrier pulsewidth. A state-of-the-art novel method has been developed to measure pico-second optical carrier pulses using highly nonlinear optical fiber. This method is based on the nonlinear optical loop mirror (NOLM), with consideration focused on the third order nonlinearity. Silicon is considered to be one of the most attractive materials for photonics integrated circuit technology (PIC) due to its compatibility with complementary metal oxide semiconductor (CMOS). As such, the method has been applied to the SOI platform Mach-Zehnder interferometer (MZI), also by considering the third order nonlinearity. In the NOLM approach, the picosecond optical data carrier pulsewidth is measured by using an optical power meter. Simulations for both the self-phase and cross-phase modulation schemes are carried out, and as expected, the cross phase modulation gives an increment in the sensitivity twice that of the self-phase modulation. Due to very high repetition rates of the order 10 GHz, the effect of counter propagating non-linear interactions in the NOLM are also considered in the theoretical evaluation. In the experimental validation, the pulses from an active fiber mode-locked laser at a repetition rate of 10 GHz were incrementally temporally dispersed using an SMF-28 fiber. The optical data carrier pulses over a range of 2-10 ps were successfully measured with a resolution of 0.25 ps. By extrapolating the theoretical evaluation and by selecting different physical parameters for the setup, the method was found to exhibit an extended range of 0.25 to 40 ps.;The concept described above is then extended to the investigation of nonlinear SOI devices using an MZI, thus miniaturizing the setup. In this investigation, the silicon waveguide has been simulated for self-phase and cross-phase modulation by solving the nonlinear Schrodinger equations using the split step method. Silicon has strong two photon absorption at telecommunication wavelengths, i.e. 1550 nm, and therefore all nonlinear losses (i.e. TPA and free carriers generated through TPA) are included in the split step simulations. The results obtained show that the on-chip nonlinear MZI (based on the SOI platform) can also be used for the measurement of optical data carrier pulse-widths of up to 10 ps. In the last part of this doctoral dissertation, a novel design for a temperature insensitive MZI is presented. Temperature dependence is one of the main challenges in the design of the SOI platform due to the large thermo-optic coefficient of its core material. A change in temperature can cause the device properties to deviate significantly, and can also alter the nonlinear properties of the device. Therefore, a design of an all-passive athermal MZI device based on the SOI platform has been developed and investigated. The MZI's temperature compensation is achieved by optimizing the relative length of the wire and subwavelength grating arms, and by tailoring the thermal response of the subwavelength structure. The simulation results of the athermal MZI design indicated that an overall temperature sensitivity of 7.5 pm/K could be achieved over a 100 nm spectral range near the 1550 nm region.This doctoral dissertation investigates the use of nonlinear photonics in targeted Lightwave communication applications. Different highly nonlinear optical materials have been considered for the investigation of Lightwave communications data carriers, with a focus on the optical carrier pulsewidth. A state-of-the-art novel method has been developed to measure pico-second optical carrier pulses using highly nonlinear optical fiber. This method is based on the nonlinear optical loop mirror (NOLM), with consideration focused on the third order nonlinearity. Silicon is considered to be one of the most attractive materials for photonics integrated circuit technology (PIC) due to its compatibility with complementary metal oxide semiconductor (CMOS). As such, the method has been applied to the SOI platform Mach-Zehnder interferometer (MZI), also by considering the third order nonlinearity. In the NOLM approach, the picosecond optical data carrier pulsewidth is measured by using an optical power meter. Simulations for both the self-phase and cross-phase modulation schemes are carried out, and as expected, the cross phase modulation gives an increment in the sensitivity twice that of the self-phase modulation. Due to very high repetition rates of the order 10 GHz, the effect of counter propagating non-linear interactions in the NOLM are also considered in the theoretical evaluation. In the experimental validation, the pulses from an active fiber mode-locked laser at a repetition rate of 10 GHz were incrementally temporally dispersed using an SMF-28 fiber. The optical data carrier pulses over a range of 2-10 ps were successfully measured with a resolution of 0.25 ps. By extrapolating the theoretical evaluation and by selecting different physical parameters for the setup, the method was found to exhibit an extended range of 0.25 to 40 ps.;The concept described above is then extended to the investigation of nonlinear SOI devices using an MZI, thus miniaturizing the setup. In this investigation, the silicon waveguide has been simulated for self-phase and cross-phase modulation by solving the nonlinear Schrodinger equations using the split step method. Silicon has strong two photon absorption at telecommunication wavelengths, i.e. 1550 nm, and therefore all nonlinear losses (i.e. TPA and free carriers generated through TPA) are included in the split step simulations. The results obtained show that the on-chip nonlinear MZI (based on the SOI platform) can also be used for the measurement of optical data carrier pulse-widths of up to 10 ps. In the last part of this doctoral dissertation, a novel design for a temperature insensitive MZI is presented. Temperature dependence is one of the main challenges in the design of the SOI platform due to the large thermo-optic coefficient of its core material. A change in temperature can cause the device properties to deviate significantly, and can also alter the nonlinear properties of the device. Therefore, a design of an all-passive athermal MZI device based on the SOI platform has been developed and investigated. The MZI's temperature compensation is achieved by optimizing the relative length of the wire and subwavelength grating arms, and by tailoring the thermal response of the subwavelength structure. The simulation results of the athermal MZI design indicated that an overall temperature sensitivity of 7.5 pm/K could be achieved over a 100 nm spectral range near the 1550 nm region

Measurement of optical pulsewidth in the picosecond regime using a non-linear fiber and power meter

A technique for the characterization of picosecond pulse widths is presented, based a non-linear optical fiber loop mirror and power meter measurement. Pulse-widths in the 2-10ps range are successfully recovered with a resolution of 0.25p

Model for the Prediction of Rain Attenuation Affecting Free Space Optical Links

A model for the prediction of the attenuation induced by rain on free space optical (FSO) links is presented. The proposed methodology is developed, starting from the accurate simulation of the interaction between FSO links (path length up to 5 km) and precipitation maps, from which analytical expressions are then proposed. The model is simple as it only receives the local rainfall statistics for the site of interest as the input, but it is also accurate, as it takes into account the dependence of the attenuation on the drop size distribution (DSD), as well as multiple scattering effects associated with rain, which contributes to reducing the overall predicted attenuation. The proposed model represents a useful tool to dimension FSO links in areas where fog is negligible, i.e., where rain has the highest impact on the link performance

Analysis of carrier frequency offset suppression techniques in SC-FDMA communication system

Single Carrier Frequency Division Multiplexing is a promising technique for high data rate uplink communication in 3GPP Long Term Evolution. It has a major advantage of low peak to average power ratio as compared to conventional Orthogonal Frequency Division Multiplexing systems. However, the problem of Carrier Frequency Offset (CFO) exists in both the systems. Different CFOs of the received signal damage orthogonality of the subcarriers. The two main reasons for this problem are misalignment of oscillator and Doppler shift effect. It results in overall performance degradation by causing inter carrier interface and multi user interference. Many solutions to this problem have been proposed in literature by estimating and suppressing CFO. This paper briefly discusses important CFO suppression techniques including Feedback Method, Inverse Pilot Matrix Method and Linear Parallel Interference Cancellation. The effect of CFO on these systems has been discussed and algorithms behind these techniques have also been explained. The review of CFO suppression and estimation algorithm is useful to select particular technique according to system requirements for improving overall performance

A generalized approach to analyze broadband arrow-shaped loaded-stub phase shifters

This paper discusses a simple and analysis-efficient approach to develop and design wideband loaded-stub (WB-LS) phase shifters. The WB-LS phase shifter achieves a uniform phase shift of 130°±5° over a relatively wide bandwidth by utilizing a transmission line loaded with one or two arrow-shaped open stubs and a reference line