Ultrashort optical pulses are widely and increasingly used in many diverse fields of science and technology. By providing high temporal resolution they enable investigation and measurement of fundamental physical, chemical and biological phenomena that occur on picosecond time scales or shorter. In addition ultrashort pulses are an essential enabling tool for high speed optical communications and data processing technologies, as well as in advanced manufacturing and photomedicine applications. In all of these areas precise measurement and control of ultrashort optical pulses is vital - advances in ever shorter pulse generation must be accompanied by new methods to characterise and manipulate them.
This thesis presents work on the ongoing development of an ultrashort pulse measurement and manipulation technique known as time-to-space conversion. Time-to-space conversion uses sum-frequency generation between spectrally resolved ultrashort pulses to transfer information from the time domain to the space domain; in other words to create the real-time spatial image of an ultrashort pulse. Mapping the pulse temporal intensity envelope and phase onto a quasi-static spatial image allows high resolution measurement of these quantities, overcoming the difficulty of optoelectronic detection of ultrashort pulses directly in the time domain. Furthermore, the spectrally resolved nature of time-to-space conversion results in a large time window of operation. This enables a series of ultrashort pulses to be simultaneously transferred to spatially separated locations via interaction with a single reference pulse, thereby performing an all-optical demultiplexing operation.
The two main developments introduced in this thesis are: a) greater feasibility of time-to-space conversion for all-optical demultiplexing of a high speed optical communications channel by demonstrating the technique in a planar nonlinear waveguide and b) the demonstration of full-field characterisation of ultrashort pulses by using interferometric detection after the time-to-space conversion. The practicality of time-to-space conversion for all-optical demultiplexing depends on minimising its optical power consumption. This can be achieved by implementation of the conversion process in the guided-wave regime, as opposed to the free-space regime in which it has previously been demonstrated. The first three papers presented in this thesis describe the preliminary steps towards this goal, namely the demonstration of non wavelength-degenerate and background-free collinearly phase-matched time-tospace conversion and the demonstration of time-to-space conversion in a planar nonlinear waveguide. Full-field characterisation of ultrashort pulses by time-to-space conversion is enabled by the quasi-monochromaticity of the output sum-frequency signal, a feature which follows from the unique geometry of the oppositely dispersed waves of the pulse to be measured and the reference pulse. The quasi-monochromatic converted signal can be mixed with a narrow linewidth local oscillator for interferometric measurement of the ultrashort pulse field amplitude and phase. The final two papers included here describe the first time demonstration of full-field measurement of bandwidth-limited and chirped pulses by time-to-space conversion and of single-shot coherent detection of a phase modulated ultrashort pulse train.
Taken together, the work presented in this thesis has achieved an increase in the utility of time-to-space conversion as an ultrashort optical pulse measurement and manipulation technique, with potential applications in optical communications and data processing and in the field of ultrashort pulse measurement.
A novel approach for a multi-port Wavelength Selective Switch (WSS) is shown in this work. The switching is performed from a series of 8 input fibers to a series of 24 output fibers. This device can be useful in reducing the complexity of optical communication nodes based on conventional switch with only one input fiber
The multi-port switching is based on a spatial separation, of light beams, according to input port and wavelength channel on a dynamic steering device – LCoS (Liquid Crystal on Silicon) SLM (Spatial light modulator).
The LCoS SLM was extensively characterized in order to understand the capabilities and limitations for better system design.
The system design is extensively discussed in this paper and a proof of concept experiment demonstrates that indeed this concept can be realized.
A record performance metric arrayed waveguide grating (AWG) design with a 200 GHz free spectral range (FSR) capable of resolving sub-one GHz resolution spectral features is developed for a fine resolution photonic spectral processor (PSP). The AWGʼs FSR was designed to support sub-channel add/drop from a super-channel of 1Tb/s capacity. Due to fabrication imperfections we introduce phase corrections to the light beams emerging from the 250 waveguides of the AWG output using a liquid crystal on Silicon (LCoS) phase spatial light modulator (SLM) placed in an imaging configuration. A second LCoS SLM is located at the Fourier plane, for arbitrary spectral amplitude and phase manipulations. The PSP is utilized in different experiments, such as flexible spectral shaping and sub-carrier drop demultiplexer with sub-GHz spectral resolution.
Accurate amplitude and phase measurements of ultrashort optical waveforms are essential for their use in a wide range of scientific disciplines. Here we report the first demonstration of full-field optical reconstruction of ultrashort waveforms using a time-to-space converter, followed by a spatial recording of an interferogram. The algorithm-free technique is demonstrated by measuring ultrashort pulses that are widely frequency chirped from negative to positive, as well as phase modulated pulse packets. Amplitude and phase measurements were recorded for pulses ranging from 0.5 ps to 10 ps duration, with measured dimensionless chirp parameter values from −30 to 30. The inherently single-shot nature of time-to-space conversion enables full-field measurement of complex and non-repetitive waveforms.
Phase modulated sub-picosecond pulses are converted by a time-to-space processor to quasi-monochromatic spatial beams that are spatially demultiplexed and coherently detected in real-time. The time-to-space processor, based on sum-frequency generation (SFG), serves as a serial-to-parallel converter, reducing the temporal bandwidth of the ultrashort pulse to match the bandwidth of optoelectronic receivers. As the SFG process is phase preserving, we demonstrate homodyne coherent detection of phase modulated temporal pulses by mixing the demultiplexed SFG beam with a narrow linewidth local oscillator (LO) resulting in single-shot phase detection of the converted pulses at a balanced detector. Positively and negatively phase-modulated signal pulses are individually detected and LO shot noise limited operation is achieved. This demonstration of real-time demultiplexing followed by single-shot full-field detection of individual pulses, highlights the potential of time-to-space conversion for ultrahigh bit rate optical communications and data processing applications.
Variable optical attenuation (VOA) for three-mode fiber is experimentally presented, utilizing an amplitude spatial light modulator (SLM), achieving up to −28dB uniform attenuation for all modes. Using the ability to spatially vary the attenuation distribution with the SLM, we also achieve up to 10dB differential attenuation between the fiber’s two supported mode group (LP01 and LP11). The spatially selective attenuation serves as the basis of a dynamic mode-group equalizer (DME), potentially gain-balancing mode dependent optical amplification. We extend the experimental three mode DME functionality with a performance analysis of a fiber supporting 6 spatial modes in four mode groups. The spatial modes’ distribution and overlap limit the available dynamic range and performance of the DME in the higher mode count case.
We generate transform-limited WDM optical sampling pulse bursts by filtering ultrashort pulses from a mode-locked laser. A phase spatial light modulator (SLM) is used in a biased pulse shaper to circumvent the need to modulate with 2π phase wraps, which are known to limit the phase response. The arrangement compresses and retimes user-selectable bandwidths from the optical short pulse source with precise control of pulse bandwidth, pulse stream rates, and duty cycle.
We introduce a next-generation long-reach access optical network (35 dB loss budget 2 dB margin) delivering up to 40G/40G per passive 1∶256 optical distribution network, supportingsymmetrical1 Gb∕s rates perhome user or up to 40 Gb∕s for business users (e.g., enterprises, antenna sites). The proposed system is based on a novel spectrally efficient orthogonal frequency division multiplexing/ wavelength division multiplexingOFDM/WDMarchitecture symmetrically using 16-QAM OFDM polarization diversity in both the downstream and upstream in order to serve low-cost energy-efficient symmetric 1 Gb∕s optical network units (ONUs), which are self-coherent, laserless, colorless, and tunable-filter-free. Each ONU comprises a standard semiconductor optical amplifier (SOA), a silicon-based photonic integrated circuit (PIC), and mixed-signal electronic integrated circuits (ICs) performing the signal processing at a relatively slow rate as compared with the overall passive optical network (PON) throughput: digital to analog converters (DACs) and analog to digital converters (ADCs) at 417 MS∕s for the home user ONUs.
We report a two-span, 67-km space-divisionmultiplexed (SDM) wavelength-division-multiplexed (WDM) system incorporating the first reconfigurable optical add–drop multiplexer (ROADM) supporting spatial superchannels and the first cladding-pumped multicore erbium-doped fiber amplifier directly spliced to multicore transmission fiber. The ROADM subsystem utilizes two conventional 1 × 20 wavelength selective switches (WSS) each configured to implement a 7 × (1 × 2) WSS. ROADM performance tests indicate that the subchannel insertion losses, attenuation accuracies, and passband widths are well matched to each other and show no significant penalty, compared to the conventional operating mode for the WSS. For 6 × 40 × 128-Gb/sSDM-WDMpolarization-multiplexed quadrature phaseshift- keyed (PM-QPSK) transmission on 50 GHz spacing, optical signal-to-noise ratio penalties are less than 1.6 dB in Add, Drop, and Express paths. In addition, we demonstrate the feasibility of utilizing joint signal processing of subchannels in this two-span, ROADM system.
We propose a new type of photonic analog-to-digital converter (ADC), designed for high-resolution (>7 bit) and high sampling rates (scalable to tens of GS/s). It is based on encoding the input analog voltage signal onto the phase of an optical pulse stream originating from a modelocked laser, and uses spatial oversampling as a means to improve the conversion resolution. This paper describes the concept of spatial oversampling and draws its similarities to the commonly used temporal oversampling. The design and fabrication of a LiNbO3/silica hybrid photonic integrated circuit for implementing the spatial oversampling is shown, and its abilities are demonstrated experimentally by digitizing gigahertz signals (frequencies up to 18GHz) at an undersampled rate of 2.56GS/s with a conversion resolution of up to 7.6 effective bits. Oversampling factors of 1-4 are demonstrated.