Simultaneous ultrafast optical pulse train bursts generation and shaping based on Fourier series developments using superimposed fiber Bragg gratings

We propose an all-fiber method for the generation of ultrafast shaped pulse train bursts from a single pulse based on Fourier Series Developments (FDSs). The implementation of the FSD based filter only requires the use of a very simple non apodized Superimposed Fiber Bragg Grating (S-FBG) for the generation of the Shaped Output Pulse Train Burst (SOPTB). In this approach, the shape, the period and the temporal length of the generated SOPTB have no dependency on the input pulse rate.

Pulse shaping by phase-modulated fiber gratings in transmission

We propose a novel approach to pulse shaping using a phase-modulated fiber Bragg gratings (FBGs) in transmission. This enables the simplification of the device fabrication while retaining the substantial advantages of FBGs in transmission. © 2013 OSA.

Proposal and design of phase-modulated fiber gratings in transmission for pulse shaping

A novel approach to pulse shaping using a phase-modulated fiber Bragg grating (FBG) in transmission is proposed and designed. We show that phase-modulated FBGs can provide transmission responses suitable for pulse shaping applications, offering important technological feasibility benefits, since the coupling strength remains basically uniform in the grating. Moreover, this approach retains the substantial advantages of FBGs in transmission, such as optimum energy efficiency, no requirement for an optical circulator, and robustness against fabrication errors.

Theory of modulational instability in Bragg gratings with quadratic nonlinearity

Modulational instability in optical Bragg gratings with a quadratic nonlinearity is studied. The electric field in such structures consists of forward and backward propagating components at the fundamental frequency and its second harmonic. Analytic continuous wave (CW) solutions are obtained, and the intricate complexity of their stability, due to the large number of equations and number of free parameters, is revealed. The stability boundaries are rich in structures and often cannot be described by a simple relationship. In most cases, the CW solutions are unstable. However, stable regions are found in the nonlinear Schrodinger equation limit, and also when the grating strength for the second harmonic is stronger than that of the first harmonic. Stable CW solutions usually require a low intensity. The analysis is confirmed by directly simulating the governing equations. The stable regions found have possible applications in second-harmonic generation and dark solitons, while the unstable regions maybe useful in the generation of ultrafast pulse trains at relatively low intensities. [S1063-651X(99)03005-6].

Actively modelocked dual-wavelength fibre laser with ultra-low inter-pulse-stream timing jitter

We demonstrate a novel dual-wavelength erbium-fiber laser that uses a single nonlinear-optical loop mirror modulator to simultaneously modelock two cavities with chirped fiber Bragg gratings as end mirrors. We show that this configuration produces synchronized soliton pulse trains with an ultra-low RMS inter-pulse-stream timing jitter of 620 fs enabling application to multiwavelength systems at data rates in excess of 130 Gb/s.

Environmentally stable high-power soliton fiber lasers that use chirped fiber Bragg gratings

Environmentally stable high-power erbium fiber soliton lasers are constructed by Kerr or carrier-type mode locking. We obtain high-energy pulses by using relatively short fiber lengths and providing large amounts of negative dispersion with chirped fiber Bragg gratings. The pulse energies and widths generated with both types of soliton laser are found to scale with the square root of the cavity dispersion. Kerr mode locking requires pulses with an approximately three times higher nonlinear phase shift in the cavity than carrier mode locking, which leads to the generation of slightly shorter pulses with as much as seven times higher pulse energies at the mode-locking threshold.

Design and fabrication of fiber Bragg gratings with V-shaped dispersion profile

We propose a new type of fiber Bragg grating (FBG) with a V-shaped dispersion profile. We demonstrate that such V-shaped FBGs bring advantages in manipulation of optical signals compared to conventional FBGs with a constant dispersion, e.g., they can produce larger chirp for the same input pulsewidth and/or can be used as pulse shapers. Application of the proposed V-shaped FBGs for signal prechirping in fiber transmission is examined. The proposed design of the V-shaped FBG can be easily extended to embrace multichannel devices.

The use of loop mirrors and chirped fibre Bragg gratings in actively-modelocked fibre lasers

The development of an all-optical communications infrastructure requires appropriate optical switching devices and supporting hardware. This thesis presents several novel fibre lasers which are useful pulse sources for high speed optical data processing and communications. They share several attributes in common: flexibility, stability and low-jitter output. They all produce short (picosecond) and are suitable as sources for soliton systems. The lasers are all-fibre systems using erbium-doped fibre for gain, and are actively-modelocked using a dual-wavelength nonlinear optical loop mirror (NOLM) as a modulator. Control over the operating wavelength and intra-cavity dispersion is obtained using a chirped in-fibre Bragg grating.Systems operating both at 76MHz and gigahertz frequencies are presented, the latter using a semiconductor laser amplifier to enhance nonlinear action in the loop mirror. A novel dual-wavelength system in which two linear cavities share a common modulator is presented with results which show that the jitter between the two wavelengths is low enough for use in switching experiments with data rates of up to 130Gbit/s.

Ultrashort pulse generation from diode laser devices

This thesis presents a detailed, experiment-based study of generation of ultrashort optical pulses from diode lasers. Simple and cost-effective techniques were used to generate high power, high quality optical short pulses at various wavelength windows. The major achievements presented in the thesis is summarised as follows. High power pulses generation is one of the major topics discussed in the thesis. Although gain switching is the simplest way for ultrashort pulse generation, it proves to be quite effective to deliver high energy pulses on condition that the pumping pulses with extremely fast rising time and high enough amplitude are applied on specially designed pulse generators. In the experiment on a grating-coupled surface emitting laser (GCSEL), peak power as high as 1W was achieved even when its spectral bandwidth was controlled within 0.2nm. Another experiment shows violet picosecond pulses with peak power as high as 7W was achieved when the intensive electrical pulses were applied on optimised DC bias to pump on InGaN violet diode laser. The physical mechanism of this phenomenon, as we considered, may attributed to the self-organised quantum dots structure in the laser. Control of pulse quality, including spectral quality and temporal profile, is an important issue for high power pulse generation. The ways to control pulse quality described in the thesis are also based on simple and effective techniques. For instance, GCSEL used in our experiment has a specially designed air-grating structure for out-coupling of optical signals; hence, a tiny flat aluminium mirror was placed closed to the grating section and resulted in a wavelength tuning range over 100nm and the best side band suppression ratio of 40dB. Self-seeding, as an effective technique for spectral control of pulsed lasers, was demonstrated for the first time in a violet diode laser. In addition, control of temporal profile of the pulse is demonstrated in an overdriven DFB laser. Wavelength tuneable fibre Bragg gratings were used to tailor the huge energy tail of the high power pulse. The whole system was compact and robust. The ultimate purpose of our study is to design a new family of compact ultrafast diode lasers. Some practical ideas of laser design based on gain-switched and Q-switched devices are also provided in the end.

Fabrication and applications of fibre Bragg gratings

The consequences of fabricating Bragg gratings in various fibres, with or without hydrogen loading, and with varying laser power levels are explored. Three new techniques for fabricating chirped gratings are presented. Beams with dissimilar wavefront curvatures are interfered to give chirped gratings. With the same aim techniques of writing gratings on tapered fibres and on deformed fibres are also covered. With these techniques, a wide variety of gratings has been fabricated from the 'superbroad' (with bandwidths of up to 180 nm), small to medium bandwidth gratings with linear chirp profiles and quadratic chirped gratings. It is demonstrated that chirped grating can be concatenated to form all-fibre Fabry-Perot and Moiré resonators. These are further concatenated with chirped gratings to produce filters with narrow passbands and very broad stopbands. A number of other applications are also addressed. The use of chirped fibre gratings for dispersion compensation and femtosecond chirped pulse amplification is demonstrated. Chirped gratings are used as dispersive elements in modelocked fibre lasers producing ultrashort pulses. A chirped fibre grating Fabry-Perot transmission filter is used in a continuous wave laser that exhibits eleven simultaneously lasing wavelengths. Finally, the use of grating-coupler devices as variable reflectivity mirrors for laser optimisation and gain clamping is considered.

Design and fabrication of fibre Bragg gratings with V-shaped dispersion profile for multi-channel signal processing

We propose a new type of fiber Bragg grating (FBG) with a V-shaped dispersion profile. We demonstrate that such V-shaped FBGs bring advantages in manipulation of optical signals compared to conventional FBGs with a constant dispersion, e.g., they can produce larger chirp for the same input pulsewidth and/or can be used as pulse shapers. Application of the proposed V-shaped FBGs for signal prechirping in fiber transmission is examined. The proposed design of the V-shaped FBG can be easily extended to embrace multichannel devices.

Grating design of oppositely chirped FBGs for pulse shaping

In this letter, we analyze and develop the required basis for a precise grating design in a scheme based on two oppositely chirped fiber Bragg gratings, and apply it in several examples which are numerically simulated. We obtain the interesting result that the broader bandwidth of the reshaped pulse, the shorter gratings required.