The authors employ a novel PIC to demonstrate athermalised transmission under direct modulation. The unique architecture of the chip enables compensation of thermal redshift of laser wavelength through control of self-heating. The PIC consists of 2 lasers in a master-slave configuration. Such a structure improves the performance of a free running laser (slave) by optical injection from another laser (master). The benefits reaped from such a PIC structure implementing optical injection include chirp minimisation, linewidth reduction, and improved modulation response. Experimental demonstration of 2.5 Gb/s OOK data transmitted over 37 km with error-free performance over the temperature range 10-45°C is presented.
The authors propose a novel, multifunctional approach to the problem of demultiplexing closely spaced channels sourced from an optical frequency comb. This solution, based on an externally injected laser, combines the functionality of a tunable demultiplexer, an ultra-low noise amplifier and a modulator, using a single device. Such a device can serve as an optimum transceiver for next generation broadband access (wired and wireless) networks.
Optical sources for the forthcoming terabit/s era of optical communications and networking will require multiple frequency-locked carriers, each with low phase noise, in order to minimize the spectral occupancy of the overall channel bandwidth. One method to construct a highly reconfigurable version of such a source is to use an optical frequency comb from a gain-switched laser to simultaneously injection-lock many different single mode lasers. The outputs from the single-mode lasers are all mutually frequency locked and possess the same low-phase noise properties of the gainswitched comb. In this submission, we present numerical simulation results from the entire system of simultaneously injection-locked single mode lasers by firstly simulating an optical frequency comb from the gain-switched laser and then using that frequency comb to injection lock the single-mode lasers. The simulation approach is to use lumped rate equations with the appropriate stochastic Langevin terms for spontaneous carrier recombination and for spontaneous emission. The inclusion of the stochastic terms are vital when identifying the locked states of the entire system. Using the simulator we are able to identify important criteria to maximize the frequency locking range that suppresses the cross talk from adjacent comb lines to greater than 30 dB, and avoiding the carrier-photon resonance of the single mode lasers is vital to achieve this. The relative simplicity of the simulator has the advantage of being exploited within optical communication simulators to predict the communication system performance when using these sources, which would be of advantage to designers of such systems.
Discrete Mode Laser Diodes (DMLDs) present an economic approach with a focus on high volume manufacturability of
single mode lasers using a single step fabrication process. We report on a DMLD designed for operation in the 1550 nm
window with high Side Mode Suppression Ratio (SMSR) over a wide temperature tuning range of -20 °C < T < 95 °C.
Direct modulation rates as high as 10 Gbit/s are demonstrated at both 1550 nm and 1310 nm. Transmission experiments
were also carried out over single mode fibre at both wavelengths. Using dispersion pre-compensation transmission from
0 to 60 km is demonstrated at 1550 nm with a maximum power penalty measured at 60 km of 3.6 dB.
KEYWORDS: Semiconductor lasers, Modulation, Receivers, Digital signal processing, Temperature metrology, Heterodyning, Phase shift keying, Waveguides, Modulators, Signal detection
We report on discrete mode laser diodes designed for narrow linewidth emission and demonstrate a linewidth as low as
96 kHz. A discrete mode laser diode with a minimum linewidth of 189 kHz was also characterised in a coherent
transmission setup using quadrature phase shift keying modulation. Similar performance to an external cavity laser is
demonstrated at baud rates as low as 2.5 Gbaud. The effect of increased linewidth on transmission performance is also
investigated using lasers with linewidths up to 1.5 MHz.
Slow light generation through four wave mixing is experimentally investigated in a non-linear semiconductor optical
amplifier (SOA). The mechanism of slow-light generation is analyzed through gain saturation behavior of the SOA. The
delay of the probe beam is controlled optically by pump-probe detuning. A delay of 260 ps is achieved for sinusoidal
modulation at 0.5 GHz corresponding to a RF phase change of 0.26π.
We revisit the use of nonlinear pulse compression for ultrafast pulse train generation in terms of the evolution dynamics
of analytic breather solutions of the nonlinear Schrödinger equation. We discuss to what degree the analytic formalism
of Akhmediev Breather solutions can provide improved insight into the compression process, providing a useful
complement to the more widely employed approach to pulse train optimization using numerical simulations. We also
report experiments where nonlinear reshaping of a directly modulated DFB laser diode signal at 1550 nm in standard
single mode fibre is used to generate a train of sub-20 ps compressed pulses at 11.7 GHz. Characterization using a
Picosolve sampling scope reveals directly the expected compressed pulse and pedestal features.
The wavelength spectra of ridge waveguide Fabry Perot lasers can be modified by perturbing the effective refractive
index of the guided mode along very small sections of the laser cavity. One way of locally perturbing the effective index
of the lasing mode is by etching features into the ridge waveguide such that each feature has a small overlap with the
transverse field profile of the unperturbed mode, consequently most of the light in the laser cavity is unaffected by these
perturbations. A proportion of the propagating light is however reflected at the boundaries between the perturbed and the
unperturbed sections. Suitable positioning of these interfaces allows the mirror loss spectrum of a Fabry Perot laser to be
manipulated. In order to achieve single longitudinal mode emission, the mirror loss of a specified mode must be reduced
below that of the other cavity modes. Here we review the latest results obtained from devices containing such features.
These results clearly demonstrate that these devices exceed the specifications required for a number of FTTH and
Datacomms applications, such as GEPON, LX4 and CWDM. As well as this we will also present initial results on the
linewidth of these devices.
The authors investigate the propagation of picosecond pulses through Semiconductor Optical Amplifiers using the measurement technique of Frequency Resolved Optical Gating by applying pulses of varying peak power, pulse width and shape. Frequency Resolved Optical Gating is a relatively new measurement system which provides complete characterisation of the pulses in both the temporal and spectral domains. We examine the pulses before and after amplification through the Semiconductor Optical Amplifier. The work shows pulse broadening, the formation of large pulse pedestals, and the generation of significant frequency chirp across the pulse in the temporal domain. In the spectral domain results exhibit spectral broadening and a shift to longer wavelengths. These properties of the output pulses would cause serious degradation in high-speed communications systems employing Wavelength Division Multiplexing and Optical Time Division Multiplexing. The resulting physical properties occurring to the pulses due to propagation through the Semiconductor Optical Amplifier are a result of Self Phase Modulation, which is due to gain saturation induced by carrier depletion and carrier heating.
The authors present the idea of using direct modulation of a laser for generating multiple RF carriers for hybrid radio/fiber systems. External light injection is used in order to increase the modulation bandwidth of the laser and at the same time to improve the overall system operation. The experiments show a 16 dB enhancement in the performance of the system used for distributing 155 Mb/s data signals on 18.6 and 19 GHz carriers.
It is obvious that the development of a wavelength tunable source of short optical pulses will be of paramount importance for future Wavelength Division Multiplexing (WDM), optical time division multiplexed (OTDM), and hybrid WDM/OTDM optical communication systems. Self-seeding of a gain-switched Fabry-Perot (FP) laser is one of the most reliable techniques available to generate wavelength tunable optical pulses. An important characteristic of these self-seeded gain-switched (SSGS) sources is the variation in the side-mode-suppression-ratio (SMSR) as the wavelength is tuned, as this may ultimately affect their usefulness in optical communication systems. A quantitative analysis of how mode partition noise and side mode suppression ratio affects a wavelength division multiplexed system is investigated here. Bit Error Rate measurements have been carried out on a 4-channel wavelength division multiplexed set-up using tunable self-seeded gain-switched pulse sources. These measurements demonstrate the degradation in overall system performance, due to mode partition noise, as the side mode suppression ratio of the self-seeded gain-switched optical pulse sources is reduced. The results also show that the constraints on the minimum side mode suppression ratio required increase with the number of channels in the system.
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