The millimeter-wave (mm-wave) frequency band has emerged as a means to overcome current radio frequency spectral limitations and represents an interesting solution to fulfill the bandwidth and networking requirements of fifth generation (5G) mobile communications and beyond. Photonic generation of these frequencies holds advantages over electronic methods in terms of cost and effective network distribution. Due to their coherent nature, optical frequency combs (OFC) are a promising solution for the efficient generation of mm-wave frequencies. The work outlined examines the use of OFCs in a mm-wave radio-over-fiber (RoF) heterodyne system with regard to the specific requirements of a 5G candidate waveform, universally filtered orthogonal frequency division multiplexing. Through experimentation and simulation, the key limitations of linewidth, effective path length difference, and relative intensity noise (RIN) are explored. Results are presented, in terms of error vector magnitude (EVM), for a wide range of system parameters highlighting important considerations to be taken in designing future mm-wave RoF systems employing OFCs. Performance of ~5% EVM using single sideband modulation is achieved for optimized system conditions and an RIN level of -132 dB/Hz.

We demonstrate, through experimental results and supporting numerical simulations, the optimum conditions to generate an optical comb via gain-switching of a single-mode DFB semiconductor laser. The DFB laser is biased appropriately and gain-switched with a large amplitude sinusoidal current, modulating the gain that is sufficient to turn the laser ON and OFF. This results in the generation of a train of short optical pulses albeit with a relatively large amount of temporal jitter. The jitter is a result of the varying turn on time delay, which breaks the periodicity of the pulses leading to an optical spectrum with no discernible tones. However, we find that the conditions for creating an optical frequency comb exist when the modulation frequency is greater than or equal to the relaxation oscillation frequency of the laser. Corresponding optical pulses are shown to exhibit lower levels of temporal jitter at these optimum points of operation. The spectral purity of the comb tones is analyzed, and we show that the FM-noise of the comb tones decreases with increasing modulation frequency with the lower limit set by operating the laser in CW conditions. All of the simulation results were obtained using standard laser rate equations.

We present a continuously tunable optoelectronic oscillator with an intra cavity slow light element based on coherent population oscillations in a semiconductor optical amplifier. The performance of the oscillator is characterized in terms of its frequency tuning range and phase noise. These two characteristics can be traded for each other by choosing the operating conditions of the slow light element and the oscillator fiber length.

Reflective semiconductor optical amplifier (RSOA) fiber cavity lasers are attractive colorless, self-seeded, self-tuning, and directly modulatable sources for passive optical networks (PONs). They comprise of an RSOA in the optical network unit as the active element, a distribution fiber as the laser cavity, a waveguide grating router, and a common reflective mirror with the latter two positioned at the remote node. In this paper, we introduce a model and perform simulations to elucidate the recently discovered successful operation of this new PON source. The results are in agreement with experiments; the formation of a narrow laser spectrum with a relatively constant output power is seen despite a relatively broad passband window of the waveguide grating router. We further study mode competition and mode partition noise. It is shown that proper chromatic dispersion management can overcome mode partition noise limitations. The quality of the RSOA fiber cavity laser does not degrade when being directly modulated and as a result these highly multimode lasers offer an economic way to transport Gbit/s upstream data over kilometers of fiber in a wavelength division multiplexing-PON.

We present detailed numerical simulations of the laser dynamics that describe optical frequency comb formation by injection-locking a gain-switched laser. The typical rate equations for semiconductor lasers including stochastic carrier recombination and spontaneous emission suffice to show the injection-locking behavior of gain switched lasers, and we show how the optical frequency comb evolves starting from the free-running state, right through the final injection-locked state. Unlike the locking of continuous wave lasers, we show that the locking range for gain switched lasers is considerably greater because injection locking can be achieved by injecting at frequencies close to one of the comb lines. The quality of the comb lines is formally assessed by calculating the frequency modulation (FM)-noise spectral density and we show that under injection-locking conditions the FM-noise spectral density of the comb lines tend to that of the maser laser.

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