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We demonstrate a novel directly modulated transmitter, based on a six-section photonic integrated circuit. The device uses a unique master-slave configuration with a variable optical attenuator in between, allowing decoupling of cavities and independent control of the injection power. The VOA also provides the ability to find an optimum injection level that balances the trade-off between the extinction ratio and the chirp of the transmitted signal. Using the device, an error-free transmission of a 10 Gb/s non-return-to-zero signal over 25 km and 50 km standard single mode fibre is achieved.

Here, the authors generate dissipative Kerr solitons with stable repetition rates and low optical power threshold. They achieve this by actively switching the bias current of injection-locked III-V semiconductor lasers and pulse-pumping crystalline and integrated microresonators with picosecond laser pulses.

We propose a novel scheme for the expansion and comb densification of gain-switched optical frequency combs (GS-OFC). The technique entails mutual injection locking of two gain-switched lasers with a common master to generate a wider bandwidth OFC. Subsequently, the OFC is further expanded and/or densified using a phase modulator with optimum drive conditions. We experimentally demonstrate the generation of an OFC with 45 highly correlated lines separated by 6.25 GHz with an expansion factor ~3. In addition, operating in comb densification mode, the channel spacing of the OFC is tuned from 6.25 GHz to 390.625 MHz. Finally, a detailed characterization of the lines, across the entire expanded comb, is reported highlighting the excellent spectral purity with linewidths of ~40 kHz, a relative intensity noise better than −152 dB/Hz, and a high degree of phase correlation between the comb lines. The proposed method is simple, highly flexible and the architecture is suitable for photonic integration, all of which make such an OFC extremely attractive for the employment in a multitude of applications.

The performance of higher order modulation formats such as 16-quadrature amplitude modulation (QAM) coherent optical communication systems are limited due to several linear and nonlinear impairments such as phase noise, frequency offset, chromatic dispersion, and fiber nonlinearities. For a satisfactory system performance, these impairments need to be eliminated either by all-optical and/or electronic means. In this paper, we use the extended Kalman filter (EKF) algorithm to jointly mitigate laser phase noise, frequency offset, and nonlinear channel impairments, namely self phase modulation and nonlinear phase noise in polarization multiplexed 400 Gb/s 16-QAM and 200 Gb/s quadrature phase shift keying (QPSK) systems. We consider a two-state EKF with phase as one state and frequency offset as another. Simulation results for transmission over 1000 km (10 $\times$ 100 km) of standard single mode fiber show a Q-factor of 12 dB for PM-16-QAM at frequency offset of 1 GHz and laser linewidth of 100 kHz and a Q-factor of 18 dB for PM-QPSK at 1 MHz linewidth. Further, phase trajectories corresponding to tracked laser phase noise and frequency offset show that EKF can suppress laser phase noise up to 10 MHz and frequency offset up to 5 GHz, thereby eliminating the requirement of using separate algorithms for phase noise and frequency offset estimation.

A multi-section active photonic integrated circuit (PIC) is characterized in detail to gauge its suitability as a transmitter for short reach applications. The PIC is 1.5 mm long and consists of two lasers integrated in a master-slave configuration, which enables optical injection locking (OIL) of the slave laser. The beneficial impact of the injection is characterized by static and dynamic measurements. The results show a reduction of the optical linewidth from 8 MHz to 2 MHz, a relative intensity noise (RIN) value as low as −154.3 dB/Hz and a 45% improvement of the slave laser modulation bandwidth from 9.5 GHz to 14 GHz. This frequency response enhancement allows the direct modulation of the slave gain section at a data rate of 10.7 Gb/s and an error-free transmission over 25 km of standard single-mode fiber (SSMF). Transmission performance of the injected case shows a 2 dB improvement in the minimum optical power required to achieve a bit error rate of 3.8×10−3 (hard decision forward error correction limit). These results demonstrate that the multi-section PIC can serve as an attractive cost-efficient transmitter in a wide variety of low-cost short-reach data communication applications.