Tam N. Huynh

Delayed Self-Heterodyne Phase Noise Measurements With Coherent Phase Modulation Detection

We present coherent phase modulation detection for laser phase noise measurements with delayed self-heterodyne method. The technique is demonstrated for the first time with the distributed feedback laser and external cavity laser. The results are within 15% of self-homodyne measurements using an optical coherent receiver.

Effects of phase noise of monolithic tunable laser on coherent communication systems

We investigate the effects of different phase noise processes of SGDBR laser on coherent systems. The SGDBR device operated well with QPSK modulation at 5 Gbaud, while the performance of 16-QAM was significantly degraded due to excess noise. The white FM noise mainly defines the ultimate performance of coherent reception, but the low frequency excess noise can potentially degrade the performance of systems that employ 16-QAM format at 5 Gbaud.

Phase Noise Characterization of SGDBR Lasers Using Phase Modulation Detection Method With Delayed Self-Heterodyne Measurements

This study employed the coherent phase modulation (PM) detection method to characterize the phase noise of a Sampled-Grating Distributed Bragg Reflector (SGDBR) laser via delayed self-heterodyne (DSH) measurements. We derived the formula for phase-error variance from the FM noise spectrum of the laser and performed measurements on the Distributed Feedback (DFB) and SGDBR lasers. The results confirmed that the DFB laser phase noise is well described by white FM noise while the SGDBR laser has additional frequency fluctuations below 400 MHz. The measurement and simulation results also demonstrated that the low frequency excess noise significantly broadens the linewidth of SGDBR lasers and can degrade the performance of coherent optical communication systems.

Phase noise analysis of injected gain switched comb source for coherent communications.

We present experimentally and analytically the phase noise characterization of an externally injected gain switched comb source. The results reveal the residual high frequency FM noise in the comb lines, which stays unnoticed in the optical linewidth value but leads to an increased phase-error variance. The potential impact of the residual phase noise is investigated in a 10.7 GBaud optical DQPSK system where a 2 dB power penalty is recorded at BER of 10(-9). In a 10.7 GBaud digital coherent QPSK system no penalty is observed but with 5 GBaud 16-QAM format a 3 dBpenalty exists at the FEC limit of 4.4e-3.

Simulations of an OSNR-Limited All-Optical Wavelength Conversion Scheme

We present simulations of a scheme to perform wavelength conversion of signals that eliminates phase-noise transfer from the pump to the converted signal. Nondegenerate four-wave mixing in a semiconductor optical amplifier is used to convert the signal to a new wavelength, and if an optical comb generator is used as the multiple-pump source, then the signal can be converted without incurring any phase-noise transfer from the pumps. We highlight the capabilities of this scheme by simulating the conversion of 16-QAM signals at 10 GBd and showing that errors due to phase-noise accumulation are eliminated, thus enabling conversion whose only impairment would be the total additive optical noise.

Dynamic Linewidth Measurement Method via an Optical Quadrature Front End

This letter describes a dynamic linewidth characterization method using an optical quadrature front end. The phase noise of the laser is recorded using a real-time oscilloscope in the time domain and the linewidth of the laser can be estimated statistically offline. The major advantage of this technique compared with conventional linewidth measurements in the frequency domain, is that this method enables the dynamic phase noise characterization which is increasingly important for fast wavelength tunable and switched networks employing advanced modulation formats. The dynamic linewidth of an sampled grating distributed Bragg reflector (SG-DBR) laser is characterized by using this method.

BER Performance of Coherent Optical Communications Systems Employing Monolithic Tunable Lasers With Excess Phase Noise

We analyze the bit error rate (BER) performance of optical coherent communication systems employing multi-section monolithic tunable lasers with large excess phase noise. The BER of the coherent system utilizing a second-order decision-directed digital phase-locked loop (DD-PLL) for phase tracking was computed numerically. Experimental results have also been demonstrated with a 16-quadratic-amplitude modulation coherent system deploying a sampled-grating distributed Bragg reflector laser at 16 Gbaud that confirms the performance of the second-order DD-PLL in tracking the excess phase noise.

Narrow-linewidth discrete-mode laser diodes for coherent communication applications

Lasers with narrow-linewidth emission are a key component for higher-order modulation formats. We report on discrete-mode laser diodes designed for narrow-linewidth emission and demonstrate linewidths less than 80 kHz. Using these devices in quadrature phase shift keying and 16-quadrature amplitude modulation transmission setups, similar performance to that of an external cavity laser is demonstrated.

Pilot-tone-aided transmission of high-order QAM for optical packet switched networks

We investigate novel pilot-aided transmission of high-order quadrature amplitude modulation (QAM) modulation formats for optical packet-switched networks. The proposed modulation scheme employs a pilot tone operating at the baud rate for noncoherent reception that is highly immune to laser phase noise. The results show that the system performance equivalent to static operation is achieved 5 ns after wavelength switching of a sampled grating distributed Bragg reflector (SG-DBR) tunable laser.

Simple analytical model for low-frequency frequency-modulation noise of monolithic tunable lasers

We employ simple analytical models to construct the entire frequency-modulation (FM)-noise spectrum of tunable semiconductor lasers. Many contributions to the laser FM noise can be clearly identified from the FM-noise spectrum, such as standard Weiner FM noise incorporating laser relaxation oscillation, excess FM noise due to thermal fluctuations, and carrier-induced refractive index fluctuations from stochastic carrier generation in the passive tuning sections. The contribution of the latter effect is identified by noting a correlation between part of the FM-noise spectrum with the FM-modulation response of the passive sections. We pay particular attention to the case of widely tunable lasers with three independent tuning sections, mainly the sampled-grating distributed Bragg reflector laser, and compare with that of a distributed feedback laser. The theoretical model is confirmed with experimental measurements, with the calculations of the important phase-error variance demonstrating excellent agreement.