Paul A. Morton

Actively mode-locked semiconductor lasers

Measurements of actively mode-locked semiconductor lasers are described and compared to calculations of the mode-locking process using three coupled traveling-wave rate equations for the electron and photon densities. The dependence of pulse width on the modulation current and frequency are described. A limitation to minimum achievable pulse widths in mode-locked semiconductor lasers is shown to be dynamic tuning due to gain saturation. Techniques to achieve subpicosecond pulses are described, together with ways to reduce multiple pulse outputs. The amplitude and phase noise of linear- and ring-cavity semiconductor lasers were measured and fond to be tens of dB smaller than YAG and argon lasers and limited by the noise from the microwave oscillator. High-frequency phase noise is only measurable in detuned cavities, and is below -110 dBc (1 Hz) in optimally tuned cavities. The prospects for novel ways to achieve even shorter pulses are discussed. >

Comparison of timing jitter in external and monolithic cavity mode‐locked semiconductor lasers

A comprehensive timing jitter comparison is made for mode‐locked semiconductor lasers using active, passive, and hybrid mode‐locking techniques in both external and monolithic cavity configurations. Active mode locking gives the lowest residual rms timing jitter of 65 fs (150 Hz‐50 MHz), followed by the hybrid and passive mode‐locking techniques. It is found that monolithic cavity devices with all active waveguides have higher timing jitter levels than the comparable external cavity case.

Wide-bandwidth continuously tunable optical delay line using silicon microring resonators

We demonstrate a distortion free tunable optical delay as long as 135 ps with a 10 GHz bandwidth using thermally tuned silicon microring resonators in the novel balanced configuration. The device is simple, easy to control and compact measuring only 30 µm wide by 250 µm long.

Microwave Photonic Delay Line With Separate Tuning of the Optical Carrier

Significant performance improvement for photonic delay lines in microwave-photonics based on a new concept of separately tuning the optical carrier is proposed and analyzed. Optical microresonator delay lines using a balanced SCISSOR design with additional resonators to separately tune the carrier phase delay to provide a true-time-delay for the broadband microwave signal are presented. Significant improvements in broadband tunable delay and devices losses are predicted.

Dynamic detuning in actively mode-locked semiconductor lasers

The authors describe a new limit on the achievable pulsewidth from actively mode-locked semiconductor lasers which is due to dynamic detuning. Dynamic detuning sets a higher limit on pulsewidth than the effects of finite gain bandwidth and dispersion, agreeing with experimental results which show pulsewidths much longer than expected if dynamic detuning is neglected. The dynamic detuning mechanism gives rise to the multiple-pulse output seen for all measurements of subpicosecond pulses and can lead to an unstable output waveform if a perfect antireflection coating is used. The analysis uses the traveling-wave rate equations to include a spatial variation in carrier and photon densities along the laser cavity and also includes the nonzero reflectivity on the antireflection-coated facet. The effects of phase at the antireflection-coated facet and dynamic carrier heating are included in the model. >

Tunable wideband optical delay line based on balanced coupled resonator structures

An optical tunable delay line based on a side-coupled integrated spaced sequence of resonator (SCISSOR) structure in which pairs of resonances are tuned in opposite directions around the signal frequency is proposed and analyzed. It is shown that this balanced SCISSOR design mitigates the deleterious effects of group delay dispersion and provides both wide bandwidth and continuously tunable long delays without distortion.

High Coupling Efficiency Etched Facet Tapers in Silicon Waveguides

We demonstrate a platform based on etched facet silicon inverse tapers for waveguide-lensed fiber coupling with a loss as low as 0.7 dB/facet. This platform can be fabricated on a wafer scale enabling mass-production of silicon photonic devices with broadband, high-efficiency couplers.

Fast Thermal Switching of Wideband Optical Delay Line With No Long-Term Transient

We present results for a broad bandwidth continuously tunable optical delay line based on the balanced side-coupled integrated space sequence of resonators scheme. A tunable delay of up to 345 ps is obtained without distortion of the optical signal. Fast thermal switching speed under 10 μs is achieved without any measurable long-term transient by utilizing a novel balanced thermal tuning scheme.

Generation of picosecond pulses with a gain‐switched GaAs surface‐emitting laser

Pulses shorter than 4 ps (deconvolved) have been obtained by optically gain switching a GaAs multiple quantum well vertical‐cavity surface‐emitting laser with a picosecond dye laser. Pulse width and relative peak delay were measured as a function of pump power. A theoretical model of the large signal response agrees well with the measured data. The model predicts the minimum achievable pulse width and pulse delay for this device structure. Experimental results and calculated values indicate that very high modulation rates are possible with vertical‐cavity surface‐emitting lasers.

Linearized silicon modulator based on a ring assisted Mach Zehnder inteferometer

We demonstrate a Linearized Ring Assisted Mach-Zehnder Interferometer (L-RAMZI) modulator in a miniature silicon device. We measure a record high degree of linearization for a silicon device, with a Spurious Free Dynamic Range (SFDR) of 106dB/Hz²/³ at 1GHz, and 99dB/Hz²/³ at 10GHz.