A. Braun

Low-threshold InGaAsP ring lasers fabricated using bi-level dry etching

We report a novel bi-level etching technique that permits the use of standard photolithography for coupling to deeply etched ring resonator structures. The technique is employed to demonstrate InGaAsP laterally coupled racetrack ring resonators laser with record low threshold currents of 66 mA. The racetrack laser have curved sections of 150-/spl mu/m radius with negligible bending loss. The lasers operate continuous-wave single mode up to nearly twice threshold with a 26-dB side-mode-suppression ratio. Bi-level etching is of interest for fabrication of mesoscopic or microcavity photonic resonator structures without relying on submicrometer processing.

Noise in fundamental and harmonic modelocked semiconductor lasers: experiments and simulations

Electric-field correlation measurements of fundamental and harmonic modelocked external cavity semiconductor lasers are presented. Based on these results, an empirical model of a harmonic modelocked pulsetrain is constructed. Using this model, the equivalence between the time-interleaved pulsetrains picture and the supermode picture of a harmonic modelocked pulsetrain is shown. Simulations based on the model are presented showing the key characteristics of modelocked pulsetrains in radio frequency (RF) and optical domains. The fundamental relationship between longitudinal mode linewidth and RF phase-noise corner frequency is delineated. The generated results point to fundamental limitations in timing jitter in modelocked lasers.

High-quality photonic sampling streams from a semiconductor diode ring laser

We report on the development of an ultralow-noise, external-cavity, actively mode-locked semiconductor diode laser for application in next-generation photonic sampling systems. A summary of harmonically mode-locked noise characteristics in a 65-MHz ring cavity is presented through the range of pulse repetition frequencies between 130 MHz and 8.3 GHz (2nd-128th harmonic). Important implications regarding the use of gain-versus-loss modulation as the active modelocking mechanisms are discussed. We also report what are, to our knowledge, the lowest noise characteristics achieved to date for a semiconductor diode laser operating at 10 GHz. Individually optimized results of 0.12% rms amplitude noise (10 Hz-10 MHz), and 43 fs rms residual phase jitter (10 Hz-10 MHz) provide a theoretical resolution of 8.6 bits in a 10-GSPS optical analog-to-digital converter. We have also achieved dispersion-compensated pulsewidths; as short as 1.2 ps, and shown successful operation of a novel phase-locked-loop capable of reducing the rms; residual phase noise by as much as 91% within its response bandwidth. Finally, the first measurements of residual phase noise out to the Nyquist frequency (5 GHz) are presented, providing an upper bound on the rms residual phase jitter of 121 fs (10 Hz-5 GHz).

Nonlinear propagation of negatively chirped pulses: Maximizing the peak intensity at the output of a fiber probe

A simple phenomenological scaling behavior is found for the power dependence of the pulse width for negatively pre-chirped pulses propagating in a normally dispersive fiber; the consequences for maximizing nonlinear signals such as two-photon fluorescence excited at the fiber output are considered.

10-GHz ultralow-noise optical sampling stream from a semiconductor diode ring laser

We report, what is to our knowledge, the lowest-noise pulsetrain produced to date with an actively mode-locked external-cavity semiconductor diode laser. Operating characteristics at 10 GHz include dispersion-compensated pulsewidths as short as 1.2 ps, amplitude noise as low as 0.12% rms, and residual phase noise as low as 43-fs rms. Potential application of such a laser in a next-generation optical analog-to-digital converter would theoretically provide as much as 8.6 bits of resolution, while sampling a 5-GHz waveform at the Nyquist frequency.

Measurement of residual phase noise and longitudinal-mode linewidth in a hybridly mode-locked external linear cavity semiconductor laser

We report measurements of the residual phase-noise knee position and longitudinal-mode linewidth of a hybridly mode-locked external linear cavity semiconductor laser as a function of laser cavity length. Excellent agreement between these measurements suggests a direct relationship between rms pulse-to-pulse timing jitter and average longitudinal-mode linewidth. This relationship leads to a fundamental limit in the timing jitter of mode-locked lasers.

Universality of mode-locked jitter performance

It is shown experimentally that the jitter of actively mode-locked laser pulses is determined by two factors: first, by spontaneous noise associated with cavity loss, and second, by round-trip propagation time. As the round-trip time is increased, a characteristic frequency which defines the high-frequency limit of phase noise decreases. For a comparable round-trip time and cavity loss, the jitter of mode-locked lasers based on diverse gain media, whether semiconductor or erbium ion is universal and independent of the upper-state transition lifetime.

Ultra low noise and supermode suppression in an actively modelocked external-cavity semiconductor diode ring laser

Using active harmonic modelocking at 10 GHz with extensive polarization control in a mirrorless ring geometry, we have achieved what are, to our knowledge, the lowest noise characteristics yet reported for an actively-modelocked external-cavity semiconductor diode laser. Residual RMS jitter as low as 18 fs (10 Hz-10 MHz) and 94 fs (10 Hz-5 GHz) has been measured, as well as RMS amplitude noise as low as 0.05% (10 Hz-10 MHz) and 0.74% (10 Hz-5 GHz). In addition, using a high-finesse fiber-Fabry-Perot intracavity etalon, we have demonstrated nearly complete supermode suppression out to the Nyquist limit, while favorably retaining a low-offset noise knee.

Ultrahigh-stability photonic sampling streams from an actively-modelocked semiconductor diode ring laser

Summary form only given. Future analog-to-digital conversion needs have inspired recent focus on hybrid optoelectronic systems. The use of short optical pulses for sampling temporal signals promises more accuracy than high-speed electronic comparators. Additional requirements for achieving high precision in high-speed sampling streams are ultralow amplitude and phase noise. Semiconductor optical amplifiers (SOAs) have exhibited proficiency in each of these areas. We report the experimental achievement of optical sampling streams capable of resolving 8.6 bits at a 10 GHz repetition rate. Such accuracy results from ultrashort pulsewidths (1.2 ps), ultralow amplitude noise (0.12% RMS), and ultralow residual jitter (43 fs RMS). A phaselocked-loop (PLL) tested at 2 GHz has further reduced jitter by 91% (from 88 fs to 8 fs).

Multiwavelength generation at 1.55 μm from an external cavity semiconductor laser

Optical analog to digital conversion schemes require a sampling source of high repetition rate, low temporal jitter, low amplitude noise, and short pulse duration to achieve the desired sampling rate and number of bits of resolution. We report on the development of an actively mode-locked semiconductor external cavity laser system where the emission is comprised of multiple wavelengths nominally centered around 1.55 microns. Cavity design includes an intra-cavity grating to produce a spatially dispersed optical spectral filtering plane. Amplitude filtering in this spectral plane serves to flatten the effective gain and a rectangular aperture array selects those wavelengths which are allowed to lase. Modelocked at 311 MHz and producing 8 spectral lines, the laser provides a sampling rate of approximately 2.5 GHz. Temporal interleaving of the pulse train by factor 4 increases the sampling rate to 10 GHz.