Joseph H. Abeles

Toward a photonic arbitrary waveform generator using a modelocked external cavity semiconductor laser

We propose an architecture for photonic arbitrary waveform generation where the phase-locked longitudinal modes of a 12.4-GHz fundamentally modelocked external cavity semiconductor laser are individually modulated. We report photonic synthesis of microwave tones at 37.2 GHz (limited by photodetector bandwidth) with linewidth < 100 Hz and dynamic range 50 dB at 100-Hz resolution bandwidth. We show photonically synthesized 12.4-GHz sine waves with superimposed sinusoidal and pulsed RF modulations, demonstrating the potential for photonic arbitrary waveform generation with the proposed architecture.

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.

Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope

An atomic force microscope (AFM) with an ultrasharp tip was used to directly measure the sidewall profile of InP/InGaAsP waveguide structures etched using an inductively coupled plasma reactive ion etching (ICP-RIE) in Cl2-based plasma. A special staircase pattern was devised to allow AFM tip to access the etched sidewall of the waveguides in the normal direction. Statistical information such as correlation length and rms roughness of the sidewall profile obtained through three-dimensional imaging by AFM has been presented. rms roughness as low as 3.45 nm was measured on the sidewall of 4-μm-deep etched InP/InGaAsP heterostructures.

High-power separate-confinement heterostructure AlGaAs/GaAs laser diodes with broadened waveguide

AlGaAs/GaAs graded-index separate-confinement heterostructure single quantum well (GRINSCH-SQW) lasers with different waveguide thickness have been analyzed experimentally and compared with results from modeling using transverse optical field distributions. We have found that for GRINSCH lasers the halfwidth of near-field and far-field patterns depends very weakly on the waveguide thickness due to the focusing of the optical field in the transverse direction by the graded-index waveguide. At the same time, the mode intensity in the cladding layers is reduced by two orders of magnitude as the waveguide thickness is increased from 40 nm to 1200 nm. As a result, a 20% improvement in the differential quantum efficiency ((eta) d) is realized, while the threshold current density remains unchanged. Differential quantum efficiency as high as 78% and output power exceeding 4 W cw have been obtained for broadened waveguide lasers.

Effect of H2 on the etch profile of InP/InGaAsP alloys in Cl2/Ar/H2 inductively coupled plasma reactive ion etching chemistries for photonic device fabrication

This study demonstrates etch profile engineering of InP, In1−xGaxAs1−yPy, and In0.53Ga0.47As heterostructures results from adding H2 to standard Cl2/Ar inductively coupled plasma-reactive ion etching chemistries. Etch rate curves of bulk InP, In1−xGaxAs1−yPy, and In0.53Ga0.47As show a general parabolic trend as a function of the H2 component of the Cl2/Ar/H2 ratio. Three distinct etching profiles of InP/InGaAsP layers were realized by varying the Cl2/Ar/H2 ratio. Highly anisotropic profiles result for Cl2/Ar/H2 ratios between 2/3/1 and 2/3/2. Waveguiding structures fabricated using this technology are presented with a loss as low as 2 dB/cm. An InP racetrack resonator with a quality factor (Q)>8000 is also presented.

Enhanced electro-optic effect in GaInAsP–InP three-step quantum wells

We report on the enhanced electro-optic coefficient of GaInAsP three-step quantum wells (3SQW) for high power electrorefraction modulator applications. Measured electro-optic coefficient of the 3SQW is nearly three times higher than the conventional rectangular quantum well (RQW) at λ=1.55 μm. The enhanced electro-optic effect, combined with a low optical absorption coefficient α<1 cm−1 in the 3SQW increases a modulator figure of merit by nearly 36 times, and decreases the power consumption by nearly one order of magnitude compared with a conventional RQW design.

Polycrystalline-metal-ferromagnetic optical waveguide isolator (POWI) for monolithic-integration with diode-laser devices

Optical Faraday rotation in polycrystalline or amorphous thin films of ferromagnetic materials can be used to obtain true-optical isolators and nonreciprocal-TE-TM-mode converters which cover a wide wavelength range. These can be integrated with semiconductor lasers and other integrated optic devices because the films do not require epitaxial deposition. A detailed description and analysis of an integrable isolator with better than 30-dB isolation ratio and low insertion loss in the 1.5-/spl mu/m wavelength region is given.

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.