Martin H. Kwakernaak

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.

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.

Highly linear and efficient phase modulators based on GaInAsP-InP three-step quantum wells

Highly linear and efficient phase modulators based on three-step quantum wells are reported. The spatial separation of electron and hole wave functions in the three-step quantum well leads to enhancement of the linear electro-optic component. In parallel, the quadratic electro-optic component is suppressed using a method based on tailored doping profile. Measured modulation efficiency is 48°∕mm V, and the ratio of linear to quadratic components of the phase modulation is 640 at λ=1560nm. The efficiency is similar to the best reported values for semiconductor modulators at this wavelength, while the linearity is more than one order of magnitude higher.

Characterization of sidewall roughness of InP/InGaAsP etched using inductively coupled plasma for low loss optical waveguide applications

The effects of etch depth on the sidewall roughness (SWR) of InGaAsP/InP waveguides fabricated utilizing two types of masks, NiCr/SiO2 and SiO2/NiCr/SiO2, were investigated with an atomic force microscopy. All the waveguides were etched in an inductively coupled plasma–reactive ion etching to depths ranging from 4 to 8 μm. The root-mean-square (rms) sidewall roughness values of the waveguides etched to depths of 4, 6, and 8 μm with SiO2 remasking layer were measured to be 2.97, 3.45, and 3.64 nm, respectively. Also the rms SWR values of the waveguides etched without the remasking layer were 3.2, 3.65, and 3.89 nm, respectively. The SiO2 thin remasking layer deposited on NiCr/SiO2 mask structure reduced the SWR of the waveguides. Measurements indicated that SWR increased with etch time, which is ascribed to an increase in mask erosion during etching.

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.

Observation of low optical overlap mode propagation in nanoscale indium phosphide membrane waveguides

The authors have developed a nanoscale, rib-loaded waveguide that propagates a low optical overlap mode (LOOM) in which less than 1% of the modal field energy resides in the semiconductor material. Because of the small modal fill factor, the potential for extremely low waveguide propagation loss, on the order of 0.001dB or less, is predicted. Elevated membrane waveguides, 50nm thick with a 50nm thick rib, have been fabricated in InP using a multistep microelectromechanical release process. Both transverse electric and transverse magnetic LOOM propagations have been observed and measurements are compared to theoretical predictions.

Study of the evolution of nanoscale roughness from the line edge of exposed resist to the sidewall of deep-etched InP∕InGaAsP heterostructures

The evolution of line edge roughness and sidewall roughness was monitored during the fabrication of deep-etched optical waveguides in InP∕InGaAsP heterostructures. Scanning electron microscopy was used to extract line edge profiles of the electron beam exposed resist and the lifted-off NiCr metal mask. Atomic force microscopy with an ultrasharp tip was utilized to extract the sidewall profiles of InP∕InGaAsP mesa waveguides that were etched using inductively coupled plasma reactive ion etching. The processing step that critically influences the roughness of the etched waveguides was determined by studying the evolution of the roughness.

Multi-frequency laser monolithically integrating InGaAsP gain elements with amorphous silicon AWG

We demonstrate a photonic integrated circuit using a novel monolithic integration platform combining InGaAsP gain elements and index matched amorphous silicon waveguide devices. The AWG based multi-frequency laser emits eight 100-GHz-spaced wavelengths near 1550 nm.

Compact high-power low-jitter semiconductor mode-locked laser module for photonic A/D converter applications

Low-capacitance, two-section, curved-waveguide gain elements were packaged with lensed polarization-maintaining fiber within standard-sized butterfly-style packages and shown to produce low-jitter pulses when used within a harmonically modelocked sigma cavity laser (jitter = 25 fs; 10 Hz - 10 MHz). Incorporation of a high finesse etalon filter into the sigma-cavity loop resulted in greater than 25 dB suppression of the supermode spurs while maintaining low integrated phase noise (jitter = 30 fs; 10 Hz - 10 MHz). A module containing the in-line sigma-cavity modelocked laser source and packaged semiconductor optical amplifiers was developed to create a configurable low jitter pulse source.

End-resonance clock and all-photonic clock

The end-resonance clock uses strong hyperfine end transition to stabilize the frequency of the local oscillator. Comparing to the conventional 0-0 atomic clock, end resonance has very small spin-exchange broadening effect. The spin-exchange rate is proportional to the number density of the alkali-metal atoms. By using the end resonance, we are able to use very high dense vapor to obtain a much better signal to noise ratio. On the other hand, the end resonance suffers from the first-order magnetic field dependence. This problem, however, can be solved by simultaneously using a Zeeman end resonance to stabilize the magnetic field. Here, we report the most recent result of the end-resonance clock. In addition, we report a whole new technique, push-pull laser-atomic oscillator, which can be thought as all-photonic clock. This new clock requires no local oscillator. It acts like a photonic version of maser, which spontaneously generates modulated laser light and modulated voltage signals. The modulation serves as the clock signal, which is automatically locked to the ground-state hyperfine frequency of alkali-metal atoms.