Bala Pesala

High-Index-Contrast Grating (HCG) and Its Applications in Optoelectronic Devices

We review recent advances in subwavelength high-index-contrast gratings (HCGs) and a variety of applications in optoelectronic devices, including vertical-cavity surface-emitting lasers (VCSELs), tunable VCSELs, high-Q optical resonators, and low-loss hollow-core waveguides (HWs). HCGs can serve as broadband (Delta lambda/lambda ~ 35%), high-reflectivity (>99%) mirrors for surface-normal incident light, which is useful to replace conventional distributed Bragg reflectors in optical devices. HCGs can also be designed as high-Q resonators with output coupling in the surface-normal direction. Finally, we discuss a novel design of HCG as shallow angle reflectors and HWs.

A novel ultra-low loss hollow-core waveguide using subwavelength high-contrast gratings

We propose a novel ultra-low loss single-mode hollow-core waveguide using subwavelength high-contrast grating (HCG). We analyzed and simulated the propagation loss of the waveguide and show it can be as low as 0.006 dB/m, three orders of magnitude lower than the lowest loss of the state-of-art chip-scale hollow waveguides. This novel HCG hollow-core waveguide design will serve as a basic building block in many chip-scale integrated photonic circuits enabling system-level applications including optical interconnects, optical delay lines, and optical sensors.

Tunable ultraslow light in vertical-cavity surface-emitting laser amplifier

We report the experimental demonstration of tunable ultraslow light using a 1.55 um vertical-cavity surface-emitting laser (VCSEL) at room temperature. By varying the bias current around lasing threshold, we achieve tunable delay of an intensity modulated signal input. Delays up to 100 ps are measured for a broadband signal with modulation frequency of 2.8 GHz. With a VCSEL design optimized for amplification and leveraging the scalability of VCSEL arrays, delays of multiple modulation periods are feasible.

Monolithically integrated multi-wavelength VCSEL arrays using high-contrast gratings.

We propose a novel design for multi-wavelength arrays of vertical cavity surface-emitting lasers (VCSELs) using high-contrast gratings (HCGs) as top mirrors. A range of VCSEL cavity wavelengths in excess of 100 nm is predicted by modifying only the period and duty-cycle of the high-contrast gratings, while leaving the epitaxial layer thickness unchanged. VCSEL arrays fabricated with this novel design can easily accommodate the entire Er-doped fiber amplifier bandwidth with emission wavelengths defined solely by lithography with no restrictions in physical layout. Further, the entire process is identical to that of solitary VCSELs, facilitating cost-effective manufacturing.

THz-bandwidth tunable slow light in semiconductor optical amplifiers

We report tunable fractional delays of 250% for 700 fs pulses propagating in a 1.55 mum semiconductor optical amplifier at room temperature. This large fractional delay is attributed to a spectral hole created by the propagating pulses for pulses with duration shorter than the carrier heating relaxation time. Delay can be tuned electrically by adjusting the current with low amplitude variation across the tuning range.

Experimental demonstration of slow and superluminal light in semiconductor optical amplifiers

Tunable delays in semiconductor optical amplifiers are achieved via four wave mixing between a strong pump beam and a modulated probe beam. The delay of the probe beam can be controlled both electrically, by changing the SOA bias, and optically, by varying the pump power or the pump-probe detuning. For sinusoidal modulated signal at 0.5 GHz, a tunable delay of 1.6 ns is achieved. This corresponds to a RF phase change of 1.6 pi. For 1.3 ns optical pulses propagating through the SOA a delay of 0.59 ns is achieved corresponding to a delay-bandwidth product exceeding 0.45. For both the cases, slow light and superluminal light are observed as the pump-probe detuning is varied.

Demonstration of piezoelectric actuated GaAs-based MEMS tunable VCSEL

We report the first experimental demonstration of a novel piezoelectric actuated microelectromechanical systems (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL). A large physical deflection was obtained with the piezoelectric actuated MEMS cantilever beam monolithically integrated with the VCSEL distributed Bragg reflector. Single-mode emission and continuous tuning were achieved at room temperature under continuous-wave operation

Chirp-enhanced fast light in semiconductor optical amplifiers

We present a novel scheme to increase the THz-bandwidth fast light effect in semiconductor optical amplifiers and increase the number of advanced pulses. By introducing a linear chirp to the input pulses before the SOA and recompressing at the output with an opposite chirp, the advance-bandwidth product reached 3.5 at room temperature, 1.55 microm wavelength. This is the largest number reported, to the best of our knowledge, for a semiconductor slow/fast light device.

Dispersion properties of high-contrast grating hollow-core waveguides

We present unique dispersion characteristics of high-contrast grating (HCG) hollow-core waveguides and show that slow light can be facilitated using internal resonances developing inside the waveguide walls. In addition, we show a fast and precise method of inferring the dispersion information from the waveguide angular reflectivity spectrum.

Electrically tunable fast light at THz bandwidth using cascaded semiconductor optical amplifiers.

Ultra fast non-linear processes are used to achieve an advance of 2 ps for a 600 fs pulse propagating through two SOAs in series. This corresponding 3.3-pulse advance is tuned continuously by changing the current applied to the devices. We propose an experimental scheme that uses a single SOA in a loop to emulate the propagation of pulse through multiple cascaded SOAs.