Charles N. Ironside

We report on photo-detectors consisting of an optical waveguide that incorporates a resonant tunneling diode (RTD). Operating at wavelengths around 1.55 μm in the optical communications C band we achieve maximum sensitivities of around 0.29 A/W which is dependent on the bias voltage. This is due to the nature of RTD nonlinear current-voltage characteristic that has a negative differential resistance (NDR) region. The resonant tunneling diode photo-detector (RTD-PD) can be operated in either non-oscillating or oscillating regimes depending on the bias voltage quiescent point. The oscillating regime is apparent when the RTD-PD is biased in the NDR region giving rise to electrical gain and microwave self-sustained oscillations Taking advantage of the RTD’s NDR distinctive characteristics, we demonstrate efficient detection of gigahertz (GHz) modulated optical carriers and optical control of a RTD GHz oscillator. RTD-PD based devices can have applications in generation and optical control of GHz low-phase noise oscillators, clock recovery systems, and fiber optic enabled radio frequency communication systems.

(2012). More is Different: Fifty Years of Condensed Matter Physics, edited by N. Phuan Ong and Ravin N. Bhatt. Contemporary Physics: Vol. 53, No. 4, pp. 389-389.

We report a novel approach for increasing the output power in passively mode locked semiconductor lasers. Our approach uses epitaxial structures with an optical trap in the bottom cladding that enlarges the vertical mode size to scale the pulse saturation energy. With this approach we demonstrate a very high peak power of 9.8 W per facet, at a repetition rate of 6.8 GHz and with pulse duration of 0.71 ps. In particular, we compare two GaAs/AlGaAs epilayer designs, a double quantum well design operating at 830 nm and a single quantum well design operating at 795 nm, with vertical mode sizes of 0.5 and 0.75 μm, respectively. We show that a larger mode size not only shifts the mode locking regime of operation toward higher powers, but also produces other improvements with respect to two main failure mechanisms that limit the output power, catastrophic optical mirror damage and catastrophic optical saturable absorber damage. For the 830-nm material structure, we also investigate the effect of nonabsorbing mirrors on output power and mode locked operation of colliding pulse mode locked lasers.

We investigate optoelectronic oscillator (OEO) configurations based on a laser diode driven by resonant tunnelling diode (RTD) optical waveguide photo-detector (PD) oscillators, with an optical fiber feedback loop carrying a fraction of the laser diode optical output that is re-injected into the OEO through the optical waveguide of the RTD-PD. In the configurations reported here, we take advantage of the RTD negative differential resistance to provide electrical highbandwidth. The optical fiber loop acts as a high quality optical energy storage element with low transmission loss. The RTD based OEO can produces stable and low-phase noise microwave signals with attractive applications in photonics and communication systems, mainly in fiber-optic based communication links since the RTD-OEO can be accessed both optically and electrically.