Optical square wave sources are particularly important for applications in high speed signal processing and optical communications. In most realizations, optical square waves are generated by electro-optic modulation, dispersion engineering of mode-locked lasers, polarization switching, or by exploiting optical bi-stability and/or optical delayed feedback in semiconductor diode lasers, as well as vertical-cavity surface-emitting lasers (VCSELs). All such configurations are bulky and cause significant timing jitters. Here we demonstrate the direct generation of optical square waves from a polarization-maintaining figure-eight nonlinear amplifying loop mirror (NALM) configuration that uses an embedded high index glass micro-cavity as the nonlinear element. Such a NALM mimics the behavior of a saturable absorber and has been used to reach passive mode-locking of pico- and even nano-second pulses. In our method, the NALM, including a high-Q micro-ring resonator, acts as an ultra-narrowband spectral filter and at the same time provides a large nonlinear phase-shift. Previously we have demonstrated that such a configuration enables sufficient nonlinear phase-shifts for low-power narrow-bandwidth (~100 MHz FWHM) passive mode-locked laser operation. Here we demonstrate the switching of stable optical square wave pulses from conventional mode-locked pulses by adjusting the cavity properties. In addition, the square wave signal characteristics, such as repetition rate and pulse duration, can be also modified in a similar fashion. The source typically produces nanosecond optical square wave pulses with a repetition rate of ~ 120 MHz at 1550nm. In order to verify the reach of our approach, we compare our experimental results with numerical simulations using a delay differential equation model tailored for a figure-eight laser.
Complex optical quantum states based on entangled photons are essential for investigations of fundamental physics and are the heart of applications in quantum information science. Recently, integrated photonics has become a leading platform for the compact, cost-efficient, and stable generation and processing of optical quantum states. However, onchip sources are currently limited to basic two-dimensional (qubit) two-photon states, whereas scaling the state complexity requires access to states composed of several (<2) photons and/or exhibiting high photon dimensionality. Here we show that the use of integrated frequency combs (on-chip light sources with a broad spectrum of evenly-spaced frequency modes) based on high-Q nonlinear microring resonators can provide solutions for such scalable complex quantum state sources. In particular, by using spontaneous four-wave mixing within the resonators, we demonstrate the generation of bi- and multi-photon entangled qubit states over a broad comb of channels spanning the S, C, and L telecommunications bands, and control these states coherently to perform quantum interference measurements and state tomography. Furthermore, we demonstrate the on-chip generation of entangled high-dimensional (quDit) states, where the photons are created in a coherent superposition of multiple pure frequency modes. Specifically, we confirm the realization of a quantum system with at least one hundred dimensions. Moreover, using off-the-shelf telecommunications components, we introduce a platform for the coherent manipulation and control of frequencyentangled quDit states. Our results suggest that microcavity-based entangled photon state generation and the coherent control of states using accessible telecommunications infrastructure introduce a powerful and scalable platform for quantum information science.
Phase sensitive amplification (PSA) is an attractive technology for integrated all-optical signal processing, due to it's potential for noiseless amplification, phase regeneration and generation of squeezed light. In this talk I will review our results on implementing four-wave-mixing based PSA inside integrated photonic devices. In particular I will discuss PSA in chalcogenide ridge waveguides and silicon slow-light photonic crystals. We achieve PSA in both pump- and signal-degenerate schemes with maximum extinction ratios of 11 (silicon) and 18 (chalcogenide) dB. I will further discuss the influence of two-photon absorption and free carrier effects on the performance of silicon-based PSAs.