[Show abstract][Hide abstract] ABSTRACT: We consider design optimization of passively mode-locked two-section
semiconductor lasers that incorporate intracavity grating spectral filters. Our
goal is to develop a method for finding the optimal wavelength location for the
filter in order to maximize the region of stable mode-locking as a function of
drive current and reverse bias in the absorber section. In order to account for
material dispersion in the two sections of the laser, we use analytic
approximations for the gain and absorption as a function of carrier density and
frequency. Fits to measured gain and absorption curves then provide inputs for
numerical simulations based on a large signal accurate delay-differential model
of the mode-locked laser. We show how a unique set of model parameters for each
value of the drive current and reverse bias voltage can be selected based on
the variation of the net gain along branches of steady-state solutions of the
model. We demonstrate the validity of this approach by demonstrating
qualitative agreement between numerical simulations and the measured
current-voltage phase-space of a two-section Fabry-Perot laser. We then show
how to adapt this method to determine an optimum location for the spectral
filter in a notional device with the same material composition, based on the
targeted locking range, and accounting for the modal selectivity of the filter.
[Show abstract][Hide abstract] ABSTRACT: We discuss optimization of mode-locked semiconductor lasers with intracavity grating spectral filters. Our method employs an experimentally calibrated model for the material susceptibility and a large signal accurate delay-differential laser model.
Integrated Photonics Research, Silicon and Nanophotonics; 01/2014
[Show abstract][Hide abstract] ABSTRACT: We present distributed feedback grating filter designs that define discrete combs of transmission resonances. Practical structures with transmission channels matched to defined wavelength grids with 40 GHz spacing are demonstrated numerically.
Integrated Photonics Research, Silicon and Nanophotonics; 01/2014
[Show abstract][Hide abstract] ABSTRACT: Integrated quantum photonic circuits are becoming increasingly complex.
Accurate calibration of device parameters and detailed characterization of the
prepared quantum states are critically important for future progress. Here we
report on an effective experimental calibration method based on Bayesian
updating and Markov chain Monte Carlo integration. We use this calibration
technique to characterize a two qubit chip and extract the reflectivities of
its directional couplers. An average quantum state tomography fidelity of
93.79+/-1.05% against the four Bell states is achieved. Furthermore, comparing
the measured density matrices against a model using the non-ideal device
parameters derived from the calibration we achieve an average fidelity of
97.57+/-0.96%. This pinpoints non-ideality of chip parameters as a major factor
in the decrease of Bell state fidelity. We also perform quantum state
tomography for Bell states while continuously varying photon distinguishability
and find excellent agreement with theory.
New Journal of Physics 06/2013; 15(6). · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quantum key distribution (QKD) is moving from research laboratories towards
applications. As computing becomes more mobile, cashless as well as cardless
payment solutions are introduced, and a need arises for incorporating QKD in a
mobile device. Handheld devices present a particular challenge as the
orientation and the phase of a qubit will depend on device motion. This problem
is addressed by the reference frame independent (RFI) QKD scheme. The scheme
tolerates an unknown phase between logical states that varies slowly compared
to the rate of particle repetition. Here we experimentally demonstrate the
feasibility of RFI QKD over a free-space link in a prepare and measure scheme
using polarisation encoding. We extend the security analysis of the RFI QKD
scheme to be able to deal with uncalibrated devices and a finite number of
measurements. Together these advances are an important step towards mass
production of handheld QKD devices.
New Journal of Physics 05/2013; 15(7). · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the observation of on-off intermittency in an optically injected dual-mode semiconductor laser. It is shown that quasi-single-mode chaotic dynamics of the injected mode are accompanied by intermittent and irregular bursts of the intensity of the uninjected mode. We define a threshold intensity of the uninjected mode to distinguish laminar and bursting states of the system. For small values of the threshold parameter we observe excellent agreement with the predictions of theory for the distribution of the laminar phase durations. For larger values of the threshold parameter, a gap appears in the distribution of laminar phase durations. Numerical simulations demonstrate that this gap is a consequence of the fact that in this case the on states of the system define large intensity spikes, which can belong either to the same or to distinct bursts away from the single-mode manifold.
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 05/2012; 85(5). · 2.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The design and measured performance characteristics of a range of index-patterned diode laser sources are presented. These devices incorporate slotted regions etched into the laser ridge waveguide, which are formed in the same fabrication step as the ridge, thus avoiding the requirement for complex lithography and regrowth steps. We first demonstrate that the index profile of single and multimode devices can be obtained directly from an inverse problem solution based on a perturbative calculation of the threshold gain of the longitudinal modes of the cavity. Measurements of temperature stability, linewidth, and modulation bandwidth of single-mode devices obtained in this way are presented. It is then shown that the design of multimode devices including two-color and pulsed mode-locked devices designed to support a discrete comb of modes is also possible. We finally demonstrate a tunable source based on a multisection design defined using etched features. This device is shown to have wide tunability with narrow linewidth modes and fast wavelength switching speed.
IEEE Journal of Selected Topics in Quantum Electronics 01/2012; · 3.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A numerical study of threshold gain and modal dispersion in integrated semiconductor laser optical frequency comb sources is presented. We consider an example device where one of the cleaved facets of the laser is replaced by a short Bragg grating section and show that as many as 16 modes can be selected at the first harmonic of the underlying Fabry-Perot cavity. An intracavity approach to limiting the grating-induced dispersion that can be implemented directly through the grating profile is demonstrated.
[Show abstract][Hide abstract] ABSTRACT: We report on the characterization of the timing stability of passively mode-locked discrete mode diode laser sources. These are edge-emitting devices with a spatially varying refractive index profile for spectral filtering. Two devices with a mode-locking frequency of 100 GHz are characterized. The first device is designed to support a comb of six modes and generates near Fourier limited 1.9 ps pulses. The second supports four primary modes resulting in a sinusoidal modulation of the optical intensity. Using a cross-correlation technique, we measured a 20 fs pulse to pulse timing jitter for the first device, while, for the second device, a mode-beating (RF) linewidth of 1 MHz was measured using heterodyne mixing in a semiconductor optical amplifier. Comparison of these results with those obtained for an equivalent Fabry-Perot laser indicates that the spectral filtering mechanism employed does not adversely affect the timing properties of these passively mode-locked devices.
[Show abstract][Hide abstract] ABSTRACT: We present successful integration of superconducting single photon detectors with optical microcavities based on GaAs/AlAs Bragg mirrors. Cavity enhancement of the optical absorption, which results in 18% detection efficiency at λ=1300nm and T=4.2K, is reported.
[Show abstract][Hide abstract] ABSTRACT: We report the first waveguide single-photon detectors. They are based on superconducting nanowires patterned on top of GaAs/AlGaAs waveguides and are suitable for monolithic integration with single-photon sources and passive quantum optical networks. The combination of single-photon sources, passive optical circuits and single-photon detectors enables important functionalities in quantum communications, such as quantum repeaters (1) and qubit amplifiers (2), and also forms the basis of all-optical quantum gates (3) and of linear-optics quantum computing (4). However, present implementations are limited to few qubits, due to the large number of optical components required and the corresponding complexity and cost of experimental set-ups. The monolithic integration of quantum photonic components and circuits on a chip is absolutely required to scale implementations of optical quantum information processing to meaningful numbers of qubits. The integration of passive circuits has been demonstrated in the silica- on-silicon platform (5), but an approach to the simultaneous integration of sources, detectors and passive circuitry is still missing. The integration of detectors is particularly challenging, as the complex device structures associated to avalanche photodiodes are not easily compatible with the integration with low-loss waveguides and even less with sources. Here we report a simple approach to the realization of single-photon detectors on optical waveguides in the GaAs/AlGaAs material system. It enables the first demonstration of waveguide single-photon detectors and is fully compatible with the monolithic integration of detectors with single-photon sources (based on InAs quantum dots) and passive optical circuits on a single chip. Our waveguide single-photon detectors (WSPDs) are based on the principle of photon-induced hot-spot creation in ultranarrow superconducting NbN wires, which is at the basis of nanowire single-photon detectors (SSPDs) (6) and can provide ultrahigh sensitivity at telecommunication wavelengths, high counting rates, broad spectral response and high temporal resolution due to low jitter values. For WSPDs, the wires are deposited and patterned on top of a GaAs/AlGaAs ridge waveguide, in order to sense the evanescent field on the surface (Fig. 1). The calculated modal absorption coefficient for the fundamental transverse-electric mode of a 1.85 μm-wide waveguide, in the presence of four 4 nm-thick and 100 nm-wide NbN wires is as high as 450 cm
[Show abstract][Hide abstract] ABSTRACT: We demonstrate low timing jitter 100 GHz passive mode-locking of a discrete mode (DM) laser. This Fabry-Perot device incorporates a spatially varying effective index selecting modes. Near Fourier limited 2.5 ps pulses were obtained.
[Show abstract][Hide abstract] ABSTRACT: We report on efficient nanowire NbN superconducting single photon detectors (SSPDs), integrated with an optical cavity based GaAs/AlAs distributed Bragg reflectors, showing a detection efficiency of 18.3% at 4.2K at 1.3 μm wavelength. Superconducting nanowire single photon detectors (SSPDs) are an emerging technology that offers ultrahigh sensitivity at telecommunication wavelengths, high counting rates, broad spectral response and high temporal resolution due to low jitter values. All these features are interesting for a variety of applications such as quantum key distribution (QKD), ultra-long distance optical communication, biomedical imaging and VLSI circuit testing. SSPDs consist of an ultrathin (4 to 5 nm) NbN nanowire patterned as a meander with 100 nm typical width. The detection mechanism is based on the hot-spot process: if the nanowire is in the superconducting state and it is current-biased near its critical current IC, the absorption of a single photon can drive an entire cross-section of the nanowire to the normal state (1). Until recently, SSPDs have been fabricated only on sapphire, MgO and silicon substrates. The integration of NbN nanowires with III-V semiconductor heterostructures (e.g. GaAs-based) could open novel avenues for their application: On one hand, advanced optical structures such as microcavities and waveguides can be realized to increase the absorption and therefore the efficiency. On the other hand, it may lead to the monolithic integration of all key functionalities needed for photonic quantum information processing (photon generation, processing and detection), and therefore to integrated quantum photonic chips. Here we report the successful operation of a SSPD based on NbN nanowires grown on a GaAs substrate, and demonstrate its monolithic integration with an optical microcavity based on a GaAs/AlAs Bragg mirror. The GaAs/AlAs Bragg technology can provide very high reflectivity >99%, and allows the use of the top-illumination, as opposed to the top-mirror approach reported in ref. (2).
[Show abstract][Hide abstract] ABSTRACT: In their previous works (O'brien et. al, 2010), the authors applied a spectral filtering technique that enables the selection of a finite number of lasing modes in a Fabry-Perot (FP) cavity to the problem of passive mode-locking. This enabled the authors to fabricate mode-locked devices that generate both sinusoidal and pulsed intensity output. In this paper, this technique was illustrated by describing a device designed to support a discrete comb of six modes. This device incorporates 48 slotted regions etched into the ridge waveguide of the laser.
[Show abstract][Hide abstract] ABSTRACT: It is shown that optical synthesis of terahertz and millimeter-wave frequencies can be achieved using two-mode and mode-locked discrete mode diode lasers. These edge-emitting devices incorporate a spatially varying refractive index profile, which is designed according to the spectral output desired of the laser. We first demonstrate a device that supports two primary modes simultaneously with high spectral purity. In this case, sinusoidal modulation of the optical intensity at terahertz frequencies can be obtained. Cross saturation of the material gain in quantum-well lasers prevents simultaneous lasing of two modes with spacings in the millimeter-wave region. We show finally that by mode locking devices that are designed to support a minimal set of four primary modes, we obtain a sinusoidal modulation of the optical intensity in this frequency region.
IEEE Transactions on Microwave Theory and Techniques 12/2010; · 2.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mode-locking at 100 GHz with 2 ps pulse duration is demonstrated in a quantum-well diode laser designed to support a discrete comb of modes. Spectral filtering was achieved using a non-periodic patterning of the cavity effective index.
Optical Communication (ECOC), 2010 36th European Conference and Exhibition on; 10/2010
[Show abstract][Hide abstract] ABSTRACT: We demonstrate efficient nanowire superconducting single photon detectors (SSPDs) based on NbN thin films grown on GaAs. NbN films ranging from 3 to 5 nm in thickness have been deposited by dc magnetron sputtering on GaAs substrates at 350 °C. These films show superconducting properties comparable to similar films grown on sapphire and MgO. In order to demonstrate the potential for monolithic integration, SSPDs were fabricated and measured on GaAs/AlAs Bragg mirrors, showing a clear cavity enhancement, with a peak quantum efficiency of 18.3% at λ = 1300 nm and T = 4.2 K.
[Show abstract][Hide abstract] ABSTRACT: We describe how the multimode spectrum of a Fabry-Perot diode laser can be tailored using a non-periodic patterning of the cavity effective index. The cavity geometry is obtained from the solution of an inverse problem based on a perturbative calculation of the threshold gain of the longitudinal modes of the cavity. Experimental measurements are presented that demonstrate an all-optical memory element based on the injection locking bistability of a two-mode device. We also demonstrate passive harmonic mode-locking of a device designed to support a comb of six modes. Near-transform limited pulsed output with 2 ps pulse duration at 100 GHz repetition rate was obtained. Prospects for the extension of our approach to locking of larger numbers of modes over wider bandwidths are discussed. Similarities between the effective index profiles found in these devices and those of related devices and grating structures are also highlighted.
Photonics and Nanostructures - Fundamentals and Applications 09/2010; · 1.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the first nanoscale (down to approximately 50 x 50 nm(2)) detector displaying single-photon sensitivity and a nanosecond response. This type of nanodetector can also be operated in multiphoton mode, where the detection threshold can be set at N = 1, 2, 3, or 4 photons, thus allowing the mapping of photon number statistics on the nanoscale. Its operation principle based on that of hot-spot formation in superconducting nanowires allies the temporal resolution and sensitivity of superconducting single-photon detectors with subwavelength resolution and photon number discrimination. Such detectors can be of great interest for the study of nanophotonic devices at low temperature.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate passive harmonic mode locking of a quantum-well laser diode designed to support a discrete comb of Fabry-Perot modes. Spectral filtering of the mode spectrum was achieved using a nonperiodic patterning of the cavity effective index. By selecting six modes spaced at twice the fundamental mode spacing, near-transform-limited pulsed output with 2 ps pulse duration was obtained at a repetition rate of 100 GHz.