Bright luminescence from upconversion nanocrystals can be achieved by combining high-excitation irradiance with a high activator concentration. The enhanced brightness allows a single nanocrystal to be tracked, which can be used in bioimaging applications, for example.

Single-nanocrystal sensitivity achieved by enhanced upconversion luminescence

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

We experimentally demonstrate mid-infrared (MIR) supercontinuum (SC) generation spanning ∼2.0 to 15.1 μm in a 3 cm-long chalcogenide step-index fiber. The pump source is generated by the difference frequency generation with a pulse width of ∼170  fs, a repetition rate of ∼1000  Hz, and a wavelength range tunable from 2.4 to 11 μm. To the best of our knowledge, this is the broadest MIR SC generation observed so far in optical fibers. It facilitates fiber-based applications in sensing, medical, and biological imaging areas.

Mid-infrared supercontinuum generation spanning 2.0 to 15.1 μm in a chalcogenide step-index fiber.

A high-performance airborne water vapor differential absorption lidar has been developed during the past years. This system uses a four-wavelength/three-absorption line measurement scheme in the 935 nm H2O absorption band to cover the whole troposphere and lower stratosphere simultaneously. Additional high spectral resolution aerosol and depolarization channels allow precise aerosol characterization. This system is intended to demonstrate a future space-borne instrument. For the first time, it realizes an output power of up to 12 W at a high wall-plug efficiency using diode-pumped solid-state lasers and nonlinear conversion techniques. Special attention was given to a rugged optical layout. This paper describes the system layout and technical realization. Key performance parameters are given for the different subsystems.

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The airborne multi-wavelength water vapor differential absorption lidar WALES: system design and performance

The development of inverse design, where computational optimization techniques are used to design devices based on certain specifications, has led to the discovery of many compact, nonintuitive structures with superior performance. Among various methods, large-scale, gradient-based optimization techniques have been one of the most important ways to design a structure containing a vast number of degrees of freedom. These techniques are made possible by the adjoint method, in which the gradient of an objective function with respect to all design degrees of freedom can be computed using only two full-field simulations. However, this approach has so far mostly been applied to linear photonic devices. Here, we present an extension of this method to modeling nonlinear devices in the frequency domain, with the nonlinear response directly included in the gradient computation. As illustrations, we use the method to devise compact photonic switches in a Kerr nonlinear material, in which low-power and high-power pul...

Adjoint Method and Inverse Design for Nonlinear Nanophotonic Devices

We report efficient cascaded Raman scattering of near-IR nanosecond pulses in large-core (65 μm diameter) As2S3 and As2Se3 optical fibers. Raman scattering dominates other spectral broadening mechanisms, such as four-wave mixing, modulation instability, and soliton dynamics, because the fibers have large normal group-velocity dispersion in the spectral range of interest. With ∼2 ns pump pulses at a wavelength of 1.9 μm, four Stokes peaks, all with peak powers greater than 1 kW, have been measured.

Cascaded Raman shifting of high-peak-power nanosecond pulses in As_2S_3 and As_2Se_3 optical fibers

We present improvements to fluorescence sensing in soft-glass microstructured optical fibers that result in significantly improved sensitivity relative to previously published results. Concentrations of CdSe quantum dots down to 10 pM levels have been demonstrated. We show that the primary limitation to the sensitivity of these systems is the intrinsic fluorescence of the glass itself.

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Driving down the detection limit in microstructured fiber-based chemical dip sensors.

An injection-seeded optical parametric oscillator (OPO), based on periodically poled KTiOPO4 and pumped by a frequency-doubled, nanosecond-pulsed Nd:YAG laser, generates continuously tunable, single-longitudinal-mode, pulsed output at approximately 842 nm for high-resolution spectroscopy. Optical-heterodyne measurements show that the OPO frequency chirp increases linearly with detuning from the free-running (unseeded) OPO frequency and can be maintained as low as 10 MHz. Other factors affecting chirp are identified.

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Pulsed injection-seeded optical parametric oscillator with low frequency chirp for high-resolution spectroscopy

We evaluate ways to analyze optical-heterodyne measurements of frequency chirp in pulsed, single-longitudinal-mode output from lasers (or other coherent light sources) that operate on nanosecond time scales. The instantaneous frequency is extracted from the beat signal generated between a continuous-wave reference beam and the output of the pulsed source. Three analysis techniques are tested: Fourier-transform, direct curve fitting, and electronic mixing. We use synthetic beat waveforms based on actual experimental parameters to evaluate the three methods and apply these chirp-measurement techniques to an injection-seeded optical parametric oscillator system.

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Control of frequency chirp in nanosecond-pulsed laser spectroscopy. 1. Optical-heterodyne chirp analysis techniques

Optical-heterodyne measurements are made on ~842-nm signal output of an injection-seeded optical parametric oscillator (OPO) based on periodically poled KTiOPO4 pumped at 532 nm by long (~27-ns) pulses from a Nd:YAG laser At low pump energies (≤25 times the free-running threshold), the narrowband tunable OPO output is single-longitudinal-mode (SLM) and frequency chirp can be <10 MHz, much less than the transform-limited optical bandwidth (~175 MHz) We explore the transition from SLM operation to multimode operation as pump energy or phase mismatch are increased, causing unseeded cavity modes to build up later in the pulse

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Richard T. White

Papers

Cascaded Raman shifting of high-peak-power nanosecond pulses in As_2S_3 and As_2Se_3 optical fibers

Driving down the detection limit in microstructured fiber-based chemical dip sensors.

Pulsed injection-seeded optical parametric oscillator with low frequency chirp for high-resolution spectroscopy

Control of frequency chirp in nanosecond-pulsed laser spectroscopy. 1. Optical-heterodyne chirp analysis techniques

Transition from single-mode to multimode operation of an injection-seeded pulsed optical parametric oscillator.