Chromo-modal dispersion for optical communication and time-stretch spectroscopy.

Dispersion management is critical in many optical applications, whether to reduce impairments in fiber optic communication or chirp pulse amplification, or to create time stretch instruments for single-shot continuous recording of fast phenomena. The most common solutions for achieving large dispersion with low loss include dispersion compensation fiber, fiber Bragg grating, and diffraction grating pairs. Such dispersive elements have finite operational bandwidth, limited total dispersion, or insufficient power handling. In this Letter, we demonstrate a new, to the best of our knowledge, implementation of the chromo-modal dispersion device based on a silicon waveguide slab that addresses these limitations. The device provides extremely large dispersion with a widely tunable spectrum. We also propose a new time-stretch spectrometer where the absorption cell simultaneously provides spectrum-to-time mapping for fast single-shot spectroscopy.

Coherent time-stretch transformation for real-time capture of wideband signals.

Time stretch transformation of wideband waveforms boosts the performance of analog-to-digital converters and digital signal processors by slowing down analog electrical signals before digitization. The transform is based on dispersive Fourier transformation implemented in the optical domain. A coherent receiver would be ideal for capturing the time-stretched optical signal. Coherent receivers offer improved sensitivity, allow for digital cancellation of dispersion-induced impairments and optical nonlinearities, and enable decoding of phase-modulated optical data formats. Because time-stretch uses a chirped broadband (>1 THz) optical carrier, a new coherent detection technique is required. In this paper, we introduce and demonstrate coherent time stretch transformation; a technique that combines dispersive Fourier transform with optically broadband coherent detection.

Spectrally encoded angular light scattering.

The angular light scattering profile of microscopic particles significantly depends on their morphological parameters, such as size and shape. This dependency is widely used in state-of-the-art flow cytometry methods for particle classification. We introduce a new spectrally encoded angular light scattering method, with potential application in scanning flow cytometry. We show that a one-to-one wavelength-to-angle mapping enables the measurement of the angular dependence of scattered light from microscopic particles over a wide dynamic range. Improvement in dynamic range is obtained by equalizing the angular dependence of scattering via wavelength equalization. Continuous angular spectrum is obtained without mechanical scanning enabling single-shot measurement. Using this information, particle morphology can be determined with improved accuracy. We derive and experimentally verify an analytic wavelength-to-angle mapping model, facilitating rapid data processing. As a proof of concept, we demonstrate the method's capability of distinguishing differently sized polystyrene beads. The combination of this technique with time-stretch dispersive Fourier transform offers real-time and high-throughput (high frame rate) measurements and renders the method suitable for integration in standard flow cytometers.

Digital broadband linearization of optical links.

We present a digital postprocessing linearization technique to efficiently suppress dynamic distortions added to a wideband signal in an analog optical link. Our technique achieves up to 35 dB suppression of intermodulation distortions over multiple octaves of signal bandwidth. In contrast to conventional linearization methods, it does not require excessive analog bandwidth for performing digital correction. This is made possible by regenerating undesired distortions from the captured output, and subtracting it from the distorted digitized signal. Moreover, we experimentally demonstrate a record spurious-free dynamic range of 120 dB·Hz(2/3) over a 6 GHz electrical signal bandwidth. While our digital broadband linearization technique advances state-of-the-art optical links, it can also be applied to other nonlinear dynamic systems.

Time-stretch oscilloscope with dual-channel differential detection front end for monitoring of 100 Gb/s return-to-zero differential quadrature phase-shift keying data.

Optical performance monitoring of high-capacity networks is one of the enabling technologies of future reconfigurable optical switch networks. In such networks, rapid performance evaluation of data streams becomes challenging due to the use of advanced modulation formats and high data rates. The time-stretch enhanced recording oscilloscope offers a potential solution to monitoring high-rate data in a practical time scale. Here we demonstrate an architecture with a differential detection front end for simultaneous I/Q data monitoring of a 100 gigabits/s return-to-zero differential quadrature phase-shift keying signal. This demonstration shows the potential of this technology for rapid performance monitoring of high-rate optical data streams that employ advanced modulation formats.

Digitally synthesized beat frequency-multiplexed fluorescence lifetime spectroscopy.

Frequency domain fluorescence lifetime imaging is a powerful technique that enables the observation of subtle changes in the molecular environment of a fluorescent probe. This technique works by measuring the phase delay between the optical emission and excitation of fluorophores as a function of modulation frequency. However, high-resolution measurements are time consuming, as the excitation modulation frequency must be swept, and faster low-resolution measurements at a single frequency are prone to large errors. Here, we present a low cost optical system for applications in real-time confocal lifetime imaging, which measures the phase vs. frequency spectrum without sweeping. Deemed Lifetime Imaging using Frequency-multiplexed Excitation (LIFE), this technique uses a digitally-synthesized radio frequency comb to drive an acousto-optic deflector, operated in a cat's-eye configuration, to produce a single laser excitation beam modulated at multiple beat frequencies. We demonstrate simultaneous fluorescence lifetime measurements at 10 frequencies over a bandwidth of 48 MHz, enabling high speed frequency domain lifetime analysis of single- and multi-component sample mixtures.