We present new work demonstrating the single-shot measurement of the intensity and phase of sub-picosecond optical pulses and waveforms without ambiguity at measurement times of 1 microsecond. While the technique has an optical front-end, back-end detection is done using 70 GHz electronics effectively extending the bandwidth of the electronics by a factor of 20-50. Measurements were made using both multishot and single-shot approaches. Multishot pulses were compared to separate cross-Frequency Resolved Optical Gating (X-FROG) measurements. Generation of the test pulses, as well as measurement results, will be presented.
Doppler Raman (DR) spectroscopy is a coherent Raman technique that combines impulsive Raman excitation with novel frequency shift detection to enable high-sensitivity Raman spectroscopy in the biological fingerprint region (500cm-1-1500cm-1) and the low frequency regime from 10cm-1 – 500cm-1. Using DR, we demonstrate nonresonant Raman spectroscopy on a suite of biologically significant targets involved in cell respiration including cytochrome c, adenosine triphosphate (ATP), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide (NADH). High-sensitivity detection of low-to-medium frequency Raman vibrational modes may provide a tool to monitor states of cell respiration along with large molecular structural changes such as protein conformational dynamics.
Sonogram encoding is utilized to measure ultrafast optical pulses at 1550 nm. The optical waveform is divided into narrow-bandwidth frequency bins, which are time multiplexed. The recombined single-shot sonogram signal is acquired using high bandwidth electronics and processed to recover the pulse intensity and phase. Measurement of optical pulses with a variety of phase distortions introduced using a pulse shaper or dispersive optical fiber is demonstrated. One sonogram is acquired within less than 1 microsecond, which allows single-shot pulse measurement at a rate of up to 1 MHz. Capabilities and detailed specifications of the method will be presented.
Optical microscopy and spectroscopy are widely used in multiple research areas relating to biology. Label-free spectroscopy and imaging are valuable tools that permit interrogation of biological samples without the need for exogenous labels, allowing for investigation of unperturbed biological systems. We demonstrate a coherent Raman technique called Doppler Raman (DR) spectroscopy which combines impulsive excitation with a novel frequency shift detection scheme for rapid, high sensitivity detection of low to medium frequency vibrational modes from 10-1800cm-1. Briefly, the DR spectroscope is a pump-probe system where the pump beam generates a time-varying index of refraction proportional to the Raman response of the sample. The time-delayed probe beam undergoes a frequency shift in the sample due to the time-varying index of refraction that is resolved using a novel high-sensitivity detection scheme. Other coherent Raman techniques such as Stimulated Raman Scattering (SRS) and Coherent Anti-Stokes Raman Spectroscopy (CARS) have been used to provide sensitive, label-free contrast for an array of biological targets, but their ability to detect low frequency vibrational modes is limited. Biologically significant targets like cytochrome c (740-760cm-1), DNA (782, 788, 1095cm-1), hydroxyapatite, and numerous pharmaceutical drugs exhibit rich Raman spectra across a range of low frequency modes below the well-known “fingerprint region”. Additionally, many proteins like hemoglobin, insulin, and bovine serum albumin have breathing modes below 50cm-1. Sensitive detection of low-frequency Raman vibrational modes unlocks a suite of potential biological and chemical dynamics like protein conformational changes and protein super complex formation.
Ultrafast time-dependent optical reflection and/or transmission spectroscopy can be used to measure time responses of materials and time arrivals between two unrelated ultrafast pulses. For example, a pump pulse, such as an x-ray pulse, excites a material, changing its refractive index. A spectrogram monitoring the change of intensity of a reflected or transmitted optical probe pulse can be used to indirectly monitor the refractive index change. Standard spectrogram deconvolution methods can be used to extract characteristics of both the material response and the probe pulse, but care must be taken to consider any experimental artifacts.
Frequency Resolved Optical Gating (FROG) uses the ultrafast laser pulse to interrogate itself for full characterization;
that is, no reference pulse is used to obtain the full intensity and phase of the input ultrafast laser pulse. Crosscorrelation FROG (X-FROG) cross-correlates a known ultrafast laser pulse with an unknown ultrafast laser pulse to characterize the unknown pulse allowing wider wavelength ranges of ultrafast laser pulses as well as more complex
ultrafast laser pulses to be characterized. We present a new X-FROG algorithm, based on the principal components
generalized projections (PCGP) algorithm that is fast, robust, and simple.
Mina Bionta, Doug French, James Cryan, James Glownia, Nick Hartmann, David Nicholson, Kevin Baker, Christoph Bostedt, Marco Cammarrata, Matthieu Chollet, Yuantao Ding, David Fritz, Steve Durbin, Yiping Feng, Marion Harmand, Alan Fry, Daniel Kane, Jacek Krzywinski, Henrik Lemke, Marc Messerschmidt, Daniel Ratner, Sebastian Schorb, Sven Toleikis, Diling Zhu, William White, Ryan Coffee
A recently demonstrated single-shot measurement of the relative delay between x-ray FEL pulses and optical laser pulses has now been improved to ~10 fs rms error and has successfully been demonstrated for both soft and hard x-ray pulses. It is based on x-ray induced step-like reduction in optical transmissivity of a semiconductor membrane (Si3N4). The transmissivity is probed by an optical continuum spanning 450 - 650 nm where spectral chirp provides a mapping of the step in spectrum to the arrival time of the x-ray pulse relative to the optical laser system.
Interest in quantum dot mode-locked lasers (QD MLLs) has grown in recent years since their first demonstration in 2001
as applications for optical time domain multiplexing, arbitrary waveform generation, and optical clocking are
anticipated. Ultrafast pulses below 1 ps have been reported from QD MLLs using intensity autocorrelation techniques,
but so far detailed characterization examining the pulse shape, duration, chirp, and degree of coherence spiking in these
lasers has not been carried out. We describe the first direct frequency-resolved optical gating (FROG) measurements on
a QD MLL operating at a repetition rate of 5 GHz.
Real-time video-rate imaging using harmonically detected Fourier domain OCT is demonstrated using an 800 nm light
source and a silicon line scan camera. At an imaging rate of 11.7 B-scans (1024 pixels × 256 pixels) per second, the
measured complex conjugate artifact suppression is 30-35 dB, the sensitivity is 121 dB, and the dynamic range is about
60 dB.
The two-harmonic FD-OCT method, where the quadrature components of the spectral interferogram are obtained by
simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is used for complex-conjugate-
resolved imaging of biological samples. The method is implemented using sampling of the phase modulated
interferogram with an integrating detector array followed by digital demodulation at the first and second harmonics. A
complex conjugate rejection ratio as high as 70 dB is achieved.
Fourier domain optical coherence tomography (FD-OCT) is an interferometric imaging technique that allows imaging to depths of a few mm in scattering biological tissues with high resolution of the order of 1-10 μm. However, the usefulness of FD-OCT is limited by background and autocorrelation interference terms that reduce the sensitivity and by phase ambiguity that halves the useful imaging depth range. These limitations can be overcome by obtaining the full, complex spectral interferogram. Simultaneous detection of the imaginary and real terms is obtained by phase modulating the reference arm of the interferometer and detecting at the first and second harmonics. A mathematical derivation of harmonically detected FD-OCT and experimental measurements showing that phase ambiguity artifacts can be suppressed by up to 70 dB are presented. The method provides efficient suppression of the complex conjugate, dc, and autocorrelation artifacts and has low sensitivity to phase noise. Beyond the removal of artifacts, the ability to obtain the full, complex interferogram is key to the development of spectrally resolved FD-OCT which would add depth-resolved spectroscopic detail to the structural information.
I demonstrate the closed-loop control of a programmable liquid-crystal spatial light modulator using real-time SHG FROG. Control theory states that proportional only controllers are almost never suitable for very tight control because any calibration errors can result in offsets between the input (desired result) and output. In this work, the pulse output from the real-time FROG software is sent to proportional-integral controller for maintaining the spectral phase of a pulse. The additional integral control term removes the constant offset that plagues proportional only controllers. The response time of pulse shaping system is a few seconds, limited only by the speed of the pulse shaper.
Visible/near-infrared diode lasers are well-suited for use as spectroscopic light sources in detection of a wide variety of gases by optical absorption. The high spectral resolution of these devices permits the selective detection of targeted species, while their characteristics of low cost, room temperature operation, and compatibility with fiber optics make them attractive for instrument development. A partial list of industrially or environmentally significant gases that may be measured by near-IR diode laser spectroscopy includes oxygen, water vapor, methane, acetylene, carbon monoxide, carbon dioxide, hydrogen halides, ammonia, hydrogen sulfide, and nitrogen oxides. This paper describes recent work at Southwest Sciences in development of diode laser-based instrumentation for industrial or environmental monitoring applications. Instrumentation utilizing a 1.393 micrometers DFB diode laser for measurement of trace moisture contamination in high purity process gases is described. In addition, recent laboratory studies to characterize the performance of new types of diode lasers in gas sensing applications are discussed, including vertical cavity surface emitting lasers in the 650 to 960 nm region and antimonide-based lasers in the 2.6 micrometers region.
We report and demonstrate a new technique to measure the full intensity and phase of a single femtosecond laser pulse. The technique, which we call 'frequency resolved optical gating' (FROG), is inexpensive, easy to implement, and provides an output that graphically displays the instantaneous frequency vs. time of the pulse. Using almost any instantaneous nonlinear- optical interaction of two replicas of the ultrashort pulse to be measured, FROG involves measuring the spectrum of the signal pulse as a function of delay between the two replicas. The resulting trace of intensity vs. frequency and delay yields an intuitive display of the pulse, similar to the pulse spectrogram. We show that the problem of inverting the FROG trace to obtain the pulse intensity and phase is a two-dimensional phase-retrieval problem. As a result, the FROG trace yields, in principle, an essentially unique pulse intensity and phase. In this work, we show that this is the case in practice, also. In addition, we present an iterative- Fourier-transform algorithm for inverting the FROG trace.
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