Fully referenced single-comb interferometry using optical sampling by laser-cavity tuning.

The correction of setup and laser instabilities in a single-comb interferometric measurement using optical sampling by laser-cavity tuning is investigated. A two-reference solution that allows full correction of the interferogram is presented. The technique is compared to a slightly simpler one-reference correction. For the one-reference case, all the subtleties involved in this partial correction and the dependence between the achievable measurement accuracy and the setup parameters are highlighted. The parameters considered are the comb bandwidth, the laser-frequency noise, the required spectral resolution, the cavity scan speed, and the length of the delay line. For both referencing approaches, experimental results using a fiber delay line of 10 km and a 100 MHz mode-locked laser with its repetition rate swept at 500 Hz are shown.

Dual-comb spectroscopy using frequency-doubled combs around 775 nm.

Two frequency-doubled combs are generated by nonlinear frequency conversion to realize spectroscopic measurements around 775 nm. Frequency-doubled interferograms are corrected in real-time by monitoring the relative instabilities between the combs at their fundamental frequency. Rubidium absorption lines are used to demonstrate the technique's accuracy and serve as absolute references to calibrate the frequency grid of computed spectra. The method allows frequency-doubled interferograms to be averaged without distortion during long periods of time. The calibrated frequency grid is validated by the measurement of the oxygen A-band. Moreover, the measurement analysis of the acetylene ν(1) + 3ν(3) overtone band has revealed some discrepancies with previous publications.

Continuous real-time correction and averaging for frequency comb interferometry.

Interferograms from a dual-comb spectrometer are continuously corrected and averaged in real-time. The algorithm is implemented on a field-programmable gate array (FPGA) development board. The chosen approach and the algorithm are described. Measurements with high signal-to-noise ratio, resolution and bandwidth are shown to demonstrate the accuracy of the optical referencing and the processing algorithm with 24 hours of averaging time, reaching a signal to noise ratio of 10,750,000 (>21 bits) in the interferogram and 316,000 in the spectrum at 100 MHz resolution. An interferogram where signal dominates the noise over the full delay range imposed by the 100 MHz repetition rate is reported for the first time.

Whispering gallery mode sensing with a dual frequency comb probe.

Silica microspheres are probed with a dual comb interferometry setup. The impulse responses of these microresonators are measured with a temporal resolution smaller than 400 fs over more than 200 ps. The amplitudes and phases of the impulse responses are interpreted as providing sensing information. The more familiar transmission spectra corresponding to the measured impulse responses are also calculated and shown. Sensing is demonstrated by varying the concentration of isopropanol in de-ionized water surrounding the microsphere and by binding bovine serum albumin on the silanized microsphere surface.

Predictive analog-to-digital converter for Fourier-transform spectrometers.

This paper proposes the use of predictive analog-to-digital converters (ADC) to handle dynamic range issues in Fourier-transform spectrometers. Several predictive approaches are proposed, and one is implemented experimentally to show that the technique works. A system was implemented with 16 bit (13 bits effective) ADCs and digital-to-analog converters (DACs) operated at 8 bits to provide a comparison basis. Measurements of a blackbody at 900 °C performed using the setup show a 13 bit effective performance, limited by the input noise of the data acquisition card.

Reducing the effect of integrating sphere speckle when characterizing the instrument line shape of a Fourier-transform hyperspectral imager.

The contribution of integrating sphere speckle pattern on the instrument line shape of a Fourier-transform hyperspectral imager is investigated. A new measurement technique that minimizes the speckle effect is presented. This technique consists of agitating the sphere while integrating with the instrument camera. Experimental results are presented that show the speckle effect on the instrument line shape and how it can be removed. This work is motivated by the constant goal of better identification and correction of the instrument line shape.

Software field widening of a Fourier-transform spectrometer using a large focal plane array.

Software field widening of a Fourier-transform spectrometer is investigated with a large multielement focal plane array detector. Experimental results are presented that stem from previous work in instrument line-shape correction. Here, pixels with calibrated wavenumber scales are binned to emulate a large-area single-pixel detector. The field of view and the signal-to-noise ratio are accordingly increased. A monochromatic source is used to characterize signal-to-noise ratio gain, and limitations are discussed. This work is motivated by the emergence of affordable infrared integrating cameras, which enable Fourier-transform spectrometers to perform massively parallel spatial sampling.

Fast line-shape correction procedure for imaging Fourier-transform spectrometers.

An instrument line-shape correction method adapted to imaging Fourier-transform spectrometers is demonstrated. The method calibrates all pixels on the same spectral grid and permits a direct comparison of the spectral features between pixels such as emission or absorption lines. Computation speed is gained by using matrix line-shape integration formalism rather than properly inverting the line shape of each pixel. A monochromatic source is used to characterize the spectral shift of each pixel, and a line-shape correction scheme is then applied on measured interferograms. This work is motivated by the emergence of affordable infrared CCD cameras that are currently being integrated in imaging Fourier-transform spectrometers.