High-resolution Broadband Spectroscopy Research Articles

Frequency comb generation from the ultraviolet to mid-infrared region based on a three-stage cascaded PPLN chain

Optical frequency combs (OFCs) have enabled significant opportunities for high-precision frequency metrology and high-resolution broadband spectroscopy. Although nonlinear photonics chips have the capacity of frequency expansion for OFCs, most of them can only access the limited bandwidths in the partial infrared region, and it is still hard to satisfy many measurement applications in the ultraviolet-to-visible region. Here, we demonstrate a compact broadband OFC scheme via the combination of three χ(2) nonlinearities in a three-stage periodically poled lithium niobate (PPLN) chain. With a supercontinuum spectrum OFC delivered into the PPLN chain, the intra-pulse diffidence frequency generation, optical parametric amplification, and high-order harmonic generation were carried out in sequence. It is crucial that the harmonics of the 1st–10th orders are simultaneously obtained with an offset-free OFC spectrum from 0.35 to 4.0 μm. In view of the great potential for integration and spectral expansion, this wideband frequency comb source will open a new insight for the valuable applications of two-dimensional material analysis, biofluorescence microscopy, and nonlinear amplitude-phase metrology.

Ultrahigh-Resolution Direct-Frequency-Comb Spectrometer

High-resolution broadband spectroscopy is crucial for studying complex molecular structures and accurately retrieving the line shapes associated with their energy levels. These line shapes carry vital information about a molecule's thermodynamic properties such as temperature, pressure, and concentration, which are desirable for $e.g.$ the petroleum industry, environmental monitoring, and medical breath analysis. This study shows how to obtain a molecular spectrum with about 80 kHz of resolution and 7.86 MHz of spectral sampling. The promising high-resolution technique allows complete gas characterization by simultaneously measuring thermodynamic properties and analyzing composition.

High-resolution broadband spectroscopy using externally dispersed interferometry at the Hale telescope: part 2, photon noise theory

High-resolution broadband spectroscopy at near-infrared (NIR) wavelengths (950 to 2450 nm) has been performed using externally dispersed interferometry (EDI) at the Hale telescope at Mt. Palomar, with the TEDI interferometer mounted within the central hole of the 200-in. primary mirror in series with the comounted TripleSpec NIR echelle spectrograph. These are the first multidelay EDI demonstrations on starlight. We demonstrated very high (10×) resolution boost and dramatic (20× or more) robustness to point spread function wavelength drifts in the native spectrograph. Data analysis, results, and instrument noise are described in a companion paper (part 1). This part 2 describes theoretical photon limited and readout noise limited behaviors, using simulated spectra and instrument model with noise added at the detector. We show that a single interferometer delay can be used to reduce the high frequency noise at the original resolution (1× boost case), and that except for delays much smaller than the native response peak half width, the fringing and nonfringing noises act uncorrelated and add in quadrature. This is due to the frequency shifting of the noise due to the heterodyning effect. We find a sum rule for the noise variance for multiple delays. The multiple delay EDI using a Gaussian distribution of exposure times has noise-to-signal ratio for photon-limited noise similar to a classical spectrograph with reduced slitwidth and reduced flux, proportional to the square root of resolution boost achieved, but without the focal spot limitation and pixel spacing Nyquist limitations. At low boost (∼1×) EDI has ∼1.4× smaller noise than conventional, and at >10× boost, EDI has ∼1.4× larger noise than conventional. Readout noise is minimized by the use of three or four steps instead of 10 of TEDI. Net noise grows as step phases change from symmetrical arrangement with wavenumber across the band. For three (or four) steps, we calculate a multiplicative bandwidth of 1.8:1 (2.3:1), sufficient to handle the visible band (400 to 700 nm, 1.8:1) and most of TripleSpec (2.6:1).

A Flexible Phase-Insensitive System for Broadband CW-Terahertz Spectroscopy and Imaging

We present a compact, robust, and flexible contin-uous-wave (CW) terahertz system, ideally suited for both imaging and high-resolution broadband spectroscopy. The setup employs an incoherent detection scheme: A photomixer transmitter is combined with a zero-bias Schottky diode on the receiver side. The useable bandwidth extends to 1500 GHz, with a spectral resolution on the 10 MHz level. In proof-of-principle measurements, we apply our setup to imaging of objects within a paper envelope as well as transmission and reflection-mode spectroscopy, taking advantage of the high spectral resolution of the terahertz source and the broad bandwidth and efficiency of the Schottky receiver.

High-resolution broadband spectroscopy using externally dispersed interferometry at the Hale telescope: Part 1, data analysis and results

High-resolution broadband spectroscopy at near-infrared wavelengths (950 to 2450 nm) has been performed using externally dispersed interferometry (EDI) at the Hale telescope at Mt. Palomar. Observations of stars were performed with the “TEDI” interferometer mounted within the central hole of the 200-in. primary mirror in series with the comounted TripleSpec near-infrared echelle spectrograph. These are the first multidelay EDI demonstrations on starlight, as earlier measurements used a single delay or laboratory sources. We demonstrate very high (10×) resolution boost, from original 2700 to 27,000 with current set of delays (up to 3 cm), well beyond the classical limits enforced by the slit width and detector pixel Nyquist limit. Significantly, the EDI used with multiple delays rather than a single delay as used previously yields an order of magnitude or more improvement in the stability against native spectrograph point spread function (PSF) drifts along the dispersion direction. We observe a dramatic (20×) reduction in sensitivity to PSF shift using our standard processing. A recently realized method of further reducing the PSF shift sensitivity to zero is described theoretically and demonstrated in a simple simulation which produces a 350× times reduction. We demonstrate superb rejection of fixed pattern noise due to bad detector pixels—EDI only responds to changes in pixel intensity synchronous to applied dithering. This part 1 describes data analysis, results, and instrument noise. A section on theoretical photon limited sensitivity is in a companion paper, part 2.

High-resolution broadband spectroscopy with a resonator-based phase modulator

A method for significant enhancement of the spectral resolution of a Fabry-Perot resonator in transmission and absorption measurements is proposed. In the method, a laser with ultrashort pulses is used as the optical source. A dispersive element is placed in front of the Fabry-Perot resonator and a phase modulator is incorporated into the resonator. The spectrum of the laser pulse transmitted through the system is approximately periodic with ultranarrow peaks. The sample transmission spectrum is measured by scanning the output pulse spectrum. It is demonstrated, in numerical simulations, that for realistic parameters of the phase modulator, the finesse of the Fabry-Perot resonator is increased from 72 to 1900 and a resolution of 1 MHz is achieved. A method for increasing the spectral range of measurements with scanning the periodic spectra is also proposed. The method is based on the use of a waveguide array of Mach-Zehnder interferometers or a single discretely tunable interferometer. The measurement of the sample transmission spectrum within 33 free spectral ranges of the resonator is numerically demonstrated. The spectral range of the measurement can be increased up to 10 THz resulting in the equivalent finesse of the system of 10(7) for a 100 fs laser pulse.