Mid-Infrared Frequency Comb Generation and Spectroscopy with Few-Cycle Pulses and χ(2) Nonlinear Optics

Abstract

Mid-infrared laser frequency combs are compelling sources for precise and sensitive metrology with applications in molecular spectroscopy and spectro-imaging. The infrared atmospheric window between 3-5.5 µm in particular provides vital information regarding molecular composition. Using a robust, fiber-optic source of few-cycle pulses in the near-infrared, we experimentally demonstrate ultra-broad bandwidth nonlinear phenomena including harmonic and difference frequency generation in a single pass through periodically poled lithium niobate (PPLN). These χ (2) nonlinear optical processes result in the generation of frequency combs across the mid-infrared atmospheric window which we employ for dual-comb spectroscopy of acetone and carbonyl sulfide with resolution as high as 0.003 cm −1 . Moreover, cascaded χ (2) nonlinearities in the same PPLN directly provide the carrier-envelope offset frequency of the near-infrared driving pulse train in a compact geometry.Coherent laser sources in the mid-infrared (MIR, 3-25 µm) have long been recognized as important tools for both fundamental and applied spectroscopy and sensing. Recently, significant interest has focused on developing laser frequency combs in the MIR spectral region [1]. Of many promising applications, molecular spectroscopy using optical frequency combs benefits from a unique combination of high spectral resolution and broad bandwidth. This is particularly useful for the simultaneous measurement of spectral absorption fingerprints for a wide range of molecular compounds. The infrared atmospheric window between 3-5.5 µm exhibits reduced atmospheric attenuation while demonstrating strong absorption coefficients for greenhouse gases and pollutants such as methane, ethane, carbon dioxide, and formaldehyde [2-9], making this spectral range useful for climate research and atmospheric monitoring. Further, the same spectral window contains important molecular structure information pertaining to the C-H and O-H functional groups which can be used in the characterization of complex biochemical molecules [10,11] and spectro-imaging of biological samples [12,13].Based on these motivations, multiple approaches to MIR frequency comb generation have been pursued. Examples include optical parametric oscillators (OPOs) [14][15][16][17][18], supercontinuum generation [19][20][21][22], difference frequency generation (DFG) [23][24][25][26], direct generation with quantum cascade lasers (QCL) [27][28][29], mode-locked fiber lasers [30][31][32], and microresonator frequency combs [33]. Despite significant progress, many of these frequency comb sources require additional resonant cavities (OPOs) or careful spatio-temporal alignment of two femtosecond pulses (DFG) that increases complexity. Others lack absolute frequency calibration and have large modespacings (microresonator combs and QCLs) that are mis-matched to the spectroscopy of small molecules.In this Letter, we introduce a simple and powerful method for generating frequency combs across the MIR atmospheric window using int...

Broadband ultraviolet-visible frequency combs from cascaded high-harmonic generation in quasi-phase-matched waveguides

“…We model the HHG processes using a 1+1D approach based on the single-mode nonlinear analytical envelope equation (NAEE) [29,30], accounting for quadratic nonlinearity, dispersion, absorption, and self-steepening. The NAEE so far has been previously applied primarily to supercontinuum generation [33], conventional harmonic generation [29], and broadband difference frequency generation [32,34], but to our knowledge this is the first publication applying it to HHG. To emphasize the important length scales for cascaded HHG, we first recast the NAEE in the dimensionless form:…”

Architecture for microcomb-based GHz-mid-infrared dual-comb spectroscopy

“…Because the two combs sample absorption spectra with a resolution set by their line spacing (or repetition rate), analysis of the corresponding comb of radio frequencies reveal these spectra in a multiplexed fashion without the use of scanning gratings or interferometers 1 – 3 . Comb generation in the mid-infrared (mid-IR) has traditionally used methods that rely upon mode-locked pulse generation, including difference-frequency-generation (DFG), optical parametric oscillation, and supercontinuum generation 3 , 4 ; and there is considerable progress using such systems for mid-IR DCS 5 – 14 . More recently, mid-IR comb generation by DFG using electro-optic frequency combs (EO-comb) has also been demonstrated 15 , 16 .…”

Nanophotonic supercontinuum-based mid-infrared dual-comb spectroscopy

“…It is also well known for accessing a broad spectral range yet maintaining a high sensitivity comparable with traditional spectroscopic approaches such as wavelength or frequency modulation spectroscopy [9,10]. DCS has today seen significant advances [11] and has also been successfully applied to an increasing portion of the mid-IR spectrum using a number of mid-IR comb sources [12], including quantum cascade lasers (QCLs) [13], microresonator Kerr frequency combs [14], difference frequency generation (DFG) [15][16][17][18], cascaded quadratic nonlinear process [19][20][21][22][23], and optical parametric oscillators (OPO) [24][25][26][27]. Yet to date, generating phase locked mid-IR frequency combs that exhibit high brightness, broad bandwidth, and fine resolution remains challenging.…”