We present a high bandwidth piezoelectric-actuated mirror for length stabilization of an optical cavity. The actuator displays a transfer function with a flat amplitude response and greater than 135 • phase margin up to 200 kHz, allowing a 180 kHz unity gain frequency to be achieved in a closed servo loop. To the best of our knowledge, this actuator has achieved the largest servo bandwidth for a piezoelectric transducer (PZT). The actuator should be very useful in a wide variety of applications requiring precision control of optical lengths, including laser frequency stabilization, optical interferometers, and optical communications.

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Mid-infrared femtosecond optical frequency combs were produced by difference frequency generation of the spectral components of a near-infrared comb in a 3-mm-long MgO:PPLN crystal. We observe strong pump depletion and 9.3 dB parametric gain in the 1.5 m signal, which yields powers above 500 mW (3 W/mode) in the idler with spectra covering 2.8 m to 3.5 m. Potential for broadband, high-resolution molecular spectroscopy is demonstrated by absorption spectra and interferograms obtained by heterodyning two combs.

Probing matter with light in the mid-infrared provides unique insight into molecular composition, structure, and function with high sensitivity. However, laser spectroscopy in this spectral region lacks the broadband or tunable light sources and efficient detectors available in the visible or near-infrared. We overcome these challenges with an approach that unites a compact source of phase-stable, single-cycle, mid-infrared pulses with room temperature electric field–resolved detection at video rates. The ultrashort pulses correspond to laser frequency combs that span 3 to 27 μm (370 to 3333 cm−1), and are measured with dynamic range of >106 and spectral resolution as high as 0.003 cm−1. We highlight the brightness and coherence of our apparatus with gas-, liquid-, and solid-phase spectroscopy that extends over spectral bandwidths comparable to thermal or infrared synchrotron sources. This unique combination enables powerful avenues for rapid detection of biological, chemical, and physical properties of matter with molecular specificity.

We demonstrate high resolution coherent control of cold atomic rubidium utilizing spectral phase manipulation of a femtosecond optical frequency comb. Transient coherent accumulation is directly manifested by the enhancement of signal amplitude and spectral resolution via the pulse number. The combination of frequency comb technology and spectral phase manipulation enables coherent control techniques to enter a new regime with natural linewidth resolution.

Infrared spectroscopy is a powerful tool for basic and applied science. The molecular "spectral fingerprints" in the 3 µm to 20 µm region provide a means to uniquely identify molecular structure for fundamental spectroscopy, atmospheric chemistry, trace and hazardous gas detection, and biological microscopy. Driven by such applications, the development of low-noise, coherent laser sources with broad, tunable coverage is a topic of great interest. Laser frequency combs possess a unique combination of precisely defined spectral lines and broad bandwidth that can enable the above-mentioned applications.Here, we leverage robust fabrication and geometrical dispersion engineering of silicon nanophotonic waveguides for coherent frequency comb generation spanning 70 THz in the mid-infrared (2.5 µm to 6.2 µm). Precise waveguide fabrication provides significant spectral broadening and engineered spectra targeted at specific mid-infrared bands. We use this coherent light source for dual-comb spectroscopy at 5 µm.Spectroscopy has been a primary scientific tool for studying nature, leading to seminal advances in astronomy, quantum physics, chemistry and biology. The coherent light from a laser provides a powerful spectroscopic tool with the properties of high spectral resolution, wavelength tunability, and a well-defined Gaussian beam enabling high intensity focusing and long-distance propagation. Frequency comb lasers combine the above qualities in addition to a broad spectrum of precisely defined optical lines (the "comb") that can be absolutely referenced to radio frequencies (RF) or atomic frequency standards 1-3 . This has led to a variety of new spectroscopic advances 4-13 .While frequency combs were initially developed for the visible and near-infrared spectral regions, more recent research has focused on extending their coverage to the mid-infrared (mid-IR) 14 . This spectral region is of great interest because it is where many molecules including greenhouse gases, poisonous agents, explosives, and organics show distinctive ro-vibrational absorption fingerprints 14 . The development of a practical, broadband, and low-noise mid-IR frequency comb with moderate power could dramatically improve frequency precision, sensitivity, and data acquisition rates compared to conventional techniques such as Fourier-

We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-χ nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive wave generation in the 2.5-3 μm region and intrapulse difference frequency generation in the 4-5 μm region. By engineering the quasi-phase-matched grating profiles, tunable, narrowband MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment-and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.

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...