Broadband mid-infrared frequency upconversion and spectroscopy with an aperiodically poled LiNbO3 waveguide.

We convert a mid-infrared frequency comb to near-infrared wavelengths through sum-frequency generation with a 1.064 µm CW laser in an aperiodically poled ZnO:LiNbO(3) waveguide. Upconversion of light in the range of 2.5-4.5 µm to 0.76-0.86 µm is demonstrated in a single device, and the efficiency of the conversion is measured across this bandwidth. We additionally characterize the spatial mode of the upconverted light. We then use this upconversion technique to detect and resolve individual lines from a methane gas sample with a common near-infrared optical spectrum analyzer. The stability of the spectrum of the upconverted light is analyzed with the goal of evaluating this technique for precise spectroscopic measurements.

Mid-infrared virtually imaged phased array spectrometer for rapid and broadband trace gas detection.

We present and characterize a two-dimensional (2D) imaging spectrometer based on a virtually imaged phased array (VIPA) disperser for rapid, high-resolution molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8 µm. We demonstrate detection of CH4 at 3.1 µm with >3750 resolution elements spanning >80 nm with ~600 MHz resolution in a <10 µs acquisition time. In addition to broadband detection, we also demonstrate rapid, time-resolved single-image detection by capturing dynamic concentration changes of CH4 at a rate of ~375 frames per second. Changes in absorption above the noise floor of 5×10(-4) are readily detected on the millisecond time scale, leading to important future applications such as real-time monitoring of trace gas concentrations and detection of reactive intermediates.

High-power, hybrid Er:fiber/Tm:fiber frequency comb source in the 2 µm wavelength region.

We present a 2 µm frequency comb based on a reliable mode-locked Er:fiber laser with 100 MHz repetition rate. After shifting the spectrum of the amplified Er:fiber comb to longer wavelengths, a single-clad Tm/Ho:fiber is used as a self-pumped pre-amplifier to generate a coherent and broadband spectrum centered at 1.93 µm. Subsequently, a cladding-pumped Tm:fiber amplifier boosts the system to a maximum output power of 4.8 W at 1.96 µm. After compression in a compact grating compressor, our amplified Er:fiber/Tm:fiber hybrid system delivers as much as 2.9 W with a pulse duration of 141 fs. The system's comb properties are examined via heterodyne measurement.

Quantum-noise-limited optical frequency comb spectroscopy.

We achieve a quantum-noise-limited absorption sensitivity of 1.7×10(-12) cm(-1) per spectral element at 400 s of acquisition time with cavity-enhanced frequency comb spectroscopy, the highest demonstrated for a comb-based technique. The system comprises a frequency comb locked to a high-finesse cavity and a fast-scanning Fourier transform spectrometer with an ultralow-noise autobalancing detector. Spectra with a signal-to-noise ratio above 1000 and a resolution of 380 MHz are acquired within a few seconds. The measured absorption line shapes are in excellent agreement with theoretical predictions.

Mid-infrared Fourier transform spectroscopy with a broadband frequency comb.

We present a first implementation of optical-frequency-comb-based rapid trace gas detection in the molecular fingerprint region in the mid-infrared. Near-real-time acquisition of broadband absorption spectra with 0.0056 cm(-1) maximum resolution is demonstrated using a frequency comb Fourier transform spectrometer which operates in the 2100-to-3700-cm(-1) spectral region. We achieve part-per-billion detection limits in 30 seconds of integration time for several important molecules including methane, ethane, isoprene, and nitrous oxide. Our system enables precise concentration measurements even in gas mixtures that exhibit continuous absorption bands, and it allows detection of molecules at levels below the noise floor via simultaneous analysis of multiple spectral features.

Cavity-enhanced direct frequency comb spectroscopy: technology and applications.

Cavity-enhanced direct frequency comb spectroscopy combines broad bandwidth, high spectral resolution, and ultrahigh detection sensitivity in one experimental platform based on an optical frequency comb efficiently coupled to a high-finesse cavity. The effective interaction length between light and matter is increased by the cavity, massively enhancing the sensitivity for measurement of optical losses. Individual comb components act as independent detection channels across a broad spectral window, providing rapid parallel processing. In this review we discuss the principles, the technology, and the first applications that demonstrate the enormous potential of this spectroscopic method. In particular, we describe various frequency comb sources, techniques for efficient coupling between comb and cavity, and detection schemes that utilize the technique's high-resolution, wide-bandwidth, and fast data-acquisition capabilities. We discuss a range of applications, including breath analysis for medical diagnosis, trace-impurity detection in specialty gases, and characterization of a supersonic jet of cold molecules.

Phase-stabilized, 1.5 W frequency comb at 2.8-4.8 microm.

We present a high-power optical-parametric-oscillator (OPO) based frequency comb in the mid-IR wavelength region. The system employs periodically poled lithium niobate and is singly resonant for the signal. It is synchronously pumped by a 10 W femtosecond Yb:fiber laser centered at 1.07 microm. The idler (signal) wavelength can be continuously tuned from 2.8 to 4.8 microm (1.76 to 1.37 microm) with a simultaneous bandwidth as high as 0.3 microm and a maximum average idler output power of 1.50 W. We also demonstrate the performance of the stabilized comb by recording the heterodyne beat with a narrow-linewidth diode laser. This OPO is an ideal source for frequency comb spectroscopy in the mid-IR.

Attosecond relative timing jitter and 13 fs tunable pulses from a two-branch Er:fiber laser.

We present what is believed to be the first direct measurement of the relative timing jitter between the two parallel pulse trains of a two-branch femtosecond erbium-doped fiber laser, operated without active stabilization. The system provides independently tunable pulses in the near infrared with durations down to 13 fs. Using an interferometric optical cross-correlator, the phase-noise spectral density is measured with high sensitivity in a range from 1 Hz up to the Nyquist frequency of 24.5 MHz. We find an integrated jitter of 11 attoseconds directly after the amplifier stages and 43 as after propagation through free-space optics and nonlinear fibers for frequency conversion.

Highly efficient second, third and fourth harmonic generation from a two- branch femtosecond erbium fiber source.

We report on highly efficient second, third and fourth harmonic generation from a femtosecond erbium-doped fiber source operating at 98 MHz repetition rate. By use of quasi-phase-matching in fan-out poled MgO:LiNbO(3), we generate pulses at 770 nm, 520 nm and 390 nm, with corresponding average powers of 120 mW, 55 mW and 6 mW, respectively. Our device can be employed as a two-color source providing radiation from ultraviolet to near infrared.

Long term comparison of two fiber based frequency comb systems.

Mode-locked erbium-doped fiber lasers are ideal comb generators for optical frequency metrology. We compare two fiber frequency combs by measuring an optical frequency independently with both combs and comparing their results. The two frequency measurements agree within 6x10-16. This is to our knowledge the first direct comparison between two fiber based frequency combs.

Widely tunable sub-30-fs pulses from a compact erbium-doped fiber source.

Coupling femtosecond light pulses from an all-fiber Er:laser system into a dispersion-shifted and highly non-linear fiber, we generate output spectra exhibiting two broadband and mutually coherent maxima. Depending on the chirp of the input pulse, the spectral separation is easily tunable over a wide range up to values exceeding 100 THz. In this way, the source provides access to an ultrabroadband wavelength interval from 1130 to 1950 nm. Because of soliton effects, the long-wave component exhibits a transform-limited pulse width of 40 fs directly after the nonlinear element. The high-frequency part propagating in the dispersive regime is recompressed to pulse durations as short as 24 fs with an optimized prism sequence.

Phase-locked two-branch erbium-doped fiber laser system for long-term precision measurements of optical frequencies.

We present a highly versatile approach to the application of femtosecond Er:fiber lasers in optical frequency metrology. Our concept relies on the implementation of two parallel amplifiers, seeded by a single master oscillator. With the comb spacing locked to a frequency of 100 MHz, we apply the output from the first amplifier to generate a feedback signal to achieve a simultaneous phase-lock for the comb offset frequency. The output of the independently configurable second amplifier enables precision frequency measurements in the visible and near-infrared. As a first application, we continuously measure the absolute frequency of a resonator-stabilized diode laser over a period of 88 hours.