The optical frequency comb fibre spectrometer.

Optical frequency comb sources provide thousands of precise and accurate optical lines in a single device enabling the broadband and high-speed detection required in many applications. A main challenge is to parallelize the detection over the widest possible band while bringing the resolution to the single comb-line level. Here we propose a solution based on the combination of a frequency comb source and a fibre spectrometer, exploiting all-fibre technology. Our system allows for simultaneous measurement of 500 isolated comb lines over a span of 0.12 THz in a single acquisition; arbitrarily larger span are demonstrated (3,500 comb lines over 0.85 THz) by doing sequential acquisitions. The potential for precision measurements is proved by spectroscopy of acetylene at 1.53 µm. Being based on all-fibre technology, our system is inherently low-cost, lightweight and may lead to the development of a new class of broadband high-resolution spectrometers.

Scanning micro-resonator direct-comb absolute spectroscopy.

Direct optical Frequency Comb Spectroscopy (DFCS) is proving to be a fundamental tool in many areas of science and technology thanks to its unique performance in terms of ultra-broadband, high-speed detection and frequency accuracy, allowing for high-fidelity mapping of atomic and molecular energy structure. Here we present a novel DFCS approach based on a scanning Fabry-Pérot micro-cavity resonator (SMART) providing a simple, compact and accurate method to resolve the mode structure of an optical frequency comb. The SMART approach, while drastically reducing system complexity, allows for a straightforward absolute calibration of the optical-frequency axis with an ultimate resolution limited by the micro-resonator resonance linewidth and can be used in any spectral region from UV to THz. We present an application to high-precision spectroscopy of acetylene at 1.54 µm, demonstrating performances comparable or even better than current state-of-the-art DFCS systems in terms of sensitivity, optical bandwidth and frequency-resolution.

Frequency-comb-assisted precision laser spectroscopy of CHF3 around 8.6 µm.

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 µm, using a CW quantum cascade laser phase-locked to a mid-infrared optical frequency comb. This latter is generated by a nonlinear down-conversion process starting from a dual-branch Er:fiber laser and is stabilized against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several transitions falling in the υ5 vibrational band are recorded with a frequency resolution of 20 kHz. Due to the very dense spectra, a special multiple-line fitting code, involving a Voigt profile, is developed for data analysis. The combination of the adopted experimental approach and survey procedure leads to fractional accuracy levels in the determination of line center frequencies, down to 2 × 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided.

Frequency-noise measurements of optical frequency combs by multiple fringe-side discriminator.

The frequency noise of an optical frequency comb is routinely measured through the hetherodyne beat of one comb tooth against a stable continuous-wave laser. After frequency-to-voltage conversion, the beatnote is sent to a spectrum analyzer to retrive the power spectral density of the frequency noise. Because narrow-linewidth continuous-wave lasers are available only at certain wavelengths, heterodyning the comb tooth can be challenging. We present a new technique for direct characterization of the frequency noise of an optical frequency comb, requiring no supplementary reference lasers and easily applicable in all spectral regions from the terahertz to the ultraviolet. The technique is based on the combination of a low finesse Fabry-Perot resonator and the so-called "fringe-side locking" method, usually adopted to characterize the spectral purity of single-frequency lasers, here generalized to optical frequency combs. The effectiveness of this technique is demonstrated with an Er-fiber comb source across the wavelength range from 1 to 2 µm.

Direct phase-locking of a 8.6-µm quantum cascade laser to a mid-IR optical frequency comb: application to precision spectroscopy of N2O.

We developed a high-precision spectroscopic system at 8.6 µm based on direct heterodyne detection and phase-locking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The ∼30 dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of ∼500 kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz-5 MHz), for an ultimate resolution of ∼21 kHz, limited by the measured linewidth of the mid-IR comb. The system was used to perform absolute measurement of line-center frequencies for the rotational components of the ν2 vibrational band of N2O, with a relative precision of 3×10(-10).

Narrow-linewidth quantum cascade laser at 8.6 µm.

We report on a narrow-linewidth distributed-feedback quantum cascade laser at 8.6 µm that is optical-feedback locked to a high-finesse V-shaped cavity. The spectral purity of the quantum cascade laser is fully characterized using a high-sensitivity optical frequency discriminator, leading to a 1 ms linewidth of less than 4 kHz and a minimum laser frequency noise spectral density as low as 0.01 Hz2/Hz for Fourier frequencies larger than 100 kHz. The cumulative standard deviation of the laser intensity is better than 0.1% over an integration bandwidth from 2 Hz to 100 MHz.

Er/Tm:fiber laser system for coherent Raman microscopy.

We present a novel architecture for a fiber-based hybrid laser system for coherent Raman microscopy, combining an amplified Er:fiber femtosecond oscillator with a Tm:fiber amplifier boosting the power of the 2-µm portion of a supercontinuum up to 300 mW. This is enough to obtain, by means of nonlinear spectral compression, sub-20-cm(-1) wide pump and Stokes pulses with 2500-3300 cm(-1) frequency detuning and average power at the 100-mW level. Application of this system to stimulated Raman scattering microscopy is discussed.

High-power frequency comb in the range of 2-2.15 µm based on a holmium fiber amplifier seeded by wavelength-shifted Raman solitons from an erbium-fiber laser.

We demonstrate a room-temperature high-power frequency comb source covering the spectral region from 2 to 2.15 µm. The source is based on a femtosecond erbium-fiber laser operating at 1.55 µm with a repetition rate of 250 MHz, wavelength-shifted up to 2.06 µm by the solitonic Raman effect, seeding a large-mode-area holmium (Ho) fiber amplifier pumped by a thulium (Tm) fiber laser emitting at 1.94 µm. The frequency comb has an integrated power of 2 W, with overall power fluctuations as low as 0.3%. The beatnote between the comb and a high-spectral-purity, single-frequency Tm-Ho laser has a linewidth of 32 kHz over 1 ms observation time, with a signal-to-noise ratio in excess of 30 dB.

Single-clad Tm-Ho:fiber amplifier for high-power sub-100-fs pulses around 1.9 µm.

A Tm-Ho:fiber amplifier based on single-clad geometry is demonstrated for the generation of high-power femtosecond pulses around 1.9 µm. The amplifier is seeded by the low-power Raman soliton generated by an Er:fiber femtosecond laser. Pulse trains at a repetition rate of 250 MHz tunable from 1.84 to 1.92 µm with corresponding powers from 2.6 to 3 W and durations from 80 to 105 fs have been obtained. Beating with a single-frequency Tm laser has shown that the pulse coherence is highly preserved. The overall power fluctuations have been measured to be as low as 0.6%.

Milliwatt-level frequency combs in the 8-14 µm range via difference frequency generation from an Er:fiber oscillator.

We report on the generation of mid-infrared (mid-IR) pulses with a maximum average optical power of 4 mW and wide tunability in the 8-14 µm range via difference frequency generation (DFG) in GaSe from an Er:fiber laser oscillator. The DFG process is seeded with self-frequency shifted Raman solitons that are shown to be phase coherent within the whole tuning range, from 1.76 to 1.93 µm. Interference measurements between adjacent pulses at the idler wavelengths attest coherence transfer to the mid-IR.

100 kHz linewidth Cr2+:ZnSe ring laser tunable from 2.12 to 2.58 µm.

We demonstrate a single-frequency, room temperature Cr2+:ZnSe laser widely tunable in the mid-infrared spectral region from 2.12 to 2.58 µm. Using a compact unidirectional ring cavity configuration, we obtain a maximum output power of 160 mW with an emission linewidth of ∼100 kHz in a 1 ms observation time. As a result, we can accurately report amplitude- and frequency-noise characterizations.