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We report on a compact optical frequency comb, operating in the wavelength range from 670 to 1500 nm, based on diode-pumped low-noise femtosecond Yb:CALGO amplified laser system. Both the carrier-envelope offset and repetition rate are phase-locked to reference synthesizers. A full characterization of the frequency comb, in terms of frequency stability, phase noise analysis, and optical beating against a single-frequency non-planar ring oscillator Nd:YAG laser, is presented, showing the excellent properties of the Yb:CALGO comb.

A fiber laser system for standoff detection of chemical and biological species by coherent anti-Stokes Raman scattering is presented. The system is based on an ytterbium fiber laser and a hollow-core photonic crystal fiber for generation of broadband pump/Stokes pulses. High-resolution Raman spectra encompassing the fingerprint region ( 600 − 1600 c m − 1 ) are obtained for toluene, and two simulants of chemical and biological warfare agents, specifically dimethyl methylphosphonate and sodium dipicolinate. The system is operated at standoff distances of 2 m and integration times of 8 ms. The fiber technology makes the approach suitable for implementation as a compact standoff detection and identification system.

We demonstrate nonlinear compression of pulses at 1.03 µm and repetition rate of 200 kHz generated by a ytterbium fiber laser using two cascaded all-solid-state multipass cells. The pulse duration has been compressed from 460 to 22 fs, corresponding to a compression factor of ∼21. The compressed pulse energy is 15.6 µJ, corresponding to an average power of 3.1 W, and the overall transmission of the two compression stages is 76%. The output beam quality factor is M2 ∼1.2 and the excess intensity noise introduced by nonlinear broadening is below 0.05%. These results show that nonlinear pulse compression down to ultrashort durations can be achieved with an all-solid-state approach, at pulse energies much higher than previously reported, while preserving the spatial characteristics of the laser.

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., An ideal optical frequency-comb system should combine both single-line spectral resolution and a bandwidth broad enough to cover as many lines as possible. Here, the authors incorporate a fibre spectrometer to detect approximately 500 comb-lines with an instrument resolution of 120 megahertz.

Modulation transfer spectroscopy is used to demonstrate absolute frequency stabilization of an 8.6-µm-wavelength quantum cascade laser against a sub-Doppler absorption of the C H F 3 molecule. The obtained spectral emission properties are thoroughly characterized through a self-referenced optical frequency comb, stabilized against either a GPS-disciplined Rb clock or a 1.54-µm Er-fiber laser locked to a high-finesse ultra-low-expansion optical cavity. Fractional long-term stability and accuracy at a level of 4 × 10 − 12 (at 100 s) and 3 × 10 − 10 , respectively, are demonstrated, along with an emission linewidth as narrow as 10 kHz for observation times of 0.1 s.

The need of a fs-scale pulsed, high repetition rate, X-ray source for time-resolved fine analysis of matter (spectroscopy and photon scattering) in the linear response regime is addressed by the conceptual design of a facility called MariX (Multi-disciplinary Advanced Research Infrastructure for the generation and application of X-rays) outperforming current X-ray sources for the declared scope. MariX is based on the original design of a two-pass two-way superconducting linear electron accelerator , equipped with an arc compressor, to be operated in CW mode (1 MHz). MariX provides FEL emission in the range 0.2–8 keV with 1 0 8 photons per pulse ideally suited for photoelectric effect and inelastic X-ray scattering experiments. The accelerator complex includes an early stage that supports an advanced inverse Compton source of very high-flux hard X-rays of energies up to 180 keV that is well adapted for large area radiological imaging, realizing a broad science programme and serving a multidisciplinary user community, covering fundamental science of matter and application to life sciences, including health at preclinical and clinical level.

We report on efficient supercontinuum generation in tapered suspended-core A s 39 S e 61 fibers pumped by a femtosecond mode-locked Cr:ZnSe laser. The supercontinuum spectrum spans the mid-infrared spectral region from 1.4 to 4.2 µm, and its spectral coherence is proved by heterodyning with a single-frequency narrow-linewidth Er-fiber laser at 1.55 µm, measuring a beat note with 27-dB signal-to-noise ratio in a resolution bandwidth of 100 kHz. The intensity stability of the supercontinuum radiation is also characterized by relative intensity noise measurements.

We demonstrate a Fourier transform (FT) coherent anti-Stokes Raman scattering (CARS) spectroscopy system based on fiber technology with ultra-broad spectral coverage and high-sensitivity. A femtosecond ytterbium fiber oscillator is amplified and spectrally broadened in a photonic crystal fiber to synthesize pulses with energy of 14 nJ at 1040 nm, that are compressed to durations below 20 fs. The resulting pulse train is coupled to a FT-CARS interferometer enabling measurement of high-quality CARS spectra with Raman shifts of ~3000 cm−1 and signal to noise ratio up to 240 and 690 with acetonitrile and polystyrene samples, respectively, for observation times of 160 µs; a detection limit of one part per thousand is demonstrated with a cyanide/water solution. The system has the potential to detect trace contaminants in water as well as other broadband high-sensitivity CARS spectroscopy applications.

We present a mid-IR high-precision spectrometer capable of performing accurate Doppler-free measurements with absolute calibration of the optical axis and high signal-to-noise ratio. The system is based on a widely tunable mid-IR offset-free frequency comb and a Quantum-Cascade-Laser (QCL). The QCL emission frequency is offset locked to one of the comb teeth to provide absolute-frequency calibration, spectral-narrowing, and accurate fine frequency tuning. Both the comb repetition frequency and QCL-comb offset frequency can be modulated to provide, respectively, slow- and fast-frequency-calibrated scanning capabilities. The characterisation of the spectrometer is demonstrated by recording sub-Doppler saturated absorption features of the CHF3 molecule at around 8.6 μm with a maximum signal-to-noise ratio of ∼7 × 103 in 10 s integration time, frequency-resolution of 160 kHz, and accuracy of less than 10 kHz.

We demonstrate a fiber-format system for dual-comb coherent anti-Stokes Raman scattering spectroscopy. The system is based on two ytterbium fiber (Yb) femtosecond lasers at repetition frequencies of 94 MHz, a Yb amplifier, and a photonic crystal fiber for spectral broadening and generation of pulses with a central wavelength of 1040 nm and durations in the sub-20-fs regime. We observed Raman spectra of acetonitrile and ethyl acetate with spectral coverage from 100 to 1300 cm−1, resolution of 8 cm−1, and a signal-to-noise ratio of around 100, when averaging over 10 acquisitions. The design is suitable for implementing portable dual-comb coherent Raman spectrometers.

We report the highest slope efficiency from a Cr2+:CdSe laser. The sample was multilayered dielectric coated with TaO 2 and SiO 2 to achieve this performance. The output beam was a diffraction limited TEM 00 gaussian mode.

We report on metrological-grade saturated absorption spectroscopy of CHF3 at 8.63 μm based on CW distributed-feedback QCL laser and a mid-IR self-frequency referenced optical comb. The experimental setup for the saturation spectroscopy of CHF3 is shown in Fig. 1. A CW DFB-QCL with a maximum output power of ∼40 mW tunable in the wavelength range from 8.55 to 8.65 μm, in coincidence with the us (asymmetric FCF bending) vibrational band of CHF3, is used to probe the gas sample. The QCL output beam is focused at the input of a 30-dB optical isolator to a diameter of 1.6∼mm, by using antireflection-coated aspheric lens mounted onto the laser housing. Then the laser beam passing through a 20-cm plano-convex lens and a 50% beam splitter is coupled to a 25-cm-long stainless steel cell containing the CHF3 gas (98% purity). The transmitted beam first passes through a second 50% beam splitter to detect the single-pass CHF3 absorptions and then is back-reflected into the gas cell to implement the double pass configuration (collinear pump and probe saturated absorption technique). The probe beam is detected exploiting the reflection on the first beam splitter by a four-stage electric cooled MCT detector (10∼MHz bandwidth). By modulating the QCL driving current at a frequency _/m=100 kHz also a wavelength modulation method is implemented to retrieved the first derivative signal by means of a first-harmonic coherent demodulation. To absolute calibrate the QCL emission frequency a difference frequency generation optical frequency comb, covering the 8–14 μm spectral region, is used [1]. The mid-IR comb is properly tuned to a central wavelength of 8.6 μm (average output power of 4∼mW in an optical bandwidth of 0.8 μm) and its repetition frequency is stabilized against a RF synthesizer locked to a GPS-disciplined Rb frequency standard (Allan deviation σ(τ)=10−11τ−1/2 and fractional frequency accuracy of 10−13, respectively, where τ is the integration time).

We perform a back-to-back comparison between two nonlinear vibrational imaging techniques: stimulated Raman scattering (SRS) and balanced detection Raman-induced Kerr effect (BD-RIKE). Using a compact fiber-based laser system for generation of pump and Stokes signals, we image polymer beads as well as human hepatocytes under the same experimental conditions. We show that BD-RIKE, despite the slightly lower signal levels, consistently offers an improved signal-to-noise ratio with respect to SRS, resulting in significantly higher image quality. Importantly, we observe that such quality is not affected by the static birefringence of the sample, which makes BD-RIKE a robust and attractive alternative to SRS. We also highlight a unique advantage of the technique, which is its capability to easily access both the real and imaginary parts of the nonlinear susceptibility, thus allowing for vibrational phase imaging. The phase information can be readily obtained from BD-RIKE with minimal experimental effort and provides an additional chemical selectivity channel for coherent Raman microscopy. Copyright © 2014 John Wiley & Sons, Ltd.

We developed a high-precision spectroscopic system at 8.6 mu m based on direct heterodyne detection and phaselocking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The similar to 30 dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of similar to 500 kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz-5 MHz), for an ultimate resolution of similar to 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 nu(2) vibrational band of N2O, with a relative precision of 3 x 10(-10). (C) 2015 Optical Society of America

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the υsub5/subvibrational band are investigated at around 1158.9 cmsup-1/supwith a fractional frequency precision of 8.6·10sup-12/supat 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10sup-11/supis mainly limited by the reproducibility of the frequency measurements.

We report on a Yb-pumped optical parametric oscillator (OPO) that delivers 30 fs pulses with spectral coverage from 680 to 910 nm and an average output power of up to 1.1 W. The resulting peak power is similar to 0.5 MW, which is, to the best of our knowledge, the highest ever demonstrated in a femtosecond OPO. The intensity noise remains at a level of 0.2% rms, and rapid wavelength tuning is obtained by simply scanning the resonator length. The performances of the OPO are promising for a variety of applications in nonlinear microscopy and ultrafast spectroscopy. (C) 2017 Optical Society of America

We report on a room-temperature Kerr-lens mode-locked Cr:ZnSe femtosecond laser operating at around 2.4 μm emission wavelength. Self-starting nearly transform-limited pulse trains with a minimum duration of 47 fs, corresponding to six optical cycles, and average output power of 0.25 W are obtained with repetition frequencies in the range from 140 to 300 MHz. The femtosecond pulse train is characterized by high-spectral purity and low time jitter.

The paper reports the characterization of a dysprosium doped ZBLAN fiber amplifier intended to be used in mid infrared applications. Using a single-frequency 1064-nm laser pump with 1.5 W, two different fiber lengths were characterized for their losses, coupling efficiency, absorption and enhancement.

Differential phase-shift keying (DPSK) demodulations operated by a structured fiber Bragg grating (FBG) filter and by a Mach–Zehnder delay interferometer (MZDI) in a single-ended configuration are compared. Experimental measurements at 10 Gb/s demonstrate that a specially designed FBG outperforms an integrated-optic MZDI of ∼4 dB and ∼3.5 dB in back-to-back and after 25-km propagation, respectively. Both demodulators show low polarization sensitivity and signal frequency detuning dependence, but only MZDI operating point requires a thermal control. FBG filter proves an interesting solution for DPSK demodulation in low-cost applications and, moreover, can be designed to match colorless requirements of wave division multiplexed passive optical network (WDM-PON) applications.

We report on a narrow-linewidth distributed-feedback quantum cascade laser at 8.6 mu m 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 Hz(2)/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. (C) 2014 Optical Society of America

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.

Accurate frequency measurements of molecular transitions around $2 \ensuremath{\mu}\mathrm{m}$ are performed by using a direct-frequency-comb spectroscopy approach that combines an ${\mathrm{Er}}^{+}$ frequency-comb oscillator at $1.5 \ensuremath{\mu}\mathrm{m}$, a Tm-Ho fiber amplifier, and a Fabry-Perot-filter, high-resolution dispersive spectrometer optical multiplex-detection system. This apparatus has unique performances in terms of a wide dynamic range to integrate the intensity per comb mode, which allows one to measure molecular absorption profiles with high precision. Spectroscopic information about transition frequencies and linewidths is very accurately determined. Relative frequency uncertainties of the order of a few parts in ${10}^{\ensuremath{-}9}$ are achieved for rovibrational transitions of the ${\mathrm{CO}}_{2}$ molecule around $5100 {\mathrm{cm}}^{\ensuremath{-}1}$. Moreover, tiny frequency shifts due to molecular collisions and interacting laser power using direct comb spectroscopy are investigated in a systematic way.

An integrated single-sideband modulator is used as the sole wide-bandwidth frequency actuator in a Pound-Drever-Hall locking loop. Thanks to the large modulation bandwidth, the device enables a locking range of +/- 75 MHz and a control bandwidth of 5 MHz without the need for a second feedback loop. As applied to the coupling of an extended-cavity diode laser at 1.55 mu m to a high-finesse optical cavity, the in-loop frequency noise spectral density reaches a minimum of 1 mHz/Hz(1/2) at 1 kHz. (C) 2015 Optical Society of America

We report on a comprehensive characterization of Cr2+-doped CdSe single crystal as an efficient active material for tunable laser applications in the mid-infrared spectral region. Optical gain, thermo-optical behavior, power efficiency, scalability and wavelength tunability have been thoroughly investigated. Using an antireflection-coated crystal pumped by a Tm-fiber laser at at 1.94 μm, 1-W CW output power and 50% slope efficiency at 2.65 μm emission wavelength have been obtained in a diffraction-limited output beam. An output peak power of 2.5 W has been achieved without significant beam distortion in a quasi-CW regime. Exploiting an intra-cavity diffraction grating in a Littrow configuration, a maximum tuning range of 900 nm from 2.22 to 3.12 μm, limited by the finite bandwidth of resonator components, has been demonstrated with an uncoated crystal.

Optical amplifiers operating in the spectral region around 2 μm are interesting for environmental monitoring, high resolution molecular spectroscopy, frequency metrology, and medicine. Thulium-holmium-doped fibers employed as gain materials in amplifiers operating near 2 μm are attractive for their high emission cross-sections and spectrally broad transition, and alignment-free operation allowed by all-fiber technology.

Summary form only given. Wide bandwidth coherent emission in the mid-infrared (mid-IR) region (2-10 μm) is of great interest in different fields, such as high-resolution molecular spectroscopy, frequency metrology, and remote sensing of environmental gases and pollutants in the atmosphere. In fact, in this spectral interval, the so called “fingerprint” region, inorganic and organic molecules show the strongest absorption features associated with fundamental ro-vibrational transitions. Among the different generation schemes, such as semiconductor lasers (quantum cascade lasers and lead salt compounds or gallium-antimonide) and nonlinear optical techniques (namely difference frequency and optical parametric generation), impurity doped crystalline lasers constitute another viable route for mid-IR generation [1]. In the last years, particular attention has been devoted to the development of Cr2+:ZnSe lasers due to excellent lasing and optical properties of this material, which represents the mid-IR analog of Ti:sapphire, and different operating regimes have been subsequently demonstrated [2].Here we report on two different and complementary approaches for the generation of highly-coherent radiation in the wavelength range from 2.1 to 2.6 μm based on the use of room-temperature Cr2+:ZnSe gain medium. In the first implementation, we demonstrate single-frequency Cr2+:ZnSe laser widely tunable in the spectral region from 2.12 to 2.58 μm. Using a compact unidirectional ring cavity configuration (see Fig. 1a), a maximum output power of 160 mW with an emission linewidth of ~100 kHz in 1-ms observation time is obtained. The low frequency and intensity noises allows the use of this oscillator in high-resolution and highprecision molecular spectroscopy in the mid-IR spectral. The second example consists of a frequency comb source with a maximum average output power up to 1.2 W, tunable in the 2.2-2.6 μm wavelength region, based on a Cr2+:ZnSe multipass solid-state amplifier seeded by the output of an actively-stabilized optical parametric oscillator, synchronously pumped by a commercial 250-MHz Er:fiber laser. The amplifier, based on an Er-fiberpumped Cr2+:ZnSe crystal in a 3-pass configuration (see Fig. 1b), boosts the output power per comb-mode up to 150 μW. Phase relationship between idler, signal, and pump waves is exploited to perform frequency comb stabilization in the whole 2.2-2.6 μm mid-infrared spectral region. This phase-locking, together with the available power level, makes the system an ideal tool for direct comb spectroscopy in the spectral region from 2 to 3 μm.

We present a frequency comb source with power level up to 150 μW per comb mode, tunable in the 2.2-2.6 μm wavelength region, based on a Cr(2+):ZnSe multipass solid-state amplifier seeded by the output of an actively stabilized optical parametric oscillator, synchronously pumped by a commercial 250 MHz Er:fiber laser. Phase relationship between idler, signal, and pump waves is exploited to perform frequency comb stabilization in the whole 2.2-2.6 μm mid-infrared spectral region.

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.

We demonstrate the passive mode-locking operation of an in-band-pumped Ho:YLiF(4) laser at 2.06 μm using a semiconductor saturable absorber mirror based on InGaAsSb quantum wells. A transform-limited pulse train with minimum duration of 1.1 ps and average power of 0.58 W has been obtained at a repetition frequency of 122 MHz. A maximum output power of 1.7 W has been generated with a corresponding pulse duration of 1.9 ps.

Solid-state amplifiers operating in the spectral region around 2 µm are interesting for environmental monitoring, high resolution molecular spectroscopy, frequency metrology, and medicine [1]. Holmium-doped crystals employed as gain materials in amplifiers operating near 2 µm are attractive for their high emission cross-sections and spectrally broad transition.

We report for the first time on laser action of a diode-pumped Yb:BaY(2)F(8) crystal. Both CW and femtosecond operations have been demonstrated at room-temperature conditions. A maximum output power of 0.56 W, a slope efficiency of 34%, and a tunability range from 1013 to 1067 nm have been obtained in CW regime. Transform-limited pulse trains with a minimum duration of 275 fs, an average power of 40 mW, and a repetition rate of 83 MHz have been achieved in a passive mode-locked regime using a semiconductor saturable absorber mirror.

Modeling a real laser diode stack based on Zemax ray tracing software that operates in a nonsequential mode is reported. The implementation of the model is presented together with the geometric and optical parameters to be adjusted to calibrate the model and to match the simulated intensity irradiance profiles with the experimental profiles. The calibration of the model is based on a near-field and a far-field measurement. The validation of the model has been accomplished by comparing the simulated and experimental transverse irradiance profiles at different positions along the caustic formed by a lens. Spot sizes and waist location are predicted with a maximum error below 6%.

Solid-state lasers emitting in the 2 µm spectral region find important applications in remote sensing and gas detection, in high-resolution molecular spectroscopy, in frequency metrology, in medicine and in optical communications. Thulium-doped crystals employed as active materials in lasers operating near 2 am are attractive especially for their wide tunability and high efficiency [1], besides the advantage of direct diode pumping at 785 nm which allows the implementation of compact, reliable and rugged laser sources.

Passive mode locking of a diode-pumped Yb3+:KYF4 laser is demonstrated using a semiconductor saturable absorber mirror. A high-stability, transform-limited pulse train. with a repetition rate of 57 MHz is generated. Solitonlike pulses with maximum average output power of 250 mW, minimum pulse duration of 170 fs, and rms time jitter of 360 fs were obtained. (C) 2008 Optical Society of America.

We demonstrate passive mode-locking by means of a semiconductor saturable-absorber mirror in a diode-pumped Yb:YLF laser. We present crystal growth process, spectroscopic measurements, and investigation of mode-locking performance. Pulse trains with minimum duration of 196 fs, average power of 54 mW and a repetition rate of 55 MHz were obtained. The optical spectrum, centered at 1028 nm, has a 7.1-nm bandwidth leading to nearly transform-limited pulses. (C) 2008 Optical Society of America.

We report on a cw diode-pumped Tm laser, with wide tunability around 1.93 μm, based on a novel Tm-doped LiLuF4 active crystal (Tm:LiLuF). Remarkable features of LiLuF are low phonon energy, which makes it suitable for IR transitions, reduced thermal lensing and birefringence, and low up-conversion losses, which results in efficient laser action. Maximum output power in excess of 1 W and wavelength tunability range of ˜210 nm were demonstrated.

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 mu m, 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 nu(5) 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 x 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided. (C) 2015 AIP Publishing LLC.

A spectroscopic characterization of Tm-Ho:KYF4 crystals with different doping levels is presented. A comparative investigation based on diode-pumped 2-μm laser experiments shows that the best trade-off in terms of cw output power characteristics and wavelength tuning ranges is obtained with 5% Tm-0.5% Ho active crystal.

We report on a compact scheme for absolute referencing and coherent averaging for dual-comb based spectrometers, exploiting a single continuous-wave (CW) laser in a transfer oscillator configuration. The same CW laser is used for both absolute calibration of the optical frequency axis and the generation of a correction signal which is used for a real-time jitter compensation in a fully electrical feed-forward scheme. The technique is applied to a near-infrared spectrometer based on a pair of free-running mode-locked Er: fiber lasers, allowing to perform real-time absolute-frequency measurements over an optical bandwidth of more than 25 nm, with coherent interferogram averaging over 1-s acquisition time, leading to a signal-to-noise ratio improvement of 29 dB over the 50 mu s single shot acquisition. Using 10-cm single pass cell, a value of 1.9 x 10(-4) cm(-1) Hz(-0.5) noise-equivalent-absorption over 1 s integration time is obtained, which can be further scaled down with a multi-pass or resonant cavity. The adoption of a single CW laser, together with the absence of optical locks, and the full-fiber design makes this spectrometer a robust and compact system to be employed in gas-sensing applications. (C) 2014 AIP Publishing LLC.

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-mu m 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. (C) 2014 Optical Society of America

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.

A compact and versatile femtosecond mid-IR source is presented, based on an optical parametric oscillator (OPO) synchronously pumped by a commercial 250-MHz Er:fiber laser. The mid-IR spectrum can be tuned in the range 2.25-2.6 \mu m (signal) and 4.1-4.9 \mu m (idler), with average power from 20 to 60 mW. At 2.5 \mu m a minimum pulse duration of 110 fs and a power of 40 mW have been obtained. Active stabilization of the OPO cavity length has been achieved in the whole tuning range.

Wide-bandwidth phase lock between the tooth of a frequency comb and a CW extended-cavity diode laser at 1.55 μm is achieved by the use of an acousto-optical frequency shifter in a feed-forward configuration. The coherence properties of the comb are efficiently transferred to the CW laser, whose linewidth is narrowed down to the ∼10 KHz comb level. A maximum control bandwidth of ∼0.6 MHz has been experimentally achieved, limited by the transit time of the acoustic wave inside the frequency shifter.

The low-frequency tail of an octave-spanning supercontinuum (SC) generated by an Er:fiber comb is enhanced by a multipass Ho:YLF amplifier and used in a sum-frequency-generation scheme to obtain absolute referencing of a single-mode Tm-Ho:YAG laser tunable around 2.09 μm. By tuning the comb repetition frequency, the probing laser is scanned across the absorption lines of a CO(2) gas sample and highly accurate absorption profiles are measured. This approach can be readily scaled to any wavelength above ~2 μm.

Continuous-wave laser action of a Tm-doped GdLiF(4) (GLF) crystal pumped by a laser diode is reported at room temperature. A comparative analysis of laser performance using GLF crystals with doping concentrations of 8 at. % and 12 at. % of Tm(3+) has been carried out. A maximum output power of 1.47 W with 57% slope efficiency and a wide tunability range from 1826 to 2054 nm have been obtained at 8% Tm doping level.

We report the frequency stabilization of a cw extended-cavity diode laser against saturated absorption lines of the H(2)O17 isotopologue of water vapor at around 1.384 microm. The saturation of rotovibrational transitions is achieved by filling a high-finesse optical resonator with H(2)O17 at low pressure and by locking the laser frequency to the resonator by using the Pound-Drever-Hall technique. Absolute frequency stabilization is obtained, locking the cavity resonance to the center of the sub-Doppler line by means of the wavelength modulation method. A relative frequency stability of sigma(y)(tau)=10(-13)(0.1tau(-2)+0.9)(1/2) is demonstrated for integration times in the range 4 mstau30 s.

Accurate frequency measurements of CO 2 transitions around 2 μm are performed by direct frequency-comb Vernier spectroscopy. Measurements of pressure line shifts at the ppb. accuracy level demonstrates the application of comb sources in precision molecular spectroscopy.

A high-power optical frequency comb at 2.06 mu m has been generated using a Ho:YLF multipass amplifier seeded by the long wavelength supercontinuum tail of an octave-spanning Er: fiber comb source. The Ho:YLF amplifier showed a net gain larger than 30 dB from 2048 to 2068 nm, allowing the generation of a 20 nm bandwidth comb with a mode spacing of 100 MHz and a power per mode ranging from 20 to 370 mu W. In the time domain, the amplified comb corresponds to a pulse train with 1.6 W total power and 508 fs transform-limited pulse duration. Using a self-referencing f - 2f interferometer and a phase-locking loop, spectral narrowing of the offset frequency down to less than 17 Hz has been achieved. (c) 2011 Optical Society of America

We report on the first demonstration of passive mode-locking laser operation of a diode-pumped Tm:GdLiF 4 crystal. Pulse trains with 20-ps duration, average power of ∼100 mW, and repetition rate of 75 MHz are obtained.