Yuchen Wang

Absolute frequency measurements of CHF 3 Doppler-free ro-vibrational transitions at 8.6 μm

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 υ5 vibrational band are investigated at around 1158.9  cm-1 with a fractional frequency precision of 8.6·10-12 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10-11 is mainly limited by the reproducibility of the frequency measurements.

Versatile mid-infrared frequency-comb referenced sub-Doppler spectrometer

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.

47-fs Kerr-lens mode-locked Cr:ZnSe laser with high spectral purity

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.

Femtosecond laser written diamond waveguides: A step towards integrated photonics in the far infrared

Abstract A first demonstration and complete characterization of mid-infrared waveguides in single crystal diamond are reported. Waveguides were designed for 2.4 μm and 8.6 μm waveguiding, with their group velocity dispersion analyzed using femtosecond pulses at 2.4 μm wavelength propagated through the waveguide and the bulk substrate. The total dispersion was found to be dominated by the bulk material rather than the waveguide, and was on the range of 275 fs2/mm, demonstrating that femtosecond laser written modifications in diamond introduce negligible perturbations to the pristine material.

1 Watt, 59 % slope efficiency, 2.65 pm Cr 2+ :CdSe laser with tenability of 560 nm

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

Metrology-grade sub-Doppler spectroscopy of CHF 3 at 8.6 μm

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).

Thermo-optical and lasing characteristics of Cr 2+ -doped CdSe single crystal as tunable coherent source in the mid-infrared

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.

Dy 3+ doped ZBLAN fiber amplifier pumped by a single frequency 1064 nm laser for mid infrared applications

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.