1.6-W self-referenced frequency comb at 2.06 μm using a Ho:YLF multipass amplifier

A high-power optical frequency comb at 2.06 μm 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 μW. 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.     

Absolute frequency measurement of a water-stabilized diode laser at 1.384?��m by means of a fiber frequency comb

We report on the absolute frequency measurement of an extended-cavity diode laser stabilized against a Doppler-free vibration?Crotation transition of the H₂ ¹⁸O isotopologue near 1.384???m. Absolute determination is performed by comparing the water-stabilized laser frequency with respect to a GPS-disciplined Rb microwave standard by means of a self-referenced fiber-based optical frequency comb. The line center frequency of the 2_2,1??2_2,0 transition of the H₂ ¹⁸O ??₁+??₃ combination band is found to be 216?519?045?955(13)?kHz.     

Saturation spectroscopy of CO2 and frequency stabilization of an optical parametric oscillator at 2.77 μm

We report the frequency stabilization of a CW single-frequency, singly resonant optical parametric oscillator (OPO) to the saturation absorption center of the (12)C(16)O2[10°1,02°1](II)>←00°0 P(14) line at 2.77 μm. The CO2 molecules were excited by the OPO idler wave, and the absorption signal was monitored through the fluorescence at 4.3 μm using a gold-coated longitudinal cell. The idler frequency was stabilized onto the line center by wavelength modulation method. The linewidth of the saturation dip was estimated to be 4.7 MHz, and the achieved frequency stability was 3.9 kHz (3.6×10(-11)).     

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.     

Fiber-based optical frequency comb at 3.3 μm for broadband spectroscopy of hydrocarbons [Invited]

A 125 MHz fiber-based frequency comb source in the mid-infrared wavelength region is presented. The source is based on difference frequency generation from a polarization-maintaining Er-doped fiber pump laser and covers a spectrum between2900 cm~(-1) and 3400 cm~(-1) with a simultaneous bandwidth of 170 cm~(-1) and an average output power up to 70 m W. The source is equipped with actuators and active feedback loops, ensuring long-term stability of the repetition rate, output power, and spectral envelope. An absorption spectrum of ethane and methane was measured using a Fourier transform spectrometer to verify the applicability of the mid-infrared comb to multispecies detection. The robustness and good long-and short-term stability of the source make it suitable for optical frequency comb spectroscopy of hydrocarbons.     

Generation of 400 μW at 17.5 μm using a two-color Yb fiber chirped pulse amplifier

We have demonstrated the generation of 400 μW of power at ~18 μm by difference-frequency mixing the 1038 and 1105 nm from a two-color, chirped pulse amplification Yb fiber system. A two-color seed is selected from a continuum source and amplified to 300 mW of total power in a two-stage Yb fiber amplifier.     

Measurements of CO2 in a multipass cell and in a hollow-core photonic bandgap fiber at 2 μm

Recently, hollow-core photonic bandgap fibers (HC-PBFs) for use in the 2 μm wavelength region have become available. We have employed tunable diode laser absorption spectroscopy (TDLAS) to quantify CO₂ in nitrogen, injected into a HC-PBF. Our spectrometer contains both an HC-PBF-based absorption cell and an astigmatic Herriott multipass gas cell. The Herriott cell was used for comparison with the HC-PBF cell. The HC-PBF cell’s sensitivity and limit of detection were calculated to be 3.5×10^?4 cm^?1?Hz^?1/2 and 59 ppm?m, respectively. To substantiate the spectrometer performance, a measurement was done in the Herriott cell probing a reference gas mixture with nominal 400 μmol/mol CO₂ in N₂. The spectrometric results were in good agreement with the reference value. The relative standard uncertainty of the spectrometric result was found to be at the ±2 % level.     

Dependence of Er:Yb-codoped 1.5 μm amplifier on wavelength-tuned auxiliary seed signal at 1 μm wavelength

We report on experiments performed with a cladding-pumped single-mode Er:Yb-codoped single-frequency fiber amplifier simultaneously seeded by a distributed-feedback diode at 1556 nm and a tunable external-cavity diode laser emitting at a wavelength of about 1 μm wavelength. The influence of the output wavelength of the external-cavity laser on amplification and reabsorption behavior of the Yb emission as well as the amplifier performance at a wavelength of 1556 nm is examined experimentally.     

Intensities and H2-broadened half-widths of Germane lines around 4.7 μm at temperatures relevant to Jupiter's atmosphere

Absolute intensities and H₂-broadened half-widths of R(0) and R(1) of ⁷²GeH₄ have been measured at 119.5, 150.3, 199.0, 250.0, and 296.0 K using a tunable diode laser and the sweep integration technique. The combined intensity of the v₃ bands of all of the five isotopic species of GeH₄ as well as that of the Q-branches alone have been measured independently at 294 K using the Wilson-Wells-Penner-Weber technique.     

Short-term and long-term frequency characteristics of 2 μm single frequency Tm:YAG laser at room temperature

Placing one 0.1 mm YAG F-P etalon at nearly Brewster angle and combined use 1 mm fused silica in the cavity, a diode-pumped linear-polarized single-longitudinal-mode (SLM) Tm:YAG laser operating at 2 μm is achieved. This paper is focused on the stability of the linear-polarized SLM laser, including power stability, long-term frequency stability and short-term frequency stability. And the factors affecting the frequency characteristics of laser were also analyzed. The instability of the linear-polarization SLM laser is less than 1%. The long-term frequency stability is in the range from 1.16 × 10⁻⁷ to 1.75 × 10⁻⁷ monitored by the wave meter. And the short-term frequency stability is 97 Hz/μs measured with the self-beating heterodyne detection method.     

Hydrogen and nitrogen broadening of the lines of C2H2 at 14 μm

Collision broadening of 14 μm acetylene transitions was studied using a diode laser spectrometer. Fifteen P(J) and Q(J) transitions in the v₅ fundamental band, broadened by hydrogen and nitrogen, were used in the study. The reduced Lorentz-widths per unit pressure obtained from the observed Voigt-widths were fitted to a model quadratic in J, i.e. γ⁰_L = a + bJ + cJ². The results for the nitrogen-broadened lines are 0.0853 cm^-1-atm^-1 for the collision broadening coefficient a at J = 0, b = -1.63 x 10^-3, and c = 3.92 x 10^-5 cm^-1-atm^-1. The results for the hydrogen broadened lines in cm^-1-atm^-1 are 0.0812, -1.16 x 10^-3, and 2.93 x 10^-5, respectively.     

Frequency stabilization of a single-frequency volume Bragg grating-based short-cavity Tm:Ho:YLF laser to a CO2 line at 2.06?μm

This study presents a 2.06???m single-frequency tunable volume Bragg grating (VBG)-based short-cavity Tm:Ho:YLF laser. A mode-hop-free tuning range of 10?GHz at a few hundred mW output power was achieved. The first-derivative spectrum of the CO₂ 20013??00001 R(4) absorption line with a signal-to-noise ratio (SNR) of 1,000/Hz^1/2 was obtained using frequency modulation and fluorescence detection. The laser frequency can be stabilized to the CO₂ line with a stability of 3?×?10^?9.     

Passive mode locking of an in-band-pumped Ho:YLiF4 laser at 2.06 μm

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.     

Picosecond optical switching of a 1.5 μm active birefringent optical fiber loop filter using 1.3 μm control optical pulses and a 1.3 μm semiconductor optical amplifier

Less than 100ps, polarization-independent switching operation of an active birefringent optical fiber loop filter using 1.3 μm control optical pulses as well as a 1.3 μm semiconductor optical amplifier (SOA) has been demonstrated. In the proposed SOA-based active birefringent filter operating at 1.55 μm wavelength, 1.3 μm SOA is employed to control the polarization-mode dispersion in the loop part. By injecting 1.3 μm ps gain-switched optical control pulses into the SOA, 1.5 μm input signals can be switched from the transmission port to the reflection port with less than 100 ps rise time.     

Diode-pumped room temperature single longitudinal mode lasing of Tm,Ho:YLF microchip laser at 2050.5 μm

Room-temperature operation of a single longitudinal-mode c-cut Tm(6%), Ho(0.4%):YLF microchip laser is reported. An incident pump power of 713 mW is used to generate the maximum single-frequency output power of 17 mW at 2050.5 nm, which corresponds to the slope efficiency of 10%.     

Performance of a diode end-pumped Cr,Er:YSGG laser at 2.79 μm

We present a diode end-pumped Cr,Er:YSGG laser at 2.79 μm operated in the continuous-wave (CW) and pulsed modes. Er³⁺ ions are directly pumped into the upper laser state ⁴ I _11/2 using a 967-nm diode laser, which avoids various nonradiative losses. The laser produces 50.2 mW of power in the CW mode. An average power of 169 mW is obtained at a repetition rate of 100 Hz and a pulse duration of 2 ms. The maximum laser energy achieved is 1.95 mJ at a repetition rate of 50 Hz, corresponding to a peak power of 970 mW and a slope efficiency is 7.6%. The far-field divergence angle of the laser is measured to be 8 mrad.     

Thermal Lensing, Laser Performance at 1.06 μm and 0.53 μm of a NYAB Laser End-pumped by a Diode-laser-array

The thermal lensing, output characteristics of an end-pumped NYAB laser with high pump power levels and operated at the fundamental wavelength, and the manner of self-frequency-doubling, have been investigated. The thermal lensing in the NYAB crystal was found to be significantly stronger than that in Nd:YVO4 crystal. A maximum output of 3.2 W at 1.06 μm was achieved with an optical conversion efficiency of 29.1％ and an average slope efficiency of 37.5％. These results were found to be slightly inferior to those of Nd:YVO4 crystal. When operated in the fashion of selffrequency-doubling, 23 mW green output was obtained at the incident pump power of 5 W, such inefficient operation was attributed to the large mode size in the NYAB crystal.     

Effects of co-dopant concentrations and excitation conditions on the 2 μm fluorescence dynamics in Tm,Ho:YLF crystals

Results of spectroscopic studies carried out on Tm, Ho:YLF crystals are reported. In particular, we have investigated the effects of the co-dopant concentrations on the time evolution of the fluorescence at 2 m emitted in response to short-pulse laser excitation at 792 nm. The fluorescence intensity profiles are analyzed in terms of their temporal and amplitude characteristics over a wide range of excitation conditions.     

Generation of passively Q-switched fiber laser at 1 μm by using Mo SSe as a saturable absorber

Passively Q-switched ytterbium-doped fiber lasers(YDFLs) incorporating a molybdenum sulfide selenide(Mo SSe)-based saturable absorber(SA) are demonstrated. The modulation depth and saturation intensity of the Mo SSe-based SA are measured to be approximately 25.0% and 0.002 MW∕cm2, respectively, using the twin detector technique. The YDFL's output has a center wavelength of 1038.5 nm with a top pulse width and energy of 1.2 μs and 18.9 nJ, respectively, at a pump power of 333 mW. The Mo SSe-based SA has a good linear optical response, providing significant opportunity for use in applications over an ultra-broadband spectrum, particularly spectroscopy and biomedical diagnostics.     

Efficient high-power Ho:YAG laser directly in-band pumped by a GaSb-based laser diode stack at 1.9 μm

An efficient high-power Ho:YAG laser directly in-band pumped by a recently developed GaSb-based laser diode stack at 1.9 μm is demonstrated. At room temperature a maximum continuous wave output power of 55 W at 2.122 μm and a slope efficiency of 62% with respect to the incident pump power were achieved. For narrow linewidth laser operation a volume Bragg grating was used as output coupler. In wavelength stabilized operation a maximum output power of 18 W at 2.096 μm and a slope efficiency of 30% were obtained. In this case the linewidth is reduced from 1.2 nm to below 0.1 nm. Also spectroscopic properties of Ho:YAG crystals at room temperature are presented.