A widely tunable (5-12.5 μm) continuous-wave mid-infrared laser spectrometer based on difference frequency generation in AgGaS2

A widely tunable (5-12.5 μm) continuous-wave (cw) mid-infrared (mid-IR) laser spectrometer based on difference frequency generation (DFG) by mixing an external-cavity diode laser (ECDL) with a Ti:Sapphire laser in an AgGaS₂ crystal is described. The wide tunability was achieved by tuning laser wavelength associated with crystal angle tuning under type II phase matching condition. A maximum output power of about 66 nW was obtained at 8.06 μm. High resolution spectrum of methane (CH₄) over more than 10 cm⁻¹ near 7.7 μm has been recorded to evaluate the performance of the developed DFG-based mid-IR laser spectrometer.     

A novel 2-μm pulsed fiber laser based on a supercontinuum source and its application to mid-infrared supercontinuum generation

A new method to achieve 2-μm pulsed fiber lasers based on a supercontinuum(SC) is demonstrated. The incident pump light is a pulsed SC which contains a pump light and a signal light at the same time. The initial signal of the seed laser is provided by the incident pump light and amplified in the cavity. Based on this, we obtain a 2-μm pulsed laser with pulse repetition rate of 50 kHz and pulse width of 2 ns from the Tm-doped fiber laser. This 2-μm pulsed laser is amplified by two stages of fiber amplifiers, then the amplified laser is used for mid-infrared(mid-IR) SC generation in a 10-m length of ZrF4–BaF2–LaF3–AlF3–NaF(ZBLAN) fiber. An all-fiber-integrated mid-IR SC with spectrum ranging from 1.8 μm to4.3 μm is achieved. The maximal average output power of the mid-IR SC from the ZBLAN fiber is 1.24 W(average output power beyond 2.5 μm is 340 mW), corresponding to an output efficiency of 6.6% with respect to the 790-nm pump power.     

1.55-μm coherent lidar based on SPA sinusoidal frequency demodulation techniques

A 1.55-μm all-solid frequency-modulated continuous-wave (FMCW) coherent lidar based on the sinusoidal frequency demodulation technique for range and velocity measurement is presented. Both the nonlinearity of linear modulation waveform and the difficulty in measuring the frequency of sinusoidal modulation sys- tem are circumvented by utilizing segmental-processing-average (SPA) techniques. The results demonstrate that the resolutions of range and velocity measurement are better than 2 mm and 0.1 mm/s, respectively. The system has numerous practical and potential applications in space missions.     

Design of efficient single-stage chirped pulse difference frequency generation at 7 μm,driven by a dual wavelength Ti:sapphire laser

We present simulations for a design of a high-energy single-stage mid-IR difference frequency generation adapted to a two-color Ti:sapphire amplifier system. The optimized mixing process is based on chirped pulse difference frequency generation (CP-DFG), allowing for a higher conversion efficiency and reduced two-photon absorption losses. The numerical start-to-end simulations include stretching, chirped pulse difference frequency generation and pulse compression. Realistic design parameters for commercially available nonlinear crystals (GaSe, AgGaS₂, LiInSe₂, LiGaSe₂) are considered. Compared with conventional unchirped DFG directly pumped by Ti:sapphire technology, we predict a threefold increase in the quantum efficiency. Our CP-DFG scheme provides up to 340 μJ pulse energy directly at 7.2 μm when pumped with 8 mJ and supports a bandwidth of up to 350 nm. The resulting 240 fs mid-IR pulses are inherently phase stable.     

Raman continuum generation at 1.0–1.3 μm in passively mode-locked fiber laser based on nonlinear polarization rotation

We experimentally demonstrate cascaded Raman scattering continuum generation at 1.0–1.3 μm with pulse energy of 47 nJ by utilizing a commercially available all-normal-dispersion single-mode fiber (SMF) with a low threshold pump power. We achieve passively mode-locked all-fiber lasing by nonlinear polarization rotation technique. Continuous Raman gain is obtained by the same SMF in every roundtrip because of the ring-cavity configuration, which makes it possible to achieve high-order Raman Stokes waves by a short fiber length. Limited by the pump power, the maximum output average power and peak power are 87.4 and 339 mW, but it has great potential to be improved. These make it an ideal continuum source for many applications, such as optical coherence tomography.     

2-μm mode-locked nanosecond fiber laser based on MoS_2 saturable absorber

We demonstrated a 2-μm passively mode-locked nanosecond fiber laser based on a MoS_2 saturable absorber(SA).Owing to the effect of nonlinear absorption in the MoS_2 SA, the pulse width decreased from 64.7 to 13.8 ns with increasing pump power from 1.10 to 1.45 W. The use of a narrow-bandwidth fiber Bragg grating resulted in a central wavelength and 3-dB spectral bandwidth of 2010.16 and 0.15 nm, respectively. Experimental results show that MoS_2 is a promising material for a 2-μm mode-locked fiber laser.     

Displacement sensor based on polarization mixture of orthogonal polarized He-Ne laser at 1.15μm

Displacement sensor based on the polarization mixture and the cavity tuning of the orthogonal polarized He-Ne laser 1.15μm is presented.The power tuning curves of He-Ne laser are irregular,and it is difficult to measure the change in cavity length.The distortion of the curves is caused by the higher relative excitation compared with the He-Ne laser at 633 nm.In view of its potential for the wider displacement measuring range,a new method of displacement sensing is developed.Experiments show that displacement measuring stability based on the method of the polarization mixture is better than that of the power tuning curves. The displacement sensor achieves the measuring range of 100 mm,resolution of 144 nm,and linearity of 7×10-6 .     

Mid-IR frequency comb source spanning 4.4-5.4 μm based on subharmonic GaAs optical parametric oscillator

Broadband mid-IR output suitable for producing 1000-nm-wide frequency combs centered at 4.9?μm was achieved in a degenerate subharmonic optical parametric oscillator (OPO) based on 500-μm-long Brewster-angled orientation-patterned GaAs crystal. The OPO was synchronously pumped at 182 MHz repetition rate by 100 fs pulses from a Cr2?:ZnSe laser with the central wavelength of 2.45?μm and the average power of 100 mW.     

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.     

Broadband terahertz light source pumped by a 1 μm picosecond laser

We obtained a frequency tunable, low-coherence, picosecond, terahertz (THz) output with a high repetition rate from a picosecond Nd:YVO₄ bounce laser in combination with tandem periodically poled stoichiometric lithium tantalate and 4′-dimethylamino-N-methyl-4-stilbazolium tosylate crystals. The frequency of the THz output was tunable in the range 2.1–7.1 THz with a linewidth of ~3.5 THz at 2.2 THz. The THz output had a maximum peak power of ~180 mW and an average power of ~0.65 μW at 3.9 THz. This system has the potential to realize ultra-high speed, THz coherence tomography.     

Optical frequency comb generation based on three parallel Mach–Zehnder modulators with recirculating frequency shifting loop

We theoretically and numerically study an approach for optical frequency comb (OFC) generation, by utilizing recirculating frequency shifting (RFS) loop based on three parallel Mach–Zehnder modulators (MZMs). Our results show that three parallel MZMs can generate a single-side-band (SSB) signal with 36 dB optical carrier suppression (OCS) ratio. Furthermore, the 60-tone OFC signal with 30 dB side-mode suppression ratio (SMSR) and 4 dB maximum power fluctuation is achieved, and 20 of the OFC signal possess the power fluctuation of less than 1 dB. Our approach provides a novel way of generating OFC with excellent SMSR and good power fluctuation.     

High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator

We describe a tunable broadband mid-IR laser source based on difference-frequency mixing of a 100?MHz femtosecond Yb:fiber laser oscillator and a Raman-shifted soliton generated with the same laser. The resulting light is tunable over 3.0?μm to 4.4?μm, with a FWHM bandwidth of 170?nm and maximum average output power up to 125?mW. The noise and coherence properties of this source are also investigated and described.     

Fourier transform spectroscopy around 3 μm with a broad difference frequency comb

We characterize a new mid-infrared frequency comb generator based on difference frequency generation around 3.1 μm. High power per comb mode (>10^?7 W/mode) is obtained over a broad spectral span (>750 nm, >790 cm^?1). The source is used for direct absorption spectroscopy with a Michelson-based Fourier transform interferometer.     

Performance of 2 μm Tm:YAP pulse laser based on a carbon nanotube absorber

We demonstrated the first use of carbon nanotube as a saturable absorber in the Q-switched and Q-switched mode-locking of a diode pumped Tm:YAP operating at 2 μm. At the incident pump power of 8.64 W, the minimum Q-switched pulse width of 255.1 ns, and the maximum peak power 53.1 W can be obtained with the corresponding repetition rate of 21.76 kHz. The performance of a diode-pumped passively Q-switched mode-locked Tm:YAP laser with high repetition rate formed with a folded cavity. As high as 780 mW average output power was produced in QML laser. The repetition rate of the mode-locked pulse inside the Q-switched envelope was 244.1 MHz. The dependence of the operational parameters on the pump power was also investigated experimentally.     

Supercontinuum generation of highly nonlinear fibers pumped by 1.57-μm laser soliton

Highly nonlinear fibers(HNLFs) are crucial components for supercontinuum(SC) generation with laser solution.However, it is difficult to exactly estimate the structure of produced SC according to material parameters. To give a guideline for choosing and using HNLFs for erbium-fiber-based optical applications, we demonstrate SC generation in five types of HNLFs pumped by 1.57-μm laser solitons. All five fibers output a SC exceeding 1000 nm. Three different SC formation processes were observed in the experiment. By comparing optical parameters of these fibers, we find the zero dispersion wavelength(ZDW) of fiber has an important influence on the SC structure and energy distribution for a given pump source.     

Influence of laser linewidth on external-cavity frequency doubling efficiency of a 1.56μm master oscillator fiber power amplifier

By using an external-cavity frequency-doubling master oscillator fiber power amplifier (MOPA), a 700~mW continuous-wave single-frequency laser source at 780~nm is produced. It is shown that the frequency doubling efficiency is improved when the seed diode laser is optically locked to a resonant frequency of a confocal Fabry--Perot (F-P) cavity. This phenomenon can be attributed to the narrowing of the 1.56~μ m laser linewidth and explained by our presented theoretical model. The experimental results are found to be in good agreement with the theoretical predictions.     

The CO fundamental-band laser as secondary frequency standard at 5 µm

We present a very high-resolution heterodyne spectrometer based on a CO laser which operates down to fundamental-band transitions of the molecule. This allows us to detect saturated absorption signals on these transitions at very low pressure (0.4 Pa) and laser intensity (< 1 mW/cm²), yielding a linewidth of about 250 kHz. With the CO fundamental-band laser stabilized to these saturation signals we have measured the transition frequencies of the fundamental bands of three isotopic species to an accuracy of typically 20 kHz (v/v 3 × 10^–10), referenced to the CO₂ frequency standard. Together with additional frequency measurements of the first hot bands, these provide the first heterodyne frequency data of sub-Doppler accuracy for transitions in low lying bands of CO. They now represent the most accurate secondary frequency standard in the spectral region around 5 µm (60 THz).     

1 μm wavelength swept fiber laser based on dispersion-tuning technique

We report a wavelength swept fiber laser at the 1 μm region based on an actively mode-locked dispersion-tuning technique. The ring-cavity laser uses a 70 cm ytterbium-doped fiber as a gain medium. Mode locking is achieved by the direct modulation of the amplitude modulator, and a ～1000 m single-mode fiber is used to provide the desired intracavity dispersion. By sine-modulating the modulation frequency, a wavelength swept laser with a range of ～30 nm can be achieved at a sweeping rate of 50 Hz. The characteristics of the laser, such as its singlewavelength tuning range, tuning sensitivity, static linewidth and sweeping rate, are also studied experimentally.