220 μJ monolithic single-frequency Q-switched fiber laser at 2 μm by using highly Tm-doped germanate fibers

We report a unique all fiber-based single-frequency Q-switched laser in a monolithic master oscillator power amplifier configuration at ~1920 nm by using highly Tm-doped germanate fibers for the first time. The actively Q-switched fiber laser seed was achieved by using a piezo to press the fiber in the fiber Bragg grating cavity and modulate the fiber birefringence, enabling Q-switching with pulse width and repetition rate tunability. A single-mode polarization maintaining large core 25 μm highly Tm-doped germanate fiber was used in the power amplifier stage. For 80 ns pulses with 20 kHz repetition rate, we achieved 220 μJ pulse energy, which corresponds to a peak power of 2.75 kW with transform-limited linewidth.     

Operation of Tm3+ fiber laser at λ = 2.3 μm coupled to a silica fiber Bragg grating

A wavelength tunable all fibre laser which utilizes Tm³⁺-doped fluoride fibre as a gain mechanism emitting around a wavelength of = 2.3 m is reported. Tm³⁺-doped fluoride fibre was coupled to a fibre Bragg grating inscribed in silica. This laser was evaluated by employing it as a wavelength tunable source in a methane gas optical sensor. A minimum gas detectivity of 100 ppm m limited by the ability to prepare low concentrations of gases was achieved. Emission wavelength control was implemented by thermally tuning the grating, although this method may prove to be too slow and cumbersome for practical use. A model describing the coupling between the silica fibre Bragg grating and the Tm³⁺-doped fibre was developed and integrated with a fibre laser model. This theoretical framework was used to examine the optimum silica and fluoride fibre parameters to achieve the maximum coupling between the fibres, with the aim of further reducing the fibre laser threshold.     

Kilowatt-peak-power, single-frequency, pulsed fiber laser near 2 μm

We generated single-frequency pulses at kilowatt peak power from an all-fiber Tm-doped master oscillator power amplifier system, which is the first report of this kind (to the best of our knowledge) of a laser in the 2 μm region. Compared with the laser linewidth of seed pulses, spectral broadening by a factor of 3 was observed with the amplified pulses. This was attributed to self-phase modulation in passive pigtail fibers of the components (isolator and wavelength division multiplexing) that were placed after the fiber amplifier. The short pulse width (~7 ns) of the kilowatt-level pulses prevents an onset of stimulated Brillouin scattering in the long fiber. When launching the pulses into several-meter single-mode fiber, significant nonlinear spectral broadening occurs due to modulation instability in the fiber. This reaction is beneficial for generation of a mid- and long-wavelength IR supercontinuum in nonlinear IR fibers.     

Dissipative dispersion-managed soliton 2 μm thulium/holmium fiber laser

We report a first dissipative dispersive-managed soliton fiber laser operating at 2?μm. The cavity comprised of all-anomalous-dispersion fiber employs chirped fiber Bragg grating, which ensures net-normal cavity dispersion and semiconductor saturable absorber for mode-locking.     

High-power diode-pumped fiber laser operating at 3 μm

A high-power diode-cladding-pumped Ho3?-doped fluoride glass fiber laser operating in cascade mode is demonstrated. The ?I?→?I? and ?I?→?I? laser transitions produced 0.77 W at a measured slope efficiency of 12.4% and 0.24 W at a measured slope efficiency of 5.2%, respectively. Using a long fiber length, which forced a large threshold for the ?I?→?I? transition, a wavelength of 3.002 μm was measured at maximum output power, making this system the first watt-level fiber laser operating in the mid-IR.     

Q-switched fiber laser operating at 1.5 μm based on WTe_2

Compared with the extensively studied MoS2 and WS2, WTe_2 owns a smaller bandgap, which is applicable to a near-infrared system in photodetectors, communications, and ultrafast optics. In this work, the WTe_2 saturable absorber(SA) with the tapered fiber structure is prepared by the magnetron-sputtering technology, which enables the prepared SA to be low in cost and have strong nonlinearity. The modulation depth of the prepared WTe_2 SA is measured as 31.06%. The Q-switched fiber laser operating at 1.5 μm is successfully investigated by incorporating the proposed SA into the prepared ring cavity. To the best of our knowledge, this is the first attempt of WTe_2 in the Q-switched fiber laser at 1.5 μm.     

Mid-infrared Er:ZBLAN fiber laser reaching 3.68 μm wavelength

We report a continuous-wave Er:ZBLAN fiber laser with the operation wavelength reaching 3.68 μm.The midinfrared Er:ZBLAN fiber laser is pumped with the dual-wavelength sources consisting of a commercial laser diode at 970 nm and a homemade Tm-doped fiber laser at 1973 nm.By increasing the launched pump power at1973 nm,the laser wavelength can be switched from 3.52 to 3.68 μm.The maximum output power of 0.85 W is obtained with a slope efficiency of 25.14% with respect to the 1973 nm pump power.In the experiment,the laser emission at 3.68 μm is obtained with a significant power of 0.62 W,which is the longest emission wavelength in free-running Er:ZBLAN fiber lasers.     

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.     

High-quality, large-area monolayer graphene for efficient bulk laser mode-locking near 1.25 μm

High-quality monolayer graphene as large as 1.2×1.2?cm² was synthesized by chemical vapor deposition and used as a transmitting saturable absorber for efficient passive mode-locking of a femtosecond bulk solid-state laser. The monolayer graphene mode-locked Cr:forsterite laser was tunable around 1.25?μm and delivered sub-100?fs pulses with output powers up to 230?mW. The nonlinear optical characteristics of the monolayer graphene saturable absorber and the mode-locked operation were then compared with the case of the bilayer graphene saturable absorber.     

All-fiber wavelength-swept laser near 2 μm

We report, for the first time to our knowledge, an all-fiber wavelength-swept Tm-doped laser based on a fiber Fabry-Perot tunable filter in the 2 μm spectral region. The laser wavelength can be continuously tuned over 200 nm from 1840 to 2040 nm in a short period of time. The demonstrated tuning speed was 12.5 μm/s with a tuning efficiency of 17.5 nm/V. The spectral linewidth of the laser was measured to be approximately 300 MHz or 0.01 cm(-1). This kind of laser can find potential applications in both high-resolution laser spectroscopy and tunable mid-IR generation via nonlinear frequency conversion.     

Gain dynamics of the 4.3 μm CO2 laser

A detailed study of the gain dynamics of the pulsed, optically pumped 4.3 m CO₂ laser is described. Small-signal gain coefficients as high as 14%/cm are measured in a 4.3 m amplifier using low-power pulses from a 4.3 m probe laser. The measurements are compared with a rate-equation model and good quantitative agreement is obtained. The model, which uses no adjustable parameters, is described in detail. Gain is studied as a function of optical pumping power, gas mixture, gas pressure and discharge excitation of the 4.3 m amplifier. Optimization of the gain is discussed.     

12 W Q-switched Er:ZBLAN fiber laser at 2.8 μm

A diode-pumped, actively Q-switched 2.8 μm fiber laser oscillator with an average output power of more than 12 W has been realized through the use of a 35 μm core erbium-doped ZBLAN fiber and an acousto-optic modulator; to our knowledge, this is the first 3 μm pulsed fiber laser in the 10 W class. Pulse energy up to 100 μJ and pulse duration down to 90 ns, corresponding to a peak power of 0.9 kW, were achieved at a repetition rate of 120 kHz.     

Mode-locked 2 μm fiber laser with a multi-walled carbon nanotube as a saturable absorber

We propose and demonstrate a passively mode-locked fiber laser operating at 1951.8 nm using a commercial thulium-doped fiber(TDF) laser, a homemade double-clad thulium–ytterbium co-doped fiber(TYDF)as the gain media, and a multi-walled carbon nanotube(MWCNT) based saturable absorber(SA). We prepare the MWCNT composite by mixing a homogeneous solution of MWCNTs with a diluted polyvinyl alcohol(PVA) polymer solution and then drying it at room temperature to form a film. The film is placed between two fiber connectors as a SA before it is integrated into a laser ring cavity. The cavity consists of a 2 m long TDF pumped by a 800 nm laser diode and a 15 m long homemade TYDF pumped by a 905 nm multimode laser diode. A stable mode-locking pulse with a repetition rate of 34.6 MHz and a pulse width of 10.79 ps is obtained when the 905 nm multimode pump power reaches 1.8–2.2 W, while the single-mode 800 nm pump power is fixed at 141.5 m W at all times. To the best of our knowledge, this is the first reported mode-locked fiber laser using a MWCNT-based SA.     

Generation of 12th order harmonic mode-locking in a Nd-doped single-mode all-fiber laser operating at 0.9 μm

Based on the Nd-doped single-mode fiber as the gain medium, an all-fiber 12th harmonic mode-locked(HML) laser operating at the 0.9 μm waveband was obtained for the first time, to the best of our knowledge. A mandrel with a diameter of 10 mm was employed to introduce bending losses to suppress mode competition at 1.06 μm, which resulted in a suppression ratio of up to 54 dB. The 1st–12th order HML pulses with the tunable repetition rate of 494.62 kHz–5.94 MHz were obtained in the mode-locked lase...     

Actively Q-switched and mode-locked Tm3+-doped silicate 2 μm fiber laser for supercontinuum generation in fluoride fiber

A diode-pumped actively Q-switched and actively mode-locked Tm3+-doped double-clad silicate fiber laser is reported providing up to 5 W of average output power at ~60 kHz Q-switch envelope repetition rate and ~8 μJ subpulses with up to 2.4 kW peak power. Using this source as a pump laser for supercontinuum generation in a ZBLAN fiber, over 1080 mW of supercontinuum from 1.9 μm to beyond 3.6 μm was obtained at an overall efficiency of 3.3% with respect to the diode pump power.     

Graphene mode-locked femtosecond laser at 2 μm wavelength

We experimentally demonstrated a passively mode-locked femtosecond laser by using a graphene-based saturable absorber mirror (graphene SAM) in the spectral region of 2 μm. The graphene SAM was fabricated by transferring chemical-vapor-deposited, high-quality, and large-area graphene on a highly reflective plane mirror. Stable mode-locked laser pulses as short as 729 fs were obtained with a repetition rate of 98.7 MHz and an average output power of 60.2 mW at 2018 nm.     

Waveguide CW 118.6 μm H2O laser

This paper reports results of an experimental study of a cw 118.6 m H₂O waveguide laser. The laser cavity is a 3 cm inner diameter, 3.2 m long Pyrex tube, with plane reflectors against its ends, one of them being a copper mesh coupler. The active medium is a H₂O:H₂ gas mixture. Successive optimizations lead to a reliable output power of 50 mW, for the EH₁₁ mode. The power density is the highest ever reported, and the stability is such that the power drifts by less than 2% per hour. This laser is competitive with the optically pumped 118.8 m CH₃OH laser.     

A comparative study of laser tissue interaction at 2.94 μm and 10.6 μm

Laser cutting of gelatin and tissue with Er and CO₂ lasers is explained by combined action of evaporation, ejection of liquid and elastic deformation of the region of radiation impact. It is shown that the ejection mechanism is more pronounced at 2.94 m than at 10.6 m. The use of high speed photography has revealed the influence of the temporal pulse shape. The experimental results are explained by a thermo-mechanical model.     

Welding of polymers using a 2 μm thulium fiber laser

Absorber-free transmission and butt-welding of different polymers were performed using thulium fiber laser radiation at the wavelength 2 μm. The relations between the laser process conditions and the dimensions and quality of the seam were investigated by means of optical and phase-contrast microscopy. Mechanical properties of the weld joints were studied in tensile strength tests. Laser-welded polyethylene samples revealed a tensile strength of greater than 80% of the bulk material strength. Transmission welding of different polymer combinations featured the formation of different joint classes depending on the spectral properties. The experiments demonstrate new application areas of mid-IR fiber laser sources for materials processing.