Flat-top supercontinuum and tunable femtosecond fiber laser sources at 1.9-2.5 μm

We report the high-energy flat-top supercontinuum covering the mid-infrared wavelength range of 1.9-2.5 μm as well as electronically tunable femtosecond pulses between 1.98-2.22 μm directly from the thulium-doped fiber laser amplifier. Comparison of experimental results with numerical simulations confirms that both sources employ the same nonlinear optical mechanism - Raman soliton frequency shift occurring inside the Tm-fiber amplifier. To illustrate that, we investigate two versions of the compact diode-pumped SESAM mode-locked femtosecond thulium-doped all-silica-fiber-based laser system providing either broadband supercontinuum or tunable Raman soliton output, depending on the parameters of the system. The first system operates in the Raman soliton regime providing femtosecond pulses tunable between 1.98-2.22 μm. Wide and continuous spectral tunability over 240 nm was realized by changing only the amplifier pump diode current. The second system generates high-energy supercontinuum with the superior spectral flatness of better than 1 dB covering the wavelength range of 1.9-2.5 μm, with the total output energy as high as 0.284 μJ, the average power of 2.1 W at 7.5 MHz repetition rate. We simulate the amplifier operation in the Raman soliton self-frequency shift regime and discuss the role of induced Raman scattering in supercontinuum formation inside the fiber amplifier. We compare this system with a more traditional 1.85-2.53 μm supercontinuum source in the external highly-nonlinear commercial chalcogenide fiber using the Raman soliton MOPA as an excitation source. The reported systems1 can be readily applied to a number of industrial applications in the mid-IR, including sensing, stand-off detection, medical surgery and fine material processing

Supercontinuum generation in mid-IR using chalcogenide and germanate nonlinear fiber

We demonstrate mid-infrared (mid-IR) supercontinuum generation with bandwidth from 2 to 2.8 μm at 20 dB below the peak in nonlinear step-index chalcogenide fiber using femtosecond mid-IR pulses directly from the oscillator. We compare the results with a supercontinuum generated in a silica-based high germanium content fiber. Supercontinuum generation occurs at 90 mW of launched average pump power that is equal to the 0.9 nJ pulse energy. The distinctive feature of the obtained supercontinuum is its stability and coherence due to the deterministic supercontinuum generation by the femtosecond pump pulses

Vapor Deposited Cr-doped ZnS Thin Films: Towards Optically Pumped Mid-Infrared Waveguide Lasers

Compact, affordable mid-IR lasers require the development of gain materials in waveguide form. We report on the high vacuum deposition of Cr:ZnS films with concentration ranging from 1018-1020 dopants/cm3 . At low concentrations, films display well-isolated absorption associated with substitutional Cr2+ ions in the lattice. Spatial modulation of the dopant concentration suppresses the absorption associated with this substitution. Lateral crystallite sizes less than 30 nm are associated with the lowest substrate temperatures (\u3c50 °C) used during deposition, and waveguide losses as low as 8dB/cm are observed. These materials are promising candidates as gain media for fabrication of waveguide mid-IR lasers

Kerr-lens mode-locked Cr:ZnS laser

We report the soft-aperture Kerr-lens mode-locked Cr:ZnS laser, generating 550 mW of 69 fs nearly transform-limited pulses at 2.39 μm wavelength. The pulse energy reached 3.8 nJ at 145 MHz repetition rate, limited by the onset of double-pulsing. This corresponds to the shortest-pulse and highest-energy direct femtosecond laser source in the mid-infrared. Dispersion compensation was achieved by a single chirped mirror and a thin sapphire plate, making the laser design simple, compact and very stable, and operating at ambient air and room temperature. The superb thermal and mechanical properties of Cr:ZnS, exceeding those of Cr:ZnSe and many established femtosecond laser crystals, should allow for further scaling of output power

Enhancement and shape control of weak molecular absorption signal with chirped-pulse mid-IR lasers

Detection of weak narrowband absorption by molecular lines is usually a topic of linear-optical technology. In this paper we demonstrate nonlinear-optical enhancement and shape control of atmospheric absortion lines using a broadband ultrashort-pulse oscillator and nonlinear fiber propagation. The absorption signal is generated inside the passively mode-locked mid-IR Cr:ZnS laser. The nonlinear interaction with the broadband ps-long pulse causes the narrow-band ns-long absorption signal to acquire a 90° phase shift with respect to the pulse [1]. The signature of such an absorber on the output spectrum is a dispersion-like modulation, which can be much stronger than absorption modulation itself. This effect has been experimentally observed using the atmospheric water vapour absorption lines around 2.5 μm in Cr:ZnSe laser [1, 2]. Additional modulation increase has been predicted to occur near the spectrum edge if the laser is operating in a chirped-pulse (dissipative soliton) regime

High-energy mid-infrared Cr:ZnS chirped-pulse oscillator

Femtosecond coherent light sources emitting in the mid-infrared are of particular interest for a number of applications like environmental sensing, medicine, metrology, material processing and telecommunications. Among the alternatives, Cr-doped chalcogenide-based crystalline lasers are the most compact, robust and cheap.For decade pulse energy of these lasers were limited at the level of 1-2 nJ. Finally Cr:ZnS laser with 3.8 nJ pulse energy and 550 mW average output power producing 69-fs pulses was demonstrated very recently [1]. Power scaling of femtosecond crystalline oscillators is not a simple task. The pulse energy of these systems is limited by the pulse breakup due to high third-order optical nonlinearity of the active medium. Increasing the output power over the certain limit result in either multiple-pulsing or harmonic mode-locking regimes. In order to overcome that, the intracavity power density inside the active medium should be reduced. That could be done by using the technique of chirped-pulse-oscillator (CPO). This technique is well-established in fiber and Tisapphire lasers, and has recently been demonstrated with the Cr:YAG and Yb-doped thin-disk lasers. For Cr:ZnSe the analytical theory [2] predicts pulse energies up to 0.5 μJ [3] and initial demonstration of CPO technique has already been performed for Cr:ZnSe as well as Cr:ZnS lasers [4]. We report the successful demonstration of Cr:ZnS CPO pulse energy over 8 nJ. The laser was build on the basis of a classic 4-mirror astigmatically-compensated cavity and was pumped by 1.61 μm cw polarized 5-W Er:fiber laser. The mode-locking was achieved using soft-aperture Kerr lens effect. The compensation of the group-delay dispersion was performed by the chirped mirror. The pulses having the duration ranging from 0.8 to 2 picoseconds were obtained at the pulse repetition rate of 105 MHz. The laser average output power reached 0.88 W with 26% slope efficiency. At output powers over 750 mW we observe leakage of 3-5 % of energy into higher-order modes, visible as the narrow spectral lines. The input-output curve, typical autocorrelation trace, and laser emission spectra for different output pulse energies are shown in the Figure 1

1 Watt femtosecond mid-IR Cr:ZnS laser

A room-temperature Kerr-Lens modelocked (KLM) Cr:ZnS laser generates <70 fs pulses duration (about eight optical cycles) with 5.6 nJ pulse energy and over 100 nm FWHM spectral width at 105-157 MHz repetition rates. The laser produces 1 W average output power at 20% optical efficiency, limited by the available Er:fiber pump. For further pulse energy scaling we also realized the chirped-pulse regime, with 0.8-2 ps pulse durations. The demonstrated applications of such mid-IR source range from extra- and intra-cavity spectroscopy to subharmonic OPO pumping. For environmentally-protected delivery we suggest and realize duration-preserving soliton delivery in a ZBLAN fiber. Further bandwidth increase is demonstrated by 2.0-2.8 μm supercontinuum generation in a chalcogenide fiber

Graphene Mode-locked Cr:ZnS Laser with 41 fs Pulse Duration

We report the ultrashort-pulse Cr:ZnS laser mode-locked by graphene-based saturable absorber mirror. Using the combination of bulk material and a chirped mirror, we demonstrate the shortest reported so far mid-IR pulses of only 5.1 optical cycles (41 fs) centered at 2.4 µm with 190 nm spectral bandwidth. The pulse spectrum almost completely fills the water-free atmospheric window. The output parameters reach 2.3 nJ pulse energy and 250 mW average output power at 108 MHz repetition rate

Ceramic Cr:ZnS Laser Mode-Locked by Graphene

The final publication is available via https://doi.org/10.1364/CLEO_SI.2014.STu2E.7.We report a high-power graphene mode-locked ceramic Cr:ZnS-laser, producing 3.9 nJ, 140 fs pulses with 45 nm spectral bandwidth at 270 MHz repetition rate, at output power for the first time exceeding 1 W level.Austrian Science Funds (FWF)NFRNANO 2021FP

Enhancement and shape control of weak molecular absorption signal with chirped-pulse mid-IR lasers

Detection of weak narrowband absorption by molecular lines is usually a topic of linear-optical technology. In this paper we demonstrate nonlinear-optical enhancement and shape control of atmospheric absortion lines using a broadband ultrashort-pulse oscillator and nonlinear fiber propagation. The absorption signal is generated inside the passively mode-locked mid-IR Cr:ZnS laser. The nonlinear interaction with the broadband ps-long pulse causes the narrow-band ns-long absorption signal to acquire a 90° phase shift with respect to the pulse [1]. The signature of such an absorber on the output spectrum is a dispersion-like modulation, which can be much stronger than absorption modulation itself. This effect has been experimentally observed using the atmospheric water vapour absorption lines around 2.5 μm in Cr:ZnSe laser [1, 2]. Additional modulation increase has been predicted to occur near the spectrum edge if the laser is operating in a chirped-pulse (dissipative soliton) regime