High-power frequency comb at 2 μm wavelength emitted by a Tm-doped fiber laser system

Kerr-lens mode-locked 49-fs Tm3+:YScO3 single-crystal laser at 2.1 µm

We report on a continuous wave (CW) and Kerr-lens mode-locked (KLM) Tm3+:YScO3 single-crystal laser centered at 2.1 µm. Efficient CW laser operation with a maximum slope efficiency of 51% was achieved under in-band pumping by an Er:Yb fiber master oscillator power amplifier (MOPA). In KLM operation, pulses as short as 49 fs corresponding to seven optical cycles were achieved at a repetition rate of 96.7 MHz with an average output power of 126 mW. Such short pulse durations are enabled by the inhomogeneously broadened emission spectrum of Tm3+:YScO3 extending to above 2200 nm.

Ultra-short pulse burst laser around 1.98 µm obtained through an all-fiber nonlinear wavelength converter and Tm-doped fiber amplifier

We report an all-fiber ultra-short pulse burst laser operating at around 1.98 µm that is obtained through a nonlinear wavelength converter and Tm-doped fiber amplifier. A mode-locked Er-doped fiber laser was first built and then amplified in subsequent amplifiers to an average power of 1.3 W. Ultra-short pulse burst output was achieved through a pulse multiplier and a fiber-pigtailed acousto-optic modulator. It was then injected into an all-fiber nonlinear wavelength converter constructed from P-doped fiber and Tm-doped fiber, obtaining an ultra-short pulse burst laser of 540 mW around 1.98 µm. Its average output power was then amplified to 4.33 W in a Tm-doped fiber amplifier with an intra-burst pulse repetition frequency of 0.9 GHz, a burst repetition frequency of 200 kHz, and a duty cycle of 2%, corresponding to about 200 pulses within each burst. This 1.98 µm pulse burst laser has enormous potential to be applied in bio-medical areas.

10 GHz regeneratively mode-locked thulium fiber laser with a stabilized repetition rate

GHz pulsed thulium-doped fiber laser with stabilized repetition rate can enable a wide range of applications. By employing regenerative mode-locking and cavity stabilization technique, we have for the first time demonstrated a 10 GHz polarization-maintaining thulium-doped fiber laser, which has a long-term repetition-rate stabilization and picosecond timing-jitter. In our experiment, a RF circuitry is designed to extract the 10 GHz longitudinal clock signal so that stable regenerative mode-locking is achieved. A piezo actuator-based phase-lock-loop is used to lock the regeneratively mode-locked pulses to a local reference synthesizer. The regeneratively mode-locked pulses with picosecond pulse width exhibit a high super-mode suppression ratio of 60 dB. In addition, the repetition rate of the laser shows good long-term stability with a variation of 8 Hz in 8 hours, corresponding to a cavity free spectral range fluctuation of less than 16 mHz. Meanwhile, the Allan deviation of the stabilized 10 GHz regeneratively mode-locked pulses is measured to be as low as 2 × 10^-12 over 1000 s average time, which is only limited by the stability of the reference synthesizer. Such an ultra-stable 10 GHz pulsed thulium fiber laser may find potential application in 2 µm optical communication, material processing and spectroscopy.

570 MHz harmonic mode-locking in an all polarization-maintaining Ho-doped fiber laser

In this paper, we demonstrate a 570.0 MHz harmonically mode-locked all-polarization-maintaining Ho-doped fiber laser based on semiconductor saturable absorbed mirror. Firstly, the laser operates in the 15.4 MHz fundamental mode-locked soliton regime, emitting 2051.5 nm, 1.62 ps soliton pulse without Kelly sidebands. And then, the stable 37th-order harmonic mode-locked soliton with maximum repetition rate up to 570.0 MHz at 2053nm is generated. Moreover, colorful soliton rain behaviors are also discussed.

Low noise all-fiber amplification of a coherent supercontinuum at 2 µm and its limits imposed by polarization noise

We report a low noise, broadband, ultrafast Thulium/Holmium co-doped all-fiber chirped pulse amplifier, seeded by an Erbium-fiber system spectrally broadened via coherent supercontinuum generation in an all-normal dispersion photonic crystal fiber. The amplifier supports a − 20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The total relative intensity noise (RIN) integrated from 10 Hz to 20 MHz is 0.07%, which to our knowledge is the lowest reported RIN for wideband ultrafast amplifiers operating at 2 µm to date. This is achieved by eliminating noise-sensitive anomalous dispersion nonlinear dynamics from the spectral broadening stage. In addition, we identify the origin of the remaining excess RIN as polarization modulational instability (PMI), and propose a route towards complete elimination of this excess noise. Hence, our work paves the way for a next generation of ultra-low noise frequency combs and ultrashort pulse sources in the 2 µm spectral region that rival or even outperform the excellent noise characteristics of Erbium-fiber technology.

All-fiber frequency comb at 2 µm providing 1.4-cycle pulses

We report an all-fiber approach to generating sub-2-cycle pulses at 2 µm and a corresponding octave-spanning optical frequency comb. Our configuration leverages mature erbium:fiber laser technology at 1.5 µm to provide a seed pulse for a thulium-doped fiber amplifier that outputs 330 mW average power at a 100 MHz repetition rate. Following amplification, nonlinear self-compression in fiber decreases the pulse duration to 9.5 fs, or 1.4 optical cycles. The spectrum of the ultrashort pulse spans from 1 to beyond 2.4 µm and enables direct measurement of the carrier-envelope offset frequency. Our approach employs only commercially available fiber components, resulting in a design that is easy to reproduce in the larger community. As such, this system should be useful as a robust frequency comb source in the near-infrared or as a pump source to generate mid-infrared frequency combs.

High power, twin-band mid-infrared PPMgLN optical parametric oscillator pumped at 1.679 µm

We report on a high-power, twin-band mid-infrared (MIR) periodically poled MgO-doped ${{\rm LiNbO}_3}$LiNbO₃ (PPMgLN) optical parametric oscillator (OPO) pumped by a near-infrared pulsed OPO with 20 kHz of repetition rate. The pump source has a central wavelength of 1.679 µm and a linewidth of 0.12 nm. The MIR OPO can be tuned from 2.74 to 2.80 µm for signal and from 4.34 to 4.19 µm for idler through raising the oven temperature from 30°C to 200°C, respectively. Under 100°C oven temperature, the OPO yielded maximum total output power of 15.4 W (2.76 µm signal plus 4.29 µm idler) with single-pass pumping configuration. The beam quality ${M^2}$M² was measured to be $ \lt {2.5}$<2.5 for signal and $ \lt {4}$<4 for idler.

Characterization of e-beam evaporated Ge, YbF3, ZnS, and LaF3 thin films for laser-oriented coatings

Thin films of Ge, ZnS, YbF₃, and LaF₃ produced using e-beam evaporation on ZnSe and Ge substrates were characterized in the range of 0.4-12 µm. It was found that the Sellmeier model provides the best fit for refractive indices of ZnSe substrate, ZnS, and LaF₃ films; the Cauchy model provides the best fit for YbF₃ film. Optical constants of Ge substrate and Ge film as well as extinction coefficients of ZnS, YbF₃, LaF₃, and ZnSe substrate are presented in the frame of a non-parametric model. For the extinction coefficient of ZnS, the exponential model is applicable. Stresses in Ge, ZnS, YbF₃, and LaF₃ were estimated equal to (-50)MPa, (-400)MPa, 140 MPa, and 380 MPa, respectively. The surface roughness does not exceed 5 nm for all films and substrates.

Ultra low-noise coherent supercontinuum amplification and compression below 100 fs in an all-fiber polarization-maintaining thulium fiber amplifier

We report an ultra-low noise, polarization-maintaining, ultrafast Thulium-doped all-fiber chirped pulse amplifier, seeded by a polarized all-normal dispersion (ANDi) supercontinuum (SC) driven by an ultrafast Erbium-fiber laser. The system comprises only polarization-maintaining fibers and delivers 96 fs pulses with 350 mW output power at 100 MHz, centered at 1900 nm. The integrated relative intensity noise (RIN) in the range of 10 Hz - 10 MHz is only 0.047% at the amplifier output, which is virtually identical to the RIN of the Erbium-fiber laser driving the SC. Therefore, neither the SC generation nor the amplification process introduce significant excess noise. The RIN of our system is an order of magnitude lower than similar systems previously seeded with Raman solitons. This highlights the superior noise properties of ANDi SC and their potential as ultra-low noise seed sources for broadband, high power ultrafast fiber amplifiers and frequency combs.

Middle-IR frequency comb based on Cr:ZnS laser

We report, to the best of our knowledge, the first fully referenced Cr:ZnS optical frequency comb. The comb features few cycle output pulses with 3.25 W average power at 80 MHz repetition rate, spectrum spanning 60 THz in the middle-IR range 1.79-2.86 µm, and a small footprint (0.1 m²), The spectral components used for the measurement of the comb's carrier envelope offset frequency were obtained directly inside the polycrystalline Cr:ZnS laser medium via intrinsic nonlinear interferometry. Using this scheme we stabilized the offset frequency of the comb with the residual phase noise of 75 mrads.

Broadband dispersive Ge/YbF3 mirrors for mid-infrared spectral range

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5-11.5 μm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF₃ layers, which have not been used before for manufacturing of multilayer dispersive optics. The design and production processes are described; mechanical stresses of the coatings are estimated based on experimental data; and spectral and phase properties of the produced mirrors are measured. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse.

CEP-stable high-energy ytterbium-doped fiber amplifier

We report on the carrier-envelope phase (CEP) stabilization of a Yb-doped fiber amplifier system delivering 30 μJ pulses at 100 kHz repetition rate. A single-shot, every-shot measurement of the CEP stability based on a simple f-2f interferometer is performed, yielding a CEP standard deviation of 320 mrad rms over 1 s. Long-term stability is also assessed, with 380 mrad measured over 1 h. This level of performance is allowed by a hybrid architecture, including a passively CEP-stabilized front-end based on difference frequency generation and an active CEP stabilization loop for the fiber amplifier system, acting on a telecom-grade integrated LiNbO3 phase modulator. Together with recent demonstrations of temporal compression down to the few-cycle regime, the presented results demonstrate the relevance of the Yb-doped high repetition rate laser for attoscience.

Mid-infrared long-pass filter for high-power applications based on grating diffraction

A gold-coated silicon grating has been developed, enabling efficient spatial separation of broadband mid-infrared (MIR) beams with wavelengths >5  μm from collinearly propagating, broadband, high-power light in the near-infrared (NIR) spectral range (centered at 2 μm). The optic provides spectral filtering at high powers in a thermally robust and chromatic-dispersion-free manner such as that necessary for coherent MIR radiation sources based on parametric frequency downconversion of femtosecond NIR lasers. The suppression of a >20  W average-power, 2 μm driving pulse train by three orders of magnitude, while retaining high reflectivity of the broadband MIR beam, is presented.

Watt-scale 50-MHz source of single-cycle waveform-stable pulses in the molecular fingerprint region

We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6-18 μm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 μm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-μm pulses in GaSe.