High-power, hybrid Er:fiber/Tm:fiber frequency comb source in the 2 μm wavelength region

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.

All-normal dispersion supercontinuum vs frequency-shifted solitons pumped at 1560 nm as seed sources for thulium-doped fiber amplifiers

We present a direct comparison between two types of femtosecond 2 µm sources used for seeding of an ultrafast thulium-doped fiber amplifier based on all-normal dispersion supercontinuum and soliton self-frequency shift. Both nonlinear effects were generated in microstructured silica fibers, pumped with low-power femtosecond pulses at 1.56 µm originating from an erbium-doped fiber laser. We performed a full characterization of both nonlinear processes, including their shot-to-shot stability, phase coherence, and relative intensity noise. The results revealed that the solitons show comparable performance to supercontinuum in terms of relative intensity noise and shot-to-shot stability, despite the anomalous dispersion regime. Both sources can be successfully used as seeds for Tm-doped fiber amplifiers as an alternative to Tm-doped oscillators. The results show that the sign of chromatic dispersion of the fiber is not crucial for obtaining a stable, high-quality, and low-noise spectral conversion process when pumped with sub-50 fs laser pulses.

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.

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.

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

We report on the generation of a high-power frequency comb in the 2 μm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30  fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz-30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.

Mode-locked Ho-doped laser with subsequent diode-pumped amplifier in an all-fiber design operating at 2052 nm

We present a mode-locked holmium-doped all-fiber soliton laser operating in the 2052 nm wavelength range. The ultrashort pulse oscillator is simultaneously self-providing 1950-nm radiation for efficient in-band pumping in a subsequent thulium-/holmium-doped fiber tandem amplifier. More than 76 nJ-pulses for Ho:YLF or Ho:YVO₄ amplifier seeding have been achieved.

Generation of watt-class, sub-50 fs pulses through nonlinear spectral broadening within a thulium-doped fiber amplifier

We demonstrate direct generation of sub-50 fs pulses from a thulium-doped fiber amplifier. Broad spectra are obtained by exploiting nonlinear effects within the amplifier fiber itself. High fractional inversion densities of thulium ions achieved by a core-pumping scheme helped to extend spectra into the shorter wavelength region around 1.7 µm. Pulses with a duration of 48 fs are obtained at an average power of 2.5 W directly after the amplifier fiber, i.e., without using a compressor.

1 μJ, sub-300 fs pulse generation from a compact thulium-doped chirped pulse amplifier seeded by Raman shifted erbium-doped fiber laser

We present a compact thulium-doped chirped pulse amplifier producing 241 fs pulses with 1 μJ energy. The system is seeded with the Raman shifted soliton generated by the combination of an erbium-doped femtosecond laser and a nonlinear fiber. The Tm-doped large mode area fiber yields output power of 71 W, corresponding to pulse energy of 2.04 μJ, with a slope efficiency of 52.2%. The amplified pulses have been compressed to a duration time of 241 fs, using a folded Treacy grating setup. The pulse energy is measured to be 1.02 μJ, corresponding to a peak power of ~3 MW. To the best of our knowledge, this is the highest average power and pulse energy generated from an all-fiber, Raman shifted soliton seeded thulium-doped chirped pulse amplifier system.

Sub-100 fs passively mode-locked holmium-doped fiber oscillator operating at 2.06 μm

We demonstrate a simple and compact Holmium-doped fiber femtosecond oscillator, in-band pumped by a commercial Tm-doped fiber laser. The oscillator operates in the dispersion managed soliton regime at net zero intracavity dispersion and delivers >1  nJ pulse energy at 35 MHz repetition rate. The pulse duration directly at the oscillator output is 160 fs FWHM, close to the Fourier-limit of 145 fs FWHM. Using an additional nonlinear compressor stage, sub-100 fs FWHM pulse durations could be achieved. The nonlinear fiber compressor is implemented by a solid core highly nonlinear fiber for spectral broadening and a single mode fiber for pulse compression.

Tunable, stable source of femtosecond pulses near 2 μm via supercontinuum of an Erbium mode-locked laser

A source of ultrashort pulses of light in the 2 μm region was constructed using supercontinuum broadening from an erbium mode-locked laser. The output spectrum spanned 1000 nm to 2200 nm with an average power of 250 mW. A pulse width of 39 fs for part of the spectrum in the 2000 nm region, corresponding to less than six optical cycles, was achieved. A heterodyne measurement of the free-running mode-locked laser with a narrow-linewidth continuous wave laser resulted in a near shot noise-limited beat note with a signal-to-noise ratio of 45 dB in a 10 kHz resolution bandwidth. The relative intensity noise of the broadband system was investigated over the entire supercontinuum, and the integrated relative intensity noise of the 2000 nm portion of the spectrum was 1.7 × 10(-3). The long-term stability of the system was characterized, and intensity fluctuations in the spectrum were found to be highly correlated throughout the supercontinuum. Spectroscopic limitations due to the laser noise characteristics are discussed.

Er/Tm:fiber laser system for coherent Raman microscopy

We present a novel architecture for a fiber-based hybrid laser system for coherent Raman microscopy, combining an amplified Er:fiber femtosecond oscillator with a Tm:fiber amplifier boosting the power of the 2-μm portion of a supercontinuum up to 300 mW. This is enough to obtain, by means of nonlinear spectral compression, sub-20-cm(-1) wide pump and Stokes pulses with 2500-3300  cm(-1) frequency detuning and average power at the 100-mW level. Application of this system to stimulated Raman scattering microscopy is discussed.

Two-color optically synchronized ultrashort pulses from a Tm/Yb-co-doped fiber amplifier

A method of producing high quality, optically synchronized two-color ultrashort pulses in an active thulium-doped fiber is proposed. We show that sech-shaped femtosecond pulses with essentially different wavelengths can be generated directly from a Tm/Yb-co-doped amplifier: one pulse at about 2 μm and the second pulse with a tunable wavelength up to 2.3 μm, which covers the pump and gain regions of Cr:ZnSe and Cr:ZnS amplifiers. The shortest pulses with durations of 145 fs at 2.25 μm and 125 fs at 2 μm were measured by the FROG (frequency-resolved optical gating) technique.

High-power frequency comb in the range of 2-2.15 μm based on a holmium fiber amplifier seeded by wavelength-shifted Raman solitons from an erbium-fiber laser

We demonstrate a room-temperature high-power frequency comb source covering the spectral region from 2 to 2.15 μm. The source is based on a femtosecond erbium-fiber laser operating at 1.55 μm with a repetition rate of 250 MHz, wavelength-shifted up to 2.06 μm by the solitonic Raman effect, seeding a large-mode-area holmium (Ho) fiber amplifier pumped by a thulium (Tm) fiber laser emitting at 1.94 μm. The frequency comb has an integrated power of 2 W, with overall power fluctuations as low as 0.3%. The beatnote between the comb and a high-spectral-purity, single-frequency Tm-Ho laser has a linewidth of 32 kHz over 1 ms observation time, with a signal-to-noise ratio in excess of 30 dB.

All-PM coherent 2.05 µm Thulium/Holmium fiber frequency comb source at 100 MHz with up to 0.5 W average power and pulse duration down to 135 fs

We report on a dual output all-PM fiber laser system running at 100 MHz repetition rate offering coherent broadband and narrowband pulses centered at 2.05 µm with a spectral FWHM bandwidth of 60 nm and 1.5 nm at up to 360 mW and 500 mW, respectively. The broadband pulses are compressed down to 135 fs. The multi-stage double-clad amplifier based on Tm/Ho codoping is seeded by a supercontinuum light source, spanning from around 1 µm up to 2.4 µm.

Single-clad Tm-Ho:fiber amplifier for high-power sub-100-fs pulses around 1.9 μm

A Tm-Ho:fiber amplifier based on single-clad geometry is demonstrated for the generation of high-power femtosecond pulses around 1.9 μm. The amplifier is seeded by the low-power Raman soliton generated by an Er:fiber femtosecond laser. Pulse trains at a repetition rate of 250 MHz tunable from 1.84 to 1.92 μm with corresponding powers from 2.6 to 3 W and durations from 80 to 105 fs have been obtained. Beating with a single-frequency Tm laser has shown that the pulse coherence is highly preserved. The overall power fluctuations have been measured to be as low as 0.6%.