Andreas Wienke

Ultrafast, stretched-pulse thulium-doped fiber laser with a fiber-based dispersion management

An ultrafast thulium-doped fiber laser with stretched-pulse operation has been realized and investigated. The passively mode-locked oscillator emitted 119 fs pulses at a peak wavelength of 1912 nm. A normal-dispersion fiber with a high numerical aperture and small core was used for intracavity dispersion management and external compression. Numerical simulations were performed and are in good agreement with the experimental results.

152 W average power Tm-doped fiber CPA system.

A high-power thulium (Tm)-doped fiber chirped-pulse amplification system emitting a record compressed average output power of 152 W and 4 MW peak power is demonstrated. This result is enabled by utilizing Tm-doped photonic crystal fibers with mode-field diameters of 35 μm, which mitigate detrimental nonlinearities, exhibit slope efficiencies of more than 50%, and allow for reaching a pump-power-limited average output power of 241 W. The high-compression efficiency has been achieved by using multilayer dielectric gratings with diffraction efficiencies higher than 98%.

700 MW peak power of a 380 fs regenerative amplifier with Tm:YAP.

We report on a high power ultrashort pulse regenerative amplifier system, entirely based on thulium-doped laser materials operating around 1.94 μm. At a repetition rate of 1 kHz the Tm:YAP regenerative amplifier emits pulse energies > 700 μJ, only limited by the damage threshold of the Tm:YAP crystal. The pulses can be compressed to 380 fs at an efficiency of 50 %. Purging of the regenerative amplifier cavity with nitrogen is necessary due to atmospheric absorptions causing long ps pedestals in the autocorrelation.

Comparison between Tm:YAP and Ho:YAG ultrashort pulse regenerative amplification

We compare the performance characteristics of Tm:YAP and Ho:YAG in ultrashort pulse regenerative amplification. Both systems follow the same amplification concept and use nearly the same experimental setup reaching similar output energies of >700 µJ.

Fiber based dispersion management in an ultrafast thulium-doped fiber laser and external compression with a normal dispersive fiber

Ultrashort pulses in a thulium-doped fiber laser were generated with a fiber based dispersion management. The laser emitted sub-500 fs pulses which were compressed externally with a normal dispersive fiber below 130 fs.

High energy, femtosecond fiber laser source at 1750 nm for 3-photon microscopy (Conference Presentation)

We present an ultrafast fiber laser system at a central wavelength of 1750 nm for imaging applications, in particular 3-photon microscopy. It generates an output pulse train with an adjustable repetition rate ranging from 1 MHz to 21 MHz. After temporal compression the pulse duration is 220 fs and the maximum achieved pulse energy is 20 nJ. The laser system consists of a polarization maintaining (PM) Erbium-doped fiber oscillator which emits a stable output pulse train at a fixed repetition rate of 42 MHz. The oscillator generates soliton pulses centered at a wavelength of 1560 nm and a spectral width of 7 nm. Mode-locking is initiated and stabilized by a semiconductor saturable absorber mirror. The output pulses are picked in a PM fiber coupled acousto-optic modulator to an adjustable repetition rate of 1 – 21 MHz. A consecutive Erbium-doped PM fiber amplifier (EDFA) boosts the energy of the soliton pulses from pJ to nJ level. The directly emitted pulses have a duration of 2 ps which can be compressed to a pulse duration of 115 fs by using a passive standard fiber. The uncompressed pulses are soliton-self-frequency shifted by Raman scattering to wavelengths longer than 1700 nm in 7 m of passive PM1550 fiber at a pulse energy of 1.1 nJ. The central wavelength can be adjusted by the pump power of the EDFA. To boost the pulse energy of the wavelength shifted pulses, the Raman stage is followed by a single-clad Thulium-doped fiber (TDF) amplifier. It consists of a 1560/1750 nm wavelength division multiplexer (WDM) and 0.9 m of TDF. To diminish nonlinear effects during amplification, the pulses are stretched with 25 m of normal dispersion fiber (NDF) inserted between the WDM and the TDF. Although on the very short wavelength amplification band, the pulses are amplified up to more than 40 nJ of pulse energy at an injected pump power of 4.1 W. After the fiber amplifier, the pulses are coupled out and propagate through a spectral filter, a triplet of l/4, l/2, and l/4 waveplates, an isolator, and a grating compressor. As the WDM, NDF, and TDF are not PM, the polarization state has to be readjusted to linear with the waveplates before entering the isolator. The added group delay dispersion of 2.17 ps2 by the NDF is compensated in a free space standard grating compressor built of two 600 lines/mm gratings. The transmission of the grating compressor is 60 %. To achieve optimum compression to a pulse duration of 220 fs at a pulse energy of 20 nJ, the compressor in combination with spectral filtering around 1750 nm has to be carefully adjusted. The maximum output pulse energy of 20 nJ is constant ranging from 1 MHz to 7 MHz, but is reduced at higher repetition rates down to 8.7 nJ. The output pulse duration is nearly constant at 220 fs for all repetition rates. Further amplification of the pulses is currently under investigation. This system will be used in future for the application of 3-photon microscopy.

Ultrafast thulium fiber laser operating at 1750 nm

Passively mode-locked femtosecond fiber lasers operating in the telecom band around 1.5 μm are well discovered since many years. Thulium- and Holmium-based hght sources in 2 μm region have greatly gained interest in recent years. Because of the absence of optical transitions in fibers doped with rare-earth ions, the gap between these two spectral regions, namely laser sources at wavelengths around 1750 nm, have hardly been investigated, even though this wavelength region has some interesting applications, such as multi-photon fluorescence imaging [1] or medical applications [2].

Sub-700fs pulses at 152 W average power from a Tm-doped fiber CPA system

Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tmdoped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.

Regenerative Amplification of Ultrashort Pulses at 2 µm with a Thulium-doped YAP Crystal

Seed pulses at nJ-level of a fiber-based pre-amplified Thulium oscillator are amplified to μJ-level at 1 kHz repetition rate in a regenerative amplifier based on a Thulium-doped YAP crystal with 355 fs compressed pulse duration.

Tm-Doped Fiber CPA System with 152 W Average Power and Sub-700fs Pulse Duration

Thulium-doped photonic crystal fibers exhibit cross relaxation with slope efficiencies of up to 55% and enabled a high power fiber CPA system emitting a record compressed average output power of 152 W and 4 MW peak power.