Few-cycle laser pulse characterization on-target using high-harmonic generation from nano-scale solids.

We demonstrate high-harmonic generation for the time-domain observation of the electric field (HHG-TOE) and use it to measure the waveform of ultrashort mid-infrared (MIR) laser pulses interacting with ZnO thin-films or WS2 monolayers. The working principle relies on perturbing HHG in solids with a weak replica of the pump pulse. We measure the duration of few-cycle pulses at 3200 nm, in reasonable agreement with the results of established pulse characterization techniques. Our method provides a straightforward approach to accurately characterize femtosecond laser pulses used for HHG experiments right at the point of interaction.

Sub-100†fs all-fiber polarization maintaining widely tunable laser at 2†µm.

An all-fiber polarization maintaining (PM) laser tunable over 170 nm via Raman soliton self-frequency shift (SSFS) delivering sub-100 fs pulses with a central wavelength ranging from 1880 to 2050 nm is presented. The 40 MHz laser emits power ranging from 15 to 34 mW with pulse duration as short as 80 fs. It is based on exclusively commercially available standard fibers and has been designed to produce sub-100 fs pulse durations over the whole spectral range without the need for adjusting neither nonlinearities nor dispersion. The laser is versatile and a robust potential alternative to Tm and Tm:Ho oscillators.

Mid-infrared frequency domain optical parametric amplifier.

We report on an optical architecture delivering sub-120 femtosecond laser pulses of 20 µJ tunable from 5.5 µm to 13 µm in the mid-infrared range (mid-IR). The system is based on a dual-band frequency domain optical parametric amplifier (FOPA) optically pumped by a Ti:Sapphire laser and amplifying 2 synchronized femtosecond pulses each with a widely tunable wavelength around 1.6 and 1.9 µm respectively. These amplified pulses are then combined in a GaSe crystal to produce the mid-IR few-cycle pulses by means of difference frequency generation (DFG). The architecture provides a passively stabilized carrier-envelope phase (CEP) whose fluctuations has been characterized to 370 mrad RMS.

Multi-gigahertz femtosecond pulses from linear and nonlinear propagation of a phase-modulated laser.

We propose and demonstrate a non-mode-locking approach to generating multi-gigahertz repetition rate, femtosecond pulses in burst mode by shaping a continuous-wave (CW) seed laser in an all-fiber configuration. The seed laser at 1030 nm is first phase modulated and de-chirped to low-contrast, ∼2 ps pulses at a 17.5 GHz repetition rate, then carved to bursts at a 60 kHz repetition rate, and finally shaped to <2 ps clean pulses by a Mamyshev regenerator. This prepared high-quality picosecond source is further used to seed an Yb-doped fiber amplifier operating in the highly nonlinear regime, delivering output pulses at 23 nJ/pulse and $20\,\mathrm{\mu}$J/burst, compressible to ∼100 fs level. The system eliminates the need for mode-locked cavities and simplifies conventional ultrafast electro-optic combs to using only one phase modulator, while providing femtosecond pulses at multiple gigahertz repetition rate, enhanced pulse energy in burst mode and the potential of further power/energy scaling.

High-power nonlinear amplification of an ultrafast electro-optic frequency comb with flexible GHz repetition rate.

We report on an all-fiber 200 W widely tunable GHz electro-optic (EO) frequency comb operating in the nonlinear regime. The EO comb pulses at 1030 nm are initially pre-compressed to sub-2 ps, then power amplified to 2.5 W, and finally boosted to 200 W in a newly designed large-mode-area, Yb-doped photonic crystal fiber. Continuously tunable across 12-18 GHz, the picosecond pulses experience nonlinear propagation in the last amplifier, leading to output pulses compressible down to several hundreds of femtoseconds. To push our system deeper into nonlinear amplification regime, the pulse repetition rate is further reduced to 2 GHz, enabling significant spectral broadening at 200 W. Characterization reveals sub-200 fs duration after compression. The present EO-comb seeded nonlinear amplification system opens a new route to the development of high-power, tunable GHz-repetition-rate, femtosecond fiber lasers.

Efficient few-cycle Yb-doped laser oscillator with Watt-level average power.

So far, the operation of ultrafast bulk laser oscillators based on Yb-doped gain materials and directly emitting few-cycle pulses have been restricted to low optical-to-optical efficiencies and average output powers of only a few milliwatt. This performance limitation can be attributed to the commonly-applied standard collinear pumping scheme in which the optical pump is transmitted through a dichroic mirror whose spectral transmission and dispersion properties severely perturb the oscillating pulse when its optical spectrum extends towards the pump wavelength. In this study, we report on a novel pumping scheme relying on cross polarization that overcomes this challenge. In our concept, the pump transmitting mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond. In contrast to a standard thin-film polarizer featuring similar polarization dependent properties, it provides a low and flat dispersion profile over a broad spectral range for the s-polarization. Implementing this pumping scheme in a soft-aperture Kerr-lens mode-locked bulk laser oscillator based on the gain material Yb:CALGO, we achieve clean 22-fs soliton pulses at 729 mW of average output power and an optical-to-optical efficiency of 25%. In a second configuration optimized for the highest average output power, we demonstrate a high optical-to-optical efficiency of 36.6%, which was obtained for 31-fs pulses at 1.63 W of average output power. In a third configuration we experimentally confirm the limiting effect of a dichroic mirror commonly used in the standard collinear pumping scheme. All the results presented here and obtained in the first and second configuration generate pulses with a center wavelength ranging from 1030 nm to 1056 nm, well within the spectral region of high gain cross sections of Yb:CALGO. While this initial demonstration was realized using a commercial diffraction-limited fiber laser as pump source, the pump geometry appears also well suited for pumping with laser diodes coupled into multimode fibers. This novel approach opens up new opportunities for compact and cost-efficient high-power few-cycle bulk laser oscillators based on Yb-doped gain materials and can be applied to any gain material with small quantum defect.

Polarization-independent pulse retrieval based on frequency resolved optical switching.

We demonstrate experimentally that the frequency resolved optical switching (FROSt) method is independent of the polarization direction of the pulse to be characterized. In this perspective, it is employed to characterize two or three co-propagating pulses linearly polarized in orthogonal directions, enabling to retrieve simultaneously their temporal intensity and phase profiles together with their group delay. This technique is also applied to track a simple nonlinear process involving different polarization states: type-I second harmonic generation (SHG). We are able to characterize the depleted fundamental pulse along with the generated second-harmonic pulse, thus demonstrating that the FROSt technique is a practical and powerful tool to observe nonlinear processes both in the temporal and spectral domains even if it involves different polarization states.

Electro-optic comb pumped optical parametric oscillator with flexible repetition rate at GHz level.

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic comb at 1.03 µm, delivering sub-picosecond signal pulses across 1.5-1.7 µm from a MgO-doped periodically poled LiNbO3 crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 µm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach-Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from one to 14, allowing the OPO to operate with a flexible repetition rate from 1 to 14.2 GHz. A strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.

High-power amplified spontaneous emission pulses with tunable coherence for efficient non-linear processes.

We report on a detailed study of an amplified spontaneous emission source operated in a pulsed regime with particular attention paid to the influence of high-intensity chaotic temporal events on the generation of nonlinear processes. To this aim, we have developed a monolithic high-power fiber system delivering partially coherent pulses of adjustable coherence. We also have demonstrated a non-linear method to characterize the stochastic properties of the source mitigating the bandwidth limitation of linear techniques. Measured parameters of the source for various configurations are presented. An enhanced classical model has been established to reproduce the statistical properties of the source and predict the behaviour when exciting non-linear processes. Finally, a non-linear process (second harmonic generation) is investigated comparing the efficiency when the process is pumped by a pulsed beam with maximal and low coherence.

Multi-GHz repetition rate, femtosecond deep ultraviolet source in burst mode derived from an electro-optic comb.

We present a femtosecond, 11.48 GHz intra-burst repetition rate deep UV source at 258 nm based on forth-harmonic generation (FHG) of an electro-optic (EO) comb operating in burst mode. Second-harmonic generation (SHG) of the burst-mode EO comb in LiB3O5 (LBO) leads to 3.7 W average power and 242 fs root-mean-square pulse duration. A second stage of SHG is further performed using two separate ß-BaB2O4 (BBO) crystals, delivering deep UV pulses at 523 mW and 294 mW, with estimated pulse durations of half-ps and sub-300 fs, respectively. At divided pulse repetition rates of 5.7 GHz and 2.9 GHz, FHG is also demonstrated, highlighting the potential of flexible repetition rate operation at the GHz level.

Single-shot phase-matching free ultrashort pulse characterization based on transient absorption in solids.

The frequency-resolved optical switching (FROSt) method developed for ultrashort pulse characterization is implemented for single-shot measurements. In this basic demonstration, the delay axis of the spectrogram is spatially encoded by the pump beam having a small incident angle with the photoexcited material. We present the calibration procedure for spectrograms acquired in single-shot and the temporal characterization of 44 fs pulses with central wavelength at 800 nm both in scanning and single-shot FROSt configurations. The retrieved pulses are compared by means of the root-mean-square field error. Finally, the pulses are propagated through a known dispersive material to measure the added group-delay dispersion.

Carrier-envelope-phase measurement of few-cycle mid-infrared laser pulses using high harmonic generation in ZnO.

High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

39 W narrow spectral linewidth monolithic ytterbium-doped fiber MOPA system operating at 976 nm.

We report on a monolithic narrow spectral linewidth master oscillator power amplifier (MOPA) delivering up to 39 W around 976 nm with very high contrast. The amplifier is based on an ytterbium-doped large mode area (LMA) octagonal double clad (DC) active fiber with parameters optimized for long living three-level operation.

Single-shot CEP drift measurement at arbitrary repetition rate based on dispersive Fourier transform.

This paper presents a single-shot technique for measuring CEP. The Temporal dispersion based One-shot Ultrafast Carrier envelope phase Analysis method (TOUCAN) is an arbitrary repetition rate single-shot CEP drift measurement technique based on dispersive Fourier transformations and has been experimentally tested at 100 kHz. TOUCAN was validated by a direct comparison of decimated data with an independent traditional CEP drift measurement technique. The impact of a temporal jitter on the CEP drift measurement is investigated and a new mitigation technique is shown to produce high accuracy jitter-free CEP drift extraction.

Highly stable, 15 W, few-cycle, 65 mrad CEP-noise mid-IR OPCPA for statistical physics.

We demonstrate a 100 kHz optical parametric chirped-pulse amplifier delivering under 4-cycle (38 fs) pulses at ~3.2 µm with an average power of 15.2 W with a pulse-to-pulse energy stability <0.7% rms and a single-shot CEP noise of 65 mrad RMS over 8h. This source is continuously monitored, by using a fast 100 kHz data acquisition device, and presents an extreme stability, in the short and long terms.

High-power widely tunable ps source in the visible light based on four wave mixing in optimized photonic crystal fibers.

We present a detailed study on the generation of widely tunable visible light through four wave mixing in specifically designed micro-structured fibers. The fiber's properties are optimized for an efficient conversion to the visible and near infrared with a combined tunability from 620 to 910 nm of a picosecond Yb-doped tunable source for biomedical applications.

Femtosecond versus picosecond laser pulses for film-free laser bioprinting.

We investigate the properties of microjets in the context of film-free laser induced forward transfer in the femtosecond and picosecond regimes. The influence of the pulse duration (ranging from 0.4 to 12 ps) and the energy (ranging from 6 to 12 µJ) is systematically studied on the height, diameter, speed, volume, and shape of the jets. The 400 fs pulses generate thin and stable jets compatible with bioprinting, while 14 ps pulses generate more unstable jets. A pulse duration around 8 ps seems, therefore, to be an interesting trade-off to cover many bio-applications of microjets generated by lasers.

Study of middle infrared difference frequency generation using a femtosecond laser source in LGT.

We demonstrate phase-matched difference frequency generation in the emerging nonlinear crystal La3Ga5.5Ta0.5O14. Tunable wavelengths between 1.4 and 4.7 µm are generated by using femtosecond sources. We also report on the measurements of the optical damage threshold in the femtosecond regime and on the nonlinear refractive index n2.

100 kHz Yb-fiber laser pumped 3 µm optical parametric amplifier for probing solid-state systems in the strong field regime.

We report on a laser source operating at 100 kHz repetition rate and delivering 8 µJ few-cycle mid-IR pulses at 3 µm. The system is based on optical parametric amplification pumped by a high repetition rate Yb-doped femtosecond fiber-chirped amplifier. This high-intensity ultrafast system is a promising tool for strong-field experiments (up to 50 GV/m and 186 T) in low ionization potential atomic and molecular systems, or solid-state physics with coincidence measurements. As a proof of principle, up to the sixth harmonic has been generated in a 1 mm zinc selenide sample.

4-W, 100-kHz, few-cycle mid-infrared source with sub-100-mrad carrier-envelope phase noise.

We demonstrate an optical parametric chirped-pulse amplifier delivering 4-cycles (38-fs) pulses centered around 3.1 µm at 100-kHz repetition rate with an average power of 4 W and an undersampled single-shot carrier-envelope phase noise of 81 mrad recorded over 25 min. The amplifier is pumped by a ~1.1 ps, Yb-YAG, thin-disk regenerative amplifier and seeded with a supercontinuum generated in bulk YAG from the same pump pulses. Carrier-envelope phase stability is passively achieved through difference-frequency generation between pump and seed pulses. An additional active stabilization at 10 kHz combining 2f-to-f interferometry and a LiNbO3 acousto-optic programmable dispersive filter achieves a record low phase noise.