Femtosecond Mamyshev fiber oscillator started by a passively Q-switched microchip laser.

A femtosecond Mamyshev fiber oscillator in normal dispersion mode at 1 µm was started reliably and safely by an inexpensive diode-pumped passively Q-switched monolithic microchip laser emitting 300-ps pulses. Four-wave mixing spectral broadening is shown to play a pivotal role in starting the Mamyshev oscillator, owing to the random short and intense temporal fluctuations allowed by its ∼10-nm bandwidth. Systematic studies of the starting dynamics show that a success rate of 100% of the attempts is achieved with modest seed energy, as low as ∼30 nJ from the sub-nanosecond laser, corresponding to ∼100 pJ for the total four-wave mixing signal required to start the oscillation.

Few-optical-cycle pulse generation based on a non-linear fiber compressor pumped by a low-energy Yb:CALGO ultrafast laser.

Pulse compression in a short, normal dispersion photonic-crystal fiber is investigated with a Yb:CaGdAlO4 laser pumped by a low-power fiber-coupled single-mode diode that delivers 70-fs pulses at 1050 nm central wavelength, with 45-mW average power at 60 MHz repetition rate. A simple and power-efficient compressor based on a ∼15-cm long, low-cost commercial nonlinear fiber, with normal dispersion at the laser wavelength, produces pulses as short as 14.9 fs, corresponding to ∼4.25 optical cycles, with 29 mW average power after a prism-pair compressor in double pass configuration. Pulse quality was investigated with frequency resolved optical gating (FROG) analysis. Furthermore, a comparative analysis of noise properties of the oscillator, pump laser and compressed pulses has been performed.

Jitter investigation of narrow-bandwidth passively Q-switched Nd:YAG unidirectional ring laser.

A passively Q-switched Nd:YAG undirectional ring laser with external feedback is reported. The laser generates 50 ns single-axial-mode pulses up to 6 kHz, with energy 34 µJ, M2<1.05, and pulse jitter <50 ns rms, which is quite remarkable for this class of devices. Jitter was effectively minimized by using relatively high-peak-power pump pulses of 10 W, in agreement with a model considering both pump fluctuations and spontaneous emission noise. This represents an improvement by a factor of 8 with respect to a similar laser device we recently reported.

Multi-watt amplification in a birefringent Yb:LiLuF4 single crystal fiber grown by micro-pulling-down.

We present, to the best of our knowledge, the first demonstration of a single crystal fiber solid-state amplifier based on a birefringent Yb3+-doped thin crystal grown by the micro-pulling-down method. We measured a small signal gain >30 in a four-passes Yb:LiLuF4 amplifier pumping with a 120-W maximum power fiber-coupled laser diode. At an absorbed pump power of 80 W, a maximum output power of 8.4 W was obtained seeding the 42-mm-long, 2%Yb3+-doped single crystal fiber sample with 0.8 W at 1021 nm. Even at the maximum incident pump power, the amplified laser beam polarization and spatial quality were excellent (M2=1.15×1.06).

Passively Q-switched single-frequency Nd:YVO4 ring laser with external feedback.

A compact diode-pumped Nd:YVO4 ring laser was developed for generation of relatively long (few tens nanoseconds) single-frequency pulses for high-spectral-resolution LIDAR applications. Exploiting the feedback from an external mirror and Cr:YAG passive Q-switching with pulsed pump, unidirectional single-frequency operation with high quality ∼50-ns, 80-µJ TEM00 pulses was achieved from 100 Hz to 10 kHz. Amplitude, duration and repetition rate stability of the pulses was better than 1%.

Laser investigation of Yb:YLF crystals fabricated with the micro-pulling-down technique.

Fiber rods of 10% doped Yb:YLF were fabricated with the micro-pulling-down technique and characterized. The crystal a axis was oriented along the rod length, allowing polarized emission with the largest cross section available in the c direction. Laser experiments showed that these fiber samples perform similarly to crystals grown by the standard Czochralski method. Intrinsic slope efficiency of ∼50% was measured in both cases, with small comparable intracavity losses, proving the good quality of the fiber material.

Continuous-wave and SESAM mode-locked femtosecond operation of a Yb:MgWO4 laser.

We present a detailed continuous-wave regime characterization and, for the first time to the best of our knowledge, SESAM mode-locked femtosecond operation with a monoclinic, Yb3+-doped, MgWO4 crystal. Pumping with a low-power, single-mode fiber-coupled laser diode emitting at 976 nm, we demonstrate threshold for continuous-wave (cw) operation as low as 50 mW (absorbed pump power) and slope efficiency up to ~60% (with respect to the absorbed pump power) for two of the principal emission polarizations. The output wavelength in the cw regime is continuously tunable over a ∼50 nm broad range. Mode-locking the laser with a SESAM, we achieve almost Fourier-transform limited pulses with a duration of 132 and 125 fs depending on the polarization.

Threshold reduction and mode selection with uncoated Raman crystal acting as a low-finesse cavity.

We report a stimulated Raman scattering threshold reduction by a factor of ∼4 using a simple uncoated SrWO4 crystal with plane/parallel faces pumped by 1.5 mJ, 11 ns single-frequency pulses at 532 nm. The weak etalon effect is sufficient to shape both the spatial and spectral characteristics of the output Raman beam. The Raman signal tends to be single-frequency close to threshold, which might be an interesting way to further seed Raman amplifiers for high-energy Fourier-limited pulses at particular visible or near-infrared wavelengths.

Ultrafast, solid-state oscillators based on broadband, multisite Yb-doped crystals.

A detailed performance comparison of new interesting Yb-doped crystals in the same oscillator setup, with single-mode fiber-coupled diode laser pump is reported. We intended to assess the shortest pulses achievable with available SESAM technology, running a fair comparison with laser crystals Yb:KLuW, Yb:SSO, Yb:CALGO, Yb:CALYO and Yb:CaF2, very likely including the most promising choices for the next generation of commercial bulk ultrafast solid-state systems.

Approximate solution for high-frequency Q-switched lasers.

A simple approximation for the energy, pulse width, and build-up time valid for high-repetition-rate Q-switched lasers is discussed. This particular regime of operation is most common in industrial applications where manufacturing time must be minimized. Limits of validity and some considerations for the choice of the most appropriate laser system for specific applications are briefly discussed.

87 fs pulse generation in a diode-pumped semiconductor saturable absorber mirror mode-locked Yb:YLF laser.

A Yb:YLF crystal has been investigated in a femtosecond oscillator pumped by two 400 mW single-mode fiber-coupled diodes emitting at 976 nm and mode locked with a semiconductor saturable absorber mirror. Almost Fourier transform-limited pulses with durations of 87 and 107 fs were demonstrated for extraordinary and ordinary polarizations, respectively. This is, to the best of our knowledge, the first demonstration of sub-100 fs pulses with Yb:YLF, and it proves the potential for ultrashort pulse generation and amplification with this material.

Difference-frequency generation of ultrashort pulses in the mid-IR using Yb-fiber pump systems and AgGaSe(2).

We employ AgGaSe(2) for difference-frequency generation between signal and idler of synchronously-pumped picosecond / femtosecond OPOs at 80 / 53 MHz. Continuous tuning in the picosecond regime is achieved from 5 to 18 µm with average power of 140 mW at 6 µm. In the femtosecond regime the tunability extends from 5 to 17 µm with average power of 69 mW at 6 µm. Maximum single pulse energies of >1 nJ in both cases represent the highest values at such high repetition rates.

Performance of Yb:Sc2SiO5 crystal in diode-pumped femtosecond oscillator and regenerative amplifier.

Yb:Sc2SiO5 has been investigated in a low-power laser femtosecond oscillator pumped by 400-mW single-mode fiber-coupled diode at 976 nm. Pulses as short as 71 fs were achieved. The same crystal was later employed in a regenerative amplifier, with an output power as high as 4.7 W at 500 kHz and sub-300-fs pulses.

Sub-50-fs widely tunable Yb:CaYAlO(4) laser pumped by 400-mW single-mode fiber-coupled laser diode.

Yb:CaYAlO(4) has been investigated spectroscopically and compared to better known Yb:CaGdAlO(4). It turns out that both materials show very similar spectroscopic parameters relevant to ultrafast lasers design. Employing single-mode fiber-coupled 400-mW laser diode at 976 nm we measured pulses as short as 43 fs, and broad tunability of 40 nm with a simple single-prism setup.

Narrow bandwidth, picosecond, 1064 nm pumped optical parametric generator for the mid-IR based on HgGa2S4.

We report on optical parametric generation in a mercury thiogallate (HgGa2S4) crystal pumped by 16 ps, 1064 nm pulses at 250 kHz. A broad tuning range extending from 1.19 to 1.47 µm (signal) and from 3.85 to 10 µm (idler) is achieved. Narrow bandwidth, quasi-Fourier limited operation with high beam quality for the signal/idler pulses and >14% pump depletion is obtained by continuous wave seed injection at 1.29 µm.

Mid-infrared rotated image singly resonant twisted rectangle optical parametric oscillator based on HgGa(2)S(4) pumped at 1064 nm.

We compare linear, planar ring, and rotated image singly resonant twisted rectangle (RISTRA) type nanosecond optical parametric oscillator cavities using HgGa2S4 nonlinear crystal pumped by 8 ns pulses at 1064 nm from a low beam quality pump source. The input-output characteristics and the output idler beam quality at 6300 nm are compared for two values of the pump beam diameter presenting different cavity Fresnel numbers and magnitudes of the spatial walk-off effect due to birefringence. The RISTRA cavity ensures in all cases a circular output beam profile but is advantageous in terms of beam quality with respect to the planar ring only at a large pump beam diameter.

Narrow-bandwidth, ~100 ps seeded optical parametric generation in CdSiP2 pumped by Raman-shifted pulses at 1198 nm.

Low-threshold, efficient optical parametric generation at ~4.64 µm is demonstrated using CdSiP2 nonlinear crystal pumped by 150 ps Raman shifted pump pulses at 1198 nm in noncritical configuration at 1 kHz repetition rate. Maximum single pulse idler energy of 6 µJ and total conversion efficiency of 30% are achieved. Seeding at the signal wavelength with a distributed feedback laser diode enables ~25 fold narrowing of the bandwidths down to ~10 GHz, resulting in a Fourier product of ~1 for the ~100 ps long signal (1615 nm) and idler (4.64 µm) pulses.

Sub-100-fs Cr:YAG laser mode-locked by monolayer graphene saturable absorber.

We report on mode-locking of a Cr:YAG laser at 1516 nm using a monolayer graphene-based saturable absorber of transmission type generating 91 fs pulses with a Fourier product of 0.38 at an average output power exceeding 100 mW. Stable single-pulse mode-locked operation without any sign of Q-switching instabilities or multiple pulses is achieved.

40-fs Yb3+:CaGdAlO4 laser pumped by a single-mode 350-mW laser diode.

We report the results of the investigation on a passively mode-locked Yb(3+):CaGdAlO(4) laser, pumped by a single transverse mode laser diode emitting 350 mW at 980 nm. This particular pump source allows efficient pumping with a nearly TEM(00) beam and minimal thermal load, making the optimization of the mode-locking performance more straightforward than with higher-power multimode beams. Indeed, using a semiconductor saturable absorber mirror and extra-cavity dispersion compensation, pulses as short as 40 fs (31-nm spectrum) have been measured, tunable across 20 nm with 15-mW output power. Slightly longer Fourier-limited 46-fs pulses with 33 mW output power directly from the oscillator have been achieved, using a different saturable absorber mirror. Such overall performance, especially considering these are among the shortest pulses generated in diode-pumped ytterbium lasers, confirms the excellent qualities of Yb(3+):CaGdAlO(4).

Nd:YVO4 amplifier for ultrafast low-power lasers.

An Nd:YVO4 amplifier consisting of two modules end pumped at 808 nm at 30 W total absorbed power has been designed for efficient, diffraction-limited amplification of ultrafast pulses from low-power seeders. We investigated amplification with a 50 mW, 7 ps Nd:YVO4 oscillator, a 2 mW, 15 ps Yb fiber laser, and a 30 mW, 300 fs Nd:glass laser. Output power as high as 9.5 W with 8 ps pulses was achieved with the 250 MHz vanadate seeder, whereas the 20 MHz fiber laser was amplified to 6 W. The femtosecond seeder allowed extracting Fourier-limited 4 ps pulses at 7 W output power. To our knowledge, these are the shortest pulses from any Nd:YVO4 laser device with at least 7 W output power. This suggests a novel approach to exploit the gain bandwidth of vanadate amplifiers with high output power levels. Such amplifier technology promises to offer an interesting alternative to high-power thin disk oscillators at few picoseconds duration, as well as to regenerative amplifiers with low-repetition-rate fiber seeders.