35-fs diode-pumped mode-locked ytterbium-doped multi-component alkaline-earth fluoride laser

Li Z., Lin Z., Loiko P., Zeng H., Zhang G., Nie H., Normani S., Benayad A., Camy P., Mateos X., Petrov V., Chen W.

We present sub-50 fs soliton pulse generation from a diode-pumped Kerr-lens mode-locked laser based on an Yb3+-doped BaF2 crystal. Utilizing a spatially single-mode, fiber-coupled InGaAs laser diode at 976 nm as a pump source, the Yb:BaF2 laser generates pulses as short as 46 fs at 1060.1 nm with an average output power of 45 mW at a pulse repetition rate of ~65.6 MHz via soft-aperture Kerr-lens mode locking. To the best of our knowledge, this represents the first demonstration of Kerr-lens mode-locked operation of the Yb:BaF2 crystal, as well as the shortest pulse duration ever achieved from any diode-pumped mode-locked laser based on an Yb3+-doped alkaline-earth fluoride crystal.

Su X., Wang Y., Su L., Liu S., Zheng L., Zhang B., He J., Zhang B.

Yb-doped fluoride has been demonstrated to be potential crystals for application in efficient ultrafast lasers. However, the trade-off between the shorter pulses with higher efficiencies is a challenge. In this work, using Yb,Gd:CaSrF2 crystal, we report on a sub-50-fs Kerr-lens mode-locked oscillator with an optical efficiency up to 44%. Pumped by a 976-nm diffraction-limited fiber laser and using chirped mirrors combined with prism pairs for the dispersion compensation, a pulse as short as 46 fs was obtained with 620-mW output power, corresponding to an optical efficiency more than 40%. Stable Kerr-lens mode-locking with RMS of output power lower than 0.3% and beam quality factors M2<1.14 were achieved. Moreover, a maximum output power of 780 mW was obtained in continuous-wave operation with 55.3% optical efficiency. To the best of our knowledge, the results in this work represent the shortest pulses generated from Yb-doped fluoride lasers as well as the highest optical efficiencies ever reported in sub-100 fs Yb bulk lasers.

Xue W., Loiko P., Lin Z., zeng H., Ge Z., Benayad A., Normani S., Camy P., Mateos X., Petrov V., Wang L., Chen W.

We report on sub-50 fs pulse generation from a passively mode-locked Yb:SrF2 laser pumped with a spatially single-mode, fiber-coupled laser diode at 976 nm. In the continuous-wave regime, the Yb:SrF2 laser generated a maximum output power of 704 mW at 1048 nm with a threshold of 64 mW and a slope efficiency of 77.2%. A continuous wavelength tuning across 89 nm (1006 – 1095 nm) was achieved with a Lyot filter. By implementing a SEmiconductor Saturable Absorber Mirror (SESAM) for initiating and sustaining the mode-locked operation, soliton pulses as short as 49 fs were generated at 1057 nm with an average output power of 117 mW at a pulse repetition rate of ∼75.9 MHz. The maximum average output power of the mode-locked Yb:SrF2 laser was scaled up to 313 mW for slightly longer pulses of 70 fs at 1049.4 nm, corresponding to a peak power of 51.9 kW and an optical efficiency of 34.7%.

Su X., Wang Y., Li G., Su L., Liu S., Zheng L., Zhang B., Zhang B.

In this Letter, using a Yb,Gd:CaSrF2 co-doping mixed crystal, an ultra-low threshold and efficient Kerr-lens mode-locked femtosecond oscillator is realized with a mode-locking threshold as low as 150 mW. With a 200-mW pump power, the shortest pulses are obtained with a pulse duration of 57 fs. A maximum mode-locked output power of 185 mW is observed under a 500-mW pump power, corresponding to an optical-to-optical efficiency of up to 37%. To the best of our knowledge, the 150-mW threshold is the lowest pump power to realize Kerr-lens mode-locking operation in Yb-doped bulk lasers. Furthermore, an optical efficiency of 37% is the highest efficiency in Yb-doped fluoride bulk lasers to date. Our results provide a new basis for high-efficiency and low-threshold Yb-doped ultrafast bulk lasers.

Xue W., Lin Z., Zeng H., Zhang G., Loiko P., Basyrova L., Benayad A., Camy P., Petrov V., Mateos X., Wang L., Chen W.

We report on a continuous-wave (CW) and passively mode-locked operation of a fluorite-type Yb:BaF2 crystal. Pumped with a spatially single-mode, fiber-coupled InGaAs laser diode at 976 nm, the Yb:BaF2 laser generated a maximum CW output power of 512 mW at 1054.4 nm, corresponding to a laser threshold of 36.5 mW and a slope efficiency of 65.0%. A continuous wavelength tuning across 85 nm (1007–1092 nm) was achieved. By implementing a semiconductor saturable absorber mirror for initiating and sustaining the soliton pulse shaping, near Fourier-transform-limited pulses as short as 52 fs were generated at 1058.2 nm with an average output power of 129 mW at a pulse repetition rate of ∼79.5 MHz. To the best of our knowledge, this is the first report on the passively mode-locked operation of the Yb:BaF2 crystal.

Jiang B., Zheng L., Jiang D., Yin H., Zheng J., Yang Q., Cheng G., Su L.

Ytterbium and rare earth ions (RE = Y, Gd, La) codoped CaF 2 -SrF 2 single crystals (3 at% Yb,6 at% RE:CaF 2 -SrF 2 ) were fabricated by temperature gradient technology (TGT). All the space groups remain the same Fm 3 m as that of Yb:CaF 2 -SrF 2 . The lattice parameter a , unit cell volume V , as well as bond length of Ca/Sr–F and F–F increase in the sequence of rare-earth ions radius Y 3+ < Gd 3+ < La 3+ . The segregation coefficients of Yb ions are 0.87 in Yb,La:CaF 2 -SrF 2 and Yb,Gd:CaF 2 -SrF 2 , which are larger than 0.85 in Yb,Y:CaF 2 -SrF 2 and 0.80 in Yb:CaF 2 -SrF 2 . Absorption spectra in the range of 200 and 400 nm were analysed with Yb 2+ contents. The absorption and emission cross-sections in the range of 900–1100 nm were determined together with fluorescence lifetime. The saturation pump density I sat , minimum pump density I min and gain cross-section σ g were analysed. Yb,La:CaF 2 -SrF 2 has a relatively higher optical parameter ( σ em × τ , 0.52 × 10 −20 cm 2 ·ms), lower I sat (3.68 kW/cm 2 ) and I min (0.50 kW/cm 2 ) at 1038 nm indicating the potential application in high power laser. Low phonon energy of CaF 2 -SrF 2 is 302 cm −1 which is located between those of CaF 2 and SrF 2 as measured by Raman spectra. It is believed that ytterbium and rare earth ions (RE = Y 3+ , Gd 3+ , La 3+ ) codoped CaF 2 -SrF 2 eutectic solid-solution is promising for high-power and wavelength-tunable solid-state lasers. The phonon energy in Yb,La:CaF 2 -SrF 2 was measured to be 302 cm −1 which is red-shifted as compared with 286 cm −1 in Yb,La:SrF 2 and blue-shifted as compared with 320 cm −1 in Yb,La:CaF 2 .

Ushakov S.N., Uslamina M.A., Nishchev K.N., Fedorov P.P., Kuznetsov S.V., Osiko V.V.

A concentration series of Ca(1 − X)SrXF2:4 mol % YbF3 crystals with Х varying from 0 to 1 has been grown. The absorption spectra of Yb3+ ions in these crystals are measured at temperatures of 300 and 77 K. It is shown that the intensity ratio of the absorption lines of clusters and tetragonal centers in the congruent crystals is higher than that in the other CaF2–SrF2 solid solution crystals. Analysis of the absorption profiles of Yb3+ optical centers in the concentration series of the solid solution crystals shows that it is energetically favorable for the Sr2+ ion to enter the local environment of both the tetragonal center and the cluster.

Zou Z., He Y., Yu H., Pang S., Wu Y., Liu J., Su L.

3 at.%Yb: Sr1-xGdxF2+x (x = 0,0.01,0.03,0.06,0.09) single crystals are grown by the temperature gradient technique (TGT), where the Gd3+ ions were used as a lattice structural modifier to form a disordered lattice site. The Yb: Sr1-xGdxF2+x crystals maintain cubic structure Fm3m. Specifically, the absorption cross section of 3 at.%Yb: Sr0.97Gd0.03F2.03 crystal was increased by more than 10% compared with that of 3 at.%Yb:SrF2. The emission cross section σem in 3 at.%Yb: Sr0.97Gd0.03F2.03 was 1.21 × 10−20 cm2 at a central wavelength of 976 nm with an enhancement of 11% as compared with that of 3 at.%Yb:SrF2. A direct continuous-wave diode pumped 3 at.%Yb: Sr0.97Gd0.03F2.03 laser could generate an output power of 3.8 W at 1052 nm with an optic-optic efficiency of 46.5% by using output coupler with a transmission T = 3%.

Kitajima S., Yamakado K., Shirakawa A., Ueda K., Ezura Y., Ishizawa H.

Highly transparent ceramic is an attractive gain medium for high-power lasers due to its high fracture toughness, homogeneity, and size scalability. Here we report the first Yb3+-doped CaF2-LaF3 ceramics laser. Codoping of La3+ ion can reduce the formation of Yb2+ ions and enhance the laser efficiency. In the laser experiment, the maximum output power of 4.36 W and the maximum slope efficiency of 69.5% were obtained with a 3% La, 2% Yb sample and a 2% La, 1% Yb sample, respectively. Due to the combined properties of Yb:CaF2 and a ceramic laser gain medium, Yb:CaF2-LaF3 ceramic is a promising gain medium for a high-power ultrashort pulse laser and amplifier.

Wu Y., Su L., Wang Q., Li H., Zhan Y., Jang D., Qian X., Wang C., Zheng L., Chen H., Xu J., Ryba-Romanowski W., Solarz P., Lisiecki R.

The spectroscopic properties of Yb:CaF2–YF3 disordered crystals obtained by the vertical Bridgman method were studied. The 3 at.% Yb, 3 at.% Y:CaF2 crystal absorption cross section σabs centered at 974 nm reached 7.2 × 10−21 cm2 and the fluorescence lifetime was 3.54 ms at room temperature. Experimental results show that co-doping Y3+ ions at different Y:Yb ratios can modulate the spectroscopy and lengthen the luminescence lifetime of Yb3+ ions in the CaF2 lattice. There are three main kinds of lattice in the 3 at.% Yb, 3 at.% Y:CaF2 crystal, as concluded from the low-temperature spectra.

Chernyshev V.A., Nikiforov A.E., Nazemnikh A.D.

The electronic structure and g factors of simple impurity centers and hexamer ytterbium clusters in fluorites have been calculated in terms of the exchange charge model. Preliminarily, their local crystal structures have been calculated in the framework of the shell model with the inclusion of lattice distortions near the impurity. Crystal structure and g factors have been calculated of linear cluster Yb–F–Yb–F oriented along a trigonal axis in BaF2.

Ricaud S., Georges P., Camy P., Doualan J.-., Moncorgé R., Courjaud A., Mottay E., Druon F.

We report what we believe to be the first Yb:SrF2 regenerative femtosecond amplifier. The regenerative amplifier produces 325-fs pulses at 100-Hz repetition rate with an energy before compression of 1.4 mJ. The interest of Yb:SrF2 in such regenerative amplifiers and its complementarity to its well-known isotype Yb:CaF2 is also discussed.

Druon F., Ricaud S., Papadopoulos D.N., Pellegrina A., Camy P., Doualan J.L., Moncorgé R., Courjaud A., Mottay E., Georges P.

We present an overview of laser results we obtained with Yb-doped calcium fluoride and its isotype strontium fluoride. In order to study the laser performance in femtosecond and high power regimes, spectral and thermal properties are first discussed including the potential of these crystals at room and cryogenic temperatures. Experimental demonstrations of high-power and ultrashort pulse oscillators and amplifiers are presented and analyzed.

Mouchovski J.T., Temelkov K.A., Vuchkov N.K.

A comprehensive analysis is implemented concerning the growth, properties, and applications of doped–co-doped single and mixed alkali earth fluoride systems. Calcium–strontium fluoride solid solutions with a Sr content proportion varying widely between 0.007 and 0.675 mol.% are obtained as a batch of axis-symmetrical boules grown by a Bridgman–Stockbarger (BS) method. The crystallization front (CF) can be controlled to retain a convex CF-shape that is favourable for normal growth of single crystals . This achieved using a broad adiabatic furnace zone (AdZ) independently of the boules’ composition. The influence of the thermal field distribution on the CF and the real crystallization rate (CR), which are both critically decisive in controlling crystal quality, were originally assessed using empirically derived formulas. The optical characteristics of the grown boules were monitored by measuring the external transmittance t and calculating the total losses following light irradiation of optical windows that were prepared from sections of the boules that had been cut parallel to one another. The t -measurements were performed by two different techniques and the comparative analysis of the results reliably indicates any inhomogeneity in the grown boules. A simple supercooling criterion proved to closely relate the morphological stability of the CF enabling one to set up the optimum growth conditions. Thus the normal growth criterion outlines the concentration bounds where the isotropic growth mechanism is replaced by cellular anisotropic growth. A procedure has been established for provisioning researchers with optical quality calcium–strontium fluoride crystals with widely varying composition grown under practically identical conditions. As a consequence one can explore possible reasons that can affect the growth mechanism for this or any other systems with a fluoride structure and so provide scope aimed at the future improvement of the crystal quality thereby enlarging the field of mixed fluoride systems’ applications.

Ricaud S., Druon F., Papadopoulos D.N., Camy P., Doualan J., Moncorgé R., Delaigue M., Zaouter Y., Courjaud A., Georges P., Mottay E.

We present a diode-pumped regenerative amplifier based on an Yb:CaF2 crystal optimized to produce short pulses for various repetition rates ranging from 100Hz to 10kHz. The shortest pulse duration generated is 178fs, and the corresponding energy is 1.4mJ before compression (620μJ after), at a repetition rate of 500Hz for 16W of pump power. The bandwidth is 10nm centered at 1040nm. Higher repetition rate regimes have also been explored, allowing an optical-to-optical efficiency up to 10% at a high repetition rate.