An efficient soliton mode-locking femtosecond Yb3 + : Y2SiO5 (Yb:YSO) laser at 1059 nm assisted with semiconductor saturable absorber mirror has been successfully realized and demonstrated. Nearly Fourier transform-limited pulse duration as short as 236 fs has been achieved with a pulse repetition frequency of 79.6 MHz and an average output power of 903 mW, corresponding to maximum pulse energy of 11.34 nJ and pulse peak power of 48 kW, respectively. The dependence of pulse duration on the intracavity pulse energy in soliton mode-locking regime has been further investigated and discussed, which is fitted by the soliton mode-locking theory and indicates the nonlinear refractive index n2 of the Yb:YSO crystal to be around 2.1 to 4.8 × 10 − 16 cm2 / W.
Nd:Ca<sub>1-x</sub>Y<sub>x</sub>F<sub>2+x</sub>(x=0%,2%,5%,10%) crystals were grown by Temperature gradient technique. The emission spectrum and absorption spectrum were tested. The emission cross section, emission bandwidth and absorption cross section were calculated. The influence of codoping ions on the spectral performance was compared and analyzed. These crystals have better spectral parameters than Nd:CaF<sub>2</sub> crystals. By increasing the concentration of Y<sup>3+</sup>, spectral properties could be optimized. Among these crystals, the crystal with x=10% has the longest fluorescence lifetime (283.5 μs). The crystal with x=5% has the largest emission cross section(2.90× 10<sup>-20</sup> cm<sup>2</sup>) and the largest absorption cross section(2.18× 10<sup>-20</sup> cm<sup>2</sup>).
The effect of doping concentration of Nd<sup>3+</sup> and co-doping Y<sup>3+</sup> on the spectroscopic properties are investigated systematically. Due to the particular clustering effect, the quench effect was demonstrated in lightly doped Nd<sub>x</sub>Gd<sub>0.03</sub>Sr<sub>0.97-x</sub>F<sub>2.03+x</sub> (x=0.0005,0.0015,0.0065,0.01) crystals. For a 3% Gd:SrF<sub>2</sub> crystal, the fluorescence lifetime at 1054 nm decrease from 380.9 to 159.8 μs by doping Nd<sup>3+</sup> from 0.15 at.% to 1 %at.%, while the emission cross section decreases to 4.12 × 10<sup>−20</sup>cm<sup>2</sup> at 1054 nm. However, the absorb cross section were increased when the concentration of Nd3+ increase from 0.5 % to 0.65 %. Thus, there is an optimum doping concentration of Nd<sup>3+</sup>. According to the research, the optimum doping concentration of Nd<sup>3+</sup> is 0.15 % in 3% Gd:SrF<sub>2</sub> crystals.
The spectroscopic properties of the 1.0 at.%Pr:Ca<sub>0.97</sub>R<sub>0.02</sub>F<sub>2.03</sub>(R=Y, La, Gd) crystals are investigated. X-diffraction and room temperature absorption spectra have been registered and analyzed. The emission spectra and decay curves of the crystals were obtained at room temperature. The photoluminescence intensity in the visible region is significantly enhanced by co-doping R<sup>3+</sup> ions in Pr:CaF<sub>2</sub> crystal. The different effects among the R<sup>3+</sup> (Y<sup>3+</sup>, La<sup>3+</sup> and Gd<sup>3+</sup>) regulating ions on the crystals were observed and compared. Pr:Ca<sub>0.97</sub>La<sub>0.02</sub>F<sub>2.03</sub> and Pr:Ca<sub>0.97</sub>Y<sub>0.02</sub>F<sub>2.03</sub> crystals have substantially strong emission at blue and orange region, while the Pr:Ca<sub>0.97</sub>Gd<sub>0.02</sub>F<sub>2.03</sub> crystal is more suitable for the red emission emitting.
Laser radiation in the wavelength range around 2 μm is required for its specific properties - it is very suitable for medical
applications, remote sensing, or pumping of optical parametric oscillators to generate ultrafast pulses in the mid-IR
region further exploited in nonlinear optics. Crystals as YLF, YAG, LLF, and GdVO4 doped by holmium were already
investigated and found suitable for the tunable laser generation around 2.1 mμ. Only a few works are devoted to the laser
operation of holmium-doped fluorides as CaF<sub>2</sub>. In this work, pulsed and continuous-wave laser operation of a modified-
Bridgman-grown Ho:CaF<sub>2</sub> active crystal at room temperature is reported. A commercial 50 W 1940 nm Tm-fiber laser
was used to pump a laser oscillator based on a novel 10 mm long 0.5 at.% Ho:CaF2 active crystal placed in the Peltiercooled
holder. In the pulsed regime (10 ms, 10 Hz), the laser slope efficiency of 53 % with respect to the absorbed pump
power was achieved. The laser generated at the central wavelength of ~2085 nm with the maximum mean output power
of 365 mW corresponding to the power amplitude of 3.65 W. In the continuous wave regime, the maximum output
power was 1.11 W with the slope efficiency of 41 % with respect to the absorbed pump power. To our best knowledge
this is the first demonstration of this laser active material operating in the CW regime at room temperature. The tuning
range over 60 nm from 2034 to 2094 nm was achieved using a birefringent filter showing the possibility to develop
a mode-locked laser system generating pulses in the sub-picosecond range.
Fluoride-type crystals (CaF<sub>2</sub>, SrF<sub>2</sub>) doped with neodymium Nd<sup>3+</sup> and codoped with buffer ions for breaking clusters of active ions and increasing fluorescence efficiency, present interesting alternative as laser active media for the diode-pumped mode-locked lasers. In comparison with widely used materials as Nd:YAG or Nd:YVO<sub>4</sub>, they have broad emission spectra as well as longer fluorescence lifetime, in comparison with Nd:glass, SrF<sub>2</sub> and CaF<sub>2</sub> have better thermal conductivity. In spite of the fact, that this thermal conductivity decreases with Nd<sup>3+</sup> doping concentration, these crystals are alternative for the Nd:glass in subpicosecond mode-locked laser systems. In this paper we review the basic results reported recently on these active materials and in the second part we present our results achieved in low power diode pumped passively mode locked lasers with Nd,La:CaF<sub>2</sub> and Nd,Y:SrF<sub>2</sub> crystals. The pulses as short as 258 fs at wavelength of 1057 nm were obtained in the first case, while 5 ps long pulses at 1065 nm were generated from the second laser system.
A polycrystalline ceramic based on an Nd,Y:SrF2 single crystal was successfully fabricated and its laser performance was experimentally investigated. We obtained dual-wavelength continuous-wave operation at the wavelengths of 1050.5 and 1058.0 nm. The maximum output power and slope efficiency were 750 mW and 31.5%, respectively. In the passively Q-switched operation, the shortest pulse with a 169-ns duration was also obtained, and the corresponding maximum repetition rate and single pulse energy were 7.3 kHz and 19.2 μJ, respectively.
The uniform-quality, large-area, monolayer graphene saturable absorber (SA) with sandwich structure was fabricated, tested, and successfully applied for the generation of diode-pumped Yb:Y2SiO5 mode-locked laser. Without extra negative dispersion elements, the shortest pulse with duration of ∼883 fs was obtained at 1042.6 nm with an output power of ∼1 W. These promising experimental results suggested that the low-cost, high-quality graphene SA could potentially be employed in practical, high-power, ultrafast mode-locking laser systems.
Fluoride-type crystals (CaF<sub>2</sub>, SrF<sub>2</sub>) doped with neodymium Nd<sup>3+</sup> present interesting alternative as a laser active media for the diode-pumped mode-locked laser systems mainly because of their broad emission spectra as well as longer fluorescence lifetime in comparison with well-known materials as Nd:YAG or Nd:YVO<sub>4</sub>. In comparison with Nd:glass active material, SrF<sub>2</sub> and CaF<sub>2</sub> have better thermal conductivity. In spite of the thermal conductivity decreases with doping concentration, these crystal might be interesting alternative for the Nd:glass mode-locked laser systems. In this contribution we present the first results of the Nd,Y:SrF2 mode-locked laser diode-pumped at 796nm. Mode-locking operation using SESAM was successfully achieved in the pulsed pumping regime (pulse-duration 1.5 ms, frequency 10 Hz) with the overall average output power of 2.3 mW (corresponding to the power amplitude of 153 mW) in one output beam at the wavelength of ~1055 nm. The actual pulse-duration was 87 ps.
A reflective graphene saturable absorber mirror (SAM) was successfully fabricated by chemical vapor deposition technology. A stable diode-pumped passively mode-locked Yb3+:Sc2SiO5 laser using a graphene SAM as a saturable absorber was accomplished for the first time. The measured average output power amounts to 351 mW under the absorbed pump power of 12.5 W. Without prisms compensating for dispersion, the minimum pulse duration of 7 ps with a repetition rate of 97 MHz has been obtained at the central wavelength of 1063 nm. The corresponding peak power and the maximum pulse energy were 516 W and 3.6 nJ, respectively.
In this contribution we present spectroscopic and laser properties of TGT (temperature gradient technique) grown
Nd,Y:SrF<sub>2</sub> crystals with neodymium concentration of 0.4, 0.65 and 0.8 at.%. The absorption cross-section, fluorescence spectra and fluorescence decay time were measured. For the laser experiments, the noncoated crystal samples 3.5 or 5 mm thick were pumped by a 796 nm laser diode matching the Nd:SrF2 absorption peak. Several output couplers with
reflectivity ranging from 70 to 98 % at the generated wavelength were tested. In the pulsed pumping regime (pulseduration
2 ms, frequency 10 Hz), the maximum average output power of 75 mW was obtained with the slope efficiency
as high as 48 % and the optical-to-optical efficiency of 42 % with respect to the absorbed pump power. The output beam
spatial profile was nearly Gaussian in both axes, oscillations started at the wavelength of 1057 nm. At higher pumping
levels, the second emission line at 1050 nm appears corresponding to our fluorescence measurements. Wavelength
tuning using birefringent filter from 1048 to 1070 nm is probably given by crystal-field splitting of the <sup>4</sup>F<sub>3/2</sub> manifold in Nd<sup>3+</sup>. True-CW laser operation was also successfully obtained at lower pumping level with the maximum output power of 90 mW using output coupler reflectivity of 98 %.
Efficient diode-pumped Yb:LuY2SiO5 laser mode locked by single walled carbon nanotube saturable absorber is reported for the first time. Under the absorbed pump power of 12.83 W, continuous wave mode-locked pulses were generated with the maximum average output power of 1.50 W. We obtained pulses as short as 5.2 ps around a center wavelength of 1058.6 nm. The peak power and the single pulse energy of the mode-locked laser were up to 2.9 kW and 15.0 nJ, respectively.
We reports on a diode-pumped passively mode-locked Yb:SSO laser with a SESAM. Pulses
duration as short as ~2 ps with a repetition rate of 53 MHz were generated. The output power
achieved ~1.9 W at a pump power of 11.5 W.
Diode-pumped soliton and non-soliton mode-locked Yb:(Gd<sub>1-x</sub>Y<sub>x</sub>) <sub>2</sub>SiO<sub>5</sub> (x=0.5) lasers have been demonstrated together
for the first time to the author's knowledge. For the non-soliton mode locking, output power could achieve ~1.2 W, and
pulse width was about 20ps. For the soliton mode-locked operation, the pulse width was 1.4ps at the wavelength of
1056nm and 375fs at the wavelength of 1042nm, with a pair of SF10 prisms as the negative dispersion elements. The
repetition rate was 48 MHz. The critical pulse energy in the soliton-mode locked operation against the Q-switched mode
locking was much lower than the value in non-soliton mode-locked operation.
A new alloyed crystal, Yb:LYSO, has been grown by the Czochralski method in our institute for the first time, and its
effective diode-pumped cw tunable laser action was demonstrated. The alloyed crystal retains excellent laser properties
of LSO with reduced growth cost, as well as the favorable growth properties of YSO. With a 5-at.% Yb:LYSO sample,
we achieved 2.84 W output power at 1085 nm and a slope efficiency of 63.5%. And its laser wavelength could be tuned
over a range broader than 80nm, from 1030nm to 1111 nm. This is the broadest tunable range achieved for Yb:LYSO
laser, as far as we know.