Fengkai Ma

Generation of 103 fs mode-locked pulses by a gain linewidth-variable Nd,Y:CaF 2 disordered crystal

We have demonstrated a diode-pumped passively mode-locked femtosecond Nd,Y:CaF2 disordered crystal laser for the first time to our knowledge. By choosing appropriate Y-doping concentration, a broad fluorescence linewidth of 31 nm has been obtained from the gain linewidth-variable Nd,Y:CaF2 crystal. With the Nd,Y:CaF2 disordered crystal as gain medium, the mode-locked laser generated pulses with pulse duration as short as 103 fs, average output power of 89 mW, and repetition rate of 100 MHz. To our best knowledge, this is the shortest pulse generated from Nd-doped crystal lasers so far. The research results show that the Nd,Y:CaF2 disordered crystal will be a potential alternative as gain medium of repetitive chirped pulse amplification for high-peak-power lasers.

Nd,Y:CaF2 laser crystals: novel spectral properties and laser performance from a controlled local structure

A series of Nd,Y:CaF2 single crystals grown using the TGT method were studied systematically by absorption, excitation and luminescence spectroscopy, and the extended X-ray absorption fine structure (EXAFS) technique. The incorporation of Y3+ substantially manipulated the local structure of Nd ions in the CaF2 crystal lattice. Subsequently, the changes in the local structure of the Nd3+ ions induced substantial variations of the photoluminescence parameters, including FWHM, cross sections, lifetimes and quantum efficiency. Preliminary laser experiments were carried out with 0.5% Nd,10% Y:CaF2, 0.8% Nd,5% Y:CaF2 and 0.8% Nd,10% Y:CaF2 crystals under LD pumping; true CW laser operation was achieved with slope efficiencies of 30.1%, 19.3% and 24.9%, respectively.

Pulsed and continuous-wave laser operation of TGT-grown Nd,Y-codoped :SrF2 single crystal

In this letter we present laser properties of temperature gradient technique (TGT) grown Nd,Y : SrF2 crystals with Nd3+ concentrations of 0.4, 0.65 and 0.8 at% and Y3+ concentration of 10 at%. The noncoated crystal samples, 3.1 or 5 mm long, were pumped by the 796 nm laser diode matching the Nd,Y : SrF2 absorption peak. In the pulsed pumping regime (pulse-duration 2 ms, frequency 10 Hz), maximum average output power of 75 mW (corresponding to peak power of 3.75 W) was obtained with slope efficiency as high as 51% and 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 1051 nm appears corresponding to our fluorescence measurements. Wavelength tuning using a birefringent filter from 1048 to 1070 nm is probably given by the crystal-field splitting of the 4F3/2 manifold in Nd3+. True-continuous-wave laser operation was also successfully obtained at lower pumping levels with maximum output power of 380 mW and slope efficiency of 28% at the wavelength of 1057 nm.

Dual-wavelength mode-locked operation on a novel Nd 3+ ,Gd 3+ :SrF 2 crystal laser

We report on efficient continuous-wave (CW) and dual-wavelength mode-locked operations on a novel Nd3+,Gd3+:SrF2 crystal laser for the first time. Codping Gd3+ in the Nd3+:SrF2 crystal significantly increased the fluorescence intensity and aroused and inhomogeneously broadened the emission spectrum. A CW operation with an output power of 992 mW and corresponding slope efficiency as high as 42.7% was obtained. We also obtained dual-wavelength synchronously mode-locked operation, with pulse duration of 1.9 ps. The mode-locked spectrum separated distinctly at 1052.2 nm and 1059.4 nm. Autocorrelation trace clearly showed a 417 fs beating period, and 1.94 THz repetition rate.

Laser performance of diode-pumped Nd, Y-codoped CaF 2–SrF 2 mixed crystal

A disordered Nd, Y-codoped CaF2?SrF2 mixed crystal was obtained by the temperature gradient technique (TGT). The absorption and fluorescence spectra of the crystal were measured at room temperature. Diode-pumped continuous-wave (CW) and Q-switched laser operations were demonstrated at 1056?nm with a 0.65?at.% Nd, 10?at.% Y-codoped crystal, for the first time to our knowledge. The CW output power of 724?mW was obtained in a compact linear cavity. Also the Q-switched pulse characteristics of Nd, Y:CaF2?SrF2 laser crystal were reported based on Cr4+:YAG saturable absorbers in a folded cavity. The shortest pulse width of 110?ns and the highest peak power of 383?W were obtained when the initial transmission of the Cr4+:YAG crystals was 90%. The dependence of the operational parameters on the pump power was also investigated experimentally.

Femtosecond mode-locked Nd,La:CaF 2 disordered crystal laser

A diode-pumped femtosecond mode-locked Nd,La:CaF2 disordered crystal laser was reported for the first time. By appropriately choosing the Nd and La-doping concentration, stable mode-locked femtosecond laser pulses were obtained by using a semiconductor saturable absorber mirror (SESAM). The laser produced 633 fs pulses at the central wavelength of 1061 nm with an average output power of 200 mW at 82 MHz repetition rate.

Spectral properties and highly efficient continuous-wave laser operation in Nd-doped Sr(1-x)Y(x)F(2+x) crystals.

Spectral properties of Nd:Sr(1-x)Y(x)F(2+x) crystals were investigated. Compared with Nd:SrF2, the spectral parameters of Nd:Sr(1-x)Y(x)F(2+x) (x=0.05,0.1) were altered in a large scale. LD-pumped true CW laser has been demonstrated in the crystals. The slope efficiency up to 43.5% in 0.43% Nd:Sr0.95Y0.05F2.05 was achieved. The system is a promising candidate for highly efficient lasers.

Efficient continuous-wave and 739 fs mode-locked laser on a novel Nd3+, La3+ co-doped SrF2 disordered crystal

A novel Nd3+, La3+ co-doped SrF2 disordered crystal was successfully fabricated and its laser performance was investigated for the first time. We obtained a continuous-wave (CW) laser with maximum average output power of 1.234 W at 1061 nm, with the corresponding slope efficiency as high as 50.6%. In mode-locked operation, a pulse as short as 739 fs was also obtained without any measures compensating for the dispersion, which suggests that Nd3+, La3+:SrF2 disordered crystal is a promising candidate to generate high-energy ultra-short pulse laser at 1 µm.

Mode-locking operation of quasi-continuous diode pumped TGT-grown Nd,Y-codoped:SrF2 crystal

Fluoride-type crystals (CaF2, SrF2) doped with neodymium Nd3+ 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:YVO4. In comparison with Nd:glass active material, SrF2 and CaF2 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.

Femtosecond diode-pumped mode-locked neodymium lasers

Fluoride-type crystals (CaF2, SrF2) doped with neodymium Nd3+ 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:YVO4, they have broad emission spectra as well as longer fluorescence lifetime, in comparison with Nd:glass, SrF2 and CaF2 have better thermal conductivity. In spite of the fact, that this thermal conductivity decreases with Nd3+ 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:CaF2 and Nd,Y:SrF2 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.