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We report on the first passively mode-locked femtosecond-laser operation of a disordered Yb:Ca3Gd2(BO3)4 crystal using a SEmiconductor Saturable Absorber Mirror (SESAM). Pumping with a single-transverse mode fiber-coupled laser diode at 976 nm, nearly Fourier-transform-limited pulses as short as 96 fs are generated at 1045 nm with an average output power of 205 mW and a pulse repetition rate of ~67.3 MHz. In the continuous-wave regime, high slope efficiency up to 59.2% and low laser thresholds down to 25 mW are obtained. Continuous wavelength tuning between 1006–1074 nm (a tuning range of 68 nm) is demonstrated. Yb:Ca3Gd2(BO3)4 crystals are promising for the development of ultrafast lasers at ~1 μm.

Yb 3+ (Yb)-doped monoclinic oxoborates Ca 4 REO(BO 3 ) 3 where RE stands for Gd or Y (denoted as GdCOB and YCOB, respectively) feature the advantages of extremely flat and smooth gain profile due to the significant inhomogeneous broadening related to their disordered structure, large ground state splitting of >1000 cm -1 , good thermo-optic properties as well as polarized emission due to the natural birefringence (acentric monoclinic symmetry), which makes them attractive for the development of tunable and sub-100-fs solid-state lasers in the 1-μm spectral region [1] - [3] and self-frequency doubling. In mixed Yb:Ca 4 Gd x Y 1-x O(BO 3 ) 3 (shortly Yb:GdYCOB) crystal with a partial substitution of Y 3+ by Gd 3+ ions is expected to provide additional broadening of the spectral bands due to the compositional disorder [4] , [5] . Here, we report on a Kerr-lens mode-locked (KLM) operation of Yb:GdYCOB laser generating pulses as short as 43 fs with an average power of 84 mW at 1036.5 nm and a pulse repetition rate of ~70 MHz.

We present a sub-50-fs diode-pumped Kerr-lens mode-locked laser employing a novel “mixed” monoclinic Yb:Ca4(Gd,Y)O(BO3)3 (Yb:GdYCOB) crystal as a gain medium. Nearly Fourier-limited pulses as short as 43 fs at 1036.7 nm are generated with an average power of 84 mW corresponding to a pulse repetition rate of ∼70.8 MHz. A higher average power of 300 mW was achieved at the expense of the pulse duration (113 fs) corresponding to an optical-to-optical efficiency of 35.8% representing a record-high value for any Yb-doped borate crystal. Non-phase-matched self-frequency doubling is observed in the mode-locked regime with pronounced strong spectral fringes which originate from two delayed green replicas of the fundamental femtosecond pulses in the time domain.

End-pumped by a 976 nm diode laser, a high-repetition-rate Er:Yb:YAl3(BO3)4 microchip laser passively Q-switched by a Co2+:MgAl2O4 crystal is reported. At a quasi-continuous-wave pump power of 20 W, a 1553 nm passively Q-switched laser with the repetition rate of 544 kHz, pulse duration of 8.3 ns, and pulse energy of 3.9 µJ was obtained. To the best of our knowledge, the 544 kHz is the highest reported value for the 1.5 µm passively Q-switched pulse laser. In the continuous-wave pumping experiment, the maximum repetition rate of 144 kHz with the pulse duration of 8.0 ns and pulse energy of 1.7 µJ was obtained at the incident pump power of 6.3 W.

We demonstrate a comprehensive characterization of the diode-pumped Yb:Bi4Si3O12 laser operating in the continuous-wave and soliton mode-locked regimes. Pumping with a 650 mW, single-transverse mode, fiber-coupled laser diode, a maximum continuous-wave output power amounted to 213 mW with a slope efficiency up to 57.6%. A broadband wavelength tuning range of more than 70 nm was achieved in CW regime with a fused silica prism. Applying a SESAM as mode locker, nearly transform-limited pulses as short as 113 fs were generated for a maximum average power of 53 mW and a pulse repetition rate of ∼106 MHz. To the best of our knowledge, this is the first report on passively mode-locked operation with the Yb:Bi4Si3O12 crystal.

We reported an electro-optically cavity-dumped Q-switched Er:Yb:YAl3(BO3)4 pulse laser for the first time. A 1531.1 nm pulse laser with an average output power of 521 mW, energy of 10 µJ, and a duration of 3.1 ns was achieved at a repetition rate of 100 kHz under the quasi-continuous-wave pumping. The pulse characteristics of the laser were investigated in detail. The result shows that the depolarization effects and the length of high voltage (HV) time applied to the EO-switch had significant impacts on the pulse characteristics.

We present an efficient femtosecond SESAM mode-locked oscillator based on a novel, ytterbium-doped “mixed” calcium oxoborate Ca4Gd0.24Y0.67O(BO3)3 (GdYCOB) crystal. With the usage of a single transverse-mode fiber-coupled diode laser emitting at 976 nm, near Fourier transform limited pulse duration of 69 fs centered at 1049.9 nm was obtained under the pulse repetition rate of 109 MHz.

We report on the first femtosecond pulse generation from a SESAM mode-locked Yb:Bi4Si3O12 (Yb:BSO) laser pumped with a single-mode fiber-coupled diode laser at 976 nm. Near Fourier transform limited 165-fs pulses centered at 1037.8 nm were obtained.

The structure of a new nonlinear optical crystal, Ca9Y0·5La0·5(VO4)7, was solved, for first time to our knowledge. It belongs to trigonal system with space group R3c, its unit-cell parameters are: a = 10·8717(2) A, c = 38·0753(14) A, V = 3897·34(18) A3, Z = 6, dc = 6·3279 g cm−3. The thermal expansion, the special heat capacity and the thermal conductivity of the crystal were investigated in detail. The crystal’s relative intensity for second harmonic generation is about 1·8 times as large as that of KH2PO4. Its transmittance exceeds 70% in the range of 400–2700 nm. From the measured principal refractive indices, the Sellmeier equations were derived. The results show that Ca9Y0·5La0·5(VO4)7 is expected to be a promising new frequency-doubling laser crystal.

A novel monoclinic magnesium tungstate crystal, Tm:MgWO 4 , was grown and characterized in terms of structure, spectroscopy and laser generation. The output power was 772 mW at 2017–2029 nm with a slope efficiency of 39%.

The Cr3+/RbAl(MoO4)2 crystal was grown by top seeding solution growth method, and its thermal and spectral properties were investigated. Based on the absorption and photoluminescence spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ħω and the Huang–Rhys factor S were calculated. The results of spectroscopic characterisation show that the Cr3+ ions occupy medium crystal field sites in Cr3+/RbAl(MoO4)2 and Cr3+/RbAl(MoO4)2 crystals may be a potential candidate for tunable laser applications.

The present paper reports the growth and spectroscopic properties of KYb(WO4)2 crystal. The KYb(WO4)2 crystal with dimensions of 32 × 29 × 12 mm was grown from a mixed flux of K2WO4 and KF by the TSSG method. The spectroscopic properties of KYb(WO4)2 crystals were investigated. The absorption and emission cross-sections of KYb(WO4)2 crystal are 4˙32 × 10–20 cm2 at 981 nm and 3˙69 × 10–20 cm2 at 1024 nm for E//X polarisation respectively. The laser parameters of KYb(WO4)2 crystal were evaluated. These results suggest KYb(WO4)2 crystal as a potential candidate for microchip laser medium.

The Yb3+/Na+:CaMoO4 crystal with dimensions of o20 × 30 mm3 has been grown by Czochralski method from a flux of Na2MoO4. The absorption and emission spectra of Yb3+/Na+:CaMoO4 crystals were investigated. The absorption bands at 934 nm have a FWHM of 59 nm for π polarisation and 65 nm for σ polarisation respectively. The absorption cross-sections σ a are 1˙73 × 10–20 cm2 for π polarisation and 1˙44 × 10–20 cm2 for σ polarisation at 934 nm respectively. The emission bands have a FWHM of 68 nm at 1022 nm for π polarisation and 65 nm at 1023 nm for σ polarisation respectively. The emission cross-sections σ e are 1˙14 × 10–20 cm2 at 1022 nm for π polarisation and 1˙27 × 10–20 cm2 at 1023 nm for σ polarisation respectively. The Yb3+/Na+:CaMoO4 crystal may be considered as a potential laser gain material.

The spectroscopic characteristics of Er3+/Yb3+:LiLa(WO4)2 crystal were investigated. Based on the Judd–Ofelt theory, the following intensity parameters Ωλ were obtained: Ω2=9˙03 × 10–20 cm2, Ω4=2˙02 × 10–20 cm2 and Ω6=0˙59 × 10–20 cm2. The upconversion emission of Er3+/Yb3+:LiLa(WO4)2 crystal was investigated under excitation with the 976 nm diode laser. The Er3+/Yb3+:LiLa(WO4)2 crystal has the intensity parameters Ωλ and a strong emission of 562 nm wavelength. Er3+/Yb3+:LiLa(WO4)2 crystal may be regarded as an upconversion laser material for the green light.

The Tm3+/Yb3+:LiGd(MoO4)2 crystal with dimensions of Φ21 × 45 mm was grown by the Czochralski method. The spectroscopic properties of Tm3+/Yb3+:LiGd(MoO4)2 crystals were investigated. Based on the Judd–Ofelt theory, the oscillator strength parameters Ωt are: Ω2=16·468 × 10−20 cm2, Ω4=3·376 × 10−20 cm2, Ω6=2·613 × 10−20 cm2 for π-polarisation and Ω2=19·3 × 10−20 cm2, Ω4=1·582 × 10−20 cm2, Ω6=2·397 × 10−20 cm2 for σ-polarisation. The absorption cross-section σab are 4·17 × 10−20 cm2 at 796 nm and 5·31 × 10−20 cm2 at 797 nm for π- and σ-polarisation respectively. The emission cross-section σem are 1·72 × 10−20 cm2 at 1606 nm for π-polarisation and 2·00 × 10−20 cm2 at 1609 nm for σ-polarisation respectively. The Tm3+/Yb3+:LiGd(MoO4)2 crystals may be regarded as a potential infrared laser crystal.

Aγ–Yb3+ : LaSc3(BO3)4 with a dimension of 30 × 20 × 10 mm has been grown from a mixed flux of Li6B4O9 and LiF by the top seeded solution growth method. The spectral properties have been investigated. The absorption and emission cross-sections of γ–Yb3+ : LaSc3(BO3)4 crystals are 1˙74 × 10−20 cm2 at 972˙3 nm and 8˙03 × 10−21 cm2 at 1026 nm respectively. The radiative lifetime and fluorescence lifetime of γ–Yb3+ : LaSc3(BO3)4 crystals are 0˙8 and 1˙658 ms respectively. The β min, l sat and l min are 9˙8%, 0˙7 kW cm−2 and 7˙1 kW cm−2 respectively. To sum up the spectral properties of γ–Yb3+ : LaSc3(BO3)4 crystal, it was suggested as a potential laser gain material.

A Yb3+:Y0·8La0·2Ca4O(BO3)3 crystal with the dimensions of o16 × 20 mm has been grown by the Czochralski method. The absorption and emission cross-sections of the Yb3+:Y0·8La0·2Ca4O(BO3)3 crystals are 1·37 × 10−20 cm2 at 977 nm and 0·64 × 10−20 cm2 at 1033 nm respectively, which are larger than that of Yb3+:YCa4O(BO3)4 and Yb3+:GdCa4O(BO3)4 crystals. It shows that the spectroscopic properties of Yb3+:Y0·8La0·2Ca4O(BO3)4 crystal can be improved by substituting the part La3+ ions for the Y3+ ions in YCa4O(BO3)4 to form mixed YxLa1−xCa4O(BO3)3.

This paper reports on the spectral parameters of Er3+ in Er3+/ Yb3+: Ca3Y2(BO3)(4), Er3+/ Yb3+: Ca3Gd2(BO3)(4) and Er3+/ Yb3+: Ca3La2(BO3)(4) crystals. The results show that the oscillator strength parameters Omega(t), fluorescence quantum efficiency and emission cross-sections of Er3+/ Yb3+: Ca(3)Ln(2)(BO3)(4) ( Ln=Y, Gd, La) crystals are larger than those of Er3+: Ca(3)Ln(2)(BO3)(4) (Ln=Y, Gd, La) crystals, and the codoped Er3+/ Yb3+ can enhance the optical transition probability of Er 3z in the crystals and improve the fluorescence quantum efficiency and emission cross section of the (4)/(13/2) -> (4)/(15/2) transition of Er3+ in Er3+/ Yb3+: Ca(3)Ln(2)(BO3)(4) ( Ln=Y, Gd, La) crystals.

A Nd3+/Yb3+ : Sr3Gd2(BO3)(4) crystal 40 mm in diameter and 15 mm in height was grown by the Czochralski method. The spectroscopic characterization of Nd3+/Yb3+:Sr3Gd2(BO3)(4) crystal was investigated. The emission crosssection of Yb3+ in the Nd3+/Yb3(+):Sr3Gd2(BO3)(4) crystal is 1.57 x 10(-20) cm(2) at 1014 nm, and the radiation lifetime tau(rad) of F-2(5/2) of Yb3+ is 881.6 mu s. The result shows that the energy transfer occurs from F-4(3/2) of Nd3+ to F-2(5/2) of Yb3+ in the Nd3+/Yb3+:Sr3Gd2(BO3)(4) crystal, which should enhance the laser efficiency of Yb3+ in the Nd3+/Yb3+:Sr3Gd2(BO3)(4) crystal.

A Yb3+ :Sr2La0·667(VO4)2 crystal with dimensions 18 × 15 × 30 mm3 has been grown by the Czochralski method. The absorption cross-section is 1·45×10−20 cm2 at 976 nm wavelength. The fluorescence spectrum exhibits a broad emission band from 925 nm up to 1125 nm and the emission cross-section is 0·64 × 10−20 cm2 at 1029 nm. The radiative and fluorescence lifetimes are 0·66 and 1·13 ms, respectively. In comparison with other Yb3+-doped materials, the value of I min of the Yb3+ :Sr2La0·667(VO4)2 crystal is lower, which implies that the Yb3+ :Sr2La0·667(VO4)2 crystal may possess a lower laser threshold.