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We report for the first time on laser action of a diode-pumped Yb:BaY2F8 crystal. Both CW and femtosecond operations have been demonstrated at room-temperature conditions. A maximum output power of 0.56 W, a slope efficiency of 34%, and a tunability range from 1013 to 1067 nm have been obtained in CW regime. Transform-limited pulse trains with a minimum duration of 275 fs, an average power of 40 mW, and a repetition rate of 83 MHz have been achieved in a passive mode-locked regime using a semiconductor saturable absorber mirror.
©2010 Optical Society of America
1. Introduction
Novel solid-state lasers in the wavelength range around 1 μm are currently under investigation due to a large number of significant applications, such as remote sensing, precision optical measurements, and frequency metrology. In this spectral range Yb-doped materials provide efficient, tunable, and ultrafast high-power laser sources because of their broad absorption and emission bandwidths and low thermal loading [1, 2, 3]. For these reasons the current interest in developing novel Yb-doped materials is still growing.
In this paper, we report on the laser action of a diode-pumped BaY2F8 (BYF) crystal doped with Yb3+ ions (nominal doping level 5 at.%). The scope of this work is to analyze the 1-μm lasing properties of Yb:BYF, which, to the best of the authors’ knowledge, have not been experimentally investigated yet. Like other fluoride hosts, the BYF crystal has favorable thermooptical properties (low thermal induced lensing and birefringence), a low phonon energy useful for operation in the IR region, and a wide and intense emission cross section [4]. Both CW and femtosecond operations have been investigated at room temperature conditions. In CW regime a maximum output power of 0.56 W, a slope efficiency of 34%, and a tunability range from 1013 to 1067 nm have been obtained. This performance is comparable to the results obtained with Yb:LiYF4, the most commonly used fluoride laser crystal [5, 6]. Up to now, slope efficiencies of ~80% have been demonstrated in cw regime using Yb-doped sesquioxides (Yb:Y2O3 [7], and Yb:Lu2O3 [8]), and tungstates (Yb:KY(WO4)2 and Yb:KGd(WO4)2 [9]). Under passive mode-locking operation, 47-fs pulse duration has been reported in a disordered Yb:CaGdAlO4 crystal with average output power of 40 mW at 3-W incident pump power [10]. Higher efficiencies were demonstrated in Yb:KGd(WO4)2 crystal, with maximum average output power of 1.1-W at 4-W incident pump power and a pulse duration of 176 fs [11]. Sub-300-fs pulse trains with an average output power of 40 mW and a repetition rate of 83 MHz have been obtained with the Yb:BYF laser operated in a passive mode-locking regime using a semiconductor saturable absorber mirror. The minimum pulse width which has been obtained (275 fs) is slightly longer compared with the mode-locking operation of other fluoride hosts such as Yb:CaF2 (150 fs, 880 mW at 15 W pump power) [12], Yb:YLF (190 fs, 54 mW at 3.9 W pump power) [13], and Yb:KYF4 (170 fs, 130 mW at 1.6 W pump power) [14].
2. Growth process and spectroscopic characterization of Yb:BYF crystal
BaY2F8 crystal has a monoclinic structure and a symmetry group C2/m. The unit-cell parameters are a = 6.935 Å, b = 10.457 Å, and c = 4.243 Å, with an angle between a axis and c axis of 99.7°, and the unit cell contains two molecules. The rare-earth dopant substitutionally enters the Y3+ sites [15].
The crystal is grown in a home-made computer-controlled resistive Czochralski furnace (see Ref [4] for further details). The Yb:BYF nominal Yb doping level is 5 at.% and is referred to the melt composition (the effective segregation coefficient of Yb in fluorides is ~1 [16]). The crystal was oriented by X-ray backscattering Laue technique and was properly cut in the following samples: 4.4 mm×4.5 mm×3.7 mm (width, height, thickness) with the edges parallel to x,y,z principal axes [17] for spectroscopy measurements and 6.6 mm×4.3 mm×4.4 mm Brewster cut for laser experiments, to exploit the E∥y and H∥x emission.
BYF is a biaxial crystal and the magnetic contribution to transitions between Yb3+ energy levels in not negligible. For this reason six different spectra are required to fully characterize the sample [17, 18]. The measured room-temperature Yb 2 F5/2 lifetime was 2.4 ± 0.3 ms.
Figure 1(a) and Fig. 1(b) show the polarized absorption coefficient and emission cross section, respectively. This figure puts in evidence the strong anisotropy of the sample with respect to the crystallographic axes of the crystal. The strongest absorption peak, centered at 959.9 nm, provides an absorption coefficient of 6.3 cm−1 (corresponding to σa,peak = 9.8 × 10−21 cm2) for E∥x and H∥y polarization. This peak appears to be particularly suitable for diode pumping, due to a relatively wide bandwidth of ~ 10 nm (full width at half maximum). The emission spectra consist of several broad peaks located in the wavelength region from 950 to 1050 nm. In particular, the maximum peak value is σa,peak = 6.3 × 10−21 cm2 at 1016.5 nm for E∥y and H∥x polarization. It should be noted the extremely wide fluorescence bandwidth, 56 nm, which has the potential for generating sub-100 fs laser pulses. Table 1 summarizes the main spectroscopic properties of different Yb-doped crystals.
3. Diode-pumped CW laser operation
Figure 2 shows the scheme of the optical resonator and pump system used for CW laser experiments. The laser cavity is based on two high-reflectivity (HR) mirrors and a plane output coupler (output couplings 1%, 2.5%, and 4.5%). The plane input mirror has a special coating design (dichroic) with high reflectance, R > 99.9%, in the wavelength range from 1020 to 1100 nm and high transmittance, T > 80%, for wavelengths shorter than 980 nm. The curved
Fig. 1. Room-temperature polarized (a) absorption coefficient and (b) emission cross section of Yb(5%):BYF crystal (calculated by the β-τ [19] method).
Table 1. Main spectroscopic properties of different Yb-doped crystals.
folding mirror, with a radius of curvature of 75-mm, has a high-reflectance coating (R > 99.9%) in the spectral interval 1010-1100 nm. The Yb:BYF crystal, mounted on a copper support without any temperature control, is placed at the Brewster angle close to the input mirror and is longitudinally pumped by a fiber-coupled diode with a maximum output power of 5 W at the emission wavelength of ~960 nm. The pump beam at the output of the 100 μm core diameter multimode fiber (0.22 NA) is collimated and then focused into the active crystal by using a pair of antireflection-coated plano-convex lenses with focal lengths of f1=38 mm and f2=50 mm. The slight astigmatism introduced by the Brewster interfaces is partially compensated by folding the cavity by an angle of ~ 10°. The laser is linearly polarized along the TM direction with an electromagnetic field orientation inside the laser crystal E∥y and H∥x. Wavelength tuning of the laser emission is achieved by an intracavity birefringent filter, based on a 2-mm thick quartz plate with the optical axis lying on the crystal plane. The birefringent filter, inserted at Brewster angle in the longest arm of the resonator, has a free-spectral range of ~65 nm in the 1-μm spectral region.
Figure 3(a) shows the single transverse mode TEM00 output power as a function of the incident pump power for three different values of the output coupling. A maximum output power of 0.56 W and a slope efficiency of 34% have been obtained for an incident pump power of 4.1 W (maximum available power after the pump optics and dichroic input mirror) and an output coupling of 2.5%. If the absorbed pump power is considered (pump absorption ~85%), the slope efficiency rises to 40%. Slightly lower output power was obtained with 1% and 4.5% output couplers, indicating that the optimum output coupling for the available pump power is close to 2.5%. Without any intracavity wavelength selector the laser oscillates on many longitudinal modes (typical five longitudinal modes spaced by 0.05 nm) centered at around 1050 nm, 1034 nm, and 1032 nm, for increasing values of the output coupling.
Fig. 2. Scheme of the CW diode-pumped Yb:BYF laser cavity. LD1: laser diode; f1 and f2: plano-convex lenses; M1: HR plane mirror; M2: HR curved folding mirror; BF: birefringent filter; OC: output coupler.
Fig. 3. (a) Output power versus incident pump power (b) output power versus emission wavelength of Yb:BYF laser for different output couplings.
Fig. 4. Scheme of the mode-locked Yb:BYF laser cavity and pump system. LD1, LD2: laser diodes; f1, f2, f3, f4: plano-convex lenses; M1, M2: folding mirrors; M3, focusing mirror; M4, M5: chirped mirrors; OC: output coupler; SESAM: semiconductor saturable-absorber mirror.
The laser tunability curve was measured for an incident pump power of ~4 W. Figure 3(b) shows the laser output power versus emission wavelength. The widest tunability range of 54 nm, from 1013 to 1067 nm, was obtained with 1% output coupling. No laser action is observed at emission wavelengths shorter than ~1010 nm, due to the limited bandwidth of the dichroic mirrors. Reduction of the wavelength tuning range was observed by increasing the output coupling, in accordance with the quasi-three level nature of the Yb:BYF laser.
4. Diode-pumped passive mode-locking operation
The scheme of the resonator and the pump system adopted for femtosecond pulse generation is shown in Fig. 4. Two different pump sources are used: LD1 is the same laser diode used for the CW laser experiments, whereas the laser diode LD2 has a maximum power of 2.8 W at 968 nm at the output of an optical fiber with a core diameter of 50 μm and a NA=0.22. The pump beams coming from LD1 and LD2 are first collimated and then focused onto the active crystal by two pairs of antireflection-coated plano-convex lenses, specifically f1=f3=38 mm and f2=f4=50 mm. The calculated spot size radii inside the active crystal, taking into account the defocalization due to the curved cavity mirrors M1 and M2, are ~166 μm and ~83 μm for the beams from LD1 and LD2, respectively. The Yb:BYF crystal is placed inside the resonator between the curved high-reflectivity (HR, reflectivity¿99.9%) folding mirrors M1 and M2 with 75-mm radius of curvature (ROC). Also in this case the crystal, mounted in a copper holder, is oriented at Brewster angle to minimize Fresnel losses and to exploit the electromagnetic field orientation E∥y and H∥x. The mode radius inside the gain medium, calculated by ABCD matrix formalism, is 21 μm×29 μm. The folding angles provided by mirrors M1 and M2 are set to ~ 11° close to the optimum astigmatism compensation condition. To start passive mode-locking we used a semiconductor saturable absorber mirror (SESAM; Batop, Germany) designed for operation at 1040 nm, with 10-ps relaxation time constant, 1% saturable absorption and saturation fluence of 90 μJ cm−2. The cavity mode is focused onto the SESAM with a waist of ~50 μm by the M3 curved mirror (100-mm ROC). Two HR chirped mirrors M4 and M5 were used for dispersion compensation (optimum group delay dispersion ≃ – 2600 fs2 per round-trip) in the resonator arm containing the output coupler.
Self-starting cw soliton mode-locking operation was achieved using output couplings of 1% and 2.5%, with a pulse repetition rate of 83 MHz. Soliton mode-locking was identified as the pulse generation mechanism, owing to the pulse duration which increases linearly with the negative intracavity dispersion and the appearance of transform-limited, self-starting pulses ~ 35 times shorter than SESAM relaxation time. Figure 5 shows the average output power versus incident pump power of the mode-locked Yb:BYF laser. A small jump of the output power is observed in correspondence to the transition from Q-switched mode-locking to cw mode-locking regime, due to saturation of the SESAM losses. Above the transition point cw mode-locking process was self starting. The calculated pulse fluence incident on the SESAM corresponding to mode-locking threshold is ~1050 μJ cm−2 (~10 times the SESAM saturation fluence). The maximum average output power was 48 mW at 6.4-W incident pump power using the 2.5% output coupler, and the pulse duration was 643 fs with a bandwidth of 1.8 nm centered at ~ 1025 nm. Changing the output coupling to 1%, the spectral bandwidth of the pulse increased to 3.8 nm (centered at ~1028 nm), and shorter pulse durations of 275 fs were observed with a maximum average power of 40 mW. In this case the incident pump power was limited to 6.1 W owing to the tendency for multiple-pulse generation observed at higher pump rates. The reduction of the average output power in mode-locking operation as compared to cw regime is due to the following main effects: i) increase of the intracavity losses due to the presence of SESAM (2%) and of three additional cavity mirrors (0.5%); ii) different resonator and pump configuration leading to a lower overlapping between the laser mode and the pump beam areas in the active crystal (a factor ~5 lower); iii) increase of the crystal temperature, due to lower extraction efficiency, leading to additional reabsorption losses. The autocorrelation traces and emission spectra of the 275-fs pulse train are reported in Fig. 6. The intensity autocorrelation is well fitted assuming a sech2-pulse shape, yielding to estimation of the time-bandwidth product of 0.33, close to the Fourier-transform limit of 0.32.
Fig. 5. Average output power versus incident pump power of the mode-locked Yb:BYF laser with output couplings of 1% (circles), and 2.5% (squares). Mode-locking thresholds are indicated by vertical dashed lines.
Fig. 6. (a) Autocorrelation trace and (b) optical spectrum of the mode-locked Yb:BYF laser with 1% output coupler. The dotted red lines represent fits assuming sech2 pulse shape.
5. Conclusion
CW laser action and passive mode-locking of a diode-pumped Yb-doped BaY2F8 fluoride crystal was demonstrated. The CW laser has a maximum output power of 0.56 W with a slope efficiency of 40% relative to the absorbed pump power, and a wide tuning range of 54 nm. In the passive mode-locking regime stable Fourier-transform limited pulse trains were obtained, with minimum pulse duration of 275 fs, 3.8 nm bandwidth centered at 1028 nm, 40 mW average output power and 83 MHz repetition rate. Improvement of the laser performance is expected from optimization of the Yb doping level and length of the BaY2F8 crystal, which has not been carried out in these preliminary laser experiments. Moreover, a pump source with higher brightness (i.e. lower beam quality factor M2) should also improve mode-locking performance in terms of pulse duration and slope efficiency.
Acknowledgements
The authors acknowledge the technical assistance of Eugenio Rizzo for laser experiments and of Ilaria Grassini for crystal preparation.
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