1. Introduction

Yb lasers have been receiving increasing attention in the last 15 years [1,2]. The very simple energy-level scheme of Yb³⁺ minimizes parasitic spectroscopic processes. The low quantum defect as a result of the close pump and laser wavelengths reduces the thermal load if compared to Nd³⁺. The capability of being efficiently pumped with laser diodes around 980 nm is an essential advantage of Yb-lasers. Depending on the host, Yb-doped materials exhibit broad absorption and emission bands. This is advantageous for the development of diode-pumped mode-locked laser sources. Compact Yb-based femtosecond laser sources emitting in the 1-μm spectral range have been developed for a great number of hosts.

In LaSc₃(BO₃)₄, hereafter LSB, the Yb³⁺-ion can substitute the Sc³⁺-ion in its two different sites as well as the La³⁺-ion in its unique site, resulting in important inhomogeneous broadening and consequently a very broad and smooth emission spectrum, particularly suitable for sub-100 fs pulse generation [3]. Recently, continuous-wave (CW) laser operation of Yb-doped LSB crystals was studied. Initially, 0.5 W of output power were obtained using a 10 at. % (with respect to La) Yb:LSB crystal under Ti:sapphire laser pumping reaching a slope efficiency of 64% with respect to the absorbed power [4]. Diode-pumping in a thin-disk (200–300 μm) configuration employing 10 and 25 at. % Yb-doped LSB produced up to 40 W of output power [5]. In this setup, a tuning range extending from 991 nm to 1085 nm was obtained for π-polarization using an intracavity birefringent filter. The large and smooth tunability observed confirms the high potential of Yb:LSB crystals for ultrashort pulse generation.

In the last years, mode-locked laser operation of Yb³⁺ based on several borate crystals was reported which illustrates their high potential for short pulse generation. Pulse durations of 198 and 90 fs were obtained with the non-centrosymmetric Yb:YAl₃(BO₃)₄ (Yb:YAB) and Yb:Ca₄GdO(BO₃)₃ (Yb:GdCOB) crystals, respectively [6,7]. Using a Yb:Li₆Y(BO₃)₃ (Yb:LYB) crystal, pulse duration of 355 fs was achieved [8] while pulses as short as 69 fs were obtained with Yb:Sr₃Y(BO₃)₃ (Yb:BOYS) [9]. Here we report, what we believe to be the first demonstration of a mode-locked laser based on Yb:LSB.

2. Spectroscopic properties

LSB is related to YAB, belonging to a more general class of borate compounds with the chemical formula LnM₃(BO₃)₄ where Ln is lanthanide or Y, and M=Al, Ga, Sc [10]. However, while YAB has trigonal huntite structure with symmetry R32, LSB crystallizes in the centrosymmetric monoclinic structure C2/c. Moreover, for ytterbium doping levels above 30 at. % the structure of Yb:LSB changes to R32 [10]. For monoclinic LSB crystals which are optically negative, it was found that the two larger refractive indices (n_x and n_y) almost coincide and can be substituted by n_o; the principal optical axis y coincides with the monoclinic b-axis, while the z-axis corresponds to a three-fold pseudosymmetry axis (denoted as crystallographic c-axis in analogy with the R32 symmetry) and the extraordinary index n_e<n_o [10]. In accordance with this, the spectroscopic properties of Yb³⁺ for polarization parallel to the x- and y(≡b)-axes were found to differ by less than 10% and it was suggested that for simplification the crystal could be considered as pseudo-trigonal [10]. The polarized absorption and emission spectra for the σ (E⊥c) and π (E∥c) polarizations are quite different [5]. The σ-absorption is characterized by an intense and broad peak around 981 nm with a cross-section of ~9.8×10^-21 cm². The emission band of the π-polarization is very broad with a maximum cross section of σ_em~2.8×10^-21 cm² at 1041 nm. From the emission cross sections for Yb:LSB [5], one can expect wider tunability for π-polarization, which was also demonstrated. In order to estimate the potential gain bandwidth for mode-locked operation for both, σ- and π-polarization, the gain cross section σ_gain=βσ_e-(1-β)σ_a for different population inversion rates β (ratio of the excited ion density to the total Yb-ion density) was calculated and can be seen in Fig. 1. For realistic inversion levels of around 0.1, the gain cross sections are similar and rather flat for both polarizations; consequently, no favorable orientation for mode-locked operation could be deduced from these plots.

 

Fig. 1. Gain cross section σ_gain for σ- (left) and π- (right) polarization of Yb:LSB and different population inversion rates β.

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3. Experimental setup

High quality Yb:LSB crystals were grown by the Czochralski method and supplied by S. A. Kutovoi. A 15 at. % Yb-doped, uncoated plate of LSB with a thickness of 3 mm was available for the present study of mode-locking. The crystal was placed in an astigmatically compensated Z-type cavity similar to that described in previous work [11]. The orientation of the sample was chosen for emission and pumping in π-polarization in accordance with the previously achieved wide tunability [5]. The axis z≡c was identified using crossed polarizers and measuring the small signal absorption. The sample was positioned under Brewster angle on a copper block without active cooling. Two pump sources, a Ti:sapphire and a diode laser, were applied. The Ti:sapphire laser emitted up to 1.8 W near 973 nm (peak absorption). The diode pump laser consisted of a 50 μm broad-stripe single emitter. It generated as much as 4.5 W of output power at 973 nm with a spectral linewidth of about 4 nm. The astigmatic emission of the broad-stripe diode laser was collimated by two crossed cylindrical micro-lenses. For the collimated beam an M² of about 12 was measured. Both pump lasers were linearly polarized and their output beams were focused by an f=62.8-mm spherical lens through one of the folding mirrors (radius of curvature, RC=-10 cm) of the resonator.

Passive mode-locking was achieved by a saturable absorber mirror [12] employed as an end reflector on which the beam was focused using a third curved mirror of RC=-15 cm. The saturable absorber mirror (SAM) was grown by metalorganic vapor phase epitaxy (MOVPE). The distributed Bragg mirror on a GaAs substrate contained 25-AlAs/GaAs quarterwave layer pairs. Its high reflection band with R>99% extended from 1000 to 1080 nm. The absorbing part on top of the Bragg mirror was a 10 nm thick single InGaAs quantum well (QW) embedded in a GaAs layer. To accelerate the saturable absorber relaxation the QW was implanted with As-ions. The relaxation time was measured by the pump-probe technique to be less than 5 ps. The saturable absorption amounted to ~0.5% and the saturation fluence was 20 μJ/cm². The non-saturable losses were negligible. The two SF10 prisms used for dispersion compensation in the arm containing the output coupler [11] had a tip-to-tip separation of 38 cm.

4. Mode-locked operation

At first, we employed the Ti:sapphire laser to pump the Yb:LSB sample at the absorption peak for the π-polarization. The absorption in the lasing state was quite constant for all input powers and amounted to ~42%, which is consistent with the calculated small signal absorption of 53% related to the spectroscopic data.

The cavity was designed for the purpose of generating the shortest pulses. Using a 1% output coupler and adjusting the prisms, pulses as short as 72 fs (that were fitted rather well assuming a sech²-pulse shape) were obtained directly from the oscillator at a repetition rate of 90 MHz. The average output power amounted to 79 mW for an absorbed pump power of 510 mW. The output spectrum was centered at 1053 nm and had a FWHM of 22 nm corresponding to a time-bandwidth product of 0.428. The total chirp could not be fully compensated by the intracavity prism pair, as can be deduced from the time-bandwidth product being somewhat larger than the Fourier limit for a sech²-pulse shape (0.315). To compensate for the residual chirp, additional external compression was performed. A second SF10 prism pair with a separation of 40 cm was used. After passing the external compressor, a pulse duration as short as 58 fs was achieved at an average power of 73 mW. With the preserved spectral bandwidth of 22 nm, the corresponding time-bandwidth product equal to 0.345 was already close to the Fourier limit. The intensity autocorrelation trace together with the corresponding fit and the spectrum of the shortest pulses are shown in Fig. 2(a). The extracavity prism sequence further acts as a compensator for the spatial dispersion of the oscillator output. The achieved pulse durations are among the shortest pulses achieved with Yb lasers. Shorter pulses of 47 fs (with a CW component) and 53 fs were obtained up to now only using Yb:CaGdAlO₄ [13] and Yb:NaY(WO₄)₂ [14], respectively. For both host materials, external compression was also required. Without external compression, pulse duration of 58 fs was achieved using phosphate glass [15] and LuVO₄ [11]. The pulse duration of 58 fs obtained in this work is, to the best of our knowledge, the shortest for mode-locked borate crystal lasers as compared to the above mentioned 69 fs reported for Yb:BOYS [9].

 

Fig. 2. Autocorrelation traces and spectra (insets) of the shortest pulses obtained from the Yb:LSB oscillator with external compression under a) Ti:sapphire and b) diode laser pumping (τ: FWHM of the pulse).

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Implementing the diode laser as a pump source, we achieved pulse durations of 82 fs at a repetition rate of 90 MHz for the Yb:LSB oscillator. The 18.5 nm broad spectrum (FWHM) could support significantly shorter pulses which means that the obtained pulse duration exceeds the Fourier limit by a factor of about 1.5. By supplementary external compression similar to that performed with Ti:sapphire laser pumping, the chirp of the output pulses was considerably reduced. The deconvolved FWHM of the shortest pulses was 67 fs (Fig. 2(b)). Here as well, the time-bandwidth product of 0.335 denotes a pulse width close to the Fourier limit. The average output power reached 47 mW directly at the output of the oscillator and 39 mW after the external prism compressor for an input pump power of 2 W, comparable with the Ti:sapphire pump level. These results indicate that a proper selection of the diode pump parameters and careful alignment allow similar mode-locked laser performance with Ti:sapphire- or diode-laser pumping. The somewhat lower output power obtained in the case of diode-pumping can be explained by the relatively narrow absorption peak of Yb:LSB near 973 nm for π-polarization [5]. The use of antireflection coated crystals and normal incidence geometry would, however, allow the pumping in σ-polarization by preserving the laser emission parallel to the π-polarization due to the slightly higher gain [5]. In this case, the broader and higher absorption for σ-polarization will not only relax the requirements to the pump diode but will allow one to use lower doping levels. The latter is of special importance in the case of LSB where optimum crystal quality can be expected at lower Yb-concentration where the crystal structure remains stable.

 

Fig. 3. Output power versus absorbed pump power showing the regions of CW and mode-locked operation for an output coupler of 1% (η: slope efficiency).

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In order to study the stability range of the passive mode-locking, the average output power was monitored versus the absorbed power for Ti:sapphire laser pumping, Fig 3. Mode-locking in the sub-100 fs range was stable for absorbed pump powers between 203 mW and the maximum of 594 mW. Below the mode-locking threshold, the laser switched to the CW regime. For the maximum absorbed pump power, an output power of 113 mW was measured with an output coupler of 1%. The slope efficiency with respect to the absorbed power for the CW and mode-locked regimes was 21% and 23%, respectively. The low threshold and broad mode-locked operation range indicate the high quality of the Yb:LSB crystal as well as the low losses and appropriate parameters of the SAM.

 

Fig. 4. Tunability of the Yb:LSB laser in the mode-locked regime (a), and related average output power and pulse duration plotted versus the central wavelength (b).

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With the same objective of studying the stability of the mode-locked regime, the short pulse spectral tunability was examined for the Yb:LSB oscillator at an incident pump power of 1.5 W. Initially, the central wavelength was simply tuned by means of the intracavity prisms from 1041 to 1057 nm (Fig. 4). At longer wavelengths the laser switched to multi-pulse operation or a CW peak occurred. This is illustrated in Fig. 4(a) for a central wavelength of 1057.5 nm where a CW-peak at 1079 nm was present. To tune the laser to shorter wavelengths, a slit was placed between the second prism and the output coupler. In this way, the tunability was extended to central wavelengths as short as 1025 nm achieving a total tuning range of 32 nm. The spectra corresponding to stable tunable mode-locked operation obtained with and without slit are shown in Fig. 4(a). Sub-200 fs pulse durations were obtained for almost the entire tuning range from 1028 to 1057 nm, as depicted in Fig. 4(b). Below 1028 nm the pulse duration increased to 264 fs. The average output power increased almost linearly from 31 mW at 1025 nm to 79 mW at 1057 nm.

A possible reason for the limited tunability on the short wavelength side could be the presence of reabsorption losses in the quasi-three-level laser scheme of Yb³⁺ due to the overlap of the absorption and emission bands and the cutoff wavelength of the dichroic pump mirror. With special design of this mirror and lower Yb-doping one can expect further extension of the tunability of the Yb:LSB laser and bandwidths in the mode-locked regime supporting even shorter pulse durations.

5. Conclusion

In conclusion, we have demonstrated what we believe to be the first Yb:LSB mode-locked oscillator. The laser generated pulses as short as 58 and 67 fs for Ti:sapphire and diode-laser pumping, respectively. The passively mode-locked Yb:LSB laser was tunable from 1028 to 1057 nm in the sub-200 fs range. The very high power (40 W) achieved previously by some of the present authors using thin disks in the CW regime [5] and the short pulse durations obtained in the present work make us confident that high average power femtosecond sources on the basis of Yb:LSB could be developed in the near future.