1. Introduction

Ultrafast diode-pumped ytterbium lasers emitting around 1 µm wavelength have become a very active research subject in the last years and have been successfully employed in several commercial products. The possibility of being directly diode-pumped by highly reliable laser diodes emitting around 980 nm wavelength, the broad bandwidth and the small quantum defect make these laser systems effective for both high average power as well as for ultrashort pulse generation. Average output power as high as 141 W with a pulsewidth of 738 fs has been reported [1] from a thin-disk Yb³⁺:Lu₂O₃ mode-locked laser oscillator.

Regarding the minimization of the pulsewidth, rather than extreme power upscaling, several promising Yb³⁺-doped laser materials have been investigated in recent years. Considering only the reports about lasers mode-locked by semiconductor saturable absorber mirrors (SESAMs), which is presently the most reliable technology for generation of ultrafast pulses in solid-state lasers, several laser media have been proved effective for the generation of pulses with duration near or below 50 fs: Yb³⁺:glass [2], Yb³⁺:LuVO₄ [3], Yb³⁺:Sc₂O₃ [4], Yb³⁺:CaGdAlO₄ [5] and Yb³⁺:YCa₄O(BO₃)₃ [6]. Among these, Yb³⁺:CaGdAlO₄ (Yb³⁺:CALGO) and Yb³⁺:YCa₄O(BO₃)₃ (Yb³⁺:YCOB) are the most promising candidates both for high-power femtosecond laser oscillators and for generation of pulses shorter than 50 fs, owing to their remarkably flat and broad emission bandwidth and their relatively high thermal conductivity. An Yb³⁺:YCOB mode-locked oscillator was recently reported to generate 35-fs pulses through extra-cavity compression [6] whereas an output power of 100 W was demonstrated for a thin-disk setup [7] in cw operation, although the mode-locking performance was limited to 5-W output power. Also Yb³⁺:CALGO has excellent thermo-mechanical properties and furthermore has been proved quite easy to grow. Output power as high as 12.5 W and 94-fs pulses for 28-W absorbed pump power was reported [8], whereas generation of pulses as short as 47 fs was shown earlier by Zaouter et al. [5]. No tuning performance of ultrafast Yb³⁺:CALGO lasers have been reported to date, and the earlier reports were mainly focussed on pulse width minimization rather than on efficiency optimization.

In order to investigate the potential for generation of ultrashort pulses with Yb³⁺:CALGO, an experiment with a low-power diffraction-limited pump beam is more suitable. This allows to get optimized ultrashort pulse generation for a given pump power, without any significant thermal effect perturbing or limiting the mode-locking performance.

In this Letter we report on the generation of stable mode-locking pulses as short as 40 fs with optical-to-optical efficiency an order of magnitude higher than in the first sub-50 fs Yb³⁺:CALGO laser [5], employing a set up pumped by a single-mode 980-nm laser diode emitting 350 mW.

2. Experiments

The pump laser diode (Axel Photonics, Inc. model M9-980-0350-S50) was collimated by a 6.24-mm focal aspheric lens, magnified horizontally with a × 3 cylindrical telescope and focused in the active medium with a 75-mm focal spherical lens. The pump spot in the focal plane was optimized to eliminate astigmatism by finely tuning the cylindrical beam expander. The waist radii were measured by longitudinal scanning with a ccd camera to be w_x × w_y = 15 × 11 μm² (M²_x × M²_y = 1.2 × 1.1) along the horizontal and vertical directions, respectively. The waist radius of the cavity mode could be changed in the range 15-20 μm.

We investigated 2%-doped Yb³⁺:CALGO crystals, with lengths of 2.5, 3, 4, 5 mm. The samples were antireflection-coated for both the pump and laser wavelength. Owing to the low pump power, no special care was taken to cool the laser crystal or to control its temperature. Indeed, in all our experiments we found little dependence on the room temperature variations of ± 3 °C.

The resonator was X-folded as in Fig. 1 , with angles of incidence of about 3° degrees at the curved mirrors, limited only by the available mechanical mounts. The pump power on the laser crystal was controlled with a variable attenuator (PBS + HWP).

 

Fig. 1 Setup of the femtosecond oscillator. LD: laser diode; L1: aspheric lens; C1, C2: cylindrical telescope; L2: pump focussing lens; HWP: half-wave plate; PBS: polarization beam splitter; M1: 50-mm radius-of-curvature mirror (mounted with 100-mm focal lens in contact); M2: 100-mm radius-of-curvature mirror; OC: output coupler.

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In continuous wave regime the laser performed best with the 3-mm crystal (Fig. 2 ), yielding 113 mW at 240 mW absorbed power with a 2.4% output coupler (slope efficiency = 56.5%). The laser was widely tunable over nearly 70 nm, from 1015 nm to 1085 nm with a 1.6% output coupler.

 

Fig. 2 Results of operation in cw regime for different output coupler transmissions T_OC, with a dielectric high-reflectivity mirror replacing the SESAM and no prisms in the cavity. Inset: tuning results in cw regime with a single intracavity prism.

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Mode-locking operation was accomplished with either a SESAM-1 from BATOP (0.6% modulation loss, 60 μJ/cm² saturation fluence) or a second SESAM-2 from High Q Laser (3% modulation loss, 140 μJ/cm² saturation fluence). Two SF10 prisms spaced by 280 mm were used for dispersion compensation. The total resonator length was 710 mm.

A non-collinear type-I BBO second-harmonic autocorrelator was used to measure pulse durations, whereas the spectrum was recorded with an ANDO 6315B optical spectrum analyzer and a single-mode optical fiber.

The laser was readily mode-locked with both SESAMs, although with SESAM-1 it was not always self-starting. Mode sizes were optimized by tuning the separation between the curved mirrors. With this setup, we achieved pulses as short as 46 fs at the output coupler with 25.6-nm spectrum (Fig. 3 ). This yields a time-bandwidth product of 0.32, very close to the ideal factor of 0.315 for a sech² intensity pulse shape.

 

Fig. 3 Output pulse autocorrelation and optical spectrum with SESAM-1 (inset).

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This result was obtained without any additional extracavity compensation, clearly indicating that the output pulse was basically chirp-free. The output power was 33 mW, thus the optical-to-optical efficiency was 14%, a value which compares favourably with that reported in Ref [5], where the output power was similar but the absorbed pump power was 2.7 W (more than ten times higher than in our case).

Replacing SESAM-1 with SESAM-2, which had a larger modulation depth, the output power dropped to 15 mW but the mode-locking became completely self-starting.

As reported earlier for similar ultrafast ytterbium lasers mode-locked by SESAMs [5,6], the 70-fs pulse obtained with SESAM-2 was significantly chirped since the optical spectrum was 31-nm wide (time-bandwidth product = 0.58). Attempts to minimize further the pulsewidth through intracavity dispersion optimization and other adjustments were unsuccessful. An extracavity prism pair of SF10 glass was necessary to compress the pulse near the Fourier limit, yielding pulses as short as 40 fs (Fig. 4 ).

 

Fig. 4 Chirped pulse autocorrelation at the output coupler (dashed line) and de-chirped compressed pulse autocorrelation with SESAM-2. Inset: optical spectrum.

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We note that this result was accomplished with only a single additional extracavity prism pair with the same prism separation as the intracavity pair, serving to compensate positive chirp and to remove spatial dispersion. Inserting a vertical slit in front of the output coupler, the laser could be tuned across 20 nm (Fig. 5 ).

 

Fig. 5 Wavelength tuning with SESAM-2.

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3. Discussion and conclusions

It is interesting to note the remarkable difference in the output pulse chirping observed with the different SESAMs.

As first remarked by Curley et al. [9], femtosecond soliton mode-locking is expected to produce Fourier-limited pulses at the end-mirrors of a linear laser resonator, although the pulse duration and chirp may undergo significant variations along the propagation inside the cavity. Chilla and Martinez [10] modelled numerically a Ti³⁺:Al₂O₃ Kerr-lens mode-locked (KLM) laser using the space-time matrices developed earlier by Kostenbauder [11]. They calculated the pulse eigenmode at the steady state, both spatial and temporal, at each position inside the resonator.

In particular, they confirmed that the pulse temporal phase was nearly constant (almost chirp-free) at the end mirrors, just like the Gaussian modes of a stable linear resonator have flat wavefronts on plane end-mirrors.

Actually, the simulation results indicated a small positive pulse chirp at the end mirrors. We think this could help explain our observation. Just like an apodizing aperture produces a curved spatial wavefront on the plane end mirrors of a stable resonator with lenses and mirrors, a “temporal apodization” (e.g. instantaneous amplitude modulation) can do the same on the pulse wavefront, producing chirp on the output pulse at the output coupler.

In the KLM laser the small chirp effect on the output pulse was due to the fast electronic nonlinearity driving the amplitude modulation in the gain element.

Given the comparable intracavity pulse intensity in the Yb³⁺:CALGO crystal with both SESAMs, it is unlikely that the significant difference in pulse chirp observed in our setup could be explained by KLM amplitude modulation in the laser medium. Furthermore, it is worth noticing that it was the 3% SESAM that produced significant chirp.

Another possibility is that the SESAM itself might contribute some fast amplitude modulation effect besides the usual relatively slower (: 100 fs) component [12] required for starting and stabilizing the soliton-like pulse. For example, optical Stark effect in SESAMs was recently exploited for fast modulation of semiconductor disk lasers [13].

In conclusion, by using low-power single-mode pumping we have demonstrated that Yb³⁺:CALGO can generate efficiently chirp-free pulses as short as 46 fs, with a single-mode pump laser diode. Employing a SESAM with stronger modulation enabled the generation of moderately chirped 70-fs pulses which were easily compressed to 40 fs, among the shortest pulse durations reported to date in diode-pumped Yb³⁺-doped lasers. Therefore, Yb³⁺:CALGO proves to be an excellent laser material for efficient generation of sub-50 fs pulses around 1 μm. Furthermore, we point out that the compact and efficient low-power ultrafast laser investigated in this Letter is very attractive for amplifier seeding applications as well as for multi-photon imaging and terahertz applications.