We report a highly efficient diode-pumped femtosecond Yb:KYW laser with a compact three-element resonator. Near-transform-limited pulses of 107fs duration at a centre wavelength of 1056nm are produced at a pulse repetition frequency of 294MHz by utilising soft-aperture Kerr-lens mode locking. The femtosecond operation had a mode-locking threshold at a pump power of 250mW and the laser was tunable from 1042nm to 1075nm. The optical-to-optical conversion efficiency exceeded 50% in this femtosecond-pulse regime.
© 2004 Optical Society of America
Yb3+-ion based solid-state femtosecond lasers have developed rapidly over past few years due to their attributes of efficiency, low cost, relative compactness and robustness [1–6]. The most promising results to date have been obtained with the Yb-doped double tungstates [notably, Yb3+:KY(WO4)2 (Yb:KYW) and Yb3+:KGd(WO4)2 (Yb:KGW)] [7,8]. Indeed, femtosecond laser operation has been demonstrated successfully in both Yb:KYW and Yb:KGW media using semiconductor saturable absorber mirrors (SESAMs) for passive mode locking. The first reported femtosecond laser based on a Yb:KGW gain crystal was pumped by a broad stripe diode laser and produced pulses of 176fs duration with an average output power of 1.1W and the shortest pulses of 112fs were generated at an output power level of 200mW . Klopp et.al. have described a passively mode-locked Yb:KYW laser pumped by a high quality beam from a 2W tapered diode laser where 101fs pulses were obtained at an average output power of 100mW . Interestingly, a thin-disc femtosecond Yb:KYW laser produced 22W of output power with 240fs pulse duration . Recently, we demonstrated a highly efficient femtosecond Yb:KYW laser pumped by an InGaAs narrow-stripe laser diode and which incorporated a SESAM for passive mode locking . Pulse durations of 123fs were generated with 107mW of output power from only ~300mW of pump. This result has been achievable mainly through the use of a near-diffraction-limited pumping technique that facilitates a good spatial overlap between pump and laser beams that in turn serves to overcome drawbacks that arise from the quasi-three-level character of Yb-doped materials.
Kerr-lens mode locking (KLM) is a well-developed technique for the generation of ultrashort pulses from lasers that are efficient, compact and have reduced intracavity component counts . Reducing the intracavity losses by excluding a SESAM (non-saturable losses) and/or pair of prisms can also lead to a substantial enhancement of the optical efficiency of such a femtosecond laser. Recent research has shown that Yb-doped tungstates are promising laser crystals for low-threshold and efficient KLM operation [13–15]. The nonlinear refractive indices n 2 of Yb:KYW and Yb:KGW were measured to be 8.7×10-16cm2/W  and 20×10-16cm2/W , respectively, compared with 3.1×10-16cm2/W for Ti:sapphire. Additionally, single-mode diode pumping avoids the limitations that arise when KLM lasers are pumped by multimode diode lasers. Multimode pump beams having M2 factors that are more than ten times beyond the diffraction limit from a broad-stripe, high brightness laser diode can not be tightly focussed into a gain medium and for this reason diode-pumped KLM lasers generally include an intracavity hard aperture.
Here we report the development of a low-threshold and highly efficient KLM femtosecond Yb:KYW laser having a narrow-stripe, fibre-coupled single mode InGaAs laser diode as the pump source. The cavity design is simplified to a three-element resonator that incorporates a prismatic output coupler for dispersion compensation . Mode locking was achieved by exploiting a soft-aperture Kerr lens effect and near-transform-limited pulses of 107fs in duration at a centre wavelength of 1056nm were produced at a pulse repetition frequency of 294MHz. Impressively, the optical-to-optical conversion efficiency exceeded 50% during femtosecond-pulse operation for which the mode-locking threshold was achieved at a pump power of 250mW.
2. Experimental set-up
A schematic of the laser set-up is shown in Fig. 1(a). For these experiments a 10 at.% Yb3+-doped KYW crystal in plane/Brewster geometry was employed and was oriented in the cavity for propagation along the b(Np) axis with polarisation parallel to the crystallo-optic Nm-axis (Fig. 1(b)). Due to the plane/Brewster design of the crystal it was possible to vary the length of active medium from 0.3mm to 2mm by a simple horizontal translation of the gain crystal. The plane facet of the crystal was coated for high transmission at 981nm (≈92%) and high reflection at wavelengths in the 1025–1100nm range. No special provision was made for good thermal contact or cooling of the sample. Our pump source was a 470-mW single mode fiber-coupled (mode field diameter of 6.6µm) polarisation-maintaining InGaAs diode laser (JDS Uniphase) emitting at 980.5nm. Aspherical (f=11mm) and spherical (f=63mm) lenses were used to collect and focus the pump beam into the gain medium and the pump beam spot radius was measured to be 17µm (1/e2 intensity). The folding mirror M2 with a radius of curvature of 50mm reflected the beam to the SF10 Littrow prism which was used as the terminating element of the cavity. The back face of this prism acted as the output coupler of the system with transmission of ~1% in the 1020–1100nm range. Using an ABCD matrix formalism, the folding angle 2θ was estimated to be in range 16–20° for astigmatic compensation. The laser mode size on the plane facet of the crystal was calculated to be 20-µm radius for a resonator operating near the mid-point of its stability range and the long arm was set to a length around 48cm.
3. Experimental results
3.1 CW performance
The three-element Yb:KYW laser was first optimized for cw operation. Due to the quasi-three-level nature of the Yb-doped materials, re-absorption losses always arise and so optimization of the thickness of the gain medium is important. In the three-element cavity set-up, where Yb:KYW crystal has wedged configuration, the length of the crystal can be adjusted by simply changing of transverse position of the crystal. Fig. 2 depicts slope efficiencies of the Yb:KYW laser during cw operation as a function of the crystal length. A slope efficiency as high as 80% was obtained when the thickness of the crystal was around 0.7mm. The threshold for laser oscillation was measured to be 55mW and an output power of 285mW was produced at an incident pump power level of 430mW. An increase in the length of the crystal leads to the decrease in the slope efficiency that can attributed to the increase in intracavity losses (reabsorption and scattering losses in the gain medium). During laser operation, the Yb:KYW crystal absorbed more than 99% of the pump radiation when the thickness was around (or exceeded) 1mm. The cw Yb:KYW laser could be tuned conveniently because of the presence of the Littrow prism in the cavity. The maximum output power was achieved at a wavelength around 1035nm and the laser was continuously tunable from 1030nm to 1080nm. Some variation in tuning range took place depending on the length of crystal used. Although the Yb:KYW laser produced maximum output power at crystal thicknesses in the 0.5–0.7mm range, the broadest tuning range was obtained for crystal lengths around 1mm which has been employed for the further KLM experiments.
3.2 Kerr-lens mode locking
A map of stability was taken to depict the variation of output power during cw laser operation as a function of distance between Brewster surface of the crystal and folding mirror (see Fig. 3(a)). It can be seen that there is a gap in output power where the cw operation is unstable and output beam profile implies strong aberrations (Fig. 3(b), upper part). Further adjustment of the cavity at this point led to the soft aperture Kerr-lens mode locking. This was accompanied by a transformation in the output beam shape to a near-diffraction-limited profile (Fig. 3(b), bottom part) and an increase in the output power by up to 30% in comparison with cw operation. A self-starting KLM operation was sometimes observed, but generally mode locking was initiated by a small mechanical perturbation. In mode-locked operation the laser was tunable from 1042nm to 1075nm by tilting of Littrow prism together with minor adjustment of the M2 folding mirror position (Fig. 4). The shortest, near-transform limited pulses with duration of 107fs (sech2 pulse shape assumed), were generated at around 1055nm with the corresponding spectral bandwidth of 10.6nm (time-bandwidth product, 0.32) and with an average output power of 126mW. The tuning of the laser to longer/shorter wavelengths leads to an increase in pulse duration such that pulses of 140fs and 210fs duration were generated at the red and blue edges of the tuning curve, respectively (Fig. 4). The increase in pulse duration when tuned to longer wavelengths can be accounted for by the decrease in the strength of the Kerr lens due to a reduced intracavity power. Longer pulses in the 1042nm wavelength region and an absence of Kerr-lens mode locking operation at wavelengths shorter than 1040nm can be explained by increased reabsorption losses in the cavity. At a fixed distance between mirror M2 and the Littrow prism and for a given length of crystal, the intracavity group velocity dispersion was controlled by the transverse position of the SF10 Littrow prism. No multi-pulse operation was observed within the entire range of the operating wavelengths, and the pulse repetition rate was measured to be 294MHz. A maximum output power of 227mW was obtained when the laser operated near 1042nm with 430mW of incident pump power and this corresponded to an optical-to-optical conversion efficiency of 53%. The KLM threshold was measured to be 250mW of incident pump power when the laser operated around 1050nm and produced up to 25mW of output power. The shortest pulses obtained from our set-up (~110fs) are longer than those reported in Ref. 14 (71fs). This may be attributed to the Yb:KYW crystal having been designed for lasing along the Nm crystallo-optic axis where the absorption and emission cross sections are highest. However, the Yb:KYW crystal possesses a broader luminescence spectrum along the Np(b) direction which was employed in Ref. 14 and potentially this can support shorter pulses during mode locking. During femtosecond operation the output beam was characterised by slightly elliptical spatial mode with a 1.15:1 ratio of ellipticity and this can be explained by non-ideal astigmatic compensation for KLM which required operation near the edge of the cavity stability region. An M2 factor of output beam was measured to be 1.2 and 1.4 in sagittal and tangential directions respectively.
In conclusion, we have demonstrated a low threshold and highly efficient diode-pumped femtosecond Yb:KYW laser. The pump source was a 470-mW single mode fiber-coupled InGaAs diode laser. A compact three-element laser geometry was used and mode locking was achieved with a soft-aperture Kerr lens effect. Stable femtosecond operation was obtained in the 1040–1075nm spectral range with a pulse repetition frequency of 294MHz. Near-transform-limited pulses of 107fs duration (10.5nm FWHM spectral width) at a centre wavelength of 1054nm were produced at an average mode-locked power of 126mW. Maximum output power of 227mW was obtained during femtosecond operation when the laser operated near 1042nm and this corresponded to an optical-to-optical conversion efficiency of 53%. We believe that this represents the highest reported efficiency for any femtosecond laser to date. The development of such a compact, highly efficient and low cost femtosecond source should permit its application in many fields such as three-photon microscopy, ultrafast spectroscopy, harmonic generation and the pumping of optical parametric oscillators for more broadly tunable IR sources.
The authors would wish to acknowledge the funding of this work by the UK Engineering and Physical Sciences Research Council (EPSRC), through the Ultrafast Photonics Collaboration project.
References and links
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