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The development and characterization of a diode-pumped ultrashort pulse Yb:KGd(WO4)2 laser oscillator is reported. The laser was pumped by a 25W fiber-coupled diode laser module operating at 980 nm wavelength. In the mode-locked regime, 296 fs duration pulses centered around 1031 nm were generated at a repetition rate of 61 MHz with a total average output power of up to 3.7W, corresponding to 205 kW of peak power and 60 nJ of energy per pulse. Compensation of positive intracavity dispersion was realized using a single chirped dielectric mirror.
©2006 Optical Society of America
1. Introduction
Directly diode-pumped Yb3+-ion doped solid-state lasers form ideal platforms for the development of efficient mode-locked femtosecond oscillators and amplifiers [1–12]. Among a variety of the Yb-doped crystalline laser materials, the crystals of Yb-doped double tungstates Yb:KGd(WO4)2 and Yb:KY(WO4)2 (Yb:KGW and Yb:KYW, respectively) are known to have broad amplification bandwidths, high laser emission and pump absorption cross sections, and small quantum defects [2, 3]. These properties make them particularly suitable for ultrashort pulse generation with efficient diode pumping using commercially available InGaAs laser diodes in the 930–980 nm region [4–6].
Previously, output powers on the order of 1W were demonstrated from longitudinallypumped Yb:KGW lasers both in continuous wave (CW) as well as in mode-locked regimes [4, 5]. Power scaling of such laser systems is very attractive for applications in, for example, nonlinear microscopy, spectroscopy, and frequency conversion. Unfortunately, higher output powers usually come at the expense of simplicity and cost. In one case, the use of a more sophisticated and bulky thin-disk geometry produced more than 22W of average power in 240 fs duration pulses [6]. In another experiment, 10W of average power was generated with slightly longer pulses by the use of two polarization-coupled diode bars [7]. The Brewster’s angle incidence of pump radiation on the dichroic mirrors makes the construction and service of this setup quite complicated.
On the other hand, power scaling of laser systems based on classical delta-cavity designs and fiber-coupled diode pump modules presents a promising alternative in terms of simplicity and cost-effectiveness [8]. In this paper we demonstrate a simple design of a high power femtosecond Yb:KGW laser and show that its output power can be scaled to reach multi-Watt levels.
2. Experimental setup and results
Initially, a high-power CW diode-pumped Yb:KGW laser was developed and characterized. Since optical pumping at 980 nm provides a smaller quantum defect than pumping at 930–940 nm, and results in lower heat deposition inside the crystal, a laser diode operating near this wavelength was used.
2.1 Continuous-wave operation
An experimental setup of the CW Yb:KGW laser is presented in Fig. 1. A 25W diode module (Apollo Instruments Inc.) coupled into a 200 µm core diameter (0.22 NA) optical fiber was employed for optical pumping. The output of the laser diode was imaged by two antireflection (AR) coated spherical lenses (40 and 50 mm focal lengths) onto a 4-mm-long and 2-mm-thick AR-coated plane/plane Yb:KGW crystal (Eksma) with 5% Yb-doping. The crystal had a slab design to facilitate efficient heat removal and was cut along the Np-axis to access the high-gain polarization parallel to the crystallographic Nm-axis [9]. The pump beam passed through a dichroic mirror M1, coated for high transmission (>95%) at the pump wavelength and high reflection (>99.9%) in the 1020–1080 nm region (Laseroptik GmbH). The pump spot inside the crystal was ~250 µm in diameter, corresponding to a confocal parameter of ~2.9 mm. The crystal was wrapped in indium foil and held between thermoelectric coolers which kept it at a constant temperature of about 10 °C.
The laser cavity was formed by the dichroic mirror M1, a highly reflective concave mirror M2 (radius of curvature r=200 mm) and a plane output coupler (OC) with transmission in the range between 1.5 and 6.0%. The cavity was configured to provide a mode size diameter of 240 µm inside the crystal. The folding angle of the cavity was kept as small as possible (~4°) in order to minimize introduced beam astigmatism.
The performance of the laser in the CW regime is shown in Fig. 2. The highest output power was achieved with 1.5% OC. At the maximum incident pump power of 23W the laser delivered up to 6.0W of CW radiation (polarized along the Nm-axis), corresponding to a laser slope efficiency of 41% with respect to the incident pump power. For comparison, laser power achieved using a 4.5% OC is also shown in Fig. 2. The pump absorption in the crystal was measured to be 70–80% under non-lasing conditions depending on the pump power level. The laser always operated in the fundamental transverse mode.
To tune the laser wavelength, an SF10 dispersive prism was introduced between the folding mirror M2 and the OC (Fig. 1). In this configuration, the laser could be continuously tuned over 41 nm, between 1035 and 1076 nm, providing between 1.3 and 4.5W of output power [10], as shown in Fig. 3. When the OC was replaced with a highly reflective mirror, the tunability range became 1030–1090 nm, which should be expected owing to the lower cavity losses.
2.2 Mode-locked operation
To obtain femtosecond pulse generation, the continuous-wave cavity was changed to the cavity configuration illustrated in Fig. 4. The pump focusing arrangement, the dichroic mirror, as well as the laser crystal were kept the same. The concave mirrors M2 and M4 had radii of curvature of 500 mm, and the mirror M1 was flat. All the mirrors had highly reflective coatings in the 1020–1080 nm wavelength range. The cavity mode spot size inside the crystal was also kept the same.
A single Gires-Tournois interferometer (GTI) mirror (Layertec GmbH) was used to compensate for the positive dispersion introduced by the cavity elements. With the help of mirror M1, up to three reflections off the GTI mirror could be accommodated in the laser setup. The mirror was designed to have a dispersion of -1300 ± 50 fs2 per reflection. As a mode locking element, a semiconductor saturable absorber mirror (SESAM, Batop GmbH) designed for 1040 nm wavelength with a specified modulation depth of 1% was inserted at one end of the cavity. The cavity focusing mirror M4 provided a beam spot size on the SESAM of approximately ~350 µm in diameter. The round-trip cavity length was ~4.9 m, giving a pulse repetition frequency of 61 MHz.
At first, the optimum performance in the mode-locked regime was obtained with a 6.0% output coupler (OC1 in Fig. 4) and three reflections on the GTI mirror. The lower number of bounces on the GTI mirror resulted only in multiple pulse mode locking. Above the mode locking threshold, corresponding to ~550 mW of average output power (or 11.2W of pump power), operation of the laser changed from the Q-switched regime to a mode-locked one to produce stable trains of femtosecond pulses. With 18.5W of pump power the laser generated as high as 2.4W of average output power, which corresponded to an optical-to-optical efficiency of 13%.
Further increase in pump power resulted in pulse splitting. The spacing between the pulses in the cavity depended on the pump power, laser alignment and ranged from 2–3 ps to half the cavity round-trip time. The pulse splitting was most likely caused by the uncompensated excess self-phase modulation. Special care was taken to verify single pulse operation using a time-correlated single photon counting technique with temporal resolution of ~30 ps.
The duration of the generated laser pulses was characterized with a second order autocorrelator based on two-photon absorption in a red light-emitting diode. The corresponding pulse spectrum, centered at 1038 nm, and an autocorrelation trace of 230 fs pulses measured at 2.4W of average output power are presented in Fig. 5. The peak-tobackground ratio of the autocorrelation was checked to be 3:1, confirming proper mode locking (not shown in Fig. 5). The time-bandwidth product was calculated to be 0.35, indicating that the pulses were almost transform-limited (0.32 assuming a sech2 temporal profile). This discrepancy could in part be caused by the limited resolution of our spectrometer, which is on the order of 1 nm, uncompensated self-phase modulation and higher-order dispersion. The pulse duration did not appreciably change with the output power. Taking into account the pulse repetition rate of 61 MHz, the laser generated approximately 39 nJ of energy per pulse. Therefore, the peak power delivered by the pulses can be estimated to be 170 kW.
To avoid pulse splitting due to excess self-phase modulation, the amount of intracavity power was lowered by increasing the output coupling of the laser. Since an OC with transmission higher than 6.0% was not available at the time of experiments, a second OC was introduced in the cavity, which acted as a folding mirror and resulted in two additional output beams (OC2 in Fig. 4). Optimum performance was achieved with a 4.5% transmission OC2, corresponding to a net useful loss of 15.0%. At 22.8W of pump power the laser generated a total of 3.7W of average output power, with a higher optical-to-optical efficiency of 16%. The mode locking threshold and pulse duration increased to 1.1W of output power and 296 fs respectively. The pulse width measurements and monitoring were done in the same way as before, with corresponding spectral and temporal profile traces shown in Fig. 6. In this case the time-bandwidth product of the pulses was found to be 0.33, within 5% of the transformlimit. The maximum effective generated pulse energy can be estimated to be 60 nJ, which translates into 205 kW of peak power. A single beam output with similar performance can be readily produced if a proper output coupler with 15.0% of transmission would be available. Depending on laser alignment, pulse break up into two pulses could also be observed.
In both cases passive mode locking was not completely self-starting and often required a slight tap on any mirror mount to start. Once initiated, it could be sustained for hours of operation. The maximum energy fluence on the SESAM for 6.0 and 15.0% OC was calculated to be 660 and 415 µJ/cm2, respectively, and no damage of the device was observed. Including the pump module, the laser setup had a 90×20 cm footprint.
3. Conclusion
In conclusion, we demonstrated a simple design of a high power ultrashort pulse Yb:KGW laser which was pumped by a high-brightness fiber-coupled diode module. The laser provided up to 2.4W in 230 fs pulses or up to 3.7W (combined) in 296 fs pulses at a repetition rate of 61 MHz. The output power in our case was limited by the available pump power and therefore can be scaled to higher values. Our results compare favorably with the recently demonstrated performance of CaF2, Yb:LSO and Yb:YSO based laser systems [11, 12], which operated in the 2W regime of output power.
Acknowledgments
This research was funded by Canada Foundation of Innovation, Ontario Innovation Trust and the Natural Sciences and Engineering Research Council of Canada.
References and links
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