We demonstrate the operation of a low threshold femtosecond Yb:KYW laser, using a saturable absorber mirror for passive mode-locking and a high brightness laser diode as pumping source. Fourier-limited pulses with a duration of 101 fs are achieved at an output power of ≈100 mW. The performance of the Yb:KYW laser was also compared to that of Yb:KGW and Yb:glass in the same setup.
©2002 Optical Society of America
Yb-doped laser materials are well suited for building simple and robust diode-pumped femtosecond lasers delivering output powers in the Watt range. Yb:YAG and Yb:glass are the most common members of this material class and have been applied in lasers for ultrashort pulses [1,2]. In general, Yb-doped laser crystals and glasses exhibit broad emission spectra supporting pulse durations below 100 fs. The relatively broad absorption bands with large cross sections can be pumped by InGaAs laser diodes. Emission cross sections are large enough, so that relatively high gain is provided and in the case of mode-locked operation good stability against Q-switching can be expected because of the low saturation fluence . Fluorescence lifetimes are generally longer than in Nd-doped hosts. The small quantum defect enhances the overall efficiency and reduces the thermal load. On the other hand, a disadvantage of Yb-doped laser materials is their quasi-three-level nature. As a result, they reabsorb a part of the emitted laser light. To saturate this loss mechanism high pump intensity and a good overlap of pump and resonator mode over the whole length of the laser medium are required.
2. Yb-doped double tungstates
2.1. Material properties
Among the Yb-doped materials the double tungstates Yb:KY(WO4)2 (abbreviated: Yb:KYW) and Yb:KGd(WO4)2 (Yb:KGW) stand out because of their large cross sections. Rare earth potassium tungstates are biaxial crystals which crystallize in the monoclinic structure with space group C2/c [4,5]. The properties of Yb:KYW and Yb:KGW are very similar . The absorption and emission cross sections depend strongly on the orientation of the crystal with respect to light polarization. When the light is polarized along the m-crystallo-optic axis, the strongest absorption and emission is observed. Oriented this way, at 981 nm Yb:KYW offers an absorption cross section peak σabs = 3.7·10-20 cm2 . Fig. 1 shows that this is about four times higher than the maximum absorption cross section of Yb:YAG or Yb:glass.
In principle, the large absorption cross section permits the use of relatively short crystals at a given doping level. As a consequence a better overlap of a non-diffraction limited pump mode and the laser mode can be achieved. For light polarization parallel to the m-cristallo-optic axis, the emission cross section of Yb:KYW at 1025 nm is σem = 3·10-20 cm2 . In Fig. 1 one can see that this value is also considerably larger than the emission cross section maxima of Yb:YAG and Yb:glass. If Yb:KYW is pumped near 980 nm, an exceptionally small quantum defect can be achieved, resulting in a very low thermal load. Compared to Yb:glass the thermal conductivity of Yb:KYW is more than four times higher, although it is not as good as that of Yb:YAG.
2.2. Mode-locked Yb:KYW and Yb:KGW lasers
Operation of femtosecond lasers using Yb:KGW and, very recently, Yb:KYW was shown in  and . The former was passively mode-locked by a saturable absorber mirror (SESAM), the latter was Kerr-lens mode-locked. Both were pumped by broad stripe diodes. Here we present what is to our knowledge the first laser using Yb:KYW with a SESAM for a self-starting mode-locking process and a novel tapered diode laser as high brightness pump source. The shortest pulse duration achieved by us (101 fs) is in the range of the minimum durations achieved in  (112 fs) and  (71 fs); our maximum mode-locked output power (150 mW) does not exceed that in  (1.1 W) and  (190 mW), but our results are obtained using only 1.1 W of incident pump power (: 4 W, : 2.6 W) and by a simpler arrangement with pumping from one side only. Compared to the Yb:KYW laser in , we increase the conversion efficiency by almost a factor of two.
3. Tapered diode laser (TDL) as pump source
The best way to provide the high pump intensities required for efficient operation of quasi-three-level materials is to use a diffraction limited pump beam. In contrast to broad stripe diodes with their large M2 values, narrow stripe diode lasers and tapered diode lasers (TDL) offer a high beam quality, so that the pump mode can easily be shaped and matched to the laser mode. Therefore the effective laser mode area in the active medium can also be made smaller. This increases the stability of the mode-locked regime, since the saturation energy, defined as the product of the saturation fluence of the laser material and the effective laser mode area, is reduced . Until recently, however, the available nearly diffraction-limited pump sources around 980 nm  did not provide sufficient output power for pumping mode-locked lasers based on Yb-doped materials. For the experiments reported here a novel TDL is used, delivering an output power up to 2 W at an M2 < 3 for the slow axis emission (1/e2-value). This type of diode laser will be commercially available from a spin-off of the Ferdinand-Braun-Institute in the near future.
The TDL (Fig. 2) consists of a 1-mm long ridge waveguide and a 3-μm-long tapered amplifier section. The highly reflecting (R > 90%) facet is about 3 μm wide and the output aperture has a width of 300 μm. The diode laser is soldered epi-down on a CuW subcarrier with AuSn. The subcarrier is mounted on an open copper heat-sink (C-mount). A typical electrical-to-optical conversion efficiency of about 40% is measured for a fixed heat-sink temperature of 25°C corresponding to a diode current of 3.0 A (output power: 1.5 W). The maximum output power of 2 W is achieved near 978 nm with a sprectral bandwidth of only 1 nm. An effective far field measurement of the slow axis emission shows that about 60 % of the radiation emitted at 2 W belong to the fundamental mode (Fig. 3). The measured fast axis divergence angle of the pump diode is below 30° due to a large optical cavity (LOC) AlGaAs waveguide structure.
4. Experimental setup
Due to the excellent beam quality of the TDL relatively simple beam shaping optics are sufficient. The astigmatic emission of the tapered diode laser is formed by an aspherical and a cylindrical lens (Fig. 4). The nearly collimated pump beam is then focused by an f = 62.8-mm spherical lens through one of the folding mirrors of the Z-folded resonator. The 3-mm thick Yb:KYW crystal is used under Brewster angle for polarization almost parallel to the m-crystallo-optic axis, which turned out to be optimum for achieving maximum output power and stable mode-locking. At a doping level of 5at.% Yb about 98% of the incident 1.1 W of pump power are absorbed. Although the Yb:KYW crystal is not actively cooled, thermal problems did not occur. Two Brewster-cut SF10 prisms are used for dispersion compensation in the arm containing the output coupler. The SESAM, serving as a passive mode-locker, is placed at the other end of the resonator where an additional waist is formed by a curved mirror.
5.1. Yb:KYW femtosecond laser
Using the setup of Fig. 4 pulses as short as 101 fs (assuming a sech2-shape) are generated at a center wavelength of 1046 nm with a 5% output coupler. An average output power of 100 mW is measured at a pulse repetition rate of 95 MHz. The observed spectrum and autocorrelation signal are shown in Fig. 5. The spectrum exhibits a FWHM of 12.5 nm, so that the pulses are almost Fourier-limited with a time-bandwith product of τpΔνp = 0.34.
With a 3.5% output coupler a maximum mode-locked output power of 150 mW at 1045 nm is achieved, corresponding to an efficiency of 14% with respect to the absorbed pump power, while pulses with a duration of 134 fs are observed. Self-starting mode-locked operation in a broader wavelength range between 1040 nm and 1070 nm is achieved with a maximum output power of 45 mW at 1053 nm, when an output coupler of 1% transmission is applied. The wavelength can be tuned by adjustment of the laser cavity, basically changing the overlap of pump and resonator mode and therefore varying the reabsorption loss in the active material.
The range of usable pump and laser parameters is limited by Q-switching and multi-pulsing tendencies at the moment. It can possibly be extended by the use of SESAMs suiting our laser better than those available now. At lower intracavity powers the Yb:KYW laser tends to Q-switching operation, which can be explained by theoretical considerations about passively mode-locked short pulse lasers . Therefore an absorbed pump power lower than 1 W or output couplers of transmission higher than 5% cannot be used for stable mode-locked operation. The observed tendency towards double or multiple pulse operation, sometimes with irregular spacing between the pulses, is not unusual for SESAM-mode-locked bulk laser systems . It was reported also for the Kerr-lens mode-locked Yb:KYW laser . This behavior results from competing mechanisms of loss and gain present in the cavity whose magnitude depends on the pulse energy . Multiple pulsing is favored when the intracavity power is large and therefore can hardly be avoided in our Yb:KYW laser for an output coupler transmission lower than 3.5%.
5.2. Comparison of Yb:KYW with Yb:KGW and Yb:glass femtosecond lasers
To compare the potential of Yb:KYW with those of Yb:KGW and Yb:glass, in the same laser cavity a 3-mm long 5 at%-Yb-doped Yb:KGW crystal and a 6-mm long 5 wt%-Yb2O3-doped fluoride phosphate glass are tested. The thicknesses of the samples are chosen to get roughly the same amount of absorbed pump power (Pabs =1.1 W). The transmission of the output coupler is chosen individually for each material to obtain optimum performance, but also to avoid the described unwanted pulsing behavior. This leads to different optimum transmission values for minimum pulse duration and a value of 3.5% for maximum output power. The results are shown in table 1. In all the measurements the pulses can be best fitted by a sech2-shape and are almost Fourier-limited.
The shorter pulses supported by the Yb:glass sample probably result from the smoother net gain spectrum of the glass. The comparison of both tungstate materials shows that they give similar results, which can be expected from the similar properties. Aside from a possibly non-optimum crystal quality (crystal growth of Yb-doped tungstates is not yet elaborated), the fact that about the same output powers are measured with Yb:KYW and Yb:KGW as with Yb:glass may be an indication of stronger reabsorption at the obtained laser wavelength in the tungstates at room temperature. This reabsorption influence could be overcome by cooling of the laser crystals to reduce the population of the upper-lying ground state Stark sublevels or by absorption bleaching using more intense pump radiation. In this context the further development of powerful high brightness diode pump sources like the one presented here is very important for an effective use of Yb:KYW and Yb:KGW. It is also promising to realize higher doping levels to shorten the crystals and therefore to further improve the overlap of pump and resonator modes. Under these conditions Yb:KYW becomes preferable to Yb:KGW. KYW is the better host for high Yb:doping levels with respect to a defect-free crystal quality, as the Yb-ion radius is closer in size to the Y-ion than to the Gd-ion in KGW. Furthermore, KYW has a better thermal conductivity than KGW.
Our experiments with a 5%-doped Yb:KYW crystal, which we realize with a tapered laser diode as pump source and a SESAM as passive mode-locker, prove the potential of the material in such a laser cavity: Self-starting mode-locking with pulses of 101 fs duration is achieved. The demonstrated maximum output power of 150 mW is not a limit; output powers in the Watt range can be expected when more pump power is applied. Both the tendencies for Q-switching and multiple-pulse generation have to be reduced by the choice of more suitable saturable absorbers. Further progress could result from the efforts to improve Yb-doped tungstate crystal growing techniques , which are expected to lead to better crystal homogeneity and purity and which will also permit higher doping levels and thus more powerful laser concepts.
This work has been supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie under contract no. 13N7213.
References and links
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