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We report on a diode-pumped CW passively mode locked ceramic Nd:YAG laser with SESAM (semiconductor saturable absorber mirror), wavelength 1064nm. At a pump power of 7.6w, the pulse width was estimated to be ~8.3ps with repetition rate ~130MHz and the average output power was 1.59w. To our knowledge, this was the first demonstration that ceramic Nd:YAG was used for diode pumped CW passively mode locking.
©2005 Optical Society of America
1. Introduction
Recently many studies on the diode pumped solid-state laser have been focus on polycrystalline ceramic lasers. Ceramic laser materials have attractive characteristics. For example, Under a low level doping concentration, it was found that the efficiency of a diode end-pumped Nd:YAG ceramic laser was even higher than that of the Nd:YAG single crystal Laser crystal hosts. Ceramic samples with a high doping concentration and a large size can be easily fabricated, whereas this is extremely difficult for single crystals; multiplayer and multifunctional ceramic laser materials are possible because of the polycrystalline nature of ceramics. Potentially, because of the short period of fabrication process and because they can be mass-produced, the cost of ceramic laser materials could be much lower than that of single crystals. Further more, no complicated facilities and critical techniques are required for growth of ceramics [1,2]. Driven by these advantages, excellent quality Nd:YAG ceramic laser materials that have been developed are good alternative to the widely used Nd:YAG single crystals.
With the development of modern optical technology, many applications need ultra-short pulse with high average power, for example, large-scale Laser display, an application in which mode-locked beams facilitate nonlinear wavelength conversion to generate high-average-power beams with red, green and blue color. Passively mode locking with SESAM (semiconductor saturable absorber mirror) has become an important method for CW mode locked laser. It leads to a compliable set up, and also allows for high average power [3] and high repetition rate [4].
In this paper, we report what is to our knowledge the first CW diode-pumped passively mode-locked ceramic Nd:YAG with SESAM. We successesfully generated pulse trains with estimated pulse width 8.3ps. The repetition rate was 130MHz. The laser output was 1.59W with an optical-optical efficiency of ≈21%.
2. Experiment setup
The laser setup used in our experiment is shown schematically in Fig.1. The pump light at 808nm from a fiber-coupled laser diode bar was focus into the ceramic by some coupling lenses. The laser diode was cooled by water and the temperature of water was set at 25°C. The focused beam in the laser medium had a diameter of ~400µm. It was about half of the calculated diameter of the laser beam in the ceramic Nd:YAG. According to the theory of mode locking [5], stable mode locking laser with SESAM could only be achieved when the laser was operated with a single transverse mode. The small size of pump beam could help to obtaining high beam quality although the efficiency would be reduced. The φ 4×5mm3 ceramic sample used in our experiment has a Nd3+doping concentration of 1 at.%. Both sides of the ceramic sample were AR-coated at 1064nm to decrease the optical loss and avoid potential etalon effect. To remove the generated heat, the ceramic sample was water-cooled and the temperature of water was set at 25°C during the operation. The cavity was a Z-folded resonator with three mirrors and a SESAM. M1 was a highly reflective mirror R>99.8%, 1064nm and the radii of curvature of M3 was 200mm. M2 was a flat mirror with HR-coated at 1064nm and AR-coated at 808nm to obtain more pump efficiency. M1 was a flat mirror used as output coupler with T=10% at 1064nm. Because M1 was a spherical mirror, it would cause astigmatism that would strongly affected the operation of the mode locked laser. Then the angel α was optimized to not more than 8°.
The absorber saturation pulse energy was estimated to be about 60 µJ/cm2, which was sufficient to pulse shaping. The modulation depth, non-saturable losses, and absorption recovery time of the SESAM were 1.0 %, <0.2 %, and ~20 pico-seconds, respectively. The SESAM was attached on a copper heat sink, without other cooling devices. The spot size on the SESAM was calculated to be about 40~60µm in diameter. To insure that the real spot size was close to the calculated one, we made the position of the SESAM adjustable.
3. Result and discussion
The output power was plotted in Fig. 2 as a function of pump power. When the pumping power was increased to 7.6W, we obtained an output power 1.59W with an optical-optical efficiency of 21%. The laser also exhibited a clear regime of QML(Q-switched mode locking) at a pump power range of Ppump<5.8W with an output power of ~1W. When the pump power was increase slightly higher than 5.8W, a stable CW mode-locking state was achieved. We employed a digital oscilloscope with 500MHz bandwidth (Tektronix TDS 3052) to show the pulse trains (Fig. 3).
According to the theory of Q-switching stability limits of continuous-wave passive mode locking [6], the minimum pulse energy Ep,c for stable CW mode locking can be obtained by:
where Fsat,L=hν/σm denotes the saturation fluence of the gain medium with a lasing frequencyν, σ is the stimulated emission cross section, and m=2 is used to reflect an average over the standing-wave in a linear cavity; Asat,L denotes the spot size on the ceramic sample; Fsat,A denotes the saturation fluence of the saturable absorber with a modulation depth of R. The saturation fluence of Nd:YAG was estimated to be Fsat,L=0.1438J/cm2 ; Asat,A denotes the spot size on SESAM which was estimated to be 40~60µ m, the saturation fluence of the absorber was estimated to be Fsat,l=60µ J/cm2, and the modulation depth of the saturable absorber was ~1%. Thus the estimated minimum pulse energy for stable mode locking was around 65nJ.
The experiment minimum pulse energy for stable mode locking could be estimated by:
where frep denotes the repetition rate frequency ~130MHz, the average output power Pout was about 1W and the output coupler Toc was 10%. Finally, the minimum pulse energy was 76nJ at a pump power of Ppump=5.8W. The experiment result was a bit more than the calculated one. It may be explained that the actual value of spot size on SESAM or the ceramic sample was larger than the estimated one. When the laser was pumped a bit higher than 5.8W, we adjusted the position of SESAM slightly to change the spot size on the SESAM and found that it didn’t appear unstable. We had continuously operated for scores of minutes and instability did not appear. It showed that our mode locked laser was stable enough to against some perturbations. Assuming a Gaussian pulse profile, the pulse width was estimated to be 8.3 pico-seconds by using an autocorrelator (FR-103 autocorrelator made by Femtochrome Resarech, Inc) and the result was captured by our digital oscilloscope (Fig. 4).
Fig. 4. 8.3 psec pulse width measured by an autocorrelator. The dots indicate the experiment data and the solid line indicates the Gaussian fit data.
4. Conclusion
In summary, we reported a first demonstration of continuous-wave passively mode-locked ceramic Nd:YAG laser at λ=1064nm. A SESAM was used in the laser to generate pulses of 8.3 pico-seconds with a repetition rate of ~130 MHz. An average power of 1.59W was obtained with an optical-optical efficiency of ~21 %. The result revealed that ceramic Nd:YAG is suitable for CW mode locking.
Acknowledgments
This work is supported by the Knowledge Innovation Programme of Chinese Academy of Sciences (KJCXZ-XW-W09) and the National High Technology Research and Development Programme of China under Grant No 2002AA311040.
References and Links
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