a mode-locked radially polarized laser based on a ceramic Nd:YAG rod that was transversely pumped by LD bars. The maximum output power of 10.6 W was achieved with the frequency of 45 MHz, the average pulse width of 75 ps Abstract: By using a semiconductor saturable absorber mirror, we demonstrated the generation of and the beam quality of M2= 2.11.
© 2011 OSA
Due to many unique properties, radially polarized laser beams are attracting more attention in a variety of laser applications, such as material processing [1,2], optical trapping [3,4], particle acceleration  and high resolution microscopy . There have been many reports on the generation of radially polarized lasers inside the cavity. A conical Brewster prism was designed to directly generate a radially polarized laser beam based on the radial and tangential polarization selectivity . Using a c-cut Nd:YVO4 crystal, a radially polarized laser beam, which oscillated based on the optical path difference between an extraordinary ray and an ordinary ray induced by the birefringence of the crystal, was obtained in a simple laser resonator . Inon Moshe and his associates demonstrated a series of impressive work on the generation of the radially polarized laser [9,10]. Based on the different focus between radially and tangentially polarized light in thermally stressed isotropic laser rod, a Nd:YAG laser oscillator was developed to produce a radially polarized laser beam at a power of 180 W with M2=2.3. And through a MOPA (master oscillator power amplifier) system, they achieved a CW radially polarized laser beam with the output power of 2 KW and the beam quality of M2<10 . The radially polarized lasers were almost always operated in CW mode in the past until recently when Florian Enderli reported the emitting of the mode-locked radially polarized laser . In their scheme, a Nd:YAG rod pumped by an arc lamp was used as an active medium, and an acousto-optic modulator was used as an actively mode-locking component. A mode-locked radially polarized laser beam was achieved with the output power of 3 W, the pulse width of 140 ps, the frequency of 82 MHz and the beam quality of M2=2.1.
In this paper, we demonstrated the generation of a passively mode-locked radially polarized laser beam by using a semiconductor saturable absorber mirror (SESAM). The SESAM was a simple, self-starting passively mode-locking component, invented by Ursula Keller in 1992 when she was at Bell Labs in New Jersey, USA. From then on, Ursula Keller and her associates did a lot of work [12–15] and led the development of SESAMs. In our experiment, the radially polarized laser beam generation scheme of Inon Moshe's half symmetric cavity  integrated with a relaying-image telescope and a SESAM were applied firstly. We achieved a CW mode-locked (CWML) radially polarized laser beam with the output power of 10.6 W, the pulse width of 75 ps, the frequency of 45 MHz and the beam quality of M2=2.11. To the best of our knowledge, the laser has the highest output power and the shortest pulse width among the ever-reported mode-locked radially polarized lasers.
2. Experimental setup
Figure 1 shows the scheme of the laser cavity which consisted of a ceramic Nd:YAG rod, a plane output mirror, two high-reflection spherical mirrors, an aperture and a SESAM. For a mode-locked laser, the residual reflection of the optic components should be avoided in certain ways, such as substituting the plane components with the wedged components, not reflecting the light on the center of the spherical mirror. For a radially polarized laser, none of the components in the cavity can show polarization-dependent transmission or reflection behavior. The design of the mode-locked radially polarized laser must cater to the needs of two aspects. In our laser cavity, the output mirror is a wedged mirror coated with the antireflection film on the face outside the cavity; both parallel end faces of the laser crystal rod are polished with 3° wedged angle; and the light inside the cavity is adjusted to depart from the center of the high-reflection spherical mirrors.
The laser crystal made by our research group is a ceramic Nd:YAG rod with the length of 45 mm, the diameter of 3 mm, and the Nd3+ ions concentration of 1%. Both faces of the laser rod are coated antireflection film with less than 0.1% reflectivity at 1064 nm. The laser rod is transversely pumped by LD bars, which were manufactured by nLIGHT laser, with a CW output power not less than 60 W each. The pump module we assembled is composed of 9 LD bars, which are combined in three sections and pump the active medium from three directions. The maximum pump power is 540 W. The laser rod mounted in a quartz tube is cooled by water. The output mirror (M1) is a wedged mirror with 8% transmissivity at 1064 nm. The aperture close to the output mirror has a diameter of 0.8 mm. The highly reflective mirrors (M2 and M3) compose a reflecting relay-image telescope, which have a high-reflection coating at 1064 nm (R>99.9%). By changing the curvature radii of the two mirrors, we can simply control the mode radius on the SESAM and adjust the cavity length. In our experiment, the optimal curvature radii of M2 and M3 are 2.5 m and 0.5 m respectively. The SESAM was manufactured by BATOP optoelectronics in Germany. The device has a recovery time of 500 fs, a modulation depth of 0.5%, a nonsaturable losses of 0.5%, and a saturation fluence of 90 uJ/cm2. The SESAM mounted upon a copper heat sink is cooled by water. The total cavity length of the laser is 3.22 m.
3. Experimental results and discussions
For an isotropic solid-state rod (such as a Nd:YAG crystal rod), the volume pumped and periphery cooled configuration induces thermal lens and thermal induced birefringence which results in bipolar len, where the radial and tangential polarization components focus differently . This can be used to generate the radially and tangentially polarized laser. A ceramic Nd:YAG rod also can be seen as an isotropic solid-state rod. In fact, so as a solid-state rod which is isotropic on the section face, such as a c-cut Nd:YVO4 rod , could be used to design the laser.
After optimizing the cavity length and the aperture diameter, the maximum power of 10.6 W is achieved at the pump power of 152 W. Figure 2 shows the output power versus the pump power. When increasing the pump power, the profiles of the laser beam are detected by CCD at the same time. The laser has only one stable TEM01* mode CWML region which is between the pump power of 136 W and 152 W. Figure 3 shows the laser profiles of the TEM01* mode. Figure 3(a) is a total intensity distribution of the TEM01* mode laser. A dark spot appears at the center of the laser beam, and the beam shape imitates that of a doughnut. The intensity distributions of the beam passing through a linear polarizer are shown in igs. 3(b)-(d). The arrows indicate the direction of the linear polarizer. Two-lobe-shaped intensity distributions along the direction of the polarizer are achieved, indicating that the laser beam is radially polarized. The measured M2 value is 2.11 which is close to the theoretical value 2.0 of the radially polarized laser beam.
In the radially polarized laser beam generation scheme of the half symmetric cavity, the laser medium has only one ‘U shape’ stable region and the radially polarized laser works around the right edge of the region. In this time the focal length of the radially polarized laser is almost close to the distance between the main plane of the laser rod and the output mirror. When the distance is reduced, the laser must improve the pump power to achieve the stable radially polarized laser output. This means that the distance determines the output power of the radially polarized laser. The radially polarized laser power can be improved by reducing the distance. In our experiments, considering the radially polarized laser output power and its beam quality, we achieved the best result with the distance of 20 cm and the aperture diameter of 0.8 mm. Figure 4 shows the mode radii of the radially polarized light on the laser rod, the output mirror and the SESAM versus the dioptric power. Noticing that the mode radius of the output mirror reduces monotonously when the dioptric power is increasing, we ascertain the laser work state by detecting the beam radius close to the output mirror. When the radially polarized laser reaches the maximum output power of 10.6 W, the laser works around ‘A’ line, as shown in Fig. 4; at this point the mode radius as a function of the position in the cavity is shown in the Fig. 5 .
The pulse train shown in Fig. 6 is detected by a photodiode and analyzed with a high-speed oscillograph, which indicates that the radially polarized laser works in the CWML state. The pulse repetition rate is 45 MHz,and the pulse duration is 75 ps (Fig. 7 ), which is detected by the FR-103WS autocorrelator manufactured by FEMTOCHROME RESEARCH, INC.
In the passive mode locking of a high-power solid-state laser, the main challenge is to overcome Q-switched mode-locking (QML) instabilities introduced by the saturable absorber. For ps lasers, not operating in the soliton mode-locked regime, the intracavity pulse energy Ep has the condition for stable cw mode locking. The SESAM parameters (the saturation fluence Fsat,A and the modulation depth ΔR) must be selected carefully. The saturation fluence of the gain medium, in a standing wave cavity, is given as Fsat,L=hν/2σL, where hν is the laser photon energy and σL (σL=3.47×10−19cm2 for ceramics Nd:YAG ) is the emission cross section of the gain medium. In the mean time, the saturation parameter S of the SESAM, which is defined by , should not be larger than 20 to avoid multiple pulsing instabilities and damage to the SESAM [14,15]. In our radially polarized laser experiment, the mode area AL of the radially polarized laser in the laser rod is far larger than that of the TEM00 mode, so we choose a rather small spot AA on the SESAM to reach stable cw mode locking. In our cavity, the mode radius on the SESAM is only 158 μm by calculation, while the mode radius on the laser rod reaches 790 μm at the maximum radially polarized laser power. The SESAM is operated quite far above saturation energy 40 times, but the multiple pulsing instabilities or the SESAM damage is not observed in the experiment.
By using a SESAM, we generated a passively mode-locked radially polarized laser based on a ceramic Nd:YAG rod which was transversely pumped by LD bars. The maximum output power of 10.6 W was achieved with the frequency of 45 MHz, the average pulse width of 75 ps, and the beam quality of M2=2.11. In the future, we intend to extend the stable range of the mode-locked radially polarized laser and further improve the pulse energy.
This project was funded by Fujian Provincial Science and Technology Project of China (No. 2007HZ0004-1) and Knowledge Innovation Project of Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.
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