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Q-switched operation of a high-quality Nd:YAG optical vortex laser with the first order Laguerre-Gaussian mode and well-determined helical wavefronts using a fiber-based pump beam conditioning scheme is reported. A simple two-mirror resonator incorporating an acousto-optic Q-switch was employed, along with an etalon and a Brewster plate to enforce the particular helicity of the output. The laser yielded Q-switched pulses with ~250 μJ pulse energy and ~33 ns pulse duration (FWHM) at a 0.1 kHz repetition rate for 5.1 W of absorbed pump power. The handedness of the helical wavefronts was preserved regardless of the repetition rates. The prospects of further power scaling and improved laser performance are discussed.
© 2013 Optical Society of America
1. Introduction
Optical vortex beams with a ring-shaped intensity profile and helical wavefronts have been attracting much interest due to their possession of orbital angular momentum (OAM) [1–4] and are used in a number of application areas such as optical trapping and manipulation of particles [5–7], optical imaging [8], quantum optics [9] and laser material processing [10–12]. Recently, Omatsu et al. investigated laser ablation using high-energy optical vortex laser pulses and succeeded in fabricating chiral metal nanoneedles with a tip curvature of <40 nm providing the potential for nano-structure fabrication [10, 11]. Therefore these impressive works have prompted many researchers to attempt to generate optical vortex laser beams with high power and high quality. In particular, they have focused on generating a beam with the Laguerre-Gaussian (LG) mode since it cannot only have the azimuthal-angle dependence of the phase, i.e. the helical wavefronts [1, 2], but also form the complete basis set of orthogonal modes. The most popular scheme to generate an optical vortex beam is to transform the Hermite-Gaussian (HG) mode (TEM0n) into the required Laguerre-Gaussian mode with helical phase fronts using external mode-transforming optics (e.g. diffractive optical elements [13], spiral phase plates [14], or a pair of cylindrical lenses [15]). However, it is hard to avoid a significant degradation in beam quality and brightness with these methods. Furthermore, power scaling is rather challenging due to the power handling limitations of the optical elements. Optical vortex beams can be generated directly within the laser itself by exploiting thermal lensing [16–19], but this method requires a phase element and an aperture in the cavity for mode discrimination [16] or needs to be operated in a high power regime due to its pump-power dependent resonator configurations [17–19] that limit the flexibility of the operating conditions such as the helicity, output power levels, and repetition rates. An alternative strategy for direct generation of the optical vortex beam in an end-pumped solid state laser is to use a ring-shaped pump beam with the spatially-matched intensity distribution for the LG0n mode in the laser resonator [20–22]. This approach employs a low-loss beam conditioning element based on a capillary fiber to re-format the pump beam into a beam with an annular near-field intensity distribution, which allows for a great deal of flexibility in the choice of laser medium [20] and strong discrimination of the transverse modes [21] due to the long depth of focus in the pump beam. This method has been successfully applied to a single-frequency Nd:YAG laser to achieve the optical vortex output with the LG01 mode and well-determined helical wavefronts [22].
In this paper, we report a Q-switched Nd:YAG optical vortex laser with the first-order Laguerre-Gaussian mode (LG01) output end-pumped by a ring-shaped diode laser beam. Employing an acousto-optic Q-switch, an etalon, and a Brewster plate, the Nd:YAG laser produced Q-switched optical vortex pulses of ~250 μJ pulse energy with ~33 ns pulse width (FWHM) at a 0.1 kHz repetition rate in the LG01 transverse mode.
2. Experiment and results
The experimental set-up for the Q-switched Nd:YAG optical vortex laser is shown schematically in Fig. 1. To produce the pump beam with the required ring-shaped intensity profile, the diode pump beam at 808 nm was launched into the annular waveguide of a simple capillary (hollow-core) fiber with a pure silica inner-cladding of a 200 µm diameter and a 130 µm diameter air-hole in the center. The capillary fiber was coated with a low refractive index (n = 1.375) fluorinated polymer outer-cladding providing a calculated NA of 0.49 for the pump light in the annular waveguide. The launching efficiency was measured to be ~64%, and the beam propagation factor (M2) for the transmitted ring-shaped pump beam was measured to be ~63. The length of the capillary fiber was about 50 cm. The re-formatted pump beam exiting the capillary fiber was relay-imaged into the Nd:YAG crystal using a simple telescope and the pump beam size was carefully adjusted to excite the targeted LG01 mode in the resonator. Further details of the ring-shaped pump source can be found in Ref. 21.
Fig. 1 Schematic diagram of the Nd:YAG laser resonator. IC: input coupler, OC: output coupler, BP: Brewster plate.
We constructed a simple two-mirror Nd:YAG laser resonator comprising a plane pump in-coupling mirror (IC) with high reflectivity (>99.8%) at the lasing wavelength (1064 nm) and high transmission (T>95%) at the pump wavelength (808 nm) and a plane output coupler (OC) with 10% transmission at the lasing wavelength. A plano-convex lens of a 300 mm focal length with anti-reflection-coated faces at the pump and lasing wavelengths was placed inside the resonator to ensure that the resonator was stable over the full range of the pump powers used in our experiments. We used an anti-reflection coated 1.0 at% Nd:YAG crystal with a length of 5 mm as the gain medium and mounted it in a water-cooled aluminum heat-sink maintained at 19 °C. This was positioned in close proximity to the pump in-coupling mirror. Using this arrangement, the TEM00 mode radius on the input coupler surface was calculated to be ~275 μm, and hence the ring-shaped pump beam waist outer radius was adjusted to be ~325 μm for the LG01 mode generation [21]. The confocal parameter (2π n w 2/ λ M 2) for the pump beam inside the Nd:YAG crystal was calculated to be 17.9 mm satisfying the condition for robust mode selection that the ring-shaped pump beam profile was preserved over the gain region [20]. Under this configuration, the laser was operated in a multi-longitudinal mode without any particular helical wavefronts [22]. In order to force the LG mode output to have one particular helical wavefronts, the LG0n mode should be composed of degenerate HGn0 and HG0n modes with a locked frequency [23]. Therefore an etalon and a Brewster plate were inserted into the cavity for fixing the operating laser frequency.
The continuous-wave (cw) laser output power as a function of absorbed pump power is shown in Fig. 2. The laser yielded 1.5 W of the linearly-polarized output at 1064 nm for an absorbed pump power of 5.7 W. The corresponding slope efficiency with respect to absorbed pump power was 32%. Laser performance was also evaluated with a 5% transmission output coupler. In this case, the laser yielded a maximum output power of 1.1 W for the same maximum pump power corresponding to a slope efficiency of 23%. Thus, we used the output coupler with 10% transmission for the Q-switched laser. The near-field output beam profile was monitored at the relay-imaged position as a function of laser output power with the aid of a silicon CCD camera (Spiricon BS-USB-SP620). The output beam had an axially-symmetric ring-shaped intensity profile at all power levels, the inset in Fig. 2. The beam propagation factor (M2) was measured to be ~2.01 with the aid of a Beam Profiler (Thorlabs) and the silicon CCD camera, which is in close agreement with the theoretical value for a pure LG01 mode, 2 [24], and hence the excited mode was indeed the LG01 mode.
Fig. 2 Nd:YAG laser output power as a function of absorbed pump power in cw and Q-switched mode of operation. The inset is the cw output beam profile at an absorbed pump power of 5.1 W.
Q-switched operation of the Nd:YAG laser was achieved using a TeO2 acousto-optic (AO) Q-switch. The AO Q-switch was carefully placed and adjusted to preserve the spatial beam characteristics while maximizing the diffraction efficiency. The average output power as a function of absorbed pump power in Q-switched mode of operation is shown in Fig. 2. Figure 3 shows the dependence of the pulse energy and the corresponding peak power on the repetition rate for an absorbed pump power of 5.1 W including the oscilloscope traces of the Q-switched pulse and its pulse train for a repetition frequency of 5 kHz. At the pulse repetition frequency of 50 kHz, the pulse energy was ~22 μJ with a relatively long pulse duration of ~194 ns corresponding to a peak power of ~113 W. The pulse energy increased along with the decrease of the pulse width at lower repetition rates due to the increase of the stored energy between the pulses and hence the higher gain. Therefore, at the low pulse repetition frequencies (<500 Hz), the laser yielded a maximum pulse energy of ~250 μJ with a pulse duration (FWHM), ~33 ns, corresponding to the peak power of ~7.6 kW. The maximum pulse energy for an optimized system was calculated to be ~650 μJ [25], allowing for the possibility of the higher pulse energy by using an OC with higher transmission. The beam profile of the pulsed output was also monitored using a silicon CCD camera and Fig. 3(c)shows the beam profiles at the repetition rates of 1 kHz, 5 kHz, and 20 kHz, confirming the ring-shaped intensity profile was well-preserved in Q-switched mode of operation. The measured beam quality M2 also remained less than ~2.1 proving the generated mode was still the pure LG 01 mode.
Fig. 3 (a) Q-switched pulse energy, pulse width (FWHM), peak power and average output power versus a pulse repetition rate, (b) the single Q-switched pulse and its pulse train at the repetition rate of 5 kHz, and (c) the output beam profiles at the repetition rate of 1 kHz, 5 kHz, and 20 kHz for an absorbed pump power of 5.1 W.
In order to confirm the possession of single OAM (i.e. helical wavefronts with one particular handedness), the generated LG01 mode beam was interfered with the reference beam of the spherical wavefront in a Mach-Zehnder interferometer [2, 23]. Under the same operating conditions with Fig. 3, the resultant interference pattern had clear spiral fringes with the well-determined handedness, as shown in Fig. 4. The handedness of the interference patterns, corresponding to the sign of the OAM, could be simply controlled by tilting the angle of the etalon in the cavity [22, 26] and remained unchanged at different repetition rates. These patterns were very stable over time and did not show any dependence on the output power levels. Therefore, these results showed that the Q-switched operation of the high-quality optical vortex laser with the LG01 mode can be easily achieved by our approach proving its flexibility in mode of operation. The mechanism of the wavefront-handedness selection using the etalon is not yet understood and hence is the subject of ongoing investigations.
Fig. 4 The spiral interference patterns with left or right-handedness at the repetition rates of 1 kHz, 5 kHz, and 20 kHz for an absorbed pump power of 5.1 W.
In the present arrangement, the output power was limited by the maximum available pump power. For higher energy operation, we need to take account of the thermal problems due to the absorbed heat, especially thermal lensing and thermally-induced depolarization loss [25]. These problems can be alleviated by optimizing the resonator configuration with an additional component compensating thermally-induced birefringence [27]. Therefore, scaling to higher energies should be possible via the use of a higher power pump source in combination with an optimized resonator configuration to avoid the thermal problems.
3. Conclusions
In summary, we have demonstrated a Q-switched Nd:YAG optical vortex laser with the first-order Laguerre-Gaussian mode and helical wavefronts end-pumped by the spatially-tailored pump beam using a simple capillary fiber. The laser yielded Q-switched pulses of ~250 μJ pulse energy with ~33 ns pulse duration (FWHM) at a 0.1 kHz repetition rate in a high quality LG01 mode. It was also confirmed that the output beam had the helical wavefronts with well-determined handedness by fixing the operating laser frequency. Further optimization of the laser design combined with a high power pump source should allow higher energy Q-switched operation of the optical vortex laser. This technique has the advantages of simplicity and flexibility in resonator design and mode of operation over existing techniques for generating an optical vortex laser and hence should benefit numerous application areas requiring various beam characteristics.
Acknowledgment
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0022830).
References and link
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