Simultaneous generation of helical beams with linear and radial polarization by use of a segmented half-wave plate

Simultaneous generation of helical beams with linear and radial polarization is demonstrated by use of a segmented half-wave plate in a Ti:sapphire laser cavity. A linearly polarized Laguerre-Gaussian beam is converted to a radially polarized beam with a spiral phase shift and vice versa. In addition, these two beams coexist in the cavity, and the cavity emits one of these two beams from an output coupler.     

Single higher-order transverse mode operation of a radially polarized Nd:YAG laser using an annularly reflectivity-modulated photonic crystal coupler

We demonstrate single higher-order transverse mode operation of a radially polarized Nd:YAG laser by using a polarization-selective and reflectivity-modulated output coupler. A narrow annular region with low reflectivity fabricated in a photonic crystal mirror behaves so as to select a higher-order transverse mode of a cylindrically symmetric laser beam. A double-ring-shaped radially polarized TM(02) mode beam is stably generated.     

基于内腔布儒斯特双轴锥棱镜的掺镱光纤激光器的径向偏振模振荡

实验采用一个内腔布儒斯特双轴锥棱镜,在掺镱多模双包层光纤激光器中实现了径向偏振光振荡,并在长2 m的增益光纤中获得了高达6.2 mW的功率输出.研究揭示了从光纤激光器振荡中获得径向偏振光的新途径.然而,该激光器输出的径向偏振光的光强分布并不很均匀,且偏振消光比较低,需要进一步改进.     

Radially polarized laser beam from a Nd:YAG laser cavity with a c-cut YVO4 crystal

We demonstrate the generation of a radially polarized beam by simply inserting an undoped c-cut YVO₄ crystal into a Nd:YAG laser cavity. In a hemispherical cavity, the cylindrically symmetric, positive birefringence of the YVO₄ crystal extends the stability limit of the cavity length for an extraordinary ray (radial polarization) compared to an ordinary one (azimuthal polarization). By adjusting the cavity length, a radially polarized beam with an output power up to 1 W was selectively obtained. In addition, a higher-order transverse mode was also generated by arranging the cavity design. The method demonstrated in this paper can be readily applied to laser systems with an isotropic laser medium. PACS 42.60.Da; 42.25.Lc; 42.25.Ja     

Converging-axicon-based radially polarized ytterbium fiber laser and evidence on the mode profile inside the gain fiber

The radially polarized mode is achieved from an active Yb fiber by utilizing of an intracavity converging axicon, where the axicon acts as a TM(01) mode selector based on not only its Brewster convex surface but also the distance between its vertex and plane output coupler. The polarization state of the annular laser beam is checked by using a home-made eight-hole aperture. Furthermore, an uncoated plane glass plate is inserted into the cavity, and the reflected beam points to the existence of an annular lasing mode inside the gain fiber. The issues for developing high-power radially polarized fiber lasers also are considered.     

Generation of a radially polarized laser beam by use of the birefringence of a c-cut Nd:YVO4 crystal

We demonstrated the generation of a radially polarized laser beam from an extremely simple laser resonator including a c-cut Nd:YVO4 crystal as a laser medium. The oscillation in the radial polarization was based on the optical path difference between an extraordinary ray and an ordinary ray induced by the birefringence of the crystal. By simply adjusting the distance between two cavity mirrors, only the extraordinary ray became stable for the oscillation, resulting in the generation of a radially polarized beam. The beam was very stable even at low power output and is expected to be a promising radially polarized laser source because of its excellent simplicity.     

Self-polarized ytterbium-doped fiber laser

We demonstrate a linearly-polarized, ytterbium-doped fiber laser that uses an uncoated, undoped ceramic YAG plate as the output coupler, and the corresponding polarization extinction ratio of laser beam increases with incident pump power and then saturates at larger pump power. For comparison, the output coupler of the fiber laser is replaced by 10% reflectivity plane mirror, while the feature of the polarization of laser output is kept unchanged. The results show that the origin of the pump-dependent and self-started polarization is associated with the intensity-dependent nonlinear birefringence in the gain fiber.     

Polarization manipulated solid-state lasers with crystalline-orientations

The polarization states of 〈111〉-cut Yb:YAG crystal microchip lasers were investigated by pumped with the elliptically polarized pump beam from fiber-coupled laser-diode. The manipulated polarized lasers were achieved in laser-diode pumped Yb:YAG microchip laser by controlling the crystalline-orientations in 〈111〉-growth Yb:YAG crystal. Generally elliptically polarized lasers were obtained in laser-diode pumped Yb:YAG microchip lasers. However, crystalline-orientation manipulated linearly polarized laser was obtained when six different sites with different crystalline orientations were set to parallel to the major axis direction of the elliptically polarized pump beam. Six different sites in Yb:YAG crystal were separated with 30° and 90°, which were responsible for the linearly polarized laser oscillations. Circularly polarized lasers were observed when a Yb:YAG crystal was aligned to a special position between two sites responsible for linearly polarized laser oscillation. Effects of the polarization states of pump source on the laser polarization states of Yb:YAG microchip lasers and polarization direction of different polarized lasers with respect to Yb:YAG crystal rotation was addressed.     

LD端面泵浦Cr~4∶YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr4+∶YAG被动调Q Nd∶YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr4+∶YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr4+∶YAG晶体内光斑半径也随谐振腔长度减小,引起Cr4+∶YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

LD端面泵浦Cr4:YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr⁴⁺:YAG被动调QNd:YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr⁴⁺:YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr⁴⁺:YAG晶体内光斑半径也随谐振腔长度减小,引起Cr⁴⁺:YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

Low-cost low-threshold diode end-pumped Tm:YAG laser at 2.016?μm

We report a low-threshold continuous-wave Tm:YAG laser that can be excited near 785?nm with low-cost, single-mode AlGaAs laser diodes. Low-threshold operation was achieved using a tightly focused, astigmatically compensated x-cavity containing a 2-mm-thick Tm:YAG crystal with 5?% Tm³⁺ concentration. Two linearly polarized single-mode diodes operating at 785.8?nm were polarization coupled to end pump the resonator. With a 6?% output coupler, as high as 32?mW of output power could be obtained at 2016?nm with 184?mW of incident pump power. The output could be further tuned in the 1935?C2035?nm range. Slope efficiency measurements indicated that cross-relaxation was very effective at this doping level. With a 2?% output coupler, lasing could be obtained with as low as 32.3?mW of pump power. In the limit of vanishing output coupling, the incident threshold pump power could be reduced to as low as 25?mW. To our knowledge, this is among the lowest lasing thresholds reported to date for continuous-wave, room-temperature thulium lasers.     

Diode-end-pumped frequency-doubled Nd:YAP/Cr:YAG laser using a KTP crystal

We present a design for a compact laser-diode-end-pumped Nd:YAP/Cr:YAG q-switched laser in a plano?Cconcave cavity configuration. With an optical-to-optical efficiency of 35%, this laser provides an average output of 1.2 W with 13 mJ at 1080 nm. After that, the laser is frequency doubled by replacing the output coupler with a flat KTP crystal to achieve an output of 8 mJ in 8 ns at 540 nm. At a peak power of 1 MW, the laser emits linearly polarized output with an average power of 800 mW. The calculated M² value is 1.3, the output is very stable, and the laser operates in the TEM₀₀ mode.     

Radially polarized,actively Q-switched,and end-pumped Nd:YAG laser

In this paper, we demonstrated an actively Q-switched, radially polarized, and laser-diode end-pumped Nd:YAG laser with an acousto-optic modulator as the Q switch and a photonic crystal grating as the output coupler. The laser generated pulses of 26.4–67.2 ns duration, and the repetition rate can be continuously adjusted from 500 Hz to 9.238 kHz with peak power up to 7.75 kW. Such a radially polarized pulse would facilitate numerous applications.     

High peak power Nd:YAG laser pumped by 600-W diode laser stack

The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has been designed. The multimode (M²≈2.6) output energy of about 42 mJ was demonstrated in free running mode for110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in plane-plane cavity with output coupler of 84% transmission. The energy of 8 mJ in 2.1-ns pulse duration was obtained for near TEM₀₀ output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission, the energy of 5.1 mJ in 2.5-ns pulse duration was obtained for output beam close to TEM₀₀ mode.     

Production of radially or azimuthally polarized beams in solid-state lasers and the elimination of thermally induced birefringence effects

Production and amplification of radially and azimuthally (tangentially) polarized laser beams are demonstrated. Based on the different focusing between radially and tangentially polarized light in thermally stressed isotropic laser rods, Nd:YAG laser oscillators were developed to produce low-loss stable oscillation in a single polarization. Pure radially polarized light at 70 W with M2 = 2 and on-axis impure radially polarized light at 150 W with M2 = 2.5 were achieved. The radially polarized beams were then amplified while good beam quality and polarization purity were retained. Complete elimination of thermal-birefringence-induced aberrations was demonstrated. This should allow much better beam quality from rod-based high-power lasers.     

Generation of a radially polarized laser beam by use of a conical Brewster prism

To generate a radially polarized laser beam we designed and fabricated a new Brewster optical element that consists of convex and concave conical prisms. The lateral surface of the convex conical prism was coated with a dielectric multilayer (SiO2 and Ta2O5) to enhance polarization selectivity. By combining two prisms we obtained a conical Brewster prism without beam divergence owing to refraction. A radially polarized TEM01* (R-TEM01*) mode laser beam was demonstrated when this prism was used inside a Nd:YAG laser cavity.     

Radially polarized LG01-mode Nd:YAG laser with annular pumping

We demonstrated a radially polarized, LG₀₁-mode Nd:YAG laser by applying annular-shaped pump light. The annular profile of the pump light was originated from the mode conversion inside a conventional multimode fiber under off-focus coupling condition. This laser gave a maximum output power of 1.2 W at 1,064 nm with a slope efficiency of 28.3 %.     

Radially polarized and passively Q-switched fiber laser

We report, for the first time to our knowledge, a radially polarized and passively Q-switched Yb-doped fiber laser. By using a Cr(4+):YAG crystal as a saturable absorber and a photonic crystal grating as a polarization mirror, a radially polarized pulse is produced, which has 202 W of peak power, 75 ns duration, and ~92% polarization purity at a 56.6 kHz repetition rate. The Q-switched pulse with radial polarization from the fiber laser would facilitate numerous applications.     

Circular modes selection in Yb:YAG laser using an intracavity lens with spherical aberration

The possibility of use of ordinary short-focus lenses with strong spherical aberration for mode selection in a laser resonator is demonstrated. Plano-convex spherical lenses with foci from 2.5 to 10.0 cm placed inside the resonator of the CW laser diode pumped Yb:YAG laser were used to select circular modes of Laguerre-Gauss and Bessel-Gauss type, or to get ring-like, multi-mode off-axis oscillations with variable inner and outer diameters of rings. At the constant pump power mode selection was obtained by shifting an intracavity lens along the resonator axis relative to the Yb:YAG ceramics active element of 1.1 mm thickness. Peaked and hollow multi-ring beams with output power up to 30 mW were produced. Near diffraction free (ndf) propagation of the central part (minimum or maximum) of these beams was observed along distances over 10 m from the resonator output coupler. The mechanisms of mode selection due to spherical aberration in the resonator and ndf propagation of output beams are considered. The possibility of producing radially polarized beams from the laser with an intracavity lens and possible applications of the devised laser scheme are discussed.     

Intracavity Raman conversion for 16-μm Stokes pulse generation in para-hydrogen

16-μm Stokes pulses were directly generated for the first time to our knowledge by an intracavity configuration for the para-hydrogen Raman laser. We have analyzed Stokes field growth using a focused gain model and designed a pump/Stokes cavity to satisfy CO₂ pump power and pulse duration requirements for Raman oscillation. The CO₂ laser oscillation with circular polarization was realized by seeding externally circularly polarized CO₂ radiation. An output energy of 2.4 mJ was obtained with the output coupler of 0.5% transmittance, which indicated that 420 mJ of Stokes pulse energy was stored inside the cavity. This suggests that a much higher energy can be extracted by the optimization of cavity parameters. Received: 18 November 1998 / Published online: 26 May 1999