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Solid-State, Laser-Pumped
Tunable Pulse Dye Lasers
By: Robin Keith Elkins
Abstract - This disclosure relates to a new class of tunable lasers incorporating a solid-state Laser material designed for pulsed operation. These lasers are Laser pumped lasers which are optically pumped by short duration Laser optical pulses that match the absorption characteristics of the Laser dye solid materials. Examples are given for configurations of various types known to successfully give rise to efficient Laser operation. The Laser dye solids are available in a wide range of tunable wavelength regions and can be pumped by various pumping lasers. Tunability is achieved by the selection of the Laser dye solid type, producing either broadband or narrow line width Laser species, depending on the configuration of the laser. The improvements afforded by the use of these Laser dye solids and the associated new Laser designs hold many advantages over prior art in the field of dye Laser design.
Background - Dye liquid lasers are well known in the art. These lasers can be categorized as either one of 2 types; either flashlamp-pumped dye lasers (FPDL's), or Laser pumped dye lasers. Flashlamp-pumped dye lasers incorporate a flashlamp type optical pump with broadband spectral characteristics and intense light. However, the pump flashlamp output is incoherent light. Conversely, the optical pump employed in Laser pumped dye lasers is coherent light with very high brightness. Laser-pumped dye lasers hold advantages in that they can be designed with better control over the optical pump radiation. Since the output from a laser-pumped dye Laser closely follows the optical pump duration, in terms of pulse duration, the output pulse duration of the pulsed dye Laser can be made short in duration. Pulsed laser-pumped dye lasers with nano-second pulse durations are well known in the art. Such lasers hold advantages in numerous applications where time resolved pulses of coherent radiation are required.
Additionally, the spectral characteristics of outputs from dye lasers are often controlled with "tuners". The tunable nature of such dye lasers is well known in the art, and hold advantages for many applications of such lasers. Also, at least in principle, the spectral output or "tuning" of some dye lasers can be changed simply by changing the Laser dye, offering a different spectral range for the outputs. All of the prior art types of dye lasers utilize a dye solution (liquid dye), usually employing methanol or ethanol (alcohol) as the solvent. The dye is commonly mixed before use with pure crystals of the dye (R6G, for example) and the solvent acting as the dye solution (liquid). In practice, this presents a problem, in that such mixing of the dye solution is messy and also involves potential contact with dyes that are known carcinogens. The alcohol is also highly flammable, and therefore must be handled carefully, presenting a potential for fire hazard.
The designs presented here hold advantages over all the prior art in that they employ a solid form Laser dye material. This means that the Laser material is ready for use, and requires no mixing. The Laser does not require the use of any flammable liquids or solvents either. With the designs disclosed here, the solid dye Laser and solid Laser dye materials also provide the desired characteristics of being tunable in output wavelength, if need be. Therefore, the designs presented here have the desirable characteristics of being tunable, without the undesirable characteristics of messy and dangerous liquid dyes. Additionally, these solid Laser dye materials offer a novel feature in that they can be machined to fit into common optical mounting hardware (standard 1.00 inch Laser mirror holders, for example), or, can be machined to fit into a holder or place unavailable for use with Laser dye liquids, because of size or shape restrictions.
These solid Laser dye materials are also capable of producing high-power Laser outputs that are tunable. If we examine an example, where a 1 cubic millimeter of solid Laser dye material can produce an output of 5 millijoules of energy in a 2 nanosecond pulse at 590 nanometers, we can extrapolate that a 25.4 (1 inch) disc, 6.3 mm thick, can produce 25.4 x (PI) x 6.3 x 0.005 (joules) = 2.51 joules per pulse. Now, with 5 pulses per second, we obtain 12.56 watts of average power output from the Laser system. The peak power output per pulse = 1.255 giga-watts peak (= 1,255,000,000 watts peak) per pulse. Of course, this would require the correct design of the Laser and sufficient stimulating energy emissions from the Laser pump. Pump Laser light from Nd:YAG second harmonic (532 nanometer) q-switched pulses or pulsed nitrogen lasers (337 nanometers) are 2 examples of suitable pump lasers for solid dye Laser systems.
Description of the Designs- There are numerous arrangements of optical elements and for application of Laser pump optical radiation to facilitate Laser action with this new class of laser. Examples are given here to document the reduction to practice of actual working lasers, provided according to general provisions of the designs disclosed.
One example of a tunable solid dye Laser is shown in Figure-1. This operational system employs the following elements. Optical pump radiation source, 100, is an output from a frequency-doubled Nd:YAG pulse Laser (Elk Industries Model 310 O.E.M. laser). This Laser source produces q-switched pulses at 532 nanometers, with a beam diameter of 6.3 millimeters, duration of ~ 2 nanoseconds, and a divergence of less than 3 milliradians. It is a flashlamp-pumped pulse Nd:YAG Laser with a cavity length of ~ 1 metre, with intra-cavity frequency doubling provided by a CD*A second-harmonic generating crystal. Alternatively, a KTP frequency doubling crystal was employed in an extra-cavity configuration. When the extra-cavity frequency doubling is employed, a high reflector at 1.06 microns is utilized to eliminate the 1.06 micron component in the output Laser beam of the Model 310 laser. The Laser has a passive dye saturable absorber type Q-switch. Polarization within the Model 310 Laser is maintained by a single brewster plate.
The pulsed Laser light at 532 nanometers coming from the Model 310 laser, 100, then passes through a dichroic filter, 101, selected for passing only green light. The dichroic filter is available from EDMUND Industrial Optics, stock number 52,533. It has a diameter of 12.5 mm, which easily permits transmission of the pump Laser light pulses (532nm), but absorbs other wavelengths. The filter 101 is mounted inside an optics adapter, Model R-12.7, available from Elk Industries. This adapter allows 12.7mm or somewhat smaller optics to be accommodated and adjusted with standard 1.00 inch diameter optics mounts. The type of optical mount is a type P-100A or MM2-1A, available from Newport Research Corporation (NRC).
Next in the optical path is a plano-convex lens 102 with a diameter of 25mm and a radius of curvature of 35mm. It is available from EDMUND Industrial Optics as stock number 45,145. this optic is mounted in a P100A optics mount available from NRC. The use of the lens 102 is to focus the 6.3mm diameter beam from the Model 310 Laser down to a smaller spot size.
The next element in the optical path is a Laser mirror, 103. This mirror is 1.00 inch in diameter, and is coated for high reflectivity at 590 nanometers. It has a radius of curvature, concave, of 0.5 metres. The mirror coating is of the dielectric type, and has a very high Laser damage threshold. The coating has a low reflectivity at 532nm, the pump wavelength. Therefore, most of the pump radiation is passed through the high reflector. It can be obtained from CVI Laser Corporation, and has a part number TLM1-590-0-1037-0.5CC. This mirror is mounted in an NRC model P-100A optic mount. The combination of the lens 102 and the curved substrate of the high reflector 103 form a type of down-collimator.
Next is the Laser material itself, 104. It is a 1.00 inch diameter, 0.125" thick disc of solid dye Laser material. This one has a florescence peak at ~ 590 nanometers. The solid dye is made of cast acrylic, which was cut into a disc out of a sheet. There are common names for these types of plastic. They are "Plexiglas" or "Acrylite". The florescent characteristic of the acrylic is sometimes referred to as "neon". The appearance of this material is flat, with parallel surfaces and a pinkish-reddish-orange glow. The material is optically transparent, but absorbs strongly in the shorter wavelength region of the visible spectrum, and, into the UV (ultraviolet), as well.
The final element used in the preferred embodiment shown in Figure-1 is the output coupler, 105. The output coupler is a partially reflective Laser mirror available from CVI Laser Corporation. The CVI model number is PR1-589-80-1050 for this optic. It is 1.00 inch in diameter, and is 0.5 inches thick, with a dielectric coating on the first surface (facing the Laser material, 104), and an AR (anti-reflection) coating on it's second surface. Both faces of the output coupler 105 are flat. It is mounted in a standard NRC P-100A optic mount.
As is with most lasers, the Laser resonator consists of only 3 parts; the high reflector 103, the Laser material 104, and the output coupler 105. Pump Laser radiation100 incident on the solid dye Laser disc 104 gives rise to Laser action as the round-trip gains exceed the losses within the Laser resonator. The resultant dye Laser beam is shown as 106. The resonator is of the stable resonator type, in that the radius of curvature of the high reflector 103 is longer than the distance between the two Laser mirrors, and is not equal to the distance between the two mirrors. Thus, the stability condition is met within the resonator, and an output beam of high temporal quality is achieved. The spatial mode of the Laser of Figure-1 is TEMoo.
Now that the Figure-1 has been described, you can see the inline nature of the design. This design can be used in applications where a size or shape of the overall system needs to be kept at a minimum or within certain constraints preventing or limiting the design otherwise. In addition, coupling efficiency is kept at a maximum due to minimal insertion losses and minimal optical path lengths required for the entire optical Laser system.
Now looking to Figure-2, we notice that it is identical to Figure-1, except the elimination of pump focusing lens 102. This design may be employed in instances where the pump Laser energies are sufficient to pump the solid dye Laser material 104 with the pump Laser beam100 corresponding to a higher range of output energy, and hence, peak power density incident on the Laser material 104. The advantages of this design are having less optical hardware, simplicity of design, and reduced size requirements. Note that this design also employs the inline system layout scheme.
In looking at Figure-3, we notice that this design is identical to Figure-2, except, the output coupler 105 has been eliminated. This design holds advantages over the systems shown in Figures 1 and 2, in that it requires even less optical hardware, has greater simplicity of design, and has reduced size requirements. Note that this design also employs the inline system layout scheme.
Additionally, solid dye Laser systems employing a pump mirror have been developed using the solid dye Laser materials from Elk Industries. Also, the use of other Laser sources for pumping solid Laser dye materials available from Elk Industries are currently being explored. These and other alternative designs will become apparent to those skilled in the art.
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Dye Laser Transverse (Output) Pattern for Dye Laser of Figure 1
Dye Laser of Figure 1 in Operation
Overhead View of Dye Laser in Figure 2
Elk Industries ® - Technical Papers
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