Slab Laser Assembly

1. Field of the Invention

Laser devices in which the active laser member is in the form of a slab. 2. Description of the Prior Art

Increased demands are being placed on laser sources, both as to military and industrial applications. Use of the slab configuration for the active laser member in place of the cylindrical rod configuration, offers potential for increasing the average output power limit of such member, for any given material of which such member is composed. Furthermore, the ability to vary the average power output over wide ranges without noticeably affecting beam divergence or efficiency is an intrinsic and valuable property of slab lasers.

While a slab laser is simple in concept, its use can be complicated by practical considerations. FIGS. 1a and 1b illustrate the pertinent geometical features of a prior art laser slab of rectangular cross section. Laser radiation can enter and exit via end faces A and propagate along path P down the length of the slab by successive total internal reflections from optically polished top and bottom faces B. In operation, coolant (not shown) is passed over such faces B while side faces C are thermally insulated. Pump radiation enters the slab via the faces B. Faces A must remain unobstructed; hence, a seal D is utilized to contain the coolant flow.

Efficiency of the slab laser depends primarily on the total internal volume of the active laser medium which is swept out by the laser beam as it propagates down the length of the slab. This swept volume, in turn, is influenced by the coupling technique used to introduce and extract laser radiation from the slab and the width of the coolant seal necessary for reliable operation. In general, it has been the practice to taper the ends of the slab for Brewster angle coupling as in FIG. 1a. This gives an input and output beam path that is parallel to the longitudinal axis of symmetry of the slab and permits an in-line resonator design. It also affords flexibility in selection of the optical pump configuration, and it permits linearly polarized laser output even at high average power. Such FIG. 1a coupling configuration, however, tends to: (1) limit the volume of the slab swept by the beam, often to less than fifty percent of the total slab volume, due to seal width requirements; (2) complicate construction due to the fragility and configuration of the Brewster angle ends of a laser slab of material such as Nd:YAG (3) introduce thermal imbalances in the Brewster angle ends which can degrade laser performance due to optical distortion; and, (4) introduce asymmetry in pump enclosure geometry, due to off-set of the Brewster angle ends, with consequent reduction in pumping efficiency and added complexity of enclosure design.

The present invention preserves the advantages of the tapered-end slab of FIG. 1a without the aforementioned undesirable tendencies by utilizing a slab laser of rectangular parallelepiped configuration, coupling the input radiation beam arriving along the longitudinal axis of symmetry of the slab into the input end of the slab by an input Brewster angle prism member that transforms such beam into two criss-crossing beam paths, and coupling the two criss-crossing beam paths arriving at the output end of the slab to a single beam leaving along such axis by an output Brewster angle prism member that performs the necessary reverse transformation. This simplifies the sealing problem, eliminates the fragile and hard-to-form Brewster-angle ends on the slab, eliminates the thermal "end effects" to which such ends are prone, simplies construction and configuration of the pump enclosure for the slab, etc.

FIGS. 1a and 1b are side and end elevation views of a prior art slab laser having integrally-formed tapered Brewster-angle ends; and

FIGS. 2 and 3 are side elevation views of two slab laser assemblies embodying the present invention.

Referring to FIGS. 2 and 3, the present invention comprises a rectangular parallelepiped slab laser 1 of active laser material having flat rectangular end faces A at right angles to the longitudinal axis of symmetry of the slab for entry and exit of laser radiation, upper and lower optically polished flat rectangular faces B for accepting the usual pumping radiation from a source (not shown) and thermally insulated side faces (not shown). In operation, coolant (not shown) is passed over faces B, and seals D extend across the ends of these faces for cooperation with a housing (not shown) to contain the coolant flow. The laser slab 1 may be made of Nd:YAG, for example. This material operates with radiation at a wavelength of 1.06 microns, as may be suitable, for example, for certain military uses, such as target designation, for example. It will be appreciated that insofar as the present invention is concerned, however, other laser materials may be suitable or preferred according to use or need.

In accordance with the present invention, input radiation to the laser slab 1 arriving along a beam path P aligned with the longitudinal axis of symmetry X of such slab is divided by an input Brewster-angle prism member 2 into two criss-crossing beam paths P.sub.1 and P.sub.2 fed to the input face A.sub.1, of such slab and propagated in criss-cross fashion along the slab by multiple internal reflections from upper and lower slab surfaces B to the output slab face A.sub.2 where an output Brewster-angle prism member 3 receives the two criss-crossing beam paths P.sub.1 and P.sub.2 and recombines them into the single, straight, axis-aligned, longitudinal beam path P. In terms of swept-to-physical volume of the slab 1, the coupling prism members 2 and 3 of the present invention permit utilization of approximately ninety percent of the total static volume of laser slab 1.

In the FIG. 2 embodiment, the major portions of the coupling prism members 2 and 3 are of a wedge-shaped TIR type prism configuration and in the FIG. 3 embodiment these portions are of the knife prism configuration. The Brewster angle .phi. for the input and output faces 5 for the prism portions of members 2 and 3 are arrived at in well known manner, taking into account the wavelength of the laser radiation and the index of refraction of the material of which the prism members are made; such material being chosen primarily according to the level of laser energy being transmitted.

It will be noted that each prism member 2 and 3, has an integrally-formed straight rectangular parallelepiped coupling end portion 6 for disposition adjacent to the end faces A.sub.1 and A.sub.2 of the laser slab 1. These coupling end portions 6 serve as transformation regions wherein the two beam paths P.sub.1 and P.sub.2 assume their criss-cross relationship for introduction to the input face A.sub.1 of the slab 1 and extraction from its output face A.sub.2.

The knife prism configuration of FIG. 3 was tested with a Nd:YAG laser slab four inches long by two tenths of an inch high and one-half inch wide at its upper and lower polished pumping faces B. End faces A were anti-reflection coated as were the complementary adjacent coupling faces of the knife prism members 2 and 3, which were separated by five thousandths of an inch clearanceway. The prism members 2 and 3 were made of laser grade fused quartz, their apex angles were sixty-nine degrees and twelve minutes, their height was one quarter of an inch, their width was seven-tenths of an inch, and their length was two hundred twenty-five thousandths of an inch. The laser radiation energy was at a wavelength of 1.064 microns, and the Brewster angles .phi. were fifty-five degrees and twenty-four minutes.