Slab laser amplifier with parasitic oscillation suppression

A slab laser amplifier with parasitic oscillation suppression has a plurality of angled pump faces related to one another in order to decrease likelihood of parasitic oscillations, with internal beam incidence angles at total internal reflection that alleviate need for reflective coatings. No polished surfaces of gain material comprising the amplifier are parallel to one another. A beam path within the gain material is such that all incident angles of the beam path upon the two main faces and the common end face are greater than a critical angle required for total internal reflection, thereby alleviating need for reflective coatings. Based on an index of refraction of the gain material, and based on a diameter of the laser beam, dimensions of the gain material are selected to maximize beam overlap in a pumped volume of the gain material.

FIELD OF THE INVENTION

The present invention generally relates to slab laser amplifiers, and relates in particular to a slab laser amplifier with parasitic oscillation suppression.

BACKGROUND OF THE INVENTION

Slab amplifiers are often used to boost output of a laser system by providing extractable energy from a pumped gain medium. The large surface area of the slab allows the pump energy to be spread over a wide volume of gain material to reduce heat effects. In general, a zig-zag pattern or tightly folded resonator (TFR) design makes use of multiple beam passes through the gain material to extract energy from the pumped region of the crystal.

Referring toFIGS. 1-6, a brief overview is provided of how a laser amplifier works in accordance with the prior art. InFIGS. 1-4, a basic laser cavity22includes gain material24. The cavity22also has at least two mirrors26A and26B, such as a 100% reflective mirror26A and a 98% partially reflective mirror26B. The cavity22further has a pump energy source28, such as a laser diode or flashlamp.FIG. 1illustrates the cavity22and its components in a state of non-operation. InFIG. 2, atoms in the gain material24receive energy from the pump source28, which excites the electrons into higher energy states. When these electrons return to their original energy state they emit a photon. This phenomenon is called spontaneous emission of photons.

Turning now toFIG. 3, as the photons pass through the gain material24, they also affect the atoms in the gain material24by stimulating them to emit more photons while in an energized state. Mirrors26A and26B aligned parallel to one another at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification along the same beam path. Referring toFIG. 4, photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified. A cascade effect occurs, and soon we have propagated many, many photons. This process is called Light Amplification by Stimulated Emission of Radiation, which is where the term “laser” comes from. As a result of one of the end mirrors26B having less than 100% reflectivity, some of the photons are transmitted through this mirror, and this transmitted portion is the laser's output beam.

Turning now toFIG. 5, a laser cavity22has limitations as to how much energy can be extracted from it, depending on available pump energy from source28, the gain material24, and other components. So, one way more power can be obtained is to use a second stage amplifier30. The basic second stage amplifier30is much like the basic laser cavity22, except there are no mirrors to contain the photons. There is a gain material24and a pump source28that excites the material24into an excited state so that there are available photons being emitted. It should be noted that there can be more than one pump source28, which can be situated on opposite sides of the gain material24. The large surface area of the slab allows the pump energy to be spread over a wide volume of gain material24to reduce localized heat effects.

Referring now toFIG. 6, the laser beam from the laser cavity22enters the amplifier pumped gain material24of the second stage amplifier30. As was the case with the laser cavity22itself, photons from one atom stimulate emission of photons from other atoms and the light intensity is amplified as it passes through the amplifier gain material24. It should be readily understood that although a single straight path of the amplified beam through the gain material24is shown, many amplifiers make use of a zigzag path, or tightly folded resonator (TFR) through the gain material24to make the best use of the excited gain material24.

One problem encountered in the scenarios described above is that a polished uncoated air/glass interface has about 4% reflectivity. This property of gain material surfaces means that the polished parallel surfaces of the amplifier gain material can act as the mirrors of a laser cavity. This parasitic oscillation thereby depletes the available gain for the beam that we intend to amplify in the first place.

Most of the current solutions to the aforementioned issue make use of anti-reflection (AR) coatings to limit the 4% reflection effect that contributes to parasitic oscillation. Some of these solutions that make use of a zigzag beam path may also employ high-reflection coatings to facilitate reflecting the beam to be amplified off of the surfaces where desired, such as low angle of incidence beam reflections inside of the gain material.

SUMMARY OF THE INVENTION

In accordance with the present invention, a slab laser amplifier with parasitic oscillation suppression has a plurality of angled pump faces related to one another in order to decrease likelihood of parasitic oscillations, with internal beam incidence angles at total internal reflection that alleviate need for reflective coatings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, we are proposing to create a slab amplifier that is of a particular geometry that does not require additional optical coatings for either parasitic oscillation suppression, or for high efficiency reflection of the beam to be amplified. In particular, angular relation of the angled pump faces to one another minimizes the likelihood of parasitic oscillations. Also, the internal beam incidence angles at TIR (total internal reflection) can alleviate the need for reflective coatings on two polished main faces and a common polished end face of the amplifier used to reflectively direct a beam path of the amplified laser beam within the pumped volume of the gain material of the amplifier.

In some embodiments, the need for reflective coatings on these surfaces can be alleviated to point of complete elimination. In others, the need is alleviated so as to reduce the need respective of at least one portion of at least one of the three aforementioned polished surfaces. Accordingly, that portion, which bounds gain material volume overlapped by the amplified laser beam, does not have to be reflectively coated to achieve total internal reflection of the amplified laser beam. Yet, even in the case of complete elimination of the need for reflective coatings, it is still envisioned that these surfaces can be completely or partially reflectively coated for other reasons, or for no reason.

Turning now toFIG. 7, a face pumped slab amplifier has a geometry32such that none of the crystal polished surfaces are parallel to one another. This geometric constraint reduces the occurrence of parasitic oscillation, which can drastically reduce the energy storage available in the excited gain material24. The nominal angles shown are for a gain material24having an index of refraction of 1.5.

Turning now toFIG. 8, the geometry32ensures that the input and output beams having electric field vectors in the plane of incidence (p-polarization) enter and leave the gain material24through Brewster end faces D and E at Brewster's angle. This orientation reduces insertion loss, while maintaining preferential polarization of the laser cavity. Main faces B and C and common end face A are polished surfaces, as are Brewster end faces D and E. Face F, however, is not a polished surface.

The two main faces B and C of the gain material24are angled slightly with respect to common end face A of the gain material at angles greater than 90 degrees. A beam path34within the gain material is such that all of the incident angles of the beam in relation to sides A, B, and C are greater than the critical angle required for total internal reflection, based upon the index of refraction of the gain material24. Thus, no reflective coatings are necessary.

The beam also reflects off of common end face A of the gain material24so that the beam returns along the length of the crystal in a zigzag path opposite the one it passed through upon entering the crystal. This geometric constraint maximizes the overlap of the beam mode and the pumped regions of the gain material.

Turning now toFIG. 9, the size of the crystal is determined by maximizing beam overlap of the pumped volume inside the gain material with the incoming beam36diameter while maintaining the above mentioned geometry. Maximization of the region of beam overlap is equivalent to minimization of the total volume of regions38of beam non-overlap. The gain material height, width and length are optimally scaled for a particular beam diameter.

An example of dimensions for gain material comprised of a crystal block are now described. Assume a beam of 2 mm diameter is to be amplified. Returning toFIG. 8, the rough dimensions of the block viewed from above are 3 mm in height, 3 mm in width at side A, and roughly 12-15 mm long.FIGS. 7-9show the slab block from above, with the optical polished surfaces A-E located around the perimeter. The top and bottom surfaces of the slab block (not shown) do not have a polished surface, nor does side F, as mentioned above.

Turning now toFIG. 10, a method of manufacture for a slab laser amplifier with parasitic oscillation suppression starts with determining a diameter of a laser beam at step40. Next, dimensions of gain material comprising the amplifier are optimally scaled in view of the diameter in order to maximize beam overlap in a pumped volume of the gain material at step42. These dimensions are optimally scaled according to a geometric constraint that a predefined geometry is maintained. The predefined geometry defines a plurality of angled pump faces related to one another in order to decrease likelihood of parasitic oscillations, with internal beam incidence angles at total internal reflection that alleviate need for reflective coatings. Finally, at step44, the amplifier is produced of the gain material in accordance with the dimensions and the predefined geometry. Surfaces of the amplifier are selectively polished in accordance with the predefined geometry, which also defines which surfaces are polished and which are not.

Turning now toFIG. 11, a method of determining a geometry for a slab laser amplifier with parasitic oscillation suppression starts with selecting a gain material to comprise the amplifier at step46. Then, based on an index of refraction of the gain material, angles greater than ninety degrees are selected at step48. These angles relate two main faces of the gain material with respect to a common end face of the gain material. The common end face is oriented opposite from Brewster end faces of the gain material. In step48, the angles are selected to ensure that a beam path of a laser beam within the gain material is such that all incident angles of said beam path upon the two main faces and the common end face are greater than a critical angle required for total internal reflection. This constraint alleviates the need for reflective coatings. Also, at step50, an orientation angle for Brewster end faces relative to one another is selected to ensure that input and output beams enter and leave the Brewster end faces at Brewster's angle. This constraint reduces insertion loss while maintaining preferential polarization of a laser cavity within the gain material. Finally, at step52, any other angles, in addition to the previously selected angles, are selected to ensure that no polished surfaces of the gain material are parallel to one another, thereby reducing occurrence of parasitic oscillation in said gain material. The resulting defined geometry is then used in the method of manufacture for a slab laser amplifier with parasitic oscillation suppression ofFIG. 10. In particular, this predefined geometry can be used in step42in optimally scaling the dimensions, and further used in step44as detailed above in producing the amplifier.