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HOLOGRAPHIC YAG LASER

This project is designed to test various ideas around a holographic 532nm yag laser. There are many variations on yag design and beam quality is just one of many issues. Due to the complexity of the subject and technical information not contained herein to allow the successful design and construction of such a system, these pages are not intended to show how to construct such a system. The information is only provided to show systems that were built and tested and only some of that information is included here. These lasers involve lethal power supplies and should not be attempted or serviced by anyone not qualified. Additionally these lasers generate pulse energies of the class IV level that can cause eye damage and burns. Additionally devices must conform to safety considerations that involve electrical powered and radiological emitting devices.

 

TOTAL SYSTEM OVERVIEW

A holographic laser designed to provide average quality and made rather inexpensive if surplus parts are used. Key feature of this Nd:yag system is a compact single chassis head and integrated power supply With the additional resonator items, the laser does achieve holographic quality. Specifically, an aperture is inserted to control spatial modes. A 2 plate air spaced quartz resonant reflector is used to reduce the number of longitudinal modes (axial modes). This etalon with another single plate etalon added on a 3rd kinematic mount forms a compound 3 plate resonant reflector OC. These two etalons must be aligned precisely with the HR and each other. In affect this creates several Fabry Perot resonators that discriminate the axial modes. The two plate etalon alone is not enough to achieve single longitudinal mode. A three plate OC is about 66% reflective and a 4 plate would be about 87%. Additionally the saturable absorber q switches using a Cr4+:yag at the Brewster angle also will help in reducing the number of axial modes. Peak power reduction was also desired with an output pulse width of 20 to 30nsec preferred.

In order to achieve single mode operation, reiterate the following design parameters that help achieve this:

1.      Flashlamp pump control to reduce gain and increase the number of photon round trips needed.

2.      3 or 4 plate etalon OC (resonant reflector) and saturable absorber. This combined with item 1 help achieve long coherence length by reducing the longitudinal modes.

3.      Aperture set to a Fresnel number of 1 or less. If too large of an aperture is used, even in single longitudinal mode you are not assured of TEM00 mode. Fresnel number = Aperture radius squared / (resonator Length * Wavelength).

4.      Resonator design. Confocal resonators or there equivalent (plano-concave set to confocal distance) give best TEM mode selectivity when used with an aperture at the HR. But the shorter focal length concave HR also concentrates the beam size within the cavity for a very small mode volume and increases divergence of the laser output. Large radius concave-plano configurations will yeild a more moderate approach. Example 5m CC HR and Plano OC at meter distance. As a note regarding Fresnel number, an attempt to find largest aperture and still be "TEM00 and single longitudinal mode" can be done to achieve higher output energy, but additional beam diameter for the same resonator length also increases divergence. So a balance between acceptable divergence and energy must be made for the overall system performance.

Beam quality is related to many considerations. But an important one is the quality of the parts used. If dirty or damaged or unable to get the beam through undamaged portions of the part, then this will lead to somewhat poorer performance with regard to holograms. Spatial filter can be done at lower power levels or at higher ones in vacuums with the correct combination of pinhole material and long focal length lenses to reduce energy concentration for the pinhole. But this again adds cost and complexity to any system. Some systems have been designed to use a SBS mirror cell to help correct wavefront issues but this was not implemented as well. A simple cell of acetone and a lens could be used to build a SBS mirror if one desired to achieve a higher quality, but then rotators and 1/4 wave plates would also be needed to achieve higher quality. Sometimes the best systems are the simplest, least maintenance, and incrementally improving beam quality over time and subsequent versions as needed.

With the above features, this system does in fact achieve single longitudinal and TEM00 mode with fairly good spatial quality provided the etalon is aligned correctly with the resonant reflector.

A complete study of the longitudinal and spatial modes will be presented in version 3 and 4. Here is an example of a study done on a holographic ruby laser also built to similar techniques. r7beam.htm r7fosc.htm r7resosc.htm

The below design as mentioned in the following is a design balance between objectives and parts availability on the surplus market. Many design choices have to be made and ideal parts may not be available to achieve a specific design goal.

 Design version 1: rwhyag1.htm Intial test version.

Design version 2: rwhyag2.htm Modified power supply to higher voltage.

Design version 3: rwhyag3.htm Modified power supply to longer pulse width.

Design version 4: rwhyag4.htm KTP replaces KD*P and New q switches ordered.

Design version 5: rwhyag5.htm Higher output needed.