A concept for laser safety glasses is demonstrated where the laser intensity itself introduces phase distortion on the transmitted laser beam and is therefore self-limiting. The absorbed light in the polycarbonate glass heats it and changes its index of refraction.
A coating on the surface produces a non-uniform intensity pattern on the transmitted laser beam. This non-uniform intensity results in a non-uniform phase distortion which destroys the coherence of the laser and does not allow the cornea to focus the laser
strongly on the retina. The absorption in the glass is the same for all laser wavelengths (the glass is sometimes referred to as a neutral density filter) and therefore the safety
glasses are good for all laser wavelengths. The amount of laser absorption required to meet the ANSI standard for max allowable exposure is a transmission of 0.1% and this is
experimentally verified by the data obtained. This attenuation is sufficient to protect the
eye from damage until the heating and phase distortion kicks in. Once this happens, the
protection is better - the higher the incident intensity. Experimental demonstration of the
effectiveness of the prototype has been obtained at 488 mm (blue light) and 514 mm
(green light) from an argon ion laser with laser duration from 0.04 seconds to 0.25
seconds. The data shows that as you increase the laser intensity beyond a certain value
the intensity at the focus of a lens actually decreases, and further increases in intensity
lowers the focal plane intensity even further. For the phase distortion safety glasses, we
can calculate the transmission which meets the ANSI standard where the phase distortion
is optimum. Thus our glasses not only meet the ANSI standard but provide protection for
any higher laser intensity. The optical quality of the prototype was very good with no
fogging, or optical distortion of transmitted laser, and after the experiment, the prototype
returned to the identical optical quality with no permanent optical distortion.
A new technique is described where laser safety glasses are based on optical distortion of the phase of the laser transmitted through the safety glasses. The glass contains an absorber at the laser wavelength, which heats the glass and therefore changes its index of reflection. The uniqueness of the concept presented is the layout of the absorber and its dependence on the transverse dimensions of the safety glass. Because the absorption varies across the glass, the absorbed power varies and therefore the change in index of refraction also varies. This variation in the optical property causes distortion and a reduction of "coherence" of the laser beam. By combining the phase distortion with absorption, the laser safety glasses can be designed to protect the retina at low intensities and once the phase distortion becomes important at intermediate intensities, it dominates at all higher intensities. These Phase Distortion Safety Glasses are designed to protect the retina from quasi cw lasers at all wavelengths - the near UV, visible, and near IR and the visible light transmission is equal to three percent. Once the threshold is reached the phase distortion will never allow the retina to be damaged no matter how high the laser intensity. These glasses also allow the operator to see the laser beam that is being protected against.
The ground to space laser encounters the entire atmosphere and the laser power transmittable with good beam quality is ultimately limited by this propagation. For CW lasers, the ultimate limit is thermal blooming and this problem is addressed. The concept of a critical laser power is used which is the optimum laser power which leads to the highest far-field intensity. A perfect beacon is assumed and a perfect wave front sensor is also assumed to determine the maximum intensity achievable with adaptive optics phase conjugation of the thermal blooming. An analytic model is derived and its predicted results are compared with a wave optics computer code results. The agreement is quite good and shows the parametric dependence of the critical power on laser beam size, wavelength and atmospheric absorption and path length.
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