Laser bulk damage thresholds were measured for both single-crystal YAG and for diffusion-bonded YAG structures
using 600 picosecond pulses at 1064 nm. The tested samples included 3-layer sandwich structures with doped cores
of various thicknesses. An undoped-YAG end cap was diffusion-bonded on one end of each of the sandwiches. The
1064 nm laser source was focused to a 13 micron diameter spot at the boundary region between the core and the
undoped endcap. Measurements included the evaluation of single- and multiple-pulse damage thresholds at single
sites, as well as thresholds for continuous 90%-overlap scans. The single-site thresholds at the diffusion-bonded
boundary were close to that of single-crystal YAG. However, the continuous scans revealed isolated microscopic
sites where the damage threshold was as much as 4 times lower than that of single-crystal YAG.
Threshold measurements for Minimum Visible Lesions (MVL) at the retina are reported for 60 picoseconds (ps) and 4 nanoseconds (ns), single laser pulses in rhesus monkey eyes using a visible wavelength of 532 nanometers (nm) from a doubled Nd:YAG laser. The 50% probability for damage (ED<SUB>50</SUB>) dosages are calculated for 1 hour and 24 hour post exposures using 95% fiducial limits. For both pulsewidths, the threshold values calculated by probit analysis decrease between the 1 hour and 24 hour ophthalmoscopic evaluations. The ED<SUB>50</SUB> value determined for the 60 ps pulsewidth was less than half the value at 4 ns (0.43 (mu) J/60 ps vs. 0.90 (mu) J/4 ns at 24 hours) for both readings. Of the 136 exposures for pulse energies ranging from 0.03 to 5.0 (mu) J no hemorrhagic lesions were produced for either pulsewidth studied. However, at 6.6 (mu) J one intraretinal hemorrhagic lesion was observed for 60 ps. The slope of the probit curve was higher for 60 ps when compared with the 4 ns value (3.03 at 60 ps vs. 2.68 at 4 ns). MVL threshold doses calculated are comparable with those reported in the literature. However, the 4 ns MVL values is less than one order of magnitude (a factor 4.7) above the Maximum Permissible Exposure (MPE) level as defined by the 'American National Standard For The Safe Use Of Lasers', ANSI Z136.1-1993<SUP>2</SUP>. We present the current MVL data as it compares with previous data obtained for picosecond and femtosecond laser pulse thresholds and provide a preliminary assessment of how the ANSI MPE standard might be amended.
Recent studies of retinal damage due to ultrashort laser pulses have shown interesting behavior. Laser thresholds for retinal damage from ultrashort (i.e. <EQ 1 ns) laser pulses are produced at lower energies than in the nanosecond (ns) to microsecond (microsecond(s) ) laser pulse regime. We examine how nonlinear optical phenomena affect the characteristics of light impinging the retina and hence, changes the minimum energy required to produce damage. Nonlinear optical phenomena which occur in homogeneous materials like the ocular media include self-focusing, stimulated Brillouin scattering, supercontinuum generation, laser induced breakdown, and nonlinear absorption. We will discuss all relevant thresholds and determine which nonlinear optical phenomena play a role in mediating the reduction in energy required to produce minimum visible lesion damage to the retina.
Bubble formation in the retinal pigment epithelium by submicrosecond laser pulses may be a source of laser induced retinal damage. Heat conduction away from absorbing melanin granules requires timescales on the order of microseconds. For pulses of shorter duration, all energy absorbed is effectively absorbed as a (delta) -function in time, and energy concentration may be high enough to cause vaporization of the surrounding medium. This occurs at lower fluences than required for thermal denaturation of a significant volume of cellular material. The adiabatic nature of the absorption and subsequent expansion is used to develop expressions for the calculation of maximum bubble size as a function of laser intensity and melanosome properties such as radius and absorption coefficients. We describe the analysis that went into the development of the bubble size expression and also present the results for representative calculations of maximum bubble radius. We find that our expression leads to a threshold for the formation of bubbles in the retinal pigment epithelium that is close to the ED<SUB>50</SUB> experimentally measured for laser induced retinal damage.
Recent studies of retinal damage due to ultrashort laser pulses have shown interesting behavior. Supra-threshold hemorrhagic lesions could not previously be produced with high pulse energy, sub-100 fs pulses, leading to the speculation that nonlinear optical phenomena might mediate these effects. To determine what effect self-focusing will have on laser propagation in the eye, we include this nonlinear phenomena in a calculation of the focal plane position and spot size in the eye as a function of incident power. Light is propagated through the components of the eye using ABCD matrices. The nonlinearity is included using the values for the nonlinear refractive indices for ocular components measured in our laboratory and the diffraction term scaling transformation. We examine how nonlinear propagation changes the irradiance at the retina for nanosecond, picosecond and femtosecond laser pulses with pulse energies near the minimum visible lesion (MVL) threshold for retinal damage. We find that self-focusing can substantially decrease the spot size and increase laser irradiance at the retina. We discuss possible effects this might have on retinal damage.
The use of laser radiation for endoscopic surgery has been a significant advance in patient care. The usual modality is to use an optical fiber to deliver the light energy from the laser to the patient. However, there have been a number of incidents of the optical fiber breaking while the beam was active. Although no injuries have been reported, we investigated the hazard posed by such broken fibers to operating room (OR) personnel. Optical fibers were fractured in a manner that simulated the fractures in the ORs and the output power and divergence measured. In most cases, the laser emission was still `beam-like,' presenting a potential ocular hazard. Clearly proper use of eye protection and administrative controls are needed to avoid injuries.
A linear decrease in the 'second laser threshold' is found when two lasers are optically coupled in the bad cavity limit. The coupled lasers are modeled as on-resonance, single mode, homogeneously broadened lasers. In-phase coupling does not cause lasers operating in the good cavity limit to operate chaotically and may make such lasers more stable. The authors observed that out-of-phase coupling of lasers in the good cavity limit can lead to self-pulsing and limit cycles.
The purpose of the study is to present the basic concepts of coupled-laser devices as well as a historical summary of some of the experimental studies of laser coupling. Focus is concentrated on the experimental efforts with gas and chemical lasers. Divided output single and injection locked lasers, master oscillator and power amplifiers, coupled and multiple output resonators, nonlinear optics coupled devices, and hybrid coupled optical systems are discussed among laser-coupling techniques. Early experiments involving the coupling of carbon dioxide lasers and coupled photolytic iodine lasers are outlined, and works employing nonlinearly coupled devices, intracavity mixing, and gain medium are covered.