Small Laser Spot Sizes Research Articles

Lasers, optical systems and safety in ophthalmology: a review.

This is a review of optical methods related to biomicroscopy and laser treatment of the posterior segment of the eye. Contact lenses can be used to observe optical structures and couple laser radiation into the eye for a vast range of conditions and techniques. A small laser spot size is indispensable for photodisruptive work, though this requires a large beam diameter at the pupil and therefore optical systems and techniques such as scleral indentation which maximize the pupillary beam diameter are preferred. For coagulation work the choice of beam focusing optics is crucial for optimum safety. Vitreous replacement can be used to change the refractive power of the eye and permit new combinations of treatment and optical systems. This review covers many aspects of laser irradiation of the eye. It should be clear that, with the multitude of different procedures and optical systems involved, the laser surgeon is faced with a daunting task in assessing and meeting safety limits.

The depletion properties of silicon microstrip detectors with variable strip pitch

We have investigated the depletion properties of trapezoidal shaped silicon microstrip detectors which have variable strip pitch [1]. Four types of detectors were examined: three detectors have constant strip width and a fourth has a varying strip width. The detectors are single sided with readout performed via p + strips. The depletion properties of the devices were measured using two different methods. The first used capacitance versus voltage measurements, while the second used a 1060 nm wavelength laser coupled to a single mode fiber with a mode field diameter less than 10 μm. The small laser spot size allowed for the depletion depth to be measured in a localized area of the detector. The laser induced charge on an electrode was measured as a function of reverse bias voltage using a sensitive charge preamplifier. The depletion voltages of the detectors demonstrate a strong dependence upon the ratio of strip width to strip pitch. Moreover, these measurements show that a large value of this ration yields a lower depletion voltage and vice versa.

Increasing the sensitivity of a FACScan flow cytometer to study oceanic picoplankton

Existing limitations on the sensitivityof commercially available flow cytometers do not permit the detection of extremely dim picoplankton cells, particularly Prochlorococcus marinus in near‐surface oligotrophic ocean waters. This problem was overcome by making some simple modifications to a FACScan (Becton Dickinson) flow cytometer to increase its sensitivity by about fivefold. The fluorescence and side scatter sensitivity was increased by replacing the existing focusing lens and beam‐steering mirror with a 3× expansion telescope and a slightly shorter focusing lens, resulting in a smaller laser spot size at the interrogation point. Forward scatter sensitivity was increased by replacing the existing photodiode with a photomultiplier tube, which has a background noise level about an order of magnitude lower. The fluorescence sensitivity of the modified instrument, measured with a quantitative fluorescence microbead standards kit, is 74 molecules of equivalent soluble fluorochrome for fluorescein. The instrument can now detect extremely dim P. marinus from open‐ocean surface waters.

SHORT WAVELENGTH MO RECORDING: SYSTEM NOISE ISSUES AND PERFORMANCE OPTIMIZATION

Optical recording at 488 nm using optimized CoPt multilayer media is described. Large mark CNR values of 62 dB are achieved by ensuring that system noise is dominated by the media noise component through a combination of media desensitization, read power optimization., and, elimination of the groove noise component which usually dominates the system noise. The results indicate that it is generally not possible to decrease the laser power requirements when utilizing a small recording laser spot size despite the increased power density because of these noise constraints. Further, because of a decreased detector sensitivity at blue recording wavelengths, further desensitization beyond that necessary to compensate for the increase power density is required when shorter wavelengths are used to achieve a smaller spot size.

Clinical Strategies for Excimer Laser Therapeutic Keratectomy

Surgical therapy for superficial corneal scars, dystrophies, or surface irregularities with the 193 nm excimer laser shows great promise. Depending upon the character of the pathology to be treated, different surgical strategies are required. Treatment strategies for three patients with diverse corneal pathology are presented. Focal surface irregularities are treated with small laser spot sizes leaving the epithelium intact. Smooth, uniformly distributed subepithelial anterior stromal pathology is best removed with a large spot size through intact epithelium. When scattered scarring of variable thickness is present, selective epithelial debridement permits improved exposure of irregularities in Bowman's layer and the anterior stroma. Guidelines for selective debridement of epithelium and protection of adjacent and underlying stroma with moderate viscosity solutions enhances the surgeons's ability to obtain a smooth and clear surface while minimizing corneal thinning. Even with these strategies and use of plano disc ablation, net hyperopic shift generally occurs with broad large area therapeutic ablation. Superficial corneal opacities and irregularities can be surgically treated with the excimer laser as an alternative to the more invasive procedure of lamellar keratoplasty. Treatment failures can then go on to lamellar keratoplasty while successfully treated patients are more rapidly rehabilitated at lower total cost than a lamellar graft. Refinement of treatment strategies to minimize the hyperopic shift and facilitate masking around irregularities will further improve the results.

Measurement of ultrasound attenuation by laser ultrasonics

Laser ultrasonics is a technique which uses lasers to generate and detect ultrasound. Determination of the absolute ultrasonic attenuation is difficult since the diffraction corrections cannot be precisely calculated because of the poor knowledge of the laser beam’s intensity distributions. We show that there are two cases where this knowledge is not generally required, which allows precise determination of the attenuation. When the spherical wave approximation holds, i.e., for small laser spot sizes, thick samples, or low ultrasonic frequencies, the diffraction corrections can be precisely calculated. When the plane wave approximation holds, i.e., large laser spot sizes, thin samples, or high ultrasonic frequencies, the diffraction corrections are negligible. Both cases are illustrated with experimental data taken on ceramic and steel samples.

Laser-driven shock-wave studies using optical shadowgraphy.

The propagation of laser-induced shock waves in a transparent solid (Plexiglass) is investigated by optical shadowgraphy. The shock waves are generated by concentrating 100-J, 300-ps laser pulses from the Asterix-III iodine laser (\ensuremath{\lambda}=1.3 \ensuremath{\mu}m) onto areas 85 or 450 \ensuremath{\mu}m in diameter on the target surface---this corresponding to laser irradiances of 3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ and 2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, respectively. It is observed that the initial phase of shock-wave propagation is obscured by an electrical breakdown phenomenon, being particularly strong at the smaller laser spot size. The possible cause of this phenomenon is discussed, and it is pointed out that the field strengths of ${10}^{8}$--${10}^{9}$ V/m required for breakdown are generated as a consequence of charge separation during two-dimensional plasma expansion. The shock-wave trajectories observed at subsequent times are compared with one- and two-dimensional numerical simulations. Good agreement is found when the same input data as for the simulations of previous thin-foil acceleration experiments are used.

Surgical Lasers and Soft Tissue Interactions

The different physical and biological qualities of the carbon dioxide, argon, and neodymium-YAG lasers provide the otolaryngologist and head and neck surgeon with a surgical tool with specific qualities and applications. The specific wave lengths of laser energy produce a varied soft tissue action. The carbon dioxide laser energy is absorbed by all biological tissues, whereas argon laser energy is an effective photocoagulator and penetrates into the subepithelial plane; the Nd-YAG laser creates a coagulation effect, which penetrates deep into the tissues. Each laser has specific physical qualities. The argon laser and the Nd-YAG laser can be transmitted through a flexible fiberoptic delivery system, whereas the carbon dioxide laser currently requires a rigid optical delivery system. The Nd-YAG and carbon dioxide lasers both require a second coincident aiming beam because of the invisibility of their laser energy, whereas the argon laser does not have such a requirement. The spot size of the laser beam can be much smaller for the argon laser than for the carbon dioxide laser. Protective lenses are necessary for the argon, carbon dioxide, and Nd-YAG lasers. The carbon dioxide laser energy is not transmitted through glass; thus most media (glass, plastic) are suitable to prevent any ocular injury. The argon laser requires a special yellowish protective mechanism, and the Nd-YAG laser requires a green protective lens mechanism. With further technical advances and improvements in the fiberoptic delivery system, smaller laser spot sizes, and changes in operating laser modes and color specifications of lasers, the advantages of laser surgery over conventional surgical techniques will become increasingly more apparent.