Applied Optics

In this paper, we explore the potential benefits and limitations of using transmitter lenses in an orbital-angular-momentum (OAM)-multiplexed free-space optical (FSO) communication link. Both simulation and experimental results indicate that within certain transmission distances, using lenses at the transmitter to focus OAM beams could reduce power loss in OAM-based FSO links and that this improvement might be more significant for higher-order OAM beams. Moreover, the use of transmitter lenses could enhance system tolerance to angular error between transmitter and receiver, but they might degrade tolerance to lateral displacement.

Keratometry is currently achieved by projecting a circular mire onto the patient's cornea and analyzing the size and shape of its reflected image. The projection mires are decisive for the precision of the measurement. We have previously developed a keratometric module for slit lamps, and the development of four projection mires are presented. Mire 1 is composed of optical fibers and electrical cables; Mire 2, 48 LEDs; Mire 3, optical fibers and no electrical cables; and Mire 4, mechanical parts--cable free. Mires 2-4 provide accurate keratometry measurements at slit lamps. Mire 4 is the most adequate for the clinical environment.

Beat frequencies between the transverse modes of an unstabilized CW CO2 laser were studied by detecting the homodyne signals emitted by the laser. Frequency pulling was observed and a resulting mode spacing reduction of up to 8 percent was measured. The dependence of mode pulling on total gas pressure was measured and was found to be in fair agreement with theory. The observed effect is important in some laser tuning techniques where it leads to a reduction of the tuning range and a nonlinear dependence of the laser frequency on cavity length.