Abstract
The safety of therapeutic laser treatments could be strongly improved by introducing real-time monitoring to the fiber delivery system. Since any fiber damage leads to a significant variation of the backscatter, its detection could be exploited to monitor the system, in particular the fiber trip. To assess this potential, the backscattered modal power distribution (BMPD) from flat, bulb, and damaged fiber tips were investigated. The BMPD detection was accomplished by using a conventional beam-splitter method and two directional couplers: a prism–fiber and a fiber–fiber lapped coupler. Unlike the conventional method, use of the couplers allows for the separate detection of backscattered and forward transmitted signals. Therefore variations in the backscatter that are due to only a change in the laser pulse can be determined. Moreover, the directional couplers, because of the amplification of high-index modes, allow for an increase in sensitivity of the method. This was particularly evident in our tests on fiber tips that had been dipped into water to obtain a better simulation of the real working conditions. Finally, the influence of the target on the BMPD was investigated as a function of its distance from the fiber tip. All the tests confirm that the target must be taken into account if the distance is <1 mm, but the monitoring system can also be used when the fiber tip works in contact with the tissue wall provided that one can ensure contact by pushing the tip against the target wall.
© 1995 Optical Society of America
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