Self-mixing detection of backscattered radiation in single-mode pulse-periodic CO₂ lasers

Abstract

The self-mixing (autodyne) effect in single-mode CO(2) lasers with pulse-periodic (PP) pumping of the active medium is theoretically analyzed and experimentally investigated. A semiempirical model of the autodyne effect in CO(2) lasers of this type is developed that allows the laser beat signal to be described from the known shape of the generated pulses. The self-mixing effect in PP CO(2) lasers is shown to be identical with that in continuous-wave CO(2) lasers, except that the autodyne amplification during the laser pulse proves time-dependent. It is demonstrated that the amplitude-frequency characteristic of the autodyne amplification for PP CO(2) lasers is also of resonance nature, but its bandwidth is broadened, as compared with that in the case of continuous laser pumping. As in the case of continuous pumping, the self-mixing effect in PP CO(2) lasers can be used to detect and analyze backscattered signals, specifically for measuring the rates of destruction of materials by the 10 μm radiation and for monitoring this process.

Capabilities of Autodyne Reception in Medical СО2 Laser Devices

“…Fig. 6 shows the one laser pulse with autodyne signal on LD-1, received when radiation was scattered by the rotating disk at laser power of 7 W. The level of autodyne signal depends both on the backscatter coefficient and the autodyne amplification [4,6]. In order to measure the amplitude-frequency characteristics (AFC) of autodyne amplification of the lasers, we used an algorithm that allowed to extract the variable component from the laser pulses due to the autodyne effect.…”

“…Surgical devices based on single-mode CO2 lasers with a radiation wavelength of 10.6 μm are widely used [3] in medical practice. In order to diagnose and control the process of laser surgical intervention in real time, we have suggested [4][5][6][7] an approach based on the selfheterodyne method for such devices, wherein the operating laser radiation is simultaneously a diagnostic one, creating an optical information feedback channel. The essence of the method is that the backscatter from the moving exterior object reaches the laser cavity and initiates a modulation of the laser output power J of Biomedical Photonics & Eng 5 31 Aug 2019 © J-BPE 030301-2 (autodyne signal) at the Doppler frequency.…”

“…Therefore, this effect is sometimes called self-heterodyning and self-mixing [4,6]. The operational diagnostics of processes of laser evaporation of biological tissues in accordance with this method consists in discovering discrepancies and distinctive features of amplitude and frequency characteristics of the signal for different types of evaporated biological tissues and correcting the parameters of applied laser radiation [4][5][6][7]. This selfheterodyne process has been well studied for continuous СО 2 lasers [6].…”

“…In these lasers, the "continuous" laser radiation is a pulse train with a frequency of 5-20 kHz, where laser power is modified by changing the pump pulse duration [9]. A computational model of the self-heterodyne effect in such lasers has earlier been proposed [4]. However, there is no information about the features of use of real surgical devices with this type of lasers as feedback sensors.…”