Parametric laser-reception lidar

Parametric laser-reception lidar A. P. Godlevsky, E. P. Gordov, Ya. Ya. Ponurovskii, A. Z. Fazliev, and P. P. Sharin

A theoretical analysis and design features of parametric CO2 laser-reception lidar are reported. Stabilized regular laser radiation intensity oscillationsare shown to occur under the laser-reception schemeof the return signal.

Results are presented

for low frequency cavity mirror oscillations and high instantaneous

beat

frequency. The laser radiation modulation structure observed suggests the feasibility of deriving information about the properties of the laser beam propagation medium.

1.

referred to as parametric laser-reception lidar

Introduction

Remote atmospheric sounding is known to be confronted by the difficulty of processing low return signals. The problem has stimulated increased use of conventional heterodyne detection techniques.1 The latter exhibit very favorable characteristics, such as high noise immunity and extremely high sensitivity due to mixing of the weak received radiation with the

intense reference beam. However, the heterodyne detection scheme appears to require sophisticated lidar systems and to impose severe restrictions on the stabil-

ity ofthe reference signal hindering the widespread use of lidar.

On the other hand, it has been pointed out2 that the laser intensity may be dramatically changed by feeding a portion of the backscattered signal power into the resonator cavity. Lasers have also been considered as return signal receivers. 3 -7

Doyle3 discussed the case

where the backscatter frequency was shifted relative to that of the reference beam. When the frequency offset is a constant, heterodyne mixing is found to occur in the laser. Devices based on this principle are used for measuring the fluctuating characteristics of the atmospheric path,5 for remote gas analysis,6 and in radar applications.8 We now report a theoretical analysis and design features of lidar with a CO2 laser. The latter acts both as a probe radiation source and a frequency-shifted return signal receiver. According to the terminology

adopted in the vibration theory, this kind of device is

The authors are with Institute of Atmospheric Optics, Siberian Branch of the U.S.S.R. Academy of Sciences, 634055 Tomsk, U.S.S.R. Received 27 October 1986. 0003-6935/87/091607-05$02.00/0. © 1987 Optical Society of America.

(PLRL), heterodyne laser detection being clearly a particular case. The proposed lidar system has been used for atmospheric sounding, specifically in remote gas analysis. The experimental results obtained will be discussed later in this paper. The laser frequency was varied by changing the laser cavity length. The problem of the variable length laser cavity has been amply discussed in the literature9 10; however, the resulting solutions obtained in the form of dynamic modes appear to be too complicated and intractable for analysis of the PLRL operation. The more simple approach adopted here is essentially based on the fact that perturbations incurred both by the periodic motions of the laser mirror and by the lidar return are small. The frequency and intensity of the field produced by wave mixing within the laser resonator are found to depend on atmospheric characteristics such as the distance from the natural target, its reflectivity, the path absorption, etc. II.

Theory

The analysis of the PLRL operation proceeds from the coupled third-order Lamb equations for counterpropagating waves within the Fabry-Perot cavity: (d

c az) Ei

( - #3E

(1)

12)E,

where E+ and E_ are the complex amplitudes, a is the pump parameter, and fi is the saturation coefficient. Mirror losses are not included in a. The space variable is eliminated by the boundary conditions: E+(-l,t) =RjE_(-It), E_[a(t),t] = R 0 E+[a(t),t]+ (1 - R2)R2KE+[a(t),t-

].

(2)

Here R0, R 1, and R 2 are amplitude cavity mirror reflection coefficients (see Fig. 1); 1is the laser cavity length;

a(t) is the coordinate of the reciprocating mirror Ro; 1 May 1987 / Vol. 26, No. 9 / APPLIEDOPTICS

1607

Ro

R2

E. -E.

11L

U

-e

a(t)

Fig. 2. Sawtooth amplitude temporal dependence of Ro mirror oscillations.

Fig. 1. Schematic diagram of the PLRL system.

, (t)

and K is the factor accounting for the signal linear loss along the atmospheric path [for the simplest case K = exp(-20L),

where 0 is the attenuation

coefficient; L is

the path length]; r = (2L)/c is the time delay; and c is the velocity of light. Since v = a(t)