Normalization of Elastic Lidar Returns by Use of Raman Rotational Backscatter John Cooney

Department of Physics & Atmospheric Science, Drexel University, Philadelphia, Pennsylvania 19104. Received 29 October 1974. The purpose of this letter is to report preliminary experi­

mental findings on the use of the pure rotational Raman component of laser atmospheric backscatter as a means for normalizing the elastic (molecular plus aerosol) backscatter component of a laser radar. These data were obtained during a series of atmospheric studies conducted in May 1974 at Wallops Island, Virginia.

In an earlier publication,1 the use of the vibrational-rota-tional component of the Raman backscatter from N2 was demonstrated as a means of normalizing the elastic back-scatter. The need for such a normalization arises because in the elastic return, the fraction of the backscatter due solely to the aerosol component is undetermined. While it is true that the Rayleigh cross section and atmospheric density are quite precisely known, unless an absolute in­strument calibration can be performed with sufficient ac­curacy, the calculation of the fraction of the elastic return due to the molecular backscatter cannot be properly evalu­ated. As a consequence of this, the relative fractions of molecular and aerosol backscatter contributing to the total elastic return remain ambiguous. On the other hand, be­cause the Raman backscatter arises only from certain spe­cific molecular components of the atmosphere, it can pro­vide a measurement process capable of isolating the aerosol backscatter.

Use has been made of this normalization procedure employing the N2 vibrational backscatter, and a more ex­tended report of its usefulness is available.2

The Raman backscatter measurement in general has the net effect of eliminating the need for an absolute system calibration. Since part of such a calibration process would involve the well-known difficulty of dealing with absolute light levels, eliminating such a need results in a significant gain in accuracy of measurement.

The change from the use of the vibrational-rotational Raman spectrum to the pure rotational has a twofold ad­vantage:

1 Instead of having to extrapolate for changes in sys­tem performance as well as in atmospheric transmission over 2330 cm - 1 (or Δλ = 1340 A at ruby wavelength), a change of only 50 cm -1 is involved. For such a short

270 APPLIED OPTICS / Vol. 14, No. 2 / February 1975

wavelength difference changes in transmission or system response can be neglected with complete safety.

2 Because of a significantly larger cross section and use of more favorable photoelectronic conversion efficiencies, backscatter signals of more than an order of magnitude greater can be realized.

Monitoring the pure rotational spectrum involves col­lecting light originating from both the O2 and N2 compo­nents of the atmosphere as their rotational spectra are in­termingled. This imposes a bit more complexity at the outset, but once the effective cross section is calculated this is of no further concern. Also the pure rotational Raman spectrum can be more temperature dependent than the vi­brational-rotational spectrum. Indeed such temperature sensitivity is being employed in another connection to mea­sure the structure of atmospheric temperature profiles. However, just as it is possible to maximize the temperature sensitivity of the backscatter by optimizing the filter char­acteristics, which preferentially transmits the rotational spectral light, so it is also possible to choose filter charac­teristics that leave the rotation backscatter light essentially temperature independent. A proper choice of wavelength interval can make the temperature invariant filter subject to less than a 1% change in effective transmission over an 80 K atmospheric temperature change. As with the tem­perature sensitive filters, the temperature invariant or in­sensitive filter must provide for sufficient rejection of the Rayleigh backscatter.

Figure 1 depicts a lidar elastic backscatter trace (at 6943 A) that has been normalized by use of the Raman rotation­al backscatter return. The anti-Stokes wing of the Raman rotational backscatter was monitored in these measure­ments by employing a filter centered at 6912 A and a rejec­tion of 6943 A greater than 105. It should be noted that for

Fig. 1. Plot of the aerosol elastic backscatter normalized by the molecular backscatter, The molecular backscatter has been sub­tracted from the total elastic backscatter to yield the aerosol back-

scatter.

normalization measurements of the type referred to here, the requirement for rejection of the Rayleigh need not ex­ceed 104 for a 1% error due to the residual Rayleigh back-scatter intrusion on the rotational return.

At the time these measurements were obtained the laser radar was also being used for other forms of backscatter measurements. Because these normalization measure­ments were basically a demonstration of principle rather than a program in itself, the laser was not optimally config­ured for obtaining signals from higher altitudes. It is felt that were the ideal arrangement available for the laser radar, the tropopause and the Junge layer could be routine­ly observed from the ground. Further measurements are planned to see if this will be born out by the facts.

This research is sponsored by NSF, Grant DES71-00598A03.

References 1. J. Cooney, Nature 244, 1098 (1969). 2. M. McCormick et at, "Tropospheric Transmissivity Measure­

ments Using the Raman Lidar Technique," in Fifth Interna­tional Laser Radar Conference, Williamsburg, Va. (June 1973).

February 1975 / Vol . 14, No. 2 / APPLIED OPTICS 271