ECE532 Biomedical Optics © 1998 Steven L. Jacques, Scott A. Prahl Oregon Graduate Institute

Collection by an aperture

An aperture is essentially a hole through which light can pass to reach some detection system.

An aperture to collect collimated light.

Consider a collimated laser beam delivering a power of 1 W to a circular 1-mm-diameter aperture. The laser beam has a Gaussian beam profile with a 1/e² radius w_o = 0.5 mm. A detector sits behind the aperture and is greater than 1 mm in diameter.

What is the power collected by the aperture?

The irradiance E [W/cm²] of the Gaussian beam profile is described as a function of the radial position r [cm] from the central axis of the beam:

where

The aperture presents a collection radius a = 0.5 mm. In general, the amount of power collected, P_collected, by an aperture with radius a is:

For our case,

• P = 1 W, a = 0.5 mm and w_o = 0.5 mm.
• P_collected = P(1 - exp(-2a²/w_o)) = P(1 - 1/e²) = 0.632 P = 0.632 [W].

P_collected = 0.632 [W]