Unstable confocal optical resonators with high magnification (about 100x) are commonly used with Metal Vapor Lasers (MVL) with high-aperture active media. The main problem in modeling these devices is the large single-pass loss,  which requires an accurate resampling process. Other problems include modelling hard apertures when using Fast-Fourier Transform (FFT) calculations, as sharp edges produce a high-frequency drift of the resulting field. 

This article will demonstrate how to handle both these problems. Here is the specification of the laser to be modelled:
      
Specifications:
Laser:

• Copper vapor (CVL)
• Wavelengths: 510.6 nm and 578.2 nm
• Active medium length: 600 mm
• Active medium diameter: 14 mm

Resonator:

• Confocal with output coupler represented by afocal meniscus with reflective coating of 3 mm in diameter
• Primary mirror: R2032mm
• Secondary (output) mirror: R20.04mm
• Distance: 1005.98mm

Experimental and previous theoretical investigations have shown the output radiation to be independent of  starting beam characteristics after at least 2 passes through the resonator. Only diffraction effects and manufacturing and adjustment errors are significant . The following model may be used for system sensitivity analysis of these factors.