forward - propagates the field by direct integration

Calling Sequence

[field_out]=forward(field_in,z[,new_size,[,new_number]])

Parameters

• field_in :input field. it must be diffractive wavefront.
• z : the propagating distance in meters.
• new_size : the new size of field at distance z. if it is set to 0,then the size of field will not be changed. default new_size = 0 .
• new_number : the new number of field at distance z (must be integer). if it is set to 0,then the size of field will not be changed. default new_number = 0 .
• field_out : the output field or diffractive wavefront.

Description

forward realize direct calculation of the Fresnel-Kirchoff integrals is very inefficient in two-dimensional grids. The number of operations is proportional to pow(N,4), where N is the grid sampling. With direct integration we do not have any reflection at the grid boundary

so the size of the grid can just match the cross section of the field distribution. forward has following features: (1) arbitrary sampling and size of square grid at the input plane; (2) arbitrary sampling and size of square grid at the output plane, it means we can propagate field from a grid containing for example 52x52 points corresponding to 4.9x4.9cm to a grid containing 42x42 points and corresponding let's say 8.75x8.75 cm. (3) The direct calculation of diffraction integral in the Fresnel approximation is used, thus forward can not be used to propagate the field to a short distances. Use forvard of steps instead. (4) forward is very slow.

Examples

	f1=begin(0.01,1e-6);
	f2=rect_ap(f1,0.005);
	f3=forvard(f2,0.2);
	field_plot(f3);
	
	f4=interpol(f3,0.0075,16);
	f5=forward(f4,400,0.4);
	field_plot(f5);
 

Author

Chen jingyuan, Gan guangyong (IAPCM, Beijing,China.)

See Also

forvard ,   steps ,   lens_fresnel ,   geom_propagation ,   near_angular ,   near_fresnel ,   far_fresnel ,   fraunhoffer_grt ,   fresnel_grt ,   propagate ,