light 14 may be focused by lens 28 and 29 on end face 22 of core 23 of optical fiber 33 light 14 may propagate from ball lens 29 through air onto the end of core 23 the fresnel coefficient of back reflectance 31 of light 14 for coupler 30 may be determined with the following formula where u201c n u201d is an index of refraction of light of the subject material calculation of reflected light 31 may amount to about 4 percent of the originally emitted light 14 for an n lens glass 1 5 and n air 1 0 this amount of back reflectance light 31 is significant enough to cause unwanted fluctuations in power of light 14 from source 11 coupled to core 23 of optical fiber 33 at end face 22 and additional noise in light 14 at the output of light source 11 fig2 shows an optical layout of a two ball optical coupler 10 light 14 may be emanated by source 11 light 14 may propagate through ball lens 15 and into ball lens 16 respectively ball lens 16 may focus light 14 down to a spot at or near the surface of lens 16 where light 14 may exit lens 16 core 23 of fiber 33 may have end face 22 that is situated against the surface of ball lens 16 at that spot where the rays of light 14 converge together this arrangement may minimize reflectance of light 14 into light 32 that moves towards the direction of.