Semi-transparent monitor detector for surface emitting light emitting devices

Apparatus for monitoring emissions from vertical cavity surface emitting lasers, VCSELs, comprising:

• an amorphous silicon layer on a glass substrate, the amorphous silicon layer having selectively doped areas that provide PN junctions of photovoltaic devices, the amorphous silicon having a sufficiently small optical absorption coefficient, within an optical wavelength range, that limits absorption by the amorphous silicon layer of a corresponding small fractional amount of light being emitted by the VCSELs, the amorphous silicon layer and the glass substrate being sufficiently thin to be transmissive of such light therethrough, excluding said small fractional amount of such light being absorbed, and a separate substrate having selected areas being provided with the VCSELs, the photovoltaic devices being on top of the VCSELs, the VCSELs being further constructed and arranged to emit such light having a wavelength range that corresponds to the wavelength range at which the amorphous silicon has said sufficiently small absorption coefficient, whereby, a substantial portion of such light is transmitted through the photovoltaic devices, and a small fraction of such light is absorbed by the doped areas of the amorphous silicon layer to produce photovoltaic currents from said photovoltaic devices as a measurement of output power of said VCSELs.