an optics box 136 the optics box 136 is mounted above the can 131 partially covering the tec controller the optics box 136 contains the micro optics components shown in fig4 namely a beam splitter etalon filters a coarse filter each in its own mini package with fibre in out and the photodiodes the optics box 136 is surmounted by a thermal environment substrate 139 spaced from the optics box by a thermal spacer 138 the beam is split and the four photo detector currents are measured rapidly the linear filter output first identifies that range in the total wavelength band in which the beam wavelength lies there are typically forty fsr ranges with four sub ranges each in a spectral band of a c band wavelength meter the sub range with its slope rising 512 522 or falling 513 523 fig8 is identified output from the appropriate one of the pair of in quadrature etalon detectors is measured and the wavelength is output from that etalon 39 s look up table for power to wavelength conversion a calibration third etalon is provided having high reflectivity to give a conventional airy function response the third etalon also has an fsr that is in a ratio with the other etalons to provide a common maximum throughput at only three for example points in the spectral band of the meter an airy function response as shown in fig6 which is sharper than the responses of the fine filters produces a more defined optical bandwidth that transmits only at the reference wavelength a broadband light source provides a maximum common transmission at this reference wavelength and the pair of quadrature etalon circuits are.