Eight-element polarisation independent transmission trap detector with high attenuation

Abstract A polarisation-insensitive transmission trap detector was constructed from two identical polarisation-dependent transmission trap detectors, each containing four <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mstyle displaystyle="false" scriptlevel="0"> <mml:mtext>10 mm</mml:mtext> </mml:mstyle> <mml:mo>×</mml:mo> <mml:mstyle displaystyle="false" scriptlevel="0"> <mml:mtext>10 mm</mml:mtext> </mml:mstyle> </mml:mrow> </mml:math> silicon photodiodes. The performance of the 8-element polarisation-independent and coaxial input–output transmission trap detector has been characterised at laser wavelengths of 635 nm and 850 nm. The measured transmittances are lower than 10 −4 and are in agreement with values calculated from the reflectances of single photodiodes. The measured relative transmittances and responsivities are independent of polarisation within 0.12%. The spectral responsivity is spatially uniform within 0.1% across an area of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mstyle displaystyle="false" scriptlevel="0"> <mml:mtext>4 mm</mml:mtext> </mml:mstyle> <mml:mo>×</mml:mo> <mml:mstyle displaystyle="false" scriptlevel="0"> <mml:mtext>4 mm</mml:mtext> </mml:mstyle> </mml:mrow> </mml:math> , consistent with the expected variations due to the oxide layer thickness of the photodiodes.