Polarisation preservation in multi-mode optical quartz fibres and implications for MAX-DOAS observations

In many spectroscopic applications, optical fibres are an essential part of the instrumental setup. One of such applications is Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), which is a remote sensing technique to detect atmospheric aerosol and trace gases by analysing spectra of scattered skylight. Typically, multi-mode quartz fibres (MMQF) of > 100 µm in diameter are used in MAX-DOAS instruments to interconnect the telescope unit with a grating spectrometer. Besides acting as a light guide giving more freedom in the spatial arrangement of telescope and spectrometer and homogenizing the illumination of the spectrometer entrance slit, such fibres have depolarising properties originating predominantly from manufacturing induced birefringent effects. This property is particularly desirable in MAX-DOAS applications, since the incoming partially polarized skylight should ideally be depolarized before entering the polarisation sensitive spectrometer. The behaviour of polarised light in mono-mode fibres is well investigated and even utilized in telecommunications, whereas equivalent literature on multi-mode fibres is scarce.

We measured the depolarisation capabilities of a set of 20 MMQF of different age, length and diameter as typically used in MAX-DOAS applications. Independent of the fibre diameter, we found that in some recently manufactured (in 2018) fibres polarisation is well preserved with a decrease in total degree of polarisation (DOLP) to 1/e of its initial value at a fibre length L_e > 10 m at 450 nm, while in older fibres (> 10 years in age) L_e ≈ 1 m was found. This is probably due to improvements in the manufacturing process in the recent years. We further investigated the dependence of L_e on wavelength, on the polarisation orientation of the ingoing light and on additional birefringence induced by applying external strain (bending) to the fibres. The results are presented and discussed with regard to their implications for MAX-DOAS observations.