Project Alma Mater

Absorption of Light, Macro algae and Atmosphere

 

Marie Curie Intra European Fellowship (IEF)

 

Project No: 3012109

 

Persons in Charge: Dr. Ranjini Raghunandan, Dr. Andy Ruth

 

 

 

Background Methodology Outreach Project Output

 

          IBBCEAS FT-IBBCEAS Off-Axis CEAS

 

 

 

Fourier Transform - IBBCEAS

 

Fourier Transform Incoherent Broadband Cavity Enhanced Spectroscopy (FT-IBBCEAS) is a variant of IBBCEAS which uses a Fourier transform spectrometer/photodiode instead of the conventional monochromator/CCD in order to establish a spectrum. In this case, the absorption is determined from the Fourier Transform of the intensity of light escaping the cavity. Typical resolutions reported in the context of broadband cavity experiments are between 0.02 cm^-1 and 4 cm^-1. With the option of a much higher spectral resolution than dispersive methods, FTS in conjunction with broadband CEAS has been mainly applied in the (near) IR region, where narrow band ro-vibrational absorption features of molecular species can be exploited to achieve good selectivity. However, high resolution and high sensitivity come at the expense of numerous scans, which generally require rather long acquisition times. Thus FT-IBBCEAS is less appropriate for kinetic studies or for environmental trace gas monitoring. Moreover, high resolution Fourier transform spectrometers are neither particularly compact, nor very robust (for field applications) and can furthermore be costly. These aspects illustrate that FT-IBBCEAS is more relevant for static spectroscopic studies. It is especially useful for gas phase applications where small sample volumes are required (e.g. in discharges, combustion plasmas, flames or chemical flow reactors) or where small quantities of rare and/or expensive compounds (e.g. isotopologues) are of interest.

 

 

 

In the present setup, the IBBCEAS spectrometer consists of a supercontinuum laser as the incoherent light source, collimation optics, the absorber of interest (algae) and a Fourier transform detector. In supercontinuum (SC) sources a broad spectral emission is generated by pumping a certain length of a highly non-linear micro-structured photonic crystal fibre (PCF) with short pulses from a seed laser, generally using a high repetition rate (kHz to MHz). The well collimated fibre output has a high power density and is spectrally very broad; it typically extends from the blue region of the spectrum (> 400 nm) to the near-IR (<2.5 µm) for a pump wavelength around 1060 nm. Significant performance instabilities can be caused by optical feedback into the PCF, making operation with an optical cavity critical due to the strong back reflection from the entrance cavity mirror (especially when working around the seed wavelength) . SC sources are also prone to exhibit considerable power as well as spectral fluctuations, which have adverse effects in cavity applications.