CAVITY RING DOWN SPECTROSCOPY AYSENUR BICER

Outline What is CRD spectroscopy A simple CRDS experiment Pulsed laser CRDS versus CW-CRDS CW- CRDS experimental schema Experimental results Knife edge method

What is CRD spectroscopy CRDS is a sensitive absorption technique in which the rate of absorption in an optical cavity is measured It has significantly high sensitivity The effective absorption path length is very long The sensitivity is independent of intensity fluctuations of the light source Small fractional absorptions sub- ppm levels CO2 400 ppm (open air) A Simple CRDS Experiment A laser pulse coupled into an optical cavity The decay time is determined by measuring the time dependence of the light leaking out of the cavity By measuring the decay time the rate of absorption is determined directly providing the losses on an absolute scale

After one pass-through the cavity the intensity of the first optical pulse (Beer-Lambert’s law) The intensity of the second pulse After n complete round trip the pulse intensity behind the cavity will be

Continuous Wave CRD spectroscopy Pulsed laser CRD spectroscopy The main advantage of using CW laser radiation sources in any spectroscopic system is the increased resolution in the frequency domain (Trigger event ) In order to observe a ring down transits CW have to be switched of The bandwidth of these lasers is very small so can be only scanned over small wavelength regions each mode can have various allowed longitudinal modes associated with it The frequency spacing between two successive transverse modes is usually much smaller than the spacing between two successive longitudinal modes and depends on the characteristics of the cavity (length, mirror radii) Pulsed lasers promise Fourier transform limited line widths of the order of 100 MHz, in practice it is difficult to archive The length of the cavity, L, and the radius of the mirrors curvature of the mirrors should be chosen such that cavity is optically stable They are rather bulky, require massive amounts of electricity to run, and cost several hundred thousand dollars The pulsed lasers have the advantage of broad wavelength coverage

CRD spectroscopy Using Continuous Wave Laser Because of narrow line width of the laser and high finesse of the cavity, spectral overlap between the laser frequency and the frequency of the cavity modes are no longer obvious 1605.74nm- 1602.31nm infrared light region to solution He- Ne laser can be used The helium-neon laser (He-Ne) was the first gas laser. The most widely used laser wavelength is the red wavelength (632.8 nm) with a CW power output ranging from 1mW to 100mW and laser lengths varying from 10 to 100 cm.

AOM He-Ne laser Photodiode DFB diode laser

Diode laser controller 1.6~1.61µ m DFB diode laser Diode laser controller He-Ne laser AOM AOM Driver PC DAQ Wavemeter or OSA Scope PD preamp   L PZT driver            

First step The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature. It has elliptical beam shape The beam pass through wave plates

Second step AOM uses the acousto-optic effect to diffract and shift the frequency of light using sound waves so we can use it in CRD spectroscopy for frequency control The laser light that passes through AOM will be diffracted into multiple orders The first order diffracted beam is directed through the optical cavity Frequency of radiation from CW laser is coincident with cavity mode, power is likely build up within the optical cavity Trigger pulse is sent to AOM to switch it off The first order beam is quickly extinguish 150ns/mm

Third step The ring down signal registered by photodiode to oscilloscope.

Temperature (Celsius) 13 - 30.30 increasing by 0.10 Wavelength between 1600.566nm – 1602.534nm

R=∞ R=200cm L=60 cm W0= 0.683 mm W1= 0.816 mm zR = 916.5 mm

Knife edge method

First order diffracted beam 10.54mW First order diffracted beam 10.42mW 10.42×0.84=8.752mW 9.9775mm 10.42×0.16=10.7950mW 10.7950mm 10.7950-9.9775=0.817mm

References Berden, G., Engeln, R. (2009). Cavity ring-down spectroccopy: Techniques and applications. A John WILEY and Sons, Inc., Publication. http://massey.dur.ac.uk/resources/grad_skills/KnifeEdge.pdf