2Outline What is CRD spectroscopy A simple CRDS experiment Pulsed laser CRDS versus CW-CRDSCW- CRDS experimental schemaExperimental resultsKnife edge method
3What is CRD spectroscopy CRDS is a sensitive absorption technique in which the rate of absorption in an optical cavity is measuredIt has significantly high sensitivityThe effective absorption path length is very longThe sensitivity is independent of intensity fluctuations of the light sourceSmall fractional absorptions sub- ppm levels CO ppm (open air)A Simple CRDS ExperimentA laser pulse coupled into an optical cavityThe decay time is determined by measuring the time dependence of the light leaking out of the cavityBy measuring the decay time the rate of absorption is determined directly providing the losses on an absolute scale
4After one pass-through the cavity the intensity of the first optical pulse (Beer-Lambert’s law) The intensity of the second pulseAfter n complete round trip the pulse intensity behind the cavity will be
5Continuous 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 ofThe bandwidth of these lasers is very small so can be only scanned over small wavelength regionseach mode can have various allowed longitudinal modes associated with itThe 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 archiveThe length of the cavity, L, and the radius of the mirrors curvature of the mirrors should be chosen such that cavity is optically stableThey are rather bulky, require massive amounts of electricity to run, and cost several hundred thousand dollarsThe pulsed lasers have the advantage of broad wavelength coverage
6CRD spectroscopy Using Continuous Wave Laser Because of narrow line width of thelaser and high finesse of the cavity,spectral overlap between the laserfrequency and the frequency of thecavity modes are no longer obviousnm nminfrared light regionto solution He- Ne laser can be usedThe 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.
9First stepThe 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 shapeThe beam pass throughwave plates
10Second stepAOM 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 controlThe laser light that passes through AOM will be diffracted into multiple ordersThe first order diffracted beam is directed through the optical cavityFrequency of radiation from CW laser is coincident with cavity mode, power is likely build up within the optical cavityTrigger pulse is sent to AOM to switch it offThe first order beam is quickly extinguish 150ns/mm
11Third stepThe ring down signal registered by photodiode to oscilloscope.