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Are you getting the concept?. Calculate D a , D l , R d and s g for 1 st order diffraction under optimal conditions for the indicated 0.5 m grating with 100 m m slits. Spectral Resolution ( l/Dl ) with Small W. Diffraction-Limited Spectral bandpass: s d = R d l f W’ d
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Are you getting the concept? Calculate Da, Dl, Rd and sg for 1st order diffraction under optimal conditions for the indicated 0.5 m grating with 100 mm slits.
Spectral Resolution (l/Dl) with Small W Diffraction-Limited Spectral bandpass: sd = Rdlf W’d sd = l DaW’d Rayleigh Criteron infers: Dld ~ sd = Rdlf W’d
Are you getting the concept? A grating monochromator with a reciprocal linear dispersion of 1.2 nm/mm is to be used to separate the sodium lines at 589.0 nm and 589.6 nm. In theory, what slit width would be required?
Double and Triple Monochromators • Use Double or Triple Systems to: • increase spectral resolution • increase stray light rejection • Two modes of operation: • additive • subtractive http://architect.wwwcomm.com/Uploads/Princeton/Documents/A&S_Modes.pdf
Additive Multi-Stage Monochromators All 3 stages contribute to dispersion Grating G1 disperses light Slit S1,2 passes only a narrow portion Grating G2 further disperses light Slit S2,3 passes only a narrow portion Grating G3 disperses light before detection Total dispersion = additive dispersion of each stage Slits open relatively wide in spectrographs to permit enough light through to use the entiredetector. →significant stray light http://architect.wwwcomm.com/Uploads/Princeton/Documents/A&S_Modes.pdf
Subtractive Multi-Stage Monochromators 1st 2 stages act as a filter Grating G1 disperses light Slit S1,2 passes only a narrow portion Grating G2 recombines dispersed light Slit S2,3 passes filtered light Grating G3 disperses light before detection Very high stray light rejection Gratings G1 and G2 must match in groove density, and thus, their dispersion actions cancel – very sharp bandpass filter. http://architect.wwwcomm.com/Uploads/Princeton/Documents/A&S_Modes.pdf
Others Ways to Separate l Bandpass Filters High-pass Filters Notch Filters Low-pass Filters www.mellesgriot.com
A-Pages Discussion Questions 1. The authors state, “…the penetration depth is comparable to the wavelength…”. Calculate the penetration depth for l = 500 nm directed into a glass microscope slide (h = 1.50) in air (h = 1.00) where total internal reflection occurs at angles greater than 78°. 2. Imagine the early experiments alluded to at the beginning of the article with 1 mm thick IREs. If you were using this thick IRE with l = 1 mm, and a critical angle of 79.9°, how many times would the beam strike the top surface of the IRE (creating an evanescent wave) over a length of 6 cm (a common microscope slide)? How many times would the beam strike the top surface when the waveguide thickness is reduced to 10 mm (with all other parameters remaining the same)? How will this difference influence the experiment? 3. What are the advantages and disadvantages of using a thin IOW instead of a thick IRE? 4. Suggest an experiment that makes use of a planar optical waveguide. 1. What characteristics make a material more suitable as a core for an IR fiber optic? Why? Show a sample calculation that demonstrates your conclusion. 2. Draw a cross-section of a hollow waveguide. What advantages does a hollow waveguide offer over other approaches? 3. Compare a quantum cascade laser to the semiconductor lasers as seen in the “Deep UV” paper read for class. Why is the author of this paper so enthusiastic about quantum cascade lasers? 4. Suggest an experiment that makes use of a mid-IR fiber optic sensor.
