Chem 524 Lecture notes (Sect. 7)—2005

PDF version unlikely to show up, but test for it here

IV. Wavelength Discriminators (continued)

B. Interferometers

1. Fabry-Perot (text: Sect. 3-7, Figure 3-56)

— multiple passes between partially reflecting surfaces

— if paths differ by ml — constructive interference

— normal incidence

— if beam enters at angle lead to "fringes" as spacing changes

free spectral range:

sharpness of interference depends on reflectivity (coefficient of finesse)

use: — couple to monochromator à enhance resolution to Dli

— etalon in laser select mode — narrowline

2. Michaelson Interferometer (ref: see Griffiths & DeHaseth and/or Marshall & Verdun–note our Textbook is a little different, tied to frequency, which is not the important issue)

encode frequency (wave number) by position (Dx) of moving mirror

interference at beam splitter on recombining from moving and fixed mirrors creates signal

interpret by Fourier transform of intensity (response) vs. Dx

a. Monochromatic light — interferogram is sine wave

retardation — measure path difference waves:

constant move mirror at n:

modulates signal:

spectral response (F.T.):

if truncate scan at xm

b. Polychromatic light: interference between wave different frequency leads to envelope on interferogram oscillation — reflect spectrum

Two frequencies–get beat pattern

Broader, must integrate over all contributions, envelop decays:

low Dx à broad base line, high Dx à interference of close frequencies

Finite motion of mirror — limit resolution, bandshape

resolution — ideal: -- if D n too small, then no interference

apodization — modify bandshape by convolving D(x), lower resolution

boxcar (no alternation), triangular (ramp), others (more continuous)

c. FT Advantages

Jacquinot — no slit — throughout enhanced, but small aperture high res

Fellgett — multiplex — all frequencies simultaneously detected

Connes — frequency accuracy (compare/correct spectra)

Costs: lost 1/2 light back to source at Beam splitter

lack modulation depth (at large Dx values)–only can transform modulation

phase errors need correction/distort bandshape

d. Phase correction

Digital spectra — can miss Dx = 0:

Electronic frequency response — — "chirp"--lose symmetry at x=0

Result — S(x) has sine components — has derivative shapes

Correct — complex FT derive Re and Im component

Mertz algorithm

measure over small Dx range (assume  slow vary )

e. Alignment error and Aperture — lower resolution

solid angle accepted:

cause — loss modulation at large Dx (loss resolution — like apodize)

— loss of frequency accuracy

mirror align cause loss intensity, resolution high (favor IR)

f. Indirect measure — need F.T. — computer must be fast, accurate co-add

g. Survey of Drive systems (handouts)

Links

Fabry Perot—

Wikipedia Fabry-Perot tutorial

http://en.wikipedia.org/wiki/Etalon

Drexel laser course on Fabry Perot:

http://repairfaq.ece.drexel.edu/sam/CORD/leot/course10_mod05/mod10-05.html

FTIR oriented sites, Michelson interferometers:

A variety of FTIR links, including sampling, companies, tutorials etc. from Michael Martin, Lawrence Berkeley Lab, ALS Beamline for IR work.

http://infrared.als.lbl.gov/FTIRinfo.html

Information about use of synchrotron for IR is here:

http://infrared.als.lbl.gov/viewgraphs/

Univ. Nantes set of instructional pages on Interferometers:

http://www.sciences.univ-nantes.fr/physique/enseignement/english/theoric.html

FTIR companies

Nicolet—Thermo now owns—range of products, emphasis on analytical lab

http://www.thermo.com/com/cda/category/category_lp/1,,234,00.html

Digilab—Varian purchased after BioRad and independence—research emphasis, early developer:

http://www.varianinc.com/cgi-bin/nav?products/spectr/ftir/index&cid=IIKJLOMMFN

Bruker—German company with wide range of instruments, including high res., time resolved, microscopy:

http://www.brukeroptics.com/ftir/index.html

ABB-Bomem—Canadian manufacturer owned by ABB, has high res. and small rugged designs (process)

http://www.abb.com/analytical

MIDAC—compact rugged FTIR

http://www.midac.com/

Jasco—Japanese company with wide range of analytically oriented spectroscopy instrum., including FTIR

http://www.jascoinc.com/products/s_ftir_raman.html

Perkin-Elmer – has long history in IR and analytical lab support

http://las.perkinelmer.com/Catalog/default.htm?CategoryID=FTIR+Systems

Many others—see LBL link above for many leading sites

http://infrared.als.lbl.gov/FTIRinfo.html