CHEM 524 -- Outline (Part 5) - 2005

FOR A PDF VERSION OF THIS SET OF NOTESWITH EMBEDDED FIGURES CLICK HERE (this may not be available for download until Oct.??.)

III. C. Special Topics in optics

1. Linear Polarizers -- random polarization in - linear out (i.e. E field with specific orientation)

            a. Absorptive (or reflective -- for metal): Aligned dipole transitions select polarization

                        -- vis & uv, absorbing (Polaroid, stretched film impregnated with dye, orient by binding to polymer)

                        -- IR: reflection: wire (grid) --made like a grating (narrow spacing l > 2d), minimize d for near IR

                                    Grating best if made by hologram, highest groove density has best near IR. Al is deposited on grating ridges

            b. Reflecting (due to index change)

                        -- Brewster angle (stack of plates), each one loses some intensity from a specific polarization and transmits all of the other polarization at specific angle

                        -- Prism birefringence properties--different index two polarizations — result: total internal reflection (Glan Prism)

                                    one polarization is transmitted with some reflection loss, the other totally reflected (narrow angle of acceptance),

                         Glan Taylor has air gap, narrow angle of acceptance, capable of high power

                        Glan Thompson has cement (glue) gap, much larger acceptance angle, lower power, longer l

                        -- Beam splitting prism (Rochon, Wollaston etc.), transmit both, but divergent angle between polarizations,

                                    if beam is collimated, can separate at a distance,

                                    MgF2 used in vac-uv as Rochon, LiIO4 (goes into IR) sometimes as a Wollaston

2. Circular polarization

            a. Wave plate, slides as example of retardation

                        -- birefringence retardation, d, depends on wavelength, l, the difference in refractive index, Dn = nx-ny and thickness, z,

                                    d = (2p/l) Dn z

                        -- as light passes through crystal, shift phase of two orthogonally polarized beams (x,y), when recombine, if l/4 shift (d = p/2) then circular (left or right), if d = p (or l/2) then perpendicular linear polarization results–other values give elliptically polarized light

                        -- Soliel-Babinet compensator will do this vaiably by sliding a wedge into the beam to vary z

            b. Fresnel rhomb -- reflection retardation -- broad band circular polarization

3. Modulation

            a. Characteristics (Figure 1): depth, duty cycle, shape, frequency

            b. Styles: Chopper straight forward intensity modulation (also used to create dual beam spectrometer)

                        alternatively use: polarization, frequency modulation (grating dither)--often more sinusoidal modulation

                                    transient grating (nonlinear effect, crossed laser beams), Interference, acousto-optic

            c. Circular/Linear polarization Modulator:

                        -- Electro-optic — induce birefringence with voltage polarization

                                    (e.g. KDP typical, Pockels Cell) - use as a Q-switch in laser common

                        -- Photoelastic — periodic stress induce birefringence

                                    — any isotropic material acoustically matched to driver can be basis, wide spectra region possible,

n      periodic retardation, sine wave in nature: variable amplitude

n      slides as example of CD and LD with polarization modulation

n      wavelength of l/4 (right to left circular) or l/2 (parallel to perpendicular linear) retardation

            d. Faraday rotator — magnetic field rotate linear (not circ.) polarization to new orientation

            e. Acousto-optic - acoustic wave sets up diffraction for specific wavelength, key -- use deflected beam, maximum modulaton depth

                        can mode-lock or Q-switch laser,

                        even has been used as basis for a spectrometer

 

Homework—read in Chap. 3, and review the modulator tutorials below, problems: see assigned work section 4 

 

Links:

Polarizers:

Karl Lambrecht Corp., (local Chicago connection) calcite and other crystal polarizers, retraders etc. (has a neat little diagram)

            http://www.klccgo.com/

Polarizer applet, Michigan State

http://lectureonline.cl.msu.edu/~mmp/kap24/polarizers/Polarizer.htm

API Amaerican Polarizers, plastic sheet

http://www.apioptics.com/

Optics for Reserch, crystal polarizers

http://www.ofr.com/oc-22_uv_polarizer.htm

Opto Sigma Corp, wide variety of crystal polarizers and plates

            http://www.optosigma.com/miva/merchant.mv?Screen=CTGY&Store_Code=OS&Category_Code=Polarizers

Meadowlark, dichroic polarizers and liquid crystal retarders and modulators  

            http://www.meadowlark.com/

Modulators:

Explanation of acousto-optic modulation

http://electron9.phys.utk.edu/optics507/modules/m7/acousto.htm

Tutorial from Drexel on E-O and A-O modulators

http://repairfaq.ece.drexel.edu/sam/CORD/leot/course04_mod07/mod04-07.html

Brimrose tech sheet with AOTF, acousto-optic tunable filter description

http://www.brimrose.com/Aointro.pdf

Stanford Research Systems (chopper)

http://www.thinksrs.com/products/SR540.htm

Electro-optical Products Corp, choppers, acousto- and electro-optic modulators

http://www.eopc.com/index.html

Electro-optical components (multi company representative)

Modulators:  http://www.eoc-inc.com/electro_optic_modulators.htm

Polarizers: http://www.eoc-inc.com/polarizers_optical_components.htm

Lasermetrics, FAST Pulse, electro-optic modulators

http://www.lasermetrics.com/

(site connects to a descriptive manual of uses http://www.lasermetrics.com/technotes.html)

Hinds photo elastic polarization modulators

http://www.hindsinstruments.com/PEM_Components/default.aspx