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Free Optical Texts:
Optical component companies often supply free guides to optics in general (and
their optical components, naturally). The optical company, Melles Griot, has a
particularly good one – a 58 page pdf file which can be downloaded, copied,
printed, etc, and is found at:
http://www.cvimellesgriot.com/Products/Documents/TechnicalGuide/Fundamental-Optics.pdf
Included on the following pages are a few sample pages from this file.
Examples of Optical Systems:
There isn’t any comprehensive collection of these. (What is needed is something
like “The Art of Electronics” is to electronic design.) A typical comment from
other optics courses is:
“References
No simple reference in this field, the older books are all pre-computer and full of classical
analytic lens design while the newer books are just a catalogue of lens designs with little
or no background.”
The collection of optical systems in the “Field Guide to Geometrical Optics”
(“Optical Systems” chapter) is a good start.
Sites like “hyperphysics” have useful information:
http://hyperphysics.phy-astr.gsu.edu/hbase/ligcon.html#c1 (or google “hyperphysics”).
Hyperphysics’ “Light and Vision” top menu:
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Thin film mirrors: (Page from Hyperphysics)
Interference Condition for Thin
Films
Index
Interferenc
e concepts
Thin films Fabry-Perot geometry Snell's law
HyperPhysics***** Light and Vision
R
Nave
Go Back
Assume we have an alternating stack of high and low index materials, such as
SiO2 (n = 1.544) and Ta2O5 (n=1.801), each with an optical thickness of λ/4.
(e.g., A physical thickness, t, such that t/n = λ/4.) From wave theory we know
that light will undergo a 180 degree phase change when it reflects from a
medium of higher index than the incident medium (i.e., from SiO2, reflecting from
Ta2O5) and no phase change when reflecting from a lower index medium (in
Ta2O5 from SiO2).
Hence in the stack, the wave reflected from the bottom of a given layer travels ½
wave further than the wave reflected from the top of the layer. That, plus the 180
degree phase change of one of the reflections causes the two reflections to be in
phase, and coherently add. A modest number of layers in such a stack (~20) can
quickly achieve near perfect reflection at the resonant wavelength.
This is often called a Quarter-Wave Mirror (QWM), or just QWS (for quarter
wave stack).
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Fabre-Perot filter (‘Etalon’):
Consider the situation where two QW stacks are facing each other across a halfwave gap:
Fabry-Perot Geometry
The Fabry-Perot Interferometer makes use of multiple reflections which
follow the interference condition for thin films. The net phase change is
zero for two adjacent rays, so the condition
represents an intensity maximum.
Index
Interference
concep
Fabry-Perot resolution
Because the two reflections are out of phase (don’t forget to add the 180 deg
shift for one reflection only), they will eventually cancel each other and the
resonant wavelength will be transmitted. (The figure shows how the transmitted
wavelength will change with angle.)
Wikipedia will supply equations to calculate the transmission vs wavelength,
given the reflectivity and loss of each mirror, but a time-domain discussion will
give a better intuition for what is happening:
Let’s follow an incident ray perpendicular to the etalon, and assume that each
mirror has 99% reflectivity (and negligible loss):
1. 99% of the light is reflected from mirror 1, and 1% is transmitted
2. Most of the 1% (let’s round up to 1) is reflected from mirror 2
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3. The light reflected from mirror 2 is out of phase with the light reflected from
mirror 1, so cancels 1% of it.
4. Now only 98% of the light is reflected from mirror 1 and 2% is transmitted.
5. On the next pass, the 2% reflected from mirror 2 cancels 2% of the light
reflected from mirror 1, which now can reflect only 96% (and transmit 4%)
6. … & etc.
This progression continues until, after a short transient period, essentially no
light is reflected from mirror 1 and all is transmitted. (This simplistic analysis
ignores mirror and cavity losses.)
From this time-domain view, we can see that
1. The etalon only becomes transmissive (for the resonant wavelengths)
after a short transient period, and,
2. The intensity inside the cavity is higher (by about the ratio of reflected to
transmitted light of each mirror) than the intensity either before or after the
etalon.
Gires-Tournois Etalon:
What happens if the second mirror is 100% reflective?
Multi-cavity Fabre-Perot Etalons:
Coherent F-P cavities allow bandpass design.
Broad-band F-P (and G-T) Etalons:
Consider the effect of a varying thickness in the “QWS” layers – control of the
reflection’s effective phase and hence the effective cavity length.
Group Delay in Etalons:
Does the light “hang out” for a while? (Otherwise, how would the high internal
intensity occur.)
Applications:
1. Telecom filters for optical networks
2. Tunable optical filters
3. Ultra-thin, highly efficient solar cells.
pg. 4