In fiber optics, some people say FDM and WDM are the same thing
because of the relationship between freq and wavelength.  Fair enough.
But I keep seeing optical architectures described as incorporating
FDM and WDM. By FDM, do they really mean FMCW?  If so, how does it
work?  I know the current is varied into a laser to change the freq in
a continous fashion, but how does it work beyond that?  And if WDM is
running at the same time (i.e. with multiple lasers each with
different freq), how is it controlled so crosstalk doesn't destroy the
signal integrity?  I'm a seismologist, not an optics engineer so
please make this easy on me!

WDM is a form of FDM

FDM - Frequency Modulation is an analog technique using a carrier
 frequency and sidebands on which an analog signal is conveyed.
When multiple carrier frequencies are used on a single link you have FDM.

WDM Wavelength Division Multiplexing.
 is a digital technique where light
 pulses - representing the bits - in different colours share a fiber.
 
 The differences:  WDM is digital technique, FDM is analog
                   FDM is used for any carrier, WDM for fiber optics

 In WDM, different channels are represented by different colors. Color
is frequency and simply related to wavelenth.

 It can be argued that digital pulses are special case of analog
waveforms - in this sense WDM is a form of FDM.
http://www.wilco-telephony.co.uk/telephony.html

   WDM can be seen as a modern developement in FDM,
 both in choice of carier
(fiber) and type of modulation :

 FDM has been used already since The 1960s
http://www.k12.hi.us/~telecom/datahistory.html


 WDM is relatively new.
http://www.bell-labs.com/org/physicalsciences/timeline/photo.html
 Wavelength Division Multiplexing (WDM) Routers Based on Silicon
Optical Bench - 1991

  
hedgie

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Clarification of Answer by hedgie-ga on 11 Feb 2004 23:17 PST

Both analog and digital channels can be used on same fiber,
using different parts of the spectrum, as for example here:

N. K. Cheung, K. Nosu and G. Winzer (eds.) IEEE JSAC: Special Issue on
Dense WDM Networks. volume 8, Aug. 1990.

Efficient Routing in Optical Networks - Aggarwal, Bar-Noy, Coppersmith.. (1996)   

....1. Introduction Fiber optic networking technology using wavelength
division multiplexing (WDM) o ers the potential of building large wide
area networks capable of supporting thousands of nodes and
providing capacities of the order of gigabits per second to each node
in the network [Gre92, Ram93, CNW90]. In WDM optical networks, the
vast bandwidth available in optical ber is utilized by
partitioning it into several channels, each at a di erent optical
wavelength. Each wavelength can carry data modulated at bit rates of
several gigabits per second. ..


etc
http://citeseer.nj.nec.com/context/31415/0

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Clarification of Answer by hedgie-ga on 11 Feb 2004 23:44 PST

Apparently use of the terms vary with different groups.

Here is another example of terms WDM and FDM,
which explains the use you ask about:



WDM is conceptually similar to frequency division multiplexing (FDM),
in which multiple information signals (each corresponding to an end
user operating at electronic speed) modulate optical signals
at different wavelengths, and the resulting signals are combined and
transmitted simultaneously over the same optical fiber as shown in
Fig. 1.5. Prisms and diffraction gratings can be used to combine
(multiplex) or split (demultiplex) different color (wavelength) signals. FDM is 


Figure 1.5 Wavelength division multiplexing.

used to carry many radio channels over the air or several simultaneous
TV channels over cable. A WDM optical system using a diffraction
grating is completely passive, unlike electrical FDM, and thus
is highly reliable. Further, a carrier wave of each WDM optical
channel is higher than that of an FDM channel by a million times in
frequency (THz versus MHz). Within each WDM channel, it is possible
to have FDM where the channel bandwidth is subdivided into many radio
frequency channels, each at a different frequency. This is called
subcarrier multiplexing.

 http://www.informit.com/isapi/product_id~{A83B57CA-0E9B-4C00-887C-3EF622FF894B}/element_id~{B6E322BB-FB4F-4130-832D-CFFEA1972DE2}/st~{A51C5BA3-1071-411E-A9D3-85273226A8FA}/content/articlex.asp

thanks, but much of what was provided was basic knowledge and the
highly technical central question was not answered.

If you use the term "FDM" literally, WDM is synonomous (i.e. we are
sending multiple information streams over a single channel and
separating them by frequency). This analog-digital stuff gives me a
gas pain. At a transmission rate of 10 Gbps, when you turn on the
laser to send a 1-bit, you're sending about 20,000 cycles of light.
Each bit stream (i.e. channel) is sent using a different color or
wavelength of light, and they are turned off-and-on independently.

How they make the different colors/wavelengths is with a gizmo called
a tuned laser. Rather than varying the drive voltage, you determine
the wavelength by adjusting the size of the resonating cavity where
the light is amplified. Heating or cooling the resonating cavity
alters its refractive index which in turn defines how fast the light
travels through the material. So by heating or cooling the cavity you
can make it "appear" longer or shorter (that's a lot easier than
diddling with the physical size). The most typical design today is
called a distributed feedback (DFB) laser that uses a cooling system
to maintain a constant temperature in the resonating cavity. A single
tuned laser component costs $10,000 to $20,000, and a lot of that cost
is tied up in the cooling mechanism.

In dense WDM (i.e. 80- or 160-channels spaced at 50 GHz intervals),
the lasers must be very precise or crosstalk will result. We also have
coarse WDM where channels are spaced at 200 G or 400 GHz intervals and
those can use cheaper uncooled lasers.

If you want a real mind-bender, we also use amplifiers to increase the
power of the optical signal so we don't have to regenerate each
individual channel (an expensive proposition with $20,000 components).
That's a whole 'nother story.