Dense Wavelength Division Multiplexing (DWDM)

Using different frequencies of light, it is possible to send multiple signals in different channels down the same optical fiber while allowing each signal to retain its individuality.  These fibers must be thicker than single-mode fibers since they must allow several signals to pass simultaneously.  After passing through the fiber, the signal can be “demultiplexed” using methods of diffraction which separates the beams of different frequencies.

In order for DWDM to be possible, the optical system must have the following capacities:

1)         The optical fiber must be able to maintain a relatively constant index of refraction for a fairly wide spread of light frequencies.  If the index of refraction for the fiber or cladding varied considerably among the frequencies used, it is not likely that the same fiber could carry all the channels effectively.  Primarily, the critical angle for total internal reflection would be different for each frequency. 

sin θ_critical = n_cladding/n_fiber

Those frequencies with significantly higher critical angles would be more likely to lose light.  Thus, the signal for these channels would diminish far more quickly than those with lower critical angles.  Another important consideration is also that the fiber should have consistently low rates of absorbance for all frequencies of light used.  If the fiber had higher absorbance for some channels than others, the signal for these channels would diminish far more quickly and would require more frequent reamplification.

2)         Compact designs for transmitting and receiving signals are also important.  Since dense wavelength division multiplexing sends upwards of 40 channels through a single fiber, optical systems for practical use will have to incorporate extremely compact lasers and photodiodes at either end of the fiber so as not to take up prohibitive amounts of space.  This aspect of wavelength division multiplexing could prove to be the most confining to future expansions in throughput.  In order to accomplish throughputs of 10 Tb/s through a single fiber using DWDM, each fiber would need to carry approximately 1000 channels (based on the value of 10 Gb/s per channel).  Thus, at each end of the fiber, 1000 transmitting or receiving components would have to have access to the end of the tiny fiber.

3)         The spacing between wavelength channels and amplitude of transmission are also extremely important to the transmission of DWDM information.  When multiple channels, differing in frequency, propagate down an optical fiber, the signals can suffer from signal-to-noise degradation and cross-talk as the signals mix with each other.

            However, there are other ways to combat the problem of cross-talk.  The optical power of the signal must remain low to minimize mixing of signals.  Also, the more channels are present in a fiber, the greater the frequency separation must be to prevent cross-talk.  However, these solutions to cross-talk also have their drawbacks.  A system with low optical power will be able to travel smaller distances before the signal needs to be amplified.  If one must increase channel spacing for each additional channel added to the transmission, there will be a limit on the number of channels one can send through the fiber before cross-talk becomes a serious problem for information transfers.