Basic Concept

Telecommunication makes comprehensive use of optical techniques, where the carrier wave belongs to the optical domain. Using light as a carrier frequency transmission of analog or digital signals up to a few GHz or Gbits/s are the standards of today. The bit rate can even be increased further using several light carrier frequencies that propagate with almost no interaction between each other on the same optical fibre. In photonics, it is more common to use the term wavelength instead of frequency. This has historical reasons because the wavelength can be measured directly whereas up until now no instruments existed to measure the frequency of light directly. However, the frequency f is directly related to the wavelength λ by the speed of light c as:

This is a very large value and the optical transmission bandwith exceeds the classical microwaves ones.
Using the wavelength division multiplexer (WDM), the bandwidth will not increase, however, parallal data streams can be transfered via the same optical fibre. The basic idea of the WDM makes use of an optical grating. It is well known that such a grating can split an incoming light beam into its fundamental wavelength. The reverse process, combining two beams with wavelength, for example, λ1 and λ2 into one beam forms the base of the WDM. Of course many other beams with different wavelengths can be combined to one beam. For each wavelength a digital or analogue modulated laser source exists. The generated beam travels through the fiber and are split back by means of a de-multiplexer, however, it works in the same way as the multiplexer.

Experimental Set-up

Within this set-up, two laser sources are used. One has a wavelength of 980 nm and the other 1.550 nm. Through the digital frequency generator, both lasers can be modulated. The first WDM is connected to a fibre test segment with a length of 5 x 1 km. At the exit of the fibre the second WDM acts as de-multiplexer. The split signals are measured by the fibre coupled photodetectors. The insertion, as well as wavelength dependant losses are measured and finally the attenuation of the fibre is calculated.
The advantage of the set-up is that the individual components are provided with ST connectors so that a variety of other investigations can be performed by patching the slot-in modules in different ways with the provided patch cables.