Examples of investigation and measurement

Measurements at the fibre
Before starting the measurements, the light has to be coupled into the fibre. For reasons of simplification this is done with the diodelaser in the modulation mode so that an oscilloscope in the AC-mode can be used at high amplification. The highest amplification is also chosen for the photodiode amplifier within the controller unit. That way light can be detected at the exit of the fibre even for weak coupling. By means of further adjustment, a maximum of coupling efficiency can be reached. After this has been achieved, the coupling efficiency, the fibre losses (by shortening of the fibre) and the numerical aperture can be measured at the exit of the fibre. The angle-resolved intensity distribution informs about the transmitted modes.

Modulation of the diodelaser
If an external modulator is available, the diodelaser can be modulated by any signal. For this reason, the controller has an input for external modulation. The characteristic curve of the diodelaser is determined and the optimum working point is chosen for the modulation or data transmission. The external modulation input can directly process TTL-signals. Negative voltages are not permitted.

Transit time measurements at the fibre
A further interesting experiment is the measurement of the transit time of the laser light within the fibre. Due to the fibre length of 100 m, significant transit time differences can already be measured. In this case, the diodelaser is modulated by the internal modulator and the injection current as well as the photo detector signal are represented on the oscilloscope. First the transit time of the complete system is measured - that means the transit time due to electronics and the transit time of light. After this value has been determined, the fibre is eliminated so that the light falls directly on to the photo detector. Now the transit time without the fibre is determined. From the difference of both measurements, the transit time through the fibre is derived. Finally, the length of the fibre can be determined if the refractive index of the fibre is considered to be known or the refractive index if the length of the fibre is known.