To make the amplitude-time plots, the RFSA is replaced with an oscilloscope. The sen-sitivity of the scope is rather low (2mV/div), so maximum optical power is preferred to obtain acceptable pictures.
6.2.1 FP based receiver in a back-to-back configuration
The amplitude-time plot shows the number of transmission peaks traversed during the optical frequency sweep. The sweep frequency time period is equal to Tsw = 1/fsw. In the sweep period time Tsw, two times the number of peaks will be displayed at the oscilloscope because the sweep signal passes the optical filter curve back and forth. The principe on how the time signal would look like is depicted in Figure 6.13.
Figure 6.13: Shape of the electrical time signal for an arbitrary fsw and Psw choice.
The optical frequency is swept linearly through a number of optical periodic filter transmission (FSR) peaks. Both the sweep power Psw and the sweep frequency fsw deter-mine the optical sweep frequency span Fsw,opt. The shape of the time signal is for half a period Tsw/2 nearly equal to the shape of the optical transmission curve, depending on the absolute optical sweep wavelength start and stop point. The shape depends on the type os sweep as well (e.g. triangularly, sinusoidally). If the time signal has a lot of discontinuities in stead of a more sine shape, probably more higher harmonics will be generated. The filter and the sweep power can be adjusted such, that no discontinuities exists in the time signal as shown in Figure 6.14.
6.2. RF AMPLITUDE-TIME MEASUREMENTS 61
Figure 6.14: Shape of the electrical time signal for a chosen fswand Pswsuch that an electrical time signal is obtained without discontinuities.
The discontinuities disappear by choosing the wavelength sweep start and stop points equal to the wavelengths where the filter’s transmission peaks and valleys are located. So, the correct optical frequency span can be controlled by controlling the sweep power Psw
while maintaining the sweep frequency fsw to keep the generated RF carrier frequencies fixed. The FP filter is moved into the position where the start and stop points of the swept frequency are located in the filters transmission peaks and valleys by rotating the filter in the nearly parallel beam (subsection 4.1.2). The resulting time signal is shown in Figure 6.14. What can clearly be seen from this figure is the origin of the factor 2 in the formula for the generation of the microwave carrier fmm = 2 · N · fsw, because here N=1 peak.
The measuring setup for measuring the time signals is depicted in Figure 6.15.
Figure 6.15: Measurement setup for measuring the time plot of a FP receiver in a back-to-back configuration.
The RFSA is replaced by an oscilloscope, and the microwave bandpass filter is removed, to obtain the time signal consisting out of all electrical harmonic components. The mea-sured time plot with the maximum electrical sweep power Psw = -6 dBm is shown in Figure 6.16.
Figure 6.16: Time output signal of the FP based receiver in a back-to-back configuration with Psw = -6 dBm
Because the sweep frequency fsw = 1.9111104 GHz, the sweep period time Tsw = 1/fsw= 0.52 ns. In this time span, two times the number of swept FP transmission peaks are shown. The number of transmission peaks is therefore N ≈ 2 − 2.5. The peak-to-peak voltage is about Vpp ≈ 3 mV. Now we try to change the electrical sweep power Psw in such way that only one FP transmission peak is swept. During the change of the electrical sweep power, we rotate the FP filter carefully to find the correct wavelength sweep start and stop points. This measurement result is shown in Figure 6.17.
6.2. RF AMPLITUDE-TIME MEASUREMENTS 63
Figure 6.17: Time output signal of the FP based receiver in a back-to-back configuration with Psw = -13.8 dBm
The electrical sweep power is lowered to Psw = -13.8 dBm. The sweep period time is still Tsw = 0.52ns. From this picture we can clearly see two electrical peaks within the Tsw = 0.52ns which indicates that we are sweeping through only one FP transmission peak.
6.2.2 FBG based receiver in a back-to-back configuration
We want to see the possible reduction of the contrast ratio when a Fabry-Perot is used in a multimode link. Because of the different arrival times of the propagating modes, constructive and destructive interference due to the multiple reflections between the FP mirrors will not occur for all modes at the same time. This effect will probably decrease the contrast ratio. This effect is measured by measuring the time signals at the photode-tector’s output in a back-to-back configuration and with the POF link. In Figure 6.18 the measuring setup is shown.
Figure 6.18: Measurement setup for measuring the time plot of a FBG receiver in a back-to-back configuration.
The measured optical peak output power Ppeak is -16.54 dBm. The measured time signal at the output of the photodetector, after amplification can be seen in Figure 6.19.
Figure 6.19: Time output signal of the FBG based receiver in a back-to-back configuration
A peak-to-peak voltage Vpp ≈ 10 mV is measured. We use this result to compare the Vpp if the FP-FBG is used in combination with the POF link.
6.2.3 FBG based receiver with the POF link
The generated peak-to-peak voltage for FBG based receiver in a back-to-back configuration is measured to be Vpp ≈ 10 mV with an optical peak power at the photodetector Ppeak
= -16.54 dBm. If we want to compare the influence on finesse due to the large number of propagating modes, we need to keep the optical power at the photodetector equal to the optical power in a back-to-back system. The measuring setup for measuring the time signal in a FBG based receiver with the POF link is shown in Figure 6.20.
Figure 6.20: Measurement setup for measuring the time plot of a FBG receiver with the POF link.
The power of the tunable laser is increased to 4 dBm. The SOA current is increased to 97 mA as well while the attenuator and isolator are removed. The peak optical power entering the photodetector Ppeak = -16.55 dBm is then nearly the same as for the FBG based receiver in a back-to-back configuration. The measured time response at the output of the photodetector after amplification is shown in Figure 6.21.