In Fig. A.3 is shown the setup used in order to make the measurements of the
n2/Ae f f for the ITU Round Robin. The black circles indicate the points in which a
splice was made and in which it is necessary to make measurements of power in order to estimate the losses of our interferometer. Note that a description of the components is not anymore necessary due to the fact that it is reported in Fig. 2.3.
First of all we have to find out how much is the power we have at the exit of the laser . To do this we use a power meter and we measure the power at the point 1 of the IF. Note that the laser is working in pulsed mode so the measured power is the average power. To convert to the peak power we have to use the formula
Wavg =Wpeak·τ·ν
where Wavg is the average power, Wpeak the peak power, τ the pulse width andν the repetition rate of the pulse. Now we have to make different measurements of power changing bothτandνsuch that their product is constant (i.e. the peak power). In this way we can find out if our power meter gives consistent results. The measurement
Figure A.1: In the figure is shown the picture of the interferometer used to make the measurements of the n2/Ae f f for the ITU Round Robin. Black circles represent the splice points.
Figure A.2: In the figure is shown the picture of the setup used to make the measurements of the n2/Ae f f for the ITU Round Robin.
of power we have made, are done with a frequency of 10kHz and a pulse width of 20 nsec. From this values we can find out what is the peak power and later calibrate our fast photodiode. An error at this point can introduce a systematic error on all the measurements and consequently on the n2/Ae f f values.
Now we build the setup as shown in Fig. A.3. We measure the power at 1 (exit of an angled connector) and then we enter into the EDFA. The fiber cable connecting the laser to the EDFA has to be fixed with tape on the table. From the EDFA we have to clean the signal (too much ASE coming from the EDFA) so we enter the optical circulator splicing the fiber coming out from the EDFA to the fiber at the entry of the circulator (point 2). Here we go into the fiber Bragg grating splicing at point 3. The reflection (i.e. the laser signal) is then measured at point 4. Here we have to measure the peak power value. In principle it is not necessary to know how much power we have lost in the splice and connections before, because the power at 4 is the power we consider as the entry power of the interferometer.
Due to the fact that the laser is not exactly matched on the fiber Bragg grating we have to tune its wavelength with the thermoelectric unit. This can be done taking the light at point 4, splicing the fibers at a pigtailed fiber, entering the fast photodiode and monitoring the pulse shape on the scope as a function of the set temperature on the thermoelectric unit. The value used during the measurements is 31.5 Celsius.
After this we have to enter the first coupler (50/50). We measure the power at points 5 and 7 and find out what are the losses and the coupling ratio of the coupler.
Then we consider arm 5/6. We have to insert here a delay line in order not to make overlap the pulses (see chapter 2.2). We measure the power at 5 and at 6 and find the loss of the delay line plus splice 5. Then we splice at 6 the 90 entry of the 90/10 coupler and we measure the power at 9 and 10. Then we measure the power at 7 and 8 and we find the loss of the polarization control and splice 7. Then we enter the 10 entry of the 90/10 coupler. We measure the power at 10 and 9. Using the information of before we find the losses and coupling ratio of the 90/10 coupler and losses of splices 6 and 8.
Then we have to characterize the losses of the faraday mirror. To do this we splice the FM directly at 10 without any FUT and measure the power at 13. The loss of the FM (β00) is equal to 1.77 dB.
The results for the interferometer once built up are the following. First of all the losses in the arms are symmetric. Second the power at 10 is equal to 1/2.23 the power measured at 4. From now on we never measure the power at point 4, because we should rebuild the setup every time. But instead we measure the power at point 10 and from this we calculate the power at the entry 4. Point 13 is connectorized.
All the fibers and components have to be attached onto a robust plate in order not to introduce any change in the polarization. Once built the setup is stable over months.
Note that the coupler’s fibers are characterized by different colors. Regarding the 50/50 coupler splice 4 at fiber A. Splice 13 at fiber B. Splice 5 at fiber C. Splice D at fiber 7. For what concern the 90/10 coupler, splice 6 at fiber ORANGE. Splice 8 at fiber WHITE-ORANGE. Splice 10 at fiber BLUE. Splice 9 at fiber WHITHE-BLUE.
1
2 3
4 13
5 7
6 8
9
10 11
12
Figure A.3: In the figure is shown the setup used to make the measurements of the n2/Ae f f for the ITU Round Robin. The black circles indicate the points in which is crucial to make loss measurements.