RESULTADOS Y DISCUSIÓN
4.1 Diagnóstico de las capacidades en la resolución de problemas
4.1.1 Análisis de resultados de la prueba de diagnóstico de capacidades
In the two previous Sections, the experimental results o f the OPLL and OIL systems were presented, using the set-up, semiconductor lasers and loop filter described in Chapter 5. The next step is to incorporate OPLL and OIL systems together and analyse the experimental results o f an OIPLL configuration. In Chapter 4, it was seen that the main problem in the OIPLL implementation is the differential phase resulting from the difference in the optical paths from master laser to photodetector and from master laser to slave laser and slave laser to photodetector. In Section 5.1.2, a method o f path m atching was proposed by configuring the OIPLL set-up as a M ichelson interferometer. W ith the master laser biased at a value o f current corresponding to 2.75xlth, the laser frequency was varied by m eans of tem perature tuning. The long itu d inal m ode frequency variation was observed using the Fabry-P erot interferom eter while the oscillations in optical pow er were observed on the optical power meter. For long path mismatches, it was possible to observe the longitudinal mode o f the master laser vary in intensity with the master laser tuning in the way predicted by eq. (5.1.1). The same oscillatory behaviour was observed in the optical power meter readings. The maximum optical power achieved was 220 |xW. After each observation, the path mismatch was adjusted and the slave laser tuning repeated. The path m ism atch was corrected by means o f adjusting the drive and piezoelectric elements located in the translation stage of the mirror mount. The master laser operating temperature was 22°C and the temperature tuning interval was ±8°C, giving beyond 170 GHz o f frequency scan. After repeating the procedure many times, path matching to less than 20 |im was achieved, which, from the results of Section 4.5, can guarantee a very good degree of path matching, as the influence o f the differential phase
Oaijf
is kept below 5 deg for more than 100 GHz of master laser frequency variation (Fig. 4.17).The alignment of the OIPLL system follows the procedures described in Section 5.1, for the alignment of the single facet set-up. Section 5.3, for the alignment o f the balanced receiver, and Section 6.1, for the introduction o f the monitoring path. The system is the same as that for the OPLL experiment, but, in this case, the half-wave plate betw een isolator and polarising beam splitter of Fig. 6.1 is rotated to allow injection to occur. During the experiment, the parameters o f the loop and the injection locking process (laser bias, injection ratio, loop gain, etc.) were kept the same to allow com parison among the three locking technique results. This is possible as the three experiments can be performed in succession using the same experimental set-up.
W ith the master laser biased, the injection level is checked by means o f the slave laser photocurrent. After that, the half-w ave plate betw een the isolator and
polarising beam splitter is rotated to prevent injection and the slave laser is biased. The photodetectors are aligned and balanced and the fine wavefront overlap is executed by means o f the piezoelectric adjustments o f the m irror tilting. The loop circuit is incorporated to the system. The half-wave plate between the isolator and polarising beam splitter is rotated to the angular positions used during the OIL experiment in order to provide the same values of injection ratio. The measurements were made using the lightwave signal analyser, to monitor the power spectral density associated with the OIPLL process, and the Fabry-Perot interferometer, to allow the measurement o f the frequency variation of the slave laser output.
Fig. 6.14 shows the sequence of power spectral density plots measured by the lightwave signal analyser for different injection ratios. Unfortunately, measurements at higher values o f injection ratio than the ones presented were not possible due to the noise floor o f the lightwave signal analyser. It can be seen that the low injection measurem ent presents a characteristic very similar to that presented by the OPLL system. That is expected as the injection influence is small and the system still presents the cycle slipping problem of the phase-lock technique. However, as the injection ratio is increased, the influence of the master laser injected power contributes to the laser frequency synchronisation process, reducing the phase noise level.
Fig. 6.15 shows the comparison between the OPLL and the OIPLL results for the same loop parameter as those described in section 6.1 and injection ratio -31.4 dB. This Figure also shows the expected theoretical plot for the OIPLL system. It can be seen that the addition o f the injection locking path improves the poor phase noise suppression of the OPLL system. Also, there is a good agreement between the modelled and experimental OIPLL results, as, in this case, the linear assumptions made during the theoretical analysis are valid. In order to relate the pow er spectral density measurements with the equivalent values of phase error spectral density, the procedure described in Section 6.1 for the OPLL system can be used. This allows the calibration o f the plot of Fig. 6.15, as in the figure caption [6.12].
The values of the phase error variance were measured for different frequency detunings between m aster and slave laser. This was a way to check the locking bandwidth that can be achieved with the OIPLL system and to observe how the phase noise suppression o f the system is distributed throughout the locking range. As theoretically predicted, it was observed that the OIL system presented an unstable region for certain values o f detuning between master and slave laser frequencies above a critical level of injection, due basically to a decrease in the damping o f the SL relaxation oscillations. Fig. 6.16 to 6.18 shows the phase error variance measured in 500 M Hz bandwidth as a function o f the detuning between master and slave laser for
Chapter 6; Experimental Results R B W = 3 0 0 k H z ST = 166.7 ms I I I I I I I I I I I I * 7 p I U j l H k 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 F r e q u e n c y (M H z ) (a) - 7 0 1 1 -8 0 1 -9 0 g 1 -1 0 0 1 -1 1 0 Ô: - 1 2 0 R B W = 3 0 0 kH z V B = 3 0 k H z ■ i i ST = 166.7 ms ATT = 0 ciB i