Anexo 1. Directiva para la liquidación de obras
2.4 OBLIGACIONES DE EL PROYECTO
In this Chapter, the main characteristics of the components of the experimental O IPLL system have been described. The first section introduced an O IPLL configuration that could compensate for mismatches between the optical paths. It was seen that a single-facet bulk optics OIPLL gives path mismatch compensation based on M ichelson interferometric properties as well as a good control over the amount of injected power into the slave laser active area. In order to match the polarisation states correctly and direct the master and slave laser beams throughout the system, the utilisation o f polarising optical components is necessary. The operation of these components and their configuration in the OIPLL were described.
The next step in the OIPLL design was to determine the characteristics of the semiconductor lasers. From the analysis of Chapter 4 it was seen that the requirements placed on the linewidths o f the lasers used in OIPLL systems are far more relaxed than those for OPLL systems. Thus, two conventional InGaAsP buried heterostructure DFB lasers, each of linewidth below 20 MHz at 55 mA bias, were chosen to implement the OIPLL system. The laser wavelengths were matched to within 0.6 nm (or 76 GHz) for equal bias and temperature operation and can easily be brought to oscillate at the same frequency by tem perature tuning. A m ethod for FM response m easurem ent was described and the dynamic tuning characteristics of the laser obtained. It was seen that the lasers do not present an uniform response and this should be taken into account during the OIPLL design.
In Sections 2.1 and 4.1, the equations for the OPLL and OIPLL systems were presented and the contribution of the DC term produced by the photodetection was discarded during the analysis. For homodyne applications, the beat frequency between the lasers is zero and the loop can not distinguish between the contribution due to the laser incident power and the contributions of the actual phase error signal, which is only a fluctuation around the DC term. Therefore, any variation in the total optical power reaching the photodetector would cause the loop circuit to mistakenly consider the intensity changes as detected phase error and the locking process could be severely compromised. In order to compensate for intensity changes o f the detected beams, a balanced detector system was designed based on the polarisation properties o f the light, achieving better than 30 dB of suppression o f the DC term of the photocurrent. Also, the origin o f the term representing the loss in the efficiency of detection introduced into the analysis since Section 2.1 was discussed. It was seen that the misalignment between the laser wavefronts being mixed on the active area of the photodetector can reduce the photodetector gain factor and, therefore, reduce the overall loop gain. For instance, 0.65° o f m isalignm ent betw een the w avefronts can generate up to 10 dB o f photodetector gain factor degradation.
Finally, the design of the loop electronics was presented. In Chapter 4, it was seen that the OIPLL system is more resilient to the effects of the loop propagation delay then the equivalent OPLL and that its phase noise is suppressed, at high frequencies, by the injection locking contribution and, at low frequencies, by the feedback loop contribution. Therefore, the loop electronics can be simplified and the utilisation o f components with conventional bandwidths is possible. A three stage feedback loop was designed using operational amplifiers. The first stage consists o f an am plifier in summing configuration, providing the coupling for the two photodetector outputs. The second stage is the loop filter itself. The loop filter was designed to be a second order
Chapter 5: Optical and Electrical D esign
active filter, built with a high DC gain and wide bandwidth operational amplifier. The last stage com bines the loop feedback signal w ith the laser bias. For that, a transconductance configuration composed o f two operational amplifiers operating in parallel, in a non-inverting summing configuration, that can provide up to 70 mA to the load was designed. The overall characteristics of the circuit, including its frequency response was presented and the effect of the real loop filter and laser FM responses on predicted OPLL and OIPLL performance studied. The results will be used during the experimental investigation related in Chapter 6.
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