2.2 BASE O SUSTENTO TEÓRICO
2.2.25 Órgano de Control Institucional
As seen in the previous Sections, the loop bandwidth and the gain of the loop are im portant parameters in the loop design regarding tracking capability. The first order loop was seen not to be the best filter configuration as both parameters can not be set independently. Second and higher order filters, however, can offer extra control of the loop characteristics by means of changes in the values of the loop filter components.
The analyses carried out until now have assum ed ideal conditions: the amplifiers and loop components having infinite bandwidth, the phase error being small enough to allow the phase detector to have a linear response and the characteristics of the FM response of the slave oscillator being perfect. However, these assumptions are not true for real systems. The amplifiers and loop components have a finite bandwidth, which would require more complex loop filters in order to cover the whole bandwidth required for a particular system. A complex but effective solution to the bandwidth limitations of loop filter components is the implementation of split path loop filters [2.10.2.19].
As seen in the Bode and Nyquist diagrams in this Chapter, the loop delay alters the dynamics of the loop and can cause instability and prevent the loop from acquiring lock if the loop gain and loop bandwidth are not limited. The limitations imposed by the loop delay in relation to gain and bandwidth result in loops with poor phase noise suppression. If the real response of either the phase detector, in the case of a heterodyne system, or the photodetector, in the case of a homodyne system, is taken into account, it will be seen in Chapter 4 that locking performance o f OPLLs also depends on the amount of phase noise that the laser sources present, that is upon the laser line widths, which must be limited to a maximum value for a given loop delay. Split path loop filters w ould also be a good solution for the loop propagation delay problem
[2.10.2.19], allowing the utilisation of non-passive components in the loop filter.
In the particular case o f OPLLs using sem iconductor lasers, wide loop bandwidths are necessary to deal with the laser phase noise. As discussed above, the control of the loop parameters is desirable to cope with the problems brought in by a finite loop delay. Therefore, a second order loop would be necessary. That would be the best choice as this type of loop can offer better tracking and acquisition than a modified first order loop. However, the second order loop itself is more complex than a modified first order loop and can introduce longer delays in the system, decreasing the phase margin and requiring extremely narrow linewidth lasers. As will be discussed in Chapter 4, the modified first order loop, despite the dependence between gain and bandwidth, is more tolerant of loop propagation delay, offering superior phase noise
Chapter 2: Optical Phase-Lock Loop Theory
suppression when longer loop delays are involved [2.10,2.17,2.18], and, therefore, is more suitable for OPLL implementation with wide linewidth lasers.
References
[2.1] L. G. KAZOVSKY and D. A. ATLAS: "A 1320-nm experim ental optical
phase-locked loop: performance investigation and PSK homodyne experiments at 140 Mb/s and 2 Gb/s",
J. Lightwave Technol,
vol. 8, no. 9, pp. 1414-1425, 1990.[2.2] C. H. SHIN and M. OHTSU: "Heterodyne optical phase-locked loop by
confocal Fabry-Perot cavity coupled AlGaAs lasers",
IEEE Photon. Technol.
Lett, y
vol. 2, no. 4, pp. 297-300, 1990.[2.3] J. M. KAHAN, A. H. GNAUCK, J. J. VESELKA, S. K. KOROTKY and B. L.
KASPER: "4-Gb/s PSK homodyne transmission system using phase-locked semiconductor lasers",
IEEE Photon. Technol. Lett.,
vol. 2, no. 4, pp. 285-287,1990.
[2.4] M. KOUROGI, C. SHIN and M. OHTSU: "A 134 MHz bandwidth homodyne
optical phase-locked-loop of semiconductor laser diodes",
IEEE Photon.
Technol. Lett.,
vol. 3, no. 3, pp. 270-272, 1991.[2.5] D. ATLAS and L. G. KAZOVSKY: "Optical PSK synchronous heterodyne
experiments at 560 Mbit/s through 4 Gbit/s",
J. Opt. Commun.,
vol. 12, no. 4, p p . 130-137, 1991.[2.6] R. T. RAMOS and A. J. SEEDS: “Fast heterodyne optical phase-lock loop using double quantum well laser diodes”.
Electron. Lett.,
vol. 28, no. 1, pp. 82- 83, 1992.[2.7] U. GLIESE, T. N. NIELSEN, M. BRUUN, E. L. CHRISTENSEN, K. E.
STUBKJAER, S. LINDGREN and B. BROBERG: “A wideband heterodyne optical phase-locked loop for generation of 3-18 GHz m icrowave carriers” ,
IEEE Photon. Technol. Lett.,
vol. 4, no. 8, pp. 936-8, 1992.[2.8] F. M. GARDNER: "Phaselock Techniques", second edition, John W iley & Sons Inc., New York, USA, 1979.
[2.9] A. BLANCHARD: "Phase-Locked Loops", first edition, John W iley & Sons Inc., New York, USA, 1976.
[2.10] R. T. RAMOS: "Optical Phase-Lock Loops Using Sem iconductor Lasers", PhD. thesis. Dept, of Electronic and Electrical Engineering, University College London, London, England, 1992.
[2.11] M. OHTSU: "Highly Coherent Semiconductor Lasers", first edition, Artech House Inc., Boston, USA, 1992.
[2.12] M. A. GRANT, W. C. MICHIE and M. J. FLETCHER: "The performance of optical phase-locked loops in the presence of non negligible loop propagation delay",
J. Lightwave Technol,
vol. LT-5, no. 4, pp. 592-597, 1987.[2.13] T. G. HGDGKINSQN: "Phase-locked-loop analysis for pilot carrier coherent optical receivers".
Electron. Lett.,
vol. 21, no. 25/26, pp. 1202-1203, 1985.[2.14] G. H. HOSTETTER, C. J. SAVANT Jr. and R. T. STEFANI: "Design of
Feedback Control Systems", CBS College Publishing, New York, USA, 1982.
[2.15] I. D. BLANCHFLOWER: "Optical Control Techniques for Microwave Phase
Locking A pplications", PhD. thesis. Dept, of Electronic and E lectrical Engineering, University College London, London, England, 1992.
[2.16] U. GLIESE: "Optical G eneration o f M icrow ave Signals", PhD. thesis. Electrom agnetics Institute, Technical U niversity of D enm ark, Lyngby, Denmark, 1992.
[2.17] R. C. STEELE, M. J. CREANER, G. R. W ALKER and N. G. W ALKER:
"Optical transm ission experim ent at 565M bit/s incorporating an endless polarisation control system", SPIE, vol. 988, Components for Fibre Optic Applications and Coherent Lightwave Communications, 1988.
[2.18] R. T. RAMOS and A. J. SEEDS: "Comparison between first-order and second- order optical phase-lock loops",
IEEE Microwave and Guided Wave Lett.,
vol. 4, no. 1, pp. 6-8, 1994.[2.19] C. J. WARD: "Delay reduction techniques in phase-locked loop amplifiers".