• the low coefficient of power entry under standard conditions (less than 5% of
optical power),
• low threshold of optical power, at which optical nonlinear effects begin to
become apparent, and
• strong temperature dependence of resonator characteristics.
Resonators on the optical distributed fiber Bragg gratings (FBG), for which the Qfactor of Q ¼ 1.2 Â 10
7 is achieved, have got a special attraction in laser systems
(Figs. 2.6, 2.7, 2.8, and 2.9).
If the phase-locked loop system for the laser optical frequency adjustment uses
the Bragg grating as the optical discriminator (Fig. 2.8) (at the grating step of 100 nm
and at the discriminator cell length of 34 mm), then we may implement the system, in
which the QWLD emission happens with the narrow band (on half-power level) of
the resonance peak of 15 MHz (Fig. 2.10) [37–41].
Experimental results of the phase noise measurement for QWLD emission with
the discriminator on the base of high-Q diffraction grating (Fig. 2.9) are shown in
Fig. 2.10. We can clearly see from these results that discriminator utilization in the
negative feedback loop allows reduction of the laser phase noise by À15 to À20 dB/
Hz at offset 0.1–10 kHz [37, 38].
Fig. 2.7 Forms of optical resonator and delay optical structures applied in lasers and in fiberoptical systems
2.7 Modern Elements of OEO: A Laser, the Optical Fiber, and a Photodetector
41