激光原理英文版ch5完整.pptxVIP

Ch.5 Passive Optical ResonatorsYong-Chang HanDepartment of PhysicsDUTCh. 5 Passive Optical Resonators5.1 INTRODUCTION5.2 EIGENMODES AND EIGENVALUES5.3 PHOTON LIFETIME AND CAVITY Q5.4 STABILITY CONDITION5.5 STABLE RESONATORS5.6 UNSTABLE RESONATORS5.1 INTRODUCTIONpassive optical resonatorsno active medium is present within the cavityLL may range from a few centimeters to a few tens of centimeters, while the mirror dimensions range from a fraction of a centimeter to a few centimetersLaser resonators thus differ from those used in the microwave field (Maser) in two main respects:(1) The resonator dimensions are much greater than the laser wavelength.A laser cavity with length comparable to the wavelength would then generally have too low a gain to allow laser oscillation.(2) Resonators are usually open, i.e. no lateral surfaces are used.This drastically reduces the number of modes which can oscillate with low loss.Open resonators have inevitably some losses due to diffraction of the e.m. field, which leads to some fraction of the energy leaving the sides of the cavity (diffraction losses).Strictly speaking, therefore, the mode definition given in Ch2 (Blackbody Radiation) cannot be applied to an open resonator and true modes (i.e. stationary configurations) do not exist for such a resonator. We therefore define a mode with some losses.the decay time of the square of the electric field amplitudevarious possible resonators(1) Plane – Parallel (or Fabry–Perot) ResonatorTo a first approximation the modes of this resonator can be thought of as the superposition of two plane e.m. waves propagating in opposite directions along the cavity axis.RECALL:Within this approximation, the resonant frequencies can be readily obtained by imposing the condition that the cavity length L must be an integral number of half-wavelengthsIn other words, (standing-wave configuration)the phase shift of a plane wave due to one round-trip through the cavity must equal an integral number times 2π, i.