Cavity mirrors

Photons of energy hcv are generated initially in the cavity through spontaneous emission. Those that strike the cavity mirrors at 90° are retained within the cavity causing the photon flux to reach a level which is sufflciently high to cause stimulated emission to occur, and the active medium is said to lase. 

Directionality. The laser beam emerging from the output mirror of the cavity is highly parallel, which is a consequence of the strict requirements for the alignment of the cavity mirrors. Divergence of the beam is typically a few milliradians. 

Infrared laser lines involving. .. 2p 5s —. .. 2p 4p transitions in the 3.39 pm region are not particularly usefiil. However, they do cause some problems in a 632.8 nm laser because they deplete the populations of the. ., 2p 5s states and decrease the 632.8 nm intensity. The 3.39 pm transitions are suppressed by using multilayer cavity mirrors designed specifically for the 632.8 nm wavelength or by placing a prism in the cavity orientated so as to deflect the infrared radiation out of the cavity. 

It is well known that by inserting an optical amplifier obtained by population inversion in an optical cavity, one can realize sources of coherent radiations, namely lasers. One can operate in the same way with parametric amphfication as shown on Fig. 1. A nonlinear crystal illuminated by an input pump is inserted in an optical cavity. This cavity is represented for convenience as a ring cavity but consists usually of a linear cavity. An important difference with the laser is that there are three different fields, insfead of one, which are presenf in the amplifying medium, all these fields being able to be recycled by the cavity mirrors. One obtain thus different types of "Optical Parametric Oscillators" or OPOs. 

In this 0-switching technique, one of the cavity mirrors is effectively removed during pumping and then suddenly replaced. The build-up time of the giant pulse is determined by the switching speed and the initial gain of the pumped laser. 

Second, after an appropriate time interval to allow the gas pulse to reach an optimum position between the cavity mirrors, a 1 qs pulse of monochromatic microwave radiation is introduced into the cavity, which is itself tuned to the correct matching resonant frequency. The pulse carries with it a band of frequencies Av 1 MHz, centred at the resonant frequency v of the cavity. The cavity has a bandwidth of approximately 1 MHz, so that the microwave radiation density is high. If the molecular species under investigation has one or more resonant frequencies within this bandwidth, an appreciable macroscopic polarisation is induced, corresponding classically to a phase-coherent oscillation of the molecular electric dipole moments. The microwave pulse must arrive at the correct time interval after the gas pulse. 

ECDL External Cavity Diode Laser OI Optical Isolator L Lens AOS Acousto-Optic Switch PZT Piezo-Electric actuator Me Cavity mirror APD Avalanche Photodiode PC Pressure Controller P Pump... 

Fig. 11. Chemical laser arrangement with flash initiation. 1 laser tube. 2 100% reflecting cavity mirror, 3 coupling mirror, 4 He-Ne laser for resonator alignment, 5 to gas manifold, 6 infrared detectors, 7 monochromator, 8 diode providing a triggering signal for the oscilloscope, 9 single spectral line, 10 total emission signal6)...

Fig. 21. Schematic representation of a subsonic C02 laser with purely chemical excitation (after Cool82)). A He and Fg injectors, H CO2 and NO inlet, C construction detail shown in B, L D2 mixing array, K part of the D2 inlet system which is shown in detail in J, D sodium chloride window, E totally reflecting cavity mirror with long focal length, M, F beam-folding (plane) mirrors, O partially reflecting cavity mirror for output coupling, N laser beam, G resonator housing flushed with nitrogen...

This glitch is caused by interaction between the cavity and the sample resonance profiles. It may be converted into an apparent frequency shift of the cavity resonance that is directly related to the absolute absorption coefficient. The linewidth can be determined fi-om the distance between the peaks of the glitch. This phenomenon became even more marked with the Mark II confocal Fabry-Perot cavity (Section 5.3), when it could be observed as a glitch in the correction voltage applied by the servo amplifier to the piezoelectric actuator of the moveable cavity mirror. Figure 4.7 shows a spectrum for the water line obtained in this manner. 

The small amount of radiation that leaks out of the cavity is measured with an appropriate detector. The decay lifetime is affected by the presence of an absorbing species within the cavity. The high finesse, resulting from extremely well fabricated high reflective coatings on the cavity mirrors, translates into an effective path length of tens of meters. If a molecular beam expansion is performed within the cavity, the direct absorption by molecular clusters can be observed. Considerable attention has been paid to the theory associated with the use of both pulsed and cw lasers. This method, while still in its infancy, promises to be another useftil tool in the characterization of hydrogen-bonded neutral molecular clusters. 

Searching for unknown molecular lines consists of stepping the cavity mirror separation, hence its resonant frequency, and following along with the microwave oscillators. Since the cavity bandwidth is around 1 MHz, the step size is 500 KHz or less. 

U = Undulator M = Optical Cavity mirror B = Bending magnets and focusing elements C = Radiofrequency cavity. 

In order to convert the undulator or optical klystron set-up to a PEL (Pig. 1), cavity mirrors of exceptional quality are required in the visible and ultraviolet regions. The technical problems of producing adequate, radiation resistant, mirrors have not yet been solved as will be seen shortly. 

Laser operation necessitates excellent synchronism between the electron bunch revolution frequency in the storage ring and the light pulse round trip frequency in the optical resonator. It is better to fine tune by modification of the RF frequency rather than by mirror translation, so as to avoid backlash and misalignment of the cavity mirrors. 

In order to attain a high sensitivity for the determination of a, the decay times should be as long as possible i.e., the reflectivity of the cavity mirrors should be as high as possible. The uncertainty Sr is mainly caused by the noise in the decay curves. A good signal-to-noise ratio therefore increases the accuracy of a. 

Fig. 6.4 Experimental realization of Q-switching by a rapidly spinning cavity mirror Ml...

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