The emission wavelength of active part of laser cavity depends upon the bandgap of material, while the allowed oscillation frequencies of passive part depends upon cavity length. How these 2 phenomena decide the actual laser output wavelength?
The emission wavelength of active part of laser cavity depends upon the bandgap of material, while the allowed oscillation frequencies of passive part depends upon cavity length. Answer : SPECTROGPAF, Spectrometr. + Specrt of Fe.
Regdrds,
The specific wavelengths/frequencies of the output beam within the gain bandwidth are determined by the longitudinal modes of the cavity.. Whichever oscillating frequencies in the cavity have maximum overlap with gain spectrum of the gain medium, they will be survived or pronounced, based on the following expression.
Nλ = 2 × Cavity Length
See the related article here. https://www.photonics.com/Articles/Lasers_Understanding_the_Basics/a25161
It depends on the energy band gap and the refractive index of the active layer
All the above answers are basically correct. But it is @Vincent Lecoeuche's mention of noise that gets to the real action in a multimode laser cavity (and even in a nominally single mode laser cavity.) Here, I'm thinking mainly of semiconductor lasers of the type used in telecom. The noise that Lecoeuche mentions is the random spontaneous emission that is amplified by the material gain and that resonates inside the cavity. We can model the resonant cavity modes to be independent or can account for their competition for material gain. If they compete significantly, one mode can reach lasing threshold and deplete the material gain that would have fed the other mode in a case referred to as spectral hole-burning. The lasing mode eats the part of the gain spectrum that other modes would have used to reach threshold. Or, in cases of so-called single mode lasers (e.g., semicondutor distributed feedback lasers), one mode can reach threshold and spatially deplete gain (a process known as spatial hole-burning) thereby starving other nearby modes that would have used the gain from the depleted par of the laser cavity. The practical impact of modal competition is relevant in telecommunication lasers that pulse rapidly. Each successive pulse causes a new competition between modes that have nearly the same material gain. Noise can cause mode A to win over mode B for one pulse and then the opposite for the next pulse. If these pulses propagate through glass fiber, dispersion can cause the color of mode A to reach the end of the fiber in a different amount of time compared to a pulse of mode B. This fiber dispersion can corrupt the optical signal. To guard against data errors caused by fiber dispersion, single mode lasers are preferred over multi-mode lasers for long-haul fiber spans. But noise causes occasional (rare) errors even in single mode lasers when a preferred mode hic-cups and a different mode lases for a pulse. Therefore, your question on what determines the lasing wavelength is fundamental for fiber-optic telecommunications, especially in conventional systems that pulse lasers on/off.
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