Why there is a minimum lasing threshold? Can someone help me understand why it is often mentioned that for certain lasing media there is a certain minimum lasing threshold without mentioning resonator design & doping concentration?
As far as I see it - it should be possible to lower lasing threshold by using more reflective mirrors (like 99.9% first and 99.8% second with just 0.1% transparency), lowering dopant concentration and filtering out parasitic luminescent photons flying in wrong directions.
Why these methods does not allow to lower lasing threshold to almost 0?
I am specifically interested here in Ruby and Dye(Rhodamine) lasers.
 A: The fundamental reason for lasing thresholds is the loss of energy via fluorescence in all directions. Only a small fraction of the emitted photons will couple to your cavity modes. You must overcome this energy loss to achieve population inversion. Thresholds can be reduced significantly through resonator design (often at the cost of slope efficiency).
Although it is a special case, a thresholdless laser is achievable. This is done through the use of what is called a microcavity. From my understanding this restricts all spontaneous emission to occur in useful cavity modes. Although I think the gain media in these lasers are microscopic in size.
REF: http://www.rp-photonics.com/thresholdless_lasers.html
A: Agree with Adam.
If the active medium is placed inside a microcavity in a photonic crystal, then spontaneous emission can be forbidden when the working frequency is well inside a photonic band gap.
A: You can lower the threshold to some extent. However why would you do all the hard work if you typically need your laser to have some noticeable power anyway?
