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I am struggling to find the expected reflection spectrum of a triangular cavity, both with and without round-trip losses.

This is a simple scheme: Schematic of the cavity setup

Laser light is being coupled in the cavity through mirror M1, which has a reflectivity R1 lower than the other mirrors of the cavity (which we assume to be perfect - R = 100%), such that after a round-trip the main escape route of the light is through the same mirror. The cavity is stable at a fixed length, and the laser frequency is tuned across the cavity resonance.

  1. What should I then see at the point A if I monitor the intensity? In the above case I would assume that you would see nothing at all, with at most a small dip due to the impedance mismatch coming from mirror M1 having some loss.

  2. What happens then if not all the light couples in the cavity? Will I see some interference between the light reflected at M1 and that leaves the cavity after a round trip? This could be confusing, since normally I would align a cavity by maximising the reflection dip, but here I could instead be decreasing the mode-matching.

  3. And finally, what if there is additional intracavity loss (say I put some absorbing material in the cavity)? Will it just add to the impedance mismatch of point 1?

I am asking these questions as I am confused but what we see experimentally. In such a cavity, with a R1 = 97.5% we see almost a full dip when the laser is resonant with the cavity, extinguishing >85% of the light, even though the cavity is empty.

Qmechanic
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Samuele Grandi
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1 Answers1

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For any laser you can "extend" the cavity externally through the use of full or partially reflecting mirrors. I.e. if you take a laser and place a 100% R mirror perfectly reflecting back into the laser you can effectively turn it off, power consumption goes to near zero. Classically we would say the light has "interfered" and cancelled itself .... but that is a violation of conservation of energy. In your experiment you have reflected back into the cavity a certain percentage of light .... in a laser the exit mirror could have external transmission of ~~10% ...enough to keep population inversion and lasing high in the cavity.

Somehow you have upset this delicate ratio. Also you should be monitoring laser current .... and what method are you using to see this "dip"?

PhysicsDave
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