I How does a laser start?

  • Thread starter kelly0303
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Hello! I read a bit about how lasers work and I am confused about the initiation process. From what I read, the amplification process (i.e. oscillations in the cavity at the right frequency, once the population inversion is created) is started by spontaneous emission.

A photon that happens to be along the cavity axis will be able to produce stimulated emission and amplify (as it has constructive interference), while the photons emitted at random direction will leave the cavity after a few round trips. However, even if the photons emitted through stimulated emission will have the same phase, photons created by spontaneous emission will not have the same phase among themselves.

So if several photons will be emitted (spontaneously) along the cavity axis, each photon will all create coherent radiation with itself, but not necessary with the others. So if 2 spontaneously emitted photons are out of phase by half a wavelength, they would cancel each other (basically the photon they create through amplification will interfere destructively), even if they are along the cavity axis. What am I missing here? Thank you!
 
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.Scott

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In the unlikely event that the first two spontaneous emissions exactly cancel each other out, there will still be a lot of stimulated emissions waiting to happen. So a third spontaneous emission will do it.
 
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In the unlikely event that the first two spontaneous emissions exactly cancel each other out, there will still be a lot of stimulated emissions waiting to happen. So a third spontaneous emission will do it.
Thank you for your reply! My example with exact cancelation was a bit extreme. What I meant was that you can have lots of photons spontaneously emitted along the axis of the cavity, which interfere randomly. They might not cancel each other, but they will interfere in a random way. Is the amplitude of the laser field some sort of average over all the possible phases? And what is the phases of the field leaving the cavity?
 

.Scott

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Is the amplitude of the laser field some sort of average over all the possible phases?
Once a photon is emitted - either spontaneously or stimulated, it can stimulate further emission. So you quickly get most opportunities for stimulation than you need. So, the amplitude is ultimately limited by how much power is being pumped in.

And what is the phases of the field leaving the cavity?
The phase relative to what?
 
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Once a photon is emitted - either spontaneously or stimulated, it can stimulate further emission. So you quickly get most opportunities for stimulation than you need. So, the amplitude is ultimately limited by how much power is being pumped in.


The phase relative to what?
What I mean is, assume you get 2 spontaneously emitted photons, along the cavity axis, such that they differ in phases by some very small amount (they don't cancel each other, but they don't add up perfectly, I assume they create some sort of beats). Each of them will create amplification (as they both have the right frequency and direction), but the amplified beams will have different phase, right? So does the amplitude of the beam leaving the laser will be a superposition of these 2 types of oscillations? Will it look like a beat?
 

.Scott

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You would only get a "beat" if there was more than one frequency. Combining two oscillators at the same frequency will not generate a beat.

Also, allowing for wear, when you turn a CW laser on, you get the same steady beam every time.
 

DrDu

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Single photons don't have a phase.
 
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Single photons don't have a phase.
I am not sure about naming... in the books I looked over they say something along the line: during stimulated emission a new photon is emitted in phase and direction with the incident one.
 

DrDu

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There is an uncertainty relation between the number of photons and the phase. For a state with definite number of photons, the phase is undefined.
 

DrDu

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Thinking about this question, I would say that this is a continuous process where the initial state with e.g. all atoms in the excited state evolves into one where the atoms are in a superposition of being excited or in the ground state and this state is entangled with a coherent laser field whose mean intensity grows in time.
 

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