# How far do photons travel inside common lasers?

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• jg167
In summary, the number of trips that photons take through the medium in a laser depends on the specific setup being used. The average number of reflections can range from single digits to thousands, with 10s to 1000s being a close estimate. The output coupler, which is typically a partially reflective mirror, determines the reflectance and transmittance of the laser beam and is usually determined by the gain of the laser medium. In some cases, the gain is low and the beam must make hundreds of passes through the medium for sufficient gain, while in others, the gain is high and only a few passes are needed. The output coupler can range from being 99% reflective to as low as 4% reflective. The

#### jg167

TL;DR Summary
Lasers work by (wildly simplistically) photons being reflected between mirrors gaining energy on each trip through the medium until they break through the mirror on the output end. How many trips does that typically result in? 10s, millions? trillions?
So (wildly simplistically) lasers work by photons being reflected between mirrors gaining energy on each trip through the medium until they break through the mirror on the output end. How many trips does that typically result in? 10s, millions? And thus the total distance is that count * the length of the cavity (1-10 inches I'd guess in common cases).

It depends entirely upon the exact setup used for the laser. Per wiki's article on output couplers:

In its most common form, an output coupler consists of a partially reflective mirror, sometimes called a beamsplitter. The reflectance and transmittance of the mirror is usually determined by the gain of the laser medium. In some lasers the gain is very low, so the beam must make hundreds of passes through the medium for sufficient gain. In this case the output coupler may be as high as 99% reflective, transmitting only 1% of the cavity's beam to be used. A dye laser has very high gain compared to most solid-state lasers, so the beam needs to make just a few passes through the liquid to reach its optimum gain, thus the output coupler is typically around 80% reflective. In others, such as an excimer laser, the 4% reflectivity of uncoated glass provides enough of a mirror, transmitting nearly 96% of the intracavity beam.

The average number of reflections could be in the single digits, or as high as thousands.

Twigg and berkeman
10s to 1000s is close enough. thanks.

berkeman
Photons do not “gain energy” in a laser. They cause more photons to be released as they pass through the cavity.
You must acknowledge that using the word “photon” is not a guarantee of a better explanation. It is impossible to distinguish between photons in a beam of light. It’s a meaningless idea as they don’t have individual labels.

Nik_2213
"Photons do not “gain energy” in a laser. They cause more photons to be released as they pass through the cavity.". sure that was sloppy wording on my part.
As for you other comment, I'm not sure what you mean by a "better explanation". I could have pheased the question as to how many reflections are needed to break though the output mirror and not used "photon" at all, but my question was interpreted as I had intended by Drakkaith so it seems to have been sufficiently clear.

Nik_2213
jg167 said:
but my question was interpreted as I had intended by Drakkaith so it seems to have been sufficiently clear.
Hmm. It was 'clear' that the meaning of what you wrote was flawed, whether or not someone else picked you up on it. You now seem to have acknowledged the error so that's fine.

There is a massive issue with any idea of photons getting more energy and it is not sustainable. The Energy of a photon is hF and the frequency light in a laser is very highly defined. There is a photon interaction with each atom in an excited state but there is no similar model for what goes on in the cavity itself (or anywhere in space). The output of a laser consists of a very narrow band of frequencies and that gives a high coherence beam. The bandwidth of the spectrum relates to the bandwidth of resonances in the cavity and the bandwidth of a resonance indicates the time for it to build up or decay. This is true for any resonator. You choose what your application is - whether you want a long coherence length (for holography, for instance) or a wide bandwidth (for fast signalling). Once you specify what you want, that will tell you the necessary width of the resonance peak (1/Q factor).
jg167 said:
how many reflections are needed to break though the output mirror
I don't know why there should be a "breaking through" of any threshold. The reflectivity of the output mirror is the same all the time (except, possibly if it gets very hot??) and the light that isn't reflected will pass out of the laser tube. At startup, I imagine there is a finite time for the output to reach its desired level. That would be when there is equilibrium between production of photons and the light flux escaping. It relates to the Q of the resonance.

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Drakkith said:
It depends entirely upon the exact setup used for the laser. Per wiki's article on output couplers:

In its most common form, an output coupler consists of a partially reflective mirror, sometimes called a beamsplitter. The reflectance and transmittance of the mirror is usually determined by the gain of the laser medium. In some lasers the gain is very low, so the beam must make hundreds of passes through the medium for sufficient gain. In this case the output coupler may be as high as 99% reflective, transmitting only 1% of the cavity's beam to be used. A dye laser has very high gain compared to most solid-state lasers, so the beam needs to make just a few passes through the liquid to reach its optimum gain, thus the output coupler is typically around 80% reflective. In others, such as an excimer laser, the 4% reflectivity of uncoated glass provides enough of a mirror, transmitting nearly 96% of the intracavity beam.

The average number of reflections could be in the single digits, or as high as thousands.

Or as low as zero. With sufficiently high gain, a pulse can be created in a single pass through the gain channel: no mirror required. Of course that pulse is not tunable, highly directional, nor long duration, but it is possible.

Drakkith

## 1. How far do photons travel inside common lasers?

Photons can travel a significant distance inside common lasers, ranging from a few centimeters to several meters. The exact distance depends on the type of laser and its design, as well as the specific properties of the photons being emitted.

## 2. Do photons travel at the same speed inside lasers as they do in a vacuum?

Yes, photons travel at the speed of light inside both lasers and a vacuum. However, the speed of light can be slowed down in certain materials, which can affect the distance the photons travel inside the laser.

## 3. Can photons travel in both directions inside a laser?

Yes, photons can travel in both directions inside a laser. In fact, most lasers use mirrors to reflect the photons back and forth multiple times, allowing them to travel a longer distance and creating a more intense beam of light.

## 4. How does the distance that photons travel inside a laser affect its power?

The distance that photons travel inside a laser can greatly affect its power. The longer the distance, the more times the photons can interact with the laser's medium, resulting in a stronger and more concentrated beam of light.

## 5. Is there a limit to how far photons can travel inside a laser?

Technically, there is no limit to how far photons can travel inside a laser. However, the longer the distance, the more likely it is for the photons to interact with the laser's medium and lose energy, which can decrease the power of the laser beam.