# The length of laser's optical cavity. Multiple of wavelength?

• deccard
In summary, the coherence of laser light does not depend on the length of the optical cavity between the mirrors being a multiple of the wavelength of the laser light. The mirrors are used to intensify the light, and the coherence comes from stimulated emission from atoms in the gain medium. The operating frequency of the laser adjusts itself to find a standing wave mode, making the length between mirrors less critical. However, if the cavity is too short, the gain spectrum may not align with the longitudinal modes and the laser may not lase.
deccard
In order to produce coherent laser light is it necessary that the length of optical cavity between the mirrors is a multiple of the wavelength of laser light? I cannot see how otherwise the laser been could bee coherent. On the other hand I don't see how it is possible to place the mirrors with nanometer precision so that for example thermal expansion doesn't change the length between the mirrors.

as i know, no relationship. The mirrors for intensify the light.

Yes the mirrors are used to intesify the laser beam. And the coherence of the laser beam comes from stimulated emision from atoms in gain medium. But the question in hand raised when I thought how the light is reflected back and forth. But now I'm starting to be pretty confident that it doesn't matter whether the length between mirrors is or is not multiple of the laser waveleangth in order to retain the coherence of light. Especially if the electromagnetic field has to be zero at the reflection surface of mirror.

Actually, there is some "give" in the frequency of the laser emission. It adjusts itself to find an operating point which involves a standing wave mode. So it turns out to be an integer number of wavelengths no matter what the actual length of the cavity.

To elaborate on what monish said, the gain spectrum of a laser is not really perfectly discrete, and every peak will have a finite linewidth. As long as the longitudinal modes of a Fabry-Perot resonator are spaced sufficiently close together, the system will lase at one of them. You are right, though, in that if one were to construct a system whose linewidth was small compared to the mode spacing (e.g., by making an extremely short cavity), then it is possible for the gain spectrum to "miss" all of the longitudinal modes.

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## 1. What is the purpose of measuring the length of a laser's optical cavity?

The length of a laser's optical cavity is an important factor in determining the properties and behavior of the laser. It impacts the laser's mode structure, spectral characteristics, and overall performance.

## 2. How is the length of a laser's optical cavity measured?

The length of a laser's optical cavity is typically measured using interferometry techniques. This involves splitting the laser beam into two paths, one of which is reflected off a movable mirror. The interference pattern between the two beams is then used to determine the length of the cavity.

## 3. Why is the length of a laser's optical cavity often a multiple of the laser's wavelength?

This is due to the fact that in order for a laser to produce coherent light, the laser cavity must support standing waves that are an integer multiple of half the laser's wavelength. This ensures that the waves within the cavity are in phase and can produce a strong, coherent laser beam.

## 4. How does the length of a laser's optical cavity affect the laser's output?

The length of a laser's optical cavity can affect the laser's output in several ways. It can determine the laser's mode structure, which impacts the beam size and shape. It can also affect the spectral characteristics of the laser, such as the linewidth and wavelength stability. Additionally, the length of the cavity can affect the laser's output power and efficiency.

## 5. Can the length of a laser's optical cavity be adjusted?

Yes, the length of a laser's optical cavity can be adjusted by moving one of the cavity's mirrors. This can be done manually or with the use of a piezoelectric transducer, which can make precise adjustments in the length of the cavity. By adjusting the cavity length, the laser's properties and output can be optimized for specific applications.

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