# Coherence length of monochromatic laser

• elif
In summary, the conversation discusses the topic of measuring coherence length in a monochromatic laser. The suggested method is to use a Michelson interferometer with a less-than-ideal laser. The teacher also requests to use a high coherence length. One suggestion is to use a spectrometer to measure bandwidth and calculate the coherence length, but the resolution of the spectrometer needs to be very high. The coherence length of He-Ne lasers can range from several cm to several dm, and high-quality lasers can have up to 100 m coherence length. Some recommended resources to read about coherence length are provided. However, the conversation also includes a request for more information on using a spectrometer to measure bandwidth and which one to use. The suggested procedure
elif

## Homework Statement

Hi.I need an experiment to study and present it.the subject is how to measure coherence length of a monochromatic laser ? lm also search on google.However,is there any advice from you ?

## The Attempt at a Solution

You can use a Michelson interferometer and see how long is the path difference when the interference is not blurred yet.
But lasers can have very long coherence lengths. Choose a not too good laser.

ehild said:
You can use a Michelson interferometer and see how long is the path difference when the interference is not blurred yet.
But lasers can have very long coherence lengths. Choose a not too good laser.
But the teacher ask for also long coherent length.I learn that I can use a spectrometry to find bandwith and then calculate the coherent length.But I couldn't find how can I get bandwith measurement? And I wonder up to what coherent length I can use michelson interferometer
thank you so much

To measure the spectral bandwidth of the laser radiation you need very high resolution of the spectrometer. The resolution must be much better than the bandwidth of the laser.

The coherence length L is c/Δf, where Δf is the bandwidth, difference of the frequencies where the power is half the maximum.

The coherence length of He-Ne lasers are in the range of several cm - several dm. But high-quality lasers can have even 100 m coherence length.

http://en.wikipedia.org/wiki/Coherence_length
http://www.rp-photonics.com/coherence_length.html

ehild said:
To measure the spectral bandwidth of the laser radiation you need very high resolution of the spectrometer. The resolution must be much better than the bandwidth of the laser.

The coherence length L is c/Δf, where Δf is the bandwidth, difference of the frequencies where the power is half the maximum.

The coherence length of He-Ne lasers are in the range of several cm - several dm. But high-quality lasers can have even 100 m coherence length.

http://en.wikipedia.org/wiki/Coherence_length
http://www.rp-photonics.com/coherence_length.html
I think I read all of them before asking that question :) But I need to describe how can use a spectrometer to measure bandwith ? what is the procedure...etc.For michelson I know what I need to do.Also do you know which spectrometer should I use ?

elif said:
I think I read all of them before asking that question :) But I need to describe how can use a spectrometer to measure bandwith ? what is the procedure...etc.For michelson I know what I need to do.Also do you know which spectrometer should I use ?

Sorry, I can not help as it would need a very-high resolution spectrometer. In principle, you have to scan the source spectrum around the nominal wavelength and find the frequency, where the intensity falls to half of the maximum.
You can find the linewidth for given coherence length at the page http://www.rp-photonics.com/coherence_length.html. The resolution must be much better then the linewidth.

## What is the coherence length of a monochromatic laser?

The coherence length of a monochromatic laser refers to the distance over which the laser beam maintains its coherence, or the ability of all the photons in the beam to maintain a fixed phase relationship with each other.

## How is the coherence length of a monochromatic laser determined?

The coherence length of a monochromatic laser is determined by the spectral bandwidth of the laser, which is the range of wavelengths contained in the laser beam. A narrower spectral bandwidth results in a longer coherence length.

## Why is coherence length important in laser applications?

The coherence length of a monochromatic laser is important in applications such as interferometry and holography, where the interference of light waves is used to create precise measurements or images. A longer coherence length allows for more accurate and detailed results.

## Can the coherence length of a monochromatic laser be increased?

Yes, the coherence length of a monochromatic laser can be increased by using techniques such as cavity enhancement, where the laser beam is reflected back and forth between two mirrors to amplify the coherence. Additionally, using a laser with a narrower spectral bandwidth can also increase the coherence length.

## Does the coherence length of a monochromatic laser change over time?

Yes, the coherence length of a monochromatic laser can change over time due to factors such as temperature fluctuations and vibrations, which can affect the spectral bandwidth of the laser. It is important to carefully control these factors in order to maintain a stable coherence length for precise applications.

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