How to Calculate Coherence Length

Lasers exhibit a high degree of coherence.

According to the RP Photonics consulting company, coherence is a property of electromagnetic radiation displayed when the electric fields of the radiation are all related to each other either through time or space. Electromagnetic radiation's most common form is visible light, but also include microwaves, x-rays and common radio waves. A single frequency laser beam is a form of visible radiation that displays a large amount of coherence with respect to time over a given length, which is called the coherence length.

Things You'll Need

• Paper
• Pencil

Instructions

1. Determine the optical bandwidth of your laser in hertz (Hz). The optical bandwidth is the range of frequencies at which the laser operates. As an example, say the optical bandwidth is 5000 Hz.

2. Multiply the bandwidth by the number pi. To four decimal places, pi = 3.1415. This leads to 5000 Hz x 3.1415, which is 15,707.5 Hz.

3. Divide the speed of light by the product of the bandwidth and pi to get the coherence length in meters. The speed of light is 3.0 x 10^8 m/s, where "10^8" means ten to the power of 8. Continuing the example, you have (3.0 x 10^8 m/s) / (15,707.5 Hz), which gives a coherence length of 19,099.2 meters.

References

• RP Photonics: Coherence
• HyperPhysics at Georgia State: The Electromagnetic Spectrum
• RP Photonics: Coherence Length
• "Physics for Scientists and Engineers with Modern Physics"; Raymond A. Serway and John W. Jewett; 2009