- 1). Get the speed of light, c, in metric units. The speed of light has a constant value in a vacuum, given by 299,792,458 m/s. Note that light always travels more slowly through a material than in a vacuum. The speed of light in air is nearly the same as the speed of light in a vacuum and is given by (the speed of light in a vacuum)/1.0003.
- 2). Calculate the bandwidth, w. The coherence time, t, is equal to the reciprocal of the bandwidth. Bandwidth is the difference between the upper and lower values of the frequencies of the light source and is typically measured in hertz. A noncoherent light source, such as a light bulb, has a very broad range of frequencies.
- 3). Rewrite the formula for the coherence length in terms of the bandwidth. The coherence length, L, is given by L=c*t, where, to recap, c is the speed of light and t is the coherence time. The coherence time is related to the bandwidth, w, as follows: t=1/w, so the coherence length is L=c/w.
- 4). Calculate the coherence length with the formula L=c/w m. For example the bandwidth of a light emitting diode is 1.7 GHz which makes the coherence length L= (299,792,458 m/s) / (1.7e9 GHz) = 0.176 m, where the number 1.7e9 has been written in exponential notation. Exponential notation is a form of short hand where the number of digits after the first is represented as a number after the letter "e." For example, the number 100 has two digits after the first and can be represented as 1.0e2, and likewise, 5789 can be represented as 5.789e3.
next post