Question

A He-Ne laser has a coherence length of 10m. What is the coherence time?

a) 3.33 X 10^-8 sec

b) 2.8 X 10^-8 sec

c) 3.32 X 10^-9 sec

d) 3.33 X 10^-9 sec​

Answer 1

Explanation:

Option d is the answer........

Answer 2

Answer:

The coherence time is 3.33×10⁻⁸sec.i.e.option (a).

Explanation:

The coherence time is the time duration over which the channel impulse response is considered to be not varying. Its relationship with coherence  length is given as,

$\Delta t_c=\frac{l_c}{c}$       (1)

Where,

$\Delta t_c$=coherence time

$l_c$=coherence length

$c$=speed of light in vaccum=3×10⁸m/s

From the question we have,

$l_c$=10m

By substituting the value of coherence length value in equation (1) we get;

$\Delta t_c=\frac{10}{3\times 10^8}=3.33\times 10^-^8sec$    

Hence, the coherence time is 3.33×10⁻⁸sec.i.e.option (a).