# Consider a laser which emits photons with a wavelength of 1.55 micrometers. The laser has a lower electron level Ei and an upper electron level Eu. A power supply pours electrons into the upper energy level and these electrons fall down in a time T= 0.1 nanosecond to the lower level emitting photons. The laser is biased to produce 100 milliwatts of optical power.

What is the current flowing in the laser if the efficiency of the laser to convert electrical energy to photon energy is 95 %?

Make an argument that the electrical current being converted to light is eN/T where N
is the electron number in the upper level.

## To find the current flowing in the laser, we can use the equation:

Power = Current * Voltage

First, let's calculate the voltage. Since the laser is biased to produce 100 milliwatts of optical power and the efficiency of the laser is given as 95%, we can express the electrical power input as:

Electrical Power = Optical Power / Efficiency

Therefore, the electrical power input is:

Electrical Power = (100 milliwatts) / 0.95

Now, we need to convert this value into watts by multiplying by 10^-3:

Electrical Power = (100 * 10^-3) / 0.95

Next, let's find the number of electrons in the upper level (N). The time (T) given is the time taken for the electrons to fall down from the upper level to the lower level (in this case, T = 0.1 nanoseconds).

The argument states that the electrical current being converted to light is eN/T, where e is the elementary charge (1.6 * 10^-19 Coulombs).

Therefore, the current flowing in the laser is:

Current = (e * N) / T

Substituting the given values:

Current = (1.6 * 10^-19 Coulombs * N) / (0.1 * 10^-9 seconds)

Simplifying:

Current = (1.6 * N) / (0.1 * 10^-10)

Note: We do not have enough information to determine the value of N.