### Operational efficiency of Laser diode

1. External  quantum efficiency :

The operational efficiency of the semiconductor laser is differential external quantum efficiency. It is the ratio of the increase in photon output rate for a given increase in the number of injected electrons.

ȠD = dpe \ dI( Eg)

Where,
pe = Optical power emitted from a device
I = Current
Eg = Band gap energy expressed in terms of electron volts

2. Internal  quantum efficiency :

The internal  quantum efficiency for a semiconductor laser is defined as ɳ  given below :

ɳ = Number of photons generated in the laser cavity / Number of injected electron

The optimal value of internal quantum efficiency Ƞi is ranging between around 50 to 100 %.

3. Total efficiency :

The total efficiency is defined as :

ɳT = Total number of output electron / Total number of injected electrons

ɳT = Pe / I. Eg

Pe / I  = ȠT . Eg

ɳT = ȠD ( 1 - Ith/I )

Where Ith - threshold current
I - injection current

4. External power efficiency

It is in converting electrical input to optical output is expressed as

Ƞep = Pe / P  * 100

Ƞep = Pe / IV * 100

Ƞep = ȠT ( Eg / V ) * 100

The Ƞep is also called as device efficiency.  