Operational efficiency of Laser diode

This article is very useful to know or learn about how to find different types of efficiency in the 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 :

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

ɳ_T = Pe / I. Eg

 Pe / I  = ȠT . Eg

ɳ_T = 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.

Power current characteristics of laser diode

To shown in the figure the output optic power versus forwarding input current characteristics is plotted in the figure for a typical laser diode. 

To shown in the figure below the threshold current only spontaneous emission is emitted hence there is a small increase in optic power with drive current. 

At threshold when lasing conditions are satisfied. At that time when the optical power increases sharply after the lasing threshold because of stimulated emission.

The lasing threshold optical gain is related by threshold current density for stimulated emission by the expression  is given below :

 Gth =  β  Jth 

Where β is constant for device structure.

Power current characteristics of a laser diode

Optical characterstics of laser diode

The output of the laser depends on the drive current passing through it. In laser diode operating at low drive current, It operates as an inefficient LED full form, when driving current crosses threshold value, lasing action begins. 

Here to shown in figure graph comparing optical power of LED operation and LASER operation.

Optical characterstics of laser diode and LED