Question

Assume the injection minority carrier lifetime of an LED is 5 ns and that the device has an optical output of 0.30mW when a constant DC drive current is applied. Plot the optical output power when the LED is modulated at frequencies ranging from 20 to 100 MHz. Note what happens to the LED output power at higher modulation frequencies.

Answer 1

According to the given problem we have to deduce the optical power of the LED when it is modulated and operated within a certain frequency range. In order to get the optical power at first we need to understand the frequency response of the optical source, which basically dictates how fast the input drive signal can vary the output level of the light. Suppose if the DC drive current is modulated at a frequency ω, then the output optical power of the device is given by the following relation:

where P(ω) is the output optical power of the device at ω frequency, P_o is the power emitted at zero modulation frequency or power when a constant DC drive current is applied and T_i is the carrier lifetime of the LED respectively.

NOTE: The above equation is considered to be valid only if we neglect the effect of parasitic capacitance(which is negligible) that can cause a delay of the carrier injection into the active junction which in turn could have delayed the optical output.

In this problem we have been given the following data/s respectively:

P_o= 0.30 mW ; ω= 2πf (where f= 20 to 100 MHz) ; T_i= 5 nS

Using the formula of the output optical power we plot the representative values of P(ω) at 20, 40, 60, 80 and 100 MHz frequency intervals respectively which is as follows:

1) Value of P(ω) for f= 20 MHz:

The optical output for 20 MHz frequency becomes,

2) Value of P(ω) for f= 40 MHz:

The optical output for 40 MHz frequency becomes,

3) Value of P(ω) for f= 60 MHz:

The optical output for 60 MHz frequency becomes,

4) Value of P(ω) for f= 80 MHz:

The optical output for 80 MHz frequency becomes,

5) Value of P(ω) for f= 100 MHz:

The optical output for 100 MHz frequency becomes,

To summarize the results we can tabulate and plot Optical Power vs Frequency Graph as follows:

Finally to conclude we can see from the table that the LED output power continues to decrease with the increase in frequency at higher modulation rates which is as per our expectations for a normal functional/working LED.