To Measure the Optical Power Emitted by the LED





Objective / Aim of the Experiment

To measure the optical power emitted by the LED.

Equipment Required

Power Supply PSU or PS!
Testing Module MCM-40
Multimeter
Optical Power Meter

Theory

The commonest optical sources are light-emitting diodes(LED) and laser diodes (LD). Both these diodes can be used to generate radiations at different wavelengths, corresponding to the windows where fibers show the minimum attenuation.

The LED is a particular diode which emits light through process of recombination of the electron-hole pairs due to a forward bias of the junction The optical power emitted is a function of the forward driving current. At present the LEDs in the 1st windows are made of gallium arsenide or of the ternary compound with aluminum (ALGaAs/GaAs), the LEDs in the 2nd and 3rd windows are made of indium gallium-arsenide-phosphide (InGaAsP/InP).

The most significant parameters of LED are:

Output wave length
Output spectral width
Output optical power: it ranges in some tens of µW, and depends on the forward driving current.
Frequency response.

Optical Power Emitted by LED

Procedure

Optical power emitted by LEDs

1. Power the module.

2. isconnect the jumper j11-j13 and connect the jumper j12b, so that the circuit can be arranged as shown in Figure 1. this configuration includes the LED at 660nm, forward polarized through the bias trimmer (p4).

3. Measure the voltage v10 across the resistor of 10Ω connected in the series of LED (between TP15 and ground). the forward current If crossing the Led in expressed by the following formula:

If = V10 / 10 [V10 in mv, If in ma]

Observe the intensity of the light emitted by the LED.

Power increase as current increase.

Characteristic Curves of LEDs

Disconnect the jumper j11-j12 and connect the jumper j13b, so that circuit can be arranged as shown in Figure 2. this configuration includes the LED at 820nm, forward polarized through the bias trimmer (p4)

Measure the voltage Vf across the LED (between TP14 and TP15) and the voltage V10 across the resistor of 10Ω connected in the series of LED (between TP15 and ground). the forward current If crossing the Led in expressed by the following formula:

If = V10 / 10 [V10 in mv, If in ma]

Connect the LED to optical power meter through cable3 (200/230)

Vary the BIAS trimmer P4 and measure Vf , V10 , If and optical power Pout

Plot the curve for the optical power of LED versus If and of If versus Vf

Change cable 3 with cable4 (50/125) and then with cable5 (10/125) and observe the readings of optical power.

Observation

S. No. Vf (mV) V10 (mV) If = V10/10 (mA) Pout (dBm)

Result

Characteristic curves of LED source is observed.

By changing the fiber optic cables it was observed that the optical power decreases as the Numerical Aperture of the cable decreases




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