Intensity Modulation

INTENSITY MODULATION OF LASER OUTPUT THROGH AN OPTICAL FIBER AIM: The aim of the experiment is to study the following ac characteristics of a linear intensity modulation system: (i) gian characteristics of FO linear intensity modulation system Vin(ac) Vs Vo(ac) for fixed carrier power Po and signal frequency F0. EQUIPMENT REQUIRED: S.No EQUIPMENT 1. LASER Diode transmitter Kit 2. LASER Diode receiver Kit 3. Optical Fiber cable 4. Function Generator 5. CRO

QUANTITY 1 1 1 1 1

CIRCUIT DIAGRAM: LASER Diode Transmitter Unit

LASER Diode Receiver Unit

LASER SET Po

Function Generator

OF Cable

Fo PIN

Oscilloscope

THEORY: Linear Intensity Modulator: slection of optimum carrier power is essential to minimize distortion.Limiting depth of modulation also ensures distortion free transmission. (a) Select an IF(dc) =10mA(corresponding to carrier power Po of -14dbm to 17 dbm) as dc operating point. (b) (b) For Vin(ac) = 200mv(P-P), we get IF(ac)=2mA(p-P) corresponding to variations from 9mA to 11mA.this is well within the linear region.The modulation

may be distortion free for a Vin(ac) of 1V(p-p).One may compute values of IF(dc) or Po(CW) (optical carrier power) for different peak to peak input signals,Vin(ac). (c) The loss due to the optical fiber cable may be overcome by proper selection of demodulator again. PROCEDURE: 1. Connect one end of cable1 to the Fo port of analog transmitter. First connected to FO pin to set the carrier power level ,Po .After setting Po, it is removed and connected to FO PT to study the system. 2. Connect Vin(ac) and GND on the Transmitter unit to a function generator(10Hz to 500 KHz sinewave O/P,10mV to 2Vp-p O/P) 3. Connect Vout of receiver unit to CH2 of a dual trace oscilloscope. Give the function generator output to CH1. 4. plug the AC mains. Adjust the SET Po Knob to set the optical carrier power to 15dbm with Vin disconnected from the function generator .Disconnect cable1 from Fo PIN port and connect to FOPT. 5. Connect Vin to function generator. Set signal frequency and amplitude to 2KHZ and 100mv respectively .Observe the transmitted (Vin) and received(Vout) signals on the oscilloscope with identical range settings. Coincide the two waveforms.A djust SET Rin suitably to obtain the unity gain for system. 6. Next,set Vin o suitable values from 10mV to 2Vp-p and note down values of Vout.Tabulate and plot a graph Vout Vs Vin .Compute Vout/Vin. RECORD OF OBSERVATIONS: Frequency,fo= 2KHz,carrier level Po=-15dbm,System Gain=1.0 Vin (mV) Vout(mV) 20 40 : 2000

Gain=Vout/Vin

MODEL GRAPH: Vout

Vin

RESULT: Thus gain characteristics are obtained.