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LASER_MODULATION_WITH_CURRENT_SOURCE

2015-03-19 10:39  
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LASER_MODULATION_WITH_CURRENT_SOURCE
Because atmospheric turbulence can become bothersome, any low-frequency baseband intensity modulation will become corrupted over long distances. To overcome this limitation, the information must be frequency-modulated up to 200 kHz in this manner. To obtain a more desirable current modulator for subcarrier modulation, use a transistor as a variable-current source to vary the current through the laser. Here the quiescent operating current through the laser is set at 4.1 mA by a potentiometer in the base bias circuit. Assuming a 0.7-Vdc drop across the transistor base-emitter junction and a 1000-Ω resistor in the emitter lead, the quiescent bias voltage from base to ground is 4.8 Vdc. The input voltage must vary 1.4 V p-p to cause a ±0.7-mA current variation through the laser. To effect this, the modulation signal source must provide up to a 1.4-V p-p signal into the laser current modulator circuit.