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How to Add Analog Modulation to a Diode Laser

One of the most useful properties of lasers is their ability to transmit information. The maximum data rate that any wave can carry is limited by its frequency, and the frequency of light is about a million times higher than radio waves. This means that light can carry information about a million times faster than radio waves. There are many ways to modulate a laser, including a diode laser. The most straightforward method is direct modulation, and it̵7;s difficult to implement.

Things You'll Need

Diode laser
Optical power meter
Controllable-current power supply
Drive circuit with modulation input

Instructions

- 1
  Direct the diode laser output into the optical power meter.
- 2
  Connect the power supply to the diode laser. Set the power supply to provide current just below the threshold current listed in the diode specification.
- 3
  Record the current and measure and record the optical output of the laser diode.
- 4
  Increase the current in steps up to the maximum current rating for the diode laser. Record the optical power output and the current at each step. To start with, you might want to try eight or 10 current steps.
- 5
  Plot the laser diode power as a function of current. Examine the graph to determine the linear region of the curve -- that is, the region over which a specific increase in current generates the same specific increase in optical power. Identify the current limits at each end of the linear range.
- 6
  Adjust the drive circuit to operate between the current limits identified. This will keep the laser above threshold but change the intensity of the light as the drive current is modulated.

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