How to Add Analog Modulation to a Diode Laser

Modulating a laser beam means changing the output in response to an input signal.

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 it’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.

Tips & Warnings

• The drive circuit will convert a voltage input to a current. You will adjust the circuit so that the minimum voltage corresponds to the minimum current in the linear range, and the maximum voltage corresponds to the maximum current. Any value between the minimum and maximum will then result in a specific analog output level.

• When you operate this circuit, you will see some of the disadvantages to analog modulation. Intensity variations from mist or dirt will be interpreted as changes in modulation; the noise at the lower intensity levels may dominate the signal, and you can drive the laser into instability. On the other hand, this is the conceptually simplest method of laser modulation.

• Lasers can damage your eyes — temporarily and/or permanently. Be careful and avoid exposure to the direct or indirect laser beam.

References

• Drexel University: Diode Lasers
• All About Circuits: Voltage-to-Current Signal Conversion