22 • General Operation NEW FOCUS, Inc.
Modulating the Current
To modulate the current, you can use either the DC-coupled Current
Modulation BNC input on the back of the controller or the high-speed
SMA input on the back of the laser head.
Since changes in the current affect the laser frequency, you may want to
modulate the current in order to achieve fine-frequency modulation.
The frequency modulation is due to changes in the index of refraction
of the laser-gain medium (the semiconductor diode) as a function of
laser current. The degree to which the current affects the wavelength
depends on the specific laser head you are using, but it is in the range of
25–150 MHz/mA.
Using the DC-Coupled Current Modulation Input
You can modulate the laser current (amplitude) using an externally
generated low-level signal.
To modulate the current, connect the line for the input signal to the
Current Modulation connector on the back of the Vortex controller.
Make sure the signal conforms to the following specifications:
The DC-coupled Current Modulation input is NOT current limited. To prevent
damage to the laser diode DO NOT modulate the current above the operating
current specified in the Acceptance Test Data Sheet.
The front-panel current readout does not reflect the modulation input.
You can calculate the actual current by adding the current shown on
the front-panel display to the modulation input.
Current Modulation Input Guidelines (DC-coupled)
Connector Type BNC
Max. Voltage ±10 V
Input Frequency Range DC–1 MHz
Impedance 5 kΩ
Modulation 0.2 mA/V
Vortex6000.book Page 22 Tuesday, February 19, 2002 3:25 PM
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