VXI SM8000 - Manual (Page 27)

VXI Technology, Inc.
SM8000 Series Operation 27
SM8101 / SM8102 - Optical Attenuators
The Optical Attenuators are based on precise-resolution stepper motors, which mechanically
position a beam block. See Figure 3-3 for the basic concepts.
The attenuator stepper motor is attached to an off-axis cam. A pair of fiber collimators is
positioned on either side of the cam, with a short open-air beam path between them. As the motor
rotates, the cam is driven slowly into the beam path, attenuating the beam. See Figure 3-2.
When the cam is fully rotated out of the beam path, the attenuator is in park, or reset position.
When in park position, the loss is limited to the intrinsic loss of the two collimators and the air
gap, known as Insertion or Residual loss.
As the cam rotates into the beam path, attenuation increases. The relationship between motor step
position and attenuation is not linear. The incremental increases in attenuation per motor step are
very small at first. In the low range (0 dB to 5 dB), the incremental attenuation per step is
approximately 0.05 dB. In the high range (50 dB to 60 dB), attenuation increases much more
quickly at approximately 0.25 dB per step.
F IGURE 3-3: A TTENUATOR D IAGRAM