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Why is the setpoint power lower than the laser power?

Problem

   When I type the command >>ldd info, the result shows the setpoint power value to be lower than the laser power.

Symptoms

   For example, when I type >>ldd info, I get the following:
TH3 > ldd info
LDD: Control Mode=Calibrate
LDD: Control State=Power
LDD: Laser Mode=Dual
LDD: Setpoint Power=42.6 Watts
LDD: Calibrate Power=42.7029 Watts
LDD: Laser Power=51.8 Watts
LDD: Laser Setpoint A=55.494 Amps
LDD: Laser Current A=55.4315 Amps
LDD: Laser Temperature A=25.3643 C
LDD: Laser On Time A=1327.04 Hours
LDD: Laser Setpoint B=55.494 Amps
LDD: Laser Current B=55.3385 Amps
LDD: Laser Temperature B=26.2811 C
LDD: Laser On Time B=1327.04 Hours
*** Command Success ***

Solution

   Why is the Setpoint power 42.6 W, while the Laser Power is 51.8 W?

The power readings of interest from the monitor command ldd info are:
LDD: Setpoint Power=42.6 Watts
LDD: Calibrate Power=42.7029 Watts
LDD: Laser Power=51.8019 Watts

The setpoint power is the power that has been requested, the calibrate power is the power measured using the beam-balance sensor, and the laser power is the power measured using an internal sensor.

When the laser is turned on, the power is calculated using the beam-balance sensor. However, imaging requires the beam-balance shutter to be open, so during the plot the laser power is maintained using the internal sensor. In this case, there is roughly 9 Watts lost between the internal sensor and the beam-balance sensor.
Technical contact(s)
  

Kurt Shelton
Submitted by
  

Kyle Good
Date submitted
  

17 Nov 2005

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