Before And After Service Product Safety Checks; Laser Head Block Diagram Description - HP 5501B Operating And Service Manual

8-16. Before and After Service Product Safety Checks

8-17. Visually inspect the interior of the serviced instrument for any signs of abnormal internally
generated heat, such as discolored printed circuit boards or components, damaged insulation,
or evidence of electrical arcing. Determine and remedy the cause of any such condition.

8-18. LASER HEAD BLOCK DIAGRAM DESCRIPTION

8-19. The major structures of the HP 5501B are shown on the HP 5501B Laser Head Block Diagram in
Figure 82 and include the control electronics, laser assembly, sampler assembly, reference
receiver, and the high voltage power supply. All of the necessary control signals for the
operation of the HP 5501B are generated internally. Power requirements are ±15 Volts DC from
external system power supplies.
8-20. Overall, the purpose of the laser head is to emit a collimated, dual frequency laser beam with a
high degree of stability. A portion of the emitted beam (about 10%) is directed to the sampler
assembly to establish a reference frequency while the main portion of the beam is directed to
external optics and returned to an external measurement receiver to develop a measurement
frequency. The system electronics compare the reference and measurement frequencies to
calculate the relative displacement of the optics.
8-21. The control electronics determine the tuning of the laser assembly to ensure an accurate laser
wavelength for making measurements. Two phases of tuning are used, warmup mode and
optical mode. When the desired characteristics of the laser beam are obtained during the optical
mode, the system is ready to make measurements.
8-22. The laser assembly optics ensures correct laser frequency polarizations and also collimates the
laser beam. Collimating minimizes variations in the diameter of the beam as it travels away from
the laser head. Laser frequency F2 is polarized in a plane parallel to the base of the HP 5501B
chassis. The other laser frequency F1 is polarized perpendicular to F2. The difference between
the two laser frequencies F1 and F2 is small compared to their optical frequencies. The exit
aperture shutter of the turret assembly has three positions. The first blocks the laser beam
entirely; the second allows a small diameter laser beam to exit for optical alignment purposes;
and the third passes the entire laser beam.
8-23. Before the laser light is emitted from the HP 5501B, a portion of it is sampled by the sampler
assembly. Most of this sample feeds into the reference receiver while the remainder is used to
control laser tuning. The reference receiver generates the reference frequency signal by mixing
the two laser frequencies. The reference frequency is a 1.5 to 2.0 MHz, TTL level square wave.
When the laser tuning stabilizes, the reference frequency is sent to the system electronics.
8-24. During a measurement, the laser beam follows a path through external measurement optics and
to the HP 10780A Receiver. If the optics remain stationary, the measurement frequency and the
reference frequency are the same. Relative motion between the measurement optics result in a
change of the measurement frequency. The electronics compare the measurement frequency to
the reference frequency and calculate the displacement of the optics. The measurement
frequency output from the HP 10780A system receiver requires proper alignment of
measurement optics in addition to stable laser tuning.
8-25. The High Voltage Power Supply (HVPS) requires -15 Vdc as input. This supply generates up to
12 kilovolts DC at power-on. After the laser emission starts, the power supply output drops to
approximately 1.2 kilovolts DC.
HP5501B
Service
8-5