technical reference
33
Synrad
Series 48Series 48
Series 48Series 48
Series 48 operator’s manual
Technical overview
As shown in Figure 3-1, the PWM on-time percentage (PWM duty cycle) exerts a non-linear power
function as power saturation is approached, flattening out at approximately 95% duty cycle. We recom-
mend using a 95% maximum PWM signal since little or no increase in laser output power occurs between
95% and 100% PWM duty cycle. It is safe to operate at 100% duty cycle by eliminating all PWM control
and simply applying on/off gating; however, you can expect a 5% increase in power draw and heat load.
Plasma section
The laser consists of an RF-excited plasma tube with an adjustable mirror on each end, mounted together
with the RF drive assembly in a single aluminum chassis. The plasma tube is made of two-inch square
cross-section extruded aluminum tubing with pre-machined ends welded on. The mechanical and
electrical arrangement of the internal electrode structure is shown schematically in Figure 3-2. The RF
drive power is applied between the lower electrode and the plasma tube. The internal resonant circuit
induces RF drive on the upper electrode that is 180 degrees out of phase with that of the lower electrode.
Thus the voltage between the two RF electrodes is roughly twice that on either electrode, causing the
plasma to form only in the 4.8 mm square bore region. The two sidewalls confine the plasma but carry
negligible current. The RF electrodes are anodized to assure uniform distribution of RF power throughout
the excitation volume. Waste heat is conducted away by all four metal sides of the bore to the outer walls
of the plasma tube, where it is transferred to the chassis. In contrast to waveguide lasers that have a
closed bore periphery, Series 48 lasers have four slots (small gaps) extending longitudinally along the
length of the bore (shown in Figure 3-2). These slots provide electrical insulation between the two pairs
of orthogonal electrodes; however, the slots are also effective for diffusion cooling of the laser gas.
Figure 3-2 Cross section of tube
Optical resonator
The optical resonator consists of a three meter radius of curvature total reflector and a flat Zinc Selenide
(ZnSe) output coupler with reflectivities of 95% or 92%. The mirrors are held on with Viton (fluorocar-
bon) elastomeric o-rings for factory adjustment by means of three Torx head 4–40 screws. No epoxy is
used for sealing. The screws are secured by adhesive after alignment.
The 4.8 mm bore, in conjunction with the mirror curvature selected, limits the output beam to TEM
00
modes when the mirrors are properly aligned. Small variations in output power (up to 10%) are seen
during warm-up as the cavity mirror spacing changes due to thermal expansion of the plasma tube.
ANODIZED
ALUMINUM
RF ELECTRODES
GAS
BALLAST
ALUMINUM
HOUSING
SMALL
GAPS
GROUND
SPACERS
LASER
PLASMA
COIL IN
RF RESONATOR
RF FEED THROUGH
AND GAS FILL PORT
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