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 The
optical resonator is the most important structure affecting the overall
power, frequency and beam-pointing stability of the laser. The resonator
must hold the plasma tube, the laser mirrors and all other optical
components in precise alignment.
The Lexel
Solid Invar® Rod Resonator Structure with its patented mirror
mounts is the ultimate laser resonator design for all ion laser
applications. It is a rugged lightweight structure with all the precision
and stability of the finest Fabry-Perot Interferometer.
Ultra-low thermal expansion Invar®
The Lexel
Resonator uses Invar with a thermal expansion of less than 0.9 x 10-6/
°C which results in a basic resonator frequency stability of better than 0.5
GHz/°C. This stability is comparable to quartz tube resonator systems and is
more than 25 times better than the best aluminum resonator.
Solid rod construction
Lexel's
straightforward design utilizes solid one-piece rods that run the full
length of the resonator, to provide unmatched mechanical rigidity.
Other low
expansion resonator designs are complex structures which must combine quartz
rods or tubes inside of an aluminum tubing structure. Transverse movement or
vibration between the quartz and aluminum can cause undesired frequency
jitter.
Only
Lexel's solid rod construction provides the excellent transverse rigidity
required to reduce frequency jitter to its very minimum.
Stress-isolated resonator mounting
 The Lexel optical resonator is mounted
to the base of the laser head with an advanced kinematic mounting system
that prevents thermal or mechanical stresses from detuning the mirrors or
plasma tube alignment. The mounting system consists of a transverse flex
plate, a spherical bearing, and an axial sliding pin to provide positive kinematic positioning with complete stress isolation between the case and
the resonator.
Kinematic mirror suspension system
Lexel's
patented mirror suspension system permits stable and repeatable mountings of
the laser mirrors to the resonator structure. Three reentrant-type springs
provide the strong compressive force between the three pivot points and the
ends of the resonator rods.
 The
reentrant springs also complete the kinematic mounting by preventing lateral
or rotary movement of the tuning plate. Positive orthogonal tuning with
outstanding repeatability is always assured.
The
reentrant compression springs do not rub or move in contact with the optical
resonator, thus the optical tuning "noise" associated with coil springs is
not present in Lexel lasers. Micrometer pitch tuning screws are used to
provide smooth precise mirror tuning.
The Lexel
resonator and mirror mount system will withstand shock and vibration that
would misalign other laser designs. Lexel lasers are shipped all over the
world and in virtually every case, they arrive in precise alignment.
Unmatched beam-pointing stability
Most ion
lasers exhibit angular beam drift caused by thermal mismatches in resonator
or optical elements. The combination of Lexel's low temperature
conductioncooled plasma tube,
the thermally-matched Invar resonator system and the Temperature Compensated
Prism Wavelength Selector results in beam-pointing stability from Lexel
lasers of better than ±1.5 arc sec/ °C (± 7.25 µ rad/°C). This performance
is repeatedly verified by scanning and film recording users in critical
alignment systems and is reportedly the best available from commercial ion
lasers.
Long-term power stability
The
ultimate test of a resonator's stability is the extent of long-term power
drift when the laser is operating at a constant current level. Lexel lasers
have the best power stability available in commercial ion lasers. They are
capable of maintaining a given power level within 2% for 10 or more hours.
In long-term industrial applications Lexel lasers frequently operate for
months without requiring any retuning.
Sealed contamination-free cavity space
 The most common and troublesome cause
of power fall-off and mode distortion in ion lasers comes from contamination
of the optical surfaces and air space making up the optical cavity. We have completely eliminated this problem by the use of its patented dust cover
assembly which seals and protects the optical components and cavity space
between the mirrors and the Brewster windows of the plasma tube.
The Lexel
dust cover assembly uses O-rings and Teflon® seals to provide a
completely sealed intracavity space. All of the dust cover parts are made of
materials carefully selected for compatibility with the UV radiation and
resulting ozone atmosphere existing in the optical cavity. Each part is
thoroughly cleaned, chemically-treated and air-baked just before being
assembled into the resonator.
Periodic optical cleaning eliminated
The Lexel
Optical Cavity dust cover has proven so effective that, if left sealed, the
mirrors and windows rarely have to be cleaned. Lexel lasers commonly operate
for one to two years even in dirty industrial environments without needing
cleaning.
To our
knowledge no other ion laser manufacturer has even come close to this
performance, since weekly or monthly cleaning of optics is a standard
procedure required to keep most other lasers operating correctly.
 Sealed
light sampler and photocell
The beam
splitter and photocell which sample a portion of the laser output are also
sealed to prevent dust, dirt and even stray room light from disturbing the
performance or accuracy of the Light Regulator and Power Meter.
Efficient optical design eliminates aperture
The Lexel
optical design uses the full plasma tube bore as the limiting aperture of
the optical system. The mirror radius is chosen to provide optimal TEM00
operation in conjunction with the bore aperture. When other mode
configurations are required they can be obtained simply by changing the
mirror. Thus maximum optical efficiency is always obtained since the full
bore diameter is always used. The problems associated with an extra intracavity aperture, such as misalignment and optical contamination, are
totally eliminated with Lexel's superior optical design.
Easy mirror changing
The
mirrors in Lexel lasers are easily and quickly removed and reinstalled.
Since the front surfaces of the mirrors are aligned by three precision
balls, both repeatability and correct orientation are always assured.
 Magnetically-coupled
beam attenuator
The Lexel
beam attenuator consists of a counter-balanced shutter located inside the
optical cavity near the output mirror. A magnet attached to the operating
lever holds the intracavity shutter either fully open or completely closed.
The magnet coupling provides positive operation of the shutter without
breaking the dust cover seal. The intracavity shutter design eliminates the
cumbersome heat dissipating structures needed for external-type beam
terminators.
Threaded output aperture
The output
mirror retainer contains a ¾"-32 thread for mounting accessories to the
output aperture. This thread will accept standard optical accessories such
as polarization rotators, beam expanders or dye laser coupling tubes.
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