|
 The
Lexel Model 503 Temperature-Controlled Etalon Assembly, when installed
in a Lexel ion laser, provides extremely stable single frequency operation
for applications requiring long coherence length and very narrow linewidth.
For holography and interferometry, it can provide coherence lengths of over
100 meters. For Brillouin scattering and high resolution spectroscopy, it
provides a linewidth of approximately 10-4 cm-1 (3
MHz).
To view
a close-up picture and diagram showing the Model 503 installed in a laser,
click the photo at right.
The Lexel
Model 503 Etalon Assembly utilizes a solid fused-silica etalon in a
precisely temperature-controlled oven to provide the ultimate in frequency
stability and controlled mode tuning. The temperature stabilization circuit
maintains the etalon frequency within ±20 MHz regardless of the change in
ambient temperature, thus assuring virtually no mode hopping. The
temperature tuning feature allows the selection of any single longitudinal
resonant mode within the Doppler-broadened gain bandwidth without the need
to readjust the etalon tilt angle.
The etalon
therefore operates at its maximum conversion efficiency, and the laser user
may easily select the single mode that yields maximum power. The potentiometer
controlling the etalon temperature is adjustable without removing the laser
cover. A flashing LED indicates that the electronic temperature regulating
circuit is operating properly.
The etalon
heater is hermetically isolated from the optical cavity -- thus completely
eliminating the possibility of the heater contaminating the etalon surfaces
or other optical components.
The Model
503 Etalon Assembly is kinematically mounted on the laser Invar®
resonator structure using the same stable suspension system as is used on
the Lexel mirror mounts. This effectively integrates the etalon assembly
with the total optical resonator structure for maximum mechanical and
thermal stability.
The tilted
etalon has long been recognized as a simple but effective method for
producing single frequency output from ion lasers. An etalon, when placed in
the laser cavity and tilted slightly, acts as a bandpass transmission
filter. The etalon will pass frequencies close to its transmission peak and
reject, by reflection, frequencies outside the etalon passband. The normal
frequency spectrum of an ion laser is made up of 20 to 40 individual
longitudinal modes covering a bandwidth of approximately 5 GHz. The
longitudinal mode spacing (c/2L) is 188 MHz in the case of a laser with a
0.8 meter mirror spacing, and 150 MHz with a 1 meter cavity.
The
installation of the etalon will force the laser into the single longitudinal
mode closest to the center of the etalon's passband.
Maximum
single frequency power is obtained only when the peak of the etalon passband
coincides with the longitudinal mode at the center of the laser gain
bandwidth. Therefore, the etalon must be frequency-tuned to achieve the
optimum single frequency performance. In order to tune an etalon it is
necessary to vary its effective length either by increasing the etalon tilt
angle or by changing the index of refraction. An air-spaced etalon has a
fixed optical length and index of refraction and can be tuned only by
increasing the tilt angle. Such angular tuning, however, is accompanied by
an increasing walk-off loss which can seriously limit the single frequency
power.
Fortunately, the solid fused-silica etalon is easily tuned by making use of
the change in index of refraction with temperature (dn/dT). The same
temperature-controlled heater system that is used to stabilize the etalon
can be used for the temperature tuning. The etalon passband peak will change
approximately 4 GHz/°C (or 40 MHz/.01 °C).
The Lexel
Model 503 Etalon Assembly can be precisely tuned over the full 5 GHz laser
bandwidth, and the longitudinal mode having the maximum power easily
selected. The etalon tilt angle, which is preset for maximum efficiency,
does not have to be changed. Conversion efficiencies (single longitudinal
mode power)/ (multilongitudinal mode power) of more than 75% can be achieved
with Lexel argon lasers in the strong 488.0 nm and 514.5 nm lines.
The
original solid fused-silica etalons were used without temperature
stabilization. This resulted in considerable mode hopping and frequency
drift. The airspaced etalon consisting of two thin windows bonded to a
hollow, thermally stable spacer was developed to overcome this difficulty.
Although the air-spaced etalon produced good thermal stability, the walk-off
loss that accompanies the required angular tuning and the additional
reflection losses caused by the two extra optical surfaces often result in
reduced laser power.
 The Lexel
Model 503 Etalon Assembly is unquestionably the most superior etalon system
available today. Its advanced temperature control circuit stabilizes the
efficient solid fused-silica etalon to a degree unequaled by any other type
of etalon and, at the same time, provides the flexibility of convenient
temperature mode tuning.
Click on the photo above to see an enlarged view of a
typical strip chart test record, showing the frequency stability that can be
achieved with the Model 503.
Operation of the Model 503 Etalon Assembly
The
operation and tuning of the etalon is extremely simple. The tilt angle of
the etalon is set at the factory for maximum efficiency. Although this
adjustment is easily accessible at the back of the laser head, it should
never have to be readjusted even after tuning to another laser wavelength.
All etalon
tuning is accomplished by adjusting the Etalon Temperature Tuning control at
the side of the laser head. Successive longitudinal modes can be selected by
turning this control approximately ¼ turn for each mode. The effect of the
tuning on the single frequency performance can be easily monitored using a
scanning spectrum analyzer and a laser power meter such as the Lexel Model
504. The Oven Indicator is a flashing red LED which shows the status of the
electronic temperature regulating circuit.
The
frequency stability of a mechanically stable, single-frequency laser depends
primarily on the temperature stability of (1) the etalon length and (2) the
laser cavity length. Each of these two parameters, if allowed to vary with
changes in ambient temperature, produces a different effect on the laser
frequency stability.
 (1)
Etalon length changes. If the etalon is not temperature
stabilized, the effective length of the etalon and the resulting etalon
transmission frequency will drift with the ambient temperature. When the
etalon passband moves to the point where it interacts with the next
longitudinal mode, the laser frequency will jump to this new mode. This is
the well-known "mode hopping" phenomenon. Mode hopping causes stepwise
changes in the laser frequency which can result in frequency excursions of
several thousand MHz.
Since the Lexel Model 503 Etalon Assembly is
temperature stabilized to within 0.01 °C (±20 MHz) regardless of ambient
temperature changes, there is virtually no mode hopping.
 (2)
Cavity length changes. Thermal expansion of the optical resonator will
change the cavity length. This results in the laser
frequency varying along the etalon transmission curve. When a new mode
enters the curve, the initial mode disappears, and the frequency excursion
starts back at its initial point. Cavity lengthening causes a sawtooth
change in laser frequency with a maximum frequency excursion of ±(c/4L);
for example, ±95 MHz for a 0.8 meter cavity, ±75 MHz for a 1 meter cavity.
The total
frequency stability of a single-frequency ion laser depends on a number of
characteristics of both the laser's physical construction and the operating
environment. The major variables are the resonator's coefficient of thermal
expansion, the ambient temperature variation, and the time period involved.
The
standard Lexel Invar Resonator has a thermal expansion of less than 0.9 x 10-6/°C
with a resulting frequency stability of better than 0.5 GHz/°C. This is over
25 times better than the best aluminum resonator. The optional 7510
Thermally Compensated Resonator Length reduces the effective expansion to
less than 10-7/°C with a frequency stability of better than 50
MHz/ °C for the most stringent long-term performance.
With
either resonator Lexel single frequency lasers are capable of frequency
stability better than ±75 MHz for periods of up to 10 hours when operated in
a good laboratory environment. Short-term frequency stability of better than
±5 MHz has been achieved.
The
frequency stability of a single frequency ion laser is best measured by
using an iodine absorption cell. The absorption curve of iodine vapor is
shown below in relation to the frequency bandwidth of the 514.5 nm argon
laser line.
 On the
sharp linear portion of the absorption curve very small changes in frequency
result in large changes of power absorption. If a constant amplitude output
from the single frequency laser is passed through the iodine cell into a
power meter, the changes in measured power provide a direct and accurate
indication of the frequency shift. An
iodine cell frequency monitoring system allows the long-term frequency
stability to be recorded very accurately. Frequency variations of less than
± 5 MHz can be resolved with such a system.
Every
Lexel argon laser that is equipped with a Model 503 Etalon Assembly is
checked for frequency stability using the iodine absorption cell technique.
A ten-hour strip chart record, is made on each unit prior to its shipment.
We have available a very convenient iodine vapor absorption cell for use in
measuring the frequency stability of single-frequency argon lasers. The
Lexel Model 505 Iodine Cell has a 150 mm absorption path through the sealed,
vacuum-processed, cell chamber. The ends of the iodine cell are terminated
with high quality Brewster's angle windows which match the polarized laser output for minimum reflection losses.
|
Single
frequency 
Lexel
