|
Ion lasers produce a large number
of high-power lasing wavelengths ranging from the ultraviolet, through the
visible, into the near infrared portion of the spectrum. Ion lasers are
compact for the amount of laser power they generate relative to other types
of visible lasers.
Almost all commercial ion lasers manufactured
today are CW argon or krypton lasers. We specialize in refining these
types to yield extremely reliable long-life ion lasers with the
best optical stability, the lowest optical noise and the maximum wavelength
range, power and beam versatility obtainable.
Argon laser characteristics
Argon-ion lasers produce the highest visible
power levels and have up to ten lasing wavelengths in the blue and green
portion of the spectrum. The chart below shows the typical lasing
wavelengths and relative power levels obtainable from a 4-watt size argon
laser.
Argon lasers are normally rated by the power
level produced by the six simultaneously lasing wavelengths from 514.5 nm to
457.9 nm. The most prominent and most used wavelengths in the argon laser
are the 514.5 nm green line and the 488.0 nm blue line. The wavelengths
outside of the standard visible range, including a highly stable infrared
line at 1090 nm, are available simply by changing mirrors. The UV
wavelengths are produced from double-ionized transitions which require
higher than normal laser current levels, and are therefore available only
from the highest power models.
Krypton laser characteristics
Krypton-ion lasers are almost identical in
construction, reliability and operating life to argon lasers. Under some
conditions krypton lasers can produce wavelengths over the full visible
spectrum with lines in the red, yellow, green and blue. The 647.1 nm and
676.4 nm red lines are the strongest and result in the best performance.
Krypton lasers are normally rated by the
power level produced at 647.1 nm. This wavelength is the most frequently
used because it can produce more red laser light than can be obtained from
other types of lasers.
(For additional information about
wavelengths and lasers, see the Laser
wavelength charts page.)
The basic multiline laser
 In
its simplest configuration an ion laser is a multiline laser producing a
number of simultaneously lasing wavelengths. The figure to the right shows
the optical configuration of a basic multiline argon laser. The mirror
arrangement consists of a rear High Reflector and an output Transmitter
aligned with the plasma tube to produce lasing.
With standard mirror coatings the output beam
of an argon laser consists of six discrete wavelengths emitted together.
They can be separated into their individual lines by using an external prism
or other dispersive elements as illustrated. The approximate distribution of
the output power among the six wavelengths of a multiline argon laser
operating at full rated power is given in this chart:
|
Wavelength |
Percent of Total Power |
| 514.5 nm |
43 |
| 501.7 nm |
5 |
| 496.5 nm |
12 |
| 488.0 nm |
20 |
| 476.5 nm |
12 |
| 457.9 nm |
8 |
Single line laser output
Most laser applications require that only one laser wavelength be
produced at a time. Single line operation is achieved by replacing the
multiline rear mirror with a Prism Wavelength Selector, as shown in the
lower part of the diagram above. This assembly consists of an internal prism
aligned to properly deflect the intracavity optical path to the High
Reflector.
Because of the dispersive properties of the prism, only one wavelength at
a time will be properly aligned and produce lasing. The wavelength selector
thus allows easy tunability and selection of any of the individual lasing
wavelengths. The power available from a single line using a prism wavelength
selector is usually greater than the power that can be obtained from the
same wavelength by splitting a multiline beam with an external prism. See
the detailed wavelength and
power table, to find the single line power available from a Lexel laser.
Transverse modes
The
transverse electromagnetic mode (TEM) structure of a laser beam describes the
power distribution across the beam. Most laser applications require a
fundamental mode beam (TEM00) with a
Gaussian power distribution across the beam as shown to the right. This
fundamental mode results in the smallest beam diameter and beam divergence
and is capable of being focused to the smallest possible spot size.
 Other applications profit from the increased power available in the first
order mode (TEM01*) or even higher
order modes. Laser output having a mode structure above the fundamental is
commonly referred to as multitransverse mode (MTM). The mode structure
produced by the laser can be changed simply by changing mirrors. We provides the proper mirrors for either fundamental or multimode operation.
Beam diameter and
divergence
The diameter of a Gaussian laser beam is conventionally measured at the 1
/e2 power point, i.e., it is the
diameter of an aperture stop that will pass 86.5% of the total laser power
at the plane of the output mirror. The beam divergence is usually given as
the full angle divergence measured in the far field. Both parameters are
directly related to the laser wavelength, mirror spacing and curvature of
the mirrors.
Diameter and divergence values for selected ion laser wavelengths are available
for the Lexel 85/95 series,
the Lexel 85-SHG and
for the Lexel 95-SHG. A full
discussion of laser beam parameters is given by Kogelnik and Li, "Laser
Beams and Resonators", Applied Optics, Vol. 5, page 1550, Oct. 1966.
Single frequency operation
 The output of a laser operating on a single wavelength has a very narrow
linewidth and extremely good coherence compared to any other type of light.
However, the laser line is actually made up of a large number of
longitudinal modes spaced over a frequency bandwidth of approximately 5 GHz.
These modes are related to the distance between the two mirrors
making up the optical cavity. The frequency spacing between these
longitudinal modes is c/2L, where c is the velocity of light
and L is the mirror spacing. Thus a 1 meter cavity length has a 150
MHz longitudinal mode spacing.
The "coherence length", i.e., the path distance over which the laser
wavefront remains in phase and usable for interferometric effects, is
approximately given by c/Δv, where
Δv is the frequency bandwidth of the laser
line. The normal multilongitudinal mode (MLM) output of an ion laser,
therefore, has a coherence length of about 60 mm.
 Many applications such as holography and long-path interferometry or
Brillouin scattering require a much longer coherence length and a very
narrow linewidth. This is accomplished in an ion laser by installing an
etalon, such as the
Lexel Model 503 Etalon, in the laser cavity as shown in the
figure above. A properly designed intracavity etalon will reject all of the
longitudinal modes except one and cause laser power to be concentrated in
this single mode. Since a single longitudinal mode has a width of less than
3 MHz the resulting coherence length can be more than 100 meters.
Laser output using an etalon is known as Single Longitudinal Mode (SLM)
or Single Frequency Operation.
Beam-pointing stability
The ability of the laser to maintain a precise angular beam position is
very important for most applications. This requires a very stable
optical resonator such as
Lexel's Solid Invar® Rod Resonator
Structure.
The major additional factor affecting beam-pointing stability is the
thermal change in the index of refraction of the quartz used in the prism
wavelength selector. The properties of quartz are such that, if left
uncorrected, it will detune the laser at a rate of 0.2 nm/ °C and cause an
angular change in the beam-pointing of 11 arc sec/ °C. This is enough to
completely detune the laser with as little as a 10°C change in ambient
temperature.
A Temperature Compensated Prism
Wavelength Selector is needed to eliminate thermal detuning and provide
the best possible beam-pointing stability.
Beam polarization
The windows of the laser
plasma tube through which the laser light must pass are aligned at
Brewster's angle to eliminate the high reflective losses from the surfaces
of the windows. This results in the additional benefit of the laser output
beam being highly polarized in the vertical plane. If it is necessary to
have polarization at any other direction, the laser head may be turned 90
degrees and operated on its side. Or a polarization rotator may be attached
to the output aperture of the laser. This will allow the output beam
polarization to be changed to any desired plane.
Laser power control and stabilization
n
Current Control The output power of an ion laser can easily be
set at any level between full output and the threshold of lasing by
adjusting the level of the
power supply's laser current through the plasma tube. Current
Control permits good power stabilization for many applications but it
requires manual compensation for any optical variations that may cause
slight changes in the laser power level.
 n
Light Control For the ultimate in laser power stabilization, a
Light Regulator circuit is used. Under this type of light control a
small portion of the output beam is split off and measured by a photocell.
The output of the photocell is amplified and compared to a reference signal
in a differential circuit that automatically readjusts the laser current
level to maintain the desired laser power. The operator can easily set the
laser power to any desired level and be assured that the Light Regulator
will continue to maintain that level throughout extended operation.
|
