4
SolsTiS User Manual v10.0
2.4
Background
– Principles of Operation
This section provides an overview of the design principles and operation of
the SolsTiS Ti:S laser. (The description below covers all SolsTiS models, so
this overview may discuss configurations and performance you have not
selected.)
Ti:Sapphire
– A Broadly Tunable Gain Medium
Ti:S has a very broad emission spectrum, and this property underpins the
wide tuning range possible with SolsTiS. Ti:S is a mechanically robust
crystalline material, produced by substituting a small percentage of Al
3+
ions
with Ti
3+
in an Al
2
O
3
crystal host in a melt process. The Ti
3+
ions are
responsible for the laser emission in Ti:S.
Optical absorption in Ti:S occurs over a broad wavelength range from ~400nm
to ~600nm. Similarly, emission is possible over a broad wavelength range
extending from ~670 to >1000nm. Lasing action below ~670nm is precluded
by an overlap between the long wavelength end of the absorption band and
the short wavelength end of the emission band, and in addition, weak
absorption due to Ti
4+
ions.
Given the absorption characteristics of Ti:S, a neodymium-based DPSS laser,
lasing at ~1µm and intracavity frequency doubled to ~0.5µm is, for example, a
highly efficient, ideal pump source for Ti:S. SolsTiS can be pumped by any
commercially available CW “green” DPSS laser (via optional factory
configuration for the selected pump source).
Ultimately, the lasing tuning range achieved is determined by a number of
factors including: cavity mirror coatings (loss), pump power and pumping
efficiency (gain), number and type of intracavity elements (loss), and
absorption due to atmospheric absorption lines (loss).
In SolsTiS, the use of proprietary broadband mirror coatings, the reduced
number of intracavity tuning elements needed to tune a small cavity, the
sealed design to mitigate atmospheric absorption effects, and the use of a
diode-pumped pump laser with high beam quality, all ensure that the available
SolsTiS tuning range and output power is maximized for any given
pump power.
Laser Cavity Modes
When the Ti:S gain medium is placed in a suitable resonant laser
cavity and supplied with enough pump power to exceed lasing
threshold, laser oscillation occurs. Unless designed otherwise,
lasers will typically oscillate on a number of well-defined, equally
spaced cavity frequencies (longitudinal modes), as can be seen in
Figure 10.
Figure 10
Simplified diagram of the longitudinal frequency
modes of a laser cavity. (L = half round-trip cavity
length; c = speed of light.)
Laser gain
envelope
Longitudinal cavity
modes
Frequency
Ga
in
Mode spacing (c/2L)