Multiple frequency, visible laser outputs are obtained from a 1064 nanometer input by tripling the output of an Nd:YAG laser and using the 355 nanometer radiation for downconversion by pumping a Tm:LiYF.sub.4 crystal.

BACKGROUND OF THE INVENTION 
This invention relates to laser systems and, more particularly, to 
frequency shifting of lasers. 
Presently, there are many optical systems using 532 nanometer laser 
radiation which is typically obtained from a doubled Nd:YAG laser. The 
military, particularly, has many applications for such systems. Many of 
these applications require wavelength diversity in the visible portion of 
the electromagnetic spectrum. In particular, it is a requirement for 
particular military scenarios that the systems be capable of emitting two 
different wavelengths both of which are visible in order to impart to the 
systems greater resistance to countermeasures from an enemy. In such 
systems, it is of major import that the wavelengths can be varied such 
that an enemy does not become accustomed to particular wavelengths and, 
thus, provide equipment responsive to the particular wavelengths. To 
employ separate lasers at different wavelengths is impractical because of 
the accompanying increase in cost and complexity of such systems, 
particularly in systems where space is of a premium. 
To alleviate the necessity of employing additional lasers, frequency 
diversity is currently being providing by down-shifting the 532 nm output 
of double Nd:YAG with laser dyes. In many cases, this has proved an 
unsatisfactory solution to the problem of frequency diversity since the 
use of dyes has many disadvantages. For example, they readily degrade by 
bleaching, thereby shortening the stability of such systems. The dyes must 
also be used in solvents which are flammable and toxic. The dyes 
additionally have poor thermal properties which leads to degradation of 
beam quality, and require plumbing to flow the liquids. 
Accordingly, it is an object of this invention to provide improved laser 
frequency diversity. 
It is another object of this invention to provide laser frequency diversity 
by using a solid state laser downconverter. 
SUMMARY OF THE INVENTION 
Briefly, in one embodiment, laser frequency diversity is provided by 
employing a tripled Nd:YAG laser, which outputs laser radiation at 355 
nanometers, to pump a Tm:LiYF.sub.4 crystal which emits at other 
frequencies. Since the military is replete with doubled Nd:YAG lasers in 
its inventory, additional frequency diversity is achieved by merely 
employing the 1064 nanometer and doubled 532 nanometer outputs of the 
doubled Nd:YAG lasers and mixing them to provide a 355 nanometer output 
which is used to pump a Tm:LiYF.sub.4 crystal. One particular frequency of 
interest from the Tm:LiYF.sub.4 crystal is 453 nanometers. By employing 
355 nanometer radiation to pump the Tm:LiYF.sub.4 crystal, a 453 nanometer 
output will be provided at relatively high efficiency. 
By providing a cavity for the Tm:LiYF.sub.4 laser including a mirror coated 
for an output at 453 nanometers, the laser system will provide both 453 
and 355 nanometer radiation. Since Tm:LiYF.sub.4 has a 'D.sub.2 emission 
at 453 nm, 520 nm, 670 nm, 760 nm and 800 nm compatible with tripled 
Nd:YAG, additional frequency diversity is achieved merely by changing the 
coating on the mirror of the Tm:LiYF.sub.4 laser for outputs at other 
frequencies. 
In an alternative embodiment, a prism is inserted in the Tm:LiYF.sub.4 
laser cavity and a rotatable broadband mirror employed to provide 
frequency diversity by merely positioning the broadband output mirror such 
that it will receive the desired output from the prism.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 1 of the drawings, there is illustrated thereby a 
first embodiment of the invention. The laser oscillator comprises a laser 
10 including a crystal 12 disposed between a pair of mirrors 14 and 16 
forming an optical resonator. If desired, reflective materials can be 
deposited on the ends of crystal 12 in place of mirrors 14 and 16 as is 
conventional. In this embodiment, crystal 12 is a Lithium Yttrium Fluoride 
(LiYF.sub.4) host doped with Thulium (Tm.sup.3+). 
Pump energy for the Tm:LiYF.sub.4 crystal is supplied by a second laser 
rather than a flash lamp. In this embodiment, pump energy is derived from 
a Nd:YAG laser 18. The output (1064 nm) is coupled to a doubling crystal 
20 to provide outputs at 1064 nm and 532 nm. These two wavelengths are 
applied to a mixing crystal 22 to generate pump energy at 355 nm. The pump 
energy from the mixing crystal 22 is applied to the Tm:LiYF.sub.4 crystal 
to cause a population inversion and cause an output from mirror 16 of the 
optical resonator. 
In the preferred embodiment mirror 14 is configured to be highly 
transmissive at 355 nm and highly reflective at 453 nm. Mirror 16 is 
configured so that it is partially transmissive at the desired output 
wavelength, 453 nm for example. This system fulfills all of the objects of 
the invention in that it provides two distinct visible wavelengths, 355 nm 
and 453 nm. The system is also adaptable for providing further visible 
outputs in that the mirrors 14 and 16 can be otherwise configured as for 
example by applying coatings thereon to provide outputs from the laser 10 
at 520 nm, 670 nm, 760 nm and 800 nm. The system is highly desirable for 
military applications in that the doubled Nd:YAG laser is the most common 
of lasers employed by the military and merely by mixing the 1064 and 532 
nm outputs therefrom, radiation at 355 nm is obtained for easy pumping of 
the Tm:LiYF.sub.4 crystal because the tripled output from the Nd:YAG laser 
(355 nm) coincides with the ' D.sub.2 absorption of the Tm:LiYF.sub.4 
crystal 12. 
Frequency diversity is provided at minimal cost increase because the only 
requirement is an add-on to conventional Nd:YAG systems, the add-on 
comprising the mixer 22 and the laser 10. The output of the system is 
capable of providing many frequencies since a choice of specific lines can 
be generated by proper mirror selection. A particular frequency is 
obtained by the proper coating of mirrors 12 and 14 as mentioned above. 
An alternative embodiment of this invention is shown in FIG. 2 of the 
drawings, wherein the 355 nm radiation is applied to a laser 24 including 
a resonant cavity having a first mirror 26 and a second mirror 28. 
Intermediate, the mirrors 26 and 28 is a Tm:LiYF.sub.4 crystal 30 and a 
prism 32. In this embodiment, mirror 28 is disposed in such a manner that 
it is rotatable as shown by line 34, so as to receive radiation from 
different portions of prism 32 depending upon the position of the mirror 
28 with respect to prism 32. In this embodiment, the mirrors are broadband 
and prism 32 is employed as the wavelength selecting device. 
While specific embodiments employing 355 nm radiation from a tripled Nd:YAG 
laser to pump a Tm:LiYF.sub.4 laser have been described, it is to be 
understood that the embodiments shown are illustrative only, and that many 
variations and modifications may be made without departing from the 
principles of the invention herein disclosed and defined by the appended 
claims.