Compact reservoir system for dye lasers

A dye solution reservoir system for a dye laser comprises a dye solution reservoir, main fluid circuit means for circulating dye solution between the reservoir and a dye laser, and a by-pass dye replenishment loop between the reservoir and the laser for (1) diverting a portion of the solution from the main circuit, (2) filtering substantially all solute, including the dye and other particles from that portion of the solution, (3) replenishing the solution by adding fresh dye solute, and (4) returning the replenished solution to the main circuit. In a preferred embodiment, the solution is filtered by a first filter for removing the dye solute and a second filter for removing degradation by-products and particulate matter. Reverse osmosis may be advantageously used as a part of this filtrate system. The advantage of such systems is a substantial reduction in the required reservoir capacity and enhanced dye recoverability.

FIELD OF THE INVENTION 
This invention relates to reservoir systems for liquid dye lasers. More 
specifically, it relates to a dye solution reservoir system including a 
by-pass dye replenishment loop for diverting a portion of the dye 
solution, filtering substantially all solute and replenishing the solution 
by adding fresh dye. 
BACKGROUND OF THE INVENTION 
Liquid dye lasers offer considerable promise for many applications, 
including isotope separation, photochemistry, pollution detection and 
spectroscopy. Dye lasers differ from the more familiar gas and solid state 
lasers primarily in that they employ, as an active medium, a fluid dye 
solution. The dye solution comprises a solvent such as an organic solvent 
and/or water and a solute of an organic dye material. Typical dyes have a 
molecular weight in the range between 200 and 1000 and are used in 
concentrations on the order of 10.sup.-4 moles per liter. A primary 
advantage of dye lasers over conventional gas and solid state lasers is 
that the dye laser output can be tuned over a broad range from the near 
ultraviolet to the near infrared. 
Dye reservoir systems are required with liquid dye lasers in order to 
continuously provide the laser with replenished dye solution. Radiation 
from the laser excitation source decomposes both the solvent and the 
solute of the solution, producing by-products which degrade the laser 
performance. Specifically, the by-products, which typically have molecular 
weights in excess of about 100, degrade the laser performance by (1) 
absorbing laser light, (2) quenching excited states, and (3) absorbing 
excitation light. In order to avoid such degradation, the laser is 
provided with a reservoir of dye solution, and fresh dye solution is 
continuously pumped through the laser cavity. The exposed solution is 
filtered to selectively remove by-products, and it is then returned to the 
reservoir. Such filtration systems must distinguish between dye molecules 
and other contaminates, removing the latter but retaining the former in 
the solution flow system. Any loss of dye is thus to be avoided since the 
system is provided with only a specific amount of dye at a predetermined 
concentration. 
The difficulty with typical prior art reservoir systems, however, is that 
they require large reservoirs of dye solution. In many applications, such 
as in portable systems, space and weight are at a premium. However, 
conventional dye solution reservoirs, which can require a capacity of ten 
gallons or more, are the most voluminous part of the dye laser system. 
Accordingly, there is a need for a more compact reservoir system. 
SUMMARY OF THE INVENTION 
In accordance with the teaching of the present invention, a filtration 
system is provided for purifying the fluent lasing material such as a dye 
solute in a water or organic solvent, in a dye laser circulating loop in 
which a by-pass loop (1) separates from the main circulation loop a 
fraction of the fluent material, (2) substantially purifies the solvent by 
removing all contaminants along with the dye solute to leave relatively 
pure solvent, and (3) restores the dye concentration level from a dye 
concentrate source before return of the purified solvent and dye to the 
main dye laser fluid circulating system. 
By thus continuously purifying a small portion of the fluent material an 
acceptable level of purity can be continuously maintained within the dye 
laser circulation system. Because the solvent comprises the overwhelming 
bulk of the laser fluent material, by retaining the solvent through 
purification and rejecting the dye and other contaminants as waste 
material, greater efficiency can be achieved along with a substantial 
reduction in the bulk of the filtration system. Because of the continuous 
diversion of the small portion of the fluent material for continuous 
purification, the reservoir capacity can be substantially reduced with the 
same level of purity of fluent material applied to the laser. It is also 
preferable that the main laser fluid circulating system include its own 
filter for removing contaminants without removal of dye molecules to 
provide optimal purity of the dye and solvent applied to the laser head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention contemplates a material-efficient and space-minimized 
filtration system for dye laser fluid which comprises a continuous 
purification loop for fluent material acting as a by-pass of a small 
portion of the fluent material in the main circulating loop supplying the 
dye laser head. In particular, the by-pass filtration acts by removal of 
all solvent components to produce substantially pure solvent to which a 
metered amount of dye concentrate is restored to establish the operating 
dye concentration in the by-passed material after which it is restored to 
the main fluid circulating loop. 
The drawing illustrates such a dye laser system wherein a dye solution 
reservoir 10 is coupled to a dye laser 11 and in particular the flow 
channel of the laser head, by main fluid circuit means such as tubing 12 
and a pump 13 for circulating dye solution through the tubing between the 
reservoir and the laser. A filter 9 is preferably included to remove large 
degradation particles and air bubbles. 
In accordance with the invention, a by-pass dye replenishment loop 14 is 
coupled to the main circuit between the reservoir and the laser for 
diverting, filtering and replenishing a portion of the solution from the 
main circuit. Specifically, the preferred by-pass loop comprises tubing 15 
coupled to the main circuit, preferably following the pump 13 outlet, for 
diverting a portion of the solution--typically, but not critically, 
10%--from the main fluid circuit. Filter arrangement 16 is provided in the 
loop circuit for filtering substantially all solute and particles from the 
solution and dye addition means 17 is provided for replenishing the 
solution by adding fresh dye to the filtered solvent. The tubing 18 then 
returns the replenish dye solution to the main circuit, preferably to 
reservoir 10. 
The filter arrangement 16 can be comprised of, for example, reverse osmosis 
membranes, activated charcoal bed filters, silica gel filters or molecular 
filters. The function of the filter, in contrast with conventional 
arrangements which selectively remove only the by-products, is to remove 
substantially all solute including the dye from the solution. In addition, 
a preferred filter arrangement is constructed to provide a sufficient 
throughput that the makeup rate of the replenished solution balances the 
rate of solution degradation in the laser at a desired purity of solution 
applied to the laser head. 
The preferred filter arrangement comprises a pair of filters 19 and 20. The 
first filter 19 is preferably a filter, such as a charcoal bed filter, for 
selectively removing the dye solute, and the second filter 20 is 
preferably a molecular filter, such as a reverse osmosis filter, for 
removing contaminates such as degradation by-products. In addition, filter 
20 will stop all particulates that might be introduced by the charcoal 
filter. The primary advantage of this filter arrangement is that the 
solvent, forming the majority of the fluid material bulk is salvaged while 
the dye is collected separately from the degradation by-products, thereby 
facilitating dye recovery. 
The preferred dye addition means 17 comprises a dye reservoir 21 for 
containing a quantity of dye concentrate and a metering pump 22, both 
connected to the feedback loop through tubing to add fresh dye to the 
filtered solvent at a preselected rate to provide a freshened dye solution 
at the desired concentration for lasing. 
Because the dye is concentrate reservoir 21 can be highly concentrated, 
typically by two orders of magnitude as compared to the solution 
concentration of typically 10.sup.-4 moles per liter, the dye concentrate 
reservoir 21 can be very small in volume as compared to the dye solution 
reservoir 10. 
Because the solvent in this system is continuously separated and completely 
purified in the by-pass loop, the amount of solvent required for operation 
is small as compared to that required for unby-passed systems. 
Accordingly, the size of the dye solution reservoir can be substantially 
reduced with substantial savings of space and weight. As compared with a 
conventional portable dye laser system using a ten gallon reservoir, a 
comparable system in accordance with the invention requires only a one 
gallon reservoir. 
Additional advantages include reduced operating cost due to reduced solvent 
consumption, reduced problems with solvent waste disposal, and ease of dye 
recovery. 
As an illustrative specific embodiment of the invention, the following 
specific components can be used to form a system in accordance with the 
invention: 
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Component Description 
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Dye Rhodamine 6G 
Solvent Water/Methanol 
Dye solution concentration 
1 .times. 10.sup.-4 M 
Filter 9 Polypropylene, 
0.25 micron nominal 
Dye solution reservoir 10 
Stainless steel, 1-5 gallons 
Tubing 12 Stainless steel (e.g. 304) 
Pump 13 Stainless steel centrifugal pump 
Dye laser 13 Flashlamp pumped 
By-pass loop tubing 15 and 23 
Stainless steel 
Percent diversion 1% of pump 13 flow rate 
Dye filter 19 Reverse osmosis 
By-products filter 20 
Molecular sieves 
Dye reservoir 21 0.1-0.5 gallons 
Metering pump 22 Positive displacement 
Dye concentrate 5 .times. 10.sup.-3 M - 1 .times. 10.sup.-2 M 
Metering rate 0.1% of pump 13 flow rate 
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All wetted parts can be stainless steel (e.g. 304) or any other 
noncontaminating material compatible with the overall design and safety 
requirements of the system (e.g. polypropylene or teflon). 
While the invention has been described in connection with but one specific 
example, it is to be understood that this is merely illustrative of many 
other specific embodiments which can also utilize the principles of the 
invention. Thus numerous and varied devices can be made by those skilled 
in the art without departing from the spirit and scope of the invention.