Professor Mark Csele's Homebuilt Lasers Page

Flashlamp-Pumped Dye Lasers

The Basics ...

The laser itself resembles any other optically-pumped laser such as a YAG or ruby except the rod is replaced with a dye cell. The laser tube is made of quartz glass of about 5 mm inside diameter. It is capped with two windows also of quartz and is filled with a solution of an organic dye. The dye is slowly flowed throug the cell to equalize temperature and remove thermal gradients which serve to defocus light passing through the cell and inhibit laser action. The laser is optically pumped from a linear flashlamp also made of quartz which sits about 2 cm away and parallel to the laser tube coupled with an elliptical reflector in which the lamp and dye cell are each at a foci of the ellipse. The dye is pumped with visible and UV light so the reflector may be fabricated from aluminum.

First Step: See the Scientific American Article in the February 1970 edition in the Amateur Scientist column [1] which describes a homebrew laser of this type. My laser, described on this page, follows this basic design with modifications to the tuning mechanism.

The flashlamp is a simple device filled with air at low pressure. A vacuum of no lower than 20 torr is needed which may be generated from an old fridge compressor run in reverse. The tricky part to this laser is the capacitor which stores energy to be discharged through the lamp. To make this laser work a capacitor of low internal inductance must be used. Ordinary electrolytic capacitors will not work. Consider this screen capture of an oscilloscope trace below which shows the current and voltage characteristics of a typical photoflash discharge through a xenon lamp. In this case the capacitor was rated at 350uF and was connected directly to the lamp with no external inductance inserted into the circuit (However be aware that photoflash caps have a relatively large internal inductance designed so that the impedance of the L/C circuit matches that of the lamp). Some researchers have used extreme measures such as coaxial capacitors [2] with the flashlmp housed in the center of the capacitor disc. The parameters of a suitable flashlamp design are covered in reference [3].

Xenon lamp (Photoflash) discharge The rise time of the discharge is about 100uS and the length of the light pulse about 1.1mS. This is far too long for a dye laser which usually requires rise times on the order of microseconds and pulse lengths under 100uS. The intrinsic inductance of electrolytic capacitors will not allow the fast discharge times required for this type of laser. Many solid-state lasers (such as the YAG, described on another page on this site) can use a relatively slow discharge such as this but not flashlamp-pumped dye lasers. The best bet here is to either make a capacitor of foil and plastic sheets or obtain a suitable low-inductance type. The capacitor required (as per the Amateur Scientist design) is rated at about 15 uF at 3000 Volts. Instead of a single capacitor one may use several smaller oil-filled capacitors (of about 4 uF each) in parallel. These are the type used for high-voltage power supplies such as those used for kilowatt-plus radio transmitters.

Mirrors are required for the laser resonator cavity. Being a high-gain laser, ordinary first-surfaced aluminum mirrors will work here. The rear mirror must be fully-reflecting and the front mirror partially reflecting (a 70% reflecting aluminum beamsplitter may be used here as in the nitrogen-laser pumped dye laser). Tuning of the wavelength of light emitted from the laser may be accomplished by substituting a diffraction grating for the rear mirror.

Details Of My FLP Dye Laser ...

The most difficult part of building this laser was finding a capacitor which must store over 100J and discharge extremely quickly. The unit for this laser was scavenged from a surplus defibrillator and was capable of energies of 400J! The system is a compact all-in-one unit and is capable of firing about one pulse every 10 seconds. I cannot overstate how important low self-inductance capacitors are in order to make this laser work - A discharge time of a few microseconds is required. Electrolytic types will discharge far too slowly. It is possible to use oil-filled radio transmitting capacitors (obtained as surplus from amateur radio suppliers). Several small units may be paralleled to increase capacitance and hence energy. A threshold energy of between 30 and 50 J can be expected.

As well as a fast capacitor (i.e., one with low internal inductance) other parts of the discharge path between the capacitor and the lamp must also have low inductance. Im my case, I connected the capacitor to the 1/4-inch brass bolts connected to the lamp via 1.5cm wide copper strap. The strap must be kept as short as possible.

One idea to get a faster discharge is to use a lower capacitance and higher voltage (described lower on this page). Lasers with discharge times in the nanoseconds have been made using a spark through air (atmospheric pressure) operating at 50kV [4]. As well, this idea may lend itself well to amateur construction since the capacitor may be homebuilt at low cost using plates of aluminum and mylar dielectric.

Mirror alignment is very critical. Unlike the nitrogen-laser pumped dye laser which may be 'pushed' into superradiant operation to make alignment easy these mirrors must be aligned correctly the first time or it does not work. The laser is not CW so it cannot be 'rocked' like most gas lasers can. Since my laser used a flat output coupler and a flat grating in the rear, alignment was difficult at best! This is done using an autocollimator or a small HeNe laser.

This photo shows a flashlamp-pumped dye laser firing. Visible here is the 15cm long quartz lamp (with an ID of 7 or 8mm) and the dye cell parallel to it just behind. The lamp could be fired with 400J of energy supplied by a large capacitor underneath the base. In a variation of the Amateur Scientist design the lamp was constructed with stainless-steel end caps with tungsten electrode inserts. Visible to the left of the dye cell is the output coupler on an adjustable aluminum mount. The total reflector for this laser cavity is a diffraction grating off to the right side of the cell.

A cut-away view of the electrodes reveals the tungsten electrode wires inside the stainless-steel end caps. Quartz tubing is sealed into the end cap with silicone sealant and is supported via a 1/4" diameter brass bolt which also carries current to the lamp. One electrode (ground) has a hole through it through which the lamp is evacuated.

The top view of the laser shows details of the cavity in the lower left as well as the grating-tuning system. The grating (on the mount in the lower right) was adjusted by the step motor above it. The fan atop the cavity is used for forced-air cooling of the lamp. Two Erlenmeyer flasks are visible in the upper-left which are used as dye reservoirs. The dye is pumped through the cell by pressurizing the flasks slightly with an aquarium pump. This ensures a clean, contamination-free way of handling the dyes.

The top panel is the actual laser control while the bottom is the tuning control. The laser control allows the user to select the charge on the main capacitor (in Joules) then charges the cap. to that level quickly by applying power to the transformer via an opto-driver block. The tuning control on the bottom controls the step-motor for the diffraction grating. It features a digital display which counts step offset in order to calculate grating angle and hence wavelength selected.

The Idea Corner ...
On the Making Of Simpler Flashlamp-pumped Dye Laser

Food for thought: The critical component in building a flashlamp-pumped dye laser is the capacitor. Regular electrolytic caps discharge far too slowly for such a laser which requires discharge times in the microseconds to work. What about using a capacitor (homebuilt) of high voltage and low capacitance? Since E=0.5*C*V2, employing a high voltage allows the use of a small capacitance and hence (potentially) faster discharge times. It would be easy to build and would guarantee a fast enough discharge to get the laser to work.

An article I came across took this approach. The authors used a 7nF, 50 kV capacitor (Only 9 Joules of energy). The "lamp" wasn't a lamp at all but rather a confined spark in open air (enticing for an amateur again since no vacuum pump is required). The result is a super-fast pump (70 nS discharge) resulting in a very low threshold energy.

It may be possible to use lower voltages (say 20 kV) with an air flashlamp at a lower-than-atmosphere pressure (say 500 to 760 torr) to make the device triggerable (using a high voltage trigger like any other flashlamp). Since the voltage decreases, the capacitance (and hence discharge time) will increase and a larger cap is required (Threshold energy will increase). My original thought was to build a capacitor from alternating plates of aluminum foil and mylar all immersed in a bath of insulating oil to make a compact capacitor unit.

Described in Fast rise spark pump for organic dye lasers, F. Aussenegg and J. Schubert, Physics Letters, Vol. 30A, No. 9, 29 Dec. 1969, is a simple scheme in which the lamp sits directly atop a flat-plate capacitor. The 'lamp' is really an enclosed spark, 50mm in length, with excessively fast risetimes. The authors go on to say that they used a low-inductance capacitor of 7nF capacitance running at 50kV. The capacitor consists of three plates of aluminium, isolated by 1.5 mm dielectric layers. To obtain the lowest possible inductance the lamp is mounted directly at the capacitor (cap and laser form a compact unit reseminling that of Sorokin's design in the Amateur Scientist). The laser cell was 3mm ID, and a cavity consisting of a 100% reflecting HR and 70% reflecting OC was used. Total energy was 9 Joules and the discharge time was an incredibly short 70 ns ! The resulting laser output (using Rhodamine-6G) was 300-500ns (a longer decay time of the lamp caused by the afterglow).

One might look at the Amateur Scientist article An Air Flash Lamp Advances Color Schlieren Photography (August, 1974) which describes a simple 300ns flashlamp using a guided spark in air. The technique could likely be adapted for use with a dye laser.

A few key references:

These are 'normal' flashlamp pumped dye lasers. All designs use air flashlamps (great for amateur construction).

[1] A tunable laser using organic dye is made at home for less than $75
Scientific American, Amateur Scientist Column, February 1970
A must-read article. Describes a simple laser using a homemade air flashlamp. Very inexpensive but the hardest part is obtaining a suitable capacitor) ordinary electrolytics will not work here.

[2] Flashlamp-pumped organic dye lasers
P. Sorokin, et al.
The Journal Of Chemical Physics, Vol 48, Number 10, 15 May 1970
Describes the use of ultra-low inductance disk capacitors and coaxial air flashlamps to pump the laser.>

[3] Optical pumps for organic dye lasers
H. W. Furumoto and H.L. Ceccon
Applied Optics, Vol 8, Number 8, August 1969
An excellent study of ultra-fast flashlamps used to pump dye lasers.

These following articles describe less 'traditional' designs including the use of a fast spark to pump the laser and producing a superradiant dye laser (i.e. no mirrors required). A few good ideas here if nothing else!

[4] Fast rise air spark pump for organic dye lasers
F. Aussenegg and J. Schubert
Physics Letters, Vol 30A, Number 9, 29 Dec. 1969
A novel approach using high-voltage/low capacitance to pump a spark as a source. Might lend itself well to amateur construction since the capacitor must be handmade with plates of aluminum.

Waveguide superradiant dye laser
P. Burlamacchi and D. Pratesi
Applied Physics Letters, Vol 22, Number 7, 1 April 1973
Apparently with a small-enough capillary and a flashlamp-pumped dye laser can be made to operate superradiantly! Enticing stuff for the amateur.

A simple, reliable waveguide dye laser for ophthalmological applications
P. Burlamacchi, et al.
Review Of Scientific Instruments, Vol 46, Number 3, March 1975
Using the same approach as the above article, the authors build a superradiant flashlamp-pumped dye laser using two slabs of glass as a waveguide.