This laser works by focussing the UV beam from a nitrogen (or excimer) laser to a thin line on the surface of a cell containing a dye solution using a cylindrical lens. The dye is excited along a line on the inside surface of the cell, the excited 'shaft' of dye molecules forming the gain medium from which a beam emerges from the ends of the dye cell. The dye cell itself is made of quartz glass to pass the 337.1nm radiation without attenuation (although regular glass will work with reduced efficiency). The cell is typically 5mm dia. inside and only 1 cm in length.
Depending on the dyes used, mirrors may be omitted (at least for initial testing). With a small (100 kW) nitrogen laser of the type described elsewhere on this site, I was able to make a solution of the common dye 7-diethylamino-4-methylcoumarin lase superradiantly! This dye is used in fabric detergent as a whitening agent. You must use a pure solution of the dye in methyl or ethyl alcohol for this laser. The dye itself may be obtained from a large lab chemical supply house such as Sargent-Welch Scientific of from a source such as Lambda-Physik or Exciton. The dyes are not overly expensive as only a small amount (much less than 1 gram) is required for a solution with one litre of alcohol. I normally use methanol as it is cheap and readily available in very pure (> 99.99% pure) form - most chemical supply houses carry methanol in pure form (im my case 'cleanroom grade').
The dyes used in these lasers are highly toxic and so must be handled with proper precautions. Some interesting dyes I have experimented with are:
- 7-diethylamino-4-methylcoumarin: A very easy to lase dye (high gain) producing laser output in the blue and green portion of the spectrum.
- Rhodamine 6-G: Another high-gain dye which produces laser beams tunable in the green-yellow to red portion of the spectrum. Recent experiments show this dye, in a methanol solution, is the most powerful dye in a nitrogen-laser-pumped laser.
- Sodium Fluorescein: A lower-gain dye producing light in the green to yellow region. Easy to obtain since it is used extensively in biology labs.
- 4-methylUmbelliferone: A weak dye producing light from deep blue through violet and into the near-UV portions of the spectrum.
Details Of My N2-Laser Pumped Dye Laser ...
My very first dye laser was built was precarious-looking for a laser. Built on a
piece of 2-by-4 lumber, the laser itself was only 15 cm long. The dye cell,
shown to the left, consisted of five pieces of cut microscope slides siliconed
together. The cell was 1cm square by about 2.5cm tall. It was simply filled with
an appropriate dye solution and placed on the base of the laser. Light from the
nitrogen pump laser was focussed to a 'line' on this cell using an inexpensive
glass cylindrical lens. The lens itself was made of quite cheap glass and likely
absorbed a good deal of the UV pump light as well - the laser still worked despite
this though, the spoils of having a very large pump laser!
This first dye laser consisted simply of a dye cell with two flat mirrors on either
side acting as a cavity - all components were mounted on a 2-by-4 wood base.
Optics for this first laser consisted of an front-surface aluminum mirror and a 70% reflecting / 30%
transmitting beamsplitter mirror for the output coupler. Each mirror was placed
in a 3-point mount which allowed adjustment. Adjustment was surprisingly easy though
since the dye first used (7-diethylamino-4-methylcoumarin) will lase superradiantly!
The dye was made to lase first by adjusting the focus of the pump beam. Mirrors
were then aligned until the output beam appeared much brighter. The pump beam
may then be defocussed to cease superradiant operation (i.e. laser normally
with mirrors). Lasing is verified by blocking the path to the rear mirror alone
which stops the output beam.
My first research laser, constructed in 1983, consisted of a 1cm long quartz glass tube filled with solutions
of organic dyes such as 7-diethylamino-4-methycoumarin and rhodamine-6G. UV
light from the nitrogen laser is focussed onto the dye to excite it and stimulate
laser action. A cylindrical lens mounted on an old microscope focussing
attachment allows the UV light to be focussed to a fine line on the dye surface
(which is required for lasing action).
The dye cell itself was a 1cm long piece of quartz tubing into the side of which I
filed a hole to admit the dye. I attached a standoff of glass tubing
using clear silicone to permit filling of the cell (see photo near the end of this section).
The mirrors were the same inexpensive ones used in my first dye laser. The rear mirror
was cut from an aluminum, first-surface mirror made for use in telescopes (these are
easily obtained and quite flat!) and the O.C. a 70%R/30%T beamsplitter (which I
cut myself as evident from the crude fracture visible in the photo.
When the UV light from an N2 laser is focussed very tightly onto
the dye the laser operates superradiantly and a beam appears regardless of mirror
alignment (or if they aren't even present). This effect makes mirror alignment
easy since one may now align the mirrors until they are parallel. When the
mirrors are in proper alignment the output beam becomes much brighter as well
as collimated (doesn't spread as much). Now the UV light may be defocussed onto
the dye cell and the laser will be operating in 'normal' mode. To prove proper
laser operation a card is inserted between the cell and the rear mirror in which
case lasing should cease. It is necessary to set the laser to NOT lase
superradiantly when tuning the laser's wavelength via it's optics (with a diffraction
grating as the rear reflecting element).
The dye cell for my research laser, which was tunable, was inside a cavity consisting of a partially
reflective mirror on the front (the output coupler) and a diffraction grating at the rear.
The output coupler was an aluminized 70%R/30%T beamsplitter cut from a larger piece.
The grating was an 'experimental grade' grating - a commercial grade with slight
scratches -and worked quite well. Both optics were purchased from Eftonscience
in Canada (Edmund Scientific in the US). The
grating and beamsplitter were both priced under $20. (The high gain of this laser
allows the use of such low-grade optics ... don't try using this quality of
components on a low-gain gas laser or it will not work).
By changing the angle of the diffraction grating, the wavelength of the laser light could
be changed. The optical path includes a lens to expand the intra-cavity beam as
well as a slit to select a certain wavelength and hence reduce the spectral width
of the output.
Here UV light at 337.1 nm wavelength from
the nitrogen laser is focussed by a plane-convex lens to a line on a 1cm dye cell made
of 7mm quartz glass. The intensely excited dye molecules at the focus of the UV 'pump' light
lase readily - When the pump beam was focussed sharply on the dye cell it lased
superradiantly (with no external mirrors!).
Clearly visible here is the dye cell in operation. To the left the output coupler is
visible. The bluish light on either side of the excited dye is silicone used
to seal the cell.
Here are the dyes I used in my experiments. They are lit via a UV lamp from above and hence are fluorescing brightly in this photo. The strongest dyes used were Rhodamine-6G and 7-diethylamino-4-methylcoumarin. Other dyes used included Sodium Fluorescein and 4-methyl Umbelliferone. All dyes are dissolved in either methanol or ethanol.
The dyes used were laboratory grade and not as pure as those normally used for
dye lasers. These are considerably cheaper, however, no doubt there was a price
to pay in terms of efficiency of the laser
The New Niagara College Dye Laser
In 2004 a group of photonics students built a dye laser for our laser lab. This laser employs a quartz fluorimeter cell (with four clear optical sides) of 10mm * 10mm dimensions and is pumped by a Lumonics-500 excimer laser running a mix of 2% N2 and 98% He at a pressure of 5 psig. Pump light from the Lumonics (UV) is not visible here except for some fluorescence as it passes through the cylindrical lens and bright fluorescence at the cell. An aluminum HR is visible in this photo (wth two protruding adjustment screws) and a 50% reflecting beamsplitter for an OC. Rhodamine-6G in a 1*10-3 Molar solution of methyl alcohol is seen here operating in the yellow-green with the beam made visible by fog. The solution is made by dissolving 0.0457g of Rhodamine-6G into 100ml of methanol (The molecular weight of Rhodamine is 457g so that 0.457g per liter of alcohol is required) as measured using a precision microbalance scale.
Fluorimeter cells are expensive at $250 each however the quality of the cell is much higher than homebuilt cells and corners of the cell are almost perfectly square (unlike many cheaper cells in thich the corners are rounded). Being made of quartz, of course, helps since it is quite transparent to UV radiation. The cylindrical lens in this case is an inexpensive one made from BK-7 glass. BK-7 has a transmission of over 90% at 337.1 nm making it an economical choice for focussing pump power onto the dye cell - quartz would be the best choice but it was difficult to locate a quartz cylindrical lens.
All opto-mechanical components here are standard including mirror mounts with angular adjustment screws, a universal prism mount used to hold the dye cell, and a lens mount for the cylindrical lens. The mounting post for the cylindrical lens is attached to a base with slots allowing the entire lens to be moved closer or further from the dye cell. When 'tightly' focussed the cell lases superradiantly and hence is not tunable. Normally the laser is made superradiant, optics aligned carefully, then the pump lens is 'backed-off' until the laser is no longer superradiant. To prove the laser is operating normally the optical path between the cell and each optic is blocked at which point lasing ceases.
The laser is seen here with a diffraction grating in place of the HR making it tunable. In this animated GIF the laser
is seen operating at two wavelengths, orange and yellow-green, as seen on the wall where the output beam strikes.
The SOP (Standard Operating Procedure) for this laser can be found on the SOP Page. The SOP contains annotated photos of the laser as well as procedures for safe operation
A few key references:
Dye laser stimulation with a pulsed N2 laser line at 3371A
J.A. Myer, et al.
Applied Physics Letters, Vol 16, Number 1, 1 January 1970, pp. 3
Describes the use of a pulsed-N2 laser to drive a dye laser along with cylindrical lens to focus the UV light to a line on the dye cell.
Tuned nitrogen laser pumped dye laser
G. Capelle and D. Phillips
Applied Optics, Vol. 9, No. 12, Dec. 1970
An excellent article which discussed tuning a dye laser using a grating as well as dye cell design.
Repetitively pulsed tunable dye laser for high resolution spectroscopy
T. W. Hansch
Applied Optics, Vol. 11, No. 4, April 1972
Describes the finer points of tuning a dye laser using both a grating and an intra-cavity etalon.