Sunita Singh and G. Sridar
Laser & Plasma Technology Division
B.A.R.C., Trombay, Mumbai
A tunable, narrow-band CW dye
oscillator pumped by a commercial Argon ion laser has been indigenously
developed at the Laser and Plasma Technology Division, BARC.
The schematic of the standing wave CW dye laser that utilizes
a three-mirror non-collinear folded cavity is shown in Fig.1.
Mirror M1 (Rc = 50mm, R>99%) and M3 (Rc = , R=96%) form the end mirror and the output coupler respectively whereas, mirror M2 (Rc = 75mm) constitutes the folding mirror in the resonator cavity. A low divergence (10.5mR) commercial Argon ion laser (6W all lines) was
used to pump the CW dye laser
oscillator. The pump beam was focused into a tiny spot of diameter
~10 m in the dye gain medium with a fourth mirror M4
(Rc = 75mm). The gain medium was in the form of a thin
horizontal laminar sheet of free flowing dye jet stream with a
high velocity of 15 m/s. The jet stream is preferred over conventional
dye cells with windows as the tightly focused pump beam can produce
high enough intensities causing optical damage of the windows.
The dye jet nozzles are indigenously designed  and generate
an optical quality laminar flow of Rh6G dye dissolved in viscous
solvent ethylene glycol.
Lasing action in the CW dye
laser was achieved by initially focussing the Argon beam into
the horizontal fast flowing dye jet as mentioned above. The moderate
power beam from the CW pump laser must be focused very tightly
concentrating its power in a small volume, to reach laser threshold.
The upper laser level has a lifetime of few nanoseconds so high
power density is needed to reach threshold. In pulsed lasers
short pump pulses with high peak powers make the dye laser a high
gain system that can tolerate high cavity loss. However, the CW
pumped dye laser remains a low gain system that requires low cavity
loss. The intracavity losses were minimized by positioning the
dye jet at Brewster angle. Careful alignment of the folding mirror,
the pump mirror and end mirror was done. (The size of the fluorescent
spots of the dye jet focused by the folding mirror and end mirror
were minimized and coincided). The threshold pump power was 0.5W
(power density=600 kW/cm2) at which the lasing action
The wavelength selective element
used for tuning was a three- plate Birefringent filter. It consists
of a crystalline quartz plate cut parallel to the optic axis.
It is positioned at Brewster angle inside the cavity. Tuning
is achieved by rotating the plate around an axis normal to the
plate. The Birefringent filter changes polarization to an extent
dependent on wavelength. If the cavity allows oscillation only
of the linearly polarized light, the laser can oscillate only
at the wavelength at which the Briefringent filter transmits linearly
polarized light. The laser was tunable over the wavelength range
567 - 632nm (Rh6G conc = 1g/L). Efficiency defined as the ratio
of output power to input power was measured at the peak of the
tuning curve to be 15% with Birefringent filter. At 5.5 W of
multiline blue-green pump power, the output power measured was
The output of the broad band
CW dye laser is a group of discrete frequencies whose envelope
has an effective linewidth. The linewidth of the CW dye laser
consisting of 3- plate Birefringent tuner was measured to be ~20
GHz using a Fabry-Perot etalon of FSR ~100 GHz. The broad line
width of ~20 GHz was frequency narrowed by inserting a pair of
thin solid etalons in the CW dye laser cavity. The subsequent
linewidth measurements were done with a Spectrum Analyser (FSR=7.5
GHz, F=240) interfaced to the computer. A software program "SPECMASTER"
enabled an on-line display of the frequency spectrum on the monitor.
The results obtained show a linewidth of 400 MHz for etalons
of thickness 0.5mm and 10mm and the output power of 200 mW was
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