Laser News, Vol.9, No.3, July 1998

An Indigenous Narrow Band CW Dye Laser Oscillator

Sunita Singh and G. Sridar

Laser & Plasma Technology Division

B.A.R.C., Trombay, Mumbai


Dye lasers are excellent tunable light sources due to the broad fluorescence spectrum of the active medium. In particular, tunable narrow-band dye lasers find applications in the field of spectroscopy [1], photochemistry and isotope separation [2,3].

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 [4] and generate an optical quality laminar flow of Rh6G dye dissolved in viscous solvent ethylene glycol.

Lasing Action

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 just starts.

Tuning And Efficiency


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 800 mw.

Linewidth Narrowing & Measurement

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 obtained.


References


[1] T.W.Hansh, I.S.Shahin, A.L.Schawlow : Nature 253, 63 (1972).

[2] R.V.Ambartzumian, V.S.Letokhov, Appl. Optcs, 11, 354 (1972).

[3] V.S.Letokhov : Science 180, 451 (1973).

[4] S.Singh, R.Khare, L.G.Nair, U.K.Chatterjee Proc. NLS, BARC, (Jan 17-19, 1996).