DYE LASERS Mehmet Mustafa KARABULUT

TABLE OF CONTENTS 1. Working Principles 2. CW and Pulse Modes 3. Applications 4. Properties 5. Recent Improvements 2

1. Working Principles Lasing medium is organic dye mixed with a liquid solvent Liquid solvent is generally ethyl or methyl alcohol, glycerol, or water Wider range of wavelengths compared with gasses and solid state mediums. Suitable for tunable lasers and pulsed lasers. 3

1. Working Principles The dye solution is usually circulated at high speeds to avoid triplet absorption A high energy source of light is used to 'pump' the liquid A fast discharge flash lamp or an external laser is usually used for pumping purpose 4

1. Working Principles Mirrors are needed to oscillate the light produced by the dye’s fluorescence The light is amplified with each pass through the liquid. A prism or diffraction grating is usually mounted in the beam path, to allow tuning of the beam. 5

2. CW and Pulse Modes Dye lasers can be operated pulsed or cw Pulsed laser action has been obtained from very many different dyes by using one of the following pumping schemes: ◦ Fast and intense flashlamps, with pulse duration usually less than 100 microseconds ◦ Short light pulses from another laser. 6

2. CW and Pulse Modes In both cases, the short pulse duration serves the purpose of producing laser action before an appreciable population has accumulated in the triplet state And before the onset of refractive-index gradients in the liquid. 7

2. CW and Pulse Modes For flashlamp pumping, linear lamps in an elliptical-cylinder pumping chamber have been used The liquid containing the active medium flowing through a glass tube placed along the second focal line of the ellipse. 8

2. CW and Pulse Modes 9

For pulsed laser pumping, nitrogen lasers are used, its UV output beam being suitable for pumping many dyes that oscillate in the visible range. To obtain more energy and higher average power, the more efficient excimer lasers (in particular KrF and XeF) are being increasingly used as UV pumps 10

2. CW and Pulse Modes For dyes with emission WL longer than nm, Nd:YAG laser or the green and yellow emissions of a copper vapor laser are being increasingly used. For these visible pump lasers, the conversion efficiency from pump laser to dye laser output is rather higher (30-40%) than that obtained with UV laser pumping (10%). 11

2. CW and Pulse Modes Furthermore, dye degradation due to the pump light is considerably reduced. 12

2. CW and Pulse Modes This is a transverse pump configuration. The direction of the pump beam is orthogonal to the resonator axis 13

2. CW and Pulse Modes The laser pump beam is focused by the lens L, generally a combination of a spherical and cylindrical lens, to a fine line along the axis of the laser cavity. 14

2. CW and Pulse Modes The length of the line focus is made equal to that of the dye 15

2. CW and Pulse Modes Transverse dimensions are generally less than 1mm. to tune the output wavelength within the wide emission bandwidth of a dye (30–50 nm) 16

2. CW and Pulse Modes To tune the output WL within the wide emission bandwidth of a dye (30–50 nm) diffraction grating is inserted in the laser cavity 17

2. CW and Pulse Modes Tuning is achieved by rotating the mirror labeled as mirror 2. 18

2. CW and Pulse Modes For continuous laser pumping, Ar + lasers (and sometimes also Kr + lasers) are often used. To achieve a much lower threshold, as required for cw pumping, the near-longitudinal pumping configuration is used. 19

2. CW and Pulse Modes The liquid dye mediumis in the form of a thin jet stream ( 200 micro mm thickness) freely flowing in a plane orthogonal to the plane of the figure and inclined at Brewster’s angle relative to the dye-laser beam direction 20

2. CW and Pulse Modes The laser beam is linearly polarized with its electric field in the plane of the figure. For laser tuning, a birefringent filter may be inserted within the laser cavity. 21

2. CW and Pulse Modes The laser beam is linearly polarized with its electric field in the plane of the figure. For laser tuning, a birefringent filter may be inserted within the laser cavity. 22

2. CW and Pulse Modes For femtosecond pulse generation a colliding pulse mode locked (CPM) laser configuration is generally used 23

3. Applications By virtue of their wavelength tunability, wide spectral coverage, and the possibility of generating femtosecond laser pulses, organic dye lasers have found an important role in many fields: Medicine: Treatment of diabetic retinopathy or treatment of several dermatological diseases Spectroscopy: Narrow band, down to single mode, tunable source of radiation for high-resolution frequency-domain spectroscopy, or as femtosecond-pulse generators for high resolution time-domain spectroscopy 24

3. Applications Astronomy (as laser guide stars) Atomic vapor laser isotope separation Manufacturing 25

4. Properties Laser dyes usually belong to one of the following classes: 1. Polymethine dyes, which provide laser oscillation in the red or near infrared.0.7–1.5 m/. 2. Xanthene dyes, whose laser operation is in the visible. 3. Coumarin dyes, which oscillate in the blue- green region (400–500 nm). 26

5. Recent Improvements Dye lasers are not widely used lasers. Therefore there is no improvements actually. 27