In dye laser, organic dyes are used as an active medium. Organic dyes are colored substances having the ability to impart colors to liquids, gases, and solid. Examples of organic dye molecules are rhodamine 6G (Xanthene dye), Coumarin, oxazines, anthracenes, etc.
Although organic dye molecules are very large, only the small part of the molecule consisting of an alternating structure of single and double carbon to carbon Bond ( called chromophore ) is used as the active or lasing medium in a laser. The part of the dye molecule used as a lasing or active medium is represented as :
C=C – C=C – C
Laser dyes are dissolved in solvents such as water, benzene, methanol, toluene, Acetone, etc. The ratio of organic dye molecule to the solvent is about 1:10000 or more so that each dye molecule is surrounded by the solvent molecules.
Construction and Working of Dye Laser
Construction of Dye Laser
The molecules of an organic dye laser have a singlet ( S0, S1, and S2) and triplet ( T1 and T2 ) Electron states. Each electronic state consists of many Vibrational states. While each vibrational state contains several rotational States. The dense collection of rotational States from a continuum of Levels between the vibrational states.
Optical Pumping in the dye molecules from the ground state to the higher vibrational rotational Levels of singlet state S1. Due to thermal redistribution in the state S1, most of the dye molecules jump down to the lowest vibrational state of singlet state S1.
This happens in about 10–11. When the molecules jump from the lowest vibrational state of S1 to the highest vibrational state of S0, radiation is emitted which is known as fluorescence. The energy level diagram of an organic dye laser in solution is in the figure.
A non-radioactive transition can also take place from state S1 to state T1. This is known as Intersystem crossing. This transition process determines the laser action.
It is because
- This transition reduces the population of the molecules in state S1 which is the upper laser level.
- The overlapping of the absorption spectrum T1 – T2 on the emission spectrum S1 – S0 causes the loss at the wavelength corresponding to laser emission. The radiative process from T1 to the ground state S0 is called Phosphorescence. This transition cannot be used for laser emission due to strong triplet-triplet absorption.
Therefore, the threshold level should be achieved for a good laser action before a significant number of molecules drop to the state T1. This condition can be achieved by using a flash lamp as a source of excitation. A flash lamp excites molecules from S0 to S1much faster than non-radiative transition to state T1 from S1.
Working of Dye Laser
To operate a dye laser, an elliptical resonant cavity or resonator is used. The dye cell is placed at one of the foci and the flash lamp is placed at the other foci in the elliptical resonant cavity. The light emitted by the flash lamp is focused on the cell.
The lights falling on the dye cell causes stimulated emission inside the dye. The emission of radiation due to stimulated emission exists in all directions but the radiation ( i.e photons ) is only amplified along the axis of the cavity formed between highly reflecting mirror and semi-transparent mirror.
Tuning of Dye laser Can be done using various techniques: One of the commonly used techniques is to send a selective wavelength through the Dye. In this case, the totally reflecting mirror of the cavity is replaced by the diffraction grating.
The arrangement for tuning the wavelength of dye is shown in the figure. The light from a nitrogen laser is made to fall on the front window of the dye cuvet. The concentration of the dye is adjusted in such a way that all the light falling on it is absorbed within a few Millimetres of the front window of the dye cuvet.
The fluorescence from this small area is then reflected back into the dye cell by the diffraction grating and the partially reflecting mirror. As a result, Laser emission takes place. The wavelength of the dye laser is selected by adjusting the diffraction grating with the help of a micrometer.
This wavelength of light amplifies in the resonant cavity formed by the diffraction grating and partially reflecting mirror. The desired wavelength comes out of the partially reflecting mirror which is refracted by the completely reflecting mirror. Hence, the desired wavelength of the dye laser is obtained.
Characteristics of Dye Laser
|2.||Active medium||Organic dyes are used as an active medium.|
|3.||Wavelength of output||Vary from 390 to 1000 nm.|
|4.||Output power||1 watt.|
|5.||Output beam diameter||0.5mm.|
|6.||Beam divergence||0.8 to 2 milliradians.|
Applications of dye laser
- The main advantage of these lasers is that they can generate a laser of any frequency in the Infrared, visible, or near-ultraviolet region. Hence, they are named tunable laser.
- Organic dye laser being tunable have a number of applications in Spectroscopy, holography, and biomedical science. The most important application of these tunable lasers in isotope separation.
Advantages of Dye Laser
- Beam diameter is very less.
- Construction is very simple.
- High output power.
- Higher efficiency of 25%.
- Its beam divergence is very less.
- It is available in visible form.
Disadvantages of Dye Laser
- The cost is very high.
- To tune at a single frequency, the laser is used birefringent or filter making it more costly.
- It has a complex chemical formula.