why dye lasers are also known as tunable lasers

The most useful feature of dye lasers is their tunability.  The tunability means that the lasing wavelength for a dye may be varied over a wide range.  Due  to this   reason, dye lasers are also called tunable lasers. Tuning over 500 angstrom has been obtained.

One of the teachniques to obtain tuning is to replace one of the mirrors  of the resonant cavity with a diffraction  grating. Thus a dye cell is usually placed inside a cavity consisting of a partially  reflective  mirror  on
the  front and a diffraction grating on the rear. A source light is focused onto the  dye to excite it and stimulate laser action.

By rotating the diffraction grating , wavelength of laser output can be altered. Thus tuning is obtained. Therefore, this combination of partially reflective mirror and diffraction grating will act as optical resonator system. For radiation to be reflected back along the  laser cavity axis, the angle θ that the normal to the diffraction grating makes with the cavity must satisfy the condition.

2dsinθ =nλ  (n = 1, 2, 3, …)

Where  d is grating spacing

Λ is wavelength of radiation.

By rotating the grating, angle θ will be changed and thus the output wavelength will be changed that is tuning of output wavelength will be achieved.

Reference: This article is referred from my authored book “Optics and lasers” having ISBN 978-81-272-3833-9. In case of any doubt in this article or any article of Physics, kindly post in the comment section. Try to make the construction figure. I will explain the working in the next articl

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