Archive for May, 2011
Last time I have discussed about the holography and written that it is a two stage process. The first stage is recording of hologram in the form of interference pattern and in the second stage, the hologram acts as a diffraction grating for the reconstruction beam and the image of the object is reconstructed for the hologram.
Do you know what is the difference between hologram and photograph? If no, then let us discuss:
Difference between Holography and Photography More >
Last time I have discussed the basic definition of holography and hologram. Today I will discuss the process with the help of which hologram is formed or complete picture is recorded:
1. Recording of hologram. The recording of hologram is based on the phenomenon of interference. It requires a laser source, a plane mirror or beam splitter, an object and a photographic plate. A laser beam from the laser source is incident on a plane mirror or beam splitter. As the name suggests, the function of the beam splitter is to split the laser beam. One part of splitted beam, after reflection from the beam splitter, strikes on the photographic plate. This beam is called reference beam. More >
The word holography originates from the Greek words “holos” (complete) and “graphos” (writing). Thus, it is the technique to record the complete picture of an object. The technique was proposed by Gabor in 1947.
An ordinary photograph records the two dimensional image of the picture because it records only the amplitude or intensity distribution. But in holography technique, both, the intensity as well as phase of the light wave is recorded.
In holography, the light waves reflected from an object is recorded. These light waves consist of intensity and phase and the record is called a hologram. The hologram has no resemblance to the original object but it contains all the information about the object in a optical code. More >
Lasers have applications in almost every field like medicine, industry, communication and science and technology. These applications are due to the directional, coherent and monochromatic properties of lasers.
a) Holography: Holography is a technique to record the complete picture of an object, that is it will produce the three dimensional picture. The process of holography will be discussed in detail later on.
b) Measurement of long distance: The beam spreading in the laser light is very small, laser can travel along distances, without appreciable spreading. The time taken by laser pulse to travel from laser source to a given target and back is measured. As the velocity of light is known, the distance of the target can be calculated using the relation 2d = c x t where d is the distance of the target and c is the velocity of light. More >
In my earlier articles I have discussed the basics of Q-switching and three of its techniques known as mechanical shutters, rotating reflector method and passive shutters. Toady I will discuss the one more following techniques of Q-switching:
To obtain faster switching, the suitable electro-optical effects of altering the refractive index of a cell by applying an electic field is used. Two such effects are:
i) Pockels effect
ii) Kerr effect
i) Pockels effect. More >
Last time I have discussed the basics of Q-switching and one of its techniques known as mechanical shutters. Toady I will discuss two more following techniques of Q-switching:
The rotating reflector method.
In this method of Q- switching, one of the end mirrors of the cavity is replaced with a total reflection prism which spins rapidly around its axis set at right angle to the resonator axis. As the prism revolves, it faces the cavity with its reflecting side and makes the laser cavity quality factor Q high for a short time. When the prism is out of this position, the Q value drops. As it revolves on further rotation, Q value drops to minimum. More >
Q-switching is a technique used to produce a high output pulse. It is accomplished by using a device to prevent the reflection of photons back and forth in the active medium. This produces a higher population inversion in the metastable state. Then suddenly the optical cavity is opened to permit a large fraction of stored energy to be emitted in the form of very intense pulse of laser radiation. Q-switched lasers produce pulses of 10 to 250 nanoseconds.
Q-switching is also known as Q-spoiling.
As the quality factor Q of a laser cavity shows the ability of the cavity to store energy, thus, high Q means that high energy can be stored in the cavity and a low Q means that the cavity will rapidly dissipate its energy. As the technique of Q-switching involves switching the optical cavity quality factor Q from a low to a high value, therefore, it is known as Q-switching.
Techniques of the Q-Switching : More >
I have already discussed the construction and working of dye laser. Let us discuss the output and applications of dye lasers.
Output: The dye laser provides 3nsec pulses in the spectral range of 360 nm to 950 nm. The typical peak powers are on the order of about 10kW to 20kW. Dye lasers can be operated in both pulsed and continue wave (CW) modes. If a flash lamp is used to pump the dye laser, the output will be pulsed one whereas if the laser is pumped by a continuous wave laser like argon-ion laser, the dye laser will also be continuous. More >
Last time I have discussed the construction of dye lasers. Let us discuss the working of dye laser:
The molecules have singlet as well as triplet states. Each electronic state comprises of several vibrational states and each vibrational state comprises of several rotational levels.
Due to absorption of light from pumping source, dye molecules get excited from the ground state E1 to upper vibrational rotational levels of excited state E2 which is upper laser level. Most of the dye molecules decay to the lowest vibrational level L of E2 in a time of about 10-11 seconds. This process is due to thermal redistribution in level E2, thus, it is a non-radiative process. Population inversion is achieved at level L. More >
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