1. What is the pumping source in Ruby laser?

1. Electrical Pumping
2. Optical
3. Chemical
4. None of the above

II. What is the output in wavelength of ruby laser?

1. 6943 angstroms
2. 6328 angstroms
3. 5400 angstroms
4. 8000 angstroms

III. Is Ruby laser a gas laser?

1. True
2. false

IV. What is the pumping source in Helium – Neon laser?

1. Electrical Pumping
2. Optical
3. Chemical
4. None of the above

V. What is the output in wavelength of Helium-Neon laser?

1. 6943 angstroms
2. 6328 angstroms
3. 5400 angstroms
4. 8000 angstroms

VI. Ruby laser is a ___________ level laser.

1. Three
2. Four
3. Five
4. Two

VII. Helium-Neon laser is a ___________ level laser.

1. Three
2. Four
3. Five
4. Two

VIII. In LASER, S stands for:

1. Spontaneous
2. Stimulated
3. Simultaneously
4. None of the above

IX. Laser medium in Ruby laser is:

1. Aluminium oxide
2. Chromium oxide
3. Chromium oxide doped with aluminium ions
4. Aluminium oxide doped with chromium ions

X. Which laser has continuous output?

1. Ruby laser
2. Helium-Neon laser
3. Both
4. None of the above

Answers: I. (2), II (1), III(2), IV(1), V(2), VI(1), VII (2), VIII (2), IX (4), X(2) The above are the 10 important MCQs of laser, Ruby laser and Helium-Neon laser.

The post 10 important MCQs of laser, ruby laser, and helium-neon laser first appeared on Winner Science.

As we have discussed already that how a hologram is formed and there are two processes recording and reconstruction during this process. As recording process in holography is based on the principle of interference. For recording and for sustained interference, the path difference between various interfering light waves should always be less than the longitudinal coherence length. For ordinary light sources like mercury the coherence length is very small (≈3cm). The path difference introduced between light waves reflected from different points of an object can be much more than this value. Thus interference patterns cannot be recorded. While coherence length for laser source can be as high as 600 km. As a result, sustained interference patterns will be recorded on the hologram. Thus holograms cannot be made without a laser source.

              Further, if a hologram is reconstructed using ordinary light, then the reference beam, converging rays (forming real image), and diverging rays (forming virtual image), all will be in the same direction. This creates a problem in observing the three-dimensional images, as we have to see through the other two beams. Holography using ordinary light is called ‘Inline Holography’. If a Laser source is used instead, then the three emerging beams will be in different directions. This is called ‘Off-Line Holography’. It allows us to observe one kind of beam at a time.

This is the reason that a laser is needed in holography.

The post Why laser is needed in holography first appeared on Winner Science.

In one of my articles, I have discussed holograms, how a hologram is made and what is holography? Today, we will discuss two types of the hologram, a) Reflection Hologram b)Transmission Hologram. Let us discuss them one by one:

Reflection hologram:

If recording material in the hologram is placed such that reference beam and object beam approach it from two opposite sides, then hologram formed is called Reflection Type hologram. The interference fringes are usually parallel to the surfaces of the recording medium. When such a hologram is reconstructed, then the reference beam and object beam lie on the same side of the hologram.

 Transmission hologram:

If the recording material in the hologram is arranged such that the reference beam, as well as the object beam, approaches it from the same side, then the hologram formed is called Transmission Hologram. The interference fringes are generally perpendicular to the surfaces of the recording material. When such a hologram is reconstructed, then reference and object beam lie on opposite sides of the hologram.

I hope so, you have understood the types of holograms. Please go through the article related to the recording and reconstruction process of holograms also to understand the complete process of hologram and holography.

The post Reflection and Transmission Hologram first appeared on Winner Science.

Let N₁ be the number of atoms per unit volume in the ground state E₁ and these atoms exist in the radiation field of photons of energy E₂-E₁ =h v such that the energy density of the field is E.

Einstein Coefficient for Stimulated Absorption:

Let R₁ be the rate of absorption of light by E₁ -> E₂ transitions by the process called stimulated absorption (Refer below figure):

This rate of absorption R₁ is proportional to the number of atoms N₁ per unit volume in the ground state and proportional to the energy density E of radiations.

That is                                    R₁∞ N₁ E

Or                                          R₁ = B₁₂N₁ E                                               (1)

Where B₁₂ is known as the Einstein’s coefficient of stimulated absorption and it represents the probability of absorption of radiation. Energy density e is defined as the incident energy on an atom as per unit volume in a state.

Einstein Coefficient for Spontaneous Emission:

Now atoms in the higher energy level E₂ can fall to the ground state E₁ automatically after 10^-8 sec by the process called spontaneous emission (Refer below figure).

The rate R₂ of spontaneous emission E₂-> E₁ is independent of energy density E of the radiation field.

R₂ is proportional to number of atoms N₂ in the excited state E₂ thus

R₂∞ N₂

R₂=A₂₁ N₂ (2)

Where A₂₁ is known as Einstein’s coefficient for spontaneous emission and it represents the probability of spontaneous emission.

Einstein Coefficient for Stimulated Emission:

Atoms can also fall back to the ground state E₁ under the influence of the electromagnetic field of an incident photon of energy E₂-E₁ =hv by the process called stimulated emission (Refer below Figure):

Rate R₃ for stimulated emission E₂-> E_1­ is proportional to energy density E of the radiation field and proportional to the number of atoms N₂ in the excited state,thus

R₃α N₂ E

Or                   R₃=B₂₁N₂ E                                                       (3)

Where B₂₁ is known as the Einstein coefficient for stimulated emission and it represents the probability of stimulated emission.

Einstein Coefficient Relation Derivation:

In steady-state (at thermal equilibrium), the two emission rates (spontaneous and stimulated) must balance the rate of absorption.

Thus                                                         R₁=R₂+R₃

Using equations (1,2, and 3) ,we get

N₁B₁₂E=N₂A₂₁+N₂B₂₁E

Or            N₁B₁₂E –N₂B₂₁E=N₂A₂₁

Or         (N₁B₁₂-N₂B₂₁) E =N₂A₂₁

Or           E= N₂A₂₁/N₁B₁₂-N₂B₂₁

= N₂A₂₁/N₂B₂₁[N₁B₁₂/N₂B₂₁ -1]

[by taking out common N₂B₂₁from the denominator]

Or                        E=A₂₁/B₂₁ {1/N₁/N₂(B₁₂/B₂₁-1))                                         (4)

Einstein proved thermodynamically, that the probability of stimulated absorption is equal to the probability of stimulated emission. thus

B₁₂=B₂₁

Then equation(4) becomes

E=A₂₁/B₂₁(1/N₁/N₂-1)                                                       (5)

From Boltzman’s distribution law, the ratio of populations of two levels at temperature T is expressed as

N₁/N₂=e^(E₂–^E₁^)/KT

N₁/N₂=e^hv/KT

Where K is the Boltzman’s constant and h is Planck’s constant.

Substituting value of N₁/N₂in equation (5) we get

E=  A₂₁/B₂₁(1/e^hv/KT-1)                                                        (6)

Now according to Planck’s radiation law, the energy density of the black body radiation of frequency v at temperature T is given as

E =    8πhv³/c³(1/e^hv/KT)                                                                  (7)

By comparing equations (6 and 7),we get

A₂₁/B₂₁=8πhv³/c³

This is the relation between Einstein’s coefficients in laser.

Significance of Einstein coefficient relation: This shows that the ratio of Einstein’s coefficient of spontaneous emission to the Einstein’s coefficient of stimulated absorption is proportional to the cube of frequency v. It means that at thermal equilibrium, the probability of spontaneous emission increases rapidly with the energy difference between two states. If you want to learn derivation of Einstein Coefficient relations, then following is our youtube video also:

The post Einstein Coefficient Relation first appeared on Winner Science.

As already discussed in my earlier articles, the full form of laser is “Light amplification by stimulated emission of radiation”. Therefore it means that laser light is amplified through a process called stimulated emission. Thus it means that stimulated emission is that process through which laser can be achieved. That is the reason the stimulated emission is known as the principle of laser.

Concepts to learn how does a laser work:

1. Stimulated absorption: In this stimulated means “with the help of external source” and absorption means “to gain”. Thus in this process the atoms in the ground state absorb energy and get excited after absorption of energy. The stimulated absorption process with proper figure is already discussed in detail.

2. Spontaneous emission: Spontaneous means “by its own” and emission means “to release”. The excited atom will de-excite naturally after approximately 10^-8 seconds. Whenever the atom de-excited, it released the energy (gained during stimulated absorption) in the form of photons. The photons emitted will be in a random direction. After how much time and what is the spontaneous emission, then read the article of spontaneous emission process with the proper figure.

3. Stimulated emission: stimulated means “with the help of external source” and emission means “to release”. The excited atom will de-excite with the help of an external source after some time. Whenever the atom de-excited, it released the energy (gained during stimulated absorption) in the form of photons. But here the photons will be released with the help of an external source and that external source is photon emitted during spontaneous emission. This photon and the emitted photon will be in the same direction. This process will continue and the population of photons is increased with the help of an optical resonator system. If you want to know why the Stimulated emission is the principle of laser, then read the article of stimulated emission process with proper figure.

4. Types of laser:

a) Ruby laser (The construction of ruby laser and working of ruby laser)

b) Helium-neon laser (The construction of helium-neon laser and working of helium-neon laser)

c) carbon-dioxide laser (The construction of carbon dioxide laser and working of carbon dioxide laser)

d) Semiconductor laser (The construction of semiconductor laser and working of semiconductor laser)

e) Dye laser

There are other types of lasers also.

5. Three main components of laser device are (construction of laser):

a)      Active medium:

For example in ruby laser, the active medium is aluminum oxide doped with .05 percent of chromium ions.

In helium-neon laser it is the combination of helium and neon in the ratio of 10 :1.

b)      Pumping source (How pumping is done):

For example in ruby laser, the pumping source is optical pumping that is helical xenon flash tube.

In helium-neon laser, electrical pumping that is electric discharge is used for stimulated absorption.

c)      Optical resonator system:

In most of the systems, it consists of a system of two mirrors. One mirror is fully reflective and the other is partially reflective.

5. The examples of how does a laser work:

How can a laser work be better understood with the help of construction of ruby laser and working of ruby laser with proper figures?

For revision one can also read the construction of helium-neon laser and working of helium-neon laser or construction of carbon dioxide laser and working of carbon dioxide laser with proper figures.

I hope after reading the articles, you will know to be able to understand and explain how does a laser work.

In case of any doubt, please post in the comment section.

The post How does a laser work first appeared on Winner Science.

Do you know what is the difference between holograms and photographs? If not, then let us discuss the difference between a photograph and a hologram:

Difference between Holography and Photography

1. In photography, only intensity is recorded so photography produces a two-dimensional picture of the object whereas in holography, both intensity, as well as phase of light wave, is recorded, thus holography gives a three-dimensional picture of the object.

2. Negative is prepared first in photography whereas in holography no negative is required. The hologram is negative and the image it gives is positive.

3. If the hologram is broken into parts, each part is capable of reconstructing the entire object. But in photography, the destruction of even a very small portion of negative or photography results in an irreparable loss of information.

4. Holography has high information capacity as compared to photography.

These are the major differences between a hologram and a photograph. If you know more, please share in the comments section.

The post Difference beween holography and photography first appeared on Winner Science.

1. Recording of the hologram.

The recording of holograms 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 a reference beam. While other part of splitted beam  (transmitted from beam splitter) strikes on the photographic plate after suffering reflection from the various points of the object. This beam is called an object beam.

The object beam reflected from the object interferes with the reference beam when both the beams reach the photographic plate. The superposition of these two beams produces an interference pattern (in the form of dark and bright fringes) and this pattern is recorded on the photographic plate. The photographic plate with a recorded interference pattern is called a hologram. The photographic plate is also known as the Gabor zone plate in honor of Denis Gabor who developed the phenomenon of holography.

Each and every part of the hologram receives light from various points of the object. Thus, even if the hologram is broken into parts, each part is capable of reconstructing the whole object.

Figure: Recording process in holography: Construction of Hologram

2. Reconstruction of image

In the reconstruction process, the hologram is illuminated by a laser beam and this beam is called a reconstruction beam. This beam is identical to the reference beam used in the construction of the hologram.

The hologram acts as a diffraction grating. This reconstruction beam will undergo a phenomenon of diffraction during passage through the hologram. The reconstruction beam after passing through the hologram produces a real as well as a virtual image of the object.

Figure: Reconstruction process in Holography

One of the diffracted beams emerging from the hologram appears to diverge from an apparent object when projected back. Thus, a virtual image is formed behind the hologram at the original site of the object and a real image in front of the hologram. Thus an observer sees light waves diverging from the virtual image and the image is identical to the object. If the observer moves round the virtual image then other sides of the object which were not noticed earlier would be observed. Therefore, the virtual image exhibits all the true three-dimensional characteristics. The real image can be recorded on a photographic plate.

Kindly Read This Also :

Note from winner science: If you want the e-notes of all the laser documents that will include the basics of lasers like stimulated absorption, the difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking, then please contact winnerscience@gmail.com. You can post your queries also there.

The post Recording and reconstruction process in hologram first appeared on Winner Science.

An ordinary photograph records the two-dimensional image of the picture because it records only the amplitude or intensity distribution. But in the holography technique, both, the intensity as well as phase of the light wave is recorded.

In holography, the light waves reflected from an object are 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 an optical code.

The formation of the hologram is done by a process called the recording process. The formation of three- a dimensional image from a hologram is done with a process called the reconstruction process.

Thus  holography consists of two processes :

I  Recording of hologram

II Reconstruction of image

Note: Next time I will discuss these 2 processes in detail.

The post Meaning of Holography first appeared on Winner Science.

a) Holography:

Holography is a technique to record the complete picture of an object, that is it will produce a 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, the laser can travel long distances, without appreciable spreading. The time taken by a 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.

c) Applications in scientific research:

Due to the coherent nature of laser light, many new optical phenomena have been observed using a laser. Using laser light we investigate the basic laws of interaction of atoms and molecules with electromagnetic waves.

d) Application in communication:

 In the fiber communication system, the laser beam is used. The rate at which information is transmitted is proportional to the bandwidth of the information carrier signal.

The bandwidth is proportional to the frequency of the carrier. Since the frequency range of laser signals is quite high compared to microwaves, large bandwidth can be obtained using the optical region as compared to the microwave region.

e) Applications in Industry:

Due to the high intensity of the laser beam, the laser can be used in welding, cutting, and in producing very high temperatures. The other advantage of a laser is that the beam can be focused onto a fine spot. The small spot size implies that high energy densities are possible. Lasers are also found suitable for machining and drilling holes.

f) Lasers in Isotope separation:

The light emerging from a laser is extremely monochromatic. When laser light falls on a mixture of two isotopes, the laser light excites the atoms of only one of the isotopes thus separating it from the other isotope.

In addition to the high monochromaticity, the high intensity of the laser is also responsible for its application for isotope separation because with low-intensity beams the separation rate would be too low for practical use.

g) Applications in Medicine:

i) Laser Surgery: The focused laser beam is capable of bloodless surgery since the beam not only cuts but also welds blood vessels being cut. Laser surgery is painless because operations are very fast and there is not enough time for the patient to respond to the incision and sense pain.

ii) Lasers in Opthalmology:  Lasers are used for several years to treat the detachment of the retina. The beam is focused on a certain point of the retina after it has passed through the lens of the eye and vitreous chamber without being absorbed in them. The green beam of laser is strongly absorbed by the red blood cells of the retina and the consequent thermal effects lead to reattachment of the retina. The operation is carried out by a 0.01-sec pulse and being very short, is virtually painless. Other illnesses treated by the focused laser beam are cataracts, tumors, and glaucoma.

iii) Laser Therapy:  He-Ne laser has produced a curing effect on trophic ulcers, poorly healing wounds, and bone fractures. The laser can also be guided through optical fiber into blood vessels to remove the clothing, in the case of heart patients, through heating. It has also found application in treating decaying teeth. The laser can replace dental drills.

The above are important applications of lasers in different fields, if you know more, please share in the comments section.

The post Applications of lasers first appeared on Winner Science.

Electro-optical Shutters.

To obtain faster switching, the suitable electro-optical effects of altering the refractive index of a cell by applying an electric field is used. Two such effects are:

i)   Pockels effect

ii)  Kerr effect

i) Pockels effect.

A crystal cell like potassium dihydrogen phosphate (KDP) is a device which when subjected to an applied DC voltage, becomes birefringent. Birefringent crystal is those crystal which split the light wave into two waves travelling at different velocities that is the crystal offers  different refractive  indices for different  polarized light. One wave is called ordinary wave having refractive index n₀ and other is called extraordinary wave having refractive index n_e. In Pockels effect, the refractive index of material varies linearly with the applied electric field.

In the laser arrangement designed for Q-switching with a Pockels cell  there, in between the active medium and the output reflecting  mirror, there are two polarisers (P₁ and P₂ ) oriented in the same sense that is transmitted light is polarized in the same plane S. The pockels cell is placed between the polarisers P1 and P2.

In the polarizer-Pockels cell-polariser combination, the light  travelling to the cell has to pass a polariser P1 and becomes polarised in the S plane.  When no voltage is applied to the Pockels cell that is when the cell is OFF, the light traverses the polariser and the cell without any loss  except the loss suffered due to absorption.

When voltage is applied to the Pockels cell that is when the cell is ON, then the cell becomes birefringent and rotates the polarisation vector  of the light  passing  through it by 90⁰.

S1 is the new plane of polarisation and is perpendicular to the orientation of polariser P2.  Thus, it will not allow the  light to pass through output reflecting mirror.

Therefore, when the cell is ON, the passage of light is blocked. In other  words, laser cavity Quality factor Q is maximum with the Pockels cell OFF and minimum when the cell is ON.

The switching rate depends on the frequency of the voltage applied to the cell. A Pockel cell requires a small voltage of 1-5 KV. The switching time is about 10^-9 sec which is smaller than that with the mechanical shutters. Pulses of 10^-8 sec duration with peak power  of 10⁶ to 10⁹ watts can be generated  with this method.

Above discussion shows that the Pockels cell is used as shutter that is why the name of the process is electro-optical shutter.

ii) Kerr effect.

This effect is different from the Pockel effect in the sense that in Kerr  effect the  birefringence  is  proportional to the square of the applied electric field or in other words, in Kerr effect, the refractive index varies as the square of the electric field intensity.

The process of Q-switching is same as discussed in  Pockels  effect.

Here as the name suggests, Kerr cell is used as electro- optical shutter.

The voltage requires for operation is about 10-20 KV.

Example of the Kerr cell is Nitrobenzene cell.

This is about Electro-optical shutters technique in q switching. I hope you have understood the same.

The post Electro-optical shutters technique in q switching first appeared on Winner Science.