Q Switching

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 : Continue reading “Q Switching”

Ouput and applications of dye laser

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. Continue reading “Ouput and applications of dye laser”

Working of dye laser

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. Continue reading “Working of dye laser”

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

Dye laser and its construction

Dye lasers use liquid organic dyes. These organic dyes are dissolved in solvents like water, ethyl  alcohol, methanol. The most widely used dye is rhodamine-6G, also known as Xanthene dye. These lasers are discovered by Sorokin and his colleagues. Dye lasers operate without the intervening metastable state.

CONSTRUCTION

The dye laser consisted of a 1cm long quartz glass tube filled with solutions of organic  dyes such as rhodamine- 6G.
Active Medium. Rhodamine-6G dissolved in a suitable liquid like water, ethyl alcohol or methanol is used as the active medium. The dye solution used in the dye lasers typically has a concentration in the range of 10-2 to 10-4 M. Rhodamine-6G emits in yellow-red region.

Pumping source: Energy to excite the dye is supplied by a strong light source that may be a flash lamp or another laser like N2 laser or argon-ion laser. Thus, optical pumping is used to excite the dye and to achieve population inversion.

Optical resonator System. 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 article.

Kindly Read This Also :

Note from winnerscience: If you want the e-notes of all the laser documents that will include the basics of lasers like stimulated absorption, 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.

Output, use and disdavantages of semiconductor laser

Output of semiconductor laser: The output powers of about 10mW are achieved in continuous wave operation and in pulsed opration the peak power runs to 100W. It has output wavelength from 8200 to 9000 angstroms. So the wavelength of semiconductor laser lies in the infrared region.

Advantages/Applications: Semiconductor lasers are compact and have efficiency of about 50-60%. They can be used as sources for light wave communication systems. They  expected to find applications in optical radar equipment and space communications.

Disadvantages. It is difficult to control the mode pattern and mode structure of the semiconductor laser action due to small size of laser region that is junction region.

As in semiconductor, the laser emission occurs between two bands of energies instead of two well defined energy levels (like in He-Ne laser), thus the laser emission is not as monochromatic as that from a gas laser.

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.

Working of semiconductor laser

In my earlier articles, I have explained the construction of semiconductor laser. Toady I will discuss the working of semiconductor laser.

Achievement of population inversion: When p-n junction diode is forward biased, then there will be injection of electrons into the conduction band along n-side and production of more holes in valence band along p-side of the junction. Thus, there will be more number of electrons in conduction band comparable to valence band, so population inversion is achieved.

Figure: Energy level diagram of semiconductor laser (No biasing)

Figure: Energy level diagram of semiconductor laser (with biasing) Continue reading “Working of semiconductor laser”

Construction of semiconductor laser

In the last article, I have explained the basics of semiconductor. Today I will explain the construction of a semiconductor laser.

Example of semiconductor laser: One of the examples of semiconductor lasers is gallium arsenide (GaAs). It is heavily doped semiconductor. Its n-region is formed by heavily doping with tellurium in a concentration of 3 x 1018 to 5 x 1018 atoms/cm3 while its p-region is formed by doping with zinc in concentration around 1019 atoms/cm3.

Active medium:

Continue reading “Construction of semiconductor laser”

Semiconductor laser

Before discussing the construction and working of semiconductor laser, let us discuss the review of semiconductors as it will be necessary for you to write the basics of semiconductors in case of exams:

A semiconductor is a material whose conductivity lies between those of conductor and insulator. Semiconductors are of two types:

a) Intrinsic semiconductors or pure semiconductors

b) Extrinsic semiconductors or doped semiconductors

Extrinsic semiconductors are further classified into two types depending upon the type of majority carriers:

i) n-type semiconductors where electrons are majority carriers.

ii) p- type semiconductors where holes are majority carriers.

When a p-type semiconductor and a n- type semiconductor is joined by special techniques, there will be flow of electrons from n side to p side and flow of holes from p side to n side. After some time, an electric field will be created which will oppose this flow and flow stops. Thus, there will be formation of depletion region. This region is called so because it is depleted from charge carriers.

Note: In next article I will explain the construction of semiconductor laser. In case of any problem in this article or any other physics article, kindly post in the comment section.

Reference: This article is referred from my authored book “optics and lasers” having ISBN 978-81-272-2948-2

wavelength, output and applications of carbon dioxide laser

Why helium is doped in carbon dioxide laser:

The CO2 molecules in the states E4 and E3 deexcite to state E2 through inelastic collision with unexcited CO2 molecules. This process is very fast so there will be accumulation of CO2 in this level and they can break the population inversion in upper levels because there is probability of excitation of molecules from E2 to E3 and/ or E4.

To stop the accumulation of CO2 molecules in E2 special additives like He and water vapors are added into the gas mixture. CO2 molecules return to the ground state E1 through collisions with the He to which it transfers the excitation energy. Continue reading “wavelength, output and applications of carbon dioxide laser”