Q switching in laser

Q switching in laser is a technique of reducing giant Plus ( i.e short duration high energy plus). It is achieved by pumping the gain or active medium in the absence of the mirrors of the cavity or resonator ( i.e the feedback mechanism ) and then the mirrors of the cavity are switched on to get a giant of short duration. The mechanism can be understood as follows:

When a gain medium is pumped and there is no feedback system (i.e Mirrors), the population inversion in the gain medium keeps on a building by raising the atom from the ground state to the excited state. When the value of population inversion is considerably high and the mirror of the resonator or cavity are placed in their positions, then the spontaneous emission is reflected back and forth by the mirrors and passes through the gain medium.

Q switching in laser
Q switching in laser

Because of the large value of population inversion, the amplification provided by the gain medium is one round trip is very large as compared to the loss suffered in one round trip. Therefore, the power of the laser beam grows very fast with every pass in the gain medium. As a result of the population inversion decrease quickly and consequently the power of laser beam decreases. Hence, a giant pulse is generated when the mirrors of the resonator or cavity are switched on suddenly. Under this condition, the Q factor of the resonator or cavity is switched from a low value to high value, and hence the technique is known as Q switching.

In actual practice, a shutter is placed between one of the mirror and one end of the gain medium of the resonator, until the population inversion is much more than the threshold population inversion required for the same laser. Then the shutter is opened rapidly to allow a Large fraction of the stored energy to be released in the form of a single short pulse known as a giant pulse. In fact, opening a shutter switches on the mirror of the resonator for the start of lasing action. 

After generating the giant pulse, the shutter is closed automatically. Again the same process is repeated to generate another giant pulse. The shutter used in this technique can be an electro-optic shutter such as a Pockels cell or a Kerr cell

Laser pulse to which two switching technique is used is known as Q switched laser.

 Types of Q switching in laser

1. Mechanical switching starter

Q switching in laser
mechanical Q switching in laser

In this technique, a totally reflecting mirror of the resonant cavity rotates rapidly about 24000 revolutions about an axis perpendicular to the axis of the cavities, While a partially transparent mirror is kept fixed. The flash of light is allowed to fall on the active medium at the instant when both the mirror are parallel to each other. At this stage, the maximum population inversion takes place and hence giant pulse is produced.

2. Electro-optical switching shutter

This technique makes us and electro-optical effects such as the Kerr effect and Pockels effect to produce a giant pulse.

Kerr effect

John Kerr, a Scottish physicist discovered in 1875 that some isotropic media become birefringent in the presence of an electric field. That is, light polarized in One Direction has a different speed in the medium to the light polarized in a plane perpendicular to that direction. This effect is known as the Kerr effect.

An isotropic medium like nitroBenzene ( C6 h5 NO2 ) becomes birefringent with different refractive Indices for light polarized parallel to or perpendicular to the direction of an applied electric field E.

The difference in refractive index  Δn is given by

                               Δn = KλE2

Where K is Kerrconstant, Lambda is the wavelength of the light and E is the magnitude of the applied field electric field E.

When a material which becomes birefringent under the influence of an electric field is placed between two perpendicular line polarizers, no light will be transmitted if the electric field is switched off. On the other hand, almost all the light Bil be transmitted for a certain value of the electric field. 

A cell containing nitrobenzene between two parallel plates separated by a small distance is known as Kerr cell. This cell operates at a very high voltage (~20,000 volts).

Production of Giant pulse using a Kerr cell

The experimental setup for producing a Giant pulse using a Kerr cell is shown in the figure:

Q switching in laser

The light is polarized by the polarizer. The linearly polarized light falls on the Kerr cell. The birefringent material is in the Kerr cell rotates the linearly polarized light through 90 degrees and the reflected light is blocked by the polarizer. 

At this state, the losses in the resonant cavity will be very large. Now, the voltage applied across the Kerr cell is disconnected, and hence the birefringent material in the cell losses its birefringence property. 

As a result, the cell does not rotate the polarization and hence the cell act as an open shutter. The light is, therefore, transmitted as a giant pulse.

 Pockels effect

The production of birefringence in an optical medium by the influence of a constant or electric field is known as the Pockels effect.

In the case of Pockels effects, the difference in refractive indices of the material for two light waves is directly proportional to the magnitude of the applied electric field E.

A cell containing a material [ say potassium dihydrogen phosphate (KDP) ] obeying Pockels effect is known as Pockels cell. The cell operates at low voltage.

Production of giant Pulse using Pockels cell.

The experimental setup for reducing agent pulse using a Pockels cell is shown in the figure:

Q switching in laser

The light from the laser materials passes through the polarizer P1 and hence it is polarised. The plane-polarized light has two components which are perpendicular to each other. The orientations of the Pockels cell and the polarizer P1 are adjusted in such a way that the plane polarised light becomes elliptically polarized after passing through the cell.

The electric field across the cell is adjusted in such a way that light again becomes plane polarised. But the plane of polarization is perpendicular to the initial plane of polarization. Hence the light does not pass through the polarizer P2. 

At this stage, the cell acts as a closed setter. Now the electric field is switched off, the plane polarised light passes through the cell, and the polarizer P2 without any loss. At this stage, the cell act as an open shutter. The light passes through the partially transparent mirror as a giant pulse. 

Applications

  1. Q switched laser is used in 3D optical data storage and 3D microfabrication.
  2. Q switched laser is also used in metal cutting or pulsed holography.
  3. Used for measurement purposes such as distance measurement (range finding).
  4. Q switched lasers are also used to remove tattoos.
  5. Q switched is used to treat skin-related issues like acne, pigmentation, dark spots, and fixes for anti-aging.

Mode locking

Mode locking is a technique to generate Ultrashort pulses of light of a very short duration [ of the order of the Pico sec or Femto sec ] from a laser beam.

The basic principle of mode-locking techniques is to induce a fixed phase relationship between the long longitudinal modes of the resonant cavity of the laser. Then, the laser is known as phase-locked or mode-locked

In a laser, the number of Modes oscillates independently. That is, large number of modes having different frequencies do not have a fixed phase relationship with each other. In this case, the output is almost the sum of the intensity of each individual mode and the laser output consists of random fluctuation in the industry as shown in the figure.

If there is a fixed phase relationship among various modes, then the mode of a laser will interfere constructively with one another periodically and hence intense pulses of light are produced in other words, if the phases of oscillating modes are locked ( i.e to bring all the modes in phase at any instant ) and maintain this phase relationship, then the modes will interfere constructively with one another periodically. 

Therefore intense pulses of light very short duration are produced. Such a laser is known as mode-locked or phase-locked. The output intensity variation with time of ultra-short pulses of laser is shown in the figure.

TECHNIQUES FOR MODE LOCKING

The techniques for mode-locking are classified as 

  • Active Mode Locking, in which an external signal is used to induce a modulation of the interactive light. 
  • Passive Mode Locking, in which no external signal is used to produce pulses.

Active Mode Locking

For active mode locking, the acousto – optic is used as shown in the figure.

Q switching in laser

Acousto of optic effect is based on the change of the refractive index of the medium in the presence of sound waves in the medium. The acousto optic interaction modulates the laser beam by switching on and off the acoustic field ( or waves). When a caustic field is off, the laser beam remains undeviated and the intensity of the beam at the diffraction angle is zero. However, when the acoustic field is on, the laser beam diffracts, and the intensity of the beam at the diffraction angle increases.

In an acoustic Optic modulator, the wavelength of the frequency of the acoustic or sound wave is kept fixed, and the drive power is changed to vary the amount of light in the diffraction beam.

The acousto optic modulator consists of a polished quartz block that is driven into acoustic resonance by the piezoelectric effect (piezoelectric effect is the phenomenon of developing unlike electric charges on the face of certain crystal like quartz when these crystal are mechanically distorted in particular direction).

 Passive Mode Locking

Passive Mode Locking Passive Mode Locking does not require an external signal to the laser for the production of pulses. This type of mode locking can be obtained by a saturable absorber ( which consists of an organic dye dissolved in an appropriate solvent ). A saturable absorber is a device which behave differently for the light of different intensity passing through it. For passive mode locking, a saturable absorber placed in a laser cavity absorbs the light of low intensity and transmits the light of high intensity.

FAQ on Q switching in laser

1 ) What is Q switching in laser?

Q switching in laser is a technique of reducing giant Plus ( i.e short duration high energy plus). It is achieved by pumping the gain or active medium in the absence of the mirrors of the cavity or resonator ( i.e the feedback mechanism ) and then the mirrors of the cavity are switched on to get a giant of short duration.

2 ) Does Q switch laser help acne?

Yes Q switched laser help in acne. It is used by beautician around the world to treat skin related issues.

3 ) What are Q switched lasers used for?

  1. Q switched laser is used in 3D optical data storage and 3D microfabrication.
  2. Q switched laser is also used in metal cutting or pulsed holography.
  3. Used for measurement purposes such as distance measurement (range finding).
  4. Q switched lasers are also used to remove tattoos.
  5. Q switched is used to treat skin-related issues like acne, pigmentation, dark spots, and fixes for anti-aging.

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