Explaining the concept of Q - switching and the various methods of Q - switching

 In this blog post, we delve into the fascinating world of Q-switching, a technology that increases the power and efficiency of lasers. Join us as we uncover the concept of Q-switching and explore the diverse methods employed to achieve this extraordinary laser pulsing phenomenon.

Topics covered:

Explain the concept of Q-switching in detail.

Conditions that need to be satisfied for Q-switching. 

The methods of Q-switching - Rotating Mirror Method, Electro-Optic Shutter

Types of Electro-Optic Shutter - Kerr cell, Pockel's cell and Acoustic optic shutter.

        1.     (a) Explain the concept of Q-switching in detail.

As soon as when the laser is turned on, the pumping begins due to which the population inversion reaches the steady state. Meanwhile, the laser beam grows and reaches the saturation intensity. Then it begins to extract the energy from the medium.

If the gain increases than the cavity losses that is been present then by the stimulated emission the potential density is been reduced. And the population inversion reaches new steady state. As the gain is reduced and becomes equal to cavity losses, the time required for the process is (T_s).

In most gain media, the upper laser level lifetime (τ_s) is less than the time required for the gain to reduce and become equal to cavity loss (T_s).

(τ_s) ˂ (T_s)

In the Solid-State laser, the expected upper laser level lifetime (τ_s) is larger than the time required for the gain to reduce and become equal to cavity loss (T_s).

When a solid-state laser operates at laser cavity, the output reaches the saturation intensity long before. The potential density reaches maximum potential. Here, the gain is used is lower than it can be achieved by pumping without cavity. Therefore, it is possible to pump a solid-state laser where the upper laser level lifetime (τ_s) without cavity. There can be a sudden bring back cavity to its position. Here, the highest possible gain is achieved. The gain pulse is the output. Since, the gain increases as per the steady state condition there is a reduction observed by the stimulated emission. Such technique is possible in Q-switching because the cavity changes from low Q to high Q. Where the Q is the ratio of energy stored to energy dissipated in cavity.

Following are the conditions that need to be satisfied for Q – switching;

            (1)   The cavity build up time should be lesser than the upper laser level lifetime.

            (2)   The cavity build up time should be lesser than the pumping flux duration (t_p)

            (3)   The initial cavity loss should be greater than the amplifier gain.

The methods of Q-switching:

            (1)   Rotating Mirror Method : This method of Rotating Mirror was first used for Q-switching. It consists of hexagonal shaped mirror, laser cavity, output mirror and rotating shaft.

The hexagonal shaped mirror is assembled on a rotating shaft which are aligned facing the laser cavity. On every 6^th rotation they are aligned back facing the laser cavity.

The output mirror is located at other end of laser cavity.

The rotating mirror serves as rear mirror to laser cavity. It rotates at frequency related to upper-level laser. The rotating mirror also acts as a shutter for Q- switching.

(2) Electro-Optic Shutter:  Electro-Optic Shutter will be represented as EOS. In the laser cavity the EOS serves as a very fast optical device. it switches from high loss to low loss. The Shutter act as Electro-Optic Crystal, when electric field is applied it becomes bifringent. for operation of EOS there must be a polarizing element which operates by rotating polarization beam. these lasers operate with only one polarization.

When the Voltage is ON, the shutter rotates the polarization beam by 45^o. while passing through the cell it gets reflected from mirror and while passing through cavity in the opposite direction the shutter again rotates by 45^o. The polarization element of amplifier rejects the polarization by 90^o. on removing the cavity it introduces high loss in cavity.

When the Voltage if OFF, the shutter does not rotate the beam, or no polarization rotation happens. as it passes through cell, there is a reflection from mirror, and it passes through the amplifier with no reflective loss. further it has been amplified in gain medium.

Types of EOS - Kerr cell, Pockel's cell and Acoustic optic shutter.

 (a) Kerr cell - when the electric field is applied to normal of the isotropic liquid, to align molecule and to make it doubly refractive. By Kerr electro- optic effect, we can be provided with rotation of laser beam. when the electric field is applied in the direction of the transverse of optical beam. for the Kerr cell we use Nitrobenzene which is considered to be good medium and the main advantage of Kerr cell is it requires less voltage than other cell. 

(b) Pockel's Cell - When the electric field is applied the refractive index changes due to which the pockel's effect is produced. Pockel's Cell provides rotation of polarizing beam. when the electric field is applied in direction, the optical axis of crystal and optical axis of beam changes. this is directly proportional to the thickness of the cell and to the magnitude of the electric field.  In the Pockel's cell, KDP and CDA are used. there is a requirement of lesser voltage to produce Pockel's cell.

(c) Acoustic Optic Shutter (AOS) - Acoustic Optic Shutter uses the Quartz crystal which is installed within cavity. the cavity is situated in Brewster's angle. the crystal in AOS act as piezoelectric transducer. When Radio frequency (RF) is applied to transducer, there are strong acoustic wave which propagates within the crystal. Amplifier is further pumped, where the laser beam is deflected out of laser cavity by the diffraction grating.

when the signal is turned OFF, the beam is allowed to pass through the cavity undeflected which is the main reason for the development of Q-switching pulses. The RF used here, ranges from 25 to 50MHz.

 

NOTE: On the further post we will be learning more about Mode locking, Pulse Shortening technique which also includes the physical interpretation of SPM. 

Questions covering on next post:

• Explain the concept of mode locking.
• Obtain the separation between two successive mode locked pulses.
• What is the significance of ultrashort laser pulses? 
• Explain self phase modulation. How can it be used for pulse shortening?
•  What is group velocity dispersion? Obtain an expression for it and explain its significance for pulse shortening or pulse lengthening.