Q switching phenomena study in laser systems
- 1. Q switching Reference: Chapter 8 Principles of Lasers By Orazio Svelto
- 2. Contents • Introduction to control of laser output beam • Q-switching and giant pulses • Methods of Q-switching 1. Active Q-switching (mechanical Q-switching, acousto-optic Q- switching, electro-optic Q-switching) 2. Passive Q-switching (saturable absorber, cavity dumping)
- 3. Q switching • A method for obtaining energetic pulses from lasers by modulating the intracavity losses. • Q-switching, sometimes known as giant pulse formation is a technique by which a laser can be made to produce a pulsed output beam. • The technique allows the production of light pulses with extremely high (gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a continuous wave (constant output) mode.
- 4. Concept • We have seen that, under cw operation, the population inversion gets clamped to its threshold value when oscillation starts. Even under the pulsed operating conditions considered, the population inversion can only exceed the threshold value by a relatively small amount due to the onset of stimulated emission. • Suppose now that a shutter is introduced into the laser cavity. If the shutter is closed, laser action is prevented and the population inversion can then reach a value far in excess of the threshold population for the case where the shutter is absent. If the shutter is now opened suddenly, the laser will exhibit a gain that greatly exceeds losses and the stored energy will be released in the form of a short and intense light pulse. • Since this operation involves switching the cavity Q factor from a low to a high value, the technique is usually called Q-switching. The technique allows the generation of laser pulses of duration comparable to the photon decay time (i.e. from a few nanoseconds to a few tens of nanoseconds) and high peak power (in the megawatt range).
- 5. Methods of Q-Switching • There are several methods that have been developed to achieve Q switching of the cavity. • The most commonly used are: (i) Electro-optical shutters (ii) Rotating prisms (iii) Acousto-optical switches (iv)Saturable absorbers. These devices are generally grouped into two categories, active and passive Q-switches. • In an active Q-switching device, one must apply some external active operation to this device (e.g. change the voltage applied to the electro-optical shutter) to produce Q-switching. • In a passive Q-switch, the switching operation is automatically produced by the optical nonlinearity of the element used (e.g. saturable absorber).
- 6. Electro-Optical Q-Switching • These devices make use of a cell exploiting an electro-optical effect, usually the Pockels effect, to induce the Q-switching. • A cell based on the Pockels effect (Pockels cell) consists of a suitable nonlinear crystal, (such as KD*P or lithium niobate for the visible-to-near-infrared region, or cadmium telluride for the middle-infrared, ) in which an applied dc voltage induces a change in the crystal’s refractive indices. This induced birefringence is proportional to the applied voltage. • Figure 8.5a shows a Q-switched laser using a suitable combination of polarizer and Pockels cell. • The Pockels cell is oriented and biased in such a way that the axes x and y of the induced birefringence are lying in the plane orthogonal to the axis of the resonator. • The polarizer axis makes an angle of 45 degrees to the birefringence axes.
- 7. Process • Consider now a laser beam propagating from the active medium toward the polarizer-Pockels-cell combination with a polarization parallel to the polarizer axis. • Ideally, this beam will be totally transmitted by the polarizer and then incident on the Pockels cell. • The E-field of the incoming wave will thus be at 45 degrees to the birefringence axes x and y of the Pockels cell and can be resolved into components Ex and Ey (Fig. 8.5b) with their oscillations in phase. • After passing through the Pockels cell, these two components will have experienced different phase shifts, giving rise to a phase difference
- 8. Contd..
- 11. Rotating Prisms • The most common mechanical means of Q-switching involves rotating one of the end mirrors of the laser resonator about an axis perpendicular to the resonator axis. • In this case, the high-Q condition is reached when the rotating mirror passes through a position parallel to the other cavity mirror. To simplify the alignment requirements, a 90 degrees roof-top prism with roof edge perpendicular to the rotation axis is often used instead of an ordinary mirror (Fig. 8.6). • Such a prism has the property that, for light propagating orthogonal to the roof edge (see Fig. 8.6), the reflected beam is always parallel to the incident beam regardless of any rotation of the prism about its roof edge. • This ensures that the alignment between the prism and the other cavity mirror is always achieved in the plane orthogonal to the roof. • The effect of rotation is then to bring the prism into alignment in the other
- 12. contd.. • Rotating-prism Q-switches are simple and inexpensive devices and can be made for use at any wavelength. • They are rather noisy, however, and, due to the limited speed of the rotating motor, they generally result in slow Q-switching. For a typical multi-transverse-mode solid state laser, for instance, the beam divergence is around a few mrad. • The high-Q situation then corresponds to an angular range of 1mrad around the perfect alignment condition. • Thus, even for a motor rotating at the fast speed of 24,000 rpm (400Hz), the duration of the high Q-switching condition is about 400 ns. This slow switching time can sometimes result in the production of multiple pulses.
- 14. Acousto-Optic Q-Switches • An acousto-optic modulator consists of a block of transparent optical material (e.g., fused quartz in the visible to near infrared and germanium or cadmium selenide in the middle-far infrared) in which an ultrasonic wave is launched by a piezoelectric transducer bonded to one side of the block and driven by a radiofrequency oscillator (Fig. 8.7a). • The side of the block opposite to the transducer side is cut at an angle and has an absorber for the acoustic wave placed on its surface (see Fig. 8.7b).With back reflection of the acoustic wave thus suppressed, only a traveling acoustic wave is present in the medium. The strain induced by the ultrasonic wave results in local changes of the material refractive index through the photoelastic effect. • This periodic change of refractive index acts then as a phase grating with period equal to the acoustic wavelength, amplitude proportional to the sound amplitude, and which is traveling at the sound velocity in the medium (traveling-wave phase grating). Its effect is to diffract a fraction of the incident beam out of the incident beam direction. • Thus, if an acousto-optic cell is inserted in a laser cavity (Fig. 8.7b), an additional loss will be present, due to beam diffraction, while the driving voltage to the transducer is applied. If the driving voltage is high enough, this additional loss will be sufficient to prevent the laser from oscillating. The laser is then returned to its high-Q condition by switching off the transducer voltage.
- 16. Saturable Absorber • The three Q-switching devices considered so far fall in the category of active Q-switches since they must be driven by an appropriate driving source (Pockel cell voltage power supply, rotating motor or rf oscillator). • We now consider a case of passive Q-switching exploiting the non-linearity of a saturable absorber, this being by far the most common passive Q-switch in use so far. • A saturable absorber consists of a material which absorbs at the laser wavelength and which has a low value of saturation intensity. • It is often in the form of a cell containing a solution of a saturable dye in an appropriate solvent (e.g., the dye known as BDN, bis 4-dimethyl-aminodithiobenzil-nickel, dissolved in 1,2-dichloroethane for the case of Nd:YAG). • Solid state (e.g., BDN in a cellulose acetate, F2:LiF, or Cr4C:YAG, again for a Nd:YAG laser) or gaseous saturable absorbers (e.g., SF6 for CO2 lasers) are also used.
- 17. Concept about Saturable absorber • Definition: light absorbers with a degree of absorption which is reduced at high optical intensities • More specific terms: saturable Bragg reflectors, semiconductor saturable absorber mirrors, artificial saturable absorbers • A high efficiency is at least possible if • (a) the saturation energy of the absorber is well below that of the laser gain medium, • (b) the absorber exhibits negligible non-saturable losses, and • (c) no additional loss channels such as ASE come into play. • However, significant non-saturable losses are often encountered in real absorbers.
- 19. Active Q switching • Here, the Q-switch is an externally controlled variable attenuator. • This may be a mechanical device such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity, or (more commonly) it may be some form of modulator such as an acousto–optic device, a magneto- optic effect device or an electro-optic device – a Pockels cell or Kerr cell. • The reduction of losses (increase of Q) is triggered by an external event, typically an electrical signal. The pulse repetition rate can therefore be externally controlled. • Modulators generally allow a faster transition from low to high Q, and provide better control. • An additional advantage of modulators is that the rejected light may be coupled out of the cavity and can be used for something else.
- 20. Passive Q switching • In this case, the Q-switch is a saturable absorber, a material whose transmission increases when the intensity of light exceeds some threshold. • The material may be an ion-doped crystal like Cr:YAG, which is used for Q-switching of Nd:YAG lasers, a bleachable dye, or a passive semiconductor device. • Initially, the loss of the absorber is high, but still low enough to permit some lasing once a large amount of energy is stored in the gain medium. As the laser power increases, it saturates the absorber, i.e., rapidly reduces the resonator loss, so that the power can increase even faster. Ideally, this brings the absorber into a state with low losses to allow efficient extraction of the stored energy by the laser pulse. After the pulse, the absorber recovers to its high-loss state before the gain recovers, so that the next pulse is delayed until the energy in the gain medium is fully replenished.
- 21. Passive Q switching (contd.) • For passive Q switching (sometimes called self Q switching), the losses are automatically modulated with a saturable absorber. Here, the pulse is formed as soon as the energy stored in the gain medium (and thus the gain) has reached a sufficiently high level. • In many cases, the pulse energy and duration are then fixed (assuming complete recovery of the absorber between the pulses), and changes of the pump power only influence the pulse repetition rate.
- 22. Cavity dumping • Particularly for high pulse repetition rates, it can be difficult to obtain very short pulses, because then lower pulse energy leads to a weaker temporal modulation of the net gain. • This problem can be solved by using the method of cavity dumping. Instead of using an ordinary output coupler mirror, the pulse generation phase is effectively done with a “closed” low-loss resonator. • Once most of the stored energy has been transferred into the circulating pulse, the energy is suddenly released with the cavity dumper, which is a fast optical switch. In that way, the optical energy in the resonator can be extracted within one resonator round-trip time, independent of the time required for pulse build-up