Laser lecture 08
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This document discusses different modes of laser operation, including continuous wave (CW), pulsed, Q-switching, and mode-locking modes. It describes free running laser pulse mode, where the pulse width is controlled by the pumping pulse. For Q-switching, a device is inserted to make the quality factor vary between minimum and maximum, producing very short, high power pulses. Mode-locking synchronizes multiple cavity modes, resulting in ultrashort intensity spikes spaced by the cavity roundtrip time.
Laser lecture 08
- 1. 14/06/2015 1 412 PHYS Lasers and their Applications Department of Physics Faculty of Science Jazan University KSA Pulsed laser operation Lecture-8 Modes of operations - CW operation - Pulsed operation: pulsed mode operation can be categorized into three types: 1- Free running laser pulse mode 2- Q-switching mode 3- Mode-locking mode
- 2. 14/06/2015 2 1- Free running laser pulse mode In this mode laser is designed mainly as pulsed laser due to its active medium and pumping scheme requirements such as: Short life time of the upper laser level - difficulties in depopulating laser lower level- heat build up by pumping- fast pumping scheme or pulsed pumping scheme - In this mode the width of the laser pulse is controlled by the pumping pulse Pulse width of this operation mode 100 s few ms 2- Q-switched operation In this type a device is inserted in the laser cavity for the purpose of making a fast variation of the oscillation feedback from a minimum to a maximum values and vise versa and that is described by the quality factor (Q-factor) The Q-factor is used to describe any resonant cavity ; It defines the ability of a cavity to store the radiant energy In the process of Q-switching: the Q-facto blocks (switched-off) the feedback between the laser mirrors until a maximum energy is pumped to the active medium and then switched –on to allow for a maximum feedback to occur….As a result very short laser pulses with very high peak powers are generated
- 3. 14/06/2015 3 Concept of Q-switching Let’s assume a laser cavity as shown in figure which is a Fabry-Perot resonator The Q-factor is expressed as Q Assuming that the reflection is the main source of losses and neglecting the losses due to absorption and diffraction, we can define the Q-factor as the ratio between the stored energy to the dissipated energy per cycle in the cavity TotalstoredEnergy 22 Energydissipatedpercycle WQ W The total stored energy The rate of dissipated energy Frequency of oscillation Frequency band width of the laser output W h W Note: The photon has to make passes per cavity length before it leaves the cavity → this means that it has to traverse a distance of in a velocity of light and hence the life time of the photon in the cavity equals R1 1 R L 1 2 2 1 L c R Therefore, the rate of energy dissipation 1 2 h c R W L The Q-factor is then given by 4 2 2 1 1 h L Q L h c R R
- 4. 14/06/2015 4 ( 1 )L m Assume m 6328.0 0.95R 8 4 10Q And hence the bandwidth 6 10 1Hz MHz More analysis The output Power of Q-switched laser ( ) /L c cP h V n p c L p c p c c n V h n V h c P The output Peak Power of Q-switched laser
- 5. 14/06/2015 5 Example: A Q-Switched Nd:YAG laser with a wavelength of 1064 nm. Its active medium is a cylindrical solid of length of 20 cm and cross-sectional diameter of 5 cm and refractive index of (1.8). Assume the two mirrors of reflectivities of 95% & 100% are attached to the ends of the medium. If the peak density of the oscillated photons is 1020 m-3 Find the output peak power? Sol. Let’s first calculate the photon life time inside the cavity 8 8 1 2 2 2 1.8 0.2 5 10 50 ln( ) 3 10 ln(0.95) c n L s ns c R R 2 2 2 4 3 ( / 2) 3.14(2.5 10 ) 0.2 4 10cV d L m The effective volume 20 4 34 8 9 9 5 10 4 10 6.63 10 3 10 1064 10 50 10 1.5 10 p c L p c p c c n V h n V hc P W The output peak power Methods of Q-switching 1. Rotating prism or mirror Rotating mirrors or prisms w ~ 10,000 rpm
- 6. 14/06/2015 6 2. The electro-optic Q-switching 3. Acousto-optic Q switching 4. Photochemical Q-switching 5. Exploding film Q-switching
- 7. 14/06/2015 7 Mode Locking Resonant cavity modes and the gain spectrum of a laser. Part (a) shows single-mode lasing, where the gain envelope overlaps with just one cavity resonance. Part (b) shows multimode lasing, where the gain envelope overlaps with several cavity resonances. (a) No phase coherence between the multiple modes (80 modes simulated), (b) 5 phase coherent modes, and (c) 80 phase coherent modes. The pulse train emerges as ultrashort intensity spikes (fs scale) spaced by longer ns intervals (roundtrip time of the cavity).
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- 10. 14/06/2015 10 1. Active Mode-Locking Methods of Q-Switching 2. Passive Mode-Locking