1374.4.Q-SWITCHINGImageremovedduetocopyrightrestrictionsPleasesee:Keller,U.,UtrafastLaserPhyics,InstituteofQuantumElectronics,SwissFederalInstituteofechnologyETH Honggerberg—HPT, CH-8093 Zurich, Switzerland.Figure 4.5: Gain and loss dynamics of an actively Q-switched laser.the laser:As a rough orientation for a solid-state laser, the following relationfor the relevant time scales is generally valid(4.32)TL》TR》Tp4.4.1Active Q-SwitchingFig. 4.5 shows the principle dynamics of an actively Q-switched laser. Thelaser is pumped by a pump pulse with a length on the order of the upper-state lifetime, while the intracavity losses are kept high enough, so thatthe laser can not reach threshold. Therefore, the laser medium acts as anenergy storage.The energy only relaxes by spontenous and nonradiativetransitions. Then suddenly the intracavity loss is reduced, for example bya rotating cavity mirror.The laser is pumped way above threshold and thelight field builts up exponentially with the net gain until the pulse energycomes close to the saturation energy of the gain medium. The gain saturatesand is extracted, so that the laser is shut off by the pulse itself
4.4. Q-SWITCHING 137 Figure 4.5: Gain and loss dynamics of an actively Q-switched laser. the laser. As a rough orientation for a solid-state laser, the following relation for the relevant time scales is generally valid τ L À TR À τ p. (4.32) 4.4.1 Active Q-Switching Fig. 4.5 shows the principle dynamics of an actively Q-switched laser. The laser is pumped by a pump pulse with a length on the order of the upperstate lifetime, while the intracavity losses are kept high enough, so that the laser can not reach threshold. Therefore, the laser medium acts as an energy storage. The energy only relaxes by spontenous and nonradiative transitions. Then suddenly the intracavity loss is reduced, for example by a rotating cavity mirror. The laser is pumped way above threshold and the light field builts up exponentially with the net gain until the pulse energy comes close to the saturation energy of the gain medium. The gain saturates and is extracted, so that the laser is shut off by the pulse itself. Keller, U., Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Hönggerberg—HPT, CH-8093 Zurich, Switzerland. Image removed due to copyright restrictions. Please see:
138CHAPTER 4.LASERDYNAMICS (SINGLE-MODE)A typical actively Q-switched pulse is asymmetric: The rise time is pro-portional to the net gain after the Q-value of the cavity is actively switchedto a high value. The light intensity growths proportional to 2go/Tr. Whenthe gain is depleted, the fall time mostly depends on the cavity decay timeTp.For short Q-switched pulses a short cavity length, high gain and a largechange in the cavity Q is necessary. If the Q-switch is not fast, the pulsewidth may be limited by the speed of the switch. Typical electro-optical andacousto-optical switches are 10 ns and 50 ns, respectivelyImageremoveddueto copyright restrictions.Please see:Keller,U.,UltrafastLaser Physics, Institute ofQuantum Electronics,Swiss Federal Institute ofTechnologyETHHonggerberg—HPT,CH-8093Zurich,SwitzerlandFigure4.6:Asymmetric activelyQ-switchedpulseFor example, with a diode-pumped Nd:YAG microchip laser [6] using anelectro-0ptical switch based on LiTaO3 Q-switched pulses as short as 270 psat repetition rates of 5 kHz, peak powers of 25 kW at an average power of34 mW, and pulse energy of 6.8 μJ have been generated (Figure 4.7)
138 CHAPTER 4. LASER DYNAMICS (SINGLE-MODE) A typical actively Q-switched pulse is asymmetric: The rise time is proportional to the net gain after the Q-value of the cavity is actively switched to a high value. The light intensity growths proportional to 2g0/TR. When the gain is depleted, the fall time mostly depends on the cavity decay time τ p. For short Q-switched pulses a short cavity length, high gain and a large change in the cavity Q is necessary. If the Q-switch is not fast, the pulse width may be limited by the speed of the switch. Typical electro-optical and acousto-optical switches are 10 ns and 50 ns, respectively Figure 4.6: Asymmetric actively Q-switched pulse. For example, with a diode-pumped Nd:YAG microchip laser [6] using an electro-optical switch based on LiT aO3 Q-switched pulses as short as 270 ps at repetition rates of 5 kHz, peak powers of 25 kW at an average power of 34 mW, and pulse energy of 6.8 µJ have been generated (Figure 4.7). Keller, U., Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Hönggerberg—HPT, CH-8093 Zurich, Switzerland. Image removed due to copyright restrictions. Please see:
1394.4.Q-SWITCHINGImageremovedduetocopyrightrestrictionsPlease see:Kafka,J.D.,andT.Baer."Mode-lockederbium-dopedfiberlaserwithsolitonpulseshaping."OpticsLetters14 (1989):1269-1271.Figure 4.7:Q-switched microchip laser using an electro-optic switch.Thepulse is measured with asampling scope [8]Similar results were achieved with Nd:YLF [7] and the correspondingsetup is shown in Fig. 4.8outputlaserA/O Q-switchcouplercrystaldiodelaserfocussingacousticpartiallycoatingopticstransducerreflectiveHR-laserlcoatingHT-diodelFigure4.8:Set-upofanactivelyQ-switchedlaser
4.4. Q-SWITCHING 139 Figure 4.7: Q-switched microchip laser using an electro-optic switch. The pulse is measured with a sampling scope [8] Similar results were achieved with Nd:YLF [7] and the corresponding setup is shown in Fig. 4.8. focussing optics coating: HR - laser l HT- diode l partially reflective coating laser crystal diode laser A/O Q-switch acoustic transducer output coupler Figure 4.8: Set-up of an actively Q-switched laser. Kafka, J. D., and T. Baer. "Mode-locked erbium-doped fiber laser with soliton pulse shaping." Optics Letters 14 (1989): 1269-1271. Image removed due to copyright restrictions. Please see:
140CHAPTER4.LASERDYNAMICS (SINGLE-MODE)4.4.2Single-Frequency Q-Switched PulsesQ-switched lasers only deliver stable output if they oscillate singlefrequency.Usually this is not automatically achieved. One method to achieve this is byseeding with a single-frequency laser during Q-switched operation, so thatthere is already a population in one of the longitudinal modes before thepulse is building up. This mode will extract all the energy before the othermodes can do, see Figure4.gImageremovedduetocopyright restrictionsPleasesee:Keller,U.,UltrafastLaserPhysics,InstituteofQuantumElectronics,SwissFederalInstituteofTechnologyETH Honggerberg—HPT,CH-8093Zurich,Switzerland.Figure 4.9:Output intenisity of a Q-switched laser without a)and withseeding b).Another possibility to achieve single-mode output is either using an etalonin the cavity or making the cavity so short, that only one longitudinal modeis within the gain bandwidth (Figure 4.10). This is usually only the case ifthe cavity length is on the order of a view millimeters or below.The microchiplaser [6][11][10] can be combined with an electro-optic modulator to achieve
140 CHAPTER 4. LASER DYNAMICS (SINGLE-MODE) 4.4.2 Single-Frequency Q-Switched Pulses Q-switched lasers only deliver stable output if they oscillate single frequency. Usually this is not automatically achieved. One method to achieve this is by seeding with a single-frequency laser during Q-switched operation, so that there is already a population in one of the longitudinal modes before the pulse is building up. This mode will extract all the energy before the other modes can do, see Figure 4.9 Figure 4.9: Output intenisity of a Q-switched laser without a) and with seeding b). Another possibility to achieve single-mode output is either using an etalon in the cavity or making the cavity so short, that only one longitudinal mode is within the gain bandwidth (Figure 4.10). This is usually only the case if the cavity length is on the order of a view millimeters or below.The microchip laser [6][11][10] can be combined with an electro-optic modulator to achieve Keller, U., Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Hönggerberg—HPT, CH-8093 Zurich, Switzerland. Image removed due to copyright restrictions. Please see:
Q-SWITCHING1414.4.very compact high peak power lasers with sub-nanosecond pulsewidth (Figure4.7).Image removed due to copyright restrictions.Pleasesee:Keller,U.,UltrafastLaserPhysics,InstituteofQuantumElectronics,SwissFederal InstituteofTechnologyETH Honggerberg—HPT, CH-8093Zurich, Switzerland.Figure 4.10: In a microchip laser the resonator can be so short, that there isonly one longitudinal mode within the gain bandwidth
4.4. Q-SWITCHING 141 very compact high peak power lasers with sub-nanosecond pulsewidth (Figure 4.7). Figure 4.10: In a microchip laser the resonator can be so short, that there is only one longitudinal mode within the gain bandwidth. Keller, U., Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Hönggerberg—HPT, CH-8093 Zurich, Switzerland. Image removed due to copyright restrictions. Please see: