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Unformatted text preview: ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Laser Dynamics and Pulsed Lasers Pulsed Laser Characterization ECE 455 Optical Electronics Pulsing Methods Q-Switching Mode Locking Tom Galvin Gary Eden ECE Illinois Introduction ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods In this section, the following subjects will be covered: Non-steady state behavior of lasers Motivations for pulsing lasers Q-Switching Mode Locking Methods for pulsing lasers Starting a Laser I ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization High Reflector PUMP Pulsing Methods Q-Switching Mode Locking Gain Medium Output Coupler Starting a Laser II ECE 455 Lecture 5 1 Before the cavity has begun lasing, the cavity has defined discrete opitcal modes which may oscillate. 2 When the laser is started these modes are seeded by photons emitted via spontaneous emission. 3 All modes which are above threshold when the gain is γ0 grow exponentially, but the mode with the highest net gain will grow more rapidly than all others. 4 The mode with the highest gain will reach a level where it saturates the medium before the other modes. The gain for all modes falls until only one is above threshold. 5 The other modes decay away and a single mode is left lasing. Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Starting a Laser Level Diagram ECE 455 Lecture 5 τ32 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods R3 N3 N2 τ21 Laser Q-Switching Mode Locking τ10 N1 N0 Throughout the analysis that follows, we’ll analyze a four-level system as shown above Starting a Laser III Seeding Rate ECE 455 Lecture 5 Laser Dyamics The first question to answer is how long will it take for a cavity mode above thresold to become seeded? The density of states for photons per unit volume and frequency is: Pulsed Lasers ρ(ν )d ν = Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking 8π n3 ν 2 dν c3 (1) Spontaneous emission can emit into any of these modes. But only one of these modes is the mode with the highest gain dφ dt 1 N2 Vc g (ν21 )∆ν21 τ21 Vc ρ(ν )∆ν21 N2 1 g (ν21 ) = τ21 ρ(ν ) N2 = ηseed τ21 = (2) (3) (4) Starting a Laser IV Buildup Time ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods How long will it take for a single photon to stabilize into laser output? The small signal gain form of the gain equation may be used because saturation effects can be ignored before laser oscillation has begun. Isat = Q-Switching Mode Locking hν (γ0 −γth )ct /n e Aτ21 (5) Therefore the buildup time is τbuildup = n ln (γ0 − γth )c Isat Aτ21 hν (6) Starting a LaserV Buildup Time II ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking There are many ways to estimate the buildup time. The net gain per round trip is GRT = R1 R2 exp [2γ0 Lg ] (7) The number or round trips necessary to reach a total gain of G is logG N= (8) logGRT The round trip time tRT = 2nLg + 2(L − Lg ) c (9) Therefore the buildup time is τbuildup = 2nLg + 2(L − Lg ) logG c logGRT (10) Example: Laser Buildup Time ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Problem: Estimate the buildup time of a Pulsing Methods Q-Switching Mode Locking Solution: Starting a Laser: Alternative View ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Laser dyanamics can be approximated with the following model d ∆N g2 =− 1+ dt g1 Lg c φ N2 g2 N1 σse ∆N − + +R3 (t ) (11) L nV τ2 g1 τ1 Lg c dφ φ N2 = φσse ∆N − + ηseed dt Ln τc τ21 (12) There is no simple analytic solution to this non linear system of equations; they must be solved numerically. Laser Spiking ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Relaxation Oscillations ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Relaxation oscillations occur in lasers where τ21 Pulsed Laser Characterization φ = φss + φ(t ) Q-Switching φ(t ) ≈ exp − (13) ∆N = ∆Nss + ∆N (t ) Pulsing Methods Mode Locking τc (14) σse c φss t sin σse c (φss ∆Nss )1/2 t 2 (15) Why Pulse a Laser? ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Physical parameters of gain medium Increase peak output power Nonlinear optical processes scale as I n , where n is the order of the nonlinearity and I is the intensity of the optical field. Extreme pumping requirements for threshold gain Increase laser bandwidth Time resolved spectroscopy Example: Pumping Requirements of an Excimer ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Problem: Predict the threshold pumping rate for a KrF laser. The laser has the following properties: R1 = 0.99, R2 = 0.04, L = 1 m, Amode = 5.25 cm2 , λ = 248 nm, τ2 = 5 ns and σse = 2.6 ˚2 . Only 15% of pump energy will go into upper state A formation. Solution: The first step is to calculate the threshold gain: Q-Switching Mode Locking γth = − 1 ln(R1 R2 ) = 0.016 cm−1 2L This requires: ∆Nth = γth = 6.21 × 1013 cm−3 σse Example: Pumping Requirements of an Excimer ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching The required volumetric pumping rate at threshold is: R= 1 hc ∆Nth = 66.3 kW-cm−3 η λ τ2 which means the total pump power must be: Mode Locking P = RV = (66.3 kW-cm−3 ) · (100 cm) · (5.25 cm2 ) = 34.8 MW Characterizing Pulsed Lasers ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Two parameters are commonly used to characterize pulsed lasers. The first is the average output power: Pave = Epulse frep (16) The second is the peak power, which may be approximated as follows: Epulse (17) Ppeak = ∆t Where Epulse is the energy per pulse and ∆t is the FWHM of the pulse. Pulse Laser Characterization Example ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Problem: LOPE’s femtosecond laser produces 40 fs pulses containing up to 4.5 mJ of energy at a repetition rate of 1 kHz. Find the peak and average power of the laser. Solution: Pave = (4.5 mJ) · (1 kHz) = 4.5 W (18) Mode Locking 4.5 mJ = 112.5 GW !!! 40 fs For comparison, summer electricity demand in the US is 783 GW. Ppeak ≈ (19) The Idea ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization In steady state the round trip gain must be equal to one for a laser. Pulsing Methods Therefore in steady state ∆N is locked to ∆Nth Q-Switching This limits the rate at which energy can be extracted Mode Locking Nothing prevents ∆N ∆Nth on a transient basis. Q-Spoiling (Pump and Dump) ECE 455 Lecture 5 The process: 1 Create an extremely high Q cavity and pump continuously Pulsed Lasers 2 Allow the CW intensity to build up inside the cavity Pulsed Laser Characterization 3 Suddenly lower the cavity Q by increasing the strength of the output coupling. Laser Dyamics Pulsing Methods Q-Switching Mode Locking Discussion: Intracavity intensity is much greater than output-coupled light Minimum pulse duration limited by round trip time of cavity Maximum intensity limited by cavity losses Don’t try this on Wall St. Pulse the Pump ECE 455 Lecture 5 Laser Dyamics The process: Pulsed Lasers 1 Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Rapidly switch on a high-powered pumping mechanism 2 Wait for a pulse to come out Minimum pulse duration is limited by cavity build up time or speed of pump This approach requires fast, high power electronics. This is the most primitive method. It is commonly used in conjuction with Q-Switching. Q-Switching ECE 455 Lecture 5 Laser Dyamics The process: 1 Pump at low rate with cavity Q spoiled to prevent oscillation 2 Generate a large population inversion ∆N > ∆Nth 3 Quickly restore Q to a high value to allow laser oscillation 4 Laser pulse extracts energy from inversion, driving ∆N < ∆Nth (absorption) 5 Laser pulse terminates 6 Turn off Q Switch and repeat Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Two types of Q-Switches are: Rotating mirror Pockells cell Q-Switching Methods: Rotating Mirror ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Gain Medium Q-Switching Mode Locking Rotating Mirror Q-Switching Methods: Acousto-Optic Modulator (AOM) ECE 455 Lecture 5 Scattering loss Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Light Q-Switching Mode Locking Acoustic density waves Q-Switching Methods: Pockels Cell ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods M1 Gain PBS QWP PC M2 PC Off Q-Switching Mode Locking PC On PBS = Polarizing Beam Splitter; QWP = Quarter Wave Plate; PC = Pockels Cell Q-Switching Methods: Saturable Absorber ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Q-Switch: Energy Storage ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods The length of time that energy can be stored is limited by the lifetime of the of upper state. The lifetime sets an upper limit on the maximum useful pump duration. Energy storage in an amplifier or laser is limited by the onset of parasitic oscillations Q-Switching Mode Locking Small stimulated emission cross sections keep the and allow a large population in the upper state An ideal amplifier material has a high fluoresence lifetime and a small stimulated emission cross section Energy Storage Example ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Problem: Nd:YAG and Nd:YLF both lase at similar wavelengths (1064 nm and 1053 nm respectively). Calculate the maximum amount of energy that can be stored in both a Nd:YAG and Nd:YLF amplifier before parasitic oscillations begin. Assume the amplifier crystal is 8 cm long and that 1% of radiation is scattered back at each interface. For Nd:YAG σse = 2.8 × 10−19 cm2 For Nd:YLF σse = 1.8 × 10−19 cm2 Solution: The first step is to calculate the threshold gain: γth = − 1 1 ln(R1 R2 ) = − ln(.01 · 0.1) = 0.576 cm−1 2L 2 · 8cm Energy Storage Example Continued ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization From the threshold gain, we can calculate the threshold inversions of both of these lasers. From the inversion density, we can easily calculate the energy storage desnity. ∆Nth (YAG ) = Pulsing Methods Q-Switching γ0 0.576cm−1 = = 2.06 × 1018 cm−3 σse 2.8 × 10−19 cm2 ρ(YAG ) = Mode Locking ∆Nth (YLF ) = hc ∆Nth = 0.384J · cm−3 λ γ0 0.576cm−1 = = 3.20 × 1018 cm−3 σse 1.8 × 10−19 cm2 hc ∆Nth = 0.597 J-cm−3 λ This analysis is of course approximate. ρ(YLF ) = Q-Switch Discussion ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Minimum pulse duration limited by cavity build up time Maximum energy of pulse is limited by energy storage density inside medium and how efficiently it can be extracted Pulsing Methods: Medium Behavior Outside Cavity ECE 455 Lecture 5 Pump Pulse Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Exponential Decay Linear Rise Nth t Pulsing Methods: Pulsing the Pump ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pump Pulse Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Gain Below Threshold Laser Pulse Nth t Pulsing Methods: Q-Switch ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pump Pulse Q-Switch Activated Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Laser Pulse Nth t Pulsing Methods: Poorly Timed Q-Switch ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pump Pulse Q-Switch Activated Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Laser Pulse Nth t Keep Track of Assumptions ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Pump pulse much faster than relaxation time Pumping rate will not have a nice flat-top Saturation of the pumping process has been ignored Spatial effects (transverse and logitudinal) have been ignored Fourier Series ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Recall that any periodic function may be represented as a weighted sum of complex exponentials ∞ cn · exp ın a (t ) = Pulsing Methods n=−∞ 2π t T (20) 2π t dt T (21) Q-Switching Mode Locking where cn = 1 2T T a(t )exp −ın 0 Properties of Fourier Series ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking The fundamental frequency of a(t ) is f0 = 1 T The fourier spectrum of a(t ) comprises delta functions separated by f0 The more rapid the variations in a(t ), the more terms will be needed in the fourier series to approximate a(t ) Optical Fourier Series ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Recall that the frequencies of the longitudinal modes of a cavity are c (22) νq = q 2Lopt If several of these modes are oscillating simultaneously, the electric field may be written Q-Switching Mode Locking |Eq |exp [ıφq ] · exp [ıq 2πνq t ] E (t ) = (23) q |Eq |exp [ıφq ] · exp ıq 2πνq t (24) = exp [ı2πν0 t ] q where νq ≡ νq − ν0 Properties of Optical Fourier Series ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Relatively low frequency envelope modulated by optical carrier frequency ν0 The fundamental frequency of the envelope is f0 = c 2Lopt The fourier spectrum E (t ) comprises delta functions separated by f0 and offset from the axes by ν0 The more rapid the variations in the envelope, the more terms will be needed in the fourier series to approximate the pulse Mode Locking I ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking In our discussion of homogenously broadened media, the mode with the highest net gain would oscillate to the exclusion of other modes. What if a coherent superposition of cavity modes could have lower loss (and thus higher net gain) than any individual longitudinal mode? This is the idea behind the technique known as modelocking. Mode Locking II ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking The cavity dispersion must go to zero. There must be a mechanism to lock the phases of the various longitudinal modes. The shortest pulse possible is limited by one of four factors: The The The The recovery time of the mode locking mechanism bandwidth of the lasing transition reflectivity and dispersion of the cavity optics frequency of the optical carrier wave Pulse Propagation in a Material Medium ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods The actual condition for resonance of the qth longitudinal cavity resonance is φRT = q · 2π (25) The phase shift from propogating through the gain medium once is Q-Switching Mode Locking φ(ω ) = = 2π Lg n(ω ) (26) λ 2π ∂n 1 ∂2n Lg n(ω0 ) + (ω − ω0 ) + (ω − ω0 )2 + . . . λ ∂ω 2 ∂ω 2 Which terms are important? Pulse Propagation in a Material Medium ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods The fourier transform E (ω ) = A(ω )e ıω0 t (27) where A(ω ) is the fourier transform of the envelope and e ıω0 t is the optical carrier frequency. Q-Switching Mode Locking The fourier transform of the pulse after is has propagated through a material medium is E (ω ) = A(ω )e ıω0 t e ıφ(ω) (28) Propagation Through Sapphire 120 fs Pulse ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Propagation Through Sapphire 40 fs Pulse ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Propagation Through Sapphire 10 fs Pulse ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Lessons ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Pulse dispersion is more sevfere for shorter pulses We need a mechanism to counteract the dispersion present in the cavity Prisms Chirped mirrors Photonic Crystals Propagation through materials is bad. Use reflective optics. Multiple Longitudinal Modes: Random Phase N = 30 ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Multiple Longitudinal Modes: Zero Phase N = 30 ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Multiple Longitudinal Modes: Zero Phase N = 90 ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Active Mode Locking: Acousto-Optic Modulator ECE 455 Lecture 5 Laser Dyamics An acoustic transducer is coupled to a crystal Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking The acoustic waves forms a standing wave pattern The periodic variation in density forms a grating, which scatters the laser beam This can be modulated rapidly. The modulation must be synchronized with the cavity round trip time. PICTURE OF AOM Modelocking Shutter Picture ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Shutter Pulse t Passive Mode Locking: Saturable Absorber ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching An absorber is placed inside the laser cavity Low intensity light, such as noise is absorbed by the aborber High intensity ’bleaches’ the medium, making it transparent. Mode Locking It only works if the recovery time of the medium is much less than the round trip time. Passive Mode Locking: Kerr Lens ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Relies on a third order nonlinearity known as self focusing n = n1 + n2 I The higher peak intensity (pulsed) mode suffers lower diffraction losses than any individual longitudinal mode. Kerr lensing is a nonlinear optical process. Hence it is (approximately) instantaneous Discovered by graduate students who forgot to turn the AOM on A Mode-Locked Oscillator ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking D. E. Spence et al. Optics Letters 16, 42 (1991) CW and Modelocked Operation ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Figure: A Ti:Sapphire laser operating CW. Figure: The same laser modelocked. One More Option ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Figure: A Ti:Sapphire laser in modelocked operation with continuous wave breakthrough Example: Shortest Possible Pulses ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode locking produces the shortest pulses. As a first estimate, the shortest possible pulse possible is: ∆t ≈ 1 ∆f Mode Locking where ∆f is the bandwidth FWHM. (29) Example Shortest Pulse ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Problem: As a gain medium, Ti:Sapphire exhibits gain from roughly 650 nm to 1100 nm. What is the short possible pulse which a Ti:Sapphire laser can generate? Solution: ∆f = c c − = 1.89 × 1014 Hz 650 nm 1100 nm Therefore, an estimate for the shortest pulses possible is: Q-Switching Mode Locking ∆t ≈ 1 = 5.29 fs ∆f For comparison, a single optical cycle at 800 nm (the peak of Ti:Sapphire gain spectrum) is T= 800 nm = 2.67 fs c Example: Finding Number of Modes Locked Together ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Problem: A mode-locked Ti:Sapphire oscillator has mirrors which are 1.8 m apart. Consider the spectrum in Figure 2, how many longitudinal modes are oscillating simultaneously? Solution: The separation between longitudinal modes is simply the free spectral range of the cavity. c FSR = = 83.3 MHz 2nL To determine the number of modes oscillating simulatenously, we take the entire spectrum bandwidth, not the FWHM. This bandwidth is: c c ∆f = − = 6 × 1013 Hz 730 nm 855 nm The number of modes oscillating simultaneously is therefore: ∆f = 721000 FSR Regenerative Amplification I ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization The pulse energy possible with mode locking is limited because the high repition rate of oscillators would require a very high power pump laser. In order to generate high powered short pulses, a technique known as regnerative amplication is used. The strategy is as follows: 1 Create low-energy high-repetition rate pulses from an oscillator 2 Use an optical switch to lower the repetition rate 3 Temporally stretch pulses by using diffraction gratings to ’chirp’ the pulses 4 Amplify the pulse by passing it through another crystal multiple times. 5 Use another diffraction grating to remove the chirp introduced by the stretcher and cavity dispersion Pulsing Methods Q-Switching Mode Locking A Regnerative Amplifier ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking A02/G - Pump Laser; FI - Faraday Isolator; PC - Pockells Cell; TFP - Thin Film Polarizer I. Matsushima et al. Optics Letters 31, 2066 (2006) ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization From the examples given, you may get the impression that Ti:Sapphire is the only medium for mode locking, not so! Dye Lasers Pulsing Methods Nd:YAG Q-Switching Cr:LiCAF and Cr:LiSAF Mode Locking Er and Yb doped fiber Semiconductor Lasers Summary ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characterization Pulsing Methods Q-Switching Mode Locking Lasers are ’seeded’ from spontaneous emission Full models of laser dynamics are quite complicated Pulsing lasers can result in a dramatic increase in peak intensity Mode-locking produces the shortest laser pulses, but due to the high rep rate, the energy per pulse is low ...
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This note was uploaded on 02/28/2012 for the course ECE 455 taught by Professor Eden,j during the Fall '08 term at University of Illinois, Urbana Champaign.

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