Pulsed Solid State Laser with Passive Q-switch Seminar

Transcription

1 University of Ljubljana Faculty of Mathematics and Physics Department of Physics Pulsed Solid State Laser with Passive Q-switch Seminar Author: Marko Kozinc Mentor: doc. dr. Rok Petkovšek Ljubljana, February 2015 Abstract In this seminar we present the structure, operating regime and use of solid state laser with passive Q- switch. We give a description and comparison of two basic concepts of optical pumping, the lamp pumping and the diode pumping. Finally we present the Q-switch theory and more precisely passive Q-switch theory.

2 Table of contents 1 Introduction Solid State Laser with Passive Q-switch Pumping of SSL Lamp-pumped SSL Diode Pumped SSL Q-Switch Theory Passive Q-Switches Conclusion Bibliography... 12

3 1 Introduction Solid-state lasers (SSL) are lasers based on solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions. Beside fiber lasers they provide one of the most versatile radiation sources in terms of output characteristics when compared to other laser systems. A large range of output parameters, such as average and peak power, pulse width, pulse repetition rate and wavelength, can be obtained with these systems. Today we find solid-state lasers in industry as tools in many manufacturing processes, in hospitals and in medicine as radiation sources for therapeutic, cosmetic, and surgical procedures, in research facilities as part of the diagnostic instrumentation and in military systems as rangefinders, target designators, and infrared countermeasure systems. Other types of lasers that employ solid-state gain media are semiconductor lasers and optical fiber lasers. However, since these lasers employ very specialized technologies and design principles, they are usually treated separately from conventional solid-state lasers [1]. 2 Solid State Laser with Passive Q-switch Lasers can operate in continuous or pulsed regime. In some cases pulsed lasers are valuable when peak power rather than average power is most important. One of the most applied techniques to produce a pulsed output beam is Q-switch technique. It allows the production of light pulses with extremely short, duration and high peak power, much higher than can be produced by the same laser operating in continuous wave mode. There are two main types of Q-switching - active and passive. For active Q switching the losses are modulated with an active control element. It is an acousto-optic (AO) [2], electro-optic modulator (EO) [3] or a single crystal photo-elastic modulator (SCPEM) [4], where the pulse is formed shortly after an electrical trigger signal arrives. There are also mechanical Q switches such as spinning mirrors or prism, used as end mirrors of laser resonators. In all case, the achieved pulse energy and pulse duration depends on the energy stored in the gain medium, on the pump power and the pulse repetition rate. A passive Q-switch is an optical element, such as doped crystal, a cell filled with organic dye or a passive semiconductor device. The characteristic of such material is that transmission increases when the intensity of light exceeds some threshold. If such a material with high absorption at the laser wavelength is placed inside the laser resonator, it will initially prevent laser oscillation. As the gain increases during a pump pulse and exceeds the round-trip losses, the intra cavity power density increases dramatically causing the passive Q-switch to saturate. Under this condition the losses are low and a Q-switch pulse builds up. Figure 1: Simple concept of a solid state laser with passive Q-switched.

4 Figure 1 shows the simple scheme of solid state laser with passive Q-switch. The edge of laser resonator consists of two mirrors, output coupler (OC) and high reflected mirror (HR). Between OC and HR lay active medium laser crystal and passive Q-switch crystal. Important part of laser system is also a pumping mechanism. 3 Pumping of SSL The process by which atoms are raised from lower level to upper level is called pumping. In the context of lasers or laser amplifiers, the goal is to achieve a population inversion in the gain medium and thus to obtain optical amplification via stimulated emission for some range of optical frequencies. Inversion by optical pumping becomes possible when using a three-level and a fourlevel system. Solid-state lasers are optically pumped with lamps and laser diodes. 3.1 Lamp-pumped SSL The pump source in lamp-pumped solid state lasers is some kind of gas discharge lamp or in rare cases, tungsten halogen lamps, which are not gas discharge lamps but rather similar to ordinary bulbs. Discharge lamps used for laser pumping are grouped in two categories, arc lamps and flash lamps. Arc lamps are optimized for continuous-wave operation, whereas flash lamps produce pump pulses for either free-running or Q-switched lasers. Both types of lamps essentially consist of a glass tube, filled with some gas (e.g. krypton) and having a metallic electrode at each end. Figure 2: Schematic of the flash pumped solid state laser [5]. The laser crystal of a lamp-pumped laser is usually a relatively long side-pumped rod, adapted to the length of the lamp. In many cases, laser rod and lamp are placed within an elliptical pump chamber with reflective walls, so that a larger percentage of the generated pump light can be absorbed in the laser rod. Excess heat is removed by cooling water, and an additional filter glass may be used to protect the laser rod from ultraviolet light emitted by the lamp. The most common type of lamp-pumped laser is the Nd: YAG laser. Krypton-filled lamps are mostly used in this case, because the krypton emission is strong in the region between 750nm and 900nm, where Nd: YAG has strong absorption lines. Figure 3 represents absorption spectrum of Nd: YAG. The absorption peaks around 870 nm are caused by a transition from the ground level directly to the upper laser level. The absorption around 808 nm is the result of the transition in to the lowest pump band. The absorption around 750 nm is the result of the manifold immediately above the lowest pump band. Other neodymium-doped gain media are also suitable. These have relatively broad absorption bands and are four-level laser media, so that they can be used with moderate pump intensities and

5 utilize a significant part of the lamp spectrum. Less common lamp-pumped lasers are based on alexandrite, Ti:sapphire, Cr:LiSAF, or laser dyes. Flash lamp pump source are relatively cheap and can provide very high powers. They are fairly robust, e.g. immune to voltage or current spikes. The lifetime of lamps is very limited, normally some hundred or up to a few thousand hours. The wall-plug efficiency of the laser is low, typically at most a few percent. Consequences of that are not only higher electricity consumption, but also a higher heat load, making necessary a more powerful cooling system, and strong thermal lensing, making it more difficult to achieve a good beam quality. Electric power supplies for lamp-pumped lasers involve high voltages, which raise additional safety issues. The low pump brightness, compared with that achievable with diode lasers, and the broad emission wavelength range exclude many solid-state gain media. Lamps are relatively noisy pump sources, leading to higher levels of laser noise. 3.2 Diode Pumped SSL Diode pumped solid state (DPSS) lasers are solid state lasers made by pumping a solid gain media with a laser diode. DPSS lasers have advantages in compactness and efficiency over other types of lasers. High power DPSS lasers have replaced flashlamp pumped lasers in many industrial, medical and scientific applications. Figure 3: Absorption spectrum of Nd: YAG and the emission spectra of a diode laser and a flash lamp [6]. Figure 3 shows the absorption spectrum for doped Nd: YAG and the emission spectra of a diode laser and a flashlamp. The flashlamp emits radiation at all wavelengths while the diode laser emits radiation at essentially a single wavelength that can be tuned to a particular absorption line. For Nd: YAG crystal is a peak absorption value at 808 nm, thus most of the broadband flashlamp energy passes through the material without being absorbed. Flashlamps convert electrical energy to optical energy more efficiently than diode laser, but, because of the inefficient absorption of pump radiation, lamp pumped lasers typically less efficient than diode pumped lasers. Lamp pumped system have low typically 1% electrical to optical efficiency [7], and the lamps need replacement after approximately 200 hours. Diode laser pump source allow operation at about 10 20%

6 efficiency and longer life, approximately hours. Fiber lasers has a wall-plug efficiency of > 30% [8], and has an operating lifetime in excess of 10,000 hours. The main disadvantage of diode laser as pump sources is because it is much more expensive than flashlamps or arc lamps. There are many advantages of using diode lasers to pump a solid state laser, instead of using the diode laser output directly. The output of the solid state lasers can produce higher peak power, have higher radiance and is more coherent than the diode laser pump source. Solid state laser store the pump power from a diode laser and this stored energy can be released in 10 ns pulses by Q- switching, which leads to a peak output power 10 4 times greater than the diode laser [6]. The most common DPSS laser in use is the 532 nm wavelengths green laser pointer. A powerful 808 nm wavelength infrared GaAlAs laser diode pumps a Nd: YAG or a Nd: YVO 4 crystal which produce 1064 nm wavelength light. This light is then frequency doubled using a nonlinear optical process in a KTP crystal (KTiOPO 4 ), so we get a 532 nm green light. Such DPSS lasers are usually around 20% efficient or even up to 35% [7]. For example, a green DPSS laser using 2,5 W pump diode would be expected to output around mw of green light. In optimal conditions, Nd: YVO 4 has a conversion efficiency of 60%, while KTP has a conversion efficiency of 80% [7]. A green DPSS laser can theoretically have an overall efficiency of 48%. But high power output can cause damage on the KTP crystal. Thus, high power DPSS lasers generally have a larger beam diameter, as the beam is expended before it reaches the KTP crystal, reducing the irradiance from the infrared light. In order to obtain high conversion efficiency with KTP crystal, the phase vectors of input beams and generated beams have to be matched. The phase matching occurs when a constant phase relationship is maintained between the generated and propagating waves. The output power P out of laser as a function of the pump power P p can be determined from the relationship [9] P out = η s (P p P th ), (1) where η s is the slope efficiency of generation and P th is the threshold power. The parameters P th and η s are determined with properties of active medium and its excitation efficiency I s P th A m ε, η exc (2) η s η exc λ p λ g T ε, (3) where I s = hν g σ e τ f is the saturation power, ε are total resonator losses, T is the mirror transmission, A m is the averaged medium area, σ e is the stimulated emission cross section, τ f is the lifetime on upper laser level. Laser output power is determined by material parameters σ e and τ f, resonator losses ε and T, average pump wavelengths and generation wavelength λ p λ g and efficiency of excitation energy transfer from pump to active material dopant η exc. Excitation efficiency includes all losses occurring during pumping process and it is expressed as η exc = η r η pro η abs η q η ext, (4)

7 where η r is the radiation efficiency determining part of electrical energy delivered to pump changing into radiant energy, η pro is the projection efficiency determining a part of energy radiated by a pump that reaches laser active material, η abs is the absorption efficiency determining value of energy absorbed by active ions of laser medium, η q is the quantum efficiency determining a part of excide ions reaching higher laser level, η ext is the efficiency of energy excitation determining a part of energy accumulated in excited ions that is emitted as a laser radiation. For lamp pumping the main sources of losses are connected with non-effective transfer of excitation energy from a lamp to active medium and lack of matching between emission spectrum of a lamp pump and absorption bands of active medium. Projection and absorption efficiencies of diode pump can be as high as 95 98%. The total efficiency η = P out P p of lamp-pumped Nd: YAG lasers usually is of 1 3%. Diode pumping efficiency is up to 8 30%, in dependence on medium geometry, method of its excitation, regime of operation and generated power. Table 1: All parameters determining efficiency of diode pumped and lamp pumped system for Nd: YAG medium [9]. Pump source Lamp Diode laser Radiation efficiency 50% 40% Projection efficiency 35% 95% Absorption efficiency 50% 98% Quantum efficiency 40% 76% Excitation efficiency 3,5% 28,3% There are two types of diode pumping geometry, longitudinal or end pumping and transverse or side pumping. End pumping is much more complicated from technological point of view because it requires adequate shape of pumping beam and adequate layers should be deposited on the endsurfaces of active materials what ensures high transmission of pumping radiation and simultaneously low resonator losses. The lasers with an end pump configuration are more efficient when side pumped system are used in high power lasers. The set of high power pumping diodes have relatively large dimensions. Transformation of radiation from such large surfaces into small pumping volume for end pumping becomes difficult problem. Figure 4: Basic schemes of diode pumping: a) end-pumping and b) side pumping [9].

8 Diode-pumped active laser materials undergo rapid thermal changes. Such changes especially affect the media being in form of rods with circular cross-section, particularly end pumped ones. They are heated near their axes from pump radiation and generation, but cooled through their sides. As a result of this effect is thermally deformed laser rod, in result of changes in its geometry and thermal dispersion of refractive index, starts to act as a lens and resonator can be out of its stability range for extreme case. Thermal deformation also generates stresses inside medium that cause changes in polarisation of transmitted radiation [7, 9]. 4 Q-Switch Theory With Q-switch technique pulsed operation of laser is achieved by variable Q factor of the optical resonator. The quality factor Q is defined as the ratio of the energy stored in the resonator cavity to the energy loss per cycle. The Q-switched pulse duration is so short that we can neglect both spontaneous emission and optical pumping. Rate of change of the photon density within the laser resonator is then [1] φ t = φ (cσ en l L ε t r ) (5) and inversion population density n t = γnφσ ec, (6) where c is speed, σ e cross section for stimulated emission, l length of laser rod, L length of resonator, ε losses in resonator cavity, t r = 2L the round-trip time of a photon in the resonator, c γ = 1 + g 2 and g g i states density. The losses in a cavity can be represented by 1 ε = lnr + δ + ζ(t, φ), (7) where the first term represents the output coupling losses determined by mirror reflectivity R, second term contains all the incidental losses such as scattering, diffraction and absorption, and last term represent the cavity loss introduced by the Q-switch. Q-switching is accomplished by making ε an explicit function of time (e. g. rotating mirror or Pockels cell) or a function of the photon density (e. g. saturable absorber). In many instances Q-switches are so fast that ζ can be approximated by step function. In this case we assume that at t = 0 the laser has an initial population inversion n i, and the radiation in the cavity as some small but finite photon density φ i, laser is being pumped and the cavity losses are ε max = ln R + δ + ζ max as illustrated in Figure 5. The losses are suddenly reduced to ε min = ln R + δ. The photon density rises from φ i, reaches a peak φ max many orders of magnitude higher than φ i, and then declines to zero. The population inversion is a monotone decreasing function of time starting at the initial inversion n i and ending at the final inversion n f. At n t the photon flux is maximum and the rate of change of the inversion dn dt is still large and negative, and n falls below the threshold value n t and finally reaches the value n f. If n i is not too far above n t, that is, the initial gain

9 is close to threshold, then the final inversion n f is about the same amount below threshold as n i is above and the output pulse is symmetric. Figure 5: Development of a Q-switched laser pulse. The resonator loss a), population inversion b), and photon flux c) as a function of time are shown [1]. On the other hand, if the active material is pumped considerably above threshold, the gain drops quickly in a few cavity transit times t r to where it equalizes the losses. After the maximum peak power is reached at n t, there are enough photons left inside the laser cavity to erase the remaining population excess and drive it quickly to zero. In this case the major portion of the decay proceeds with a characteristic time constant τ c, which is the cavity time constant. The output energy of the Q-switched laser is [1] E out = hνa 2σ e γ ln (1 R ) ln (n i n f ), (8) where hν is the laser photon energy and A is the effective beam cross-section area. The initial and final population inversion densities, n i and n f are related by transcendental equation n i n f = n t ln ( n i n f ), (9) where n t is the population inversion density at threshold, that is

10 n t = 1 2σ e l (ln 1 R + δ). (10) The pulse width of the Q-switch pulse can also be expressed as a function of the inversion levels n i, n f and n t n i n f t p = t r n i n t [1 + ln( n i nt )]. (11) 4.1 Passive Q-Switches The passive Q-switch is switched by the laser radiation itself and it does not require high voltage, fast electro-optic driver, or RF generator as an active methods. The passive Q-switch offers the advantage of an exceptional simple design, which leads to very small, robust, and low-cost systems. The major drawbacks of a passive Q-switch are the lack of a precision external trigger capability and a lower output compared to electro-optic or acousto-optic Q-switched lasers. Figure 6: Energy levels of a saturable absorber with excited-state. σ gs is the ground-state absorption cross section and σ es is the excited-state absorption cross sections. τ is the upper state lifetime Napaka! Vira sklicevanja ni bilo mogoče najti.[10]. A simple energy-level scheme of saturable absorber is shown in Figure 6. Absorption at the wavelength of interest occurs at the 1 3 transition. We assume that the 3 2 transition is fast. The ground-state absorption cross section has to be large and, simultaneously, the upper state lifetime, the life time of level 2, has to be long enough to enable considerable depletion of the ground state by the laser radiation. When the absorber is inserted into the laser cavity, it will look opaque to the laser radiation until the photon flux is large enough to depopulate the ground level. If the upper state is sufficiently populated the absorber becomes transparent to the laser radiation, a situation that is similar to a three-level laser material pumped to a zero inversion level. A passive Q-switch requires a material which exhibits saturation of the ground-state absorption. However, most materials also exhibit absorption from an excited state. This is illustrated in Figure 6 by the transition from the upper state, level 2 to some higher level 4 which has an energy level corresponding to the laser transition. As the ground state is depleted, absorption takes place increasingly between levels 2 and 4. Excited-state absorption results in a residual loss in the resonator when the ground-state absorption has been saturated. The 2 4 transition does not

11 saturate because of the fast relaxation of level 4. A saturable absorber is useful for Q-switching only as long as σ gs > σ es, where σ es is the cross section for excited-state absorption [10]. Solutions of the rate equation lead to an absorption coefficient which is intensity dependent α 0 (E) = α E i Es, (12) where α 0 is the small-signal absorption coefficient and E s is a saturation fluence E s = hν σ gs, (13) where σ gs is the absorption cross section for the 1-3 transition. Important characteristics of a saturable absorber are the initial transmission T 0, the fluence E s at which saturation becomes appreciable, and the residual absorption which results in a T max of the fully bleached absorber. The small signal transmission of the absorber is T 0 = exp( α 0 l s ) = exp( n g σ gs l s ) (14) where l s is the thickness of the bleachable crystal and n g is the ground state density. In order to calculate the transmission as a function of fluence, the photon flux and population density must be considered as a function of position within the absorbing medium. The energy transmission T i of an ideal saturable absorber as a function of input fluence E i is given by T i = E s ln [1 + (e Ei Es 1) T E 0 ]. i (15) A saturable absorber with excited-state absorption can be described by a four-level model. In this case, maximum transmission T max is given by T max = exp( n g σ es l s ). (16) For a nonideal absorber the transmission T n can be approximated by T n = T 0 + T i T 0 1 T 0 (T max T 0 ), (17) where T i is the transmission of an ideal absorber, T 0 and T max are the lower and upper limits of the transmission. The most common passive Q-switch material is Cr 4+ : YAG crystal. The Cr 4+ ions provide the high absorption cross section of the laser wavelength and the YAG provides suitable chemical, thermal and mechanical properties. The crystal Cr 4+ : YAG has broad absorption bands centred at 410 nm, 480 nm, 640 nm and 1050 nm. The typical values of cross sections are σ gs = cm 2 for ground-state absorption and σ es = cm 2 for excited-state absorption at the Nd: YAG wavelength. The excited-state lifetime (level 2) is 4,1 μs and the lifetime of the higher

12 excited state (level 4) is 0.5 ns. With hν = J at 1.06 μm and the above value for σ gs one obtains a saturation fluence of E s = 27 mj cm 2 for Cr 4+ : YAG [10]. Commercially available Cr 4+ : YAG passive Q-switches are specified by the low-power transmission at the laser wavelength. Typical transmission values range from 30 50%, and the crystal thickness is usually between 1 5 mm. Values of the small signal absorption coefficient α 0 vary from 3 6 cm 1. For example, for α 0 = 4cm 1 and l s = 2mm the low-power transmission is T 0 = 45%. The pulse repetition rate can only indirectly be controlled, e.g. by varying the laser's pump power and the amount of saturable absorber in the cavity. Direct control of the repetition rate can be achieved by using a pulsed pump source as well as passive Q-switching. 5 Conclusion A passively Q-switched laser contains a saturable absorber instead of the modulator. For continuous pumping, a regular pulse train is obtained, where the timing of the pulses usually cannot be precisely controlled with external means, and the pulse repetition rate increases with increasing pump power. In the medical fields solid-state Q-switch lasers have found applications in ophthalmology for vision correction and photocoagulation, skin resurfacing, and as replacements for scalpels in certain surgical procedures. In medicine we need pulsed lasers with low repetition rate and high energy. A low energy and high repetition laser system was developed for applications in industry for material processing. This is easier to achieve by active Q-switching techniques. Particularly for low pulse repetition rates, lamp pumping can be an economically favourable option, since discharge lamps are much cheaper than laser diodes for a given peak power. For average powers, however, diode pumping becomes more attractive, also because thermal effects in the laser crystal are strongly reduced. With a wide range of wavelengths, short pulse durations, high average powers and high pulse energies, compact and cost-effective DPSS lasers have a promising future in medical device manufacturing. One reason limiting their implementation is high price of pumping diodes. However, market analysis shows that their price decrease and the main obstacle in development and expansion of DPSS lasers in medicine and industry will disappear. 6 Bibliography 1. Koechner, W., Solid-State Laser Engineering. 2006, New York: Springer. 2. Plaessmann, H., et al., Subnanosecond pulse generation from diode-pumped acoustooptically Q-switched solid-state lasers. Applied Optics, (33): p El-Sherif, A.F. and T.A. King, High-energy, high-brightness Q-switched Tm3+-doped fiber laser using an electro-optic modulator. Optics Communications, (4 6): p Bammer, F. and R. Petkovsek, Q-switching of a fiber laser with a single crystal photo-elastic modulator. Optics Express, (10): p Wikipedia. Laser construction. Available from: 6. Fan, T.Y., Diode-Pumped Solid State Lasers. Lincoln Laboratory Journal, (3): p Davarcioglu, B., An Overview of Diode Pumped Solid State (DPSS) Lasrers. International Archive of Applied Sciences and Technology, (2).

13 8. Richardson, D.J., J. Nilsson, and W.A. Clarkson, High power fiber lasers: current status and future perspectives [Invited]. Journal of the Optical Society of America B, (11): p. B63-B Z. Jankiewicz, K.K., Diode-pumped solid state lasers. Opto-Electronics Review, (1): p Burshtein, Z., et al., Excited-state absorption studies of Cr⁴⁺ ions in several garnet host crystals. Quantum Electronics, IEEE Journal of, (2): p