Vật lý Laser 2013 - Chương IV: Các loại laser và ứng dụng

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Vật lý Laser 2013 - Chương IV: Các loại laser và ứng dụng

  1. 1. LASER VÀ ỨNG DỤNG TS. Nguyễn Thanh Phương Bộ môn Quang học và Quang điện tử
  2. 2. Chương IV: Các loại laser và ứng dụng
  3. 3. Chương IV: Các loại laser và ứng dụng 04/12/2013 3
  4. 4. Chương IV: Các loại laser và ứng dụng Nhắc lại: những yếu tố cấu thành laser • tương tác giữa ánh sáng và vật chất • đảo mật độ tích lũy • môi trường khuếch đại thích hợp • buồng cộng hưởng quang học • tương tác giữa một buồng cộng hưởng quang học và khuếch đại bên trog BCH: - phương trình tốc độ của laser - ngưỡng phát laser - bão hòa khuếch đại - so sánh mode và lọc lựa mode 04/12/2013 4
  5. 5. Chương IV: Các loại laser và ứng dụng IV.1. Laser rắn
  6. 6. IV.1. Laser rắn - Dựa trên dịch chuyển của các ion kim loại (e.g.Cr3+) hoặc ion đất hiếm (Nd3+, Ho3+). Các ion này phân bố trong môi trường tinh thể hoặc thủy tinh với mật độ ~1019/cm3. - Dịch chuyển laser xảy ra chủ yếu giữa các trạng thái điện tử nội, chúng ít bị ảnh hưởng bởi tương tác giữa tạp chất cũng như mạng tinh thể.  Suy giảm không bức xạ không đáng kể, dịch chuyển tương đối “sắc nét”, vạch phổ không bị nở rộng do tương tác với tinh thể.  Giảm ngưỡng bơm - Bơm quang học: Nd-lasers bơm bằng flash lamps (xung) hoặc laser bán dẫn. cw-Ti:Sa-lasers are typically bơm bằng Ar+ lasers hoặc Nd:YAG laser nhân đôi tần số (532 nm). 04/12/2013 6
  7. 7. IV.1. Laser rắn - Bên cạnh laser bán dẫn, laser rắn ngày nay được sử dụng rộng rãi và đạt hiệu quả thương mại nhất • Nhỏ gọn, thuận tiện (1W Nd:YAG lasers không yêu cầu làm lạnh bằng nước) • Hiệu suất cao (nếu được bơm bằng laser bán dẫn) • Công suất quang ra cao (TW), (e.g. Nd:glass lasers) • Dịch chuyển bước sóng trong dải rộng (Ti:Sa: 660 to 986 nm) • Xung cực ngắn (fs-Ti:Sa lasers) • Tính ổn định cao (công suất, tần số) 04/12/2013 7
  8. 8. IV.1. Laser rắn Laser Ruby 04/12/2013 8
  9. 9. IV.1. Laser rắn • Laser ruby - Tinh thể ruby: Cr3+ pha tạp trong tinh thể sapphire (Al2O3) - Truyền qua vùng màu hồng - Chế tạo lần đầu năm 1960, laser đầu tiên trong lịch sử! Không được sử dụng nhiều nữa - Hoạt động cả ở chế độ xung và liên tục - dài ~ 5-20 cm  ~ 5-10 mm - hiệu suất tổng cộng ~1% 04/12/2013 9
  10. 10. II.2.4. Một số loại khuếch đại laser • Laser ruby (tiếp) ... Là 1 laser rắn, đại diện hệ 3 mức năng lượng. - Mức 1 là trạng thái cơ bản - Mức 2 là kết hợp 2 mức năng lượng rất gần nhau, trạng thái thấp nhất tương ứng với bước sóng đỏ 694,3 nm. - Mức 3 là kết hợp của 2 dải có bước sóng trung tâm tương ứng 550 nm và  ~ 50ns 400nm. energy converted into heat  ~ 3ms 2A thermalization (~1ns) E ~400nm ~550nm R2 R1 electric dipole allowed only because of interaction with crystal 04/12/2013 10
  11. 11. IV.1. Laser rắn • Laser ruby (tiếp) ... Dùng 1 đèn flash (ánh sáng trắng) kích thích Cr3+ từ 1 -> 3. Cr3+ phân rã từ 3 -> 2 với thời gian 32 cỡ ps. Các nguyên tử này nằm lại ở 2 với thời gian tsp  3 ms. Dịch chuyển không bức xạ được bỏ qua. Dịch chuyển này nở rộng vạch đồng nhất với Dn  330 GHz.  ~ 50ns energy converted into heat  ~ 3ms 2A thermalization (~1ns) E ~400nm ~550nm R2 R1 electric dipole allowed only because of interaction with crystal 04/12/2013 11
  12. 12. IV.1. Laser rắn • Laser ruby (tiếp) ... Loại Bước sóng (nm) Công suất đỉnh Độ rộng xung Xung bình thường 694,3 100 kW < 0,5 ms Q-switch 694,3 10 – 50 MW 10 – 20 ns Mode-locking 694,3 ~ GW 10 – 30 ps CW 694,3 1 mW  04/12/2013 12
  13. 13. IV.1. Laser rắn … Ruby (Rubin) continued optical and laser properties of ruby at room temperature 04/12/2013 13
  14. 14. IV.1. Laser rắn Neodymium Lasers 04/12/2013 14
  15. 15. IV.1. Laser rắn • first realized with glass1961, with YAG 1964. • crystal - Nd:YAG is the most important material used for solid state laser systems. YAG stands for YttriumAluminum-Garnet, Y3Al2O12, a colourless, isotropic crystal. For a Nd:YAG laser rod ~1% of the Y3+ ions is replaces by Nd3+ ions. The YAG-structure is very stable from lowest to highest temperature, its mechanical stability and workability (growing, grinding, polishing) as well as the achievable optical quality are good. absorption spectrum of Nd:YAG 04/12/2013 15
  16. 16. IV.1. Laser rắn … Nd:Lasers continued - level scheme of Nd:YAG fast, nonradiative decay  ~240µs fast, nonradiative decay - Nd:YAG is a four-level laser, it is homogeneously broadened - lasing is mainly supported by the R2 sub-level of the 4F3/2 level. At room temperature ~40% of 4F3/2 atoms are in R2 ( Boltzmann). - "strongest" laser transition at 1064.1 nm - lower laser level is 4I11/2 with various sub-levels, which all give slightly different emission wavelength. - lower laser levels are thermally not populated, so inversion can easily be achieved, even for cwoperation. 04/12/2013 16
  17. 17. IV.1. Laser rắn • applications - material processing (cw and pulsed lasers) welding, marking, writing, drilling (sizes of few µm possible), cutting - medical, especially ophthalmology - illumination and ranging (military) - pumping of other lasers (e.g. frequency doubled Nd:YAG for pumping of Ti:Sa lasers) and non-linear optics (e.g. frequency doubling [532 nm], tripling [355 nm], quadrupling [256 nm], parametric conversion). - Nd:glass lasers and corresponding amplifiers are also used for laser fusion experiments 04/12/2013 17
  18. 18. IV.1. Laser rắn • discharge pumping - cw-lasers are pumped by diode lasers at ~ 810nm or various types of discharge lamps or filament lamps, pulsed lasers by flash lamps. - energy corresponding to non-radiative decays limits quantum efficiency to ~ 76%. Excess power (~24%) is converted into heat, which has to be dissipated. Light not absorbed by the pump bands is also partially converted into heat arc lampe Wolframlamp Diode laser array 2 kW 500 W 1W Useful power 100 W 5W 0.2 W Laser power 8W 0.23 W 0.06 W Conversion efficiency 0.4% 0.04% 6% lifetime 400 h 100 h 5000 h Total electrical pumping power 04/12/2013 18
  19. 19. IV.1. Laser rắn … discharge pumping continued - discharge lamps (cw) arc lamps - typical electrical power: ~ 1…10 kW, ~ (100V, 50 A) - arc length ~ 50mm - lifetimes few 10h … ~ 1000 h - discharge/filament tube is mounted inside a flow tube which carries the coolant (liquid). filament lamps - typical electrical power: ~ 1kW, - filament length ~ 50mm - lifetimes ~100h 04/12/2013 19
  20. 20. IV.1. Laser rắn … discharge pumping continued - discharge lamps (pulsed) • typical (small linear lamp): 60mm long, 4mm diameter, 10J input over 300µs @ 10pps for a 60mm long, 6 mm diameter rod  Q-switched output 100…200mJ, lamp lifetime ~106 shots • typical (large linear lamp): 16cm long, 13mm diameter, ~2kJ input over 1ms @ ~1pps, lamp lifetime ~105 shots 04/12/2013 20
  21. 21. IV.1. Laser rắn … discharge pumping continued - thermal loading pulsed Nd:YAG lasers as well as other solid-state systems can provide very high peak powers (many GW) and large pulse energies (many joules). Especially if lamps (~ 10 kW electric power each) are used for pumping, thermal loading of the crystal is a serious, power-limiting issue. Absorption of pump plight outside the pump band, and heating due to nonunity quantum efficiency • will induce thermal lensing through temperature dependence of the index of refraction. This modifies the resonator geometry dynamically! • thermal stress causes birefringence and can even lead to damage of the crystal. Reduction of problems arising from thermal loading requires • uniform pumping • good heat removal 04/12/2013 21
  22. 22. IV.1. Laser rắn … discharge pumping continued - thermal birefringence Light transmitted through a pair of crossed polarizers with a Nd:glass rod in between. Light is injected by a second laser, and only a single flash lamp pulse is applied.  polarization of light is dynamical and spatially dependent, i.e. light is depolarized 04/12/2013 22
  23. 23. IV.1. Laser rắn … discharge pumping continued - slab geometry a slab geometry provides a number of advantages over rod-designs: • pumping is more homogeneous • larger surface per volume (better heat removal) • temperature gradients only in y-direction.  cartesian symmetry helps to avoid thermal stress induced depolarization problems (laser emissions is already polarized in the y-z plane due to Brewster cut of crystal) 04/12/2013 23
  24. 24. IV.1. Laser rắn … discharge pumping continued - different slab geometries exist single (dual) flash lamp design multiple flash lamp design 04/12/2013 24
  25. 25. IV.1. Laser rắn … discharge pumping continued - disk geometry beneficial for ultra-high power pulsed solid-state laser systems like those used for laser fusion (at Lawrence Livermore National Laboratory): - better cooling, - larger aperture ( >70cm ) - better gain uniformity - better beam quality 04/12/2013 25
  26. 26. IV.1. Laser rắn • diode pumping - high pumping efficiency, because diode lasers at 810nm match Nd:YAG absorption bands very well  reduction of thermal load problems (thermal lensing, thermal birefringence)  improved total electrical-to-optical efficiency - better pump beam quality: pump laser light can be focused into the gain volume (especially for end-pumped systems) - longer MTBF (mean time between failure): typically 10.000 h for diode lasers vs. a few hundred h to about 1000 h for discharge lamps. - operation simplified: reduced cooling requirements, no high voltage "spikes", no UV-light which degrades crystal, optics and coolant. - a single diode laser can provide a few W cw-power (typically not fundamental mode). Single transverse mode laser diodes with ~0.1 W up to 1 W output power exist. Sometimes broad stripe diode lasers, 1Darrays ("bars") or 2D-arrays can be used. 04/12/2013 26
  27. 27. IV.1. Laser rắn … diode pumping continued 04/12/2013 27
  28. 28. IV.1. Laser rắn … diode pumping continued 04/12/2013 28
  29. 29. IV.1. Laser rắn … diode pumping continued - there are several geometries for optical pumping with laser diodes • end pumped systems (single and double) - pump light can be matched to mode volume 04/12/2013 29
  30. 30. IV.1. Laser rắn … diode pumping continued • side pumping of a rod - direct coupling (diodes close to amplifier) - coupling with optics - fiber coupling (!) • achievable: optical cw-pumping at ~10kW, cw-output typical 100W, up to ~1kW 04/12/2013 30
  31. 31. IV.1. Laser rắn … diode pumping continued • side pumping of a slab - applies 2D-arrays or densely-packed 1D-arrays - diodes very close to slab, no optics required 04/12/2013 31
  32. 32. IV.1. Laser rắn … Nd:Lasers continued - physical, optical, thermal properties of Nd:YAG A MISER oscillator (Monolithic Isolated Single-mode End-pumped Ring), or alternatively, an NPRO (Non-Planar Ring-Oscillator): the crystal itself constitutes the amplifier, optical resonator, and optical diode to enforce unidirectional oscillation. T. J. Kane and R. L. Byer, Opt. Lett. 10 (2), 65 (1985) ; I. Freitag et al., Opt. Commun. 115, 511 (1995) 04/12/2013 32
  33. 33. IV.1. Laser rắn • other solid state lasers Name Chemical form Center wavelength (nm) Range (nm) Temperature Pumping source Efficiency (%) 04/12/2013 33
  34. 34. IV.1. Laser rắn … other solid state lasers continued Tuning range for various transition metal solid state lasers large tuning range of Ti:Sa is basis for ultra-short pulse operation 04/12/2013 34
  35. 35. IV.1. Laser rắn cw-Ti:Sa laser system Coherent MBR 110, tuning range - further information regarding Ti:Sa lasers see 2.3.4 04/12/2013 35
  36. 36. Chương IV: Các loại laser và ứng dụng IV.1. Laser rắn IV.2. Laser khí
  37. 37. IV.2. Laser khí • gas lasers: general - gas lasers are very important historically. (Besides the ruby laser) they where the first lasers to be used, and they have been cultivated very well. There are still some advantages over other types of lasers, but most of the gas lasers are being replaced by semiconductor or solid state lasers. - advantages • gas lasers can be powerful (few 10 W cw in the optical domain, e.g. ArI laser) • work well down (even cw) into the deep blue or UV (ArI, copper laser) • gas lasers exist for many different wavelength, and different gas species may be combined to give a large variety of laser types - problems all gas lasers have very low efficiency (typically 10-3 or less), poor stability (power and frequency), can not easily be frequency controlled, and typically have poor beam quality (all compared to solid-state or semiconductor lasers) 04/12/2013 37
  38. 38. IV.2. Laser khí • pumping there are different methods of pumping. - most common type is based on a continuous or pulsed discharge - optical pumping (with another laser) - gas-dynamical lasers: through fast adiabatic expansion, the gas is transferred to a non-equilibrium state. It approaches a new equilibrium at lower temperature, but for some gases and transitions the lower laying rotational vibrational states re-thermalize faster than some excited rotational vibrational state: transient inversion between rotationalvibrational states is generated. - chemical lasers: inversion is generated through a chemical reaction • general features - gas lasers are among the most powerful (cw and pulsed) lasers. However, the beam profile, linewidth, stability, and tuneability can typically not compete with dye lasers, solid state lasers, or diode lasers. 04/12/2013 38
  39. 39. IV.2. Laser khí • the role of collisions collisions (between electrons and laser atoms, between gas atoms or between laser atoms and the containing walls) play an important role for gas lasers - collisions with e- transfer atomic population into the upper laser level. - collisions between atoms can transfer energy from one atom of some other atomic species to the laser atoms ("collisions of the second kind") A*  BLaser  A  B* LASER These processes are effective if the collision is almost resonant, i.e. the laser atom needs about the same amount of energy for excitation as the atom A can deliver through de-excitation during the collision. - collisions with the wall can help to transfer atoms from the lower laser level to the electronic ground state if a spin-flip is required (which can not be provided by a fast radiative (i.e. electric dipole) transition). 04/12/2013 39
  40. 40. IV.2. Laser khí HeNe Lasers 04/12/2013 40
  41. 41. IV.2. Laser khí • level scheme - He and Ne are mixed at a ratio of ~ 5:1 in a discharge tube - He is excited to high lying states, from which it decays rapidly to the 1s2s level - He 1s2s states are meta-stable, as transition to the ground state corresponds to a l=0l=0 tran-sition, S=1S=0 corresponds to a spin-flip, i.e. is strongly forbidden.  population accumulates in the He 1s2s state - He 1s2s collides with ground state Ne and transfers the full excitation energy to the Ne atom in a nearly resonant exchange collision. 04/12/2013 41
  42. 42. IV.2. Laser khí … gas lasers: HeNe continued - some transitions and laser lines in HeNe not allowed not allowed 04/12/2013 42
  43. 43. IV.2. Laser khí … gas lasers: HeNe continued • typical setup - HeNe lasers are typically based on capillary discharge tubes. The small diameter provides effective de-excitation to the electronic ground state through collisions with the wall. It further also provides transversal mode selection, so that HeNe lasers typically run in fundamental Gaussian mode. - two concepts for discharge tubes exist: (i) smaller tubes are typically sealed with the end caps formed by the mirrors. There is no user access to the mirrors! (ii) Alternatively separate discharge tubes with Brewster windows are used, which provide "polarization selection". - the shortest HeNe lasers (~20cm) provide single axial mode oscillation! 04/12/2013 43
  44. 44. IV.2. Laser khí … gas lasers: HeNe continued - HeNe laser lines - laser activity covers many lines between 543 nm (green) 3.39µm (IR) with output powers of up to a few mW. Popular and commercially available lines are: 543 nm 594 nm 612 nm 633 nm 1523 nm and 04/12/2013 44
  45. 45. IV.2. Laser khí … gas lasers: HeNe continued • common HeNe laser parameters wavelength transition Typ. Output power 04/12/2013 45
  46. 46. IV.2. Laser khí … gas lasers: HeNe continued • application Red HeNe lasers have many industrial and scientific uses. - They are widely used in laboratory, because of relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence (a single mode gaussian beam) and long coherence length - however since about 1990 semiconductor lasers have offered a lower cost alternative for many such applications. - A consumer application of the red HeNe laser is the LaserDisc player, made Pioneer. The laser is used in the device to read the optical disk. 04/12/2013 46
  47. 47. IV.2. Laser khí Ar-Ion (Ar+) Lasers 04/12/2013 47
  48. 48. IV.2. Laser khí • gas lasers: ArI (Argon-Ion laser) - inversion is achieved by a twostep mechanism 1. ionization of Ar-atoms in the discharge tube: Ar  e   EKIN   Ar   e   slow  (fast) collisions 2. excitation of Ar-ions in the discharge tube: Ar   e   E KIN   Ar   excited   e   slow  - the ArI-laser is a four level laser, it provides cw-operation - ArI lasers provide output powers of up to a few 10 W (multiline), and can be operated single line in the green (514 nm) and in the blue (488 nm) and at other slightly different wavelength 04/12/2013 48
  49. 49. IV.2. Laser khí • gas lasers: ArI (Argon-Ion laser) continued - common ArI laser lines [nm]: 454, 457, 465, 472 477, 483, 488, 496, 502, 514, 520, 568 04/12/2013 49
  50. 50. IV.2. Laser khí • gas lasers: ArI (Argon-Ion laser) continued 04/12/2013 50
  51. 51. IV.2. Laser khí • gas lasers: ArI (Argon-Ion laser) continued 04/12/2013 51
  52. 52. IV.2. Laser khí • gas lasers: ArI (Argon-Ion laser) continued • applications - Ar+ lasers have been extensively used as pump lasers for dye lasers and cw-TiSa lasers. They are now being replaced by all-solid-state laser system, which are based on frequency doubled NdYag lasers (1064 nm  532 nm), that are more compact, much more efficient, cheaper, more stable and typically provide better beam profiles. - holography (but see comment above) - medical applications (but see comment above) - laser light shows (but see comment above) 04/12/2013 52
  53. 53. IV.2. Laser khí • emission wavelength of various nobel gas lasers 04/12/2013 53
  54. 54. IV.2. Laser khí Excimer Lasers 04/12/2013 54
  55. 55. IV.2. Laser khí • excimers - excimers are diatomic molecules which do not posses a stable electronic ground state. They only exist as excited dimers. - because the electronic ground state is unstable, the laser atoms dissociate immediately after they have reached the lower laser level. Excimer lasers are effectively four level lasers with a very fast (~ps) decay from the lower laser level to & laser emission the system ground state (i.e. two atoms). The lower laser level is effectively unpopulated. - many dimers provide gas laser activity, e.g. ArF, KrF, XeF, HgCl, NaXe, Xe2Cl, … 04/12/2013 55
  56. 56. IV.2. Laser khí … excimer lasers continued - excimer lasers are typically pumped by (i) an electron beam (current 5-50 kA, 5-500 A/cm2) (ii) a pulsed gas discharge (power densities of discharge ~ 200 MW/dm3, 1 dm3 typical discharge volume) and emit pulses with temporal width on the order of 10 ns. - repetition rates are in the few Hz to ~100 Hz range - excimer lasers are based on molecular electronic transitions. Excimer lasers therefore provide tuneable laser activity in the deep blue-to-UV wavelength range (down to below 100 nm) 04/12/2013 56
  57. 57. IV.2. Laser khí … excimer lasers continued - excimer lasers provide large amplification (~0.1/cm). Typically, excimer lasers provide large peak power (MW-GW) and pulse energy (~J). - due to large amplification excimer lasers do not require low loss cavities. Consequently the emission features poor beam profile quality and modest coherence length. - excimer lasers are or have been used for • pump sources for pulsed dye lasers • LIDAR systems (Light Detection And Ranging) • material processing and surface cleaning • due to the large peak powers and energies excimer lasers have also been used in non-linear optics to generate deep-UV coherent radiation through high-order frequency conversion in laser generated plasmas. Today these lasers can often be replaced ultra-short (fs) pulse laser systems (e.g. Ti:Sa-based) which provide significantly higher peak powers because their pulses are much shorter. 04/12/2013 57
  58. 58. IV.2. Laser khí … excimer lasers continued • excimer parameters 04/12/2013 58
  59. 59. IV.2. Laser khí … excimer lasers continued • common excimer laser parameters 04/12/2013 59
  60. 60. IV.2. Laser khí N2 Lasers 04/12/2013 60
  61. 61. IV.2. Laser khí • N2 laser active medium - N2 is a gas-laser medium which provides three different types of laser activity described below • lasers based on transitions between different electronic states (emission in the blue-to-UV range) • vibration-rotation lasers based on transitions between different vibrational states of the same electronic state (emission: 3µm 300µm) • rotation lasers based on transitions between different rotational states of the same vibrational and electronic state (emission: 25µm – 1mm) 20 04/12/2013 61
  62. 62. IV.2. Laser khí • N2 laser active medium - all transitions to the A 3S+u state (0.75µm…1.24µm) are self-terminating: transition to the electronic ground state X 1S+u is not dipole allowed (intercombination line). Lasers based on the A 3S+u as the lower laser level can therefore only be operated as pulsed lasers. - visible laser activity can be observed between the C 3Pu state and the B 3P state. The lower laser level is long lived (~30 µs, increase to ~10 ms g due to interaction with atoms) so that these lasers are self-terminating as well. - if the laser active gas medium is not quickly exchanged between subsequent laser pulses the repetition rate has to be limited to ~100 Hz in order to allow the population to decay back to the electronic ground state. 04/12/2013 62
  63. 63. IV.2. Laser khí • N2 lasers - N2 lasers provide very large amplification (2.2 / cm for the 337 m line), so that the inversion is fully depleted by a single trip through the amplifier. Therefore these lasers can be operated without mirrors (typically, at least one mirror is used). - typical N2 laser parameters - lasers are typically operated with pure N2 at a pressure between a few 1 mbar and ~1 bar. - beam profile quality and coherence length is poor. - applications: pump laser for dye lasers, spectroscopy 04/12/2013 63
  64. 64. IV.2. Laser khí CO2 Lasers 04/12/2013 64
  65. 65. IV.2. Laser khí • CO2 laser active medium - CO2 is a gas-laser medium which provides vibrational-rotational laser activity. In can be operated in cw- as well as in pulsed mode. CO2 lasers are the most powerful cw lasers at all (~100 kW cw !!) - CO2 normal modes 1351.2 cm-1 (7.5 µm) 672.2 cm-1 (14.9 µm) two-fold degenerate, upper index gives resulting angular momentum, l=n2, n2-2,…1 or 0 2396.4 cm-1 (4.2 µm) 04/12/2013 65
  66. 66. IV.2. Laser khí … CO2 laser active medium continued - the CO2 laser is discharge pumped, discharge contains also N2 and He. excitation through N2 is very efficient (it is almost resonant, corresponding N2 state is meta-stable) upper laser level life time: 1µs … 1 ms lower laser level is depleted through collisions, especially through collisions with He. - CO2 laser feature very high efficiency (quantum efficiency: 45%, electrical-to-optical efficiency: up to 30%) 04/12/2013 66
  67. 67. IV.2. Laser khí … CO2 laser active medium continued - many different technical realizations of CO2 lasers exist. • lasers with a slow, longitudinal N2-flow (~80 W / m) flow removes dissociation products (CO, O2) • "sealed-off" lasers (up to 60 W/m, many 1000 h of continuous operation) addition of H2O + H2 or H2 + O2 causes CO to react to CO2. • lasers with a fast N2-flow (few 10 kW cw): a fast (~300m/s) flow guarantees fast exchange of active volume. This is important especially for high power lasers: the fast flow provides convection to remove the heat which would otherwise limit the excitation density 04/12/2013 67
  68. 68. IV.2. Laser khí … CO2 laser active medium continued • waveguide lasers (20 W / m) "optical" fields can be guided in waveguides rather than between mirrors. This allows a compact setup. • transversally excited atmospheric pressure (TEA) laser uses a short (<1µs) electric pulse between transversally oriented electrodes. This allows high pressure and consequently larger power (peak power MW to GW, energy many 10 J / liter ). 04/12/2013 68
  69. 69. IV.2. Laser khí … CO2 laser active medium continued • applications - cutting and welding - lower power level lasers are used for engraving. - Some examples of medical uses are laser surgery, skin resurfacing ("laser facelifts") , treat certain skin conditions - fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. - Because the atmosphere is quite transparent to infrared light, CO 2 lasers are also used for military rangefinding using LIDAR techniques.CO2 lasers are used in the Silex process to enrich uranium. 04/12/2013 69
  70. 70. IV.2. Laser khí … CO2 laser active medium continued • typical CO2 laser parameters 04/12/2013 70
  71. 71. Chương IV: Các loại laser và ứng dụng IV.1. Laser rắn IV.2. Laser khí IV.3. Laser bán dẫn
  72. 72. IV.3. Laser bán dẫn Semiconductor Lasers Courtesy of Sacher Lasertechnik 04/12/2013 72
  73. 73. Laser Focus World, Issue 1, Vol. 47, Jan 2011 Semiconductor lasers  Communication & Optical Storage (transmitter lasers, R/W lasers)  Medical equipment (hair removal, surgery, dentistry, ophthalmology, PDT)  Material processing (welding of plastic materials, soldering and annealing of metals)  Lighting & Display applications  Measuring equipment (incl. sensors)  Pumping of solid-state and fiber lasers  ...  Science and research
  74. 74. Application (II) Laser Transmitter modules for Fiber Optics Communication Diode laser for optical data storage
  75. 75. Application (III) Material processing Plastics welding with diode lasers [ Leister Process Technologies] Heat Treatment of Metal
  76. 76. Application (IV) Diode lasers for optical pumping system
  77. 77. Application (V)
  78. 78. Application (VI) Red LD for Photo-dynamic Therapy Low-cost surgery module @ 980nm Diode Laser accupunture ...
  79. 79. Application (VII) Frequency Stabilized Diode Laser @ 670 nm for Shifted Excitation Raman Difference Spectroscopy
  80. 80. History (I) Light Amplification by Stimulated Emission of Radiation Ch. Townes N.G. Basov A. M. Prokhorov Zh. Alferov H. Kroemer Nobel prize winners (pioneers of semiconductor lasers) 1962: Groups at GE, IBM, and MIT's Lincoln Lab. simultaneously develop GaAs laser and first semiconductor laser desmontrated (in cryogenically cooled, pulsed operation). Oct. 1962: N. Holonyak Jr. (GE Co. Lab. in Syracuse, N.Y) publishes his work on the "visible red" GaAsP laser diode.
  81. 81. History (II) 1963: H. Kroemer of the University of California, Santa Barbara, R. Kazarinov & Zh. Alferov team of the Ioffe Physico-Technical Institute in St. Petersburg, independently propose ideas to build semiconductor lasers from semiconductor heterostructures. Spring 1970: Zh. Alferov’s group at the Ioffe Physico-Technical Institute St. Peterburg, Russia and M. Panish and I. Hayashi at Bell Lab. produce the first CW room-temperature semiconductor lasers, paving the way toward commercialization of fiber optics communications. 1972: Ch. Henry at Bell Lab. invents the QW laser, which has very low lasing threshold than conventional diode lasers, and more efficient. 04/12/2013 81
  82. 82. History (III) 1975: Engineers at Laser Diode Labs Inc. develop the first commercial CW semiconductor laser operating at room temperature. 1975: First QW laser operation made by Jan P. Van der Ziel, R. Dingle, R. C. Miller, W. Wiegmann, and W.A. Nordland Jr. The lasers are actually developed in 1994. 1976: First demonstration, at Bell Labs, of a CW semiconductor laser at room temperature at a wavelength beyond 1 µm, the forerunner of sources for long-wavelength lightwave systems. 1994: The first quantum cascade laser (QCL) - is invented at Bell Lab. by J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson and A. Y. Cho (changing the thickness of the semiconductor layers can change the laser’s wavelength; room-temperature operation and power and tuning ranges features are ideal for remote sensing of gases in the atmosphere.
  83. 83. History (III) 1994: The first demonstration of a quantum dot laser with high threshold density was reported by N. N. Ledentsov of A.F. Ioffe Physico-Technical Institute in St. Petersburg Jan. 1997: S. Nakamura, S. P. Den Baars and J. S. Speck at University of California, Santa Barbara, announce the development of GaN laser emitting at blue-violet light in pulsed operation. Sep. 2006: J. Bowers and colleagues at the University of California, Santa Barbara, and M. Paniccia, director of Intel’s Photonics Technology Lab. in Santa Clara, California, announced that they have built the first electrically powered hybrid silicon laser using standard silicon manufacturing processes. The breakthough could lead to lowcost, terabit-level optical data pipes inside future computers.
  84. 84. IV.3. Laser bán dẫn • general - semiconductor lasers rely on solid state physics. Most common type is diode laser, which applies physics of semiconductor diode (pn-junction) - "pro's" of semiconductor lasers • simple pumping: current injection • high efficiency: typically the differential efficiency ( DPout/DPin above threshold) is ~50% • very compact: typical dimension is 100µm  100µm  500µm for typical 10mW …100mW (single transverse mode) or up to few 10 W for transverse multimode lasers • available at almost all wavelength between ~400nm and ~2µm 04/12/2013 84
  85. 85. IV.3. Laser bán dẫn … pro's of semiconductor lasers continued • diode lasers are relatively cheap: diode "chip" ranges between few Euro (i.e. for consumer electronics) and few 1000 Euro, mostly depending on (i) production volume, (ii) wavelength, (iii) power. • to make a diode laser from a laser diode, current and temperature stabilization electronics as well as opto-mechanics have to be added (total cost between 10.000 and 20.000 Euro for a scientific diode laser) • good tuneability: typically, diode lasers are tuneable by a few % of the central wavelength • very agile: fast frequency modualtion via current modulation (up to GHz) 04/12/2013 85
  86. 86. IV.3. Laser bán dẫn - "con's" of semiconductor lasers - pour beam quality: elliptic, e.g. 1x3 or larger aspect ratio, and astigmatic, distortion, side lobes - large line width: ~MHz typically, can be reduced by orders of magnitude; active stabilization requires large (~MHz) control bandwidth - very sensitive to optical, electrical, and electrostatic damage (anyone who has ever build a diode laser has "killed" a laser diode) - strong dependence on current and temperature (e.g. ~100 GHz/K and 30 GHz / mA for a single transverse laser diode at 850nm): for a spectroscopy laser temperature stabilization at mK level is required and the current source has to be ultra-low noise (typically few µARMS at diode currents of 100mA for a laser diode with few mW output)  most spectroscopy stabilization applications require active frequency 04/12/2013 86
  87. 87. IV.3. Laser bán dẫn • principle of operation - based on the recombination between electrons pumped into the conduction band and holes in the valence band. During this process a photon is spontaneously emitted, or is created by a stimulated emission process. quasi-Fermi-energy of … … conduction band … valence band 04/12/2013 87
  88. 88. IV.3. Laser bán dẫn … principle of operation continued - in thermodynamical equilibrium the (quasi-) Fermi energy related to the electrons in the conduction band (FL) and of the holes in the valence band (FV) are identical. For the conduction band the quasi Fermi-energy gives the energy of highest laying level which is populated by an electron (T=0 K). For the valence band the quasi Fermi-energy gives the energy of highest laying level which is populated by a hole (T=0 K). If the Fermi-energy lays in between the conduction and valence band, an undoped "semiconductor" is an isolator. 04/12/2013 88
  89. 89. IV.3. Laser bán dẫn … principle of operation continued - pn-junction lasers • with no voltage applied the quasi-Fermilevels are degenerate. No inversion is achieved (at T=0K) • with voltage applied in forward direction thermal non-equilibrium is established and the degeneracy of quasi-Fermi-levels is removed in the junction zone. pn-junction, no bias If FL-FV>Eg inversion is generated in the junction zone, and electrons in the conduction band and holes in the valence band can recombine. pn-junction, forward bias • typical and common semiconductors are GaAlAs (~800nm), InGaAsP (1.3µm -1.5µm), GaInP (670 nm) 04/12/2013 89
  90. 90. IV.3. Laser bán dẫn … principle of operation continued p-n homojunction & heterojunction
  91. 91. IV.3. Laser bán dẫn … principle of operation continued Biased p-n homojunction & heterojunction
  92. 92. IV.3. Laser bán dẫn … principle of operation continued • front and rear end of the semiconductor can provide the optical feedback if appropriately reflection coated. Then the laser diode provides laser operation and can be considered a diode laser. • especially in spectroscopy applications at least one of the ends is AR-coated and feedback is provided by external, frequency selective elements. Then, the chip functions as an amplifier only and should be termed laser diode 04/12/2013 92
  93. 93. IV.3. Laser bán dẫn • pumping different methods of pumping semiconductor lasers exist - optical pumping - electron beam pumping (with high energy electrons generated by electron gun) - current injection (term: injection laser or diode laser) This is the most common application Inversion can be created in a thin layer (~1µm) of the pn-junction. Laser emission therefore always features large divergence angles (few 10 deg HWHM) at least in the direction normal to the junction. 04/12/2013 93
  94. 94. IV.3. Laser bán dẫn • homojunction lasers - consist of p- and n-doped zones of identical semiconductor material; first laser diodes realized - threshold current density of homojunction lasers is ~100kA/cm2 at room temperature, at room temperature operation therefore only in pulsed mode Homojunction lasers can be operated in cw-mode at low temperatures (few 10 K) Homojunction lasers were soon replaced by heterojunction lasers, where different host material was used for the different layers 04/12/2013 94
  95. 95. IV.3. Laser bán dẫn • gain-guided double heterostructure lasers - two structure boundaries are used: Ga1-xAlxAs-GaAs and GaAs-Ga1-yAlyAs to reduce threshold current (density) - this design (i) avoids diffusion of electrons and holes out of the active area so that the active zone is better localized  threshold current is reduced (ii) provides a wave guide like confinement for the vertical direction due the relatively larger index of refraction of Ga1-xAlxAs optical loss in non-active region is reduced  threshold current is reduced threshold current density ~1 kA/cm 2 (iii) thin (~10µm) wide top electrodes confine the gain region in horizontal direction, so that transverse single mode operation can be achieved (gain-guiding) 04/12/2013 95
  96. 96. IV.3. Laser bán dẫn … gain-guided double heterostructure lasers continued 04/12/2013 96
  97. 97. IV.3. Laser bán dẫn • index-guided double heterostructure lasers - the active region is confined in horizontal direction by a diode oriented such that it is biased in reverse direction under operating conditions. This forces the injection current into the active region, and it provides wave guide like confinement in the horizontal direction. Both decreases threshold current (density) Index-guided heterostructure lasers have proven to work well. They provide threshold currents as low as 10 mA and feature transverse single mode operation. 04/12/2013 97
  98. 98. IV.3. Laser bán dẫn • quantum well heterostructure lasers - quantum well lasers use a multilayer sandwich of very thin heterostructure layers (e.g. GaAs-Ga1-xAlxAs, each structure ~5nm). This way the active area is confined vertically to ~30nm, which is less than the de-Broglie wavelength of the electrons This design further reduces the threshold current. Threshold current is less dependent on temperature, so that quantum well lasers can also provide high output powers (~100mW, single transverse mode) at room temperature. 04/12/2013 98
  99. 99. IV.3. Laser bán dẫn • Distributed Bragg Reflector (DBR) lasers - transverse single mode operation of diode lasers can be achieved by transverse and horizontal confinement (gain-guiding, index-guiding, ridge waveguides) - DBR lasers use an on-chip periodic structure outside the active region which acts like a volume phase grating (Bragg diffraction) and selects one wavelength (longitudinal mode) for operation. typical emission spectrum reflector section burried grating gain section metallization ridge waveguide 04/12/2013 99
  100. 100. IV.3. Laser bán dẫn • DFB and DBR lasers - transverse single mode operation of diode lasers can be achieved by transverse and horizontal confinement (gain-guiding, index-guiding, ridge waveguides) - DBR (Distributed Bragg Reflector) laser use an on-chip periodic structure outside the active region which acts like a grating and selects one wavelength (longitudinal mode) for operation - DFB (Distributed Feed Back) laser use an on-chip periodic structure inside the active region which acts like a grating and selects one wavelength (longitudinal mode) for operation. - DFB and DBR provide single mode operation without any additional external elements, but they can by far not be coarsly tuned as well as "regular" lasers. 04/12/2013 100
  101. 101. IV.3. Laser bán dẫn • Distributed Feedback (DFB) lasers - DFB (Distributed Feed Back) laser use an on-chip periodic structure inside the active region which acts like a grating and selects one wavelength (longitudinal mode) for operation. metallization typical emission spectrum burried grating ridge waveguide - DFB and DBR provide single mode operation without any additional external elements, but they can by far not be coarsely tuned as well as extended cavity diode lasers (sometimes also called: “external cavity DL” ). 04/12/2013 101
  102. 102. IV.3. Laser bán dẫn • single-frequency diode lasers: Littrow lasers - for spectroscopic applications lasers have to be operated in singlefrequency mode, i.e. in single axial and transverse mode. Gain-guided and index guided double heterostructure lasers guaranty TEM00 mode operation, but single axial mode operation has to established through frequency selective components in an extended (also: “external”) cavity. - the most simple approach is the "extended cavity" diode laser design, where light is fed back from a grating in Littrow configuration ( 6.2) grating in Littrow configuration short focal length (few mm) lens with large numerical aperture (0.5..0.6) grating has large line density so that • only one diffraction order exists • for a given orientation of the grating the first diffraction order is diffracted right back into the diode laser only for one specific wavelength (typically 30%) • 0-order provides laser output 04/12/2013 102
  103. 103. IV.3. Laser bán dẫn … single-frequency diode lasers: Littrow lasers continued - the Littrow wavelength follows from the grating equation  d  sin  IN  sin OUT where  denotes the wave length, d the distance between two adjacent grating lines, and QIN and QOut the incidence angle and the exit angle of the first diffraction order. For a Littrow setup the geometry requires  IN  OUT so that   sin  ,  IN  OUT 2d so that, if Q~45 deg is required, then d ~ 2 For a 850 nm laser this corresponds to 1660 lines / mm 04/12/2013 103
  104. 104. IV.3. Laser bán dẫn … single-frequency diode lasers: Littrow lasers continued - the extended cavity design reduces the laser linewidth: as the cavity length is increased (from few 100µm to few cm) the intrinsic laser linewidth is decreased according to DnLASER ~ 1/L3 A typical grating laser line width is 100 kHz – 1 MHz (over 1..10 ms), the intrinsic linewidth is significantly smaller (in the kHz range) - continuous tuning range of AR-coated laser diode in a grating setup is a few GHz, with special mechanical design 50 GHz…100 GHz. With special care taken for mechanical tuning, tuning of current (and temperature) the diode laser can provide continuous scanning all through its gain bandwidth. - absolute tuning range of AR-coated laser diodes in a grating setup depends on laser diode, but will typically be ~1-5% of central wavelength. Non-AR coated laser diodes have to be temperature tuned for wavelength tuning, and achieve significantly smaller absolute tuning ranges. 04/12/2013 104
  105. 105. IV.3. Laser bán dẫn … single-frequency diode lasers: Littrow lasers continued Courtesy of Sacher Lasertechnik 04/12/2013 105
  106. 106. IV.3. Laser bán dẫn • single-frequency diode lasers: Littman lasers - the Littman design is an alternative external cavity design. In the Littman setup the first diffraction order is retro-reflected by an additional mirror. Frequency tuning is now achieved by tilting the planar retro-mirror. - Littman setups • make use of the grating twice, i.e. the grating provides larger selectivity • the output beam does not move as the laser is tuned 04/12/2013 106
  107. 107. IV.3. Laser bán dẫn … single-frequency diode lasers: Littrow lasers continued Courtesy of Sacher Lasertechnik 04/12/2013 107
  108. 108. IV.3. Laser bán dẫn • single-frequency diode lasers: diode laser with resonant optical feedback - grating lasers have relatively large line width because the corresponding cavity finesse is low (“effective reflection" from grating 30% max) - one can use an external, resonant cavity with much higher finesse as optical reference. Light coupled into the cavity will be coupled back to the diode once per round trip (resonant optical feedback). D C E M G laser diode collimator etlaon curved cavity mirror glass plate to pick off some (4%) of light Thanks to the phase-intensity coupling ( large line width enhancement factor, Henry's alpha-parameter) the laser emission will phase lock to the reflected light (self-injection locking) 04/12/2013 108
  109. 109. IV.3. Laser bán dẫn … diode laser with resonant optical feedback continued - note that the cavity only feeds back light in case of resonance - the cavity also acts like a low pass filter which suppresses high-frequency phase noise. For fast phase noise it acts like a fly wheel, to which the laser is locked / locks itself The laser frequency is very close to one of D laser diode C collimator the cavity resonance frequencies E etlaon - these lasers provide narrower linewidth (few M curved cavity mirror G glass plate to pick off 10 kHz), and reduced phase noise at high some (4%) of light Fourier frequencies. They are more easy to phase lock, but they are harder to operate and they provide smaller absolute (few nm) as well as continuous tuning ranges (~100 MHz). 04/12/2013 109
  110. 110. IV.3. Laser bán dẫn • single-frequency diode lasers: grating enhanced external cavity diode laser D laser diode COL collimator GRT volume holographic transmission grating OD optical diode HWP half wave plate MF curved cavity mirror MP MC planar coupling mirror HCD Hänsch-Couillaud detector G stabilization electronics - this setup combines good absolute and continuous tuneability of grating diode lasers with narrow linewidth of diode lasers with resonant optical feedback. 04/12/2013 110
  111. 111. IV.3. Laser bán dẫn • grating enhanced external cavity diode laser 04/12/2013 111

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