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Fiber Lasers and their Applications

Fiber Lasers and their Applications. Prof. Dr Ir Patrice MÉGRET Faculté Polytechnique de Mons Electromagnétisme et Télécommunications Boulevard Dolez 31 7000 MONS. Basic principles. Output. Noise. Amplifier. +. Feedback. A laser is an oscillator and thus needs three ingredients.

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Fiber Lasers and their Applications

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  1. Fiber Lasers and their Applications Prof. Dr Ir Patrice MÉGRET Faculté Polytechnique de Mons Electromagnétisme et Télécommunications Boulevard Dolez 31 7000 MONS

  2. Basic principles

  3. Output Noise Amplifier + Feedback A laser is an oscillator and thus needs three ingredients • Amplifying medium (need external power) • Noise (to start) • Feedback  resonator

  4. Optical Amplification

  5. Three interaction mechanisms are always simultaneously present • (stimulated) absorption • spontaneous emission • stimulated emission • a) ==> optical detectors • b) ==> LED (incoherent) • c) ==> LD (coherent) from Senior, "Optical Fiber Communications", Prentice Hall, 1992

  6. Population inversion is needed to build a amplifier • to produce the population inversion, it is necessary to excite atoms from level 1 to level 2. • This process is called pumping and is achieved using an external energy source (which can be electrical, optical, chemical, ...) from Senior, "Optical Fiber Communications", Prentice Hall, 1992

  7. Three and four level systems are commonly used the terminal level is an intermediaire state ==> moderate pumping the terminal level is the ground state ==> high pumping necessary from Senior, "Optical Fiber Communications", Prentice Hall, 1992

  8. pump pump Laser Effect in a Single-Mode Fiber signal signal Light amplification in fiber is an old story Introduction • Optical amplification in a Neodymium-doped fiber [C. Koester, E. Snitzer, 1963] • Fiber laser at 1.3 µm [J. Stone, C. Burrus, 1974] • Erbium-doped fiber [Southampton University, 1985] • Erbium-doped fiber amplifier [Southampton University, 1986] • ...

  9. The two types of optical fiber amplifier have common features • Erbium-Doped Fiber Amplifiers • 3rd telecommunication window (1.55 µm) • now a mature technology • Praseodymium-Doped Fluoride Fiber Amplifiers • 2nd telecommunication window (1.31 µm) • rely on fluoride fiber progresses but commercial devices available • Main features of optical fiber amplifiers • High optical intensities achievable in singlemode fibers • Geometrical compatibility with fiber links • High gain, large bandwidth, high output power • Quantum limit noise, high linearity, absence of crosstalk • Transparency to bit rate and data format

  10. Er+3 or Pr3+ ions absorb and emit light in different bands

  11. Light is amplified through stimulated emissions hn hns hn hns hnp hn hns Pumping Stimulated absorption of pump photons (Ground State Absorption) Amplification Stimulated emission of signal photons that are coherent (E,f,k) with incident photons Noise Spontaneous emission of photons which are notcoherent but can be amplified by stimulated emission (Amplification of Spontanteous Emission)

  12. lp = 980 nm Excited state lp = 1480 nm Metastable state ls Fundamental state (c) (d) (a) (b) Erbium-doped fiber amplifier is a 3-level laser system p= 980 (1480) nm s= 1530 nm • Rapid non-radiative desexcitation from 3 to 2 : N3=0 (two-level laser system) • Rather long lifetime of ions in the metastable level : 21 =10 ms

  13. Isolator Pin Pout WDM WDM Doped fiber Pump Source An optical fiber amplifier is a rather compact device Pump Source • Pump and signal are injected into rare-earth doped fiber using WDM couplers • Forward, backward or bidirectional pumping schemes • Single-pass or double-pass (with a mirror) amplification schemes

  14. Pout Erbium doped fiber Pin WDM Mirror for signal and/or pump Pump laser diode at 980 nm or 1480 nm EDFA: double pass configuration

  15. Feedback = resonator

  16. Optical waves interfere when they are present simultaneously in the same region of space Case of two monochromatic waves of the same frequencywi Complex amplitudes : Depending on j = j2-j1 : constructive or destructive interference In the case of I1 = I2 = I0 : • I = 4 I0 when j = 0 • I = 0 when j = p

  17. Interferometers can measure small variations of distance, refractive index, wavelength Mach-Zehnder Michelson Sagnac

  18. Fiber Isolator Coupler Optical fibre ring cavity In an optical resonator, light is confined and stored at certain resonance frequencies • filter • spectrum analyser • generation of pulsed or CW laser light (with active medium inside the cavity) • Light circulates or is repeatedly reflected within the cavity • Wavelength selectivity is due to optical feedback Mirror Fabry-Perot cavity

  19. Fabry-Perot cavity is the simplest planar resonator • Resonator modes as standing waves • Resonator modes as travelling waves mode = sol. of Hemholtz eq. satisfying boundary cond. mode = wave that reproduces itself after a single round trip condition of positive feedback nF is the mode spacing no loss r=100 % jm=p d n nr-1 nr nr+1

  20. Losses in a real cavity are not zero Let r² be the intensity attenuation factor introduced by the two mirror reflections and by the absorption in the medium during a round trip (phase shift j) Finesse of the resonator

  21. Large value of F means sharp resonance peaks

  22. M1 M2 E+(z) r'1 E-(z) r'2 Fabry-Perot with an active medium has a threshold for amplification

  23. Continuous Wave Fiber Lasers

  24. Output Noise Amplifier Fiber + Isolator pump Active Fiber Feedback output Optical fiber ring cavity Active Fiber output Output 2 Output 1 50:50 pump Active Fiber pump Optical fiber FP cavity Figure 8 cavity fiber laser A lot of structures have been used Polarization controler

  25. SOUDURE RESEAU DE BRAGG (R= 99%) ISOLATEUR ISOLATEUR MULTIPLEXEUR POMPE RESEAU DE BRAGG (R= 20%) Bras à 980 nmBras à 1550/980 nm Bras à 1550 nm Fibre dopée à l’erbium Fiber laser with two FBG, 5 m of doped fiber and a total length of 13 m (realized by students)

  26. /4 B Input light Mirror Faraday rotator Output light Polarization beam splitter and Faraday rotator are some key elements Input light • Faraday rotation mirror (FRM): a 45° Faraday rotator followed by a conventional mirror • After reflection and double-pass through the rotator, light is returned at the input port (the only port of the FRM) with a 90° polarization rotation • Polarizing beamsplitter (PBS): two prisms from the same anisotropic (uniaxial) material cemented with orthogonal optic axes • [Saleh, Fundamentals of Photonics] • Different refraction angles at the interface for both polarization components • Spatially separates orthogonal polarization states

  27. Single-polarization isolator l/2 PBS PBS l/2 Polarization-independent isolator Optical isolator is based on Faraday rotator and can be polarization independent [Saleh, Fundamentals of photonics] Single-polarization isolator

  28. Pumping is realized at 980 nm and creates amplification at 1550 nm

  29. Two Bragg gratings are used for feedback (same wavelengths but different reflectivities) 1st grating 2nd grating R=99% R=20% • Tuneability is achievable by: • Temperature tuning of FBG • Strain tuning of FBG

  30. Optical spectra at the two outputs

  31. Efficiency is of the order of 23.5%

  32. Pulsed Fiber Lasers

  33. How to get pulses from a laser? • External modulation : CW laser + external switch or modulator • energy is blocked during the off-time of the pulse train • peak pulse power < CW power • Internal modulation : turning the laser itself on and off • energy is stored during the off-time of the pulse train • peak pulse power >> CW power • different methods : • gain switching : gain control by turning the laser pump on and off • Q-switching : periodic loss increase (absorber inside the resonator) • cavity dumping : loss modulated by altering mirror transmittance • mode locking : coupling laser modes and locking their phases

  34. In a free-running laser, modes normally oscillate independently nF n nr-1 nr nr+1 Dn Free-running modes a comb of equally spaced modes (nF) of random phases => train of identical bursts of incoherent light, spaced by trep= tF= 1/ nF trep= 1/ nF t

  35. Coupling modes and locking their phases force them to oscillate together nF n nr-1 nr nr+1 Dn Locked modes a comb of equally spaced modes (nF) in phase => train of very intense and short bursts of light, spaced by trep= tF= 1/ nF trep= 1/ nF 1/trep : repetition rate tp=1/Dn t

  36. tF= 100 ns Peak sharpness increases with the number of locked modes • period of pulse train = round trip time = tF (repetition rate = mode spacing = nF) • pulse width = tp=1/Dn (for Er3+:silica Dn= 4 THz =>tp= 250 fs) • peak intensity (M²|A|²) is M times higher than average intensity (M|A|²)

  37. Frequency domain Time domain cavity loss fmod= nF tF laser output n nr-1 nr nr+1 t phase information of a mode is passed to its neighbours through the modulation sidebands pulse builds up after each round trip because cavity loss is minimum at each passage of the pulse How the modes can be locked together ? • Passive mode locking : use of a saturable absorber • Active mode locking : use of an AM or FM modulator (e.g. electro-optic mod.) with modulation frequency equal to (or a multiple of) the mode spacing nF

  38. - N nF nF + N nF n nr-1 nr nr+1 Harmonic mode locking allows to get high repetition rates with reasonable fibre length • For high repetition rates, the fibre length that is required is too short in practice • if fmod= nF then nF = c0/(nL) = 1 GHz => L = 20 cm • Harmonic mode locking can be used for high repetition rates • if fmod= N nF and N = 100 then nF = c0/(nL) = 1 GHz => L = 20 m • N pulses per round trip trep= (1/N)tF • N supermodes are susceptible to oscillate together => beating between supermodes=> amplitude fluctuations in the pulse train

  39. Electro-optic effects are useful to realize some devices Electro-optic effect = small change in refractive index dn induced by a DC or low frequency electrical field E applied to the material • dn(E) proportional to E = (linear) Pockels effect • dn(E) proportional to E² = (nonlinear) Kerr effect • Electrically controllable optical devices useful in optical communication and optical signal-processing • lens with controllable focal length • phase modulator, dynamic wave retarder • intensity modulator, switch

  40. Applied electric field Ii V 0 I0 A phase modulator in a Mach-Zehnder interferometer ... LiNbO3 waveguide

  41. ... can act as a linear intensity modulator or as an optical switch Linear intensity modulator (j0=p/2) Switch (j0=2p)

  42. Principle: linear electro-optic effect (Pockels effect): Dn  V 50/50 50/50 (AM + PM) OUT (AM only) OUT 2 OUT 1 To cancel PM modulation: = and opposite voltages applied to the arms Dual-output MZM : Output Y-coupler Output X-coupler DC bias DC bias RF input RF input +V -V IN IN RF-driven Mach Zehnder electro-optic intensity modulator is the key element for pulsed fiber lasers

  43. Pump laser diode WDM coupler Erbium-doped fiber DC Amplitude modulator Optical isolator rf amplifier rf generator Output coupler Optical filter Optical pulse train Er-doped fiber lasers as an alternative to semiconductor lasers for pulse train generation Structure of an actively mode-lockederbium-doped fiber ring laser (AML-EDFL): Advantages: • Intracavity pulse shaping (e.g., solitons) • External reference available • Flexibility Drawback: Very sensitive to perturbations, several noises affect the pulse train

  44. Stabilization scheme OC 1 3 Pump LD 2 Polarizing beamsplitter DC WDM Coupler 90° splice PM- FBG RF OUT 2 Piezo drum 8.4 m DCF/ 200 m DSF Er-doped fiber Faraday mirror AM Modulator Optical isolator Output Coupler Optical filter Optical filter Non-PM section PM section Sigma cavity includes both a PM ring and a non-PM branch • Unidirectional, PM ring: • Modulator • Isolator • Filter... • Double-pass, non-PM branch: • Er-doped fiber • DCF / DSF • Fiber under strain • Filter...

  45. PBS PBS transmits reflects /2 splice FRM Optical isolator L = LR + 2LB The sigma laser: a virtually polarization-maintaining cavity • PM fiber (e.g.: PANDA) : linear polarization along one of the polarization axes is maintained • Standard fiber : Intrinsic + stress-induced refractive index anisotropies • Polarization changes randomly during propagation • However: Thanks to the 90° polarization rotation at the FRM, both orthogonal polarization components experience the same delay after one round-trip in the non-PM branch • Hence, initially linear polarization is returned linear to the PBS, rotated by 90°

  46. Length of Er-doped fiber ring lasers ~ 10 - 100 m => FSR ~ 1 - 10 MHz << GHz repetition rates Solution: Modulation frequency fm (= repetition rate) = NFSR with N >> f How to pass round the biggest drawback of fiber lasers => Modes no longer locked to their closest but to their Nth closest neighbors (N ~ 103-104) = Harmonic Mode Locking (HML)

  47. 2f m FSR Rational harmonic mode locking for repetition rate multiplication The pulse train repetition rate fp can be multiplied P times  modulation frequency fm if fm is detuned from optimal HML frequency by a fraction of the FSR : fm = (N+R/P)FSR => fp = Pfm = (NP+R)FSR RHML2 (P = 2) : fm = (N+1/2)FSR Equal pulses [RK et al., OL 25, p. 1439, 2000] RHML3 or + : Pulse-to-pulse fluctuations

  48. Bias doubling is another technique to double the repetition rate

  49. T OUT 2 OUT 1 Glue 1 P 0.1 s Intensity modulation (a) 0.5 (b) (c) 0 t 0 t V /2 -V /2 DL V p p Glue t 0.1 s t RF driving voltage (a) (b) (c) Cavity length is stabilized by minimizing average interpulse noise • This noise is minimal for optimal cavity length tuning (DL = 0) • Fiber has some elasticity => can be adjusted thanks to a piezoelectric crystal • Average interpulse noise is measured at output 2 of the dual-output Mach-Zehnder modulator

  50. 10-Hz dithering HVA DC PR RF Pump LD Polarizing beamsplitter WDM coupler 90° splice Piezo drum Erbium-doped fiber 1 2 AM modulator Optical isolator Output coupler Optical filter Implementation of the feedback loop • Detuning is detected through the measurement of average interpulse noise • A 10-Hz dithering is applied to the piezo in order to determine the sign of the correction • The stabilization scheme operates also in RHML2 regime [Kiyan et al., OL 24, p. 1029, 1999] Faraday mirror

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