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Fiber Lasers and their Applications
Prof. Dr Ir Patrice MÉGRET Faculté Polytechnique de Mons Electromagnétisme et Télécommunications Boulevard Dolez 31 7000 MONS
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Basic principles
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A laser is an oscillator and thus needs three ingredients
Amplifying medium (need external power) Noise (to start) Feedback resonator Noise Amplifier Feedback + Output
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Optical Amplification
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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
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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
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Three and four level systems are commonly used
the terminal level is the ground state ==> high pumping necessary the terminal level is an intermediaire state ==> moderate pumping from Senior, "Optical Fiber Communications", Prentice Hall, 1992
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Light amplification in fiber is an old story
pump Laser Effect in a Single-Mode Fiber signal 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] ... Optical amplification was demonstrated for the first time in a Neodymium-doped fibre in 1963. In 1974, a fibre laser operated at 1.3 µm for the first time. In 1985, the first Erbium-doped fibre was fabricated at Southampton University and one year after the Erbium-Doped Fibre Amplifier was born.
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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 Erbium-Doped Fibre Amplifiers are designed to operate in the thirs telecommunication window (1.55 µm) while Praseodymium-Doped Fluoride Fibre Amplifiers are designed to operate in the second telecommunication window (1.31 µm). EDFAs are now a mature technology while PDDFAs still rely on the technological progresses of fluoride fibres. However commercial PDFFAs already exist.
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Er+3 or Pr3+ ions absorb and emit light in different bands
Erbium-doped fibres can be pumped in three wavelength bands : pumping at 980 nm is the most usual; pumping at 1480 nm is called resonant pumping; pumping at 810 nm is not efficient because of Excited State Absorption of the pump. Praseodymium-doped fibre are usually pumped around 1015 nm.
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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 not coherent but can be amplified by stimulated emission (Amplification of Spontanteous Emission) The pumping process excites the ions from the ground level to the pump level through stimulated absorption. It realizes the inversion of population. The desexcitation of ions from the metastable level occurs through stimulated emission of photons that are coherent with the incoming signal photons. This produces the amplification of the signal. The generation of noise in optical amplifiers is an effect of the spontaneous desexcitation of the ions. Such spontaneously emitted photons have no coherence with the incoming signal photons and are multiplied by stimulated emission. This amplified spontaneous emission (ASE) is the background noise of the optical amplifier.
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Erbium-Doped Fibre Amplifier is a 3-level laser system
hnp hns 4I11/2 (pump level) 4I13/2 (metastable level) 4I15/2 (ground level) N3=0 N2 N1 lp= 980 (1480) nm ls= 1530 nm t12 Rapid non-radiative desexcitation from 3 to 2 : N3=0 (two-level laser system) Rather long lifetime of ions in the metastable level : t21 =10 ms Local population densities Ni(r,f,z,t) [ions/m³] are given by atomic rate equations Steady-state operation is assumed : Ni(r,f,z,t)=Ni(r,f,z) are time-independent Erbium-Doped Fibre Amplifier is a 3-level laser system in which the terminal level of the laser transition is the ground level. Moreover, the rapid non-radiative desexcitation from pump level to metastable level allows to consider the EDFA as a two-level system.
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Erbium-doped fiber amplifier is a 3-level laser system
lp = 980 nm ls lp = 1480 nm (a) (b) (c) (d) Excited state Metastable state Fundamental state 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
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An optical fiber amplifier is a rather compact device
Doped fiber Pin WDM Pump Source Pout Isolator Pump Source An optical fibre amplifier is a rather compact device. It consists of : one (or two) pump sources (usually high-power laser diodes), input and output optical isolators, WDM (Wavelength Division Multiplexing) couplers to inject pump and signal into the doped fibre. 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
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EDFA: double pass configuration
Erbium doped fiber Pin Pump laser diode at 980 nm or 1480 nm WDM Mirror for signal and/or pump Pout
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Feedback = resonator
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Case of two monochromatic waves of the same frequency wi
Optical waves interfere when they are present simultaneously in the same region of space Case of two monochromatic waves of the same frequency wi Complex amplitudes : Depending on j = j2-j1 : constructive or destructive interference Interference cannot be explained on the basis of ray optics since it is dependent on the phase relationship between the spatially superposed waves. In the case of I1 = I2 = I0 : I = 4 I0 when j = 0 I = 0 when j = p
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Interferometers can measure small variations of distance, refractive index, wavelength
In the Sagnac interferometer, the optical paths are identical but opposite, so that the rotation of the interferometer results in a phase shift proportional to the angular frequency of rotation (e.g. gyroscope). Mach-Zehnder Michelson Sagnac
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In an optical resonator, light is confined and stored at certain resonance frequencies
Light circulates or is repeatedly reflected within the cavity Wavelength selectivity is due to optical feedback Optical fibre ring cavity Fiber Coupler Isolator Mirror Fabry-Perot cavity filter spectrum analyser generation of pulsed or CW laser light (with active medium inside the cavity)
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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
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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
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Large value of F means sharp resonance peaks
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Fabry-Perot with an active medium has a threshold for amplification
E+(z) E-(z) r'1 r'2
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Continuous Wave Fiber Lasers
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A lot of structures have been used
Noise Amplifier Feedback + Output pump Isolator Optical fiber ring cavity Fiber output Active Fiber pump Optical fiber FP cavity Output 1 Output 2 Active Fiber pump output Figure 8 cavity fiber laser Active Fiber 50:50 Polarization controler
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Fiber laser with two FBG, 5 m of doped fiber and a total length of 13 m (realized by students)
MULTIPLEXEUR RESEAU DE BRAGG (R= 20%) RESEAU DE BRAGG (R= 99%) ISOLATEUR Bras à 980 nm Bras à 1550/980 nm Bras à 1550 nm Fibre dopée à l’erbium POMPE SOUDURE
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Polarization beam splitter and Faraday rotator are some key elements
Mirror Input light Output /4 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
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Optical isolator is based on Faraday rotator and can be polarization independent
Single-polarization isolator PBS Polarization-independent isolator [Saleh, Fundamentals of photonics] Single-polarization isolator
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Pumping is realized at 980 nm and creates amplification at 1550 nm
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Two Bragg gratings are used for feedback (same wavelengths but different reflectivities)
1st grating 2nd grating R=99% Tuneability is achievable by: Temperature tuning of FBG Strain tuning of FBG R=20%
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Optical spectra at the two outputs
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Efficiency is of the order of 23.5%
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Pulsed Fiber Lasers
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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 Gain switching is a rather direct approach in which the gain is controlled by turning the laser pump on and off. In Q-switching, the laser output in turned off by increasing the resonator loss (i.e. its quality factor Q) periodically by means of a modulated absorber inside the resonator. During off-times, the energy is stored in the form of an accumulated population inversion (pump is always on). During on-times, this large accumulated population inversion is released, generating intense short pulses of light. Cavity dumping is based on storing photons in the resonator during off-times and releasing them during on-times. By contrast with Q-switching, the resonator loss is here modulated by altering the mirror transmittance. Photons being stored in the resonator during off-times, the mirror is suddenly removed altogether, increasing its transmittance to 100% during the on-times. The result is a strong pulse of laser light. In mode locking, the resonator modes are coupled together and their phases are locked to each other. When mode locking is achieved (e.g. by modulating the losses inside the resonator), the modes behave like the Fourier components of a periodic function and therefore form a periodic pulse train.
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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
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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
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Peak sharpness increases with the number of locked modes
tF= 100 ns 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)
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Analytical expression of a mode-locked pulse train
nF is the mode spacing, tF =1/nF=2d/c is the round trip time |Ar| are determined by gain spectrum of active medium and resonator loss arg(Ar) are random since modes interact in an inhomogeneous broadened medium If M=2S+1 modes have the same phase and amplitude (Ar=A) then
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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 Frequency domain n nr nr+1 nr-1 fmod= nF phase information of a mode is passed to its neighbours through the modulation sidebands Time domain tF t cavity loss laser output pulse builds up after each round trip because cavity loss is minimum at each passage of the pulse A saturable absorber is a medium whose absorption coefficient decreases as the intensity of the light passing through it increases (it transmits intense pulses with relatively little absorption and absorbs weak ones).
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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 n nr nr+1 nr-1 nF - N nF + N nF
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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 Light transmitted through a transparent plate of controllable refractive index undergoes a controllable phase shift. This principle is used in optical phase modulators. Using an interferometric scheme, phase modulation can be converted into intensity modulation. This principle is used in optical intensity modulators and optical switches. Electro-optic effects in an anisotropic material change its effect of polarized light.
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A phase modulator in a Mach-Zehnder interferometer ...
V Ii Applied electric field I0 LiNbO3 waveguide
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... can act as a linear intensity modulator or as an optical switch
Linear intensity modulator (j0=p/2) Switch (j0=2p)
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RF-driven Mach Zehnder electro-optic intensity modulator is the key element for pulsed fiber lasers
(AM + PM) 50/50 Principle: linear electro-optic effect (Pockels effect): Dn V To cancel PM modulation: = and opposite voltages applied to the arms DC bias RF input IN Output Y-coupler +V -V OUT (AM only) Dual-output MZM : OUT 1 Output X-coupler OUT 2 IN DC bias RF input
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The pulse envelope is assumed to be slowly varying
complex wavefunction GVD b0 = b(w0) mode-propagation constant central frequency complex envelope assumed to be slowly varying in comparison to central frequency ~ Fourier transform A(z,n) is a narrow function of with width Dn << n0 n b(n) Dn n0
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Chromatic dispersion is the frequency dependence of the refractive index n(w)
GVD within Dn << n0 : Group Velocity (GV) Group index Group Velocity Dispersion (GVD) GV : pulse shape is not altered as pulse envelope propagates at speed vg GVD : since vg is itself frequency dependent, different spectral components associated with the pulse travel at different speeds leading to pulse broadening
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Normal and anomalous dispersion regimes
GVD D (ps/(nm*km)) l (nm) lD : zero dispersion wavelength (1.3 or 1.5 µm) Normal dispersion D<0, b2>0 Anomalous dispersion D>0, b2<0 Lower frequency (red-shifted) components travel faster than higher frequency (blue-shifted) components Higher frequency (blue-shifted) components travel faster than lower frequency (red-shifted) components
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GVD does not affect the pulse spectrum but modifies the pulse shape
z, t T0 P0 T normalized amplitude GVD Propagation equation in a loss-free linear dispersive medium Fourier GVD changes the phase of each spectral component by an amount that depends on the frequency and the propagated distance Such phase changes do not affect the pulse spectrum but can modify the pulse shape
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Gaussian unchirped pulse becomes chirped when it propagates
broadening T1>T0 GVD Broadening is independent on the sign of b2 for an unchirped pulse and is larger for smaller initial pulse width T0 Time-dependent phase implies that instantaneous frequency ni differs across the pulse from central frequency n0 => linear frequency chirp dw=ni-n0 which depends on the sign of b2
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Pulse broadening and dispersion-induced chirp in normal dispersion regime (b2>0)
GVD Red-shift Blue-shift Red-shift Blue-shift Up-chirp : red-shifted components (leading edge of the pulse) travel faster than blue-shifted components (tailing edge of the pulse) in normal dispersion regime
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Gaussian pulse with initial linear chirp remains gaussian when it propagates
GVD initial linear chirp C chirp broadening T1>T0 Without chirp (C=0), spectral widthDw is Fourier-transform-limited (DwT0=1) With linear chirp (C>0 (<0) : up-(down) chirp), spectral width Dw is enhanced by (1+C²)1/2 For an initially chirped pulse, broadening depends on the relative signs of b2 and C b2C >0 : pulse broadens monotically, b2C <0 : initial narrowing stage
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Pulse compression is possible if b2C<0
GVD Compression occurs when the dispersion-induced chirp compensates for the initial chirp (b2C<0 is satisfied)
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SPM does not affect the pulse shape but modifies the pulse spectrum
Propagation equation in a nonlinear medium with loss but no dispersion SPM gives rise to intensity-dependent phase shift, increasing with propagated distance, while the pulse shape (|U(z,T)|²) remains unchanged Time-dependent phase implies nonlinear frequency chirp new frequency components are continuously generated as the pulse propagates Actual shape of the pulse spectrum is obtained by taking the Fourier transform
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Case of a Gaussian pulse
SPM dw<0 (red-shift) near the leading edge while dw>0 (blue-shift) near the tailing edge For Gaussian pulses, SPM-induced chirp is linear and positive (up-chirp) over a large central region of the pulse Shape of the SPM-broadened spectrum depends on pulse shape and initial chirp
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Different propagation regimes
Dispersion length Nonlinear length L << LD, L << LNL : neither dispersive nor nonlinear effects play a significant role (only fibre loss) L > LD, L << LNL : pulse evolution governed by GVD L << LD, L > LNL : pulse evolution governed by SPM L > LD, L > LNL : GVD and SPM act together (fibre can support solitons through the interplay of GVD and SPM effects)
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Er-doped fiber lasers as an alternative to semiconductor lasers for pulse train generation
Structure of an actively mode-locked erbium-doped fiber ring laser (AML-EDFL): Advantages: Optical filter Erbium-doped fiber rf amplifier rf generator Optical isolator Pump laser diode Output coupler WDM coupler Amplitude modulator DC Optical pulse train Intracavity pulse shaping (e.g., solitons) External reference available Flexibility Drawback: Very sensitive to perturbations, several noises affect the pulse train
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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 PM section Non-PM section 1 3 2 PM- FBG OC DC RF Optical filter Er-doped fiber isolator Pump LD Output Coupler WDM AM Modulator Piezo drum Faraday mirror Polarizing beamsplitter Stabilization scheme 90° splice 8.4 m DCF/ 200 m DSF OUT 2
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The sigma laser: a virtually polarization-maintaining cavity
FRM Optical isolator PBS /2 splice PBS transmits reflects L = LR + 2LB 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°
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How to pass round the biggest drawback of fiber lasers
Length of Er-doped fiber ring lasers ~ m => FSR ~ MHz << GHz repetition rates Solution: Modulation frequency fm (= repetition rate) = NFSR with N >> f => Modes no longer locked to their closest but to their Nth closest neighbors (N ~ ) = Harmonic Mode Locking (HML)
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Rational harmonic mode locking for repetition rate multiplication
FSR 2f m 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 = Pfm = (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
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Bias doubling is another technique to double the repetition rate
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Cavity length is stabilized by minimizing average interpulse noise
/2 -V t T 1 0.5 RF driving voltage Intensity modulation OUT 1 OUT 2 DL P (a) (c) (b) 0.1 s t Glue 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
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Implementation of the feedback loop
Optical filter Erbium-doped fiber isolator Pump LD Output coupler WDM AM modulator Piezo drum Polarizing beamsplitter DC RF 90° splice PR 10-Hz dithering HVA 1 2 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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Stabilization feedback loop is needed
Optical filter Erbium-doped fiber isolator Pump LD Output coupler WDM AM modulator Piezo drum Faraday mirror Polarizing beamsplitter DC RF 90° splice PR 10-Hz dithering HVA 1 2
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Pulse train measurement and characterization is complicated
90:10 coupler 50:50 coupler Photodiode OSA Sampling oscilloscope ESA VSA EDFA Auto correlator Electronics Polarization controller Computer Power splitter (a) 0.2 0.4 0.6 0.8 1 Time (ns) HML RHML2 Sampling scope traces (fm = 3 GHz)
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Several typical noise contributions are identified in the radio-frequency spectrum
-30 -20 -10 10 20 30 -100 -80 -60 -40 Frequency offset (kHz) Power (dBm) Relaxation oscillations Amplitude jitter Power fm 5fm Frequency 2.95 3 3.05 -100 -80 -60 -40 -20 Frequency (GHz) Power (dBm) Supermode noise Amplitude jitter (Phase jitter) 101 102 103 104 -110 -100 -90 -80 -70 -60 Frequency offset (Hz) Power (dBm) Low-frequency noise Amplitude jitter Phase jitter Pulse width jitter
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Optical spectrum and autocorrelation trace
1540 1540.5 1541 1541.5 1542 1542.5 -70 -65 -60 -55 -50 -45 Wavelength (nm) Power (dBm) HML RHML Dn (FWHM) Optical spectra of 3-GHz and 6-GHz pulse trains fm = 3 GHz Resolution = 0.02 nm Background-free optical autocorrelation -10 -5 5 10 0.2 0.4 0.6 0.8 1 Delay (ps) Relative intensity qAC (FWHM) 2 4 6 8 -4 -3 -2 -1 qAC/2 gaussian solitonic qAC q 0.648 2/2 qDn 0.315 0.441 Transform-limited (chirp-free)
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Exotic configurations
Pulsed fiber lasers with Brillouin mirrors
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Q-switched fiber lasers
Diode pump ~976 nm AOM Yb-Doped fiber Pulses 0.1 – 1 µs Such fiber lasers could be realized from the use of rare-earth-doped fibers in combination with non-fiber elements (AOM, EOM…) [J.A. Alvarez-Chavez et al. “High-energy, high-power ytterbium-doped Q-switched fiber laser”, Opt.Lett. 25, 37-39(2000)].
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Q-switched fiber lasers: passive & all-fiber solutions
Self-pulsing in Er-laser (due to quenching): 0.1-ms-pulses with ms-period [P. Le Boudec et al., Opt.Lett. 18, 1890 (1993)] Q-switching in Yb-laser due to a saturable absorber (Sm-doped fiber): 0.5-µs pulses with ~20 µs period [A. A. Fotiadi et al., CLEO-Europe (2005)] Q-switching with Brillouin mirror: the most effective!!! 1-ns-pulses with 200-µs-period [S.Chernikov et al., Opt.Lett.22, 298, 1997]; Ns-pulse generation Peak/average power contrast >105 Universal method Pulse-to-pulse stability is rather poor
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Q-switched lasers: principle of operation
Close Q - switcher ( Tr 0 ) rod mirror TQ mirror Open Gain R1 R2 Tr ~ 1 Q - switcher ( Tr 1 ) rod mirror mirror Q-switch parameters important for ns-pulse generation: Switching time TQ in ns-range. Switching contrast Tro / Trc >>1 (for long cavities)
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Specifics of fiber lasers
Q Cavity length is long pulse duration is long also. for example: L ~ 10 m nL/c ~100ns AOM mechanism, µs [J.A. Alvarez-Chavez et al. Opt.Lett. 25, 37-39(2000).] One-pass gain in the fiber can be very high: G > 20 dB if provided by high switching contrast: Tropen / Trclose > 40 dB
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Q-switched fiber lasers: ns-pulse generation
Q-switcher: ns switching time high contrast >40dB RE-doped fiber Q one-pass gain ~ 20 dB mirror Signal with high contrast >40dB Laser = MOPA configuration RE-doped fiber Low-signal one-pass gain ~ 40 dB
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Stimulated Brillouin scattering is a good candidate for Q-switching
SBS is low threshold process: ~10 Wm Switching time ~ hypersound delay time: ~10 ns High switching contrast (up to ~ exp(~20) ~ 85 dB) ~ exponential intensity grows with power
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Rayleigh scattering supports SBS
- fast varying function Fiber laser RS mirror Traditional single-longitude-mode solid-state laser GAIN GAIN Optical fiber mirror mirror Set of parallel quartz plates RS mirror causes strong narrowing of the laser line
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Self-Q-switched fiber laser (simplest configuration)
Rare - earth doped fiber amplifier Single mode optical fiber Cavity mirror Laser cavity Output Principle of operation is Rayleigh-SBS mechanism Has been used for Raman Q-switched laser
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Self-Q-switched fiber laser (configuration with a ring mirror)
Principle of operation is Rayleigh-SBS mechanism For resonance ring frequencies it is equivalent to previous
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Self-Q-switched fiber laser
~80% ~20% 1480nm ~120 mW 5/95 ~1.25m ~1m ~5 m 2 ms/div. All-fiber integrated format Standard telecom components Low-pump power (~120mW) Peak/average power contrast: 500W/25 mW (up to ~105) Poor pulse-to-pulse stability
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Pulse shape and spectrum
band Pulse Peak/average power ~500W/25mW Pulse duration ~10ns Spectrum Linewidth ~0.25nm 3 Brillouin components (~11GHz)
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Dynamics of Q-switched laser (Rayleigh-Brillouin mechanism)
:FBG
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Effect of FBG bandwidth
Spectrum below lasing threshold Low Pump power Pump ~60mW No Lasing ASE FBG band
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Effect of FBG bandwidth
Spectrum near lasing threshold More power ! Pump ~70mW 1 component Low signal/ASE contrast ASE
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Effect of FBG bandwidth
Spectrum just above threshold More ! Pump ~80mW 2 components Shift ~11 GHz (SBS) Low signal/ASE contrast
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Effect of FBG bandwidth
Spectrum much above threshold More !!! Pump ~110mW >10 components Shift ~11 GHz (SBS) High signal/ASE contrast: ~30dB
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Synchronization of SBS components
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Pulses and spectra (FBG width 12.5 GHz)
~15ns
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Experiment and simulations (FBG width 12.5 GHz)
Power, W ~300 W Output 2 Output 1 40 ns/div. 40 ns/div. [A.A.Fotiadi, P.Mégret, M.Blondel, Opt.Lett., Vol.29, N10, 2004, pp ]
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SBS noise limits laser stability (FBG width 12.5 GHz)
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900-1800 nm, nanosecond pulse fiber source
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Tunable nanosecond pulse sources
Until now such supercontinuum sources have been available only from the use of conventional solid-state or microchip Q-switched lasers and in combination with PCF [L. Provino et al., “Compact broadband continuum source based on microchip laser pumped microstructural fibre”, Electronic Letters 37, (2001).] [W. J. Wadsworth et al. “Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibres“, Optics Express 12, (2004).]
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Low-power all-fiber solution
We propose the use of a low-power Er/Brillouin oscillator for external SC generation in conventional DSF [A.A. Fotiadi et al., “Dynamics of self-Q-switched fiber laser with Rayleigh – stimulated Brillouin scattering ring mirror”, Opt.Lett. 29, (2004).] High-power (2W) Yb/Brillouin laser has already demonstrated its ability to generate SC intracavity. [S.V. Chernikov et al., “Supercontinuum self-Q-switch Ytterbium laser,” Opt.Lett., 22 (1997).]
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Principle configuration
Concrete realization Pump laser wavelength 1480nm; pump power ~120mW Er/Brillouin laser ~1556 nm, linewidth ~0.25 nm, peak power ~500W DSF: Zero of GWD ~1547 nm; Slope ~0.074 ps·nm-2 ·km-1 D=0.67 ps·nm-1 ·km-1 (at ~1556nm) The source has been characterized within nm. Fast detector operation range is nm.
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Er/Brillouin Q-switched laser
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Pulse train and spectrum
2 ms/div. 1556nm Average power ~25mW Pulse train Period ~200µs Stability ~15% Spectrum Signal/ASE contrast >30dB
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Pulse shape and spectrum
band Pulse Peak/average power ~500W/25mW Pulse duration ~10ns Spectrum Linewidth ~0.25nm 3 Brillouin components (~11GHz)
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SC generation in DSF
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Pulse train and spectrum
2 ms/div. 1556nm Average power ~25mW Pulse train Period ~200µs Much better stability Spectrum 900-over 1800nm Average power ~25mW
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Pulse shape Pulse ~80% power is over 1700nm Before DSF After DSF
Two peaks ~100W Duration ~25ns ~80% power is over 1700nm Out of the detector range (1000nm – 1700nm)
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Evolution of the spectrum in DSF.
Modulation instability Raman Modulation instability
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Evolution of the spectrum in DSF.
Broadening of spectrum Four-wave mixing (FWM) of Brillouin components
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Effects of bending losses
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Spectrum correction by coiling of fiber on cylinders
8 10 12 6 4 0-8 turns; diameter 12mm 8 turns; diameters: 4, 6, 10, 12mm.
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Modification of the pulse shape
0 turns 2 turns 4 turns 10 ns/div. 10 ns/div. 10 ns/div. 6 turns 0, 2, 4, 6 turns; diameter: 12mm. 10 ns/div.
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Example Pulse Spectrum 8 turns;diameters: 12 mm. Peak power ~10W
Average power: ~200µW Pulse Peak power ~10W Duration ~7ns Spectrum Width >500nm 8 turns;diameters: 12 mm.
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Example Pulse Spectrum 8 turns;diameters: 10 mm. Peak power ~7W
Average power: ~150µW Pulse Peak power ~7W Duration ~5ns Spectrum Width ~400nm 8 turns;diameters: 10 mm.
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Example Pulse Spectrum Peak power ~2W Central wavelength 1.05µm
Average power: 25µW Pulse Peak power ~2W Duration ~2ns Infinitive on/off contrast Spectrum Central wavelength 1.05µm Width <50nm
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Average power only 25 µW: Raman conversion
Average power ~25 µW ~1062 nm Spectrum Wavelength 1053nm Linewidth ~50nm ~1105 nm Pulses Peak power ~2W Duration ~2ns Raman shift: ~43nm
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Amplification in an Yb-amplifier
Average power ~25 µW ~20cm Peaks ~500W Spectrum Wavelength 1053nm Linewidth ~50nm Input Pulses Peak power ~2W Duration ~2ns Infinitive contrast Output Pulses Peak power ~500W Duration ~1-2ns Power ~30mW Input Pulses have incredibly high on/off contrast!
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Conclusions Concept of fiber lasers with Brilouin mirror:
ns-switching time with ~40dB switching contrast Peak/average power contrast ~105 with ns-pulses Instability is determined by SBS noise and FBG profile The simplest way to realize a nanosecond pulse source operating within 900-over 1800 nm Low-power, all-fiber format, low-cost solution Multi-cascade Brillouin process; four-wave-mixing of Brillouin components; modulation instability;Raman Unique pulse performance Watt-peak-power level (for band >25nm) 2-10 ns duration Infinitive on/off contrast
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Other All-fiber Sources
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All-Fiber Frequency Doubled Er/Brillouin Laser
In collaboration with Optoelectronics Research Centre (ORC), University of Southampton (submitted to CLEO’2006, USA)
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Principle configuration
Concrete realization Pump laser wavelength 1480nm; pump power ~120mW Er/Brillouin laser ~1556 nm, linewidth ~0.25 nm, peak power ~500W Periodically poled silica fiber The ~10-cm long PPSF was fabricated by C. Corbary from a twin-hole fiber by point-to-point UV-erasure of the second-order nonlinearity induced by uniform thermal poling (T=250 oC, ~30 minutes with ~4 kV applied).
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Pulse train and spectrum
Period ~200µs Stability ~15% Spectrum Second harmonic Forth harmonic
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Pulse dynamics 2ns width Up to ~25W peak power
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All-Fiber Ytterbium Laser Employing Samarium Fiber as Saturable Absorber
In collaboration with USTL, Lille, France and FORC, Moscow, Russia (EU patent is applied for, CLEO-EUROPE; submitted to CLEO’2006, USA)
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Absorption spectrum of Sm-doped fiber
Isolator Pump/signal filter Saturable absorber ( nm) ~1064 nm ~1085 nm Suitable for many other wavelengths 1064nm 976nm 1085nm Yb Yb, absorption Yb, emission
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Principle configuration
All fiber spliced configuration! Pump up to ~20W at ~976nm FBG: at 1085nm and 1064nm The use of a Sm-doped fiber as an optical isolator 1-3 m as a saturable absorber 10 cm Yb-doped fiber Inner clad: 125x125 µ2 Core Ø: ~6 µ Sm-doped fiber Clad Ø: ~125 µ
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Pulse train All-fiber pulsed laser source Passive pulse operation
High stability Performance characteristics Wavelength: 1085 nm Pump: ~7 W Average power: ~2.5 W Peak power: ~30 W Pulse duration: ~600 ns Period: ~8 µs Similar behavior is observed at nm
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Mode-locked fiber laser (based on nonlinear polarization rotation in a fiber)
In collaboration with USTL, Lille, France and FORC, Moscow, Russia
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Pulse train 500-fs pulse train at 1535nm
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Spectrum wavelength
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Thank you for your kind attention
Olivier Deparis*, Roman Kiyan*, Andréi Fotiadi, Olivier Pottiez*, Gautier Ravet, Marc Wuilpart, Christophe Caucheteur, Sébastien Bette, Véronique Moeyeart
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