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

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 the ground state
==> high pumping necessary
the terminal level is an intermediaire state
==> moderate pumping
from Senior, "Optical Fiber Communications", Prentice Hall, 1992

8. 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.

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
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.

10. 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.

11. Light is amplified through stimulated emissionshn
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.

12. 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.

13. 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

14. 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

15. 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

16. Feedback = resonator

17. 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

18. 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

19. 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)

20. 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

21. 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

22. Large value of F means sharp resonance peaks

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

24. Continuous Wave Fiber Lasers

25. 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

26. 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

27. 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

28. 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

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

30. 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%

31. Optical spectra at the two outputs

32. Efficiency is of the order of 23.5%

33. Pulsed Fiber Lasers

34. 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.

35. In a free-running laser, modes normally oscillate independentlynF 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

36. Coupling modes and locking their phases force them to oscillate togethernF 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

37. 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)

38. 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

39. 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).

40. 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

41. 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.

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

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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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

51. 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

52. Pulse compression is possible if b2C<0
GVD
Compression occurs when the dispersion-induced chirp compensates for the initial chirp (b2C<0 is satisfied)

53. 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

54. 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

55. 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)

56. 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

57. 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

58. 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°

59. 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) = NFSR with N >> f
=> Modes no longer locked to their closest but to their Nth closest neighbors (N ~ )
= Harmonic Mode Locking (HML)

60. 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 = 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

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

62. 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

63. 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

64. 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

65. 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)

66. 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

67. 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
qDn 
0.315
0.441
Transform-limited (chirp-free)

68. Exotic configurations
Pulsed fiber lasers with Brillouin mirrors

69. 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)].

70. 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

71. 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)

72. Specifics of fiber lasersQ
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

73. 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

74. 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

75. 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

76. 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

77. 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

78. 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

79. Pulse shape and spectrum
band
Pulse
Peak/average power ~500W/25mW
Pulse duration ~10ns
Spectrum
Linewidth ~0.25nm
3 Brillouin components (~11GHz)

80. Dynamics of Q-switched laser (Rayleigh-Brillouin mechanism)
:FBG

81. Effect of FBG bandwidth
Spectrum below lasing threshold
Low Pump power
Pump ~60mW
No Lasing
ASE
FBG
band

82. Effect of FBG bandwidth
Spectrum near lasing threshold
More power !
Pump ~70mW
1 component
Low signal/ASE contrast
ASE

83. Effect of FBG bandwidth
Spectrum just above threshold
More !
Pump ~80mW
2 components
Shift ~11 GHz
(SBS)
Low signal/ASE contrast

84. Effect of FBG bandwidth
Spectrum much above threshold
More !!!
Pump ~110mW
>10 components
Shift ~11 GHz
(SBS)
High signal/ASE contrast: ~30dB

85. Synchronization of SBS components

86. Pulses and spectra (FBG width 12.5 GHz)
~15ns

87. 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 ]

88. SBS noise limits laser stability (FBG width 12.5 GHz)

89. 900-1800 nm, nanosecond pulse fiber source

90. 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).]

91. 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).]

92. 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.

93. Er/Brillouin Q-switched laser

94. Pulse train and spectrum
2 ms/div.
1556nm
Average power ~25mW
Pulse train
Period ~200µs
Stability ~15%
Spectrum
Signal/ASE contrast >30dB

95. Pulse shape and spectrum
band
Pulse
Peak/average power ~500W/25mW
Pulse duration ~10ns
Spectrum
Linewidth ~0.25nm
3 Brillouin components (~11GHz)

96. SC generation in DSF

97. 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

98. 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)

99. Evolution of the spectrum in DSF.
Modulation instability
Raman
Modulation instability

100. Evolution of the spectrum in DSF.
Broadening of spectrum
Four-wave mixing (FWM) of Brillouin components

101. Effects of bending losses

102. Spectrum correction by coiling of fiber on cylinders8 10 12 6 4
0-8 turns;
diameter 12mm
8 turns;
diameters: 4, 6, 10, 12mm.

103. 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.

104. 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.

105. 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.

106. 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

107. 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

108. 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!

109. 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

110. Other All-fiber Sources

111. All-Fiber Frequency Doubled Er/Brillouin Laser
In collaboration with
Optoelectronics Research Centre (ORC),
University of Southampton
(submitted to CLEO’2006, USA)

112. 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).

113. Pulse train and spectrum
Period ~200µs
Stability ~15%
Spectrum
Second harmonic
Forth harmonic

114. Pulse dynamics
2ns width
Up to ~25W peak power

115. 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)

116. 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

117. 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

118. 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

119. Mode-locked fiber laser (based on nonlinear polarization rotation in a fiber)
In collaboration with
USTL, Lille, France
and FORC, Moscow, Russia

120. Pulse train
500-fs pulse train at 1535nm

121. Spectrum
wavelength

122. Thank you for your kind attention
Olivier Deparis*, Roman Kiyan*,
Andréi Fotiadi, Olivier Pottiez*,
Gautier Ravet, Marc Wuilpart,
Christophe Caucheteur, Sébastien Bette, Véronique Moeyeart