1. Ultra-short pulse operation of all-optical fiber passively mode-locked
Yb lasers
Vladimir Kalosha
Fiber Optics Group
Dept. of Physics, U. of Ottawa
to be published in Optics Express (May 2006)

2. Research Chair Program Acknowledgment to:
L. Chen
X. Bao
Coauthors:
CIPI
NSERC
Research Chair Program
Acknowledgment to:

3. Outline Motivation Advantages: Yb- vs Er-doped fiber lasers
Mode-locked Yb fiber lasers: SESAM vs. NPE
Round-trip Model
Results on dynamics modeling
Particular emphasis: How to generate ultimate shortest pulses directly from the cavity?
Discussion
Conclusion

4. Yb- vs Er-doped fiber lasers

5. Absorption & Emission cross-sections of Yb+3 in Ge:SiO2 glass

6. Passive mode-locking in Yb-doped fiber lasers
Tuennermann et al. OE 13, 9346 (2005): SESAM, self-similar pulsed
regime
Wise et al. OE 11, 3550 (2003): NPE, stretched-pulsed regime,
36-fs, 2-nJ pulses by extra-cavity
compression
Wise et al. OE 13, 3460 (2005): NPE, PCF, sub-100-fs pulses
Okhotnikov et al. OE 14, 4368 (2006): SESAM, both regimes …
The aim: to create an environmentally robust all-fiber Yb fiber laser
generating high-power sub-ps pulses directly from the cavity,
with intra-cavity dispersion compensation by PCF,
with mode-locking by SESAM
Theory: to study generated pulse parameters in dependence on
Laser parameters for both regimes

7. Schematic of passive mode-locked Yb-doped fiber laser
YDF: Yb-doped fiber
SMF: single-mode fiber
DCF: photonic crystal fiber
as a dispersion-compensating fiber
SBR: saturable Bragg reflector/SESAM
OUT: output coupler
PUMP: pump coupler/WDM
nm: YDF, SMF b2>0
DCF b2<0

8. Round-trip model of the laser generation: evolution equations in the intra-cavity fibers…
YDF:
GVD b2>0
Loss
Kerr
nonlinearity
Saturated
bandwidth-limited
gain
SMF & DCF:
GVD b2>0 & b2<0

9. …and in the intra-cavity passive modulator
Fast response
to pulse intensity
SESAM:
parameters as for SESAM from BATOP
Slow response
to pulse energy
Ref.: Kutz et al. JOSA B 14, 2681 (1997)

10. Intra-cavity fiber parameters used in the simulations of the laser dynamics

11. Characteristics of silica fibers used in the simulations of the laser dynamics
YDF, SMF: Analytics for step-index fiber
DCF: Vectorial FEM for photonic crystal fiber (inset)
Normal GVD of YDF & SMF and anomalous GVD of 1050 nm

12. Complete intra-cavity dispersion compensation in the case w/o fiber nonlinearity
Steady-state spectrum, spectral peak location and duration vs. DCF length
LYDF=LSMF=0.7m; LDCF=60…115cm
LDCF=115cm
dechirped
pulse
generated
LDCF=60cm
A complete compensation of
intra-cavity GVD!
Is it possible in the presence of Kerr nonlinearity in the fibers?

13. Importance of Kerr nonlinearity in intra-cavity DCF
“Similariton”:
tparabolic top
tsteep edges
tlinear chirp 3 3 1 1 2 2
1, no DCF
2, DCF with GVD w/o Kerr nonlinearity
3, DCF with GVD and Kerr nonlinearity
N.B.: vertical scale for frequency and delay should be relative to 0th
intensity levels

14. Pulse duration dependence on intra- cavity DCF with Kerr nonlinearity
Long,
phase-modulated,
red-shifted pulses
‘similaritons’
LDCF=84cm
t=2.70/0.13ps
Normal net GVD
=85cm
=1.96/0.09ps
Zero total dispersion
=85.6cm
Shorter,
less phase-modulated,
Stretched-pulse regime
=1.30/0.06ps
Anomalous net GVD
=88cm
=0.29/0.09ps
Multi-pulsed regime
Unstable regime
LDCF

15. Pulse duration dependence on laser gain
Normal net GVD
Anomalous net GVD
Generated pulse duration
Dechirped Pulse duration
Stretched-pulse regime
Similar-pulse regime

16. Discussion Different mechanisms of ultra-short pulse generation at
PML in fiber lasers:
Dispersion and nonlinearity compensation (soliton)1
Interrelation of finite bandwidth gain and positive
dispersion (similariton)2
Stretched pulse mechanism (dispersion-managed
solitons)3
For shortest-pulse generation these effects
should be combined with pulse shortening by
passive modulator and effect of finite gain
bandwidth
1EL 27, 544 (1991)
2PRL 84, 6010 (2000)
3OL 18, 1080 (1993)

17. Pulse transformation along the cavity
Anomalous net GVD
LDCF=87cm, g0=0.6dB/m
Normal net GVD
LDCF=86cm, g0=0.5dB/m

18. Conclusion
Generation model for all-fiber Yb laser includes interrelation of PML/SESAM + saturable finite bandwidth gain + dispersion + Kerr nonlinearity
Dispersion compensation by intra-cavity photonic crystal fiber is shown
Laser parameters were found provided generation of sub-ps phase-modulated pulses
Development of an all-fiber passively mode-locked Yb laser system is under way