Vladimir KaloshaVladimir Kalosha

Ultra-short pulse operation of Ultra-short pulse operation of

all-optical fiber passively mode-locked all-optical fiber passively mode-locked

Yb lasersYb lasers

Fiber Optics GroupFiber Optics Group

Dept. of Physics, U. of OttawaDept. of Physics, U. of Ottawa

to be published in to be published in Optics ExpressOptics Express (May 2006) (May 2006)

CoauthorCoauthors:s:

L. L. ChenChen

X. BaoX. Bao

Acknowledgment to:Acknowledgment to:

CIPICIPI NSERCNSERC Research Chair ProgramResearch Chair Program

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

Yb- vs Er-doped fiber lasersYb- vs Er-doped fiber lasers

Generation

975…1200nm

Generation

975…1200nm

Pump

860-1064 nm

by diodes

Pump

860-1064 nm

by diodes

No ESANo ESA

No

concentration

quenching

No

concentration

quenching

Higher

pump

conversion

Higher

pump

conversion

Broader gain

band

Broader gain

band

Ultra-short

high-energy

pulses

Ultra-short

high-energy

pulses

Positive GVD

Photonic Crystal Fibers

Absorption & Emission cross-Absorption & Emission cross-sections of Ybsections of Yb+3+3 in Ge:SiO in Ge:SiO22

glassglass

Passive mode-locking in Yb-doped fiber Passive mode-locking in Yb-doped fiber laserslasers

Tuennermann Tuennermann et al.et al. OE 13, 9346 (2005): OE 13, 9346 (2005): SESAM,SESAM, self-similar self-similar pulsed pulsed

regimeregime Wise Wise et al.et al. OE 11, 3550 (2003): OE 11, 3550 (2003): NPE,NPE, stretched-pulsed stretched-pulsed

regime,regime,

36-fs, 2-nJ pulses by extra-36-fs, 2-nJ pulses by extra-cavitycavity

compressioncompression Wise Wise et al.et al. OE 13, 3460 (2005): OE 13, 3460 (2005): NPE, PCF, sub-100-fs pulsesNPE, PCF, sub-100-fs pulses Okhotnikov Okhotnikov et al.et al. OE 14, 4368 (2006): OE 14, 4368 (2006): SESAM, both regimesSESAM, 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 SESAMTheory: to study generated pulse parameters in dependence on Laser parameters for both regimes

Schematic of passive mode-locked Yb-Schematic of passive mode-locked Yb-doped fiber laserdoped fiber laser

YDF: Yb-doped fiberSMF: single-mode fiberDCF: photonic crystal fiber as a dispersion-compensating fiberSBR: saturable Bragg reflector/SESAMOUT: output couplerPUMP: pump coupler/WDM

Dispersion @ 1050 nm: YDF, SMF DCF

Round-trip model of the laser Round-trip model of the laser generation:generation:

evolution equations in the intra-cavity fibers…evolution equations in the intra-cavity fibers…

YDF:

SMF & DCF:

GVD2>0 & 2<0

GVD 2>0Kerr

nonlinearity

Loss

Saturated bandwidth-limited

gain

……and in the intra-cavity passive modulatorand in the intra-cavity passive modulator

SESAM:

Ref.: Kutz et al. JOSA B 14, 2681 (1997)

Fast response to pulse intensity

Slow responseto pulse energy

parameters as for SESAM from BATOP

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

Characteristics of silica fibers used in Characteristics of silica fibers used in the simulations of the laser dynamicsthe simulations of the laser dynamics

YDF, SMF: Analytics for step-index fiberDCF: Vectorial FEM for photonic crystal fiber (inset)

Normal GVD of YDF & SMF and anomalous GVD of DCF @ 1050 nm

Steady-state spectrum, spectral peak location and duration vs. DCF lengthLYDF=LSMF=0.7m; LDCF=60…115cm

Complete intra-cavity dispersion compensationComplete intra-cavity dispersion compensationin the case w/o fiber nonlinearityin the case w/o fiber nonlinearity

LDCF=115cm

A complete compensation ofintra-cavity GVD!

Is it possible in the presence of Kerr nonlinearity in the fibers?

LDCF=60cm

dechirpedpulse

generated

3

Importance of Kerr nonlinearity Importance of Kerr nonlinearity in intra-cavity DCFin intra-cavity DCF

1, no DCF2, DCF with GVD w/o Kerr nonlinearity3, DCF with GVD and Kerr nonlinearity

2

1 1

2

3

N.B.: vertical scale for frequency and delay should be relative to 0th intensity levels

“Similariton”: parabolic topsteep edgeslinear chirp

Pulse duration dependence on intra-Pulse duration dependence on intra-cavity cavity DCF with KerrDCF with Kerr nonlinearity nonlinearity

LDCF=84cm

=85cm

=85.6cm

=88cm

=1.96/0.09ps

=1.30/0.06ps

=0.29/0.09ps

=2.70/0.13ps

LDCF

Shorter,less phase-modulated,Stretched-pulse regime

Long,phase-modulated,red-shifted pulses‘similaritons’

Zero total dispersion

Norm

al net

GV

DA

nom

alo

us

net

GV

D

Multi-pulsed regimeUnstable regime

Normal net Normal net GVDGVD

Pulse duration dependence on laser gainPulse duration dependence on laser gain

Anomalous net GVDAnomalous net GVD

Similar-pulse regimeSimilar-pulse regime Stretched-pulse regimeStretched-pulse regime

Generated pulse duration

Dechirped Pulse duration

DiscussionDiscussionDifferent mechanisms of ultra-short pulse generation at Different mechanisms of ultra-short pulse generation at

PML in fiber lasers:PML in fiber lasers: Dispersion and nonlinearity compensation (Dispersion and nonlinearity compensation (solitonsoliton))11

Interrelation of finite bandwidth gain and positive Interrelation of finite bandwidth gain and positive

dispersion (dispersion (similaritonsimilariton))22 Stretched pulse mechanism (Stretched pulse mechanism (dispersion-managed dispersion-managed

solitonssolitons))33

For shortest-pulse generation these effects For shortest-pulse generation these effects

should be combined with pulse shortening byshould be combined with pulse shortening by

passive modulator and effect of finite gain passive modulator and effect of finite gain

bandwidthbandwidth 1EL 27, 544 (1991)2PRL 84, 6010 (2000)3OL 18, 1080 (1993)

Pulse transformation along the cavityPulse transformation along the cavity Normal net GVDNormal net GVD LLDCFDCF=86cm, =86cm,

gg00=0.5dB/m=0.5dB/m

Anomalous net GVDAnomalous net GVD LLDCFDCF=87cm, g=87cm, g00=0.6dB/m=0.6dB/m

ConclusionConclusion

Generation model for all-fiber Yb laser includes Generation model for all-fiber Yb laser includes interrelation of PML/SESAM + saturable finite interrelation of PML/SESAM + saturable finite bandwidth gain + dispersion + Kerr nonlinearitybandwidth gain + dispersion + Kerr nonlinearity

Dispersion compensation by intra-cavity photonic Dispersion compensation by intra-cavity photonic crystal fiber is shown crystal fiber is shown

Laser parameters were found provided generation Laser parameters were found provided generation of sub-ps phase-modulated pulsesof sub-ps phase-modulated pulses

Development of an all-fiber passively mode-locked Development of an all-fiber passively mode-locked Yb laser system is under wayYb laser system is under way