Mode-locked picosecond pulse generation from an octave-spanning supercontinuum

0.18 MB·English

by  D. Kielpinski

Mode-locked picosecond pulse
generation from an octave-spanning
supercontinuum
D.KielpinskiandM.G.Pullen
1
CentreforQuantumDynamics,GriffithUniversity,NathanQLD4111,Australia
1
0
J.CanningandM.Stevenson
2
InterdisciplinaryPhotonicsLaboratories(iPL),SchoolofChemistry,MadsenBuildingF09,
n UniversityofSydney,Sydney,NSW,2006,Australia
a
J P.S.WestbrookandK.S.Feder

3 OFSLaboratories,Somerset,NewJersey08873,USA
2 d.kielpinski@griffith.edu.au

]
s
c Abstract: We generate mode-locked picosecond pulses near 1110 nm
i
t by spectrallyslicing and reamplifyingan octave-spanningsupercontinuum
p
sourcepumpedat1550nm.The1110nmpulsesareneartransform-limited,
o
with1.7psdurationovertheir1.2nmbandwidth,andexhibithighinterpulse
.
s coherence.Boththesupercontinuumsourceandthepulsesynthesissystem
c
i are implemented completely in fiber. The versatile source construction
s
suggests that pulse synthesis from sliced supercontinuummay be a useful
y
h techniqueacrossthe1000-2000nmwavelengthrange.
p
© 2011 OpticalSocietyofAmerica
[

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1. Introduction
The advent of optical frequency counting techniques using octave-spanning supercontinuum
(SC) generation [1, 2] suggests that similar technology can prove useful in the synthesis
of optical radiation. Much useful work has been done to show the possibilities of optical
frequency synthesis by phase-locking a stable single-frequencylaser to an optical frequency
counter.Such techniquescan achieveultimate frequencyresolutionof a partin 1019 [3] with
broad and fast tunability over wide frequency ranges [4, 5]. More direct approaches obtain
highlycoherentlightbyamplifyinga singlecombline[6] ora bandofcomblines[7],or by
injection-lockinganexternallaseroscillatorwithasinglecombline[8].
Onthetime-domainside,spectralslicingofrelativelynarrowband(<200nm)supercontinua
hasbeenusedtogeneratepicosecondpulsetrainsinhundredsofwavelengthchannelsatonce
[9,10]andthesepulsesareknowntohavecoherencetimesgreaterthanthepulseduration[11].
Ramanself-frequencyshiftingalsoallowsthegenerationofpspulsesatvariouswavelengths,
andagainthecoherencetimeisknowntobegreaterthanthepulseduration[12].However,the
opticalfrequencysynthesisapplicationsenabledbyoctave-spanningSCrequirepulse-to-pulse
phasecoherenceovertimesmuchlongerthantherepetitionperiodofthepulsetraintoensure
that the mode-locked pulse train gives rise to a stable optical frequency comb. Our work is
thefirsttoinvestigatethetemporalcoherenceofslicedoctave-spanningsupercontinuumover
timescalesgreaterthanthepulseduration.
Herewedescribethegenerationofmode-lockedpicosecondpulsesfromanoctave-spanning
supercontinuum (SC) source pumped at 1550 nm. The 1110 nm wavelength is of particular
interestforourexperimentsonquantumcomputingwithtrappedYb+ions,butthearchitecture
demonstrated here can be adapted to a wide variety of wavelengths in the 1000-2000 nm
range of our SC source. The characteristics of our 1110 nm source are similar to those of a
picosecondmode-lockedYbfiberlaserinallrespectsbutwavelength(see,e.g.,refs.[13,14]).
Thepicosecondoutputpulsesarechirpedbutcanbecompressednearlytothetransformlimit,
while the pulse energy of ∼100 pJ and the 40 MHz repetition rate are similar to those of a
typicalmode-lockedfiberlaser.Toourknowledge,nosuchlaserhasbeenreportedat1110nm,
showingthatourarchitecturecanexpandthewavelengthrangeofmode-lockedfibersources.
Ourresultsindicatethatthe1110nmlightinheritsthecoherencepropertiesoftheSCsource.
In the future, absolute frequency stabilization of our SC source will enable us to coherently
synthesize picosecond pulses at desired wavelengths, representing a major step toward an
arbitraryopticalfieldsynthesizer.
2. Octave-spanningsupercontinuum
We generate supercontinuum (SC) in a highly nonlinear fiber Bragg grating pumped by
femtosecondpulses at 1550nm (Fig. 1(a)). The 1550 nm signal originatesin a mode-locked
fiber laser (Precision Photonics) with 40 MHz repetition rate and is amplified to ∼120 mW
in an Er-doped fiber amplifier (IPG Photonics). The lengths of single-mode fiber (SMF) at
the input and output of the power amplifier are optimized to give a minimum pulse duration
of 90 fs at the amplifier output. Another 20 cm length of dispersion-shifted fiber (NZ-DSF)
compressestheamplifiedpulsefurther,to55fs,thoughthesolitoncompressionleavesbehinda
pedestalofseveralpsdurationcontainingabout50%ofthepulseenergy.Theamplifiedpulses
areinjectedintoahighlynonlinearfiberBragggrating(HNLF-FBG)withbandgapnear1108
nm. A Bragg grating was written into the HNLF using a 248 nm excimer beam and a phase
mask.The6mmUVbeamwasscanneduniformlyfor27mm.Theresultinggratinghadacore
modereflectionbandwidthextendingfrom1104.4nmto1108.4nm.TheHNLFextendsanother
30 cm past the FBG region.Fig. 2 shows the SC spectrumin the 900- 1700nm wavelength
range,asmeasuredwithanopticalspectrumanalyzer(OSA).WhiletheOSArangeislimited
towavelengthsbelow1700nm,furthermeasurementswithanIRmonochromatorindicatethat
theSCplateauextendsto>2000nm.SplicelossesandlinearlossintheFBG reducetheSC
averagepowerto40mWattheHNLFoutput.TheSCpowerspectraldensityisenhancednear
1110nm,slightlytotheredoftheFBGbandgap[15].TheSCsourcemaintainshighspectral
brightness(>10m W/nm)overnearlytheentire1000-2000wavelengthrange.Thisspectral
brightness is sufficient for generation of mode-locked picosecond pulses at mW power lev-
elsoversubstantialportionsoftheSCspectrum,asexemplifiedinthepresentworkat1110nm.
aa))
11111100 nnmm rreemmaaiinniinngg
XXX cm XXX cm HNLF-FBG drop filter SC
mode-locked laser SMF-28e NZ-DSF
EDFA
1550 nm, 40 MHz
bb))
fused-
pump pump fiber pump
WDM 10 m YDF WDM 10 m YDF filters WDM 10 m YDF
CFBG
pump pump pump
ddiiooddee ddiiooddee ddiiooddee
output
Fig. 1. Schematic of the mode-locked 1110 nm source. Supercontinuum generation (a)
providesaspectrallyslicedseedforreamplification(b).
10
dB]dB] HNLF-FBG
y [y [ 0 enhancement
sitsit
nn
dede -10
al al
ctrctr
ee -20
pp
ss
er er
ww -30
oo
pp
e e
ativativ -40
elel
rr
-50
900 1000 1100 1200 1300 1400 1500 1600 1700
wavelength [nm]
Fig.2.Spectrumofsupercontinuumgeneratedbythe1550nmpumpshowingcoverageof
the1000-1700nmwavelengthrange.
3. Spectralslicingandreamplification
TheSC sourceisinitially spectrallysliced bycustomfused-fibercouplersto isolatethe1000
-1300nmband.ThispartoftheSCservesastheinputtoahome-builtytterbium-dopedfiber
amplifier (YDFA) system (Fig. 1(b)) that preferentially amplifies input light near 1110 nm.
While the gain peak of short lengths of ytterbium-dopedfiber is near 1030 nm, excited-state
absorption at this wavelength is also relatively high, so that longer lengths of more highly
doped fiber exhibit gain peaks at relatively longer wavelengths. Our YDFA stages use 10-20
meter lengths of fiber with absorption coefficient of 1200 dB/m at 980 nm and exhibit gain
peaks in the 1080 - 1110 nm range. Each stage is backward-pumpedwith up to 400 mW of
976 nm diode laser light. The first YDFA stage amplifies a relatively broad SC band from
1060-1120nm.An opticalisolator separatesthe first two YDFA stages, minimizingparasitic
lasing. Custom fused-fiber couplersfilter out wavelengthsbelow 1085 nm at the input to the
second YDFA stage. A fiber circulator passes the second-stage output through to the third
YDFAstage.Afterafirstpassthroughthisstage,theamplifiedSCencountersachirpedfiber
Bragggrating(CFBG).ThisCFBG(L∼4mm,D l ∼1.1nm,R∼7.4dB)wasfabricated
chirp
using a computer controlled, scanning free space Sagnac interferometer configuration where
the 244nm output of a frequency doubled Ar+ laser is split mostly into the two first order
diffraction orders through a chirped phase mask (pitch 10678.5nm) designed for 1554nm
(chirp ∼3nm/cm). By tilting the interferometermirrors, the translated chirped interferogram
hasareducedpitchwhichisimagedontothecustommadegermanosilicateopticalfibre(cutoff
wavelength<700nm)bytheSagnacinterferometer.ThefinalBraggwavelengthiscenteredat
∼1108.5nmafterannealing.TheCFBGreflectionbandwidthisapproximately1nm(FWHM)
around 1108.5 nm, which serves to filter out the parasitically amplified wavelengths below
1105nm.ThelightreflectedfromtheCFBGpassesagainthroughthethirdYDFAstage,being
furtheramplifiedontheway,andemergesattheoutputportofthecirculator.Thisnarrowband
double-passingof the final YDFA stage amplifies the 1110nm signal by an additional4 dB,
whilereducingtheamplifiedSCbackgroundby6dB,leadingtoanoverallimprovementof10
dBinspectralselectivityaround1110nm.
Figure 3 shows the optical power spectra at the YDFA system’s input and output. While
the input spectrum extends far outside the displayed limits, the wavelength range amplified
by the YDFA is set by the 1 nm bandwidth of the CFBG. From the measured ∼ 200m W
powercontainedinthe5nmwideSCenhancementpeak,weseethatthein-bandpowerinput
to the YDFA is only 40m W. Total output powers of up to 5.5 mW are achieved, with a net
amplificationof21dB.
b.]b.] 1
arar0.9
sity [sity [0.8
nn
ee0.7
dd 0.55 nm
al al 0.6 x30
ectrectr0.5 FWHM
pp
er ser s00..44
ww0.3
oo
pp
e e 0.2
vv
atiati0.1
elel
rr 10080 1090 1100 1110 1120 1130 1140
wavelength [nm]
Fig. 3. SC input (dotted line) and YDFA output (solid line) power spectral density near
1110 nm. The input spectrum is multiplied by a factor of approximately 30 for ease of
comparison.
4. Pulsepropertiesandcoherence
The1110nmpulsesareinitiallyhighlychirpedbutcanbecompressedtonearlythetransform
limit. Measured at the output of the YDFA, the pulses have full-width at half-maximum
(FWHM)durationof∼6ps.(Weassumesech2 pulseshapethroughout.)Weuseafree-space
grating compressorto compensate the normalgroup-velocitydispersion (GVD) accumulated
in the YDFA. Figure 4 shows an autocorrelation trace for optimal pulse compression. The
correspondingpulse duration is 1.7±0.1 ps FWHM. We must be cautious in estimating the
time-bandwidth product because of the asymmetric source spectrum (Fig. 3); the spectral
FWHM is only 0.7 nm, yielding a time-bandwidth product smaller than the transform limit.
Numericallypredictingthetransform-limitedpulsedurationforthemeasuredoutputspectrum,
we find that the 1110 nm pulses are within a factor of 1.6 of the transform limit. No pulse
pedestalisvisiblewithinthe50psscanrangeoftheautocorrelator,showingthatthepulsesare
ofhightemporalquality.
The1110nmpulsesare“mode-locked”onlyifthetemporalcoherenceoftheelectricfield
ismaintainedfromonepulsetothenext.HerewerelyontheSCgenerationprocesstotransfer
the coherenceof the 1550 nm pump pulses to the derived1110 nm pulses. Unless the pump
pulse duration is short (<100 fs) and the soliton number in the HNLF is low (<15), the
coherence of the SC process can easily be lost through fundamental mechanisms such as
photonshotnoiseandspontaneousemission[16].Ourpumpsourcehasa55fspulseduration
andanestimatedsolitonnumberof∼9,sohighSCcoherenceisexpected.
Wemeasurethetemporalcoherencebetweensuccessive1110nmpulseswithanunbalanced
Mach-Zehnder interferometer implemented in fiber. A piezoelectric stretcher in one arm of
the interferometersweeps the path length differenceD L, and we observeinterferencefringes
by photodetectionwhen D L is close to the distance L =c/n between successive pulses.
rep rep
700
b.]b.]
arar600
al [al [500 FWHM
nn
gg
sisi 2.6 ps
n n 400
oo
atiati300
elel
orrorr200
cc
oo
autaut100
0
-6 -4 -2 0 2 4 6
delay [ps]
Fig.4.Autocorrelationtraceofthecompressed1110nmpulses.Points:data;line:sech2
fit.Theautocorrelationsignalisoffsetfromzeroforeaseofviewing.
Thefringevisibilityislimitedbythedifferentialdispersionoftheinterferometerarms.Weuse
500cmofCorningSMF28efiber,withdispersionof−22ps/(nmkm)[17],toimplementthe
distanceL .NumericallysimulatingthedispersiveeffectofthefiberonthemeasuredYDFA
rep
spectrum, we obtain a maximum fringe visibility of approximately 55%, so the measured
visibilityof49±1%indicatesahighdegreeofinterpulsecoherence.
The RF spectrum produced by photodetection of a mode-locked laser source contains a
strong, narrow line at the laser repetition rate n , indicating low timing jitter between the
rep
envelopes of successive pulses. Figure 5 shows the RF spectrum of the 1110 nm source as
measuredbyafastphotodiode.TheRFspectrumisindistinguishablefromthatoftheoriginal
1550nmoscillatoratourminimumresolutionbandwidthof1kHz.
5. Outlook
We have generated mode-locked picosecond pulses at 1110 nm by spectral slicing and
reamplification of an octave-spanning supercontinuum source. The resulting light source is
similar to that of a standard mode-locked fiber oscillator, except that such oscillators have
not,to ourknowledge,beendemonstratedatthiswavelength.Muchofthe complexityof our
amplifier design arises from the low gain at our target wavelength of 1110 nm, exacerbated
by the relatively low quality of fiber components at this unusual wavelength. Hence our
experiment demonstrates the feasibility of all-fiber mode-locked ps pulse generation right
across the ytterbium-doped fiber gain bandwidth of 1010 - 1150 nm. Pressure tuning of a
suitablewavelength-selectingCFBGwouldenabledynamictuningofthepspulsewavelength
over>100nm[18].AmplificationofaspectrallybroaderSCseedshouldpermitgenerationof
femtosecondpulses,aslongasthenonlinearanddispersivepulsepropagationintheamplifier
isproperlytakenintoaccount.
-10
-20
m]m]-30
BB
dd
e [e [-40
otot
atnatn-50
ee
bb
-60
-70
40.5 40.52 40.54 40.56 40.58
RF frequency [MHz]
Fig.5.RFspectrumofthe1110nmsource.Resolutionbandwidth:1kHz.
More broadly,fiber and semiconductoramplifiers can be used in place of our YDFA over
mostofthe1000-2000nmwavelengthrange.Takingthe1110nmsourcebandwidthof∼1.5
nm as typical, we see that several hundred wavelength channels of mode-locked ps pulses
can be simultaneouslyderivedfrom a single SC source, as shown in other work at lower SC
bandwidth. As we have shown, each channel can be made to inherit its phase behavior from
the SC source, passively phase-locking all channels together. Stabilizing the SC source in
absolute frequency, combined with independent modulation of each channel, would allow
phase-coherentsynthesisofhigh-resolutionopticalfieldwaveforms.
6. Acknowledgments
This work was supported by the US Air Force Office of Scientific Research under contract
FA4869-06-1-0045,byDIISRISLGrantCG130013,andbytheAustralianResearchCouncil
(ARC) underDP0773354(Kielpinski),DP0770692(Canning),and Prof. Howard Wiseman’s
FederationFellowshipFF0458313.WethankGoe¨ryGenty,O¨merIlday,andNathanNewbury
forhelpfuldiscussions.