Generation of a broadband single-mode supercontinuumin a conventional dispersion-shifted
fiber by use of a subnanosecond microchip laser
Arnaud Mussot, Thibaut Sylvestre, Laurent Provino, and Hervé Maillotte
Laboratoire d’Optique P. M. Duffieux, Unité Mixte de Recherche, Centre National de la Recherche Scientifique/Université deFranche-Comté 6603, 25030 Besançon cedex, France
In view of its potential for many applications suchas optical metrology, spectroscopy, biomedical optics,and optical communications, supercontinuum (SC)generation in optical f ibers has been the object of ex-tensive research during the past two decades. Firstattempts made with conventional optical fibers havegiven rise to continuua with a spectral extent limitedto a maximum of 400 nm.1 – 4 The advent of speciallydesigned optical f ibers such as photonic-crystal f ibers5
(PCFs) and tapered fibers6 has recently stimulatedrenewed interest in the generation of supercontinua,because of the strong nonlinearities and unique specif-ically controlled dispersion profiles of these f ibers. Inparticular, their zero-dispersion wavelength (ZDW)near 800 nm permitted, for the f irst time to ourknowledge, the generation by means of femtosecondpulses of ultrabright supercontinua with spectra thatspan nearly 2 octaves �.1200 nm�. In this pulseregime, which is by far the most widely studied,recent theoretical and experimental works have shownthat SC generation in PCFs comes from an intricatefrequency conversion process involving a combinationof many nonlinear phenomena, namely, self-phase andcross-phase modulation, four-wave mixing (FWM),stimulated Raman scattering (SRS), soliton self-frequency shifting, f ission of higher-order solitons,and dispersive wave generation. These effects havedifferent weights according to the wavelength, energy,and duration of the pump pulse and to the fiber dis-persion characteristics.7 – 9 But it is signif icant thatoperating in the vicinity of the f iber’s ZDW facilitatesthe operation of efficient phase-matched parametricprocesses and plays a prominent role in enhancingthe spectral extension and smoothness of the SC spec-trum. In the nanosecond and picosecond regimes,fewer experiments in the normal-dispersion domainalso showed the efficient generation of SCs in PCFsby spectrally shifting the initial pump energy nearthe ZDW of the fiber by the Raman effect.10,11 Again,phase-matched FWM near the ZDW is the key processfor obtaining a large and smooth SC.
0146-9592/03/191820-03$15.00/0
The techniques discussed above produce impressiveresults, but they require special optical fibers anda bulky laser source in most cases. In this Letterwe emphasize that a simple experimental setupmade from a conventional telecommunication fiberand a low-cost microchip laser also allows one togenerate a bright and wideband SC. Indeed, wepresent experimental results obtained with a longdispersion-shifted fiber (DSF) that show the genera-tion of a spatially single-mode, visible–infrared SCof more than 1100 nm with a spectral mean powerdensity of �5 mW�nm. With this f iber the dynamicsof SC formation cannot rely on the combination ofnonlinear processes near the ZDW, as occurs in PCFs,because of the huge gap between the pump wavelength�l � 532 nm� and the ZDW of the DSF �l � 1550 nm�.Instead, the SC in our experiment results from aninterplay between multimode phase-matched para-metric wave mixing near the pump wavelength andsubsequent double-cascade SRS that further evolvesinto a spatially single-mode SC.
The fiber used in our experiment was a 650-mlong DSF with a core radius of 2.8 mm and a cutoffwavelength at 1020 nm. Note that GeO2 dopingof the fiber core favors the large growth of SRSthat is responsible for continuum formation. Thepump pulses were produced by a frequency-dou-bled, passively Q-switched Nd:YAG microchip laserat a repetition rate of 6.7 kHz. The mean outputpower at 532 nm was 15 mW, and the full widthat half-maximum pulse duration was 0.4 ns. Thelinearly polarized single-mode output beam wasfocused into the DSF by a 103 microscope objectivewith a coupling efficiency exceeding 70%, givingrise to a peak intensity inside the fiber of as much as10 GW�cm2. The fiber output spectrum was recordedby means of an optical spectrum analyzer, and weanalyzed the SC dynamics by tuning the launchedpower by means of a l�2 plate and a Glan polarizer.Figure 1 shows the spectrum of the SC obtainedwhen the launched mean power reached its maximum
Fig. 1. Output spectrum of the SC at P0 � 10.5 mW(resolution, 0.1 nm). Pump wavelength P at 532 nm isindicated by an arrow.
value, P0 � 10.5 mW. As can be seen, most of theincident energy at 532 nm was unilaterally trans-ferred to higher wavelengths. The resultant SCstretched over more than 1100 nm, from 650 beyond1750 nm at least, which was the upper detection limitof the optical spectrum analyzer.
Figure 2 details the beginning of the SC formationfor increasing input powers. For Fig. 2(a) the pumppower exceeds the Raman threshold, which yields astrong, 13.2-THz-shifted, first-order Raman Stokesband �S1� at 546 nm that emerges well above the noisef loor at � 2 75 dBm. Additionally, another strongStokes wave �PP � shifted by 5 nm from the pump(P) is spontaneously generated through multimodephase-matched parametric mixing, as detailed below.Parametric wave PP acts as a second pump andgenerates its own first-order Raman Stokes bandSP1 , as illustrated in Fig. 2(b), in addition to theordinary second Stokes order �S2� issued from S1.Then Fig. 2(c) shows the occurrence of a second-orderStokes wave SP2 at 563 nm, Raman shifted from SP1 .The initial pump pulse at 532 nm and the parametricpump at 537 nm are thereby responsible for thegeneration of two simultaneous Raman cascades. InFigs. 2(c) and 2(d) the double cascade merges into awider hybrid Raman Stokes wave SH1 that in turngenerates higher-order bands SH2,3, ...,N , thus leading tothe ultrabroadband continuum of Fig. 1.
Let us examine in more detail the steps in SCformation. First, because of the multimode natureof the DSF and its strong group-velocity dispersionat 532 nm, wave PP results from a well-known multi-mode phase-matched FWM process.12 To identify themodal composition of this parametric mixing, we cutthe f iber back to 1 m, keeping the optimized launchingconditions, and the output of the f iber was dispersedby a diffraction grating. Thus we found that theinteracting modes are the LP01 and LP11 modes. Asillustrated in Fig. 3(a), pump P is distributed in thesetwo modes, giving rise to an LP01 anti-Stokes wave [la-beled AS1 in Fig. 3(b)] and an LP11 Stokes wave �PP �.The same modal distribution appears in Fig. 3(b) fora second FWM process involving S1 as a mixed-modepump. Hence SP1 , which is generated in the LP11mode, results from a combination of multimode para-metric wave mixing from S1 and Raman gain fromPP . Meanwhile, SRS unbalances the parametricenergy transfer, leading to a lower LP01 anti-Stokescomponent, AS2 near S1. Note that in the 650-m-longfiber the anti-Stokes waves associated with the two
FWM processes near P and S1 vanish as a result ofRaman-induced absorption [Figs. 2(a)–2(d)].
To compare the measured FWM shift with theory12
we considered the linear phase-matching relationshipfor mixed-mode, single-polarization excitation, whichcan be expressed as
�b101 2 b1
11� 3 V 1 b2 3 �V2�2� � 0 , (1)
where the f irst and second terms represent the contri-butions of modal dispersion (group-velocity differencebetween the LP01 and the LP11 modes) and of materialgroup-velocity dispersion b2, respectively, to the phasemismatch, where V is the frequency shift. In Eq. (1),b1
01 and b111 are the f irst-order derivatives of the
propagation constant for LP01 and LP11, respectively,at the pump frequency. We did not take into accountthe power-dependent nonlinear contribution (the re-sult of cross-phase modulation) to the phase mismatchbecause it is negligible with respect to the linear terms.By tuning the pump power in the 1-m-long fiber weobserved that frequency shift V indeed remainedunaffected. To calculate the first- and second-orderderivatives of the propagation constants we modeledthe DSF as a step-index fiber. This simple modelyielded V � 6 THz (5.7 nm), in good agreement withthe measured value (the parameters are a 2.8-mm coreradius, a core–cladding index difference of 0.0054,b2 � 6.6 3 10226 s2�m, b1
01 � 4.9713 3 1029 s�m,b1
11 � 4.9738 3 1029 s m21).
Fig. 2. Output spectra for increasing pump power from(a) P0 � 1.72 mW to (d) P0 � 1.83 mW.
Fig. 3. Modal distribution of the FWM spectrum for(a) the first multimode parametric process near P and(b) both parametric processes near P and S1. AS1 andAS2 are the corresponding anti-Stokes waves.
Fig. 4. (a) Modal distribution of the SC in the spectral do-main recorded with a CCD camera (the spatial transversedimension is along the vertical axis). Spatial far-field out-put intensity distribution (b) without and (c) with chro-matic filtering.
As we explained above for the 650-m-long DSF, theinterplay of SRS and multimode FWM leads to thedouble Raman cascade whose successive Stokes ordersprogressively broaden, as Figs. 2(c) and 2(d) show.For a simple Raman cascade it was shown in Ref. 3that the spectral width of a given Raman order SNis typically twice that of the preceding order SN21because Raman gain in optical fibers has a broadbandwidth. Such spectral broadening is even moreaccelerated in our case of a double Raman cascade,which enhances the merging into the hybrid ordersSHN and the evolution toward the broadband super-continuum of Fig. 1.
Although the early steps in SC formation rely intrin-sically on multimode FWM and SRS, another impor-tant property of the SC is its transverse evolution intothe fundamental LP01 mode from 650 nm. Figure 4(a)depicts this property by displaying the beginning ofthe SC spectrum dispersed on a diffraction grating.The doughnut-shaped distribution shown in Fig. 4(a)for discrete components P to approximately SH2 fea-tures mode overlap in long optical fibers. Then themodal distribution evolves progressively toward thefundamental LP01 mode, which appears to be the onlyexcited mode from SH6 , i.e., at the beginning of thecontinuum. Indeed, the SC that is formed froml � 650 nm to l � 1750 nm is generated entirely inthe fundamental mode. To verify this property, inFigs. 4(b) and 4(c) we show the far-field spatial outputintensity distribution recorded by a CCD camerawithout and with a frequency low-pass chromaticfilter at 650 nm. The spot size reduction in thefiltered image as well as its homogeneous intensityprofile [Fig. 4(c)] conf irms that the SC is effectivelysingle mode. This interesting property is due tothe progressive mode coupling, along the 650-m-longfiber, that yields selective filtering by the Raman gainduring propagation and favors SRS growth on the fun-damental mode, as was shown previously by Chiang13
[Fig. 4(a)]. The quite surprising coincidence betweenthe beginning of the SC at 650 nm and the evolutionin the fundamental mode from this wavelength seemsin fact correlated. Indeed, as all waves evolve in theLP01 mode from 650 nm, all the power confined inonly this mode may lead to a dramatic enhancement ofthe effective nonlinear coeff icient. Therefore, Raman
orders can saturate faster and can induce, along withthe mutual action of self- and cross-phase modulationand, possibly, FWM between Raman orders,14 a dra-matic broadening that completes the merging of thediscrete double Raman cascade into the SC.
To conclude, a nearly 2-octave-spanning visible–IRsupercontinuum has been generated in a conven-tional dispersion-shifted f iber by use of an inexpensivemicrochip nanosecond laser. To the best of our knowl-edge, this configuration is the simplest setup thatallows one to generate such an extended SC in opticalfibers. Let us emphasize that, once the launchingconditions have been optimized, the SC output re-mains actually highly stable on a day-to-day basis.The SC’s spectral extent, brightness, and spatiallysingle-mode distribution are directly relevant topractical applications such as spectroscopy, optical co-herence tomography, white-light interferometry, andmicroscopy, with the additional advantages of com-pactness, attractive cost of the setup, and immediatecompatibility with the conventional connecting compo-nents. Also, as this SC dynamic relies on an originalcombination of nonlinear processes that are differentfrom that which is involved in photonic-crystal fibers,these results contribute to a further understanding ofthe complexity of SC generation in optical f ibers.
The authors thank Gilbert Tribillon for help-ful discussions. A. Mussot’s e-mail address [email protected].
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