Visible supercontinuum generation in photonic crystal ... · Mussot, M. Beaugeois, M. Bouazaoui,...

Visible supercontinuum generation inphotonic crystal fibers with a

400 W continuous wave fiber laser

J. C. Travers, A. B. Rulkov, B. A. Cumberland,S. V. Popov and J. R. Taylor

Femtosecond Optics Group, Physics Department, Prince Consort Road,Imperial College, London SW7 2AZ, UK

[email protected]

http://www.femto.ph.ic.ac.uk

Abstract: We demonstrate continuous wave supercontinuum generationextending to the visible spectral region by pumping photonic crystal fibersat 1.07 μm with a 400 W single mode, continuous wave, ytterbium fiberlaser. The continuum spans over 1300 nm with average powers up to50 W and spectral power densities over 50 mW/nm. Numerical modellingand understanding of the physical mechanisms has led us to identify thedominant contribution to the short wavelength extension to be trapping andscattering of dispersive waves by high energy solitons.

© 2008 Optical Society of America

OCIS codes: (060.4370) Nonlinear optics, fibers; (140.3510) Lasers, fiber.

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1. Introduction

Continuous wave (CW) pumping of optical fibers has led to the highest spectral power, andsome of the smoothest, supercontinua demonstrated to date [1–7]. The resulting sources areuseful for a wide range of applications ranging from biomedical imaging to chemical sensing.The fundamental mechanisms, based on modulation instability (MI) leading to soliton forma-tion and subsequent soliton dynamics, are also intrinsically interesting. A long standing issueis the lack of generation of blue-shifted spectra from the commonly used infrared CW lasers.In this paper we demonstrate the first CW pumped supercontinuum to reach the visible spectralregion. We also scale the average power to an unprecedented level of 50 W, leading to spectralpower densities of over 50 mW/nm. To achieve this we use a 400 W CW pump source andtherefore these experiments are in a novel pump regime compared to previous CW pumped su-percontinua. The generation of frequency up-shifted components from a CW pump source hasbeen previously observed from 1.55 μm pump wavelengths in conventional optical fibers [8],but some confusion has arisen as to the physical mechanism for this occurrence, with referencesbeing made to fission of high order soliton solutions [8,9], a process which appears to be in con-flict with the basic MI dynamics. In this work we consider the fundamental processes involvedin the CW supercontinuum development and find that the blue-shifting spectral components aredue to trapping of dispersive waves by high energy solitons, a process widely exploited in pulsepumped conditions [10, 11].

Continuous wave supercontinua were first generated in the late 1990s [12, 13], but signifi-cant development did not occur until high nonlinearity fibers and high power single mode CWlasers became more widely available. The first demonstration in photonic crystal fiber (PCF) [1]showed that multi-watt average powers are achievable in a simple experimental configuration.Subsequently a large body of work has been produced showing results pumped at 1.55 μm inconventional optical fibers [2, 3, 6, 14] and at 1.06 μm in PCF [1, 4, 5, 7]. In the former case,due to favorable dispersion curves, continua extending to wavelengths short of the pump wave-length have been observed [8], along with the usual Raman-soliton continuum extending tolonger wavelengths. In PCFs pumped at 1.06 μm this has not been observed until now.

The reason for this fact is due to competing requirements on the dispersion curves for efficientRaman-soliton continuum generation to long wavelengths, and for short wavelength extension.For the former, it is important that |β2|/γ (where β2 is the group velocity dispersion and γ thenonlinear coefficient) stays relatively constant with wavelength, to allow the solitons to shiftas far as possible to longer wavelengths before broadening so much that Raman self-scatteringno longer occurs. For the latter, we need to pump close to the zero dispersion wavelength,to allow MI or soliton coupling to dispersive waves to transfer power to the normal dispersionregion. These two requirements conflict as pumping close to the zero generally implies a steeperdispersion slope.

We have recently taken two approaches to achieving short wavelength generation for1.07 μm pumping, with the aim of generating a continuous wave visible supercontinuum,which would have numerous applications. In one approach we have optimized the dispersionto precisely accommodate the relevant phase and group velocity matching conditions to reachthe visible, sacrificing somewhat the Raman-soliton continuum [15]. As an alternative, in thepresent work, we show how by power scaling the pump, using an industrial class continuouswave fiber laser, we can also extend to the visible spectral region.



This paper is organized as follows. In Section 2 we explain the experimental setup and theresults we have achieved. In Section 3 we discuss our model of nonlinear propagation in opticalfibers, including some details on the initial conditions. In Section 4 we discuss the physicalmechanisms involved along with some simulation results and note that further extension to theblue should be possible using cascaded or tapered fibers.

2. Experiment

2.1. Setup

Fig. 1. Experimental setup.

Figure 1 shows the experimental setup. The industrial class pump laser was supplied by IPGPhotonics. It emitted up to 432 W of average power at 1.07 μm, with random polarization and aspectral linewidth of 3.6 nm. The single mode output of the laser was interfaced to a collimatingunit producing a 7 mm (1/e2) collimated beam. Correspondingly, we were unable to splice thelaser output directly to the setup and therefore used a bulk lens in order to couple this beaminto a series of mode matching single mode fibers to reduce the mode field diameter, beforefinally splicing to the PCF we were using. The free-space coupling was typically greater than70% efficient, the total free-space to PCF efficiency was between 30% and 50% depending onthe splice loss to the PCF. The coupling at full power was stable, and did not require constantmonitoring or adjustment, at least for time periods exceeding 1 hour, however, in initial experi-ments we reduced the thermal load on the coupling lens and input fiber by modulating the laserwith a duty cycle of between 1 and 40. Final results were taken with no modulation in orderto scale the average power. References to equivalent power refer to the peak or equivalent CWpower with no modulation.

Table 1. Parametersa of the photonic crystal fibers.

Fiber Name Λ / μm d/Λ λ0 / μm Dp / ps nm−1 km−1 γp / W−1 km−1

HF1050 3.4 0.47 1.05 4.3 11HF840 4.3 0.90 0.84 73 44HFDBL 1.6b 0.49b 0.81b, 1.73b 65b 43b

a the parameters are: pitch Λ, air hole diameter d, zero dispersion wavelength λ0, dispersion at the pumpwavelength Dp and nonlinear coefficient at the pump wavelength γp.

b these values are estimated as described in [7] and are uncertain.

We pumped several PCFs to probe the affect of dispersion profile and nonlinearity on theresulting supercontinuum shape. The PCF parameters and the names we use to refer to themare given in Table 1. HF1050 is similar to PCFs commonly used for visible supercontinuum



HF1050 HFDBL

HF840

Fig. 2. Calculated dispersion curves for our fibers. HF1050 (blue), HF840 (red), HFDBL(green).

generation with picosecond and nanosecond lasers at 1.06 μm [16,17]. It has a zero dispersionwavelength near to 1.05 μm and a low anomalous dispersion at the pump wavelength. Thedispersion curve for this and the other PCFs we used are shown in Fig. 2.

HF840 has been previously used for continuous wave supercontinuum generation as it has alow water loss value [5]. It has a single zero dispersion wavelength around 0.84 μm. HFDBLhas two zero dispersion wavelengths at around 0.81 and 1.73 μm, and has also been usedpreviously for high power supercontinuum generation [7].

We pumped a number of lengths of each fiber and show below results for the approximatelyoptimal length, judged by the smoothness and extent of the continuum produced. The outputpower of each fiber changes considerably depending on their different attenuation curves andthe particular spectral extent of each continuum.

2.2. Results

Figure 3 shows the results of pumping 17 m of HFDBL with our setup. A maximum of 170 Wwas coupled into the PCF forming a continuum spanning from 1.06 to 1.9 μm with ∼10 dBflatness. The role of the second zero dispersion wavelength is clearly evidenced by the dip inspectral power around 1.7 μm. The effect of water loss is also apparent as the spectral powerreduces after 1.4 μm. The qualitative form of the spectra is very similar to those we previouslyachieved in [7] with 50 W pumping, but with the higher powers accessible in this work we havesignificantly increased the power transfered to the dispersive wave formed beyond the secondzero dispersion wavelength. The spectral power densities between 1.06 and 1.4 μm are over50 mW/nm as the output powers were around 27 W.

The supercontinuum spectra obtained at the output of 20 m of HF840 are shown in Fig. 4for a pump power of 170 W. The total average output power was over 50 W and the continuumextended from 1.06 to 2.2 μm. The 10 dB width of the spectrum was over 900 nm, between 1.14and 2.05 μm. Across this range the spectral power exceeds 10 mW/nm, with half of the rangebetween 50 and 100 mW/nm. Water loss causes a significant fall off in spectral power, althoughthe power density at the long edge of the continuum around 2.1 μm is still over 5 mW/nm,which is sufficient for most applications.

These results clearly demonstrate the power scaling potential of using an industrial class



Power/dB(relative)

50 W

85 W

170 W

Fig. 3. Measured spectra out of 17 m HFDBL for 50 W (blue), 85 W (red) and 170 W(green) equivalent coupled pump power on a relative power scale.

Fig. 4. Measured spectra out of 20 m of HF840 for 170 W equivalent pump power, normal-ized to the total average output power of 50 W.

laser, but pumping HFDBL and HF840 has not lead to a visible supercontinuum spectrum. Thereasons for this are analyzed in detail below, but stem from the fact that the zero dispersionwavelength is too far from the pump wavelength. To generate visible continua there must bea mechanism to transfer power to the normal dispersion region. This can be achieved eitherthrough the generation of dispersive waves from the solitons formed from the MI process whichinitiates the continuum, or directly from widely separated MI sidebands, where one MI side-lobe overlaps with the normal dispersion region. Both of these processes require the pumpwavelength to be sufficiently close to the zero dispersion.

HF1050 has a suitable dispersion curve. The result of pumping 50 m of this fiber with 230 Wfrom our setup is shown in Fig. 5. The total average output power was 28 W. It is clear thatthe supercontinuum extends to the visible spectral region, down to 0.6 μm and the continuumappeared bright red to the eye. The long wavelength edge of the continuum was 1.9 μm. The



spectral powers were over 2 mW/nm in the short wavelength side, which is competitive withthe highest power pulse pumped visible supercontinua, and over 10 mW/nm in the infraredregion, with substantial spectral regions between 20 and 30 mW/nm. This result shows thatvisible supercontinua similar to those obtained with pulse pumped systems can be achievedwith extremely high power CW pump conditions, but with additional benefits due to scalablespectral power and flatness. We should be able to extend the continuum to the blue, and scalethe power further by carefully designing the fiber dispersion and pump conditions, as discussedin the following sections.

Fig. 5. Measured spectra out of 50 m of HF1050 for 230 W equivalent pump power, nor-malized to the total average output power of 28 W.

3. Modelling

3.1. Propagation equation

To gain insight into the supercontinuum dynamics we modelled the propagation of a model CWfield through the PCFs. The complex field envelope E(ω ,z) at angular frequency ω and axialfiber position z was calculated using the generalized nonlinear Schrodinger equation, modifiedto include the dispersion of the mode field profile [18, 19],

∂zE(ω ,z) =− i(β (ω)−β (ω0)− ∂ωβ (ω)|ω0 Ω)E(ω ,z)

− α(ω)2

E(ω ,z)

− in2ω

cA(1/4)e f f (ω)

∫dω1E(ω1,z)R(ω1 −ω)

×∫

dω2E(ω1 −ω2 −ω ,z)E(ω2,z),

(1)

with

E(ω ,z) =E(ω ,z)

A(1/4)e f f (ω)

. (2)

In Eq. (1) β (ω) is the mode propagation constant, Ω = ω −ω 0 is the frequency shift with re-spect to a chosen reference frequency ω0, n2 = 2.74×10−20 m2 W−1 is the nonlinear refractive



index [20], c the speed of light and Ae f f the effective mode area. The wavelength dependentloss is included in α(ω). The first term on the right hand side of the response function,

R(ω1 −ω) = (1− fr)+ frhr(ω1 −ω), (3)

represents the Kerr effect and the second term represents the Raman effect, where h r is theRaman response function in the frequency domain [21] and the factor f r = 0.19 determines theRaman contribution to n2. There is some variation in the literature about whether a factor of 2/3should be included as a prefix to the Raman part of R(ω 1 −ω). This was reported in [22], andarises from ignoring a cross term when deriving Eq. (1). It turns out, however, that a thoroughanalysis of the ways which n2 and gr (the Raman gain coefficient) are experimentally measured,along with a self-consistent analysis of their relation leads to Eq. (3) being the correct definitionfor use in our propagation equation [23].

Equation (1) can be integrated directly after a change of variables, or more commonly it issolved using the split-step Fourier method. Here we use the Runge-Kutta in the interaction pic-ture method [24]. The step size was chosen automatically based on the relative local error [25],which was held below 1×10−6 for the simulations discussed below.

Modelling CW phenomena is made difficult by the time scales involved. To make the sim-ulations tractable we can only simulate a snapshot of the field as it propagates. We thereforehave to carefully choose a time window which contains sufficient information to accuratelyreproduce experimental observations. The consensus from the current literature is that a timewindow of several hundred picoseconds with the periodic boundary conditions inherent to thesplit-step Fourier method is sufficient [9, 26, 27]. In the simulations described below we used218 grid points over a time window of 256 ps. To visualize the results we use spectrograms orXFROG traces, computed with a windowed Fourier transform of the field envelope:

S(t,ω ,z) = 10log10

∣∣∣∣∫ ∞

−∞Ere f (t ′ − t)E(t ′,z)exp[−iωt ′]dt ′

∣∣∣∣ (4)

Here Ere f is an envelope of a reference pulse, in our case a 3 ps Gaussian.The propagation constants and effective areas of the PCF modes were computed from scan-

ning electron micrographs of the fiber cross section using a free software package [28]. Theeffective areas were calculated from the modal fields using a vectorial method accounting ac-curately for the air-holes [29]. For HFDBL the very high water loss at 1.38 μm was includedwith a spectrally dependent α(ω) term in Eq. 1, derived directly from the measured loss spec-trum of the fiber.

3.2. Initial conditions

There is great difficulty in modelling the initial conditions of the pump CW fields for supercon-tinuum generation as accurate single-shot diagnostics of CW lasers are difficult to obtain. Thisproblem is compounded when considering CW fiber lasers as the laser cavities are both non-linear and highly dispersive, leading to more complicated field evolution than their bulk lasercounterparts. This problem is also of great importance as modulation instability, the precursorof CW supercontinua, is highly dependent on the input noise conditions. We have performeda comparison of the various models previously used with careful experimental results, and de-signed our own model. These results are reported elsewhere [30,31]. Here we briefly summarizethe main points.

One of the simplest models is that of a CW field with no temporal or spectral phase oramplitude fluctuations, with quantum noise [26]. This leads to a spectrum with a very narrowspike at the central laser frequency and is approximately comparable to a single frequencylaser. Such narrow pump spectra are not comparable to those observed from a high power fiber



laser. To improve on this, a number of models have been based on the phase diffusion concept,where the temporal phase is modelled as a Gaussian noise process, which leads to Lorentzianshaped laser spectra [32, 33]. This model neglects temporal amplitude fluctuations, which ina nonlinear dispersive cavity with high powers are certain to exist. Also, Lorentzian spectrawith the bandwidths equivalent to our pump lasers contain significant power outside of the gainregion of the fiber, beyond experimental observation. An alternative model is to represent thelaser field as a collection of longitudinal modes with no phase relationship [9, 34]. This modelstarts from the measured spectral power of the pump laser with a random spectral phase addedto each frequency bin. This leads to very strong intensity fluctuations in the time domain ofthe order of the inverse spectral width of the pump (coherence time). However the resultingfluctuations in this case are too high as dispersion and self-phase modulation are completelyneglected, although they should be significant in a fiber laser cavity. A careful comparisonbetween these models and real experimental results with CW fiber lasers has shown them to besomewhat limited even, in some cases, for qualitative comparison [30, 31].

Our approach was to model the effects of dispersion and nonlinearity in the fiber lasers bymodelling the whole laser itself. We start with quantum noise represented as two photons permode. We then amplify through a fiber with spectrally dependent gain, gain saturation, nonlin-earity and dispersion. Bragg gratings are modelled at the end of each amplification pass simplyby a suitable spectral filter. We iterate the field through such a cavity until the average outputpower is at the desired level. The resulting field exhibits many of the expected characteristics:temporal and spectral amplitude and phase fluctuations and a triangular shaped spectrum on alog scale. The power fluctuations are much weaker than with the random spectral phase modeldescribed above. Comparisons with experimental results have shown this model to be an im-provement on the other proposals [30, 31]. We modelled our 400 W laser in this way and usedit to produce the simulated results reported below.

4. Discussion

4.1. Long wavelength generation: the Raman-soliton continuum

The initiation of a continuous wave supercontinuum in the low anomalous dispersion regionis due to modulation instability [35, 36]. Noise fluctuations of an otherwise CW or quasi-CWfield become self-trapping due to the Kerr effect. This process is fundamentally linked to theexistence of solitons, which are maintained by the same trapping effect. Fundamental solitonsare a stable condition — adiabatic amplification simply leads to shorter fundamental solitons— and therefore the process of MI does not naturally lead to higher order soliton solutions.

The early stages of the CW continuum formation are clearly identifiable in the simulatedspectrograms of our pump laser in HFDBL, shown in Fig. 6. In Fig. 6(a) is shown the pumpfield with the modelled initial conditions as described in Section 3. In Fig. 6(b) we see theemergence of solitons, which we attribute to MI. Here we can clearly see that the solitons formearlier and with higher peak power at the peaks of the input power fluctuations. Note also thatthe solitons being formed are much shorter in duration than the input power fluctuations. Thedependence of MI on the noisy initial conditions means that we create a train of solitons with adistribution of energies, this leads to a smooth continuum.

Once formed, the solitons may shift due to Raman self-scattering [37,38]. To do so they musthave a short enough duration so that their bandwidth is broad enough to self amplify throughRaman. The frequency shift of a soliton is proportional to the fourth power of the soliton energyand so the shape of the soliton energy distribution strongly affects the resulting spectrum, whichhas been called a Raman-soliton continuum [39]. In Fig. 6(c) we see how Raman self-scatteringhas started to shift the highest energy solitons to longer wavelengths, beginning the continuumformation, while new solitons are forming from the remaining pump field. We should note again



(a) (b)

Delay / ps

aele

/

(c)

Delay / ps

(d)

Fig. 6. Simulated spectrograms of the supercontinuum development through HFDBL for170 W pump power. The spectrograms are calculated for fiber lengths of: (a) 0 m, (b) 1.5 m,(c) 3.0 m, (d) 7.0 m. The full color scales and axis figure scales change for each sub-figureand range over 40 dB.

that these are single shots of 200 ps duration from a continuous process. The summation of allof these solitons leads to a very smooth spectrum on average.

In addition to the above process, inelastic collisions between solitons can transmit signifi-cant energy from higher frequency solitons to those with a lower frequency. Solitons collidewhen they overlap in time (i.e. align vertically in these spectrograms) as long as they are closeenough spectrally for the Raman process to occur (up to about 40 THz). As solitons with lowerfrequency have clearly Raman shifted further, they tend to have higher energies and so thehighest energy solitons get further excited. This enhances the continuum bandwidth. In thecontext of CW supercontinua this was discussed in [33], and was originally identified in pi-cosecond pumped supercontinua [40]. In Fig. 6(d) we see evidence of soliton collisions. Thenon-solitonic traces left behind by the solitons is the radiation shed by a pair of solitons afteran inelastic collision mediated through the Raman process occurred. Both pulses involved willhave energies mismatched from the required soliton energy, therefore some energy is shed asthe pulse adapts back to the soliton condition.

Limitations to the continuum bandwidth are caused by at least four mechanisms. Firstly,there is a finite length of fiber, and so the maximum shift is that achieved by the most energeticsoliton through the necessarily limited nonlinear medium. Secondly, losses become significant,which reduces the soliton energy and can slow or stop their shift. Thirdly, the balance of non-linearity and dispersion, which maintains the soliton shape can change very significantly andthus broaden the soliton temporally so that it no longer has the spectral bandwidth for Raman



self-scattering [7]. Finally, the Raman-soliton continuum can be limited by a second zero dis-persion wavelength. As we noted in [7], HFDBL has a second zero dispersion which preventsthe continued shift of the solitons as the anomalous dispersion region is limited [41]. This isseen clearly in Fig. 6(d) where the solitons gather around 1.6 to 1.7 μm, before the zero disper-sion point, and phase-matched dispersive waves are generated beyond this point, in the normaldispersion region. The dispersive waves are clearly being chirped as they propagate through thefiber. These features were clearly identifiable on the experimental spectra shown in Fig. 3.

4.2. Short wavelength generation

There are two main mechanisms available for generating wavelengths short of the pump in theCW regime. Either solitons formed from MI have a spectral overlap with the normal dispersionregion, and can therefore directly excite phase-matched dispersive waves [42, 43], or the anti-Stokes MI sidelobe is itself in the normal dispersion region causing a growth of power there.The excitation of phase-matched dispersive waves is proportional to the spectral amplitudeof the soliton spectrum at the phase-matched wavelength, and as the soliton spectral powerdrops off exponentially, pumping close to the zero is required. Similarly, for MI extension tothe normal dispersion region, a low anomalous dispersion is required for wide Stokes shifts andcloseness to the zero dispersion is required to achieve overlap into the normal dispersion region.In fact, these two processes are intimately linked. Therefore it is clear how pumping close tothe zero dispersion wavelength is essential, as we observed experimentally in Section 2.

After this initial stage, further extension to the short wavelength region can be gained eitherthrough four wave mixing between the soliton continuum and the dispersive waves, or throughthe soliton trapping of dispersive waves [10, 11]. In both cases we must generate significantpower in the normal dispersion region for further extension to occur. In the four wave mixingcase this is essential to achieve phase-matching to yet shorter wavelengths. In the soliton trap-ping case it is inherent to the process that the trapped waves are group-velocity matched to thesolitons, thus requiring that a dispersion zero must be between the soliton and dispersive wavefrequencies. It should be noted that some authors have attributed short wavelength generationfrom 1.5 μm CW pumped continua to soliton fission processes [8, 9]. However, soliton fissionrequires high order soliton solutions which, as noted above, are not naturally generated fromthe MI process; we therefore believe that they cannot play a role in the continuum mechanism.

To identify which mechanism (four wave mixing, or soliton trapping) dominates in the caseof HF1050 we can plot the phase and group velocity matching curves for the two processes andcompare with our experimental results, as shown in Fig. 7. For the four wave mixing phase-matching curve it is assumed that the required pump wavelengths (in the normal dispersionregime), are made available in the initial supercontinuum stages, this was verified by exper-iment. At small Stokes/anti-Stokes shifts from the pump, the two matching curves are quitesimilar, preventing unambiguous differentiation between the processes from experimental data,but at large Stokes shifts the curves diverge considerably. Marked by a pair of horizontal andvertical lines on Fig. 7 are the longest Stokes and shortest anti-Stokes wavelengths generatedin our experiments. It is clear that they cross almost precisely on the calculated group-velocitymatching curve. This implies that the soliton trapping of dispersive waves is the dominantmechanism for our short wavelength extension.

To verify this we ran numerical simulations of our pump laser propagating through HF1050.The results are shown in Fig. 8. The spectrogram of the pump is shown in Fig. 8(a) and exhibitstemporal power fluctuations as described in Section 3. After 3 m of propagation (Fig. 8(b)) MIhas led to the formation of solitons and dispersive waves have been formed at wavelengths shortof the pump. We estimate that the MI sidelobe separation for our pump and fiber conditions is∼20 nm which is not large enough for significant generation of power in the normal dispersion



GVM

PM

Fig. 7. Calculated phase (PM) and group velocity (GVM) matching curves for HF1050.The solid yellow line indicates the long wavelength edge in the supercontinuum output ofHF1050 and the solid green line indicates the short wavelength edge.

region. Instead the dispersive waves are excited by the very short duration (and hence widebandwidth) solitons formed through the MI process. Fig. 8(c) shows that as the solitons shift tolonger wavelengths via the Raman effect, the dispersive waves maintain their relative positionsto individual solitons. This is characteristic of the soliton trapping effect [10, 11]. In addition,examination of the precise locations of individual soliton-dispersive wave pairs is in agree-ment with the group velocity matching curve shown in Fig. 7. The short wavelength extensionthrough this process is understood as described in [10,11]. The soliton modulates the refractiveindex such that the dispersive wave cannot escape in one direction. The soliton then chirps thedispersive wave towards the blue, through cross-phase modulation, where the group velocity islower. It then shifts to longer wavelengths through Raman, in doing so it is decelerated as longerwavelengths have lower group velocity, and therefore falls back onto the dispersive wave, andthus the process can repeat. This trapping effect therefore leads to a cascade of scattering eventsfor the dispersive wave pushing it further and further towards the blue. In Fig. 8(d) we see thatthe solitons have pushed the dispersive waves as far short as 0.6 μm, in agreement with ourexperiments.

This process will be limited by either the breaking of the group velocity matching or by thehalting of the red-shifting solitons, which can occur by any of the reasons described in Sec-tion 4.1. Further extension to the blue should be possible if we use cascaded or tapered fibers toextend the group velocity matching of the anti-Stokes components, as previously demonstratedin the picosecond pump regime [17, 44].

5. Conclusion

We have demonstrated the extension of a continuous wave supercontinuum to the visible spec-tral region and analyzed the physical mechanisms enabling this process. We have identified thetwo requirements to generate a short wavelength continuum: pumping close to the zero dis-persion with sufficient power to generate dispersive waves, and met them by using a selectedphotonic crystal fiber and pumping with an industrial class 400 W, continuous wave, ytterbiumfiber laser. The high power available with this laser enabled the scaling of the supercontinuumaverage power to 50 W and spectral power densities of over 50 mW/nm over wide supercon-



(a) (b)

Delay / ps

aele

/

(c)

Delay / ps

(d)

Fig. 8. Simulated spectrograms of the supercontinuum development through HF1050 for170 W pump power. The spectrograms are calculated for fiber lengths of: (a) 0 m, (b) 3 m,(c) 9 m, (d) 25 m. The full color scales and axis figure scales change for each sub-figureand range over 40 dB.

tinua spanning over 1300 nm. Further optimization of the fiber design, and the use of cascadedor tapered fibers should allow extension to the blue spectral region as has been shown withpicosecond pump conditions.



Visible supercontinuum generation in photonic crystal ... · Mussot, M. Beaugeois, M. Bouazaoui,...

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