IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 15, AUGUST 1, 2012
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Continuous Wave Supercontinuum Generation Through Pumping in the Normal Dispersion Region for Spectral Flatness Ben Howard Chapman, Sergei V. Popov, and Roy Taylor
Abstract— We demonstrate the generation of a spectrally flat continuous wave supercontinuum by pumping deep in the normal dispersion region. A Raman cascade forms up to the anomalous dispersion region, where modulation instability supercontinuum dynamics are initiated. The Raman cascade seeds the frequency up-shifted part of the continuum, leading to a spectral flatness of 5 dB from 1.15 to 2.15 µm.
Fig. 1. Experimental set-up. LMAF: large mode area fiber. SMF: single-mode fiber. HNLF: highly nonlinear fiber.
Index Terms— Fiber lasers, fiber nonlinear optics, optical fiber dispersion, Raman scattering, supercontinuum generation.
I. I NTRODUCTION
C
ONTINUOUS wave (CW) pumping of supercontinua is an established route to the generation of high power, spectrally flat supercontinua. The CW supercontinuum mechanism is driven by the temporal breakdown of the pump line to solitons through modulation instability and so is inherently reliant on pumping in the anomalous dispersion regime, although power can subsequently be transferred into the normal dispersion regime through the interaction of solitons with normally dispersive radiation [1]–[4]. Pumping at a normally dispersive wavelength, however, can give rise to supercontinuum generation where power is transferred to an anomalously dispersive wavelength through Raman scattering generating a cascade of discrete components, with anomalously dispersive components of the cascade pumping the supercontinuum. This has been previously demonstrated with both pulse- [5]–[7] and CW- [8] pumped schemes, although in this case we pump the supercontinuum significantly further away from the zero dispersion wavelength (ZDW). Here we demonstrate that in certain circumstances it is possible to utilize the cascaded Raman components in the normal dispersion regime to act as seeds to enhance spectral flatness. In this letter, we demonstrate the generation of a supercontinuum of 5 dB flatness spanning from 1.15 μm to 2.13 μm, results which are comparable to others in the literature using CW or quasi-CW pump schemes in the anomalous dispersion region of photonic crystal fibers [9]–[11]. In this letter, however, the supercontinuum is generated in a graded index depressed
Manuscript received December 6, 2011; revised May 16, 2012; accepted May 28, 2012. Date of publication June 5, 2012; date of current version July 5, 2012. The authors are with the Physics Department, Femtosecond Optics Group, Imperial College, London SW7 2AZ, U.K. (e-mail: ben.chapman05@ imperial.ac.uk;
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2203011
cladding fiber, pumped far from the zero dispersion wavelength in the normal dispersion region. This illustrates that it is possible to exploit the availabilty of high-power CW ytterbium fiber lasers operating around 1 μm without relying on microstructured fibers to generate supercontinua with good spectral flatness. We used a 300 m length of highly non-linear fiber (HNLF) produced by Sumitomo Corporation, details of which can be found in [12]. The fiber had a mode field diameter (MFD) of 3.5 μm at the pump wavelength of 1.06 μm and a nonlinear parameter, γ, of 21 W−1 km−1 . The dispersion curve for the fiber is shown in Fig. 2, and the ZDW of the fiber was measured to be located at 1553 nm. We pumped the fiber using a high power ytterbium fiber laser (IPG Photonics) with a collimated free-space output. A schematic of the experimental set-up is shown in Fig. 1. The pump radiation was coupled into a large mode area fiber (LMAF), a single mode step-index fiber with MFD of 12.5μm, using an aspheric lens. Coupling into a LMAF enables a greater degree of stability on input coupling against fluctuations due to thermal loading and a lower coupling loss. The LMAF was spliced to an intermediary single mode fiber (MFD 6.4 μm) and a 0.4% fused fiber coupler (to monitor the power coupled into the fiber) which was spliced onto the HNLF. This cascade of fiber types with decreasing MFD enables a lower overall splice loss between the LMAF and the HNLF. These splices introduced a total loss of ∼1 dB, with low-loss splices achieved using an arc fusion splicer with empirically derived splicing profiles, generally with lower arc current (and hence fiber end temperature) than that used for splicing standard single mode fiber, and with longer duration arc, generally on the order of 10–15 seconds. To avoid damage to inline components and at splices, the pump source was on-off modulated at 34 Hz and a switched-on time of 0.64 ms (giving a duty factor of 46), in this way the fiber was pumped in the CW regime and all powers (pump and output) are given in terms of the switched-on power. The output from the fiber was collimated and the spectrum
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Fig. 2.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 15, AUGUST 1, 2012
Dispersion curve of HNLF. Fig. 4. Total (instantaneous) output power from 300-m length of HNLF with increasing pump power.
Fig. 3. Evolution of the Raman cascade pumped supercontinuum along 300 m of highly nonlinear fiber with 140-W pump power.
recorded using a monochromating spectrometer and an indium antimonide (InSb) detector, which is responsive across the 1–5 μm region. After coupling losses, the power coupled into the HNLF was 140 W. At this pump power a supercontinuum spanning from 1.15–2.15 μm was observed (see Fig. 3). A cut-back across the fiber was performed to reveal the process by which the continuum was formed, with spectra recorded for 200 and 100 m lengths of fiber (also shown in Fig. 3), and total output powers recorded so that spectra could be plotted in terms of spectral power (dBm/nm). The continuum formation process is revealed by the spectrum for 100 m length of fiber, in that a clear cascade of Stokes Raman shifts, characteristically separated by 13 THz, are apparent in the normal dispersion regime whilst in the anomalous dispersion region, no such cascade is visible as the first Raman line to be generated in the anomalous region at 1590 nm has acted as a source of solitons that evolve from modulation instability. Once formed, the solitons experience Raman gain, collisions and self Raman interactions leading to long wavelength broadening of the supercontinuum. With increasing fiber length we see that power is transferred along the successive Raman lines into the anomalous
dispersion region, pumping the supercontinuum. On the shortwavelength side of the ZDW (the normal dispersion region) we observe the ‘filling in’ of the Raman lines which is a combination of the broad Raman gain bandwidth and amplification of the signals, but also as the residual Raman lines will seed the dispersive-solitonic interactions which are responsible for the generation of frequency up-shifted radiation in supercontinua [13]. This leads to the generation of a spectrally flat supercontinuum, with flatness of 5 dB over the 1.15–2.15 μm region, with average spectral power across this region of 12.8 mW/nm. The supercontinuum is limited by loss at the long wavelength edge, as the attenuation in silica rises dramatically above ∼2 μm. This results in the observed sharp long wavelength edge to the supercontinuum spectrum. As the short wavelength part of the spectrum is generated through group velocity matched interactions between the solitons making up the long wavelength (anomalously dispersive) components of the continuum, and the dispersive radiation in the short wavelength (normally dispersive) part of the continuum, the spectrum is also limited on the short wavelength edge by the limits imposed on the long wavelength edge. As such, the continuum extends only as far as 1.15 μm, with a discrete line at 1.06 μm corresponding to undepleted pump radiation. Indeed, as the continuum extent is fundamentally limited by the loss at the long wavelength edge the total supercontinuum output power does not indefinitely scale with pump power. Fig. 4 shows the total supercontinuum output power with increasing pump power in the 300 m length of fiber. The greatest output power is seen for 85 W of pump power, after which the output power starts to decrease. This can be understood through the dependence on the rate of soliton selffrequency on the peak power of the solitons in the continuum. Higher power pumping will lead to the generation of higher peak power solitons from the break up of the pump line, which will shift to the opaque (> 2 μm) spectral regions. Fig. 5 shows the supercontinuum produced through 85 W of pumping in the three fiber lengths examined. This illustrates that after 300 m of fiber, the continuum has reached the long wavelength opacity limit in the fiber, with a greater total output power, and thus a greater average spectral power of
CHAPMAN et al.: CONTINUOUS WAVE SUPERCONTINUUM GENERATION THROUGH PUMPING
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Fig. 5. Evolution of the Raman cascade pumped supercontinuum along 300 m of highly nonlinear fiber with 85-W pump power.
16.5 mW/nm. Pumping at lower powers is, however, detrimental to the spectral flatness of the supercontinuum - in this case 9 dB over the 1.15 to 2.15 μm region. This illustrates that over-powered pumping of the supercontinuum can lead to a decrease in the output power, but can enhance the spectral flatness (in this case by 4 dB). II. C ONCLUSION In conclusion we have demonstrated that it is possible to achieve supercontinuum generation whilst pumping far in the normally dispersive region of a non-microstructured fiber, after wavelength conversion of the pump radiation through successive Raman scattering. Furthermore, such a pump scheme may be desirable to enhance spectral flatness as it will populate the normally dispersive region with dispersive radiation which will seed the soliton-dispersive wave interactions responsible for short-wavelength extension of the supercontinuum.
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