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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 6 — Mar. 25, 2013
  • pp: 7712–7725
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Adaptive broadband continuum source at 1200–1400 nm based on an all-fiber dual-wavelength master-oscillator power amplifier and a high-birefringence fiber

L. A. Vazquez-Zuniga, Hong Sig Kim, Youngchul Kwon, and Yoonchan Jeong  »View Author Affiliations


Optics Express, Vol. 21, Issue 6, pp. 7712-7725 (2013)
http://dx.doi.org/10.1364/OE.21.007712


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Abstract

We experimentally analyze the stimulated Raman scattering characteristics of a high-birefringence fiber (HBF), which presents an extraordinary level of spectral broadening incurred by the strong nonlinear interaction between the pump and Stokes pulses via the polarization-mode dispersion and group-velocity dispersion of the fiber. We also investigate the impact of the inter-pulse time-delay on the additional spectra broadening when dual-wavelength pump pulses are used. Exploiting these unique SRS properties of the HBF, we develop a novel Raman continuum source based on an all-fiber dual-wavelength master-oscillator power amplifier that can generate a dip-free spectrum in the 1200−1400-nm spectral range. We finally obtain a broadband continuum having an average power of ~840 mW and a 3-dB bandwidth of ~240 nm centered at 1200−1400 nm, which also represents a good spectral flatness and conversion efficiency. This type of source is very useful and important for optical coherence tomography applications, for example.

© 2013 OSA

1. Introduction

In general, it is necessary to pump an optical fiber at wavelengths within an anomalous dispersion regime or close to its zero-dispersion wavelength in order to generate efficient SC [4

4. J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]

10

10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

]. While there have been a remarkable amount of SC generation results based on novel types of PCFs in a variety of regimes both in continuous-wave (CW) and in pulsed regimes [4

4. J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]

10

10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

], such sources are critically dependent on the development of PCFs which must have the right dispersion profile for the specific spectral band desired. Apart from the fact that PCFs are less commonly available compared to conventional types of optical fibers, the use of PCFs frequently leads to a significant level of difficulty when they are spliced to other optical fibers [11

11. L. Xiao, M. S. Demokan, W. Jin, Y. Wang, and C. L. Zhao, “Fusion splicing photonic crystal fibers and conventional single-mode fibers: Microhole collapse effect,” J. Lightwave Technol. 25(11), 3563–3574 (2007), doi: . [CrossRef]

]. Thus, it must be of great interest if there is an alternative way to generate a broad continuum utilizing readily available, conventional types of optical fibers.

The motivations for this approach are also supported from the following aspects: While it is possible to generate SC utilizing a customized PCF with the right dispersion characteristics for obtaining the 1200−1400-nm spectral range when pumped with a source at ~1.1 μm, in general the level of requirement for the pump source and fiber (PCF) is relatively tight and high [7

7. J. H. V. Price, W. Belardi, T. M. Monro, A. Malinowski, A. Piper, and D. J. Richardson, “Soliton transmission and supercontinuum generation in holey fiber, using a diode pumped Ytterbium fiber source,” Opt. Express 10(8), 382–387 (2002), doi:. [CrossRef] [PubMed]

10

10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

]. Furthermore, in many cases it is very hard to avoid the unwanted spectral conversion into the shorter wavelengths [7

7. J. H. V. Price, W. Belardi, T. M. Monro, A. Malinowski, A. Piper, and D. J. Richardson, “Soliton transmission and supercontinuum generation in holey fiber, using a diode pumped Ytterbium fiber source,” Opt. Express 10(8), 382–387 (2002), doi:. [CrossRef] [PubMed]

, 10

10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

] so that the overall conversion efficiency to the target spectral range cannot be as high as desired. On the contrary, the SRS process itself basically does not require specific dispersion characteristics of the nonlinear conversion fiber nor ultrashort-pulsed sources [7

7. J. H. V. Price, W. Belardi, T. M. Monro, A. Malinowski, A. Piper, and D. J. Richardson, “Soliton transmission and supercontinuum generation in holey fiber, using a diode pumped Ytterbium fiber source,” Opt. Express 10(8), 382–387 (2002), doi:. [CrossRef] [PubMed]

, 10

10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

]. That is, one can readily utilize conventional types of optical fibers for obtaining the spectral range of 1200−1400 nm even with a fiber-based amplifier source seeded by a well-controllable, nanosecond-pulsed diode laser at ~1.1 μm. In addition, the spectral conversion by SRS is advantageously unidirectional from the pump to the Stokes lines, which invariably leads to down-conversion except for some very special cases matched for the anti-Stokes generation [6

6. S. Coen, A. H. L. Chau, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers,” J. Opt. Soc. Am. B 19(4), 753–764 (2002). [CrossRef]

], so that one can simply eliminate the unwanted loss due to the up-conversion to the short-wavelength side if the Raman-active fiber is pumped at ~1.1 μm.

However, the continuum spectra normally obtained through the SRS process in the normal dispersion regime tend to consist of discrete peaks of multi-orders of Stokes lines spaced by ~13.2 THz (in case of silica-based optical fibers) through cascaded Raman effects [19

19. K. K. Chen, S.-U. Alam, P. Horak, C. A. Codemard, A. Malinowski, and D. J. Richardson, “Excitation of individual Raman Stokes lines in the visible regime using rectangular-shaped nanosecond optical pulses at 530 nm,” Opt. Lett. 35(14), 2433–2435 (2010). [CrossRef] [PubMed]

, 20

20. C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express 18(23), 24126–24139 (2010). [CrossRef] [PubMed]

] since soliton effects are in principle absent [4

4. J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]

10

10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

]. This may be a serious challenge to SRS-based continuum sources if normal-dispersion fibers are used. However, one can resolve this by utilizing a multi-wavelength pump [20

20. C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express 18(23), 24126–24139 (2010). [CrossRef] [PubMed]

23

23. L. F. Mollenauer, A. R. Grant, and P. V. Mamyshev, “Time-division multiplexing of pump wavelengths to achieve ultrabroadband, flat, backward-pumped Raman gain,” Opt. Lett. 27(8), 592–594 (2002). [CrossRef] [PubMed]

]. This technique was initially developed to support wavelength-division-multiplexed (WDM) telecom systems operating at ~1.5-μm spectral range [21

21. H. Masuda and S. Kawai, “Wide-band and gain flattened hybrid fiber amplifier consisting of an EDFA and a multiwavelength pumped Raman amplifier,” IEEE Photon. Technol. Lett. 11(6), 647–649 (1999). [CrossRef]

23

23. L. F. Mollenauer, A. R. Grant, and P. V. Mamyshev, “Time-division multiplexing of pump wavelengths to achieve ultrabroadband, flat, backward-pumped Raman gain,” Opt. Lett. 27(8), 592–594 (2002). [CrossRef] [PubMed]

] where most optical fibers are already in the anomalous regime. Nevertheless, it can also be exploited for different spectral ranges having normal dispersion [20

20. C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express 18(23), 24126–24139 (2010). [CrossRef] [PubMed]

].

Here, we investigate the adaptive generation of a broadband Raman continuum in the spectral range of 1200−1400 nm via utilizing a conventional high-birefringence fiber (HBF) together with an all-fiber, dual-wavelength nanosecond-pulsed master-oscillator power amplifier (MOPA) source. The use of multi-level, triple-wavelength pulses has been investigated in a Raman amplifier, which was based on a highly nonlinear fiber (HNF) of 2 km in counter-propagating pump regime, controlling the Raman gain spectrum across the multiple Stokes lines [20

20. C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express 18(23), 24126–24139 (2010). [CrossRef] [PubMed]

]. However, to the best of our knowledge, there has been no report on the detailed SRS characteristics of HBFs and their applications to the “broadband continuum” generation in the nanosecond-pulsed regime. Therefore, we firstly discuss the unique SRS characteristics of the HBF in comparison with other types of optical fibers via investigating its spectral behavior for different polarization states of the input pump. Secondly, we investigate the impact of the inter-pulse time-delay and synchronization on the resultant SRS spectrum when dual-wavelength pump pulses at 1060 and 1080 nm are utilized. Building upon these results, we finally develop an all-fiber Raman continuum source having an average power of ~840 mW and a bandwidth of ~240 nm in full-width at half maximum (FWHM).

2. SRS characteristics of an HBF

Figure 1(a)
Fig. 1 (a) Spectra of the Raman Stokes lines generated from various different Raman-active fibers. (b) Schematic representation of the walk-offs of the Stokes pulses against the pump pulses in an HBF for two different cases: The input polarization of the pump pulse is aligned along the slow axis in Case 1 and at 45° to the slow axis in Case 2, respectively.
shows the output spectra obtained through four different types of conventional optical fibers pumped by a nanosecond-pulsed source at 1060 nm having various peak powers up to a sub-kilowatt level, which include a standard single-mode fiber (SMF-28, Corning), a non-zero dispersion-shifted fiber (NZDSF, OFS), a large effective area fiber (LEAF, Corning), and an HBF (HB1500, Fibercore). One can see that discrete peaks of multi-orders of Stokes lines spaced by ~13.2 THz are formed through the fibers since all of them have normal dispersion characteristics at the pump wavelength up to ~1300 or ~1500 nm. However, one can notice that for the case of the HBF the spectral dips between the two adjacent Stokes lines are much shallower than those for the other fibers, as indicated by the downward arrows in Fig. 1(a). This spectral behavior of the fiber seems very useful for forming a continuum spectrum with a good spectral flatness if it is controllable.

Motivated by this rather extraordinary feature of the HBF, we would like to understand and explain it qualitatively via Fig. 1(b), which depicts two cases that might happen in the HBF when pumped with a linearly polarized pulse, for example. (It should be noted that the spectrum shown in Fig. 1(a) was not necessarily generated by a linearly polarized pulse.) In the first case, we consider a pump pulse Px linearly polarized along the slow axis (x-axis) of the HBF. While the pump pulse Px can initially generate both Stokes pulses in the co- (Sx) and cross- (Sy) polarizations, Px will predominantly amplify Sx rather than Sy through the SRS process because the Raman gain coefficient for the co-polarized Stokes is larger than that for the cross-polarized Stokes by approximately a factor of ten [24

24. D. J. Dougherty, F. X. Kärtner, H. A. Haus, and E. P. Ippen, “Measurement of the Raman gain spectrum of optical fibers,” Opt. Lett. 20(1), 31–33 (1995). [CrossRef] [PubMed]

]. Thus, the impact of Sy is quite negligible. Sx might initially contain broad spectral components from the spontaneous Raman scattering process; however, the resultant spectrum of Sx after propagation through a distance will become narrowed down because of the successive SRS process although a slight spectral broadening would accompany because of the walk-off effect [25

25. R. H. Stolen and A. M. Johnson, “The effect of pulse walkoff on stimulated Raman scattering in fibers,” IEEE J. Quantum Electron. 22(11), 2154–2160 (1986). [CrossRef]

] between Px and Sx via the chromatic group-velocity dispersion (GVD) of the HBF. It should be noted that Sx will travel slightly ahead of Px, being up-chirped, i.e., having the leading edge red-shifted and the trailing edge blue-shifted as a result of self-phase modulation (SPM) together with the normal dispersion condition (which will be the case throughout this paper). Considering the pulse overlap between Px and Sx, one can see that the trailing blue-shifted part will be better overlapped with Px, thereby seeing higher Raman gain than the leading red-shifted part. It is noteworthy that in normal dispersion regime the trailing blue-shifted part will always see the higher Raman gain than the leading red-shifted part [25

25. R. H. Stolen and A. M. Johnson, “The effect of pulse walkoff on stimulated Raman scattering in fibers,” IEEE J. Quantum Electron. 22(11), 2154–2160 (1986). [CrossRef]

]. In addition, since Sx will also generate the next-order Raman Stokes via the cascaded Raman effect [19

19. K. K. Chen, S.-U. Alam, P. Horak, C. A. Codemard, A. Malinowski, and D. J. Richardson, “Excitation of individual Raman Stokes lines in the visible regime using rectangular-shaped nanosecond optical pulses at 530 nm,” Opt. Lett. 35(14), 2433–2435 (2010). [CrossRef] [PubMed]

, 20

20. C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express 18(23), 24126–24139 (2010). [CrossRef] [PubMed]

], we can readily expect that the trailing blue-shifted part of Sx will undergo more power depletion because of seeing the higher Raman gain all the time, thereby giving rise to a “red-shift” in the resultant spectrum of Sx. Somehow, the amount of spectral gain broadening and shift is dependent on the amount of walk-off between Px and Sx via the GVD of the HBF, which is in general quite limited in the cases of non-birefringence fibers as seen in Fig. 1(a).

3. Experimental results

3.1. Experimental arrangement

The schematic of the broadband source pumped by an all-fiber, dual-wavelength MOPA is shown in Fig. 2
Fig. 2 Schematic of the broadband continuum source based on a dual-wavelength MOPA seeded at 1060 and 1080 nm via a Raman-active fiber.
. The system consists of two single-mode, 14-pin butterfly laser diodes (LDs) with operating wavelengths of 1060 and 1080 nm, modulated by two custom-made synchronized LD controllers (Notice Korea). Thus, the two LDs are capable of generating pulses of ~2 ns, with different repetition rates and inter-pulse time-delays. The signals from both LDs pass through their own isolators and are combined into a fiberized 1060/1080 WDM combiner. The common port of the WDM combiner is then spliced to a tap coupler (99:1). The 99-% port seeds the first ytterbium-doped fiber amplifier (YDFA) of the system, while the 1-% port is used for monitoring purposes. The YDFA comprises 6 m of a double-clad ytterbium-doped fiber (YDF, CorActive) cladding-pumped with a 975-nm high-power multimode LD through a tapered fiber bundle (TFB). The YDF has a core of ~8 μm in diameter with a numerical aperture (NA) of 0.08 and a cladding of ~125 μm in diameter with an NA of 0.45, respectively. Its small-signal cladding absorption rate is ~6.8 dB/m at 976 nm. The output of the amplifier is spliced to an isolator to protect the first YDFA against any back reflection from the second YDFA. The signal is then launched into the second YDFA through another tap coupler (99:1) for monitoring purposes. The second YDFA comprises a 9 m of the same YDF used for the first YDFA and is also cladding-pumped with a 975-nm high-power multimode LD. Another isolator is spliced to the output end of the second YDFA, followed by a tap coupler (99:1) for monitoring the final output signal and a polarization controller (PC) for controlling the output polarization state, respectively. In addition, it is noteworthy that the inclusion of the two 1060/1140 WDM couplers between the first and second YDFAs and after the last tap coupler is to bypass the unwanted backscattered Stokes lines, i.e., mainly the first Stokes lines at 1100−1150 nm of the Raman continuum generated in the Raman-active fiber that is to be spliced to the output end of this dual-wavelength MOPA source. We initially investigate an HBF as the Raman-active fiber, motivated by the unique SRS property observed in Fig. 1.

In addition, the single-mode LDs operate in a pulsed-mode, generating pulse widths of ~2 ns at a repetition rate of 500 kHz throughout the whole experiment. In the given condition, the average output power of the combined two-LD signal is ~0.2 mW and is amplified up to ~70 mW through the first YDFA and up to ~1.2 W through the second YDFA, respectively. This yields a peak power of ~1.2 kW when the two-wavelength pulses are fully overlapped. This will be the operating condition throughout our experiment unless stated otherwise.

3.2. Analysis of the polarization-dependent SRS of an HBF

Since the all-fiber MOPA source is not based on PM fibers and PM components, an additional setup is prepared for the sake of analyzing in detail the polarization-dependent SRS in an HBF. The setup is as follows: The output end of the MOPA before the Raman-active fiber (see Fig. 2) is flat-cleaved, and its output beam is collimated in free space and passes through a broadband polarizing beam splitter (PBS, PBS-1), thereby resulting in a linearly polarized beam as shown in Fig. 3
Fig. 3 Schematic prepared for analyzing the polarization-dependent SRS in an HBF. AL: aspheric lens; RM: rotation mount.
. The polarizing angle of PBS-1 is fine-tuned via a rotation mount. Then, the output beam through the HBF is again collimated and passes through another broadband PBS (PBS-2). The transmitted beam from PBS-2 is coupled into a standard single-mode fiber (SMF-28) and monitored with an optical spectrum analyzer (OSA). The rotation of PBS-2 allows us to analyze the state of polarization (SOP) of the Raman continuum formed in the HBF. It is noteworthy that the HBF is fixed to a fiber holder with its slow and fast axes aligned parallel to x and y axes in terms of the laboratory coordinates. In addition, the polarization extinction ratios of both PBSs are better than 20 dB, and the reflected beams by PBS-1 and PBS-2 are terminated into individual beam dumps. The HBF (HB1500, Fibercore) investigated in our experiment has a core NA of 0.16, a mode field diameter of 7.9 μm at 1550 nm, and a cut-off wavelength of 1255 nm. Its length is ~60 m. In particular, its beat length is given by 1.9 mm at 1060 nm (2.7 mm at 1550 nm), yielding a PMD of ~1.88 ps/m at 1060 nm [28

28. G. Millot and J. M. Dudley, “Polarization-mode dispersion measurements in high-birefringence fibers by means of stimulated Raman scattering,” Appl. Opt. 41(13), 2589–2591 (2002). [CrossRef] [PubMed]

] while the chromatic dispersion parameter is given by β2 = −13.7 ps2/km at ~1550 nm [29

29. M. Meissner, C. Marquardt, J. Heersink, T. Gaber, A. Wietfeld, G. Leuchs, and U. L. Andersen, “All-fibre source of amplitude squeezed light pulses,” J. Opt. B Quantum Semiclassical Opt. 6(8), S652–S657 (2004). [CrossRef]

]. Given the dispersion parameters, we estimate that the walk-offs due to GVD and PMD are comparable, and the walk-off variation due to the PMD against the GVD is given by ± 113 ps in 60 m of the HBF. This variation may be regarded as relatively small in comparison with the duration of the input pump pulse, which is ~2 ns. However, it should be noted that if higher-order Stokes pulses are generated, they will undergo significant duration-narrowing because of the inherent nature of the nonlinear conversion process [30

30. Y. Wang, “Dynamics of stimulated Raman scattering in double-clad fiber pulse amplifiers,” IEEE J. Quantum Electron. 41(6), 779–788 (2005). [CrossRef]

]. Therefore, we cannot simply underestimate the spectral gain-broadening due to the walk-off effect. Given that the walk-offs due to GVD and PMD are comparable, the maximum spectral shift can be as far as the amount of Stokes shift, although the overall shift will be limited because the walk-off is gradually built up over the fiber length. In addition, it should be noted that although the HBF supports higher-order modes (HOMs, including TE01, TM01, and HE21) below the cut-off wavelength, we carefully arranged for the free-space optics, thereby exciting only the fundamental mode (HE11) into the HBF at the pump wavelengths [31

31. M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef] [PubMed]

]. The total power launched into the HBF was reduced to roughly half the full output power of the MOPA mainly due to the insertion of PBS-1.

We first operated the MOPA source with a single-wavelength input either at 1060 or 1080 nm, varying the input polarization angle via PBS-1 with respect to the slow-axis (x-axis). It should be noted that in this case we temporarily took PBS-2 out of the setup in order to monitor the whole output. Since the peak power of the input pulse was sufficiently high, one can easily see the nonlinear spectral evolution of the output spectrum by cascaded SRS as shown in Fig. 4
Fig. 4 Measured optical spectra (upper) and their spectrum-prints (lower) of the Raman Stokes lines generated through 60 m of an HBF for different angles of rotation of PBS-1 for input wavelengths of (a) 1060 nm and (b) 1080 nm, respectively.
. The spectral powers are normalized to the maximum power levels, and for visual aids “spectrum-prints” plotted in color-coded lines in dB scales are also accompanied in the bottom. The results indicate that when the input polarization was aligned either to the slow or fast axis of the HBF, the Raman Stokes lines had relatively deeper spectral valleys. (See the cases of the angle of PBS-1 aligned at 0° or 90°). In these cases, the differences between the adjacent peaks and valleys of the spectral power were given by at least 20 dB. However, one can notice that when the input polarization was aligned at 45° to the slow axis, the spectral power level at the valleys significantly increased by more than 5 dB in most cases, and the Raman Stokes lines were red-shifted relative to the cases aligned at 0° or 90°. This experimental observations support our explanation offered in Section 2 with Fig. 1(b), verifying that the spectral broadening and red-shift of the Raman Stokes lines occur when cross-polarized Stokes pulses of non-negligible power are provided together with the pump pulse.

Next, we analyzed the SOP of the output beam by restoring the insertion of PBS-2 in the setup. We initially investigated the co-linear polarization case in which the input polarization was matched to either of the two principal axes of birefringence of the HBF. Figure 5
Fig. 5 Spectrum-prints of the Raman Stokes lines generated through the HBF when the input polarization was aligned either to the (a) x-axis or (b) y-axis for input wavelengths of 1060 (upper) and 1080 nm (lower), respectively. The angle of rotation denotes the angle of PBS-1 (shown in Fig. 3) with respect to the slow axis (x-axis).
represents the corresponding spectrum-prints for the input wavelengths of 1060 and 1080 nm, respectively. The angle of rotation on the vertical axis shown in Fig. 5 indicates the polarizing angle of PBS-2 measured from the slow axis (x-axis) of the HBF. One can see that as discussed in Fig. 1(b), the Stokes lines are predominantly in the same polarization as that of the input pump with extinction ratios of more than 30 dB. In addition, the extinctions ratios did not show any noticeable spectral dependence.

On the contrary, one can see significant difference in the polarization-dependent spectral behavior for the case of the input polarization aligned at 45° to the slow axis (x-axis) of the HBF, as shown in Fig. 6
Fig. 6 Spectrum-prints of the Raman Stokes lines generated from the HBF when the input polarization was aligned at 45° to the slow axis (x-axis) for the input wavelengths of (a) 1060 and (b) 1080 nm, respectively. The angle of rotation denotes the angle of PBS-2 (shown in Fig. 3) with respect to the slow axis (x-axis).
. For this experiment, we monitored the spectral power through PBS-2, while rotating its polarizing angle from the x-axis. It is noteworthy that there are two noticeable features: One is that the overall Raman conversion strength was maximized when the Stokes lines were polarized in the slow axis (0° in terms of the rotation angle), i.e., Sx, as denoted in Fig. 1(b). The other is that although the Stokes lines polarized in the fast axis ( ± 90° in terms of the rotation angle), i.e., Sy, as denoted in Fig. 1(b), had less power than Sx, their spectral broadening and red-shift were much more significant: One can see that the spectral dips become shallower and shifted towards longer wavelengths as the angle of PBS-2 increased from 0° to 90°. These experimental observations again fully support our explanation offered in Section 2. That is, for the pump excitation at 45° to the slow axis (x-axis), Sy will represent more spectral broadening and red-shift while Sx will represent more overall Raman conversion. In fact, these consequences are due mainly to the effects of the combination of the PMD and GVD of the HBF as discussed in Fig. 1(b).

3.3. Impact of the inter-pulse time-delay between the dual-wavelength pump pulses

Since we use the dual-wavelength pump pulses, it must be worthwhile to investigate the impact of the inter-pulse time-delay between the dual-wavelength pump pulses when they are turned on at the same time. This analysis will allow us to understand the pulse-to-pulse Raman interaction and its influence on the spectral shaping of the resultant Stokes lines.

Figure 7
Fig. 7 Spectrum-prints of the Raman Stokes lines generated from the HBF with the dual-wavelength pulses at 1060 nm and 1080 nm when the input polarization was aligned to (a) the slow axis (x-axis) or (b) fast axis (y-axis), varying the inter-pulse time-delay from −2.5 ns to + 2.5 ns. The time traces of (c)−(g) depict the input dual- wavelength pulses with different inter-pulse time-delays.
shows the evolution of the output spectra for different values of the inter-pulse time-delay where we kept the input polarization of the pump pulses aligned either to the slow axis (x-axis) or fast axis (y-axis). In such conditions, the SRS will predominantly occur between the pump and Stokes pulses in the same polarization, as discussed in Fig. 5, as well as in Fig. 1(b), so that we can see the impact of the inter-pulse time-delay separately from the other effects, e.g., SRS effects between cross-polarized pumps and Stokes. We varied the time-delays from −2.5 ns to + 2.5 ns, where the positive sign indicates that the 1060-nm pulse was ahead of the 1080-nm pulse. The minimum resolvable step of the time-delay was 0.1 ns. One can see that for the cases where the time-delays are close to zero [see Fig. 7(e)], the pulse-to-pulse Raman interaction became stronger with an increase of more than 10 dB of the spectral power level in the range between 1200 nm and 1300 nm, compared to the cases having the time-delays of ± 2.5 ns. Actually, this consequence can be explained in the similar manner with the case previously discussed in Fig. 6, if we regard the cross-interaction between the two pulses of different wavelengths as the cross-interaction between the two pulses of different polarizations. In fact, the 1060-nm pump pulse can work as an auxiliary pump for the Stokes lines generated by the 1080-nm pulse, and vice versa. Therefore, one can readily expect that the overall SRS will be boosted up as the overlap between the two pulses is increased. On the other hand, if there is a certain level of time-delay between the two pulses which is shorter than the individual pulse width, an intermediate part of one pulse, which may be leading or trailing, will eventually see the peak of the other pulse, so that the spectral components in the slope of the one pulse will consequently see “boosted” Raman gain. Thus, this walk-off effect will also lead to spectral broadening [25

25. R. H. Stolen and A. M. Johnson, “The effect of pulse walkoff on stimulated Raman scattering in fibers,” IEEE J. Quantum Electron. 22(11), 2154–2160 (1986). [CrossRef]

].

3.4. Spectral shaping with an HBF in conjunction with an HNF

The resultant output spectra obtained through the HBF for three different input pump conditions are shown in Fig. 8
Fig. 8 Output spectra and their spectrum-prints of the Raman continuum generated through the HBF: The blue (A) and red traces (B) are for the cases before and after optimizing the polarization state of the input pump, and the green trace (C) is for the case when the peak power of the 1080-nm seed pulse was individually reduced by ~10%.
. The blue and red traces are for the cases before and after optimizing the SOP of the input pump, respectively. It should be noted that the SOP of the input pulse was adjusted via the fiberized PC shown in Fig. 2. (The input pulses to the HBF might well be elliptically polarized and contain a fraction of power in higher-order modes.) One can see that both the level of spectral power and the spectral flatness in the range of 1200 to 1300 nm were significantly improved by the adjustment. The green trace is for the case when the peak power of the 1080-nm seed pulse was individually reduced by ~10%, where one can see the further improvement both in the spectral power level and in the spectral flatness. In this case, the resultant Raman continuum became as broad as ~150 nm (FWHM) centered at ~1250 nm. The average output power was ~930 mW, yielding the input-to-output conversion efficiency of ~78%.

However, we observed that the spectral power of the Raman continuum dropped by more than 10 dB at wavelengths longer than 1320 nm, regardless of the optimization of the input pump. In order to enhance the spectral power in the longer wavelengths, it was necessary to increase either the power of the pump lines or the length of the Raman-active fiber. The maximum output power of the MOPA source was actually limited by the safe operation range of the fiberized components, including the output isolator and tap/WDM couplers at the output end, which were graded to operate below ~1.5 W. Consequently, we considered splicing an additional length of another Raman-active optical fiber to the HBF. We chose an HNF (HNDS1310B, Sumitomo) since it has a relatively high nonlinear coefficient (γ = 15 W−1km−1 at 1300 nm) while it has a moderate normal chromatic dispersion at 1300 nm (D = −8 ps/nm/km at 1300 nm). In addition, its cut-off wavelength is given by 1020 nm. Next, its length was optimized to ~10 m via the cut-back method in order to obtain the Raman continuum well-centered at 1200−1400 nm. (It should be noted that if the HNF is too long, the spectral range of the continuum will exceed over 1500 nm at the expense of the spectral power at ~1200 nm, which is not desirable in our case.)

5. Conclusion

We have investigated the SRS in an HBF for various conditions of the input pump pulse, paying particular attention to its spectral broadening and shaping. We found that the PMD of the fiber in conjunction with its GVD plays a crucial role via the interaction between the pump and Stokes pulses, which is a unique property that cannot be seen with other non-birefringence optical fibers. That is, the walk-off, i.e., the inter-pulse time-delay between the pump and Stokes pulses incurred by the PMD and GVD of the HBF gives rise to extraordinary temporal shifts of the Raman-gain peaks, thereby leading to the correspondent broadening of the overall Raman gain spectrum [25

25. R. H. Stolen and A. M. Johnson, “The effect of pulse walkoff on stimulated Raman scattering in fibers,” IEEE J. Quantum Electron. 22(11), 2154–2160 (1986). [CrossRef]

]. In addition, the extraordinary spectral broadening and shift can also be boosted up in case parametric processes via FWM and XPM among cross-polarized HOM pump and Stokes pulses are involved. This spectral broadening effect can further be exploited via varying the inter-pulse time-delay if a multi-wavelength pump regime is considered. When we used the input pump comprised with dual-wavelength pulses at 1060 and 1080 nm with a variable inter-pulse time-delay, we could observe another significant level of spectral broadening in the resultant Raman Stokes lines.

Exploiting these unique properties, we have developed a Raman continuum source based on an all-fiber MOPA synchronously seeded at 1060 and 1080 nm with a variable inter-pulse time-delay followed by a 60-m Raman-active fiber of an HBF. This source could emit a Raman continuum with a good spectral flatness having an average power of ~930 mW and a bandwidth of ~150 nm (FWHM) centered at ~1250 nm. It should be noted that the spectral shape and flatness were formed by the adaptive control of the polarization, inter-pulse time-delay, and peak powers of the dual-wavelength pulses generated from the MOPA. With the inclusion an HNF of 10 m in addition to the HBF, we generated a Raman continuum having an average power of ~840 mW and a bandwidth of ~240 nm (FWHM) in the 1200−1400-nm spectral range. Both results represented good input-to-output conversion efficiency of in excess of 70%, having the spectrum well confined in 1200−1400 nm. Broadband continuum sources in this spectral range are, in particular, of great interest for OCT imaging applications [17

17. A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography–principles and applications,” Rep. Prog. Phys. 66(2), 239–303 (2003). [CrossRef]

, 18

18. P. L. Hsiung, Y. Chen, T. H. Ko, J. G. Fujimoto, C. J. S. de Matos, S. V. Popov, J. R. Taylor, and V. P. Gapontsev, “Optical coherence tomography using a continuous-wave, high-power, Raman continuum light source,” Opt. Express 12(22), 5287–5295 (2004). [CrossRef] [PubMed]

]. For example, the 240-nm bandwidth demonstrated here is capable of yielding an axial resolution of ~3 μm when utilized as a light source for a spectral-domain OCT system [17

17. A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography–principles and applications,” Rep. Prog. Phys. 66(2), 239–303 (2003). [CrossRef]

]. In this regard, Raman continuum sources based on nanosecond-pulsed MOPAs combined with HBFs or other conventional types of optical fibers could be attractive alternatives to SC sources based on the ultrashort-pulsed lasers and/or PCFs which may suffer from strong spectral modulation or low optical conversion efficiency.

Acknowledgment

This work was supported by Samsung Advanced Institute of Technology.

References and links

1.

D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]

2.

J. Nilsson and D. N. Payne, “Physics. High-power fiber lasers,” Science 332(6032), 921–922 (2011). [CrossRef] [PubMed]

3.

A. Tünnermann, T. Schreiber, and J. Limpert, “Fiber lasers and amplifiers: an ultrafast performance evolution,” Appl. Opt. 49(25), F71–F78 (2010). [CrossRef] [PubMed]

4.

J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]

5.

J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25(1), 25–27 (2000). [CrossRef] [PubMed]

6.

S. Coen, A. H. L. Chau, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers,” J. Opt. Soc. Am. B 19(4), 753–764 (2002). [CrossRef]

7.

J. H. V. Price, W. Belardi, T. M. Monro, A. Malinowski, A. Piper, and D. J. Richardson, “Soliton transmission and supercontinuum generation in holey fiber, using a diode pumped Ytterbium fiber source,” Opt. Express 10(8), 382–387 (2002), doi:. [CrossRef] [PubMed]

8.

J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]

9.

J. C. Travers, A. B. Rulkov, B. A. Cumberland, S. V. Popov, and J. R. Taylor, “Visible supercontinuum generation in photonic crystal fibers with a 400 W continuous wave fiber laser,” Opt. Express 16(19), 14435–14447 (2008), doi:. [CrossRef] [PubMed]

10.

K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]

11.

L. Xiao, M. S. Demokan, W. Jin, Y. Wang, and C. L. Zhao, “Fusion splicing photonic crystal fibers and conventional single-mode fibers: Microhole collapse effect,” J. Lightwave Technol. 25(11), 3563–3574 (2007), doi: . [CrossRef]

12.

Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express 12(25), 6088–6092 (2004), doi:. [CrossRef] [PubMed]

13.

D. B. S. Soh, S. W. Yoo, J. Nilsson, J. K. Sahu, K. Oh, S. Baek, Y. Jeong, C. A. Codemard, P. Dupriez, J. Kim, and V. N. Philippov, “Neodymium-doped cladding pumped aluminosilicate fiber laser tunable in the 0.9 μm wavelength range,” IEEE J. Quantum Electron. 40(9), 1275–1282 (2004). [CrossRef]

14.

Y. Jeong, S. Yoo, C. A. Codemard, J. Nilsson, J. K. Sahu, D. N. Payne, R. Horley, P. W. Turner, L. M. B. Hickey, A. Harker, M. Lovelady, and A. Piper, “Erbium:ytterbium codoped large-core fiber laser with 297-W continuous-wave output power,” IEEE J. Sel. Top. Quantum Electron. 13(3), 573–579 (2007). [CrossRef]

15.

P. F. Moulton, “Power scaling of high-efficiency Tm-doped fiber lasers,” in Proc. LASE 2008, paper 6873–15 (2008).

16.

E. M. Dianov, V. V. Dvoyrin, M. V. Mashinsky, A. A. Umnikov, M. V. Yashkov, and A. N. Gur’yanov, “CW bismuth fibre laser,” Quantum Electron. 35(12), 1083–1084 (2005). [CrossRef]

17.

A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography–principles and applications,” Rep. Prog. Phys. 66(2), 239–303 (2003). [CrossRef]

18.

P. L. Hsiung, Y. Chen, T. H. Ko, J. G. Fujimoto, C. J. S. de Matos, S. V. Popov, J. R. Taylor, and V. P. Gapontsev, “Optical coherence tomography using a continuous-wave, high-power, Raman continuum light source,” Opt. Express 12(22), 5287–5295 (2004). [CrossRef] [PubMed]

19.

K. K. Chen, S.-U. Alam, P. Horak, C. A. Codemard, A. Malinowski, and D. J. Richardson, “Excitation of individual Raman Stokes lines in the visible regime using rectangular-shaped nanosecond optical pulses at 530 nm,” Opt. Lett. 35(14), 2433–2435 (2010). [CrossRef] [PubMed]

20.

C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express 18(23), 24126–24139 (2010). [CrossRef] [PubMed]

21.

H. Masuda and S. Kawai, “Wide-band and gain flattened hybrid fiber amplifier consisting of an EDFA and a multiwavelength pumped Raman amplifier,” IEEE Photon. Technol. Lett. 11(6), 647–649 (1999). [CrossRef]

22.

S. Namiki and Y. Emori, “Ultrabroad-band Raman amplifiers pumped and gain-equalized by wavelength-division multiplexed high-power laser diodes,” IEEE J. Sel. Top. Quantum Electron. 7(1), 3–16 (2001). [CrossRef]

23.

L. F. Mollenauer, A. R. Grant, and P. V. Mamyshev, “Time-division multiplexing of pump wavelengths to achieve ultrabroadband, flat, backward-pumped Raman gain,” Opt. Lett. 27(8), 592–594 (2002). [CrossRef] [PubMed]

24.

D. J. Dougherty, F. X. Kärtner, H. A. Haus, and E. P. Ippen, “Measurement of the Raman gain spectrum of optical fibers,” Opt. Lett. 20(1), 31–33 (1995). [CrossRef] [PubMed]

25.

R. H. Stolen and A. M. Johnson, “The effect of pulse walkoff on stimulated Raman scattering in fibers,” IEEE J. Quantum Electron. 22(11), 2154–2160 (1986). [CrossRef]

26.

G. Van der Westhuizen and J. Nilsson, “Fiber optical parametric oscillator for large frequency-shift wavelength conversion,” IEEE J. Quantum Electron. 47(11), 1396–1403 (2011). [CrossRef]

27.

P. Dupriez, F. Poletti, P. Horak, M. N. Petrovich, Y. Jeong, J. Nilsson, D. J. Richardson, and D. N. Payne, “Efficient white light generation in secondary cores of holey fibers,” Opt. Express 15(7), 3729–3736 (2007), doi:. [CrossRef] [PubMed]

28.

G. Millot and J. M. Dudley, “Polarization-mode dispersion measurements in high-birefringence fibers by means of stimulated Raman scattering,” Appl. Opt. 41(13), 2589–2591 (2002). [CrossRef] [PubMed]

29.

M. Meissner, C. Marquardt, J. Heersink, T. Gaber, A. Wietfeld, G. Leuchs, and U. L. Andersen, “All-fibre source of amplitude squeezed light pulses,” J. Opt. B Quantum Semiclassical Opt. 6(8), S652–S657 (2004). [CrossRef]

30.

Y. Wang, “Dynamics of stimulated Raman scattering in double-clad fiber pulse amplifiers,” IEEE J. Quantum Electron. 41(6), 779–788 (2005). [CrossRef]

31.

M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef] [PubMed]

32.

F. Poletti and P. Horak, “Dynamics of femtosecond supercontinuum generation in multimode fibers,” Opt. Express 17(8), 6134–6147 (2009), doi:. [CrossRef] [PubMed]

OCIS Codes
(140.3510) Lasers and laser optics : Lasers, fiber
(290.5910) Scattering : Scattering, stimulated Raman
(140.3615) Lasers and laser optics : Lasers, ytterbium

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 15, 2013
Revised Manuscript: March 12, 2013
Manuscript Accepted: March 13, 2013
Published: March 21, 2013

Virtual Issues
Vol. 8, Iss. 4 Virtual Journal for Biomedical Optics

Citation
L. A. Vazquez-Zuniga, Hong Sig Kim, Youngchul Kwon, and Yoonchan Jeong, "Adaptive broadband continuum source at 1200–1400 nm based on an all-fiber dual-wavelength master-oscillator power amplifier and a high-birefringence fiber," Opt. Express 21, 7712-7725 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-7712


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References

  1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B27(11), B63–B92 (2010). [CrossRef]
  2. J. Nilsson and D. N. Payne, “Physics. High-power fiber lasers,” Science332(6032), 921–922 (2011). [CrossRef] [PubMed]
  3. A. Tünnermann, T. Schreiber, and J. Limpert, “Fiber lasers and amplifiers: an ultrafast performance evolution,” Appl. Opt.49(25), F71–F78 (2010). [CrossRef] [PubMed]
  4. J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett.21(19), 1547–1549 (1996). [CrossRef] [PubMed]
  5. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett.25(1), 25–27 (2000). [CrossRef] [PubMed]
  6. S. Coen, A. H. L. Chau, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers,” J. Opt. Soc. Am. B19(4), 753–764 (2002). [CrossRef]
  7. J. H. V. Price, W. Belardi, T. M. Monro, A. Malinowski, A. Piper, and D. J. Richardson, “Soliton transmission and supercontinuum generation in holey fiber, using a diode pumped Ytterbium fiber source,” Opt. Express10(8), 382–387 (2002), doi:. [CrossRef] [PubMed]
  8. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys.78(4), 1135–1184 (2006). [CrossRef]
  9. J. C. Travers, A. B. Rulkov, B. A. Cumberland, S. V. Popov, and J. R. Taylor, “Visible supercontinuum generation in photonic crystal fibers with a 400 W continuous wave fiber laser,” Opt. Express16(19), 14435–14447 (2008), doi:. [CrossRef] [PubMed]
  10. K. K. Chen, S.-U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express18(6), 5426–5432 (2010), doi:. [CrossRef] [PubMed]
  11. L. Xiao, M. S. Demokan, W. Jin, Y. Wang, and C. L. Zhao, “Fusion splicing photonic crystal fibers and conventional single-mode fibers: Microhole collapse effect,” J. Lightwave Technol.25(11), 3563–3574 (2007), doi: . [CrossRef]
  12. Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express12(25), 6088–6092 (2004), doi:. [CrossRef] [PubMed]
  13. D. B. S. Soh, S. W. Yoo, J. Nilsson, J. K. Sahu, K. Oh, S. Baek, Y. Jeong, C. A. Codemard, P. Dupriez, J. Kim, and V. N. Philippov, “Neodymium-doped cladding pumped aluminosilicate fiber laser tunable in the 0.9 μm wavelength range,” IEEE J. Quantum Electron.40(9), 1275–1282 (2004). [CrossRef]
  14. Y. Jeong, S. Yoo, C. A. Codemard, J. Nilsson, J. K. Sahu, D. N. Payne, R. Horley, P. W. Turner, L. M. B. Hickey, A. Harker, M. Lovelady, and A. Piper, “Erbium:ytterbium codoped large-core fiber laser with 297-W continuous-wave output power,” IEEE J. Sel. Top. Quantum Electron.13(3), 573–579 (2007). [CrossRef]
  15. P. F. Moulton, “Power scaling of high-efficiency Tm-doped fiber lasers,” in Proc. LASE 2008, paper 6873–15 (2008).
  16. E. M. Dianov, V. V. Dvoyrin, M. V. Mashinsky, A. A. Umnikov, M. V. Yashkov, and A. N. Gur’yanov, “CW bismuth fibre laser,” Quantum Electron.35(12), 1083–1084 (2005). [CrossRef]
  17. A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography–principles and applications,” Rep. Prog. Phys.66(2), 239–303 (2003). [CrossRef]
  18. P. L. Hsiung, Y. Chen, T. H. Ko, J. G. Fujimoto, C. J. S. de Matos, S. V. Popov, J. R. Taylor, and V. P. Gapontsev, “Optical coherence tomography using a continuous-wave, high-power, Raman continuum light source,” Opt. Express12(22), 5287–5295 (2004). [CrossRef] [PubMed]
  19. K. K. Chen, S.-U. Alam, P. Horak, C. A. Codemard, A. Malinowski, and D. J. Richardson, “Excitation of individual Raman Stokes lines in the visible regime using rectangular-shaped nanosecond optical pulses at 530 nm,” Opt. Lett.35(14), 2433–2435 (2010). [CrossRef] [PubMed]
  20. C. Farrell, C. A. Codemard, and J. Nilsson, “Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier,” Opt. Express18(23), 24126–24139 (2010). [CrossRef] [PubMed]
  21. H. Masuda and S. Kawai, “Wide-band and gain flattened hybrid fiber amplifier consisting of an EDFA and a multiwavelength pumped Raman amplifier,” IEEE Photon. Technol. Lett.11(6), 647–649 (1999). [CrossRef]
  22. S. Namiki and Y. Emori, “Ultrabroad-band Raman amplifiers pumped and gain-equalized by wavelength-division multiplexed high-power laser diodes,” IEEE J. Sel. Top. Quantum Electron.7(1), 3–16 (2001). [CrossRef]
  23. L. F. Mollenauer, A. R. Grant, and P. V. Mamyshev, “Time-division multiplexing of pump wavelengths to achieve ultrabroadband, flat, backward-pumped Raman gain,” Opt. Lett.27(8), 592–594 (2002). [CrossRef] [PubMed]
  24. D. J. Dougherty, F. X. Kärtner, H. A. Haus, and E. P. Ippen, “Measurement of the Raman gain spectrum of optical fibers,” Opt. Lett.20(1), 31–33 (1995). [CrossRef] [PubMed]
  25. R. H. Stolen and A. M. Johnson, “The effect of pulse walkoff on stimulated Raman scattering in fibers,” IEEE J. Quantum Electron.22(11), 2154–2160 (1986). [CrossRef]
  26. G. Van der Westhuizen and J. Nilsson, “Fiber optical parametric oscillator for large frequency-shift wavelength conversion,” IEEE J. Quantum Electron.47(11), 1396–1403 (2011). [CrossRef]
  27. P. Dupriez, F. Poletti, P. Horak, M. N. Petrovich, Y. Jeong, J. Nilsson, D. J. Richardson, and D. N. Payne, “Efficient white light generation in secondary cores of holey fibers,” Opt. Express15(7), 3729–3736 (2007), doi:. [CrossRef] [PubMed]
  28. G. Millot and J. M. Dudley, “Polarization-mode dispersion measurements in high-birefringence fibers by means of stimulated Raman scattering,” Appl. Opt.41(13), 2589–2591 (2002). [CrossRef] [PubMed]
  29. M. Meissner, C. Marquardt, J. Heersink, T. Gaber, A. Wietfeld, G. Leuchs, and U. L. Andersen, “All-fibre source of amplitude squeezed light pulses,” J. Opt. B Quantum Semiclassical Opt.6(8), S652–S657 (2004). [CrossRef]
  30. Y. Wang, “Dynamics of stimulated Raman scattering in double-clad fiber pulse amplifiers,” IEEE J. Quantum Electron.41(6), 779–788 (2005). [CrossRef]
  31. M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett.23(1), 52–54 (1998). [CrossRef] [PubMed]
  32. F. Poletti and P. Horak, “Dynamics of femtosecond supercontinuum generation in multimode fibers,” Opt. Express17(8), 6134–6147 (2009), doi:. [CrossRef] [PubMed]

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