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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 23 — Nov. 5, 2012
  • pp: 25461–25470
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Mode evolution in long tapered fibers with high tapering ratio

Juho Kerttula, Valery Filippov, Vasily Ustimchik, Yuri Chamorovskiy, and Oleg G. Okhotnikov  »View Author Affiliations


Optics Express, Vol. 20, Issue 23, pp. 25461-25470 (2012)
http://dx.doi.org/10.1364/OE.20.025461


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Abstract

We have experimentally studied fundamental mode propagation in few meters long, adiabatically tapered step-index fibers with high numerical aperture, core diameter up to 117 μm (V = 38) and tapering ratio up to 18. The single fundamental mode propagation was confirmed by several techniques that reveal no signature of higher-order mode excitation. It can be, therefore, concluded that adiabatic tapering is a powerful method for selective excitation of the fundamental mode in highly multimode large-mode-area fibers. Annular near field distortion observed for large output core diameters was attributed to built-in stress due to thermal expansion mismatch between core and cladding materials. The mechanical stress could be avoided by an appropriate technique of fiber preform fabrication and drawing, which would prevent the mode field deformation and lead to reliable diffraction-limited fundamental mode guiding for very large core diameters.

© 2012 OSA

1. Introduction

Fiber mode area enlargement is one of the key means for power scaling and nonlinearity mitigation. Currently, there are three main approaches for development of near single mode (SM) large mode area (LMA) fibers, namely, low numerical aperture (NA) fibers, microstructured fibers such as large-pitch photonic crystal fibers (PCF) and leakage-channel fibers (LCF), and selective mode excitation in multi-mode (MM) fibers.

In low-NA fibers, controlled bending is often utilized to induce large losses for the high-order modes (HOM) [1

1. J. P. Koplow, D. A. V. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25(7), 442–444 (2000). [CrossRef] [PubMed]

], and the method was used to realize the first kW laser [2

2. Y. Jeong, J. Sahu, D. 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). [CrossRef] [PubMed]

] and recently a 10-kW amplifier [3

3. V. Fomin, M. Abramov, A. Ferin, A. Abramov, D. Mochalov, N. Platonov, and V. Gapontsev, “10 kW single-mode fiber laser,” presented at the Fifth International Symposium on High-Power Fiber Lasers and Their Applications, St. Petersburg, Russia, 28 Jun. - 1 Jul. 2010.

]. However, the core diameter usually remains limited to below 50 μm due to the minimum core/cladding refractive index difference (typically NA = 0.05-0.06) achievable by modified chemical vapor deposition (MCVD) technology. In addition, the guiding properties of these fibers are weak due to the low NA, making them highly sensitive to environmental perturbations, especially uncontrolled bends.

An alternative technology for mode area scaling is based on different types of microstructured fibers. Active PCF with small hole-diameter-to-pitch ratio d/Λ (< 0.1) was demonstrated in [4

4. J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “Low-nonlinearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier,” Opt. Express 12(7), 1313–1319 (2004). [CrossRef] [PubMed]

], and core diameter as large as 70 μm has been achieved [5

5. O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J. Limpert, S. Ermeneux, P. Yvernault, F. Salin, and A. Tünnermann, “Millijoule pulse energy Q-switched short-length fiber laser,” Opt. Lett. 32(11), 1551–1553 (2007). [CrossRef] [PubMed]

]. Recently, the core diameter of a large-pitch PCF was further increased to 135 μm [6

6. F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, “26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality,” Opt. Lett. 37(6), 1073–1075 (2012). [CrossRef] [PubMed]

]. Similarly to low-NA fibers, PCFs of this type possess weak guiding properties, and they are realized as unbendable rod-type fibers; furthermore, their mode content is strongly dependent on pump power [6

6. F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, “26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality,” Opt. Lett. 37(6), 1073–1075 (2012). [CrossRef] [PubMed]

]. The basic principle of these fibers, the delocalization of higher order modes out of the guiding core, is also utilized in the LCF design [7

7. L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. B 24(8), 1689–1697 (2007). [CrossRef]

]. In LCFs, the single mode LMA core is formed by channels introduced in the cladding to make the core waveguide leaky for all modes. Such fibers can potentially be several meters long and bendable to a small diameter, although high-power experimental results are yet to be demonstrated.

Finally, mode area scaling can be achieved by selective excitation of a certain mode in a multimode fiber. Using a lens system for selective excitation of the fundamental mode in a multimode fiber with a core diameter of 45 μm and NA of 0.13, single-mode propagation over 23.5 m was demonstrated in [8

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

]. The excitation of the single LP07 mode by using a long period fiber Bragg grating (LPG) in a multimode fiber with an 86-μm core was demonstrated in [9

9. S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett. 31(12), 1797–1799 (2006). [CrossRef] [PubMed]

]. Furthermore, fundamental mode propagation through a 20-cm straight fiber with 300 μm core diameter and NA = 0.39 was demonstrated in [10

10. C. D. Stacey, R. M. Jenkins, J. Banerji, and A. R. Davies, “Demonstration of fundamental mode only propagation in highly multimode fibre for high power EDFAs,” Opt. Commun. 269(2), 310–314 (2007). [CrossRef]

], and propagation through 2 m in 100-μm core fiber was demonstrated in [11

11. S. Hurand, L.-A. Chauny, H. El-Rabii, S. Joshi, and A. P. Yalin, “Mode coupling and output beam quality of 100-400 μm core silica fibers,” Appl. Opt. 50(4), 492–499 (2011). [CrossRef] [PubMed]

] with output M2 of 1.6. The results of [8

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

11

11. S. Hurand, L.-A. Chauny, H. El-Rabii, S. Joshi, and A. P. Yalin, “Mode coupling and output beam quality of 100-400 μm core silica fibers,” Appl. Opt. 50(4), 492–499 (2011). [CrossRef] [PubMed]

] allow concluding that mode coupling induced by macro-bends in fibers with a sufficiently large cladding diameter is small, and after excitation of a single mode (fundamental or another), the lone mode can propagate in a highly multimode core over long distances without significant mode conversion.

In addition to the free-space [8

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

, 10

10. C. D. Stacey, R. M. Jenkins, J. Banerji, and A. R. Davies, “Demonstration of fundamental mode only propagation in highly multimode fibre for high power EDFAs,” Opt. Commun. 269(2), 310–314 (2007). [CrossRef]

, 11

11. S. Hurand, L.-A. Chauny, H. El-Rabii, S. Joshi, and A. P. Yalin, “Mode coupling and output beam quality of 100-400 μm core silica fibers,” Appl. Opt. 50(4), 492–499 (2011). [CrossRef] [PubMed]

] and LPG [9

9. S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett. 31(12), 1797–1799 (2006). [CrossRef] [PubMed]

] approaches, adiabatic tapering can be used for fundamental mode excitation in multimode fibers by using the SM end as a launching port. In this paper, we investigate the fundamental mode evolution in long, adiabatic, high-NA tapers with high tapering ratio. Although mode area scaling by tapering a multimode optical fiber has been reported [12

12. N. Amitay, H. Presby, F. DiMarcello, and K. Nelson, “Optical fiber tapers–A novel approach to self-aligned beam expansion and single-mode hardware,” J. Lightwave Technol. 5(1), 70–76 (1987). [CrossRef]

14

14. Y. Jung, G. Brambilla, and D. Richardson, “Efficient higher-order mode filtering in multimode optical fiber based on an optical microwire,” in Asia Optical Fiber Communication and Optoelectronic Exposition and Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper SuB4.

], only short (few-cm) tapers with up to ~50 μm core diameters have been studied experimentally. Previously, we have proposed the use of long active tapered fiber as a gain medium for fiber lasers and amplifiers [15

15. J. Kerttula, V. Filippov, Y. Chamorovskii, V. Ustimchik, K. Golant, and O. G. Okhotnikov, “Theoretical and experimental comparison of different configurations of tapered fiber lasers,” in Conference on Lasers and Electro-Optics/European Quantum Electronics Conference, Technical Digest (CD) (Optical Society of America, 2011), paper CJ1_6.

]. Experimentally, we have shown that the wide side core diameter can be scaled up to 40-60 μm (NA 0.11) with a diffraction-limited beam, whereas further increasing the core diameter often leads to deterioration of the output beam quality [16

16. V. Filippov, Y. Chamorovskii, J. Kerttula, K. Golant, M. Pessa, and O. G. Okhotnikov, “Double clad tapered fiber for high power applications,” Opt. Express 16(3), 1929–1944 (2008). [CrossRef] [PubMed]

18

18. J. Kerttula, V. Filippov, Y. Chamorovskii, K. Golant, and O. G. Okhotnikov, “Actively Q-switched 1.6-mJ tapered double-clad ytterbium-doped fiber laser,” Opt. Express 18(18), 18543–18549 (2010). [CrossRef] [PubMed]

]. The main aim of this paper is a detailed experimental study of the evolution of the beam characteristics (modal composition, state of polarization, transversal mode field distribution, and beam quality) after mode area expansion in a long (several meters) tapered passive fiber with high NA, high tapering ratio, and core diameter up to 117 μm.

2. Experimental results

2.1 Mode field distribution and beam quality

Single-mode excitation

The dependence of output beam quality on the cut-back was characterized by measuring the output beam divergence and M2 (4σ-method, average of two measured orthogonal axes) at various cut lengths for both coiled and straight 7-m taper, with the results shown as a function of core diameter shown in Fig. 3
Fig. 3 The output beam divergence (a) and M2 (b) versus core diameter for straight and coiled 7-m taper.
. For the 20-m taper, the beam quality was measured for the uncut fiber with output core diameter 86 μm, with the results M2 = 1.31 for R = 100 cm and M2 = 1.42 for R = 15 cm. The output beam profiles are shown in Fig. 2(c).

Multi-mode excitation

Prior to the cut-back measurement, 532-nm and 1064-nm emission were launched into the narrow end of the 7-m taper from a frequency-doubled diode laser. At the wide end output, a dichroic filter was used to separate the two wavelengths. Figure 4(a)
Fig. 4 (a) The far field beam profiles from 117-μm output core for narrow-to-wide propagated green (left) and infrared (right) emission through the 7-m taper. (b) The near field beam profiles after propagation through the 30-cm fiber section cut from the 7-m taper with core diameter increasing from 90 μm to 117 μm. The butt-coupled SM input launching was centered for lowest order mode excitation achievable (left) or off-center core excitation (right).
shows the wide output far field distribution for both wavelengths. As evident from Fig. 4(a), green excitation of the narrow input core (few-mode for green) led to apparent multi-mode output, whereas for the 1-μm excitation the far field had a Gaussian shape. The output divergence for both wavelengths was measured to be 10 mrad.

Furthermore, 1060-nm SM diode emission was launched into a highly multi-mode 30-cm taper section cut from the wide end with core diameter increasing from 90 μm to 117 μm. The SM diode pigtail was butt-coupled to the 90-μm core and the beam profiles, divergence, and polarization state were measured at the output. The output beam distributions are shown in Fig. 4(b) for centered and arbitrary excitation of the large core. The 1/e2 divergence measured after the 30 cm propagation was found to be 50 mrad and 86 mrad for centered and off-centered excitation, respectively.

2.2. State of polarization

The evolution of the state of polarization (SOP) during narrow-to-wide propagation through the long tapered fibers was studied with the 1060-nm SM diode emission. The Stokes parameters were determined after propagation through different taper sections shown in Table 1

Table 1. Polarization measurements.

table-icon
View This Table
, and the degree of polarization (DOP), ellipticity ε, and azimuth θ of the SOP were calculated from the measured parameters. It should be noted that although ellipticity and azimuth are sensitive to perturbations such as bending of the pigtail, the DOP is not. The transmission through a polarizer for the 1060-nm SM diode pigtail output (99% DOP) and for the 7-m taper output is shown in Fig. 5
Fig. 5 (a) Transmission through a polarizer for the SM diode emission, and after narrow-to-wide propagation through coiled and uncoiled 7-m taper.
. The degradation of DOP during propagation through the entire length of either taper was found negligible, whereas propagation through the 30-cm MM taper piece with core diameter 90 μm to 117 μm excited by butt-coupled SM fiber caused significant depolarization (Table 1).

2.3. Mode content

Changes in mode content were experimentally studied for propagation through the entire 20-m taper based on the S2-method [19

19. J. W. Nicholson, A. D. Yablon, S. Ramachandran, and S. Ghalmi, “Spatially and spectrally resolved imaging of modal content in large-mode-area fibers,” Opt. Express 16(10), 7233–7243 (2008). [CrossRef] [PubMed]

]. Emission from a 1-μm broadband source was launched into the narrow end of the fiber, and a SM fiber acting as a spatial filter was butt-coupled to the wide output core. The SM fiber was not scanned over the entire output field, but only a few aperture locations were studied. The optical spectra at the source and at the output of the spatial filter were Fourier transformed to derive an intermodal group delay spectrum. The measurement was performed for various taper coiling radii and for different locations of the spatial aperture at the output core; however, coiling and aperture location had little effect on the beat spectrum. A typical group delay spectrum (R = 15 cm) is shown in Fig. 6
Fig. 6 Intermodal group delay spectrum normalized to fiber length for narrow-to-wide propagated broadband input. The insets show the optical spectra of the source before and after the 20-m taper, and an example location of the SM fiber (spatial filter) butt-coupled to the wide end taper core.
. The spectrum of the broadband source exhibited some initial modulation resulting in harmonics in the Fourier domain; however, significant new harmonics could not be observed in the spectrum after propagation through the taper for any bending or aperture location.

2.4. Impact of built-in mechanical stress

Built-in mechanical stresses in optical fibers may lead to significant changes in the refractive index profile of the core [20

20. G. W. Scherer, “Stress-induced index profile distortion in optical waveguides,” Appl. Opt. 19(12), 2000–2006 (1980). [CrossRef] [PubMed]

, 21

21. O. E. Shushpanov, A. N. Tuzov, I. V. Alexsandrov, S. P. Vikulov, M. E. Zhabotinskii, V. V. Romanovtzev, and S. J. Feld, “An automated system for measurement of mechanical stresses in optical fiber preforms with polarization-optical method,” Radiotechnika 43, 67–72 (1988) (in Russian).

]. We investigated the existence of such stresses in the studied tapers and their possible influence on the transverse field distribution of the output beam. A 37 mm long section with 1.6 mm outer diameter cut from the wide end of the 7-m taper was excited with a white light source to study the presence of mechanical stress. White light was launched into the fiber piece through a linear polarizer, and the output beam profile was measured as is, and through another (crossed) polarizer. The same measurement was performed after annealing the taper section for 4 h at 1000 °C. The observed output beam profiles are shown in Fig. 7
Fig. 7 The spatial distribution of linearly polarized white light propagated through a 37-mm taper section (117 μm core diameter) cut from the wide end of the 7-m taper (top left), and the same output after a crossed polarizer (top right). Bottom row: the same measurements after annealing of the fiber piece for 4 h at 1000 °C.
.

The cross-like structure resulting from mechanical stress is clearly visible in the spatial distribution at the taper output (Fig. 7, top left). The top right image in Fig. 7 shows the same output after transmission through a cross-state polarizer, illustrating the spatial distribution of mechanical stress in the transversal plane of the fiber. Two ring-shaped stress zones can be identified: a broad zone in the cladding, and a narrow zone at the core-cladding boundary. Thermal annealing led to almost complete elimination of the stress (Fig. 7, bottom row).

Furthermore, a quantitative measurement of the intrinsic stress in the taper was carried out. A 1.6-mm diameter fiber segment was cut from the wide part of the 7-m taper and longitudinal mechanical stress was measured directly using a polarimetric setup [21

21. O. E. Shushpanov, A. N. Tuzov, I. V. Alexsandrov, S. P. Vikulov, M. E. Zhabotinskii, V. V. Romanovtzev, and S. J. Feld, “An automated system for measurement of mechanical stresses in optical fiber preforms with polarization-optical method,” Radiotechnika 43, 67–72 (1988) (in Russian).

, 22

22. M. R. Hutsel, R. Ingle, and T. K. Gaylord, “Accurate cross-sectional stress profiling of optical fibers,” Appl. Opt. 48(26), 4985–4995 (2009). [CrossRef] [PubMed]

] with a spatial resolution of 20 μm. The result of this measurement shown in Fig. 8
Fig. 8 The axial stress component (left axis) and the corresponding lower limit estimation of the stress-induced refractive index change (right axis).
reveals two ring-like stress areas: a broad one in the cladding and a narrower one with an opposite sign at the core-cladding boundary, in agreement with the pictorial result presented in Fig. 7.

It should be noted that the stress measurement in Fig. 8 could only determine the axial (longitudinal) stress. The relevant radial refractive index change in an optical fiber in cylindrical coordinates is defined as Δnr = -B2*Sr-B1*(Sθ + Sz), where B1 and B2 are the stress-optical coefficients for fused silica, and Sr, Sθ, Sz are the radial, tangential, and axial stress components, respectively [20

20. G. W. Scherer, “Stress-induced index profile distortion in optical waveguides,” Appl. Opt. 19(12), 2000–2006 (1980). [CrossRef] [PubMed]

]. Because B1, B2 > 0, B2 << B1, and the three stress components are of the same order of magnitude, B1*Sz can be taken as a reasonable lower limit estimate for Δnr (Fig. 8, right axis).

3. Discussion

The solitary fundamental mode propagation has been confirmed by several experiments. First, the measured large-diameter output beam divergence of 10 mrad (Fig. 3a) closely corresponds to the theoretical limit 1.22λ/D, where λ is the wavelength and D is the output core diameter. Second, the propagation of SM diode emission through the studied tapers did not cause noticeable degradation of DOP (Table 1). For comparison, propagation through a 30-cm highly-MM taper section caused up to 20% depolarization and increased divergence to 86 mrad for arbitrary excitation. Third, the 7-m taper was simultaneously excited in the narrow end by 1064-nm (V = 1.5) and 532-nm (V = 4.6) emission, revealing apparent MM output for green and a Gaussian far field for infrared. The output divergence for both wavelengths was the same (10 mrad), although the theoretical limit for green is 50% less. Finally, the near constancy of the mode composition of the 1060-nm SM input propagating through the 20-m taper was confirmed by a simplified S2 method.

With increasing core size, the originally Gaussian field distribution gradually assumed a top-hat shape (~50 μm core), followed by an annular profile at larger diameters (Fig. 2(a)). The evolution of the near field distribution is accompanied by moderate increase in M2 (Fig. 3(b)). Interestingly, the progressive mode field distribution change is characteristic only to the near field, whereas far field distribution remains nearly Gaussian for any core diameter (Fig. 2(b), 2(c)). Usually, such an annular near field shape is associated with a higher order mode (LP02) excitation or a central dip in the core index profile. Since the mode content did not change, and the core index profile was uniform, this behavior was ascribed to built-in mechanical stress, which has a ring structure (Figs. 7, 8), modulates the magnitude of the refractive index by several percent, and distorts the fundamental mode field.

The internal mechanical stress associated with inhomogeneous cooling of the fiber structure during drawing (quenching stress), the mismatch of the thermal expansion coefficients of core and cladding materials [22

22. M. R. Hutsel, R. Ingle, and T. K. Gaylord, “Accurate cross-sectional stress profiling of optical fibers,” Appl. Opt. 48(26), 4985–4995 (2009). [CrossRef] [PubMed]

] or external stress due to bends [27

27. D. Donlagic and B. Culshaw, “Propagation of the fundamental mode in curved graded index multimode fiber and its application in sensor systems,” J. Lightwave Technol. 18(3), 334–342 (2000). [CrossRef]

] can lead to a distortion of the mode field and finally limit the output beam quality. Furthermore, it was recently shown that the presence of rare-earth dopants may lead to additional stress [28

28. F. Just, H.-R. Müller, S. Unger, J. Kirchhof, V. Reichel, and H. Bartelt, “Ytterbium-Doping Related Stresses in Preforms for High-Power Fiber Lasers,” J. Lightwave Technol. 27(12), 2111–2116 (2009). [CrossRef]

]. As can be seen from Fig. 2(a), the distortion of the mode shape becomes significant around 700 μm outer diameter (~50 μm core diameter), likely determined by the magnitude of built-in stress induced by the specific dynamics of fiber cooling during the drawing process. Although the preform of the studied fibers was ensured to have a uniform core index profile, stress compensation or mitigation methods were not applied. By utilizing annealing procedures, modifying the chemical composition of the core and cladding to match their thermal expansion coefficients, and varying the geometry of the pulled fibers, it should be possible to avoid the observed deformation of the fundamental mode field.

In addition to the built-in stress, mechanical stresses due to bending exist in a long fiber which inevitably needs to be coiled for practical purposes [27

27. D. Donlagic and B. Culshaw, “Propagation of the fundamental mode in curved graded index multimode fiber and its application in sensor systems,” J. Lightwave Technol. 18(3), 334–342 (2000). [CrossRef]

]. As shown by our experiments, bending of the studied tapers leads to an asymmetric deformation of the beam. Whereas the cross-section of the beam after propagation through a straight taper has a symmetric ring structure (Fig. 2(a)), propagation through a coiled taper leads to a crescent-shaped near field profile (Fig. 2(c)) and results in moderate degradation of the beam quality (Fig. 3(b)). However, coiling did not induce changes in the output divergence (Fig. 3(a)), the DOP (Table 1), or mode composition (Fig. 6), which allows concluding that although the bend-induced stress in a coiled taper distorts the fundamental mode field, its effect on mode coupling is small. As in any LMA fiber, small-radius bending of the taper will eventually limit the achievable mode area [29

29. J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express 16(17), 13240–13266 (2008). [CrossRef] [PubMed]

] even if the built-in stress can be avoided, unless bend-compensation methods are utilized. However, with the single fundamental mode guiding, additional complications associated with HOM content can be avoided.

4. Conclusion

In this paper, robust single-mode propagation without depolarization in few-meter long, step-index, high-NA adiabatic tapers with core diameter up to 117 μm (V = 38) and tapering ratio up to 18 was experimentally demonstrated. Nevertheless, the field distribution of the fundamental mode was found to exhibit annular distortion in the near field for large core size. The distortion was attributed mainly to built-in stress in the cladding and at the core-cladding boundary, likely due to inhomogeneous cooling during drawing and mismatched thermal expansion coefficients of the core and cladding materials.

The robust fundamental mode propagation practically without mode coupling illustrates the potential of long adiabatic tapers particularly for traveling-wave LMA amplifiers and high power delivery. Avoiding the built-in stress by careful design of core and cladding materials, doping, and fiber geometry is expected to allow scaling to very high core diameters with diffraction-limited output.

Acknowledgments

The authors would like to thank O. E. Shushpanov for measurements of mechanical stresses in the fiber and G. A. Ivanov and I. L. Vorob’ev for help with the preform preparation. This research was supported in part by the Finnish Funding Agency for Technology and Innovation (Tekes) project MARTEC, the Graduate School in Electronics, Telecommunications and Automation (GETA), and the Walter Ahlstrom Foundation.

References and links

1.

J. P. Koplow, D. A. V. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25(7), 442–444 (2000). [CrossRef] [PubMed]

2.

Y. Jeong, J. Sahu, D. 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). [CrossRef] [PubMed]

3.

V. Fomin, M. Abramov, A. Ferin, A. Abramov, D. Mochalov, N. Platonov, and V. Gapontsev, “10 kW single-mode fiber laser,” presented at the Fifth International Symposium on High-Power Fiber Lasers and Their Applications, St. Petersburg, Russia, 28 Jun. - 1 Jul. 2010.

4.

J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “Low-nonlinearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier,” Opt. Express 12(7), 1313–1319 (2004). [CrossRef] [PubMed]

5.

O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J. Limpert, S. Ermeneux, P. Yvernault, F. Salin, and A. Tünnermann, “Millijoule pulse energy Q-switched short-length fiber laser,” Opt. Lett. 32(11), 1551–1553 (2007). [CrossRef] [PubMed]

6.

F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, “26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality,” Opt. Lett. 37(6), 1073–1075 (2012). [CrossRef] [PubMed]

7.

L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. B 24(8), 1689–1697 (2007). [CrossRef]

8.

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

9.

S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett. 31(12), 1797–1799 (2006). [CrossRef] [PubMed]

10.

C. D. Stacey, R. M. Jenkins, J. Banerji, and A. R. Davies, “Demonstration of fundamental mode only propagation in highly multimode fibre for high power EDFAs,” Opt. Commun. 269(2), 310–314 (2007). [CrossRef]

11.

S. Hurand, L.-A. Chauny, H. El-Rabii, S. Joshi, and A. P. Yalin, “Mode coupling and output beam quality of 100-400 μm core silica fibers,” Appl. Opt. 50(4), 492–499 (2011). [CrossRef] [PubMed]

12.

N. Amitay, H. Presby, F. DiMarcello, and K. Nelson, “Optical fiber tapers–A novel approach to self-aligned beam expansion and single-mode hardware,” J. Lightwave Technol. 5(1), 70–76 (1987). [CrossRef]

13.

Y. Jung, Y. Jeong, G. Brambilla, and D. J. Richardson, “Adiabatically tapered splice for selective excitation of the fundamental mode in a multimode fiber,” Opt. Lett. 34(15), 2369–2371 (2009). [CrossRef] [PubMed]

14.

Y. Jung, G. Brambilla, and D. Richardson, “Efficient higher-order mode filtering in multimode optical fiber based on an optical microwire,” in Asia Optical Fiber Communication and Optoelectronic Exposition and Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper SuB4.

15.

J. Kerttula, V. Filippov, Y. Chamorovskii, V. Ustimchik, K. Golant, and O. G. Okhotnikov, “Theoretical and experimental comparison of different configurations of tapered fiber lasers,” in Conference on Lasers and Electro-Optics/European Quantum Electronics Conference, Technical Digest (CD) (Optical Society of America, 2011), paper CJ1_6.

16.

V. Filippov, Y. Chamorovskii, J. Kerttula, K. Golant, M. Pessa, and O. G. Okhotnikov, “Double clad tapered fiber for high power applications,” Opt. Express 16(3), 1929–1944 (2008). [CrossRef] [PubMed]

17.

V. Filippov, Y. Chamorovskii, J. Kerttula, A. Kholodkov, and O. G. Okhotnikov, “600 W power scalable single transverse mode tapered double-clad fiber laser,” Opt. Express 17(3), 1203–1214 (2009). [CrossRef] [PubMed]

18.

J. Kerttula, V. Filippov, Y. Chamorovskii, K. Golant, and O. G. Okhotnikov, “Actively Q-switched 1.6-mJ tapered double-clad ytterbium-doped fiber laser,” Opt. Express 18(18), 18543–18549 (2010). [CrossRef] [PubMed]

19.

J. W. Nicholson, A. D. Yablon, S. Ramachandran, and S. Ghalmi, “Spatially and spectrally resolved imaging of modal content in large-mode-area fibers,” Opt. Express 16(10), 7233–7243 (2008). [CrossRef] [PubMed]

20.

G. W. Scherer, “Stress-induced index profile distortion in optical waveguides,” Appl. Opt. 19(12), 2000–2006 (1980). [CrossRef] [PubMed]

21.

O. E. Shushpanov, A. N. Tuzov, I. V. Alexsandrov, S. P. Vikulov, M. E. Zhabotinskii, V. V. Romanovtzev, and S. J. Feld, “An automated system for measurement of mechanical stresses in optical fiber preforms with polarization-optical method,” Radiotechnika 43, 67–72 (1988) (in Russian).

22.

M. R. Hutsel, R. Ingle, and T. K. Gaylord, “Accurate cross-sectional stress profiling of optical fibers,” Appl. Opt. 48(26), 4985–4995 (2009). [CrossRef] [PubMed]

23.

A. W. Snyder, “Coupling of modes on a tapered dielectric cylinder,” IEEE Trans. Microw. Theory Tech. 18(7), 383–392 (1970). [CrossRef]

24.

D. Marcuse, “Mode conversion in optical fibers with monotonically increasing core radius,” J. Lightwave Technol. 5(1), 125–133 (1987). [CrossRef]

25.

P. M. Shankar, L. C. Bobb, and H. D. Krumboltz, “Coupling of modes in bent biconically tapered single-mode fibers,” J. Lightwave Technol. 9(7), 832–837 (1991). [CrossRef]

26.

L. C. Bobb, P. M. Shankar, and H. D. Krumboltz, “Bending effects in biconically tapered single-mode fibers,” J. Lightwave Technol. 8(7), 1084–1090 (1990). [CrossRef]

27.

D. Donlagic and B. Culshaw, “Propagation of the fundamental mode in curved graded index multimode fiber and its application in sensor systems,” J. Lightwave Technol. 18(3), 334–342 (2000). [CrossRef]

28.

F. Just, H.-R. Müller, S. Unger, J. Kirchhof, V. Reichel, and H. Bartelt, “Ytterbium-Doping Related Stresses in Preforms for High-Power Fiber Lasers,” J. Lightwave Technol. 27(12), 2111–2116 (2009). [CrossRef]

29.

J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express 16(17), 13240–13266 (2008). [CrossRef] [PubMed]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.2400) Fiber optics and optical communications : Fiber properties

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 2, 2012
Revised Manuscript: October 9, 2012
Manuscript Accepted: October 18, 2012
Published: October 25, 2012

Citation
Juho Kerttula, Valery Filippov, Vasily Ustimchik, Yuri Chamorovskiy, and Oleg G. Okhotnikov, "Mode evolution in long tapered fibers with high tapering ratio," Opt. Express 20, 25461-25470 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-23-25461


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References

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  11. S. Hurand, L.-A. Chauny, H. El-Rabii, S. Joshi, and A. P. Yalin, “Mode coupling and output beam quality of 100-400 μm core silica fibers,” Appl. Opt.50(4), 492–499 (2011). [CrossRef] [PubMed]
  12. N. Amitay, H. Presby, F. DiMarcello, and K. Nelson, “Optical fiber tapers–A novel approach to self-aligned beam expansion and single-mode hardware,” J. Lightwave Technol.5(1), 70–76 (1987). [CrossRef]
  13. Y. Jung, Y. Jeong, G. Brambilla, and D. J. Richardson, “Adiabatically tapered splice for selective excitation of the fundamental mode in a multimode fiber,” Opt. Lett.34(15), 2369–2371 (2009). [CrossRef] [PubMed]
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  15. J. Kerttula, V. Filippov, Y. Chamorovskii, V. Ustimchik, K. Golant, and O. G. Okhotnikov, “Theoretical and experimental comparison of different configurations of tapered fiber lasers,” in Conference on Lasers and Electro-Optics/European Quantum Electronics Conference, Technical Digest (CD) (Optical Society of America, 2011), paper CJ1_6.
  16. V. Filippov, Y. Chamorovskii, J. Kerttula, K. Golant, M. Pessa, and O. G. Okhotnikov, “Double clad tapered fiber for high power applications,” Opt. Express16(3), 1929–1944 (2008). [CrossRef] [PubMed]
  17. V. Filippov, Y. Chamorovskii, J. Kerttula, A. Kholodkov, and O. G. Okhotnikov, “600 W power scalable single transverse mode tapered double-clad fiber laser,” Opt. Express17(3), 1203–1214 (2009). [CrossRef] [PubMed]
  18. J. Kerttula, V. Filippov, Y. Chamorovskii, K. Golant, and O. G. Okhotnikov, “Actively Q-switched 1.6-mJ tapered double-clad ytterbium-doped fiber laser,” Opt. Express18(18), 18543–18549 (2010). [CrossRef] [PubMed]
  19. J. W. Nicholson, A. D. Yablon, S. Ramachandran, and S. Ghalmi, “Spatially and spectrally resolved imaging of modal content in large-mode-area fibers,” Opt. Express16(10), 7233–7243 (2008). [CrossRef] [PubMed]
  20. G. W. Scherer, “Stress-induced index profile distortion in optical waveguides,” Appl. Opt.19(12), 2000–2006 (1980). [CrossRef] [PubMed]
  21. O. E. Shushpanov, A. N. Tuzov, I. V. Alexsandrov, S. P. Vikulov, M. E. Zhabotinskii, V. V. Romanovtzev, and S. J. Feld, “An automated system for measurement of mechanical stresses in optical fiber preforms with polarization-optical method,” Radiotechnika43, 67–72 (1988) (in Russian).
  22. M. R. Hutsel, R. Ingle, and T. K. Gaylord, “Accurate cross-sectional stress profiling of optical fibers,” Appl. Opt.48(26), 4985–4995 (2009). [CrossRef] [PubMed]
  23. A. W. Snyder, “Coupling of modes on a tapered dielectric cylinder,” IEEE Trans. Microw. Theory Tech.18(7), 383–392 (1970). [CrossRef]
  24. D. Marcuse, “Mode conversion in optical fibers with monotonically increasing core radius,” J. Lightwave Technol.5(1), 125–133 (1987). [CrossRef]
  25. P. M. Shankar, L. C. Bobb, and H. D. Krumboltz, “Coupling of modes in bent biconically tapered single-mode fibers,” J. Lightwave Technol.9(7), 832–837 (1991). [CrossRef]
  26. L. C. Bobb, P. M. Shankar, and H. D. Krumboltz, “Bending effects in biconically tapered single-mode fibers,” J. Lightwave Technol.8(7), 1084–1090 (1990). [CrossRef]
  27. D. Donlagic and B. Culshaw, “Propagation of the fundamental mode in curved graded index multimode fiber and its application in sensor systems,” J. Lightwave Technol.18(3), 334–342 (2000). [CrossRef]
  28. F. Just, H.-R. Müller, S. Unger, J. Kirchhof, V. Reichel, and H. Bartelt, “Ytterbium-Doping Related Stresses in Preforms for High-Power Fiber Lasers,” J. Lightwave Technol.27(12), 2111–2116 (2009). [CrossRef]
  29. J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express16(17), 13240–13266 (2008). [CrossRef] [PubMed]

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