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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 5 — Mar. 1, 2010
  • pp: 4411–4416
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Optically induced long-period fiber gratings for guided mode conversion in few-mode fibers

N. Andermahr and C. Fallnich  »View Author Affiliations


Optics Express, Vol. 18, Issue 5, pp. 4411-4416 (2010)
http://dx.doi.org/10.1364/OE.18.004411


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Abstract

We propose and demonstrate guided mode conversion by an optically induced long-period fiber grating (OLPG). The beating of two guided modes excited by a nanosecond pulsed laser beam is used to induce a refractive index grating via the Kerr effect. This grating converts a counter-propagating continuous-wave beam from the LP01 to the LP11 mode. Numerical simulations using a beam propagation method show that a full conversion is possible. Experimentally, a mode conversion with an efficiency of about 50% is firstly demonstrated.

© 2010 Optical Society of America

1. Introduction

Long-period fiber gratings (LPGs) arise from periodic variations of the refractive index [1

1. K. Hill, B. Malo, K. Vineberg, F. Bilodeau, D. Johnson, and I. Skinner, “Efficient mode conversion in telecommunication fibre using externally written gratings,” Electron. Lett. 26(16), 1270–1272 (1990). [CrossRef]

]. If the grating period equals the beating length of two transverse modes, a conversion can be obtained, e.g. the fundamental mode can be converted to a higher-order either guided or leaky mode. The grating periods of LPGs typically are in the range between few hundred μm and a few mm. Various applications for LPGs have been shown, ranging from absorption filters, temperature, strain and refractive index sensors [2

2. V. Bhatia and A. M. Vengsarkar, “Optical fiber long-period grating sensors,” Opt. Lett. 21(9), 692–694 (1996). [CrossRef] [PubMed]

] to optical switches [3

3. H. C. Nguyen, D.-I. Yeom, E. C. Mägi, L. B. Fu, B. T. Kuhlmey, C. M. de Sterke, and B. J. Eggleton, “Nonlinear long-period gratings in As2Se3 chalcogenide fiber for all-optical switching,” Appl. Phys. Lett. 92(10), 101127 (2008). [CrossRef]

]. Mode conversion in few-mode fibers offers a high potential for many applications [4

4. S. Ramachandran, “Dispersion-Tailored Few-Mode Fibers: A Versatile Platform for In-Fiber Photonic Devices,” J. Lightwave Technol. 23(11), 3426 (2005). [CrossRef]

] like signal power management or dispersion compensation [5

5. M. Schultz, O. Prochnow, A. Ruehl, D. Wandt, D. Kracht, S. Ramachandran, and S. Ghalmi, “Sub-60-fs ytterbium-doped fiber laser with a fiber-based dispersion compensation,” Opt. Lett. 32(16), 2372–2374 (2007). [CrossRef] [PubMed]

]. Different generation techniques have been demonstrated: written in photosensitive fibers by UV pulses [6

6. B. Malo, K. A. Vineberg, F. Bilodeau, J. Albert, D. C. Johnson, and K. O. Hill, “Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change,” Opt. Lett. 15(17), 953–955 (1990). [CrossRef] [PubMed]

], written by femtosecond pulses [7

7. Y. Kondo, K. Nouchi, T. Mitsuyu, M. Watanabe, P. G. Kazansky, and K. Hirao, “Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses,” Opt. Lett. 24(10), 646–648 (1999). [CrossRef]

], or induced by stress [8

8. S. Savin, M. F. Digonnet, G. S. Kino, and H. J. Shaw, “Tunable mechanically induced long-period fiber gratings,” Opt. Lett. 25(10), 710–712 (2000). [CrossRef]

], acoustic waves [9

9. K. J. Lee, I.-K. Hwang, H. C. Park, and B. Y. Kim, “Axial strain dependence of all-fiber acousto-optic tunable filters,” Opt. Express 17(4), 2348–2357 (2009). [CrossRef] [PubMed]

] or electro-optical effects [10

10. M. Kulishov, P. Cheben, X. Daxhelet, and S. Delprat, “Electro-optically induced tilted phase gratings in waveguides,” J. Opt. Soc. Am. B 18(4), 457–464 (2001). [CrossRef]

].

In this letter, we propose and demonstrate a LPG induced by the optical Kerr effect. A high power beam, which will be called writing beam, temporally induces a grating into the fiber making use of the beating between the LP01 and the LP11 mode. A counter-propagating low power beam, denoted as probe beam, is diffracted from this grating, i.e. it is converted to another mode. Compared to other techniques this optically induced LPG (OLPG) is only induced for transient periods, as a pulsed laser beam is used to achieve the necessary peak power of the writing beam. The necessity of a high power writing beam is limiting for some applications, but may be reduced by a high-nonlinearity fiber as for example used in [3

3. H. C. Nguyen, D.-I. Yeom, E. C. Mägi, L. B. Fu, B. T. Kuhlmey, C. M. de Sterke, and B. J. Eggleton, “Nonlinear long-period gratings in As2Se3 chalcogenide fiber for all-optical switching,” Appl. Phys. Lett. 92(10), 101127 (2008). [CrossRef]

]. As the proposed OLPG is not side-written but induced by a guided beam, it potentially allows an easy generation of very long LPGs, which offer a very small bandwidth for LPG-based absorption filters. A tunability of LPGs has for example been shown with mechanical, acoustical and tilted gratings [8

8. S. Savin, M. F. Digonnet, G. S. Kino, and H. J. Shaw, “Tunable mechanically induced long-period fiber gratings,” Opt. Lett. 25(10), 710–712 (2000). [CrossRef]

, 9

9. K. J. Lee, I.-K. Hwang, H. C. Park, and B. Y. Kim, “Axial strain dependence of all-fiber acousto-optic tunable filters,” Opt. Express 17(4), 2348–2357 (2009). [CrossRef] [PubMed]

, 10

10. M. Kulishov, P. Cheben, X. Daxhelet, and S. Delprat, “Electro-optically induced tilted phase gratings in waveguides,” J. Opt. Soc. Am. B 18(4), 457–464 (2001). [CrossRef]

]. The proposed OLPG is optically tunable via the properties of the writing beam.

2. Basic setup

Figure 1 illustrates the experimental setup. A pulsed laser beam, which serves as the writing beam, is coupled to a few-mode fiber in the way that two guided modes (LP01 and LP11) are excited. Due to different propagation constants the relative phase between the modes changes during the propagation, what causes a grating-like intensity distribution with the period equal to the beating length. A counter-propagating continuous-wave (cw) probe beam is carefully aligned to excite only the fundamental mode (LP01). After the transmission through the few-mode fiber the probe beam is coupled to a single-mode fiber (SMF) serving as a spatial filter, so that for the case of a mode conversion a drop of the power, which is detected behind the SMF, is expected. We used the same wavelength (λ = 1.064μm) for the writing and the probe beam, which is not a mandatory choice, but ensures that the grating period naturally matches the beating between the LP01 and the LP11 of the probe beam.

Fig. 1. Experimental setup: The pulsed laser induces a grating that diffracts the counter-propagating continuous-wave (cw) laser beam. NPBS/PBS: non-/polarizing beam splitter, SMF: single-mode fiber, PD: photodiode.

The combination of a half-wave plate and a polarizing beam splitter is used to control the power of the writing beam. As the fiber itself as well as the induced index change are in general birefringent a half-wave plate and a quarter-wave plate are used to optimize the mode coupling. The light is coupled into the SMF with a non-polarizing beam splitter in order to prevent the unintended detection of polarization changes.

3. Numerical simulation

Fig. 2. Numerical simulation of the OLPG: (a) Refractive index change induced in the fiber by the mode beating of a 200kW quasi-cw beam, (b) intensity of the probe beam that is propagated through the fiber with modified refractive index, and (c) normalized modal power of cw beam, the fundamental mode (LP01) is converted to the LP11 mode.

4. Experimental details and results

In the experiments we used a Q-switched Nd:YAG laser in order to induce the LPG. The repetition rate was frep = 10Hz, the maximum used pulse energy Ep = 70μJ and the pulse duration T = 14ns, which corresponds to a pulse length of 2.9m in the fiber. Compared to the simulations such a long pulse is necessary as the maximum peak power of the Gaussian-shaped pulse was only about 5kW. The beam profile of the available laser was not diffraction-limited (M2 ≈ 2), so that the coupling efficiency into the fiber was only about 35%. Due to this beam profile always both modes, the fundamental as well as the higher-order mode were excited. The few-mode fiber had a step index profile, a core diameter of d = 25μm, a numerical aperture of NA = 0.06 and a length of L = 8m. The fiber had a V-number V=πλdNA of 4.4 and therefore nominally supports the propagation of LP01, LP11, LP21 and LP02. The fiber was coiled to a diameter of 15cm, what accomplishes suppression of LP21 and LP02. In former experiments [13

13. N. Andermahr and C. Fallnich, “Interaction of transverse modes in a single-frequency few-mode fiber amplifier caused by local gain saturation,” Opt. Express 16(12), 8678–8684 (2008). [CrossRef] [PubMed]

] we measured only a very small excitation of LP21 and LP02 in the same type of fiber. The fiber had a high index acrylate coating, so that light in the cladding was not guided. In order to reduce the reflections of the pulse into the detection branch the fiber ends were angle-polished and the optical components were slightly inclined. The beam of a cw non-planar ring oscillator with diffraction-limited beam quality and a power of P = 40mW served as the probe beam. The probe beam alignment was carefully adjusted, while the writing beam was blocked, to excite only the fundamental mode. The coupling efficiency was about 70%. In order to measure only the power in the fundamental mode a part of the transmitted probe beam was reflected at a non-polarizing beam splitter, coupled into the SMF (actually a strongly coiled part of the few-mode fiber) and detected with a photodiode. In this way the SMF serves as a spatial mode filter, whose efficiency can be described by the suppression of the LP11, which depends on the precision of the alignment. With a perfect alignment LP11 is completely suppressed. Assuming a maximum misalignment of 3μm the transmission of the LP11 has been calculated for the used fiber to be still smaller than 10%.

Turning on the cw and the pulsed beam at pulse energies greater than 3μJ strong back-reflections were observed for some pulses due to seeded stimulated Brillouin scattering (SBS) [14

14. M. Dämmig, G. Zinner, F. Mitschke, and H. Welling, “Stimulated Brillouin scattering in fibers with and without external feedback,” Phys. Rev. A 48(4), 3301–3309 (1993). [CrossRef] [PubMed]

]. SBS occurs if a forward propagating beam is scattered back by an acoustic wave, which itself is formed by the interference of the forward and backward propagating wave. The threshold of this nonlinear process can be lowered by an additional backward propagating wave with similar frequency. However, as the writing and the probe laser oscillated independently from each other, the seeding of SBS occurred randomly, so that we chose pulses without appearance of SBS for the measurement. An explicit avoidance of SBS was possible by detuning the frequencies of the writing and the probe laser.

Fig. 3. (a-h) Power of the probe beam behind the SMF for different pulse energies, (i) dip minimum over pulse energy with a third-order fit, and the calculated decrease of the fundamental mode in dependence on the pulse energy.

In order to confirm that the decrease of detected probe power is caused by a guided mode conversion the spatial dependence of the converted and unconverted probe beam was measured. As we had no camera available, which was fast enough to resolve the conversion, the SMF was scanned over the beam. Therefore, it was considered that a superposition of the LP01 and the LP11 modes causes a shift of the beam center depending on the relative phase. Fig. 4 shows the calculated intensities of (a) the fundamental mode, (b) an example of the superposition of LP01 and LP11 with equal powers, and (c) the calculated transmission through a SMF scanned over these two beam profiles. Experimentally, the incoupling end of the SMF was laterally shifted and the transmitted power of the converted and unconverted beam was measured. The results (each data point is a result of averaging over six pulses) are shown in Fig. 4(d) and (e). A third-order polynomial was fitted only to guide the eye. While the scan across the unconverted beam resulted in a symmetric decrease of the transmission, the scan over the converted beam is shifted to the left (Fig. 4(d)). After horizontally displacing the writing beam a shift to the right was observed (Fig. 4(e)). Depending on the incoupling of the writing beam it was also possible to shift the probe beam center up or down. The shift of the beam center is an evidence for the occurrence of the first higher-order mode [15

15. P. Kwee, F. Seifert, B. Willke, and K. Danzmann, “Laser beam quality and pointing measurement with an optical resonator,” Rev. Sci. Instrum. 78(7) (2007). [CrossRef] [PubMed]

]. From these scanning experiments we estimate that about 50% of the fundamental mode power have been converted to the LP11 mode. The transmission measurement suggests a slightly higher efficiency of about 60%, but this value includes possible conversions to further higher-order modes.

Fig. 4. Spatial dependence of the converted beam: Calculated intensity profiles of (a) the fundamental mode and (b) a superposition of LP01 and LP11 with equal powers; (c) calculated transmission scanning a SMF across (i) the fundamental mode and across (ii) the mode superposition; (d) measured transmission of the converted and unconverted probe beam across the position over the SMF (lines are third-order polynomial fits to guide the eye); (e) analogous to (d) after a horizontally shifted incoupling of the writing beam.

5. Conclusion and outlook

Acknowledgments

The reported investigations were partially supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 407.

References and links

1.

K. Hill, B. Malo, K. Vineberg, F. Bilodeau, D. Johnson, and I. Skinner, “Efficient mode conversion in telecommunication fibre using externally written gratings,” Electron. Lett. 26(16), 1270–1272 (1990). [CrossRef]

2.

V. Bhatia and A. M. Vengsarkar, “Optical fiber long-period grating sensors,” Opt. Lett. 21(9), 692–694 (1996). [CrossRef] [PubMed]

3.

H. C. Nguyen, D.-I. Yeom, E. C. Mägi, L. B. Fu, B. T. Kuhlmey, C. M. de Sterke, and B. J. Eggleton, “Nonlinear long-period gratings in As2Se3 chalcogenide fiber for all-optical switching,” Appl. Phys. Lett. 92(10), 101127 (2008). [CrossRef]

4.

S. Ramachandran, “Dispersion-Tailored Few-Mode Fibers: A Versatile Platform for In-Fiber Photonic Devices,” J. Lightwave Technol. 23(11), 3426 (2005). [CrossRef]

5.

M. Schultz, O. Prochnow, A. Ruehl, D. Wandt, D. Kracht, S. Ramachandran, and S. Ghalmi, “Sub-60-fs ytterbium-doped fiber laser with a fiber-based dispersion compensation,” Opt. Lett. 32(16), 2372–2374 (2007). [CrossRef] [PubMed]

6.

B. Malo, K. A. Vineberg, F. Bilodeau, J. Albert, D. C. Johnson, and K. O. Hill, “Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change,” Opt. Lett. 15(17), 953–955 (1990). [CrossRef] [PubMed]

7.

Y. Kondo, K. Nouchi, T. Mitsuyu, M. Watanabe, P. G. Kazansky, and K. Hirao, “Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses,” Opt. Lett. 24(10), 646–648 (1999). [CrossRef]

8.

S. Savin, M. F. Digonnet, G. S. Kino, and H. J. Shaw, “Tunable mechanically induced long-period fiber gratings,” Opt. Lett. 25(10), 710–712 (2000). [CrossRef]

9.

K. J. Lee, I.-K. Hwang, H. C. Park, and B. Y. Kim, “Axial strain dependence of all-fiber acousto-optic tunable filters,” Opt. Express 17(4), 2348–2357 (2009). [CrossRef] [PubMed]

10.

M. Kulishov, P. Cheben, X. Daxhelet, and S. Delprat, “Electro-optically induced tilted phase gratings in waveguides,” J. Opt. Soc. Am. B 18(4), 457–464 (2001). [CrossRef]

11.

D. Yevick, “A guide to electric field propagation techniques for guided-wave optics,” Opt. Quantum Electron. 16(3), 185–197 (1994). [CrossRef]

12.

N. Andermahr and C. Fallnich, “Modeling of transverse mode interaction in large-mode-area fiber amplifiers,” Opt. Express 16(24), 20,038–20,046 (2008). [CrossRef]

13.

N. Andermahr and C. Fallnich, “Interaction of transverse modes in a single-frequency few-mode fiber amplifier caused by local gain saturation,” Opt. Express 16(12), 8678–8684 (2008). [CrossRef] [PubMed]

14.

M. Dämmig, G. Zinner, F. Mitschke, and H. Welling, “Stimulated Brillouin scattering in fibers with and without external feedback,” Phys. Rev. A 48(4), 3301–3309 (1993). [CrossRef] [PubMed]

15.

P. Kwee, F. Seifert, B. Willke, and K. Danzmann, “Laser beam quality and pointing measurement with an optical resonator,” Rev. Sci. Instrum. 78(7) (2007). [CrossRef] [PubMed]

OCIS Codes
(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers
(190.2055) Nonlinear optics : Dynamic gratings

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: December 3, 2009
Revised Manuscript: December 23, 2009
Manuscript Accepted: January 11, 2010
Published: February 18, 2010

Citation
N. Andermahr and C. Fallnich, "Optically induced long-period fiber gratings for guided mode conversion in few-mode fibers," Opt. Express 18, 4411-4416 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-5-4411


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References

  1. K. Hill, B. Malo, K. Vineberg, F. Bilodeau, D. Johnson, and I. Skinner, "Efficient mode conversion in telecommunication fibre using externally written gratings," Electron. Lett. 26(16), 1270-1272 (1990). [CrossRef]
  2. V. Bhatia and A. M. Vengsarkar, "Optical fiber long-period grating sensors," Opt. Lett. 21(9), 692-694 (1996). [CrossRef] [PubMed]
  3. H. C. Nguyen, D.-I. Yeom, E. C. Mägi, L. B. Fu, B. T. Kuhlmey, C. M. de Sterke, and B. J. Eggleton, "Nonlinear long-period gratings in As2Se3 chalcogenide fiber for all-optical switching," Appl. Phys. Lett. 92(10), 101127 (2008). [CrossRef]
  4. S. Ramachandran, "Dispersion-Tailored Few-Mode Fibers: A Versatile Platform for In-Fiber Photonic Devices," J. Lightwave Technol. 23(11), 3426 (2005). [CrossRef]
  5. M. Schultz, O. Prochnow, A. Ruehl, D. Wandt, D. Kracht, S. Ramachandran, and S. Ghalmi, "Sub-60-fs ytterbium-doped fiber laser with a fiber-based dispersion compensation," Opt. Lett. 32(16), 2372-2374 (2007). [CrossRef] [PubMed]
  6. B. Malo, K. A. Vineberg, F. Bilodeau, J. Albert, D. C. Johnson, and K. O. Hill, "Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change," Opt. Lett. 15(17), 953-955 (1990). [CrossRef] [PubMed]
  7. Y. Kondo, K. Nouchi, T. Mitsuyu, M. Watanabe, P. G. Kazansky, and K. Hirao, "Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses," Opt. Lett. 24(10), 646-648 (1999). [CrossRef]
  8. S. Savin, M. F. Digonnet, G. S. Kino, and H. J. Shaw, "Tunable mechanically induced long-period fiber gratings," Opt. Lett. 25(10), 710-712 (2000). [CrossRef]
  9. K. J. Lee, I.-K. Hwang, H. C. Park, and B. Y. Kim, "Axial strain dependence of all-fiber acousto-optic tunable filters," Opt. Express 17(4), 2348-2357 (2009). [CrossRef] [PubMed]
  10. M. Kulishov, P. Cheben, X. Daxhelet, and S. Delprat, "Electro-optically induced tilted phase gratings in waveguides," J. Opt. Soc. Am. B 18(4), 457-464 (2001). [CrossRef]
  11. D. Yevick, "A guide to electric field propagation techniques for guided-wave optics," Opt. Quantum Electron. 16(3), 185-197 (1994). [CrossRef]
  12. N. Andermahr and C. Fallnich, "Modeling of transverse mode interaction in large-mode-area fiber amplifiers," Opt. Express 16(24), 20,038-20,046 (2008). [CrossRef]
  13. N. Andermahr and C. Fallnich, "Interaction of transverse modes in a single-frequency few-mode fiber amplifier caused by local gain saturation," Opt. Express 16(12), 8678-8684 (2008). [CrossRef] [PubMed]
  14. M. Dämmig, G. Zinner, F. Mitschke, and H. Welling, "Stimulated Brillouin scattering in fibers with and without external feedback," Phys. Rev. A 48(4), 3301-3309 (1993). [CrossRef] [PubMed]
  15. P. Kwee, F. Seifert, B. Willke, and K. Danzmann, "Laser beam quality and pointing measurement with an optical resonator," Rev. Sci. Instrum. 78(7), 073103 (2007). [CrossRef] [PubMed]

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