uv-induced bragg grating inscription into single-polarization all-solid hybrid microstructured...

UV-induced Bragg grating inscriptioninto single-polarization all-solid hybrid

microstructured optical fiber

Ryuichiro Goto,1,∗ Ihsan Fsaifes,2 Assaad Baz,2 Laurent Bigot,2

Katsuhiro Takenaga,3 Shoichiro Matsuo,3 and Stuart D. Jackson1

1Institute of Photonics and Optical Science, School of Physics, University of Sydney,New South Wales 2060, Australia

2IRCICA-PhLAM, CNRS Universite Lille1, Parc Scientifique de la Haute Borne,59658 Villeneuve d’Ascq, France

3Optics and Electronics Laboratory, Fujikura Ltd., Sakura, Chiba 285-8550, Japan

*[email protected]

Abstract: We demonstrate a single-polarization all-solid hybrid mi-crostructured optical fiber with a UV-induced Bragg grating. A strong(∼20 dB) UV-induced Bragg grating was inscribed within the 30 nm-widesingle-polarization window of the fiber, producing polarized Bragg reflec-tion. The sharp band-edge cutoff allows a large polarization-extinctionratio of the Bragg reflection. The hybrid structure of the fiber enabledminimal UV exposure to the high-index regions and the location of thesingle-polarization window was maintained after the grating was inscribed.

© 2011 Optical Society of America

OCIS codes: (060.3735) Fiber Bragg gratings; (060.4005) Microstructured fibers.

References and links1. J. Simpson, R. Stolen, F. Sears, W. Pleibel, J. MacChesney, and R. Howard, “A single-polarization fiber,” J.

Lightwave Technol. 1, 370–374 (1983).2. R. Goto, S. D. Jackson, and K. Takenaga, “Single-polarization operation in birefringent all-solid hybrid mi-

crostructured fiber with additional stress applying parts,” Opt. Lett. 34, 3119–3121 (2009).3. A. Cerqueira. S. Jr., D. G. Lona, I. de Oliveira, H. E. Hernandez-Figueroa, and H. L. Fragnito, “Broadband

single-polarization guidance in hybrid photonic crystal fibers,” Opt. Lett. 36, 133–135 (2011).4. R. Goto, R. J. Williams, N. Jovanovic, G. D. Marshall, M. J. Withford, and S. D. Jackson, “Linearly polarized

fiber laser using a point-by-point Bragg grating in a single-polarization photonic bandgap fiber,” Opt. Lett. 36,1872–1874 (2011).

5. M. L. Aslund, N. Jovanovic, N. Groothoff, J. Canning, G. D. Marshall, S. D. Jackson, A. Fuerbach, and M. J.Withford, “Optical loss mechanisms in femtosecond laser-written point-by-point fibre Bragg gratings,” Opt.Express 16, 14248–14254 (2008).

6. L. Jin, Z. Wang, Q. Fang, Y. Liu, B. Liu, G. Kai, and X. Dong, “Spectral characteristics and bend response ofBragg gratings inscribed in all-solid bandgap fibers,” Opt. Express 15, 15555–15565 (2007).

7. L. Bigot, G. Bouwmans, Y. Quiquempois, A. L. Rouge, V. Pureur, O. Vanvincq, and M. Douay, “Efficient fiberBragg gratings in 2D all-solid photonic bandgap fiber,” Opt. Express 17, 10105–10112 (2009).

8. S. Johnson and J. Joannopoulos, “Block-iterative frequency-domain methods for Maxwell’s equations in aplanewave basis,” Opt. Express 8, 173–190 (2001).

9. N. M. Litchinitser, A. K. Abeeluck, C. Headley, and B. J. Eggleton, “Antiresonant reflecting photonic crystaloptical waveguides,” Opt. Lett. 27, 1592–1594 (2002).

10. T. T. Alkeskjold, “Large-mode-area ytterbium-doped fiber amplifier with distributed narrow spectral filtering andreduced bend sensitivity,” Opt. Express 17, 16394–16405 (2009).

#146737 - $15.00 USD Received 4 May 2011; revised 14 Jun 2011; accepted 15 Jun 2011; published 28 Jun 2011(C) 2011 OSA 4 July 2011 / Vol. 19, No. 14 / OPTICS EXPRESS 13525

1. Introduction

Single-polarization (SP) fibers and fiber Bragg gratings (FBGs) are two important fiber-baseddevices for the production of high-power linearly polarized output from an all-fiber laser cav-ity. SP fibers select a polarization and FBGs provide wavelength-selective reflection in the lasercavity. Fiber-based devices are often superior to bulk-optic devices for high-power fiber lasersbecause they offer low-loss and robust coupling between devices by fusion splicing. In addi-tion, integrating several functions into one fiber is a major advantage of fiber-based devices,e.g., a FBG inscribed into a SP fiber enables a polarized Bragg reflector. One common way toproduce SP fibers is to introduce a cutoff in a birefringent fiber [1]. The cutoff wavelength ispolarization dependent in a birefringent fiber, and the fiber operates as a SP fiber between thetwo cutoff wavelengths (SP window). Large birefringence increases the width of the SP win-dow, and hence it is an essential characteristic. Hybrid microstructured optical fibers (MOFs) –which guide core modes by both the photonic bandgap (PBG) effect and total internal reflec-tion (TIR) – provide large birefringence for enabling SP fibers [2, 3]. We have demonstrated alinearly polarized fiber laser using a point-by-point FBG inscribed into a SP hybrid MOF [4].The insertion loss induced by scattering [5] may however ultimately limit the power scalabil-ity of the laser. UV-induced FBGs provide a low insertion loss, however, a major issue withinscribing a UV-induced FBG into a SP hybrid MOF is that the high-index regions are typi-cally highly germanium doped, thus highly UV photosensitive. The high-index regions sufferan increased refractive index when they are exposed to UV light; and the transmission win-dows shift to long wavelengths [6]. The high-index regions made by phosphorous doping arenot UV photosensitive (at wavelengths commonly used for inscribing UV-induced FBGs), thuseliminate the long-wavelength shift [7]; however, difficulties associated with the fabrication ofa phosphorous doped preform resulted in a relatively large propagation loss (0.84 dB/m).

In this paper, we demonstrate an alternative method to inscribe a UV-induced FBG into aSP hybrid MOF. UV exposure to the high-index regions was minimized by launching UV lightfrom the low-index cladding, and the location of the SP window was maintained after the FBGinscription. A strong (∼20 dB) Bragg grating was inscribed within the SP window, enablingpolarized Bragg reflection with a large polarization-extinction ratio (PER).

2. Fiber structure and fabrication

The cross section of the SP hybrid MOF used in our experiment is shown in Fig. 1. The fiberdiameter was 203 µm. The fiber was fabricated by the stack-and-draw method; glass rods werestacked hexagonally and seven central rods were replaced by one larger rod to form the core.The inner section (50% of the radius) of the core rod was doped with germanium and fluorine.Germanium doping makes the core UV photosensitive and the raised refractive index by thegermanium doping was compensated by fluorine co-doping. Three types of microstructures– germanium-doped high-index regions, fluorine-doped low-index regions, and boron-dopedstress-applying parts – were embedded in pure silica background and comprised the cladding.The high-index regions guide core modes by the PBG effect, and the low-index regions andstress-applying parts guide core modes by TIR. The high-index regions and stress-applyingparts possess a larger thermal expansion coefficient compared to the low-index regions, thusinduce large asymmetric thermal stress to the core after fiber drawing. The relative refractiveindex and rod-to-pitch ratio of each microstructure, which were measured at the glass rod stagebefore the stacking process, are summarized in Table 1. Figure 2(a) displays the calculatedphotonic band structure in the cladding (red), and the fundamental and higher-order modes(blue and green) inside the first and second PBGs (yellow and orange). We used the plane-wave expansion method [8] and did not include stress birefringence. The photonic bands inthe high-index regions define multiple PBGs between the photonic bands, and the core modes


experience cutoffs at the edges of each transmission window (band-edge cutoff). The core andhigh-index regions are anti-resonant inside the PBGs and core modes are guided, while they areresonant inside the photonic bands and core modes leak out through the high-index regions [9].The photonic band in the low-index regions and stress-applying parts lies under the core modesand restricts the number of higher-order modes (HOMs).

Our aim is to operate the fiber as a SP fiber at the short-wavelength edge of the first PBG usinglarge stress birefringence. Stress birefringence splits the two orthogonal fundamental modes[vertically in Fig. 2(a)], thus the band-edge cutoff becomes polarization dependent and a SPwindow appears between the two cutoff wavelengths [2]. A sharp cutoff is desirable because itprovides a sufficient PER over the entire SP window; a sharp cutoff requires a sharp resonancebetween the core and high-index regions. Reducing the coupling between the core and high-index regions – by enlarging the core size and increasing the spacing between the core and high-index regions – enables a sharper resonance; this characteristic has been observed in other large-mode-area hybrid MOFs [10]. The large (> 20 µm) core diameter of our fiber however allowedthe propagation of HOMs, thus this fiber is not strictly single-mode within the SP window.In [10], it is demonstrated that a hybrid MOF is naturally single-mode at the short-wavelengthedge of a transmission window, because the band-edge cutoff wavelength is always longer forHOMs. In our fiber, however, some HOMs weakly propagate inside the photonic band [dashedline in Fig. 2(a)]. Typical calculated power distributions of the fundamental modes and HOMsare shown in Fig. 2(b). The calculation suggests that HOM B does not show a clear band-edgecutoff, whereas the fundamental mode A and HOM C show clear band-edge cutoffs [A1 and C2in Fig. 2(b)]. This is because the stress-applying parts prevented the resonance between HOM Band the high-index regions, due to the large rod-to-pitch ratio and low refractive index. HOM Bcan be suppressed by precisely controlling the size of the low-index regions. In the followingfiber transmission measurements, we selectively excited the fundamental mode using a single-mode step-index fiber (λcutoff ∼ 0.92 µm).

Fig. 1. Cross section of the fiber.

Table 1. Refractive Index and Size of Microstructures in the Cladding

Type of relative refractive index difference rod-to-pitch ratiomicrostructure Δ (%) d/Λ

Germanium-doped high-index region 1.5 0.4Fluorine-doped low-index region -0.3 0.65Boron-doped stress-applying part -0.6 0.8


Fig. 2. Calculated photonic band diagram of the fiber, showing (a) the photonic band andcore modes, and (b) the calculated power distribution of the fundamental and higher-ordermodes at the points indicated in (a). We did not include material dispersion and the refrac-tive index of pure silica was set to 1.45 in the calculation.

3. Single-polarization window and UV-induced Bragg grating inscription

We measured the SP window of the fiber by measuring polarization-dependent cutoffs at theshort-wavelength edge of the first PBG using the same experimental setup described in [2].Figure 3 shows the polarization-dependent transmission spectrum of a 2 m-long loosely coiledfiber, showing a 30 nm-wide SP window. The SP window is almost the same as our previousreport [2], however, the sharp cutoff enabled a large PER over the entire SP window. Theattenuation of the fiber was about 12 dB/km at 1.31 µm, which is nearly 50-times smaller thanthe fiber in [7]. Figure 4(a) and 4(b) show the measured near-field power distributions of a 2m-long loosely coiled fiber when polarized light at 1095 nm was launched from a single-modefiber with offset. Figure 4(a) shows that the fundamental mode was predominantly excited whenthere was no offset; offset launching however resulted in distorted power distributions. Thedistortion is mainly in the Y-direction, suggesting the distortion is attributed to the excitation ofHOM B. By comparing Fig. 4(a) and 4(b), it is clear that the propagation of the core mode wassuppressed in the fast axis.

Fig. 3. Polarization-dependent cutoff and SP window of the fiber. The peak at 1064 nm isdue to the residual pump of the supercontinuum source used for the measurement.

We inscribed a FBG within the SP window in order to produce polarized Bragg reflection.The germanium-doped core enables UV-induced FBG inscription, however UV exposure to the


Fig. 4. Measured near-field power distribution of the fiber at 1095 nm for the (a) slow and(b) fast axes.

high-index regions would result in shifting the transmission windows to longer wavelengths.This long-wavelength shift would be particularly problematic for our polarized FBG inscrip-tion, because the SP window in our fiber is at the short-wavelength edge of a transmissionwindow and the PBG guidance at the Bragg wavelength may be lost. We minimize the UVexposure to the high-index regions by focusing UV light into the core through the low-indexregions. Figure 5 shows the Lloyd interferometer setup we used for FBG inscription, and theorientation of the high-index regions relative to the UV light. The UV (244 nm) beam was col-limated in one direction (top view), and half of the beam was reflected by a dielectric mirrorsuch that an interference pattern was created by the reflected and non-reflected beams. The UVbeam was focused in the other direction (side view) into the core by a cylindrical lens. In orderto allow the launch of the UV beam through the low-index regions, we launched visible light(He-Ne laser) transversely into the core prior to FBG inscription and adjusted the orientation ofthe high-index regions using the diffraction image created by the microstructures. We inscribeda 2 mm-long FBG into a hydrogen-loaded (140 atm, 90 °C, 4 days) hybrid MOF.

Fig. 5. Lloyd interferometer used for Bragg grating inscription, and orientation of the high-index regions.

The fiber transmission spectrum before and after the FBG inscription is shown in Fig. 6(a)and 6(b), indicating the inscription process hardly shifted the transmission windows of the fiber,nor induced a further insertion loss. Figure 7 shows the polarization-dependent transmissionspectrum of the fiber including the grating, measured after hydrogen had diffused out of thefiber. A 2 nm-bandwidth and strong Bragg resonance (∼20 dB) was measured within the SPwindow, indicating the Bragg reflection is polarized. By comparing Fig. 7 with Fig. 3, weconclude that the SP window was maintained after the FBG inscription. The weak transmissionextending to short wavelengths (<1080 nm) in Fig. 7 may be due to the slight difference of thecoiling (compared to the coiling in Fig. 3), because in this fiber, (1) the cutoff was sensitive tocoiling, and (2) the direction of the coiling, relative to the direction of the high-index regions,also strongly affects the cutoff. The slightly decreased sharpness of the cutoff could be due to


some UV exposure to the high-index regions, and may be eliminated by optimizing the size ofthe focused beam in the interferometer setup.

(a) Wide-band measurement (b) Narrow-band measurement

Fig. 6. Transmission spectrum of the fiber before and after the grating inscription, showing(a) the first and second PBGs and (b) around the Bragg wavelength. The polarization wasaligned to the slow axis within the SP window. The two separate Bragg resonances in (b)may be due to a low signal-to-noise ratio.

Fig. 7. Polarization-dependent transmission spectrum of the fiber with a Bragg grating.

4. Conclusion

In conclusion, we have demonstrated a UV-induced polarized FBG using a SP hybrid-MOF. Astrong (∼20 dB) UV-induced FBG was inscribed within the SP window of the fiber, enablingpolarized Bragg reflection; the sharp cutoff of the fiber enabled the large PER of the Braggreflection. The SP window was maintained after the grating inscription even though the high-index regions were UV photosensitive. Our approach enables the integration of a polarizer andBragg reflector into one fiber, providing a robust platform for linearly polarized fiber lasers.Other characteristics of all-solid PBG fibers, such as spectral filtering and certain dispersioncharacteristics, can also be integrated to enhance the functionality of such fiber lasers.

Acknowledgments

Yasuhito Saitoh is thanked for preparation of the preform. R. Goto acknowledges the Depart-ment of Education, Employment and Workplace Relations, Australia for financial support. Thiswork was supported from funds provided by the Australian Research Council through its Dis-covery Projects program.


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