We demonstrate a single-polarization all-solid hybrid microstructured optical fiber with a UV-induced Bragg grating. A strong (∼20 dB) UV-induced Bragg grating was inscribed within the 30 nm-wide single-polarization window of the fiber, producing polarized Bragg reflection. The sharp band-edge cutoff allows a large polarization-extinction ratio of the Bragg reflection. The hybrid structure of the fiber enabled minimal UV exposure to the high-index regions and the location of the single-polarization window was maintained after the grating was inscribed.
© 2011 OSA
Single-polarization (SP) fibers and fiber Bragg gratings (FBGs) are two important fiber-based devices for the production of high-power linearly polarized output from an all-fiber laser cavity. SP fibers select a polarization and FBGs provide wavelength-selective reflection in the laser cavity. Fiber-based devices are often superior to bulk-optic devices for high-power fiber lasers because they offer low-loss and robust coupling between devices by fusion splicing. In addition, 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 to produce SP fibers is to introduce a cutoff in a birefringent fiber . The cutoff wavelength is polarization dependent in a birefringent fiber, and the fiber operates as a SP fiber between the two cutoff wavelengths (SP window). Large birefringence increases the width of the SP window, 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 reflection (TIR) – provide large birefringence for enabling SP fibers [2, 3]. We have demonstrated a linearly polarized fiber laser using a point-by-point FBG inscribed into a SP hybrid MOF . The insertion loss induced by scattering  may however ultimately limit the power scalability of the laser. UV-induced FBGs provide a low insertion loss, however, a major issue with inscribing a UV-induced FBG into a SP hybrid MOF is that the high-index regions are typically highly germanium doped, thus highly UV photosensitive. The high-index regions suffer an increased refractive index when they are exposed to UV light; and the transmission windows shift to long wavelengths . The high-index regions made by phosphorous doping are not UV photosensitive (at wavelengths commonly used for inscribing UV-induced FBGs), thus eliminate the long-wavelength shift ; however, difficulties associated with the fabrication of a 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 a SP hybrid MOF. UV exposure to the high-index regions was minimized by launching UV light from the low-index cladding, and the location of the SP window was maintained after the FBG inscription. A strong (∼20 dB) Bragg grating was inscribed within the SP window, enabling polarized 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 fiber diameter was 203 μm. The fiber was fabricated by the stack-and-draw method; glass rods were stacked 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 the germanium doping was compensated by fluorine co-doping. Three types of microstructures – germanium-doped high-index regions, fluorine-doped low-index regions, and boron-doped stress-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 and stress-applying parts guide core modes by TIR. The high-index regions and stress-applying parts possess a larger thermal expansion coefficient compared to the low-index regions, thus induce large asymmetric thermal stress to the core after fiber drawing. The relative refractive index and rod-to-pitch ratio of each microstructure, which were measured at the glass rod stage before the stacking process, are summarized in Table 1. Figure 2(a) displays the calculated photonic 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  and did not include stress birefringence. The photonic bands in the 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 and high-index regions are anti-resonant inside the PBGs and core modes are guided, while they are resonant inside the photonic bands and core modes leak out through the high-index regions . The photonic band in the low-index regions and stress-applying parts lies under the core modes and 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 using large 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 SP window appears between the two cutoff wavelengths . A sharp cutoff is desirable because it provides a sufficient PER over the entire SP window; a sharp cutoff requires a sharp resonance between 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 . The large (> 20 μm) core diameter of our fiber however allowed the propagation of HOMs, thus this fiber is not strictly single-mode within the SP window. In , it is demonstrated that a hybrid MOF is naturally single-mode at the short-wavelength edge of a transmission window, because the band-edge cutoff wavelength is always longer for HOMs. In our fiber, however, some HOMs weakly propagate inside the photonic band [dashed line in Fig. 2(a)]. Typical calculated power distributions of the fundamental modes and HOMs are shown in Fig. 2(b). The calculation suggests that HOM B does not show a clear band-edge cutoff, whereas the fundamental mode A and HOM C show clear band-edge cutoffs [A1 and C2 in Fig. 2(b)]. This is because the stress-applying parts prevented the resonance between HOM B and the high-index regions, due to the large rod-to-pitch ratio and low refractive index. HOM B can be suppressed by precisely controlling the size of the low-index regions. In the following fiber transmission measurements, we selectively excited the fundamental mode using a single-mode step-index fiber (λcutoff ∼ 0.92 μm).
3. Single-polarization window and UV-induced Bragg grating inscription
We measured the SP window of the fiber by measuring polarization-dependent cutoffs at the short-wavelength edge of the first PBG using the same experimental setup described in . Figure 3 shows the polarization-dependent transmission spectrum of a 2 m-long loosely coiled fiber, showing a 30 nm-wide SP window. The SP window is almost the same as our previous report , however, the sharp cutoff enabled a large PER over the entire SP window. The attenuation of the fiber was about 12 dB/km at 1.31 μm, which is nearly 50-times smaller than the fiber in . Figure 4(a) and (b) show the measured near-field power distributions of a 2 m-long loosely coiled fiber when polarized light at 1095 nm was launched from a single-mode fiber with offset. Figure 4(a) shows that the fundamental mode was predominantly excited when there was no offset; offset launching however resulted in distorted power distributions. The distortion is mainly in the Y-direction, suggesting the distortion is attributed to the excitation of HOM B. By comparing Fig. 4(a) and (b), it is clear that the propagation of the core mode was suppressed in the fast axis.
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 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 inscription, because the SP window in our fiber is at the short-wavelength edge of a transmission window and the PBG guidance at the Bragg wavelength may be lost. We minimize the UV exposure to the high-index regions by focusing UV light into the core through the low-index regions. Figure 5 shows the Lloyd interferometer setup we used for FBG inscription, and the orientation of the high-index regions relative to the UV light. The UV (244 nm) beam was collimated in one direction (top view), and half of the beam was reflected by a dielectric mirror such that an interference pattern was created by the reflected and non-reflected beams. The UV beam was focused in the other direction (side view) into the core by a cylindrical lens. In order to 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 of the high-index regions using the diffraction image created by the microstructures. We inscribed a 2 mm-long FBG into a hydrogen-loaded (140 atm, 90 °C, 4 days) hybrid MOF.
The fiber transmission spectrum before and after the FBG inscription is shown in Fig. 6(a) and (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 transmission spectrum of the fiber including the grating, measured after hydrogen had diffused out of the fiber. A 2 nm-bandwidth and strong Bragg resonance (∼20 dB) was measured within the SP window, indicating the Bragg reflection is polarized. By comparing Fig. 7 with Fig. 3, we conclude that the SP window was maintained after the FBG inscription. The weak transmission extending to short wavelengths (<1080 nm) in Fig. 7 may be due to the slight difference of the coiling (compared to the coiling in Fig. 3), because in this fiber, (1) the cutoff was sensitive to coiling, 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 of the focused beam in the interferometer setup.
In conclusion, we have demonstrated a UV-induced polarized FBG using a SP hybrid-MOF. A strong (∼20 dB) UV-induced FBG was inscribed within the SP window of the fiber, enabling polarized Bragg reflection; the sharp cutoff of the fiber enabled the large PER of the Bragg reflection. 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 and Bragg 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 dispersion characteristics, can also be integrated to enhance the functionality of such fiber lasers.
Yasuhito Saitoh is thanked for preparation of the preform. R. Goto acknowledges the Department of Education, Employment and Workplace Relations, Australia for financial support. This work was supported from funds provided by the Australian Research Council through its Discovery Projects program.
References and links
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