We discuss the effect of UV laser exposure time, repetition rate, and heating by a heating coil on the resonance peak depth and polarization-dependent loss (PDL) of long-period fiber gratings (LPFGs). The LPFGs were fabricated with a KrF excimer laser and an amplitude mask. We observed an initial increase of the resonance peak depth and PDL as the UV exposure time was increased, which eventually decreased in response to over-coupling. With the total UV fluence kept constant, the peak depth continued to increase as the repetition rate was increased beyond 10 Hz, whereas the maximum PDL decreased when the repetition rate was higher than 17 Hz. This is believed to be a thermal effect caused by the rapid delivery of UV laser pulses. We observed a similar reduction of the maximum PDL from 1.35 to 0.25 dB when the fiber was heated by an adjacent heating coil.
©2003 Optical Society of America
Optical fiber gratings are used as basic components in optical communication and sensing systems such as optical filters, fiber sensors, and erbium-doped fiber amplifier (EDFA) gain-flattening filters [1–3]. Optical fiber gratings are often fabricated by irradiation of a UV laser beam onto a single side of optical fiber, and this causes photoinduced birefringence and thus polarization dependence of the fiber-based devices and components. Such characteristics can be used for certain applications such as single-mode operation of an erbium-doped fiber laser and in-line wave retarders [4,5]. However, they become increasingly troublesome at high bit rates, and the penalties of the polarization dependence of optical components can lead to the degradation of overall system performance [6,7]. In the case of long-period fiber gratings (LPFGs), this effect is manifested as polarization-dependent loss (PDL) in the transmission spectrum . Such anisotropy of the photoinduced index change is also observed, for instance, during the writing of rocking filters with visible light propagating in the core of a birefringent fiber .
In the study of Erdogan and Mizrahi, the polarization of the writing UV beam was shown to have a strong effect on the photoinduced birefringence in Ge-doped silica optical fibers . The measure of birefringence was dependent on both the fiber type and exposure conditions, and they grew in proportion to the total induced index change. Vengsarkar et al. reported that birefringence in grating-based fiber devices was caused mainly by the side-writing process . After exposure to UV light, a single side of the photosensitivity fiber was cleaved and etched. The atomic force microscope (AFM) images of the fiber end faces showed that geometrically asymmetric UV exposure was the main cause of the asymmetric index change. On the basis of this observation, birefringence was reduced by use of the dual side-exposure technique: After the first UV exposure, the fiber was rotated by 180° to expose the other side to UV irradiation. On the other hand, Poirier et al. showed that the luminescence bleaching in Ge-doped fiber is anisotropic, which confirms that the defects aligned with the polarization of the UV beam are preferentially excited and bleached . Dossou et al. investigated the contribution of the transverse asymmetry of the index-change profile to the photoinduced birefringence through numerical simulation . These previous studies indicate that the photoinduced birefringence of optical fiber has two main contributions: the polarization of the UV writing beam and the asymmetric index change in the transverse plane.
We discuss the effect of UV laser exposure time, repetition rate, and heating by a heating coil on the transmission characteristics and PDL of LPFGs fabricated with an amplitude mask, a KrF excimer laser, and UV side exposure. Thermal treatment of the optical fiber through heating or exposure to CO2 laser irradiation reportedly enhances photosensitivity [14,15]. In this connection, we observed a similar effect when using a UV excimer laser with high repetition rate. The enhanced photosensitivity can serve to reduce the PDL of LPFGs, since the refractive-index change upon irradiation of the UV laser is faster at the initial stage and the refractive-index change tends to saturate as the cumulative UV fluence increases. This effect leads to the reduction of the birefringence caused by the induced index changes and hence to the reduction of the PDL. In this study we used two methods for thermally treating the fiber: (1) increasing the repetition rate of the UV excimer laser pulses, and (2) heating the optical fiber with an adjacent heating coil. The measurement results obtained with both methods confirmed reduction of the PDL because of the thermal treatment of the optical fiber during the grating fabrication process.
We can describe the principle of LPFGs by applying the coupled-mode theory to the core mode and the cladding modes. The derivations can be found in numerous articles . With the photoinduced birefringence taken into consideration, the polarization eigenmodes of the core mode are no longer degenerate and attain two distinct effective indices for the slow and fast axes. The resonance wavelengths can be obtained from the phase-matching condition
where the index i denotes the slow or fast axis. The index difference resulting from the photoinduced birefringence is of the order of 10-6 [10,13]. The slightly separated transmission spectra of the two eigenpolarization modes and the PDL curve are shown in Fig. 1. The transmission spectra were obtained analytically with the parameters L=2.5 cm and Λ=450 µm. The birefringence induced by UV exposure was taken to be 4×10-6. Since the coupling coefficient κ is proportional to the index change and the field overlap between the core mode and the cladding mode, it is not sensitive to small index changes, and we employed the same κ for the two polarization modes. For the simulation result shown in Fig. 1, we used κ=4×10-5. Even though the spectral shift between the two polarization modes is only a few nanometers and is hardly observable with an unpolarized light source, it can be clearly seen through PDL measurements.
3. Experimental results
In this section we discuss our experimental measurements on the temporal change of the transmission characteristics and the PDL of LPFGs fabricated with an amplitude mask and a KrF excimer laser. The setup for fabrication of the LPFGs and measurements is shown in Fig. 2. The period of the amplitude mask was 450 µm, and the length of the LPFG was 2.5 cm. The UV laser energy was 146 mJ per pulse, and the repetition rate was 15 Hz. The photosensitivity fiber was made at the Kwangju Institute of Science and Technology (K-JIST) and was treated with hydrogen loading under 90 bar for 4 days. The specifications of the fiber are as follows: cutoff wavelength, λc=1002 nm; Δ=1.1%; N.A.=0.22; core and cladding diameters, 3.6 µm and 125 µm, respectively.
The transmission spectra and PDL were measured with an optical spectrum analyzer (OSA) and a PDL meter. The PDL meter was equipped with a motorized polarization controller to scan the states of polarization (SOP). The measurement results of the peak depth and the maximum PDL are plotted in Fig. 3. The peak depth increases initially with increase of exposure time and then begins to decrease after ~230 s of exposure time. Even though the coupling constant κ increases monotonically with increase of exposure time, the peak depth eventually begins to decrease because of overcoupling. The PDL also shows rapid growth before overcoupling occurs, which is consistent with a previous study that predicted quadratic increase of the birefringence in proportion to the index change .
The relationship between the UV laser repetition rate and the transmission characteristics such as the resonance peak depth and the PDL of LPFGs was investigated. The same experimental setup as depicted in the previous paragraph was used, except that the UV laser repetition rate was varied. A total of 1200 pulses were delivered for fabrication of each LPFG. The PDL and the resonance peak depth are shown in Fig. 4. Even though the total fluence is the same for the fabricated LPFGs, we observed a shift of the resonance peak to the longer wavelength and an increase of the peak depth as the repetition rate increased. This can be interpreted to mean that the refractive-index change in the core region by the photosensitivity effect becomes larger with higher repetition rate. On the other hand, the PDL initially increased as the repetition rate increased but eventually decreased after peaking at ~17 Hz as the repetition rate was further increased.
The origin of the observed effect is the thermal heating of the optical fiber resulting from the rapid delivery of the UV laser beam energy onto the fiber, and it is analogous to the effect observed with the heat treatment by high temperature or CO2 laser irradiation [14,15]. The peak depth does not change appreciably when the repetition rate is below 10 Hz but begins to increase at 10 Hz, whereas the resonance wavelength continues to shift to the longer wavelength even when the repetition rate is below 10 Hz. This measurement result can be explained by considering that the heat diffusion occurs simultaneously as the laser beam energy is deposited in the optical fiber. When the time interval between the laser pulses is larger than the heat diffusion time, the heat treatment effect will be small. Conversely, when the time interval between the laser pulses is shorter than the heat diffusion time, the photosensitivity will be significantly enhanced because it is heated in a manner similar to the CO2 laser treatment . The photosensitivity enhancement is closely tied to the reduction of the PDL as shown in Fig. 4. When the heating effect is small at a low repetition rate, the birefringence due to the asymmetric exposure to the UV laser beam increases the PDL. In the regime of high repetition rate beyond approximately 17 Hz, the fast increase of the index change in the region with low exposure to the laser beam—combined with the saturation of the refractive-index change in the region with high exposure—contributes to the reduction of the PDL. This explanation is supported by the simulation of Dossou et al.; i.e., the PDL level increased with increasing saturation depth but decreased after a certain level .
To verify that the origin of the photosensitivity enhancement is the thermal effect, we performed separate experiments using a heating coil placed near the optical fiber during the grating fabrication process. The heating coil was nickel–chrome solenoid type, 5 mm in diameter and 2 cm in length. It was connected to a power supply that could be controlled with a voltage knob. The resistance of the heating coil was 9.3 Ω, and it was separated from the optical fiber by approximately 7 mm. The pulse repetition rate of the UV laser was kept at 10 Hz, and we fabricated LPFGs with different voltage settings to observe the effect of heating on the PDL. We adjusted the beam exposure times so that the resonance peak depth was approximately 5 dB for all cases. The maximum PDL is shown in Fig. 5(a), and the exposure time taken to reach the 5-dB peak depth is shown in Fig. 5(b). As expected, the peak depth grows more rapidly and the PDL is reduced as the applied voltage is increased. The resonance wavelength also shifts to the longer wavelength as the applied voltage is increased. We observed that the grating did not grow further when the voltage was higher than 15 V. This may be due to the out-diffusion of the hydrogen molecules loaded in the fiber. Heating the fiber also had a pronounced effect on the PDL of the LPFGs. We measured the temperature at the fiber as the applied voltage was varied, and Fig. 5(a) shows that the PDL decreased from 1.35 dB at 18 °C (0 V) to 0.25 dB at 211 °C (10 V). The efficiency of grating fabrication, i.e., the reciprocal of the exposure time taken to reach the 5-dB peak depth, and thus the measure of the photosensitivity, also strongly depends on the amount of heating as seen in Fig. 5(b). These data confirm the conjecture that the heat generated under the high repetition rate of the UV laser pulses is the cause of the enhanced photosensitivity and the lower PDL.
In this paper we have discussed the effect of the UV laser exposure time, repetition rate, and heating by a heating coil on the transmission characteristics and PDL of LPFGs. The LPFGs were fabricated with a KrF excimer laser and an amplitude mask. We observed that the resonance peak depth and the maximum PDL increased initially with the UV laser exposure time and then decreased as a result of overcoupling. Two methods were used for thermal treatment of the optical fiber: (1) increasing the repetition rate of the UV excimer laser pulses, and (2) heating the optical fiber with an adjacent heating coil.With the total number of pulses kept constant, the resonance wavelength shifted to a longer wavelength and the resonance peak depth increased as the repetition rate increased. Under the same condition, the PDL increased with the repetition rate up to 17 Hz but decreased at higher rates. When the optical fiber was heated during the grating fabrication by use of an adjacent heating coil, the PDL decreased from 1.35 dB at 0 V to 0.25 dB at 10 V. The writing efficiency also increased as the heating voltage increased. This effect is directly related to the enhancement of photosensitivity by thermal treatment [14,15]. These results indicate that the PDL of LPFGs can be reduced by simply heating the optical fiber during the grating fabrication process with the two methods proposed and demonstrated in this study. Compared with other fabrication methods designed to reduce the PDL, these methods are effective and relatively simple to implement.
References and links
1. A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long-period fiber gratings as band-rejection filters,” J. Lightwave Technol. 14, 58–65 (1996). [CrossRef]
3. S. Yoshida, S. Kuwano, and K. Iwashita, “Gain-flattened EDFA with high Al concentration for multistage repeated WDM transmission systems,” Electron. Lett. 31, 1765–1767 (1995). [CrossRef]
4. P. A. Morton, V. Mizrahi, T. Tanbun-Ek, R. A. Logan, P. J. Lemaire, H. M. Presby, T. Erdogan, S. L. Woodward, J. E. Sipe, M. R. Phillips, A. M. Sergent, and K.W. Wecht, “Stable single-mode hybrid laser with high power and narrow linewidth,” Appl. Phys. Lett. 64, 2634–2636 (1994). [CrossRef]
5. T. Meyer, P. A. Nicati, P. A. Robert, D. Varelas, H. G. Limberger, and R. P. Salathe, “Reversibility of photoinduced birefringence in ultra-low birefringence fibers,” Opt. Lett. 21, 1661–1663 (1996). [CrossRef] [PubMed]
6. M. Schiano and G. Zaffiro, “Polarization mode dispersion in chirped fiber gratings,” in Proceedings of the European Conference on Optical Communications (IEEE, New York, 1998), Vol. I, pp. 403–404.
7. E. Ciaramella, E. Riccardi, and M. Schiano, “System penalties due to the polarization mode dispersion of chirped gratings,” in Proceedings of the European Conference on Optical Communications (IEEE, New York, 1998), Vol. I, pp. 515–516.
8. Y. Zhu, P. Berini, E. Simova, and C. P. Grover, “Wavelength-dependent PDL measurements in fiber gratings,” in Conference on Lasers and Electro-Optics, CLEO 99 (Optical Society of America, Washington, D.C., 1999).
9. P. St. J. Russell and D. P. Hand, “Rocking filter formation in photosensitive high birefringence optical fibers,” Electron. Lett. 26, 1846–1848 (1990). [CrossRef]
10. T. Erdogan and V. Mizrahi, “Characterization of UV-induced birefringence in photosensitive Ge-doped silica optical fibers,” J. Opt. Soc. Am. B 11, 2100–2105 (1994). [CrossRef]
11. A. M. Vengsarkar, Q. Zhong, D. Inniss, W. A. Redd, P. J. Lemaire, and S. G. Kosinski, “Birefringence reduction in side-written photoinduced fiber devices by a dual-exposure method,” Opt. Lett. 19, 1260–1262 (1994). [CrossRef] [PubMed]
12. M. Poirier, S. Thibault, J. Lauzon, and F. Quellette, “Dynamic and orientational behavior of UV-induced luminescence bleaching in Ge-doped silica optical fiber,” Opt. Lett. 18, 870–872 (1993). [CrossRef] [PubMed]
13. K. Dossou, S. LaRochelle, and M. Fontaine, “Numerical analysis of the contribution of the transverse asymmetry in the photoinduced index change profile to the birefringence of optical fiber,” J. Lightwave Technol. 20, 1463–1470 (2002). [CrossRef]
14. M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” in Bragg Gratings, Photosensitivity, and Poling in Glass Fibers and Waveguides, OSA Technical Digest (Optical Society of America, Washington, D.C., 1999), paper PD4.
15. G. Brambilla, V. Pruneri, L. Reekie, and D. N. Payne, “Enhanced photosensitivity in germanosilicate fibers exposed to CO2 laser radiation,” Opt. Lett. 24, 1023–1025 (1999). [CrossRef]
16. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15, 1277–1294 (1997). [CrossRef]