FIBRE BRAGG GRATING FOR TELECOMMUNICATIONS APPLICATIONS: TUNEABLE THERMALLY STRESS ENHANCED OADM.

Transcription

1 32 FIBRE BRAGG GRATING FOR TELECOMMUNICATIONS APPLICATIONS: TUNEABLE THERMALLY STRESS ENHANCED OADM. P. S. André 1,2, J. L. Pinto 1,2, I. Abe 3, H. J. Kalinowski 3,1, O. Frazão 5, F. M. Araújo 4,5 1 Instituto de Telecomunicações - Pólo de Aveiro, Aveiro, Portugal. 2 Departamento de Física, Universidade de Aveiro, Aveiro, Portugal. 3 Centro Federal de Educação Tecnológica do Paraná, Curitiba, Brazil. 4 Departamento de Física da Faculdade de Ciências, Universidade do Porto, Porto, Portugal. 5 INESC Porto - Unidade de Optoelectrónica e Sistemas Electrónicos, Porto, Portugal. Abstract This paper presents some aspects of the production, characterization and implementation of fibre Bragg grating as a key element for selective filtering in optical communications. Theory, fabrications and applications of the Fibre Bragg grating for a specific tuneable optical add-drop multiplexer (OADM) are presented. The OADM performance on a Gbit/s dense wavelength division multiplexing optical communications system with 95 km is reported. The power penalty for the removed channels was less than 0.1 db. We obtain good agreement between the FBG behaviour the OADM performance with the numerical simulations results. Keywords: Fibre Bragg Grating, Optical Telecommunications, Optical Add Drop Multiplexer, Dense Wavelength Division Multiplexing. I - INTRODUCTION The enormous growth in the demand of bandwidth is pushing the utilization of fibre infrastructures to their limits. To fulfil this requirement the constant technology evolution is substituting the actual single wavelength systems connected in a point-to-point topology by dense wavelength division multiplexing (DWDM) systems, creating the foundations for the Optical Transport Network (OTN) [1]. The objective is the deployment of an optical network layer with the same flexibility as the equivalent synchronous digital hierarchy (SDH), because it is more economical and allows a better performance in the bandwidth utilization. Optical add-drop multiplexers (OADM) are the simplest elements to introduce wavelength management capabilities by enabling the selective insertion (add) and removal (drop) of optical channels. A wavelength tuneable OADM, giving access to all the wavelengths of the WDM signals provides more flexibility to satisfy reconfiguration requirements and to enhance network protection [2].

2 33 The significant discovery of photosensitivity in optical fibres [3] lead to the development of a new class of in-fibre component, called the fibre Bragg grating (FBG). Fibre Bragg gratings are revolutionising the telecommunications technology due to their intrinsic integration with fibres and the large number of device functionalities that they can facilitate, such as filtering, chromatic dispersion compensation, and optical amplifiers gain flattening. The most distinguishing feature of the FBG is the flexibility that they offer for achieving desired spectral characteristics, due to the broad range of variation in their determining physical parameters [4]. In this work we present experimental, and simulated results of the FBG filtering characteristics, and we also study the optical spectra of a specific FBG written for selective and tuneable spectral filtering. II THEORY AND FABRICATION OF FBG A FBG is a periodic modulation of the core refractive index in a single mode optical fibre, written by exposure to ultraviolet (UV) light in the region around 248nm [4,5]. This fabrication process is based on the photosensitive mechanism, which is observed in Ge-doped optical fibres [6]. If broadband light is travelling through an optical fibre containing such a periodic structure, its diffractive properties promote that a very narrow wavelength band is reflected back (see Fig. 1). The centre wavelength of that band can be represented by the well-known Bragg condition: λ B = 2 neff Λ (1) where λ B is the centre wavelength, n eff is the effective index of the guided mode and Λ is the period of the index modulation. Broadband spectra Transmitted spectrum n cladding Períod Λ n core Reflected spectrum Optical fibre with grating Figure 1 - Schematic representation of a fibre Bragg grating.

3 34 Fabrication techniques have been subjects of much research interest owing to the driving force arising from communications and sensing applications. A number of schemes have been demonstrated, which has led to successful commercialisation of the FBG fabrication. Essentially, there are two methods to fabricate FBGs namely the two-beam interferometer method and the phase mask method. The first technique has been demonstrated by Meltz [5] and uses an interferometer to generate the optical fringe pattern necessary to writing the grating structure into the fibre core. The second process has been reported as an improved method for the fabrication of FBG [7]. A phase-mask is a diffractive element that can be used to form an interference pattern laterally, i.e. Bragg grating pitch, with the light beams which are spatially phase modulated and diffracted by the phase-mask, as shown in (see Fig. 2). This interference pattern is then used to photo imprint a refractive index modulation in the photosensitive fibre immediately behind the phase mask in proximity and parallel. While the first method is more flexible to adjust the spatial characteristics of the refractive index profile written in the fibre, the second one has several advantages which include: The Bragg wavelength of an FBG is determined by the pitch of phase mask and is independent of wavelength of the UV laser; the phase mask method offers a high potential for mass production with good repeatability at low cost; this single beam writing method improves the mechanical stability of the FBG writing apparatus; the requirement for the coherence of the UV laser is reduced and hence low spatial and temporal coherence excimer lasers can be used. UV light Optical Fibre Λ Silica phase mask Λ/ Figure 2 - Schematic diagram of the phase mask writing process. The grating used in this work was manufactured by illuminating an optical fibre exposed to a phase-mask spatially modulated UV (248 nm) writing beam originated from a KrF excimer laser. The standard single mode fibre (SMF) has been previously kept under high-pressure hydrogen atmosphere in order to enhance its photosensitivity due to hydrogen diffusion into the glass matrix. This process is reliable and gives excellent results in the reduction of the writing time of the grating [6]. Optical characterizations after the writing revealed a reflection band centred at 1548 nm with full width half maximum of 0.3nm. The grating was covered with a polymeric resin for protection and then bonded inside an aluminium tube (external diameter 1mm, wall thickness 0.1mm). The lead fibre lengths were protected by sheath and tubing material and

4 35 finally mounted in FC/PC fibre connectors. The Aluminium tube with the grating inside was assembled over a thermoelectric Peltier element, so that its temperature could be easily set and controlled (see Fig. 3 for a photograph of the assembled device). Figure 3 - Photograph of the assembled device. The FBG is glued inside the Aluminium tube protuberant from the brass block. III - THERMALLY STRESS ENHANCED TUNING OF BRAGG GRATINGS The wavelength tuning capacities of FBG are related with their capacity to shift the central reflection wavelength. There are two main methods to obtain such effect: to shift the Bragg grating central wavelength peak by modifying the fibre refractive index or by changing the grating period. These variations can be dynamically induced either by temperature or by mechanical stress. The strain response arises due to both the physical elongation of the grating and the change in fibre index due to the photo elastic effect, whereas the thermal response arises due to the thermal expansion of the fibre material and temperature dependence of the refractive index. The first allows a broad tuning range (>36 nm) and high tuning speed (>10 nm/ms), but has low reproducibility and low reversibility. On the other hand, thermal tuning presents high reproducibility, high reversibility and built-in temperature compensation, but it is limited by a narrow tuning range (~ 1 nm) and a slow tuning speed (~ 1 nm/s). We present a hybrid method based on a thermal-stress thermally enhanced actuation on a FBG, with the advantages of the thermal tuning and a higher tuning range. The used configuration is schematically illustrated in Fig. 4. The enhancement of the temperature sensitivity of wavelength in this configuration arises from the use of an Aluminium tube, which has a positive thermal expansion coefficient, where the FBG is bonded. When heated, the Aluminium expands, thereby inducing a strain in the FBG, the temperature also rapidly increases in the FBG due to a thermal conductive compound which fill all the tube and keep the FBG an the tube in a thermal equilibrium.

5 36 Al FBG TCC B Figure 4 - Enhanced thermal tuning configuration. Al - Aluminium tube, TCC Thermal conductive compound The Bragg wavelength of a FBG, λ B, given by Eq. 1, depends on the effective refraction index of the fibre and on the index modulation period, therefore the variation of the peak reflection wavelength as function of the temperature and stress is [8]: neff Λ neff λb = 2 Λ + neff ε + 2 Λ ε ε T + n eff Λ T T (2) The first term in Eq. 2 is related with the stress applied to the grating and can be expressed by: λ = λ (1 p ) ε (3) BS B e z where p e is the silica photo elastic coefficient with a value of The second term in that equation is related with the temperature applied to the grating and is given by: λbt = λb ( α+ ξ) T (4) where α is the thermal expansion coefficient and ξ is the thermo optic coefficient, in the case of silica these coefficients have a value of K -1 and K -1, respectively. The induced strain on the grating will depend on the change of temperature of the Aluminium tube, T, and on the thermal expansion coefficient of the Aluminium, α CTE, which have a value of K -1. ε = α T (5) Z CTE Substituting Eqs. 3, 4 and 5 on the expression given in Eq. 2 we arrive to the shift of the FBG reflection wavelength, given by:

6 37 [(1 p) ( )] λ= λ α + α+ ξ T (6) B e CTE For a typical wavelength in the region of 1550 nm the tuning coefficient calculated by Eq. 6 is 41.1 pm K -1. Experimental results for the dependence of the reflected wavelength with the temperature of the FBG and Aluminium tube are presented in the next section; from that data we obtain a value of pm K -1 for the tuning coefficient. The faster temperature dependence and easy management of the grating peak are required for devices intended to dynamically allocated wavelength multiplexed networks, such as tuneable filters, optical add-drop multiplexers, wavelength converters. The increase of temperature and stress applied on the FBG, results on a lowering of the reflectivity and an increase of the bandwidth, however these alteration on the FBG performance does not affect significantly the performance of the device in an OADM. In section 5 we discuss that performance. IV - OPTICAL AND THERMAL CHARACTERIZATION The Optical spectra of the produced FBG were measured by Optical Spectrum Analyser with better than 0.07 nm resolution. Grating spectra were also measured with a hybrid fibre air Michellson Interferometer [9], the acquired interferograms are transformed with a Fast Fourier Transform routine to obtain the associated frequency spectra. As optical sources to illuminate the FBG either an ELED (75 nm full width at half maximum) or the Amplified Spontaneous Emission of an EDFA were used. Light reflected by the grating is fed to the measuring apparatus through an optical coupler, as depicted on Fig.5. Care was taken to index match the unused ports of the grating fibre and of the optical coupler, to avoid light reflected at the fibre air interfaces. Figure 5 - Basic set-up for the optical characterization of fibre Bragg gratings. A set of optical spectra is presented in Fig. 6. As it can be seen, the spectral behaviour with respect to changes in temperature shows a single spectral band at room temperature, with peak positioned at 1547 nm and full width at half maximum (FWHM) bandwidth of 1 nm.

7 38 Figure 6 - Optical spectra of the FBG as the temperature changes. As the temperature increases this reflection band shifts initially to longer wavelengths, while at higher temperatures (T~40ºC) it can be seen that a second peak is partially distinguishable at the shorter wavelength tail of the spectrum. The main peak can be followed as temperature changes, giving the results presented in Fig. 7. In the same picture it is also shown the graph of the peak position versus temperature for the grating when isolated from the Aluminium enclosure, so that the enhancement due to the strain imposed by the metallic thermal expansion process is clearly visible Peak Wavelength (nm) Temperature Temperature and Strain kthermal=44.37 pm/ºc kthermal=9.98 pm/ºc Temperature (º C) Figure 7 - Variation in the peak position of the Bragg grating reflection band as a function of temperature. Higher slope: assembled device. Lower slope: grating only.

8 39 The slopes, respectively pm/k and 9.98 pm/k, show that the strain imposed by Aluminium expansion is the main agent in the spectral shift of the reflection band for the assembled device. The slope and range of the spectral shift are adequate for devices intended for WDM applications, as discussed in section 5. It shall be mentioned that the measured slope when the grating is glued to the Aluminium tube is close to the theoretical estimate we presented in section 3. Numerical simulations carried on using Optiwave s IFO-Grating software predicted a thermally enhanced slope close to that figure. Other measurements were made using different gratings (both commercial or from other research groups) glued to different Aluminium supports like, e.g., small plates. Results for the slope of the peak position versus temperature curve gave results from 36.8 pm/k to 39.8 pm/k, probably more influenced by the geometry of the metallic support than by the (fibre) grating characteristics. Fig. 8 depicts measurements of the FWHM bandwidth and reflectivity of the FBG as the temperature changes. These parameters are of capital importance for the intended use of the device as an OADM in WDM systems. As it can be seen from that figure, the 3dB bandwidth has a smaller change with temperature, while the same parameter measured by the 20 db point shows extended change for higher temperatures. This is mainly due to the asymmetry of the band as temperature increases, when a secondary lobe becomes more important in the lower wavelength range of the spectrum. Reflectivity measurements also reflect such behaviour, as the reflected power is spread over a broader range of wavelengths. Bandwidth (nm) db Bandwidth - 3 db Bandwidth Reflectivity Wavelength (nm) Reflectivity (db) Figure 8 - FWHM bandwidth and reflectivity of the assembled Bragg grating as temperature changes, measured by the peak position. Further measurements, taken some weeks after, revealed the single band optical spectrum at room temperature. However, that spectrum presented a split when the temperature increased. The measurements showed that, after splitting, the peak at higher wavelengths shifted with higher rate, followed by the peak at lower wavelength after an initial delay. When time is given to the temperature to stabilize, both peaks collapsed again. A second set of optical spectra of the grating at different temperatures is shown in Fig. 9, taken a few months after. At room temperature two peaks are clearly identified, with peak wavelength of nm and nm, respectively, and individual FWHM of 0.20 nm.

9 40 As temperature increases, the peak on the right of the spectrum shifts to longer wavelengths with a measured slope of 35 pm/k. The peak on the left starts to shift with the same slope, but soon that shift becomes undistinguishable as the reflection band starts to broaden until it spreads completely. At the maximum attained temperature in the experiment (~70 ºC), that band has almost vanished from the spectrum. During the broadening it is possible to see that extra peaks appear and disappear in the measured spectrum. The change in the spectrum is also perceived through the frequency spectra of the grating reflection band, measured directly by the interferometer [10]. INTENSITY (Arbitrary) C 40 C 50 C 60 C 70 C WAVELENGTH (nm) Figure 9 - Optical response of the same FBG taken a few months after. Details are given in text. The reflection bands are normalized to enhance the spectral changes. We believe that the differences in the behaviour of the optical spectra of the FBG as the temperature changes is due to an improper gluing of the fibre inside the capillary tube used to impose the stress on the grating. It is probable that the fibre region, where the grating is written, touches the Aluminium wall. As temperature changes this region is subject to an initial strain and thermal gradient different from the other regions of the grating, giving in result an optical spectrum that shows two (or more) reflection bands. The above insight is also supported by the change in the spectra presented in Fig. 6 and Fig. 9. In between the measurements the device was transported from one laboratory to the other. The landing and take off acceleration of the airplane might have changed the position of the fibre in the region touching the wall, giving in result such deep changes in the spectra at room temperature and very diverse thermal behaviour. V - A TUNEABLE OADM FOR OPTICAL COMMUNICATIONS A basic and well-studied architecture for an OADM is shown in Fig. 10, and consists of a fibre Bragg grating (FBG), an optical circulator, and a power coupler. At first, N multiplexed wavelengths are led to the Bragg grating through the circulator, then the filtered signal is reflected and go back to the circulator where is removed. The remaining channels are coupled with the added channel in the power coupler [11].

10 41 Circulator FBG Coupler In Out Drop λ Add λ Figure 10 - Basic OADM architecture using a fibre Bragg grating. The functionality of the OADM is demonstrated using the experimental network in Fig. 11. Three distributed feedback lasers (DFB) based at the ITU grid of 50 GHz ( 0.4 nm) spacing, , and THz, were externally modulated through Ti:LiNbO3 Mach-Zehnder intensity modulators, at Gbit/s (STM-16) with a non-return to zero (NRZ) pseudo random bit sequence (PRBS). In the WDM experimental transmission link, 70 km of single mode standard fibre, with 0.19 db/km optical attenuation are used. Two Erbium doped fibre amplifiers (EDFA) having saturated output powers of 13 and 17 dbm and noise figures smaller than 4 db are employed to provide the required power to compensate the link and OADM losses. Figure 11 - Schematic of the implemented OADM and experimental DWDM transmission set - up. The optical spectra at the OADM input are show in Fig. 12.

11 Power (dbm) Wavelength (nm) Figure 12 - Three channel optical spectra at the OADM input. It is possible to access and remove the 3 adjacent wavelengths, with a 20 º C temperature range, Fig. 13 displays the optical spectra when a channel is removed in the OADM for a FBG temperature of 30 ºC, while in Fig. 14 is shown the optical spectra with an other removed channel, for a FBG temperature of 50 ºC Power (dbm) Wavelength (nm) Figure 13 - Optical spectra of the channel 1 dropped for a FBG temperature of 30 ºC.

12 Power (dbm) Wavelength (nm) Figure 14 - Optical spectra of the dropped channel 3, when the FBG is set to 50 C. The performance of our network is assessed by the BER measurements on the removed channel. The BER performance against the receiver optical power, for the back to back operation (0 km), for the two dropped channels on the OADM after propagation on 50 km of fibre and for the same channels at the OADM input, are show on Fig. 15, the BER floor (measurement limit) and the 10-9 BER are also indicated. The power penalty measured for the two removed channels at 10-9 BER is inferior to 0.1 db, when compared with the same channel at the OADM input after propagation on 50 km of fibre. This power penalty is due to heterodyne cross talk induced by the leak of the signal power from the other neighbour s signal components. These residual components interfere with the detection process, resulting in noise addition on the detector Back-to-Back Channel 1 50 km of fibre Channel 3 Log (BER) BER=10-9 BER Floor Power (dbm) Figure 15 - BER performance for the dropped channel.

13 44 Fig. 16 shows the direct detection (without electrical reshaping filter) eye diagram of the drop channel 1 for a 10-9 BER. In there it is possible to observe a high signal-to-noise ratio (wide vertical eye opening), indicating a small signal quality degradation, due to cross talk and a high spectral selectivity which reduces the ASE noise at the receiver and consequently the noise-noise and noise-signal beating. Also it s clear the inexistence of inter symbols interference and timing jitter (wide horizontal eye opening is present). Figure 16 - Eye diagram for the channel 1. The implemented network was also simulated using a commercial photonic transmission numerical simulator, VPI Transmission Maker from Virtual Photonics, which allow us to confirm the good OADM operation performance. VI - CONCLUSIONS We reviewed the essential aspects related with the manufacturing and characterization of fibre Bragg gratings. The theory, fabrications and applications of fibre Bragg grating for a specific tuneable optical add-drop multiplexer was also presented. Finally, we have reported an OADM solution for DWDM systems using a FBG. The OADM performance was demonstrated in a 50 GHz, 3 channels WDM system, working at STM-16 bit rate. The power penalty due to the presence of the OADM in the network is negligible, within the experimental uncertainty; therefore the cascading capacity of this OADM configuration is high, when compared with others OADM configurations. The investigated OADM configuration is promising, due to its good spectral characteristics which results in low homodyne cross talk, and consequently potential high cascability. Acknowledgments Work financed by the Portuguese scientific program PRAXIS XXI (PRAXIS XXI/BD/17227/98) through the DAWN project and by Portugal Telecom Inovação through the O-NODE project (P114). This work also received financial support from CAPES, CNPq

14 45 and PRONEX (Brazilian Agencies), being part of the activities of the CAPES/ICCTI project 58/00. REFERENCES [1] Charles A. Brackett, Foreword Is There an Emerging Consensus on WDM Networking?, IEEE J. Lightwave Technol., 14, (1996). [2] K. I. Sato, S. Okamoto, H. Hadama, Network performance and integrity enhancement with optical path layer technologies, IEEE J. Select. Areas Commun., 12, (1994). [3] K.O. Hill, Y. Fujii, D.C. Johnson and B.S. Kawasaki, Photosensitivity in optical waveguides: Application to reflection filters fabrication, Appl. Phys. Lett. 32(10), 647 (1978). [4] K.O. Hill and G. Meltz, Fiber Bragg Gratings Technology and Overview, IEEE J. Lightwave Technol. 15, (1997). [5] Meltz, G., Morey, W.W., and Glenn, W.H. Formation of Bragg gratings in optical fibers by transverse holographic method, Opt. Lett. 14, (1989). [6] K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, Photosensitivity in Optical Fibers, Annu. Rev. Mater. Sci. 23, (1993). [7] K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, and J. Albert, Bragg Gratings Fabricated in Monomode Photosensitive Optical Fiber by UV Exposure through a Phase Mask, Appl. Phys. Lett., 62, (1993). [8] Andreas Othonos, Kyriacos Kalli, Fiber Bragg Gratings Fundamentals and Applications in Telecomunications and Sensing, Artech House London (1999). [9] I.Abe, Interferômetro híbrido para análise de espectros óticos, M.Sc. Thesis, CPGEI/CEFET-PR, (1998). [10] I. Abe et al, Fibre Brag Gratings for Optical Communications, Proc. ConfTele rd Conference on Telecommunications, (2001). [11] Shien-Kuei Liaw, Keang-Po Ho, Chinlon Lin, Sien Chi, Experimental Investigation of Wavelenftg Tunable WADM and OXC Devices Using Strain Tunable Fiber Bragg Grattings, Optics Commun. 169, (1999).