first circulator, and FBG. A wavelength-tuneable laser source (Agilent 81600B) was used to transmit a specific wavelength channel at 1 mW through the CMC.
Wavelength-tuneable add/drop multiplexer using broadband transmission filters and a narrowband reflection filter Ronnie Kritzinger, André Booysen Centre for Optical Communications and Sensors University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, RSA ABSTRACT We present an optical add/drop multiplexing device utilising a pair of broadband transmission filters and a narrowband reflection filter, designed for operation in a dense wavelength-division multiplexing network. Evanescent field coupling in an optical fibre-based cladding-mode coupler allow broadband light to be transferred between two fibres containing non-uniform long-period fibre gratings. An erbium-doped fibre amplifier was used to restore the power level of the output signal obtained from the cladding-mode coupler, to its original level. A tuneable Bragg grating is used to select a specific wavelength channel from the broadband light routed through the cladding-mode coupler, and is able to work over a linear tuning range of 5.3 nm, covering ~13% of the C-band. Keywords: Add/drop multiplexing, transmission filters, optical fibre, cladding-mode coupler, tuneable Bragg grating.
1. INTRODUCTION As data rates increase beyond 40 Gb/s with the introduction of broadband technologies, the need for optical add/drop multiplexers (OADMs) that are wavelength-tuneable and economically viable increases. Long-period fibre gratings (LPGs) and fibre Bragg grating (FBGs) are in-fibre spectral filters that contribute a great deal to the sensing and DWDM industry1. These optical filters exhibit low insertion losses, rapid response times, and are relatively easy to manufacture, making it for ideal for high-speed fibre networks. In this paper we present an OADM device illustrated in Fig. 1, incorporating a broadband cladding-mode coupler (CMC) and a narrowband FBG. The CMC routes a wide range of wavelength channels from one fibre to another optical fibre, and a tuneable FBG is used to select a specific signal. One distinct advantage of this OADM device is the drop of several wavelengths between two fibres, compared to other OADM topologies where dropped channels have to be pre-selected2.
2. EXPERIMENTAL LAYOUT OF THE BROADBAND OADM SYSTEM Since the multiplexing part of the OADM device illustrated in Fig. 1 is the mirror image of the de-multiplexing part, only results obtained for the de-multiplexing part are presented. The de-multiplexing part of the OADM consists of the CMC, first circulator, and FBG. A wavelength-tuneable laser source (Agilent 81600B) was used to transmit a specific wavelength channel at 1 mW through the CMC. The transmitted wavelength range is determined by the non-uniform LPGs used in the CMC. The non-uniform LPGs were fabricated to transmit wavelengths between 1544.40 nm and 1580.03 nm, which cover half of the C-band and part of the L-band. An erbium-doped fibre amplifier (EDFA) is used to restore the output power level obtained from the CMC to the original input power level. The FBG is tuned to reflect the particular wavelength signal, and the results are analysed on an optical spectrum analyser (Yokogawa AQ6317C). The non-resonant wavelengths travelling beyond the FBG is routed to the multiplexing part of the OADM. All the signals exiting the second circulator are pre-amplified before multiplexed on the original optical fibre link. 2.1 Design and fabrication of broadband transmission filters The non-uniform LPGs used in the CMC were fabricated in PS1500 single-mode fibre (SMF) using a TEM01* - mode carbon-dioxide laser source (Edinburgh Instruments PL2-M). An automated point-by-point LPG fabrication system was used to manufacture azimuthally-symmetric non-uniform LPGs3. During the manufacturing of a LPG, a tungsten halogen 19th International Conference on Optical Fibre Sensors, edited by David Sampson, Stephen Collins, Kyunghwan Oh, Ryozo Yamauchi, Proc. of SPIE Vol. 7004, 70042O, (2008) 0277-786X/08/$18 doi: 10.1117/12.786013
Proc. of SPIE Vol. 7004 70042O-1 2008 SPIE Digital Library -- Subscriber Archive Copy
Fig. 1. Broadband OADM designed for DWDM networks.
broadband source (Ocean Optics LS-1) was used as light source when measuring the output spectrum with an OSA. Fig. 2a illustrates the transmission spectra after two 40 mm non-uniform LPGs were manufactured with a grating period of 457 µm. In Fig. 2a, the spectrum is shown for coupling to the 5th cladding mode. The LPGs were designed using a genetic algorithm (GA), and the induced index change exhibited a hyperbolic-tangent apodization profile as shown in Fig. 2b. A programmable shutter device was used to control the laser during LPG fabrication. The power measured before the shutter device was 1.15 W. A shutter time of 409 ms was used for a single exposure of the fibre to the CO2 laser. The lowest index change that could be achieved within the fibre core for a single exposure of the CO2 laser was approximately 1 × 10−4 . However, a minimum core index change of 3.2 × 10−7 was necessary to realise the reconstructed index modulation profile obtained from the GA. This was not possible, since our index change value was ~311 times higher than the minimum value required. We then obtained a new index modulation profile from the GA generated index change profile, taking into account the 1 × 10−4 index change value obtained for a single CO2 laser exposure on the fibre. 10
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Fig. 2. Transmission spectrum and index modulation profile for the fabricated non-uniform LPGs used in the CMC device. Fig. 2a illustrates the reconstructed spectra obtained with a GA, and the spectra measured after the fabrication of the two complex LPGs. Fig. 2b illustrates the target index profile reconstructed using a GA, and the implemented index profile during LPG fabrication.
2.2 Cladding-mode coupler incorporating non-uniform LPGs The CMC consisted of the two non-uniform LPGs fabricated which is placed in parallel and in close contact, without the need of fusion. Each fibre containing a LPG structure was fitted on a separate mild steel plate containing v-grooves, and
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the fibre was held in place using small magnets. The width of the v-grooves was ~250 µm. The CMC layout was oriented in such a way that the fibres lay in a vertically position. Each plate was fixed on a XYZ translation stage, such that the two parallel fibres could be aligned with each other. In the CMC an index-matching immersion liquid (Cargille 4550) was used for the coupling region between the fibres, in order to expand the coupling region between the fibres and minimize coupler losses. The CMC experimental setup relies on gravity to keep the fibres hanging or standing straight, where after surface tension of the chemical compound will then draw the fibres closer to each other once it is applied. If air acts as the coupling medium between the parallel-placed fibres the losses were found to be ~48 decibels. The immersion liquid used had approximately the same refractive index as the index of the cladding (n=1.4441 at 1550 nm) of the PS1500 fibre used in the CMC. We set the wavelength channel to be centred at 1549.25 nm during our experiments, which is similar to the true resonant wavelength of the FBG illustrated in Fig. 1. The strongest power coupling was obtained when the two LPGs were placed 23 mm apart along the axes of the fibres as illustrated in Fig. 3a and 3b. This resulted in a peak output power of -19.97 dBm at room temperature. When the two LPGs were placed closer than 17 mm, the power coupling results did not improve. When light is injected into the CMC the guided core mode in the first fibre couples to a desired cladding mode centred at a specific wavelength, where after this cladding mode travels in the cladding region and the coupling region between the fibres, exciting a similar cladding mode in the cladding of the second fibre. The LPG in the second fibre allows the cladding mode to couple to the core of the second fibre. -19.6 0
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Fig. 3. Power spectra measured for the strongest and weakest power coupling between fibres when the Cargille 4550 liquid was used. in the coupling region. The signal bandwidth was 20 pm.
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Fig. 4. Power spectra measured for the strongest and weakest coupling between fibres when the distances between the LPGs are increased.
3. AMPLIFICATION OF CLADDING-MODE COUPLER OUTPUT SIGNAL Sufficient light transfer was obtained in the CMC, but the strongest coupling obtained was 19.94 dB weaker than the original input power level. An EDFA was used to restore the CMC output signal to the original input power level. The EDFA consisted of a 980 nm pump diode, a WDM coupler, and a 4.2 m Er3+-doped fibre link. Another 980 nm WDM coupler was used to get rid of the unwanted 980 nm wavelength that was only used during the amplification phase. Fig. 4 illustrates the amplified CMC output power spectra measured when the pump diode power was increased with small increments from 0–85 mW. From Fig. 4 it can be seen that the power spectra exhibit some noise. The noise of the measured power spectra is a combination of the noise from the light source and EDFA that increased during amplification. We were able to obtain the required EDFA gain of 19.94 dB at a pump power of 65.8 mW as shown in Fig. 5.
4. WAVELENGTH SELECTION USING A TUNEABLE REFLECTION FILTER The tuneable reflection filter illustrated in Fig. 1 consists of a 5mm Bragg grating, which is stretched using a custommade traction device. A polymer coating was placed around the fibre containing the FBG to protect it during the experiments. Axial strain sensitivity of a FBG is governed by the equation: ∆λ/λR = (1 - Pe)εax, where ∆λ is a wavelength shift, λR is the FBG resonant wavelength, εax = ∆Lax/Lax is the applied axial strain, and Pe is the effective photoelastic
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Fig. 4. Amplified CMC output signal using an EDFA.
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Fig. 5. Measured EDFA gain as a function of pump power.
constant4. The applied strain is calculated using the axial displacement (∆Lax) and the stressed length (Lax) values. The photoelastic constant is approximately 0.21 for silica SMF4. The FBG used in the experiments reflected 37.2% of the light power centred at its resonant wavelength. When the wavelength of the input light were varied, such that it fell within the bandwidth region of the LPGs used in the CMC, the FBG could be tuned to select a specific wavelength channel as illustrated in Fig. 6 and 7. The FBG bandwidth at full width half maximum was 540 pm. 5
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Fig. 6. Reflection spectra of injected wavelength channels.
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Fig. 7. FBG wavelength shift versus applied strain.
5. CONCLUSION We have demonstrated a OADM device that routes a wide-range of wavelengths from one fibre to another using a broadband CMC, and a tuneable FBG is used to select a specific wavelength channel in a 5.3 nm bandwidth region.
REFERENCES 1 2 3 4
R. Kashyap, Fibre Bragg gratings, Chapters 6–8, Academic Press, New York, 1999. Y. Zhu, C. Lu, B. M. Lacquet, P. L. Swart, S. J. Spammer, “Wavelength-tunable add/drop multiplexer for DWDM using long-period gratings and fibre stretchers,” Opt. Comm. 208, 337-344 (2002). R. Kritzinger, P. L. Swart, A. Booysen, “Design and fabrication of low-loss broadband transmission filters for dense wavelength-division multiplexing networks,” AFRICON ‘07, Windhoek, 2007. A. Iocco, H. G. Limberger, R. P. Salathé, L. A. Everall, K. E. Chisholm, I. Bennion, “Bragg grating fast tunable filter for wavelength division multiplexing,” J. Lightwave Technol. 17(7), 1217-1221 (1999).
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