IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004
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Tunable Dispersion and Dispersion Slope Compensator Using Novel Gires–Tournois Bragg Grating Coupled-Cavities S. Doucet, R. Slavík, and S. LaRochelle
Abstract—We report on the first realization of lattice-coupled all-fiber Gires–Tournois etalons for chromatic dispersion (CD) compensation. CD tunability is obtained by cascading two fiber Bragg grating superstructures. We show that the use of one superstructure with lattice-coupled cavities leads to a 60% tunability increase compared to single cavity designs. Dispersion slope compensation is proposed and demonstrated using a temperature gradient. Index Terms—Chromatic dispersion (CD) compensation, fiber Bragg gratings, Gires–Tournois, multichannel dispersion compensators, optical fiber devices, optical fiber filters. Fig. 1. Device configuration.
I. INTRODUCTION
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IGH-BIT-RATE dense wavelength-division-multiplexing optical systems require tunable multichannel chromatic dispersion (CD) compensators. Current state-of-the-art devices include tunable higher order mode-based CD compensating fibers [1], integrated-optics coupled ring microresonators [2], superimposed chirped fiber Bragg gratings [3], and Gires– Tournois etalons (GTEs) including all-fiber-based devices [4] and thin film multicavity devices [5]. Compared to other technologies, all-fiber GTE are compact and low-loss devices with tuning capabilities of CD and CD slope [6]. A GTE tunable CD compensator is formed with a set of at least two GTE elements placed in cascade through circulator(s). For two GTEs, both GTEs have the same absolute value, but opposite sign, of CD slope. The total CD is then constant over the channel bandwidth, with a value dependent on the relative spectral position of the GTEs. All-fiber distributed Gires– Tournois etalon (DGTE) compensators, based on two superimposed chirped fiber Bragg gratings (CFBGs), were proven to be promising devices with third-order CD compensation capabilities. However, the main drawback of these devices is the low values of CD compensation achievable with a single set of two GTE elements ( 200 ps/nm [4]). A cascade of a higher number of GTEs increases complexity, loss, and cost. More advantageous would be direct lattice-coupling of DGTEs Manuscript received May 6, 2004; revised June 23, 2004. This work was supported by the Quebec Government, by the Canada Research Chair Program, and by TeraXion Inc. S. Doucet and S. LaRochelle are with the Centre d’Optique, Photonique et Laser (COPL), Département de Génie Électrique et de Génie Informatique, Université Laval, QC G1K 7P4, Canada (e-mail:
[email protected]). R. Slavík was with the Centre d’Optique, Photonique et Laser (COPL), Département de Génie Électrique et de Génie Informatique, Université Laval, QC G1K 7P4, Canada. He is now with IREE, Academy of Sciences of the Czech Republic, 182 51 Praha 8, Kobylisy, Czech Republic. Digital Object Identifier 10.1109/LPT.2004.834882
on the same piece of fiber by superimposing a larger number of CFBGs. However, one must overcome technological difficulties originating in the required phase synchronism among the lattice-coupled cavities formed by the superimposed CFBGs. In this letter, we demonstrate an all-fiber multichannel dispersion compensator based on lattice-coupled DGTEs realized by the superimposition of three CFBGs. We also report on dispersion slope compensation using temperature gradient tuning. II. DESIGN AND PRINCIPLE OF OPERATION A GTE is basically a Fabry–Pérot etalon with a 100% backreflector. The GTE is, thus, an all-pass filter, operating in reflection, that has spectrally periodic group delay variations with a free spectral range (FSR) fixed by the cavity length. The amplitude and shape of the group delay response, and consequently the CD, are both determined by the reflectivity of the input mirror. With only one degree of freedom, optimization of the device is limited, and the requirement for low group delay ripples (GDR, 3 ps [4]) generally fixes the maximum CD compensation that can be achieved. In order to circumvent this limitation, lattice-coupled cavities might be used [7]. In such configurations, one or several additional mirrors are added, thus offering greater flexibility in the design. In this work, we consider a DGTE set in which one DGTE is formed by two coupled-cavities [DGTE (a)] while the other DGTE of the cascade is single-cavity device [DGTE (b)] (Fig. 1). The structure is designed such that DGTE (b) has linear CD over 50% of the FSR, while the other one has opposite CD slope covering 90% of the FSR (Fig. 2). DGTE (b) is then anchored to a 50-GHz frequency grid and DGTE (a) provides CD tuning. To obtain opposite CD slope, the operating bandwidth of each channel corresponds respectively to a maximum of the group delay (frequencies resonating in the DGTE) for DGTE (b) and to a minimum for DGTE (a). Fig. 2 shows the result of cascading
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004
Fig. 2. (a) and (b) Operating principle with uniform temperatures and (c) with temperature gradient.
the two DGTEs. Fig. 1 shows also two thermoelectric elements (TE) on the DGTE (a) holder. This allows the application of different temperature settings to the DGTE (a), with or without . A temperature gradient causes a temperature gradient slight variation in the FSR. As pointed-out in [6], FSR variations in the DGTEs set leads to third-order CD compensation as the relative spectral position of the DGTE elements varies across the successive channels [Fig. 2(c)]. The design was performed in two steps. First, we fitted the desired CD response with a characteristic polynomial [7] using a simplex optimization algorithm. Then, using the step-down algorithm [7], we found the required CFBG’s reflectivities and the corresponding refractive index modulations. Then, the relative positions of the CFBGs were sought by fitting the DGTE response to the characteristic polynomial. This is necessary because the phase of a CFBG reflection coefficient depends on the strength and chirp of the grating. In the second step, we calculated the response of the DGTE cascade, tuned the coupledcavities GTE, and determined the GDR. We then adjusted the maximum CD value, with the CD slope parameter, to keep the GDR amplitude below 5 ps. We found that with a two-cavity DGTE (a), we can get CD tuning as high as 330 ps/nm and for three cavities it was 460 ps/nm. For both configurations, DGTE (b) in the set is only a single-cavity GTE. To achieve a dispersion tuning range of 1000 ps/nm, the cascade would need two DGTEs with five and three stages, respectively. This would be very challenging for this technology based on superimposed FBGs. The main application of this device is, therefore, compensation of the residual dispersion caused by unmatched dispersion slopes at the end of dispersion-managed fiber links. III. EXPERIMENT In the experimental demonstration, we made the DGTE structures shown in Fig. 1 using the following design parameters: reflectivities of 95.0%, 38.4%, and 1.6% for DGTE (a) and 99.0% and 7.0% for the DGTE (b). The total grating length was 25 mm, which covered a 7.5-nm spectral range or 18 channels. The stronger gratings of both DGTE were apodized with a tanh function over the first and last 1.7 mm (10%–90%). The selected two-cavity design has in-band amplitude variations less than 0.7 dB at the maximum CD setting. The maximum insertion loss of the grating cascade over the whole tuning range was 1.45 dB and the three-port circulator loss was 1.5 dB for each device. A stronger grating in DGTE (a) would reduce amplitude variations to 0.15 dB and make it a truly all-pass filter but the present design
Fig. 3. Group delay measured (circles) and calculated (solid line) of one period of the coupled-cavity DGTE (a).
Fig. 4. CD temperature tuning for all channels (from fits on 25-GHz spectral windows).
was chosen in order to be in a writing regime where the optical fiber photosensitive response is mostly linear with exposure time. The total insertion loss of the cascaded device, including circulators, could be reduced to 2.5 dB by using deuterium loaded fibers, a 20-dB backreflector, and a four-port circulator (commercially available with 1.8-dB loss). The cavity length was 2 mm to get a 50-GHz FSR with a 35-nm length difference between the two cavities of DGTE (a). To write CFBGs, we used a scanning method [8] with a chirped phase mask (2.5 nm/cm, TeraXion Inc.). Between the successive exposures, the phase mask was moved by 2 mm and this displacement was monitored with a precision of 10 nm. The strongest gratings are written first in the fibers. To fabricate the lattice-coupled structure, we perform calibrations of the exact phase mask displacement on a grating segment outside the bandwidth of interest. During the calibration step, the writing process is stopped and the group delay measurement is performed. Once the phase mask position has been set, writing of the weaker grating is performed. Fig. 3 shows the measured group delay response over one channel and the theoretical fit calculated with reflectivities of 95%, 46.3%, and 1.8%. The GDR due to fabrication imperfections was 4 ps for DGTE (a) and 3 ps for DGTE (b). Fig. 4 shows CD values of the first ten channels with different uniform temperature settings (without gradient). The responses of the last eight channels suffered from phase distor-
DOUCET et al.: TUNABLE DISPERSION AND DISPERSION SLOPE COMPENSATOR
Fig. 5. Measured response of ten channels (a) GD with T =z = 1 C/mm, (b) dispersion with T =z = 1 C/mm, (c) GDR with T =z = 1 C/mm, and (d) FSR between the channels.
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sion due to fabrication errors and are not displayed here. At the maximum setting of 330 ps/nm, the CD value varies between 300–450 ps/nm for the ten channels. Fig. 5 demonstrates CD slope tunability with temperature gradients. The CD and the maximum GDR of each channel were evaluated over a bandwidth of 25 GHz. Peaks 3 and 5 are perturbed by CFBGs imperfections, which consequently affect the optical cavity lengths leading to variations of FSR and CD. The tunability is limited by the maximum temperature gradient of our experimental setup ( 1 C/mm). To get higher values, one should increase the temperature gradient or use CFBGs with smaller chirp, which would increase the spatial distance between the resonating fields corresponding to neighboring channels. Tunability could also be improved with more elaborate temperature control, which could apply an arbitrary temperature profile resulting in a channel-perchannel tunable CD compensator. Bit-error-rate (BER) measurements of 10-Gb/s pseudorandom nonreturn-to-zero data ( nm) were performed to evaluate the CD compensator. The signal was propagated in 80 km of Corning SMF-28 fiber (CD of 1360 ps/nm). Without any compensation, the BER penalty at was of dB (Fig. 6). Subsequently, we adjusted the DGTE compensator to ps/nm and compared the result with an available 100-ps/nm dispersion-compensating fiber (DCF) module. Fig. 6 shows that the performance of the DGTE (penalty reduced to dB) is comparable to the DCF module (penalty reduced to dB). We further increased the CD of the DGTE to the maximum CD compensation ( ps/nm) and the BER penalty was reduced to dB. IV. CONCLUSION A tunable multichannel CD compensator was fabricated using three superimposed CFBGs to create a coupled-cavity Gires–Tournois. This design increases the dispersion tuning range by 60% compared to approaches based on single-cavity elements. The experiments showed that the reflectors’ coupling strengths and the cavities’ phases can be simultaneously and
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Fig. 6. BER measurement for propagation through 80 km of SMF-28 fiber. Points: Back-to-back. Cross: No compensation. Square: DCF module ( 100 ps/nm). Triangle and diamond: DGTE with temperature of 44 C ( 100 ps/nm) and 50 C ( 330 ps/nm), respectively.
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adequately controlled during the writing of the Bragg grating superstructure. This first demonstration of all-fiber lattice-coupled cavities Gires–Tournois open the door to more complex designs in which a larger number of cavities could be used to achieve higher values of CD compensation. Although these structures are challenging to prepare using a simple chirped phase mask, they could be produced in volume using more complex state-of-the-art phase mask. We also demonstrated an attractive approach to achieve thirdorder CD compensation by applying a temperature gradient to the tunable DGTE. Due to its distributed nature, this device offers more tuning flexibilities compared to its bulk-optics counterpart or planar waveguide resonators. This could allow flexible CD compensation on a per-channel basis using more complex temperature profiles along the fiber axis. REFERENCES [1] S. Ramachandran, B. Mikkelsen, L. C. Cowsar, M. F. Yan, G. Raybon, L. Boivin, M. Fishteyn, W. A. Reed, P. Wisk, D. Brownlow, R. G. Huff, and L. Gruner-Nielsen, “All-fiber grating-based higher order mode dispersion compensator for broad-band compensation and 1000-km transmission at 40 Gb/s,” IEEE Photon. Technol. Lett., vol. 13, pp. 632–634, June 2001. [2] G. Lenz and C. K. Madsen, “General optical all-pass filter structures for dispersion control in WDM systems,” J. Lightwave Technol., vol. 17, pp. 1248–1254, July 1999. [3] R. Lachance, S. Leliévre, and Y. Painchaud, “50 and 100 GHz multichannel tunable chromatic dispersion slope compensator,” in Proc. Optical Fiber Communication (OFC 2003), GA, Mar. 2003, Paper TuD3, pp. 164–165. [4] X. Shu, K. Sugden, P. Rhead, J. Mitchell, I. Felmeri, G. Lloyd, K. Byron, Z. Huang, I. Khrushchev, and I. Bennion, “Tunable dispersion compensator based on distributed Gires–Tournois etalons,” IEEE Photon. Technol. Lett., vol. 15, pp. 1111–1113, Aug. 2003. [5] D. J. Moss, M. Lamont, S. McLaugthlin, G. Randall, P. Colbourne, S. Kiran, and C. A. Hulse, “Tunable dispersion and dispersion slope compensators for 10 Gb/s using all-pass multicavity etalons,” IEEE Photon. Technol. Lett., vol. 15, pp. 730–732, May 2003. [6] X. Shu, K. Chisholm, and K. Sugden, “Design and realization of dispersion slope compensators using distributed Gires–Tournois etalons,” in Proc. Eur. Conf. Optical Communication (ECOC 2003), Rimini, Italy, Sept. 2003, Paper We4.P57, pp. 670–671. [7] C. K. Madsen and J. H. Zhao, Optical Filter Design and Analysis. New York: Wiley, 1999, ISBN-0-471-18373-3. [8] F. Ouellette, J. F. Cliché, and S. Gagnon, “All-fiber devices for chromatic dispersion compensation based on chirped distributed resonant coupling,” J. Lightwave Technol., vol. 12, pp. 1728–1738, Oct. 1994.
