Open Access
Add to BibSonomyAbstract
To realise novel, low-cost, photonic technologies that can support 100Gb/s Ethernet in next-generation dense wavelength-division-multiplexed metro transport networks, we are developing arrayed photonic integrated circuits that leverage colourless reflective modulators. Here, we demonstrate a single-channel, hybrid reflective electroabsorption modulator-based device, showing error-free 25.3Gb/s duobinary transmission with bit-error rates less than 1 × 10−12 over 35km of standard single-mode fibre. We further confirm the modulator’s colourless operation over the ITU C-band, with a 1.2dB variation in required optical signal-to-noise ratio over this wavelength range.
©2013 Optical Society of America
1. Introduction
The rapid growth in bandwidth-intensive applications and exponentially-increasing energy demands are driving the need for low-cost, high-capacity, energy-efficient network architectures and photonic technologies to ensure the future scaling of access and metropolitan networks. Dense wavelength-division-multiplexing (DWDM) approaches offer terabit-scale fibre capacities in current state-of-the-art infrastructures. However, growing traffic demands are requiring optical DWDM components with reduced power consumption, footprint, and consequently low cost-per-bit.
In order to address these challenges, a potential solution involves the simplification and redesign of the network architecture to reduce the number of costly optical/electrical/optical (O/E/O) conversions. In existing metro nodes, these operations occur when grey (i.e. uncoloured) switch/router interfaces are connected to DWDM transponders to allow for long-reach transmission and wavelength conversion. One possible redesign approach is to realise DWDM interfaces with tuneable lasers; however, host devices do not currently support the tuning functionality to appropriately and dynamically set the wavelength of the transmitters’ lasers. Thus, instead, we adopt a more straightforward, power-efficient IP over DWDM (IPoDWDM) solution [1] that is based on fully colourless, reflective transmitter modules located directly on the routers’ linecards.
Our approach enables wavelength-agile network interfaces with low-cost, colourless, compact photonic integrated circuits (PICs) and low-loss, high-port-count piezoelectric optical matrix switches [2]. The PIC transmitter modules leverage reflective electroabsorption modulator (REAM)-based phase and amplitude modulators seeded with continuous wave (CW) optical carriers, which are generated by a multi-frequency laser (MFL). Assuming that the MFL and the network’s wavelength multiplexers operate athermally and with the same frequency grid, this reflective approach does not require the active wavelength control of expensive tuneable laser sources on the router linecards. Complete reconfigurable optical add-drop multiplexer (ROADM) functionality is provided by the optical matrix switches.
With the aim of realising the targeted low cost, reduced power consumption, and compact footprint goals, the reflective transmitters will use hybrid array-integrated PICs with custom low-power electronic driver arrays [3]. The PICs will support per-lane/wavelength line rates of 25Gb/s or higher, in order to achieve cost-effective 4 × 25Gb/s approaches to 100Gb/s channel provisioning for: (a) short-reach, inter-datacentre links, and (b) wavelength-dynamic IPoDWDM metro transport networks. Error-free operation, denoted by attaining bit-error rates (BERs) less than 1 × 10−12, is a specific requirement for (a), since interfaces in point-to-point links typically do not incorporate forward error correction.
The envisioned system will be required to operate up to the metro-reach distances of ~500km (with compensated links). As a result, the PICs should support modulation formats with greater dispersion tolerance and narrower spectra (consequently enabling higher information spectral density), than the non-return-to-zero on-off keying (NRZ-OOK) format traditionally used in access networks. Duobinary (DB) modulation [4] is suitable for these metro-scale applications, owing to its use of simple direct detection receivers. Conventionally, LiNbO3 Mach-Zehnder modulators (MZMs) have been used to generate DB modulation; however, due to their relatively large size and travelling wave electrode design, MZM components are unsuitable for our targeted compact reflective designs. This approach instead uses array-integrated PIC modules based on REAM devices to enable efficient DB transmission. Thus, for the intended colourless metro applications, we are developing a novel hybrid REAM-based PIC that can support DB modulation at the required single-channel bit rates. This device can then be integrated to yield a high-throughput, multi-channel EAM-based transmitter array to achieve 100Gb/s aggregate rates (i.e. using 10 × 11Gb/s or 4 × 25Gb/s REAM arrays).
Using the single-channel REAM-based PIC, we have demonstrated error-free transmission of 10Gb/s DB signals over 215km of standard single-mode fibre (SSMF) [5]. Here, we show that the same device can also support the transmission of 25.3Gb/s DB modulated signals over 35km of SSMF, with error-free operation (measured BERs less than 1 × 10−12) [6]. The reflective transmitter should also exhibit uniform performance over the targeted wavelength range of the MFL, i.e. colourless operation over the International Telecommunications Union (ITU) C-band. The modulator PIC’s colourless operation is thus verified, demonstrating comparable performance over the anticipated operating frequency range; a small 1.2dB change in required optical signal-to-noise ratio (OSNR) (at a BER of 10−10) is obtained.
Similar hybrid reflective devices have been shown to support differential quadrature phase shift keying (DQPSK) modulation formats [7]. However, to the best of our knowledge, this is the first demonstration of error-free 25Gb/s DB transmission using such a reflective component. These results indicate that our reflective approach has good potential for realising multi-channel arrayed PICs for 100Gb/s data rates, with adequate dispersion tolerance and signal quality performance for our targeted applications.
2. Reflective Metro Node Architecture
To enable dynamic IPoDWDM networks, we have previously shown the architecture of a colourless reflective metro node (Fig. 1 ) that avoids the need for tuneable lasers. Simple, colourless REAM-based transmitter PIC modules are directly mounted on densely-packed router linecards. The MFL generates all the required optical carriers and feeds the reflective modulators. An arrayed waveguide grating (AWG) and N-degree optical matrix switch, connected to all the reflective transmitters, facilitates wavelength and optical path selection. The node supports ‘express’ wavelength channels, minimising the number of unnecessary O/E/O conversions by avoiding router traversals. A colourless, directionless, non-blocking optical space switch [8] provides full ROADM capabilities, including the wavelength ‘add’ and ‘drop’ functionalities. The complete metro node design creates a colourless, wavelength-agile interface with similar functionality as currently available in state-of-the-art nodes.
Fig. 1 High-level schematic of the colourless reflective metro node architecture (AWG: arrayed waveguide grating; EDFA: erbium-doped fibre amplifier; MFL: multi-frequency laser; Rx: receiver; Tx: transmitter).
3. Device description
The reflective DB PIC (Fig. 2 ) features an array of InGaAs/InAlAs multiple quantum well (MQW) REAMs [9], which is based on a ridge waveguide structure and incorporates mode expanders to allow for hybrid integration with silica waveguides. The MQW core provides high modulation efficiency with minimum saturation impairments at high input powers. The silicon REAM daughter board also uses an integral load resistor array.
The DB PIC also includes a silica planar motherboard, which contains a hybrid Michelson interferometer circuit with a 2 × 2 multimode interference (MMI) coupler, as well as alignment structures for the input/output fibres. The phase difference between the two interferometer arms is adjusted using heater elements, which are deposited on the motherboard. The silica motherboard features 0.75% Δn (refractive index contrast) optical waveguides with dimensions of 6μm × 6μm, which yields a mode that is larger than the one from the REAM chip. Thus, the coupling losses associated with the current PIC prototype are relatively high. In future generations of the device, the coupling loss can be addressed by moving to 1.5% Δn waveguides, in order to provide a better match to the EAM mode and minimise overall loss. This would also result in a more compact PIC, since the higher Δn would allow the use of tighter waveguide bends with radii of 2-3mm.
The relatively small size of the REAM array chip (approximate length of 0.2mm) permits further integration of multiple DB transmitters on a single compact board. Additionally, the reflective design benefits from shorter high-speed drive traces, as these are taken from the edge of the motherboard, hence minimising RF losses and crosstalk. Figure 2 shows the block schematic of the DB modulator PIC and a photograph of the current packaged prototype.
The current prototype PIC is packaged with GPO connectors for RF signal drive purposes, as well as polarisation-maintaining fibre with FC/APC connectors. The polarisation of the input CW carrier is fixed by aligning to this fibre’s axis. In this reflective IPoDWDM approach, the carrier source is local to the reflective transmitter, so a means of polarisation control is feasible, unlike in remote modulation applications such as WDM-PON. However, a future goal is to enable fully polarisation-independent operation of the modulator PIC by optimising the device and package birefringence. The polarisation dependence of the PIC can be reduced using a number of techniques; for example, by using a material with the appropriate thermal expansion coefficient to tailor the overcladding of the planar lightwave circuit, the birefringence of the resulting optical planar waveguides can be minimised [10]. We anticipate that the dominant issue for polarisation control will then arise from the active components, i.e. the REAM arrays. The REAM array chip can be designed for low polarisation by tailoring the tensile strain in the quantum wells. Further, it is necessary to optimise the coupling between the REAM chip and the silica planar motherboard, since the mode size can differ slightly for the two polarisation states. A fibre-to-REAM measurement shows that the dynamic polarisation dependence of the REAMs used in the current PIC prototype is less than 1dB. In future designs, if the measured polarisation variation is too large, polarisation diversity on chip [11] can be employed where the transmitter splits the signal using a compact polarisation splitter and the two orthogonal polarisation components are modulated separately. Such a circuit can still be realised within a small footprint PIC and may offer cost advantages in enabling higher yields for the REAM array device compared to a fully polarisation-insensitive design.
4. Experimental setup
The experimental setup for the modulator PIC to generate 25.3Gb/s DB modulated signals is shown in Fig. 3 . An external cavity laser (‘C-band CW source’) generates a CW carrier, which is injected into the device. The optical input power is maintained constant at + 8dBm.
Fig. 3 Experimental setup to evaluate the performance of the modulator PIC’s 25.3Gb/s DB signal (AWG: arrayed waveguide grating; BERTs: bit-error-rate testers; CRU: clock recovery unit; DEMUX: demultiplexer; EDFA: erbium-doped fibre amplifier; LPF: low-pass filter; MUX: multiplexer; OSA: optical spectrum analyser; PD: photodiode; PPGs: pulse pattern generators; VOA: variable optical attenuator).
The heater element in one arm of the interferometer is biased to result in an overall π phase shift to the signal in one arm of the device compared to the other after a double pass through the interferometer. The 25.3Gb/s drive signal is generated by electronically multiplexing four 6.325Gb/s NRZ signals (27-1 pseudo-random bit sequence (PRBS)) from four pulse pattern generators (PPGs). One REAM is driven with the 25.3Gb/s output data from the multiplexer (MUX) (with a 3Vpp output swing from a RF amplifier), while the second REAM is driven with the MUX’s inverted 25.3Gb/s data signal. In this way, the modulator operates in ‘push-pull’ configuration. Before modulating the REAMs, the 25.3Gb/s data signals are filtered using fourth-order Bessel-Thomson low-pass filters (LPFs) with a −3dB bandwidth of 7GHz (conventionally, LPFs to create duobinary signals are set at a quarter of the aggregate bit rate). The quasi-three-level waveforms generated by low-pass filtering are used to drive the REAMs between their high, intermediate, and low reflecting states. Device emission is suppressed in the intermediate (‘zero’-bit level) case due to destructive interference at the MMI. Owing to the relatively high insertion loss of the current prototype device (approximately 25dB), the DB modulated signal is then amplified by an erbium-doped fibre amplifier (EDFA). In order to increase the reach in next-generation designs, this high loss could be reduced by optimising coupling losses (as discussed above) and monolithically integrating the REAM structures with semiconductor optical amplifiers (SOAs) to achieve lossless performance [12]. The modulator’s transmission performance is assessed using SSMF lengths varying from 0km (back-to-back (B2B)) to 41km.
On the receiver side, a second EDFA acts as a preamplifier to overcome fibre losses. The output of this EDFA is connected to one input arm of a 2 × 2 coupler, with a third EDFA connected to the other input arm. The third EDFA acts as an amplified spontaneous emission (ASE) source to degrade the OSNR at the receiver; the OSNR is controlled in the setup by adjusting a variable optical attenuator (VOA). One output arm of the 2 × 2 coupler is connected to an optical spectrum analyser (OSA) for OSNR measurements (with 0.1nm reference bandwidth), while the other arm is connected to an AWG with an average −3dB optical bandwidth of 22.1GHz per channel, and 50GHz channel spacing across the C-band. The selected wavelength channel is then coupled to a PIN photodiode (PD) with 50GHz bandwidth (the mismatch between the receiver bandwidth and the signal bit rate is due to limited available hardware). The input power to the PD is kept constant and sufficiently high above the thermal noise floor such that the OSNR degradation is the dominant impairment in the system. The PD’s 25.3Gb/s electrical output signal is demultiplexed using a 1:4 demultiplexer (DEMUX) to return the signal to the four 6.325Gb/s tributary channels, which are connected to four bit-error-rate testers (BERTs) to monitor the BER of the entire system. The PPGs and MUX are synchronised using a common clock, while the DEMUX and BERTs use a 25.3Gb/s clock recovery unit (CRU) to extract the clock for BER analysis.
5. Experimental results
Using the above setup, the device first generates 25.3Gb/s DB modulated signals with the CW carrier set at λ = 1542.54nm. Figure 4(a) provides the bandwidth-resolution-limited (0.01nm) optical spectra of the CW carrier and of the 25.3Gb/s duobinary signal after the modulator.
Fig. 4 (a) Optical spectra of the CW carrier and of the 25.3Gb/s duobinary modulated signal (λ = 1542.54nm). (b) The measured BER with respect to the OSNR for the DB modulator operating at λ = 1549.65nm.
The laser is then set to generate a CW carrier at λ = 1549.65nm, which is injected in the DB modulator. The OSNR is degraded by increasing the ASE power from the third EDFA, and the BER is measured with respect to the OSNR for a representative set of transmission lengths using several spans of SSMF (Fig. 4(b)). The measured OSNR penalty for the B2B case at a BER of 10−3 is within 2dB of the expected theoretical value for DB modulation assuming the use of a MZM and with similar receiver bandwidth [13]. This penalty is likely attributed to minor drive waveform imperfections, causing slight patterning and amplitude spread on the ‘one’-bit level and thereby resulting in some closure in the optical eye diagram.
As expected, we observe the reduction in required OSNR associated with DB transmission as the distance increases initially from 0km to 16km, and the subsequent increase in OSNR for distances beyond 16km. Error-free operation (attaining an average BER less than 1 × 10−12) is maintained for distances up to 35km. A significant change in the slope of the BER curve is observed at 41km, indicating the likely onset of an error floor below a 10−9 BER. This measurement was limited by the maximum achievable OSNR in the experimental setup. This performance degradation arises from chromatic dispersion, which causes intersymbol interference and subsequent eye closure. This value is consistent with previous results obtained at 10Gb/s using the same device [5], since the dispersion-limited reach at 25.3Gb/s is reduced by approximately the square of the bit rate.
The optical eye diagrams of the 25.3Gb/s DB modulated signals for six transmission lengths of SSMF (λ = 1549.65nm) is shown in Fig. 5 . A substantial eye opening is achieved at all distances, with the exception of 41km, where the transmitted eye is significantly degraded.
Fig. 5 Optical eye diagrams of the modulator at 25.3Gb/s after the indicated SSMF transmission lengths (λ = 1549.65nm).
Similarly, the modulator PIC’s colourless nature is confirmed by evaluating the BER (at 25.3Gb/s) as a function of the measured OSNR at the greatest error-free distance (35km), for several C-band wavelengths on the 50GHz ITU grid. The BER curves for four example wavelength channels between 1535.04nm and 1560.61nm are provided in Fig. 6(a) . Further, Fig. 6(b) provides the OSNR values required to obtain a BER of 1 × 10−10 for eight uniformly-spaced channels. Comparable performance is achieved within the device’s anticipated operating spectrum, with a small 1.2dB differences in required OSNR for this greater than 25nm span. Measurements at wavelengths shorter than 1535nm were not feasible due to the vicinity of the band edge of the REAM, which results in the device’s high insertion loss.
Fig. 6 (a) The measured BER with respect to OSNR at 35km for the DB modulator at four wavelength channels spanning the C-band. (b) The OSNR (at BER = 1 × 10−10) as a function of wavelength, validating the DB modulator’s colourless operation at 35km.
6. Conclusion
Simplifying the network architecture and avoiding the need for expensive, thermally-stabilised tuneable lasers will result in significant energy savings for future data-centric applications. We adopt a novel reflective approach that can effectively enable 100Gb/s Ethernet by leveraging array-integrated transmitters with custom power-efficient driver circuitry [3]. A key enabling technology for the envisioned colourless transmitter is a hybrid REAM-based duobinary modulator. Here, we report on a single-channel duobinary modulator PIC that can support 25.3Gb/s transmission with error-free operation over 35km of SSMF. Relatively uniform performance with respect to the required OSNR over the ITU C-band is demonstrated, attesting to the device’s colourless operation. This work is the first step to realising energy-efficient, multi-lane, arrayed PICs for wavelength-dynamic, low cost-per-bit 100GbE services and applications in next-generation metro transport and access networks.
Acknowledgments
This work was supported by the European Commission through the C3PO project (contract 257377) under the FP7 ICT Programme. Tyndall also acknowledges funding by Science Foundation Ireland (Grant 06/IN/I969).
References and links
1. P. D. Townsend, A. Clarke, P. Ossieur, D. W. Smith, A. Borghesani, D. G. Moodie, I. F. Lealman, X. Z. Qiu, J. Bauwelinck, X. Yin, K. Grobe, B. T. Teipen, R. Jensen, N. Parsons, and E. Kehayas, “Towards colourless coolerless components for low power optical networks,” in Proc. 37th European Conf. on Optical Commun. (ECOC), Geneva, Switzerland, Sep. 2011, paper Tu.LeSaleve.4.
2. A. M. Clarke, A. Borghesani, D. W. Smith, P. Ossieur, P. D. Townsend, R. Jensen, and N. Parsons, “Demonstration of wavelength agile metro node using reflective colorless components,” in Proc. Optical Fiber Commun. Conf. (OFC), Los Angeles, CA, Mar. 2011, paper OMN2.
3. R. Vaernewyck, J. Bauwelinck, X. Yin, R. Pierco, J. Verbrugghe, G. Torfs, Z. Li, X. Z. Qiu, J. Vandewege, R. Cronin, A. Borghesani, and D. Moodie, “A 113 Gb/s (10 × 11.3 Gb/s) ultra-low power EAM driver array,” in Proc. 38th European Conf. on Optical Commun. (ECOC), Amsterdam, Netherlands, Sep. 2012, paper Mo.2.B.2.
4. D. Penninckx, M. Chbat, L. Pierre, and J.-P. Thiery, “The phase-shaped binary transmission (PSBT): A new technique to transmit far beyond the chromatic dispersion limit,” IEEE Photon. Technol. Lett. 9(2), 259–261 (1997). [CrossRef]
5. A. Naughton, P. Ossieur, C. Antony, D. W. Smith, A. Borghesani, D. G. Moodie, G. Maxwell, P. Healey, and P. D. Townsend, “Error-free 10Gb/s duobinary transmission over 215km of SSMF using a hybrid photonic integrated reflective modulator,” in Proc. Optical Fiber Commun. Conf. (OFC), Los Angeles, CA, Mar. 2012, paper OW4F.3.
6. C. P. Lai, A. Naughton, P. Ossieur, C. Antony, D. W. Smith, A. Borghesani, D. G. Moodie, G. Maxwell, P. Healey, A. Poustie, and P. D. Townsend, “Demonstration of error-free 25Gb/s duobinary transmission using a colourless reflective integrated modulator,” in Proc. 38th European Conf. on Optical Commun. (ECOC), Amsterdam, Netherlands, Sep. 2012, paper We.1.E.4.
7. I. Kang, S. Chandrasekhar, C. Kazmierski, M. Rasras, and N. Dupuis, “1650-km transmission of 50-Gb/s NRZ and RZ-DQPSK signals generated using an electroabsorption modulators-silica planar lightwave circuit hybrid integrated device,” in Proc. Optical Fiber Commun. Conf. (OFC), San Diego, CA, Mar. 2010, paper OMJ4.
8. R. Jensen, A. Lord, and N. Parsons, “Colourless, directionless, contentionless ROADM architecture using low-loss optical matrix switches,” in Proc. 36th European Conf. on Optical Commun. (ECOC), Turin, Italy, Sep. 2010, paper Mo.2.D.2.
9. D. G. Moodie, D. C. Rogers, P. J. Cannard, A. Borghesani, C. W. Ford, R. Firth, R. Cronin, M. J. Robertson, D. W. Smith, L. Ponnampalam, C. Renaud, A. J. Seeds, M. Thakur, T. Quinlan, and S. Walker, “Photodiodes and reflective electroabsorption modulators for mm-wave and UWB applications,” in Proc. European Workshop on Photonic Solutions for Wireless, Access, and in-House Networks, Duisburg, Germany, May 2009.
10. A. Kilian, J. Kirchof, G. Przyrembel, and W. Wischmann, “Birefringence free planar optical waveguide made by flame hydrolysis deposition (FHD) through tailoring of the overcladding,” J. Lightwave Technol. 18(2), 193–198 (2000). [CrossRef]
11. S. Smolorz, E. Gottwald, H. Rohde, D. Smith, and A. Poustie, “Demonstration of a coherent UDWDM-PON with real-time processing,” in Proc. Optical Fiber Commun. Conf. (OFC), Los Angeles, CA, Mar. 2011, paper PDPD4.
12. D. Smith, I. Lealman, X. Chen, D. Moodie, P. Cannard, J. Dosanjh, L. Rivers, C. Ford, R. Cronin, T. Kerr, L. Johnston, R. Waller, R. Firth, A. Borghesani, R. Wyatt, and A. Poustie, “Colourless 10Gb/s reflective SOA-EAM with low polarization sensitivity for long-reach DWDM-PON networks,” in Proc. 35th European Conf. on Optical Commun. (ECOC), Vienna, Austria, Sep. 2009, paper 8.6.3.
13. P. J. Winzer and R.-J. Essiambre, “Advanced optical modulation formats,” Proc. IEEE 94(5), 952–985 (2006). [CrossRef]
