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We present a novel parallel-ring-resonator tunable laser monolithically integrated with an InGaAlAs electroabsorption modulator. The fabricated tunable laser exhibits stable wavelength tuning with a step of 200 GHz over a wide tuning range of 35 nm, achieved with a single electrode control. The variation in the laser output power with wavelength tuning is less than 1 dB even at an injected tuning current of 20 mA. Clear eye openings at 25 Gbit/s with a dynamic extinction ratio of more than 10 dB are demonstrated over a wavelength range of 25.7 nm with a constant voltage swing of 2 V at 45°C. Error-free operation is confirmed under the same operating conditions.
©2012 Optical Society of America
1. Introduction
With the explosive growth of the internet and its related services, the excessive increase in power consumption of large-capacity routers has become a major concern. Optical packet-switched networks (OPS-NWs) that rely on optical packet routers are considered a potential solution that can maximize the flexibility and throughput of the network owing to the packet-level granularity [1,2]. We have previously constructed an 8 × 8 hybrid optoelectronic router (HOPR) prototype that combines the strength of optical and electrical technologies [3]. We demonstrated error-free routing of 10 Gbit/s asynchronous arbitrary-length optical packets together with high-level functions such as QoS and multicasting, while achieving low power consumption and low latency.
One of the key building blocks of HOPR is an N × N optical switch that routes the optical packets to the desired output ports [4,5]. Figure 1 shows a schematic of the wavelength-routing switch to handle 100-Gbit/s (25-Gbit/s × 4λ) optical packets. Within the switch, the label-swapped packet first enters a tunable wavelength converter (TWC) consisting of a tunable laser and an array of four burst-mode receiver front-ends (APD-TIAs), together with driver amplifiers, and EAMs. The tunable laser output is divided into four parts, modulated by the EAMs, and connected to four planes of wavelength-cyclic AWGs. Control signals from the scheduler tune the wavelength of the laser to enable the desired path across the AWGs. Having the AWGs equipped with a tunable laser at each input port allows non-blocking N × N wavelength-based switching. The packet data encoded onto the tunable laser output is routed to the desired AWG output port, then received by a fixed wavelength converter (FWC) consisting of the APD-TIA equipped with a driver amplifier, and an EA-DFB laser, to convert the signal wavelength back to the original input wavelength.
Within the TWC, the wavelengths of optical packets are changed on a packet-by-packet basis, thus the lasing wavelength of the tunable laser should be tuned within several nanoseconds. So far, several types of tunable lasers have been demonstrated, including sampled-grating /super-structure-grating distributed-Bragg-reflector (SG/SSG DBR) lasers [6,7], digital supermode (DS) DBR lasers [8], modulated grating Y-branch (MGY) lasers [9], and grating coupler with rear sampled grating reflector (GCSR) lasers [10]. These lasers have a common tuning mechanism based on the free-carrier plasma effect produced by current injection, which changes the refractive indices of the tuning sections within a few nanoseconds. However, the transient thermal effect that causes the lasing wavelength to drift remains a problem [11]. The wavelength drift has a much longer response time (millisecond order) compared to the mechanism employed for fast tuning. In our previous work, a double-ring-resonator tunable laser (DRR-TL) [12] was used as a high-speed tunable laser in HOPR [5]. Compared to the DBR-type lasers [6–10], the ring-resonator filters employed in the cavity of the DRR-TL exhibit superior filtering characteristics in addition to the advantage of having a compact structure [13,14]. These characteristics include a narrower transmission bandwidth with a Lorentzian-type filter response and an infinite number of resonant peaks in the transmission spectrum. The rapid (less than 11 ns) and stable wavelength tuning (less than 5 GHz wavelength drift) achieved by the low tuning current of the DRR-TL enables the realization of a fast, scalable, and low-power N × N optical switch [12]. A shortcoming of the DRR-TL is that the output power varies during tuning due to the free-carrier absorption caused by current injection into the tuning section.
In this paper, we present a novel compact tunable transmitter equipped with a new TL that provides a stable output power during wavelength tuning. The transmitter consists of a monolithically-integrated parallel-ring-resonator tunable laser (PRR-TL) and an InGaAlAs EAM. The PRR-TL provides a small power variation of less than 1 dB over a wide tuning range of 35 nm. Under semi-cooled conditions, a dynamic extinction ratio of more than 10 dB at 25 Gbit/s is obtained with a constant voltage swing of 2 V over a tuning range of 25.7 nm. Error free operation is demonstrated under these conditions.
2. Laser structure and fabrication
Figure 2 shows a photograph of the fabricated PRR-TL. The device size is 1.1 x 0.35 mm2. The laser consists of separate sections for gain, phase, and filtering. The laser cavity of the PRR-TL is defined by a cleaved-facet used as the front mirror and a reflective filter section. In the filter section, two ring resonators are placed in parallel with slightly different free spectral ranges (FSRs). Each ring resonator plays the role of a wavelength selective filter as well as a loop-mirror in a Mickelson interferometer that is a part of the laser cavity.
In our previous work [12], two ring resonators were cascaded as shown in Fig. 3(a) . In that DRR filter, the wavelength tuning range Δλ is expanded by the Vernier effect. Δλ is expressed as
where FSR1 and FSR2 are the FSR of Ring1 and Ring2, respectively. M and N are the number of resonant peak spacings within Δλ of Ring1 and Ring2, respectively, and are written as,
Fig. 3 Two filter configuration. (a) Double-ring resonator (DRR). (b) Parallel-ring resonator (PRR).
On the other hand, in the PRR configuration (Fig. 3(b)), the filter produces constructive (in-phase) and destructive (out-of-phase) interferometric characteristics in the reflection spectrum as the ring resonator raises a phase difference of π between its peak wavelengths. Fig. 4 shows the calculated reflection spectrum from the PRR filter section. In this calculation, FSR1 and FSR2 are assumed to be 200 GHz and 222 GHz, respectively. The interferometric characteristics can be seen repeated every 2Δλ due to the difference between M and N as given in Eq. (4). Thus, the tuning range is twice that of the DRR filter with the same FSRs.
In the PRR filter, the light passes through the ring resonator only once compared to four times in the DRR filter. Therefore, the PRR filter is expected to have a larger reflectivity than the DRR filter. Figure 5(a) shows the calculated filter loss for the PRR and DRR filters as a function of the propagation loss of the waveguide composing their ring resonators. In this calculation, FSR1 is assumed to be 200 GHz with an M of 10 for the DRR filter, and an M of 5 or 10 for the PRR filter. The filter loss increases with propagation loss for both the DRR and PRR filters, however the filter loss is clearly lower for the PRR filter. One possible disadvantage of using the PRR configuration may be the low wavelength selectivity due to the small count of light pass through the ring resonator. The peak-reflectivity difference ΔR between the resonant peak and its adjacent peak (Fig. 4) must be high enough for stable lasing, because small ΔR results in a small side-mode suppression ratio (SMSR) of the tunable laser. Figure 5(b) shows the calculated ΔR value of the PRR and DRR filters as a function of the propagation loss of their waveguides. FSR1 and M are assumed to be the same in the calculation shown in Fig. 4(a). Compared to the DRR filter, the PRR filter with the same M has a small ΔR. However, the PRR filter, which provides twice the tuning range of the DRR filter, can be set to have half the value of M of the DRR filter while keeping the same Δλ as could be seen from Eq. (1). In this case, the PRR filter provides a lager ΔR than the DRR filter as shown in Fig. 5(b), hence improving the filter performance.
Fig. 5 Calculated (a) the filter loss and (b) ΔR of the PRR and DRR filter as a function of the propagation loss of the waveguide section.
For the PRR-TL section, a stack-layer structure is used [12], in which an InGaAsP MQW layer with upper and lower separate-confinement hetero-structure (SCH) layers for the active section are grown on top of a 0.3 μm-thick InGaAsP layer (λg = 1.4 μm). This enables the fabrication to be done with a single re-growth step. The length of the gain section is 400 μm. The gain and phase sections have a shallow-ridge waveguide structure whereas the ring resonators have a deep-ridge one [12]. After the structure has been fabricated, the device is coated with benzocyclobutene (BCB) and etched back for planarization. The electrodes are then formed by a liftoff process.
3. Laser characteristics
Figure 6 shows the superimposed lasing spectra of the fabricated PRR-TL. The FSRs of the two ring resonators are set at 200 and 214 GHz. In this experiment, the device is CW-operated at 25°C. The current injected to the gain section and Ring1 are kept constant at 100 and 0 mA, respectively. When the injection current for both ring resonators was 0 mA, lasing was observed at 1552.7 nm. The injection current of Ring2 was then increased from 0 to 16 mA while keeping the current in Ring1 at 0 mA. When the injection current was increased up to 13 mA, a corresponding increase in the lasing wavelength was observed up to 1585.9 nm. At an injection current of 16 mA, the lasing wavelength jumped abruptly to 1551.4 nm. All the lasing channels exhibited an SMSR larger than 35 dB.
The lasing wavelength versus the current in Ring2 of the PRR-TL is shown in Fig. 7(a) together with the lasing wavelength of a DRR-TL fabricated on the same substrate, where the rings of the DRR-TL have FSRs of 400 and 444 GHz. Wavelength tuning with a 200 GHz step is achieved by a single electrode control over a wide tuning range of more than 35 nm. The lasing output power versus Ring2 current for both the PRR-TL and DRR-TL are shown in Fig. 7(b). The PRR-TL exhibits a higher output power than the DRR-TL; moreover the power variation with IRing2 is less than 1 dB even when the current injected to Ring2 has been 20 mA. In the DRR-TL, the power variation is more than 3 dB. Thus, the PRR-TL provides a stable lasing output power during wavelength tuning.
4. Monolithic integration with InGaAlAs EAM
Figure 8 shows a photograph of the fabricated tunable transmitter consisting of the PRR-TL and an EAM. The device size is 1.4 x 0.35 mm2. The laser cavity of the PRR-TL is defined by an etched front mirror and the PRR filter section. More about the structure and design of the etched mirror can be found in Ref [15].
To achieve 25-Gbit/s operation and a sufficient extinction ratio over a wide wavelength range simultaneously, InGaAlAs multiple-quantum-wells (MQWs) are used in the EAM section. These MQWs have a large conduction band offset and a small valence band offset compared with InGaAsP MQWs, which results in steep extinction curve and large E/O frequency bandwidth [16]. The EAM section composed of InGaAlAs MQWs is directly butt-jointed to the PRR-TL section. This enables fabrication with only an additional re-growth step to the PRR-TL and allows the independent design of the PRR-TL and EAM. The PRR-TL and EAM sections were designed with epitaxial structures optimized to have the transmitter operating in the C-band. The EAM section has the same shallow-ridge waveguide structure as the gain and phase sections. The length of the EAM section is 150 μm. The EAM section is buried with BCB to reduce the capacitance of the EAM section, and thereby extended the frequency bandwidth up to 39 GHz [16].
Figure 9(a) shows a lasing spectrum of the fabricated transmitter at 45°C. In the measured device, the FSRs of the two ring resonators were set at 200 and 222 GHz. The current injected to the gain section and Ring2 were 100 and 3.2 mA, respectively. The SMSR was larger than 45 dB. The static extinction ratio (SER) characteristics of the fabricated transmitter are shown in Fig. 9(b). The lasing wavelength was tuned from 1544.3 to 1570.0 nm by injecting currents into the ring resonators. An SER of over 15 dB with steep extinction curve is obtained over a tuning range of 25.7 nm.
Figure 10(a) shows non-return-to-zero (NRZ) eye diagrams obtained at 25 Gbit/s with a pseudorandom bit stream (PRBS) of 231-1 over the full tuning range of the transmitter. A dynamic ER greater than 10 dB was achieved for wavelengths up to 1570 nm with DC bias levels ranging from −0.8 to −1.5 V while a constant voltage swing of 2.0 V was maintained at all wavelengths. The bit-error-rate (BER) measurement results are shown in Fig. 10(b). Error-free operation at 25 Gbit/s was confirmed over the tuning range with the same operating condition.
5. Conclusion
A novel tunable laser based on parallel-ring resonator (PRR) was demonstrated. Theoretical analysis revealed that the PRR filter provides a tuning range that is twice the tuning range of the conventional double-ring-resonator (DRR) filter with the same ring resonators. In addition, the calculation indicates that for PRR and DRR filters of the same tuning range, the PRR filter provides smaller filter loss and larger side-mode suppression ratio. The fabricated device exhibited stable wavelength tuning with a step of 200 GHz over a wide tuning range of more than 35 nm by a single electrode control. The output power variation with wavelength tuning at an injected current of 20 mA was a small value of less than 1 dB compared to more than 3 dB in the conventional DRR-TL. A tunable transmitter monolithically integrated with the PRR-TL and InGaAlAs electroabsorption modulator was developed. Under semi-cooled conditions of the transmitter, clear eye openings are demonstrated at 25 Gbit/s over a tuning range of 25.7 nm. Moreover, a dynamic extinction ratio of more than 10 dB is obtained with a constant voltage swing of 2 V. Error-free operation at 25 Gbit/s was confirmed over the tuning range. With these features, the device is very promising as a compact and energy-efficient functional-unit that supports 100-Gbit/s optical packet-switched networks.
Acknowledgment
This work was partially supported by the National Institute of Information and Communications Technology (NICT).
References and links
1. S. J. B. Yoo, “Optical packet and burst switching technologies for the S. J. B. Yoo, “Optical packet and burst switching technologies for the future photonic internet,” J. Lightwave Technol. 24(12), 4468–4492 (2006). [CrossRef]
2. R. Takahashi, T. Nakahara, K. Takahata, H. Takenouchi, T. Yasui, N. Kondo, and H. Suzuki, “Ultrafast optoelectronic packet processing for asynchronous, optical-packet-switched networks,” J. Opt. Netw. 3(12), 914–930 (2004). [CrossRef]
3. H. Takenouchi, R. Urata, T. Nakahara, T. Segawa, H. Ishikawa, and R. Takahashi, “First demonstration of a prototype hybrid optoelectronic router,” in Proceedings of 35th European Conference on Optical Communication (ECOC), 2009, PD3.2.
4. J. Gripp, M. Duelk, J. E. Simsarian, A. Bhardwaj, P. Bernasconi, O. Laznicka, and M. Zirngibl, “Optical Switch Fabrics for Ultra-High-Capacity IP Routers,” J. Lightwave Technol. 21(11), 2839–2850 (2003). [CrossRef]
5. R. Urata, T. Nakahara, H. Takenouchi, T. Segawa, H. Ishikawa, A. Ohki, H. Sugiyama, S. Nishihara, and R. Takahashi, “4×4 optical packet switching with a prototype 4×4 label processing and switching sub-system,” in Proceedings of 35th European Conference on Optical Communication (ECOC), 2009, 6.3.1.
6. V. Jayaraman, Z. M. Chuang, and L. Coldren, “Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings,” IEEE J. Quantum Electron. 29(6), 1824–1834 (1993). [CrossRef]
7. H. Ishii, Y. Tohmori, Y. Yoshikuni, T. Tamamura, and Y. Kondo, “Multiple-phase-shift super structure grating DBR lasers for broad wavelength tuning,” IEEE Photon. Technol. Lett. 5(6), 613–615 (1993). [CrossRef]
8. D. J. Robbins, G. Busico, E. Barton, L. Ponnampalam, J. P. Duck, N. D. Whitbread, P. J. Williams, D. C. J. Reid, A. C. Carter, and M. J. Wale, “Widely tunable DS-DBR laser with monolithically integrated SOA: design and performance,” IEEE J. Sel. Top. Quantum Electron. 11(1), 32–39 (2005).
9. R. Laroy, G. Morthier, T. Mullane, M. Todd, and R. Baets, “Stabilisation and control of widely tunable MG-Y lasers with integrated photodetectors,” IET Optoelectron. 1(1), 35–38 (2007). [CrossRef]
10. M. Oberg, S. Nilsson, K. Streubel, L. Backborn, and T. Klinga, “74 m wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflector,” IEEE Photon. Technol. Lett. 5(7), 735–737 (1993). [CrossRef]
11. P. Kozodoy, T. A. Strand, Y. A. Akulova, G. Fish, C. Schow, P.-C. Koh, Z. Bian, J. Christofferson, and A. Shakouri, “Thermal effects in monolithically integrated tunable laser transmitters,” IEEE Trans. Compon. Packag. Tech. 28(4), 651–657 (2005). [CrossRef]
12. T. Segawa, S. Matsuo, T. Kakitsuka, T. Sato, Y. Kondo, and R. Takahashi, “Semiconductor double-ring-resonator-coupled tunable laser for wavelength routing,” IEEE J. Quantum Electron. 45(7), 892–899 (2009). [CrossRef]
13. B. Liu, A. Shakouri, and J. E. Bowers, “Wide tunable double ring resonator coupled lasers,” IEEE Photon. Technol. Lett. 14(5), 600–602 (2002). [CrossRef]
14. T. Chu, N. Fujioka, and M. Ishizaka, “Compact, lower-power-consumption wavelength tunable laser fabricated with silicon photonic-wire waveguide micro-ring resonators,” Opt. Express 17(16), 14063–14068 (2009). [CrossRef] [PubMed]
15. T. Segawa, S. Matsuo, T. Kakitsuka, Y. Shibata, T. Sato, Y. Kawaguchi, Y. Kondo, and R. Takahashi, “Monolithically integrated wavelength-routing switch using tunable wavelength converters with double-ring-resonator tunable lasers,” IEICE Trans. Electron. E 94-C(9), 1439–1446 (2011).
16. W. Kobayashi, M. Arai, N. Fujiwara, T. Fujisawa, T. Tadokoro, K. Tsuzuki, Y. Kondo, and F. Kano, “Design and fabrication of 10-/40-Gb/s, uncooled electroabsorption modulator integrated DFB laser with butt-joint structure,” J. Lightwave Technol. 28(1), 164–171 (2010). [CrossRef]
