25-Gbit/s burst-mode optical receiver using high-speed avalanche photodiode for 100-Gbit/s optical packet switching Masahiro Nada,1,* Makoto Nakamura,2 and Hideaki Matsuzaki1 2
1 NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198 Japan Present affiliation: Dep. of Electrical, Electronic and Computer Engineering, Gifu Univ., 1-1 Yanagido, Gifu, 5011193 Japan * [email protected]
Abstract: 25-Gbit/s error-free operation of an optical receiver is successfully demonstrated against burst-mode optical input signals without preambles. The receiver, with a high-sensitivity avalanche photodiode and burst-mode transimpedance amplifier, exhibits sufficient receiver sensitivity and an extremely quick response suitable for burst-mode operation in 100-Gbit/s optical packet switching. ©2014 Optical Society of America OCIS codes: (040.1345) Avalanche photodiodes (APDs); (060.6719) Switching, packet.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
T. Nakahara, R. Urata, T. Segawa, Y. Suzaki, H. Takenouchi, and R. Takahashi, ”Hybrid optoelectronic router prototype for asynchronous optical packet switched networks,” in Proceedings of PS 2010, PTuC1 (2010). T. Segawa, W. Kobayashi, S. Matsuo, T. Sato, R. Iga, and R. Takahashi, “Parallel-ring-resonator tunable laser integrated with electroabsorption modulator for 100-Gb/s (25-Gb/s × 4) optical packet switching,” in Proceedings of ECOC 2012, Mo.1.E.1 (2012). D. van Veen, V. E. Houtsuma, P. Winzer, and P. Vetter, ” 26-Gbps PON transmission over 40-km using duobinary detection with a low cost 7-GHz APD-based receiver,” in Proceedings of ECOC 2012, Tu.B.1 (2012). T. Yoshimatsu, M. Nada, M. Oguma, H. Yokoyama, T. Ohno, Y. Doi, I. Ogawa, and E. Yoshida, “Compact and high-sensitivity 100-Gb/s (4 × 25 Gb/s) APD-ROSA with a LAN-WDM PLC demultiplexer,” in Proceedings of ECOC 2012, Th.3.B (2012). M. Nada, Y. Muramoto, H. Yokoyama, T. Ishibashi, and S. Kodama, “InAlAs APD with high multiplied responsivity-bandwidth product (MR-bandwidth product) of 168 A/W·GHz for 25 Gbit/s high-speed operations,” Electron. Lett. 48(7), 397–399 (2012). M. Nada, Y. Muramoto, H. Yokoyama, N. Shigekawa, T. Ishibashi, and S. Kodama, “Inverted InAlAs/InGaAs avalanche photodiode with low-high-low electric field profile,” Jpn. J. Appl. Phys. 51(2), 02BG03 (2012). M. Nada, H. Yokoyama, Y. Muramoto, T. Ishibashi, and S. Kodama, “Lateral scalability of inverted p-down InAlAs/InGaAs avalanche photodiode,” in Proceedings of IPRM2012 (IEEE, 2012), pp. 215–218. T. Nakahara, R. Takahashi, T. Yasui, and H. Suzuki, “Optical clock-pulse-train generator for processing preamble-free asynchronous optical packets,” IEEE Photonics Technol. Lett. 18(17), 1849–1851 (2006). C. Lenox, H. Nie, P. Yuan, G. Kinsey, A. L. Homles, Jr., B. G. Streetman, and J. C. Campbell, “Resonant-cavity InGaAs–InAlAs avalanche photodiodes with gain-bandwidth product of 290 GHz,” IEEE Photonics Technol. Lett. 11(9), 1162–1164 (1999).
1. Introduction The rapid growth of data traffic in optical fiber communications systems is now a serious issue for electrical routers from the viewpoint of power consumption and scalability. As an alternative to electrical routers, optical packet switching (OPS) has been proposed. Nakahara et al. have successfully demonstrated 10-Gbit/s OPS against asynchronous optical packets with a hybrid optoelectronic router [1]. In order to extend the capacity of OPS to 100 Gbit/s, increasing the serial baud-rate to 25-Gbit/s is required. As an important component for 100-Gbit/s OPS, Segawa et al. have reported a 25-Gbit/s optical transmitter integrated with an electro-absorption modulator (EAM) and tunable laser (TL) [2]. Therefore, now an optical receiver that performs with high speed and high sensitivity for 100-Gbit/s OPS is expected.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 443
Future PON systems also anticipate 25-Gbit/s high-sensitivity optical receivers. As a PON for the next generation, a 40-Gbit/s WDM/TDM PON, called “NG-PON2”, is being discussed. For further in the future, a 100G(E)-PON system with a serial baud rate of 25-Gbit/s is being discussed and is now in the research stage [3]. Previously, we have reported a high-speed avalanche photodiode (APD) for 100-Gbit/s Ethernet systems [4, 5] and successfully demonstrated 100-Gbit/s 50-km transmission (25 Gbit/s × 4 lanes) using the 25-Gbit/s APD. In this study, we applied our high-sensitivity 25-Gbit/s APD to a burst-mode optical receiver to study the feasibility of our optical receiver for OPS. The fabricated burst-mode optical receiver successfully performed error-free operation for a BER of 10−12 against burstmode optical input signals. This is, as far as we know, the first demonstration of 25-Gbit/s burst-mode error-free operation. 2. Optical receiver requirements for 100-Gbit/s OPS Figure 1 shows the architecture for a 100-Gbit/s OPS using four wavelengths of 25 Gbit/s. The optical input signal is launched into a tunable wavelength converter (TWC) consisting of a TL, burst-mode optical receiver, driver, and modulator. The modulated TL output signal is incident to a cyclic arrayed waveguide grating (AWG). The wavelength of the signal is tuned by a control signal so that it can pass through its desired path across the AWG. A fixed wavelength converter (FWC) consisting of a burst-mode receiver, a driver, and an electroabsorption distributed feedback (EA-DFB) laser then receives the input data to convert it back to its original input wavelength.
Fig. 1. Architecture of optical switch for 100-Gbit/s (25-Gbit/s × 4 lanes).
The burst-mode optical receivers provide the electrical modulation signal to modulators in each TWC or to the EA-DFBs in each FWC. In addition to 25-Gbit/s operation, the optical receiver for 100-Gbit/s OPS must operate with high sensitivity, because the optical losses of the modulator in a TWC and AWG are relatively high, typically 3 and 10 dB, respectively. Assuming launch power of 0 dBm, sensitivity of at least −13 dBm is required for the optical receiver in the FWC. Different from some applications in optical fiber communications systems such as Ethernet, the optical receiver must also provide burst-mode operation to accept optical packet signals on a packet-by packet basis like OPS. In order to improve bandwidth utilization efficiency, an extremely short rise time is also necessary. The last condition for that is that the receiver decodes the first 1 bit of the received signal, because no preamble signals are needed for the packet signal. A rapid rise time against an input signal is therefore important for the receiver. The requirements for an optical receiver for 100-Gbit/s OPS are summarized in Table 1.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 444
Table 1. Requirements for optical receiver Baud rate
25 Gbit/s
Signal type
Burst
Receiver sensitivity
At least −13 dBm at BER = 10−12
Rise time
As fast as rise from 1st 1 bit
3. APD, TIA and receiver-module designs The optical receiver consists of an APD and trans-impedance amplifier (TIA). For the APD, we need 25-Gbit/s operation with high sensitivity. Thus, we used an APD with an inverted pdown structure [6] (Fig. 2). The p-contact, p-InGaAs absorption, undoped-InGaAs absorption, p-field control, InAlAs avalanche, n-field control, edge-field buffer, and n-contact layers are grown on the semi-insulating InP substrate by the MOCVD method. The APD has an absorption layer with a large thickness of 1 µm and a 100-nm-thick InAlAs avalanche layer, which enables high-speed and high-responsivity operation. The details of the APD structure and its design advantages are described elsewhere [6]. The inverted p-down structure has double-mesa geometry consisting of a smaller mesa formed with an n-contact layer and a larger mesa formed with the other layers below the n-contact layer. The active area of the device is defined only by the area of the smaller mesa [7]. The diameter of the smaller mesa is 20 µm, which is sufficiently large for optical coupling with a fiber. To relax the tolerance of optical coupling when mounting the APD to the receiver, we use a vertical illumination structure.
Fig. 2. Schematic cross section of APD.
We designed and fabricated the TIA IC using 0.13-µm SiGe BiCMOS technology. The cutoff frequency (ft) and the maximum frequency (fmax) are 200 and 270 GHz, respectively, which are high enough for 25-Gbit/s operation. For high-sensitivity and high-speed operation as a receiver, the TIA is designed to allow a photocurrent as large as 0.3 mA from the APD. In order to ensure 25-Gbit/s burst-mode operation for 100-Gbit/s OPS, it is better that the electric signal rise as fast as the first 1 bit is decoded against burst-mode input signal [8]. For this purpose, all interfaces, such as those between the APD, TIA, and packages, are DCcoupled. As a receiver package, we chose a GPPO-connected box-type one (Fig. 3).
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 445
Fig. 3. Photograph of optical receiver.
4. Results and discussion Figure 4(a) shows the measured 3-dB bandwidth (f3dB) against multiplied responsivity (MR) for the fabricated APD. The APD starts to output RF signal when the MR reaches 4.1 A/W, and then the f3dB gradually decreases with increasing MR due to the limitation of the gainbandwidth (GB) product. Generally speaking, the required f3dB for operation at a baud rate of 25 Gbit/s is 17 GHz. Our APD chip exhibits sufficient f3dB of 20 GHz for 25-Gbit/s operation to the MR as large as 9.1 A/W as shown in Figs. 4(a) and 4(b). Assuming the responsivity at a unity gain of 0.9 A/W, the MR of 9.1 A/W corresponds to the multiplication factor (M) of about 10. Such a large f3dB at M = 10 is provided by the estimated GB-product of 235 GHz. The large GB-product is successfully obtained thanks to the thin InAlAs avalanche layer [9].
Fig. 4. (a): 3-dB bandwidth against multiplied responsivity of fabricated APD, (b): frequency dependences of O/E response of fabricated APD chip at MR = 9.1.
To evaluate our 25-Gbit/s burst-mode optical receiver, we measured its O/E response. Figure 5 shows the frequency characteristics of the O/E response of a fabricated 25-Gbit/s burst-mode optical receiver consisting of an APD and TIA for MRs of the APD of 4.1, 5.1, and 5.9 A/W. The optical receiver exhibits a large f3dB of 17 GHz at the minimum MR of 4.1
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 446
A/W, and this value is maintained up to the larger MR of 5.9 A/W. The obtained frequency characteristics are sufficient for the 25-Gbit/s operation. We chose the MR of 5.9 A/W as a receiver operation condition, considering the responsivity of the APD and bandwidth penalty.
Fig. 5. Frequency characteristics of 25-Gbit/s optical receiver.
Figures 6(a) and 6(b) show the burst-mode optical input signal, and Fig. 6(c) shows the electrical output signal from the burst-mode optical receiver. The burst-mode optical input signal was generated by 1550-nm light and modulated by a 25.00-Gbit/s burst-mode pulsepattern generator and lithium-niobate (LN) modulator. Both the guard time and packet length were set to 150 ns. The electrical output signal is clearly decoded against the burst-mode optical input signal. Note that the electrical signal rises from the first 1 bit with a small penalty of less than 0.4 dB as compared with the following second bit. After the second bit, the high level of the output voltage is almost flat. This indicates that the fabricated optical receiver requires no preamble signals in burst-mode operation, which will make it possible to further improve the bandwidth utilization efficiency. Finally, we investigated the minimum receiver sensitivity for the optical receiver against the burst-mode optical input signal. For comparison, we also measured the receiver characteristics against the continuous optical input signal. Both the burst-mode and continuous optical input signal had a baud rate of 25.00 Gbit/s with a wavelength of 1550 nm. The pseudo random bit sequence (PRBS) was set to 231-1. The extinction ratios are 8.7 dB for the burst-mode optical input signal and 11.76 dB for the continuous optical input signal. Figures 7(a) and 7(b) show eye diagrams of the burst-mode optical input signal and electrical output signal. Figures 7(c) and 7(d) show eye diagrams of the continuous optical input signal and electrical output signal. For the eye diagram of the electrical output shown in Figs. 7(b) and 7(d), the operation condition of the APD was set so that the MR is 5.9 A/W. As shown in Figs. 7(b) and 7(d), the electrical output signal shows clear eye opening against both the burst-mode and the continuous optical input signal. Note that the electrical signal against the burst-mode optical input signal exhibits a good eye pattern without significant degradation of the eye opening as compared with the electrical output signal against continuous optical input signal.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 447
Fig. 6. (a): Optical input signal. The horizontal axis is 100 ns per division. (b): Enlarged optical input signal. The horizontal axis is 100 ps per division. (c): Electrical output signal from optical receiver.
Fig. 7. (a): Eye diagram of 25-Gbit/s burst-mode optical input signal. (b): Eye diagram of electrical output signal from the optical receiver against 25-Gbit/s burst-mode signal. (c): Eye diagram of 25-Gbit/s continuous optical-input signal. (b): Eye diagram of electrical output signal from optical receiver against 25-Gbit/s continuous signal.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 448
Figure 8 shows the BER characteristics of the fabricated optical receiver for the burstmode and continuous optical input signal. The bias condition of the APD was set to be the same as that for measurement of the eye diagram. A minimum receiver sensitivity of −15.5 dBm at a BER of 10−12 is achieved against the 25-Gbit/s burst-mode optical-input signal. Although there is about 1-dB penalty compared with the continuous optical input signal, the obtained sensitivity is sufficient for 100-bit/s OPS applications. The penalty between the burst-mode and continuous optical input signal can be suppressed to less than 0.4 dB if we express the receiver sensitivity with optical modulation amplitude (OMA). It seems that the penalty between the burst-mode and the continuous signal is just due to degradation of the extinction ratio of the optical input signal, and the optical receiver exhibits almost the same performance against both the burst-mode and continuous signals.
Fig. 8. BER characteristics of fabricated optical receiver against 25-Gbit/s burst signal and continuous signal.
5. Conclusion We successfully demonstrated 25-Gbit/s high-sensitivity burst-mode operation using a highsensitivity APD and burst-mode TIA. The fabricated optical receiver can respond to the first bit against burst-mode optical input signal. The minimum receiver sensitivity of −15.5 dBm for a BER of 10−12 is also achieved. Our optical receiver has sufficient performance for 100-Gbit/s OPS with four lanes of 25 Gbit/s. It also has the potential for application to future PON systems, which require 25-Gbit/s burst-mode operation. Acknowledgments The authors thank Y. Suzaki, R. Takahashi, T. Segawa, H. Koizumi, Y. Muramoto, T. Ishibashi, H. Yokoyama, and T. Akeyoshi for valuable discussions, and K. Murata for his continuous encouragement. This work was partially supported by the National Institute of Information and Communication Technology (NICT).
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 449
1 NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198 Japan Present affiliation: Dep. of Electrical, Electronic and Computer Engineering, Gifu Univ., 1-1 Yanagido, Gifu, 5011193 Japan * [email protected]
Abstract: 25-Gbit/s error-free operation of an optical receiver is successfully demonstrated against burst-mode optical input signals without preambles. The receiver, with a high-sensitivity avalanche photodiode and burst-mode transimpedance amplifier, exhibits sufficient receiver sensitivity and an extremely quick response suitable for burst-mode operation in 100-Gbit/s optical packet switching. ©2014 Optical Society of America OCIS codes: (040.1345) Avalanche photodiodes (APDs); (060.6719) Switching, packet.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
T. Nakahara, R. Urata, T. Segawa, Y. Suzaki, H. Takenouchi, and R. Takahashi, ”Hybrid optoelectronic router prototype for asynchronous optical packet switched networks,” in Proceedings of PS 2010, PTuC1 (2010). T. Segawa, W. Kobayashi, S. Matsuo, T. Sato, R. Iga, and R. Takahashi, “Parallel-ring-resonator tunable laser integrated with electroabsorption modulator for 100-Gb/s (25-Gb/s × 4) optical packet switching,” in Proceedings of ECOC 2012, Mo.1.E.1 (2012). D. van Veen, V. E. Houtsuma, P. Winzer, and P. Vetter, ” 26-Gbps PON transmission over 40-km using duobinary detection with a low cost 7-GHz APD-based receiver,” in Proceedings of ECOC 2012, Tu.B.1 (2012). T. Yoshimatsu, M. Nada, M. Oguma, H. Yokoyama, T. Ohno, Y. Doi, I. Ogawa, and E. Yoshida, “Compact and high-sensitivity 100-Gb/s (4 × 25 Gb/s) APD-ROSA with a LAN-WDM PLC demultiplexer,” in Proceedings of ECOC 2012, Th.3.B (2012). M. Nada, Y. Muramoto, H. Yokoyama, T. Ishibashi, and S. Kodama, “InAlAs APD with high multiplied responsivity-bandwidth product (MR-bandwidth product) of 168 A/W·GHz for 25 Gbit/s high-speed operations,” Electron. Lett. 48(7), 397–399 (2012). M. Nada, Y. Muramoto, H. Yokoyama, N. Shigekawa, T. Ishibashi, and S. Kodama, “Inverted InAlAs/InGaAs avalanche photodiode with low-high-low electric field profile,” Jpn. J. Appl. Phys. 51(2), 02BG03 (2012). M. Nada, H. Yokoyama, Y. Muramoto, T. Ishibashi, and S. Kodama, “Lateral scalability of inverted p-down InAlAs/InGaAs avalanche photodiode,” in Proceedings of IPRM2012 (IEEE, 2012), pp. 215–218. T. Nakahara, R. Takahashi, T. Yasui, and H. Suzuki, “Optical clock-pulse-train generator for processing preamble-free asynchronous optical packets,” IEEE Photonics Technol. Lett. 18(17), 1849–1851 (2006). C. Lenox, H. Nie, P. Yuan, G. Kinsey, A. L. Homles, Jr., B. G. Streetman, and J. C. Campbell, “Resonant-cavity InGaAs–InAlAs avalanche photodiodes with gain-bandwidth product of 290 GHz,” IEEE Photonics Technol. Lett. 11(9), 1162–1164 (1999).
1. Introduction The rapid growth of data traffic in optical fiber communications systems is now a serious issue for electrical routers from the viewpoint of power consumption and scalability. As an alternative to electrical routers, optical packet switching (OPS) has been proposed. Nakahara et al. have successfully demonstrated 10-Gbit/s OPS against asynchronous optical packets with a hybrid optoelectronic router [1]. In order to extend the capacity of OPS to 100 Gbit/s, increasing the serial baud-rate to 25-Gbit/s is required. As an important component for 100-Gbit/s OPS, Segawa et al. have reported a 25-Gbit/s optical transmitter integrated with an electro-absorption modulator (EAM) and tunable laser (TL) [2]. Therefore, now an optical receiver that performs with high speed and high sensitivity for 100-Gbit/s OPS is expected.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 443
Future PON systems also anticipate 25-Gbit/s high-sensitivity optical receivers. As a PON for the next generation, a 40-Gbit/s WDM/TDM PON, called “NG-PON2”, is being discussed. For further in the future, a 100G(E)-PON system with a serial baud rate of 25-Gbit/s is being discussed and is now in the research stage [3]. Previously, we have reported a high-speed avalanche photodiode (APD) for 100-Gbit/s Ethernet systems [4, 5] and successfully demonstrated 100-Gbit/s 50-km transmission (25 Gbit/s × 4 lanes) using the 25-Gbit/s APD. In this study, we applied our high-sensitivity 25-Gbit/s APD to a burst-mode optical receiver to study the feasibility of our optical receiver for OPS. The fabricated burst-mode optical receiver successfully performed error-free operation for a BER of 10−12 against burstmode optical input signals. This is, as far as we know, the first demonstration of 25-Gbit/s burst-mode error-free operation. 2. Optical receiver requirements for 100-Gbit/s OPS Figure 1 shows the architecture for a 100-Gbit/s OPS using four wavelengths of 25 Gbit/s. The optical input signal is launched into a tunable wavelength converter (TWC) consisting of a TL, burst-mode optical receiver, driver, and modulator. The modulated TL output signal is incident to a cyclic arrayed waveguide grating (AWG). The wavelength of the signal is tuned by a control signal so that it can pass through its desired path across the AWG. A fixed wavelength converter (FWC) consisting of a burst-mode receiver, a driver, and an electroabsorption distributed feedback (EA-DFB) laser then receives the input data to convert it back to its original input wavelength.
Fig. 1. Architecture of optical switch for 100-Gbit/s (25-Gbit/s × 4 lanes).
The burst-mode optical receivers provide the electrical modulation signal to modulators in each TWC or to the EA-DFBs in each FWC. In addition to 25-Gbit/s operation, the optical receiver for 100-Gbit/s OPS must operate with high sensitivity, because the optical losses of the modulator in a TWC and AWG are relatively high, typically 3 and 10 dB, respectively. Assuming launch power of 0 dBm, sensitivity of at least −13 dBm is required for the optical receiver in the FWC. Different from some applications in optical fiber communications systems such as Ethernet, the optical receiver must also provide burst-mode operation to accept optical packet signals on a packet-by packet basis like OPS. In order to improve bandwidth utilization efficiency, an extremely short rise time is also necessary. The last condition for that is that the receiver decodes the first 1 bit of the received signal, because no preamble signals are needed for the packet signal. A rapid rise time against an input signal is therefore important for the receiver. The requirements for an optical receiver for 100-Gbit/s OPS are summarized in Table 1.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 444
Table 1. Requirements for optical receiver Baud rate
25 Gbit/s
Signal type
Burst
Receiver sensitivity
At least −13 dBm at BER = 10−12
Rise time
As fast as rise from 1st 1 bit
3. APD, TIA and receiver-module designs The optical receiver consists of an APD and trans-impedance amplifier (TIA). For the APD, we need 25-Gbit/s operation with high sensitivity. Thus, we used an APD with an inverted pdown structure [6] (Fig. 2). The p-contact, p-InGaAs absorption, undoped-InGaAs absorption, p-field control, InAlAs avalanche, n-field control, edge-field buffer, and n-contact layers are grown on the semi-insulating InP substrate by the MOCVD method. The APD has an absorption layer with a large thickness of 1 µm and a 100-nm-thick InAlAs avalanche layer, which enables high-speed and high-responsivity operation. The details of the APD structure and its design advantages are described elsewhere [6]. The inverted p-down structure has double-mesa geometry consisting of a smaller mesa formed with an n-contact layer and a larger mesa formed with the other layers below the n-contact layer. The active area of the device is defined only by the area of the smaller mesa [7]. The diameter of the smaller mesa is 20 µm, which is sufficiently large for optical coupling with a fiber. To relax the tolerance of optical coupling when mounting the APD to the receiver, we use a vertical illumination structure.
Fig. 2. Schematic cross section of APD.
We designed and fabricated the TIA IC using 0.13-µm SiGe BiCMOS technology. The cutoff frequency (ft) and the maximum frequency (fmax) are 200 and 270 GHz, respectively, which are high enough for 25-Gbit/s operation. For high-sensitivity and high-speed operation as a receiver, the TIA is designed to allow a photocurrent as large as 0.3 mA from the APD. In order to ensure 25-Gbit/s burst-mode operation for 100-Gbit/s OPS, it is better that the electric signal rise as fast as the first 1 bit is decoded against burst-mode input signal [8]. For this purpose, all interfaces, such as those between the APD, TIA, and packages, are DCcoupled. As a receiver package, we chose a GPPO-connected box-type one (Fig. 3).
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 445
Fig. 3. Photograph of optical receiver.
4. Results and discussion Figure 4(a) shows the measured 3-dB bandwidth (f3dB) against multiplied responsivity (MR) for the fabricated APD. The APD starts to output RF signal when the MR reaches 4.1 A/W, and then the f3dB gradually decreases with increasing MR due to the limitation of the gainbandwidth (GB) product. Generally speaking, the required f3dB for operation at a baud rate of 25 Gbit/s is 17 GHz. Our APD chip exhibits sufficient f3dB of 20 GHz for 25-Gbit/s operation to the MR as large as 9.1 A/W as shown in Figs. 4(a) and 4(b). Assuming the responsivity at a unity gain of 0.9 A/W, the MR of 9.1 A/W corresponds to the multiplication factor (M) of about 10. Such a large f3dB at M = 10 is provided by the estimated GB-product of 235 GHz. The large GB-product is successfully obtained thanks to the thin InAlAs avalanche layer [9].
Fig. 4. (a): 3-dB bandwidth against multiplied responsivity of fabricated APD, (b): frequency dependences of O/E response of fabricated APD chip at MR = 9.1.
To evaluate our 25-Gbit/s burst-mode optical receiver, we measured its O/E response. Figure 5 shows the frequency characteristics of the O/E response of a fabricated 25-Gbit/s burst-mode optical receiver consisting of an APD and TIA for MRs of the APD of 4.1, 5.1, and 5.9 A/W. The optical receiver exhibits a large f3dB of 17 GHz at the minimum MR of 4.1
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 446
A/W, and this value is maintained up to the larger MR of 5.9 A/W. The obtained frequency characteristics are sufficient for the 25-Gbit/s operation. We chose the MR of 5.9 A/W as a receiver operation condition, considering the responsivity of the APD and bandwidth penalty.
Fig. 5. Frequency characteristics of 25-Gbit/s optical receiver.
Figures 6(a) and 6(b) show the burst-mode optical input signal, and Fig. 6(c) shows the electrical output signal from the burst-mode optical receiver. The burst-mode optical input signal was generated by 1550-nm light and modulated by a 25.00-Gbit/s burst-mode pulsepattern generator and lithium-niobate (LN) modulator. Both the guard time and packet length were set to 150 ns. The electrical output signal is clearly decoded against the burst-mode optical input signal. Note that the electrical signal rises from the first 1 bit with a small penalty of less than 0.4 dB as compared with the following second bit. After the second bit, the high level of the output voltage is almost flat. This indicates that the fabricated optical receiver requires no preamble signals in burst-mode operation, which will make it possible to further improve the bandwidth utilization efficiency. Finally, we investigated the minimum receiver sensitivity for the optical receiver against the burst-mode optical input signal. For comparison, we also measured the receiver characteristics against the continuous optical input signal. Both the burst-mode and continuous optical input signal had a baud rate of 25.00 Gbit/s with a wavelength of 1550 nm. The pseudo random bit sequence (PRBS) was set to 231-1. The extinction ratios are 8.7 dB for the burst-mode optical input signal and 11.76 dB for the continuous optical input signal. Figures 7(a) and 7(b) show eye diagrams of the burst-mode optical input signal and electrical output signal. Figures 7(c) and 7(d) show eye diagrams of the continuous optical input signal and electrical output signal. For the eye diagram of the electrical output shown in Figs. 7(b) and 7(d), the operation condition of the APD was set so that the MR is 5.9 A/W. As shown in Figs. 7(b) and 7(d), the electrical output signal shows clear eye opening against both the burst-mode and the continuous optical input signal. Note that the electrical signal against the burst-mode optical input signal exhibits a good eye pattern without significant degradation of the eye opening as compared with the electrical output signal against continuous optical input signal.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 447
Fig. 6. (a): Optical input signal. The horizontal axis is 100 ns per division. (b): Enlarged optical input signal. The horizontal axis is 100 ps per division. (c): Electrical output signal from optical receiver.
Fig. 7. (a): Eye diagram of 25-Gbit/s burst-mode optical input signal. (b): Eye diagram of electrical output signal from the optical receiver against 25-Gbit/s burst-mode signal. (c): Eye diagram of 25-Gbit/s continuous optical-input signal. (b): Eye diagram of electrical output signal from optical receiver against 25-Gbit/s continuous signal.
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 448
Figure 8 shows the BER characteristics of the fabricated optical receiver for the burstmode and continuous optical input signal. The bias condition of the APD was set to be the same as that for measurement of the eye diagram. A minimum receiver sensitivity of −15.5 dBm at a BER of 10−12 is achieved against the 25-Gbit/s burst-mode optical-input signal. Although there is about 1-dB penalty compared with the continuous optical input signal, the obtained sensitivity is sufficient for 100-bit/s OPS applications. The penalty between the burst-mode and continuous optical input signal can be suppressed to less than 0.4 dB if we express the receiver sensitivity with optical modulation amplitude (OMA). It seems that the penalty between the burst-mode and the continuous signal is just due to degradation of the extinction ratio of the optical input signal, and the optical receiver exhibits almost the same performance against both the burst-mode and continuous signals.
Fig. 8. BER characteristics of fabricated optical receiver against 25-Gbit/s burst signal and continuous signal.
5. Conclusion We successfully demonstrated 25-Gbit/s high-sensitivity burst-mode operation using a highsensitivity APD and burst-mode TIA. The fabricated optical receiver can respond to the first bit against burst-mode optical input signal. The minimum receiver sensitivity of −15.5 dBm for a BER of 10−12 is also achieved. Our optical receiver has sufficient performance for 100-Gbit/s OPS with four lanes of 25 Gbit/s. It also has the potential for application to future PON systems, which require 25-Gbit/s burst-mode operation. Acknowledgments The authors thank Y. Suzaki, R. Takahashi, T. Segawa, H. Koizumi, Y. Muramoto, T. Ishibashi, H. Yokoyama, and T. Akeyoshi for valuable discussions, and K. Murata for his continuous encouragement. This work was partially supported by the National Institute of Information and Communication Technology (NICT).
#198939 - $15.00 USD (C) 2014 OSA
Received 4 Oct 2013; accepted 25 Nov 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000443 | OPTICS EXPRESS 449
