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We present here an all-integrated ETDM 40 Gbit/s optoelectronic transmitter module, which has been developed by the French RNRT ERMIONE consortium. This transmitter integrates the final three high speed functions — namely, the electrical multiplexer, the driver and the electro-absorption modulator - in a single compact and low-cost module. This device has permitted a 40 Gbit/s single channel error-free transmission over 300 km of Teralight™ fiber.
©2004 Optical Society of America
1. Introduction
In order to reach very high capacity (several Tbit/s) in DWDM optical transmission systems in a near future, 40 Gbit/s appears to be an appropriate bit rate per wavelength, because it would permit both simpler network management and smaller equipment footprint. A pre-requisite for such transmission systems is to succeed in producing compact and low-cost opto-electronic devices. Electrical Time Division Multiplexing (ETDM) using available very high speed Integrated Circuits (ICs) in SiGe, GaAs and InP has to enable to reach this aim. Such a choice has been made within the scope of the French RNRT ERMIONE project, with the objective to realize low cost micro-optoelectronic integrated 40 Gbit/s transmitter and receiver modules, based on III–V technologies.
In this paper, we present the ERMIONE transmitter integrating, in a single module, a 2:1 electrical multiplexer (MUX), a high gain GaAs driver [1] and an InP 40 Gbit/s electro-absorption modulator (EAM) [2]. The device has been evaluated in transmission over 300 km of Teralight™ fiber, and compared to the performances of a commercial LiNbO3 40 Gbit/s modulator in back-to-back as well as in transmission.
2. Main realizations of the ERMIONE project
As the bit rate increases, connections between the different modules in the emitter and receiver become a limiting factor of the performances of the whole system. In this respect, the integration of the final three high speed functions of the transmitter - namely the multiplexer, the driver and the optical modulator - in a single module seems a promising solution for 40 Gbit/s applications. Indeed, it suppresses connections at high bit rate, which are the most critical in terms of performances and cost. However, such integration induces difficulties, such as resonances in the module, which have to be overcome. The main objective of the ERMIONE project consisted then in succeeding this integration as well as demonstrating ability of the final module to generate 40 Gbit/s signals able to propagate over sufficiently long distances. We detail below the different steps of the project.
Module specifications were established with taking into account industrial insights on components.
Some realisations of key devices were made during the project, namely: MUX 2:1, driver IC’s, optical component such as EAM and finally packages. A receiver, integrating a DEMUX 1:2, a preamplifier and a PIN photodiode, has been also developed, but is not presented in this paper.
At 40 Gbit/s, dimensions of modules are near to RF wavelength. Therefore, a rigorous and global 3D-ElectroMagnetism analysis have been achieved for predicting package parasitic modes [3,4].
In addition, transmission system simulations have been conducted [5]. The aim of this task was to optimize performances of the whole foreseen 40 Gbit/s fiber optic link. It was also a way to validate choices made for key devices incorporated in the optical link. Individual components were measured (MUX, driver), and modeled (EAM), before being integrated in the transmission chain.
The final tasks consist in packaging and assembling the different elements of the module. Figure 1 shows the transmitter along with its cold-start alimentation card.
The EAM is based on a strained InGaAsP 14-MQW structure on InP. To reduce its overall capacitance (only 100 fF), polyimide is used under the bond-pad and the 75 µm-long modulator has a deep-ridge structure. Its static extinction ratio is typically 15 dB on 3 V, and optical losses are 10 dB at 1550 nm. This large optical attenuation is mainly due to coupling losses: the EAM waveguide is not tapered resulting in about 3 dB coupling losses at each input or output face. Finally, the 3-dB bandwidth has been measured at 40 GHz at -2 V.
The driver is based on a D01PH technology and consists in a two-chip double distributed broadband amplifier: namely a preamplifier including a unique gate and two output drain lines, and an amplifier including two gates and a single drain line. The preamplifier can be considered as a power divider-amplifier and the amplifier as a power combiner. The preamplifier has about 7 dB gain over 55 GHz bandwidth while the amplifier has a typical gain of 19 dB over 53 GHz bandwidth; the total gain of the cascaded two-chips is 26 dB over 52 GHz bandwidth with a total group delay variation of less than 10 ps over 30 GHz bandwidth. This driver is able to output 7.5 Vpp into 50 Ohms. Its architecture is detailed on the Fig. 2. The overall module consumption is about 3.2 W outside the cold-start card and its size is 100×45×20 mm3.
Fig. 2. GaAs P-HEMT 40 Gbit/s driver based on a double distributed architecture, permitting to double the output peak-to-peak voltage.
As the three ICs are integrated in the same package, a wall with a ceramic feed-through was introduced to separate the MUX circuit from the EAM+driver chipset and thus to limit the package modes. The feed-through, which operates at 40 Gbit/s, has been tested accordingly. Figure 3 presents the mounting-test (the feed-through with its 2 access alumina). The result presented on the Fig. 3 (S21 parameter versus frequency) shows that the performances of this transition are compatible with our 40 Gbit/s application. The resonance observed at 48 GHz is probably coming from inside the volume of the feedthrough.
The 2:1 MUX is based on an optimised high frequency design, which achieves the best state-of-the-art performance level in terms of reached bit-rate versus transistor transition frequency ratio. Compared to a more classical approach [6], the circuits presented in this paper have been significantly improved by the introduction of design concepts not reported before for this kind of circuit. Transport of the signal has progressed in terms of quality owing to electromagnetic optimization of inter-stage interconnects and introduction of micro-inductances. Additionally, the selector circuit at the heart of the 2:1 MUX function, which, up to now, limited the bandwidth, now uses a “cascode” configuration which improves it. Finally, the MUX output stage also integrates an original boosting principle [7] which allows a strong improvement of the rise and fall times to less than 7 ps (this figure is thus halved with respect to our previous design [6]). Beside these design improvements, simplified applications are also targeted: a new output stage was designed which includes offset, amplitude, and peaking tunings. Most of the introduced features take advantage of the availability of both enhancement and depletion mode transistors in the process. The MUX was processed using ED01AH 0.13 µm enhancement/depletion (E/D) mode technology, which gives transition frequencies (Ft) of 90 GHz and 70 GHz, respectively. This E/D GaAs PHEMT technology compares favorably in terms of reduced mask count with bipolar or HBT technologies.
The MUX circuit, represented on Fig. 4, was measured on-wafer up to 50 Gbit/s and showed rise and fall times of about 7 ps, input sensitivity of 500 mVpp and produced 700 mVpp output on 50 Ω. The consumption is 1.7 W under -5.2 V. The MUX was also mounted in a module for tests, using a 70 GHz oscilloscope. The result on Fig. 4 shows a very open eye with rise and fall times of 7 ps and a jitter of less than 1.2 ps RMS (which was the test setup jitter limitation).
To our knowledge, the integration of the realized 2:1 MUX IC in a single chip 4:1 version is completely possible without technical bottleneck. The main requirement is a higher level of design manpower, which was not available in the scope of this work. A hybrid MUX was therefore constructed using an assembly of three 2:1 MUX chips in a single module.
3. Experimental results
The experimental set-up is depicted on Fig. 5. The transmitter was composed of a distributed feedback laser at 1550 nm modulated at 40 Gbit/s by the ERMIONE module. ETDM was used to generate 40 Gbit/s non-return to zero pseudo-random bit sequences of length 215-1. For longer sequences (not necessarily representative of the real traffic), some error floor appear due to imperfections on the electro-optical response of the overall module. Indeed, higher is the sequence length, denser is the line spectrum, and more sensitive is the signal to the in-band ripple of the transmitter, which in turn induces eye-diagram imperfections and error floors. No forward error correction was used in the experiment. The channel was pre-compensated (+125 ps/nm) and launched into the transmission line, which contained three spans of Teralight™ fiber compensated by higher order mode modules (HOM) [8]. HOM modules counteracted half of the cumulated dispersion of the fiber spans. Such dispersion map reduced intra-channel non-linear effects because signal pulses were transmitted in pseudo-linear regime: in all points of the line, pulses were largely broadened and their peak power was very low.
The PMD of the transmission line was lower than 2 ps and the average span loss was 20 dB. The loss of each span was counterbalanced by a 1455-nm counter-propagating-pump distributed Raman amplifier which supplied an optimized backward on/off gain of 19 dB. The compensating modules were located before one EDFA. The channel average optical powers, launched respectively in the fiber span and HOMs, were +1 and 0 dBm. Large effective area (80 µm2) and high figure of merit (170 ps/nm/dB) of HOMs enabled to reduce the impact of nonlinearities [8] in the compensating modules. At the transmission end, the channel was recovered by a 0.6 nm flat-top optical filter. The net residual dispersion of the channel was compensated to near 0 ps/nm, owing to a Virtually-Imaged Phased-Array (VIPA) dynamic dispersion compensator [9], which permitted fine adjustment of the dispersion residue at the transmission end. Bit error rate (BER) was measured over the four 10 Gbit/s tributaries, and the 40 Gbit/s BER was their average.
Figure 6 represents, on the left hand side, optical eye diagram at the EAM output. On the right hand side, the optical eye-diagram at emission is at the top, optical eye-diagram at the transmission end in the middle, and electrical eye diagram at 10 Gbit/s after 1:4 electrical demultiplexing at the bottom. These measurements were made with a 32 GHz HP photodiode coupled to a 50 GHz sampling head. The measured dynamic extinction ratio on the NRZ 40 Gbit/s signal is equal to 9.7 dB (see Fig. 6). The vertical and horizontal eye openings are respectively equal to 80% and 82%, whereas the RMS jitter is only of 830 fs. From our knowledge, this level of performance is remarkable for a 40 Gbit/s EAM.
Fig. 6. Eye diagram measured at the output of the EAM with main features mentioned (left), eye diagrams measured in main points of the set-up (right).
Figure 7 shows BER measurements as a function of the received optical signal-to-noise ratio (OSNR) in 0.1 nm (OSNR was in fact measured with a resolution bandwidth of 1 nm), for both back-to-back and 300-km transmission configurations. BER performances of the ERMIONE module were also compared with BER obtained when it was replaced by a commercial 40 Gbit/s LiNbO3 modulator. Note that this LiNbO3 modulator was equipped with an automatic bias control loop, which permitted to measure the BER during a long time because of excellent transmitter stability.
Fig. 7. BER as a function of OSNR measured in 0.1 nm at the receiver, for the ERMIONE module and for a commercial 40 Gbit/s LiNbO3 modulator.
When no further attenuation is added at the transmitter side, OSNR at the transmission end is 30 dB. Corresponding BER is near to 10-11 after 300 km. From our knowledge, it represents the largest distance for such EAM-based integrated 40 Gbit/s transmitter module.
BER performances comparison between our EAM-based module and the commercial 40 Gbit/s LiNbO3 modulator shows that the Lithium Niobate modulator is better than the EAM of approximately 4 dB in OSNR (for a BER=10-9), in back-to-back as well as in transmission. However, this difference, essentially due to electro-optical band imperfections, eye diagram defects (crossing points measured at 30 % with the EAM instead of 50 % with the LiNbO3 modulator), and extinction ratio reduction (9.7 dB with the EAM instead of 13.6 dB with the LiNbO3 modulator), is not redhibitory to the use of such modules in long-haul transmission systems or metro applications. Finally, the very low penalty due to 300 km transmission (<0.5 dB) proves that our dispersion map enables propagation in a quasi-linear regime: OSNR determines for a large part the performance of the transmission line.
4. Conclusion
In this paper, a new 40 Gbit/s compact transmitter module, integrating a 2:1 MUX, a driver, and an EAM, has been realized and evaluated over a 300-km long transmission line. It results from a cooperative action including electromagnetic and system simulations, devices development, packaging, modules characterization and lab trials. Beyond the feasibility shown for such an original module, eye diagrams and especially BER analysis demonstrated the excellent operation of this integrated module. Further investigations will be conducted in the very next months to evaluate the performance of the whole transmitter/receiver chain.
Acknowledgments
This work was partly supported by French Ministry of Industry through RNRT project ERMIONE. Authors are very grateful to Fujitsu for providing the VIPA and to all people involved in the ERMIONE project, both for their work and support and listed below:
From Alcatel/Opto+: F. Blache, J.R. Burie, D. Delagrandière, M. Goix, F. Jorge, W. Mouzannar, N. Sahri, and S. Vuye.
From France Telecom R&D: S. Gosselin, L. Billès, J-Y. Guilloux.
From IRCOM: S. Verdeyme.
From Université Limoges/GESTE: J.L Verneuil.
From AVANEX: C. Gomes.
From Philips OMMIC: D. Smith.
From EGIDE: C. Tavernier.
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