Discussion10 Gbit/s error-free DPSK modulation using a push–pull dual-drive silicon modulator
Introduction
In order to cope with the growing bandwidth demand of next generation communication networks, optical transmission systems with low cost, high spectral efficiency and high per-channel rate are required. Advanced data modulation formats have become quite attractive for optical communication systems because they enable the demanded increase of the optical network capacity [1]. When compared with on–off keying, phase-shift keying (PSK) modulation format offers several advantages, namely 3-dB receiver sensitivity improvement when combined with balanced detection, enhanced tolerance to fiber non-linearities and improved spectral efficiency via higher order modulation formats such as quadrature PSK (QPSK) [2].
Silicon-photonics based optoelectronics devices have been shown to be the best candidates to provide high-bandwidth communications with low power consumption and low cost due to their compatibility with complementary metal-oxide-semiconductor (CMOS) manufacturing processes. Hence, it is expected that the use of silicon based optical devices will allow large scale integration relieving the existing bandwidth bottleneck.
Silicon modulators based on microring structures have been proposed to achieve phase modulation [3], [4]. Experimental demonstrations of 5 Gbit/s error-free differential PSK (DPSK) modulation [5] and 20 Gbit/s QPSK modulation, but without successful error-free demonstration, have been recently reported [6]. Ring based modulators have unique features in terms of small footprint and low drive voltage. However, the optical bandwidth or range of useful wavelengths for modulation is much lower compared to conventional Mach–Zehnder modulators (MZMs) that in turn implies that the modulator performance is more sensitive to fabrication tolerances. Thus, some tuning mechanism, which is usually based on the thermo-optic effect [6], becomes mandatory, increasing power consumption and complexity of the transmitter. Very recently, the first works dealing with phase modulation in silicon MZMs have also been reported. In principle, a single MZI is capable of covering any phase and amplitude modulation format. However, MZMs are typically arranged in a nested configuration to independently modulate in the two quadratures. A silicon based dual-drive nested MZM for QPSK modulation was firstly demonstrated at 20 Gbit/s [7]. However, a poor system constellation was achieved due to the low extinction ratio and unbalanced output optical power at the MZMs. A higher modulation speed, 50 Gbit/s QPSK, has also been demonstrated by using a single-drive nested MZM but no error-free modulation was achieved [8].
In this work, we experimentally demonstrate error-free DPSK modulation up to 10 Gbit/s using a silicon dual-drive MZM. Furthermore, DPSK modulation up to 20 Gbit/s is also achieved.
Section snippets
Design and fabrication
Fig. 1(a) and (b) shows the GDS design and fabricated MZM. Multimode interference couplers (MMI) were used as input/output 3 dB couplers. The silicon waveguide core has a height of 220 nm, a width of 450 nm, and a slab thickness of 100 nm, as illustrated in Fig. 1(c). Optical phase modulation is achieved by depleting the majority carriers from a reverse biased p–n junction [9] with doping concentrations of 1.6 QUOTE 1017 cm−3 in the p-type region and 8 QUOTE 1017 cm−3 in the n-type region.
The
Experiments and results
Next, we characterized the DPSK modulation using the measurement setup shown in Fig. 3. The input light emitted by an external cavity laser (ECL) is coupled from a standard single mode fiber to the chip via grating couplers. The polarization was optimized and set to a TE polarization using a polarization controller (PC). Before being launched onto the chip, the optical signal was amplified by an erbium-doped fiber amplifier (EDFA), and filtered by a 3 nm wide tunable optical filter. Digital data
Conclusion
In summary, we have successfully demonstrated error-free DPSK modulation at 5 Gbit/s and 10 Gbit/s using a dual-drive silicon MZM. Furthermore, we have also shown the feasibility of the proposed MZM for 15 Gbit/s and 20 Gbit/s DPSK modulation. The obtained results validate the potential to achieve higher order modulation formats, such as QPSK, by arranging the MZM in a nested configuration. Furthermore, a fully integrated silicon transceiver could be implemented by combining the proposed MZM with
Acknowledgments
Financial supports from HELIOS (Photonics Electronics Functional Integration on CMOS) FP7-224312 and Generalitat Valenciana under PROMETEO-2010-087 R&D Excellency Program (NANOMET) are acknowledged. M. Aamer and P. Sanchis thank Dr. Javier Herrera for his useful help. D.J. Thomson, F.Y. Gardes and G.T. Reed are supported by funding received from the UK EPSRC funding body under the grant “UK Silicon Photonics”.
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