A silicon photonic tunable optical dispersion compensator (TODC) is demonstrated based on a series of 5 thermally tunable Mach-Zehnder interferometers. The TODC has a 2.8mm×5.0 mm foot-print with continuously tunable dispersion from 0ps/nm to 2000ps/nm with a low tuning power of 80mW. This TODC is used to extend the reach of a 10Gb/s link from 85km to 150km.
© 2007 Optical Society of America
The reach of an optical communication link is fundamentally determined by its loss and chromatic dispersion. For systems running at 2.5 Gb/s loss is the dominant limitation, and is compensated by using optical amplifiers. For higher data rates dispersion becomes important and must be militated against, as the dispersion limited reach varies inversely as the square of the data rate . As an example the dispersion limited reach for a 2.5Gb/s optical link is ~1000km, for a 10Gb/s system this drops to 60km and for a 40Gb/s system to 4km!
Dispersion compensating fiber (DCF) is the most common form of dispersion management used in optical communication systems today due to its wide operating bandwidth. This means a single DCF module can compensate for the dispersion in a DWDM system across all channels simultaneously. Unfortunately DCFs have the drawback of high loss, the ability to compensate only for fixed amount of fiber dispersion and do not compensate exactly over their entire bandwidth for the dispersion slope resulting in a residual dispersion towards the edges of a DWDM system. In addition DCFs have high polarization mode dispersion (PMD) and high latency which limits their use in high speed and storage area networks [2, 3].
Electronic dispersion compensation (EDC) is an alternative to DCF with the advantage that it can be implemented into an existing network on a per channel basis, providing tunable mitigation of chromatic dispersion as well as other intersymbol degradations such as modal and polarization mode dispersion . However, as electronic dispersion compensation is performed after photodetection it cannot distinguish the phase of the incoming light and so suffers signal-to-noise penalty when compensating for large amount of dispersion . Currently EDC chips are also power hungry resulting in significant challenges scaling to higher speeds.
Planar optical dispersion compensators (ODC) have been demonstrated in many different device architectures, mainly fabricated from silica glass. These include wide  and narrow  bandwidth tunable Bragg gratings, cascaded etalons  and planar waveguide based devices [9–13]. The benefits of this type of ODC are that they are tunable, both in terms of wavelength and dispersion value, and can be implemented on a per channel basis similar to EDC chips easing the upgradeability of existing optical networks. While most planar ODCs have limited bandwidth, working only over a single WDM channel, they can be designed with periodic transfer functions allowing compensation of any single channel in the network easing device inventory. Most importantly, unlike EDCs, optical compensators can directly compensate for phase as well as amplitude distortions. In this paper a silicon tunable optical dispersion compensator (TODC) is described which can be tuned from 0ps/nm to +/-2000ps/nm extending the reach of a 10Gb/s NRZ optical link from 80km to 150 Km. This TODC device epitomizes the strengths of silicon photonics in that it has a 50x smaller footprint, and 5x lower tuning power compared to similar devices fabricated from silica [12, 13].
2. Device structure
The silicon TODC is based on the cascaded Mach-Zehnder interferometer (MZI) design suggested previously by Gehler et al and Doerr, et al., [12, 13]. This architecture has a key advantage over other cascaded designs in that it may be tuned with a single control voltage, and so requires less complex drive circuitry and lower electrical power consumption. A schematic of the TODC device is shown in Fig. 1(a).
The TODC consists of three asymmetric MZIs in series, separated by two symmetric MZIs which act as tunable couplers and control the dispersion of the device. When the couplers are tuned to their bar-state, i.e. , the device acts as a large symmetric MZI with flat amplitude and 0ps/nm dispersion response. The maximum dispersion compensation is obtained when the couplers are tuned to have 50/50 splitting ratio. In this state the outer asymmetric MZIs act as a demultiplexer/multiplexer pair with the center asymmetric MZI forming a wavelength dependent delay which is the basis of the device’s dispersion. By tuning the splitting ratio of the symmetric MZIs in tandem the dispersion of the device can be continuously tuned using a single control bias .
The device is composed of silicon rib waveguides fabricated on the (100) surface of an SOI substrate using standard photolithographic patterning and reactive ion etching techniques. A cross-section scanning electron microscope (SEM) image is shown in Fig. 1(b). The SOI waveguide has a height of 1.55um, a width of 0.9um and a slab height of 0.85um. To reduce coupling loss the input and output waveguides are tapered laterally over a distance of 500um to a width of 4um.
The MZIs comprise evanescent couplers formed by placing two 80um long waveguides 0.6um apart. To ensure the coupler gap is filled correctly without voids boron phosphorous silicon glass (BPSG) is used as the upper cladding and reflowed after deposition. Prior to depositing the BPSG, a thin, 80nm, layer of low temperature oxide was deposited to prevent boron and phosphorous diffuse into the silicon rib area. After the BPSG reflow the BPSG was polished to the level of the thermal oxide above the rib as seen in the image, and a 1.5um thick low temperature oxide deposited and used to control the separation of the metal heater from the waveguide. The 2um wide metal heaters are composed of a 0.5um thick layer of aluminum followed by a 0.2um layer of titanium. To increase the tuning efficiency thermal isolation trenches are etched through the silicon epi-layer down to the buried oxide 4um away from the edge of the silicon waveguide. The symmetric MZIs are fabricated with 700µm long heaters deposited on the 1mm long arms. The asymmetric MZIs are formed with a ΔL of 3.44mm to produce a device with a free spectral range (FSR) of 25GHz; the heaters are placed on the 400um radius curved sections of the waveguides with a length of 800µm.
After fabrication die facets are polished and anti-reflection coated (~1%) to allow coupling to and from the chip using lensed single mode fibers. The final device size is 2.8mm by 5mm.
3.1 Device level testing
The TODC is tuned using the thermo-optic effect using the Aluminum metal heaters placed above the waveguides. The tuning efficiency of the heaters was measured by fabricating MZI coupled ring resonators  and monitoring the coupling as a function of electrical power applied to the heater. Figure 2(a) shows the induced phase shift as a function of the electrical power dissipated in the thermal heater, the tuning efficiency is measured to be 91mW/π. Figure 4(b) shows the total electrical drive power needed to tune the TODC to achieve a certain dispersion value. As can be seen by this figure the initial biasing of the device to spectrally align the three asymmetric MZIs and obtain 0 ps/nm dispersion is the main contributor to the drive power. Once biased at 165mW the device is tuned by changing the bias on the symmetric MZIs and requires an additional 80mW to tune the dispersion from 0ps/nm to 2,000 ps/nm.
The current device is designed to be single polarization and has a measured polarization dependent loss (PDL) of greater than 25dB transmitting predominantly TM polarization. The fiber to fiber insertion loss of the TODC is shown in Fig. 3(a) for a variety of different bias powers. The insertion loss is comprised of 2.5dB coupling loss per facet and 2.5dB on-chip propagation loss. The insertion loss ripple varied with the bias on the device and had a maximum value of 0.35dB; during use this ripple did not affect the overall performance of the dispersion compensator.
The measured group delay response of the TODC is shown in Fig. 3(b) for the same bias conditions shown in Fig. 3(a). At a bias of 166mW the device is tuned to obtain a flat group delay and insertion loss. The peak dispersion of the device, obtained with a bias power of 240mW, is measured to be 2,000ps/nm over a bandwidth of 3.5GHz.
3.2 System level testing
The performance of the TODC was tested in the experimental optical link test bed shown schematically in Fig. 4. The output of a tunable external cavity laser (ECL) was externally modulated with a 10Gb/s LiNbO3 modulator being driven by a 223-1 non-return to zero (NRZ) pseudo random bit sequence (PRBS). The input power into the link was set at 0dBm and an EDFA was placed every ~60 km. After transmission the signal was passed through a polarization controller to set it’s polarization to TM before being coupled into the TODC. A 1nm bandwidth ASE optical filter was placed before the receiver and the electronic signal measured using a bit error rate tester (BERT). A variable attenuator before the detector allowed the bit error rate (BER) to be measured as a function of power for different link lengths. The power penalty is defined as the additional power required to achieve a BER of 10-9 compared to the back to back case, and is shown in Fig. 5(a) as a function of link length for a link with and a link without the silicon photonic TODC. For each link length the TODC was tuned to optimize the received BER, for example the BER of the 160km link was optimised with the TODC set at 2,000ps/nm dispersion at a tuning power of 240mW. The optical signal to noise ratio (OSNR) was measured for the 160km link, with 3 EDFAs, to be 34dB.
The link performance as a function of distance is measured by the power penalty which is the ratio of the received optical power needed to obtain a BER of 10-9 compared to the back-to-back link. The optical link without TODC has a 2dB power penalty after traversing 85km of fiber; the TODC extends the link to ~150km. Also shown in the Fig. 5(b) are two eye diagrams after 160km of fiber, which show the dramatic improvement in the optical data transmission obtained with the silicon photonic based TODC.
The wavelength stability required to use this device was measured by detuning the wavelength of the laser about its set point and measuring the power penalty for a 120km long link. As can be seen from Fig. 6 the TODC can withstand ±3GHz of wavelength instability without significantly affecting its performance.
In conclusion a silicon photonic tunable optical dispersion compensator capable of extending the reach of a 10Gb/s optical link to 150km is demonstrated. In comparison to similar silica based tunable dispersion compensators [12, 13] this silicon TODC with a footprint of 14 mm2 has nearly a 50x smaller size, and a 5 x reduction of tuning power. One exciting benefit of a silicon platform is the ability to integrate the TODC with other devices such as an on-chip photo-detector. Here a germanium based detector  could be integrated with the dispersion compensator to fabricate a dispersion tolerant receiver. This would be the equivalent of electronic dispersion compensators used today with the advantage of the additional phase control available in the optical domain.
The authors thank T. Mader, A. E. Willner and L. Paraschis for technical discussions; J. Jimenez, J. Ngo and Natalie Ziharev for assistance in device fabricating and sample preparation, and Walid Mathlouthi for help with optical testing.
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