10-Gb/s transmission and beyond

The authors outline obstacles encountered in the development of 10-Gb/s (STM-64, OC-192) systems. Technologies to overcome these obstacles are presented and compared, taking into account real field environments. A perspective on 40-Gb/s systems technologies is also given     

Evanescently coupled photodiodes integrating a double-stage taper for 40-Gb/s applications-compared performance with side-illuminated photodiodes

The design, fabrication, and performance of double-stage taper photodiodes (DSTPs) are reported. The objective of this work is to develop devices compatible with 40-Gb/s applications. Such devices require high efficiency, ultrawide band, high optical power handling capability, and compatibility with low-cost module fabrication. The integration of mode size converters improves both the coupling efficiency and the responsivity with a large fiber mode diameter. Responsivity of 0.6 A/W and 0.45 A/W are achieved with a 6-/spl mu/m fiber mode diameter and cleaved fiber, respectively, providing relaxed alignment tolerances (/spl plusmn/1.6 /spl mu/m and /spl plusmn/2 /spl mu/m, respectively), compatible with cost-effective packaging techniques. DSTPs also offer a wide bandwidth greater than 40 GHz and transverse-electric/transverse-magnetic polarization dependence lower than 0.2 dB. Furthermore, a DSTP saturation current as high as 11 mA results in optical power handling greater than +10 dBm and a high output voltage of 0.8 V. These capabilities allow the photodiode to drive the decision circuit without the need of a broad-band electrical amplifier. The DSTP devices presented here demonstrate higher responsivities with large fiber mode diameter and better optical power handling capabilities and are compared with classical side-illuminated photodiodes.     

WDM PON using 10-Gb/s DPSK downstream and re-modulated 10-Gb/s OOK upstream based on SOA

A signal remodulation scheme of 10-Gb/s differential phase-shift keying(DPSK) downstream and 10-Gb/s on-off keying(OOK) upstream using a semiconductor optical amplifier(SOA) and a Mach-Zehnder intensity modulator(MZ-IM) at the optical networking unit(ONU) side for wavelength division multiplexed passive optical network(WDM PON) is proposed.Simulation results indicate that error-free operation can be achieved in a 20-km transmission,and the receiver sensitivity of return-to-zero differential phase-shift keying(RZ-DPSK) is higher than nonreturn-to-zero differential phase-shift keying(NRZ-DPSK) in the proposed scheme.     

A CMOS multichannel 10-Gb/s transceiver

We describe a CMOS multichannel transceiver that transmits and receives 10 Gb/s per channel over balanced copper media. The transceiver consists of two identical 10-Gb/s modules. Each module operates off a single 1.2-V supply and has a single 5-GHz phase-locked loop to supply a reference clock to two transmitter (Tx) channels and two receiver (Rx) channels. To track the input-signal phase, the Rx channel has a clock recovery unit (CRU), which uses a phase-interpolator-based timing generator and digital loop filter. The CRU can adjust the recovered clock phase with a resolution of 1.56 ps. Two sets of two-channel transceiver units were fabricated in 0.11-/spl mu/m CMOS on a single test chip. The transceiver unit size was 1.6 mm /spl times/ 2.6 mm. The Rx sensitivity was 120-mVp-p differential with a 70-ps phase margin for a common-mode voltage ranging from 0.6 to 1.0 V. The evaluated jitter tolerance curve met the OC-192 specification.     

Nonlinearity Limitations When Mixing 40-Gb/s Coherent PDM-QPSK Channels With Preexisting 10-Gb/s NRZ Channels

We investigate the penalties onto a 40-Gb/s polarization-division-multiplexing (PDM)-quadrature phase-shift keying caused by PDM, wavelength-division multiplexing and 10-Gb/s nonreturn-to-zero neighbor channels. Besides, we optimize the carrier phase estimation process and introduce bandgaps in the multiplex in order to contain limitations caused by cross nonlinear effects.     

10-Gb/s Operation of RSOA for WDM PON

We report on the 10-Gb/s operation of the reflective semiconductor optical amplifier (RSOA) for the next-generation wavelength-division-multiplexed passive optical network (WDM PON). The bandwidth of the RSOA used in this experiment is merely 2.2 GHz. Nevertheless, a clear eye opening is obtained at 10 Gb/s by using the electronic equalizer processed offline. We investigate the impacts of the network's operating conditions (such as the injection power to the RSOA and the fiber length) on the performances of these equalizers. The results show that the RSOA-based WDM PON is operable at 10 Gb/s and the maximum reach can be extended to ${>}$ 20 km with the help of the forward error correction codes.      

10-Gb/s modulator drivers with local feedback networks

A novel intrinsic collector-base capacitance (C/sub CB/) feedback network (ICBCFN) was incorporated into the conventional cascode and series-connected voltage balancing (SCVB) circuit configurations to implement 10-Gb/s modulator drivers. The drivers fabricated in 0.35-/spl mu/m SiGe BiCMOS process could generate 9 V/sub PP/ differential output swings with rise/fall time of less than 29 ps. Also, the ICBCFN was modified as an intrinsic drain-gate capacitance feedback network (IDGCFN) to implement drivers with differential output swing of 8 V/sub PP/ in 0.18-/spl mu/m CMOS process. The power consumption is as low as 0.6 W. The present work shows that the driving capability is greater than that of the currently reported silicon-based drivers.     

10-Gb/s Inductorless CDRs With Digital Frequency Calibration

Two 10-Gb/s inductorless clock and data recovery (CDR) circuits using different gated digital-controlled oscillators (GDCO) are presented. A digital frequency calibration is adopted to save the power consumption and chip area. They have been fabricated in 0.18-$mu{hbox{m}}$ CMOS process. By using the complementary gating technique, the first CDR circuit occupies an active area of 0.16 ${hbox{mm}}^{2}$ and draws 36 mW from a 1.8 V supply. The measured rms jitter and peak-to-peak jitter is 8.5 ps and 42.7 ps , respectively. By using the quadrature gating technique, the second CDR circuit consumes an active area of 0.25 ${hbox{mm}}^{2}$ and its power consumption of 56 mW. The measured rms jitter and peak-to-peak jitter is 3.4 ps and 21.8 ps, respectively. The power of the second CDR circuit is higher than that of the first one but its jitter is reduced.      

Over-10-Gb/s IC's for future lightwave communications

This paper reviews research and development in NTT Laboratories on IC's faster than 10 Gb/s for future optical communication systems. Novel design and circuit techniques achieve such high-speed IC's and stable operation even in packages and modules. High-bit-rate operation of 10 Gb/s (10-GHz equalizing amplifier circuit, a 10-GHz clock recovery circuit, 10-Gb/s decision circuits, and 10-Gb/s multiplexers and demultiplexers) is obtained. 20-Gb/s operation is also achieved for some IC's. Future improvements using advanced device and circuit technologies are discussed, and bit rates over 40 Gb/s are predicted     

Packaging technology for a 10-Gb/s photoreceiver module

We designed a 10-Gb/s photoreceiver module integrating a flip-chip avalanche photodiode (APD), a Si-preamplifier IC, and a slant-ended fiber (SEF). Flip-chip bonding minimizes parasitic reactances in the interconnect between the photodiode and the preamplifier IC. The optical coupling system consists of a slant-ended fiber and a microlens monolithically fabricated on the photodiode. This gives a flat IC-package assembly, which enables stripline interfaces to extract high-speed signals, increases misalignment tolerances, and lowers package height. Tolerances of over ±9 μm were obtained in every direction, which matched our theoretical predictions. To attach and hermetically seal the optical coupling, the fiber ferrule was directly laser-welded to the package wall with a double ring structure. The module withstood shock and vibration tests and had a 10-GHz bandwidth and -23-dBm minimum photosensitivity at 10 Gb/s     

10-Gb/s external modulation in optical CPFSK format

We demonstrated a synchronous control technique for external optical modulation in a format of continuous-phase frequency-shift keying (CPFSK) at 10 Gb/s. In this method, the FSK signal in the upper- or lower-sideband state synchronously shifts to the other state at the timing when their phases are the same. We investigated the accuracy of the timing control required for the synchronous control. Experimental results show that the allowable timing misalignment to keep power penalty of the receiver sensitivity less than 1 dB was more than 25 ps, 25% of each bit period.     

120-Gb/s VCSEL-based parallel-optical interconnect and custom 120-Gb/s testing station

A 120-Gb/s optical link (12 channels at 10 Gb/s/ch for both a transmitter and a receiver) has been demonstrated. The link operated at a bit-error rate of less than 10/sup -12/ with all channels operating and with a total fiber length of 316 m, which comprises 300 m of next-generation (OM-3) multimode fiber (MMF) plus 16 m of standard-grade MMF. This is the first time that a parallel link with this bandwidth at this per-channel rate has ever been demonstrated. For the transmitter, an SiGe laser driver was combined with a GaAs vertical-cavity surface-emitting laser (VCSEL) array. For the receiver, the signal from a GaAs photodiode array was amplified by a 12-channel SiGe receiver integrated circuit. Key to the demonstration were several custom testing tools, most notably a 12-channel pattern generator. The package is very similar to the commercial parallel modules that are available today, but the per-channel bit rate is three times higher than that for the commercial modules. The new modules demonstrate the possibility of extending the parallel-optical module technology that is available today into a distance-bandwidth product regime that is unattainable for copper cables.     

Burst-Mode Electronic Equalization for 10-Gb/s Passive Optical Networks

Using computer simulations, it is shown how burst-mode electronic equalization in the optical line termination of a passive optical network (PON) allows 10 Gb/s in the upstream direction with directly modulated distributed-feedback lasers. This allows achieving 10 Gb/s using cost-effective components at the optical network unit. Fast convergence of the equalizer coefficients is achieved during the preamble of each burst using the recursive least squares algorithm instead of the least mean squares algorithm. With a nine-tap feed-forward equalizer, two-tap decision feedback equalizer, a physical reach of 38 km and a splitting factor of 32 can be achieved in the 1.5-$mu$m window, exceeding requirements for deployed PONs.      

Si bipolar chip set for 10-Gb/s optical receiver

Three Si bipolar ICs, a preamplifier, a gain-controllable amplifier, and a decision circuit, have been developed for 10-Gb/s optical receivers. A dual-feedback configuration with a phase adjustment capacitor makes it possible to increase the preamplifier bandwidth up to 11.2 GHz, while still retaining flat frequency response. The gain-controllable amplifier, which utilizes a current-dividing amplifier stage, has an 11.4-GHz bandwidth with 20-dB gain variation. A master-slave D-type flip-flop is also operated as the decision circuit at 10 Gb/s. On-chip coplanar lines were applied to minimize the electrical reflection between the ICs     

An integrated direct-coupled 10-Gb/s driver for common-cathode VCSELs

An all- npn integrated driver for directly modulating common-cathode vertical-cavity surface-emitting lasers (VCSELs) at high speeds (such as 10 Gb/s) is proposed and experimentally demonstrated. Special biasing techniques allow the output transistors to operate with small collector-emitter voltages while maintaining their fast current-switching capabilities. A current-splitting technique in the output stage minimizes the transients through the bias source and reduces jitter and overshoot.     

10-Gb/s millimeter-wave signal generation using photodiode bias modulation

We present a simple 120-GHz-band millimeter-wave (MMW) modulation method that uses the bias-voltage dependence of unitraveling-carrier-photodiode output power, which we call photodiode (PD) bias modulation. We investigated the dependence of the output-power-saturation mechanisms on the bias voltage. We used a lowpass filter in the bias circuit to increase the modulation bandwidth, and the 3-dB modulation bandwidth was over 7 GHz. We demonstrated the modulation of 120-GHz MMW signals at a data rate of 10 Gb/s using PD bias modulation.     

A 10-Gb/s silicon bipolar IC for PRBS testing

A 10 Gb/s silicon bipolar IC for pseudorandom binary sequence (PRBS) testing was fabricated and tested. The IC features PRBS generation of the sequences of length 2¹⁵-1 and 2²³-1 b up to 10 Gb/s according to CCITT recommendations. Furthermore, the IC is capable of analyzing PRB sequences of the same length and generation polynomials so that a full test of components is possible. In addition, a new PRBS test word synchronization can be provided between two chips for external multiplexing of the sequences up to 40 Gb/s. The IC can be connected to a standard PC, so evaluation of the error test data can be performed in a flexible way. The IC was fabricated with the HP25 process of Hewlett Packard company, the chip size is 32 mm², and it consumes 6.2 W at the nominal supply voltage of -5 V     

Chirp characteristics of 10-Gb/s electroabsorption modulatorintegrated DFB lasers

We present a complete large-signal dynamic model of electroabsorption modulator integrated (EAMI) distributed feedback (DFB) lasers using the time-dependent transfer matrix method. With this model, it is possible to analyze dynamic characteristics depending on optical feedback and spatial hole burning. Also, we can separately calculate the laser and modulator chirp including the voltage-dependent modulator chirp parameter, the grating phase at the end of the laser section, the length of the waveguide region, and electrical coupling. Therefore, our model can provide better predictions regarding the laser and modulator chirp. The calculated large-signal chirp using our model has similar characteristics to the measured large-signal chirp for 10-Gb/s EAMI-DFB lasers     

Performance Comparison of Duobinary Formats for 40-Gb/s and Mixed 10/40-Gb/s Long-Haul WDM Transmission on SSMF and LEAF Fibers

Duobinary formats are today considered as being one of the most promising cost-effective solutions for the deployment of 40-Gb/s technology on existing 10-Gb/s WDM long-haul transmission infrastructures. Various methods for generating duobinary formats have been developed in the past few years but to our knowledge their respective performances for 40-Gb/s WDM transmission have never been really compared. In this paper, we made an extensive numerical evaluation of the robustness of these different types of duobinary transmitter to accumulation of ASE noise, chromatic dispersion, PMD but also to single-channel and WDM 40-Gb/s transmission impairments on standard single-mode fiber. A numerical evaluation of the ability of duobinary format for mixed 10/40-Gb/s WDM long-haul transmission with 50-GHz channel spacing is also led, on both standard single-mode and LEAF fibers, and compared to DQPSK format. In order to clearly identify the limiting transmission effects on each of these two fiber types, the assessment of the performance of a 50-GHz spaced WDM 40-Gb/s long-haul transmission using either duobinary or DQPSK channels only is implemented at last.      

Rayleigh Backscattering Noise-Eliminated 115-km Long-Reach Bidirectional Centralized WDM-PON With 10-Gb/s DPSK Downstream and Remodulated 2.5-Gb/s OCS-SCM Upstream Signal

We propose a novel scheme of Rayleigh backscattering noise-eliminated, long-reach, single-fiber, full-duplex, centralized wavelength-division multiplexed passive optical network with differential quadrature phase-shift keying (DPSK) downstream and remodulated upstream using an optical carrier-suppressed subcarrier-modulation (OCS-SCM) technique and optical interleaver. The error-free transmissions of 10-Gb/s downstream and 2.5-Gb/s upstream signals are experimentally demonstrated over 115-km single-fiber bidirectional SMF-28 with less than 0.5 and 1.9 dB power penalties, respectively.