Aidan Daly

Directly PAM-4 Modulated 1530-nm VCSEL Enabling 56 Gb/s/

We demonstrate a 56 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission using direct detection and a long-wavelength 18-GHz bandwidth vertical-cavity surface-emitting laser as directly modulated light source for short-reach inter- and intra-connects in datacenters and short-reach networks. Error-free transmission over 2 km at 7% hard-decision forward-error correction threshold is achieved by applying powerful equalization schemes at the receiver side. Three equalization schemes, i.e., a maximum likelihood estimation (MLSE), a feed-forward equalizer (FFE), and a combination of the FFE and the MLSE are thoroughly investigated, and the performance comparison between them is carried out.

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Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications, gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing). The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of 6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation bandwidth of 5.5GHz with an output power of 0.8mW.

Data-Center Interconnects

In this article we describe a cost-effective approach for hybrid laser integration, in which vertical cavity surface emitting lasers (VCSELs) are passively-aligned and flip-chip bonded to a Si photonic integrated circuit (PIC), with a tilt-angle optimized for optical-insertion into standard grating-couplers. A tilt-angle of 10° is achieved by controlling the reflow of the solder ball deposition used for the electrical-contacting and mechanical-bonding of the VCSEL to the PIC. After flip-chip integration, the VCSEL-to-PIC insertion loss is -11.8 dB, indicating an excess coupling penalty of -5.9 dB, compared to Fibre-to-PIC coupling. Finite difference time domain simulations indicate that the penalty arises from the relatively poor match between the VCSEL mode and the grating-coupler.

Tuneable VCSEL aiming for the application in interconnects and short haul systems

We demonstrate a 1530 nm VCSEL that can operate error-free without DSP or FEC to 56 Gb/s. At 50 Gb/s, error-free operation is attained up to 2 km of SMF. A two-tap FFE driver is used to precompensate the response of the VCSEL. The optical spectrum of the VCSEL under equalization at 50 Gb/s is analyzed and the chirp properties are reported. The low latency of FEC-free NRZ and the distance of 2 km makes these technology suitable for optical links in both high performance computing and large data centers.

Flip-chip integration of tilted VCSELs onto a silicon photonic integrated circuit.

We propose and experimentally demonstrate the use of a novel-design long-wavelength VCSEL with high direct-modulation speed to implement 40-Gb/s transmission suitable for optical interconnects and short-reach scenarios. The single-mode operation, together with the use of a common optical bandpass filter, enables a fiber reach in the kilometer range. The VCSEL is characterized and directly modulated at 40 Gb/s with a power consumption of 18.5 mW. Bit-error-rate (BER) performance has been analyzed for a pseudorandom bit sequence (PRBS) of length 27 - 1 as commonly done in most of similar studies, as well as for increasing the PRBS lengths up to 231 - 1 to understand the system limitations when applying more realistic data, so as to induce slower dynamics physical effects such as thermal effects. BER values below common forward error correction threshold are measured without an optical amplification up to 1 km of a single-mode fiber. In particular, error-free operation (BER <; 10-10) with 1.6 dB of penalty with respect to the backto-back case was measured for the PRBS length of 27 - 1.

Error-free 56 Gb/s NRZ modulation of a 1530 nm VCSEL link

We report on the photoresponse of an asymmetrically doped p(-)-Ge/n(+)-Si heterojunction photodiode fabricated by wafer bonding. Responsivities in excess of 1 A/W at 1.55 μm are measured with a 5.4 μm thick Ge layer under surface-normal illumination. Capacitance-voltage measurements show that the interfacial band structure is dependent on both temperature and light level, moving from depletion of holes at -50 °C to accumulation at 20 °C. Interface traps filled by photo-generated and thermally-generated carriers are shown to play a crucial role. Their filling alters the potential barrier height at the interface leading to increased flow of dark current and the above unity responsivity.

Performance Analysis of 40-Gb/s Transmission Based on Directly Modulated High-Speed 1530-nm VCSEL

We demonstrate the transmission of 28 Gb/s NRZ-OOK over DCF-free links up to 10 km using a monolithic VCSEL, direct detection and an MLSE-based receiver for low-cost, short-reach interconnects. DSP complexity is also investigated in terms of performance optimization.

Ge/Si heterojunction photodiodes fabricated by low temperature wafer bonding.

We demonstrate the generation and transmission of a 28-Gb/s intensity modulated optical signal over single mode fiber (SMF) links up to 1, 2, 5, and 10 km employing a 18-GHz 3-dB bandwidth monolithic vertical-cavity surface-emitting laser (VCSEL) based on buried tunnel junction technology with regrown n-doped InP material. Inexpensive technologies such as nonreturn-to-zero on-off keying modulation format, direct detection as well as digital signal processing-based receivers make the transmission feasible for chromatic dispersion (CD) uncompensated SMF links up to 10 km at the 7% hard-decision (HD) forward error correction (FEC) limit. Three different equalizers for the receiver side are investigated in this paper in terms of performance optimization, i.e., the maximum likelihood sequence estimation equalizer (MLSE), the feed-forward equalizer (FFE) and a hybrid scheme that comprises of an FFE and an MLSE equalizer (FFE/MLSE) in cascaded form. Our experimental results indicate that MLSE scheme outperforms the other two counterparts in all transmission scenarios. Performance optimization in terms of MLSE complexity is also presented. Less complex implementations like FFE equalizer can provide reliable transmission below the 7% HD-FEC limit for links up to 5 km. The FFE/MLSE scheme provides similar performance as MLSE for links up to 5 km, while its performance deteriorates for 10 and 15 km links as signal distortion due to CD becomes too severe to recover the signal. A timing recovery unit is employed before the equalizers in order to compensate for phase offsets and improve the signal quality. Our proposed scheme proves a promising candidate to enable VCSEL-based short reach optical interconnects in data centers and metro-access area.

28 Gb/s NRZ-OOK using 1530-nm VCSEL, direct detection and MLSE receiver for optical interconnects

The performance of two equalization techniques is experimentally compared using a 1530-nm VCSEL, 56Gb/s/A PAM-4, and direct detection over 2 km links for datacenter interconnect applications. Results on the performance evaluation in terms of DSP implementation and complexity are presented.

1.55-μm Long-Wavelength VCSEL-Based Optical Interconnects for Short-Reach Networks

The stability of an optically injected laser is considered theoretically with an emphasis on the understanding of the locked phase whereas previous works focus primarily on the frequency detuning limits. Exemplary photon and carrier number curves for regions within and outside stable locking are presented. The dependence of the phase limits on injection ratio naturally divides into three regions with qualitatively different descriptions for the phase boundaries in each. Frequency detunings at which the locked phase is zero for different injection ratios are investigated. Using this zero phase point it is shown that the coupling rate between the injected and internal field as well as the linewidth enhancement factor can be determined in a single voltage measurement under weak injection. The modulation response parameters at these detunings are analysed and shown to be strongly interconnected.