Paul D. Townsend

High-speed operation for upstream passive optical network (PON) applications enabled by bias-controlled operation of silicon photonics integrated semiconductor optical amplifiers (SOA) is demonstrated in a continuous-mode back-to-back configuration. The InP-based C-band SOAs are heterogeneously integrated onto a 400 nm thick silicon waveguide using micro-transfer printing technology. Optimized apodized grating couplers are designed and incorporated to minimize fiber-to-chip coupling loss (2.5 dB). The integrated SOAs exhibit small-signal on-chip gains of 15–17 dB at a bias current of 120 mA. When the integrated SOA is employed as a receiver-side preamplifier in a 100 Gb/s four-level pulse-amplitude modulation (PAM-4) upstream PON application, bias optimization across received power levels extends the SOA operating input power range to 22 dB while mitigating pattern-dependent distortion arising from SOA gain saturation. A minimum SOA input power of −23 dBm is achieved at the hard-decision forward error-correction threshold, referenced to the on-chip SOA input and excluding input grating-coupler loss. These results demonstrate the potential of heterogeneously integrated, bias-controlled SOAs as compact preamplifiers for extending the operating input power range of next-generation high-speed upstream PON receivers.

Camera-on-tip surgical endoscopes for cardiovascular imaging capable of real-time hyperspectral imaging require high speed optical communication links that operate within the strict power and footprint constraints at the distal end of a surgical guidewire. To address these constraints a half-duplex reflective link was previously demonstrated in which a Surface Normal Electroabsorption Modulator (SNEAM) served as a dual-functional optical transceiver at the distal tip requiring 5 V bias voltage, which is nevertheless incompatible with low power CMOS driver technology. Additionally, practical half-duplex operation requires time coordinated gating of the up- and down-stream channels. Adopting direct laser gating introduces thermally induced wavelength chirp, producing a time-varying power penalty that erodes the available link margin. This increases the possibility of corrupted camera image frames in the upstream channel and risks misdiagnosis. In this work, a SNEAM with an optimized Fabry-Perot (FP) cavity has been designed and fabricated. The device achieves ∼10 dB extinction ratio with a 3 V _pp swing and the operating bias is reduced to below 3 V, with ∼1 mW device power consumption at the distal end. A novel asymmetric gating strategy is proposed and experimentally validated, in which only the downstream laser is gated while the upstream laser operates continuously. Error free downstream operation (BER < 10 ^-9 ) was experimentally validated at 0 dB downstream to upstream optical isolation. This asymmetric gating strategy improves link robustness against variable bending loss with only ∼0.2 mW additional static power overhead during the downstream time slot.

Emerging camera-on-tip surgical endoscopes supporting hyperspectral imaging with high frame rates to minimize motion artifacts will require alternatives to conventional read-out electrical wiring. We propose an optical communication link utilizing an asymmetric reflective surface-normal electro-absorption modulator (SNEAM) at the distal end of an endoscope, functioning as both a modulator and detector. The light for the communication channel is delivered from a proximal source via an optical fiber, eliminating the need for a power-hungry laser at the distal tip. We designed and fabricated a transfer-printed SNEAM, operating as the modulator at ∼1561 nm, and implemented a half duplexing protocol, demonstrating bidirectional transmission at 320 Mb/s over single-mode fiber on a benchtop setup with device power consumption of about 0.5 mW. The fundamental SNEAM cavity resonance can provide ∼6 dB extinction for a 5 V_pp swing, and a device bandwidth of ∼8 GHz was measured at -6 V bias. By leveraging the enhancement from a cavity side resonance at 1314 nm, the photocurrent responsivity was significantly improved. This, combined with the low optical fiber loss due to the short, few-meter, link length, enabled error-free operation (BER < 10⁻⁹) without transimpedance amplifier or forward error correction, minimizing footprint and power consumption at the distal end.

Research on passive optical networks (PONs) capable of reaching speeds of 100 Gb/s per wavelength and beyond has become critical in recent years. One cost-effective solution is 4-level pulse amplitude modulation (PAM4), which utilizes affordable, low-bandwidth devices. The use of semiconductor optical amplifiers (SOAs) as receiver preamplifiers is being investigated to compensate the high PAM4 signal to noise ratio requirement and address PON loss budgets of up to 29 dB. The most recent ITU-T 50G standard specifies an expected loud-soft packet dynamic range of 20.5 dB, currently managed by burst mode trans-impedance amplifiers (BM-TIAs). However, conventional BM-TIAs face limitations in dynamic range and bandwidth when processing high-speed burst-mode upstream signals. To overcome these challenges, we propose burst power equalization in the optical domain to simplify the burst-mode receiver’s design. This paper introduces two novel SOA-based equalization techniques referred to here as: (A) variable bias and (B) control light injection. These methods not only equalize the burst packets but mitigate nonlinear SOA patterning effects in high-power bursts. Our experimental results demonstrate that these SOA-based equalizers extend the dynamic range to 24 dB in a 100 Gb/s PAM4 system, maintaining bit error rates below the soft-decision forward error correction threshold. These findings show that SOA-based equalization reduces the need for high-gain BM-TIAs, thereby preventing any bandwidth limitations associated with the TIA. Furthermore, we show that complex digital signal processing is unnecessary to achieve high dynamic range using an SOA preamplifier, and linear equalization is sufficient to compensate low-cost 25G class receiver bandwidth limitations and relevant fiber dispersion effects. Our approach reduces system complexity while enhancing performance, and therefore represents a cost-effective and scalable solution, positioning SOA-based equalization as a critical innovation for future-proofing high-speed optical networks.

In this paper, we propose an original adaptive neural network equalizer (NNE) algorithm named SkipNet, which is suitable for rapid training on a packet-by-packet basis for burst-mode non-linear equalization in upstream PON transmission. SkipNet uses the simple LMS algorithm and avoids complex neural network training algorithms such as backpropagation and mini-batch training. We demonstrate SkipNet on captured continuous mode 100 Gbit/s PAM4 signals using an SOA preamplifier to achieve the challenging 29 dB PON optical loss budget. The adaptive SkipNet equalizer is shown to overcome combinations of severe SOA patterning effects and fiber dispersion impairments to achieve >29dB dynamic range back-to-back and >22.9dB dynamic range for up to 81.6 ps/nm accumulated dispersion. It can adapt in as little as 250 training symbols to each impairment scenario, which is equivalent to existing FFE/DFE solutions, while matching the non-linear performance of previously proposed static NNE solutions. To the best of our knowledge, SkipNet is the first ever adaptive NNE framework that can realistically be trained and adapted on a packet-by-packet basis and within strict PON packet preamble lengths.

A high-speed InP-based electro-absorption modulator (EAM) on 220 nm silicon-on-insulator (SOI) is designed, fabricated, and measured. The III–V device is heterogeneously integrated to the SOI using transfer printing, with direct bonding. The printing accuracy of the device was within ±0.5 μm. This design evanescently couples light between the III–V waveguide and the SOI via a taper region in the InP ridge for high transmission. This method is a flexible and robust method of transferring an InP EAM to SOI, where multiple device variations have been transferred. At 1550 nm, the printed EAM has a measured electrical bandwidth of up to 40 GHz, an extinction ratio (ER) of 30 dB from 0 to −6 V, and an insertion loss of 6.5 dB, which reduces with longer wavelengths. An ER of 25 dB is obtained over a spectral bandwidth of 30 nm with biasing to −8 V. Open-eye diagrams were measured up to 50 Gbps in a back-to-back measurement. This device is suitable for applications in high-speed communications and sensing, leveraging the added advantage of III–V absorption modulation on a silicon photonics platform.

Spectrally-sliced coherent receivers (SSRx) are potentially attractive for future high-speed access networks, as they allow the use of low-cost, low-bandwidth commodity components with minimal impact on performance. In this letter, we present such a receiver, and show that it is capable of detecting a single carrier, high bandwidth 100 Gb/s PDM-QPSK optical signal using two parallel coherent receivers with reduced front-end bandwidths as low as 6.5 GHz. The receivers utilize frequency separated local oscillators derived from an integrated injection-locked gain-switched comb laser assembly (iCLA) followed by an all-active $1\times 4$ demultiplexer photonic integrated circuit (PIC) to derive the spectral signal slices. The high mutual coherence of the comb lines is then exploited to enable signal reconstruction in the digital domain. The use of the active demultiplexer PIC is a key advance compared to previous work using passive comb demultiplexers as it delivers sufficient local oscillator power without the need for bulky, external optical amplifiers to compensate for demultiplexer losses. Furthermore, a minimal performance penalty is observed (< 1dB at the HD-FEC limit), compared to a conventional high-bandwidth receiver. Importantly, the active iCLA and demultiplexer PIC solutions demonstrated here are key building blocks required for a fully integrated spectrally-sliced coherent receiver for future coherent access networks.

In multiple-input multiple-output (MIMO) systems, channel estimation is of crucial importance to guarantee reliable recovery of ultra-high-speed MIMO signals. This paper proposes a novel channel estimation algorithm for the emerging MIMO-based orthogonal chirp-division multiplexing (OCDM) systems by utilizing the unique features of OCDM signals. In the proposed algorithm, a set of pilot signals is designed based on the Fresnel basis, which is essentially a family of orthogonal linear chirps. The pilots are assigned to different antennas for transmission occupying the same time slot and bandwidth. According to the convolution-preservation theorem of the Fresnel transforms, the transfer matrices of MIMO-OCDM systems can be readily estimated at the receiver without any inter-antenna interference, even if the pilots overlap in both the time and frequency domains. The proposed algorithm avoids bandwidth waste in conventional channel estimators, in which silent pilots will be required in time and/or frequency to ensure the received MIMO pilots separable. We show that the proposed algorithm is unbiased for the unique OCDM pilots and has better estimate accuracy and system performance. Finally, analysis and numerical results are provided to validate its advantages as a promising algorithm for emerging wireless access technology based on MIMO-OCDM.

A highly flexible wavelength and space switched analog radio-over-fiber (ARoF) fronthaul transmission of a millimeter-wave (mmWave) emerging 6G waveform over a centralized/cloud radio access network (C-RAN) is experimentally demonstrated in this work. A spread spectrum multiplexing technique — orthogonal chirp division multiplexing (OCDM) — which is highly resilient to inter-channel interference and enables enhanced channel estimation is utilized in the fronthaul transmission demonstration. The flexible properties of a low noise silicon photonic (SiP) microring resonator (MRR) based tunable laser and a low cross-talk $4\times 4$ SiP optical wavelength/space switch are combined to form a reconfigurable 10 km fronthaul system enabling 64-QAM OCDM transmission at 24 GHz with consistent performances ~5% EVM across all test wavelengths/ports. A signal constituting Wi-Fi and 5G NR standard compatible 64-QAM orthogonal frequency division multiplexing (OFDM) bands, at 10 GHz and 24 GHz respectively, are also transmitted and evaluated in the proposed system with EVM performances below 64-QAM EVM limit (8%) achieved, thus demonstrating the system’s potential in a future converged multi-service environment.

The spectrally-sliced coherent receiver is an attractive solution for future optical access network applications where cost is a sensitive factor, as it enables reduced bandwidth of the analog front-end components and analog-to-digital convertors when compared to a conventional intradyne coherent receiver. In a proof-of-concept experiment, we demonstrate a single-polarization 50 Gb/s QPSK C-band transmission experiment over 40 km of standard (G.652) fiber using receiver bandwidths as low as 6.25 GHz and also utilize an array of local oscillator lines derived from an injection locked gain-switched optical frequency comb, which can be realized as a compact photonic integrated circuit. This paper also analyses the influence of optical frequency comb properties on the quality of the reconstructed signal. The critical parameters considered in this study include comb power and flatness, mutual comb line coherence, and the comb line suppression ratio after demultiplexing the comb lines. We have shown that with practically achievable comb parameter values, the receiver can operate with less than a 1 dB sensitivity penalty compared to a conventional intradyne receiver. The scheme has the potential to provide a low-cost coherent receiver solution for next-generation access networks.