The design of application-specific integrated circuits (ASIC) is at the core of modern ultra-high-speed transponders employing advanced digital signal processing (DSP) algorithms. This manuscript discusses the motivations for jointly utilizing transmission techniques such as probabilistic shaping and digital sub-carrier multiplexing in digital coherent optical transmissions systems. First, we describe the key-building blocks of modern high-speed DSP-based transponders working at up to 800G per wave. Second, we show the benefits of these transmission methods in terms of system level performance. Finally, we report, to the best of our knowledge, the first long-haul experimental transmission – e.g., over 1000 km – with a real-time 7 nm DSP ASIC and digital coherent optics (DCO) capable of data rates up to 1.6 Tb/s using two waves (2 × 800G).

/pdf/800g-dsp-asic-design-using-probabilistic-shaping-and-digital-2yhfdgnk8t.pdf

800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing

Fiber-optical networks are a crucial telecommunication infrastructure in society. Wavelength division multiplexing allows for transmitting parallel data streams over the fiber bandwidth, and coherent detection enables the use of sophisticated modulation formats and electronic compensation of signal impairments. In the future, optical frequency combs may replace multiple lasers used for the different wavelength channels. We demonstrate two novel signal processing schemes that take advantage of the broadband phase coherence of optical frequency combs. This approach allows for a more efficient estimation and compensation of optical phase noise in coherent communication systems, which can significantly simplify the signal processing or increase the transmission performance. With further advances in space division multiplexing and chip-scale frequency comb sources, these findings pave the way for compact energy-efficient optical transceivers.

/pdf/phase-coherent-lightwave-communications-with-frequency-combs-53udh8qr46.pdf

Phase-coherent lightwave communications with frequency combs.

We present a 16-Gb/s transmitter composed of a stacked voltage-mode CMOS driver and periodic-loaded reverse biased pn junction Mach–Zehnder modulator. The transmitter shows 9-dB extinction ratio and 10.3-pJ/bit power consumption and operates with 1.3 μm light. Penalties as low as 0.5 dB were seen as compared to a 25-Gb/s LiNbO3 transmitter with both a monolithic metal–semiconductor–metal receiver and a reference receiver at 16-Gb/s operation. We also present an analytic expression for relative transmitter penalty (RTP), which allows one to quickly assess the system impact of design parameters such as peak-to-peak modulator drive voltage, modulator figure of merit, and transmitter extinction ratio to determine the circumstances under which a stacked CMOS cascode driver is desirable.

Demonstration of a High Extinction Ratio Monolithic CMOS Integrated Nanophotonic Transmitter and 16 Gb/s Optical Link

A paradigm shift in optical communication networks is proposed, with the introduction of a new ecosystem of devices and components with the capability of transforming current point-to-point optical networks (with their entailed, limiting, electrical aggregation) into flexible, scalable and cost-effective point-to-multipoint networks. In the new architecture, which better aligns with the hub-and-spoke traffic patterns observed in today's metro and access network segments, interoperability across a variety of transceivers operating at different speeds is achieved using individually routed, digitally generated subcarriers. The first comprehensive demonstration of the technical feasibility of the proposed point-to-multipoint architecture based on digital subcarrier multiplexing is presented, along with the remarkable cost savings and simplification of the network it enables.

Point-to-Multipoint Optical Networks Using Coherent Digital Subcarriers

Acknowledging the predominantly hubbed traffic profile in the metro, we apply digital subcarrier multiplexing techniques to 400Gb/s coherent pluggable optics, enabling a point-to-multipoint architecture which shows TCO savings of 76% over a five-year period compared to a traditional architecture based on ROADMs and point-to-point transponder

CAPEX Savings Enabled by Point-to-Multipoint Coherent Pluggable Optics Using Digital Subcarrier Multiplexing in Metro Aggregation Networks

A novel low complexity 16QAM feed-forward carrier recovery is provided based on blind phase search. It is discovered that 8 out of 16 states generate double minimum in the algorithm. A second modification leads to significant reduction in tracking error.

Novel 16QAM carrier recovery based on blind phase search

A system, method, and apparatus is disclosed for interpolation of an output of an analog to digital converter (ADC) to enable operation of the ADC at a sampling rate that is independent of the sampling rate for a DSP core so as to efficiently enable operation at higher date rates. According to one of the embodiments, an interpolation circuit is coupled between the ADC and DSP core and receives a first plurality of samples of data at the first data rate from the ADC and supplies a plurality of samples of second data at a second data rate to the DSP core; the second data rate being less than the first data rate. According to one of the embodiments, the interpolation circuit includes a memory and a FIR filter circuit having filter tap coefficient values selected to provide attenuation at high frequencies to reduce aliasing noise.

Method, system, and apparatus for interpolating an output of an analog-to-digital converter

Consistent with the present disclosure, an encoder circuit is provided at a transmit side of an optical fiber link that maps an input sequence of bits of fixed length k a sequence of symbols of a codeword of length n, such that the symbols of the codeword define a predetermined transmission probability distribution. Preferably, each symbol of the codeword is generated during a corresponding clock cycle, such that after n clock cycles, a complete codeword corresponding to the input bit sequence is output. On a receive end of the link, a decoder is provided that outputs the k-bit sequence every n clock cycles. Accordingly, buffers need not be provided at the output of the encoder and the input of the decoder, such that processing of the input sequence, codewords, and output sequence may be achieved efficiently without large buffers and complicated circuitry. Moreover, the input sequence, with any binary alphabet may be matched to a desired output distribution with any arbitrary alphabet. Accordingly, probabilistic constellation shaping may be achieved over constellations of arbitrary size. In addition, relatively long codewords, may be encoded and decoded with the apparatus and method disclosed herein. Accordingly, for a fixed SNR a higher SE (more bits per symbol) can be achieved. Alternatively, for a fixed SE, a lower SNR may be sufficient. Moreover, the resulting SE may be finely tailored to a particular optical link SNR to provide data transmission rates that are higher than the low order modulation formats that would otherwise be employed for optical signals carried by such links.

Distribution matching for probabilistic constellation shaping with an arbitrary input/output alphabet

In one implementation, a device may include a voltage regulator circuit, a data processing circuit, and an error correction circuit, where the error correction circuit may correct errors in data processed by the data processing circuit to obtain error-corrected data and output an error-corrected version of the processed data. Additionally, an error monitor circuit may output an error signal indicative of a level of the errors in the processed data. A control circuit may receive the error signal and control the voltage regulator circuit to adjust, based on the error signal, the supply voltage to the data processing circuit. In some implementations, the control circuit may also base its decision to control the voltage regulator circuit based on available timing margins in the data processing circuit.

Sandy Thomson

Papers

800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing

Novel 16QAM carrier recovery based on blind phase search

Method, system, and apparatus for interpolating an output of an analog-to-digital converter

Distribution matching for probabilistic constellation shaping with an arbitrary input/output alphabet

Adaptive voltage scaling based on the results of forward error correction processing