1. Introduction

2. Services and requirements for F6G

3. Photonics-based subsystems for all-optical processing

3.1. All-optical coherent & simplified coherent solutions: Challenges & Opportunities

Recently, interest has arisen in the implementation of analogue-based coherent receivers to substantially reduce the power consumption at the receiver side, by eliminating the power-hungry DACs/ADCs and DSP. Along these lines, several new techniques have been proposed to implement spectrally-efficient and low-power consumption links, based in particular on Stokes vector detection, Self-Coherent (SC), and Krammers-Kronig (KK) techniques. Self-coherent systems have been proposed for DCI due to their promise of offering low complexity, higher spectral efficiency, and increased dispersion tolerance [21], enabling partial attainment of the advantages of fully-coherent detection. Several DSP-based approaches have been proposed to linearize self-coherent systems (mitigating the signal-to-signal beat noise component), however, the KK algorithm turns out the most effective. Stokes vector detection can exploit up to three degrees of freedom and circumvent the local oscillator (LO) required in coherent systems, but it is also based on power-hungry ADCs and DSP [22]. A quantitative comparison of four low cost/power consumption TRxs for short reach interconnects is presented in the following table.

Table 2. Comparison of various parameters of four low -cost/-power transceiver schemes for short-reach transmission [23].

Table 2. Comparison of various parameters of four low -cost/-power transceiver schemes for short-reach transmission [23].

	PAM-4	Duobinary	DP-QPSK	KK
LO requirements	DFB	DFB	Electrical PLL	None
Tx complexity	MZM	MZM / DML	I/Q modulator	Quad MZM for SSB
DSP at Rx	NO	NO	NO	YES (Heavy)
Spectral efficiency (b/s/Hz)	1	1.1	2	5
Filter-less networks	YES	YES	YES	NO
Signaling rate	25 Gbs/s/λ	25 Gbs/s/λ/pol	112 Gbs/s/λ/pol	224 Gbs/s/λ/pol
Reach	20 km	20 km	20 km	100 km
Rx sensitivity (at FEC limit)	-17 dBm	-37.3 dBm	-26.5 dBm	24 dB OSNR
Application	Intra-DC	Intra-DC	Short reach	Inter-DC

A notable alternative, the all-optical receiver, has been initially presented in [23] (see Figure 5 for its high-level design), indicating the possibility to replace the DSP chip with an all-optical analogue front-end that can achieve most of the required functionality.

The novel required Rx Blocks of this scheme are:

Analog optical carrier phase recovery utilizing an Optical Phase-Locked Loop (OPLL) based on a low-linewidth Laser [20,24], to optically decouple the two I and Q quadratures, and

Analog optical polarization demultiplexing circuit [25], decoupling all-optically the two signal polarizations.

The OPLL, could be implemented, at a bandwidth of ~10-100MHz, based on a high-bandwidth photodetector and a low-linewidth semiconductor laser, operating the local oscillator laser as a Current Controlled Oscillator (CCO). The Analogue Polarization Demultiplexing (PolDMUX) circuit block, depicted in Figure 5 in the polarization controller inset, consists of a cascade of a several 2x2 matrix stages and it is implemented based on directional couplers and phase modulators (PMs), as it experimented in [21]. It is noted that slow (~10kHz rate) precise tuning control of the phase alignments of the PMs in the cascade is required as an essential enabler for the practical polarization demux functionality of this block to which we refer as Analog Optical Signal Processor (AOSP). Due to the all-optical nature of signal processing in the 10KHz-100MHz building blocks of this transceiver, which are transparent to the ultra-high-speed optical signals modulated onto the light flowing along it, the efficient operation of the AOSP polarization demux is essentially independent of the baud-rate, hence the supported bit-rate is scalable to > 800 Gb/s rates per wavelength/space lane. Moreover, multiple optical wavelengths may be served at once (polarization demultiplexed together by a single AOSP) to further scale the offered capacity. The AOSP is still followed by ADCs (slicers), albeit at much reduced Effective Number Of Bits (ENOB), and the DSP is much reduced in cost, footprint and power consumption, as the XY polarizations and IQ quadratures are effectively decoupled ahead of the four ADCs. Other receiver operations such as timing recovery and detection may be performed with a high-speed analog electronics stage using conventional clock and data recovery (CDR) techniques. Alternatively, these functions may be relegated to the scaled-down DSP.

Figure 5. Block diagram of DSP-free coherent receiver (PM-QPSK) based on AOSP [26].

Figure 5. Block diagram of DSP-free coherent receiver (PM-QPSK) based on AOSP [26].

For basic proof of principle feasibility purposes, the operation of such AOSP receiver has been demonstrated via preliminary simulations and the results are shown in Figure 6. In this particular example, the operation at an aggregate rate of 400 Gb/s was considered [23]. In the simulation platform, the AOSP was constructed by considering a low-linewidth local oscillator laser and a cascade of discrete Mach-Zehnder Modulators (MZMs) and PMs for the Pol-Demux, which is not as effective as in the case where integrated elements in a Photonic Integrated Circuit (PIC) are employed. Following the all-optical AOSP-based polarization demultiplexing and phase/frequency recovery operations, the four coherently recovered analogue lanes Ix, Qx, Iy, Qy, display clear bipolar Pulse Amplitude Modulation 4 (PAM4) eye diagrams which are detectable using conventional CDRs (based on all-analogue electronic symbol timing recovery & 4-level slicing implemented in merchant Silicon circuity). These initial simulation results at 400 Gb/s [23], are significant, as they validate the feasibility of the all-optical analogue signal processing based coherent transceiver.

The aforementioned all-optical solutions, besides the sufficient transmission performance, can also scale-up the capacity capabilities in order to meet the ever-growing traffic demands of the coming decade. Furthermore, they can serve as important solutions for optical interconnects in datacenters and 5G telecom infrastructures, which are the network domains wherein the cost (measure in $/Gbps) and power consumption (W/Tb/s) must be further reduced [23].

Figure 7, presents a high-level comparison between the AOSP based transceiver and other popular transceiver designs for 400 Gb/s interconnects. Evidently, the cost and power consumption using conventional coherent solutions are prohibitively large for such applications. A straightforward solution could consist of the combination of parallel fibers exploiting lower performance transceivers in order to achieve the desired capacity. Unfortunately, the scalability of such idea, once benchmarked against alternatives, is limited by the number of fibers that can be deployed in parallel. Another potential solution is Wavelength Division Multiplexing (WDM), however WDM is constrained in cost and power consumption due to the high complexity of deployment. As a consequence, it is becoming evident that for short-reach applications (below 10 km) the AOSP transceiver may be a competitive option as it concurrently addresses the desirable characteristics of low cost and power consumption, high data rate, low latency, high scalability and reach of up to at least 10 km.

Other notable options are classified in the category of ‘simplified coherent’ solutions. These solutions entail various tradeoffs between the extremes of direct-detection and coherent detection, exploiting both the amplitude and the phase of the optical signal to increase the transmitted data rate, albeit with lower complexity compared with conventional coherent transceivers, e.g. comprising a smaller number of optical elements. The main advantage in coherent technology is the benefit from up to 4-times higher bit-rate, while keeping transceiver cost and power consumption at bay. To achieve this goal, new all-optical techniques are envisaged [27,28,29]. Using these alternatives, the cumbersome DSP components are no longer required. These techniques target extremely low-cost applications where the requirements cannot be simply met via using direct detection techniques, whereas the cost and power consumption of fully-coherent solutions would be prohibitive, whereas the coherent performance would be an ‘overkill’.

In this category of solutions, Erkilinc et al. in [27] proposed a single polarization scheme that incorporates a polarization-time block-code signal, circumventing polarization tracking, driving a dual-polarization modulator at the transmitter. In this context, both intradyne and heterodyne architectures have been investigated and resulted to massive simplification of the receiver side, discarding polarization rotators, beam splitters, two balanced Photo-Diodes (PD) as well as ADCs. Next, Cano et al [28] adopted another simplified version, first introduced by Kazovsky et al. [29], using a 3x3 coupler followed by three single-ended PDs and 3 ADCs for the intradyne reception. The main advantage of this solution is the polarization scrambling technique [30], attaining polarization-independent detection avoiding polarization tracking devices at the receiver. The third approach by Ciaramella [31] utilized polarization-independent reception with a polarization beam splitter (PBS) and a symmetric 3x3 coupler followed by three single-ended PDs. The 3x3 coupler incorporated the intermixing of 3 signals: a) one horizontal signal b) one vertical polarized signal and c) the incoming signal. The horizontal and vertical signals are output by the polarization beam splitter. In this architecture, ADCs and DSP can be completely removed; therefore, only On Off Keying (OOK) or PAM4 signal can be detected by the receiver. The most recent work in the literature is called quasi-coherent [32,33], also classified in the category of simplified coherent approaches. Here, the signal and LO are mixed using a coupler. Then passing them through a PBS, the two resulting optical signals are fed into two PDs and next, by performing envelope detection, they are inserted into the electronic amplifier. Based on these aforementioned techniques, we conclude that dual-polarization schemes or Quadrature Amplitude Modulation (QAM) formats can be dispensed with under certain conditions, for the sake of minimization of optical and/or electrical components, albeit the reduced data rates. That is why these coherent-like approaches are best suited for extremely low-cost and low-power consumption solutions in short-reach applications, but do not typically qualify for either long-haul transmission or for high-end (inter) datacenter interconnects.

It is for this reason, that new coherent approaches emerge in order to cover the white areas of optical communication market. Based on the complexity which directly impacts the cost and power consumption, Figure 8 maps the different options with the network domain where they can be applied.

4. Optical transmission: key challenges and proposed directions

4.1. Capacity scaling: what to expect in the F6G

According to Cisco, the traffic demands will keep increasing at a rate of about 100% every 3 years (Figure 16). The reason for this traffic increase, as mentioned in section 2, is the need for meeting the ever-increasing demand for “fresh” services which has been accelerated due to the increased tele-working following the COVID-19 pandemic. So, in order to satisfy the requirements of the upcoming F6G services while at the same time avoiding the 100 Tb/s capacity crunch of the Standard Single Mode Fiber (SSMF), network providers and vendors have to investigate and deploy novel techniques aiming at increasing of at least, one order of magnitude the overall transportation capacity compared with F5G systems. The general trend of transportation capacity per single fiber over time can be seen in Figure 17, according to which during the past five decades, the transportation capacity within an optical fiber started from about some tens of Mb/s and has now reached about 100 Tb/s. This value is the maximum nominal rate of the Single Mode Fiber (SMF) in long-haul transmission. This figure highlights an increase of six orders of magnitude in less than fifty years and clearly underlines the need to find ways in order to exceed 1 Pb/s by 2030.

Recently, various ways have been proposed in order to overcome this 100 Tb/s capacity crunch, such as: a) to exploit more spectrally efficient modulation formats, like probabilistic shaping and coded modulation, b) to exploit the entire low-loss attenuation spectrum of the optical fiber (< 0.4 dB/km) which extends from 1260-1625 nm by transmitting either a higher number of channels or by increasing the bandwidth of each channel (migrating e.g. to higher baud rates) and c) to use SDM, mainly by either considering bundles of SMFs or by using multi-core / mode fibers. In all these three cases, the overall transportation capacity can be extended beyond 100 Tb/s and eventually satisfy the target of 1 Pb/s for 2030 for commercial systems and the target of 10 Pb/s for research works by 2030, as it is illustrated in Figure 17.

Another observation which can be extracted from this illustration, is that the research technology is about 8-10 years ahead of commercial systems. This is expected, as the results of the huge effort of the researchers around the globe, need to be refined and distilled in order to produce the most cost and at the same time, performance wise methods/technologies/components which will then proceed to commercialization. In order to attain Pb/s rates, novel optical components need to be introduced, such as band filters, cost-efficient optical amplifiers (especially for O and E-bands), band switches and new generation pump sharing SDM based amplifiers which can provide both increased redundancy and reduced costs [60]. Another key technology which can aid the F6G networks to transmit data in even larger distances and/or exploit higher cardinality modulation formats, increasing in this way the transported capacity, is the ML-based signal processing [61]. For example, Deep Neural Networks (DNNs) using digital backpropagation (DBP) methods can achieve a low-complexity nonlinearity compensation. Their performance is similar to those of conventional DBP modules but they encounter relaxed computational complexity.

Figure 17. Evolution of transported capacity within a single mode fiber over the last 50 years along with forecasts for up to 2030 [62,63,64].

Figure 17. Evolution of transported capacity within a single mode fiber over the last 50 years along with forecasts for up to 2030 [62,63,64].

Next, we study the evolution of the capacity per channel over the time interval 1970-2020 in order to understand the general trend and elaborate on the capacity needs for the next decade. Figure 18 highlights the need for fresh commercial systems able to reach at least 2 Tb/s per port by the end of the decade while in [65] the need for up to 6.4 Tb/s by 2031 is also underlined. These rates can be achieved using multiple optical channels packed together forming a super-channel. Today, the highest commercially available rate is 800 Gb/s (Table 4) and can be realized mainly with two options. The first one is to employ a baud rate much larger than those of 100/200G cases, e.g. 90 Gbaud, and the second is to employ more efficient modulation formats, e.g. probabilistic constellation shaping and/or employ a higher cardinality modulation. Each of these alternatives has its own trade-offs, e.g. lower cardinality modulation formats such as 16QAM, require a lower Optical Signal to Noise plus Interference Ratio (OSNIR) in order to attain the same Bit Error Rate (BER) compared with higher cardinality formats, e.g. 64QAM. This, eventually leads to a higher reach for the lower cardinality modulation formats; however, this reach is traded for a higher number of channels, as the lower cardinality formats consume a wider bandwidth for the same data rate. In this case, capacity is traded for connectivity, and the optimal solution is based solely on the high-level design set by the network engineer.

Another observation that can be derived from Table 4 is that the evolution on the baud rate and modulation format from 100G to 800G rates clearly indicates that F6G systems need to adopt transceivers with baud rates of 256 Gbaud and even higher in order to exceed 2 Tb/s. The most recent research works demonstrate baud rates of 168 Gbaud with PM-16QAM format [66] and 220 Gbaud with OOK format [67]. Further, the interplay between attainable reach and bit rate is shown in [68]. In particular, in [68] it is shown that when using 130 Gbaud while considering different modulation formats, rates of 930 Gb/s can be transmitted over 1105 km and rates of 1.28 Tb/s can be transmitted over 452.4 km. Finally, rates of 1.6 Tb/s can be transmitted in a link length of 153.4 km [68]. This analysis clearly designates that in the near future, the technology will show significant progress towards this direction both by stressing the electronics to exceed 512 Gbaud and by using DSPs which will be more tailored to system parameters, than current DSPs, allowing to increase the transmission reach (for longer links). However, for short-reach communication the use of oDAC approaches, as presented in the previous section, seems an excellent approach to scale the data rates in a low cost and low power consumption way.

Figure 18. Evolution of transported capacity per single channel for commercially available systems over the last 50 years along with forecasts for up to 2030 [62,63,64,69,70].

Figure 18. Evolution of transported capacity per single channel for commercially available systems over the last 50 years along with forecasts for up to 2030 [62,63,64,69,70].

Table 4. Details of commercially available transceivers for regional and long-haul transmission [71].

Table 4. Details of commercially available transceivers for regional and long-haul transmission [71].

Type	Modulation Format	Baud Rate	Channel Spacing	Data Rate	Required OSNIR (for BER=10−3)
100G	PM-QPSK	32Gbaud	37.5 GHz	100 Gb/s	9.8 dB
200G	PM-16QAM	32Gbaud	37.5 GHz	200 Gb/s	16.55 dB
400G	PM-16QAM	63Gbaud	75 GHz	400 Gb/s	16.55 dB
	PM-64QAM	42Gbaud	50 GHz	400 Gb/s	22.5 dB
	PCS-16QAM	80-95Gbaud	100 GHz	400 Gb/s	varies
800G	PM-16QAM	128Gbaud	150 GHz	800 Gb/s	16.55 dB
	PM-32QAM	96Gbaud	112.5 GHz	800 Gb/s	19.5 dB
	PM-64QAM	80Gbaud	100 GHz	800 Gb/s	22.5 dB
	PCS-64QAM	90Gbaud	100 GHz	800 Gb/s	N/A
200–800G	Probabilistic Shaping	60-95Gbaud	75-100 GHz	200-800 Gb/s	varies

As the rates per channel can reach up to 800 Gb/s with currently available solutions, the attainable rates in the access part of the network are significantly smaller. For example, the most recent PON standard, which is 50G-EPON [72], can provide access to the network with a downstream rate of 50 Gb/s. In the F6G, there is a strong need to advance to an 100G-PON standard or directly to 200G-PON in order to support the various capacity demanding services which were presented in section 2. To further understand this need, Figure 19 illustrates the progress over time of the downstream rate for the various PON standards. Figure 19 also shows the main advantages of each PON standard over its predecessor. These advances on bit rate from standard to standard are a result of the advances on the transceiver operational parameters, such as modulation format, baud rate, operational wavelength and number of wavelengths. It goes with a saying that as the access part of the network is the most cost and power consumption sensitive, direct detection technology stills qualifies to increase the overall rate.

Towards the access rate evolution beyond 50 Gb/s, several options for realizing the 100G-PON have been demonstrated. In the first option, which was presented by Huawei in the BT’s Innovation 2017 exhibition, the system exploited a ValkyrieBay chassis equipped with the unique Xena Loki-100G-5S-1P 5-speed dual-media test module in order to generate Ethernet traffic through the 100G PON. The main purpose of this test was the verification of the overall implementation [73]. More specifically, the system exploited WDM, as four wavelengths were employed to carry 4x25G data channels over a 10 km feeder fiber. This implementation is highly scalable as it can support different rates: 25G, 50G or 100G. The second option, is a 100G PON Prototype developed by ZTE [74] which is an integration test platform featuring many capabilities as it can simulate various modulation formats, like Non-Return-to-Zero (NRZ), Duobinary, PAM4, and DMT (Discrete Multitone) whilst various DSP algorithms can be employed. Moreover, it considers four 25G wavelengths supporting the following four combinations: asymmetric single-wavelength uplink 10G/downlink 25G, symmetric single-wavelength uplink/downlink 25G, asymmetric four-wavelength uplink 40G/downlink 100G, and symmetric four-wavelength uplink/downlink 100G. The feasibility of this prototype was verified at MWC 2017 in Barcelona. The third 100G PON solution was developed by Nokia Bell Labs and validated in Vodafone’s Competence Centre in Eschborn, Germany [75]. This option is based on a single wavelength using a combination of 25G optics and advanced DSP techniques to reach 100 Gb/s. As the 100G PON commercialization is planned for the second half of decade, the next 100G PON standard is expected to be the result of a significant amount of work and conveyed knowledge between the academia, operators, vendors and standardization organizations in order to adopt the most cost and energy efficient solution.

One of the main aspects for the adoption of a PON technology is the maximization of spectral efficiency. Ideal candidates towards achieving this target are Nyquist wavelength division multiplexing PON (Nyquist-WDM-PON) and orthogonal frequency division multiplexing WDM PON (WDM-OFDM-PON). Both solutions can a) provide a finer granularity compared with Time Division Multiplexing (TDM)-PON schemes, b) enable the bandwidth assignment in an as-needed basis and c) maximize spectral efficiency as they have a rectangular spectrum.

In particular, Orthogonal Frequency Division Multiple Access (OFDMA) PON is a well-established technology in wireless and fixed access systems such as WiMAX, long-term evolution (LTE), Asymmetric Digital Subscriber Line (ADSL) and Wi-Fi and can be efficiently incorporated in the PON context to provide significant advantages [76,77,78]. Next, OFDM can be used both for coherent and direct detection schemes. This is important, because when the coherent scheme is adopted, the overall performance can be greatly boosted, and on the other hand, when a DD scheme is considered, low-cost components, such as Directly Modulated Laser (DML), Vertical Cavity Surface Emitting Laser (VCSEL) and low-bandwidth electronics along with DMT can be used, suppressing the overall costs. Finally, OFDMA-PON enables the convergence between wireless and fixed networks allowing to manufacture universal components for both uses.

Further, Nyquist-WDM-PON [79] is a capable solution for PON which aims to minimize the overall number of transceivers, by aggregating multiple transceivers onto a single hub transceiver. The hub transceiver uses coherent technology and multiple combined subcarriers based on Nyquist criterion into a single optical wavelength. In this way, a 100G or a 400G wavelength can be divided into 4 x 25Gb/s or 16 x 25Gb/s subcarriers, respectively or even into a combination of different number of subcarriers and line rates. Using Nyquist-WDM-PON, the network is migrating from point-to-point architecture towards point-to-multipoint, decreasing the overall number of optical interfaces from 2N to N+1, where N is the number of access nodes. This solution provides significant advantages compared with legacy PON systems. The most important are a) decreased number of transceivers due to the use of hub transceivers, b) better routing efficiency, density, and simplicity, as the overall number of ports is significantly decreased, c) better alignment of CapEx with actual bandwidth requirements and quick adaptation on changing bandwidth demands and traffic patterns, d) lower OpEx in terms of power consumption, footprint, number of aggregation sites and support. The concept of Nyquist-WDM-PON has been already commercialized in a version with the name “XR-Optics” [80].

Regarding the physical layer performance of Nyquist-WDM-PON and WDM-OFDM-PON, a comparative analysis is illustrated in [81] and shown in Figure 20. This analysis shows that the physical layer performance is highly dependent on the linewidth, resulting in considerable phase noise. In [81], two phase noise compensation methods were considered, namely, common phase error (CPE) and orthogonal basis expansion (OBE). Nyquist-WDM-PON showed a greater performance compared with WDM-OFDM-PON. This happened as the inter-carrier-interference (ICI) in WDM-OFDM-PON cannot be mitigated by CPE. Next, when OBE is used, WDM-OFDM-PON attains a greater performance than Nyquist-WDM-PON under the two cases, especially at small laser-linewidth. WDM-OFDM-PON is vulnerable to phase noise due to the long OFDM symbol period, but with the aid of effective phase noise suppression method, it can attain a lower BER compared with Nyquist-WDM-PON and eventually qualify as a cost-effective solution for access networks.

5. Optical Switching: key challenges and proposed directions

6. Conclusions

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