Carrier-assisted differential detection
Light: Science & Applications volume 9, Article number: 18 (2020) AbstractTo overcome power fading induced by chromatic dispersion in optical fiber communications, optical field recovery is a promising solution for direct detection short-reach applications, such as fast-evolving data center interconnects (DCIs). To date, various direct detection schemes capable of optical field recovery have been proposed, including Kramers−Kronig (KK) and signal−signal beat interference (SSBI) iterative cancellation (IC) receivers. However, they are all restricted to the single sideband (SSB) modulation format, thus conspicuously losing half of the electrical spectral efficiency (SE) compared with double sideband (DSB) modulation. Additionally, SSB suffers from the noise folding issue, requiring a precise optical filter that complicates the receiver design. As such, it is highly desirable to investigate the field recovery of DSB signals via direct detection. In this paper, for the first time, we propose a novel receiver scheme called carrier-assisted differential detection (CADD) to realize optical field recovery of complex-valued DSB signals via direct detection. First, CADD doubles the electrical SE compared with the KK and SSBI IC receivers by adopting DSB modulation without sacrificing receiver sensitivities. Furthermore, by using direct detection without needing a precise receiver optical filter, CADD can employ cost-effective uncooled lasers as opposed to expensive temperature-controlled lasers in coherent systems. Our proposed receiver architecture opens a new class of direct detection schemes that are suitable for photonic integration analogous to homodyne receivers in coherent detection.
Coherent detection has profoundly impacted optical communications due to its superior capability of recovering both optical intensity and phase, namely, field recovery1,2. Distinct from conventional intensity modulation with direct detection, field recovery enables in-phase/quadrature (IQ) modulation, increasing the transmission spectral efficiency (SE). Moreover, optical field impairments such as chromatic dispersion and polarization mode dispersion can be digitally compensated by accessing the field information. However, coherent transceivers are relatively costly due to the hardware complexity and tight specifications for laser. To address hardware complexity, photonic integration has become a promising solution7, but the issue of precise frequency control between the local oscillator and transmitter laser is inevitable. Consequently, coherent detection remains a suitable solution for medium- to long-haul transport8, while direct detection is still dominant for short-reach applications, such as data center interconnects9,10. For direct detection, loss of field recovery is the main obstacle to digitally compensating chromatic dispersion, limiting the transmission reach for conventional IM/DD systems. To bridge the gap between direct and coherent detection, a self-coherent scheme has attracted extensive research interest, in which a strong carrier is inserted at the transmitter and propagated along with the information-bearing signals. After square-law detection using a single-ended photodiode (PD), signals can be extracted from the signal-carrier beating term, and the optical field is reconstructed without using a local oscillator. Since direct detection generally provides only intensity information, until now, signals have been mainly restricted to the single sideband (SSB) modulation format in various proposed intensity-only detection schemes14. For such detection schemes, signal−signal beating interference (SSBI) is the dominant limitation. To mitigate SSBI, a frequency gap, which is commonly as wide as the signal bandwidth, can be placed between the carrier and signals15. To overcome the poor SE of the above approach, a self-coherent scheme without a frequency gap has been proposed in which SSBI can be estimated and then subtracted in an iterative manner16,17,18. In recent years, the Kramers−Kronig (KK) receiver has been proposed to effectively mitigate SSBI without using iterations. Via KK relations, the phase of signals is obtained using the intensity information. Since the SSB modulation format is adopted for KK receivers, twin-SSB20,21 and WDM22,23-based KK receivers implemented with optical filters have been proposed to fully utilize the optical spectrum. Compared to the optical SE, however, a high electrical SE is a more dictating factor for short-reach applications. For KK or SSBI iterative cancellation receivers, the electrical SE is intrinsically limited by the SSB modulation format. Since one sideband is unfilled, half of the electrical SE is lost. Apart from the electrical SE, SSB signals suffer from noise folding due to the square-law detection of the photodiode. Consequently, rather than SSB signals, it is highly desirable to investigate the direct detection of complex-valued double sideband (DSB) signals with field recovery. Although there are some demonstrations of DSB direct detection via block-wise phase switching, the effective SE is reduced by half due to the repetition of data.
To reconstruct the optical field, a carrier is necessary to obtain the desired carrier-signal beating term. We denote the carrier and signal field as C and S, respectively. Assuming that the responsivity of the photodiode equals 1 for simplicity, after square-law detection, the received photocurrent I can be expressed as:
See: https://www.nature.com/articles/s41377-020-0253-8
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