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Orthogonal time division multiplexing (OrthTDM) interleaves sinc-shaped pulses to form a high baud-rate signal, with a rectangular spectrum suitable for multiplexing into a Nyquist WDM (N-WDM)-like signal. The problem with generating sinc-shaped pulses is that they theoretically have infinite durations, and even if time bounded for practical implem...
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We present a 10-GHz granularity reconfigurable WDM superchannel filter in a photonic integrated circuit. The filter uses a ring-resonator-assisted tapped-delay-line topology. An example configuration with two ring resonators and four tapped delay lines provides a spectral filter with a flat-top passband, a roll-off factor of 8%, a tunable central w...
Citations
... Meanwhile, optical orthogonal frequency division multiplexing (OFDM) enables flexible allocation of different subcarriers within a single mode [6][7][8][9][10][11]. While electrical discrete Fourier transform (DFT) faces significant computational load as the number of subcarriers increases [12], optical DFT can significantly reduce the processing time, using an arrayed waveguide grating (AWG) that optically generates and processes a set of DFT subcarriers simultaneously [13][14][15][16][17]. However, using adjacent optical subcarriers can cause significant signal quality degradation due to crosstalk, and various solutions have been proposed to address this issue, including frequency domain filtering through cascaded AWGs [18] and sparse subcarrier multiplexing [19]. ...
... On the other hand, signal propagates in optical devices at the speed of light, so that the processing time of optical DFT is negligible. Arrayed waveguide grating (AWG) configurations have been proposed and fabricated to implement the optical DFT [18][19][20]. Using on-off keys (OOK) modulation, analog-todigital conversion is not required, and the optical crosstalk can be reduced by optical filtering and by an AWG cascade configuration [21]. ...
An arrayed waveguide grating (AWG) configuration can simultaneously perform the optical discrete Fourier transform and multiplex and demultiplex (MUX/DeMUX) two optical modes, to optically generate/process orthogonal frequency division multiplexing (OFDM) signals in future scalable, converged two-mode optical communication systems. We use a single AWG-based MUX/DeMUX, inter-mode and intra-optical-subcarrier symbol interleaving to mitigate the mode/subcarrier crosstalk. We analyze the performance of a 2-mode × 8 optical-subcarrier × 10 Gbit/s, on-off keying (OOK), differential phase shift keying (DPSK), and differential quadrature phase shift keying (DQPSK) mode division multiplexing (MDM)-OFDM system, that can be used in high-speed, large-capacity short-range optical link, using only passive all-optical signal processing. We numerically demonstrate that it possible to achieve error-free transmission for an 8 subcarrier × 10 Gbit/s × 2 mode system, using OOK and DPSK modulation and for a 4 subcarrier × 10 Gbit/s × 2 mode system, using DQPSK modulation, using intra-optical subcarrier symbol interleaving.
... Nyquist signals can be generated optically by either filtering modulated signals [6,7,[20][21][22][23] or by modulating sinc-shaped optical pulses [3,[24][25][26]. These reduce the requirements on DSP and DACs, while being able to operate over very high bandwidths [22]. ...
Insufficient receiver bandwidth destroys the orthogonality of Nyquist-shaped pulses, generating inter-symbol interference (ISI). We propose using an optical pre-sampler to alleviate the requirement on the receiver bandwidth through pulse re-shaping. Experiments and simulations using an optically shaped 40-Gbaud Nyquist-shaped on-off-keying signal (N-OOK) show receiver sensitivity improvements of 4- and 7.1-dB under 18- and 11-GHz receiver electrical bandwidths, respectively.
... Optical communication systems are usually deployed over fiber-optic links [1][2][3], free-space optical (FSO) links [4][5][6], or hybrid FSO-fiber links [7]. Recent advances in photonics integrated circuits (PIC) have been greatly pushing forward the worldwide application of optical communications [8][9][10]. Although they have enabled a capacity-approaching communication [11][12][13], fiber-optic links may be too fragile or costly to be deployed in some environments, e.g., seismic belts. ...
In this paper, we experimentally investigate the turbulence mitigation methods in free-space optical communication systems based on orbital angular momentum (OAM) multiplexing. To study the outdoor atmospheric turbulence environment, we use an indoor turbulence emulator. Adaptive optics, channel coding, Huffman coding combined with low-density parity-check (LDPC) coding, and spatial offset are used for turbulence mitigation; while OAM multiplexing and wavelength-division multiplexing (WDM) are applied to boost channel capacity.
... Optical communication systems are usually deployed over fiber-optic links [1-4] and free-space optical links [5][6][7]. Recent advances in photonics integrated circuits (PIC) have been greatly pushing forward the worldwide application of optical communications [8][9][10]. Although have enabled a capacity-approaching communication [11][12][13], fiber-optics links may be too fragile or costly to be deployed in some environments, e.g., seismic belts. ...
We review recent progress in high-speed orbital angular momentum (OAM) multiplexed free-space optical communication systems. The outdoor atmospheric turbulence is emulated by an indoor turbulence emulator, which is based on split-step beam propagation method. Adaptive optics, channel coding, Huffman coding combined with LDPC coding, and spatial offset are used for turbulence mitigation; while OAM multiplexing and wavelength-division multiplexing (WDM) are applied to boost aggregate capacity.
... Although they offer tuning flexibility, these filters have a trade-off between performance and complexity. In particular, for WDM superchannel applications, tens to hundreds of delay lines, with lengths of tens of centimeters, would be required to achieve a filter passband roll-off factor <10% and a frequency resolution in the order of sub-GHz [32], [33]. This raises challenges for device fabrication with respect to waveguide uniformity and loss, which have a significant impact on the filter's bandwidth and shape, together with the risk of lower yield per wafer due to the large chip size. ...
We present a 10-GHz granularity reconfigurable WDM superchannel filter in a photonic integrated circuit. The filter uses a ring-resonator-assisted tapped-delay-line topology. An example configuration with two ring resonators and four tapped delay lines provides a spectral filter with a flat-top passband, a roll-off factor of 8%, a tunable central wavelength, and a selectable free spectral range, i.e. 2° or 4° the 3-dB passband bandwidth. The device was fabricated in a silicon nitride (Si3N4/SiO2) waveguide with a chip area of 0.36 cm2 and has a fiber-to-fiber insertion loss of 5 dB. We experimentally demonstrate the filter for selecting a subchannel from a WDM superchannel comprising 10-GHz subchannels with 10% and zero guard bands. Owing to the infinite impulse responses of ring resonators, this filter requires a significantly smaller area and fewer tuning elements compared with designs based on only tapped-delay-line finite-impulse-response filters. This work contributes to the creation of chip-scale all-optical WDM superchannel transceivers and reconfigurable optical add/drop multiplexers (ROADMs).
... A practical application is to create a high-rate optical channel using lower-rate optical modulators [78]. In [79], we proposed a direct implementation of a near-Nyquist WDM filter, using an FIR with amplitude weighting of each of its waveguides to give a truncated-sinc (TS) impulse response. The filter is fed with OTDM pulses, and the output is Nyquist-OTDM signal. ...
... The filter is fed with OTDM pulses, and the output is Nyquist-OTDM signal. In this case, the reconfigurable filter is working as a FIR filter, considering the FSR of the filter and receiver speed limitation, we tuned the filter to have a near rectangular frequency response, with a nominal bandwidth, B, of 40 GHz and its impulse response is a truncated sinc with pulse width 25 ps, which can be expressed as [79] ...
Chip-scale integrated optical signal processors promise to support a multitude of signal processing functions with bandwidths beyond the limit of microelectronics. Previous research has made great contributions in terms of demonstrating processing functions and device building blocks. Currently, there is a significant interest in providing functional reconfigurability, to match a key advantage of programmable microelectronic processors. To advance this concept, in this work, we experimentally demonstrate a photonic integrated circuit as an optical signal processor with an unprecedented combination of two key features: reconfigurability and terahertz bandwidth. These features enable a variety of processing functions on picosecond optical pulses using a single device. In the experiment, we successfully verified clock rate multiplication, arbitrary waveform generation, discretely and continuously tunable delays, multi-path combining and bit-pattern recognition for 1.2-ps-duration optical pulses at 1550 nm. These results and selected head-to-head comparisons with commercially available devices show our device to be a flexible integrated platform for ultrahigh-bandwidth optical signal processing and point toward a wide range of applications for telecommunications and beyond.
... For example, if the chirp is from redder to bluer (both, of course, infra-red), a standard single mode fiber will speed up the bluer parts relative to the redder parts, providing pulse compression. This can be used to create sinc-like pulses from chirped OFDM subcarriers, which we call 'quasi-NWDM' [79]. Multiple sets of sinc pulses can be interleaved if they sets are delayed relatively to one another. ...
... These frequency offsets mean that the sets of pulses will acquire different delays in the dispersive fiber, so that they become interleaved in time. We have previously shown that the associated spectrum of chirped-OFDM/quasi-NWDM is not suitable for creating superchannels as it has wide tails: a better spectrum can be obtained using an optical-time division multiplexer (OTDM) as a preprocessor, connected to a single input to the first slab coupler, as in Fig. 17(b) [79]. This creates a spectrum with a Fresnel shape, which is analogous to the far-field pattern of an aperture illuminated by a point source behind it. ...
... The width of the spectrum can be reduced further if the chirping is forgone, for example, if the sinc pulses are generated (with some truncation) using a finite-impulse response filter, as shown in Fig. 17(c). In this case the spectrum is a rectangular function convolved with a sinc function [79]. Nagashima et al. have shown that fiber nonlinearity can be mitigated using fractional Fourier transforms, by letting the signal format oscillate between chirped-OFDM and quasi-NWDM along a dispersion managed fiber [80]; for example, by ensuring that the signal has a low PAPR just after each optical amplifier. ...
Photonic circuits are the key to advanced functionality in future optical systems, as they efficiently process terabit/s data streams. This paper reviews how photonic circuit topologies have evolved to support high-spectral efficiency modulation formats, including: all-optical optical frequency division multiplexing (AO-OFDM), discrete Fourier transform spread OFDM (DFT-S-OFDM), Nyquist wavelength division multiplexing, (NWDM), orthogonal time division multiplexing (OrthTDM, OTDM), chirped-OFDM, and quasi-Nyquist signals. The importance of the order of components such as modulators, delays, and filters within these circuits is stressed, as simple changes to the circuit topology have significant impacts on the spectra of the signals, and the required bandwidths of the modulators.
... Electrical shaping can provide quite a good roll-off factor, but the baud-rate is restricted by the speed of the digital to analogue converter (DAC). Alternately, optical pulse shaping methods can circumvent the limitation of electrical speed, the methods include: modulating externally modulated lasers to generate mode locked combs [8], using a liquid crystal spatial modulator [9] or an arrayed waveguide grating router (AWGR) [10][11][12] to shape the Gaussian pulse from a mode-locked laser. Among all the proposed devices, photonic integrated circuits (PICs) have advantages of: compact size, low cost, robustness, and can integrate many functions onto one chip. ...
... In [10], a chirped AWGR has been proposed for implementing a fractional Fourier transform (FrFT), which produces chirped OFDM signals that can be converted to quasi N-WDM channels using a dispersive fiber (DF). However, we have shown that these channels have much wider spectra than necessary [11], so require large guard bands for a given level of inter-channel-interference (ICI) when multiplexed into an N-WDM system. Similarly, a modified chirped AWGR has been demonstrated to achieve a narrower overall spectrum, but a guard band is still needed to reduce crosstalk [11]. ...
... However, we have shown that these channels have much wider spectra than necessary [11], so require large guard bands for a given level of inter-channel-interference (ICI) when multiplexed into an N-WDM system. Similarly, a modified chirped AWGR has been demonstrated to achieve a narrower overall spectrum, but a guard band is still needed to reduce crosstalk [11]. A direct FIR filter is proposed with amplitude weighting of each of its waveguides to give a truncated-sinc impulse response and near rectangular spectrum; however, this requires large number of splitters with designed splitter ratios [11]. ...
We propose and demonstrate by simulations a novel Nyquist-WDM (N-WDM) superchannel transmitter based on an arrayed waveguide grating router (AWGR). This approach can generate Nyquist pulses at multiple wavelengths using a single AWGR. Results for a 3-channel 960-Gbit/s QPSK superchannel system show that a 10% guard band reduces the inter-channel interference (ICI) sufficiently. The design introduces less than 0.16-dB penalty when the waveguide loss is 2 dB/cm and 0.73-dB penalty when the standard deviation of phase error is 10°. Such Nyquist pulse shapers can be realised on a chip scale using photonic integrated circuits technology, and could be compactly integrated with other functional components to create single-chip N-WDM superchannel transmitters.
