Ultra-long-haul digital coherent PSK Y-00 quantum stream cipher transmission system.

A record-long 10,118-km fiber transmission with physical layer encryption is demonstrated utilizing a Y-00 cipher based on signal masking by quantum (shot) noise. The Y-00 cipher enables symmetric-key data encryption to ensure the security of the physical layer of optical communications. Irreducible secrecy without significant negative impact on transmission performance is achieved by the synergistic effect of combining seed-key-based high-order modulation and truly random shot noise inevitable in optical detection. This paper reports a comprehensive study of applying a phase-shift-keying (PSK) Y-00 cipher for ultra-long haul fiber transmission. Theoretical analysis shows that security-enhanced transmission over transoceanic-distance (>10,000 km) fiber is feasible when the quadrature PSK data signal is encrypted by converting to a PSK signal with 218 levels. Subsequently, 10,118-km standard single-mode fiber transmission of 48-Gbit/s line-rate dual-polarization PSK Y-00 cipher with 218 levels is experimentally demonstrated. An adequate signal quality above the Q-factor threshold of soft decision forward error correction is achieved together with sufficient signal masking by shot noise, yielding balanced transmission performance and high security in an ultra-long-haul PSK Y-00 cipher transmission system.

Experimental demonstration of a 4,294,967,296-QAM-based Y-00 quantum stream cipher template carrying 160-Gb/s 16-QAM signals.

We demonstrate a 4,294,967,296-quadrature amplitude modulation (QAM) based Y-00 quantum stream cipher system carrying a 160-Gb/s 16-QAM signal transmitted over 320-km SSMF. The ultra-dense QAM cipher template is realized by an integrated two-segment silicon photonics I/Q modulator.

SiN/Si double-layer platform for ultralow-crosstalk multiport optical switches.

We experimentally demonstrate a double-layer platform of silicon nitride and silicon for ultralow-crosstalk multiport optical switches. By using a silicon nitride overpass with a large gap of 1.5 µm, we achieve a crosstalk of less than -50 dB and -45 dB almost entirely in the C-band for 4 × 4 and 16 × 16 switches, respectively. To demonstrate the scalability of the platform, we also measured a 32 × 32 passive test device and show that a worst-case crosstalk of less than -50 dB is feasible with appropriate gate switches.

Single-channel 48-Gbit/s DP PSK Y-00 quantum stream cipher transmission over 400- and 800-km SSMF.

A record-large product of data rate and transmission distance in quantum-noise-assisted cipher systems for physical layer security of fiber-optic transmission was demonstrated. The cipher system is based on symmetric-key direct-data encryption utilizing signal masking by quantum (shot) noise. The encryption is achieved by 216-level phase randomization of quadrature phase-shift keying data signal, resulting in 48-Gbit/s dual polarization Y-00 cipher with 218 phase levels. Successful transmission of the Y-00 cipher over 400- and 800-km standard single-mode fibers was achieved without significant negative impact on transmission quality. The product of the data rate and distance was 40 Gbit/s (net rate) × 800 km = 32,000 Gbit/s·km. The system achieves masking of 217 phase levels by shot noise, which promises irreducible and unchanged security based on the quantum nature of coherent light.

Digital coherent PSK Y-00 quantum stream cipher with 217 randomized phase levels.

We report a 10-Gbaud fiber-optic cipher transmission system by using a phase-shift keying (PSK) Y-00 quantum stream cipher. The PSK Y-00 cipher is a symmetric-key direct data encryption technique based on extremely high-order random phase modulation using a pre-shared short key. Neighboring signal phases following encryption are masked by quantum (shot) noise, which provides security based on shot noise's inherent effects. To implement such a system, we utilize coarse-to-fine phase modulation with two cascaded phase modulators and digital decryption incorporated into digital signal processing (DSP) for intra-dyne coherent detection. We demonstrate 10-Gbaud PSK Y-00 cipher transmission over a 400-km standard single-mode fiber (SSMF). The coarse-to-fine phase modulation achieves 217 phase levels for signal masking by shot noise. The DSP with decryption realizes detection of the cipher without penalties. Masking 167 signal phase levels by shot noise is achieved at a bit-error ratio defined by a hard-decision forward-error correction threshold (3.8 × 10-3) in the transmissions over the 400-km SSMF.

Integrated silicon photonic wavelength-selective switch using wavefront control waveguides.

A wavelength selective switch (WSS) can route optical signals into any of output ports by wavelength, and is a key component of the reconfigurable optical add/drop multiplexer. We propose a wavefront control type WSS using silicon photonics technology. This consists of several arrayed waveguide gratings sharing a large slab waveguide, wavefront control waveguides and distributed Bragg reflectors. The structure, design method, operating principle, and scalability of the WSS are described and discussed. We designed and fabricated a 1 × 2 wavefront control type WSS using silicon waveguides. This has 16 channels with a channel spacing of 200 GHz. The chip size is 5 mm × 10 mm. The switching operation was achieved by shifting the phase of the light propagating in each wavefront control waveguide, and by controlling the propagation direction in the shared large slab waveguide. Our WSS has no crossing waveguide, so the loss and the variation in loss between channels were small compared to conventional waveguide type WSSs. The heater power required for switching was 183 mW per channel, and the average extinction ratios routed to Output#1 and Output#2 were 9.8 dB and 10.2 dB, respectively.

Reconfigurable all-optical on-chip MIMO three-mode demultiplexing based on multi-plane light conversion.

We present, to the best of our knowledge, the first experimental demonstration of reconfigurable all-optical on-chip multi-input-multi-output three-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable three-mode demultiplexing is experimentally realized with wavelength-dependent loss of less than 3 dB and modal crosstalk of less than -10 dB over a 23 nm optical bandwidth. Error-free mode demultiplexing of 40 Gbps non-return-to-zero signal is also demonstrated.

Non-duplicate polarization-diversity 8 × 8 Si-wire PILOSS switch integrated with polarization splitter-rotators.

We demonstrate a fully integrated polarization-diversity 8 × 8 thermo-optic Si-wire switch that uses only a single path-independent insertion loss (PILOSS) switch matrix. All input/output ports of the PILOSS switch matrix are uniquely assigned for polarization diversity without switch duplication. To integrate polarization splitter-rotators on a chip, we propose a compact path-length-equalized polarization-diversity switch configuration. Polarization-dependent loss (PDL) and differential group delay (DGD) are minimized. The 8 × 8 switch is fabricated by the CMOS-compatible fabrication process on 300-mm diameter wafer and additional etching of upper cladding after dicing. The chip size is 7 × 10.5 mm2. A PDL of 2 dB and a DGD of 1.5 ps are achieved. The crosstalk in the worst-case scenario is -20 dB in the full C-band.

Broadband silicon photonics 8 × 8 switch based on double-Mach-Zehnder element switches.

We fabricated and characterized a silicon photonics 8 × 8 strictly non-blocking optical switch based on double-Mach-Zehnder (MZ) element switches. The double-MZ switches, each of which consisted of an intersection and two asymmetric MZ switches, enabled the suppression of crosstalk across a wide wavelength range. The 8 × 8 switch exhibited an average fiber-to-fiber insertion loss of 11.2 dB and -20 dB crosstalk in a bandwidth wider than 30 nm. Furthermore, we constructed an 8 × 8 polarization-diversity switch by using two 8 × 8 switches and demonstrated 32-Gbaud dual-polarization, quadrature-phase-shift-keying, four-channel wavelength-division-multiplexed signal transmission without significant signal degradation.

Novel polarization diversity without switch duplication of a Si-wire PILOSS optical switch.

We demonstrate the compact polarization diversity based on the bidirectional full-port use of a path-independent-insertion-loss (PILOSS) optical switch. A polarization-diversity 4 × 4 strictly non-blocking optical switch is developed using a single thermooptic PILOSS Si-wire switch and fiber-based polarization beam splitters (PBSs) and combiners (PBCs). We measure characteristics of the switch and confirm that the proposed configuration demonstrates the performance in the insertion loss, polarization-dependent loss (PDL), and differential group delay (DGD) comparable with that of a conventional polarization-diversity 4 × 4 PILOSS switch using double switch elements. On the other hand, higher crosstalk is observed. The crosstalk increase is associated with the backward crosstalk at a waveguide intersection based on a directional coupler. The effect of the backward crosstalk on the total crosstalk is estimated, and future prospects are discussed.

Ultra-compact 32 × 32 strictly-non-blocking Si-wire optical switch with fan-out LGA interposer.

We demonstrate a 32 × 32 path-independent-insertion-loss optical path switch that integrates 1024 thermooptic Mach-Zehnder switches and 961 intersections on a small, 11 × 25 mm2 die. The switch is fabricated on a 300-mm-diameter silicon-on-insulator wafer by a complementary metal-oxide semiconductor-compatible process with advanced ArF immersion lithography. For reliable electrical packaging, the switch chip is flip-chip bonded to a ceramic interposer that arranges the electrodes in a 0.5-mm pitch land grid array. The on-chip loss is measured to be 15.8 ± 1.0 dB, and successful switching is demonstrated for digital-coherent 43-Gb/s QPSK signals. The total crosstalk of the switch is estimated to be less than -20 dB at the center wavelength of 1545 nm. The bandwidth narrowing caused by dimensional errors that arise during fabrication is discussed.

Ultra-high-extinction-ratio 2 × 2 silicon optical switch with variable splitter.

We demonstrate a record-high extinction-ratio of 50.4 dB in a 2 × 2 silicon Mach-Zehnder switch equipped with a variable splitter as the front 3-dB splitter. The variable splitter is adjusted to compensate for the splitting-ratio mismatch between the front and rear 3-dB splitters. The high extinction ratio does not rely on waveguide crossings and meets a strong demand in applications to multiport circuit switches. Large fabrication tolerance will make the high extinction ratio compatible with a volume production with standard complementary metal-oxide semiconductor fabrication facilities.

Ultra-compact 8 × 8 strictly-non-blocking Si-wire PILOSS switch.

We report on a path-independent insertion-loss (PILOSS) 8 × 8 matrix switch based on Si-wire waveguides, which has a record-small footprint of 3.5 × 2.4 mm2. The PILOSS switch consists of 64 thermooptic Mach-Zehnder (MZ) switches and 49 low-crosstalk intersections. Each of the MZ switches and intersections employs directional couplers, which enable the composition of a low loss PILOSS switch. We demonstrate successful switching of digital-coherent 43-Gbps QPSK signal.

Compact 2 × 2 polarization-diversity Si-wire switch.

A polarization-independent 2 × 2 switch based on silicon-wire waveguides has been realized with a compact size of 600 × 500 µm². Polarization-independent operation was achieved with a polarization-diversity technique which implements polarization splitters, TE-TM intersections, and Mach-Zehnder switches. The extinction ratios of the 2 × 2 switch for TE, TM, and a mixed polarization at a wavelength of 1550 nm were measured to be larger than 30 dB, 25 dB, and 30 dB, respectively. The measured switching powers for the TE and TM polarizations were 25 and 55 mW, respectively. The measured polarization-dependent loss was lower than 1 dB. The differential group delay (DGD) between the TE and TM modes was also evaluated using the Mueller matrix method, which was in good agreement with the values estimated from the path lengths for each mode. A path-length-compensated switch was fabricated, whose DGDs for all paths were indeed as small as ~2 ps, mainly from the access waveguides. The switch could provide an important route to develop ultra-compact polarization-independent integrated circuits based on silicon-wire waveguides.

Sub-millisecond timing-jitter-free tuning of parametric dispersion compensator.

We demonstrate sub-millisecond tuning of a prototype parametric tunable dispersion compensator (P-TDC) based on cascaded polarization-diverse four-wave mixing (FWM) process with a fast tunable and highly wavelength-stable pump light source. The pump light source is developed using a tunable distributed amplification chirped sampled grating distributed reflector laser that is fully wavelength tunable by on-chip heaters with a 3-dB frequency response of 45 kHz, resulting in fast dispersion tuning of less than 50 µs without additional timing jitter. The P-TDC is developed as the first prototype to satisfy essential requirements for practical network uses: stable input-polarization diversity, input-wavelength preservation, and seamless dispersion tunability for entire C-band input wavelengths are simultaneously achieved.

Compact and phase-error-robust multilayered AWG-based wavelength selective switch driven by a single LCOS.

A novel liquid crystal on silicon (LCOS)-based wavelength selective switch (WSS) is proposed, fabricated, and demonstrated. It employs a multilayered arrayed waveguide grating (AWG) as a wavelength multiplex/demultiplexer. The LCOS deflects spectrally decomposed beams channel by channel and switches them to desired waveguide layers of the multilayered AWG. In order to obtain the multilayered AWG with high yield, phase errors of the AWG is externally compensated for by an additional phase modulation with the LCOS. This additional phase modulation is applied to the equivalent image of the facet of the AWG, which is projected by a relay lens. In our previously-reported WSS configuration, somewhat large footprint and increased cost were the drawbacks, since two LCOSs were required: one LCOS was driven for the inter-port switching operation, and the other was for the phase-error compensation. In the newly proposed configuration, on the other hand, both switching and compensation operations are performed using a single LCOS. This reduction of the component count is realized by introducing the folded configuration with a reflector. The volume of the WSS optics is 80 × 100 × 60 mm3, which is approximately 40% smaller than the previous configuration. The polarization-dependent loss and inter-channel crosstalk are less than 1.5 dB and -21.0 dB, respectively. An error-free transmission of 40-Gbit/s NRZ-OOK signal through the WSS is successfully demonstrated.

No guard-band wavelength translation of Nyquist OTDM-WDM signal for spectral defragmentation in an elastic add-drop node.

We demonstrate a seamless spectral defragmentation in an elastic all-optical add-drop node based on wavelength division multiplexing (WDM) channels of Nyquist optical time division multiplexing (OTDM) signal. A 172 Gbaud Nyquist OTDM signal occupying a 215 GHz range is elastically shifted adjacent to its neighboring channel, completely filling a variable spectral gap caused by the dropped channel. The frequency shift is done in a dual-stage polarization-diversity four wave mixing-based converter using polarization-maintaining highly nonlinear fiber. The spectrally defragmented signals are successfully transmitted over a 80 km fiber link with BER<10(-9).

Baud-rate flexible clock recovery and channel identification in OTDM realized by pulse position modulation.

We propose a novel scheme of OTDM utilizing pulse position modulation, where optical null headers (ONH) are inserted between the signal pulses periodically to allow channel identification. The ONH also achieves in-band clock distribution through the generation of high contrast pilot tone on the signal power spectra, enabling baud-rate flexible clock recovery. Using the novel scheme, clock recovery with a timing jitter of less than 200 fs is achieved at different baud rates up to 344 Gbaud. We demonstrate stable clock recovery with channel identification in 344-Gb/s OTDM transmissions over dispersion managed 3-km SMF.

Pattern-effect-free all-optical wavelength conversion using a hydrogenated amorphous silicon waveguide with ultra-fast carrier decay.

Ultra-fast carrier decay, recently discovered in a hydrogenated amorphous silicon waveguide, can be exploited for pattern-effect-free all-optical signal processing based on optical Kerr nonlinearity. In this study, we utilized a 10 Gbit/s RZ-OOK data stream as a pump for degenerate four-wave mixing in a low-loss hydrogenated amorphous silicon waveguide. The propagation loss of the waveguide used was 1.0±0.2 dB/cm at 1550 nm. Unlike crystalline silicon waveguides, no noticeable difference was observed in the BER characteristics between the cases of PRBS 2(7)-1 and 2(31)-1.

Parametric tunable dispersion compensation for the transmission of sub-picosecond pulses.

Parametric tunable dispersion compensator (P-TDC), which allows format-independent operation owing to seamlessly wide bandwidth, is expected to be one of the key building blocks of the future ultra-high speed optical network. In this paper, a design of ultra-wide band P-TDC is presented showing that bandwidth over 2.5 THz can be achieved by compensating the chromatic dispersion up to the 4th order without employing additional method. In order to demonstrate the potential application of P-TDC in the Tbit/s optical time division multiplexing transmissions, 400 fs optical pulses were successfully transmitted through a dispersion managed 6-km DSF fiber span.