Multiple optical carrier generation using frequency mixing in damage resistant multiple QPM device.

We devised a method for the measurement of the phase-matching curve of multiple quasi-phase-matched (QPM) LiNbO3 waveguide under conditions of high-power second-harmonic generation. The data obtained revealed that the phase-matching condition can be preserved due to the high damage resistance of the directly bonded LiNbO3 waveguide. Based on this evaluation, we tried to generate multiple optical carriers using multi-stage frequency mixing in the multiple QPM device. The multiple optical carriers have mutual phase correlation, which is suitable for coherent wavelength division multiplexing (WDM) transmission. We also demonstrated 20 Gb/s quadrature phase shift keying (QPSK) signal generation using multiple optical carriers in order to ensure signal quality.

A coherent Ising machine for 2000-node optimization problems.

The analysis and optimization of complex systems can be reduced to mathematical problems collectively known as combinatorial optimization. Many such problems can be mapped onto ground-state search problems of the Ising model, and various artificial spin systems are now emerging as promising approaches. However, physical Ising machines have suffered from limited numbers of spin-spin couplings because of implementations based on localized spins, resulting in severe scalability problems. We report a 2000-spin network with all-to-all spin-spin couplings. Using a measurement and feedback scheme, we coupled time-multiplexed degenerate optical parametric oscillators to implement maximum cut problems on arbitrary graph topologies with up to 2000 nodes. Our coherent Ising machine outperformed simulated annealing in terms of accuracy and computation time for a 2000-node complete graph.

Simultaneous nonlinearity mitigation in 92 × 180-Gbit/s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation.

We experimentally demonstrated the simultaneous nonlinearity mitigation of PDM-16QAM WDM signals using complementary-spectrally-inverted optical phase conjugation (CSI-OPC). We achieved reserved-band-less, guard-band-less, and polarization independent OPC based on periodically poled LiNbO3 waveguides. By employing the CSI-OPC, 2.325-THz-band (93 × 25 GHz) complementary spectral inversion was achieved while retaining the original WDM bandwidth. A Q2-factor improvement of over 0.4 dB and a 5120 km transmission with a Q2-factor above the FEC limit were confirmed using a 10-channel WDM transmission at the signal band center and signal band edge. We then demonstrated the mitigation of the nonlinear impairments in a 3840 km long-haul WDM signal transmission for all 92-channel 180-Gbit/s PDM-16QAM quasi-Nyquist-WDM signals.

Rapid spectrum measurement at 3 µm over 100 nm wavelength range using mid-infrared difference frequency generation source.

We demonstrate a broadband rapid scanning light source in the 3-µm region by using difference frequency generation (DFG). The DFG source consists of a module with quasi-phase-matched LiNbO3 ridge waveguides, a 1-µm-band wide swept range laser for the pump source, and a 1.5-µm continuous wave laser for the signal source. The sweep rate and the tuning bandwidth of this source are 20 kHz and 100 nm, respectively. This source enables us to evaluate the temperature dependence of absorbance of methane gas.

Dual wavelength 3.2-µm source for isotope ratio measurements of (13)CH(4)/(12)CH(4).

Difference frequency generation using one 1.58-µm and two 1.06-µm distributed feedback Bragg-grating laser diodes and a ridge-type PPLN alternately provide two 3.2-µm coaxial waves resonant with individual isotopic transitions separated by 13 cm(-1). The ν(3) band R(6) A(2) allowed transition of (13)CH(4) and the ν(3) band R(6) A(2) weakly allowed transition of (12)CH(4) are an ideal pair for isotope ratio measurements. The (13)CH(4)/(12)CH(4) isotope ratio is determined for three sample gases with a relative uncertainty of 0.7 ‰, and it is confirmed that the temperature dependence is smaller than the uncertainty.

Integrated quasi-phase-matched second-harmonic generator and electro-optic phase modulator for low-noise phase-sensitive amplification.

We propose a quasi-phase-matched second-harmonic generator integrated with an electro-optic phase modulator in a directly bonded LiNbO3 (DB-LN) waveguide to obtain high signal-to-noise ratio (SNR) pump light for a phase-sensitive amplifier (PSA). This integrated device exhibits 1-MHz modulation and 1-W second-harmonic-generation properties sufficient for phase-locking between the signal and pump and for PSA gain, respectively. A novel PSA configuration based on the high-input-power tolerance of the device helps to suppress the noise from the erbium-doped fiber amplifier used for pump-light generation and leads to an improvement of the SNR of the pump light. The SNR improvement was confirmed by comparing the noise figure of a PSA employing the DB-LN waveguide with that of a PSA using a Ti-diffused LN waveguide modulator.

In-line phase-sensitive amplification of QPSK signal using multiple quasi-phase matched LiNbO3 waveguide.

Phase-sensitive amplifiers (PSA) using periodically poled (PPLN) LiNbO3 waveguides are promising as low-noise optical amplifiers. However, it is difficult to realize in-line operation for multi-level phase modulated signals using a PPLN based PSA with the conventional configuration. In this paper, we report a PPLN based in-line PSA that can regenerate quadrature phase shift keying (QPSK) signals. Multi-stage frequency mixing in a multiple quasi-phase matched LiNbO3waveguide allows carrier phase recovery from a QPSK signal. Non-degenerate parametric amplification enables the phase-sensitive amplification of a QPSK signal. Amplitude and phase regeneration is examined utilizing gain saturation and phase squeezing capability.

Properties of power dependence on low-crosstalk waveband conversion with an apodized multiperiod-QPM LiNbO3 device.

Quasi-phase-matched (QPM) LiNbO3 devices having a multiperiod-periodically-poled structure, which we call multiperiod-QPM LiNbO3 devices, are capable of shifting the idler waveband during waveband conversion via cascaded difference-frequency generation (cascaded DFG). However, these multiperiod-QPM devices have a problem that they produce extra ripples between QPM peaks in the phase-matching curve. These ripples cause crosstalk between wavebands arising from sum-frequency generation (SFG) between the signal and idler wavebands and subsequent DFG between the SFG wavelength and the signal waveband. To decrease the size of the ripples and thus that of the crosstalk, an apodized multiperiod-QPM device is developed. In demonstrating waveband conversion for low crosstalk with this device, we measure the dependence of the idler power, the crosstalk power, and their ratio on the signal power. This measurement shows that it agrees well with theoretical prediction and that the obtained feature of crosstalk reduction is kept even for decreased signal power.

Real-time N2O gas detection system for agricultural production using a 4.6-µm-band laser source based on a periodically poled LiNbO3 ridge waveguide.

This article describes a gas monitoring system for detecting nitrous oxide (N2O) gas using a compact mid-infrared laser source based on difference-frequency generation in a quasi-phase-matched LiNbO3 waveguide. We obtained a stable output power of 0.62 mW from a 4.6-µm-band continuous-wave laser source operating at room temperature. This laser source enabled us to detect atmospheric N2O gas at a concentration as low as 35 parts per billion. Using this laser source, we constructed a new real-time in-situ monitoring system for detecting N2O gas emitted from potted plants. A few weeks of monitoring with the developed detection system revealed a strong relationship between nitrogen fertilization and N2O emission. This system is promising for the in-situ long-term monitoring of N2O in agricultural production, and it is also applicable to the detection of other greenhouse gases.

In-line phase sensitive amplifier based on PPLN waveguides.

We demonstrate a χ(2)-based in-line PSA with a carrier-recovery and phase-locking system for a phase shift keying (PSK) signal. By doubling the signal phase using a wavelength conversion technique, the carrier was recovered from a PSK signal. The carrier phase was synchronized to a local oscillator using optical injection locking. Phase sensitive amplification with a wide phase sensitive dynamic range of 20 dB was achieved using degenerate parametric amplification in a periodically poled LiNbO(3) (PPLN) waveguide. The phase regeneration effect was examined for a degraded signal by means of constellation analyses and bit-error rate measurements. The in-line PSA also operated successfully as a repeater amplifier in a 160 km fiber link without a power penalty. Finally, we demonstrate the regeneration of non-linear impairments induced by fiber non-linearity.

Hybrid optoelectronic buffer using CMOS memory and optical interfaces for 10-Gbit/s asynchronous variable-length optical packets.

A hybrid optoelectronic buffer consisting of a complementary metal-oxide-semiconductor (CMOS) memory and optical/optoelectronic interface devices is described. The interface devices: an all-optical serial-to-parallel converter (SPC), electrical-to-optical parallel-to-serial converter (PSC), and optical clock pulse-train generator (OCPTG) enable write-in/read-out of preamble-free asynchronous high-speed optical packets to/from CMOS memory, and consequently, flexible and highly functional processing of the packets with CMOS circuitry. A prototype hybrid optoelectronic buffer subsystem using a field programmable gate array (FPGA)-based memory and modules for the interface devices is developed and error-free write-in and read-out operation for 10-Gbit/s asynchronous variable-length optical packets is demonstrated.

4x4 optical packet switching of asynchronous burst optical packets with a prototype, 4x4 label processing and switching sub-system.

We report a prototype, 4x4 (4 input/4 output) label processing and switching sub-system for 10-Gb/s asynchronous burst variable-length optical packets. With the prototype, we perform a 4x4 optical packet switching demonstration, achieving error-free (BER<10(-12)) label processing and switching operation for all possible input/output combinations (16 switching paths) simultaneously. Power consumption and latency of the entire, self-contained sub-system is 83 W (includes fan power) and 300 ns, respectively.

Photonic encoder and decoder for optical code-division multiplexing with time-to-space converters and angle-multiplexed holograms.

A photonic encoder-decoder pair for optical code-division multiplexing (OCDM) that uses time-to-space converters and angle-multiplexed holograms is proposed. The encoder converts the pulse from each input port into a specific code and multiplexes input signals into the output port. The hologram in the decoder generates a correlation waveform between the transmitted code and the recorded code. The performance of the OCDM system with the encoder-decoder pair is estimated. The maximum spectral efficiency for 8-bit length orthogonal codes in the worst case at a bit-error rate of 10(-9) is 0.17 (bits/s)/Hz when the number of channels is 8.