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We achieved degenerate phase-sensitive amplification with the gain of over 30 dB under a CW condition using a periodically poled LiNbO3 waveguide module, which uses second- harmonic pump light to directly amplify the signal.

A PPLN-based in-line phase sensitive amplifier with a highly sensitive carrier-recovery system based on two-stage SHG/DFG parametric conversion achieves multi-stage amplification with a low noise figure of 3 dB and a 20-dB gain.

We developed in-line PSA for the simultaneous amplification of wavelength-multiplexed and polarization-multiplexed QAM signals. We achieved >20-dB gain and a 12-nm bandwidth for 16-channel PDM-16QAM signals with an inherent PSA characteristic of a 5.1-dB SNR advantage.

We demonstrate the first polarization-independent phase-sensitive amplification of a polarization-division multiplexing (PDM) signal. A novel polarization-diversity loop configuration using periodically poled LiNbO3 waveguides is proposed to achieve the polarization-independent amplification of both a single-polarized signal and a PDM signal. The proposed configuration provides two independent optical parametric amplification processes for two orthogonal polarization components in the loop, while preventing the amplification of the reflection noise by a counter-propagating pump. Polarization-independent error-free operation was confirmed with a bit-error-rate measurement for a 40-Gb/s quadrature phase-shift keying (QPSK) signal. We then demonstrated the phase regenerative amplification of an 80-Gb/s PDM-QPSK signal with artificial phase noise.

Techniques for stabilizing the carrier-envelope offset (CEO) frequency of an optical frequency comb are prerequisites for precision spectroscopy. A 2f-to-3f self-referencing interferometer (SRI) requires only a 2/3-octave bandwidth of supercontinuum (SC) light; however, it has not been regarded as a practical method because the conversion efficiency of third-harmonic generation is much lower than that of the second-harmonic generation. To solve this problem, we utilized a dual-pitch periodically poled lithium niobate (PPLN) ridge waveguide that has two different quasi-phase matching pitch sizes [Fig. 1(a)] [1]. Here, we report the advantage of using the dual-pitch PPLN ridge waveguide over using a single-pitch one, i.e., reduction of the pulse energy for stabilizing the CEO frequency.

We estimated the pulse energy required for frequency stabilization with dual-pitch periodically poled lithium niobate ridge waveguides in a 2f-to-3f self-referencing interferometer. The required pulse energy is 0.51 nJ with a dual-pitch waveguide, which is less than half compared with the f-to-2f method.

We have observed an ultra-broadband frequency comb with a wavelength range of at least 0.35 to 4.4 μm in a ridge-waveguide-type periodically poled lithium niobate device. The PPLN waveguide is pumped by a 1.0–2.4 μm wide frequency comb with an average power of 120 mW generated using an erbium-based mode-locked fiber laser and a following highly nonlinear fiber. The coherence of the extended comb is confirmed in both the visible (around 633 nm) and the mid-infrared regions.

Taking the pulse of optimization Finding the optimum solution of multiparameter or multifunctional problems is important across many disciplines, but it can be computationally intensive. Many such problems defined as computationally difficult can be mathematically mapped onto the so-called Ising problem, which looks at finding the minimum energy configuration for an array of coupled spins. Inagaki et al. and McMahon et al. show that an optical processing approach based on a network of coupled optical pulses in a ring fiber can be used to model and optimize large-scale Ising systems. Such a scalable architecture could help to optimize solutions to a wide range of complex problems. Science , this issue pp. 603 and 614

We fabricated a periodically poled and ZnO-doped LiNbO3 ridge waveguide by employing direct bonding and dry etching techniques. We obtained a second-harmonic generation (SHG) conversion efficiency of 2400%/W, and converted 92% of the pump light into SH light at a pump power of 160 mW. We developed a fiber-coupled module using the fabricated ridge waveguide. The high conversion efficiency and high damage resistance of the ridge waveguide result in the parametric amplification of the signal and converted signal lights. The low insertion loss of the module (-4 dB) and sufficient parametric conversion gain (+8 dB) enable us to achieve a wavelength converter with + 4 dB fiber-to-fiber gain, which means the wavelength converter operates without loss.

There have been many reports on engineered quasi-phase matched (QPM) wavelength converters that enable multiple wavelength generation. Most of these converters make it possible to realize multiple equally spaced QPM peaks. However, for gas sensing using several absorption lines, it is necessary to generate unequally spaced mid-infrared wavelength peaks. To achieve this, we propose a new phase modulated QPM device capable of realizing multiple unequally spaced QPM peaks. By suppressing the unwanted QPM peaks we were able to obtain the desired peaks flexibly. We also studied the fabrication tolerance of the device. We fabricated a waveguide device for 3 ?m difference frequency generation (DFG) based on the design. The phase matching characteristics of the fabricated device agreed well with the numerical design. Using the 3 ?m DFG device and a tunable laser diode array as signal source, we constructed a mid-infrared light source that can generate multiple unequally spaced wavelengths. We employed the wavelength switching capability to demonstrate the detection of multiple hydrocarbon gases, namely CH4, C2H4, and C2H6.

We fabricated direct-bonded quasi-phase-matched (QPM) LiNbO3 (LN) ridge waveguides for difference frequency generation (DFG) in the 2.7-µm wavelength range, and obtained a conversion efficiency of 87%/W. We showed that dips in the DFG tuning curve originated from the absorption of H2O in air and the O–H absorption in the LN crystal exhibited no influence on DFG when we used the type-0 phase-matching condition. We confirmed that the direct-bonded QPM-LN ridge waveguide is suitable for achieving high-efficiency DFG in the 2.7-µm range.

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.

We succeeded in generating a coherent optical frequency comb covering more than 3 octaves from visible to mid-infrared with a waveguide-type PPLN. The generated comb is demonstrated to be useful for metrology and spectroscopic applications.

We propose a new multiple quasi-phase-matched wavelength converter based on the continuous phase modulation of a /spl chi//sup (2)/ grating for use in variable wavelength conversion. A numerical study shows that the proposed device exhibits a high conversion efficiency, flexible design, and robust fabrication tolerance. A waveguide device fabricated by annealed proton exchange agrees well with the numerical design. Fine-tuning the device enabled us to demonstrate variable wavelength conversion between signals on the standard optical frequency grid. Using the device, we also demonstrated fast (

Wavelength converters using quasi-phase matched LiNbO3 (QPM-LN) waveguides are recognized as key devices for future wavelength division multiplexing (WDM) systems. These converters provide unique characteristics such as the simultaneous conversion of WDM channels, a large signal bandwidth, and transparency as regards modulation format. In this review, we describe waveguide fabrication using the annealed-protonexchange and direct-bonding techniques. A direct-bonded ridge waveguide shows a high resistance to photorefractive damage and a -4.6 dB wavelength conversion using a χ(2)-cascaded pumping scheme is successfully demonstrated. Polarization independent wavelength conversion using a LPE-LN substrate is also described.

We have fabricated a periodically poled ridge waveguide using a liquid phase epitaxy (LPE)-grown Zn:LiNbO/sub 3/ substrate. The waveguide can confine both the transverse electric and transverse magnetic mode. Its strong resistance to photorefractive damage enables stable parametric amplification. We demonstrate polarization-independent wavelength conversion using polarization diversity in a single waveguide.

The degradation mechanism of a vertical-cavity surface-emitting laser (VCSEL) with an air-post structure is analyzed for stable optical parallel interconnection communication. It is clarified that the degradation is caused by the behavior of latent defects in the GaAs active layer. Decreasing the defects in the active region as well as decreasing the threshold current is important for obtaining a long-lifetime VCSEL.

The pulse energy for stabilizing the carrier-envelop offset frequency of an Er-doped fiber laser was reduced by a dual-pitch (DP-) periodically poled lithium niobate (PPLN) ridge waveguide implemented in a 2f-to-3f self-referencing interferometer (SRI). The pulse energy requirement was less than half that for a single-pitch PPLN ridge waveguide implemented in an f-to-2f SRI. We also found that environmental noise could be reduced by adjusting the pulse energy for frequency stabilization with the DP-PPLN ridge waveguide, as estimated from the phase noise of an out-of-loop interferometer.

We review our recent results for phase sensitive amplifiers based on periodically poled LiNbO3 waveguides. We also cover their optical communication applications such as regenerative repeater amplification and multilevel signal amplification.

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.

We review the capabilities of phase sensitive amplifiers based on PPLN waveguides for optical communication. Specifically, we discuss their unique low noise amplification, phase and amplitude regeneration in multi-span transmissions, and multilevel phase coding signal amplification.

We demonstrate dynamic grouped-wavelength conversion using multiple quasi-phase-matched LiNbO3 modules and a tunable laser diode array. A high signal-to-noise ratio of larger than 45 dB is obtained for the converted signals. The power penalty of the wavelength converter is confirmed to be less than 0.2 dB.

We fabricated a 50-mm-long ZnO-doped quasi-phase-matched LiNbO ridge waveguide by employing the direct bonding and dry etching techniques. We obtained a second-harmonic generation conversion efficiency of 2400%/W and a second-harmonic output power of 148 mW with a pump power of 160 mW. We demonstrated 1.55-m-band wavelength conversion based on a cascade scheme. A parametric conversion gain of 5 dB was successfully achieved.

We fabricate a 763-nm laser module based on second-harmonic generation using a direct-bonded quasi-phase-matched LiNbO 3 ridge waveguide. We obtained a 0.84-mW output of 763 nm light using a 1526-nm distributed-feedback laser diode. We also demonstrate O 2 gas detection using the module output.

We fabricate a direct-bonded quasi-phase-matched LiNbO3 ridge waveguide for 2.3-µm-band difference frequency generation. The waveguide exhibits a 100 nm bandwidth for a 0.937-µm pump and a 1.55-µm-band signal. A 100%/W conversion efficiency and 0.4 mW output are obtained. We also discuss the temperature dependence of the quasi-phase-matched wavelength.

We fabricate a 50-mm-long ZnO-doped quasi-phase matched LiNbO/sub 3/ ridge waveguide by employing the direct bonding method. A second-harmonic generation conversion efficiency of 560%/W and an output power of 110 mW with a pump power of 190 mW are achieved. We demonstrate 1.55-/spl mu/m band wavelength conversion based on a /spl chi//sup (2)/ cascade scheme. An internal wavelength conversion efficiency of -0.3 dB is successfully achieved.

We report the experimental distribution of a time-bin entangled photon pair over 300 km of optical fiber, which is a record for long-distance quantum communication. The observed two-photon interference fringe exhibited a visibility of 84%.

We have investigated 4-μm-band SO3 absorption lines for in situSO3 detection using a mid-infrared laser source based on difference frequency generation in a quasi-phase-matched LiNbO3 waveguide. In the wavelength range of 4.09400–4.10600 μm, there were strong SO3 absorption lines. The maximum absorption coefficient at a concentration of 170 ppmv was estimated to be about 3.2×10−5 cm−1 at a gas temperature of 190°C. In coexistence with H2O, the reduction of the SO3 absorption peak height was observed, which was caused by sulfuric acid formation. We discuss a method of using an SO3 equilibrium curve to derive the total SO3 molecule concentration.

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.

The noise figure (NF) of a phase sensitive amplifier (PSA) based on a periodically poled LiNbO3 (PPLN) waveguide was evaluated in the optical and electrical domains. Phase sensitive amplification was realized using degenerate parametric amplification in the PPLN waveguide, which was pumped by the second harmonic frequency of the signal. Second harmonic pumping enables direct observation of the intrinsic amplified spontaneous emission (ASE), which determined the NF of the PSA. An NF below the 3 dB quantum limit was obtained by observing the intrinsic ASE. The low NF was also confirmed via the noise floor measurement of a cascaded PSA and erbium doped fiber amplifier in the electrical domain. The PSA was used as a preamplifier for detecting a 40 Gbit/s phase shift keying signal. The low noise characteristics were confirmed by the improved sensitivity.

We constructed the first CW pumped degenerate parametric amplifier based on periodically poled and ZnO-doped LiNbO3 ridge waveguides. An in-phase gain of + 11 dB was achieved owing to the high conversion efficiency and high damage resistance of the waveguide obtained by employing direct bonding and dry etching techniques. Nearly identical amplification and deamplification were obtained owing to a sufficient spatial and temporal overlap between the pump and signal beams. No secondary wavelength conversion process was observed, and a maximum output of 22 dBm was obtained. We also successfully demonstrated the phase sensitive amplification of a modulated signal light.

We constructed the first CW pumped degenerate parametric amplifier based on PPLN waveguides. We successfully demonstrated an in-phase gain of +11 dB and the phase sensitive amplification of a modulated signal.

The photorefractive effect in annealed proton-exchanged waveguides in periodically poled nondoped, MgO- and ZnO-doped LiNbO3 was evaluated by monitoring the quasiphase matching (QPM) wavelength shift induced by a 0.784-μm-irradiating light. The QPM wavelength shift was reduced at room temperature by a factor of 3–6 in ZnO- and MgO-doped samples compared with the nondoped samples within a 104–105-W/cm2-irradiation intensity range. The doped samples exhibited no significant wavelength shifts when the temperature was raised to slightly above room temperature (50–60 °C).

We demonstrate flexible waveband conversion covering the entire C-band using 8-peak-multiple-QPM-LN waveguides to solve the problem of wavelength contention in a waveband-switching node. The phase-transparent conversion of a 43 Gb/s RZ-DQPK waveband signal is also confirmed.

Entangled photon pairs are one of the most important resources for the development of quantum communication technologies. In order to produce pulsed photon pairs in the telecommunication band from 1.5 mum pump light, most of the previous experiments used two successive periodically poled lithium niobate (PPLN) waveguides. We report what we believe to be a new method using cascaded second-order nonlinearity in a single PPLN waveguide to produce high-purity time-bin entangled photon pairs. We confirmed the generation of entanglement through a two-photon interference experiment showing a coincidence fringe with a visibility of up to 97%.

We developed an apodized multiple quasi-phase-matched (QPM) LiNbO(3) device to reduce the cross talk that occurs during waveband conversion. Since we found that the cross talk arises from ripples between multiple QPM peaks in the phase-matching curve, we reduced the amplitude of the ripples by employing the apodized structure. Using this device, we succeeded in demonstrating waveband conversion with low cross talk.