Vertical plasmonic resonance coupler.

Efficient wavelength-selective coupling of lights between sub-wavelength plasmonic waveguides and free space is theoretically investigated. The idea is based on a new type of vertical resonance coupling devices built on plasmonic metal/insulator/metal (MIM) waveguides. The device structure consists of a vertical grating coupler in a resonance cavity formed by two distributed Bragg reflectors (DBRs). With the metal loss included, maximum coupling efficiency around 50% can be obtained at the 1550 nm wavelength with a filtering 3 dB bandwidth around 20 nm (7 nm for the lossless case), demonstrating the feasibility of the idea for achieving high efficiency wavelength-selective vertical coupling through optical resonance. By utilizing this coupler, a plasmonic add-drop device is proposed and theoretically demonstrated. This kind of compact wavelength selective coupling devices shall have the potential to open up a new avenue of photonics circuitry at nanoscale.

Relative carrier-envelope phase stabilization of hybridly synchronized ultrafast Yb and Er fiber-laser systems with the feed-forward scheme.

Stabilization of the relative carrier-envelope (CE) phase for hybridly synchronized two-color fs Yb and Er fiber-laser systems is demonstrated for the first time by utilizing the feed-forward scheme based on an acousto-optic frequency shifter. The slow drift issues arising from the feed-forward scheme are solved by adding the in-loop relative CE frequency coarse stabilization via modulating the laser pump current. Sub-fs timing locking between the two-color pulses is still maintained due to the fast response and large locking range of hybrid synchronization. The approach provides an alternative way to obtain phase-stable synchronized two-color pulses with higher pulse energies.

Investigation of associations between NR1D1, RORA and RORB genes and bipolar disorder.

Several genes that are involved in the regulation of circadian rhythms are implicated in the susceptibility to bipolar disorder (BD). The current study aimed to investigate the relationships between genetic variants in NR1D1 RORA, and RORB genes and BD in the Han Chinese population. We conducted a case-control genetic association study with two samples of BD patients and healthy controls. Sample I consisted of 280 BD patients and 200 controls. Sample II consisted of 448 BD patients and 1770 healthy controls. 27 single nucleotide polymorphisms in the NR1D1, RORA, and RORB genes were genotyped using GoldenGate VeraCode assays in sample I, and 492 markers in the three genes were genotyped using Affymetrix Genome-Wide CHB Array in sample II. Single marker and gene-based association analyses were performed using PLINK. A combined p-value for the joining effects of all markers within a gene was calculated using the rank truncated product method. Multifactor dimensionality reduction (MDR) method was also applied to test gene-gene interactions in sample I. All markers were in Hardy-Weinberg equilibrium (P>0.001). In sample I, the associations with BD were observed for rs4774388 in RORA (OR = 1.53, empirical p-value, P = 0.024), and rs1327836 in RORB (OR = 1.75, P = 0.003). In Sample II, there were 45 SNPs showed associations with BD, and the most significant marker in RORA was rs11639084 (OR = 0.69, P = 0.002), and in RORB was rs17611535 (OR = 3.15, P = 0.027). A combined p-value of 1.6×10-6, 0.7, and 1.0 was obtained for RORA, RORB and NR1D1, respectively, indicting a strong association for RORA with the risk of developing BD. A four way interaction was found among markers in NR1D1, RORA, and RORB with the testing accuracy 53.25% and a cross-validation consistency of 8 out of 10. In sample II, 45 markers had empirical p-values less than 0.05. The most significant markers in RORA and RORB genes were rs11639084 (OR = 0.69, P = 0.002), and rs17611535 (OR = 3.15, P = 0.027), respectively. Gene-based association was significant for RORA gene (P = 0.0007). Our results support for the involvement of RORs genes in the risk of developing BD. Investigation of the functional properties of genes in the circadian pathway may further enhance our understanding about the pathogenesis of bipolar illness.

Familiality and clinical outcomes of sleep disturbances in major depressive and bipolar disorders.

OBJECTIVE: Sleep disturbances are frequently observed in major depressive (MDD) and bipolar disorder (BD). This study reported sleep profiles of patients and their relatives versus controls, and examined the familiality of sleep features in mood disorder families. We also evaluated the influences of sleep disturbance on patients' quality of life (QOL), functional impairment, and suicidality. METHODS: We recruited 363 BD and 157 MDD patients, 521 first-degree relatives, and 235 healthy controls, which completed a diagnostic interview, Pittsburgh Sleep Quality Index (PSQI), and QOL questionnaire. The magnitude of heritability of sleep features was calculated and familiality was evaluated by mixed regression models and intraclass correlation coefficient (ICC). The associations between sleep problems and clinical outcomes were examined using multiple regression models. RESULTS: More than three-quarters of mildly-ill patients were classified as "poor sleepers". MDD patients had significantly worse sleep quality as compared to BD patients. Moderate but significant familial aggregation was observed in subjective sleep quality, sleep latency, disturbance, daytime dysfunction, and global score (ICC=0.10-0.21, P<.05). Significant heritability was found in sleep quality (0.45, P<.001) and sleep disturbance (0.23, P<.001). Patients with good sleep quality had better QOL and less functional impairment (P<.05) than poor sleepers. Poor sleep quality and nightmares further increased the risk for suicidal ideation (ORadj=2.8) and suicide attempts (ORadj=1.9-2.8). CONCLUSION: Subjectively measured sleep features demonstrated significant familiality. Poor sleep quality further impaired patients' daily function and QOL, in addition to increasing the risk of suicidality, and thus requires special attention in related clinical settings.

Subfemtosecond hybrid synchronization between ultrafast Yb and Er fiber laser systems by controlling the relative injection timing.

We report experimental as well as theoretical investigation of the key factors that influence the relative timing jitter between hybrid synchronized ultrafast Yb and Er fiber laser systems. Experimental results show that, within the achievable synchronization range, the synchronization performance varies significantly with the relative injection timing between the 1 µm master and 1.5 µm slave pulses. This observation is in agreement with the insights obtained from the theoretical analysis, which identifies the retiming effect as a function of the initial condition of the master-slave pulse collision. By controlling the relative injection timing with a low-bandwidth intracavity feedback, relative timing jitter as low as 0.87 fs (within 1.9 MHz bandwidth) is successfully obtained.

Repetition frequency pulling effects in asynchronous mode-locking.

Repetition frequency pulling effects are found to exist in asynchronous harmonic mode-locked fiber soliton lasers. The deviation frequency of asynchronous mode-locking is found to be dependent on the active modulation depth, not wholly determined by the difference between the intracavity active modulation frequency and the cavity harmonic repetition frequency. Transition from asynchronous to synchronous mode-locking will also occur when the modulation depth is above the threshold. Independent repetition frequency control of asynchronous mode-locked laser can be achieved through the effects.

Direct observation of two-color pulse dynamics in passively synchronized Er and Yb mode-locked fiber lasers.

We report direct experimental observation of interesting pulse synchronization dynamics in a cavity-combined Er and Yb mode-locked fiber lasers by measuring the relative position between the two-color pulses in the shared fiber section. The influence of the 1.03 µm pulse on the 1.56 µm single pulse as well as bound soliton pairs can be clearly identified as an effective phase modulation through the XPM effect with the walk-off effect taken into account. For the 1.56 µm single pulse under synchronization, the dependence of the relative position variation and the center wavelength shift on the cavity mismatch detuning is found analogous to the typical characteristics of FM mode-locked lasers with modulation frequency detuning. Moreover, depending on the cavity mismatch, the passively synchronized 1.56 µm bound soliton pairs are found to exhibit two different dynamical behaviors, i.e., phase-locked (in-phase) as well as non-phase-locked. The physical origins for these two kinds of bound soliton pairs are investigated experimentally by disclosing their locations with respective to the copropagating 1.03 µm pulse.

Highly efficient femtosecond pulse stretching by tailoring cavity dispersion in erbium fiber lasers with an intracavity short-pass edge filter.

We demonstrate highly efficient pulse stretching in Er(3+)-doped femtosecond mode-locked fiber lasers by tailoring cavity dispersion using an intracavity short-pass edge filter. The cavity dispersion is preset at around zero to obtain the shortest pulsewidth. When the cutoff wavelength of the short-pass edge filter is thermo-optically tuned to overlap the constituting spectral components of mode-locked pulses, large negative waveguide dispersion is introduced by the steep cutoff slope and the total cavity dispersion is moved to normal dispersion regime to broaden the pulsewidth. The time-bandwidth product of the mode-locked pulse increases with the decreasing temperature at the optical liquid surrounding the short-pass edge filter. Pulse stretch ratio of 3.53 (623.8 fs/176.8 fs) can be efficiently achieved under a temperature variation of 4 °C.

Entangled quantum nonlinear Schrödinger solitons.

Considered as a multipartite quantum system, time-multiplexed nonlinear Schrödinger solitons after collision are rigorously proved to become quantum entangled in the sense that their quadrature components of suitably selected internal modes satisfy the inseparability criterion. Clear physical insights for the origin of entanglement are given, and the required homodyne local oscillator pulse shape for optimum entanglement detection is determined.

Passive synchronization between a self-similar pulse and a bound-soliton bunch in a two-color mode-locked fiber laser.

We report the first (to our knowledge) experimental observation of passive synchronization between a 1.03 microm self-similar pulse and a 1.56 microm bound-soliton bunch in a two-color mode-locked fiber laser. The self-similar pulse can form an effective potential well of several picoseconds to trap the copropagating bound solitons in the shared-fiber section. Such synchronization mechanism is supported by the experimental evidence of smaller time separation between the bound solitons under synchronization. In addition, the self-similar pulse and bound solitons can still remain locked under the periodic collision of another 1.56 microm unlocked pulse train.

Influence of depressed-index outer ring on evanescent tunneling loss in tapered double-cladding fibers.

A tapered fiber with a depressed-index outer ring is fabricated and dispersion engineered to generate a widely tunable (1250-1650 nm) fundamental-mode leakage loss with a high cutoff slope (-1.2 dB/nm) and a high attenuation for stop band (>50 dB) by modification of both waveguide and material dispersions. The higher cutoff slope is achieved with a larger cross angle between the two refractive index dispersion curves of the tapered fiber and surrounding optical liquids through the use of depressed-index outer ring structures in double-cladding fibers.

Double-pass high-gain low-noise EDFA over S- and C+L-bands by tunable fundamental-mode leakage loss.

We demonstrate a high-gain low-noise double-pass tunable EDFA over S- and C+L-bands by discretely introducing fundamental-mode leakage loss in a 16-m-long standard C-band Er(3+)-doped fiber. The amplified spontaneous emission at the wavelengths of longer than 1530 nm can be substantially attenuated by the ASE suppressing filters to maintain high population inversion and to squeeze out the optical gain for S-band signals. When the filters are disabled, the gain bandwidth immediately returns back to the C+L-bands. Under S-band operation, a 37 dB small signal gain and a minimum 4.84 dB noise figure at 1486.9 nm are achieved with a 980 nm pump power of 154 mW.

Towards the short-wavelength limit lasing at 1450 nm over 4I13/2 ? 4I15/2 transition in silica-based erbium-doped fiber.

The transition rate of the stimulated emission at the higher energy levels of the excited states in a silica-based erbium-doped fiber can be enhanced by introducing fundamental-mode cutoff filtering mechanism. The electrons excited by optical pumping can more occupy the higher energy levels of the excited states when the transition rate for the lower energy levels (longer wavelengths) of the excited states is substantially suppressed. The achieved lasing wavelength can thus be moving toward the shorter wavelengths of the gain bandwidth. The laser transition between 4I13/2 ? 4I15/2 multiplets of the silica-based erbium-doped fiber is known to emit fluorescence with the shortest wavelength around 1450 nm. We, for the first time, experimentally demonstrate a widely tunable fiber laser at the wavelength very close to 1450 nm by using a standard silica-based C-band erbium-doped fiber. The tuning range covers 1451.9-1548.1 nm, with the best temperature tuning efficiency as high as 57.3 nm/ degrees C, by discretely introducing tunable fundamental-mode cutoff tapered fiber filters along a 16-m-long erbium-doped fiber under a 980 nm pump power around 200 mW. The signal-ASE-ratio can be higher than 45 dB whereas the FWHM of the laser lasing lights can be reduced below 0.2 nm by using an additional Fabry-Perot filter.

Tunable Er3+-doped fiber amplifiers covering S and C+L bands over 1490-1610 nm based on discrete fundamental-mode cutoff filters.

We demonstrate thermo-optically tunable Er3+-doped fiber amplifiers covering S and C+L bands (1490-1610 nm) using fundamental-mode cutoff filters discretely located in a 17.5 m long standard Er3+-doped fiber. The material dispersion and waveguide structure of the tapered fibers are locally modified to produce high-cutoff-efficiency filters that make the unwanted amplified spontaneous emission highly attenuated at the longer wavelengths, and the optical gain is thus moved toward the shorter wavelengths. The maximal signal gains are measured to be 18.92, 37.18, and 15.19 dB with 980 nm pump power of 135 mW in the S, C, and L bands, respectively.

Stable new bound soliton pairs in a 10 GHz hybrid frequency modulation mode-locked Er-fiber laser.

We report what is, to our knowledge, the first experimental observation of stable new bound soliton pairs at the 10 GHz repetition rate in a hybrid FM harmonic mode-locked Er-fiber laser (1177 soliton pairs simultaneously in the laser cavity). The two solitons in the soliton pair have the identical pulse shape and are with the antiphase (pi phase difference). Their time separation is about three times the FWHM soliton width and varies with the phase modulation strength. The corresponding mechanism for explaining the formation as well as the superior stability of these closely bound soliton pairs is also given.

Long-term stabilization of a 10 GHz 0.8 ps asynchronously mode-locked Er-fiber soliton laser by deviation-frequency locking.

Without using high speed RF feedback electronics, we successfully demonstrate a novel and economic long-term stabilization scheme for a 10 GHz 0.8 ps asynchronously mode-locked Er-fiber soliton laser by controlling the cavity length to lock the deviation frequency at 25 kHz. The required deviation frequency between the cavity harmonic frequency and the modulation frequency can be directly obtained from the low frequency electronic sideband of the laser output. The same feedback control unit is also useable for higher modulation frequencies, because the suitable deviation frequency always remains within the range of 15~40 kHz.

Photon-number fluctuation and correlation of bound soliton pairs in mode-locked fiber lasers.

Quantum photon-number fluctuation and correlation of bound soliton pairs in mode-locked fiber lasers are studied on the basis of the complex Ginzburg-Landau equation model. We find that, depending on their phase difference, the total photon-number noise of the bound soliton pair can be larger or smaller than that of a single soliton, and the two solitons in the soliton pair have a corresponding positive or negative photon-number correlation. It is predicted for the first time to our knowledge that out-of-phase soliton pairs can exhibit less noise as a result of negative correlation.

Direct generation of a 10 GHz 816 fs pulse train from an erbium-fiber soliton laser with asynchronous phase modulation.

By employing the technique of asynchronous mode locking, we have successfully demonstrated direct generation of stable 10 GHz 816 fs pulse trains with a supermode-suppression ratio >70 dB from a hybrid mode-locked Er-fiber laser. When the modulation frequency deviates from the cavity harmonic frequency by 15-40 kHz, stable femtosecond soliton pulses are formed. Our results demonstrate that asynchronous mode locking can act as an effective mechanism for achieving a shorter pulse width and for stabilizing high-repetition-rate pulse trains in soliton fiber lasers.

Fiber Bragg grating sequential UV-writing method with real-time interferometric side-diffraction position monitoring.

This work presents a new sequential UV-writing procedure for fabricating long fiber Bragg grating (FBG) devices. To real-time accurately align the position of every exposed FBG section prior to UV exposure, a single-period reference fiber grating with strong refractive index modulation is probed by applying an interferometric side-diffraction method to measure the grating phase as the position reference. In this way the overlapped FBG sections can be connected section-by-section without obvious phase errors, even when the written index-modulation is weak.

Relaxation of a field-unwound cholesteric liquid crystal.

We have numerically investigated the homogeneous-planar and homeotropic-planar transitions, respectively, in a planar-aligned cholesteric liquid crystal by using our multidimensional software based on the finite element method. When the unwinding field is turned off abruptly, the relaxation process of a field-unwound cholesteric liquid crystal is accompanied by an elastic-induced Helfrich deformation without introduction of defects, which will continuously convert into a stable planar texture with natural pitch and domain wall.