Light Sources For Optical Communications Research Articles

Donor-acceptor substituted thiophene dyes for enhanced nonlinear optical limiting

Designed, synthesized and characterized here two thiophene dyes 3-(4-(diethylamino)phenyl)-2-(thiophen-2-yl)acrylonitrile (ET), 3-(4-(diphenylamino)phenyl)-2-(thiophen-2-yl)acrylonitrile (PT) along with their -CHO substituted derivatives 3-(4-(diethylamino)phenyl)-2-(5-formylthiophen-2-yl)acrylonitrile (ET-CHO) and 3-(4-((4-formylphenyl) (phenyl)amino)phenyl)-2-(5-formylthiophen-2-yl)acrylonitrile (PT-2CHO) for optoelectronic devices towards protecting human eyes and optical sensors, as well as for stabilizing light sources in optical communications. Using the open aperture Z-scan method in tandem with linear absorption spectroscopy, emission spectroscopy and density functional theory (DFT) calculations, the nonlinear absorption and optical limiting behavior under nanosecond (5 ns) Nd: YAG laser excitation at 532 nm wavelength was demonstrated. The observed nonlinearity in all the samples was found to be arising from two-photon absorption process. The DFT calculations at a B3LYP/6-31G(d) level for the geometry optimization have shown that the HOMO levels of ET and PT dyes are delocalized throughout the molecule, while for ET-CHO and PT-2CHO dyes, they occupy all over the molecule except -CHO group present on thiophene ring. Also, LUMO levels are delocalized through the aromatic bridge and shift towards thiophene ring for all dyes. This change in the band structure has tuned the effective nonlinear absorption coefficients (βeff) and PT-CHO possesses a maximum value of βeff = 2.9 × 10−11 m/W. The excellent optical limiting performance of these chromophores makes them useful in photonic or optoelectronic devices for protecting human eyes and optical sensors, as well as for stabilizing light sources in optical communications.

3-kHz Spectral Linewidth Laser Assembly With Coherent Optical Negative Feedback

We demonstrate the stable operation of a narrow-linewidth semiconductor laser source based on a coherent optical negative feedback system. The linewidth was reduced to 3 kHz from 13.5 MHz. The power spectral density of frequency modulation (FM) noise was reduced by 35 dB, while the relative intensity noise was low enough for a light source for optical communications (less than -140 dB/Hz). Additionally, the system showed stable operation more than an hour. Our results indicate that the instability of a laser system is caused by fluctuations in the feedback loop length due to the mechanical vibrations, which can be suppressed by the robust integration of the system.

Ultrafast Graphene Light Emitters.

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

Controllable and stepwise synthesis of soluble ladder-conjugated bis(perylene imide) fluorenebisimidazole as a multifunctional optoelectronic material.

By a controllable and stepwise strategy, a soluble ladder-conjugated perylene derivative BPI-FBI as the only product has been synthesized, which avoids the tough work to isolate regioisomers generated by a conventional one-step condensation method. BPI-FBI exhibits broad absorption spectra covering the whole visible region from 300 to 700 nm because of the large π-conjugation skeleton and has a low LUMO level inheriting the prototype PDI. In the steady-state space-charge-limited current (SCLC) devices, BPI-FBI exhibits an intrinsic electron mobility of 1.01 × 10(-5) cm(2) V(-1) s(-1). With a high two photon absorbing activity in the near-infrared region from 1200 to 1400 nm, BPI-FBI also exhibits good optical limiting performance, which will be useful for sensor or human eye protection and stabilization of light sources for optical communications.

Controlling the spectrum of photons generated on a silicon nanophotonic chip.

Directly modulated semiconductor lasers are widely used, compact light sources in optical communications. Semiconductors can also be used to generate nonclassical light; in fact, CMOS-compatible silicon chips can be used to generate pairs of single photons at room temperature. Unlike the classical laser, the photon-pair source requires control over a two-dimensional joint spectral intensity (JSI) and it is not possible to process the photons separately, as this could destroy the entanglement. Here we design a photon-pair source, consisting of planar lightwave components fabricated using CMOS-compatible lithography in silicon, which has the capability to vary the JSI. By controlling either the optical pump wavelength, or the temperature of the chip, we demonstrate the ability to select different JSIs, with a large variation in the Schmidt number. Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range.

Two-photon absorption and optical power limiting properties of ladder-type tetraphenylene cored chromophores with different terminal groups

A series of soluble ladder-type tetraphenylene cored chromophores with different terminal groups have been synthesized and their structure–property relationship with regards to various linear optical and nonlinear optical properties has been established. By using the two-photon excited fluorescence method and the nonlinear transmission method, the two-photon absorption (2PA) properties of these chromophores were determined, and they were found to be strongly dependent on the electron-richness of the ladder-type tetraphenylene core, as well as the terminal groups. The introduction of strong electron donors (N-alkyl) in both the central core and the terminals led to a chromophore with a high 2PA cross-section value of 2137 GM. Optical limiting behaviors of the synthesized chromophores in THF were investigated by using a femto-second ultra-fast laser. 2PA coefficients for these chromophores in THF (5 mM) ranged from 0.131–0.256 cm GW−1. The ladder-type tetraphenylene cored chromophore with the highest 2PA cross-section value exhibited the best optical limiting performance, as evidenced by light transmission as low as 19.1% at 770 nm under an intensity of 99.6 GW cm−2. The excellent optical limiting performance of these chromophores makes them useful in photonic or optoelectronic devices for protecting human eyes and optical sensors, as well as for stabilizing light sources in optical communications.

Transient property of optically pumped organic film of different fluorescence lifetimes

The authors have investigated the direct relationship between the fluorescence lifetime (FL) and the transient photoluminescence (PL) response of organic materials to examine the applicability of organic light-emitting diodes as light sources for optical communications. Transient PL responses of organic materials with different FLs were measured as a frequency dependence of PL intensity pumped by a modulated violet laser diode. The authors have revealed that the cutoff frequency of PL intensity is significantly related to FL of an organic film. The −3dB cutoff frequency of 1,4-bis[2-[4-[N,N-di(p-tolyl)amino]phenyl]vinyl]benzene (DSB) has reached about 160MHz, which is much higher than that of other organic materials. This is because the FL of DSB is shortest among all the organic materials used in this study.

Microcavity Effect from a Novel Terbium Complex Langmuir-Blodgett Film

Two important microcavity effects—fluorescence intensity enhancement and lifetime shortening—are demonstrated to be exhibited by new microcavities consisting of a pure terbium complex sandwiched between silver mirrors. The advantages of the use of the fluorescent rare-earth complex as the emitting material and the Langmuir–Blodgett technique for the construction of microcavities are discussed. Microcavities acting as optical resonators have potential applications as high-density light sources for optical communications and color displays.

Er-Doped GaAs for High Speed Photoconductor Applications

ABSTRACTThe observation of 1.54 μm luminescence in erbium-doped-GaAs (GaAs:Er) and AlxGal-xAs, has stimulated research efforts because of their potential application to light sources in optical communications. In this paper, we study a different aspect of the photoresponse behavior of this material. The dependence of the carrier lifetime on the doping concentration of Erbium is investigated in GaAs:Er grown by MBE. A reduction in the carrier lifetime down to ∼1 ps is observed for the highest doping (∼5×1019 cm−3) investigated. Together with the high-resistivity observed for the higher doping values, this material serves as a novel photoconductor material for high-speed optoelectronics.

3元系固溶半導体 (Pb<SUB>1−<I>x</I></SUB>Cd<I><SUB>x</SUB></I>)S<SUB>1−<I>y</I></SUB>の組成<I>x</I>と化学量論的組成からの偏差<I>y</I>の同時制御

Attention is recently paid to the new materials for tunable laser diodes, lasing at the wavelength from 2 to 4 μm. The laser diodes are expected as the light source for optical communications in the next generation and also as the real-time IR-photospectrometer for detecting quickly and exactly the kind and the concentration of hydrocarbon pollutants in air such as methane and ethane.Multinary solid solution semiconductors such as ternary (Pb1−xCdx)S1−y and quaternary (Pb1−xCdx) (S1−zSez)1−y are best suited for the above purposes. To realize the continuous laser emission of tunable laser diodes near room temperature, it is necessary that the structure of the diode is a double-heterojunction with a matched lattice parameter and the composition (x, z) of each layer is controlled to a high degree.For this purpose, simultaneous control of the composition (x, z) and the deviation y from stoichiometry is indispensable for each constituent layer.In this study, a new method for simultaneously controlling the composition x and the deviation y from stoichiometry was proposed for the (Pb1−xCdx)S1−y solid solution. That is, the activities of sulfur (PS2) and CdS (aCdS) were controlled independently in the heat-treatment process of the solid solution by providing the respective reservoir chambers in the heat-treatment tube.As a result, it was found that the composition x of the solid solution is seriously effected by the cadmium partial pressures (PCd) dissociated from CdS. From this fact, it was suggested that the deviation y from stoichiometry could also be controlled simultaneously by regulating PS2, keeping PCd to a specific value corresponding to the target composition x.Based on this principle, the diagrams of P(PCd)-x and P(PS2)-T in composition x=0.20 were obtained successfully for this solid solution.In conclusion, the exploited method is applicable to a simultaneous control of other ternary or quaternary solid solutions involving two metallic components.