Long-wavelength Optical Communication Research Articles

Characterization and Optimal Design of Silicon-Rich Nitride Nonlinear Waveguides for 2 μm Wavelength Band

Optical communication using the 2 μm wavelength band is attracting growing attention for the sake of mitigating the information ‘capacity crunch’ on the way, where on-chip nonlinear waveguides can play vital roles. Here, silicon-rich nitride (SRN) ridge waveguides with different widths and rib heights are fabricated and measured. Linear characterizations show a loss of ~2 dB/cm of the SRN ridge waveguides and four-wave mixing (FWM) experiments with a continuous wave (CW) pump reveal a nonlinear refractive index of ~1.13 × 10−18 m2/W of the SRN material around the wavelength 1950 nm. With the extracted parameters, dimensions of the SRN ridge waveguides are optimally designed for improved nonlinear performances for the 2 μm band, i.e., a maximal nonlinear figure of merit (i.e., the ratio of nonlinearity to loss) of 0.0804 W−1 or a super-broad FWM bandwidth of 518 nm. Our results and design method open up new possibilities for achieving high-performance on-chip nonlinear waveguides for long-wavelength optical communications.

Ga originated kink-and-tail Zn diffusion profiles in InGaAsP and InGaAlAs alloys during MOVPE regrowth

We investigated the diffusion characteristics of Zn in ternary and quaternary alloys of InGaAsP and InGaAlAs, which are important materials in long-wavelength optical communication devices. The measured Zn diffusion profiles of InGaAs, InGaAsP, and InGaAlAs showed kink-and-tail shapes in which Zn concentration fell abruptly at first and then decreased slowly, whereas those of InP and InAlAs showed only abrupt decreases. Thus, only Ga-containing alloys had tail-like profiles. Since this tail was well described by the group-V vacancy related defect model, we deduced that its mechanism is closely related with group-V vacancies in Ga-related bonds such as GaP or GaAs. Furthermore, we demonstrated the possibility that many more group-V vacancies originated from GaP bonds than from GaAs bonds, indicating the difficulty in crystal growth of high quality alloys that have GaP components.

Nanophotonic Devices Based on Semiconductor Quantum Nanostructures

SUMMARY In this study, our recent research activities on nanophotonic devices with semiconductor quantum nanostructures are reviewed. We have developed a technique for nanofabricating of high-quality and high-density semiconductor quantum dots (QDs). On the basis of this core technology, we have studied next-generation nanophotonic devices fabricated using high-quality QDs, including (1) a high-performance QD laser for long-wavelength optical communications, (2) high-efficiency compound-type solar cell structures, and (3) single-QD devices for future applications related to quantum information. These devices are expected to be used in high-speed optical communication systems, high-performance renewable energy systems, and future high-security quantum computation and communication systems.

Soliton-based ultrafast multi-wavelength nonlinear switching in dual-core photonic crystal fibre

Systematic experimental study of ultrafast multi-wavelength all-optical switching performance in a dual-core photonic crystal fibre is presented. The focus is on nonlinearly induced switching between the two output ports at non-excitation wavelengths, which are generated during nonlinear propagation of femtosecond pulses in the anomalous dispersion region of a dual-core photonic crystal fibre made of multicomponent glass. Spatial and spectral characteristics of the fibre output radiation were measured separately for both fibre cores under various polarization and intensity conditions upon selective, individual excitation of each fibre core. Polarization-controlled nonlinear switching performance at multiple non-excitation wavelengths was demonstrated in the long-wavelength optical communication bands and beyond. Depending on the input pulse polarization, narrowband switching operation at 1560 nm and 1730 nm takes place with double core extinction ratio contrasts of 9 dB and 14.5 dB, respectively. Moreover, our approach allows switching with simultaneous wavelength shift from 1650 to 1775 nm with extinction ratio contrast larger than 18 dB. In addition, non-reciprocal behaviour of the soliton fission process under different fibre core excitations was observed and its effect on the multi-wavelength nonlinear switching performance was explained, taking into account the slight dual-core structure asymmetry. The obtained results represent ultrafast all-optical switching with an extended dimension of wavelength shift, controllable with both the input radiation intensity and the polarization by simple propagation along a 14 mm long fibre.

GaN-based waveguide devices for long-wavelength optical communications

Refractive indices of AlxGa1−xN with different Al concentrations have been measured in infrared wavelength regions. Single-mode ridged optical waveguide devices using GaN/AlGaN heterostructures have been designed, fabricated, and characterized for operation in 1550 nm wavelength window. The feasibility of developing photonic integrated circuits based on III-nitride wide-band-gap semiconductors for fiber-optical communications has been discussed.

Metamorphic graded bandgap InGaAs-InGaAlAs-InAlAs double heterojunction p-i-I-n photodiodes

High-speed metamorphic double heterojunction photodiodes were fabricated on GaAs substrates for long-wavelength optical fiber communications. The high quality linearly graded quaternary InGaAlAs metamorphic buffer layer made possible the growth of excellent InGaAs-InGaAlAs-InAlAs heterostructures on GaAs substrates. The use of a novel double heterostructure employing an InGaAlAs optical impedance matching layer, a chirped InGaAs-InAlAs superlattice graded bandgap layer (SL-GBL), and a large bandgap i-InAlAs drift region enabled photodiodes to achieve a low dark current of 500 pA, a responsivity of 0.6 A/W, and a -3 dB bandwidth of 38 GHz at -5 V reverse bias for 1.55 /spl mu/m light. The effect of accumulated charges at the InGaAs-InAlAs heterointerface was examined through a comparison of the dark currents of InGaAs-InAlAs and InGaAs-InP abrupt single heterojunction photodiodes; to photodiodes with chirped InGaAs-InAlAs SL-GBLs. The charge accumulation effects observed in abrupt heterojunction devices were suppressed by including a chirped InGaAs-InAlAs SL-GBL between the InGaAs absorption layer and InAlAs drift layer. The effect of passivation techniques was evaluated by comparing dark currents of unpassivated photodiodes and photodiodes passivated with either polyimide or SiN/sub x/. The enhancement of photodiode bandwidth through the inclusion of a transparent large bandgap I-InAlAs drift region was verified by comparing the bandwidths of the P-i-I-N photodiodes that have I-InAlAs between i-InGaAs photoabsorption layer and N/sup +/ InAlAs cathode to conventional P-i-N photodiodes without a drift region.

Over 1.5 μm light emission from InAs quantum dots embedded in InGaAs strain-reducing layer grown by metalorganic chemical vapor deposition

We demonstrated the 1.52 μm light emission at room temperature from self-assembled InAs quantum dots embedded in the In0.45Ga0.55As strain-reducing layer. By capping InAs quantum dots with an InGaAs strain-reducing layer instead of GaAs, the photoluminescence peak of InAs quantum dots can be controlled by changing the indium composition of the InGaAs strain-reducing layer. The full width at half maximum is as narrow as 22 meV. The wavelength of 1.52 μm is the longest wavelength so far achieved in self-assembled InAs quantum dots, which would be promising to quantum-dot lasers on GaAs substrate for application to light sources in long-wavelength optical communication systems.

Single-mode semiconductor lasers for long-wavelength optical fiber communications and dynamics of semiconductor lasers

Research activities devoted to long-wavelength optical fiber communications, especially for single-mode semiconductor lasers at the long-wavelength range of 1.5 /spl mu/m and the dynamics of their lasing wavelength properties, which limit the transmission bandwidth of a single-mode fiber, are reviewed. Advanced semiconductor lasers based on the single-mode operating nature, such as wavelength tunable lasers and photonic integrated circuits are also reviewed.

Ultra-wide-band long-wavelength photodetectors

Developments and performances of ultra-wide-band long-wavelength photodetectors are outlined. Theoretical consideration shows the competence of the waveguide p-i-n photodetector for ultra-wide-band long-wavelength optical fibre communication. A newly proposed multimode waveguide p-i-n photodetector with mushroom-mesa structure can relax the parasitic limits, and reaches a record performance of 110 GHz bandwidth and 50% external quantum efficiency at a 1.55 μm wavelength. A high-speed monolithically integrated photoreceiver using the waveguide p-i-n photodetector is also presented.

A monolithically integrated frequency mixing photoreceiver

A monolithically integrated photoreceiver/mixer for long-wavelength optical communication and switching is discussed. The circuit uses a narrow gate junction field effect transistor as a voltage tunable feedback resistor to achieve high-efficiency low-distortion mixing. An overall opto-electronic conversion gain of 20 dB and a second-order frequency product of -26 dBc have been measured. >

Giga-bit rate receiver OEICs grown by OMVPE for long-wavelength optical communications

Receiver optoelectronic integrated circuits (OEICs) in which AlInAs/GaInAs HEMTs and GaInAs pin PDs were integrated monolithically on InP substrates, were studied. Good uniformity of device characteristics over a 2-inch whole wafer was obtained. Three different types of receiver circuits, i.e. high-impedance type, trans-impedance type, and straightforward termination type, were fabricated and are discussed. The sensitivity of −30.4 dBm was obtained for 1.2 Gbit/s NRZ signals from the high-impedance type OEIC, into which an equaliser was also integrated. The receiver operated up to 1.6 Gbit/s. The dynamic range of more than 23 dB was obtained using the straightforward termination type OEIC. These performances are good enough for practical use in optical communication systems operating at gigabit/s data rates.

Monolithic InGaAs p-i-n InP metal-insulator-semiconductor field-effect transistor receiver for long-wavelength optical communications

We have fabricated a monolithically integrated p-i-n field-effect transistor (FET) receiver consisting of an InGaAs/InP p-i-n detector and a high-speed InP metal-insulator-semiconductor field-effect transistor. The receiver sensitivity of the p-i-n FET is −18.2 dBm at a bit rate of 2.4 Gb/s and a wavelength of 1.55 μm.

Tunable infrared modulator and switch using Stark shift in step quantum wells

A tunable infrared modulator and/or switch using intersubband Stark shift in a step quantum well is discussed. The device utilizes the intersubband absorption and the large change of separation between energy levels in the step quantum well under an applied electric field. The incident infrared beam on the device is either absorbed or transmitted depending on the energy separation of the levels, and thus the modulation can be achieved by adjusting the energy levels with an applied electric field. The extremely short lifetime of the intersubband transition makes this modulator suitable for application in high-speed long-wavelength optical communications. >

Low-noise current optoelectronic integrated receiver with internal equalizer for gigabit-per-second long-wavelength optical communications

An equalizer, which is essential in order to improve the sensitivity of receiver optoelectronic integrated circuits (OEICs) at a gigabit-per-second data rate, has been monolithically integrated on an InP substrate with a p-i-n photodiode and a high-impedance high-electron-mobility-transistor (HEMT) amplifier. The receiver operated up to 1.6 Gb/s and showed low noise current characteristics. The minimum noise current is less than 4 pA/ square root Hz. The sensitivity calculated from the noise current characteristics is -28.4 dBm for 1.6-Gb/s signals. The receiver chip, which was assembled on a ceramic mount, exhibited a sensitivity of -30.4 dBm at 1.2 Gb/s and 1.3- mu m wavelength. The performance is as good as those of receiver OEICs with an external equalizer and sufficient for practical use in gigabit-per-second optical communication system. >

Simple, very low dark current, planar long-wavelength avalanche photodiode

We have demonstrated a novel planar, avalanche photodiode (APD) for use in long-wavelength (0.95–1.65 μm) optical communication systems. The device is a separate absorption and multiplication region APD utilizing p+ guard rings which are concentric with, but not attached to the central diffused p+-n junction region. Since no contact is made to the rings, their potential is allowed to ‘‘float’’ at a value somewhat less than that established by the externally applied voltage. The APD, which is fabricated in a manner identical to simple p-i-n photodiodes, eliminates edge breakdown effects while greatly reducing the electric field at the insulator/semiconductor interface. A 60-μm-diam junction device grown by vapor phase epitaxy is observed to have a primary dark current of <300 pA, and a capacitance of 290 fF at 90% of the breakdown voltage. Uniform gains as high as 11 have been observed.

Twelve-channel individually addressable InGaAs/InP p-i-n photodiode and InGaAsP/InP LED arrays in a compact package

We report on monolithically integrated 1 × 12 arrays of In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> As p-i-n detectors and InGaAsP LED's for use in long-wavelength optical communication system applications. The detectors are sensitive in the wavelength region of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95-1.65 \mu</tex> m, and the LED's emit at 1.30 μm. The devices utilize Si V-block fiber array connectors with one end polished at a 45° angle with respect to the fiber axis such that the optical path is bent by 90° to afford coupling laterally to the photonic device. The resultant structures are built in a 14-pin dual-in-line (DIP) compact metal package, and are capable of operation at least up to 200 Mbit/s.

The sensitivity of photoconductor receivers for long-wavelength optical communications

We calculate the sensitivity of In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> As photoconductor receivers for use in moderate to high bit-rate lightwave transmission applications. It is found that the noise of photoconductive receivers is dominated at all bit rates <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B &lt; 4</tex> Gbit/s by Johnson noise in the conductive channel. Nevertheless, the total noise current decreases approximately linearly with photoconductive gain, and therefore the sensitivity of photoconductive receivers can be comparable to high-sensitivity p-i-n photodiode receivers. The sensitivity of practical photoconductive receivers compares most favorably with p-i-n receivers in the bit-rate range of 500-2 Gbit/s. However, receivers employing high-speed In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> As/InP avalanche photodiodes are expected to be more sensitive than photoconductive receivers over the entire bit-rate range considered. In this analysis, we consider the effects of slow photoconductor response on receiver sensitivity, and find that the limited gain-bandwidth product of practical photoconductors increases the complexity of the receiver circuit by necessitating equalization, resulting in a decrease in receiver sensitivity and dynamic range.

Long-wavelength optical fiber communication

Recent research on long-wavelength lightwave communication utilizing the wavelength region between 1.3 and 1.6 µm is reviewed with an eye toward future system development. The attraction of the long-wavelength region is the availability of the ultimately low-loss and wide-band features of the silica fiber, where minimum loss is 0.27 dB/km at a wavelength of 1.3 µm and 0.16 dB/km at 1.55 µm. The single-mode fiber has found its first significant applications in long-wavelength systems. The specific characteristics of lightwave components are discussed with focus on physical fundamentals. The practical performance of fibers and lightwave devices is surveyed. The dynamic properties of long-wavelength laser diodes are discussed in relation to fiber characteristics. The noise characteristics of long-wavelength detectors are considered for the purpose of specifying the repeater spacing. Some system studies are reviewed, for example, 1.3-µm-wavelength lightwave systems, which have demonstrated bandwidth-distance products of about 40 GHz ċ km. Various approaches to extend the capacity of long-wavelength lightwave transmission are given. In the future, the 1.5-µm wavelength system could operate at the lowest loss wavelength region extending from 1.5 to 1.65 µm. Much higher performance, for example, bandwidth-distance products of 185 GHz ċ km, achieved by further continuation of research and development on lightwave sources as well as fibers. Because of the author's familiarity with work in Japan, that work is emphasized and most frequently cited.

Sensitivity of Avalanche Photodetector Receivers for Long-Wavelength Optical Communications

We consider, in detail, the potential improvement in receiver sensitivity that can be realized using an avalanche photodiode (APD) rather than the conventional p-i-n diode in long-wavelength optical communications systems. Numerical computations are used to determine optimum gains and receiver sensitivities for several values of ionization coefficient ratios and dark currents. Sensitivities are considered for transmission bit rates of 45 Mb/s, 90 Mb/s, and 274 Mb/s — values characteristic of present long-wavelength systems. We find that general relationships and scaling laws between receiver sensitivity and the other critical parameters can be formulated if the sensitivity is calculated in units relative to the quantum limit. An important result is that the improvement in APD sensitivity depends strongly on dark current, but only weakly on the ionization coefficient ratio. Our calculations are compared with recent results obtained for In0.53Ga0.47As/InP APDs sensitive in the λ = 0.95 μm to 1.6 μm wavelength region. We also include a brief discussion comparing APD sensitivities with those obtained using phototransistors and majority carrier devices.

Low dark-current, high-efficiency planar In0.53Ga0.47As/InP P-I-N photodiodes

We have fabricated high quantum efficiency, high speed, low dark-current In 0.53 Ga 0.47 As/InP p-i-n planar photodiodes Which meet the performance requirements of long-wavelength optical communications systems. The diodes employ plasma deposited SiN x as a mask to Zn diffusion. The dark-current density at 10 V is J = 5 × 10-5A/cm2with a corresponding capacitance of C = 5 × 10-9F/cm2.