Grating-assisted Codirectional Coupler Research Articles

InP/InGaAsP grating-assisted codirectional coupler tunable receiver with a 30 nm wavelength tuning range

The authors demonstrate a widely tunable grating-assisted codirectional coupler (GACC) based receiver in the InP/InGaAsP materials system. With current injection of only 20 mA into each filter section, the integrated receiver can be continuously tuned over a 30 nm wavelength range. To the best of the authors knowledge, this is the highest tuning range reported so far for an InP/InGaAsP GACC tunable receiver.

Wavelength-selective grating-assisted codirectional coupler tunable receiver on InP/InGaAsP

The authors demonstrate the first grating-assisted codirectional coupler (GACC) based tunable the InP/InGaAsP materials system. Initial results using reverse bias show a receiver full-width half-maximum of 4 nm and a tuning range of 3 nm. From numerical simulations the authors found the material uniformity played a very important role in the performance of GACC filters.

Enhanced wavelength tuning in grating-assisted codirectional coupler filter

Wavelength-tunable devices based on grating-assisted codirectional coupler filters have been shown to possess enhanced wavelength tunabilities and are promising for use in multiwavelength communication networks. It was found theoretically that the enhancement factor can be estimated by the mismatch of group velocities between the two coupled models. We have confirmed this prediction via measurements of wavelength tuning for both the coupler filter and the Fabry-Perot filter in reverse bias. These measurements are accomplished with photonic-integrated devices. An enhancement factor of approximately 11 is measured on our device, which agrees very well with the theory. >

Photonic integrated tunable receivers with optical preamplifiers for direct detection

We propose and demonstrate a photonic integrated tunable receiver which cascades a traveling-wave optical preamplifier, a passive tunable grating-assisted codirectional coupler filter, and a waveguide photodetector. The devices contain three strained InGaAs quantum wells and operate around λ=0.98 μm. The spectral responses have been measured using the broadband spontaneous emission from the integrated amplifier. Optical filters ∼4-nm wide, ∼10-nm apart with grating pitches ranging from 7.4 to 10.6 μm were observed. Wavelength selective photodetection of a channel ∼3-nm wide, tunable over 4.3 nm with a reverse bias is in relatively good agreement with theory.

Bidirectional eigenmode propagation for large refractive index steps

The bidirectional eigenmode propagation (BEP) combines the advantages of the transfer-matrix method for propagation and the mode-matching method (satisfying Maxwell's boundary conditions) at longitudinal large discontinuities for forward and backward travelling eigenvalues including discretized radiation modes. The calculated examples include a grating-assisted codirectional coupler for which 20% radiation losses have been found, and a cleaved waveguide facet with 39% reflected intensity and recapture of radiation modes due to a taperlike structure. The BEP is applicable to various types of optical passive (directional coupler, butt-coupling, taper) and active (DBR-, DFB-, and VCSE-laser, detector) devices with large refractive index differences.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A photonic integrated tunable receiver/optical preamplifier in a vertical coupler filter structure

The authors propose and demonstrate a novel integrated tunable receiver which integrates a traveling-wave optical preamplifier, a two-stage passive grating-assisted codirectional coupler (GACC) filter, and a photodetector. The GACC filter has been shown to have a wavelength tunability an order of magnitude wider than that of a distributed-Bragg-reflector (DBR) filter. The transmissive style of the filter makes it possible to cascade several optical elements to achieve the high functionality of photonic integration. The device was fabricated on GaAs substrate using strained InGaAs quantum wells for optical gain and GaAs for the index tuning material operating around lambda =0.98 mu m. With zero bias applied to the filter electrodes, the receiving passband was measured to be centered at approximately 0.98 mu m with a FWHM bandwidth of approximately 3 nm. A differential signal gain of 12 dB was obtained when the amplifier current increased from 20 mA to 34 mA.

On the spectral properties of widely tunable lasers using grating-assisted codirectional coupling

The analysis of mode-suppression-ratio (MSR) and wavelength tunability of widely tunable semiconductor lasers using grating-assisted codirectional-coupler (GACC) filters is considered. A tradeoff is found between tuning range and spectral purity when using this type of filter as an intracavity mode selector. It is shown that in order to improve the tuning range by a factor of 10 compared to a conventional tunable laser using distributed feedback reflectors (DBRs), one has to tolerate a MSR around 23 dB at 1 mW output optical power/facet for an ideal cavity design. Limitations imposed by the materials are illustrated with design curves based on a vertical-twin-guide structure in both GaAs and InP systems for lasing wavelengths at 0.98 and 1.55 mu m, respectively. >

A transfer matrix approach based on local normal modes for coupled waveguides with periodic perturbations

A simple transfer matrix method is used to analyze power coupling and scattering in optical waveguide structures with periodic index perturbations. The transfer matrix is determined by a mode-matching technique for the local normal modes of the structures. This approach accounts for the change in the field patterns along the waveguide axis and is more rigorous than coupled-mode theory based on ideal modes. The distinct advantages of the method are that (1) it can be applied to structures with relatively strong index perturbations caused by large grating height and/or large index difference; (2) the TM modes whose transverse electric field is perpendicular to the index interface can be properly treated; and (3) the radiation loss due to scattering at the index discontinuities along the waveguide axis can be estimated. Analytical expressions for the power coupling in a grating-assisted codirectional coupler are derived, and numerical results are shown for some typical structures. >