The coupled-mode theory (CMT) for optical waveguides is reviewed, with emphasis on the analysis of coupled optical waveguides. A brief account of the recent development of the CMT for coupled optical waveguides is given. Issues raised in the debates of the 1980’s on the merits and shortcomings of the conventional as well as the improved coupled-mode formulations are discussed. The conventional coupled-mode formulations are set up in a simple, intuitive way. The rigorous CMT is established on the basis of a linear superposition of the modes for individual waveguides. The cross-power terms appear logically as a result of modal nonorthogonality. The cross power is necessary for the self-consistency of the CMT for dissimilar waveguides. The nonorthogonal CMT, though more complicated, yields more-accurate results than the conventional orthogonal CMT for most practical applications. It also leads to the prediction of cross talk in directional couplers. The conventional orthogonal CMT is, however, reliably accurate for describing the power coupling between two weakly coupled, nearly identical waveguides. For dissimilar waveguides, a self-consistent orthogonal CMT can be derived by a redefinition of the coupling coefficients, and it predicts the coupling length and therefore the power exchange between the waveguides accurately if the two waveguides are far apart. Three typical coupler configurations—the uniform, the grating-assisted, and the tapered—are examined in detail. The accuracy, scope of validity, limitations, and extensions of the coupled-mode formulations are discussed in conjunction with each configuration. To verify the arguments in the discussions, comparisons with the exact analytical solutions and the rigorous numerical simulations are made.

Coupled-mode theory for optical waveguides: an overview

The authors have recently demonstrated 57 nm of tuning in a monolithic semiconductor laser using conventional distributed-Bragg-reflector (DBR) technology with grating elements removed in a periodic fashion. They describe the theory and design of these so-called sampled-grating tunable lasers. They calculate sample-grating reflectivity, and present normalized design curves which quantify tradeoffs involved in a sampled-grating DBR laser with two mismatched sampled-grating mirrors. These results are applied to a design example in the InP-InGaAsP system. The design provides 70-nm tuning while maintaining >30-dB mode suppression ratio (MSR), with fractional index change Delta mu / mu >

Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings

After over two decades of exploration, tunable diode lasers are beginning to find significant applications, driven largely by the huge demand for bandwidth that is guiding many developments in the optical fiber communication business today. In the paper, some of the history and key developments that have led to the technologies available today are reviewed from the perspective of the author. After discussion of some of the early work, the focus shifts to widely tunable diode lasers, which would appear to be key enablers for future dense wavelength-division multiplexing and optical switching and networking systems. The distinguishing characteristics of the current technological alternatives are summarized.

Monolithic tunable diode lasers

Intermixing the wells and barriers of quantum well structures generally results in an increase in the band gap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing has been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer; such processes offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICS). Recent progress in QWI techniques is reviewed, concentrating on processes which are compatible with PIC applications, in particular the achievement of low optical propagation losses.

https://iopscience.iop.org/article/10.1088/0268-1242/8/6/022/pdf

Quantum well intermixing

Vertical-cavity surface-emitting lasers (VCSELs) are now key optical sources in optical communications. Their main application is currently in local area networks using multimode optical fibers. VCSELs are also being rapidly commercialized for single-mode fiber metropolitan area and wide area network applications. The advantages of VCSEL include simpler fiber coupling, easier packaging and testing, and the ability to be fabricated in arrays. In addition, VCSELs have an inherent single-wavelength structure that is well suited for wavelength engineering. All these advantages promise to lead to cost-effective wavelength-tunable lasers, which are essential for the future intelligent, all-optical networks. The author reviews the advances on wavelength-tunable VCSELs. She summarizes some of the early breakthroughs in wavelength engineering of VCSELs and then concentrates on the designs and properties of micromechanical tunable VCSEL.

Tunable VCSEL

The authors report all-optical 100 Gbit/s wavelength conversion employing cross-phase modulation for the first time

100 Gbit/s all-optical wavelength conversion with integrated SOA delayed-interference configuration

We have integrated a broadly tunable grating‐assisted vertical coupler as an intracavity filter to demonstrate a novel monolithic multiple‐quantum‐well InGaAsP/InP laser. The narrow, bandpass intracavity filter results in an extended cavity laser with a measured electrical tuning range of 57 nm with single‐frequency operation over most of that range.

Broadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57‐nm tuning range

Long distance (over 500 km) transmission of 10 Gbit/s based wavelength division multiplexed signals using semiconductor optical amplifiers (SOAs) has been successfully demonstrated with a bit error rate (BER) better than 10/sup -14/ utilising the wavelength modulation technique. With the normal transmission scheme. There is an error floor near the BER of 10/sup -4/ regardless of the transmission distance. Which is due to the cross gain modulation effect in the SOA.

10 Gbit/s based WDM signal transmission over 500 km of NZDSF using semiconductor optical amplifier as the in-line amplifier

The balanced operation of a multiple-quantum-well balanced heterodyne receiver photonic integrated circuit (PIC) is described. Using only SMA-connected 50 Omega commercial electronics, a free-space beam sensitivity of -42.3 dBm at 108 Mb/s and -39.7 dBm at 200 Mb/s for NRZ FSK (frequency-shift keying) reception has been achieved. This represents a 14 dB improvement over any previous heterodyne receiver PIC sensitivity. In addition to providing the multichannel benefits of heterodyne reception, this is also the highest sensitivity yet reported for any OEIC (optoelectronic integrated circuit) receiver. >

Balanced operation of a GaInAs/GaInAsP multiple-quantum-well integrated heterodyne receiver

By controlling the thickness of the grating depth with chemical beam epitaxy (CBE) growth time, we report in this letter the design and performance of an integrated tunable detector. A carefully designed tunable active filter, which allows only one below threshold Fabry–Perot mode for operation, is integrated with a waveguide detector. The full tuning range of this kind of tunable device can now be utilized for system applications.

C. A. Burrus

Papers

100 Gbit/s all-optical wavelength conversion with integrated SOA delayed-interference configuration

Broadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57‐nm tuning range

10 Gbit/s based WDM signal transmission over 500 km of NZDSF using semiconductor optical amplifier as the in-line amplifier

Balanced operation of a GaInAs/GaInAsP multiple-quantum-well integrated heterodyne receiver

InGaAs/InGaAsP integrated tunable detector grown by chemical beam epitaxy