Fiber optics communication systems require compact light emitting sources capable of generating single-mode, tunable, narrow linewidth radiation in the 1.3-1.56 .mu.m wavelength range. Some of the existing semiconductor lasers, for example, InGaAsP DFB lasers can meet requirements for high power and proper wavelength, but a high dynamic single mode yield is difficult to achieve. Conventional index coupled CFB lasers employing an index corrugation have an inherent problem in existence of two longitudinal modes with an equal threshold gain which results in poor single mode operation as shown, for example, in the article by H. Kogelnik and C. V. Shank "Coupled-Wave theory of distributed feedback lasers", J. Appl. Phys., vol. 43, no. 5, pp. 2327-2335, 1972. In complex coupled DFB lasers, a periodic optical gain or loss modulation in the presence or absence of conventional index corrugation along the laser cavity effectively breaks the mode degeneracy between the two Bragg modes around the stop band of the DFB lasers, and thus avoids a serious and inherent problem for conventional index coupled DFB lasers, as shown in publications by Y. Luo, Y. Nakano, K. Tada et al. "Purely gain-coupled distributed feedback semiconductor 35 lasers", Appl. Phys. Lett., vol. 56, pp. 1620-1622, 1990 and G. P. Li, T. Makino, R. Moore et al. "Partly gain-coupled 1.55 .mu.m strained layer multi-quantum well DFB lasers", IEEE J. Quantum Electronics, vol. QE-29, pp. 1736-1742, 1993. With introduction of even a small amount of gain or loss coupling, the dynamic single mode yield of complex coupled DFB lasers increases drastically, whether or not there is index coupling. It effectively provides lasing predominantly on a preferred and fixed Bragg mode among the two originally degenerate ones around a stop band, regardless of random distribution of unknown laser facet phases. In in-phase gain coupled DFB lasers, the higher index region within a grating period has a higher optical modal gain, resulting in lasing mainly of the right Bragg mode with the lasing wavelength longer than the Bragg wavelength. Both theory and experiments have confirmed this conclusion. For anti-phase loss coupled DFB lasers, the higher index region within a grating period experiences higher optical loss, resulting primarily in lasing of the left Bragg mode with the lasing wavelength shorter than the Bragg wavelength.
Previously known gain coupled or loss coupled DFB lasers utilize only one type of complex coupling which results in a high dynamic single mode yield for one of predetermined and dominant Bragg mode only. The random distribution of cleaved or HR-coated laser facets is usually the major reason why the opposite Bragg mode across the stop band sometimes actually lases as the dominant mode. Nevertheless, with an increase of complex coupling strength, the single mode yield increases quickly, and an excellent dynamic single mode behavior with a high side mode suppression ratio (SMSR) can be obtained, as reported in the publication by B. Borchert, K. David, B. Stegmuller et al. "1.55 .mu.m gain-coupled quantum-well distributed feedback lasers with high-single-mode yield and narrow linewidth", IEEE Photon. Technol. Lett., vol. 3, no. 11, pp. 955-957, 1991.
Complex coupled DFB lasers have been studied intensively by different scientific groups. It was shown that they constitute a good candidate for practical applications in fiber optics communication systems. Some of the achievements are reported in the following publications: W. T. Tsang, F. S. Choa, M. C. Wu et al. "Semiconductor distributed feedback lasers with quantum well superlattice gratings for index or gain-coupled feedback", Appl. Phys. Lett., vol. 60, pp. 2580-2582, 1992 and J. Hong, K. Leong, T Makino et al. "Impact of random facet phase on modal properties of partly gain-coupled DFB lasers", J. Selected Topics on Quantum Electronics, vol. 3, no. 2, pp. 555-568, 1997.
Nevertheless, a rapid advance in high speed and large capacity dense wavelength division multiplexing (DWDM) fiber optics systems has been constantly demanding semiconductor lasers having not only single mode properties but also a wide continuous tuning range for practical and cost effective applications. A number of techniques have been used in attempt to obtain a widely tunable DFB laser with dynamic single mode performance comparable with standard DFB laser generating at fixed wavelength. First, conventional index coupled tunable lasers followed by complex coupled lasers were tried. The latter appeared to be better in comparison with index coupled lasers because of maintaining good single mode properties within a wide wavelength tuning range. However, similar to other single contact DFB lasers, a wavelength range of complex coupled DFB lasers is limited either by the injection current or base temperature. A multi-sectional laser with separate electrodes for independent current injection was introduced next, providing an effective mode selection and phase control. Some of the experimental implementations of the idea are described in the following publications: Y. Yoshikuni, K. Oe, G. Motosugi, T. Matsuoka "Broad Wavelength tuning under single-mode oscillation with a multi-electrode distributed feedback laser", Electronic Lett., vol. 22, no. 22, pp. 1153-1154; I. Kim, R. C. Alferness, U. Koren et al. "Broadly tunable vertical-coupler filtered tensile-strained InGaAs/InGaAsP multiple quantum well laser", Appl. Phys. Lett., vol. 64, pp. 2764-2766, 1994; M. Oberg, S. Nilsson, K. Streubel et al. "74 nm wavelength tuning range of an InGaAsP/In vertical grating-assisted codirectional coupler laser with rear sampled grating reflector", IEEE Photonics Technol. Lett., vol. 5, pp. 735-737, 1993; H. Ishii, Y. Tohmori, Y. Yoshikuni et al. "Multiple-phase-shift super structure grating DBR lasers for broad wavelength tuning", IEEE Photonics Technol. Lett., vol. 5, pp. 613-615, 1993; M. Okai, I. F. Lealman, L. J. Rivers et al. "In-line Fabry-Perot optical waveguide filter with quasi-chirped gratings", Electron. Lett., vol. 32, pp. 108-109, 1996. When conventional index coupled DFB lasers are used in conjunction with multi-electrodes to achieve wider wavelength tuning range, current injection into two different electrodes of DFB lasers has to be well controlled and the current ratio among the electrodes needs to be precisely maintained. Due to the inherent Bragg mode degeneracy in conventional index coupled DFB lasers, each DFB section has its own stop band. The two Bragg modes around the related stop band compete equally for lasing as the dominant mode. Which one of the Bragg modes will tend to lase depends largely on the unknown grating facet phase and the injection currents in the two different sections. When two sections of the laser are biased independently, two pairs of Bragg modes, four in total, tend to compete against each other. Normally it results in a low SMSR or fast degradation of SMSR during the wavelength tuning within a certain current injection range. To overcome these problems, a complicated and tedious injection current control is normally required. As a result, a high yield device with a high SMSR, wide controlled tuning range and long term stable operation is difficult to obtain. Moreover, from device to device, the random facet phase usually causes a random distribution of lasing mode between the two pairs of degenerated Bragg modes, and the final single mode yield with a wide wavelength tuning range is low. Even if a preselected SMSR is initially high, it can degrade rapidly with a small injection current variation within a certain range due to the sensitivity of index coupled DFB lasers to initial phase variations.
Among other multi-sectional lasers, complex coupled lasers appear to be the best candidates for a wide range tuning. Complex coupled DFB lasers are preferably used as building blocks for tunable lasers to ensure a high SMSR and provide high single mode yield and a small degradation of SMSR over injection current variation. Nevertheless, the tuning range of these lasers is still limited which results in a continued demand for further increasing a tuning range of DFB lasers while maintaining other essential characteristics of the lasers, and development of effective methods of their operation.