| Abstract | Wavelength-division-multiplexed passive optical networks (WDM-PONs) are an attractive solution for future broadband access networks because of various advantages including large capacity, network security, protocol transparency, and upgradeability. However, the WDM-PON is still an expensive solution to implement mainly due to the high cost of WDM sources. To construct a cost-effective and multichannel WDM system, a low-cost multiwavelength laser (MWL) source is a promising candidate because it replaces multiple discrete laser arrays and reduces the management overhead. There have been many approaches to obtain MWLs such as based on rare-earth-doped fiber amplification, quantum-well (QW) semiconductor optical amplifiers and stimulated Raman scattering. However, those resulting MWLs have not performed well in term of channel number, intensity uniformity, size and stability mostly due to the use of homogeneous gain materials. The Fabry-Perot (F-P) based semiconductor laser as a MWL in WDM-PONs has been something of a ‘holy grail,’ but noisy longitudinal lasing modes in bulk or QW semiconductor FP lasers due to their strong longitudinal mode competitions made them a ‘no-go.’ In recent years, InAs/InP quantum-dot (QD) gain materials operating at the most important telecom C-band wavelength range from 1530 nm to 1566 nm offers many new possibilities for QD lasers in terms of broader spectral bandwidth, higher temperature stability, and lower power consumption than QW or bulk lasers. The suitability of QD lasers for multi-wavelength operation with better performance lies in the facts such as spectral hole-burning in broad inhomogeneous gain of QDs and spatial hole-burning in an F-P cavity. Its inhomogeneous broadening of gain spectrum stems from statistically distributed sizes and geometries of self-assembled QDs, and its 3-dB bandwidth of up to 150 nm could be easily achievable, which provides a base for uniform and stable multi-channel operation. Each of lasing modes selected by an F-P cavity extracts only electrons in QDs resonant with the wavelength of that mode, depletes electrons in these QDs with the corresponding dot sizes. Because QDs are spatially isolated and only interact via wetting layers, the supply of electrons that remain in the material surrounding QDs helps the realization of ultrafast gain recovery to suppress gain fluctuation. Consequently, each mode consumes population inversion of differently localized carriers. This fast-recovery ultra-wide inhomogeneous broadening, as well as traditional spatial hole-burning inside a standing-wave cavity, will principally support stable multi-wavelength operation with high channel number and high uniformity of channel intensities |
|---|
