The bandwidth of a distributed feedback (DFB) directly modulated laser (DML) is limited by its carrier–photon resonance (CPR) frequency. A viable approach to break the bottleneck is to introduce a photon–photon resonance (PPR), since the PPR can happen at a much higher frequency than the CPR. Among the many structures that can possibly generate the PPR, the dual-sectional push–pull modulated (PPM) DFB is of particular interest for its fabrication cost-effectiveness as no regrowth is required. The PPR in the PPM DFB, however, usually shows a rapid roll-off on both edges, which brings in an indentation on the lower frequency side of the PPR peak and, consequently, cuts off the bandwidth. To compensate for this dip, we introduce a detuned PPR and restart the CPR response by exploiting a time delay between the differential signals applied to the PPM DFB. Our simulation result shows that the broadened PPR peak and the restarted CPR response indeed mitigate the dip and effectively expand the PPM-DFB’s bandwidth to approximately 50 GHz, a value double that of the conventional (single-sectional) DFB DML.

Conventional push-pull modulated (PPM) distributed feedback (DFB) lasers exploit structures with symmetry, which wastes a significant amount of the optical power output from the rear facet. To improve the power efficiency, PPM DFB lasers with asymmetrically coated facets can be considered. However, the resulted imbalanced push-pull modulation causes a dip in device small-signal intensity modulation response near the carrier-photon resonance (CPR) frequency, which distorts the waveform and even cuts off the bandwidth. This work proposes an asymmetric section length design in conjunction with the asymmetric facet coating to offset the aforementioned effect on device intensity modulation response. With a delayed push-pull modulation (DPPM) scheme further incorporated, our simulation result shows that the PPM DFB laser with optimized asymmetric structure can enhance the power efficiency and maintain a smooth modulation bandwidth up to around 50 GHz.

Disordered waveguides mediated by transverse Anderson localization (TAL) can be used as mode division multiplexing system. The whole cross-section of the disordered waveguides is covered by all localized modes, which function as independent space channels. The localized modes are independent of the boundary and external excitation. The numerical results reveal that the average width tends to be steady in one particular realization of the disordered waveguide with a sufficient number of localized modes. In this paper, we proposed that localization length can be measured by the average width of localized modes. Besides, we explore the impact of the disordered waveguide design parameters, such as refractive index contrast, fill-fraction, and feature size, on the average width of localized modes.