Research On Mode Partition Noise And Optimal Design Of FP Laser For Short-reach Applications

Optical fiber transmission technologies have been established in broadband communication networks for decades.The explosive growth of Internet traffic and multimedia applications lead to increasing demands for communication bandwidth.At the same time,the size,power and cost requirements of the optical links are also imposed.In telecommunications,the gigabit-capable passive optical network(GPON)has been one of the leading optical access technologies eliminating the bandwidth bottleneck in the last mile.On the other hand,data center networks today are also facing new challenges in supporting data-intensive applications.The Fibre Channel(FC)protocol for optical interconnections among computers standardized by the Fibre Channel Technical Committee is a solution based on an open technology that allows data to be transmitted at extremely high speed through optical fiber.In addition,taking advantage of high-speed,stable and good EMI performance of fiber communication,the active optical cable can realize the low-cost and high-speed data transmission without changing the traditional electrical interface.A high performance-to-cost ratio solution is therefore highly needed for the lasing source in cost-sensitive applications.Compared to the expensive distributed feedback laser,the simple low-cost multi-longitudinal mode FP laser is a more appropriate light source.However,the low-cost FP laser is usually limited to relatively low-speed applications due to its relatively large spectral width and intrinsic mode partition noise(MPN).To our best knowledge,there is no suitable low-cost solution to reduce MPN in the existing technologies.In this paper,aiming at reducing the MPN-induced power penalty,the characteristics of MPN have been studied thoroughly.The research achievements and contributions are summarized as the followings:(1)Four ways to characterize the MPN with their advantages and disadvantages are exhibited.The principles for MPN are discussed.Several methods used to reduce MPN are also summarized.The basic knowledge for MPN has been summarized to lay a theoretical foundation for the study of MPN.(2)The modified multi-mode quantum-well rate equation model and the broadband traveling wave model are proposed for the simulation of laser and its related MPN characteristics.A comprehensive numerical fiber link model including the laser model,the fiber model and the receiver model,is established to realize the simulation of the whole physical process of optical signal's emission,transmission and reception.In the numerical models used to characterize the MPN,we combine methods,such as the probability distribution method and the k-factor,with the fiber link model to provide numerical tools to study MPN.(3)Based on the numerical model,the influence of the nonlinear gain coefficient on the MPN in the quantum well multimode laser is studied.The results show that the MPN can be significantly suppressed by increasing the self-saturation gain.The asymmetric crosssaturation gain can change the spectral shape,but its influence on the MPN is weak.On the other hand,we have also investigated the variation of the MPN with the number of longitudinal modes within a given spectral width,as well as with the spectral width under a fixed number of longitudinal modes.(4)Based on the principle of MPN,we have proposed a narrow spectral width FP laser with built-in bandpass filter,which can significantly reduce the spectral width of the laser as well as the MPN-induced power penalty.The proposed device is a ridge waveguide FP laser with a widened ridge section at one end.The effective index of the widened ridge will be higher than that of the non-widened one.Hence,a new FP cavity is formed in the widened ridge section.Due to the bandpass characteristic of the new FP cavity's reflection spectrum,the output spectrum is compressed.The parameters of the device,such as the introduction of the tapering section,the length of the widened and non-widened ridge section,have been optimized,and the optimal parameters are obtained.The chip-level and system-level performance of the optimized proposed device are also simulated by the fiber link model.The results show that compared to conventional FP lasers,the proposed device has a narrower spectral width and a lower MPN power penalty.What's more,the maximum transmission distance of the proposed device is twice that of the conventional FP laser.(5)We also carried out experimental research on the fabrication process of the device.In the fabrication process,no extra burden is introduced to the conventional ridge waveguide FP laser fabrication process other than the replacement of the mask pattern in the standard photolithography process.Hence,the cost of the device is greatly reduced.(6)According to the optimal design,we have fabricated the 1310 nm In AlGaAs/InP multiple-quantum-well ridge waveguide FP laser with a built-in BPF.The influences of the widened ridge section and the tapering region on the characteristics of the propoed device have been studied.The adjustment method of the current in the two separated electrodes have been further examined.Finally,the proposed device is compared with the reference FP laser fabricated on the same wafer.The results show that the propoed device can effectively narrow the root-mean-square width of the optical spectrum approximately by half.Further more,the proposed laser exhibits noticeably better transmission performances over the conventional FP laser.The proposed compact and cost-effective FP laser design is promising to be applied in the short-reach applications such as PON and FC systems.