Coupled Cavity Semiconductor Laser Based On Deep-etched Trenches

Since the invention of semiconductor laser and optical fiber in 1960s,optical fiber communication has made tremendous development.Especially in recent few decades,fast-growing internet traffic and booming development of data center has resulted in the great demand for high speed optical transmission systems.As the key component in optical communication system,low-cost single mode tunable laser has become a focus for reaserch institutes and companies.Several technologies havebeen developed for realizing single mode laser.Distributed feedback laser which is cost-effective and can realize high side mode suppression ratio,is the mainstream technology in optical communication,however,the tuning range is limited with only near 10 nm because of the thermal tuning,for larger tuning range,DFB array with optical combiner is always needed.For wideband wavelength tuning application,several device categories,such as SGDBR laser,DS-DBR laser or MGY laser are widely used.All these lasers involve grating-based mode selection and optical integration,which needs multiple epitaxial regrowth and high cost.Microring based tunable laser is another research hotspot,the transmission spectrum can be much flatter,but it also has problems such as high precision fabrication and thermal sensitive spectrum shift.Recent years,several single mode laser based on novel mode selection mechanism are proposed,such as half-wave coupled V-cavity laser,trench coupled slotted FP laser,dark-state laser and parity-time symmetric laser.Widely tunable half-wave coupled V-cavity laser has been proposed since 2008,which has high side mode selection and large tunability by introducing half-wave coupler for mode selection and Vernier effect for tuning range expansion.It has mulpiple advantages over grating based tunable laser,simple fabrication process without expitaxial regrowth,compact and low-cost.Several V-cavity laser based modules or integration devices have also been developed.Traditional frequency domain numerical analysis based on transfer matrix method and threshold gain equation is mature,however,it can hardly reflect the operation mechanism for mode lasing.This paper analyzed two basic kinds of couplers with physical importance,quarter-wave coupler and half-wave coupler,by establishing a theory model based on time domain coupled mode theory.According to transfer matrix method analysis,quarter-wave coupled laser shows dual wavelength emission,while half-wave coupled laser shows single mode emission properties.By employing tCMT theory,we give a complete analytic solution of mode lasing properties for quarter-wave coupled and half-wave coupled laser.Besides,analytical optimum solution for coupled caivity laser with arbitrary coupling phase was also given,giving a full analytical solution which is coincide with the established half-wave coupler theory.At last,the relationship between half-wave coupled laser and "dark state" laser or Parity-Time(PT)symmetric laser has also been deduced by tCMT method,showing that they are equivalent in optical field distribution.Based on the previous deduced time-domain half-wave coupled mode theory,we proposed a novel three-section coupled-cavity laser using lossy half-wave trenches.For simplification,we first analyze a basic two-section coupled-cavity laser using multi-mode rate equations considering spatial "hole burning"(SHB)induced mode competition.The influence of coupling phase and loss for single mode emission has been analyzed in detail,which shows a large coupling phase tolerance for single mode emission in coupled cavity laser.Based on the two-section coupled-cavity laser analysis,three-section coupled-cavity laser are proposed,which coupling loss are introduced for mode selection,and the injection gain or facet HR film coating of the filter section is to increase the mode selectivity.Compared to quarter-wave coupled laser,half-wave coupled laser can provide much higher mode selectivity.By measuring the wavelength tuning property for each gain or filter section,we give a specific tuning mechanism for the lossy coupled cavity laser,which shows application in DWDM communication system.Finally,electrical tuning of 12 channels covering as large as 84 nm tuning range with side-mode suppression ratio up more than 45 dB has been achieved.The large wavelength tuning range is a result of large thermal induced gain spectrum shift,which has been verified by calibration experiment.Mass of lasers with different trench lengths are also tested,showing large trench length tolerance.Dynamic property measurement shows a linewidth distribution ranging from 3 MHz to 13 MHz,and resonant frequency of larger than 8 GHz in small signal response spectrum and RIN spectrum is achieved.The laser can be directly modulated up to 5 GHz,with eye clearly opened when transmitted up to 25 km.Positive and negative chirping is observed under different bias conditions during the error rate measurement,and the theoretical explaination is given finally.At last,we also present the device fabrication and experimental results of a dual wavelength quarter-wave coupled semiconductor laser for photonic millimeter-wave generation.The threshold current of the laser is about 35 mA.Dual-mode lasing with wavelength difference around 0.35 nm was realized with SMSR better than 28 dB.By tuning the bias currents of the laser sections,the wavelength detuning of the doublet mode can be continuously tuned from 0.3 nm to 0.42 nm with SMSR better than 25 dB,corresponding to a frequency difference between 37.5 GHz and 52.5 GHz.