Objective A tunable laser is one of the crucial technologies in optical networks due to its advantage of flexibly switching output wavelengths.After years of research,tunable semiconductor lasers have developed various solutions.Super-structure grating distributed Bragg reflector(SSGDBR)tunable lasers,modulated grating Y-branch distributed Bragg reflector(MGY-DBR)lasers,and chirped sample grating distributed reflector(CSGDR)tunable lasers can achieve wide-range tuning.External cavity lasers(ECL),distributed feedback(DFB)lasers,and vertical cavity surface emitting lasers(VCSELs)are typical structures used for tunable lasers.The cavity length of ECL can be very long,resulting in a very narrow linewidth.However,its large volume makes it difficult to integrate with other devices on a single chip.The tuning range of a single DFB laser is only 3-5 nm,which cannot meet the requirements for wide-range tuning.To achieve wide-range tuning,multiple DFB lasers with different center wavelengths can be integrated into a DFB array.Although the manufacturing process of DFB lasers is mature,ensuring the integration of multiple lasers on the same chip remains challenging.VCSELs have a short cavity length and high reflectivity,resulting in a large longitudinal mode spacing and good single longitudinal mode performance.However,they exhibit multiple transverse modes and a larger linewidth,which makes integration with other devices challenging.To address the limitations of the aforementioned lasers,we design and propose a monolithic integrated tunable laser-multi-channel interference(MCI)laser.This laser achieves wavelength tuning based on the thermal optical effect by selecting modes through arm interference of different lengths.Unlike the lasers mentioned earlier,the MCI laser does not require a grating for mode selection.Additionally,each arm can independently change the phase and control the laser wavelength,significantly simplifying the laser production process and increasing production tolerance.Through multiple rounds of design and iteration,our study has successfully verified the feasibility of the scheme and achieved transmitter optical subassembly(TOSA)packaging and miniaturized integrated components for this laser.Methods MCI laser includes an active section for optical gain,a semiconductor optical amplifier(SOA)section for amplifying output optical power,and a multi-channel interference region for mode selection.The multi-channel interference section is composed of a common phase section for tuning longitudinal modes and a 1×8 multi-mode interferometer(MMI)for beam splitting,with 8 arms of different lengths.Each arm end is equipped with a multimode interference reflector(MIR)that reflects the total light to the active section,and a metal thermal electrode is positioned above each arm.The 8 arms independently adjust the phase of the light field based on the thermal optical effect,enabling interference to obtain a reflection spectrum dominated by a single reflection peak.Adjusting the phase interference of the 8 arms enhances coarse wavelength adjustment while adjusting the longitudinal mode of the common phase zone facilitates fine wavelength adjustment.The shape of the entire reflection spectrum can be optimized by adjusting the differences in arm lengths.Optimizing these differences is crucial for achieving better single-mode performance of MCI lasers.During the optimization of arm length differences,it is essential to consider suppressing adjacent longitudinal modes and other modes far from the main reflection peak simultaneously.We propose a method using particle swarm optimization(PSO).The PSO algorithm is combined with the multi-mode rate equation to optimize the laser arm lengths and obtain the optimal arm length design method,thereby achieving a better reflection spectrum across the entire wavelength band.The signal-to-mode suppression ratio(SMSR)of the wavelength at 1515 nm,at the edge of the gain spectrum,serves as the optimization index for the PSO algorithm,ensuring it exceeds 45 dB across the entire band and controls the half-width of the main peak to suppress adjacent longitudinal modes.After iterative analysis,a set of eight arm lengths is determined:140.00,268.28,384.64,521.80,526.80,674.00,681.25,and 750.41 μm.Results and Discussions The MCI laser achieves a tuning range of 48 nm covering the C++band(1524-1572 nm).It boasts an SMSR exceeding 46 dB and a fiber-coupled output power exceeding 16.5 dBm.Both SMSR and output power depend on the gain of the active region.The peak of the gain spectrum maximizes gain,minimizes threshold current,maximizes SMSR,and optimizes output power.Moving away from this peak increases threshold current,reducing SMSR and output power accordingly.The gain spectrum steepens towards shorter wavelengths,accelerating reductions in SMSR and output power in that direction.Frequency deviation from the international telecommunication union(ITU)standard remains within±0.5 GHz across all channels and is wavelength-independent.Further optimization can be achieved by adjusting the TEC1 operating temperature.Wavelength drift is less than±1 pm,demonstrating excellent temperature control and wavelength locking performance.Output optical power for 120 wavelengths remains fixed within different values(12-16 dBm),and the power deviation is maintained within±0.1 dB.The reverse bias extinction ratio(ER)in the SOA region exceeds 40 dB,and all wavelength channels achieve a narrow linewidth of less than 150 kHz.The ITLA's total power consumption is less than 3 W under 75 ℃ environment.Conclusions A new type of monolithic wide-range tunable semiconductor laser—MCI laser,is proposed.This device utilizes thermal-optical effects and 8-arm interference enhancement to adjust reflection peaks and longitudinal modes,achieving wavelength tuning akin to a distributed Bragg reflector(DBR).With the absence of grating structures,it offers advantages in fabrication simplicity and tolerance.The integration of offset quantum wells technology for active and passive components reduces regrowth complexity.The MCI laser features a wavelength tuning range exceeding 48 nm,SMSRs greater than 46 dB,Lorentzian linewidths less than 150 kHz,and consumes less than 50 mW for total thermal tuning power.The MCI laser chip,thermoelectric cooler(TEC),and wavelength locking device are packaged into a TOSA box,achieving an integrated tunable laser assembly(Nano-ITLA)measuring only 25.0 mm× 15.6 mm× 6.5 mm.Fiber-coupled output powers exceed 16.5 dBm,covering 120 ITU channels with frequency deviations less than±0.5 GHz in the C++band.Under wavelength locking and power balancing control,wavelength drifts are less than±1 pm,power jitters are less than±0.1 dB,and the reverse-biased SOA section exhibits extinction ratios greater than 40 dB.The MCI wide-range tunable laser demonstrates excellent potential for applications in coherent optical communication.
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