Objective The high-speed tunable optical transmitter is a key component of the wavelength division multiplexing(WDM)system.Due to its compactness,high modulation efficiency,and low power consumption,the electro-absorption modulator(EAM)based on the quantum-confined Stark effect is a preferred choice for high-speed transmitters.Since standard single-mode fibers(SMFs)feature zero dispersion at wavelengths close to 1.3 µm,O-band electro-absorption modulated lasers(EMLs)are more competitive than C-band EMLs for short-distance applications due to the absence of dispersion penalties.However,an O-band tunable laser integrated with an EAM has not been reported before.EMLs are generally considered costly due to the critical regrowth process required by the commonly adopted butt-joint integration scheme.Meanwhile,common tunable lasers are mainly based on complex grating structures.Such a structure requires high-resolution lithography and epitaxial regrowth,making the EMLs even more expensive.Therefore,costs have become an important obstacle to their deployment in cost-sensitive applications such as 5G and access networks.To overcome such shortcomings,we present a regrowth-free electro-absorption modulated widely tunable V-cavity laser(VCL)using an identical epitaxial layer(IEL)integration scheme.Similar to that of Fabry-Perot(FP)lasers,the fabrication process of this laser requires no grating or epitaxial regrowth.The IEL integration scheme,which employs the same multiple quantum well(MQW)structure for both the laser and EAM sections,can be used to simplify device fabrication significantly.Moreover,to achieve a larger modulation bandwidth without increasing the complexity of the process,we optimized the electrode design of the EAM to reduce the parasitic capacitance.In this paper,we report a widely tunable 25 Gb/s transmitter that integrates a VCL and an EAM in the O-band and demonstrate its dispersion-penalty-free transmission over 25 km standard SMFs.Methods The device consists of a VCL and an EAM,which are connected via a deep-etched trench.Shallow-etched ridge waveguides with a width of 2 µm are applied for both the VCL and EAM.The laser includes two different-length FP resonators coupled by a reflective half-wave coupler.This coupler is designed to induce a π phase difference between the bar-coupling coefficient and the cross-coupling coefficient at the operational wavelengths,thereby ensuring a high SMSR.The length of the short cavity is designed to be 235 µm,and the corresponding resonant frequency spacing is 200 GHz.The long cavity is 5%longer so that the Vernier effect can be used to extend the tuning range and achieve a large free spectral range(FSR)of about 20 channels,approximately 22 nm in the O-band.The deep trench acts as a partially reflective mirror for the VCL,providing sound electrical isolation for the EAM.The trench is designed with a width of 1 µm to achieve high reflectivity,which decreases the threshold current of the VCL.The EAM waveguide,with a length of 80 µm,is designed with an 8° tilt angle to minimize end-face reflection.By employing deep etching(etching to the semi-insulating substrate layer),the doped Indium Phosphide(InP)beneath the original EAM pad is eliminated,resulting in the EAM pad being slightly lower than the n-InP cladding layer.The depth of the deep etching is about 4.2 µm,and the thicknesses of the metal and SiO2 are about 0.6 and 0.5 µm,respectively.Consequently,the EAM pad does not form a conventional parallel-plate capacitor with the ground plane,effectively reducing parasitic capacitance.The deep etching of the pad section can be fabricated simultaneously with the deep-etched trench of the laser without any additional fabrication processes.Results and Discussions The coupler electrode was injected with a continuous wave current of around 50 mA,while the bias voltage of the EAM was set to 0 V.The current of the short cavity electrode and the long cavity electrode changes from 20 to 50 mA.The temperature,controlled by the thermal electrical cooler(TEC),varies from 40 to 60℃during tuning.We obtained the superimposed lasing spectra of 20-channel 100-GHz-spaced wavelengths for the transmitter(Fig.2).The channel wavelengths spanned from 1305.72 to 1316.61 nm,aligning with the ITU-T grids and covering a range of 11 nm.The fiber coupled output power ranges from-1 to 1 dBm with the SMSR over 37 dB.The normalized optical transmissions of the EAM at four different channels were measured as a function of the reverse-bias voltage(Fig.3).The optical transmission curves only exhibit minor differences across various wavelength channels,with the extinction ratio ranging between 9 and 10 dB under a bias voltage of-2.5 V.For the dynamic characteristics of the tunable EML,we measured the small signal response and compared it with that obtained from a conventional device(Fig.4).The 3 dB bandwidth of the integrated EAM increased from 13.0 to 17.5 GHz,indicating that this improved structure effectively reduces the parasitic capacitance of the pad.Finally,we evaluated the large-signal transmission characteristics and obtained back-to-back(BtB)eye diagrams from channels 1,7,13,and 20,with a dynamic extinction ratio exceeding 5 dB(Fig.5).The bit error rate(BER)curves for the four channels were also measured for both BtB and 25-km fiber transmission scenarios(Fig.6).Utilizing a BER threshold of 5×10-5,typical for 25 Gb/s transmission with forward error correction(FEC),the received power sensitivity for all measured channels in BtB case ranges from-21.5 to-22 dBm.The BER performance after 25-km fiber transmission is slightly better than the BER performance of BtB.Conclusions We have developed an O-Band tunable transmitter based on a V-cavity laser monolithically integrated with a 25 Gb/s EA-modulator using the IEL integration scheme.The transmitter shows a wavelength tuning of 20 channels from 1305.72 to 1316.61 nm with 100 GHz spacing.By applying such an innovative electrode design,we have successfully expanded the 3 dB bandwidth of the electrical-optical response to 17.5 GHz without additional processes.All channels exhibit distinct eye diagram openings at a rate of 25 Gb/s with a dynamic extinction ratio surpassing of 5 dB at a consistent peak-to-peak driving voltage of 2 V.Penalty-free transmission over 25 km standard SMF at 25 Gb/s is demonstrated for all channels.The results show that the O-band tunable transmitter is promising for next-generation high-capacity WDM optical communication systems.
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