Daibing Zhou

High-speed directly modulated widely tunable two-section InGaAlAs DBR lasers

We report widely tunable two-section distributed Bragg reflector (DBR) lasers, which have InGaAlAs multiple quantum wells (MQWs) as the gain material. By butt-jointing InGaAsP, which has a photoluminescence wavelength of 1.4 μm as the material of the DBR section, a wavelength tuning range of 12 nm can be obtained by current injection into the DBR section. The direct modulation bandwidth of the lasers is greater than 10 GHz over the entire wavelength tuning range up to 40°C. Compared with InGaAsP DBR lasers having the same structure, the InGaAlAs lasers have smaller variations in both the threshold current and slope efficiency with the temperature because of the better electron confinement in the InGaAlAs MQWs. Moreover, the DBR-current-induced decreases in the modulation bandwidth and side mode suppression ratio (SMSR) of the optical spectra are notably smaller for the InGaAlAs lasers than for the InGaAsP lasers.

Fabrication of 32 Gb/s Electroabsorption Modulated Distributed Feedback Lasers by Selective Area Growth Technology

A 32 Gb/s monolithically integrated electroabsorption modulated laser is fabricated by selective area growth technology. The threshold current of the device is below 13 mA. The output power exceeds 10 mW at 0 V bias when the injection current of the distributed feedback laser is 100 mA at 25°C. The side mode suppression ratio is over 50 dB. A 32Gb/s eye diagram is measured with a 3.5Vpp nonreturn-to-zero pseudorandom modulation signal at −2.3 V bias. A clearly opening eyediagram with a dynamic extinction ratio of 8.01 dB is obtained.

A monolithically integrated dual-mode laser for photonic microwave generation and all-optical clock recovery

We demonstrate a monolithically integrated dual-mode laser (DML) with narrow-beat-linewidth and wide-beat-tunability. Using a monolithic DFB laser subjected to amplified feedback, photonic microwave generation of up to 45 GHz is obtained with higher than 15 GHz beat frequency tunability. Thanks to the high phase correlation of the two modes and the narrow mode linewidth, a RF linewidth of lower than 50 kHz is measured. Simulations are also carried out to illustrate the dual-mode beat characteristic. Furthermore, using the DML, an all-optical clock recovery for 40 Gbaud NRZ-QPSK signals is demonstrated. Timing jitter of lower than 363 fs (integrated within a frequency range from 100 Hz to 1 GHz) is obtained.

A high-power tapered and cascaded active multimode interferometer semiconductor laser diode

A high power semiconductor laser diode with a tapered and cascaded active multimode interferometer (MMI) cavity was designed and demonstrated. An output power as high as 32 mW was obtained for the novel laser diode with a tapered and cascaded active MMI cavity, being much higher than the 9.8 mW output power of the conventional single ridge F-P laser with the same material structure and the same device length due to the larger active area; and also being higher than the 21.2 mW output power of the rectangular and cascaded active MMI laser diode with nearly the same structure, except for the shape of the MMI area. In addition, the tapered and cascaded active multimode interferometer laser showed stable single mode outputs up to the maximum output power.

Fabrication of InP-based monolithically integrated laser transmitters

InP-based photonic integrated circuits (PICs) have aroused great interest in recent years to meet the needs of future high-capacity and high-performance optical systems. With the advantages of small size, low power consumption, low cost, high reliability, InP-based PICs are promising solutions to replace the multiple discrete devices used in various systems. In this paper, we will review the design, fabrication, key integration technology and performance of several kinds of InP-based monolithically integrated transmitters developed in our group in recent years. Particular attention will be paid to the electro-absorption modulated laser (EML), multi-wavelength distributed feedback (DFB) laser arrays, widely tunable distributed reflector (DBR) lasers and their arrays, integrated amplified feedback lasers (AFL), and few-mode transmitters.

Widely Tunable Two-Section Directly Modulated DBR Lasers for TWDM-PON System

Wavelength tunable and directly modulated distributed Bragg reflector (DBR) lasers with butt-joint technology are designed, fabricated and characterized. The DBR laser consists of a gain section and a DBR section. To increase the electrical isolation between the gain section and the DBR section, parts of a p-doped material in the isolation region are etched off selectively. Over 2 kΩ isolation resistance is realized ultimately without the need of ion implantation, which simplifies the fabrication process. The laser exhibits high speed modulation with a large tunable range. The 3 dB direct modulation bandwidth of the device is over 8 GHz in a 12 nm tunable range. This widely tunable DBR laser with the simple structure is promising as a colorless light source for the next-generation time and wavelength division multiplexed passive optical network (TWDM-PON) systems.

Synchronized operation of a monolithically integrated AWG-based multichannel harmonically mode-locked laser

We report a novel AWG-based monolithically integrated multichannel mode-locked laser. Four wavelength channels output of synchronized nearly transform-limited pulse trains operating at the fifth harmonic repetition rate of 12.7 GHz was demonstrated under hybrid mode locking.

Dual-wavelength distributed feedback laser for photonic microwave generation

We present a compact and simple approach to realize dual-wavelength distributed feedback laser by introducing two distributed feedback (DFB) lasers, in series, with different ridge widths and uniform gratings. Each laser emits single longitudinal mode and an electro-absorption modulator (EAM) has been integrated within the laser cavity in order to stabilize the beating signal by nonlinear four-wave mixing and lock phase of the two optical signals. A stable 74.9GHz beating signal was obtained within a wide range of bias parameters in terms of drive currents to two DFB lasers. By down-conversion technique, the beating signal has been observed by electrical spectrum analyzer.

Modeling and Experiment Verification of Lateral Current Spreading Effect in Ridge Waveguide Electroabsorption Modulators

We investigate the lateral current spreading effect and its influences on the transmission characteristics of ridge waveguide electroabsorption modulators. An RF equivalent circuit based on a transmission line model explicitly accounting for this effect is proposed. The equivalent circuit parameters in our model extracted from S11 curve fitting are used to simulate the S21 response (electro-optical response (E/O) response), which matches well with the measured direct S21 small-signal modulation response. The physical significance of our model parameters can be well explained based on the secondary ion mass spectroscopy doping profiling and the simulation of the current spreading effect in our device structure.

The effect of zinc diffusion on extinction ratio of MQW electroabsorption modulator integrated with DFB laser

Monolithically integrated electroabsorption modulated lasers (EML) are widely being used in the optical fiber communication systems, due to their low chip, compact size and good compatible with the current communication systems. In this paper, we investigated the effect of Zinc diffusion on extinction ratio of electroabsorption modulator (EAM) integrated with distributed feedback laser (DFB). EML was fabricated by selective area growth (SAG) technology. The MQW structure of different quantum energy levels was grown on n-type InP buffer layer with 150nm thick SiO2 parallel stripes mask by selective area metal-organic chemical vapor deposition (MOCVD). A 35nm photoluminescence wavelength variation was observed between the laser area (λPL=1535nm) and modulator area (λPL=1500nm) by adjusting the dimension of parallel stripes. The grating (λ=1550nm) was fabricated in the selective area. The device was mesa ridge structure, which was constituted of the DFB laser, isolation gap and modulator. The length of every part is 300μm, 50μm, and 150μm respectively. Two samples were fabricated with the same structure and different p-type Zn-doped concentration, the extinction ratio of heavy Zn-doped device is 12.5dB at -6V. In contrast, the extinction ratio of light Zn-doped device is 20dB at -6V, that was improved for approximate 60%. The different Zn diffusion depth into the MQW absorption layer was observed by Secondary ion mass spectrometer (SIMS). The heavy Zn-doped device diffused into absorption layer deeper than the light Zn-doped device, which caused the large non-uniformity of the electric field in the MQW layer. So the extinction ratio characteristics can be improved by optimizing the Zn-doped concentration of p-type layer.