A novel design of two-mode (DE)MUX based on multimode interference (MMI) couplers on InP substrate is proposed. A phase shifting section based on the thickness variation of the core layer is introduced in the (DE)MUX to realize a 100% mode conversion efficiency and multiplexing. The total length of the structure is only 549 μm, much shorter than other InP based mode (DE)MUXs. Simulations show that, the device crosstalk is below -20 dB and the insertion loss is lower than 1 dB for both of the fundamental mode and the first order mode within the whole C band. This new structure can be potentially integrated with other devices based on InP substrate to serve as a monolithic few-mode transmitter/receiver.
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.
High power single-mode ridge waveguide 1060-nm semiconductor lasers are reported. The lasers consist of
compressively strained double InGaAs/GaAs quantum wells and a GaAs/AlGaAs separate confinement vertical structure.
A super large vertical optical cavity is employed to have a low internal loss, large optical spot size and low vertical
optical divergence angle. The material composition and thickness of waveguide layers and claddings layer are optimized
systematically. The active layer is detuned from center of the waveguide and thickness of cladding layers is optimized to
guaranty single mode lasing of the large optical cavity. The large vertical cavity laser structure with thickness of 4 μm
allows the lasers have a low internal loss of less than 0.6 /cm, a large optical spot size about 1μm and a vertical
divergence angle about 20 degree. For lateral optical confinement, a double trench ridge waveguide is employed to
maintain single-lateral-mode operation. Based on the optimization, 1.5 W continue wave optical power is achieved for
broad area lasers with 1mm longitude cavity length. Narrow stripe ridge waveguide lasers of 1mm cavity length with
single mode current and optical power of 700 mA and 340 mW is obtained. Suggestions for further improvements in
terms of single mode power and applications of the high power semiconductors are discussed.
A comprehensive design optimization of 1.55-μm high power InGaAsP/InP board area lasers is performed aiming at
increasing the internal quantum efficiency (IQE) while maintaing a low internal loss of the device as well. The P-doping
profile and separate confinement heterostructure (SCH) layer band gap are optimized respectively with commercial
software Crosslight. Analysis of lasers with different p-doping profiles shows that, although heavy doping in P-cladding
layer increases the internal loss of the device, it ensures a high IQE because higher energy barrier at the SCH/P-cladding
interface as a result of heavy doping helps reduce the carrier leakage from the waveguide to the InP-cladding layer. The
band gap of the SCH layer are also optimized for high slope efficiency. Smaller band gap helps reduce the vertical
carrier leakage from the waveguide to the P-cladding layer, but the corresponding higher carrier concentration in SCH
layer will cause some radiative recombination, thus influencing the IQE. And as the injection current increases, the
carrier concentration increases faster with smaller band gap, therefore, the output power saturates sooner. An optimized
band gap in SCH layer of approximately 1.127eV and heavy doping up to 1e18/cm3 at the SCH/P-cladding interface are
identified for our high power laser design, and we achieved a high IQE of 94% and internal loss of 2.99/cm for our design.
We have investigated 1.3-μm InGaAsP strained multi-quantum-well (MQW) lasers on InP substrate for direct
modulation applications using the commercial laser simulator PIC3D. The physical mechanisms affecting the laser
dynamic characteristics such as nonradiative recombination losses and vertical electron leakage effect are considered in
our simulation. The number of wells is optimized because increasing the number of QWs can decrease the nonradiative
recombination losses and increase the modal differential gain, nevertheless, the carrier distribution between wells
become more non-uniform with too many QWs numbers resulting in uneven simulated recombination rate and
increasing Auger recombination. The influence of barrier height is analyzed and a tradeoff has to be determined because
too high barriers results in more nonuniform carrier distribution in the active regions, increasing the Auger
recombination rate severely while the vertical current leakage outside the QWs will increase dramatically at lower barrier
height. The 1.3-μm FP laser with the MQWs of 6 wells, 1.15 Q barriers bandgap and 8 wells, 1.1 Q barrier bandgaps is
fabricated and characterized. The FP laser with MQWs structures composed of 8 compressive strain quantum wells and 9
barriers with the optimized bandgap 1.1 Q shows better properties. The threshold current is around 19 mA and the
resonance frequency of 9.5 GHz and 3-dB bandwidth in excess of 13.3 GHz at 120 mA injection current. This
modulation frequency is suitable for 10 Gbits/s optical data transmission.
We report systematic modelling of 1310 nm InGaAsP/InP electroabsorption modulators. The modulator is a reverse
biased p-i-n diode, in which the MQW structure is composed of several InGaAsP/InGaAsP quantum wells. By a 3D
finite element software PICS3D, we have comprehensively investigated the internal physical mechanism of the
modulator, which includes the red shift of the absorption edge with the reverse bias and the absorption intensity, which
could be derived from the normalized overlap integral between the energy levels for the electrons and the holes. The
absorption spectrum on wavelength and the reverse bias voltage is analyzed, which provide us with both the extinction
ratio and the transimision loss for a special operating wavelength. Key design parameters such as barrier height and
quantum well width are optimized for extinction ratio, and confirmed by parallel experimental studies. What’s more, the
RF performance has been investigated in detail. The junction capacitance, the series resistance and the parasitic
capacitance (mostly the bonding pad) are studied systematically. A ridge structure model is analyzed for high speed
performance, in which the important parameters, such as the ridge width, the cavity length, the area of the bonding pad
and the thickness of polyimide (or BCB) under the bonding pad, are optimized for over 20GHz 3dB bandwidth. The
cavity length is optimized by making compromise between the extinction ratio and the RF performance. In conclusion,
the design parameter space of the 1310nm InGaAsP/InP EAM have been systematically explored. Our work should
provide a firm basis for 1310nm InGaAsP/InP EAM device design optimization for optical datacom applications.
A high power single-lateral-mode double-trench ridge waveguide semiconductor laser is reported. The laser has a compressively strained double quantum-well (DQW) and an GaAs/AlGaAs separate confinement structure. The ridge waveguide is defined by two trenches of finite width on either side of the ridge, which will result mode radiation towards outside of the trenches. The relationship between the leakage loss and the waveguide geometry of the each lateral mode is studied with effective index method. The relationship under different bias condition is evaluated. Based on the simulation, lasers with various trench width, trench depth and ridge width are fabricated and tested. With optimized geometry parameters, a laser of 1.5-mm cavity length with a maximum single-lateral-mode operation current of 550 mA is obtained. The threshold current and the slope efficiency of the laser is 30 mA and 0.72 W/A, respectively. The maximum single-lateral-mode power is up to 340 mW.