Liangshun Han

Two-mode de/multiplexer based on multimode interference couplers with a tilted joint as phase shifter

A novel design of a two-mode de/multiplexer (DE/MUX) based on multimode interference (MMI) couplers is presented. Instead of the phase shifter (PS) in the shape of a narrow strip waveguide, which needs tight design and fabrication requirements, a tilted joint is used as a PS in the proposed device, so that the effects of the fabrication errors of the PS on the performance of the device can be reduced greatly. Simulations show that while the size of the device is as small as 39.54 μm, which is more compact than other MMI-based DE/MUX, the fabrication tolerance is larger than ±25  nm. Within the entire C-band wavelength range, the de-multiplexing crosstalk of the device is lower than -28  dB and the insertion loss is below 1.0 dB.

Multichannel DFB Laser Arrays Fabricated by Upper SCH Layer SAG Technique

Multiwavelength laser arrays (MWLAs) fabricated by a novel upper separate confinement heterostructure (SCH) layer selective area growth (SAG) technique are studied in comparison with laser arrays fabricated by the conventional multiquantum well (MQW) SAG technique. For MWLAs by the MQW SAG, the range of the laser emission of the array is smaller than the span of the PL wavelengths of the MQW active layers. In addition, the material quality of the MQW layer degrades greatly as the width of the SAG strips is increased. Thus, the MWLAs fabricated by the method have scattered L-I characters and large fluctuation of wavelength spacings. In contrast, the MWLAs fabricated by the novel SAG method have much better wavelength spacing uniformity than those by MQW SAG, which is comparable to or better than the MWLAs fabricated by E-beam direct writing. Furthermore, the upper SCH layer SAG technique is suitable for the fabrication of MWLAs with wavelength interval, which is a challenge even for E-beam technique. In the SCH layer SAG, the MQW layer is untouched, and the same is true for all the laser elements in an array. The metal organic vapor phase epitaxies ability to control the material properties with high accuracy can be fully utilized for the fabrication of MWLAs. Combined with partially gain-coupled distributed feedback structures, the technique is promising for the fabrication of low-cost, high-quality MWLAs.

A High Extinction Ratio Polarization Beam Splitter With MMI Couplers on InP Substrate

A high extinction ratio (ER) polarization beam splitter with three different multimode interference couplers on InP substrate was designed by a three-dimensional beam propagation method. Simulations show that the polarization ER (PER) is over 34 dB and internal loss is <;0.8 dB for both TE and TM polarizations. The PER imbalance between the two polarization states has been reduced to 22 dB. The total length of the device is <;950 μm. The effects of the width of the input port on the performance of the device are discussed in detail.

Simultaneous Wavelength- and Mode-Division (De)multiplexing for High-Capacity On-Chip Data Transmission Link

We present designs of wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) optical links using mode de/multiplexers (DE/MUXs) based on multimode interference (MMI) couplers with a tilt joint as a phase shifter. The properties of WDM-MDM links with three wavelengths and two optical modes are numerically studied by using 3-D beam propagation method. In the first design, the wavelength combiner is also an MMI coupler type. The insertion loss of the design is around 6 dB because of the inherent loss of the MMI combiner. The size of the main passive optical parts (mode DE/MUXs and wavelength combiner) of the design is only 10.4 × 114.2 μm2, making the design promising for future compact and high-capacity optical interconnection applications. In another design, arrayed waveguide gratings are used for wavelength multiplexing, leading to a lower insertion loss of the optical connections. For both designs, the mode crosstalk between the two different modes for the same wavelength are below -22 dB.

Fabrication and characterization of deep ridge InGaAsP/InP light emitting transistors.

Deep Ridge InGaAsP/InP Light Emitting Transistors (LET) with ~1.5 μm light emissions have been fabricated and characterized. In the deep ridge LETs, all the light emissions are from the intrinsic base area, which makes them more suitable for high speed direct modulation. A collector emitter voltage (V CE) dependent output power, which has been predicted numerically, is observed experimentally for the first time and may facilitate the use of LETs in optoelectronic integrations. A novel trend of self-heating related saturation of light power with base current is also observed, which is explained by the three port operation of the device. Further, an abnormal common-emitter current-voltage (I-V) characteristic of the deep ridge LETs is shown and is attributed to the non-radiative recombination centers at the ridge side walls. With the good quality of the quantum wells, laser operation at near room temperature is achieved in the deep ridge LET with 800 μm cavity length. With proper surface passivation techniques and device optimizations, performance of the deep ridge transistor based optoelectronic devices can be further enhanced greatly and ultra low power consumption which is highly desirable can be expected.

Monolithically Integrated 4-Channel-Selectable Light Sources Fabricated by the SAG Technology

The monolithic integration of four 1.5-μm InGaAsP/InP distributed feedback (DFB) lasers with a 4 × 1 multimode-interference (MMI) optical combiner using the selective area growth and butt-joint regrowth technologies is proposed and demonstrated. A dry-etching stop layer is grown to guarantee the etching depth of MMI. The four channels could match the ITU wavelength grid of 200-GHz well with a simple integrated thin-film heater, covering the wavelength range of 1552.3-1556.3 nm. The average output power of LD is 2.8 mW with a 150-mA current, and the threshold current is 20-22 mA at 25°C. The four channels can operate separately or simultaneously.

DBR Laser With Over 20-nm Wavelength Tuning Range

We report a widely tunable distributed Bragg reflector (DBR) laser, in which InGaAsP with 1.4-μm photoluminescence wavelength is butt-jointed as the DBR section material. A titanium (Ti) thin-film heater is integrated in the DBR section of the device. With the help of the tuning effect of the heater, an over 20-nm wavelength tuning range is obtained. The DBR laser is a promising light source for future wavelength division multiplexing passive optical networks.

Fabrication of Low-Cost Multiwavelength Laser Arrays for OLTs in WDM-PONs by Combining the SAG and BIG Techniques

We present the fabrication of a low-cost multiwavelength laser array monolithically integrated with a passive optical combiner for optical line terminals (OLTs) in wavelength-division multiplexing (WDM) passive optical networks (PONs). By combining the upper separate confinement heterostructure layer selective area growth technique and the bundle integrated guide technique, both multiwavelength emission with highly uniform spacing and low-loss passive-waveguide material can be obtained in a single metal-organic chemical vapor deposition (MOCVD) growth step, which greatly simplifies the fabrication of the device. A prototype laser array, which has four distributed feedback (DFB) laser elements and a multimode interference coupler as combiner, is successfully fabricated. The shallow ridge structure of the DFB lasers and the deep ridge structure of the passive waveguides are obtained by a single dry etching step, which further eases the device fabrication. The properties of the device are measured and discussed. The results indicate that our method is promising for fabricating cost-effective OLT light sources for WDM-PONs.

Continuous-wave operation up to 20°C of deep ridge npn-InGaAsP/InP multiple quantum well transistor laser emitting at 1.5-μm wavelength

We report continuous-wave (CW) operation up to 20 °C of 1.5-μm wavelength npn-InGaAsP/InP multiple quantum well (MQW) transistor laser (TL) with a deep-ridge structure. With CW laser emission, the common emitter current gain of the device can be over 3.5, which is significantly larger than those of the previously reported long wavelength TLs. It is found that at low base current, the laser operation occurs on the first excited state of the MQWs. At high base current, however, the device shows stimulated emissions on the ground state transition. The trend is contrary to what has been observed in the GaAs based TLs and is explained by the change of carrier flow at different base currents.

DWDM source based on monolithic side-wall sample grating DFB laser array

We report side-wall sampled grating and quantum well intermixing techniques to fabricate a full function 4-channel DWDM source with excellent wavelength precision (residuals <;0.13 nm) and high yield. Output power was >10 mW.