Junjie Xu

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.

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.

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.

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.

A Dual-Grating InGaAsP/InP DFB Laser Integrated With an SOA for THz Generation

We report a dual-mode semiconductor laser that has two gratings with different periods below and above the active layer. A semiconductor optical amplifier (SOA), which is integrated with the dual-mode laser, plays an important role in balancing the optical power and reducing the linewidths of the emission modes. A stable two mode emission with the 13.92-nm spacing can be obtained over a wide range of distributed feedback and SOA injection currents. Compared with other types of dual-mode lasers, our device has the advantages of simple structure, compact size, and low fabrication cost.

Fully integrated multi-optoelectronic synthesizer for THz pumping source in wireless communications with rich backup redundancy and wide tuning range.

We report a monolithic photonic integrated circuit (PIC) for THz communication applications. The PIC generates up to 4 optical frequency lines which can be mixed in a separate device to generate THz radiation, and each of the optical lines can be modulated individually to encode data. Physically, the PIC comprises an array of wavelength tunable distributed feedback lasers each with its own electro-absorption modulator. The lasers are designed with a long cavity to operate with a narrow linewidth, typically <4 MHz. The light from the lasers is coupled via an multimode interference (MMI) coupler into a semiconductor optical amplifier (SOA). By appropriate selection and biasing of pairs of lasers, the optical beat signal can be tuned continuously over the range from 0.254 THz to 2.723 THz. The EAM of each channel enables signal leveling balanced between the lasers and realizing data encoding, currently at data rates up to 6.5 Gb/s. The PIC is fabricated using regrowth-free techniques, making it economic for volume applications, such for use in data centers. The PIC also has a degree of redundancy, making it suitable for applications, such as inter-satellite communications, where high reliability is mandatory.

Laser Arrays With 25-GHz Channel Spacing Fabricated by Combining SAG and REC Techniques

Multiwavelength laser arrays with 25-GHz channel spacing have been fabricated by upper separate confinement heterostructure (SCH) layer selective area growth (SAG) technique. The reconstruction equivalent chirp (REC) technique is used to introduce an equivalent phase shift into the grating structure, so that a high single longitude mode yield of lasers can be ensured. Because different emission wavelengths are realized through thickness modulation of the upper SCH layer by SAG, the sampling period in the REC technique is the same for all the lasers in an array, which may alleviate the effects of the facet phase on the emission wavelength. The fabricated laser arrays have less than 0.051 nm standard deviation of wavelength deviations. Only conventional photolithography process is needed, helping to lower the fabrication cost of the laser arrays.

Transistor Laser With a Current Confinement Aperture in the Emitter Ridge

We propose a transistor laser with a current confinement aperture (a-TL) formed in a reverse junction layer in the middle of emitter ridge. The a-TLs are studied numerically in comparison with the deep ridge TLs. Data show that in a-TLs, the effects of nonradiative recombination centers on the side walls of the emitter ridge can be reduced greatly due to the confinement of carrier flow in the center region of the emitter ridge. Furthermore, the reverse junction layer can be used as an etching stop layer for the emitter ridge formation, which facilitates both the design and the fabrication of the device. Together with the fact that in a-TLs, the effects of p-type material are alleviated effectively by placing the multi-quantum wells above the base layer, high performance can be expected, especially for long-wavelength devices.

Passively Mode-Locked Quantum-Well Laser With a Saturable Absorber Having Gradually Varied Bandgap

We report a novel passively mode-locked quantum-well laser, which have an asymmetric colliding pulse mode-locked structure. The saturable absorber (SA) multi-quantum wells (MQWs) of the laser have bandgap, which is gradually varied along the ridge direction realized by selective area growth technique, forming a broadband absorber. As a result, light pulses are obtained at a repetition frequency of 226 GHz with a minimum pulse width of 605 fs under an appropriate current bias. Because of the continuous variation of the SA MQW bandgap, the SA absorption peak covers a wide range of wavelength, which makes the device more immune to SA bandgap change resulted from SA reverse bias or light absorption-induced heating of the SA, compared with traditional MQW mode-locked lasers.