The ability to control the polarization state with high speed is of substantial interest in optical communication systems such as those using high-speed complex digital signal processing (DSP) to manipulate the TE-TM polarization state of light or Stokes vector (SV) modulation and direct detection (SVM/DD) systems. For these applications, there is a growing interest in integrating a polarization controller with the light source, the detectors, and other components in a photonic integrated circuit (PIC). As an increasing number of devices such as laser diodes (LDs) and electro-absorption modulators utilize multiple-quantum-well (MQW) structures as the active region, it is desirable to design a polarization controller compatible with MQW structures. Furthermore, high-speed polarization imaging is a compelling application for material characterization(especially for biomedical studies and clinical applications) and dynamic observation of living cells and biological processes.
Polarization mode converters (PMC) using asymmetric waveguides and exploring various material platforms have become a hot research topic in the field of polarization control in recent years. PMCs based on bulk materials such as InGaAsP, silicon-on-insulator (SOI), etc. have achieved nearly 100% TE-to-TM polarization conversion efficiency (PCE). However, integrating bulk materials with quantum well devices often requires a complex butt-joint technique. PMC based on Fabry-Perot laser epitaxial structures have been designed using the RIE-lag effect, which only achieves a maximum PCE of 80%.
Recently single wavelength distributed feedback (DFB) lasers based on AlGaInAs multiple quantum well materials have found widespread application in very-short-reach (VSR) systems, passive optical networks (PONs), wavelength-division multiplexing (WDM), AlGaInAs laser have the advantages of a wide emission wavelength range, strong high-temperature performance and high output power.
PIC research group led by Prof. Lianping Hou from University reported an AlGaInAs multiple-quantum-well photonic integrated circuit device which can control the state of polarization of the output light source, consisting of a PMC, differential phase shifter (DPS) and a side wall grating distributed-feedback (DFB) laser. The relevant research results were published in Photonics Research, Volume 11, No. 4, 2023 (Xiao Sun, Song Liang, Weiqing Cheng, Shengwei Ye, Yiming Sun, Yongguang Huang, Ruikang Zhang, Jichuan Xiong, Xuefeng Liu, John H. Marsh, Lianping Hou. Regrowth-free AlGaInAs MQW polarization controller integrated with a sidewall grating DFB laser[J]. Photonics Research, 2023, 11(4): 622).
Two kinds of polarization controllers – DFB-PMC and DFB-PMC-DPS are shown in Fig. 1(a), the DFB-PMC device achieved the conversion of the TE mode light to TM mode, and the DFB-PMC-DPS device monolithically integrated a DPS behind the PMC to achieve phase modulation by applying a bias voltage. SEM images of the DFB grating, PMC, and DFB-PMC device are presented in Fig.1(b), For the DFB-PMC device with a designed Bragg wavelength at 1550 nm, the average S1 parameter was –0.968 representing a PCE of 98.4% for 140 mA < IDFB < 190 mA, as shown in Fig.1(c), For the DFB-PMC-DPS device, IDFB was fixed at 170 mA and VDPS was gradually changed from 0 V to −3 V. It is found that the SV rotates along the S2-S3 plane, and the measured rotation angle Δθ as a function of VDPS is presented in Fig.1(d). A phase shift of nearly 60° is seen as VDPS is changed from 0.0 V to –3.0 V in steps of –0.2 V.
Fig.1 (a) Schematic of the monolithic DFB-PMC device (left), and DFB-PMC-DPS device (right). (b) SEM image of DFB laser with sidewall gratings, PMC, and DFB-PMC device. (c) measured SV at PMC side of DFB-PMC device as a function of IDFB. (c) measured SV at DPSC side of DFB-PMC-DPS device as a function of VDPS.
It is worth mentioning that the realization of the monolithic integration of the PMC with the DFB laser and DPS based on the identical epitaxial layer (IEL) PIC scheme, is at the heart of high-capacity data and high-speed optical communication systems. Our optimized structure reduces the inherent birefringence of the MQW, which increases the PCE to 98.4%. A major advantage of the design is that only a single MOVPE step and two dry-etch steps are required to fabricate the device, significantly reducing complexity and cost.
Furthermore, the research group will reduce the exciton absorption close to the photoluminescence (PL) wavelength to improve the output power using quantum well intermixing (QWI) techniques.
