Indirect stress and air-cavity displacement measurement of MEMS tunable VCSELs via micro-Raman and micro-photoluminescence spectroscopy

We employ micro-Raman spectroscopy to optically infer the stress experienced by the legs of a bridge-type microelectromechanical systems (MEMS) used in high contrast gratings tunable vertical cavity surface emitting lasers (VCSELs). We then employ micro-photoluminescence (PL) spectroscopy to indirectly measure the air cavity displacement of the same MEMS structure. Results from micro-Raman show that electrostatically actuating the MEMS with a DC bias configuration yields increasing residual stress on the endpoints of the MEMS with values reaching up to 0.8 GPa. We simulated a finite element model via Comsol Multiphysics which agrees with the trend we observe based on our micro-Raman data. Our micro-photoluminescence (PL) spectroscopy showed that change in the air cavity of the VCSEL structure results in a change in the full width of the PL peak emitted by the layer consisting of 4 pairs of Distributed Bragg Reflectors (DBRs). The change in the full width of the PL peak was due to the change in the optical cavity induced by displacing the MEMS via externally applied bias and agrees with our transfer matrix convolution simulation. These optical characterization tools can be used for failure analysis, MEMS design improvements, and monitoring of MEMS tunable VCSEL devices for mass production and manufacturing.

A two-way 224-Gbit/s PAM4-based fibre-FSO converged system

A two-way 224-Gbit/s four-level pulse amplitude modulation (PAM4)-based fibre-free-space optical (FSO) converged system through a 25-km single-mode fibre (SMF) transport with 500-m free-space transmission is successfully constructed, which adopts injection-locked vertical-cavity surface-emitting lasers with polarisation-multiplexing mechanism for a demonstration. Compared with one-way transmission, two-way transmission is an attractive architecture for fibre-FSO converged system. Two-way transmission over SMF transport with free-space transmission not only reduces the required number of fibres and the setups of free-space transmission, but also provides the advantage of capacity doubling. Incorporating dual-wavelength four-level pulse amplitude modulation (PAM4) modulation with polarisation-multiplexing mechanism, the transmission capacity of fibre-FSO converged system is significantly enhanced to 224 Gbit/s (56 Gbit/s PAM4/wavelength × 2-wavelength × 2-polarisation) for downlink/uplink transmission. Bit error rate and PAM4 eye diagrams (downstream/upstream) perform well over 25-km SMF transport with 500-m free-space transmission. This proposed two-way fibre-FSO converged system is a prominent one not only because of its development in the integration of fibre backbone with optical wireless extension, but also because of its advantage in two-way transmission for affording high downlink/uplink data rate with good transmission performance.

LDLA14: a 14 Gbps optical transceiver ASIC in 55 nm for NICA multi purpose detector project

This paper presents the design and test results of a 14 Gbps optical transceiver ASIC (LDLA14) fabricated in a 55 nm CMOS technology for NICA Multi Purpose Detector (MPD) project. The LDLA14 is a single-channel bidirectional (1Tx + 1Rx) optical transceiver ASIC, including a Laser Driver (LD) module and a Limiting Amplifier (LA) module. It would drive the Vertical Cavity Surface Emitting Laser (VCSEL) of Transmitter Optical Sub-Assembly (TOSA) and receive signals from Photo Diode (PD) of Receiver Optical Sub-Assembly (ROSA), respectively. In the LDLA14, a novel structure of capacitive coupling pre-emphasis is proposed in the output driver of LD to obtain peaking effect without sacrifice the modulation current swing. A shared inductor technology and a Continuous Time Linear Equalizer (CTLE) pre-emphasis structure are added in the output buffer of LA to improve the quality of the output eye diagram. The dimension of LDLA14 is 1.5 mm × 1.3 mm, and the power consumption is 178 mW. The Peak-to-Peak Jitter (PPJ) and Root-Mean-Square Jitter (RMSJ) of the 14 Gbps optical eye diagram of LD in the Tx direction are 22.5 ps and 3.5 ps, respectively. The PPJ and RMSJ of the 14 Gbps electrical eye diagram of LA in the Rx direction are 23.1 ps and 4.7 ps, respectively. The BER tests have been conducted in Tx, Rx directions and the Tx-Rx loop condition, and the BER less than 10−12 is achieved in all tests.

Performance analysis of wavelength division multiplexing MDM-PON system using different advanced modulations

Mode division multiplexing (MDM) is very competent next generation multiplexing technique and is becoming popular among researchers these days. In this research article, an integrated passive optical network (PON) using MDM and wavelength division multiplexing (WDM) is proposed at 25 Gbps over 3 km multimode fiber (MMF) link distance. For MDM, diverse Laguerre–Gaussian (LG) such as LG12, LG15, LG18, LG111 and LG114 are incorporated and also for cost reduction, vertical cavity surface emitting laser (VCSEL) is located in optical line terminal (OLT). Performance of diverse advanced modulations such as compressed spectrum return to zero (CSRZ), duo-binary return to zero (DRZ) and modified duo-binary return to zero (MDRZ) is evaluated and compared with non-return to zero (NRZ) in terms of Bit error rate (BER) at varied MMF link lengths. Results revealed that CSRZ performance stand out and NRZ provide worst performance.

Displacement damage effects in proton irradiated vertical-cavity surface-emitting lasers

The displacement damage effects of vertical-cavity surface-emitting lasers (VCSELs) irradiated by 3 and 10 MeV protons in the range of Ф = 6.7×1012 p/cm2 to Ф = 1.6×1014 p/cm2 were investigated. The threshold current exhibited consistent degradation at the same displacement damage dose, as did the series resistance. Additionally, the external quantum efficiencies of 850 and 680 nm VCSELs were degraded by 2% and 21%, respectively. Further, the threshold current of the 850 nm VCSEL was restored by 14% after annealing at 20 mA, which is remarkably higher than that achieved by annealing only at high temperatures. These results support the applicability of VCSELs to both data communication and instrumentation applications in harsh radiation environments.

Highly reflective and conductive AlInN/GaN distributed Bragg reflectors realized by Ge-doping

We report on the realization of highly conductive and highly reflective n-type AlInN/GaN distributed Bragg reflectors (DBR) for use in vertical cavity surface emitters in a metalorganic vapor phase epitaxy process. While Ge-doping enables low-resistive n-type GaN/AlInN/GaN heterostructures, very high Ge doping levels compromise maximum optical reflectivities of DBRs. Simulations of the Bragg mirror's reflectivities together with structural analysis by X-ray diffraction reveal an increased absorption within the doped AlInN layers and interface roughening as major causes for the observed reduction of the optical reflectivity. By adjusting the Ge doping level in the AlInN layers, this structural degradation was minimized and highly conductive, 45-fold AlInN/GaN DBR structures with a maximum reflectivity of 99 % and vertical specific resistance of 5x10-4 Ωcm2 were realized.

Electrical-stress driven oxidation in 940 nm oxide-confined VCSEL

In this work, accelerated stress tests have been performed on oxide confined vertical cavity surface emitting LASER arrays to study the formation of defects degrading the performance of the device. One such defect is an additional oxide volume forming at the oxide aperture edge, which is used for optical and electrical confinement. After producing an additional oxide volume the sample was investigated using transmission electron microscopy to estimate the oxidation speed. To produce further insights into the formation process, the temperature during such a stress test was estimated by experimentally measuring the thermal resistance, and by a thermodynamic transport simulation. Both methods produced very similar results showing a temperature increase of around 22 K for a dissipated power of 3.5 mW per emitter. However this temperature rise is very small when compared to oxidation models found in literature and should not be enough to promote the oxidation. This indicates the presence of a new enhanced oxidation mechanism, which could be connected to corrosion based failure mechanisms reported in literature.