VCSELs for optical wireless communication

Accumulation of all advantageous properties VCSELs are famous for, like low power consumption, circular low divergent beam profile, high modulation bandwidth, and scalability of monolithic arrangements, results in two-dimensional (2D) VCSEL arrays that appear as key components to reach highest aggregate bandwidths of tomorrow's parallel optical transceivers. We report on 2D VCSEL arrays, substrate emitting although operating at 850 nm and prepared for flip-chip bonding, that are well suited for the customer's needs in terms of speed, power consumption, reliability and compact integration. Based on advanced technology, our arrays target the requirements of transceivers in the OC-192 VSR and 10 Gigabit Ethernet arena. In this paper we present the basic technology, static and dynamic device characteristics as well as reliability data for a 4x12 850 nm bottom-emitting VCSEL array. A13

Interest in Quantum Dot Lasers stem mainly from the low threshold current which can result due to quantization of energy levels and change in density of state function. In the most used lasers, separate confinement heterostructures, the nanoscale active region is ‘built into’ the waveguide region (optical confinement layer, OCL) based on a wide gap semiconductor material.The most important characteristic of quantum dot laser is the amount of light it emits as current is injected into the device. The dependence of laser power output on current, cavity length, and various other parameters has been estimated based on the rate equations model. Linear relation has been obtained between power and current and as the cavity length increases, the slope of the power–current characteristic decreases.The external quantum efficiency indicates the efficiency of a laser device in converting the injected electron–hole pairs (input electric charges) to the photons emitted from the device (output light). External differential quantum efficiency decreases linearly with increasing cavity length. As the internal loss increases, the slope of external differential quantum efficiency versus cavity length increases. The internal quantum efficiency is independent of the geometrical properties of the laser device, such as the cavity length or the stripe width. Internal quantum efficiency is one of the main figures of merit that should be used in assessing the quality of the semiconductor wafer from which the quantum dot laser is manufactured.One of the most important device characteristics of a laser diode is the efficiency of conversion of the input electric power into output optical power. Power conversion efficiency is found to increase with increasing drive current, gets its maximum value and after that it decreases slightly with increasing drive current. Power conversion efficiency also depends on different parameters like cavity length, internal efficiency, and threshold current.

The design and optimization of two-dimensional VCSEL arrays operating with duty-cycles from 2 to 5% (quasicontinuous- wave or QCW) operating at 940 nm are presented. Designs for nominal 8 W and 100 W peak power using TriLumina’s flip-chip-bondable, back-side-emitting VCSELs are reviewed. Performance as a function of duty cycle, including peak power output, spectral width and beam divergence are presented. Performance from -40°C to 125°C, corresponding to automotive grade 1 requirements, is reviewed. Optimization of the VCSEL arrays as a function of the number of emitters per chip is analyzed for trends in wall-plug efficiency, slope efficiency and operating conditions.

On the photon counting error probability and its application in optical wireless communications

The silicon photonic optical phased array (OPA) featured by high integration and agile beam steering exhibits greatly potential application in optical wireless communication (OWC). In this article, we demonstrate high-data-rate and long-range OWC using an integrated Silicon-Silicon Nitride OPA chip with 12 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> large aperture. The OPA chip constituted by 128 waveguide grating antennas at nonuniform spacing is employed for wide-steering and small-divergence solid beam steering. In the experiment, we demonstrated 100° wide range communications in horizontal direction via antenna phase tuning. To our best knowledge, 100° is the widest cover range in the reported OPA-based OWC systems. Moreover, the OPA chip is also utilized to achieve up to 32 Gb/s low BER transmission of non-return-to-zero (NRZ) signal over 54 m free space distance. In addition, we also successfully demonstrated more than 50 Gb/s PAM4 signal transmission by measuring clear eye diagrams at the horizontal direction of 0° and 50° over 10 m. Thanks to agile beam steering of the OPA, switching time among arbitrary positions in multi-targets communication can be less than 27 μs. By virtue of tiny beam divergence 0.021° × 0.029° emitted from the OPA, data transmission switching between two targets spaced at 3.5 cm over 20 m can be achieved accurately. The experimental measurement shows the sidelobe crosstalk from the adjacent target channel is less than −25.2 dB. Our demonstrations validate the feasible implementation of the silicon-based OPA in the high-data-rate and wide-steering-range OWC system.

Non-uniform series resistance design of VCSEL arrays is studied to improve thermally induced current bifurcation based on an electro-opto-thermal model of VCSEL arrays. Taking an 850nm VCSEL array with 4×4 cells for example, the impact of series resistance on current bifurcation is investigated. Increasmg series resistance is helpful to enhance the critical current values of current bifurcation point (Irc) and hence delay the current bifurcation phenomenon. For VCSEL array with non-uniform series resistance, Irc is increased by 28.6% and the total output optical power is enhanced by 14.3% when compared with that of VCSEL array with uniform series resistance. Therefore, non-uniform series resistance design is a better method for delaying the current bifurcation phenomenon and enhancuig the output optical power of VCSEL arrays.

The high responsivity and broad spectral sensitivity of organic photodetectors (OPDs) present a bright future of commercialization. However, the relatively high dark current density still limits its development. Herein, two novel nonpolar p-type conjugated small molecules, NSN and NSSN, are synthesized as interface layers to enhance the performance of the OPDs, which not only can tune energy alignments and increase the reverse charge injection barrier but also can reduce the interfacial trap density. Moreover, benefiting from the smoother surface morphology and enhanced conductivity, the NSN exhibited superior charge transport and collection properties. Consequently, the OPD with NSN achieved a dark current density of 0.37 nA cm-2 and a high specific detectivity of 2.77 × 1013 Jones at -2 V. More importantly, the optimized OPDs can be successfully integrated into optical communication systems, demonstrating precise digital signal communication without obvious distortion, showing promising application potential in the wireless transmission system.

With the fast development of power grid, there is an urgent need for low delay, wide bandwidth, high reliable, and extensive connected information services, which is planned to be implemented by the emerging 5G technology for future smart grid. 5G has special advantages of providing Enhanced Mobile Broadband (eMBB), Ultra-reliable and Low Latency Communications (uRLLC), and Massive MachineType Communication (mMTC). Based on current situation, the 5G upgrading involves the modifications of business applications, network support, security, management, and other essential aspects, and it will roughly take 3-5 years. The deployment of 5G equipment also requires a great deal of investment and it may not be economically beneficial in some places where power enterprises incomes is not satisfactory. Therefore, this paper proposes the optical wireless communications (OWC) as an supplement technology for it will be able to provide as satisfactory networking performance as 5G while it has advantages of fast deployment and low cost which work as a good supplement to 5G. This paper describes the general deployment method of OWC in power grid systems and analysis its performance via Monte Carlo simulation (MCS).

High-performance self-powered photodetectors are in high demand for many applications in wireless sensor networks, Internet-of-things devices, wearable electronics, optical communication, and biomedical sensing. The self-powered photodetector design requires a type-II staggered heterojunction between two semiconductors. Hence, conventional heterojunction-free self-powered photodetector designs remain a challenging task. Here, we report a self-powered photodetector of significantly high performance using an all-inorganic ${\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_{3}$ monocrystal grown in situ directly over the substrate with prepatterned interdigitated gold ($\mathrm{Au}$) electrodes. Interestingly, we achieved an extremely low dark current of about 1 fA at room temperature, a dark-to-light current ratio of ${10}^{7}$, a responsivity of 2 A/W, and a specific detectivity of ${10}^{14}$ Jones in self-powered mode. The self-powered nature of the $\mathrm{Au}/{\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_{3}/\mathrm{Au}$ photodetector with asymmetric contact geometry is investigated theoretically using a back-to-back Schottky diode model and experimentally using the Kelvin-probe force microscopy technique. Both approaches converged to predict the existence of a sufficient contact potential difference between the metal contacts due to having an interfacial area difference, which remains the source of built-in potential in the device to dissociate photogenerated excitons effectively for self-powered operation. Our innovative approach in this study opens up an inventive direction for developing a cost-effective heterojunction-free self-powered photodetector.

We have demonstrated high density, 2D (4x12) VCSEL arrays operating at an aggregate data rate of over 480Gb/s in an aerial density of 1400x3750 &mu;m², or 9.14 Tbs/cm². These flip-chip, bottom-emitting 990nm VCSELs have low drive voltage, low electrical parasitics, improved thermal impedance and 2D scalability over their wire-bonded top emitting counterparts. Excellent high speed performance was obtained through the use of 1) compressively strained InGaAs MQW active region 2) low parasitic capacitance oxide-confined VCSEL structures and 3) low series resistance, high index contrast AlGaAs/GaAs mirrors. 10Gb/s operation was obtained with low operating current density of ~6kA/cm² at 70C. Our best results to date have achieved data rates greater than 12.5Gb/s @70C at a current density less than 10kA/cm². The device results show good agreement with theoretically calculated/simulated values. This work was partially supported by DARPA under contract MDA972-03-3-0004.

In power-limited trans-oceanic submarine systems, the electrical-to-optical (E/O) power conversion efficiency determines the amount of optical signal power generated by repeaters, which governs the cable capacity. In contrast to conventional submarine systems, recent single mode fiber spatial division multiplexing (SDM) systems operate at a lower channel power and support a larger number of optical fibers. Moreover, pump sharing technology is used in SDM systems to enhance cable capacity and system reliability, where a group of pump lasers are combined to feed a group of fiber pairs. The repeater E/O power conversion efficiency is directly related with cable capacity, as well as capacity per power and cost per capacity analysis. Therefore, it is important to understand the characteristics of repeater E/O efficiency. We developed a model to analyze the repeater E/O efficiency in submarine SDM systems. Experimental measurement data of pump lasers and numerical simulation results of Erbium doped fiber amplifiers (EDFAs) were used as input information to the model to analyze repeater E/O efficiency in various system designs. Based on the E/O efficiency model results, we investigated cable capacity using the Gaussian noise (GN) model. Moreover, cable capacity per power was calculated and compared based on three different power metrics: (i) electrical supply power, (ii) pump optical power and (iii) EDFA output power. With the aid of the repeater E/O efficiency model, one can analyze the efficiency of submarine systems using the direct metric of cable capacity per electrical supply power.

Perovskite photodetectors (ePDs) have shown significant promise for applications in imaging and optical communications due to their excellent optoelectronic properties. Dark current density (Jd) plays a crucial role in determining the performance of 3D PePDs based on poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Herein, a novel viologen derivative, 1‐allyl‐1′‐(2‐phosphonoethyl)‐viologen (APV), is introduced into PEDOT:PSS to improve the crystalline quality of the perovskite film and effectively suppress dark current. Consequently, the optimized 3D PePD achieves an ultra‐low Jd of 5.75 × 10−7 mA cm−2 at −0.5 V and a maximum specific detectivity of 2.08 × 1013 Jones at 705 nm, positioning it among the top high‐performance 3D PePDs for visible photodetection. Furthermore, the optimized PePD exhibits a fast response time of 256 ns and a large bandwidth of 1.5 MHz. Upon successful integration into an optical wireless communication (OWC) system as the signal receiver, it demonstrates a data rate of up to 12.5 Mbps with minimal distortion. We believe that APV modification provides a universal strategy to realize sensitive PePDs, potentially revolutionizing the applications of OWC and imaging.image

An optoelectronic parallel computing system called OCULAR-H will be shown. It has a multilayer architecture that eliminates data transfer bottlenecks between optoelectronic processors that have two-dimensional VCSEL and PD array by reconfigurable free-space optical interconnections.

In the past terrestrial optoelectronic distance measurement in the kilometer range was only possible by marking targets with reflector prisms or by increasing the optical output power far beyond the eyesafety limits. These requirements are unacceptable for many measurement tasks and this paper describes how they can be avoided by using a low power semiconductor laser and modern signal processing techniques. 1. DESIGN GOALS A number of manufacturers have fielded optoelectronic rangefinders which must be supported by reflector prisms mounted on the target if distances beyond 100 m are to be measured. These devices based on semiconductor lasers are very accurate (to the order of mm) and because of their low optical output power they are generally eyesafe. Devices are also available for measurements without reflector prisms (non-cooperative targets) to large distances beyond 5000 m where poor target reflectivity and atmospheric attenuation are overcome by high power output usually from an NdYAG crystal laser. The very high intensity laser radiation however is dangerous to the human eye and therefore these devices are constrained to operate under rigorous safety measures. Between these two extremes there are numerous applications where not extreme range or accuracy but eyesafety non-cooperative targets small size and low cost are of primary importance. This is the case not only for civilian applications but also for military use e. g. in force-on-force exercises or special operations. Other design goals

Visible light communication and positioning (VLCP) systems have been rarely researched to sense and model the obstacles. However, the accumulating database containing the signal amplitudes and receiver positions can be exploited to detect and reconstruct the positions and shapes of the obstacles. In this work, we propose an algorithm to model the obstacles in a VLCP system, where several light-emitting diodes (LEDs) are set at known positions, and the users equipped with photodiodes (PDs) randomly move to receive the LEDs' signals at different positions. Based on the received signals, the blocked links between the users and LEDs can be distinguished, and thus we can reconstruct the obstacle information. Simulation and analytical results show that the algorithm can estimate the positions and shapes of obstacles. Such an algorithm expand the services of the VLCP systems, and can sense the three-dimensional (3D) information of the environment through the light channel coefficients so as to circumvent the expensive devices, such as laser radars. The proposed algorithm can also support navigation and prediction of wireless channel fading.