High coupling efficiency, highly robust optical fibre passive alignment

Silicon wafer-scale processing promises to provide a manufacturable approach for high volume, low cost, opto-electronic packaging. A silicon interposer with precise mechanical features is presented which accurately positions optical fibers with laser shaped fiber endfaces to Vertical Cavity Surface Emitting Laser (VCSEL) and optical photodetector array die. These optical array die and corresponding electrical driver and receiver array die are designed to be flip-chip mounted onto the top surface of silicon interposer sites. The electrical and optical die are passively aligned and electrically interconnected by simultaneous reflow of solder pads used for the flip chip mounting. Input and output electrical traces are routed on the silicon interposer top surface and transferred to the interposer back surface by through wafer vias. After dicing the populated silicon interposer sites from the wafer, termination is achieved with optical fibers that feature a multi-facet laser shaped endface. This optical fiber termination step also features passive alignment using precision placement of v-grooves for the optical fibers and a set of flip chip Au Sn solder pads formed on the top surface of the silicon interposer. The VCSEL and photodetector arrays have a matching set of solder pads that are precisely located relative to optical apertures on each die. When the wafer-scale solder reflow process is performed, surface tension of the molten solder pulls the two pad arrays into precise alignment. This places apertures of the optical devices into an aligned position over the v-grooves that receive the optical fibers. The result is a right angle optical coupling arrangement, which has a very compact structure. The optical coupling efficiency is enhanced by laser forming a multi-facet mirror directly on the endface of the optical fibers. This is achieved using a pulsed CO2 laser, which is programmed to cut the desired shape onto the fiber endface, and application of a reflective coating. One facet of the shaped fiber endface is allocated to provide a mechanical stop surface. This stop surface properly positions the fiber endface mirror relative to the VCSEL / detector optical apertures when it physically contacts the end facet of the Si interposer v-groove during fiber termination. The opposite end of the v-groove is widened to accommodate the coating of the optical fiber in order to provide a strain relief where the fiber exits the interposer. Finally, electrical termination is made to the back side of the silicon interposer using a high speed flex circuit. The result is a compact high speed Chip-to-World opto-electronic interconnect designed for greater than 10 Gbps operation giving optical coupling efficiency of less than 1 dB loss. Optical simulation using Zemax® is used to calculate coupling efficiency with respect to manufacturing tolerances.

High-speed and “green” ~850 nm vertical-cavity surface-emitting lasers (VCSELs) have lately attracted lots of attention due to their suitability for applications in optical interconnects (OIs). To further enhance the speed and its maximum allowable linking distance of VCSELs are two major trends to meet the requirement of OI in next generation data centers. Recently, by use of the advanced 850 nm VCSEL technique, data rate as high as 64 Gbit/sec over 57m and 20 Gbit/sec over 2km MMF transmission have been demonstrated, respectively. Here, we will review our recent work about 850 nm Zn-diffusion VCSELs with oxide-relief apertures to further enhance the above-mentioned performances. By using Zn-diffusion, we can not only reduce the device resistance but also manipulate the number of optical modes to benefit transmission. Combing such device, which has excellent single-mode (SMSR >30 dB) and high-power (~7mW) performance, with advanced modulation format (OFDM), record-high bit-rate-distance-product through MMF (2.3 km×28 Gbit/sec) has been demonstrated. Furthermore, by selective etching away the oxide aperture inside Zn-diffusion VCSEL, significant enhancement of device speed, D-factor, and reliability can be observed. With such unique VCSEL structure, >40 Gbit/sec energy-efficient transmission over 100m MMF under extremely low-driving current density (<10kA/cm2) has been successfully demonstrated.

Remarkable progress made in performance characteristics and reliability of high-speed (> 10 GHz) 850-nm multi-mode (MM) oxide-confined vertical cavity surface emitting lasers (VCSELs) during the last decade has led them to find applications in space satellite systems. The main advantage of deploying high-speed VCSELs in space satellites over directly modulated 850-nm edge emitting lasers is the absence of COMD (catastrophic optical mirror damage). In recent years, leading VCSEL manufacturers introduced higher speed (~ 20 GHz or 25 Gbps) VCSELs with encouraging characteristics. However, little has been reported on reliability and failure modes of these state-of-the-art VCSELs although it is crucial to understand failure modes and degradation mechanisms in these VCSELs through physics of failure investigation and subsequently develop VCSELs that exceed lifetime requirements for space satellite systems. For the present study, we performed short-term and long-term accelerated life-tests on 25 Gbps oxide-confined MM VCSELs to study reliability of these devices. Our goal is to extract credible reliability model parameters (thermal activation energy and current exponent factor) from these life-tests to determine suitability of these lasers for future space satellite systems. We also performed failure mode analysis on VCSELs at different stages of degradation using various techniques. We employed nondestructive techniques including optical beam induced current (OBIC) and electron beam induced current (EBIC) techniques as well as destructive techniques including focused ion beam (FIB) and high-resolution TEM techniques. Our detailed reliability and failure mode analysis results are reported along with our understanding on the physical origin of degradation in high-speed VCSELs with strained InGaAs quantum wells.

The future of optical interconnection needs for high optical coupling efficiency for high density multi-layer printed wiring board (PWB). Hence, a self-written waveguide (SWW) optical pin is implemented between surface device and multi-layer channel as optical coupling for reaching high coupling efficiency. A comparatively studies of three configurations of coupling methods was carried out by ray-tracing simulation. The correlation between optical coupling efficiency and mirror angle was analyzed as well as the relationship between optical coupling efficiency and photodiode (PD) positional. A maximum value of optical coupling efficiency with optical pin is obtained for both correlations. The 1 dB down-tolerance width of the correlation between optical coupling efficiency and the mirror angle for vertical pin (case 1) is larger compared to case 2 and case 3 (without vertical pin). For the relationship between optical coupling efficiency and PD positional error, the 1 dB down-tolerance widths for case 1, case 2 and case 3 were ±13μm, ±19μm and ±20μm, respectively. From this result, it is found that vertical pin is well-suited to satisfy both coupling efficiency and tolerance for shifting mirror angle and PD positional error.

We presented 980-nm oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with a 16-μm oxide aperture. Optical power, voltage, and emission wavelength are measured in an ambient temperature range of 5 °C–80 °C. Measurements combined with an empirical model are used to analyse the power dissipation in the device and the physical mechanism contributing to the thermal rollover phenomenon in VCSEL. It is found that the carrier leakage induced self-heating in the active region and the Joule heating caused by the series resistance are the main sources of power dissipation. In addition, carrier leakage induced self-heating increases as the injection current increases, resulting in a rapid decrease of the internal quantum efficiency, which is a dominant contribution to the thermal rollover of the VCSEL at a larger current. Our study provides useful guidelines to design a 980-nm oxide-confined VCSEL for thermal performance enhancement.

The strong (>20 nm) wavelength detuning technique has been demonstrated to enhance the modulation speed and high-temperature characteristics (at 85 °C), as well as lower the required driving current density performance of oxide-relief 850-nm vertical-cavity surface-emitting lasers (VCSELs) for >40 Gbit/s operation. By increasing the wavelength detuning from 15 to 20 nm, a significant improvement in the electrical-to-optical (E-O) bandwidth (20 to 27 GHz) of the VCSEL can be observed. This detuning design (∼20 nm) is incorporated along with a Zn-diffusion structure into our oxide-relief VCSEL with a miniaturized oxide-relief aperture (∼3 μm). Highly single-mode, high-speed (26 GHz) operation, and moderate differential resistance (100 Ω) values can be simultaneously achieved. In addition, it is found that devices with a further larger detuning wavelength (>20 nm) and enlarged oxide-relief apertures (∼8 μm) can sustain the same maximum E-O bandwidth (26 GHz) as that of a miniaturized (∼3 μm) VCSEL, resulting in the lower driving current density (8 versus 18.8 kA/cm2) required for high-speed performance. Excellent transmission performance, which includes an extremely low energy-to-data rate ratio (EDR: 228 fJ/bit; over 100 m OM4 fiber) and record-low driving-current density (8 kA/cm2; 3.5 mA) has been successfully achieved for 41 Gbit/s error-free transmission for these VCSELs.

Multimode VCSEL(Vertical Cavity Surface Emitting Laser) coupling into MMF (Multimode Optical Fiber) and its influence on MMF link has been numerically studied. Considering the transverse misalignment between VCSEL and MMF, coupling efficiency between VCSEL higher-order modes and modes of MMF is calculated and the Mode Power Distribution in the MMF is obtained. Simulation of MMF link with multimode VCSEL indicates that the higher-order VCSEL modes lead to decreasing of the eye opening of received signal. The transverse misalignment of VCSEL between MMF greatly deteriorates the eye opening. With 300m OM3-MMF link , the EOP (eye-opening-penalty) is greater than 14dB when the misalignment between VCSEL and MMF is lager than 5&mu;m.

Based on the chaos synchronization between two pairs of corresponding linear polarization modes in two 1550 nm vertical-cavity surface-emitting lasers (1550 nm-VCSELs) subject to optical injection of common chaotic signals, a novel bidirectional and dual-channel long-distance chaos secure communication system is proposed. The chaotic synchronization characteristics, bidirectional dual-channel communication performance, and the influences of the fiber channel on the message transmission are numerically investigated. Results show that, driven by a common chaotic signal generated from a driver VCSEL (D-VCSEL), two pairs of responding linear polarization modes in two response 1550nm-VCSELs (R-VCSELs) can be synchronized completely and isochronally. Meanwhile, the synchronization coefficients between the two pairs of responding linear polarization modes in D-VCSEL and R-VCSELs are low. Based on the high quality chaos synchronization between two pairs of responding linear modes in two R-VCSELs, bidirectional and dual-channel chaos secure communication can be achieved. After adopting a single mode polarization-maintaining fiber (or dispersion-shifted polarization-maintaining fiber), for 2.5 Gbit/s messages, the Q factor of the decryption signals after transmitting 60 km (or 200 km) can be more than 6.

We report on a high-performance oxide-confined 850-nm vertical-cavity surface-emitting laser (VCSEL) believed to have the highest modulation bandwidth at low bias currents for datacenter applications. This article has extensively discussed the device's characteristics, including static performance, optical spectra, small-signal response, relative intensity noise (RIN), and large-signal performance. The VCSEL with an oxide aperture of 4 μm delivers a 3.7 mW optical power, low spectral width, below −150 dB/Hz VCSEL RIN, and a breakthrough 33.4 GHz frequency response at room temperature (25 °C). Designed for high-performance short-reach communication networks and telecommunication applications, this high-speed 850-nm VCSEL supports data rates of up to 64 Gb/s non-return-to-zero (NRZ) and 106.25 Gb/s (53.125 Gbaud) 4-level pulse amplitude modulation (PAM-4) in multi-mode fiber (MMF) without the need for feed-forward equalization (FFE), decision feedback equalizer (DFE), or any offline digital signal processing (DSP).

High-frequency discrete and distinct mode-beating noise (MBN) spectra are an anomalous phenomenon observed in the related intensity noise spectrum from the vertical-cavity surface-emitting laser (VCSEL), which reveals strong correlation with fine-splitting transverse modes caused by non-circular emission aspect of the VCSEL with imperfect true-roundness. Such a phenomenon is demonstrated for the first time, which seriously degrades the transmission performance of high-speed and broad-band encoding data carried by the multi-mode (MM) VCSEL biased under a large DC bias. In this work, the 4-ary quadrature-amplitude modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and the non-return-to-zero on–off keying (NRZ-OOK) transmissions with the direct modulation of different 850-nm MM VCSELs, as suffered from the MBN-induced notches of the signal-to-noise-ratio (SNR) spectra, are characterized under their high-level lasing conditions with multi-mode numbers. The significant SNR degradation on the receiving and decoding performances caused by the cross-correlation between MBN and data spectra is also examined by tuning the lensed fiber along the transverse (X/Y)-axis of the VCSEL (perpendicular to the surface normal) when collimating the output beam, which finds the disorderly shifted MBN spectra, as attributed to the collection of light emitted from the overlapped gain region of different transverse modes. In contrast, the MBN reveals invariant spectral features while the lensed fiber is either back-and-forth tuning along the vertical (Z)-axis (parallel with the surface normal) or tilting its angles from the surface normal under the same horizontal position and distance concerning the MM VCSEL. Such MM VCSEL exhibits several MBN spikes to decay the SNRs of NRZ-OOK and QAM OFDM data encoded around 21 and 34 GHz. For other MM VCSELs with reducing numbers of their transverse lasing modes and deforming roundness of their circular emitting aspects, the corresponding MBN spectral peaks can also be respectively observed at 14/17/24 GHz and 4.5/10.5/13 GHz to degrade the SNRs of the encoded data at such frequencies. Even though pre-emphasized encoding technology is employed, such an MBN effect is strongly correlated with the asymmetric circular aspect of the VCSEL with a degraded roundness ratio, which still affects the data modulation performance of the MM VCSEL for data center communication.

To determine the feasibility of coupling the output of a single-mode optical fiber into a single- mode rib waveguide in a temperature varying environment, a theoretical calculation of the coupling efficiency between the two was investigated. Due to the complex geometry of the rib guide, there is no analytical solution to the wave equation for the guided modes, thus, approximation and/or numerical techniques must be utilized to determine the field patterns of the guide. In this study, three solution methods were used for both the fiber and rib guide fields; the effective-index method, Marcatili's approximation, and a Fourier method. These methods were utilized independently to calculate the electric field profile of each component at two temperatures, 20 degree(s)C and 300 degree(s)C, representing a nominal and high temperature. Using the electric field profile calculated from each method, the theoretical coupling efficiency between an elliptical-core optical fiber and a rib waveguide was calculated using the overlap integral and the results were compared. It was determined that a high coupling efficiency can be achieved when the two components were aligned. The coupling efficiency was more sensitive to alignment offsets in the y direction than the x, due to the elliptical modal field profile of both components. Changes in the coupling efficiency over temperature were found to be minimal.

By the need for high-speed data transmission in PCB, the studies on the optical PCB has been conducted with optical interconnection and its packaging technology. Particularly, the polymer-based optical interconnection has been extensively studied with the advantages such as cost-effective and ease of process. For high-efficiency and passive alignment, the studies were performed using the 45 degree mirrors, MT connector, and etc. In this work, integrated PLC device and fiber alignment array block was fabricated by using imprint technology to solve the alignment and array problem of optical device and the optical fiber. The fabricated integrated block for optical interconnection of PLC device has achieved higher precision of decreasing the dimensional error of the patterns by optimization of process and its insertion loss has an average value of 4.03dB, lower than criteria specified by international standard. In addition, a optical waveguide with built-in lens has been proposed for high-efficiency and passive alignment. By simulation, it was confirmed that the proposed structure has higher coupling efficiency than conventional no-lens structure and has the broad tolerance for the spatial offset of optical waveguide.

We propose a novel passive alignment method for multiple silica-based planar lightwave circuit (PLC) interconnections. The method employs grooves for fitting formed on a silica-based PLC with optical fibers. We achieved low-loss PLC interconnections with the method. The method is a promising way of realizing cost-effective and pluggable multi-PLC integration.

The optical solder effect of the self-written waveguide (SWW) using Rhodamine 6G dye-dispersed ultraviolet (UV) curable resin by a green laser is demonstrated. This enables formation of SWW from the end of polyimide waveguide having strong UV absorption. High efficient coupling between an optical fiber and a vertical-cavity surface-emitting laser is successfully achieved

Vertical-cavity surface-emitting laser (VCSEL) has received much attention due to its high modulation response bandwidth, two linear polarization modes, and easy integration. The fundamental characteristics of lasers can be observed by simple optical feedback. Here, we experimentally investigate the dynamical characteristics in a 1550-nm VCSEL subjected to mirror optical feedback. The distribution of dynamics for different bias currents and feedback strength is in detail unveiled. Four different dynamic routes to laser chaos are observed at different bias currents. As the bias current increases, the route to chaos becomes complex. Apart from the well-known stable state, quasi-periodic state, chaotic state, and two previously reported dynamics switching phenomena, the dynamics switching between intermittently quasi-periodic state and quasi-periodic state is observed at a high bias current. Additionally, the laser chaos at different bias currents is also comparatively studied. The chaos bandwidth can reach 18 GHz. The systematic investigation provides the basis for utilizing the broadband laser chaos and understanding the nonlinear dynamics in VCSELs.