Optical waveguide amplifiers for heterogeneous integration in optical backplanes

The optical amplifier performance of Nd³⁺-doped polymer and amorphous Al₂O₃ channel waveguides with single-mode and multi-mode behavior around 880 nm is compared. Internal net gain in the wavelength range 865-930 nm is investigated under continuous-wave excitation near 800 nm, for Nd³⁺ dopant concentrations typically in the range of 0.6- 1.0 × 10²⁰ cm^-3. A peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long polymer waveguide at a launched pump power of 25 mW. The small-signal gain measured in a 1-cm-long sample is 2.0 dB/cm. In Al₂O₃, a peak gain of 1.57 dB/cm in a short and 3.0 dB in a 4.1-cm-long waveguide is obtained at 880 nm. Tapered multi-mode Nd³⁺-doped amplifiers are embedded into an optical backplane and a maximum 0.21 dB net gain is demonstrated in a structure consisting of an Al₂O₃:Nd³⁺ amplifier placed between two passive polymer waveguides on an optical backplane. The gain can be further enhanced by increasing the pump power and improving the waveguide geometry, and the wavelength of amplification can be adjusted by doping other rare-earth ions.

Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -complex-doped polymer channel waveguide amplifiers with various lengths and Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> concentrations are fabricated by a simple procedure. Internal net gain at 840-950 nm and 1064 nm is experimentally and theoretically investigated under continuous-wave excitation at 800 nm. Internal net gain in the range 865-930 nm is observed and a peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long waveguide with a Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> concentration of 0.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> at a launched pump power of 25 mW. The small-signal gain measured in a 1-cm-long sample with a Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> concentration of 1.03 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> is 2.0 dB/cm and 5.7 dB/cm at 873 nm and 1064 nm, respectively. By use of a rate-equation model, the internal net gain at these two wavelengths is calculated and the macroscopic parameter of energy-transfer upconversion as a function of Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> concentration is derived. Ease of fabrication, compatibility with other materials, and low cost make such rare-earth-ion-doped polymer waveguide amplifiers suitable for providing gain in many integrated optical devices.

Rare-earth-ion-doped polymer waveguide amplifiers [1–2] have been studied due to their potential use in integrated optical applications. Neodymium can be of great interest for high-speed amplifiers in optical interconnects, because it emits around 850–930 nm on the 4F 3/2 → 4I 9/2 ground-state transition. Polymers are promising host candidates in this application due to their low cost, ease of fabrication, and potential integration with polymer waveguides in integrated optical systems, e.g. optical backplanes. In this work, Nd3+-complex-doped polymer channel waveguide amplifiers have been realized and gain at 850-930 nm has been demonstrated.

The optical true-time-delay line is a key building block for modern broadband phased-array antennas, which have become one of the most critical technologies for both military and civilian wireless communications. We present our research results in developing an optical polymer-based waveguide true-time-delay module for multilink phased- array antennas by incorporating wavelength-division multiplexing (WDM) technology. The demonstrated optical polymeric waveguide circuits can provide a large number of optical true-time delays with a dynamic range of 50 ns and a time resolution of 0.1 ps. Various fabrication techniques are investigated for producing ultralong low-loss (0.02 dB/cm) polymeric channel waveguides with tilted waveguide grating output couplers. Fast photodiode arrays are fabricated and rf signals with frequencies of 10 to 50 GHz are generated through the optical heterodyne technique. A detailed study of waveguide amplification to achieve loss-less polymeric waveguide is conducted. The optical amplification of 3.8 dB/cm is achieved at a wavelength of 1064 nm in a Nd3+-doped polymeric waveguide. WDM techniques are also employed for potential multilink applications. The presented methodologies enable hybrid integration with a reduced cost in optoelectronic packaging and an increased reliability and decreased payload for the next generation of phased-array antennas.

We present an array of electro-optic Bragg gratings for on-chip optical waveguide interconnect applications. These electro-optical gratings can serve as switchable couplers to couple high-speed optical signals from a vertical cavity surface emitting lasers (VCSEL) to on-chip polymeric waveguides and from waveguides to photodetectors. The unique switching feature provides a promising solution to facilitate dynamically reconfigurable on-chip optical interconnects architecture for intra-chip, chip-to-chip and board-to-board data communications. We demonstrate on-chip switched optical waveguide interconnect using electro-optic grating and polymeric channel waveguides on a silicon substrate.

The optical properties and characteristics of rare earth- doped polymers have been studied to evaluate their viability for use in optical amplifiers. Rare earth ions are encapsulated in organic, covalently bonded chromophores. The optical properties of various rare earth chromophores doped into polymers are measured and calculated and are then used in numerical simulations of amplifiers and lasers. The result provide an estimate of their potential device performance and establish the fundamental bases for applications in photonics. Owing to their distinct advantages, such as chromophore energy transfer effects, high rare earth ion concentrations, shielding of the ion form high energy vibrations of the host, enhanced optical transition moments and controllable decay rates and branching ratios, rare earth-doped polymers are found to be promising candidates for various device applications. Numerical simulations for samarium and europium doped polymers indicate that gains about 10 dB and greater are achievable in relatively short polymer optical fiber and waveguide amplifiers. Studies of the dependence of metastable state lifetime of rare earth doped polymer systems on doping concentrations reveal that rare earth chromophores dissociation occurs at low concentrations. These results are used to optimize the parameters of our rare earth doped polymer optical waveguide amplifiers.

Large polarization dependent loss (PDL) as high as 15 dB was observed in the rib-type channel waveguide fabricated using a birefringent polymer. The PDL mechanism was explained by the mode-mode coupling between the TM mode of a rib and the TE mode of a slab waveguides. This TM-TE mode conversion is due to nonuniform off-diagonal component of the dielectric susceptibility tensor which is generated by a thermal stress during the curing process.

Optical technologies have received large interest in recent years for use in board-level interconnects. Polymer multimode waveguides in particular, constitute a promising technology for high-capacity optical backplanes as they can be cost-effectively integrated onto conventional printed circuit boards (PCBs). This paper presents the first optical backplane demonstrator based on the use of PCB-integrated polymer multimode waveguides and a regenerative shared bus architecture. The backplane demonstrator is formed with commercially-available low-cost electronic and photonic components onto conventional FR4 substrates and comprises two opto-electronic (OE) bus modules interconnected via a prototype regenerator unit. The system enables interconnection between the connected cards over four optical channels, each operating at 10 Gb/s. Bus extension is achieved by cascading OE bus modules via 3R regenerator units, overcoming therefore the inherent limitation of optical bus topologies in the maximum number of cards that can be connected to the bus. Details of the design, fabrication, and assembly of the different parts of this optical bus backplane are presented and related optical and data transmission characterisation studies are reported. The optical layer of the OE bus modules comprises a four-channel three-card waveguide layout that is compatible with VCSEL/PD arrays and ribbon fibres. All on-board optical paths exhibit insertion losses below 13 dB and intra-channel crosstalk lower than -29 dB. The robustness of the signal distribution from the bus inputs to all respective bus output ports in the presence of input misalignment is demonstrated, while 1 dB input alignment tolerances of approximately ±10 μm are obtained. The electrical layer of the OE bus modules comprises the essential driving circuitry for 1×4 VCSEL and PD arrays and the corresponding control and power regulation circuits. The interface between the optical and electrical layers of the bus modules is achieved with simple OE connectors that enable end-fired optical coupling into and out of the on-board polymer waveguides. The backplane demonstrator achieves error-free (BER <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> ) 10 Gb/s data transmission over each optical channel, enabling therefore, an aggregate interconnection capacity of 40 Gb/s between any connected cards.

A novel optical backplane relying on standard MM 12-fiber ribbons and MPO connectors is proposed for high-capacity ICT apparatus. A modular, scalable and full-mesh optical interconnection between optoelectronic boards is guaranteed thanks to an optimized fiber ribbon layout routing that divides the overall backplane into different and independent optical sub-circuits. The automated assembly of fiber ribbons into sub-circuits with a robotic work-cell is described. System validation of the optical backplane performed with commercial MM 12-fiber transceivers @10Gb/s proved the feasibility of the proposed solution for future optical interconnections with terabit overall capacity.

Current high performance switching and computing systems rely on an electrical interconnection network to transport information throughout the system for chip-to-chip and board-to-board communications [1]. This network however is limited relative to the bandwidth, connectivity, power consumption, and latency it offers. Because the aggregate bandwidth of the integrated circuits inside these systems continues to increase, so must the system's interconnection capabilities [2], The intrinsic limitations of current and projected electrical interconnection networks has lead system designers to consider optical interconnects deployed "inside the box" as a way of increasing system performance [3]. An optical backplanes is an example of such a system. The identification of critical issues in free-space optical backplanes is being pursued in the form of system demonstrator experiments [4-10]. Through the construction of several system demonstrators an iterative design, analysis, and implementation procedure has been used to address the critical issues associated with deploying optical interconnections at the board-to-board level. This talk will highlight the challenges and choices associated with the demonstrators.

We demonstrate reactively sputtered Al2O3:Er3+ waveguide amplifiers with an erbium concentration of 3.9 × 1020 ions/cm3, capable of achieving over 30 dB small signal net gain at 1532 nm using bidirectional pumping at 1480 nm. We observe on chip output powers of 10.2-13.6 dBm of amplified signal power at 1532 nm for a 12.9 cm waveguide amplifier considering -25.4 dB of lumped coupling losses per facet. Annealing was used to improve the performance of the devices, which were patterned using electron beam lithography and reactive ion etching. This result, to our knowledge, represents record breaking on-chip internal net gain for Al2O3:Er3+ waveguide amplifiers, which show promise over other technologies due to wafer scalability and promise of easy monolithic integration with other material platforms to support a wide variety of applications.

Optical gains at 637 nm wavelength using light‐emitting diodes (LEDs) instead of traditional semiconductor lasers as pumping sources are demonstrated in the organic molecule 2,6‐bis[4‐(diphenylamino)phenyl]‐9,10‐anthracenedione (AQ(PhDPA)2)‐doped polymethylmethacrylate (PMMA) and SU‐8 polymer waveguides. Under excitation of four blue‐violet LEDs with different central wavelengths, fluorescence in the red band is observed owing to the transition of AQ(PhDPA)2from the excited states to the ground state based on the thermally activated delayed fluorescence (TADF) mechanism. Channel waveguides with a cross‐section of 6 µm × 5 µm are fabricated. The relative gains of 5.0 and 4.0 dB cm−1are obtained in rectangular waveguides with active core layers as AQ(PhDPA)2doped PMMA and SU‐8, respectively. Optical gains have also been achieved in waveguides based on evanescent‐wave coupling. By using the vertical top pumping mode of a 450 nm LED, a gain of 2.6 dB cm−1is obtained in a passive SU‐8 waveguide with AQ(PhDPA)2doped PMMA as the upper cladding. In contrast, the replacement of SU‐8 with AQ(PhDPA)2doped SU‐8 as the active material, triples the optical gain, which reaches to 7.8 dB cm−1owing to the presence of stimulated excitation in both channel waveguide and upper cladding.

We report on recent developments of our board-level optical interconnect technology towards polymer waveguide flexes and on the adaption of connectorization and electro-optical assembly methods to be used in optical backplanes and high-density optical subassemblies.

Polymer materials offer a number of interesting features for the fabrication of optical waveguides when compared with other materials such as silica glass. Organic polymers have attracted a lot of attention due to their ease of fabrication and structural flexibility. These are important factors with regard to their use as packaging and optical interconnects. In addition, the thermo-optic (TO) coefficient of polymers is an order of magnitude larger than that of silica glass and some non-aromatic polymers have very low birefringence. These are desirable characteristics in terms of their application to such optical integrated devices as TO switches. Polymeric waveguide applications are therefore divided into two categories. The first comprises optical packaging and interconnects and the second, optical integrated devices. Nevertheless, polymeric materials have certain disadvantages. An optical waveguide must retain its optical and physical properties under a variety of environmental conditions including high temperature and high humidity. However, polymer materials do not have sufficient reliability and thermal stability except for polyimide and the like. To solve these problems, we have worked to develop new materials, processes and packaging.

Er/sup 3+/-doped tellurite and Er/sup 3+/-doped alumina optical waveguide amplifiers are analyzed both as single amplifiers and as elements of 16-channel wavelength-division multiplexing (WDM) systems; their performances are compared with that from Er/sup 3+/-doped Al/P silica waveguide amplifiers. The amplifier model is based on propagation and population-rate equations and includes both uniform and pair-induced up-conversion mechanisms. It is solved numerically by combining finite elements and the Runge-Kutta algorithm. The analysis predicts that Er/sup 3+/-doped tellurite waveguides exhibit improved gain characteristics compared with alumina and Al/P silica waveguides. Using Er/sup 3+/-doped tellurite waveguide amplifiers, it is suggested that 16 WDM channels may be transmitted to a maximum distance of 4250 km. By using in-line notch gain equalizing filters, the maximum transmission distance can be increased to 5250 km.