Nd-doped waveguide amplifiers for heterogeneous integration in optical backplanes

Large-core, tapered Al 2 O 3 :Nd3+ channel waveguides have been fabricated and optical amplification at a wavelength of 880 nm has been characterized. Amplification in optical backplanes has been demonstrated by inserting such Al 2 O 3 :Nd3+ channel waveguides in between two polymer waveguides. A maximum 0.21-dB internal net gain has been demonstrated in an Al 2 O 3 :Nd3+ waveguide coupled in between two polymer channel waveguides. The gain can be improved by increasing the pump power and adjusting the waveguide geometry and dopant concentration for the chosen pump power. Use of such rare-earth-ion-doped waveguide amplifiers can provide a solution for compensating the losses occurring in optical interconnects.

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.

Piezoelectric acoustic devices that are integrated with semiconductors can leverage the acoustoelectric effect, allowing functionalities such as gain and isolation to be achieved in the acoustic domain. This could lead to performance improvements and miniaturization of radio-frequency electronic systems. However, acoustoelectric amplifiers that offer a large acoustic gain with low power consumption and noise figure at microwave frequencies in continuous operation have not yet been developed. Here we report non-reciprocal acoustoelectric amplifiers that are based on a three-layer heterostructure consisting of an indium gallium arsenide (In0.53Ga0.47As) semiconducting film, a lithium niobate (LiNbO3) piezoelectric film, and a silicon substrate. The heterostructure can continuously generate 28.0 dB of acoustic gain (4.0 dB net radio-frequency gain) for 1 GHz phonons with an acoustic noise figure of 2.8 dB, while dissipating 40.5 mW of d.c. power. We also create a device with an acoustic gain of 37.0 dB (11.3 dB net gain) at 1 GHz with 19.6 mW of d.c. power dissipation and a non-reciprocal transmission of over 55 dB.

We have proposed and demonstrated a band rejection filter based on grating-assisted forward coupling between an electrooptic polymer waveguide and a passive polymer waveguide. A wavelength tuning by the electrooptic effect induced by poling has been also demonstrated.

Post-fabrication tuning of a polymeric grating-assisted codirectional coupler filter by photobleaching

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.

Linear optical switching in a FLC/waveguide composite device

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.

We report a high-performance erbium-doped tantalum pentoxide (Er3+: Ta2O5) large-mode-area (LMA) waveguide amplifier fabricated by direct etching of the gain medium. The device achieves strong optical confinement and high pump-signal overlap (>98.6%) while maintaining propagation loss of 0.7 dB/cm after annealing. Under 140 mW total pump power at 980 nm, the 2.75-cm-long amplifier delivers a maximum on-chip signal enhancement of 38.1 dB at 1530 nm and 24.1 dB at 1550 nm, corresponding to net gains of 8.6 and 9.6 dB, respectively, with gain exceeding 3 dB/cm across the C-band. Large-signal measurements show a maximum output power of 2.4 mW (1.25 dB net gain), confirming stable high-power operation enabled by the LMA design. A minimum noise figure of 5.68 dB is achieved at 1550.5 nm. This device not only overcomes the fabrication challenges associated with Er3+: Ta2O5 and validates its amplification potential, but also demonstrates excellent large-signal amplification performance, highlighting its strong potential for compact, high-output, and low-noise on-chip optical amplification in integrated photonic systems.

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.

10 Gb/s data transmission with TO-packaged multimode GaAs VCSELs over 1 m long polymer waveguides for optical backplane applications

Optical backplanes are attractive components for systems with high data rates between subsystems and a large number of interconnects. An optical backplane which uses multimode polymer waveguides was originally developed for avionic applications but can be used in telecom switching systems as well. For transmission distances in the range of 100cm and data rates up to 10Gbps, the modal dispersion can be negelected. The waveguides are fabricated on large substrates (aluminum, FR4 and others) by a direct writing technique. Splitters and couplers can be fabricated with the same technique. The waveguides have a low loss (0.04dB/cm) and high temperature stability (up to 250&deg;C) and are used with 840nm vertical cavity lasers. The waveguide cross section can be chosen between approx. 250&mu;m x 250&mu;m and 50&mu;m x 50&mu;m. We have successfully transmitted up to 10Gbit/s over multimode polymer waveguides with lengths of 100cm. A free space, expanded beam coupling is used for the board-backplane transition, resulting in high alignment tolerances. The overall insertion loss for a backplane connection is typically between 2 and 8 dB, depending on waveguide length, radius of curvature, number of waveguide crossings etc. A typical transceiver power budget of 15-20dB allows the integration of star couplers with up to 16 ports. Several test systems with different interconnection schemes have been realized and tested. Tests include mechanical stability (vibration), thermal stability (cycles, shocks and accelerated aging) and gamma irradiation as well as optical power levels, signal integrity and bit error rates.

The system performance of data- and telecommunication equipment must keep up with the increasing network speed. Optical interconnections technology is a promising alternative for high-throughput systems. We demonstrate the optical backplane system using a waveguide-embedded optical backplane and two processing boards. The transmitter and receiver modules were prepared for optical printed circuit boards (PCBs), which consists of the metal optical bench, the driver chips, vertical-cavity surface-emitting lasers (VCSELs), photodiodes, and a tapered polymeric waveguide. We report high-speed transmission of 27–1 pseudorandom bit sequence (PRBS) nonreturn to zero (NRZ) data up to 10 Gbits/s through the optical backplane system. The results demonstrate that the optical backplane system can be practical and valuable for the future high-throughput systems by using metal optical bench and precisely machined optical plug-adaptor structure to achieve stable board-to-board interconnection.

In our work on the amplification of Bleustein-Gulyaev waves in photoconducting CdS crystals, we have found that the gain as a function of electric field is quite symmetric about the synchronous value. We have therefore been able to achieve amplification with a segmented structure consisting of an interdigital electrode. The unequal spacing between fingers produces amplification in the region where the field is positive, which is in excess of the attenuation in the region where the field is negative. The spacings between adjacent electrodes are 500 and 250 μm, giving a periodicity of 850 μm; the length of the active region is 6 mm. The experimental electronic gains obtained were 19 dB at 43 MHz and 32 dB at 86 MHz with an applied voltage of 130 V; 4 dB net gain was obtained at 86 MHz. It has been observed that the electronic gain is sensitive to the photoexciting light wavelength. We give an interpretation of this phenomenon based on the relative penetration depth of the mechanical wave, of the photoexcited carriers, and of drifting electric field.

SnO2-based glass-ceramics activated by rare earth ions have been extensively investigated because of the need to develop reliable fabrication protocols and clarify some interesting optical, structural, and spectroscopic features of the system. There is one important weakness in glass photonics when the rare earth ions are employed as luminescent sources. This is the low absorption cross section of the electronic states of the rare earth ions. A sensitizer is therefore requested. In the last years, we demonstrated that SiO2-SnO2 glass ceramics, presenting a strong absorption cross section in the UV range due to the SnO2 nanocrystal, are effective rare earth ions sensitizers. Another interesting property of the SiO2-SnO2 system is its photorefractivity. The high photorefractivity of sol-gel-derived SnO2-SiO2 glass-ceramic waveguides has been demonstrated in several papers published by our consortium. It has been shown that the UV irradiation induces refractive index change allowing the direct writing of both channel waveguides and Bragg gratings. The results presented in this communication not only demonstrate the viability and outstanding properties of the SiO2- SnO2 glass-ceramics for photonic applications but also put the basis for the fabrication of solid state and integrated lasers. The next steps of the research are the fabrication of the channels and mirrors exploiting the photorefractivity as well as to draw glass ceramic fiber, checking the lasing action and corresponding functional characteristics. Finally, it is worth noting that the dynamic of the energy transfer from the nanocrystals to the rare earth ions is still an exciting open question.