A novel 16-channel optical backplane bus with volume holographic optical
elements (VHOEs), operating as diffraction grating beam alignment guides, was designed
and fabricated for a high-performance computing system multi-slot bus. These thin film
VHOEs were fabricated to diffract light beams for each bus slot into a glass wave-guiding
plate (refractive index 1.52) for total internal reflection to other slot positions. Slot-to-slot
optical alignment issues, including channel crosstalk and beam alignment tolerances, were
computer modeled to optimize a low cost and simple optical packaging structure. For each
slot position, a 4 × 8 element optical packaging plate was then fabricated to allow insertion
of 16 VCSELs and 16 Photodiodes, each in an individual TO-46 can.
Through the VHOE, the slot-to-slot fan-out received beam intensities were
experimentally measured for each of the 16 channels and found to be in the range of 90 &mgr;W
~ 150 &mgr;W. This 90 &mgr;W minimum fan-out power is 5dB greater than the receiver sensitivity
requirement. In this study, the maximum 10 Gbps single channel bandwidth was tested and
a 1.6 Gbps aggregate bandwidth was also demonstrated through a three slot 16-channel
optical backplane bus. This aggregate bandwidth was limited by an electronic element in
the receiver circuit (155 Mbps PD-TIA) and processor (100 Mbps FPGA). With the
system's measured optical isolation of greater than 80dB, and suitably fast receiver
electronics, simulation modeling indicates that Terabit per second bus data rates can be
achieved in inexpensive, mechanically robust and reliable form factors.
A 3-slot optical backplane bus demonstrator based on glass substrate with photopolymer volume gratings array (PVGA)
on top surface is built to allow 16 channels of data to be broadcast from central slot to two daughter slots or uploaded from
any daughter slot to central slot. VCSELs and photodetectors packaged in the form of TO-46 can are assembled on top of
each PVG and interleaved to reduce the crosstalk to below noise level. By carefully aligning the fabrication system, the
incident angle deviation from Bragg condition is reduced to below 0.1° to maximize optical power delivery. The
orientation and period of hologram fringes are uniform in the active area by collimating recording beams.
Above 4.8Gbps aggregated data transmission is successfully demonstrated using the multi-channel system. Three
computer mother boards using FPGA are made to verify the data transmission among the slots. Interface boards between
the FPGA boards and optical transceivers are designed and fabricated to separate the implementation of digital layer and
optical layer. Single channel transmissions with 3.2Gbps and even 10Gbps data rate are also tested with above 100uW
input power, showing the potential to improve the total two-way bandwidth to above 102.4Gbps. Alignment tolerance of
the optical interconnect system is investigated theoretically and experimentally. By analyzing the diffractive
characteristics, the bandwidth limit of the optical layer is determined to be in the order of Terahertz. Design and
fabrication issues are discussed for future optical backplane bus to make terahertz bandwidth into reality. Based on the
experiments for Bit-interleaved Optical Backplane bus and Multi-channel optical backplane bus demonstrators,
theoretical analysis of the bandwidth limit of the optical backplane bus using photopolymer volume gratings has been
carried out.
The primary technical challenge for optical backplanes involve the alignment and optical isolation of multiple data channels. Since most backplanes require data transfer rates greater than a single optical channel can costeffectively provide, multiple data channels is the common solution for higher aggregate transfer rates. Established optical alignment and isolation techniques include spatial separation of optical channels, use of lensing elements to focus specific transmitter outputs to specific receiver areas, use of differing wavelengths for adjacent channels with appropriate frequency filtering on receivers, and the use of "light guide tubes" for each channel. This presentation will examine another promising option, the use of "matched" Holographic Optical Elements (HOEs) to provide both cross channel optical isolation and to significantly relax traditional optical alignment requirements. Matched HOEs can both induce upon a transmitted optical stream, and then filter upon a received optical stream, a number of distinguishing characteristics such as wavelength, polarization, phase, and amplitude. Thus the use of a unique "matched HOE" pair with each transmitterreceiver pair of multiple optical data channels can provide an efficient mechanism to isolate individual data streams even when they may be physically coincident, such as in a length of fibre optic or when multiple free space data transmitters illuminate several channel's receiver elements. Thus, the alignment issue is relaxed from the usual constraint of attempting to physically separate channels to one where, as long as the receiver is within the optical cone of it's matched transmitting element, cross channel interference can be effectively eliminated.
Optical backplane bus based on glass substrate with volume holographic gratings on top surface possesses a great ability to broadcast information. This feature is utilized to accomplish a bit-interleaved optical interconnect system. In this system, each daughter board sends only one bit per round and the bit pulses from different boards can cascade in a designed series when the transmitters are distributed in an appropriate manner. In this way, even slow electronic chips can be coordinated to generate an aggregate bandwidth up to 10Gbps, which is impossible to achieve with a multi-drop electrical bus. Besides the benefits of high data rate and low crosstalk, such a bit-interleaved architecture provides a secure data storage method. Each daughter board only stores a quarter bits of any byte, so that no single board has the entire information and security is enhanced. Alignment tolerance and power budget of the proposed optical interconnect system is theoretically calculated and experimentally verified. With collimating lenses, the packing density of transceivers is more than 4/cm2, and thus the signal density can be above 40Gbps/cm2/board. The insertion loss due to misalignment and beam divergence is measured to be approximately 3dB. The bit error rate (BER) of 10Gbps receivers with -12dBm sensitivity is estimated to be below 10-12.
A high performance polymer waveguide array with 45° micromirrors was fabricated by soft molding to achieve fully embedded board-level optoelectronic interconnects. One-step-transferring of a 3-D polymer structure is demonstrated. Low-loss and thermally stable UV curable polymers based on fluorinated acrylate are chosen as waveguide core and cladding materials. A 45° total interior reflection (TIR) micromirror was formed by two methods: blade cutting and mechanical polishing. And the surface roughnesses are further improved by using a focused ion beam (FIB) technique. The high-quality 45° micromirror was obtained to provide surface-normal light coupling between waveguide and the optoelectronic devices. The measured propagation loss of the multimode waveguide was 0.156dB/cm at 850nm wavelength. The excess loss of the mirror was less than 1.5dB.
We demonstrate a flexible optical waveguide film with integrated Vertical-cavity surface-emitting laser (VCSEL) and positive-intrinsic-negative (PIN) photodiode arrays for fully embedded board level optical interconnects. The optical waveguide circuits with 45° micro-mirror couplers are fabricated on a thin flexible polymeric substrate by soft molding. 45° micro-mirrors on waveguide array for fully embedded board level optical interconnections are investigated both theoretically and experimentally. Smooth mirror surface fabrication is demonstrated by using microtome blade. Thin film VCSEL arrays and PIN photodiode arrays are directly integrated on to the waveguide film. Measured propagation loss of the waveguide was 0.3dB/cm at 850nm.
Short-range optical interconnection is more emphasizing in high performance systems. Multimode waveguide array is considered as a major interconnection medium due to the relatively easy packaging with devices. The multimode fiber array conjunction with VCSEL and Pin photodiode array is widely used in board to board and/or system to system interconnection. We demonstrate a flexible optical waveguide film which was composed of VCSEL, photodiode array, multimode waveguide array and 45 degree micro mirror couplers. The flexible waveguide film has many potentials such as it can be integrated with typical rigid electronic board and free from geometrical constraint. The waveguide film with 45° mirror was fabricated on a flexible transparent substrate using soft molding technique and then thin film VCSEL and photo-detector array are integrated. Master structure of the waveguide, which has multimode waveguide array and 45° mirror structure was fabricated using conventional lithography and microtome technique.
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