Abstract:
In accordance with an aspect of the invention, a system has a transmitter and a receiver, where the transmitter includes a beam source and an optical element. The beam source produces a beam that represents information, and the optical element alters the beam so that the beam has a uniform intensity over a cross-sectional area. The receiver is separated from the transmitter by free space through which the beam propagates and includes an active area positioned to receive a portion of the beam that the receiver converts into a received signal. To accommodate possible misalignment, the cross-sectional area of the beam is larger than the active area by an amount that accommodates a range of misalignment of the receiver with the transmitter.

Description:
BACKGROUND 
       [0001]    High data rate signal transmission is a concern in many systems. Current server systems, for example, often use a set of rack mounted components or user-selected blades that work together to provide data storage, processing, and communications, and the individual components often need to communicate with each other at high data rates to provide the desired system performance. In a server system using blades, the blades, e.g., server blades and storage blades, are mounted in a common enclosure and share components such as cooling fans, power supplies, and enclosure management. In general, data signals transmitted among the blades must collectively provide high data rate transmissions, and with current technology, each data signal may have a bandwidth of about 10 Gbit/s or more. 
         [0002]    Electrical signals generally oscillate at high frequencies to provide high data transmission rates, and the high frequency oscillations can present impedance and noise problems for electrical signals transmitted over copper wires. Optical signaling can avoid many of these problems, but optical signaling may still require complex waveguide systems or dealing with loose optical cables or ribbons. Optical signaling also requires circuits for conversion between optical and electrical signals, and the requirements for such circuits may present challenges. For example, a receiver converting a high data rate optical signal to an electrical signal for an electronic component generally requires a small-area photodiode because of the high frequency of the electrical signal, and directing the optical signal onto the small area of the photodiode generally requires use of precise and reliable alignment systems. Other methods and systems for transmitting high data rate signals that avoid the problems associated with wires and optical fibers or waveguides are sought. 
       SUMMARY 
       [0003]    In accordance with an aspect of the invention, a system has a transmitter and a receiver, where the transmitter includes a beam source and an optical element. The beam source produces a light beam that represents information, and the optical element alters the beam so that the beam has a uniform intensity over a cross-sectional area. The receiver is separated from the transmitter by free space through which the beam propagates and includes an active area positioned to receive a portion of the beam that the receiver converts into a received signal. To accommodate possible misalignment, the cross-sectional area of the light beam is larger than the active area of the receiver by an amount that accommodates a range of misalignment of the receiver with the transmitter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  shows a server system in accordance with an embodiment of the invention employing alignment-tolerant free space data channels for communications among system planes or blades. 
           [0005]      FIG. 2  shows a free space data channel according to an embodiment of the invention employing an optical element to produce a beam with a spatially uniform intensity over a width required for alignment tolerant transmissions. 
           [0006]      FIG. 3  shows a free space data channel according to an embodiment of the invention employing a photodiode and a grating system to enhance collected optical signal strength. 
           [0007]      FIG. 4  is a cross-sectional view of a system in accordance with an embodiment of the invention using reflectors for fan out of an alignment tolerant optical signal. 
       
    
    
       [0008]    Use of the same reference symbols in different figures indicates similar or identical items. 
       DETAILED DESCRIPTION 
       [0009]    In accordance with an aspect of the invention, an optical transmitter for a high data rate signal can use an optical element such as a diffuser to spread the intensity of a free space beam uniformly over an area sufficient to compensate for expected misalignment between the transmitter and a target receiver. The area of the optical beam will typically be large relative to the size of a photodiode in a receiver. As a result, the photodiode will receive uniform power even if the transmitter and receiver are misaligned or move relative to each other because of mechanical vibrations or translations caused by thermal expansion or contractions. In one particular embodiment, separate components such as circuit boards or blades in a server that are mounted roughly parallel to each other can communicate at high data rates using free space optical signals. 
         [0010]      FIG. 1  illustrates a server system  100  in accordance with an embodiment of the invention. System  100  includes a set of blades  110  that are mounted on a shared backplane  120 . Additional components  130  such as power supply transformers and cooling fans can also be connected to backplane  120 , and the entire assembly would typically be contained in a shared enclosure (not shown). A user interface and sockets for external connections to server system  100  may be provided through the shared enclosure. 
         [0011]    Some or all of blades  110  in system  100  may be substantially identical or of differing designs to perform different functions. For example, some blades  110  may be server blades or storage blades. Each blade  110  includes one or more subsystems  112  that implement the particular functions of the blade  110 . Subsystems  112  may be mounted on either one or both sides of each blade  110  in the manner of components on a printed circuit board, or blades  110  may include enclosures with subsystems  112  in the interior of the blade  110 . Typical examples of such subsystems  112  include hard drives or other data storage and processor subsystems containing conventional computer components such as microprocessors, memory sockets, and integrated circuit memory. Subsystems  112  and the general features of blades  120  may be of conventional types known for server systems using blade architectures, such as the c-class architecture of sever systems commercially available from Hewlett-Packard Company. 
         [0012]    Each blade  110  additionally includes one or more optical transceivers  114  or  116 . Each transceiver  114  is positioned on a blade  110  to be nominally aligned with a corresponding transceiver  116  on a neighboring blade  110  when the blades  110  are properly mounted on backplane  120 . Transceivers  114  and  116  may otherwise be substantially identical to each other. In a typical configuration for server system  100 , there may be about 5 cm of free space between corresponding transceivers  114  and  116 , and each aligned pair of transceivers  114  and  116  may be subject to misalignment on the order of about 500 to 1000 μm due to variations in the mechanical mounting of blades  110 . Additionally, the alignment of transceivers  114  and  116  may be subject to variations on the order of 40 to 50 μm due to temperature variations and/or mechanical vibrations, for example, from the operation of cooling fans. 
         [0013]    The receiver section of each transceiver  114  or  116  generally includes a photodiode having a light sensitive area of a size selected according to the data rate of the transmitted signal. For a data rate of 10 Gb/s or larger the width of light sensitive area generally needs to be less than about 40 μm across. An optical communication channel between a pair of transceivers  114  and  116  is made tolerant of the misalignment through use of a beam  118  having a uniform intensity over an area that is sufficient to provide a consistent power to the photodiode regardless of the misalignment. As an example, if the power at the transmitter is around 1 mW and the receiver sensitivity is around 10 μW, the transmitted beam can be expanded to 1 mm 2 , and the receiver only needs to capture 0.1 mm 2  of the transmitted beam to receive 10 μW of power. Typically, a 10-Gbps photodetector has a receiving aperture of around 40 microns, but a small optical element, such as a lens, on the receiving side can collect and focus the received power from a 0.1-mm 2  area into the active area of the photodetector in the receiver. In this example, expansion of the light beam relaxes the required alignment accuracy of transceivers  114  and  116  from a few microns (since the beam must be focused down to a small spot on the photodetector to avoid slow tails) up to a few hundred microns. 
         [0014]      FIG. 2  illustrates a system  200  implementing a misalignment tolerant optical communication channel in accordance with an embodiment of the invention. System  200  includes a beam source  210  and an optical element  220  that may be in an optical transmitter or the transmission part of an optical transceiver such as transceiver  114  or  116  of  FIG. 1 . System  200  also includes a photodiode  230  that may be in an optical receiver or the receiver part of an optical transceiver such as transceiver  116  or  114  of  FIG. 1 . 
         [0015]    Beam source  210  produces a beam  212  that that is modulated in to encode data for transmission. In an exemplary embodiment, beam source  210  includes a laser diode such as a VCSEL and a drive circuit that varies the drive power to the laser diode as needed to produce an amplitude modulation of beam  212 . Alternatively, an acousto-optic modulator or other device can be positioned to encode data in a constant power beam from a laser diode. Beam  212  generally has a non-uniform intensity distribution, which is characteristic to beam source  210 . For example, a typical beam  212  from a laser diode may have an energy or intensity distribution  214  with an approximately Gaussian dependence on radial distance from the center of beam  212 . Additionally, a typical numerical aperture of a beam from a VCSEL is around 0.3. 
         [0016]    Optical element  220  operates as a diffuser to collimate and spread the energy in a beam  222  evenly over an area having a width A 1 . Diffractive optical elements that operate as engineered diffusers suitable for producing a uniform intensity distribution  224  for beam  222  are commercially available through sources such as Thorlabs, Inc, and Suss MicrOptics. However, optical element  220  could alternatively include refractive optics that similarly generate a flat top intensity distribution. For example Dickey et. al. “Beam Shaping: A Review,” in Laser Beam Shaping Applications, Dickey, Holswade and Shealy, CRC p. 269-307, describe systems using microoptical lens arrays to convert an input laser beam into multiple beamlets while a second lens array in combination with a spherical lens superimposes the images of each of the beamlets in the first array into the homogenization plane. The cross-sectional area of uniform intensity produced by optical element  220  can be of any desired shape, e.g., circular, rectangular, or square, but is preferably chosen to cover an expected range of misalignment and/or movement of photodiode  230  relative to beam source  210 . With such a configuration, the photoactive area of photodiode  230  will remain in an area of uniform beam intensity and receive a constant beam power, even when photodiode moves as a result of vibrations. 
         [0017]    System  200  when applied in a server system such as illustrated in  FIG. 1  can transmit data at about 10 Gbit/s. At such frequencies, photodiode  230  preferably has a small photoactive area, e.g., with a width A 2  on the order of about 40 to 50 gm. Beam  222  should have a width of about 100 μm for the expected maximum misalignment of about 40 to 50 μm resulting from mechanical vibrations, thermal variations, and fixed alignment error expected in a conventional blade server system. In this configuration, photodiode  230  receives about 25% of the optical power in beam  222 . Thus with a 1 mW intensity laser, the receiver receives about 250 μW of power at the photodetector even with a 50 μm misalignment. Expanding the beam to about a width of about 1 mm, for greater misalignment tolerance, will yield a received power of 2.5 μW at the 50-μm detector, but adding a lens  240  at the receiver allows capture of more power while still providing substantial misalignment tolerance. In general, the amount of received power required to detect the transmitted data can be traded off with misalignment tolerance. 
         [0018]      FIG. 3  illustrates a system  300  using a beam source  210  and an optical element  220  as in system  200  of  FIG. 2  to produce a beam  222  having a uniform intensity distribution across a cross-section of beam  222 , but system  300  further uses a grating structure  350  to enhance collection of light from beam  222  into photodiode  230 . Such grating structures are known and further described, for example, by Yu et al., “Design of Midinfrared Photodetectors Enhanced by Surface Plasmon on Grating Structures,” Appl. Phys. Lett. 89. 115116 (2006). Grating structure  350  concentrates beam  222  on photodiode  230  to provide a stronger signal and reduce the power wasted from beam  222 . 
         [0019]    A free space communication channel similar to that illustrated in  FIG. 3  can alternatively use grating structure  350  to collect signal energy from a non-uniform signal beam. The optical channel would still be tolerant of misalignment provided that area of grating structure  350  is sufficient that the portion of the signal beam incident on grating structure  350  for the range of expected misalignments of photodiode  230  and beam source  210  is sufficient to detect the transmitted data. 
         [0020]      FIG. 4  illustrates a system  400  in accordance with an embodiment of the invention using a free space optical signal to distribute communications from a transmitting circuit unit  412  in a first system plane  410  to circuit units  414  in the same system plane  410  or circuit units  424  and  426  in another system plane  420 . System planes  410  and  420  may be circuit boards, blades in a server system such as illustrated in  FIG. 1 , or similar systems having components with electrical circuit units  412 ,  414 ,  424 , and  426  that may require high data-rate communications. System planes  410  and  420  can be mounted on a shared base or back plane  430  so that system planes  410  and  420  are substantially parallel to each other and substantially perpendicular to back plane  430 . 
         [0021]    In operation, transmitting circuit unit  412  in system plane  410  controls a beam source  210  to encode desired data in a light beam  212  that beam source  210  transmits through a diffractive element  220 . The diffractive element  220  distributes beam energy to produce a uniform intensity beam  222  that is transmitted through free space to system plane  420 . Beam  222  has a uniform intensity across an area such that a photodiode  230  on system plane  420  receives uniform power for any position of photodiode  230  with a range of expected misalignment of beam source  210  and photodiode  230 . Photodiode  230  can thus receive and convert a portion of beam  222  into an electrical signal provided to circuit unit  424  in system plane  420 , and the magnitude of the electrical signal is not subject to unacceptable variations due to alignment errors or mechanical vibrations. The mechanical vibrations are at extremely low frequencies, typically less than 1 kHz while the transmitted data is on the order of a few hundred megahertz to several or tens of gigahertz, thus the movement of the beam due to vibrations will not degrade the received data so long as the portion of the transmitted beam does not deviate outside the photodiode area. If the expected trajectory of the mechanical vibrations from the transmitting board is known, the receiving aperture, which includes either a grating or lens, can be patterned to match the trajectory of the transmitted beam, thus improving the alignment tolerance. 
         [0022]    A reflector  435  around photodiode  230  is designed and positioned to reflect a portion of beam  222  back through free space toward system plane  410 . When photodiode  230  is small relative to the area of beam  222 , reflector  435  can direct most of beam  222  back to system plane  410 . Reflector  435  can be a flat mirror or focusing element such as a parabolic mirror that directs beam  222  onto a diffractive element  440 . Diffractive element  440  is a diffuser that is positioned and engineered to produce a collimated beam  442  having uniform intensity across an incident area on plane  410  sufficient to contain a photodiode  450 . Photodiode  450  can then convert a portion of beam  442  into an electrical signal representing the data from transmitting circuit unit  412  and provide the electrical signal to circuit unit  414  in system plane  410 . Use of optical beams  222  and  442  to relay data from transmitting circuit  412  to receiving unit  414  avoids impedance and noise issues that might arise if data were transmitted in system plane  440  a significant distance, e.g., several cm or more, using a high-frequency electrical signal. In this way, system  400  can “re-use” the transmitted beam to broadcast the data to other portions of system plane  410  or to another adjacent system plane. The amount of the “drops” will depend again on the receiver sensitivity and the losses of the system. 
         [0023]    A reflector  455  is around photodiode  450 , so that of beam  442  is incident on reflector  455 . Reflector  455  may be flat or curved as needed to reflect a portion of beam  442  through a diffractive optical element  460 . Diffractive element  460  then produces a beam  462  having a uniform intensity across a desired cross-section. The area of uniform intensity is of a size sufficient to tolerate relative misalignment of a second photodiode  470  on system plane  420 . Photodiode  470  can then convert a portion of beam  462  into an electrical signal representing the data from transmitting circuit unit  412  and provide the electrical signal to circuit unit  426  in system plane  420 . 
         [0024]    Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.