Free space optical interconnect system

Details Free space optical interconnect system Free space optical interconnect system

1999-07-16

2004-08-10

Sedighian, M. R. (Department: 2633)

Optical communications

Transmitter and receiver system

Including compensation

C398S119000, C398S122000, C398S151000, C398S156000, C398S159000, C398S167000

Reexamination Certificate

active

06775480

ABSTRACT:

FIELD OF THE INVENTION
Background of the Invention
Free space optical interconnect systems have long been proposed to deliver fast, highly parallel data transfer. These systems have the potential to obviate limitations of electrical interconnects, which are not capable of supporting data throughputs beyond a capacity of several hundred Gb/s, and to increase the capacity up to the Terabit/s range. Thus free space interconnect systems are promising and attractive alternatives for various telecommunication and computing applications.
However, the most important challenge preventing the current acceptance of free space interconnect systems is alignment. Two issues are of concern: the precision to which it is possible to align the system, and the precision to which it is necessary to maintain this alignment during operation. For practical applications it is necessary to establish and maintain alignment of circuit boards carrying transmitters and receivers, which may comprise an array of pixels, to within 10's of microns over a distance of meters. Such a system requires extremely expensive highly precision optomechanics, and to date has been implemented only in a controlled laboratory environment. In real product usage, when vibrations, temperature fluctuations and temperature gradients are encountered, the optical links move out of alignment and data is not correctly transferred.
Therefore, the goal of providing some alignment tolerance for optical links is to ensure the correct operation of all of the pixels on each array at the highest possible speed. Correct operation is defined as the correct reception of a logic 1 or logic 0 signal. Once the laser power, the receiver sensitivity and the detector area have been defined, the probability of correct reception of the logic bits is mainly a function of optical beam misalignment. Misalignment mechanisms can be due solely to mechanical movements, but in practice, optical effects can also contribute. Six degrees of freedom of the mechanical movements: translation in x, y, and z (&Dgr;x, &Dgr;y, &Dgr;z) and rotation about the x, y, and z axes (&thgr;
x
, &thgr;
y
, &thgr;
z
), where x and y axes define the plane of an optical module in its nominal alignment position, with z axis being perpendicular to this plane, result in a number of optical effects. These include an image shift (&Dgr;x, &Dgr;y), image rotation (&thgr;
z
), defocus (&Dgr;z) and image tilt (&thgr;
x
, &thgr;
y
) Image shift and rotation are basically lateral translation effects, and defocus and image tilt introduce defocus effects. Contributors to the overall lateral misalignment effects include:
mechanical misalignment in x and y;
mechanical rotation about the z axis;
mismatches in focal lengths;
wavelength shifts and laser mode-hops caused by temperature fluctuations and resulting in beam deflections introduced by diffractive elements;
distortions of the image of an array of sources by the interconnect lens system, and
telecentricity, when defocus, in addition to increasing spot size, introduces lateral misalignments in nontelecentric systems.
Contributors to the overall defocus effects include:
source array tilt;
image tilt;
curvature of the plane of best focus;
mechanical tilt about x and y axes;
misalignment along z axis.
Numerous attempts have been made to increase alignment tolerance for optical interconnect systems which may be categorized as passive, active, or dynamic strategies.
However, passive alignment of dense, high speed free space optical interconnects for distances of more than 1 cm require mechanical support structures that are too expensive, difficult to align, and insufficiently stable for commercial applications, see, e.g., “Optoelectronic ATM switch employing hybrid silicon (MOS/GaAs) FET-SEEDS”, A. L. Lentine et al., SPIE Proceeding, vol. 2692, pages 110-108, 1996; and “Optical bus implementation system using Selfoc lenses”, K. Namanaka, Optics Letters, Vol. 16, No. 16, pp. 1222-1224, August, 1991. Passive alignment is done before any devices are powered up. This alignment technique is used in almost all electrical connectors, and most optical fiber connectors are passive. Recently, solder bump techniques have been applied to certain free space optical interconnect components, and preliminary reports indicate the potential for submicron alignment in all 6 degrees of freedom over a scale of up to 1 cm, J. W. Parker “Optical Interconnection for Advanced Processor Systems: A Review of the ESPRIT II OLIVES Program”, L. Lightwave Technology 9 (12), 1764-1773, 1991.
Active alignment requires some feedback about the quality of the alignment. Usually the feedback is achieved by illuminating the system and monitoring the alignment either visually or by measuring a photocurrent in the detectors. Real-time active alignment is necessary if the alignment tolerances are tight or the system stability is poor so that the system will not remain aligned for a reasonable length of time. In this case, the feedback and alignment actuators must be integrated into the system to ensure permanent alignment. For example, CANON manufacturer uses image recognition and active beam-steering using a liquid filled variable angle prism in a single channel 155 Mb/s link product, which currently costs $100K. The product uses built in viewing cameras and optical pattern recognition techniques to define the system alignment, the complexity and cost of such a system clearly limiting widespread application. Alternatively, NTT has a system using actively controlled variable angle liquid filled prisms for board to board parallel free space optical interconnect, see. e.g. “Optical beam direction compensating system for board-to-board free space optical interconnection in high-capacity ATM switch”, K. Hirabayashi et al., Journal of Lightwave Technology, Vol. 15, No. 5, May 1997. Cost, size, environmental ruggedness and reliability of these systems remain concerns.
Additionally, to develop both a marketable and reliable system, devices have to be packaged in a useful and reliable manner. For large systems including cumbersome and bulky mechanical parts providing alignment, this could involve an significant amount of physical space just to house all the individual components.
Recently, a proposal for avoiding high precision mechanics in free space interconnect systems by use of redundant detectors has been put forward by F. A. P. Tooley in IEEE Journal of Selected Topics in Quantum Electronics April 1996, vol. 2, No. 1, pp. 3-13 and in Digest, IEEE Summer Topical Meetings, Aug. 5-9, 1996, p. 55-56. This system increases tolerance to misalignment by providing an array of detectors in place of a single detector and electrically re-routing the misaligned optical data to the correct channel, or, alternatively, by replicating the signal a number of times. The overhead associated with increasing the alignment tolerance requires a control and router circuit, which adds electrical power consumption.
Therefore a need exists for development of alternative structures for free space optical interconnect systems which would avoid high precision mechanics, while providing precise alignment combined with simple design, reliability, low power consumption and compact packaging.
SUMMARY OF THE INVENTION
Thus, the present invention seeks to provide an optical interconnect system and method which avoid or reduce the above-mentioned problems.
Therefore, according to one aspect of the present invention there is provided a free space optical interconnect system comprising:
a transmitter and a receiver, at least one of the transmitter and the receiver comprising a plurality of elements arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted;
means for identifying a misalignment between the transmitter and the receiver; and
means for re-routing data from the cluster which is misaligned to a redundant cluster providing data transmission through the system, the re-routing being perform

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