Optical communications – Optical transceiver – Including optical fiber or waveguide
Reexamination Certificate
2000-06-02
2003-07-01
Pascal, Leslie (Department: 2633)
Optical communications
Optical transceiver
Including optical fiber or waveguide
C398S141000, C398S183000, C398S128000, C398S201000, C398S212000
Reexamination Certificate
active
06585432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optoelectronic communication system, and more particularly to an optoelectronic communication system in a turbulent medium that employs a receiver having an array of optical fibers and photodetectors.
2. Description of the Prior Art
The telecommunications industry is switching rapidly to a hybrid platform which utilizes both electronics and photonics to increase the operating bandwidth of the communication system. Today's communication systems consist of optical fiber networks, fiber amplifiers, optical diode transmitters, and high speed semiconductor receivers. This architecture works well in the confines of optical fibers, however, free space propagation of these signals which is necessary for remote applications have problems in matching optical fiber network bandwidths.
Free space propagation of the signal through the atmosphere, water or other turbulent media introduces fluctuating distortions and aberrations. These fluctuating distortions prevent focusing the signal beam onto the small area high speed detectors typically utilized in optical communication systems. An optical transmitter, for example, including a 0.05 m collection dish with a focal length of 1 m would concentrate the light into a diameter of 200 &mgr;m for a 100 times diffraction limited optical communication beam. Aberrations similar to this could be incurred by atmospheric propagation at a wavelength of 1.5 &mgr;m. This area is approximately 300 times larger than that of the high speed semiconductor photodiode detectors employed in conventional communication systems. One approach is to correct for the fluctuating distortions with adaptive optics or phase conjugation techniques in order to obtain a near diffraction limited signal beam which allows focusing onto a small high speed receiver. These techniques suffer from the slow response time, limited phase front correction or high signal intensities required for efficient conjugation. Another approach is to use a large area detector in the optical receiver so that a significant fraction of the distorted signal beam can be collected by the receiver. This method has many advantages but has proven difficult to implement since the detector temporal response and the detector area are often inherently coupled.
What is needed, therefore, is an optoelectronic communication system with integrated high speed small area detectors in an optoelectronic receiver architecture which provides a large effective area receiver while it retains the large intrinsic bandwidth of the individual detector elements.
In addition, it is desirable for the optoelectronic receiver to develop time compensated electrical signals.
SUMMARY OF THE INVENTION
Transmission of an optical signal through a turbulent media, such as the atmosphere, produces a fluctuating spatial intensity pattern due to optical distortions and aberrations. With respect to
FIGS. 1A
,
1
B, and
1
C, three views are shown of an optical signal being transmitted through a turbulent media at three instants of time, t
1
, t
2
and t
3
, respectively. These time varying distortions make it impossible to focus the signal beam onto a single small high speed detector illustrated by the numeral
10
typically utilized in optical communication systems. The present invention involves collecting either a large enough subarray of the distorted signal (shown by the numeral
12
) or the entire distorted signal (shown by the numeral
14
and encompassing the periphery in
FIGS. 1A
,
1
B and
1
C) with a detector array. The collected signal is invariant to the fluctuating distortions, thereby eliminating problems in free space propagation of optically transmitted high bandwidth signals.
The invention involves combining high speed small area photodetectors utilizing an optoelectronic interconnect, preferably with a time compensated reading (probe) beam methodology into a receiver to increase the effective detector area while maintaining the temporal response (high bandwidth). The large effective area detector captures all of the signal beam thereby eliminating problems with regard to optical signal transmission through turbulent media. This detector array directly measures temporal pulses, however, it is applicable to both temporally, phase or frequency encoded signal sources. Phase encoded information would require an optical interferometer before the photodetector array to convert the phase modulation into temporal information.
In another aspect, this invention also involves using a separate transmission beam and reading (probe) beam. A decoupled architecture allows each of the optical wavelengths to be optimized for their own individual function. For example, the transmission wavelength can be chosen to increase signal throughput through the turbulent media while the reading beam could allow for dispersion free fiber propagation, the highest photodetector efficiency or the fastest temporal response.
The described construct utilizes optoelectronic sampling to combine the electric fields from each of the photodetectors in the array. Optoelectronic sampling utilizes an electro-optic material (for example LiTaO
3
, LiNiO
3
, GaAs, or birefringent polymers) with the birefringent axis properly oriented with respect to the electric field and an optical probe beam. The probe beam can either be continuous wave to provide real time signal processing or pulsed (i.e., mode-locked, Q-switched) to provide a high speed time gated detector capability. Depending on the orientation of the electric field and optical probe beam polarization relative to the birefringent axis of the electro-optic crystal this field can be made to induce a time dependent polarization rotation or a time dependent phase change on the optical probe beam. This change varies with the electric field strength and therefore with the light intensity incident on the photodetector. Typically the electro-optic response can be made to be either quadratic (homodyne configuration) or linear with the electrical field strength (heterodyne configuration) depending on the probe beam configuration. The temporal resolution of electro-optic sampling is limited by the propagation delay of the optical probe beam across the region where the electric field exists and by the optical phonon band which is materially dependent and typically occurs in the 3-10 THz range. This corresponds to 100-300 fs, respectively, for a full width period at half maximum frequency. For the small area photodetectors having a diameter of approximately 15 &mgr;m that are utilized in today's high bandwidth communications systems, the propagation delay across individual elements would be of the order of 100 fs. Depending upon the temporal response of the individual photodetector elements, the effective area of the photodetector array and the required temporal response (bandwidth) of the array different time compensation configurations can be adopted for the receiver.
Calculations of the effective areas of photodetector arrays for the following architectures, assuming an intrinsic detector element response time of 5 ps and a detector array requirement of less than 6 ps temporal resolution indicate that (1) Without time compensation—the effective detector area can be increased by 33 times; (2) For one dimensional time compensation where an optical element is utilized to compensate the signal across each row (or column) of fibers so one element of each row (or column) is time coincident with the probe beam, the effective detector area can be increased by 1000 times. The optical element could be an optical wedge or stacked optical plates; and (3) For two dimensional time compensation (i.e., element by element time compensation) where an optical element is utilized to compensate the signal at each photodetector element, there is no limit to the effective receiver area with regard to temporal resolution. This two dimensional optical element could be a stepped optical wedge or the time compensating could be accomplished
Keller Robert W.
Northrop Grumman Corporation
Pascal Leslie
Singh Dalzid
LandOfFree
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