US007230227B2
`
`(12) United States Patent
`US 7,230,227 B2
`(10) Patent No.:
`Wilcken et al.
`(45) Date of Patent:
`Jun. 12, 2007
`
`(54)
`
`(75)
`
`LENSLET/DETECTOR ARRAY ASSEMBLY
`FOR HIGH DATA RATE OPTICAL
`COMMUNICATIONS
`
`Inventors: Stephen K. Wilcken, Seattle, WA (US);
`Jonathan M. Saint Clair, Seattle, WA
`(US)
`
`(73)
`
`Assignee: The Boeing Company, Chicago, IL
`(US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 8 days.
`
`(21)
`
`Appl. No.: 10/961,173
`
`(22)
`
`Filed:
`
`Oct. 8, 2004
`
`(65)
`
`(51)
`
`(52)
`
`(58)
`
`(56)
`
`Prior Publication Data
`
`US 2006/0076473 A1
`
`Apr. 13, 2006
`
`Int. Cl.
`
`(2006.01)
`H03F 3/08
`US. Cl.
`.............................. 250/214 A; 250/208.2;
`398/202; 330/308
`Field of Classification Search ............ 250/214 A;
`398/202; 330/308
`See application file for complete search history.
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,282,527 A
`4,477,814 A
`5,034,997 A
`5,214,438 A
`5,327,149 A
`5,343,033 A *
`5,479,595 A
`5,760,942 A *
`
`8/1981 Winderman et a1.
`10/1984 Brumbaugh et a1.
`7/1991 Iwasaki
`5/1993 Brusgard et a1.
`7/1994 Kuffer
`8/1994 Cain ....................... 250/208.2
`12/1995 Israelsson
`6/1998 Bryant
`....................... 398/208
`
`6,049,593 A
`6,285,481 B1
`6,307,521 B1
`6,567,200 B1 *
`
`4/2000 Acampora
`9/2001 Palmer
`10/2001 Schindler et a1.
`5/2003 Pammer et a1.
`
`............. 398/202
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`W0
`
`WO 02/32020
`
`4/2002
`
`OTHER PUBLICATIONS
`
`Tao, et a1. “Wideband fully differential CMOS transimpedance
`preamplifier,” Electronics Letters 39(21): Oct. 16, 2003; 2 pages.
`
`(Continued)
`
`Primary ExamineriThanh X. Luu
`Assistant ExamineriStephen Yam
`(74) Attorney, Agent, or FirmiTimothy K. Klintworth;
`Wildman, Harrold, Allen & Dixon, LLP
`
`(57)
`
`ABSTRACT
`
`An assembly is provided that may be used in high data rate
`optical communications, such as free-space communication
`systems. The assembly may include a main optical receiver
`element and a lenslet array or other optical element disposed
`near the focal plane that collects an optical signal and
`focuses that signal as a series of optical signal portions onto
`a photodetector array, formed of a series of lnGaAs photo-
`diodes, for example. The electrical signals from the photo-
`detectors may be amplified using high bandwidth transim-
`pedance amplifiers connected to a summing amplifier or
`circuit that produces a summed electrical signal. Altema-
`tively, the electrical signals may be summed initially and
`then amplified via a transimpedance amplifier. The assembly
`may be used in remote optical communication systems,
`including free-space laser communication environments, to
`convert optical signals up to or above 1 Gbit/s or higher data
`rates into electrical signals at 1 Gbit/s or higher data rates.
`
`25 Claims, 4 Drawing Sheets
`
`
`
`CLOCK/DATA RECOVERY
`CIRCUIT
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`114
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`116
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`APPLE 1042
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`1
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`APPLE 1042
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`

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`US 7,230,227 B2
`Page 2
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`U.S. PATENT DOCUMENTS
`
`9/2003 Stann ........................ 356/509
`6,618,125 B2 *
`6,834,165 B2* 12/2004 Feng
`398/202
`
`1/2006 Buckman et al.
`6,983,110 B2 *
`398/212
`
`2001/0026390 A1* 10/2001 Braun .................. 359/189
`.. 250/214 SW
`2002/0109076 A1*
`8/2002 Tochio et al.
`
`....... 398/202
`2005/0047801 A1*
`3/2005 Schrodinger
`
`............ 250/214 A
`2005/0218299 A1* 10/2005 Olsen et al.
`
`OTHER PUBLICATIONS
`
`Ambundo, et al. “Fully Integrated Current-Mode Subaperture
`Centroid Circuits and Phase Reconstructor,” 10th NASA Syrnp.
`VLSI Design, Albuquerque, NM Mar. 2002.
`Ribak, et al. “A fast modal wave-front sensor,” Optics Express
`9(3):152-157 (2001).
`“New Paint Compounds Provide Early Detection of Corrosion to
`Aircraft”; AFSOR: Research Highlights Jul/Aug. 1999.
`Ballard, et al., “MTI Focal Plane Assembly Design and Perfor-
`mance” SPIEilmaging Spectrometry V, Denver, CO (US), Jun. 17,
`1999.
`
`Oh, et al. “A 2.5Gb/s CMOS Transimpedance Amplifier Using
`Novel Active Inductor Load,” 27th European Solid-State Circuits
`Conference, Villach, Austria, Sep. 18-20, 2001.
`
`* cited by examiner
`
`2
`
`

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`U.S. Patent
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`Jun. 12, 2007
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`Sheet 1 0f4
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`US 7,230,227 B2
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`
`
`CLOCK/DATA RECOVERY
`CIRCUIT
`
`116
`
`FIG. 2
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`104
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`104
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`104
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`104
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`1
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`1
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`TIA ASSEMBLY
`
`3
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`

`

`U.S. Patent
`
`Jun. 12, 2007
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`Sheet 2 0f 4
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`US 7,230,227 B2
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`4
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`

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`U.S. Patent
`
`Jun. 12, 2007
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`Sheet 3 0f 4
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`US 7,230,227 B2
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`FIG. 4
`
`410
`
`414
`
`4E
`
`420
`
`404
`/
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`408 q ‘ y w}
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`410 =
`
`412
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`416
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`=
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`414
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`=
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`422
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`424
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`40844 . y
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`410:
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`412
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`416
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`414
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`408-=r A w
`}
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`408~——r A w
`>
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`5
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`

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`U.S. Patent
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`Jun. 12, 2007
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`Sheet 4 0f 4
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`US 7,230,227 B2
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`FIG. 7
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`706
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` 702
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`710
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`Assembly
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`Optical Collection
`Element
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`Optical
`Element!
`Detector
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`6
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`US 7,230,227 B2
`
`1
`LENSLET/DETECTOR ARRAY ASSEMBLY
`FOR HIGH DATA RATE OPTICAL
`COMMUNICATIONS
`
`FIELD OF THE INVENTION
`
`The present disclosure generally relates to optical com-
`munications and, more particularly, to optical detector arrays
`that convert an unfocused or blurred spot comprising an
`optical signal into an electrical signal by summing signals
`from detectors forming the array.
`
`BACKGROUND OF THE RELATED ART
`
`Optical communication is becoming the preferred method
`for secure, high-bandwidth communications. Fiber-based
`communication systems, for example, are used in environ-
`ments where the access points are known and fixed, and
`free-space communication systems are used in remote appli-
`cations where access points may vary. For the latter systems,
`compact, lightweight, field deployable receivers are desired.
`Yet,
`the size of existing communications equipment has
`limited the effectiveness of such devices.
`
`To understand the problems with free-space communica-
`tions systems, one may look to the environments in which
`these systems operate. In a standard configuration, a laser
`transmitter produces an information carrying laser beam that
`transmits that signal through air. A remote user then uses an
`optical receiver to detect and demodulate that signal, to
`obtain an electrical rendition of the original laser signal.
`Over great travel distances, however, the original laser beam
`will expand and distort in response to anomalies in the air
`medium through which the beam travels. Turbulence in the
`atmosphere, for example, may distort the laser beam and
`produce a twinkling or blurring effect
`that
`represents
`changes in intensities and phase across the laser beam
`wavefront.
`
`Additionally, atmospheric turbulence and poor optical
`quality receivers prevent the laser beam from being focused
`to a point at the remote location. Rather, the laser beam is
`only focused down to a blurred spot by the receiver. In other
`words, although the optical beam may originate from a laser
`point source,
`in free-space communication systems,
`that
`laser point source is imaged to a two-dimensional blur spot
`at the remote receiver.
`
`As a result of this blur spot, a larger detector is needed to
`collect the available energy in the laser signal. In fact, in
`remote applications, there is so much signal intensity loss
`over the propagation path that it is desirable to collect as
`much of the received optical signal as possible, which means
`that larger diameter optical receivers must be used. Larger
`optical receivers, however, increase weight and reduce port-
`abilityitwo things undesirable for
`remote deployable
`receivers. Larger detectors also slow receiver responsive-
`ness, because the intrinsic capacitance of larger detectors is
`larger, and scales with the area of the detector which means
`larger parasitic effects and longer response times. These
`performance limitations also adversely affect the bandwidth
`(and thus operating data rates) of optical receivers, prevent-
`ing them from being used in high data rate applications.
`
`SUMMARY OF THE INVENTION
`
`An embodiment of the invention is an optical device for
`converting optical energy extending over a two-dimensional
`spot into electrical energy. The optical device may include:
`a focal plane assembly; a photodetector array having a
`
`2
`
`plurality of photodetectors each positioned to collect at least
`a portion of the optical energy from the focal plane assem-
`bly; a transimpedance amplifier assembly; and a summing
`amplifier assembly coupled to the transimpedance amplifier
`assembly to produce a summed electrical signal represen-
`tative of the optical energy over the two-dimensional spot.
`Another embodiment of the invention includes a method
`
`of converting a high-data rate optical signal extending over
`a two-dimensional spot into a high-data rate electrical sig-
`nal. The method may include: focusing the optical signal
`onto a photodetector array; converting the optical signal into
`a plurality of electrical signals; disposing a transimpedance
`amplifier assembly to amplify each of the plurality of
`electrical signals; and summing each of the amplified elec-
`trical signals, to produce a summed electrical signal repre-
`sentative of the optical signal over the two-dimensional spot.
`A further embodiment of the invention includes a method
`
`of converting a high-data rate optical signal extending over
`a two-dimensional spot into a high-data rate electrical sig-
`nal. The method may include: focusing the optical signal
`onto a photodetector array; converting the optical signal into
`a plurality of electrical signals; disposing a summing ampli-
`fier assembly to sum each of the plurality of electrical
`signals; and disposing a transimpedance amplifier assembly
`to amplify the sum of each of the plurality of electrical
`signals, to produce a summed electrical signal representative
`of the optical signal over the two-dimensional spot.
`Some embodiments provide an optical receiver assembly
`that may be used to collect the largest practical amount of
`laser energy transmitted across a free-space region from a
`transmitter. Near the focus of the receiver, the assembly may
`include an optical element or array of optical elements
`placed near the detector array that have a size sufficient to
`capture and further concentrate the optical energy contained
`in the blurred optical spot. The captured optical energy is
`focused to a photodetector array that has a series of small
`photodetection elements of relatively low circuit capaci-
`tance. In some examples, InGaAs photodiodes are used. In
`some examples, the electrical signals from these photode-
`tectors are collected and either summed and then amplified
`or amplified individually and then summed by a series of
`high bandwidth transimpedance amplifiers. Different types
`of transimpedance amplifiers architectures may be used. For
`example, CMOS fabricated singled-ended or differential
`transimpedance amplifiers may be used to achieve high
`bandwidths and thus high data rates, for example 1 Gbit/s or
`higher. In some examples, the summing circuitry and tran-
`simpedance amplification can support data rates high
`enough to support high-definition modulated information to
`produce an optical communication that can receive free-
`space high definition video signals modulated on a laser
`signal.
`Optical assemblies may be used in remote communication
`environments where signal intensities are generally quite
`low. Further, the optical assemblies may be used in portable,
`field-deployed applications along with optical receivers and
`wavefront correction elements.
`
`The features, functions, and advantages can be achieved
`independently in various embodiments of the present inven-
`tion or may be combined in yet other embodiments.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates a diagrammatic view of an optical
`device including a focal plane assembly lenslet array, tran-
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`US 7,230,227 B2
`
`3
`simpedance amplifier assembly, and summing amplifier
`assembly that may be used in high data rate optical com-
`munications.
`
`FIG. 2 illustrates a diagrammatic view of another optical
`device similar to that
`illustrated in FIG. 1, but with a
`summing pre-amplifier assembly and a transimpedance
`amplifier.
`FIG. 3 illustrates an example focal plane assembly (e.g.,
`a lenslet array) and a photodetector array.
`FIG. 4 illustrates a circuit diagram of an example imple-
`mentation of the transimpedance amplifier assembly and
`summing amplifier assembly of FIG. 1.
`FIG. 5 illustrates a circuit diagram of an example imple-
`mentation of the summing amplifier assembly and transim-
`pedance amplifier assembly of FIG. 2.
`FIG. 6 illustrates a circuit diagram of an example differ-
`ential transimpedance amplifier.
`FIG. 7 illustrates a hybridized photodetector array with a
`read-out integrated circuit, in accordance with an example.
`FIG. 8 illustrates a diagrammatic view of an optical
`system capable of collecting high data rate optical commu-
`nication signals and detecting those signals Via an optical
`element detector apparatus such as that illustrated in FIGS.
`1 and 2.
`
`DETAILED DESCRIPTION OF AN EXAMPLE
`
`To detect optical radiation like laser signals, an optical
`device may have a focal plane assembly (FPA) able to
`collect
`the optical energy and direct
`that energy to an
`optoelectronic converter, such as a photodetector array. The
`electrical signals from the photodetector array may then be
`applied to amplifiers and analog or digital summing ele-
`ments to produce a rendition of the original laser signal
`captured by the FPA. FIGS. 1 and 2 illustrate two different
`configurations of optical devices that may be used in such
`high data rate optical communication systems.
`FIG. 1 illustrates an optical device 100 that may be used
`to collect and convert optical signals having data rates of
`approximately 1 Gbit/s or higher, and in some examples
`approximately 10 Gbit/s or higher. The device 100 may be
`used to collect optical energy having a frequency of typically
`between approximately 1 GHz and 50 GHz, for example.
`The device 100 includes an FPA in the form of a lenslet
`
`array 102 that may collect optical energy from a laser
`transmitter, not shown. The array 102, discussed in further
`detail below, may be a two-dimensional array of lenslets 104
`adj acently abutting one another to maximize the amount of
`optical energy collected by the array 102. The lenslets 104
`may be formed of any suitable optically-transparent mate-
`rial. Materials will depend on the wavelength, for example,
`quartz for visible wavelengths, or silicon for shortwave
`infrared (SWIR) wavelengths. In the illustrated example, a
`photodetector element 106 is positioned below each lenslet
`104, for example, at a focal distance and performs optical-
`to-electrical conversion. As explained in further detail
`below, the photodetectors 106 may be semiconductor PIN or
`avalanche photodiodes, such as InGaAs, SiGe, InP and
`InGaP, for example.
`The photodetector elements 106 may be formed into a
`photodetector array 108 that is coupled to a read out inte-
`grated circuit (ROIC) 109 which includes transimpedance
`amplifier assembly 110 and a summing amplifier assembly
`112. A transimpedance amplifier (TIA) assembly 110 may
`include an array of transimpedance amplifiers, one for each
`photodetector element 106. Example transimpedance ampli-
`fiers are described below and, yet, others will be known to
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`the TIA
`persons of ordinary skill in the art. In general,
`assembly 110 comprises high data rate transimpedance
`amplifiers with an RC time constant low enough to amplify
`electrical signals having Gbit/s data rates while still provid-
`ing a large enough gain value to produce a summed signal
`detectable by downstream circuits, such as those used in
`optical communication systems. To further reduce the RC
`time constant of the individual transimpedance amplifiers
`and further improve response time, the size of the corre-
`sponding photodetector elements 106 may be decreased to
`reduce input capacitance at the transimpedance amplifiers.
`The TIA assembly 110 is coupled to the high-data rate
`summing amplifier assembly 112. Each assembly 112 col-
`lects (receives or senses) and adds electrical signals from the
`TIA assembly 110 together to produce a summed electrical
`signal 114 representative of all optical energy collected by
`the array 102. The summed signal 114, therefore, may have
`an electrical energy proportional to the optical energy col-
`lected by the array 102 and a high date rate, the same, or
`substantially similar, to the data rate of that optical energy.
`The summed signal 114 may be coupled to a display not
`shown. The ROIC 109 may provide the output signal 114 to
`a clock/data recovery circuit 116, for example, a micropro-
`cessor-based circuit or application specific integrated circuit
`(ASIC) that may strip modulated data from the electrical
`signal 114.
`FIG. 2 illustrates an optical device 200 similar to device
`100 and therefore like reference numerals are used. The
`
`lenslet array 102 and photodetector array 108 are positioned
`to produce electrical signals 202a, 202b, 2020, and 202d,
`each representative of an optical energy collected by one of
`the lenslets 104. The signals 20211720251 are collected and
`summed in a ROIC 203 that, in the illustrated example,
`includes a summing pre-amplifier 204, for example, a cur-
`rent summing circuit. The summing pre-amplifier 204 pro-
`duces a summed electrical signal 206 and couples it to a TIA
`assembly 208, in the ROIC 203 in the illustrated example,
`that provides high gain and high data rate amplification of
`the summed electrical signal 206 in the form of an amplified
`summed electrical signal 210, which may be provided to the
`clock/data recovery circuit 116, as described above.
`An example lenslet array and photodetector assembly 300
`is illustrated in FIG. 3. The assembly includes an optical
`array 302 which may be a plurality of lenslets 304 collec-
`tively formed using standard monolithic microlens array
`fabrication techniques and mechanically mounted to a car-
`rier 306. The lenslets 304 may be spherical, elliptical,
`cylindrical, or a spherical lenses. Alternatively, lenslets may
`be Fresnel lenses or graded index-lenses that collect optical
`energy. Alternatively still,
`lenslets may be holographic
`focusing elements that direct collected optical energy to a
`photodetector. Further, although the lenslets 304 are illus-
`trated as disposed on a top surface 310 of the carrier 306,
`they may be disposed on a bottom surface of the carrier 306.
`Even further still, additional focusing elements may be used,
`either above or below the carrier 306.
`
`In the illustrated example, the collected optical energy is
`coupled to a photodetector array 308 of substrate 309. The
`photodetector array 308 is formed of a plurality of photo-
`detectors 312, which may be InGaAs photodetectors having
`an operating wavelength range above 1 pm, for example.
`More generally, however, the photodetectors 312 may be
`any semiconductor PIN or avalanche photodiode, such as an
`InGaAs, SiGe, InP, and InGaP, or HngTe device depending
`on the operating optical wavelength. The photodiodes may
`operate over standard optical communication wavelengths
`such as the 1.55 mm region, although the examples
`
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`

`

`US 7,230,227 B2
`
`5
`described herein are not limited to these particular wave-
`lengths. For example, military communications may operate
`at longer wavelengths, such as 10.6 mm, for which HngTe
`detectors may be used. The apparatus 300 is depicted as a
`4x4 array, by way of example. The apparatuses described
`herein may take the form of any M><M or M><N array.
`To keep the input capacitance low for the high data rate
`amplifiers used in the example optical devices, the photo-
`detectors 312 may be fabricated using semiconductor
`growth and photolithography processes to a size of approxi-
`mately 100 pm or below, depending on the input capacitance
`desired. Merely by way of example, an InGaAs detector of
`30 um in diameter may produce a CD of approximately 100
`fF, and a 400 um detector may produce a CD of approxi-
`mately 32 pF, where the smaller the CD, the higher the
`bandwidth and the faster the response time of the associated
`summing transimpedance amplifiers. For 10 Gbit/s data
`rates, a 30 um diameter photodetector may be preferred, for
`example.
`FIG. 4 illustrates an example circuit 400 including a
`transimpedance array 402 and summing amplifier 404 that
`may be used in the configuration of FIG. 1. The array 402
`includes a plurality of transimpedance amplifiers 406, which
`in the illustrated example are identical, and thus labeled with
`common reference numerals. Each amplifier 106 may be
`part of a receiver element 408 that also includes a photo-
`detector element, e.g., a photodiode 410. Each photodiode
`410 is coupled to a terminal of an operational amplifier 412
`and shares a node with a feedback resistor 414. The polarity
`of the terminals on the amplifiers may depend on whether
`the amplifier is inverting or not. In the illustrated example,
`the output from each amplifier 412 is coupled through a
`resistor 416 to a common node 418 providing an input to the
`summing amplifier 404, which has a feedback resistor 420
`coupled across operational amplifier 422. Buffer operational
`amplifier 424 is also provided at an output terminal in the
`illustrated example to provide a summed electrical signal to
`decision circuit or other circuit (not shown).
`FIG. 5 illustrates another example circuit 500 in which
`current summing of the signals from the photodiodes (simi-
`larly referenced 410) occurs before transimpedance ampli-
`fications. Each photodiode is coupled to a resistor 502
`coupled to node 504 providing an input to an operational
`amplifier 506. A feedback resistor 508 provides gain to a
`summing amplifier 510 formed of elements 506 and 508. A
`summed electrical signal from the amplifier 510 is provided
`to a TIA 512 formed of an operational amplifier 514 and
`feedback resistor 516. In the illustrated example, an optional
`bias signal may be input to the amplifier 514 for controlling
`gain in the amplifier. Additionally, a DC restore feedback
`518 and AC coupling element 519, or impedance matching
`or line balancing element, form a feedback to the amp 514.
`The output from the TIA 512 is provided to a buffer amplifier
`520 prior to provision to the decision circuit or other circuit.
`The transimpedance amplifier may be implemented in a
`variety of different ways, examples of which are illustrated
`in FIGS. 4 and 5. FIGS. 4 and 5 illustrate example transim-
`pedance amplifiers having a single-stage, or single-ended
`configuration. Example
`implementations may include
`single-stage buffered amplifiers, with or without additional
`resistive loads, single-stage cascaded amplifiers, and current
`amplifiers. Differential transimpedance configurations are
`also contemplated, such as that
`illustrated by way of
`example in FIG. 6, where a two-stage differential transim-
`pedance amplifier 600 is formed of first and second cascaded
`differential operational amplifiers 602 and 604 and feedback
`resistors 606 and 608, respectively.
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`The circuit schematics illustrated are examples and that
`circuit elements may be removed, substituted, or augmented
`with additional circuit elements. For example, a bias signal
`may be used on any operational amplifier described herein
`to control or adjust gain. Further still, a circuit equalizer,
`such as an inductive load, may be used to reduce the RC time
`constant in a transimpedance amplifier, thereby increasing
`bandwidth response and operating data rates.
`Various fabrication techniques may be used to form the
`transimpedance amplifier, summing amplifier, and/or clock/
`data recovery circuit. For example, commercial off-the-shelf
`elements may be used. Preferably, however, various circuit
`elements may be formed with CMOS fabrication technol-
`ogy. For example, the photodetector array may be hybrid-
`ized with a CMOS transimpedance amplifier layer and
`CMOS summing amplifier to reduce noise and manufactur-
`ing costs. An example optical device 700 is illustrated in
`FIG. 7.
`
`In the illustrated example of device 700, a photodetector
`array substrate 702 of InGaAs, or other semiconductor
`material, includes photodiodes 704 that are disposed adja-
`cent a window 706 of a hermetically sealed package 708.
`The substrate 702 is hybridized with a CMOS TIA layer 710
`having a plurality of TIAs 712 formed therein. The CMOS
`TIA layer 710 has been fabricated along with a CMOS
`summing amplifier layer 714, with the entire apparatus
`mounted on a substrate 716, for example a heat sink. The
`layers 710 and 714 form a ROIC 718. Aplurality of pins 720
`(only two of which are shown in the illustrated perspective)
`extend from a bottom of the package 708 for coupling the
`device 700 and various layer components to control cir-
`cuitry, not shown.
`In an example fabrication, InGaAs detector elements on
`the order of 50 pm or below die size and 500 um height may
`be fabricated into a substrate using semiconductor process-
`ing techniques. The photodiode substrate may then be
`hybridized with a transimpedance amplifier layer (single
`amplifier or arrayed) and a summing amplifier layer (single
`amplifier or arrayed) designed using either 0.18 pm or 0.13
`pm standard CMOS fabrication processes. The fabrication
`process may combine multiple layers into a single fabrica-
`tion layer to improve performance and reduce parasitic
`effects, as desired. For example, the transimpedance ampli-
`fier layer and the summing amplifier layer may be combined
`into a single CMOS processed layer.
`The optical-to-electrical conversion devices described
`herein may be used in numerous applications,
`including
`free-space optical communications. An example optical
`receiver 800 including an optical collection element 802 is
`shown in FIG. 8. The optical collection element 802 may be
`a focusing lens or antenna system with a focusing mirror. An
`example deployable antenna system with a focusing mirror.
`An example deployable antenna system that may he used by
`remote personnel
`is described in further detail
`in US.
`application Ser. No. 10/885 ,553, filed on Jul. 6, 2004, and
`entitled “Hybrid RF/Optical Communication System with
`Deployable Optics and Atmosphere Compensation System
`and Method” incorporated herein by reference. To correct
`for turbulence or other affects in the free-space propagation
`region between the laser transmitter and the receiver 800 and
`to correct for errors caused by the surfaces of the antenna
`system or lens, a wavefront correction system 804 is used.
`The system 804 may include a spatial light modulator and
`hologram correction film, for example. Example wavefront
`correction systems are described in US. application Ser. No.
`10/885,553. The system 804 may produce a near diffraction-
`limited beam capable of being focused by a lens 806 to a
`
`9
`
`

`

`US 7,230,227 B2
`
`7
`spot having a diameter of preferably 50 um or below. The
`lens 806 focuses the corrected laser energy from the wave-
`front correction system 804 to an optical element and
`detector array circuit 808, like those described above.
`Numerous example devices and techniques are described,
`some of which are described in relation to example envi-
`ronments, provided for explanation purposes. The example
`devices and techniques may be implemented in various
`ways, beyond the disclosed examples. For example,
`although free-space optical communication systems are
`described,
`the optical elements and photodetector arrays
`may be used in waveguide-based communication systems.
`Furthermore, although examples are described in the context
`of detecting single-wavelength modulated or un-modulated
`laser energy,
`the described techniques may be used in
`multiplexed environments with multiple laser signals, such
`as, wavelength division multiplexing (WDM) environments.
`In WDM environments, for example, a prism may be used
`to disperse different wavelengths to difference receiver
`arrays.
`Although certain apparatus constructed in accordance
`with the teachings of the invention have been described
`herein, the scope of coverage of this patent is not limited
`thereto. On the contrary, this patent covers all embodiments
`of the teachings of the invention fairly falling within the
`scope of the appended claims either literally or under the
`doctrine of equivalents.
`
`What is claimed is:
`
`1. An optical device for converting optical energy extend-
`ing over a two-dimensional spot into electrical energy, the
`optical device comprising:
`a focal plane assembly, wherein the focal plane assembly
`comprises a lenslet array including a plurality of adja-
`cently-positioned lenslet elements;
`a photodetector array having a plurality of photodetectors
`each positioned to collect at least a portion of the
`optical energy from the focal plane assembly;
`a transimpedance amplifier assembly; and
`a summing amplifier assembly coupled to the transim-
`pedance amplifier assembly to produce a summed
`electrical signal representative of the optical energy
`over the two-dimensional spot.
`2. The optical device of claim 1, wherein the transimped-
`ance amplifier assembly comprises a plurality of transim-
`pedance amplifiers each associated with a different one of
`the plurality of photodetectors.
`3. The optical device of claim 2, wherein the plurality of
`transimpedance amplifiers are coupled to a common input
`node of the summing amplifier assembly.
`4. The optical device of claim 3, wherein the summing
`amplifier assembly comprises an operational amplifier in a
`transimpedance amplifier configuration.
`5. The optical device of claim 2, wherein at least one of
`the plurality of transimpedance amplifiers is in a single-
`ended configuration.
`6. The optical device of claim 2, wherein at least one of
`the plurality of transimpedance amplifiers is in a differential
`configuration.
`7. The optical device of claim 1, wherein at least one of
`the plurality of photodetectors is an lnGaAs photodiode.
`8. The optical device of claim 1, wherein at least one of
`the plurality of photodetectors is an lnGaAs, SiGe, lnP, or
`lnGaP photodiode.
`9. The optical device of claim 1, wherein at least one of
`the plurality of photodetectors is an HngTe photodiode.
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`55
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`60
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`65
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`8
`10. The optical device of claim 1, wherein at least one of
`the plurality of photodetectors is sensitive over at least the
`visible or near infrared wavelength regions.
`11. The optical device of claim 1, wherein at least one of
`the plurality of photodetectors is sensitive over at least the
`shortwave infrared, medium-wave infrared, or long-wave
`infrared wavelength regions.
`12. The optical device of claim 1, wherein the summing
`amplifier assembly is coupled to the plurality of photode-
`tectors to produce the summed electrical signal, and wherein
`the transimpedance amplifier assembly comprises at least
`one transimpedance amplifier disposed to amplify the
`summed electrical signal.
`13. The optical device of claim 12, wherein the summing
`amplifier assembly comprises a current summing amplifier.
`14. The optical device of claim 13, wherein the current
`summing amplifier compnses an operational amplifier in a
`transimpedance amplifier configuration.
`15. The optical device of claim 1, wherein the transim-
`pedance amplifier assembly is a first CMOS assembly and
`wherein the summing amplifier assembly is a second CMOS
`assembly.
`16. The optical device of claim 15, wherein photodetector
`array is hybridized with the first and second CMOS assem-
`blies.
`
`17. The optical device of claim 1, wherein the transim-
`pedance amplifier assembly and the summing amplifier
`assembly have a bandwidth large enough to convert optical
`energy having a data rate above approximately 1 Gbit/s.
`18. The optical device of claim 1, wherein the transim-
`pedance amplifier assembly and the summing amplifier
`assembly have a bandwidth large enough to convert optical
`energy having a data rate above approximately 10 Gbit/s.
`19. A method of converting a high-data rate optical signal
`extending over a two-dimensional spot into a high-data rate
`electrical signal, the method comprising:
`forming an lenslet array including a plurality of lenslets
`for collecting the optical signal;
`forming a photodetector array including a plurality of
`photodiodes, wherein the plurality of photodiodes are
`respectively associated with the plurality of lenslets;
`focusing the optical signal onto the photodetector array;
`converting the optical signal into a plurality of electrical
`signals;
`disposing a transimpedance amplifier assembly to amplify
`each of the plurality of electrical signals; and
`to
`summing each of the amplified electrical signals,
`produce a summed electrical signal representative of
`the optical signal over the two-dimensional spot.
`20. The method of claim 19, wherein disposing the
`transimpedance amplifier assembly comprises disposing a
`plurality of transimpedance amplifiers such that at least one
`of the plurality of transimpedance amplifiers is associated
`with at least one of the plurality of photodiodes.
`2