WO2017019298A1

WO2017019298A1 - Frequency measurement focal plane array input circuit

Info

Publication number: WO2017019298A1
Application number: PCT/US2016/041971
Authority: WO
Inventors: Allen W. Hairston; Gary M. Madison
Original assignee: BAE Systems Information and Electronic Systems Integration Inc
Current assignee: BAE Systems Information and Electronic Systems Integration Inc
Priority date: 2015-07-30
Filing date: 2016-07-13
Publication date: 2017-02-02
Anticipated expiration: 2018-01-30
Also published as: US20170031012A1; US10073168B2

Abstract

The invention measures the frequency of a heterodyne laser radar (LADAR) system signal in the input cell of a focal, plane array (FPA). Embodiments amplify the return signal, and drive it into a counter for a fixed period of time. The frequency is the number of counts divided by the count time. An example design amplifier amplifies the return of a single photon response of an avalanche photodiode with a gain of 100 into a digital signal level at a 200 MHz rate with only 84 µW, demonstrating the feasibility of the approach.

Description

FREQUENCY M EA S UREMENT FOCAL P LANE ARRAY INPUT

C I RCUIT

FIELD OF THE INVENTION

[0001] The invention relates to a circuit and method for measuring the frequency of a heterodyne laser radar (LADAR) system signal in the input cell of a focal plane array (FPA).

BACKGROUND OF THE INVENTION

[0002] LAser raDAR (LADAR) combines laser-focused imaging with radar's ability to calculate distances by measuring the time for a signal to return. Such sensors are used for detection, identification, and precision measurement of the range to a target. Data generated by the sensor can be used for targeting, command, and control computers of automated and semi-automated systems. The data can be used for precision short and long-range target detection, identification, and ranging measurements for command, control, and navigation systems of autonomous, semi-autonomous, and remotely operated manned and unmanned vehicles. In operation, light reflected by backscattering is received at the detector. LADAR detection can be by direct energy detection (amplitude measurement) or by coherent detection. Coherent systems generally use optical heterodyne detection, which, being more sensitive than direct detection, allows them to operate at a much lower power but at the expense of more complex transceiver requirements. LADAR systems can employ two types of pulse models: micropulse and high energy. Micropulse systems use considerably less laser energy, typically on the order of one microjoule. Common wavelengths range from 10 microns to 250 nm (UV). S UMM ARY OF THE INVENTION

[0003] An embodiment provides a device for measuring a frequency of a heterodyne laser radar (LADAR) system signal in an input cell of a focal plane array (FPA) comprising a detector; an AC coupl ed ampli fier input; a counter; and a latch outpulting a read out. In embodiments the counter and the latch comprise 1 4 to 1 6 bits . In other embodiments, the detector is an avalanche photodiode (APD). In subsequent embodiments the AC coupled ampli fier input comprises gain and band width adj ustment. For additional embodiments the gain is about 500 and bandwidth from about 25 to 200 M Hz. In another embodiment, the APD is a HgCdTe detector. A following embodiment further comprises a LADAR range detector measuring a time of arrival of a return pulse using an externa l reference clock as a counter clock, and storing a counter value corresponding to the time of arrival of the return LADAR pulse. Subsequent embodiments further comprise a photon counting focal plane array, where the gain of the APD and amplifier is large enough for individual photons to generate a signal large enough to operate the counter. Additional embodiments further comprise a narrow laser cold filter.

[0004 ] Another embodiment provides a method for measuring a frequency (FM) of a heterodyne laser radar (LADAR) system signal in an input cell of a focal plane array (FPA) comprising providing a LADAR FM FPA circuit; counting a beat frequency, whereby detection and frequency measurement are accompl ished; counti ng photons during a period, whereby photons are passively counted; and clocking start or stop times of counter, whereby ranging is directly detected. In included embodiments a fixed counting period is an entire frame. In yet further embodiments a fixed counting period is less than a frame. In related embodiments a fixed counting period is more than a frame. Further embodiments comprise a range gate. In ensuing embodiments pulses are longer than two times a frame rate. For yet further embodiments, a minimum of count periods is chosen as a count period with least noise counts. For more embodiments, passi ve and active counting are simultaneously accompli shed. Continued embodiments include a faster frame rate is employed for shorter pulses. For additional embodiments, the system has single photon sens i tivity, wherein the LADAR FM FPA circuit comprises an avalanche photodiode (APD) having a gain of about 100 into a digital signal level at a 200 MHz rate with 84 .

[0005 ] A yet further embodiment provides a system for measuring a frequency (FM) o f a heterodyne laser radar ( LADAR) system signal in an input cell of a focal plane array (FPA) comprising an avalanche photodiode (APD); an AC coupled amplifier comprising gain and bandwidth adjustment input; a counter; a latch outputting a read out; counting a beat frequency, whereby detection and frequency measurement are accompli shed ; counting photons during a period, whereby photons are passively counted ; and clocking start or stop times of counter, whereby ranging is directly detected, wherein the FPA dimensions are 256 by 256, pixel dimensions are about 30 μπι, and an avalanche photo diode gain is about 1 00, adjustable per-pixel .

[0006] The features and advantages described herein are not al l- inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skil l in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readabil ity and instructional purposes, and not to limit the scope o f the inventive subj ect matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a depiction o f a LADAR system environment configured in accordance with one embodiment of the invention .

[ 0008] Figure 2 is a LADAR system block diagram configured in accordance with one embodiment of the invention . [0009] Figure 3 is a frequency measurement ( FM) focal plane array ( FPA ) input circuit block diagram configured in accordance with one embodiment of the inventio n.

[0010] Figure 4 is a transient ana lysis graph configured in accordance with one embodiment of the invention.

(001 1 ] Figure 5 is a methods of operation diagram configured in accordance with one embodiment of the invention.

| 0012 f Figure 6 i s an operation flow chart configured in accordance with one embodiment of the inventi on.

D ETAILED DESCRIPTION

J 0013] The invention measures the frequency of a heterodyne laser radar (LADAR) system signal in the input cel l of a focal plane array (FPA). Embodiments amplify the return signal, and drive it into a counter for a fixed period of time. The frequency is the number of counts di vided by the count time. An ampli fier is designed to ampli fy the return of a single photon response of an avalanche photodiode with a gain of 100 into a digital signal level at a 200 MHz rate with only 84 μ\ν.

10014 ] FIG. 1 depicts a LADAR system environment 100. Platform 105 (not necessarily airborne) incorporates G PS locating information 1 10 , and supports laser scanner system 1 15 to scan target area 120.

[ 0015 ] FIG. 2 is a LA DAR system block diagram 200. Components comprise laser 205 ; beam expander 210; transmitting optics 215; receiving telescope 220; target 225 ; photodetector 230; A/D converter 235; signal and data processor 240 ; and output device 245.

[0016] Embodiments of the invention support focal pl ane arrays with formats up to 256 x 256. This FPA architecture can al so support alternate F PA operating modes, allowing it to do direct detect ranging LADAR or passive photon counti ng. The ci rcuit can also count photons for very low flux applications or perform direct detect ranging by counting reference clock cycles until return pulse is detected. | 00 1 7 ] FI G. 3 depicts a frequency measurement focal plane array input circuit 300 for heterodyne LADAR Systems. Elements include l inear ava lanche photodiodc (APD) 305 wi th DCOM - 1 0V input; 500X AC coupled amplifier 31 0 ; gain and bandwidth adj ustment input 3 1 5 ; 14 - 1 6 bit counter 320 ; optional XCK in for ranging 325; 1 4 - 1 6 bit latch 330; and read out 335. LADAR Frequency Measurement FPA embodiments count fast pulses for multiple functions. They measure heterodyne LADAR presence and frequency by counting cycles in a known period. The frequency is equal to the number of counts per count period, assuming the signal is present for the entire count period. Embodiments count passively generated photons for low flux levels. In embodiments, this may require detectors operating at T<80K. When there is sufficient gain to count photons, pulse counting functionality is no di fferent for FPAs. Direct range detection can be performed by counting external reference clock cycles for the return time.

[ 00 18 ] For additional functionality, detection starts or stops the count of an external ly supplied reference clock. For embodiments, the counter has 14 to 16 bits. A 1 0 KHz frame rate is employed with all digital input and output for embodiments . Embodiments use HgCdTe linear avalanche photo diode (APO) detectors, nominally at 80K operation . They al so have input fuse high voltage protection on the read out integrated circuit (ROIC) for shorted detectors. Embodiments are fabricated in 1 30 nm CMOS processes. The high speed amplifier has programmable gain and bandwidth control . This gain is high enough to count indi vidual photons at 200 MHz (G APD = 100), and gain control is programmable by pixel. Using nonlinear gai n a llows the counting o f very large signals at high gain with no saturation. Array sizes up to 256 x 256 can operate at less than 6W of power; ex isting cryo coolers are adequate for 6W of cooling. In embodiments, pixel size is nominal ly 30 μηι.

(001 9 ] LADAR FM FPA embodiments provide four functions: detection, frequency measurement, pho ton counting, and di rect range detection. Detection and Frequency Measurement

[0020] Detection and frequency measurement functions for heterodyne LADA R systems count the beat frequency cycles per fixed period to measure the frequency and indicate detection. A fixed period is nominal ly an entire frame, but could al so employ multiple and/or shorter periods, with a faster frame rate for shorter pulses. A fixed period could be less than a frame time with a range gate. Multiple period / frame capability comprises a register and digital comparator, with >2X logic. For pulses shorter than the frame time, the maximum of count periods is chosen, where the count period is <= pulse/2. For pulses longer than twice the frame time, the minimum of count periods is chosen as the count period with the least noise counts. For cases where noise dark counts are with photon counts, the higher output wi ll indicate detection of the signal and the frequency is recovered with signal processing.

Passive Photon Counting

[0021 ] For pass ive photon counting, i f the APD FPA has single photon sensitivity (gain > threshold), then it can also count photons (and dark electrons) for very high sensitivity passive applications. However, this is only accurate i f the probabi l ity of "simultaneous" photons is low. If the dark count rate is low, then passive and active functions can be accomplished at the same time (a small number of counts are passive photons plus noise, and high number of counts are active frequency measurements) . A narrow laser cold filter would let few passive photons through, but sufficient for operation.

Direct Detect Ranging

[0022 ] For direct detect ranging, if the counter is supplied with an external reference clock and some simple logic, thi s i nput circuit can also do direct detect LADAR ranging with a digital output. Embodiments stop or start the count usi ng an external reference clock on detection of a pulse. " Start" minim izes counting during detection. A read out reference counter starts at the beginning of the range gate, and stops with the all the other counters for delay calculation. In embodiments, there are a few test pixels in the corners of the FPA .

[0023 ] FIG. 4 is a transient analysis graph 400 demonstrating the feasi bi lity of reasonable performance with circuits that could fit in an FPA. 4A depicts 0 to 50 ns, and 4B depicts 50 - 1 00ns. The five traces are: outO 405; out l 410; out2 4 15; out3 420; and O P 425. High Speed Detection in FPA pixels provides full scale output with input equivalent to si ngle photons into the APD at 200 M Hz. FIG. 4 depicts this transient analysis for o f approximately 50 fF, f = 200MHz, and 100 electrons . Embodiments include 84 μ\ν pixel power that is compatible with FPA sizes up to 256 x 256 with 6W total FPA power. Gain is greater than 500, and the bandwidth is from 25 to 200M Hz. Gain and bandwidth are selectable with bias and control signal changes. P ixel level programming contro ls the overall threshold and compensates fo r non-uniform ity. In embodiments, the overall pixel c i rcuit fits within a 30 μιη pixel.

[0024 ] FIG 5 is a methods o f operation diagram 500. FM FPA LADAR systems 505 have four functions comprising detection 510; frequency measurement 515; direc t range detection 520; and photon counting 525.

[0025 ] FIG 6 is an operation flow chart 600. Embodiments comprise the steps of providing a LADAR FM FPA circuit 605; counting a beat frequency, whereby detection and frequency measurement are accompl ished 610; counting photons during a period, whereby photons are passively counted 615; and clocking start or slop times of counter, whereby ranging is directly detected 620. Steps can be performed i n alternate orders.

[0026] The foregoing description of the embodiments of the invention has been presented for the purposes of i llustration and description . I t is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light o f this disc losure it is to be understood that other simi lar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviati ng therefrom. I t is intended that the scope of the invention be l imited not by this detai led description, but rather by the claims appended hereto.

Claims

CLA IMS What is claimed is:

1 . A device for measuring a frequency of a heterodyne laser radar (LADAR) system signal in an input cel l of a focal plane array (FPA) comprising: a detector; an AC coupled amplifier input; a counter; and a latch outputting a read out.

2. The device of claim I , wherein said counter and said latch compri se 1 4 to 1 6 bits.

3. The device of claim 1 , wherein said detector is an avalanche photodiode ( APD).

4. The device of claim 1 , wherein said AC coupled amplifier input comprises gain and bandwidth adjustment.

5. The device of claim 1 , wherein said gain is about 500 and bandwidth from about 25 to 200 MHz.

6. The device of claim 1 , wherein said APD is a HgCdTe detector.

7. The device of claim 1 , further comprising a LA DAR range detector measuring a time of arrival of a return pulse using an external reference clock as a counter clock, and storing a counter value corresponding to said time of arri val of said return LADA R pulse.

8. The device of claim 3 , further comprising a photon counting focal plane array, wherein a gain of said APD and amplifier is large enough for i ndividual photons to generate a si gnal large enough to operate said counter.

9. The device of claim 1 , further compri si ng a narrow laser cold filter.

1 0. A method for measuring a frequency (FM) of a heterodyne laser radar (LADAR) system signal in an input cell of a focal plane array (FPA) compri sing : providing a LA DAR FM FPA circuit; counti ng a beat frequency, whereby detection and frequency measurement are accomplished; counting photons during a period, whereby photons are passively counted; and clocking start or stop times of counter, whereby ranging is directly detected.

1 1 . The method of claim 1 0 wherein a fixed counting period is an entire frame.

12. The method of claim 1 0 wherein a fixed counting period is less than a frame.

1 3. The method of claim 1 0 wherein a fixed counting period is more than a frame.

1 4. The method of claim 1 2 comprising a range gate.

1 5. The method of claim 1 0 wherein pulses arc longer than two times a frame rate.

1 6. The method of claim 1 5 wherein a minimum of count periods is chosen as a count period with least noise counts.

1 7. The method of claim 1 0 wherein passi ve and active counti ng are simultaneously accomplished.

1 8. The method of claim 1 0, wherein a faster frame rate is employed for shorter pulses.

1 9. The method of claim 1 0 wherein said system has single photon sensitivity, wherein said LADAR FM FPA circuit comprises an avalanche photodiodc (A PD) having a gain of about 100 into a digital signal level at a 200 MHz rate with 84 pW.

20. A system for measuring a frequency (FM) of a heterodyne laser radar (LADAR) system signal in an input cell of a focal plane array (FPA ) comprising: an avalanche photodiode (APD); an AC coupled amplifier comprising gain and bandwidth adjustment input; a counter; a latch outputting a read out; counting a beat frequency, whereby detection and frequency measurement are accomplished ; counting photons during a period, whereby photons are passively counted; and clocking start or stop times of counter, whereby ranging i s directly detected, wherein said FPA dimensions are 256 by 256, pixel dimensions are about 30 μηι, and an avalanche photo diode gain is about 1 00, adjustable per-pixel.