Focal Plane Array Input Circuit for LADAR Frequency Measurement
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Solution Overview
Problem
Current heterodyne laser radar (LADAR) systems face challenges in accurately measuring frequency signals in focal plane arrays due to limitations in sensitivity and complexity, particularly in low-power operations and high-sensitivity detection requirements.
Innovation Solution
A circuit and method utilizing an avalanche photodiode (APD) with an AC coupled amplifier, counter, and latch, along with a photon counting focal plane array, enable frequency measurement by counting beat frequencies and photons, with adjustable gain and bandwidth, and employing a narrow laser cold filter for improved sensitivity and direct range detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coherent detection is used in LADAR systems, then sensitivity is improved, but device complexity increases
Solution Approach 1:
The focal plane array is divided into multiple independent pixels, each capable of performing coherent detection and frequency measurement separately. This segmentation allows the system to achieve high sensitivity through coherent detection while managing complexity by distributing functionality across multiple simple, identical units rather than requiring a single complex system.
Solution Approach 2:
Each pixel in the focal plane array is designed to perform multiple functions: coherent detection, frequency measurement, and ranging. This multi-functionality reduces overall system complexity by eliminating the need for separate dedicated circuits for each function, while maintaining high sensitivity through the coherent detection capability inherent in each pixel.
2Use of energy by moving object
If micropulse systems are used, then laser energy consumption is reduced, but signal strength decreases
Solution Approach 1:
The system replaces direct amplitude detection with coherent detection and frequency measurement. By detecting the frequency of beat signals generated by mixing local oscillator light with returned light, the system can achieve high sensitivity and detect weak signals from micropulse transmissions without requiring high laser energy, as the frequency measurement capability provides superior signal discrimination.
Solution Approach 2:
The system changes the detection parameter from amplitude to frequency. By measuring the frequency of heterodyne beat signals rather than the amplitude of returned light, the system can detect signals from low-energy micropulse transmissions with high sensitivity, as frequency measurement is less susceptible to noise and interference than amplitude detection at low signal levels.
3Measurement precision
If frequency measurement is performed in each pixel, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The frequency measurement function is segmented and implemented independently in each pixel through simple counter circuits that count beat frequency cycles. This segmentation achieves high measurement precision through multiple independent measurements while keeping individual pixel complexity low, as each pixel uses identical simple counter logic rather than complex centralized processing.
Solution Approach 2:
Each pixel performs self-contained frequency measurement using its own integrated counter and latch circuits. The pixels autonomously count beat frequency cycles and store results without requiring external intervention or complex inter-pixel communication. This self-service capability achieves precise frequency measurement while minimizing overall system complexity by eliminating the need for complex external measurement equipment.
4Measurement precision
If photon counting is implemented, then sensitivity is improved, but noise increases
Solution Approach 1:
The system replaces direct photon counting with coherent detection and frequency measurement of beat signals. By mixing local oscillator light with returned light and measuring the frequency of the resulting beat signal, the system achieves single-photon sensitivity without the noise associated with direct photon counting, as the frequency measurement approach provides superior signal-to-noise ratio through coherent signal processing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves single-photon sensitivity and efficient frequency measurement with reduced noise, enabling precise detection and ranging capabilities in LADAR systems, even at low flux levels, with a compact and power-efficient design suitable for large focal plane arrays.
Implementation Method 1
an avalanche photodiode (APD)... an avalanche photo diode gain is about 100
Implementation Method 2
AC coupled amplifier input... AC coupled amplifier comprising gain and bandwidth adjustment
Data Source
AI summary
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.