Sub-Clock Pulse Generator for Precise Laser Pulse Width Control
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Solution Overview
Problem
There is a need for precise control of laser pulse width in augmented reality headsets that use picoprojectors to ensure proper functionality and user comfort, as existing techniques adequately control laser power but not pulse width.
Innovation Solution
A delay locked loop system that includes a control loop, delay chain, and pulse decoder to generate a pulsed output signal with a pulse width equal to a desired fraction of a reference clock signal, using a delay chain with N delay cells and biasing voltages to adjust the pulse width accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional laser control techniques are used, then laser power can be precisely controlled, but laser pulse width cannot be precisely controlled
Solution Approach 1:
The reference clock signal period is segmented into N equal intervals using a delay chain with N delay cells. Each delay cell introduces a delay of T/N, where T is the clock period. This segmentation allows precise selection of pulse width as a fraction (M/N) of the reference clock period by choosing M specific delay taps, thereby achieving precise laser pulse width control while maintaining reliable laser operation.
2Ease of operation
If laser pulse width is not precisely controlled, then device complexity remains low, but user comfort deteriorates
Solution Approach 1:
A feedback mechanism is implemented where the output of the Mth delay cell is fed back to control the selection of delay taps. The feedback signal ensures that exactly M delay cells are activated to generate the pulse width, providing precise control over the laser pulse duration. This feedback-based approach enhances user comfort by preventing eye discomfort while maintaining manageable device complexity through systematic control.
3Measurement precision
If a delay chain with N delay cells is used, then pulse width control precision is improved, but device complexity increases
Solution Approach 1:
The system utilizes periodic action by synchronizing the delay chain operation with the reference clock signal period. The delay chain processes signals in discrete periodic steps corresponding to clock cycles, with each delay cell representing a fixed time increment. This periodic operation allows precise pulse width control through simple tap selection while maintaining regular, predictable system behavior that reduces overall complexity.
Data Source
AI summary
A delay locked loop includes a control loop receiving reference and feedback clock signals, and generating biasing voltages therefrom. A delay chain receives the reference clock signal and generates N successively delayed versions thereof, each at a successive tap thereof. The Nth delayed version is the feedback clock signal. The control loop has a phase detector asserting an up signal when a phase of the feedback clock signal lags that of the reference clock signal, asserting a down signal when the phase of the feedback clock signal leads that of the reference clock signal. A digital filtering block compares a number of assertions of the up signal during the period of the reference clock signal to those of the down signal, and asserts an up or down command signal based thereupon. A biasing voltage generation circuit receives the up and down command signals and generates the biasing voltages therefrom.