Laser Driver Pulse Scaling for Short-Pulse Speckle Reduction
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Solution Overview
Problem
Traditional pulse generators for laser displays are limited by coil charging time, which restricts pulse frequency and pulse duration, failing to achieve the specific duration required for mitigating speckle patterns effectively.
Innovation Solution
A pulse scaling circuit within a laser driver that includes transistors, capacitors, and resistors to generate output pulses of shorter duration, converting input pulses of 0.7 to 6.5 ns to output pulses of 0.25 to 2 ns, using asymmetric charge and discharge cycles to achieve a spread spectrum effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If traditional pulse generators are used with coil charging time, then the power stage can operate, but the pulse frequency is restricted and pulse duration cannot be shortened below 0.7 ns
Solution Approach 1:
The circuit is segmented into distinct functional blocks: pulse generator circuit for creating voltage pulses, pulse scaling circuit for duration transformation, and power stage circuit for current conversion. This segmentation allows each block to be optimized independently, enabling the pulse scaling circuit to reduce pulse duration without being constrained by the power stage's coil charging time.
Solution Approach 2:
The pulse scaling circuit acts as an intermediary between the pulse generator and power stage. It receives voltage pulses from the pulse generator, scales their duration to the required 0.25-2 ns range, and then passes them to the power stage. This intermediary function resolves the contradiction by decoupling the pulse duration control from the power stage's inherent time constraints.
2Reliability
If pulse duration is extended to 0.7-6.5 ns for traditional pulse generators, then the power stage can charge coils adequately, but speckle patterns cannot be effectively mitigated
Solution Approach 1:
The pulse scaling circuit changes the temporal parameter of the voltage pulses, specifically reducing their duration from the traditional 0.7-6.5 ns range to 0.25-2 ns. This parameter change enables the laser to operate in a regime where the pulse duration is too short to induce significant thermal effects in the laser crystal, thereby reducing speckle pattern formation while still allowing the power stage to function reliably.
3Object-affected harmful factors
If pulse duration is reduced to 0.25-2 ns for spread spectrum effect, then speckle patterns are mitigated, but traditional pulse generators cannot generate such short pulses
Solution Approach 1:
The pulse scaling circuit employs asymmetric transistor configurations (different transistor types, resistor values, and capacitor arrangements) to create unequal charging and discharging time constants. This asymmetry enables the circuit to transform longer input pulses into significantly shorter output pulses, achieving the 0.25-2 ns duration required for spread spectrum operation and effective speckle mitigation.
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 laser driver effectively reduces coherence artifacts like speckle patterns by emitting light in a spread spectrum, improving efficiency and reducing speckle effects through short current pulses.
Implementation Method 1
a capacitor having an electrode connected to a common terminal of a pair of resistors connected in series with the first transistor
Data Source
AI summary
A laser driver includes a pulse generator circuit, a pulse scaling circuit, and a power stage circuit. The pulse generator circuit generates a first voltage pulse of a first duration. The pulse scaling circuit includes a first transistor with a first gate electrode receiving the first voltage pulse, a capacitor having an electrode connected to a common terminal of a pair of resistors connected in series with the first transistor, and a second transistor with a second gate electrode connected to the first gate electrode and a second drain electrode coupled to a supply voltage via a resistor. Responsive to the reception of the first voltage pulse, a second voltage pulse of a second duration shorter than the first duration is generated at the second drain electrode. The power stage circuit converts the second voltage pulse into a current pulse driving at least one emission element of a laser display.