Actuation Circuit for Laser Diode Non-Linearity Compensation
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Solution Overview
Problem
Existing gas concentration measurement methods using transmission measurements face challenges with quantum noise and non-linear semiconductor lasers, particularly in terrestrial conditions, where the strong non-linear characteristic of tunable semiconductor lasers complicates absorption determination.
Innovation Solution
An actuation and evaluation circuit that subtracts a reference signal from the transmission signal to compensate for the non-linear characteristic of the semiconductor laser, using analog subtraction and digital memory for storing characteristics, and integrates the output signal to measure gas concentration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a broadband light source is used with a photodiode to measure gas concentration, then the measurement can be performed, but quantum noise increases proportionally to the root of the photocurrent
Solution Approach 1:
The patent uses a tunable semiconductor laser to provide highly localized spectral illumination precisely at the absorption line wavelength, rather than broadband illumination. This concentrates the measurement energy exactly where needed (at the absorption line) while avoiding illumination at other wavelengths that would generate unnecessary quantum noise in the photodiode.
Solution Approach 2:
The patent segments the spectral range by using a tunable laser to isolate and measure only the specific absorption line wavelength, separating it from other spectral components. This allows the measurement to focus on the relevant absorption signal while excluding other wavelengths that contribute to noise.
2Object-affected harmful factors
If an optical filter is used to limit the spectral range to the absorption line, then quantum noise is reduced, but the relative alteration of the photodiode current by absorption becomes too small
Solution Approach 1:
The patent changes the parameter of light source spectral width from broadband to narrowline by using a tunable semiconductor laser. This narrowline laser provides high spectral density at the absorption line wavelength, generating a strong absorption signal that produces a large relative alteration in photodiode current while maintaining low quantum noise.
3Adaptability or versatility
If tunable semiconductor lasers are used for gas analysis, then the laser line can be varied to match absorption lines, but the strongly non-linear characteristic of the laser must be subtracted out by fitting
Solution Approach 1:
The patent employs feedback by using the derivative signal (obtained through differentiation of the transmission signal) as a reference to compensate for the laser's non-linear characteristic. The zero-crossing point of the derivative signal provides a feedback reference that automatically tracks the absorption line center, eliminating the need for complex fitting procedures.
Solution Approach 2:
The patent replaces the mechanical/mathematical fitting process with a signal processing approach using differentiation. By differentiating the transmission signal with respect to laser current, the non-linear laser characteristic is transformed into a derivative signal whose zero-crossing directly indicates the absorption line center, simplifying the measurement process.
4Measurement precision
If the laser current is increased to improve signal strength, then the absorption signal becomes stronger, but the non-linear characteristic of the semiconductor laser becomes more pronounced
Solution Approach 1:
The patent uses periodic modulation of the laser current to scan through the absorption line in a controlled manner. By applying a sinusoidal or triangular modulation signal to the laser current, the system periodically traverses the absorption line, allowing the derivative signal to be generated and processed to extract the absorption information while compensating for non-linearities.
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
This approach effectively compensates for the non-linear laser characteristic, reducing noise and improving the accuracy of gas concentration measurements, making the method suitable for terrestrial applications while maintaining cost-effectiveness.
Implementation Method 1
The alteration of the laser line is particularly due to an alteration of the refraction index as a result of a higher temperature of the semiconductor crystal
Implementation Method 2
the light transmitted through a measuring volume is measured by a photodiode
Implementation Method 3
By the absorption the gas in the measuring volume is periodically heated, so that initially temperature differences are created, pressure differences as a result thereof, and thus sound
Data Source
AI summary
The invention relates to an actuation and evaluation circuit for a laser diode (1) and a photodiode (3) for determining the concentration of a gas. The laser diode can generate light in the range of an absorption line of the gas. The circuit comprises a driver (10, 11, 12, 13) for generating a driving signal (17) for the laser diode (1), an assembly (8, 9) for generating a reference signal (20), and a subtractor (5) for subtracting the reference signal (20) from the signal (21) supplied by the photodiode. The invention further relates to a measuring device for determining the concentration of a gas by means of such an actuation and evaluation circuit. Finally, the invention relates to a corresponding method.