Optical Spectrum Line Width Calculation Using Phase Iteration
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Solution Overview
Problem
The self-delay heterodyne/homodyne method for optical spectrum line width measurement is limited by the need for a long delay fiber, restricting the evaluation of lasers with narrow line widths, as it requires a delay difference significantly longer than the coherence time, making it impractical for lasers with line widths of 1 kHz or less.
Innovation Solution
The method calculates the phase and power spectrum of optical interference signals using equations that increase the delay time difference, allowing for the calculation of spectrum line width without requiring a long delay fiber, by iteratively calculating phases X1(t) and XN(t) and determining the spectrum line width from the half width at half maximum of the power spectrum, which converges to a Lorentz function.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a long delay fiber is used to achieve sufficiently long delay difference (τ>>τc), then measurement precision is improved, but device complexity and practical feasibility deteriorate
Solution Approach 1:
The patent changes the parameter of delay difference from a fixed physical constraint (fiber length) to a variable that can be effectively increased through signal processing. By calculating phase XN(t) from X1(t) using the equation XN(t) = ΣX1(t-nτ), the effective delay difference is multiplied by N times, achieving the required τ>>τc condition without physically extending the delay fiber.
Solution Approach 2:
The patent replaces the mechanical/physical solution (extending delay fiber length) with a signal processing approach. Instead of physically increasing the delay path length, the system uses phase calculation and spectrum analysis to achieve the equivalent effect, substituting optical/mechanical extension with computational processing.
2Adaptability or versatility
If delay fiber length is increased to evaluate lasers with narrower line widths, then measurement capability is improved, but ease of operation and practical implementation worsen
Solution Approach 1:
The patent enables evaluation of lasers with narrower line widths by changing the effective delay parameter through signal processing rather than physical modification. The phase calculation method allows the system to achieve sufficiently long effective delay for narrow line width lasers without reconfiguring the physical apparatus.
Solution Approach 2:
The patent makes the measurement apparatus universally applicable to lasers with various line widths. By using the phase calculation method XN(t) = ΣX1(t-nτ), the same apparatus configuration can evaluate both wide and narrow line width lasers, eliminating the need for different fiber lengths for different laser types.
3Measurement precision
If the delay difference is extended to improve measurement resolution, then measurement precision is improved, but loss of time and processing complexity increase
Solution Approach 1:
The patent performs preliminary phase calculation to extract X1(t) from the interference signal, which then serves as the basis for calculating XN(t). This preliminary processing step enables subsequent rapid computation of the enhanced delay effect without requiring actual physical delay extension, reducing overall measurement time.
Solution Approach 2:
The patent substitutes time-consuming physical delay fiber extension with computational phase processing. The calculation XN(t) = ΣX1(t-nτ) is performed digitally, which is much faster than physically extending and adjusting long delay fibers, thereby reducing the time loss associated with achieving high resolution.
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 enables higher resolution line width measurements without altering the apparatus configuration, allowing for the evaluation of lasers with narrower line widths and improving measurement accuracy by signal processing alone.
Implementation Method 1
two light beams having a delay difference τ are generated from light emitted from a laser to be measured 200 using the Mach-Zehnder interferometer 110, and an optical spectrum line width σ is obtained from a power spectrum shape of an optical interference signal obtained by multiplexing the two light beams
Implementation Method 2
In the heterodyne method, one of the light beams having the delay difference τ is given any optical frequency shift fb to cause interference
Data Source
AI summary
Provided is an optical spectrum line width calculation method, apparatus, and program capable of calculating a spectrum line width of a laser to be measured from an optical interference signal generated by an optical interferometer having a delay line, based on a phase of the optical interference signal having a delay time longer than a delay time due to the delay line. The optical spectrum line width measurement apparatus includes a Mach-Zehnder interferometer, an optical receiver that receives an optical interference signal emitted from the Mach-Zehnder interferometer, an A/D converter that converts an analog electric signal output from the optical receiver into a digital electric signal, and a processing apparatus that processes the digital electric signal. Two light beams having a delay difference τ are generated from light emitted from the laser to be measured, and an optical interference signal is generated by multiplexing the two light beams.