KEYWORDS: Signal processing, Digital signal processing, Optical testing, Spectroscopy, Channel projecting optics, Pulsed laser operation, Laser optics, Analog electronics, Linear filtering, Fourier transforms
The Fourier transform spectrometer is based on a scanning-type Michelson interferometer with two optical-beams
including reference laser and measurement light. The optical spectrum of measurement light can be obtained by colleting
the analog signal of measurement light with the sampling time-scale based on interference fringes pulse of reference
laser. The optical interferogram signal of the reference laser is converted to pulse signal, and it is subdivided into 16
times. So the sampling rate is constant only depending on frequency of reference laser and irrelative to the
scanning-motor velocity. This means the sampling rate of measurement channel signal is on a uniform time-scale.
In order to get optical spectrum, the analog signal of measurement channel should be collected and Fourier-transformed.
Digital Signal Processor (DSP) TMS320-F2812 is used. Its internal 12-bit Analog-to-Digital Converter (ADC) for
measurement channel is triggered by the 16-times pulse signal of reference laser. The DSP is used to process optical
interferogram signal with ADC sampling rate up to about 1.5MHz. The optical spectrum of measurement channel signal
is processed with 1024 points-Fast Fourier Transform (FFT) method. Then, the ZOOM-FFT is adopted. By digital
frequency-shift conversion and low-pass filter, these digital signals are processed with 1024 points-FFT once again to
improve the frequency measurement resolution. Finally, the optical spectrum of measurement channel will be
displayed on LCD with high frequency-resolution.
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