Simulation Results of POLYNICES FMCW THz Spectrometer for Single Layer Sample Thickness Measurements

The THz spectrometer is used for single-layer thickness determination. It includes two CW lasers: a static laser with a fixed frequency and a frequency modulated one, according to a sawtooth wave. The two fields are superimposed in a 3 dB coupler become amplified with an SOA and split again into two optical arms by a second 3 dB coupler. The one output illuminates the THz emitter and propagates at the THz path where it interacts with the sample at the reflection geometry. At the receiver, the THz signal is mixed with the optical signal of the second arm. After filtering, the generated photocurrent (I_IF_tot) consists of a superposition of sinusoidal signals. Each of them has a constant frequency in the kHz regime and corresponds to a different reflected component from the sample. Consequently, the FFT of the current will result in multiple peaks. The thickness of the sample can be extracted from the distance of adjcacent peaks after multiplying with a normalization factor: " c/(2*ns*slope)"

--Parameters
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Speed of light: c = 3*10^8 m/s
Slope of the sawtooth wave for FM: slope = 4*10^14 Hz/s
Modulation frequency: f_mod = 100 Hz
Modulation period: T_mod = 1/f_mod = 10 ms
Number of simulated periods: N_period = 1
Starting frequency: f_start = 2 THz
Frequency of static laser: f_static = 1 THz
Sampling rate: fs = 24 Msamples/s
Refractive index of sample: ns
Thickness of sample: ls

--Examples
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Case 1: ns = 2.42, ls = 61.98
Case 2: ns = 2.42, ls = 92.9
Case 3: ns = 2.5 , ls = 60
Case 4: ns = 2.5 , ls = 75