## 3 Experimental Results
Different orders of resonances have different pressure sensitivity levels; the resonances shift through different wavelengths and separate gradually as the MBR's internal pressure changes. The pressure values are 0.387, 0.411, and 0.579 bar in Figure 2b,e,f, respectively. The pressure tuning proves that the asymmetrical spectrum contains two resonances. Figure 2f plots the two separate resonances (blue scattered line), the IQ curves (black scattered line), fitted IQ curves, replotted resonances (red solid line), fitted resonances, and replotted IQ curves (yellow dashed line). To verify the fitting results above, we fitted the IQ curve, replotted the resonance, fitted the resonance, and replotted the IQ curve. The fitting and replotting results (\(g=0\) MHz) listed in Table 1 agree with the previous results. The CODs of the IQ and the transmitted replotted curve are 0.862 and 0.863, respectively. The IQ curve of the low-quality resonance features noise, so its COD value is not significantly better. The process proves the validity of the OD-PDH technique in analyzing overlapping asymmetrical spectra. Note that we obtained \(g=0\) MHz. This naturally reveals that the two resonances simply overlap. Figure 2: (**a**) Standard single resonance transmitted spectrum (blue scattered line), its IQ curve (black scattered line), fitted IQ curve (red solid line) and replotted resonance (magenta solid line). (**b**) Experimental (black scattered line) and fitted (red solid line (\(g=0\)), yellow dashed line (\(g=1.19\times 10^{4}\) MHz) spectrum at inner pressure of 0.387 bar. (**c**) Experimental spectrum (blue scattered line) and IQ curve (black scattered line), fitted resonances, and replotted IQ curves (red solid line (\(g=0\) MHz) and yellow dashed line (\(g=1.19\times 10^{4}\) MHz). (**d**) Experimental spectrum (blue scattered line) and IQ curve (black scattered line), fitted IQ curves and replotted resonances (red solid line (\(g=0\) MHz), and yellow dashed line (\(g=3.76\times 10^{3}\) MHz). (**e**) and (**f**) Experimental resonances (blue scattered line) and IQ curve (black scattered line) at inner pressure of 0.411 bar and 0.579 bar, respectively, fitted and replotted curves (red solid line and yellow dashed line). Figure 3 gives another example of the asymmetrical mode (black scattered line). Again, we can fit the asymmetrical resonance (overlapping, red solid line; coupled, yellow dashed line) in Figure 3a with the two-resonance expression and derive two group parameters as listed in Table 2 (CODs \(>\) 0.991). Figure 3b plots the spectrum in Figure 3a (blue scattered line) and its IQ curve (black scattered line), fitted resonances and replotted IQ curves (overlapping, red solid line; coupled, yellow dashed line). Using the two group parameters to reproduce the IQ curve, the group of parameters with \(g=1.20\times 10^{4}\) MHz in Figure 3b fit the spectrum well (replotted coupled curve COD, 0.997; replotted overlapping curve COD, 0.645), which means that the two modes couple. Figure 3c plots the spectrum in Figure 3a (blue scattered line) and its IQ curve (black scattered line), fitted IQ curves and replotted resonances (overlapping, red solid line; coupled, yellow dashed line). We can also fit the IQ curves and generate two groups of parameters (COD \(>\) 0.998), and then use those parameters to reproduce the spectrum in Figure 3c. This result also demonstrates that the group of parameters with \(g=1.20\times 10^{4}\) MHz fits the spectrum. (replotted coupled transmitted spectrum COD, 0.994; replotted overlapping transmitted spectrum COD, 0.893). To prove that the asymmetrical resonance is a coupled resonance, as we did above, we injected air into the MBR to tune the resonance; Figure 3d plots the resonance when the pressure ranged from 0.215 to 0.519 bar. A single example of asymmetrical resonance may be coupled and overlapping resonance. After pressure tuning, the resonance did not separate and the resonance shifted and maintained its asymmetry.