Experimental implementation of FM absorption in buffer gas cell

Chapter VI: Frequency Modulation Spectroscopy of YbOH

6.4 Experimental implementation of FM absorption in buffer gas cell

Tunable

Laser EOM

Buffer Gas Cell

Photodiode 25kHz-125 MHz

40 dB Amp

41-58 MHz Bandpass

24 dB Amp RF Oscillator

50/50 Splitter

25 dB Amp

1 dB Attenuator

Phase

Shifter I & Q Demodulator LO

RF Q I

1.9 MHz Low Pass

1.9 MHz Low Pass SRS SR560

3 kHz Low Pass 20x Amp

SRS SR560 3 kHz Low Pass 20x Amp

𝐼_𝐹𝑀(𝜔) 𝑄_𝐹𝑀(𝜔)

50.3 MHz

Figure 6.7: Diagram of experimental FM absorption spectroscopy setup. The green lines indicate the laser path and the black lines indicate the rf (or DC after demodulation) signal path. Detailed descriptions of the components and their functions are given in the text.

reference to be tuned, which allows the phase angle,𝜃, of the𝐼_{𝐹 𝑀}(𝜔)and𝑄_{𝐹 𝑀}(𝜔) signals to be adjusted. We generally operate at a phase shifter voltage where the 𝐼_{𝐹 𝑀}(𝜔)and𝑄_{𝐹 𝑀}(𝜔) signals are approximately equal in magnitude.

After sidebands are applied to the laser with the EOM, the laser beam passes through the buffer gas cell and is detected with an AC-coupled fast photodiode (New Focus 1801 photoreceiver) with a 25 kHz - 125 MHz bandwidth. The resulting AC signal is then amplified with a 40 dB low-noise amplifier (Mini-Circuits ZKL-1R5+) and passed through a 41-58 MHz band pass filter before being input into the RF port of the I and Q demodulator. Multiple combinations of a second amplifier and additional bandpass, low-pass, and high-pass filters were tried, none resulted in improved SNR compared to the single amplifier and bandpass filter.

The I and Q demodulator is essentially two rf mixers and a 90^◦ phase shifter. The output of the I port is the in-phase demodulated DC signal resulting from the mixing of the photodiode signal (RF port) and the rf reference (LO port). The output of the Q port is the in-quadrature demodualted DC signal resulting from the mixing of the photodiode signal (RF port) and the rf reference (LO port) with a 90^◦phase shift. The outputs of both the I and Q ports are passed through 1.9 MHz low-pass filters and input into SRS SR560 low noise pre-amplifiers. The SR560s are set to

have a 12dB/oct 3 kHz low-pass filter and 20x amplification. Due to the fact that the molecular pulse is ∼1 ms long, setting the low-pass filter cutoff any lower begins to filter out the DC FM signal. The outputs of both of the SR560s are the measured 𝐼_{𝐹 𝑀}(𝜔)and𝑄_{𝐹 𝑀}(𝜔) signals.

It is important to note that before we used the rf circuit described above, we originally tried to accomplish FM detection using an SRS SR 844 rf lock-in amplifier. This rf lock-in had all the same features as the discrete rf circuit in one package and with the ability to tune the filter cutoffs and amplification. However, we found that the lock-in introduced a large amount of∼60 Hz and ∼120 Hz line noise which could not be easily filtered out. Therefore, we abandoned the rf lock-in and moved to the discrete rf circuit as it provided much better SNR.

2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90

Frequency -17680 cm

¹

6 4 2 0 2 4

Int eg ra te d I

FM

S ign al (a rb .)

Nomalized FM In Phase

2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90

Frequency -17680 cm

¹

0.075 0.050 0.025 0.000 0.025 0.050 0.075 0.100

Integrated OD (arb.)

Normalized DC Absorption

Figure 6.8: DC absorption and in-phase FM absorption of two lines of the [17.68] band of YbOH. For this scan four ablation shots were taken at each frequency step and averaged. A frequency step of 9 MHz was used. Both the integrated DC optical depth and the integrated 𝐼_{𝐹 𝑀} signal were normalized by the integrated OD from a normalization probe fixed to the^𝑅𝑅₁₁(0)line of the ˜𝐴²Π_1/2(0,0,0) −𝑋˜²Σ⁺(0,0,0) band of YbOH [116].

The implementation of FM absorption spectroscopy in the cryogenic buffer gas cell provided a factor of ∼10 improvement in the SNR. This improvement in SNR is illustrated in Fig. 6.8 and 6.9 where comparisons of DC and FM absorption signals of spectral features of the weak [17.68] band of YbOH7 are shown. In Fig. 6.8 and 6.9, both the DC (integrated OD) and FM (integrated 𝐼_{𝐹 𝑀} signal) signals are

7The spectrum of this band is discussed later in Section 6.6.

normalized to the integrated OD from a normalization probe fixed to the ^𝑅𝑅₁₁(0) line of the ˜𝐴²Π_1/2(0,0,0) −𝑋˜²Σ⁺(0,0,0)band of YbOH [116]. Fig. 6.8 shows a slow scan over two isolated lines from in the[17.68]band. Here the laser frequency was stepped in 9 MHz intervals and the average of four ablation shots at each frequency step was taken. The resulting DC OD and in-phase FM signal,𝐼_{𝐹 𝑀}, were integrated over the time of the molecular pulse to obtain the spectrum in Fig. 6.8.

The DC absorption spectrum in Fig. 6.8 has a SNR of 1.6 while the in-phase FM spectrum has a SNR of 17.2, a factor of 10.7 improvement. Here SNR is defined as the ratio of the DC or FM signal amplitude8 to the max positive amplitude of the noise.

Another portion of the[17.68]spectrum recorded at a faster scanning speed, a speed more typically used when taking broadband high-resolution spectra, is shown in Fig.

6.9. For this faster scan, the laser was continuously scanned at∼10 MHz/sec, data collected at a repetition rate of 8.7142 Hz and every eight neighboring data points averaged to provide DC and FM signals every∼10 MHz. In this case, essentially no spectral features are visible in the DC absorption spectrum. However, the drastically improved SNR from the FM absorption allows clear identification of many spectral features including very weak features such as the one observed at∼17681.77 cm⁻¹. It is clear that the implementation of FM absorption spectroscopy in the buffer gas cell allows the measurement of weak transitions which are otherwise impossible to measure via more traditional means, such as DC absorption.

8In the case of the FM signals this is the maximum of the positive peak of the FM signal.

1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90

Frequency -17680 cm ¹

0.60.4 0.20.0 0.20.4 0.6

Integrated IFM (arb.)

Nomalized FM In Phase

1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90

Frequency -17680 cm ¹

0.10 0.05 0.00 0.05 0.10 0.15

Integrated OD (arb.)

Normalized DC Absorption

Figure 6.9: DC absorption and in-phase FM absorption of a portion of the [17.68]

band of YbOH. Here the laser was continuously scanned, ablation shots were taken every∼1.3 MHz, and sets of the 8 nearest ablation shots were averaged resulting in data points every∼10 MHz. Both the integrated DC optical depth and the integrated 𝐼_{𝐹 𝑀} signal were normalized by the integrated OD from a normalization probe fixed to the^𝑅𝑅₁₁(0)line of the ˜𝐴²Π_1/2(0,0,0) −𝑋˜²Σ⁺(0,0,0)band of YbOH [116].

6.5 FM spectroscopy of 𝐴˜²Π_1/2(1,0,0) −𝑋˜²Σ⁺(3,0,0)band of¹⁷⁴YbOH