Standalone optical frequency-offset locking electronics for atomic physics

Abstract

We present a standalone frequency-offset locking system for controlling narrow-linewidth lasers using off-the-shelf electronic components. We lock two frequency-doubled 1560 nm lasers to a stable primary laser operating at 780 nm via their optical beat note. This radio-frequency beat note is fed through a broadband variable divider, a frequency-to-voltage converter, and a proportional-integrator controller to lock each follower laser to a tunable offset frequency relative to the primary. This architecture provides a large capture range ($> 1$ GHz), fast response times ($< 1$ ms), and high linearity. We achieve a frequency resolution of 1.9 kHz and a short-term fractional frequency instability $10^{-11}/\sqrt{τ\rm (s)}$ at 780 nm without the need for a dedicated, precise clock reference. We perform high-resolution spectroscopy of cold $^{87}$Rb atoms to demonstrate the tunability and precision of our locking system. We designed the system to be modular and extensible, making it applicable to a wide variety of atomic physics experiments, including laser cooling, spectroscopy, and quantum sensing with atoms, ions, and molecules.

Figures (8)

• Figure 1: Block diagram showing the different subsystems of our laser setup. F1: follower 1 laser, F2: follower 2 laser, PBS: polarizing beam splitter, $\lambda/4$: quarter-waveplate, $\lambda/2$: half-waveplate, PD: photodiode, FPD: fast photodiode, AOM: acousto-optic modulator, EOM: electro-optic phase modulator, MOT: magneto-optical trap. FPD1 and FPD2 measure the beat notes between the primary laser and follower lasers 1 and 2, respectively. FPD3 monitors the beat note between the two follower lasers.
• Figure 2: Top: beat note spectra measured between each pair of lasers. Data are fit to a Voigt distribution, which yielded full widths at half maximum of $1.412(9)$ MHz, $1.361(9)$ MHz, and $0.332(7)$ MHz, respectively. Bottom: fit residuals for each beat note spectrum.
• Figure 3: PCB layout (top) and the corresponding block diagram (bottom) of the locking system. PCB tiles for each stage are mounted to a support board that provides power, inputs for the beat note signal and user control voltage, and outputs for the error signal, laser feedback, and FVC monitor.
• Figure 4: (a) Prescaler division ratios $D$ for the seed values $S$ within our constrained 8-bit range. The approximate capture range of the lock is shown on the right-hand vertical axis. (b) Open-loop frequency sensitivity of the locking system as a function of $D$. In both plots, the shaded regions indicate the range accessible by our laser controllers ($\sim 2$ GHz capture range determined by a $\pm 1 ~\rm{V}$ current modulation range and 1.67 mA/V current sensitivity). Solid squares indicate the division ratios used in this work: $D = 712$, 936, and 1424.
• Figure 5: Closed-loop calibration of locking systems for followers 1 and 2. (a) Measurements of the optical beat frequency as a function of the control voltage. Linear fits to these data yield slopes of $-92.572(11)$ MHz/V for follower 1 ($D = 936$) and $-71.238(10)$ MHz/V for follower 2 ($D = 712$). (b) Fit residuals. The excursion near 1 V is due to the FVC nonlinearity, which is $< 0.1\%$ for both systems. (c) FVC output voltage as a function of beat frequency. Linear fits yield slopes of 0.010590(13) V/MHz for follower 1, 0.013864(21) V/MHz for follower 2.