In coherent population trapping (CPT) atomic clocks, a clock cell is usually filled with a buffer gas in addition to an alkali metal to obtain a narrow clock transition line, owing to the Dicke effect. However, this results in a frequency shift and broadening of the electric dipole optical transition. A Doppler-free spectrum cannot be obtained, causing difficulties in laser frequency locking. Usually, a separated alkali metal vapor cell without a buffer gas, referred to as the reference cell, is employed to obtain the Doppler-free spectrum via the saturation absorption, polarization, and modulation transfer spectra. Moreover, a frequency-shift device, such as an acoustic-optic modulator (AOM), is used to compensate for this optical frequency shift. However, this increases the size, weight, power, complexity, and cost of clock systems. Here, we report a compact laser frequency-offset locking scheme, denoted as dual modulation, which can be realized with the Doppler-free absorption-enhanced peak obtained after the interaction between the dual-modulated multichromatic laser and ⁸⁷Rb atomic ensemble in the reference cell.

First, based on half-wave modulation (HWM) in traditional CPT atomic clocks, a 200 MHz radio frequency (RF) signal is combined with a 3.417 GHz microwave signal through an electronic power combiner. Then, these signals are added to the direct current biasing device (bias-tee). Then, the laser driving the current forms dual-modulation in which the ±1 sidebands from a coherent bichromatic light for successive CPT resonance. Subsequently, the dual-modulated light is split into two arms, one of which is used as a pump light and sent to a reference cell (ref cell), where only pure ⁸⁷Rb is present. Because of the mirror, the pump transmission is reflected as a probe light, which overlaps the pump light and is incident on the cell again. The D₁ line of the ⁸⁷Rb spectra with the dual-modulated laser is observed in the reference cell to realize laser frequency offset locking. The other is sent to a clock cell for the CPT experiments, where the ⁸⁷Rb isotope-enriched vapor and buffer gas are filled. Finally, the dual-modulated light-passed clock cell is converted into an electrical signal by using a photodetector (PD), sampled using an analog-to-digital converter (ADC), and sent to a computer for processing.

First, the dual-modulated light interacts with the ⁸⁷Rb atom in the reference cell. When the RF frequency is equal to the buffer gas-induced frequency shift of the clock cell, a Doppler-free spectrum with enhanced absorption generated by the interaction with the RF-modulated sideband is obtained. The resonance peak frequency generated by the interaction between the first sideband of the 200 MHz RF modulation added to the HWM laser and ⁸⁷Rb atomic ensemble in the reference cell, coincides with the CPT involved 52S1/2,F=1,2→52P1/2,F'=2 optical transitions from the clock cell. Thus, the buffer gas-induced frequency shift of the involved optical transition in a clock cell is compensated, relative to a pure ⁸⁷Rb reference cell. Then, 200 MHz laser frequency offset locking is realized through dual modulation, and laser frequency noise suppression with this method is impressive, compared with that of a free-running laser. Its contribution to the short-term frequency stability of the CPT clock is at the 2.5×10^-14 level through a frequency modulation to amplitude modulation (FM-AM) effect. Subsequently, the CPT signal of the dual modulation was compared with that of the scheme by using an AOM to compensate for the frequency offset. We obtained a CPT clock transition of dual modulation with a contrast (C) of 1.5%, full width at half maximum (H_FWHM) of 210 Hz, and quality factor (Q) of 71 MHz^-1, which is comparable to the case of the frequency shift by the AOM.

In this study, we report a laser frequency-offset locking scheme in which the collision frequency shift produced by the buffer gas in the CPT-involved optical transition is compensated. The scheme additionally adds RF modulation to the laser diode through an electronic method in which the RF frequency equals the frequency offset owing to the collision frequency shift. The Doppler-free absorption enhancement peak, whose frequency corresponds to the optical transition from the clock cell, can be obtained through the interaction between the dual-modulated light and the ⁸⁷Rb in the reference cell. The laser noise is effectively suppressed using this spectrum to lock the laser frequency. The contrast of the (0↔0) transition signal is 1.5%, and its H_FWHM is 210 Hz. The effect is similar to that of the traditional scheme. Without the need for a bulky, expensive, and power-hungry AOM, our method can be used to implement compact and high-performance CPT and POP atomic clocks. It is also compatible with other laser locking methods, such as polarization spectroscopy and modulation transfer spectroscopy.