一种用于超稳激光的数字控制锁频电路

Digitally Controlled Frequency⁃Locked Circuit for Ultrastable Laser

超稳激光具有超高的频率稳定度和极窄线宽等优点,广泛应用于各种精密测量物理实验。为了确保不引入额外开关噪声,其频率锁定电路通常采用模拟电路实现,但是模拟控制电路存在锁定参数调节不方便、难以实现自动锁定和远程控制等方面的不足。针对这些问题,提出一种基于数字控制的通用型模拟锁频电路。该电路可以通过数字开关实现对锁定参数的大范围调节,通过数字算法实现自动重锁,同时该电路具有USB通信功能,可以通过电脑远程调整锁频参数和实现频率锁定。利用该电路实现了激光器频率到高精细光学谐振腔的长期锁定,锁定后激光频率稳定度1 s积分时间4.6×10^(-16),2~10 s积分时间小于4.2×10^(-16),接近10 cm超稳腔的热噪声极限。这种长期稳定运行的超稳激光系统将有利于基于超稳激光的精密测量物理实验的开展和光钟走向应用。

Objective Ultrastable lasers offer the benefits of ultrahigh-frequency stability and extremely narrow linewidths.They are crucial in atomic clocks,optical-frequency transmission,gravitational-wave detection,Lorentz-invariance testing,and other applications.Typically,an ultrastable laser is created using the Pound-Drever-Hall (PDH) technique to lock the laser frequency to an ultrastable Fabry-Perot (F-P) cavity.Owing to the continuous progress and development of science and technology,the demand for scientific tasks is increasing.Simultaneously,higher requirements are imposed on the stability and long-term locking ability of ultrastable lasers.When the laser frequency is locked,circuits or mechanical disturbances may cause the laser to be unlocked.Once this occurs,the ultrastable laser must be relocked promptly.Analog feedback circuits are commonly used to implement frequency-locked to avoid introducing additional digital noise.However,the conventional analog circuits present some disadvantages,including inconvenient adjustment of locking parameters,difficulty in automatic locking,and necessity for remote control.Hence,this study proposes a universal analog frequency-locked circuit with digital control.Methods A digitally controlled analog frequency-locked circuit was designed to stabilize the frequency of various types of lasers,such as Nd∶YAG,fiber,and external-cavity diode lasers.To satisfy the requirements of different lasers and cavities,the proportionalintegration-differentiation (PID) parameters of the circuits were adjusted from hundreds of hertz to hundreds of kilohertz.Additionally,a microcontroller,digital switches,and digital potentiometers were integrated into the circuit to enable the digital control of the locking parameters and locking switches.To determine whether the digital chips imposed additional bandwidth limitations to the circuit,the transmission characteristics of the frequency-locked circuit in the open loop were measured using a vector network analyzer.An Agilent 34401A digital multimeter was used to measure the voltage of the error signal with a null input after locking.Subsequently,the stability of the error signal was calculated and compared to test whether the circuit stability and noise level were affected by the digital-control chips.Two identical frequency-locked circuits were applied to two ultrastable laser systems developed by our team.After the locking parameters were optimized,the laser frequency was set to off-resonance and the auto-relock function was activated to verify the automatic relocking effect of the frequency-locked circuit.Finally,the frequency stability of the locked laser was assessed by measuring and analyzing the beat frequencies of two sets of ultrastable lasers.Results and Discussions The introduction of digital-control chips does not affect the feedback bandwidth of the frequency-locked circuit,as shown in Fig.3.This implies that the digital switches do not delay the signal.The stability of the error signal improved compared with that afforded by our previous purely analog design,as illustrated in Fig.4.This is because the circuit structure is optimized and the digital circuit remains in a silent state when the laser is frequency-locked,thus preventing digital noise caused by changes in the digital-circuit state.In the experiment,when the circuit’s relock function is activated,the laser frequency can be swept to the resonance and locked.The laser is obstructed when the laser frequency is locked.After the laser obstructer is removed,the laser frequency shifts rapidly (see Fig.6).After being locked,the laser frequency stability is 4.6×10^(-16) at an integration time of 1 s,whereas it is less than 4.2×10^(-16) from 2 s to 10 s (Fig.7),which is similar to the thermal-noise limit of the 10 cm ultrastable cavity.Conclusions A digital-control analog frequency-locked circuit is developed.This circuit not only preserves the benefits of analog circuits,such as high feedback bandwidth,low noise,and high offset stability,but also achieves the digitization of frequency-locked parameters and control.As such,it enhances the adjustment flexibility and reliability of frequency-locked.It comprises a wide range of adjustable parameters and satisfies the frequency-locked requirements of various lasers.Additionally,the circuit incorporates a frequency-relock function that can automatically relock the laser frequency when it becomes unlocked,thus ensuring that the laser frequency remains locked for an extended period.The introduction of digital control chips does not increase the amount of additional noise in the frequency-locked circuit or reduce the feedback bandwidth.The feedback bandwidth of the frequency-locked circuit is approximately 20 MHz,and the locking-error stability of the circuit is 100 n V.Two circuits are used to lock the two customdeveloped ultrastable lasers and evaluate the stability of the beat frequency.The short-term stability of the ultrastable laser is measured to be 4.6×10^(-16) at 1 s of integration time,less than 4.2×10^(-16) from 2 s to 10 s of integration time,and 4.0×10^(-16) at 4 s of integration time,which is similar to the thermal-noise limit of the 10 cm cavity.