Digitally Controlled Frequency-Locked Circuit for Ultrastable Laser
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 proportional-integration-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 nV.Two circuits are used to lock the two custom-developed 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.
ultra-stable laserPound-Drever-Hall frequency stabilizationdigital controlled frequency-locked circuitauto-relock
万方数据
维普
