| Narrow linewidth lasers are widely used in various research fields due to their advantages such as good monochromaticity,high stability,and long coherence length.However,the linewidth of these free-running lasers still cannot meet some of the more demanding scientific researches and industrial productions,such as coherent communication,precision measurement,optical frequency standards,absorption spectrum measurement,and researches on the interaction between light and matter,which requires a narrower linewidth laser to further increase detection sensitivity.The surface shape error of the optical element is an important parameter for evaluating its quality,and it is of great significance to improve the detection accuracy of the surface shape error.Optical interference techniques can achieve accurate online measurement of optical surfaces.The linewidth of the laser source limits the measurement accuracy of the surface shape,so the linewidth of the laser source needs to be further narrowed.In order to further narrow the linewidth of various lasers,the feedback control technique is usually used to stabilize the laser frequency to certain frequency references,such as the center frequency of the atomic transition line,some external frequency standards etc.In this paper,a self-designed ultra-stable cavity is used as a frequency reference,and the laser frequency is locked to the resonance frequency of the ultra-stable cavity to achieve an effective narrowing of the linewidth of the 632.8 nm external cavity diode laser(ECDL).The developed narrow linewidth laser generation system mainly includes four parts: super-stable cavity design,optical path design,laser frequency control,and system integration.The super-stable cavity adopts a two-mirror Fabry-Perot(F-P)cavity structure.The cavity body is made by a glass-ceramic with an expansion coefficient of about 10-6 K-1.The cavity mirrors are a pair of flat and concave mirrors with their reflectivities of 99.9885(± 0.0035)%.In order to minimize the influence of external mechanical vibration,acoustic noise,air flow,temperature change and other factors on the length and stability of the FP cavity,we adopted vacuum sealing,vibration isolation treatment,temperature control and other corresponding measures.Placing the F-P cavity into a vacuum environment can effectively reduce the effects of sound and air flow on the cavity mode.A molecular pump unit is used to control the vacuum of the chamber at 10-5 torr.In order to effectively isolate the mechanical vibration,the F-P cavity is separated from the vacuum chamber by a double-layer silicon rubber material.Although the expansion coefficient of the F-P cavity body is extremely low and has little effect on temperature,in order to further stabilize the length of the F-P cavity,a temperature control system is designed to control the temperature of the whole vacuum chamber to make the F-P cavity in a constant temperature environment.Considering the convenient transportation and transfer after system integration,we have designed a compact layout and stable structure of the optical path system.The ECDL used in this system is the DL pro series laser from Toptica Company,which has two frequency control terminals,includes piezoelectric ceramics(PZT)and current modulation,and the response bandwidths are 1k Hz and 100 MHz,respectively.The frequency control of the laser adopts Pound-Drever-Hall(PDH)frequency stabilization technique.The modulation frequency of 18 MHz is loaded to the laser current modulation port.The reflected signal from the F-P cavity is demodulated to obtain an error signal.After the frequency lock is performed,a feedback bandwidth of 1MHz is obtained.Finally,we narrowed the laser linewidth with its original value of about 300 k Hz in the free-running state to the order of 10 k Hz,and the frequency drift in 12 consecutive hours was about 30 MHz.The 632.8 nm narrow linewidth laser source developed in this paper can effectively improve the measurement accuracy of the surface error of optical elements by interference technique. |
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