As an important wavelength reference, the 543 nm frequency stabilized laser offers a distinct alternative to the commonly used 633 nm stabilized He-Ne laser, playing a significant role in high-precision length and wavelength metrology. This paper presents a novel method for generating and stabilizing a 543.5 nm laser that differs fundamentally from traditional stabilized He-Ne lasers. The approach leverages an acousto-optic modulator (AOM) to phase-lock a single-frequency, narrow-linewidth 1087 nm fiber laser to an optical frequency comb, and subsequently generates a highly stable 543.5 nm laser through second-harmonic generation (SHG). This approach enables the frequency comb's high precision to be transferred seamlessly and without degradation to a continuous-wave laser. The paper analyzes the SHG process in a periodically poled lithium niobate (PPLN) crystal for generating the 543.5 nm output, and discusses the advantages of locking the CW laser to the optical frequency comb using an optical phase-locked loop (OPLL). Experimental results show that this method yields 48 mW of 543.5 nm laser output, with frequency fluctuations <1 Hz and a relative uncertainty of 1.0 & times; 10(-)& sup1;& sup3; after locking to the frequency comb. Compared with traditional 543 nm iodine-stabilized He-Ne lasers, this new method significantly enhances both the output power and frequency stability of the laser. The 543.5 nm laser was then used to measure the wavelength of a low-power thermally stabilized 543 nm He-Ne laser, thereby achieving direct traceability of laser wavelength measurements to primary frequency standards. This result highlights the method's potential as a new-generation wavelength reference in the green spectral region, contributing significantly to the advancement of precision optical metrology.

Objective The 1.5 mu m laser is widely used in modern fiber-optic communicatiObjective The 1.5 mu m laser is widely used in modern fiber-optic communication systems due to its low propagation loss in optical fibers, which effectively supports long-distance transmission. Additionally, the 1.5 mu m laser plays a crucial role in gas analysis, environmental monitoring, and remote sensing. Furthermore, a high-performance 1.5 mu m frequency-stabilized laser can serve as a reliable wavelength reference source for dense wavelength division multiplexing (DWDM), precision fiber-optic sensing, and other high-accuracy measurement applications. However, the conventional 1.5 mu m acetylene gas absorption frequency stabilization systems are often complex and bulky. To address this issue, this study develops a low-cost and structurally simple 1.5 mu m distributed feedback semiconductor laser diode (DFB-LD) frequency stabilization system, integrating acetylene gas absorption characteristics with current modulation techniques. By employing a self-developed digital control circuit and the modulation-demodulation algorithm, the laser wavelength is accurately locked. Methods A self-developed digital control system is used to regulate the laser output frequency via current feedback, achieving precise and stable laser frequency control. The system transmits the real-time laser status to a host computer through a serial communication protocol, enabling monitoring and adjustments. By combining acetylene gas absorption characteristics with current modulation techniques, the system utilizes a microcontroller unit (MCU) to perform laser wavelength tuning and demodulation of the modulation signal. The laser undergoes 2.5 kHz modulation, generating a 60 MHz modulation depth, and is split into two parts by a 1 & ratio;9 optical coupler, where 90%of the output light is fed into the optical heterodyne detection system and the remaining 10%is used for frequency stabilization. The stabilization beam passes through an acetylene gas cell and is subsequently converted into an electrical signal by a photodetector. This signal is processed by a bandpass filter and sent to an analog-to-digital converter (ADC) chip for high-precision sampling before being transmitted to the MCU for further processing. The MCU applies a cross-correlation algorithm to analyze the collected signal, extracting the error signal, which represents the deviation of the laser frequency from the reference absorption peak. This error signal is fed into a proportion-integration-differentiation (PID) control circuit, which calculates the required current correction value based on the error signal. The correction current is subsequently injected into the semiconductor laser, forming a current control signal, thereby stabilizing the laser output frequency and ensuring precise frequency locking at the acetylene gas absorption peak. This process forms a closed-loop control system, ensuring long-term frequency stability and high-precision locking. By optimizing a few PID and pre-divider parameters, the system enables flexible frequency tuning. Additionally, the high signal-to-noise ratio (SNR) harmonic demodulation allows for accurate identification of the absorption peak and rapid frequency locking. Results and Discussions The heterodyne beat signal of the semiconductor laser is measured and compared under free-running and feedback-controlled conditions. By comparing the heterodyne results with those from an optical wavelength reference device, it is confirmed that the proposed frequency stabilization system effectively enhances laser frequency stability. With current feedback and temperature control, the 1 s frequency stability is optimized from 8.8x10(-9) to 3.2x10(-9,) while the long-term 1000 s stability is optimized from 7.1x10(-8 )to 9.7x 10(-11), achieving an optimization of nearly three orders of magnitude. The system also demonstrates a reproducibility of 1.1x10(-8) fully validating the effectiveness of the feedback control system in frequency stabilization. By optimizing the laser structural layout and the digital control strategy, the system achieves miniaturization and integration while maintaining high precision. This meets the requirements of precision measurement, gas analysis, LIDAR, and other applications for a high-accuracy laser wavelength reference source. Conclusions This study investigates a digital frequency stabilization control method for a 1.5 mu m distributed feedback semiconductor laser, achieving laser frequency stabilization through a self-developed digital control system with current feedback regulation. The digital stabilization system transmits the real-time laser status to a host computer via a serial communication protocol, enabling real-time monitoring of the scanning or locking state of the laser, thereby enhancing system interactivity and monitoring capabilities. The digital frequency stabilization technology proposed in this study successfully overcomes the limitations of traditional on systems due to its low propagation loss in optical fibers, which effectively supports long-distance transmission. Additionally, the 1.5 mu m laser plays a crucial role in gas analysis, environmental monitoring, and remote sensing. Furthermore, a high-performance 1.5 mu m frequency-stabilized laser can serve as a reliable wavelength reference source for dense wavelength division multiplexing (DWDM), precision fiber-optic sensing, and other high-accuracy measurement applications. However, the conventional 1.5 mu m acetylene gas absorption frequency stabilization systems are often complex and bulky. To address this issue, this study develops a low-cost and structurally simple 1.5 mu m distributed feedback semiconductor laser diode (DFB-LD) frequency stabilization system, integrating acetylene gas absorption characteristics with current modulation techniques. By employing a self-developed digital control circuit and the modulation-demodulation algorithm, the laser wavelength is accurately locked. Methods A self-developed digital control system is used to regulate the laser output frequency via current feedback, achieving precise and stable laser frequency control. The system transmits the real-time laser status to a host computer through a serial communication protocol, enabling monitoring and adjustments. By combining acetylene gas absorption characteristics with current modulation techniques, the system utilizes a microcontroller unit (MCU) to perform laser wavelength tuning and demodulation of the modulation signal. The laser undergoes 2.5 kHz modulation, generating a 60 MHz modulation depth, and is split into two parts by a 1 & ratio;9 optical coupler, where 90%of the output light is fed into the optical heterodyne detection system and the remaining 10%is used for frequency stabilization. The stabilization beam passes through an acetylene gas cell and is subsequently converted into an electrical signal by a photodetector. This signal is processed by a bandpass filter and sent to an analog-to-digital converter (ADC) chip for high-precision sampling before being transmitted to the MCU for further processing. The MCU applies a cross-correlation algorithm to analyze the collected signal, extracting the error signal, which represents the deviation of the laser frequency from the reference absorption peak. This error signal is fed into a proportion-integration-differentiation (PID) control circuit, which calculates the required current correction value based on the error signal. The correction current is subsequently injected into the semiconductor laser, forming a current control signal, thereby stabilizing the laser output frequency and ensuring precise frequency locking at the acetylene gas absorption peak. This process forms a closed-loop control system, ensuring long-term frequency stability and high-precision locking. By optimizing a few PID and pre-divider parameters, the system enables flexible frequency tuning. Additionally, the high signal-to-noise ratio (SNR) harmonic demodulation allows for accurate identification of the absorption peak and rapid frequency locking. Results and Discussions The heterodyne beat signal of the semiconductor laser is measured and compared under free-running and feedback-controlled conditions. By comparing the heterodyne results with those from an optical wavelength reference device, it is confirmed that the proposed frequency stabilization system effectively enhances laser frequency stability. With current feedback and temperature control, the 1 s frequency stability is optimized from 8.8x10-9 to 3.2x10-9, while the long-term 1000 s stability is optimized from 7.1x10-8 to 9.7x 10-11, achieving an optimization of nearly three orders of magnitude. The system also demonstrates a reproducibility of 1.1x10-8, fully validating the effectiveness of the feedback control system in frequency stabilization. By optimizing the laser structural layout and the digital control strategy, the system achieves miniaturization and integration while maintaining high precision. This meets the requirements of precision measurement, gas analysis, LIDAR, and other applications for a high-accuracy laser wavelength reference source. Conclusions This study investigates a digital frequency stabilization control method for a 1.5 mu m distributed feedback semiconductor laser, achieving laser frequency stabilization through a self-developed digital control system with current feedback regulation. The digital stabilization system transmits the real-time laser status to a host computer via a serial communication protocol, enabling real-time monitoring of the scanning or locking state of the laser, thereby enhancing system interactivity and monitoring capabilities. The digital frequency stabilization technology proposed in this study successfully overcomes the limitations of traditional analog control methods, such as tedious tuning and low intelligence levels. It significantly improves system accuracy and stability while optimizing operational procedures. This method aligns with the trend of digital and intelligent transformation in metrology and exhibits broad application prospects in precision gas analysis, LIDAR, remote sensing, and other high-precision measurement fields. It provides an efficient, accurate, and intelligent solution for modern metrology and high-precision measurements.

Calibration of the grating pitch typically requires metrological instruments or diffraction techniques. The traceability and stability demands for laser wavelength in these techniques present substantial challenges for onsite grating calibration. This paper introduces a novel traceability approach to grating pitch calibration using a grating pitch comparator that utilizes the transition frequency of the chromium. The chromium transition frequency is converted into a chromium grating through atom lithography, establishing a self-traceable length standard dCr = 212.7787 +/- 0.0049 nm. Calibration of the test grating's pitch is achieved by comparing the phase shift caused by the simultaneous movement of the chromium grating and the test grating. By using the dCr, this approach combines the high-accuracy of fundamental physical constants with the environmental stability of grating interferometers, eliminating the need for measuring laser wavelength, air refraction, diffraction angles. It significantly enhances the precision and makes more straightforward and portable, crucial for facilitating traceable on-site grating measurements.

Objective The 633-nm frequency-stabilized helium - neon (He-Ne) laser has been extensively employed in diverse fields. In nanoscale-length metrology, laser interferometric displacement measurement systems, which serve as traceable instruments for precise length determination, exhibit high measurement accuracies. When the refractive-index fluctuations are negligible, the ultimate precision of the long-range length measurements is fundamentally constrained by the relative frequency stability and absolute accuracy of the laser source. Contemporary ultra-precision measurement systems, requiring multi-axis and multi-channel capabilities, impose stringent demands on light sources, necessitating simultaneous realization of exceptional frequency stability (10(-11)), absolute frequency accuracy, and a high output power (e.g., total dual-beam power >= 800 mu W for interferometric applications). Nevertheless, conventional 633-nm frequency-stabilized He-Ne lasers exhibit performance limitations. Power-balanced thermal frequency-stabilized configurations provide adequate output power (>500 mu W) but insufficient frequency stability (10(-8)-10(-9)). In contrast, iodine-stabilized lasers achieve metrological-grade frequency stability (10(-11)) and accuracy at the cost of reduced power (similar to 200 mu W) as well as residual frequency modulation unsuitable for high-speed interferometry. To address these constraints, in this study, we have implemented an optical phase-locked loop (OPLL) architecture to lock the frequency of a fully intracavity, thermally stabilized He-Ne laser to both an iodine-stabilized frequency standard and external-cavity diode laser (ECDL). This hybrid approach synergistically combines the high-power capability of thermal stabilization (>600 mu W) with the exceptional frequency stability of reference sources (<10(-11)), thereby fulfilling the dual requirements of modern ultra-precision measurement systems while reducing the density of modulation artifacts inherent in direct iodine-stabilized outputs. Methods First, a complete system architecture comprising three principal subsystems, i.e., beat signal acquisition, locking logic execution, and result recording/analysis, was established. A parametric model for the loop filter was subsequently developed based on the error signal characteristics. The initial frequency locking of the slave laser to the iodine-stabilized reference laser enabled the acquisition of beat frequency data. Second, a systematic investigation of the degradation mechanisms underlying its long-term stability was performed. This step included rigorous documentation of the correlation between the laser tube temperature and PID parameters during the locking procedures. To validate these findings, the reference source was substituted with an ECDL, and relocation experiments were conducted. The resultant beat frequencies exhibited enhanced stability metrics, with no observable deterioration in long-term stability performance. Because the reference laser exhibited non-negligible frequency fluctuations, the measured beat frequency stability inherently represented only the relative stability between the slave and reference lasers. Consequently, a comprehensive characterization of the slave laser's optical frequency stability must account for the residual fluctuations in the reference source. The direct determination of the intrinsic frequency stability of a slave laser theoretically requires beat frequency analysis against a secondary reference laser with superior stability; however, such metrological-grade references are often unavailable in practical scenarios. Although the root-sum-square combination of the individual laser stabilities provides a nominal estimate, this approach introduces systematic uncertainties. Therefore, a precise evaluation of the absolute frequency stability of the slave laser necessitates computational reconstruction of its real-time frequency fluctuations using advanced signal decomposition techniques rather than relying solely on relative beat frequency measurements. Results and Discussions The experimental results demonstrate that the frequency stability achieved by the proposed method, while delivering an output power of 1 mW, is superior to that of power-balanced thermal frequency-stabilized lasers. When employing an iodine-stabilized laser as the reference, the slave laser attains short-term optical frequency stability comparable to that of the reference source, reaching 2.6x10(-11) at tau =1 s. To improve long-term stability, the fully intracavity, thermally stabilized He-Ne laser is frequency-locked to an ECDL, achieving optical frequency stability of 3.8x10(-12) at tau =1 s (Fig. 7), with a relative standard measurement uncertainty of 4.0x10(-12) (Table 1). Extended integration time testing reveals further stability enhancement (1.9x 10(-13) achieved at tau =1000 s; Fig. 8). These metrics confirm the efficacy of the ECDL-based locking scheme in suppressing long-term drift while maintaining metrological-grade precision. Statistical results from multiple experimental trials confirm that when implementing frequency-offset locking with high-stability references, computational correction for reference laser fluctuations becomes less critical. Conversely, applying the method for calculating the synthesized frequency stability is essential when employing lower-stability references such as iodine-stabilized lasers, as evidenced by the observed diverging stability metrics in such configurations. Conclusions This paper presents a method and experimental implementation for enhancing laser frequency stability through OPLL-based frequency-offset locking of a fully intracavity, thermally stabilized He-Ne laser to reference sources. The method simultaneously, systematically accounts for the influence of reference laser frequency fluctuations on the actual optical frequency stability of the slave laser via synthetic computation. Experimental and computational results demonstrate that when locked to an ECDL, the composite Allan deviation curve indicates that the performance of the laser system improves by nearly an order of magnitude compared to that of conventional iodine-stabilized laser systems. In addition, its output power surpasses that of iodine-stabilized systems by more than four-fold. Nevertheless, the long-term stability achieved by locking iodine-stabilized lasers remains suboptimal. To address this limitation, future studies will be focused on replacing thermal actuation with a piezoelectric transducer (PZT)-based cavity length control mechanism. This modification is anticipated to eliminate the thermal hysteresis and equilibrium instability inherent in thermal regulation, thereby further improving the frequency stability of fully intracavity He-Ne lasers in frequency-offset locking configurations with iodine-stabilized references.

Nanometrology serves as the foundation of nanomanufacturing and underpins progress in frontier scientific research and advanced manufacturing. To achieve high-precision displacement measurements, the establishment of a reliable traceability chain is essential. Traditionally, displacement metrology relies on a traceability chain based on the 632.8 nm iodine-stabilized laser wavelength. Alternatively, atomic lithography offers a novel route by enabling the fabrication of chromium atom lithography gratings (Cr ALGs), whose grating pitch is directly traceable to the atomic transition frequency of chromium, thereby forming a new, simplified traceability chain in nanometrology. To validate the accuracy and reliability of this emerging traceability chain, this work adopts the 632.8 nm wavelength as the measurement reference and employs a diffraction-based method to determine the large-area average pitch of Cr ALGs, thereby enabling a direct comparison between the two traceability routes. A frequency-doubled 632.8 nm laser system was developed and combined with a longitudinal deviation method and twice diffraction alignment technique, achieving a Littrow diffraction angle resolution of 0.1 arcseconds. This allows for precise measurement of grating pitches ranging from 500 l/mm to 5000 l/mm. The system's accuracy was verified by comparing the measured pitch of a Physikalisch Technische Bundesanstalt-calibrated standard grating. Using this system, the calibrated pitch of the Cr ALG was determined to be (212.7779 +/- 0.0021) nm (k = 1), which shows picometer-level agreement with the theoretical pitch derived from the Cr atomic transition frequency. This large-area, laser-referenced calibration approach effectively reflects the pitch values and uncertainties under practical application conditions, thereby confirming the accuracy and reliability of value transfer within the simplified traceability chain based on Cr ALGs.

Objective With the rapid development of the aerospace and microelectronics industries,the demand for ultra precision measurement is also increasing.He-Ne lasers are widely used in mechanical and ultra precision measurement fields due to their excellent coherence and other characteristics.Among them,the thermally stabilized He-Ne laser is suitable as a wavelength scale laser for laser interferometry due to its high frequency stability,good beam quality,and low cost.However,traditional thermally stabilized lasers have poor frequency stability and reproducibility,which cannot further meet the requirements of high-precision laser interferometry for frequency stability and accuracy.This article reports a frequency biased locking system for thermally stable He-Ne laser based on a combination of an acousto-optic modulator and an optical phase-locked loop.This system combines the high-frequency response characteristics of an acousto-optic modulator with the high sensitivity characteristics of an optical phase-locked loop,enabling fast and accurate frequency locking of a thermally stable He-Ne laser. Methods This article reports an optical phase-locked loop bias locking system based on an acousto-optic modulator.An iodine stabilized frequency laser is chosen as master laser,and a thermally stabilized He-Ne laser as the slave laser.The beam of the slave laser is modulated by an acousto-optic modulator and locked onto the master laser.The reference signal for frequency offset locking is a 30 MHz signal generated by a signal generator.Data are collected using a frequency counter.The locking result is shown(Fig.9). Results and Discussions In the experiment,a highly stable He-Ne laser based on intracavity saturation absorption stabilization was used as the wavelength reference source for thermal stabilization laser locking.Through beat frequency measurement with iodine stabilized laser wavelength reference,the results show that the 1 s wavelength stability of the iodine stabilized laser is 1.3×10-11,reaching 4.1×10-13 in 1 000 s,reproducibility better than 1.0×10-11.The frequency jitter of the laser beat frequency after the system is locked is shown(Fig.10).As a comparison,the figure shows the drift of the beat frequency under free operation.In the experiment,a frequency counter was used to count the beat frequency signal for 30 min in the open-loop state of the optical phase-locked loop.Then,the reference frequency was set to 30 MHz to lock the thermal stabilized frequency laser to the iodine stabilized frequency laser,and the beat frequency after the loop locking was continued to be counted for 180 min.The beat frequency was locked at a bias frequency of 30 MHz,with a fluctuation range below 0.2 Hz.We have achieved high stability frequency locking of thermally stabilized lasers compared to iodine stabilized lasers.The relative Allen variance of the frequency offset of the optical phase-locked loop is shown(Fig.11).Among them,the relative Allen variance of the integration time of 1 s and 1000 s is 3.3×10-9 and 1.4×10-12 respectively. Conclusions This article introduces a high stability laser frequency stabilization method based on the combination of an acousto-optic modulator and an optical phase-locked loop.An experiment was conducted using a self-developed optical phase-locked loop system to lock the bias of a thermally stable all cavity He-Ne laser to an iodine stable frequency laser.The signal-to-noise ratio of the beat frequency signal was increased to over 40 dB(Fig.9)through a beat frequency signal detection unit based on an acousto-optic modulator.A digital frequency discriminator and PI control circuit were used to feedback control the acousto-optic modulator,achieving closed-loop control of the optical phase-locked loop.The frequency stability of the thermally stable He-Ne laser is significantly improved,enabling it to meet the requirements for laser frequency stability and accuracy in the fields such as ultra precision interferometry and ultra sensitive spectral detection.

Objective Based on the principle of iodine molecule saturation absorption for frequency stabilization, the 633-nm He-Ne laser holds considerable application value in geometric metrology, precision interferometric measurements, atomic spectroscopy, and gravity measurements. Recently, digital metrology has emerged as the future direction of metrology. Although 633-nm iodine-stabilized He-Ne lasers based on analog control systems offer high frequency stability and cost-effectiveness, their further development is constrained by limitations in intelligence and digitization levels. The 633-nm iodine-stabilized He-Ne laser, which is based on a digital control system, not only satisfies the demands of digitization but is also more suitable for miniaturization. However, due to the precision limitations of the employed analog-digital (AD) and digital-analog (DA) converters, the frequency stabilization performance of the 633-nm iodine-stabilized He-Ne laser based on digital circuits is often less ideal, with frequency stability typically lower than that of iodine-stabilized He-Ne lasers based on purely analog control systems. To address the aforementioned issues, in this study, a 633-nm iodine-stabilized laser is proposed based on a modulo-mixed control approach. It combines the advantages of high levels of digitization and stability. Methods The structure of the laser system is depicted in Fig. 1. The laser comprises two main components: the laser head and control system. The laser head incorporates a self-developed high-power iodine-stabilized He-Ne laser head, comprising a laser tube, iodine cell, high-reflectivity mirror, piezoelectric ceramic, photodetector, and thermoelectric cooling element. The control system consists of two parts: analog circuitry and digital circuitry. The analog circuitry section consists of three parts: a sinusoidal signal generator, an optical power signal demodulator, and a proportional-integral-differential (PID) controller. The sinusoidal signal generator employs a Wien bridge sinusoidal signal generation circuit. By selecting low-temperature drift precision components and finely tuning the component parameters, high-quality output can be realized. The optical power signal demodulator is realized via a mixer. The demodulated third harmonic signal used for locking realizes a satisfactory level, with a signal-to-noise ratio of 8:1. The PID controller allows for gain adjustment through a digital potentiometer, which is managed by an mirco-controller unit (MCU), expanding the adaptability of laser frequency locking to different scenarios. The digital circuitry section, employing two absorption peak recognition algorithms, achieves automatic locking of absorption peaks. It enables the uploading of laser operational data to a computer, enhancing the digitization level of the laser system. Combining the aforementioned sections, the final design of the laser system is successfully implemented. Results and Discussions To assess the automatic peak-locking functionality of the laser, the newly developed laser is compared with the iodine-stabilized He-Ne laser, a national length standard, with both locked onto the d-peak and g-peak. The frequency difference between the two peaks is set at 39.422 MHz. In the experiment, the newly developed laser is intentionally unlocked, and the locking process is monitored via beat frequency. The experimental results are depicted in Fig. 10. The results demonstrate that the system can successfully relock, with a relocking duration of 30 s. After locking, the laser remains positioned in a stable manner at the target absorption peak. To assess the long-term stability of the laser wavelength, a 14-h beat frequency experiment is conducted using an optical frequency comb as the reference laser. The experimental results are presented in Fig. 11. The findings indicate that over the 14-h measurement period, the average vacuum frequency value obtained is 473612353616 kHz, with a deviation of 12 kHz from the internationally recommended value by the International Committee for Weights and Measures. Without altering the analog PID parameters, the laser overall PID gain is adjusted through a digital potentiometer. The wavelength stability (Allan standard deviation) of the laser, as determined by the beat frequency with an optical frequency comb, before and after PID gain adjustment, is illustrated in Fig. 12. Maintaining the same PID parameters and appropriately adjusting the overall PID gain after locking contribute to an improvement in frequency stability. After gain adjustment, the frequency stability is measured as 7.3x10(-11) for 0.1 s, 1.4x10(-11) for 1 s, 3.0x10(-12) for 10 s, 8.5x10(-13) for 100 s, 3.1x10(-13) for 10(3) s, and 2.0x10(-13) for 10(4) s. The experimental results demonstrate that the laser exhibits a high level of frequency stability. Conclusions The 633-nm iodine-stabilized He-Ne laser proposed in this study, based on the modulo-mixed control method, exhibits crucial digital features such as automatic peak recognition. It satisfies the urgent demand in the metrology industry for digitally-enabled length measurement standards. Simultaneously, leveraging analog circuitry technology, the laser achieves high-precision locking of absorption peaks. Experimental results demonstrate a frequency stability of 1.4x10(-11) for 1 s and 3.1x10(-13) for 10(3) s, satisfying the metrology industry requirements for high stability in 633-nm iodine-stabilized He-Ne lasers. The laser system presented in this study provides essential technical support for the application of the 633-nm iodine-stabilized He-Ne laser in precision measurements and digital metrology.

Objective The precise wavelength measurement of a 543-nm He-Ne laser traditionally relies on iodine-stabilized 543-nm He-Ne laser as the wavelength reference source,which is used to measure the thermally stabilized 543-nm He-Ne laser wavelength via beat frequency beating method.Usually,thermally stabilized 543-nm He-Ne laser employs as a secondary laser wavelength standard,which is widely used in precise measurement.Furthermore,iodine-stabilized 543-nm He-Ne laser is the internationally recommended standard laser frequency reference with its relative standard uncertainty of 4.5×10-11.The iodine-stabilized 543-nm He-Ne laser utilizes the intracavity saturation absorption frequency stabilization method,requiring the insertion of the iodine cell into the laser resonant cavity.However,the gain of the Ne atom at 543 nm is notably low,approximately 1％of that at 633 nm.Consequently,the output power of the single longitudinal mode is limited,reaching only approximately 50 μW.Influences,such as mechanical drift,device aging,and other factors,can lead to detuning of the laser resonator cavity,and mismatches with the iodine cell.Hence,prolonged absence in light might be experienced by the 543-nm iodine-stabilized laser,compromising its utility as a laser wavelength reference.Thus,the development of a new wavelength measurement method for the 543-nm He-Ne laser is deemed essential. Methods In this study,a new method for accurately measuring the wavelength of the 543-nm He-Ne laser with an optical frequency comb is reported.This method employs offset locking technology to lock the 1086-nm distributed feedback single frequency fiber laser to the optical frequency comb.A frequency-stabilized 543-nm laser is generated by the nonlinear crystal PPLN via a frequency doubling process.The beat notes,detected via the generated CW 543 nm laser and measured 543-nm thermally stabilized He-Ne laser,can achieve accurate wavelength measurement of the measured laser. Results and Discussions The innovative approach for accurately measuring the wavelength of the 543-nm He-Ne laser using an optical frequency comb addresses the challenges of low output power and low signal-to-noise ratio typically observed in beat frequency measurements at 543 nm.Experimental data indicates a signal-to-noise ratio of 41 dB between the 1086-nm single-frequency fiber laser and optical frequency comb.After locking the 1086-nm laser,a frequency fluctuation range of 0.2 MHz is observed over 3 h,with a standard deviation of 0.03 MHz.A frequency jitter range of 1.3 MHz is noted between the 543-nm laser,produced by frequency doubling,and tested He-Ne laser,with a standard deviation of 0.2 MHz.The short-term(1 s)frequency stability and long-term(1000 s)frequency stability of the assessed 543-nm He-Ne laser outperform with values better than 2.1 × 10 10 and 1.1 × 10-10,respectively.These findings closely match the direct measurement outcomes of the iodine-stabilized laser,which exhibits a frequency difference of 2.9 MHz. Conclusions In this study,the method for precise wavelength measurement of 543-nm He-Ne laser via optical frequency comb is reported.This method utilizes offset locking technology to lock the 1086-nm single frequency fiber laser to the optical frequency comb.Furthermore,a frequency-stabilized 543 nm laser,generated via frequency doubling with a PPLN crystal,serves as the reference for gauging the wavelength of the thermally frequency-stabilized He-Ne laser.Within 3 h after offset locking,the beat frequency fluctuation range between the single-frequency fiber laser and optical frequency comb is 0.2 MHz,and the standard deviation is 0.03 MHz.The frequency jitter difference between the 543-nm laser,generated by frequency doubling,and measured thermally stabilized He-Ne laser is 1.3 MHz,standard deviation is 0.2 MHz,short-term(1 s)frequency stability is better than 2.1× 10-10,and long-term(1000 s)frequency stability is better than 1.1 ×10-10.This method measuring the wavelength of 543 nm is equivalent to the direct beat frequency measurement of iodine stabilized laser.In this method,the low-power 543-nm He-Ne frequency stabilized laser wavelength measurement is realized via frequency doubling.The link between continuous laser and optical frequency combs is established and the laser wavelength measurement is directly traced to the time and frequency reference,which effectively ensures the accuracy of the measurement results.

Objective In length measurement, the laser interferometer is used as the length-based standard at all levels, and the measured length value is calculated according to the emission wavelength of its light source. The stable and reliable output laser frequency (or wavelength) characteristics of various frequency stabilized lasers as the light source of the interferometer are the basic working conditions to ensure the normal operation of the interferometer. The 633 nm internal-mirror He-Ne laser with the double longitudinal mode power balance frequency stabilization principle is controlled based on the central frequency of the laser medium gain curve. However, the central frequency of the gain curve is affected by the air pressure in the tube. Therefore, the laser frequency after frequency stabilization is related to the gas pressure in the tube. For the full-cavity He-Ne laser tube, the change of the pressure in the cavity is mainly caused by the temperature change, and the pressure in the cavity increases linearly with the increase of temperature. Therefore, the frequency of the laser output is closely related to the temperature. As the frequency stabilized He-Ne laser is used more and more widely, there are higher requirements for the environmental adaptability of the frequency stabilized lasers. For example, the frequency stabilized He-Ne laser used in the laser interferometric absolute gravimeter needs to work at different ambient temperatures. Therefore, the frequency reproducibility of the current frequency stabilized He-Ne laser at extreme temperatures is greatly restricted. Methods In this paper, the free-running characteristics of the 633 nm internal-mirror He-Ne laser tube at - 20-40 V are studied. According to these characteristics, a preheating and frequency stabilization control scheme of the laser system is designed, and the frequency stability of the 633 nm internal-mirror He-Ne laser is realized. When the room temperature is about 24 , the locked thermally stabilized laser beats with the high-precision iodine stabilized laser, and the relative standard uncertainty and Allen variance of the thermally stabilized laser are obtained. In addition, 6 sets of frequency reproducibility experiments are carried out in 3 months. The ambient temperature is T = (24 +/- 2) , and the measurement time is more than 1 h each time. The frequency reproducibility of the thermal frequency stabilization control system of the 633 nm internal-mirror He-Ne laser is investigated, and the influence of ambient temperature on the frequency stabilization results is studied. Different ambient temperatures for frequency stabilization experiments are set up. The ambient temperature is in - 20-40 with an interval of about 20 . The temperature and beat values of the laser tube wall after frequency stabilization are measured at different ambient temperatures. At the same time, the pressure estimation model is used to estimate the frequency drift with ambient temperature after frequency stabilization, which is compared and verified with the experimental results. Results and Discussions The free-running process of the laser tube at the ambient temperature of - 20-40 is studied. The experimental results show that when the laser tube reaches the approximate thermal equilibrium state at different ambient temperatures, the voltage difference of the two longitudinal modes changes for 40 cycles (Fig. 5) , and the thermal expansion rate of the cavity length is also the same, which shows that the ratio of heating to heat dissipation of the laser system is constant in the thermal equilibrium state. In addition, there is a linear relationship between the wall temperature of the laser tube and the initial ambient temperature after the system reaches the thermal equilibrium (Fig. 6 ) , which shows that even if the laser is at different ambient temperatures, the heat exchange capacity between the laser system and the external environment is approximately unchanged, so the difference between the temperature of the wall of the laser tube and the initial ambient temperature is fixed when the system finally reaches the thermal equilibrium. In addition, the influence of ambient temperature on the frequency stabilization results is investigated. The experimental results show that the temperature of the laser tube wall changes with the ambient temperature after frequency stabilization, and the relationship between them is linear (Fig. 11). At the same time, the drift of the laser output frequency with the ambient temperature after frequency stabilization is about 293 kHz/t , which is consistent with the drift value of 268 kHz/t calculated by the pressure estimation model. After linear interpolation calibration, the difference between the actual frequency value and the reference output frequency value is less than 1.81 MHz (Fig. 12). Conclusions In this paper, the free-running characteristics of the 633 nm internal-mirror He-Ne laser tube at the ambient temperature of - 20-40 degrees C are studied. The experimental results show that when the laser tube reaches the thermal equilibrium at different ambient temperatures, the difference between the laser tube temperature and the ambient temperature is fixed, the expansion rate is also the same, and the voltage difference of the double longitudinal mode changes for the same period. According to these characteristics, the preheating and frequency stabilization control scheme of the laser system is designed, and the frequency stabilization of the 633 nm internalmirror He-Ne laser is realized. When the room temperature is about 24 degrees C the beat frequency results of the locked thermally stabilized laser and the high-precision iodine stabilized laser show that the relative standard uncertainty of frequency within 3 h is u = 6.4 x10(-9). When the sampling time tau = 1 s, the corresponding Allen variance is 7.0 x 10(-11). When 0.1 s <=tau <= 2000 s, the Allan variance is better than 4.3 x 10(-10), and the frequency reproducibility within 3 months is better than 4.6 x 10(-9). This paper also studies the frequency drift law of the output laser after locking at the ambient temperature of - 20-40 degrees C. The experimental results show that the temperature of the laser tube wall changes linearly with the ambient temperature after frequency stabilization. At the same time, the drift of the laser output frequency with the ambient temperature after frequency stabilization is about 293 kHz/t , which is consistent with the drift value of 268 kHz/t calculated by the pressure estimation model. In this regard, when the laser is working in a large temperature range (such as - 20-40 degrees C), interpolation calibration can be used to obtain a more accurate reference output frequency.

光学锁相环(OPLL)根据其锁定的两束激光间是否存在频差可分为零差光学锁相环和外差光学锁相环.主要介绍了外差光学锁相环的研究进展,它是一种通过鉴频鉴相方式使激光间的频率差保持相对稳定的偏频锁定方法.相较于其他激光偏频锁定方法,光学锁相环具有结构简单、伺服频率带宽大、频率偏置范围宽、锁定准确度高等优势,在原子相干、冷原子系统、相干功率合成以及外差干涉测量等领域都得到了越来越广泛的应用.首先介绍了激光偏频锁定的主要方法及光学锁相环的特点,其次介绍了光学锁相环的基本模型,并分析了光学锁相环的误差反馈过程.按照光学锁相环实现方法的不同详细介绍了其采用的关键技术和研究进展,对近年来光学锁相环在不同领域的应用进展做了简要介绍,最后对该方法的发展路线进行了总结和展望.

利用光学拍频法可实现稳频He-Ne激光波长稳定度的测量以及波长不确定度的评定.报道了一种可应用于测量现场的便携式633nm激光波长评价系统,介绍了拍频测量技术原理、激光波长评价系统的基本构成以及与非便携式的传统拍频系统测量激光波长的比对结果.波长评价系统对传统光学拍频光路部分进行了改进并集成为一个便携测量系统,系统采用了全新设计的集成式光学拍频模块、小型化高精度频率计以及频谱监测仪器实现了高信噪比拍频信号的产生、计数与监测,并自主开发了拍频测量数据采集、处理与分析软件.实验结果表明该系统能够在测量现场实现激光波长的稳定度以及不确定度评价,提升了现场测量的便捷性以及测量效率.

报道了一种基于光学锁相环的高稳定度激光稳频方法,用于提高可调谐外腔半导体激光器(TECDL)的频率稳定度和准确度.自行研制的光学锁相环电路采用数字鉴相与差分运算相结合的方式获得高灵敏度的鉴频鉴相误差信号,并通过高速模拟PID实现整个系统的闭环锁定.利用该光学锁相环系统进行了TECDL偏频锁定至光学频率梳(OFC)的实验,实验结果表明环路锁定后拍频频率波动在±0.3 Hz范围内,偏置频率为50 MHz时,光学锁相环系统在1 s和1 000 s积分时间的相对阿伦方差分别为1.5×10-9和8.5×10-13.系统锁定后,拍频线宽由500 kHz压缩至2 kHz.该研究表明采用基于光学锁相环的激光稳频方法可以实现亚Hz级的激光频差控制,通过将TECDL偏频锁定至高稳定度的参考激光源可显著提高其频率稳定度,使其能够满足超精密测量、冷原子/离子干涉测量等领域对激光频率稳定度和准确度的要求.

光辐射计量基标准是国家计量体系的重要组成部分,是相关量值的溯源源头.光辐射计量正在朝着量值复现量子化、计量基标准量值传递扁平化方向发展. 根据发展趋势,中国计量科学研究院牵头承担了国家重点研发计划"光辐射计量基标准研究"项目(项目编号:2016YFF0200300),在光辐射计量基标准体系中的波长、功率以及它们与时间、空间量组合的量值方面开展研究.项目研究了光辐射测量量子化技术、高稳定度激光波长复现技术、高温固定点黑体辐射技术、光腔衰荡法气体成份测量技术、太赫兹关键参数计量技术.

计算电容是电学阻抗单位复现的基准装置,是电容、电感和交流电阻的溯源依据.它是电磁计量领域内除量子电压、量子电阻基准之外准确度最高的装置.计算电容装置实现微小电容值(0.2至0.6 pF)精确测量的关键在于对其屏蔽电极位移的高准确度测量.从计算电容屏蔽电极位移测量的基本原理出发,系统分析了屏蔽电极位移测量结果的影响因素,具体包含Fabry-Perot干涉仪的锁定、稳频激光波长的实时校准、残余气体折射率影响、测量余弦误差以及Gouy相位修正,结合计算电容测量结果的复现性,得到屏蔽电极位移测量的相对标准不确定度为4.7×10-9.

A compact digital control system based on an all-programmable system-on-chip iodine-stabilized laser is presented for realization of the meter. The system is composed of ZYNQ7000, peripheral circuits, and human-computer interaction, which can operate independently. An nth-harmonic extraction algorithm with less resource consumption is used in this system. The digital system overcomes the problems of complex debugging, large volume, and manual locking. Additionally, customers can set up, calibrate, and upgrade the system by themselves. Its stability is similar to that of the current analog system, with long-term stability of up to 10-13. The repeatability of the two lasers with the digital system is approximately 1.5×10-11, and the absolute frequencies satisfy the international recommendation.

Accurate iodine spectroscopy measurement plays important role in wavelength metrology, precise spectrum detection and interferometry measurement. In all the spectrum of iodine, the saturation absorption lines at 633 nm are the most widely used with the iodine stabilized He-Ne laser. Due to its weak output power, it is difficult to directly measure the absolute frequency with femtosecond frequency comb. In this paper, we present a new optical beating note system based on polarization maintaining fibers, which is employed to measure the weak iodine stabilized laser successfully. The experimental results show that a standard deviation of the frequency fluctuation of 12 kHz and a relative stability of 1.4×10-11 at 1 s are obtained, respectively. The reproducibility of the iodine stabilized laser is also studied, and a confident result of 3.8 kHz in peak to peak value is obtained with six months.

In order to meet the requirement of high output power of the laser monochromatic light source in the precision measurement, a high-power iodine stabilized He-Ne laser system with a fully enclosed, integrated structure was developed. The principle of saturation spectral detection, the method of absorption peak recognition and locking and the frequency stability of iodine stabilized laser were studied. Firstly, the basic principle of detecting saturation absorption spectrum of iodine molecular using the three harmonic method was introduced, and its mechanism of eliminating the power background was analyzed. Then, the stability of the integrated resonant cavity in the iodine stabilized laser was demonstrated, and the effects of axial expansion and lateral asymmetric deformation on the output power were discussed in detail. After that, the correspondence between the profile of laser output power and the iodine molecular saturation absorption peaks was presented, the feasibility of using the secondary harmonic signal to achieve absorption peak recognition was introduced, and the long-term locking ability of high-stability resonant cavity was demonstrated. Finally, the wavelength stability and reproducibility of high-power iodine stabilized He-Ne laser were analyzed. The experimental results showed that the standard deviation for the frequency jitter of high-power iodine stabilized He-Ne laser was 33 kHz, the stability at 1 000 s and the reproducibility in three months were 4.1×10−13 and 3.3 kHz (7.0×10−12), respectively. Its absolute frequency was 3.0 kHz lower than the recommended value by the International Committee for Weights and Measures (CIPM).

This paper illustrates the improvements on the laser interferometer in the calculable capacitor at nim, including the fiber coupling optical layout, mechanical drift and aging monitor, and parasitic interference reduction. Using the new fiber coupling, we can more flexible to change the laser source without break the beam layout. And the reduction of parasitic interference is more than one order in amplitude with the benefit of antireflection coatings.

The experiments were carried out aimed to compare the capacitance measured in terms of the new calculable capacitor standard to the capacitance measured in terms of the von Klitzing constant ( RK) using the quantized Hall resistance (QHR). The experiment systems such as the calculable capacitor, the quantized Hall resistance standard, and the impedance measurement chain are briefly presented. The recent two key comparison results on capacitance and resistance standards are also introduced.

原子光刻光栅的研制存在光栅上纳米周期结构覆盖面积小和纳米条纹高度不一致的问题,限制了原子光刻光栅的应用.本文介绍通过椭圆光斑激光汇聚铬原子沉积大面积一维光栅的技术,将纳米周期结构的覆盖范围扩大到1.5mm×1.5mm,同时,沉积条纹高度的一致性得到有效改善.用椭圆光斑驻波场进行原子光刻研究对实验装置调整的精细度和对准提出了更高的要求.并分析了影响大范围原子沉积的几个关键因素.