ObjectiveOptical frequency combs, which are critical for bridging microwaves and optics, represent significant advancements in optical frequency metrology. They are of paramount importance for high-precision spectral analysis, accurate absolute distance measurement, and the high-precision frequency standards establishment. Several metrology institutions in China have established optical wavelength and frequency standard devices based on optical frequency combs, providing traceability to microwave frequency standards for precise laser sources like frequency-stabilized lasers, ensuring accurate and reliable measurement of related quantities in industrial and research fields. As standard devices, optical frequency comb systems must undergo the same traceability process—comparison and calibration—to ensure accurate and reliable measurement values. However, due to their complex structure and large volume, the optical frequency comb devices used in practice are challenging to move for calibration. Therefore, the authors proposes a method of optical frequency comb comparison based on optical fiber network and optical "common-view method," mainly for the calibration of remote optical frequency combs, thus achieving regular traceability and calibration of optical frequency combs.
MethodsA method of calibrating remote optical comb using optical fiber networks is proposed. The optical combs including the standard comb and those to be calibrated, and the transmission laser to be measured are located at different nodes and connected to the network through optical fiber interfaces (
Fig.1) respectively. During the comparison or calibration process, the absolute frequency of the transmission laser is measured simultaneously at different locations using the standard optical comb and other optical combs under test. The measurements obtained are then processed and corrected for errors. The main error sources in optical comb calibration are experimentally evaluated. Initially, the noise of optical fiber link 1 (
Fig.3) was measured with the self-heterodyne loopback method. Subsequently, it also involves the experimental assessment of synchronization counting errors using manual and program-controlled counting modes respectively. It was also investigated whether there was consistency between the precision and stability of the rubidium clock and the optical frequency measurement results obtained using the optical comb under test, which was referenced to the rubidium clock. Finally, the calibration results of the optical combs are given comprehensively by correcting the above errors and absolute frequency measurement results.
Results and DiscussionsThe experiment measured the noise of the optical fiber link and the noise floor of the optical frequency transfer devices. The results were shown in
Fig.4. The transmitted frequency stability was measured to be 5.1×10
-15 at 1 s, and 7.0×10
-15 at 300 s respectively. Meanwhile, a frequency offset of -0.07 Hz was introduced by the fiber link. Because the rubidium clock utilized as the reference of the optical comb under test, the fiber link noise in the experiment can still meet the requirements of optical comb calibration without active fiber noise-cancelled. The authors also studied different counting methods and their accuracy differences, as shown in
Fig.6 and
Fig.7. The relative frequency offset is 6.55×10
-15 due to the manual counting method and 2×10
-16 due to by the programmed counting method, respectively. Both methods can meet the comparison requirements. Considering the objects and application scenarios of the comparison method, the manual counting method is recommended. The calibration result of the optical comb’s accuracy is shown in
Fig.9, where the triangle represents the result of 6 relative frequency offsets, and the red line is their mean value: 2.08×10
-10; measurement uncertainty is 3.16×10
-13; The gray area is the relative uncertainty of SRS FS725 rubidium clock frequency value used in the experiment. Therefore, it can be seen from the
Fig.9 that the optical absolutely frequency measurement results are within the rubidium clock frequency deviation range. The measurement stability of the comb of under test is investigated also, as shown in
Fig.10, and is consistent with the rubidium clock stability.
ConclusionsThe authors demonstrate remote optical comb calibration and comparison via an optical fiber network, a valuable contribution to meeting the practical requirements. Initially, the authors introduce the principles of remote calibration and comparison with fiber network. Subsequently, by using the internal fiber network and ultra-stable laser, the measurement accuracy and stability of the optical comb under test, referenced to the rubidium clock, were calibrated using a standard optical comb that was referenced to a
H maser, and provided the corresponding calibration values. Additionally, the optical frequency calibration results were compared with the performance of referenced rubidium clock, and the validity of the calibration work was verified. Meanwhile, the authors investigate the influence of optical frequency transmission noise due to fiber link, remote synchronous counting method, and frequency drift elimination method of measured laser using "common-view method" on remote calibration and comparison of optical comb, and quantitative evaluation results are given. This technique can be further expanded for application in the precision comparison and calibration of optical comb systems, referencing superior frequency standards such as
H maser.