Frequency-modulated continuous-wave (FMCW) interferometry is a pivotal technique for high-resolution, noncontact distance measurement. However, enhancing its resolution typically requires a substantial laser tuning range, and a systematic framework linking signal quality to ultimate resolution remains underdeveloped. This article introduces an Arcsine (Asin) phase demodulation method based on peak feature reconstruction to achieve high-resolution FMCW interferometry with a relatively compact tuning range. The proposed method reconstructs the phase via peak detection and an arcsine operation, significantly improving the accuracy of demodulation per unit tuning range. Experimental validation demonstrates that with a tuning range of only 26 GHz, our approach enables absolute distance measurement over 0-1 m with submicrometer resolution and an accuracy of 0.86 mu m.

Thin-film lithium niobate (TFLN) photonics has emerged as an appealing platform for on-chip systems. To achieve precise spectral control in various optical applications, such as dispersion compensation, pulse shaping, and optical signal processing, the accurate and efficient prediction of the spectral response for the TFLN chirped Bragg grating (CBG) is of great significance. The on-chip two-mode CBG with mode selective conversion capabilities offers an efficient way to extract the reflected light, eliminating the use of an optical circulator. However, this configuration poses a challenge in the design methodology. The finite-difference time-domain (FDTD) method is renowned for its high accuracy in numerical simulations but is extremely time-consuming when dealing with long CBGs. The transfer matrix method (TMM) can remarkably enhance computational efficiency for long-length CBG designs. Nevertheless, the predicted spectral responses are severely distorted. This is primarily due to the neglect of higher-order mode reflections at the discontinuous interface within the grating period. To address this issue, we propose a modified TMM considering the higher-order mode reflections in the transfer matrix, and the simulated results are in excellent agreement with those predicted by the FDTD method. Finally, an on-chip two-mode TFLN CBG is experimentally characterized in relation to the modified TMM design. The fabricated device exhibits a bandwidth of 12 nm, a group delay of 13.8 ps, and a low ripple of 0.9 ps, validating the correct and efficient function of the proposed modified TMM during the design of multimode TFLN devices.

In this paper, we propose and experimentally demonstrate an enhanced hybrid distributed multi-parameter fiber sensing based on distributed Brillouin amplification. By utilizing a continuous optical chirp chain (OCC) as a Brillouin pump, the power of the probe pulse can be amplified uniformly, thus improving the signal-to-noise ratio (SNR) of Rayleigh backscattering at the end of the fiber. Meanwhile, the temperature information can be extracted from the Brillouin loss spectra of OCC. In the experiment, OCC with a 60 ns segment duration and a 250 MHz sweeping range is employed. Benefiting from the Brillouin loss scheme, the SNR of phase-sensitive optical time-domain reflectometry (φ-OTDR) at the end of the fiber can be enhanced by approximately 10 dB. Distributed acoustic and temperature sensing is achieved with a strain sensitivity of 64 pε/Hz1/2, a temperature precision of 0.2 ℃, and a spatial resolution of 6 m over 50 km single-mode fiber.

On-chip photonic glucose sensors are important for future development in biosensing, disease diagnosis, and environmental monitoring, where a compact and high-sensitivity sensing head is the key component. In this paper, we demonstrate a high-sensitivity sensing head for glucose concentration monitoring using a Mach-Zehnder Interferometer (MZI) formed polymer waveguides with a microstructure gap on the sensing arm. By optimizing the length and gap of the sensing arm, the sensor achieves a sensitivity of glucose concentration of 2.37 nm/(g/L) (corresponding to 2651 nm/RIU) with a gap length of only 50 mu m. To validate the idea, we fabricate the on-chip sensor using an in-house microfabrication process. The fabricated sensing chip shows a sensitivity of glucose concentration of 2.32 nm/(g/L) (corresponding to 2554 nm/RIU) with low-temperature crosstalk, as the temperature sensitivity is only 31 pm/degrees C. The proposed on-chip sensor offers high sensitivity, low crosstalk, and low cost, making it a promising candidate for detection applications requiring a high-sensitivity sensing head.

In this Letter, we propose a method to improve strain sensing accuracy across the full-scale distance at the 100 m level in optical frequency domain reflectometry (OFDR) by minimizing residual phase noise (RPN), which significantly degrades strain accuracy. We derive a quantitative relationship between RPN variance and interferometer delay and design a coded delay fiber module (OPEM) that dynamically provides delay fibers of optimal length. By quantitatively analyzing RPN and guiding the output configuration of the OPEM to suppress RPN at the hardware level, we achieved strain sensing with a spatial resolution of 2 mm and a strain accuracy better than 1.5 με (2σ) across the full-scale distance at the 100 m level, approaching the theoretical accuracy limited by optical shot noise.

Multi-turn optical fiber coils, as the key sensing components of Fiber Optic Disc Accelerometers (FODAs), significantly affect the FODA performances due to their layered structure. This paper explores the strain distribution characteristics of the optical fiber coils by utilizing the inherent properties of the FODA, supported by Finite Element Method (FEM) and high-precision Optical Frequency Domain Reflectometry (OFDR). The results indicate that the simulation outcomes are largely consistent with the OFDR measurements, accurately reflecting the strain distribution within the optical fiber coils of the FODA. This analysis dose not requires additional external force application devices and carries no risk of damaging the FODA. Based on FEM, the simulation model is employed to optimize the existing structure, proposing a multi-turn optical fiber coil fabricated from ultra-fine diameter fiber. By altering the structure of optical fiber coil to make the internal strain distribution trend uniform, the sensitivity of the FODA is increased to approximately 150% of its original value.

The degradation of optical symmetry caused by the thermal strain of the fiber coil results in the thermal bias drift of fiber optic gyro (FOG). This paper presents a method for quantitative analysis of the optical symmetry of the fiber coil and accurate prediction of thermal bias drift in FOG. Firstly, a thermal strain model is established to illustrate the strain characteristics of the layer structure in the fiber coil under temperature fields, emphasizing the importance of thermal strain on optical symmetry. Then, an equivalent asymmetric index (alpha(Tk)) is introduced to quantitatively evaluate the optical symmetry of the fiber coil. By deriving equations for optical symmetry and thermal bias drift caused by thermal strain, a novel method is proposed to predict and optimize the thermal bias drift. This method utilizes high-performance OFDR to measure temperature and strain in a 3 km fiber coil to predict thermal bias drift. It also discusses the relationship between the length of the tail fiber and optical symmetry, predicting the precise length required to optimize thermal bias drift. Subsequently, the same fiber coil is placed in the FOG to verify the accuracy of the thermal bias drift prediction. Adjusting the length of the tail fiber reduces the thermal bias drift from 0.33 degrees/h to 0.13 degrees/h.

We proposed a novel, to the best of our knowledge, chirp-pulse pair phase-sensitive optical time-domain reflectometry (CPP-φOTDR) technique, enhanced by an adaptive filtering algorithm. This technique utilizes a pair of chirp pulses: one with a low chirp rate and another with a high chirp rate, with their Rayleigh backscattering (RBS) processed through a low-pass (LP) electrical filter. The adaptive filtering algorithm effectively preserves the extensive measurement range afforded by the high chirp rate pulse while enhancing the sensitivity provided by the low chirp rate pulse. Consequently, the CPP-φOTDR enables vibration measurements over a wide dynamic range and broad frequency bandwidth without incurring additional acquisition costs. In the experiments, we employed a chirp-pulse pair featuring bandwidths of 500 MHz and 8 GHz, utilizing only the receiver's 500 MHz bandwidth to retrieve the vibrational signal. The dynamic range of the CPP-φOTDR was enhanced by 25.1 dB, with the assistance of adaptive filtering of the acoustic waveform demodulated with RBS of the chirp-pulse pair. The proposed method could be utilized for monitoring the ocean in marine science and for analyzing seismic waves in geophysics.

Interferometric optical vector analysis (OVA) technique, employing orthogonal polarization interrogation and polarization diversity detection, provides comprehensive spectral response characterization of optical devices, though with exacting polarization alignment demands. To overcome this intrinsic limitation, we introduce an unbalanced orthogonal polarization-interrogated OVA scheme that operates without active polarization control. Besides, our method achieves performance parity with leading commercial instrument.

Objective Long-distance polarization-maintaining fiber is mainly used in submarine cables,optical fiber sensor networks,navigation and positioning,geophysical surveys,and other fields.One of the key concerns in the measurement field is the polarization crosstalk of the core component of the optical fiber gyroscope used for navigation and positioning.The optical frequency domain polarimetry(OFDP)is a new polarization measurement method that has advantages such as long measurement distance,high sensitivity,and small measurement times.It can be widely used to accurately characterize polarization-maintaining fibers and their devices.The main performance limitation of OFDP is interference phase error.While some parts of this error,such as the tunable light source's intrinsic phase noise,sweep frequency nonlinearity,and ambient noise,have been effectively suppressed,residual interference phase noise still exists.This residual noise can degrade the accuracy of polarization crosstalk measurements and is mainly caused by dispersion in the test optical path and birefringent dispersion in the device under test.Existing optical fiber dispersion compensation methods are mostly applied to absolute distance measurements and do not meet the requirements of distributed and transmitted optical polarimeters.We propose an OFDP optical path scheme(SR-OFDP)based on a self-reference interferometer.The accuracy of polarization crosstalk measurement is enhanced through the application of distributed iterative dispersion compensation technology.We hope that the chromatic dispersion verification method and the concept of distributed iterative dispersion compensation proposed in this study will contribute to the advancement of distributed dispersion compensation techniques in the optical frequency domain. Methods We first review the basic principles and testing scheme of OFDP.We then theoretically analyze the phase distortion caused by chromatic and birefringent dispersion and discuss the corresponding suppression schemes.Notably,due to the similarity between the phase term introduced by chromatic dispersion and the swept nonlinearity of the light source,it is possible to match the dispersion coefficient by adjusting the length of the delay ring in the interferometer.Then,we employ the SR-OFDP optical path scheme,where phase noise induced by chromatic dispersion in the optical path is eliminated through interpolation resampling.To mitigate the phase error caused by birefringent dispersion in the long-distance polarization-maintaining fiber ring,we propose a distributed iterative dispersion compensation scheme based on optimal criteria.The core idea is to use the criterion function to obtain the total second-and third-order dispersions of the measured fiber,and construct the corresponding dispersion compensation convolution kernel to convolve with the compensated signal,and finally,the phase error of wave number domain is compensated by dispersion in space domain. Results and Discussions In the OFDP accuracy optimization scheme,the experimental design and results for dispersion suppression are shown as follows.Firstly,experiments are designed to identify the source of chromatic dispersion in the optical path.The original single-mode fiber inside the auxiliary interferometer is replaced with a dispersion compensation fiber with a dispersion coefficient of-100 to-200 ps/(nm/km),simulating the case of mismatched dispersion coefficients in the interferometer.When the dispersion coefficients are approximately the same,a 500-meter polarization-maintaining fiber is tested.The amplitude accuracy of the polarization crosstalk peak at the main peak position improves from-25.5 dB to 0 dB,and the spatial resolution increases from 22.41 m to 9.6 cm(Fig.4),confirming that the dispersion in the measured optical path is caused by differences in the interferometer's dispersion coefficient.Subsequently,the SR-OFDP optical path scheme is employed to suppress this dispersion(Fig.5).Based on this optical path,distributed iterative dispersion compensation technique is used to obtain the distributed polarization crosstalk results of a 9.5 km PMF with a sensitivity of-105 dB in 2 s.After dispersion compensation,the amplitude accuracy of the end peak significantly improves by 20 dB(Fig.6).The forward and backward alignment results of measured fiber dispersion compensation indicate that the dispersion compensation algorithm has good spatial accuracy(Fig.7).In addition,after dispersion compensation,10 groups of repeated test results show that the standard deviation of each position along the fiber length is as low as 0.3 dB(Table 1),indicating that the results after the dispersion compensation algorithm exhibit good stability.Conclusions In the present study,the dispersion compensation method described is used to improve the accuracy of the polarimeter in the optical frequency domain,filling the gap in fiber dispersion compensation methods for optical frequency domain interferometry.Compared with traditional polarization measurement technologies(such as OCDP),which only satisfy a single index,the SR-OFDP technology with dispersion compensation capability offers distributed measurement,high sensitivity,and long-distance testing.The test speed can reach the order of seconds,and its excellent comprehensive performance is incomparable to other technologies.In the future,OFDP technology will play a key role in the production and fault analysis of high-precision optical fiber gyroscopes,provide more accurate testing means for optical fiber devices,components,and optical paths,and promote the further development of distributed polarization crosstalk testing technology.

This paper proposes a distributed birefringence measurement method for long-distance polarization maintaining fiber (PMF) based on polarization optical frequency domain reflectometry (P-OFDR). Distinguished from the existing ways, the method characterizes birefringence by the internal strain of the PMF, and cuts off the dependence of birefringence measurements on the wavelength of the interrogated laser. It obtains the internal strain through the cross-correlation of the Rayleigh backscattering (RBS) spectra from the two orthogonal axes of polarization. The measured internal strain is converted to birefringence according to the parameters of the PMF. In addition, a delay correction method is proposed for overcome the degradation of the RBS spectra correlation due to birefringence delay, and extends the measurement distance. In the experiments, the distributed birefringence measurement in 5020m PMF is achieved with a spatial resolution of 10cm and a measurement accuracy of 1.4×10-7. The single sweep is faster at only 1 second and offers excellent stability. Finally, we demonstrate the application of the method in a fiber coil. The measured distributed birefringence of the coil is symmetric. Stress concentration due to temperature variations and winding defects is clearly visible in the results. The method is helpful for quality evaluation of polarization maintaining devices and PMF, as well as potentially applying to birefringence-based distributed sensing.

As the core sensing elements of ultra-long fiber interferometer, the distributed thermal strain difference of the fiber rings can cause extra noise of the flexural disk, resulting in a penalty of the deterioration accuracy. In this paper, the thermal strain distribution characteristics of the fiber ring are firstly analyzed by the finite element method (FEM), and the distribution result is consistent with that demonstrated by the Rayleigh optical frequency-domain reflectometry (R-OFDR) strain measurement. The interferometer phase noise caused by the distributed strain difference is further studied by constructing a fully symmetric polarization-maintaining fiber-ring Mach-Zehnder interferometer (MZI) with an arm length of over 100 meters. The results show that the distributed thermal strain difference of two fiber rings will cause additional phase fluctuation, which leads to higher low-frequency noise. Therefore, a dual-fiber-ring MZI with matched distributed thermal strains is proposed to suppress the phase noise caused by the thermal strain, and the best suppression is as high as 45.6 dB. This is very important for the research and design of low noise fiber seismometer.

Optical frequency domain reflectometry (OFDR) can provide a powerful tool for fiber components and devices diagnosis and characterization. However, apart from laser source phase noise, the impact of the chromatic dispersion effect on the system is severely increased with the measurement length and laser tunable range. We propose a distributed chromatic dispersion compensation method for the device under the test (DUT) with complex structures containing different dispersive media connections in OFDR. Based on the mismatch factor (which is related to the difference in dispersion coefficient between the reference fiber and DUT), chromatic dispersion errors in the signal from different dispersion mediums can be compensated segment by segment by constructing the dispersion phase error signal. The compensation method is evaluated by experiments on DUT with multiple material dispersion (including reduced-cladding single-mode fiber (RC SMF) and dispersion compensation fiber (DCF)) and various measurement distances. Finally, we experimentally analyzed the internal reflection of a multifunctional integrated optical chip (MFIOC). Sweeping the laser source by 160 nm, the input coupling, output coupling, beam splitting, and defect points are individually revealed with a high resolution better than 15 mu m. We believe that this approach enables high-precision quantitative measurements as well as accurate identification and localization of fault diagnosis for optical fibers and devices.

Significance Distributed fiber sensing and measurement techniques have been given attractive attention in recent decades due to high sensitivity, high resolution, and large capacity. They have found a wide range of applications in the structural health monitoring of civil infrastructures such as bridges and dams, power-transmission line monitoring, oil-gas extraction and pipeline leakage detection, marine geophysical exploration, dynamic measurement, fiber-optic device characterization, fault diagnosis, etc. On the one hand, distributed measurement techniques can be categorized in principle into scattering effects (including Rayleigh backscattering, Brillouin scattering, and Raman scattering) and coupling effects (polarization crosstalk). On the other hand, these techniques can be divided into optical time domain reflectometry (OTDR), optical frequency domain reflectometry (OFDR), and optical coherence domain reflectometry (OCDR). OTDR employs the short and high power light pulse for interrogation, which is an effective tool for long distances. However, the tradeoff between sensing length and spatial resolution restricts the measurements to only meter-level spatial resolutions. OCDR utilizes the low coherence light from a broadband light source. They can offer a micrometer-level spatial resolution, whereas the measurement range is less than a few meters. OFDR is a distributed optical fiber measurement method based on the frequency-modulated continuous wave principle in the optical domain. It obtains the characteristics, such as scattering/reflection/loss and polarization features, along the optical fiber according to the mapping relationship between the Fourier transformation frequency of the interference signal and the characteristic location. In addition, the distribution of external physical fields, such as temperature/stress/strain sensing, can be further acquired. Unlike distributed measurement methods based on time-domain or coherent-domain, OFDR offers superior comprehensive properties, including high spatial resolution, high measurement sensitivity, long measurement distance, broad dynamic range, and high-speed response. However, due to the influence of phase noise, amplitude noise, and environment noise, the performance of OFDR in practice is not satisfactory. In the past few years, various methods have been proposed to compensate for the laser source noise and environment noise to improve the performance of the OFDR. Distributed sensing based on OFDR is also developing towards high performance and multi-parameters. With the continuous expansion and deepening of the application field, OFDR is facing more daunting challenges, which put forward higher requirements for its measurement performance and anti-interference ability. Therefore, it is of great importance and necessary to provide an overview of recent research progress in existing high-performance OFDR tests and sensing techniques to guide the future development direction. Progress We first review the measurement principle of OFDR and summarize key technologies to enhance OFDR system performance, such as the noise sources in distributed measurement (Fig. 1), the degradation mechanisms of the spatial point spread function (Fig. 3), and the error or noise compensation techniques. Then, the measurement limit of distributed sensing based on OFDR is derived, and several methods for improving the sensing accuracy and measurement distance are analyzed (Fig. 14). Subsequently, an outline of the current development status of domestic and foreign OFDR instruments is given (Table 6). Besides, application examples are given in measuring integrated waveguide devices, polarization maintaining fibers, and inside stress sensing of optical fiber coil. Finally, several future research directions of OFDR are prospected. Conclusions and Prospects OFDR systems can provide a good performance of high spatial resolution, high speed, and long measurement and sensing length. This technique can be widely applied to the fields of high-performance fiber optic component measurement and high-precision multi-parameter sensing. In the future, OFDR will continue to develop toward the goal of higher performance, stronger environmental adaptability, and higher measurement cost-effectiveness. The mixed modulation technology such as multi-domain localization (including time, frequency, and coherent domain) and multi-dimensional modulation (including amplitude, phase, and polarization modulation) can provide an effective way to break through the measurement limits and realize the ultra-high performance of OFDR technology. Furthermore, the high-precision OFDR sensing technology should be stepped up to meet the demands of multi-parameter decoupling and anti-interference ability improvement. Correspondingly, for the noise compensation algorithms at present, artificial intelligence and advanced algorithms are all important means for noise suppression capability enhancement and demodulation accuracy improvement. Besides, new requirements are put forward for the small size, low power consumption, and low cost of the core modules in OFDR instruments. With the continuous innovation of OFDR technology theory and the progress of technology development, China's current overall technology level has achieved international parallelism. However, the typical application fields of OFDR technology need to be continuously expanded, and the advantages of the technology need to be continuously emphasized. In this context, the development of domestic OFDR technology should be highly valued and vigorously developed to realize OFDR technology independent control and localization of hardware, including continuous mode-hopping-free tunable laser source, high-speed and high-precision optoelectronic conversion, and data acquisition module. Moreover, the OFDR technology should gradually move towards engineering applications in the field rather than being confined to laboratory measurements. The environmental adaptability of OFDR instruments should be enhanced to ensure that the core technical indicators of distributed testing and sensing are not degraded in different scenarios. Finally, a highperformance distributed specialized measurement and quantitative sensing methodology should be proposed to promote application development in core fields and typical scenarios, which provides a solid foundation and strong support for satisfying the requirements of applications such as testing of military devices, exploration of oil and gas resources, and power and energy monitoring.

Anti-resonant fiber (ARF) works well in a relatively strong magnetic field due to its weak Faraday effect, which results from the fundamental mode mainly transmitting in the air core. Accurately measuring the Faraday effect strength, i.e., the effective Verdet constant, of an ARF determines its applicable scenarios. However, the effective Verdet constant of ARF is ~3 orders of magnitude lower than that of a standard single-mode fiber, which is very difficult to measure. In this paper, we reveal that intermodal interference is the main obstacle to measuring the ultralow effective Verdet constant of ARF and propose using a narrow-band low-coherence light to suppress it. The measured effective Verdet constant of ARF is 0.423 ± 0.005 mrad/T/m at 1550 nm.

In this paper, we propose and demonstrate a spectral splicing method (SSM) for distributed strain sensing based on optical frequency domain reflectometry (OFDR), which can achieve km level measurement length, µɛ level measurement sensitivity and 104 µɛ level measurement range. Based on the traditional method of cross-correlation demodulation, the SSM replaces the original centralized data processing method with a segmented processing method and achieves precise splicing of the spectrum corresponding to each signal segment by spatial position correction, thus realizing strain demodulation. Segmentation effectively suppresses the phase noise accumulated in the large sweep range over long distances, expands the sweep range that can be processed from the nm level to the 10 nm level, and improves strain sensitivity. Meanwhile, the spatial position correction rectifys the position error in the spatial domain caused by segmentation, which reduces the error from the 10 m level to the mm level, enabling precise splicing of spectra and expanding the spectral range, thus extending the strain range. In our experiments, we achieved a strain sensitivity of ±3.2 µɛ (3σ) over a length of 1 km with a spatial resolution of 1 cm and extended the strain measurement range to 10,000 µɛ. This method provides, what we believe to be, a new solution for achieving high accuracy and wide range OFDR sensing at the km level.

A 260 m polarization multiplexing fiber interferometer with disc-type vibration pickup structure was built for acceleration sensing. The response coefficient was corrected to reduce the impact of temperature fluctuations by 207 times and suppress cross-interference.

Optical frequency domain polarimetry (OFDP) is an emerging distributed polarization crosstalk rapid measurement method with an ultrawide dynamic range. However, interferometric phase noise induced by the laser source and ambient noise results in a trade-off between measurement length and dynamic range. In this Letter, we solve this problem with a self-referenced unbalanced Mach-Zehnder interferometer. The features of long distance (9.8 km), ultrawide dynamic range (107.8 dB), short measurement time (2 sec), and signal-to-noise ratio improvement against ambient noise are experimentally demonstrated. The method makes it possible to evaluate a long polarization-maintaining fiber in an environment whose state changes rapidly.

Radial gradient index (GRIN) lens is widely used in optical path measurement based on white light interferometry (WLI), yet the additional error caused by the GRIN lens when the sample under test (SUT) is not at the ideal position has rarely been discussed. To deal with this problem, a ray-tracing model is established to reveal the influence of angular tilt and axial offset of SUT when GRIN lens is applied. Simulations based on WLI are done using the parameters of a real GRIN lens. The results show that the maximum additional optical path change is as larger as $0.99~\mu \text{m}$ within the range of ±0.1° angular tilt and $\pm 2000~\mu \text{m}$ axial offset of SUT, which can be regarded as additional error if the influence of GRIN lens is ignored. Experimental results of optical path change and insertion loss at different tilt angles of SUT simultaneously agree well with one set of simulation.

Simultaneous measurement of thickness and refractive index is an important quality control scheme in high-end fabrication. However, the precise measurement of large-range thickness is still technically difficult. In this article, we propose a high-precision simultaneous measurement method of thickness and group refractive index using differential white light interferometry. Using the bi-directional white light interferometer with a high dynamic range, we can get access to the optical paths on both sides of the specimen. We evaluate the performance of the proposed method by measuring specimens of three different materials, with different thicknesses ranging from 200 to $2000 \ \mu \text{m}$ . The measurement data are in good agreement with the nominal thickness, and the standard deviations of repeated measurements are $\le 0.009 \ \mu \text{m}$ and $\le 1.63\times 10^{-4}$ refractive index unit (RIU) for thickness and group refractive index, respectively. In addition, we evaluate the impact of the temperature change on the group refractive index. A quasi-linear relationship between group refractive index and temperature is observed, and the slope of the linear fit of silicon (Si) specimen around 20 °C is about $3.29\times 10^{-4}$ RIU/°C at 1310 nm. We also analyze and summarize the main factors which may introduce uncertainty in the measurement results. The expanded uncertainties ( $k =2$ ) of measured thickness and group refractive index are $\le 4.21\times 10^{-2} \ \mu \text{m}$ and $\le 7.84\times 10^{-4}$ RIU, respectively. The repeatability and uncertainty of group refractive index measurement both decrease as the thickness of the specimen increases.