Fourier transform-limited optical frequency-modulated continuous-wave interferometry over several tens of laser coherence lengths

Modeling and optimization of an unbalanced delay interferometer based OPLL system

We present and establish a versatile analytical model that allows overall analysis and optimization for the phase noise performance of the delay interferometer based optical phase-locked loop (OPLL). It allows considering any type of lasers with arbitrary frequency noise properties while taking into account the contributions from various practical noise sources, thus enabling comprehensive investigation for the complicated interaction among underlying limiting factors. The quantitative analysis for their evolution along with the change of the delay of the interferometer unveils the resulting impact on the fundamental limit and dynamics of the output phase noise, leading to a well-balanced loop bandwidth and sensitivity thus enabling the overall optimization in terms of closed-loop noise performance. The tendencies observed and the results predicted in terms of coherence metrics in numerical verification with different lasers have testified to the precision and effectiveness of the proposed model, which is quite capable of acting as a design tool for the insightful analysis and overall optimization with guiding significance for practical applications.

Compact multifunction digital OFDR system without using an auxiliary interferometer

This paper describes an integrated, accurate, and inexpensive semiconductor laser -based optical frequency domain reflectometry (OFDR) system design. The system utilizes the fiber under test for both sensing and frequency sweep linearization functions, allowing the system to mitigate and compensate for phase errors without the need for an auxiliary interferometer, as is the case for traditional OFDR systems. Benefiting from the unique and embedded design, this system reaches the minimal OFDR system with only one optical interferometer and its corresponding optic-electric components without sacrificing accuracy. In addition, conventional design requires an external auxiliary interferometer, which may experience different noises from the main measuring interferometer, deteriorating the overall performance. Experimental results demonstrate the enhanced performance of the compact design as compared with the former methods, as well as the reduced complexity and improved cost-effectiveness.

Dual-frequency Doppler velocimeter based on delay interferometric optical phase-locking

We present a dual-frequency laser Doppler velocimeter (DF-LDV) relying on a DF laser source (DFLS) generated by optical phase-locking two individual lasers to a common unbalanced Mach-Zehnder interferometer, which allows achieving high stability regardless of the DF separation of the lasers. This DFLS is evaluated using an optical frequency comb, testifying to the generation of DFLS with large DF separation up to terahertz with flexible tunability and high stability. Demonstration of DF-LDV using the DFLS of ${\sim}1.024\; {\rm THz}$ separation has achieved $1.62 \times {10^{- 2}}$ mm/s velocity resolution even for a slow velocity of $1.8\; {\rm mm}/{\rm s}$ in a mere 5 s acquisition time, confirming the high resolution and efficient speckle noise suppression enabled by the proposed DF-LDV. Featuring high precision, flexibility, and robustness, this method is particularly attractive from the practical point of view.

Optical linear frequency sweep based on a mode-spacing swept comb and multi-loop phase-locking for FMCW interferometry

We report on the generation of a highly coherent broadband optical linear frequency sweep (LFS) using mode-spacing swept comb and multi-loop composite optical phase-locked loop (OPLL). We exploit a specially designed agile opto-electronic frequency comb as a sweeping reference, whose mode-spacing is capable of arbitrary frequency sweep while preserving a stable phase and power distribution per mode. By locking a continuous-wave (CW) laser to any of its modes using composite OPLL with a large loop bandwidth, it allows the extraction of the optical LFS at high-order modes in a coherent manner with a multiplied sweep range and rate. With such capability, only intermediate frequency LFS with smaller bandwidth is required to yield a broadband LFS while inheriting the coherence and precision from the comb. We achieve optical LFS of 60 GHz at 6 THz/s sweep rate with a nine-folded sweep bandwidth of the driving signal. Fourier transform-limited spatial resolution at more than 80 times of the intrinsic coherence length of the CW laser is demonstrated in an OFMCW interferometry, verifying the high coherence with more than 4 orders of magnitude improvement in spatial resolution. The characteristics in terms of agility, coherence, and precision are discussed together with the potential limitations. The proposed method is capable of generating arbitrary frequency-modulated optical waveforms with a multiplied bandwidth, showing attractive potential in future metrology applications.

Novel RF-source-free reconfigurable microwave photonic radar

A novel reconfigurable microwave photonic (MWP) radar has been proposed and experimentally demonstrated. At a transmitting end, a microwave signal with a large bandwidth and ultra-low phase noise is generated by a Fourier domain mode locking optoelectronic oscillator. At a receiving end, photonics-based de-chirp processing is implemented by phase-modulating light waves in a dual-drive Mach-Zehnder modulator and mixing the modulated light waves at a photodetector. Without the requirement of external RF sources, the developed photonics-assisted programmable radar is capable of generating and processing microwave signals with adjustable format, bandwidth and central frequency. The proposed radar working from X to Ku band with an instantaneous bandwidth of 2 GHz is demonstrated. The reconfiguration of the radar is theoretically analyzed. The tunability of radar bandwidth and central frequency is investigated. Microwave imaging of a pair of trihedral corner reflectors based on the developed MWP radar is achieved.

Nonlinearity Correction in OFDR System Using a Zero-Crossing Detection-Based Clock and Self-Reference

Tuning nonlinearity of the laser is the main source of deterioration of the spatial resolution in optical frequency-domain reflectometry (OFDR) system. In this paper, we develop methods for tuning nonlinearity correction in an OFDR system from the aspect of data acquisition and post-processing. An external clock based on a zero-crossing detection is researched and implemented using a customized circuit. Equal-spacing frequency sampling is, therefore, achieved in real-time. The zero-crossing detection for the beating frequency of 20 MHz is achieved. The maximum sensing distance can reach the same length of the auxiliary interferometer. Moreover, a nonlinearity correction method based on the self-reference method is proposed. The auxiliary interferometer is no longer necessary in this scheme. The tuning information of the laser is extracted by a strong reflectivity point at the end of the measured fiber. The tuning information is then used to resample the raw signal, and the nonlinearity correction can be achieved. The spatial resolution test and the distributed strain measurement test were both performed based on this nonlinearity correction method. The results validated the feasibility of the proposed method. This method reduces the hardware and data burden for the system and has potential value for system integration and miniaturization.

Low-cost optical fiber physical unclonable function reader based on a digitally integrated semiconductor LiDAR

This paper introduces an integrated fiber physical unclonable function (PUF) verification system based on a semiconductor laser source at substantially lower complexity and cost than existing alternatives. A source sub-section consisting of a linear frequency-swept semiconductor laser is used in combination with an optical frequency domain reflectometry (OFDR)/LiDAR-based measurement sub-section in order to conduct fiber identification via measurement of the unique Rayleigh reflection pattern of a section of optical fiber. When using these Rayleigh reflection patterns as PUFs, this technique results in a maximum equal error rate (EER) of 0.15% for a 5-cm section of optical fiber and an EER of less than 1% for a 4-cm section. These results demonstrate that the system can serve as a robust method fiber identification for device and communication verification applications.

Phase-noise model for actively linearized frequency-modulated continuous-wave ladar

Understanding the effects of laser phase noise on frequency-modulated continuous-wave distance measurements is important in evaluating ranging accuracy. The standard white-frequency-noise assumption is commonly used to predict the ranging performance. However, other noise sources are typically present that can further degrade the heterodyne beat signal and make this assumption invalid. In addition, many ranging systems employ active sweep linearization techniques that can impact the phase noise. Here, we present a phase-noise model for assessing the accuracy of a phase-locked swept laser source.

Micron-precision measurement using a combined frequency-modulated continuous wave ladar autofocusing system at 60 meters standoff distance

In this paper, we propose a novel combined frequency-modulated continuous wave (FMCW) ladar autofocusing system and a fast compensation method for dispersion mismatch, which could allow high-precision ranging to be performed at a long distance. By using the dual-beam laser autofocusing system based on a liquid lens, this system can quickly complete a measurement with high-precision. The experimental results showed that the precision was below 126 μm in a range up to 60m, corresponding to a relative precision of 2.1 × 10 ^-6, compared to a reference interferometer.

Waveform measurement technique for phase/frequency-modulated lights based on self-heterodyne interferometry

A novel technique is proposed and demonstrated for measuring the temporal waveforms of phase/frequency-modulated lights based on self-heterodyne interferometry with a delay time much shorter than the modulation period and on the unwrapped phase detection of heterodyne beat signals with real-time vector signal analysis. The technique makes use of an approximated relationship between the beat signal phase and the instantaneous frequency of modulated lights. The results of waveform measurements are presented for directly frequency-modulated and externally phase-modulated lights, which have been commonly employed for FWCW-LIDAR and serrodyne frequency translation, respectively. The temporal waveforms of triangular modulation are successfully measured with a frequency deviation as large as 15 GHz and the detailed investigation is presented on the deviation of measured waveform from ideal ones.