Optical Frequency Shift Research Articles

Cycle-to-cycle analysis for high-repeatability optical-heterodyne interferometry

Optical-heterodyne interferometry enables high-precision measurement of displacement, surface topography, and retardation via the introduction of an optical frequency shift. However, certain types of frequency-shifters including rotating half-waveplates may induce repetitive intensity variation, resulting in precision degradation. To address this issue, the heterodyne signals are split at the local minima during analysis. Using this approach, a single-shot retardation repeatability of λ/380, 000 is achieved at 80 Hz sampling. The proposed method applies to other types of optical-heterodyne interferometry to address challenges such as residual amplitude modulation of an electro-optic modulator to facilitate more precise measurement.

Photonic scheme for simultaneous measurement of Doppler- frequency-shift and Angle-of-arrival based on optical frequency shift heterodyne and power mapping

Photonic scheme for simultaneous measurement of Doppler- frequency-shift and Angle-of-arrival based on optical frequency shift heterodyne and power mapping

Comparison of ultra-low lattice thermal conductivity of the full-Heusler compound Li2Rb(Cs)Bi after considering strong quartic anharmonicity

This paper conducts a detailed study on the thermal transport and thermoelectric properties of Li2Rb(Cs)Bi and analyzes the optical phonon frequency shift caused by considering anharmonicity. We mainly focus on studying the microscopic mechanism of the difference in lattice thermal conductivity (κL) of the two materials. By calculating the group velocity, scattering rate, scattering phase space and scattering sub-process, it is concluded that κL is mainly dominated by the acoustic branch. Due to its small group velocity and large scattering rate, Li2CsBi has a low κL, which is 0.60 W m−1K−1 at 300 K. Research results show that n-type Li2CsBi has a higher ZT value of about 2.1 at T = 900 K, while p-type Li2RbBi has a higher ZT value of about 1.5 at the same temperature. These results provide an important theoretical basis for the application of Li2Rb(Cs)Bi in the field of thermoelectric conversion.

Composite Raman-Nath heterodyne interferometry with relevance for precise spectroscopy

Atomic state manipulation by laser radiation requires a stringent control of the light phase noise for precise spectroscopy and quantum computation. In this work we describe a novel interferometer recently employed to estimate the phase noise induced by an optical frequency shifter based on serrodyne modulation for optical frequency standards (Barbiero et al., 2023). The interferometer is generated by an acousto-optic actuator working in the Raman-Nath regime. Using the heterodyne detection between the unshifted optical mode with the first higher order optical components, we are able to detect the differential phase between the two optical path arms with a reduction of the phase noise induced by the actuator. We provide a theoretical treatment for the actuator phase suppression and we estimate the detectable phase noise limit of this interferometer. Finally, we discuss the possible applications in the field of precision measurements.

InP-based integrated optical frequency shifter for heterodyne detection applications

We present the design and characterization of an integrated optical frequency shifter based on an IQ modulator featuring standard building blocks from a generic photonic integration Indium Phosphide (InP) foundry. The device is conceived to be used in applications with low bandwidth requirements such as dual-comb spectroscopy or integrated low-cost LiDARs. An initial analytical study of the system was carried out and subsequently validated by simulations. Then, we obtained experimentally the most relevant parameters of the system and evaluated its performance under frequency shifter operation. Good performance up to 250 MHz is achieved, thus proving its suitability for the intended applications.

Correlations between the optical phase modulation and the optical frequency and phase shifts in ultrasound-modulated optical tomography

In ultrasound-modulated optical tomography, ultrasound causes the phase of incident light to vary periodically with ultrasound. The periodic variation in phase is known as phase modulation. The phase modulation causes the modulated light intensity to vary periodically with the ultrasound, which is called ultrasonic modulation of light. As is well known, incident light is shifted in frequency and phase by ultrasound in acousto-optic effect, and the tomography is based on the effect. However, the correlations between the phase modulation and the frequency and phase shifts in the ultrasonic modulation of light have been ignored. In this paper, the correlations are investigated theoretically and experimentally in detail. Studies reveal that the modulated light is phase-modulated by the frequency and phase shifts, and the frequency shift is the fundamental cause for the ultrasonic modulation of light. Studies show that the frequency shift, rather than the phase shift, causes the modulated light intensity to vary periodically with the ultrasound. Additionally, the modulated light intensity signal is composed of cosine signals with frequencies Ω, 2Ω, 3Ω, etc, and the amplitude of the cosine signals depends on the amplitude of the phase modulation. Then, the modulated light intensity signal contains relatively more cosine signals with high frequency as the amplitude of the phase modulation increases. At last, for the ultrasound with lower power, the amplitude ratio of cosine signals with frequencies of 2Ω and Ω increases as the scattering coefficient of turbid media increases. Studies find that both the frequency-shifted light and the amplitude ratio can be used to image turbid media.

Control of quality factor of atomic force microscopy cantilever by cavity optomechanical effect

The effective quality factor of the cantilever plays a fundamental role in dynamic mode atomic force microscopy. Here we present a technique to modify the quality factor of an atomic force microscopy cantilever within a Fabry–Perot optical interferometer. The experimental setup uses two separate laser sources to detect and excite the oscillation of the cantilever. While the intensity modulation of the excitation laser drives the oscillation of the cantilever, the average intensity can be used to modify the quality factor via optomechanical force without changing the fiber-cantilever cavity length. The technique enables users to optimize the quality factor for different types of measurements without influencing the deflection measurement sensitivity. An unexpected frequency shift was observed and modelled as temperature dependence of the cantilever’s Young’s modulus, which was validated using finite element simulation. The model was used to compensate for the thermal frequency shift. The simulation provided relations between optical power, temperature, and frequency shift.

Dynamic range expansion for optical frequency shift detection based on multiple harmonics

Dynamic range expansion for optical frequency shift detection based on multiple harmonics

Photonic Measurement for Doppler Frequency Shift and Angle of Arrival Based on Integrated Dual-Parallel Dual-Drive Modulator

A microwave photonic Doppler frequency shift (DFS) and angle of arrival (AOA) measurement method based on a dual-parallel dual-drive Mach–Zehnder modulator (DP-DDMZM) is proposed and demonstrated. A sawtooth wave signal is used to drive the DC port of the modulator to realize the optical frequency shift, and thus the direction discrimination of DFS is realized. Due to single-sideband modulation, the proposed system can avoid periodic power fading and the separation of the remote antenna unit (RAU) and central office (CO) can be achieved. In the experiment, the microwave DFS is estimated with a clear direction and a maximum measurement error of 0.25 Hz over an ultrawide operation frequency from 6 to 36 GHz. The experiment also proves that the phase error of AOA measurement is less than 1.5 degrees. Compared with the traditional electronic microwave measurement scheme, the proposed scheme has great competitive advantages in future broadband electronic applications due to the features of multifunction, large bandwidth and anti-interference.

Integrated heterodyne laser Doppler vibrometer based on stress-optic frequency shift in silicon nitride

We demonstrate a compact heterodyne Laser Doppler Vibrometer (LDV) based on the realization of optical frequency shift in the silicon nitride photonic integration platform (TriPleX). We theoretically study, and experimentally evaluate two different photonic integrated optical frequency shifters (OFSs), utilizing serrodyne and single-sideband (SSB) modulation. Both OFSs employ stress-optic modulators (SOMs) based on the non-resonant piezoelectrical actuation of lead zirconate titanate (PZT) thin-films, deposited on top of the silicon nitride waveguides with a wafer-scale process. To improve the modulation bandwidth of the SOMs we investigate a novel configuration of the electrodes used for the actuation, where both electrodes are placed on top of the PZT layer. Using this top-top electrode configuration we report frequency shift of 100 kHz and 2.5 MHz, and suppression ratio of the unwanted sidebands of 22.1 dB and 39 dB, using the serrodyne and the SSB OFS, respectively. The best performing SOM structure induces 0.25π peak-to-peak sinusoidal phase-shift with 156 mW power dissipation at 2.5 MHz. We use the SSB-OFS in our compact LDV system to demonstrate vibration measurements in the kHz regime. The system comprises a dual-polarization coherent detector built in the PolyBoard platform, utilizing hybrid integration of InP photodiodes (PDs). High quality LDV performance with measurement of vibration frequencies up to several hundreds of kHz and displacement resolution of 10 pm are supported with our system.

Optical frequency shifted FMCW Lidar system for unambiguous measurement of distance and velocity

A new frequency-modulated continuous-wave (FMCW) Lidar system is proposed to measure distance and velocity unambiguously by using a frequency-shifted probe light. The optical frequency shift is induced by an acousto-optic modulator (AOM). The generated beat signal is up converted to high frequency, sparing sufficient RF bandwidth to avoid signal aliasing caused by high Doppler frequency shifts. The aliasing-free beat signal thus can be used to extract distance and velocity information unambiguously. Here, we experimentally demonstrated real-time measurement on a disk spinning up to 26.43 m/s at 1.267 m away. The precision of the measured distance and velocity are 16 mm and 0.037 m/s respectively. Furthermore, the proposed scheme is capable of discriminating the direction of rotation based on a single Fast Fourier Transform analysis. In addition, we apply a free space circulator to the system to weaken the mirror-scattered signals and thus to achieve continuous measurement of beats signal over wide frequency range. Finally, a three-dimensional image of the spinning disk is reconstructed.

Cells in the mechanical spotlight

Brillouin microscopy, a technique that visualizes the transfer of energy from photons of light to phonons of mechanical energy (1), has emerged as a useful tool in the study of biological mechanics. Unlike other approaches to mechanical characterization, such as optical tweezers, atomic force microscopy (AFM), or rheology, Brillouin microscopy does not require physical contact between a probe and the specimen. Instead, the mechanical properties of the specimen are characterized by observing optical frequency shifts in the Brillouin scattered light from the specimen.

Photo-Elastic Enhanced Optomechanic One Dimensional Phoxonic Fishbone Nanobeam

We investigated the strength of acousto-optical (AO) interaction in one-dimensional fishbone silicon nanobeam computationally. The structure can generate phononic and photonic band gaps simultaneously. We use defect cavity optical mode and slow light mode to interact with acoustic defect modes. The AO coupling rates are obtained by adding the optical frequency shifts, which result from photo-elastic effect and moving-boundary effect disturbances. The AO coupling rates are strongly dependent on the overlap of acoustic and optical mode distribution. The strength of AO interaction can be enhanced by choosing certain acoustic defect modes that are formed by the stretching of wings and that overlap significantly with optical fields.

Precise characterization of VCSEL in the 1550 nm band having a wavelength tuning range of 12 nm within 10 µs as light sources for fast sensor systems

Abstract. A wavelength tuning range up to 12 nm has been achieved over 12 µs pump current pulses within a usable time of 10 µs applied to VCSELs operating in the near infrared around 1550 nm. The instantaneous optical frequency shift has been measured with an interferometric setup; the overall tuning range has then been verified with an optical spectrum analyzer. The repetition rate of the current pulses can be as high as 20 kHz with almost no influence on the sweep parameters. These results can be used for fast spectroscopic gas analysis or for rapid interferometric swept-source distance sensors.

Photonics-Based Simultaneous Angle of Arrival and Frequency Measurement System With Multiple-Target Detection Capability

A photonics-based system for simultaneous angle of arrival (AOA) and frequency measurement is proposed and demonstrated. In the proposed system, an acousto-optic modulator (AOM) based optical frequency shift loop (OFSL) is used to generate an optical frequency-stepped local oscillator (LO). After balanced in-phase and quadrature (I/Q) down-conversion with the frequency-stepped LO, the frequency to be measured is obtained. Thanks to the balanced I/Q down-conversion, the image frequencies and undesired frequency-mixing components can be suppressed, making the system have better adaptability to multiple-target scenarios. In addition, the AOA from different RF sources can also be estimated based on the Van Cittert–Zernike theorem, even if the working bands of these targets overlap with each other. In the proof-of-concept experiment, the measurement of multiple targets with different frequencies is achieved by frequency detection with 0.179 MHz resolution. Moreover, incoherent microwave sources at the same frequency band can also be distinguished in the angular direction, achieving the capacity of simultaneous frequency and AOA measurement of multiple-target.

Performance Analysis of a Multi-Function Mach-Zehnder Interferometer Based Photonic Architecture on SOI Acting as a Frequency Shifter

A photonic frequency shifter based on generalized Mach-Zehnder interferometer (GMZI) architecture is presented and experimentally validated. The circuit consists of four Mach-Zehnder modulators (MZM) in a 4 × 4 network bounded by two 4 × 4 multimode interference couplers and functionally equivalent to two parallel dual-parallel MZM (DP-MZM). The circuit can offer static bias free operation, virtual connectivity control of the components, and spatial separation of up- and down-converted carriers, which can be collected from separate ports without using any optical demultiplexing filters. Thus, the design permits remote heterodyning (advantages which cannot be obtained using a commercial DP-MZM or filter based optical frequency shifter). Experimental investigation shows deviation from ideal performance due to possible fabrication error and poor fiber-chip coupling. A carrier suppression of >20 dB and spurious sideband suppression >12 dB relative to the principal harmonics is achieved without any tuning for bias adjustment. In addition to the frequency conversion, the integration feasible circuit can also perform as a sub-carrier generator, IQ modulator, and frequency multiplier.

Coherent Random-Modulated Continuous-Wave LiDAR Based on Phase-Coded Subcarrier Modulation

A coherent random-modulated continuous-wave (RMCW) LiDAR transmits a lightwave modulated by a pseudo-random binary sequence (PRBS). The lightwave backscattered from targets is received and used to reconstruct the PRBS. Then, the time-of-flight is extracted by correlating the reconstructed PRBS and the original PRBS. We propose a coherent RMCW LiDAR based on phase-coded subcarrier modulation, in which the impacts of internal reflection and optical Doppler frequency shift (DFS) are mitigated. A continuous lightwave is amplitude-modulated by an RF signal which is phase-coded with a PRBS. Coherent detection is used in the receiver. A beat signal that consisted of a low-frequency signal and a high-frequency signal is obtained by a single balanced photodetector (BPD). The optical DFS can be directly extracted from the low-frequency signal. It is used to compensate for the frequency offset of PRBS, which is extracted from the high-frequency signal. In addition, the background noise caused by internal reflection is suppressed by averaging over successive measurement spots. In this paper, the performance of a coherent RMCW LiDAR is firstly analyzed by numeric simulations and demonstration experiments. Then, line-scanning measurements for moving targets are implemented to demonstrate the 3D imaging capability of the proposed coherent RMCW LiDAR.

Optical Fiber Frequency Shift Characterization of Overburden Deformation in Short-Distance Coal Seam Mining

Overburden deformation is an important concern for the safe and green mining of coal resources. Similarity simulation testing is the main approach used to study the deformation characteristics of the overburden in coal mining. In the application of Brillouin optical time-domain analysis (BOTDA) in similarity simulation tests, the capability of distributed optical fiber sensing (DOFS) to detect the characteristics of the overburden deformation and the evolution is a key factor affecting the testing accuracy. In this study, the relationships between DOFS and overburden deformation and the face impact pressure under geological conditions in short-distance coal seam mining were explored. The results show that DOFS can be used to monitor the strain conditions of the overburden during the entire mining process and can provide the peak positions of the advance support pressure on the face. A DOFS characterization model for investigating the spatial and temporal evolutions of overburden deformation was established. A new method of characterizing the face impact pressure based on the fiber frequency shift variation was developed. The method was demonstrated to be effective through comparison of monitored results of impact pressure counts detected using pressure sensors. The characteristics of the face impact pressure in short-distance coal seam mining were obtained. The results of this study provide valuable guidance for the development of similarity simulation testing and intelligent mining engineering techniques.

Estimation of reducing the impact of the optical lattice spectrum on the optical clock frequency shift

In this paper the method for reducing the shift of the “clock” resonance by the example of 87Sr has been described. This shift arises due to the presence in the optical spectrum of the lattice laser of the incoherent component. This component is a traveling wave. It is shown that the effective decrease of the impact on the shift of the “clock” resonance by introducing the interference filter is achieved. This filter was inserted directly into the optical scheme after the laser source. Examples of the optical lattice spectra with and without filter are considered. The estimates of the efficiency ratio of the use of the interference filters to improve the final uncertainty of the optical reference are made. It is shown that the filter reduces the relative uncertainty from the impact of the incoherent component of the optical radiation of the optical lattice by two orders of magnitude. It should help to reduce the lower bound for the total uncertainty limit of the optical frequency clocks down to 10−18.

All-optical serrodyne frequency shifter.

We describe and demonstrate a method to induce a single-sideband serrodyne-like optical frequency shift in an all-fiber device. The time-varying phase-shift is generated by cross-phase modulation in a nonlinear fiber. Unlike electro-optic based serrodyne methods, which require a high-fidelity and high-bandwidth analog electrical ramp modulation signal, the proposed optical method uses a simple pulsed pump and can be made simultaneously both low-loss and polarization-independent. We demonstrate the technique by shifting a 1550 nm optical signal 150 MHz in a 1.5 dB insertion-loss polarization-independent frequency shifter and show a pathway to multi-GHz shifting frequencies.