Local-field engineering in slot waveguide for fabricating on-chip Bragg grating filters with high reflectivity across a flat broadband.

On-chip Bragg gratings with high reflectivities have been found to have widespread applications in filters, resonators, and semiconductor lasers. However, achieving strong Bragg reflections with flat response across a broad bandwidth on the popular 220 nm silicon-on-insulator (SOI) platform still remains a challenge. In this paper, such a high performance device is proposed and fabricated, which is based on a slot waveguide with gratings etched on the inner sidewalls of the slot. By manipulating the local field in the slot region using a chirped and tapered grating-based mode transition, the device achieves a flat response with ultra-high reflection and low transmission for the TE mode across a broad operating bandwidth. Leveraging the ultra-high birefringence of the SOI waveguide, the device functions both as a TE slot waveguide reflector and a TM pass polarizer. Simulation results demonstrate that the device exhibits an ultra-high rejection of more than 50 dB and a reflectivity exceeding 0.99 for the TE mode across a 91 nm wavelength range, while maintaining a high transmittance of larger than 0.98 for the TM mode. Experimental results validate that the device performance is consistent with the simulation results. A fabricated device based on such a gratings exhibits a low insertion loss (<0.8 dB) and high polarization extinction ratio (>30 dB) over 100 nm bandwidth (1484 nm-1584 nm), demonstrating that the performance of the present design is competitive with that of the state-of-the-art SOI Bragg gratings.

2†µm band four-wavelength-switchable narrow linewidth fiber laser enabled by fs-laser direct-written polarization-dependent parallel fiber Bragg gratings.

We propose and experimentally demonstrate a four-wavelength-switchable single-longitudinal-mode (SLM) narrow linewidth thulium-holmium co-doped fiber laser (THDFL) using two polarization-dependent parallel fiber Bragg gratings (PD-PFBGs). The PD-PFBGs, fabricated using femtosecond (fs) laser direct-writing technology in a standard single-mode fiber (SMF) via a point-by-point method, are used as a four-channel reflection filter. Two FBGs are inscribed in parallel in the fiber core along the axial direction and are uniquely positioned symmetrically on either side of the centerline. This configuration enables polarization-dependent multi-channel filtering capability, which further allows for polarization-control-based four-wavelength-switchable operations of the THDFL. SLM lasing is accomplished by utilizing a simple dual-ring sub-cavity filter. An exceptional output performance of the THDFL is achieved, including an optical signal-to-noise ratio (SNR) of >72 dB, maximum power and wavelength fluctuations of 0.350 dB and 0.024 nm, respectively, and a linewidth of <2 kHz, for all four single-wavelength operations lasing at ∼2000 nm. These performance indicators suggest that the THDFL can be applied in free-space optical communication, atmospheric monitoring, and Lidar.

Relative error's quadratic dependence on the electric current and the methods for its compensation in fiber optic current sensor systems.

We find that the relative error of a fiber optic current sensor (FOCS) increases quadratically with the electric current to be measured, causing unacceptable inaccuracy for direct current (DC) measurements beyond 100 kA. We prove analytically and confirm experimentally that such a nonlinear relative error escalation (REE) mainly originates from the residual linear birefringence of the spun fiber used in the FOCS. We propose and demonstrate that by first measuring residual linear birefringence, together with the circular birefringence of the spun fiber, the REE of the FOCS can be significantly reduced from -1.22% to -0.15% at 200 kA DC by a compensation scheme using the measured birefringences in the quadratic expression we derived. Further reduction of the REE to -0.02% at 200 kA DC can be obtained if the quadratic relation between the REE and the current under test is experimentally obtained. Our work points to a new direction for drastically improving the accuracy of FOCS at large currents and shall prove beneficial for scientists and engineers working in the field of current sensing.

Dual-loop diode-tuned Fourier domain harmonically mode-locked opto-electronic oscillator with over 50†dB side-mode spur reduction.

We propose and demonstrate a dual-loop harmonic Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) for drastically reducing the side-mode spurs. The frequency domain mode-locking is achieved by synchronizing the scanning period of the filter to an integer fraction of the round trip times of the two loops with a self-made low cost diode-tuned RF filter. We found, for the first time to the best of authors' knowledge, that the frequency scanning bandwidth (FSBW) of the mode-locked output is strongly affected by the length mismatch between the two fiber loops. By using the phase noise of FDML OEO's delayed self-heterodyne signal as a performance indicator, we found experimentally that both the locking bandwidth and the FSBW of the device are inversely proportional to the length mis-match of the two loops. Finally, with dual-loop fiber lengths of 2041m and 2449.2m, including 2039m common fiber loop, we successfully obtained linearly chirped microwave signals around 9GHz with a phase noise of -127dBc/Hz at 10kHz offset from the 9GHz carrier, a FSBW of 0.4GHz, and a chirp rate of 200THz/s at 500.38 kHz repetition rate. More impressively, the side-mode spur ratio of the linear frequency modulated (LFM) signal is reduced to less than -83dB, the lowest ever achieved for a FDML OEO to the best of authors' knowledge, which is more than 50dB improvement over that achieved with a single loop FDML OEO reported previously.

Quantitative phase imaging based on polarization encoding.

Most optical characterization methods rely on measuring the complex optical fields emerging from the interaction between light and material systems. Nevertheless, inherent scattering and absorption cause ambiguities in both interferometric and noninterferometric attempts to measure phase. Here we demonstrate that the complete information about a probe optical field can be encoded into the states of polarization, and develop a topography measurement method by blindly varying the ambient refractive index surrounding the sample in a wedged cuvette, which is capable of simultaneously measuring the thickness and the ambient refractive index of the sample in real time, as well as extending the measurement range of the sample thickness. With the method, we have successfully measured the topography of a 136.7 µm thick coverslip by blindly changing the ambient refractive index by 0.001246, resulting in the thickest sample characterization ever achieved by quantitative phase imaging, to the best of our knowledge. An efficient and complete characterization of optical fields is critical for any high-resolution imaging approach and the technique demonstrated here should prove attractive for applications ranging from microscopy to remote sensing. Thanks to the high precision and fast response speed, this method may pave a new way for measuring the topography of the thick samples, such as biological tissues.

Compact and ultra-broadband all-silicon TM-pass and TE-reflected polarizer using grating based weakly coupled nanowires.

On-chip silicon polarizers with broad operating bandwidth and compact footprint have recently attracted increasing attention for their applications in large capacity and high density integrated optical systems. However, strong waveguide dispersion usually limits the bandwidth of the silicon polarizers, especially for the TM-pass polarizers. In this paper, we overcome the bandwidth limit of the TM polarizer by utilizing a novel waveguide structure composed of two weakly coupled nanowires with gratings sandwiched in between. Such a structure can effectively enlarge the bandgap for the undesired TE polarized light, while act as a low loss subwavelength metamaterial for TM polarized light over an extremely large wavelength range. In simulation, we obtain a compact polarizer of 13.6 µm × 1.3 µm in size with an ultra-broad operating bandwidth of ∼362 nm for extinction ratios (ERs) >21 dB and insertion losses (ILs) <1 dB, which covers E-, S-, C-, L-, and U-bands and part of O-band. The measurements of fabricated devices show that the device performed well in the test wavelength range from 1300 to 1600 nm with an ER >15 dB and an average IL ∼1 dB, consistent with the simulation results. This work paves a new way for designing compact and ultra-broadband on-chip polarizers.

High-performance polarization management devices based on thin-film lithium niobate.

High-speed polarization management is highly desirable for many applications, such as remote sensing, telecommunication, and medical diagnosis. However, most of the approaches for polarization management rely on bulky optical components that are slow to respond, cumbersome to use, and sometimes with high drive voltages. Here, we overcome these limitations by harnessing photonic integrated circuits based on thin-film lithium niobate platform. We successfully realize a portfolio of thin-film lithium niobate devices for essential polarization management functionalities, including arbitrary polarization generation, fast polarization measurement, polarization scrambling, and automatic polarization control. The present devices feature ultra-fast control speeds, low drive voltages, low optical losses and compact footprints. Using these devices, we achieve high fidelity polarization generation with a polarization extinction ratio up to 41.9 dB and fast polarization scrambling with a scrambling rate up to 65 Mrad s-1, both of which are best results in integrated optics. We also demonstrate the endless polarization state tracking operation in our devices. The demonstrated devices unlock a drastically new level of performance and scales in polarization management devices, leading to a paradigm shift in polarization management.

Noninvasive examination of the cardiac properties of insect embryos enabled by optical coherence tomography.

Understanding the cardiac properties of insect embryos at different development stages is important, however, few works have been conducted probably due to the lack of effective tools. Using locust embryos as an example, here we show, for the first time, that optical coherence tomography (OCT) is capable of obtaining detailed information of embryos' heart activities and irregularities, such as the heart rate, cardiac cycle, diastolic and systolic diameters, hemolymph pumping rate and ejection fraction at different stages of embryonic development and at different temperatures. We develop algorithms and mathematical methods for extracting and analyzing cardiac behavior information of locust embryos. We discover that locust embryos experienced suspended development (quiescence) caused by cold storage have a heart rate 20% more than that of embryos without experiencing quiescence and that the hemolymph pumping rate of the two types of embryos behaves differently as the embryos grow. In addition, using OCT as an accurate cardiac activity examination tool, we show that the heart rates of locust embryos are effectively reduced due to nitric oxide synthase gene silencing by RNA interference, indicating potential application of using locust embryos as a good model organism to study cardiovascular diseases, including the congenital heart disease and arrhythmia. Finally, the capabilities offered by OCT in the studies of locust embryonic development may also prove helpful to promote locust reproduction for nutritions or restrain locust reproduction for pest control.

Clamping-force induced birefringence in a single-mode fiber in commercial V-grooves investigated with distributed polarization analysis.

V-grooves are critical components for attaching fiber pigtails to photonics integrated circuits and for holding fibers in optical devices. Improper choice of V-groove angles and clamping method may cause large birefringence in the fibers and degrade polarization related performance of the devices. In this paper, we theoretically analyze the clamping-force induced birefringence of a single-mode (SM) fiber clamped in a V-groove by a flat-lid or by two identical V-grooves, respectively. We build a distributed polarization analyzer with complete Muller matrix analysis capability, which enables us to accurately measure local birefringence values in the fiber induced by clamped V-grooves of different angles. We find that for a SM fiber clamped in a commercial V-groove made with ZrO2 or SiO2 by a flat-lid, the zero-birefringence (ZB) angle is almost exactly 60°, suggesting that the friction coefficient in the V-groove can be safely ignored. In contrast, previous studies either indicated that the ZB V-groove angles clamped by a flat-lid never existed or significantly deviated from 60° due to the friction coefficient. More importantly, we also find, for the first time, both theoretically and experimentally, that a SM fiber clamped by two identical commercial V-grooves has a ZB when the V-groove angle is 90°. The methods and results reported in this paper shall prove beneficial for the fiber optic component industry to optimize polarization related performances of devices for sensing, communication, and instrumentation applications.

Low loss and high extinction ratio all-silicon TM-pass polarizer with reflection removal enabled by contra-mode conversion Bragg-gratings.

Bragg-gratings have been frequently used to design compact and high extinction ratio (ER) on-chip polarizers. However, the strong reflection of the unwanted polarization may deteriorate the performance of the light source or cause unwanted interferences. In this paper, we propose a Bragg-grating-based all-silicon TM-pass polarizer with low reflection, low insertion loss (IL) and high ER. Unlike previously reported polarizers based on single mode waveguides, we construct the Bragg grating with a multimode waveguide, which not only acts as a Bragg reflector, but also a mode-order converter to convert the reflected TE light into higher order modes to be eventually filtered out by utilizing a tapered transition. On the other hand, the grating has little adverse influence on the TM input light since it works at sub-wavelength-guided wave propagation regime. Finally, the polarizer obtained has a length of 30µm, an ER of 51.83dB, an IL of 0.08dB, and an operating bandwidth of ∼61nm for ER > 30dB at the wavelength of 1.55µm. More importantly, the reflection of the unwanted polarization is suppressed to -12.6dB, which can be further lowered via additional design optimization. Our work points to a new direction for making better on-chip polarizers.

Wavelength-switchable ultra-narrow linewidth fiber laser enabled by a figure-8 compound-ring-cavity filter and a polarization-managed four-channel filter.

We propose and demonstrate a high-performance wavelength-switchable erbium-doped fiber laser (EDFL), enabled by a figure-8 compound-ring-cavity (F8-CRC) filter for single-longitudinal-mode (SLM) selection and a polarization-managed four-channel filter (PM-FCF) for defining four lasing wavelengths. We introduce a novel methodology utilizing signal-flow graph combined with Mason's rule to analyze a CRC filter in general and apply it to obtain the important design parameters for the F8-CRC used in this paper. By combining the functions of the F8-CRC filter and the PM-FCF assisted by the enhanced polarization hole-burning and polarization dependent loss, we achieve the EDFL with fifteen lasing states, including four single-, six dual-, four tri- and one quad-wavelength lasing operations. In particular, all the four single-wavelength operations are in stable SLM oscillation, typically with a linewidth of <600 Hz, a RIN of ≤-154.58 dB/Hz@≥3 MHz and an output power fluctuation of ≤±3.45%. In addition, all the six dual-wavelength operations have very similar performances, with the performance parameters close to those of the four single-wavelength operations, superior to our previous work and others' similar work significantly. Finally, we achieve the wavelength-spacing tuning of dual-wavelength operations for photonic generation of tunable microwave signals, and successfully obtain a signal at 23.10 GHz as a demonstration.

Widely tunable single-/dual-wavelength fiber lasers with ultra-narrow linewidth and high OSNR using high quality passive subring cavity and novel tuning method: erratum.

We provide a corrected equation of our previous publication [Opt. Express24, 19760 (2016)10.1364/OE.24.019760].

Distributed transverse-force sensing along a single-mode fiber using polarization-analyzing OFDR: erratum.

We correct an inaccurate statement on the birefringence measurement using the PA-OFDR in our previous publication [Opt. Express28, 31253 (2020)10.1364/OE.405682].

Two-dimensional correlation (2D) method for improving the accuracy of OCT-based noninvasive blood glucose concentration (BGC) monitoring.

The optical scattering coefficient (µs) in the dermis layer of human skin obtained with optical coherence tomography (OCT) has shown to have a strong correlation with the blood glucose concentration (BGC), which can be used for noninvasive BGC monitoring. Unfortunately, the nonhomogeneity in the skin may cause inaccuracies for the BGC analysis. In this paper, we propose a 2D correlation analysis method to identify 2D regions in the skin with µs sensitive to BGC variations and only use data in these regions to calculate µs for minimizing the inaccuracy induced by nonhomogeneity and therefore improving the accuracy of OCT-based BGC monitoring. We demonstrate the effectiveness of the 2D method with OCT data obtained with in vivo human forearm skins of nine different human subjects. In particular, we present a 3D OCT data set in a two-dimensional (2D) map of depth vs. a lateral dimension and calculate the correlation coefficient R between the µs and the BGC in each region of the 2D map with the BGC data measured with a glucose meter using finger blood. We filter out the µs data from regions with low R values and only keep the µs data with R values sufficiently high (R-filter). The filtered µs data in all the regions are then averaged to produce an average µs data. We define a term called overall relevancy (OR) to quantify the degree of correlation between the filtered/averaged µs data and the finger-blood BGC data to determine the optimal R value for such an R-filter with the highest obtained OR. We found that the optimal R for such an R-filter has an absolute value (|R|) of 0.6 or 0.65. We further show that the R-filter obtained with the 2D correlation method yields better OR between µs and the BGC than that obtained with the previously reported 1D correlation method. We believe that the method demonstrated in this paper is important for understanding the influence of BGC on µs in human skins and therefore for improving the accuracy of OCT-based noninvasive BGC monitoring, although further studies are required to validate its effectiveness.

Scan-less 3D optical sensing/Lidar scheme enabled by wavelength division demultiplexing and position-to-angle conversion of a lens.

We propose a novel scheme for 3D sensing or Lidar without the need for beam scan or 2D photo-imaging. The scheme is enabled by the combination of a lens' position-to-angle conversion and the wavelength division multiplexing/demultiplexing (WDM) commonly used in optical fiber communication systems. However, unlike in a WDM system where different wavelengths carry different data channels, here lights of different wavelengths are demultiplexed into different waveguides or fibers with their exiting ends placed in the focal plane of the lens, which converts the exiting lights into beams of different angles to form a 1D or 2D beam array according to the relative position of the fiber ends with respect to the optical axis of the lens for illuminating the targets and finally sensing the light reflected from different directions. The returned signals are then demultiplexed into different photodetectors to determine the distances of the reflections in different directions. We show that the scheme has the potential to be implemented in photonics integrated circuit (PIC) for low cost production. We successfully demonstrate the scheme with the off-the-shelf discrete fiber optic components using 4 WDM channels and time-of-flight (ToF) technique for distance measurement, although hundreds wavelength channels from a photonic integrated microcomb may be used in practice. Finally, we demonstrate that the angular resolution of the beam array of different wavelengths can be improved by dithering the fiber array or the lens. We believe this new scheme provides an attractive alternative to the MEMS and optical phased array based beam scanning and can be explored further to enable low cost and high speed 3D sensing, particularly Lidar systems.

Fourier domain mode-locked opto-electronic oscillator with a diode-tuned bandpass filter.

We report what we believe to be the first Fourier domain mode-locked (FDML) opto-electronic oscillator (OEO) without using a tunable signal source to implement, such as a tunable laser or a tunable microwave source as described in the previous reports. We designed and fabricated a tunable microwave filter with individually packaged microwave components, in which a low cost diode-tuned phase shifter was used to rapidly tune the filter center frequency. We successfully realized Fourier domain mode-locking of an OEO using the diode-tuned filter and obtained linearly chirped microwave signals around 9 GHz with a chirp rate of 36 MHz/µs and a frequency tuning range of 0.4 GHz, which can be extended to 142 MHz/µs and 1.56 GHz, respectively, with a filter circuit using chip sized components. We found, for the first time to the best of authors' knowledge, that the phase noise of FDML OEO's delayed self-heterodyne signal is an excellent indicator for mode-locking frequency optimization, which had a "U" shape dependence on the detuning of the mode-locking frequency with a locking range of over 40 Hz. We also investigated harmonic mode-locking of the FDML up to 5th harmonics and achieved a chirp rate of 180 MHz/µs using the tunable filter of individually packaged components. Compared with the previous FDML OEO's implemented with tunable signal sources, our diode tuned FDML OEO has the advantages of low cost, compact size, excellent frequency tuning linearity, easy implemention and immunity to laser frequency drift and noise for achieving better frequency repeatability and lower phase noise.

PM fiber based sensing tapes with automated 45° birefringence axis alignment for distributed force/pressure sensing.

Polarization maintaining (PM) fibers can be used for distributed force/pressure sensing in which the birefringence axis of the PM fiber should preferably be oriented 45° from the direction of the force/pressure for the maximum sensitivity. However, it is a challenge to achieve such 45° axis orientation for a long length of PM fiber in practice. In this paper, we report the development of what we believe the first equipment and process for making PM fiber based sensing tapes, capable of automatically adjusting the fiber axis orientation 45° with respect to the tape surface. In particular, we develop a machine vision system with the ability of continuously determining fiber axis orientation in real time as the fiber passes by and feeding back the orientation information to a fiber rotation apparatus to automatically adjust its orientation before fixing the fiber on a transparent PET tape with UV epoxy. We show the results of a successfully fabricated 70-m-long PM fiber sensing tape achieving an axis orientation accuracy of 45 ± 3° throughout the whole length of the tape, which is further validated with a distributed polarization crosstalk analyzer (DPXA). Finally, we demonstrate distributed transversal load sensing with 14 force applying weights randomly distributed along the sensing tape using the DPXA, with a polarization crosstalk measurement uniformity of 0.62 dB (standard deviation) using the same applied weight of 100 grams. The same sensing tape can also be used for pressure sensing with properly designed fixtures.

Optical coherence tomography as a noninvasive 3D real time imaging tool for the rapid evaluation of phenotypic variations in insect embryonic development.

Noninvasive visualization of embryos at different development stages is crucial for the understanding of the basic developmental biology. It is therefore desirable to have an imaging tool capable of rapidly evaluating the effects of gene manipulation or genome editing in developing embryos for the studies of gene functions and genetic engineering. Here, we propose and demonstrate a novel use of optical coherence tomography (OCT) to noninvasively exam the embryonic development of the migratory locusts in real time with 3-dimensional (3D) view capability. In particular, we obtain the sufficiently high spatial resolution tomographic 2D and 3D images of live locust embryos throughout their development processes. We show that not only we are able to noninvasively observe all previously known forms of blastokinesis as an embryo develops, such as anatrepsis, katatrepsis, revolution, rotation and diapauses, and determine their precise occurring time or duration, but also discover an unreported rotation form we named "twist." In addition, with the OCT images we determined the exact occurring time of diapauses of the locusts from Tibetan plateau for the first time. Finally, we demonstrate that OCT systems can be used to rapidly capture the development defects of genetically modified embryos in which certain genes essential for embryonic development were suppressed by RNA interference. Our work shows that OCT is an enabling imaging tool with sufficient spatial resolution for the rapid evaluation of embryonic variations of small animals.

Four-wavelength-switchable SLM fiber laser with sub-kHz linewidth using superimposed high-birefringence FBG and dual-coupler ring based compound-cavity filter.

We propose and demonstrate a four-wavelength-switchable erbium-doped fiber laser (4WS-EDFL) with a four-channel superimposed high-birefringence fiber Bragg grating (SI-HBFBG) and a dual-coupler ring based compound-cavity (DCR-CC) filter. Both for the first time, a SI-HBFBG as a four-channel reflective filter is used in a multi-wavelength switchable fiber laser to define wavelength channels and a DCR-CC filter is used to select a single mode from dense longitudinal-modes in a fiber laser. We present in detail how to design, fabricate, and characterize the DCR-CC filter with both theoretical analysis and experimental results, which we believe is the first systematic approach for making a compound-cavity based filter used for selecting single-longitudinal mode (SLM) in a fiber laser. The enhanced polarization hole burning effect in a 2.9 m long erbium-doped fiber, coiled inside a three-loop polarization controller, and the polarization-mismatch-induced losses are introduced into the laser cavity to achieve wavelength-switching operations. We show that the 4WS-EDFL can be switched among fifteen lasing states, including four single-wavelength operations, six dual-wavelength operations, four three-wavelength operations and one four-wavelength operation, all with high stability. For demonstration, in switchable single-wavelength operations, the four SLM lasing outputs measured are all with an optical signal to noise ratio of >80 dB, a linewidth of <700 Hz, a relative intensity noise of ≤-156.7 dB/Hz at frequencies over 3 MHz, an output power fluctuation of ≤0.555 dB and excellent polarization characteristics.

Polarimetry fiber optic gyroscope.

We report a different mechanism for rotation sensing by analyzing the polarization of light exiting from a Sagnac loop. Unlike in an interferometric fiber optic gyroscope (I-FOG), here the counter-propagating waves in the Sagnac loop are orthogonally polarized at the loop exit and, consequently, cannot directly interfere with each other when recombined at the exit. We show that the Stokes parameters s2 and s3 of the combined waves are simply the cosine and sine functions of the phase difference between the counter propagation waves, which is linearly proportional to the rotation rate, allowing precise determination of the rotation rate by polarization analysis. We build such a proof-of-concept polarimetry FOG and achieved key performance parameters comparable to those of a high-end tactical-grade gyroscope. In particular, the device shows a bias instability of 0.09°/h and an angular random walk of 0.0015°/h, with an unlimited dynamic range, demonstrating its potential use for rotation sensing. This new approach eliminates the need for phase modulation required in I-FOGs, and promotes easy photonics integration, enabling the development of low-cost FOGs for price-sensitive applications, such as autonomous and robotic vehicles.