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Two-wavelength interferometers can extend an unambiguous measurement range; however, they suffer from complex optical configurations. To simplify the optical setup for a two-wavelength common-path off-axis interferometer, we propose a dual-channel two-wavelength interferometer using a polarized cube beam splitter. In contrast with the previously presented two-wavelength common-path off-axis interferometer, the proposed method has a simple setup, in which only one polarized cube beam splitter is inserted into the 4f system. With the help of polarization modulation, two single-wavelength interferograms can be captured simultaneously. Several experimental results are presented to demonstrate the advantages and effectiveness of the proposed method.

A robust in-fiber tunable acousto-optic Mach–Zehnder interferometer with a taper-shaped sandwich-like fiber structure is proposed and characterized experimentally, based on which tunable dual-wavelength lasers are demonstrated. The fiber structure was prepared by two-step etching methods, which could be used to fabricate either a symmetric structure for a continuous tuning dual-wavelength laser or an asymmetric structure for a switchable one. The proposed structure has advantages of low cost, low driving power, and robustness. The method for preparing the fiber structure is agile, which paves the way for its applications.

Dual-wavelength arbitrary phase-shifting digital holography with automatic phase-shift detection is first proposed in this study. Holograms with two wavelengths and the interference fringes used to detect the phase-shifting amount for each wavelength were simultaneously recorded in one image using the space-division multiplexing technique. Compared with conventional methods, the proposed approach can achieve simultaneous phase shifting of the reference beams of two wavelengths, which substantially reduces recording time and does not require excessive phase-shifting device precision. The proposed and conventional methods were quantitatively evaluated with numerical simulations, and a dynamic deformation measurement was obtained using the system. In the quantitative evaluation of the simulation, the root-mean-square errors of amplitude and phase images reconstructed by the proposed method were reduced by 12% and 19% compared to the conventional method, respectively. Both numerical simulations and experiments verified the effectiveness of the proposed method.

Traditionally, there is a trade-off between the numerical aperture and field of view for a microscope objective. Diffractive lens arrays (DLAs) with overlapping apertures are used to overcome such a problem. A spot array with an NA up to 0.83 and a pitch of 75 µm is produced by the proposed DLA at a wavelength of 488 nm. By measurement of the fluorescence beads, the DLA-based confocal setup shows the capability of high-resolution measurement over an area of 3 m m × 3 m m with a 2.5 × 0.07 N A objective. Further, the proposed fluorescence microscope is insensitive to optical aberrations, which has been demonstrated by imaging with a simple doublet lens.

We develop a watt-level 725 nm continuous-wave tunable narrow linewidth injection-locked Ti:sapphire laser. The seed laser is frequency-locked on a Fabry–Perot cavity to narrow the linewidth and stabilize the frequency. Because the wavelength of the seed laser is located at the edge of the gain profile of the Ti:sapphire crystal, it is difficult to injection-lock the Ti:sapphire laser at 725 nm. A cavity mirror, which has a long-pass-filter coating with a sharp edge, is used in the Ti:sapphire cavity to suppress mode competition from the long wavelength side. This method effectively reduces the power requirement of the seed laser at 725 nm, and the Ti:sapphire laser can be injection-locked at higher output power. As a result, a 1 W output power with a 27% slope efficiency is achieved in the injection-locked laser, and a 210 mW UV laser is obtained in a subsequent second-harmonic generation stage. To the best of our knowledge, this is the shortest wavelength for the watt-level injection-locked continuous-wave Ti:sapphire laser. This laser will be used in a mercury optical lattice clock in the future.

A compact silicon on-chip wavelength triplexer engineered by double bridged subwavelength gratings (BSWGs) is proposed based on cascaded symmetric directional couplers (SDCs). The SDC consists of a pair of symmetric and identical BSWGs with another SWG waveguide implemented in the middle of the SDC section. It functions well provided that the coupling lengths can match even or odd times of beat lengths of the three wavelengths. Relying on deliberately tailoring the structural dimensions of SWGs at different positions of SDC, the two lowest-mode refractive indices and beat lengths can be efficiently tuned, drastically reducing device footprint. The results show a total device length of 42.2 µm, which is ∼ 10 % of its conventional counterpart based on SDCs. An insertion loss (IL) lower than 0.67 dB, reflection loss (RL) below − 21.4 d B , and extinction ratio (ER) as high as 34.5 dB are also obtained in the results. The bandwidths around wavelength bands and fabrication tolerances to dimensional variations are investigated and analyzed. The field evolutions for the three injected wavelength bands are presented.

Design parameter optimization of a chip based on a heterostructure with a surface diffraction grating for laser beam angle modulation was carried out. It was found that the widest field of view is achieved at outcoupling angles close to the total internal reflection. The highest energy efficiency is provided by using epitaxy Bragg reflector on the substrate side. It is shown that using a multiple quantum well that gives electro-optic refractive index modulation by 0.007 and second-order surface diffraction grating with a period of 317 nm provides laser beam steering in a range of 5°, with resolution of 130 points at a wavelength of 782 nm.

A supercontinuum (SC) light source enables multispectral photoacoustic imaging at excitation wavelengths in the visible-to-near-infrared range. However, for such a broad optical wavelength range, chromatic aberration is non-negligible. We developed a multispectral optical-resolution photoacoustic microscopy (MS-OR-PAM) setup with a nanosecond pulsed SC light source and a reflective objective lens to avoid chromatic aberration. Chromatic aberrations generated by reflective and conventional objective lenses were compared, and the images acquired using the reflective objective were not affected by chromatic aberration. Hence, MS-OR-PAM with the reflective objective was used to distinguish red blood cells from melanoma cells via spectral subtraction processing.

A novel, to the best of our knowledge, configuration of a Littrow–Offner spectrometer that is off-axis in both the meridian and sagittal planes is proposed. Through theoretical analysis, the relationship between the distance of each element and the aperture is obtained, and the theoretical basis of the sagittal plane off-axis is given. Extra aberrations are corrected by a spherical lens with a convex grating. An example of design has been presented with an f -number of 4 working in 300–500 nm, according to the theory. Its spectral resolution achieves a 0.1 nm at central wavelength, and its volume decreases by 14.5% compared with the traditional Offner spectrometer.

A new, to the best of our knowledge, multi-purpose spectral-polarizing beam splitter (SPBS) design is described that simultaneously isolates multiple identical reflected and transmitted polarized passbands to permit passive simultaneous polarimetry in a common instantaneous field of view (FOV). The SPBS design can be implemented in wavelength regions between ∼ 260 n m and ∼ 1700 n m and consists of fewer than 30 layers. Degree of linear polarization values are ∼ 0.99 . FOV area coverage in a swath as wide as 6° is accomplished by object space scanning. A polarization calibrator/stability monitor source is integrated with the optical system. Estimated system polarimetric error is < 2 % . Applications range from biomedical instrumentation to remote sensing.

A deep-ultraviolet (DUV) narrowband tunable filter with a central wavelength of 225.0 nm is designed, fabricated, and demonstrated experimentally. The simulation results showed a transmittance of about 42.1% at 225.0 nm and a full width at half-maximum (FWHM) of ∼ 1.2 n m . The experimental results for normal incidence light revealed a corresponding central wavelength of 225.0 nm, a transmittance of 27.7%, and a FWHM of 1.9 nm. By changing the incident angle, the central wavelength can be tuned from 225.0 to 224.5 nm. This DUV narrowband tunable filter has been shown to demonstrate good performance and has broad applicability.

A novel, to the best of our knowledge, device scheme for a silicon-nanowire flat-spectral-band wavelength optical filter is proposed and theoretically demonstrated. The proposed wavelength filter is composed of cascade-connected multiple delayed interference optical delay lines, together with several multimode interference couplers with symmetric and asymmetric splitting ratios. Theoretical calculations based on analytic and numerical simulations exhibit flatband spectra over a wavelength range of >80 nm with potentially better production yield for arbitrary channel spacing.

Metalens is a planar optical component that uses nanostructures with a thickness on the order of the wavelength to manipulate the wavefront of the incident light. A key problem, especially in color imaging and display applications, is the correction of chromatic aberration, which is an inherent effect caused by the dispersion of periodic lattices and resonance modes. However, the current achromatic metalenses either use the PB phase method that is only valid for circularly polarized light or nanostructures with complex cross sections that are difficult to manufacture. Here, we designed a broadband achromatic metalens for linearly polarized light from 450 to 800 nm. Rectangular titanium dioxide nanofins of various lengths and widths were applied to modulate the phase and dispersion of the incident light. The metalens can fulfill three target phases simultaneously by using an optimization method. The designed metalens has a stable focus from 450 to 800 nm with an average focusing efficiency of 64%. It can be potentially applied in microscopes, lithography machines, sensors, and displays.

In order to improve the performance and simplify the structure of an optical antenna for a space laser communication system, we design a free-form off-axis three-mirror optical antenna with an integrated primary/tertiary-mirror structure. The adoption of the integrated primary/tertiary-mirror structure improves efficiency of light energy utilization and reduces the complexity of optical processing and assembly. The introduction of free-form optical elements and optical structure constraints helps to correct the off-axis aberration and realize a large field of view. The obtained optical antenna has the magnification of five times and field of view of 2.4 ∘ × 2.4 ∘ . The image quality obtained here reaches the diffraction-limited level. At the communication wavelength of 808 nm, the wavefront error is better than λ / 22 , and the system has a high energy concentration. The proposed optical antenna could not only improve tracking accuracy of the optical antenna in space but also greatly reduce the complexity of the laser communication system. It has reference significance and application value for free-space laser communication.

For an echelle spectrometer based on a digital micromirror device and photomultiplier, a lens group is placed in front of the digital micromirror device and used to converge the two-dimensional spectrum and correct aberration. The matching spectral reduction model is deduced by establishing the mathematical models of the imaging mirror and lens group. According to the known wavelength and corresponding coordinates, the refractive index of the prism is fitted, and the spectrum is divided into three parts to calibrate. Experimental results of a mercury lamp show that the maximum deviations of x and y coordinates are 3.4 pixels and 2.3 pixels in the corrected spectral reduction model. The deviations are in the range of the errors permitted, which means the extraction of order and wavelength satisfy the requirement of the spectrometer.

Mid-infrared (mid-IR) multispectral microscopy, especially operating at the wavelength of 5–11 µm, is an effective tool for detecting, identifying, and quantifying the structure and composition of biological tissues. Compared with that based on the optical lens, the mid-infrared microscope composed of off-axis parabolic (OAP) mirrors is low cost, simple, and suitable for longer range of wavelength without chromatic aberrations, while keeping the optical transmission efficiency. Here we report a compact and versatile mid-infrared multispectral confocal microscope based on off-axis parabolic mirrors. We also perform numerical calculations based on the vectorial diffraction theory on OAP mirrors and analyze the typical aberrations and misalignment of the OAP-based optical system. Finally, we perform multispectral imaging of the epiretinal membrane of the human eyes with the spectrum selected according to its resonance absorption peak. The system is designed to perform multispectral or even hyperspectral imaging to identify and predict potential disease.

In order to obtain a composite achromatic wave plate with adjustable retardation, temperature insensitivity, and achromatic bandwidth of 500 nm, a five-element composite achromatic quartz wave plate is designed based on particle swarm optimization. The total phase retardation can be adjusted by rotating the azimuth angle of the central wave plate. The results show that the total phase retardation is adjustable with the range of 90°–180° when the range of temperature and the achromatic wavelength is − 20 − 80 ∘ C and 750–1250 nm, respectively. The absolute value of the relative deviation of the maximum phase retardation is less than 3.6%, which meets the design requirements of the wave plate. The temperature insensitivity of the five-element wave plate is better than the three- and four-element wave plates. This method is of great reference value for designing this kind of composite achromatic wave plate.

Cascaded stimulated Raman scattering (SRS) of carbon disulfide ( C S 2 ) was investigated by a pulsed Nd:YAG laser with a wavelength of 532 nm. The fourth-order Stokes and second-order anti-Stokes lines were generated when the pump laser energy was about 1.909 mJ in one sample cell ( C 1 ) only. However, the same result was obtained in the second sample cell ( C 2 ) with a pump energy of 0.883 mJ. At the same time, the fifth-order Stokes line was produced in C 2 when the pump energy increased to 1.208 mJ, and the coherent radiation wavelength ranged from 498 to 644 nm. The result was attributed to the resonance enhancement effect, where the frequency difference between the pump laser and the Stokes light emitted from the working medium ( C S 2 ) self-matched with the vibrational energy level of C = S , which resulted in the generation of the cascaded broadband SRS. The anti-Stokes SRS was attributed to four-wave mixing. Simultaneously, the pulse durations of the Stokes and anti-Stokes were compressed to about 380 ps by SRS and laser-induced breakdown. The resonance effect not only reduced the threshold, but it also generated broadband and short-pulse SRS.

Utilizing the near-field enhancement effect of a polystyrene microsphere, direct ablation of nanohole arrays by a temporal-shaping femtosecond (fs) laser pulse is presented. The nanohole arrays, which are circular, regular, and free of cracks, were processed without extra post-processing, and their average diameter decreased gradually, as the double-pulse delay increased until 2500 fs. The simulated results by a plasma model and finite difference time domain solution demonstrate that the size decrease of the structure is attributed to the increase of the ablation threshold of silicon. Through fs laser near-field fabrication, the FWHM of nanoholes can be reduced to approximately 50 nm ( λ / 16 ) and even to 23 nm when using the second harmonic laser at a wavelength of 400 nm.

In non-rotationally symmetric optical systems, chromatic aberrations must be defined in a generalized way with reference to the optical axis ray and optimal tilted parabasal image plane of the central wavelength. In this paper, the definition of generalized chromatic aberrations is clarified. The mathematical calculation in optical systems with arbitrary symmetry and surface shape is realized by ray- and wavefront-based methods, respectively, that are originally identical. The additivity of wavefront after each surface ensures the surface decomposition of chromatic aberrations. In the end, the influence of pupil aberration, which is of a higher order, is discussed. The consistency of our methods with Seidel aberrations in the lowest aberration order of the rotationally symmetric system and the application of our methods in diverse non-rotationally symmetric refractive systems will be addressed in detail in Part II.

We report a widely tunable optically pumped vertical-external-cavity surface-emitting laser. The multiple quantum wells in the active region of the gain chip are generally designed to form the resonant periodic gain structure, and three different methods are used to tune the oscillating wavelength. The maximum wavelength coverage of 45 nm is obtained when a 2 mm thickness birefringent filter is introduced in a straight-line cavity, while the tuning range of 8 nm is performed by employing a 0.15 mm thickness uncoated Fabry–Perot etalon. For the first time, to the best of our knowledge, we present an 11 nm tuning range by the use of an inserted blade as the tuning element, and the related wavelength tuning mechanism is analyzed.

A polymer/silica hybrid 3D waveguide thermo-optic (TO) mode switch based on cascaded asymmetric directional couplers (ADCs) is theoretically designed and simulated, where the spatial modes of a few-mode silica waveguide can be switched to various single-mode polymer waveguides placed above the few-mode silica waveguide. A beam propagation method is employed to optimize the dimensional parameters of the mode switch to convert the L P 11 a and L P 11 b modes of the few-mode silica waveguide to the L P 01 mode of two single-mode polymer waveguides using the cascaded ADC 1 and ADC 2. The coupling ratios are higher than 96.4% (93.4%) and 95.1% (92.8%) for the ADC 1 and ADC 2, respectively, under the TE (TM) polarization within the wavelength range from 1530 to 1570 nm, which shows good wavelength independence. Furthermore, the monolayer graphene is introduced as the heating electrode and buried on the surface of the polymer core to increase the heating efficiency and reduce the power consumption. The power consumption for ADC 1 and ADC 2 is 16.69 mW and 17.35 mW, respectively. Compared to the traditional TO switch with an aluminum (Al) heating electrode, the heating efficiency of the presented device can be improved by ∼ 30 % . Moreover, the response speed of the TO mode switch with a 3D waveguide structure was also significantly improved. Compared to the device with Al electrodes, the introduced graphene electrodes can improve the switching speed of the device by ∼ 60 % . The presented TO mode switch with its small size and easy integration should find applications in reconfigurable mode division multiplexing systems.

A single-frequency polarization-maintaining holmium-doped fiber master oscillator power amplifier operating at signal wavelengths of 2054 n m and 2090 n m is presented. The two-stage setup delivers up to 240 W peak power and 6.7 µ J pulse energy for a pulse width of 30.2 n s at a repetition rate of 100 k H z . The first amplifier stage is designed by simulation, tailored for high gain at the signal wavelength range, favoring amplification at 2090 n m . The design is discussed, and the measured values are compared with the simulation. The second stage is investigated regarding the efficiency for co- and counter-pumping. Stimulated Brillouin scattering was found to be the limiting factor for pulse peak power scaling in the second stage. The measured output pulse shapes are discussed and compared to pulse shapes derived with the Frantz–Nodvik model.

The progression of secondary waves emanating from scattering centers within a non-absorbing material in the visible part of the electromagnetic spectrum (wavelength 0.500 µm) is traced using two modeling schemes: the scattering order formulation (SOF) of the discrete dipole approximation (the original SOF) and the SOF with the Twersky approximation, according to which the path of the secondary waves in a successive scattering chain does not loop through the same scattering center more than once (the SOF-Twersky). It is shown that for smaller submicron-sized scatterers (radius 0.100–0.200 µm), the scattering phase function in the backscattering hemisphere converges after a small number of orders of scattering, as does the scattering phase function in the forward scattering hemisphere. For larger submicron-sized to micron-sized scatterers (radius 0.300–0.500 µm), however, the scattering phase function in the backscattering hemisphere deviates significantly from the correct scattering phase function for a small number of orders of scattering, even as the scattering phase function in the forward scattering hemisphere remains at reasonable values. These results point to the particular importance of higher orders of scattering in reducing backscattering by particles composed of non-absorbing materials. Likewise, the results imply that a possible source for the lack of convergence in the original SOF lies in a certain lack of destructive interference of secondary waves propagating in particular into the backscattering hemisphere. The extent to which “downward recursion” can ameliorate this lack of destructive interference of secondary waves propagating into the backscattering hemisphere within the SOF is also examined. In the SOF with downward recursion, looping of secondary scattered waves is allowed, unlike the other versions of the SOF; however, the scattering centers situated the largest distance from one another within a given scatterer interact with one another first, and the interactions proceed to smaller and smaller scattering center separations.

A photonic instantaneous frequency measurement receiver based on frequency to optical power mapping is proposed and experimentally demonstrated. One channel of a dense wavelength-division multiplexer (DWDM) is used as an optical filter to establish a power ratio function related to the frequency of the microwave signal. Different from most optical filters, the DWDM filter features smooth and quasilinear roll-off over a wide bandwidth. With the help of a laser of good wavelength stability and a bias controller, large measurement range and high accuracy are simultaneously achieved without multi-step operation. The instanstaneous frequency measurement receiver can measure the frequency with an accuracy of 0.2% of the signal frequency in the range of 1–40 GHz over 1.5 h in the experiment.

A wavelength-tunable Q-switched erbium-doped fiber laser based on a digital micromirror device (DMD) is experimentally demonstrated. The Q-switched pulses are generated by incorporating a saturable absorption device made of graphene oxide. Stable Q-switched pulses at 1.5 µm band are obtained at a low threshold of 20 mW, corresponding to the pulse width of 7.1 µs and the repetition rate of 43.3 kHz. The maximum output power and the maximum pulse energy of the Q-switched pulses are 260.1 µW and 3.97 nJ, respectively. By controlling the DMD, the center wavelength of the Q-switched pulses can be tuned from 1528.2 to 1559.3 nm, with a tuning range of about 31 nm. The fine tunable accuracy can reach 0.08 nm by precisely controlling the DMD. Combining the filtering characteristics of the DMD with the saturable absorption characteristics of nanomaterials, the Q-switched laser with tunable wavelength is realized, which, we believe, is reported for the first time and has broad application prospects.

Gaseous Raman lasers cover a range of wavelengths but lack wavelength tunability. Here, a 1 c m − 1 linewidth 532 nm laser was used as a pump laser, and with a narrow-linewidth seed laser injection, a narrow-linewidth first Stokes (S1) Raman laser was achieved. By tuning the wavelength of the seed laser, a tuning range of S1 up to 1 c m − 1 was obtained. The wavelengths of the first anti-Stokes and second Stokes lasers could also be tuned. A theoretical model was developed, and spectral profiles of Raman lasers from experiments and simulations agreed well; further simulation predicted that the linewidth of S1 could be compressed to as narrow as 0.01 c m − 1 under optimal conditions. A universal method of fine-tunable Raman lasers is presented that can be utilized in several important applications.

We demonstrate an optical phased array that consists of two subarrays based on the silicon on insulator (SOI) platform, each subarray including 16 independent channels. The demonstrated field of view of the optical phased array is 36.6 ∘ × 32.6 ∘ with a spot size of 1.68 ∘ × 0.0673 ∘ . A steering range of 32.6° is achieved by combining two subarrays with different periods and tuning the wavelength from 1500 nm to 1600 nm. In another dimension, the steering is realized by introducing phase differences between channels.

Terahertz spectroscopy, combined with chemometric methods, has proved to be an effective tool in the quantitative analysis of many substances. However, current research has mainly focused on comparing different algorithms under the full spectrum. In fact, the full spectrum is not only composed of characteristic features of the samples, but also many types of noises. Hence, the accuracy of the quantitative analysis may be unsatisfactory if the full spectrum is selected. In this paper, a wavelength selection method based on partial least squares and absorption peak was proposed and an efficient frequency band was determined in the quantitative analysis of three types of pesticides, i.e., 6-benzylaminopurine, 2, 6-dichlorobenzonitrile, and imidacloprid. By introducing two parameters, the sum of peak intervals ( S i ) and peak width, the most efficient peak was selected from multiple peaks and the specific peak width was given with the aid of particle swarm optimization. We concluded that the most efficient absorption peak for quantitative analysis corresponding to the largest S i and full width near one-half maximum could characterize full spectrum information precisely. Comparing before and after wavelength selection, the correlation coefficient ( R ) of the three pesticides have increased from 0.9671, 0.9705, 0.9884 to 0.9921, 0.9934, and 0.9957. In conclusion, the proposed wavelength selection method was demonstrated to be very efficient for the quantitative analysis of the pesticide mixtures, which also could be helpful in the analysis of other multicomponent mixtures with absorption peaks.

We present the inscription of narrow-linewidth fiber Bragg gratings (FBGs) into different types of multicore fibers (MCFs) using ultrashort laser pulses and the phase mask technique, which can act as notch filters. Such filters are required, e.g., to suppress light emitted by hydroxyl in the Earth’s upper atmosphere, which disturbs ground-based observation of extraterrestrial objects in the near infrared. However, the inscription into a commercially available seven-core fiber showed a quite large core-to-core deviation of the resonance wavelength of up to 0.45 nm. Two options are presented to overcome this: first, we present the photo-treatment of the FBGs to tune the resonance wavelength, which allows for sufficient resonance shifts. Second, adapted MCFs containing 12 cores, arranged on a circle, are fabricated. For this, two different fabrication procedures were investigated, namely, the mechanical drilling of the preform for a rod-in-tube version as well as a stack-and-draw approach. Both adapted MCFs yielded significant improvements with core-to-core wavelength variations of the FBGs of only about 0.18 nm and 0.11 nm, respectively, sufficient to fulfill the requirements for astronomical filter applications as discussed above.

We numerically investigated a sandwiched photonic crystal fiber (S-PCF) based on triangular lattice air holes that can be used for broadband dispersion compensation. By adjusting two layers of air holes diameter in the cladding, the S-PCF performs a large negative dispersion curve that can compensate for the dispersion of a standard single-mode fiber (SMF). The kappa value of the proposed PCF is also consistent with that of the SMF at the wavelength of 1.55 µm. In addition, the dispersion compensation multiple reaches up to 25.37. We have successfully obtained an effective compensated dispersion with the value of − 0.5 to + 0.1 p s / n m / k m in the wavelength of 1.46–1.67 µm, which covers the S + C + L + U wavelength bands. Moreover, we considered the manufacturing tolerance of the dispersion and effective compensated dispersion with ± 2 % and ± 5 % variation.

A distributed feedback (DFB) fiber laser and fiber Bragg gratings (FBGs) are configured to demodulate the wavelength shifts of FBG dynamic strain sensors. The FBG sensors act as sensing units to detect the dynamic strain and the demodulators while the DFB fiber laser only acts as a narrow-linewidth light source. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the output is subsequently converted into a corresponding intensity change and detected directly by a photodetector. The 0.2 nm linewidth FBG sensor can detect the impact signal with a frequency of up to 300 kHz with a maximum of 29.17 µɛ, which is comparable with the detecting result of the piezoelectric transducer sensor. Moreover, the directional response of the FBG sensor is maximized when the direction of acoustic wave propagation is parallel to the optical fiber. The relation between the sensitivity and the FBG spectrum linewidth is presented, and the detectable strain range versus different FBG linewidths is also discussed.

A silicon-based engineered hybrid plasmonic waveguide with ultra-low dispersion is proposed. The ridge-shaped structure of the nanophotonic waveguide enables nano-scale confinement with electrically tunable characteristics using the plasma dispersion effect in silicon. The waveguide exhibits ultra-low dispersion of 1.28 p s 2 / m at telecommunication wavelength (1550 nm) in C band together with dual flatband dispersion over a wavelength range of 370 nm. The hybrid plasmonic mode is made to be confined in 15 nm thick S i O 2 with a propagation loss of 15.3 dB/mm utilizing the engineered ridge structure comprising Si, S i O 2 , and gold. In addition, the proposed waveguide shows six zero-dispersion wavelengths. The imaginary and real parts of the effective refractive index of the guided hybrid plasmonic mode are reported to be tunable with the applied voltage. The reported numerical results can pave the way for achieving intensity modulators and other electrically tunable devices at telecommunication wavelengths. The ultra-low dispersion and electrical tuning make this nanophotonic waveguide an absolute contender for applications including efficient nonlinear signal processing such as wide wavelength conversion based on four-wave mixing, supercontinuum generation, and other nanoscale integrated photonic devices.

We present a method for calibration and data extraction for a Stokes polarimeter working with three different wavelengths simultaneously. In the Stokes polarimeter considered in this work, we use two liquid crystal variable retarders (LCVRs) combined with a Glan–Thompson linear polarizer. A recently developed fitting calibration procedure is used. We use the same calibration samples and LCVR voltages for all three wavelengths, giving simultaneous measurement and calibration. We compare the performance of the polarimeter, after calibration, using four or six calibration samples in our experiment. To generate the four known calibration beams, we use a linear polarizer oriented at 130° and 30° with respect to the horizontal, a horizontal linear polarizer followed by a half-wave plate (at 632 nm) with its fast axis at 30°, and a horizontal linear polarizer followed by a quarter-wave plate (at 632 nm) with its fast axis at 30°. For calibration with six reference beams, we add two known calibration beams by setting the fast axis of the half- and quarter-wave plates at 130°. Experimental results show good agreement with the expected results, with the fitting calibration procedure giving an approximately 50% reduction in total RMS error with four calibration samples. There is a negligible reduction in the error when six calibration samples are used compared to the case with four samples.

The wavelength demultiplexer (deWMUX) is an indispensable component for the wavelength diversity system. In this paper, we propose a subwavelength-grating-assisted 1 × 2 deWMUX based on the principle of multimode interference, which aims to output 1.55 and 2 µm wavelengths from two different channels. The simulation results show that the contrast of the designed deWMUX at both wavelengths is greater than 24 dB, the insertion loss is less than 0.5 dB, and the 1 dB bandwidth is wider than 100 nm. In addition, the manufacturing tolerances of the device are also studied.

We present design and performance data of two diode-pumped Alexandrite lasers developed explicitly as laser emitters in mobile potassium resonance lidar systems. The lasers yield an output power of up to 1.75 mJ at a repetition rate of 500 Hz with a beam quality of M 2 < 1.1 in both spatial directions. Reliable single longitudinal mode operation with a unrivaled narrow linewidth of 3.3 MHz at a potassium resonance line at 769.898 nm is achieved. The wavelength can be switched from pulse to pulse in a range of several gigahertz so the potassium line can be scanned. The lasers are finally integrated in highly efficient lidar systems with a power consumption of 500 W for the whole lidar system. The extremely high spectral requirements are investigated and the performance for different working points regarding repetition rates and pump durations is investigated. Several weeks of remotely controlled operation of the prototype in a field campaign were conducted without changes of the output parameters. Approximately 1000 h of reliable single longitudinal mode operation was achieved during the campaign and measurements of Doppler–Mie wind observations in the stratosphere and of the potassium layer in the mesopause were conducted simultaneously even at daytime.

A single-frequency (SF) fiber laser is widely applied in many fields. A superior ultra-narrow-bandwidth fiber filter, which consists of an in-series commercial common fiber Bragg grating (FBG) filters with a partial overlap in their reflection spectrum, is presented in this paper. The bandwidth of the filter with a center reflection wavelength of 1549.83 nm reaches 0.024 nm at 3 dB reflection, a remarkable reduction of approximately 300% by combination usage narrower than a single FBG used bandwidth. We also demonstrate a method to further narrowing of the filter bandwidth by increasing the usage times of filters, and the bandwidth can reach 0.0025 nm at 3 dB reflection. A long linear cavity single-frequency fiber laser with an entire cavity of 30 cm is implemented with the linewidth of 941 Hz at 3 dB, which is about 210% narrower than the previous record of SF fiber laser based on FBG filters in a linear cavity. This research may open a new door toward ultra-narrow-bandwidth filters and SF fiber lasers.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2019-07-08, 資料番号: PA1910042000

Many vortex-generation techniques have been developed to address a range of potential applications, exploiting their unique amplitude and phase profiles and their possession of orbital angular momentum. In this work, we present what may be the simplest method of vortex beam generation, requiring only a wedged optic: the wedge-plate shearing interferometer (WPSI). We show that the WPSI can reflect a first order Laguerre–Gaussian vortex beam ( L G 01 ) with a theoretical purity of > 99 % from an input fundamental Gaussian beam, with 98% L G 01 purity experimentally demonstrated. We demonstrate 1% power conversion with a route to 14%. The monolithic WPSI is a simple, compact, and highly stable device, which can operate at any wavelength that the material is transparent to. We anticipate that it will be useful where sampling a robust, high-purity vortex beam from a Gaussian laser beam is required, including low-cost vortex generation for metrology or education.

Near-infrared wavelength observations are crucial for understanding numerous fields of astrophysics, such as supernova cosmology and positronium annihilation detection. However, current ground-based observations suffer from an enormous background due to OH emission in the upper atmosphere. One promising way to solve this problem is to use ring-resonator filters to suppress OH emission lines. In this work, we discuss our optimization of ring-resonator filter performance from five perspectives: resonance wavelength matching, polarization-independent operation, low insertion loss, low-loss coupling to astronomical instruments, and broadband operation. In the end, we discuss next steps needed for reliable supernova and positronium observations, thus providing a roadmap for future advances in near-infrared astronomy.

A dual-wavelength locking technique for coherent 2-µm differential absorption lidar (DIAL) applications has been developed for simultaneously measuring water vapor ( H 2 O ) and radial wind velocity profiles. The two wavelengths for DIAL measurement were stabilized by sidebands of an electro-optic modulated laser, which was locked to the C O 2 R30 absorption line. We selected wavelengths of 2050.550 and 2051.103 nm to minimize the effects of height-dependent temperature and pressure variations. The long-term wavelength stability of the two locked lasers of < 0.2 p m , which corresponds to 14 MHz, was achieved. The DIAL instrument using the developed technique meets the requirement of measuring the H 2 O concentration with a systematic error < 5 % below an altitude of 5 km.

Spectroscopic single-molecule localization microscopy (sSMLM) generates super-resolution images of single molecules while simultaneously capturing the spectra of their fluorescence emissions. However, sSMLM splits photons from single-molecule emissions into a spatial channel and a spectral channel, reducing both channels’ precisions. It is also challenging in transmission grating-based sSMLM to achieve a large field-of-view (FOV) and avoid overlap between the spatial and spectral channels. The challenge in FOV has further significance in single-molecule tracking applications. In this work, we analyzed the correlation between the spatial and spectral channels in sSMLM to improve its spatial precision, and we developed a split-mirror assembly to enlarge its FOV. We demonstrate the benefits of these improvements by tracking quantum dots. We also show that we can reduce particle-identification ambiguity by tagging each particle with its unique spectral characteristics.

The SPICA far-infrared instrument (SAFARI), one of instruments of the space infrared telescope for cosmology and astrophysics, requires a calibration source assembly to calibrate the transition edge sensor readout circuits. A high-performance integrating sphere working at SAFARI wavelength (34–230 µm) is essential. A novel process for preparing terahertz integrating spheres was developed. The aluminum surfaces after sandblasting, wet-etching, and gold plating processes demonstrate rough morphology but high reflectance of 0.91 in 1–10 THz. The spatial distribution of the measured output power excellently agrees with the numerical simulation results based on the assumption of uniform surface radiation at the output port.

An ultra-narrow-bandwidth near-infrared perfect plasmonic absorber with a periodic structure composed of metal–insulator–metal configuration is numerically designed and analyzed for a refractive index sensor. A perfect absorptivity of 99.99% and a rather narrow bandwidth of 2 nm are observed in the near-infrared wave band when the light is vertically incident in the structure. The ultra-high absorption and ultra-narrow bandwidth are explained as local surface plasmon resonance stimulated in the structure. The absorber is put into use as a plasmonic refractive index sensor with wavelength sensitivity as high as 1500 nm/RIU and figure of merit as high as 750, which significantly surpass those of many plasmonic sensors proposed in the past few decades. The designed absorber is tunable by changing geometry parameters. Because of the excellent sensing properties mentioned above, the proposed structure can be widely applied in fields such as photodetectors, biosensors, and chemical molecule detection.

The development of miniaturized multi-channel infrared filters based on plasmonic metasurfaces is attracting growing attention, driven by its potential applications in infrared imaging, photodetectors, and spectroscopy. However, the advance of such filters in long-infrared wavelengths has rarely been reported. This paper reports our recent progress on developing multi-channel spectral filters based on micrometer metallic hole arrays in the long-infrared band of 10–15 µm. The effects of structural parameters and the shapes of metallic hole arrays on filtering performance are investigated by numerical simulations with the finite-difference time-domain method and then experimentally verified by optical characterizations of fabricated filters using electron beam lithography. The transmission peaks of the filter on a zinc selenide substrate were optimized with a maximum transmittance of 63%. A comparison of the hole shapes shows that elliptical holes give rise to sharper transmission peak quality than round ones by 28%. The progress achieved in this work should be a promising step in the development of metallic hole-based spectral filters with miniaturized dimensions.

Rapid, nondestructive and accurate detection of internal qualities of the apple is an important research interest. In this study, the brix, acidity and brix/acidity ratio of the apple were rapidly detected by visible and near-infrared spectroscopy (VIS-NIRS). By scanning spectra and measuring the reference values of brix and acidity of apple samples, the relationship models between the spectra and brix, acidity, brix/acidity ratio were, respectively, established. Sample division, characteristic wavelength optimization, and modeling methods were compared systematically, and the optimal prediction model of each quality index was determined. The experimental results show that the competitive adaptive reweighted sampling method can effectively select characteristic wavelengths, which not only improves the prediction speed, but also greatly enhances the prediction accuracy. The established partial least squares models based on these selected characteristic wavelengths all have high accuracy and robustness for the three quality indices. The determination coefficients of the models are 0.9899, 0.9615, 0.9535, and the relative percent deviation are 9.9269, 5.0987, 4.6374, respectively. All this work proves that VIS-NIRS can be used for rapid and nondestructive detection of the internal qualities of an apple.

Phase change material G e 2 S b 2 T e 5 (GST) has recently emerged as a highly promising candidate for photonic device applications owing to its high optical contrast, self-holding bi-stability, and fast material response. Here, we propose and analyze a 1 × 2 tunable switch using a GST embedded silicon microring resonator exploiting high optical contrast during GST phase change and a high thermo-optic coefficient of amorphous phase GST. Our device exhibits high extinction ratios of 25.57 dB and 18.75 dB at through and drop ports, respectively, with just a 1 µm long GST layer. The two states of the switch are realizable by electrically inducing phase change in GST. For post phase change from amorphous to crystalline and vice versa, the fall time down the 80% of phase transition temperature is ∼ 66 n s and ∼ 45 n s , respectively. The resonance wavelength shift per unit active length is 0.661 nm/µm, and the tuning efficiency is 1.16 nm/mW. The large wavelength tunability (4.63 nm) of the proposed switch makes it an attractive option for reconfigurable photonic integrated circuits.

The possibility of distributed wide-range strain and temperature measurements in a 100 km long optical fiber using tunable-wavelength low-coherence optical time-domain reflectometer (OTDR) is demonstrated. The specified distance range is provided by employing two narrowband microelectromechanical system (MEMS) spectral filters tuned synchronously as well as by taking advantage of Raman amplification and amplification by remotely pumped erbium-doped fiber segments built into the fiber under test. With the time of a single measurement of 10 min and the spatial resolution of about 1 m, the measurement range reached 1000 µɛ in strain units, which is equivalent to the temperature range of 110°C.

Digital holography (DH) with two wavelengths (TW) that are close to each other was applied to height measurement of solder bumps having spherical specular surfaces with diameters of ∼ 20 µ m and heights of ∼ 20 µ m . We employed the parallel phase shifting method for instantaneous image capturing, and we improved the spatial resolution of our TW-DH system having two beams with different wavelengths that traveled in opposite directions in the interferometer. It gave 74-times higher repetition and 2.4-times higher spatial resolution than those in our previous DH system based on the Fourier transform method.

International audience; The purpose of this work is to provide theoretically grounded assessment on both the field-of-view and the bandwidth of a lensless holographic setup. Indeed, while previous works have presented results with super-resolution and field-of-view extrapolation, there is no well established rules to determine them. We show that the theoretical field of view can be hugely large with a spatial-frequency bandwidth only limited by the wavelength leading to an unthinkable number of degrees of freedom. To keep a realistic field of view and bandwidth, we propose several practical bounds based on few setup properties: namely the noise level and the spatio-temporal coherence of the source.