Showing papers in "Chinese Optics Letters in 2023"

Advances in multipass cell for absorption spectroscopy-based trace gas sensing technology [Invited]

Abstract: In the field of absorption spectroscopy, the multipass cell (MPC) is one of the key elements. It has the advantages of simple structure, easy adjustment, and high spectral coverage, which is an effective way to improve the detection sensitivity of gas sensing systems such as tunable diode laser absorption spectroscopy. This invited paper summarizes the design theory and the research results of some mainstream types of MPCs based on two mirrors and more than two mirrors in recent years, and briefly introduces the application of some processed products. The design theory of modified ABCD matrix and vector reflection principle are explained in detail. Finally, trends in its development are predicted.

Understanding wide field-of-view flat lenses: an analytical solution [Invited]

Abstract: Wide field-of-view (FOV) optics are widely used in various imaging, display, and sensing applications. Conventional wide FOV optics rely on complicated lens assembly comprising multiple elements to suppress coma and other Seidel aberrations. The emergence of flat optics exemplified by metasurfaces and diffractive optical elements (DOEs) offers a promising route to expand the FOV without escalating complexity of optical systems. To date, design of large FOV flat lenses has been relying upon iterative numerical optimization. Here, we derive, for the first time, to the best of our knowledge, an analytical solution to enable computationally efficient design of flat lenses with an ultra-wide FOV approaching 180°. This analytical theory further provides critical insights into working principles and otherwise non-intuitive design trade-offs of wide FOV optics.

Enhanced Fano resonance for high-sensitivity sensing based on bound states in the continuum

Abstract: Although previously reported terahertz absorbers can achieve high-sensitivity refractive index sensing, the resonant peak is too broad, which leads to a low figure of merit (FOM). Transmissive sensors based on bound states in the continuum (BIC) can achieve high FOM, but they have some limitations in high sensitivity. Herein, we propose a periodic triple parallel metal bars structure to obtain high quality, a strong field, and multiple hot spots by the Friedrich–Wintgen BIC. Numerical results show the sensitivity and FOM can reach 1877 GHz/RIU and 665, respectively. Compared to the previously reported transmissive sensors based on BIC, the sensitivity has been greatly improved.

Simultaneous temperature and magnetic field measurements using time-division multiplexing

Abstract: Nitrogen-vacancy color centers can perform highly sensitive and spatially resolved quantum measurements of physical quantities such as magnetic field, temperature, and pressure. Meanwhile, sensing so many variables at the same time often introduces additional noise, causing a reduced accuracy. Here, a dual-microwave time-division multiplexing protocol is used in conjunction with a lock-in amplifier in order to decouple temperature from the magnetic field and vice versa. In this protocol, dual-frequency driving and frequency modulation are used to measure the magnetic and temperature field simultaneously in real time. The sensitivity of our system is about 3.4 nT = (cid:1)(cid:1)(cid:1)(cid:1)(cid:1) Hz p and 1.3 mK = (cid:1)(cid:1)(cid:1)(cid:1)(cid:1) Hz p , respectively. Our detection protocol not only enables multifunctional quantum sensing, but also extends more practical applications.

All-fiber telecom band energy-time entangled biphoton source

Abstract: We report an all-fiber telecom-band energy-time entangled biphoton source with all physical elements integrated into a compact cabinet. At a pump power of 800 µW, the photon pairs generation rate reaches 6.9 MHz with the coincidence-to-accidental ratio (CAR) better than 1150. The long-term stability of the biphoton source is characterized by measuring the Hong–Ou–Mandel interference visibility and CAR within a continuous operation period of more than 10 h. Benefiting from the advantages of compact size, light weight, and high stability, this device provides a convenient resource for various field turnkey quantum communication and metrology applications.

Laser frequency locking method for Rydberg atomic sensing

Abstract: Based on the Rydberg cascade electromagnetically induced transparency, we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature. The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator, realizing real-time feedback for both lasers. We studied the effect of the different probe and coupling laser powers on the error signal. In addition, the Allan variance and a 10 kHz amplitude-modulated signal are introduced to evaluate the performance of the laser frequency stabilization. In principle, the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

50 m/187.5 Mbit/s real-time underwater wireless optical communication based on optical superimposition

Abstract: In this paper, an optical pulse amplitude modulation with 4 levels (PAM-4) using a fiber combiner is proposed to enhance the data rate of a field-programmable gate-array-based long-distance real-time underwater wireless optical communication system. Two on – off keying signals with different amplitudes are used to modulate two pigtailed laser diodes, respectively, and the generated optical signals are superimposed into optical PAM-4 signals by a fiber combiner. The optical PAM-4 scheme can effectively alleviate the nonlinearity, although it reduces the peak-to-peak value of the emitting optical power by 25%. A real-time data rate of 187.5 Mbit/s is achieved by using the optical PAM-4 with a transmission distance of 50 m. The data rate is increased by about 25% compared with the conventional electrical PAM-4 in the same condition.

Passively Q-switched operation of a 1.94 µm thulium-doped solid-state laser based on MXene V2CTx

Abstract: MXene V2CTx has great practicability because it is not easy to degrade under ambient conditions. In this paper, a V2CTx saturable absorber (SA) was firstly applied to a passively Q-switched (PQS) laser, to the best of our knowledge. The V2CTx-SA was prepared by the spin-coating method. The linear absorption of the V2CTx-SA in the 1000–2200 nm region and the nonlinear absorption near 2 µm were studied. With the V2CTx-SA, a typical PQS operation at 1.94 µm was realized in a Tm:YAlO3 laser. The minimum pulse width produced by the PQS laser was 528 ns, and the peak power, repetition rate, and average output power were 10.06 W, 65.9 kHz, and 350 mW, respectively. Meanwhile, the maximum pulse energy was 6.33 µJ. This work demonstrates that the V2CTx can be used as an effective SA to obtain nanosecond pulses with high peak power and high repetition rate simultaneously.

Highly efficient transient stimulated Raman scattering on secondary vibrational mode of BaWO4 crystal due to its constructive interference with self-phase modulation

Abstract: An exceptionally high stimulated Raman scattering (SRS) conversion efficiency to the first Stokes component associated with the secondary (low-frequency and low intensity) vibrational mode ν2 (∼330 cm-1) was observed in a BaWO4 crystal in a highly transient regime of interaction. The effect takes place in the range of pump pulse energy from ∼0.1 to ∼0.5 µJ with maximum energy conversion efficiency up to 35% at 0.2 µJ. The nature of the observed effects is explained by interference of SRS and self-phase modulation, where the latter is related to a noninstantaneous orientational Kerr nonlinearity in the BaWO4 crystal.

NH3 laser THz emission under optical pumping by “long” (∼100 µs) CO2 laser pulses

Abstract: Terahertz (THz) NH3 lasing with optical pumping by electron-beam-sustained discharge “long” (∼100 µs) CO2 laser pulses was obtained. The NH3 laser emission pulses and the “long” pulses of the CO2 pump laser were simultaneously measured with nanosecond response time. The NH3 lasing duration and its delay with respect to the pump pulse were measured for various CO2 laser pulse energies. For the CO2 laser pump line 9R(30), three wavelengths of 67.2, 83.8, and 88.9 µm were recorded. For the CO2 laser pump line 9R(16), only a single NH3 laser line with a wavelength of 90.4 µm was detected.

Photon pair generation in lithium niobate waveguide periodically poled by femtosecond laser

Abstract: Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols. The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching. Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW). Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source. This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.

High color saturation and angle-insensitive ultrathin color filter based on effective medium theory

Abstract: An ultrathin angle-insensitive color filter enabling high color saturation and a wide color gamut is proposed by relying on a magnesium hydride-hydrogenated amorphous silicon (MgH 2 -a-Si:H) lossy dielectric layer. Based on effective medium theory, the MgH 2 -a-Si:H layer with an ultrathin thickness can be equivalent to a quasi-homogeneous dielectric layer with an effective complex refractive index, which can be tuned by altering the thickness of MgH 2 to obtain the targeted value of the imaginary part, corresponding to the realization of high color saturation. It is verified that the proposed color filter offers highly enhanced color saturation in conjunction with a wide color gamut by introducing a few-nanometer thick MgH 2 layer. As the MgH 2 -a-Si:H layer retains the advantages of high refractive index and tiny thickness, the proposed color filter exhibits large angular tolerance up to ±60°. In addition, MgH 2 with an unstable property can interconvert with Mg under a dehydrogenation/hydrogenation reaction, which empowers the proposed color filter with dynamically tunable output color. The proposed scheme shows great promise in color printing and ultracompact display devices with high color saturation, wide gamut, large angular tolerance, and dynamic tunability.

InAs/GaAs quantum dot laterally coupled distributed feedback lasers at 1.3 μm

Abstract: We report the InAs/GaAs quantum dot laterally coupled distributed feedback (LC-DFB) lasers operating at room temperature in the wavelength range of 1.31 μ m. First-order chromium Bragg gratings were fabricated alongside the ridge waveguide to obtain the maximum coupling coefficient with the optical field. Stable continuous-wave single-frequency operation has been achieved with output power above 5 mW/facet and side mode suppression ratio exceeding 52 dB. Moreover, a single chip integrating three LC-DFB lasers was tentatively explored. The three LC-DFB lasers on the chip can operate in single mode at room temperature, covering the wavelength span of 35.6 nm.

Cascaded metasurface for separated information encryption [Invited]

Abstract: For a conventional cascaded metasurface, the combination channel and each single channel are mutually dependent because the phase modulation of a cascaded metasurface is the sum of each single one. Here we propose a cascaded metasurface that can independently encode information into multiple channels. Based on the orientation degeneracy of anisotropic metasurfaces, each single metasurface can produce a quick-response (QR) image in the near field, governed by the Malus law, while the combined channel can produce a holographic image in the far field, governed by geometric phase. The independent and physically separated trichannel design makes information encryption safer.

Modulation of epsilon-near-zero wavelength and enhancement of third-order optical nonlinearity in ITO/Au multilayer films

Abstract: We report the modulation of epsilon-near-zero (ENZ) wavelength and enhanced third-order nonlinearity in indium tin oxide (ITO)/Au multilayer films. The samples consisting of five-layer 40 nm ITO films spaced by four-layer ultrathin Au films of different thickness, i.e., ITO(40 nm)/[Au(x)/ITO(40 nm)]4, were prepared by magnetron sputtering at room temperature. The ENZ wavelength in the multilayer films is theoretically calculated and experimentally confirmed. The nonlinear refractive index and nonlinear absorption coefficient of the samples of x=0, 2, 3, 4 nm were determined using the Z-scan method at a wavelength of 1.064 µm. The large nonlinear refractive index n2=1.12×10-13 m2/W and nonlinear absorption coefficient β=-1.78×10-7 m/W in the sample of x = 4 nm are both four times larger than those in the single-layer ITO film. The large optical nonlinearity due to the ENZ enhancement and carrier concentration is discussed. The results indicate that the ITO/Au multilayer films are promising for advanced all-optical devices.

Optical scrambler using WGM micro-bottle cavity

Abstract: Pengfa Chang (常朋发), Chen Wang (王 晨), Tao Jiang (蒋 涛), Longsheng Wang (王龙生), Tong Zhao (赵 彤), Hua Gao (高 华), Zhiwei Jia (贾志伟), Yuanyuan Guo (郭园园), Yuncai Wang (王云才), and Anbang Wang (王安帮) 1 Key Laboratory of Advanced Transducers & Intelligent Control Systems, Ministry of Education and Shanxi Province, College of Physics & Optoelectronic Engineering, Taiyuan University of Technology, Taiyuan 030024, China 2 Guangdong Provincial Key Laboratory of Photonics Information Technology, School of Information Engineering, Guangdong University of Technology, Guangzhou 510006, China

Temperature-sensing scheme based on a passively mode-locked fiber laser via beat frequency demodulation

Abstract: In this paper, we propose a temperature-sensing scheme utilizing a passively mode-locked fiber laser combined with the beat frequency demodulation system. The erbium-doped fiber is used in the laser ring cavity to provide the gain and different lengths of single-mode fibers inserted into the fiber ring cavity operate as the sensing element. Different temperature sensitivities have been acquired in the experiment by monitoring the beat frequency signals at different frequencies. The experimental results indicate that the beat frequency shift has a good linear response to the temperature change. The sensitivity of the proposed sensor is about − 44 kHz/°C when the monitored beat frequency signal is about 10 GHz and the ratio of the sensing fiber to the overall length of the laser cavity is 10 m/17.5 m, while the signal-to-noise ratio (SNR) of the monitored signal is approximately 30 dB. The proposed temperature-sensing scheme enjoys attractive features such as tailorable high sensitivity, good reliability, high SNR, and low cost, and is considered to have great potential in practical sensing applications.

Quasiperiodic photonic crystal fiber [Invited]

Abstract: In the fields of light manipulation and localization, quasiperiodic photonic crystals, or photonic quasicrystals (PQs), are causing an upsurge in research because of their rotational symmetry and long-range orientation of transverse lattice arrays, as they lack translational symmetry. It allows for the optimization of well-established light propagation properties and has introduced new guiding features. Therefore, as a class, quasiperiodic photonic crystal fibers, or photonic quasicrystal fibers (PQFs), are considered to add flexibility and richness to the optical properties of fibers and are expected to offer significant potential applications to optical fiber fields. In this review, the fundamental concept, working mechanisms, and invention history of PQFs are explained. Recent progress in optical property improvement and its novel applications in fields such as dispersion control, polarization-maintenance, supercontinuum generation, orbital angular momentum transmission, plasmon-based sensors and filters, and high nonlinearity and topological mode transmission, are then reviewed in detail. Bandgap-type air-guiding PQFs supporting low attenuation propagation and regulation of photonic density states of quasiperiodic cladding and in which light guidance is achieved by coherent Bragg scattering are also summarized. Finally, current challenges encountered in the guiding mechanisms and practical preparation techniques, as well as the prospects and research trends of PQFs, are also presented.

Hundred-watt level high-order mode all-fiberized random distributed feedback Raman fiber laser with high mode purity

Abstract: An all-fiberized random distributed feedback Raman fiber laser (RRFL) with LP 11 mode output at 1134 nm has been demonstrated experimentally, where an intracavity acoustically induced fiber grating is employed for modal switching. The maximum output power of LP 11 mode is 93.8 W with the modal purity of 82%, calculated by numerical mode decomposition technology based on stochastic parallel-gradient descent algorithm. To our best knowledge, this is the highest output power with high purity of LP 11 mode generated from the RRFL. This work may pave a path towards advanced fiber lasers with special temporal and spatial characteristics for applications.

W-band radio-over-fiber transmission system with delta-sigma modulation and direct detection

Abstract: We experimentally built a W-band photonics-aided millimeter-wave radio-over-fiber transmission system and demonstrated the delivery of up to 8192-ary quadrature amplitude modulation (QAM) signal. Discrete multitone signals are converted into 1-bit data streams through delta-sigma modulation and then modulated onto a 76.2 GHz carrier. An envelope detector is used at the receiver side for direct detection. The results prove that our proposed system can support 2048QAM and 8192QAM transmission while meeting the hard decision forward error correction threshold of 3.8×10-3 and the soft decision forward error correction threshold of 4.2×10-2, respectively. We believe this cost-effective scheme is a promising candidate for future high-order QAM millimeter-wave downlink transmission.

Preliminary study on direct measurements and diagnostics for chemical reaction dynamics of NOx by using laser wavelength modulation spectroscopy

Abstract: Studies on the kinetics of gas-phase chemical reactions currently rely on calculations or simulations and lack simple, fast, and accurate direct measurement methods. We developed a tunable laser molecular absorption spectroscopy measurement system to achieve direct measurements of such reactions by using wavelength modulated spectroscopy and performed online measurements and diagnostics of molecular concentration, reaction temperature, and pressure change during the redox reaction of ozone with nitrogen oxides (NO x ) with 0.1 s temporal resolution. This study provides a promising diagnostic tool for studying gas-phase chemical reaction kinetics.

Improve the throughput of M-to-1 free-space optical systems by employing uniquely decodable codes

Abstract: This paper utilizes uniquely decodable codes (UDCs) in an M-to-1 free-space optical (FSO) system. Benefiting from UDCs’ nonorthogonal nature, the sum throughput is improved. We first prove that the uniquely decodable property still holds, even in optical fading channels. It is further discovered that the receiver can extract each source’s data from superimposed symbols with only one processing unit. According to theoretical analysis and simulation results, the throughput gain is up to the normalized UDC’s sum rate in high signal-to-noise ratio cases. An equivalent desktop experiment is also implemented to show the feasibility of the UDC-FSO structure.

Characteristic extraction of soliton dynamics based on convolutional autoencoder neural network

Abstract: In this article, we use a convolutional autoencoder neural network to reduce data dimensioning and rebuild soliton dynamics in a passively mode-locked fiber laser. Based on the particle characteristic in double solitons and triple solitons interactions, we found that there is a strict correspondence between the number of minimum compression parameters and the number of independent parameters of soliton interaction. This shows that our network effectively coarsens the high-dimensional data in nonlinear systems. Our work not only introduces new prospects for the laser self-optimization algorithm, but also brings new insights into the modeling of nonlinear systems and description of soliton interactions.

Atomic structures and carrier dynamics of defects in a ZnGeP2 crystal

Abstract: ZnGeP2 (ZGP) crystals have attracted tremendous attention for their applications as frequency conversion devices. Nevertheless, the existence of native point defects, including at the surface and in the bulk, lowers their laser-induced damage threshold by increasing their absorption and forming starting points of the damage, limiting their applications. Here, native point defects in a ZGP crystal are fully studied by the combination of high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and optical measurements. The atomic structures of the native point defects of the Zn vacancy, P vacancy, and Ge-Zn antisite were directly obtained through an HAADF-STEM, and proved by photoluminescence (PL) spectra at 77 K. The carrier dynamics of these defects are further studied by ultrafast pump-probe spectroscopy, and the decay lifetimes of 180.49, 346.73, and 322.82 ps are attributed to the donor Vp+ → valence band maximum (VBM) recombination, donor GeZn+ → VBM recombination, and donor–acceptor pair recombination of Vp+ → VZn-, respectively, which further confirms the assignment of the electron transitions. The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results. Our work is helpful for understanding the interaction mechanism between a ZGP crystal and ultrafast laser, doing good to the ZGP crystal growth and device fabrication.

Photo-reconfigurable and electrically switchable spatial terahertz wave modulator [Invited]

Abstract: Spatial terahertz wave modulators that can arbitrarily tailor the electromagnetic wavefront are in high demand in nondestructive inspections and high-capacity wireless communications. Here, we propose a liquid crystal integrated metadevice. It modulates the terahertz wave based on the adjustable electromagnetically induced transparency analog when spatially changing the environmental refractive index. The functions of the device can be arbitrarily programmed via photo-reorienting the directors of liquid crystals with a digital micromirror device-based exposing system. The thin liquid crystal layer can be further driven by an electric field, and thus the function can be rapidly switched. Amplitude modulation and the lens effect are demonstrated with modulation depths over 50% at 0.94 THz.

Q-switched Er3+/Dy3+ codoped ZrF4 fiber laser: continuously tunable pulse generation from 3.06 to 3.62 µm

Abstract: In this Letter, we report on widely tunable pulse generation from a red-diode-clad-pumped mid-infrared (mid-IR) Er3+/Dy3+ codoped ZrF4 fiber laser, for the first time, to the best of our knowledge. Using a Fe2+:ZnSe crystal, continuously tunable Q-switched pulses across the range of 3.06–3.62 µm have been attained, which not only represents the widest range (in wavelength domain) from a pulsed rare-earth-doped fiber laser at any wavelength, but also almost entirely covers the strong absorption band of C-H bonds in the mid-IR, providing a potential way for gas detection and polymer processing. In addition, the commercial InAs quantum-well-based saturable absorbers (SAs) have been employed instead, and the obtained longest Q-switching wavelength of 3.39 µm is slightly shorter than 3.444 µm determined by its nominal direct bandgap of 0.36 eV.

Experimental damage thresholds of a laser suppression imaging system using a cubic phase plate

Abstract: The development of laser systems leads to an increasing threat to photoelectric imaging sensors. A cubic phase plate wavefront coding imaging system is proposed to reduce the risk of damage owing to intense laser radiation. Based on the wavefront coding imaging model, the diffracted spot profile and the light intensity distribution on the observation plane are simulated. An experimental device is set up to measure the laser-induced damage thresholds and investigate the morphology of laser-induced damage patterns of the conventional and the wavefront encoding imaging system. Simulations and experimental results manifest the superior laser suppression performance of the proposed method, which can help diminish the undesirable effects of laser irradiation on an imaging sensor.

Real-time suppression of random phase drift for laser ranging with high-frequency intermode beats

Abstract: Laser ranging with frequency comb intermode beats (IMBs) has been suffering from random phase drifts (RPDs) for two decades. In this study, we reveal the influence of signal transmission path on the RPDs and propose a real-time suppression method using two IMBs of similar frequencies from different combs. As the two IMBs obtain similar RPDs during their transmission through same signal paths, the RPD of the original probing signal IMB is suppressed by deducting the RPD of the newly added local IMB in real time. In our experiments, a real-time suppression of RPDs is achieved using IMBs of 1001 and 1000 MHz. For the sampling time of 100 s, the effect of 19-fold suppression has been achieved. The proposed method provides a new solution for the long-standing phase drift problem in laser ranging with comb IMBs.