#### Results in Journal IEEE Photonics Technology Letters: 18,775

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, Marco Malinverni, Marco Rossetti, Marcus Duelk, Christian Velez
Published: 24 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3115603

Abstract:
We report on AlInGaN-based blue and green edge-emitting lasers designed for low-current operation and low power consumption. By properly optimizing cavity length and mirror coatings, threshold currents below 5 mA are achieved on single-mode blue laser diodes (LDs) emitting at 460 nm. For output power levels below 10 mW, the LDs with an optimized chip design exhibit a decreased power consumption and an increased power stability against temperature variations compared to LDs with a standard chip design. Extended to green emitters, the new design results in single-mode LDs emitting at 515 nm with threshold currents close to 15 mA.
, F. Boitier, E. Awwad, P. Ramantanis, M. Lonardi, P. Ciblat
Published: 24 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3115627

Abstract:
We propose a method to estimate the amplitude of an unexpected power loss which, leveraging on a calibration, enables the real-time monitoring of a network link. It is based on an existing fiber-longitudinal power profile evaluation technique. The reliability of the method is assessed experimentally. When the anomaly is located at 0 km from the beginning of the span, the estimation bias is smaller than 0.2 dB for losses up to 10 dB. When the anomaly is located at 25 km from the beginning of the span, the same estimation bias is observed but for losses up to 5 dB. In both cases, the standard deviation of the estimation is smaller than 0.2 dB.
Zengting Ge, Ye Xiao, Tengfei Hao, Wei Li, Ming Li
Published: 24 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3113775

Abstract:
We experimentally demonstrate Tb/s fast random bit generation from random signals generated in a broadband random optoelectronic oscillator. The broadband random optoelectronic oscillator is used as photoelectron entropy source to generate wideband random signal. We sample the random waveform with a resolution of 10 bits and a sampling rate of 128 GS/s. All 10 bits can be preserved and a generation rate of 2.5 Tb/s (128 GS/s × 10 bits × 2 data) is achieved by using time-shift bit-order-reverse bitwise exclusive-or operation as a more complicated post-processing method. The randomness of random bit sequences is verified by using NIST Special Publication 800-22 statistical tests.
, Rong Zhang, Hong Gao, Yinggang Liu, Xueguang Qiao, Yinglong Lin
Published: 23 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3115249

Abstract:
An in-fiber Michelson interferometer (MI) formed by seven-core fiber (SCF) for low-temperature sensing is proposed and experimentally demonstrated. The sensor head is constructed by a section of seven-core fiber splicing a single-mode fiber (SMF) with a taper. When the external temperature changes, the interference spectrum of the MI will shift due to thermal expansion and thermo-optic effects. By measuring the wavelength variations of the interference dip, the external temperature can be determined. Experimental results proves that the sensor exhibits a maximum linear temperature sensitivity of -0.0775 nm/°C in the temperature range of -60 °C to 15 °C, and the standard deviation is 5% in stability test. The sensor possesses high sensitivity and easy fabrication, which makes it could be a good candidate in low-temperature sensing application.
Kangmei Li, Xin Chen, Jaekwon Ko, Jason E. Hurley, Jeffery S. Stone, Kyung Jun Park, Byoung Yoon Kim, Ming-Jun Li
Published: 22 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3114397

Abstract:
Mode division multiplexing (MDM) has attracted great attention in recent years to increase the network capacity. To apply MDM in data center applications, a low-cost and low power consumption system is desired. In this paper, we utilize 850-nm single-mode VCSEL as the laser source and report mode division multiplexed transmission over a standard single-mode fiber, which is two-mode at 850 nm. Due to the small spot size and low numerical aperture, single-mode VCSEL can be coupled into single-mode fibers with low loss. The mode multiplexer and de-multiplexer devices are designed and fabricated based on fiber mode selective couplers using Hi780 fiber as the 850-nm single-mode launch and receiving fiber and standard single-mode fiber as the few-mode transmission fiber. The MDM system supports two channels, corresponding to the LP01 and LP11 mode in the few-mode transmission fiber. Detailed characterizations of the multiplexer and de-multiplexer are conducted including the insertion loss and crosstalk measurements. System transmission at 25 Gb/s shows error free performance over 1-km standard single-mode fiber for both channels. The utilization of VCSEL and standard single-mode fiber can potentially provide a low-cost solution for MDM systems in data center applications.
, Keiji Shimada, Ryosuke Matsumoto, Takashi Inoue, Shu Namiki, Masahiko Jinno
Published: 22 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3114063

Abstract:
In the next-generation flexible point-to-multipoint optical networks such as optical access systems, it is desirable for one transceiver to handle various Internet-of-Things traffic streams. A symbol rate-adaptive multiband-coherent-transceiver, that can transmit and receive wavelength division multiplexing (WDM) signals arranged at the Nyquist spacing after Nyquist spectrum shaping, achieves flexibility by changing the number of subbands (SB) and the symbol rate per SB. Band-asymmetric coherent access systems with wide bandwidth optical transceivers on the optical-line-terminal side and narrowband optical transceivers on the optical-networking-unit side have been reported for uplink and downlink transmissions. We also examined the multiband-coherent-transceiver for ultra-dense WDM signals with a narrow analog-device bandwidth and narrow SB spacing to improve cost and spectral efficiency. To the best knowledge of the authors, there is no other research on a Nyquist spectrally shaped WDM-passive optical network system that applies these two functions and flexibly changes the symbol rate per SB with a SB spacing less than the Nyquist spacing.
Yunpeng Ge,
Published: 22 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3114336

Abstract:
An optical fiber Mach-Zehnder interferometer based on an inner air-cavity with an open micro-channel structure is presented. The device is fabricated by using femtosecond laser to inscribe a cavity structure in the optical fiber core, followed by discharging the cavity area with a fusion splicer to form an inner air-cavity in the fiber core, and finally create an open micro-channel with the help of hydrofluoric acid corrosion. The fiber in-line interferometric device is miniature, robust, and stable in operation and exhibits a high pressure sensitivity of ~3351 pm/MPa, with an extremely low temperature cross-sensitivity of 2.58 kPa/°C.
, Hong-Lin Lin, Jun Da Ng, Aaron J. Danner
Published: 22 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3114993

Abstract:
We propose and demonstrate a Mach-Zehnder modulator in Z-cut lithium niobate thin film (LNTF) with a vertical electric field structure. By placing the metal electrodes on top and bottom of the waveguide rather than the usual lateral configuration, the electric field is fully overlapping the optical field. Such a configuration reduces the critical requirement of electrode alignment as needed in X/Y-cut LN based devices. Both the simulation and process details to realize the proposed device are demonstrated. The measured static performance accords well with the simulations. Additionally, we developed a new method to accurately characterize the dynamic performance of the Z-cut LNTF modulator, and the measured tuning efficiency is around 8.84 pm/V. Our proposed device validates the feasibility of integrated Z-cut LNTF based modulators, and will likely extend the research area of integrated lithium niobate photonics.
Published: 20 September 2021
IEEE Photonics Technology Letters, Volume 33, pp 1113-1114; https://doi.org/10.1109/lpt.2021.3103351

, W. Zhang, W. Cao, D. J. Thomson, C. G. Littlejohns, X. Yan, D. T. Tran, M. Banakar, G. T. Reed
Published: 20 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3114384

Abstract:
In this paper, we present an experimental demonstration of a Si optical modulator based on Fano-like resonance, which is the first of its kind in the literature. The Fano-like resonance is obtained through weak coupling between two optical resonators in the prototype device and a PN junction is designed for carrier-depletion type electro-refractive modulation. 20 Gb/s data rate on-off keying (OOK) is obtained experimentally on the prototype device. Compared with a ring resonator modulator with a similar optical resonance linewidth, a Fano-like resonance based modulator can potentially have a higher extinction ratio (ER) because the Fano-like resonance has an asymmetric spectrum line shape and the line shape on one side can be sharper than the Lorentz resonance line shape of a ring resonator.
, Soha Namnabat, Stephanie Arouh, Robert A. Norwood, Stanley Pau, Nasser Peyghambarian
Published: 16 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3112971

Abstract:
Despite significant advances in silicon photonics, optical packaging and interconnection of photonic integrated circuits constitutes a large portion of the associated manufacturing costs. Optical fibers are the dominant approach to optical interconnection. However, techniques that rely on fibers lack scalability, have low efficiency and show poor misalignment tolerance. Proposed alternative solutions using polymer waveguides have shown improved coupling efficiency, misalignment tolerance and scalability. Here we present the design, simulation and the experimental confirmation of an adiabatic silicon nitride taper to polymer waveguide coupler with only 0.52 dB of TE coupling loss at 1550-nm wavelength, excluding material loss. The coupler demonstrates ±4 μm of lateral misalignment tolerance with a 1-dB penalty which, to the best of our knowledge, is the highest reported tolerance for silicon nitride to polymer waveguide coupling.
Anshika Srivastava, ,
Published: 16 September 2021
IEEE Photonics Technology Letters, Volume 33, pp 1193-1196; https://doi.org/10.1109/lpt.2021.3113459

Abstract:
This letter reports a Pentacene and Er-doped ZnO (EZO) nanocomposite (NC) based wideband photodetector working in ultraviolet (UV), visible and a part of the near-infrared (NIR) regions. The proposed device consists of Al/Pentacene:EZO/PEDOT:PSS/Indium Tin Oxide (ITO) structure using a low-cost solution method. The Pentacene:EZO NC shows a wide absorption spectrum covering UV-visible-NIR regions. At a reverse bias of –1 V, the measured responsivity values (A/W) of the proposed device are 23.36, 18.41, and 206.97 at 300 nm (UV), 660 nm (visible) and 980nm (NIR), respectively. The reproducibility of the proposed structure is also analyzed in the paper.
Kai Tu, Zhanwu Xie, Wenlong Zeng, , Haitao Yan
Published: 15 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3112569

Abstract:
A high temperature real-time accurate monitoring system based on photoelectric oscillation had been proposed and demonstrated. We use the narrow bandwidth characteristics of phase-shifting fiber Bragg grating (PS-FBG) to generate stable photoelectric oscillation signals. After the PS-FBG is ceramic packaged to adapt to and apply to 500 °C high temperature, the wavelength shift caused by temperature change is precisely responded by narrow linewidth PS-FBG, so the change of photoelectric oscillation signal can be monitored in real time with high precision. The experimental results show that the sensitivity is -1.24 GHz/°C and the linearity is -0.99953. The system has a resolution of 0.0083 °C. The test system has excellent potential application value in precise control of high temperature environment such as metal smelting, crystal growth, chemistry and pharmaceutical engineering.
, Pengcheng Guo, Hai Lu, Dong Zhou, Jin Wu, Weifeng Sun
Published: 15 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3113181

Abstract:
Published: 14 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3112685

Abstract:
This letter reports the fabrication and characterization of a ZnO nanorods / p-type poly[[4,8-bis [(2 ethylhexyl) oxy] benzo [1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl] thieno [3,4-b] thiophenediyl]](PTB7) polymer hybrid heterojunction based high performance ultraviolet-visible (UV-Vis) wideband photodetector. Under a low bias voltage of -1 V, the device shows the responsivities, detectivities and external quantum efficiencies (EQEs) measured at 380 nm (UV) and 640 nm (visible) wavelengths are ~ 307.18 A/W and ~ 33.64 A/W; ~1.56x1013 Jones and ~1.7x1012 Jones; and ~100.23x103 % and ~6.51x103 % respectively.
Published: 14 September 2021
IEEE Photonics Technology Letters, Volume 33, pp 1189-1192; https://doi.org/10.1109/lpt.2021.3112485

Abstract:
Optical sinc-shaped Nyquist pulses are promising in photonic signal processing and high-speed optical data transmission due to their orthogonality and inter-symbol interference (ISI) free characteristics. Nonetheless, the presence of non-idealities results in a deteriorated system performance, especially for the metrological assessment of sampling precision. In this work, we explore the impact of the optical filter roll-off factor on Nyquist pulses for a three-line comb with a frequency spacing of 8.4 GHz using a wideband on-chip silicon Mach-Zehnder modulator. Furthermore, we characterize the integrated modulator and compare the sinc-shaped Nyquist pulse quality with the recently reported works.
, Yuangang Lu, Zelin Zhang, Zhengnan Wu, Yuyang Zhang
Published: 14 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3112761

Abstract:
A novel fast measurement method based on Brillouin gain spectrum (BGS) and Brillouin beat spectrum (BBS) in a complex index profile fiber has been proposed to realize simultaneous measurement of distributed temperature and strain. An interesting phenomenon that the temperature/strain sensitivities of the power on the BGS slope and the power on the BBS peak are very different has been discovered and utilized for temperature and strain sensing. By means of only measuring the power trace of the BGS slope and the power trace of one BBS peak along the sensing fiber, the distributed temperature and strain can be measured by using the Brillouin optical time-domain reflectometry (BOTDR) without scanning the Brillouin spectrum. The temperature and strain measurement uncertainties of the proposed method are one order of magnitude lower than that of the BBS-based measurement method. In a proof-of- concept experiment, the temperature and strain of the 2 km large effective area fiber (LEAF) are measured at a spatial resolution of 3 m within 18 s.
Lizhuo Zheng, Zhiyi Zhang, Mable P. Fok, Zhiyang Liu,
Published: 14 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3112703

Abstract:
, Qiang Shu, Ru-Mao Tao, Qiu-Hui Chu, Yun Luo, Dong-Lin Yan, Xi Feng, Yu Liu, Wen-Jie Wu, Hao-Yu Zhang, et al.
Published: 13 September 2021
IEEE Photonics Technology Letters, Volume 33, pp 1181-1184; https://doi.org/10.1109/lpt.2021.3112270

Abstract:
A high-power monolithic continuous wave (CW) Yb doped fiber amplifier at 1064 nm has been demonstrated based on traditional large mode area step-index fiber, which generated 5.07 kW narrow line-width laser with near diffraction-limited beam quality. At the maximal output power, >35dB OSNR has been achieved with line-width being 370pm, and the measured beam quality is M2x=1.252, M2y=1.322. The influence of cooling temperature on SRS has been investigated in high power fiber lasers, which shows that the SRS at 4kW has been suppressed 24 dB by lowering the temperature from 20°C to 8°C. To the best of our knowledge, this is the highest narrow line-width laser power generated from the traditional large mode area step-index monolithic fiber amplifier.
, J. Mu, I. Roumpos, K. Fotiadis, A. Manolis, C. Vagionas, M. Dijkstra, S.M. Garcia-Blanco, T. Alexoudi, K. Vyrsokinos
Published: 10 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3111894

Abstract:
On chip waveguide optical amplifiers have been extensively studied over the last years, with a wide variety of materials tested and proposed for different applications. Among the most prominent solutions for on-chip amplification, erbium doped waveguide amplifiers (EDWAs) are able to offer attractive performance metrics that can exceed SOA-based amplification solutions in traditional single and multi-channel systems. In this paper, we experimentally demonstrate a record high 8×40 Gbps non return to zero (NRZ) wavelength division multiplexing (WDM) data amplification through a 5.9 cm long on-chip amplifier consisting of an erbium-doped aluminum oxide spiral waveguide monolithically integrated on the Si3N4 platform. Experimental results show more than 12.7 dB amplification per channel for low saturation total input power of -2.75 dBm, and clear eye diagrams and bit-error rate values below the KR4-FEC limit of 2×10-5 for all eight channels without any digital signal processing (DSP) applied to the signal to the receiver or transmitter side. The high losses from the fiber to chip interfaces, however, prevented achieving device net gain.
Seyed Amin Khatami, Pejman Rezaei, Mohammad Danaie, Amir Habibi Daroonkola
Published: 10 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3109633

Abstract:
In this paper, a 180° ring-shaped hybrid based on the photonic crystal structure (PCS) is presented. The ring hybrid is designed for the operation wavelength of 1550 nm. Initially, a 180° hybrid is a design based on conventional microwave procedures. Different wave behavior inside the structure in THz and microwave frequency band has caused the wave propagation in this structure to change especially in terms of phase. By analyzing the field distribution within a structure, the rod arrangement is modified. In proposed structure, the phase difference between the output ports is greatly improved (about 90°). Also, by reducing the leakage and improving the matching, the output power of the ports tends to half power. Moreover, 32 nm wavelength shift is improved. The proposed structure’s reflectance value is less than 20 dB at the desired wavelength. The power splitting ratio between the two output ports is improved up to -3.7 dB and the power of the isolated port is less than -20 dB. This work has been analyzed using the FDTD method.
Zhipeng Dong, Honggang Sun, Yimin Zhang, Jinhai Zou, Lixin Xu,
Published: 9 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3111191

Abstract:
In this paper, we report the first demonstration of a visible-wavelength-tunable, vortex-beam fiber laser based on a Sagnac-loop fiber filter and efficient long-period fiber grating. A 30 cm Pr3+/Yb3+-codoped ZBLAN fiber pumped by a 445 nm laser diode provides ~ 635 nm strong optical gain. A homemade dielectric mirror on the fiber end-facet and a Sagnac-loop fiber filter are used to build a wavelength-tunable fiber laser with a wavelength range of 633.89 to 636.02 nm. A self- fabricated long-period fiber grating acts as a mode converter in the cavity, and one-order vortex-beam including cylindrical vector beams (CVBs) and orbital angular momentum beams (OAMs) with a high mode purity are successfully obtained. Furthermore, we also measured the speckle contrast of the visible-wavelength fiber laser, and the speckle contrast is reduced by 17.13% through CVBs. This work could offer an effective approach to develop compact, wavelength-tunable vortex fiber lasers in the visible spectrum for applications to high-resolution imaging, laser display, visible light communications, and scientific research.
Published: 3 September 2021
IEEE Photonics Technology Letters, Volume 33, pp 1059-1060; https://doi.org/10.1109/lpt.2021.3103398

Abstract:
, Toshimasa Umezawa, Atsushi Kanno, Naokatsu Yamamoto, Tetsuya Kawanishi
Published: 3 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3110258

Abstract:
Lunbin Zhou, Teng Zhang, Bin Xu, Xiaodong Xu, Andrey Lyapin, Zhenfang Yu, Jun Xu
Published: 3 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3109882

Abstract:
All solid-state microchip lasers having the advantage of short cavity lengths give them tremendous potential as pulsed devices for short pulse laser generation. In this work, by constructing a compact microchip-like laser resonator, we have demonstrated continuous-wave (CW) Pr,Mg:SrAl12O19 (SRA) lasers at 622 nm and 643 nm, respectively with maximum output powers of 0.69Wand 1.18Wwith excellent beam qualities, which is believed to be the highest powers that have achieved in Pr3+ microchip lasers. Based on this CW opeation, we, by using a Co:LMA saturabel absorber, have further demonstrated passively Q-switched operation of the two lasers. The shortest pulse widths reached about 28.5 ns (for the 622 nm laser) and 31.9 ns (for the 643 nm laser), which are relatively short pulse time durations for passively Q-switched Pr3+ lasers thanks to the compact laser configuration. This work has proposed a simple route for achieving compact high-power CW and short-pulse Q-switched visible lasers.
, Vladimir Shulyak, Benjamin White, Chee Hing Tan, Jo Shien Ng
Published: 3 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3110123

Abstract:
Avalanche photodiodes (APDs) are used in optical receivers of high-speed optical communication systems to improve signal-to-noise ratio over conventional photodiodes. Low excess noise characteristics are crucial for APDs to preserve the benefits associated with high internal gains. In this work, we presented room temperature data of avalanche gain and excess noise factors of Al0.85Ga0.15As0.56Sb0.44 APDs using pure and mixed carrier injection profiles. Using pure electron injection, the best possible excess noise performance for a given avalanche width was measured with an excess noise factor < 2 for gains up to 25. This is the lowest excess noise reported for this material system at high gain. Two other injection profiles with increased portion of injected holes worsened the excess noise performance, confirming the need to use pure electron injection for the best possible APD noise performance. The data reported in this article is available from the ORDA digital repository (https://doi.org/10.15131/shef.data.15082455).
Jianxing Li, , Junwei Shi, Ying-Jiang Guo, Anxue Zhang
Published: 1 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3109612

Abstract:
A coplanar waveguide (CPW) based spoof surface plasmon polariton (SSPP) waveguide with strong field confinement is proposed, which has a low-pass filtering response. Then, two half-wavelength slot resonators are loaded on both sides of the SSPP waveguide to introduce a notch in the passband. Meanwhile, the SSPP waveguide with two ring slot resonators is simulated, whose results are consistent with the SSPP low-pass filter. Finally, two PIN diodes are bridged across the slits of the two resonators to realize the dynamic switchability of the notch. The waveguide functions as a notch filter when the diodes are in on-state, and a low-pass filter when the diodes are in off-state. To verify the feasibility of the design, a SSPP waveguide example is fabricated and measured, where the simulated and measured results are in reasonably good agreement.
, Chun-Nien Liu, Wei-Chih Cheng, Wood-Hi Cheng
Published: 1 September 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3109620

Abstract:
We demonstrate co-doped phosphor-in-glass (PiG) for high performance white light-emitting diodes (WLEDs) employing reduction of red/green phosphors interaction and antireflection coating (ARC). In this study, the novelties are a suitable selection of the 650-nm red and 540-nm green phosphor materials to reduce the mutual interaction between the emission of green phosphor and the absorption of red phosphor, evidenced by photoluminescence spectroscopy, and then an ARC on both sides of the PiG to reduce Fresnel reflection loss. Therefore, the PiG with high luminous efficiency of 63 lm/W, high color rendering index (CRI) of 96, and less amount of phosphor material are obtained. The overall luminous efficiency of the PiG-based WLED is improved by about 15% and the total amount of phosphors is reduced by about 58%. This proposed of co-doped PiG with excellent performance and less amount of phosphor material usage is essential to meet the economic requirement for high-quality WLEDs indoor lighting applications.
, Peiji Song, Chun-Kit Chan
Published: 30 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1127-1130; https://doi.org/10.1109/lpt.2021.3109029

Abstract:
We propose and experimentally demonstrate a novel encryption scheme to enhance the physical layer security in broad-cast passive optical networks (PONs). We exploit, for the first time, a new dimension using non-orthogonality for encryption. Different from the conventional masking of data, the proposed chaotic non-orthogonal matrix (CNOM) can also dynamically scramble the number of subcarriers in an orthogonal frequency division multi-plexing (OFDM) signal while maintaining the same bandwidth to further deceive potential eavesdroppers. By simultaneously using faster-than-Nyquist signaling and redundant precoding, the proposed scheme can achieve the compromise between spectral efficiency (SE) and resilience against system impairments, while increasing the overall key space. We then carry out a proof-of-concept experiment to verify the feasibility of the proposed CNOM-based encryption. Results showed that the transmission performance could be improved in addition to the security enhancement.
Xu Yang, , Feng Li, Xiaoyang Li, Bincheng Li, Chao Geng, Xinyang Li
Published: 30 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1-1; https://doi.org/10.1109/lpt.2021.3108419

Abstract:
We demonstrate a technique to realize fine target tracking and pointing using target-in-the-loop (TIL) concept via a coherent beam combining (CBC) system. A tracking and pointing system is composed of a densely packed array of 7 phase-control elements with simultaneous piston and tilt controls of the outgoing beams wavefront phases. A photo-detector (PD) with a pinhole is used as target detection and installed on a translation stage. Energy detected by the mobile PD is used for evaluating the beam combining and scanning performance, and to provide the feedback signal for controls of piston and tilt phases of the transmitted beams by using the stochastic parallel gradient descent (SPGD) algorithm for merit function maximization.
Xinran Huang, Liuming Zhang, Weisheng Hu, J. P. Turkiewicz, Erich Leitgeb,
Published: 30 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1139-1142; https://doi.org/10.1109/lpt.2021.3108832

Abstract:
A physical-layer secure data encryption algorithm is proposed and demonstrated using chaotic constellation mapping associated with probabilistic shaping (PS) for orthogonal frequency division multiplexing passive optical networks (OFDM-PON). The constellation of the encrypted data is uniformly distributed after chaotic diffusion in quadratic-amplitude modulation (QAM) mapping, to eliminate the statistical characteristics, where a key space of ~10231 is created by a 4-dimensional hyper digital chaos. Moreover, PS is introduced to improve the OFDM signal transmission. A 9.4 Gb/s encrypted PS-16-QAM OFDM signal transmission is successfully demonstrated over 20 km standard single-mode fiber (SSMF), where the gain of receiver sensitivity is ~2 dB (bit-error rate @ 10-3) using PS.
, Zhangqi Dang, Zexu Liu, Zhenming Ding, Zhifang Yang, Xiaodong Zhang, Xinhong Jiang, Ziyang Zhang
Published: 26 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1135-1138; https://doi.org/10.1109/lpt.2021.3108240

Abstract:
A multiport thermo-optic switch based on parallel polymer waveguides and microheater array is proposed. The basic unit is a 1 × 3 switch for Up Transfer, Isolation, and Down Transfer operations. By introducing refractive index perturbation along the selected waveguides through microheaters, the Transfer /Isolation condition can be switched locally, and light is then steered into different paths. A 1 × 5 switch is demonstrated experimentally as an example. Colorless (1.50 μm to 1.62 μm) and polarization independent operation is achieved with crosstalk suppression ratio larger than 10 dB, which can be further optimized. The design requires less waveguides and tuning sections than conventional Mach-Zehnder interferometer-based switches. 6 microheaters are placed but only 3 need to be powered on simultaneously to reach any of the 5 output ports. The maximal heater power is 33.8 mW over a coupling length of 1331.6 μm. This design can easily scale up to form an arbitrary cascaded M × 1 and 1 × N switch network for high-density photonic integration.
, Jiexuan Gu, Guiyao Zhou, Jinghua Sun
Published: 26 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1115-1118; https://doi.org/10.1109/lpt.2021.3108153

Abstract:
Recent progress in designing optimized microstructured optical fiber spreads an application scenario of optical fiber sensing. In this paper, we demonstrate a novel three-core microstructured optical fiber (TMOF) spliced with single mode fibers (SMFs). Due to low crosstalk and different effective refractive index between three cores in TMOF, multi-beam interference is achieved and verified based on both of experiments and theoretical analysis. In addition, the bending response could also be recorded by the transmission spectrum of TMOF because of the asymmetry of three core effective refractive index. Owing to the consistency of cores responding to temperature changes, the TMOF can effectively suppress the influence of temperature. Theses excellent sensing characteristics indicate the TMOF splicing with SMFs is promising in the field of bending, integrated Mach Zehnder devices and multi-parameter sensing measurement under different temperature.
, W. Xie, B. Li, X. Bu, A. Song, X. Luo, G.-Q. Lo
Published: 25 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1131-1134; https://doi.org/10.1109/lpt.2021.3107865

Abstract:
Low-loss, compact silicon nitride and silicon waveguide bends are demonstrated using COMS-compatible multi-layer PECVD SiN-on-SOI integration platform over the whole C band. The curvature of advanced waveguide bend is designed with linearly changing to reduce bending loss. The bending loss for PECVD SiN reduces from 0.087 dB/90° for normal bend to 0.037 dB/90° for advanced bend with the radius of 30 μm. It indicates a 60% footprint reduction with same loss level compared with the normal bend with 50-μm radius. Wafer-level distribution and uniformity of the bending loss are characterized, showing the stable performance. Similarly, we demonstrate a 3 μm-radius silicon waveguide bend in the same platform, showing the bending loss of only 0.0065 dB/90° bending loss, which is even smaller comparing to 0.012 dB/90° bend loss of a 5 μm-radius normal bend. Such advanced bend design with significantly reduced loss and footprint paves a way for high-dense large-scale photonic integrated circuits.
, Yusuke Kohno, Yoshiaki Nakano, Takuo Tanemura
Published: 24 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1123-1126; https://doi.org/10.1109/lpt.2021.3107277

Abstract:
High-speed and robust optical beam-steering device will be the key component for various applications, such as LiDAR (light detection and ranging) and free-space optical communication. Optical phased arrays (OPAs) integrated on semiconductor chips have recently received increasing attention due to the high-speed operation, compactness, and low cost. In particular, indium phosphide (InP)-based OPAs are advantageous for high-output-power applications at 1.55-μm eye-safe wavelength, owing to their capability of monolithically integrating active components, such as high-power lasers and optical amplifiers. In this letter, we design and fabricate, to the best of our knowledge, the largest-scale InP-based OPA and experimentally demonstrate its beam-steering operations. The fabricated OPA consists of 100 waveguides with carrier-injection-based phase shifters, densely integrated on 7 mm × 5 mm footprint. A focused beam width of 0.11° is steered across the free spectral range of 8.88°, corresponding to more than 80 resolvable points. This is the largest number of resolvable points obtained by a monolithic InP OPA. The response time of the OPA is confirmed to be less than 16 ns, which is limited by the driver circuit. This work paves the way for realizing compact beam-steering modules for high-speed and high-output-power imaging applications.
Yang-Jeng Chen, Yi-Hsin Fang, Rih-You Chen, Cong-Long Chen, Bo-Hong Chen,
Published: 24 August 2021
IEEE Photonics Technology Letters, pp 1-1; https://doi.org/10.1109/lpt.2021.3107356

Abstract:
3D hybrid integration based on III-V/Si waveguides by selective undercut wet etching (SUWE) of the lower Si layer has been demonstrated on a silicon-on-insulator (SOI) template. The process was performed on an adhesive-bonding material structure, defined by the upper InP/InAlGaAs (InAlGaAs quantum well) p-i-n heterostructure and the lower 220 nm SOI. After the top p-i-n III-V tapered waveguide was first fabricated and covered by Si3N4, KOH was then used for SUWE of Si layer from SiO2 and Si3N4, forming the bottom SOI single-mode waveguide. In comparison to monolithic III-V waveguide structure (InP based material), the simulated confinement factor of active shows an enhancement from 10.3% to 16.7%. SOA/EAM devices were also processed and integrated in III-V layer, showing 7.2 dB optical gain at 70 mA injection current in SOA and 21.7 dB extinction ratio within 0 -2 V swing voltage in EAM. The strong quantum confined Stark effect (QCSE) in MQW confirms the improved confinement factor. It suggests a vertical self-alignment scheme could be used for realizing compact and submicron scale heterogeneous integration in a Si photonics template.
Yuan Wang, Yaotian Zhao, Jinlong Xiang, , Jianji Dong, Xinliang Zhang, Yikai Su
Published: 24 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1109-1112; https://doi.org/10.1109/lpt.2021.3107307

Abstract:
We propose an ultra-compact band-pass and bandstop filter with bandwidth tunability based on the loop-cascaded nanobeam structure in simulation, with a small coupling region of 41 × 1 μm2. It features low insertion loss, large bandwidth tunable range and high sidelobe suppression ratio. By two-stage cascading, the bandwidth of the band-pass filter can be adjusted in the range of 6 nm-24 nm. We also experimentally demonstrate the loop-cascaded nanobeam band-pass and band-stop filter with a coupling region of only 17.5 × 1 μm2.
, , , Zhiguo Yu, Manqing Tan, Zhiyong Li
Published: 23 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1093-1096; https://doi.org/10.1109/lpt.2021.3106642

Abstract:
In this letter, we experimentally demonstrate the first slow-light Mach-Zehnder modulator (MZM) on a hybrid ithin-film Lithium Niobate platform. The Bragg grating waveguides are served as phase shifters, which enhance the modulation efficiency with the slow-light effect. Compared with large-signal modulation efficiency ( $\text{V}_\pi \cdot \text {L} =0.67\,\,\text{V}\cdot$ cm) and an ultra-compact footprint (0.3 mm $\times1.2$ mm). Data transmission up to 60 Gbps and an on-chip insertion loss of 1.9 dB are also achieved. This modulator provides a potential solution for high-intensity integration, low-power transmission, and optical biosensing.
Published: 23 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1101-1104; https://doi.org/10.1109/lpt.2021.3106670

Abstract:
In this letter, a photonic technique with wavelength tunable multiplication factor is proposed to generate odd multiple of modulating frequency. It is based on the cascading of two Mach-Zehnder modulators, super structured fiber Bragg grating, and a power combiner involving coherent heterodyne technique. Frequency tripling and quintupling of 10 GHz modulating signal have been demonstrated by tuning the wavelength of laser source and RF power control.The proposed scheme provides a simple method for photonic generation of widely tunable RF frequencies, and it has ability to change the multiplication factor by adjusting the wavelength of the laser source instead of tuning the passband frequency of the optical filter. The even multiplication capability of the proposed system can also be explored by changing the wavelength of the laser source.
, Xiang Li, Yue Liu, Wenhuan Chen, Huiwen Niu, Qi Yan, Shengjia Wang, Song Li, Tao Geng, Weimin Sun, et al.
Published: 19 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1105-1108; https://doi.org/10.1109/lpt.2021.3106146

Abstract:
An optical fiber Mach-Zehnder interferometer (MZI) coated by magnetic fluid (MF) is fabricated for magnetic field measurement. The interferometer is composed of multimode fiber-taper single mode fiber-multimode fiber (MTSM). The two MMFs play the role of a beam splitter and a coupler. The TSMF induces the generation of high-order cladding mode and enhances the recoupling ability of the interferometer. The transmission spectrum and cladding modes with different taper waist diameter are theoretically analyzed. Interference dips are sensitive to the alteration of magnetic field intensity when the taper waist diameter of interferometer is 34 μm. The highest sensitivity is up to -0.81 nm/mT at the magnetic field range of 8-17 mT.
Lei Liu, , Haojie Wei, Yu He, Song Hu
Published: 16 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1097-1100; https://doi.org/10.1109/lpt.2021.3105262

Abstract:
Structured illumination microscopy (SIM) has attracted many attentions due to high-precision, high-efficiency and strong adaptability. In most SIM, two essential parameters need to be accurately obtained to restore the 3D shape of the object which are the focus position of each pixel $\mathbf {z}_{\mathbf {max}}$ and the scan step $\boldsymbol {\Delta z}$ . For years, researchers have constantly improving the structure of SIM system to obtain more accurate $\mathbf {z}_{\mathbf {max}}$ to realize optimized measurement results. Meanwhile, the preset value of $\boldsymbol {\Delta z}$ is directly substituted into calculation. However, the actual value of $\boldsymbol {\Delta z}$ is very likely inconsistent with the preset value due to various factors in practical such as nonlinear motion of scan actuator, which will introduce great error into the measurement result. In this letter, a self-calibrated measurement method is proposed to resolve this problem. As we know, the full width at half maximum (FWHM) of the modulation curve is uniquely determined when the optical measurement system is determined. In this letter, a self-calibrated measurement method based on SIM (SC-SIM) is proposed and the determined FWHM is utilized to calculate the actual value of $\boldsymbol {\Delta z}$ to realize more accurate and reliable measurement result. Simulation and experiment are carried out to demonstrate the feasibility of the proposed method.
Published: 13 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 975-976; https://doi.org/10.1109/lpt.2021.3101387

Abstract:
, Yungui Nie, Min Liu, Yufeng Du, Ruisi Liu, , , Binbin Zhu
Published: 13 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1081-1084; https://doi.org/10.1109/lpt.2021.3104618

Abstract:
We propose and experimentally demonstrate a distributed digital pre-equalization (DPE) technique for orthogonal frequency division multiplexing (OFDM)-based bandlimited visible light communication (VLC) systems. In the VLC system applying distributed DPE, the subcarriers are divided into two bands, where the bandwidth and the power of each band can be flexibly adjusted to maximize the achievable data rate of the system. Hence, distributed DPE exhibits much higher tolerance against light-emitting diode (LED) nonlinearity than conventional centralized DPE. Experimental results verify the superiority of distributed DPE for OFDM-based bandlimited VLC systems. More specifically, a data rate of 976.6 Mbit/s is achieved by using distributed DPE, which corresponds to an achievable rate improvement of 25% in comparison to centralized DPE.
Kun Xu, Jie Zhang, Xiang Zhou, Chao Lu, Lena Wosinska, Yongli Zhao
Published: 13 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 977-977; https://doi.org/10.1109/lpt.2021.3101660

Abstract:
We are pleased to provide readers with the newest and important technologies presented in Asia Communications and Photonics Conference (ACP) and International Conference on Information Photonics and Optical Communications (IPOC) 2020 (ACP/IPOC 2020), through this Special Issue published by the IEEE Photonics Technology Letters (PTL) journal. Technologies of photonics and optical communications (POCs) have developed rapidly in the last decades and have evolved to become a key-enabling component for the applications of high-speed telecommunications, advanced microwave signal generation and processing, astronomy observations, and even for quantum information processing and computing. The field of POC continues attracting a great deal of attention and keeps progressing in several important directions. Based on the nowadays matured exploration methods, current research is mainly directed towards implementations in integrated formats, such as high-speed optical transmission, big-capacity optical switching, automated optical-network/system evolution, and so on. In addition, intensive research is being conducted toward the utilization of POC across the abovementioned practical application areas and others.
, Ruohe Yao, Kai Wang, Zijing Xie,
Published: 13 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1077-1080; https://doi.org/10.1109/lpt.2021.3104645

Abstract:
We systemically investigated the effect of chip structures on the optical characteristics of micro-size LEDs. The total internal reflection (TIR) can be improved by selecting a suitable chip structure, thereby increasing the light efficiency. The saturation light output power (LOP) of the micro-size LED using pentagon-type structure is 44.78 mW, an increase of 6.08% compared with that of micro-size LED using circular-type structure. Simulation and experimental results indicate that chip structure of pentagon-type can effectively enhance the light extraction from the top or side of the micro-size LED.
Lucas Silva Schanner, Jose Helio da Cruz Junior, ,
Published: 12 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1089-1092; https://doi.org/10.1109/lpt.2021.3104508

Abstract:
We propose a method for estimation of power profile in optical transmissions by employing machine learning optimization to a digital back-propagation model. The method allows to estimate the absolute power values along the link and it requires solely the coherently acquired data at the receiver-side. The estimated values are validated using experimental results from an unrepeatered transmission, employing remote and Raman amplification.
Yanna Ma, Xin Jing, Fuxing Gu
Published: 11 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 1085-1088; https://doi.org/10.1109/lpt.2021.3104034

Abstract:
Broadband and tunable microwave signals are widely used in modern communication systems and sensing systems. In this work, we demonstrate a stable and tunable optoelectronic oscillator (OEO) scheme based on external low-power beat note injection of the stimulated Brillouin signal and a phase modulation sideband of the original pump signal. The injection-locked system reduces the high-frequency disturbance induced by the beat note signal, and thus the power required only meets the OEO locking threshold requirement. In addition, the external stimulated Brillouin signal generator avoids the excessive length of the resonant cavity and guarantees the frequency stability, especially the mode stability in the OEO with a total path length of ~12 m. By varying the stimulated Brillouin signal frequency and selecting different pump modulation sidebands, a continuously tuned beat note is generated, and a beat note injected OEO with a maximum frequency up to 11.5 GHz is achieved, which is the intrinsic upper frequency limit of the devices used, even for an injection beat frequency power as low as −14 dBm ( $\sim 40~ {\mu } \text{W}$ ). Our work provides an important solution for the realization of widely tunable microwave systems.
Published: 11 August 2021
IEEE Photonics Technology Letters, Volume 33, pp 929-930; https://doi.org/10.1109/lpt.2021.3100215

Abstract:
Published: 11 August 2021
IEEE Photonics Technology Letters, Volume 33; https://doi.org/10.1109/lpt.2021.3085058

Abstract:
Presents a listing of the editorial board, board of governors, current staff, committee members, and/or society editors for this issue of the publication.
Published: 11 August 2021
IEEE Photonics Technology Letters, Volume 33; https://doi.org/10.1109/lpt.2021.3087983

Abstract:
Presents the front cover for this issue of the publication.
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