Showing papers in "IEEE Photonics Technology Letters in 2015"

A 71-Gb/s NRZ Modulated 850-nm VCSEL-Based Optical Link

Abstract: We report error free ( $\textrm {BER} ) operation of a directly non-return-to-zero modulated 850-nm vertical cavity surface-emitting laser (VCSEL) link operating to 71 Gb/s. This is the highest error free modulation rate for a directly modulated laser of any type. The optical link consists of a 130-nm BiCMOS driver IC with two-tap feed-forward equalization, a wide bandwidth 850-nm VCSEL, a surface illuminated GaAs PIN photodiode, and a 130-nm BiCMOS receiver IC.

Surface Plasmon Resonance Photonic Crystal Fiber Biosensor: A Practical Sensing Approach

Abstract: We propose a simple, two rings, hexagonal lattice photonic crystal fiber biosensor using surface plasmon resonance phenomenon. An active plasmonic gold layer and the analyte (sample) are placed outside the fiber structure instead of inside the air-holes, which will result in a simpler and straight forward fabrication process. The proposed sensor exhibits birefringent behavior that enhances its sensitivity. Numerical investigation of the guiding properties and sensing performance are conducted by finite element method. Using wavelength and amplitude interrogation methods, the proposed sensor could provide maximum sensitivity of 4000 nm/RIU and 320 RIU $^{-1}$ , respectively. The resolutions of the sensor are $2.5 \times 10^{-5}$ and $3.125 \times 10^{-5}$ RIU for wavelength and amplitude interrogation modes. The proposed sensor design shows promising results that could be used in biological and biochemical analytes detection.

Artificial Neural Network Nonlinear Equalizer for Coherent Optical OFDM

Abstract: We propose a novel low-complexity artificial neural network (ANN)-based nonlinear equalizer (NLE) for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) and compare it with the recent inverse Volterra-series transfer function (IVSTF)-based NLE over up to 1000 km of uncompensated links. Demonstration of ANN-NLE at 80-Gb/s CO-OFDM using 16-quadrature amplitude modulation reveals a Q-factor improvement after 1000-km transmission of 3 and 1 dB with respect to the linear equalization and IVSTF-NLE, respectively.

Femtosecond Laser Inscribed Bragg Gratings in Low Loss CYTOP Polymer Optical Fiber

Abstract: We report on the first inscription of fiber Bragg gratings (FBGs) in cyclic transparent optical polymer (CYTOP)-perfluorinated polymer optical fibers (POFs). We have used a direct write method with a femtosecond laser operating in the visible. The FBGs have a typical reflectivity of 70%, a bandwidth of 0.25 nm, a 3-mm length, and an index change of $\sim 10^{-4}$ . The FBGs operate in the $C$ -band, where CYTOP offers key advantages over polymethyl methacrylate optical fibers, displaying significantly lower optical loss in the important near-infrared (NIR) optical communications window. In addition, we note that CYTOP has a far lower affinity for water absorption and a core-mode refractive index that coincides with the aqueous index regime. These properties offer several unique opportunities for POF sensing at NIR wavelengths, such as compatibility with existing optical networks, the potential for POF sensor multiplexing and suitability for biosensing. We demonstrate compatibility with a commercial Bragg grating demodulator.

The Development of an $\Phi $ -OTDR System for Quantitative Vibration Measurement

Abstract: Phase sensitive optical time domain reflectometry is suitable for vibration detection due to its high sensitivity. However, for conventional amplitude-measuring method, the strain-amplitude relation is nonlinear and the monotonic interval of strain-amplitude transfer function is very small (corresponding to less than $2\pi $ phase shift in light duration). Thus, the vibration detection is qualitative and it is difficult to distinguish different kinds of vibrations. In this letter, a quantitative vibration measurement by analyzing the differential phase in the interrogated fiber separation is proposed. A statistics calculating method is present to suppress the uncertainty in phase-measuring. Dynamic strain of 400 $\text{n}\varepsilon $ (peak-to-peak, corresponding to phase shift of 17 rad) was reproduced with high quality by proposed method.

Topological Insulator Solution Filled in Photonic Crystal Fiber for Passive Mode-Locked Fiber Laser

Abstract: We first reported that the topological insulator (TI) nanosheets solution filled in photonic crystal fiber can operate as an effective saturable absorber (SA) with the merits of low-insertion loss ( $\sim 0.42$ dB), long interaction length (>10 cm), and high-power tolerance. This SA device exhibited a saturable intensity of 14.9 MW/ $\mathrm{cm}^{2}$ , modulation depth of 19.1%, and nonsaturable loss of 25% at 1060 nm. Upo employing, this device rendered us to establish an ytterbium-doped all-fiber laser oscillator, where stable evanescent wave mode-locking operation has been achieved. This letter provided a new way of utilizing the unique nonlinear optical property of TI.

Nonlinear Frequency Division Multiplexed Transmissions Based on NFT

Abstract: We experimentally demonstrate the successful modulation and error-free detection of three eigenvalue nonlinear frequency division multiplexed (NFDM) signals over 1800 km based on the recently developed nonlinear Fourier transform theoretical framework with digital coherent receivers The three eigenvalues are located on the upper-half complex plane and are modulated by independent ON–OFF keying signals, thus forming 3-bit NFDM symbols for nonlinear fiber transmissions

Beyond 100-Gb/s Indoor Wide Field-of-View Optical Wireless Communications

Abstract: Optical fiber communication networks can provide terabit aggregate capacities to buildings and offices within modern cities. Practical wireless systems are orders of magnitude below this capacity. In this letter, we describe an indoor optical bidirectional wireless link with an aggregate capacity over 100 Gb/s. The link operates over ~3 m range at 224 Gb/s (6 x 37.4 Gb/s) and 112 Gb/s (3 x 37.4 Gb/s) with a wide field of view (FOV) of 60° and 36°, respectively. To the best of our knowledge, this is the first demonstration of a wireless link of this type with a FOV that offers practical room-scale coverage.

Passively $Q$ -Switched Erbium-Doped Fiber Laser Based on Few-Layer MoS 2 Saturable Absorber

Abstract: We demonstrate a passively $Q$ -switched erbium-doped fiber laser (EDFL) based on few-layer MoS2 as a saturable absorber (SA). Few-layer MoS2 is prepared by the chemical vapor deposition method. The prepared MoS2 is transferred onto the end face of a fiber connector to form a fiber-compatible MoS2-based SA. The saturation intensity and modulation depth of the MoS2 SA are measured to be 0.43 MW/cm $^{\mathrm {\mathbf {2}}}$ and 33.2%, respectively. The $Q$ -switched EDFL has an all-fiber linear cavity with two fiber Bragg gratings as the end mirrors. By inserting the MoS2 SA into the laser cavity, stable $Q$ -switched operation is achieved at $1.55~\mu $ m. The laser has a pump threshold of 20.4 mW, a pulse repetition rate tunable from 10.6 to 173.1 kHz, and a minimum pulse duration of $1.66~\mu $ s. Our results show that few-layer MoS2 is a promising SA for $Q$ -switching laser operation.

Asymmetrical hybrid optical OFDM for visible light communications with dimming control

Abstract: This letter proposes an asymmetrical hybrid optical orthogonal frequency division multiplexing (AHO-OFDM) scheme for dimmable visible light communication systems. In the proposed scheme, either asymmetrically clipped optical OFDM (ACO-OFDM) or pulse-amplitude-modulated discrete multitone (PAM-DMT) signal is inverted and then both the signals are combined for transmission, where pulsewidth modulation is no longer required for dimming control. The power of ACO-OFDM and PAM-DMT signals is adjusted so that the amplitude of the combined AHO-OFDM signal is asymmetrical, which could utilize all the available subcarriers as well as the entire dynamic range of light-emitting diodes with various dimming levels. Simulation results show that the proposed scheme could achieve a wide dimming range with a small throughput fluctuation.

W-Band 8QAM Vector Signal Generation by MZM-Based Photonic Frequency Octupling

Abstract: We propose a novel scheme for photonic multiamplitude quadrature-amplitude-modulation (QAM) vector signal generation at microwave/millimeter-wave bands enabled by Mach-Zehnder modulator (MZM)-based adaptive photonic frequency multiplication. In order to attain an electrical millimeter-wave vector signal displaying multiamplitude QAM modulation, such as 8QAM, after square-law photodiode detection, the driving radio-frequency (RF) signal, carrying multiamplitude QAM transmitter data, should be both amplitude- and phase-precoded before used to drive the MZM. We experimentally demonstrate 8QAM vector signal generation at W-band adopting photonic frequency octupling (x8) enabled by our proposed scheme. The MZM is driven by a 12-GHz precoded RF signal carrying 1-GBaud 8QAM transmitter data. The generated 1-GBaud 8QAM vector signal at W-band is air transmitted over 2-m distance. To the best of our knowledge, it is the first time to realize the generation and reception of multiamplitude QAM vector signal by one external modulator at W-band.

Graphene-Based Conducting Metal Oxide Coated D-Shaped Optical Fiber SPR Sensor

Abstract: We report a graphene-based indium tin oxide coated surface plasmon resonance sensor built on a D-shaped optical fiber for near infrared. Different parameters of the proposed sensor have been optimized to obtain a maximal phase matching between the core guided mode and the plasmon mode using finite element method. Wavelength sensitivity of the proposed structure is as high as 5700 nm/RIU with a maximum resolution of $1.754 \times 10^{-5}$ RIU. The proposed sensor can also be used for bio layer thickness monitoring with a maximum resolution of 62.5 pm in wavelength interrogation.

Optical OFDM With Single-Photon Avalanche Diode

Abstract: In this letter, an optical orthogonal frequency division multiplexing (OFDM) system based on a single-photon avalanche diode (SPAD) receiver is presented. SPAD detectors do not require a transimpedance amplifier as they operate in the Geiger mode. This poses challenges on detecting continuous data carrying signals. However, in this letter, we show that OFDM signals can indeed be detected, and the bit error ratio performances of both dc-biased optical OFDM and asymmetrically clipped optical OFDM are investigated. The investigations are carried out in the absence of background light. The results are compared with those of standard optical OFDM with convetional photodiode receivers. The SPAD-based OFDM system shows superior power efficiency over the state-of-the-art counterpart, and it can reach receiver sensitivities that are comparable with existing radio frequency systems.

Surface-Grating-Coupled Low-Loss Ge-on-Si Rib Waveguides and Multimode Interferometers

Abstract: Germanium-on-silicon is a highly promising platform for planar photonics for the midinfrared, due to germanium’s wide transparency range. In this letter, we report Ge-on-Si waveguides with record low losses of only 0.6 dB/cm, which is achieved using a 2.9- $\mu \text{m}$ thick germanium layer, thus minimizing mode interaction with dislocations at the germanium/silicon interface. Using these waveguides, multimode interferometers with insertion losses of only 0.21 ± 0.02 dB are also demonstrated.

Dual-Chirp Microwave Waveform Generation Using a Dual-Parallel Mach-Zehnder Modulator

Abstract: A photonic approach to generating a dual-chirp microwave waveform using a single dual-parallel Mach-Zehnder modulator (DPMZM) is proposed and experimentally demonstrated. A dual-chirp microwave waveform can be used in a radar system to improve its range-Doppler resolution. In the proposed approach, a baseband single-chirp waveform is applied to one sub-Mach-Zehnder modulator (sub-MZM) in the DPMZM and a microwave carrier is applied to the other sub-MZM. By biasing the two sub-MZMs at the minimum transmission point to suppress the optical carrier, a dual-chirp microwave waveform with a central frequency upconverted to the frequency of the microwave carrier is generated. A theoretical analysis is performed, which is then verified by a proof-of-concept experiment. A dual-chirp microwave waveform at 6 GHz with a tunable bandwidth at 200 MHz and 2 GHz is generated.

Network Architecture of a High-Speed Visible Light Communication Local Area Network

Abstract: A novel ultrahigh-speed LED visible light communication (VLC) local area network to provide beyond 10-Gb/s optical wireless access based on star topology architecture is proposed and experimentally demonstrated for massive users. Fiber link is used as the backbone of the bidirectional VLC network. The hybrid access protocol is utilized: 1) frequency division multiplexing for downlink and uplink fiber transmission and 2) time division multiplexing for bidirectional VLC transmission. A full-duplex VLC network for eight VLC access points (VAPs) has been successfully demonstrated with the total throughput of 8 Gb/s. Each VAP is offered 500-Mb/s downstream and 500-Mb/s upstream. The measured bit error rates of downlink and uplink for all the VAPs are under 7% FEC limit of $3.8\times 10^{-3}$ over 25-km standard single mode fiber and 65-cm free space, clearly validating the promising potential of the proposed VLC network architecture for future 40- and 100-Gb/s wireless access.

Temperature-Insensitive Mode Converters With CO 2 -Laser Written Long-Period Fiber Gratings

Abstract: We fabricate mode converters by directly writing long-period gratings in a two-mode fiber with a CO2 laser. These mode converters allow the fundamental LP01 mode to be converted to any of the four higher order cylindrical vector modes or the LP11 modes. The transmission characteristics of these devices are insensitive to temperature variations and the mode conversion efficiency is insensitive to the polarization state of the input light. One of our typical gratings, which contains 15 grating periods, can provide a conversion efficiency >99% over a bandwidth of 34.0 nm in the C-band. These robust mode converters could find applications in mode-division-multiplexing systems and other applications that require cylindrical vector modes.

Distributed Strain and Vibration Sensing System Based on Phase-Sensitive OTDR

Abstract: A system based on phase-sensitive optical time domain reflectometry is proposed for simultaneously strain and vibration sensing. The strain of fiber is detected by comparing the patterns of signal for different laser frequencies, and the vibration of fiber is detected simultaneously from the signals for any certain laser frequency. During the measurement, frequencies of the probe optical pulses are modulated sequentially in ascending or descending order. Using the signals generated by optical pulses with the same frequency, the vibration of fiber is detected with fast response speed; using that with different frequencies, the strain of fiber is detected with high resolution. In our experiment, a sensing system with 2-m spatial resolution, up to 1-kHz frequency measurement range and 10- $\text{n}{{\varepsilon }}$ strain resolution is realized for a 9-km sensing fiber length.

Ultrafast Pulsed All-Fiber Laser Based on Tapered Fiber Enclosed by Few-Layer WS 2 Nanosheets

Abstract: We demonstrate all-fiber mode-locked laser based on a tapered optical fiber saturable absorber (SA) enclosed in tungsten disulfide (WS2) nanosheets. Tapered fibers were fabricated using the standard flame brushing method to an interaction length of 3 mm with waist diameters of 10 and 15 $\mu \text{m}$ . WS2 nanosheets were prepared via a liquid phase exfoliation method to form a uniform dispersion. Subsequently, the WS2 nanosheets were optically deposited along the interaction length of the tapered fibers by evanescent field interactions. We built a ring laser including the fabricated mode-lockers. The SA with a 10- $\mu \text{m}$ taper diameter delivers the pulses with a pulse duration of 369 fs and 3-dB spectral bandwidth of 7.5 nm; on the other hand, the output pulses using the mode-locker with 15- $\mu \text{m}$ waist diameter were found to have 563-fs pulse duration and 5.2 nm of 3-dB bandwidth. It is shown that the smaller waist diameter of tapered fiber causes wider spectral bandwidth of the ultrafast pulses and narrower 3-dB bandwidth.

Compact Silicon Nitride Arrayed Waveguide Gratings for Very Near-Infrared Wavelengths

Abstract: In this letter, we report a novel high-index-contrast silicon nitride arrayed waveguide grating (AWG) for very near-infrared wavelengths. This device is fabricated through a process compatible with a complementary metal–oxide–semiconductor fabrication line and is therefore suitable for mass fabrication. The large phase errors that usually accompany high-index-platform AWGs are partly mitigated through design and fabrication adaptions, in particular the implementation of a two-level etch scheme. Multiple devices are reported, among which a 0.3-mm $^{2}$ device which, after the subtraction of waveguides loss, has a −1.2 dB on-chip insertion loss at the peak of the central channel and 20-dB crosstalk for operation $\sim 900$ nm with a channel spacing of 2 nm. These AWGs pave the way for numerous large-scale on-chip applications pertaining to spectroscopy and sensing.

QAM Vector Signal Generation by Optical Carrier Suppression and Precoding Techniques

Abstract: We numerically and experimentally investigate and compare photonic constant- and multi-amplitude quadrature-amplitude-modulation (QAM) vector signal generation at radio frequency (RF) bands enabled by a Mach-Zehnder modulator (MZM)-based optical carrier suppression (OCS) modulation. In order to attain an electrical vector RF signal displaying multi-amplitude QAM modulation, such as 8 QAM and 16 QAM, after square-law photodiode detection, the driving RF signal carrying multiamplitude QAM transmitter data, should be both amplitude- and phase-precoded before used to drive the MZM. However, for constant-amplitude QAM modulation, such as quadrature-phase-shift-keying (QPSK), only phase precoding is needed. We experimentally demonstrate 1-GBd vector signal generation at 12 GHz enabled by an MZM-based OSC modulation, adopting QPSK, 8 QAM, and 16 QAM modulation, respectively. We also experimentally compare the bit error rate performance of the three different vector signals, and the QPSK case is the best, while the 16 QAM case is the worst.

Magnetic Field Sensor Based on Photonic Crystal Fiber Taper Coated With Ferrofluid

Abstract: A novel and compact magnetic field sensor based on a tapered photonic crystal fiber (PCF) coated with ferrofluid is proposed. It consists of a section of tapered PCF, which is spliced between two single-mode fibers with a waist diameter of $24~\mu $ m. The ferrofluid is filled in the capillary to coat the PCF taper. Experimentally, the refractive index (RI) of the ferrofluid increased under increasing magnetic field intensity ( ${H}$ ) with a sensitivity of $\mathbf {4 \times 10}^{\mathbf {-5}}$ RIU/Gs and the RI sensitivity in the evanescent field is 401 nm/RIU. Therefore, the interference spectrum is shifted as the change of $\boldsymbol {H}$ ranged from 100 to 600 Gs with a sensitivity of 16.04 pm/Gs and resolution of 0.62 Gs. The proposed magnetic field sensor is attractive due to its compact size, low cost, and immunity to electromagnetic interference beyond what conventional magnetic field sensors can offer.

37-GHz Modulation via Resonance Tuning in Single-Mode Coherent Vertical-Cavity Laser Arrays

Abstract: We show a significant improvement of modulation bandwidth from $2\times 1$ photonic crystal vertical-cavity surface-emitting laser arrays. Control of injection bias conditions to array elements enables resonance tuning of each element with variation of the phase relation and coherence of the array, resulting in the ability to tailor the modulation response. A bandwidth of 37 GHz is obtained under highly single-mode coherent operation with narrow spectral width and increased output power while the laser array is biased at low current density. Lasers with such performance characteristics may greatly enhance high-rate data transfer in computer server, data center, and supercomputer applications with potentially long device lifetime.

Silicon Photonic Segmented Modulator-Based Electro-Optic DAC for 100 Gb/s PAM-4 Generation

Abstract: We report on the design and characterization of a silicon-on-insulator traveling-wave multi-electrode Mach–Zehnder modulator (MZM). The 2-bit electro-optic (EO) digital-to-analog converter is formed by dividing a series push–pull MZM into two segments, one for each bit. The EO bandwidth of the longer segment of the MZM is measured to be 48 GHz at 0 V reverse bias. We operate the device at speeds up to 50 GBd to create a four-level pulse amplitude modulation signal, and thus generating 100 Gb/s on a single wavelength without signal processing at the transmitter or the receiver. The pre-forward error correction (FEC) bit error rate is estimated to be lower than the hard-decision FEC threshold of $3.8 \,\, \times \,\, 10^{-3}$ over 1 km of standard single-mode fiber, and thus leading to error-free transmission at 100 Gb/s.

Chaos-Based Partial Transmit Sequence Technique for Physical Layer Security in OFDM-PON

Abstract: We propose and demonstrate a physical layer security scheme via chaotic partial transmit sequence (PTS) technique in orthogonal frequency-division multiplexing passive optical network (OFDM-PON). A 4-D hyper chaos is employed to generate the chaotic partition information, the chaotic phase weighing factors in PTS as well as the chaotic training sequence for OFDM symbol synchronization, which provides a huge key space to enhance the physical layer confidentiality. Transmission of 8.9-Gb/s 16 quadrature amplitude modulation encrypted OFDM signals is demonstrated over 20-km standard single-mode fiber, which shows reliable robustness against exhaustive attacks. Moreover, the proposed scheme improves the OFDM transmission performance due to the effective reduction of the peak-to-average power ratio of the OFDM signals by chaotic random PTS.

Dual-Wavelength Single-Longitudinal-Mode Tm-Doped Fiber Laser Using PM-CMFBG

Abstract: We have demonstrated a dual-wavelength thulium-doped fiber laser with single-longitudinal-mode (SLM) operation at $\sim 2$ - $\mu \text{m}$ region. A polarization-maintaining chirped Moire fiber Bragg grating was employed as a polarization-dependent narrow-band filter, to suppress multilongitudinal-mode oscillation. Single-and dual-wavelength switchable operation was achieved by simply adjusting the polarization controller. We also showed that the laser lines retained SLM operation with the additional benefit of single polarization. Meanwhile, the dependence of each laser wavelength on the pump power was also investigated in detail.

Space-Collaborative Constellation Designs for MIMO Indoor Visible Light Communications

Abstract: In conventional multi-input multioutput (MIMO) visible light communication (VLC) techniques such as repetition coding (RC), spatial multiplexing (SMP), and spatial modulation (SM), the transmitted symbols through different light-emitting diodes located different space are usually generated from unipolar pulse amplitude modulation constellations independently. In this letter, we propose a novel constellation design for MIMO indoor VLC systems, in which the two symbols from two spaces collaborate so closely that the average optical power is minimized under the constraint that the minimum Euclidean distance is fixed. Therefore, this new constellation is called space-collaborative constellation (CC). Computer simulations show that the CC always has better error performance than SM and SMP, and also has better error performance than RC for most of good channels as well as for some really bad channels.

140-Gb/s 20-km Transmission of PAM-4 Signal at 1.3 $\mu \text{m}$ for Short Reach Communications

Abstract: We experimentally demonstrated 140-Gb/s transmission over 20-km standard single-mode fiber employing pulse-amplitude modulation (PAM)-4 and direct detection (DD) at 1.3 $\mu \text{m}$ . DD faster than Nyquist is employed to compensate for channel impairments. The optimal length of taps of decision directed least mean square and the optimal tap coefficient for digital postfilter are investigated. A receiver sensitivity of −5.5 dBm at bit error rate of $3.8\times 10^{-3}$ is realized for 140-Gb/s PAM-4 signal after 20-km transmission. To the best of our knowledge, this is the highest reported baud rate (70 GBd) of direct detected PAM-4 signal and the highest per channel bit rate with single polarization and DD for short reach communications.

A Refractive Index Nanosensor Based on Fano Resonance in the Plasmonic Waveguide System

Abstract: A novel and compact refractive index sensor based on Fano resonance in the plasmonic waveguide system, which comprises with a stub and groove resonator coupled with a metal–insulator–metal waveguide, is proposed and investigated by the finite-element method. Due to the interaction of the narrow discrete resonance and a broad spectrum caused by the stub resonator and the groove, respectively, the transmission spectrum exhibits a sharp asymmetrical profile. Simulation results show that the Fano resonance can be easily tuned by changing the parameters of the structure. These characteristics offer flexibility to design the devices. This nanosensor yields a sensitivity of $\sim 1260$ nm/RIU and a figure of merit of $\sim 2.3\,\times \, 10^{\mathrm {\mathbf {4}}}$ . This letter is significant for design and application of the sensitive nanoscale refractive index sensor.

Direct Modulation at 56 and 50 Gb/s of 1.3- $\mu $ m InGaAlAs Ridge-Shaped-BH DFB Lasers

Abstract: Direct modulation at 56 and 50 Gb/s of 1.3- $\mu $ m InGaAlAs ridge-shaped-buried heterostructure (RS-BH) asymmetric corrugation-pitch-modulation (ACPM) distributed feedback lasers is experimentally demonstrated. The fabricated lasers have a low threshold current (5.6 mA at 85 °C), high temperature characteristics (71 K), high slope relaxation frequency (3.2 GHz/mA $^{\mathrm {\mathbf {1/2}}}$ at 85 °C), and wide bandwidth (22.1 GHz at 85 °C). These superior properties enable the lasers to run at 56 Gb/s and 55 °C and 50 Gb/s at up to 80 °C for back-to-back operation with clear eye openings. This is achieved by the combination of a low-leakage RS-BH and an ACPM grating. Moreover, successful transmission of 56- and 50-Gb/s modulated signals over a 10-km standard single-mode fiber is achieved. These results confirm the suitability of this type of laser for use as a cost-effective light source in 400 GbE and OTU5 applications.