Optical Intensity Modulation Research Articles

Long range dynamic displacement: precision PGC with sub-nanometer resolution in an LWSM interferometer

We propose a precision phase-generated-carrier (PGC) demodulation method with sub-nanometer resolution that avoids nonlinear errors in a laser wavelength sinusoidal modulation fiber-optic interferometer for long range dynamic displacement sensing. Using orthogonal detection and an AC-DC component extraction scheme, the PGC carrier phase delay (CPD) and laser intensity modulation phase delay can be obtained simultaneously to eliminate the nonlinear error from accompanied optical intensity modulation and CPD. Further, to realize long range displacement sensing, PGC phase modulation depth (PMD), determined by the laser wavelength modulation amplitude and the working distance of the interferometer, is required to maintain an optimal value during measurement, including initial position and dynamic movement. By combining frequency sweeping interference and modified PGC-arctan demodulation to measure real-time working distance, adaptive PMD technology is realized based on proportion control. We construct a fiber-optic Michelson and SIOS commercial interferometer for comparison and perform experiments to verify the feasibility of the proposed method. Experimental results demonstrate that an interferometer with sub-nanometer resolution and nanometer precision over a large range of 400 mm can be realized.

10-Gb/s data transmission using optical physical layer encryption and quantum key distribution

A 10-Gb/s data transmission using the optical physical layer encryption and quantum key distribution (QKD) is developed. The physical layer encryption basing on the spectrum phase encoding technology, and the quantum key distribution relying on optical intensity modulation (IM) Y-00 cipher, are together performed to secure optical transmissions at 10 Gb/s. Especially, We propose a quantum key distribution method in which the encryption key generated at a update rate of up to 10 MHz is used for the parameters of the physical layer encryption with the spectral phase encoding. Compared with traditional quantum key distribution, our key rate is out of reach. A 10-Gb/s secure prototype is developed basing on this technique, which is experimentally applied in the 20-channel wavelength division multiplexing (WDM) network over 100 km fiber. The experimental results show that this method can ensure reliable transmission of cooperative users and prevent information from eavesdropping. The power penalty caused by data encryption is less than 3 dB for improving security of the data transmission. Furthermore, the bit error rate (BER) is analyzed, a BER above 0.48 for eavesdropper is verified.

(Invited) Selective Epitaxy of Submicron Ge Wire Structures for Photodetectors and Optical Modulators in Si Photonics

A selective-area epitaxial growth of Ge on the submicron scale is studied using chemical vapor deposition (CVD) on Si in terms of the fabrication of integrated photonic devices operating at the optical communication wavelengths of around 1.55 μm. A mesa structure of Ge on the micron scale, having a vertical pin junction, is effective for fabricating a waveguide-integrated photodetector in the optical receiver. A 3-dB cutoff frequency is obtained up to approximately 50 GHz. For a higher frequency operation, a narrower structure of submicron-wide Ge wire, having a lateral pin junction, is required. Such a wire structure is also important for the application to an electro-absorption optical intensity modulator in the optical transmitter. Structural and optical properties are investigated for a submicron-wide Ge wire on Si selectively grown by CVD.

Real-time channel conditional distribution tracking for intelligent decoding of optical IMDD signals.

In this Letter, we propose a real-time machine learning scheme of a tracking optical intensity-modulation and direct-detection (IMDD) system's conditional distribution using linear optical sampling and inline Gaussian mixer modeling (GMM) programming. End-to-end conditional distribution tracking enables an adaptive decoding of optical IMDD signals, with robustness to the bias point shift of the optical intensity modulator. Experimental demonstration is conducted over a 20-Gbits/s optical pulse amplitude modulation-4 (PAM-4) modulation system. Optical PAM-4 signals are optically down-sampled by short pulses to 250 Msa/s. Then, statistical characters of signal distribution can be estimated using inline GMM processing. Due to the real-time learned distribution, intelligent decoding of received signals exhibits a perfect adaptation to the changing bias point of a Mach-Zendner intensity modulator, enhancing the communication reliability with bit error rate (BER) below 3.8⋅10-3. In addition, the proposed scheme also provides the possibility of practical implementation to other machine learning signal decoding methods.

Integrated optical RF intensity modulators transfer function traits

The most common integrated optical intensity modulators are made according to the Mach-Zehnder interferometer scheme. It leads to specific interconnection between the interferometer arms phase difference and the output spectrum. The description and the experimental demonstration of this traits are presented.

Optical Modulation via Coupling of Distributed Semiconductor Heterojunctions in a Si -ITO-Based Subwavelength Grating

A mechanism of optical intensity modulation is proposed by utilizing the electro-optic coupling in distributed semiconductor heterojunctions of p-type silicon ($\mathrm{Si}$) and n-type indium tin oxide (ITO) in the form of the subwavelength grating in a rib waveguide. The coupled multiple semiconductor heterojunctions of $\mathrm{Si}$-ITO are made to exhibit efficient optical intensity modulation via electrically tunable permittivity of ITO. The subwavelength grating is a nanophotonic element that not only provides a way to couple multiple heterojunctions, but it also gives rise to efficient optical (fiber to chip) coupling at a wavelength of 1550 nm. Lateral coupling of distributed heterojunctions via depleted charge density distributed along vertical and horizontal directions enable the device to show a high extinction ratio of 24 dB. Also, electrical tuning of the coupling efficiency for an 80-\textmu{}m long device is reported, which exhibits the multifunctional nature of the proposed nanophotonic device. The proposed modulation scheme, with a modulation efficiency of 0.34 V/mm and energy consumption of 36 pJ, may open pathways for energy-efficient compact devices and circuits for large-scale optoelectronic integration. The proposed mechanism of optical modulation takes advantage of distributed semiconductor heterojunctions and enables electrically tunable inherent optical coupling with a nanophotonic element called a subwavelength grating, which further improves the modulation performance compared with a conventional $\mathrm{Si}$-ITO heterojunction.

Precise Optical Modulation and Its Application to Optoelectronic Device Measurement

Optoelectronic devices which play important roles in high-speed optical fiber networks can offer effective measurement methods for optoelectronic devices including optical modulators and photodetectors. Precise optical signal modulation is required for measurement applications. This paper focuses on high-speed and precise optical modulation devices and their application to device measurement. Optical modulators using electro-optic effect offers precise control of lightwaves for wideband signals. As examples, this paper describes frequency response measurement of photodetectors using high-precision amplitude modulation and wavelength domain measurement of optical filters using fast optical frequency sweep. Precise and high-speed modulation can be achieved by active trimming which compensates device structure imbalance due to fabrication error, where preciseness can be described by on-off extinction ratio. A Mach-Zehnder modulator with sub Mach-Zehnder interferometors can offer high extinction-ratio optical intensity modulation, which can be used for precise optoelectronic frequency response measurement. Precise modulation would be also useful for multi-level modulation schemes. To investigate impact of finite extinction ratio on optical modulation, duobinary modulation with small signal operation was demonstrated. For optical frequency domain analysis, single sideband modulation, which shifts optical frequency, can be used for generation of stimulus signals. Rapid measurement of optical filters was performed by using an optical sweeper consisting of an integrated Mach-Zehnder modulator for optical frequency control and an arbitrary waveform generator for generation of a source frequency chirp signal.

Hybrid Coding and Filtering Technique for Optical IM-DD Link With Robustness to Multipath Interference and Bandwidth Limitation

In this work, a hybrid code division multiplexing (CDM) coding and digital filtering method for optical intensity modulation and direct detection (IM-DD) link is investigated to address multipath interference (MPI) and channel bandwidth limitation. CDM signaling already provides robustness to MPI through suppressing the unsynchronized paths by orthogonal (de)coding. As for digital filtering based on square root raised cosine (SRRC) waveforms, its advantage to IM-DD link includes robustness to channel bandwidth limitation due to pulse shaping. Additionally, matching filters achieve the best orthogonality only when they are synchronized well, thus robustness to MPI can be expected on the basis of CDM. We study the performance of such hybrid coding and filtering method over IM-DD link in the presence of MPI and bandwidth limitations through simulation. This is followed by experimental demonstrations of single point-to-point transmission with capacity at 112 Gbps. BER comparison between CDM only and hybrid filtering and coding method is performed by emulating MPI using a Mach-Zehnder interferometer. Furthermore, bidirectional transmission over a single fiber verifies its performance advantage and demonstrates potentials for reducing implementation costs of end-to-end optical communications.

Si racetrack optical modulator based on the III-V/Si hybrid MOS capacitor.

We have fabricated a Si racetrack optical modulator based on a III-V/Si hybrid metal-oxide-semiconductor (MOS) capacitor. The III-V/Si hybrid MOS optical phase shifter was integrated to a Si racetrack resonator with a coupling length of 200 µm and a coupling gap of 700 nm. The fabricated Si racetrack resonator demonstrated a small VπL of 0.059 Vcm. For 10-dB optical intensity modulation, the Si racetrack resonator showed a 60% smaller driving voltage than a Mach-Zehnder interferometer modulator with the same phase shifter, leading to a better balance between high energy efficiency and large modulation bandwidth.

Time Domain Discrete Fourier Domain Mode Locked Laser With k-Space Uniform Comb Lines

Frequency comb swept lasers or wavelength stepped swept lasers with comb filters have been proposed in optical domain subsampling or circular-ranging optical coherence tomography (OCT) to achieve high axial scan rate and extended imaging range with a given acquisition bandwidth. However, the comb filters lack flexibility in varying the free spectral range (FSR). In this article, we propose and demonstrate a novel discrete Fourier domain mode locked (FDML) laser by using intracavity optical intensity modulation. Swept signals with comb lines uniformly distributed in the frequency domain is generated with tailor-made pulse patterns according to the sweep trace of the swept filter. Discrete k -space uniform swept signals with different FSRs and pulse durations are generated in the same laser. The FDML laser discretized by temporal modulation is more stable than that with a comb filter. The discrete FDML lasers will be potential sources of OCT and the fast-developing swept source LiDAR.

SOP Change Robust Optical Modulation Based on Dual Polarization Modulation for Multi-Dimensional Optical Transmission

In optical transmission technique using optical polarization, the change of state of polarization (SOP) and the rotation of SOP (RSOP) during optical transmission has a significant impact on the transmission performance. We propose polarized intensity rotational frequency shift keying (PIR-FSK) as a novel modulation technique in change of SOP and RSOP-tolerant optical transmissions. The proposed PIR-FSK signal does not affect the entire intensity of the optical carrier by modulating the sinusoidal signals on each X and Y-polarized optical carrier. Therefore, the proposed PIR-FSK modulation and optical carrier intensity modulation can operate at the same time. Moreover, since the proposed technique does not modulate the signal using the SOP unlike the PolSK, PIR-FSK is not affected by SOP changes that occur during transmission, and the signal does not be degraded even with RSOP. We demonstrated that the signals modulated using the proposed PIR-FSK modulation have higher signal capacity and efficiency compared to the signal modulated using PolSK modulation.

Optical-intensity modulator with InSb nanosheets

Optical-intensity modulators have increasingly important applications in optoelectronic field. However, it still remains a challenge to find an optical intensity modulation material with stable chemical properties, high efficiency and low cost in the infrared region. Here, we present a liquid phase exfoliation-based programme for the preparation of InSb nanosheets. Experimentally, the mechanically exfoliated InSb nanosheets possess preferable morphology and stable chemical properties, which means this programme is reliable and efficient. However, InSb nanosheets have completely different Raman peaks compared with bulk. Afterwards, the InSb nanosheets were used to realize switchable dual-wavelength mode locking around 1565 nm. This work has high practical value and will lay a solid foundation for the large-scale study of optical-intensity modulator based on Sb compounds.

Phase modulation depth setting technique of a phase-generated-carrier under AOIM in fiber-optic interferometer with laser frequency modulation.

The phase modulation depth (PMD) in phase-generated-carrier demodulation is determined by the laser frequency modulation amplitude and working distance of a fiber-optic interferometer and must be set at a certain value. Active setting of the amplitude is unsuitable, especially for high-speed modulation, owing to variations in the laser source tuning coefficients. Existing calculation schemes for passive setting cannot work both owing to carrier phase delay (CPD) and the accompanied optical-intensity modulation (AOIM). Herein, a modified phase modulation depth calculation and setting technique is proposed. Double photoelectric detection and signal division are optimized to eliminate AOIM using a fiber delay chain and phase-locked amplifier module. Fast Fourier-transform and look-up table methods are used to calculate phase modulation depth without adding the carrier, which is unaffected by CPD. A fiber-optic Michelson interferometer is constructed to verify the feasibility of the proposed method. The experimental results show that AOIM can be eliminated; moreover, PMD can be calculated and set precisely. The displacement deviation is less than 1.03 nm. The resolution of measurement is considerably lesser than 1 nm and nanoscale accuracy is achieved in displacement measurement.

Cavity resonance tunability of porous silicon microcavities by Ar+ ion irradiation

Present work reports the effect of low-energy Ar+ ion beam irradiation on the optical properties and surface morphology of porous silicon (PSi) based optical microcavities. These microcavities were realized by an active λ/2 spacer layer sandwiched between two Distributed Bragg Reflectors fabricated by galvanostatic electrochemical etching. Low-energy 10 keV Ar+ ions at various fluences were used for this irradiation study for these microcavities and characterized. The peak in the reflectance spectra of resonant cavity shows a red-shift after ion irradiation. These experimental observations are in good agreement with transfer matrix simulations. The Raman studies show that the crystalline Si is modified in nanostructured PSi and subsequently, the structural features of Si-Si and Si-O bonds. The simulated depth profiles suggest that the penetration of Ar+ ion is limited to top two PSi layers and the reduction of effective refractive index of these two layers. These experimental findings, analytical simulations, and cavity modelling provide the evidence of resonent cavity peak tunability, optical field intensity modulation, and photon confinement within the microcavity. This approach could be useful to tune the optical properties of PSi and an effective method for producing widely tunable optical structures in Si-based photonic architectures.

Coupled Electromagnetic and Reaction Kinetics Simulation of Super-Resolution Interference Lithography.

Inspired by the ability of super-resolved fluorescence microscopy to circumvent the diffraction barrier, two-color super-resolution interference lithography exploits nonequilibrium kinetics in materials to achieve large-area nanopatterning while using visible light. Periodic patterns with super-resolved features down to tens of nanometers have been demonstrated in thin films and monolayers. Extending these advances to the bulk nanopatterning of thick films requires a quantitative understanding of the time-dependent interactions of optical dynamics, including absorption, diffraction, and intensity modulation at two wavelengths, with the photoactivated and inhibited reaction kinetics. Here, we develop an efficient electromagnetic (EM) perturbation theory approach that facilitates for the first time fully coupled simulations of EM and chemical kinetics in two-color interference lithography. Applied to a spirothiopyran-functionalized photoresist system, these simulations show that diffraction and absorption effects are negligible (<0.1%) for depths up to 10 μm, and that tuning exposure time and intensities can lead to concentration contrast up to 80%. We investigate multiple exposure strategies to reduce the pitch of the line pattern including sequential exposures with different times to achieve uniform lines and multiplexed exposures with equal periods. This capability to rapidly and accurately predict the coupled optical and chemical dynamics facilitates the computational design of high-precision patterns in two-color interference lithography.

Controllable Soliton Pairs in Few-Layer FePSe3 Nanosheets-Based Optical-Intensity Modulators

Optical-intensity modulators based on two-dimensional (2D) materials play a significant role in the realization of ultrashort pulses. FePSe3, as a new 2D material, has shown great potential in this...

Visible light communication with efficient far-red/near-infrared polymer light-emitting diodes

Visible light communication (VLC) is a wireless technology that relies on optical intensity modulation and is potentially a game changer for internet-of-things (IoT) connectivity. However, VLC is hindered by the low penetration depth of visible light in non-transparent media. One solution is to extend operation into the “nearly (in)visible” near-infrared (NIR, 700–1000 nm) region, thus also enabling VLC in photonic bio-applications, considering the biological tissue NIR semitransparency, while conveniently retaining vestigial red emission to help check the link operativity by simple eye inspection. Here, we report new far-red/NIR organic light-emitting diodes (OLEDs) with a 650–800 nm emission range and external quantum efficiencies among the highest reported in this spectral range (>2.7%, with maximum radiance and luminance of 3.5 mW/cm2 and 260 cd/m2, respectively). With these OLEDs, we then demonstrate a “real-time” VLC setup achieving a data rate of 2.2 Mb/s, which satisfies the requirements for IoT and biosensing applications. These are the highest rates ever reported for an online unequalised VLC link based on solution-processed OLEDs.

Correction of nonlinear errors from PGC carrier phase delay and AOIM in fiber-optic interferometers for nanoscale displacement measurement.

In fiber-optic interferometers with laser frequency modulation, carrier phase delay and accompanied optical intensity modulation (AOIM) in phase-generated-carrier (PGC) demodulation inevitably produce nonlinear errors that can seriously hamper displacement measurement accuracy. As for the existing improved PGC scheme, they are only capable to compensate for one of these effects. As the only method that is effective in eliminating the two effects simultaneously, typical ellipse fitting methods require target movements λlaser/4, and fail when the PGC carrier phase delay is proximate to certain values (e.g., nπ +π/4, nπ +π/2). Herein, a modified nonlinear-error correction method for errors due to PGC carrier phase delay and AOIM is proposed. Active laser-wavelength scanning by constant variation of the laser drive temperature is used to replace the target movement. A fiber-optic Michelson interferometer is constructed and experiments are performed to verify the feasibility of the proposed method. The experimental results show that after correction, the nonlinear error is reduced to less than 1nm, and nanoscale displacement measurement is achieved.

Self-assembly plasmonic metamaterials based on templated annealing for advanced biosensing.

In this paper, we introduce a novel method for the fabrication of self-assembly plasmonic metamaterials by exploiting fluid instabilities of optical thin films. Due to interplay between template reflow and spinodal dewetting, two metal nanoparticles of different sizes are generated on the top mesas of free-standing porous anodic aluminum oxide (AAO) template, which results in the apprearance of double resonant peaks in the extinction spectrum. These two resonant peaks possess refractive index resolution 3.27 × 10-4 and 2.53 × 10-4 RIU, respectively. This optical intensity modulation based plasmonic nanoplatform shows a dramatically surface sensing performance with outstanding detection capacity of biomolecules, because of the very small decay length of electric field at dual-modes. The detection ability for concanavalin A (Con A) demonstrats that the limit of detection of dual-modes reaches as small as 68 and 79 nM, respectively.

Chromatic dispersion mitigation using a SEFDM-based diversity technique for IM/DD long reach optical links.

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.