Compact Optical Module Research Articles

Analysis of SiNx Waveguide-Integrated Liquid Crystal Platform for Wideband Optical Phase Shifters and Modulators

In this study, the potential of employing SiNx (silicon nitride) waveguide platforms to enable the use of liquid-crystal-based phase shifters for on-chip optical modulators was thoroughly investigated using 3D-FDTD (3D finite-difference time-domain) simulations. The entire structure of liquid-crystal-based Mach–Zehnder interferometer (MZI) optical modulators, consisting of multi-mode interferometer splitters, different tapering sections, and liquid-crystal-based phase shifters, was systematically and holistically investigated with a view to developing a compact, wideband, and CMOS-compatible (complementary metal-oxide semiconductor) bias voltage optical modulator with competitive modulation efficiency, good fabrication tolerance, and single-mode operation using the same SiNx waveguide layer for the entire device. The trade-off between several important parameters is critically discussed in order to reach a conclusion on the possible optimized parameter sets. Contrary to previous demonstrations, this investigation focused on the potential of achieving such an optical device using the same SiNx waveguide layer for the entire device, including both the passive and active regions. Significantly, we show that it is necessary to carefully select the phase shifter length of the LC-based (liquid crystal) MZI optical modulator, as the phase shifter length required to obtain a π phase shift could be as low as a few tens of microns; therefore, a phase shifter length that is too long can contradictorily worsen the optical modulation.

Adaptive optical third-harmonic generation microscopy for in vivo imaging of tissues.

Third-harmonic generation microscopy is a powerful label-free nonlinear imaging technique, providing essential information about structural characteristics of cells and tissues without requiring external labelling agents. In this work, we integrated a recently developed compact adaptive optics module into a third-harmonic generation microscope, to measure and correct for optical aberrations in complex tissues. Taking advantage of the high sensitivity of the third-harmonic generation process to material interfaces and thin membranes, along with the 1,300-nm excitation wavelength used here, our adaptive optical third-harmonic generation microscope enabled high-resolution in vivo imaging within highly scattering biological model systems. Examples include imaging of myelinated axons and vascular structures within the mouse spinal cord and deep cortical layers of the mouse brain, along with imaging of key anatomical features in the roots of the model plant Brachypodium distachyon. In all instances, aberration correction led to enhancements in image quality.

All-optical modulator with photonic topological insulator made of metallic quantum wells

Abstract All-optical modulators hold significant prospects for future information processing technologies for they are able to process optical signals without the electro-optical convertor which limits the achievable modulation bandwidth. However, owing to the hardly-controlled optical backscattering in the commonly-used device geometries and the weak optical nonlinearities of the conventional material systems, constructing an all-optical modulator with a large bandwidth and a deep modulation depth in an integration manner is still challenging. Here, we propose an approach to achieving an on-chip ultrafast all-optical modulator with ultra-high modulation efficiency and a small footprint by using photonic topological insulators (PTIs) made of metallic quantum wells (MQWs). Since PTIs have attracted significant attention because of their unidirectional propagating edge states, which mitigate optical backscattering caused by structural imperfections or defects. Meanwhile, MQWs have shown a large Kerr nonlinearity, facilitating the development of minimally sized nonlinear optical devices including all-optical modulators. The proposed photonic topological modulator shows a remarkable modulation depth of 15 dB with a substantial modulation bandwidth above THz in a tiny footprint of only 4 × 10 µm2, which manifests itself as one of the most compact optical modulators compared with the reported ones possessing a bandwidth above 100 GHz. Such a high-performance optical modulator could enable new functionalities in future optical communication and information processing systems.

Hybrid plasmonic valley-Hall topological insulators

Abstract The emerging field of photonic topological insulators offers promising platforms for high-performance optical communication, computing, and sensing. However, conventional photonic topological insulator designs typically operate within the diffraction limit due to their dielectric nature. This limitation imposes constraints on device miniaturization, reduces light–matter interaction, and decreases overall device sensitivity. Introducing a new valley-Hall hybrid plasmonic topological insulator, we overcome this limitation by exploiting the coupling of surface plasmon oscillations with the optical modes of a dielectric photonic crystal, allowing for sub-diffraction vertical confinement of light. Deep-subwavelength chiral edge states can, therefore, be generated and robustly guided along disordered Z-shaped topological boundaries with much lower propagation loss compared to purely plasmonic platforms. Such extreme manipulation of light on an integrated chip platform maximizes light–matter interaction and opens the door for truly compact and efficient optical modulators, molecular sensors, and next-generation nanophotonic and quantum devices.

Folded beam path architecture for highly efficient filter-based spectral sensors.

This paper demonstrates a method to significantly enhance the detection efficiency of filter-based spectral sensors without the use of additional dichroic optics for spectral preselection. The fundamental principle is that light reflected from one interference filter or filter segment can be used consecutively, reducing the overall system losses. The proof-of-concept is presented using two compact optical modules. The first module uses 10 individual filters between 520 and 800nm, and the second is capable of continuous spectrum acquisition between 450 and 825nm using a linear variable filter (LVF) as a key element. An efficiency increase factor of up to approximately 100 compared to a common system, where the entire LVF is directly illuminated, was demonstrated.

Four-channel CWDM transmitter chip based on thin-film lithium niobate platform

Multi-lane integrated transmitter chips are key components in future compact optical modules to realize high-speed optical interconnects. Thin-film lithium niobate (TFLN) photonics have emerged as a promising platform for achieving high-performance chip-scale optical systems. Combining a coarse wavelength-division multiplexing (CWDM) devices using fabrication-tolerant angled multimode interferometer structure and high-performance electro-optical modulators, we demonstrate monolithic on-chip four-channel CWDM transmitter on the TFLN platform for the first time. The four-channel CWDM transmitter enables high-speed transmissions of 100 Gb/s data rate per wavelength channel (i.e., an aggregated date rate of 400 Gb/s).

Proposal of compact multimode interference optical modulator with high extinction ratio

We proposed a compact 1 × 1 multimode interference (MMI) optical modulator with a high extinction ratio. A triply coupled quantum well structure is designed for the core layer for the enhancement of the quantum-confined Stark effect, and the MMI waveguide with isolation trenches is symmetrically designed. The length of the MMI modulator is reduced by 30%, and the wavelength dependence was reduced compared with that observed in the previously reported MMI modulator. Owing to both the electroabsorption effect and the electric-field induced refractive index change in a multiple-quantum-well core layer, the extinction ratio is expected to be approximately 45 dB at an operating voltage of 3.3 V.

Gap-plasmon-driven spin angular momentum selection of chiral metasurfaces for intensity-tunable metaholography working at visible frequencies

AbstractTunable metasurfaces can replace conventional bulky active optical modules to realize practical flat optical devices such as lenses, LiDAR, holography, and augmented reality. However, tunable metasurfaces have generally been limited to switching between two distinct states. Here, we present liquid crystal (LC) integrated chiral metasurfaces, of which the metahologram intensity can be adjusted continuously between fully ‘on’ and ‘off’ states. The chiral metasurface consists of a gap-shifted split ring resonator (SRR), and exhibits spin angular momentum selection that reflects left-circularly-polarized light but perfectly absorbs right-circularly-polarized light (99.9%). The gap-shifted SRR realizes spin angular momentum selection using a metal–dielectric–metal multilayer structure and thereby induces a strong gap-plasmonic response, achieving the maximum calculated circular dichroism in reflection (CDR) of 0.99 at the wavelength of 635 nm. With the chiral metasurface, metaholograms are demonstrated with tunable intensities using LCs that change the polarization state of the output light using an applied voltage. With the LC integrated chiral metasurfaces, 23 steps of polarization are demonstrated for the continuous tuning of the holographic image intensity, achieving measured CDR of 0.91. The proposed LC integrated spin-selective chiral metasurface provides a new resource for development of compact active optical modules with continuously-tunable intensity.

The ordinary negative changing refractive index for estimation of optical confinement factor

Abstract The electro-optic effect is considered very important in optical communication systems. The small optical confinement factor is attributed to the weak overlap between the electric field and optical wave and hence the optical signal is not efficiently modulated. In this paper, the problem of the small optical confinement factor in the Mach–Zehnder modulator based on lithium niobate (LN) which is deeply studied. The data were analyzed through a proposed mathematical model to explain the relationship between the change in the ordinary negative refractive index and the confinement factor. The system is improved using a small length of the modulator arm as only 3 to 8 µm, low driving power of about 4 V/µm, a large change in the negative ordinary refractive index of about—0.2 × 10−7, and a compact optical modulator. This can reflect a strong optical confinement factor when the electric field is applied to the electrodes of the optical modulator.

Ultra-Compact Efficient Thermally Actuated Mach-Zehnder Modulator Based on VO2

Vanadium dioxide (VO₂) has emerged as a prominent optical phase change material (OPCM) for creating high performance devices based on hybrid silicon platforms. However, realizing an efficient and compact optical modulator required for Mach-Zehnder interferometer (MZI) structures, still remains a major challenge in active Si-platforms enabled by VO₂. This is mainly due to the simultaneous variation of both real and imaginary parts of the refractive index during the phase transition process, which is a significant issue. A modified MZI structure is proposed in this paper while the refractive index variation issue is overcome by operating in the wavelength range between 1.5 to 1.6 &#x03BC;m including the optical C-band. An indium tin oxide (ITO) layer is considered as the microheater for the thermal excitation. An optimized triggering signal with an amplitude of 12.5 V along with an arm length of 2.35 &#x03BC;m of the MZI device (<i>V</i>_&#x03C0;<i>L</i>_&#x03C0; = 30 V&#x2219;&#x03BC;m) established a &#x03C0;-shift at the output of the device. The proposed device consumes &#x007E;26 pJ for modulating a single bit with a delay of 3.5 ns.

Design of high extinction ratio, low loss, and compact optical modulators and switches based on GST phase change material

In this paper, an electrically driven optical modulator and switch based on micro-ring resonators using the phase change material (PCM) of G e 2 S b 2 T e 5 (GST) is designed and their optical properties are investigated. The change in refractive index between two states in PCMs is orders of magnitude larger than conventional electro-optic and free-carrier dispersion effects. These extraordinary variations make PCMs excellent candidates for optical active devices such as modulators and switches. In our design, GST is used as the active material because it is far more stable and results in lower insertion loss compared to other PCMs such as vanadium dioxide ( V O 2 ) (at λ = 1.55 µ m , the imaginary part of the refractive index is 1.49 for GST and 2.53 for V O 2 is their metallic phases). By optimizing the GST dimensions, the device figure of merit is maximized. Three-dimensional finite-difference time-domain simulations show a very high extinction ratio of 37.3 dB with a low insertion loss of 0.427 dB for the proposed modulator and high extinction ratios of 19 dB and 10 dB in off and on states and insertion losses of 1.52 dB and 2.02 dB in off and on states, respectively, of the proposed 1 × 2 switch.

An adaptive optics module for deep tissue multiphoton imaging in vivo.

Understanding complex biological systems requires visualizing structures and processes deep within living organisms. We developed a compact adaptive optics module and incorporated it into two- and three-photon fluorescence microscopes, to measure and correct tissue-induced aberrations. We resolved synaptic structures in deep cortical and subcortical areas of the mouse brain, and demonstrated high-resolution imaging of neuronal structures and somatosensory-evoked calcium responses in the mouse spinal cord at great depths in vivo.

QAM-GFDM of Dual-Mode VCSEL Mixed 28-GHz MMW Carrier for Fiber-Wireless Link

An optical heterodyned 28-GHz carrier generation is demonstrated for synthesizer-free fifth-generation (5G) millimeter wave over fiber (MMWoF) link by using orthogonally polarized dual-mode vertical-cavity surface-emitting laser (VCSEL) as the frontend transmitter, and the power envelope detection is employed for self-heterodyne down-conversion of the generalized frequency division multiplexing (GFDM) data stream. The demonstrated MMWoF link exhibits superior immunity to the residual frequency and phase noises as well as the inter-carrier interference induced by the free-running dual-mode carrier. For back-to-back optical downstream wireline transmission, the receiving GFDM data optimizes its bit error rate (BER) to 2.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> by adjusting the bias current of the dual-mode VCSEL to 8 mA (3 I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ), and by grouping the GFDM data matrix with total symbols of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N = K×M</i> with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> = 2 subcarriers and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> = 24 timeslots. After a long-reach delivering the optical data stream over 50-km dispersion-managed single-mode fiber (DM-SMF), the qualified receiving power sensitivity is −12 dBm, whereas the decoding GFDM data reveals improved receiving sensitivity with an extremely low power penalty of only 0.3 dB. For the wireless transmission after optical downstream receiving at the remote node, the self-heterodyned MMW carrier exhibits peak power of −62.3 dBm and carrier-to-noise (CNR) of 26.8 dB over long-reach 50-km DM-SMF downstream optical link. In comparison with the mutual heterodyned from two incoherent optical carriers, the self-heterodyned frequency fluctuation can be suppressed from >500 to <330 MHz with significantly suppressed phase noise. After wireless delivering over 2 m in free space, the self-heterodyne down-conversion by power envelope detection guarantees the down-converted quadrature amplitude modulation GFDM (QAM-GFDM) data stream with an average power of -48.4 dBm after optimizing the impedance matching by selecting the appropriate frequency response of the power envelope detector. As a result, the wireless MMW transmission of 8-Gbit/s 4-QAM GFDM data stream is qualified with error vector magnitude (EVM) of 16.9%, signal-to-noise ratio (SNR) of 8.5 dB, and BER of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . By combining the free-running dual-mode source with two advanced techniques including mode-power equalization and self-heterodyned power envelope detection, the proposed synthesizer-free MMWoF network has demonstrated excellent immunity to the phase/frequency noise, inter-carrier interference which is particularly suitable for building up the compact and simplified optical transmitter and microwave receiver modules with cost-effective MMW self-heterodyne scheme.

Electrical crosstalk suppression for a compact optical segmented modulator.

Advanced coding formats can improve the spectral efficiency in optical transmission systems, while the generation can be expensive and power hungry when electrical digital-to-analog converts (DACs) are utilized. Optical segmented modulators can supersede electrical DACs with the merits of low cost and power efficiency. However, due to their compact size, the leakage current between the adjacent segments results in considerable electrical crosstalk, which impairs the linearity of the modulators and distorts the modulated signal. Here, we propose and demonstrate an electrical crosstalk suppression scheme for optical segmented modulators by introducing a complementary doped region as an insulator. Two depletion regions with high impedances are formed, resulting in the decrease in leakage current and crosstalk. Qualitative and quantitative analysis are performed, and experimentally, in a ring based segmented modulator, more than 5 dB crosstalk improvement is successfully achieved within the 30 GHz range.

Analysis of Optical Modulator Based on Silicon Waveguide using FDTD

We propose a novel and compact optical modulator based on a silicon microring resonator with a side-coupled circular cavity. The proposed modulator has been numerically and theoretically inspected using the finite-difference time-domain (FDTD) method. To benchmark of the proposed structure, with Firely optimization algorithm, the effect of geometrical parameters to the performance parameters including modulation depth, full with at half the maximum (FWHM), loss, and bandwidth are explored. Then the suitable parameters of an efficient structure are determined in the light of the former studies. The obtained results show that the resonance dips of the modulator have a high transmission quality ratio and that the resonance wavelengths have a reverse relationship with the radius of the in cavity. Also, the numerical results achieved from the transmission spectra are used to analyze the modulation characteristic of the structure. The bandwidth can be tuned to a value as high as 70 GHz with a modulation depth of 58% around the resonance wavelength of 1550 nm and an extinction rate of 18 dB utilizing the novel configuration and by optimizing the structural parameters. Besides, the coupling modulation phase characteristic of the modulator for different input FWHM is also discussed in this paper. The obtained results are compared with other theoretical results and present good agreement. This model can constitute a handy tool for both theoreticians and experimentalists in future research on related topics.

Compact optical module to generate arbitrary vector vortex beams

We demonstrated a compact optical module that is capable of efficiently generating vector vortex beams (VVB). With this device, a linearly polarized input beam can be converted into a vector beam with arbitrary spatial polarization and phase distributions, accompanied by an energy utilization up to 61%. Equally important, the area utilization of the spatial light modulator, a key component in the device, is as high as 65.5%. With the designed vector-vortex-beam-generation module, several types of VVBs with different vortex topological charges and spatial polarization distributions were created experimentally. This device may find applications in optical tweezers, laser machining, and so on.

Compact optical zoom camera module based on Alvarez elements

A compact optical zoom camera module with Alvarez freeform elements is reported. A 3 × magnification in the optical zoom system was achieved by means of lateral shifting two pairs of Alvarez lenses. The optical design performances, profiles of the freeform surfaces, tolerance analysis results, and Monte Carlo yield simulation for mass production of the optical zoom system are presented. The optomechanical structures of the whole camera module including the barrels, actuators, and the slide-guiding systems are also described. The Alvarez elements and aspheric lenses were fabricated by precise injection molding and the movable elements are actuated by voice coil motors with a stroke of 3 mm. A camera module was developed with a size of 25 mm (width) × 25 mm (length) × 6 mm (height). The zooming and imaging capabilities of this Alvarez zoom lens were demonstrated experimentally.

Predicting Onset of Combustion Instability in Gas Turbines Using Fiber Optic Sensors

A data driven method based on cross-entropy derived from symbolic time series analysis of the chemiluminescence and pressure variations has recently been found to be robust and computationally efficient among existing methods for predicting instability. In this article, we report the development and testing of compact and field-deployable optical fiber-based modules for sensing chemiluminescence and pressure variations in the combustor. The time-series data obtained from the above sensor modules are used to deduce the D-Markov cross-entropy for different protocols corresponding to typical combustor operation conditions. Such cross-entropy data is compared with similar data obtained from standard high speed camera and PZT-based pressure sensors. It is found that the fiber optic sensor modules compare favorably with the conventional sensors thereby providing an exciting new pathway for field-deployable solutions to monitor the onset of thermo-acoustic instability in combustors.

Hybrid Si-VO2 modulator with ultra-high extinction ratio based on slot TM mode.

Nowadays, the development of modern optical systems relies on optical device size minimization and operating power reduction. Optical modulator based on silicon on insulator (SOI) platform is a key element in different optical systems. Therefore, the optical modulator with compact size and low insertion loss could improve the optical system efficiency. In this work, a novel compact optical modulator based on hybrid plasmonic/silicon layers is introduced. The full vectorial finite element method (FV-FEM) is used to numerically analyze the proposed design. Vanadium dioxide (VO2) is also utilized as a cap layer to control the modulation process. The insertion loss (IL) and extinction ratio (ER) of the suggested modulator are equal to 2.1 dB/µm and 28 dB/µm, respectively, at the operating wavelength 1.55 µm. Consequently, high figure-of-merit (FoM) =ER/IL = 13.5 is achieved with an optical bandwidth (ER > 3 dB) greater than 1 µm, which is large in comparison to pervious designs.

Slow light enabled high-modulation-depth graphene modulator with plasmonic metasurfaces.

Graphene has attracted the interest of researchers seeking to develop compact optical modulators with the flexible tunability of graphene conductivity by tuning the Fermi level. Plasmonic structures have provided a robust way to enhance the modulation depth (MD) of graphene optical modulators, but the available schemes suffer from low MD, fabrication complexities, or both. Here, an ultra-thin plasmonic metasurface structure capable of guiding slow surface plasmons (SPs) is proposed to construct graphene-based optical modulators. The designs take advantage of the strong field enhancement of slow SP modes as well as the orientation match between the electric field and the graphene plane. A typical 0.96-μm-long metasurface-based graphene modulator presents a significantly improved MD of 4.66dB/μm and an acceptable insertion loss of 1.4dB/μm, while still having ease of fabrication.