Free Spectrum Range Research Articles

Fundamental mode hybridization in a thin film lithium niobate ridge waveguide.

The high performance of thin film lithium niobate on insulator (LNOI) platform shows potential for electro-optical signal processing and nonlinear optics systems. To realize precise polarization management for sub-wavelength devices, we theoretically and experimentally investigate fundamental transverse electric (TE) and transverse magnetic (TM) mode hybridization in an x-cut LNOI ridge waveguide. Sudden jumps in the free-spectrum-range (FSR) of these modes in a fabricated microring resonator demonstrate the mode hybridization. The measured Q-factor of the lithium niobate (LN) microring is 1.78 million near the critical coupling condition. The hybridization wavelength was designed at 1562 nm and observed at 1537 nm. Potential applications include fundamental mode conversion, polarization rotation, polarization splitter, and polarization insensitive waveguides in optical receiver module.

High accuracy temperature sensing system exploiting a Sagnac interferometer and an optoelectronic oscillator

A temperature sensing system with high measurement accuracy based on a Sagnac interferometer (SI) and an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The polarization maintaining fiber (PMF) constituting the SI, is employed as the sensing unit such that the free spectrum range (FSR) of the SI changes when the PMF experiences temperature variations, which results in the frequency shifts of the oscillating signal. The frequency response of the OEO with respect to the temperature is investigated, and the sensitivity of the proposed temperature sensing system is measured to be −2660.6 kHz/°C. The stability of the OEO is also discussed, and the high measurement accuracy of 0.0173 °C implemented by a fiber ring resonator (FRR) is obtained. Furthermore, the combination of traditional SI and OEO allows for the high-speed interrogation of the proposed system and extends the applications of traditional fiber optic sensors.

Reconfigurable double C-shape metamaterial (DCM) for a terahertz resonator

We present a reconfigurable terahertz (THz) resonator using double C-shape metamaterials (DCM) that can be used as filter and single-/dual-resonance switch. By changing the position of the C-shape metamaterial along x-axis and y-axis directions, the resonances can be modulated from single-resonance to dual-resonance in TE mode and the corresponding free spectrum range (FSR) can be changed from 0.19 THz to 0.09 THz. These results indicate the proposed DCM can be used as a single-/dual-resonance switch and polarization switch. To increase the tunability, flexibility, and applicability of DCM, the resonant frequency could be tuned by changing the gap between DCM with the dual-layer. The resonances are blue-shifted 0.04 THz from 0.22 THz to 0.26 THz (1st resonance) and 0.11 THz from 0.36 THz to 0.47 THz (2nd resonance) in TE mode. The relationship of resonance and gap variation is quite stable and linear. This design of DCM provides a potential possibility of feasible opto-electronics applications in the THz frequency range.

Single Wavelength Resonator Based on $\pi$ Phase-Shifted Antisymmetric Bragg Grating

The principle, design, and characterization of a resonator on the silicon-on-insulator (SOI) platform based on the $\pi $ phase-shifted antisymmetric Bragg grating (ASBG) are presented. With the aid of an asymmetric directional coupler (ADC), the resonance circuit between the two ASBGs is formed by the forward first-order transverse electric (TE1) mode and the backward fundamental transverse electric (TE0) mode. Only negligible amount of the backward TE0 mode can be coupled to the bus waveguide, which is different from the conventional Fabry–Perot (F-P) resonator. Compared with the widely used microring resonator, it can realize single wavelength resonance. The single resonance resonator with an extinction ratio of 20.0 dB and a quality factor ( ${Q}$ -factor) of 9888 has been experimentally demonstrated. Moreover, multiple wavelength resonances with a free spectrum range (FSR) of 0.63 nm is obtained when the phase shifter length is $200~\mu \text{m}$ .

Actively electromagnetic modulation of IHI-shaped terahertz metamaterial with high-efficiency switching characteristic

We present terahertz (THz) metamaterial with multi-configurations, which are composed of H-shaped, II-shaped, and IHI-shaped configurations, respectively. By changing the geometrical dimension of H-shaped metamaterial, the electromagnetic response exhibits switch and bandwidth modulation characteristics between 0.20 THz and 0.30 THz. For II-shaped metamaterial, the electromagnetic response shows the single-resonance and dual-resonance switching and tunable free spectrum range (FSR) characteristics. The superimposed structures of H- and II-shaped metamaterials to form IHI-shaped metamaterial, which exhibits the combinations of H- and II-shaped metamaterials characterizations. The resonances could be switched at 0.25 THz, 0.55 THz, and 0.95 THz depending on the metamaterial pattern. To increase the flexibility of proposed device, there are composed of dual-layer IHI-shaped metamaterial on Si substrate. The FSR could be tuned from 0.16 THz to 0.05 THz by changing the height of top IHI-shaped metamaterial. These optical properties of IHI-shaped metamaterial open an avenue to be used in widespread applications with high portability, flexibility, and compatibility.

Microring Switching Control Using Plasmonic Ring Resonator Circuits for Super-Channel Use

The multi-wavelength selection and switching system using the hybrid plasmonic add-drop ring resonator (HPARR) for optical communication is proposed for multi-carrier super-channel-based designed. The plasmonic polariton technique applied in the ring resonator mode to the alternate waveguide interferometer switches the multi-wavelength laser emission in the various ranges. The combination of curvature-coupled plasmon ring and substances with different refractive index allows switching the multi-wavelength emission to shorter the free spectrum range (FSR) and specific wavelengths, without an applied pump signal or adjusted the ring size. It is suitable for the super-channel of wavelength division multiplex (WDM) in the future optical network.

Ultrasensitive Temperature Sensor Based on Fiber-Optic Fabry–Pérot Interferometer with Vernier Effect

We propose and demonstrate an ultrasensitive temperature sensor based on a fiber-optic Fabry–Perot interferometer (FPI) with the Vernier effect. The sensor is prepared by splicing a section of silica tube and single-mode fiber (SMF) to a section of SMF in sequence, which formed two-cascaded FPIs. Their superimposed spectrum can produce the Vernier effect and form the interference spectrum envelope due to a similar free spectrum range (FSR). The shift of the interference spectrum envelope is much larger than that of a single FPI, when the temperature changes. Experimental results show that the designed sensor can provide a high temperature sensitivity of 183.99 pm/°C, which is almost 220 times higher than that of a single air cavity (0.86 pm/°C) and about 20 times higher than that of a single silica cavity (9.14 pm/°C). The sensor designed has compact structure (< 1 mm) and high sensitivity, providing a prospect for successful applications.

MXene Ti3C2Tx: A Promising Photothermal Conversion Material and Application in All‐Optical Modulation and All‐Optical Information Loading

AbstractAll‐optical modulators, where one light modulates the parameters of another light (intensity, phase, frequency, etc.) without external electronic control, have drawn extraordinary attention in all‐optical information processing. In this contribution, by taking advantage of the two‐pass structure of Michelson interferometer and the strong photothermal conversion efficiency of MXene nanomaterials, an all‐optical modulator is demonstrated with a modulation depth of &gt;27 dB, a free spectrum range of 16.8 nm and a pump‐induced phase shift slope of 0.043 π mW−1. Moreover, the all‐optical modulator is successfully employed to switch on/off the intensity of the signal light and load information from one channel to another with comparable response time and baudrate. It is believed that the MXene‐based all‐optical modulator will open a door to practical applications of 2D materials‐based all‐optical devices in all‐optical signal processing.

Fast-Switching Microwave Photonic Filter Using an Integrated Spectrum Shaper

A fast-switching single passband microwave photonic finite impulse response filter (MPF) using an incoherent broadband optical source (BOS) and an integrated spectrum shaper is proposed and experimentally demonstrated. The key component of our proposed MPF is the silica-on-silicon integrated spectrum shaper, which is used to slice the spectrum of the BOS. The performance of the integrated spectrum shaper is measured in terms of time delay, power consistency, and switching speed. A number of spectral shapes with the free spectrum range from 1.88 to 0.058 nm can be flexibly switched by changing the driving voltage of the optical switches. With the dispersion compensating fiber as dispersion medium, a fast-switching MPF with the switching speed up to 1.4-KHz is demonstrated in a wide tunable range from 1 to 12 GHz and the sidelobe suppression is up to 35 dB, which shows a great agreement with the theoretical result.

Tunable terahertz metamaterial by using three-dimensional double split-ring resonators

We present an optically tunable terahertz (THz) metamaterial by using three-dimensional double split-ring resonators (3D-DSRR), which shows superior properties in adjustability. They exhibit tunable single-band and dual-band resonances by changing height between inner and outer SRRs. The maximum tuning range of dual-band resonance is 0.55 THz from the resonance of 0.45 THz shifted to the resonance of 1.00 THz. The corresponding tuning range of free spectrum range (FSR) spanned two resonances can be tuned from 0.20 THz to 0.75 THz. By changing radius of inner SRR from 14.5 μm to 22.5 μm, the tuning range of resonance is 0.40 THz. While changing the height between inner and outer SRRs and kept the radiuses of inner and outer SRRs as constant as 22.5 μm, 3D-DSRR can be tuned to have two resonances and then merge into one resonance. 3D-DSRR shows a tunable and switchable resonance for single-band and dual-band characteristics. These characterizations are very suitable for the use of tunable multi-resonance applications. To further investigate the stability of 3D-DSRR, the rotation angles of inner and outer SRRs are modified from 0° to 180°, respectively. The corresponding resonances are identical. Such results indicate the proposed device with high stability to allow manufacturing tolerance and potentially to be used in wearable electronic devices, pressure sensors, and other fields.

Ultra-high sensitivity optical sensors based on cascaded two Fabry-Perot interferometers

In this paper, a novel high-sensitivity optical sensor based on cascaded two Fabry-Perot interferometers (FPIs) has been investigated theoretically and experimentally. In order to improve the sensitivity, the two FP interferometers were designed with slightly different free spectrum ranges (FSRs) to produce Vernier effect. The experimental results showed that the sensitivity of the sensor is 23,794.6 nm/RIU for the wavelength interrogation and 4602.3 dB/RIU for the intensity interrogation, much higher than that of the conventional sensors for the bulk refractive index (RI) sensing. Finally, the reaction between Streptomycin monoclonal antibody and Streptomycin-BSA was measured, showing a great potential for low-cost and portable systems in biochemical sensing.

Double-efficiency photonic channelization enabling optical carrier power suppression.

A novel double-efficiency photonic channelization scheme with optical carrier power suppression (OCS) is proposed for the first time, to the best of our knowledge. In this scheme, a tunable optical frequency comb generator is used to efficiently generate radio-frequency (RF) carriers, and a Fabry-Perot filter (FPF) is introduced to split a broadband signal into multiple narrowband signals. With the well-designed frequency of RF carriers, the wavelength spacing of optical carriers, and the free spectrum range of the FPF, double-channelized efficiency can be obtained. A proof-of-concept system is demonstrated to verify the feasibility of this double-efficiency channelization scheme, in which a 5GHz baseband signal is channelized into five 1GHz sub-channels, and about 35dB OCS is obtained. Moreover, the performance of the double-efficiency channelization scheme is analyzed based on a 5Gbit/s baseband signal with an on-off keying (OOK) format in which the influence of the 3rd order term interference is discussed.

Energy-Efficient Dynamic Lightpath Adjustment in a Decomposed AWGR-Based Passive WDM Fronthaul

To cope with the explosion of mobile traffic demand, cloud radio access networks (C-RAN) have been recently proposed. In C-RANs, the two main elements of traditional base stations (BSs), i.e., the baseband unit (BBU) and the remote radio head (RRH), are separated, and the BBU can be virtualized and located in a remote location together with BBUs of other BSs. High-bandwidth digitized baseband signals must be exchanged between BBUs and RRHs through a large-capacity fronthaul network. Wavelength division multiplexing (WDM) technology is an efficient solution to provide sufficient fronthaul bandwidth with a large number of optical transceivers. With the fixed connections between BBUs and RRHs in the traditional C-RAN, the utilization of optical transceivers as well as BBUs is inefficient. In this paper, we propose a decomposed arrayed waveguide grating router (AWGR)-enabled passive WDM fronthaul, and we formulate the minimal energy consumption problem as integer linear programming, with the objective of minimizing the energy consumption caused by optical transceivers and BBUs, etc. We also design two dynamic lightpath adjustment heuristics by light-path reconfiguration to improve energy efficiency. In addition, we take AWGR decomposition into consideration to reduce the tuning range of tunable transceivers and free spectrum range of AWGR, hence lowering the deployment cost of transport network components. Numerical evaluation shows that our proposed schemes achieve significant energy savings compared to fixed lightpath allocation.

Observation of Wavelength Tuning and Bound States in Fiber Lasers

We report an experimental observation of wavelength tuning and bound states in fiber lasers. A Mach-Zehnder interferometer (MZI) is adopted as an intra-cavity tunable filter to realize large-scale wavelength tuning and bandwidth controlling. By finely manipulating the MZI and intra-cavity polarization state, continuous wavelength-tunable operation from 1550.7 nm to 1580.8 nm is achieved. Meanwhile, the spectral bandwidth varying from 1.85 nm to 3.41 nm is also controlled by broadening the free spectrum range (FSR) of the MZI. Additionally, with modest polarization adjustment, both tightly and loosely bound states are experimentally observed, which can be validated by the numerical simulations. The results indicate that the proposed fiber laser is attractive for telecommunication systems, on account that the tuning feature can be applied to wavelength-division multiplexer (WDM) and the various soliton bound states could contribute to the high-level modulation format.

Design and demonstration of ultra-high-Q silicon microring resonator based on a multi-mode ridge waveguide

We present the design and experimental demonstration of the ultra-high-Q-factor silicon microring resonator based on a multi-mode ridge waveguide. The multi-mode ridge waveguide is designed to decrease the propagation loss and to improve the Q factor. The ultra-high Q factor of 1.1×106 is experimentally demonstrated, with the free spectrum range of 0.208nm. The single-mode ridge waveguide is used in the coupling region to reduce the dimension of the microring resonator, and the bend radius is only 20μm. To precisely control the resonance wavelength, a small heater is implemented on the silicon microring resonator with the tuning efficiency of 7.1 pm/mW. The degenerate four-wave mixing of the silicon microring resonator is investigated, and the conversion efficiency is measured to be -15.5 dB without optimizing the dispersion of the microring resonator and carriers extraction.

Research on a Micro-Nano Si/SiGe/Si Double Heterojunction Electro-Optic Modulation Structure

The electro-optic modulator is a very important device in silicon photonics, which is responsible for the conversion of optical signals and electrical signals. For the electro-optic modulator, the carrier density of waveguide region is one of the key parameters. The traditional method of increasing carrier density is to increase the external modulation voltage, but this way will increase the modulation loss and also is not conducive to photonics integration. This paper presents a micro-nano Si/SiGe/Si double heterojunction electro-optic modulation structure. Based on the band theory of single heterojunction, the barrier heights are quantitatively calculated, and the carrier concentrations of heterojunction barrier are analyzed. The band and carrier injection characteristics of the double heterostructure structure are simulated, respectively, and the correctness of the theoretical analysis is demonstrated. The micro-nano Si/SiGe/Si double heterojunction electro-optic modulation is designed and tested, and comparison of testing results between the micro-nano Si/SiGe/Si double heterojunction micro-ring electro-optic modulation and the micro-nano Silicon-On-Insulator (SOI) micro-ring electro-optic modulation, Free Spectrum Range, 3 dB Bandwidth, Q value, extinction ratio, and other parameters of the micro-nano Si/SiGe/Si double heterojunction micro-ring electro-optic modulation are better than others, and the modulation voltage and the modulation loss are lower.

Highly-sensitive optical biosensor based on equal FSR cascaded microring resonator with intensity interrogation for detection of progesterone molecules

We report a refractive index sensing system with a high sensitivity of 1579 dB/RIU and a low refractive index detection limit of 9.7 × 10−6 RIU. The sensing system was based on cascaded-microring resonators (CMRR) using intensity interrogation. The free spectrum range (FSR) for the reference ring was designed to be equal to the sensing ring. The sensing ring of the CMRR was modified with molecularly imprinted polymers (MIPs) for the detection of progesterone. The achieved limit of detection (LOD) was as low as 83.5 fg/mL.

Utilizing wheel-ring architecture for stable and selectable single-longitudinal-mode erbium fiber laser

To achieve a steady single-longitudinal-mode (SLM) erbium-doped fiber (EDF) laser, the wheel-ring architecture is proposed in the laser cavity. According to Vernier effect, the proposed wheel-ring can produce three different free spectrum ranges (FSRs) to serve as the mode-filter for suppressing the densely multi-longitudinal-mode (MLM). Here, to complete wavelength-tunable EDF laser, an optical tunable bandpass filter (OTBF) is utilized inside the cavity for tuning arbitrarily. In addition, the entire output performances of the proposed EDF wheel-ring laser are also discussed and analyzed experimentally.

Using multi-ring structure for suppression of mode competition in stable single-longitudinal-mode erbium fiber laser

In this demonstration, a stable and tunable single-longitudinal-mode (SLM) erbium-doped fiber (EDF) laser with multiple-ring configuration is proposed and investigated. The proposed compound-ring structure can create different free spectrum ranges (FSRs) to result in the mode-filter effect based on the Vernier effect for suppressing the other modes. Additionally, the output stabilization of power and wavelength in the proposed EDF multiple-ring laser are also discussed.

Magnetically tunable high-Q spin-wave optoelectronic single-loop resonator

Resonant properties of a spin-wave optoelectronic single-loop resonator are experimentally investigated. The resonator is composed of a spin-wave delay line and a microwave photonic delay line based on a single-mode optical fiber. This circuit is equivalent to a comb filter. The microwave photonic delay line provides the main contribution to the delay time and defines mostly the free spectra range (FSR). The spin-wave delay line realizes the reconfiguration of resonator bandwidth and its frequency tuning. Influences of delay time and amplifier gain coefficient on quality-factor are analyzed.