Free Spectrum Range Research Articles

Tunable Terahertz Free Spectra Range Using Electric Split-Ring Metamaterial

A design of electric split-ring metamaterial (eSRM) with tunable terahertz (THz) free spectra range (FSR) characteristic is presented. The proposed eSRM is composed of double semicircle-shaped ring resonators with two connected metallic bars. By changing the radius of the semicircle-shaped ring resonators, the tuning ranges of FSR are 0.187 THz and 0.024 THz in transverse electric (TE) and transverse magnetic (TM) modes, respectively, which indicates eSRM could operate stably in TE and TM modes. Furthermore, eSRM exhibits switching and polarization-dependent characteristics by exploiting comb driver to control the distances between ring resonators along ${x}$ -and ${y}$ -axis directions. The electromagnetic responses could be tuned and switched between dual- and quad-resonance in both TE and TM modes. eSRM shows polarization angle-sensitive characteristic and the corresponding FSR could be tuned from 919 GHz to 336 GHz by changing the polarization angle of incident THz wave. In view of the above optical characteristics of eSRM, it is used for the sensing application exposed on different surrounding refraction index. The sensitivity, quality factor (Q-factor), and figure of merit (FOM) are calculated. The sensitivity is 0.457 THz/RIU and the average Q-factor and FOM are 40.65 and 35.47, respectively. These results indicate the proposed eSRM could be used in widespread applications, such as dual-/quad-resonance switches, polarization switches, and high-efficient environmental sensors. [2021-0006]

Design and optimization of a microwave photonic filter exploiting differential mode group delay of a multi-mode fiber

A reconfigurable and application-specific tunable microwave photonic filter having finite impulse response is proposed. The filter exploits the differential mode group delays of linearly polarized modes in a multi-mode fiber to achieve single and multiple high-frequency passband response. The filter taps are realized using a single continuous wave laser source and differential delays among modes that are propagated through a single spool of graded index multi-mode fiber. Depending upon the application requirement, different values of free spectral range of the microwave photonic filter are obtained by employing fiber spools of different lengths. The reconfigurability of the microwave photonic filter is achieved by varying the number of linearly polarized modes. It has been shown that the passband ripple can be reduced by employing higher order spatial modes and unequal power ratio among different modes. The proposed filter enables application-specific free spectra range tunability and reconfigurability along with added advantages of easy implementation and passband optimization.

Dual Demodulation of Temperature and Refractive Index Using Ring Core Fiber Based Mach-Zehnder Interferometer.

We report the ring core fiber spliced with single mode fiber and no core fiber which is used for temperature and refractive index (RI) sensing. The Mach-Zehnder interferometer (MZI) is formed with this kind of sandwich fiber structure and the maximum extinction ratio of the interference spectra reaches 27 dB with the free spectra range of 12 nm. The MZI fiber sensor is applied for temperature sensing with the sensitivity of 69 pm/°C and 0.051 dB/°C. The RI sensitivity reaches 182.07 dB/RIU and −31.44 nm/RIU with the RI ranging from 1.33 to 1.38. The RI value can be directly demodulated with the interference dip intensity which shows insensitivity to temperature. The demodulation of temperature can be achieved by using the linear equations between dip wavelength shift with the variation of temperature and RI.

Cascaded Sagnac Loops Embedded With Two Polarization Maintaining Photonic Crystal Fibers for Highly Sensitive Strain Measurement

A strain sensor based on cascaded Sagnac loops with two polarization-maintaining photonic crystal fibers (PMPCFs) has been experimentally demonstrated. Air holes distributed on the cross section of PMPCF go through the whole fiber, which makes the special fiber more sensitive to ambient force compared with conventional optical fiber. The lengths of the two PMPCFs are close, but not the same, to form envelopes on the total output spectrum and then obtain high resolution to the detected parameter by the Vernier effect. The influences of the difference in length of the two PMPCFS on spectral envelopes are discussed. As the length of the PMPCF in reference loop increases, free spectrum range of the cascaded Sagnac loops (or envelope period) and magnification decrease. The sensitivities of this strain sensor are calculated by tracking upper and lower envelopes. Results reveal that its average sensitivity is up to 45.15 pm/με and the corresponding resolution is 0.44 με as strain varies from 0 to 2000 με. Compared with a single Sagnac loop, the sensitivity magnification is about 28.94. The sensing characteristics of a conventional polarization-maintaining fiber (PMF) are studied to compare with the PMPCF. Moreover, reversibility has also been proved to possess low error rate. In short, the fiber strain sensor possesses high sensitivity and great reversibility.

A novel microring resonator based on multi-Mach–Zehnder interferometers

A microring resonator (MRR) with an extended free spectrum range (FSR) based on multi-Mach–Zehnder interferometers (MZI) is proposed and the simulation is demonstrated. In the MRR, two straight bus waveguides are replaced by multi-MZIs to extend the FSR. The transfer matrix method and coupled-mode theory are adopted to derive the mathematic model of the multi-MZIs based MRR. A numerical simulation is carried out by the finite-difference time domain (FDTD) method. The simulation results show that the resonance peaks are suppressed at certain intervals due to multi-MZIs, an octuple quasi-FSR is obtained by a tri-MZIs based MRR. The key advantage of the proposed multi-MZIs based MRR is that the extended FSR is realized without having to add complicated auxiliary components or increasing the number of ring resonators.

Broadband Coupler Manipulation Through Particle Swarm Optimization for Arrayed Waveguide Grating Based Optical Coherence Tomography

The state-of-the-art optical coherence technology (OCT) has its limitation, such as bulky and quite expensive. Miniaturization and integration of the OCT system are promising to get a compact, less costly, and more stable device. In this paper, we report the influence of optical power splitting on the sensitivity and axial resolution in spectral-domain OCT (SD-OCT) on silicon chip using tandem Mach-Zehnder directional coupler (MZDC) based broadband coupler through particle swarm optimization (PSO). A less-flat wavelength response from a broadband coupler in an interferometer for SD-OCT on-chip will decrease the sensing sensitivity. The arrayed waveguide grating (AWG) as a spectrometer in SD-OCT typically suffers from the axial resolution degradation due to the non-flat free spectrum range (FSR). To avoid the higher optical loss from the large FSR, the PSO manipulation in MZDC is then proposed to demonstrate U-shaped wavelength compensation on the non-flat AWG spectrum. With the broadband coupler configuration, a SD-OCT on-chip can maintain the sensitivity and axial resolution performance.

Multiwavelength Mode-Locked Fiber Laser Based on an All Fiber Lyot Filter

We report the experimental demonstration of a multiwavelength passively mode-locked fiber laser based on an all-fiber Lyot filter (AFLF). The Lyot filter is composed of two 45°-titled fiber gratings (45°-TFGs) and a length of polarization-maintaining fiber (PMF). The free spectrum range (FSR) and the 3-dB bandwidth of the Lyot filter are 7.5 nm and 3.6 nm, respectively. The laser generates 5-wavelength mode-locked pulses, which have 7.49 nm channel spacing, 192.6 kHz fundamental repetition rate and over 50 dB signal-to-noise ratio (SNR). As increasing the pump power from 178 mW to 407 mW, the pulse width and the average output power increased linearly from 10 ns to 23 ns and from $256\mu \text{W}$ to $621\mu \text{W}$ , respectively. In addition, the fluctuations of center wavelength and laser power are < 0.5 nm and < 0.5 dB respectively within a duration of 1 hour.

Microwave Photonic Interrogation of a High-Speed and High-Resolution Temperature Sensor Based on Cascaded Fiber-Optic Sagnac Loops

A high-speed and high-resolution temperature sensor based on two cascaded fiber-optic Sagnac loops (FSLs) interrogated using a microwave photonic (MWP) method is proposed and experimentally demonstrated. In the proposed system, an FSL is used as a sensor element. When it is experiencing temperate change, its spectrum is shifted, which leads to the shift in the spectrum of the cascaded FSLs. Instead of measuring the wavelength shift of the spectrum by using an optical spectrum analyzer (OSA), which has a low wavelength resolution and a slow scanning speed, we convert the optical spectrum to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping, and use a digital signal processor (DSP) to extract the sensing information. In our experiment, two cascaded FSLs with two different free spectrum ranges (FSRs) are employed. Due to the Vernier effect, multiple dips in the envelope of the optical spectrum are introduced. By passing a broadband frequency-chirped optical pulse generated by a frequency-swept laser source to the two cascaded FSLs, the sensing information is encoded in the optical spectrum and it is converted to the time domain due to SS-WTT mapping. By detecting the optical waveform at a photodetector (PD), a temporal microwave waveform with its shape identical to the optical spectrum is generated. By using a DSP, the time shift of a dip is measured and the sensing information is precisely demodulated. Experimental result shows that the proposed fiber-optic temperature sensor can provide a temperature resolution of 1.33 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> °C at a sensing speed of 23.497 kHz.

High resolution relatively short fiber length measurement system based on microwave photonic link

Abstract A high resolution relatively short fiber length measurement system is proposed, which based on the phase modulation microwave photonic link. It is based on analysis of the free spectrum range (FSR) of the microwave photonic link with high resolution vector network analyzer. Compared with the traditional method, there's no need costly and high-performance laser for the system, no dead zone and can improve the precision millimeter. The smallest relative length of fiber is 3 mm with the existed high resolution vector network analyzer, and the error can be controlled within 0.05 mm when the relative fiber length is 100 m.

A High-Performance Microwave Photonic Phase Shifter Based on Cascaded Silicon Nitride Microrings

A microwave photonic phase shifter based on four cascaded tunable silicon nitride microring resonators is proposed. The phase shifts and RF power variations of the microwave photonic phase shifter were investigated theoretically and experimentally. Good overall performances with full phase tuning range of 400°, low RF power variations of ± 1.5 dB with a broad bandwidth of 11 GHz, fiber to fiber insertion loss about 5.2 dB were achieved. The maximum frequency can be extended to about 40 GHz, which is limited by the MRR’s free spectra range.

A High-Strength Strain Sensor Based on a Reshaped Micro-Air-Cavity.

We demonstrate a high-strength strain sensor based on a micro-air-cavity reshaped through repeating arc discharge. The strain sensor has a micro-scale cavity, approximate plane reflection, and large wall thickness, contributing to a broad free spectrum range ~36 nm at 1555 nm, high fringe contrast ~38 dB, and super-high mechanical robustness, respectively. A sensitivity of ~2.39 pm/με and a large measurement range of 0 to 9800 με are achieved for this strain sensor. The strain sensor has a high strength, e.g., the tensile strain applied the sensor is up to 10,000 με until the tested the single-mode fiber is broken into two sections. In addition, it exhibited low thermal sensitivity of less than 1.0 pm/°C reducing the cross-sensitivity between tensile strain and temperature.

Tunable Terahertz Metamaterial Using an Electric Split-Ring Resonator with Polarization-Sensitive Characteristic

We present a tunable terahertz (THz) metamaterial using an electric split-ring resonator (eSRR), which exhibits polarization-sensitive characteristics. The proposed eSRR is composed of double symmetrical semicircles and two central metal bars. By changing the lengths of two metal bars, the electromagnetic responses can be tuned and switched between dual-band and triple-band resonances in transverse magnetic (TM) mode. Furthermore, by moving the bottom metal bar to change the gap between the two metal bars, the first resonance is stable at 0.39 THz, and the second resonance is gradually blue-shifted from 0.83 to 1.33 THz. The tuning range is 0.50 THz. This means that the free spectrum ranges (FSR) could be broadened by 0.50 THz. This proposed device exhibits a dual-/triple-band switch, tunable filter, tunable FSR and polarization-dependent characteristics. It provides an effective approach to perform tunable polarizer, sensor, switch, filter and other optoelectronics in THz-wave applications.

Graphene Quantum Dots Assisted Optically Tunable In-Core Mach-Zehnder Interferometer

An optically tunable in-core Mach-Zehnder interferometer (MZI) based on the photoexcitation dynamics of nitrogen-doped graphene quantum dots (NGQDs) is demonstrated. The tunable MZI is fabricated by depositing NGQDs into a femtosecond laser ablated microgroove in the core. Combining both the high sensitivity to the external refractive index change of the MZI itself and the quantum confinement effect in quantum dots, a tuning range of ~90 nm is achieved as the pump power density of an ultraviolet (UV) LED varies from 0 to 41.1 mW/cm2, which accounts for 85% of the free spectrum range (FSR) of the MZI with a length of $205~\mu \text{m}$ . Such an optically tunable MZI with high integration, tuning efficiency and wide FSR can be suitably used in edge filter demodulation and tunable laser.

Highly sensitive temperature sensor based on cascaded HiBi-FLMs with the Vernier effect

We propose and experimentally demonstrate a highly sensitive temperature sensor based on cascaded high birefringence fiber loop mirrors (HiBi-FLMs) with the Vernier effect. The two HiBi-FLMs have almost the same free spectrum range (FSR), which can be regarded as two scales of different periods of an optical Vernier-scale, and act as the fixed part and the sliding part, respectively. A Gauss fitting algorithm is introduced to fit the Vernier spectrum envelope to accurately trace the spectral shift of the envelope. The temperature sensitivity and temperature resolution of the cascaded configuration are much larger than those of the individual HiBi-FLMs due to the Vernier effect. The experimental result shows that the temperature sensitivity of the proposed sensor can be improved from − 1.723 n m / ∘ C (single HiBi-FLM) to − 43 n m / ∘ C (cascaded configuration) by employing the Vernier effect, and the temperature resolution is enhanced from ± 0.029 ∘ C to ± 0.001162 ∘ C . The amplification factors for temperature sensitivity and temperature resolution are both 24.96, which shows good agreement with the theoretical predictions. The proposed sensor with huge temperature sensitivity, high resolution, and simple configuration may be a promising candidate for some applications that need precise temperature control.

Label Free All-Fiber Static Pressure Sensor Based on Vernier Effect With Temperature Compensation

This paper reported a label free, all-fiber static pressure sensor constructed by cascading two single mode fiber(SMF) – few-mode fiber(FMF) — single mode fiber(SFS) critical wavelength structures. Vernier effect was adopted to magnify the static pressure wavelength sensitivity of the interference peaks and the superimposed transmission spectrum has an envelope which the free spectrum range varies with wavelength and reaches maximum when approaching the envelope critical wavelength (CWLE). Thus, the envelope peaks near the CWLE in the transmission spectrum with the modified Vernier effect is label free, which is easy to recognize and entirely different from the envelope peaks in a transmission spectrum with uniform free spectrum range. Both theoretical and experimental results show that the envelope peaks near the CWLE exhibit different static pressure and temperature wavelength sensitivities, and the wavelength sensitivity increase significantly when the envelope peak approaching the CWLE. The experimental results show that the left envelope peak 1 and the left envelope peak 2, which numbered from the peak on the left side and closest to the CWLE, have different static pressure sensitivities of 4.072 nm/MPa and 2.649 nm/MPa, respectively, and temperature sensitivities of 1.753 nm/°C and 1.045 nm/°C, respectively. The performances of this proposed approach can be further improved by optimizing the physical lengths of the FMF employed in the sensing and reference SFS structures, to satisfy the requirements in practical applications, especially the extreme conditions and two-parameter measurement of ultra-high pressure and high temperature.

Tunable multi-resonance of terahertz metamaterial using split-disk resonators

We present three tunable multi-resonance of terahertz (THz) metamaterials. They are composed of single-, dual-, and triple-split-disk resonators (SDRs) on Si substrates, which are denoted as SDR-1, SDR-2, and SDR-3, respectively. They exhibit extraordinary electromagnetic characteristics. SDR-1 exhibits polarization-dependence owing to the asymmetrical SDR structure. To increase the flexibility and applicability of SDR configuration, SDR-2 and SDR-3 are presented to modify the distances between the SDR layers. By moving the top SDR layer of SDR-2, a controllable resonance with a 0.32 THz shifting and tunable free spectrum range (FSR) of 0.15 THz at transverse magnetic mode is achieved, while an electromagnetically induced transparency-like effect appears at the transverse electric mode. The spectral bandwidth of SDR-3 can be tuned to 0.10 THz, and the resonant intensity becomes controllable by moving the middle SDR layer of SDR-3. Furthermore, by moving the top SDR layer of SDR-3, the tuning ranges of resonance, FSR, and bandwidth of SDR-3 are 0.23 THz, 0.20 THz, and 0.08 THz, respectively. Such designs of SDR configurations provide a high-efficient THz resonator in the THz-wave applications such as filters, switches, polarizers, sensors, imaging, and so on.

Actively tunable terahertz metamaterial with single-band and dual-band switching characteristic

We present a design of tunable terahertz (THz) metamaterial (TTM) with single-band and dual-band filtering and switching characteristics. The proposed device is composed of face-to-face split-ring resonator (SRR) and a central bar. By moving the SRRs to change the gap between SRRs and central bar, the free spectrum ranges (FSR) could be bi-directionally broadened 0.14 THz and narrowed 0.19 THz at TE and TM modes, respectively. To control the coplanar moving central bar, the electromagnetic response of TTM device exhibits polarization-dependent characteristic. TTM shows the switching characteristic between single-band to dual-band resonance at TE mode and exhibits resonance-insensitive to the displacement of central bar at TM mode. These results open an avenue to be potentially used for detector, sensor, and switch in the THz-wave applications.

An Ultra-Wideband Microwave Photonic Channelizer Based on Coherent Optical Frequency Combs and Image-Reject Down-Conversion

Radio frequency (RF) channelization receiver is one of the most effective methods for processing ultra-wideband RF signals. In this paper, an ultra-wideband microwave photonic channelizer based on coherent optical frequency combs (OFCs) and image-reject down-converter is proposed and demonstrated. The OFCs used in the system are generated by two dual parallel Mach–Zehnder modulators (DPMZM), and the free spectrum range (FSR) is flexibly adjustable. The image-reject down-converter can not only effectively achieve image separation but also suppress the seven-order distortion and DC offset using balanced detection. A K- and Ka-band RF signal covering the frequency band from 25 to 30 GHz is sliced into 5 sub-channels with 1-GHz bandwidth, two-tone signals with frequencies of 27.5 and 27.51GHz are used to measure the spurious free dynamic range (SFDR) which is 116.8 dB $\cdot $ Hz2/3, the image-rejection is over 66.4 dB, and the error vector magnitude (EVM) is 4.2%.

Tunable terahertz metamaterial for high-efficiency switch application

We present two types of tunable terahertz (THz) metamaterials (TTM-1 and TTM-2) to explore their extraordinary optical properties. The proposed TTMs are composed of Au layers with 300 nm in thickness on Si substrates. The designs of TTMs exhibit superior properties in adjustability for high-efficiency THz switching characteristic. By changing the geometrical dimensions of TTMs, the corresponding electromagnetic responses could be tuned and switched between single-band and dual-band resonances. TTM-1 exhibits three switching windows with higher switching ratios by embedding different materials into the cavity underneath the complementary metamaterial. TTM-2 can be tuned to have two resonances and then merge into one resonance by increasing the height between the inner and outer rings. The transmission intensity of TTM-2 can be tuned from 0 to 0.7 at 0.57 THz by changing the sizes of inner and outer rings. TTM-2 exhibits tunable filter, single-/dual-band switch, tunable free spectrum range, and tunable bandwidth characteristics by varying the radius of the inner and outer rings. This study paves a way to the possibility of tunable high-efficiency switch, filter, polarizer, and other THz applications.

Temperature sensor of high-sensitivity based on nested ring resonator by Vernier effect

A temperature sensor based on nested ring resonator (NRR) by Vernier effect is proposed and theoretically demonstrated with larger sensitivity. This configuration, on a standard silicon-on-insulator (SOI) chip, consists of a single ring resonator acting as the reference, and a NRR acting as the sensor, which can simultaneously achieve higher quality factor (Q) and larger free spectrum range (FSR) compared with single ring. By adjusting parameters of the NRR, the sensitivity of the whole structure can be 10.611 nm/K, two orders of magnitude larger than that of the single NRR.