Sagnac Loop Reflectors Research Articles

Fully tunable Fabry-Pérot cavity based on MEMS Sagnac loop reflector with ultra-low static power consumption

The Fabry-Pérot interferometer, a fundamental component in optoelectronic systems, offers interesting applications such as sensors, lasers, and filters. In this work, we show a tunable Fabry-Pérot cavity consisting of tunable Sagnac loop reflectors (SLRs) and phase shifters based on electrostatic microelectromechanical (MEMS) actuator. The fabrication process of the device is compatible with the standard wafer-level silicon photonics fabrication processes. This electrostatic actuation mechanism provides well-balanced, scalable pathways for efficient tuning methodologies. The extinction ratio of the continuously tunable SLRs’ reflectivity is larger than 20 dB. Full 2π phase shifting is achieved, and response times of all the components are less than 25 μs. Both actuators have extremely low static power, measuring under 20 fW and the energy needed for tuning is both below 20 pJ.

A high-power narrow-linewidth microlaser based on active-passive lithium niobate photonic integration

We demonstrate an on-chip integrated Fabry-Perot (FP) resonator laser with high output power and narrow linewidth. The laser chip is fabricated on monolithically integrated active (i.e., Er3+-doped) and passive (i.e., non-doped) thin film lithium niobate (TFLN) substrate in which the laser cavity is constructed with two Sagnac loop reflectors fabricated on the passive TFLN. The monolithically integrated FP resonator laser achieves an output power of 126.35 μW and a linewidth of 47.5 kHz.

Integrated Electro‐Optically Tunable Narrow‐Linewidth III–V Laser

Herein, an integrated electro‐optically tunable narrow‐linewidth III–V laser with an output power of 738.8 μW and an intrinsic linewidth of 45.55 kHz at the C band is demonstrated. The laser cavity is constructed using a fiber Bragg grating (FBG) and a tunable Sagnac loop reflector (TSLR) fabricated on thin‐film lithium niobate. The combination of the FBG and the electro‐optically tunable TSLR offers the advantages of single spatial mode, single frequency, narrow linewidth, and wide wavelength tunability for the electrically pumped hybrid integrated laser, which features a frequency tuning range of 20 GHz and a tuning efficiency of 0.8 GHz V−1.

On-chip electro-optically tunable Fabry-Perot cavity laser on erbium doped thin film lithium niobate

We demonstrate an on-chip electro-optically tunable Fabry-Perot (FP) cavity laser on Er3+-doped thin film lithium niobate (Er: TFLN). The FP cavity consists of two Sagnac loop reflectors at the two ends with a loaded quality factor of 1.3 × 105 and a free spectral range of 68 pm. The fabricated Er: TFLN FP laser structure is integrated with microelectrodes designed for electro-optically tuning, and a continuous laser wavelength tuning with 24 pm around 1544 nm is achieved by applying a driving voltage from −6 V to 6 V.

On-chip single-mode thin-film lithium niobate Fabry-Perot resonator laser based on Sagnac loop reflectors.

We demonstrate an on-chip single-mode Er3+-doped thin-film lithium niobate (Er:TFLN) laser which consists of a Fabry-Perot (FP) resonator based on Sagnac loop reflectors (SLRs). The fabricated Er:TFLN laser has a footprint of 6.5 mm × 1.5 mm with a loaded quality (Q) factor of 1.6 × 105 and a free spectral range (FSR) of 63 pm. We generate the single-mode laser at 1544 nm wavelength with a maximum output power of 44.7 µW and a slope efficiency of 0.18%.

Reconfigurable optical filter based on microring resonator assisted by tunable Sagnac reflector

To meet the demands for various applications in optical filtering and microwave signal processing, integrated silicon photonic filters are required to be multifunctional, reconfigurable and tunable. In this work, an integrated multi-functional optical filter is proposed, which is designed based on a tunable Sagnac loop reflector and a microring resonator. The through port and drop port of an add-drop microring resonator are connected with the two ports of a tunable reflector. By controlling the thermal phase shifters in different scenarios, the device can be reconfigured into a reflective-type asymmetric Mach-Zehnder interferometer filter, a reflective-type all-pass microring resonator filter and self-interference microring resonator filters. An analytical model is established based on the transfer matrix. The simulation results show that the device can achieve the following functions: sinusoidal spectral filtering with four different free spectral ranges, Lorentzian spectral filtering toggling between band pass and band stop, and spectral reconfigurations of Fano resonance, electromagnetically induced transparency, and electromagnetically induced absorption. Each spectrum mentioned above can be tuned fast and widely. Reflection provides a new degree of freedom in design, breaks through the inherent footprint limit, and achieves a wide range of free spectral ranges. Our proposed tunable Sagnac loop reflector assisted microring resonator provides a new scheme for realizing flexible, tunable and multi-functional reconfigurable integrated photonic filters, and has broad applications in the integrated photonic analog signal processing and microwave photonics.

Hybrid integrated external cavity laser with a 172-nm tuning range

Chip-scale lasers with a wide tunable range and a narrow linewidth have rich applications in optical communications, sensing, and light detection and ranging systems. External cavity lasers (ECLs) have the advantage of a wide tuning range and a narrow linewidth compared with their counterparts such as distributed feedback lasers and distributed Bragg reflector lasers because the passive filter separated from the gain section can be widely tuned and the low-loss external cavity extends the photon lifetime. However, so far, the tuning range achieved by the chip-scale ECLs is in the range of 80–120 nm, smaller than that of the bulk free-space ECL (typically >160 nm). The bottleneck in reaching an ultrabroad tuning range for chip-scale ECLs lies in the unsuppressed side modes of the external cavity. Here, we demonstrate a hybrid-integrated ECL consisting of a broadband III–V reflective semiconductor optical amplifier and a low-loss Si3N4 wavelength-selective reflector. A record 172-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio is achieved beyond the free spectral range of the Vernier ring filter due to the enhanced lasing mode selection from a tunable Sagnac loop reflector. The Si3N4 platform enables a low-loss external cavity, facilitating laser linewidth reduction below 4 kHz over the full tuning range. The on-chip output power reaches 26.7 mW at a wavelength of 1550 nm.

Integrated Coherent Tunable Laser (ICTL) With Ultra-Wideband Wavelength Tuning and Sub-100 Hz Lorentzian Linewidth

This paper describes the design, fabrication, and record performance of a new class of ultra-wideband wavelength tuning, ultra-low noise semiconductor laser, the Integrated Coherent Tunable Laser (ICTL). The ICTL device is designed for, and fabricated in, a CMOS foundry based Silicon Photonics platform, utilizing heterogeneous integration of III-V material to create the integrated gain section of the laser–enabling high-volume mass-market manufacturing at low cost and with high reliability. The ICTL incorporates three or more ultra-low loss micro-ring resonators, with large ring size, in a Sagnac loop reflector geometry, creating exceptional laser reflector performance, plus an extended laser cavity length that enables highly-coherent output; ultra-low linewidth and phase noise. This paper describes record integrated laser performance; 118 nm wavelength tuning, covering S-, C- and L-bands, with Lorentzian linewidth <100 Hz, and with excellent relative intensity noise (RIN) of ≤ −155 dBc/Hz. The remarkable performance of the ICTL device, coupled with the high volume/low cost capability of the Silicon Photonics platform enables next-generation applications including ultra-wideband WDM transmission systems, fiber-optic and medical-wearable sensing systems, and automotive FMCW LiDAR systems utilizing wavelength scanning.

Third-Order-Reflective Hybrid Interleaver Based on Fabry-Perot Assisted Michelson Interferometer

We present the design and experimental demonstration of a 3rd-order-reflective hybrid interleaver based on Fabry-Perot (FP) cavity assisted Michelson interferometer (FPA-MI), in which the FP cavity consists of two cascade Sagnac loop reflectors (SLRs). Through comparison, it can be seen that the flatness and rectangularity of interleaver are increased with the increase of order. When the transmission coefficient of the variable SLR is 0.1975 or 0.98, the performance of the interleaver is the best under the lossless condition. The influence of loss and transmission coefficient on the flatness of the interleaver can be evaluated by the dispersion model when the loss is considered, and when the dispersion range is 0.911, the flatness of the interleaver is the best. The reflective FPA-MI interleaver, due to its superior performance, will provide a more promising application in dense wavelength division multiplexing optical communication systems.

Spectral Shaping Based on Coupled Sagnac Loop Reflectors Formed by a Self-Coupled Wire Waveguide

We propose and theoretically investigate integrated photonic filters based on coupled Sagnac loop reflectors (SLRs) formed by a self-coupled wire waveguide. By tailoring coherent mode interference in the device, three different filter functions are achieved, including Fano-like resonances, wavelength interleaving, and varied resonance mode splitting. For each function, the impact of device structural parameters is analyzed to facilitate optimized performance. Our results theoretically verify the proposed device as a compact multi-functional integrated photonic filter for flexible spectral shaping.

Three Waveguide Coupled Sagnac Loop Reflectors for Advanced Spectral Engineering

We theoretically investigate advanced multi-functional integrated photonic filters formed by three waveguide coupled Sagnac loop reflectors (3WC-SLRs). By tailoring the coherent mode interference, the spectral response of the 3WC-SLR resonators is engineered to achieve different filter functions with good performance parameters. These include optical analogues of Fano resonances that yield ultrahigh spectral extinction ratios (ERs) and slope rates, resonance mode splitting with high ERs and low free spectral ranges, and classical Butterworth, Bessel, Chebyshev, and elliptic filters. A detailed analysis of the impact of the structural parameters and fabrication tolerances is provided to facilitate device design and optimization. The requirements for practical applications are also considered. Our results theoretically verify the effectiveness of using 3WC-SLR resonators as multi-functional integrated photonic filters for flexible spectral engineering for a range of applications such as optical filtering, switching, and sensing.

Tunable optical filter design with ring resonator based Sagnac loop

We theoretically investigate an advanced tunable photonic filter implemented by a novel structure, termed ring resonator based Sagnac loop reflector (RR-SLR). The ring resonator is rotated by electrostatic actuation to provide Sagnac phase shift and hence tunability. We investigate mode splitting in this filter and demonstrate its high performance in achieving diverse filtering functions. By changing the reflectivity of the Sagnac loop reflector, and ring resonator power coupling and tuning the phase shift through actuator, we tailor coherent mode interference to achieve resonance mode splitting with high extinction ratio, low insertion loss, and wide free spectral range. A detailed analysis of the impact of the structural parameters is presented to facilitate device optimization and design. The filtering performance is compared with other reported works. These results theoretically confirm the effectiveness of using RR-SLR as a multifunctional photonic filter for flexible spectral engineering.

Advanced Multi-Functional Integrated Photonic Filters Based on Coupled Sagnac Loop Reflectors

We theoretically investigate integrated photonic resonators formed by two mutually coupled Sagnac loop reflectors (MC-SLRs). Mode interference in the MC-SLR resonators is tailored to achieve versatile filter shapes with high performance, which enable flexible spectral engineering for diverse applications. By adjusting the reflectivity of the Sagnac loop reflectors (SLRs) as well as the coupling strength between different SLRs, we achieve optical analogues of Fano resonance with ultrahigh spectral slope rates, wavelength interleaving / non-blocking switching functions with significantly enhanced filtering flatness, and compact bandpass filters with improved roll-off. In our designs the requirements for practical applications are considered, together with detailed analyses of the impact of structural parameters and fabrication tolerances. These results highlight the strong potential of MC-SLR resonators as advanced multi-functional integrated photonic filters for flexible spectral engineering in optical communications systems.

Scanning Spectrometer-on-a-Chip Using Thermo-Optical Spike-Filters or Vernier-Comb Filters

This theoretical modeling and simulation paper presents designs and projected performance of an on-chip scanning spectrometer using thermo-optical (TO) spike- or Vernier-comb filters operating around 3600 nm for the germanium-on-nitride technological platform. Raman spectra and absorption spectra are targeted. The spike filter consists of a thermo-optically tunable waveguide Bragg grating resonator (WBGR). The length of the WBGR is chosen to realize a Lorentzian resonant spectral profile with a bandwidth changing between 0.1 and 1 nm. The Vernier-comb filter is obtained using the cascade of two multi-Sagnac loop Mach–Zehnder interferometers, in which through the spectrum of the second MZI is shifted along the wavelength axis by means of a low-power TO heater stripe atop each loop-connector. The sagnac loop reflectors length is designed according to the wavelength-channel spacing requirements in order to induce the Type-1 Vernier effect. Time-scanned discrete sampling of the unknown input spectrum is obtained by driving the TO heater stripe with a periodic ramp current. We examined the device performance in terms of signal reconstruction in the mid-infrared region. The investigation demonstrated that high quality spectrum reconstruction is obtained for arbitrary input signals with an observation wavelength range from 23.4 to 77.4 nm. We also show that spectrum reconstruction of arbitrary signals is feasible in the germanium on nitride chips even in presence of additive noise.

Ultra-sharp asymmetric Fano-like resonance spectrum on Si photonic platform.

In this paper, we report the generation of an ultra-sharp asymmetric resonance spectrum through Fano-like interference. This generation is accomplished by weakly coupling a high-quality factor (Q factor) Fabry-Pérot (FP) cavity and a low-Q factor FP cavity through evanescent waves. The high-Q FP cavity is formed by Sagnac loop mirrors, whilst the low-Q one is built by partially transmitting Sagnac loop reflectors. The working principle has been analytically established and numerically modelled by using temporal coupled-mode-theory (CMT), and verified using a prototype device fabricated on the 340 nm silicon-on-insulator (SOI) platform, patterned by deep ultraviolet (DUV) lithography. Pronounced asymmetric resonances with slopes up to 0.77 dB/pm have been successfully measured, which, to the best of our knowledge, is higher than the results reported in state-of-the-art devices in on-chip integrated Si photonic studies. The established theoretical analysis method can provide excellent design guidelines for devices with Fano-like resonances. The design principle can be applied to ultra-sensitive sensing, ultra-high extinction ratio switching, and more applications.

On-Chip Detection of Trace Gases Using Photonic Matched Filters

This theoretical modeling and simulation the design and projected performance of an on-chip detector of trace gases using photonic matched filters, with operation in the 3-5-μm mid-infrared range. The thermo-optically tuned multi-Sagnac loop Mach-Zehnder interferometer (MZI), based on the germanium-on-silicon nitride technological platform, has been adopted to detect the signature of gases with regularly spaced and narrow absorption lines. The MZI arms are comprised of an N-cascaded connection of Sagnac loop reflectors, each pair separated by a waveguide whose length is designed according to the line spacing of the gas under detection. Detection of one or two gas absorption bands using one filter band is achieved at the Drop port of an MZI filter having on each arm the apodized Hamming Bragg waveguide grating. We performed parametric analysis on the GON platform to find the design rules for the waveguide gas cell and the Hamming or multi-Sagnac filters targeted at CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> or CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O or SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or CO. The analysis predicted good sensor performance.

Multiple-Sagnac-Loop Mach–Zehnder Interferometer for Wavelength Interleaving, Thermo-Optical Switching and Matched Filteri

This theoretical modeling and simulation paper presents design and projected performance of an on-chip thermo-optical multi Sagnac loop Mach–Zehnder interferometer (MZI), operating at C-band and 3300 nm for silicon-on-insulator and germanium-on-silicon technological platforms, respectively. The MZI arms are comprised of an N -cascaded connection of Sagnac loop reflectors, each pair separated by a waveguide whose length is designed according to the wavelength-channel spacing requirements. N is chosen according to the Butterworth filter technique to provide one spectral response having maximal flatness within the desired bandwidth. Switched, spatial routing of odd and even channels is achieved by shifting the Through and Drop MZI spectra along the wavelength axis by means of a low-power thermo-optical heater stripe atop each loop-connector that changes its effective refractive index appropriately. We examined the SOI device performance for wavelength-division multiplexed wavelength (de)interleving. The analysis predicted good performance in terms of bandwidth, insertion loss, and channel isolation for 200, 50, and 25 GHz channel spacing. For the Ge-on-Si Sagnac MZI operating in the mid infrared, we showed that a comb filter is feasible whose lines exactly match the spectral signature of methane for trace-gas sensing applications.

Controllable trapping and releasing of nanoparticles by a standing wave on optical waveguides.

Based on the balance between the scattering force and the trapping force of an evanescent field of a standing wave on silicon waveguides, we propose a structure for controllable trapping and releasing of nanoparticles, which can act as pause operation for nanoparticle flow control. The design is realized by the cascade of an optical switch with a structure of a ring-assisted Mach-Zehnder interferometer (RAMZI) and a Sagnac loop reflector which connects to one output of the switch. Through thermal tuning, with a tiny refractive index change of 4.3×10-4 on a ring resonator, the output of a RAMZI can be switched between two ports. As for the release state of the nanoparticle flow, the light is guided to the port without a reflector. There is no standing wave or traps formed on a waveguide. Therefore, the scattering force dominates, which drives particles moving forward to output ports. Otherwise, for trapping a state, the light will be reflected by the Sagnac loop and form a stationary standing wave which provides an array of traps for nanoparticles. Most importantly, the structure can switch its state to trap or sequentially release particles without losing the control of samples which, to the best of our knowledge, has not been realized before. With the statistical description of particle motion, the balance between trapping and releasing is distinguished by the trapping time and tuned by reflectance. The feasibility of our design is verified using the three-dimensional finite-difference time domain and Maxwell stress tensor methods. Our structure possesses the merits of high compactness and time effectiveness and, thereby, it is highly suitable for on-chip optical manipulation of nanoparticle flow control, which brings great potential in integrated on-chip optofluidics.

Advanced photonic filters based on cascaded Sagnac loop reflector resonators in silicon-on-insulator nanowires

We demonstrate advanced integrated photonic filters in silicon-on-insulator (SOI) nanowires implemented by cascaded Sagnac loop reflector (CSLR) resonators. We investigate mode splitting in these standing-wave (SW) resonators and demonstrate its use for engineering the spectral profile of on-chip photonic filters. By changing the reflectivity of the Sagnac loop reflectors (SLRs) and the phase shifts along the connecting waveguides, we tailor mode splitting in the CSLR resonators to achieve a wide range of filter shapes for diverse applications including enhanced light trapping, flat-top filtering, Q factor enhancement, and signal reshaping. We present the theoretical designs and compare the CSLR resonators with three, four, and eight SLRs fabricated in SOI. We achieve versatile filter shapes in the measured transmission spectra via diverse mode splitting that agree well with theory. This work confirms the effectiveness of using CSLR resonators as integrated multi-functional SW filters for flexible spectral engineering.

Micro-ring resonator quality factor enhancement via an integrated Fabry-Perot cavity

We propose and experimentally demonstrate the enhancement in the filtering quality (Q) factor of an integrated micro-ring resonator (MRR) by embedding it in an integrated Fabry-Perot (FP) cavity formed by cascaded Sagnac loop reflectors. By utilizing coherent interference within the FP cavity to reshape the transmission spectrum of the MRR, both the Q factor and the extinction ratio (ER) can be significantly improved. The device is theoretically analyzed and practically fabricated on a silicon-on-insulator wafer. Experimental results show that up to 11-times improvement in the Q factor, together with an 8-dB increase in the ER, can be achieved via our proposed method. The impact of varying structural parameters on the device performance is also investigated and verified by the measured spectra of the fabricated devices with different structural parameters.