Thermo-optic Research Articles

Highly Efficient Silicon Photonic Microheater Based on Black Arsenic–Phosphorus

AbstractWhile black phosphorus has shown excellent optoelectronic properties in many aspects, the environmental instability ultimately places limits on its practical applications. Black arsenic–phosphorus (b‐AsP), an alloy of black phosphorus with arsenic atoms, has emerged as a new 2D material with better stability. Recently, the heterostructure via integration of 2D material with nanophotonic waveguides is opening an avenue for many on‐chip applications of phosphorene. However, the thermo‐optic (TO) properties, which are widely used for waveguide heaters, are not studied on b‐AsP yet. Herein, the TO effects of a b‐AsP/silicon van der Waals heterostructure are first investigated and its application as a transparent waveguide heater is demonstrated. A high efficiency of 0.74 nm mW−1 is achieved with a nonresonant and broadband heater based on 20 nm thick b‐AsP, which eliminates the need for enhancement by cavity‐assisted light–matter interaction, and thus allows for compact footprint, broadband operation, and low latency.

High-performance graphene-integrated thermo-optic switch: design and experimental validation [Invited

The extraordinary optical properties of single-layer graphene have spurred the development of a variety of photonic components. We have previously demonstrated a scalable and versatile platform to facilitate the integration of graphene and other 2-D materials with chalcogenide glass-based planar photonics. In this paper, we detail the design criteria and optimization guidelines towards high-performance graphene-integrated thermo-optic (TO) switches based on the chalcogenide glass-on-graphene platform. Notably, absorption loss of graphene can be reduced to < 20 dB/cm when it is sandwiched inside photonic structures capitalizing on the anisotropic absorption property of graphene. We quantify energy efficiency of the TO switch, showing that the choice of cladding materials plays a critical role in improving device efficiency. Furthermore, we report a record TO switching efficiency of 10 nm/mW via judicious engineering of the overlap between optical mode and thermal profile.

Metal-Printing Defined Thermo-Optic Tunable Sampled Apodized Waveguide Grating Wavelength Filter Based on Low Loss Fluorinated Polymer Material

In this work, thermo-optic (TO) lateral shift apodized sampled waveguide grating for 1550 nm wavelength is designed and fabricated by the metal-printing technique based on fluorinated epoxy-terminated polycarbonates (FBPA-PC EP) and fluorinated epoxy resin (FSU-8) materials. The optical characteristics and thermal stability of the FBPA-PC EP and FSU-8 materials are analyzed. To realize periodic wide-spectrum filtering and suppress the side-lobes of grating, a lateral shift apodized sampled waveguide grating is proposed. The 3 dB bandwidth and wavelength spacing can reach 4.8 nm and 9.7 nm. The side-lobe suppression ratio of proposed device can reach 22.6 dB, which is much better than traditional Bragg grating (6.1 dB). Driving electrical powers of 42.4 mW and 87.2 mW can produce blueshifts of 1.8 nm and 3.5 nm in the measured reflection spectrum, respectively. This device realizes the aim of multiple functions, including periodic filtering, wide-spectrum filtering, and high side-lobe suppression. The device is applicable of realizing signal processing and wavelength division multiplexing (WDM )systems.

Thermo-optic Aided Tunability of Sr0.6Ba0.4Nb2O6 Thin Film-based Electro-optic Modulator Using Waveguide Coupled SPR Modes

Present article demonstrates the effective tuning of thermo-optic (TO) properties with the Electro-Optic (EO) modulation characteristics of SBN60 thin films utilizing Waveguide coupled surface plasmon resonance (WCSPR) technique. Electro-optic (EO) coefficient (r) was in turn tuned from a value of 594 pm/V to 621 pm/V with increasing ambient temperature from 296 to 373 K. Tunable percent modulation index varying from 24 to 55% was obtained with increasing ambient temperature at a fixed applied electric field. The obtained results clearly illustrate an efficient thermally tunable EO modulator based on SBN thin film utilizing a simplistic approach with $$ {\left.\kern0em \left(\frac{\partial n}{\partial T}\right)\right|}_{E=0}=88.3\times {10}^{-4}{K}^{-1} $$.

Silicon Non-Blocking 4 × 4 Optical Switch Chip Integrated With Both Thermal and Electro-Optic Tuners

We experimentally demonstrate an integrated strictly non-blocking silicon 4 × 4 optical switch chip that can be operated in both thermo-optic (TO) and electro-optic (EO) switching modes. It is based on the double-layer network (DLN) architecture and consists of twelve 2 × 2 Mach-Zehnder interferometer (MZI) switch elements. TO phase shifters based on TiN microheaters and EO phase shifters based on p-i-n diodes are embedded in both waveguide arms of the MZI elements. The power consumption for TO and EO switching is 34 mW/π and 7 mW/π, respectively. The on-chip insertion losses are 1.74 ± 0.59 dB and 3.79 dB ± 1.32 dB for TO and EO switching, respectively. Due to the merits of the DLN architecture and the optimized performance of the switch elements, the chip possesses low crosstalk of -29.1 dB and -19.4 dB for TO and EO switching, respectively. Quadrature phase-shift keying (QPSK) optical signals with a data rate of 64 Gb/s are transmitted through the switch with no observable deteriorations. Such an optical switch is a promising candidate for both optical circuit switching and optical packet switching for a variety of applications.

Thermo Optic Coefficients and Electronic Polarizabilities of Tungsten Bronzes Using Ellipsometry

The values of dn/dT for thermally evaporated WO 3 thin films were found to be negative either in the heating cycle (295-373 K) or in the cooling cycle (100-300 K) and were found to be of order of 10 -5 K -1 . On the other hand thermo optic coefficients (TOCs: dn/dT and dk/dT values) of tungsten bronzes (H + , Li + , Na + ) with different concentrations were found to be positive and negative, respectively in both the heating and cooling cycles and they were found to be in the order of 10 -4 K -1 . Heating cycles show hysteresis loops of n and k for tungsten bronzes, which are good for electrochromic devices. On heating the bronzes at temperature higher than 400 K, there might be a reduction in the porosity but an irreversible disordering of hydrogen, lithium or sodium atoms could occur because of anomalous dispersions produced in the optical data. For cooling cycles, the calculated TOCs were again of the order of 10 -4 but there was an increase in dn/dT and a decrease in dk/dT values due to more amorphousness built up in the bronzes during the cooling cycles. This change in the values of TOCs for tungsten bronzes created small decrease in electronic polarizabilities (α e ) which were calculated in the range from 8.003 to 8.02 × 10 -24 cm 3 in the cooling cycles.

On-chip integrated optical switch based on polymer waveguides

Optical polymer is one of the promising materials for photonic integrated devices that can be processed with simple, flexible and semiconductor compatible technology. Especially, optical polymer with the advantages of high thermo-optic (TO) and electro-optic (EO) coefficients has been widely applied in optical waveguide switches. However, most of the current optical switches are independent with single function. It is essential to develop an on-chip integrated device with multilayer for signal manipulation. In this paper, we propose an on-chip 3-dimensional integrated optical switch based on the EO polymer-clad waveguide incorporate with the TO tunable vertical coupler. The proposed integrated optical switch could realize the function-integration of the adjustable 3D optical connection and the high-speed modulation. A typical fabricated switch shows an insertion loss of 16.3 dB and an extinction ratio of 16.0 dB at the driving power of 48.2 mW. The dynamic characteristics of TO and EO switching functions of the device were also measured in this work.

Low power consumption thermo-optic switch formed by an integrated processing method.

In this paper, an integrated processing method was demonstrated to fabricate the polymer-based thermo-optic (TO) switch with low power consumption. The characteristic parameters of the switch were carefully designed and simulated. The air trench structure was exploited to reduce the power consumption, which can be formed with the waveguide simultaneously by the integrated processing method. Moreover, the introduced polymer/silica hybrid waveguide structure can also improve the response time of the device. A typical fabricated switch presented a low switching power of 5.2mW. The measured switching rise time and fall time are 192.2 and 201.1μs, respectively.

Metal-printing polymer waveguide thermo-optic switches compatible with 650 and 532 nm visible signal wavelengths for plastic optical fiber systems.

In this work, thermo-optic (TO) waveguide switches for 650 and 532nm visible wavelengths are designed andfabricatedby the metal-printing technique based on poly (methyl methacrylate-glycidyl methacrylate) [P(MMA-GMA)] material. The optical characteristics and thermal stability of the P(MMA-GMA) material are analyzed. Optical transmission modes in the core waveguide for different visible wavelengths are simulated, and the thermal field distribution from the self-heating electrode structure is calculated, respectively. The structural parameters of the devices compatible with 650 and 532nm visible wavelengths are designed optimally. For 650 and 532nm signal wavelengths, the insertion loss of the actual TO switch fabricated is less than 3.2dB, and the response time of the device is about 367.4μs at 100Hz square wave electrical signals. The driving electrical power of the device for the 650nm signal wavelength is 15.2mW and 14.0mW for the 532nm signal wavelength, respectively. The extinction ratio of the visible TO switch for 650nm is 15.1dB and 18.5dB for 532nm, respectively. The technique is suitable for realizing plastic optical fiber system applications.

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.

Refractive index sensing based on Mach–Zehnder interferometer with a hybrid silica/polymer waveguide

Abstract We propose a refractive index sensing structure based on Mach–Zehnder interferometer (MZI) with a hybrid silica/polymer waveguide. Due to the thermo-optic (TO) effects of polymer and silica, the refractive index of sample can be measure by an integrated circuit. We discuss the design principle of the refractive index sensing waveguide and give its equivalent circuit to explain the sensing process. The sensitivity of MZI waveguide achieves 5556 °C/RIU which has a compact size of 10 μm × 6 mm. The sensing structure can be applied in a highly compact biosensing system and possess a great potential in point-of-care diagnostics and healthcare applications.

Ultra-compact silicon photonics switch with high-density thermo-optic heaters.

Miniaturization of silicon photonics switches is essential for both dense integration and low-loss operation. However, it has remained unclear how small the switches can be made while using thermo-optic (TO) element switches. In this paper, the minimum possible distance between adjacent TO phase shifter arms was first examined. Next, the architecture for a switch matrix for the high-density arrangement of TO switches that includes multi-layer electrical wirings for compact electrical wire-out was proposed and demonstrated. As a result, we achieved 1/23 miniaturization of an 8 × 8 silicon photonics switch for the PIC part when compared with our previous design.

Compact 2×2 parabolic multimode interference thermo–optic switches based on fluorinated photopolymer**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0402502), the National Natural Science Foundation of China (Grant Nos. 61575076, 61475061, 61605057, and 61675087), and the Jilin Provincial Industrial Innovation Special Fund Project, China (Grant No. 2016C019).

In this work, a dual-side parabolic structural (DSPS) multimode interference (MMI) thermo–optic (TO) waveguide switch is designed and fabricated by using novel low-loss fluorinated photopolymer materials. Comparing with the traditional dual-side linear structural (DSLS) MMI device, the effective length of the MMI coupling region proposed can be effectively reduced by 40%. The thermal stability of the waveguide material is analyzed, and the optical characteristics of the switching chip are simulated. The actual performances of the entire MMI switch are measured with an insertion loss of 7 dB, switching power of 15 mW and an extinction ratio of 28 dB. In contrast to the traditional MMI optical switch, the new type of parabolic structural MMI TO waveguide switch exhibits lower power consumption and larger extinction ratio. The compact fluorinated polymer MMI TO switches are suitable well for realizing miniaturization, high-properties, and lower cost of photonic integrated circuits.

Buried graphene electrode heater for a polymer waveguide thermo-optic device.

We propose the use of graphene as the electrode heater material for a polymer waveguide thermo-optic (TO) device. Because a graphene electrode can be buried in a polymer waveguide without introducing a significant loss to the transverse magnetic polarized light, we can do away with the buffer layer that is required in a conventional TO device to isolate the metal electrode heater from the waveguide and, hence, reduce the driving electric power of the device. To demonstrate the principle, we fabricate and compare two polymer waveguide TO mode switches based on the configuration of a balanced Mach-Zehnder interferometer, which are identical except that one uses a buried graphene electrode and the other uses an aluminum electrode deposited on the waveguide surface. Our experimental device that uses a graphene electrode has a switching power almost four times lower and also responds faster. The use of buried graphene electrodes is an effective approach to reducing the power consumption of TO devices.

High-efficiency Ge thermo-optic phase shifter on Ge-on-insulator platform.

We report on a Ge thermo-optic (TO) phase shifter on a Ge-on-insulator (GeOI) platform for mid-infrared (MIR) integrated photonics. Numerical analysis showed that the Ge TO phase shifter can realize three times higher modulation efficiency than a Si TO phase shifter, owing to the large TO coefficient and refractive index of Ge. The Ge TO phase shifter, operating at a wavelength of 1.95 μm fabricated on a GeOI wafer, achieved an operating power of 7.8 mW for a phase shift of π, which was less than half of that in a previously reported Si TO phase shifter operating at a wavelength of 1.55 μm. Thus, the Ge TO phase shifter is promising for high-performance and low-power MIR photonic integrated circuits for various sensing and communication applications.

On-Chip Digital Fourier-Transform Spectrometer Using a Thermo-Optical Michelson Grating Interferometer

This theoretical modeling and simulation paper presents designs and projected performance of an on-chip digital Fourier transform spectrometer using a thermo-optical (TO) Michelson grating interferometer operating at ∼1550 and 2000 nm for silicon-on-insulator and for germanium-on-silicon technological platforms, respectively. The Michelson interferometer arms consist of two unbalanced tunable optical delay lines operating in the reflection mode. They are comprised of a cascade connection of waveguide Bragg grating resonators (WBGRs) separated by a piece of straight waveguide with lengths designed according to the spectrometer resolution requirements. The length of each WBGR is chosen according to the Butterworth filter technique to provide one resonant spectral profile with a bandwidth twice that of the spectrometer bandwidth. A selectable optical path difference (OPD) between the arms is obtained by shifting the notch in the reflectivity spectrum along the wavelength axis by means of a low-power TO heater stripe atop the WBGR, inducing an OPD that depends on the line position of the WBGR affected by TO switching. We examined the device performances in terms of signal recostruction in the radio-frequency (RF) spectrum analysis application at 1 GHz and at 1.5 GHz of spectrometer resolution. The investigation demonstrated that high-quality spectrum reconstruction is obtained for both Lorentzian and arbitrary input signals with a bandwidth up to 40 GHz. We also show that spectrum reconstruction of 100–200 GHz RF band input signals is feasible in the Ge-on-Si chips.

Metal-print-defining thermo-optic tunable chirped waveguide Bragg gratings using organic-inorganic hybrid PMMA materials

In this work, thermo-optic (TO) tunable chirped waveguide Bragg gratings based on organic-inorganic hybrid PMMA material are achieved using the metal-print-defining technique. The molecular structural characteristics and thermal stabilities of the polymer grafting materials are analyzed. Structural parameters of the chirped waveguide gratings and self-electrode heaters are designed and performances of the entire device are simulated. The contrast value of the reflection band is about 15 dB between 1530 and 1570 nm. The actual thermo-optic sensitivity of the chirped grating is 0.2 nm/K. The time delay is obtained as 112 ps and the group velocity dispersion is measured as 2.1 ps/nm. This technique is very beneficial for achieving the dispersion-compensating optical communication system.

Reconfigurable optical-microwave filter banks using thermo-optically tuned Bragg Mach-Zehnder devices.

A new reconfigurable, tunable on-chip optical filter bank is proposed and analyzed for the silicon-on-insulator platform at the ~1550 nm wavelength. The waveguided bank is a cascade connection of 2 x 2 Mach-Zehnder interferometer (MZI) filters. An identical standing-wave resonator is situated in each MZI "arm." Using the thermo-optic (TO) effect to perturb this waveguide's index, the TO heater stripes provide continuous tuning of the filter by shifting the resonance smoothly along the wavelength axis. To reconfigure and program the cascade array, a broadband 2 x 2 MZI-related switch is inserted between adjacent filters. The novel TO switch, described here, can provide either single or double interconnection of 2 x 2 filters. The filter resonator is a new in-guide array of N closely coupled phase-shifted Bragg-grating resonators that provide one resonant spectral profile with 5 to 100 GHz bandwidth. The length of each grating cavity in the N group is chosen according to the Butterworth filter technique, and this gives high peak transmission for the composite. The predicted spectral profiles of a three-stage cascade show two-or-three peaks, or two-or-three notches with movable wavelength-locations as well as tunable wavelength-separations between those features. A tunable notch within a wider movable passband is also feasible. Potential applications include microwave photonics, wavelength-selective systems, optical spectroscopy and optical sensing.

Mach-Zehnder crossbar switching and tunable filtering using N-coupled waveguide Bragg resonators.

This theoretical modeling-and-simulation paper presents designs and projected performance of ~1500-nm silicon-on-insulator 2 x 2 Mach-Zehnder interferometer (MZI) optical crossbar switches and tunable filters that are actuated by thermo-optical (TO) means. A TO heater stripe is assumed to be on the top of each waveguided arm in the interferometer. Each strip-waveguide arm contains an inline set of N-fold coupled, phase-shifted Bragg-grating resonators. To implement accurate and realistic designs, a mixed full-vectorial mathematical model based upon the finite-element, coupled-mode, and transfer-matrix approaches was employed. The Butterworth-filter technique for grating length and weighting was used. The resulting narrowband waveguide-transmission spectral shape was better-than-Lorentzian because of its steeper sidewalls (faster rolloff). The metrics of crossbar switching, insertion loss (IL) and crosstalk (CT), were evaluated for choices of grating strength and TO-induced change in the grating-waveguide refractive index. The predicted ILs and CTs were quite superior to those cited in the literature for experimental and theoretical MZI devices based upon silicon nanobeam resonators. This was true for the Type-I and Type-II resonator addressing discussed here. Finally, we examined the TO-tunable composite filter profiles that are feasible by connecting two or more Type-I MZIs in an optical series arrangement. A variety of narrow filter shapes, tunable over ~2 nm, was found.

Polarization-independent and colorless switching operation of three-dimensional 4 × 4 polymer optical switch with two-layered waveguide structure

We fabricated a three-dimensional (3D) 4 × 4 polymer optical interconnection switch with a two-layered waveguide structure equipped with four Mach–Zehnder interferometer optical switches (MZ-OSs), two vertical directional couplers (VDCs), and a multimode interferometer (MMI) coupler. The experimental result for the device shows polarization-independent and colorless switching operation by the thermo-optic (TO) effect in the 1550-nm-wavelength range with a switching power of about 18 mW.