Low-power all-optical switch based on a graphene-buried polymer waveguide Mach-Zehnder interferometer.

We propose a low-power all-optical switch based on the structure of a graphene-buried balanced Mach-Zehnder interferometer (MZI), where the signal light is switched between the two output ports of the MZI by the heat generated from graphene's absorption of the pump light. We use orthogonal polarizations for the pump and the signal light to maximize pump absorption and minimize graphene-induced signal loss. Our experimental device fabricated with polymer waveguides buried with 5-mm long graphene shows a pump absorption of 10.6 dB (at 980 nm) and a graphene-induced signal loss of 1.1 dB (at 1550 nm) and can switch the signal light with a pump power of 6.0 mW at an extinction ratio of 36 dB. The actual pump power absorbed by graphene for activating switching is estimated to be 2.2 mW. The rise and fall times of the switch are 1.0 and 2.7 ms, respectively. The switching characteristics are weakly sensitive to ambient temperature variations. Our device can be butt-coupled to single-mode fibers and could find applications in fiber-based and on-chip all-optical signal processing.

Thin-film lithium-niobate modulator with a combined passive bias and thermo-optic bias.

It is essential to bias a thin-film lithium-niobate Mach-Zehnder electro-optic (EO) modulator at the desired operation condition to ensure optimal performance of the modulator. While thermo-optic (TO) control can solve the problem of bias drift, it consumes significant electric power. In this paper, we propose a technique to largely reduce bias power consumption by combining passive bias and TO bias. In our design, waveguide sections with different widths are introduced in the two arms of the MZ modulator to produce a desired phase difference of π/2 rad (the desired operation condition), and local heating with electrode heaters placed on the waveguides is employed to provide compensation for any phase drift caused by fabrication errors and other effects. As the TO control only serves to compensate for small errors, the electric power required is low and the response is fast. To demonstrate our technique experimentally, we fabricate several modulators of the same design on the same chip. Our experimental modulators can operate up to ∼40 GHz with a half-wave voltage of ∼2.0 V over a wide optical bandwidth, and the performances are insensitive to ambient temperature variations. The TO bias powers required range from 1 mW to 15 mW, and the thermal rise and fall times are 47 µs and 14 µs, respectively. Our technique can facilitate the development of practical high-speed EO modulators on the lithium-niobate-on-insulator platform.

All-optical mode switching with a graphene-buried polymer waveguide directional coupler.

We demonstrate all-optical mode switching with a graphene-buried polymer waveguide asymmetric directional coupler (DC) by using the photothermal effect of graphene, where TE-polarized pump light and TM-polarized signal light are employed to maximize pump absorption and minimize graphene-induced signal loss. Our experimental device, which uses a graphene length of 6.2 mm, shows a pump absorption of 3.4 dB (at 980 nm) and a graphene-induced signal loss of 0.1 dB. The device can spatially switch between the fundamental mode and the higher-order mode with extinction ratios larger than 10 dB (at 1580 nm) and switching times slightly shorter than 1 ms at a pump power of 36.6 mW. Graphene-buried polymer waveguides offer many new possibilities for the realization of low-power all-optical control devices.

Ultralow-loss fusion splicing between negative curvature hollow-core fibers and conventional SMFs with a reverse-tapering method.

Negative curvature hollow-core fibers (NC-HCFs) can boost the excellent performance of HCFs in terms of propagation loss, nonlinearity, and latency, while retaining large core and delicate cladding structures, which makes them distinctly different from conventional fibers. Construction of low-loss all-fiber NC-HCF architecture with conventional single-mode fibers (SMFs) is important for various applications. Here we demonstrate an efficient and reliable fusion splicing method to achieve low-loss connection between a NC-HCF and a conventional SMF. By controlling the mode-field profile of the SMF with a two-step reverse-tapering method, we realize a record-low insertion loss of 0.88 dB for a SMF/NC-HCF/SMF chain at 1310 nm. Our method is simple, effective, and reliable, compared with those methods that rely on intermediate bridging elements, such as graded-index fibers, and can greatly facilitate the integration of NC-HCFs and promote more advanced applications with such fibers.

Electro-optic reconfigurable two-mode (de)multiplexer on thin-film lithium niobate.

We propose and demonstrate a compact electro-optic reconfigurable two-mode (de)multiplexer using the configuration of cascaded Mach-Zehnder interferometers formed on thin-film X-cut lithium niobate on silica. Our fabricated device, which is 9.5-mm long, can spatially switch between the two transverse-electric modes with an efficiency higher than 98% from 1530-1560 nm and beyond at an applied voltage of 6.5 V. The switching speed is faster than 30 ns. Our proposed mode switch could find applications in fiber-based and on-chip mode-division-multiplexing systems.

Electro-optic mode-selective switch based on cascaded three-dimensional lithium-niobate waveguide directional couplers.

We propose an electro-optic mode-selective switch based on cascaded three-dimensional lithium-niobate waveguide directional couplers fabricated with a single-step annealed proton-exchange process. To compensate for discrepancies due to uncertainties in the fabrication process, we develop a post-tuning technique to improve the performance of the coupler by means of depositing a layer of titanium oxide (TiO2) onto one of the waveguides of the coupler. By integrating two cascaded dissimilar directional couplers, we experimentally demonstrate switchable (de)multiplexing of the LP01, LP11a, and LP11b modes, where the LP11a mode can be switched at an efficiency over 75% from 1530 nm to 1612 nm with an applied voltage varying between -9 V and +30 V, and the LP11b mode can be switched at an efficiency higher than 90% from 1534 nm to 1577 nm with an applied voltage varying between -21 V to 0 V. The switching times are 230-300 ns. Our proposed waveguide platform could be employed to develop advanced switches for applications in areas where high-speed switching of spatial modes is required, such as reconfigurable mode-division-multiplexing communication.

Electrically generated optical waveguide in a lithium-niobate thin film.

This paper reports an electrically generated optical waveguide for the transverse-magnetic wave. The waveguide is formed in a z-cut single-crystal lithium-niobate (LN) thin film by the electro-optic effect, where the extraordinary refractive index (RI) of the LN film is increased by a voltage applied to patterned electrodes that define the waveguide geometry. Such a waveguide can be made to exist or disappear by turning on or off the applied voltage. A straight waveguide and an S-bend waveguide with an RI contrast of ∼0.004 are generated at a voltage of 200 V. The propagation loss of the generated waveguide measured at the wavelength 532 nm is 1.8 dB/cm. Electrically generated optical waveguides could fulfill useful functions in photonic integrated circuits, such as reconfigurable cross connect and switching that require wavelength-independent and mode-independent operation.

Optical modulation in hybrid antiresonant hollow-core fiber infiltrated with vanadium dioxide phase change nanocrystals.

We present a study of optical modulation by the effect of temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO2) nanocrystals deposited in an antiresonance hollow-core fiber (AR-HCF). We fabricate such a VO2-coated fiber by embedding alkylsilane functionalized VO2 nanocrystals into the air holes of an AR-HCF. With this fiber, we achieve an optical loss modulation of ∼60% at a temperature above ∼53∘C over an ultrabroad spectral range that encompasses the S+C+L band.

Polarization-insensitive mode-independent thermo-optic switch based on symmetric waveguide directional coupler.

We propose a thermo-optic switch based on a symmetric directional coupler formed with two parallel identical two-mode waveguides, where the two modes in one waveguide can be simultaneously switched to the corresponding modes in the other waveguide. We design and fabricate such a device with polymer materials. Our fabricated device has a total length of 22.5 mm and operates at a switching power of 128 mW. The extinction ratios measured across the C-band are higher than ∼18 dB and ∼13 dB for the fundamental mode and the higher-order mode, respectively. The switching time is ∼1 ms. The performance of the device is insensitive to the polarization state of light. Our proposed mode-independent switch could find applications in reconfigurable mode-division-multiplexing transmission systems.

Externally pumped low-loss graphene-based fiber Mach-Zehnder all-optical switches with mW switching powers.

We propose and experimentally demonstrate an all-optical switch based on a graphene-coated fiber Mach-Zehnder interferometer, where the phase of the signal light in one arm of the interferometer is changed by the heat generated from external pump light absorption by the graphene coating. The external pumping scheme allows efficient pump absorption with multiple layers of graphene coated on an ordinary fiber or a slightly tapered fiber without introducing significant additional signal loss. Without using any wavelength multiplexer/demultiplexer, the switch can be pumped at any convenient wavelength or even with broadband light. Our experimental device, which is based on a standard 125-µm-diameter single-mode fiber with a 5-mm-long graphene coating, can be switched with a pump power of 5.3 mW at an extinction ratio of 19 dB with no additional signal loss. The switching power is insensitive to the graphene coating's length and can be reduced to 4.8 mW, with the fiber tapered to 40 µm. The measured switching powers agree well with the theoretical values obtained by treating the graphene coating as a uniform sheet of heat source without thickness. The switch's response time decreases with the fiber diameter and inversely with the graphene coating's length. The switch's rise and fall times, based on a 40-µm tapered fiber with a 20-mm-long graphene coating, are 30 ms and 50 ms, respectively.

All-optical loss modulation with graphene-buried polymer waveguides.

We present an analysis of all-optical loss modulation in a graphene-buried waveguide based on the Pauli blocking effect. We show that, to realize effective loss modulation, both the signal light and the co-propagating control light must be polarized along the direction parallel to graphene's surface, and the loss-modulation efficiency is given by the ratio of the loss coefficients of the signal light and the control light. To demonstrate the principle, we fabricate two polymer waveguide samples, one with a 0.6-mm-long graphene film buried in the center of the waveguide core and the other with a 10.0-mm-long graphene film placed on the top surface of the core. We achieve a loss modulation to 1550-nm signal light from 5.0 dB to 0.4 dB with a 980-nm control power varying from 6.5 dBm to 12. 5 dBm for the first sample, and from 8.0 dB to 0.5 dB with a control power varying from 14.5 dBm to 19.5 dBm for the second sample. The experimental results agree well with the theoretical analysis. A graphene-buried waveguide offers much flexibility as a platform for the realization of all-optical devices, such as optical switches, optical samplers, and optically tunable attenuators.

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.

Three-dimensional long-period waveguide gratings for mode-division-multiplexing applications.

We propose a three-dimensional (3D) long-period grating structure that has a controllable grating width and depth and can be formed at any chosen position on the surface of a waveguide core with a single photolithography process. The process relies on the partial etching of small structures on the surface of a polymer waveguide through a waveguide mask with narrow apertures that define the grating pattern. The 3D grating structure allows the design of mode converters for any nondegenerate guided modes of a waveguide, regardless of their symmetry properties, and thus relaxes the design constraint of conventional two-dimensional waveguide gratings. To show the flexibility of the 3D grating structure, we present several mode converters fabricated with this structure. The mode-conversion efficiencies achieved are higher than 90% at the resonance wavelengths. In addition, we demonstrate a three-mode multiplexer by integrating a grating-based mode converter with two asymmetric directional couplers. The proposed grating structure together with the fabrication process can greatly facilitate the development of grating-based devices, especially for MDM applications.

Graphene electrodes for lithium-niobate electro-optic devices.

We propose and demonstrate the use of graphene electrodes for lithium-niobate electro-optic (EO) devices to exempt the need of incorporating a buffer layer between the waveguide and the electrodes. Using graphene electrodes, our experimental mode converter, based on an EO-generated long-period grating in a LiNbO3 waveguide, shows a reduction in the half-π voltage by almost three times, compared with the conventional electrode design using metal. With the buffer layer exempted, the device fabrication process is also significantly simplified. The use of graphene electrodes is an effective approach to enhancing the efficiency of EO devices and, at the same time, reducing their fabrication cost.

Reconfigurable broadband mode (de)multiplexer based on an integrated thermally induced long-period grating and asymmetric Y-junction.

We propose a reconfigurable broadband mode (de)multiplexer based on a thermally induced long-period grating integrated with an asymmetric Y-junction. Either of the two spatial modes of a two-mode waveguide launched into the grating end of the device can be switched into either of the two output ports of the Y-junction by controlling the electric power applied to the electrode heater that induces the grating. Our typical device fabricated with polymer material that has a length of ∼14 mm shows a mode selectivity higher than 12 dB over the C+L band at a switching power of 198 mW. The device could find applications in reconfigurable mode-division-multiplexing systems.

Fast and low-power thermo-optic switch based on organic-inorganic hybrid strip-loaded waveguides.

We design and fabricate a thermo-optic waveguide switch based on the configuration of a balanced Mach-Zehnder interferometer optimized for fast operation. The structure of the waveguides consists of two core layers with the lower layer formed with the high-index organic-inorganic hybrid material DR1/SiO2-TiO2. The hybrid material allows the mode field to be confined in an area for fast heat removal, and the formation of index tapers to expand the mode field at the two ends of the device to facilitate butt-coupling with single-mode fibers. Our typical fabricated device shows the same rise time and fall time of 80 µs and low switching power of 8.9 mW at the wavelength of 1550 nm.

Ultra-broadband mode converters based on length-apodized long-period waveguide gratings.

We propose an ultra-broadband mode converter based on the structure of a length-apodized long-period grating, where π-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP01-LP11a and an LP01-LP11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP01-LP11a and LP01-LP11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general grating-based mode-coupling devices for a wide range of applications.

Ultra-broadband mode multiplexers based on three-dimensional asymmetric waveguide branches.

We propose a three-dimensional waveguide mode multiplexer structure that operates on the principle of adiabatic mode transition along multilayer asymmetric waveguide branches. Using this structure, we designed and fabricated a three-mode and a four-mode multiplexer with polymer material that can operate over the C+L band and beyond with small modal crosstalk (<-10 dB) and negligible polarization dependence and be directly connected to fibers with low loss. These multiplexers could be used as mode-selective devices for ultra-broadband mode-division multiplexing based on few-mode fibers.

Ultra-broadband mode filters based on graphene-embedded waveguides.

We propose spatial-mode filters based on embedding graphene films in optical waveguides. To demonstrate the effectiveness and the flexibility of this approach, we design and fabricate several mode filters where graphene films of different widths and lengths are placed in different locations of polymer waveguides. Our mode filters are easy to make and can provide high mode extinction ratios that are insensitive to the operation wavelength. Such mode filters could find applications in broadband mode-division-multiplexing transmission systems and other areas that involve mode selection or stripping.

Broadband mode switch based on a three-dimensional waveguide Mach-Zehnder interferometer.

We propose a mode switch that operates on modulating the optical phases of a three-dimensional balanced four-arm waveguide Mach-Zehnder interferometer. We design and fabricate the device with polymer material to achieve thermo-optic switching between any two of the E11, E21, E12, and E22 modes of the waveguide. Our experimental device shows an extinction ratio higher than 14 dB and a switching time shorter than 3.7 ms, measured with the E11 mode switched to any of the other modes at 1550 nm. This mode switch can operate over a wide range of wavelengths with weak polarization dependence and could be used in reconfigurable fiber-based mode-division-multiplexing systems where mode routing is required.