Tunable Wavelength Filter Research Articles

Tunable narrow band-rejection and band-pass filter based on long-period waveguide grating on a thin-film lithium niobate rib waveguide

We propose and demonstrate experimentally an electro-optic (EO) and thermo-optic (TO) tunable wavelength filter with band-rejection and band-pass dual-function. Our proposed filter is based on a long-period waveguide grating (LPWG) formed on a lithium niobate on insulator (LNOI) rib waveguide with a channel-shaped polymer cladding waveguide. The LPWG formed on the surface of the LNOI core enables efficient mode coupling between the two fundamental modes of the LNOI waveguide and the polymer cladding waveguide and hence dual-function filtering. For the z-/x-polarized input light, our fabricated best filter shows an 18.1-dB/8.2-dB band-rejection contrast and a 15.7-dB/17.4-dB band-pass contrast at 1565.26/1594.24 nm wavelength, with a relatively narrow 3-dB bandwidth of 2.5/2.3 nm, respectively. In addition, the fabricated filter also shows an EO tuning efficiency of ∼32 pm/V and a TO tuning efficiency of 0.465 nm/°C, respectively. Our proposed filter could find applications in wavelength division multiplexing, temperature sensing, and other fields of optical communication and optical information processing.

Easy wavelength-tunable passive mode-locked fiber laser through a Lyot filter with a SESAM

In this work, we present an easy wavelength-tunable passive mode-locked fiber laser with a semiconductor saturable absorber mirror as a mode-locker. The cavity includes a Lyot filter as a wide wavelength-tunable filter that can be tuned by simply increasing the retardance of a variable wave retarder. The Lyot filter consists of the variable wave retarder and highly birefringent fiber inserted between two linear polarizers oriented at the same angle. The highly birefringent fiber has 8.196 cm length and linear birefringence ∆n=4.805x10−4 which provides a 61 nm period. Continuous mode-locking was experimentally achieved from 1561.9 to 1546.1 nm by simply increasing the retardance from 0.0778π to 0.4778π rad delivering sub-ps pulses width at 10.57 MHz repetition rate.

Multi-pulse random fiber laser with wide tuning range in L-band based on tunable band-pass filter and electro-optic modulator

In this paper, a L-band random fiber laser (RFL) with wide tuning range based on tunable band-pass filter (TBF) and electro-optic modulator (EOM) is proposed and studied. The L-band erbium-doped fiber amplifier (L-EDFA) is added to output L-band laser, and a wavelength tunable band-pass filter is used to generate tunable wavelength output. When the pump power is increased from 50mW to 350mW, the output spectra are all chaotic multi-wavelengths. After the filter being added, a stable narrow-band single-wavelength laser can be output in the range of 1564–1618 nm. An EOM is added to introduce actively Q-switched. When the pump power is 50mW, a stable single pulse output can be observed on the oscilloscope. When the pump power is 100mW, the output pulse is double pulse. When the pump power exceeds 100mW, three pulses or four pulses phenomenon appears. The adjustable multi-pulse laser has a wide application prospect in pulse coding, secure communication and other fields.

Electroluminescence hyperspectral imaging of light-emitting diodes using a liquid crystal tunable filter

We demonstrate the use of a low-cost liquid-crystal-based wavelength-tunable filter and CMOS video camera to add hyperspectral imaging capabilities to a probe station equipped with a simple optical microscope. The resultant setup is used to rapidly resolve the spectral and spatial variations in electroluminescence typically observed for InxGa1−xN/GaN light-emitting diodes. Applying standard statistical analyses of variation within the multivariate datasets, such as moments and principal components, we observe inhomogeneities on a spectral scale significantly smaller than the bandwidth of the tunable filter. The resultant tool offers an alternative to scanning beam luminescence techniques for high-throughput hyperspectral analysis of optoelectronic devices.

Performance Analysis of Scalable Optical Circuit Switch Employing Fast-Tunable AMZI Filters for Coherent Detection

High-port-count optical switches are expected to resolve the envisaged bandwidth and power crunch in intra-data center networks stemming from the slowdown of Moore's Law. This paper briefly reviews electrical switching technologies in current intra-data centers and discusses requirements for optical circuit switches in future data centers. We highlight an optical switch architecture that combines the two independent dimensions of space and wavelength to realize large port counts and fast switching. Wavelength routing is implementable with receiver-side channel selection using wavelength tunable filters (TFs). Coherent detection necessitates a fast-tunable local oscillator (LO) at the receiver. It can be cost-effectively realized using an LO bank that consists of fixed-wavelength laser sources and Silicon-photonic asymmetric Mach-Zehnder interferometer (AMZI) filters. Colorless detection that removes a TF at the front of the receiver is possible when the number of wavelength-division multiplexing (WDM) channels is small, however, as the number exceeds a certain limit the TF is needed to prevent power saturation of the receiver. We present an effective cooperative filtering scheme for transmission signals and LO channels sourced from an LO bank; it can detect C-band WDM coherent signals while easing the receiver's power limitation. The scheme is analyzed to clarify receiver performance and the switch port-count bounds under various operating conditions. We demonstrate that the cooperative filtering scheme effectively works from 60 (typical of coherent receivers) to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt; $</tex-math></inline-formula> 116 wavelength channels, which is verified by experiments for 1,856 × 1,856 optical switching with the short switching time of 3.2 μs.

EDF laser displacement sensor based on bending characteristics of polarization-independent double-pass cascaded-chirped long-period fiber grating

In this study, we propose an Erbium-doped fiber (EDF) laser displacement sensor using bending characteristics of a cascaded-chirped long-period fiber grating (C-CLPG), in which the C-CLPG assisted by a Faraday rotator mirror (FRM), namely double-pass C-CLPG, shows a channeled spectrum and plays both the roles of wavelength tunable filter and sensor element. In the sensing scheme, the spectral shift of the channeled spectrum due to the displacement-induced bending deformation is utilized. Due to the FRM reflection, the double-pass C-CLPG has characteristics suitable for sensing: the polarization-independent channeled spectrum and the compensation effect for polarization variation in the sensor section. In the experiment, it is successfully demonstrated that the EDF laser in a sigma-type cavity configuration (EDFσL) using the double-pass C-CLPG enables precise displacement measurement based on the oscillation wavelength shift corresponding to the spectral shift of the double-pass C-CLPG. Further, the advantage of this EDFσL is confirmed by comparison with the conventional EDF ring laser using the single-pass C-CLPG.

Output Stabilization of Wavelength-Swept Laser Based on Closed-Loop Control of Fabry-Pérot Tunable Wavelength Filter for Fiber-Optic Sensors.

The output of a wavelength-swept laser (WSL) based on a fiber Fabry–Pérot tunable filter (FFP-TF) tends to shift the peak wavelength due to external temperature or heat generated by the FFP-TF itself. Therefore, when measuring the output of WSL for a long time, it is very difficult to accurately measure a signal in the temporal domain corresponding to a specific wavelength of the output of the WSL. If the wavelength variation of the WSL output can be predicted through the peak time information of the forward scan or the backward scan from the WSL, the variation of the peak wavelength can be compensated for by adjusting the offset voltage applied to the FFP-TF. This study presents a successful stabilization method for peak wavelength variation in WSLs by adjusting the offset voltage of the FFP-TF with closed-loop control. The closed-loop control is implemented by measuring the deviation in the WSL peak position in the temporal domain using the trigger signal of the function generator. The feedback repetition rate for WSL stabilization was approximately 0.2 s, confirming that the WSL output and the peak position for the fiber Bragg grating (FBG) reflection spectrum were kept constant within ±7 μs at the maximum when the stabilization loop was applied. The standard deviations of WSL output and reflection peak positions were 1.52 μs and 1.59 μs, respectively. The temporal and spectral domains have a linear relationship; the ±7 μs maximum variation of the peak position corresponded to ±0.035 nm of the maximum wavelength variation in the spectral domain. The proposed WSL system can be used as a light source for temperature or strain-dependent sensors as it compensates for the WSL wavelength variation in applications that do not require a fast scanning rate.

Wavelength and bandwidth tunable filter and its application in a dissipative soliton fiber laser.

A wavelength and bandwidth tunable filter and its application in a dissipative soliton (DS) Yb-doped fiber laser are demonstrated. The spectral filter consisting of six cascaded temperature-sensitive long-period fiber gratings is designed and fabricated for the first time, to the best of our knowledge. The corresponding spectral characteristics of the filter are also characterized with temperature. Its 3-dB bandwidth can be adjusted from 4.84 to 11.02 nm, and the center wavelength is continuously adjustable from 1036 to 1045 nm. The sensitivity of the variation of the center wavelength and the linearity of the center wavelength variation are 32 pm/°C and 99.53%, respectively. This home-made spectral filter has two notable features: i) with regard to the tunable spectrum, the 3-dB bandwidth of the spectrum filter can be unchanged; ii) with regard to the spectral tunability, the 3-dB bandwidth of the spectral filter can also be quantitatively changed as needed by changing the heating mode. The home-made spectral filter is used in the DS fiber laser to further realize the continuous adjustment of DS with a tuning accuracy of 0.03 nm by a step size of 1°C. Such a wavelength-tunable DS fiber laser greatly enhances the design flexibility of the coherent anti-Stokes Raman scattering source.

Design and verification of a LO bank enabled by fixed-wavelength lasers and fast tunable silicon ring filters for creating large scale optical switches.

The fast and widely tunable wavelength bank is a key enabler in creating wavelength-routing optical switches that do not use fast wavelength tunable lasers. A cost-effective design criterion needs to be developed before it can be applied to intra data center networks. In this paper, we develop a systematic method for designing a wavelength bank that yields high port-count and fast wavelength-routing optical switches for intra data center application. The wavelength bank is created with fixed-wavelength laser sources and wavelength-tunable filters with rapid wavelength selectivity. To optimize the optical switching system that uses the wavelength bank for supplying local oscillator (LO) lights for coherent detection, various parameters are analyzed, including effective bandwidth, laser output power, loss distribution, splitter port count, and optical amplifier gain. We carry out numerical simulations for optimizing the tradeoff between system performance and cost. To verify the designed wavelength bank, a silicon ring filter is newly fabricated with an average fiber-to-fiber insertion loss of 5.3 dB over a 22-nm bandwidth. Using 256-Gb/s DP-QPSK signals, experiments demonstrate a 1,024×1,024 optical switch that uses a fabricated silicon ring filter. The effectiveness of the scalable and fast-tunable LO bank is verified by achieving 262.1-Tb/s switch throughput with switching time under 18 µs.

AuNPs assisted all-fiber compact phase modulator and its application in erbium-doped fiber laser

A compact, low-loss, simply prepared all-fiber phase modulator based on the plasmonic absorption of gold nanoparticles decorated on the surface of non-adiabatic microfiber was demonstrated By applying 808 nm pump laser of different powers to the phase modulator, the effective refractive index changes resulting in phase shift of the interference spectrum at 1550 nm. The proposed device can also be utilized as an all-optical switch with the rising and falling edge time of 4.69 ms, 1.99 ms, respectively Furthermore, a tunable wavelength erbium-doped fiber laser was designed by inserting the proposed AuNPs-based phase modulator as a wavelength-tunable filter into a ring cavity. The output center wavelength of the laser was tuned from 1560.23 nm to 1559.29 nm. The proposed AuNPs-based phase modulator has advantages of compact size, low-loss, and simple preparation, which make it can be anticipated to have great potential in the application of all-optical systems.

Widely wavelength-tunable mode locked Er-doped fiber oscillator from 1532 nm to 1594 nm with high signal-to-noise ratio

A widely wavelength tunable mode locked Er-doped fiber laser oscillator based on nonlinear polarization rotation (NPR) effect was demonstrated. By intentionally introducing a position-and-width adjustable slit and a pair of reflective gratings into the cavity, which worked as a wavelength and bandwidth tunable spectral filter, continuous wavelength tunable mode locked operation from 1532 nm to 1594 nm was achieved, corresponding to a tuning range of 62 nm, which is a new record for mode locked continuously wavelength-tunable single mode Er-doped fiber oscillators based on nonlinear polarization rotation, to the best of our knowledge. During the wavelength tuning, the pulse durations for different wavelengths changed slightly and closed to the Fourier-transform limit. The signal-to-noise ratio (SNR) of the radio frequency (RF) spectrum of the output pulses is higher than 89 dB over the entire wavelength tuning range, superior to the typical SNR of 50–70 dB reported in most papers. In addition, the pulse duration could be tuned from 904 fs to 1339 fs by adjusting the width of the slit, for which the corresponding spectral bandwidth changed from 3.4 nm to 1.7 nm.

Wide wavelength-tunable passive mode-locked Erbium-doped fiber laser with a SESAM

In this work we present a simple polarization-maintaining wavelength-tunable passive mode-locked Erbium-doped fiber laser with a semiconductor saturable absorber mirror (SESAM) as a mode locker. The cavity includes a Sagnac interferometer-based fiber optical loop mirror (FOLM) as a wide wavelength-tunable filter. Tunable mode-locking was experimentally achieved in the range of 1543.2 nm to 1569.5 nm by thermally adjustment of FOLM wavelength reflection. The output pulses have a repetition rate of 11.16 MHz with pulse duration about 0.9 ps. The experimental results were confirmed by numerical simulations.

Polymeric tunable wavelength filter with two-stage cascaded tilted Bragg gratings.

Wavelength-division multiplexed optical communication systems used in 5G networks require tunable wavelength filters with narrow bandwidth for 100 GHz channel spacing, wide wavelength range to cover 16 channels, and a side mode suppression ratio (SMSR) exceeding 30 dB. To fabricate wavelength filters satisfying these specifications, tunable Bragg grating filters based on polymeric optical waveguides are proposed. The combination of mode-sorting waveguide and tilted Bragg grating enables the extraction of Bragg reflected signals to another path, without using an external circulator. Moreover, the double reflection by the two-stage cascaded structure produces narrower reflection bandwidth, improved SMSR characteristics, and reduced adjacent-channel crosstalk through the suppression of undesired mode coupling. The proposed device exhibits a 20 dB bandwidth of 1.0 nm and SMSR of 35 dB, over the entire wavelength-tuning range.

A Multipurpose and Highly-Compact Plasmonic Filter Based on Metal-Insulator-Metal Waveguides

A multipurpose and ultra-compact nanoplasmonic wavelength filter based on stub structure in a metal-insulator-metal (MIM) waveguide is suggested and numerically investigated. A novel approach of connecting two stepped-like apertures to both input and output ports is applied to form Fabry-Perot (FP) cavities, which enabled the structure to act as a dual band-pass filter at wavelengths 1310 nm and 1550 nm. It is shown that the variation in cavities’ length allows to realize a long-wavelength cutoff filter, and cutoff wavelength can be easily tuned by adjusting the length of the cavities. Furthermore, it is revealed that increasing the gap between the stepped-like apertures and the cavities provides a triple band-pass at telecom wavelengths, e.g., 1267.5 nm, 1414.19 nm, and 1644.7 nm. The tunable broadband high-pass wavelength filter is then achieved while the lengths of stepped-like apertures and stub resonators are set to be identical. Finally, a tunable nearly perfect absorber can be obtained by varying the width of stub resonators. Therefore, because of functionality, size, as well as efficiency the proposed plasmonic filter may greatly contribute to miniaturization of next generation of photonic integrated circuits (PICs), and find applications in on-chip integration and wavelength-division multiplexing (WDM) in optical communication systems.

Liquid Crystal Based Rib Waveguide

We report electrically controlled liquid-crystal-core rib waveguide on fused silica substrate, in which the modal characteristics of waveguide can be tuned over a wide range of wavelengths. The coplanar electrode structure is proposed for controlling the LC molecules direction in the waveguide. Electrically controlled refractive index contrast and profile of waveguide is used for single and dual mode operations of the waveguide. We experimentally demonstrate transition from single mode to dual mode operation by externally applied voltage. In single mode regime the externally applied voltage has been used to tune the mode field diameter of the modes from 11.6 to 9.2 μm along the horizontal direction and 7.8 to 6.4 μm along the vertical direction. In the dual mode regime, long period waveguide grating (LPWG) based mode converter has been numerically demonstrated, and more than 99.9% power conversion from TE00 mode to TE01 mode has been obtained. Further, we show electrically tunable wavelength filters using LPWG in an LC core rib waveguide. We have obtained a tuning range of 140 nm for such electrically tunable wavelength filters. The proposed liquid crystal based rib waveguide has the possibility of on-chip low-cost fabrication.

Wavelength-Division Demultiplexing Enhanced by Silicon-Photonic Tunable Filters in Ultra-Wideband Optical-Path Networks

With the recent growth of network traffic demands, a high-capacity optical-path network based on reconfigurable optical add-drop multiplexers (ROADMs) is highly desirable. Among the various technologies available, extending the useable frequency band is one of the most promising solutions since it can enlarge the network capacity with relatively small signal-quality degradation. However, increasing the number of wavelength signals detracts flexible signal-drop capabilities of the ROADM because the effective dynamic range of the digital coherent receiver is restricted by the number of wavelength signals input to the receiver. To overcome this difficulty, we adopt the ROADM structure that sets an optical wavelength-tunable filter in front of each receiver. This scheme retains the effective receiver dynamic range even if a large number of wavelength signals are multiplexed. In this paper, we demonstrate flexible signal drop in a C+L-band optical-path network by using an optical wavelength-tunable filter implemented on a single silicon-photonic chip. The prototype filter is monolithically implemented in conjunction with an input-port selector so as to reduce the total system loss. The filter comprises multiple asymmetric Mach-Zehnder interferometers for wavelength tuning and symmetric Mach-Zehnder interferometers for input-port selection. Its effectiveness is experimentally confirmed by measuring the bit-error ratios of 32-Gbaud/100-Gbps dual-polarization quadrature-phase-shift-keying signals and 32-Gbaud/200-Gbps dual-polarization 16-ary quadrature-amplitude-modulation signals. A single wavelength signal is extracted out of 285 wavelength signals multiplexed over the C+L band and successfully demodulated with negligible penalty in terms of the optical signal-to-noise ratio. The net fiber capacity reaches 57 Tbps.

Design of lithium niobate phase-shifted Bragg grating for electro-optically tunable ultra-narrow bandwidth filtering.

Tunable optical wavelength filters are indispensable components in optical communication systems. In this paper, an ultra-narrowband tunable filter inscribed in lithium niobate (LN) wafer is designed. Two sections of Bragg grating and a pair of electrodes are fabricated in the LN waveguide. The operation wavelength could be tuned by adjusting the electric voltage applied on the electrode pair owing to the electro-optic effect of LN crystal. Simulation results indicate that the filtering bandwidth of the proposed phase-shifted Bragg grating is determined by grating lengths no longer than 1pm. The tuning sensitivity is dependent on the distance between the two gratings. In addition, the proposed structure overcomes the difficulty in controlling the phase shift of the grating pair, ensuring its potential applications in optical sensing systems.

Wavelength Tunable Filter Based on Acousto-Optic Modulation of a Double-Core Fiber

A tunable notch filter is acoustically modulated in the transmission spectrum of a double-core optical fiber. Flexural acoustic waves dynamically change the coupling power and the wavelength of the propagating modes, allowing to switch and tune wavelengths between the two cores. A maximum modulation depth of 5.4 dB and a tuning wavelength range of 8.4 nm are achieved with the present configuration. The results indicate new possibilities for application in space division multiplexing systems, dynamic couplers, gain equalizers, and fibers lasers.

Graphene filled optical fiber Fabry‐Pérot cavity tunable filter

AbstractBefore saturated absorption, graphene possesses optical nonlinear refractive index, which is a photoinduced refractive index effect. Taking advantage of this effect, a wavelength tunable filter is realized based on a graphene filled all optical fiber Fabry‐Pérot microcavity. Experimental results show that the interference wavelength of the filter exhibits linear red‐shift with the increase of the tunning optical power and tunning efficiency increases with the increase of cavity length and filled graphene thickness. In this study, the maximum wavelength vs power tunning efficiency can get 66 pm/mW. This filter has the advantages of high tunning efficiency, simple structure, antienvironmental interference good mechanical performance, and repeatability.

Liquid crystal-based widely tunable integrated optic wavelength filters

We report widely tunable wavelength filters using liquid crystal (LC)-based long-period waveguide gratings (LPWGs). The high birefringence and electrically tunable refractive index of the LC have been used to achieve a tuning range covering most of the S, C, and L bands of an optical communication system. E7 LC has been used as a cover layer in an LPWG rib waveguide structure. The LPWG structure couples light from the fundamental mode to the slab modes of the waveguide. This results in transmission dips at resonance wavelengths. The mode constants of the coupling modes can be changed by controlling the refractive index of the LC and hence the resonance wavelengths by using the externally applied electric field. We have carried out numerical simulations to analyze the performance of E7 LC-based LPWGs on various substrates such as ion-exchanged BK7 glass and proton-exchanged LiNbO3 and silicon-on-insulator and silicon nitride substrates. The potential applications of the device are electrically tunable wavelength filters for optical communication systems.