Tunable Attenuator Research Articles

Empirical Analysis of Widely Tunable Fused Fiber Coupler Assisted by External Medium of High Thermo-Optic Coefficient

This article reports the fused fiber coupler whose transmission spectrum is widely tuned or switched with temperature. With a proper choice of the external medium applied on the fused region of the coupler, the tuning range could be expanded up to the free spectral range of the coupler itself. A polymer with a matched refractive index and high thermo-optic coefficient was used as the external medium, and a micro-coil heater was installed to tune or switch the coupler. As the elongation length of the coupler increased in the fabrication process, the tuning range increased also. With the polymer having a thermo-optic coefficient of −1.8 × 10−4/°C and a 1.440 room temperature refractive index, wavelength tuning as wide as 210 nm was experimentally obtained. The fabrication process of the tunable fused coupler and the temperature-dependent spectral properties of the implemented couplers in terms of the elongation length and the polymer index are presented in detail. The feasibility of the proposed device as a tunable filter, optical switch, and variable optical attenuator are also presented.

Switchable, dual-wavelength passively mode-locked ultrafast fiber laser based on a single-wall carbon nanotube modelocker and intracavity loss tuning

We demonstrate a dual-wavelength passively mode-locked soliton fiber laser based on the single-wall carbon nanotube saturable absorber. By using a simple scheme of adjusting the intracavity loss, the gain profile of the erbium-doped fiber laser is effectively controlled. Besides operating at a single wavelength, the laser is able to simultaneously generate sub-picosecond pulses at both ~1532 and 1557 nm wavelength. The mode-locking wavelength can also be quickly switched from one wavelength to the other by changing the intracavity loss with a tunable attenuator.

Photonic Crystal Fabry-Perot Self-Collimation Interferometer by Liquid Crystal Infiltration

A Fabry-Perot self-collimation interferometer (FPSI) constructed in a two-dimensional photonic crystal (2D PhC) by liquid crystal infiltration has been proposed and demonstrated theoretically. The resonant cavity of FPSI is infiltrated with a nematic liquid crystal (LC) 5CB with ordinary and extraordinary refractive indices 1.522 and 1.706, respectively. The transmission spectrum of the FPSI has been investigated with the 2D finite-difference time-domain method. Calculation results show that resonant transmission peaks have nearly equal frequency spacing 0.0090c/a. When the effective refractive index neff of the liquid crystal is increased from 1.522 to 1.706, the peaks shift to the lower frequencies over 0.0071c/a while the peak spacing is almost kept unchanged. Thus this FPSI by LC infiltration can work as a tunable attenuator or an optical switch. For the central operating wavelength around 1.55m, its dimensions are only about tens of microns. Thus this device may be applied to photonic integrated circuits.

Dynamic spectral line-by-line pulse shaping by frequency comb shifting

Traditionally, reconfigurable optical pulse shaping is performed by using programmable spatial light modulators or tunable optical attenuators and phase shifters. We propose a technique to achieve tunability in the line-by-line pulse-shaping regime by controlling the offset frequency of an optical frequency comb. The method requires a high-spectral-resolution line-by-line shaper with channel spacing equal to a submultiple of the comb spacing. As a particular example, we numerically analyze tunable pulse-repetition-rate multiplication by grating-based line-by-line pulse shaping using a fixed phase-only mask.

1 ×3 Silicon Oxynitride Tunable Optical Waveguide Attenuators Based on the Multimode Interference Effect

The silicon oxynitride (SiON) films of various refractive indices achieved through the careful adjustments of oxygen-to-nitrogen contrast ratios were deposited on silicon substrates using the plasma-enhanced chemical vapor deposition (PECVD) method. The refractive indices of SiON films spanning from 1.47 to 1.94 were realized by manipulating the flow rates of pertinent gaseous precursors. The Fourier transform infrared (FTIR) spectra were gathered in order to verify that a significant reduction in the infrared absorption of the N–H bond could in fact be achieved with the thermal annealing process. Afterward, the integrated 1 ×3 SiO2/SiON/SiO2 tunable multimode interference (MMI) optical waveguide attenuators were then designed and simulated using the beam propagation method (BPM), and the optimal structures were then proceeded for device fabrication. The positive thermooptic (TO) effect of SiON was employed to change the self-imaging light pattern as generated by the side-heated MMI region for purposes of optical steering and beam attenuation. The experiment results showed that a 22-dB attenuation could be realized with the heating power of 2.2 W. Finally, when the heating power of ∼1.73 W was applied, the rise and fall times of 1 ×3 tunable MMI optical waveguide attenuators were obtained as 455 and 420 µs, respectively.

Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer

A novel lateral force sensor based on a core-offset multi-mode fiber (MMF) interferometer is reported. High extinction ratio can be obtained by misaligning a fused cross section between the single-mode fiber (SMF) and MMF. With the variation of the lateral force applied to a short section of the MMF, the extinction ratio changes while the interference phase remains almost constant. The change of the extinction ratio is independent of temperature variations. The proposed force sensor has the advantages of temperature- and phase-independency, high extinction ratio sensitivity, good repeatability, low cost, and simple structure. Moreover, the core-offset MMF interferometer is expected to have applications in fiber filters and tunable phase-independent attenuators.

Tunable attenuator based on polymer microstructured optical fibers

Based on polymer microstructured optical fibers (PMOF) and selective liquid crystal injection technology, a novel temperature control tunable attenuator is designed theoretically in this paper. In our experiments, PMMA microstructured optical fibers with high performance are drawn successfully and the selective injection of liquid crystal 5CB into microholes is realized. The tunable attenuator is fabricated with an extinction ratio of over 60dB, which agrees well with the design.

Characterization of Memory Effects in Power Amplifiers Using Digital Two-Tone Measurements

In this paper, a novel method to measure the amplitude and phase of two-tone third-order intermodulation products generated in a high-power amplifier is presented. The method is based on the sampled input and output signals of the amplifier. The presented measurement setup and the associated algorithms for the calculation of the amplitudes and phases are considerably faster and simpler than the current methods. By making use of the sampled input and output signals of the amplifier and the signal processing techniques, the need for a nonlinear reference, a tunable attenuator, and a phase shifter in the existing measurement setups is eliminated, which makes it simple and easy to use. Hence, this is a substantial simplification of the measurement setup compared to what has been reported earlier. The proposed measurement setup is also suitable for fast automated measurements, which is of interest for many applications that are both laboratory and production oriented. In addition, a method to increase the bandwidth of the measured signal is used to overcome the bandwidth limitation set by the Nyquist criteria for sampled systems. Measurements are done on two base-station high-power amplifiers and are found to be in agreement with theory and reference measurements.

Continuously tunable fibre attenuator operating in the wavelength range near 1.5 μm

A fibre attenuator is fabricated for the telecommunication wavelength range near 1.5 μm in which a single-mode silica fibre with side polishing is used. The fibre surface is covered by a layer of fluorine-containing polymer with a large thermooptic coefficient. The principle of attenuator operation is based on a change in the conditions of total internal reflection for a guided mode in the polished region due to thermally induced variation in the refractive index of the fluoropolymer layer. The attenuator is insensitive to light polarisation, it has a continuously variable attenuation coefficient in the range 0.2–27 dB, and can be easily incorporated into fibreoptic links.

Tunable optical fiber devices based on broadband long-period gratings and pumped microfluidics

This letter describes classes of tunable microfluidic fiber (μFF) devices that use specially designed long-period gratings in which the phase matching condition is satisfied over a wide spectral range. Dynamic tuning is achieved by electrowetting-based pumping of microfluidic plugs back and forth over the gratings. As specific examples, we demonstrate dynamically tunable broadband attenuators and filters with adjustable profiles by using fluids with different refractive indices. These devices have attractive features that include in-fiber design and polarization-independent behavior together with low-power, nonmechanical, fully reversible, and latchable tuning.

Automatic tunable and reconfigurable fiberoptic microwave filters based on a broadband optical source sliced by uniform fiber Bragg gratings

We demonstrate an automatic tunable transversal notch filter based on uniform fiber Bragg gratings and a broadband optical source. High tunability can be performed by stretching the fiber with the gratings written in series. Also, high sidelobe supression can be achieved by introducing tunable attenuators in a parallel configuration of the gratings.

Tunable fiber attenuators for channel power equalization in WDM systems

A tunable fiber attenuator is proposed for channel power equalization in wavelength-division-multiplexed (WDM) optical communication systems. This fiber attenuator consists of two identical fiber Bragg gratings, which are subject to bending in opposite directions by an electrically controlled mechanical driver. The attenuation level at the desired wavelength depends on the amount of bending applied to the gratings. A device with a continuous adjustment of attenuation over a range of 32 dB and a driving voltage of less than 2 V has been demonstrated. The performance of these tunable attenuators for channel power equalization in a two-channel WDM system has been measured. Excellent spectral flattening (+/−0.1 dB) has been achieved for static channel power equalization. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 1–4, 2000.

Tunable fiber attenuators for channel power equalization in WDM systems

A tunable fiber attenuator is proposed for channel power equalization in wavelength-division-multiplexed (WDM) optical communication systems. This fiber attenuator consists of two identical fiber Bragg gratings, which are subject to bending in opposite directions by an electrically controlled mechanical driver. The attenuation level at the desired wavelength depends on the amount of bending applied to the gratings. A device with a continuous adjustment of attenuation over a range of 32 dB and a driving voltage of less than 2 V has been demonstrated. The performance of these tunable attenuators for channel power equalization in a two-channel WDM system has been measured. Excellent spectral flattening (+/−0.1 dB) has been achieved for static channel power equalization. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 1–4, 2000.

Optically tunable millimeter-wave attenuator based on layered structures

A new type of optically controllable, millimeter-wave (MMW) attenuator based on high-resistivity (high-R) silicon (Si) wafers and a layered structure is developed. A high-R float-zone Si wafer is a lossless dielectric material at microwave frequency without optical excitation. When an Si wafer is optically excited, free carriers are generated, and the Si wafer becomes a lossy dielectric. This property is combined with a layered structure to develop a simple optically tunable MMW attenuator. A more than 20 dB attenuation with a 10% bandwidth of the center frequency is obtained at W-band. The proposed structure is useful for developing low-cost attenuators and switches in the MMW region. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 9–13, 2000.

Polymeric tunable optical attenuator with an optical monitoring tap for WDM transmission network

A polymeric tunable optical attenuator has been fabricated using asymmetric Y-branch waveguides integrated with an optical monitoring tap. One arm of the branch serves as the main output port, while the other arm helps remove the residual optical power resulting from attenuation. An optical power tap used for the monitoring port is introduced to take a fraction of the main port power. This monitoring port is useful for observing the main output continuously. Furthermore, it can be fed back to the electrical driver to maintain the attenuated output regardless of variations in input light polarization and power. By utilizing the modal evolution effect in asymmetric branches, the attenuator has enhanced fabrication tolerance and reduced wavelength sensitivity. Also, no electrical bias is needed to achieve maximum optical transmission. The attenuator exhibited a dynamic range of more than 20 dB at 1550 nm and a response time of about 1 ms. The polarization dependent loss was about 0.9 dB, and the wavelength uniformity was less than 1.2 dB over the range of 1530-1560 mm.

RF tunable attenuator and modulatorusing high Tc superconducting filter

A small path for both control current and radiofrequency (RF) current has been introduced into a high Tc superconducting filter. The interaction between the control current and RF current made the insertion loss of the filter sensitive to the amplitude of the control current. This structure has been used as an RF tunable attenuator and a modulator. 5 dB attenuation range was achieved. The maximum modulation frequency was ~3 MHz.

Area-efficient self-calibration technique for pipe-lined algorithmic A/D converters

An area-efficient self-calibration technique for pipe-lined A/D converters is presented. It consists of trimming the reference voltage to each stage by means of a tunable MOSFET attenuator. This simplifies the calibration circuit in each stage and shifts most of the calibration task to a hardware that is shared by all stages.

Microwave devices using the proximity effect between superconducting and normal metals

Superconductors coated with normal metals present more strongly temperature or field dependant characteristic lengths (penetration depth, coherence length on the N side …) than bare superconductors. These properties can be used in ‘stripline’ transmission line structures. The authors describe some experimental results obtained with resonant structures at a frequency of 4 GHz, using mainly PbCu, PbSn, or SnAu contacts. The use of the proximity effect in magnetically tunable filter cells or attenuators, and the intrinsic limitations of these devices are discussed.