Tunable Duty Cycle Research Articles

Photonic generation of high-accuracy triangular waveform with tunable duty cycle based on a dual-wavelength I/Q modulation

Photonic generation of high-accuracy triangular waveform with tunable duty cycle based on a dual-wavelength I/Q modulation

Regulation of laser-induced nanogratings by tuning the Marangoni-plasmon-coupled effect.

Laser-induced subwavelength nanogratings on films find widespread applications in enhancing a spectrum through surface plasmon excitation. It is challenging to achieve high uniformity, diversity, and controllability due to the intricate interplay between two basic mechanisms in laser nanostructuring: the Marangoni effect and surface plasmon polaritons (SPPs). We tune the coupled effect on Ge2Sb2Te5 films by adjusting the laser polarization, whose component controls the two effects' strength ratio. The Marangoni effect dominates when the SPPs' direction mismatches with the growing direction of nanogratings. Tuning this competition relationship helps to create nanogratings with tunable duty cycle and distribution, which are significant for light modulation applications. A highly efficient direct writing method with a line-shaped laser beam is employed to create large-area regular nanogratings by enhancing the effect tuning. We demonstrate diverse Au nanogratings with the aid of a lift-off operation and apply them in surface plasmon-coupled emission (SPCE), showcasing exceptional enhancement and narrowing performance.

Photonic generation of rectified cosinusoidal and sinusoidal shaped microwave waveforms with tunable duty cycle

Photonic generation of rectified cosinusoidal and sinusoidal shaped microwave waveforms with tunable duty cycle

A dynamic grating with tunable duty cycle and height

Grating plays a pivotal role in various optical systems owing to its unique dispersion properties. However, the traditional grating has fixed functionalities once fabricated. In this paper, a tunable grating based on a hybrid interdigitated electrodes–dielectric–graphene​ structure is proposed. The carrier on the graphene exhibits a periodic distribution when the voltage is applied on the electrode, which makes the graphene behave like a grating. Herein, the duty cycle and height of the grating can be dynamically controlled by the voltage. The numerical result shows that the absorption in graphene changes by 10.2% at the wavelength of 7.4μm when the designed structure is utilized to excite the surface plasmon polaritons (SPPs) since the excitation efficiency of SPPs varies with the parameters of grating. This new type of grating will have a potential application in future modulators, photodetectors and sensors.

Photonic Generation of Rectangular and Triangular Microwave Waveforms With Tunable Duty Cycle

We propose a microwave photonic scheme to generate rectangular and triangular waveforms by external modulation. Different from previous works, the duty circle of the generated waveforms in our scheme is freely tunable. The main structure is based on a Mach-Zehnder modulator and a tunable optical delay line. By power splitting and time-delay tuning on two polarization branches, triangular or rectangular waveforms can be obtained by the superposition of beating signals. Moreover, the RF modulation index in this work is unfixed, which increases the flexibility of the system. The proposed scheme is theoretically analyzed and experimentally verified. The triangular and rectangular waveforms at repetition rates of 4 and 5 GHz are successfully generated.

A Dynamic Grating with Tunable Duty Cycle and Height

A Dynamic Grating with Tunable Duty Cycle and Height

Repetition-Frequency-Doubled Transform-Limited Optical Pulse Generation Based on Silicon Modulators

Compact and integrated optical pulse generation with a high repetition rate and a small duty cycle can benefit various applications such as integrated microwave photonics. In this work, a method to generate a repetition-frequency-doubled narrow optical pulse train based on two cascaded silicon Mach–Zehnder modulators is proposed and demonstrated. The generated frequency comb has even-order sidebands with linear phase and suppressed odd-order sidebands, which results in the transform-limited pulses in the time domain. The tunability of the repetition rate from 4.0 to 6.6 GHz and the duty cycle from 10.6% to 14.0% has been achieved. An analytic model that considers the non-ideal properties of the silicon modulators is also established and is able to reproduce the key features in the experiment. Our work shows the potential of an integrated silicon chip to generate high-speed optical pulse trains with tunable duty cycles demanded in integrated microwave photonics and integrated photonic signal processing.

A 30-MHz Voltage-Mode Buck Converter Using Delay-Line-Based PWM Control

A 30-MHz voltage-mode buck converter using a delay-line-based pulse-width-modulation controller is proposed in this brief. Two voltage-to-delay cells are used to convert the voltage difference to delay-time difference. A charge pump is used to charge or discharge the loop filter, depending on whether the feedback voltage is larger or smaller than the reference voltage. A delay-line-based voltage-to-duty-cycle (V2D) controller is used to replace the classical ramp-comparator-based V2D controller to achieve wide duty cycle. A type-II compensator is implemented in this design with a capacitor and resistor in the loop filter. The prototype buck converter was fabricated using a 0.18- ${\mu }\text{m}$ CMOS process. It occupies an active area of 0.834 mm2 including the testing PADs. The tunable duty cycle ranges from 11.9%–86.3%, corresponding to 0.4 V–2.8 V output voltage with 3.3 V input. With a step of 400 mA in the load current, the settling time is around 3 ${\mu }\text{s}$ . The peak efficiency is as high as 90.2% with 2.4 V output and the maximum load current is 800 mA.

A novel linear square/triangular wave generator with tunable duty cycle

A novel circuit of square/triangular wave generator consists of single multiple output dual-X current conveyor transconductance amplifier (MO-DXCCTA), one grounded resistor and one grounded capacitor is proposed. One square wave in current mode and one triangular wave in voltage mode are simultaneously available from the proposed generator. The proposed circuit is fully electronically controllable facilitating the feature of duty cycle adjustment. The duty cycle is electronically tunable via bias current. Moreover, a wide sweep of oscillation frequency is possible via grounded capacitor with very good linearity. The tuning of oscillation frequency does not affect the amplitudes of output waveforms. Furthermore, the oscillation frequency can also be controlled electronically and independently via bias current for a fixed duty cycle of 50%. A prototype of MO-DXCCTA using commercial ICs (AD844 and LM13700) is used to carry out the experimental results. Measured results show very good variation of frequency (up to 245.5 kHz) against capacitor with nonlinearity less than 2%.

Single Active Element-Based Tunable Square/Triangular Wave Generator with Grounded Passive Components

This paper presents a novel circuit of tunable square/triangular wave generator based on single active element and grounded passive components. The structure of the proposed generator is simple as it consists of one multiple-output differential voltage current conveyor transconductance amplifier (MO-DVCCTA) as active element and one grounded resistor and one grounded capacitor as passive components. The proposed generator provides triangular wave in voltage mode and square wave in both voltage mode and current mode simultaneously. The proposed generator enjoys the features of availability of square wave in both current mode and voltage mode, electronically tunable duty cycle via DC current over a 93–7% range, electronically and independently adjustable DC level of voltage-mode square wave and triangular wave via an additional DC current, independent and electronic control of amplitude of current-mode square wave by bias current and independent tunability of oscillation frequency by grounded capacitor. Moreover, the operation of the proposed generator at 50% duty cycle exhibits the electronic and independent control of oscillation frequency via bias current also. As an additional feature, the proposed generator exhibits <1% nonlinearity up to a frequency of 123.9 kHz. The maximum temperature coefficient of oscillation frequency over a temperature range of 0–50 \({^{\circ }}\)C is found 185 ppm/\({^{\circ }}\)C which is quite good in view of temperature stability of the oscillation frequency. The prototype of MO-DVCCTA based on commercial ICs has been used in the realization of circuit of the proposed generator. The PSPICE simulation results and experimental results are given to validate the proposed generator.

Square-wave oscillations in a semiconductor ring laser subject to counter-directional delayed mutual feedback.

Square-wave (SW) switching of the lasing direction in a semiconductor ring laser (SRL) is investigated using counter-directional mutual feedback. The SRL is electrically biased to a regime that supports lasing in either counter-clockwise (CCW) or clockwise (CW) direction. The CCW and CW modes are then counter-directionally coupled by optical feedback, where the CCW-to-CW and CW-to-CCW feedback are delayed by τ1 and τ2, respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of T≈τ1+τ2. When τ1=τ2, symmetric SWs with a duty cycle of 50% are obtained, where the switching time and electrical linewidth of the SWs can be reduced to, respectively, 1.4 ns and 1.1 kHz by strengthening the feedback. When τ1≠τ2, asymmetric SWs are obtained with a tunable duty cycle of τ1/(τ1+τ2). High-order symmetric SWs with a period of T=(τ1+τ2)/n can also be observed for some integer n. Symmetric SWs of order n=13 with a period of T=10.3 ns are observed experimentally.

Experimental investigation of all-optical nonreturn-to-zero differential phase-shift keying to return-to-zero DPSK format conversion based on nonlinear polarization rotation of semiconductor optical amplifier

A novel all-optical nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) to return-to-zero DPSK (RZ-DPSK) format conversion scheme is proposed and experimentally demonstrated. This conversion is based on nonlinear polarization rotation of a semiconductor optical amplifier. Experimental results show that a 10 Gb/s RZ-DPSK signal with an extinction ratio over 10 dB can be converted with a tunable duty cycle from 33% to 66%, and the ER of the converted signal decreases with the increase in the duty cycle. For all cases of different duty cycles, the converted signals experience a -0.4 to -1.2 dB power penalty at a bit error rate of 10(-9) compared with the original signal. In addition, the spectra show that this format conversion is a wavelength-preserved operation.

Uniform and Duty Cycle Tunable Photonic Crystal LEDs Fabricated by Intermediate Mask Layer: Soft UV Nanoimprint Lithography

As an improvement of the conventional nanoimprint lithography, an intermediate mask layer process is invited for photonic crystal pattern transfer on LEDs. Benefit from the smooth effect of the under-layer resist, uniform photonic crystal structure is successfully transferred to the p-GaN surface. The intermediate SiO2 layer has a high etching selectivity to the under-layer resist, which can be used to fabricate nanostructures with tunable duty cycle via one single initial mold. The photonic crystal LED fabricated by the process of intermediate mask layer nanoimprint lithography shows a 3.31 fold PL enhancement to that of the un-patterned LED.

Photonic crystal structures on nonflat surfaces fabricated by dry lift-off soft UV nanoimprint lithography

The surface nonflatness induced from the material itself or the production atmosphere can lead to serious non-uniformity consequences in nanoimprint lithography (NIL) which is used for providing a low cost and high throughput nano-fabrication process. In this paper, soft UV NIL (SUNIL) processes are used for photonic crystal (PC) pattern transfer of a GaN-based light-emitting diode (LED) with patterned sapphire substrate (PSS). The results reveal a significant incompatibility between the conventional SUNIL and the nonflat p-GaN surface. Ellipse-shaped rather than circle-shaped PC structure is obtained on the p-GaN surface due the deformation of the soft mold in nonflat NIL. A dry lift-off (DLO) SUNIL is proposed to overcome the non-uniformity issue in nonflat NIL as well as the collapse problem of the free-standing pillar-shaped resist in wet lift-off. The photoluminescence enhancements of the LED fabricated by the DLO SUNIL method compared to those with conventional SUNIL and unpatterned LED are 1.41 fold and 3.48 fold, respectively. Further study shows that the DLO SUNIL is applicable in the fabrication of the PC structure with tunable duty cycle via one single initial PC mold.

4 × 10 Gb s−1 wavelength multicasting with tunable NRZ-to-RZ format conversion using nonlinear polarization rotation in an SOA

We experimentally demonstrate simultaneous 4 × 10 Gb s−1 all-optical wavelength multicasting and non-return-to-zero (NRZ)-on-off-keying (OOK) to return-to-zero (RZ)-OOK format conversion with a tunable duty cycle using nonlinear polarization rotation in a semiconductor optical amplifier (SOA). The experimental results show that the duty cycle of four converted RZ-OOK signals can be tuned by adjusting the orientation of a polarizer placed at the SOA output. Four-channel NRZ-OOK-to-RZ-OOK conversion with a full width at half maximum of 33–67 ps can be simultaneously obtained with an extinction ratio over 10 dB. Moreover, it is experimentally verified that such a wavelength multicasting scheme with simultaneous NRZ-OOK-to-RZ-OOK conversion is insensitive to the wavelength of the input signal, indicating that such a scheme can be operated in the whole C-band with less than 0.18 dB power penalty at a bit error ratio level of 10−9. The device can facilitate the cross-connection between optical transmission networks employing different modulation formats.

14.4: Polychromatic High‐Frequency Steady‐State Visual Evoked Potentials for Brain‐Display Interaction

AbstractAn oncoming interactive platform integrated with LCD provides another option not only for handicapped but public to make our daily lives more convenient. Steady‐state visual evoked potentials (SSVEPs) are human brain responses to visual stimulation at specific light flashing frequencies. With the improvement of brain‐computer interface (BCI) applications in mind, this paper proposed effective and comfortable SSVEP stimuli of high‐frequency and polychromatic encoded lights with tunable duty cycles and phases.

4 × 10 Gb/s wavelength multicasting with tunable NRZ-to-RZ pulse format conversion using time- and wavelength-interleaved pulses

We demonstrate 4×10Gb/s simultaneous wavelength multicasting and NRZ-to-RZ pulse format conversion with tunable duty cycle. Multicasting is achieved by four-wave mixing of the input signal with a time- and wavelength-interleaved laser source, while the format conversion is obtained through the use of a pulsed probe. The input data are copied to four multicast outputs with a common relative delay time. Error-free operations have been obtained in all the outputs with power penalties ranging from − 0.5 to 0.5dB. Output duty cycles with a tuning range of 4.3ps have been achieved.

Optical signal processing based on semiconductor optical amplifier and tunable delay interferometer

In this paper, several applications in all-optical signal processing based on a semiconductor optical amplifier (SOA) and variable delayed interferometers (DIs) have been experimentally demonstrated. Wavelength converter based on a nonlinear polarization switch (NPS) and a DI is proposed and presented for the wavelength conversion of nonreturn-to-zero (NRZ) signals. An all-optical nonreturn-to-zero to return-to-zero (NRZ-to-RZ) format converter with tunable duty cycles is achieved by the DI with variable delays. The 40 Gb/s reconfigurable optical OR/NOR gate in a single SOA, followed a tunable optical bandpass filter (OBF) and a DI, optical 2R regeneration using an SOA-DI are investigated. It is found that this combinative realization of filters has been endowed with great flexibility and quality for 40 Gb/s optical logic and 2R regeneration.

Investigation of all-optical nonreturn-to-zero-to-return-to-zero format converter based on a semiconductor optical amplifier and a reconfigurable delayed interferometer

A simple and robust all-optical nonreturn-to-zero to return-to-zero (RZ) format converter is demonstrated and investigated using a semiconductor optical amplifier (SOA) and a reconfigurable delayed interferometer. The scheme can be used to convert input signals at different bit rates and obtain converted signals with tunable duty cycles. Format conversion at 10 Gb/s is achieved with a negative power penalty of -4.9 dB at a bit-error rate of 10(-9). Transmission of the converted RZ signal over a dispersion-managed fiber link of 80 km results in only a small power penalty of 0.9 dB. In addition, the format converter is found to be insensitive to the power fluctuation of the optical clock, as the SOA works in the saturation regime.

All-optical NRZ-OOK-to-RZ-OOK format conversions with tunable duty cycles using nonlinear polarization rotation of a semiconductor optical amplifier

We experimentally demonstrate an all-optical 10Gb/s format conversion from non-return-to-zero (NRZ) on-off-keying (OOK) to return-to-zero (RZ)-OOK with tunable duty cycle in the whole C-band using nonlinear polarization rotation (NPR) arising in an semiconductor optical amplifier (SOA). The experimental results show that, by tuning the polarizer at the SOA output, an RZ signal with tunable duty cycle from 33% to 66% could be obtained with an extinction ratio(ER) over 10dB. In addition, we show that the NRZ-to-RZ conversion with duty cycle of 33–66% can be obtained with less than 1dB power penalty at the bit error ratio (BER) of 10−9. The device can facilitate the cross-connection between optical transmission networks employing different modulation formats.