Variable Optical Delay Line Research Articles

Coherence revival multiplexed, buffered swept source optical coherence tomography: 400 kHz imaging with a 100 kHz source.

The effective speed of a swept source optical coherence tomography (SSOCT) imaging system was quadrupled using efficient sweep buffering along with coherence revival and spatial multiplexing. A polarizing beam splitter and fold mirror assembly were used to create a dual spot sample arm with a common objective designed for near-diffraction-limited retinal imaging. Using coherence revival, a variable optical delay line allowed for separate locations within a sample to be simultaneously imaged and frequency encoded by carefully controlling the optical path length of each sample path. This method can be used to efficiently quadruple the imaging speed of any SSOCT system employing a low duty-cycle laser that exhibits coherence revival. The system was used to image the retina of healthy human volunteers.

High-energy, tunable-wavelengths, Q-switched pulse laser

An all fiber, wavelength tunable, Q-switched erbium-doped fiber laser (EDFL) based on a graphene saturable absorber (SA) and an unbalanced Mach–Zehnder interferometer (UMZI) has been demonstrated. The wavelengths of the Q-switched laser can be accurately tuned over a range of ~35nm by inserting into the cavity an UMZI with a variable optical delay line (OVDL) in one arm, which acts as an optical beam filter into the cavity. The Q-switched pulse duration and the pulse energy have all been characterized. The maximum pulse energy of 36.9nJ was obtained.

Switchable and tunable microwave photonic filter based on reflective semiconductor optical amplifier

ABSTRACTA microwave photonic filter, which can be switched between a high‐Q bandpass filter and a notch filter, is proposed and experimentally demonstrated. It is based on a recirculating delay line loop with a reflective semiconductor optical amplifier, and negative taps are generated employing wavelength conversion. The filter can be also tuned by adjusting an optical variable delay line. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:198–201, 2014

Addressing fiber Bragg grating sensors using all-fiber low coherence interferometry

In this paper we propose an interrogating system for the quasi-distributed measurement of strain or temperature. The system basically consists of an all-fiber low coherent interferometer that includes the cascaded fiber Bragg gratings used in its test arm. The variable optical delay line with movable mirror traditionally used as the reference arm is here replaced by a set of fibers with different optical lengths all connected to an optical switch. This switch makes it possible to set the desired length for the reference arm. Second proposed solution is to replace variable delay line by fiber with multiple reflection points placed at positions close to those used at reference arm. Temperature and strain resolutions are 0.8°C and 10μm/m, respectively. The advantages of the system are the possibility to interrogate identical FBGs and to interrogate sensors placed far apart. The system does not use tunable laser source or movable elements.

An Optoelectronic Oscillator Based on Carrier-Suppression-Effect-Free Single Bandpass Microwave Photonic Filter

A novel optoelectronic oscillator based on a single bandpass microwave photonic filter (MPF) without any electronic microwave filter is proposed and experimentally demonstrated. The MPF, which is composed by a broadband optical source, an inline interferometer based on a variable optical delay line (VODL), an intensity modulator, and a dispersive element, serves as an oscillating mode selector. The oscillation frequency can be tuned by changing the differential time delay via the VODL. Due to the non-carrier-suppression effect of the selector, the oscillator can generate transmission-null-free microwave signal with widely tunable frequency range. An experiment is performed, and a tunable frequency range of 6.8 GHz is achieved. The generated microwave signal at ~ 6.5 GHz ( ~ 1.06 GHz) exhibits a good phase noise performance about -95 dBc/Hz ( -110 dBc/Hz) at an offset of 10 kHz.

Tunable multi-tap microwave photonic filter with complex coefficients using a dual-parallel Mach–Zehnder modulator

We propose and demonstrate a tunable multi-tap microwave photonic filter (MPF) with complex coefficients. It is based on addition of carrier suppressed single sideband (CS-SSB) signal and phase-controllable optical carrier. Instead of using the technique of optical filtering, the CS-SSB signal is generated by a dual-parallel Mach–Zehnder modulator (DPMZM) in our scheme. Frequency tuning of the filter is achieved by changing the phase of the optical carrier, which can be controlled by a variable optical delay line (VDL). The proposed structure features high flexibility and wideband RF response, and the experimental results demonstrate that it can be tuned continuously over one free spectral range (FSR) without changing the spectral shape of the filter.

A high-Q microwave photonic filter by using an SOA-based active mode-locked fiber ring laser

A high-Q microwave photonic filter using a semiconductor optical amplifier (SOA)-based mode-locking fiber ring laser is proposed, analyzed and experimentally demonstrated. The proposed microwave photonic filter can realize a high-Q frequency response, it is compact without an optical source, and it can be easily tuned by adjusting an optical variable delay line in a ring cavity. A result with a Q-factor of about 236 and a rejection ratio of about 45 dB is obtained. The measured results and the theoretical estimations agree very well.

Four-tap microwave photonic filter with tunable center frequency and reconfigurable transfer function

A four-tap microwave photonic filter with tunable center frequency and reconfigurable transfer function is proposed.It is based on two Mach-Zehnder interferometers (MZIs) cascaded with a variable optical coupler (OC) in between. By simultaneously adjusting the variable optical delay line incorporated in each MZI, center frequency of the filter can be tuned continuously. Reconfiguration of the filter is achieved by changing the coupling ratio of the variable OC. Experimental result demonstrates a tunable four-tap filter with sidelobe suppression in excess of 25 dB. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1740–1743, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26877

폴리머 결합 링 공진기 기반 가변 광신호 지연기의 설계 및 제작

본 논문에서는 결합 링 공진기 구조 기반의 가변 광신호 지연기를 고 굴절률 차이를 갖는 폴리머 물질을 이용하여 설계 및 제작하였다. 링들의 FSR은 100 GHz 로 설계하였고, 8 개의 링들이 결합된 구조를 사용하였다. 버스 도파로에 가까운 두 개의 링들이 공진하는 경우 약 100 ps, 4 개의 링들이 공진하는 경우 180 ps 정도의 지연시간이 측정되었으며, 이는 이론적인 결과에 가까움을 확인할 수 있었다. In this paper, a variable optical signal delay line based on coupled ring resonators is designed and fabricated in high-index contrast polymer material. The free spectral ranges (FSR) of the rings are designed to be 100 GHz, and 8 coupled rings are used. When two rings near a bus waveguide are in resonance, the optical delay is measured to be about 100 ps. When four rings are in resonance, the measured delay is about 180 ps. Both are close to the theoretical calculations.

Microwave photonic bandpass filter based on spectrumslicing and phase modulator

A tunable microwave photonic bandpass filter with high mainlobe-to-sidelobe ratio (MSR) based on a phase modulator and a dispersive device is proposed. The multi-tap characteristics of the filter are realized by slicing a broadband source using a Mach-Zehnder interferometer (MZI) which results in a high MSR of 25 dB. The tunability of the filter is realized by an optical variable delay line (OVDL) in one arm of the MZI, which changes the wavelength spacing of the sliced broadband source and results in a tunable free spectrum range (FSR) of the filter. The central frequency of the bandpass filter is tunable from 10.7 GHz to 27 GHz by changing the wavelength spacing from 0.145 nm to 0.054 nm.

Emulation of an Optical Flexible Delay Line by Parallel Variable Optical Delay Lines

In this letter, we consider the discrete-time setting and propose a construction of an optical flexible delay line by parallel variable optical delay lines (VODLs) under a very simple packet routing scheme. We show that to exactly emulate an optical flexible delay line with maximum delay d (time slots), where d is a positive integer, it is necessary and sufficient to have at least [(2d+1)/3] VODLs with maximum delay d (time slots) in our proposed architecture. We also show that under a uniform traffic model, the average number of busy VODLs is approximately d+1-(2d+1)(d/(d+1)) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sup> , which is quite close to our simulation result. Through simulations, we further show that if we can tolerate a loss probability of, say 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , then we only need at most 1.7 times of the average number of busy VODLs, which is much smaller than the worst-case bound [(2d+1)/3].

A tunable single-passband microwave photonic filter with positive and negative taps using a fiber Mach–Zehnder interferometer and phase modulation

In this paper, a tunable single-passband microwave photonic filter with positive and negative taps based on phase modulation and a fiber Mach–Zehnder interferometer (FMZI)) is presented, theoretically analyzed and experimentally demonstrated. The FMZI as the slicing filter results in a single-bandpass characteristic for the presented filter, and by using phase modulation, a number of negative taps are generated, thus making the baseband resonance suppressed, which is desirable for microwave photonic filter's application. The wavelength spacing of the FMZI can be tuned by employing an optical variable delay line, making the present filter continuously tunable. A tunable single-passband microwave photonic filter without baseband resonance is achieved in the experiment. The present filter shows an extinction ratio of up to ∼20 dB and good tuning linearity.

A simple microwave photonic notch filter based on a semiconductor optical amplifier

A novel microwave photonic notch filter structure is proposed and experimentally demonstrated. The structure is based on a recirculating delay line loop consisting of an optical variable delay line, a semiconductor optical amplifier (SOA) and a tunable narrowband optical filter. Negative tap is generated using wavelength conversion based on cross-gain modulation of an amplified spontaneous emission spectrum in the SOA. A narrow bandpass response with negative coefficients and a broadband all-pass response are combined to achieve a narrow notch response with flat passband which can eliminate interference with minimal impact on the expected signal. Experimental results show good agreement with theoretical analysis.

Switchless hybrid analog-digital variable optical delay line for radio frequency signal processing

The author proposes the design of a novel switchless variable optical delay line (VFODL) for radio frequency (rf) signals that combines an analog mode VFODL with a discrete-state VFODL. This hybrid VFODL proposes the use of a laser with smart, fast (e.g., 1 ns) wavelength tuning in combination with flat-top passband design spatially and temporally dispersive optical elements to simultaneously provide both long and short time delays, thus finely covering a wide nanoseconds delay range. An example design indicates a 9.6-ns time delay range with 6-ps increments, giving 1600 independent delays using a no-moving-parts signal-processing structure. VFODL applications include rf antenna array processing, radar testing, and precision electrical timing systems.

All-optical microwave notch filter with flat passband based on semiconductor optical amplifier

An all-optical filter structure for interference mitigation of microwave signals is proposed and demonstrated. The structure is based on a recirculating delay line loop (RDLL) and a fiber Bragg grating. The RDLL is comprised of a semiconductor optical amplifier (SOA) followed by a tunable narrowband optical filter and an optical variable delay line. Negative tap is generated using wavelength conversion based on the cross-gain modulation of amplified spontaneous emission spectrum of the SOA. A negative band-pass filter and a broadband all-pass filter are synthesized to achieve a narrow notch filter with flat passband which can excise interference with minimal impact on the wanted signal over a wide microwave range. Experimental results show the notch rejection ratio of 30 dB, good shape factor and tunability.

Continuously tunable incoherent microwave photonic filter using a tunable Mach‒Zehnder interferometer as the slicing filter

AbstractA continuously tunable incoherent microwave photonic filter employing a tunable Mach‐Zehnder interferometer (MZI) as the slicing filter is studied. By incorporating the optical variable delay line (OVDL), the frequency spacing of the MZI can be tuned continuously, which in turn makes the whole microwave photonic filter continuously tunable in a wide frequency range. The frequency response characteristics are studied and some simulation results are also given to optimize the present microwave photonic filter. Experimental results show the present filter has good frequency response characteristics and tuning linearity. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2382–2386, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22732

A tunable and reconfigurable microwave photonic filter based on a Raman fiber laser

A tunable and reconfigurable microwave photonic filter is demonstrated by employing a tunable Mach–Zehnder interferometer (MZI) and a wavelength-controllable Raman fiber laser. The free-spectral-range and spectral response of the microwave photonic filter are tuned and reconfigured by controlling the optical variable delay line in one arm of the MZI and the powers of the pump LDs. Experimental results are given to show the enhanced flexibility of the present microwave photonic filter.

Wavelength-spacing continuously tunable multi-wavelength SOA-fiber ring laser based on Mach–Zehnder interferometer

A Mach–Zehnder interferometer (MZI) which is used as a wavelength-spacing tunable comb filter in a fiber ring laser is built by employing an optical variable delay line (OVDL). Stable multi-wavelength semiconductor optical amplifier (SOA)-fiber ring laser based on an SOA and the MZI comb filter is achieved. Wavelength spacing can be continuously tuned by adjusting the OVDL and, as an example, multi-wavelength lasing with the wavelength spacing of 0.4, 0.8, or 1.6 nm is demonstrated. The output of the proposed multi-wavelength SOA-fiber ring laser is quite stable at room temperature and the output spectrum can be adjusted by controlling the bias current of the SOA.

A reconfigurable architecture for continuously variable optical slow-wave delay lines

A novel reconfigurable architecture based on slow-wave propagation in integrated optical ring resonators is proposed for the realization of variable optical delay lines. A continuously variable delay is achieved by combining a coarse discrete (digital) delay, provided by a coupled resonator slow-wave structure, with a fine continuous (analog) delay given by a cascaded ring- resonator phase-shifter. The reflective configuration of the structure enables a simple, accurate and robust tuning of the delay and provides a footprint reduction by a factor 2 with respect to conventional coupled resonator optical waveguides. Proof-of-concept devices realized in 4.4% silicon oxynitride waveguides and activated by a thermal control are discussed. Experimental results demonstrate, in both spectral and time domain, a continuously variable delay, from zero to 800 ps (2 bit fractional delay), on a 2.5 Gbit/s NRZ signal, with less than 8 dB insertion loss and less than 5 mm2 device footprint.

Channel-spacing-tunable multi-wavelength fiber ring laser with hybrid Raman and Erbium-doped fiber gains

A multi-wavelength fiber ring laser of tunable channel spacing is proposed by employing an optical variable delay line (OVDL) in a Mach-Zehnder interferometer. Stable lasing is achieved at room temperature with the hybrid gains of a single mode fiber (as the Raman gain medium) and an Erbium-doped fiber (EDF) in a ring structure. The channel spacing of the present multi-wavelength fiber ring laser can be continuously tuned by adjusting the computer-controlled OVDL.