Tunable Optical Delay Research Articles

On-Chip Digital Fourier-Transform Spectrometer Using a Thermo-Optical Michelson Grating Interferometer

This theoretical modeling and simulation paper presents designs and projected performance of an on-chip digital Fourier transform spectrometer using a thermo-optical (TO) Michelson grating interferometer operating at ∼1550 and 2000 nm for silicon-on-insulator and for germanium-on-silicon technological platforms, respectively. The Michelson interferometer arms consist of two unbalanced tunable optical delay lines operating in the reflection mode. They are comprised of a cascade connection of waveguide Bragg grating resonators (WBGRs) separated by a piece of straight waveguide with lengths designed according to the spectrometer resolution requirements. The length of each WBGR is chosen according to the Butterworth filter technique to provide one resonant spectral profile with a bandwidth twice that of the spectrometer bandwidth. A selectable optical path difference (OPD) between the arms is obtained by shifting the notch in the reflectivity spectrum along the wavelength axis by means of a low-power TO heater stripe atop the WBGR, inducing an OPD that depends on the line position of the WBGR affected by TO switching. We examined the device performances in terms of signal recostruction in the radio-frequency (RF) spectrum analysis application at 1 GHz and at 1.5 GHz of spectrometer resolution. The investigation demonstrated that high-quality spectrum reconstruction is obtained for both Lorentzian and arbitrary input signals with a bandwidth up to 40 GHz. We also show that spectrum reconstruction of 100–200 GHz RF band input signals is feasible in the Ge-on-Si chips.

Tunable optical delay line based on integrated grating-assisted contradirectional couplers

Tunable optical delay lines are one of the key building blocks in optical communication and microwave systems. In this work, tunable optical delay lines based on integrated grating-assisted contradirectional couplers are proposed and experimentally demonstrated. The device performance is comprehensively improved in terms of parameter optimization, apodization analysis, and electrode design. Tunable group delay lines of 50 ps at different wavelengths within the bandwidth of 12 nm are realized with a grating length of 1.8 mm. Under thermal tuning mode, the actual delay tuning range is around 20 ps at 7.2 V voltage. At last, a new scheme adopting an ultra-compact reflector for doubling group delay is proposed and verified, achieving a large group delay line of 400 ps and a large dispersion value up to 5.5×106 ps/(nm·km) within bandwidth of 12 nm. Under thermal tuning mode, the actual delay tuning range is around 100 ps at 8 V voltage.

Low-Loss Continuously Tunable Optical True Time Delay Based on Si3N4 Ring Resonators

We design, fabricate, and characterize ultra-low loss continuously tunable optical true time delay devices based on Si3N4 ring resonators in a side-coupled integrated spaced sequence of resonators (SCISSOR) structure. A large tunable delay range up to 3.4 ns is demonstrated using the Balanced SCISSOR delay tuning scheme, for a record loss of only 0.89 dB/ns of delay. By optimizing the coupler design a device delay bandwidth of over 10 GHz is achieved with over 0.5 ns maximum time delay. This ultra-low loss delay line can enable distortion free operation for RF signals used in RF beamforming and other applications. Additionally, this device has the potential to be integrated with active photonic components to be part of a fully integrated photonic integrated circuit in an electronically scanned phased array system; utilizing a silicon photonics platform including heterogeneous integration.

Reconfigurable monitoring and control system for tunable optical delay lines.

In this Letter, we propose a monitoring and control system (MCS) for operating tunable optical delay lines (TODLs), regardless of their operation principle and implementation technology. The monitoring system resorts to two out-of-band pilot tones added to the input optical signal. The amplitude and phase difference between tones are retrieved to the control system, which calculates and applies the TODL control signals. The MCS was validated using a Mach-Zehnder delay interferometer-based TODL, implemented in three different silicon photonic integrated circuits (PICs). The three PICs resort to different kinds of phase shifters based on thermo-optic, carrier-injection, and carrier-depletion effects. The proposed MCS enabled tuning the delay within the entire range of the TODL in all tested PICs. The scalability of the MCS for large-scale photonic beamformers is discussed.

Phase-shifted Solc-type filter based on thin periodically poled lithium niobate in a reflective geometry.

Configurable narrow bandwidth filters are indispensable components in optical communication networks. Here, we present an easily-integrated compact tunable filtering based on polarization-coupling process in a thin periodically poled lithium niobate (PPLN) in a reflective geometry via the transverse electro-optic (EO) effect. The structure, composed of an in-line polarizer and a thinned PPLN chip, forms a phase-shift Solc-type filter with similar mechanism to defected Bragg gratings. The filtering effect can be dynamically switched on and off by a transverse electric filed. Analogy of electromagnetically induced transparency (EIT) transmission spectrum and electrically controllable group delay is experimentally observed. The mechanism features tunable center wavelength in a wide range with respect to temperature and tunable optical delay to the applied voltage, which may offer another way for optical tunable filters or delay lines.

Tunable Optical Buffer through an Analogue to Electromagnetically Induced Transparency in Coupled Photonic Crystal Cavities

Tunable on-chip optical delay has long been a key target for the research community, as it is the enabling technology behind delay lines, signal retiming and other applications vital to optical signal processing. To date, the field has been limited by high optical losses associated with slow light or delay structures. Here, we present a novel tunable delay line, based on a coupled cavity system exhibiting an electromagnetically induced transparency-like transmission spectrum, with record low loss, around 15 dB/ns. By tuning a single cavity, the delay of the complete structure can be tuned over 120 ps, with the maximum delay approaching 300 ps.

Design of an ultra-compact graphene-based integrated microphotonic tunable delay line.

The design of a continuously tunable optical delay line based on a compact graphene-based silicon Bragg grating is reported. High performance, in terms of electro-optical switching time (tswitch < 8 ns), delay range (Δτ = 200 ps), and figure of merit FOM = Δτ/A = 1.54x105 ps/mm2, has been achieved with an ultra-compact device footprint (A ~1.3 x 10-3 mm2), so improving the state-of-the-art of integrated optical delay lines. A continuous and complete tunability of the delay time can be achieved with a very low delay loss ( = 0.03 dB/ps) and a weak power consumption ( = 0.05 mW/ps). A flat bandwidth B = 1.19 GHz has been calculated by exploiting the slow-light effect in the device. This performance makes the proposed optical delay line suitable for several applications in Microwave Photonics (MWP), such as beamsteering/beamforming, for which large delay range, flat and wide bandwidth and small volume are required.

Fiber-type difference frequency generation infrared optical frequency comb based on the femtosecond pulses generated by a mode-locked fiber laser

Optical frequency comb (OFC) is a new type of high-quality laser source. The visible and near-infrared OFCs have become mature, and it has been widely used in optical frequency metrology, time/frequency transfer, precision laser spectroscopy and other fields. Since the mid and far-infrared spectral regions contain a large number of baseband absorption lines for molecules and the absorption intensities are several orders of magnitude higher than those in the visible and near-infrared spectral region, one has made great efforts to develop the mid and far-infrared OFCs in recent years. Although a variety of approaches to achieving infrared OFCs directly have been proposed, the method of difference frequency generation (DFG) infrared OFC based on the optical rectification technique is still more efficient. DFG infrared OFCs with widely tuning ability have been demonstrated based on fiber lasers so far. However, how to obtain the broadband spectrum for a DFG infrared OFC with widely tuning ability still needs to be solved. In this paper we report a fiber-type DFG infrared OFC by using the femtosecond pulses from a mode-locked erbium-doped fiber laser as the fundamental light. Based on the self-developed mode-locked fiber laser oscillator with repetition rate locked, the two-color fundamental pulse trains with the central wavelengths of 1.5 and 2.0 m are respectively achieved after the chirped pulse fiber amplification and all-fiber supercontinuum (SC) generation techniques have been utilized. With a time-domain synchronous detection system based on the intensity autocorrelation principle, the accurate synchronization with the fundamental two-color pulses is obtained by optimizing the OFS compensated fiber length and adjusting a tunable optical delay line. Finally, by using the optical rectification technique, a fiber-type DFG infrared OFC is successfully generated with the help of a suitable designed GaSe nonlinear crystal. Our experimental results also show that the spectral location of the DFG infrared OFC can be tuned by controlling the spectral shape of the SC combined with the adjustment of the phase-matching for the nonlinear crystal. The measured tuning range of the DFG infrared OFC is from 6 to 10 m, and the maximum spectral width is 1.3 m. This fiber-type DFG infrared OFC may play an important role in the molecular spectroscopy, the atmospheric environmental monitoring, and other fields.

Tunable low dispersion optical delay line using three coupled micro-resonators

There is a large demand for integrated photonic devices capable of producing controllable pulse delays of optical signals. Normally, the achievable delay is proportional to the length of the optical path making it difficult to achieve significant delays in chip based devices. We propose a tunable optical delay line consisting of a waveguide coupled in an all pass geometry to a serial array of three micro-ring resonators. The coupling between the whispering gallery modes of the rings leads to a triplet of absorption resonances in between which the group delay is very large while pulse dispersion and attenuation is very small. Moreover, the delay and bandwidth are widely tunable by the coupling between the resonators with delays and bandwidths up to several hundred picoseconds and several tens of GHz, respectively. Although attenuation between the resonances is small, we show that including a modest amount of gain in the second and third resonators eliminates the attenuation regardless of how large the losses are in the first resonator.

1-m tunable optical delay line using microfluid sliding in a hollow-core fiber: Feasibility study

A novel variable optical delay line based on a hollow-core photonic bandgap fiber is proposed. The device incorporates microfluid, the end surface of which serves as an optical reflector, in the hollow-core of the fiber. The position of the fluid end is controlled by a syringe pump to change the optical delay of the reflected beam. We demonstrate wide tunability of the optical delay up to 1 m with a scan speed of several mm/s. The return loss and beat pattern in the reflected signal is studied and the potential of the device as an ultra-long delay line is discussed.

Continuously tunable ultra-thin silicon waveguide optical delay line

As light cannot be stopped or directly stored in any media, optical delay lines are usually used to temporally trap the optical signals. We report a wide-range continuously tunable optical delay line chip consisting of a ring resonator array and a Mach–Zehnder interferometer (MZI) switch array on the 60-nm-thick silicon waveguide platform. The ring delay line provides continuous delay tuning of more than 10 ps with a push–pull differential tuning method. The MZI switchable delay line provides digitally programmable delay tuning with a resolution of 10 ps upon reconfiguration of the MZI switches to establish different optical routing paths. Dual-stage MZI switches are used to ensure low crosstalk with an improved signal-to-noise ratio. The delay line chip can generate a maximum delay of >1 ns with an on-chip insertion loss of 12.4 dB. Optical pulse time-division multiplexing and quasi-arbitrary waveform generation are realized based on the delay line chip. These results represent a significant step towards the realization of highly reconfigurable optical signal processors enabled by optical delay manipulation with broad applications for optical communications and microwave photonics.

Demonstration of Over-the-Air RF Self-Interference Cancellation Using an Optical System

We demonstrate an analog optical self-interference cancellation system for over-the-air multipath cancellation in radio-frequency communications. Self-interference cancellation is achieved by subtracting a matched copy of the self-interference signal from the corrupted received signal. The system uses optical signal processing instead of traditional radio-frequency approaches to achieve wide instantaneous bandwidth and high precision. Channel matching is performed entirely in the optical domain with variable optical attenuators and tunable optical delays. A singlemode to multimode combiner enables the cancellation of up to six multipath components using just two optical sources. The system achieved 35 dB of narrowband cancellation and 30 dB of cancellation across 20 MHz of instantaneous bandwidth, with multipath cancellation contributing more than 10 dB of cancellation at the signal of interest frequency.

Proposal for all-optical switchable and tunable ultrawideband monocycle generation utilizing SOA wavelength conversion and time delay

An all-optical ultrawideband monocycle generator based on wavelength conversion in a semiconductor optical amplifier (SOA) and optical tunable delay in an optical delay line (ODL) is proposed and simulated. The system achieves optically switchable in pulse polarity and tunable in both the pulsewidth and radio frequency (RF) spectrum.

Low coherence technique to interrogate optical sensors based on selectively filled double-core photonic crystal fiber for temperature measurement

In this paper, an optical fiber sensing system based on low coherence interferometry (LCI) is proposed and demonstrated to interrogate sensors comprised of selectively filled double-core photonic crystal fiber (SFDC-PCF). The sensor used here is made by selectively filling about 1/3 area of air holes in the cladding of photonic crystal fiber with distilled water. So the dual-core in the sensor has different effective refractive indices, resulting in a phase delay between two lights transmitting in the fiber. The phase delay of the sensor can be compensated by a Mach-Zehnder interferometer with a scanning optical tunable delay line in one arm of the interferometer, namely temporal interrogation. By tracking the value of phase delay, the change of the measurand can be detected. Temperature measurement is carried out to testify the system performance. An average sensitivity of 0.9µm/°C is achieved within the temperature range of 29–92°C. This work provides a new thinking for fiber sensing technology based on LCI. The proposed all-fiber sensing system, with the merits of cost-effective, stability, and flexibility, can demodulate the SFDC-PCF sensor signals well. Further improvements such as better sensitivity, larger measurement range and multiplexing efficiency can be realized by tailoring the PCF sensor's structure.

A Tunable Focus Lens-Based Variable Optical Delay Line for Free-Space and Fiber-Optic Applications

We present a novel variable optical delay line (VODL) design for free-space and fiber-optic applications. The design employs multiple electronically controlled tunable lenses (ECTLs) to vary the optical path of a beam between two fixed locations. The use of ECTLs entirely eliminates bulk motion of optical elements as variable signal delay is achieved by electronically tuning the focal lengths of the ECTLs. For each unique set of focal length values the beam follows a different path through the system which results in an ECTL-induced variable signal delay between the VODL input and output ports. The design provides a repeatable digitally controlled short variable signal delay as well as a small signal delay step size for short delay measurements.

폴리머 단일 링 Add/Drop 필터와 지연 도파로로 구성된 튜닝 가능 광 신호 지연기

튜닝 가능한 광 신호 지연기를 설계, 제작 및 특성 측정을 하였다. 광 신호 지연기는 네 개의 폴리머 링 공진기 add/drop 필터들과 그 사이에 배치된 지연 도파로들로 구성되었다. 폴리머 도파로는 한변의 길이가 <TEX>$1.8{\mu}m$</TEX> 인 정사각형 매립 구조이고, 코어와 클래딩의 굴절율은 각각 1.48과 1.37이다. 이와 같은 도파 구조로 인하여 매우 작은 반경의 곡선도파로를 활용함으로써, 콤팩트한 소자를 실현할 수 있다. 각각의 add/drop 필터의 링 공진기 상에 전극을 형성하여 열 광학 효과에 의한 튜닝이 가능하도록 하였다. 측정 결과, 각각의 add/drop 필터를 튜닝함으로써 지연 도파로의 수에 비례하는 지연 시간인 110 ps, 225 ps, and 330 ps를 확인할 수 있다. A tunable optical delay line is designed, fabricated, and characterized. The tunable delay line consists of four polymer-ring add/drop filters with delay waveguides between adjacent ones. The polymer waveguide is a buried structure, designed to be square with core width and height of <TEX>$1.8{\mu}m$</TEX>. The refractive indices of the core and cladding polymer are 1.48 and 1.37 respectively. The large index difference and small cross section of the waveguide enable us to realize a compact device using a small radius of curvature. Four pairs of electrodes are evaporated above the add/drop filters to provide heating currents for thermal tuning. In measurements we can identify variable time delays of 110, 225, and 330 ps in proportion to the number of delay lines.

Photonic True-Time-Delay Beamformer for a Phased Array Antenna Receiver based on Self-Heterodyne Detection

In this paper, we propose a novel photonic true-time delay beamforming system for a phased array antenna receiver. The beamformer relies on simple tunable optical delay lines and also on self-heterodyne coherent detection. Each tunable delay line is implemented by a MachZehnder delay interferometer with tunable coupling ratio, which allows varying the delay from 0 up to the delay of the interferometer. Self-heterodyne coherent detection allows achieving maximum sensitivity inherent to coherent detection, and also laser phase noise cancellation, photonic RF phase shifting, and photonic RF frequency downconversion. The proposed system is analytically described and numerically validated. Numerical results show that, considering a beamsteering range of $ \pm 30^\circ $ , the system is able to beamform the signals received by ${\boldsymbol{N}}$ antenna elements with negligible distortion, thereby proving to be very promising for high-end phased array antenna receivers operating at high RF frequencies.

Cost-effective fiber multiplexing system based on low coherence interferometers and application to temperature measurement

Based on the low-coherence interferometric principles, a cost-effective all-fiber Mach-Zehnder multiplexing system is proposed and demonstrated. The system consists of two interferometers: sensing interferometer and demodulation interferometer. By scanning an optical tunable delay line back and forth constantly with a stable speed, sensing fibers with different optical paths can be temporal interrogated. The system is experimentally proved to have a high performance with a good stability and low system noises. The multiplexing capacity of the system is also investigated. An experiment of measuring the surrounding temperature is carried out. A sensitivity of 12 μm/°C is achieved within the range of 20°C to 80°C. This low cost fiber multiplexing system has a potential application in the remote monitoring of temperature and strain in building structures, such as bridges and towers.

Tunable optical delay line for optical time-division multiplexer

A novel three-stage all-pass filter (APF) is proposed as a tunable optical delay line to construct an optical time division multiplexer (OTDM), with which, we can get ultrahigh bit rates with several low-speed channels. The proposed design mitigates the deleterious effects of group delay dispersion and provides wide bandwidth with small ripples and continuously tunable long delays achieved with small variations in the effective refraction index, making it suitable for high-speed optical networks on chip.

Continuously tunable femtosecond delay-line based on liquid crystal cells.

We introduce a new device for group and phase delay steering of femtosecond pulse trains that makes use of cascaded, electrically driven, nematic liquid-crystal cells. Based on this approach we demonstrate a continuously tunable optical delay line. The simple collinear implementation with no moving parts enables to shape the achievable temporal range with sub-femtosecond accuracy. By appropriately choosing the bias voltages applied to the cascaded cells, the imparted group delay can be made either positive or negative and precisely adjusted. Moreover, independent control of the group delay and the phase of femtosecond pulses is demonstrated.