Microwave Photonics Applications Research Articles

Development of fiber Bragg gratings technology and their complex structures for sensing, telecommunications and microwave photonics applications

Abstract In this paper research on the development of the fiber Bragg grating (FBG) technology which has been conducted at the Institute of Electronic Systems (IES), Warsaw University of Technology (WUT) since 2004 is presented. In particular the directions in the development of advanced set-ups employing the phase mask inscription scheme are discussed and supported with the descriptions of structures designed and fabricated with the use of the laboratory stages constructed at the IES. The novelty of the presented solutions is based on the combination of numerous techniques of the external modification of the interferometric patterns (projected onto cores of photosensitive fibers to modulate their refractive index) as well as application of the modification of the internal properties of the waveguides themselves by the means of introducing strain or tapering. The development of these sophisticated set-ups resulted in the inscription of FBGs with precisely designed spectral characteristics which found application in telecommunications and sensor technology are also illustrated here. The paper is summarized with the specification of the most important achievements attained at the lab and drafting of the possible directions of further research.

Barium fluoride whispering-gallery-mode disk-resonator with one billion quality-factor.

We demonstrate a monolithic optical whispering-gallery-mode resonator fabricated with barium fluoride (BaF₂) with an ultra-high quality (Q) factor above 10⁹ at 1550 nm, and measured with both the linewidth and cavity-ring-down methods. Vertical scanning optical profilometry shows that the root mean square surface roughness of 2 nm is achieved for our mm-size disk. To the best of our knowledge, we show for the first time that one billion Q-factor is achievable by precision polishing in relatively soft crystals with mohs hardness of 3. We show that complex thermo-optical dynamics can take place in these resonators. Beside usual applications in nonlinear optics and microwave photonics, high-energy particle scintillation detection utilizing monolithic BaF₂ resonators potentially becomes feasible.

High-Power and High-Linearity Photodetector Modules for Microwave Photonic Applications

We demonstrate hermetically packaged InGaAs/InP photodetector modules for high performance microwave photonic applications. The devices employ an advanced photodiode epitaxial layer known as the modified uni-traveling carrier photodiode (MUTC-PD) with superior performance in terms of output power and saturation. To further improve the thermal limitations, the MUTC-PDs were flip-chip bonded on high thermal conductivity substrates such as Aluminum Nitride (AlN) and Diamond. Modules using chips with active area diameters of 40, 28, and 20 μm were developed. The modules demonstrated a 3-dB bandwidth ranging from 17 GHz up to 30 GHz. In continuous wave mode of operation, very high RF output power was achieved with 25 dBm at 10 GHz, 22 dBm at 20 GHz, and 17 dBm at 30 GHz. In addition, the linearity of the modules was characterized by using the third order intercept point (OIP3) as a figure of merit. Very high values of OIP3 were obtained with 30 dBm at 10 GHz, 25 dBm at 20 GHz and more than 20 dBm at 30 GHz. Under short pulse illumination conditions and by selectively filtering the 10 GHz frequency component only, a saturated power of >21 dBm was also measured. A very low AM-to-PM conversion coefficient was measured, making the modules highly suitable for integration in photonic systems for ultralow phase noise RF signal generation.

High quality-factor optical resonators

Various resonators are investigated for microwave photonic applications. Micro-sphere, disk and fiber ring resonators were designed, realized and characterized. Obtained quality factors are as high as Q = 1010.

Laser sources for microwave to millimeter-wave applications [Invited

We present several laser sources dedicated to advanced microwave photonic applications. A quantum-dash mode-locked laser delivering a high-power, ultra-stable pulse train is first described. We measure a linewidth below 300 kHz at a 4.3 GHz repetition rate for an output power above 300 mW and a pulse duration of 1.1 ps after compression, making this source ideal for microwave signal sampling applications. A widely tunable (5–110 GHz), monolithic millimeter-wave transceiver based on the integration of two semiconductor distributed feedback lasers, four amplifiers, and two high-speed uni-traveling carrier photodiodes is then presented, together with its application to the wireless transmission of data at 200 Mb/s. A frequency-agile laser source dedicated to microwave signal processing is then described. It delivers arbitrary frequency sweeps over 20 GHz with high precision and high speed (above 400 GHz/ms). Finally, we report on a low-noise (below 1 kHz linewidth), solid-state, dual-frequency laser source. It allows independent tuning of the two frequencies in the perspective of the implementation of a tunable optoelectronic oscillator based on a high-Q optical resonator.

Long-Term Frequency Stabilization of an Optoelectronic Oscillator Using Phase-Locked Loop

We demonstrate an approach of long-term frequency stabilization of a single-loop optoelectronic oscillator (OEO) with a phase-locked loop (PLL). The OEO is frequency locked to a highly stable microwave reference via a PLL circuit with a PID regulator. With this approach, we obtained a highly stable 3.035-GHz microwave signal with phase noise of −65 dBC/Hz at 10-Hz offset frequency, and the frequency stability has reached 6.98 × 10−14 at average time of 1000 s. These experimental results show that the PLL-locked OEO is greatly improved in both long-term frequency stability and phase noise performance at low offset frequencies. Therefore, the PLL-locked OEO can be a strong candidate of highly stable, low phase noise microwave generators for microwave photonics applications.

Monolithically Integrated Optical Phase Lock Loop for Microwave Photonics

We present a review of the critical design aspects of monolithically integrated optical phase lock loops (OPLLs). OPLL design procedures and OPLL parameters are discussed. A technique to evaluate the gain of the closed loop operating system is introduced and experimentally validated for the first time. A dual-OPLL system, when synchronised to an optical frequency comb generator without any prior filtering of the comb lines, allows generation of high spectral purity signals at any desired frequency from several GHz up to THz range. Heterodyne phase locking was achieved at a continuously tuneable offset frequency between 2 and 6 GHz. Thanks to the photonic integration, small dimensions, and custom-made electronics, the propagation delay in the loop was less than 1.8 ns, allowing good phase noise performance with OPLLs based on lasers with linewidths less than a few MHz. The system demonstrates the potential for photonic integration to be applied in various microwave photonics applications where narrow-bandwidth tuneable optical filters with amplification functionality are required.

High Power Photodiodes for Microwave Applications

High Power Photodiodes for Microwave Applications

Rate equation analysis of injection-locked quantum cascade lasers

The modulation properties of optical injection-locked quantum cascade lasers (QCLs) are investigated theoretically via a simple low dimensional rate equation model. It is found that both strong injection level and positive optical frequency detuning increase the modulation bandwidth, while a large linewidth enhancement factor (LEF) contributes to the enhancement of the peak magnitude in the intensity modulation (IM) response. As opposed to conventional injection-locked interband lasers, it is demonstrated that no dip occurs in the QCL's IM response, which is beneficial for a series of broadband microwave photonic applications. Computations also show that the value of the LEF can critically modify both the locking and stability regions on the optical frequency detuning injection level map.

A superhigh-frequency optoelectromechanical system based on a slotted photonic crystal cavity

We develop an all-integrated optoelectromechanical system that operates in the superhigh frequency band. This system is based on an ultrahigh-Q slotted photonic crystal (PhC) nanocavity formed by two PhC membranes, one of which is patterned with electrode and capacitively driven. The strong simultaneous electromechanical and optomechanical interactions yield efficient electrical excitation and sensitive optical transduction of the bulk acoustic modes of the PhC membrane. These modes are identified up to a frequency of 4.20 GHz, with their mechanical Q factors ranging from 240 to 1730. Directly linking signals in microwave and optical domains, such optoelectromechanical systems will find applications in microwave photonics in addition to those that utilize the electromechanical and optomechanical interactions separately.

Microwave Photonics Applications of Multicore Fibers

This is the first time, to our knowledge, that the application of multicore fibers (MCFs) to the field of microwave photonics (MWP) has been proposed. The implementation of sampled optical delay lines for RF signals by means of heterogeneous MCFs is addressed as a generic building block and discussion is then centered on several selected applications including filtering, optical beamforming, and arbitrary waveform generation.

Robust and environmental insensitive fiber optic Sagnac interferometer for microwave photonic applications

A technique that allows a fiber optic Sagnac interferometer based microwave photonic device to be implemented using non-polarization maintaining components inside the Sagnac loop while still obtaining an output that is insensitive to changes in environmental conditions is presented. It is based on inserting the non-polarization maintaining components in between a polarization beam combiner and a Faraday rotator mirror inside the loop. The technique also introduces a phase bias to the light propagating inside the loop. Experimental results demonstrate that the discretely and continuously tunable Sagnac loop based signal processors implemented using non-polarization maintaining components have an environmentally insensitive frequency response.

Generalized Tensor Analysis Model for Multi-Subcarrier Analog Optical Systems

We propose and develop a general tensor analysis framework for a subcarrier multiplex analog optical fiber link for applications in microwave photonics. The goal of this work is to construct an uniform method to address nonlinear distortions of a discrete frequency transmission system. We employ tensorial representation, i.e., multidimensional arrays, to unify the multi-subcarrier intermodulation effect. Based on the tensorial expression of the optical field in a subcarrier vector space, various kinds of modulation methods, dispersive transmission as well as detection schemes are specified. In addition, it is demonstrated that each corresponding tensor is formally determined by device structures, which allows for a synthesized study of device combinations more systematically. For implementing numerical methods, the practical significance of the tensor model is it simplifies the derivation details compared with series-based approaches by hiding the underlying multi-fold summation and index operation. The integrity of the proposed methodology is validated by investigating the classical intensity modulated system. Furthermore, to give an application model of the tensor formalism, we make a study of two multi-subcarrier systems with detailed performance discussions. We believe the tensor model provides us not only a consolidated notation, but also an alternative numerical approach to effectively analyze multi-subcarrier analog optical systems.

Dynamic Microwave Photonic Filter Using Separate Carrier Tuning Based on Stimulated Brillouin Scattering in Fibers

Dynamic reconfiguration of a microwave photonic filter by tuning its basic delay based on stimulated Brillouin scattering-induced slow light and optical phase shift of the optical carrier is experimentally implemented. The measurements confirm that the free spectral range of the filter changes when a Brillouin pump is applied. These results demonstrate the potential of the separate carrier technique in microwave photonics applications.

Slow and Fast Light Effects and Their Applications to Microwave Photonics Using Semiconductor Optical Amplifiers

We provide a comprehensive review of the application of slow and fast light (SFL) techniques to the field of microwave photonics. Basic principles leading to the implementation of phase shifting and true time delay operations which are instrumental in this field are first considered. We then focus on the description of the main results obtained by our groups in the implementation of broadband, full 360 phase shifting using coherent population oscillations in semiconductor waveguides. Next, attention is given to the evaluation of the system impairments implied by these devices when included in analog links. Finally, the main results obtained for several microwave photonic applications such as filtering, arbitrary waveform generation and optoelectronic oscillators (OEOs) are reviewed, and other directions for future research in the field are discussed.

Magnesium Fluoride Whispering Gallery Mode Disk-Resonators for Microwave Photonics Applications

We have manufactured a high-Q whispering gallery mode resonator with magnesium fluoride for microwave photonics applications in the gigahertz frequency range. This crystal is scarcely used for resonator fabrication despite its numerous advantages, that are mainly high mechanical hardness, low sensitivity to water vapor pollution, and low sensitivity to photo- and thermo-refractive fluctuations at optimal temperatures. Using a customized machining procedure, we have successfully fabricated and characterized a 5-mm resonator with a surface rugosity of the order of 1 nm (from 3 to 12 atoms). Cavity ring-down measurements enabled us to determine that the resonator has a quality factor Q = 3.4 × 108 at 1550 nm.

Orthogonally polarized optical single sideband modulation for microwave photonics processing using stimulated Brillouin scattering

We present a novel technique to generate orthogonally polarized optical single sideband modulated signals. The modulation scheme is based on all optical stimulated Brillouin scattering processing of the optical carrier of an optical single sideband modulated signal, by means of the polarization state dragging induced by this non-linear effect. This modulation technique can be used in several microwave photonics applications, such as antenna beamforming or microwave photonics filters. In order to perform a proof-of-concept experiment, the orthogonal modulator is deployed for the implementation of an RF phase-shifter.

Experimental investigation of relative intensity noise in Brillouin fiber ring lasers for microwave photonics applications

Intensity noise characteristics of a single-mode Brillouin fiber ring laser are experimentally analyzed from 100 kHz up to 18 GHz. The Stokes wave is shown to be shot-noise limited to -155 dB/Hz for a 1 mA detected photocurrent over the whole spectral range 100 MHz-18 GHz. The pump-to-Stokes noise filtering efficiency is evaluated by artificially increasing the pump intensity noise. It evidences that a shot-noise-limited Brillouin laser could be realized by using a narrow-linewidth semiconductor laser pump, for stringent microwave photonics applications.

Controlling Microwave Signals by Means of Slow and Fast Light Effects in SOA-EA Structures

We present a novel scheme for the control of microwave signals in the optical domain. We propose the use of alternating amplifying and absorbing sections to implement phase control by using fast and slow light effects in semiconductors. The potential benefits from the proposed semiconductor optical amplifier and electroabsorber structures are the high tuning speed, the continuous scan of the phase delay that brings antenna angular continuing scanning, the small size, the capability of integration, the low insertion losses, and the low bias voltage. We have obtained phase changes of almost 60deg around 10 GHz using commercially available components that were not optimized for this purpose. These results indicate a potential for several microwave photonic applications including the change of the direction of the radiation pattern of photonic phase-array antennas or the implementation of fast tunable microwave photonic filters.

Slow Light in a Semiconductor Waveguide for True-Time Delay Applications in Microwave Photonics

We have investigated the slow and fast light properties of a semiconductor waveguide device employing concatenated gain and absorber sections. This letter presents the experimental results as well as theoretical modeling. A large phase shift of 110deg and a true-time delay of more than 150 ps are demonstrated. The combination of amplitude and phase control of the modulated signal shows great promise for applications within microwave photonics.