Research Interests

Aerospace and Defense

Riza’s Defense Optics activities started from his Caltech Ph.D. student days in the mid-80’s where he showed for the first time how Acousto-Optics could be used to control transmit and receive mode RF phased array radars/antennas (Applied Optics 1991) including simultaneous multiple beamforming implementing RF spatial beam multiplexing (IEEE PTL 1992). Later work by Riza at General Electric  revolved around developing next generation highest phase stability acousto-optic (AO) RF signal processors such as convolvers, correlators, and spectrum analyzers for wide instantaneous bandwidth Electronic Warfare (EW) applications (Applied Optics 1992, 1994; IEEE PTL 1995). Additional AO signal processing innovations included powerful Range-Doppler (SPIE 1998) and Nulling filter (SPIE 1994, 1996) optical implementations suited for extreme RF bandwidth and signal complexity radar signal processing. These Riza invention later underwent prototyping and testbed tech transfers at US Air Force Rome Labs and Photonics Systems, Inc., Melbourne, Florida. In 1992, Riza also invented a 2-D Spatial Light Modulator (SLM)-based powerful high tap and weight count time-delay-based transversal Radio Frequency (RF) filter (US Patent 5329118). In 1995 for tech transfer & commercialization, Lockheed Martin acquired Riza’s GE patents on radar controls. In 1999, Riza wrote the Chapter on AO signal processing for RF signals in the Wiley Encyclopedia of Electrical & Electronics Engineering. In 2004, Riza introduced a compact polarization independent version of his original RF filter (Applied Optics).

Another aspect of the Riza group innovations while at GE and later in academia includes a series of pioneering photonic beamforming control systems for Phased Array Radars and Antennas for both narrowband and wideband operations. These innovations formed the most diverse designs for RF beamformer proposed as they engaged a diverse tool set of device technologies including phase control liquid crystals (LCs), polarization rotation LCs, polarization sensitive LC gratings, optical MEMS, AOs, liquid lens, fibers, fiber gratings, wavelength tunable and broadband sources, wavelength tunable filters including multi-wavelength selection, wavelength division multiplexers and solid-optics (in IEEE, OSA, SPIE, Elsevier Publications & US Patents). These works also included the first high speed designs and improvements to realize multiple simultaneous phased array antenna beams in space (IEEE 1992, SPIE &1EEE 2003). In 1997, Riza edited the most comprehensive book in the field of Photonics for Antenna/Radar Controls (SPIE Press). In 2001, Riza invented the most flexible photonic beamformer interface for phased array antennas using LC on silicon SLM device (IEEE 2001, Elsevier 2003). In 2004, Riza invented new Hybrid Photonic Signal Processing technique that for the first time combines the worlds of analog and digital photonic signal processing leading to world record numbers of 16 bits RF time delay controls with both long time delays and at high time resolutions (IEEE JLT). In 2009, Riza proposed the first silicon photonics wideband RF phased array antenna beamformer design (SPIE 2010) and a decade later, this subject continues to be a hot topic of research.

In 2002, Riza also pioneered a novel hybrid optical-RF phased array antenna control system that used the same optical hardware to both control an RF array antenna and a steered freespace optical beam (US SBIR 2002). Riza is also a pioneer in the use of optics for security systems as in 1994 (IEEE 1995, SPIE 1996), he pioneered the first optical security systems using multi-dimensional optical ID cards including use of interferometric sensing and RF coded card ID generation.

Health and Imaging

Riza’s interest in optics for health and imaging goes back to his 1989 General Electric (GE) days with the development of the simplest control liquid crystal electronic lens motivated by vision applications. In 1995, Riza’s group introduced the use of optics for phased array ultrasound controls including the concept of intracavity imaging using optical wavelength-to-image space mapping using a single optical fiber probe and wavelength coding of the spatial array space. Since 1995, the group has also proposed several world first novel optical imaging systems such as in 2001 with the first Digital Micromirror Device (DMD)-based agile pixel imager using a point photo-detector (2002 Applied Optics) (often called single pixel imager), in 2002 with the first acousto-optics-based Optical Coherence Tomography (OCT) system (Applied Optics 2003),  and in 1999 with a first wavelength coded optical imager with transmit/receive (T/R) optical amplification and agile wavelength switching & gain controls with tunable or broadband light sources for wavelength-to-space 1-D, 2-D and 3-D W-MOS (Wavelength Multiplexed Optical Scanner) sensing and endoscopy, as well as optical energy delivery for ablation, cutting/surgery (IEEE 1999). The WMOS imager or spectrally encoded optical imager was also extended by Riza in 2000 (SPIE 2000) to the Interferometric WMOS or I-WMOS imager design for higher sensitivity imaging including 3-D imaging where depth information can be discriminated using variable phase/optical delay control in the interferometer reference arm (SPIE 2000, IEEE 2002, Agile High Sensitivity Optical Sensor, US Pat. Appl. 2003 ). The Riza lab wavelength coded imaging techniques have been extended by others using optical wavelength to time sequential mapping via fiber dispersion (also called time stretch) to form the STEAM ultrafast imager (AIP 2008). In addition, the Riza lab spectrally encoded imager design including the high sensitivity I-WMOS or interferometric spectral imager invention (SPIE 2000) have been extended and demonstrated for numerous successful medical imaging applications including medical endoscopes such as by leading medical schools (e.g., Harvard Univ.).

Other pioneering works include Agile Ultrasound Modulated Optical Tomography Techniques using Smart Fiber-Optics (SPIE 2004), Angstrom sensitivity high speed scanning interferometric imagers (AIP 1996), fiber-based scanning imaging endoscopes (SPIE 2000), confocal programmable spectral microscopes (AIP 2005), and world first Electronically controlled Variable Focal Length Lens (ECVFL) or E-lens based agile no-moving parts microscopes (SPIE 2004), endoscopes (OSA 2002) and human vision testers (GE 1990).

Riza is also a pioneer in the use of the Digital Micromirror Device (DMD) to design a variety of high speed programmable broadband spectrum operation optical interferometers. In 1997 (SPIE Proc. 3160, 1997), Riza proposed the use of programmed gratings on the DMD to spatially steer light. In 2009,  Riza proposed the design of a variety of DMD-based wavefront splitting interferometers such as the classic Young’s Double slit interferometer with programmable slit sizes and slit gaps (N. A. Riza, “Agile optical image sensing, control, and measurement modules,” USA Patent Application No.61/258,344, 2009; also see 2009 patent appl. Design figures in SPIE Proc. 9896, 2016; later experimentally demonstrated this system in SPIE 2016) and a compact symmetric paths multi-channel Michelson-style programmable wavefront interferometer (also in 2009 US Patent Appl. and SPIE Proc.9896, 2016). Later several other labs have independently used the 2009 proposed Riza DMD-based Young’s double slit interferometer design to conduct a variety of spatial coherence measurements for light sources (see 2014 Opt Lett., 2017 Opt. Express, 2021 Optik, JOSAA 2022).

A more recent world first is the invention of the Optical Array Device based agile pixel full spectrum (UV-SWIR) imager (e.g., using the DMD and/or LED array) to form the Active-Passive mode Coded Access Optical Sensor (CAOS) camera for extreme linear dynamic range smart imaging (JEOS 2015). The CAOS camera set a world record of 177 dB linear dynamic range (IEEE 2019) and is uniquely designed on the principles of time-frequency RF coding deployed in multi-access RF wireless phone networks. Such a design inherent provides extreme noise suppression and linearity via RF coding and frequency (Hz) domain filtering (e.g., via DSP methods). One CAOS design features a triple coding feature (Applied Optics 2021) use space-frequency-code manipulations giving a world first extreme embedded security camera. The smart CAOS camera features the hybrid CAOS-CMOS/CCD/FPA mode delivering a novel Pixels of Interest (POI) extreme linear dynamic range camera that combines the best features of CAOS with classic CMOS/CCD/FPA multi-pixel sensors (OSA 2016). Calibrated white light color imaging comparing the CAOS and CMOS cameras has also been conducted proposing how the CAOS camera can overcome the limitations of a CMOS camera when viewing a high linear dynamic range color image (MDPI 2021). The Riza lab in 2019 also pioneered a novel calibration-based minimalistic multi-exposure digital sensor camera robust linear high dynamic range enhancement technique that can be applied to all classic CMOS/CCD/FPA multi-pixel cameras (IS&T & Elsevier 2021).

Communications

Riza and his group’s contributions in the field of communications include pioneering works in RF wireless communications, optical wireless communications and fiber-optic communications.

Riza is a pioneer in the use of MEMS for fiber-optic switching (SPIE 1992, US Patent 1993). In the fiber-optic signal attenuation arena, the group has pioneered the most diverse set of variable optical attenuator (VOA) modules using optical MEMS, Acousto-Optic (AO), liquid and Liquid Crystal (LC) technologies (IEEE 1998). Riza invented the fault-tolerant all-digital paradigm for designing fiber-optic signal power conditioning and routing modules that include multi-wavelength equalizer modules (OSA 1999). This technique provides 100% repeatable and energy efficient controls for light flow controls in fiber communications. Riza pioneered the design  of fiber-optic signal conditioning and routing modules using the reliable Texas Instruments (TI) Digital Light Processing (DLP) technology, namely using the Digital Micromirror Device (DMD). The group for example proposed and demonstrated using the DMD, the world’s first fault-tolerant variable attenuators, all-digital fiber-optic switches, spectral equalizers, tunable laser designs and wavelength sensitive add-drop filters. Riza also invented the Hybrid MEMS/LC design for fiber-optic applications including the hybrid analog-digital design for producing world record performance VOAs (IEEE 2005). Other Riza inventions include super low noise LC switches and low loss MEMS, LC, and hybrid MEMS-LC 3-D cross-connect switches. Specifically, Riza invented the world’s first N x N large port count crossconnect design using 3-D beamforming optics that allowed inherent light power attenuation control at localized fiber ports and also proposed and demonstrated the exceptionally low near zero coupling loss fiber-to-freespace-to-fiber connection designs such as using the self-imaging fiber-to-free-space-to-fiber design (IEEE 1998, US Patent 2000, SPIE 2000 and 2003, OSA 1999 and 2003). These methods have been deployed and extended in commercially deployed fiber-optic modules including the very large crossconnect optical circuit switches (e.g., Calient Technologies) that are performing ultra-low loss optical circuit switching operations  in many data centers around the world with over 750,000 deployed fiber terminations. It is important to note that Riza’s near zero coupling loss fiber-to-freespace-fiber self-imaging designs reduced optical power losses with over an order of magnitude impact and in fact, with larger freespace propagation distance between fibers such as needed in crossconnect circuit switches, created many orders of magnitude reduction in optical losses. Thus vital energy savings occurred with such low loss Fiber-optic signal controls designs enabling acceleration and development of the modern internet for the world high bandwidth data communications. In effect, greener and wide spread operations for the internet data services were possible as the energy saved due to lower loss optical designs was energy well spent elsewhere such as in fiber-optical amplifiers that require additional pump laser energy to boost data signals in the fiber transmission lines typically placed every 40 km. Thus the Riza low losss designs also created in-direct and critical impact for the overall infrastructure of the growing and evolving internet.

Many of the Riza’s fiber-optical design inventions underwent prototyping and commercialization at Nuonics, Inc., a 1998 startup company founded by Riza. The Nuonics design technology underwent acquisition and sale to a global commercial corporation. This DMD-based design technology and its extension is deployed in many commercialized products (e.g., Newport Corp., Cidra Corp., Nistica Corp/NTT Electronics/Fujikura/Molex) such as fiber-optic multiwavelength equalizers and routers across the world fiber-optic communications and internet infrastructure.

In 1992, Riza & his GE colleagues pioneered spatial optical CDMA, a space division multiplexing (space MUX) method to transmit multiple simultaneous data channels, both for optical wireless and multi-fiber bundle optical data communications (SPIE 1992). The method combined space-time coding to realize a multiaccess optical network. A decade later, many efforts evolved around the world to deploy Space Mux in few mode fiber and multicore fiber to increase data carrying capacity of fiber networks. Realizing that coupling freespace interconnected light between single mode fibers (SMFs) can be a challenge, Riza in 1994 proposed the first use of a variable focus lens for SMF fed optical wireless short range interconnects (IoP 1994) and later extended the work to incorporate 3-D beamforming for robust SMF-freespace-to-SMF coupling controls (SPIE 1997).

Continuing with the same theme of SMF-freespace coupling, the group also developed the most advanced theoretical models for freespace-to-SMF lens coupling (OSA 1999, 2003) that has realized near zero loss SMF-to-freespace-to-SMF optical power transfer for short distances as well as for longer range optical wireless link distances (Elsevier 2006), including the use of smart optical 3-D beamforming combining cascaded strong and weak lensing via the self-imaging mechanism to form ultra-low loss point-to-point optical datacom links as well as for low loss optical wireless power transfer (IEEE 2012, US Patent Appl. 2014). These smart 3-D beamforming works have been incorporated around the world to maximize received link optical power for adequate signal-to-noise (SNR) to set optical wireless data transmission records for terrestrial, indoor and underwater links as well as highly efficient optical wireless power transfer links.

Riza introduced and demonstrated the first smart indoor multiple beams optical wireless concept (IEEE 1999) using agile 3-D optical beamforming-based reconfigurable light (for both laser and LED light) to produce  both Line of Sight (LOS) and Non Line of Sight (NLOS) (SPIE 2013) robust and high efficiency indoor optical wireless communications, including its first application for data centers (JEOS 2011, IEEE 2012). Others around the world have used the Riza multibeam reconfigurable indoor optical wireless system to set world indoor optical wireless data records for eye safe telecom C-band operations and develop systems for IEEE Industry 4.0 Standard. In addition, designers have used the Riza lab SMF-freespace ultra-low loss coupling models and techniques to commercially deploy millions of SMF-freespace/solid optic-based components that are deployed across the world optical communications and internet infrastructure.

In the free-space optical wireless domain, Riza invented Multiplexed Optical Scanner Technology (MOST), a new powerful method using polarization (P-MOS), space (S-MOS), wavelength (W-MOS), and code (C-MOS) multiplexing, including hybrid mux-methods to realize no-moving parts three dimensional steering of light for optical wireless and imaging applications (IEEE 1999). These optical scanners set world records for large aperture 4-pi steradian coverage inertia less optical scanning (OSA 2004). Riza also invented the first de-centered variable focus electronically controlled lens including MEMS lens and liquid lens-based optical beam steering method (SPIE OE 2004, Elsevier 2009) and combined it with liquid lens focus/defocus methods to make smart search and lock-in low loss optical wireless data links (JEOS 2011) as well as fiber-optic attenuation and switching components (Elsevier 2000,  2009, 2010). In 2018, Riza introduced his newest invention, the camceiver, the world’s first combined camera plus data transceiver module with applications in data centers and indoor user ID tag locating systems (IEEE 2018). The camceiver features full spectrum UV to SWIR operations including designs for T/R operations with high speed fiber-optic data links.

Riza is also a pioneer in multiple simultaneous RF beams optical beamforming systems for RF wireless communications phased array antennas and his 1989 Caltech Ph.D. work proposed such smart beamforming systems. In particular, in 1995, he was the first to propose the use of optical multi-beamforming systems for RF wireless Base-Station antennas for cellular systems including for mm-wave (today called 5G and 6G) high data rate wireless phased array antennas on a variety of fixed and mobile platforms including automobiles (SPIE 1995). He proposed the first design for frequency reuse and smart frequency allocations within a cell using spatially multiplexed antenna beams via a multi-beams optical beamformer. In addition, in the same 1995 SPIE paper to improve multi-user wireless data transfer performance, he proposed the first design combining the use of carrier/sub-carrier frequency multiplexing and spatial multiplexing of multiple simultaneous beams for RF cellular base-station phased array antennas (PAA). This approach was much like the 1996 proposed Orthogonal Frequency Division Multiplexing-Multiple Input Multiple Output (OFDM-MIMO) or spatial-spectral multiplexing MIMO PAA method that today is an IEEE standard for RF wireless communications.  Several world labs have pursued both the combined frequency-space Multiplexing multi-beam PAA idea as well use of optical controls approach for mm-wave wireless communication array antennas.

In addition, in the same paper in 1995 (SPIE 1995), he extended the mm-wave RF multibeam multi-user beamforming approach for the optical wireless application creating the first use of multiple simultaneous beams for multiple users using passive as well as electronically programmable beamforming optics such as liquid crystal optical devices. Effectively, he pioneered Spatial Multiplexing via Optical Multi-beamforming and Spatial-Frequency Multiplexing for both the RF mm-wave and optical wireless domains of multi-access communications.

Extreme Environment Systems

Generation and control of clean energy such as via combined cycle gas-fired power plants is an important research area for greener economies. Such power plants operate in extreme and hazardous environments and require operations at the newer higher temperatures ranges for better efficiencies and cleaner operations. Riza and co-workers have demonstrated novel extreme environment optical sensors needed for these extreme temperature (> 1500 deg-C) Zero Emissions fossil fuel-based power generation systems. These Riza group inventions include Silicon Carbide-based extreme temperature hybrid design sensors to measure temperature and pressure. To design these Silicon Carbide (SiC) optical sensors, Riza lab conducted the first single crystal SiC thermo-optic coefficient material characterization and experimental measurements exceeding 1000 deg-C (AIP 2005). Riza and co-researchers invented a world first SiC pyrometry-laser interferometry hybrid freespace-fiber-optic design extreme temperature sensor for direct measurement of gas temperature in a combustor (IEEE 2006). Temperature sensing experiments using this silicon carbide thermometer placed in a commercial rig reached world record high temperatures of 1600 deg-C (ASME 2010). The Riza lab also conducted the first experiments using a SiC optical sensor to measure cryogenic temperatures (IEEE 2006).

The Riza group has also proposed and developed the world first variable focus lens (or E-lens) based non-contact highest transverse spatial resolution 3-D shape sensors based on spatial processing to measure shapes of 3-D objects in extreme environments (OSA 2009). The work included using these sensors for caustic liquid level sensing [Elsevier 2010] as well as active depth from defocus sensing (Elsevier 2015) and the earliest works in distance sensing using machine learning (Elsevier 2017). Riza also pioneered the use of variable focus lens-based low loss optical power transfer optical wireless links using LEDs and lasers (IEEE 2012, US Patent Appl. 2014).

In the late 1980s while doing his Caltech Ph.D. research, Riza invented an electronically controlled, self-aligning, vibrations robust, low noise free-space laser beams Acousto-Optic (AO) architecture using double Bragg diffraction. This design innovation allowed precision amplitude, frequency, and phase control of laser beams, including Riza lab’s world record Angstrom scale sensitivity scanning heterodyne laser interferometry demonstration (OSA 2003). A unique feature of the architecture is that despite Radio Frequency (RF) steered beam motion, the final AO modulated output beam is stationary allowing coupling to a fixed optical fibre or point detector (AIP 1996, 2005). Thus, a noise immune robust laser beam amplitude/frequency/phase modulation architecture is realized that can operate with many types of lasers with different power levels and spectral characteristics, including infrared band and with laser frequency shifting and RF generation from MHz to several GHz. Many worldwide national metrology labs (e.g., France’s Laboratoire Commun de Metrologie LNE-CNAM and USA’s NIST) have adopted this robust programmable AO architecture to set records in measurement science (e.g., 1 picometer wavelength stability 650 nm to 1000 nm spectroradiometer for extreme precision temperature metrology done by French lab, AIP 2016) as well as design precision instruments for absorption spectroscopy, ion spectroscopy, laser cooling and atom clocks.

Another world first is the agile pixel imager including the Coded Access Optical Sensor (CAOS) camera for extreme linear dynamic range as well as bright light smart imaging. See Health and Imaging section for relevant light irradiance measurement instrumentation inventions.