Silicon-on-sapphire devices are attractive for the mid-infrared optical applications up to 5 microns due to the low loss of both silicon and sapphire in this wavelength band. Designing efficient couplers for silicon-on-sapphire devices presents a challenge due to a highly confined mode in silicon and large values of refractive index of both silicon and sapphire. Here, we present design, fabrication, and measurements of a mode-converting coupler for silicon-on-sapphire waveguides. We utilize a mode converter layout that consists of a large waveguide that is overlays a silicon inverse tapered waveguide. While this geometry was previously utilized for silicon-on-oxide devices, the novelty is in using materials that are compatible with the silicon-on-sapphire platform. In the current coupler the overlaying waveguide is made of silicon nitride. Silicon nitride is the material of choice because of the large index of refraction and low absorption from near-infrared to mid-infrared. The couplers were fabricated using a 0.25 micron silicon-on-sapphire process. The measured coupling loss from tapered lensed silica fibers to the silicon was 4.8dB/coupler. We will describe some challenges in fabrication process and discuss ways to overcome them.

RF photonic channelizers can overcome limitations of conventional electronic methods for analysis of wideband RF spectral content. Here, we will present a recent progress on the RF photonic channelizer systems that are based on optical parametric combs. These systems can analyze very wide RF bandwidths exceeding 100GHz, therefore providing essential capability for the applications demanding a wide-bandwidth spectral analysis. The RF channelizers being presented utilize parametric processes in the highly non-linear fiber mixers to generate a large number of RF signal copies in the optical domain. Two different implementations for generation of RF signal copies will be presented and compared: one using a parametric multicasting and another utilizing a direct comb modulation. Generation of optical combs spanning more than 10THz will be shown. We will also present two distinct system architectures for RF photonic channelizer system: one employing a periodic optical filter such as Fabry-Perot etalon to select channels from the signal comb, and another one utilizing a coherent detection between a frequency-locked signal comb and a parametrically generated local oscillator (LO) comb. The second scheme gives benefit of providing both in-phase and quadrature (I/Q) information on channelized intermediate frequency (IF) signals. We will present a system with 32 implemented channels using a filtered scheme and a 32-channel coherent system with a full-field detection implemented on one tunable channel. Sensitivity and dynamic range as well as benefits of both system architectures will be discussed.

An essential capability in many applications, ranging from commercial, surveillance and defense, is to analyze the spectral content of intercepted microwave and millimeter-wave signals over a very wide bandwidth in real-time and with high resolution. A range of photonic schemes have been introduced for the real-time processing of wideband signals to overcome limitations of current conventional electronic frequency measurement approaches. Here, a novel microwave/millimeter-wave channelizer is presented based on a RF photonic front-end employing parametric wavelength multicasting and comb generation. This new technology enables a contiguous bank of channelized coherent I/Q IF signals covering extremely wide RF instantaneous bandwidth. High channel counts and wide RF instantaneous bandwidth are enabled by use of parametrically generated frequency-locked optical combs spanning >4 THz. Full field analysis capabilities of the coherent detection system are demonstrated by frequency domain analysis of 18 contiguous 1.2 GHz IF channels covering 15.5 GHz to 37.1 GHz input frequency range, and time and spectral domain analysis of a 75 GHz harmonically generated input signal. Sensitivity and dynamic range of the system are analyzed and discussed.

A staring wideband RF channelization using parametric multicasting and spectral slicing via a periodic optical filter with 32 implemented channels is presented. Parametric multicasting is technique which replicates and spectrally translates an RF signal via optical nonlinear processes. Injection locking of the pump and signal lasers to a single master laser was implemented to improve performance and stabilize pump and signal spacings. Sensitivity and dynamic range in each channel is also discussed.

A coherent detection RF-IF down converting link utilizing injection-locked distributed feedback (DFB) lasers is demonstrated. This photonic link design provides several RF performance advantages. Coherent detection allows unambiguous recovery of full time-domain I/Q IF signals. Injection locked laser design relaxes the tuning range requirement on RF local oscillators, and provides high optical power for efficient RF-IF conversion while minimizing degradation to link sensitivity due to phase noise. A cascaded injection locked configuration suppresses LO related spurious tones in the IF band is also demonstrated. Sensitivity and dynamic range data from RF C-band to Ka-band are presented.

An integrated optical compound ring filter for use in a multicast and slice channelizer/spectrum analyzer is presented. We discuss design and implementation of a SiO 2 based photonic filter with 250 MHz resolution bandwidth, a flattened passband, and finesse greater than 100, enabling channelization of instantaneous bandwidths above 10GHz.

Fiber optic links are commonly employed for high bandwidth applications. Analog RF photonic link applications often have critical sensitivity and linearity requirements, as such the noise figure (NF) and spurious free dynamic range (SFDR) are key performance parameters for these links. There has been considerable research dedicated to developing improved components and link designs to yield improvement in NF and SFDR. A single output intensity modulator based on an AlGaAs polarization modulation (PolM) design with half wave voltage Vπ≅3 V at 1 GHz, 3 dB roll off at 40 GHz, and optical insertion loss ≅3 dB has been reported and is commercially available. [1,2] If the linear polarizer at the output of this device is replaced with a polarizing beam splitter (PBS), dual complementary outputs similar to a dual output Mach Zehnder modulator (MZM) are provided. This allows for an intensity modulated direct detection balanced link (IMDD-BL) design to be implemented which provides multi-octave operation and common mode suppression (CMS) of laser relative intensity noise (RIN) to improve the link NF.[3] Such a IMDD-BL using a PolM, a single fiber from transmitter to receiver, and the PBS preceded by a polarization controller located at the receiver has been demonstrated at 2 GHz showing common mode suppression of RIN similar to a link using a standard lithium niobate (LN) MZM.[4]

Sensitivity and dynamic range data is presented for a wideband staring RF spectrum analyzer based on parametric multicasting and spectral slicing with a periodic optical filter.

We present the design and characterization of tapped delay line filters fabricated in two-level photonic integrated circuits. Phase error correction techniques are evaluated to enable spectrum analysis of a 4GHz band with 300MHz resolution.

Higher order curved planar-waveguide Bragg gratings are realized by periodic width modulation. Linear chirp is achieved by width tapering. Devices were fabricated in low loss silica waveguide platform. Characterization results will be presented.

A new class of photonic channelized radio-frequency (RF) receiver is proposed and demonstrated. The new device relies on generation of high fidelity signal copies by wavelength multicasting in a self-seeded, two-pump parametric mixer. Signal copying to widely spaced wavelengths enables channelization of the full RF bandwidth using a single periodic filter. The channelization uses freely tunable frequencies of newly generated copies and eliminates the need for construction of a dense, narrowband filter bank. The new concept is demonstrated by channelization of four subcarrier channels with 1-GHz spacing and greater than 20-dB extinction ratio between extracted channels.

We present the design of an optical domain staring RF spectrum analyzer based on parametric multicasting and spectral slicing with a periodic filter. An implementation covering an 18GHz band with 250MHz resolution bandwidth is presented.

A photonic integrated circuit is designed containing a long grating with spiral geometry. Waveguide width and index modulation are studied as methods to form the grating structure. Both methods require an initial mask step for waveguide formation. In the case of width modulation the grating is formed in the same step as the waveguide, whereas a second mask step is required for index modulation. Thus width modulation removes the alignment tolerances associated with a two step process. The spiral geometry enables a long grating (~1 m) to be realized in a small area (1 cm2). The ability to form the grating in such a small area enables the use of current lithography mask / projection equipment. Thus, the requirements for mechanical/optical precision in a customized long fiber Bragg grating fabrication system is transferred to the precision of commercial lithography mask fabrication and projection equipment.

The superior distance-bandwidth product of optical fiber communications has led to the development of a wide array of active and passive optical/optoelectronic components for the digital data communication industry. The cable television (CATV) industry has been the primary driver for development of high performance analog fiber optic links. RF-over-fiber offers advantages for military RF signal distribution, primarily antenna remoting, as such there is significant interest in leveraging the commercial developments noted above for these military applications. Because the link distances for many of these military applications are often so short that the RF performance is poorer than a design using copper coax or waveguide, the adoption of RF-over-fiber in deployed systems has been limited. This has motivated continuing development of link components and designs to improve link performance. This paper will summarize system-level metrics relevant to Navy antenna remoting applications, discuss an example deployed system, and conclude with an update on recent improvements in analog link components/performance.

RF-over-fiber technology for military antenna remoting is discussed including system level metrics and examples of deployed systems. The design of a compact RF spectrum analyzer based on optical domain RF channelization is introduced.

An optically clocked track-and-hold (TH) circuit for improved TH linearity and noise performance is presented. Results with f/sub in/=1.0073 GHz and sample rate f/sub s/=1.003 GS/s show 11.8 SFDR bits and 9.6 SNR bits.