Zhenhai Fu

Shrinkage-corrected volume holograms based on photopolymeric phase media for surface-normal optical interconnects

The film shrinkage effect of photopolymeric phase media failed to provide the desired volume holograms for point-to-point optical interconnects. In this letter, we report a compensation method to physically correct the shrinkage effect that resulted from the holographic recording and the postbaking. Dupont photopolymer HRF-600X001 is studied. The correction of the Bragg diffraction angle shift of 1°21′, which is induced by a 5.25% film shrinkage, is successfully demonstrated with the surface-normal configuration. A shrinkage-corrected volume hologram with 80% diffraction efficiency is experimentally confirmed. The methodology reported herein is applicable to other phase media when the associated film shrinkage data are experimentally determined.

High packing density 2.5 THz true-time-delay lines using spatially multiplexed substrate guided waves in conjunction with volume holograms on a single substrate

A 3-bit true-time-delay lines device having a packing density of 5 lincs/cm/sup 2/ with a minimum delay step of /spl sim/100 ps is designed, fabricated and demonstrated. This device is based on substrate guided wave propagation combined with slanted photopolymer volume phase gratings. In this paper, we report the delay and bandwidth measurements for the 3-bit delay lines fabricated on BK-7 glass substrates with a substrate bouncing angle of 45/spl deg/. The power fluctuation among the outputs due to the cascading fanout effect (a serious drawback for real system applications) is experimentally investigated as well. A power fluctuation controlled to within /spl plusmn/10% is achieved. A femtosecond laser pulse is sent through the device and a bandwidth measurement of up to 2.5 THz is obtained. The delay step is measured by employing an ultrafast photodetector together with a sampling scope. The true-time-delay device presented herein has the potential to be integrated with photodetector arrays due to its planar structure on a single substrate together with the surface normal fan-in and fan-out features.

Waveguide-hologram-based wavelength-multiplexed pseudoanalog true-time-delay module for wideband phased-array antennas

A pseudoanalog true-time-delay (TTD) module based on substrate-guided waves and wavelength-division multiplexing is presented. A 1-to-32 (5-bit) even fan-out is demonstrated by use of a two-dimensional waveguide hologram array. This module has a packing density of 2.5 lines/cm(2) and very compact packaging (8 cm x 4 cm x 8 mm). It also reduces TTD system complexity by providing continuously tuned delay signals to parallel-control the whole phased-array antenna system. The device has a measured bandwidth of as high as 2.4 THz. The delay signal can range from tens of picoseconds to several nanoseconds.

Ultra-low-loss polymeric waveguide circuits for optical true-time delays in wideband phased-array antennas

The optical true-time-delay line is a key building block for modern broadband phased-array antennas, which have become one of the most critical technologies for both military and civilian wireless communications. We present our research results in developing an optical polymer-based waveguide true-time-delay module for multilink phased- array antennas by incorporating wavelength-division multiplexing (WDM) technology. The demonstrated optical polymeric waveguide circuits can provide a large number of optical true-time delays with a dynamic range of 50 ns and a time resolution of 0.1 ps. Various fabrication techniques are investigated for producing ultralong low-loss (0.02 dB/cm) polymeric channel waveguides with tilted waveguide grating output couplers. Fast photodiode arrays are fabricated and rf signals with frequencies of 10 to 50 GHz are generated through the optical heterodyne technique. A detailed study of waveguide amplification to achieve loss-less polymeric waveguide is conducted. The optical amplification of 3.8 dB/cm is achieved at a wavelength of 1064 nm in a Nd3+-doped polymeric waveguide. WDM techniques are also employed for potential multilink applications. The presented methodologies enable hybrid integration with a reduced cost in optoelectronic packaging and an increased reliability and decreased payload for the next generation of phased-array antennas.

Compact broadband 5-bit photonic true-time-delay module for phased-array antennas

Photonic true-time-delay (TTD) lines offer many advantages over their electronic counterparts and are attracting more and more research effort. We demonstrate a device with 32 TTD lines (5-bit) based on substrate guided-wave propagation combined with slanted photopolymer volume phase gratings on a quartz substrate. System design, device fabrication, optimization of fan-out intensity uniformity, and device performance evaluation are addressed as well. The device has a measured bandwidth of up to 2.4 THz and a measured fan-out delay step of 50 ps. The fan-out beam intensity uniformity is within +/-10% . The packing density is 2.5 delay lines/cm(2), which is to our knowledge the highest demonstrated thus far.

Linearity of volume hologram out-coupling for wavelength-division demultiplexing

Close-form expressions are used to analyze the spatial and angular linearity of the out-coupling volume holograms in wavelength division multiplexing/demultiplexing (WDM/WDDM). Optimal spatial linear out-coupling regimes are located. Some design criteria for volume holographic WDDM applicable to 800nm, 1300nm, and 1550nm optical wavelength window are addressed. As a design example, we deploy these criteria to design a passive surface normal input/output wavelength division demultiplexer working in the wavelength range of 768-864 nm. Coupling of the demultiplexed optical signal from the substrate modes to a linear multi-mode fiber array is verified with experiment. The importance of the spatial linearity of the out-coupling in volume holographic WDDM structure is highlighted and possible coupling of the signal to linear single-mode fiber array is mentioned.

Dispersion correction of surface-normal optical interconnection using two compensated holograms

Optical interconnects have advantages over electrical interconnects in applications where low transmission loss, electromagnetic interference immunity, low power budget, and high bandwidth requirements are critical. 1‐3 The need for optically-interconnected memories and processors in multichip modules is imperative due to the rapid increase of clock speed and of data throughput. Free-space interconnects must overcome packaging vulnerability in order to be practical. Guided-wave interconnects based on silica or polymeric thin films attract more attention since they can be fabricated in two-dimensional arrays using a standard very large scale integrated ~VLSI! microfabrication process. Coupling light into and out of waveguides efficiently becomes more important. Coupling methods using gratings, end-face joints, and prisms have been reported. 4,5 Surface-normal transmission holographic gratings are widely used to couple light into and out of waveguides due to their high diffraction efficiency and planarized packaging. In this letter, we investigate the light dispersion of a surface-normal input volume holographic grating. Experimental data of grating dispersion characteristics are obtained using a mode-locked femtosecond laser. A compensation method is developed to eliminate the wavelength-induced dispersion, and we demonstrate surface-normal input and output optical interconnect structures which automatically correct the dispersion resulting from the laser wavelength chirping and therefore greatly enhance the interconnection bandwidth. The basic structure of a surface-normal input and output optical interconnect using volume holographic gratings and substrate-guided waves is shown in Fig. 1. The grating structure induced by the refractive index modulation is slanted, having a tilt angle f. The grating spacing is L. Due to the high diffraction efficiency of the volume hologram and the low propagation loss due to total internal reflection, the surface-normal optical interconnect configuration is useful in photonics applications such as backplane buses, computer clock signal distribution, and waveguide-based wavelength

Polymer-based optical waveguide circuits for photonic phased-array antennas

Photonic phased array antennas represent one of the most critical technologies for both national defense and civilian wireless communications. In this paper, we present a novel compact detector-switched polymeric waveguide true-time- delay module, which is a crucial building block for advanced wideband photonic phased array antennas. The photolithographically defined ultra-low-loss polymeric waveguides provide us an attractive solution for achieving ultra long delay time over tens of nsec with ultra fine resolution of less than 1 ps. The 2D distributed waveguide grating couplers tap the optically encoded microwave signal, propagating along the polymeric channel waveguide, to high- speed photodetectors. These photodetectors can be electrically switched on and off independently for selecting different delay times. The appropriate delay time is equal to the time of flight along the waveguide. We have demonstrated that such optical true-time-delays can be implemented in such a device scheme with the RF spectrum of 11 GHz to 40 GHz. The optically encoded microwave signals are obtained by using semiconductor-laser-based optical heterodyne technique. Such a monolithic integrated module not only reduces the cost associated with optoelectronic packaging, but also reduces the system payload with an improved reliability.

Five-bit substrate guided wave true-time delay module working up to 2.4 Thz with a packing density of 2.5 lines/cm2 for phased array antenna applications

A 5-bit true-time delay module is demonstrated. The combi- nation of a 1-to-4 fiber beamsplitter and four 1-to-8 collinear guided-wave fanouts provides us with the required 32 surface-normal fanout beams with a delay step of 50 ps. The system design, device fabrication, and optimization of the fanout intensity uniformity down to 610% among 32 fanouts are addressed. The packing density of this device is 2.5 (delay lines)/cm 2 , which is the highest demonstrated thus far. A bandwidth of 2.4 THz is confirmed experimentally by employing a femtosecond laser and an ultrafast detector.

Polarization sensitivity of photopolymer-based volume holograms for one-to-many surface normal optical interconnects

Polarization dependence of photopolymer-based volume ho- lograms for substrate-guided-wave optical interconnects with surface normal configuration is investigated. Energy optimization and the trade- off between energy equalization and polarization insensitivity of one-to- many cascaded interconnects are discussed. DuPont photopolymer film HRF 600X001-20 was employed in our experiment. A volume hologram with diffraction efficiency of 91.5%61.5% is achieved for different lin- early polarized optical waves. A 1-to-9 fanout device with 65% energy fluctuation for a randomly polarized input optical wave at 632.8 nm has been fabricated for evaluating the trade-off under p and s waves. As the Mylar substrate of the photopolymer films has strong effects in changing incident linearly polarized light into elliptically polarized light, the energy uniformity of a practical device can be improved significantly. The devel- oped theory is applicable to any volume hologram.