Demonstration of Analog-to-Digital Conversion Using Spatial-Spectral Holography

Abstract

Proof-of-concept demonstrations for an optical approach to broadband analog-to-digital conversion are shown with 800 MHz bandwidth and >26dB dynamic range. Models suggest that this frequency-domain stretched processor could achieve 10-bit performance over 20 GHz bandwidths.OCIS codes: (070.1170) Analog optical signal processing; (090.2900) Optical storage materials IntroductionWith the advent of software defined radios, ultra-wideband communication protocols, and extremely high frequency carriers (>90 GHz), modern microwave systems are becoming more complex and diverse. In addition to flexible carrier frequency and modulation formats, microwave systems continue to increase their signal bandwidths for enhanced performance. While electronic analog-to-digital converters (ADCs) have seen steady improvement in performance over the past several years, there are still no single chip ADCs that can directly capture signals with 10 effective vertical bits of resolution beyond 2 GHz of bandwidth. The tracking of single chip ADC performance improvements over the past several years has shown that only modest improvements in performance can be expected in the near term [1]. Due to this, there has been an increased interest in photonic based ADC systems [2], to try to enhance signal capture bandwidths and vertical resolution. In particular, photonic-assisted ADCs have shown significant promise arising from the capability to modulate high bandwidth electronic signals directly onto an optical carrier and to utilize lower bandwidth digitizers with high dynamic range. Recently, time-stretch photonic assisted ADCs have shown effective sampling rates in excess of 1 terasample per second with 5 vertical bits of resolution [3], however, time apertures remain limited due to time bandwidth product (TBP) limitations.Recently an approach to photonics assisted ADC has been proposed [4], which utilizes spatial spectral holography (SSH) in inhomogeneously broadened optical absorbers. Simulations have shown that this approach could provide direct conversion of signals at 20 GSPS with over 10 bits of vertical resolution in the near term with existing components and materials. This approach may also be extendable to bandwidths over 300 GHz in newly available materials. Rather than stretching a signal in the time-domain, the SSH-ADC stretches a signal's frequency domain representation. Like time-stretched processors, the SSH-ADC exploits the high performance of lower bandwidth ADCs and wide bandwidth telecom grade modulators. However, the potential TBP of the SSH-ADC is several orders of magnitude greater than currently demonstrated optical time-stretching techniques. This paper presents the first ever proof-of-concept demonstrations of the SSH-ADC technique on signals with equipment limited bandwidths up to 800 MHz and >26 dB of dynamic range (4.4 vertical bits). These demonstrations also show the system's ability to provide direct digitization of signals with large 6.5 GHz microwave carrier frequencies.2. Spatial-Spectral Holographic Analog-to-...