Previous research has shown that temperature gradients along a fiber can broaden the Stimulated Brillouin Scattering
(SBS) gain profile and thereby increase the SBS threshold. However, within practical temperature ranges this method
has been limited to SBS thresholds of a few hundred Watts. It is also well known that strain gradients applied to a fiber
can broaden the SBS resonance. To suppress the SBS threshold to kW levels in fiber amplifiers of length ~5 m requires
broadening of the SBS resonance width to ~1 GHz, which can be achieved with a strain of 1 - 2%. Although tensile
strain is generally limited by fiber failure to less than ~1%, compressive strain has been employed to the level of many
percent in a number of applications in the tuning of fiber Bragg gratings. We demonstrate the effect of SBS gain
broadening and suppression by strain gradients at high power (~ 190 W) for the first time to our knowledge, and explore
scaling of this method to kW output levels.
A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a single-mode fiber-amplifier chain. Phase control of each element is achieved with a Lithium-Niobate phase modulator.
A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channel's phase modulator.
This effort represents the results of a multi-year effort to achieve high power from a single element fiber amplifier and to understand the important issues involved in coherently combining many individual elements to obtain sufficient optical power for directed energy weapons. Northrop Grumman Corporation and the High Energy Laser Joint Technology Office jointly sponsored this work.
A four-element fiber array has been constructed to yield 8 watts of coherently phased, linearly polarized light energy in a single far field spot. Each element consists of a 2-W single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channel's phase modulator. The stability of the optical train is an essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed using nano-positioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a common far field spot. This system proved to be highly robust and handled any acoustic perturbations.
The demand for higher data capacity and reduced levels of interference in the communications arena are driving dtat links toward high carrier frequencies and wider modulation bandwidths. Circuitry for performing intermediate frequency processing over these more demanding ranges is needed to provide complex signal processing. We have demonstrated photonics technologies utilizing Bragg Grating Signal Processing (BGSP), which can be used to perform a variety of RF filter functions. The desirable benefits of multiple-tap adaptive finite impulse response (FIR) filters, infinite impulse response (IIR) filters, and equalizers are well known; however, they are usually the province of digital signal processing and demand preprocessor sample rates that require high system power consumption. BGSPs provide these functions with discrete optical taps and digital controls while only requiring bandwidths easily provided by conventional RF circuitry. This is because the actual signal processing of the large information bandwidths is performed in the optical regime, while control functions are performed at RF frequencies compatible with integrated circuit technologies. To realize the performance benefits of photonic processing, the Bragg grating reflectors must be stabilized against environmental without unduly taxing the RF control circuitry. We have implemented a orthogonally coded tap modulation technique which stabilizes the transfer function of the signal processor and enables significant adaptive IF signal processing to be obtained with very low size, weight, and power. Our demonstration of a photonic proof-of-concept architecture is a reconfigurable, multiple-tap FIR filter that is dynamically controlled to implement low-pass, high-pass, band-pass, band-stop, and tunable filters operating over bandwidths of 3 Ghz.
The demonstration of RF photonic links with bandwidths of 100 GHz or more is expected in the near future, due to rapid and continuing progress in modulator and detector technology. Since it is very difficult to electronically process such a large bandwidth after photodetection, photonic approaches that reduce the burden on the electronics are increasingly relevant.One such approach is an optical channelizer, where an RF modulator optical carrier is optically dispersed onto a detector array. Each element of the array only sees a portion of the original wideband RF spectrum. We analyze the RF performance of optical channelizers in terms of crosstalk and uniformity of response. Both direct detection and heterodyne channelizers will be considered. Analysis of this kind is necessary for our application, since the usual parameters of a dispersive optical system, such as resolution, resolving power or filter linewidth do not provide enough information to determine the RF performance. The analysis has shown that a Fabry-Perot filter based channelizer cannot provide adequate RF performance, while a grating-based channelizer can. For channelizer to 1 GHz channels with a -70 dB crosstalk specification, a total grating length of roughly 80 cm is required, so a multiple bounce geometry is necessary to obtain a reasonably compact system.
KEYWORDS: Signal processing, Fiber Bragg gratings, Filtering (signal processing), Control systems, Linear filtering, Digital signal processing, Optical filters, Servomechanisms, Modulators, Electronic filtering
Higher data capacity demands and lower interference requirements in the wireless communications arena are exploiting higher carrier frequencies and wider modulation bandwidths. Circuitry which can perform intermediate frequency processing over these more demanding ranges is needed to provide complex signal processing without commercial penalties. Photonics technologies utilizing Bragg Grating Signal Processing (BGSP) can bridge the gap between the very high frequency RF millimeter wave integrated circuit domains at the antenna interface and the CMOS digital signal processor sat the base band frequency interface. The desirable benefits of multiple; tap adaptive finite impulse response (FIR) and infinite impulse response filters and equalizers are well known; however, they are usually the province of digital signal processing and force the sample rates prior to these processors to a higher overall system power consumption level. BGSP provides these functions with discrete taps and digital controls but at the bandwidths usually reserved for RF circuitry because the actual processing occurs at optical frequencies and at wave lengths which are compatible with integrated circuit technologies. The high performance benefits of photonic processing can be realized if the stability control of the Bragg grating is derived from the same metric which induces in photonics its sensitivity to drift. We will present a orthogonally coded tap modulation technique which stabilizes the transfer function of the signal processor and enables significant adaptive IF signal processing to be obtained with very low size, weight, and power. Our demonstration of a photonic proof-of-concept architecture is a reconfigurable multiple tap FIR filter that is dynamically controlled to perform low pass, high pass, band pass, and band stop filters operating over bandwidths of 3 GHz.
The Diode Array Pumped Kilowatt Laser (DAPKL) has demonstrated more than an order of magnitude increase in brightness and average power for short pulse diode-pumped solid-state lasers since its inception in 1991. Significant advances in component technology has been demonstrated, including development of a diffusion bonding process for producing large slabs of Nd:YAG laser material. Phase conjugation by stimulated Brillouin scattering has been demonstrated with high reflectivity and fidelity in a simple focused geometry with input powers of 100 W. Pulse energies at 1.06 μm of up to 10 J per pulse have been demonstrated with a beam quality of 1.25 times diffraction limited at 33 Hz. An average power of 940 Watts at 100 Hz has been obtained with two times diffraction limited beam quality. Efficient frequency doubling with an average power of 165 W has been demonstrated with 5 J per pulse at 0.53 μm. The system has been packaged in a compact brassboard for long term stability and reliability of operation.
A fiber-optic approach for low-loss true time delay of wideband RF signals for phased-array-antenna beamsteering is presented. An optical carrier modulated by the RF signal of interest is launched into a delay-line fiber composed of optical Bragg reflection gratings written holographically into the core of a single-mode fiber. The desired beam steering is realized by tuning the optical carrier wavelength for reflection from the appropriate grating. Radiation testing of various fibers with Bragg gratings has been performed indicating preferable fiber types. True time delay offers much reduced beam squint and sharper antenna superior nulling compared to phase shift scanned antenna. Examples of applications of this concept showing its advantages are presented.
We discuss the development of a fiber-optic approach for low-loss true time delay of wideband RF signals for phased-array-antenna beamsteering and general analog signal-processing applications. The device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber to reflect light of different optical wavelengths. An optical carrier modulated by the RF signal of interest is launched into this delay-line fiber. The desired RF time delay is realized by runing the optical carrier wavelength for reflection from the appropriate grating. Unlike conventional switching schemes, this time- delay element has an RF insertion loss that is independent of the number of time delays (bit resolution) provided. The wavelength selectable nature of the TTD device can simplify beamsteering control and channel multiplexing in phased-array antennas. Experimental results are reported for optical and RF characterization of the Bragg-grating element. Both time and frequency-domain RF data are shown that demonstrate the wavelength-selectable time delay concept.
We present a novel concept, the fiber optic Bragg grating true- time-delay (TTD) element, for implementing true time delay in the distribution network of an optically fed phased array antenna. the device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber at various positions along its length. An optical carrier is modulated by the RF signal of interest and launched into this delay-line fiber. The desired RF time delay may be realized by wavelength-selectable nature of the TTD device offers the possibility for simplified beamsteering control and channel multiplexing.
We have assembled and tested a diode-pumped, phase conjugated Nd:YAG master oscillator power amplifier (PC MOPA) operating at an average power of 100 Watts. One joule per pulse has been extracted at a repetition rate of 100 Hz with a beam quality (BQ) of 1.1 x diffraction limited (D.L.). This combination of average power and beam quality makes this the brightest short pulse solid-state laser reported to date. The optical efficiency of 22% and the overall efficiency of 9.4% also represent record performance for high energy short pulse lasers. Excellent spatial uniformity and a pulse length of 7 ns make this laser ideal for frequency doubling and parametric conversion.
CCD holography is to be used to measure the phase conjugation fidelity of an SBS experiment at TRW. The diagnostic has been shown capable of phase measurements with an RSS accuracy of 1/80 (lambda) . The technique and experiments designed to quantify its accuracy are described. The application of this technique to the SBS experiment and the peculiarities of doing the experiment in the IR are also discussed.
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