Optical delay lines of a proposed type would be made from rods of such dielectric materials as calcium fluoride, fused silica, or sapphire. These would offer advantages over prior optical delay lines, as summarized below.
Optical delay lines are key components of opto-electronic microwave oscillators, narrow-band opto-electronic microwave filters, evanescent-field optical biochemical detectors, and some Fourier-Transform spectrum analyzers. Heretofore, optical delay lines used in such applications have been of two types: resonators and coiled long optical fibers, both of which have disadvantages:
- Resonators are compact, but excitation must be provided by narrow-band lasers. Wide-band (including noisy) laser light cannot be coupled efficiently to narrow-band resonators.
- When light is coupled into a narrow-band resonator from a source of reasonably high power, a significant amount of optical energy circulates within the resonator, causing nonlinear loss and significant noise.
- Typically, a coil-type optical delay line is made of fused-silica fiber, which exhibits fundamental loss. To overcome the limit imposed by the optical loss in fused silica, it would be necessary to use fibers having crystalline cores.
- Although space is saved by winding fibers into coils, fiber-coil delay lines are still inconveniently bulky.
The proposed compact dielectric-rod delay lines would exploit the special class of non-diffracting light beams that are denoted Bessel beams because their amplitudes are proportional to Bessel functions of the radii from their central axes. High-order Bessel beams can have large values of angular momentum. They can be generated with the help of whispering-gallery-mode optical resonators, as described, for example, in "Simplified Generation of High-Angular-Momentum Light Beams" (NPO-42965) NASA Tech Briefs, Vol. 31, No. 3 (March 2007), page 8a. In a delay line according to the proposal, the dielectric rod would be dimensioned to function as a multimode waveguide. Suitably chosen high-angular-momentum modes in such a waveguide exhibit low group velocity (hence, long delay) and no resonance. Such a delay line could perform well at any wavelength or range of wavelengths within the transparency wavelength band of the dielectric material, and the maximum possible group delay achievable through suitable design would be limited only by the optical loss in the rod material.
This work was done by Lute Maleki, Andrey Matsko, Anatoliy Savchenvkov, and Dmitry Strekalov of Caltech for NASA's Jet Propulsion Laboratory.
NPO-43459
This Brief includes a Technical Support Package (TSP).
Compact Dielectric-Rod White-Light Delay Lines
(reference NPO-43459) is currently available for download from the TSP library.
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Overview
The document discusses the development of compact dielectric-rod white-light delay lines, as detailed in NASA's Technical Support Package NPO-43459. It addresses the limitations of existing delay line technologies used in optical and microwave engineering, specifically focusing on two major types: resonators and long fiber coils.
Resonators, while compact, require narrowband sources for effective operation, which poses challenges when trying to couple wideband noisy lasers efficiently. This inefficiency can lead to significant energy storage within the resonator, resulting in nonlinear losses and increased noise. On the other hand, long fiber coils, although capable of handling broader bandwidths, are bulky and suffer from fundamental losses inherent in fused silica fibers.
To overcome these challenges, the document proposes the use of dielectric rods made from transparent materials such as calcium fluoride (CaF2), fused silica (SiO2), or sapphire. These rods serve as the core element for the white-light delay line. The innovation lies in utilizing a high momentum Bessel-beam mode within the dielectric rod, which exhibits low group velocity and no resonance frequency. This characteristic allows the delay line to effectively delay light pulses across any frequency within the transparency window of the material used.
The compact design of the proposed delay line eliminates the need for complex components such as atomic cells, heaters, or electrical systems, making it simpler and more efficient. The maximum group delay achievable is limited only by the optical loss in the material, allowing for versatile applications across various optical frequencies.
Overall, the compact dielectric-rod white-light delay line represents a significant advancement in optical delay line technology, offering a solution that combines high performance with a straightforward design. This innovation has potential implications for a range of technological, scientific, and commercial applications, particularly in fields requiring precise control of light propagation and timing. The document serves as a resource for further exploration of these developments and their applications in aerospace and beyond.
