The mercury linear ion trap frequency standard (LITS) at JPL has been demonstrated to have multiple potential applications in NASA deep space missions. An increase of the deep ultraviolet (DUV)/vacuum ultraviolet (VUV) light output from the plasma discharge lamp light source used in trapped ion clock atomic state preparation and detection would improve the clock signal-to-noise ratio (SNR) and decrease optical pumping times. Both lead to an improvement in clock short-term stability and/or enable the use of a local oscillator having lower cost and performance. A solution for higher intensity light generation is needed.

A cross-section of the HCPCF mercury lamp integrated with a DUV fiber at one end. The purple part is the mercury micro-plasma. On the left side, the hollow core is vacuum-sealed by high-temperature flame. A cross-section of the air hole structure of the PCF is shown. The dark brown part on the right is the higher index layer of the DUV fiber, where the light is finally collected for output.

The functionality of mercury micro-plasma generated in a sub-mm scale capillary tube and the associated fiber-based optical interface were previously demonstrated [“Deep UV Discharge Lamps in Capillary Quartz Tubes with Light Output Coupled to an Optical Fiber” (NPO-48845), NASA Tech Briefs, Vol 38, No. 6 (June 2014), p. 55]. This work extends the concept to use hollow-core photonic crystal fiber (HCPCF) in the micro-plasma generation process as a replacement for the capillary tube. An ability to generate, collect, and guide the VUV light with an intensity at least one order of magnitude higher than the capillary lamps has been estimated.

The light of the micro-plasma generated in the capillary lamp is collected via the fiber tip at the end of the capillary. As the length of the capillary increases, less additional light is coupled due to the decrease of collecting (solid) angle.

Using fused quartz with ultra-low loss at 194 nm, HCPCF works at the first order band-gap (low loss) with a plasma diameter of 1 mm. Light generation is simulated at the fiber end for different types of lamp while the length is increased. Due to the fact that HCPCF serves both as a plasma generator and a DUV/VUV waveguide, the light intensity is more than ten times that of the capillary lamp. Furthermore, due to plasma generation within the HCPCF, further improvements can be achieved with longer length.

The HCPCF lamp works at the submillimeter range, where the plasma is categorized as a micro-plasma. The light generated can be applied to trapped mercury ions for optical pumping and detection. Conversely, a trapped ion clock can be used as a spectroscopic probe for the mercury micro-plasma in DUV/VUV wavelengths. The trapped mercury ions provide a well-defined atomic ensemble that is de-coupled from major environmental parameters such as room temperature, magnetic field, etc.

This is the first HCPCF lamp design. It keeps the virtues of the capillary-fiber optical system and provides higher-output light intensity for application to mercury ion clocks. By utilizing this type of HCPCF lamps, the short-term stability of current ground-based trapped ion clocks is expected to improve.

The Hg+ ion trap and the related spectroscopic experimental apparatus may serve as a high-resolution probe to study the micro-plasma physics in HCPCF lamps. This may broaden microplasma applications into fields such as lithography, biotech sensors, medical treatment, environmental monitoring, etc.

This work was done by Lin Yi, Robert L. Tjoelker, Eric A. Burt, and Shouhua Huang of Caltech for NASA’s Jet Propulsion Laboratory.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel, NASA Management Office–JPL.

Refer to NPO-49310.

This Brief includes a Technical Support Package (TSP).
Document cover
Hollow-Core Fiber Lamp for Mercury Ion Clocks and Micro-Plasma Studies

(reference NPO-49310) is currently available for download from the TSP library.

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