Tapered ultra-high numerical aperture optical fiber tip for nitrogen vacancy ensembles based endoscope in a fluidic environment
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-03-16 |
| Journal | Applied Physics Letters |
| Authors | Dewen Duan, Vinaya Kumar Kavatamane, Sri Ranjini Arumugam, Yan-Kai Tzeng, Huan-Cheng Chang |
| Institutions | Stanford University, Max Planck Institute for Biophysical Chemistry |
| Citations | 13 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research presents a novel design for a Nitrogen-Vacancy (NV) center based endoscope tip, utilizing a tapered and end-sealed microstructured optical fiber (MOF-taper) to enable high-efficiency sensing in fluidic environments.
- Core Innovation: Replaced standard Multimode Fiber (MMF) tapers with an ultra-high Numerical Aperture (NA) MOF-taper whose internal air capillaries are sealed at the tip.
- Fluidic Immunity: The sealed air capillaries isolate the fiber core from the surrounding liquid medium, preserving the high Refractive Index (RI) contrast (silica core vs. air capillaries). This maintains high Fluorescence (FL) collection efficiency regardless of the external liquidâs RI (up to n = 1.51).
- Performance Contrast: Standard MMF-tapers experienced nearly 100% FL loss when immersed in oil (RI = 1.51), whereas the sealed MOF-taper retained significantly higher collection capability.
- Spatial Resolution: Achieved high spatial resolution (less than 10 ”m) due to the small diamond size and tapered tip geometry.
- Sensing Demonstration: Demonstrated magnetic field sensing stability in oil, achieving an optically detected magnetic resonance (ODMR) demodulation stability of 0.3 MHz, corresponding to a sensitivity of approximately 10.7 ”T.
- Versatility: This design overcomes the major limitation of previous NV endoscopes, making them viable for broad applications in biological processes and chemical reactions occurring in liquids.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| MOF Type | MM-HNA-35 | NKT Photonics | Ultra-high NA Microstructured Optical Fiber |
| MOF Core Diameter | 35 ± 2 | ”m | Un-tapered MOF |
| MOF Cladding Diameter | 125 ± 3 | ”m | Un-tapered MOF |
| MOF NA | > 0.6 | - | At 880 nm wavelength |
| Silica RI (ns) | â 1.456 | - | Fiber constituent material |
| Water RI (nw) | 1.33 | - | Liquid environment test medium |
| Oil RI (no) | 1.51 | - | Immersion oil test medium (RI > ns) |
| MMF Type | GIF625 | - | Graded-index fiber used for comparison |
| MMF-Taper Tip Core Dia. | ~10.2 | ”m | Used for comparison tests |
| MOF-Taper Tip Core Dia. | ~13.2 | ”m | Used for comparison tests |
| Diamond Size (Sensing) | ~8.6 | ”m | Fixed on end-sealed MOF-taper apex |
| FL Collection Improvement (Air) | ~3 | times | MOF-taper vs. un-tapered MOF |
| FL Loss in Oil (MMF-Taper) | Almost 100% | % | Due to RI matching (no â ns) |
| FL Loss in Oil (Sealed MOF-Taper) | ~71.5 | % | Loss relative to performance in air |
| Laser Power (CW Green) | ~5.86 | mW | Used for NV excitation |
| ODMR Stability | 0.3 | MHz | In 2.7 mT field environment |
| Magnetic Field Sensitivity | ~10.7 | ”T | Calculated from ODMR stability |
| Critical Sealing Length (t) | < 2.4 | ”m | Required for ~8 ”m diamond on MOF-taper (Ideal condition) |
Key Methodologies
Section titled âKey MethodologiesâThe fabrication and testing focused on creating a robust, high-NA fiber tip capable of operating efficiently in high-RI liquids.
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MOF Tapering:
- The ultra-high NA MOF (MM-HNA-35) was heated using a flame and simultaneously stretched.
- The fiber was cut at the taper waist to produce the MOF-taper tip.
-
Air Capillary End-Sealing (Fusion Splicing):
- The MOF-taper (or MOF) was fusion spliced to a section of MMF-taper (or MMF).
- The fiber was then cleaved near the spliced joint, ensuring a short section of the capillaries collapsed region remained on the MOF-taper tip.
- Note: The sealing technique used resulted in a sealed region length (t) longer than the calculated critical length, leading to some unavoidable FL loss.
-
Diamond Integration:
- NV-rich diamond crystals (~8.6 ”m diameter) were fixed onto the apex of the end-sealed MOF-taper using UV-curing glue.
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Optical Setup (Excitation and FL Collection):
- A continuous wave (CW) green laser (~5.86 mW) was used for NV excitation.
- The same fiber (MOF-taper) was used for both excitation delivery and Fluorescence (FL) collection (endoscope configuration).
- Collected FL was filtered (615-nm long-pass filter and 512-nm clear-up filter) and measured using a spectrometer.
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Performance Testing in Fluids:
- FL collection efficiency was measured for MMF, MMF-taper, sealed MOF, and sealed MOF-taper tips.
- Measurements were conducted sequentially in air, water (RI = 1.33), and microscopy immersion oil (RI = 1.51).
-
Magnetic Sensing Test (ODMR):
- The end-sealed MOF-taper with the diamond was immersed in oil.
- The ODMR signal stability was measured using a microwave synthesizer and a lock-in amplifier setup.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology significantly enhances the utility of NV-based quantum sensors, particularly in environments previously inaccessible to fiber-optic endoscopes.
- Biomedical and In-Vivo Sensing: Enables magnetic field, temperature, and electric field sensing within biological tissues, cells, or organs where high-RI physiological fluids are present.
- Chemical Reaction Monitoring: Allows for real-time, localized sensing of chemical processes in liquid solutions, including high-RI solvents.
- Microfluidics and Lab-on-a-Chip: Provides a compact, high-resolution sensing platform for analyzing samples within microfluidic channels.
- Environmental Monitoring: Applicable for sensing parameters in complex aqueous or oil-based environmental samples.
- High-Resolution Quantum Microscopy: Used as a compact probe for high spatial resolution (less than 10 ”m) sensing in fluidic media.
View Original Abstract
Fixing a diamond containing a high density of Nitrogen-Vacancy (NV) center ensembles on the apex of a multimode optical fiber (MMF) extends the applications of NV-based endoscope sensors. Replacing the normal MMF with a tapered MMF (MMF-taper) has enhanced the fluorescence (FL) collection efficiency from the diamond and achieved a high spatial resolution NV-based endoscope. The MMF-taperâs high FL collection efficiency is the direct result of multiple internal reflections in the tapered region caused by silica, which has a higher refractive index (RI) than the surrounding air. However, for applications involving fluidic environments whose RI is close to or higher than that of the silica, the MMF-taper loses its FL collection significantly. Here, to overcome this challenge and achieve a high spatial resolution NV-based endoscope in a fluidic environment, we conceptually proposed a tapered ultra-high numerical aperture microstructured optical fiber (MOF) whose air capillaries at the tapered end are sealed. Since the end-sealed air capillaries along the tapered MOF (MOF-taper) have isolated the MOF core from the surrounding medium, the core retains its high FL collection and NV excitation efficiency in liquids regardless of their RI values. Replacing the MMF-taper with the MOF-taper will achieve a versatile NV-based endoscope that could potentially find widespread applications in fluidic environments where many biological processes and chemical reactions occur.