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Assembly process and sub-Doppler spectroscopy of end-capped photonic micro-cell

Thomas Billotte, Matthieu Chafer, Martin Maurel, Foued Amrani, Frédéric Gérôme, Benoît Debord, Fetah Benabid

Published Aug 30, 2020• 0 citations
OpenAlex

Abstract

Generation of sub-Doppler spectral lines in compact devices is very useful in bringing functionalities such as laser-frequency stabilization, slow light application or quantum sensing to a wider and friendlier use. An all-fiber Photonic MicroCell (PMC), which consists of a gas-filled hollow core photonic crystal fiber (HCPCF) hermetically spliced to a solid optical fiber [1], is one of the most promising of such devices because of its ultra-long interaction length. Thus far, the PMC assembly processes are either limited to a short range of HCPCF core diameter [2], or entails the use of glue [3], or exhibit high injection losses (~10dB) [4]. Here, we report on a 7-meter contaminant free and single-mode C2H2-PMC. The process consists of end-capping a 80µbar acetylene-filled HCPCF via heat-collapsing a fused borosilicate sleeve on the two HCPCF tips. The process requires no helium gas [2] nor glue [3], which are potential pollution sources, and keeps the physical integrity of the HCPCF intact. The used HCPCF exhibits a novel cladding lattice and operates in a robust single-mode fashion [5]. The fiber has a 30 µm core diameter, a 10dB/km transmission loss at 1540 nm. The resulted PMC has polished and mounted ends to FC/APC connectors for integration (Fig. 1(a)), exhibits a total insertion loss of ~2dB and a motion-resistant single-mode guidance (see reconstructed near field in Fig. 1 (b)). The PMC was then used to generate saturable absorption spectroscopy (SAS) measurement (Fig. 1 (b-c))). The observed saturable absorption peak reaches 25% contrast and 20MHz linewidth (FWHM) for ~190mW of pump power that is the highest observed contrast in a fully sealed PMC [6]. The evolution of the SA peak characteristics over pump power are represented in Fig. 1 (e), and shows for pump power of less than 100 mW a transit-time limited linewidth (19MHz), thus indicating no additional collisional dephasing that could be induced by contaminant during the assembly process. The linewidth was monitored over 3 months for 23 mW pump power with no significant variation (Fig.1(f)), demonstrating thus its hermeticity, and its suitability for long time and mobile applications. Fig. 1. (a) FC/APC patch-cord like final PMC. (b) Near field intensity profile at the output of polished PMC. (c) Normalized SAS measurement on P9 absorption line of acetylene (1530.371nm) in purple line, simulation of SAS signal for 31 µbar pressure in dashed line. (d) SA contrast zoomed in SA peak with numerical calculation in dashed line. (e) Experimental (dots) and calculated (line) SA transparency peak contrast and linewidth evolution with pump power. The dashed line correspond to the transit time limit. (f) Evolution of SA FWHM during 95 days for a pump power of 23mW.

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Generation of sub-Doppler spectral lines in compact devices is very useful in bringing functionalities such as laser-frequency stabilization, slow light application or quantum sensing to a wider and friendlier use. An all-fiber Photonic MicroCell (PMC), which consists of a gas-filled hollow core photonic crystal fiber (HCPCF) hermetically spliced to a solid optical fiber [1], is one of the most promising of such devices because of its ultra-long interaction length. Thus far, the PMC assembly processes are either limited to a short range of HCPCF core diameter [2], or entails the use of glue [3], or exhibit high injection losses (~10dB) [4]. Here, we report on a 7-meter contaminant free and single-mode C2H2-PMC. The process consists of end-capping a 80µbar acetylene-filled HCPCF via heat-collapsing a fused borosilicate sleeve on the two HCPCF tips. The process requires no helium gas [2] nor glue [3], which are potential pollution sources, and keeps the physical integrity of the HCPCF intact. The used HCPCF exhibits a novel cladding lattice and operates in a robust single-mode fashion [5]. The fiber has a 30 µm core diameter, a 10dB/km transmission loss at 1540 nm. The resulted PMC has polished and mounted ends to FC/APC connectors for integration (Fig. 1(a)), exhibits a total insertion loss of ~2dB and a motion-resistant single-mode guidance (see reconstructed near field in Fig. 1 (b)). The PMC was then used to generate saturable absorption spectroscopy (SAS) measurement (Fig. 1 (b-c))). The observed saturable absorption peak reaches 25% contrast and 20MHz linewidth (FWHM) for ~190mW of pump power that is the highest observed contrast in a fully sealed PMC [6]. The evolution of the SA peak characteristics over pump power are represented in Fig. 1 (e), and shows for pump power of less than 100 mW a transit-time limited linewidth (19MHz), thus indicating no additional collisional dephasing that could be induced by contaminant during the assembly process. The linewidth was monitored over 3 months for 23 mW pump power with no significant variation (Fig.1(f)), demonstrating thus its hermeticity, and its suitability for long time and mobile applications. Fig. 1. (a) FC/APC patch-cord like final PMC. (b) Near field intensity profile at the output of polished PMC. (c) Normalized SAS measurement on P9 absorption line of acetylene (1530.371nm) in purple line, simulation of SAS signal for 31 µbar pressure in dashed line. (d) SA contrast zoomed in SA peak with numerical calculation in dashed line. (e) Experimental (dots) and calculated (line) SA transparency peak contrast and linewidth evolution with pump power. The dashed line correspond to the transit time limit. (f) Evolution of SA FWHM during 95 days for a pump power of 23mW.
We report on the development of all-fiber stand-alone iodine-filled photonic microcells demonstrating record absorption contrast at room temperature. The microcell’s fiber is made of inhibited coupling guiding hollow-core photonic crystal fibers. The fiber-core loading with iodine was undertaken at 10 −1 -10 −2 mbar vapor pressure using what, to the best of our knowledge, is a novel gas-manifold based on metallic vacuum parts with ceramic coated inner surfaces for corrosion resistance. The fiber is then sealed on the tips and mounted on FC/APC connectors for better integration with standard fiber components. The stand-alone microcells display Doppler lines with contrasts up to 73% in the 633 nm wavelength range, and an off-resonance insertion loss between 3 to 4 dB. Sub-Doppler spectroscopy based on saturable absorption has been carried out to resolve the hyperfine structure of the P(33)6-3 lines at room temperature with a full-width at half maximum of 24 MHz on the b4 component with the help of lock-in amplification. Also, we demonstrate distinguishable hyperfine components on the R(39)6-3 line at room temperature without any recourse to signal-to-noise ratio amplification techniques.
A photonic microcell (PMC) is a length of gas-filled hollow core-photonic crystal fiber (HC-PCF) which is hermetically sealed at both ends by splicing to standard single mode fiber. We describe advances in the fabrication technique of PMCs which enable large core Kagome-lattice HC-PCFs to be integrated into PMC form. The modified fabrication technique uses fiber-tapering to accommodate the large dimensions of the fiber and enables low loss splices with single mode fiber by reducing mode field mismatch. Splice losses as low as 0.6 dB are achieved between 1-cell defect Kagome HC-PCF and single mode fiber. Relative to the previously reported PMCs, which were based on photonic bandgap HC-PCF, the present Kagome HC-PCF based PMC provides broad optical transmission, surface mode-free guidance and larger core at the cost of slightly increased fiber attenuation (~0.2 dB/m). Therefore, the integration of this fiber into PMC form opens up new applications for PMC-based devices. The advantage of the large core dimensions and surface mode free guidance for quantum optics in gas-filled HC-PCF are demonstrated by generation of narrow sub-Doppler features in an acetylenefilled large core PMC.
This thesis describes developments in the fabrication and applications of photonic microcells (PMCs). A PMC is a length of gas-filled hollow core-photonic crystal fibre (HC-PCF) that is hermetically sealed by splicing both ends to standard single mode fibre. A PMC enables integration of gas-filled HC-PCF into all-fibre systems with low insertion loss. PMCs have applications in coherent optics and metrology, where specific HC-PCF designs are advantageous. Two types of state-of-the-art HC-PCF, double bandgap HCPCF and large-pitch Kagome fibre, are fabricated. The double bandgap HC-PCF extends the usable bandwidth of the fibre by providing low loss guidance across two transmission bands. The large-pitch Kagome HC-PCF has a record low attenuation of 0.3 dB/m at 800 nm, while maintaining broadband guidance, which is partially attributed to the core shape. Three distinct developments in the field of PMCs are described. Firstly, a record length 20 metre acetylene-filled PMC is fabricated which is the key component in the first demonstration of an all-fibre slow and fast light system based on electromagnetically induced transparency. Secondly, a technique based on fibre tapering is presented which enables low loss integration of large core Kagome HC-PCFs into PMC form. Thirdly, micromirrors are developed and integrated with HC-PCF to confine light in the longitudinal dimension, providing a means to fabricate multi-pass PMCs. Two uncoated micromirrors are used to form a low finesse microcavity in Kagome HC-PCF, with record high fringe visibility using reflections from a silica/air junction. In collaboration with Kansas State University, an acetylene optical reference with sub-10 KHz accuracy and the first acetylene laser based on population inversion are demonstrated using Kagome HC-PCF.Finally, this thesis reports on rubidium vapour loading in HC-PCF with the ultimate aim being the production of a rubidium-filled PMC for applications in metrology. Preliminary results highlight the limited loading distance of the current technique and modified loading schemes are outlined accordingly.
We report detailed experimental results on the optical nutation effect in acetylene-filled hollow-core photonic crystal fibers (HC-PCFs). This coherent quantum optical effect is manifested by the appearance of attenuated oscillations in the profiles of transmitted step-like coherent optical pulses. The experiments used 15 ns optical pulses with peak powers up to ∼4 W and centered at 1530.37 nm, which corresponds to the acetylene (12C2H2) vibration-rotation resonance absorption line P9. The all-fiber cell made from a HC-PCF with internal diameter ∼10.3 μm contained a gas pressure of about 0.12 Torr. The Maxwell-Bloch equations are used to analyze the influence of the homogeneous relaxation, the inhomogeneous Doppler broadening of the absorption line, and the random orientation of the acetylene molecules. By comparing the experimental results with numerical simulations, we obtain the longitudinal and the transverse relaxation times to be close to 10 ns and find that the transition dipole moment of the acetylene P9 absorption line is ∼1.36×10−32 Cm.
This thesis reported on atomic spectroscopy for the development of alkaline atomic vapor photonic microcell (PMC). The work is motivated by reproducing the outstanding laboratory environment based performances achieved in the fields of frequency standard and coherent optics in highly compact and stand-alone devices that can be accessible to a wider scientific community or to a commercial market. In our case these future devices will be based a Hollow-core photonic crystal fiber (HC-PCF) filled with a gas phase material to form a PMC, and outstands with an ultra-long long interaction length and micrometric modal area. However, the micrometric scale of the fiber core harboring the atoms raises several technical and scientific challenges. Among the technical challenges, we list the development of efficient process for atom loading inside long hollow fiber with small core diameter, the suppression or mitigation of physio-chemical reactivity of the atoms (i.e. Rubidium) with the fiber core inner-surface silica etc. In parallel, the large surface-to-volume ratio of the fiber-core raises questions like the coherence relaxation dynamics and the nature and effect of the atom-surface interaction. The thesis reports on fiber-core inner surface coating with different materials to mitigate the physio-chemical reactions of the confined atoms with the surface, on tapering large core inhibited coupling Kagome HC-PCF, and splicing technique that ensures low splice loss and no atomic reactivity during the splicing process. In parallel, the thesis reports on a set of spectroscopy experiments to assess the relaxation dynamics of atoms inside HC-PCF and to report on novel sub-Doppler transparencies.
) accuracy. Using an earlier Q-tipping technique, 37% off-resonant transmission and 5 MHz accuracy were achieved in finding the line center, but a large 13% etalon-like effect appears on the wings of the optical depth. The etalon-like effect is reduced to less than 1% by using the tapering method. In both cases, the microcells could be connectorized, albeit with a reduction in off-resonant transmission efficiency, for integration into multimode fibers or free-space optical systems. Although contamination is introduced during both fabrication techniques, the P13 PMC line center shifts are small with respect to the sub-Doppler line center. This shows that the PMC can be used for moderate-accuracy frequency measurements. Finally, repeatable measurements show that PMCs are stable in terms of total pressure over approximately one year.