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Insertion loss of SMF-28-NANF interconnect using OM1 (AR coated, blue circles, solid blue line represents fitted measurement data) and OM2 with (red crosses) and without (black diamonds) AR coating GRIN MFAs.
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We demonstrate halving the record-low loss of interconnection between a nested antiresonant nodeless type hollow-core fiber (NANF) and standard single-mode fiber (SMF). The achieved interconnection loss of 0.15 dB is only 0.07 dB above the theoretically-expected minimum loss. We also optimized the interconnection in terms of unwanted cross-coupling...
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We report simultaneous low coupling loss (below 0.2 dB at 1550 nm) and low back-reflection (below −60 dB in the 1200-1600 nm range) between a hollow core fiber and standard single mode optical fiber obtained through the combination of an angled interface and an anti-reflective coating. We perform experimental optimization of the interface angle to...
Citations
... GRIN fiber that is typically 1/4 pitch long (hundreds of micrometers to millimeters long) was shown to work as a mode-field adapter (MFA) between an SMF and an HCF [18], [19], Fig. 3 (a). The desired mode-field size adaptation can be controlled coarsely via choice of the GRIN fiber. ...
... Consequently, the Fourier transform of Eq. 5 contains peaks at zero frequency (its amplitude represents s 2 1 s 2 2 ) and the beating frequencies (i.e., 2π λ ∆n m L), enabling us to separate amplitude of the beating between the fundamental mode and each of the HOMs. Using n ef f0 and n ef fm obtained from simulations [18], we can then identify which peak corresponds to which HOM. As follows from Eq. 5, this beating is proportional to s 1 s 2 c m1 c m2 , while transmission power is proportional to s 2 1 s 2 2 . ...
... As follows from Eq. 5, this beating is proportional to s 1 s 2 c m1 c m2 , while transmission power is proportional to s 2 1 s 2 2 . This analysis is an extension of that presented in [18], where only symmetric situation (c m1 = c m2 , s 1 = s 2 ) was considered. ...
We propose a hollow-core fiber (HCF) end-cap that incorporates a short segment of a single-mode fiber (SMF) that serves as a modal filter. To adapt the end-cap input and output beam to the desired size, the SMF was fusion spliced with short segments of mode-field adapting graded index (GRIN) fibers on both sides. The end-cap is anti-reflective coated to minimize insertion loss and parasitic reflections. The presented proof-of-concept experiments show its ability to suppress coupling into HCFs' higher-order modes. For example, without any end-cap, the extinction ratio between the LP
$_{11}$
and the fundamental mode was found to be as low as 9 dB when coupling light from a free-space beam that was misaligned by as little as 1.1
$^{\circ }$
. This was improved to 23 dB when inserting the developed end-cap. Such small angle misalignment often exists when aligning the input beam with 3-axis (x,y,z) stages only (rather than 5-axis that also include pitch and yaw). Finally, we glued the end-cap with the HCF, providing hermetic sealing to the HCF input/output. This is of interest for stable operation in applications that use free-space light launch or require HCF output into free space.
... This method of achieving low-loss interconnects between SMF and HCF is well explored, and is summarised along with other low-loss interconnect methods in Ref. [35]. Of particular note are past achievements in the telecommunication band, where interconnect losses as low as 0.15 dB have been achieved [36]. ...
We present a low-loss, compact, hollow core optical fibre (HCF) cell integrated with single mode fibre (SMF). The cell is designed to be filled with atomic vapour and used as a component in photonic quantum technologies, with applications in quantum memory and optical switching. We achieve a total insertion loss of 0.6(2) dB at 780 nm wavelength via graded index fibre to ensure efficient mode matching coupled with anti-reflection coatings to minimise loss at the SMF-HCF interfaces. We also present numerical modelling of these interfaces, which can be undertaken efficiently without the need for finite element simulation. We encapsulate the HCF core by coupling to the SMF inside a support capillary, enhancing durability and facilitating seamless integration into existing fibre platforms.
... The length of the GIF is chosen so that the light pattern which exits is expanded (and typically, collimated) so that this mode more closely matches the fundamental mode of the HCF. This scheme is pictured in Fig. 4 (a), and has been employed to achieve extremely low insertion losses of 0.15 dB [90]. ...
... However, it should be noted that the coupling efficiency of these interconnects is fundamentally limited by the differing shapes of the SMF and HCF modes. A GIF lens does not change the overall shape of the mode, it only alters its width, and so the fundamental limit is 0.07 dB [90]. Other interconnection techniques will have to be developed to go below this limit. ...
Recent progress in the development and applications of microstructured optical fibres for quantum technologies is summarised. The optical nonlinearity of solid-core and gas-filled hollow-core fibres provides a valuable medium for the generation of quantum resource states, as well as for quantum frequency conversion between the operating wavelengths of existing quantum photonic material architectures. The low loss, low latency and low dispersion of hollow-core fibres make these fibres particularly attractive for both short- and long-distance links in quantum networks. Hollow-core fibres also promise to replace free-space optical components in a wide range of atomic experiments.
... In addition, the implementation of the capillary sleeve enables the effective separation of the MH-ECF and AR-HCF, which can improve the optical coupling efficiency. Since the mode-field diameter between Photonics 2024, 11, 301 5 of 10 the MH-ECF and AR-HCF is different, as shown in Figure 1b,d, it is necessary to keep a certain gap to ensure that the mode overlap integral is as large as possible [27][28][29]. It should be noted that the curing process is performed with the help of a microscope. ...
Laser absorption spectroscopy for gas sensing basically employs an air pump located at the gas cell probe to draw in ambient gases, and the on-site gas sample is subsequently delivered for laboratory non-real-time analysis. In this study, an in situ all-fiber remote gas sensing strategy is proposed. The anti-resonant hollow-core fiber (AR-HCF) is used as the sensing fiber, and a 20 m middle-hole eccentric-core fiber (MH-ECF) is used as the conducting fiber. The remote ambient gases can be inhaled into the AR-HCF as a result of the negative pressure transmitted through the MH-ECF when pumping gas at the interface of the MH-ECF. Since the real-time monitoring of greenhouse gas emissions in industrial processes holds immense significance in addressing global climate change, the detection of CO2 is achieved with the TDLAS-WMS method, and the gas sensing performance of an all-fiber remote gas sensing structure (RGS) is experimentally validated. The response time t90 under the pumping condition is about 456 s, which is about 30 times faster than that of free diffusion. Allan deviation results for more than one hour of continuous monitoring indicate that the lowest detection limit for the all-fiber RGS is 0.0373% when the integration time is 184 s. The all-fiber remote gas sensing strategy also possesses the benefits of being applicable to multiplex, hazardous gas environment passive monitoring.
... Suslov successfully made interconnector with coupling loss of 0.15 dB. 11 But this design is relatively large in size. ...
In this study, a novel freeform surface lens coupler is designed to realize the coupling functions in the hollow‐core photonic crystal fiber (HCPCF) resonator optical gyroscope. One surface of the lens is an ellipsoid. Coupling of the two HCPCF loop's ports can be directly accomplished via reflection on the ellipsoid surface. The other surface of the lens is the Biconic Zernike surface. Transmitting through the above two surfaces, the coupling between the intracavity and extracavity is completed. In this paper, simulations were performed based on two types of HCPCFs, a hollow‐core photonic bandgap fiber of HC19‐1550 and a state‐of‐the‐art nested antiresonant nodeless fiber. The volume of the lens coupler is calculated to be approximately 4 × 2 × 4 mm³. The simulation results show that the intracavity coupling loss can be as low as 0.011 dB, and the coupling loss between the intracavity and extracavity can be as low as 0.4 dB. Compared with the traditional fusion scheme between hollow‐core fiber and polarization‐maintaining fiber, the freeform surface lens coupler in this paper can achieve lower coupling loss and higher resonance fineness. Compared with other spatial coupling schemes, it has lower coupling loss and fewer coupling components.
... On the other hand, Z. Zhang et al presented the ultralow-loss HCF interconnection technique, but it seems that it cannot be used as a connector since the HCF end face is exposed [12]. D. Suslov et al realized low loss interconnection between standard SMF and HCF technology using the mode field adaptation [13]. One solution of the HCF interconnection is to attach a normal PC connector to the SMF side that is spliced to the HCF. ...
... It also features short coupling length and wide bandwidth and has attracted widespread attention in recent years. HCF uses air as the transmission medium and possesses characteristics such as low latency [12], low dispersion [13], and minimal nonlinearity [14]. It has the potential to overcome the bottleneck of communication system capacity posed by inherent limitations of solid core fibers. ...
Apolarization beam splitter based on a dual hollow-core anti-resonant fiber is proposed. In the horizontal direction, elliptical shapes are introduced to form two hollow cores, with an air gap between the ellipses serving as a mode-coupling channel. Using the finite element method, a specific analysis is conducted on the influence of various structural parameters of the dual hollow-core anti-resonant fiber on the coupling length of the x-polarized and y-polarized modes, as well as the extinction ratio of higher-order modes.Optimizing the structural parameters of the fiber has led to the design of a dual hollow-core anti-resonant fiber polarization beam splitter that is exceptionally compact, measuring just 1.306 cm in length. This device achieves a broad bandwidth of 420 nm ranging from 1.53 µm to 1.95 µm and maintains superior single-mode characteristics within this wavelength spectrum. The novel dual hollow-core anti-resonant fiber is poised to play a significant role in the optical communication systems and optical network technologies.
... Two HOM groups need particular attention, including LP 11 mode group because it is often the lowest loss HOM group, and LP 02 mode group because by virtue of its shared symmetry with the fundamental mode, it often receives the highest power at interconnection points. It has been shown previously [8], that LP 11 mode launch is ultimately limited by the symmetry of the fabricated HCF (with ideal structure showing zero excitation), with reports showing unwanted cross-coupling below -35 dB [8]. The situation with LP 02 mode is different, even though it has the same symmetry as LP 01 mode. ...
... Two HOM groups need particular attention, including LP 11 mode group because it is often the lowest loss HOM group, and LP 02 mode group because by virtue of its shared symmetry with the fundamental mode, it often receives the highest power at interconnection points. It has been shown previously [8], that LP 11 mode launch is ultimately limited by the symmetry of the fabricated HCF (with ideal structure showing zero excitation), with reports showing unwanted cross-coupling below -35 dB [8]. The situation with LP 02 mode is different, even though it has the same symmetry as LP 01 mode. ...
... Coupling into this mode is generally caused by the mismatch between the mode-field size and shape of the launching beam. It has been shown that for an input Gaussian beam that has a modefiled diameter (MFD) identical to that of the LP 01 mode, this coupling is -24 dB [8]. Experimentally, a value of -22 dB was demonstrated [8]. ...
... We refer to this loss as limited by the "mode-field shape mismatch" between the SSMF and HCF. Several reports compared the finite-element simulated results with the best-achieved experimentally when coupling from free space (0.22 dB in [12]) and from SSMF (0.15 dB in [13]). However, the best-achieved experimental results were, at best, about twice as large as those predicted by simulations (0.074 dB [11]). ...
... It should also provide a flat phase profile across the beam cross section (such as obtained in a Gaussian mode in its focus/waist), as the HCF fundamental guided mode has also a flat phase profile in the core region. Fiber-compatible mode-field adaptation using SSMF tapering [18], SSMF thermally-expanded core [13], reverse tapering [19], and splicing of a short segment of a graded index (GRIN) fiber [13] have been proposed. The last-mentioned method requires specific GRIN parameters to achieve the desired MFD with the required flat phase profile. ...
... It should also provide a flat phase profile across the beam cross section (such as obtained in a Gaussian mode in its focus/waist), as the HCF fundamental guided mode has also a flat phase profile in the core region. Fiber-compatible mode-field adaptation using SSMF tapering [18], SSMF thermally-expanded core [13], reverse tapering [19], and splicing of a short segment of a graded index (GRIN) fiber [13] have been proposed. The last-mentioned method requires specific GRIN parameters to achieve the desired MFD with the required flat phase profile. ...
We propose an approach to interconnect a hollow-core fiber (HCF) of arbitrary core size with standard single-mode fiber with perfect mode-field size adaptation and experimentally achieve for the first time insertion loss agreeing with that predicted by simulations. We demonstrate this using three low-loss HCFs, including 1 st window nested antiresonant nodeless fiber (NANF), 2 nd window NANF and the state-of-the-art double NANF (DNANF). The connection with a minimum achieved insertion loss of 0.079 dB was permanently secured via gluing and did not degrade during 4 weeks of continuous measurement. To the best of our knowledge, this is the lowest reported value and is comparable to or lower than the connection between dissimilar single-mode fibers (e.g., standard single-mode fiber and dispersion-compensating fiber). We also show that such connection leads to excellent suppression of higher-order modes coupling, of importance to all applications sensitive to multi-path interference. Importantly, obtaining agreement between simulations and experiments validates for the first time the accuracy of the simulations and opens the door to further optimization via simulations with the ability to subsequently achieve the same result experimentally.
... Several works have investigated various practical methods of optimizing launch into hollow-core fibres (HCFs): either via free-space coupling into bare HCFs [18], splicing end-caps to HCFs [19], direct splicing of HCFs and commercial SCFs [20], gluing intermediate graded-index elements [21], [22], or more delicate ones, such as employing tapered nanospikes [23], or silica balls [24]. However, no study has hitherto established the fundamental limits governing this coupling efficiency (CE) and whether it can be optimized through the design of the fibre cross-section itself. ...
We theoretically explore the fundamental limits on the efficiency of coupling light into hollow-core antiresonant fibres. We study in particular the coupling of a free-space Gaussian beam to the guided modes of one of the most successful antiresonant fibre (ARF) geometries, the the nested antiresonant nodeless fibre (NANF). Through finite element simulations, we study the effect of the geometrical parameters of the fibre on the coupling efficiency, showing that coupling into the fundamental LP
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-like mode is typically maximized around 96-98 % when the incident beam waist is about 70 % of the core diameter. We find that due to the nature of antiresonance guidance, higher coupling efficiencies are achieved for fibres operating in the second antiresonant window (or generally even-numbered windows) than in those operating in the fundamental antiresonant window (or generally odd numbered windows), although the difference between even and odd decreases with the order of the window. We verify this theoretical finding experimentally with precise measurements of coupling efficiency into two NANFs operating in first and second windows, respectively. Our results which consistently show a steady 1.4 percentage point higher coupling efficiency for the second window fibre imply that such fibers may be the most suitable candidates for applications such as laser delivery which require up to a few hundred meters of fiber.
