1. Introduction

In recent years, several notable efforts have been reported to guide the light along the liquid core in composite optical waveguides enabling a direct and long interaction between the guided light and the liquid for advanced optofluidic applications. Previously reported liquid core waveguide structures include: polydimethylsiloxane (PDMS) based waveguide [1], coreless photonic crystal fiber [2], Teflon based waveguide [3], anti-resonant reflecting optical waveguides (ARROWs) [4], nanoporous waveguides [5], Fresnel optical fiber [6] and thin film waveguides [7], to name a few. These liquid core waveguides have demonstrated various optoelectronic applications including optical switching [8], on-chip integration [9], and electrowetting lens in microfluidic channels [10]. A microfluidic 2x2 optical switch and optofluidic 1x4 switch [11] have been also reported for add/drop and optical cross connect applications by using soft lithography replication process [12]. Supercontinuum generation has been also realized in a toluene core silica waveguide [13].

In most of aqueous solutions the refractive index is lower than silica which is used in conventional optical waveguides, and there have several types of alternative cladding structures with a refractive index sufficiently lower than that of aqueous core. A patterned substrate and air cladding have shown good liquid core guidance with a low scattering loss [14]. Plastic microchannels [15], PDMS opt fluidic channel [11], and Teflon AF-coated microchannels [16, 17] have been attempted to provide the light guidance along the liquid core. However, the above mentioned attempts have not fully considered light polarization fidelity and its control, which is a critical parameter for advanced polarimetric sensing applications. And these prior microchannel waveguides were limited to a short light propagation length, which inevitably shortened the light-liquid interaction length. These prior waveguides also suffer from the light coupling issues, especially coupling with conventional single mode fibers (SMFs), which hindered a compact fiber input-output packaging.

In this study, we theoretically proposed and experimentally demonstrated a new type of liquid core optical waveguide that can provide not only a highly confined light propagation through the liquid core but also a very flexible birefringence control using an open V-groove silica fiber over a macroscopic length exceeding 47cm long, for the first time to the best knowledge of the authors. The proposed V-groove liquid core fiber (VLCF) was fusion spliced to conventional SMFs at both ends to provide a compact all-fiber packaging. The modal guidance and its unique birefringence in VLCF were theoretically analyzed by using a full-vectorial finite element method (FEM) with the perfect matched layer (PML) boundary condition. In order to implement the proposed structure in reality, silica VLCF was successfully fabricated only by conventional optical fiber drawing without requiring additional complicated processes, which can be further applied to other cladding materials. For the fabricated VLCFs, light propagation through the liquid core was tested at the wavelength of 635, 850, and 1550 nm and the output light polarization was also analyzed at λ = 850 nm using a polarimeter to confirm the birefringence.

Schematic diagrams of the proposed open V-groove liquid core fiber (VLCF) are shown in Fig. 1. The waveguide consists of the liquid core and the open V-groove cladding, whose refractive index is lower than that of the liquid to ensure direct light guidance through the liquid core. Light coupled into the liquid core can be guided axially due to total internal reflection on the liquid-cladding and the air-liquid interfaces as shown in Fig. 1(a). Key structural parameters are shown in Fig. 1(b) and they are: the liquid thickness of T, the liquid refractive index n_L, the cladding has the refractive index of n_C, opening angle of α and its apex with the curvature radius of R and the fiber outer diameter of D. In VLCF we assumed n_L>n_C as in Fig. 1(c), which is an asymmetric waveguide structure providing an inherent birefringence. In experiments, we tested this VLCF concept is silica fiber, which was drawn to an outer diameter to match that of conventional SMF. This allowed unique fusion splicing capability between VLCF and SMF, as shown in the inset micro-photograph, to make a compact all-fiber packaging with SMF input and output, which has been impossible in prior structures.

 

Fig. 1 A schematic diagram of the proposed open V-groove liquid core fiber (VLCF). (a) direct light guidance through the liquid core (b) cross-sectional view of VLCF and key structural parameters: T, liquid core thickness, fiber diameter, D, opening angle, α, and the apex curvature radius, R. (c) perspective view of VLCF refractive index profile with liquid refractive index of n_L, refractive index of clad, n_C, and air refractive index of n_air. (d) integration with single mode fibers (SMFs) by splicing them at both ends of VLCF.

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Similar asymmetric cladding structure has been implemented in exposed core micro-structured optical fibers [18–20], where part of the cladding was removed by elaborated micro-scale etching and milling processes to expose the solid silica core to the exterior environment.

In comparison to prior arts, our proposed structure can offer practical advantages in the following notions: fiber fabrication, liquid filling, modal guidance control, birefringence, and interconnection. In prior micro-structured liquid core fibers [21–24], most of the structures have been based on hexagonal air-hole arrays [25]. In order to fabricate these fibers, highly elaborated sequential processes had to be followed [26]. In addition to these meticulous steps, some of the prior reports further required additional post-processing such as sub-micron scale etching and milling [18–20,27]. Our V-groove fiber, in contrast, significantly reduced the number of process steps: slotting on the glass rod, and directly fiber drawing with a tension control, which will provide much improved reliability in fabrication processes.

There also have been liquid core fibers in a capillary structure [28–30] but all of those are operated in the multimode regime due to either the large core or the large index difference between core and cladding. In contrast, our open V-groove fiber structure allows a very flexible mode control by varying either the liquid height or V groove angle, not to mention the liquid refractive index. Note that this flexible mode control has not been reported in prior capillary liquid core fibers, and our open V groove structure can provide a definite advantage in modal interference sensing schemes. In prior capillary structures, it is impossible to obtain birefringence due to their circularly symmetric hole fixed by the manufacturer, yet our structure will allow a high birefringence in the fundamental mode, which could be further applied to polarimetric sensing schemes overcoming prior capillary structure’s limitations.

In recent liquid core fibers, in order to fill the hole with the liquid, bulky liquid reservoirs in multiple locations have been required as well as pressure control pumps [28, 29, 31]. Furthermore, in order to selectively fill a certain hole, various processes should be preceded before actual filling [32]. In contrast, our filling method required only a hollow capillary tube whose end was tapered with a well-known acid etching, along with a conventional liquid dispenser, which can obviate both liquid reservoirs and pumps required in prior arts.

In addition to these requirements, prior arts demanded complicated liquid-inlet and out-let ports in a package to deliver the liquid and launch the light through the liquid core [21–24]. In contrast, our technique is based upon the open V-groove structure such that it has inherently built-in inlet and outlet for the liquid. Furthermore, our V-groove in silica material can be directly spliced to SMFs at both ends such that the liquid core is already aligned to SMF core, which is another practical advantage over prior arts.

2. Numerical analysis

In order to analyze optical properties of V-groove liquid core waveguide, we used the commercial full vectorial finite element method (FEM) package [33]. In the simulation, we assumed the cylindrical perfectly matched layer (PML) absorbing boundary condition to obtain the complex effective indices n_eff in the eigenvalue equation for magnetic field propagating along the z direction.

Here ω is the angular frequency of the light, β is propagation constant of the guided mode, and $k_0=2\pi /\lambda$ is the free space wave number. The coordinate system used in the waveguide analyses is shown in Fig. 2(a) along with the refractive index profiles along x and y axes. Note that the refractive index profiles along the x and y axes are different due to the open V-groove nature, which results in unique and inherent birefringence. We assumed a parabolic segment at the apex as shown in Fig. 2(b) with the liquid core thickness of T and the segment length of 2b.

 

Fig. 2 (a) Coordinate system for VLCF and refractive index profile along x and y direction. (b). a parabolic segment at the apex with the liquid core thickness of T and the segment length of 2b (c) Electric field and Intensity distribution of fundamental $H{F}_{11}^x$ mode in x direction. (d) Electric field and Intensity distribution of fundamental $H{E}_{11}^y$ mode in y direction

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Modal birefringence, B, is given by the effective index difference between the $H{E}_{11}^x$ and the $H{E}_{11}^y$ modes, $\text{B}=\left|n_{eff}^x-n_{eff}^y\right|$. In this study, we used silica glass as the cladding material and its dispersion expressed in Sellmeier equations [34] were used to evaluate precise refractive index for the cladding. We used commercially available optical liquid form Cargill [35] as the core material and its refractive index was evaluated using Cauchy equations [35]. The parameters used in the simulations are summarized in Table 1. Intensity distributions of the $H{E}_{11}^x$ and the $H{E}_{11}^y$ modes at λ = 850nm are schematically shown in Figs. 2(c) and 2(d), respectively. Note that these modes are linearly polarized similar to conventional single mode fibers (SMFs), but they are not non-degenerate to result in a unique birefringence.

Table 1. Structural parameters of VLCF used in numerical analyses

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2.1. Propagation and birefringence modeling

We carried out optical wave propagation analyses with the variation of opening angle (α), the thickness of liquid core (T), and the refractive index of liquid (n_L). Figures 3(a) and 3(b) summarize the modal guidance regions at the wavelength 850 nm and 1550 nm, respectively. The refractive index of liquid and difference refractive index $(\Delta =(n_{core}-n_{clad})/n_{clad})$ were set to n_L = 1.4536 and Δ = 7.86 × 10⁻⁴ at λ = 850 nm as shown in Fig. 3(a), and n_L = 1.4496 and Δ = 39 × 10⁻⁴ at λ = 1550 nm as shown in Fig. 3(b). In the α versus T planes, the modal guidance region is clearly divided into three segments: no propagation mode (gray area), single mode (green area), and multimode (white area) sections. The single mode area decreases with α at both and 1550 nm. In comparison to the case at λ = 850, the T range for the single mode guidance increased at λ = 1550 nm.

 

Fig. 3 Modal guidance conditions for the single mode (green area), multimode (white area), and no core mode propagation (gray area) on the opening angle (α) versus liquid core thickness (T) plane (a) at λ = 850 nm, and (b) at λ = 1550 nm. Refractive index information is described in the text. Birefringence for various opening angle (α) in the spectral range (c) λ = 800-1000nm and (d) 1450-1650nm.

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Our V groove fiber used in experiments had the opening angle α~40° and at this angle, the single mode range for the liquid core thickness was T = 7~15 μm at λ = 850 nm and T = 6~12 μm at λ = 1550 nm. Birefringence variations in the spectral range of λ = 800-1000 nm and λ = 1450-1650 nm are summarized in Figs. 3(c) and 3(d), respectively. The birefringence in our VLCF was comparable to conventional polarization maintaining fibers (PMFs) [36–38]. The birefringence of VLCF increased with the light wavelength, which is consistent with prior reports [39, 40]. It is also noted that the birefringence showed a consistent increase with the opening angle α, but the corresponding T should be appropriately decreased to meet the guidance conditions suggested as the green zones in Figs. 3(a) and 3(b). Therefore, optimal values for the opening angle α, and the liquid core thickness T should be sought after to simultaneously provide a high birefringence and good modal guidance for given liquid refractive index n_L.

We further carried out modal analyses in the liquid core index (n_L) versus liquid core thickness (T) plane for the opening angle α = 40°, the approximate experimental value in this study. Figures 4(a) and 4(b) show the modal guidance regions at λ = 850 and 1550 nm, respectively. The area for the single mode guidance at λ = 850 nm was significantly smaller than that of λ = 1550 nm, especially for higher values of n_L. We also calculated the corresponding birefringence and the results are summarized in Figs. 4(c) and 4(d) for the spectral range of λ = 800-1000 nm and λ = 1450-1650 nm, respectively. Birefringence analyses were carried out for the VLCFs satisfying the single mode guidance conditions in Figs. 4(a) and 4(b). VLCF with the higher liquid refractive index showed the higher birefringence, which is consistent with a prior report [39–42]. It is noted that birefringence in the spectral range λ = 800-1000nm shown in Fig. 4(c) did not change significantly, but in the spectral range λ = 1450-1650 nm in the Fig. 4(d), the birefringence monotonically increased with the wavelength. The maximum birefringence is plotted as a function of the liquid index in Fig. 4(e). The slope of the birefringence increment was steeper at λ = 850 nm than at λ = 1550 nm. It is highly noteworthy that our VLCF can provide a useful birefringence range in the order of ~10⁻⁴ comparable to those of conventional PMFs in a very wide spectral range, λ = 850 to 1550nm, by simply varying the liquid refractive index and its thickness without modifying the fiber structure, which was not possible in prior PMFs.

 

Fig. 4 Modal guidance conditions for the single mode (green area), multimode (white area), and no core mode propagation (gray area) on the liquid core refractive index (n_L) versus liquid core thickness (T) plane (a) at λ = 850 nm and (b) at λ = 1550 nm. Here the groove opening angle was set to α = 40°. Birefringence for various liquid core refractive index in the spectral range of (d) λ = 800-1000 nm and (c) λ = 1450-1650 nm. (e) Birefringence as a function of the liquid core refractive index at λ = 850 nm and 1550 nm.

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3. V-groove fiber fabrication and liquid core injection

Our V-groove fiber fabrication includes two simple steps: slotting on a preform, and drawing it into the fiber. The process started with a commercial cylindrical silica rod with the outer diameter of 15 mm and the length of 100 cm (Heraeus F300). On this rod, we made a slot of 1mm width and ~7.5mm depth by traversing a diamond saw in the axial direction, as shown in Fig. 5(a). This preform was placed in a conventional optical fiber drawing tower equipped with a graphite resistance furnace and a fiber take-up system. The furnace was heated at the temperature of 1800 °C and the fiber diameter was controlled using the fiber take-up system and preform feed. The slot of the preform opened from outside as it was heated up, to form a V-groove with an opening angle α, which is then frozen to fiber as shown in the third diagram in Fig. 5(a). The drawn V-groove fiber had the cross-sectional diameter, D = 130 µm, a groove depth, h = 50 µm, and opening arc length, d = ~37 µm, and opening angle α = ~40°.

 

Fig. 5 (a) Changes in the cross section of the V-groove fiber during the fabrication processes. (b) A schematic diagram of injecting the liquid in V-groove using a micro syringe pump and a tapered hollow optical fiber (inserted picture).

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In order to form the liquid core uniformly along the V-groove, we used a commercial micro syringe dispenser (Harvard micro syringe) [43] that can be set a specific flow rate or target volume allowing accurate delivery of fluids. We filled our V-groove fiber with index matching liquids form Cargill [35] in the refractive index range from 1.450 to 1.465, which showed similar filling lengths. Liquids having proximate micro-fluidic properties as these liquids could be directly used in our silica V-groove fibers. These types of liquids from Cargill are being widely used in the immersion lithography in semiconductor manufacturing sites where slow evaporation requirements have been already met. For more volatile liquids, additional packaging using a polymer tube could be implemented to further control the liquid evaporation. Other important liquids with a refractive index higher than silica such as dimethyl sulfoxide (DMSO), glycerol, lipids, and especially cholesterols are being pursued by the authors. Other than these liquids, there have been recent reports [44–46] for bio-solutions that have high refractive indices, which are readily applicable to our silica V-groove fiber. However, the present silica V groove fiber would not be suitable for water based solutions in the refractive index range around 1.33, which is lower than silica refractive index. But the same design and fabrication technique could be applied to lower refractive index polymers such as Teflon or Fluorinated PMMA, which can allow direct light guidance through those aqueous liquids.

The liquid was filled from the bottom of the V-groove without any bubbles by using a conically etched capillary fiber. Figure 5(b) shows the schematic diagram of micro syringe pump that connected to the conically etched capillary fiber. The capillary fiber was drawn from a blank silica tube and it had the inner hole diameter of ~10 μm, and the outer cladding diameter of ~125 μm. Its end was conically tapered by isotropic chemical etching in hydrofluoric acid solution (HF 49%) as used in conventional wet etching. The conical capillary was connected to the micro syringe and positioned to VLCF by using a conventional mechanical x-y-z stage to fill the liquid from the bottom of the V-groove.

Recently, open V-groove structures have been intensively investigated in experiments for microfluidic applications [47–49]. Many of recent investigations have been based on a theoretical model that has been set-up for the flow of a series of alcohols with various surface tension-to-viscosity ratios, γ/μ, spreading in open V-shaped grooves cut in a copper plate [50]. In the model, the liquid filling length and the height were expressed in an analytic formula, which included the contact angle of the liquid on the V-groove surface as well as geometrical parameters of V-grooves and fluidic properties of the liquid. In the reference [50], groove angle was in the range of 21 to 58° and the liquid height in the range of 32.8 to 104 μm, which resulted in alcohol filling distance in the range of 1.2 to 2.2 cm. In our study, the liquid core thickness was also observed to be uniform but smaller by a factor of 3-10, and the filling distance was ~10 cm per single liquid injection, which indicates that the surface interaction between the index matching liquid and open silica V-groove surface would be significantly different from those in the reference [50].

Detailed theoretical modeling of optical liquid wetting on silica V-grooves has been very scarce, and data for surface interaction between the index matching liquid and silica V groove surfaces have not been reported yet, to the best knowledge of the authors. Exact modeling of liquid dynamics within silica V groove would require dedicated microfluidic measurements followed by proper numerical modelling, which is beyond the scope of this paper. Therefore, we focused on experimental measurements of optical properties and light guidance analyses to deliver clear and concise findings.

As shown in Fig. 2(b), the liquid core is approximated as a segmented parabolic with the following parameters: the radius of curvature R, the liquid core thickness T, and the liquid width 2b. The area of a segmented parabola is A = 4bT/3 and the liquid volume for the V-groove fiber with the length of L is given by V = 4LbT/3 [51]. In the case of the parabolic segment, T is proportional to b² such that volume is directly related to T for a given radius of curvature R. This volume corresponds to the amount of liquid that should be injected and we used a micro syringe dispenser that can control the volume and speed of liquid injection.

4. Experimental setup for light guidance and birefringence measurements

In order to characterize the light guidance along the fabricated VLCF, we measured the near-field of the guided mode using an experimental set-up shown in Fig. 6(a). Firstly, one end of V-groove silica fiber was spliced to conventional SMF using a commercial arc fusion splicer, as shown in Fig. 1(d). The arc conditions were optimized to leave the V-groove intact at the interface. Then the liquid core with n_L = 1.455 was injected to form a ~10cm long VLCF as described in Fig. 5(b). The laser operating at the wavelength of 635, 850, and 1550 nm were coupled to VLCF through SMF and the near-field patterns out of VLCFs were captured using a CCD camera. Figure 6(c) shows the near filed pattern of VLCF with T = 20μm at λ = 635 nm, which showed multimode characteristics with a non-Gaussian intensity profile. We also measured the modal patterns at λ = 850 nm, for the VLCF with T = 12μm as in Fig. 6(d). The far-field at λ = 1550 nm through the VLCF with T = 7μm is shown in Fig. 6(e). Consistent to theoretical analyses summarized in Figs. 4(a) and 4(b), the light guidance for the case of Figs. 6(d) and 6(e) was in the single mode, which was confirmed with our modal intensity measurements showing a Gaussian profile for the fundamental mode. The intensity profiles along the x and y axes are shown in the insets. We could confirm that thickness of the liquid core can directly change the modal guidance by measuring near filed pattern measurements, as expected from theoretical analyses. In our open VLCF, experimentally we found the length of liquid core was ~10 cm long for a single injection, and the injection process was repeated in ~10 cm interval as in Fig. 6(b) to obtain VLCF length of ~47 cm. In the experiment, the splice points between SMF and the V groove waveguide were mechanically fixed by a pair of optical fiber holder as shown in the Fig. 1(d), which protected V-groove from transversal tilting. The V groove waveguide was maintained in a steady state after the liquid core injection in order to keep the core thickness uniform in the longitudinal direction during the measurements.

 

Fig. 6 (a) experimental setup for measuring far field intensity pattern of the guided mode in the actual VLCFs. (b) Schematic diagram of seamlessly extending the liquid core simply by repeating the liquid injection along the V-groove fiber at ~10cm interval. (c) Near field intensity pattern of the guided mode at λ = 635 nm for a VCLF with T = 20 μm, (d) near field intensity pattern of the guided mode at λ = 850 nm, for a VLCF with T = 12 μm, (e) near field intensity pattern of the guided mode at λ = 1550 nm, for a VLCF with T = 7 μm. Here we used n_L = 1.455.

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In experiments, the V-groove opening angle at α = 40° showed the highest uniformity in geometrical parameters for the fiber outer diameter of ~130µm. Note that this diameter is required to match those (~125 µm) of the conventional SMFs. Our liquid filling system in Fig. 5(b) provided the liquid thickness T in the range of 8 to 15 μm for the index matching liquid (n_L = 1.455). Experimentally we found the length of liquid core was ~10cm long for a single liquid injection. In order to match the core diameter of SMFs in the range of 9~10 μm, we experimentally found the T = ~12 µm was optimal providing the lowest splice loss in the SMF-VLCF-SMF assembly. For the given liquid volume optimized for T = ~12 µm, we did not observe any significant deviation in the liquid core thickness in the repeated filling processes. Along a 10 cm long V-groove, T was measured to be within experimental error of ~ ± 1μm. We could experimentally confirm that the liquid thickness in an open groove is mainly determined by the liquid’s fluidic properties (viscosity, volume) and the liquid-surface interaction (surface tension, contact angles), and the geometry of the groove consistent to prior reports [47–50]. The way the liquid was injected did not play a decisive role as long as it provided a steady flow of liquid filling up from the bottom of the groove.

The uniformity in the thickness of the liquid core along our V-groove was estimated using a well-known conventional image processing technique [50], and the variation was within experimental error of ~ ± 1μm, except at both ends where the V-grooves were spliced to SMF. The open surface of SMF’s facets provided additional surface interaction with the liquid such that the liquid core was thicker by a factor of 1.3 within the length of ~10μm from the splice points than the rest of V-groove.

We measured the average propagation loss along the assembly with the VLCF length of 10cm at λ = 850nm and the loss was about 0.3 dB/cm comparable to prior reports in [52–54]. We used a conventional cut-back method [38], where the transmission power (P₁) through a ~10cm long V-groove liquid core fiber was measured and then that of a shorter length of ~5cm (P₂) was measured to estimate the loss α~-log₁₀(P₂/P₁)/5cm. We maintained the splice conditions and laser launching conditions identical for two measurements. Measured loss was attributed to the spatial mismatch of the guided modes between SMF and VLCF, and scattering loss at the V-groove surface.

For these experimentally optimized parameters, extension of the liquid core length was successfully achieved as schematically shown in Fig. 6(b). Owing largely to unique microfluidic natures of open V-grooves [47–50], we could seamlessly extend the liquid core up to 47 cm with a uniform thickness variable in the range of 7 to 12 μm by simply repeating the same liquid injection process as shown in Fig. 5(b) at multiple spots along the silica V-groove with a ~10cm interval. Note that in contrast to prior arts our VLCF length is not limited to this specific value but can be further increased flexibly by considering the trade-off between the light-liquid interaction length and the accumulated scattering loss there within.

We further characterized polarization states of the guided modes in the fabricated silica VLCF using the experimental set-up schematically shown in Fig. 7(a). In this case, we spliced two SMFs at both ends of the V-groove fiber and then filled the liquid core. We used a laser diode at λ = 850nm, and the VLCF had L = ~10cm, T = 12μm, α = 40°, and n_L = 1.455, where the single mode condition is satisfied. The incident light polarization was varied using a two paddle mechanical polarization controller. The polarization ellipse resented by two angles, ellipticity (η) and azimuth (θ), is shown in Fig. 7(b) The azimuth angle (θ) is the angular deviation of the ellipse from the x-axis. The ellipticity (η) is calculated from the ratio of the semi-minor (b) to the semi-major (a) axis, tan(η) = b/a. In its sign, η>0 and η<0 represent the right-handed and left-handed polarization, respectively. If η = 0, the light is linear polarized [55,56]. These two angles were experimentally measured using a polarimeter to characterize the polarization characteristics.

 

Fig. 7 (a) Experimental setup used to measure light polarization through SMF-VLCF-SMF assembly with liquid core length L = 10cm, T = 12μm, and n_L = 1.455, α = 40°. (b) Polarization Ellipse represented by ellipticity (η) and azimuth (θ) angles. Here a and b are the semi-major axis and semi-minor axis, respectively (c) ellipticity (η) and azimuth (θ) angle versus time measured by the polarimeter using a laser at λ = 850 nm.

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Figure 7(c) summarizes the polarization state through the fabricated VLCF, resented by two angles η and θ, as a function of the elapsed time. In the first 10 seconds, peddles of the polarization controller were not touched. In the next 10 seconds, the first peddle was rotated in its full range. In the last 10 seconds, the second peddle was rotated in its full range. It is noted that the output polarization state was not affected at all by the incident polarization states, which experimentally demonstrated the strong polarization maintenance of our VLCF due to its high birefringence. Figure 7(c) also shows that the ellipticity angle (η) was maintained to zero, which is consistent with our theoretical analyses that the fundamental mode is linearly polarized as shown in Fig. 2. The azimuth angle (θ)) was maintained near 85ᵒ. From these measurements, the birefringence of the VLCF was experimentally estimated to be ~0.1 × 10⁻³, which agreed well with theoretical analyses in Figs. 3 and 4.

5. Conclusion

A versatile V-groove liquid core fiber (VLCF) was proposed and experimentally demonstrated with a unique polarization preserving characteristics and seamlessly extendable liquid core length exceeding 47cm, which has not been achievable in prior arts. Unique asymmetric open V-groove with air-liquid-silica interfaces provided a flexible control of light guidance and magnitude of birefringence, by changing waveguide parameters such as the liquid index (n_L), liquid core thickness (T), and air opening angle (α). By thorough modal analyses, single mode guidance ranges were mapped on the α versus T plane and n_L versus T plane at two wavelengths λ = 850nm and 1550nm. In experiments, we obtained homogeneous and uniform liquid core fiber using a silica V-groove fiber with an optimal opening angle of α ~40°, T~12μm, and the outer diameter of ~130μm along with an index matching liquid with n_L = 1.455. The fundamental mode guidance was experimentally confirmed by measuring the near-field pattern of the guided light. High polarization maintenance in the linear polarization state was also experimentally confirmed using a polarimeter. The birefringence of ~0.1 × 10⁻³ was measured at λ = 850nm, which was consistent to theoretical estimation. At both ends of a VLCF with the liquid core length of ~10cm long, conventional single mode fiber input and output were successfully spliced to make a compact all-fiber package. The insertion loss was 0.3dB/cm, which could be further reduced with optimal splicing conditions and reduction of surface scattering. Our VLCF can open immediate applications in biochemical sensing with a high refractive index and nonlinear liquid optics. This structure could be further implemented with a low refractive index cladding material for low refractive index liquid as well, which is being pursued by the authors.