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Abstract
We demonstrate a broadband -based fiber coupler operating up to the 5.4 μm wavelength range developed by using a fused biconical tapering technique. During the manufacturing process, real-time data monitoring of the coupling ratio was at 2.64 μm. The measurement of minimal excess loss was at less than 1 dB in the range of 5–5.4 μm. Also, fiber bend loss was numerically analyzed to determine optimal coupler geometric parameters.
© 2019 Optical Society of America
1. INTRODUCTION
Over the last decade, mid-IR photonics has garnered significant interest due to its potential applications in optical spectroscopy, chemical sensing, mid-IR light sources, thermal imaging, and mid-IR comb generation [1–7]. The essential components for mid-IR photonic devices are ring resonators, Mach–Zehnder interferometers, wavelength division multiplexing (WDM) couplers, and power splitters operating in the broad mid-IR range with predictable coupling ratio and excess loss [8–10]. There are several conventional approaches for developing mid-IR photonic devices. These include planar semiconductor waveguide structures [6], laser inscription waveguides in glass [10], and mid-IR fiber devices [11]. Planar waveguides typically have significant optical losses (0.1–1 dB/cm) [4,6,7] that limit the application range to small-scale devices. Fiber-optic components are preferable for developing mid-IR high-power lasers, supercontinuum light sources, and spectroscopic instruments for remote gas sensing [7,12], which leads to increased demand in mid-IR fiber technologies.
There are three main types of transparent materials in the mid-IR that are applied to fiber-optic devices: heavy metal fluoride glasses [13–15], halide polycrystals [16], and chalcogenide glasses [17–19]. Chalcogenide fibers present low optical loss up to the 16 μm range (Te-based chalcogenide fiber), which makes them a prospective candidate as a platform for all-fiber structure mid-IR optical devices [20].
The key element of most fiber-optic devices is the optical coupler (for distributing the optical signal or power), which splits the power between multiple output channels from one input or mixes different incoming wavelengths from several inputs to one output. In the mid-IR range, the first work on the development of optical couplers dates back to 1995 [21]. In that study, a coupler made of step-index -based chalcogenide-glass multimode fibers with a coupling ratio of 15:85 developed by using the fused biconical tapering (FBT) technique was described and characterized. Other techniques for coupler manufacturing, including side-polishing and side-etching, are also possible. Side-polishing was demonstrated in [22], and the side-etching technique was studied in [23]. However, side-polishing requires a high-quality surface between the fibers and etching leads to a surface degradation; thus, we believe that the FBT technique is more reliable, repeatable, and technologically advanced than side-polishing and side-etching.
Recently, a breakthrough with creating a single-mode optical coupler based on chalcogenide fibers with variable coupling ratio and loss as low as 0.1 dB was demonstrated [24,25], whereby two single-mode glass fibers were brought into contact and then heated and stretched while being in parallel. Near-IR broadband sources were used to characterize coupler performance, and the measurements were limited to the 2.2 μm range in the above-mentioned works. In a study by Tavakoli et al. [25], the fiber coupler transmission in the 4 and 10.6 μm range was numerically simulated. To our knowledge, the experimental investigation of the broadband coupling in the 2.2–10 μm range has not yet been conducted.
In the present work, we focused on developing a low-loss fiber coupler in the 3–5 μm wavelength range, corresponding to an IR atmospheric window for application in laser spectroscopy based on novel high-output-power rare-earth-doped fluoride fiber lasers [26] and transition metal (TM)-doped II-VI chalcogenide lasers [27]. We demonstrate a broadband -based fiber coupler operating up to 5.4 μm with minimal loss of around 1 dB in the range of 5–5.4 μm, developed by using the FBT technique. Real-time coupling ratio measurements were conducted at 2.64 μm.
Practical applications of fiber couplers require them to be mechanically robust. For this purpose, we performed the twisting of the fibers, which increases infusion degree and maintains the fibers in contact. The approach is described in detail in a study by Stepanov et al. [28]. However, the twisting of fibers leads to additional bend loss. The study and analysis of bend loss behavior in -based fibers were carried out in the present investigation.
2. OPTICAL FIBER CHARACTERIZATION
A. Fiber Manufacturing
In the present work, the chalcogenide glass for fiber drawing was synthesized by melting purified arsenic monosulfide with the required amount of chalcogens. The synthesized arsenic sulfide glass had a content of metals and silicon less than 0.5 ppm wt. when measured by utilizing the activation method, gas chromatography, and laser mass spectrometry. The carbon content, which was determined by gas chromatography and the activation method, was approximately 0.6 ppm wt. The hydrogen and oxygen content, which was determined by a tandem time-of-flight laser mass spectrometry and IR spectroscopy in the bulk samples and optical fibers, was 0.02–0.2 ppm wt., and the heterophase inclusions of 100 nm size in the glass matrix were less than . Single-mode optical fiber was fabricated by using the double crucible method. The fiber was protected by fluoroplastic coating with 10 μm thickness. The core and cladding glass of single-mode fibers had the following composition: –core, –cladding. Core and cladding diameters of the fiber were 6.3 and 123 μm, respectively; numerical aperture (NA) was measured as 0.17 in the 2.64 μm range.
B. Fundamental Mode Properties and Optical Losses
We assumed the core refractive index to be equal to 2.4186 for 2.64 μm to calculate the fundamental mode dispersion and bend loss [29]. The difference between core and cladding refractive indices () was obtained from measurements of the fiber’s NA. This allowed us to calculate the effective mode index of the fundamental mode, .
During tapering, the effective mode index drops significantly. Figure 1 shows the effective mode index of the fundamental mode dependence on the waist coefficient , where is the initial core radius, and is core radius after stretching. One of the main loss paths for the light in the fundamental mode is due to bending. Bending arises due to the twisting of the helix structure of the fiber coupler. The bending radius, which is equal to the inverse curvature, is , where is the distance between the fibers, which is, in turn, equal to the cladding diameter, and is the helix pitch. To estimate the overall loss, we implement a simplified loss expression [30]:
Here, denotes optical loss ( is loss measure in dB/length), is effective bending radius, is core radius, is the fiber number, is the wavevector projection along the fiber symmetry axis, , and , , .
Fig. 1. Dependence of the effective refractive index on the waist coefficient of the fundamental mode of chalcogenide fiber.
The resulting loss value is presented in Fig. 2(a). Equation (1) is known for exaggerating losses; therefore, the lower bending loss limit may be obtained by adding to the in Eq. (1) [30]. Further, we consider the mean value between the upper and lower limits as the optimal estimation of bending losses. Bending loss [Fig. 2(b)] on waist coefficients and fiber curvature radius [Fig. 2(a)], and on waist coefficients and helix pitch [Fig. 2(b)] were calculated.
Fig. 2. (a) Bending loss of chalcogenide fibers. (b) Bending loss for the helix structure of chalcogenide fibers, depending on the waist coefficient and helix pitch .
Bending loss per mm significantly decreases with an increase of the helix pitch [Fig. 2(b)]. For a helix pitch greater than 5 mm, the loss does not exceed 0.05 dB/mm. Based on this result, we chose a pitch of 5 mm and a total length of 20 mm for the experimental coupler model in order to maintain the total bending loss below 1 dB.
C. Experimental Setup for the FBT Method
The fused biconical tapering (FBT) technique implies placing two independent optical fibers in contact, followed by heating and stretching. Cross infusion and narrowing of fiber diameter occurs in the heated region, allowing power coupling between two fiber modes.
Figure 3 shows the scheme of the experimental setup. The experimental setup consisted of two motorized stages (Zaber X-LSM), an in-house-built heating device, 2.64 μm DFB laser diode (Nanoplus), and a pair of TE-cooled InAs photodiodes (IoffeLED PD27). Angled fiber connectors were installed on the fiber ends and carefully polished to improve the free space coupling and to avoid back-reflectance. Before placing the fibers on the experimental setup, 100 mm of each fiber were cleaned of the fluoroplastic coating using acetone. Then, the fibers were twisted and set on the motorized stages. We made four turns on the 20 mm long region, which is enough to keep bend loss below 1 dB. The twisted region was fixed on both sides with photosensitive epoxy to maintain tight contact between fibers. After that, local heating was implied in the twisted region. To avoid the oxidization of the fiber surface, the heating was carried out under heated argon flow. The temperature in the heated region was measured to be around 200°C. No additional pre-fusion or pre-tapering was carried out. Laser emission was coupled to input port 1. Output signals were measured on both output ports 3 and 4 during the tapering process. The taper slope was approximately 1 mm in length. The fiber diameter in the tapered region was approximately 42 μm, and the waist region length was 20 mm. After that, the fiber coupler was relocated for broadband measurements without recoating the tapered region.
Fig. 3. Experimental setup for (a) in situ coupling ratio measurement. (b) Broadband coupler characterization. LD, laser diode; IL, input lens; TS, translation stage; OL, output lens; PD, photodiode; SM, spherical mirror.
Broadband coupler measurements were performed by using the Bruker IFS 113 v Fourier transform spectrometer with a spectral resolution of . SiC Globar was utilized as the broadband emission source. Fiber ends were adjusted to the maximum of the signal passing through the fiber with three axis stages and detected using a highly sensitive liquid nitrogen-cooled InSb detector.
3. RESULTS AND DISCUSSION
Power coupling performance is usually described by the coupling ratio (CR) parameter. CR is equal to the ratio of output power from one channel to the total output power from both channels:
The sum is equal to unity by definition . The evolution of the coupling ratio over time during the tapering process is shown in Fig. 4(a). At the beginning of the heating, equals unity, and equals zero. The graph indicates the attainment of a 50:50 coupling ratio for a heating process lasting 16 min with stretching speed equal to 5 μm/s. After cooling, the coupling ratio stabilized at the level of 53:47. The excess loss was calculated as , where is an optical power coupled from the laser to the chalcogenide fiber, and , are optical powers measured at each of the output ports.Fig. 4. (a) Coupling ratio over time on 2.64 μm. (b) Broadband coupling ratio measurements in the 2–2.7 μm wavelength range. (c) Broadband coupling ratio measurements in the 3.3–5.4 μm wavelength range.
Broadband coupling ratio [Figs. 4(b) and 4(c)] and excess loss [Figs. 5(a) and 5(b)] measurements were carried out in the 2–2.7 μm wavelength range and the 3.3–5.4 μm wavelength range. Due to intense water vapor absorption in the 2.7–3.3 μm wavelength range, the signal-to-noise ratio (SNR) was below detector sensitivity in that range. Small peaks in broadband coupling ratio around 3.4 and 4.1 μm [Fig. 4(c)] are due to O-H and S-H absorption in the fiber, which leads to change in refractive index according to the Kramers–Kronig relations. For the same reason, significant (around 5 dB) signal attenuation was observed at 4.1 μm [Fig. 5(b)]. The minimal loss was measured to be around 1 dB in the range of 4.5–5.5 μm.
Fig. 5. Broadband excess loss measurements (a) in the 2–2.7 μm wavelength range and (b) in the 3.4–5.5 μm wavelength range.
4. CONCLUSIONS
We have demonstrated a broadband fiber coupler operating in the mid-IR range, which was developed by using the fused biconical tapering technique. We have theoretically investigated the influence of the twisting process on total fiber losses. We have shown that, for a coupler helix pitch greater than 5 mm, the bend loss does not exceed 0.05 dB/mm. The coupling ratio was monitored during the tapering process at 2.64 μm to obtain a 50% level. We measured broadband coupling ratio and excess loss of the fiber coupler in the 2–2.7 μm wavelength range and the 3.3–5.4 μm wavelength range. In the 2.7–3.3 μm wavelength range, significant absorption was observed due to the evanescent wave interaction of fiber modes with atmospheric water vapor in the tapered region.
Funding
Russian Science Foundation (15-12-20040-P).
Acknowledgment
We are thankful to A. I. Ignatov from the Dukhov Research Institute of Automatics (VNIIA) for valuable help and fruitful discussions and to I. I. Vinogradov from the Space Research Institute (IKI RAS) for providing the 2.64 μm laser.
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