A wrapping-on-a-rod technique is presented and demonstrated successfully to realize broadband microfiber-based highly birefringent (Hi-Bi) devices with 3D geometry. By wrapping a circular microfiber (MF) on a Teflon-coated rod (2 mm in diameter), a large and broadband birefringence can be obtained utilizing a rod-microfiber-air (RMA) structure. Wavelength scanning method is used to measure the birefringence of the device. Results show that group birefringence as high as 10−3 can be achieved over 400 nm wavelength range. This compact element presents great potential in sensing and communication applications, as well as lab-on-a-rod devices.
©2012 Optical Society of America
Maintaining the polarization state of transmission is of great importance in some systems where random mode coupling of the propagating signal can lead to serious deterioration in system reliability and performance. To overcome this problem, researchers have developed highly birefringent (Hi-Bi) fibers. Conventionally, the internal birefringence of Hi-Bi fibers can be produced by introducing stress applying parts around the fiber core (e.g., commercial PANDA , Bow-Tie fibers  and internal elliptical cladding fiber ) or by geometrical effect of the core (e.g., D-shaped fiber  and elliptical core fiber ). All these optical fibers support two orthogonally polarized modes and the birefringence is on the order of 10−4 - 10−5. Recently, Hi-Bi photonic crystal fibers (PCFs) have also been fabricated . However, all these bulky 125-um-diameter fibers are not suitable for integrating in future micro/nano-photonics. Moreover, the fabrication cost for Hi-Bi PCFs remains a serious problem.
On the other hand, following the ever-growing development of micro/nano-photonics, microfibers (MFs) are of great interest for researchers because of their low loss, large evanescent fields, strong confinement, configurability, and robustness. They have found potential applications in a wide range of fields from telecommunications to sensors, and lasers [7–17]. Among them, Hi-Bi MFs have been proposed or demonstrated exploiting different structures, such as flat fiber with rectangle cross section [12, 13], MF with elliptical cross section , MF drawn from commercial PANDA fiber  and MF with a slot inside .
In this paper, we present a simple and effective wrapping-on-a–rod technique for the fabrication of miniaturized compact broadband MF Hi-Bi devices. Without complex and expensive micromachining facility, MF is drawn from standard single-mode fiber and then wrapped on a low-index rod. We both theoretically predict and experimentally demonstrate a MF-based Hi-Bi device by such a technique utilizing a rod-microfiber-air (RMA) structure. The device with compact 3D geometry shows a large birefringence (> 10−3) over 400 nm bandwidth. This compact device presents great potential in sensing and communication applications, as well as lab-on-a-rod devices.
2. Schematic model and theoretical analysis
We first numerically investigate the structure by employing finite element method (FEM). A schematic model in the black dashed box of Fig. 1 is used to study the properties of the stereo RMA structure, which can be obtained by wrapping a circular MF on a rod pre-coated with low-index polymer, such as Teflon used here. The rod only acts as a supporting element which can be any kind of material (polymethyl methacrylate, i.e. PMMA in our experiment) as long as the surface is smooth. Because the thickness of the Teflon film is tens of micrometers, little field will penetrate into the PMMA rod which will be neglected in our calculation. The blue, pink and yellow part in Fig. 1 is the MF, Teflon and supporting rod, respectively. Different coils of the MF are kept away from each other in order to prevent mutual field coupling. If adjacent coils are close enough, a microfiber coil/loop/knot resonator can be realized where the polarization-related coupling effect in the coupling region has to be considered . The rod is 2 mm in diameter which is much larger than the diameter of the MF (~1 - 3 μm). And, as a result, the bending-induced birefringence is not taken into consideration in our cases .
We numerically investigate the wave guiding properties of the RMA structure. The cross-section of the scheme is shown in Fig. 2 . Attributed to the asymmetric refractive index distribution in the two orthogonal directions (x and y in Fig. 2), light with different polarization experiences different effective refractive index, which is similar to that of a D-shaped fiber [19, 20]. And this lays the foundation of the principle employed in this paper. The difference between the effective refractive index is defined as phase birefringence (Bphase), result of which is presented in Fig. 3 . In our calculation range, it is easily found that a smaller rMF and a longer operation wavelength (λ) results in a larger Bphase. As λ decreases, the MF has greater ability to confine the light in the silica region and less evanescent field will penetrate into air or Teflon. It is the same case for increasing rMF, because of which Bphase falls. Moreover, in a wide operation wavelength range (1200 - 1650 nm) and MF diameter range (1.2 - 3 μm), Bphase remains larger than 4 × 10−4 which is the typical value for conventional polarization maintaining fibers .
3. Experimental fabrication and measurement
First, we use flame-brushing technique to draw the MF from conventional single mode fiber (SMF) which is cost-effective. Prior to the main experiment, we test the polarization properties of the MFs and they show no birefringence. Then, MFs with diameters ranging from 1 μm to 3 μm are wrapped around the PMMA rod (2 mm in diameter) that is pretreated with a coating of Teflon (Teflon® AF 601S1-100-6, a production of DuPont), tens of micrometers in thickness. Due to the high refractive index of PMMA, a thin film of low-index (1.36 @ 1550 nm) Teflon is indispensable in order to prevent the light field from suffering high loss. Because the thickness of the Teflon film is tens of micrometers, little field will penetrate into the PMMA rod which will be neglected in our calculation. To eliminate the field coupling between coils of the MF, distance is kept from coil to coil as shown in the inset of Fig. 4 . The total insertion loss of the device is about - 5 dB (blue line in Fig. 4), including loss from the MF with a large evanescent field and bending-induce loss. It can be further minimized by optimizing the tapering process and working environment.
The birefringence of the Hi-Bi RMA samples is measured by wavelength scanning technique as shown in Fig. 1 [12, 14]. Light from a broadband source (NKT, SuperK Versa) is linearly polarized by the first polarizer, exciting two orthogonal polarization modes that propagate through the device and then recombine at the second polarizer, forming interference pattern. The associated transmission spectrum of various samples is recorded by an optical spectrum analyzer (Yokogawa, AQ6370C). The transmission spectrum of our RMA structure can be predicted byFig. 4. As can be seen, large extinction ratio (16 dB - 32 dB) covering 400 nm bandwidth is obtained which indicates excellent polarization maintaining properties. However, noise in the spectrum increases when the wavelength is below ~1300 nm attributed to the fact that at these wavelengths, the fiber becomes multi-mode. Moreover, according to previous literature, bending-induced birefringence of our scheme is on the order of 10−6 which is negligible . In order to confirm this, we also examine the spectrum by immersing the RMA structure in Teflon solution. Results show that birefringence disappears after the asymmetry is broken.
Limited by the length of the MF, our samples contain one or two coils of MF and this results in large free spectrum range (FSR). In addition, the radius of the MF is not uniform in the region that wrapped on the rod. Thus, we can only obtain an average group birefringence (Bgroup) of our RMA samples. The results are calculated by Eq. (2) with experimental data from the spectrum.
L is the length of the MF wrapped around the rod. Solid asterisks in Fig. 5 show Bgroup of our samples with different rMF (measured at the thinnest waist of the MF) calculated from the spectrum. It is clear that a large Bgroup ~10−3 which is more than one order of magnitude than in conventional Hi-Bi fibers is achieved over the measured range and reaches ~4 × 10−4 at ~1415 nm for one particular sample (red asterisks in Fig. 5). In our measured range, a longer wavelength sees a larger Bgroup when rMF > 0.9 μm.
The high birefringence of the RMA structure can also be confirmed by theoretically calculating Bgroup, which is related to Bphase by
Bgroup exhibits a behavior notably different from Bphase. For MF with diameter less than the operation wavelength, Bgroup is small. However, it rises steeply to reach its peak value that is significantly larger than Bphase. The reason for this difference is the derivative of Bphase with respect to the wavelength, which is large at small MF diameter. After crossing over the peak value, Bgroup decreases at a tendency similar to that of Bphase. Comparing the results from the experimental data and that from FEM method, we find that they agree well with each other when rMF > 1.1 μm. Although rMF is not uniform and λ is just an average value, in this region, Bgroup changes slowly with respect to λ and rMF. However, it is not the case around the region where Bgroup reaches its peak value. In fact, Bgroup turns out to be a fast-varying function of λ and rMF around this region, especially in the longer wavelength and bigger radius side. Considering that rMF is not uniform along the coil on the rod and that the radius labeled in Fig. 5 is the thinnest measured, the difference between the results from the experiment and that from theory is reasonable. The radius of the MF is directly related to the spread of the evanescent field into the Teflon coating and air which determines the birefringence. Thus, controlling the radius of the MF is a key point in obtaining a large birefringence.
In this paper, we both theoretically exploit and experimentally realize a new kind MF-based Hi-Bi device which incorporates wrapping a MF around a rod that is pretreated with Teflon coating. This compact and cost-effective device could operate over a broad bandwidth with birefringence higher than 10−3. The fabrication technique of wrapping-on-a-rod is effective and low-cost. More novel and compact lab-on-a-rod devices can be explored by flexible design of rod material or coil profile. Our investigation of it would put insight into the application of MF in future fiber communication, sensing and micro/nano-photonics.
This work is supported by National 973 program under contract No. 2012CB921803 and 2011CBA00205, NSFC program No. 11074117 and 60977039, Natural Science Foundation of Jiangsu Province of China under contract No. BK2010247. The authors also acknowledge the support from PAPD and the Fundamental Research Funds for the Central Universities.
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