Design and development of a trenched optical fiber with ultra-low bending loss

Pramod R. Watekar; Seongmin Ju; Won-Taek Han

doi:10.1364/OE.17.010350

1. Introduction

With growing interests among researchers in fiber-to-the-home (FTTH) applications driven by increasing number of internet users, the research and the development of optical fibers having bend insensitivity are in much demand [1–4]. In contrast to currently used single mode optical fibers (SMFs) that cause a huge power penalty due to bendings at corners of a home, bend-insensitive optical fibers (BIF) offer cost savings and compactness of fiber boxes. Specifically, two kinds of bend-insensitive optical fibers are available: (a) low-index trenched BIFs, and (b) Mode field diameter (MFD) optimized BIFs.

In the low-index trenched BIF approach, the bend insensitivity is achieved by putting a low index trench in the cladding, thereby reducing the cladding effective index and preventing the loss of power due to bendings [1,2]; the commercialization of these BIFs have been already achieved [3–5]. In the extended version of the low-index trenched optical fiber, the optical fiber with bending loss of 0.03 dB/loop has been reported at 1550 nm for the bending radius of 5 mm where a nano-engineered low-index ring around the core has been used [5]. Thus, this approach of adding low-trenches or low-index nano-rings in the cladding region has successfully addressed the issue of reducing the bending loss. Typical bending loss for the low-index trenched optical fiber is about 0.67 dB/loop [4] and 0.03 dB/loop [5] for the low-index trenched and nano-engineered optical fibers, respectively, at 1550 nm for the bending radius of 5 mm. In the second approach to achieve bend insensitivity in the optical fiber, the MFD of optical fiber core can be optimized such that the mode field is confined inside the optical fiber core, which then limits the bending loss upon sharp bends. Experimentally measured bending loss reported with such technique of bending loss optimization has been restricted to about 1 to 1.7 dB/loop at 1550 nm for the bending radius of 5 mm because MFD cannot be reduced below 6–7 µm, otherwise incompatibility of these fibers with existing single mode fibers (SMFs) arise [6–8].

Making the low index trench in the optical fiber needs a high skill of fiber manufacture. The low-index trench in the optical fiber can be added by co-doping silica with fluorine or boron. If we define the trench index difference as

Δ n_{Trench} = n_{clad} - n_{Trench}

where n _Trench is the trench index and n _clad is the cladding index, its typical value is around 0.002 for the fluorine doping (in the modified chemical vapor disposition (MCVD) system), but this index difference value of the trench is not sufficient to achieve the desired bend insensitivity [6]. Another way to make low refractive index of the trench is to use the boron doping, but then the control of trench width becomes an issue due to the diffusion of boron atoms in the surrounding regions. Therefore, the fluorine doping is preferred to the boron doping. Considering the skill required and cost related factors in manufacturing the low-index trenched optical fiber, we combined (a) the shallow low-index trench to be fabricated using the fluorine doping and (b) the MFD optimization to achieve the lowest bending loss for any optical fiber reported so far. It should be strongly emphasized that since the optical fiber fabricated by using the MCVD technique usually shows central dip in the core and therefore to design the experimental BIF fiber, we chose the worst case of central index dip (i.e., the dip index reaching the cladding index value). It is noted that occurrence of the worst case central index dip is usual if no measures are taken to reduce it (for example, removing the innermost layer, or additional supply of GeO2 just before sealing of the preform; that needs a skilled experimentalist). Therefore, if the current methodology is followed, no central dip removing technique is necessary; limitations on the dispersion and the MFD will be taken care by the design. In the current communication, firstly, we discuss a theoretical design of the bend-insensitive optical fiber by adopting the MFD and the trench optimization technique, and secondly, we report the development of optical fiber at tolerable limits to achieve the lowest bending loss ever reported for the single mode optical fiber.

2. Design

To theoretically design the bend-insensitive SMF, the MFD of fiber was calculated from the following relation:

MFD = 2 \sqrt{\frac{{2 π (\int_{r} E^{2} r d r)}^{2}}{{π \int}_{r} E^{4} r d r}}

The macrobending loss in units of dB/km was calculated using [9,10]:

α_{macro} = \frac{10}{{Log}_{e} 10} {(\frac{π V^{8}}{16 a R_{b} W^{3}})}^{1 ⁄ 2} \exp (\frac{- 4 R_{b} Δ W^{3}}{3 a V^{2}}) \frac{{[\int_{0}^{\infty} (1 - g) E r d r]}^{2}}{\int_{0}^{\infty} E^{2} r d r}

where E is the radial field of fundamental mode, a denotes the fiber core radius, R_b is the bend radius, n_max and n_min are maximum and minimum values of the refractive index and other parameters appearing in Eq. (3) are given by:

g = \frac{n {(r)}^{2} - n_{min}^{2}}{n_{max}^{2} - n_{min}^{2}}; V = k_{0} a \sqrt{n_{max}^{2} - n_{min}^{2}}

We used the commercial FiberCAD code to solve propagation equations and to calculate the bending loss of optical fiber. It is noted that to address the issue of a central index dip (typical for the optical fiber made with the MCVD process) the refractive index profile was modeled as:

n (r) = n_{max} \sqrt{1 - 2 Δ {(1 - \frac{r}{a})}^{2}}

where n _max is the maximum value of core refractive index, w is a width of dip region, and Δ(n ² _max-n ² _clad)/(2n ² _max). We choose 100% central dip (i.e., the dip approaching the value of cladding index; n(0)=n_clad) for designing the optical fiber.

Design criteria are listed in Table 1 that followed the ITU-T recommendation for G652 for the optical fibers except that the MFD requirement was relaxed to be above 7.5 µm [8]. With regards to the cutoff wavelength of SMF, the ITU-T recommendation tells that the cable-cutoff wavelength should not be more than 1260 nm. Though there is no exact relationship available between the cable cutoff wavelength and the fiber cutoff wavelength, with our own observations it can be stated that the mentioned cable cutoff wavelength is roughly equals to 1350 nm to 1450 nm of the optical fiber cutoff wavelength. The optimum parameters that we obtained were subjected to these cutoff wavelength constraints. The design methodology adopted was as follows. Step-1: we obtained the optimum core refractive index and radial parameters of the bend-insensitive single mode optical fiber that followed the design criterion-1. Step-2: by using the core parameters obtained with Step-1, a trenched SMF was designed for various trench parameters that followed the design criterion-2. By using these two different design steps, we were able to obtain the trench parameters that resulted in the minimum bending loss possible within said limits of design criteria.

Table 1. Design criteria for the bend- insensitive single mode optical fiber.

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Following the design criterion-1, calculated refractive index and core radius values of the bend-insensitive single mode optical fiber without any trench are listed in Table 2(a) for the core without central index dip, and in Table 2(b) for the core with central index dip. Parameters that pose limitations on core radius values at the given index, n _max, are shown in brackets. It can be observed that for the bend-insensitive single mode optical fiber to follow the design criterion-1, there is limited freedom to choose core parameters. As an example, typical curves used to get data in Table 2(b) are illustrated in Fig. 1 for the core refractive index of 1.452, where it can be observed that the MFD at 1550 nm is always more than 7.5 µm for any value of the core radius and therefore it has been mentioned as ‘Any’ in Table 2(b). It can also be noted that for parameters mentioned in Fig. 1, the bending loss decreases with increasing the core radius and therefore we need the core radius above certain value (i.e., above ‘minimum a’) to get the bending loss less than 2 dB/loop at 1550 nm and the bending radius of 5 mm.

Table 2. (a). Optical parameters of the single mode optical fiber without central index dip. These parameters were obtained by following limitations on the bracketed parameter as listed in the design criterion-1.

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Core radius a is in µm, Bending loss is in dB/loop at 1550 nm for 5 mm of the bending radius.

Table 2. (b). Optical parameters of the single mode optical fiber with central index dip. These parameters were obtained by following limitations on the bracketed parameter as listed in the design criterion-1.

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Core radius a is in µm, Bending loss is in dB/loop at 1550 nm for 5 mm of the bending radius.

Fig. 1. Dependence various parameters of the single mode fiber on its core with the central index dip. The core index was 1.452.

Parameter	Design criterion-1	Design criterion-2
Bending loss (dB/loop)	<=2	Lowest
Loop radius=5 mm	at 1550 nm	at 1550 nm
Absolute dispersion (ps/km.nm)	<=18.5	<=18.5
Dispersion slope (ps/km.nm²)	<=0.095	<=0.095
Mode field diameter (µm)	>=7.5	>=7.5

n _max		Step-index profile (core radius a, in µm)
	(MFD) a	(Dispersion) a	(Bending loss) Minimum a	Permitted a Cutoff=1350 nm	Permitted a Cutoff=1450 nm
1.45	Any	1.325–4.465	5.355	-	-
1.451	Any	1.205–4.270	4.135	-	-
1.452	Any	1.108–4.112	3.440	-	3.440–3.646
1.453	>=2.308	1.039–3.952	2.983	2.983–3.201	2.983–3.437
1.454	>=3.428	0.973–3.824	2.653	-	-
1.456	>=3.752	0.877–3.608	2.201	-	-

Wavelength (nm)	MFD (µm)	Maximum Bending loss (R_b =5 mm) dB/loop
1310	5.9	Negligible
1550	7.7	0.014
1625	7.9	0.044

Bending loss (dB/loop) at wavelength → Fiber type:	1300 nm	1550 nm	1625 nm
SMF	7.92	39.42	60.24
Trenched BIF [3]	1.98	9.55	15.60
Trenched BIF [4]	1.16	0.67	1.02
Nano-engineered BIF [5]	-	0.03	-
MFD optimized BIF [7]	-	1.17	3.30
MFD optimized+Trenched BIF [Present work]	Negligible	0.014	0.044

n _max		Step-index profile with central dip (core radius a, in µm)
	(MFD) a	(Dispersion) a	(Bending loss) Minimum a	Permitted a (Cutoff=1350 nm)	Permitted a (Cutoff=1450 nm)	Permitted a (Cutoff=1500 nm)
1.45	Any	1.88–4.69	-	-	-	-
1.452	Any	1.63–4.79	5.345	-	-	-
1.454	Any	1.46–4.39	3.691	-	-	-
1.455	Any	1.39–4.43	3.245	-	-	3.245–3.41
1.456	Any	1.33–4.56	2.911	2.911–2.93	2.911–3.05	2.911–3.263
1.457	>=2.738	>=1.28	2.652	2.738–2.823	2.738–2.922	2.738–3.135
1.458	>=2.85	>=1.23	2.446	-	-	2.85–3.02
1.46	>=3.1	>=1.15	2.172	-	-	-

n _max		Step-index profile with central dip (core radius a, in µm)
	(MFD) a	(Dispersion) a	(Bending loss) Minimum a	Permitted a (Cutoff=1350 nm)	Permitted a (Cutoff=1450 nm)	Permitted a (Cutoff=1500 nm)
1.45	Any	1.88–4.69	-	-	-	-
1.452	Any	1.63–4.79	5.345	-	-	-
1.454	Any	1.46–4.39	3.691	-	-	-
1.455	Any	1.39–4.43	3.245	-	-	3.245–3.41
1.456	Any	1.33–4.56	2.911	2.911–2.93	2.911–3.05	2.911–3.263
1.457	>=2.738	>=1.28	2.652	2.738–2.823	2.738–2.922	2.738–3.135
1.458	>=2.85	>=1.23	2.446	-	-	2.85–3.02
1.46	>=3.1	>=1.15	2.172	-	-	-

Abstract

1. Introduction

2. Design

3. Experiments

4. Results and discussion

5. Summary

Acknowledgments

References and links

Cited By

Figures (15)

Tables (5)

Equations (7)

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