Improvement and evenness of the side illuminating effect of side emitting optical fibers by fluorescent polyester fabric

Abstract

A strong and even side illuminating effect is required for plastic optical fibers (POFs) in illuminating applications. In consideration of good flexibility and illumination, side emitting POFs with 2 and 3 mm core diameters are preferred, especially in active illuminating safety textiles. However, the side illumination intensity of side emitting POF varies significantly along the fiber length. Fluorescent polyester (PET) fabric rather than traditional surface modifications is employed to enhance and even the side illuminating effect of POFs based on the emitting principle of phosphors. Two testing methods of side illumination intensity are carried out on semi-automatic devices. The results indicate that 2 mm side emitting POFs might take the place of 3 mm side emitting POFs by using fluorescent PET fabric, with a similar side illuminating effect in applications under certain circumstances.

Keywords

side emitting optical fiber side illumination intensity fluorescent polyester fabric bending radius

Plastic optical fiber (POF), an optical waveguide, is well known as a medium of light transportation from one place to another. The achievement of light transportation in POF is based on the total internal reflection.¹ Therefore, POF consists of two layers at least: core and cladding. The most popular core materials of POF are poly(methyl methacrylate) (PMMA), polystyrene (PS) and polycarbonate (PC) based on the high transparence of light and good fiber or film forming ability.² Apart from the above two features for core materials, there is another vital requirement for cladding materials. In order to guide light rays through POF, the refractive index of fiber cladding is slightly smaller than that of the fiber core according to Snell’s law. Another layer, known as the jacket, is generally recommended to improve ultimate fiber performances under external environmental influences, such as heat, acid, ultraviolet (UV) light, scratches, etc.

POF was initially introduced as a substitute for glass optical fiber in short haul data communications links in the 1960s.³ POF has only received enough attention in the 1990s due to the achievement of low attenuation^4–7 and successive improvements in both transparency and bandwidth of POF.⁸ Since then, POF has been widely utilized in daily lives, such as in fiber optic networks, electronics and sensors, light and solar energy communication, etc. Compared with other optical mediums, POF has many advantages, for instance, it is light weight, flexural, non-conducting, anti-impact, easy to connect and cheap.² The applications of POF have been even extended in fields such as therapy and textiles nowadays.

In general, there are two major purposes of POF integrated textiles: shining and sensing. In the former case, POFs could be either integrated into weft knits, weaves or embroideries for information communications and visual enjoyment. POF fabrics have been widely utilized in indoor and outdoor lighting,^9,10 active safety,¹¹ medical technology^12–14 and flexible displays,¹⁵ as well as fashion and design.¹⁶ The first POF-based luminous textile product was presented by Sächsisches Textilforschungsinstitut (STFI) in 2000. In the latter, POFs could serve as POF fabric sensors in geo-textiles or biomedical fields in order to transfer signals to processor units for detection or monitoring based on the mechanical fluctuations or geometrical optical alterations of POFs.

At present, luminous fabrics are receiving more attention. There are many advantages of POF-based luminous fabrics.¹⁷ Firstly, POFs make fabrics luminous. POF fabrics could emit light on fabric surfaces or at required places. In contrast to general electrical products, POF fabrics are immune to electromagnetic interference (EMI) and free of electricity and heat. In addition, POF fabrics can still keep the textile appearance. The dimension of the luminous area is flexible, which could be small in centimeters in embroideries or large in meters in weaves and weft knits.

On the other hand, many potential uses of POF fabrics have been restricted by the properties of POF itself, which not only influence the illumination properties of POF fabrics, but also limit the integration of POFs into fabrics. For example, the possibility to apply POFs into traditional fabric structures is obviously lower with thicker POFs, whereas it is still problematic to commercially manufacture side emitting POFs with diameter less than 0.2 mm due to the complicated manufacturing processing and poor transmission rate of light rays. Moreover, the bendability of POFs, manufacturing techniques, illuminating effect and drapability of POF fabrics are influenced by POF thickness. In fact, the diameter of POF used in traditional textile structures varies from 0.2 to 1.0 mm,¹⁸ which limits the light intensity markedly. Sufficient light intensity of POF fabrics could be only achieved by either macro-bends of POFs formed in fabrics through certain structure designs or additional treatments.

Much effort has been devoted to the enhancement of side illumination of POF by surface modifications. Polymeric materials can be chosen for POF core and cladding, or fluorescent additives can be added into POF core or cladding during manufacturing¹⁹ to change the critical angle of total internal reflection. Notches created on the fiber surface²⁰ can let a part of light rays emit out. Surface abrasion by the polishing method^21,22 can damage the outer layer of fiber and increase its surface roughness. Laser treatment²³ or weave structure designs^24,25 can also give opportunities to light rays to escape. However, these methods might also increase optical loss and the unevenness of the side illuminating effect simultaneously.

Generally, two main types of POFs are employed in shining applications: end emitting POF that transmits light between two ends; side emitting POF in which light can be emitted from both the fiber surface and the fiber end. Side emitting POFs thicker than 2 mm could be especially useful in safety to highlight the profiles or some parts of textile products for sportsmen, firemen, policemen, etc., due to their good flexibility and side illumination intensity.

This contribution is an attempt to investigate the side illuminating effect of side emitting POF by using fluorescent polyester (PET) fabric based on the presence of phosphors, with respect to the integration of POFs into textiles in safety applications, rather than provide a new manufacturing technique of POF fabric, or develop a standard method to evaluate the side illumination intensity of POF. Two testing methods of side illumination intensity were introduced for samples under both straight and bending states. The effects of POF diameter, fluorescent PET fabric, bending radius and bending angle on side illumination intensity were investigated and analyzed. The results from both testing methods indicate that the fluorescent PET fabric might improve and even the side illuminating effect of side emitting POF to some extent.

Experimental materials and methods

Materials

Side emitting POFs with 2 and 3 mm core diameters were prepared by Grace POF Co., Ltd, Taiwan. Both POFs possessed the same structures and material components. The ratios of core to cladding were 10:1 for 2 mm core diameter POF and 12:1 for 3 mm core diameter POF. The core material was made of acrylic resin, with 1.475 refraction index. The outer layer combined with cladding and anti-UV coating was made of fluorine-resin material, with 1.34 refraction index. More technical data of POFs are given in Table 1.

Table 1.

Introduction of technical data of naked side emitting plastic optical fiber (POF)

Parameters	Values
Fiber type based on core refraction index distribution	Step-index POF
Numerical aperture	0.6
Maximal input angle [°]	75
Wavelength [nm]	380–780
Minimal optical bending radius [mm]	8 times fiber diameter
Storage temperature [℃]	–40 to +120

All tested samples wrapped with fabric were prepared by Stap a. s., Czech Republic, as shown in Figure 1 (POF with 3 mm core diameter). The fabric with plain weave structure was made of fluorescent PET yarns with the linear density of 330.47 denier. The PET fabric thickness was around 0.2 mm. After wrapping, the external diameters of POFs with 2 and 3 mm core diameters were approximate 3 and 4 mm, respectively. Based on this, the tightness of wrapping was controlled during manufacturing. An additional part in this fabric was designed to be stitched into textile products for better integration.

Figure 1.

Samples of fluorescent polyester (PET) fabric wrapped plastic optical fiber (POF) with 3 mm core diameter: (a) top view; and (b) side view; A: fabric with POF; B: fabric without POF; C: enlarged image of fabric structure.

Testing methods

Straight state testing

The investigation of side illumination intensity of the sample under straight state was performed on the apparatus shown in Figure 2. The sample (14) connected to the light source (13) was held straight by two sets of rollers and the detector (15) connected to the computer was mounted on a tunnel (10), which provided the same background for all measurements. Both the sample and light source were moved together by a stepper motor (9) and a control unit that manipulates the drive roller (8). The side illumination intensity of sample for each step was recorded. The distance between detector and light source at the initial testing point for all tests should be as small as possible and remain the same, which was 103 mm.

Figure 2.

Apparatus for measuring the side illumination intensity of a sample under the straight state. 1: mainboard; 2: spacers (4×); 3: console; 4: head roller; 5: rear roller; 6 pressing roller bracket; 7: storage rod; 8: drive roller; 9: stepper motor; 10: tunnel; 11: spring; 12: drive belt; 13: light source; 14: sample; 15: detector.

Bending state testing

In bending state testing, one end of the sample (6) was connected to the light source (4) which was in the tangent line of the circular wheel (5) (the tangent point at the beginning was set as 0°) and another end was fixed by a clamp, which was also located in the tangent line of the circular wheel (the tangent point was 180°), as shown in Figure 3. The movement of the detector in place of the light source was conducted by both the stepper motor (2) and the control unit (3). The distance between the detector and the light source at 0° bending angle for all tests should be as small as possible and remain the same, which was 84 mm. The radii of 10 wheels were 10, 15, 20, 25, 50, 75, 100, 125, 150 and 175 mm.

Figure 3.

Apparatus (a) and corresponding flowchart (b) for measuring the side illumination intensity of a sample under the bending state. 1: detector; 2: stepper motor; 3: control unit; 4: light source; 5: clamp; 6: sample; 7: wheel.

Figure 4.

Power fitting curve for fluorescent polyester fabric wrapped plastic optical fiber with 3 mm core diameter.

In both testing methods, the end of the sample that was connected to the light source should be polished smoothly to optimize the accepted light intensity. All measurements should be performed in a dark circumstance to eliminate light influence from the testing environment. Each sample was tested 10 times, the results at each step were averaged and the standard errors of mean were calculated (given as error bars in Figures 5 –7). The corresponding testing parameters are given in Table 2.

Figure 5.

Side illumination intensity of samples under straight state testing. POF: plastic optical fiber.

Figure 6.

Side illumination intensity of samples under bending state testing: (a) 2 mm plastic optical fiber (POF) without fluorescent polyester (PET) fabric; and (b) 2 mm POF with fluorescent PET fabric.

Figure 7.

Side illumination intensity of samples under bending state testing: (a) 3 mm plastic optical fiber without fluorescent polyester (PET) fabric; and (b) 3 mm POF with fluorescent PET fabric.

Table 2.

Testing parameters of side illumination intensity in two testing methods

Parameters	Values
Parameters	Straight state testing	Bending state testing
Step size [mm]	10	5
Initial testing length [mm]	103	84
Measured length [mm]	690	360
Bending angle [°]	0	0–180
Initial distance between detector and light source [mm]	103	84

Evaluation methods

Generally, there is an exponential relationship between light intensity and transmitted distance for end emitting optical fiber under the straight state, as expressed in Equation (1) ^26,27

P (L) = P (0) 10^{- a / 10} = P (0) 10^{- a_{L} \cdot L / 10}

(1)

where P(L) is the output light intensity or the side illumination intensity at the distance of L, P(0) is the input (received) light intensity from the light source or the side illumination intensity at the point where L = 0, a is the attenuation and a_L is the attenuation coefficient.

However, the exponential fitting model cannot give a good fit for side emitting POF.²³ Taking the wrapped POF with 3 mm core diameter as an example, the result from Figure 4 also shows that the standard power fitting model is not effective. In the primary period, the side illuminating effect of side emitting POF is more distinct, which means that the optical loss is more significant than that in the subsequent period. Thereby, the model as a linear spline with a single knot at L = L_c is employed

P (L) = P (0) + a_{1} \cdot L + a_{2} \cdot (L - L_{c})_{+}

(2)

\begin{matrix} P (L) = {\begin{matrix} P_{1} + a_{1} \cdot L, L \leq L_{c} \\ P_{2} + a_{2} \cdot L, L \geq L_{c} \end{matrix} \end{matrix}

(3)

When L = L_c

P (L_{c}) = P_{1} + a_{1} \cdot L_{c} = P_{2} + a_{2} \cdot L_{c}

(4)

where a₁ and a₂ are the slopes of the first and second linear lines, the related absolute values are mean attenuation coefficients, L_c is the length related to the critical point between two linear curves and P₁ and P₂ are the intercepts of the first linear line and the second linear line, respectively.

Results and discussion

Straight state testing

Figure 5 presents the results from straight state testing and Table 3 gives the corresponding fitting parameters. It is observed that the linear spline fitting method gives a good fit for the experimental data.

Table 3.

Evaluation parameters of side illumination intensity by linear spline fitting method

	3 mm POF		2 mm POF
	Without fabric	With fabric	Without fabric	With fabric
Minimal residual sum of squares [W/m²]²	122.52E-03	41.19E-03	3.78E-03	9.15E-03
Critical point [W/m²]	0.12	0.2	0.12	0.13
Slope 1 [Wm⁻²mm⁻¹]	–4.26E-03	–7.14E-03	–1.24E-03	–1.35E-03
Slope 2 [Wm⁻²mm⁻¹]	–5.40E-05	–10.52E-05	–9.96E-05	–18.62E-03
Intercept 1 [W/m²]	0.83	1.44	0.34	0.45
Intercept 2 [W/m²]	0.10	0.15	0.12	0.18
Index coordinate	8	9	10	14
L_c [mm]	172.98	182.88	192.77	233.06

POF: plastic optical fiber.

The results of samples with the same POF diameters reveal that POFs with fluorescent PET fabric exhibit a stronger side illuminating effect than naked POFs in measured distance. When the POF diameter is 3 mm, POF with fluorescent PET fabric shows 1.74 times larger input light intensity and 1.67 and 1.95 times higher attenuation coefficients in the first and second linear lines, respectively, than naked POF. In the case of 2 mm POF diameter, the input light intensity of POF with fabric increases by 1.33 times, while the values of attenuation coefficients in the first and second linear lines increase by factors of 1.09 and 1.87, respectively.

The major reason for increased input light intensity is ascribed to the existence of phosphors in fluorescent PET fabric, which could store energy from light rays and then release it afterwards. When POF with fluorescent PET fabric is connected to the light source continuously, the side illumination intensity from the sample surface increases accordingly. However, the PET fabric itself can absorb a part of light rays from the POF surface, resulting in high light attenuation. Considering the values of critical length (L_c) in the meantime, it indicates that the fluorescent PET fabric might facilitate the side illuminating effect of side emitting POF in the measured length.

The effect of POF diameter on side illumination intensity is significant. No matter whether the sample is naked or covered by fluorescent PET fabric, thick samples show larger side illumination intensity in measured length than thin ones. This could be explained by the enhanced ability of acceptance of light rays in thick POF. As observed in Figure 5, the values of side illumination intensity of thick POFs with and without fabric at the starting point are 3.18 and 2.43 times the values of thin POFs with and without fabric, respectively. On the other hand, the attenuation plays a vital role in the side illuminating effect, and determines the reducing rate of light intensity according to Equation (2). Under the condition without fabric, compared with thin naked POF, thick naked POF presents a 3.44 times larger attenuation coefficient in the first linear curve and a 1.84 times smaller attenuation coefficient in the second linear curve. Similarly, the attenuation coefficient in the first linear curve increases by 5.28 times and the attenuation coefficient in the second linear curve decreases by 1.77 times in thick POF with fabric. Taking the values of critical length into account, this implies that thick samples could exhibit a strong side illuminating effect not only in measured length but also in longer distance.

It is surprising to find that 2 mm POF with fluorescent PET fabric displays larger side illumination intensity than 3 mm naked POF in measured length from 125 to 635 mm. This demonstrates that the fluorescent PET fabric might have a greater influence on side illumination intensity than POF diameter under the above testing conditions, and thin naked POF could substitute thick naked POF for specific length applications with respect to a good side illuminating effect by using fluorescent PET fabric.

Bending state testing

Figures 6 and 7 illustrate the curves of side illumination intensity versus bending angle for all samples under 10 bending states. The results from bending state testing reveal similar phenomena of effects of fluorescent PET fabric and POF diameter on side illumination intensity as the results from straight state testing. Both fluorescent PET fabric and thick POF could benefit the side illuminating effect for bent samples.

Theoretically, there is no significant difference among values of side illumination intensity for all samples at 0° bending angle. However, it is observed that the values at 0° bending angle fluctuate evidently. This might be attributed to the different heights of the 10 wheels. In fact, the wheels with 10, 15 and 20 mm bending radii have the same height, larger than the same height of the remaining wheels. The detection area is close to the middle part of the sample on thick wheels, instead of facing the upper part of the sample on thin wheels. Consequently, thick wheels lead to large side illumination intensity at the beginning of testing. Another reason behind this might be the large number of light rays reflected back from the bending area to the front part of the sample on small wheels, leading to an increase in side illumination intensity in the initial testing period.

The curves of side illumination intensity versus bending angle vary under different bending states. For thin samples (Figure 6), when the bending radius is equal to or less than 20 mm, the side illumination intensity increases as the bending radius decreases, and the increasing rate changes in the same manner. When the bending radius is more than 20 mm, there is no significant difference in the values of side illumination intensity. Similar phenomena can be observed with thick samples (Figure 7), but the critical bending radius shifts to 25 mm. This could be explained by the limit of bending radius of POF, which is given as one of the technical data and is about eight times the POF diameter. When POF is bent, the incident angle increases in the bending area and more light rays could be refracted into the POF cladding, and both side illumination intensity and attenuation increase. At the bending area, if the bending radius of POF is smaller than its technical bending limit, there would be a significant growth in attenuation, and the side illumination intensity would rise markedly.

It is interesting to investigate the relationship between side illumination intensity and the ratio of wheel diameter (D) to fiber diameter (d). Take 3 mm naked POF as an example, as shown in Figure 8. At the same bending angle, the side illumination intensity decreases with the increase in wheel diameter, the decreasing rate increases from 0° to 90° bending angle and decreases from 95° to 175° bending angle. In the investigation with the same wheel, if the bending radius is less than 25 mm or the value of D/d is less than 16.7, the higher the bending angle in the range of 0–90°, the larger the side illumination intensity, and the corresponding increasing rate is inversely proportional to the wheel diameter. On the contrary, the higher the bending angle in the range of 95–175°, the smaller the side illumination intensity, and the corresponding decreasing rate is inversely proportional to the wheel diameter.

Figure 8.

Curves of side illumination intensity versus D/d for 3 mm naked plastic optical fiber: (a) 0–90° bending angle; and (b) 95–175° bending angle.

There is a decaying exponential relation between the peak of side illumination intensity and the wheel diameter, and a growing exponential relation between the peak of side illumination intensity and the bending angle, as shown in Figure 9. When the bending radius declines from 175 to 10 mm, the peak rises in the range of 1–12 W/m² and is located at the bending angle shifting from 0° to 100°. This means that the highest value of the peak might be found with the smallest bending radius and 100° bending angle. Besides, there are the same peaks between 2 mm POF with fluorescent PET fabric and 3 mm naked POF when the D/d value is more than 20. The similar values of side illumination intensity might be obtained between thick naked POF with a high bending radius and thin POF with fluorescent PET fabric with a low bending radius at 0–30° bending angle.

Figure 9.

Relations among peak, bending angle and D/d.

Conclusions

The fluorescent PET fabric was applied to improve the side illuminating effect of side emitting POF. The measurements of side illumination intensity of samples were conducted under both straight and bending states. The results from two testing methods indicate that the employment of fluorescent PET fabric might enhance the side illuminating effect due to the existence of phosphors. It is also found that the large POF diameter could lead to increased side illumination intensity according to the high capacity of light rays in thick POF.

Meanwhile, in straight state testing, the linear spline fitting model is effective to evaluate the results of side illumination intensity due to various optical losses in front and rear parts of measured length. In bending state testing, the limit of the optical bending radius of POF, eight times the POF diameter, has a critical effect on the side illuminating effect. When the bending radius is less than this limit, the increasing rate of side illumination intensity goes up with decreasing bending radius; the side illumination intensity goes up firstly and declines afterwards when the bending angle shifts from 0° to 180°. The highest value of the peak of side illumination intensity is located at the middle part of bending area.

Overall, the employment of fluorescent PET fabric for 2 mm POF might take the place of 3 mm naked POF in terms of a similar side illuminating effect when the measured length is in the range of 125–635 mm in straight state applications, or the D/d value is larger than 20 and the bending angle is smaller than 30° in bending state applications.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Studentské Grantové Soutěže (SGS) project 2014 by the Technical University of Liberec.

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