[ieee 2012 symposium on photonics and optoelectronics (sopo 2012) - shanghai, china...

Metal-based Asymmetric Hollow Waveguide and Y-Branch Plastic Optical Fiber Couplers

Abang Annuar Ehsan Institute of Microengineering and Nanoelectronics

Universiti Kebangsaan Malaysia Bangi, Selangor, Malaysia

aaehsan@eng.ukm.my

Mohd Kamil Abd Rahman Faculty of Applied Science

Universiti Teknologi MARA Shah Alam, Selangor, Malaysia

Abstract— Metal-based asymmetric plastic optical fiber couplers based on hollow waveguide and Y-Branch structures have been developed. Non-symmetrical coupling ratios were obtained by controlling the tap line width of the metal hollow waveguide whereas a simple attenuation technique based on lateral displacement of two fibers is used for the Y-branch. The asymmetric hollow waveguide coupler fabricated on metallic block shows a tap-off ratio or TOFR variation of 10.7% to 89.3%, for a tap width of 500 um to 1 mm. The asymmetric Y-branch design provide a range of coupling ratios from 14.23% to 85.77% for an output fiber lateral displacement of 0.1 mm to 4.4 mm. The excess loss of these devices varies from 9.49 dB to 11.51 dB for the asymmetric hollow coupler and 6.3 dB to 8.3 dB for the Y-branch coupler. Keywords- asymmetric, coupler, plastic optical fiber, POF, waveguide taper, Y-branch

I. INTRODUCTION Passive components for plastic optical fiber (POF)

technology especially POF couplers are of great interest for applications in short length networks, such as in-home and in vehicle network. The fiber-based Y-branch couplers, constructed by polishing two fibers and gluing them together would be the cheapest and easiest technique of producing low cost POF couplers. Nevertheless, the coupling ratio is fixed at 50:50 ratio and non-symmetrical coupling ratio and variable type couplers would be difficult to manufacture.

Planar waveguide-based symmetrical Y-branch POF couplers with core diameter of 1000 µm have been reported by H. Mizuno et.al. [1], T.Klotzbuecher et.al. [2] and Y.Takezawa et.al. [3]. In addition to the symmetrical ratio couplers, there have been several reported designs on the planar waveguide based Y-branch coupler with asymmetric coupling ratios. The first is the device reported by S.Suzuki et.al. [4] which is a singlemode Y-branch coupler with the center axis of the branching output waveguide and the center axis of the taper waveguide shifted from each other. The second asymmetric coupler design is a multimode Y-branch waveguide device reported by H.Kurokawa et.al. [5] which uses reflection at the reflecting surface of the waveguide to divide the optical power. Another asymmetric coupler which has been proposed by H.B. Lin et.al. [6] is a singlemode Y-branch coupler using microprisms. Another version of the

asymmetric waveguide coupler has been reported by J.D.Love et.al. [7] using asymmetric multimode Y-junction splitter. The power splitting ratio is controlled by the geometry size of the output branch.

In this paper, two designs of metal-based asymmetric POF coupler are presented. The first POF coupler design is constructed using metal hollow structure. The device utilizes asymmetric multimode Y-junction splitter design but using a novel metal hollow structure. The second device is a design using a simple Y-branch with a hollow waveguide taper. The non-symmetrical coupling ratios are obtained by using a simple theory of attenuation caused by the lateral displacement of two fibers. The hollow structure design has a reflective surface on the inner walls of the waveguide where light rays propagate along the waveguide solely by reflection on all of the metallic inner-surfaces. So, theoretically there is no way for rays to escape from this block because all rays which hit the block will reflect back into the hollow region. This structure allows more flexibility in guiding light rays without the constraint of the material’s refractive index. The devices are fabricated on: aluminum substrates using high speed engraving machine. The fabricated devices are then assembled and characterized.

II. DEVICE DESIGN The first of the non-symmetrical coupler design is the

metal-based asymmetric hollow waveguide coupler. In this design, the whole waveguide is made hollow where light rays propagate by reflection on all of the metallic inner-surfaces. The asymmetric coupler design utilized a simple TOFR (Tap Off Ratio) technique to tap off power from the main bus line. The theory behind this is the use of asymmetric 1x2 Y-junction splitter which is discussed in details by J.D. Love [7]. This design is achieved by varying the size of the tap-off line or tap line. This design was part of our own work on an asymmetric POF coupler for a portable optical access-card system which was published [8]. The TOFR is given by equation (1) [7]:

xyyTOFR+

= (1)

In the device design, this TOFR is achieved by varying the width size of the tap line. Fig. 1 illustrates the 1x2 asymmetric waveguide coupler design with the bus and tap line.

978-1- -0911-1/12/$31.00 ©2012 IEEE4577

Fig. 1 Asymmetric waveguide coupler design

In the device design, the split angle, θ has been set to 6º.

This design also includes a linear taper structure to allow the tap line with smaller core size to be coupled to a large core POF fiber. Fig. 2 shows the plot of the design TOFR against the tap width. These results are obtained by using equation (1), from y=300 m until y=1000 m with a bus line width, x of 1000 m.

Fig. 2 TOFR vs Tap Width for an asymmetric hollow waveguide coupler

The second asymmetric coupler design is based on a

simple 1x2 Y-branch structure. The Y-branch structure is selected as it is the simplest optical splitting device which allows optical signal to be split into two symmetrically. The non-symmetrical coupling ratio is obtained using this structure by utilizing a simple concept of attenuation caused by the lateral displacement of two fibers. The theory behind the non-symmetrical coupling lies in the principle of attenuation caused by the lateral displacement of two fibers. The loss associated to it is given by the following relationship [9].

⎥⎦

⎤⎢⎣

⎡−−=

dnAS N

321log10α (2)

where S is the separation between the two fibers, AN is the numerical aperture of the fibers, n is the refractive index of the fibers and d is the diameter of the fibers.

Fig. 3 Generic Y-branch coupler with movable output fiber

Fig. 3 shows a generic design structure for the proposed asymmetric Y-branch POF coupler. The device consists of a block with Y-branch structure engraved on it. POF fibers are then slotted into this structure. The input fiber is a non-movable fiber. The output fibers, however are divided into two sections: non-movable and movable fibers. The fibers after the middle splitting junction are short non-movable fibers. Another section of the output fibers are defined as movable. The middle waveguide taper is a hollow region where light rays propagate solely by reflection on the metallic inner-surfaces. Fig. 3 shows the lateral movement of the output fibers giving a example lateral displacement of S1 and S2. In the asymmetric Y-branch coupler design, only one output fiber is movable. Based on Fig. 3, a relationship between the coupling ratios and the lateral displacement (S1 and S2) can be obtained. In the asymmetric design structure, we assume that only one output fiber is moved whereas the second output fiber remain unmovable. The following relationship is obtained for the coupling ratios and lateral displacement [10].

( )[ ]21 21)1(

1 SRCRCRR

S −−−

= (3)

where AN, n and d are defined earlier, R is defined

asdn

AR N

32= , CR is the coupling ratio given by the following

relationship [4],

21

1

PPPCR+

= (4)

The relationship in equation (3) can be simplified in terms of the coupling ratio with S2=0,

1

1

21

SRSR

CR−−

= (5)

Using the standard value of a step index (SI) POF fiber, where AN = 0.5, n =1.49 and d = 1 mm, gives R= 223.71. The use of a symmetrical Y-branch coupler will ensure that the output power is divide equally by the waveguide taper in the middle.

Fig. 4 Coupling ratio against output fiber lateral displacement for asymmetric Y-branch coupler

Fig. 4 shows the design plot of the non-symmetrical Y-branch coupler. The plotted coupling ratio was obtained by using the coupling ratio equation (5). The coupling ratios vary from 0.8% to 99.2% for lateral displacement of the output fiber (S1) varying from S1 = 0.1 mm to S1 = 4.4 mm. Fig. 5 shows the CAD design layout device showing the individual components for the device construction. The device in Fig. 5 is composed of an input fiber, middle fibers, movable output fibers, device block which includes the middle hollow waveguide taper and open space region, and the top metallic block. The input and middle fibers are non-movable fibers whereas the output fibers are movable fibers. The open space region in the form of a rectangular shaped void is placed after the middle non-movable fibers.

Fig. 5 CAD design for metal-based asymmetric Y-branch POF coupler

III. DEVICE FABRICATION One of the major advantage of the proposed device is the

simplification of the fabrication steps. The designed structures are engraved onto the aluminum blocks using CNC machining tool. Roland’s EGX-400 desktop CNC machine has been utilized to engrave the device design onto the device blocks. Milling tool size of 1.0 mm is used and spindle speed of 30,000 rpm has been utilized. After the device block has been fabricated, a top metal block is positioned and the connecting screws are secured. Finally, short POF fibers are inserted into the input and output ports. The devices have been tested using Advanced Fiber Solution FF-OS417 optical source at 650 nm and optical power meter OM210. The effective input power is 0 dBm.

Fig. 6(a) is the metallic device block for the asymmetric hollow waveguide coupler whereas Fig. 6(b) is the assembled device structure with the input and output POF fibers. Fig. 6(c) shows the planar view of the asymmetric branch or tap line.

(a) (b) (c) Fig. 6 Fabricated asymmetric hollow waveguide coupler: (a) device block (b) assembled device (c) close view of tap line

Fig. 7 is the plot of the TOFR vs Tap Width for the fabricated metal-based asymmetric hollow coupler. The simulation and design tap lines refer to the values for the coupling ratios of the simulated device and the TOFR of the designed devices respectively. The Tap Width on the x-axis refers to the size of the tap line. In this plot, the coupling ratio varies from as small as 10.7% and up to 47.7% by a simple variation of the tap line width. The range of coupling ratios that that can be obtained from this device is from 10.7% at the tap line and to 89.3 % at the bus line. The excess loss of this device varies from 9.49 dB to 11.51 dB.

The fabricated device shows some similarities in the coupling ratios or TOFR variation against the tap line width as those of the simulated device. However, the design TOFR deviates from the simulated and fabricated devices. Hence, a suitable factor needs to be included in the original TOFR equation given by equation (1). A good multiplying factor would be the ratio of the tap line over the bus line. Therefore, a new structured TOFR for the 1x2 asymmetric metal hollow device will be as follows.

TOFR (new) = )(

2

xyxy

xy

xyy

+=⎟

⎠⎞

⎜⎝⎛

+ (6)

Fig. 7 TOFR vs Tap Width for asymmetric hollow waveguide coupler (design, simulated and fabricated devices).

Fig. 8 New TOFR vs Tap Width for asymmetric hollow waveguide coupler (design, simulation and fabricated devices)

Fig. 8 is the new plot of the TOFR obtained using equation (6). In this figure, Design refer to the original TOFR based on equation (1), Design (New TOFR) is the curve for the TOFR based on the new TOFR ratio as in equation (6), simulation is based on the simulated data while fabricated is based on the data collected from the experiment. The characteristics of the designed TOFR is now much closer to those of the simulated TOFR and fabricated device hence, improved the reproducibility of the 1x2 asymmetric coupler devices.

Similarly, the asymmetric Y-branch coupler has been fabricated and characterized. Fig. 9(a) shows the fabricated device block and Fig. 9(b) shows how the POF fibers are inserted and positioned into the U-groove slots of the device block. The insertion loss of the device when both the movable output ports are not shifted is 6.3 dB with an excess loss of 3.3 dB while the coupling ratio is about 50:50. The output fibers are moved laterally using a single axis miniature translation stage with a 250 µm displacement per revolution. Fig. 10 is the plot of the coupling ratios against the output fiber lateral displacement for the fabricated device. The design port refers to the coupling ratio obtained for the designed device whereas simulation port refers to the coupling ratio obtained for the simulated device. The fabricated device shows coupling ratios variation from 46.27% down to 14.23% for port 1 and from 53.73% to 85.77% for port 2. Hence, the range of coupling ratios that can be obtained from this device is from 14.23% to 85.77%. The excess loss of this device varies from 6.3 dB to 8.3 dB.

(a) (b)

Fig. 9 Fabricated asymmetric Y-branch POF coupler (a) device block (b) device with fibers inserted into U-groove slots.

Fig. 10 Coupling ratios against fiber lateral displacement (designed, simulated and fabricated devices)

The excess losses of the fabricated devices are high due to the structure imperfection of the devices. A perfectly constructed hollow structure with high reflective surfaces will not allow any light ray to escape from the hollow region. These light rays will be confined in that hollow region and will propagate from the input fiber to the receiving output fibers. This is made possible in the model when the device is a piece of metal block with hollow structure in the middle of the block. However, the fabricated and assembled device is actually a two-piece metal blocks sandwiched together. Due to the limitation of the raw metallic samples that we have obtained where they do not have a 100% flat surface but rough surfaces, there will be air gaps between the top block and the bottom device block, when these structures are sandwiched. These air gaps enable light rays to escape from the hollow region and resulted in a high radiation loss.

IV. CONCLUSIONS In this paper, we have successfully developed two metal-based asymmetric POF couplers. The first asymmetric hollow coupler, based on simple variation of tap line width provides a TOFR or branching ratios of 10.7% to 89.3%. A second device based on an asymmetric Y-branch design provides much simpler device design and construction. The proposed simple attenuation technique caused by the lateral displacement of two fibers has been proven to provide the required non-symmetrical coupling ratios. The fabricated device provides coupling ratios from 14.23% to 85.77%. The asymmetric Y-branch coupler provide better excess loss compare to that of the asymmetric hollow coupler.

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