Contact to Nakahara Opto-Electric Labs., Inc.

Headquarters and
Mito business unit

4282-3, Sakado-chou, Mito city, Ibaraki310-0841 Japan


Capillaries for fiber arrays

2-1.Fiber arrays

Fig.2-1 Fiber array assembly.

Fig.2-2 An example of a fiber array.

Fig.2-3 Typical example of a 1x8 splitter bonded by fiber arrays.

Fig.2-4 Fiber to the home (FTTH). 

Fiber arrays are components that connect optical waveguides and optical fibers. Conventionally, the most common type of fiber array is the V-groove type, as shown in Fig. 2-1. V-grooves are prepared by machining or wet etching on a glass plate that is several millimeters in thickness having a precision of ±1 μm. The optical fibers are arrayed along the grooves and fixed by an adhesive agent. The end surface is optically polished. Figure 2-2 shows an example of the fiber array.

There are various types of optical waveguides such as planar lightwave circuits, silicon photonic waveguides and lithium niobate waveguides. Regardless of types, channel numbers and pattern complexity of the waveguides, each waveguide must be connected to external components and devices via optical fibers.

Waveguides are prepared via techniques such as semiconductor lithography, and hence, the waveguide dimensions are controlled at submicron precision regardless of the number of waveguides (e.g., 8, 32 or 48). Therefore, the optical fibers to be attached to the waveguides also need to be arrayed at submicron precision. Fiber arrays used for such purposes are shown in Fig. 2-3.

Current representative optical waveguides used in conjunction with fiber arrays are 1×8 splitters for fibers to the home (FTTH) (Fig. 2-4) and arrayed waveguide grating (AWG) for dense wavelength divisions and multiplexing (DWDM).

2-2.Issues of V-groove fiber arrays

V-groove fiber arrays are widely used for optical communication and datacenter wiring applications. However, there remains numerous issues that require circumventing for present fiber arrays, including:

  1. Limited possibility for miniaturization

  2. Difficulty of 2-dimensional (2D) use

  3. Difficulty in arraying fibers that can maintain polarization

Currently, further miniaturization, integration, and energy savings are recognized as the biggest challenges in the improvement of optical communication equipment. These challenges also apply to fiber arrays used for optical communication equipment, and furthermore, the demand for miniaturization is increasing.

For V-groove fiber arrays, the V-grooves, covers, and optical fibers are fixed by an adhesive agent. Accordingly, miniaturization of the fiber arrays reduces the adhesive area and increases the chance of cover peel off. This uncovering subsequently leads to an immediate risk of communication disconnection and is unacceptable in terms of reliability. Furthermore, miniaturization requires increased levels of complexity for assembly, which further poses challenges in terms of cost.

Spatial optical coupling has often been employed in recent years as a wavelength selective switch and multicast switch. This technique, in some cases, requires 2D fiber arrays. Lamination of V-groove fiber arrays is not practical as improvements to the precision in the laminating direction are not obtained.

Recently, the need for fiber arrays that maintain polarization has also increased. For V-groove arrays, each polarization-maintaining fiber must be held and fixed at three points, namely, both sides of the V-groove and a lid contact point. This requirement applies uneven stresses to the fiber, which will impair the extinction ratio.

2-3.Capillary-type fiber arrays

A capillary-type fiber array is made by inserting fibers into the holes of capillary, fixing fibers with an adhesive, and polishing the end face (Fig.2-5). The capillary hole diameters and the hole pitches are controlled with a precision of ±1 μm (Figs. 2-6).

Fig.2-5. Assembly of capillary fiber arrays.

Fig.2-6. Cross-section of a two-channel capillary

In this case, assembling the fiber array requires only a capillary and fibers, no lid is required. Hence, the theoretical issues related to peel off are no longer a consideration. As fibers that maintain polarization are inserted into the circular hole, the stress applied to the fibers is uniform and the extinction ratio is not impaired. Single-core and dual-core capillary-type fiber arrays have already been used in numerous communication components. Dual-core arrays are recognized as highly reliable miniature fiber arrays for micro-integrated coherent receivers (µ-ICR) that are state-of-the-art digital coherent systems. NOEL has already commercially manufactured and distributed such type of capillaries in large quantities.

NOEL also offers capillaries that vary in pitch, multi-hole capillaries (Fig. 2-7) and 2D capillaries, which are expected to be applied in next-generation optical communications, datacenter wiring and silicon photonics.

In particular, capillary-type fiber arrays have a feature that the E/R characteristics of polarization fibers are superior to those of V-groove-type fiber arrays because the stress applied to optical fibers is small due to the thin adhesive layer and is applied symmetrically and uniformly. Furthermore, capillary-type fiber arrays are extremely superior as optical input/output components for silicon photonics because there is little change or degradation of optical characteristics during the solder reflow process. (Nakahara Optoelectronics Laboratory, Inc. has also developed a three-core fiber array with a polarization-maintaining fiber in the center and single-mode fibers on the left and right sides (Fig. 2-7-d).





Fig.2-7 Capillaries: (a) and (d) three-channel, (b) four-channel, (c) two-channel

For silicon photonics applications, we have also been able to realize fiber arrays with mode field (MFD) conversion function. The MFD = 10 μm of a normal single-mode fiber is converted inside the capillary to 3.5 μm, a value close to the MFD of a silicon waveguide, for miniaturization and high reliability. Figure 2-8 shows a comparison of three-core fiber arrays with MFD = 10 μm and MFD = 3.5 μm. The space between each core of these three-core capillary fiber arrays is precisely controlled within ±0.5μm. In general, the outer diameter of a High NA PANDA fiber with an MFD of 3.5 μm fluctuates by about ±2 μm. Capillary-type fiber arrays can absorb this fluctuation and are extremely superior as fiber arrays for silicon photonics.

Fig.2-8 Comparison of MFDs of capillary type fiber array facets
(Upper: MFD=3.5μm, Lower: MFD=10μm)

For details, please feel free to contact us at: Contact Form.