Technique Auto-Aligns Fiber to a Waveguide

Breck Hitz

The high cost of actively aligning components in photonic devices is one of the major barriers to a generation of inexpensive, mass-produced photonic equipment. Indeed, Intel Corp.’s Photonics Technology Lab of Santa Clara, Calif., has identified the development of passive alignment techniques as one of the six major building blocks of its silicon photonics program (see “The Quest to Siliconize Photonics,” Photonics Spectra, November, page 53).

Figure 1. A socket was etched in the planar waveguide by immersing it in a HF acid bath. Images ©OSA.

Recently, scientists in Australia demonstrated a passive technique of aligning an optical fiber with a planar waveguide. They believe the method will lend itself well to the bulk manufacture of photonic devices.

Developed by scientists at the University of Melbourne in Victoria and at Australian National University in Canberra, the technique uses a plug-and-socket approach that intrinsically aligns the core of the fiber with the planar waveguide, ensuring a mechanically sound connection. The first step is to etch a socket into the planar waveguide by immersing it in HF acid (Figure 1). Because the germanium-doped core is etched more rapidly than the pure-silica cladding, the socket is etched into the core and not into the cladding. In their demonstration, the investigators etched a 20-µm-deep, 10-µm-wide socket by leaving the waveguide in a room-temperature, 48 percent HF bath for 15 minutes.

Figure 2. The end of an optical fiber was etched to form a plug that fit into the socket in the planar waveguide.

The second step is to etch the core of an optical fiber down to a size that fits into the socket in the planar waveguide. The scientists did this by etching the fiber through its permeable acrylate coating (Figure 2). They immersed the coated fiber in an HF acid bath, allowing the acid to diffuse through the coating and etch the silica beneath, thinning the fiber. The 3M fiber they selected for the demonstration was single-mode at 633 nm and had a core diameter of 3.4 µm. After being immersed in a 48 percent HF bath for nearly four hours, the tip of the fiber was etched to a 9-µm diameter.

The researchers inserted the etched tip of the fiber into the socket of the planar waveguide, forming a passively aligned connection. The core of the optical fiber is intrinsically aligned with the center of the waveguide because the socket is precisely defined by the chemical etching to be centered on the waveguide, and the core of the fiber is likewise defined to be at the center of the plug.

According to Brant C. Gibson of the University of Melbourne, the first stage of this project focused primarily on optimizing the mechanical joining of the fiber to the waveguide. The next will focus on the insertion loss of the connection and mode-matching between the fiber and the waveguide. The waveguide’s dimensions and materials can be selected so that the modes of the fiber are matched to those of the waveguide.

“Due to an intrinsically aligned connection, it is our expectation that the optical loss will be equal to or lower than that of conventional fiber-to-waveguide pigtailing techniques,” Gibson said.