1590 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 20, NO. 8, AUGUST 2002 Low-Loss Deep Glass Waveguides Produced With Dry Silver Electromigration Process Ricky W. Chuang, Member, IEEE, Member, OSA, and Chin C. Lee, Fellow, IEEE Abstract—We report an effectively simple dry silver electro- migration technology designed to fabricate low-loss deep channel waveguides on a specially chosen BF450 glass substrate with re- fractive index of 1.472. The simplicity is achieved by replacing the gold or aluminum film electrodes commonly deposited on the glass substrate with two stainless-steel electrodes to facilitate the electro- migration process. In contrast to earlier ion exchange waveguides reported, a relatively high electrical field of 545 V/mm was ap- plied to the glass to speed up the migration and also to prevent the silver ions that were driven into the glass from reducing into silver atoms, a major contributor to waveguide loss. The deep channel waveguides thus fabricated showed no discolors or cracks, of which the attenuation losses of less than 0.1 dB/cm were later measured using our 0.6328 m He–Ne laser edge-coupling setup. Lastly, the scanning electron microscope equipped with an energy-dispersive X-ray (EDX) detector was adopted to obtain the concentration pro- file of silver ions distributed in a channel waveguide region after the exchange. The EDX measurements were then utilized along with the Gladstone–Dale relation to deduce the refractive index profile, of which a nearly step-like profile was deduced from every deep channel waveguide fabricated. Index Terms—Channel waveguides, electromigration, energy dispersive X-ray (EDX) spectroscopy, ion exchange, scanning electron microscopy (SEM), silver. I. INTRODUCTION G LASS integrated optics represents a viable approach to the development of cheap and robust passive components for communication, signal processing, and sensing systems. Bi- nary ion exchange, in turn, is one of the most widespread tech- nologies for fabricating optical waveguides on glass substrates. Among the number of variations of this technique reported, the dry Ag –Na ion exchange electromigration process appears to be the most appealing one, especially for certain applications that require large index changes in the waveguiding regions. In fact, the success of fabricating reproducible low-loss single- and multimode planar and channel waveguides on different glass substrates has already been achieved [1]–[5]. Notice that the type of glass substrates chosen may critically affect the perfor- mance of any waveguide devices constructed. Hence, adopting a suitable glass substrate to fabricate waveguide devices with the lowest attenuation losses shall be a top priority for people engaging in research and development on integrated photonics. Sodalime glass, at the very least, is the most common and affordable commercial substrate for prototyping wide varieties Manuscript received November 27, 2001; revised March 28, 2002. The authors are with Electrical and Computer Engineering, Materials Science and Engineering, University of California, Irvine, CA 92697-2625 USA (e-mail: cclee@uci.edu). Digital Object Identifier 10.1109/JLT.2002.800337 of passive integrated optical devices. However, the qualities of passive devices built were often compromised by its relatively high intrinsic optical loss. Therefore, it is in this regard that optical quality BK7 glass is frequently chosen instead for wave- guide fabrication. Even though it has been widely documented in many research journals that BK7 glass often merits itself for rendering high-performance shallow waveguides, as has already succeeded in the past [2], our previous research effort to achieve extremely low-loss deep waveguides (more than 50 m deep) fabricated on BK7 substrate has proven fruitless. In fact, we have attempted in the past to vary the process parameters, including the fabrication time and temperature, in order to improve the waveguide performance; nonetheless, no major improvements were significant enough to bring the attenuation loss below 2 dB/cm. To circumvent this techni- cally challenging bottleneck, extensive research on the number of commercially available glass substrates was conducted to determine their eligibilities, among which a BF450 glass sub- strate with refractive index of 1.472 was eventually chosen for deep waveguide fabrication. Besides selecting a suitable glass substrate, an ability to tailor and properly deduce the refractive index profiles of fabricated waveguides is critically important for integrated optics appli- cations. Among all of the processes that have been employed to build glass waveguides, a solid-film electromigration tech- nique stands out for its flexibility and excellent reproducibility. The method not only provides waveguides with low propaga- tion losses; most of all, it offers waveguides with an exceptional match to conventional single- and multimode fibers in such a fashion that the coupling losses are minimized. In other words, the refractive index profile in the waveguide region can be well controlled if the dry film technique is adopted. Hence, it is well conceived that the matching between waveguides and optical fibers would not be able to be realized unless the refractive index profiles of waveguides were obtained beforehand. To measure the index profiles of waveguides fabricated, several techniques are available, including interferometry [6], two-dimensional reflectivity method [7], and the inverse Wentzel–Kramers–Brillouin (WKB) approximation [8], [9], of which the calculation relies on the mode indexes data. Although the first two methods provide waveguides with index distributions in two dimensions, the techniques are gener- ally time consuming, destructive, and prone to errors unless absolute measurements of illumination are undertaken. On the other hand, the combination of the inverse WKB method and prism coupler technique would comfortably provide the index profiles of waveguides with accuracies approaching 1 10 . However, this procedure has an inherent flaw due to its failure to properly determine the refractive index near 0733-8724/02$17.00 © 2002 IEEE