Optical Waveguide End Facet Roughness and Optical Coupling Loss

Abstract

This paper investigates the end facet roughness of multimode polymer channel waveguides fabricated on FR4 printed circuit boards, PCBs, when cut at right angles to their optical axis by milling routers for optical butt-coupling connectors and compares it with that resulting from dicing saws and polishing and proposes a novel end facet treatment. RMS surface roughness of waveguide end facets, measured by AFMs, are compared for a range of rotation speeds and translation speeds of a milling router. It was found that one-flute routers gave significantly less rough surfaces than two or three-flute routers. The best results were achieved for a one-flute router when the milling bit was inserted from the PCB side of the board with a rotation speed of 15,000 rpm and a translation speed of 0.25 m/min which minimized the waveguide core end facet RMS roughness to 183 ± 13 nm and gave input optical coupling loss of 1.7 dB ± 0.5 B and output optical coupling loss of 2.0 dB ± 0.7 dB. The lowest RMS roughness was obtained at chip loads of 16 µm/revolution. High rotation speeds should be avoided as smearing of the end facet occurs possibly due to polymer heating and softening. For the first time to our knowledge, channel waveguide optical insertion loss is shown to be linearly proportional to the ratio of the waveguide core end facet RMS roughness to its autocorrelation length. A new fabrication technique for cut waveguide end facet treatment is proposed and demonstrated which reduces the insertion loss by 2.60 dB ± 1.3 dB which is more than that achieved by the closest available index matching fluid which gave 2.23 dB ± 1.2 dB. The new fabrication method gives a more robust end facet for use in commercial products.

Multi-criteria optimization in CO2 laser ablation of multimode polymer waveguides

“…For instance, it is noted by Yoon [28] that insertion loss of multimode polymer waveguide is significantly influenced by the sidewall roughness and the insertion loss decreases linearly as surface roughness improves. However, here the overall effect of surface roughness on the insertion loss of the fabricated waveguides was not investigated [29][30][31]. Moreover relevant experimental dataset pertaining to effect of sidewall roughness along with considered parameters was not available for GRA.…”

Printing and preparation of integrated optical waveguides for optronic sensor networks

“…In combination with a constant feed rate of the tool the milling process generates chips of material as the cutting edge moves through the solid body. We chose a single flute end mill as this configuration offers a minimized surface roughness of the work piece [16]. In this investigation the up milling process is employed in which the material's feeding direction is opposite to the cutting edge movement in the initial moment of chip formation [17].…”

Large Optical Backplane With Embedded Graded-Index Glass Waveguides and Fiber-Flex Termination

“…This was done using a CO2 laser cutting system [17] with the ability to produce substantially higher optical end facet qualities than could be achieved by diamond reel cutting. With four-fold increase in the edge strength and trace stability along with an RMS surface roughness of 40 nm, the glass edges processed by this cutting method guarantees low loss optical coupling [18] to the waveguides without post-processing steps, such as polishing.…”