A 60-GHz optical frequency shifter using coupled inverted slot lines

A millimeter-wave optical frequency shifter (OFS) using coupled inverted sl ot lines (ISL's) described here is integrated on an X-cut LiNbO3 substrate. Theoretical analysis shows Chat the use of the coupled ISL's makes it poss ible to decrease the distance between the arms of a Mach-Zehnder interferom eter for optical frequency shifting. The ISL coupling characteristics measu red at 60 GHz coincide well with the calculated ones, A 60-GHz frequency sh ifting of a 1.3-mu m optical carrier is experimentally demonstrated using t wo- and four-branch OFS's. The theoretical analysis and experimental result s of the two-branch OFS show that the ratio of the frequency-shifted signal intensity to the optical image intensity is in proportion to the differenc e between the applied de-bias voltage and the half-wave voltage. The four-b ranch OFS consists of two two-branch OFS's excited 180 degrees out of phase , and can suppress the optical image and original carrier simultaneously. T he measurements of the four-branch OFS reveal the optical image suppression of over 12 dB and the original carrier suppression of 5 dB below the desir ed 60-GHz frequency-shifted signal.