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.