160GHz two-tone lightwave generation using high extinction-ratio optical modulation

Abstract

By using a high extinction-ratio optical Mach-Zehnder modulator biased at a maximum transmission point, we demonstrated generation of a two-tone lighwave consisting of the second order sideband components whose frequency separation was 160GHz. IntroductionTwo-tone lightwave signal generation has played important roles in radio-on-fibre links, radio astronomy, etc [1,2,3], where the two-tone lightwave consists of two phase-locked spectral components. A pair of laser sources with an optical phase-locked loop (OPLL) for feedback control can generate a two-tone lightwave, however, it is not easy to obtain stable operation of OPLL. On the other hand, LiNbO3 MachZehnder (MZ) optical intensity modulators can generate stable upper sideband (USB) and lower sideband (LSB) components, by feeding sinusoidal rf signals. The output spectrum depends on the dc bias voltage applied on the electrode in the MZ structure. When the bias is set to a minimum transmission point, double-sideband suppressed carrier (DSB-SC) optical modulation can be achieved, where the output has the first order LSB and USB. The frequency separation between the two spectral components is precisely equal to double the modulating signal frequency [1]. The spectral components generated by optical modulation are always phase-locked, so that we can easily construct robust systems without using complicated feedback control techniques. However, the modulation frequency is limited by the frequency response of the modulator. A typical 3dB bandwidth of a modulator is 30 GHz [4], so that the frequency upper limit of the two-tone signal generated by DSB-SC modulation can not be larger than 100 GHz. Recently, a novel optical modulation scheme for highfrequency millimetre wave generation was reported, where 76GHz millimetre wave generation from 19GHz was demonstrated [5]. A MZ modulator whose bias was set to a maximum transmission point was used to generate the second order LSB and USB, where the output also contained the input carrier frequency component. A high-frequency two-tone signal of the fourth order harmonic of the signal fed to the modulator can be generated by filtering out the carrier component. The first sideband components are suppressed in the modulator, however, the suppression ratio depends on the extinction ratio of the MZ modulator. Some additional filters are needed to obtain high suppression ratio. However, the optical frequencies of the first order sidebands vary with the modulation frequency, so that we need high performance tuneable optical filters to obtain the

New Estimation Method of Millimeter‐Wave Power by Dealing Optical Power

“…In this way, the resulting optical signals are made up only of +/− secondary sideband waves, generating optical 2‐tone signals with the frequency interval which is fourfold (4 f m ) of the input RF signals ( f m ). This optical 2‐tone signal was input to a photodetector (PD) and its beat signal is converted into electrical signal to generate the millimeter‐wave signal with frequency of 4 f m .…”