Abstract:We demonstrate a waveguide-based frequency shifter on the silicon-organic hybrid (SOH) platform, enabling frequency shifts up to 10 GHz. Spurious side-modes are suppressed by more than 23 dB using temporal shaping of the drive signal.
IntroductionIntegrated electro-optic frequency shifters are key elements for various applications such as interferometric metrology and are also highly interesting for optical communications [1,2]. Currently, acousto-optic modulators (AOM) are used for frequency shifting, providing high side-mode suppression ratio (SMSR) and conversion efficiency (CE). However, operation frequencies of these devices are limited, and integration into planar waveguide circuits proves to be difficult, especially when material systems with vanishing second-order nonlinearities such as silicon-on-insulator (SOI) are to be used as an integration platform. On the other hand, waveguide-based phase modulators are standard building blocks in silicon photonics. Serrodyne frequency shifters can be realized by driving a single phase modulator with a sawtooth signal, but this concept poses stringent requirements on the electrical drive signal to obtain large SMSR and good CE [3]. A serrodyne frequency shifter has recently been demonstrated on the silicon photonic platform [4], resulting in a remarkable SMSR of 39 dB, while the frequency shift was limited to 1 kHz due to the speed of the deployed thermo-optic phase shifters. Another approach is based on combining the output of two Mach Zehnder modulators (MZM) with a π/2 phase shift, hence realizing a device that acts independently on the in-phase and quadrature component of the optical signal. When driving the two MZM of such an IQ modulator by a sine and a cosine signal, frequency shifting by means of single side band (SSB) modulation can be achieved [5,6]. However, this concept suffers from a trade-off between SMSR and CE: For high SMSR, the MZM need to be driven in the small-signal regime, where the amplitude transmission depends linearly on the applied voltage. High CE, on the other hand, requires large signals to drive the device to maximum transmission, but comes along with spurious side-modes in the optical spectrum due to the nonlinear transfer function of the MZM. Moreover, SSB modulation with high SMSR is hard to achieve with state-of-the-art silicon photonic devices: Current phase modulators are based on free-carrier dispersion in SOI waveguides, which leads to an inherent coupling of amplitude and phase modulation and thereby also generates unwanted side-modes.In this paper, we demonstrate that SSB frequency shifters can be realized on the silicon photonic platform. We use silicon-organic hybrid (SOH) phase modulators that combine SOI slot waveguides with electro-optic (EO) organic materials, thereby enabling efficient and broadband phase modulation without introducing unwanted amplitude-phase coupling [7,8]. Using SOH phase modulators of only 1 mm length, we demonstrate frequency shifting of up to 10 GHz. Furthermore we introduce a concept...