Photonic integrated circuit implementation of a sub-GHz-selectivity frequency comb filter for optical clock multiplication

Abstract

We report a photonic integrated circuit implementation of an optical clock multiplier, or equivalently an optical frequency comb filter. The circuit comprises a novel topology of a ring-resonator-assisted asymmetrical Mach-Zehnder interferometer in a Sagnac loop, providing a reconfigurable comb filter with sub-GHz selectivity and low complexity. A proof-of-concept device is fabricated in a high-index-contrast stoichiometric silicon nitride (SiN/SiO) waveguide, featuring low loss, small size, and large bandwidth. In the experiment, we show a very narrow passband for filters of this kind, i.e. a -3-dB bandwidth of 0.6 GHz and a -20-dB passband of 1.2 GHz at a frequency interval of 12.5 GHz. As an application example, this particular filter shape enables successful demonstrations of five-fold repetition rate multiplication of optical clock signals, i.e. from 2.5 Gpulses/s to 12.5 Gpulses/s and from 10 Gpulses/s to 50 Gpulses/s. This work addresses comb spectrum processing on an integrated platform, pointing towards a device-compact solution for optical clock multipliers (frequency comb filters) which have diverse applications ranging from photonic-based RF spectrum scanners and photonic radars to GHz-granularity WDM switches and LIDARs.

Programmable optical processor chips: toward photonic RF filters with DSP-level flexibility and MHz-band selectivity

“…This function is of high interest to be used together with integrated optical pulse train sources [85] that provide the desired high device compactness and stability but lack pulse rate changing capability. In [84], two experiments of five-time multiplications were successfully demonstrated, that is, from 2.5 to 12.5 Gpulses/s and from 10 to 50 Gpulses/s. As another key result of this work, the measured filter shape exhibits high performance across the entire telecommunication C-band, covering a usable bandwidth wider than 4 THz, which proves an important capability of the waveguide technology that is highly desired by many applications in fiber communication systems and networks [11,81].…”

“…We presented a processor chip comprising an RAMZI incorporated in a Sagnac loop [84] as shown in Figure 15. While the circuit variation is as simple as adding a connecting waveguide between two complementary ports of the RAMZI, this design equals a two-stage RAMZI nested in a lattice structure and enables a comb filter with a record small 3-dB passband bandwidth of 600 MHz for the optical filters of this kind [48].…”

Picosecond optical pulse processing using a terahertz-bandwidth reconfigurable photonic integrated circuit

“…To verify the optical clock rate multiplication function [75], we configured our processor chip into two different tapped delay line filter topologies: (A, B) a 2-tap filter with a FSR of 20 GHz and (C, D) a 4-tap filter with a FSR of 40 GHz, by selectively switching on a number of arms. The measured filter responses optimized for equalsuppression stopband nulls are shown in Figure 5B and D, respectively.…”

Programmable RF Receiver Related On-Chip Photonic Processor

“…The first and most extensive type is based on Mach-Zehnder interferometers (MZI), which includes designs based on cascaded MZIs [7], [8], ring resonator-assisted MZI (RAMZI) [9]- [12], and MZIs combined with other tunable components [13], [14] and so on. Notably, RAMZI contains fundamentally ring resonators embedded in a Mach-Zehnder interferometer.…”

“…It allowed a flat-top passband response tuned from 2.5 to 5.5GHz. Other topologies like combining MZI and semiconductor optical amplifiers (SOAs) [13] and Sagnac loops [14] can also be employed to implement reconfigurable filters.…”