Optical frequency combs (OFCs) have played an important role over the past years for optical frequency metrology and synthesis, for astronomy, telecommunications, and spectroscopy. Among the different methods that have been studied for OFC generation, electro-optic frequency combs (EOFCs) using electro-optical modulators show a large flexibility in comb repetition rate, making it a solution of choice for absorption spectroscopy where a fine sampling in the frequency domain is usually required. Silicon photonics is a promising platform for EOFC generation, thanks to its high volume production and strong light confinement allowing to achieve small footprint photonic integrated circuits (PICs). Additionally, silicon PICs benefit from a direct compatibility with complementary metal-oxide semiconductor (CMOS) fabrication process. Carrier depletion-based modulators have already proved to be efficient and to achieve high bandwidth operation, which makes them suitable for EOFC generation. In this work we will show the first dual comb spectroscopy experiment using silicon optical modulators. As a proof of concept for spectroscopy applications, beating of two silicon EOFCs with slightly different repetition rates is observed in the RF domain using a multi-heterodyne detection technique. Each EOFC is generated from a silicon push-pull Mach-Zehnder modulator and shows typically 12 equally separated lines. The comb repetition rate is swept from 500 MHz to 12.5 GHz, thanks to the inherent flexibility of EOFCs, while their relative offset is kept steady (4 MHz). This technique is used to recover the transfer function of an optical band-pass filter without any tunable laser.
Optical frequency combs (OFCs) are involved in a large diversity of applications such as metrology, telecommunication or spectroscopy. Different techniques have been explored during the last years for their generation. Using an electrooptical modulator (EOM), it is possible to generate a fully tunable OFC for which the optical repetition rate is set by the frequency of the applied electrical radio frequency (RF) signal. In order to realize on-chip OFC generators, silicon photonics is a well-suited technology, benefiting from large scale fabrication facilities and the possibility to integrate the electronics with the EOM. However, observing OFCs with a repetition rate lower than 10 GHz can be challenging since such spacings are smaller than the typical resolution of grating-based optical spectrum analyzers. To overcome this issue, two alternative solutions based on heterodyne detection techniques are used to image the OFC on the electrical RF domain. The first technique consists in applying two frequencies close to each other simultaneously on the modulator, and observing the beating between the resulting two combs. Another method consists in observing the beating between the OFC and the input laser, once the frequency of this input laser has been shifted from the center of the OFC by means of an acousto-optic modulator. Based on both measurement techniques, OFCs containing more than 10 lines spaced with repetition rates from 100 MHz to 15 GHz have been observed. They are generated using a 4-mm long silicon depletionbased traveling-wave Mach-Zehnder modulator (MZM) operating at a wavelength of 1550 nm.
Silicon photonics is a promising solution for next generation of short-range optical communication systems. Silicon modulators have driven an important research activity over the past years, and many transmission links using on-off keying modulation format (OOK) were successfully demonstrated with a large diversity of modulator structures. In order to keep up with the demand of increasing bitrates for limited bandwidths in Datacom applications, higher modulation formats are explored, such as quadrature phase shift keying (QPSK) or 4-level pulse amplitude modulation (PAM-4). However, driving the modulators to generate PAM-4 signals commonly require expensive and power-hungry electronic devices such as digital-to-analog converters (DACs) for pulse-shaping and digital signal processors (DSP) for nonlinearity compensation. Lastly, new solutions were studied to overcome this issue, including new driving methods based on the use of two different input binary sequences applied directly on the modulator. While most of the reported works are focused on the C-band of communication, the O-band can present a definitive advantage due to the low dispersion of standard single-mode (SSMF) fiber. For those reasons, we demonstrate the generation of a 10-Gbaud DAC-less PAM-4 signal in the O-band using a depletion-based silicon traveling wave Mach-Zehnder modulator (TWMZM). An open eye diagram was obtained, and a bit error rate (BER) of 3.8×10-3 was measured for a received optical power of about -6 dBm.
This paper review our recent work on silicon modulators based on free carrier concentration, working in the O-band of optical communications (1260 nm - 1360 nm) for short distance applications. 25 Gbit/s OOK modulation is obtained using a driving voltage of 3.3 Vpp , and QPSK dual-drive Mach-Zehnder modulator (DDMZM) operating in the O-band is demonstrated for the first time.
Space division multiplexing (SDM) is currently widely investigated in order to provide enhanced capacity thanks to the utilization of space as a new degree of multiplexing freedom in both optical fiber communication and on-chip interconnects. Basic components allowing the processing of spatial modes are critical for SDM applications. Here we present such building blocks implemented on the silicon-on-insulator (SOI) platform. These include fabrication tolerant wideband (de)multiplexers, ultra-compact mode converters and (de)multiplexers designed by topology optimization, and mode filters using one-dimensional (1D) photonic crystal silicon waveguides. We furthermore use the fabricated devices to demonstrate on-chip point-to-point mode division multiplexing transmission, and all-optical signal processing by mode-selective wavelength conversion. Finally, we report an efficient silicon photonic integrated circuit mode (de)multiplexer for few-mode fibers (FMFs).
We report on direct numerical calculations and experimental measurements of the group-index dispersion in a photonic crystal waveguide fabricated in silicon-on-insulator material. The photonic crystal is defined by a triangular arrangement of holes and the waveguide is carved out by introducing a one-row line defect. Both the numerical and experimental methods are based on the time of flight approach for an optical pulse. An increase of the group index by approximately 45 times (from 4 to 155) has been observed when approaching the cutoff of the fundamental photonic bandgap mode. Numerical 2D and 3D simulations of pulse dynamics in the waveguide made by the time-domain method shows excellent agreement with measured data in most of the band. These group index values in a photonic crystal waveguide are to the best of our knowledge the largest numbers reported so far by direct tracking of pulse propagation.
We demonstrate a 57.6-km-long linear photonic crystal fiber (PCF) transmission experiment using a recirculating loop with a 19.2-km PCF spool. A 10-Gbit/s non-return-to-zero signal was transmitted over PCF transmission fiber without dispersion compensation.
We demonstrate all-optical label encoding and updating for an orthogonally labeled signal in combined IM/FSK modulation format utilizing semiconductor lasers, semiconductor optical amplifiers and electro-absorption modulators. Complete functionality of a network node including two-hop transmission and all-optical label swapping is also experimentally demonstrated with overall penalty of less than 2 dB, proving the orthogonal IM/FSK labeling scheme to be a feasible solution for future optically labeled networks.