Optically Enabled ADCs and Application to Optical Communications

Electrical-optical signal processing has been shown to be a promising path to overcome the limitations of state-of-the-art all-electrical data converters. In addition to ultra-broadband signal processing, it allows leveraging ultra-low jitter mode-locked lasers and thus increasing the aperture jitter limited effective number of bits at high analog signal frequencies. In this paper, we review our recent progress towards optically enabled time- and frequency-interleaved analog-to-digital converters, as well as their monolithic integration in electronic-photonic integrated circuits. For signal frequencies up to 65 GHz, an optoelectronic track-and-hold amplifier based on the source-emitter-follower architecture is shown as a power efficient approach in optically enabled BiCMOS technology. At higher signal frequencies, integrated photonic filters enable signal slicing in the frequency domain and further scaling of the conversion bandwidth, with the reconstruction of a 140 GHz optical signal being shown. We further show how such optically enabled data converter architectures can be applied to a nonlinear Fourier transform based integrated transceiver in particular and discuss their applicability to broadband optical links in general

Integrated CMOS-Compatible Mode-Locked Lasers and Their Optoelectronic Applications

Mode-locked lasers producing femtosecond pulsesshow ultra-low timing jitter. Upon photo detection the harmonicsof the photo current at multiples of the pulse repetition rate, showideally ultra-low phase noise. Here, we review the scaling behindthis ultra-low jitter sources and their potential impact in optoelectronic systems. Therefore, integrated CMOS-compatiblemode-locked lasers producing femtosecond pulses are expected todeliver trains of ultrashort pulses with unprecedented low timingjitter, and enable microwave signals at every harmonic of thefundamental repetition rate with ultra-low phase noise afterdetection in a very compact format. In combination with tunablelasers and frequency comb technology also high precision opticalsignals can be synthesized for miniaturized optical clocks andoptical spectroscopy systems. We review recent progress towardschip-scale mode-locked lasers in the femtosecond regime usingrare-earth doped gain media. Current limitations, can beovercome with improved low-loss integrated waveguides andnovel gain deposition technology. In addition, a suite of integratedoptical components in silicon photonics technology is discussedthat enables the implementation of integrated frequency combs,and, therefore optical synthesizers or ultra-low noise microwavesources based on direct optical frequency division

Low-Drift Optoelectronic Oscillator Based on a Phase Modulator in a Sagnac Loop

A low-drift optoelectronic oscillator (OEO) is developed and experimentally shown. In the proposed OEO, a fiber Sagnac interferometer, including an optical phase modulator (PM) and a nonreciprocal bias unit, functions as an intrinsically drift-free intensity modulator. The phase noise of the proposed OEO is modeled in phase space, which is verified by experiments. Phase noise and frequency stability of the photonically generated microwave signals are measured. The single-sideband phase noise of the generated microwave signal is -106.6 dBc/Hz at 10-kHz offset from the 10.833-GHz carrier, with 120-fs rms timing jitter integrated from 1 kHz to 10 MHz. Frequency drift measurements show ±0.85-ppm maximum fractional frequency deviation over 35 h, mainly caused by drift of the fiber delay line

Tunable Low-Jitter Low-Drift Spurious-Free Transposed-Frequency Optoelectronic Oscillator

We propose and theoretically and experimentally demonstrate a novel tunable spurious-free single-loop optoelectronic oscillator (OEO) with low drift and low-phase noise. In the proposed transposed-frequency OEO (TF-OEO), a nonreciprocal bias unit and an optical phase modulator in a fiber Sagnac interferometer function jointly as an intrinsically drift-free intensity modulator, which improves the long-term drift. Besides, a transposed-frequency low-noise filtered amplifier is used which replaces the conventional radio frequency (RF) bandpass filter (BPF) and RF amplifier with an intermediate frequency (IF) BPF, an ultralow phase noise IF amplifier, and a tunable local oscillator, to attain frequency tuning and single-frequency selection with ultralow phase noise at the same time. The quality of the generated microwave signals is theoretically investigated and verified by experiments. Preliminary phase noise, frequency stability, spurious noise levels, and frequency tunability of the photonically generated microwave signal are also investigated. A microwave signal with a frequency tunable range of 15 MHz around 10.833 GHz is generated with no spurs. The generated microwave oscillation has a single-sideband phase noise of -120 dBc/Hz at 10 kHz offset from 10.833 GHz carrier, with 36 fs RMS timing jitter integrated from 1 kHz to 10 MHz. Long-term frequency stability measurements show ±0.05 ppm maximum fractional frequency deviation over 60 h, which is mainly limited by drift of the fiber delay line. The measured results show the long-term frequency stability (in terms of overlapping Allan deviation) within 8.7 × 10-9 at 1000 s averaging time

A CEP-stable, femtosecond 8.5 μm source based on intrapulse DFG of 2.1 μm pulses

Few-cycle carrier-envelope phase (CEP) stable laser pulses enable the precise control of strong-field electron dynamics, such as high-harmonic generation in gases and solids or electron emission in nano-structures [1]. The intrapulse difference-frequency generation (DFG) pumped by a broadband pulse is a reliable method of producing passively CEP-stable pulses. Mid-infrared (mid-IR) pulse generation via intrapulse DFG has been demonstrated with near-IR pump pulses. For example, the intrapulse DFG of Ti:sapphire laser pulses can generate ~2 μm [2] or ~5 μm pulses [3], while spectrally broadened and compressed pulses of Yb:YAG laser were used to cover the region from 6.8 to 16.4 μm [4]. Since the efficiency and output pulse energy is rather low for intrapulse DFG, these pulses typically seed further optical parametric amplifiers. If the pump wavelength is shifted to ~2 μm region, the mid-IR intrapulse DFG at >5 μm becomes more efficient due to a lower quantum defect and the excellent phase matching characteristics of mid-IR nonlinear crystals based on non-oxide materials