Lithium niobate (LN) has experienced significant developments during past decades due to its versatile properties, especially its large electro-optic (EO) coefficient. For example, bulk LN-based modulators with high speeds and a superior linearity are widely used in typical fiber-optic communication systems. However, with ever-increasing demands for signal transmission capacity, the high power and large size of bulk LN-based devices pose great challenges, especially when one of its counterparts, integrated silicon photonics, has experienced dramatic developments in recent decades. Not long ago, high-quality thin-film LN on insulator (LNOI) became commercially available, which has paved the way for integrated LN photonics and opened a hot research area of LN photonics devices. LNOI allows a large refractive index contrast, thus light can be confined within a more compact structure. Together with other properties of LN, such as nonlinear/acousto-optic/pyroelectric effects, various kinds of high-performance integrated LN devices can be demonstrated. A comprehensive summary of advances in LN photonics is provided. As LN photonics has experienced several decades of development, our review includes some of the typical bulk LN devices as well as recently developed thin film LN devices. In this way, readers may be inspired by a complete picture of the evolution of this technology. We first introduce the basic material properties of LN and several key processing technologies for fabricating photonics devices. After that, various kinds of functional devices based on different effects are summarized. Finally, we give a short summary and perspective of LN photonics. We hope this review can give readers more insight into recent advances in LN photonics and contribute to the further development of LN related research.
Terahertz polarization control has a wide range of applications in imaging, communication, biology, and analytical chemistry. Polarization rotator is of great significance as one of the main units of a polarization controller. In this paper, we propose and simulate a terahertz polarization rotator based on silicon asymmetrical structure, whose polarization extinction ratio reaches up to 35 dB and 26 dB for transverse electronic and transverse magnetic modes, respectively. Its 15-dB bandwidth is 40 GHz for a conversion length of 4.4 mm. The results show that the integrated scheme can achieve the function of polarization rotation with high conversion efficiency. We believe it will play an important role in terahertz polarization management.
A novel concept for a compact SOI device used to achieve the conversions between the polarization division multiplexing (PDM) and the mode division multiplexing signals (MDM) is proposed and simulated by utilizing a structure combining a 2D grating coupler and a two-mode multiplexer. The detailed design of the proposed device is given and the simulations show the good performance of the device. The proposed device can be useful for accommodating the future fiber optic communication networks which utilizing multiple multiplexing techniques.
KEYWORDS: Signal processing, Optical fibers, Picosecond phenomena, Modulation, Radio frequency circuits, Optical signal processing, Signal generators, Single mode fibers, Gaussian pulse, Binary data
We propose a low cost solution to generate power efficient ultra-wideband (UWB) radio frequency (RF) signal based on the hybrid of optical fiber and RF circuits. In our solution, any return-to-zero (RZ) optical pulse with enough bandwidth is transmitted over optical fiber first, then converted to a power-efficient UWB pulse by an electrical bandpass filter (EBPF) with a passband of 3.1-10.6 GHz. The transmission and modulation of UWB signal is processed in optical domain, whereas the generation of UWB is processed in electrical domain. This solution embodies the advantages of both low-loss long-haul transmission of optical fiber and low-cost mature RF circuits. Both UWB modulations of on-off keying (OOK) and binary phase shift keying (BPSK) are experimentally demonstrated. The impacts of RZ pulsewidth and the EBPF bandwidth on the UWB waveforms are also discussed.
All-optical programmable logic arrays (PLAs) based on canonical logic units (CLUs), i.e. minterms and maxterms, are presented. We experimentally demonstrated the full set of two-input and three-input minterms as well as maxterms using the cross-gain modulation in semiconductor optical amplifiers (SOAs). Maxterms can be easily obtained based on minterms. The reconfigurability and scalability of the system are largely enhanced compared to our previous work. Correct and clear temporal waveforms are achieved for all the canonical logic units. The measured extinction ratios of two-input and three-input CLUs are ~ 15 dB and ~ 11 dB, respectively. Four important logic functions, including multiplier, multiplexer, demultiplexer and decoder, are presented as examples to show that the canonical logic units-based programmable logic array (CLUs-PLA) can be reconfigured to perform different logic functions.
We have experimentally demonstrated a 40Gb/s all-optical binary-coded-decimal (BCD) decoder for the first time,
utilizing delay interferometers (DIs) and cascading semiconductor optical amplifiers (SOAs) without any assisting light.
Extinction ratios (ERs) of the intermediate results after the first SOA are all over 11dB which ensures the capability to
cascade to the second one. The final results are in the form of return-to-zero (RZ) format with correct and clear temporal
waveforms. The proposed scheme could be extended to 1-of-16 decoder, logic minterms and read only memory (ROM).
We demonstrated experimentally 40 Gbit/s all-optical format conversions between return-to-zero (RZ) and nonreturn-to-
zero (NRZ) using a fiber delay interferometer (FDI) and a single semiconductor optical amplifier (SOA). Firstly, 40 Gbit/s data format conversion from RZ to NRZ is realized using a FDI with temperature control and an optical bandpass filter (BPF). Then, 40 Gbit/s data format conversion from NRZ to RZ is implemented, using four-wave mixing (FWM)
effect of SOA, by injecting synchronously NRZ signal and clock pulses into a single SOA. Presented method has some distinct advantages including multi-channel parallel processing, easy integration, convenient tuning, good stability, and
so on, which has potential to be used in future optical networks that could combine wavelength division multiplexing
(WDM) and optical time domain multiplexing (OTDM) transmission techniques.
We demonstrate multi-channel regenerative non-return-to-zero (NRZ) to return-to-zero (RZ) conversions with tunable
output pulse-width and single-to-dual function, using a phase modulator and an array waveguide grating (AWG).
Transmission and bit error ratio (BER) show a good performance for the converted RZ signal compared with
conventional one.
We propose and demonstrate a multi-level all-optical format conversion from NRZ signal to RZ signal,
using a semiconductor optical amplifier (SOA) and a detuned optical filter. The three-level NRZ signal
and a synchronous optical clock signal are injected into an SOA, which acts as a nonlinear element and
causes the spectrum of the input NRZ signal to be broadened due to the cross phase modulation (XPM)
effect. A detuned filter is thus used to extract the specific part of the broadened spectrum to obtain
three-level RZ signal. The proposed conversion can be achieved with error free at 10G baud rate.
We propose and demonstrate a simple and flexible all-optical
NRZ-DQPSK receiver based on optical
detuned filtering. The proposed demodulator is quite flexible to input bit rates, which means it can be
used for different bit rates without any change. With two given optical band-pass filters, simulation and
comparison show that NRZ-DQPSK signals at 20, 40 and 80Gb/s (10, 20 and 40G symbol rate) can be
demodulated to in-phase and quadrature data successfully by adjusting the detuning of two given
filters.
We propose and theoretically demonstrate multi-channel all-optical non-return-to-zero (NRZ) format to
return-to-zero (RZ) format conversions for wavelength division multiplexing (WDM) network. By using two fiber
based delay interferometers (DIs) with free spectral range (FSR) of 20GHz and 40GHz, 8*10Gb/s (200GHz
spacing on ITU grid) RZ signals can be converted to corresponding NRZ signals with small penalty. The
principle and simulation are analyzed in both time and frequency domains. Format conversions are
demonstrated for multi-channel RZ signals with duty cycle of 33%, 50% and 66% respectively. Q factors before
and after the multi-channel RZ-to-NRZ conversions show a good performance and thus the practicability of the
proposed format converter.
We propose a novel and simple non-return-to-zero differential phase shift keying (NRZ-DPSK) wavelength division
multiplexing (WDM) system which can simultaneously demultiplex and demodulate multiple wavelengths. The phaseto-
intensity demodulation principle is based on detuned filtering, which is achieved by using a single commercial array
waveguide grating (AWG) in our scheme. By properly choosing appropriate AWG channels at the transmitter, the AWG
at the receiver can act as both the de-multiplexer and the demodulator of the DPSK signals. Simulations at 10, 20 and
40Gb/s show good flexibility and performance for the proposed system.
We demonstrate a fiber ring laser with a dispersion compensation fiber (DCF) and a delayed
interferometer (DI), which is able to switch eleven wavelengths one by one. In ring cavity,
DCF supplies different effective cavity lengths for different wavelengths, DI generates a
wavelength comb corresponding to the ITU grid, a flat-gain erbium-doped fiber amplifier
(EDFA) provides uniform gain for each lasting wavelength, and a semiconductor optical
amplifier (SOA) not only acts as active modulator, but also alleviates homogeneous
broadening effect of EDFA. Stable pulse trains with a pulsewidth about 40 ps at 10 GHz have
been obtained by injecting external optical control signals into the laser. Wavelength switching
process among eleven wavelengths is achieved by merely tuning an intracavity optical delay
line.
All-optical clock recovery (CR) from 20-Gbit/s nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signal is
demonstrated experimentally by using Polarization-Maintaining Fiber Loop Mirror Filter and semiconductor optical
amplifier (SOA) fiber ring laser. Only by adjusting polarization controller (PC), NRZ-DPSK signal were conveniently
and fast converted to pseudo return-to-zero (PRZ) signal via PMF-LMF. Then the PRZ signal is injected into the SOA
fiber laser for CR. The recovered clock signals with the extinction ratio of 10 dB and the root-mean-square timing jitter
of 850 fs is achieved under 231-1 pseudorandom binary sequence NRZ-DPSK signals measurement.
We propose and demonstrate simultaneous multiple dense wavelength division multiplexing (DWDM) channels
optoelectronic non-return-to-zero (NRZ) to return-to-zero (RZ) regenerative format conversions based on a single phase
modulator (PM) and a fibre delay-interferometer (DI). The PM is driven by a local RF clock signal, and the DI with free
spectral range (FSR) equals to the channel spacing is used to extract the chirps induced by the phase modulation, for all
the channels at the same time. Since the original carriers are suppressed to some extent while the chirps are extracted,
thus the NRZ-to-RZ conversions can be achieved with regeneration. The proposed multi-channels format conversions are
successfully demonstrated at 16*10 Gb/s, with channel spacing of 100GHz. Bit error ratio (BER) measurements show
3.5 and 4.2 dB penalty improvements for 50 and 75 km transmission without dispersion compensation, respectively.
We propose and demonstrate all-optical format conversion from nonreturn-to-zero differential phase-shift-keying
(NRZ-DPSK) to return-to-zero binary PSK (RZ-BPSK) at different bit-rates, using a tunable band-pass filter, a
semiconductor optical amplifier (SOA) fiber ring laser and an SOA based Mach-Zehnder interferometer (SOA-MZI).
The filter acts as the DPSK demodulator and also as the differential decoder. It converts the NRZ-DPSK into the RZ
signals, which is equivalent to the data information before differential encoding. The SOA fiber ring laser is used to
recover the clock signals from the demodulated RZ signals at different bit-rates. In the subsequent SOA-MZI, the RZ
signal is used to modulate the recovered clock signal which is synchronous to the demodulated RZ signal. Thus, the
amplitude information is encoded onto the phase of the clock signal, through cross gain modulation (XGM) and cross
phase modulation (XPM) effects in the SOA-MZI. The converter can be operated at flexible bit-rates and used as the
interface between long-haul WDM and OTDM systems.
We demonstrate a simple configuration for clock recovery from nonreturn-to-zero (NRZ) signal, which is preprocessed
by a narrow-band filter. Clock component is enhanced evidently after the filter. Compared with previous preprocessing
schemes, the single filter is simple and suitable for different bit-rates. The output performances related to the bandwidth
and the detuning of the filter are analyzed. By cascading a clock recovery unit with semiconductor optical amplifier
based fiber ring laser, clock signal can be extracted from the preprocessed signal with extinction ratio over 10 dB and
root mean square timing jitter of 900 fs, at 10 to 40 Gb/s. By simply using a filter with larger bandwidth, much higher
bit-rate operation can be achieved easily.
All-optical format conversions from return-to-zero (RZ) and carrier suppressed RZ (CS-RZ) to non-return-to-zero (NRZ)
are demonstrated with a temperature-controlled all-fiber delay interferometer (DI) and narrow-band filters, at 40 Gb/s.
The format conversion processes are analyzed in frequency domain. By properly controlling the DI and the tunable
filters, for the RZ input, NRZ signal can be achieved at the carrier wavelength from one of the DI output ports; and for
the CS-RZ input, NRZ signals can be achieved at two different wavelengths simultaneously from two output ports of the
DI. The operation principle is analyzed with spectra evolution analysis. The power penalties are 1.5 dB and 1.8 dB, for
RZ and C-RZ to NRZ conversions, respectively.
A complete analytical model of the polarization characteristics of a semiconductor laser amplifier in a loop mirror (SLALOM) is presented. Based on this model, various kinds of output for all-optical conversion of non-return-to-zero (NRZ) to pseudo-return-to-zero (PRZ) signals, based on a simple SLALOM scheme, are analyzed theoretically. Simulation results were in good agreement with experimental results on 20-Gbit/s all-optical NRZ-to-PRZ conversion. The results show that the theoretical model proposed in this paper is right, and proper polarization-state control is helpful for improving the output characteristics in all-optical pattern conversion.
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