Different modulation formats may be required in different optical communication networks. Format conversion from
intensity format to phase format is necessary to ensure the functions between long-haul transmission networks and
metropolitan area networks. Cross-phase modulation (XPM) in silicon waveguides provides a promising way to realize
all-optical integrated format conversion since a nonlinear phase shift is induced to the probe by the incident signal power.
An on-off keying (OOK) signal can be converted to differential phase-shift keying (DPSK) signal if nonlinear phase
shift of π is achieved. We numerically investigate the nonlinear phase shift caused by XPM in silicon waveguides by
considering the influences of the walk-off effect, group-velocity dispersion, and nonlinear losses including two-photon
absorption (TPA) and free-carrier absorption (FCA). The nonlinear phase shift is tried to be enhanced through
waveguide design and wavelength management. The walk-off effect can be minimized by carefully choosing the zero
dispersion wavelength of the used silicon waveguide and setting the signal and probe wavelengths symmetrically. Low
and flat dispersion is beneficial to acquiring a large nonlinear phase shift. TPA and FCA will greatly reduce the
nonlinear phase shift obtained from XPM and they should be effectively suppressed in order to realize high-quality
format conversion.
With a transparency window up to 6 μm, sapphire can serve as a platform to support silicon photonic integrated circuit in
MWIR. Planar waveguide devices based on silicon-on-sapphire (SOS) are emerging as a bridge between MWIR and
SWIR through frequency band conversion process. While these devices are widely proposed to amplify MWIR signals
and generate MWIR source, it can also be inversely utilized to achieve MWIR light detection. Here MWIR signals are
down-converted to telecommunication wavelength (1.55 μm) through SOS waveguides and indirectly detected by SWIR
detectors. Since detectors at telecommunication wavelengths exhibit superior performances in terms of speed, noise and
sensitivity, the indirect detection scheme can be a promising candidate to improve the detection performance. In this
report, we analyze performance of the indirect detection of MWIR signals by wavelength conversion in SOS
waveguides. Particularly we modeled and compared the noise performance of the indirect detection with direct detection
using state-of-the-art MWIR detectors. We show that, in addition to advantages of room temperature and high speed
operation, the proposed indirect detection can improve the electrical signal-to-noise ratio up to 50dB, 23dB and 4dB
compared to direct detection by PbSe, HgCdTe and InSb detectors respectively. The improvement is more pronounced
in detection of weak MWIR signals.
In this study, we demonstrate method for quasi phase matched silicon-on-sapphire waveguides suitable for MWIR
wavelength conversion to achieve higher conversion efficiency than that can be achieved in uniform waveguide
geometries. In particular we show that periodic change in waveguide width by 0.5μm and hence periodic change in
waveguide dispersion can to reset phase accumulation and provide ever-increasing gain profile. With the fabrication
flexibility of large cross-section of MWIR waveguides, the possibility of using quasi-phase-matching can provide >30dB
conversion efficiency enhancement and increase the conversion bandwidth by 2 times. Such improvement may facilitate
the fabrication of parametric oscillators that can improve the conversion efficiency by 50dB.
Four-wave mixing (FWM) in silicon waveguides is considered to be a promising effect to realize the wavelength
conversion function for wavelength-division-multiplexing optical communication systems. Compared to the degenerate
FWM with a single pump, the nondegenerate FWM with two pumps shows more flexibility in phase-matching condition
and has more opportunities to acquire broader conversion bandwidth. The bandwidth enhancement is theoretically
analyzed for the two-pump FWM and an enhancement of 25% is experimentally demonstrated. Also, an ultra-broadband
wavelength conversion is presented based on two-pump FWM by fixing one pump near the signal and scanning the other
pumps.
Wavelength tunable synchronous pulse sources are highly desirable for spectroscopy and optical diagnostics. The
common method to generate short pulses in the fiber is the use of nonlinear induced spectral broadening which result in
soliton shaping in anomalous dispersion regime. However, to generate ultra-short pulses, broadband gain mechanism is
also required. In recent years, Raman fiber lasers have retrieved strong interest due to their capability of serving as pump
sources in gain-flattened amplifiers for optical communication systems. The fixed-wavelength Raman lasers have been
widely studied in the last years, but recently, much focus has been on the multi wavelength tunable Raman fiber lasers
which generate output Stokes pulses in a broad wavelength range by so called cascaded stimulated Raman scattering. In
this paper we investigate synchronous 1st and 2nd order pulsed Raman lasers that can achieve frequency spacing of up to
1000cm-1 that is highly desired for CARS microscopy. In particular, analytical and numerical analysis of pulsed stability
derived for Raman lasers by using dispersion managed telecom fibers and pumped by 1530nm fiber lasers. We show the
evolution of the 1st and 2nd order Stokes signals at the output for different pump power and SMF length (determines the
net anomalous dispersion) combinations. We investigated the stability of dispersion managed synchronous Raman laser
up to second order both analytically and numerically. The results show that the stable 2nd order Raman Stokes pulses
with 0.04W to 0.1W peak power and 2ps to 3.5ps pulse width can be achieved in dispersion managed system.
Nonlinear silicon photonics has been an immense research subject in the past several years with promising prospects of
delivering chip scale signal modulation, shaping and characterization tools. In particular, broadband parametric process
has been considered for applications ranging from wideband light amplifiers to signal characterization and signal
shaping tools. Although underlying nonlinear effect, Kerr phenomena, in silicon has generated promising result of
wavelength conversion, the success of these devices have been challenged by the presence of nonlinear losses such as
two photon absorption and the two photon generated free carrier absorption. Experimental demonstrations were limited
to conversion efficiencies below -10dB. Here, we present the prospect of ultra wide discrete band conversion schemes
and the prospect of parametric process at mid-infrared wavelengths where nonlinear losses are not present. In particular,
we explore the parametric wavelength conversion scheme at mid-wave infrared wavelength (2μm~6μm) by four-wavefixing
process in silicon waveguides with new cladding materials, such as sapphire, that can provide transparency up to
6μm and facilitate phase matching condition for discrete wavelength bands as far as 60THz away from each other.
Design criteria include the optimization of mode overlap integrals and dispersion engineering for an ultra-wide band
signals. The particular results of wavelength conversion between 2μm bands and 5μm bands, and between 1.8μm bands
and >4μm bands will be presented. Prospects of frequency band conversion in generation of new infrared signals and
low noise, room temperature detection of mid-infrared signals will also be discussed.
KEYWORDS: Microwave radiation, Signal generators, Single mode fibers, Signal processing, Frequency conversion, Modulation, Radio optics, Heterodyning, Optical fibers, Scattering
Microwave signal generation and processing such as frequency conversion are proposed and experimentally
demonstrated based on stimulated Brillouin scattering (SBS) in single-mode fibers (SMFs). A simple scheme to generate
microwave/millimeter-wave sources is realized by utilizing the optical heterodyne technique between an optical carrier
and its Stokes light. Stable microwave sources with the frequencies of about 10.56 and 21.71 GHz are experimentally
realized using the first- and second-order SBS frequency shifts based on standard SMFs. A microwave frequency
up/down conversion method is presented without premodulation for bi-direction radio-over-fiber systems by using fiber
SBS. The microwave signal of 1.5 GHz is experimentally up-converted to 9.06 and 12.06 GHz, and the microwave
signal of 9 GHz is down-converted to 1.56 GHz, respectively.
A database is founded to estimate the nonlinear impairments of single channel return-to-zero (RZ) transmission systems with arbitrary parameters. Our link setup consists of multi-span transmission fiber and dispersion compensation component, with dispersion slope full compensating. The total transmission distance and the transmission fiber could be arbitrary, i.e. with arbitrary nonlinear coefficient, attenuation and dispersion. The parameters of input signal could also be arbitrary, i.e. with arbitrary transmission rate, input power, and any duty cycle of RZ pulse. Even the amplification schemes could be single-stage erbium-doped fiber amplifier (EDFA), or double-stage EDFA, or hybrid EDFA-Raman amplification. The database is obtained by numerous numerical simulations, neglecting the amplified spontaneous emission noise, polarization mode dispersion. The performance of transmission system impaired by nonlinear effects is represented by eye opening penalty (EOP). One can use the database to estimate the EOP of single channel transmission systems with bit rate up to 160 Gb/s, and can also evaluate the maximum transmission distance for given system impairment conveniently. For high-bit-rate transmission systems, intra-channel nonlinearity dominates the nonlinear impairments, therefore, the database to asses the nonlinear impairment of single channel transmission link is also valid for high-bit-rate wavelength-division multiplexed (WDM) systems. The accuracy and the limitation of the database are also discussed.
We demonstrate that all the inter- and intra-channel nonlinear impairments can be eliminated simultaneously by optical phase conjugation (OPC) in a power-symmetry system. However, for practical systems without power-symmetry, it is found that the effects of OPC on various nonlinearities are different in the same link. Even some nonlinearities are suppressed and some are enhanced. Therefore, optimizing the transmission link with OPC to suppress the dominant
nonlinearity is demanded. By using 1-km-long highly nonlinear fiber (HNLF) and tuning the pump wavelength near the zero dispersion wavelength of the HNLF, we experimentally generate the phase conjugation of the dispersed ~300 fs pulses. OPC with conversion efficiency of about -16 dB and conversion bandwidth of about 38 nm is obtained.
We investigate response flatting of cascaded sum- and difference- frequency generation (SFG/DFG)-based wavelength conversion in quasi-phase-matched (QPM) periodically poled lithium niobate waveguides for wavelength division multiplexing optical communication systems. The cascaded SFG/DFG-based configuration shows more robustly combined properties than conventional DFG-based and cascaded second-harmonic generation and DFG (SHG/DFG)-based configurations. The response fluctuation, generated while enhancing the conversion bandwidth, can be efficiently reduced by the method of pump detuning or QPM period detuning.
We investigate wavelength conversions based on chirped optical superlattices, such as linearly chirped and sinusoidally chirped optical superlattices, through cascaded second-order nonlinear processes for wavelength division multiplexing application. They are compared with periodic optical superlattices in terms of bandwidth, efficiency and response flatness. The sinusoidally chirped optical superlattice shows more excellent overall performance than the periodic and linearly chirped optical superlattices.
We investigate the four-wave mixing (FWM) effect in a dispersion-managed transmission line. The dispersion-managed line consists of many repeated fiber spans, each of which includes a positive- and a negative-dispersion fiber. The analytical expression of the FWM is obtained. The influence of the channel spacing, the fiber spans’ number and the compensating dispersion parameter on the FWM effect is analysed. The dispersion-managed transmission line is optimized to depress the generated FWM noise.
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