KEYWORDS: Intermodulation, Modulation, Semiconductor lasers, Transmitters, Radio optics, Radio over Fiber, Signal detection, Nonlinear optics, Phase shifts, Telecommunications
In typical radio-over-fiber (RoF) systems, sub-carrier multiplexing is achieved by using directly modulated laser diode
(LD). However, the conventional LD for optical communication system has non-linear characteristics such as, 3rd order
intermodulation distortion (IMD3), which degrade overall analog RoF system performance. In addition, when the
multi-channel input signals are modulated directly, 5th order intermodulation distortion (IMD5) signals act as a noise on
overall systems. In this paper, we experimentally demonstrate an opto-electrical predistortion optical transmitter to
enhance IMD3 & IMD5 for radio-over-fiber systems. To reduce the 3rd and 5th order intermodulation (IM3 & IM5),
master laser diode (MLD) and slave laser diode (SLD) with similar performance are used, and the matching circuits of
two LDs are designed with equal properties. The amplitude and phase balances as well as bias current of two LDs are
carefully considered for the enhancements. The experimental results show that reductions of about 30 dB in the IM3
and about 12 dB in the IM5 are achieved at 2.2 GHz.
KEYWORDS: Semiconductor lasers, Signal detection, Attenuators, Modulation, Phase shifts, Intermodulation, Signal generators, Radio over Fiber, Distortion, Transmitters
We propose and demonstrate a pre-distortion method for reducing the third-order intermodulation distortion (IMD3) of
the distributed-feedback laser diode. The pre-distortion scheme consists of two loops and uses two laser diodes (LD), a
photodetector (PD), RF attenuators, RF combiners/dividers, and an RF amplifier. In our scheme, the error signal is
generated by the slave LD through controlling the phase and attenuation adjustment in the inner loop and then drives the
master LD to reduce nonlinear property in the outer loop. The experimental results show that the significant reduction
of about 30 dB in the third-order intermodulation distortion (IMD3) is achieved at 2.4 GHz in the implemented system.
For the broad frequency range from 1.97 GHz to 2.48 GHz (510 MHz), IMD3 is enhanced by more than 10 dB.
We have experimentally demonstrated the transmission performance of a feedforward optical transmitter using an
external light injection technique. The feedforward compensation method shows 31 dB intermodulation distortion
suppression and 2.2 dB noise reduction. A high side-mode suppression ratio (SMSR) exceeding 35 dB of the
wavelength-locked FP-LD was obtained over 12 nm ranges. The suppression characteristics of the intermodulation
distortion for wavelength differences and various transmission lengths were measured and analyzed as the evaluation
criteria for the transmission performance in WDM/SCM based radio-over-fiber systems.
We designed and fabricated traveling-wave photodetector with enhanced bandwidth. Because the saturation velocity of
hole is smaller than that of electron, bandwidth limitation in conventional symmetric TWPD results from the difference
in electron and hole transit times. For solving this problem, we designed a new structure with asymmetric intrinsic region
to equalize the carrier transit times. The intrinsic region on the epitaxial layer consists of InGaAs core and 1.3Q InGaAsP
cladding regions. In the whole i-layer thickness with 1 &mgr;m, the core region is 0.2 &mgr;m thick and the thickness of cladding
region is asymmetrically made up. As the thickness of upper cladding region to p-side is decreased, the transit lengths of
electron and hole are matched and the bandwidth of TWPD is enhanced. By fabricating TWPD's from three kinds of
epitaxial wafers, we prove the enhanced bandwidth of TWPD with asymmetric intrinsic region.
Latching optical switches and optical logic gates with AND or OR, plus the INVERT functionality are demonstrated for
the first time by the monolithic integration of a single and differential typed vertical cavity lasers with depleted optical
thyristor (VCL-DOT) structure with a low threshold current of 0.65 mA, a high on/off contrast ratio of more than 50 dB,
a high slope efficiency of 0.38 mW/mA, and high sensitivity to input optical light. By simply changing a reference
switching voltage, this single-typed device operates as two logic functions, optical logic AND and OR. The differential-typed
VCL-DOTs operate also as all logic gates, AND/NAND, OR/NOR, and INVERT function by simple change of a
reference input light power. The thyristor laser fabricated by using the oxidation process shows a high optical output
power efficiency and a high sensitivity to the optical input light.
A feedforward over an analog optical link is known as difficult technique in spite of its superior linearization. We proposed a broadband analog feedforward optical transmitter using a wideband 180° hybrid coupler instead of conventional frequency-sensitive phase shifter with the narrow bandwidth property. Using the wideband 180° hybrid coupler, the wideband linearization technique enhances the wide operation frequency and simple adjustment of the feedforward optical transmitter. Our experimental results show the 3rd-IMDs enhancement more than 10 dB at 385 MHz range (1.375~1.76 GHz).
We propose the optical transceiver having reference clock generator and CDR with delayed data topology in this paper. The 125MHz reference clock of optical transmitter have been extracted from 10 × 250 Mb/s data arrays. The clock extraction of reference clock generator is achieved by summing the edge information of the each data. Moreover, our optical transmitter includes 2-stacks NMOS serializer scheme rather than 3-stacks conventional scheme to achieve high speed operation. In optical receiver design, we employ a novel CDR with delayed data topology to overcome the problems in conventional CDR such as instability in locking state, nonlinearity output proportional to phase difference, false locking at harmonic frequency. The optical transceiver is designed by using of 0.35μm CMOS technology.
At low light intensities (linear regime), the absorption coefficient is independent of the light intensity; however, at sufficiently high intensities, the absorption becomes a nonlinear function. In case that incident optical power is sufficiently high to saturate the absorption coefficient, the optical power which is not absorbed is transmitted to the next region in the absorption layer if the photodetector is in longitudinal structure. Therefore this type of light-absorption behavior would be critical in analyzing the nonlinearity of a traveling-wave photodetector (TWPD). The transmitted optical power is absorbed as a nonlinear function of absorption coefficient, and the generated photocurrent due to the saturated absorption also shows the nonlinear characteristics. In this paper, we calculated the photocurrent of a TWPD considering the effect of the absorption saturation caused by high input intensity, and compared with the results using the linear function of absorption coefficient. For a different incident optical power, the nonlinear characteristics of photocurrent are analyzed for two-tone input signals, and thereby the inter-modulation distortions (IMD's) and spurious-free dynamic range (SFDR) are obtained.
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