KEYWORDS: Analog electronics, Sensors, Interferometry, Signal detection, Signal processing, Switching, Semiconductor lasers, Computer intrusion detection, Photodiodes, Linear filtering
We present a self-mixing interferometer (SMI) vibrometer developed around a laser diode with a simple and compact analog processing of the fringe signal, and show that this instrument can readily detect ambient vibrations—which adversely affect mechanical material-working lathes. In addition, we find out that the vibrometer is an excellent intrusion detector, capable of discovering a man stepping several meters away from the sensor, thus hinting at an SMI application as an antithief sensor.
Photonic microwave down-conversion using period-one nonlinear dynamics of semiconductor lasers is proposed, which provides high conversion efficiency and requires no local oscillators. Experimental demonstration of microwaves at 33.7GHz down-converted to a frequency ranging from 10 to 14 GHz is presented.
A supercontinuum generation system is developed, which consists of an erbium-doped fiber ring laser, an erbium-doped
fiber amplifier, and a 100-m highly nonlinear fiber. Through nonlinear polarization rotation, the fiber ring laser generates
a train of noise-like pulses in the form of repetitive picosecond pulse packets consisting of femtosecond noise-like fine
temporal structures. The noise-like pulses are amplified before being sent into the highly nonlinear fiber. As a result, an
octave-spanning supercontinuum from 1177 nm to 2449 nm is obtained, which has a 20-dB spectral width of 980 nm.
Because of the nonlinearity of the fiber amplifier, the duration of the noise-like pulses is shortened while their average
power is enhanced. However, the enhanced pulse energy makes the key contribution to the spectral broadening of the
resulting spuercontinuum in this study since the highly nonlinear fiber is so long that the effect of the pulse compression
on supercontinuum generation is weak.
When a semiconductor laser is subject to an incoming optical carrier, equivalently an external optical injection, it
can enter nonlinear period-one dynamics through Hopf bifurcation due to the radical modification in field-carrier
coupling of the injected laser which results from the dynamical competition between injection-imposed laser oscillation
and injection-shifted cavity resonance. Equally-separated spectral components appear, of which intensity
and frequency depend strongly on the injection level and frequency. This suggests that a dynamical amplitude
or frequency variation of the incoming optical signal, such as amplitude-shift keying (ASK) or frequency-shift
keying (FSK), respectively, would lead to corresponding dynamical variation in amplitude and frequency of each
spectral component. Therefore, by properly selecting the optical frequency of the output optical carrier and
by minimizing the residual ASK and FSK modulation, both ASK-to-FSK and FSK-to-ASK conversions can be
achieved, where bit-error ratio down to 10-12 is achieved with a slight power penalty. Only a typical semiconductor
laser is necessary as the key conversion unit. In addition, frequency shifts of the optical carrier can also
be achieved, which allows a simultaneous frequency conversion of the optical carrier if required.
KEYWORDS: Frequency conversion, Data conversion, Four wave mixing, Semiconductor lasers, Modulation, Optical amplifiers, Wavelength division multiplexing, Semiconductor optical amplifiers, Signal to noise ratio, Frequency converters
Frequency conversion is considered as a key functionality for wavelength division multiplexing systems, which
converts an incoming optical carrier of one frequency to an outgoing optical carrier of another frequency while
preserving the quality of carried data. The all-optical approach is promising for such a purpose because not only
the system complexity and power consumption of frequency converters are greatly reduced but also the flexibility
and reconfigurability are highly improved. Four-wave mixing of solitary semiconductor lasers has been proposed
to achieve all-optical frequency conversion. Although the conversion efficiency is relatively high, however, the
conversion bandwidth is small, limiting the conversion range, and the efficiency flatness is poor, distorting the
data signal. In this study, we propose to use four-wave mixing of injection-locked semiconductor lasers instead.
When a semiconductor laser is subject to a strong optical injection, it can enter into stable locking dynamics
before undergoing Hopf bifurcation. An incoming optical carrier of one frequency perturbs the injection-locked
laser, generating an outgoing optical carrier of another frequency through four-wave mixing. Tens to hundreds
of gigahertz of frequency conversion can be achieved, increasing the conversion bandwidth by 3 folds. The
conversion efficiency varies within 5 dB over the enhanced bandwidth, improving the efficiency flatness by at
least 10 dB. Greatly improved eye-diagrams and bit-error ratios are thus obtained. The input power dynamic
range of the incoming optical carrier is greatly enhanced, increasing the flexibility of the proposed system.
Photonic transmission of microwave signals from a central office to remote base stations is a key functionality
in broadband radio-over-fiber access networks. Because of chromatic dispersion, a strong fluctuation of the
microwave power along fiber transmission happens to microwave-modulated optical carriers with double-sideband
features. Therefore, optical single-sideband modulation characteristics are preferred. Direct modulation of
a semiconductor laser is the simplest scheme for photonic microwave generation and transmission. However,
the symmetric property of the laser in the modulation sideband intensity makes the scheme unattractive for
radio-over-fiber applications. In this study, we apply the injection locking technique to the laser for optical
single-sideband generation. Proper optical injection can drive the laser to the stable-locking dynamical state
before entering the Hopf bifurcation. The field-carrier coupling of the injected laser is radically modified due to
the dynamical interaction between the injection-shifted cavity resonance and the injection-imposed oscillation.
Therefore, the relaxation resonance sidebands of the injected laser are considerably shifted in frequency and
asymmetrically modified in intensity, the extent of which depends strongly on the injection condition. Under the
range of our study, direct modulation of the injected laser can thus generate microwave signals that are broadly
tunable up to 4 times its free-funning relaxation resonance frequency and are highly asymmetric up to 20 dB
in modulation sidebands. The microwave frequency can be tuned over a broad range while keeping a similar
level of modulation sideband asymmetry, or different levels of modulation sideband asymmetry can be obtained
while keeping a similar microwave frequency. This adds the flexibility and re-configurability to the proposed
system. No optical phase-locking electronics, no high driving voltages, and no narrow-bandwidth optical filters
are necessary as in many other systems.
There has been much effort devoted to frequency conversion technology due to strong demand for optical communication
systems. A frequency converter converts an incoming optical carrier of one frequency to an outgoing
optical carrier of another frequency while preserving the quality of carried data. The all-optical approach is
promising for such a purpose because not only the complexity and power consumption of a converter are much
reduced but also the flexibility and reconfigurability are greatly improved. However, most proposed methods,
such as applying cross gain modulation in semiconductor optical amplifiers, suffer from the need of a probe or
a pump beam besides the incoming optical carrier, making systems complicated and costly. In this study, we
propose to use semiconductor lasers as frequency converters instead, where no probe or pump beam is necessary.
When a semiconductor laser is subject to an incoming optical carrier, equivalently an external optical injection,
it can enter into period-one dynamics through Hopf bifurcation. By taking advantage of the dynamics, tens of
gigahertz of frequency conversion can be achieved, which can be continuously and dynamically tuned by controlling
the injection level and frequency. The conversion efficiency and transmission efficiency can also be varied
through the change of both injection parameters. Their behaviors as functions of the parameters, however, are
opposite to each other. High conversion efficiency is observed to achieve under low levels of injection, where
strong filtering of frequency may not be necessary and significant signal amplification can be achieved. Low
bit-error-rate and a 3-dB penalty are also observed, suggesting the quality of carried data is preserved.
Nonlinear dynamics of semiconductor lasers has found many interesting applications in microwave photonics
technology. In particular, a semiconductor laser under optical injection of proper strength and optical frequency detuning
can enter into the dynamical period-one (P1) state through Hopf bifurcation. The resulting optical output carries a
broadly tunable high-speed microwave modulation without employing any expensive microwave electronics. It is
therefore a desirable source for radio-over-fiber (RoF) applications. The P1 state can also be adjusted to have a nearly
single sideband (SSB) optical spectrum. It is an advantageous property for long distance fiber transmission because it
minimizes the microwave power penalty that is induced by chromatic dispersion. In this work, we investigate in detail
the properties of the P1 state and the effect of fiber dispersion as a function of the injection conditions. Based on a well-established
rate equation model, the results show that the generated microwave frequency can be several times higher
than the intrinsic relaxation resonance frequency of the laser. With a large injection strength and an injection detuning
frequency higher than that required for Hopf bifurcation, the generated microwave power is nearly constant and the
optical spectrum is close to SSB. We simulate the effect of fiber chromatic dispersion and the result shows a maximum
microwave power penalty of less than 2 dB. The characterization of the P1 state is useful in guiding the design of RoF
systems based on optically injected semiconductor lasers.
Subcarrier multiplexed fiber-optic systems using direct modulation of semiconductor lasers have attracted much attention for analog and digital broadband services. Since analog modulation is adopted, the system performance can be degraded seriously due to inherent nonlinearities of semiconductor lasers. In this study, the injection-locking technique is applied to reduce the nonlinearities in directly modulated semiconductor lasers. In particular, the characteristics of the second harmonic distortion (SHD) and the third harmonic distortion (THD) are experimentally investigated. First of all, at a modulation frequency of 5 GHz, both SHD and THD in the injection-locked semiconductor laser are observed to reduce significantly and equally for a large range of modulation power, which are 15dB- and 23 dB-decreased, respectively. Second, at a fixed modulation power of 6 dBm, the reduction of SHD and THD is, however, found to vary with modulation frequency. The reduction of SHD is more substantial at around one-half of the relaxation resonance frequency of the free-running laser. The decrease in THD is more significant at around one-half and one-third of the relaxation resonance frequency of the free-running laser. Finally, how the reduction of harmonic distortions varies with the operational parameters of the laser system is also investigated. The results demonstrate the feasibility of the injection-locking method in reducing harmonic distortions for high-speed and high-power analog modulation applications.
The characteristics of period-one oscillations in semiconductor
lasers subject to optical injection is experimentally and
quantitatively investigated. The changes in the frequency
separation and in the magnitude difference between the principal
oscillation and the sideband of the injected laser are studied
as a function of experimentally accessible parameters, the detuning frequency and the injection strength of the injection signal. The frequency separation decreases as the injection strength and the detuning frequency decrease. The magnitude of the principal
oscillation decreases with the decreasing injection strength and the
increasing detuning frequency, while that of the sideband grows
at the same time. At some operating conditions, these characteristics
leads to a situation that the magnitude of the sideband becomes larger than that of the original principal oscillation, resulting in a frequency shift of the principal oscillation from the injection frequency to the sideband.
A directly modulated semiconductor laser when subject to optical injection locking is experimentally investigated. The modulation bandwidth is observed to increase when the laser is injection-locked to an external optical signal. It enhances with increasing injection strength and positively increasing detuning frequency. A more than three fold of bandwidth enhancement is observed experimentally. The frequency chirping is observed to reduce in the injection-locked laser system. It reduces significantly at low modulation frequency and increases with modulation frequency, approaching to that of the laser in free-running condition at high modulation frequency. More importantly, simultaneous bandwidth enhancement and chirp reduction is experimentally achieved.
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