Embedded sensors in large civil structures for structural health monitoring applications require data communication
capabilities between sensor nodes. Conventional communication modalities include electromagnetic
waves or acoustical waves. However, ultrasonic guided elastic waves that can propagate on solid structures such
as pipes for a great distance have rarely been studied for data communication purposes. The multi-modal and
dispersive characteristics of guided waves make it difficult to interpret the channel responses and to transfer useful
information along pipes. Time reversal is an adaptive transmission method that can improve the spatial and
temporal wave focusing. Based on the focusing effect of time reversal, we have developed a data communication
technique using guided waves in a highly dispersive pipe environment.
In this paper, we experimentally demonstrate the data communication using time reversal pulse position
modulation (TR-PPM). Three-step laboratory tests have been performed using piezoelectric transducers in a
pitch-catch mode. We first measure the channel responses between the transmitter and the receiver on a pipe.
We then carry out the time reversal transmission by reversing the sounding signal and feeding it back to the
same channel. Finally, we perform the time reversal communication experiment by sending the modulated time
reversal signals as a stream of binary bits at a given data rate. A series of experiments are conducted on steel
pipes. Experimental results demonstrate that time reversal pulse position modulation for data communications
can be achieved successfully using guided elastic waves.
Piezoelectric sensors that are embedded in large structures and are inter-connected as a sensor network can provide
critical information regarding the integrity of the structures being monitored. A viable data communication
scheme for sensor networks is needed to ensure effective transmission of messages regarding the structural heath
conditions from sensor nodes to the central processing unit. In this paper we develop a time reversal based data
communication scheme that utilizes guided elastic waves for structural health monitoring applications. Unlike
conventional data communication technologies that use electromagnetic radio waves or acoustical waves, the
proposed method utilize elastic waves as message carriers and steel pipes as transmission channels. However,
the multi-modal and dispersive characteristics of guided waves make it difficult to interpret the channel responses
or to transfer correctly the structural information data along pipes. In this paper, we present the basic principles
of the proposed time reversal based pulse position modulation and demonstrate by simulation that this method
can effectively overcome channel dispersion, achieve synchronization, and delivery information bits through
steels pipes or pipe-like structures correctly.
We present an investigation of spectral dynamics of an ultrashort-pulse Er-doped fiber laser mode-locked by a
semiconductor saturable absorber mirror (SESAM). The SESAM used has a saturable absorption modulation depth of
18%, a saturation fluence of 70 μJ/cm2 and a relaxation time of 10 ps at a wavelength of 1550 nm. Detailed pulse
dynamics of the laser are measured at different pumping levels, and the laser operation is linked to the characteristics of
the SESAM. It is observed that, as the pump power is increased, the laser operation changes from cw lasing, to self-Q
switching, Q-switched mode-locking, and then cw mode-locking. Self-starting and stable mode-locking operation is
achieved at a repetition rate of 12 MHz. The pulses with a width of 650 fs are produced at 1562-nm wavelength with a
pump power of 80 mW, and the corresponding spectral width is about 5.0 nm (FWHM). Based on the complex
Ginzburg-Landau equation, self-starting mode-locking process in the fiber laser is simulated, which confirms that
ultrashort pulses can be achieved. The calculated spectral characteristics of the mode-locked pulses are consistent with
the experimental observations.
This work is motivated by the need to ultra-short optical pulses for realizing OTDM/WDM. As an attractive ultra-short pulse source, the mode-locked fiber laser attracts more and more attentions. In this paper, we proposed a novel method to design passively mode-locked fiber laser by combining lumped-gain amplification and high output couple ratio. On the one hand, through using lumped-gain amplifier as the amplifying units in the cavity, the mode-locked fiber laser can effectively reduce the periodic energy fluctuation of intra-cavity and offer a very good suppression to the spectral sidebands. On the other hand, a coupler with high output couple ratio is used in the cavity to enhance the output single-pulse energy. The characteristics of the presented laser have been investigated theoretically and experimentally in detail. With this method, a passively mode-locked Er3+-doped fiber laser with high spectrum quality and high single pulse energy is obtained. The experimental results show that its spectral sidebands suppression ratio is more than 20dB, the maximum average output power is about 18mW and the single pulse energy is about 1.2nJ with pulse-width 200fs at a repetition of 14.2MHz.
Ytterbium-doped silica fibers exhibit very broad absorption and emission bands, from 800nm to 1064nm for absorption and 970nm to 1200nm for emission. Therefore wide band lasers can be obtained using a wide variety of pump lasers. In this paper, the characteristics of high-doped Yb3+ fiber are analyzed and verified by experiment and a highly-doped Yb3+ fiber ring laser with short cavity has been presented. Comparing with normal Yb3+doped fiber, the relationship between the important characteristics of the Yb3+doped fiber laser such as threshold power, output power and laser parameters such as pump power, fiber length, output couple ratio is analyzed. Numerical results are coincident with the experiment phenomenon very well. A 1053 nm pulse has been achieved in our fiber laser. The output power is 6mW as pump power is 110mW and the slope efficiency is 17%. The Yb3+ fiber laser we produced can be used as a stable source in obtaining ultrafast pulse, fiber sense and optical communications.
Theory and experimental results on the self-starting passive mode-locked Yb fiber ring laser generating short pulse are reported. The relations between the laser cavity parameters and mode-locked pulse characters are discussed. 980nm LD pumped laser is used as the pump source and high concentration Yb3+-doped fiber is adopted as gain medium. Using the nonlinear polarization rotation (NPR) effect of the fiber, self-starting stable mode-locked pulse is obtained, with center wavelength of 1046nm, 3dB bandwidth of 6.01nm and 20dB bandwidth of 16nm. The mode-locked threshold power is 150mW and output power is 26mW with 50MHz repetition rate.