Conventional lasers rely on multiple passes through a gain medium to reinforce a preselected frequency, thereby obtaining near-monochromatic output. For many purposes, such as optical arbitray waveform generation, it is desirable to have light spread over a broader range of frequencies. One means of fulfilling that objective is to introduce a frequency shifting element into the laser feedback loop such that successive passes of a wavepacket take place with different carrier frequencies. The operation of such a frequency shifting feedback (FSF) laser has been investigated by several research groups [1–46] and it has been used for a number of practical applications, including the ultrahigh repetition rates pulse, optical real-time Fourier transformation, RF-chirped waveform generation. In the early 1970’s, the concept of frequency shifted feedback loop incorporating an active medium was introduced for pulsed lasers as a possibility for electronic frequency control. The prediction was that cavity modes were chirped because of the frequency shift introduced during each transit through the cavity. By satisfying a resonant condition between the applied frequency shift and the loop fundamental frequency, it was possible to generate a pulsed output with a repetition rate equal to the modulator frequency. And a theoretical work has shown the possibility of further increasing the repetition rate by adjusting the frequency shift and the loop fundamental frequency as the ratio of two coprimes. Then some scholars provided an experimental demonstration of the idea by injecting a dye FSF cavity with a dye single-mode seed laser: the generation of 6-ps Fourier-transform-limited pulses was demonstrated with repetition rates tunable by steps of 80 MHz between 0.24 GHz and 36.6 GHz, this is, over two orders of magnitude. Finally, franch scholar H.Guillet de Chatellus explains this result in depth by providing an extensive description of the fractional Talbot effect in cw-seeded FSF lasers. Then we called the resonant condition as the integer Talbot condition and the fractional resonant condition as the fractional Talbot condition. When the modulating frequency is slightly detuned away from the integer Talbot case, the FSF loop seeded by a single CW laser is demonstrated to generate the RF chirp wavform. Based on these unique properties of the FSF laser, I propose some new methods to enhance the accuracy of the velocity and distance measurement by using the frequency shifting loop. Besides, I also do some researches on the dual-sidebands frequency-shifting loop (FSL) and find some originally results, such as pulse doublets, optical real-time Fourier transformation and temporal fractional Talbot effect. In the following, I will present these academic researches systematically. The investigation of the coherent dual-frequency lidar-radar system. We construct a frequency-shift Mach-Zehnder Interferometer (MZI) to generate a fibered dual-frequency laser source and apply it to range measurement. Different from the traditional Mach-Zehnder interferometer in the case of long delay time, we theoretically investigates the beat phase noise under the case that delay length is far shorter than the laser coherent length. The effect of laser phase noise and acousto-optic frequency shifter (AOFS) noise in the beat note phase noise of the dual-frequency laser is theoretically investigated. General expressions are presented for the power spectral density and phase spectral density of the beat note. From the simulations, we find that high AOFS noise introduces a much greater phase jitter into the beat note noise conversion. Based on the theoretical results, an outdoor experiment is conducted to demonstrate the ability of the phase-shift method in the absolute distance measurement and beat note Doppler frequency-shift method in the velocity measurement. By conducting the experiment in the outdoor environment, we finally obtain 8 mm range resolution at a distance of 1.5 km and the velocity resolution is better than 0.5 m/s. The investigation on the RF up-conversion and waveform generation using a frequency-shifting amplifying fiber loop, application to Doppler velocimetry. RF-modulated laser based on the frequency-shifting amplifying loop has the ability of generating high-order harmonics. Here we focus on a direct application of this source: a velocity measurement with different order harmonics. The emitted wave from the frequency-shifting amplifying loop is directed through a fiber coupler with a splitting ratio of 1:99. The 99% output is connected to a fiber circulator, and the other one (1%) is directly detected by photodetector PD2 (3.5 GHz bandwidth) and used as a monitor signal. The light beam emitted by the circulator is collimated by a 10-cm-diameter lens and transmitted to the moving target. The target moves toward and away from the laser in one cycle at a speed ν adjustable from 0.1 m/s to 0.8 m/s. We find (i) a good linear dependency of the frequency shift versus motor velocity, and (ii) the expected n-fold enhancement of the slopes. This measurement confirms that RF up-conversion permits to significantly improve the experimental accuracy at low velocities. The investigation on a novel hybrid TOF/phase-shift method for absolute distance measurement using a falling-edge RF-modulated pulsed laser. Traditional range-finding techniques often take advantage of two methods: pulsed TOF laser ranging and phase-shift laser ranging. The pulsed TOF laser ranging technique is appropriate for long-distance measurement because of the long unambiguous distance and high peak power. The range resolution, which is determined by the pulse width, is typically on the order of meters. In comparison, phase-shift laser ranging offers good precision but exhibits "2π" phase ambiguity, making it suitable for measurements of incremental displacement. To address the limited ambiguous distance, two or more measurement rulers are usually used in the phase-shift rangefinder. Therefore, the TOF method has the characteristic of a long unambiguous distance, and the phase-shift method has the characteristic of a high precision. They are thus complementary in terms of the unambiguous distance and accuracy. Combining the characteristics of the long, unambiguous distance of the pulsed TOF method and the accuracy of the phase-shift measurement, a hybrid TOF/phase-shift method may achieve long-distance and high-accuracy absolute distance measurement. Then we investigate a falling-edge RF-modulated pulsed laser by using a frequency-shifted amplifying loop. A single-frequency fiber laser (seed laser) is coupled into a fiber link through coupler 1. The output port of coupler 1 is sent through an amplifier, followed by an acousto-optic (AO) switch back to the input of coupler 2. The second coupler allows us to extract a fraction of the optical field in the pulsed frequency-shifted amplifying loop. The AO switch is used as a frequency shifter and beam chopper. A Yb3+-doped fiber amplifier is inserted to compensate for the loss in the loop. The RF modulation within the pulse results from the interference of the frequency-shifted pulse with the seed laser. Then we directly use the falling-edge RF-modulated pulsed laser to measure the object distance. To verify the capability for long-range absolute distance measurement, we make a distance measurement inside the fiber by inserting a 2-km fiber in the receiving port. The experimental results show that the TOF/phase-shift method can compensate for the timing error caused by the TOF method and significantly improve the distance precision. Finally, we show experimentally that the hybrid TOF/phase-shift method is a promising technique that achieves 3 mm precision in the 1.5 km measurement range (in fiber), corresponding to a relative precision of 2×10-6. The investigation on the temporal and spectral properties of frequency-shifting loop. Frequency-shifting loops usually rely on the use of an acousto-optic frequency-shifter. While it features high frequency conversion efficiency in the sub-100 MHz range, AOFS have limited efficiency in the GHz range, and offer limited tunability. In this respect, EOM offer much higher modulation frequency and bandwidth. Besides, EOM are compact and easy to integrate with other fibered devices. Thus we propose to investigate an all-fibered frequency-shifted feedback loop when a widely tunable common electro-optic amplitude or phase modulator (EOAM or EOPM) is employed. Instead of the single side-band AO frequency-shift, the EO loop will produce at each round-trip two side-bands with opposite frequency-shifts. The carrier will also circulate together with the multiple frequency-shifted sidebands. In order to predict the time response of the FSL, a general time-delayed interference model is deduced under the cases of the integer Talbot condition and fractional Talbot condition. When the integer Talbot condition is satisfied (f_m⁄f_c =p), the output traces show a periodic pulse. However, different modulators also show some differences. The phase-modulating FSL is demonstrated to achieve the optical real-time Fourier transformation. And the amplitude-modulating FSF loop features double-pulse regime which the interval between the two pulses can be continuously adjusted by changing the static phase retardance and the modulation depth. Besides, when the fractional Talbot condition is satisfied (f_m⁄(f_c=p⁄q)), the time response of the amplitude-modulating FSF loop could observe the temporal fractional Talbot effect, that is a periodic pulse with the repetition rate qf_s=pf_c. Phase-modulating FSF loop features the arbitrary waveform generation due to the destructive interference of higher-order harmonics. Experiments involving the temporal and spectral properties of the amplitude-modulating FSL and the phase-modulating FSL are conducted to demonstrate the above results.