We systematically discussed the principle of the dual-comb ranging system and built a theoretical model. To correct the phase distortion caused by femtosecond frequency comb noise, a correction approach is proposed and numerical simulations are conducted subsequently. In the simulations, the performance of the method under noise of repetition rate and offset frequency is analyzed respectively. The results indicate that ranging accuracy can be effectively improved by the method. It is verified by the experiments.

In frequency-sweep polarization-modulation ranging, distance is determined by the frequency of modulated waves and the corresponding wavelength multiple when emitted and returned waves are in phase. However, measurement of the frequency and the wavelength multiple is affected by thermally induced phase delay of the polarized wave. In this article we systematically discuss the principle of the ranging method and analyze the influences of thermally induced phase delay. New approaches to measurement are proposed to eliminate the impact on frequency and the wavelength multiple. Theoretical analysis and experimental results proved the efficiency and applicability of the methods.

Recently,multi-wavelength pulsed lasers have become a research hotspot due to their versatile applications,such as precision spectroscopy,microwave/terahertz photonics,optical signal processing,and wavelength division multiplexed optical fiber communication systems.As a promising candidate,passively mode-locked fiber laser has the advantages of ultrashort pulse,ultrahigh peak power,compact structure and low-cost.In the existing multi-wavelength passively mode-locked fiber lasers,multi-wavelength mode-locked operation is achieved by adjusting the intracavity modulators to a proper state after laser has worked.It is inconvenient for practical use,so,its application scope is restricted.In this paper,a new method to achieve dual-wavelength mode-locked operation in an erbium-doped fiber laser is proposed. For an erbium-doped fiber,the peaks of both absorption and emission spectra overlap in the 1530 nm-region.So the emission light in the 1530 nm-region will be re-absorbed by the erbium-doped fiber with low pump power or long gain fiber.Utilizing the emission re-absorption effect,the gain spectrum can be modified by different lengths of gain fiber. In the experiment,an all-fiber ring cavity is adopted and a transmission-type semiconductor saturable absorber is used as a modelocker.The cavity consists of ～3.2-m-long single mode fiber and an erbium-doped fiber.Gain fibers with different lengths are used in the cavity to reveal the dependence of emission re-absorption on both gain spectrum and mode-locked output spectrum.According to the experimental results,there are two humps in the amplified spontaneous emission spectrum located in the 1530 nm-region and 1560 nm-region,respectively.With the gain fiber length increasing, gain spectrum in the 1530 nm-region is suppressed,and gain intensity in the 1560 nm-region gradually surpasses that in the 1530 nm-region.Based on the experimental results,self-starting dual-wavelength mode-locked operation is achieved with a 31-cm-long gain fiber.The two spectral peaks with close intensity are located at 1532.4 nm and 1552.3 nm, respectively.The maximum output power is 4.8 mW at a repetition rate of 58.01 MHz and a signal-to-noise ratio of 58.2 dB.This self-starting dual-wavelength mode-locked erbium-doped fiber laser is convenient for practical use and can meet the requirements for many potential applications.

Laser tracker is an important optical instrument for precise geometric coordinate measurement in large scale. Position sensitive detector (PSD) is one core miss-distance detecting sensor of laser tracker to achieve precise cooperation target tracking. The tracking parameter calibration accuracy based on PSD has remarkably direct influence on the performance of tracking and measurement. Firstly, a laser tracking measurement system is presented, and the working principle of the system's tracking unit based on PSD is introduced. Then the hardware and software design of miss-distance detecting module is described on the basis of PSD according to the actual demand. The experimental result shows that the stability of spot position is better than±2.0μm in the square effective area (4×4mm 2). Finally, from precise detecting principle and system structure analysis, the forming mechanisms of tracking zero point and phenomena of laser spot rotating with azimuth axis rotation are analysed. Their influence on target tracking is discussed, and two novel calibration methods of tracking zero point and phase angle of laser spot rotating coordinate are proposed. By these calibration methods and precision displacement experiment the relationship model in tracking state between PSD displacement and target miss-distance based on data fitting and interpolation is set up. Experimental results indicate that the detection module design and the tracking parameter calibration method can well meet the demand of the precise and fast target tracking.

A self-starting simple structured dual-wavelength passively mode-locked (ML) erbium-doped fiber (EDF) laser is proposed in this Letter. An all-fiber ring cavity is adopted and a transmission-type semiconductor saturable absorber is used as modelocker. In this laser, there are two gain humps located at the 1530 nm region and the 1550 nm region, respectively. Along with the length of EDF increasing, the intensity of the hump at 1530 nm region is gradually suppressed because of the re-absorption of emission by the ground state. With the proper length of EDF, the gain intensities of two regions are very close. When the pump power is above the ML threshold, the self-starting dual-wavelength ML operation is achieved easily without manual adjustment. The two spectral peaks with close intensities are located at 1532 and 1552 nm, respectively. The effect of intracavity dispersion on the output spectrum is also experimentally demonstrated.