Research On The Dual-wavelength Femtosecond Laser Based On Spectral Filtering Effect

Dual-comb technology with low noise has become an advanced method in precision measurement fields,e.g.absolute distance measurement and spectroscopy,due to its large measurement range,high update rate and high accuracy.However,in dual-comb technology,complex phase-locked loops are required to suppress the noise of femtosecond lasers,which restrict the practical application of dual-comb technology.To this end,the repetition rate and carrier envelope phase of the two lasers are locked to optical references or radio frequency references.Recent years,the complexity and cost of the dual-comb system are greatly reduced by using a dual-repetition-rate femtosecond laser as the source.In this dissertation,the dual-wavelength femtosecond laser is studied based on the spectral filtering effect.The main research works are as the following:1.The working principle and transmittance function of Lyot filter and Sagnac filter that are commonly used in fiber lasers are demonstrated.An all-polarizationmaintaining(all-PM)Sagnac filter is obtained based on the traditional Sagnac filter.The wavelength separation is solely determined by a specific piece of polarizationmaintaining fiber.2.Preparation method of the carbon nanotubes(CNT)saturable absorber films and analyze of their optical absorption properties are demonstrated.A CNT mode-locked fiber laser is built.Based on Lyot filter,a dual-wavelength CNT mode-locked fiber laser is successfully built.This laser can operate at dual-wavelength mode-locking state(1532 nm and 1556 nm).To this end,the laser directly outputs two femtosecond pulse trains with inherent offset repetition rate attributed to the net cavity dispersion.Since the two pulse trains oscillating in the same laser cavity,the common-mode noise will be effectively suppressed.3.The mode-locking principle of nonlinear amplifying loop mirror(NALM)is elaborated.An all-PM mode-locked fiber laser based on reflective NALM is achieved.Based on the newly proposed all-PM Saganc filter,an all-PM dual-wavelength modelocked fiber laser is constructed.The central wavelengths of the dual-wavelength modelocking operation are switched among 1570/1581 nm,1581/1594 nm and 1570/1594 nm.The all-PM structure of the laser cavity makes the dual-wavelength mode-locking state unaffected by environmental disturbance and has excellent long-term stability.