2.8 μm Pulsed Fiber Laser Based On Graphene And Its Heterostructures Saturable Absorber

The mid-infrared laser is widely used in the fields of national defense and military,material processing,biomedicine,metrological statistics,fiber-optic communications and so on,due to its excellent properties such as good beam quality,high tunability and high peak power.At present,the mid-infrared ultrafast laser mainly relies on passive Q-switching and mode-locking technology based on saturable absorbers,but the commercial semiconductor saturable absorber mirror（SESAM）have limited operating wavelength range,high cost and complex preparation processes,which restricts the development of the mid-infrared ultrafast laser.The two-dimensional material represented by graphene,with ultrafast carrier relaxation time and wide band saturable absorption characteristics,has become one of the ideal saturable absorbers for generating mid-infrared ultrashort pulses,and has a simple structure and easy to be highly integrated,which is a key material to promote the development of the mid-infrared ultrafast fiber laser.More importantly,by combining graphene with other two-dimensional materials to construct vertical heterojunctions,better saturable absorption properties can be achieved by tuning the interlayer carrier and energy transfer processes,thus providing a new idea for the modulation of ultrashort pulse mid-infrared laser.In this paper,focusing on the Er³⁺:ZBLAN ultrafast fiber laser design in the 2.8μm,high-performance graphene and graphene/Mo₂C heterojunction saturable absorbers were developed,and two different types of laser cavities,transmission and reflection,were constructed to achieve mid-infrared Q-switched and Q-switched mode-locked pulse output.The main research contents of this paper are as follows:1.Graphene films are directly grown on calcium fluoride（Ca F₂）optical glass substrate by low-temperature plasma enhanced chemical vapor deposition（PECVD）method,and a transferable graphene saturable absorption mirror（GSAM）suitable for the mid-infrared band has been prepared.The modulation depth of GSAM is 3.9%,the saturation intensity is 1MW/cm²,and the non-saturated absorption loss is 9.2%.The transmission type 2.8μm passive Q-switched laser pulse output is realized based on GSAM,and the central wavelength is 2796nm.The average output power is 142 m W,the pulse width is 300.2 ns,the peak power is 7.76W,the repetition frequency is 61 k Hz,and the slope efficiency is 17.6%,when the pump power was only 0.8 W.The results show that the transfer-free GSAM can effectively avoid the additional optical loss caused by material pollution,achieve high peak power output at low pump power,and ensure the working stability of the laser cavity.2.The in-situ growth of graphene/Mo₂C（Gr/Mo₂C）vertical heterojunction and the preparation of saturable absorber mirrors have been realized by high-temperature chemical vapor deposition（CVD）.By varying the experimental conditions to regulate the microstructure,such as the size and thickness of the heterojunction,we obtain the Gr/Mo₂C heterojunction saturable absorber with a large nonlinear absorption coefficient and modulation depth.The Gr/Mo₂C heterojunction exhibits excellent saturable absorption properties at 2.8μm,the modulation depth of Gr/Mo₂C is 6%,the saturation intensity is 1.5 MW/cm²,and the non-saturated absorption loss is 4%.The Gr/Mo₂C heterojunction is transferred onto the surface of the Ca F₂ optical glass substrate to prepare transmission-type Gr/Mo₂C SAM,which realizes the output of 2.8μm passive Q-switched laser pulse,with an average output power of 258 m W,pulse width of 200 ns,peak power of 14.1 W,the repetition rate of 101 k Hz,and slope efficiency of 22.9%.In addition,the Gr/Mo₂C heterojunction has been transferred onto the surface of the gold mirror substrate to prepare the reflection-type Gr/Mo₂C SAM,which has been applied to the mid-infrared reflection-type fiber laser cavity to achieve the output of 2.8μm passive Q-switched mode-locked laser pulse,with a narrowest pulse width of 6 ns,a repetition rate of 28.4MHz,and an average output power of 330 m W.By inserting a lens to concentrate the light beam,the reflection-type laser cavity can further increase the peak power density of the incident laser on the SAM surface,while decreasing the output laser threshold.These works have opened up a new path for promoting the development of high-performance mid-infrared fiber mode-locked lasers.