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A ring cavity fiber laser was proposed by using graphene as a saturable absorber for fiber optic communication and material processing.The principle how the grapheme could generate pulse trains and the laser output characteristics were studied experimentally.Then the laser induced deposition method was used to transfer the graphene to the fiber end face and place it in the ring laser cavity.By taking a 974 nm semiconductor laser as the pumping source and the Er3+-doped fiber as the gain medium,the laser generated stable pulse trains by changing the orientations of a polarizer above the threshold pump power.The experiment shows that the center wavelength of laser is 1 560.1 nm with a full width at half maximum spectrum of 0.27 nm and the pulse repetition rate is 7.89 MHz with a pulse width of 14.7 ps.It means that the graphene is expected to replace single-walled carbon nanotubes in pulsed laser applications as a novel mode-locked material because of its good saturable absorption properties and a higher damage threshold.

Long Period Fiber Gratings(LPFGs) were inscribed by using a 800 nm femtosecond laser and their high-temperature properties were verified in this paper.Firstly,an inscription system for the laser into fiber cores was designed using a camera and an electric moving platform.With controlling a pulse laser of 1.3 mW,a LPFG was inscribed in a non-hydrogen-loaded bare fiber using point by point technique,and the resonant peak of the LPFG is about-17 dB in the 1 200~1 700 nm band.Then,high temperature sensing experiments were performed on the LPFG by a high temperature box,obtained results show that the linearity of the resonance peak changes is about 0.992 and temperature sensitivity is 0.056 nm/℃ at 300-800 ℃.It demonstrates that the inscription technology for LPFGs by using the 800 nm femtosecond laser is feasible and obtained LPFGs have good response characteristics and stability,and are suitable for the high-temperature measurement.

Based on a wavelength scanning technique and the characteristics of ultra-narrow-linewidth laser,a kind of cavity enhanced absorption gas detection system with high sensitivity is designed.A semiconductor laser with ultra-narrow-linewidth and tunable wavelength is used as a light source,and an optical cavity which consisted of two mirrors with high reflectivity is also used as an absorption cell in this system.The laser frequency overlaps with one of cavity modes via scanning the cavity length and the input laser is switched off by using the laser detuning technique.Then,the ring-down detection of acetylene with low concentration is achieved.With the cavity enhanced absorption technology,spectra of acetylene near the region of 6 518.824 cm-1 and laser ring-down time at different concentrations are measured and also analyzed.The experiments for concentration measurement indicate that the relative error is less than 2.5%,relevance coefficient is 0.999 and the lowest detection limit is 2×10-6.By filling a volume of acetylene into the gas cell gradually,the system dynamic response time is less than 10 s.Obtained results show that the system has advantages of good accuracy,high sensitivity and quick response,which can be used in the fault gas on-line monitoring for power transformers in real time.

On the basis of the wavelength scanning and spectrum modulation techniques of an ultra-narrow-linewidth semiconductor laser,a kind of multi-component gas online detection system with high precision was designed.A semiconductor laser with ultra-narrow-bandwidth and tunable wavelength was utilized as a light source and a novel series gas cell was also used to compress the horizontal space and increase the optical path of the system.By taking the sawtooth signal to modulate the spectrum of laser and using space division multiplexing(SDM) and time division multiplexing(TDM) techniques,the detection of multi-component gases(CO,CO2,CH4) based on ultra-narrow-linewidth laser characteristics was achieved.The experiments indicate that the relative measurement error is less than 2% and linearity relevance coefficient is 0.99 for the three kinds of gases.Furthermore,the system dynamic response time is less than 10 s by filling a volume of multi-component gas into the gas cell gradually.Finally,the result of long time stability monitoring shows that the maximum concentration fluctuation is less than 0.02%.The system has advantages of high accuracy,good stability and quick response,and is suitable for the fault gas monitoring of power transformers in real time.