Research Of A New Dual-wavelength Erbium-doped Fiber Ring Cavity Laser And Its Application

Dual-wavelength fiber ring laser based on erbium-doped fiber, using erbium-doped fiber as the gain medium, the optical fiber as the guide medium, the ring cavity as the resonator of the laser beams, containin two laser beams with different wavelength in the same ring cavity stability at the same time, has advantages such as low threshold、wide gain bandwidth、narrow linewidth、wide tuning range、simple and compact structure、light weight. The laser has broad application prospects in the field of communications and sensing.Based on the two areas above, we research design and application of dual-wavelength erbium-doped fiber ring laser. In the field of communication, we focus on the optical heterodyne method to produce high frequency microwave signal; In the field of sensing, our focus is the erbium-doped fiber ring laser gyroscope.In recent years, with the development of microwave photonics, especially Radio on fiber (ROF) technology, demands of the microwave frequency increased rapidly. Optical-heterodyne technique is a new optical means to generate a high frequency microwave signal. Dual-wavelength fiber ring laser is a kind of potential laser source of optical-heterodyne technique,.Erbium-doped fiber ring laser gyro is a kind of inertial devices combinded by laser gyro technology and fiber optic technology. Use erbium-doped fiber ring laser as sensitive components, erbium-doped fiber ring laser gyro detect the angular velocity by measuring the frequency difference between the two laser beams transmitting along the CW and CCW direction. The erbium-doped fiber ring laser gyro has many advantages such as light weight, high stability and sensitivity, wide dynamic range. However, similar to other laser gyro, the erbium-doped fiber ring laser gyro is disturbed about frequency locking and zero frequency drift.Erbium-doped fiber is homogeneously broadened media at room temperature. As gain media in dual-wavelength laser, when the wavelength of the two beams are close, cross gain saturation are produced by inhomogeneous broadening effect, which will lead to disappear of one the beams. As a kind of solid media, spatial hole burning will be introduced in erbium-doped fiber laser due to reflection of the laser beams, which lead to multiple longitudinal mode oscillation, or instability of the power. Therefore, to produce a dual-wavelength erbium-doped fiber ring laser with little wavelength space, inhomogeneous broadening effect and spatial hole burning effec of the erbium-doped fiber must be weaken.This paper is mainly around the applications of dual-wavelength erbium-doped fiber ring laser in communication and sensing fields. According to the two different application background, we research the design of dual-wavelength erbium-doped fiber ring laser respectively.1) reseach of photonic generation of microwave based on optical heterodyne methodIn this paper, how do the laser beams impact on the beat signal is discussed first. According to the discussion, design principles of laser was proposed. When be used as laser source of optical-heterodyne technique for frequency tunable microwave signal, in dual-wavelength erbium-doped fiber ring laser, the wavelength space between the two laser beams should be adjusted in a very wide range, and the state of single longitudinal mode is maintained during the adjustment.A direction related polarizer is proposed. The polarizer is composed by cascading two polarization beam splitter with the same polarization. Due to the polarizer, polarization burning and opposite direction transmission is introduced into our erbium fiber ring laser to suppress the mode competition owing to the gain homogeneous broaden. What is more, The polarizer brings two different linearly polarized laser beams into erbium-doped fiber to weaken the spatial hole burning.Two unite filters, composing by optical thin film filter and M-Z comb filter respectively, are used on every beam to enhance the stability of wavelength. The two unite filter have different pass band, make the wavelength spacing of the two laser beams tunable.According to design scheme, laser experiment system was made, the wavelength difference of the two laser beams range from0.019nm to15nm. during the process of the adjustment, the single longitudinal mode state of the two beams are invariably maintained.On the basis of dual wavelength laser above, a frequency tunable microwave signal source is realized by using optical heterodyne. Restricted to observing conditions, we observed that the frequency of the microwave signal range from the minimum value of472.5MHz to about26GHZ which is close to millimeter wave band in microwave band. If there is proper beat conditions and observation method, the frequency of the beat frequency signal can be extend to the THZ band.2) Study of erbium-doped fiber ring laser gyroTo solve the frequency locking and zero frequency drift in erbium-doped fiber ring laser gyro, the dual-wavelength fiber ring laser which has the following characteristics:the two beams transmit along the CW and CCW direction respectively, the frequency difference between the two beams can be adjusted to very small amont, and the state of single longitudinal mode is maintained during the adjustment.Two bi-directional circular polarizers are introduced into erbium-doped fiber ring laser in this paper. Two circular polarization laser beams are induced by the circular polarizers, have same sense of rotation, transmit along opposite directions. Due to the characteristic of the circular polarization, there is no standing wave in erbium-doped fiber, the frequency locking caused by spatial hole burning effect is weaken. Besides that, polarization burning is introduced in erbium-doped fiber to enhance the stability of the beams, and the complexity of the polarization regulating process of the laser is diminished significantly.A Faraday rotator with45°rotation angle is used to introduce nonreciprocal effect into the laser to reduce the coupling of the two laser beams due to back-reflection effect. For this reason, the frequency locking caused by back-reflection effect is weaken. The unite filter composed by optical thin film filter and band-stop fiber ring cavity filter is used to enhance the frequency stability and maintain the single longitudinal mode state of the laser beams.Laser experiment system is produced According to design structures. However, due to the intense competition between the two laser beams, the laser fail to get the two laser beams at the same time.This paper analysis the cause of this phenomenon from the perspective of the device,. Because of device fabrication process, the generation effect and maintaining effect of circularly polarized light was not ideal in the laser, thus the compitition of the two laser beam was introduced within the laser, which leaded to the unstability of the beams.