We propose a new LD-fiber-coupling-method for bidirectionally-pumped erbium-doped fiber amplifier (EDFA) using
two mutually-pumped phase-conjugate mirrors (MPPCMs). MPPCM is formed by two mutually incoherent beams in
photorefractive crystal (PRC) and can convert each incident beam into each phase-conjugate beam of them. Owing to
mode conversion by phase-conjugate characteristics, the coupling loss due to mode mismatching between a pump beam
and a fiber is improved. Therefore, precision lens and lensed fiber for mode matching are not required. Additionally,
because photorefractive index gratings that compose a MPPCM are rewritable in real time, MPPCM can dynamically
adapt to a variety of system error factors, such as misalignment of components, mode conversion generated by LDs or
fibers with defects. In our system, the overall coupling efficiency of pump beams is mainly determined by diffraction
efficiency of MPPCMs that depends on the photorefractive coupling strength and the intensity ratio of two incident
beams. However, achievable diffraction efficiency in this system is restricted by optical loss between two MPPCMs.
This is because the transmitted pump beams passing through the EDF are reused for forming the opposite MPPCM. To
achieve a high signal amplification gain, absorption loss of pump beam powers in the EDF is unavoidable. Therefore,
we analytically and experimentally evaluate the acceptable loss between two PRCs to form both MPPCMs.
In the bidirectional pumped Erbium Doped Fiber Amplifier (EDFA), we propose the new method with two Mutually Pumped Phase Conjugate Mirrors (MPPCMs) to couple the pump beam into the fiber. This method makes it possible to couple the pump beam into the propagation mode of the fiber without high precision adjustment by the characteristic of MPPCM. Additionally, high precision readjustment is not required when a LD, fiber or lens is exchanged due to failures. In our method, it is important to obtain high diffraction efficiency of MPPCM for high coupling efficiency between the pump beam and the fiber. Diffraction efficiency of MPPCM depends on the intensity ratio of two incident beam intensities inside the PRC. We analyze the coupling efficiency for the intensity ratio of two pump beams, and show the optimum intensity ratio of two pump beams. Then, we perform an experiment to couple the pump beam (Ar<sup>+</sup> laser 514.5nm) to the fiber (multimode fiber of 62.5μm core diameter). In the experiment, coupling efficiency higher than 35% was confirmed. Moreover, in our method, the time to complete the coupling depends on the beam intensities in the early stage of the process. Therefore, we also propose the 4f imaging system to achieve a faster coupling at the early stage, and design the optical system to improve the time to complete the coupling in our method.