Development of a 7kW pulse light source for single mode fibers having 1.55μm band output

摘要

In recent years, the quantity of information to be transmitted through optical fibers is growing, increasing the use of the wavelength division multiplexing (WDM) system. In an WDM transmission, transmission lines and optical parts are more likely to be damaged by the increase of the optical power from the pulse beams brought on by the large quantity of information, compared to conventional fiber transmission. Presently, reliability of the damaged optical parts is evaluated by inducing W level continuous wave (CW) laser beam. However, the evaluation for a single unit is time consuming, making it impossible to evaluate the entire system. To solve this problem, a high power pulse light source was developed as a light source apparatus for accelerating the generation of optical damage. For the high power pulse light source, pulse amplifying characteristics of erbium-doped fiber amplifier (EDFA) was conveniently used. The gain recovers around the generation of the pulse when a repeating pulse beam which has had ample recovery time is induced. This enables the generation of a pulse beam having peak power at the kW level. This high power pulse light source also uses optical band pass filter (OBPF), polarization controller (PC), and a new optimized EDF structure to enhance the peak power. The OBPF eliminates amplified spontaneous emission (ASE) generated between pulses in the first EDFA. Therefore the ASE amplification between pulses in the second EDFA is subdued, enabling a sufficient inverted population state. The PC controls the polarized light of a signal beam from the second EDFA output which generates high peak power. By controlling the state of the polarized light, nonlinear optical effects can be suppressed which is the cause for weakening the peak power of the signal beams around the second EDFA output. The new structure EDF uses enlarged mode field diameter (MFD) to suppress the nonlinear optical effect. With the enlarged MFD, optical pulses with equal peak power can be transmitted at a lower power density than conventional EDFs, suppressing the nonlinear optical effect in the EDF. By applying the above improvements of OBPF, PC and new structure EDF, the high power pulse light source introduced in this paper has succeeded in achieving a maximum 7kW peak power. Additionally, this high power pulse light source is a transverse single mode output from a single mode fiber which allows a high quality M{sup}2≈1 beam that can be made to have high convergence characteristics. This makes the light source ideal to be used not only in reliability evaluations for accelerated tests but also in micro machining metallic surfaces. Furthermore, the eye safe 1.55μm bandwidth signal beam allows such uses as in laser range finder and in laser radar.