By Chang Sun
All-fiber supercontinuum (SC) laser source has important applications in various fields such as laser radar, fiber communications, biomedical imaging and spectroscopy. In particular, hundreds of milliwatts SC source can be applied in biomedical imaging like fluorescence lifetime imaging, spectroscopy and optical coherence technology because it can improve the resolution of the image compared with other optical sources. In these areas, requirements of hundreds of milliwatts SC source are focused on bandwidth and flatness of spectrum, while the power in general application remains only hundreds of milliwatts. Currently, however, a supercontinuum that meets such requirements is reported rarely, because the nonlinear effects in PCF (photonic crystal fiber) causing supercontinuum generation is not easily produced at low power.
A solution that we are presently researching could solve the above problem. The all-fiber laser system uses SESAM (semiconductor saturable absorber mirror) as mode-locked device to compose linear mode-locked resonator. Through selecting appropriate type of SESAM and increasing the cavity length, ultra-short pulse with high peak power is achieved. At low power, high peak power pulse can still cause high nonlinear effect in PCF (photonic crystal fiber) to broaden the bandwidth and optimize the flatness of supercontinuum spectrum. Then the achieved ultra-short pulse is propagated into the designed PCF to generate a supercontinuum source with output of 307mw, bandwidth spanning at least 500nm~1700nm and flatness below 15dB. This result can meet many applications in biomedical imaging.