CHARACTERIZATION OF AN ERBIUM-DOPED FIBER AMPLIFIER AS A LIGHT-SOURCEAND DEVELOPMENT OF A NEAR-INFRARED SPECTROPHOTOMETER BASED ON THE EDFA AND AN ACOUSTOOPTIC TUNABLE FILTER

A novel light source for the near-infrared region which has the highes t intensity and widest spectral bandwidth of all near-IR light sources has been developed. The system is based on a single-mode fiber (about 18 m long) doped with Er3+ ion, The doped ion produces amplified spon taneous emission (ASE) in the near-IR region (from 1500 to 1600 nm) wh en it is excited by a diode laser at 980 mn, Because the diode laser i s fusion-spliced directly to the doped fiber, the system is compact, a ll-solid-state, reliable, and stable and requires little maintenance, Its ASE output intensity was found to be comparable with those of diod e lasers currently available for this near-IR region send is much high er than those of conventional halogen-tungsten lamps and the so-called (high-intensity) superluminescent light emitting diodes (SLEDs), Its spectral bandwidth is, however, much wider than those of the diode las ers and the SLEDs. Even higher intensity can be obtained from the dope d fiber when a low-intensity (1 mW) light from a 1550-nm laser diode i s introduced into the doped fiber, The intensity is enhanced (up to 7 times compared to the ASE) because the input light is amplified by the doped fiber. Furthermore, the output intensity of this erbium-doped f iber amplifier (EDFA) can be appropriately adjusted to provide relativ ely higher output intensity at any range of wavelengths (within this 1 500-1600-nm region) by simply changing the temperature and/or the driv en current of the input diode laser. Subsequently, an acoustooptic tun able filter was used to provide a means to spectrally tune the EDFA ra pidly and to develop an all-solid-state, compact near-IR spectrophotom eter which not only is very sensitive, stable, and reliable but also h as a very high throughput. This spectrophotometer can detect water in ethanol at a limit of detection of 10 ppm, More importantly, the high throughput makes it possible to use the instrument to measure spectra of highly absorbing samples (e,g,, absorption spectrum of 1.0 M Pr3+ a queous solution through four sheets of paper); measurements which are currently not possible with halogen-tungsten lamp-based spectrophotome ters.