The present invention provides an apparatus and method for high resolution spectroscopy using a narrow light beam source such as a superluminescent diode (SLD) and a tunable optical filter (TOF) for analyzing a formation fluid sample downhole and at the surface to determine formation fluid parameters. The SLD and TOF have a matching etendue. The analysis comprises determination of gas oil ratio, API gravity and various other fluid parameters which can be estimated after developing correlations to a training set of samples using a neural network or a chemometric equation.