A general theoretical description of modulated laser fields and power spectra for a current modulated single-mode laser is derived, taking into account both the intensity and frequency modulation (IM and FM) of the emitted light. The theory relies on an explicit knowledge of the modulus as well as the phase of the current-to-frequency modulation transfer function for the laser. Numerical examples are presented for sinusoidal, sawtooth, and square wave modulation considering broad-band and narrow-band FM cases with various amounts of IM. The IM causes a significant distortion of the pure FM spectrum, strongly dependent on the modulus and the phase of the current-to-frequency modulation transfer function. In general, it causes the FM spectrum to become asymmetrical with a change of the relative sideband level. The theoretical results have been confirmed experimentally by Fabry-Perot interferometer measurements on a temperature stabilized CSP injection laser. In the interpretation of the measurement results, the detailed characteristics of the interferometer, and the detection system are taken into account. The measurements include narrow-band and broad-band sinusoidal modulation as well as broad-band saw-tooth and square wave modulation.