In this letter we demonstrate a new kind of spectrometer capable of resolving the modes of a 1.3 &mgr;m quaternary laser diode. The device uses the nonlinear sum frequency generation of light in the semiconductor waveguide to map out the frequency spectrum of the input beam. This device was used to obtain the spectrum of a multimode Fabry–Perot 1.3 &mgr;m laser diode. Furthermore, the same device was used to observe the shift in the individual Fabry–Perot mode as well as the shift in the overall spectrum of the laser diode as a function of current. These observations indicate that the waveguide spectrometer can be useful for applications like implementing frequency division multiplexing communication systems by allowing the determination of absolute and relative frequencies of different channels in a compact fashion.