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Mixing of two 155-μm laser waves in millimeter frequency range for optical components test

We describe a method for generating an amplitude modulated optical wave in the range of telecommunication frequencies. The principle is based on optical heterodying of two free DFB lasers. Frequency modulation of the optical wave resulting from beat is equal to the difference of the two laser wave frequencies. Control in temperature of laser sources ensure their wavelength stability. We theoretically study and also simulate the influence of the spectral width of the laser rays on spectral purity of the generated signal in a quadratic receiver. We then describe the experimental set up which permits to obtain a tunability of the source up to 275 GHz. We present the results obtained in reception by beat of two identical lasers in a 60 Ghz bandwidth photodiode. The signal generated in quadratic receiver is in terms of spectral width (FWHM) of about 37 MHz for a power level of - 35 dBm. Study of power and frequency stability of the generated signal in photodiode is carried out. This work permits to test large bandwidth photodiodes, it would also allow to characterize in bandwidth passive optical components.