Abstract
Accurately characterizing third order intermodulation distortion (IMD3) in high-linearity photodiodes is challenging. Two measurement techniques are evaluated-a standard two-tone measurement and a more complicated three-tone measurement technique to measure IMD3. A model of the measurement system is developed and used to analyze the limitations of the two techniques in determining the distortion of highly linear photodiodes. Experimental validation is provided by comparing the simulation trends with IMD3 results measured on two types of waveguide photodiodes: 1) an InP based uni-traveling-carrier (UTC) photodiode and 2) a Ge n-i-p waveguide photodetector on Silicon-on-Insulator (SOI) substrate.
Highlights
Microwave nonlinearities in photodiodes arise from the complex interaction of many different physical effects [1]
Recent efforts to develop high current photodetectors with high linearity have resulted in surface illuminated photodiodes with over 700mA of photocurrent [4] and 3rd order Output Intercept Points (OIP3) in excess of 50dBm [5]
As mentioned in the previous section, the change in input radio frequency (RF) power essentially corresponds to a change in optical modulation index, which is experimentally determined to be between 20 and 30%
Summary
Microwave nonlinearities in photodiodes arise from the complex interaction of many different physical effects [1]. An alternative to optical heterodyning is using external intensity modulators to generate two RF tones by modulating the output of two c.w. lasers This greatly simplifies the measurement setup, it introduces nonlinearities into the measurement through the intensity modulators and RF signal generators. An alternate approach to the two tone measurement technique is to use three tones to measure IMD3 [8,10]. In this technique, some of the third order non linear distortion components generated in the device under test (DUT) are independent of the harmonics originating in the optical modulators and signal generators. This establishes the three-tone measurement technique as the preferred technique for measuring very linear photodiodes
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