Measurement of Second Harmonic Distortion of the Detected Signal in an IM-DD Fiber Optic Link when the input IM is second harmonic contaminated

Abstract

Abstract-In this letter, we have measured a second harmonic distortion of the detected optical signal in an IM-DD fiber optic link when the input intensity modulation RF signal contains a small amount of second harmonic distortion. The plot of output vs. input second harmonic distortion is linear. The output second harmonic distortion is smaller than the input second harmonic distortion. This is due to the existence of a small negative slope in the modulation response of the FP laser diode over the 100MHz÷200MHz frequency band.With the rapid development of an optical communication system in the 21 st century, the research on high speed optical modulation and detection is being carried out all over the globe. The simplest, widely used and commercially viable fiber optic communication is the IM-DD communication. The modulation format used here is the intensity modulation (IM) and the detection scheme employed is the direct detection (DD) in a photodiode. In any communication system, message signal distortion at the output is an important parameter for assessing the reliability of the communication link.Survey of relevant literature indicates that message signal distortion related studies have been done by some authors [1][2][3][4][5]. Investigation of harmonic distortion, intermodulation distortion , signal clipping and effect of RIN on subcarrier multiplexed IM-DD optical link has been carried out by some authors [1][2][3]. Harmonic distortion and intermodulation distortion in direct modulation arises from the nonlinearity of light power vs. Laser diode (LD) bias current characteristics (L-I curve). No investigation seems to be carried out when the input RF IM signal is itself second harmonic contaminated.The experimental set up using a laser test bench supplied by Thorlabs is shown in Fig. 1 (optical power meter: PM100D, pigtailed LD mount: LM9LP, PD detector: DEC01CFC).The Fabry-Perot laser diode (FPLD) lasing at the central wavelength of 1557.1 nm operates at 23 o C which is maintained by a TEC controller (ITC4001). The DC bias current (I dc ) and the RF modulation current (I mod ) are applied to the LD through a bias tee. The LD output is intensity modulated (IM). The intensity modulated lightwave is fed to an InGaAs photodiode (PD) having a bandwidth of 1.2GHz. The PD output current is proportional to the input optical power. The output of the PD is connected to a load resistance R L . The IM lightwave is detected by a PD and its power spectrum is displayed in an Agilent microwave spectrum analyzer (E4403B, 9kHz-3 GHz). Input and output second harmonic distortion can be measured using this spectrum analyzer. The fundamental modulation frequency is 100MHz and the second harmonic is at 200MHz. The 1557.1nm wavelength falls in the C-band (1525nm÷1565nm) of the IR spectrum. Fiber loss is minimal at this wavelength. Also, optical amplifiers like EDFA are available at this wavelength and DWDM communication can be implemented over this band. Since 1557.1nm wavelength falls in the telecommunication band, message sig...