Optoelectronic oscillator controlled by photodiode-based optoelectronic chromatic dispersion and FBG integration

. High Optoelectronic Chromatic Dispersion in Ge PN-type photodetectors affects the output of the Optoelectronic Oscillator. This is utilized to achieve high sensitivity wavelength monitoring and strain sensing. The sensitivity is enhanced for higher oscillating mode numbers and lower cavity lengths.


Introduction
Commercial PN photodiodes can exhibit a very large effective chromatic dispersion, known as optoelectronic chromatic dispersion (OED).The OED sensitivity of PN photodetectors is measured with sinusoidal modulated light and is defined as the wavelength-dependent change in RF phase-shift   = /.It has been shown to be dependent on an absorption spectrum parameter of the semiconductor:  −1 (  ⁄ ).We have demonstrated that germanium possesses a huge   value in the C-and Lband owing to a very large value of the absorption parameter [1].For example, with a 4 MHz sinusoidal illumination, a commercial Ge PN photodiode (GPD Optoelectronics, GM3) exhibits a wavelength-dependent RF phase-change of 1 / in the telecommunication C-band.To achieve comparable dispersion in SMF28 optical fiber would require 400  in length [2].As a result, PN photodiodes have potential in high-sensitivity wavelength monitoring and spectral sensing.By integrating a Ge PN photodiode with an FBG interrogation system, a wavelength-shift resolution   = 1.25 /√ was achieved.Furthermore, the OED sensitivity of the photodetector was enhanced by RF interferometry-based phase-shift amplification of  = 410 4 , resulting in femtometer-resolution wavelength monitoring [3].OED was also utilized for high sensitivity spectral sensing of ethanol in water [4].
In this work we show that the high OED in the photodiode affects the operation of the Optoelectronic Oscillator (OEO).Besides altering the resonant frequencies of the oscillator, we demonstrate applications in wavelength sensing and FBG interrogation.

Theory
Fig 1 is a schematic of a single-cavity OEO.The feedback loop can generate self-sustained oscillations if its overall gain is larger than the loss and the circulating waves add up constructively in phase.While the former condition can be achieved by adding an electrical/optical amplifier to the system, the latter can be achieved by controlling the phase using a long fiber delay line [5].The Q-factor of a long fiber-delay line is given as  =   , where   is the oscillator frequency and the τ is the delay given by / with  being the optical length of the fiber and  the speed of light.The relative spectrally-dependent time delay in a fiber is given as  =  (where  is the dispersion due to the fiber).We show that by the addition of a high OED Ge-PN photodetector in the OEO system, the overall time delay is modified to  = ( +   ) (where   is the OED dispersion from the PN photodetector).

Experiment
We measured the m=1 frequency for two different photodiodes.According to equation ( 7), a high OED photodetector in the OEO cavity will give an additional dispersion.A commercial Ge PN-type biased photodetector (Thorlabs, PDA 50B-EC) operated at zero gain was used.With increasing wavelength, the primary resonant frequency in the OEO cavity decreases.However, no significant change in the resonant frequency is observed when a low OED InGaAs PIN photodiode is integrated in the OEO cavity.These results are shown in Fig. 2. In the next experiment, the fiber cavity length was 5 , and the fundamental oscillating mode displays a decrease in RF frequency shift of approx.218 / for increase in wavelength, as plotted in Fig. 3a.This response is inversely proportional to the fiber length, as seen in Fig. 3b, as predicted by theory.In a third experiment, an FBG illuminated with broadband LED light is inserted in place of the tuneable laser in Fig 1 , and the reflected signal of the FBG acts as the optical source of the OEO.The axial strain applied on the FBG can be measured as the change in frequency of the oscillations generated in the OEO loop.We measured an RF frequency shift of 0.2645 / on the fundamental oscillating mode, which gave a minimum strain resolution of 38 .Higher modes and shorter cavity lengths can enhance the sensitivity to the sub-microstrain regime.This improved strain resolution can be achieved by incorporating appropriate RF filters.

Conclusion
In this work, we show for the first time that the photodiode OED can significantly affect the RF oscillation frequencies in an OEO system.The huge value of OED in germanium photodiodes can be utilized for a variety of spectral sensing applications.We demonstrate an application in FBG-based strain sensing.

Fig 2 :
Fig 2: Oscillating mode  = 1 RF frequency shift dependence on wavelength for (a) InGaAs-PIN and (b) Ge-PN photodetectors integrated with an OEO system of fiber length 1 .

Fig 3 :
Fig 3: (a) RF frequency with wavelength for mode m=1 for different cavity lengths.Longer cavity lengths show a flatter response.(b) RF frequency shift with wavelength for different modes at cavity length L=5m.