Integrated optoelectronic oscillator frequency tuning based on a variable optical time delay line in a loop-back

An optoelectronic oscillator, a simple and cost-effective electro-optical system, is known for its ability to generate ultra-high-frequency signals with an ultra-low phase noise level. This system can produce high-frequency signals up to microwaves, often surpassing their electric analogs in these parameters. Some scientific groups have even developed optoelectronic oscillators based on integrated photonics to enhance size and mass characteristics. However, these generators do have their drawbacks, including long-term frequency instability and a significant frequency step (about tens of kilohertz). The first drawback is due to the high sensitivity of optoelectronic oscillators to environmental parameters, particularly ambient temperature. While the thermal stabilization problem of photonic integrated circuits has been addressed, stabilizing the output frequency and its control in integrated optoelectronic oscillators remains a relevant and ongoing challenge in the field. The paper explores the potential for controlling the output frequency of an optoelectronic oscillator by adjusting the optical delay line in the feedback loop. We introduce a scheme for an optoelectronic oscillator based on the silicon-on-insulator platform and provide a mathematical model for the oscillator with a tunable optical delay line in the feedback loop. The tuning step for the output frequency is 50 MHz/ps, calculated analytically. We conducted numerical simulations using Ansys Lumerical Interconnect software to verify the mathematical model. The results show that the output frequency can be tuned in the range of 4.28 GHz, with a frequency step ranging from 30 to 60 MHz/ps. The maximum frequency deviation from the simulation is 20 MHz, approximately 0.2% of the output frequency. The obtained results demonstrate the effectiveness of the proposed frequency control method for various applications of the optoelectronic oscillator. The linear mathematical model we have developed can be used to calculate the output frequency of the optoelectronic oscillator while accounting for nonlinear errors. However, creating a comprehensive mathematical model that considers the nonlinear effects of integrated photonics will require further elaboration and development in future studies. The frequency tuning approach we have presented can be implemented using discrete optical components. However, in this case, the minimum frequency tuning step is limited by the discrete time delay in the optical delay line. On the other hand, an integrated optical delay line offers similar time delay control. This time delay tuning provides additional flexibility for controlling the output frequency within the optoelectronic oscillator eigenfrequencies, which are constrained by the frequency response of the notch filter.