Monolithically Integrated Optical Phase Lock Loop for Microwave Photonics

Abstract

We present a review of the critical design aspects of monolithically integrated optical phase lock loops (OPLLs). OPLL design procedures and OPLL parameters are discussed. A technique to evaluate the gain of the closed loop operating system is introduced and experimentally validated for the first time. A dual-OPLL system, when synchronised to an optical frequency comb generator without any prior filtering of the comb lines, allows generation of high spectral purity signals at any desired frequency from several GHz up to THz range. Heterodyne phase locking was achieved at a continuously tuneable offset frequency between 2 and 6 GHz. Thanks to the photonic integration, small dimensions, and custom-made electronics, the propagation delay in the loop was less than 1.8 ns, allowing good phase noise performance with OPLLs based on lasers with linewidths less than a few MHz. The system demonstrates the potential for photonic integration to be applied in various microwave photonics applications where narrow-bandwidth tuneable optical filters with amplification functionality are required.

Optical phase lock loop as high-Q filter for optical frequency comb line selection

“…The RF power level at the input to the phase detector can be variable and precisely adjusted by the variable gain of the RF amplifier. These changes, according to the OPLL theory [7], have an effect on the OPLL bandwidth which can be observed and measured on the electrical spectra of the heterodyne signal, as demonstrated in Fig. 4.…”

“…However, big frequency offset between the two lasers may not constitute a major requirement for many applications. For instance, when the OPLL is meant to be used to select one of the multiple lines of the OFCG, the highest frequency offset does not need to be greater than a half of the frequency spacing between the comb lines [7]. It is worth reminding that many combs have their lines separated by a dozen or so GHz, which is often limited by the frequency bandwidth of the modulator and the driving synthesizer.…”

Coherent frequency tuneable thz wireless signal generation using an optical phase lock loop system

“…Even thought, the continuous tuneability of the offset frequency can be achieved up to 12 GHz in the OPLL sub-system the maximum offset frequency required equals the half of the frequency spacing between the comb lines. The instability of locking the slave laser will occur if the offset frequency falls exactly in middle between the comb lines [12]. In our case, the frequency comb lines are spaced by 15 GHz.…”

Monolithically integrated optical phase lock loop with 1 THz tuneability

“…The OPLL is realised using a foundry fabricated photonic integrated circuit (PIC) and commercially available RF components, which significantly reduce the cost of the OPLL sub-system. Two such OPLLs could be used to select two lines of an optical comb [6], so that a high-purity and broadly tuneable GHz to THz signal could be generated through photomixing in a broad bandwidth photodiode [7]. The advantage of such a photonic-based mm-and THz-wave generation is that broad frequency tuneability of the laser is directly translated onto tuneability of the electrical signal which is generated through heterodyning.…”