NTT and Denki achieve frequency stabilization of optical comb

23 Aug 2023

Novel linewidth source expected to be applied to high-speed, large-capacity optical communications.

NTT Corp. (NTT) and Tokyo Denki University, both based in Japan, have succeeded in further stabilizing the output of an optical frequency ruler generated by modulating the intensity and phase of laser light.

The partners say that, by this result, it is possible to obtain a light source with a narrow linewidth whose frequency interval is 25 GHz. This is expected to be applied usefully to high-speed and large-capacity optical communications such as next-generation digital coherent transmission.

In addition, microwaves with significantly reduced phase noise can be generated, says the team, which is expected to improve the accuracy of microwave generation and evaluation equipment. This research was published in Scientific Reports, and presented in an invited talk at PIERS2023, in Prague, Czechia, in July.

NTT and their partners have developed a detection and control mechanism for the CEO signal (fceo), which has been difficult for electro-optic modulation-based optical frequency combs.

An optical frequency comb is an optical signal that has a comb-like emission line spectrum on the frequency axis and is used for optical frequency measurement and precise frequency conversion from visible light to microwaves.

Optical communications

Frequency-stabilized OFCs have been realized using fiber combs and Ti:sapphire lasers. However, the typical frequency intervals of these OFCs are tens to hundreds of MHz, which makes them difficult to use in optical communication applications because each comb mode cannot be individually separated and controlled.

To address this problem, the research group has studied the OFC (EO comb) that generates side bands by feeding a CW laser into an electro-optic modulator. Because the frequency interval of EO combs is up to several tens of GHz, the comb modes can be individually separated and controlled, making them suitable for optical communication applications.

In addition, by selecting the microwave frequency to be applied to the EO modulator, the frequency interval can be easily changed, expanding the range of applications. On the other hand, as the EO comb moves away from the seed light source on the frequency axis, microwave noise is superimposed and the frequency becomes unstable, so the frequency must be stabilized.

NTT and Denki University have been researching technology to stabilize the frequency of EO comb. Although the previous method used two lasers with different frequencies, there was room for improvement in the stability of the EO comb due to frequency drift between the lasers.

The research group succeeded in further stabilizing the EO comb by using a single laser as the seed light source, detecting the CEO frequency (“fceo”), which had been difficult in the EO comb and feeding it back to the microwave frequency applied to the EO modulator to keep the fceo constant.

Future development

In their joint statement the partners noted, “We have demonstrated that even EO combs with a large frequency interval can stabilize the CEO frequency. In addition, EO comb linewidth is sufficiently narrower than the laser linewidth required for digital coherent communications.

“As communication speeds increase further, more precise control of the laser linewidth is required. In this study, it was possible to provide multiple light sources for optical communication in a single experimental system, and it is expected to enable high-speed, large-capacity optical communications using EO comb.”

Furthermore, NTT said it had succeeded in reducing the phase noise of microwaves to below the measurement limit of a highly accurate phase noise analyzer. This is expected to improve the accuracy of microwave generation and evaluation equipment and be used for timekeeping in places where GPS signals are difficult to reach.

NTT is aiming to further improve the frequency stability and convenience of the EO comb: “We will also develop technologies that provide high-precision microwave signals, reduce errors in position measurements and time stamps and realize technologies that contribute to areas that require real-time, accurate data, such as for road and air traffic control, and financial transactions.”