NTT Succeeds at Frequency Stabilization of an Electro-Optic Modulation-Based Optical Frequency Comb
NTT Corporation (NTT) and Tokyo Denki University have succeeded in further stabilizing the graduated frequency of an optical frequency ruler generated by modulating the intensity and phase of laser light in an electrical signal. By using this result, it is possible to obtain a light source with a narrow linewidth whose frequency interval is 25 GHz, and it is expected to be applied 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, which is expected to improve the accuracy of microwave generation and evaluation equipment. This research was published in the British scientific journal, Scientific Reports on May 30, 2023, and was presented in an invited talk at PIERS2023, an international conference on photonics, electromagnetics and information communications, on July 3, 2023.
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Wavelength conversion using a dual-pitch PPLN waveguide (Photo: Business Wire)
Frequency-stabilized OFCs have been realized using fiber combs  and Ti:sapphire lasers . However, the typical frequency intervals (frep) 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 sidebands by feeding a CW laser  into an electro-optic modulator (EO 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 Tokyo 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.
Key Points of Technology:
Because EO combs have a low energy per pulse, they typically use long, highly nonlinear fibers to generate broadband OFC. On the other hand, this method reduces the signal-to-noise ratio of the OFC and makes it difficult to detect the CEO frequency (see Technical Detail 1 for detection methods). This time, NTT used seven phase modulators  to broaden the sidebands, and then used short, highly nonlinear fibers to generate EO combs with high bandwidth and high signal-to-noise ratio.
(2) Use of the 2f-3f self-referencing interferometer, in which the CEO frequency can be detected by an OFC in the 2/3-octave bandwidth.
In a conventional f-2f self-referencing interferometer, the OFC is broadened from the frequency f to 2f to generate a second harmonic light at the frequency of f and to receive the interference signal at a frequency close to 2f. In this 2f-3f self-referencing interferometer, the OFC is broadened from the frequency f to 3f/2, to generate a third harmonic light at the frequency of f and a second harmonic light at the frequency of 3f/2, to receive the interference signal at a frequency close to 3f. This reduces the OFC bandwidth required to detect the CEO frequency. The research group used NTT's proprietary 2f-3f self-referencing interferometer with a dual-pitch PPLN waveguides.
The stability of the EO comb was also evaluated by measuring the phase noise  of the stabilized microwave. An experimental result shows that the phase noise of microwave (frequency: frep = 25 GHz) was reduced to below the measurement limit of the highly accurate phase noise analyzer . In particular, compared to when the news was released in 2016, the noise level has reduced by less than a tenth. Based on these results, the microwave stability was estimated to be comparable to that of commercial high-precision hydrogen masers .
Furthermore, the linewidth of the EO comb was evaluated by taking the interference signal between the narrow-linewidth laser and the frequency-stabilized EO comb at a wavelength of 1397 nm. An experimental result shows that the linewidth of the 811th EO comb counted from the seed light source (frequency: fs) was 300 Hz, which is about 3 orders of magnitude smaller than the laser linewidth required for digital coherent communication. This result is considered to provide a light source for optical communication that can support even higher communication speed.
Furthermore, NTT 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. In the future, NTT aims 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 (e.g., traffic control, air traffic control, financial transactions, etc.).
2. 2f-3f self-referencing interferometer with dual-pitch PPLN waveguides:
3. Applications of optical frequency measurement:
Information on publication of papers and presentations at international conferences:
Research Title: "Optical-Referenceless Optical Frequency Counter With Twelve-Digit Absolute Accuracy"
Author: Atsushi Ishizawa, Tadashi Nishikawa, Kenichi Hitachi, Tomoya Akatsuka & Katsuya Oguri
International Conference: PIERS 2023
Announcement Title: Low-Phase-Noise Frequency-Tunable Microwave and Millimeter-Wave Generation Using an Electro-Optic-Modulation Comb"
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