May 17, 2016
Nippon Telegraph and Telephone Corporation
Tokyo Denki University
Nippon Telegraph and Telephone (NTT) Corporation (Chiyoda-ku, Tokyo, Japan; Hiroo Unoura, President and CEO) and Tokyo Denki University(Adachi-ku, Tokyo, Japan; Hiroshi Yasuda, University President)has succeeded in greatly reducing the noise of commercially available microwave and millimeter-wave generators by using a laser source “optical ruler”, or optical frequency comb*1. With the optical frequency comb used as a high-sensitivity detector the noise of 25-GHz millimeter-wave signal is, for example, reduced to -110 dBc/Hz at a 1-kHz offset frequency from the center frequency. This noise value is one-hundred times lower than the lowest noise in commercially available microwave and millimeter-wave generators. Our method should contribute to a high-speed and high-capacity wireless communications.
This achievement is reported in Scientific Reports in May 17, 2016.
This work was supported by JSPS KAKENHI Grant Numbers 24360143 and 26286067.
In the advanced information society, microwave and millimeter-wave signal generators play an important role. They are widely used in electronic devices, such as those for wireless communications and radar measurements. Recently, a demand has arisen for low-noise microwave and millimeter-wave signals in radar measurements, wireless communications, and high-precision spectroscopy. Commercially available microwave and millimeter-wave signal generators are based on a quartz oscillator (fundamental frequency of 10 MHz). Therefore, the noise from them increases as the output frequency increases. For example, the noise at 1 GHz becomes10, 000 times higher than that at 10 MHz.
Reducing the noise in microwave and millimeter-wave signals requires both noise-detection and feedback techniques. GPS signal, which is widely used for car navigation systems, is a high-accuracy microwave with a frequency of 10 MHz. However, the several tens of gigahertz signal needed for some applications has a lot of noise. NTT Basic Research Labs (NTT BRL) focused on an optical frequency comb*1, which acts as an optical ruler and developed one with 25-GHz mode spacing (Fig. 1). Most frequency combs in use today are based on a mode-locked laser. In contrast, ours is based on an electro-optic modulator (EOM)*2, which is advantageous for easily generating an optical pulse train at more than 10 GHz and for variable repetition rates. However, a disadvantage is that the noise increases and the spectral linewidth widens with distance from the frequency comb’s center wavelength (Fig. 2).
We demonstrated that an EOM frequency comb can work as a noise booster and a high-sensitivity detector for detecting the magnified noise from microwave and millimeter-wave signal generators. Our method can generate low-noise and continuously frequency-variable microwave and millimeter-wave signals. In addition, we can overcome the disadvantage of the electro-optic-modulator-based frequency comb (Fig. 3). The noise in 25-GHz millimeter-wave signals can be reduced to -110 dBc/Hz*3 at a 1-kHz offset frequency from the center frequency. The noise with our method is one-hundred times lower than the lowest noise reported for commercially available signal generators. Even lower noise will be achieved if we use a reference laser that is further away from the center wavelength of the semiconductor laser. In addition, the output frequency of microwave and millimeter-wave signal generators can be expanded for their universal use. We succeeded in generating low-noise signals at continuously variable frequencies over the range from 6 to 72 GHz.
In this study, we generated a short optical pulse train because our method required an optical frequency comb with a broadband spectrum. First, the output light from a semiconductor laser (linewidth of 800 Hz) was modulated with six EOMs at 25-GHz modulation frequency and the spectral bandwidth was increased. The short optical pulses were generated by controlling the velocity of each wavelength of the optical frequency comb with an optical fiber. To further shorten the pulse laser, we used a technique to increase spectral bandwidth step by step in an optical amplifier. As a result, we successfully generated a 200-fs (fs: 10-15 s) pulse laser, and then we were able to generate broadband spectrum with highly nonlinear fiber.
The interference signal between an optical frequency comb and reference laser with narrow linewidth (i.e. low noise) includes information about the noise from microwave and millimeter-wave signal generators. The noise information is converted into an electrical signal and detected with high sensitivity. Finally, by using the electrical signal, the noise from microwave and millimeter-wave signal generators is reduced with a feedback circuit (Fig. 3). The spectral linewidth in the EOM frequency comb reflects the noise from the microwave and millimeter-wave signal generator. The further the wavelength is away from the center wavelength of the semiconductor laser, the wider the spectral linewidth becomes. We demonstrated that our method can greatly reduce noise if we use an optical frequency comb that is far away from the center wavelength and detect the noise from a microwave and millimeter-wave signal generator (Fig. 4). Figure 4 shows the noise in 25-GHz millimeter-wave signals when we use the 10th and 278th optical frequency comb modes from the center wavelength of the semiconductor laser (0th optical frequency comb). When we use the 278th optical frequency comb, the noise with our method becomes much lower than the lowest noise reported for commercially available signal generators (GPS synchronization). Furthermore, the noise in 25-GHz millimeter waves can be reduced to -110 dBc/Hz at 1 kHz offset frequency when we use a narrow linewidth laser (linewidth of 1 Hz) instead of the semiconductor laser. Our simulation shows that our method has a potential to generate microwave and millimeter-wave signals with 10,000 times lower noise.
We generated microwave and millimeter-wave signal at continuously variable frequencies with voltage-controlled oscillators (VCO) *4. By using one VCO at 25 GHz and another with a wider variable rage, the microwave and millimeter-wave signals can be generated over the entire range from 6 to 72 GHz. Our method shows that the noise from 6 to 72 GHz can be greatly reduced (Fig. 5).
The noise in microwave and millimeter-wave signal with our method is one-hundred times lower than that in commercially available microwave and millimeter-wave generators. In the near future, we will demonstrate the generation of the microwave and millimeter-wave signal with 10,000 times lower noise by using a reference laser that is much further away from the center wavelength of the semiconductor laser. We will continuously improve our method toward high-speed and large-capacity wireless telecommunications and high-accuracy time and frequency synchronization.
A. Ishizawa, T. Nishikawa, T. Goto, K. Hitachi, T. Sogawa, and H. Gotoh, “Ultralow-phase-noise millimetre-wave signal generator assisted with an electro-optics-modulator-based optical frequency comb” Scientific Reports (2016)
Nippon Telegraph and Telephone (NTT) Corporation
Science and Core Technology Laboratory Group, Public relations
Tokyo Denki University
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