2016/05/17

- Toward high-speed and large-capacity wireless telecommunications with high-accuracy microwave and millimeter waves -

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Background

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, 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), 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.

Achievement

We demonstrated that an EOM frequency comb^*2 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.

Technical Features

1. Broadband light generation with a 25-GHz optical pulse train [NTT and Tokyo Denki University]
   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.
2. Noise reduction of microwave and millimeter-wave signal generators [NTT]
   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).

Future Plans

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.

Publication information

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)

Glossary

*1 ... Optical frequency comb
The optical frequency comb, for which Prof. Theodor Hモ渡sch and Dr. John Hall received the 2005 Nobel Prize in Physics, has been applied in a wide range of fields from national standards of length to astronomy. A pulsed laser has regularly spaced line spectra in the frequency domain. These spectra look like a comb or lines on a ruler; hence, they are referred to as an optical frequency comb or optical ruler (Fig. 1).

*2 ... EOM frequency comb
An optical frequency comb uses a mode-locked titanium sapphire laser, mode-locked erbium fiber laser, or EOM laser as a light source. The widely used mode-locked laser has a fixed resonant cavity. Therefore, the repetition rate is low at about 100 MHz and is almost fixed. On the other hand, an EOM is an optical device that can change the phase of a laser source. The EOM frequency comb can easily generate an optical pulse train at several tens of gigahertz and provides variable repetition rates because it does not need a resonant cavity (Fig. 22).

*3 ... Noise unit [dBc/Hz]
縲oise in microwave and millimeter-wave signal is expressed in dBc/Hz. The value is normalized in 1-Hz bandwidth at the offset frequency away from the signal frequency.

*4 ... Voltage-controlled oscillator
A voltage-controlled oscillator (VCO) is an electric oscillator whose oscillation frequency is controlled by a voltage input. Since the diode capacitance is controlled by the voltage across the diode, the resonant frequency can be changed and the VCO frequency becomes variable.