微波产生

• 为原子频率参照提供低相位噪声微波信号
• 长期高稳定性及可靠性
• 对雷达，遥感，通信，导航以及高速电子器件也十分重要
• 频率梳使得超低噪声微波信号产生成为可能

具有长期稳定性和可靠性的低相位噪声微波信号对于当今的原子频率参考是非常重要的，因为询问振荡器的相位噪声可影响到原子钟的稳定性（迪克协同效应）。性能更加优异稳定的微波源对于雷达遥感技术，通讯及导航，高速电子器件，超长基线干涉技术，以及高精度时间参考分配和同步等都有十分重要的意义。

直到最近，能够达到低噪声表现的最好的微波源是超低噪声石英振荡器或低温蓝宝石振荡器。后者的噪声比石英振荡器小几个数量级，然而由于必须在低温物理状态下进行操作，这大大地提高了实现的复杂性以及类似微波源的拥有成本。

通过使用频率梳作为“光学分频器”，可以将超稳光学振荡器的稳定特征延展至微波范畴，这样的振荡器包括高精细度腔体稳定激光等。频率梳的一条梳状线被相位锁定至腔体稳定激光，相应重复率的谐波即可被探测到。由于同腔体稳定激光锁相，频率梳的重复率与激光信号在相位上相干，然而从频率上却是其10⁵-10⁶分之一 。通过该方法可获得极低的相位噪声。

掺铒光纤频率梳的坚固性及易用性有利于实现紧凑型可移动微波源。由于掺铒光纤光梳在电信波段操作，因此他们也有利于大规模脉冲分布。利用光纤频率梳已经实现低至−111 dBc / Hz（11.55GHz载波，1Hz偏移）或−175 dBc ∕ Hz（10GHz载波，10MHz偏移）的相位噪声^[1,2]。光脉冲交错是实现10GHz光纤频率梳重复率的稳定有效的方法，在这种方法中，光脉冲串被分开，加以半周期延迟后重新组合。该技术增加光检测微波信号的信号强度，从而提高信噪比^[2,3]。

通过光学手段产生的微波信号在将来的低噪声应用中将会起到重要的作用。如今，铯喷泉钟，全球时间参考标准UTC（协调世界时），已经转向利用频率合成手段产生的"光学微波信号"作为原子钟信号询问^[4]。图示为德国布伦瑞克联邦物理技术研究院（PTB）的铯喷泉钟。

• Generating low-phase-noise microwave signals for atomic frequency standards
• High long-term stability and reliability
• Also important for radar, remote sensing, communications, navigation and high-speed electronics
• Frequency combs enable generation of microwave signals with ultra-low noise

Low-phase-noise microwave signals with high long-term stability and reliability are very important for today’s atomic frequency standards where the phase noise of the interrogation oscillator can affect the stability of the clock (Dick effect). But also technologies like radar and remote sensing, communications and navigation, high-speed electronics, very long baseline interferometry, and high-precision timing distribution and synchronization benefit from better and more robust microwave sources.

Best microwave sources

Until recently, the best microwave sources achieving low noise performance were ultralow-noise quartz oscillators or cryogenic sapphire oscillators. The latter one is outperforming the quartz oscillators by orders of magnitude. But it comes with the need to operate at cryogenic temperatures, which significantly increases the complexity and the cost of ownership of such sources.

Using a frequency comb as "optical frequency divider" one can transfer the stability of ultra-stable optical oscillators like high finesse cavity-stabilized lasers from the optical regime into the microwave regime. A comb line of the frequency comb is phase locked to the cavity-stabilized laser and a harmonic of the repetition rate is detected. Because of the phase lock, the repetition rate of the frequency comb is phase coherent with the signal from the cavity-stabilized laser but divided down in frequency by a large factor on the order of 10⁵-10⁶. This method achieves an exceptionally low phase noise.

The robustness and ease of use of Er:fiber frequency combs is advantageous for the implementation of compact, mobile microwave sources. Since Er:fiber combs operate in the telecom wavelength range they are also beneficial for largescale pulse distribution. Values as low as −111 dBc / Hz at 1 Hz offset from a 11,55 GHz carrier or −175 dBc ∕ Hz at 10 MHz offset from a 10 GHz carrier have been achieved with fiber combs^[1,2]. Optical pulse interleaving, where an optical pulse train is split and recombined with a half-period delay, is a stable and efficient method to achieve repetition rate multiplication of fiber frequency combs to values on the order of 10 GHz. This technique increases the signal level of the photodetected microwave signal and thereby improves the signal to noise level^[2,3].

Today's use

Optically generated microwave signals will play an important role in many future low noise applications. Today, microwave signals used for the interrogation of some primary cesium fountain clocks, which contribute to the worldwide reference time scale UTC (Coordinated Universal Time) have already been migrated towards frequency synthesis schemes based on “optical microwaves”^[4]. The picture above shows the cesium fountain clocks operated at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany.