Takuma Nakamura

Offset-free broadband Yb:fiber optical frequency comb for optical clocks

We demonstrate a passively offset-frequency stabilized optical frequency comb centered at 1060 nm. The offset-free comb was achieved through difference frequency generation (DFG) between two portions of a supercontinuum based on a Yb:fiber laser. As the DFG comb had only one degree of freedom, repetition frequency, full stabilization was achieved via locking one of the modes to an ultra-stable continuous wave (CW) laser. The DFG comb provided sufficient average power to enable further amplification, using Yb-doped fiber amplifier, and spectral broadening. The spectrum spanned from 690 nm to 1300 nm and the average power was of several hundred mW, which could be ideal for the comparison of optical clocks, such as optical lattice clocks operated with Sr (698 nm) and Hg (1063 nm) reference atoms.

Stable CW laser based on low thermal expansion ceramic cavity with 4.9 mHz/s frequency drift

We describe a CW laser stabilized to a low thermal expansion ceramic cavity which has a lower frequency drift rate than cavities based on ultralow-expansion glass (ULE), which are widely used as optical references. Two identical optical cavities with spacers of different material, ceramic and ULE, were assembled and the optical frequencies locked to each of these cavities were compared. The optical frequency drifts of both CW lasers were measured to within a precision of 10-11 in one second over the course of one year. The ceramic cavity had a long-term frequency drift rate of 4.9 mHz/s and the ULE cavity had one of 23 mHz/s.

Beyond 500-kHz bandwidth piezo-electric transducers for GHz-comb applications

Lasers with high optical frequency stability have played a significant role in numerous applications including metrology and spectroscopy. A laser with better frequency stability allows shorter integration time and higher accuracy. To achieve this, one of the most crucial factors is the stability of the cavity length of the laser. The length of the cavity is always disturbed by environmental noises such as acoustic or vibrational ones, and we often utilize cavity length modulators to compensate it. Piezo-electric transducers (PZTs) and electro-optic modulators (EOMs) are commonly used for this purpose. PZTs typically work up to several kHz ranges, whereas EOMs have much broader feedback bandwidth (more than several hundred kHz) and lead to much higher stability. However, we cannot apply EOMs in many cases, such as laser cavities with high-power, high-quality factor or broadband spectrum, because EOMs are used in transmissive configurations. PZTs with a mirror can be used in such cases thanks to their reflective configurations. Therefore PZTs with broad feedback bandwidth have been in great demand. The bandwidth of a PZT is restricted by mechanical resonances lying around 10 kHz to several hundred kHz, which are caused by vibration coupling with a mirror mount. To damp these resonances, various structures of the mirror mount were demonstrated [1]. However, resonances lying over the 200-kHz region have not been damped, and the resulting feedback bandwidth has not exceeded 200 kHz as well.

Magneto-optic modulator for high bandwidth cavity length stabilization

We propose a novel magneto-optical approach for the repetition frequency stabilization of optical frequency combs. We developed a Yb:fiber mode-locked laser with a fiber-based magneto-optic modulator used to stabilize one of the longitudinal modes to an optical reference with sub-hundred mrad residual phase noise. This modulator does not induce mechanical resonances and as such has the potential to achieve much broader feedback bandwidths than conventional modulators used for cavity length stabilization.