Isao Ito

Direct 15-GHz mode-spacing optical frequency comb with a Kerr-lens mode-locked Yb:Y 2 O 3 ceramic laser

A 15-GHz mode spacing optical frequency comb based on a Kerr-lens mode-locked Yb:Y2O3 ceramic laser has been developed. Individual modes were clearly resolved by a commercial spectrometer. To demonstrate the long-term operation of the optical frequency comb, a single longitudinal mode was phase-locked to a frequency-stabilized continuous wave laser and the repetition rate to a radio frequency standard. To the best of our knowledge, 15 GHz is the largest reported mode spacing (repetition rate) for both a Kerr-lens mode-locked laser and a direct femtosecond laser based-optical frequency comb.

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.

Phase-matched frequency conversion below 150 nm in KBe 2 BO 3 F 2 .

Sum frequency mixing has been demonstrated below 150 nm in KBeBO3F2 by using the fundamental with its fourth harmonic of a 6 kHz Ti: sapphire laser system. The wavelength of 149.8 nm is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm, respectively. The phase matching angles measured from 149.8 to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The measured transmission spectra of KBeBO3F2 crystals support the generation of coherent radiation below 150 nm.

Injection locking of Yb-fiber based optical frequency comb

We demonstrated the synchronization of offset and repetition frequency between two independent Yb-doped fiber mode-locked lasers by injection locking. By injecting master-laser pulse-train into slave laser cavity, stability and accuracy of master frequency comb are transferred to slave comb. Passive stabilization of frequency comb offers robust and convenient way to duplicate frequency comb that can be applied to long-distance comb transfer. Injecting master pulse would also help to initiate and stabilize mode-locking of high repetition rate or ultrabroadband frequency combs. Additionally, we also demonstrated even more robust synchronization of combs can be achieved with the help of active stabilization of relative offset frequency difference.

149.8 nm, the shortest wavelength generated by phase matching in nonlinear crystals

Narrow band light sources in the vacuum ultraviolet (VUV) region are attractive for photo lithography and high resolution photoelectron spectroscopy. Phase matching is essential to generate high power VUV lights by using a narrow band, low peak intensity and nanosecond pump source. In this research, sum frequency mixing has been demonstrated below 150 nm in KBe2BO3F2 by using the fundamental with its fourth harmonic of a 6 kHz Ti:sapphire laser. The laser system we have developed in this research, consists of a Ti:sapphire laser system and a frequency conversion stage. We generated 149.8-nm radiation, which is the shortest wavelength ever obtained to our knowledge by phase matching in nonlinear crystals. The fifth harmonic output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm, respectively. The phase matching angles measured from 149.8 nm to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The optical transmission spectra of some KBBF crystals were measured by the spectrophotometer. The transmittance near the absorption edge supports the generation of coherent radiation below 150 nm. The improvement of a prism-coupled device contributed to the generation of coherent radiation below 150 nm. Another reason for the present break through to the shorter wavelength is the use of the short pulse driving source compared with our previous research.

Status of R&D efforts toward the ERL-based future light source in Japan

The energy recovery linac is a very promising synchrotron light source in future. We are contemplating to realize a ERL_based next generation light source in Japan, under a collaboration between KEK, JAEA, ISSP, and other SR institutes. To this end, we started R&D efforts on its key technologies, including a low-emittance photocathode gun and superconducting cavities. We also plan to assemble these technologies into a small test ERL, and to demonstrate their operations. We report our R&D status.

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.

Development of Copper Coated Chamber for Third Generation Light Sources

We describe the development of the copper coated chamber, which is suitable for the 3rd generation light sources. This chamber aims to reduce the resistive-wall impedance[1]. However, this coating might affect the ultra-high vacuum condition worse. In order to check the validity of this chamber for the ultra-high vacuum condition, we have produced the copper coated chamber and built the test bench. The measured outgassing from the inner surface of the newly developed copper coated chamber is sufficiently small to utilize in ultra-high vacuum condition.

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.