Atsushi Onae

Long-term measurement of optical frequencies using a simple, robust and low-noise fiber based frequency comb

We have developed a fiber-based frequency comb system consisting of a simple mode-locked fiber laser and a backward pumping amplifier combined with a highly nonlinear fiber with a short zerodispersion wavelength. As a result, the signal to noise ratio of the obtained carrier-envelope-offset frequency beat is larger than 45 dB at a bandwidth of 100 kHz. Furthermore, we have succeeded in measuring the optical frequencies of a 1542-nm acetylene-stabilized laser and a 532-nm iodinestabilized Nd:YAG laser continuously for more than one week using the fiber-based comb system. The long-term measurement revealed that the frequency stability of the iodine-stabilized laser was 5.7 x 10(-15) with 100 000 s averaging.

A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator.

We demonstrate that fiber-based frequency combs with multi-branch configurations can transfer both linewidth and frequency stability to another wavelength at the millihertz level. An intra-cavity electro-optic modulator is employed to obtain a broad servo bandwidth for repetition rate control. We investigate the relative linewidths between two combs using a stable continuous-wave laser as a common reference to stabilize the repetition rate frequencies in both combs. The achieved energy concentration to the carrier of the out-of-loop beat between the two combs was 99% and 30% at a bandwidth of 1 kHz and 7.6 mHz, respectively. The frequency instability of the comb was 3.7x10(-16) for a 1 s averaging time, improving to 5-8x10(-19) for 10000 s. We show that the frequency noise in the out-of-loop beat originates mainly from phase noise in branched optical fibers.

Broad-spectrum frequency comb generation and carrier-envelope offset frequency measurement by second-harmonic generation of a mode-locked fiber laser

Frequency comb spanning more than one octave has been achieved by injecting the second harmonic generation (780 nm) of a mode-locked fiber laser (1.56 /spl mu/m) into a photonic crystal fiber. We propose and realize a novel interferometric scheme for observing the carrier-envelope offset frequency.

Frequency metrology with a turnkey all-fiber system

The repetition-rate and carrier envelope offset frequency of a turnkey, all-fiber-based continuum generator are phase-locked to a highly-stable atomic clock, H-maser. The performance of the system is evaluated and compared to a traditional Ti:sapphire-based comb.

Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm

The absolute frequency of an acetylene-stabilized laser is measured using femtosecond combs based on mode-locked Ti:sapphire and fiber lasers. The acetylene-stabilized laser serves as an important optical frequency standard for telecommunication applications.

Phase-locked widely tunable optical single-frequency generator based on a femtosecond comb

We present an arbitrary optical single-frequency generator based on a femtosecond optical frequency comb. The functions of this device are comparable to those of a radio-frequency synthesizer. However, this device operates at hundreds of terahertz. The absolute frequency accuracy of this synthesizer is approximately 1 kHz at a 282 THz carrier frequency. The stability is approximately 2 x 10(-14) at 100 s, and the tuning speed exceeds 30 GHz/s. This source demonstrates the integration of a phase-locked optical comb into a versatile and easy-to-use system for the generation of tunable, absolute optical frequencies. By using downconversion, one could generate tunable terahertz frequencies that are phase locked to a microwave reference, such as a Cs atomic clock, and high-precision interferometry could benefit greatly from the stability and accuracy of this widely tunable source.

Optical frequency link between an acetylene stabilized laser at 1542 nm and an Rb stabilized laser at 778 nm using a two-color mode-locked fiber laser

Abstract We have demonstrated optical frequency link of two frequency-stabilized laser diodes operating at 1542 nm (a rotational-vibrational line of isotope acetylene molecule) and 778 nm (an Rb two photon transition) using a two-color mode-locked fiber laser. The laser produces a frequency comb (300 fs pulse) in the 1560 nm region and, through the second harmonic generation (SHG) process, another frequency comb (100 fs pulse) in the 780 nm region. Beat note signals have been observed between each of the stabilized laser radiation and a mode of the mode-locked fiber laser in the vicinity of the stabilized laser frequency. The S/N ratios were 25 dB and 30 dB with 100 kHz resolution bandwidth for beat notes at λ=1542 nm and λ=778 nm, respectively. Preliminary result has shown the potential of the system for frequency measurements with the accuracy of below 100 kHz. This limitation will be overcome by improving the frequency counting system using a phase-locked tracking oscillator and installing a PZT element in the fiber laser cavity. Supposing the accuracy of the mode spacing is similar to the case of a mode-locked Ti:sapphire laser, the accuracy of kHz level is very presumable.

Towards an accurate frequency standard at 1.5 /spl mu/m based on the acetylene overtone band transition

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s.

Narrow linewidth comb realized with a mode-locked fiber laser using an intra-cavity waveguide electro-optic modulator for high-speed control

We have developed an optical frequency comb using a mode-locked fiber ring laser with an intra-cavity waveguide electro-optic modulator controlling the optical length in the laser cavity. The mode-locking is achieved with a simple ring configuration and a nonlinear polarization rotation mechanism. The beat note between the laser and a reference laser and the carrier envelope offset frequency of the comb were simultaneously phase locked with servo bandwidths of 1.3 MHz and 900 kHz, respectively. We observed an out-of-loop beat between two identical combs, and obtained a coherent δ-function peak with a signal to noise ratio of 70 dB/Hz.

Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer.

We report on a displacement metrology setup that provides sub-pm resolution in air. The setup is based on a Fabry-Perot cavity. However, unlike current Fabry-Perot cavity based displacement setups we incorporate a novel fs-laser based arbitrary wavelength synthesizer that provides efficient suppression of atmospheric disturbances while providing very wide and precise tuning of the output wavelength. The wavelength synthesizer provides sub-10 attometer wavelength resolution. The setup provides subpm length stability for integration times of up to one minute and sub-10 pm for up to half an hour without airtight enclosure of the Fabry-Perot cavities.