Fiber-based Frequency Comb Research Articles

Hybrid mode-locked Er-doped fiber femtosecond oscillator with 156 mW output power

We report on an Er-doped fiber oscillator that produces 146 fs pulses with 156 mW average power at a repetition rate of 49.9 MHz. The pulse energy reaches 3.13 nJ, surpassing the conventional power limit in the dispersion-managed soliton regime. Such high pulse power is obtained by devising a hybrid mode-locking scheme that combines saturable absorption with nonlinear polarization evolution. The oscillator also offers excellent temporal purity in the generated pulses with high power, providing a robust fiber-based frequency comb well suited for industrial uses.

Characterizing a fiber-based frequency comb with electro-optic modulator

We report on the characterization of a commercial- core fiber-based frequency comb equipped with an intracavity free-space electro-optic modulator (EOM). We investigate the relationship between the noise of the pump diode and the laser relative intensity noise (RIN) and demonstrate the use of a low-noise current supply to substantially reduce the laser RIN. By measuring several critical transfer functions, we evaluate the potential of the EOM for comb repetition rate stabilization. We also evaluate the coupling to other relevant parameters of the comb. From these measurements, we infer the capabilities of the femtosecond laser comb to generate very-low-phase-noise microwave signals when phase-locked to a high-spectral-purity ultra-stable laser.

Low noise erbium fiber fs frequency comb based on a tapered-fiber carbon nanotube design

We report on a low noise all-fiber erbium fs frequency comb based on a simple and robust tapered-fiber carbon nanotube (tf-CNT) design. We mitigate dominant noise sources to show that the free-running linewidth of the carrier-envelope offset frequency (fceo) can be comparable to the best reported performance to date for fiber-based frequency combs. A free-running fceo linewidth of ~20 kHz is demonstrated, corresponding to an improvement of ~30 times over previous work based on a CNT mode-locked fiber laser [Opt. Express 18, 1667 (2010)]. We also demonstrate the use of an acousto-optic modulator external to the laser cavity to stabilize fceo, enabling a 300 kHz feedback control bandwidth. The offset frequency is phase-locked with an in-loop integrated phase noise of ~0.8 rad from 10Hz to 400kHz. We show a resolution-limited linewidth of ~1 Hz, demonstrating over 90% of the carrier power within the coherent fceo signal. The results demonstrate that the relatively simple tf-CNT fiber laser design can provide a compact, robust and high-performance fs frequency comb.

光コムによる精度19桁級の周波数合成

光コムによる精度19桁級の周波数合成

Time-domain optical reflectometry measurements using a frequency comb interferometer

We characterize the temporal response of fiber-optic components using a fiber-based frequency comb interferometer; measurements are compared and validated against a commercial instrument. The main advantage of the instrument lies in the absence of moving parts or a tunable laser, leading to very fast scanning. A measurement of a mechanical distortion, cycled at 130 Hz, on a fiber Bragg grating (FBG) is presented. A complete profile of the mechanical distortion is taken every 2.5 ms (400 Hz scanning speed) and each "snapshot" is taken in 200 micros. This scanning speed was arbitrarily chosen, and the instrument could be set to scan much faster, up to hundreds of kilohertz. With high-reflectivity FBGs, the same instrument could scan simultaneously the profile of 140 wavelength-multiplexed FBGs at 2 kHz.

Fiber-comb-stabilized light source at 556 nm for magneto-optical trapping of ytterbium

A frequency-stabilized light source emitting at 556 nm is realized by frequency-doubling a 1112-nm laser, which is phase-locked to a fiber-based optical frequency comb. The 1112-nm laser is either an ytterbium (Yb)-doped distributed feedback fiber laser or a master-slave laser system that uses an external cavity diode laser as a master laser. We have achieved the continuous frequency stabilization of the light source over a five-day period. With the light source, we have completed the second-stage magneto-optical trapping (MOT) of Yb atoms using the 1S0 - 3P1 intercombination transition. The temperature of the ultracold atoms in the MOT was 40 uK when measured using the time-of-flight method, and this is sufficient for loading the atoms into an optical lattice. The fiber-based frequency comb is shown to be a useful tool for controlling the laser frequency in cold-atom experiments.

Sub-100 attoseconds stability optics-to-microwave synchronization

We use two fiber-based femtosecond frequency combs and a low-noise carrier suppression phase detection system to characterize the optical to microwave synchronization achievable with such frequency divider systems. By applying specific noise reduction strategies, a residual phase noise as low as −120 dBc/Hz at 1 Hz offset frequency from a 11.55 GHz carrier is measured. The fractional frequency instability from a single optical-to-microwave frequency divider is 1.1×10−16 at 1 s averaging down to below 2×10−19 after only 1000 s. The corresponding rms time deviation is lower than 100 attoseconds up to 1000 s averaging duration.

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.

Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633 nm

A fiber-based frequency comb has been developed to measure the frequency of a 633-nm iodine-stabilized laser at the National Metrology Institute of Japan (NMIJ). The measured frequency was consistent with the previous results measured with a Ti:sapphire-based comb. The NMIJ comb was shipped to Australia for the validation of the measurement capability of the National Measurement Institute, Australia (NMIA) and NMIJ combs using a common microwave reference and a common optical frequency. Consequently, the frequency consistency of the two combs was approximately 8 times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> . Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs.

The Stability of an Optical Clock Laser Transferred to the Interrogation Oscillator for a Cs Fountain

We stabilize a microwave oscillator at 9.6 GHz to an optical clock laser at 344 THz by using a fiber-based femtosecond laser frequency comb as a transfer oscillator. With a second frequency comb, we independently measure the instability of the microwave source with respect to another optical clock laser frequency at 456 THz. The total fractional frequency instability of this optic-to-microwave and microwave-to-optic conversion resulted in an Allan deviation sigmay of 1.2 times 10-14 at 1 s averaging time (bandwidth of 50 kHz). The residual phase noise density is -97 dBc/Hz at a 10 Hz offset from the 9.6 GHz carrier. Replacing the existing quartz-based interrogation oscillator of the Physikalisch-Technische Bundesanstalt (PTB) caesium fountain CSF1 with this optically stabilized microwave source will reduce the instability contribution due to the Dick effect from the 10-13 level at 1 s averaging time to an insignificant level at the current status of CSF1. Therefore, this new microwave source can be an alternative to cryogenic sapphire-loaded cavity oscillators to overcome the limitations of state-of-the-art quartz oscillators.

Comparison of Fiber-Based Frequency Comb and Ti:Sapphire-Based Frequency Comb

For the first time we compare two kinds of optical frequency combs, one of which is based on a Ti:sapphire femtosecond laser and the other is based on a mode-locked erbium-doped fiber laser. The comparison is performed by measuring an optical frequency standard with these two combs simultaneously. The two frequency measurements agree within 1.8 Hz ( $3.8{\times}10^{-15}$ ) with the uncertainty of 17.2 Hz ( $3.6{\times}10^{-14}$ ), from which it can be concluded that the Ti:sapphire-based frequency comb and the fiber-based frequency comb have no systematic discrepancy at this level of uncertainty.

Comparison of Fiber-Based Frequency Comb and Ti:Sapphire-Based Frequency Comb

Comparison of Fiber-Based Frequency Comb and Ti:Sapphire-Based Frequency Comb

Optical Frequency Synthesis From a Cryogenic Sapphire Oscillator Using a Fiber-Based Frequency Comb

The authors have demonstrated optical frequency synthesis using a fiber-based frequency comb system, referenced to an ultrastable microwave oscillator. The oscillator is based on a high Q-factor cryogenic sapphire resonator cooled with liquid helium. The 100-MHz signal synthesized from the 10.8-GHz oscillation frequency of the cryogenic sapphire oscillator (CSO) was used to stabilize the repetition frequency of the mode-locked fiber laser. When the synthesized optical frequency was compared with a rubidium two-photon stabilized laser at 778 nm, the measured fractional frequency stability was ~6 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14 </sup> tau <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> for the averaging times tau between 1 and 100 s, and the best frequency stability was 3.0 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> for an averaging time of 1280 s

Octave-Spanning Comb Generation and Optical Frequency Measurement Using Mode-Locked Fiber Lasers

Recent developments in fiber based frequency combs have made them the preferred link between optical and microwave frequencies. This is due to their robustness and cost-effectiveness. We have developed fiber based frequency combs at NMIJ (National Metrology Institute of Japan) and used them to measure a range of wavelengths, including stabilized lasers with wavelengths at 633, 1064, 1542nm. Our comb has continuously worked for over 1 and half years in our laboratory. In addition, one of our fiber combs was recently shipped to the Australian National Measurement Institute to perform a comb comparison with the NMIA (National Measurement Institute, Australia) fiber comb. In this paper, we firstly survey the octave-spanning comb generation and optical frequency measurement using mode-locked lasers. We subsequently describe a development of the fiber comb at NMIJ, and the results of the comb comparison performed in Australia. The consistency of the frequency measurements of the comb comparison reached to the 10-16 level.

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.

Long term comparison of two fiber based frequency comb systems

Mode-locked erbium-doped fiber lasers are ideal comb generators for optical frequency metrology. We compare two fiber frequency combs by measuring an optical frequency independently with both combs and comparing their results. The two frequency measurements agree within 6x10-16. This is to our knowledge the first direct comparison between two fiber based frequency combs.

Circumvention of noise contributions in fiber laser based frequency combs

We investigate the performance of an Er:fiber laser based femtosecond frequency comb for precision metrological applications. Instead of an active stabilization of the comb, the fluctuations of the carrier-envelope offset phase, the repetition phase, and the phase of the beat from a comb line with an optical reference are synchronously detected. We show that these fluctuations can be effectively eliminated by exploiting their known correlation. In our experimental scheme, we utilize two identically constructed frequency combs for the measurement of the fluctuations, rejecting the influence of a shared optical reference. From measuring a white frequency noise level, we demonstrate that a fractional frequency instability better than 1.4 x 10(-14) for 1 s averaging time can be achieved in frequency metrology applications using the Er:fiber based frequency comb.