Frequency-doubled Fiber Research Articles

Modulation transfer spectroscopy offset laser frequency stabilization laser

One of the key parts of laser system of atomic gravimeter based on cold atoms is investigated. In order to construct optical system of atomic gravimeter the laser offset frequency stabilization is needed. Method for frequencydoubled fiber laser’s offset frequency stabilization is suggested and developed. This method is based on modulation transfer spectroscopy and the usage of fiber electro-optical modulator. The experimental scheme of this method is described. The error signals for offset frequency stabilization of frequency-doubled fiber laser are obtained. In order to maximize error signal’s amplitude, experimental parameters such as cell’s temperature, pump and probe beam intensities and electrooptical modulators signal amplitudes are optimized. The influence of polarization on error signal’s amplitude is investigated. It is shown that circular polarization allows to achieve error signal with higher amplitude. The achieved results can be applied to the construction of quantum gravimeter, quantum frequency standards and to the laser cooling experiments.

Optical frequency generation using fiber Bragg grating filters for applications in portable quantum sensing.

A method for the agile generation of the optical frequencies required for laser cooling and atom interferometry of rubidium is demonstrated. It relies on fiber Bragg grating technology to filter the output of an electro-optic modulator and was demonstrated in an alignment-free, single-seed, frequency-doubled fiber laser system. The system was capable of frequency switching over a 30 GHz range in less than 40 ns, with ∼0.5W output power and amplitude modulation with a ∼15ns rise/fall time and an extinction ratio exceeding 80 dB. The technology is ideal for enabling high-bandwidth, mobile industrial, and space applications of quantum technologies.

30 TW and 33 fs pulses delivered by a Ti:Sa amplifier system seeded with a frequency-doubled fiber laser.

We report a full experimental comparison study on the injection of a Ti:Sa multi-terawatt amplifier chain with a standard 15 fs Ti:Sa oscillator and 35 fs frequency-doubled fiber oscillator. The study highlights that the Ti:Sa oscillator, with high performance in terms of pulse duration and spectral width, can be replaced by the frequency-doubled fiber oscillator to seed Ti:Sa amplifier chains almost without any compromise on the output pulse duration and picosecond contrast. Finally, we demonstrate for the first time to our knowledge a 30 TW and 33 fs Ti:Sa amplifier injected by a fiber oscillator.

Raman-shifted wavelength-selectable pulsed fiber laser with high repetition rate and high pulse energy in the visible.

A high-pulse-energy, diffraction-limited, wavelength-selectable, visible source, based on Raman frequency shifting of a frequency-doubled Yb-doped fiber laser, has been studied. The relative length-scaling laws of Raman gain and self-phase modulation push the design towards short fiber lengths with large core size. It is experimentally demonstrated that the Raman clean-up effect in a graded-index multi-mode fiber is not sufficient to obtain diffraction-limited beam quality in the short fiber length. Thus, a large-core photonic crystal fiber is used to maintain diffraction-limited performance and output pulse energies of ~1 μJ, at a 1-MHz repetition rate and 1.3-ns pulse-width are successfully achieved. This step-tunable visible source should find applications in photoacoustic microscopy.

Absolute frequency measurement of the molecular iodine hyperfine transitions at 548 nm

We report absolute frequency measurement of the molecular iodine P(28) 24-0 a1, a10, and a15 hyperfine transitions at 548 nm. The light source is based on a frequency-doubled fiber amplifier system seeded by an external cavity diode laser. Saturated absorption is performed by modulation transfer spectroscopy, and the absolute transition frequency is measured by an optical frequency comb. The effects of pressure shift and pressure broadening are discussed. Our determination of the line centers reaches a precision of better than 20 kHz. This light source can be used as a reference laser for lithium ion spectroscopy.

135 W average-power femtosecond pulses at 520 nm from a frequency-doubled fiber laser system

We present a high-average-power femtosecond laser system at 520 nm central wavelength. The laser system delivers sub-500 fs pulses with 135 W average power at a pulse repetition rate of 5.25 MHz. Excellent beam quality is provided by high power fiber amplifiers and maintained during frequency doubling, resulting in a beam quality factor of M2<1.2. To our knowledge, the system presented here is the highest average power green laser source generating femtosecond pulses with diffraction-limited beam quality.

Frequency-Doubled Fiber Lasers for RF Spectrum Analysis in Spectral-Hole-Burning Media

We demonstrate an RF spectrum analyzer based on spectral-hole burning (SHB) that operates with unity probability of intercept and resolution under 100 kHz. An SHB crystal, which consists of rare-earth ions doped into a crystal host, records the power spectrum of an RF signal modulated onto an optical carrier as a series of spectral holes that persist for about 10 ms. While the crystal's homogeneous and inhomogeneous linewidths place the fundamental limits on resolution and bandwidth, respectively, the practical limits depend on the lasers used to interrogate the record stored in the crystal's absorption profile. Up to now, SHB spectrum analyzers have used chirped beams from externally modulated, stabilized lasers, which have linewidths of under 10 kHz but cannot chirp over much more than octave bandwidths, or directly modulated diode lasers, which can chirp over more than 20GHz but have linewidths of about 1 MHz. Switching to chirped fiber lasers, which have natural linewidths of under 2 kHz and chirping linewidths on the order of 10 kHz, produces a measurement with fine resolution without any laser stabilization. In addition, by chirping the fiber laser with a sufficiently fast piezo, the resulting chirp could extend over tens of gigahertz in under 10 ms, yielding both fine resolution and broad bandwidth without extraordinary stabilization schemes.

Efficient white light generation in secondary cores of holey fibers

We report the generation of white light from a picosecond pump by efficient four-wave mixing processes. A 530 nm green source based on a frequency-doubled Yb-doped fiber laser generates strong red and blue sidebands in the secondary cores of a holey fiber with large air-filling factor. Phase matching is attributed to birefringence within the submicrometer-sized secondary cores induced by non-symmetric deformation during the fiber drawing.

Alexandrite-pumped alexandrite regenerative amplifier for femtosecond pulse amplification

We demonstrate a regenerative amplif ier incorporating alexandrite as the gain medium that is pumped by an alexandrite laser. Temperature-altered gain permitted the 728-nm alexandrite pump laser, operating at room temperature, to pump a 780-800-nm alexandrite laser that was maintained at elevated temperatures. 200-fs pulses from a Ti:sapphire oscillator were amplif ied to the millijoule level. This system also amplif ied femtosecond pulses from a frequency-doubled Er-doped fiber laser.

Annealing experiments in frequency-doubling fibers

Frequency-doubling fibers were annealed at various temperatures. The conversion efficiency was monitored and found to drop by orders of magnitude at temperatures of ~200–400 °C. An activation energy of 0.28 ± 0.04 eV was inferred from these measurements for a Ge-doped fiber and of 0.40 ± 0.08 eV for a Ge- and P-doped fiber. The decay of the second-harmonic conversion efficiency in time was also studied.

Grating evolution in frequency-doubling fibers

The position and the width of the chi((2)) grating in a frequency-doubling fiber were studied during the preparation process. The peak of the grating moves several centimeters from the input end to the output end of the fiber before the conversion efficiency saturates. On saturation, the grating moves toward the input end and becomes narrower.

Preparation of frequency-doubling fibers under UV excitation

Germanium-doped silica fibers were prepared for frequency doubling of 1.06-microm wavelength radiation under the simultaneous presence of the first, second, and fourth harmonics of the laser light. It was found that the preparation rate for frequency doubling was dramatically increased by the presence of the fourth harmonic. The experiments also showed that a metastable excited state plays a significant role in the process.

Phase measurement in frequency-doubling fibers

Fibers were prepared for second-harmonic generation by injecting a frequency-doubled (seed) signal with the fundamental light from a Nd:YAG laser. The relative phase shift between the seed and the second-harmonic light generated by the fiber was measured to be close to 90 degrees ( approximately 99 +/- 9.2 degrees ). A water cell was used to sweep the relative phase of the waves with interferometric precision. Some physical consequences of a pi/2 phase shift are pointed out.