Three centuries after Newton's experiments on the decomposition of white light into its spectral components and the synthesis of white light from various colors, nonlinear-optical transformations of ultrashort laser pulses have made it possible to produce an artificial white light with unique spectral properties, controlled time duration, and a high spectral brightness. Owing to its broad and continuous spectrum, such radiation is called supercontinuum. The laser generation of white light is an interesting physical phenomenon and the relevant technology is gaining in practical implications — it offers novel solutions for optical communications and control of ultrashort laser pulses, helps to achieve an unprecedented precision in optical metrology, serves to probe the atmosphere of the Earth, and suggests new strategies for the creation of compact multiplex light sources for nonlinear spectroscopy, microscopy, and laser biomedicine. Here, we provide a review of physical mechanisms behind the laser generation of white light, examine its applications, and discuss the methods of generation of broadband radiation with controlled spectral, temporal, and phase parameters.

https://iopscience.iop.org/article/10.1070/PU2006v049n06ABEH005975/pdf

Let there be white light: supercontinuum generation by ultrashort laser pulses

Microstructure fibers have opened a new phase in nonlinear optics. Due to their unique properties, fibers of this type radically enhance all the basic nonlinear-optical phenomena, offering new strategies for frequency conversion, spectral transformation, and control of ultrashort laser pulses. These fibers allow supercontinuum radiation to be efficiently generated using nano- and subnanojoule femtosecond pulses. Here, we analyze the physical mechanisms behind the enhancement of nonlinear-optical interactions of ultrashort pulses in microstructure and hollow photonic-crystal fibers and discuss applications of microstructure fibers for highly efficient supercontinuum generation and frequency conversion of femtosecond laser pulses.

https://iopscience.iop.org/article/10.1070/PU2004v047n01ABEH001731/pdf

Nonlinear optics of microstructure fibers

Enhancement of nonlinear optical interactions in the core of a photonic-crystal fiber allows several χ(3) processes to be simultaneously observed in the field of unamplified 30-fs pulses of a Cr:forsterite laser. Subnanojoule fundamental-radiation pulses of this laser experience spectral broadening arising from self-phase modulation and generate the third harmonic at 410–420 nm. Third-harmonic pulses also appear spectrally broadened at the output of the fiber as a result of the cross-phase-modulation effect. This catalog of enhanced χ(3) processes observed in photonic-crystal fibers opens the way for using such fibers for frequency conversion of low-energy femtosecond pulses with simultaneous chirp control and subsequent pulse compression.

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Enhanced X(3) interactions of unamplified femtosecond Cr:forsterite laser pulses in photonic-crystal fibers

Studies in the field of laser science and technology have resulted in the creation of a unique passively mode-locked laser generating a continuous regular train of ultrashort pulses. The unique feature of this laser is that it combines the properties of a radiation source with the output spectrum consisting of numerous extremely narrow strictly equidistant spectral lines with the properties of a radiation source emitting ultrashort high-power laser pulses. This feature has permitted a wide range of remarkable results. The basic properties of continuous-wave passively mode-locked lasers are discussed and their main applications are considered.

Continuous-wave femtosecond lasers

A new frequency chain was demonstrated to measure an optical frequency standard based on a rovibrational molecular transition in the 28-THz spectral region accessible to a CO2 laser. It uses a femtosecond-laser frequency comb generator and two laser diodes at 852 nm and 788 nm as intermediate oscillators, with their frequency difference phase locked to the CO2 laser. The rf repetition rate of the femtosecond laser was compared with a 100-MHz signal from a hydrogen maser, located at BNM-SYRTE. The 100-MHz signal is transmitted by amplitude modulation of a 1.55-μm laser diode through a 43-km telecommunication optical fibre. As a first example, the absolute measurement of a saturation line of OsO4 in the vicinity of the P(16) laser line of CO2 is reported with a relative uncertainty of 10-12, limited by the CO2/OsO4 frequency day-to-day reproducibility. The current limit on the stability of the frequency measurement is 4×10-13 at 1 s.

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Absolute frequency measurement in the 28-THz spectral region with a femtosecond laser comb and a long-distance optical link to a primary standard

A concept of high-precision optical frequency measurements involving the use of femtosecond frequency combs with a bandwidths exceeding the frequency of a reference laser source is discussed. This relation between the frequency of the reference laser source and the bandwidth of the frequency comb allows the difference frequency of two modes in a femtosecond frequency comb to be phase-locked to the reference laser. The radio-frequency interval between two adjacent modes in the femtosecond frequency comb is also stabilized in this case, allowing high-precision frequency measurements to be performed in a very simple way. A laser system allowing this principle of high-precision frequency measurements to be implemented includes a mode-locked Ti:sapphire laser, a methane-stabilized He-Ne laser, and a tapered fiber. Special measures have been taken to improve stability characteristics of the laser system. The process of spectral broadening of femtosecond pulse trains in holey and tapered fibers is considered in terms of the Zakharov equation as a result of four-wave mixing of phase-locked equidistant spectral components. The main physical factors influencing the phase distribution and coherence properties of broadly spanning frequency combs produced with the use of this technique are discussed. Phase shifts and spectral distortions arising due to dispersion effects, modulation instabilities, and shock waves of pulse envelopes are explored, and general recipes to reduce the influence of these effects by means of fiber optics are discussed.

Femtosecond frequency combs stabilized with a He-Ne/CH4 laser: Toward a femtosecond optical clock

The spectral characteristics and stability of a frequency of intermode beats of a fs self-mode-locked Ti:sapphire laser are investigated. An active method is used to obtain high stability. The frequency stability of intermode beats not lower than 1.27×10−12 rms in 50 s is achieved. Possible applications of the setup, such as measurement of large frequency intervals in the optical range, creation of optical frequency synthesizers, etc., are proposed. The physical principles for the creation of an optical clock of a new type using a highly stable fs Ti:sapphire laser are considered.

A femtosecond self-mode-locked Ti:sapphire laser with high stability of pulse-repetition frequency and its applications

We report on the progress in development of a highly accurate optical frequency standard based on the single ion of ytterbium-171 at the Institute of Laser Physics, Novosibirsk.

https://iopscience.iop.org/article/10.1088/1742-6596/793/1/012004/pdf

Optical frequency standard with ytterbium single ion

The repetition frequency of femtosecond Ti : Al2O3 laser pulses was stabilised. A stability of at least 2 × 10-10 in 100 s was achieved. An investigation was made of the mode beat spectra.

http://iopscience.iop.org/article/10.1070/QE1997v027n04ABEH000938/pdf

Stabilisation of the repetition frequency of femtosecond Ti : Al2O3 laser pulses

The spectral characteristics and stability of a frequency of intermode beats of a femtosecond Ti:S laser are investigated.An active method is used to obtain high stability. The frequency stability of intermode beats not lower than 5.8 1014 in1 OOs is achieved. Possible applications of the setup, such as measurement of large frequency intervals in the optical range,creation of optical frequency synthesizers, etc. are proposed. The physical principles for the creation of an optical clock of anew type using highly stable femtosecond Ti:S laser are considered.PACS: 42.62.Eh; 07.60.Ly; 06.30.FtKeywords: Laser stabilization; Mode-locked laser; Optical synthesizer; Optical frequency clock 1. INTRODUCTION Recently there appeared new applications of ultrashort light pulses in addition to the traditional ones (i.e., ultrafast spectroscopy, ultrastrong optical fields). These applications are based on the operational characteristics of mode-lockedlasers, such as the wide radiation spectrum of equidistant modes and the regular sequence of short high-power light pulses inoutput radiation1'2. Lasers of this type can be used to construct optical standards of frequency, length and time, in the Fourierspectroscopy, high-precision measurements of large frequency intervals in the optical range, etc. In all cases the parametersof light pulses, especially their repetition frequency, i.e., the frequency of intermode beats, must be highly stable.The present paper is the experimental investigation of the stability and spectrum of the frequency of intermode beats of afemtosecond Ti: S laser and their improvement by an active method. In this method, the frequency of intermode beats of thelaser is phase-locked to the frequency of a highly stable radio frequency oscillator. We tested this method using a He-Nelaser in 1992, and it turned out to be highly efficient3.

S. V. Chepurov

Papers

Femtosecond frequency combs stabilized with a He-Ne/CH4 laser: Toward a femtosecond optical clock

A femtosecond self-mode-locked Ti:sapphire laser with high stability of pulse-repetition frequency and its applications

Optical frequency standard with ytterbium single ion

Stabilisation of the repetition frequency of femtosecond Ti : Al2O3 laser pulses

Application of femtosecond lasers for the frequency synthesis in radio-optical ranges and for the creation of an optical clock