This Review discusses the emerging field of mid-infrared frequency comb generation, including technologies based on novel laser gain media, nonlinear frequency conversion and microresonators, as well as the applications of these combs in precision spectroscopy and direct frequency comb spectroscopy. Laser frequency combs are coherent light sources that emit a broad spectrum of discrete, evenly spaced narrow lines whose absolute frequency can be measured to within the accuracy of an atomic clock. Their development in the near-infrared and visible domains has revolutionized frequency metrology while also providing numerous unexpected opportunities in other fields such as astronomy and attosecond science. Researchers are now exploring how to extend frequency comb techniques to the mid-infrared spectral region. Versatile mid-infrared frequency comb generators based on novel laser gain media, nonlinear frequency conversion or microresonators promise to significantly expand the applications of frequency combs. In particular, novel approaches to molecular spectroscopy in the 'fingerprint region', with dramatically improved precision, sensitivity, recording time and/or spectral bandwidth may lead to new discoveries in the various fields relevant to molecular science.

Mid-infrared frequency combs

Optical Waveguide Theory

The composition and properties of neodymium-doped (Nd-doped) phosphate glasses used for simultaneous high-energy (10 3 –10 6 J) and high-peak-power (10 12 –10 15 W) laser applications such as fusion energy research, are reviewed. The most common base glasses are meta-phosphates (O/P ∼3) with the approximate composition: 60P 2 O 5 –10Al 2 O 3 –30M 2 O/MO; K/Ba or K/Mg are typical modifiers. The spectroscopy of Nd 3+ in these glasses is well understood and laser properties can be accurately determined from measured spectroscopic properties. The major mechanisms for Nd 3+ non-radiative relaxation are reviewed and empirical expressions are presented that predict these effects in phosphate glasses. Optical and thermal–mechanical properties have been measured on a number of laser glasses and can be correlated with composition. Sub-critical crack growth rates in stress regions I, II and III have been reported for the first time in phosphate laser glasses. The mechanism for Pt inclusion formation and dissolution has been studied leading to damage resistant (Pt-inclusion-free) laser glasses.

Nd-doped phosphate glasses for high-energy/high-peak-power lasers

Rare earth ions (Tm3+, Er3+, and Yb3+)-doped cubic Gd2O3 nanocrystals were prepared by a simple sol−gel method. Raman and FT-IR spectra were measured to evaluate the vibrational feature of the samples. Under 980 nm laser excitation, blue (488 nm), green (564 nm), and red (661 nm) upconversion has been recorded in Gd2O3:Tm+Yb and Gd2O3:Er (Gd2O3:Er+Yb), respectively. A great enhancement of red emission and diminishment of green emission of Er3+ in Gd2O3:Er+Yb have been observed. Laser power and doping concentration dependence of the upconverted emissions were studied to understand the upconversion mechanisms. Excited absorption and energy-transfer processes are discussed as the possible mechanisms for the visible emissions.

Visible Upconversion in Rare Earth Ion-Doped Gd2O3 Nanocrystals

The composition and properties of neodymium-doped (Nd-doped) phosphate glasses used for simultaneous highenergy (10 3 ‐10 6 J) and high-peak-power (10 12 ‐10 15 W) laser applications such as fusion energy research, are reviewed. The most common base glasses are meta-phosphates (O/P3) with the approximate composition: 60P2O5‐10Al2O3‐ 30M2O/MO; K/Ba or K/Mg are typical modifiers. The spectroscopy of Nd 3a in these glasses is well understood and laser properties can be accurately determined from measured spectroscopic properties. The major mechanisms for Nd 3a non-radiative relaxation are reviewed and empirical expressions are presented that predict these eAects in phosphate glasses. Optical and thermal‐mechanical properties have been measured on a number of laser glasses and can be correlated with composition. Sub-critical crack growth rates in stress regions I, II and III have been reported for the first time in phosphate laser glasses. The mechanism for Pt inclusion formation and dissolution has been studied leading to damage resistant (Pt-inclusion-free) laser glasses. ” 2000 Elsevier Science B.V. All rights reserved.

Section 3. Applications Nd-doped phosphate glasses for high-energy/high-peak-power lasers

New erbium-doped phosphate glasses and fibers were fabricated. A net gain of 15.5 dB at 1535 was achieved from 5.1 cm long fiber, which represents the highest gain per unit length for fiber amplifiers to the best of our knowledge.

Net gain of 15.5 dB from a 5.1 cm long Er/sup 3+/-doped phosphate glass fiber

This paper summaries our effort to develop new Nd3+, Er3+ and Yb3+ doped phosphate laser glasses, which exhibit high strength and low thermal expansion coefficient as well as acceptable optical athermal behavior. Ion-exchange chemical strengthening processes and laser performances of these glasses are presented.

Phosphate glasses for high-average-power lasers

Broadband erbium-doped fiber amplifiers at 1.5 micrometers band, an important communication band, are generating great interest in order to increase the transmission capacity of wavelength-division-multiplexing transmission networks. The host materials for Er3+ doping are very important for obtaining an intrinsically broad amplifier bandwidth. This paper reports the spectral properties of the Er3+ doped lead halotellurite glasses, PbX2-TeO2 (X equals F, Cl, Br). The measured absorption and emission spectra were analyzed by Judd-Ofelt and McCumber theories. It was found that Er3+ doped lead halotellurite glasses have a good glass stability, high refractive indices, high absorption and stimulated emission cross sections, and a very broad bandwidth for 1.5 micrometers absorption and emission. The 1.5 micrometers band, which has a substantial magnetic dipole transition component, is a special band whose bandwidth increases significantly with the refractive index of the host material. It is expected that the halotellurite glasses will be the promising host materials for 1.5 micrometers broadband amplification.

Spectral properties of erbium-doped lead halotellurite glasses

Tellurium-enriched high-purity chalcogenide fibers were fabricated and their infrared (IR)-transmitting properties were investigated. The thermal stability of Ge–As–Te glasses could be improved significantly by introducing a small amount of selenium into the glasses, therefore allowing the production of structurally and optically homogeneous glass fibers with minimal scattering losses. The fibers with greater electrical conductivity (σ) showed higher transmission losses than those with smaller σ because of the severer free-carrier absorption in the former, which was identified by the temperature dependence of the fibers' attenuation. In an effort to reduce σ of the glass while maintaining its outstanding transmitting property in the long-wave IR region, iodine was added to replace a quantity of tellurium in the glass. The fiber made of Ge25Te58I7Se10 glass exhibited good transmitting property in the 4–12 μm region and showed losses as low as ∼1 dB/m around 9.5 μm.

Long-Wave Infrared-Transmitting Optical Fibers

This paper reports the preparation of planar waveguides in an Er3+ doped tellurite glass with a composition, 15Ns2O-25WO3-60TeO2- 1Er2O3, by Ag+-Na+ ion-exchange. The glass was chemically stable during ion-exchange process. Single-mode and multi- mode planar and buried waveguides in the tellurite glasses have been prepared, which is the first demonstrated of waveguides in tellurite glasses fabricated by ion-exchange technique to the best of our knowledge. The depths of the waveguides could be controlled by varying ion-exchange temperatures and times. The diffusion parameters, diffusion coefficients and activation energy were estimated. It is indicated that the tellurite glass is a prospective candidate for active integrated optics.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Tao Luo

Papers

Net gain of 15.5 dB from a 5.1 cm long Er/sup 3+/-doped phosphate glass fiber

Phosphate glasses for high-average-power lasers

Spectral properties of erbium-doped lead halotellurite glasses

Long-Wave Infrared-Transmitting Optical Fibers

Optical waveguides prepared in Er3+-doped tellurite glass by Ag+-Na+ ion exchange