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

A mode-locked Tm-Ho-codoped fiber laser is reported, which is the first demonstration of a mode-locked fiber laser oscillator at the wavelength beyond 2 μm . The laser was built with a piece of newly developed Tm-Ho-codoped dc silicate fiber and a Sb-based semiconductor saturable absorber mirror (SESAM). When cladding pumped with a laser diode at 0.8 μm, the fiber laser generates mode-locked solitons at 2.06 μm with a pulse energy of 0.41 nJ and pulse duration of 1.1 ps.

Mode-Locked Tm–Ho-Codoped Fiber Laser at 2.06 $\mu$ m

Spectral properties of erbium-doped lead halotellurite glasses for 1.5 μm broadband amplification

Performance of high-concentration Er/sup 3+/-Yb/sup 3+/-codoped phosphate fiber amplifiers in a copumping configuration is presented. Gain, noise figure, and output signal saturation power of fiber amplifiers with different lengths are reported. From a 3.6-cm-long fiber, 18-dB internal gain, i.e., 5 dB/cm, for small signal input at 1535 nm, was achieved. To the best of our knowledge, this is the highest gain per unit length for erbium-doped fiber amplifiers ever reported.

Performance of high-concentration Er/sup 3+/-Yb/sup 3+/-codoped phosphate fiber amplifiers

Er 3+ doped tellurite glasses with various Na 2 O/Li 2 O, Na 2 O/K 2 O and K 2 O/Li 2 O ratios (0-4.5) were prepared at 850°C using a platinum crucible. The effect of relative alkali content on absorption linewidth in erbium-doped tellurite glasses was investigated. The highest effective linewidth was obtained from glass samples with mixed alkali molar ratio of Na 2 O/Li 2 O=1, Na 2 O/K 2 O=1 and K 2 O/Li 2 O=1. Glass transition temperature, crystallization temperature, density, refractive index, Judd-Ofelt parameters and emission cross-section of Er 3+ doped tellurite glasses are also reported.

Effect of relative alkali content on absorption linewidth in erbium-doped tellurite glasses

We present an all-fiber Q-switched single-frequency laser oscillator operating in the eye-safe region at 1950 nm. It is based on the stress-induced polarization modulation in a Tm-doped distributed Bragg reflector fiber laser. The laser emits Q-switched single-frequency laser pulses with a pulse repetition rate ranging from tens of hertz to hundreds of kilohertz and an average power of several milliwatts. Pulse duration and laser spectral linewidth have been characterized. This is, to our best knowledge, the first demonstration of a Q-switched single-frequency fiber laser near 2 microm.

Tao Luo

Papers

Mode-Locked Tm–Ho-Codoped Fiber Laser at 2.06 $\mu$ m

Spectral properties of erbium-doped lead halotellurite glasses for 1.5 μm broadband amplification

Performance of high-concentration Er/sup 3+/-Yb/sup 3+/-codoped phosphate fiber amplifiers

Effect of relative alkali content on absorption linewidth in erbium-doped tellurite glasses

All-fiber Q-switched single-frequency Tm-doped laser near 2 μm