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 monolithic polarization maintaining fiber chirped pulse amplification system with 30 cm Yb3+-doped high efficiency media fiber that generates 25 µJ sub-500 fs pulses with 20 W at 1.03 µm has recently been demonstrated.

Monolithic Polarization Maintaining Fiber Chirped Pulse Amplification System for High Energy Femtosecond Pulse Generation at 1.03 μm

We demonstrate a single-frequency gain-switched Ho-doped fiber laser based on heavily doped silicate glass fiber
fabricated in house. A Q-switched Tm-doped fiber laser at 1.95μm was used to gain-switch the Ho-doped fiber laser via in-band pumping. Output power of the single-frequency gain-switched pulses has been amplified in a cladding-pumped Tm-Ho-codoped fiber amplifier with 1.2m active fiber pumped at 803nm. Two different nonlinear effects, i.e.,
modulation instability and stimulated Brillouin scattering, could be seen in the 10μm-core fiber amplifier when the
peak power exceeds 3kW. The single-frequency gain-switched fiber laser was operated at 2.05μm, a popular laser
wavelength for Doppler lidar application. This is the first demonstration of this kind of fiber laser.

Single-frequency gain-switched Ho-doped fiber laser

The paper reviews strategies for obtaining glasses with wide optical transparencies and sufficiently high glass forming ability to produce complex optical elements such as single mode fibers. These fibers find applications in many infrared technology including thermal imaging, laser guiding and most importantly, vibrational sensing of chemical and biomolecules.

Fabrication, characterization and applications of infrared transparent chalcogenide fibers

Summary form only given. A new germanate glass containing GeO/sub 2/, Al/sub 2/O/sub 3/, Na/sub 2/O, BaO and rare-earth ions was designed and fabricated. Spectral properties of rare-earth ions were characterized. Slab waveguides were fabricated by immersing the samples into a molten salt bath containing AgNO/sub 3/. A variety of ion-exchange temperatures ranging from 2300 C to 3300 C and ion exchange times ranging from a few minutes to several hours were used to characterize the diffusion process. The effective mode indices were measured using the prism-coupler technique, and the index profile was calculated using the inverse WKB method.

Germanate glass channel waveguides for infrared waveguide lasers

We report a passively mode-locked fiber laser using a 30-cm long newly developed Tm-doped silicate glass fiber. The mode-locked pulses operate at 1980 nm with duration of 1.5 ps and energy of 0.76 nJ.

Tao Luo

Papers

Monolithic Polarization Maintaining Fiber Chirped Pulse Amplification System for High Energy Femtosecond Pulse Generation at 1.03 μm

Single-frequency gain-switched Ho-doped fiber laser

Fabrication, characterization and applications of infrared transparent chalcogenide fibers

Germanate glass channel waveguides for infrared waveguide lasers

Mode-Locked Thulium-doped Silicate Fiber Laser with Saturable Absorber Mirror