Anti-Stokes fluorescence cooling of nanoparticle-doped silica fibers.

The first observation of cooling by anti-Stokes pumping in nanoparticle-doped silica fibers is reported. Four Yb-doped fibers fabricated using conventional modified chemical vapor deposition (MCVD) techniques were evaluated, namely, an aluminosilicate fiber and three fibers in which the Yb ions were encapsulated in CaF2, SrF2, or BaF2 nanoparticles. The nanoparticles, which oxidize during preform processing, provide a modified chemical environment for the Yb3+ ions that is beneficial to cooling. When pumped at the near-optimum cooling wavelength of 1040 nm at atmospheric pressure, the fibers experienced a maximum measured temperature drop of 20.5 mK (aluminosilicate fiber), 26.2 mK (CaF2 fiber), and 16.7 mK (SrF2 fiber). The BaF2 fiber did not cool but warmed slightly. The three fibers that cooled had a cooling efficiency comparable to that of the best previously reported Yb-doped silica fiber that cooled. Data analysis shows that this efficiency is explained by the fibers' high critical quenching concentration and low residual absorptive loss (linked to sub-ppm OH contamination). This study demonstrates the large untapped potential of nanoparticle doping in the current search for silicate compositions that produce optimum anti-Stokes cooling.

Millimeter wave photonics with terahertz semiconductor lasers.

Millimeter wave (mmWave) generation using photonic techniques has so far been limited to the use of near-infrared lasers that are down-converted to the mmWave region. However, such methodologies do not currently benefit from a monolithic architecture and suffer from the quantum defect i.e. the difference in photon energies between the near-infrared and mmWave region, which can ultimately limit the conversion efficiency. Miniaturized terahertz (THz) quantum cascade lasers (QCLs) have inherent advantages in this respect: their low energy photons, ultrafast gain relaxation and high nonlinearities open up the possibility of innovatively integrating both laser action and mmWave generation in a single device. Here, we demonstrate intracavity mmWave generation within THz QCLs over the unprecedented range of 25 GHz to 500 GHz. Through ultrafast time resolved techniques, we highlight the importance of modal phases and that the process is a result of a giant second-order nonlinearity combined with a phase matched process between the THz and mmWave emission. Importantly, this work opens up the possibility of compact, low noise mmWave generation using modelocked THz frequency combs.

Loss-compensated slow-light fiber Bragg grating with 22-km/s group velocity.

This Letter reports the behavior of the slow-light resonances of a strong apodized fiber Bragg grating (FBG) in which the intrinsic loss is compensated for by a small internal gain. The 6.5-mm FBG, written with a femtosecond laser in an Er-doped single-mode fiber, was pumped at ∼1475nm just below the lasing threshold to offset most of its intrinsic loss, thereby narrowing its resonances. The fundamental slow-light resonance was measured to have a linewidth of 8.5 fm, or a record group velocity of ∼22km/s, and a peak transmission near unity (-0.2dB). The measured dependencies of the linewidth and peak transmission on pump power agree well with a new model that predicts the transmission spectrum of loss-compensated FBGs in the presence of pump and signal saturation.

Ultrafast response of harmonic modelocked THz lasers.

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity's fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

Laser cooling in a silica optical fiber at atmospheric pressure.

For the first time, to the best of our knowledge, laser cooling is reported in a silica optical fiber. The fiber has a 21-µm diameter core doped with 2.06 wt.% ${{\rm Yb}^{3 + }}$Yb3+ and co-doped with ${{\rm Al}_2}{{\rm O}_3}$Al2O3 and ${{\rm F}^ - }$F- to increase the critical quenching concentration by a factor of 16 over the largest reported values for the Yb-doped silica. Using a custom slow-light fiber Bragg grating sensor, temperature changes up to $ - {50}\;{\rm mK}$-50mK were measured with 0.33 W/m of absorbed pump power per unit length at 1040 nm. The measured dependencies of the temperature change on the pump power and the pump wavelength are in excellent agreement with predictions from an existing model, and they reflect the fiber's groundbreaking quality for the radiation-balanced fiber lasers.