Widely tunable cavity-enhanced backward difference-frequency generation

The wavelength tuning and intensity modulation characteristics are presented for three types of twin-guide (TG) lasers entirely fabricated by metalorganic vapour phase epitaxy (MOVPE). A wide continuous tuning range of 5.3 nm is obtained for the tunable twin-guide (TTG) laser. The TTG lasers, however, have the drawbacks of output power reduction and spectral linewidth broadening during tuning. A new type of tunable laser, the complementary twin-active-guide (CTAG) laser, is proposed, and its tuning characteristics are presented for the first time. A wide continuous tuning range over 4 nm is obtained, although the thermal effect is more dominant than carrier effects. The CTAG structure enables suppression of the spectral linewidth broadening induced by the injected carrier recombination shot noise (IRSN) during tuning. The characteristics of the TG lasers modulated by reverse bias are presented, and the possibility of using these TG lasers for wavelength tuning is studied.

We report on the generation of mid-infrared (mid-IR) pulses with a maximum average optical power of 4 mW and wide tunability in the 8-14 μm range via difference frequency generation (DFG) in GaSe from an Er:fiber laser oscillator. The DFG process is seeded with self-frequency shifted Raman solitons that are shown to be phase coherent within the whole tuning range, from 1.76 to 1.93 μm. Interference measurements between adjacent pulses at the idler wavelengths attest coherence transfer to the mid-IR.

All-optical wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) is proposed and experimentally demonstrated in periodically poled LiNbO3 (PPLN) waveguides. The signal pulse with 40-GHz repetition rate and 1.57- ps pulse width is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. No obvious changes of the pulse shape and width, also no chirp are observed in the converted idler pulse. The results imply that single-to-multiple channel wavelength conversions can be achieved by appropriately tuning the two pump wavelengths.

A multiwavelength midinfrared quasi-phase-matched (QPM) difference frequency generation (DFG) scheme is proposed based on a segmented temperature controlling technique for the uniform grating periodically poled lithium niobate (PPLN). The QPM DFG output performances have been investigated under the fixed pump and signal wavelength conditions, respectively. Our theoretical results show that when the pump wavelength is fixed at 1.08 μm, three idler QPM bands, located at 3.46, 3.50, and 3.57 μm, have been obtained, where the temperatures of the three PPLN segments with the same length are set at 20°C, 70°C, and 100°C. However, when the signal wavelength is fixed at 1.58 μm, six idler QPM bands, located at 2.95, 3.06, 3.14, 3.63, 3.72, and 3.83 μm, have been achieved with the same crystal temperature distribution of 20°C, 70°C, and 100°C.

We propose an integrated terahertz emitter operating at room temperature between 2.4 and 6 THz. Based on difference-frequency generation in a triply resonant Au/AlAs/GaAs/AlAs/Au microcylinder, this nonlinear source is pumped by two near-IR whispering-gallery modes that are excited by InAs quantum dots embedded in the resonator. In the vertical direction, these pump modes are due to total internal reflection at GaAs/AlAs interfaces, while the terahertz mode is confined between the metallic layers. This parametric source offers potential advantages with respect to existing terahertz sources for spectroscopic applications, such as room-temperature operation and electrical pumping.

In this paper, mid-IR light generation based on difference frequency generation (DFG) in a single piece of PPLN is proposed and studied. In the studies, a Yb doped fiber laser at 1.092 μm and a tunable laser around 1.55 μm were used. The output mid-IR laser with the wavelength tunable around 3.7 μm was generated. Since compact Yb doped fiber lasers at 1.092 μm and tunable semiconductor laser diodes around 1.55 μm are available on the market, the proposed mid-IR laser is potentially portable. Our simulations show that tunable mid-IR light as broad as 800 nm can be obtained from a single PPLN chip simply by tuning temperature (from 30 ^oC to 500 ^oC) and signal wavelength (from 1.48 μm to 1.62 μm).

We demonstrate narrowband CW mid-infrared generators based on difference frequency generation in periodically poled KTP and KTA crystals. The crystals are used to nonlineary mix the beams from high-power narrowband fiber laser systems working at 1060- and 1550-nm spectral bands. We explore various techniques to control the linewidth of fiber lasers. The tunability achieved in a source based on nonlinear mixing in a KTA crystal exceeds the range of 3100–3620 nm.

We compare narrow-band CW mid-IR generators based on a difference frequency generation in periodically poled KTA and KTP crystals that are used to mix the beams from high-power fiber lasers operating in 1060nm and 1550nm spectral bands.

Nonlinear interactions such as second harmonic generation and sum and difference frequency generation are routinely used for the generation of laser wavelengths. With high power lasers it is necessary to use bulk crystals and tight beam focusing to achieve the maximum conversion efficiency. The use of Quasi- phase-matched (QPM) materials such as PPLN and PPKTP are now routinely reported; however, in this paper, a simple linear QPM crystal does not fully the conversion response. The theoretical and experimental results shows that by careful control of the crystal design it is possible to fully compensate for the phase errors associated with the focusing induced Gouy shift. However, this optimisation is only valid for linearly invariant materials, which are unable to compensate fully for the Gouy phase inherent to focused interactions.

Summary form only given. We previously reported a high-conversion-efficiency, tandem KTA-CdSe OPO source capable of covering four bands in the spectral range from 1.5 to 10 /spl mu/m. The design uses the angle-tuned, idler output of a KTA OPO to pump a non-critically phase-matched (NCPM) CdSe OPO. We use two techniques to increase the KTA OPO efficiency (1) difference-frequency generation (DFG) in a second KTA crystal placed in the KTA OPO cavity, and (2) injection-seeding of single frequency radiation at the signal wavelength into the KTA OPO cavity. We also demonstrate a tandem OPO pumped by a CW-diode-pumped, Q-switched, 4-kHz-repetition-rate, Nd:YLF laser.

The quantum cascade laser (QCL) is an important laser source in the mid-infrared and terahertz frequency range. The past twenty years have witnessed its tremendous development in power, wall plug efficiency, frequency coverage and tunability, beam quality, as well as various applications based on QCL technology. Nowadays, QCLs can deliver high continuous wave power output up to 5.1 W at room temperature, and cover a wide frequency range from 3 to 300 μm by simply varying the material components. Broadband heterogeneous QCLs with a broad spectral range from 3 to 12 μm, wavelength agile QCLs based on monolithic sampled grating design, and on-chip beam QCL combiner are being developed for the next generation tunable mid-infrared source for spectroscopy and sensing. Terahertz sources based on nonlinear generation in QCLs further extend the accessible wavelength into the terahertz range. Room temperature continuous wave operation, high terahertz power up to 1.9 mW, and wide frequency tunability form 1 to 5 THz makes this type of device suitable for many applications in terahertz spectroscopy, imaging, and communication.

Wavelength tunable picosecond pulse bunch laser around 3 µm through PPMgLN –based DFG with temperature insensitive high power scaling

The method of adiabatic frequency conversion, in analogy with the two level atomic system, has been put forward recently and verified experimentally to achieve robust frequency mixing processes such as sum and difference frequency generation. Here we present a comparative study of efficient frequency mixing using various techniques of shortcuts to adiabaticity (STA) such as counter-diabatic driving and invariant-based inverse engineering. We show that, it is possible to perform sum frequency generation by properly designing the poling structure of a periodically poled crystal and the coupling between the input lights and the crystal. The required crystal length for frequency conversion is significantly decreases beyond the adiabatic limit. Our approach significantly improves the robustness of the process against the variation in temperature as well as the signal frequency. By introducing a single parameter control technique with constant coupling and combining with the inverse engineering, perturbation theory and optimal control, we show that the phase mismatch can be further optimized with respect to the fluctuations of input wavelength and crystal temperature that results into a novel experimentally realizable mixing scheme.

In this paper, a high efficient terahertz source based on n-type gallium phosphide crystal via cryogenic process is investigated through collinear difference frequency generation pumper by 1064 nm Nd:YAG laser and its OPO system. Absorption coefficient of this crystal at THz range shows a dramatic decrease from ~ 50 cm-1 to 0.5 cm--1 as the temperature decreases from 300 k to 80 k. Four times enhancement of the terahertz emission power and much more broad spectra range (~ 0.2- 3.8 THz) has been achieved in this kind of 0.5 mm length gallium phosphide crystal during the whole varied temperature difference frequency generation from 300 k to 80 k. These results indicate that cooling down the crystal temperature is an effective way to improve the terahertz source property, such as terahertz output power and frequency range.

Owing to their ability to generate terahertz radiation at relatively high temperatures, strong interest has been shown in developing quantum-cascade lasers for imaging and spectroscopy applications. Unlike conventional semiconductor lasers, the operation of this type of device is not based on the characteristics of the conduction and valence bands, but relies on transitions between the electronic sub-bands in quantum wells, making emission possible in a frequency range of 2‐5 THz. Until very recently most of the reported terahertz quantum-cascade lasers have been edge emitters. However, small out-coupling of the light results in a strongly divergent beam and hence poor slope efficiency. This is the problem that researchers from Harvard University in the USA and the University of Leeds in the UK have teamed up to investigate 1 (/nphoton/reshigh/2006/1206/full/nphoton.2006.72.html#B1) . The approach they take is to fabricate double-metal waveguide quantum-cascade lasers — placing cladding on both sides of the active region. Previous surface-emitting structures have used single-metal waveguides, but these cannot provide the same level of light confinement. The key to the new design is the incorporation of second-order diffraction gratings: first-order diffraction is perpendicular to the plane leading to surface emission, whereas second-order diffraction is in-plane and provides the required feedback. The researchers observe output powers comparable to edge-emitting lasers but with twice the slope efficiency. The device also shows single-mode behaviour and only a small (~6 degrees) beam divergence along the laser ridge. The team also predict that the double-metal waveguides could find a use in future terahertz sources that use difference-frequency generation between two mid-infrared laser modes.