High-efficiency, broadly tunable, tandem optical parametric oscillators

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.

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.

A compact and stable terahertz (THz) source is demonstrated based on difference frequency generation (DFG) pumped by an efficient dual-wavelength acousto-optic (AO) Q-switched solid-state Nd:YLF laser with composite gain media (a-cut and c-cut) in the coaxial pumping configuration. Optimal power ratio and pulse synchronization of the orthogonal polarized 1047/1053 nm dual-wavelength laser could be realized by varying the pump focusing depth and/or pump wavelength. The total power of 2.92 W was obtained at 5 kHz pumped by 10-W laser-diode power at 803 nm. Such an efficient dual-wavelength laser demonstrated good stability and inconspicuous timing jitter benefiting from the suppressed gain competition between two resonating wavelengths. An 8-mm-long GaSe crystal was employed to generate THz waves at 1.64 THz by DFG and the maximum THz average output power was about 0.93 μW. This compact coaxial pumping method can be extended to all kinds of neodymium (Nd) doped laser crystals to produce different dual-wavelength lasers for various THz wavelength generation, which have good prospects for portable and costless applications like imaging, non-destructive inspection, etc.

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.

The optical and nonlinear optical properties of 2-(N-prolinol)-5-nitropyridine (PNP) have been investigated. The refractive indices show strong dispersion and high birefringence in the spectral range from 488 to 1064 nm. At 632 nm the main refractive indices are nx=1.990, ny=1.788, and nz=1.467. The two major nonlinear optical susceptibilities were determined (dyxx=48 pm/V, dyyy=17 pm/V). Angle-tuned phase-matched second-harmonic generation was demonstrated at 1064 nm. Type I noncritical phase matching using the nonlinear optical susceptibility dyxx is predicted for a fundamental wavelength λ=1020 nm.

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.

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).

A combined optical parametric oscillator (OPO) with continuous tuning of the radiation wavelength in the spectral range 2.5–10.8 μm, optically pumped with the radiation from a Q-switched Nd : YLF laser (1.053 μm), is developed and tested. The oscillation is provided by an OPO1 based on a MgO : PPLN ‘fan-out’ structure in the spectral region 2.5–4.5 μm and by an OPO2 based on HgGa2S4 nonlinear crystals in the spectral region 4.18–10.8 μm, respectively. The angles of phase matching are measured for the HgGa2S4 crystals in the spectral range 4.18–10.8 μm for the type II conversion (eo-e), which virtually coincide with the calculated ones. The experimental absorption spectra of a gas mixture in the range 2.5–10.8 μm obtained using a gas-filled sealed-off photoacoustic cell are presented.

The aim of this survey is to compare the performance of different nonlinear crystals used as optical parametric oscillator material when pumped at 1064nm with short pulse and low repetition rate Nd:YAG laser. Signal and Idler wavelengths have been monitored and slope efficiency viewgraphs as well as other relevant parameters displayed. Some experiments with KTP used as monolithic OPO have been performed and discussed in this paper.

Ultrafast picosecond coherent sources in the near-to-mid-infrared (IR) spectral range are of great interest for a variety of applications such as pump-probe spectroscopy, remote sensing, photobiology and novel upconversion imaging techniques. Nonlinear optics, and in particular nonlinear frequency conversion techniques, offer an efficient and effective approach towards the realization of sources emitting such radiation, as nowadays, nonlinear frequency conversion technologies are recognised to be viable and reliable sources of laser radiation with broad wavelength tunability and power scalability, without the need of cryogenic cooling. In this thesis, we have demonstrated high-power, high-repetition-rate picosecond sources based on nonlinear frequency conversion processes through optical parametric oscillators (OPOs) and difference-frequency-generation (DFG), in order to cover the near- to mid-IR wavelength region. We have developed a stable, high-repetition-rate picosecond rapidly tunable OPO based on fan-out designed grating periods in PPKTP nonlinear crystal. The OPO is synchronously pumped by a mode-locked frequency-doubled Yb-fiber laser in the green at 532 nm, and can provide stable and high-power radiation which is rapidly tunable from 749-962 nm in the signal and from 1189-1838 nm in the idler, at room temperature. Further, we have demonstrated what we believe to be the first tunable high-repetition-rate picosecond source based on OP-GaP crystal in the mid-IR. Using a single-pass DFG between a mode-locked Yb-fiber laser at 1064 nm and the tunable output from a picosecond MgO:sPPLT OPO synchronously pumped by the same laser, the source generated continuous tunable radiation across 3040-3132 nm in the mid-IR at the repetition rate of ~80 MHz, in good beam quality. Additionally, we have also presented the first high-power, high-beam-quality, idler-resonant picosecond OPO based on a multi-grating MgO:PPLN crystal tunable across 2100-4000 nm in the mid-IR. The OPO provided as much as 3.5 W of mid-IR radiation with M2 values to be better than 1.8 in both horizontal and vertical directions Las fuentes coherentes de picosegundos ultrarrápidos en el rango espectral de infrarrojo cercano a infrarrojo medio (IR) son de gran interés para una amplia variedad de aplicaciones tales como pump-probe espectroscopia, la teledetección, la fotobiología y las nuevas técnicas de upconversion imaging. La óptica no lineal, y en particular las técnicas de conversión de frecuencia no lineal, ofrecen un enfoque eficiente y eficaz para la realización de fuentes que emiten tales radiaciones, ya que hoy en día, las fuentes basadas en la conversión de frecuencia no lineal son fuentes viables y fiables de emisión de radiaciones láser con amplia sintonía de longitud de onda y escalabilidad de potencia, sin necesidad de refrigeración criogénica. En esta tesis, hemos demostrado fuentes de picosegundos de alta potencia y alta tasa de repetición basadas en procesos de conversión de frecuencia no lineales a través de osciladores ópticos paramétricos (OPO) y generación de frecuencias de diferencia (DFG) para cubrir la región de longitud de onda de IR cercano a IR medio del espectro electromagnético. Hemos desarrollado una fuente de picosegundos estable, de alta tasa de repetición, rápidamente sintonizable, basado en un cristal PPKTP con períodos de rejilla diseñados en forma de abanico. El OPO, es bombeado sincrónicamente por un mode-locked láser de fibra de Yb, doblado en frecuencia para generar verde a 532 nm, que proporciona radiación estable y de alta potencia rápidamente sintonizable de 749-962 nm en el signal y de 1189-1838 nm en el idler, a temperatura ambiente. Además, hemos demostrado lo que creemos que es la primera fuente de picosegundos sintonizable con alta tasa de repetición basada en el cristal OP-GaP en el IR medio. Usando una DFG de un solo paso entre un mode-locked láser de fibra de Yb a 1064 nm y la salida ajustable de un OPO de picosegundo basado en un cristal de MgO:sPPLT bombeado sincrónicamente por el mismo láser, la fuente generó radiación sintonizable de 3040-3132 nm en el medio-IR a la velocidad de repetición de ~80 MHz, con buena calidad de haz. Además, también presentamos el primer OPO de picosegundos de alta potencia y buena calidad de haz, con resonancia del idler, basado en un cristal MgO:PPLN de rejilla múltiple sintonizable a lo largo de 2100-4000 nm en el IR medio. El OPO proporcionó hasta 3.5 W de radiación de IR medio con valores de M2 mejores que 1.8 tanto en dirección horizontal como vertical

Difference-frequency generation (DFG) is a powerful technique for generating widely tunable infrared radiation. However, conventional phase-matching schemes may require tuning multiple parameters—such as the wavelengths, crystal temperature, crystal angle, and poling period—to achieve wide tunability, which increases the complexity of practical operation. In this work, we employ a backward quasi-phase-matching scheme with distinctive tuning characteristics and demonstrate pump-enhanced continuous-wave DFG output tunable from 1751 to 2451 nm (700 nm range) in a bulk crystal. The tuning is achieved solely by varying the pump wavelength and the signal wavelength (less than 5 nm), enabling continuous, rapid, and room-temperature operation. The tuning characteristics, power-scaling behavior, and output stability are experimentally verified with the idler wavelength set at 2000 nm. The approach offers a paradigm for widely tunable infrared radiation generation and holds promise for applications in spectroscopy and biomedical sensing.

We report a chirped-pulse optical parametric oscillator (CPOPO) based on a KTiOAsO4 (KTA) crystal. Due to the relatively low ratio between its second-order and third-order nonlinear susceptibility, a single KTA crystal could provide parametric gain and intra-cavity spectral broadening simultaneously in a CPOPO. Numerical simulations show that a signal-bandwidth of 390 nm can be obtained from a KTA-based CPOPO, and the pulses can be de-chirped with a width of ∼20fs outside the OPO cavity. Experimentally, from a fiber-laser-pumped OPO with a 3-mm-long KTA crystal, we obtained a signal wave covering 1332-1667 nm, with a -20dB bandwidth of 45.3 THz, around 12 times as much as the gain-bandwidth of the KTA crystal.

Tunable wavelength conversion of picosecond (ps)-pulses based on cascaded sum- and difference frequency generation is proposed and demonstrated by use of a fiber ring laser incorporating a periodically poled lithium niobate waveguide and two parallel-arranged tunable filters (TFs). The pulsed signal with 40-GHz repetition rate and 1.57-ps pulsewidth is employed. No external pump and control sources are required in the wavelength converter. By properly adjusting two TFs to change the lasing pump and control wavelengths, flexible wavelength conversion from a changeable input signal wavelength to a variable output idler wavelength is achieved

The subject of this study was to investigate the threshold radiant exposures for bubble formation at single porcine melanosomes in suspension and for porcine RPE cell damage when using pulse durations in the ns to microsecond(s) time regime. A frequency doubled Nd:YLF laser ((lambda) equals 527 nm) with adjustable pulse duration between 250 ns and 3 microsecond(s) and a Q- switched Nd:YAG laser ((lambda) equals 532 nm, (tau) equals 8 ns) were used for the single pulse irradiation. Fast flash photography was applied to probe vaporization around individual melanosomes while a fluorescence viability assay was used to probe cell vitality. Applying single ns laser pulses to RPE cells, an ED50 threshold radiant exposure of 84 mJ/cm2 was determined, which is close to the vaporization threshold around single melanosomes. When irradiating with pulse durations of 3 microsecond(s) , a threshold of about 223 mJ/cm2 was measured, which is only 40% lower of the vaporization threshold around the single melanosome at that pulse width. This can be explained with heat contribution from adjacent melanosomes, which increases towards longer pulse durations. Calculations are in good agreement with the experimental results when assuming a surface temperature at the melanosome of 140 degrees Celsius and an absorption coefficient of 8000 cm-1 to initiate vaporization. It can be concluded that the origin of RPE cell damage for single pulse irradiation with a duration of 8 ns results from transient microbubbles around the melanosomes, which lead to a transiently increased cell volume and subsequently a rupture of the cell structure. It is also likely that the same effect plays the major role when using pulse durations up to 3 microsecond(s) .

The absolute values and dispersion of the second harmonic generation (SHG) coefficient of the cubic wide‐gap II–VI semiconductors Zn Te, ZnSe, and ZnS are investigated within the spectral range from 1047 to 1321 nm. The fundamental beam is generated by a Nd:YLF laser and by a tunable KTP optical‐parametric oscillator (OPO) pumped by a ps‐Ti:sapphire laser system. The measured values of the SHG coefficient are significantly higher than in commercially used birefringent crystals as KDP or ADP. In a further step symmetric single crystalline ZnSe/ZnTe/ZnSe/GaAs(001) waveguides are grown by metalorganic vapour phase epitaxy (MOVPE). Using end‐fire coupling a strong phasematched signal of the 1170.5 nm fundamental wave is observed. Phase matching is achieved by coupling the TE0 fundamental mode to the TM2 second harmonic mode.