Development of coherent tunable source in 2–16 μm region using nonlinear frequency mixing processes

The generation of tunable narrow-band light in the mid-infrared region is of considerable interest in pollution monitoring, remote sensing and trace gas detection. Nonlinear difference frequency mixing (DFM) is one of several techniques which allow access to the important 3-20 μm region, where many molecules have fundamental absorptions. Recent advances in single longitudinal mode high-power diode-laser technology, coupled with new developments based on periodic poling of non-linear crystals and improved growth techniques for birefringently phasematched crystals (such as GaSe), has made DFM a promising approach for a compact broadly tunable, narrowband infrared source. A judicious selection of pump laser wavelengths and non-linear crystals will allow most of the mid-infrared spectral region to be accessible by DFM. Our approach involves DFM of two high power single longitudinal mode diode lasers in the non-linear crystal, GaSe. We combine the broad tuning-range of the diode lasers with the extended critical phasematching range of GaSe to produce a widely tunable system with which we have implemented computer controlled diode frequency tuning, automatic crystal angle positioning, and automatic alignment of the pump beams into the crystal.

The pump intensity required to achieve operation of an optical parametric oscillator based upon a resonantly enhanced four-wave mixing process is examined. The effects of a finite phase mismatch and of absorption of the generated waves are treated. Numerical calculations for an atomic system (Na) and two molecular systems (HCl, CO) reveal acceptably low pump intensity requirements and suggest that such a device is potentially an efficient, scalable source of tunable coherent radiation.

We report direct measurements of the tunable spin-flip Raman (SFR) laser linewidth, obtained by heterodyning the SFR laser output with the output from a cw gas laser. The measured linewidth for the cw SFR laser operating near 5.3 \ensuremath{\mu}m is 1 kHz and is limited by the spectrum analyzer resolution. The measured linewidth of 1 kHz is the narrowest known for any tunable source of coherent radiation in the infrared.

\Ve present a new heterodyne technique for detecting weak signals beyond the standard shot-noise limit by employingamplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from ahighly efficient (up to 60% conversion efficiency) single-pass second-harmonic generation in a nonlinear crystal (KNbO3)pumped by femtosecond (130 fs) pulses at 857.7 nm. The weak signal field is introduced from the entrance port ofthe crystalto combine with the generated squeezed LO field. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratiobeyond the shot-noise limit is directly observed for this new heterodyne detection scheme. Practical applications of proposedtechnique in laser radar systems are discussed.Keywords: second harmonic generation, squeezed light 1. INTRODUCTION Second harmonic generation (SHG) is one of the nonlinear frequency conversion methods used as a means to extend coherentlight sources to shorter wavelengths. Having at the input of the nonlinear medium a fundamental field at a given frequency(no, one can obtain a field at doubled frequency 2wo with a quite high conversion efficiency, especially in case of pulse lasersbeing used as the fundamental field for pumping. The efficiency of the SHG conversion process is dependent on suchparameters as the input pump power, traveled distance inside the nonlinear medium, and the values of the medium's nonlinearcoefficients.i When one considers SHG from ultrashort pulses (such as pulse widths below half of a picosecond) in certainnonlinear media (such as KNbO3) processes like group velocity dispersion have to be taken into account.2One of the attractive features of SHG is the resulting suppression of the amplitude noise of the generated light. Amplitude-squeezed states of light have less quantum fluctuations in photon numbers of the light field than those of the coherent field(such as lasers).3 Unlike squeezed vacuum states of light, amplitude squeezed states of light can have intense optical power4,which allows direct replacement of coherent lasers in precision optical measurements where quantum shot-noise associatedwith the laser field needs to be suppressed.It has been observed that the degree of squeezing is proportional to the efficiency of SHG process,56 where the efficiency isdefined as the ratio of the generated second-harmonic light power to the input fundamental power. Therefore, configurationsof highly efficient SHG processes are very desirable for the generation of squeezed light. Using short pulse laser to pumpSHG process has a main advantage, which is the gain of its high peak power. Since the input power is one of the parametersthat the efficiency of SHG depends on, the high peak intensities that can be reached in short laser pulses contribute to theachievement of high conversion efficiencies in the nonlinear crystal. Recently, single-pass traveling-wave (TW) SHG hasbeen proposed and experimentally demonstrated in a bulk crystal7 and in quasi-phase-matched nonlinear waveguide8.Conversion efficiencies up to 60% were obtained in the experiments with nonlinear waveguides, and O.35-dB squeezing inthe generated second harmonic light was observed. We have shown that conversion efficiencies higher than 60% andamplitude squeezing of more than 1.0 dB could be obtained in a conceptually simple system of type I TW SHG in a bulkcrystal .

Generation of ultrashort light pulses tunable in the whole visible and infrared spectral region is of considerable interest for a wide range of applications. Nonlinear frequency mixing techniques are well suited to generate such ultrashort tunable light pulses with a high repetition rate in these wavelength ranges. In particular, second-harmonic, sum, and difference frequency generation allow for the generation of tunable light pulses in a wide wavelength range depending on the laser sources used. Femtosecond pulses, e.g., have been generated in the wavelength range from 1.3 μm to 1.6 μm by difference frequency mixing of the cw mode- locked radiation of a Nd:YAG laser and a dye laser.1,2 In this contribution we report on the generation of tunable picosecond pulses in the wavelength range from 620 nm to 680 nm, 725 nm to 900 nm, and 2.6 μm to 4 μm by frequency conversion processes between the cw mode-locked radiation of a NaCl:OH color center laser and a Nd:YAG laser which is also used as the pump source for the color center laser.

Broadly tunable compact visible laser sources in the spectral region of 500–650 nm are valuable in biophotonics, photomedicine and for many applications including spectroscopy, laser projection and confocal microscopy. Unfortunately, commercially available lasers of this spectral range are in practice bulky and inconvenient in use. An attractive method for the realization of portable visible laser sources is the frequency-doubling of the infrared laser diodes in a nonlinear crystal containing a waveguide [1]. Nonlinear crystal waveguides that offer an order-of-magnitude increase in the IR-to-visible conversion efficiency also enable a very different approach to second-harmonic generation (SHG) tunability in periodically-poled crystals, promising order-of-magnitude increase of wavelength range for SHG conversion. This is possible by utilization of a significant difference in the effective refractive indices of the high-order and low-order modes in multimode waveguides [2]. The recent availability of low-cost, good quality semiconductor diode lasers, offering the coverage of a broad spectral range between 1 μιη and 1.3 μπι [3,4], in combination with well-established techniques to fabricate good quality waveguides in nonlinear crystals, allows compact tunable CW laser sources in the visible spectral region to be realized [2].

Light with helical phase structures, carrying quantized orbital angular momentum (OAM), has many applications in both classical and quantum optics, such as high-capacity optical communications and quantum information processing. Frequency conversion is a basic technique to expand the frequency range of the fundamental light. The frequency conversion of OAM-carrying light gives rise to new physics and applications such as up-conversion detection of images and generation of high dimensional OAM entanglements. Quasi-phase matching (QPM) nonlinear crystals are good candidates for frequency conversion, particularly due to their high-valued effective nonlinear coefficients and no walk-off effect. Here we report the first experimental second-harmonic generation (SHG) of an OAM-carried light with a QPM crystal, where a UV light with OAM of 100 ℏ is generated. OAM conservation is verified using a specially designed interferometer. With a pump beam carrying an OAM superposition of opposite sign, we observe interesting interference phenomena in the SHG light; specifically, a photonics gear-like structure is obtained that gives direct evidence of OAM conservation, which will be very useful for ultra-sensitive angular measurements. Besides, we also develop a theory to reveal the underlying physics of the phenomena. The methods and theoretical analysis shown here are also applicable to other frequency conversion processes, such as sum frequency generation and difference-frequency generation, and may also be generalized to the quantum regime for single photons.

In order to investigate the dielectric and magnetic parameters of soft ferrimagnet in millimeter waves, several yttrium iron garnet (YIG) and hexagonal ferrite ceramic (HFC) are measured. The complex permittivity and permeability of the YIG and HFC have been obtained in a broad-band spectrum. Measurements are realized by using a broad-band quasi-optical millimeter-wave system with a backward-wave oscillator as the source of tunable coherent radiation. Moreover, the alternating-direction-implicit finite-difference time- domain method has been used to simulate the YIG and HFC models. The experiment results are in good agreement with the numerical curves. It is shown that these results provide an accurate characterization of soft ferrites as well as hard anisotropic ferrites in the entire millimeter-wave range.

The generation of tunable coherent uv radiation by sum frequency mixing, in potassium pentaborate, the outputs of two tunable N(2) pumped dye lasers has been investigated. Tunable radiation extending from 217 nm to 208 nm has been generated in this manner. A conversion efficiency of 2.5 x 10(-4) is observed at input powers of ~2 kW in each laser beam. Improved refractive index data for potassium pentaborate are also presented.

In this paper, complex permittivity and permeability of several yttrium iron garnet (YIG) and hexagonal ferrite ceramics are investigated in a broad-band spectrum for the first time. The measurements were realized using a broad-band quasi-optical millimeter-wave (QOM) system with a backward-wave oscillator as the source of tunable coherent radiation. The dielectric and magnetic parameters of the ferrimagnets were obtained by fitting theoretical curves to the experimental spectra. Three types of YIG specimen were initially measured and compared to theoretical results, and then hexagonal ferrites were measured in the demagnetized state for the first time. It was shown that this method provides an accurate characterization of soft ferrites as well as hard anisotropic ferrites in the entire millimeter-wave range.

Crystals that are simultaneously χ(2)- and χ(3)-active offer a wide range of possibilities for the generation of new coherent wavelengths of light. Frequency conversion processes such as stimulated Raman scattering, second and third harmonic generation, or parametric sum and difference frequency mixing can be combined effectively in the same noncentrosymmetric crystal in cascaded χ(3) ↔ χ(2) lasing processes. We present several new manifestations of these effects under picosecond laser excitation in α-quartz (SiO2), the oldest nonlinear-laser crystal. Among them are 45 Stokes and anti-Stokes wavelength comb generation of more than two octaves (from 0.3692 μm to 1.5142 μm) and self-conversion of Raman-generation frequencies into the wavelength region of the second and third harmonics of one-micron pumping via many-step cascaded χ(3) ↔ χ(2) processes.

The present work is devoted to the study of the dynamics of multi-photon processes in semiconductor heterostructures. A time-dependent description is important for understanding in detail the transient response of semiconductors excited by ultrashort optical pulses. In the first part of this thesis, we set up a phenomenological model based on rate equations, in order to investigate the possibility of measuring degenerate two-photon gain in a semiconductor microcavity. The amplification predicted by the model is fairly low ( 2%) and mainly limited by the intra-band relaxation of the carriers, which leads to rapid saturation. In the second part, we develop a general theory for the dynamics of multi-photon processes in semiconductors. It will give insight into complex effects related to the coherence between the bands, which are not included in usual absorption coefficients or susceptibilities. For this purpose, we derive effective multi-band Bloch equations that include resonant multi-photon processes induced by two linearly polarized electromagnetic pulses of frequency close to the band gap and close to the half of the band gap respectively. The benefit of the proposed approach is two-fold. First, the description of the dynamics is restricted to a reduced number of bands. However, the discarded bands are not neglected, but consistently taken into account in the higher order processes. Second, all quantities appearing in the effective multi-band Bloch equations vary on the same time scale, which makes the numerical integration much more efficient. The time-dependent polarization current, as well as some susceptibilities, are derived on a consistent level of approximation, and are discussed in detail. The propagation of the electromagnetic fields is neglected. Such a model is appropriate for the description of low-dimensional quantum confined systems (e.g. quantum wells or quantum wires) excited by two colinearly propagating pulses. It accounts for various linear and nonlinear optical processes, such as one- and two-photon absorption, second-harmonic generation, difference-frequency mixing, or coherent control of photocurrent. In this thesis, the general theory is applied to the study of three specific physical situations. First, we investigate the charge and spin current in a symmetric AlGaAs/GaAs quantum well, injected by interference between one- and two-photon inter-band transitions. We identify new coherent terms in the expression of the current, which contribute significantly to the terahertz emission. The effects of the Stark shifts and the inter-valence band two-photon transitions are also calculated and discussed. Second, we calculate the anisotropic two-photon absorption spectra of an AlGaAs/GaAs V-shaped quantum wire with realistic band structure. The Coulomb interaction is taken into account within the Hartree-Fock approximation. The various excitonic peaks are identified with respect to the involved subbands and to the symmetry properties. We also show that excitons that are dark for one-photon excitation may become bright for two-photon spectroscopy, when the light is polarized perpendicularly to the growth direction, but not along a symmetry axis of the wire. Finally, the last application focuses on the optical injection of current in the presence of excitonic effects. Concentrating on the same AlGaAs/GaAs V-shaped quantum wire, we show that the Coulomb interaction within the Hartree-Fock approximation induces terahertz oscillations in the injected charge current. The oscillation frequency corresponds to the energy spacing between the two lowest excitonic resonances, slightly below the band gap, excited respectively by the laser pulse with frequency close to the band gap, and the one with frequency close to the half of the band gap.

We report a theoretical analysis on spatial noncolinear phase matching of multiwave mixing and its application to a second-harmonic-generation (SHG) experiment. From the numeric calculations, the noncolinear phase-matching properties in general situations were determined. The theory gives the applicability for all noncolinear phase matching. Fine coincidences between theoretical calculations and observed spatial loci on noncollinear phase-matching SHG were confirmed. Relations that allow the calculation of the noncollinear phase-matching angle for any case of SHG are established. As an example, the noncolinear phase-matched SHG pattern on a screen is calculated numerically in the case of SHG of 1064 nm from a Nd:YAG laser under the phase-matched condition for two organic nonlinear crystals: 1-(2-thienyl)-3-(4-methyphenyl) propene-1 (TC-28), which is biaxial, and (2-furyl) methacrylic anhydride (FMA), which is uniaxial. Experimental results compared quite favorably with the theoretical analysis. Noncolinear phase matching may be of great practical interest in optical multiwave-mixing processes, such as optical parametric oscillation and optical parametric amplification. This technique also can be used for the measurement of crystal optical constants.

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). However, a phase modulator operating at the frequency-converted wavelength would be needed, which is a non-standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators. Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error. In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated. The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm. CBC of mid-infrared DFG modules is a first step towards combining continuous wave OPOs.

Cascading refers to phenomena associated with the multiple interchange of photons between beams which interact parametrically via second order nonlinearities. [1] Although second harmonic generation (SHG) is the most common example, it also involves sum and difference frequency mixing, optical parametric amplification etc. It has been known since the early days of nonlinear optics that two successive second order interactions can contribute to a third order nonlinearity via χ(2)ijkχ(2)kmn ➔χ(2)ijmn(effective) [2,3] We have found that the effects are more subtle than this and they can give rise to many interesting effects in nonlinear optics. They are essentially of two types, one based on the nonlinear phase shift accumulated by the fundamental beam on transmission through a second order medium and the second is a consequence of the strong coupling which occurs at high intensities between the fundamental and harmonic beams. The second leads to the generation of spatial solitary waves. [4] Here we discuss our progress in studying these effects.