Ingo Breunig | Albert-Ludwigs-University of Freiburg (original) (raw)
Papers by Ingo Breunig
Optik & Photonik, 2013
A single source emitting single-frequency light with tunable color and without the need of changi... more A single source emitting single-frequency light with tunable color and without the need of changing toxic chemicals, as it is the case for dye lasers, is desired by many scientists. The Department of Microsystems Engineering IMTEK at the University of Freiburg together with the Fraunhofer Institute for Physical Measurement Techniques IPM now extend the tuning range of continuous-wave optical parametric oscillators into the visible spectrum by integrated, intra-cavity frequency doubling. Combination of these two nonlinear-optical effects enables the operator to deliberately choose any wavelength between 430 and 690 nm.
2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference (CLEO EUROPE/EQEC), 2011
Continuous-wave optical parametric oscillators (OPOs) combine single-frequency output with a huge... more Continuous-wave optical parametric oscillators (OPOs) combine single-frequency output with a huge tuning range. These properties have let them become working horses for spectroscopy in the near and mid infrared over the last decades [1]. However, singly resonant OPOs require high pump powers. Multiply resonant systems operate at much smaller pump powers, but tunability is hindered by high demands to the dielectric coatings of resonator mirrors. Whispering gallery resonators (WGRs) are an elegant solution since resonance in these monolithic spheroids arises from total internal reflection: the OPO process can be triply resonant across broad wavelength ranges, with quality factors limited by material absorption, yielding mW pump thresholds [2]. Lithium niobate (LiNbO3) exhibits low absorption and high nonlinear coefficients in the near infrared, making it a common material for nonlinear devices. Its ferroelectric domains can be structured periodically to compensate the phase mismatch between the waves involved in a nonlinear process, a standard procedure in linear resonators [3]. The optimal domain structure for use in circular WGRs is a radial pattern (Fig. 1a). Since the light passes through the domain structure repeatedly, the poling structure can be described by discrete spatial frequencies kj = j=2pR in a WGR with radius R. For a perfectly centered radial poling pattern with 2N domains, only the harmonics j = N; 3N;::: are nonzero, The Nth harmonic is the largest, with a coefficient SN = 2=p. A new degree of freedom lies in the distance dR between the center of the poling pattern and that of the resonator shape (Fig. 1b): nonzero displacements result in a chirped structure along the rim; additional spatial frequencies Sj6N appear as the largest component SN decreases. We have manufactured an R = 1:55 mm (LiNbO3) WGR from a sample with 2N = 686 radial domains. Pump light of 1 mm wavelength is coupled into the resonator by frustrating the total internal reflection with a rutile prism. With a pump threshold of a few mW, the output wavelengths of the parametric oscillation can be tuned from 1.78 to 2.5 mm by temperature changes of 40 K (Fig. 1c). The branches of signal and idler wavelengths correspond to three discrete Sj, and the branch spacing is that expected for a resonator of this size.
Quantum Sensing and Nanophotonic Devices XI, 2013
We demonstrate a whispering gallery optical parametric oscillator pumped at 488 nm wavelength. Th... more We demonstrate a whispering gallery optical parametric oscillator pumped at 488 nm wavelength. This millimetersized device has a pump threshold of 160 μW. The signal field is tunable between 707 and 865 nm wavelength and the idler field between 1120 and 1575 nm through temperature variation. Although the conversion efficiency is fundamentally limited to several percent because of absorption loss for the pump wave, the results provide evidence that such oscillators will be able to cover finally the entire visible range.
Laser Resonators and Beam Control XII, 2010
ABSTRACT Providing optical feedback by a resonator enhances the efficiency of nonlinear optical e... more ABSTRACT Providing optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 mum wavelength to signal light being tunable from 1.25 to 1.85 mum and to corresponding idler light from 2.3 to 5.3 mum. We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of 1.35 THz and a power of 2 muW. Now, the straightforward next development step is to reduce the footprint of such devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity mode for 3 hours.
Optics letters, 2015
We achieve a continuous operation of a whispering gallery optical parametric oscillator by stabil... more We achieve a continuous operation of a whispering gallery optical parametric oscillator by stabilizing the resonator temperature T on the mK level and simultaneously locking the pump frequency to a cavity resonance using the Pound-Drever-Hall technique. The millimeter-sized device converts several mW of a pump wave at 1040 nm wavelength to signal and idler waves around 2000 nm wavelength with more than 50% efficiency. Over 1 h, power and frequency of the signal wave vary by <±1% and by <±25 MHz, respectively. The latter can be tuned over 480 MHz without a mode hop by changing T over 120 mK. In order to prove the suitability for high-resolution spectroscopy, we scan the signal frequency across the resonance of a Fabry-Perot interferometer resolving nicely its 10 MHz linewidth.
Optics express, Jan 16, 2013
Optical whispering gallery modes (WGMs) of mm-sized axisymmetric resonators are well localized at... more Optical whispering gallery modes (WGMs) of mm-sized axisymmetric resonators are well localized at the equator. Employing this distinctive feature, we obtain simple analytical relations for the frequencies and eigenfunctions of WGMs which include the major radius of the resonator and the curvature radius of the rim. Being compared with results of finite-element simulations, these relations show a high accuracy and practicability. High-precision free-spectral-range measurements with a millimeter-sized disc resonator made of MgF(2) allow us to identify the WGMs and confirm the applicability of our analytical description.
Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XIII, 2014
ABSTRACT We demonstrate the continuous-wave operation of a cascade that has been successfully app... more ABSTRACT We demonstrate the continuous-wave operation of a cascade that has been successfully applied so far only for picosecond systems: A doubly-resonant optical-parametric oscillator (OPO) based on lithium niobate generates signal and idler waves close to degeneracy. Subsequently, these two light fields are converted to a terahertz wave via difference frequency mixing in an orientation-patterned gallium arsenide crystal placed inside the OPO cavity. Using this scheme, we achieved tunability from 1 to 4:5 THz frequency, a linewidth smaller than 10 MHz, and a Gaussian beam profile. The output power is of the order of tens of μW, with a scalability into the milliwatt regime.
Optisch-parametrische Oszillatoren (OPOs) sind weit abstimm- bare und dadurch variabel einsetzbar... more Optisch-parametrische Oszillatoren (OPOs) sind weit abstimm- bare und dadurch variabel einsetzbare Lichtquellen. Diese sind für den sichtbaren Spektralbereich bislang allerdings nur im gepulsten Betrieb kommerziell erhältlich. In diesem Artikel stellen wir zwei Konzepte vor, mit deren Hilfe kontinuierlich emit- tierende OPO-Systeme das sichtbare Spektrum erobern.
Continuous-wave optical parametric oscillators (OPOs) with periodically-poled lithium niobate as ... more Continuous-wave optical parametric oscillators (OPOs) with periodically-poled lithium niobate as the nonlinear medium combine a wide tuning range with single-frequency operation. Singly-resonant systems pumped by lasers with a wavelength of 1 mum have been demonstrated for more than ten years. They have pump thresholds of about 1 W, and most of them have crystal lengths of 50 mm. We show
ABSTRACT Stability and footprint of optical parametric oscillators (OPOs) strongly depend on the ... more ABSTRACT Stability and footprint of optical parametric oscillators (OPOs) strongly depend on the cavity used. Monolithic OPOs tend to be most stable and compact since they do not require external mirrors that have to be aligned. The most straightforward way to get rid of the mirrors is to coat the end faces of the nonlinear crystal. Whispering gallery resonators (WGRs) are a more advanced solution since they provide ultra-high reflectivity over a wide spectral range without any coating. Furthermore, they can be fabricated out of nonlinear-optical materials like lithium niobate. Thus, they are ideally suited to serve as a monolithic OPO cavity. We present the experimental realization of optical parametric oscillators based on whispering gallery resonators. Pumped at 1 μm wavelength, they generate signal and idler fields tunable between 1.8 and 2.5 μm wavelength. We explore different schemes, how to phase match the nonlinear interaction in a WGR. In particular, we show improvements in the fabrication of quasi-phase-matching structures. They enable great flexibility for the tuning and for the choice of the pump laser.
Recently achieved radial poling of whispering-gallery resonators (WGRs) strongly extends the capa... more Recently achieved radial poling of whispering-gallery resonators (WGRs) strongly extends the capabilities of tailoring the second-order nonlinear phenomena, such as second-harmonic generation and optical parametric oscillation, and transferring them to the range of low-power continuous-wave light sources. Owing to discreteness of the frequency spectrum, the resonance and phase-matching conditions for interacting waves cannot be fulfilled simultaneously in WGRs in the general case. Using Yariv's generic approach to the description of WGR phenomena, we analyze two closely related issues: the possibilities to achieve the resonant and phase-matching conditions using the temperature tuning and the impact of detunings and phase mismatches on the nonlinear transformation efficiencies. It is shown that the radial poling provides important necessary conditions for the subsequent fine tuning to the nonlinear resonances. The requirements to the temperature tuning, as exemplified by the case of lithium niobate, are substantially dependent on the nonlinear process in question, the actual wavelength range, and the pump intensity.
ABSTRACT Continuous-wave optical parametric oscillators (OPOs) combine single-frequency operation... more ABSTRACT Continuous-wave optical parametric oscillators (OPOs) combine single-frequency operation with a huge tuning range. Thus, they have emerged as working horses for spectroscopy in the near and mid infrared over the last decades (Dunn and Ebrahimzadeh, 1999). However, in the terahertz range prominent nonlinear materials like lithium niobate have strongly damped phonon resonances leading to huge absorption. Consequently, the generation of terahertz waves by optical parametric oscillation requires around 100 W of near-infrared pump light exceeding the power delivered by standard single-frequency lasers.
Over the past 10 years, with the advent of new crystals designs and a new generation of pump lase... more Over the past 10 years, with the advent of new crystals designs and a new generation of pump lasers, continuous-wave (cw) optical parametric oscillators (OPOs) have developed into mature monochromatic light sources. Nowadays, cw OPOs can fulfill a wide variety of criteria for sensitive molecular gas sensing. It can access the mid-infrared wavelength region, where many molecules have their fundamental rotational-vibrational transitions, with high power. This high power combined with wide wavelength tuning and narrow linewidth creates excellent conditions for sensitive, high-resolution spectroscopy. OPOs combined with robust methods, such as photoacoustic spectroscopy and cavity-enhanced spectroscopy, are well suited for field measurements and remote-sensing applications. The wide tunability of cw OPOs allows detection of larger molecules with broad absorption band structures, and its fast scanning capabilities allow rapid detection of trace gases, the latter is a demand for life-science applications. After a short introduction about the physical principle of cw OPOs, with its most recent physical developments, this review focuses on sensitive molecular gas sensing with a variety of spectroscopic applications in atmospheric and life sciences.
We report a diffraction-limited photonic terahertz (THz) source with linewidth <10 MHz that can b... more We report a diffraction-limited photonic terahertz (THz) source with linewidth <10 MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation-patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near λ = 2 µm. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030 nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10 W of 1030 nm pump, 100 W of intracavity power near 2 µm was attained with GaAs inside cavity.
Laser Resonators, Microresonators, and Beam Control XVI, 2014
ABSTRACT Whispering gallery optical parametric oscillators are millimeter-sized monolithic source... more ABSTRACT Whispering gallery optical parametric oscillators are millimeter-sized monolithic sources for tunable coherent light. Several experiments have revealed that during optical parametric oscillation the pump resonance strongly differs from a Lorentzian shape. We theoretically and experimentally analyze these line-shape distortions. It turns out that the line shape of the pump resonance strongly depends on the coupling strength of the pump light and on the loss ratio between generated light and pump light. The line-widths, i.e. the losses, for the light generated by the parametric process can be deduced without measuring them directly.
A different phase-matching period of 24.4 to 31.0 mum is used in singly-resonant optical parametr... more A different phase-matching period of 24.4 to 31.0 mum is used in singly-resonant optical parametric oscillator (OPO) leading to idler wavelengths from 2.33 to 5.32 mum. A frequency shift depending on the poling period would be a non-ambiguous proof that not a Raman effect is observed.
Superconductor Science and Technology, 2004
We have developed a scanning magnetic microscope (SMM) with 25 µm resolution in spatial position ... more We have developed a scanning magnetic microscope (SMM) with 25 µm resolution in spatial position for the magnetic features of room temperature objects. The microscope consists of a high-temperature superconductor (HTS) dc SQUID sensor, suspended in vacuum with a self-adjusting standoff, close spaced liquid nitrogen Dewar, X -Y scanning stage and a computer control system. The HTS SQUIDs were optimized for better spatial and field resolutions for operation at liquid nitrogen temperature. Measured inside a magnetic shield, the 10 pT Hz −1/2 typical noise of the SQUIDs is white down to frequencies of about 10 Hz, increasing up to about 20 pT Hz −1/2 at 1 Hz. The microscope is mounted on actively damped platforms, which negate vibrations from the environment as well as damping internal stepper motor noises. A high-resolution video telescope and a 1 µm precision z-axis positioning system allow a close positioning of the sample under the sensor. The ability of the sensors to operate in unshielded environmental conditions with magnetic fields up to about 15 G allowed us to perform 2D mapping of the local ac and dc susceptibility of the objects.
Review of Scientific Instruments, 2011
The combination of an all-optical terahertz source with a photoconductive antenna to achieve cohe... more The combination of an all-optical terahertz source with a photoconductive antenna to achieve coherent detection is presented. This approach aims to overcome the frequency limits introduced by optoelectronic terahertz sources commonly used. Here the Gaussian-shaped and linearly polarized terahertz waves are generated by a continuous wave optical parametric oscillator with a power of 3 μW at 1.4 THz. The infrared signal light of the optical parametric oscillator can be used to coherently detect the generated terahertz wave with a photoconductive antenna. As a proof-of-principle experiment we determine the thickness profile of a plastic lens using phase shifting interferometry.
Optik & Photonik, 2013
A single source emitting single-frequency light with tunable color and without the need of changi... more A single source emitting single-frequency light with tunable color and without the need of changing toxic chemicals, as it is the case for dye lasers, is desired by many scientists. The Department of Microsystems Engineering IMTEK at the University of Freiburg together with the Fraunhofer Institute for Physical Measurement Techniques IPM now extend the tuning range of continuous-wave optical parametric oscillators into the visible spectrum by integrated, intra-cavity frequency doubling. Combination of these two nonlinear-optical effects enables the operator to deliberately choose any wavelength between 430 and 690 nm.
2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference (CLEO EUROPE/EQEC), 2011
Continuous-wave optical parametric oscillators (OPOs) combine single-frequency output with a huge... more Continuous-wave optical parametric oscillators (OPOs) combine single-frequency output with a huge tuning range. These properties have let them become working horses for spectroscopy in the near and mid infrared over the last decades [1]. However, singly resonant OPOs require high pump powers. Multiply resonant systems operate at much smaller pump powers, but tunability is hindered by high demands to the dielectric coatings of resonator mirrors. Whispering gallery resonators (WGRs) are an elegant solution since resonance in these monolithic spheroids arises from total internal reflection: the OPO process can be triply resonant across broad wavelength ranges, with quality factors limited by material absorption, yielding mW pump thresholds [2]. Lithium niobate (LiNbO3) exhibits low absorption and high nonlinear coefficients in the near infrared, making it a common material for nonlinear devices. Its ferroelectric domains can be structured periodically to compensate the phase mismatch between the waves involved in a nonlinear process, a standard procedure in linear resonators [3]. The optimal domain structure for use in circular WGRs is a radial pattern (Fig. 1a). Since the light passes through the domain structure repeatedly, the poling structure can be described by discrete spatial frequencies kj = j=2pR in a WGR with radius R. For a perfectly centered radial poling pattern with 2N domains, only the harmonics j = N; 3N;::: are nonzero, The Nth harmonic is the largest, with a coefficient SN = 2=p. A new degree of freedom lies in the distance dR between the center of the poling pattern and that of the resonator shape (Fig. 1b): nonzero displacements result in a chirped structure along the rim; additional spatial frequencies Sj6N appear as the largest component SN decreases. We have manufactured an R = 1:55 mm (LiNbO3) WGR from a sample with 2N = 686 radial domains. Pump light of 1 mm wavelength is coupled into the resonator by frustrating the total internal reflection with a rutile prism. With a pump threshold of a few mW, the output wavelengths of the parametric oscillation can be tuned from 1.78 to 2.5 mm by temperature changes of 40 K (Fig. 1c). The branches of signal and idler wavelengths correspond to three discrete Sj, and the branch spacing is that expected for a resonator of this size.
Quantum Sensing and Nanophotonic Devices XI, 2013
We demonstrate a whispering gallery optical parametric oscillator pumped at 488 nm wavelength. Th... more We demonstrate a whispering gallery optical parametric oscillator pumped at 488 nm wavelength. This millimetersized device has a pump threshold of 160 μW. The signal field is tunable between 707 and 865 nm wavelength and the idler field between 1120 and 1575 nm through temperature variation. Although the conversion efficiency is fundamentally limited to several percent because of absorption loss for the pump wave, the results provide evidence that such oscillators will be able to cover finally the entire visible range.
Laser Resonators and Beam Control XII, 2010
ABSTRACT Providing optical feedback by a resonator enhances the efficiency of nonlinear optical e... more ABSTRACT Providing optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 mum wavelength to signal light being tunable from 1.25 to 1.85 mum and to corresponding idler light from 2.3 to 5.3 mum. We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of 1.35 THz and a power of 2 muW. Now, the straightforward next development step is to reduce the footprint of such devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity mode for 3 hours.
Optics letters, 2015
We achieve a continuous operation of a whispering gallery optical parametric oscillator by stabil... more We achieve a continuous operation of a whispering gallery optical parametric oscillator by stabilizing the resonator temperature T on the mK level and simultaneously locking the pump frequency to a cavity resonance using the Pound-Drever-Hall technique. The millimeter-sized device converts several mW of a pump wave at 1040 nm wavelength to signal and idler waves around 2000 nm wavelength with more than 50% efficiency. Over 1 h, power and frequency of the signal wave vary by <±1% and by <±25 MHz, respectively. The latter can be tuned over 480 MHz without a mode hop by changing T over 120 mK. In order to prove the suitability for high-resolution spectroscopy, we scan the signal frequency across the resonance of a Fabry-Perot interferometer resolving nicely its 10 MHz linewidth.
Optics express, Jan 16, 2013
Optical whispering gallery modes (WGMs) of mm-sized axisymmetric resonators are well localized at... more Optical whispering gallery modes (WGMs) of mm-sized axisymmetric resonators are well localized at the equator. Employing this distinctive feature, we obtain simple analytical relations for the frequencies and eigenfunctions of WGMs which include the major radius of the resonator and the curvature radius of the rim. Being compared with results of finite-element simulations, these relations show a high accuracy and practicability. High-precision free-spectral-range measurements with a millimeter-sized disc resonator made of MgF(2) allow us to identify the WGMs and confirm the applicability of our analytical description.
Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XIII, 2014
ABSTRACT We demonstrate the continuous-wave operation of a cascade that has been successfully app... more ABSTRACT We demonstrate the continuous-wave operation of a cascade that has been successfully applied so far only for picosecond systems: A doubly-resonant optical-parametric oscillator (OPO) based on lithium niobate generates signal and idler waves close to degeneracy. Subsequently, these two light fields are converted to a terahertz wave via difference frequency mixing in an orientation-patterned gallium arsenide crystal placed inside the OPO cavity. Using this scheme, we achieved tunability from 1 to 4:5 THz frequency, a linewidth smaller than 10 MHz, and a Gaussian beam profile. The output power is of the order of tens of μW, with a scalability into the milliwatt regime.
Optisch-parametrische Oszillatoren (OPOs) sind weit abstimm- bare und dadurch variabel einsetzbar... more Optisch-parametrische Oszillatoren (OPOs) sind weit abstimm- bare und dadurch variabel einsetzbare Lichtquellen. Diese sind für den sichtbaren Spektralbereich bislang allerdings nur im gepulsten Betrieb kommerziell erhältlich. In diesem Artikel stellen wir zwei Konzepte vor, mit deren Hilfe kontinuierlich emit- tierende OPO-Systeme das sichtbare Spektrum erobern.
Continuous-wave optical parametric oscillators (OPOs) with periodically-poled lithium niobate as ... more Continuous-wave optical parametric oscillators (OPOs) with periodically-poled lithium niobate as the nonlinear medium combine a wide tuning range with single-frequency operation. Singly-resonant systems pumped by lasers with a wavelength of 1 mum have been demonstrated for more than ten years. They have pump thresholds of about 1 W, and most of them have crystal lengths of 50 mm. We show
ABSTRACT Stability and footprint of optical parametric oscillators (OPOs) strongly depend on the ... more ABSTRACT Stability and footprint of optical parametric oscillators (OPOs) strongly depend on the cavity used. Monolithic OPOs tend to be most stable and compact since they do not require external mirrors that have to be aligned. The most straightforward way to get rid of the mirrors is to coat the end faces of the nonlinear crystal. Whispering gallery resonators (WGRs) are a more advanced solution since they provide ultra-high reflectivity over a wide spectral range without any coating. Furthermore, they can be fabricated out of nonlinear-optical materials like lithium niobate. Thus, they are ideally suited to serve as a monolithic OPO cavity. We present the experimental realization of optical parametric oscillators based on whispering gallery resonators. Pumped at 1 μm wavelength, they generate signal and idler fields tunable between 1.8 and 2.5 μm wavelength. We explore different schemes, how to phase match the nonlinear interaction in a WGR. In particular, we show improvements in the fabrication of quasi-phase-matching structures. They enable great flexibility for the tuning and for the choice of the pump laser.
Recently achieved radial poling of whispering-gallery resonators (WGRs) strongly extends the capa... more Recently achieved radial poling of whispering-gallery resonators (WGRs) strongly extends the capabilities of tailoring the second-order nonlinear phenomena, such as second-harmonic generation and optical parametric oscillation, and transferring them to the range of low-power continuous-wave light sources. Owing to discreteness of the frequency spectrum, the resonance and phase-matching conditions for interacting waves cannot be fulfilled simultaneously in WGRs in the general case. Using Yariv's generic approach to the description of WGR phenomena, we analyze two closely related issues: the possibilities to achieve the resonant and phase-matching conditions using the temperature tuning and the impact of detunings and phase mismatches on the nonlinear transformation efficiencies. It is shown that the radial poling provides important necessary conditions for the subsequent fine tuning to the nonlinear resonances. The requirements to the temperature tuning, as exemplified by the case of lithium niobate, are substantially dependent on the nonlinear process in question, the actual wavelength range, and the pump intensity.
ABSTRACT Continuous-wave optical parametric oscillators (OPOs) combine single-frequency operation... more ABSTRACT Continuous-wave optical parametric oscillators (OPOs) combine single-frequency operation with a huge tuning range. Thus, they have emerged as working horses for spectroscopy in the near and mid infrared over the last decades (Dunn and Ebrahimzadeh, 1999). However, in the terahertz range prominent nonlinear materials like lithium niobate have strongly damped phonon resonances leading to huge absorption. Consequently, the generation of terahertz waves by optical parametric oscillation requires around 100 W of near-infrared pump light exceeding the power delivered by standard single-frequency lasers.
Over the past 10 years, with the advent of new crystals designs and a new generation of pump lase... more Over the past 10 years, with the advent of new crystals designs and a new generation of pump lasers, continuous-wave (cw) optical parametric oscillators (OPOs) have developed into mature monochromatic light sources. Nowadays, cw OPOs can fulfill a wide variety of criteria for sensitive molecular gas sensing. It can access the mid-infrared wavelength region, where many molecules have their fundamental rotational-vibrational transitions, with high power. This high power combined with wide wavelength tuning and narrow linewidth creates excellent conditions for sensitive, high-resolution spectroscopy. OPOs combined with robust methods, such as photoacoustic spectroscopy and cavity-enhanced spectroscopy, are well suited for field measurements and remote-sensing applications. The wide tunability of cw OPOs allows detection of larger molecules with broad absorption band structures, and its fast scanning capabilities allow rapid detection of trace gases, the latter is a demand for life-science applications. After a short introduction about the physical principle of cw OPOs, with its most recent physical developments, this review focuses on sensitive molecular gas sensing with a variety of spectroscopic applications in atmospheric and life sciences.
We report a diffraction-limited photonic terahertz (THz) source with linewidth <10 MHz that can b... more We report a diffraction-limited photonic terahertz (THz) source with linewidth <10 MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation-patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near λ = 2 µm. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030 nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10 W of 1030 nm pump, 100 W of intracavity power near 2 µm was attained with GaAs inside cavity.
Laser Resonators, Microresonators, and Beam Control XVI, 2014
ABSTRACT Whispering gallery optical parametric oscillators are millimeter-sized monolithic source... more ABSTRACT Whispering gallery optical parametric oscillators are millimeter-sized monolithic sources for tunable coherent light. Several experiments have revealed that during optical parametric oscillation the pump resonance strongly differs from a Lorentzian shape. We theoretically and experimentally analyze these line-shape distortions. It turns out that the line shape of the pump resonance strongly depends on the coupling strength of the pump light and on the loss ratio between generated light and pump light. The line-widths, i.e. the losses, for the light generated by the parametric process can be deduced without measuring them directly.
A different phase-matching period of 24.4 to 31.0 mum is used in singly-resonant optical parametr... more A different phase-matching period of 24.4 to 31.0 mum is used in singly-resonant optical parametric oscillator (OPO) leading to idler wavelengths from 2.33 to 5.32 mum. A frequency shift depending on the poling period would be a non-ambiguous proof that not a Raman effect is observed.
Superconductor Science and Technology, 2004
We have developed a scanning magnetic microscope (SMM) with 25 µm resolution in spatial position ... more We have developed a scanning magnetic microscope (SMM) with 25 µm resolution in spatial position for the magnetic features of room temperature objects. The microscope consists of a high-temperature superconductor (HTS) dc SQUID sensor, suspended in vacuum with a self-adjusting standoff, close spaced liquid nitrogen Dewar, X -Y scanning stage and a computer control system. The HTS SQUIDs were optimized for better spatial and field resolutions for operation at liquid nitrogen temperature. Measured inside a magnetic shield, the 10 pT Hz −1/2 typical noise of the SQUIDs is white down to frequencies of about 10 Hz, increasing up to about 20 pT Hz −1/2 at 1 Hz. The microscope is mounted on actively damped platforms, which negate vibrations from the environment as well as damping internal stepper motor noises. A high-resolution video telescope and a 1 µm precision z-axis positioning system allow a close positioning of the sample under the sensor. The ability of the sensors to operate in unshielded environmental conditions with magnetic fields up to about 15 G allowed us to perform 2D mapping of the local ac and dc susceptibility of the objects.
Review of Scientific Instruments, 2011
The combination of an all-optical terahertz source with a photoconductive antenna to achieve cohe... more The combination of an all-optical terahertz source with a photoconductive antenna to achieve coherent detection is presented. This approach aims to overcome the frequency limits introduced by optoelectronic terahertz sources commonly used. Here the Gaussian-shaped and linearly polarized terahertz waves are generated by a continuous wave optical parametric oscillator with a power of 3 μW at 1.4 THz. The infrared signal light of the optical parametric oscillator can be used to coherently detect the generated terahertz wave with a photoconductive antenna. As a proof-of-principle experiment we determine the thickness profile of a plastic lens using phase shifting interferometry.