A Method to Correct the Temporal Drift of Single-Photon Detectors Based on Asynchronous Quantum Ghost Imaging.

Single-photon detection and timing has attracted increasing interest in recent years due to their necessity in the field of quantum sensing and the advantages of single-quanta detection in the field of low-level light imaging. While simple bucket detectors are mature enough for commercial applications, more complex imaging detectors are still a field of research comprising mostly prototype-level detectors. A major problem in these detectors is the implementation of in-pixel timing circuitry, especially for two-dimensional imagers. One of the most promising approaches is the use of voltage-controlled ring resonators in every pixel. Each of these runs independently based on a voltage supplied by a global reference. However, this yields the problem that the supply voltage can change across the chip which, in turn, changes the period of the ring resonator. Due to additional parasitic effects, this problem can worsen with increasing measurement time, leading to drift in the timing information. We present here a method to identify and correct such temporal drifts in single-photon detectors based on asynchronous quantum ghost imaging. We also show the effect of this correction on recent quantum ghost imaging (QGI) measurement from our group.

Temperature investigation of low SWaP thulium-doped fiber lasers.

We investigate the temperature dependence of an in-band core-pumped thulium-doped fiber laser with a low SWaP (size, weight, and power) architecture. The temperature investigation is carried out both experimentally and numerically by a simulation model. We demonstrate experimentally that the investigated setup is resistant for temperatures till 353 K. In addition, we explain the observed behavior by considering the temperature depended spectroscopic parameters of thulium-doped silica fibers. Finally, a numerical investigation is carried out for higher temperatures up to 573 K and higher output powers up to 12 W as well as for different wavelengths and show that the considered fiber lasers works still efficient at these temperature ranges. We show the reliability of the considered thulium-doped fiber laser architecture for applications in harsh environment.

High average power Q-switched Ho3+:YAG laser with a single-line emission at 2122 nm.

We present an efficient Q-switched dual-crystal Ho3+:YAG laser resonator achieving an output power of 56 W at an incident pump power of 100.7 W with a slope efficiency of 64.4%. The setup was pump power limited. Notably, by carefully tuning multiple etalons placed within the cavity, we achieved a single-line spectral emission at 2122 nm. This deviates from the typical emission wavelengths of 2090 nm or 2096 nm for Ho3+:YAG at comparable output powers in Q-switched operation. The laser exhibited very good beam quality, approaching diffraction-limited performance with an excellent M2- < 1.2. At the maximum output power, a pulse FWHM of 100 ns was measured.

3D quantum ghost imaging.

We present current results of a novel, to the best of our knowledge, type of setup for quantum ghost imaging based on asynchronous single photon timing using single photon avalanche diode (SPAD) detectors, first presented in [Appl. Opt.60, F66 (2021)APOPAI0003-693510.1364/AO.423634]. The scheme enables photon pairing without fixed delays and, thus, overcomes some limitations of the widely used heralded setups for quantum ghost imaging [Nat. Commun.6, 5913 (2015)NCAOBW2041-172310.1038/ncomms6913]. It especially allows three-dimensional (3D) imaging by direct time of flight methods, the first demonstration of which will be shown here. To our knowledge, it is also the first demonstration of 3D quantum ghost imaging at all.

Alignment-insensitive end-pumped continuous-wave crossed-Porro prism Ho3+:YAG laser.

A continuous-wave crossed-Porro prism Ho3+:YAG laser is presented and compared with a corresponding mirror resonator. A maximum output power of 30.7 W is reached with a slope efficiency of 67.4% with respect to the absorbed pump power. The laser output beam shows a very good beam quality of better than M2 < 1.2 which clearly surpasses that of the mirror resonator. In terms of alignment sensitivity, the crossed-Porro prism resonator is superior to the mirror resonator due to the retro-reflective nature of the prisms in the axis around the apex.

High-power continuous-wave Tm3+:Ho3+-codoped fiber laser operation from 2.1 µm to 2.2 µm.

A Tm3+:Ho3+-codoped free-space single-oscillator fiber laser is under investigation with special focus on the power scalability of emission wavelengths from 2.1 µm to 2.2 µm. Using a tunable diffraction grating, a 200-nm tunable laser source is built. Laser output powers above 10 W are delivered from 1990 nm up to 2190 nm, demonstrating the range for stable high-power laser operation. By replacing the diffraction grating by a highly reflective, narrow linewidth volume Bragg grating, power scaling is performed at 2.1 µm and is even enabled at a wavelength of 2.2 µm. Using a volume Bragg grating (VBG) at 2.1 µm, a slope efficiency of 49% is measured with an output power of 262 W. Using another VBG with a center wavelength of 2.2 µm, the fiber laser delivers a record power of 77 W with a slope efficiency of 29%.

Mid-infrared optical parametric oscillator pumped by a high-pulse-energy, Q-switched Ho3+:YAG laser.

A high-pulse-energy mid-infrared light source is presented, based on a zinc-germanium-phosphide optical parametric oscillator (ZGP OPO) pumped by an actively $Q$-switched high-pulse-energy ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ laser. The ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ pump laser source is capable of generating a pulse energy of 15 mJ from a single ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ rod at room temperature at a pulse repetition frequency (PRF) of 700 Hz. A maximum power of 20.1 W at a central wavelength of 2090 nm can be obtained in continuous operation, with a slope efficiency of 45.1%. A good beam quality with an ${{\rm M}^2}$ better than 1.3 was achieved in $Q$-switched operation. The presented laser architecture was used as a suitable pump source for a ZGP-based OPO. Operated at a PRF of 2 kHz and pumped with a pulse energy of 8 mJ, a low conversion threshold of 1.5 W and a maximum total output power of 6.3 W could be obtained in a linear ZGP-based OPO. At maximum power, the peak power of the generated mid-infrared radiation exceeded 120 kW, while the beam quality was affected by the strong gain lens building inside the nonlinear material as a consequence of the high-energy pump pulses.

Nanosecond pulsed single-frequency two-stage holmium-doped fiber MOPA at 2054 nm and 2090 nm.

A single-frequency polarization-maintaining holmium-doped fiber master oscillator power amplifier operating at signal wavelengths of $2054\;{\rm nm}$ and $2090\;{\rm nm}$ is presented. The two-stage setup delivers up to $240\;{\rm W}$ peak power and $6.7\;\unicode{x00B5} {\rm J}$ pulse energy for a pulse width of $30.2\;{\rm ns}$ at a repetition rate of $100\;{\rm kHz}$. The first amplifier stage is designed by simulation, tailored for high gain at the signal wavelength range, favoring amplification at $2090\;{\rm nm}$. The design is discussed, and the measured values are compared with the simulation. The second stage is investigated regarding the efficiency for co- and counter-pumping. Stimulated Brillouin scattering was found to be the limiting factor for pulse peak power scaling in the second stage. The measured output pulse shapes are discussed and compared to pulse shapes derived with the Frantz-Nodvik model.

Quantum ghost imaging using asynchronous detection.

We present first results of a novel type of setup for quantum ghost imaging based on asynchronous single photon timing using single photon avalanche diode (SPAD) detectors. This scheme enables photon pairing with arbitrary path length difference and does, therefore, obviate the dependence on optical delay lines of current quantum ghost imaging setups [Nat. Commun.6, 5913 (2015)NCAOBW2041-172310.1038/ncomms6913]. It is also, to our knowledge, the first quantum ghost imaging setup to allow three-dimensional imaging.

Fraunhofer Institute of Optronics, System Technologies and Image Exploitation: introduction to the focus issue.

This focus issue offers a glimpse into the breadth and depth of research in optics and image processing at the Fraunhofer Institute of Optronics, System Technologies and Image Exploitation (abbreviated Fraunhofer IOSB). The institute belongs to the Fraunhofer Society, which is Europe's largest organization for applied research. The society comprises 72 institutes spread throughout Germany as well as subsidiaries in several European countries, and the USA, UK, Chile and Singapore, each focusing on different fields of applied science. It is named after the Bavarian physicist Joseph von Fraunhofer (1787-1826), the inventor of the spectrometer, optical lens manufacturer and an entrepreneur, who exemplified the goals of the society, i.e.: a focus on research excellence, innovation, and commitment to market and customer-oriented research.

High pulse energy ZnGeP2 OPO directly pumped by a Q-switched Tm3+-doped single-oscillator fiber laser.

A mid-infrared ${{\rm ZnGeP}_2}$ optical parametric oscillator pumped by a ${{\rm Tm}^{3 +}}$-doped fiber laser is reported, providing pulse energies of $230 \;\unicode{x00B5}{\rm J}$, pulse widths of 40 ns, and peak powers of ${\sim}6\;{\rm kW} $ with excellent efficiency and beam quality. The pump source is an actively $Q$-switched single oscillator optimized to generate high pulse energies.

Adaptable Shack-Hartmann wavefront sensor with diffractive lenslet arrays to mitigate the effects of scintillation.

Adaptive optics systems are used to compensate for distortions of the wavefront of light induced by turbulence in the atmosphere. Shack-Hartmann wavefront sensors are used to measure this wavefront distortion before correction. However, in turbulence conditions where strong scintillation (intensity fluctuation) is present, these sensors show considerably worse performance. This is partly because the lenslet arrays of the sensor are designed without regard to scintillation and are not adaptable to changes in turbulence strength. Therefore, we have developed an adaptable Shack-Hartmann wavefront sensor that can flexibly exchange its lenslet array by relying on diffractive lenses displayed on a spatial light modulator instead of utilizing a physical microlens array. This paper presents the principle of the sensor, the design of a deterministic turbulence simulation test-bed, and an analysis how different lenslet arrays perform in scintillation conditions. Our experiments with different turbulence conditions showed that it is advantageous to increase the lenslet size when scintillation is present. The residual phase variance for an array with 24 lenslets was up to 71% lower than for a 112 lenslet array. This shows that the measurement error of focal spots has a strong influence on the performance of a Shack-Hartmann wavefront sensor and that in many cases it makes sense to increase the lenslet size. With our adaptable wavefront sensor such changes in lenslet configurations can be done very quickly and flexibly.

55 W actively Q-switched single oscillator Tm3+, Ho3+-codoped silica polarization maintaining 2.09 µm fiber laser.

A bidirectional 793 nm diode-pumped actively Q-switched Tm3+, Ho3+-codoped silica polarization-maintaining (PM) double-clad (DC) fiber laser is reported. With this fiber laser, 55 W of average output power with 100 ns pulse width at 200 kHz repetition rate and 2.09 µm wavelength is obtained. The pump power injection with end-caps fusion-spliced on fiber tips provides good power stability (< 1.1%) and beam quality factors (M2 < 1.7). The fiber laser output beam polarization factor is 97.5%. At 55 W, no thermal-induced damage is observed on any optical element, and power scaling of the laser is only pump-power-limited in the range of the total available pump power (180 W).

Mid-infrared ZGP OPO with divergence compensation and high beam quality.

Divergence compensation, optimization of the optical-to-optical efficiency, and high beam quality of signal and idler beams of a high-energy mid-infrared ZnGeP2 (ZGP) optical parametric oscillator (OPO) have been demonstrated by use of a Galilean telescope inside the nonplanar fractional-image-rotation enhancement (FIRE) ring resonator. With a small variation of the distance between the lenses of the telescope, the divergences of signal and idler beams could be adjusted. Up to 36 mJ of mid-infrared pulse energy in the 3-5 µm wavelength range is obtained with 92 mJ of pump energy on crystal. The beam quality factors M2 are < 1.5 for the resonant signal beam and the non-resonant idler beam, respectively. Actually, this is an improvement of the beam quality by a factor 3 for the signal and ~2.7 for the idler beam compared without using a telescope inside the FIRE ring resonator.

Improvement of the beam quality of a high-pulse-energy mid-infrared fractional-image-rotation-enhancement ZnGeP2 optical parametric oscillator.

The beam quality of a high-energy mid-infrared ZnGeP2 (ZGP) optical parametric oscillator has been improved by use of a negative lens inside the nonplanar fractional-image-rotation enhancement (FIRE) resonator compensating for the thermal lensing and gain guiding effects. With an f=-0.9 m CaF2 lens, the beam quality is 1.5 and ∼2 for the resonant signal beam and the non-resonant idler beam, respectively. Up to 33 mJ of mid-infrared pulse energy in the 3-5 µm wavelength range is obtained with 94 mJ of pump energy on the crystal. This is an improvement of the beam quality by a factor of 3 for the signal beam and ∼2 for the idler beam compared without using a lens inside the FIRE resonator.

Optical parametric generation by a simultaneously Q-switched mode-locked single-oscillator thulium-doped fiber laser in orientation-patterned gallium arsenide.

Optical parametric generation is demonstrated in orientation-patterned gallium arsenide, pumped by a novel single-oscillator simultaneously Q-switched and mode-locked thulium-doped fiber laser, downconverting the pump radiation into the mid-infrared wavelength regime. The maximum output energy reached is greater than 2.0 µJ per pump pulse.

High-peak-power single-oscillator actively Q-switched mode-locked Tm3+-doped fiber laser and its application for high-average output power mid-IR supercontinuum generation in a ZBLAN fiber.

A single-oscillator actively Q-switched mode-locked (QML) thulium-doped silica fiber laser is presented and used to pump a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber for mid-infrared (mid-IR) supercontinuum (SC) generation. The fiber laser provided high-peak-power levels directly from the oscillator delivering single mode-locked pulse energies up to 48 µJ, being 2-4 orders of magnitude higher than conventional continuous wave mode-locked lasers. By pumping a ZBLAN fiber specially designed for high-output-power SC generation, 7.8 W have been achieved in all spectral bands with a spectrum extending to 4.2 µm.

High-pulse-energy mid-infrared fractional-image-rotation-enhancement ZnGeP2 optical parametric oscillator.

A high-energy mid-infrared ZnGeP2 (ZGP) optical parametric oscillator (OPO) based on the nonplanar fractional-image-rotation enhancement resonator pumped by a 2.05 µm Ho3+:YLF laser is presented. Up to 120 mJ pulse energy in a rotationally symmetric beam is generated in the 3-5 µm wavelength range at 1 Hz repetition rate. Slope efficiencies of up to 78% are achieved with respect to the pump pulse energy incident onto the ZGP crystal. The OPO pulses have a duration close to 15 ns, corresponding to a maximum peak power of 8 MW. A measurement of M2 dependence on pump beam diameter is presented.

Actively mode-locked Tm(3+)-doped silica fiber laser with wavelength-tunable, high average output power.

A diode-pumped, actively mode-locked high-power thulium (Tm3+)-doped double-clad silica fiber laser is demonstrated, providing an average output power in mode-locked (continuous wave) operation of 53 W (72 W) with a slope efficiency of 34% (38%). Mode-locking in the 6th-harmonic order was obtained by an acousto-optic modulator driven at 66 MHz without dispersion compensation. The shortest measured output pulse width was 200 ps. Owing to a diffraction grating as cavity end mirror, the central wavelength could be tuned from 1.95 to 2.13 µm. The measured beam quality in mode-locked and continuous wave operation has been close to the diffraction limit.

Mid-infrared lasing from Ho(3+) in bulk InF(3) glass.

We report on an Ho3+:InF3 glass laser pumped by a Cr3+:LiSAF laser emitting at 889 nm. Ho3+:InF3 glass is a promising material for direct mid-infrared lasers around 4 µm. To evaluate the performances of this new material, we compared it to an Ho3+:BYF crystalline laser pumped by the same source. At 650 mJ pump energy, 7.2 mJ (46 mJ) was obtained with Ho3+:InF3 (Ho3+:BYF), respectively. This is, to the best of our knowledge, the first reported laser activity in this type of glass.