CW Pump Research Articles

Role of the inverse Čerenkov effect in the formation of ultrashort Raman solitons in silica microspheres.

We theoretically demonstrate a new regime, to the best of our knowledge, of the formation of ultrashort optical solitons in spherical silica microresonators with whispering gallery modes. The solitons are driven by a coherent CW pump at the frequency in the range of normal dispersion, and the energy is transferred from this pump to the solitons via two channels: the Raman amplification and inverse Čerenkov effect. We discuss three different regimes of soliton propagation and we also show that these Raman solitons can be controlled by weak coherent CW signals.

Parametric interaction of laser cavity-solitons with an external CW pump

We study the interaction of a laser cavity-soliton microcomb with an externally coupled, co-propagating tunable CW pump, observing parametric Kerr interactions which lead to the formation of both a cross-phase modulation and a four-wave mixing replica of the laser cavity-soliton. We compare and explain the dependence of the microcomb spectra from both the cavity-soliton and pump parameters, demonstrating the ability to adjust the microcomb externally without breaking or interfering with the soliton state. The parametric nature of the process agrees with numerical simulations. The parametric extended state maintains the typical robustness of laser-cavity solitons.

Visible–Telecom Photon Pair Source Based on a Photonic-Crystal Fiber under Continuous-Wave Pumping

The generation of interband photon pairs with wavelengths near 0.5 and 1.6 μm in a photonic-crystal fiber under low-power cw optical pumping by a diode laser with a central wavelength of 0.8 μm has been experimentally demonstrated. It has been found that the generation rate of entangled photons under cw pumping is comparable with values obtained with pulsed pumping by a femtosecond Ti:sapphire laser if the average cw pump power is an order of magnitude higher than the average pulsed pump power. The reached rates of photon generation are ensured by the used photonic-crystal fiber with a small effective mode area and a special dispersion profile. The reached low noise in the output signal is ensured by the separation of carrier frequencies of generated photons into different spectral bands.

Continuous-wave pumped dispersion-compensated ultrashort-pulse doubly resonant oscillator studies

We have reported theoretical analysis of broadband phase-coherent spectral generation with stable short pulse output based on optical parametric oscillator (OPOs) pumped by low-cost single-mode continuous-wave (cw) laser considering the incorporation of mode-locking element in the cw dispersion-compensated OPO in doubly-resonant oscillator (DRO) configuration. Exploiting the control of dispersion in the cw mode-locked cavity, the intracavity round-trip phase shift, phase coherence among resonating waves, carrier envelope offset phase of mode-locked output and enhancement of efficiency of system using active mode-locking element driven at high modulation frequency has been studied. The study paves the way of realization of compact, low-cost, efficient ultrashort pulse OPO under cw pumping.

Determination of exciton binding energy using photocurrent spectroscopy of Ge quantum-dot single-hole transistors under CW pumping

We reported exciton binding-energy determination using tunneling-current spectroscopy of Germanium (Ge) quantum dot (QD) single-hole transistors (SHTs) operating in the few-hole regime, under 405–1550 nm wavelength (λ) illumination. When the photon energy is smaller than the bandgap energy (1.46 eV) of a 20 nm Ge QD (for instance, λ = 1310 nm and 1550 nm illuminations), there is no change in the peak voltages of tunneling current spectroscopy even when the irradiation power density reaches as high as 10 µW/µm2. In contrast, a considerable shift in the first hole-tunneling current peak towards positive VG is induced (ΔVG ≈ 0.08 V at 0.33 nW/µm2 and 0.15 V at 1.4 nW/µm2) and even additional photocurrent peaks are created at higher positive VG values (ΔVG ≈ 0.2 V at 10 nW/µm2 irradiation) by illumination at λ = 850 nm (where the photon energy matches the bandgap energy of the 20 nm Ge QD). These experimental observations were further strengthened when Ge-QD SHTs were illuminated by λ = 405 nm lasers at much lower optical-power conditions. The newly-photogenerated current peaks are attributed to the contribution of exciton, biexciton, and positive trion complexes. Furthermore, the exciton binding energy can be determined by analyzing the tunneling current spectra.

Observation of amplified spontaneous emission in Cd1-xZnxSe/ZnSe alloyed quantum dots through CW pumping

Colloidal quantum dots (QDs) have attracted much attention due to their optical tunability and application as a unique solution processable gain medium in lasers. Many research groups over the world have successfully attained amplification from Cd-based QDs through femtosecond (fs) and nanosecond (ns) pumping. However, for the wide application of QD-based lasers, an operation in the CW regime is much more favorable. Here, we present a critical step towards this goal by demonstrating CW laser (532 nm) pumped amplified spontaneous emission (ASE) in Cd1-xZnxSe/ZnSe alloyed core QDs dispersed in toluene at room temperature. Alloyed core QDs show lower non radiative transitions, which has been demonstrated by TA dynamics in this report. The threshold level for CW ASE is observed to be 31 W cm-2. These results indicate that these QDs can sustain CW-stimulated emission at room temperature, and can be widely applicable by further optimizing the QD material quality in order to have a low amplification threshold.

Large regenerative parametric amplification on chip at ultra-low pump powers

Chip-based optical amplifiers can significantly expand the functionalities of photonic devices. In particular, optical-parametric amplifiers (OPAs), with engineerable gain spectra, are well suited for nonlinear-photonic applications. Chip-based OPAs typically require long waveguides that occupy a large footprint, and high pump powers that cannot be easily produced with chip-scale lasers. We theoretically and experimentally demonstrate a microresonator-assisted regenerative OPA that benefits from the large nonlinearity enhancement of microresonators and yields a high gain in a small footprint. We achieve 30-dB parametric gain with only 9 mW of cw pump power and show that the gain spectrum can be engineered to cover telecom channels inaccessible with Er-based amplifiers. We further demonstrate the amplification of Kerr-soliton comb lines and the preservation of their phase properties. Additionally, we demonstrate amplification by injection locking of optical parametric oscillators (OPOs), which corresponds to a regenerative amplifier pumped above the oscillation threshold. Dispersion engineering techniques such as coupled cavities and higher-order-dispersion phase matching can further extend the tunability and spectral coverage of our amplification schemes. The combination of high gain, small footprint, low pump power, and flexible gain-spectrum engineering of our regenerative OPA is ideal for amplifying signals from the nanowatt to microwatt regimes for portable or space-based devices where ultralow electrical power levels are required and can lead to important applications in on-chip optical-, and microwave-frequency synthesis and precise timekeeping.

Quantum Key Distribution with a Continuous-Wave-Pumped Spontaneous-Parametric-Down-Conversion Heralded Single-Photon Source

Quantum key distribution (QKD) protocols utilize single photons for key exchanging. Heralded single-photon sources (HSPSs) with spontaneous parametric down-conversion (SPDC) is a practical and effective way of producing single photons of high fidelity, bringing extra performance especially at long distances. A pulsed laser is usually used as the pump for HSPS QKD, but it has disadvantages in operational stability and practicality compared to a cw one. On the other hand, a precise model describing the performance of HSPS with a cw pump is still missing. It is inaccurate and unsafe to apply the key rate model of the pulse-pumped HSPS QKD directly to the cw one, and it also overlooks the timing jitter and dead time of the single-photon detectors. In this paper, we study the cw case in detail, including practical imperfections of the experimental instruments. Our model shows that a cw-pumped setup can reach a much longer transmission distance (31 % longer) than the fastest pulsed HSPS QKD system at present, together with other advantages such as higher operational stability and better spectral characteristics.

(INVITED)Investigation of intense visible defect luminescence from visible and infrared pumped barium fluorosilicate glass-core fiber

Optically active defects in silica have been studied for decades and are often indicators of network irregularities such as those that might result from optical or mechanical damage. They are well-known to be weak emitters and are usually present in relatively low concentration, thus precluding their use in a wide range of applications, including sensing and laser gain. Here, a new paradigm in intense defect emission in the visible wavelength range from a nominally passive optical fiber is presented. Optical fiber starting with 100 mol % BaF2 precursor core material and a pure silica cladding was successfully drawn utilizing the molten core method. These fibers demonstrate an intense, yet unexpected, green photoluminescence peaking near 537 nm (in addition to a second, weaker band near 704 nm) arising from relatively low-power CW pumping in the near-infrared at 976 nm. To understand the origins of this emission, absorption across the optical spectrum is analyzed and photoluminescence via excitation in both the visible and near-infrared wavelength ranges is studied. In addition, Raman spectra, decay lifetimes, magnetization curves, and temperature dependence measurements were collected. The emission spectra maintained a Pekarian-like spectral shape, suggesting an optically active defect as the mechanism behind the green emission. The results presented herein point towards the most likely origin being silanone or dioxasilyrane groups usually associated with surface defects. Importantly, such fibers, fabricated through less conventional methods and possessing novel compositions, may prove to be key in further extending the range of possibilities in defect engineering to well beyond what was previously thought possible.

150 mW Tb3+ doped chalcogenide glass fiber laser emitting at λ > 5 μm

The paper describes terbium-doped chalcogenide glass fiber lasers emitting up to 150 mW at 5.1–5.4 μm under CW pumping by a thulium 1.908 µm fiber laser. Attention is drawn to the temporal dynamics of their radiation and to its dependence on the doping level.

Kerr soliton frequency comb generation by tuning the coupling coefficient in coupled nonlinear microcavities.

Kerr soliton frequency comb generation in nonlinear microcavities with compact configurations are promising on-chip sources. Current Kerr comb generation by using a single microcavity with a tunable CW pump laser or high-power femtosecond pulse pump are difficult to be integrated on chip. In this paper, we propose an on-chip soliton comb generation scheme by tuning the coupling coefficient of two coupled microcavities instead of tuning the wavelength of the cw pump laser or using a pulsed pump laser in a single microcavity. The two microcavities are assumed to be identical. We showed by numerical simulation that Kerr comb generation is possible in both the blue and red detuned regions of the main microcavity in the coupled cavity system. We further found that the range and boundary of the soliton generation region of the couple microcavities depend on the coupling coefficient between the coupled cavities. To ensure that the modes being coupled have identical optical paths, we designed a Sagnac loop structure which couples the clockwise and counterclockwise modes in a single microcavity and demonstrated Kerr comb generation in both the blue and red detuned regions by tuning the coupling coefficient. The proposed Kerr comb generation scheme can be utilized for chip-scale integrated soliton comb sources, which will contribute to the development of on-chip applications.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

Thermal characterization of a nematic liquid crystal suited for the fabrication of NIR spectrally-tunable vertical cavity surface emitting lasers

In this work, the thermo-optical properties of a nematic liquid crystal are determined through reflectance measurements performed on a high finesse tunable filter fabricated using a polymer-based microcell technology. The final aim is to insert such material in the optical cavity of a 850 nm tunable VCSEL device, in which local self-heating due to CW pumping must be taken into account. These localized interferometric experiments are performed in the near-infra-red range and at temperatures up to 115 °C. A thermal model is derived from the acquired data. Finally, we demonstrate that the birefringence of QYPDLC-36 liquid crystal remains higher than 0.18 at 60 °C, feature well suited to real device operation.

Maiman revisited: tuneable single mode CW ruby ring laser

The cw ruby laser reveals amazing properties, which have not been expected since its invention 60 years ago. CW pumping with a 405 nm laser diode requires an overall electrical power of only 6 W to achieve more than 100 mW in TEM00 at 694 nm for a linear resonator. In a ring cavity unidirectional single frequency operation with up to 75 mW, tuneable over more than 420 GHz is demonstrated. Experimental details will be presented, and possible applications will be discussed.

Designing high-power, octave spanning entangled photon sources for quantum spectroscopy.

Entangled photon spectroscopy is a nascent field that has important implications for measurement and imaging across chemical, biology, and materials fields. Entangled photon spectroscopy potentially offers improved spatial and temporal-frequency resolutions, increased cross sections for multiphoton and nonlinear measurements, and new abilities in inducing or measuring quantum correlations. A critical step in enabling entangled photon spectroscopies is the creation of high-flux entangled sources that can use conventional detectors as well as provide redundancy for the losses in realistic samples. Here, we report a periodically poled, chirped, lithium tantalate platform that generates entangled photon pairs with ∼10-7 efficiency. For a near watt level diode laser, this results in a near μW-level flux. The single photon per mode limit that is necessary to maintain non-classical photon behavior is still satisfied by distributing this power over up to an octave-spanning bandwidth. The spectral-temporal photon correlations are observed via a Michelson-type interferometer that measures the broadband Hong-Ou-Mandel two-photon interference. A coherence time of 245fs for a 10nm bandwidth in the collinear case and a coherence time of 62fs for a 125nm bandwidth in the non-collinear case are measured using a CW pump laser and, essentially, collecting the full photon cone. We outline in detail the numerical methods used for designing and tailoring the entangled photons source, such as changing center wavelength or bandwidth, with the ultimate aim of increasing the availability of high-flux UV-Vis entangled photon sources in the optical spectroscopy community.

A Highly Inclusive Theory of Q-Switched Lasers

A highly inclusive theory of Q-switched lasers is presented which applies to both actively and passively Q-switched lasers. The theory expands on the author’s previous publications on energy optimization to include: (1) saturable absorbers exhibiting excited state absorption and therefore a finite Figure of Merit (FOM); (2) unequal beam areas in the gain and absorbing media; and (3) both pulsed and CW pumping. Design curves for optimizing the output energy for pulse pumped lasers and CW pumped lasers at a desired pulse rate are provided. As in the author’s previous work, the optimization equation is obtained utilizing Lagrange Multipliers and yields the value of the parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho = ln(n_{i}/n_{f})$ </tex-math></inline-formula> which maximizes the single pulse energy, where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{i}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{f}$ </tex-math></inline-formula> are the laser inversion population densities before and after Q-switching. Using this initial result, the associated optimum absorber transmission and mirror reflectivity, pulse energy, peak power, and pulse width can be computed sequentially via simple formulas. The aforementioned parameters can all be expressed as functions of two variables, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z_{eff}$ </tex-math></inline-formula> , which are slight modifications of the variables <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> and z previously introduced by the author for passively Q-switched lasers having infinite FOM and equal beam areas The term “plane wave theory” has sometimes been used by this and other authors in prior publications to describe a uniform pump profile in the laser medium, but we will refer to this special case more accurately as the “uniform pump theory”. The theory is extended here in the final section to include non-uniform pumping. The common case of a Q-switched laser end-pumped by a quasi-Gaussian CW laser is considered in detail and the theoretical results are found to agree well with experimental data.

On-chip octave-spanning supercontinuum generation in hybrid slot waveguide by power-efficient dual pump

We propose an on-chip cascaded structure consisting of a microresonator and a slot waveguide to generate the high-performance supercontinuum harnessing the dual pump scenario. Utilizing a single CW pump with the power of only 0.1 W, the dissipative Kerr soliton with the full width at half maximum of 69.2 fs and the peak power of 103.3 W is generated and couples out the microresonator together with the CW pump to serve as a dual pump seed for the supercontinuum generation in the followed slot waveguide. The slot waveguide built upon a silicon-organic hybrid structure is carefully designed to possess remarkable nonlinearity and appropriate dispersion profile for smooth and broadband supercontinuum generation. We have numerically achieved the 1.56 octave-spanning supercontinuum spanning 1 ~ 3 μm with the proposed dual pump scenario. The supercontinuum generated by the dual pump scenario shows flattened, broadband, and quasi-right-angled-trapezoid shape, along with good coherence. Compared with the single pump scenario of CW or soliton pulse only, the supercontinuum generated by the dual pump scenario possesses the much stronger spectrum, broader spanning, and medium coherence. Our work opens up a new path for on-chip and cost-effective supercontinuum generation with the high performance of intensity, bandwidth, and coherence.

A Direct Method for the Simultaneous Estimation of Self-Steepening and the Fractional Raman Contribution in Fiber Optics

We propose an original, simple, and direct method for the simultaneous estimation of the self-steepening parameter and the fractional Raman contribution in fiber optics. Our proposal is based on the dependence of the modulation instability gain on both parameters, as obtained from a linear stability analysis of the newly introduced photon-conserving generalized nonlinear Schrödinger equation (pcGNLSE), and requires only the CW or quasi-CW pumping of the waveguide under test and a few direct spectral measurements. Further, we demonstrate the feasibility of the estimation procedure by means of detailed simulations for typical waveguide parameters in relevant spectral ranges. Last, we discuss the range of applicability of the proposed method and compare its results, advantages, and disadvantages with a recently introduced method based on short-pulse dynamics.

Comparative investigation of lasing and amplification performance in cryogenic Yb:YLF systems

We present detailed experimental results with cryogenic Yb:YLF gain media in rod-geometry. We have comparatively investigated continuous-wave (cw) lasing and regenerative amplification performance under different experimental conditions. In the cw lasing experiments effect of crystal doping, cw laser cavity geometry and pump wavelength on lasing performance were explored. Regenerative amplification behavior was analyzed and the role of depolarization losses on performance was investigated. A recently developed temperature estimation method was also employed for the first time in estimating average crystal temperature under lasing conditions. It is shown that the thermal lens induced by transverse temperature gradients is the main limiting factor and strategies for future improvements are discussed. To the best of our knowledge, the achieved results in this study (375 W in cw, and 90 W in regenerative amplification) are the highest average powers ever obtained from this system via employing the broadband E//a axis.

Pump Wavelength Flexible, Continuous-Wave Fiber Supercontinuum Using Two-Stage Spectral Broadening

Low-power continuous-wave (cw) pumped supercontinuum sources exhibit poor spectral flatness and high sensitivity to the position of pump-wavelength with respect to the zero-dispersion of the nonlinear fiber. We propose a technique to overcome these limitations by the simple addition of a section of transmission fiber (such as Corning LEAF) before the nonlinear broadening stage. The pump incoherence introduced by this fiber in the C-band allows for a smooth, equalized, input wavelength flexible supercontinuum at low powers. With the proposed two-stage spectral broadening, we have successfully demonstrated for the first time, a cw pump wavelength agile all-fiber supercontinuum module with spectra spanning from 1300 nm to 2000 nm using relatively short lengths of nonlinear fiber. Similar supercontinuum is obtained for output powers from 80 mW up to a maximum in this work of 550 mW making it an ideal source for broadband low-power applications.