Single-mode Power Research Articles

Large-Mode-Area All-Solid Photonic Bandgap Fibers for the Mitigation of Optical Nonlinearities

There is still significant need for power scaling of fiber lasers. Large-mode-area fibers are a key for the mitigation of optical nonlinearities. In recent years, mode instability has shown itself to be an additional significant limiting factor for single-mode power scaling in the regime of a few hundred watts to kilowatts. It is better appreciated now that further power scaling requires significant high-order-mode suppression in addition to a large effective mode area in a fiber. In recent years, we have shown that all-solid photonic bandgap fibers are a superior approach due to their unsurpassed higher-order-mode suppression in large-mode-area designs, making them well suited for applications at high average powers. We will review of some of the recent progress, challenges, and prospects of all-solid photonic bandgap fibers in this invited paper.

59 fs mode-locked Yb:KGW oscillator pumped by a single-mode laser diode

In this letter we present a passively mode-locked Yb:KGW oscillator pumped by a low power single-mode laser diode. Contrary to high power operation, single-mode pumping enabled us to suppress parasitic thermal effects, while keeping the setup compact and its alignment straightforward. Undisturbed mode-locking (ML) stability was achieved without active cooling of the gain medium and the laser was entirely self-starting. Pulses 59 fs in duration were obtained in a semiconductor saturable absorber mirror (SESAM)-assisted Kerr-lens mode-locked regime. The corresponding spectrum was 20.2 nm broad at a central wavelength of 1036 nm approaching the performance limit of the crystal. To the best of our knowledge, these are the shortest pulses generated from a Yb:KGW laser.

Infrared laser induced population transfer and parity selection in (14)NH3: A proof of principle experiment towards detecting parity violation in chiral molecules.

We have set up an experiment for the efficient population transfer by a sequential two photon-absorption and stimulated emission-process in a molecular beam to prepare quantum states of well defined parity and their subsequent sensitive detection. This provides a proof of principle for an experiment which would allow for parity selection and measurement of the time evolution of parity in chiral molecules, resulting in a measurement of the parity violating energy difference ΔpvE between enantiomers of chiral molecules. Here, we present first results on a simple achiral molecule demonstrating efficient population transfer (about 80% on the average for each step) and unperturbed persistence of a selected excited parity level over flight times of about 1.3 ms in the beam. In agreement with model calculations with and without including nuclear hyperfine structure, efficient population transfer can be achieved by a rather simple implementation of the rapid adiabatic passage method of Reuss and coworkers and considering also the stimulated Raman adiabatic passage technique of Bergmann and coworkers as an alternative. The preparation step uses two powerful single mode continuous wave optical parametric oscillators of high frequency stability and accuracy. The detection uses a sensitive resonantly enhanced multiphoton ionization method after free flight lengths of up to 0.8 m in the molecular beam. Using this technique, we were able to also resolve the nuclear hyperfine structure in the rovibrational levels of the ν1 and ν3 fundamentals as well as the 2ν4 overtone of (14)NH3, for which no previous data with hyperfine resolution were available. We present our new results on the quadrupole coupling constants for the ν1, ν3, and 2ν4 levels in the context of previously known data for ν2 and its overtone, as well as ν4, and the ground state. Thus, now, (14)N quadrupole coupling constants for all fundamentals and some overtones of (14)NH3 are known and can be used for further theoretical analysis.

Widely Tunable Single-Mode Yb-Doped All-Fiber Master Oscillator Power Amplifier

A Yb-doped linearly polarized fiber ring oscillator continuously tunable from 1000 to 1099 nm is built and amplification over the whole range is investigated with emphasis on the short wavelength regime of 1000–1025 nm. With an all-fiber single-mode polarization-maintaining master oscillator power amplifier, over 10 W output from 1010 to 1090 nm and over 30 W from 1014 to 1080 nm are achieved. At wavelength <1020 nm, bandpass filters are inserted for suppressing amplified spontaneous emission. The source can be used in a variety of applications, including laser cooling of Yb-doped solids.

A Model for Doubly Clamped Piezoelectric Energy Harvesters With Segmented Electrodes

An analytical model for doubly clamped piezoelectric energy harvesters with segmented electrodes, considering the effect of strain nodes formation and charges cancellation issues, is presented. The single-mode power and voltage outputs of each of the lead zirconate titanate segments are obtained and are connected in series across a resistive load for the total outputs. The analytical solutions are found to closely agree with the Finite Element Method (FEM) simulation results obtained from COMSOL Multiphysics.

First Demonstration of Six-Mode PON Achieving a Record Gain of 4 dB in Upstream Transmission Loss Budget

The power budget and costs are the two primary concerns for access networks. A major challenge is to minimize upstream power combining loss to increase the power budget. Spatial modes multiplexing offers the possibility to minimize upstream combining loss without significant added costs. We demonstrate the first integrated six-mode passive optical network, utilizing spatial modes to eliminate upstream combining loss. A record 4-dB net gain in power budget was achieved by fusion splicing a photonic lantern with a FMF, in contrast to the traditional power combining scheme with a single-mode power combiner/splitter. BERs <10−9 were obtained for all the six modes enabling Ethernet transmission using commercial GPON equipment. The packet loss of all six modal channels was tested for 12 h. Finally, we discuss some special issues of few-mode PON regarding its practical application.

High-power actively Q-switched single-mode 1342 nm Nd:YVO4 ring laser, injection-locked by a cw single-frequency microchip laser.

In this paper we report on the realization of a single-mode Q-switched Nd:YVO4 ring laser at 1342 nm. Unidirectional and single-mode operation of the ring laser is achieved by injection-locking with a continuous wave Nd:YVO4 microchip laser, emitting a single-frequency power of up to 40 mW. The ring laser provides a single-mode power of 13.9 W at 10 kHz pulse repetition frequency with a pulse duration of 18.2 ns and an excellent beam quality (M2 < 1.05). By frequency doubling of the fundamental 1342 nm laser, a power of 8.7 W at 671 nm with a pulse duration of 14.8 ns and a beam propagation factor of M2 < 1.1 is obtained. The 671 nm radiation features a long-term spectral width of 75 MHz.

High-Power Single-Mode Tapered Terahertz Quantum Cascade Lasers

We demonstrate a series of high output power single-mode terahertz quantum cascade lasers based on semi-insulating surface-plasmon waveguide with tapered geometry structure. The taper half-angle is ranging from 0° to 3°. The tapered waveguide effects on the output power and the laser beam quality are experimentally studied. High single longitudinal mode edge-emission peak powers (92.3 mW) are realized. The peak power monolithically increases with the increasing of the taper angle. All distributed feedback (DFB) lasers lased in the temperature range of 70–75 K. The highest brightness is obtained from the 2° laser. Stable single-mode emissions are realized for DFB lasers with the side-mode suppression ratio $\sim 18$ dB.

Increasing single mode power of 1.3-μm VCSELs by output coupling optimization.

We report on the single mode emission power enhancement of 1.3-μm VCSELs by adjusting the reflectivity of the top GaAs-based DBR for output coupling optimization using selective removal of Bragg reflector layers. Devices with record single mode power of 6.8-mW at room temperature and 2.8-mW at 80°C, with more than 30 dB single mode suppression ratio, have been obtained.

13 kW monolithic linearly polarized single-mode master oscillator power amplifier and strategies for mitigating mode instabilities

We report on the high power amplification of 1064nm linearly-polarized laser in all-fiber polarization-maintained MOPA, which can operate at output power level of 1.3kW. The main amplifier was pumped with six 915nm laser diodes, and the slope efficiency is 65.3%. The beam quality (M2) was measured to be <1.2 at full power operation. The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities (MI) sets in, which reduced to about 90% after the onset of MI. Power scaling capability of strategies for suppressing MI is analyzed based on a novel semi-analytical model, the theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber is an effective and practical way in standard double-cladding large mode area fiber, and, by tight coiling of the gain fiber to the radius of 5.5cm, the MI threshold can be increased to 3 times higher than that without coiling or loose coiling. Experimental study has been carried out to verify the idea, which has proved that MI was suppressed successfully in the amplifier by tight coiling.

Single-mode master-oscillator power amplifier at 647 nm with more than 500 mW output power.

Using an AlGaInP-based truncated tapered power amplifier, it was possible to boost the output power of a 647-nm distributed Bragg reflector laser from 50mW to more than 500mW. The light source has the potential to replace bulky Kr ion lasers still in use at this wavelength.

Coupled ridge waveguide distributed feedback quantum cascade laser arrays

A coupled ridge waveguide quantum cascade laser (QCL) array consisting of fifteen elements with parallel integration was presented. In-phase fundamental mode operation in each element is secured by both the index-guided nature of the ridge and delicate loss management by properly designed geometries of the ridges and interspaces. Single-lobe lateral far-field with a nearly diffraction limited beam pattern was obtained. By incorporating a one-dimensional buried distributed feedback grating, the in-phase-operating coupled ridge waveguide QCL design provides an efficient solution to obtaining high output power and stable single longitudinal mode emission. The simplicity of this structure and fabrication process makes this approach attractive to many practical applications.

High performance, low dissipation quantum cascade lasers across the mid-IR range.

In this work, we present the development of low consumption quantum cascade lasers across the mid-IR range. In particular, short cavity single-mode lasers with optimised facet reflectivities have been fabricated from 4.5 to 9.2 μm. Threshold dissipated powers as low as 0.5 W were obtained in continuous wave operation at room temperature. In addition, the beneficial impact of reducing chip length on laser mounting yield is discussed. High power single-mode lasers from the same processed wafers are also presented.

Single-mode high-power interband cascade lasers for mid-infrared absorption spectroscopy.

For high-sensitivity absorption spectroscopy, single-mode light sources capable of emitting high optical output power in the 3 to 5 µm wavelength range are vital. Here, we report on interband cascade lasers that emit 20 mW of optical power in a single spectral mode at room temperature and up to 40 mW at 0 °C using second-order laterally coupled Bragg gratings for distributed feedback. The lasers employ a double-ridge design with a narrow 3-µm-wide top ridge to confine the optical mode and a 9-µm-wide ridge for current confinement. The lasers were developed for an integrated cavity output spectroscopy instrument for stratospheric detection of hydrogen chloride at a wavelength of 3.3746 µm and emit at the target wavelength with more than 34 mW of single-mode power.

Real-time tracking of electromechanical oscillations in ENTSO-e Continental European Synchronous Area

Small signal stability is a crucial aspect of accurately keeping control in modern interconnected power systems, in order to ensure the required security and reliability standards. Such an aspect could represent a serious limiting factor in the search for ever higher power system exploitation levels. Hence, in order to exploit the existing networks as much as possible, ensuring at the same time adequate security levels, the system operators must monitor the electromechanical oscillations continuously, estimating their fundamental parameters (e.g. frequency, damping factor/ratio, amplitude and phase), and determining the actual dynamic stability margins. Nowadays, electromechanical oscillations tracking can be performed in real time thanks to the quick development of wide area measurement/monitoring systems, and the employment of so-called measurement-based methods (a.k.a. mode meters). In this regard, the paper presents a performance comparison among several measurement-based methods currently operating on the Italian Wide Area Monitoring (WAM) platform. The main objective of this investigation is to analyze the behavior of each of them with respect to actual critical/not critical electromechanical oscillations recorded on the European Network Transmission System Operators for Electricity (ENTSO-e) Continental Synchronous European System (CESA). The considered case studies cover a large variety of situations that can be encountered in the ordinary ENTSO-e CESA operation: local and inter-area oscillations; single mode and multi-mode power signals; ambient and ringdown data; and so on. By keeping in mind that usually no best estimator exists, owing to the lack of an optimality definition, the present analysis aims at examining whether some algorithms are generally more suitable than others, with respect to the variety of aforementioned situations. In detail, the measurement-based methods considered in this work are the Tufts–Kumaresan method, Hankel Singular Value Decomposition-VarPRO, Hilbert–Huang Transform, its refined Empirical Mode Decomposition with Fourier-Based Masking Technique (R-EMD), and a recently published Hilbert Transform-based estimation algorithm. Obviously, this set of estimation techniques cannot be considered exhaustive for the research problem under analysis. Nonetheless, it represents a collection of the major, recent and most fashionable methodologies implemented in actual WAM architectures.

Short-wavelength infrared defect emission as a probe of degradation processes in 980 nm single-mode diode lasers

Infrared emission from 980-nm single-mode high power diode lasers is recorded and analyzed in the wavelength range from 0.8 to 8.0 μm. A pronounced short-wavelength infrared (SWIR) emission band with a maximum at 1.3 μm originates from defect states located in the waveguide of the devices. The SWIR intensity is a measure of the non-equilibrium carrier concentration in the waveguide, allowing for a non-destructive waveguide mapping in spatially resolved detection schemes. The potential of this approach is demonstrated by measuring spatially resolved profiles of SWIR emission and correlating them with mid-wavelength infrared (MWIR) thermal emission along the cavity of devices undergoing repeated catastrophic optical damage. The enhancement of SWIR emission in the damaged parts of the cavity is due to a locally enhanced carrier density in the waveguide and allows for an analysis of the spatial damage patterns. The figure shows a side view of a diode laser during catastrophic degradation as recorded by a thermocamera within 5 successive current pulses. The geometry of the device is given in grayscale. The position of the laser chip is indicated by the dotted line. The thermal signatures of the internal degradation of the diode laser are overlaid in color. The bi-directional spread of the damage along the laser cavity is clearly visible.

High Power Single Mode Multi-Oxide Layer VCSEL with Optimized Thicknesses and Aperture Sizes of Oxide Layers

A novel multi-oxide layer structure for vertical cavity surface emitting laser (VCSEL) structures is proposed to achieve higher single mode output power. The structure has four oxide layers with different aperture sizes and thicknesses. The oxide layer thicknesses are optimized simultaneously to reach the highest single mode output power. A heuristic method is proposed for plotting the influence of these variable changes on the operation of optical output power. A comprehensive optical-electrical thermal-gain self-consistent VCSEL model is used to simulate the continuous-wave operation of the multi-layer oxide VCSELs. A comparison between optimized VCSELs with different structures is presented. The results show that by using multi-oxide layers with different thicknesses, higher single-mode optical output power could be achieved in comparison with multi-oxide layer structures with the same thicknesses.

Intra-cavity patterning - new method of single mode emission enhancement

We demonstrate the results of numerical simulations of 1.3μm InAlGaAs/InP VCSEL with a ring pattern etched at the interface between the cavity and top DBR. We show that the proposed design offers selective confinement of the fundamental mode and strong discrimination of higher order modes. Consequently, VCSEL with ring confinement is capable to operate in a broad range of injected currents in the single mode regime, which facilitates the improvement of maximal emitted power and increase of the wavelength tuning range with very narrow spectral characteristics. High power, single-mode emitting vertical-cavity surface-emitting lasers (VCSELs) are of great interest, since they can serve as high-power, smoothly tuned with current and temperature, coherent-optical sources. Emerging applications include inexpensive and portable gas sensors, telecommunication emitters, and so on. For these applications, it is preferable to have a low-threshold, high side-mode suppression ratio (SMSR) light source. VCSELs inherently emit in a single longitudinal mode; controlling the lateral mode is far more complicated. It can be achieved using a well-established wet oxidation technology, which makes possible single-mode operation only for relatively narrow apertures (providing less than 5 mW of output power) (1). Other methods provide up to twofold higher emitted power but exploit more challenging technologies (2-5). The limitation of high power single mode operation comes from small optical apertures or multimode operation under high injection levels. This paper provides numerical analysis of the ring pattern etched in the cavity of the VCSEL which eliminates higher order mode emission. The ring pattern provides confinement for a fundamental mode and forces higher order modes to be pushed out of the active region. The analyzed structure (Fig. 1a) (6) incorporates InAlGaAs quantum wells within an InP cavity. The cavity is bounded by 35 pairs of Al0.9Ga0.1As/GaAs DBRs from the bottom and 20.5 pairs of Al0.9Ga0.1As/GaAs DBRs from the top. Laterally patterned tunnel junction (TJ) layers are responsible for tunnelling carriers into the

Inverted-relief cavity used in VCSELs to suppress higher-order transverse modes

One of the most acute VCSEL disadvantages is an excitation of higher-order transverse modes in devices emitting more intense radiation. The paper presents results of simulation of VCSEL performance with the aid of the inverted-relief cavity used to suppress those modes. This suppression method is very efficient and relatively simple from technological point of view, so it is worth to be recommended. Full Text: PDF References C. Jung, R. J&auml;ger, M. Grabherr, P. Schnitzer, R. Michalzik, B. Weigl, S. M&uuml;ller, K.J. Ebeling, "4.8 mW singlemode oxide confined top-surface emitting vertical-cavity laser diodes", Electron. Lett. 33, 1790 (1997). CrossRef W. Nakwaski, "VCSEL structures used to suppress higher-order transverse modes", Opto-Electr. Rev. 19, 119 (2011). CrossRef R. Sarzała, T. Czyszanowski, M. Wasiak, M. Dems, Ł. Piskorski, W. Nakwaski, K. Panajotov, "Numerical Self-Consistent Analysis of VCSELs", Adv. in Opt. Techn. (2012). CrossRef H. Martinsson, J.A. Vuku&scaron;ič, A. Larsson, "Single-mode power dependence on surface relief size for mode-stabilized oxide-confined vertical-cavity surface-emitting lasers", IEEE Photon. Technol. Lett. 12, 1129 (2000). CrossRef H.J. Unold, S.W.Z. Mahmoud, R. J&auml;ger, M. Grabherr, R. Michalzik, K.J. Ebeling, "Large-area single-mode VCSELs and the self-aligned surface relief", IEEE J. Sel. Top. Quantum Electron. 7, 386 (2001). CrossRef D. Supper, J. Pfeiffer, M. Ilzh&ouml;fer, Ch. Giuliani, Ch. Degen, A.P. Lima, A. Ramakrishnan, G. Ebbinghaus, G. Steinle, "Long-wavelength VCSEL with integrated relief for control of singlemode emission", Opt. Comm. 267, 447 (2006). CrossRef

10 W single-mode Er/Yb co-doped all-fiber amplifier with suppressed Yb-ASE

In this work we demonstrate a single-frequency, single-mode all-fiber master oscillator power amplifier (MOPA) source, based on erbium–ytterbium co-doped double-clad fiber emitting 10 W of continuous wave power at 1565 nm. In the power amplifier stage, the amplified spontaneous emission from Yb3+ ions (Yb-ASE) is forced to recirculate in a loop resonator in order to provide stable lasing at 1060 nm. The generated signal acts as an additional pump source for the amplifier and is reabsorbed by the Yb3+ ions in the active fiber, allowing an increase in the efficiency and boosting the output power. The feedback loop also protects the amplifier from parasitic lasing or self-pulsing at a wavelength of 1 μm. This allows one to significantly scale the output power in comparison to a conventional setup without any Yb-ASE control.