Fundamental Mode Operation Research Articles

Efficient in-band pumped Ho:LuLiF_4 2 μm laser

An efficient Ho:LuLiF(4) laser in-band pumped by a cladding-pumped Tm-doped silica fiber laser operating at 1937 nm is reported. At low-cavity output coupling, the Ho:LuLiF(4) laser yielded 5.1 W of output at a wavelength of 2066 nm for 8.0 W of absorbed pump power with a slope efficiency of 70%. At high-cavity output coupling, the lasing wavelength shifted to 2053 nm and the laser produced an output power of 5.4 W with a slope efficiency of 76%. The beam propagation factor (M(2)) was measured to be approximately 1.1 at the maximum output power confirming fundamental transverse mode (TEM(oo)) operation. The influence of resonator design on laser performance is discussed, along with prospects for further power scaling and improvement of the laser efficiency.

Strained InGaAs/GaAs Quantum-Well Laser Emitting at 1054 nm

In this paper, we present an InGaAs/GaAs strain QW laser with tilted waveguide emitting at 1054 nm that has been fabricated by Metal-Organic Chemical-Vapor Deposition (MOCVD) on a GaAs substrate. The active region consists of 6 nm QW separated by a 60-nm barrier within a two-step graded-index separated-confinement heterostructure (GRINSCH) region. The use of a Strained Buffer Layer (SBL) is shown to significantly improve the laser performance. The composition and micro-structure quality of the QW was measured by high-resolution X-ray diffraction. The strain QW laser exhibits a threshold current of 9 mA and a slope efficiency of 0.4 W/A (unused antireflection coating on the facets). Stable fundamental transverse mode operation was obtained up to 33 mW emitted power at an injection current of 100 mA. The spectral width of the strain QW laser is 1.6 nm at an injection current of 50 mA.

Novel planar Gunn diode operating in fundamental mode up to 158 GHz

We show the experimental realisation of fundamental mode operation of planar Gunn diode structures fabricated in GaAs/AlGaAs quantum wells. The electron density in the active channel is enhanced by positioning double delta-doping layers on either side. Small signal measurement shows that a typical device exhibits negative resistance up to 158 GHz. Using this device structure we have demonstrated a planar Gunn oscillator working at 115.5 GHz with an output power of -28 dBm.

Lateral current injection GaInAsP/InP laser on semi-insulating substrate for membrane-based photonic circuits

A room-temperature pulsed operation was demonstrated using lateral current injection-type lasers composed of a 400-nm-thick GaInAsP core layer with compressively strained 5 quantum wells. A threshold current of 105 mA and corresponding density of 1.3 kA/cm(2) (260 A/cm(2) per well) were obtained with the stripe width of 5.4 microm and the cavity length of 1.47 mm. A fundamental transverse mode operation was obtained with the narrower stripe device of 2.0 microm and the cavity length of 805 microm, while the threshold current and corresponding density were 49 mA and 3.0 kA/cm(2), respectively.

Extremely large mode area optical fibers formed by thermal stress

We report a strictly single-mode optical fiber with a record core diameter of 84 microm and an effective mode area of approximately 3600 microm(2) at 1 microm. We also demonstrate fundamental mode operation in an optical fiber with a record core diameter of 252 microm and a measured mode field diameter (MFD) of 149 microm at 1.03 microm, i.e. an effective mode area (Aeff) of approximately 17,400 microm(2) at 1.03 microm, an Aeff of 31,600 microm(2) at 1.5 microm. All these fibers have near parabolic index profiles with a peak refractive index difference DeltaN approximately approximately 6 x 10(-5), i.e. a record low numerical aperture (NA) of approximately 0.013 in an optical fiber. This low refractive index difference was achieved by frozen-in thermal stress as a result of two different types of glass in the fibers. When the fundamental mode was excited in the 252 microm core fiber using a 1.03 microm ASE source, the output beam was measured to have M2x = 1.04 and M2y = 1.18.

Spatially Filtered Feedback for Mode Control in Vertical-Cavity Surface-Emitting Lasers

Transverse mode control of a vertical-cavity surface-emitting laser (VCSEL) is achieved by spatially filtered optical feedback. The spatial filter consisted of a pinhole, either 42 ?m or 81 ?m, in the 5.8 × magnified VCSEL image plane of a confocal lens arrangement. The pinhole was translated to map out the voltage drop across the VCSEL from spatially-selective feedback. For low injection currents, spatial filtered feedback enabled single fundamental mode operation due to higher-order mode suppression by appropriately locating the pinhole. For higher injection currents with five laser modes, spatial selection of feedback enabled control of the relative mode intensities and wavelengths. The maximum increase in mode intensity achieved was 1.8 × for the 4 ?m pinhole and 2.4 × for the 81 ?m pinhole. The maximum frequency shift achieved was - 2.7 GHz and - 7.1 GHz with the 42 ? and the 81 ?m pinholes. The spatially selective optical feedback produced long-lived changes to the VCSEL even after the feedback was blocked-results for two separate trials where the recovery time was 38 minutes are presented (although results varied for different bias and feedback conditions).

A Tube-Core Orthogonal Fluxgate Operated in Fundamental Mode

In this paper, we suggest applying fundamental-mode operation to orthogonal fluxgates with tube cores. Excitation current in these fluxgates flows through a toroidal coil wound around the tube core, whereas in the orthogonal fluxgates with amorphous wires, it flows directly through the core. Having no excitation current inside the core reduces its heating and, hence, decreases the fluxgate thermal drift. Employing the toroidal coil also allows decreasing the excitation current by simply increasing the number of coil turns, while keeping the same intensity of the excitation field. Our experiments have shown a much higher efficiency of the new operating mode as compared with the second-harmonic mode. Adding a great enough dc bias to the ac excitation has caused a dramatic noise reduction. This effect is especially pronounced at relatively low frequencies, below 10 kHz. The fluxgate resolution in the fundamental mode, 10 pT/radicHz at 1 Hz , is by a factor of 30 better than in the second-harmonic mode. The sensitivity in the fundamental mode exceeds by a factor of 12.5 the sensitivity in the second-harmonic mode. We have also observed an about two times lower thermal drift of the fluxgate output. We have also found in this work that the phase noise of the excitation current is the main contributor to the fluxgate noise at low frequencies. It contributes about 67% to the fluxgate noise power density near the fundamental.

Planar buried crescent InP/InGaAsP/InP heterostructure on p-InP

A process is described for liquid phase epitaxy of mesa stripe buried crescent InP/InGaAsP/InP heterostructures with a p-n-p-n/ZnSe leakage current blocking structure. The process enables the fabrication of 1.3-μm light-emitting diodes with a low threshold current, stable fundamental transverse mode operation, and high output power. When mounted with the mesa stripe up, the diodes offer an emission power above 30 mW.

High-Performance 76-GHz Planar Gunn VCO

A 76-GHz Gunn voltage-controlled oscillator (VCO) with a high output power and a wide tuning-frequency range was fabricated by optimizing VCO circuits and using laser micromachining. The tuning-frequency range of the fabricated Gunn VCO was more than two times higher than that attained in our previous experiments by optimizing VCO circuits. The VCO attained a tuning-frequency range of 493MHz, out-put power variation of 1.0dB, and tuning-frequency linearity of 6.1% over a tuning-voltage range from 0 to 10V. Its power consumption was 2.0W at operation voltage of 3.6V. And it measured out-put power was 13.3dBm with DC-RF conversion efficiency of 1.0% at 76.5GHz. Moreover, under fundamental-mode operation, it achieved low phase noise of -107.8dBc/Hz at an offset frequency of 1MHz. Since laser micromachining was used in fabricating the Gunn VCO, the reproducibility of its RF performance was improved.

Single-mode 212 W tapered fiber laser pumped by a low-brightness source

With a tapered double-clad all-glass ytterbium fiber as a gain medium, a maximum output power of over 200 W at 1079 nm and a slope efficiency of over 70% were demonstrated. The tapered double-clad fiber concept allows for using low-brightness diode bars and results in cost-effective and efficient high-power fiber lasers. The adiabatic conical fiber gain waveguide combines improved pump absorption owing to enhanced mode mixing in the pump cladding, low-noise single fundamental mode operation with M(2)< or =1.02, and strong potential for significant power scaling.

An antenna-feed system with an RF drive source for a Cherenkov maser—an antenna-amplifier

The microwave transmission system of an X-band antenna-amplifier (a relativistic traveling-wave tube intended for operation in the fundamental nonaxisymmetric HE11 mode of a dielectric rod) has been investigated. A specially designed two-frequency tunable resonance microwave pulse compressor is used as an RF drive source. The microwave transmission system is incorporated into a hollow high-voltage electrode of a linear induction accelerator. It is shown that this transmission system can be reasonably matched in a frequency band of about 700 MHz, and microwave pulses with a duration of ∼4 ns and a peak power of at least up to 1.6 MW can be transmitted via it without substantial losses and envelope distortions. It is also shown that the actual configuration of the device virtually does not affect the angular distribution of the far-field radiation pattern of the rod antenna, which is an extension of the slow-wave structure of the amplifier.

On the design of electrically pumped vertical-external-cavity surface-emitting lasers

Vertical-external-cavity surface-emitting lasers (VECSELs) yield an excellent beam quality in conjunction with a scalable output power. This paper presents a detailed numerical analysis of electrically pumped VECSEL (EP-VECSEL) structures. Electrical pumping is a key element for compact laser devices. We consider the optical loss, current confinement, and device resistance. The main focus of our investigation is on the achievement of an adequate radial carrier distribution for fundamental transverse mode operation. It will be shown that a trade off between the conflicting optical and electrical optimization has to be found and we derive an optimized design resulting in guidelines for the design of EP-VECSELs which are compatible with passive mode locking.

Scanning optical homodyne detection of high-frequency picoscale resonances in cantilever and tuning fork sensors

Higher harmonic modes in nanoscale silicon cantilevers and microscale quartz tuning forks are detected and characterized using a custom scanning optical homodyne interferometer. Capable of both mass and force sensing, these resonators exhibit high-frequency harmonic motion content with picometer-scale amplitudes detected in a 2.5MHz bandwidth, driven by ambient thermal radiation. Quartz tuning forks additionally display both in-plane and out-of-plane harmonics. The first six electronically detected resonances are matched to optically detected and mapped fork eigenmodes. Mass sensing experiments utilizing higher tuning fork modes indicate greater than six times sensitivity enhancement over fundamental mode operation.

920-nm Vertical-External-Cavity Surface-Emitting Lasers With a Slope Efficiency of 58% at Room Temperature

We report a high-power fundamental transverse mode operation of an optically pumped 920-nm vertical-external- cavity surface-emitting laser. The effects of the gain related structural parameters on the laser performance are investigated. Based on the optimization of the gain structure and the thermal management, a maximum pump-limited output power of 12 W in a TEM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> mode with a slope efficiency of 58% (input power of 24 W) was achieved at room temperature.

High Modulation Bandwidth Implant-Confined Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

High-speed implant-confined photonic crystal vertical-cavity surface-emitting laser (VCSEL) diodes are fabricated and characterized. The maximum achievable 3-dB bandwidth is a function of both the index confinement provided by the etched pattern (which serves to reduce the diffraction loss and provide for stabilized fundamental mode operation) and the increased parasitic resistance caused by the removal of doped material. In optimized designs, the reduction in net loss provided by the etched pattern can more than offset the increased parasitics, resulting in a higher 3-dB bandwidth than possible in an unetched conventional implant VCSELs for both single- and multi transverse mode operation. A maximum 3-dB small-signal modulation bandwidth of 15 and 18 GHz is reported for an optimized single transverse-mode and multimode VCSEL, respectively.

High-Temperature Dynamics, High-Speed Modulation, and Transmission Experiments Using 1.3- $\mu\hbox{m}$ InGaAs Single-Mode VCSELs

High-temperature dynamics, including small and large signal modulation response, of 1.28-μm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) with highly strained InGaAs quantum wells have been investigated. The VCSELs are oxide-confined with a relatively large oxide aperture for high output power and have a surface relief for fundamental mode operation. An inverted surface relief is used to improve manufacturability and to suppress oxide modes that otherwise appear in VCSELs with a large detuning between the cavity resonance and the gain peak. The size dependence of the modulation bandwidth and bandwidth limitations are investigated. A VCSEL with an optimum combination of oxide aperture and surface relief diameters produces clear open eyes with an extinction ratio of > 6 dB at OC-48 and 10-GbE bit rates from 25 °C to 85 °C under constant drive conditions. The same VCSEL is also shown to be capable of error-free transmission (BER < 10-9) over 9 km of standard single-mode fiber under the same conditions.

Computer simulation of an operation of the GaInP/AlGaInP QW VCSELs: Excitation of various transverse LPij modes

A comprehensive fully self-consistent optical–electrical–thermal-recombination model of the 650-nm oxide-confined GaAs-based GaInP/AlGaInP vertical-cavity surface-emitting diode lasers (VCSELs) is used to determine their anticipated room-temperature (RT) continuous-wave (CW) performance characteristics. As expected, for the devices with very large active regions, higher-order transverse LPij modes exhibit the lowest lasing thresholds. However, the desired single fundamental LP01 mode operation remains dominating one for relatively large active regions of diameters up to as much as 10mm. Therefore, the 650-nm GaInP/AlGaInP QW VCSELs have been found to offer a very promising RT CW performance as sources of carrier radiation for the optical communication taking advantage of plastic (polymer) optical fibers (POFs).

GaInNAsSb/GaNAs quantum-well VCSELs: Modeling and physical analysis in the 1.50−1.55 μm wavelength range

Physical analysis of the long-wavelength room-temperature (RT) continuous-wave (cw) emission within the 1.50−1.55 μm range from the GaAs-based GaInNAsSb/GaNAs quantum-well (QW) vertical-cavity surface-emitting diode lasers (VCSELs) is carried out. To this end, a comprehensive three-dimensional optical-electrical-thermal-gain self-consistent simulation of physical processes taking place inside a laser volume is applied. While the 1.50-μm emission has been found to be reached practically without trouble, the 1.55-μm emission needs additional optimizations. For the optimal QW structure, the 1.55-μm emission in highly detuned VCSELs requires RT cw threshold currents more than 18 times higher than those of the 1.50-μm one. Some threshold reduction may be achieved by adding the second quantum well. The 3λ-cavity design has been proved to be optimized for the 1.50-μm emission ensuring stable fundamental-mode operation even at elevated temperatures whereas the 1.5λ-cavity is optimal for the 1.55-μm emission for which threshold reduction is more important. For the 1.50-μm emission, some structure optimizations have been proposed. The GaInNAsSb/GaNAs structure offers promising prospects to manufacture VCSELs emitting 1.55-μm radiation, but some technology and construction improvements are still necessary.

Fundamental-mode operation in polarization-maintaining ytterbium-doped fiber with an effective area of 1400 μm^2

We report, for the first time to our knowledge, fundamental-mode operation in polarization-maintaining ytterbium-doped fiber with an effective area of approximately 1400 micro m(2) and birefringence of 2.1 x 10(-4). The measured critical bending radius, R(c), defined as the radius at which the loss is equal to 3 dB for 1 m of fiber, is approximately 4 cm. Lasing with slope efficiency exceeding 60% and M(2) of 1.2 has also been demonstrated.

Single-mode 1.3-/spl mu/m photonic crystal vertical-cavity surface-emitting laser

We report the use of a photonic crystal in a 1.3-mum vertical-cavity surface-emitting laser to achieve single fundamental-mode operation. A focused ion beam was used to mill the photonic crystal into a top dielectric distributed Bragg reflector. The laser operated continuous-wave at room temperature with greater than 1 mW of single-mode output power observed with a sidemode suppression ratio of more than 37 dB and little change to the electrical properties of the device