Fundamental Mode Operation Research Articles

Mode-Stabilized Operation in a Microlens-Integrated 980 nm Vertical-Cavity Surface-Emitting Laser

Microlens-integrated 980 nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated and their mode-stabilized operations investigated. A stable single fundamental transverse mode operation in the microlens-integrated VCSEL was observed. Numerical modal calculations accurately explain the effects of the microlens on mode stabilization.

Effect of Index Variation in Active Layer on Transverse Mode for Vertical-Cavity Surface-Emitting Lasers

The distribution of carrier density in an active layer of a vertical-cavity surface-emitting laser (VCSEL) produces a refractive index distribution, affecting the transverse-mode behavior of the VCSEL causing high-order-mode lasing. To achieve high-power fundamental-mode operation, we investigated the reduction in this effect by elongating the cavity. Experimental data show that the high-order mode started lasing immediately above a threshold in a lambda-cavity VCSEL with an active area large than 10 µm in diameter. By introducing a long monolithic cavity, high-order-mode lasing was suppressed. Theoretical results clearly show that such high-order-mode suppression is achieved because the effect of carrier-induced index change in the active layer is reduced by elongating the cavity.

1060 nm vertical-external-cavity surface-emitting lasers with an optical-to-optical efficiency of 44% at room temperature

We report a high power fundamental transverse mode operation of an optically pumped 1060nm vertical-external-cavity surface-emitting laser. A diamond heat spreader was capillary bonded to the semiconductor surface. A 10W continuous wave operation with optical-to-optical conversion efficiency of 44% was achieved at room temperature. The thermal rollover was not found up to a heat sink temperature of 60°C. High efficiency and good thermal stability were mainly due to the optimization of epitaxial quality and the high conductivity of the diamond heat spreader. We have found that the increase of the round trip loss caused by the heat spreader was about 1%.

Bend-resistant fundamental mode operation in ytterbium-doped leakage channel fibers with effective areas up to 3160 µm2

We report fundamental mode operation in ytterbium-doped leakage channel fibers with effective areas up to 3160 microm(2). These fibers are bend-resistant. Lasers with slope efficiency ~60% and M(2) ~1.3 has been demonstrated. Bend loss and amplifier performance have also been characterized.

Five-hundred-milliwatts distributed-feedback diode laser emitting at 940 nm

A DFB laser emitting at a wavelength around 940 nm in a single longitudinal mode up to an output power of 500 mW with a side mode suppression ratio greater than 50 dB is presented. More than 4 nm tuning range was reached by varying the current between 100 and 800 mA. Up to a power of about 300 mW, the lateral far field revealed stable fundamental mode operation.

Mode-locking dynamics in electrically driven vertical-external-cavity surface-emitting lasers

We develop a novel description of electrically driven vertical-external-cavity surface-emitting semiconductor lasers (VECSELs) mode-locked by saturable absorber mirrors. Our approach is based on an analytical solution of the bidirectional traveling-wave equations for fundamental transverse mode operation. The resulting time-domain equations describe the evolution of the electric fields and carrier-densities at the quantum-well layers of the emitter and absorber structures which are coupled through delayed boundary conditions. For the design considered, we obtain stable mode-locked pulses of few tens of picoseconds at 15-GHz repetition rate in agreement with recently reported experimental results.

Active mode locking of a phase-conjugating SBS-laser oscillator

We present a flashlamp-pumped Nd:YAG laser simultaneously emitting pulse structures on microsecond, nanosecond and picosecond time scales. Within a microsecond flashlamp pump pulse a nonlinear reflector based on stimulated Brillouin scattering (SBS) generates several Q-switch pulses. The phase-conjugating effect of the SBS reflector provides a compensation of phase distortions generated inside the laser rod, resulting in transverse fundamental mode operation. Additional acousto-optic loss modulation inside the resonator leads to mode locking. As a result, each Q-switch pulse is subdivided into several picosecond pulses. Energies of up to 2 mJ for the mode-locked pulses with durations between 220 and 800 ps are demonstrated. The wide variability of the laser’s temporal output parameters as well as its high beam quality make it a splendid tool for fundamental research in laser materials processing.

Long-wavelength tunable vertical-cavity surface-emitting lasers and the influence of coupled cavities

In this paper, we present an InP-based micromechanically tunable VCSEL emitting in the 1.55microm wavelength region with a 26nm tuning range. The laser is based on a two-chip concept, allowing for a separate optimization of the curved top mirror and the amplifying component. Current confinement is achieved by a buried tunnel junction. The design of the microcavity ensures fundamental mode operation with a side mode suppression ratio exceeding 49dB even for large apertures. Simulations indicate that the tuning range is limited by coupled cavity effects and reveal important design criteria like an upper boundary regarding the device thickness.

Harmonic control for an integrated microstrip antenna with loaded transmission line

This paper proposes a harmonic-suppression technique for a square microstrip-antenna design. In this study, the possible resonance at the 2nd and 3rd harmonic frequencies were effectively suppressed by adding loads to the antenna's feeding line, and the antenna performances for fundamental-mode operation were well retained. The design and experimental results are discussed and simple design principles for harmonic suppression of the proposed antennas are presented. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 379–383, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20641

Modes-controlled slot antennas with frequency selective surface

This paper presents two designs of square-ring slot antennas to suppress the 2nd and 3rd harmonics by involving a frequency-selective surface (FSS) with square-loop-type and meandered-dipole-type elements. The transparency of the surface in the higher-order mode makes the ground plane invalid, so that the harmonics cannot resonate and emit. The experimental results show that the FSS structure not only demonstrates band-reject characteristics, but also well retains the antenna performances for fundamental-mode operation. Moreover, with the square-loop-type lattice, the impedance bandwidth was increased by about four times as much as that of the reference antenna. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 47–49, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21256

Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier

We demonstrate, theoretically and experimentally, a high power/energy 19-core Yb-doped fiber amplifier that operates in its fundamental in-phase mode. The calculated result using an improved coupled mode theory with gain shows that, with a Gaussian beam as seed, the in-phase supermode dominates. Experimentally, we use a Q-switched single-core fiber laser with single transverse mode as seed, and amplify it with a 5.8 m 19-core fiber. The measured near and far fields are close to the in-phase supermode. The measured M2 factor of the amplified beam is 1.5, which is close to the theoretical value. A pulse energy gain of 20 dB is obtained with the amplified pulse energy up to 0.65 mJ at a repetition frequency of 5 kHz. No appreciable stimulated Brillouin scattering is observed at this power level.

Single fundamental mode photonic crystal vertical cavity surface emitting laser with 9 GHz bandwidth

A record 9 GHz small signal modulation in a single fundamental mode photonic crystal vertical cavity surface emitting laser is demonstrated for the first time. The device is designed by incorporating etched photonic crystal holes in the top distributed Bragg reflector for optical confinement. Emission spectra measured during small signal modulation confirms single fundamental mode operation.

Suppression of magnetic noise in the fundamental-mode orthogonal fluxgate

A new important advantage has been discovered for the fundamental-mode operation of the orthogonal fluxgate employing an amorphous wire. It has been found that a great enough dc bias practically completely suppresses the magnetic noise generated in the fluxgate core by the ac-bias field; the fluxgate resolution becomes limited only by the excess electric noise in the ac-bias current. As a result, the magnetometer resolution increases by a factor of 60 and reaches 10 pT/ Hz at frequencies above 2 Hz. The suppression of magnetic noise can be explained by excluding magnetization reversals in the fluxgate core. In the fundamental mode, the fluxgate core is kept saturated continuously due to the unipolar bias, and magnetization varies by coherent rotation. Practically no magnetic noise is generated in this case. In the second-harmonic mode, magnetization is reversed by the bipolar bias. This causes nucleating domains and generating intensive magnetic noise in the fluxgate core.

Cascade nitride VCSEL designs with tunnel junctions

The performance of various possible designs of 400-nm nitride vertical-cavity surface-emitting lasers (VCSELs) has been analyzed with the aid of an advanced three-dimensional (3D) thermal-electrical-optical-gain self-consistent threshold simulation. It has been demonstrated that it is practically impossible to reach fundamental-mode operation in nitride VCSELs of the traditional design with two ring contacts. To enhance this desired operation, the uniformity of current injection into VCSEL active regions should be dramatically improved. Therefore, we have focused our research on designs with tunnel junctions and/or a semitransparent contact. In particular, it has been proved that a design with two cascading active regions, two tunnel junctions and a semitransparent contact may offer the most promising room-temperature performance characteristics for both pulse and continuous-wave operations. In particular, this design offers high mode selectivity with a distinct fundamental transverse mode domination. The simulations also reveal that the thickness and localization of the semitransparent contact, as well as the localization of active regions and tunnel junctions, are crucial for successful construction design.

High-power highly reliable 1.02-1.06-/spl mu/m InGaAs strained-quantum-well laser diodes

By growing the InGaAs active layer at temperatures lower than in conventional growth, we extended the lasing wavelength and presented the high reliability in InGaAs strained-quantum-well laser diodes. Equivalent I-L characteristics were obtained for 1.02-, 1.05-, and 1.06-/spl mu/m laser diodes with a cavity length of 1200 /spl mu/m. Maximum output power as high as 800 mW and fundamental transverse mode operation at up to 400 mW were obtained at 1.06 /spl mu/m and an 1800-/spl mu/m cavity. Stable operation was observed for over 14 000 h under auto-power-control of 225 mW at 50/spl deg/C for the 1.02-, 1.05-, and 1.06-/spl mu/m lasers with a 900-/spl mu/m cavity.

Narrow vertical beam divergence laser diode based on longitudinal photonic band crystal waveguide

Narrow (<5°) vertical beam divergence is realised in an InGaAs–AlGaAs double-quantum well edge-emitting laser diode. A multilayer GaAs–AlGaAs structure was used as a waveguide, acting as a longitudinal photonic band crystal and enabling stable fundamental mode operation. Longitudinal mode grouping effect is observed in the lasing spectrum and attributed to the transverse cavity-induced hole-burning effect.

Single-mode operation and transverse-mode control in VCSELs induced by frequency-selective feedback

An experimental study of a vertical-cavity surface-emitting laser with frequency-selective feedback is reported. We show that we can force the laser to emit in its fundamental transverse mode or in a higher order transverse mode and that we can also control the polarization emitted in the fundamental one. We obtain single fundamental-mode operation within a large range of injection current values (almost /spl ap/2.5 I/sub th/) and a maximum optical power of 2.7 mW. In this current range, the output is found to be stable.

Effect of hybrid structure (1–3 composite and ceramic) on the performance of sandwich transducers

By using 1–3 composite rings to replace the ceramic rings in a Langevin sandwich transducer, the non-axial and many other spurious resonances in the transducer are suppressed, and only the axial resonance modes are retained. In this study, a combination of ceramic and 1–3 composite rings, referred to as a hybrid structure, was used as the driving element to improve the performance of the transducer. Sixty-kilohertz pre-stressed sandwich transducers with hybrid structures were constructed. Various relative placements of the ceramic and composite rings in the transducers were studied in order to optimize the transducer performance and the results are discussed. The damping effects in the composite rings were evaluated from the axial vibration displacements measured by a laser vibrometer. Characteristics of hybrid transducers were compared with the full ceramic and full composite transducers. A symmetric structure in the hybrid transducer is found to be most effective for the fundamental and harmonic mode operations.

240-325-GHz GaAs CW fundamental-mode TUNNETT diodes fabricated with molecular layer epitaxy

Gallium arsenide (GaAs) transit-time diodes with tunnel injection of electrons (TUNNETT) with transit-time layer thickness of 100 and 150 nm were fabricated with molecular layer epitaxy (MLE). Continuous-wave fundamental-mode oscillation in the frequency range of 240 to 325 GHz in metal rectangular resonant 0.86 /spl times/ 0.43 mm size (WR-3) cavities was obtained. Output power of -13 dBm was generated at 322 GHz. The fundamental mode operation, as well as experiments on different impedance matching configurations, suggest that it is possible to develop fundamental mode TUNNETT generators for the frequency range of 350 GHz to 1 THz. Operation of the TUNNETTs confirms device quality of the MLE.

High brightness single-mode ridge laser utilizing buried heterostructure defined by quantum-well intermixing

The authors demonstrate a novel high brightness single-lateral mode ridge laser using quantum well intermixing to form a buried heterostructure. Increased discrimination between the fundamental and higher order modes can be achieved using the buried heterostructure to reduce the width of the gain section, enhancing fundamental mode operation. This allows the ridge width to be increased while maintaining fundamental mode operation, hence reducing the optical intensity at the facet and increasing the optical power before mirror degradation. Standard and novel buried heterostructure ridge lasers of 5-/spl mu/m width are compared; far-field beam profiles clearly show improved modal stability for the novel structure.