Single-mode Laser Operation Research Articles

Distributed feedback gallium nitride nanowire lasers

Achieving single-mode laser operation in nanowire lasers remains a challenge due to a lack of mode selection approaches. We have implemented single-mode lasing using distributed feedback by externally coupling gallium nitride nanowires to a dielectric grating to achieve mode-control. The effective periodicity of the grating experienced by the nanowire was altered using nanomanipulation to change the angular alignment between the nanowire and the grating. The effective periodicity controls the spectral location of the distributed feedback stop-band. Single-mode emission was achieved at an alignment, where the designed periodicity of the grating was experienced by the nanowire.

A laterally-coupled distributed feedback laser with equivalent quarter-wave phase shift

We report the first laterally-coupled distributed feedback (LC-DFB) laser with a quarter-wave equivalent phase shift (EPS) realized by interference lithography (IL) and conventional photolithography. A specially designed sampled grating is fabricated on both sidewalls of a ridge waveguide to provide a quarter-wave EPS at the center of the cavity. The resulting laser exhibits stable single-mode lasing operation over a wide range of injection currents, with a side mode suppression ratio (SMSR) of 41.1 dB. This provides a practical, low-cost method to fabricate quarter-wave phase shifted DFB lasers with high performance without any epitaxial regrowth or the use of electron-beam lithography, thereby simplifying the fabrication of DFB lasers with stable and precise wavelengths, as single devices or as arrays in photonic integrated circuits.

A high-Q low threshold thulium-doped silica microsphere laser in the 2 μm wavelength region designed for gas sensing applications

A high-Q and low threshold laser resonator, operating in the 2 μm wavelength region, has been demonstrated by coupling a thulium-doped silica microsphere to a tapered fibre. Microspheres with diameters ranging from fifty to a few hundred micrometres were carefully fabricated for this purpose by melting an etched-clad thulium-doped silica fibre tip using a focused beam from a CO2 laser, while the tapered fibre with waist diameter in the desired range of 2 μm was fabricated by using heating and stretching of standard single-mode telecommunication fibre. The tapered fibre served the dual purpose of transporting pump power into the sphere and allowing the extraction of the resulting laser emission. Under excitation at a wavelength of ∼1.6 μm, lasing occurred at wavelengths over the range from 1.9 to 2.0 μm. Single-mode laser operation was obtained by exciting the fundamental whispering gallery mode resonance of the microsphere, while multi-mode lasing occurred for non-fundamental mode excitation. The threshold power of the laser was measured to be about 50 μW delivered pump power, and a maximum laser power of 0.8 mW at around 1.94 μm was observed for a 6 mW pump power, operating at wavelengths around 1.6 μm. The laser was designed as a low threshold and compact source for miniaturized gas sensing devices operating over this important wavelength region.

Stable single-mode operation of surface-emitting terahertz lasers with graded photonic heterostructure resonators

Graded photonic heterostructures (GPH) can be regarded as energy wells for photons. We show that judicious engineering of such photonic wells, obtained by tailoring the grading and the slit width of the GPH resonator, allows one to ensure spectrally single-mode emission on the fundamental symmetric mode in the whole lasing dynamical range of terahertz quantum cascade lasers. Furthermore, the radiative character of the symmetric mode leads to single-mode emission with mW output power in continuous-wave operation, as well as to single-lobed far-field beam patterns. A careful combination of theoretical analysis and experimental observations reveals that the results stem from interplay between mode competition and spatial hole burning effects.

Tunable Distributed Feedback Quantum Cascade Lasers by a Sampled Bragg Grating

A novel complex-coupled distributed feedback quantum cascade laser emitting around λ ~ 4.7 μm is demonstrated by a sampled Bragg grating (SBG). The key superiorities are to utilize the +1st-order (positive first order) transmission of the SBG for laser single-mode operation, and use conventional holographic exposure combined with the optical photolithography technology to fabricate the sampled grating, which lead to improved flexibility, repeatability, and cost-effectiveness. Selective single-mode lasing with a mean side mode suppression ratio above 20 dB and wavelength coverage range of 87 nm is achieved by changing the sampling period.

Fano-Resonance Gain by Dephasing Electron–Hole Cooper Pairs in Semiconductors

A genuine many-body feature in semiconductor lasers is theoretically found in the gain spectra when a BCS-type (Bardeen–Cooper–Schrieffer) electron–hole (eh) pairing instability occurs during low-temperature operation. Owing to the finite dephasing time of eh pairs, an ultranarrow Fano-resonance gain between a bound eh pair and a continuum of unbound (plasma-like) eh pairs is found close to the phase-transition point. As a result, single-mode laser operation is strongly modified relative to the conventional laser operation using the Lorentzian-type eh plasma gain. Sharp Fano-resonance gain occurs owing to the Hopf bifurcation in the poles of the optical susceptibility.

Single-mode lasing of GaN nanowire-pairs

Stable single-mode lasing operation from a pair of coupled GaN nanowires is demonstrated through optical pumping. GaN nanowires with different lengths were placed side-by-side in contact to form a coupled cavity through nanoprobe manipulation. Unlike individual nanowire lasers, which operate in a combined multiple transverse and multiple longitude mode oscillation, a coupled nanowire-pair provides a mode selection mechanism through the Vernier effect, which can strongly enhance the free spectrum range between adjacent resonant modes and generate a stable single-mode operation with a high side-mode suppression ratio.

A heterodyne laser system to study frequency stabilized Zeeman 633 nm He-Ne lasers deficient in temperature steadiness

A heterodyne laser system is constructed to study the effect of external transverse magnetic field on the dynamic instability of internal mirror TEM00q He-Ne lasers having temperature instability. The system could provide precise detailed knowledge about the roles of both the temperature and the applied magnetic field separately. Two different internal mirror 633 nm TEM00q He-Ne lasers with frequency stability of 10−6 are studied. The applied transverse magnetic field on He-Ne lasers increased the frequency stability to be in the order of ∼10−10, in spite of temperature instability. In the same time, a single-mode operation with an enhanced laser output power is obtained. The sensitivity of the method showed that the laser exhibits a short-term frequency stability of 4.7×10−10, which after one minute, when mode collapse starts, decreased to 2.6×10−8, over the next four minutes. This increase in the frequency difference of the inter-mode beat signal that affect the stability is attributed to the uncontrolled temperature of the laser tube. The results revealed that the magnetic field plays the dominant role to achieve the maximum frequency stability, while the unsteadiness of temperature of the laser tubes limits the long term single-mode laser operation.

Mid-infrared semiconductor heterostructure lasers for gas sensing applications

An overview of the three competing mid-infrared semiconductor laser approaches, being diode, quantum cascade and interband cascade laser designs, is given. Limiting factors as well as unique possibilities of each approach are discussed. Several designs of distributed feedback structures for single-mode laser operation are presented, which is an important prerequisite for gas-sensing applications. The analysis of current state-of-the-art device data allows suggestions with respect to suitable fields of applications.

Continuous Wave Laser Generation in Proton Implanted Nd:GGG Planar Waveguides

We report on room temperature continuous wave laser generation at ~1062 nm from proton implanted Nd:GGG planar waveguides. Single-mode stable laser operation is realized with an 808-nm pump source, resulting in a laser threshold of ~49.3 mW and slope efficiency of 30%. The maximum output power of ~21.9 mW, an absorbed pump power of ~117.4 mW, leads to an optical-to-optical conversion efficiency of 18.7%.

1.55 $\mu$m InAs/InAlGaAs Quantum Dot DFB Lasers

For the fabrication of quantum dot (QD) distributed feedback (DFB) lasers, self-assembled InAs/InAlGaAs QDs were grown on the InP/InGaAs grating structures by a molecular beam epitaxy. Ridge-waveguide QD DFB lasers with a stripe width of 3 mum were fabricated. Single-mode lasing operation around the wavelength of 1.56 mum was successfully achieved under the pulsed and continuous-wave (CW) modes at room temperature. The lasing operation was also observed up to 70 degC under pulsed mode, which is the first observation on the single-mode lasing around 1.55 mum. The characteristic temperatures calculated from the temperature dependence of the threshold current density were 121.8 K from room temperature to 45 degC, which was reduced to 74.2 K from 45 degC to 70 degC.

Formation of InAs 1−xSb x quantum dots on vicinal InP(0 0 1) for 1.55-μm DFB laser applications

We have investigated the MOVPE growth of InAs 1− x Sb x quantum dots (QDs) on vicinal InP(0 0 1) substrate for DFB laser applications. Miscut orientation was adopted toward the [1 1¯ 0] direction in which InAsSb quantum wires elongate on normal InP(0 0 1) substrate. The interval between the QDs and the lateral size of the QDs in the [1 1 0] direction decreased as the Sb fraction was increased, which brought the density to as high as 1.1×10 11 cm −2 of InAsSb QDs. The InAsSb QDs were stacked with 30-nm-thick InGaAsP spacer layers for 10 periods without correlation, and had an emission of a wavelength at around 1.55 μm although the segregation of Sb atoms was slightly observed. A pn-buried-heterostructure-type DFB laser with InAsSb-QD active layer was fabricated. The lasing wavelength was 1540.6 nm at 25 °C, and CW, single-mode lasing operation was demonstrated up to 100 °C.

Demonstration of 1.51µm InAs/InP(311)B quantum dot single-mode laser operating under continuous wave

The achievement of a 1.51 µm InAs/InP(311)B quantum dot (QD) single-mode Fabry–Perot laser operating under continuous wave at room temperature is reported. A threshold current of 80 mA associated with a 0.12 W/A external efficiency is reported for as-cleaved device at room temperature. The maximum output power, the gain peak wavelength at threshold and the full width at half maximum of the spectral gain are 5 mW, 1512 nm and 60 nm, respectively. Although these performances must be improved in the future, these results constitute to our knowledge the state-of-the-art on InP(3 1 1)B substrate. These results are of great interest since QD-based lasers are expected to play a major role in the next generation telecommunication networks as low-cost devices.

Short Cavity DBR Laser Using Vertical Groove Gratings for Large-Scale Photonic Integrated Circuits

We present a novel compact distributed Bragg reflector (DBR) laser using InP-InGaAsP deep ridge waveguides with vertical groove gratings. Stable single-mode laser operation was achieved with an active cavity length down to 25 mum and a threshold current of 14 mA. The devices are promising building blocks in large-scale photonic integrated circuits because of their simple structure and low power consumption.

InAs-InAlGaAs quantum dot DFB lasers based on InP [001

The InAs-InAlGaAs quantum dot (QD) lasers with the InAlGaAs-InAlAs material system were fabricated on distributed feedback (DFB) grating structures on InP (001). The single-mode operation of InAs-InAlGaAs QD DFB lasers in continuous-wave mode was successfully achieved at the emission wavelength of 1.564 mum at room temperature. This is the first observation on the InP-based QD lasers operating around the emission wavelength window of 1.55 mum. The threshold current density of the InAs-InAlGaAs QD DFB laser with a cavity length of 1 mm and a ridge width of 3 mum, in which one of the cleaved facets was coated with 95% high-reflection, was 1.23 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (176 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> for single QD layer). The sidemode suppression ratio value of the QD DFB laser was as high as 42 dB at the driving current of 100 mA

Quasi-self-imaging planar waveguide lasers with high-power single-mode output

We describe high-power planar waveguide laser which can achieve single-mode output from a multi-mode structure. The planar waveguide is constructed with incomplete self-imaging properties, by which the coupling loss of each guided mode can be discriminated. Thermal lens effects are evaluated for single-mode operation of such high-power diode-pumped solid-state lasers.

Channel waveguide lasers in a lead silicate glass fashionedusing the extrusion technique

We report the use of extrusion for the development of planar buried channel waveguide lasers in a neodymium-doped lead-silicate glass host. The extrusion process was performed at a constant die temperature of 555±10°C, an applied pressure of 1850Ncm−2 and a viscosity of 107.8–107.2P, respectively. A planar substrate, 10mm in length cut from the extruded product, had four buried waveguides each with a core size of 8 by 2.5μm in the horizontal and vertical directions. Optical characterization of this waveguide revealed single-mode laser operation at 1058nm with a slope efficiency of 40% for an absorbed power of 59.3mW. The measured device propagation loss was ∼0.3dBcm−1.

Single-mode operation of terahertz quantum cascade lasers with distributed feedback resonators

Distributed feedback terahertz quantum-cascade lasers emitting at 4.34 and 4.43THz are presented. Mode selection is based on a complex-coupling scheme implemented into the top-contact layer by a combination of wet chemical etching and ohmic-contact deposition. Single-mode emission stable at all injection currents and operating temperatures is shown, with a side-mode suppression ratio exceeding 20dB. Peak output powers of up to 1.8mW are obtained at low temperatures.

Single-mode lasing operation using a microring resonator as a wavelength selector

We have demonstrated semiconductor lasers with microring resonators as frequency-selective elements. We show that the small resonator inside the laser makes single frequency-mode operation possible with reasonably good side-mode rejection.

Single-mode operation of coupled-cavity lasers based on two-dimensional photonic crystals

We report the fabrication of short-cavity lasers with highly reflective two-dimensional photonic crystal mirrors on an InGaAsP/InP laser structure emitting at 1.57 μm. An intracavity photonic crystal mirror creates two coupled cavities, which provide additional longitudinal mode selection for stable single-mode operation with side-mode suppression ratios exceeding 35 dB. The shortest lasers with l=100 μm overall length have a threshold current of 13 mA and provide more than 4 mW power under continuous wave operation. Longer devices with l=200 μm deliver up to 9 mW. A maximum modulation bandwidth of 7.9 GHz was determined by relative intensity noise measurements.