Second-order Grating Research Articles

Fabrication and Characterization of High Power 1064-nm DFB Lasers

A ridge waveguide distributed-feedback laser emitting at 1064 nm is demonstrated. Two low-temperature-grown In0.21Ga0.79As/GaAs quantum wells are employed as the active layer. A second-order grating is formed by holographic photolithography and wet-etching. The laser operates at a single-mode up to 255 mA, corresponding to an output power of 90 mW.

Single-mode surface-emitting distributed feedback quantum-cascade lasers based on hybrid waveguide structure

Surface-emitting distributed feedback quantum-cascade lasers operating at \lambda \approx 7.8 \mu m are demonstrated. The metal-covered second-order grating is shallow-etched into the surface of a thin InGaAs contact and cladding layer. This forms a hybrid waveguide and used to achieve relatively low waveguide losses and high coupling strengths. The devices exhibit stable single-mode operation from 90 to 130 K with a side mode suppression ratio above 20 dB. A slope efficiency of 194 mW/A is obtained at 90 K, which is twice higher than that of the Fabry-Perot counterpart.

Highly photostable solid-state organic distributed feedback laser fabricated via thermal nanoimprint lithography

A highly photostable distributed feedback (DFB) laser has been fabricated by spin-coating a polystyrene film doped with a perylenediimide derivative on a SiO 2 grating. The second-order grating, with a period of 368 nm, was fabricated via thermal nanoimprint lithography and pattern transfer. When the DFB laser is photopumped at 533 nm, the laser emission spectrum shows two peaks at 573 and 581 nm. The laser threshold ( I th) is 30 times lower than the amplified spontaneous emission threshold for a sample without grating. These devices are very photostable: the halflife is longer than 100,000 pump pulses at a pump intensity of 6 μJ/pulse.

A continuously tunable low-threshold organic semiconductor distributed feedback laser fabricated by rotating shadow mask evaporation

We report on an organic semiconductor distributed feedback laser with a continuously tunable wavelength from 621 to 657 nm on a single sample with thresholds as low as 38 μJ/cm2. Using laser interference lithography and reactive ion etching, we textured a glass substrate with a one-dimensional second-order Bragg surface grating. Optical confinement and gain is provided by a wedge-shaped layer of the organic semiconductor tris(8-hydroxyquinoline) aluminum doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyril)-4H-pyrane, which is fabricated by a rotating shadow mask evaporation. This allows for a simple, broadband continuous tuning of the laser emission wavelength.

Quasi-phase matched second-harmonic generation through thermal poling in femtosecond laser-written glass waveguides

Quasi-phase matched second-harmonic generation at 532 nm is demonstrated in a channel waveguide that is written in bulk fused silica using a femtosecond laser. The second-order nonlinear grating is fabricated using uniform thermal poling followed by periodic erasure inside an e-beam deposition system caused, by what we believe to be, x-rays. A SHG conversion efficiency of 2 x10(-5) %/W-cm(2) was obtained for a 1 cm long device, corresponding to an effective nonlinear coefficient of 0.0075 pm/V.

A hybrid light source with integrated inorganic light-emitting diode and organic polymerdistributed feedback grating

We report a compact light source that incorporates a semiconductor light-emitting diode,nanostructured distributed feedback (DFB) Bragg grating and spin-coated thin conjugatedpolymer film. With this hybrid structure, we transferred electrically generated 390 nmultraviolet light to an organic polymer via optical pumping and out-couple greenluminescence to air through a second-order DFB grating. We demonstrate the feasibility ofelectrically driven, hybrid, compact light-emitting devices and lasers in the visible range.

Above-Threshold Spectrum of the Radiation Field in Surface-Emitting DFB Lasers

This paper addresses the above-threshold analysis of the amplified spontaneous emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers with a second-order grating. To the best of the authors' knowledge, this paper is the first report in this regard. The analysis takes advantage of the time- and frequency-domain approaches. The essence of this method is obtaining the spectrum of the laser power using the fields in the frequency domain, whereas the above-threshold carrier distribution inside the cavity is obtained by the finite-difference time-domain approach. The use of this approach can be justified by the fact that the wave equations in the time and frequency domains are related by the Fourier transform. Applying this method to the surface-emitting DFB lasers with a second-order grating, the authors will demonstrate that excitation of the radiation field by the interference between the counter-propagating waves inside the cavity provides additional filtering mechanism, which makes the side-mode suppression ratio of the power emitted from the surface different than that from the edge. More importantly, it is shown that the interesting features of a properly designed quarter-wave phase-shifted surface-emitting DFB lasers with a second-order grating can be exploited in the design of transmitters for optical communications

Integration of Surface-Emitting Red Distributed Bragg Reflector Laser and Microfluidic Structure for Biomolecular Sensing

A GaInP quantum well surface-emitting red distributed Bragg reflector (DBR) laser consisting of a narrow ridge active channel, and second- and third-order curved DBR gratings is designed and fabricated for compact biomolecular sensing. From the second-order DBR grating, surface emission of 1.4 mW output power at 654 nm wavelength was obtained under CW operation. The surface-emitting DBR laser was combined with a PDMS microchannel and a photodiode. Preliminary sensing of microspheres flowing through the microchannel was demonstrated.

Orientation-Selective Adaptation to First- and Second-Order Patterns in Human Visual Cortex

Second-order textures-patterns that cannot be detected by mechanisms sensitive only to luminance changes-are ubiquitous in visual scenes, but the neuronal mechanisms mediating perception of such stimuli are not well understood. We used an adaptation protocol to measure neural activity in the human brain selective for the orientation of second-order textures. Functional MRI (fMRI) responses were measured in three subjects to presentations of first- and second-order probe gratings after adapting to a high-contrast first- or second-order grating that was either parallel or orthogonal to the probe gratings. First-order (LM) stimuli were generated by modulating the stimulus luminance. Second-order stimuli were generated by modulating the contrast (CM) or orientation (OM) of a first-order carrier. We used four combinations of adapter and probe stimuli: LM:LM, CM:CM, OM:OM, and LM:OM. The fourth condition tested for cross-modal adaptation with first-order adapter and second-order probe stimuli. Attention was diverted from the stimulus by a demanding task at fixation. Both first- and second-order stimuli elicited orientation-selective adaptation in multiple cortical visual areas, including V1, V2, V3, V3A/B, a newly identified visual area anterior to dorsal V3 that we have termed LO1, hV4, and VO1. For first-order stimuli (condition LM:LM), the adaptation was no larger in extrastriate areas than in V1, implying that the orientation-selective first-order (luminance) adaptation originated in V1. For second-order stimuli (conditions CM:CM and OM:OM), the magnitude of adaptation, relative to the absolute response magnitude, was significantly larger in VO1 (and for condition CM:CM, also in V3A/B and LO1) than in V1, suggesting that second-order stimulus orientation was extracted by additional processing after V1. There was little difference in the amplitude of adaptation between the second-order conditions. No consistent effect of adaptation was found in the cross-modal condition LM:OM, in agreement with psychophysical evidence for weak interactions between first- and second-order stimuli and computational models of separate mechanisms for first- and second-order visual processing.

Gain clamping in semiconductor optical amplifiers with second-order index-coupled DFB grating

A systematic investigation of gain-clamped semiconductor optical amplifiers (GC-SOAs) based on the second-order index-coupled DFB gratings is carried out by way of simulation. In particular, we focus on the main effects of the radiation loss caused by the first-order diffraction of the gratings on the amplifier performance. The magnitude of the total complex coupling coefficient is the main factor to determine the level of gain clamping. We demonstrate that a high-performance GC-SOA can be realized by using purely loss-coupled second-order DFB gratings with more relaxed tolerance on grating strength and period. It is shown that, in the presence of weak reflection-related coupling, the parasitic radiation loss associated with the second-order grating always helps to expand the linear amplification region and to reduce the longitudinal spatial hole burning along the cavity. Further, we demonstrate through comparison that the GC-SOAs have higher saturation power and much shorter carrier lifetime than the conventional SOAs. An improved design by longitudinal variation of the grating duty cycle is proposed such that the noise performance of the amplifier can be enhanced without much sacrifice on the linear amplification regime.

Restriction of the efficiency of photoinduced second-harmonic generation during optical poling of germanium-silicate glass

The large growth of the absorption of radiation with doubled frequency in a region of high values for the induced electric fields has been detected during the investigation of an optical poling process in germaniumsilicate glass. The absorption suppresses the writing of the second-order susceptibility grating and leads to restriction of the maximum efficiency of the photoinduced second-harmonic generation in the glass.

High-power 783 nm distributed-feedback laser

A ridge-waveguide GaAsP/AlGaAs laser, emitting an optical power of up to 200 mW in a single lateral and longitudinal mode at a wavelength of 783 nm, is presented. The distributed feedback is provided by a second-order grating, formed into an InGaP/GaAsP/InGaP multilayer structure. The laser is well suited as a light source for Raman spectroscopy.

Single-lobe, surface-normal beam surface emission from second-order distributed feedback lasers with half-wave grating phase shift

Half-wave phase shifts were fabricated in the center of second-order GaAs gratings, for use in surface-emitting, horizontal-cavity, semiconductor diode lasers (λ=0.98 μm). Incorporating such gratings in diode lasers with distributed-feedback (DFB) active regions and distributed Bragg reflectors (DBRs) is found to provide surface-normal, single-lobe beam emission, as predicted by theory. InGaAs/AlGaAs/InGaP, two-quantum-well structures are employed. A 500-μm-long GaAs/Au second-order grating with half-wave phase shift represents the DFB region, which provides feedback and unidirectional light outcoupling. GaAs/SiO2/Au, 500-μm-long, second-order gratings are the DBR regions, on either side of the DFB region, which provide both frequency-selective feedback as well as unidirectional outcoupling. Lateral-mode control is achieved via a 2.5-μm-wide ridge waveguide. Surface emission is obtained through a 80-μm-wide window stripe in the metallization on the substrate n-side. Single-frequency lasing in an orthonormally emitted, single-lobe, diffraction-limited beam is obtained to 3× threshold under pulsed-drive conditions (200 ns wide pulses, 1 kHz repetition rate).

Limitation of optical poling in germanium-silicate glasses

The giant increase of the absorption of the radiation with the doubled frequency in a region of high values of the optically-induced electric fields (about 10 5–10 6 V/cm) has been observed during the investigation of the optical poling process in germanium-silicate glass. The absorption suppresses the writing of the second-order susceptibility grating and leads to the restriction of the maximum efficiency of the photoinduced second harmonic generation in glass. We demonstrate that the observed effect is not anisotropic and may be considered as a consequence of a competition between the charge separation by the local trapping levels due to electrostricted photoinduced phonons and recombination diffusion of carriers.

An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers

We have designed and fabricated an out-of-plane coupler for butt-coupling from fiber to compact planar waveguides. The coupler is based on a short second-order grating or photonic crystal, etched in a waveguide with a low-index oxide cladding. The coupler is optimized using mode expansion-based simulations. Simulations using a 2-D model show that up to 74% coupling efficiency between single-mode fiber and a 240-nm-thick GaAs-AlO/sub x/ waveguide is possible. We have measured 19% coupling efficiency on test structures.

A Surface-Emitting Circular Grating Polymer Laser

An optically pumped circular grating polymer laser (see Figure) is obtained using a nanostructured quartz substrate covered with a thin film of methyl-substituted ladder-type poly(p-phenylene) (MeLPPP). The second-order grating serves at the same time as a 2D feedback mechanism and as an output coupler. Laser emission perpendicular to the substrate with a clear threshold behavior, an extremely narrow spectral width, and a highly directional beam is achieved.

Tapered-cavity surface-emitting distributed-Bragg-reflector semiconductor lasers: modeling and experiment

We report on the design and fabrication of tapered cavity grating coupled surface-emitting distributed Bragg reflector (DBR) lasers in the 980-nm regime. A curved second-order grating is used at the end of a tapered gain section to provide feedback as well as collimated surface out-coupling. A detailed numerical analysis shows that operation up to approximately twice the threshold is possible without significant degradation of the far field. Near diffraction-limited collimated surface-emitting output with moderate power of about 150 mW is obtained under continuous operation.

Tunable grating coupled surface-emitting tapered laser

We report on the design and fabrication of a wavelength-tunable grating coupled surface-emitting laser. A curved second-order grating is used at the end of a tapered gain section to provide feedback as well as collimated surface out-coupling. Wavelength tuning is achieved by current injection into the grating section through a film of indium-tin oxide deposited on the grating surface. Low divergence single-frequency surface emission with /spl sim/1-nm wavelength tuning is demonstrated.

Single-mode, single-lobe operation of surface-emitting, second-order distributed feedback lasers

A thin p-cladding InGaAs/InGaP/GaAs laser structure (λ=0.977 μm) with a second-order Au/air grating (70% duty cycle) and asymmetrically coated (30%, 5% reflectivity) cleaved facets emits in a diffraction-limited (0.11°) single lobe in a direction virtually normal to the chip surface. The near-field pattern corresponds to (grating) phase shifts of 10° and 40° at the low- and high-reflectivity cleaved-mirror facets. An analysis of 30%/0% coated, 500-μm-long devices shows that single-lobe surface emission occurs for a wide variation in grating phase with respect to the high reflectivity mirror, ΔφHR: 10°–80°. For 99%/0% coated devices, single-lobe emission occurs with relatively uniform near-field intensity profile and external differential quantum efficiency ηd around 20% for ΔφHR values close to π/4 (i.e., in the 45°–65° range). Single-lobe emission normal to the chip surface (i.e., symmetric-mode lasing) can then be obtained from devices without cleaved mirrors by introducing a phase shift close to π/2 in the center of the second-order grating. For the structure used, a phase shift of 90°–130° in the center of 1-mm-long gratings is found to provide single-lobe surface emission with substantially uniform near-field profile, and ηd values as high as 22%. Increasing the grating-section length increases ηd (e.g., 35% for 1.5-mm-long gratings) at some price in near-field uniformity.

40-mW 650-nm distributed feedback lasers

Using a buried second-order grating in a single-mode GaInP-AlInP laser structure, we obtain single-spatial- and longitudinal-mode operation at 652 nm. Over 40-mW continuous wave (CW) is obtained at room temperature (RT), with efficiencies and far fields comparable to conventional Fabry-Perot lasers. The devices are continuously tunable with current and temperature and exhibit no mode hops, and thus should be useful for applications where short wavelength and longitudinal mode stability is required, such as spectroscopy, interferometry, or optical storage.