Order Gratings Research Articles

Experiment demonstration of high speed 1.3 µm grating assisted surface-emitting DFB lasers.

Surface emitting lasers are attractive light sources for silicon integrated photonic circuits. High speed direct operation is of great importance for these lasers in high capacity and low cost on-chip communication system. Here, we demonstrate a 1.3 µm surface emitting ridge-waveguide distributed feedback (DFB) laser with second order grating and λ/4 phase shift grating, which can achieve a 24 Gb/s operation over a wide temperature. The fabricated lasers can achieve low threshold current as 6.8 mA, and 12.5 mA at 20, and 70°C, respectively. Stable single mode operation has been observed with high side mode suppression ratio (SMSR) > 40 dB at all temperatures (20-70 °C). Meanwhile, the surface emitting optical power can reach 1.7 mW at high temperature as 70 °C. 3 dB bandwidth of small signal response is 21 GHz and 12 GHz at 20 °C and 70 °C respectively. The far-field divergence angle of surface emitting beam is 13.4°×20.2° of 10 µm length second order grating coupler. The proposed laser may have great advantages of single mode, high speed modulation and good temperature tolerance. In addition, compared with conventional DFB lasers, the surface emitting DFB laser has no additional manufacturing process, which is simple to fabricate and easy to integrate with silicon platform.

30-Gbps Directly Modulated Semiconductor Lasers Based on Surface High-Order Gratings

Semiconductor lasers with surface high-order gratings have been fabricated and measured. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$31^{\mathrm {st}}$ </tex-math></inline-formula> -order gratings were defined by the standard contact-type i-line photolithography near the front facet and subsequently etched into the ridges of the semiconductor lasers by the inductively coupled plasma. Both the static and dynamic characteristics were investigated. The threshold current and maximum output power are 22 mA and 12 mW, respectively. Side-mode suppression ratios are larger than 35 dB at bias currents except approximately 145 mA due to mode hopping. −3 dB bandwidth of the small signal response is 13.8 GHz at 130 mA. The eye diagram at a bit rate of 30 Gbps is clearly open.

Analysis of the time domain characteristics of tapered semiconductor lasers

We use traveling wave coupling theory to investigate the time domain characteristics of tapered semiconductor lasers with DBR gratings. We analyze the influence of the length of second order gratings on the power and spectrum of output light, and optimizing the length of gratings, in order to reduce the mode competition effect in the device, and obtain the high power output light wave with good longitudinal mode characteristics.

Continuous-wave operation of a semipolar InGaN distributed-feedback blue laser diode with a first-order indium tin oxide surface grating.

A novel approach to realize DFB gratings on GaN based laser diodes is presented and continuous-wave single longitudinal mode operation is achieved. The first order gratings were fabricated on the surface of indium tin oxide (ITO) on top of the laser ridge, which combines the benefits of simplified fabrication, easy scalability to wider ridges, and no regrowth or overgrowth. Under continuous-wave operation, the laser emits with a full FWHM of 5pm, a SMSR of 29dB and output power from a single facet as high as 80mW. To the best of authors' knowledge, this is also the first demonstration of a DFB-LD on semipolar InGaN/GaN system.

High-efficiency V-shaped phase gratings to suppress high order diffractions

We propose a V-shaped phase grating to simultaneously improve the diffraction efficiency of the 1st order diffraction mode and suppression of high order modes. An analytic description is given that indicates the 0th, 2nd, 3rd, and 4th order diffractions can be completely suppressed while the 1st order absolute diffraction efficiency can reach a theoretical maximum of 27.72%, which is much higher than that reported from single order gratings, such as sinusoidal amplitude transmission gratings. The dependencies of the 1st order diffraction efficiency on the wavelength and depth deviations are also discussed. The experimental results show that the absolute efficiency of the 1st order diffraction is 25.85% while the 2nd, 3rd, and 4th order diffractions disappear completely, confirming the theoretical predictions. The diffraction patterns of a xenon lamp demonstrate the suppression of the high order diffractions from our quartz phase grating. The unique dispersive properties of the V-shaped phase grating make it attractive for broad spectroscopy and high-accuracy monochromatization applications.

Design Optimization for Semiconductor Lasers With High-Order Surface Gratings Having Multiple Periods

Semiconductor lasers that use high-order surface gratings formed with etched slots have simple fabrication. Such gratings however increase loss and have a small free-spectral range that can increase threshold, lower slope efficiency, and limit SMSR. In this paper, we look at the design of a grating and use 2, 3, and 4 period structures within a 37 ${\text{th}}$ order grating. At fixed temperature, a single period device should work best while temperature tuning requires a multiperiod device. We found that the two period grating structure works best among the multiperiod devices. A follow-on study with the scattering matrix model shows that the slope efficiency which is quite low in this study (about 0.1 mW/mA at best) can be improved to a value of upto 0.17 mW/mA by reducing the number of slots used to form the grating from the present design that has 24 slots to 18.

InGaN/GaN Laser Diodes With High Order Notched Gratings

We report on InGaN/GaN distributed feedback laser diodes with high order gratings emitting at a single wavelength around 428 nm. The 39th order notched gratings have the advantage of a simplified fabrication route with no need for overgrowth. The laser ridge and grating were formed by electron beam lithography followed by ICP etching. The as-cleaved lasers emitted in the pulsed regime with a peak single-mode output power of 15 mW. Optimization of the grating design should lead to higher power single wavelength operation.

Femtosecond laser inscription of Bragg grating waveguides in bulk diamond.

Femtosecond laser writing is applied to form Bragg grating waveguides in the diamond bulk. Type II waveguides are integrated with a single pulse point-by-point periodic laser modification positioned toward the edge of the waveguide core. These photonic devices, operating in the telecommunications band, allow for simultaneous optical waveguiding and narrowband reflection from a fourth-order grating. This fabrication technology opens the way toward advanced 3D photonic networks in diamond for a range of applications.

Increased diffraction efficiencies of DBR gratings in diode lasers with adiabatic ridge waveguides

The influence of the lateral layout on the diffraction efficiency of gratings in DBR lasers is presented. In this experimental study DBR ridge waveguide (RW) lasers with different ridge widths as well as straight and adiabatic waveguides are compared. The lasers are based on a vertical layer structure with an asymmetric super large optical cavity. DBR gratings of 3rd and 7th order are manufactured using electron beam lithography and dry etching. Their diffraction efficiencies are determined by measuring the optical output power emitted through the rear and front facets of unmounted devices. In comparison to a laser with a 30 μm ridge, the DBR diffraction efficiency in a laser with a 4 μm ridge is reduced by 46 percentage points. Implementing an adiabatic widening of the ridge width increases the diffraction efficiency from 35% to 72%. The latter is close to 81% achieved for the laser with 30 μm ridge. The new layout with enhanced DBR diffraction efficiency increases the optical output power of the narrow RW laser by a factor of 1.3. Similar results are obtained with 7th order gratings. All devices provide single-mode emission with spectral widths of 30 pm and side mode suppressions >60 dΒ. According to these results, implementing adiabatic waveguides is beneficial in terms of diffraction efficiency and performance of narrow RW lasers based on the applied vertical structure.

Highly Sensitive Waveguide Bragg Grating Temperature Sensor Using Hybrid Polymers

A waveguide Bragg grating temperature sensor implemented using a hybrid inorganic-organic material (Ormocer) with a 25-times higher temperature sensitivity than a typical silica fiber is presented. The sensor consists of second order gratings (1010-nm pitch) in 5 $\mu \text{m} \,\, \times $ 5 $\mu \text{m}$ waveguides fabricated on a planar substrate using the replication-based methods. The gratings were imprinted in the under-cladding layer, and the waveguide cores were patterned on top by capillary filling of microchannels, which were defined in a transparent and flexible mold. The somewhat larger, slightly multimode waveguides facilitate pigtailing with an optical fiber but lead to three reflection peaks corresponding to the different excited waveguide modes. The peak at the longest wavelength (Bragg wavelength at 1539 nm, corresponding to the fundamental mode) was tracked during temperature testing, and a sensitivity of −249 pm $^{\circ }\text{C}^{-1}$ was found.

Grating integrated single mode microring laser

Microring and microdisk lasers are potential candidates for small footprint, low threshold in-plane integrated lasers; however, they exhibit multimode lasing spectra and bistability. Here, we theoretically propose and experimentally demonstrate a novel approach for achieving single mode lasing in microring lasers. Our approach is based on increasing the radiation loss of all but one of the resonant modes of microring resonators by integrating second order gratings on the microrings' waveguide. We present single mode operation of electrically pumped semiconductor microring lasers whose lasing modes are lithographically selected via the second order grating. We also show that adding the grating does not increase the lasing threshold current significantly.

High-order multilayer coated blazed gratings for high resolution soft x-ray spectroscopy.

A grand challenge in soft x-ray spectroscopy is to drive the resolving power of monochromators and spectrometers from the 10(4) achieved routinely today to well above 10(5). This need is driven mainly by the requirements of a new technique that is set to have enormous impact in condensed matter physics, Resonant Inelastic X-ray Scattering (RIXS). Unlike x-ray absorption spectroscopy, RIXS is not limited by an energy resolution dictated by the core-hole lifetime in the excitation process. Using much higher resolving power than used for normal x-ray absorption spectroscopy enables access to the energy scale of soft excitations in matter. These excitations such as magnons and phonons drive the collective phenomena seen in correlated electronic materials such as high temperature superconductors. RIXS opens a new path to study these excitations at a level of detail not formerly possible. However, as the process involves resonant excitation at an energy of around 1 keV, and the energy scale of the excitations one would like to see are at the meV level, to fully utilize the technique requires the development of monochromators and spectrometers with one to two orders of magnitude higher energy resolution than has been conventionally possible. Here we investigate the detailed diffraction characteristics of multilayer blazed gratings. These elements offer potentially revolutionary performance as the dispersive element in ultra-high resolution x-ray spectroscopy. In doing so, we have established a roadmap for the complete optimization of the grating design. Traditionally 1st order gratings are used in the soft x-ray region, but we show that as in the optical domain, one can work in very high spectral orders and thus dramatically improve resolution without significant loss in efficiency.

Ultrafast laser fabrication of Bragg waveguides in chalcogenide glass.

Bragg waveguides are fundamental components in photonic integrated circuits and are particularly interesting for mid-IR applications in high index, highly nonlinear materials. In this work, we present Bragg waveguides fabricated in bulk chalcogenide glass using an ultrafast laser. Waveguides with near circularly symmetric cross sections and low propagation loss are obtained through spatial and temporal beam shaping. Using a single-pass technique, the waveguide and Bragg structure are formed at the same time. First through sixth order gratings with strengths of up to 25dB are realized, and performance is evaluated based on the modulation duty cycle of the writing beam.

Wavelength stabilized 785 nm DBR-ridge waveguide lasers with an output power of up to 215 mW

Wavelength stabilized distributed Bragg reflector (DBR) diode lasers at an emission wavelength of 785 nm will be presented. The devices have a 14 nm thick GaAsP single quantum well as active layer, which is embedded in Al0.65Ga0.35As waveguide layers and Al0.7Ga0.3As cladding layers. The DBR structures are realized as deeply etched tenth order gratings using I-line wafer stepper lithography. The devices have a stripe width of 2.2 µm and a cavity length of 3 mm including a DBR grating with a length of 500 µm. The devices are mounted p-side up on C-mounts. At a temperature of 25 °C a continuous wave output power of 215 mW and a conversion efficiency of 28% are measured. Up to 140 mW single mode operation with a small tuning range below 190 pm is observed. At 50 mW an aging test was performed showing reliable operation over 1000 h.

Exposed core microstructured optical fiber Bragg gratings: refractive index sensing

Bragg gratings have been written in exposed-core microstructured optical fibers for the first time using a femtosecond laser. Second and third order gratings have been written and both show strong reflectivity at 1550 nm, with bandwidths as narrow as 60 pm. Due to the penetration of the guided field outside the fiber the Bragg reflections are sensitive to the external refractive index. As different modes have different sensitivities to refractive index but the same temperature sensitivity the sensor can provide temperature-compensated refractive index measurements. Since these Bragg gratings have been formed by physical ablation, these devices can also be used for high temperature sensing, demonstrated here up to 800°C. The fibers have been spliced to single mode fiber for improved handling and integration with commercial interrogation units.

High-Power Distributed Feedback Lasers With Surface Gratings

We present the results for high-power broad-area distributed feedback lasers with surface gratings of 80th, 135th, and 270th Bragg orders. A maximum output power of 11 W for a laser with 80th order grating, emitting around 976 nm with a spectral width <; 1 nm has been achieved. Similar narrow linewidth operation occurs for devices with 135th Bragg order from 7- to 11-W output power. The technological approach to fabricating these devices based on stepper lithography and reactive ion etching is described.

Highly controllable ICP etching of GaAs based materials for grating fabrication

Highly controllable ICP etching of GaAs based materials with SiCl4/Ar plasma is investigated. A slow etching rate of 13 nm/min was achieved with RF1 D 10 W, RF2 D 20 W and a high ratio of Ar to SiCl4 flow. First order gratings with 25 nm depth and 140 nm period were fabricated with the optimal parameters. AFM analysis indicated that the RMS roughness over a 10 × 10 μm2 area was 0.3 nm, which is smooth enough to regrow high quality materials for devices.

λ∼3.36 μm room temperature InGaAs/AlAs(Sb) quantum cascade lasers with third order distributed feedback grating

We report on the development of strain compensated InGaAs/AlAs(Sb) quantum cascade lasers, incorporating a buried third order distributed feedback grating. Single mode operation with a side mode suppression ratio of ∼30 dB has been achieved in the wavelength range of 3.358–3.380 μm for temperatures between 270 and 360 K. The threefold increase in grating pitch size, compared with a first order grating, allows conventional photolithographic techniques to be used for single mode laser fabrication.

Comparison of pupil responses to the first and second order gratings

Comparison of pupil responses to the first and second order gratings

Simulation Analysis of Higher Order Laterally-Coupled Distributed Feedback Lasers

Laterally-coupled distributed feedback (LC-DFB) lasers use a lithographic fabrication step to define the distributed feedback grating, avoiding subsequent regrowth. Using higher order gratings can enhance the lithographic tolerance for lower resolution patterning, yielding lasers more amenable to fabrication. We show that LC-DFB lasers with higher order gratings, although requiring a higher threshold gain than those of first-order, provide a degree of longitudinal mode discrimination. Incorporating radiating partial waves, we have calculated modified coupled-mode coefficients for various duty cycles, grating orders, and grating geometries using a two-dimensional finite-element method. The modified coupled-mode equations were solved with and without a lambda/4 phase shift. The phase shift, while beneficial for first-order gratings, was found to generally diminish laser performance for lasers with higher order gratings.