Second-order Distributed Feedback Lasers Research Articles

Colorimetric Phosphate Detection Using Organic DFB Laser Based Absorption Spectroscopy.

A novel compact laser absorption spectrometer is developed for colorimetric detection. We demonstrate the realization of the system as well as example measurements of phosphate in water samples based on the malachite green (MG) method. A phosphate concentration range of to (which corresponds to a molar concentration range of to ) is investigated. This photometer demonstrates the ease of integration of organic distributed feedback (DFB) lasers and their miniaturizability, leading the way toward optofluidic on-chip absorption spectrometers. We constructed an optically pumped organic second-order DFB laser on a transparent substrate, including a transparent encapsulation layer, to have access to both emission directions of the surface-emitting laser. Using the two different surface emission directions of the laser resonator allows monitoring of the emitted light intensity without using additional optical elements. Based on these advances, it is possible to miniaturize the measurement setup of a laser absorption spectrometer and to measure analytes, such as phosphate.

High pump efficiency of a second-order distributed feedback laser based on holographic polymer dispersed liquid crystals with preferred liquid crystal molecular orientation

We report on the fabrication and characterization of a surface-emitting distributed feedback (DFB) organic semiconductor laser based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating.

Terahertz master-oscillator power-amplifier quantum cascade lasers

We report on the realization of a monolithically integrated master-oscillator power-amplifier architecture in a terahertz quantum cascade laser (THz-QCL) with a metal-metal waveguide. The master-oscillator section is a first-order distributed feedback (DFB) laser. Instead of using a thick anti-reflection coating, we exploit a diffraction grating together with an absorbing boundary in the power-amplifier section to efficiently extract the laser radiation and suppress the self-lasing in it. The devices demonstrate a stable generation and power amplification of single-mode emission. The amplification factor is about 5, and the output power is approximately twice that of the standard second-order DFB lasers fabricated from the same material. Emission beam pattern with a divergence angle of ∼18 × 40° is achieved. Our work provides an avenue for the realization of single-mode THz-QCLs with high output power and good beam quality.

Heterogeneously integrated III-V/silicon distributed feedback lasers

Heterogeneously integrated III-V-on-silicon second-order distributed feedback lasers utilizing an ultra-thin DVS-BCB die-to-wafer bonding process are reported. A novel DFB laser design exploiting high confinement in the active waveguide is demonstrated. A 14 mW single-facet output power coupled to a silicon waveguide, 50 dB side-mode suppression ratio and continuous wave operation up to 60°C around 1550 nm is obtained.

Improved performance of perylenediimide-based lasers

The aim of this work was to improve the laser performance, in terms of threshold and operational lifetime, of lasers based on polymer films doped with perylenediimide (PDI) derivatives as active media. For such purpose, we first investigated the amplified spontaneous emission (ASE) properties of perylene orange (PDI-O), when doped into polystyrene (PS) films. Lower ASE thresholds and larger photostabilities than those of similar films containing another PDI derivative (PDI-C6), recently reported in the literature, have been measured. Results have been interpreted in terms of the photoluminescence efficiency of the films, which depends on the type of molecular arrangement, inferred with the help of nuclear magnetic resonance experiments. We also show that PS films have a better ASE performance, i.e. lower thresholds and larger photostabilities, than those based on poly(methyl methacrylate), which was recently highlighted as one of the best matrixes for PDI-O. Finally, a one-dimensional second-order distributed feedback laser using PS doped with PDI-O was fabricated and characterized. This device has shown a threshold significantly lower (by around one order of magnitude) than that of a similar laser based on PDI-C6-doped PS.

Influence of the excitation area on the thresholds of organic second-order distributed feedback lasers

It is shown that the optical pump power (or energy) density thresholds required to obtain lasing from organic second-order distributed feedback lasers, increase when the excitation area (A) is smaller than a certain value (Acrit). So, in order to obtain the minimum possible thresholds and to ensure that they constitute adequate quantities for comparison purposes, the condition A > Acrit should be fulfilled. Results also indicate that when A < Acrit (Acrit ∼ 0.1 mm2 for the devices studied here), the operational device lifetime, which depends mainly on the pump power (or energy) density, becomes drastically reduced.

Film thickness and grating depth variation in organic second-order distributed feedback lasers

We report on the preparation and characterization, under optical pump, of second-order one-dimensional distributed feedback (DFB) lasers based on polystyrene films doped with a perylenediimide derivative, as active media. The DFB gratings were engraved on the substrates (SiO2) by thermal nanoimprint lithography, followed by reactive ion etching. Laser emission wavelength was tuned from 554 to 583 nm by changing film thickness (h) between 240 and 1200 nm. The effect on the performance (emission wavelength, threshold, slope efficiency, number of modes, and spectral shape) of varying the grating depth (d) from 30 to 240 nm, for the whole range of h values, has been investigated. Although there is extensive work in the literature aiming to tune the emission wavelength of organic DFB lasers by h variation, the effect of changing d systematically has not been previously studied. Experimental results have been interpreted by models that take into account the presence of the grating by averaging either h or the effective refractive index. Single-mode emission (λ0) was observed for h < 1000 nm, while for thicker films lasing appeared at two different wavelengths (λ0 and λ1). Models indicate that λ0 and λ1 correspond to the TE0 and TE1 waveguide modes, respectively. It was found that d plays an important role in determining the DFB thresholds and slope efficiencies for two h regimes: (i) For h < 350 nm, lowest thresholds and highest slopes efficiencies were obtained with the shallower gratings; and (ii) for h > 1000 nm, d affects significantly the losses associated with the TE1 mode, so single mode emission was achieved at λ0 or at λ1 for deep and shallow gratings, respectively. Finally, the shape of the emission spectra, both below and above threshold, has also been analyzed in order to clarify the physical mechanisms responsible for the existence of gain. Bragg dips were observed in the spectra below threshold only for devices with d/h larger than around 0.3 and their width increased with increasing d/h. In these cases, single-mode DFB emission appeared at the long-wavelength edge of the Bragg dip, indicating that index-coupling modulation contributes significantly to the gain process. On the other hand, for smaller d/h values, Bragg dips became too small to be detected, so gain coupling becomes the dominant mechanism accounting for the presence of gain.

Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures

Symmetric and antisymmetric band-edge modes exist in distributed feedback surface-emitting semiconductor lasers, with the dominant difference being the radiation loss. Devices generally operate on the low-loss antisymmetric modes, although the power extraction efficiency is low. Here we develop graded photonic heterostructures, which localize the symmetric mode in the device centre and confine the antisymmetric modes close to the laser facet. This modal spatial separation is combined with absorbing boundaries to increase the antisymmetric mode loss, and force device operation on the symmetric mode, with elevated radiation efficiency. Application of this concept to terahertz quantum cascade lasers leads to record-high peak-power surface emission (>100 mW) and differential efficiencies (230 mW A(-1)), together with low-divergence, single-lobed emission patterns, and is also applicable to continuous-wave operation. Such flexible tuning of the radiation loss using graded photonic heterostructures, with only a minimal influence on threshold current, is highly desirable for optimizing second-order distributed feedback lasers.

Organic solid-state lasers based on sexiphenyl as active chromophore

We report on various sexiphenyl derivatives as gain media in organic solid-state lasers. The molecules involved in this research are simple p-sexiphenyl, the laser dye molecule 2,5,2””’,5””’-tetra-t-butyl-p-sexiphenyl (TBS) and the spirolinked sexiphenyl-derivative 2,7-bis(biphenyl-4-yl)-2’,7’-di-tert-butyl-9,9’-spirobifluorene. It appears that the morphology of vacuum-deposited thin films is highly dependent on the sterical dimensions of the respective molecules. Whereas thin films based on simple p-sexiphenyl comprise large clusters which significantly deteriorate their waveguiding properties; films formed by TBS, and the spiroderivative show a dramatically improved morphology with reduced surface roughness. Therefore amplified spontaneous emission (ASE) and lasing are demonstrated in both of the last but not in films based on p-sexiphenyl. Second-order distributed-feedback lasers with TBS as the active medium have been prepared with an emission between 390 and 435 nm depending on the grating period of the Bragg reflector. While the ASE characteristics are similar in films formed by TBS and the spiroderivative, TBS exhibits even superior laser threshold densities which are as low as 45μJ∕cm2 at a wavelength of 396 nm.

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).

Analysis of surface-emitting second-order distributed feedback lasers with central grating phaseshift

An analysis of second-order distributed feedback lasers (DFB) with central grating phaseshift is performed. The devices have an active grating (i.e., DFB) section, passive grating sections (i.e., DBRs); and the active grating is formed at a metal-semiconductor interface. Coupled-mode theory and the transfer matrix method are employed. It is found that a central grating phaseshift, /spl Delta//spl phi/, of 180/spl deg/ causes the laser to radiate in a beam of symmetric near-field amplitude profile, in sharp contrast to conventional second-order DFB lasers which radiate in beams of asymmetric near-field amplitude profile. In turn the far-field profile becomes a single-lobe beam pattern. Thus, a means to fundamentally obtain surface emission in an orthonormal single-lobe beam from a second-order DFB/DBR device has been found. The orthornomal-beam emission is achieved at no penalty in device efficiency. External differential quantum efficiencies, /spl eta//sub d/, in excess of 70% can be obtained, and the guided-field intensity profile is substantially uniform. The effects of the lengths of the DFB section (L/sub DFB/) and of each of the DBR sections (L/sub DBR/) on device performance are analyzed and optimal values are found to occur for L/sub DFB/ in the 500-700 /spl mu/m range and for L/sub DBR/ in the 600-700 /spl mu/m range. One can obtain /spl eta//sub D/ values as high as 76% from devices with 80% of the energy in the central lobe, and moderate threshold gains (i.e., 40 cm/sup -1/). Threshold gains as low as 25 cm/sup -1/ can also be obtained from highly efficient devices (i.e., /spl eta//sub D//spl cong/70%), at some penalty in guided-field uniformity. In either case the intermodal discrimination is quite high (70-75 cm/sup -1/). Gratings with half-wave (i.e., /spl pi/) phaseshifts have been fabricated by using the dual-tone photoresist method, and the concept has been experimentally proven: orthonormal, single-lobe emission in a diffraction-limited beam from 1500 /spl mu/m-long devices. Extension to two-dimensional (2-D) large-aperture: 200 /spl mu/m/spl times/1500 /spl mu/m; surface emitters is quite possible, which should allow for the emission of watts of coherent CW power in a stable, single mode. The 2-D structure represents a defect-free, second-order active photonic lattice.

Color-selective semiconductor nanocrystal laser

Theoretical predictions of the benefits of three-dimensional quantum confinement have provided motivation for the development of quantum-dot lasers. Such lasers, developed in the case of self-assembled quantum dots, have not been successfully demonstrated with quantum-confined colloidal nanocrystals (NCs). Here, using recently developed NC-titania chemistry, we report the successful development of an optically pumped, NC-based distributed feedback laser, in which the narrow gain profiles of these nanoparticles have been matched with the feedback of a second-order distributed feedback laser. This laser, whose output color can be selected by choosing appropriately sized nanocrystals, operates at 80 K and at room temperature.

Solid-State Dye Laser with Photo-Induced Distributed Feedback

This paper describes the action of a solid-state dye laser utilizing a photo-induced distributed feedback (DFB) resonator. We solidified rhodamine B into a dried xerogel monolith using the sol–gel method, and used it as an active laser medium. The pump source was third-harmonic generation (THG) produced by a pulsed Nd:YAG laser. We divided the pump beam in two and irradiated the two beams on the active medium. The interference fringes simultaneously form both a spatial periodic modulation of optical gain and a refractive index in the active medium. This periodical structure functions as a DFB resonator. Without an external resonator, we achieved a laser oscillation with a narrowed spectrum. The full width at half maximum (FWHM) of the laser wavelength was approximately 1 nm. We tuned the center wavelength of the laser from 600 nm to 635 nm. In addition, we achieved a second-order DFB laser oscillation with second-harmonic generation (SHG) of a Nd:YAG laser.

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.

Axial profile of grating coupled radiation from second-order DFB lasers with phase shifts

Axial profiles of the grating-coupled radiation field emitted in a direction normal to the surface of distributed-feedback (DFB) lasers with phase-shifted second-order corrugation are theoretically analyzed. The profiles are calculated for nonreflecting DFB lasers with the following phase shifts: λ/4 shift, two lambda /8 shifts, and two 3 lambda /8 shifts. It is demonstrated that the radiation field can be controlled by changing the phase shifts. This result suggests a new method for modulating the surface emission of a DFB laser.

Effects of radiation loss on the performance of second-order DFB semiconductor lasers

The effects of radiation loss on the performance of second-order distributed-feedback (DFB) semiconductor lasers with a symmetrical grating are analyzed systematically. The threshold gains of the two lowest modes, the differential quantum efficiency, and the spectral linewidth are calculated as a function of the complex coupling coefficient. The comparison is also made between a conventional DFB and a quarter-wave phase-shifted DFB laser. It is shown that the effects of radiation loss are different in these two laser types. In addition, a simple approximate expression for threshold gain is derived. By way of example, a DFB laser with a rectangular grating is analyzed. It is shown that the performance of a second-order DFB laser is not sensitive to the grating pitch over a fairly wide range, provided that it is close to the pitch which gives the maximum coupling coefficient.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Yield dependency on length of AlGaAs/GaAs second-order distributed feedback lasers with cleaved facets

The effect of length on the yield of second-order distributed feedback lasers with cleaved facets capable of operating in a single longitudinal mode and on their characteristics has been studied experimentally. The optimum length of 200 μm was determined in reference to the coupling coefficient and the end-facet reflectivity. A yield of more than 70% was obtained for lasers 200 μm in length capable of operating at a level of at least 10 mW and over.

Yield analysis of second-order DSM DFB lasers and implications for design

The results of yield analysis of dynamic single-mode (DSM) second-order distributed feedback (DFB) lasers are presented. Design implications of this analysis are discussed. Three ways of achieving DSM operation which, depending on the design, can operate simultaneously are considered: radiation loss, asymmetric end cavity reflections, and phase shifting of a grating. The interplay among these mode selection mechanisms is analyzed under conditions of random facet cleaving. The calculated expected yield is subjected to three constraints: a minimum side-mode suppression ratio, a maximum threshold current, and a minimum external quantum efficiency. A comparison of DSM performance between first-order and second-order DFB lasers of analogous designs is also included.

Mode selectivity in DFB lasers with a second-order grating

The relative effectiveness of three methods of achieving dynamic single-mode operation of second-order distributed feedback lasers is analysed. These methods, based on phase shifting, mirror asymmetry and radiation loss are compared under conditions of random cleaving of facets.

Second-order distributed feedback lasers with mode selection provided by first-order radiation losses

We present a theoretical study of the effect of radiation losses on the mode selectivity of DFB lasers with second-order gratings. For a second-order grating, interference of the radiation due to first-order diffraction of oppositely propagating guided waves cancels the radiation loss at one of the edges of the spectrum gap. This provides threshold gain discrimination of order 10 cm-1against one of the two dominant modes occurring near the edges of the gap. This should allow fabrication of DFB lasers with properties that are nearly independent of the positions of the facets relative to the grating corrugations, which are uncontrolled. By applying antireflection coatings to the two ends, differential quantum efficiencies close to those of conventional Fabry-Perot lasers should be achievable.