Facet Coatings Research Articles

Cavity Length Scaling of Quantum Cascade Lasers for Single-Mode Emission and Low Heat Dissipation, Room Temperature, Continuous Wave Operation

Single-mode operation of Fabry-Perot quantum cascade (QC) lasers is achieved through the systematic shortening of the cavity length from 764 to 110 μm. The increased mirror loss is mitigated using highly reflective (HR) metallic facet coatings (R >; 95%). Ultrashort cavity QC lasers operate single mode with the best device having a mode-hop free current tuning range of 3.44 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Using combined heat-sink temperature and current tuning, the largest single-mode tuning range attained was 8.56 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . The heat dissipation of a 110-μm ultrashort cavity QC laser is 0.23 W at 80 K and 0.43 W at 150 K. With a 500-μm cavity and both facets HR coated with reflectivities of ~95% and 75%, room-temperature continuous wave operation is realized with heat dissipation of 1.2-1.7 W.

Room temperature interband cascade lasers

An analysis of the threshold current densities and differential slope efficiencies in interband cascade lasers with type-II active regions revealed lower Auger recombination rates than anticipated theoretically or using the data from prior experiments. Furthermore, a new design of the waveguide yielded room-temperature internal losses as low as ≈6 cm−1. The combination of these advances with improvements to the processing of narrow ridges has led operation up to 335 K in continuous-wave mode for devices emitting at 3.3–3.7 μm. An 8.7 μm-wide ridge without facet coatings produced up to 23 mW of cw power at 300 K, and displayed a maximum wall-plug efficiency of 2.0%.

Mid-infrared interband cascade lasers operating at ambient temperatures

We discuss the state-of-the-art performance of interband cascade lasers emitting in the 3–5 μm spectral band. Broad-area devices with five active stages display pulsed threshold current densities as low as 400 A cm−2 at room temperature. Auger decay rates are extracted from the analysis of threshold current densities and differential slope efficiencies of nearly 30 lasers, and found to be significantly lower than was anticipated based on prior information. New designs also produce ICLs with room-temperature internal losses as low as ≈6 cm−1. The combination of these advances with improvements to the processing of narrow ridges has led to the fabrication of a 4.4-μm-wide ridge emitting at 3.7 μm that lased to 335 K in continuous mode. This is the highest continuous-wave (cw) operating temperature for any semiconductor laser in the 3.0–4.6 μm spectral range. A 10-μm-wide ridge with high-reflection and anti-reflection facet coatings produced up to 59 mW of cw power at 298 K, and displayed a maximum wall-plug efficiency of 3.4%.

High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings

We present a method of preserving the device wall-plug efficiency by adjusting mirror losses with facet coatings for longer cavity quantum cascade lasers. An experimental study of output power and wall-plug efficiency as functions of mirror losses was performed by varying the front facet coating reflectivity with a high-reflectivity-coated rear facet. The use of optimized reflectivity coatings on 7-mm-long chips resulted in continuous-wave output power of 2.9W at 293K for thermoelectrically cooled devices mounted on AlN submounts and average and continuous-wave output power in excess of 1W for uncooled devices emitting at 4.6μm.

Multiple vertical mode high power 975 nm diode lasers restricted to single vertical mode operation through use of optical facet coatings

A new design approach has been developed for facet coating of high power diode lasers, that is shown to preferentially suppress higher-order guided optical modes, without any degradation in device performance. Repeated optical coating of reference lasers is found to controllably transform the vertical far field from a three-peaked higher-order mode to a single-peaked fundamental mode, in good agreement with simulations. 975 nm devices with such coatings reach power conversion efficiency of 65%.

Broadly tuneable, short external cavity diode laser for optical coherence tomography

The application of a broadly tuneable asymmetric multiple-quantum-well (AMQW) laser is demonstrated in an external cavity for both time-domain and Fourier-domain optical coherence tomography (OCT). The AMQW laser was custom designed and fabricated to have a large tuning range when operated with no facet coatings in a diffractive optical element (DOE) short external cavity (SXC). The depth resolution of Fourier domain OCT was found to be as short as 13 µm in air with the 117 nm tuning range (full-width-half-maximum of 80 nm) of the AMQW DOE SXC source. For time domain OCT, synthesised OCT with the AMQW sources was investigated. It was found that the depth resolution is set by the tuning range of the AMQW DOE SXC laser. The depth resolution can be improved as 117 nm is not the limit to the tuning range of AMQW lasers.

Structural analysis of life tested 1.3 μm quantum dot lasers

We present the results of an accelerated life test study of quantum dot lasers operating at 1310 nm. The devices were run at 1 and 2 kA/cm2 (∼10 and ∼70 times Ith, depending on facet coatings), at temperatures of 80 and 100 °C for 1350 h. Some devices, particularly those with higher current densities, showed significant drops in output power and increase in threshold current over this time. The devices were examined using electroluminescence, which shows nonradiative recombination centers in the active region of the device as dark spots. A clear correlation between the density of dark spots and degradation is observed. The defect structure responsible for the dark spots has been identified using conventional and high-resolution cross-section transmission electron microscopy of selected structures. The defects consist of an inverted stacking fault pyramid or microtwin enclosing the dot. The more extensive defects observed after the life test are consistent with their growth by climb, i.e., addition and/or removal of point defects. It is proposed that quantum dot devices show enhanced resistance to the growth of these defects in comparison with quantum well lasers.

Optimization of facet coating for highly strained InGaAs quantum well lasers operating at 1200 nm

The optical output power of a laser diode can be enhanced by anti-reflection (AR) and high-reflection (HR) facet coatings, respectively, at the front and back facet. AR and HR coatings also serve the purpose of protection and passivation of laser diode facets. In this work, we have designed and optimized a single layer λ/4 thick Al 2O 3 film for the AR coating and a stack of λ/4 thick Al 2O 3/ λ/4 thick Si bi-layers for the HR coating for highly strained InGaAs quantum-well edge emitting broad area (BA) laser diodes. Effect of the front and back facet reflectivities on output power of the laser diodes has been studied. The light output versus injected current ( L–I characteristics) measurements were carried out on selected devices before and after the facet coatings. We have also carried out the numerical simulation and analysis of L–I characteristics for this particular diode structure. The experimental results have been compared and verified with the numerical simulation.

Single mode cw operation of 658nm AlGaInP lasers based on longitudinal photonic band gap crystal

GaInP–AlGaInP lasers with broad waveguide based on a longitudinal photonic band gap crystal have been studied. Lasers with 10μm stripe width exhibit single transverse mode operation. The vertical beam divergence is about 8° and is insensitive to the drive current. The aspect ratio is ∼2:1. The quality factor for the lateral beam M2 is less than 2 in single mode regime under pulsed excitation. The total maximum continuous wave output power in the single mode regime at 20°C is more than 115mW (for high reflection/antireflection facet coatings), indicating a dramatic reduction in the catastrophic optical mirror damage problem.

Facet coatings for nonhermetically packaged lasers

Nonhermetic packaging is one avenue of reducing the module cost of optical devices; however, this requires the coatings applied to the facets of lasers and modulators to be highly stable and impervious to the effects of high humidity under high temperature/high power operating conditions. We have developed two types of facet coatings for nonhermetic applications. One involves electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of a SiNx cap layer and the other utilizes ion beam deposition (IBD) of a Ta2O5 cap layer, both on top of a two-layer SiO2/a-Si high-reflectance (HR) coating deposited on the facets of Fabry–Perot (F–P) ridge waveguide (RWG) lasers operating at 1310 nm. The lasers were aged under 85 °C/85% relative humidity (RH) conditions and ex situ tested using drive currents of 10–20 mA (subthreshold) and 80–120 mA operating currents (Iop). The output power–current–voltage (L–I–V) tests were conducted at room temperature and at various time intervals up to 5000 h. For comparison, tests were also done on devices aged in a dry oven.

High-temperature and high-speed operation of a 1.3-/spl mu/m uncooled InGaAsP-InP DFB laser

1.3-μm uncooled InGaAsP-InP loss-coupled distributed feedback lasers operating over 10 Gb/s and at 85/spl deg/C were successfully fabricated. In order to achieve high-speed and high-temperature operation simultaneously, the following are thoroughly investigated: modulation-doped and strain-compensated multiple-quantum-well active layers, Fe-doped buried-heterostructure, coupling coefficient of loss-coupled grating, detuning lasing wavelength from the gain peak, and facet coatings. The authors also demonstrate 10 Gb/s transmission with negligible dispersion power penalty over 20 km of nondispersion-shifted fiber at 10 Gb/s for temperatures ranging from 25/spl deg/C to 85/spl deg/C.

Material properties of ion beam deposited oxides for the optoelectronic industry

High quality, dense films of SiO2, Al2O3, Ta2O5, and TiO2 were deposited with an ion beam deposition system (IBD). IBD has significant advantages over other techniques in terms of directionality, stress control, repeatability, thermal stability, and film uniformity [J. J. Cuomo, J. M. E. Harper, C. R. Guarnieri, D. S. Yee, L. J. Atanasio, J. Angilello, C. T. Wu, and R. H. Hammond, J. Vac. Sci. Technol. 20, 349 (1982)]. To decrease the surface damage induced by ion bombardment, a multi-energy process was developed. This is especially important for laser facet coatings. The oxide films were optimized for the desired refractive index and zero absorption. Stress values of −0.2 to −0.5 GPa (compressive) and extremely good uniformity (&amp;lt;0.2%) of film thickness and refractive index were obtained. The impact of the IBD deposition angle on film stress was also investigated. These films showed excellent thermal stability with virtually no degradation of optical properties after three 1 h anneals at 350 °C (Δn&amp;lt;0.0124). With the exception of TiO2, all films remained in compressive stress after annealing. TiO2 turned tensile. With the variety of oxide materials developed, designing an anti-reflective or highly reflective stack, which satisfies requirements of stress, uniformity, deposition rate, and reflectance, becomes a matter of choosing the appropriate material set.

InGaAsP/InP Laterally Coupled Distributed Feedback Ridge Laser

A 1.3-µm-wavelength InGaAsP/InP laterally coupled distributed feedback ridge laser with improved structure and characteristics is reported in this paper. For easier fabrication of the first order grating with 198 nm period on and beside the ridge waveguide side-walls, two-step etching has been used to form a narrow ridge waveguide with a special shape. The grating is patterned by electron beam lithography and deeply transferred also by two-step etching, in order to enhance the lateral evanescent field coupling. A stable CW laser oscillation with a low threshold current of 17 mA at room temperature and a high side mode suppression ratio of 45 dB for a laterally coupled distributed feedback ridge laser has been achieved. An external slope efficiency of 0.25 W/A and an output power of about 25 mW are also available in our most recent devices with anti-reflection and high reflection facet coatings. The optical transmission of a Gbit/s order modulated signal has also been tested using our preliminary laser modules through a 1 km optical fiber.

Improved gain block for long wavelength (1.55 µm)hybrid integrated devices

A curved waveguide, strained multiple quantum well reflective amplifier with an integrated front facet mode expander has been designed and fabricated. A device without facet coatings has been found to have a chip gain of 36 dB with 6 dB of gain ripple. This equates to an effective facet reflectivity of 8/spl times/10/sup -6/.

1.3 μm room-temperature GaAs-based quantum-dot laser

Room-temperature lasing at the wavelength of 1.31 μm is achieved from the ground state of an InGaAs/GaAs quantum-dot ensemble. At 79 K, a very low threshold current density of 11.5 A/cm2 is obtained at a wavelength of 1.23 μm. The room-temperature lasing at 1.31 μm is obtained with a threshold current density of 270 A/cm2 using high-reflectivity facet coatings. The temperature-dependent threshold with and without high-reflectivity end mirrors is studied, and ground-state lasing is obtained up to the highest temperature investigated of 324 K.

1.4-μm InGaAsP-InP strained multiple-quantum-well laser for broad-wavelength tunability

InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings.

Very-low-threshold, highly efficient, and low-chirp 1.55-μm complex-coupled DFB lasers with a current-blocking grating

Low-threshold current DFB lasers have been fabricated based on a complex-coupled structure with a current blocking grating. A threshold current as low as 5 mA was obtained, which enabled high temperature operation up to 105/spl deg/C. Asymmetric facet coatings were applied to obtain a high efficiency of 0.3 W/A. The strained InGaAsP MQW active region and the antiphase coupling has resulted in a low modulation chirping. By direct modulation of the device at 2.488 Gb/s and using optical amplifiers, we have demonstrated digital transmission over a 235 km-long standard single-mode fiber with a power penalty of 1.55 dB.

Above-threshold analysis of loss-coupled DFB lasers: threshold current and power efficiency

The threshold current and power efficiency of loss-coupled DFB lasers have been calculated numerically using an above-threshold model. Compared with perfectly AR-coated devices, lasers with asymmetric facet coatings have substantially lower threshold current. However, significantly higher front-facet power efficiency is only possible at small grating-coupling coefficients. In particular, the range of loss-coupling coefficients for higher efficiency becomes narrower with larger grating duty cycle and larger index-coupling coefficient. These results can be explained by the interdependence of the reflectivity and extra loss associated with loss-coupled gratings.

Facet influence on wavelength tunability and linewidth chirp in InGaAs/InAlGaAs quantum-well distributed feedback lasers under high-bit-rate modulation

The strong influence of facet properties on wavelength shift and wavelength chirp is studied in uncoated and coated distributed feedback (DFB) lasers. A detailed comparison is performed between various experimental laser data (spectra, electronic and thermal wavelength tunability, relative intensity noise, linewidth) and the results of model calculations combining rate equations and the transfer matrix method. From experimental data of different lasers, a set of physical DFB laser parameters is determined. We succeeded in describing all the experimental data of different lasers by the same parameter set. By use of this set and a large signal analysis we found that the wavelength chirp and the wavelength shift resulting from electronic effects including spatial hole burning varies considerably for different end facet phases (EFPs) and facet coatings, but otherwise identical DFB laser geometry.

Blue-green laser diodes on ZnSe substrates

II-VI heterostructures composed of ZnMgSSe/ZnSe/ZnCdSe layers have been grown by molecular beam epitaxy on conducting ZnSe substrates and fabricated into blue and green laser diodes. These are the first laser diodes ever prepared on ZnSe. Pulsed laser emission from devices with no facet coatings has been obtained from 77 K to near room temperature. CW laser emission was also observed at 77 K.