Browsing by Author "Arslan, Seval"

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Open Access
Conservation of quantum efficiency in quantum well intermixing by stress engineering with dielectric bilayers
(Institute of Physics Publishing, 2018) Arslan, Seval; Demir, Abdullah; Şahin, S.; Aydınlı, A.
In semiconductor lasers, quantum well intermixing (QWI) with high selectivity using dielectrics often results in lower quantum efficiency. In this paper, we report on an investigation regarding the effect of thermally induced dielectric stress on the quantum efficiency of quantum well structures in impurity-free vacancy disordering (IFVD) process using photoluminescence and device characterization in conjunction with microscopy. SiO2 and Si x O2/SrF2 (versus SrF2) films were employed for the enhancement and suppression of QWI, respectively. Large intermixing selectivity of 75 nm (125 meV), consistent with the theoretical modeling results, with negligible effect on the suppression region characteristics, was obtained. Si x O2 layer compensates for the large thermal expansion coefficient mismatch of SrF2 with the semiconductor and mitigates the detrimental effects of SrF2 without sacrificing its QWI benefits. The bilayer dielectric approach dramatically improved the dielectric-semiconductor interface quality. Fabricated high power semiconductor lasers demonstrated high quantum efficiency in the lasing region using the bilayer dielectric film during the intermixing process. Our results reveal that stress engineering in IFVD is essential and the thermal stress can be controlled by engineering the dielectric strain opening new perspectives for QWI of photonic devices.
Open Access
Facet cooling in high-power InGaAs/AlGaAs lasers
(Institute of Electrical and Electronics Engineers Inc., 2019) Arslan, Seval; Gündoğdu, Sinan; Demir, Abdullah; Aydınlı, A.
Several factors limit the reliable output power of a semiconductor laser under CW operation, such as carrier leakage, thermal effects, and catastrophic optical mirror damage (COMD). Ever higher operating powers may be possible if the COMD can be avoided. Despite exotic facet engineering and progress in non-absorbing mirrors, the temperature rise at the facets puts a strain on the long-term reliability of these diodes. Although thermoelectrically isolating the heat source away from the facets with non-injected windows helps lower the facet temperature, data suggests the farther the heat source is from the facets, the lower the temperature. In this letter, we show that longer non-injected sections lead to cooler windows and biasing this section to transparency eliminates the optical loss. We report on the facet temperature reduction that reaches below the bulk temperature in high power InGaAs/AlGaAs lasers under QCW operation with electrically isolated and biased windows. Acting as transparent optical interconnects, biased sections connect the active cavity to the facets. This approach can be applied to a wide range of semiconductor lasers to improve device reliability as well as enabling the monolithic integration of lasers in photonic integrated circuits.
Open Access
Facet temperature reduction by separate pumped window in high power laser diodes
(SPIE, 2018) Arslan, Seval; Gündoğdu, Sinan; Demir, Abdullah; Aydınlı, A.
The main optical output power limitation in high power laser diodes is the catastrophic optical mirror damage (COMD) initiated by facet heating due to optical absorption, which limits the reliable power and lifetime of a single laser. Facet heating correlated with current injection near laser facets can be reduced by unpumped window structure. However, the high-power laser slope efficiency drops as the length of the window increases. In this work, separately pumped window (SPW) method is proposed and experimentally demonstrated to significantly reduce the facet temperature of the semiconductor lasers without compromising their performance. We used 5-mm long high-power laser diodes and compared its performance and facet temperature to the devices integrated with SPW facet sections, which are electrically isolated from the laser section. The slope efficiencies of the lasers with SPW and that of 5-mm lasers without SPW are comparable when SPW is pumped at its transparency current, illustrating that SPW integrated lasers preserve their slope efficiency. As the window pumping current increases, the threshold current of the laser with SPW decreases when the SPW approaches transparency. The facet temperature rise (ΔT) of the lasers were measured by the thermoreflectance method. The Î"T measured at waveguide regions of lasers was shown to be reduced by 42% implementing SPW region to conventional lasers. Therefore, SPW technique was shown to be a promising approach to increase the COMD level of the high-power laser diodes and it opens up a new avenue for reliable semiconductor laser operation at very high output power levels.
Open Access
IFVD-based large intermixing selectivity window process for high power laser diodes
(SPIE, 2018) Arslan, Seval; Şahin, S.; Demir, Abdullah; Aydınlı, A.
Catastrophic optical mirror damage (COMD) is a key issue in semiconductor lasers and it is initiated by facet heating because of optical absorption. To reduce optical absorption, the most promising method is to form non-absorbing mirror structures at the facets by obtaining larger bandgap through impurity-free vacancy disordering (IFVD). To apply an IFVD process while fabricating high-power laser diodes, intermixing window and intermixing suppression regions are needed. Increasing the bandgap difference (ΔE) between these regions improves the laser lifetime. In this report, SrF2 (versus SixO2/SrF2 bilayer) and SiO2 dielectric films are used to suppress and enhance the intermixing, respectively. However, defects are formed during the annealing process of single layer SrF2 causing detrimental effects on the semiconductor laser performance. As an alternative method, SixO2/SrF2 bilayer films with a thin SixO2 dielectric layer is employed to obtain high epitaxial quality during annealing with small penalty on the suppression effect. We demonstrate record large ΔE of 125 meV. Broad area laser diodes were fabricated by the IFVD process. Fabricated high-power semiconductor lasers demonstrated conservation of quantum efficiency with high intermixing selectivity. The differential quantum efficiencies are 81%, 74%, 66% and 46% for as grown, bilayer protected, SrF2 protected and QWI lasers, respectively. High power laser diodes using bilayer dielectric films outperformed single-layer based approach in terms of the fundamental operational parameters of lasers. Comparable results obtained for the as-grown and annealed bilayer protected lasers promises a novel method to fabricate high power laser diodes with superior performance and reliability.
Open Access
Novel concepts in high power semiconductor lasers
(2018-11) Arslan, Seval
This doctoral thesis deals with innovations to the cavity optics of high power semiconductor lasers emitting light at 9xx nm. High power laser diodes are complex electronic and photonic systems. Developments in epitaxial crystal growth techniques and the quality that ensued has been the driving force in the progress of the field. Semiconductor lasers with high output powers and high efficiencies have thus become possible. Commercial single emitters each with over 10 watts output with efficiencies reaching 60% is available.Even higher output powers have been demonstrated in the lab. High power semiconductor lasers have many applications such as acting as optical pumps in other lasers, range finding, optical storage, light sources in sensors and medical tools. The demand for higher powers and efficiencies continues. Among several possibilities, one of the main limits of maximum output power is the catastrophic optical mirror damage (COMD). At high pump currents and hence output powers, facet absorption leads to temperatures high enough to damage the cavity mirrors. This thesis is focused on novel approaches to increase the COMD threshold. We demonstrate design, fabrication and characterization of the high power strained InGaAs/AlGaAs lasers emitting light at 9xx nm. To prevent facet absorption which decreases the laser efficiency especially at high injection currents, band gaps in the vicinity of the laser facet are increased using impurity-free vacancy disordering (IFVD) while preserving the band gap in the lasing region away from the facets. A record large bandgap at the facet region, relative to that of the lasing region is achieved by thermal stress management of a bilayer dielectric structure. We demonstrate excellent optical loss and optical power output with this bilayer approach. Further, positive feedback cycle during absorption at the facets is broken by keeping the facets cold, by design. Thus, in this cold window approach, we extend the passive unpumped windows to keep the heat source from the main body of the cavity away from the facets while eliminating the additional loss incurred by biasing this section to transparency. This new biased window approach leads to much cooler facet temperatures while reducing the bulk temperatures as well. Thus, we use thermore ectance spectroscopy to measure facet temperature as a function of pump and bias current. We clearly demonstrate that, for the first time, facet temperatures have been decreased below the bulk temperature without penalty on the output power.
Open Access
Reduced facet temperature in semiconductor lasers using electrically pumped windows
(SPIE, 2019-02) Demir, Abdullah; Arslan, Seval; Gündoğdu, Sinan
The self-heating of semiconductor lasers contributes directly to facet heating and consequently to the critical temperature for catastrophic optical mirror damage (COMD) but the existing facet engineering methods do not address this issue. Targeting this problem, we report experimental and modeling results that demonstrate a new method achieving facet temperatures significantly lower than the laser cavity temperature in GaAs-based high-power semiconductor lasers by using electrically isolated and pumped windows. Owing to monolithic integration, the method does not introduce any penalty on the efficiency and output power of the laser. Thermal modeling results show that the laser output facet can be almost totally isolated from heat generated in the laser cavity and near cold-cavity facet temperatures are possible. The method can be applied to single emitters, laser bars, and monolithically integrated lasers in photonic integrated circuits to improve their reliability and operating performance.