Longitudinal modes evolution of a GaN-based blue laser diode Moch S. Romadhon, Abdulaziz Aljalal, Watheq Al-Basheer n , Khaled Gasmi Department of Physics, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia article info Article history: Received 22 August 2014 Received in revised form 15 January 2015 Accepted 26 January 2015 Keywords: Longitudinal modes Mode spacing Blue laser diode abstract Spectral emission of a single-transverse Fabry–Perot GaN-based blue laser diode was experimentally inves- tigated to study the evolution of its longitudinal modes. A 0.003-nm resolution spectrometer was employed to detect and record emission spectra of the laser diode in the wavelength range between 440 and 450 nm as a function of the operating current and temperature. The stability of the laser was investigated over continuous 14 h by monitoring variations in emitted power and central wavelength and found to exhibit excellent stability. The longitudinal mode spacing of 0.0548 nm was found to agree with corresponding calculated mode spacing. The longitudinal modes were observed to shift at rates of 0.0045 nm/mA and 0.0154 nm/1C toward longer wavelengths. Similarly, the gain was observed to shift towards longer wavelengths but with a rate of 0.0432 nm/1C. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Since the invention of the first working at cryogenic temperatures laser diode in 1962, laser diodes employment has undergone an exponential increase in numerous fields of basic and applied sciences. The main principle of a laser diode function is the electron–hole reco- mbination process in a typical semiconducting p–n junction within the active layer of the laser diode. Light of wavelength dependent on the energy gap between the electrons in the conduction band and the holes in the valence band is emitted as a result of the recombination process. Like any laser system, laser diodes require a pump source, an active medium and a resonator for optical feedback. The pump mec- hanism (population inversion) is achieved by the application of a suitable current that flows through the p–n junction while the condition of optical resonator is satisfied by simulating a Fabry–Perot cavity through cleaving the facets parallel to the semiconducting junction. Laser diode cleaved facets are essential to cause part of the generated light to oscillate back and forth within the laser cavity, due to the refractive index contrast between laser diode active layer material and air, consequently, setting the condition for formation of stable standing waves. As the cavity length is significantly longer than the resonating light wavelength, cavity resonances corresponding to maximum gain can occur at multiple wavelengths; a phenomenon known as “mode hopping”. Among few types of laser diodes, it is well-established that GaN-based laser diodes are efficient, economic and versatile sources of laser radiation with output wavelength that covers a wide spectral range from the near ultraviolet to the visible green [1–5]. In 1996, Nakamura and co-workers [1,2] introduced the first demonstration of a room-temperature nitride-based CW blue laser. Nowadays, GaN-based laser diodes have wide range of applica- tions in electronics [3], sensing [4,5], military and defense [6–8], telecommunications [9], optical lithography [10], medical treatment [11], trace gas detection [12,13] and many other fields of research in modern science and technology [14]. Up-to-date, and despite their wide applications and many land- mark enhancements concerning efficiency, output power, device life time and beam shape and quality, many fundamental characteristics of this type of semiconducting lasers are not fully understood. For example, it is commonly observed that in GaN-based lasers emission spectra, the experimentally evaluated longitudinal modes spacing is of one order of magnitude larger than that theoretically calculated from the cavity parameters [1,15–18]. When lasing, GaN-based laser diodes spectra customarily demonstrate kinks and mode hopping features that can be correlated with laser diode excitation current intensity above the threshold. Another commonly observed characteristic of this type of laser diodes emission spectra is their sensitivity to applied current (usually in the order of mA); where emitted longitudinal modes structure are customarily observed to evolve and to exhibit detectable substructure variations as a function of applied current [18]. Due to fact that many applications of the GaN-based laser diodes require high level of spectral and temporal stability of the emitted longitudinal modes, investigating laser diode longitudinal modes evolution and dynamics is crucial to understand the operation of the diode laser in any useful application. Generally, the temperature of laser diodes has direct effect on the emitted spectra wavelength and evolution as a function of applied current variation. An increase in the device internal temperature reduces the degree of population inver- sion, consequently, leading to decrease of laser's optical gain [19]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology http://dx.doi.org/10.1016/j.optlastec.2015.01.012 0030-3992/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Fax: þ966 13 860 2293. E-mail address: Watheq@kfupm.edu.sa (W. Al-Basheer). Optics & Laser Technology 70 (2015) 59–62