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Longitudinal modes evolution of a GaN-based blue laser diode
Optics & Laser Technology 70 (2015) 59–62
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
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
art ic l e i nf o
a b s t r a c t
Article history: Received 22 August 2014 Received in revised form 15 January 2015 Accepted 26 January 2015
Spectral emission of a single-transverse Fabry–Perot GaN-based blue laser diode was experimentally investigated 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.
Keywords: Longitudinal modes Mode spacing Blue laser diode
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 recombination 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 mechanism (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
n
Corresponding author. Fax: þ 966 13 860 2293. E-mail address: [email protected] (W. Al-Basheer).
http://dx.doi.org/10.1016/j.optlastec.2015.01.012 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
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 applications 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 landmark 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 inversion, consequently, leading to decrease of laser's optical gain [19].
M.S. Romadhon et al. / Optics & Laser Technology 70 (2015) 59–62
Furthermore, many applications of GaN-based laser diodes require precise knowledge of the device temperature and its explicit effect on the emitted spectra structure and evolution. In this study, a simple experimental setup was employed to observe and record longitudinal modes of a commercial CW GaN-based blue laser diode. The temporal stability of the modes was investigated by recording the diode's longitudinal modes intensity of continuously emitted spectra as a function of lasing elapsed time. Moreover, we report experimental results of investigating the evolution and stability of emitted spectra structure as a function of applied current and diode's temperature. To the best of our knowledge, this study is the first probe of GaN-based laser diode over the blue spectral range 440– 450 nm. The reported results herein are anticipated to provide scholars and users of needed information on the operation and nature of emission spectra of GaN-based blue laser diodes.
2. Experimental setup The employed experimental setup consisted of the following components, briefly; a semiconductor blue laser diode of GaN-base (model LD-0445-0050-1), purchased from Toptica Photonics, operating over the temperature range from 10.0 1C to þ60.0 1C, and lasing wavelength (440–450 nm) was used. The laser diode was housed in a TCLDM9 laser diode mount from Thorlabs with optimal output power of 50 mW. The laser diode current and temperature were controlled using Thorlabs LDC200C and TED200C controllers, respectively. A fraction of the laser output CW beam was directed to a power meter head (PM 100D from Thorlabs), while the other fraction was directed to the monochromator opening slit using a sequence of optics; namely, a highly reflective mirror, a collimator and a 10.0 cm bi-convex lens for focusing. Beam focusing was essential to minimize divergence of laser beam, due to the laser's intrinsic property of relatively high beam divergence of angles 8.301 and 22.01 for the parallel and perpendicular directions, respectively and as reported by the manufacturer. A SPEX monochromator (Model 500M) was coupled with a 3000 pixels USB 2.0 CCD line camera of 7 μm pixel width to measure laser emitted light intensity as a function of light wavelength. The dispersion of the SPEX500M monochromator is 1.6 nm/mm. Employed monochromator was calibrated by utilizing 8 known sharp emission lines from a standard He lamp over the spectral region 388.86–447.15 nm. For monochromator calibration, the He lamp was housed in a small box with a small opening for initial collimation followed by two bi-convex lenses of 5 and 15 cm foci lengths for collimation and focusing at the monochromator entrance slit, respectively. A National Instrument-USB 6251 was used as an interface between the PC and controllers with a SC-2345 signal conditioner between the PC and controllers. A homemade LabVIEW code was utilized to acquire, record and save data.
the longitudinal modes which are specified by the Arabic numbers 1, 2, 3 and 4. These kinks are caused by lower gain at these wavelengths. The asymmetry in the bundle shape are typically observed in the GaNbased lasers, especially well above the threshold current [18], and can be explained by structural variations due to oscillating light modes interference with laser diode substrate [20]. Fig. 2 shows four curves evaluating the stability of the laser diode operated at 100 mA and 20 1C over extended period of time (14 h). The four curves represent the laser output power, the fractional change in the central wavelength, the fractional change in applied current, and the fractional change in temperature. For the sake of clarity, the fractional changes in the central wavelength, current and temperature were multiplied by a constant and shifted from zero. The data in Fig. 2 were collected every 5 min. During 14 h of lasing, the laser diode output power, central peak wavelength, applied current, and temperature exhibited relatively small variation indicating high level of temporal stability for the employed blue laser diode. Fig. 3 shows a contour plot of the longitudinal modes intensity profile as a function of wavelength and applied current at fixed laser diode temperature of 20 1C. Each profile was taken at fixed current with 200 ms integration time averaged over 100 scans with a current step between two consecutive profiles of 0.5 mA. The waiting time between collecting two consecutive profiles was 5 min to ensure that the current and temperature controllers reach their equilibrium states. 0.6 0.5
Intensity (a. u.)
60
I = 60.2 mA T = 20.1 0C
0.4 0.3
3 2
0.2 0.1
4
1
0.0 446.0
446.5
447.0
447.5
448.0
Wavelength (nm) Fig. 1. A high resolution emitted spectrum of a GaN-based blue laser diode at 60.2 mA applied current and 20.1 1C temperature. The sharp peaks are the longitudinal modes, and the Arabic numbers 1, 2, 3 and 4 indicate the locations of kinks in the intensity profile of the longitudinal modes.
10
I = 100 mA T = 20 0C
8
Fig. 1 shows a typically recorded emission spectrum of used GaNbased laser diode (model LD-0445-0050-1) where a longitudinal modes bundle is exhibited with fine and well-resolved spectral substructure over the wavelength range 446.40–447.55 nm. The presented spectrum was recorded at 60.2 mA applied current and 20.1 1C temperature. The threshold current of the employed blue laser diode was observed at 20 mA applied current which agrees with the manufacturer sheet. It was observed that within the longitudinal modes bundle, that has asymmetrical profile, 21 equally spaced modes were recorded over a small spectral region of 1.15 nm which demonstrates the good resolution of the CCD camera-monochromator system. To obtain this high resolution, the width of the monochromator slit was set to 6 mm. The highest mode intensity was located at 447.20 nm, whereas a number of kinks were observed in the intensity profile of
Arbitrary unit
3. Results and discussion 6 4 2 0
0
2
4
6
8
10
12
14
Time (Hours) Fig. 2. The stability of the laser diode over time operated at 100 mA applied current and 20 1C: (a) the laser power, (b) the fractional change of the central wavelength (multiplied by 100,000 and shifted by 6 for clarity), (c) the fractional change in the applied current (multiplied by 100 and shifted by 4 for clarity), (d) the fractional change in the temperature (multiplied by 100 and shifted by 2 for clarity).
M.S. Romadhon et al. / Optics & Laser Technology 70 (2015) 59–62
61
where n is the effective refractive index of the modes. Eq. (1) above can also be given in terms of the group refractive index ngr as [19], T = 20 0C
Δλ ¼
λ2 2Lngr
;
Wavelength (nm)
where the group refractive index ngr is related to the effective refractive index n by ngr ¼ n λ
Current (mA) Fig. 3. Contour plot of the longitudinal modes intensity profile as a function of wavelength and applied current at fixed laser diode temperature of 20 1C.
Wavelength (nm)
T = 20 0C
dn : dλ
Current (mA) Fig. 4. Evolution of longitudinal modes peaks location as a function of the applied current variation at fixed 20 1C temperature. The data points of each longitudinal mode peak are fitted to a straight line.
All spectra shown in Fig. 3 were recorded at fixed laser diode temperature of 20 1C. Upon increasing the laser diode applied current over the range from 20 to 105 mA, the longitudinal modes peaks location experiences a red (bathochromic) shift in wavelength is observed. At the same time, intensity of longitudinal modes peaks were also observed to increase as a function of applied current increase. The spectra in Fig. 3 were used to plot the evolution of the wavelength location of the longitudinal mode peaks as a function of the applied current as shown in Fig. 4. The same spectra used to generate Fig. 3 were also used to track the evolution of the wavelength location of the longitudinal mode peaks as a function of the applied current as shown in Fig. 4. Each longitudinal mode evolves linearly with the current and the lines in Fig. 4 are linear fits to the data. The fitted lines are equally spaced with a mode spacing of 0:0548 7 0:002 nm and are parallel with a positive slope of 0:0045 7 0:0002 nm/mA. The measured mode spacing is typical for blue laser diodes and is relatively small compared to commonly observed mode spacing in other semiconductor lasers [19]. Theoretically, the longitudinal modes spacing Δλ can be given as a function of wavelength λ, cavity length L, and index of refraction n by the equation [22]:
λ2
2Ln
1
1
λ dn n dλ
;
ð1Þ
ð3Þ
The mode spacing Δλ can be evaluated for our laser at the wavelength λ of 447 nm by using a typical value for the group refractive index ngr of 2.90 with a cavity length of 650 μm [19]. These values yield a mode spacing of 0.0530 nm which is in excellent agreement with the experimentally measured value of 0.0548 nm. Smetanin et al. [23] suggested that the existence of electron–hole plasma oscillations in the active layer of GaN-based laser diodes can contribute to discrepancy between experimental and theoretical mode spacing. Thus, the good agreement between our experimentally measured and calculated values of the longitudinal mode spacing could be attributed to infinitesimal contribution of electron–hole plasma oscillations. The positive slope of 0.0045 nm/mA found in Fig. 4 is mainly due to resistive heating caused by the injected current as the laser is operated well above the threshold and hence the charge carrier density in the active region stays nearly constant [19]. An estimation of the change of the temperature of the laser junction as a function of the injected current can be used to find the rate of wavelength mode change with the junction temperature. This rate should be close to the rate when the current is fixed and the heat sink temperature is changed. The difference between the temperature of the junction T and the temperature of the heat sink T 0 is proportional to the difference between the input electrical power I V and the output optical power P opt T T 0 ¼ Rth ðI V P opt Þ;
Δλ ¼
ð2Þ
ð4Þ
where I is the injected current, V is the operating voltage, and Rth is the thermal resistance [19]. Thus the rate of change in the junction temperature with the injected current is dT dV dP opt ¼ Rth V þI : ð5Þ dI dI dI Putting typical values of our laser with Rth 35 K=W; V 6 V; I 70 mA; ðdV=dIÞ 50 V=A, and ðdP opt =dIÞ 0:6 W=A, gives ðdT=dIÞ 300 1C=A: Hence, dλ=dT ¼ ðdλ=dIÞðdI=dTÞ ð0:0045 nm =mAÞ ð1=300Þ A=1C ¼ 0:014 nm=1C [19,21]. This value should be compared to the value ðdλ=dTÞ ¼ 0:0154 nm=1C obtained when the heat sink temperature changed while fixing the injected current as shown in Fig. 6. Fig. 5. Shows a contour plot of the longitudinal modes intensity profile as a function of wavelength and laser diode's temperature at fixed applied current of 101 mA. It was observed that the output intensity is slowly decreasing with the diode's temperature, whereas an observed tendency for modes peaks wavelengths to experience bathochromic shift of 2.0 nm as a function of increase in temperature over the range from 5 1C to þ 55 1C. Generally, the temperature of laser diode has a direct effect on its output power as well as it emission modes wavelength locations. Microscopically, internal temperature increase of laser diode reduces the degree of optical pumping (population inversion), consequently, causing decrease in optical gain for a given charge carrier [19]. As the temperature increases, the longitudinal modes wavelengths experience a shift due to sensitivity of the laser resonator refractive index to laser diode internal temperature [19]. Fig. 6 shows the longitudinal modes peaks wavelength evolution as a function of laser diode's temperature when the applied current is kept constant at 101 mA. The same linear behavior obs-
62
M.S. Romadhon et al. / Optics & Laser Technology 70 (2015) 59–62
bathochromic shift of 0.0045 nm/mA upon increasing the laser diode applied current with a fixed temperature. Similarly, a bathochromic shift of 0.0154 nm/1C was observed upon increasing laser diode temperature at fixed current. These shifts were concluded to originate from the same physical phenomenon of resistive heating of the laser junction. In addition, the gain was found to shift to longer wavelengths at rate of 0.0432 nm/1C which is approximately three times greater than longitudinal modes wavelengths change with temperature at fixed current above threshold. Utilizing emission spectra of longitudinal modes, mode spacing was experimentally evaluated to be 0.0548 70.0020 nm which was found to excellently agree with calculated value of 0.0530 nm.
Wavelength (nm)
I = 101 mA
Acknowledgment
Temperature (0C) Fig. 5. Contour plot of the longitudinal modes intensity profile as a function of wavelength and laser diode's temperature at fixed applied current of 101 mA.
449.5
References
I = 101 mA
Wavelength (nm)
449.0 Slope = 0.0432 nm/ 0C
448.5 448.0 447.5 447.0
Slope = 0.0154 nm/ 0C
446.5 446.0 10
This work was fully supported by the Deanship of Scientific Research at King Fahd University of Petroleum & Minerals under internal research Grant numbers RG1218-1 and RG1218-2.
20
30
40
50
Temperature (0C) Fig. 6. Longitudinal modes wavelength location as function of laser diode's temperature at fixed applied current of 101 mA. The straight line with a slope 0.0432 nm/1C represents the shift in the gain while the straight lines of slope 0.0154 nm/1C slope represent the shift in modes wavelength.
erved when the temperature was fixed and current was changing, was also observed when the current was fixed and the temperature was changing. Similarly, the lines fitting the modes were parallel and equally spaced with tendency to shift to higher wavelengths with temperature. In order to realize the evolution of the diode's gain shift with temperature, a straight line fit was performed over the location of the central wavelength at each temperature. The straight line fits in Fig. 6 show clearly higher slope of 0.0432 nm/1C for the gain evolution than the slope of 0.0154 nm/1C for the modes evolution. It is typical for the blue laser diode that the gain is more sensitive to temperature than the longitudinal modes. The ratio of the gain shift rate to mode shift rate was found to be 2.8 which is comparable to the value of 2.7 obtained by Eichler et al. [17]. 4. Conclusions In summary, the longitudinal modes spectra of a commercially available GaN-based blue laser diode, Toptica Photonics model LD0445-0050-1, have been experimentally studied by employing a high resolution monochromator coupled to a line CCD camera. The laser power was found to be stable over extended period of time (14 h). The evolutions of the longitudinal modes wavelength and intensity were investigated as a function of applied current and temperature. The longitudinal modes wavelengths were observed to experience a linear
[1] Nakamura S, Senoh M, Nagahama SI, Iwasa N. Room-temperature continuouswave operation of InGaN multi-quantum-well structure laser diodes. Appl Phys Lett 1996;69:4056–8. [2] Nakamura S, Pearton S, Fasol G. The blue laser diode: the complete story. New York: Springer-Verlag; 2000. p. 215–89. [3] Nasim H, Jamil Y. Diode lasers: from laboratory to industry. Opt Laser Technol 2014;56:211–22. [4] Neukum J. Overview on new diode lasers for defense applications. Proc SPIE 2012;8547:85470X. [5] Vallon R, Li JS, Cousin J, Parvitte B, Mammez D, Joly L, et al.. Tunable diode laser measurements of self-and pressure-shifts coefficients of CO2 around 2.05 μm. In: Proceedings of the Abstract of 8th international conference TDLS-2011, paper C8. Zermatt, Switzerland, July 11–15; 2011. p. 56. [6] Ma X, Zhong L. Advances in high power semiconductor diode lasers. Proc SPIE 2007;6824:682402. [7] Najda SP. Advances in AlGaInN laser diode technology for defense applications. Proc SPIE 2013;8733:87331. [8] Pandey R, Merchen D, Stapleton D, Patterson S, Kissel H, Fassbender W, et al. Advancements in high-power diode laser stacks for defense applications. Proc SPIE 2012;8381:83810G. [9] Michalzik R, Kern A, Wahl D. Bidirectional multimode fiber interconnection. Proc SPIE 2012;8276:827617. [10] Rothenbach CA, Gupta MC. High resolution, low cost laser lithography using a Blu-ray optical head assembly. Opt Lasers Eng 2012;50:900–4. [11] Bergh AA. Blue laser diode (LD) and light emitting diode (LED) applications. Phys Status Solidi A 2004;201(12):2740–54. [12] Lou XT, Somesfalean G, Chen B, Zhang YG, Wang HS, Zhang ZG, et al. Simultaneous detection of multiple-gas species by correlation spectroscopy using a multimode diode laser. Opt Lett 2010;35:1749–51. [13] Koch J, Miclea M, Niemax K. Analysis in chlorine in polymers by laser sampling and diode laser atomic absorption spectrometry. Spectrochim Acta B: At Spectrosc 1999;54:1723–35. [14] Chen D, Fang Z, Cai H, Geng J, Qu R. Instabilities in a grating feedback external cavity semiconductor laser. Opt Express 2008;16:17014–20. [15] Jiang HX, Lin JY. Mode Spacing anomaly in InGaN blue lasers. Appl Phys Lett 1999;74:1066–8. [16] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, et al. Longitudinal mode spectra and ultrashort pulse generation of InGaN multiquantum well structure laser diodes. Appl Phys Lett 1997;70:616–8. [17] Eichler C, Schad S, Scholz F, Hofstetter D, Miller S, Weimar A, et al. Observation of temperature-independent longitudinal-mode patterns in violet-blue InGaNbased laser diodes. IEEE Photonics Technol Lett 2005;17:1782–4. [18] Ropas G, Le Floch A, Agrawal GP. Spectral and spatial dynamics in InGaN blueviolet lasers. Appl Phys Lett 2006;89:241128 (–3). [19] Scheibenzuber WG. GaN-based laser diodes: towards longer wavelengths and short pulses. Heidelberg: Springer; 2012. p. 21–3. [20] Witzigmann B, Laino V, Luisier M, Schwarz UT, Feicht G, Wegscheider W, et al. Microscopic analysis of optical gain in InGaN/GaN quantum wells. Appl Phys Lett 2006;88:021105. [21] Toptica Photonics Data Sheet for Model LD-0445-0050-1. [22] Schwarz UT, Sturm E, Wegscheider W, Kümmler V, Lell A, Härle V. Gain spectra and current-induced change of refractive index in (In/AL)GaN diode lasers. Phys Staus Solidi A 2003;200:143–6. [23] Smetanin IV, Vasil’ev PP. Enhanced longitudinal mode spacing in blue violet InGaN semiconductor lasers. Appl Phys Lett 2012;100:041113 (–041113).
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
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
art ic l e i nf o
a b s t r a c t
Article history: Received 22 August 2014 Received in revised form 15 January 2015 Accepted 26 January 2015
Spectral emission of a single-transverse Fabry–Perot GaN-based blue laser diode was experimentally investigated 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.
Keywords: Longitudinal modes Mode spacing Blue laser diode
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 recombination 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 mechanism (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
n
Corresponding author. Fax: þ 966 13 860 2293. E-mail address: [email protected] (W. Al-Basheer).
http://dx.doi.org/10.1016/j.optlastec.2015.01.012 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
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 applications 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 landmark 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 inversion, consequently, leading to decrease of laser's optical gain [19].
M.S. Romadhon et al. / Optics & Laser Technology 70 (2015) 59–62
Furthermore, many applications of GaN-based laser diodes require precise knowledge of the device temperature and its explicit effect on the emitted spectra structure and evolution. In this study, a simple experimental setup was employed to observe and record longitudinal modes of a commercial CW GaN-based blue laser diode. The temporal stability of the modes was investigated by recording the diode's longitudinal modes intensity of continuously emitted spectra as a function of lasing elapsed time. Moreover, we report experimental results of investigating the evolution and stability of emitted spectra structure as a function of applied current and diode's temperature. To the best of our knowledge, this study is the first probe of GaN-based laser diode over the blue spectral range 440– 450 nm. The reported results herein are anticipated to provide scholars and users of needed information on the operation and nature of emission spectra of GaN-based blue laser diodes.
2. Experimental setup The employed experimental setup consisted of the following components, briefly; a semiconductor blue laser diode of GaN-base (model LD-0445-0050-1), purchased from Toptica Photonics, operating over the temperature range from 10.0 1C to þ60.0 1C, and lasing wavelength (440–450 nm) was used. The laser diode was housed in a TCLDM9 laser diode mount from Thorlabs with optimal output power of 50 mW. The laser diode current and temperature were controlled using Thorlabs LDC200C and TED200C controllers, respectively. A fraction of the laser output CW beam was directed to a power meter head (PM 100D from Thorlabs), while the other fraction was directed to the monochromator opening slit using a sequence of optics; namely, a highly reflective mirror, a collimator and a 10.0 cm bi-convex lens for focusing. Beam focusing was essential to minimize divergence of laser beam, due to the laser's intrinsic property of relatively high beam divergence of angles 8.301 and 22.01 for the parallel and perpendicular directions, respectively and as reported by the manufacturer. A SPEX monochromator (Model 500M) was coupled with a 3000 pixels USB 2.0 CCD line camera of 7 μm pixel width to measure laser emitted light intensity as a function of light wavelength. The dispersion of the SPEX500M monochromator is 1.6 nm/mm. Employed monochromator was calibrated by utilizing 8 known sharp emission lines from a standard He lamp over the spectral region 388.86–447.15 nm. For monochromator calibration, the He lamp was housed in a small box with a small opening for initial collimation followed by two bi-convex lenses of 5 and 15 cm foci lengths for collimation and focusing at the monochromator entrance slit, respectively. A National Instrument-USB 6251 was used as an interface between the PC and controllers with a SC-2345 signal conditioner between the PC and controllers. A homemade LabVIEW code was utilized to acquire, record and save data.
the longitudinal modes which are specified by the Arabic numbers 1, 2, 3 and 4. These kinks are caused by lower gain at these wavelengths. The asymmetry in the bundle shape are typically observed in the GaNbased lasers, especially well above the threshold current [18], and can be explained by structural variations due to oscillating light modes interference with laser diode substrate [20]. Fig. 2 shows four curves evaluating the stability of the laser diode operated at 100 mA and 20 1C over extended period of time (14 h). The four curves represent the laser output power, the fractional change in the central wavelength, the fractional change in applied current, and the fractional change in temperature. For the sake of clarity, the fractional changes in the central wavelength, current and temperature were multiplied by a constant and shifted from zero. The data in Fig. 2 were collected every 5 min. During 14 h of lasing, the laser diode output power, central peak wavelength, applied current, and temperature exhibited relatively small variation indicating high level of temporal stability for the employed blue laser diode. Fig. 3 shows a contour plot of the longitudinal modes intensity profile as a function of wavelength and applied current at fixed laser diode temperature of 20 1C. Each profile was taken at fixed current with 200 ms integration time averaged over 100 scans with a current step between two consecutive profiles of 0.5 mA. The waiting time between collecting two consecutive profiles was 5 min to ensure that the current and temperature controllers reach their equilibrium states. 0.6 0.5
Intensity (a. u.)
60
I = 60.2 mA T = 20.1 0C
0.4 0.3
3 2
0.2 0.1
4
1
0.0 446.0
446.5
447.0
447.5
448.0
Wavelength (nm) Fig. 1. A high resolution emitted spectrum of a GaN-based blue laser diode at 60.2 mA applied current and 20.1 1C temperature. The sharp peaks are the longitudinal modes, and the Arabic numbers 1, 2, 3 and 4 indicate the locations of kinks in the intensity profile of the longitudinal modes.
10
I = 100 mA T = 20 0C
8
Fig. 1 shows a typically recorded emission spectrum of used GaNbased laser diode (model LD-0445-0050-1) where a longitudinal modes bundle is exhibited with fine and well-resolved spectral substructure over the wavelength range 446.40–447.55 nm. The presented spectrum was recorded at 60.2 mA applied current and 20.1 1C temperature. The threshold current of the employed blue laser diode was observed at 20 mA applied current which agrees with the manufacturer sheet. It was observed that within the longitudinal modes bundle, that has asymmetrical profile, 21 equally spaced modes were recorded over a small spectral region of 1.15 nm which demonstrates the good resolution of the CCD camera-monochromator system. To obtain this high resolution, the width of the monochromator slit was set to 6 mm. The highest mode intensity was located at 447.20 nm, whereas a number of kinks were observed in the intensity profile of
Arbitrary unit
3. Results and discussion 6 4 2 0
0
2
4
6
8
10
12
14
Time (Hours) Fig. 2. The stability of the laser diode over time operated at 100 mA applied current and 20 1C: (a) the laser power, (b) the fractional change of the central wavelength (multiplied by 100,000 and shifted by 6 for clarity), (c) the fractional change in the applied current (multiplied by 100 and shifted by 4 for clarity), (d) the fractional change in the temperature (multiplied by 100 and shifted by 2 for clarity).
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where n is the effective refractive index of the modes. Eq. (1) above can also be given in terms of the group refractive index ngr as [19], T = 20 0C
Δλ ¼
λ2 2Lngr
;
Wavelength (nm)
where the group refractive index ngr is related to the effective refractive index n by ngr ¼ n λ
Current (mA) Fig. 3. Contour plot of the longitudinal modes intensity profile as a function of wavelength and applied current at fixed laser diode temperature of 20 1C.
Wavelength (nm)
T = 20 0C
dn : dλ
Current (mA) Fig. 4. Evolution of longitudinal modes peaks location as a function of the applied current variation at fixed 20 1C temperature. The data points of each longitudinal mode peak are fitted to a straight line.
All spectra shown in Fig. 3 were recorded at fixed laser diode temperature of 20 1C. Upon increasing the laser diode applied current over the range from 20 to 105 mA, the longitudinal modes peaks location experiences a red (bathochromic) shift in wavelength is observed. At the same time, intensity of longitudinal modes peaks were also observed to increase as a function of applied current increase. The spectra in Fig. 3 were used to plot the evolution of the wavelength location of the longitudinal mode peaks as a function of the applied current as shown in Fig. 4. The same spectra used to generate Fig. 3 were also used to track the evolution of the wavelength location of the longitudinal mode peaks as a function of the applied current as shown in Fig. 4. Each longitudinal mode evolves linearly with the current and the lines in Fig. 4 are linear fits to the data. The fitted lines are equally spaced with a mode spacing of 0:0548 7 0:002 nm and are parallel with a positive slope of 0:0045 7 0:0002 nm/mA. The measured mode spacing is typical for blue laser diodes and is relatively small compared to commonly observed mode spacing in other semiconductor lasers [19]. Theoretically, the longitudinal modes spacing Δλ can be given as a function of wavelength λ, cavity length L, and index of refraction n by the equation [22]:
λ2
2Ln
1
1
λ dn n dλ
;
ð1Þ
ð3Þ
The mode spacing Δλ can be evaluated for our laser at the wavelength λ of 447 nm by using a typical value for the group refractive index ngr of 2.90 with a cavity length of 650 μm [19]. These values yield a mode spacing of 0.0530 nm which is in excellent agreement with the experimentally measured value of 0.0548 nm. Smetanin et al. [23] suggested that the existence of electron–hole plasma oscillations in the active layer of GaN-based laser diodes can contribute to discrepancy between experimental and theoretical mode spacing. Thus, the good agreement between our experimentally measured and calculated values of the longitudinal mode spacing could be attributed to infinitesimal contribution of electron–hole plasma oscillations. The positive slope of 0.0045 nm/mA found in Fig. 4 is mainly due to resistive heating caused by the injected current as the laser is operated well above the threshold and hence the charge carrier density in the active region stays nearly constant [19]. An estimation of the change of the temperature of the laser junction as a function of the injected current can be used to find the rate of wavelength mode change with the junction temperature. This rate should be close to the rate when the current is fixed and the heat sink temperature is changed. The difference between the temperature of the junction T and the temperature of the heat sink T 0 is proportional to the difference between the input electrical power I V and the output optical power P opt T T 0 ¼ Rth ðI V P opt Þ;
Δλ ¼
ð2Þ
ð4Þ
where I is the injected current, V is the operating voltage, and Rth is the thermal resistance [19]. Thus the rate of change in the junction temperature with the injected current is dT dV dP opt ¼ Rth V þI : ð5Þ dI dI dI Putting typical values of our laser with Rth 35 K=W; V 6 V; I 70 mA; ðdV=dIÞ 50 V=A, and ðdP opt =dIÞ 0:6 W=A, gives ðdT=dIÞ 300 1C=A: Hence, dλ=dT ¼ ðdλ=dIÞðdI=dTÞ ð0:0045 nm =mAÞ ð1=300Þ A=1C ¼ 0:014 nm=1C [19,21]. This value should be compared to the value ðdλ=dTÞ ¼ 0:0154 nm=1C obtained when the heat sink temperature changed while fixing the injected current as shown in Fig. 6. Fig. 5. Shows a contour plot of the longitudinal modes intensity profile as a function of wavelength and laser diode's temperature at fixed applied current of 101 mA. It was observed that the output intensity is slowly decreasing with the diode's temperature, whereas an observed tendency for modes peaks wavelengths to experience bathochromic shift of 2.0 nm as a function of increase in temperature over the range from 5 1C to þ 55 1C. Generally, the temperature of laser diode has a direct effect on its output power as well as it emission modes wavelength locations. Microscopically, internal temperature increase of laser diode reduces the degree of optical pumping (population inversion), consequently, causing decrease in optical gain for a given charge carrier [19]. As the temperature increases, the longitudinal modes wavelengths experience a shift due to sensitivity of the laser resonator refractive index to laser diode internal temperature [19]. Fig. 6 shows the longitudinal modes peaks wavelength evolution as a function of laser diode's temperature when the applied current is kept constant at 101 mA. The same linear behavior obs-
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bathochromic shift of 0.0045 nm/mA upon increasing the laser diode applied current with a fixed temperature. Similarly, a bathochromic shift of 0.0154 nm/1C was observed upon increasing laser diode temperature at fixed current. These shifts were concluded to originate from the same physical phenomenon of resistive heating of the laser junction. In addition, the gain was found to shift to longer wavelengths at rate of 0.0432 nm/1C which is approximately three times greater than longitudinal modes wavelengths change with temperature at fixed current above threshold. Utilizing emission spectra of longitudinal modes, mode spacing was experimentally evaluated to be 0.0548 70.0020 nm which was found to excellently agree with calculated value of 0.0530 nm.
Wavelength (nm)
I = 101 mA
Acknowledgment
Temperature (0C) Fig. 5. Contour plot of the longitudinal modes intensity profile as a function of wavelength and laser diode's temperature at fixed applied current of 101 mA.
449.5
References
I = 101 mA
Wavelength (nm)
449.0 Slope = 0.0432 nm/ 0C
448.5 448.0 447.5 447.0
Slope = 0.0154 nm/ 0C
446.5 446.0 10
This work was fully supported by the Deanship of Scientific Research at King Fahd University of Petroleum & Minerals under internal research Grant numbers RG1218-1 and RG1218-2.
20
30
40
50
Temperature (0C) Fig. 6. Longitudinal modes wavelength location as function of laser diode's temperature at fixed applied current of 101 mA. The straight line with a slope 0.0432 nm/1C represents the shift in the gain while the straight lines of slope 0.0154 nm/1C slope represent the shift in modes wavelength.
erved when the temperature was fixed and current was changing, was also observed when the current was fixed and the temperature was changing. Similarly, the lines fitting the modes were parallel and equally spaced with tendency to shift to higher wavelengths with temperature. In order to realize the evolution of the diode's gain shift with temperature, a straight line fit was performed over the location of the central wavelength at each temperature. The straight line fits in Fig. 6 show clearly higher slope of 0.0432 nm/1C for the gain evolution than the slope of 0.0154 nm/1C for the modes evolution. It is typical for the blue laser diode that the gain is more sensitive to temperature than the longitudinal modes. The ratio of the gain shift rate to mode shift rate was found to be 2.8 which is comparable to the value of 2.7 obtained by Eichler et al. [17]. 4. Conclusions In summary, the longitudinal modes spectra of a commercially available GaN-based blue laser diode, Toptica Photonics model LD0445-0050-1, have been experimentally studied by employing a high resolution monochromator coupled to a line CCD camera. The laser power was found to be stable over extended period of time (14 h). The evolutions of the longitudinal modes wavelength and intensity were investigated as a function of applied current and temperature. The longitudinal modes wavelengths were observed to experience a linear
[1] Nakamura S, Senoh M, Nagahama SI, Iwasa N. Room-temperature continuouswave operation of InGaN multi-quantum-well structure laser diodes. Appl Phys Lett 1996;69:4056–8. [2] Nakamura S, Pearton S, Fasol G. The blue laser diode: the complete story. New York: Springer-Verlag; 2000. p. 215–89. [3] Nasim H, Jamil Y. Diode lasers: from laboratory to industry. Opt Laser Technol 2014;56:211–22. [4] Neukum J. Overview on new diode lasers for defense applications. Proc SPIE 2012;8547:85470X. [5] Vallon R, Li JS, Cousin J, Parvitte B, Mammez D, Joly L, et al.. Tunable diode laser measurements of self-and pressure-shifts coefficients of CO2 around 2.05 μm. In: Proceedings of the Abstract of 8th international conference TDLS-2011, paper C8. Zermatt, Switzerland, July 11–15; 2011. p. 56. [6] Ma X, Zhong L. Advances in high power semiconductor diode lasers. Proc SPIE 2007;6824:682402. [7] Najda SP. Advances in AlGaInN laser diode technology for defense applications. Proc SPIE 2013;8733:87331. [8] Pandey R, Merchen D, Stapleton D, Patterson S, Kissel H, Fassbender W, et al. Advancements in high-power diode laser stacks for defense applications. Proc SPIE 2012;8381:83810G. [9] Michalzik R, Kern A, Wahl D. Bidirectional multimode fiber interconnection. Proc SPIE 2012;8276:827617. [10] Rothenbach CA, Gupta MC. High resolution, low cost laser lithography using a Blu-ray optical head assembly. Opt Lasers Eng 2012;50:900–4. [11] Bergh AA. Blue laser diode (LD) and light emitting diode (LED) applications. Phys Status Solidi A 2004;201(12):2740–54. [12] Lou XT, Somesfalean G, Chen B, Zhang YG, Wang HS, Zhang ZG, et al. Simultaneous detection of multiple-gas species by correlation spectroscopy using a multimode diode laser. Opt Lett 2010;35:1749–51. [13] Koch J, Miclea M, Niemax K. Analysis in chlorine in polymers by laser sampling and diode laser atomic absorption spectrometry. Spectrochim Acta B: At Spectrosc 1999;54:1723–35. [14] Chen D, Fang Z, Cai H, Geng J, Qu R. Instabilities in a grating feedback external cavity semiconductor laser. Opt Express 2008;16:17014–20. [15] Jiang HX, Lin JY. Mode Spacing anomaly in InGaN blue lasers. Appl Phys Lett 1999;74:1066–8. [16] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, et al. Longitudinal mode spectra and ultrashort pulse generation of InGaN multiquantum well structure laser diodes. Appl Phys Lett 1997;70:616–8. [17] Eichler C, Schad S, Scholz F, Hofstetter D, Miller S, Weimar A, et al. Observation of temperature-independent longitudinal-mode patterns in violet-blue InGaNbased laser diodes. IEEE Photonics Technol Lett 2005;17:1782–4. [18] Ropas G, Le Floch A, Agrawal GP. Spectral and spatial dynamics in InGaN blueviolet lasers. Appl Phys Lett 2006;89:241128 (–3). [19] Scheibenzuber WG. GaN-based laser diodes: towards longer wavelengths and short pulses. Heidelberg: Springer; 2012. p. 21–3. [20] Witzigmann B, Laino V, Luisier M, Schwarz UT, Feicht G, Wegscheider W, et al. Microscopic analysis of optical gain in InGaN/GaN quantum wells. Appl Phys Lett 2006;88:021105. [21] Toptica Photonics Data Sheet for Model LD-0445-0050-1. [22] Schwarz UT, Sturm E, Wegscheider W, Kümmler V, Lell A, Härle V. Gain spectra and current-induced change of refractive index in (In/AL)GaN diode lasers. Phys Staus Solidi A 2003;200:143–6. [23] Smetanin IV, Vasil’ev PP. Enhanced longitudinal mode spacing in blue violet InGaN semiconductor lasers. Appl Phys Lett 2012;100:041113 (–041113).