The Report of the Characteristics of Semiconductor Laser Experiment

The Report of the Characteristics of Semiconductor Laser Experiment Masruri Masruri (186520) 22/05/2008 1 Laboratory Setup The experiment consists of two kind of tasks: To measure the caracteristics of Power versus Injected Current (P vs I) of laser by varying the temperature value (10 o, 25 o, and 50 o C). Here we have to estimate the value of threshold current, I th, using the fitting method of the two segments which are the segment before the lasing and the segment after the lasing. We also estimate the conversion efficiency of the laser for the indicated temperature. To perform this task we use the Power Meter which is depicted in Fig. 1. Fig.1a: The Laboratory Setup for Estimating the Threshold Current In Fig. 1, we connect the laser which has been connected with the controller (to control the temperature and the injected current) to the Optical Sensor which has been connected with the Power Meter to measure the power. The following are the procedures to setup the experiment: 1. Make sure that laser is off. 2. Connect the interface of the fiber connector to the input of Optical Sensor. 3. Power on the power meter. 4. Power on the laser 5. Set the value of the indicated temperature, for example 10 o C and fix it. Varying the injected current of the laser and see the power output on the power meter s display for each of the injected current. Do the same procedure for the other indicated temperatures (25 o and 50 o C) Using the Optical Spectrum Analyzer (OSA) to: - measure the slope of wavelength, λ, with the varies of temperatures from 10 o and 50 o C, fixing the injected current at 100mA. - Measure the slope of wavelength, λ, with the varies of the injected current from 20 ma to 150 ma, fixing the temperature at 25 o C. 1

- Use the linear estimation to valutate the dependence from the two different parameters, T and I. To perform this task we use the Power Meter which is depicted in Fig. 2. Fig. 1b: The Laboratory Setup for Estimating the Dependence of Wavelength from Temperature and Current. In Fig. 2, we connect the laser, which has been conneted to the controller (to control the temperature and the current), to the OSA to measure the central wavelength and its peak power. The following are the procedures to setup the experiment: 1. Make sure that laser is off. 2. Connect the interface of the fiber connector to the input of OSA. 3. Power on the OSA. 4. Turn on the laser. 5. Set the parameter of the OSA. The parameters of OSA that we have to setup are: - Resolution bandwidth The ability of OSA to display two signal closely spaced in wavelength as two distinct responses is determined by the wavelength resolution. Wavelength resolution is determined by the bandwidth of the optical filter. The term of resolution bandwidth is used to describe the width of the optical filter in an OSA. - Sensitivity Sensitivity is defined as the minimum detectable signal and is defined as six times the root-mean-square noise level of the instrument. - Span The minimum wavelength and the maximum wavelength is desired to display in the monitor. 2 Fitting Method In the experiment 1, we would like to find a threshold current of the semiconductor laser. The method that we use is to divide the curve into two segments which are the segment before the lasing and the segment after the lasing. Then we make the fitting line to each of these segments. We then find the intersection point between these fitting lines. The point of I of this intersection is the estimated threshold current for the laser. Fig 3 describes the method. The following is the method to find the threshold current: 1. Divide the data from the experiment, the first group is the data before the lasing happened and the second is the data in which laser start to lase. We choose the current (which is the independent variable that we change during the the experiment) in which it start to produce the power which has considered in the range of the mw. For example the amount 0.98 mw is considered the laser to start to lase, while the amount of 0.0012 mw is considered not yet. 2. From these two groups of data we do fitting to using the Least Square Method to find the line regression of the data. 3. From the line we can find the intersection point to obtaine the threshold current. 2

3 Minimum Linear Square Fig. 2: Fitting of Two Segments From the experiment, let say that the independent variable (current) is called x i, and the dependent variable (Power) we call y i. we can find the linear approach using the equation. P (x i ) = a 1 x i + a 0 (1) We have to find a 1 and a o such that the line can pass the points with the minimum error which can be formulated as below: E(a 0, a 1 ) = y i P (x i ) With the least square method the error function is modified as below: E(a 0, a 1 ) = Subsitute to the equation (1) we obtaine E(a 0, a 1 ) = [y i P (x i )] 2 [y i (a 1 x i + a 0 )] 2 The error E(a 0, a 1 ), will be maximum/minimum if satisfies the the requirement where i = 0, 1. In this case we find E(a 0, a 1 ) a 0 = a 0 2 E(a 0, a 1 ) a i = 0 [y i (a 1 x i + a 0 )] 2 = 0 [y i a 1 x i a 0 )( 1) = 0 3

a 0.m + a 1 m x 1 = y i And E(a 0, a 1 ) a 1 = a 1 [y i (a 1 x i + a 0 )] 2 = 0 2 [y i a 1 x i a 0 )( x i ) = 0 m a 0 x 1 + a 1 m x 2 1 = y i And we obtaine a 0 = m m x2 i y i m x m iy i x i m( m x2 i ) ( m x (2) i) 2 a 1 = m m x iy i m x m i y i m( m x2 i ) ( m x (3) i) 2 Subsitute (2) and (3) to (1) to obtain the fitting equation. We will use this least square method to find the linear regression for both the experiment 1 and experiment 2. 4 Experiment I Using the fitting method which have been expained in the previous section, we find the linear equation and also the current threshold which is the intersection between the fitting lines. The results are summarized in Table 1. Temperature Line Line Current Conversion Efficiency ( C) Equation 1 Equation 2 Threshold (ma) of Laser (mw/ma) 10 0.001195776 12.90368838 20.398 0.644229549 +0.000386667x i +0.644229549x i 25 0.011535177 12.0898828 21.008 0.57669286 +0.001751647x i +0.57669286x i 50 0.102305636 9.445754605 24.1996 0.396285886 +0.010186882x i +0.396285886x i Table 1. The Fitting Lines, Threshold Current and Conversion Efficiency of Laser for the temperature 10 o, 25 o, and 50 o C. The curves for each of the temperature are depicted in Figure 4 - Figure 9. Note that for all of these curves using the legend as below: The conversion efficiency (slope efficiency) of the laser can be obtained from the slope of the curve after the lasing [8], in this case is the slope of the line equation 2 which is the equation of the 4

splitting line after the lasing. η d (I, T ) = dp (4) di From Table 1, the conversion efficiency of the laser is 0.644229549, 0.57669286, and 0.396285886 mw/ma for 10 o C, 25 o C, and 50 o C, respectively. Fig. 3 describes the conversion efficiency as a function of temperature. It tells us that the efficiency decreases with an increase in the temperature. Fig. 3: Conversion efficiency as a function of temperature Fig. 4,6,8 are the curves for each of the indicated temperature, and the threshold current we obtain by zooming the curves in are indicated in Fig. 5,7,9 for the temperatures 10 o,25 o, and 50 o, respectively. 5 Experiment II The result of experiment 2 using the least square method is described in Table 2. Description Fitting Equation slope Fixing I at 100 ma, varying T between 975.518761 + 0.312256248x i λ versus T : 0.312256248 10 o and 50 o C. (Central Wavelength) 4.569116508 0.034021823x i P peak versus T : - 0.034021823 (Peak Power) Fixing T at 25 o C, varying I between 980.2914604 + 0.027351485x i λ versus I : 0.027351485 20 ma and 150 ma (Central Wavelength) 11.49497215 + 0.12493255x i P peak versus I : 0.12493255 (Peak Power) Table 2. The fitting lines for each of the reference temperature and injected current. Fig.10 describes that by fixing the injected current, I, at 100mA the central wavelength, λ, increases as the increase of the temperature, T, between 10 o C until 50 o C. The fitting line shows 5

Fig. 4: P versus I, fixing the temperature at 10 o C Fig. 5: Estimated threshold current at 10 o C 6

Fig. 6: P versus I, fixing the temperature at 25 o C Fig. 7: Estimated threshold current at 25 o C 7

Fig. 8: P versus I, fixing the temperature at 50 o C Fig. 9: Estimated threshold current at 50 o C 8

that the λ increases linearly as the temperature increases. From the Table 1, it is shown that the slope for the dependence between λ and T is 0.312256248. Fig. 10: λ versus T, fixing I at 100mA (OSA with resolution bandwidth 0.2 nm, sensitivity HIGH 1, span 925, 1025nm) Figure 11 describes that by fixing the injected current, I, at 100mA the peak power, P peak, which is the Power (dbm) at the central wavelength, decreases as the increase of the temperature, T, between 10 o C and 50 o C. The fitting line shows that the P peak decreases linearly as the temperature increases. Table 2, it is shown that the slope for the dependence between P peak and T is -0.034021823. Figure 12 describes that by fixing the temperature, T, at 25 o C the central wavelength, λ, increases as the increase of the injected current, I, between 20mA and 150mA. The fitting line shows that the λ increases linearly as the temperature increases. From the Table 2, it is shown that the slope for the dependence between λ and I is 0.027351485. Figure 13 describes that by fixing the temperature, T, at 25 o C the peak power, P peak, increases as the increase of the injected current, I, between 20mA and 150mA. The fitting line shows that P peak increases linearly as the injected current increases. Table 2 describes that the slope for the dependence between P peak and I is 0.12493255. Figure 14 describes the spectrum of laser using OSA at the reference temperature 25 o C. Figure 15 describes the spectrum of laser using OSA as the temperature varies from 10 o, 25 o, and 50 o C, respectively. It is shown that the central wavelength is shifted to the right as the temperature increases, and the Power (dbm) decreases as the temperature increases. 6 Theoritical Analysis 6.1 Threshold Current Varies with the Temperature The lasing threshold current of injection lasers can have related exponential dependences on temperature is reported by Pankove [1]. The Pankove equation can be written by: I th = I 0 exp T T 0 (5) Where I 0, is the threshold current extrapolated to T = 0 o K and T 0 is a coefficient which is called caracteristic temperature. 9

Fig. 11: P peak vs T, fixing I at 100mA (OSA with resolution bandwidth 0.2 nm, sensitivity HIGH 1, span 925, 1025nm) Fig. 12: λ versus I, fixing T at 25 o C (OSA with resolution bandwidth 0.2 nm, sensitivity HIGH 1, span 925, 1025nm) 10

Fig. 13: P peak versus I, fixing T at 25 o C (OSA with resolution bandwidth 0.2 nm, sensitivity HIGH 1, span 925, 1025nm) Fig. 14: Spectrum Laser using OSA (resolution bandwidth 0.05nm, sensitivity HIGH 3, temperature 25 o C, current 100mA) 11

Fig. 15: Spectrum Laser OSA based on temperature (resolution bandwidth 0.05nm, sensitivity HIGH 3, temperature 25 o C, current 100mA) If we take into account in the two different temperatures let say T 1 and T 2, we can find the characteristic temperature, T 0, using the equations: [2] I th1 = I 0 exp T 1 T 0 (6) Dividing (6) by (7) gives I th2 = I 0 exp T 2 T 0 (7) I th1 I th2 = e T1 T2 T 0 (8) T 0 can then be determined by taking the natural log of both sides of (8) and rearranging T 0 = T 1 T 2 ln(i th1 /I th2 ) T 0 is a measure of the sensitivity of the laser to changes in temperature. If it is very large, the threshold current I th will not vary greatly with changes in temperature, on the other hand if T 0 is small, the threshold current varies with the temperature.researchers have investigated the factors that influence a low T 0. Some factors which has been investigated by Asada [3] are depicted in Fig. 16. The threshold condition of semiconductor lasers can be expressed as the gain being equal to the total losses. This condition determines the threshold carrier density n th since the material gain and the loss depend on the carrier density. n th and the carrier lifetime τ s, determine the threshold current I th. Thus, the temperature characteristics of I th are determined by those of the gain, the loss, and the carrier lifetime. The intervalence band absorption is related to the loss and reduces the differential quantum efficiency η d, while the nonradiative recombination (in particular, the Auger effect) and the carrier leakage over the heterobarrier determine the carrier lifetime. These relations are schematically shown in Fig. 16. Li [2] had used simple model based on his observation that researchers had evaluated three factors that cause a low T 0, which is current leakage (coef A), net optical gain (coef B), and Auger recombination (coef C). The simple model of threshold current that had been used by Li: (9) 12

Fig. 16: The process investigating the temperature characteristics of the threshold current in the paper of Asada [3] Where: q = electron charge ; t=active layer thickness ; J th qt = An th + Bn 2 th + Cn 3 th (10) If we denote the temperature sensitivity for the threshold density as dn th dt sensitivity can be written as: dj th dt = qt(a + Bn th + Cn 2 th) dn th dt then the temperature From the simulation based on the three factors explained before, Li [2] has suggested that Auger recombination and current leakage through diffusion over the barrier are considered two major paths for leakage currents responsible for low T 0 in InGaAsP lasers. The leakage current is caused by electrons and holes passing the active region without recombination. Another experiments that give the same conclusion to Li that auger recombination plays a significant role for the temperature sensitivity of the threshold current (low T 0 ) are Dutta [4], and Haug[5]. 6.2 Auger Recombination In semiconductors an Auger transition occurs when an electron and a hole recombine and release energy to another electron or hole nearby in the crystal. The energy released by the captured carrier in multiphonon emission is used to generate lattice phonons[6]. There are two types of Auger processes in semiconductors, direct auger recombination which is dominant in narrow-gap semiconductors, and phonon-asisted auger recombination which is dominant in wide-gap semiconductors. Direct auger recombination is also called phonon-less auger recombination. Further, both the phonon-less and phonon-assisted auger processes are divided by CHCC auger process (CHCC-AP) and CHHS auger process (CHHS-AP). In CHCC, energy is transfered to an electron, while in CHHS, energy is transfered in a hole (see Fig.17). Phonon-less AP are strongly temperature-dependent, in contrast to phonon-assisted AP[7]. To understand the relationship between threshold current and the auger recombination we can refer to the formula as follow: (11) I th = qn th τ c = q τ c (N 0 + 1 G N τ p ) (12) The exponential increase in the threshold current with temperature which has been explained using Pankove formula, can be understood from Eq. (12). The carrier lifetime τ c is generally N dependent because of Auger recombination and decreases with N as N 2. N is carrier population. 13

Fig. 17: CHCC-AP (left) and CHHS-AP (right) E g - band-gap; E c ( E v) - conduction (valence) band barrier offset; SO - spin-orbital splitting The rate of Auger recombination increases exponentially with temperature and is responsible for the temperature sensitivity of InGaAsP laser.[9] 6.3 Power Peak Decreases with an Increase in the Temperature Fig. 11 describes that by fixing the injected current at 100mA, the peak power increases as an increase in the temperature. This phenomenon can be explained as follow:[9] For I > I th, the photon number P increases linearly with I as The emitted power P e is related to P by the relation P = τ p q (I I th) (13) P e = 1 2 (ν gα mir )hωp (14) Equation (13) shows that P depends on the injected current, I, and threshold current, I th (as τ p and q constants). As I is fixed, in this case 100mA, P depends only on the I th. Here P decreases with an increase of I th. From Table 1 we know that the threshold current increases as an increase of the temperature. Here we can say that P decreases with an increase of the temperature. Since from equation (14), the emitted power depends on the photon number P we can conclude that an increase in the temperature decreases the emitted power. In this experiment the peak power is the emitted power of the central wavelength. 6.4 The Central Wavelength Shifts as Temperature Varies The refractive index of silica varies linearly with temperature via the thermal expansion and the thermooptic effects [10]. To explain the relationship between wavelength and the refractive index for simplicity we can use the equation for the fabry-perot with the length L.[11] λ = 2nL (15) m2 From the eq. (15), the wavelength increases linearly with an increase in the refractive index. Since the refractive index varies linerarly with temperature, this impacts that wavelength changes linearly with the temperature. The increase of injected current can also cause the heat in the active layer which change the refractive index and finally can shift the wavelength. 14

7 Conclusion We can summarized the analysis of the experiment as follow: Threshold current depends on the temperature. Threshold current increases with an increase in the temperature. Fixing the injected current, the central wavelength shifts as the increase of temperature. Fixing the injected current, the peak power decreases with an increase of temperature. Fixing the temperature, the central wavelength shifts as the increase of injected current. Fixing the temperature, the peak power increases with an increase in the injected current. References [1] J. I. Pankove, Temperature dependence of emission efficiency and lasing threshold in laser diodes, IEEE J. Quantum Electron.,Vol. QE-4, pp. 119-122, April 1968. [2] Z.-M. Li and T. Bradford, A comparative study of temperature sensitivity of InGaAsP and AlGaAs MQW lasers using numerical simulations, IEEE J. Quantum Electron., Vol. 31, pp. 1841-1847, October 1995. [3] M. Asada and Y. Suematsu, The effects of loss and nonradiative Recombination on the Temperature Dependence of Threshold Current in 1.5-1.6 pm GaInAsP/InP lasers, IEEE J. Quantum Electron.,vol. QE-19, pp. 917-923, June 1983. [4] N. K. Dutta and R. J. Nelson, Temperature dependence of the lasing characteristics of the 1.3 µm InGaAsP-lnP and GaAs AI 0.36 Ga 0.64 As DH Lasers, IEEE J. Quantum Electron.,vol. QE-18, pp. 871-878, May 1982. [5] A. Haug, Theory of the temperature dependence of the threshold current of an InGaAsP Laser, IEEE J. Quantum Electron.,vol. QE-21, pp. 716-718, June 1985. [6] F. A. Riddoch and M. Jaros, Auger recombination cross section associated with deep traps in semiconductors, J. Phys. C: Solid St. Phys.,pp. 6181-6188, June 1980. [7] A. Haug, Evidence of the importance of Auger Recombination for InGaAsP lasers, Electronic Letters 19 th, Vol. 20, pp. 85-86, January 1984. [8] U. Menzel et.all, Modelling the temperature dependence of threshold current, external differential efficiency and lasing wavelength in QW laser diodes, Semicond. Sci. Tech., pp. 1382-1392, June 1995. [9] G.P. Agrawal, Fiber-Optic Communication Sytem, 3 rd ed., John Wiley and Son, 2002. [10] M. Douay et.all, Thermal Hysteresis of Bragg Wavelengths of Intra-core Fiber Gratings, IEEE Photonics Tech. Letters, Vol. 5, pp. 1331-1334, November 1993 [11] S. Selleri. Laser a Semiconduttore. pp. 13. Universita degli Studi di Parma, 2007. 15