ICTON 2000 41 MO.D.5

REGULAR OSCILLATION IN TWO-SECTION LASER STRUCTURES BASED ON &DOPED SUPERLATTICES

D.V. Ushakov, V.K. Kononenko, and I.S. Manak

Belarussian State Universiv Fr. Scorina Pr., 4, 220050 Minsk, Belarus

E-mail: UshakovD V@rfe. bsu. unibel. by

Regular light pulse oscillators are used in devices of integrated optics, ranging, detector calibration, recording, and automatic handling. In many cases, wavelength tuning is necessary. Such a regime is reached at inhomogeneous excitation of two-section laser oscillators and in sources with wide gain spectra. Large possibilities to tune output power and spectral characteristics appear in laser structures based on doping superlattices or n - i - p i crystals.

In the doping superlattices, the tuning of the energy and spectral characteristics is determined by changing the potential profile depth 2AV and effective band gap E; versus the crystal excitation level. To obtain a wide tuning range it is necessary to use doping superlattices with a deep potential profile. The largest value of 2AV has been realized in &doped superlattices at definite superlattice periods d [ 1, 21. Besides, in the structures with &doped layers, the required gain can be obtained at enough low excitation levels since enhanced overlap of the electron and hole wave functions [3,4].

In the present work, performance of two-section laser structures with the GaAs &doped superlattice is analyzed. The superlattice structure has four periods, equal sheet donor and acceptor concentrations n d = n, = 1 OI3 cm-2, and intrinsic layers of the thickness d, = 9 nm. Calculations of the potential energy profile, energy levels and wave functions of electrons and holes have been performed by the self-consistent solving of the Schrodinger and Poisson's equations taking into account the carrier screening, band gap narrowing, and impurity concentration fluctuations [3]. The gain and radiative recombination rate are found in the k-selection rule model [ 51.

The tuning range at a fixed gain in the total spectrum maximum k(h) of the two- section laser depends on the relative length of the first r1 and second r2 sections (rI + 1-2 = l), i.e., k(h) = rlkl(h) + rzk2(h), where kl and k2 are the gain coefficients in the first and second sections. The total gain k(h) in the spectrum maximum is equal to the losses kl at the continuous wave operation. For the fixed loss coefficient, the maximum value of the tunable lasing wavelength is reached where the surface electron concentrations in the first nl and second n2 laser sections equal. At increasing the electron concentration in the first (amplifying) section, excitation level of the second (absorbing) section will be lower because of the threshold condition requirement. As a result, the lasing wavelength at the total spectrum maximum decreases. For the two-section lasers based on the GaAs &doped superlattices, the continuous tuning range of the total gain maximum can reach up to 60 nm in near ifra-red [6]. The same tuning range is obtained in the transient regime [7].

Below, possibilities of wavelength tuning in the pulsed mode regime are examined. Dynamic processes in the two-section laser are described by the following equation system [7]

0-7803-6337-X/OO/$IO.O0 02000 I EEE

Mo.D.5 42 ICTON 2000

Here, j , is the current density in the first ( i = 1) and second ( i = 2) sections, R, is the spontaneous recombination rate, qq, is the quantum yield, vg is the group velocity of light, E is the nonlinear gain parameter, /3 determines the contribution of spontaneous transitions into lasing modes. Note, that in non-stationary conditions several modes are generate. Therefore, in Eqs. (1) and (2), the photon density set S, for N modes (j = l...N) is presented The total photon density Slot is defined as a sum of all modes and the lasing tunable wavelength is determined as a value averaged by all mode intensities.

The pulsed mode regime occurs if the stationary solution of the kinetic equation system (1)-(2) is unstable. Current carriers in the absorbing section have large life-times (= 1 ps) at small excitation levels because of the potential wells for electrons and holes in the doping superlattice are space-shifted. As a result, the steady-state and pulsed mode regimes are realized at the reverse current in the absorbing laser section. At the introducing of an additional non-radiative recombination channel with the life-time constant znr2 = 1 ns, for example by the ion-defect implantation [SI, the wavelength tuning can exist at a forward current in both laser sections.

The calculations of generation regime boundaries have been performed in the case where the luminescence quantum yield in the first section 711 = 1 and it is defined in the second section by the formula

In Fig. 1 , the region of current densities j l in the first section and j2 in the second section corresponding to regular lasing oscillation is contained between curve 2 of the stationary switch-on and curve 1 of the threshold.

Fig. 1. Regions of different lasing regimes in the two-section laser. (1) threshold curve, (2) stationary threshold curve. k,= 50 cm- , zn,2 = 1 ns, E = 5x10- cm , Y I = 0.7. I 18 3

In Fig. 2, time dependencies of the electron concentration in the amplifying M I and absorbing n2 laser sections and total photon density SI,, are shown under the current step

ICTON 2000 43 MO.D.5

excitation. Calculations have been performed for inter-modal separation 1.4 meV that corresponds to the cavity length of 100 pm. Pulses with the shortest wavelength at 1017 nm are realized at small current densities of 10 A/cm2 in the absorber. At decreasing the excitation of the gain section and, correspondingly, at increasing the excitation in the absorber, the averaged regular oscillation wavelength is swiping up to 1062 nm. In addition, the period and amplitude of the pulses become smaller.

n1(10’~m-~)

8 8 - - - - i

6 . . . . . : . . . . . . . . . I . . . . . . . . . . . . . . . . . . . . . . . . . . :

t i 4 i ;

2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 10 12 14 t(ns)

(10’2m-2)

/

_ .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I I I I I 6 8 10 12 14 t(ns)

Stot

12

8

4

Stoi

6

4

117

053

I18

054

8 10 12 14 t(ns)

(b)

I I

- I 2

6 8 10 12 14 t (ns)

Fig. 2. (a, c) Dynamics of changes in the surface electron concentration n and (b, d) total photon density S,,, in the pulsed mode regime at different current densities in the first (1) and second (2) laser sections. (a, b ) j l = 300 A/cm2, j 2 = 10 A/cm2 and (c, d ) j l = 200 A/cm2,

j 2 = 390 A/cm*. Numbers at the peaks of the curves &,,(t) correspond to the averaged wavelength La”. kl= 50 cm- , T~~~ = 1 ns, E = 5x 10- cm , YI = 0.7. I 18 3

Mention, that in the regular oscillation regime of the two-section laser based on the GaAs &doped superlattice the bi-pulse generation at two separated wavelengths is possible (Fig. 2 (b)). The appearance of long-wavelenght pulse A2 after the first short-

Mo.D.5 44 ICTON 2000

wavelenght and more powerful pulse 21 is explained as followed (Fig. 3). After finishing the first pulse A1 the non-equilibrium carrier concentration increases in the absorber and it decreases in the gain laser section. The total gain coefficient at this wavelength is small and its maximum is shifted to the longer wavelength. If the total gain coefficient is larger than the losses in some wavelength range, then lasing pulse at the longer wavelenght 2 2 exists. Fo the described laser structure the bi-pulse generation occurs in the pulsed mode regime at the wavelengths 10 18 and 1054 nm. The radiation pulse duration can make up 30 ps.

Fig. 3. Spectral dependencies of the gain coefficients in (1) the first 0.7kl and (2) second 0.3k2 sections and of the total gain coefficient k (bold curves) versus time (a) t = 14.10, (b) 14.15 , and (c) 14.19 ns. The dotted lines correspond to the wavelengths at which k(h) = k,. j l = 300 A/cm*,jz= 10 A/cm2, k,= 50 cm- , ~ , , ~ 2 = 1 ns, E = 5x10- cm , r l = 0.7. 1 18 3

In summary, it is theoretically shown that in two-section laser structures based on &doped superlattices a broadened spectral tuning is realised under special requirements for the parameters and excitation conditions in the gain and absorbing sections. The tuning occurs in the stationary operation, during transient process, and regular pulsation regime as well. In the GaAs &doped superlattice lasers, the tuning range covers of up to 60 nm in near infra-red.

References

1 . 2. 3. 4.

5. 6.

7. 8.

E.F. Schubert. Surface Sci. 228 (1 990) 240-246. E.F. Schubert. Opt. Quantum Electron. 22 (1 990) S 14 1 -S 186. V.K. Kononenko, I.S. Manak, D.V. Ushakov. Proc. SPIE. 3580 (1998) 10-27. D.V. Ushakov, V.K. Kononenko, I.S. Manak. Nonlinear Phenomena in Complex Systems. (Minsk, 1999) 144- 15 1 . D.V. Ushakov, V.K. Kononenko, I.S. Manak. J. Appl. Spectr. 66 (1999) 71 1-715. D.V. Ushakov, A.A. Afonenko, I.S. Manak. Materials IV Int. Conf. on Laser Physics and Spectroscopy. 1 (Grodno, 1999) 125- 127. D.V. Ushakov, A.A. Afonenko, I.S. Manak. Lithuanian J. Phys. 39 (1999) 361-365. A.B. Juravlev, V.A. Maruschak, E.L. Portnoi, et al. Sov. Phys. Semicond. 22 (1988) 352-354.