The buried heterostructure (BH) semiconductor laser finds favour in optical transmission systems due to its stable transverse mode property. Investigations into BH structure lasers have been per-formed [1] including use of a blocking current [2] in order to realize an optimized structure design. An effective device structure for reducing leakage current in BH laser diodes with semiin-sulation doped Fe in InP blocking layers has been analyzed [3].

In this study, the leakage current and light output power as a function of the leakage current path is examined. The blocking region used follows p-n-p-n structure.

At the threshold current, the BH laser with a 0.3um leakage path, the quantum efficiency is over 20%. For higher injection currents, the leakage current increases and the light power will become saturated.

2. Device Structure

Since the BH laser examined in this article has a symmetrical structure, only the right hand side is simulated. Figure 1 shows a schematic cross-sectional view of the structure examined. This is the conventional BH laser structure; it consists of a p-n-p-n InP blocking layer by the side of the p- InGaAsP active region, giving a lasing wavelength of 1.55um. The cavity length is 250um, the thickness and width of the active InGaAsP region is 0.15um and 1um respectively. The reflectivity of the front and rear facets is 31%. To simulate the leakage path, two blocking layers are chosen to be variable whilst keeping the other blocking layers fixed. The thickness, delta t, is varied from from 0.1um to 0.5um.

Figure 1. Schematic cross-sectional view of the half BH structure.

3. Results and Discussions

To get the photon density during the current injection, Laser works in conjunction with Blaze to solve the two dimensional Helmholtz equation in order to calculate the transverse optical field profile. It also allows calculation of the carrier recombination rate, optical gain and laser output power. Poisson’s equation, equation 1, and the electron and hole continuity equations, equations 2 and 3 respectively, are solved together with the photon rate equation. All of the terms in the equations detailed in this article are described in reference 4.

(1)

(2)

(3)

The recombination term of thr carrier continuity equation includes the (non-radiative) SRH and Auger r combination rates, and the (radiative) spontaneous and stimulated recombination rates.

The optical gain of the laser links the electrical behaviour of the laser to the optical behaviour of the laser. The Laser module within ATLAS supports four gain models. The coupling between the stimulated carrier recombination rate (Rst) and the photon density is given by equation 4.

(4)

where E(x, y) is optical field distribution.

To determine the photon density S, Laser solves the photon rate equation [4], equation 5.

(5)

(6)

The photon lifetime, equation 6, is used to represent all of the losses in the laser [4].

In this study, the BH structure was chosen so that only the right hand side was modeled i.e. use was made of the symmetrical structure. The boundary condition was chosen so that at the top and bottom and along the right side of the structure the optical electric field is 0; the left side is set to mirror the boundary.

The recombination term was chosen to only consider the SRH, Auger and radiative recombination rate. The photon lifetime only considered the mirror losses. The free carrier loss and absorption loss were ignored.

The parameter of leakage current analysis of the BH laser is shown in Figure 2. The variable delta t varies from 0.1 to 0.5um. The active layer channel region was fixed at 2.5um.

Figure 2. The leakage current path variable parameter.

In Figure 3, the light output power as a function of anode current is shown for several deltat values. The dark line is the point at which the efficiency is 22.6% at threshold current. For low injection currents i.e. < 20mA, all of the ratios of drive current to emitted light are the same. For injection currents >20mA, the efficiency is to be low.

Figure 3. Light Power per Facet Anode Current characteristics.

The anode voltage and anode current characteristics are shown in figure 4. In this study the lasing threshold voltage is 1.15V. For voltages over 1.2V, the anode current is significantly dependent on the value delta t takes. To investigate a value of 0.3um for delta t was chosen since this represents a common device value. Figures 5,6 and 7 show the energy band diagram for the active region and blocking region.

Figure 4. Anode Current- Anode Voltage Characteristics.

In figure 5 at an anode voltage 1.1V, the current flow is seen to be through only the active region.

Figure 5. Current Flow distribution and Optical Field Energy
Band diagram of the Active region and Blocking Region
of the space 0.3um at anode voltage of 1.1V.

In Figure 6 at an anode voltage 1.25V, the current flow is observed to go through the blocking region. It is that the n-p-n-p blocking region was turn on to flow the current.

Figure 6. Current Flow distribution and Optical Field Energy
Band diagram of the Active region and Blocking Region
of the space 0.3um at anode voltage of 1.25V.

For increased anode voltages, Figure 7, it is seen that the current through the blocking region increases.The equivalent circuit in figure 8 shows the n-p-n-p thyristor device was combine the n-p-n transistor and p-n-p transistor. The active channel region is a p-i-n diode. The p-n-p collector and n-p-n base were shorted and connected to the anode of the p-i-n laser diode. When the leakage current flow to start to n-p- n base then, the n-p-n transistor will be turn on to flow the current it will be leakage current of the BH Laser.

Figure 7. Current Flow distribution and Optical Field Energy
Band diagram of the Active region and Blocking
Region of the space 0.3um at anode voltage of 1.5V.

Figure 8. The equivalent circuit model
for the active region and blocking
region and the leak current path.

4. Summary

The leakage current of a BH laser has been investigated. The two layer changed to simulate the I- L characteristics. At the low injection condition, the slope efficiency is the same. When the anode voltage is over 1.2V, the leakage current was seen to depend on the leakage path distance as deltat.

References

N.K. Dutta, D.P. Wilt, and R.J.Nelson, J.Lightwave Technol. LT-2, 201 (1984)
S.Seki at al., Two-dimensional numerical analysis of current blocking mechanism in InP bur-ied heterostructure lasers J. Appl. Phys. 71(7), 1 April 1992 pp.3572-3578
S.Asada et al., Analysis of Leakage Current in Buried Heterostructure Lasers with Semiinsu-lationg Blocking Layers IEEE Journal of Quantum Electronics Vol. 25, No.6, JUNE 1989 pp.1362-1368
ATLAS Users Manual Vol.1

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