52.pdf

PROCEEDINGS OF SPIE

SPIEDigitalLibrary.org/conference-proceedings-of-spie

Design of high-power ridge

waveguide 980-nm pump lasers

Michele Goano, Elena Torasso, Ivo Montrosset, Sergio

Pellegrino, M. G. Re, et al.

Michele Goano, Elena Torasso, Ivo Montrosset, Sergio Pellegrino, M. G.

Re, D. Reichenbach, "Design of high-power ridge waveguide 980-nm pump

lasers," Proc. SPIE 2148, Laser Diode Technology and Applications VI, (1

June 1994); doi: 10.1117/12.176643

Event: OE/LASE '94, 1994, Los Angeles, CA, United States

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 6/5/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Design of high power ridge waveguide 980 nm pump lasers

M. Goano, E. Torasso, I. Montrosset

Dipartimento di Elettronica, Politecmco di Tormo, Corso Duca degli Abruzzi 24, 10129 Torino

S. Pellegrino, M. Re, D. Reichenbach

Alcatel—Telettra Research Center, via Trento 30, 20059 Vimercate (MI)

ABSTRACT

The design of ridge waveguide semiconductor pump lasers poses particular simulation problems, since an accurate

modeling of the electric and thermal effects and of the optical guiding capabilities is needed. We developed a

model whose selfconsistent application allows the evaluation of, among the others, the threshold current of the

lasing mode and the gain margin of the higher order modes, the P-I characteristic, the power coupled in the fiber,

and the far field pattern. Particular attention was paid to the experimental verification of the transverse single

mode operation region, and its evolution at high injection levels. The appropriate conditions for operation in the

150—200mW single mode output power range have been found.

1 INTRODUCTION

The InGaAs quantum well (QW) lasers emitting at 980 nm have received a great attention in the last years'

and have progressed rapidly as pumping sources for Er-doped fiber amplifiers and also for application in optical

interconnection networks. Ridge waveguide (RW) Fabry-Perot lasers have been reported with good characteristics

such as low threshold current,2' 3, , but the major problem is in high power regime were single transverse mode

operation should be guaranteed in order to mantain high coupling efficiency with the fiber.

The design of such devices is not simple at all because a careful modeling of all the relevant parameters is

needed; in this paper we present a model were all the effects are introduced and simulated with various degrees of

accuracy. Since our goal was the creation of an easy-to-use software tool for the structural design of such devices

and for the verification of the experimental results, we considered more important to take into account all the

relevant effects, than to model with great care only some of them; this choice allowed to realize a computer code

running also on a PC. In particular we considered:

• the lateral current spreading above the active region;

• the carrier diffusion in the active region;

• the carrier leakage effect from the active region;

• the temperature device model;

• the temperature effect on material characteristics and waveguiding;

• the waveguide properties using the effective index (El) model;

• the transverse mode competition;

• the modal mirror reflectivities;

• the coupling of the laser beam with the fiber.

In the following we describe first how these effects have been represented in our model and then we present some

results of the simulations.

ISPIE Vol. 2148

O-8194-1443-3/94/$6.OO

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 6/5/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

2 THE ELECTRIC MODEL

The implemented electric model takes into account the lateral current spreading in the resistive layers above the

active region, the carrier diffusion and recombination in the active region, and the current leakage from the active

region.

The results for two analytic models proposed in the literature to take into account the spreading in stripe lasers

have been considered6' in our case the electrode to active layer distance is reduced to the height from the active

layer and the ridge bottom. In Fig. 1 (left) an example of the current injection distribution in the active layer

obtained with the two considered method is shown.

E 5E+008

4E+008

•0 3E-i-008

2E÷008

3E+O18

. 2E+018

Figure 1: Junction current density obtained using different spreading models (left); carrier density profile at and

far above threshold (right). In both cases the ridge width is 2 m.

The carrier density profile N(y) is obtained by solving numerically the diffusion equation:8

Deff(T)N(y)

—

R(N(y),T)

—

vggmat(N,T)P

where the term depending on the photon density P can be neglected in the small-signal operation or for below-

threshold analysis. The carrier recombination has been approximated as:

R = AN + B(T)N2 + C(T)N3

An example of carrier distribution at threshold and far above threshold is shown in Fig. 1 (right), where the effect

of the spatial hole burning is quite clearly shown.

A simple analytical model for the electron current leakage across the heterojunction interface has been also

implemented. It takes into account only the electrons and holes able to pass the barrier between the active region

and the guiding layers.

3 THETHERMALMODEL

The simple traditional model based on a constant thermal resistance:9

4T = RthPth Rth(R512 + VjIj —

P0)

SPIE Vol. 2148 I 53

7E+008

6E+008

5E+018

4E+018

no spreading

—Lengyel et al.

____%\\\\\\\\

Yonezu et al

IE÷008

.._.

IIth

—-I=lOIthL...

:t::JT

IE+018

2345678910

2 3 4

Distance

from

the

center

the

ridge

(pm)

Distance

from

the

center

of the

ridge

(pm)

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 6/5/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

where Pth 1S

the

thermal generated power, V is the junction voltage, and I is the part of the injected current

which contributes to the heat generation in the active region,9 has been modified to take into account the transverse

distribution of the heat generated by the carrier recombination in the active layer. Therefore the contributions of

Joule effect and recombination have been associated to different thermal resistances:

zT = Rth,Joule R812 + Rth,ricom(X,y) (V,I3 —

P0).

and Rth,ricom(X,Y) has been obtained by solving the heat flux equation with a finite difference scheme.'0

In Fig. 2 a map of the temperature for an actual device is shown; the corresponding thermal resistance profiles

along the junction and in the orthogonal direction are shown in Fig. 3. Since the optical field is well confined around

the active layer and Rth is an almost linear function of x, to save memory only the junction temperature profile

and the temperature derivatives perpendicular to the junction have been used in our simulator. For a defined

structure the previous data have been computed apart, and during the above threshold simulation we supposed

that the temperature distribution remains unchanged in the form but is changing its maximum value according to

the temperature variation in the center of the ridge.

Beside the influence of the temperature on the local active layer parameter characteristics (material gain,

recombination and diffusion coefficients), we considered also the effect on the "bulk" refractive index distribution

throught the simple

=11.41O5K'

ÔT T0=300K

This change affects the field confinement, and has been verified experimentally.

4 THE ELECTROMAGNETIC MODEL

The modal analysis has been carried out using the effective index method. Since the refractive index variation is

much smaller and slower in the transversal y direction we can factorize the electromagnetic problem in depth and

transversal directions. Taking y as a parameter, we solve "for every y" the Helmoltz equation along x:

182

kon2(x; )]

p,(x;

y) = ki(y)ço1(x; y)

ISPIE Vol. 2148

1.5

Distance from the center of the ndge

(micron)

Figure 2: Typical Rth map.

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 6/5/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use