Optical and Quantum Electronics 21 (1989) 347-352

Single long

, symmetricalthree-cavity GalnAsP/lnP lasers

YUSHU ZHANG, WAN ZHAO, JIAWEI SHI, DINGSAN GAOJilin University, Chang Chun, China

Received 30 June; accepted 6 September 1988

Let

When g, = 0 (g, is the gain in the short cavity L,),

- ; ; + (, , ,e

)- + 2;', e

cos 2/3, L,_

I

e r.e,

2,

e

cos 2/1, L,

(-)

15 .E =

)' [l +

+

( I + y) cos 2/3,L,a rcco t (3)

,,e

(1

sin 2fi,L,

Using r,, the symmetrical three-cavity laser, as shown in Fig . Ia, can be transformed intoan equivalent single-cavity L, which is replaced by an equivalent interface M, . It is obvious

Two ways of i e ction characteristics of d ode lasers are analysed .The mode-selection mechanism and technological processes are presented for sym-metrical three-cavity lasers, and the experimental results are in good agreement withtheoretical results .

1 . IntroductionIn order to realize long-distance signal propagation with large capacity and high efficiency .i t is necessary to use the single longitudinal mode InGaAsPjInP laser in an opticalcommunication system . Three ways can be used to strengthen the single longitudinal modeoperation : increasing the interval between two modes . raising the side-mode losses: andlocking the modes by using external injection .

2 . Mode-selection characteristics in symmetrical three-cavitylasers

The structure of the symmetrical three-cavity laser is shown in Fig . 1 . Each of the two endsof the cavity L, is connected to a cavity L, in which some parameters are the same as in thecavity L, . Considering the effective reflection of the cavity L, for the light wave in the cavityL,, we can obtain the effective reflectivity in terns of the multiple reflection theory [1, 2]

r + ' , e (I)I +

0306-8919!89 $03 .00 + .12 (' 1989 Chapman and Hall Ltd .

347

M 2

M 1

M 1

M 2L 2

L1

L 2

V2r2

ri

rt

ri

r2 r2

ret 'r°re

TQ(a)

M e ID

L 1

'71

Yushu Acing et al .

Figure 1 (a) Symmetrical three-cavity and (b)re

(b)

re

equivalent single cavity .

that methods for and results from the usual single-cavity case can be used for this equivalentsingle cavity . The cavity loss, threshold and phase conditions are given by, respectively :

a,,(i•) _ (2/L1) In (I/I1,l)

(4)G = a,,

(5)

2J, + 2/1;L, = 2nzrr m = I, 2, 3, . . .

(6)

Some questions are discussed as follows .

2 .1 . Cavity loss a R, ('i)In Equation 2 the expression for I r c.l can be changed into a simpler form when21;,L, = 2m,7r. (2m ± ;)7r, (2m, + 1)mr, and three special values am, a 2 , and 1,',, areobtained . Based on these above special values, the a r ,(i,) curve can be obtained roughly, asshown in Fig . 2 when ii, > it, > n o . It is shown that the variation of a,,,(/'.) with i possessesthe appropriate relationship of where, i,,,,_ is the mode spectrum when the cavity L, isthought of as a single cavity. The larger Aa n, is of great advantage for strengthening thesuppression of the side-mode oscillation . It can be proved that, when the parametersselected satisfy the condition

(gz - x, )L, = In

+ >>, )(n, - n0)/(h , + nn)

n1>n2>n0

AAm1

IIIIIlIIl nIIII AmtXm2+ 2 r-AXm2

I Amt

Aam - 4 II1 AM l~`ml

-A~m+{

Am _- Am+1,m

348

A

Figure 2 Cavity loss and mode spectrum for the symmetrical three-cavity .

Si•mmetrical three-cavity GaInAsPjhnP lasers

x n , is maximum . It can be seen that by varying the values of g,,1, and ii, the mode-selectioncharacteristics can be changed .

2.2. Mode spectrumFrom Equation 3 it is known that J,, a function of j3, or f,,, written as J4

27rir, ;'r., then phase condition 6 may be written as

(),(i.) + 47rn,L,?i. = 2mm )n = l, 2 . 3, . .

The relationship between the resonant wavelength i n, and the mode number is repre-sented by Equation 7 . When subtituting a series of in into Equation 7, we can evaluate aseries of 1.n, and obtain the mode spectrum. Now we discuss the case when the structureparameters satisfy the condition

ir, > ir, > n, and I.,,,I =

(8)

We can find two groups of solutions when J,. = 0, 7t in the phase condition 6 :

_ () <2flL,_

These two groups o so

? The case -low-loss cavity in which resonanc

s . and the case J,, = 7r corresponds to the icavity in which resonance is difficult under the condition of low current .

When we consider the mode separation, the following cases are noteworthy .

I . The mode spectrum

i." for the different cavity losses are illustrated in Fig . 2 . In thecase of the mode i. ; ;, with resonating low cavity loss, the mode separation was given by A). ;,, .Since Di. , is the lowest common multiple of A/.n, , and A/. n,,, we have A;, ;;,max (0% n , i , A ;,,,,) .

2. When

consists of only

the mode separation is Ai" ,, too .3 . When the modes with both the high cavity loss and low cavity loss resonate simul-

taneously, the mode separation A ;."' is half of A;. ;;,, but still larger than A) . n ,, of the isolatedmain cavity L, .

The above description and discussion for and show that the symmetricalthree-cavity provides a larger possibility of raising the losses of the side modes andincreasing the mode separation than the single cavity, so that the symmetrical three-cavitypossesses better mode-selection characteristics than the single cavity .

3. Device structure and fabrication proceduresThe structure of the device is shown in Fig . 3 . The double current confinement substratediffused inner stripe and oxide stripe is used to perform the fundamental mode operation .We etched the lasers in the lateral direction so that the longitudinal mode could be

'_rrr, 7r

2m, n

2m, 7r

(2)?r, + I)7r

rr7, + rr„

(7)

349

Cd diffused layer

light outlAu/ Zn

P- In In As PP-PIn Go As P (act)

r~n-In P

n-InPlsub)

Au/Ge/Ni

3 50

A-A cross-section

Yushu Zhang c t al.

Figure 3 GalnAsP/InP laser with symmetrical three-

cavity .

Figure 4 Photograph of the cleaved cross-section of the

epitaxial wafer .

selected . This type of laser has some advantages, namely the structure is simple and thefabrication easy .

The main fabrication process for three-cavity lasers is as follows . First, InGaAsP(cap, 0.3 to 0.71im, Zn-doped, > 2 x 10"cm ')/InP (upper clad, 1.5µm, Zn-doped,

5 x 10''cm ') .'InGaAsP (active, i. r = 1 .3/im, 0 2 to 0 .3/im, undoped),InP (lower clad,1 .5/im, Te-doped, - 1 x 10"cm ')double heterostructLire was grown on (100i)-orientedn-InP substrate by liquid phase epitaxy, and then ohmic contact was made . The metalcladding on the p- and n-sides are AuiZn and ALI/Ge'Ni, respectively . Fig. 4 is a photo-graph of the cleaved cross-section of the epitaxial wafer . The junctions have been exposedin the figure . The wafer was then cleaved into individual diodes . Selective etching was finallyperformed [3] .

4. Experimental results and discussionWe measured the spectrum of a device, as shown in Fig . 5 . The oscillation is a kind of singlelongitudinal mode with a side-mode suppression ratio of 50 : I under the lower pumping

Stvnnretrica/ Three-curia • GaInAsP,InP lasers

F-2L=L 1 +2L 2 -120 un

L2^- 10 µm170 mA

1204 1202 1200 1198

A (nm)

(a)

Figure 5 Spectrum of the device at (a) lower and (b) higher pumping current .

L

In2n2n21Ln 1n1n0

n2

n0

fv1 equivalent

L +`1~`2.~

n2

n l

n2

F-2250 mA

calculatedspectrum

&Am1=2 .1 nm IIIIIIIIIIIII Am

1AAm2a21nm

AmtLAO

AbIM'

n'

X,,I

~AAma~=10.5 nm

I

Figure 6 Calculated spectrum for the symmetrical three-cavity .

current, as shown in Fig . 5a . The result under the higher pumping current is shown inFig . 5b, where other modes appear with larger mode separation .The calculated spectrum is shown in Fig . 6. In the device structure, the section L, is a

symmetrical waveguide and section L,_ is an asymmetrical waveguide, as shown in Fig . 6 .They have different refractive indices ii, and ii,, and may be equivalent to a symmetricalthree-cavity, as shown in Fig . 6. The calculated spectra can be obtained from previoustheoretical analysis . The calculated data are close to the experimental results, and the modespectrum is shown in Fig . 6 . is given according to previous analysis . It can bepredicted that three resonance peaks may appear when G(i) rises to the high position asin Fig . 6. The calculated spectrum approaches the measured spectrum as shown in Figs 5band 6. In Fig . 5a the separation between the attenuated side peak and the main peak isconsistent with the calculated A/",,,, . This structure of the device evidently provides thecharacteristic property of mode selection .

351

In summary, the theoretical analysis is in good agreement with the experimental resultsfor the symmetrical three-cavity .

ReferencesI .

H . C. CASEY and M. B . PAN ISH . 'Heterostructure Lasers', Part A (Academic Press, New York . 1978) p . 165 .2 .

GUOGUANG MU and YUANLING ZHAN . 'Optics (Publishing House of the People's Education . Beijing .1978) p . 441 .K . L . CHEN and S. WANG, Electron . Lett . 21 (1985) 94 .

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Yu.shu Zhang et al.