DELAY TIME IN OPTICAL SWITCHING OF BISTABLE TANDEM LASERS

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DELAY TIME IN OPTICAL SWITCHING OF BISTABLE TANDEM LASERS

U. Öhlander, P. Blixt, O. Sahlén

To cite this version:

U. Öhlander, P. Blixt, O. Sahlén. DELAY TIME IN OPTICAL SWITCHING OF BISTABLE TANDEM LASERS. Journal de Physique Colloques, 1988, 49 (C2), pp.C2-157-C2-160.

�10.1051/jphyscol:1988237�. �jpa-00227654�

Colloque C2, Suppl6ment n 0 6 , Tome 49, j u i n 1988

DELAY TIME IN OPTICAL SWITCHING OF BISTABLE TANDEM LASERS

U. OHLANDER, P. BLIXT* and 0.

SAHLEN

I n s t i t u t e o f O p t i c a l R e s e a r c h and D e p a r t m e n t o f P h y s i c s I I . Royal I n s t i t u t e o f T e c h n o l o g y , S-100

44

S t o c k h o l m , Sweden ' ~ e p a r t m e n t o f A p p l i e d P h y s i c s , Royal I n s t i t u t e o f T e c h n o l o g y , S-100

44

S t o c k h o l m , Sweden

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Nous avons &die la dponse de "diodes-laser-tandem" bistables (BLD) &lenchtes optiquement Plus particulibmenf l a d amique ainsi que l'tnergie minimale de la commutation en fonction du courant d'alimentation ont tt6 t v a l u h pour &ntes longueurs d'onde de I'impulsion incidente. Des dkclenchements inftrieurs h la nanoseconde et

au picojoule ont bt6 observes. En estimant un couplage du faisceau incident de I'ordre de 10 8, la plus barse tnergie de dklenchement du bistable est de 23 fl, et le temps de montee le plus court est inftriew h 100 ps.

w -

^We report on the switching-on response of bistable tandem laser diodes (BLD's) to optical input pulses. An investigation of switching delay time and minimum switching energy as functions of current DC bias has been performed for different input pulse wavelengths. Subnanosecond, subpicojoule switching is reported. Estimating the input coupling to be ten percent, the lowest bistable switching energy recorded was 23 fJ. The shortest recorded BLD rise time was less than 100 PS.

Absorptive bistability in inhomogeneously pumped laser diodes has been investigated both theoretically 111

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¹⁴¹ and experimentally, 141

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161. An exwriment usine bistable three-section laser diodes in ovtical time-division multidexine has been donned bv workers at NEC, ref fl;. They also &rted on an experimental investigation'of the spectral dependen& for "minimum &tical pow& for bistable switch-on. This dependence was examined in more detail for different relative current bias levels in a review article by Kawaguchi 181.

However, the switching time scale in this investigaticm was not specS1ed. Recently, NEC also ned subnanosecond switch-off of three-section laser diodes by applying a subnanosecond negative current pulse to the contactedxrber, ref @I. The switching-on was done with electrical pulses in that experiment. Vay recently, fast electrical switchiig-on and off was reported by workers at the University of Tokyo 1101.

In this pap-, we investigate fast optical switching-on of bistable two-section l a s a diodes. The purpose is to explore what the switching ener y bias level and in t wavelength u i r e m t s for high tition rates in optical switching-on of inhmogeneously umped bista%lk laser d i d e s are. Two experimen3ave been done at x r e n t time scales. The f m t experiment is p e d m e d with f n s FWHM input pulses and the second expaiment with 05 ns FWHM optical input pulses.

2. Ex~erim-

In Figure 1. the set-up for the two experiments is schematically shown. All lasers in these experiments were InGaAsP (1.3 pm) buried hetemsuucture laser diodes made by Ericsson Co en6 AB. The master laser diode (MLD) was temperature controlled to enable wavelength control. The F-P was a

a-

5 sun m r ~ a b r y - ~ e r o t filrer for narrowing the input pulse spectrum. It was used in experiment 2. The two F c o a t e d le&es to the right of the F-P were used to spatially narrow the input pulse, to pass the optical isolator (Isol.) and the ut coupling lens to the bistable two-section laser (BLD). The beam splitter BS spliaed art of .the input pulse to be coupled into D s e detector D, was a0.8 ns mounted Ge-APD in expenmenl 1, and a 0.2 ns mounted &-PIN m experiment 2, feeding a 75 ps sampling head. The turnable polarizing beam splitter, PBS, was used to control the input y l s e energy. The Isol. prevented feedback into the MLD. 'llw half-wave plate (U2) served to match the polarization planes of the Input pulse and the BLD.

The BLD, ref I&, was LD 124 in experiment 1 and LD 21 in expaiment 2. The switching-on current thlesho1d.s were around 50 and 45 mA, respectively. The BLD was mt actively t e m p h r r e controlled during the experiments. The switch-on current threshold of LD 21 had a small drift when the laser was switched, of about 0.1 mA per ten minutes. In experiments, care was taken to change the current bias level to compensate forthis drift Both lasers had a hysteresis width of about 4 mA. The BLD absorber section facet was towards the MLD in both experiments.

m e detector D2 was a 0.2 ns mounted Ge-PIN feeding a 75 ps sampling head in experiment 1, and a 100 ps mounted InGaAsP-PIN, feeding a 30 ps sampling head in experiment 2. The InGaAsP-PIN was made by Ericsson Components AB. The input pulse energy was measured between the short arrows m Figure 1. This was made by a large-area (diameter

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1 cm) Ge-PIN. The wavelengths of the MLD and BLD w u e measured at the position of the lon amnvs in Figure 1. The mooochromator used for spectral measurements had a resolution of 8

A.

The repetition frequency was 0 3 8 and 2.1. Mhz in experiment 1 and 2, respectively. The electrical pulses to the two laser diodes were synchronized

3.- 1

- s : . .

Figure 2. shows an oxilloscope photo, recording bistable switchingim of the BLD with a 3 ns FWHM optical input pulse. The upper ti'ace shows the signal from detector D,, while the lower

aace

shows the signal from detector D,

It was checked that the switching was bistable by changing the oscilloscope time scale. The BLD light o u t p t switched-off sharply at the electrical reset pulse about 200 ns after the set pulse. Can was taken to synchronize the twb oscilloscope channels.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988237

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A delay between the input pulse and the switch-on of the BLD output can clearly be seen in Figure 2. We define the delay time r as the time between the peak of the input pulse and the f m t relaxation peak of the BLD output, givin a delay of about 6 ns in Figure 2. This delay was measured to decrease with increasing input pulse energy E for a constant relative

2

current bias of 0.1 mA, Figure 3.

Negative values of r can be seen for large E. This is due to the finite rise time of the input pulse. We defme the relative DC current bias A/ as the difference between the current switch-on threshold and the DC current bias. As seen in Figure 3, measurement sequences for five different MLD wavelengths were performed. No wavelength dependence could be clearly distinguished.

Also, the delay .r increased with increasing relative DC current bias A/ for a constant input pulse energy E of 23 pl, Figure 4. A linear relationship is seen. Bistable switching with delays longer than about 10 ns were not seen.

4. Exveriment 2 : Switching--with a 0.5

In Figure 5. bistable switching-on with a 0.5 ns optical input pulse is shown. The input pulse wavelength was 1300 nm, and the BLD wavelength was 1310 nm. The relative current bias level A/ was 0.1 mA. The input pulse energy E was measured to be 0.73 pJ. We see a delay r of about 0.5 ns.

In Figure 6. the measured minimum energy for switching E, is seen to increase with increasing relative DC current bias ^A/, for three different input pulse wavelengths (&); 1300, 1310 and 1314 nm. The spectral FWHM of the input pulse after having been filtered through the F-P was 6,4, and 4 nm, respectively. 7he BLD wavelength (b) was measured to be 1310 nm during switching, for all three lnput pulse wavelengths.

We define the measured minimum energy for switching as the measured input pulse energy for which switching was barely seen. This is a subjective definition. Therefore, two series of measurements with different experimentators were carried out for each wavelength in this experiment. The delay r was 1-2 ns when switching with minimum switching energy. Delays longer than 2 ns could not be systematically seen in experiment 2. This is believed to be due to a noise in the bias level of around 50 p ~ . Results from rate equation calculations imply that the bias noise must not exceed several microamperes in order to see delay times longer than tens of nanoseconds.

As can be seen in Figure 6., the sensitivity for changes in relative bias kvel was measured to be smallest when the input pulse wavelength was shorter than the wavelength of the tandem laser optical output. This sensitivity increased with increasing input pulse wavelength. The lowest measured input pulse energy for bistable switching was 0.23 pJ, at an input pulse wavelength of 1300 nm.

Estimating the input coupling lo be 10 76, this gives a lowest recorded switching energy of 23 fl.

In Figures 7-9 the dynamic res nse of the BLD is shown for the three different input pulse wavelengths mentioned above. All photos were recorded for a relative &ias current of 0.1 mk and for inout oulse energies around 5 oJ. The main aualitative features of these photos are a fast rise time of less than 0.2 ns and a ringing leading'to

a

loweringlof the BLD oitput about a &nosecond after switch-on.

The lowering of the BLD output is seen to give a zero output in the case of an input pulse wavelength longer than the wavelength of the BLD, Figure 7. This might lead to severe problems with bit errors if the component is to be used in a high-speed communication system.

However, it is seen that this problem becomes less severe for the shorter input pulse wavelength. Figure 9. This wavelength dependence of the ringing is seen also for lower input pulse energies. Figure 10 shows the dynamic response of the BLD output at the same conditions as in Figure 9, but at a faster time-scale (50 ps). The fastest d e d BLD rise time was 100 ps, the temporal resolution of the detecting system. This implies the possibility of gigahertz repetition rates, provided that the switch-off time can be lowered by using electrical switch-off of the absorber 191.

5. Conclusions

Subnanosecond bistable switching-on of tandem lasers with subnanosecond, subpicojoule optical pulses has been performed. A detector limited (100 ps) switch-on rise time has been recorded, implying the possibility of gigahertz repetition rates. Bistable switching-on with subpicojoule optical pulses is possible over a wide input ulse wavelength range (1300

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1314 nm). Estimating the input coupling to be 0.1, the lowest recorded energy for bistable switching is

%

^fJ, corresponding to a peak power of 50 pw.

For 3 ns optical input pulses, delay times up to 10 ns can be seen for bistable switching. The delay depends on input pulse energy, and on relative bias level. For 0.5 m optical input pulses bistable switching delays longer than 1 -2 ns cannot be systematically seen.

The minimum energy for bistable switchin depends on the relative bias level of the tandem laser. The sensitivity f a changes in relative bias level depends on the input pulse wavefength. The sensitivity for changes in relative bias level was measured to be smallest for the input pulse wavelength shorter than the wavelength of the tandem laser optical output. This sensitivity increased with increasing input pulse wavelength.

A large ringing occurs in the tandem laser output. This ringing lowers the tandem laser output remarkably about a nanosecond after the fust relaxation peak. This might give rise to problems from a system point of view for input pulse wavelengths matched to or longer than the wavelength of the master laser. The z m output is avoided for shorter input pulse wavelength.

6.

Acknowledeement

This work was partially supported by the Swedish Natural Science Research Council (NFR) and by the Swedish Telecom. The authors also wish to thank Lars ThylCn at Ericsson Telecom and Mats Jansson at Ericsson Components AB for their support of this work.

M L D 1 ~ ~

I' _-IPes# 1*p143+ _{_I}

_f

_Osc.

E= F-P BS ISOI. i $ ^I

Fig. 1

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Experimental set-up for experiment 1 and 2. MLD is the master laser diode, F-P is a Fably-Perot filter, BS is a beam splitter, D , and D2 are photo detectors, PBS is a @wiziig beam splitter, Isol. &.an optical isolator,,W2 is a half-wave plate, BLD is the bistable tandem laser, Osc. are the two samplln osc~lloscope channels. The Input pulse energy is measured at the short arrows. The wavelengths of the laser diodes are measured at the fong arrows.

E ( p a Fig. 2

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Bistable switching-on res se of the BBL (lower Fig. 3

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Delay times as a function of pulse energy E of the oscilloscope trace) with a 3 11s optical input pulse (upper input pulse. The measurement sequences 1-5 were for MLD

arcilloscope tr;tce). wavelengths 1309,1304,1308,1307 and 1312 nmrespective-

ly. The relative cumnt bias level Al was 0.1 mA.

Fig. 4

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Delay time T as a function of BLD relative bias level Al.

MLD and BLD wavelengths were 1308 nm and 1310 nm, respectively. The input pulse energy E was 23 pl.

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Fig. 5

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Subnanosecond bistable swirceg-on response of the BLD for subnanosecond. sub~icoioule Input wlses. The anput pulse was measured to

bk

0.73 fibefore 'he BLD input c&pling lens. The input pulse and BLD wavelengths were 1300 nm and 1310 nm, respectively. The relative current bias level was 0.1 mA.

Fig. 7

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^{Dynamic response of the BLD when A, >}

&..

The input ulse was 3.2 ,0.5 ns with a peak wavelength of 1314 n m

TR

BLD waveggth was 1310 nm.

Fig. 8 -Dynamic response of the BLD when & =&. The input pulse was 4.3 ,0.5 ns with ape& wavelength of 1310

P:

N% the same as the LD wavelength.

A

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7

2- ^{5 -}

0.

4 -

Y

Fig. 9

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Dynamic response of the BLD when & <

&.

The Fig. 10

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Switchin rise time of the BLD when Lw < bW. The input ulse was 5.8 pJ, 0.5 ns with a peak wavelength of 1300 input pulse was 5.g pJ, 0.5 ns wifh a peak wavelength of 1300

nm. &e BLD wavelength was 1310nm. nm. The BLD wavelength was 1310 nm.

A,,m=13L4m~: A

0 &=1310nm: 0

A 0 0 0

A ?,,u = 1300 nnl : 0

0 0

REFERENCES

111 Dziura, T.G. andHall, D.G., IEEE J. Quant. Elec. (1986) 1579

121 Perkins, M.C., Ormondroyd, R.F. and Prof. Rozzi, T.E., IEE Proc. Part J (1986) 283 131 Ohlander, U. and SahlCn, 0.. IEEE J. Quant Elec. (1987) 487

141 Harder, C., Lau, K.Y, Yariv, A., IEEE J. Quant Elec. -(1982) 1351 I51 Kawaguchi, H., Appl. Phys. Lett. (1982) 702

161 Ohlander, U., Sahlbn, 0. and Ivarsson, L., J. Appl. Phys. 62 (1987) 2203

171 Suzuki, S., Terakado, T., Komatsu, K., Nagashima, K., Suzuki, A. and Kondo, M., IEEE J. Lightway Tech.

D

(1986) 894 181 Kawaguchi, H., Opt and Quant. Elec.

fi

(1987) 1

191 Tomita, A., Ohkouchi, S. and Suzukj, A,, Proc. Conf. Optical Switching, Lake Tahoo, (1987) 119 1101 Liu, H-F., Hashimoto, Y. and Kayima, T., IEEE J. Quant Elec. QlU4 (1988) 43

q

X,- = 1310 nm

I

0.0 1.0 2.0 3.0

A 1

^{( m ~ )}

Fig. 6 -Measured minimum switching energy E, as a function of relative bias level A! for three different input pulse wave- lengths & : 1300,1310 and 1314 mn. The BLD wavelength

&was 1310 nm.