I 186 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 2, FEBRUARY 1995

Wavelength Shifts of 1.5-pm DFB Lasers Due to Human-Body-Model Electrostatic Discharge Followed by Accelerated Aging Experiments

Jichai Jeong, Member, IEEE, K. H. Park, and H. M. Park, Member, IEEE

Absfruct-This paper reports human-body-model (HBM) elec- trostatic discharge (ESDI-induced wavelength shifts of 1.5 pm

latent defects inside the active region. Because of ESD or EOS-induced damage, lasers could be changed in their char-

InGaASp/hP distributed feedback (Dm) lasers. Reversembias acteristics and then lase at somewhat different wavelengths.

if the lasing wavelength is shifted larger than the specified channel spacing. In this paper, we investigate behaviors of DFB lasers using HBM ESD stress and subsequent accelerated aging experiments. Stressed lasers have been subsequently aged to see their behaviors during the long-term operation. This may help understand behaviors of lasers during field operations after receiving such a stress.

has been to the lasers. Their and Optical This will be a big impact on performance of WDM systems, characteristics are significantly changed after ESD exposure. F= most of ESD lasers, the wavelength s ~ * are measured to be less than 2 di. Subsequent aging results show that the aging-induced wavelength shifts of ESD damaged lasers are less than OS hi. No significant impad On reHabd@ has been found from ESD damaged lasers. Similar results are obtained from non-ESD of the lasing wavelengths.

except for the shift

I. INTRODUCTION

UTURE lightwave communication systems using F wavelength-division-multiplexing (WDM) technology will require long-term wavelength stabilities of source lasers throughout the systems’s lifetime (usually 25 years). With erbium-doped fiber amplifiers (EDFA’S), WDM systems increase transmission capacity in future long-distance communications [I], [2]. In WDM systems, wavelength instabilities of lasers should be much smaller than the specified channel spacing during the system’s lifetime.

It was observed that the wavelength shifts of 1.5 pm DFB lasers in commercially available laser modules were very small (less than 500 MHz), compared to typical frequency division multiplexing network channel spacings [3]. However, the recent results show that the wavelength shifts in DFB laser modules and laser chips are somewhat larger than these of the previous results [4], [5]. Still, there is uncertainty of wavelength stabilities of DFB lasers operated in an used temperature for the long-term operation.

During field operations of WDM systems in near future, lasers can suffer from unwanted electrical overstress (EOS) from power surges and lightening strikes. Also, ESD exposure caused from mishandling devices can damage lasers during manufacturing WDM transmitter modules. EOS and ESD stress can not only damage laser chips but also impair system operation [6] . The stress can cause catastrophic failures or

Manuscript received Nov. 14, 1994. This work was supported by Pohang University of Science & Technology, Electronic Telecommunication Research Institute, and by KOSEF-OERC-94-02-02- 1.

J. Jeong was with the Deptartment of Electronics and Electrical Engineering, POSTECH, Kyungbuk, 790-784, Republic of Korea and is now with the Department of Radio Engineering, Korea University, Seoul, Korea.

K. H. Park and H. M. Park are with Compound Semiconductor Div., ETRI, Taejon, 302-606, Republic of Korea.

IEEE Log Number 94078157.

0733-8724/95$04

11. EXPERIMENTS

Using twenty-nine 1.5 pm InGaAsPhP DFB lasers, the lasing wavelengths were measured at 20°C with the bias current of 40 and 60 mA. All the lasers were facet-coated with high reflection (HR) and anti-reflection (AR) for the longitudinal mode control. The wavelength of each laser was measured at the constant current instead of the constant power, because the wavelength at the constant power can not be reproducibly measured due to change of light coupling into a fiber. The light, using a GRIN lens followed by an in-line isolator (available from E-tek), was coupled into a single mode fiber to measure the lasing wavelengths using a commercial wavemeter. The accuracy of the measured wavelengths is mostly dependent on reproducibility of temperature control throughout whole experiments. Five control lasers (without ESD stress) were used to check repeatability of measured wavelengths from our test setup at each measurement. From the control lasers, it was verified that the lasing wavelengths in the test setup can be reproducibly measured within 0.04 a for the best case and 0.08 A for the worst case.

Twelve lasers were exposed by an ESD simulator. A 100 pF charged capacitor with the test volatge was discharged through a 1.5 kf2 resistor in series with the test laser. Using dummy lasers, the threshold level of ESD damage were determined with increasing the applied voltage. ESD exposed lasers could be changed in optical and electrical characteristics such as the threshold current, the slope efficiency, and the reverse leakage current. In order to find out the most sensitive parameter, forward-biased and reverse-biased ESD simulations were performed. We started from the voltage of 500 V by applying 3 pulses and then increased the applied voltage up to 10 kV. Lasers suffered from ESD stress. At each increment of the voltage (100 V), lasers were characterized.

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After measuring the lasing wavelengths, the lasers produced enough light output were subsequently aged at 60°C and 3 mW for lo00 hours. Lasers without ESD exposure were simultaneouly aged at the same aging condition. Aging rates were calculated from the increased bias currents to maintain the constant light outputs as a function of time.

HI. RESULTS AND DISCUSSIONS A. ESD Damage

We did not see any big change in the characteristics of lasers with forward-biased ESD stress, up to 10 kV. On the other hand, we found dramatic change in characteristics of lasers after reverse-biased ESD stress. Most lasers started to show change in current-voltage (IV) and light-current (LI) characteristics at above 1 kV. Among the measured parameters, the reverse leakage current was found to be the most sensitive parameter to ESD damage. Lasers used in these experiments are more vulnerable to reverse-biased ESD stress. DeChiaro et al. [7] observed that InGaAsP lasers with uncoated facets often tend to exhibit lower failure voltage under forward-bias ESD than under reverse-bias ESD. They found that passivated lasers with sulfides had high ESD failure voltages. This indicates that uncoated lasers had damages at the facet due to the local heating with absorbing high optical power during forward-biased ESD exposure. However, our results &e different from their results. Since our DFB lasers were facet-coated for both mirrors, ESD damage may be generated inside the active region rather than near the AR facet. Dark spots were rarely observed on the AR facet after ESD exposure. ESD damage usually occurred at the weak spot. The weak spot in our facet-coated lasers is not in the facet but inside the active region. In these experiments, we concentrated on reverse-biased ESD exposure. The voltage of 1.4 kV was applied to twelve lasers with the reverse-bias direction. Three out of twelve lasers did not lase after ESD stress.

Electrical and optical characteristics measured from two lasers before and after reverse-biased ESD damage are shown in Figs. 1 and 2. The dashed line curves were measured after ESD stress. ESD-stressed lasers show significant changes in their characteristics. For example, the reverse leakage current and the threshold current are increased. A kink develops at above 100 mA. The output power is significantly dropped up to 60% from the original power. Because of increasing the reverse leakage current and the threshold current, reverse- biased ESD stress seems to generate dark spots andor dark lines inside the active region. The defects can contribute to the source of the leakage current and the increase of the threshold current to overcome the loss. Unfortunately, we were not able to observe ESD damage because of the lack of our facilities. A few lasers did not lase at all after ESD exposure. The side mode suppression ratio of some survived lasers were still less than 30 dB measured at below the kink currents.

B. Wavelength Shifts

Figure 3 shows the normal distribution plot of the measured wavelength shifts after reverse-biased ESD stress. Wavelength stability due to ESD stress is less than 2 8, except for one laser.

Bias Current (mA) (a)

-2.5 2.5

Fig. 1. Characteristics of a laser with ESD exposure (dashed line) and without ESD exposure (solid line): (a) light-current characteristics and (b) current-voltage characteristics. A kink develops at around 100 mA and the laser has little change in its characteristics below the kink current.

The laser has the blue-shifted lasing wavelength of 6.5 A. The electrical and optical characteristics of the laser having the blue-shifted lasing wavelength of 6.5 A are shown in Fig. 1. Even though the laser has the least change in its characteristics below the kink current, the wavelength shift is larger than any other lasers. The laser may lase at the next higher transverse electric modes. The next mode was located at 28 A below the main mode before ESD stress. After the mode jump, the lasing wavelngth will be adjusted by the gain-phase relation. The wavelength shift of 6.5 A in the laser may be related to the mode jump. As a comparision, the laser having electrical and optical characteristics before and after ESD exposure shown in Fig. 2 has the red-shifted lasing wavelength of 0.85 A.

For lasers with large changes in their threshold currents, lasing wavelengths are usually blue-shifted because of more carrier injection to reach the threshold condition. The increased carrier concentration results in a reduced refractive index ( n , f f / d N = -2.1 x lo-'' cm3) [8]. The Bragg wavelength

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188 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 2, FEBRUARY 1995

Bias Current (mA) (a)

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(b) Fig. 2. Characteristics of another laser with ESD exposure (dashed line) and without ESD exposure (solid line): (a) light-current characteristics and (b) current-voltage characteristics. The threshold current and the reverse leakage current are increased and the light output is reduced.

in DFB lasers with first order gratings can be determined from AB = 2hn,ff, where AB is the Bragg wavelength, A is the grating period, and n,ff ' is the effective index of the active layer. With only considering the carrier effects the lasing wavelength could be shifted to the shorter wavelength region (blue shift) due to the reduced n, f f . For a laser with the threshold current change of 10 mA after ESD exposure, the wavelength shift was measured to be about 0.1 nm toward the red side. In order to obtain the red shift of 0.1 nm, the change of n,ff can be calculated from ANA = An,ff/n,ff with A - = 1.54 pm. The laser should have the increased refractive index (An,,,) of 2 x for moving the lasing wavelength to the red side. In this case, the carrier concentration should be reduced by 10l6 cm-3 with the assumed carrier concentration of lola cm-3 so that the wavelength moves 0.1 nm to the red side. The threshold carrier concentration calculated from Nth = IthTs/q. where Ith is the threshold current, T~ is the carrier lifetime, and q is the electronic charge, should increase with increasing of the threshold current after ESD stress. The

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Fig. 3. Normal distribution plot of the wavelength shifts due to ESD damage with the reverse-bias volatge of 1.4 kV. The blue-shifted (-6.5 A) laser has the measured characteristics shown in Fig. 1.

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Threshold Current Change After ESD (mA)

Correlation plot between the wavelength shifts and the threshold Fig. 4. current changes due to ESD damage.

above situation can not be explained by the carrier effects. The thermal effects due to the increased threshold current should be considered to explain the red-shifted wavelengths, shown in Fig. 4. The DFB lasers normally have the red-shifted lasing wavelengths with increasing the junction temperature. m i c a l measured wavelength shift is 0.9 W C . For the threshold current change of 10 mA, the laser should overcome the carrier effects first and then move the lasing wavelength toward the longer wavelength region. With typical thermal impedance of 100°C/W in our lasers, we can estimate the junction temperature increase of 1 S°C, resulting in the wavelength shift of 1.35 8, to the red side. In our experiments, the thermal effects play a major role in the wavelength shifts.

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Quantiles of Standard Normal Fig. 5. Normal distribution plot of the wavelength shifts measured from the ESD-damaged lasers before aging (0) and after aging (A).

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C. Accelerated Aging-Induced Wavelength Shifts of ESD-Damaged Lasers

After ESD stress, five lasers produced enough light outputs and then were able to age at 3 mW and 60°C up to lo00 hours. The results are shown in Fig. 5 as a normal proba- bility distribution plot. The circles represent the wavelength shifts measured from ESD damaged lasers. All the five lasers have the red-shifted wavelengths right after ESD stress. After aging, the ESD-damaged lasers have the blue-shifted lasing wavelengths.

As a comparison, nine lasers without ESD stress were aged at the same time. After aging at 60°C and 3 mW, the lasers have generally the red-shifted wavelengths. The data is indicated as the circles (0) in Fig. 6. For the ESD- damaged lasers, the wavelength shifts are distributed below these without ESD stress.

ESD-induced defects in the active region could be annealing out or growing during aging. In most of lasers, LI and IV characteristics were slightly improved after flowing the current from 0 to 200 mA. For some lasers, LI and IV characteristics were fully recovered. This means that the defects generated from ESD stress are annealing out with flowing the current. Therefore, in the ESD exposed lasers during aging, the defect may be slowly annealing out and then the carrier effect becomes the dominent role in determining the lasing wave- lengths. The aging rates are measured to be less than O . l % k h from both group of lasers. ESD-induced defects do not signifi- cantly contribute to increase the aging rates. We could not see the reliability impact due to ESD stress. As mentioned above, we will exmanine the defect structure after equipping the proper facilities and then we can draw more clear conclusions.

IV. CONCLUSIONS The wavelength shifts of less than 2 A were measured from

1.5 pm DFB lasers after reverse-biased ESD stress. The lasing wavelengths after ESD exposure were red-shifted except for

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Fig. 6. Normal distribution plot of the wavelength shifts after aging with ESD damage (A) and without ESD damage (0).

one laser, although the threshold currents were increased. The wavelengths are ususally moved toward the lower wavelength region (blue-shift) due to injection of more carriers to over- come the loss (the portion of the threshold current change). However, almost all lasers have the red-shifted wavelengths. The heating effect due to increased carriers may be more dominant factor than the carrier effect to determine the lasing wavelength.

The ESD-damaged lasers were subsequently aged at 60°C and 3 mW for lo00 hours to see the behaviors of the wavelength shifts. After aging, all the lasers have the blue- shifted wavelengths of less than 0.5 A. The aging rates are less than O.l%/kh. The aged lasers without ESD exposure have similar aging rates and have the red-shifted wavelengths. The wavelengths are shifted to the opposite direction after aging. This implies that ESD stress affects not to the aging rate but to the lasing wavelength shifted to the reverse direction.

REFERENCES

[ l ] T. Li, “The impact of optical amplifiers on long-distance lightwave telecommunications,” Proc. IEEE, vol. 81, no. 11, pp. 1568-1579, 1993.

[2] E. Desurvire, “The golden age of optical fiber amplifiers,” Physics Today, pp. 20-27, Jan. 1994.

[3] W. B. Sessa, R. E. Wagner, and P. C. Li, “Frequency stability of DFB lasers used in FDM multi-location networks,” I992 OFC Tech. Dig. Ser., San Jose, CA, vol. 4, paper ThC3, 1992.

[4] R. S. Vodhanel, M. Krain, R. E. Wagner, and W. B. Sessa, “Long-term wavelength drift of the order of -0.01 n d y r for 15 free-running DFB laser modules,” San Jose, CA, vol. 4, paper WG5, 1994.

[5] Y. C. Chung and Jichai Jeong, “Aging-induced wavelength shifts in 1.5 pm DFB lasers,” 1994 OFC Tech. Dig. Ser., San Jose, CA, vol. 4, Paper WG6, 1994.

[6] S. P. Sim, M. J. Robertson, and R. G. Plumb, “Catastrophic and latent damage in GaAlAs lasers caused by electrical transients,” J. Appl. Phys., vol. 55, pp. 3950-3955, 1984.

[7] L. F. DeChiaro and C. J. Sandroff, “Improvements in electrostatic discharge pereformance of InGaAsP semiconductor lasers by facet passivation,” IEEE Trans. Electron Devices, vol. 39, no. 3, pp. 561-565, 1992.

[8] S. Shin and C. B. Su, “The sublinear relationship between index change and carrier density in 1.5 and 1.3 pm semiconductor lasers,” IEEE Photon. Technol. Lett., vol. 4, no. 6, pp. 534-537, 1992.

I.,

190 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 2, FEBRUARY 1995

Jichai Jeong (S'83-M87), photograph and biography not available at the time of publication. publication.

H. M, Park (h4'88), photograph and biography not available at the time of

K. H. Park, photograph and biography not available at the time of publication.

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