The Study of Relaxation Time in Test of 940 nm Semiconductor Laser

doi:10.4236/jcc.2013.17011

Paper Menu >>

Journal Menu >>

Journal of Computer and Communications, 2013, 1, 46-49

Published Online December 2013 (http://www.scirp.org/journal/jcc)

http://dx.doi.org/10.4236/jcc.2013.17011

Open Access JCC

The Study of Relaxation Time in Test of 940 nm

Semiconductor Laser*

Jiachun Li, Jianjun Li, Tao Liu, Bifeng Cui, Jun Deng, Jun Han, Linjie He, Shengjie Lin

Key Laboratory of Opto-Electronics Technology, Beijing University of Technology, Beijing, China.

Email: [email protected]

Received September 2013

ABSTRACT

Conventional test of the peak wavelength of a laser used to be applied immediately after a device is injected current.

However, the results can not be considered as an accurate description to temperature characteristic. This passage puts

forward a concept of relaxation time in wavelength texts, mainly based on the experiment of 940 nm strain quantum

well laser, confirming that under constant current, wavelength will get through a process of rising, and then, reach the

limit. This process brings th e effect on sp ectral characteristics of a d evice which cannot be ignored and the accumulated

heat in relaxation time will gradually impact the emission wavelength of the laser, even crest split to form bimodal phe-

nomenon.

Keywords: Semiconductor Laser; Test; Temperature Characteristics; Relaxation Time

1. Introduction

With the development of fiber-optical technology, the

fiber pump sources manufacture has been the critical te-

chnology to improve the signal transmission efficiency.

The strongest absorption peak of Yb-doped optical fiber

is around 976 nm [1]. However, the absorption peak is

sharp; as a result, the requirements of wavelength and

bandwidth ar e very strict, which makes it suitable for the

application of pulse signal. While the absorption peak

near 940 nm has high absorption bandwidth, it will not

appear concentration quenching phenomenon, which is

beneficial to the application of continuous signal. As a

consequence, the research of laser pump sources used by

940 nm semiconductor laser with reliable and stable high

power output has a gr e a t signifi c a nc e .

In fact, output power of laser is constantly improving,

and the problem of heating effect becomes more serious.

The quantity of heat produced by a working laser makes

the temperature of active layers rise sharply, which re-

sults in the wavelength red shift and the cavity surface or

interior to burn down [2]. So it is an important issue to

study the temperature character of the device. Nowadays,

the study on testing the heat character of the device by

controlling the heat or changing its current is major. But

it is rare to study the effect on the device itself made by

the heat which is accumulating with time and produced

by the device when it works under constant current. This

paper will study and analyze this effect, and makes it

clear that the influence on the spectral characteristic of a

device. Moreover, we put forward a concept of relaxation

time that wavelength could test after it, which can esti-

mate a device’s spectral character more accurately.

2. Put Forward the Inference

In conventional test of semiconductor laser, the test is

used to be applied immediately after a device is injected

current. However, in practical applications, the device often

works under constant current conditions, Understanding

the effect on wavelength characteristics by the device’s

heat accumulation over time will help to accurately as-

sess the thermal properties.

Wavelength change is due to heating of the device

causing the band gap widens, thereby causing the de-

crease of photon energy, and the increase of wavelength.

Heating of a device operates primarily from the P-clad-

ding layer, P-cladding layer is heavily doped, the resis-

tance rate is high, the device generates a lot of heat while

continue to work. In certain process conditions, the vo-

lume resistivity will generate heat in a certain range, with

certain cooling capacity of the package, we predicted that

if a device work under a certain current conditions, after

some time, heating degree will reach a state of equili-

brium. As a response, the temperature and wavelength of

the device will reach a state of dynamic equilibrium the

Supported by the advanced technology fund of Beijing University of

Technology, No.002000514312003.

The Study of Relaxation Time in Test of 940 nm Semiconductor Laser

Open Access JCC

same. As this process related to the change of tempera-

ture, we called it as dynamic thermal relaxation process,

the corresponding time is defined as the thermal relaxa-

tion time τT.

In order to verify the existence of relaxation time, we

carried out a set of validation experiments.

3. Experimental Verification

3.1. Chip Materials and St ructural Design

An epitaxial wafer used in the test had been grown b y the

MOCVD system, the epitaxial structures are shown in

Figure 1, the epitaxial layer were mainly in turn: GaAs

substrate; GaAs buffer layer; Al0.3GaAs, up and down

cladding layers, 700 nm; the two Al0.1GaAs/InGaAs com-

posed of strained quantum well optical waveguide layer;

GaAs ohmic contact thickness 450 nm. Center wavelength

is designed to 940 nm.

The electrode structures of the device are designed to

ridge, the ridge width of 100 μm. The current blocking

layer is grown by using PECVD systems, 200 nm of SiO2

material are obtained, P-type electrode material is Ti an d

Au, N-type electrode material is AuGeNi alloy and Au,

TO3 package with seat tube, adapter copper heat sink for

the heat sink, the package will be P type electrode lay a flip

process, the final packaged chip cavity length is 2000 μm.

3.2. Experimental Design and Installation

Experiment is using the spectral tester to test the spectral

characteristics of the instrument, first set up the device

test injection current I0 and projected peak wavelength λp,

then began spectral measurement, the instrument will

automatically λp’ centered at 40 nm spectral range scan,

the spectrum to be combined with precise measurements

to complete the rotation of the grating, spectral measure-

ments to each scanning time 25 s, 25 s intervals so expe-

riment a peak wavelength for data recording, the device

is continuously administered during the current I0.

Figure 1. Epitaxial structure.

3.3. Results and Discussion

Figure 2 shows a spectrum when the injection current I0

is 1.5A. Four curves mean four devices which fabricated

by the same process and packaged. Abscissa represents

for the time of duration injection current I0, and ordinate

represents the output peak wavelength of light λp, injec-

tion current always remains the same. The change of

peak wavelength would be obtained by the curves in

Figure 2 The peak wavelength gets to an extreme value

with the time increasing. This verifies the existence of

the thermal relaxation time. The period, which from the

beginning of the test to the peak wavelength gets a steady

data, is called the test relaxation time.

The following equation which relates to peak wave-

length λp and the relaxation time τT can be got.

λp = a/(1 + c*exp(−b*τT)) (1)

In Equation (1), a, b and c are model parameters, and

different parameters corresponding to different devices.

In this paper, it based on device No.1 as an example for

fitting. The fitting curve showed in Figure 3 and gets

that a = 950.7，b = 0.01114，and c = 0.009262.

Figure 2. I0 = 1.5A Red shift curve of peak wavelength in

relaxation time.

Figure 3. Simulation of peak wavelength changing in relax-

ation time.

Al0.1GaAs

GaAs

Al0.3GaAs

GaAs

Al0.1GaAs

GaAs

Al0.3GaAs

InGaAs

The Study of Relaxation Time in Test of 940 nm Semiconductor Laser

Open Access JCC

4. Further Study on Relaxation Time

In order to study the variation of spectral characteristics

with time in the relaxation time, we carried out the fol-

lowing experiments:

4.1. The Change of Peak Wavelength during

Relaxation Time

One device set to continuous testing with change the in-

jection current I0. Firstly, the inj ection current of 1.5A is

set. Each 25 seconds record its emission peak wavelength

and until the peak wavelength remains a steady value and

dynamic stability condition. Then set the injection cur-

rent to 1.5A and repeat the step above to measure the

data. Later in the same method the peak wavelengths is

measured at 2A and 3A Units

Figure 4 shows the spectrum chart when the injection

current at 1A, 1.5A, 2A and 3A. After relaxation time,

which keep applying test current at 1A, a limit peak wa-

velength of 944 nm and 2.5 nm red shift are obtained. At

I0 = 3A, red s hift gets to 5 nm during the relaxation time.

Figure 3 shows when larger injection current is meas-

ured the larger limit value of red shift should be got. And

increased the current value by a step 1A, the change of

wavelength most gets to steady state within 300 seconds.

It cannot ignore the heat accumulated with the increase

of time even run at a constant current. A deviation is

taken into the test results by the heat effecting and when

increasing injection current the heat effect on emission

wavelength of the device is also gradually increased.

Predictably, when high power laser continue to run at a

large current the heat produced during relaxation time

will have great influence on the output wavelength. There-

fore, to get a more accurate measure of the characteristics

the influence of relaxation time would be considered.

4.2. Device Temperature Varia tion in Relaxation

Time

In order to study the relationship between device temper-

Figure 4. Redshift curve of peak wavelength in different

current conditions.

ature and peak wavelength in relaxation time, we did a

test to observe temperature changes. We use a cooler to

keep heat sink at room temperature 25˚C, the measured

temperature with instrument is the highest temperature of

the central area of the device. Inject a constant current of

3A to the device, record the temperature and the peak

wavelength every 25 s. As shown in Figure 5, curve

slowly down after, and gradually stabilized after 200 s.

So, the temperature change as we predicted in the pre-

vious experiment. From the curve, in 3A constant current,

we can see that the device temperature changes from 302

K to 307 K, the average rate of change over time as

0.025 K/s in the relaxation time.

Figure 6 is the relationship curve between the device

temperature and the peak wavelength, the wavelength

increases with increasing temperature. As shown in data

distribution, data corresponding to the abscissa increa-

singly dense, indicating that changes of temperature the

more backward the smaller it is. The rising trend has be-

come increasingly slow curve, corresponding to the wa-

velength change it is getting smaller and.

Figure 5. I0 = 3A The temperature of device change in re-

laxation time.

Figure 6. I0 = 3A Device temperature and the peak wave-

length of the output light of the curve.

The Study of Relaxation Time in Test of 940 nm Semiconductor Laser

Open Access JCC

5. Summary

Through the test of the wavelength of 940 nm strain

quantum well laser under the constant current, the rise of

temperature and the temperature will reach the limit.

With the constant current increased, the limit value will

be greater. At room temperature, under 3A continuous

current condition, the peak wavelength of the laser will

reach steady state after 300 s. Compared with the imme-

diately measured data, the wavelength has increased by 4

nm and the temperature increased by 5˚C. And as the

current increases, the accumulated heat in relaxation time

will gradually impact the emission wavelength of the

laser, even crest split to form bimodal phenomenon. Un-

der the constant current, in order to get a well output

spectral characteristics, the accumulated heat in relaxa-

tion time must be considered. Selecting suitable operat-

ing current should be based on the device performance

after the relaxation time.

REFERENCES

[1] R. Pathak, J. M ine lly , J. Haapamaa, J. Watson, D. Schleu-

ning, H. Winhold, et al., “915 nm Laser Bar-Based High-

Performance Sources for Fiber Laser Pumping,” 2009,

Article ID: 719808

[2] X. Z. Ma, J. Huo, Y. Qu and S. L. Du, “8 Temperature

Characteristics of 808 nm Semiconductor Lasers,” Infra-

red and Laser Engineering, Vol. 12, 2010, pp. 1306-

1309.

[3] G. R. He, W. J. Shen, Q. Wang, W. H. Zheng and L. H.

Chen, “Temperature Characteristics of 980 nm High Pow-

er Vertical Ca vity Surface Emi tting Lasers,” Infrared and

Laser Engineering, Vol. 1, 2010, pp. 57-60.

[4] H. W. Qu, X. Guo, L. M. Dong, J. Deng, X. L. Da, Z. T.

Xu and G. D. Shen, “St udy on the Temperature Characte-

ristics of Vertical Cavity Surface Emitting Laser,” Infra-

red and Laser Engineering, Vol. 2, 2005, pp. 83-86.