Wavelength stabilized 980 nm uncooled pump laser modules for erbium-doped fiber amplifiers

Optics and Lasers in Engineering 43 (2005) 271–289

Wavelength stabilized 980 nm uncooled pumplaser modules for erbium-doped fiber amplifiers

Tomas Pliska*, Sebastian Arlt, Rainer B.attig, Tim Kellner,Isabella Jung, Nicolai Matuschek, Pascal Mauron, Bernd Mayer,

Stefan Mohrdiek, J .urgen M .uller, Susanne Pawlik,Hans-Ulrich Pfeiffer, Berthold Schmidt, Boris Sverdlov,

Stefan Teodoropol, J .org Troger, Bernd Valk, Christoph Harder

Bookham (Switzerland) AG, Binzstrasse 17, CH-8045 Z .urich, Switzerland

Received 28 November 2003; received in revised form 9 February 2004; accepted 13 February 2004

Available online 22 July 2004

Abstract

We review the development of wavelength stabilized 980 nm pump laser modules without

active temperature stabilization for applications in erbium-doped fiber amplifiers. Operation

over a wide temperature range with an output power exceeding 400mW at an ambient

temperature of 70�C is demonstrated. The overall reliability of uncooled modules is estimated

to be well below 500 FIT at all operating conditions. Such devices are made possible by

continuous development and steady improvement of the pump laser chip, the optimization of

the fiber Bragg grating stabilization scheme, careful design of the module package, and

extended reliability analysis on the basis of stress tests as well as field data.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Semiconductor laser; Pump laser module; Uncooled module; Erbium-doped fiber amplifier;

Fiber Bragg grating

1. Introduction

Semiconductor lasers deployed in fiber-optical data transmission networksessentially accomplish two tasks: emitting the signal in the 1520–1620 nm bands,

ARTICLE IN PRESS

*Corresponding author. Tel.: +41-1-455-85-85; fax: +41-1-455-85-86.

E-mail address: [email protected] (T. Pliska).

0143-8166/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.optlaseng.2004.02.004

and amplifying it by pumping Erbium-doped fiber amplifiers (EDFAs) in theirabsorption bands centered around 980 and 1480 nm. The periodic amplification ofthe transmitted signal is a key operation in today’s optical sub-marine, terrestriallong-haul, and metropolitan networks. EDFAs have established themselves as thepreferred devices for efficient optical amplification. Their performance dependscritically on the characteristics of the lasers used to pump the erbium atoms intoupper level energy states to enable stimulated amplification of the signal in the1.5 mm-wavelength band. Traditionally high power EDFAs use a combination of980 nm pump lasers in the front stage for low-noise pre-amplification and 1480 nmpump laser in the power stage for boosting the output power.

As the focus in optical telecommunication systems turns more and more towardsaffordability, there is a trend to produce EDFAs of lower cost, reduced powerconsumption, and increased compactness. A typical example of simplifying thepackaging and functionality of an opto-electronic device is the elimination of thethermo-electric cooler (TEC) from pump laser modules. TECs have beentraditionally used to maintain the laser heat sink at a constant temperature (usuallyaround 25�C) regardless of changes in the environmental conditions. The removal ofthe TEC enables so-called uncooled pump modules of smaller size consuming lesselectrical power, and allows for EDFA designs of reduced complexity and cost [1,2].With respect to power consumption, 980 nm pump lasers offer inherent advantagesover their 1480 nm counterparts: they are more efficient and thus dissipatesignificantly less power, facilitating thermal management of uncooled modules.

The accurate definition of the pump laser wavelength is essential for optimumEDFA performance, particularly for maintaining gain flatness over a wide band ofpumping conditions. This is especially important in uncooled modules where largevariations of the ambient temperature would cause shifts of the laser emission intowavelength regions outside the erbium absorption band. Fiber Bragg gratings(FBGs) are the preferred tools to stabilize the laser emission wavelength around980 nm, because of their versatility, good manufacturability in large volumes, andlow cost [3,4].

For 980 nm pump laser modules the target specifications defined by the newapplications may be summarized as follows: a wavelength-stabilized output power of100–200mW with a prospective increase to 300mW in future generations at a powerconsumption of less than 1W, operation at case temperatures from 0�C to 70�C, anda total module failure rate below 500 FIT, 1 FIT being defined as 1 failure in 109 h ofoperation. In this article, we review our recent progress in 980 nm pump laser andmodule development, and discuss our roadmap towards further optimization of thisnew type of product.

2. Pump laser development

The heart of any pump laser module is the laser diode chip. There has beenconsiderable activity in recent years by several organizations to obtain a stable beamfrom lasers operating at high power using sophisticated chip designs, such as master-

ARTICLE IN PRESST. Pliska et al. / Optics and Lasers in Engineering 43 (2005) 271–289272

oscillator power-amplifier (MOPA) structures, distributed feedback lasers (DFB),external cavity lasers, large mode lasers etc. using different epitaxial growthtechniques [5–8]. Our approach for realizing a high power single-mode laser chip isbased on the continuous development and improvement of the well-understood andwell-established ridge waveguide Fabry–P!erot laser concept [9]. The heterostructurefor our laser devices is grown by molecular beam epitaxy (MBE). It is based on anInGaAs quantum-well (QW) active region embedded into a thin AlGaAs opticalcavity waveguide. The progress in our laser chip development, enabling a steadyincrease of the fiber-coupled output power from about 100mW up to 1W, has beendocumented in several articles [10–17]. With our recent progress in 980 nm laserdiode development, enabling pump modules with a power exceeding 1W coupledinto a single-mode fiber, we envision future EDFAs solely pumped by 980 nmmodules.

Fig. 1 shows how design, manufacturing, and testing improvements have allowedfor a continuing increase in roll-over as well as rated operating power, starting withour generation-03 (G03) laser chip introduced in 1997 followed by the subsequentgenerations up to the G08 laser currently (October 2003) in the qualification process.The more recent generations G06, G07, and G08 form the basis for the uncooledmodules discussed in this article.

A key to progress in laser diode development is the optimization of the basicheterostructure with respect to index and doping profile for high power conversionefficiency, and of the ridge waveguide for good beam stability. The device structure isdesigned such that a balance between laser efficiency, fiber coupling efficiency,thermal properties, temperature stability, and reliability is found for optimizedoverall performance. This optimization process is particularly important for laserdiodes for uncooled applications that have to withstand harsh operating conditions

ARTICLE IN PRESS

0 500 1000 1500 2000 2500 30000

500

1000

1500

2000

G08

G07

G06

G05

G03

Po

we

r (m

W)

Current (mA)

Fig. 1. Optical output power from the laser facet as function of injection current for five different

generations of 980 nm pump lasers developed since 1997 (G03: 1997, G05: 1999, G06: 2001, G07: 2002,

G08: 2003). The laser diodes are mounted junction side up and operated under continuous-wave emission

at a heat sink temperature of 25�C.

T. Pliska et al. / Optics and Lasers in Engineering 43 (2005) 271–289 273

[18]. Simulation and design are realized using two- and three-dimensional modelingtools that simulate the laser performance by solving Poisson’s equation incombination with the carrier continuity equations, the heat transport equation,and the laser rate equations [18,19]. By this means, we have designed our recent laserstructures with improved quantum efficiency and high lateral beam stability at highinjection currents. In addition, the present design gives low divergence of the opticalfar field in both lateral and vertical directions to enable efficient coupling from thelaser into the single-mode fiber.

In Fig. 2, we plot the ex-facet and fiber-coupled output power as well as the wall-plug efficiency, generally defined as the ratio of optical output power to electricalinput power, of our most recently developed laser generation [16]. The light iscoupled into a Corning HI1060 single-mode fiber with a wedged fiber lens at the fibertip. For this temperature stabilized device, the laser power exceeds 1.2W at the drivecurrent of 1.4A, while 1W is coupled into the fiber with a coupling efficiency of morethan 80%. The wall-plug efficiency, calculated from the fiber-coupled power, is stillhigher than 40% at this point. The lateral and vertical optical far fields of the laserdiode are shown in Fig. 3 for several power levels, confirming single spatial modeoperation up to the output power of 1.2W. As the drive current changes, the farfield’s full-width half-maximum divergence angle changes from 5.5� to 8� and from19� to 20� in the lateral and vertical directions, respectively.

For use in uncooled modules it is important that the temperature dependence ofthe laser diode’s threshold current and slope efficiency is weak. In particular, asufficiently high characteristic temperature To on the order of 150K is required. Ourlaser devices satisfy this requirement, hence allowing for efficient operation of thelaser chip even at elevated temperatures.

ARTICLE IN PRESS

1200

1000

800

600

400

200

0

Pow

er (m

W)

120010008006004002000

Current (mA)

50

40

30

20

10

0

Wal

l-plu

g ef

ficie

ncy

(%)

ex-facet power

fiber output power

Fig. 2. Optical output power (ex-facet and fiber-coupled; left scale) and wall-plug efficiency (for fiber-

coupled power, right scale) at 25�C heat sink temperature as a function of injection current for a G08

pump laser chip. Here, the wall-plug efficiency is defined as the ratio of fiber-coupled optical power to

electrical input power.

T. Pliska et al. / Optics and Lasers in Engineering 43 (2005) 271–289274

3. Fiber Bragg grating stabilization

An FBG is a periodic modulation of the refractive index in the fiber written byillumination with ultra-violet or visible light [20,21]. In modern optical tele-communication systems, FBGs are used for a large variety of functions, such asfiltering, de-multiplexing, add–drop operations, dispersion compensation, gainflattening in EDFAs, wavelength stabilization, use as reflectors in Raman amplifiers,sensing, etc. [22,23]. In combination with pump lasers, the principal role of theFBG is to provide controlled optical feedback from a narrow spectral band in orderto lock the laser to a defined emission wavelength that matches the erbiumabsorption band.

The basic configuration of an FBG stabilized pump laser is schematicallyshown in Fig. 4 together with a photograph of the interior of the module housing.The FBG is usually positioned 1–2m away from the laser, beyond the lasercoherence length. It is characterized by three parameters: center wavelength, peakreflectivity, and spectral bandwidth. The center wavelength is located within theerbium absorption band around 980 nm. The typical parameter range for thereflectivity is 0.01–0.1 and for the bandwidth 0.5–2.5 nm. The rear-side mirror of thelaser has a reflectivity of 0.95. The front mirror reflectivity is adapted for everychip generation for optimized laser efficiency and reliability, depending on chiplength and epitaxial structure, and varies between 0.005 and 0.05. A monitorphotodiode mounted behind the laser back facet inside the module is sometimes usedfor power control.

The FBG enhances the effective feedback in the aforementioned narrow spectralband, thus increasing the laser effective front reflectivity. The lasing condition isgiven by

gmodðlÞ ¼ ai þ1

2Lln

1

RbRf ;eff ðlÞ

� �; ð1Þ

ARTICLE IN PRESS

200

150

100

50

0

Inte

nsity

(a.

u.)

-40 -20 0 20 40

Lateral angle (degrees)

200

150

100

50

0

Inte

nsity

(a.

u.)

-40 -20 0 20 40

Vertical angle (degrees)

Fig. 3. Lateral (left) and vertical (right) far field intensity profile at injection currents of 300, 600, 900, and

1200mA (inner to outer curves) for the same high power pump laser diode as in Fig. 2.


where gmod indicates the modal gain, ai is the internal loss coefficient of the lasercavity, L the cavity length, Rb the laser back facet reflectivity, and Rf,eff thecompound reflectivity of the laser front facet and the FBG. Eq. (1) is preferentiallysatisfied within the wavelength band defined by the FBG, hence inducing lasing atthis particular wavelength, although the laser gain maximum may be located at adifferent wavelength. The laser is locked as long as Eq. (1) is satisfied within thespectral band defined by the FBG. As the laser gain curve shifts in wavelength withchanging current and temperature, loss of locking to the FBG occurs if the detuningbetween FBG wavelength and laser gain maximum becomes large and the strength ofthe FBG is no longer sufficient to satisfy Eq. (1) at the FBG center wavelength. Theset of operating conditions in current and temperature in which locking ismaintained is referred to as locking range.

It is obvious that the knowledge of the detailed shape of the laser gain spectrumand its dependence on injection current and temperature is essential to model theproperties of the device. We measure the gain spectra of our devices by the methodsof Hakki and Paoli [24] as well as that of Cassidy [25]. More recently, we havedeveloped a method based on return loss measurement for determining the laser gainof fiber coupled devices mounted into modules [26]. This technique is particularlyuseful for long-cavity Fabry–P!erot diode lasers for which the methods by Hakki–Paoli and Cassidy are not readily applicable. An example of its application is shownin Fig. 5 where we plot the gain curves of one of our lasers as a function ofwavelength with the injected current set to B0.01, 0.45, 0.60, 0.75, and 1.06 times itsthreshold value. Performing similar measurements at constant current abovethreshold at different temperatures, we obtain the full dependence of the laser gainon current and temperature. These measured gain spectra are then used in our

ARTICLE IN PRESS

Laser chip

Fiber Bragggrating

Monitordiode

Fiber

Thermistor

Carrier platformLensedfiber

Soldered fiber

13 mm

7 mm

Fig. 4. Photograph and schematic of a fiber Bragg grating (FBG) stabilized pump laser inside a module.

The light is coupled into the fiber by means of a lens fabricated on the tip of the fiber. The metalized fiber is

directly soldered onto the carrier platform. The FBG is separated from the laser by up to a few meters of

fiber. The monitor diode at the laser rear facet can be used for power control. The thermistor is used for

temperature monitoring. The relative dimensions in the schematic are not to scale.


simulation tools to find the adequate laser and FBG parameters for locking in agiven range of operating conditions.

The ability to accurately predict the locking range is particularly critical foruncooled modules where locking to the FBG has to be maintained despite largevariations in operating temperature. The uncooled pump laser modules used inmetropolitan network systems are supposed to operate in a temperature range of0–70�C at least. The effect of wavelength stabilization by an FBG over a wide rangeof operating conditions is illustrated in Fig. 6 [27]. In Fig. 6a, we plot the wavelengthof an unstabilized G06-type laser module as a function of injection current atdifferent temperatures ranging from �15�C to 120�C. The overall variation of theunstabilized laser emission wavelength is more than 30 nm in this range of operatingconditions. This variation in lasing wavelength is essentially determined by thespectral shift of the gain curve shown in Fig. 5. The wavelength shift is compared tothe spectral dependence of the sharply peaked 4I11/2 erbium absorption band: it isobvious that the wavelength of the freely running laser does not coincide with themaximum of the erbium absorption for most of the operating conditions. The samedependence for the stabilized laser is shown in Fig. 6b. For all operating conditionsthe emission wavelength is now nearly constant and well centered at the peak of theabsorption band. The remaining wavelength shift, shown in the inset of Fig. 6b, isdue to the thermal shift of FBG center wavelength, taking a value of only 7 pm/K,two orders of magnitude smaller than the temperature shift rate of the laser gain.The emission spectra of the stabilized laser at the extreme temperatures of �15�Cand 120�C are shown in Fig. 7. More than 90% of the total power is contained in aspectral band of less than 1.5 nm width centered at the FBG wavelength of 980 nm.

Comprehensive understanding of the pump laser module’s properties is importantfor designing EDFAs. In order to simulate the properties of the stabilized lasermodule such as output power from the fiber and back facet, fiber coupling losses,

ARTICLE IN PRESS

950 960 970 980 990 1000 1010 1020 1030-20

-10

0

10

20

I/Ith<0.010.450.600.751.06

Mo

da

l ga

in (

cm-1

)

Wavelength (nm)

Fig. 5. Experimentally measured gain as a function of wavelength for a 980 nm pump laser. The

temperature is held constant, and the current is varied from B0.01 to 1.06 times the threshold value.


and threshold current, we have developed a model that is based on an extension ofthe well-known traveling-wave amplifier model [28]. The combined reflectivity of thelaser front facet and the FBG is taken into account by using an effective reflectorapproach [29]. Moreover, our modeling scheme includes possible losses in effectivefeedback due to variation of the polarization of the light propagating back and forthin a single-mode fiber forming an external cavity between diode laser and FBG [30].An example of such a simulation with comparison to experimental data is shownFig. 8 where we plot the fiber coupled and back facet power (Fig. 8a), as well as the

ARTICLE IN PRESS

0 50 100 150 200 250 300 350 400 450 500 550960

970

980

990

1000

1010

2 transition

of Er-doped fiber

100 oC120 oC

75 oC

25 oC

0 oC

-15 oC

Wa

vele

ng

th (

nm

)

Current (mA)

0 1 2 3 4 5 6 7 8 9 10 11 12 13Absorption coefficient (a.u.)

(b)

(a)

0 50 100 150 200 250 300 350 400 450 500 550960

970

980

990

1000

1010

Absorption coefficient (a.u.)

-15 oC

120 oC

4I11/2

transitionof Er-doped fiber

Wa

vele

ng

th (

nm

)

Current (mA)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

-20 0 20 40 60 80 100 120979.0

979.5

980.0

980.5

981.0

wa

vele

ng

th (

nm

)

temperature ( C)

Fig. 6. (a) Peak emission wavelength as a function of injection current of an unstabilized laser (G06-type)

operated at temperatures ranging from �15�C to 120�C. For comparison we plot the absorption band of

the erbium-doped fiber (upper scale). (b) Same as in (a) for the FBG stabilized laser. The inset shows in

detail the wavelength shift of the FBG center wavelength as a function of temperature at constant injection

current (solid line: linear fit with 7 pm/K shift).


threshold current (Fig. 8b) as a function of FBG reflectivity. As a general result, boththreshold current and output power of an FBG stabilized laser decrease as comparedto an unstabilized laser due to the increase in effective front reflectivity. However, arelatively small decrease in output power on the order of a few percent is clearly asmall penalty in comparison to the beneficial effect of wavelength stabilization by theFBG. On the other hand, the variation in back-facet power is quite significant, aresult to be taken into account when using the back facet photodiode to monitor thelasers output. The good agreement between calculation and measurement in Fig. 8confirms the capability of our model to accurately simulate the properties of a pumplaser module.

In a recently published article, we have extended our model in such a way that thedynamic behavior of the laser can be studied by using an improved set of rateequations [31]. This extended model is useful for investigating the spectral propertiesand their dynamic characteristics of the laser under arbitrary optical feedback. Inparticular, the model holds for the case of high optical feedback, as opposed tosimpler models found in the literature, such as the often used Lang–Kobayashimodel [32].

4. Module performance

Compactness and minimal power consumption are important requirements forsuccessful implementation of new generation pump laser modules. One of the maindifferences between temperature stabilized and uncooled modules is their packagingtechnology. While conventional modules using a TEC are most often based on so-called butterfly housings with a footprint of 30� 15mm2, uncooled modules use themuch more compact miniDIL packages with dimensions of only 13.2� 7.4mm2.

ARTICLE IN PRESS

950 960 970 980 990 1000 1010-40

-35

-30

-25

-20

-15

-10

-5

0

T=120 °CT=-15 °C

No

rma

lize

d p

ow

er

(dB

)

Wavelength (nm)

Fig. 7. Optical spectra of the same laser as in Fig. 6b at the temperatures of –15�C and 120�C.


The miniDIL package, shown in Fig. 4, is based on an 8-pin ceramic (Al2O3) ormetal housing without TEC inside. In order to maintain fiber coupling efficiencyover time and at varying ambient conditions, package design and materials arecarefully chosen [33,34]. The laser chip is mounted onto an Al2O3 or AlN carrierplatform that includes the back-facet photodiode, thermistor, and the fiber couplingarrangement. The platform is directly soldered to the base of the housing. The light iscoupled into the fiber by means of a cylindrical wedged fiber lens that is adapted tothe asymmetric laser far-field. The technique of direct soldering of the fiber to theplatform enables dense packaging inside the module without precisely machined,stress-free metal parts for holding and fixing the fiber.

We have reported on the development of our uncooled modules in a series ofarticles [2,27,35]. Our most recent results on FBG stabilized miniDIL modules using

ARTICLE IN PRESS

(a)

95

90

85

80

75

70

Fib

er o

utpu

t pow

er (

mW

)

543210

FBG reflectivity (%)

FBG reflectivity (%)

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

bac

k fa

cet p

ower

(b)

31

30

29

28

27

26

25

24

23

Thr

esho

ld c

urre

nt (

mA

)

543210

Fig. 8. (a) Fiber output power (solid triangles, left scale) and back facet power (open diamonds, right

scale) at constant current (I ¼ 150mA) as a function of FBG reflectivity for a G06 laser module. The full

and dashed lines indicate the respective calculated curves. The relatively large error bars for the power

measurement are a result of the uncertainty of the power measurement itself and varying splice loss, as for

every data point a new FBG had to be spliced to the module. (b) Threshold current as a function of FBG

reflectivity for the same device as in (a). The solid line indicates the calculated dependence.


the G08 laser chip are presented in Ref. [36]. In Fig. 9, we show the fiber outputpower as a function of injected current for a module wavelength-stabilized in atemperature range between 10�C and 100�C. The module’s output power exceeds500mW at a heat sink temperature of 25�C and 400mW at 70�C. At a temperatureof 100�C, an output power of still more than 200mW is obtained.

In order to control the power budget, efficient electrical-to-optical powerconversion within the module is important at all operating temperatures. Themaximum wall-plug efficiency of our laser diodes reaches 60% at 25�C and is stillnear 50% at 70�C. In Fig. 10 we show the module’s wall-plug efficiency, in this casedefined as the ratio of fiber-coupled optical power to input electrical power, as a

ARTICLE IN PRESS

0 100 200 300 400 500 600 700 800 900 1000 11000

100

200

300

400

500

600

Fib

er o

utpu

t pow

er (m

W)

Current (mA)

Fig. 9. Fiber output power as a function of injected current of a wavelength stabilized miniDIL module

using a G08-type laser chip at heat sink temperatures ranging from 10�C to 100�C.

0 100 200 300 400 500 600 700 800 900 1000 11000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

T=90 °C

T=70 °C

T=55 °C

T=25 °C

Mod

ule

wal

l-plu

g ef

ficie

ncy

Current (mA)

Fig. 10. Module wall-plug efficiency as a function of current for the same module as in Fig. 9 at the

temperatures of 25�C, 55�C, 70�C and 90�C. The wall-plug efficiency is defined as the ratio of the optical

fiber output power to the total electrical input power.


function of injected current at the temperatures of 25�C, 55�C, 70�C, and 90�C forthe same module as in Fig. 9. The maximum wall-plug efficiency for the module is36% and 30% at 25�C and 70�C, respectively. The difference in wall-plug efficiencybetween the bare laser chip and the module results from the fiber coupling losses andthe reduction of the laser efficiency due to the FBG (shown in Fig. 8a).

Fig. 11 compares the total electrical input power as a function of ambienttemperature for a typical temperature stabilized module and an uncooled moduleoperated at various optical output powers [35]. Modules using a TEC for activetemperature control require a total electrical power, including TEC drive power, ofabout 2.5W at 70�C and still 1.7W at 45�C at an optical output power of 200mW,whereas the power consumption of an uncooled module remains well below 1W.This comparison illustrates the beneficial effect of eliminating the TEC on overallpower requirements.

Yet another important aspect of FBG stabilized pump laser modules is their noiseat low frequencies (below about 50 kHz). The pump and amplification scheme oferbium in glass at 980 and 1550 nm, respectively, is based on a three-level system.Photons at 980 nm are absorbed from the 4I15/2 ground state to the 4I11/2 upper-levelstate, from which a fast non-radiative transition occurs to the 4I13/2 intermediatestate. Stimulated emission in the 1550 nm-band takes place between the 4I13/2 stateand the ground state. Because of the long lifetime (on the order of ms) of theintermediate 4I13/2 state, the erbium’s response follows a low-pass filter character-istic. Therefore, only pump power fluctuations in the kHz-regime are important andneed to be minimized, whereas high-frequency (above approximately 50 kHz) noise isfiltered by the erbium’s slow response. Power variations dP (in dB) of pump lasers

ARTICLE IN PRESS

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Ele

ctric

al in

put p

ow

er (

W)

9080706050403020100

Ambient temperature (°C)

100 mW

150 mW

200 mW

200 mW, TTEC=25 °C

200 mW, TTEC=45 °C

without TEC

with TEC

Fig. 11. Total electrical power required for operating a pump module as a function of ambient

temperature. Open squares represent measurements for temperature stabilized modules with a thermo-

electric cooler (TEC) operated at an internal temperature of 25�C and 45�C, respectively, and a fiber

output power of 200mW. In this case, the electrical input power is the sum of the laser and TEC drive

powers. Solid diamonds indicate measurements for uncooled operation at fiber output power levels of 100,

150, and 200mW.


are often defined as

dP � �10 logPav � ðPmax � PminÞ

Pav

� �; ð2Þ

where Pav, Pmax, and Pmin is the average, maximum, and minimum power,respectively, measured in a given time interval, usually seconds to minutes. Thetypically accepted power fluctuation for a 980 nm pump is on the order of 0.2 dB(5%). In Fig. 12 we plot the power variation as a function of output power for thefour operating temperatures of 0�C, 23�C, 47�C, and 70�C for a G07 laser mountedin a miniDIL module. Each data point represents a measurement at constantinjection current in a noise bandwidth of 50 kHz and a sampling time interval of 5 sin which the values of Pav, Pmax, and Pmin are determined. The power variation ofthis device is below 0.1 dB at all four operating temperatures, illustrating the goodpower stability of our uncooled modules.

5. Reliability testing

As a common practice, the reliability of an uncooled low-cost 980 nm pumpmodule is specified below 500 FIT (1 FIT=1 failure in 109 h of operation), whilepractically, it is lower. In general, one can distinguish between two failuremechanisms: sudden failure of the laser chip, and wear-out-induced changes inchip-to-fiber coupling as the most likely package failure mechanism. While in thecase of sudden laser diode failure a catastrophic breakdown of the optical outputpower occurs, package wear-out necessitates a steady increase of drive current inorder to maintain constant optical output power. Usually, a module is considered awear-out fail, if an increase in drive current by more than 20% relative to the initialdrive current is needed to compensate for the decrease in optical power. The overallmodule reliability can be estimated by adding chip and package contributions.

ARTICLE IN PRESS

0 50 100 150 200 250 300 3500.0

0.1

0.2

0.3

0.4

0.5

T=0 οC T=23 οC T=47 οC T=70 οC

Po

we

r va

ria

tion

(d

B)

Fiber output power (mW)

Fig. 12. Power variation according to Eq. (2) as a function of fiber output power for an uncooled G07-

type miniDIL module measured at heat sink temperatures of 0�, 23�, 47�, and 70�C.


Reliability testing is based on accelerated life tests with different stress conditions.The reliability of a pump laser chip is a function of its junction temperature,operating current, and optical output power. The failure rate is usually described bya phenomenological model that takes the form

FRBjxpy expð�Ea=kBTÞ ð3Þ

in which j is the current density, p the optical power, and T the junction temperature.The quantities x, y, and Ea are fit parameters derived from stress tests. While theseparameters may change with each new chip generation, the functional form is wellestablished and covers past and current generations alike. For the G06 laser chip, forexample, the respective values are x ¼ 2:2; y ¼ 1:8; and Ea ¼ 0:45 eV. Similarparameters are derived for the other laser generations. The G06 laser chip has beenqualified according to submarine standards for an operating power of 330mW witha failure rate of less than 100 FIT, while the more recent G07 laser chip is rated foran output power of 500mW with a failure rate of still less than 120 FIT.

All of our pump lasers rely on the same proven design with a thermally robustridge waveguide structure. The enabling technology for highly reliable pump lasershas been the introduction of our proprietary E2 facet passivation technique that hasessentially eliminated the effect of catastrophic optical damage (COD) on the facets,often considered as the most frequent failure mode in GaAs-based laser devices[11,37]. In the past years, we have constantly increased the output power of ourpump lasers and simultaneously improved their reliability, mainly driven by theparticularly stringent reliability requirements for sub-marine applications. By thismeans we have steadily broadened our database that also serves for reliabilityassessment of our uncooled devices [38].

While reliability testing based on high-stress tests always requires extrapolation tonormal operating conditions, field data provide direct confirmation of productreliability. We have field reliability data available for more than 200 000 devicesdeployed since 1995. Moreover, a mildly accelerated long-term life test with ninefirst-generation pump lasers was started in 1990. As outlined in Refs. [38,39], bothdata sets, the one from the field and the one from the long-term experiment, suggesta failure rate below 50 FIT for these early generation lasers operating undertemperature-stabilized conditions, in full agreement with our reliability model andextrapolation from accelerated stress tests.

Because uncooled modules are operated over an extended temperature range ascompared to submarine or temperature-stabilized terrestrial pump modules, theirreliability testing has to be performed at even higher temperatures and power levels.We have recently discussed reliability proving of uncooled pump lasers in Ref. [39].One of the most important trade-offs for the reliability of uncooled pump lasers isshown in Fig. 13 in which we plot the calculated failure rate of a G07 laser chip as afunction of module output power at two different ambient temperatures (45�C and70�C), assuming thermal conditions as in an Al2O3-based miniDIL package. Thecalculation suggests a chip failure rate of about 250 FIT even at the extremeoperating conditions of 70�C and 200mW fiber output power. In typicalenvironmental conditions, the module is operated for most of the time at


temperatures that do not exceed 45�C in which case the chip failure rate is below 120FIT at 200mW fiber output power.

Our package design, as outlined in Section 4, is key to reducing the probability ofpackage wear-out fails. Our approach is to mount the laser chip, back facet monitorphotodiode, thermistor, and the fiber coupling arrangement onto a commonplatform that is soldered directly to the package base [34]. This near planar designenhances reliability in two ways. Mismatches in the coefficients of thermal expansionare minimized, reducing thermal stress during assembly and under varying ambientconditions in the field. In addition, the optical beam height is minimized, reducingdrifts in optical coupling due to differential thermal expansion in the sensitivevertical direction. The observed changes in coupling efficiency over the range of0–70�C are below +/�3%, proving the excellent stability of our design [36].

We perform extensive reliability testing of our modules according to Telcordia

standards, including high temperature storage, thermal cycling, and lifetests.Analysis of lifetest data shows that the failure rate for the package is below 150FIT at 70�C case temperature. Two thousand five hundred-hour lifetest data for 14200-mW uncooled and FBG stabilized miniDIL modules are shown in Fig. 14 wherethe relative change in fiber output power as a function of time is plotted. Themodules are operated at 70�C heat sink temperature and 200mW output power fromthe fiber. The fiber output power varies within a band of +/�3%.

6. Conclusions and outlook

It is evident that, in the field of pump laser modules, successful productdevelopment with short time-to-market periods relies on the combination of variousactivities including laser chip design and fabrication, development of fiber Bragg

ARTICLE IN PRESS

1000

900

800

700

600

500

400

300

200

100

0

Chi

p fa

ilure

rat

e (F

IT)

300250200150100500

Fiber output power (mW)

T=45°C

T=70°C

Fig. 13. Chip failure rate in a miniDIL module as a function of module output power at operating

temperatures of 45 (dashed line) and 70�C (solid line) for a G07-type laser. The calculation is based on

chip lifetest data evaluated according to Eq. (3), assuming FBG stabilization and thermal conditions as in

an Al2O3miniDIL housing.


grating stabilization schemes, packaging technology development, and reliabilitymodeling and testing. As a result of these efforts we have successfully launched anuncooled miniDIL 980 nm pump module specified for an output power of 100mWduring the year of 2002, while our 200mW miniDIL module has been released as acommercially available product in July 2003.

Despite the demand for compact cost-effective packaging solutions, uncooledmodules are nevertheless supposed to yield high power, in particular, at hightemperatures. As a future trend we clearly expect a demand for growing outputpower from uncooled modules, similar to their temperature-stabilized counterparts,within an extended temperature range beyond 0–70�C. In anticipating this trend, ourstrategy is to continue the development of new laser diode generations with higherpower and improved far-field characteristics, as outlined in Section 2. In order toillustrate the progress in our uncooled module development, we plot in Fig. 15the measured fiber output power at 70�C of our G05 (2000), G06 (2001), G07(2002), and G08 (2003) modules, the latter emitting more than 400mW of opticalpower.

As output power increases and specifications on operating conditions areexpanded, the requirements on packaging technology become more demanding. Aparticularly critical issue to be considered in future generation uncooled modules isthe trade-off between operating power and device failure rate. As operating currentof the laser chip and hence heat load of the package increase, adequate thermalmanagement of the module is of utmost importance in order to keep the laserjunction temperature at a level that does not deteriorate the laser reliability.Therefore, in parallel to laser chip development, our activities focus also oninnovative package designs targeted at optimizing the thermal properties and, at thesame time, further simplifying the assembly and thus reducing manufacturing cost ofour future module generations.

ARTICLE IN PRESS

0 500 1000 1500 2000 2500-25

-20

-15

-10

-5

0

5

10

15

20

25

Ch

an

ge

in f

ibe

r o

utp

ut

po

we

r (%

)

Time (h)

Fig. 14. Two thousand five hundred-hour lifetest data for 14 FBG-stabilized miniDIL modules operated

at 70�C case temperature and 200mW output power.


References

[1] Hargreaves D, Lick GS, Ventrudo BF. High power 980-nm pump module operating

without thermoelectric cooler. Optical Fiber Communication Conference, San Jose, USA, February

25–March 1, 1996, Paper ThG3.

[2] Pliska T, Mohrdiek S, Harder C. Power stabilisation of uncooled 980nm pump laser modules from 10

to 100�C. Electron Lett 2001;37(1):33–4.

[3] Giles CR, Erdogan T, Mizrahi V. Simultaneous wavelength-stabilization of 980-nm pump lasers.

IEEE Photonics Technol Lett 1994;6(8):907–9.

[4] Ventrudo BF, Rogers GA, Lick GS, Hargreaves D, Demayo TN. Wavelength and intensity

stabilisation of 980 nm diode lasers coupled to fibre Bragg gratings. Electron Lett 1994;30(25):2147–9.

[5] Welch DF. A brief history of high-power semiconductor lasers. IEEE J Sel Top Quantum Electron

2000;6(6):1470–7.

[6] Verdiell J-M, Ziari M, Welch DF. Low-loss coupling of 980 nm GaAs laser to cleaved singlemode

fibre. Electron Lett 1996;32(19):1817–8.

[7] Horie H, Arai N, Mitsuishi Y, Komuro N, Kaneda H, Gotoh H, Usami M, Matsushima Y. Greater

than 500-mW CW kink-free single transverse-mode operation of weakly index guided buried-stripe

type 980-nm laser diodes. IEEE Photonics Technol Lett 2000;12(10):1304–6.

[8] Achtenhagen M, McElhinney M, Nolan S, Hardy A. High-power 980-nm pump laser modules for

erbium-doped fiber amplifiers. Appl Opt 1999;38(27):5765–7.

[9] Schmidt B, Mohrdiek S, Harder C. Pump laser diodes. In: Kaminov I, Li T, editors. Optical fiber

telecommunications IVA. New York: Academic Press; 2002.

[10] Harder C, Buchmann P, Meier H. High-power ridge-waveguide AlGaAs GRIN-SCH laser diode.

Electron Lett 1986;22:1081–2.

[11] Meier HP, Harder C, Oosenbrug A. A belief has changed: highly reliable 980-nm pump lasers for

EDFA applications. Optical Fiber Communication Conference, San Jose, USA, February 25–March

1, 1996, Paper ThG2.

[12] Schmidt B, Pawlik S, Rothfritz H, Thies A, Mohrdiek S, Harder C. 400mW 980nm-module with very

high power conversion efficiency. Optical Fiber Communication Conference, Baltimore, USA, March

7–10, 2000.

[13] Schmidt B, Pawlik S, Mayer B, Mohrdiek S, Jung I, Sverdlov B, Lichtenstein N, Matuschek N,

Harder C. Highly efficient 980 nm single mode modules with over 0.5Watt pump power. Optical

Fiber Communication Conference, Anaheim, USA, March 17–22, 2001, Paper WC1.

ARTICLE IN PRESS

0 200 400 600 800 10000

50

100

150

200

250

300

350

400

450

T=70 °C

G08

G07

G06

G05

Fib

er

ou

tpu

t p

ow

er

(mW

)

Current (mA)

Fig. 15. Comparison of fiber output power at a module heat sink temperature of 70�C as a function of

injection current for four different generations of uncooled miniDIL modules.


[14] Schmidt B, Mohrdiek S, Pliska T, Jung I, Pawlik S, Lichtenstein N, Harder C. High power

980 nm pump lasers. Integrated Photonics Research Topical Meeting, Monterey, USA, June 11–13,

2001.

[15] Schmidt B, Pawlik S, Matuschek N, M .uller J, Pliska T, Troger J, Lichtenstein N, Wittmann A,

Mohrdiek S, Sverdlov B, Harder C. 980 nm single mode modules yielding 700mW fiber coupled

pump power. Optical Fiber Communications Conference, Anaheim, USA, March 17–22, 2002, Paper

ThGG64.

[16] Sverdlov B, Schmidt B, Pawlik S, Mayer B, Harder C. 1W 980nm pump modules with very

high efficiency. European Conference on Optical Communication, Copenhagen, Denmark,

September 8–12, 2002, post-deadline paper.

[17] Schmidt B, Lichtenstein N, Sverdlov B, Matuschek N, Mohrdiek S, Pliska T, M .uller J, Pawlik S, Arlt

S, Pfeiffer H-U, Fily A, Harder C. Further development of high power pump laser diodes. SPIE

International Symposium on Information Technologies and Communications, Orlando, USA,

September 7–11, 2003;42–54. Proc SPIE 5248.

[18] Schmidt B, Pliska T, Matuschek N, Badii R, Mohrdiek S, Kellner T, Pawlik S, M .uller J, Sverdlov B,

Witzig A, Pfeiffer M. Simulation and design of pump laser diodes for un-cooled applications.

International Semiconductor Laser Conference 2002, Garmisch-Partenkirchen, Germany, September

29–October 3, 2002, Paper ThC1.

[19] Schmidt B, Lichtenstein N, Fily A, Matuschek N, Badii R, Pliska T, Sverdlov B, M .uller J, Pawlik S,

Harder C. Advances in design and modeling of pump laser diodes. International Conference on

Numerical Simulation of Optoelectronic Devices, Zurich, Switzerland, September 25–27, 2002.

[20] Hill KO, Fuji Y, Johnson DC, Kawasaki BS. Photosensitivity in optical fiber waveguides: application

to reflection filter fabrication. Appl Phys Lett 1978;32(10):647–9.

[21] Hill KO, Meltz G. Fiber Bragg grating technology fundamentals and overview. J Lightwave Technol

1997;15(8):1263–76.

[22] Giles CR. Lightwave applications of fiber Bragg gratings. J Lightwave Technol 1997;15(8):1391–404.

[23] Archambault J-L, Grubb SG. Fiber gratings in lasers and amplifiers. J Lightwave Technol

1997;15(8):1378–90.

[24] Hakki BW, Paoli TL. Gain spectra in GaAs double-heterostructure injection lasers. J Appl Phys

1975;46:1299–306.

[25] Cassidy DT. Technique for measurement of the gain spectra of semiconductor diode lasers. J Appl

Phys 1984;56:3096–9.

[26] Troger J. Measurement of gain in pump diode lasers using a low coherence source and synchronous

detection. J Lightwave Technol 2003;21(12):3441–5.

[27] Pliska T, Arlt S, Matuschek N, Schmidt B, Mohrdiek S, Harder C. High power wavelength

stabilized 980 nm pump laser modules operating over a temperature range of 135K. 14th Annual

Meeting of the IEEE Lasers and Electro-Optics Society, San Diego, USA, November 11–15, 2001,

Paper TuA4.

[28] Matuschek N, Pliska T, Sverdlov B, Mohrdiek S, Schmidt B, Harder C. Polarization effects in

wavelength-stabilized pump laser modules. 15th Annual Meeting of the IEEE Lasers and Electro-

Optics Society, Glasgow, UK, November 10–14, 2002, Paper ThS2.

[29] Achtenhagen M, Mohrdiek S, Pliska T, Matuschek N, Harder CS, Hardy A. L–I characteristics of

fiber Bragg grating stabilized 980-nm pump laser modules. IEEE Photonics Technol Lett

2001;13(5):415–7.

[30] Pliska T, Matuschek N, Mohrdiek S, Hardy A, Harder C. External feedback optimization by means

of polarization control in fiber Bragg grating stabilized 980-nm pump lasers. IEEE Photonics Technol

Lett 2001;13(10):1061–3.

[31] Badii R, Matuschek N, Pliska T, Troger J, Schmidt B. Dynamics of multimode diode lasers with

strong, frequency-selective optical feedback. Phys Rev E 2003;68:036605-1–12.

[32] Lang R, Kobayashi K. External optical feedback effects on semiconductor injection laser properties.

IEEE J Quantum Electron 1980;QE-16:347–55.

[33] Enochs S. Opto-mechanical packaging for extended temperature performance. Proc SPIE 1043, Laser

Diode Technology and Applications, Los Angeles, 1989.


[34] B.attig R, Pfeiffer H-U, Matuschek N, Valk B, Enochs S. Reliability issues in pump laser packaging.

15th Annual Meeting of the IEEE Lasers and Electro-Optics Society 2002, Glasgow, UK, November

10–14, 2002, Paper WW1.

[35] Mohrdiek S, Pliska T, Matuschek N, Arlt S, Schmidt B, B.attig R. Pump modules with ultra low

power dissipation for metro systems. European Conference on Optical Communication, Amsterdam,

The Netherlands, September 30–October 4, 2001, post-deadline Paper PD.B.1.10.

[36] Mohrdiek S, Pliska T, B.attig R, Matuschek N, Valk B, Troger J, Mauron P, Schmidt BE, Jung ID,

Harder CS, Enochs S. 400mW uncooled miniDIL pump modules. Electron Lett 2003;39(15):1105–7.

[37] Harder CS, Brovelli L, Meier HP, Oosenbrug A. Highly reliability 980-nm pump lasers for Er

amplifiers. Optical Fiber Communication Conference, Dallas, USA, February 16–21, 1997.

[38] Pfeiffer H-U, Arlt S, Jacob M, Harder C, Jung ID, Wilson F, Oldroyd T, Hext T. Reliability of

980 nm pump lasers for submarine, long-haul terrestrial and low cost metro applications. Optical

Fiber Communication Conference, Anaheim, USA, March 17–22, 2002, Paper ThN4.

[39] Arlt S, Pfeiffer H-U, Jung ID, Jakubowicz A, Schwarz M, Matuschek N, Pliska T, Schmidt B,

Mohrdiek S, Harder CS. Reliability proving of 980 nm pump lasers for metro applications.

International Semiconductor Laser Conference 2002, Garmisch-Partenkirchen, Germany, September

29–October 3, 2002, Paper ThC5.

ARTICLE IN PRESST. Pliska et al. / Optics and Lasers in Engineering 43 (2005) 271–289 289

Documents

Wavelength stabilized 980 nm uncooled pump laser modules for erbium-doped fiber amplifiers