A 4.25-Gbps Transmitter Optical Sub-Assembly (TOSA) Using an
850-nm Vertical-Cavity Surface-Emitting Laser (VCSEL)
by Norihiro Iwai * , Maiko Ariga * , Yoshihiko Ikenaga * , Hiroaki Suzuki * ,Kevin Nishikata * , Noriyuki Yokouchi * and Akihiko Kasukawa *The explosive growth of the Internet in recent years has given rise to a needfor higher data transmission speeds. And the rapid proliferation of electronic
appliances raises the hope for greater proliferation of short-haul data communications linking
various appliances in the home. Against this background an 850-nm vertical-cavity surfaceemitting laser (VCSEL) transmitter optical sub-assembly (TOSA), which can operate at 4.25
Gbps, is the target of R&D activity in institutions around the world as a light source for the
next generation of Fiber Channel. Accordingly we have optimized the structure of a VCSEL
chip, fabricated a TOSA, and evaluated its characteristics. This has enabled us to fully
satisfy specifications with respect to 4.25-Gbps operation, static characteristics, temperature
characteristics, and reliability.

ABSTRACT1.

INTRODUCTION

The explosive growth of the Internet in recent years has
given rise to a need for higher data transmission speeds.
And the rapid proliferation of electronic appliances raises
the hope for greater proliferation of short-haul data
communications linking various appliances in the home.
The 850-nm vertical-cavity surface-emitting laser(VCSEL) 1) chip is already being used in a transmitter optical sub-assembly (TOSA), as a driver for gigabit Ethernetor Fiber Channel data communications applications.
At present VCSEL TOSAs are mainly used at transmission speeds of 2.5 Gbps or less, but there is a strong
movement toward increasing Fiber Channel speed to4.25 Gbps, creating the need for a VCSEL TOSA capableof 4.25-Gbps operation.
We have therefore fabricated an 850-nm VCSEL TOSA
capable of 4.25-Gbps operation, and have evaluated its
operation at 4.25 Gbps, together with its static characteristics, temperature characteristics and reliability. As aresult we have obtained characteristics that fully satisfy
specifications, as reported in the following.

2.

by the characteristics of the VCSEL chip. In this section
we discuss the structure of an 850-nm VCSEL chip, andthe optimization of its design.
2.1 Structure of the VCSELIn the oxide-confined structure 2) developed in the work
described here, the effective confining of current and lightby means of an aluminum oxide layer has resulted in theachievement of outstanding characteristics in terms ofthreshold current, high efficiency 3), 4), etc.
What is more, the L/I (optical power/current) and V/I(voltage/current) characteristics are heavily dependent on
the diameter of the light-emitting region (oxide aperturesize) enclosed by the aluminum oxide layer referred to.
This means that it is possible to achieve the desired characteristics by controlling the oxide aperture size.

VCSEL CHIP STRUCTURE AND ITSOPTIMIZATIONTOSA characteristics, except for high-frequency
modulation characteristics, are almost entirely determined* Semiconductor R&D Center, Yokohama R&D Laboratories,
R&D Div.

Figure 1

Structural schematic of oxide-confined VCSEL.

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A 4.25-Gbps Transmitter Optical Sub-Assembly (TOSA) Using an 850-nm Vertical-Cavity Surface-Emitting Laser (VCSEL) Title

Table 125°C, DC

Threshold current (mA)43Upper spec limit

2

1

0

0

20

Figure 2

60

80

100

120

140

160

Oxide aperture size dependence of threshold current.

100
25°C, DC80

60

Upper spec limit

40

20

0

Lower spec limit

0

20

40

60

80

100

120

140

160

Aperture size (µm2)

Figure 3

Oxide aperture size dependence of differential
resistance.

5
N=2
4
3
2
1
0

0

20

40

60

80

Temperature (°C)

Specification of 4.25-Gbps VCSEL TOSA.
Item

40

Aperture size (µm2)

Differential resistance (Ω)

2.2 Optimizing the VCSEL Structure
Table 1 shows some of the specification of a 4.25-Gbps
VCSEL TOSA. In this work, from among a number ofcharacteristics, we report on two that are of the utmost
importance in transceiver design and are in a trade-off
relationship with oxide aperture size: threshold current
and differential resistance. Measurements were taken in
the wafer state at 25°C using DC drive. We also report on
the temperature dependence of threshold current, lasing
wavelength, slope efficiency and differential resistance.
Finally, although the importance of chip characteristics
are as assembled in the TOSA, it was found that there
was virtually no difference between the characteristics ofthe chip itself and those when assembled into the TOSA,
so in this work we effected the optimization in the chipstate.
Figure 2, plotting oxide aperture size dependence ofthreshold current, shows a substantially linear relationship. It can be seen that to satisfy the TOSA specificationsin Table 1, it is sufficient to set the oxide aperture size at
130 μm2 or less.
Similarly Figure 3 shows the results for differential resistance, which is in inverse proportion to oxide aperturesize. It can be seen that to satisfy the TOSA specificationsin Table 1, it is sufficient to set the oxide aperture size inthe range of 60 to 120 μm2.
Let us now proceed to discuss results for temperature

5

Threshold current (mA)

Figure 1 is a schematic diagram showing a crosssection of an 850-nm oxide-confined VCSEL 5). This structure consists of a p-type GaAs substrate onto which aregrown a bottom distributed Bragg reflector (DBR) mirror
comprising 35 pairs of p-type Al0.2Ga 0.8As/Al 0.9Ga 0.1As,
an active layer (λ cavity) comprising a GaAs/Al0.2Ga0.8As
multiquantum well (MQW), and a top DBR mirror comprising 25 pairs of n-type Al0.2Ga0.8As/Al0.9Ga0.1As. On the low
refraction index layer closest to the active layer in the bottom DBR mirror there is formed an AlAs layer (aluminumoxide layer formed by wet oxidation) for the purpose ofconfining current and light. Further, a pad electrode isformed on the polyimide for reducing parasitic capacitance.
In this work we have fabricated chips with differentoxide aperture sizes and evaluated their various characteristics, effecting the optimization of chip structure (oxide
aperture size).

Min.

Max.

Threshold current (mA)—

2.5

Differential resistance (Ω)

2550Temperature dependence of threshold current
(mA)

-1

+1

Temperature dependence of wavelength (nm/°C)

—

0.07

Temperature dependence of slope efficiency(ppm/°C)—-4500

Temperature dependence of differential
resistance (ppm/°C)

—

-3000

Figure 4

Temperature dependence of threshold current.

characteristics. The temperature dependence of threshold current, lasing wavelength, slope efficiency and differential resistance are shown in Figures 4 through 7
respectively. Measurements were taken in the wafer state
at temperatures of from 20 to 70°C using DC drive.
This resulted in a fluctuation in threshold current of ±0.2 mA, temperature dependence of lasing wavelength of
0.59 nm/°C, temperature dependence of slope efficiencyof -3300 ppm/°C, and temperature dependence of differ-

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A 4.25-Gbps Transmitter Optical Sub-Assembly (TOSA) Using an 850-nm Vertical-Cavity Surface-Emitting Laser (VCSEL)

855

Wavelength (nm)

853

SC

851
849
847
845

LC
0

20

40

60

80

Temperature (°C)

Temperature dependence of lasing wavelength.
Figure 8

0.4

0.2
N=2
0

0

20

40

60

80

Temperature (°C)

Figure 6

Temperature dependence of slope efficiency.

Differential resistance (Ω)

50
40
30
20
10
N=2
0
0

20

40

60

80

Temperature (°C)

Figure 7

Temperature dependence of differential resistance.

ential resistance of -2500 ppm/°C, confirming that for allthese characteristics the specifications shown in Table 1
were satisfied.

3.

3.1 TOSA Structure
Figure 8 shows photographs of the TOSAs developed inthis work, that with the SC barrel at top left and LC barrel
at bottom right. Normally in the 850-nm band multimodefibers with a core diameter of 50 or 62.5 μm are used—larger than single-mode fibers (diameter 7 μm), so that
coupling is easier. For this reason plastic barrels are used
at fiber couplings. Monitor photodiodes are incorporated
to enable auto power control (APC) drive. The VCSEL
chip is mounted to a TO-46 header via a submount. And
since the U.S. Food and Drug Administration specifies
that lasers of 850-nm-band wavelengths be used at aneye safety level of Class 1 or below, this structure can beused at an optical power (fiber-coupled) of not more than
350 μW.
3.2 Results of TOSA Evaluation
3.2.1 Static Characteristics
Figure 9 shows the current dependence of opticalpower, voltage and monitor current at 25°C, DC drive.
Threshold current was 1.8 mA, optical power was 450 μW@6 mA (coupling efficiency with GI-50 multimode fiberof approximately 70%, so that fiber coupling power was
300 μW), operating voltage was 1.9 V @6 mA, differential
resistance was 40Ω @6 mA, and monitor current was 400
μA @6 mA, fully satisfying specifications.
2

3

1200

2.5

1000

2

800

25° C, DC

4.25-GBPS VCSEL TOSAAt present TOSAs with speeds of 2.5 Gbps are themainstay of the market, but in the future this can be
expected to shift rapidly to 4.25 Gbps (Fiber Channel
standard). Accordingly we proceeded with development
work aimed at a maximum modulation speed of 4.25
Gbps or more. In this section we report on the structureof the TOSA, and on the results of evaluation of static
characteristics, temperature characteristics, modulation
characteristics, eye pattern and reliability.

Photograph of 4.25-Gbps TOSAs using 850-nm
VCSEL with U.S. 10-cent coin for scale.

1.5

Voltage1

1.5

Monitor
Power

1

600
400

0.5
0.5
0

0

5

10

0
15

Monitor current (µA)

0.6

Optical power (mW)

Slope efficiency (mW/mA)

0.8

Voltage (V)Figure 52000Current (mA)

Figure 9

Current dependence of optical power, voltage andmonitor current.

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A 4.25-Gbps Transmitter Optical Sub-Assembly (TOSA) Using an 850-nm Vertical-Cavity Surface-Emitting Laser (VCSEL) Title

tance, the measurement results were substantially similar
to those for the chip.

6
Threshold current (mA)

N=4
5
4
3
2
1
0
-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 10 Temperature dependence of threshold current.

N=4

0.4

10

0.3

5
Response (dB)Monitor current (mA)

0.5

3.2.3 Modulation Characteristics
As a result of the influence of wire inductance, and theinductance and capacitance of the TO header itself, themodulation bandwidth when assembled in the TOSA
configuration is more restricted in comparison with thefrequency characteristics of the chip alone. Specifically,
this influence becomes suddenly apparent if modulation
speed exceeds 2.0 GHz. Since the f 3dB band for the
VCSEL chip developed here is approximately 8 GHz,
4.25-Gbps operation presents no problem for the chipby itself, but the effect due to the assembling referred toabove imposes restrictions on the modulation bandwidth.

0.2
0.1
0
-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 11 Temperature dependence of monitor current.

3.2.2 Temperature Characteristics
In a transceiver for a data storage area network (SAN),
Fiber Channel standards are mainly used, with theoperating temperature range specified at 0 to 70°C. Whenthe temperature rise inside the transceiver is taken into
account, the TOSA mounted in such a transceiver mustdeliver stable operation from 0 to 80°C. And dependingon the application, even more severe conditions must
be contemplated—temperatures of from -40 to 95°C.
Accordingly in this work we evaluated temperature
characteristics over a wide range— - 40 to 100°C.
Figures 10 and 11 show the temperature dependence ofthreshold current and monitor current.
The amount of fluctuation in threshold current withrespect to temperatures of 0 to 80°C (the Fiber Channel
standard value), which was the main target in this work,
was extremely small—only ±0.2 mA, which fully satisfied
the specification (±1 mA). At the lower end of the -40 to
100°C temperature range (-10°C or below), however, therise in threshold current had an effect and the fluctuationin threshold current was ±1.5 mA, greater than the specification. This is because of the large value of detuning,as determined by the difference between the gain peakwavelength and the Fabry-Perot wavelength (becauseof the optimized design for the 0 to 80°C range), making
possible appropriate adjustment by means of design.
The fluctuation in monitor current was also very low—
only 0.1 %/°C—fully complying with the specification of
0.3 %/°C.
Further, in terms of the temperature dependence oflasing wavelength, slope efficiency and differential resis-

0
-5
-10
-15
-20

4-10 mA
0

2

4
Frequency (GHz)8

6

Figure 12 Small-signal modulation characteristics.

Figure 12 shows the result of measurements of smallsignal modulation characteristics. It can be seen that atbias currents of 5 mA or less the resonance peak of relaxation oscillation is below 4 GHz, raising concerns about
influence on the eye pattern.
At bias currents in excess of 6 mA, on the other hand,relaxation oscillation frequency obtained is higher than
4 GHz, and for the f3dB band is higher than 5 GHz. These
results are comfortably clear of the 3.0 GHz theoreticallyrequired for 4.25-Gbps operation. Since the thresholdcurrent of the chip used in these measurements is 1.8
mA, we may estimate that to achieve 4.25-Gbps operation would require a normalized bias level [bias current threshold current]/threshold current of approximately 2.3
or more.
3.2.4 Eye Patterns
In actual operation in a transceiver, importance attaches
to rise time, fall time and jitter in the eye pattern. These
characteristics are greatly modified by the circuitry used
for mounting in the transceiver, and by the driver IC used
to drive the VCSEL. Since the circuitry used in the transceiver, driver IC, etc. will differ according to the designs ofeach individual customer, the method used here for evaluation was a standard method using a pseudorandom bit
sequence (PRBS) from a pulse pattern generator. In orderto minimize the effect of inductance from the TO header
pins during measurement, the TOSA were soldered onto
micro striplines with the pins cut short.

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N=4Rise time (ps)

50 ps

40

20
-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 14 Temperature dependence of rise time.
Figure 13 Eye pattern diagram at 4.25 Gbps (extinction ratio
10 dB).

3.2.5 Reliability
Any discussion of reliability must be broadly classified interms of random failure or wear-out failure; but here we
report on the role of wear-out failure. Figure 17 shows
the results of a 5000-hr high-temperature aging test of
VCSEL chips. Figure 18 shows the results of a 2000-hr
high-temperature aging test of of a 4.25-Gbps VCSELTOSA. The test conditions were a temperature of 100°C,
operating current density of 10 kA/cm 2 and automatic

Fall time (ps)

N=4

70

50
-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 15 Temperature dependence of fall time.

Jitter (ps)

30

N=4

20

10
-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 16 Temperature dependence of jitter.
1.2

Normalized power

Figure 13 shows the results of measurements of eyepatterns with a 4.25-Gbps PRBS (231-1). The operating
conditions were: bias current of 6 mA, extinction ratio of
10 dB, and a modulation voltage of 0.5 V, with measurements taken at room temperature without a filter.
Despite the facts that the measurements were madeunder optimal conditions rather than actual service conditions, they yielded excellent results—rise and fall times(20~80 %) of 38 ps and 70 ps respectively and jitter of 20
ps or less—fully satisfying the specifications of 90 ps forrise time and 90 ps for fall time.
Let us now discuss the temperature dependence ofrise time, fall time and jitter as found from the results oftemperature dependence for the 4.25-Gbps PRBS (231-1)
eye pattern. These are shown for temperatures of -40
through 100°C in Figures 14 through 16. The measurements were taken at an extinction ratio of 10 dB, modulation voltage of 0.5 V, optical power fiber output of 300 μW
constant (APC control).
The result was that at temperatures from 0 to 80°C
(0~70°C for the transceiver itself and assuming an internal temperature rise of 10°C) the Fiber Channel standards
were a rise time of 35~40 ps, fall time of 40~100 ps andjitter of 18~22 ps. Even when the temperature range wasbroadened (-40 to 100°C), the maximum values were 40
ps for rise time, 78 ps for fall time and 24 ps for jitter, confirming that at a temperature range of -40 to 100°C thespecifications were fully satisfied.
But since, as has been explained above, matching withtransceiver circuitry and the driver IC is important in actual use, it is essential in the last analysis that evaluation be
conducted on chips mounted in transceivers.

90

1

0.8

ACC
100°C, 10 kA/cm2
N=20

0.6

0.4

0

1000

2000

3000

4000

5000

Aging time (h)Figure 17 Results of aging test of VCSEL chips.

current control (ACC) drive, with sample sizes of 20 and
50 respectively.
Based on an assumption for both VCSEL and TOSA
the life expectancy is reached at the point where opti-

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A 4.25-Gbps Transmitter Optical Sub-Assembly (TOSA) Using an 850-nm Vertical-Cavity Surface-Emitting Laser (VCSEL) Title

communications applications, for which a market take-off
is anticipated.
Chip structure was optimized, after which its evaluation when mounted in the TOSA was carried out, and it
was possible to achieve 4.25-Gbps operation, together
with good results in terms of static characteristics andtemperature characteristics that fully satisfied specifications. In reliability tests it was possible to obtain estimatedlife expectancies of 90 million hours at 25°C ambient and
50,000 hours at 100°C ambient.

Normalized power

1.2

1

0.8

ACC
100°C, 10 kA/cm2
N=50

0.6

0.4

0

500

1000

1500

2000

2500

Aging time (h)Figure 18 Results of aging test of TOSA.

cal power is reduced by 2 dB from the initial value, life
expectancy was estimated to be approximately 50,000 hr
at 100°C ambient. Converted to room temperature, this isequivalent to a life expectancy of 90 million hours (activation energy is 1.0 eV).

4.

INTRODUCTION TO MASSPRODUCTION

Since, unlike ordinary edge-emitting lasers, VCSELs
emit light in a direction perpendicular to the substrate,they have the characteristic that they can be tested at
the wafer level. Here we will describe a burn-in test that,taking advantage of this characteristic, is carried out at
the wafer level. Figure 19 is a photograph of a burn-in test
being carried out at wafer level. Current was applied byprobe directly to pad electrodes on the wafer. The wafer
was heated by being placed on a heater table. Sinceordinary burn-in tests are carried out after mounting on a
TO-CAN, they involve enormous costs for test equipment
and materials. In this method, by comparison, burnin testing can be carried out for an entire wafer at once,
thereby reducing the material cost when defects occur
and providing an effective means for lowering the cost ofmass production.

ACKNOWLEDGMENTS
We would like to express our thanks to Prof. Fumio
Koyama of the Tokyo Institute of Technology for his
guidance and encouragement in the conduct of thisresearch. We also wish to thank Hitoshi Shimizu,
Takeo Kageyama, Casimirus Setiagung and Tatsuyuki
Shinagawa of the Photonic Device Research Centerand to Yoon Youngduk of the Production Technology
Dept. for invaluable opinions and discussion; and to
Takeshi Hama, Tomonori Sekiguchi, Koji Hiraiwa, Kunio
Tokunaga, Natsumi Ueda and Tadayuki Sato of the
Photonic Device Research Center, and to the personnelof the Semiconductor Device Development Dept.,
Production Technology and Operations Section for taking
charge of the fabrication and evaluation of prototypes.

REFERENCE
1) K . I g a , F. Ko y a m a , a n d S . K i n o s h i t a : “ S u r f a c e e m i t t i n g
semiconductor Lasers,” IEEE. J.of Quant. Electron., 24 (1988)
1845.
2) K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, and K. M. Geib:“Low threshold voltage vertical-cavity lasers fabricated by selective
oxidation,” Electron. Lett., 30 (1994) 2043.
3) Y. Hayashi, T. Mukaihara, N. Hatori, N. Ohnoki, A. Matsutani, F.
Koyama, and K. Iga: “Record low-threshold index-guide InGaAs/
GaAlAs vertical-cavity surface-emitting laser with a native oxide
confinement structure,” Electoron. Lett., 31 (1995) 560.
4) G. M. Yang, M. H. MacDougal, and P. D. Dupkus: “Ultra low
threshold current vertical-cavity surface-emitting lasers obtainedwith selective oxidation,” Electron. Lett., 31 (1995) 886.
5) N. Ueda, M. Tachibana, N. Iwai, T. Shinagawa, M. Ariga, Y. Sasaki,
N. Yokouchi, Y. Shiina, and A. Kasukawa: “Transverse ModeControl and Reduction of Thermal Resistance in 850nm Oxide
Confined VCSELs, ” IEICE Trans. Electron., E85-C, No.1 (2002)
64.

Figure 19 Example of burn-in test at wafer level.

5.

CONCLUSION

We h a v e a d e v e l o p e d a 4 . 2 5 - G b p s T O S A u s i n g
850-nm VCSEL for mounting in transceivers for data

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