Semiconductor Optical Amplifiers with Low Noise Figure
Hideaki Hasegawa *, Masaki Funabashi *, Kazuomi Maruyama *,Kazuaki Kiyota *, and Noriyuki Yokouchi *In the multilevel phase modulation which is expected to provide the nextgeneration modulation format for optical communications, the insertion lossesof the optical transmitters and receivers tend to increase due to their structural complexity withadvancement in the modulation level. For this reason, there is a need for a compact singlechannel optical amplifier to compensate for such insertion losses, and a semiconductor opticalamplifier (SOA), which is smaller than an erbium doped fiber amplifier (EDFA) and capable ofbeing integrated with other optical devices is drawing the attention for such applications. In thisstudy, the design of SOA structures was optimized using simulation techniques, and prototypes
were fabricated. The prototyped SOA has an output power of 5 dBm or higher and a noise figure(NF) of 4 dB or lower at 100 mA of current satisfying the requirements for a single-channel opticalamplifier, whose characteristics are similar to those of an EDFA, and thus, holds a considerable
promise as an optical amplifier of compact size, low power consumption and low noise.

ABSTRACT1.

INTRODUCTION

Multilevel phase modulation schemes, which are based
typically on phase shift keying (PSK), are drawing attention as a next-generation optical communication method
for their superiority to conventional on-off keying (OOK)
schemes in terms of receiver sensitivity, spectrum efficiency and dispersion tolerance characteristics. However, theinsertion losses of the optical transmitters and receivers
tend to increase as the modulation level advances
because of the increasing number of optical components
as the modulation system uses multistage phase modulators on the transmitter side and passive delay circuits andbalanced photodetectors on the receiver side. Figure 1
shows the schematic structure of a modulator for a 16
quadrature amplitude modulation (16QAM) using four
sets of Mach-Zehnder interferometers, which results in ahigh insertion loss of about 10 dB 1). Compact SOAs arerecently drawing attention to compensate for such highinsertion losses. The SOAs can be integrated eithermonolithically with other compound semiconductorbased optical devices, or in a hybrid manner with silicabased optical waveguides of planar lightwave circuits(PLCs) with a low coupling loss of below 1 dB by using a
spot size converter (SSC) 2). Moreover, the SOAs have
many advantages such as simple current pumping, compactness, low power consumption and low cost. In order
to be used as a single channel amplifier for optical communications, on the other hand, the SOAs are required toprovide a polarization-insensitive gain and a low NF,

which are similar to those of an EDFA. The aim of thisstudy is to satisfy the SOA characteristics of both low
noise and high output, making it possible to develop an
SOA which satisfies the required characteristics for a single-channel amplifier. SOAs with low noise figure andhigh output characteristics have already been reported in
the literature 3). But this paper has not described thedependence of NF on structural parameters such as theactive layer width, the gain region length and the active
layer structure with the multi quantum well (MQW).
Accordingly in this study, we carefully investigated numerically, as well as experimentally, the dependence of NF on
the active layer width, on the gain region length and on
the active layer structure. Furthermore, we optimized theactive layer width and the gain region length to realize
SOA characteristics that meet the NF and the output
power requirements for a typical single-channel amplifier.

Phase modulator
Channel
amplifier

Laser

Phase modulator

Figure 1

Schematic structure of the 16QAM modulator.

* Photonic Device Research Center, Yokohama R&D Lab. R&D Div.

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Semiconductor Optical Amplifiers with Low Noise Figure

ANALYSIS TECHNIQUES FOR SOA
CHARACTERISTICS

Analysis techniques for the NF and output power characteristics of an SOA will be described in this section. The
analysis technique for the NF characteristics of an SOA is
introduced in the literature 4). The fundamental equation
used in this analysis is a rate equation describing the
temporal changes of carrier density as shown in Equation(1).
dN ( z, t )IΓ=− R ( N ( z , t )) −g ( N )[ Q+k (z ,t)+Q−k (z ,t)]dt
N QW dWeN QW dWL−2Γg ( N ) K [Q+j (z ,t)+Q−j (z ,t) ]N QW dW(1)

wherecarrier density;
N(z,t)
I
electric current;echarge amount;dthickness of a quantum well layer;
number of quantum wells;
NQW
W
width of active layer;Lgain region length of SOA;K	coefficient representing the multiplicity of end facet reflection;
recombination rate term;
R(N(z,t))Γ
optical confinement factor;g(N)material gain coefficient;+–
Q k(z,t), Q k(z,t)	photon density of forward and backward propagating signal lights,
respectively;+–
Q j(z,t), Q j(z,t)	photon density of forward and backward propagating amplified spontaneous emissions (ASE), respectively.
By solving this rate equation and the propagation equations of the signal light and ASE light simultaneously, thedistribution of the signal light and ASE light can beobtained. Letting G be the gain, CE be the coupling efficiency on the receiving side and PASE be the intensity of
the ASE at the output, the noise figure NF (dB) can becalculated from the following equation:

By substituting the approximated model shown inEquation (3) into Equation (1), the characteristic analysisof an MQW SOA can be carried out. In Equation (3), g0 is
the gain coefficient, N is the carrier density and N0 is thetransparency carrier density. Figure 2 compares the net
gain calculated by the approximated model and the one
measured by using the Hakki-Paoli method. The net gain
Gnet can be expressed by Equation (4).


120
100
80
60
40



NF
（dB）= 10 log 10⋅ CE

6-QW structure, at 25°C

20
Measured net gain0

Approximated model I (g0=1000cm-1)

-20
-40

1
2 PASE ν+hν ∆ν G ν
Gν

Approximated model II (g0=1650cm-1)

1

2

(2)

From Equation (2), it can be seen that the NF is determined mainly by the gain G and the power of ASE light.
In case of the laser analysis, the material gain coefficient g(N) in Equation (1) is expressed by an approximated model shown in Equation (3).
g (N)= g 0 ln (N/N 0)

3

4

5

Carrier density (1018cm-3)
Figure 2(4)

Gnet (N)=Γ g(N)−α (N)where α(N) is the loss.
Whereas, as shown in Figure 2, the approximatedmodel by Equation (3) suggests a tendency for the net
gain to rise as the injected carrier density increases, themeasured net gain tends to rapidly saturate on the highcarrier density side. While lasers operate at low-carrier
density due to carrier clamping, SOAs come to operate at
high-carrier density, so that the net gain at high-carrier
density becomes important. In this study, considering thatthe model based on Equation (3) cannot accurately
express the material gain at high carrier concentration,
the measured net gains were used in the analysis. Figure
3 shows the gain and NF characteristics calculated using
the approximated model and those calculated using actual measurement data. It can be seen that, while theapproximated model gives differences from experimental
results of 4.1 dB and 0.7 dB for the gain and NF, respectively, those differences are reduced to 1.2 dB and 0.2 dB
respectively by using the measured net gain for calculation.

Net gain (cm-1)

2.

Comparison of calculated net gains using
approximated model and measured values.

(3)

Comparison with measured net-gain andapproximated model
Figure 2

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Comparison of net gains calculated fromapproximated model with measured values.

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Semiconductor Optical Amplifiers with Low Noise Figure

20
18
16

4.1 dB

Gain (dB)

14

1.2 dB1210
8
6

By measured net gain

4

By approximated model for net gain

2

Experimental results

0

-25

-20

-15
-10
Input intensity (dBm)-50(a) Gain characteristics

6
By measured net gain
By approximated model for net gain
Experimental results

NF (dB)

5

tion direction in SOAs having different optical confinement factors. It is seen from the figure that the lower the
optical confinement factor, the more resistant to saturation is the signal light along the longitudinal direction, andthe lower the NF tends to be. In such a structure, since
the gain saturation of signal light is less likely to occur,
injected carriers are transformed more into signal lightthan into noise light, so that the NF tends to decrease.
Consequently, to realize low-noise characteristics it isimportant to design a waveguide structure in which thesignal light is less susceptible to saturation. The blue
solid line in Figure 5 (a) and (b) shows the targeted performance of 5 dBm and 4 dB for the optical output powerand chip NF, respectively; and the red solid line at 2.8 μm
shows the boundary for satisfying the condition for single
mode operation. In fact, we have fabricated prototype
SOAs with a waveguide structure that meet the targeted
optical output power, the NF and the single-mode conditions, by setting the active layer width to 2.8 μm and gain
region length to 1000 μm.

0.7 dB
0.2 dB

4

3

-25

-20

-15

-10

-5

Pumping current ( I )0

Input intensity (dBm)

L(b) NF
Figure 3

Experimental results and calculated results forgain (a), and NF (b), using net gains based on
approximated model and measurement.

W

Input
(Pin)

Signal light

Waveguide structure

3.

DESIGN OF LOW NOISE SOA

The waveguide structures of low-noise SOAs have been
Figure 3*using
Experimental
results
and calculated
results withdesigned
the analysis
techniques
described
above.
measured net-gain and approximated model.
Figure 4 illustrates the structure parameters of an SOA
considered for the design. Structure parameters to be
designed include the active layer width W, the gain region
length L, the optical confinement factor Γ and the loss α.
The device fabricated here uses an active layer of
GaInAsP, and employs buried hetero-structure for the
waveguide. Figure 5 shows the relationship between theactive layer W and gain region length L at a pumping current of 100 mA and input power of -5 dBm for a device
consisting of three active layers (3-QW) and having an
optical confinement factor Γ of 1.1%. From Figure 5, thenoise figure is seen to decrease as the active layer width
increases and the gain region length shortens. It is also
reported that NF decreases as optical confinement factor
and loss decline 6). In other words, the NF tends todecrease with the wider active layer, with the shorter gain
region length, with the lower loss and with the lower optical confinement factor. Figure 6 shows the calculated
results of the signal light distribution along the propaga-

Γ=Γ H×Γ V

ΓH

Quantum wells

ΓV
InGaAsP/InP
Multi-quantum well structure
Figure 4

Structure parameters of the SOA.

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Semiconductor Optical Amplifiers with Low Noise Figure

4

Active layer width (µm)

4.

3

15

2.8 µm10

Single-mode

5 dBm
2
0
500

1000

1500

2000

2500

3000

SOA gain region length (µm)(a) Optical output power

Active layer width (µm)

4

Low NF
4.0 dB

3.6
3

3.8

4.2

2

4.4
500

1000

1500

2000

2500

3000

SOA gain region length (µm)(b) NF
Figure 5

The output power (a) and the NF (b), dependingon an active layer width and a gain region length.

Optical output power (dBm)10
Γ=1% (NF=3.01 dB)

5

Γ=2% (NF=3.73 dB)0Γ=3% (NF=5.01 dB)

-5
-10
-15
-20
-25
-30

0

300

600

900

Propagation distance (µm)

Figure 6

The signal light distribution in SOAs with different
optical confinement factors.

FABRICATION OF PROTOTYPE LOWNOISE SOA

Prototypes were fabricated in accordance with the waveguide structure design described in Section 3, and in
addition, the number of quantum wells was increased inturn from the predetermined value of three (3-QW) to five
(5-QW) and eight (8-QW). Metal organic chemical vapor
deposition (MOCVD) was used for crystal growth, and theactive layer is made up of GaInAsP quantum wells with a
0.8% compressive strain. Buried heterostructure wasused for current confinement, and multistep separate
confinement heterostructure (SCH) was employed toreduce the optical absorption at the p-doped upper cladding layer. Further, anti-reflection coating was applied tothe end facets. Figure 7 shows the optical output power
and the NF characteristics of the 3-QW, 5-QW, and 8-QW
SOAs prototyped here. Bench-top measurements wereconducted using an automatic fiber alignment system.
The measurement wavelengths were set to 1500 nm,
1560 nm and 1560 nm, corresponding to the peak gain
wavelength of 3-QW, 5-QW, and 8-QW device, respectively. In the figure, the ,  and  plots represent the measured optical output power and the NF of a 3-QW, a
5-QW, and a 8-QW, respectively. The solid line represents
the calculated results. In the calculation, the Γ values
obtained by the mode analysis are used, and they are
1.1%, 2.3% and 4.4% for the 3-QW, the 5-QW, and the
8-QW, respectively. In terms of loss, the absorption loss at
the active layer is dominant since the SOAs operate
under the pumping conditions of a high carrier density,and the absorption loss at the active layer is known to be
proportional to Γ 7). The loss for the 3-QW structure is 8
-1
cm , which is a measured value, and those for the 5-QW
-1
-1
and 8-QW structures are 17 cm and 32 cm , respectively, which are estimated values assuming a proportional
relationship to Γ. The experimental results show that the
3-QW and the 5-QW structures have almost the same NF.
Increasing the number of wells produces two effects: oneis that since the operating carrier density decreases thegain become less likely to saturate, so that the NF
decreases--which is referred to as "effect A"; and the otheris that the absorption loss in the active layer increases,
thus increasing the NF--"effect B". It is considered that, forthe 3-QW and the 5-QW structures, these two effects cancel each other, so that the NF shows virtually no change.
On the other hand, since the effect B becomes dominant
in the case of the 8-QW, the NF rapidly increases. In
terms of output power, it is thought that since the net gainincreases in proportion to Γ as the number of wells
increases from 3-QW to 5-QW and to 8-QW, the output
power is high at the low-input power side of below -5
dBm, but it decreases due to gain saturation at the highinput power side. It can also be seen that, among thethree active layer structures prototyped here, the 5-QW
satisfies both the output power and the NF at an optical
input power of below -5 dBm that is assumed to be theoperating condition for a single-channel amplifier.

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Semiconductor Optical Amplifiers with Low Noise Figure

15

16
14

5

12

0

10

-5

8

-10

6

-15

4

-20

-25

Figure 7

-20

-15
-10
Input power (dBm)-50

NF (dB)

Output power (dBm)

105.

REFERENCES

3-QW(experimental)
5-QW
8-QW
Calculated results

2

The output power and the NF characteristics offabricated SOAs.

CONCLUSION

In this paper, design and prototyping of low-noise SOAs
have been conducted through numerical analyses that
adopt net gain data actually measured to predict the performance of such SOAs. The SOA of the 5-QW structure
prototyped here has raised the prospect of achieving a
module NF of 6 dB, which is equivalent to that of erbium
doped fiber amplifiers even if a coupling loss of about 2
dB is taken into account. Also, a chip output power ofhigher than 5 dBm has been achieved, which fully satisfies the performance required for single-channel amplifiers. These results demonstrate that the SOA is promising
as a compact, low power consumption and low noise
optical amplifier for next-generation optical communications.

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3) K. Morito, S.Tanaka, S.Tomabechi, and A. Kuramata, “A broad-band
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