Optics Communications 284 (2011) 828–832

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Optics Communications

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An approach to enhance the receiver sensitivity with SOA for opticalcommunication systems

Surinder Singh ⁎Department of Electronics and Communication Engineering, Sant Longowal Institute of Engineering and Technology, Longowal, Sangrur, Punjab, India

⁎ Tel.: +91 1672 284704.E-mail address: surinder_sodhi@rediffmail.com.

0030-4018/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.optcom.2010.09.088

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 December 2008Received in revised form 30 September 2010Accepted 30 September 2010

Keywords:Amplified spontaneous emission powerSemiconductor optical amplifierBERDPSKPIN

In this paper, we investigate an SOA (semiconductor optical amplifier) preamplifier structure by optimizingthe carrier lifetime in order to reduce the amplified spontaneous emission (ASE) noise and crosstalk, withadequate gain increase. This proposed SOA optical preamplifier has no need of optical alignment andantireflection coating. This structure of SOA eliminates the need of optical filter, and exhibits large tolerance tothe input light wavelength. The receiver sensitivity is investigated for single and multi channel transmissionlinks. The received power of −50.34 dBm is observed at bit error rate (BER) 10−12 for 10 Gb/s with PINreceiver. Further, the impact of gain, amplified spontaneous emission power and gain variation for differentcarrier lifetime with input power for OOK system is illustrated. The proposed SOA has constant gain of30.06 dB up to gain saturation for carrier lifetime 0.18 ns. It is predicted that low value of carrier lifetimesuffers less from ASE noise.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The optical switches, splitters and multiplexers are the parts ofdeveloped optical fiber telecommunication networks. These elementsintroduce an input/output insertion loss, which must be accommo-dated within the power budget of transmission systems. These lossesare compensating with receiver for greater sensitivity and bandwidth.The avalanche photodiode (APD) receivers has a high sensitivity of−29.5 dBm at 10 Gb/s and also superior over PIN receivers up toseveral Gb/s [1].

The gain bandwidth product of the APD has limited sensitivity athigher bit rates. The PIN receivers offer greater potential bandwidth,but a poor sensitivity [2]. The work regarding receiver sensitivityenhancement was firstly reported with −37.2 dBm using EDFA(erbium doped fiber amplifier) at 10 Gb/s [3] and −38.8 dBm, usingtwo-cascaded EDFA [4].

Semiconductor optical amplifiers (SOAs) are prospective device foroptical preamplifier as compared to fiber amplifiers due to theircompact size, ultra wideband gain spectrum, low power consumption,ease of integration with other devices and low cost. An SOA pre-amplifier receiver has sensitivity of−34.3 dBm at 4 Gb/s without anyoptical filter [5]. An improvement of receiver sensitivity of 14.5 dB at10 Gb/s, using high gain SOA with polarization sensitivity of 1.1 dB±0.4 dB followed by a 0.6 nm optical filter [6].

An optical preamplifier was proposed using an inverted-ASE signalfrom a saturated SOA [7], whose advantages over conventional SOAoptical preamplifiers are relaxed requirement on AR coating, fiberalignments, narrower optical band pass filter and wider wavelengthtolerance. Yamatoya et al. [8] demonstrated the minimum receiversensitivity of −32.7 dBm by including 0.1 nm optical filter using anSOA preamplifier with optical gain bandwidth product over 60 nm.

Shuangmi et al. [9] reported that SOA type-II has greater receiversensitivities achieved by increasing the carrier lifetime up to 1.4 ns inmulti channel WDM communication link. It also reported sensitivitiesimprovement by increasing the gain saturation of SOA.

This paper extended the work reported in references [6,7,9] tooptimize the SOA preamplifier structure for improvement in receiversensitivity for PIN receivers at 10 Gb/s. The paper is arranged into fivesections. In Section 2, the theoretical analysis has been reported forSOA preamplifier. In Section 3, SOA structure parameters reported andin Section 4, SOA characteristics are observed. In Section 5, SOApreamplifier is presented for both single and multi channel. Finally inSection 6, conclusions are made.

2. Theoretical analysis

The receiver sensitivity given by [5]

Psen = PinGl ð1Þ

where l is the loss between the last amplifier and the receiver. For ourproposed SOA preamplifier structure as in Section 3, Ntot total noise is

829S. Singh / Optics Communications 284 (2011) 828–832

less, because no need of optical filter and PASE amplified spontaneousemission noise power is quit low, so better sensitivity is obtained.

The small signal gain go is given by [10]

go = Γσg = V� �

Iτ = q−Ntð Þ ð2Þ

where Γ is confinement factor,σg is differential gain,V is active volume,I is bias current, τ is the carrier lifetime, q is charge of electron and Nt

is the transparency carrier density.Now, themulti-channelWDM case is considered, the rate equation

that governs the carrier density in the SOA gain region [9]

∂N t; zð Þ∂t =

Jqta

−N t; zð Þτ

−a N t; zð Þ−Ntð Þ∑M

i=1Pi t; zð Þ

hvwteffð3Þ

where t is time, z is the position along the propagation direction oflight, J is the injection current density, N(t,z) is the carrier density, ta isthe thickness of active region, τ is the carrier recombination time, a isthe differential gain, Nt is the transparency carrier density, M is thetotal number of channels, Pi(t,z) is the power in the ith channel, hv isthe energy of photon, w is the width of waveguide, teff is the effectivethickness of the waveguide and Γ=ta/teff is the confinement factor.The gain coefficient of the amplifier is given by

g t; zð Þ = N t; zð Þ−Nt½ �aΓL ð4Þ

where L is the length of active region of SOA. The crosstalk cansuppress by keeping the gain coefficient constant. There is variation ofgain

g t1; z1ð Þ = g t; zð Þ + Δg ð5Þ

Where Δg is the small dynamic variation of g(t1,z1) around g(t,z).Solving Eq. (3) using Eqs. (4) and (5) for small dynamic variation. Wehave

∂Δg∂t1

+Δgτ

P t2; z2ð Þ = aΓτ

Iτwqta

−N t; zð ÞaΓL� �

−g t; zð Þ P t2; z2ð Þτ

ð6Þ

Δg small gain variation can be found by solving first order differentialEq. (14) as reported in [11]

Δg t1; z1ð Þ = aΓIwqtaPL

expΔP1τ

� �

+1τ∑M

n=1

−1ð ÞnPnL

expΔPnτ

� �aΓLNn tn; znð Þ−gPn−1 tn−1; zn−1ð Þh i

ð7Þ

Where ΔP1 and N(t1,z1) is the time variation of power and carrierdensity. Also taken derivative power coefficient, PL=∂P(t1,z1)/∂t1 for

CW Optical Source

AmplitudeModulator

ElectricalFilter

NRZDriver

DataSource

Fig. 1. Set up for measurin

reducing complexity of the system and derivative carrier density,N̄=∂N(t1,z1)/∂t1.

Eq. (7) shows that the small variation of gain is dependent uponvariation of carrier lifetime, confinement factor and power of thesignals. From the Eq. (7), the longer the lifetime, the smaller will bethe gain variation which produce inter channel crosstalk, which issame as reported in Ref. [9]. Similar, if the bias current decreases,which results in reduced gain fluctuation, then it means less crosstalk.But by reducing the injection current, amplification factor goes ondecreasing. Also Eq. (7) shows that with increase in launched powerleads to increase the crosstalk.

3. SOA structure parameters

The standard InGaAlAs travelingwave SOAwith negligible residualfacet reflectivity is taken as the amplifier model in our simulations.After solving the above rate Eq. (7), the relevant parameters are asfollows: the length is 900 μm, the width of active layer is 2 μm, itsthickness is 0.2 μm and confinement factor is 0.3. The transparencycarrier density in the SOA is taken to be 1.08×1018 cm−3 andwith thedifferential gain of 2×10−16 cm2. The spontaneous carrier lifetime(carrier recombination time) τ at this density is evaluated 0.25 nswith saturation power Psat=34.18 mW. The injection current is to be400 mA. The optical bandwidth of SOA is 40 nm and spontaneousemission factor is considered 4. The input and output coupling lossesof SOAs are taken as 3 dB. Nonradiative recombination assumednegligible (at room temperature).

4. SOA characteristics

In order to optimize the SOA for preamplifier, the simulations havebeen carried out for SOA preamplifier parameters given in Section 3by using set up as shown in Fig. 1. The OOK (on-off keying) signalformed by encoding a continuous wave (CW) lorentzian light sourcewith different power and data in terms of NRZ format launched intoamplitude modulator. The NRZ data were pseudorandom binarysequence (PBRS) with word length 29-1 at 10 Gb/s.

The full wave half maxima (FWHM) line width of CW light sourceis 30 MHz. The low pass Bessel filter has 3 dB bandwidth of 13 GHz forgeneration of data source in terms of NRZ format. Then NRZ-OOKsignal is launched into SOA with parameters as reported in Section 3.Then output is fed into PIN receiver for detection of signal. The PINreceiver has quantum efficiency 0.7 with responsivity 0.88 A/W. Thedark current for PIN is 0.1 nA. The single pole electrical filtering with3 dB bandwidth is 20 GHz. The time domain simulation is carried outwith simulation bandwidth 0.1 THz at centre wavelength 1550 nm.

The optical gain of SOA preamplifier is measured for differentcarrier lifetime using PIN-receiver at 1550 nm as shown in Fig. 2. Withdecrease in spontaneous carrier lifetime, the small signal gain goes ondecreasing. For all carrier lifetime, the gain is almost constant up togain saturation with respect to applied power. The optical gain of

SOA

PIN Receiver

g the SOA response.

Fig. 2. Optical gain of SOA preamplifier variation with input signal power for differentcarrier lifetime using PIN receiver.

Fig. 3. ASE noise power of SOA preamplifier variation with input light power for PINreceiver at different carrier lifetime.

830 S. Singh / Optics Communications 284 (2011) 828–832

30.06 dB remains constant up to gain saturation for carrier lifetime0.18 ns.

The relationship between input light and output ASE power of theSOA preamplifier is shown Fig. 3. With increases in input light power,the ASE noise power goes on decreasing for higher carrier lifetime.This shows good agreementwith results reported in [8]. But for carrier

EDFA

OpticalTransmitter

OpticalFilter

0.2 nm

Fig. 4. System set up used to evaluate the

lifetime less than 0.18 ns, the ASE noise power is almost constant andsmall. The ASE noise power observed is less than as reported in [8]. Itis observed for carrier lifetime 0.3 ns that the ASE power is 42.27 μWwith input power 0.2 mW for PIN receiver.

Its concludes that as we increase the carrier lifetime, the ASE noisepower and SOA-induced crosstalk start increasing with increase ingain, which is same as from analysis, Eq. (7).

5. SOA preamplifier for single and multi channelstransmission links

The simulations have been carried out for system setup shown inFig. 4 with SOA preamplifier with parameters as reported in Section 3.The transmitter bandwidth is 13 GHz for generation of data source interms of NRZ format at 10 Gb/s. The NRZ data are modulated byoptical Sine square Mach Zehnder amplitude modulator. The EDFAwith noise Fig. 8 dB and large gain of 37.5 dB is used. At receiver side,the PIN receiver is used to detect the output signal, with and withoutSOA preamplifier.

The time domain simulation is carried out with simulationbandwidth of 0.1 THz at centre wavelength 1550 nm. The opticalgain bandwidth variations with input wavelength is main constrainfor optical preamplifier. The optical gain bandwidth variation withrespect the input light is observed. The gain bandwidth for input lightpower of −20 dBm, 0 dBm and 5 dBm are measured as shown inFig. 5. It is observed that the 3 dB bandwidth of the SOA preamplifier ismore than 100 nmwhich is too large as compared to [8]. It is observedthat with increase in the input light power, bandwidth of SOApreamplifier goes on decreasing.

The receiver sensitivity of PIN receiver with SOA preamplifiers ismeasured by using the same value of electrical filter bandwidth ofreceiver of Be=7.5 GHz and optical filter bandwidth of Bo=0.1 nm asreported in Ref. [8].

After performing the simulations of setup as shown in Fig. 4, thesignal is not detected without preamplifier at 10 Gb/s as shown Fig. 6.The BER is around 10−7 at high input signal. In presence of excess loss,modulation loss and loss of filter, the detected signal is around−39 dBm at the floor of bit error rate 4.5×10−7 without preamplifier.By using SOA preamplifier model without optical filter the detectedsignal power is −71.7 dBm with BER floor 3×10−10 for 10 Gb/s withcarrier lifetime 0.18 ns. With increase the input signal further, allinput signals are detected with BER less than 10−10 up to gainsaturation of SOA preamplifier. But for this carrier lifetime, the gain ofSOA is drop as from Eq. (7). Therefore small output signal is detectedaround−71.7 dBm. As the carrier lifetime increases up to 0.25 ns, thegain is improved. From results as shown Fig. 6, the improved detectedpower of −50.34 dBm is observed at BER 3.9×10−12. Also, withincrease in the input signal, the improved detected signal is observedat BER floor less than 10−12. By further increasing the carrier lifetime

PIN Receiver

Optical Band pass Filter Bo =0.1 nm

SOA

PIN Receiver

SOA in a preamplifier configuration.

1510 1520 1530 1540 1550 1560 1570 1580 1590-0.25

-0.2

-0.15

-0.1

-0.05

0

Wavelength of Input Light Signal (nm)

Gai

n V

aria

tion(

dB)

Input Light Power

-20 dBm 0 dBm 5 dBm

Fig. 5. Gain variation versus wavelength for different input light power at 10 Gb/s.

-70 -60 -50 -40 -30 -2010-16

10-14

10-12

10-10

10-8

10-6

Received Power (dBm)

Bit

Err

or R

ate

Without SOA0.18 ns 0.25 ns 0.3 ns

Fig. 6. BER versus received power for different carrier lifetime of SOA preamplifier withPIN receiver.

-40 -35 -30 -25 -20 -15 -10 -5 010-40

10-35

10-30

10-25

10-20

10-15

10-10

10-5

100

Received Power (dBm)

Bit

Err

or R

ate

0.1 ns

Carrier Lifetime (ns)

0.25 ns 0.3 ns

Fig. 8. BER versus received power for different carrier lifetime for SOA preamplifier inmulti channel WDM transmission links.

831S. Singh / Optics Communications 284 (2011) 828–832

up to 0.3 ns or more, the gain saturation occurs at lower input power.Therefore, BER observed for this carrier lifetime is more than 10−8

with improvement in received power. This is due to the input signalpower more than saturation power. At higher value of carrier lifetime,the gain increases but ASE noise power and other impairments isalso pronounced, as shown Figs. 2 and 3. The detected power of−50.34 dBm at carrier lifetime 0.25 ns shows an improvement overthe results reported in Refs. [6,8].

Now, this SOA preamplifier is considered for multi channel trans-mission system. Fig. 7 shows a schematic of 20×10 Gb/s OOK WDMsystem.

Twenty lorentzian laser sources in the wavelength range1550.38 nm to 1557.63 nm (100 GHz channel spacing) are modulatedby each optical sin2 Mach Zehnder amplitude modulator with returnto zero (RZ) format. The input of each transmission channel is from

20×10 Gb/sNRZ-OOK

Signalλ=1550 nm

SMF60 km

DCF10 km SOA1

SM60

Fig. 7. Block diagram of WDM system using SOA1 as

−20 dBm to 0 dBm. Therefore, design carries 200 Gb/s WDM RZ-OOKsignals over 350 kmwith 70 km SOA spacing. Each of spans consists ofa 60 km standard single mode fiber (SMF), one or more DCF and anSOA at the end.

The length of DCF is chosen in accordance with [10] for completecompensation (D̄=0)

L2 =−D1L1D2

Where L1, L2 are lengths of SMF & DCF and D1, D2 are dispersionparameters for SMF and DCF. For SMF, the value of dispersionparameter D at the operating wavelength is 17 ps/km-nm. Thecorresponding value for DCF is six times greater with opposite sign.The loss of SMF is 0.2 dB/km and loss of DCF is 0.55 dB/km. The timedomain simulations have been carried out for the set up, as shown inFig. 7 for different carrier lifetime with bandwidth 1.256 THz andcenter wavelength 1550.3 nm.

The plot is shown in Fig. 8, the resulting BER versus the averagedetected power per channels. As from result, if lower value ofspontaneous lifetime is taken, then detected power per channel isdrop by 15 dBwith BER greater than 10−9. The power drop occurs dueto fall in gain of SOA due to ASE noise power. Also from Eq. (7), SOA-induced cross talk is generated at lower value of carrier lifetime.Therefore variation of gain is increase at 0.1 ns carrier lifetime. So thiscauses drop in quality of signals.

As from the results, if there is increase the carrier lifetime to0.25 ns, all channels are detected with sufficient power at BER lessthan 10−18. Here gain fluctuations decreases due to increase in carrierlifetime, so SOA-induced crosstalk is also less. In this case, averagedetected channels show BER less than 10−9 up to saturation of theSOA.

Further, with increase in carrier lifetime to 0.3 ns, the SOA-inducedcrosstalk is reduced. But with increase the input signal, SOA saturatedearlier as compared to lower value of carrier lifetime. So result shows

SOA1

20×10 Gb/sNRZ-OOK

Signal λ=1550 nm

Fkm

DCF10 km

SOAPreamplifier

inline amplifiers and SOA preamplifier at end.

832 S. Singh / Optics Communications 284 (2011) 828–832

that good BER is observed at receiver for input signal up to −5 dBm.So by using carrier lifetime 0.25 ns, good improvement in detectedpower per channel is observed with high input signal. These resultsshow an improvement over results reported in [9]. The spontaneouscarrier lifetime depends upon the ASE noise. For large value of ASE,more is the carrier lifetime. It can be controlled by

a. To design and fabrication of proposed structure of SOA preampli-fier in optical networks for better accuracy.

b. There is ASE suppression device available with commercial SOA toreduce the ASE noise and improve the noise figure. [12]. So controlof ASE will also decrease the lifetime of carriers.

c. As the injection current increases, the number of photon emittedper second increases [13]. Therefore from this, the spontaneouscarrier lifetime is controlled.

Further optimization of SOA structure is also possible forimprovement of the receiver performance.

6. Conclusions

An SOA structure is proposed by optimizing the carrier lifetime forpreamplifier with PIN photo receiver. The minimum received powerdetected is−50.34 dBm at BER floor of 3.9×10−12 for PIN receiver at10 Gb/s by using carrier lifetime 0.25 ns. The impacts of ASE noisepower, optical gain, gain variation for different carrier lifetime withPIN receiver illustrated. It is observed that ASE noise power forproposed SOA preamplifier is quite low. It has been shown that gainvariation increases with increase of the input light power and it was

also observed that tolerance of input wavelength power is more than100 nm. For multi channel WDM transmission links show improve-ment in detected signal at 0.25 ns carrier lifetime.

Acknowledgement

The author acknowledges the Rsoft and Optiwave for simulationsoftware OptSim and OptiSystem.

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