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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 10, Iss. 3 — Feb. 11, 2002
  • pp: 183–189
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Optical mixing of microwave signals in a nonlinear semiconductor laser amplifier modulator

José Capmany, Salvador Sales, Daniel Pastor, and Beatriz Ortega  »View Author Affiliations


Optics Express, Vol. 10, Issue 3, pp. 183-189 (2002)
http://dx.doi.org/10.1364/OE.10.000183


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Abstract

In this paper we propose and evaluate the optical mixing of RF signals by means of exploiting the nonlinearity of a SLA modulator. The results show the potential for devices with low conversion losses (and even gain) and polarization insensitivity and reduced insertion losses.

© Optical Society of America

1. Introduction

The semiconductor laser amplifier (SLA) is a versatile device that has found multiple applications in the fields of telecommunications and optical signal processing [1

1. T. Durhuus et al, “All optical wavelength Conversión by semiconductor optical amplifiers,” IEEE/OSA J. Lightwave. Technol 14, 942–953 (1996). [CrossRef]

]. Especially interesting are those pertaining to the later class, where the fast dynamics and highly nonlinear properties of the devices are exploited to implement cross gain, cross phase and four wave mixing based wavelength converters.

The principle of operation and is illustrated in figure 1. In this case, the SLA is employed in a non linear modulator configuration [8

8. J. Mork, A. Mecozzi, and G. Eisenstein, “The modulation response of a semiconductor optical amplifier, ” IEEE Sel. To. Quantum. Electron. 5, 851–860 (1999). [CrossRef]

]. In this paper we propose and evaluate the mixing of RF signals by means of exploiting the non linearity of an SLA modulator. The results show that potential high levels of conversion efficiency are attainable with modest modulation indexes.

Figure 1: Rf signal mixer based on a SLA employed in a nonlinear modulator configuration

2. SLA Modulator propagation and rate equations

For the sake of consistency we follow the model for the SLA modulator provided by Mork and co-workers [8

8. J. Mork, A. Mecozzi, and G. Eisenstein, “The modulation response of a semiconductor optical amplifier, ” IEEE Sel. To. Quantum. Electron. 5, 851–860 (1999). [CrossRef]

] . The equations for the propagation of the photon density S=S(z,t) and the rate equation for the carrier density N=N(t,z) are given by:

Sz=Γa(NNo)SαLS
Nt=I(t,z)eVNτsvga(NNo)S
(1)

As usual, z represents the spatial coordinate along the SLA, t is the local time measured in a coordinate system moving with the group velocity vg, I(t,z) the modulation current, No the carrier density at transparency, V the active region volume, τ s the carrier lifetime, Γ the confinement factor, αL the internal loss in the SLA and a the differential gain.

We aim to investigate the SLA RF mixing response to an input current modulation given by:

I(t)=I¯ +ΔI(z,ΩLO)ejΩLOt+ΔI*(z,ΩLO)ejΩLOt
+ΔI(z,ΩS)ejΩSt+ΔI*(ΩS)ejΩSt
(2)

Where I̅ represents the constant DC current , ∆Ii) i=LO,S stand for the modulation current amplitudes at Rf frequencies Ωi , i=LO,s corresponding to the Local oscillator and the signal respectively and * denotes the complex conjugate. If a small signal modulation approach is considered taking into account up to the second order terms in S and N then the following approximation can be taken :

Y(z,t)=Y¯(z)+ΔY(z,ΩLO)ejΩLOt+ΔY*(z,ΩLO)ejΩLOt
+ΔY(z,Ωs)ejΩst+ΔY*(Ωs)ejΩst+
+ΔY(z,2ΩLO)ej2ΩLOt+ΔY*(z,2ΩLO)ej2ΩLOt+
ΔY(z,2Ωs)ej2Ωst+ΔY*(2Ωs)ej2Ωst+
+ΔY(z,ΩLOΩS)ej(ΩLOΩs)t+ΔY*(z,ΩLOΩs)ej(ΩLOΩs)t+
ΔY(z,ΩLO+Ωs)ej(ΩLO+Ωs)t+ΔY*(z,ΩLO+Ωs)ej(ΩLO+Ωs)t
(3)

Where Y(z, t) represents either S(t,z) or N(t,z) as required. Substituting (3) and (2) into (1) and retaining up to second order terms yields the following set of differential equations:

dS¯ dz=(gsatαL)S¯
(4)
dΔS(Ωi)dz=(gsatαLβ(Ωi))ΔS(Ωi)+ΓΔI(Ωi)eVvggsat
(5)
dΔS(2Ωi)dz=(gsatαLβ(2Ωi))ΔS(2Ωi)+
+(gsatβ(2Ωi)gsat){ΓaτSΔS(Ωi)Q(Ωi)xx[ΔI(Ωi)eVgsatvgΓΔS(Ωi)]}
(6)
dΔS(ΩLO±ΩS)dz=(gsatαLβ(ΩLO±ΩS))ΔS(ΩLO±ΩS)+
ΓaτS(gsatβ(ΩLO±ΩS)gsat)
{ΔS(ΩS)Q(ΩLO)(ΔI(ΩLO)eVgsatvgΔS(ΩLO)Γ)+ΔS(ΩLO)Q(*)(ΩS)(ΔI(ΩS)eVgsatvgΔS(*)(ΩS)Γ)}
(7)

In the above i=LO, s, gsat = Γgo /(1 + S̅/Ssat) is the saturated gain, Ssat =1 /(av g τs) = ΓPsat /(ħwAv g) the saturation photon density with Psat being the SLA output saturation power and A the active zone cross area, go = a(I̅τs /(eV) - No) the small signal material gain, β(Ω) = gsatQ(Ω)(S̅/Ssat) and finally, Q(Ω) = [-jΩτs + 1 + (S̅/Ssat)]-1 . Equations (5–7) with the initial conditions ∆S(0,U)=∆N(0,U)=0, for all U values, being U a frequency variable and S(0,t) = S̅(0) = Sin describe the process of second order signal mixing in a SLA modulator and have been tested by comparing with the direct numerical solution of (1)–(2). In all cases the agreement has been excellent. For example, figure 2 shows the results obtained for a L=5 mm SLA modulator with similar characteristics as those of [8

8. J. Mork, A. Mecozzi, and G. Eisenstein, “The modulation response of a semiconductor optical amplifier, ” IEEE Sel. To. Quantum. Electron. 5, 851–860 (1999). [CrossRef]

]. The parameters employed in the calculations are: Pin=1 mW, Psat=28.5 mW, τ s=100 psec, a=3.10-20 m2, No=1.1.1024 m-3, Γ=0.3, A=0.2.10-12 m2. Solid lines correspond to the results obtained by solving (4–7), whereas (*) curves represent the numerical solution of (1)–(2).The modulating RF carriers are separated by 100 MHz. The linear characteristic displays a resonant characteristic, the origin of which has been thoroughly explained [8

8. J. Mork, A. Mecozzi, and G. Eisenstein, “The modulation response of a semiconductor optical amplifier, ” IEEE Sel. To. Quantum. Electron. 5, 851–860 (1999). [CrossRef]

].

Figure 2: Relative magnitudes of linear and second order nonlinear terms for a 5mm SLA modulator with Pin=1 mW, Psat=28.5 mW, τ s=100psec, a=3.10-20m2, No=1.1.1024m-3, Γ=0.3, A=0.2.10-12 m2. Solid lines correspond to the results obtained by solving (4–7), whereas (*) curves represent the numerical solution of (1)–(2). Ω21+100MHz.

Figure 2 has been plotted for a 100 MHz frequency difference between the two RF signals. Similar behavior and efficiency of the mixing process have been detected for differences ranging from 10 MHz to 10 GHz (the SOA modulator bandwidth). Note that, the frequency response of the nonlinear process depends on the modulation bandwidth of the SOA. Notice that the resonant behaviour is enhanced in second order nonlinear terms that correspond to the sum of frequencies while it is mitigated in the nonlinear term corresponding to the frequency difference. Furthermore, in this last case a notch characteristic is displayed. All second order terms are low pass with a roll-off characteristic showing a double slope than that corresponding to the linear term. We have also obtained results for shorter amplifiers L=1 mm, where the resonant behaviour is not present even in the linear term. In this cases all the second order show analogous low-pass characteristics. In any case it is worth pointing out that second order non linearities can show high values which may indicate that SLA modulators may find an interesting application in the field of microwave mixing.

3. Mixer performance

The mixing efficiency can be obtained as usual [2–5] by comparing the RF powers of the input signal PRFin and the output powers of the up-converted PRF-up and downconverted signal PRF-down, taking as a parameter the value of the DC RF power due to the LO.

PRFin=ΔI(Ωs)2Zm2
PRFup=R(ħwvgA)ΔS(ΩLO+Ωs)2Zr2
PRFdown=R(ħwvgA)ΔS(ΩLOΩs)2Zr2
(8)

Where R is the detector responsivity and Zr, Zm represent the impedances of the receiver and the SOA modulator respectively, which we will assume equal (Zr=Zm=50 Ω).

To gain insight on the mixing potential of SLA modulators we have evaluated the conversion efficiency for the upconversion term (PRF-up / PRFin) for a device with Pin=0.5 mW, Psat=28.5 mW, τ s=100 psec, a=3.10-20 m2, No=1.1.1024 m-3, L=1mm, Γ=0.45, A=0.2.10-12 m2. Other relevant parameters are the unsaturated gain Go=23.45dB; the saturated gain G(Pin=0.5mW)=18.33 dB, I̅ = 450mA ,. The frequencies of the RF signal and the LO are 3.5 and 3.49 GHz, resulting in an up-converted frequency of 6.99 GHz. Similar results have been obtained for RF and LO signal frequencies near 15 and 20 GHZ.

Figure 3 shows the results for PRF-up vs the value of PRFin both expressed in dBm,. Taking the LO Rf power as a parameter. For moderate LO powers (12 dBm) the obtained conversion loss is 6 dB which is far smaller than the values reported for electroabsorption modulators5 (19.8 dB for PLO=10 dBm), single LiNbO3 MZ modulators 9 (15.8dB for PLO=30 dBm), and standard microwave technology mixers (21.95 dB. For PLO= 16 dBm). Furthermore, for LO powers comparable to the two later devices, the SOA based mixer produces almost no conversion loss (PLO=16 dBm) and even conversion gain (6 dB for PLO=24 dBm). Thus the proposed device can be advantageous in terms of this parameter in comparison with other technology options.

An issue of importance when dealing with SOA based devices attains to their noise performance. In our case, the limiting noise source has been found to be that caused by the beating between the signal and spontaneous noise and its value has also been plotted in figure 3. Compared to other options, the noise floor as expected is higher, however since the conversion losses are lower, there is enough margin to yet attain a considerable SNR value for the converted signal. The reader must note that total noise is almost constant with ∆PRFin since the signal spontaneous beating noise is mainly due the average optical power and not to the modulation RF sidebands.

Figure 3: PRF-up vs the value of PRFin both expressed in dBm for a standard SOA based RF mixer,. taking the LO Rf power as a parameter. (device parameters are given in the text).

The results shown are only preliminary, but outline the potential for devices with low conversion losses (and even gain) and polarization insensitivity and reduced insertion losses. It should be pointed out that this analysis has only considered SOAs with standard parameters. Enhanced performance can be expected with special designs incorporating longer active lengths and higher gain and also, with tandem SOA configurations. This is currently under investigation. To the knowledge of the authors, this is the first time this device has been proposed for this purpose and it can find applications in microwave and millimeter wave fiber-radio and fiber wireless systems, where flexible broadband RF mixing with low loss or even gain is required.

4. Summary and Conclusions

In summary, we have proposed for the first time to our knowledge and evaluated the use of a SLA modulator for the mixing of RF signals directly in the optical domain by means of exploiting the nonlinearity of the modulator. The results show that broadband devices with potential low conversion losses (and even gain), polarization insensitivity and low insertion losses can be expected with moderate LO Rf powers.

Acknowledgements

The authors wish to acknowledge the financial support of the Spanish CICYT via projects TEL 99–0437 and TIC 98–0346 and the European Union COST 267 Action. Useful comments from Dr. José Miguel Fuster are also acknowledged.

References

1.

T. Durhuus et al, “All optical wavelength Conversión by semiconductor optical amplifiers,” IEEE/OSA J. Lightwave. Technol 14, 942–953 (1996). [CrossRef]

2.

G.K. Gopalakrishnan, W.K. Burns, and C.H. Bulmer, “Microwave optical mixing in LiNbO3 modulators,” IEEE Trans. Microwave Theory Tech. 41, 2383–2391 (1993). [CrossRef]

3.

A. Lindsay, G. Knight, and S. Winfall, “Photonic Mixers for wide bandwidth RF receiver Applications, ” IEEE Trans. Microwave Theory Tech. 43, 2311–2397 (1995). [CrossRef]

4.

K.P Ho, S.K. Liaw, and C. Lin, “ Efficient photonic mixer with frequency doubling, ” IEEE Photon. Tech. Lett. 9, 511–513 (1997). [CrossRef]

5.

D.S. Shim et al., “ Optoelectronic RF signal mixing using an electroabsorption waveguide as an integrated photodetector/Mixer, ” IEEE Photon. Tech. Lett. 12, 193–195 (2000). [CrossRef]

6.

R. Helkey, J.V. Twinchel, and C. Cox, “ A down-conversion optical link with RF gain,” IEEE J. Lightwave Technol. 15, 956–961 (1997). [CrossRef]

7.

C.P Liu, A.J. Seeds, and D. Wake, “Two terminal edge-coupled InP/InGaAs heterojunction phototransistor optoelectronic mixer, ” IEEE Microwave Guide Wave Lett. 7, 72–74 (1997). [CrossRef]

8.

J. Mork, A. Mecozzi, and G. Eisenstein, “The modulation response of a semiconductor optical amplifier, ” IEEE Sel. To. Quantum. Electron. 5, 851–860 (1999). [CrossRef]

9.

J. Fuster et al, “ Fiber-optic microwave link employing optically amplified electrooptical upconverting receivers ,“ IEEE Photon Tech. Lett. 9, 1161–1163 (1997). [CrossRef]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators

ToC Category:
Research Papers

History
Original Manuscript: January 17, 2002
Revised Manuscript: February 5, 2002
Published: February 11, 2002

Citation
Jos� Capmany, Salvador Sales, Daniel Pastor, and Beatriz Ortega, "Optical mixing of microwave signals in a nonlinear semiconductor laser amplifier modulator," Opt. Express 10, 183-189 (2002)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-10-3-183


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References

  1. T. Durhuus et al., ?All optical wavelength Conversi?n by semiconductor optical amplifiers,? J. Lightwave. Technol 14, 942-953 (1996). [CrossRef]
  2. G. K. Gopalakrishnan, W. K. Burns and C. H. Bulmer, ?Microwave optical mixing in LiNbO3 modulators,? IEEE Trans. Microwave Theory Tech. 41, 2383-2391 (1993). [CrossRef]
  3. A. Lindsay, G. Knight, and S. Winfall, ?Photonic Mixers for wide bandwidth RF receiver Applications, ? IEEE Trans. Microwave Theory Tech. 43, 2311-2397 (1995). [CrossRef]
  4. K. P Ho, S. K. Liaw and C. Lin, ?Efficient photonic mixer with frequency doubling, ? IEEE Photon. Tech. Lett. 9, 511-513 (1997). [CrossRef]
  5. D. S. Shim et al., ?Optoelectronic RF signal mixing using an electroabsorption waveguide as an integrated photodetector/Mixer, ? IEEE Photon. Tech. Lett. 12, 193-195 (2000). [CrossRef]
  6. R. Helkey, J. V. Twinchel and C. Cox, ?A down-conversion optical link with RF gain,? J. Lightwave Technol. 15, 956-961 (1997). [CrossRef]
  7. C. P Liu, A. J. Seeds and D. Wake, ?Two terminal edge-coupled InP/InGaAs heterojunction phototransistor optoelectronic mixer, ? IEEE Microwave Guide Wave Lett. 7, 72-74 (1997). [CrossRef]
  8. J. Mork, A. Mecozzi and G. Eisenstein, ?The modulation response of a semiconductor optical amplifier, ? IEEE Sel. To. Quantum. Electron. 5, 851-860 (1999). [CrossRef]
  9. J. Fuster et al, ?Fiber-optic microwave link employing optically amplified electrooptical upconverting receivers,? IEEE Photon Tech. Lett. 9, 1161-1163 (1997). [CrossRef]

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