Noise Characteristics of Polarization Sensitive Optically Preamplified Receivers

Transcription

1 HEWLETT PACKARD Noise Characteristics of Polarization Sensitive Optically Preamplified Receivers D. G. Cunningham, A. N. Coles, I. H. White l Networks and Communications Laboratory lip Laboratories Bristol lipl May, 1990 optical amplifier; optical receiver; polarization Experimental results confirm SNR degradation due to uncontrolled polarization at the input to optically preamplified receivers can be minimal for gain sensitivities less than 4dB. Internal Submitted Accession to Electronics Date Only Letters. lcambridge University Engineering Department (c) Copyright Hewlett-Packard Company 1990

2 Introduction: Semiconductor optical preamplifiers can increase the sensitivity of receivers in broadband optical communication systems [1]. However, optical amplifiers generally have different gains in the TE and TM modes, and unless the input signal state of polarisation is controlled both the output signal and signal related noise powers may vary with time. In a practical system control of the input signal state of polarisation may be undesirable due to insertion loss and increased component count, or impossible when signals are received from more than one optical source. We present a theoretical and experimental investigation of the effect of variable input state of polarisation on the signal to noise ratio (SNR) at the output of an optically preamplified receiver. The semiclassical theory of light [2] is used to model the noise properties of an optical amplifier. The advantage of this method over the more commonly used photon number approach is that it naturally includes polarisation effects and leads to an intuitive, physical understanding of the noise generation process. The new expression derived for the total noise variance includes a new excess noise factor for pure travelling wave amplifier systems. Theory: (See table 1 for definition of symbols). When an optical amplifier is illuminated by a signal laser of linear polarisation, the total detected field, E, is the superposition of the amplified spontaneous emission (ASE) field, A, and the amplified laser field L. The ASE can be considered to be thermal light [3] consisting of two statistically independent components representing TE and TM polarisation modes such that the average intensity of the ASE is fa = fact E)+ fa(tai) = {(GTE - 1)+(GTM -1)]I,pc2' Since (E. E*) =(L.L*) + (A. A*) + 2~(L. A*) we may calculate the average detected intensity as 1= h + IA(TE) + IA(TM) where the statistically independent beat intensities associated with 2lR(LTE. A~E) and 2lR(LTM. A~M) average to zero. We expect the statistically independent laser and ASE intensities to contribute both quantum shot noise and excess noise, due to the classical fluctuations of the fields to the total noise variance. However, the intensities associated with the beating between the laser and the ASE fields will contribute excess noise only, since their mean intensity is zero. Hence, the variance of the detected photocurrent 2

3 may be calculated as: = 2eB[iL + ia(te) + ia(tm)] +hrinb [JA(TE) + ia(tm)][t -00 Ir(r)1 dr] + 2[(]L(TE»)A(TE) + )L(TM»)A(TM)][~1:'Y(r) dr] (1) The integral [f~oo Ir(rW dr] represents the coherence time of the ASE, r c = t [3, 4], and it is assumed that r c «T. The integral U~oo 'Y(r) dr] is also equal to r c if the laser signal is at the centre frequency of a rectangular ASE spectrum. However, this is not usually the case, and u~oo 'Y(r) dr] = xr c = 1;, where X can be thought of as a new excess noise factor for pure travelling wave amplifiers. The terms on the right hand side of equation 1 represent the combined signal and ASE shot noise, signal intensity noise, spontaneous-spontaneous beat noise and signal-spontaneous beat noise respectively. For the particular case of a signal spontaneous beat noise limited system the SNR at the detector may be written as a function of signal input polarisation angle, 8, such that: SNR(8) t; [GTE cos (0) + GTM sm (8)] ( s, ) = -m Ct-X 2 2 X -- 4 I,p GTE(GTE -l)cos (8) + GTM(GTM -l)sin (0) 2XB (2) For large gains the SNR reaches a minimum at an angle 0', where tan 2(8') = GGTIj. The value of TM this minimum, SN R(O'), relative to SNR(OO), represents the maximum SNR degradation due to uncontrolled input signal state of polarisation, and is plotted together with 8' as a function of the gain polarisation sensitivity, GGTIj. in figure 1. TM Experimental: The signal-spontaneous noise was measured experimentally using an antireflection coated 500 IJm long ridge waveguide laser. with a gain peak at 1500 nm. The input signal from an isolated 1535 nm DFB laser was coupled into the amplifier using microscope objectives. The output of the amplifier was coupled into a single mode fibre-pigtailed interference filter of 3 nm bandwidth centered at the signal wavelength and detected using a Lasertron QDFT

4 pinfet. After amplification the receiver noise was measured on a Hewlett Packard HP8568B spectrum analyser. The polarisation sensitivity of the amplifier gain at the signal wavelength was measured as 2.5 db. A linear input polarisation state was rotated between TE and TM modes, and the measured ratio of output signal to signal-spontaneous beat noise is shown in figure 2 along with the theoretical result for the device parameters. Also shown in figure 2 are results from reference [5], which confirm the theory for larger gain polarisation sensitivity (GGn 'I'M =6.8 db). Discussion and conclusions: Experimental results confirm that the maximum SNR degradation due to uncontrolled signal state of polarisation at the input to a signal-spontaneous beat noise limited optically preamplified receiver is minimal «1 db) for Si:u.. < 4 db. However, as the gain G1'M polarisation sensitivity increases, the SNR degradation grows rapidly and becomes proportional to the gain ratio GG n 'I'M. This is to be contrasted with a spontaneous-spontaneous beat noise limited receiver, where the SNR degradation as the input signal state of polarisation varies from the TE to TM mode is equal to the square of the gain ratio. Although advances have been made in the design and fabrication of semiconductor optical amplifiers with peak gain insensitive to polarisation (e.g. [6]), differences in the phase of the residual gain ripple in the TE and TM modes may still give rise to a polarisation sensitive gain at a fixed signal wavelength. For a near travelling wave amplifier with 3 db gain ripple in the TE and TM modes, the gain polarisation sensitivity will also be 3 db when these ripples are in antiphase at the signal wavelength. This will result in a maximum SNR degradation of less than 0.5 db at the output of a signal-spontaneous noise limited receiver when the input state of polarisation is uncontrolled. Acknowledgements: We thank Dr. G. Henshall of STC Technology Ltd. for supplying the optical amplifier. 4

5 References [1] o'mahony, M.J., Marshall, I.\V., Westlake, H.J., and Stallard, W.G.: Wideband 1.5 pm Optical Receiver Using Travelling Wave Laser Amplifier. Electron. Lett., 1986,22, pp [2] Henry, C.H.: Theory ofspontaneous Emission Noise in Open Resonators and its Application to Lasers and Optical Amplifiers. J. Lightwave Technology, 1986, LT-4, pp [3] Hodara, H.: Statistics of Thermal and Laser Radiation. Proc. IEEE, July 1965, pp [4] Goodman, J.W.: Statistical Optics. John Wiley & Sons, 1984, Chapters 6 & 9. [5] Walker, G.R., Steele, R.C., Walker, N.G.: Polarization dependence of semiconductor laser amplifier noise fig ure. OFC 1990, San Francisco, paper WM32. [6] Mersali, B., Gelly, G., Accard, A., Lafragette, J.-L., Doussiere, P., Lambert, M., and Fernier, B.: 1.55 urn High Gain Polarisation Insensitive Semiconductor Travelling Wave Amplifier With Low Driving Current. Electron. Lett., 1990, 26, pp

6 B C2 e GTE GTM hii 1. t, h(te) h(tm) t; -. JA(TE) JA(TM) JL JL(TE) h(tm) m RIN T r(r) fj detector bandwidth = 2~ bandwidth of the ASE =t coupling loss at amplifier input coupling loss at amplifier output electronic charge TE modal gain of the amplifier TM modal gain of the amplifier average photon energy of the ASE signal laser input intensity detected intensity of amplified laser =h(te) + h(tm) TE component of amplified laser intensity =I i c l c 2 G T E cos 2 fj TM component of amplified laser intensity =liclc 2GTM sin 2 fj 'seed' spontaneous noise of the amplifier = hiibon.p detected photocurrent due to the TE mode of the ASE detected photocurrent due to the TM mode of the ASE total detected photocurrent due to the amplified laser detected photocurrent due to the TE component of the amplified laser detected photocurrent due to the TM component of the amplified laser optical modulation index spontaneous emission factor Relative Intensity Noise averaging time of the receiver electronics complex degree of coherence of the ASE = 'Y(r) exp( - j2'1riir) laser input angle of polarisation coherence time of the ASE Table 1: Definition of symbols. 6

7 SNR RelatlYe to TE leis) Angle of maxinun SNR degradation L...L...L...L...J GaIn ratio~(db) Figure 1: Maximum signal to noise ratio degradation and input polarisation angle for maximum degradation versus gain polarisation sensitivity (S;~). SNRRelative to TE (db) , A Experimental Reference era (TM) InputSignal Polarisation Angle (degrees). Figure 2: Signal to noise ratio degrada.tion versus input polarisation angle.