eye diagram at the UN1 output. These eye diagrams are colour grade histograms which have been mapped to a greyscale image. Owing to imperfect interleaving, the input data signal showed con- siderable channel non-uniformity which was equalised at the device output, clearly demonstrating the amplitude restoration capabilities of the UNI. Fig. 2d shows the corresponding lOGbit/s eye after demultiplexing of the 80 Gbit/s regenerated signal. Fig. 3 shows BER measurements recorded at the UN1 output after demultiplexing back to 10GbiUs. Compared to the 8OGbiUs back- to-back signal the output signal suffers a power penalty of 2.7dB for 8OGbit/s regenerative wavelength conversion.

E W

0 m -

-1 8 -16 -14 -12 -10 received power, dBm

Fig. 3 BER against received power

W 10Gbitis back-to-back A 80Gbit/s back-to-back + 80-40Gbit/s conversion 0 80-80Gbit/s conversion

By using a 40GHz probe signal, 8WOGbit/s regenerative demultiplexing was also achieved. Fig. 2c shows the 4OGbit/s eye diagram at the output of the UN1 for this operation. The BER measurements plotted in Fig. 3 show a 2.2dB power penalty for

In both modes of operation, there was no noticeable depend- ence on the polarisation of the input switching signal. For 80Gbith switching, the optimised pump and probe powers, measured at the 3dB coupler in front of the SOA, were 9.1 and 5.2dBm, respec- tively, which implies a switching energy of -2OOfJ. The power pen- alties can be attributed to a combination of the finite extinction ratio of the interferometer (ascertained from the autocorrelation and demultiplexed eye diagram), residual data pattern length dependent effects, and the lower quality of the probe pulses when compared to the ML-FRL.

regenerative demultiplexing.

Conclusions; We have demonstrated error-free 80Gbit/s all-optical regenerative data switching of an SOA based interferometer. This is the highest bit rate of operation reported to date. Furthermore, the device was polarisation insensitive with respect to the input data signal, exhibited a low switching power of 200fJ and oper- ated successfully with long pattern lengths.

0 IEE 1999 Electronics Letters Online No: 19990976 DOI: 10.1049/el:19990976

A.E. Kelly, I.D. Phillips, R.J. Manning, A.D. Ellis, D. Nesset, D. G. Moodie and R. Kashyap (BT Laboratories, Martlesham Heath. Ipswich, Suffolk, IPS 3RE. United Kingdom)

E-mail: tony.kelly@bt.com

25 June 1999

References

1 MIKKELSEN, B., VAA, M., POULSEN, H.N., DANIELSEN, S.L., JOERGENSEN, C., KLOCH, A., HANSEN, P B., STUBKJAER, K.E., WUNSTEL, K., DAUB, K., LACH, E., LAUBE, G., IDLER, W., SCHILLING, M., and BOUCHOULE, s.: ‘40Gbitis all-optical wavelength converter and RZ-to-NRZ format adapter realised by monolithic integrated active Michelson interferometer’, Electron. Lett., 1997, 33, (21, pp. 133-134

LAMOULER, P, and ALARD, F.: ‘2OGbitis optical 3R regenerator using SOA based Mach-Zehnder interferometer gate’. Tech. Dig. ECOC 1997, Vol. 2, Paper Tu4B, pp. 269-272

3 PHILLIPS, I.D., ELLIS, A.D., THIELE, H.J., MANNING, R.J., and KELLY, A.E.: ‘40Gbitis all optical regeneration and demultiplexing with long pattern lengths using a semiconductor nonlinear interferometer’, Electron. Lett., 1998, 34, (24), pp. 2340-2342

4 THIELE, H.J., ELLIS, A.D., and PHILLIPS, I D : ‘Recirculating loop demonstration of 40Gbit/s all-optical 3R data regeneration using a semiconductor nonlinear interferometer’, Electron. Lett., 1999, 35, (3), pp. 230-231

5 ELLIS, A.D., KELLY, A.E., NESSET, D., PITCHER, D , MOODIE. D.G., and KASHYAP, R.: ‘Error free IOOGbitis wavelength conversion using grating assisted cross-gain modulation in 2mm long semiconductor amplifier’, Electron. Lett., 1998, 34, (20), pp. 1958-1959

6 HALL, K.L , and RAUSCHENBACH, K.A.: ‘100Gbitis bitwise logic’, Opt. Lett., 1998, 23, pp. 1271-1273

7 PATEL, N.s., RAUSCHENBACH, K.A., and HALL, K.L.: ‘40Gbitis demultiplexing using an ultrafast nonlinear interferometer (UNI)’, IEEE Photonics Technol. Lett., 1996, 8, (12), pp. 1695-1697 KELLY, A.E., MARCENAC, D.D., and NESSET, D.: ‘4OGbitis wavelength conversion over 24.6nm using FWM in semiconductor optical amplifier with optimised MQW active region’, Electron. Lett.,

2 BILLES, L., SIMON, J.C., KOWALSKI. B., HENRY, M., MICHAUD, G.,

8

1997,33, (25), pp. 2123-2124 9 MOODIE, D G . , CANNARD, P.J., DANN, A.J., MARCENAC, D.D.,

FORD, c.w., REED, J., MOORE, R.T., LUCEK, J.K., and ELLIS. A.D.: ‘Low polarisation sensitivity electroabsorption modulators for 160Gbit/s networks’, Electron. Lett., 1997, 33, (24), pp. 2068-2070

lntermodulation distortion by PD in heterodyne detection fibre-optic millimetre- wave links T. Akiyama, K. Inagaki and Y. Mizuguchi

The intermodulation distortion (IM) caused by PDs in heterodyne detection optical-fibre millimetre-wave links has been measured and analysed. The characteristics of IM in multi-carrier transmission systems are explained. It is shown that second-order IM occurs close to the signal frequency and also that it is important to reduce this 1M in order to improve the system performance.

Introduction: The fibre-optic transportation of millimetre-wave (MMW) signals would be an attractive choice for future mobile radio communications and wireless LANs because of the low transmission loss and extremely wide bandwidth of the optical fibre. Typically, intensity modulation direct detection (IM-DD) systems generate MMW subcarrier signals by an external optical modulator (EOM) [l]. However, intermodulation distortion (IM) is known to occur as a result of the nonlinear characteristics of the EOM. Second-order IM (IM2) occurs at frequencies far different from the signal frequency. On the other hand, third-order IM (IM3) occurs closer to the s ipal frequency, therefore IM3 is diffi- cult to remove. Various studies have been reported aiming to reduce IM [2] .

A fibre-optic MMW link system with heterodyne detection has shown promise [3]. This system generates MMWs equal to the fre- quency difference between two lights with heterodyne detection, and has the following advantages: only slight degradation caused by fibre chromatic dispersion [4], a high availability of light, and the availability of an optical signal processing antenna [5].

In a heterodyne system, there are several techniques for trans- mitting a large number of information signals by frequency divi- sion multiple access. For example, a large number of information signals can be multiplexed onto one subcarrier in different inter- mediate frequency (IF) signals. In this technique, only two optical

1478 ELECTRONICS LETTERS 19th August I999 Vol. 35 No. 17

carriers can handle any increase in the number of information sig- nals. However, the total data bandwidth becomes- broad, necessi- tating a broadband EOM. Another technique involves the generation of a number of optical subcarriers and the placement of one information signal on one subcarrier as the baseband sig- nal. In this technique, as the number of information signals increases, the number of lasers must also increase, but no broad- band EOM is required because the bandwidth of each EOM is sufficient for the bandwidth of one signal.

If the transmission distance is not great, we can assume the nonlinearity of the optical fibre to be negligible and the nonlinear- ity of the PD to be dominant. Some reports are available on IM caused by a photodetector (PD) in heterodyne systems. Williams et al. reported an analysis of the generation of two microwave sig- nals by two pairs of lasers [6]. They described the modelling of the pin photodetector, and the occurrence of IM2 at frequencies far different from the signal frequency in their scheme. We analysed and measured the generation of two MMW signals by three lasers, and showed that IM2 occurs close to the signal frequency in this scheme [7]. In this Letter we present an analysis of IM caused by a PD with n MMW signals transmitted by n+l lasers.

1-foPTo 2-fOPTO

millimetre-wave

Fig. 1 Schematic diagram of heterodyne system with three light inputs and two millimetre-wuve outputs

Analysis of ZM; We investigate one of the optical fibre-MMW links using heterodyne detection. Fig. 1 shows a schematic dia- gram of a heterodyne system that generates two MMW signals by three light inputs, here, laser 0 is the reference light. The portions of two laser lights (lasers 0 and 1) are combined and detected with the PD. The generated MMW frequencies at the PD are equal to the difference frequencies of each laser. The frequencies of each laser are controlled by a phase-locked loop (PLL) circuit to gener- ate the desired MMW frequency. Similarly, laser 2 is phase-locked with laser 0. In the same way, we can generate many MMW sig- nals by adding the phase-locked lasers. Fig. 2 shows the relation- ship between the light frequencies and MMW signals.

- millimetre wave -light wave

Fig. 2 Relationship between light and ntm-wave frequencies

We explain the occurrence mechanism of IM caused by the non- linear characteristics of a PD in a heterodyne system. The electric fields E, of laser i ( i is the number of lasers and the total number of lasers is n + 1) are expressed by E, = A, exp(-j2nfOP, t), where A, is the amplitude, and foPn gives the frequency of laser i. These lights are combined, and photoelectrically converted, in the PD. The total field intensity Z on the PD is obtained by

12=0 I

ELECTRONICS LETTERS 19th August 1999 Vol. 35

n . n

= CA? + AoAt C O S [ ~ T ( ~ O P T ~ - ~ O P T O ) ~ ] 2=0 i=l

n n

+ AiA, C O S [ ~ T ( ~ O P T ~ - OPT,)^] i=l j = l , z # j

1L

= (n + 1)A2 + 2A2 1 C O S [ ~ T ~ R F ~ ~ ]

2=1

n-1

+ 2A2 E(% - i ) c 0 ~ [ 2 ~ ( i f ~ ~ o ) t ] (1) z= 1

where the amplitude of each laser is assumed to be equal to A, and frequenciesfOpTl ... fOpTn are set at equal intervals. In eqn. 1,

fRFj is the frequency difference between laser 0 and i (1 2 i I n), and the desired MMW frequency. In addition to these MMW sig- nals, undesired microwaves, the frequency of which is an integral number times the frequency fRm, occur due to the frequency dif- ference between laser i and laserj ( i , j = 1, 2, ..., n, i #I] . When the input-output characteristics of the PD are nonlinear and are assumed to have a polynomial as input power I , the output photo- current i, is assumed to be i, = k , I + kzP + k3P. We examine IM occurring at frequency 2fRFI - f R n by substituting eqn. 1 for this polynomial. Second-order terms with k2 are generated by the product of two arbitrary terms in eqn. 1. The IM2s occurring at frequency 2fRFI -fRn consist of the product of terms cos2nfR,t and cos2nfR,t, CodrcfRmt and cos%2f&t, etc. From the number necessary to generate such combinations, we can calculate the fac- tor of the IM2. Similarly, third-order terms with k3 are generated by the product of three arbitrary terms in eqn. 1. From the calcu- lation results of these combinations, the output photocurrent of distortion id,,T (frequency of 2fRFI -fRm) can be expressed as

n-1

i d z s = { b z ( i - I)] i14rF2 + k 2 ( n + 1)3 z i=l

When n = 2, eqn. 2 is described as idis = (4A4k2 + 48A6k3)COS[2?T(2f~~1 -fRm)]. Therefore, the first term on the right- hand side of eqn. 2 contains the coefficient k2 in addition to k3; IM2 occurs due to the frequency difference between every laser i and laser j (i, j > 0) in the heterodyne system. In most cases, k2 is greater than k3, so when the optical power is high, IM3 will be dominant. However, when the optical power is low, IM2 will be dominant.

Results: Fig. 3 shows measured and calculated results of the signal and IM (frequency of 2fRFI - fRn) output power against the input power for each laser. In Fig. 3, each mark shows a measurement result of the generation of two MMW signals by three light inputs. Here, the light inputs were from LD pumped Nd:YAG ring lasers with a variable wavelength in the 1319nm band. Each laser power was set to the same value. A high-speed pin type PD was driven with a bias of -3V, and the PD output was directly connected with a spectrum analyser through a bias-tee. Two MMWs (38.5 and 38.6GHz) were generated, and IM occurred at 38.4 and 38.7GHz.

From these measurement results, we assumed the values for coefficients k,, kz and k3 in eqn. 2 and calculated the output power of the IM for the number n of signals. In Fig. 3, the indi- vidual lines show calculated results of the output power of IMs where the numbers of lasers were 3, 5, 11, and 21. From Fig. 3, it can be seen that as the number of signals was increased, the power of the IM also increased. Fig. 3 also shows that when the input optical power is high, the IM is influenced by the third-order term. When the input light power is low, however, the IM is influenced by the second-order term, and the dynamic range is restricted by the IM2.

No. 17 1479

- 1 - I

-30 1 m-60k c E -90

Q c a -120

-40 -20 0 input optical power, dBm

Fig. 3 Example of calculation result of signal und IM power output for input optical power of each light in heterodyne link A measured signal 4 measured IM

corresponding calculated results (9 signal .hf, (ii) IM: 21 opt., 20mmw (iii) IM: 11 out.. l0mmw (ivj IM: 5 opi., immw (v) IM: 3 opt., 2mmw

Conclusion: We have analysed the IM due to PDs in multicarrier transmission, heterodyne detection, fibre-optic MMW links. We showed that both IM2 and IM3 occur close to the signal fre- quency due to the PD in the heterodyne system. In the heterodyne system, the EOM does not generate the high MMW frequencies, so it is possible to reduce the nonlinear characteristics of the EOM by a pre-distortion technique. In this case, the system dynamic range is restricted by the IM2, and this reduction is important for improving the system performance.

0 IEE 1999 Electronics Letters Online No: 19990975 DOI: 10.1049/el:19990975

T. Akiyama, K. Inagaki and Y. Mizuguchi (ATR Adaptive Communications Research Laboratories, 2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan)

29 June 1999

References

c o x , c . , ACKERMAN, E., HELKEY, R , and BETTS, G.E.: ‘Direct- detection analog optical links’, IEEE Trans., 1997, MTT-45, pp. 1375-1383 BRIDGES, w.B., and SCHAFFNER, J.H.: ‘Distortion in linearized electrooptic modulators’, IEEE Trans., ’ 1995, MTT-43, pp. 21 84- 2197 AHMED, z , NOVAK, D., WATERHOUSE, R.B., and LIU, H.F.: ‘37-GHz fiber-wireless system for distribution of broad-band signals’, IEEE Trans., 1997, MTT-45, pp. 1431-1435 GLIESE, u., NORSKOV, s., and NIELSEN, T.N.: ‘Chromatic dispersion in fiber-optic microwave and millimeter-wave links’, IEEE Trans., 1996, MTT-44, pp. 1716-1724 JI, Y., INAGAKI, K., MIURA, R., and KARASAWA, Y.: ‘Optical processor for multibeam microwave array antennas’, Electron. Lett., 1996, 32, (9), pp. 822-824 WILLIAMS, K.J., ESMAN, R.D., and DAGENAIS, M.: ‘Nonlinearities in p- i-n microwave photodetectors’, J. Lightwave Technol., 1996, LT-14, pp. 84-96 AKIYAMA, T., and INAGAKI, K.: ‘Second-order intermodulation distortion in heterodyne detection fiber-optic millimeter-wave links’. Proc. European Microwave Conf., 1998, Vol. 1, pp. 87-92

Measurement of higher-order polarisation mode dispersion effects on 10Gbit/s system over installed non-dispersion-shifted fibre

D.A. Watley, L.M. Gleeson, K.S. Farley, E.S.R. Sikora, W.S. Lee and A.P. Hadjifotiou

Experimental results are reported demonstrating that the variation in system performance due to higher-order polarisation mode dispersion effects can be minimised by carefully managing the net chirp through the use of appropriate modulators and dispersion compensation.

Introduction: With the commercial deployment of high-capacity optical systems, polarisation mode dispersion (PMD) has received attention as a potential limit of such systems, with particular emphasis now being placed on the higher-order aspects of PMD for terrestrial systems. Higher-order PMD refers to a variation of the PMD over the spectral bandwidth of the modulated data, and as such the frequency dependence of the PMD manifests itself as a chromatic dispersion term. Recent theoretical work has indicated that second-order components of higher-order PMD only give rise to a significant additional signal distortion when the received data are chirped [l, 21. The received data chirp, which we will refer to as the net chirp of a system, results from the interaction between the transmitter chirp and the chirp induced by fibre dispersion [3]. We present results describing transmission experiments using both chirped and unchirped modulators at 10Gbitls over 78km of installed, cabled non-dispersion-shifted fibre, experimentally dem- onstrating the dependence of the higher-order distortions on the net chirp of the data.

optical dispersion

compensation

transmitter spooled fibre

D= 1368ps/nm(A.r) =Ops

Fig. I Experimental configuration

Experimental configuration: 1,O GbitJs NRZ transmission experi- ments were carried out over an installed 78km span of standard fibre with a mean differential group delay (DGD) of 11.7~s (Fig. 1). The transmitter comprised a tunable laser source and an L i m o 3 external Mach-Zehnder ’ modulator followed by optical post-amplification. Two types of modulator were used: an unchirped modulator, a = 0, and a negatively chirped modulator, a = 4 . 7 [3]. The unchirped modulator isolated the source of net chirp to being chromatic dispersion alone, allowing for a simpler measurement of the higher-order effects, while the negatively chirped modulator is more representative of typical deployment configurations for commercial STM-64/OC-192 systems operating over standard fibre. The chromatic dispersion of the link was measured to be 1169ps/nm at the operating wavelength of 1550.1 nm and measurements were taken with and without disper- sion compensation of -1400ps/nm. The compensating fibre was used in a post-compensation configuration. Using the technique of altering the polarisation coupling at several points along the trans- mission span, AT was manipulated in the manner described in [4], yielding instantaneous DGD; values in the range 8-3Ops at the operating wavelength. The best and worst system performance was then measured against input state of polarisation (SOP) after each manipulation of the polarisation coupling, both with and without dispersion compensation. System performance was quantified by measurement of the receiver sensitivity for IC9 bit error ratio (BER) operation. Through dispersion-induced chirp on the received data [3], we can regard the compensated and uncompen- sated cases as having a different magnitude of net chirp. To resolve the first- and higher-order contributions to the vanation in

1480 ELECTRONICS LETTERS 19th August 1999 Vol. 35 No. 17