Impact of thermal drifts in LED spectrally sliced WDM systems

15 March 1999

Optics Communications 161 Ž1999. 318–329

Full length article

Impact of thermal drifts in LED spectrally sliced WDM systems Ghulam Murtaza

)

Centre for Communication Networks Research, Department of Electrical and Electronic Engineering, The Manchester Metropolitan UniÕersity, Faculty of Science and Engineering, Chester Street, Manchester M1 5GD, UK Received 9 October 1998; revised 30 December 1998; accepted 7 January 1999

Abstract In LED spectrally sliced wavelength division multiplexed ŽWDM. systems the optical crosstalk caused by spectral overlapping of adjacent wavelength channels is known to depend on the source spectrum. However, the LED output is temperature dependent and therefore the crosstalk performance of such a system may be expected to vary with operating temperature drifts of the optical source. These effects are theoretically explored to estimate the subsequent performance degradation due to optical crosstalk. Analytical expressions developed indicate that the optical crosstalk noise-to-signal ratio for all spectrally sliced wavelength channels has very little dependence on the LED operating temperature. However, the optical power coupled into a spectrally sliced channel is strongly dependent on the source operating temperature which can cause a depletion of optical power levels within channels placed at the extremes of the source emission spectrum. Hence, the LED source may be employed without any temperature control when precautions are taken in the case of channels located at the tail-ends of the source emission spectrum. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction Over the last decade, the spectral slicing technique has been successfully demonstrated for several networking applications as well as being investigated for the multiplexing of optical fibre sensor systems w1–8x. One of the main reasons for the continued interest in spectrally sliced wavelength division multiplexed ŽWDM. systems is their potential lower cost which results from an economical LED transmitter design w2–8x. However, an aspect that has been generally neglected in previous studies is the possibility of LED thermal drifts w9–11x which may have a significant influence on the optical power levels in the spectrally sliced wavelength channels. The temperature-induced LED output variations are found to be non-uniform over the emission spectrum w12x and therefore, in addition to their affect on the channel coupled optical power levels, the linear optical crosstalk w13x between the adjacent channels

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E-mail: [email protected]

may vary across the wavelength channels. These issues need to be addressed in order to identify the associated practical limitations of such WDM systems and also to determine whether or not there is a requirement for temperature stabilisation of the LED sourceŽs.. The use of temperature control techniques will, however, inevitably increase the cost of such a system which mitigates against the basic objective of enabling a lower cost design. Conversely, it is essential to establish the extent of the subsequent degradations of both optical power levels in the spectrally sliced channels and also the optical crosstalk performance that may result from LED operating temperature variations. A mathematical description of the LED thermal behaviour has already been developed and reported by the author elsewhere w12x. In addition, a mathematical analysis of optical crosstalk phenomenon in LED spectrally-sliced WDM systems has also been published w14x. By combining these analytical models, a study of the performance degradations in LED spectrally-sliced WDM systems caused by LED thermal drifts is undertaken in order to develop analytical expression for the required assessment. The

0030-4018r99 r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 0 1 2 - 7

G. Murtazar Optics Communications 161 (1999) 318–329

issues of optical power levels in spectrally sliced wavelength channels, the effective LED spectral bandwidth available for spectral slicing and the optical crosstalk between the adjacent wavelength channels are analysed in this paper. Furthermore, the effect of temperature on the transmission characteristics of a channel selecting wavelength filter element is also addressed. It is noted that the temperature sensitivity of the filtering elements will generally be small in comparison with that of the LED source. Nevertheless, in the case of a system employing both wavelength multiplexing and demultiplexing components that experience different temperature conditions then significant changes could result from the differential variations of transmission characteristics within the two devices. This phenomenon is also investigated by using the same modelling process.

2. Temperature dependence of spectral sliced optical power By using a Gaussian distribution to model the spectral emission of an LED source as well as the transmission of an optical filtering element, the optical power coupled into a spectrally-sliced wavelength channel Pi has been shown to be w12x: q`

Pi s

Hy`

Fi Ž l . L Ž l . d l s AP0

ž

a2 b 2 q a2

0.5

/

=exp yDi2r Ž b 2 q a 2 .

where the following notations have been used; Fi Ž l. is the wavelength dependent transmission of the channel selecting bandpass filter, LŽ l. the LED spectral emission at the nominal room temperature of 258C, A the peak transmission of the channel selecting filter element, l the optical wavelength, l i the centre wavelength of the ith wavelength channel, Di the position of the channel on the LED spectrum as defined by Di s Ž l0 y l i ., l0 the peak emission wavelength of the LED source, a the channel spectral width at 1re points of the passband as defined by a s 0.6 lc where lc is the 3 dB spectral width, b the LED spectral width at 1re points of the emission spectrum as defined by b s 0.6 l w where l w is the 3 dB spectral width, and P0 the total fibre-coupled optical power available from the LED source. When spectral slices are much narrower than the LED spectral width then b 2 4 a 2 we.g., Ref. w15xx and therefore the fractional optical power ` i coupled into a spectrally sliced channel becomes; ` i s Ž PirP0 . f A

a

ž / b

exp yDi2rb 2

analyses presented here. The variations of the fractional optical power ` i Žand hence the slicing loss. with both the channel Žslice. width and channel position for an LED device exhibiting l w s 120 nm are illustrated in Fig. 1. Thus, as expected, the fractional optical power coupled into a spectrally-sliced channel decreases with its separation from the LED peak emission wavelength. As indicated by the shaded region in Fig. 1, a 5 nm wide Ž3 dB width. channel placed at a separation of 40 nm from the LED peak wavelength endures a slicing loss of around y15 dB which implies a channel coupled power of y30 dBm with an LED source supplying total coupled power of y15 dBm we.g., Ref. w8xx. However, a channel width of 5 nm restricts the maximum spectral width available for slicing purposes Žcalled the effectiÕe slicing width from here onwards. to less than 80 nm Žsee Fig. 1. in such a way that any channels placed outside this spectral band will fail to deliver the required optical power. But, from Eq. Ž2., the channel coupled optical power is directly proportional to the channel width. Consequently, to extend the spectral utilisation to the LED’s full spectral width of 120 nm the channel widths would need to be at least 8 nm in order to deliver the same optical power levels. It should be noted, however, that any increase of the channel width needs to be considered in conjunction with the optical crosstalk between the wavelength channels. Furthermore, the temperature-dependent LED spectral emission LŽ l,T . w12x is given by L Ž l ,T . s

Ž1.

Ž2.

For convenience, the peak transmission of wavelength filtering elements is assumed to be 100% throughout the

319

P0

ž' / b p

exp y

=exp y

ž

l y l0 b

2

/

k S2 DT 2 y 2 Ž l y l0 . k S DT b2

q k I DT

Ž3. where the amplitude of the LED emission spectrum is B s P0rb'p . Eq. Ž3. therefore allows the LED output to be determined for a operating temperature change of DT relative to the nominal room temperature Ž258C. and when the temperature coefficients of intensity and spectral shift Ž k I and k S , respectively. are also known. Following Ž1., the temperature dependence of optical power coupled into a spectrally-sliced wavelength channel can now be written Ž . Ž . as, Pi Ž DT . s Hq` y` Fi l L l,T d l which yields Pi Ž DT . s AP0 a2r Ž b 2 q a 2 .

0.5 2

=exp y Ž Di q k S DT . r Ž b 2 q a 2 . q k I DT

Ž4. When the spectral width of the slice is much smaller than the LED spectral width then Eq. Ž4. reduces to Pi Ž DT . f AP0 Ž arb . exp

y Ž Di q k S DT . b2

2

q k I DT

Ž5.

G. Murtazar Optics Communications 161 (1999) 318–329

320

Fig. 1. Fractional optical power levels coupled into a spectrally sliced wavelength channel at different channel positions and with varying selected channel width.

Therefore, the fractional optical power ` i coupled into a spectrally-sliced wavelength channel can be written: `is

ž

Pi Ž DT . P0

/

f A Ž arb . exp

y Ž Di q k S DT . b2

2

q k I DT

Ž6.

For example, the spectral bandwidth of the ABB HAFO w InGaAsP LED device 1A275 w16x operating at a peak wavelength of 1320 nm is around 120 nm while the temperature coefficients of intensity and spectral shift are typically y0.8%r8C and 0.55 nmr8C, respectively. Hence, when channel selecting bandpass filters exhibiting a spectral width of 10 nm and peak transmission of 100% Ž A s 1. are being employed, the temperature dependence of the spectrally sliced channels Ž lc s 10 nm. can be obtained by substituting the known parameters Ž b s 0.6 l w s 72 nm, k I s y0.008r8C, k S s 0.55 nmr8C and a s 0.6 lc s 0.6 = 10 s 6 nm. into Eq. Ž6.. Thus; ` i s Ž 6r72 . exp

y Ž Di q 0.55 = DT .

Ž 72. 2

temperature. At a maximum slicing loss of y15 dB the effective spectral bandwidth available at room temperature is around 140 nm which reduces to less than 130 nm with an operating temperature increase of 208C. Because the LED spectral emission shifts towards longer wavelengths with a rise in its operating temperature, the optical power levels of shorter wavelength channels are found to diminish rapidly while the longer wavelength channels show a gain. The greatest change is observed to occur for channels located at the extreme short wavelengths which is nearly 0.1 dBr8C for channels placed beyond Di s Ž l0 y li . s 60 nm. Therefore, it is possible that as a consequence of such temperature-induced optical power variations some wavelength channels located at the tail ends of the LED spectrum may cease to function satisfactorily. Moreover, some intermediate wavelength channels exhibit very small optical power level changes w17x. In this case the channels placed around Di s Ž l0 y l i . s y60 nm display a temperature sensitivity of the order of 0.01 dBr8C Žsee Fig. 2..

2

y 0.008 = DT .

Ž7. A contour plot of Eq. Ž7. is shown in Fig. 2 where it may be observed that the LED spectral width available for spectral slicing decreases with a rise in the LED operating

3. Effective LED bandwidth for spectral slicing Suppose that in a spectrally-sliced WDM system all channels will be required to exhibit a minimum optical power level Pmin and the system is to be designed to be operable over a specific temperature range. It is expected

G. Murtazar Optics Communications 161 (1999) 318–329

321

Fig. 2. Impact of the LED operating temperature changes on the channel optical power levels as well as the variations of the LED effective bandwidth for spectral slicing.

that should the LED temperature rise then the emission spectrum will shift to longer wavelengths which will lead to a depletion of the optical power levels in the shorter wavelength channels. Therefore, there will be a shortest selectable channel wavelength lS which will continue to provide the minimum level of optical channel power even after a rise of DTmax in the LED operating temperature. However, should the LED operating temperature drop below the normal room temperature conditions then some of the longest wavelength channels may cease to operate due to a loss in optical power. Therefore, a longest selectable channel wavelength lL corresponding to a drop DTmin in the LED operating temperature will also exist that will continue to provide the minimum level of optical channel power into the channel. The effective spectral bandwidth leff s Ž lH y lL . of the LED can then be defined by the limiting wavelengths which are easily found from Eq. Ž5. by setting Pmin s ` P0 s Pi Ž DT . and A s 1, thus yielding

lS s l0 q k S DTmax y b w k I DTmax q ln Ž arb` . x lL s l0 q k S DTmin q b w k I DTmin q ln Ž arb` . x

0.5

0.5

Ž8.

All wavelength channels should thus be located within the range defined by lS - l 0 - l L which in turn depends on the anticipated operating temperature range of the LED source being spectrally sliced. For a slice width of 10 nm the typical temperature dependence of the two limiting wavelengths as given by Eq. Ž8. is displayed in Fig. 2 by

shading the prohibited wavelength regions. Hence, for an operating temperature range from 58C to 658C, the effective spectral bandwidth is observed to be around 100 nm at the channel slicing loss of y15 dB. Temperature dependence of the two wavelengths for different channel coupled optical power levels is given by the corresponding contour lines in Fig. 2. Moreover, the shortest and the longest wavelengths may also be calculated directly by using Eq. Ž8.. The effective spectral bandwidth of the LED for spectral slicing will therefore, generally, be smaller than its 3 dB spectral bandwidth l w . Two important observations can therefore be made from the above. Firstly, the effective spectral bandwidth of an LED depends on the intended operating temperature of the device and it is likely to be no larger than its 3 dB spectral bandwidth l w . This inference applies to all such WDM systems that incorporate some provision to operate the LED source at a constant Žstabilised. temperature we.g., Ref. w4xx, though not necessarily at the room temperature. The effective spectral bandwidth of the LED will become smaller with operation of the device at higher stabilised operating temperatures. Secondly, therefore, when not employing LED temperature stabilisation the effective spectral bandwidth will be even smaller Žsee Fig. 2. as defined by the anticipated operating temperature range of the LED source and the corresponding limiting wavelengths. In general, the larger the allowed operating temperature range, the smaller the effective spectral width will become. Hence, in both cases, the important quantity is the effective spec-

G. Murtazar Optics Communications 161 (1999) 318–329

322

tral bandwidth of the LED which is determined by the intended operating conditions, the temperature sensitivity of the device and the optical power requirements for the wavelength channels to be provided. The design of such a system should therefore reflect the effective spectral bandwidth of the device and not its specified room temperature 3 dB spectral bandwidth.

4. Temperature dependence of interchannel crosstalk Optical crosstalk is an important parameter for all WDM systems. When designing a spectrally-sliced system, the spectral slice widths and the spectral separation between the slices need to be selected to comply with any channel optical power requirements while ensuring that the optical crosstalk performance meets the desired specification. The main cause of optical crosstalk in spectrally-sliced LED systems is spectral overlapping of adjacent wavelength channels which leads to a leakage of optical power Žthis is sometimes referred to as the intensity noise. between them w18,19x. The optical power leakage Ni j between two spectrally-overlapping wavelength channels can thus be written as w19x q`

Ni j s

Hy`

Fj Ž l . Fi Ž l . L Ž l . d l

Ž9.

where it has been assumed that the jth channel with the passband Fj Ž l. is contributing a noise level Ni j into the ith channel with the passband Fi Ž l. and LŽ l. is the spectral distribution of the LED source. When all three spectral distributions in Eq. Ž9., namely, the spectral transmissions of the two interfering channels and the spectral emission of the LED are assumed to be Gaussian w2,19x, the resulting expression then yields Ni j s

A i A j B a i a j b'p

Ž

.

(

a2i a2j q b 2 a 2i q b 2 a2j

ž

=exp y

b 2D 2 q a2i D j2 q a 2j Di2 a 2i a 2j q b 2 a 2i q b 2 a2j

/

Ž 10.

where in addition to the already defined symbols, the definitions D s Ž l j y l i . and D j s Ž l0 y l j . have been incorporated. In addition, designating k s Ž a ira j . and r 2 s b 2ra2i gives Ni j s

A i A j B Ž a i r'p .

(1 q r

2

A i B Ž a i r'p .

(1 q r

Di2 q k 2 Ž Di y D . q r 2 k 2D 2 a2i Ž 1 q r 2 q r 2 k 2 .

/

Ž 11.

2

ž

exp y

Di2

/

a 2i Ž 1 q r 2 .

Ž 12.

and by assuming that the slice widths of all channels are equal so that a i s a j s a Žwhere j s i q 1 and j s i y 1. the simplified optical crosstalk noise-to-signal ratio can be obtained by combining Eqs. Ž11. and Ž12. as follows, Ni j

ž / ž Pi

sAj

0.5

1qr2 1q2 r2

ž

=exp y

/ Ž D y Di q r 2D .

2

a2 Ž 1 q 2 r 2 . Ž 1 q r 2 .

/

.

Ž 13.

Furthermore, when the spectral slices are much narrower in comparison with the LED spectral bandwidth then r 2 4 1 and the noise-to-signal ratio due to a crosstalk contributing channel j becomes Ni j

1

ž / ž' / ž Pi

sAj

exp y

2

Ž r 2D y Di . 2 a 2r 4

2

/

Ž 14.

The approximation r 2 4 1 is particularly valid in the case of new broad spectrum sources w20–22x that are now being introduced for spectrum slicing applications. In addition, greater optical power output levels available from such devices can enable sufficient channel optical power levels with slice widths smaller than 1 nm we.g., Ref. w8xx. It should be noted that D s Ž l j y l i . changes sign depending on the position of the crosstalk contributing channel in relation to the subject channel. Moreover, Di s Ž l0 y l i . can also be positive or negative depending on the position of the subject channel within the LED emission spectrum. For a channel with crosstalk contributions from one nearest-neighbour on each side, the crosstalk C NN is given by C NN s 10 log

Ni j

ž / Pi

q jsiy1

Ni j

ž / Pi

Ž 15. jsiq1

Noise-to-signal ratios for the upper and lower wavelength crosstalk contributing channels can be obtained from Eq. Ž14. so that 1

ž / ž' / ž ž / ž' / ž Pi

2

ž

Pi s

Ni j

qr2k2

=exp y

Furthermore, rewriting Eq. Ž1. for the channel coupled optical power as

s AU

U

Ni j Pi

s AL

L

2

1

2

exp y

exp y

Ž r 2D y Di .

2

2 a2r 4

Ž yr 2D y Di . 2 a 2r 4

/ 2

/

Ž 16.

G. Murtazar Optics Communications 161 (1999) 318–329

where the subscripts U and L refer to upper and lower channel, respectively. Using Eq. Ž15. and assuming A U s A L s 1 in Eq. Ž16. gives C NN s 10 log

1

½ ž '2

= 1 q exp

exp y

Ž r 2D q Di .

2

2 a 2r 4

/

2 DDi

ž /5

Ž 17.

r 2 a2

Now, repeating the analysis from Eq. Ž9. to Eq. Ž17. for the case when LED output is temperature dependent as given by Eq. Ž3., the resulting optical crosstalk expression can be shown to be C NN s 10 log

1

½ ž '2

= 1 q exp

exp y

ž

Ž r 2D q Di q k S DT .

2

/

2 a 2r 4

2 D Ž Di q k S DT .

r 2 a2

/5

Ž 18.

5. Differential thermal effects between WDM components In the case when the spectral emission from an LED source LŽ l. is subjected to a multiplexing device at the launch-end and an identical demultiplexing device at the receiver-end then the optical power Pi received at the terminal photodetector linked to the ith wavelength channel can be written as q`

ž

Ž b 2D q a2Di q a2 k S DT . 2

2a b

q 10 log 1 q exp

ž

2

/

4

2 D Ž Di q k S DT . b2

/

Ž 19 .

It should be noted that the optical crosstalk noise-to-signal ratio given by Eqs. Ž18. and Ž19. is independent of the temperature coefficient of LED output intensity but not the temperature coefficient of spectral shift. Once again, the data for the InGaAsP LED device w16x given in Section 2 can be substituted into Eq. Ž19. to evaluate the extent of temperature-induced variations of the optical crosstalk. Thus, C NN s y1.5 y 4.34

channel within the LED emission spectrum. Thus, within the range of parameter values and approximations of this analysis, the optical crosstalk noise-to-signal ratio is found to exhibit variations of no more than 0.1 dB which is irrespective of both any LED temperature drifts and the positions of the wavelength channels on the emission spectrum of optical source. Temperature stabilisation of an LED source within such a system may therefore prove unnecessary.

Pi s

and therefore C NN s y1.5 y 4.34

ž

Ž Ž72. 2 D q a2Di q a2 = 0.55 = DT .

q 10 log 1 q exp

2 a 2 = Ž 72 .

ž

4

2 D Ž Di q 0.55 = DT .

Ž 72. 2

323

/

Hy` F Ž l. F Ž l. L Ž l. d l i

Ž 21.

i

It is assumed that the temperature sensitivity of the WDM devices is such that the resulting changes in the bandpass transmission characteristics are uniform over all the spectrally sliced wavelength channels. In order to analyse the WDM system for any differential bandpass transmission variations it is convenient to suppose that while the multiplexer is working at the normal room temperature the demultiplexing device is subjected to a different temperature which would cause a spectral shift of, say d T , in the centre wavelength of all transmitted channels. As already demonstrated in Section 4, the LED operating temperature is of little concern from the viewpoint of optical crosstalk noise-to-signal ratio and therefore it need not be included in the following analysis. Thus, the optical power Pi delivered to the ith channel photodetector element can be obtained,

2

q`

/

Ž 20.

A contour plot of optical crosstalk C NN with temperature change DT and channel wavelength positions Di is displayed in Fig. 3. The characteristics shown correspond to a channel width of lc s ar0.6 s 5 nm in conjunction with channel separations of D s 5 nm, 10 nm and 15 nm as presented in Fig. 3. It may be observed that the optical crosstalk variations with temperature are negligible in all three cases ŽFig. 3a–c.. Similarly, the optical crosstalk is found to be largely independent of the position of a

Pi s

Hy` F Ž l , d i

T

. Fi Ž l . L Ž l . d l

Ž 22 .

where d T is the wavelength displacement between the spectral transmission of the multiplexing device Fi Ž d . and the spectral transmission Fi Ž l, d T . of the demultiplexing device which is caused by the differing temperature conditions. Hence, by assuming Gaussian shaped transmission characteristics, Eq. Ž22. becomes Pi s A2i B

`

Hy` exp

=exp y

y

Ž l y li y d T . 2

Ž l y li . 2 a2i

a 2i exp y

Ž l y l0 . 2 b2

dl

Ž 23.

324

G. Murtazar Optics Communications 161 (1999) 318–329

Fig. 3. Variations of the nearest-neighbour optical crosstalk levels with the LED operating temperature changes and the position of a channel on the LED emission spectrum: Ža. when channel separation is equal to the slice width; Žb. when the channel separation is twice the slice width; Žc. when the channel separation is three times the slice width.

G. Murtazar Optics Communications 161 (1999) 318–329

325

Fig. 3 Žcontinued..

However, the optical crosstalk noise Ni following Eq. Ž9. will be `

Ni j s A i A j B

Hy` exp

=exp y

y

Ž l y lj y d T . a2j

Ž l y li y d T . a2i

2

Ni j

2

exp y

Ž l y l0 . b2

ž / Pi

2

Solving Eqs. Ž23. and Ž24. by incorporating the substitutions, B s P0rb'p , Di s Ž l0 y l i ., D s Ž l j y l i ., a i s a j s a and r 2 s b 2ra2 then yields

ž(

P0 A2i

1q2 r2

/

exp y

2 Di2 y 2 d T Di q d T2 Ž 1 q r 2 . a

2

s exp

Ž D q d T . Di b2

y

Ž D 2 y d T2 . 2 a2

Ž 27.

dl

Ž 24.

Pi s

Now, letting r 2 4 1, A j s A i s 1 and combining Eqs. Ž25. and Ž26. gives the optical crosstalk noise-to-signal ratio Ž Ni jrPi . due to one adjacent interfering channel as

2

Ž1q2 r .

Therefore, as b s 0.6 l w and a s 0.6 lc then Eq. Ž27. gives

D

Ci j s y6.02

ž /

q 12.04

lc

2

q 6.02

2

ž /

Ž D q d T . Di l2w

dT lc

Ž 28 .

Ž 25. Ni j s

P0 A i A j

ž'

1q 2 r 2

=exp

y

/

2 D i2 q D 2 Ž 1 q r 2 . y 4 d T D i q 2 d T2 y 2 D Ž D i q d T . a2 Ž 1 q 2 r 2 .

Ž 26.

With l w s 120 nm, lc s 10 nm and d T s 0 to lc , the variation of crosstalk levels for various channel positions is shown in Fig. 4 where the characteristics correspond to D s 10 nm, 20 nm, and 30 nm. It can be observed therefore that the crosstalk levels degrade as a result of increasing spectral mismatch Žcentre-to-centre wavelength separation. between the multiplexing and demultiplexing bandpass transmission characteristics. However, the optical

326

G. Murtazar Optics Communications 161 (1999) 318–329

crosstalk degradation for a 50% spectral mismatch Ža 5 nm in the case of 10 nm wide spectral slices. is no more 2 dB

at any channel position. This may be significant for small values of the normalised channel separation Žthat is, the

Fig. 4. Nearest-neighbour optical crosstalk effects due to differential temperature-induced bandpass transmission between the multiplexing and demultiplexing devices: Ža. when channel separation is equal to the slice width; Žb. when the channel separation is twice the slice width; Žc. when the channel separation is three times the slice width.

G. Murtazar Optics Communications 161 (1999) 318–329

327

Fig. 4 Žcontinued..

ratio of spectral separation to channel width. when the optical crosstalk isolation will be low. A 50% spectral mismatch at the normalised separation of greater than two, however, produces less than 10% degradation of the optical crosstalk noise-to-signal ratio Žsee Fig. 4b.. Nonetheless the optical crosstalk given by Eq. Ž28. is the contribution of one interfering channel only. For two contributing channels, one on either side of the subject channel, the optical crosstalk degradations will be even smaller because of some cancellation due to the last term in Eq. Ž28. where D takes an opposite sign for each channel. As the spectral slices become narrower the percentage mismatch increases and consequently such optical crosstalk variations may be substantial for narrower spectral slices. In order to assess the extent of the resulting changes it is noted that the temperature sensitivity of the optical bandpass filters is typically 0.02 nmr8C but for grating based WDM devices the experimental value was observed to be less than 0.005 nmr8C w23x. A temperature change of 508C can therefore cause a worst-case spectral shift of 1 nm which represents a 20% spectral mismatch in the case of 5 nm wide slices. The corresponding affect on the optical crosstalk levels is a change of less than 1 dB as observed in Fig. 4. However, in most practical cases the spectral slice widths will usually be greater than 5 nm and environmental temperature variations are not expected to be very large due to the relatively protected environments of the WDM housings. To a large degree the effects of different temperature conditions at the multiplexing and demulti-

plexing devices may thus be of very little significance in relation to the optical crosstalk performance of an LED spectrally sliced WDM system when all channels exhibit similar temperature sensitivity.

6. Discussion Several temperature-induced effects of spectrally sliced WDM systems have been addressed in Sections 2–5 through the development of analytical expressions. One important outcome from the theoretical analyses presented in this paper is that an LED can be used without any temperature control in a spectrally sliced WDM system. Although the effective spectral bandwidth of the LED available for spectral slicing may be reduced in the absence of temperature stabilisation, the optical crosstalk noise-to-signal ratio is almost independent of the source operating temperature. Similarly, it was found that moderate differences of the temperature conditions at the wavelength multiplexing and demultiplexing devices also produce only relatively marginal optical crosstalk degradations which will rarely exceed 2 dB. An independent experimental confirmation of these predictions is provided by a recent study which used a 1300 nm LED for a six channel WDM system in order to determine the optical crosstalk degradation that would result from an operating temperature difference between the multiplexing and demultiplexing devices w24x. In this investigation the maxi-

328

G. Murtazar Optics Communications 161 (1999) 318–329

mum increase in crosstalk levels was found as being no more than 2 dB while a differential temperature swing of up to 808C was allowed between the two devices. It follows from Eq. Ž2. that when the spectral slices are narrow in comparison with the LED spectral width the optical power coupled into a spectrally sliced wavelength channel will be proportional to the slice width. The channel coupled optical power, however, decreases with the separation of a channel from the LED peak wavelength. For equal optical power levels a channel located at the 3 dB points on the emission spectrum needs to be twice as wide as a channel placed at the LED peak emission wavelength. Moreover, to couple an optical power level of y30 dBm into all spectrally sliced channels with an LED source capable of delivering optical power of y15 dBm, the channel widths are required to be at least 7.5 nm Žin the case of an LED with spectral width of 120 nm. which then facilitates the use of the LED’s full spectral width Žsee Fig. 1.. The number of channels that may be extracted by spectral slicing an LED clearly depends on the desired channel optical power levels and hence the slice width. According to Eq. Ž2. the slice width needed to enable a particular optical power level into the channel is inversely proportional to the total coupled power output of the source. Therefore, the same number of channels may be extracted from two LEDs, which have an equal total optical power output but display different spectral widths, by selecting appropriate channel widths. This outcome indicates that it is not the spectral width but the total fibre-coupled optical power emitted by an LED that determines its usefulness for spectral slicing. The analysis presented in this paper has been restricted to incoherent WDM systems that employ LED spectral slicing. In such systems, the optical crosstalk for any particular channel is purely determined by the levels of the optical power leakage Žor the intensity noise levels. from the adjacent channels with respect to the optical power level present at the subject channel. However, the use of coherent or quasi-coherent optical sources Že.g., laser diodes or superluminescent diodes. can lead to additional crosstalk effects due to the coherence properties of the optical signals. Moreover, there can be configuration-dependent crosstalk contributions in both coherent and incoherent WDM systems that have also been neglected. In particular, these can include Fresnel reflections, Rayleigh scattering losses, temporal and chromatic dispersion effects and the different optical losses experienced by each wavelength channel signal due to the wavelength-dependent transmission of the system that may lead to further crosstalk effects. The width of the LED emission spectrum that may be usefully employed for spectral slicing Žcalled the effective slicing width. is found to decrease with the operating temperature of the device. Moreover, an increase in temperature causes the effective slicing width to move towards

the longer wavelengths Žsee Fig. 2.. For devices used with temperature stabilisation the operating temperature may be different to the normal room temperature. A system design should therefore be in compliance with the effective slicing width corresponding to the intended operating temperature of the specific LED source. For LED devices that are not temperature stabilised, an increase in the operating temperature could optically deplete channels placed at the shorter wavelength extreme of the effective slicing width and the same limitation may result at the longer wavelength extreme with a temperature decrease.

7. Conclusions For LED spectrally sliced WDM systems the optical crosstalk noise-to-signal ratio is found to exhibit little dependence on the operating temperature of the optical source. This occurs because the effect of LED thermal drifts for both the optical channel signals and the interchannel crosstalk noise is similar and therefore their ratio does not vary with the operating temperature of the source. Furthermore, it suggests that such systems may be deployed without any temperature stabilisation of the optical source which makes them very attractive for future commercial exploitation in areas such as the telecommunication access network w24x. However, it is apparent that without temperature stabilisation some wavelength channels placed at the extremes of the LED emission spectrum may not function satisfactorily because thermal drifts of the source can cause depletion of optical power in these regions. In the system design, a decision will therefore be required to either employ temperature stabilisation at a cost or to restrict the effective slicing width and to refrain from placing any channels at the extremes of the source emission spectrum. Alternatively, the system design could potentially be optimised with a reduced interchannel separation in order to extract a similar number of channels or a higher optical power output LED source could be utilised.

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