Improved resonance and chirp mechanisms for multigigabit/s push-pull modulated DFB lasers

B.J.Flanigan J .E. Carrol I M.C. Nowell R.G.S.Plumb

Indexing terms: Internal resistance, Push-pull modulated DFB laser

~ ~ ~~

Abstract: The internal resonance within a push- pull modulated DFB laser is investigated with numerical and analytical analysis. The classic photon-electron resonance is reduced in amplitude, and the operation of the laser is dominated by a new resonance which is dependant on the mode spacing between the main lasing mode and its nearest side mode. Simulations show that this allows modulation bandwidths well beyond the conventional photon-electron resonance limit. The interaction between the dominant modal resonance and the weaker photon-electron resonance leads to a novel time resolved chirp shape which improves the properties for transmission of a pulse along an optical fibre. This chirp shape can be tailored to further improve transmission properties by controlling the interaction between the resonances. Simulations of the time resolved chirp support published experimental results.

1 Introduction

Direct modulation of semiconductor lasers is attractive for operation in optical communication systems owing to the relative simplicity and low cost of the technique. However, as transmission rates move towards several tens of gigabith, two factors limit the suitability of con- ventional directly modulated laser diodes as light sources. Firstly, at high bit rates such sources suffer from significant chirping, causing excesses for the mod- ulated linewidths of several tens of gigahertz over the Fourier limit and this combined with the dispersion in the fibre results in severe transmission penalties. Exter- nal modulation [ 1, 21 not only can reduce the chirp but also can provide prechirp which helps to compensate for dispersion and external modulators are therefore currently the preferred option for multigigabitis sys- tems, although such techniques add to complexity and 0 IEE, 1996 IEE Proceedings online no. 19960137 Paper first received 29th June 1995 and in revised form 2nd October 1995 B.J. Flanigan, J.E. Carrol and R.G.S. Plumb are with the University of Cambridge, Department of Engineering, Trumpington Street, Cambridge CB2 1PZ, UK M.C. Nowell is with the University of Bath, School of Physics, Claverton Down, Bath BA2 7AY, UK

cost. A second problem of direct modulation comes from the frequency response, which is limited by the internal resonance between the electrons and photons within a conventional diode laser. In spite of attempts to increase this fundamental resonance frequency [3, 41, modulation bandwidths are difficult to push beyond

The push-pull modulated DFB laser has been pro- posed as a directly modulated device which may solve both these fundamental problems [5-71. Controllable chirp from push-pull modulation has been shown experimentally to provide longer transmission distances than conventional modulation over the standard opti- cal fibre. Further, simulations indicate that the modu- lation bandwidth of the device is enhanced well beyond the conventional photon-electron resonance suggesting that l00GHZ may be possible, given appropriate pack- aging.

The new work in this paper is to consider in more detail the modal structure in a DFB laser, which then explains the improved resonance and chirp that is obtained from push-pull modulation. The device per- formance is shown to be controlled by a trade-off between the enhanced resonant mechanism and the conventional photon-electron resonance. The work is illustrated using dynamic time domain modelling together with previous experimental results.

2 Push-pull modulation

Conventionally, diode lasers are directly modulated by changing the injection current, thereby changing the charge carrier density and hence changing the optical gain in the laser. However, increases in carrier density reduce the refractive index so that, as the device is switched on, the refractive index decreases, but increases as the laser switches off. The lasing frequency changes or ‘chirps’ and the modulated linewidth increases over the Fourier limit by several tens of GHz [8]. The frequency changes during the electronic tran- sients can be referred to as dynamic chirp, whereas the frequency difference between the steady-state ‘on’ and ‘zero’ levels is referred to as static chirp. Chirp may be moderated by reducing the carrier density dependence of the refractive index, characterised by the linewidth enhancement factor am Values of a, < 1 have been reported using strained layer multiple quantum well material [9] but zero aN cannot provide the appropriate prechirp that can be offered by some external modula- tors.

20-30GHz.

49 IEE Proc.-Optoelectron., Vol. 143, No. 1, February 1996

Push-pull modulation provides an alternative method of moderating and tailoring the chirp. The scheme utilises a DFB laser with a two- or three-ele- ment contact, as illustrated in Fig. 1. The outer con- tacts are modulated in antiphase with each other, so that an increasing current and carrier density at one end are compensated by a decreasing current and car- rier density at the other. In the three-contact device, the centre contact is typically DC biased. Modulation moves the carrier density from end to end as inferred from the schematic diagram shown in Figs. 3-5 for a two-contact device. Figs. 3 and 5 illustrate the carrier and optical densities where the right contact is ‘on’. The ‘off state is the mirror image about the centre in these diagrams.

rbias+Ar

c 4

Fig. 1 modulation

Schematic diagram of two contact DFB laser used for push-pull

I bias 2 AI Idc *bias 1 1 1

I I

Fig.2 modulation

Schematic diagram of three contact DFB laser used for push-pull

carrier density

distance along cavity

Fig. 3 density against length

Two section push-pull laser: schematic representation of carrier

distance along cavity

Fig. 4 ing (dashed line shows lasing frequency relative to stop bands)

Two section push-pull laser: stop bandfrequencies for Bragg grat-

I + distance along cavity

Fig. 5 Two section push-pull laser: photon density against length

The changes of refractive index with carrier density change the central frequency of the Bragg stop band along the laser as sketched in Fig. 4. A strong feedback (reflection) is provided from the left-hand section so that little light escapes from the left. Gain and phase adjustments are automatically made within the right- hand section to produce lasing near the lower edge of the stop-band (-1 mode). Symmetry between the ‘on’ and ‘off states (corresponding to the ‘ones’ and ‘zeros’) ensures that static chirp is eliminated. In addition, the mean carrier density remains almost constant during

switching of the contacts, giving a negligible change in the mean refractive index, thereby minimising the level of dynamic chirp. Thermal changes are also minimised for similar reasons so that any thermal chirp is reduced.

The modal structure of the DFB is important for push-pull modulation and has not previously been dis- cussed in detail. A uniform DFB laser with no phase shifts has two main modes: one around the upper and one around the lower frequency edges of the Bragg stopband (the (+1) and (-1) modes, respectively). As illustrated through Fig. 4, it is the lower frequency (-1) mode that must be selected for the light to experience reflection from the appropriate end of the laser (if the higher frequency (+ 1) mode were selected, the light would be reflected towards the lower bias end of the device, further depleting the carriers in that end result- ing in the structure becoming unstable and self-pulsa- tions occurring). The Bragg grating then actuaily assists in the to and fro motion of the optical energy through the reflections. Grating structures were used in our studies that at least theoretically favoured selecting this mode [lo, 111. In addition, strong reflection requires a strong coupling coefficient in the grating; KL -4 has been found to be advantageous.

Unoptimised push-pull laser devices using bulk material have demonstrated transmission over 150 km at 2.5Gbit/s with BER and giving a dispersion penalty of -0.5 dB. These experimental measurements have been reported elsewhere [7] and help to support the claim that push-pull modulation gives improve- ments over conventional modulation.

A well established dynamic large signal time domain model [12-141 has been used to simulate push-pull modulation of DFB devices. This model includes spon- taneous noise and longitudinal hole burning effects and is capable of calculating multimoded steady-state char- acteristics as well as small signal and large signal mod- ulation characteristics. The frequency dependence of the gain in a semiconductor is typically approximated as having a Lorentzian distribution about its peak. In the model this is included by using a first order infinite impulse response digital filter, the response of which has the time domain form

%+l = Ayt + (1 - A)zt (1) where yr is the output and xt is the input of the filter at time t . A is the complex filter coefficient given by

A = aexp(icp) ( 2 ) where the amplitude a controls the width of the gain curve and the phase Q controls the centre frequency. The parameters used in the simulations are given in Table 1, and are appropriate for quantum well material with a high but achievable differential gain [15]. It should be noted that initial experimental trials reported in [7] were carried out using bulk InGaAsP active region devices, but improvements in the speed of response are expected with the use of quantum well materials.

3 Enhanced resonance

With conventional modulation, any increase in the optical output has to be accompanied by a prior increase in the gain to a value above transparency. The light grows until the increased stimulated recombina- tion then depletes the carriers and reduces the gain

50 IEE Proc.-Optoelectron , Vol. 143, No. I , February I996

below transparency. The interaction between photon generation and electron depletion leads to the classic resonance with the modulated output decreasing at approximately 20dB/decade above this resonance.

Table 1: Simulation parameters

Bimolecular coeffiecient B, 10- 'o~m3/s

Auger coefficient C, 10-%m6/s

Differential gain g-N, 10-'6cm3

Linear carrier lifetime 7, ns

Transparency carrier density No, 1018cm-3

Linewidth enhancement factor aH

Absoption and scattering loss a,, cm-1

Effective phase refractive index neVO

Effective group refractive index ns

Waveguide confinement factor r Length of cavity L, p m

Coupling coefficient K, cm-I

Surface radiation loss coefficient h7, cm-l

Active layer thickness d, n m

Active layer width w, pm

Inversion factor nsp

Wavelength h, p m

Nonlinear gain coefficient E, IO-l7cm3

Facet reflectivities

Gain filter amplitude, a

Gain filter phase, $

Length of contacts (3 contacts), p m

Bias level on each contact, m A

1

1.3

12

5

1.5

3

30

3.2

3.6

0.0687

800

80

5.5

3.9

1.8

2

1.55

1

0

0.005

0

266.7

With ideal push-pull modulation, the photonic energy within the cavity is not increased or decreased but is kept constant and moved back and forth along the laser length with the output coming from one side and the bit complement from the other. The traditional photon4ectron resonance is ideally eliminated and replaced with the limitation on how fast the photonic energy may propagate back and forth in the structure. This limit is controlled by the frequency separation between the dominant (-1) lasing mode and its closest side mode, the (-2) mode (where modes at higher fre- quencies than the Bragg frequency are denoted as (+l), (+2) etc. and at lower frequencies as (-I), (-2) etc.). Here we present a novel simulation technique whereby the position and width of the gain curve is artificially controlled to elucidate the important role of these two modes.

The enhanced resonance is readily demonstrated by an examination of the small signal responses for push- pull modulation and conventional modulation. The small signal response has been examined with the numerical model using familiar control theory, by sup- pressing all stochastic noise processes and applying a current impulse. For the case of push-pull modulation this impulse is applied in antiphase across the end con- tacts. Fig. 6 illustrates the simulated small signal response for the same device driven as a two contact push-pull device, and as a conventional single contact device, whilst the phase of the modulation responses for both cases are shown in Fig. 7. The single contact response exhibits the characteristic peak at the photon- electron resonant frequency. Simply by changing the modulation scheme to push-pull modulation, the mod-

IEE Proc.-Optoelectron,, Vol. 143, No. 1, February 1996

ulation bandwidth is increased. The push-pull response is limited by a characteristic resonance frequency FR which corresponds to a new resonant mechanism. In Fig. 7 phase lags can be observed around the conven- tional photon-electron resonant frequency (for single contact) and the enhanced resonant frequency FR (for push-pull), further indicating that the laser cannot respond beyond these modulation speeds. Simulations have indicated that the push-pull resonant frequency FR is equal to the frequency separation between the dominant (-1) lasing mode the (-2) side mode, as has been presented in [6].

10-31 , , , , , , ,. , , , , , , , , . , ./ , , , , , , . , 0.1 1 10 FR 100

frequency, GHz Fig. 6 lated CIS ( i ) single contact device and (ii) three contact pmh-pull device

Comparison of magnitude of AM responses for same device modu-

Total bias level in both cases is 75mA

3, -8 c 2 1

0.1 1 10 100 frequency, GHz

Fig.7 as ( i ) single contact device and (ii) three contact push-pull device Total bias level in both cases is 75mA

Comparison ofphuse of AM responses for same device modulated

To fully understand the enhanced resonant mecha- nism, it is necessary to examine the longitudinal field profiles of the (-1) and (-2) modes. The time domain model used to simulate the push-pull device is multi- moded, and therefore the individual mode fields cannot be obtained directly. However, from eqns. 1 and 2, the width of the gain filter can be controlled by the ampli- tude coefficient a, and the centre frequency through the phase coefficient 9. By narrowing and offsetting the gain filter, the gain curve can be artificially positioned about a given mode frequency. With all noise and cur- rent sources switched off, an optical impulse field is then injected at a point along the laser cavity, and the fields in each section calculated over a number of time steps. The resulting longitudinal field profile is deter- mined by the dominant mode, which can be selected using the narrowed off-set gain filter. In this way, the longitudinal field profiles of both the (-1) and (-2) modes were obtained as illustrated in Figs. 8-10. It should be noted that the profiles obtained in this man- ner do not give quantitative values for the actual field amplitudes during modulation, but they do convey the shape of the (-1) and (-2) field profiles. It can be seen that whilst the (-1) mode is symmetrical about the cav- ity, the (-2) mode has an asymmetrical distribution.

51

Fig. 10 shows the effect of the presence of the two modes in the cavity. The beating between the modes causes a displacement of the energy back and forth, at an oscillation rate equal to the mode separation.

8

2 6 . 0

$ 4:

Fig. 8 mode

Simulated field amplitudes against normalised length for (-I)

n

4 t Fig. 9 mode

Simulated field amplitudes against normalised length for (-2)

Fi .1Q (-y) mode plw (-2) mode, (l ip (-1) mode minus (-2) mode Beating between modes causes stored energy to oscillate back and forth

Simulated field am Iitudes against normahsed length for ( I )

The mechanism of the enhanced push-pull resonance then becomes apparent. As a pulse is switched on or off, the rapid asymmetric change in the carrier density in each half of the device causes excitation of the nor- mally suppressed asymmetric (-2) mode. This mode lies below threshold, and therefore its amplitude decays with time. The resulting beating between the (-2) mode and the above threshold (-1) mode results in a damped harmonic motion of the optical energy within the cav- ity before the steady state is reached, at a frequency equal to the mode separation. This resonant mecha- nism is then the dominant limitation to the modulation speed, since the movement of energy due to the push- pull modulation cannot occur faster than these oscilla- tions. The resonance typically occurs at a higher fre- quency than the photon-electron resonance frequency.

Further evidence of the importance of the (-2) mode can be obtained by considering the artificial case of push-pull modulation when only the (-1) mode is allowed to be present in the cavity. This can be achieved by off-setting and narrowing the gain curve as before so that the (-2) mode remains strongly sup- pressed even as the pulse is turned on and off. The AM

52

responses can then be calculated for the case where both the (-1) mode and (-2) modes are present in the cavity, and the case where only the (-1) is allowed to be present. To ensure that the same operating points are being compared, the width of the gain curve is kept constant (i.e. the same filter amplitude coefficient a is used in each case), but the filter position is altered (i.e. different filter phase coefficients, $, are used) thereby allowing the (-2) mode to be suppressed without addi- tionally limiting the response by using a narrower gain curve. The AM responses for both cases are shown in Fig. 11. By suppressing the excitation of the (-2) mode, the response becomes limited by the conventional pho- ton-electron resonance (Fig. 1 l(i)), whereas when both modes are allowed to be present the response starts to fall after the photon-electron resonant frequency, but then rises again due to the resonance between the (-1) and (-2) modes (Fig. ll(ii)). This result is significant because it indicates that photon-electron resonance is not entirely eliminated with push-pull modulation, since there are still small changes in the average carrier density across the device during the transition between modulation states, although the effect of the photon- electron resonance is small compared to that of the res- onance between the (-1) and -(2) mode. Fig. 11 illus- trates that without this unique resonance mechanism involving the (-2) mode, the limit to the speed of push- pull modulation would then be the same as that of a conventionally modulated device.

0.1 1 IO 100 f requency,GHz

Fig. 11 AM response of three contact push-pull modulated laser with narrowed gain $Iter (i) Filter parameters: a = 0.8, y = 0; only (-1) mode can lase (ii) Filter parameters: Q = 0.8, y = 0.8; both (-1) and (-2) modes are present in cavity The 3dB bandwidth for (i) is 13GHz, and for (ii) is 23GHz. The bias levels in both cases are 30mA on each contact

time,ns Fi .12 Simulated output power for conventionully modulated DFB, for 01% 2 SGbit/s pattern Noise sources are suppressed for clarity The total bias level IS 75mA and the modulation depth is 40mA

Since FR is determined by mode spacing, the push- pull modulation bandwidth is entirely structurally dependent. FR can be increased by using shorter devices, since the mode spacing is roughly inversely proportional to the cavity length. However, a trade-off exists since push-pull modulation requires a high KL

IEE Proc -0ptoelectron , Vu1 143, Nu I , February 1556

product to effectively shift the optical energy along the cavity. Therefore, shorter devices require large values of coupling coefficient to achieve a high extinction ratio. With appropriate design of the structure and gain, bandwidths of greater than l00GHz have been simulated using short cavity devices (<300pn long). Experimentally, parasitics from the device packaging have limited the modulation rate to below 3 GHz, therefore the enhanced response has not yet been observed in practice.

"4 2.0, 5

tirne,ns Fig. 13 Simulated time resolved chirp of etalon measurement system for conventionally modulated DEB, for 010 2.5Gbith pattern Noise sources are suppressed for clarity. The total bias level is 75mA and the modulation depth is 40mA

'2- 2.0

.- 4 r 5 1.0

U

1

"' d Y -1.0 1 I

0 0.2 0.4 0.6 0.8 1.0 1.2 time,ns

Fig. 14 Sirnuluted response of etalon measurement system j a r conven- tionally modulated DFB, for 010 2.5GbitUs pattern Noise sources are suppressed for clarity; total bias level is 75mA and modula- tion depth is 40mA

4 Time resolved chirp

4. I Modelling Previous experimental work [7] has indicated that push-pull modulation results in a unique time resolved chirp shape which is quite different to that of a conven- tionally modulated laser. Here, we present simulations of the theoretical chirp and also of the response of the experimental chirp measurement system to explain this shape.

The theoretical chirp can be obtained directly from the output of the time domain model described in Sec- tion 2, which consists of the optical field values sam- pled in time. By differentiating the rate of change of the optical field with respect to time, the temporal behaviour of the instantaneous lasing frequency can be obtained. Fig. 12 shows the simulated chirped pulse and Fig. 13 shows the theoretical time resolved wave- length chirp for a conventionally modulated single con- tact device. The wavelength falls as the pulse is turned on and rises as the pulse is turned off, due to the car- rier density dependence of the refractive index, and oscillations occur as a result of photon-electron reso- nance. The transient intensity pulse and time resolved chirp for push-pull modulation of the same device, shown in Figs. 15 and 16, exhibit markedly different characteristics. As the pulse is initially turned on, there is a sharp decrease in wavelength due to the increase in carrier density at the emitting end of the laser. How- ever, the chirp does not then follow changes in carrier density, but instead high frequency oscillations occur during the switching. After simulating different devices

IEE Puoc.-Optoelectron., Vol. 143, No. 1, February 1996

and different bias levels, it was found that these damped oscillations occur at the enhanced resonant frequency FR indicating that the dominant chirp mech- anism comes not from the photon-electron resonance and the carrier density dependence of the refractive index, as with conventional modulation, but rather from the excitation of the (-2) mode at the point of switching. One notes also that the wavelength for the 'ones' is the same as the wavelength for the 'zeros', confirming that the symmetry of push-pull modulation eliminates static chirp, with wavelength deviations occurring only during the transition between the two states.

time.ns Fig. 15 Simulated output power for three contact push-pull modulated DFB, for 010 2.5Gbith pattern Noise sources are suuuressed for clarity. The total bias level is 75mA and the modulation depth is '4bmA

f 0 . 4 , k

time,ns Fig. 16 Simulated time resolved chirp for three contact push-pull modu- luted DFB, for 010 2.5Gbith pattern Noise sources are suppressed for clarity. The total bias level is 75mA and the modulation depth is 40mA

5. 0.41 I

i - @ -0.4 1 9 -0.6;

0 0.2 0.4 0.6 0.8 1 1.2 time,ns

Simulated res onse of etalon measurement system for three con- Fig. 17 tact push-pull modulateo!DFB, for 010 2.5Gbit/s pattern Noise sources are suppressed for clarity. The total bias level is 75mA and the modulation depth is 40mA

Previous experimental measurements of the time resolved chirp were carried out using a narrow band- width Fabry-Perot etalon filter [ 161. The experimental set-up is illustrated in Fig. 18. The chirped signal (a repetitive single bit) was passed through the 0.1 nm bandwidth etalon. This was scanned across the relevant wavelength range of the signal, and the transmitted sig- nal at each filter step measured using a sampling oscil- loscope and stored on computer. This allowed a three- dimensional plot of time, wavelength and optical power to be obtained. By calculating the centre of gravity of the power (p) produced at every filter centre wave- length (Afilter) the wavelength at a given time and hence the time resolved chirp could be obtained, i.e.

( 3 )

The response of this system was modelled using simu- lated chirped pulses produced by the time domain model described earlier. The pulses were convolved with the etalon and receiver responses for different etalon centre wavelengths to obtain a measure of the power at different optical wavelengths. Figs. 14 and 17 illustrate the simulated response of the etalon measure- ment system for single contact modulation and push- pull modulation, respectively. The simulated measured chirp for the single contact case corresponds well with the theoretical chirp whereas the measured chirp for the push-pull case exhibits some differences from the theoretical chirp. The initial sharp decrease in wave- length at the leading edge of the pulse is not detected by the measurement system. This is because at that point in time there is very little power in the pulse, therefore the etalon system, which detects changes in the transmission level, will not measure the deviation. In addition, the high speed oscillations at the resonant frequency FR are not present in the measured chirp for push-pull modulation, due to the time-bandwidth limit of the etalon response [ 171; the oscillations typically occur at frequencies greater than 20GHz and wave- length deviations at this chirp rate cannot be resolved by the etalon, therefore the measured chirp is a con- volved form of the actual chirp. The simulated response of the etalon system shown in Fig. 17 com- pares well with the experimentally measured chirp shape for push-pull modulation reported in [7].

total optical field due to the superposition of these two modes can be written as

F = A~ exp(iw1t) exp(-yt) exp( i~wt) ] (4)

where Am = a2 - al. Providing A2/A1 << 1, and using the identity (1 + x) = exp(x) (valid for small x), the phase of this expression is given approximately by

$ = wl t + - A2 exp(-yt) sin(Awt) A1 (5)

Differentiating this with respect to time gives the instantaneous lasing frequency O) (t) ,

w ( t ) = w1 + -Awexp(-yt)[cos(Awt) A2 - ysin(Awt)] (6)

Eqn. 6 predicts that the lasing frequency will undergo damped sinusoidal oscillations about the frequency of the (-1) mode wI at an oscillation frequency equal to the separation of the (-1) and (-2) modes Aw. Further, the amplitude of the oscillations, and therefore the size of the wavelength deviation, is linked to the (-2) mode amplitude A2 and hence to the extent to which the (-2) mode is excited during the turn-on and turn-off of a pulse.

Considering the simulated transient chirp shown in Fig. 16, it can be seen that these oscillations at the mode separation frequency are indeed present. In addi- tion, however, there are deviations due to the carrier density dependence of the refractive index. This indi- cates that the chirp shape is controlled by the interac- tion between the enhanced (-1)/(-2) mode resonance and the classic photon-electron resonance. This is fur- ther supported by the results described previously in Section 3 and illustrated in Fig. 11 where it was found

A1

that the dominant (-l)/(-2) mode resonance and the weaker photon-electron resonance are both present during push-pull modulation. This presents the inter-

high electrical esting possibility of controlling and altering the chirp shape by manipulating the relative strengths of the two speed amplifier

detector resonances. Initial results have alreadv indicated that

digital sampling oscilliscope

Fig. 18 The pattern generator’s DATA and outputs were used to push-pull modulate laser

Experimental set-up for time resolved chirp measurements

It is believed that the unique chirp shape produced by push-pull modulation can improve transmission performance. With conventional modulation, the two edges of the pulse travel at different speeds due to the different direction of the wavelength shifts, hence the pulse spreads apart in a dispersive fibre. With push- pull modulation, the wavelength deviation at both edges is in the same direction and the overall deviation is reduced. One expects therefore both edges to travel at similar speeds, resulting in reduced spreading of the pulse during propagation.

4.2 Theoretical explanation The oscillations in the simulated time resolved chirp at the resonant frequency FR indicate that the chirp mech- anism for push-pull must be linked to the enhanced resonant mechanism. Consider a laser with both the (- I ) and (-2) modes present in the cavity. The amplitude of the dominant lasing mode field will vary as Alexp (iolt), whereas the amplitude of the (-2) mode, which is below threshold and will therefore decay exponentially, varies as A2exp(io2t - yt), where y is the decay rate. The

by introducing a degree of asymmetry into the modula- tion, i.e. using different modulation depths on each contact, the strength of the excitation of the (-2) mode can be reduced and hence the chirp shape can be tai- lored to further improve transmission performance. The factors affecting the strength of the (-2) mode exci- tation are not yet fully understood and further work is required here, but the mechanism seems to be linked to the bias levels and the rate of change of the carrier den- sities in each half of the push-pull device at the point of switching.

The excitation of the (-2) mode is readily demon- strated by an examination of the simulated time aver- aged modulated spectrum and the side-mode suppression ratio between the (-1) and (-2) modes. The spectrum was measured over a pseudo random bit sequence, and smoothed with a Gaussian filter (Ah-0.lnm) in order to average the noise and make measurements more relevant to experimental results were similar broadening will occur due to the measur- ing instrument. The spectra for both single contact modulation and push-pull modulation of the same device are shown in Figs. 19 and 20 together with sim- ulated eye diagrams over a transmission distance of 50km, at transmission rates of 2.5Gbitis and 10Gbit/s, respectively. The stronger excitation of the (-2) mode

54 IEE Proc.-Optoelectron., Vol. 143, No. I , Febvuavy 1996

can clearly be seen in the push-pull spectra, with a reduced side-mode suppression ratio at the faster trans- mission rate due to the larger rate of change of the car- rier densities in each half of the device, resulting in a stronger (-2) mode excitation. A small side-mode sup- pression ratio is normally considered detrimental to operation, but for push-pull modulation the (-2) mode is only excited during the switching between modula- tion states and remains strongly suppressed in the steady state, hence the stability of the device is not affected. This is evident from the eye diagrams in Figs. 19 and 20; as a result of the enhanced resonant response and the reduced chirp the eye closure is rela- tively small for the push-pull case at lOGbit/s, whereas the eye closure is more severe for the single contact case.

I I I I I I , 105 1 n i

2.5 Gbls

100

. i

-7

i / ! ! -1

”-5b0 -400 -300 -200 j -100 6 100 ;OO 3100 (-2% 1)

optical frequency relative to Bragg frequency, GHz Fig. 19 Corn arison of simulated time averaged modulated spectrum for (i) conventiona fsingle contact laser and (ii) three contact push-pull modu- luted laser Bit rate = 2.5Gbitis Total bias current is 75mA and modulation depth is 40mA in both cases. Fre- quency positions of (-1) and (-2) modes are marked, and corresponding eye diagrams at transmission length of 50km are shown in inset. Although (-2) mode becomes more strongly excited at the faster transmission rate for push- pull modulation, the eye opening remains better than the single contact case

/ j i i , : : I IO’I

-500 -400 -300 -20d i 100 0 100 200 300 (- 2 I(- I 1

optical frequency relative to Bragg frequency,GHz Fig.20 Com arison of simulated time averaged modulated spectrum for (i) conventiona fsingle contact laser and (ii) three contact push-pull modu- lated laser Bit rate = lOGbitis Total bias current is 75mA and modulation depth is 40mA in both cases. Fre- quency positions of (-1) and (-2) modes are marked, and corresponding eye diagrams at transmission length of 50km are shown in inset. Although (-2) mode becomes more strongly excited at the faster transmission rate for push- pull modulation, the eye opening remains better than the single contact case

5 Discussion and conclusions

This paper has presented an analysis of the enhanced resonance and unique chirp mechanism that exist in push-pull modulated DFB lasers, highlighting the role of the (-1) and (-2) modes in determining the speed of response and the chirp shape. The classical photon- electron resonance within the device is reduced due to the symmetry of the modulation. A higher frequency

IEE Proc-Optoelectron., Vol. 143, No. I , February 1996

resonance which is structurally dependent becomes the dominant resonant mechanism. This enhanced reso- nance comes from excitation of the (-2) side mode dur- ing switching, with the resulting beating with the (-1) lasing mode leading to oscillation of the energy back and forth along the cavity. The frequency of this reso- nance, and hence the push-pull modulation bandwidth, is equal to the mode separation and so can be increased by appropriate design of the Bragg grating, giving greater control over this resonance compared to the conventional photon-electron resonance, and hence allowing the possibility of greatly increased modulation bandwidths.

To achieve these high modulation bandwidths, a strong resonant response is required and this raises a number of structural issues. A threshold power level must be reached in order to give a strong enough exci- tation of the (-2) mode. If this threshold is not reached, the enhanced resonance is weak and so the response becomes limited by the photon-electron resonance fre- quency. In general, devices with a large Id, product will reduce the bias level required to reach this threshold, because the grating is then more effective at shifting the optical power. However, both a shorter cavity and a lower coupling coefficient increase the frequency sepa- ration between the (-1) and (-2) modes, and there is therefore a trade off between achieving a large reso- nance frequency and a strong resonant response. A high differential gain also decreases the threshold bias level because a smaller change in carrier density can give a sufficiently strong excitation of the (-2) mode, and therefore the use of quantum well material is likely to be advantageous. Simulations indicate that using an appropriate design bandwidths of greater than 100 GHz are possible, given an appropriate package.

Push-pull modulation exhibits a unique time resolved chirp shape better suited to transmission than that of a conventionally modulated device. This shape is deter- mined by the interaction between the dominant (-l)/(- 2) mode resonance and the weaker photon-electron resonance. We believe that by controlling the interac- tion between the two resonances through the structure and bias conditions of the device, the chirp shape can be tailored to further improve transmission perform- ance. This, combined with the enhanced modulation bandwidth, makes push-pull modulation an attractive directly modulated source for use in future multigiga- bit/s optical systems.

6 Acknowledgments

The work was supported by EPSRC and BNR Europe. The authors thank Dr James Whiteaway of BNR Europe for his helpful discussions.

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