1 April 2007 All-optical flip flop based on a symmetric Mach-Zehnder switch with a feedback loop and multiple forward set/reset signals
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A novel all-optical set/reset flip flop (AOFF) based on a symmetric Mach-Zehnder switch with a feedback loop and multiple forward set/reset signals is presented. The proposed flip flop has a fast response, a flat output gain, and a short switching-on interval of a few hundreds of picoseconds regardless of the associated feedback-loop delay. It is shown that a high on/off constrast ratio at the AOFF output is achieved above 20 dB.



An all-optical flip flop (AOFF) is an essential component for latching functions in high-speed all-optical processing applications.1, 2 Currently, an AOFF can be realized using the coupled multimode-interference bistable laser diodes scheme3, 4 or by a symmetric Mach-Zehnder (SMZ) switch with a single-pulse counter-propagation control-signal feedback loop.5 In the former scheme, a number of wavelengths are required; whereas in the latter scheme, only a single wavelength is employed with a feedback loop (FBL) to enhance the AOFF configuration simplicity. Because the real-time signal-propagation delay associated with the FBL is hundreds of picoseconds,5 there is a lag in feedback signal (i.e., requiring a sufficient transient time equivalent to the FBL delay to fully set the AOFF in an on state) when switching the AOFF to the on state. In addition, the counterpropagation between a control and input signal in the SMZ will result in an additional delay in the rising and falling edges of AOFF output.6 As a result, these proposed AOFFs operate on the order of nanoseconds. Therefore, achieving a fast response time and an on interval that is shorter than the transient time are the issues in FBL-based AOFF employed in high-speed applications. Here, we propose a new AOFF configuration assisted by a FBL SMZ with multiple forward-control signals (set S and reset R ) to overcome these limitations.


AOFF Operation

An AOFF circuit block diagram and its operation principle are depicted in Fig. 1. The AOFF is composed of a SMZ switch5, 7 with a continuous-wave (cw) signal input; set and reset control inputs in the upper and lower control arms, respectively; and a FBL (with a signal propagation delay of TFBL ) feeding β% of power from the AOFF output (Q) to the upper control arm of the SMZ. Polarization controllers are used to introduce an orthogonal polarization between the cw and control signals, and consequently, a polarization beamsplitter is used at the output of the SMZ to separate them. In the absence of the optical pulses at control inputs, and providing that both semiconductor optical amplifiers (SOAs) are identical, the SMZ is in a balance state because the signal gain and phase profiles in both arms in the SMZ are the same; thus, the cw signal propagating in both arms will not emerge at the AOFF output (i.e., in the off state). A single set pulse will pass through a number of paths with different delays and attenuators to produce a multiplexed pulse set S in TFBL , before being applied to the upper control input of the SMZ for toggling the AOFF to the on state. The first pulse of S will saturate SOA1 , thus inducing an imbalance in gain and phase profiles between two arms and hence causing a switching cw signal to Q . To maintain the AOFF in the on state, i.e., a flat SOA gain saturation level, a portion β% of Q output power PFBL is fed back to the upper control input of the SMZ. However, since PFBL takes a TFBL to arrive at SOA1 , S pulses following the first pulse continue to maintain the SOA1 saturation, thus precluding gain from recovering to its initial value when the first pulse exits SOA1 while PFBL still has yet to arrive. Similar to the set pulse, a reset pulse, after a delay of TON (the on interval), creates R , which is applied to the lower control input of the SMZ. The first pulse of R saturates the SOA2 gain dropping it to the same level of SOA1 saturating gain (i.e., restoring the gain and phase balance between SMZ arms) and once again toggling the AOFF to its off state because cw is no longer switched to Q . Note PFBL is still in the upper control port within a subsequent TFBL period although there is no output signal at Q . To retain the same gain level in both SOAs in this period, the following pulses in R will ensure a continuous gain saturating of SOA2 for the SMZ to be in balance, thus completely turning off the Q signal during and after TFBL once the reset signal is applied.

Fig. 1

Multiple forward-control AOFF configuration.



AOFF Stability

The temporal gain of the output Q is expressed by:7


where K is an overall constant coupling factor, G1(t) and G2(t) are the temporal gain profiles of SOA1 and SOA2 , and αLEF is the SOA linewidth enhancement factor. It is noted that Q(t)=0 when G1(t)=G2(t) . The SOA gain computed over a SOA length LSOA is given by:7


where Γ is the confinement factor, g is the gain coefficient, and N(t) is the SOA carrier density. The gain profiles are, therefore, dependent on the temporal change of carrier, which is governed by the SOA rate equation with the applied average power P(t) (Ref. 8):


where Ie is the injection dc current, q is the electron charge, VSOA is the active volume, τe is the carrier lifetime, hv is the photon energy, ASOA is the cross-section area of the active region, and NT is the carrier density at transparency. To achieve operational stability in the AOFF, the feedback power is constrained to match with the average powers of both S and R signals. This will ensure the steady imbalance and balance states in SMZ during the transient durations when the AOFF is switched to the on and off states, respectively. These constraints are represented as follows:




where PS,avg(t) and PR,avg(t) are the average powers of control pulses in S and P streams, respectively, over TFBL , and M is the number of pulses in each S or R . In Eq. 4, if PFBL is smaller than the average power of the applied control signal S , the Q signal will eventually cease. However, a greater PFBL will gradually saturate the SOA gain, thus saturating AOFF-output gain. As a result, Q varies in a large intensity range, which is determined by the intensity variation ratio (IVR) between the minimum and the maximum values of the Q signal during TON . For a complete turning off in the AOFF, the applied average power of the control signal R is required to be half of PFBL , ensuring both SOAs receive the same average control power. If this power is different from PFBL , a residual signal will emerge at the output Q , which in turn unexpectedly restores the AOFF to the on state again. This residual signal will therefore deteriorate the on/off contrast ratio (CR) at Q , which is defined by the power ratio of signals in the on and off states.


Results and Discussion

The AOFF operation is validated using VPI simulation software. Simulation and SOA device parameters are given in Table 1. Note that the average power of S is 3dB greater than R because S is reduced by 3dB when coupled with PFBL to ensure that the SOAs are excited with the same set/reset powers. The TFBL is approximated as 0.2ns , equivalent to a 40-mm optical waveguide FBL.5 The SOA model is assumed to be polarization-independent, though in practical systems, polarization-gain dependence ( 1 to 2dB ) and the imperfect polarization states of the cw and set/reset signals will slightly affect AOFF operation. The flip-flop operation is illustrated in Fig. 2. A series of set and reset single pulses, shown in Fig. 2a, are applied to the AOFF in a range of TON values of 0.1, 0.2, 0.5, 1, 2, and 5ns . The resultant temporal gain profiles of SOAs corresponding with the set/reset signals are observed in Fig. 2b. During a period of TON , the SOA1 gain is kept at the same saturation level by both S and PFBL . Figure 2c displays the AOFF-output waveforms. There are ripples at the leading edge of the Q output signal in the on state during a TFBL owing to the variation in the SOA1 gain profile caused by the discrete excitations on SOA1 by pulses in S . When the AOFF is switched off, a small residual signal, lasting in TFBL , still emerges at Q . This is due to the gain variation of SOA2 caused by multiple-pulse excitations of R in contrast to a flat gain profile of SOA1 maintained by a leftover of constant PFBL within that TFBL , hence, causing ripples at the trailing edge of the Q signal. It will, therefore, result in on/off CR deterioration.

Fig. 2

(a) Set/reset pulses, (b) temporal gain profiles of SOA1 and SOA2 , and (c) AOFF output (Q) .


Table 1

Simulation and SOA device parameters.

Input power PCW 0dBm
Gaussian pulse width 20ps
Signal wavelength 1554nm
PS (peak power of first pulse) 13.5dBm
PS (peak power of following pulses) 8.5dBm
PR (peak power of first pulse) 10.5dBm
PR (peak power of following pulses) 5.5dBm
SOA linewidth enhancement factor αLEF 5
SOA length LSOA 0.5mm
SOA confinement factor Γ 0.2
SOA carrier density at transparency NT 1.4×1024m3
SOA spontaneous emission factor nsp 2
DC-bias le 150mA
FBL delay TFBL 0.2ns
Splitting factor β 15%

The graphs in Fig. 3 show that the highest achieved CR is 22dB at β=15% [AOFF total output power is 14.5dB ; see Fig. 2c] where the conditions in Eqs. 4, 5 are satisfied at TON=1ns . It is also shown that the AOFF output signal is relatively flat during TON and the observed IVR is 0.95. Beyond this optimum operation point, both CR and IVR are considerably decreased due to high residual power and improper feedback power, respectively. Note that high β results in flat-level performance in CR and IVR; however, since SOA1 gain is saturated due to high-power PFBL , their values are noticeably small.

Fig. 3

AOFF IVR and CR against β (at TON=1ns ).




A new AOFF configuration based on a SMZ with FBL and multiple-pulse forward set/reset signals is proposed. A multiple set/reset control-signal scheme fully overcomes the feedback-loop delay, thus making AOFF suitable for high-speed memory or signal processing applications where TON is required to be as small as a few hundred picoseconds regardless of the FBL delay. In addition, the forward controls enhanced the AOFF toggling response within the pulse width of the set and reset signals. On/off contrast and intensity variation ratios of 22dB and 0.95, respectively, are achieved at the optimum operating point.


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© (2007) Society of Photo-Optical Instrumentation Engineers (SPIE)
H. Le-Minh, H. Le-Minh, Fary Z. Ghassemlooy, Fary Z. Ghassemlooy, Wai Pang Ng, Wai Pang Ng, } "All-optical flip flop based on a symmetric Mach-Zehnder switch with a feedback loop and multiple forward set/reset signals," Optical Engineering 46(4), 040501 (1 April 2007). https://doi.org/10.1117/1.2721773 . Submission:

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