Semiconductor Optical Amplifier (SOA) offers all logical operations.

“XOR” has a wide range of application.

Either a clock or a continuous wave is called assist probe it is transferred to the o/p of the XOR.

INTRODUCTION

Scheme that avoid the use of probe signal based on Cross polarization rotation(XPR).

Its operation based on two i/p signal at different wavelength.

Scheme that avoid the use of probe signal based on four wave mixing(FWM).

FWM based schemes are not suitable for ON-OFF keying.

Another scheme with i/p data at same wavelength.

Two SOAs are employed

Types of SOA without using any assist probe light

Single i/p-o/p wavelength XOR operation performed by nonlinear effects of SOA.

It offers minimized number of components.

Suitable for binary to QASK format conversion.

Most of the optical schemes require active control of signal phase.

Here we demonstrate the nonlinear effect of SOA without any phase control.

Experimental setup is as shown below

Experimental setup & Operation principle

O/p of actively mode locked fiber laser (MLFL) operating at 10GHz sent to electro optical modulator (EOM) driven by a pattern generator.These o/p is modulated with a 1100data sequence and given to erbium doped fiber amplifier (EDFA).Optical filter remove out of band noise.Optical delay line generate all the possible bit combinations at the two XOR gate i/p.Each gate o/p is connected to SOA by means of optical circulators (OC).OCs retrieve the signals after they travelled through the amplifier.The signal is then recollected by the Polarization Beam Combiner (PBC).Due to the cross gain compression effect (XGC) the gain of the pulses at SOA is reduced.

Gain can be adjusted by means of variable attenuators (VA).

When a pulse travel alone the amplifier it get all the available gain.

When no data pulse is present at IN1 or IN2 the gate o/p level zero.

Truth table of XOR is shown below

The signals are recombined at the o/p of PBC.For a single polarization o/p signal a polarizer is exploited.

RESULTSThe figure shows different waveforms.OUT1 is the PBC o/p when the i/p connected to OF2.OUT2 is the PBC o/p when the i/p connected to OF1.It can be seen that optimal central wavelength of the filters were slightly changed.The action of the band pass OFs after the SOA would be more effective.A weak dependence of the relative delay of the i/p pulses was also observed.It is due to the corresponding variation in the non linear interaction length of the pulse.

The o/p spectra after OF1 & OF2 is as shown

Bit Error Rate (BER) measurements are shown in figure.

BER measurement is performed by using 1100 i/p sequence.

Two i/p signal represent LSB.

MSB of a binary sequence to be encoded in a QASK signal.

Let LSB sent to IN1 & MSB to IN2.

If LSB & MSB are zero then o/p is also zero.

For MSB=0 & LSB=1can be made arbitrary lower than the gate o/p power.

The parameter of the gate are adjusted to minimize the o/p of PBC.For MSB=LSB=1 o/p pulse from MSB o/p path is generated with o/p power.This generated a gray coded QASK signal at the o/p of the gate.

BINARY TO QASK CONVERSION

The VAs & PCs can be also used to equalize the power difference b/w the o/p levelBinary code can also be generated with this architecture.The oscilloscope traces showing the data sequence at the gate inputs (LSB and MSB), and gate output are shown.

OUT_tot represents the single polairized output signal after the polarizer at the PBC output.

OUT1 and OUT2 signals are normalized to the same levels.

The two signals amplitude can be independently adjusted in order to provide the quaternary signals at the output of the PBC.

The input/output eye diagrams, detected with a 10 GHz bandwidth photodiode are shown below.

Based on semiconductor technology the scheme can be integrated with a reduced footprint.

The lack of signal phase reduced the complexity & cost.

ADVANTAGES

Here demonstrated the operation of a novel architecture for performing all-optical XOR between data signals at the same wavelength by using a single semiconductor optical amplifier, and no assist probe signal.

Nonlinear effects are exploited to discriminate between the cases of single- and two pulses traveling the amplifier

Due to the limited nonlinear interaction length of the two pulses XPR alone is not sufficient to produce a high-quality output signal.

CONCLUSION

The BER measurements shows only 2 dB of power penalty for the XOR gate output data with respect to the input data.

The same proposed device also allows to perform binary-to-quaternary ASK format conversion (2-bit DAC) in the optical domain.

Dynamic operation without phase control of the signals is demonstrated.

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16.

“20 Gb/s all-optical XOR with UNI gate,” IEEE Photon.Technol. Lett., vol. 12, no. 7.

“Improving transmission privacy using optical layer XOR,” in Proc. CLEO 2007, CThBB6,

BIBLOGRAPHY