1884 OPTICS LETTERS / Vol. 32, No. 13 / July 1, 2007

Cascadable all-optical inverter based on anonlinear vertical-cavity semiconductor optical

amplifier

Haijiang Zhang,* Pengyue Wen, and Sadik EsenerUniversity of California, San Diego, Electrical and Computer Engineering Department, 9500 Gilman Drive, La Jolla,

California 92093-0407*Corresponding author: hzhang@soliton.ucsd.edu

Received February 2, 2007; revised April 26, 2007; accepted May 4, 2007;posted May 8, 2007 (Doc. ID 79701); published June 22, 2007

We report, for the first time to our knowledge, the operation of a cascadable, low-optical-switching-power��10 �W� small-area ��100 �m2� high-speed (80 ps fall time) all-optical inverter. This inverter employscross-gain modulation, polarization gain anisotropy, and highly nonlinear gain characteristics of an electri-cally pumped vertical-cavity semiconductor optical amplifier (VCSOA). The measured transfer characteris-tics of such an optical inverter resemble those of standard electronic metal-oxide semiconductor field-effecttransistor–based inverters exhibiting high noise margin and high extinction ratio ��9.3 dB�, making VC-SOAs an ideal building block for all-optical logic and memory. © 2007 Optical Society of America

OCIS codes: 200.4660, 190.4360, 250.5980.

All-optical inverters have attracted extensive re-search attention since the 1980s in view of potentialapplications in all-optical signal processing [1–7]. All-optical inversion has been demonstrated in variousdevices, including a light-emitting laser diode com-bined with a photodetector [1], both Fabry–Perot andtraveling-wave semiconductor optical amplifiers(SOAs) by employing cross-gain modulation [2](XGM), erbium-doped fiber amplifiers or SOAs withfeedback [3,4], and vertical-cavity surface-emittinglasers using TM switching, etc. [5–7] Existing ap-proaches, however, suffer from various drawbacks,such as the complexity of the setup [1,3,4,6,7], high-switching optical power ��1 mW� [2–5], low speed[3,4], lack of two-dimensional (2D) integration capa-bility [1–4,6,7], and, particularly, poor transfer char-acteristics [1–7], preventing cascaded inverter opera-tion and logic level restoration critical for buildingmore complex logic circuits.

In the past few years, vertical-cavity semiconduc-tor optical amplifiers (VCSOAs) have drawn increas-ing attention because of their nonlinear optical gainresponse [8–11]. Due to the small gain volume, theoptical power needed for nonlinear operations in VC-SOAs has been demonstrated to be orders of magni-tude lower than required in their in-plane counter-parts [10]. Their small device size and low powerconsumption make these devices favorable for 2D in-tegration. In this Letter we report, for the first timeto our knowledge, a novel high-performance cascad-able all-optical inverter that employs XGM, polariza-tion gain anisotropy, and nonlinear gain characteris-tics (including optical bistability) in a single VCSOA.As with SOA inverters [2], the optical inversion in aVCSOA is obtained through gain competition be-tween two optical inputs (the bias and signal). How-ever, when a VCSOA is operated in its highly nonlin-ear (and possibly bistable) region, the sharp input–output transition makes the optical inversiontransfer curve very steep, enabling logic regenerationand positive noise margins. Typically, VCSOAs ex-

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hibit two separated gain windows corresponding totwo orthogonal linearly polarized states [12]. There-fore, when the input signal and the optical bias areorthogonally polarized the inverted output becomesorthogonally polarized with respect to the input sig-nal. As a result, the polarization dependence of theVCSOA gain naturally makes the inverted outputsignal remain isolated from the amplified input sig-nal at the inverter output, enabling cascaded in-verter operation.

XGM occurs in a SOA or VCSOA subject to two in-cident optical beams (optical bias and input signal)due to the gain saturation in a homogeneous material[2]. With a constant carrier injection, the presence ofthe signal beam suppresses the gain for the biasbeam by consuming free carriers in the amplifiergain region. This effectively causes the inverted rela-tion between the output of the bias beam and the in-put of the signal beam. However, XGM-based opticalinversion typically suffers from very poor transfercharacteristics [4], eliminating the possibility toimplement more complex logic functions. For ex-ample, an optical flip-flop requires inverters to havean input–output transition slope steeper than −1 be-tween two flat logic levels, which is very difficult to

Fig. 1. Experimental setup of a VCSOA-based optical

inverter.

2007 Optical Society of America

July 1, 2007 / Vol. 32, No. 13 / OPTICS LETTERS 1885

achieve in a standard SOA. Researchers have pro-posed to introduce positive feedback to a SOA to in-crease the slope and reduce the switching power [4].However, the response time is inherently increasedby the feedback configuration and switching powerremains high ��1 mW� [4]. Previously, we reported asharp input–output transition curve observed in aVCSOA operated in its highly nonlinear (bistable) re-

Fig. 2. Polarization-dependent gain windows measured ina VCSOA. S and P represent the two orthogonalpolarizations.

Fig. 3. Output power of the signal and optical bias beamslength detuning, and the CW optical bias power is 1 �W. (

beams is 30 and 10 pm, respectively. The CW optical bias pow

gion with a single input beam [9,10]. In this case,when an optical CW bias and input signal with wave-lengths detuned from the amplifier intrinsic reso-nance are coupled into a VCSOA simultaneously, thesignal beam experiences an abrupt change in opticalgain when its power is modulated across the ampli-fier bistable switching power. By the well-knowngain-carrier relation [2], such an abrupt change inthe optical gain can imprint the inverted signal to theoutput of the optical bias beam through the change ofthe carrier density in the gain region, resulting in anextremely sharp inversion transfer curve.

The experimental setup used to demonstrate theoperation of a VCSOA-based inverter is illustrated inFig. 1. We used an electrically pumped, proton-implanted 850 nm VCSOA biased at 95% of itsthreshold. A chopper is inserted in the optical path tomodulate the input signal beam. The input signaland optical bias beams are combined and coupledinto the VCSOA with their wavelengths, relativepower levels, and polarization states defined by con-trolling the associated laser sources, optical attenua-tors, and polarizers. The amplified and inverted out-puts are separated by a beam splitter and twopolarizers aligned appropriately with respect to theinput polarization directions. Two optical powermeters and an oscilloscope are used to measure theoutputs.

The polarization-dependent gain windows in Fig. 2

us input signal power. (a), (b) Both beams have zero wave-) The wavelength detuning for the signal and optical bias

versc), (d

er is 3.5 �W.

1886 OPTICS LETTERS / Vol. 32, No. 13 / July 1, 2007

show peak wavelength separation of 27 pm, essen-tially resulting from birefringence. The slight differ-ence in their maximum gain is likely caused by thedichroism in the VCSOA [12]. In the experiment, theinput signal wavelength is chosen such that the sig-nal beam is always amplified within the gain windowwith the shorter wavelength. Figure 3 shows the out-put power of the optical bias and signal beams versusinput signal power under two different operation con-ditions. Figures 3(a) and 3(b) were obtained with zerowavelength detuning for both incident beams(840.700 nm for the signal beam, 840.727 nm for thebias beam) to ensure that only XGM occurred in thedevice, while Figs. 3(c) and 3(d) were measured with30 pm �840.730 nm� and 3 pm �840.730 nm� wave-length detuning for the signal beam and CW biasbeam, respectively. Compared with Fig. 3(b), Fig. 3(d)clearly shows a much better transfer characteristic ofoptical inversion with clear plateaus at two logic lev-els and a very sharp transition between them. This isdue to an abrupt change of the carrier density result-ing from the gain when the input signal powercrosses 10 �W. In Fig. 4, under the same operationconditions as for Fig. 3(d) but with an input signalmodulated by a chopper, the dynamic behavior of op-tical inversion at low speed is shown. An on/off ex-tinction ratio of �9.3 dB is observed when the extinc-tion ratio of input is infinite with the choppermodulation. The extinction ratio of a VCSOA inverteris mainly determined by the VCSOA gain and inputand bias detunings. Therefore, it can be enhancedfurther by optimizing the device design and operationcondition.

Fig. 4. All-optical inversion with an on/off ratio of�9.3 dB.

Fig. 5. Switching time of the optical inverter. The 20% to80% transition time is less than 80 ps.

The speed response of VCSOA inverters is a strongfunction of the optical bias and input signal intensi-ties. In addition, the fall time is governed by the gainsuppression time of the XGM process, while the risetime depends on the gain recovery time and positiveintensity feedback determined by the device struc-ture, bias current, and gain medium. To measure thefall time, we used a diode laser that is modulated de-liberately into its distortion region to provide an in-put signal with a very sharp and clean rising edge.Figure 5 shows the measured fall time of about 80 ps,which is obtained by using an Agilent digital commu-nication analyzer. We observed the rise time of VC-SOA inverters to be comparable with their fall time.When compared with the reported speed of the XGMprocess in SOAs [13], we believe the speed responseof VCSOA-based optical inverters can potentially bemade significantly faster than reported here by opti-mizing the VCSOA biasing conditions, structure, andmirror reflectivity.

In summary, a novel cascadable, all-optical in-verter based on polarization-dependent XGM,coupled with highly nonlinear gain in a VCSOA, isproposed and demonstrated experimentally. Themeasured transfer characteristics of such an opticalinverter exhibit very sharp transition region, highnoise margin, high extinction ratio ��9.3 dB�, andlow switching power ��10 �W�. Single-wavelengthoperation is also demonstrated experimentally to fur-ther reduce system complexity. We believe that sucha VCSOA inverter can be large-scale integrated onphotonic chips, enabling all-optical logic, memory,and signal-processing functions to be realized fully inthe optical domain [14].

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