All-optical signal processing at 10GHz using a photonic crystal moleculeSylvain Combrié, Gaëlle Lehoucq, Alexandra Junay, Stefania Malaguti, Gaetano Bellanca et al. Citation: Appl. Phys. Lett. 103, 193510 (2013); doi: 10.1063/1.4829556 View online: http://dx.doi.org/10.1063/1.4829556 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i19 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

All-optical signal processing at 10 GHz using a photonic crystal molecule

Sylvain Combri�e,1 Ga€elle Lehoucq,1 Alexandra Junay,1 Stefania Malaguti,2

Gaetano Bellanca,2 Stefano Trillo,2 Loic M�enager,3 Johann Peter Reithmaier,4

and Alfredo De Rossi1,a)

1Thales Research and Technology, 1 Avenue A. Fresnel, 91767 Palaiseau, France2Department of Engineering, Universit�a di Ferrara, v. Saragat 1, 44122 Ferrara, Italy3Thales Systemes Aeroport�es, 2 Av. Gay Lussac, 78851 Elancourt, France4Institute of Nanostructure Technologies and Analytics, CINSaT, University of Kassel, Heinrich-Plett-Str. 40,34132 Kassel, Germany

(Received 12 August 2013; accepted 26 October 2013; published online 8 November 2013)

We report on 10 GHz operation of an all-optical gate based on an Indium Phosphide Photonic

Crystal Molecule. Wavelength conversion and all-optical mixing of microwave signals are

demonstrated using the 2 mW output of a mode locked diode laser. The spectral separation of the

optical pump and signal is crucial in suppressing optical cross-talk. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4829556]

The strong optical confinement in resonant cavities1 ena-

bles energy-efficient and fast all-optical switching. Owing to

their very small modal volume, Photonic Crystal (PhC) cav-

ities have enabled nonlinear operation in the sub-femto Joule

range and a typical recovery time within a few tens of

picoseconds.2 The practical implementation of all-optical

switching, frequency conversion, sampling or time-domain

demultiplexing requires the suppression of the crosstalk

between the control and the data signals. Ensuring a large

spectral separation between these is a convenient solution, in

particular if polarisation diversity or spatial multiplexing are

not possible. In ring cavities, this configuration is readily

achieved by assigning the wavelengths of the optical input to

different resonances of the resonator comb.1 In photonic crys-

tals, this is achieved with larger, hence, multi-mode cavities.

For instance, optical bistability was demonstrated using a

two-mode PhC cavity.3 Kerr-based switching with spectrally

separated pump and probe was reported using different Fabry

P�erot modes in a PhC waveguide.4 A two-mode cavity capa-

ble of spectral and spatial demultiplexing has been shown

recently.5

Also, the practical use of optical switches calls for oper-

ations at high repetition rates, which implies that thermal

effects must be properly handled. A convenient solution

involves a solid substrate, acting as an optical isolator and

thermal conductor. A recent example relies on the heteroge-

neous integration of III-V PhC cavities on a silicon photonic

chip. This enabled the demonstration of all-optical process-

ing at 10 GHz.6 Here we demonstrate that all-optical process-

ing at high repetition rate is also possible in a device based

on air suspended Indium-Phosphide photonic crystal

membranes.

We use the concept of photon molecule, namely, the cou-

pling of two single-mode cavities7–9 to generate two super-

modes and two resonances. The device is made of a thin

(250 nm) membrane of InP grown by Molecular Beam

Epitaxy. The hexagonal PhC lattice has a period a¼ 450 nm

and entails two identical H0 cavities,10 coupled together

(Fig. 1(a)) in a topology similar to that reported recently in

Ref. 11. There, the two cavities have slightly different param-

eters in order to optimize the Fano-assisted switching.12–14

Importantly, only one resonance, entailing the strongly asym-

metric lineshape, is involved in the switching operation.

Here, instead, the pump and the probe (or, equivalently, the

clock and the signal) are assigned to the two resonances. The

mutual coupling of the two cavities results into a splitting of

about 10 nm (Fig. 1(b)), that is 5 times larger than in Ref. 11

and about an order of magnitude larger than the spectral

width of the optical signals (1.2 nm). Therefore, such a large

spectral separation enables the efficient suppression of any

unwanted cross-talk between the pump and the probe, such

that the all-optical switching operation is genuinely due to the

nonlinear interaction taking place in the cavity. Also, the

choice of two identical cavities maximizes the spatial overlap

of the symmetric and anti-symmetric super-modes and

thereby the nonlinear interaction.

We note that the input and the output waveguides are

directly coupled to both cavities (Fig. 1(a)). This results into a

destructive interference between the two resonances and

explains the sharp dip observed in the transmission spectrum

(Fig. 1(b)). As a result, the maximum static transmission con-

trast is about 30 dB. This spectral feature is well reproduced

by the Coupled Mode Theory (CMT). The overall device

transmission, from the input to the output fiber, which

includes coupling losses through fiber collimators, microscope

objectives, and propagation loss, is approximately 20 dB at

the resonance. This has to be compared to about 14 dB overall

transmission of a reference waveguide located on the same

chip.

The free-carrier induced index change,15 triggered by lin-

ear or nonlinear (two-photon, as in the case here) absorption,

is the dominant nonlinear process in semiconductor (III-V

alloys, specifically) optical microcavities.1,16 We investigated

the switching dynamics of the device through two different

pump-probe measurements. In both cases, the pump and the

probe are obtained by spectral slicing the output of a 100 fs

passively mode-locked fiber laser (Optisiv) operating at

36 MHz. The pump (probe) is selected using a 1.2 nm (0.8 nm)a)Electronic mail: alfredo.derossi@thalesgroup.com

0003-6951/2013/103(19)/193510/4/$30.00 VC 2013 AIP Publishing LLC103, 193510-1

APPLIED PHYSICS LETTERS 103, 193510 (2013)

wide band-pass filters, generating pulses approximately 2 ps

(3 ps) long. The relative delay is controlled using a mechanical

translation stage. We first use a homodyne technique, as in

Ref. 17, where the probe is intensity modulated and the signal

is detected via a lock-in amplifier. In addition, an Acousto-

Optic Modulator shifts in frequency the probe input, while an

optical filter (with bandwidth 1 nm) at the output is centered at

the probe wavelength, in order to suppress the pump signal.

We then implemented a heterodyne technique equivalent to

that in Ref. 18, by replacing the filter at the output with an in-

terferometer connected to the reference pulse and a balanced

photodetector. We point out that here the detected signal

shown in Fig. 2 is proportional to the probe transmission, as

the pump is not chopped and there is no cascaded lock-in.

Thus, before the arrival of the pump, the signal is proportional

to the linear transmission of the probe pulse.

Fig. 2(a) shows the heterodyne signal as a function of

the probe-pump delay, normalized to level in the off state,

namely, at negative delay, before the arrival of the pump.

The pump is spectrally matched to the resonance at

�1550 nm, while the probe is blue detuned by 1.4 nm from

the resonance at 1560 nm (Fig. 1(b)). The switch-off process

clearly reveals two time constants, obtained by fitting the

signal with two decaying exponentials. This is consistent

with experiments made with InGaAsP and InP2 PhC cav-

ities,19 and the two time constants are attributed to carrier

diffusion and recombination, respectively.2

In all our measurements, the typical value of the fast

time constant is about 12 6 1 ps, while the long time con-

stant is about 200 ps or longer, depending on the excitation

level. The switching contrast (on/off ratio of the probe sig-

nal) increases with the pump energy and reaches a maximum

of about 10 dB. In order to evaluate the switching energy, we

assume an input coupling efficiency (including waveguide

loss) of about �7 dB. Thus, the average power of 40 lWtranslates into 200 fJ of coupled energy per bit. Conversely,

a switching ratio of 5 is achieved with about 100 fJ. We note

that the homodyne technique leads to identical (within meas-

urement error) estimates for the switching contrast and decay

constants.

The energy per bit figure is well above the sub-femto

Joule level demonstrated in GaInAsP PhC,2,11 but it is still 2

orders of magnitude better than in micro-ring resonators.1 It

must be noted that the better performances in GaInAsP PhC

are entirely due to the much stronger absorption and larger

refractive index dependence on the free carrier density of

this material. We also note that the two-cavity switch shown

here has almost comparable energy performances than the

single cavity device made on the same material (InP). The

detailed modeling of this single-cavity InP switch, including

the heterodyne technique are discussed in a related paper.19

The switching contrast is related to the linear transmission

spectrum (Fig. 1(b)); however, it also depends on the band-

width of the signal and on the speed of the gate. For instance,

from Fig. 1(b) a switching contrast approaching 30 dB would

be expected, provided that the signal bandwidth is much nar-

rower and that the nonlinearly induced blue shift of the reso-

nance is made larger than in the experiment reported here.

Besides the switching contrast, the level of the transmit-

ted signal in the on state is also important, as this qualifies the

dynamical insertion losses of the device. This is shown in

Fig. 2(b), comparing the probe transmission at resonance and

when blue-detuned (i.e., configured for switching). Thus, the

switch on level reaches about 70% of the probe at resonance.

The practical use of an all-optical switch implies opera-

tion at fast (>1 GHz) repetition rate. We generate a clock at

fMLLD¼ 10.098 GHz by spectrally slicing the output of a

FIG. 1. Design of the PhC membrane entailing two coupled H0 cavities and

access waveguides. The distribution of the optical field (jH?j2), perpendicular

to the plane of the membrane, is superimposed (a). The measured, solid line,

transmission reveals two peaks k1;2 at 1550 and 1560 nm, well reproduced by

the CMT model, red-dashed (b). The pump (clock) and probe (signal) are tuned

as indicated by the arrows. The Q-factor of the resonances is also indicated.

FIG. 2. Heterodyne, frequency non-degenerate, pump-probe measurement

of the switching dynamics. Transmitted probe signal, normalized to the level

at large negative delay as a function of the probe to pump delay, at various

average input pump power levels (a). Black dashed curve is exponential fit.

Probe is blue detuned (�1.4 nm) from the resonance at 1560 nm. (b)

Homodyne measurement, level of the transmitted probe depending on the

detuning, normalized to transmission at zero detuning and large negative

delay. The pump level is 40 lW and the repetition rate is 36 MHz.

193510-2 Combri�e et al. Appl. Phys. Lett. 103, 193510 (2013)

mode-locked laser diode extending from 1545 to 1555 nm

(Alcatel-Lucent-Thales III-V Lab). This is combined with a

CW tunable laser to perform a wavelength conversion

experiment (Fig. 3(a)). The output is filtered (0.8 nm band-

pass filter centered at the CW laser wavelength) in order to

reject the pump and then amplified, using an erbium doped

fiber amplifier (EDFA) and detected with a fast photodiode

(U2t, 40 GHz). The pump is tuned at 1550 nm ðk1Þ, while the

probe is blue detuned by roughly 1 nm from the resonance at

1560 ðk2Þ, e.g., as shown in Fig. 1(a). The exact determination

of the effective detuning (i.e., considering a thermally induced

drift) is however difficult. In practice, the detuning is set to

maximize the all-optical modulation, e.g., the onset of a spec-

tral comb at the output around k2, as shown in Fig. 3(b)

(Anritsu, resolution 50 pm). In the time domain (Fig. 3(c)),

this corresponds to a peak (about 25 ps) followed by a slower

decay, which is consistent with Fig. 2(a). We conclude that

this slow time constant does not hinder the fast response, but

merely induces a spectral offset of the cavity, provided that

the excitation is periodic. The pump average level at input is

2 mW, i.e., 40 fJ per clock pulse. Assuming (based on model-

ing) that 10% of the coupled energy is absorbed, and a calcu-

lated thermal resistance of 1� 105 KW�1, we estimate the

heating of the cavity to a few Kelvin.

A nonlinear optical switch can be used as an all-optical

mixer, where two microwave signals, both on an optical car-

rier, generate sum and difference frequencies in the optical

domain. The implications are that the local oscillator does

not need to be close to the mixer and that the isolation of the

local oscillator is very good.20 All-optical mixing (up-con-

version) at 40 GHz was demonstrated using Electro

Absorption Modulators (EAM).21 Here we demonstrate the

up and down conversion of a microwave signal, with fre-

quency ranging from 0 to 20 GHz, due to mixing with the

10 GHz clock. This is shown in Fig. 4. The device is oper-

ated exactly as in the wavelength conversion experiment,

except that the CW laser is modulated at fMZ with a Lithium-

Niobate Mach-Zendher Modulator (20 GHz bandwidth). The

optical power of the signal before the device is about 1 mW

and the RF driver power is 16 dBm. After filtering the pump

ðk1Þ out and optical amplification, the modulated optical car-

rier is detected and characterized with electrical spectrum

analyzer (ESA, Rhode and Schwartz, 40 GHz). The sum and

difference of frequencies, f6 ¼ fMLLD6fMZ, are shown in

Fig. 4(b), as a result of the all-optical modulation at fMLLD of

the optical carrier modulated at fMZ. The oscilloscope trace

of the downconverted signal at f– (500 MHz) is shown in

Fig. 4(c). The dependence of the signal at f– on the power

level of the input at signal at fMZ is linear, as expected. This

is shown in Fig. 4(d). Finally, in order to appreciate the effi-

ciency of the frequency conversion process, we plot the

FIG. 3. Wavelength conversion experiment at 10.098 GHz. The set-up (a)

entails a Mode-Locked Laser Diode, a band-pass filter at 1550 nm, and a

tunable CW laser diode set around k2 ¼ 1560 nm. The sample output is ei-

ther filtered at k2 and detected with a 40 GHz photodiode, or sent to the

Optical Spectrum Analyzer (OSA). Oscilloscope trace (b) of the detected

signal with average (soild) and (c) spectra of the modulated probe, measured

with the OSA revealing modulation at the MLLD frequency.

FIG. 4. Demonstration of all-optical down-conversion of microwave signals.

Experimental set-up including the modulator (a), electrical spectra (b) of the

detected optical signal after filtering the pump out; close up near the differ-

ence frequency 500 MHz, the MLLD 10.098 GHz, and the MZ driver fre-

quency 10.598 GHz. Corresponding trace at the oscilloscope of the signal at

500 MHz (c). Power, measured at the ESA, of the downconverted signal at

f– vs. the power level of the MZ modulator driver (d) and (e) the same

(including fþ), as a function of the MZI driver frequency. Here, the power

level is relative to the output signal at f¼ fMZ.

193510-3 Combri�e et al. Appl. Phys. Lett. 103, 193510 (2013)

electrical power at f6 relative to that of the signal modulated

at fMZ in Fig. 4(e), e.g., the power levels as shown in Fig.

4(b). The conversion of f– is spectrally flat up to 18 GHz

(�3 dB). This is consistent with the results in Fig. 3(c). We

can attempt a comparison by noting that, while our device is

slower than the EAM, still it is operated with a pump power

level 3 dBm only (to be compared with 12 dBm). Moreover,

its footprint is orders of magnitude smaller, enabling the

easy integration within a monolithic photonic circuit.

Besides, speed could be improved by known techniques for

accelerating the carrier dynamics.1 Also, we note that our de-

vice is particularly efficient when driven by a mode-locked

diode laser, which, by itself, is an excellent clock. This is

related to the switching mechanism, namely, the index

change following the nonlinear generation of free carriers,

which depends strongly on the peak to average power ratio.

Therefore, even if the average power level of the optical car-

rier is as large as that of the clock (namely, our experimental

conditions), the switching dynamics is only controlled by the

clock, while the response to the signal can still be regarded

as linear. This is what is indeed observed in Fig. 4(d). Thus,

the signal dynamics is maximized, which is a desirable fea-

ture of microwave (i.e., analog) signal processing.

In conclusion, we have designed, fabricated, and demon-

strated the all-optical switching operation of an optical gate

made of a Photonic Crystal Molecule. This device entails

two coupled cavities generating two spectrally well-

separated resonances. This configuration is very convenient

for practical all-optical switching, as it eliminates any issues

related to optical cross-talk between the optical control and

the signal. The switching contrast, measured by an optical

heterodyne technique, is more than 10 dB with a fast time

constant (12 ps). The maximum switching energy (providing

the maximum contrast) is 200 fJ. We demonstrate wave-

length conversion and all-optical mixing of a microwave sig-

nal using only 2 mW of optical power at the waveguide

input. The average power coupled in

This work has been supported by the 7th framework of

the European commission through the COPERNICUS pro-

ject (www.copernicusproject.eu). We thank Alexandre Shen

(Alcatel-Lucent-Thales III-V Lab) for making us available a

mode-locked diode laser, Fabrice Raineri and Jesper Moerk

for enlightening discussions, St�ephane Xavier for assistance

with the device fabrication, Aude Martin for helping with

measurements and Gr�egory Moille for help with thermal

modeling.

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193510-4 Combri�e et al. Appl. Phys. Lett. 103, 193510 (2013)