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OPTICS LETTERS / Vol. 23, No. 23 / December 1, 1998

Record low-power all-optical semiconductor switch operation
at ultrafast repetition rates above the carrier cutoff frequency
Yoshiyasu Ueno, Shigeru Nakamura, and Kazuhito Tajima
Optoelectronics and High-Frequency Device Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan
Received July 7, 1998
The switch repetition-rate limit of all-optical semiconductor switches, for which the dynamic extinction ratio
decreases with increasing repetition rate, is examined and overcome. The extinction ratio is improved by
optimization of the interferometer phase bias, which has previously been set to p. The extinction-ratio
increase accompanies a drastic change in the output pulse spectrum. Both the output pulse prof iles andthe spectra, before and after optimization, are successfully reproduced by simulation. As a result, switch
repetition of 42 GHz (2.5 times higher than the semiconductor-carrier cutoff frequency) is achieved by use of5-ps, 1548-nm pump pulses. What are believed to be record low input pump-pulse energies of 750 aJ (peakpower, 140 mW) at 10 GHz and 6 fJ (1.1 mW) at 42 GHz are used.  1998 Optical Society of America
OCIS codes: 130.5990, 290.2200, 320.0320.

Ultrafast optical switches controlled by light pulses,that is, all-optical switches,1 have been a major application target for nonlinear optics since the early 1960’s
and are intended for use in information processing thatwill be faster than that under the electronics limit. In
the early 1990’s two similar semiconductor switches2 – 4
based on the band-f illing effect near resonance energy
decreased pump-light power to a practical level. The
semiconductor carrier lifetime of each switch, which
was believed to limit its fall time, was masked by a combination of a time delay and an interferometer incorporated into the switch structure. Consequently, the falltime as well as the rise time of the switch was determined by the pump-pulse width. In fact, switch windows as narrow as 800 fs were demonstrated,5 whichsuggested that these switches could be used at bit rates
of more than 1 terabitys.
The repetition rate of these switches is an issue
independent of the rise and fall times. Tajima
and co-workers suggested that the carrier lifetime
should not limit the repetition rate of switches
incorporating
absorptive
semiconductor
waveguides.2 For switches with semiconductor opticalamplifiers (SOA’s), Manning et al. also suggested
that the carrier lifetime should not limit the repetition rate when the carrier injection rate isincreased to suppress the carrier-density saturation.6 In fact, switch repetition of 40 to 100 GHz
was demonstrated for such switches incorporating
absorptive waveguides7 and for those with SOA’s.8 – 10In Ref. 7, however, four consecutive 25-ps-spaced
pulses instead of 40-GHz pulses were used as thepump pulses (carrier lifetime, 600 ps). In Refs. 8–10
the carrier lifetimes in the SOA’s were not described.
Thus previous experimental studies are of limited
usefulness with regard to the repetition-rate limit.
The pump-pulse average power required for theseswitches basically increases in proportion to the repetition rate. The required energy for absorption-type
switches is 4–7 pJ,2 which corresponds to a 160–
280-mW average power if the switches are operated at0146-9592/98/231846-03$15.00/0

40 GHz. When SOA’s are used in place of the absorptive semiconductor waveguides, the pump-pulse energy
is reduced owing to the stimulated amplification of thepump pulse in the SOA’s to 100–300 fJ.6,8
In this Letter it is reported that the repetition rateof all-optical semiconductor switches is limited by theratio of the switch-window width to the repetition
period. We have found that this limit can be overcome
by optimization of one of the operating conditions (thephase bias). It is also reported that this optimization
helps to reduce the input pump-pulse energy.
Figure 1 shows the all-optical switch studied
here.11 Input pump pulses switch a cw light on andoff through the switch. The mechanism that ensuresthe ultrafast rise time of this switch is the same as thatof the switches studied in Refs. 2–4: Each time theSOA amplifies a l1 pump pulse at t0 , the SOA modulates the phase of the copropagating l2 signal lightowing to the band-f illing-induced nonlinear refractiveindex change. Although the rise time of the phasechange is determined by the pump-pulse width stp d,
its recovery is determined by the carrier lifetime stc d.
The SOA also modulates the signal-light amplitude.
After the SOA, the signal is split into two components,
A and B, in the split delay. The B component [dashedcurve in Fig. 2(a)] is delayed by Dt relative to the A
component (solid curve). [Details of the calculation
and Fig. 2(b) are described below.] After the split

Fig. 1. Experimental setup: Each input pump pulseswitches the signal light owing to the band-f illing effect inthe SOA. The switch-window width is determined by thedelay time Dt of the split delay. Q, quarter-wave plate; P,polarizer; F, spectral filter; H’s, half-wave plates.
 1998 Optical Society of America

December 1, 1998 / Vol. 23, No. 23 / OPTICS LETTERS

Fig. 2. Calculated phase shifts for the two split signal
components. Solid curves, fast component; dashed curves,slow component. (a) fR ­ 4.2 GHz, tc ­ 60 ps; ( b) fR ­42 GHz, tc ­ 60 ps.

delay, the two components reach the switch output
port through a quarter-wave plate and a polarizer.
The quarter-wave plate is tuned so that in the absence of pump pulses the two components interfere
destructively after the polarizer. This tuning ensuresthat the A-component phase is biased by 1p relative
to the B component. As a result the pump pulse isbelieved to switch on the signal light at t0 and switch
it off at t0 1 Dt, because the phase difference remains
1p before t0 and after t0 1 Dt [Fig. 2(a)]. The pumppulse forms a switch window from t0 through t0 1 Dt(the switch window is square if tp ,, Dt; see Ref. 2).
This switch is useful for experiments because of its
structural simplicity and stability.
For the experiments we used a SOA with a 0.2-mmthick, 1.2-mm-wide, 800-mm-long InGaAsP bulk activelayer; a 12-mm-long calcite crystal sDt ­ 6.8 psd; 5.0-ps,1548-nm, 10.5-GHz mode-locked pulses emitted froma fiber-ring laser (Soliton Source, Pritel, Inc.); an optical multiplexer (from 10.5 to 42 GHz); and 5-mW
1560-nm cw light from a single-mode semiconductor
laser. The internal unsaturated gain of the SOA at
1548 and 1560 nm was 700 for a 200-mA current injection. The pulse gain-saturation energy and carrier
lifetime were measured to be 180 fJ and 60 ps, respectively, in separate experiments with 4.8-ps, 82-MHz
mode-locked pulses. This carrier lifetime indicatesthat the SOA’s cutoff frequency s1ytc d was 16.7 GHz.
The mode-locked pulses were amplif ied by an Er-doped
fiber amplifier before pumping of the all-optical switch.
The output pulses were amplified by another Er-doped
fiber amplif ier before being detected.
At a 10.5-GHz repetition rate, our switch generated
clean 7-ps, 1560-nm output pulses. However, when
the repetition rate was increased from 10.5 to 42 GHz(2.5 times higher than the cutoff frequency), the output
pulse extinction ratio significantly decreased [solidcurve in Fig. 3(a)]. The input pulse energy was 6.0 fJ(peak power, 1.1 mW).
We also found that optimizing the phase bias byslightly rotating the quarter-wave plate drastically
improved the extinction ratio at 42 GHz from 2 to 12[solid curve in Fig. 3(c)]. A signif icant change in theoutput spectrum was also observed [solid curves inFigs. 3(b) and 3(d)].
To clarify the mechanism behind these phenomena
we performed a simulation, using a simple model. The

1847

nonlinear refractive-index change and gain coefficientswere assumed to be proportional to the carrier density.
We took into account the carrier saturation to accountfor both the pulse gain saturation and the carrier cutoff frequency. We determined the band-f illing coeff icients at 1548 and 1560 nm by fitting the calculated
self-phase-modulated spectrum of the amplif ied pumppulse (Fig. 4) and the cross-phase-modulated spectrumof the signal light to those spectra measured understrong pump conditions. No other fitting parameters
were used.
The calculated autocorrelation trace and the spectrum before the optimization matched the measured
ones very well [Figs. 3(a) and 3(b)]. According to thesimulation, the extinction-ratio decrease at 42 GHz
originated from the phase difference between the A andthe B components before t0 and after t0 1 Dt, which decreased from approximately p at 10 GHz to 0.90 p at
42 GHz. This phase-difference drift from p can be approximately written asFAstd 2 FB std ­ Fbias 2

Dt
DFNL ,tR(1)Fig. 3. Measured (solid) and calculated (dashed) autocorrelation traces and spectra for the 42-GHz output pulses(a), ( b) before and (c), (d) after optimization. The dottedcurves in ( b) and (d ) are calculated spectra for a transformlimited 4-ps pulse for comparison.

Fig. 4. Measured (solid) and calculated (dashed) spectraof the amplified pump pulses. The input pulse energy
was 340 fJ. The nonlinear phase shift was estimatedto be 2.0 p.

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OPTICS LETTERS / Vol. 23, No. 23 / December 1, 1998

Fig. 5. Streak-camera traces of the 10-GHz output pulses.
The intensity scale was carefully calibrated so that the138-dB gain of the post-Er-doped fiber amplifier was taken
into account.

because the phase recovers almost linearly in time
[Fig. 2(b)] when the repetition rate ( fR ­ 1ytR , wheretR is the repetition period) is larger than the carrier
cutoff frequency s1ytc d considered here. Equation (1)suggests that the switch repetition rate is limitedby the factor DtytR , under the previous condition,Fbias ­ p.
Both the measured and the calculated spectra inFig. 3(b) basically consisted of 42-GHz (0.34-nm)
spaced peaks but lacked the 1560.2-nm center component. We explain this by considering the Fouriercomponents of the output field amplitude:
f sDvd ­Z

fE0 stdexpsiFbias d 1 E0 st 2 Dtdg

3 exps2iDvtddt .

transmittance was 8, which matched fairly wellwith the measured transmittance. When the input
pulse energy increased from 6 fJ, the transmittancedecreased both experimentally and theoretically because the gain decreased faster than the phase shift
increased. The nonlinear phase shift was estimatedto be 0.20 p.
The phase-bias optimization also helped to reducethe input pump-pulse energy for switching at 10 GHz.
The lowest input pulse energy at which an extinction
ratio greater than 10 was obtained was 750 aJ (Fig. 5),
in which the peak power was 140 mW s20 kWycm2 d.
The continuous noise component originated from theSOA’s amplified spontaneous emission. The nonlinear phase shift was 0.30 p. These values led
to a effective nonlinear index of refraction sn2 eff d of
1.3 3 1028 cm2 yW [x s3d of 3.5 3 1026 esu], which is several orders of magnitude larger than those of reported
x s3d - or x s2d -cascading materials used or proposed foruse in ultrafast all-optical switches.1
In summary, after considering the origin of therepetition-rate limit for all-optical semiconductorswitches, we have shown how this limit can be overcome. As a result, ultrafast switching operation at
10–42 GHz with record low input pump-pulse energies
was demonstrated.
This work was performed partially under the management of the Femtosecond Technology Research Association supported by the New Energy and Industrial
Technology Development Organization. Y. Ueno’s
e-mail address is ueno@obl.cl.nec.co.jp.
References(2)

Equation (2) shows that f sDv ­ 0d should vanish when
Fbias is set to p.
Equation (1) suggests that one can set the phasedifference FAstd 2 FB std close to p by optimizing thephase bias as Fbias ­ p 1 sDtytR dDFNL (­ 1.10 p
for the above experiment). In fact, the calculated
autocorrelation trace and spectrum with the optimized
phase bias reproduced the measured ones successfully[Figs. 3(c) and 3(d)]. The above suggestion should
be valid for overcoming the repetition-rate limit of
all switches similar to those in Refs. 2–4, including
absorption-type switches.
In the spectrum, the center component was generated. The spectrum envelope became similar in shape
to the calculated spectrum of a 4-ps transform-limited
pulse [dashed curves in Figs. 3(b) and 3(d)], in amanner similar to mode-locked pulse spectra. This
spectrum change should be helpful for tracking theoptimum phase bias of all-optical switches for longterm operation (regardless of the repetition rate) as
well as for optimizing the phase bias.
The switch transmittance for the 5-mW signal,
measured with a calibrated streak camera, was 16(owing to amplif ication) at 42 GHz. The calculated

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