APPLIED PHYSICS LETTERSVOLUME 79, NUMBER 16

15 OCTOBER 2001

5-ps, 10-GHz pulse generation from an all-optical semiconductor switch
embedded in a ring cavity
Yoshiyasu Ueno,a) Shigeru Nakamura, and Kazuhito Tajima
Photonic and Wireless Devices Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba,Ibaraki 305-8501, Japan

共Received 21 May 2001; accepted for publication 21 August 2001兲
We observed spontaneous generation of nearly transform-limited pulses 共5 ps, 10 GHz, 1558 nm兲
from a semiconductor ring oscillator. This oscillator is completely different from conventionalmode-locked lasers in that the pulse width is passively determined by a split delay time inside its
ring cavity. This is because the ring cavity includes an all-optical semiconductor switch whose
window width is determined by the split delay time. © 2001 American Institute of Physics.
关DOI: 10.1063/1.1410876兴

All-optical devices based on optical nonlinearities haverecently begun to attract attention for their ability to push up
the signal-processing speed limit, which has up until now
been limited by transistors, optoelectronic devices, and soforth. Since the mid-1990’s, the amount of research carriedout on all-optical devices based on the nonlinearity of semiconductor optical amplifiers 共SOAs兲 has increased substantially, and there are three reasons for this. The first reason isthat their speeds are much faster than they had previously
been believed to be: the speeds of SOA-based all-opticaldevices with specific interferometric structures 共optical circuits兲 are not limited by the carrier’s relaxation time, whichranges from 100 ps to 10 ns. Optical interference inside thedevices masks the relaxation time.1–5 In fact, the speed of
all-optical logic operation now exceeds 160 GHz.6 – 8 The
second reason is that their nonlinearities are extremely large
due to resonant enhancement and the pulse-amplificationinduced recombination of current-injected electron–holepairs: an optical input pulse with 10–100 fJ energy can generate a nonlinear phase shift that is sufficient for all-opticalswitching.5–9 The third reason is that SOAs have inherent
advantages in large-scale low-cost integration, which is usedin commercial applications such as ultrabroadband photonic
network systems. The SOA chips can be integrated bothmonolithically10 and in hybrid manners on silica-based planar lightwave circuits5,11 that are formed onto large siliconwafers.
A SOA can also be used as a nonlinear element for highrepetition mode-locked semiconductor ring lasers.12–14 Suchpulse lasers can be fabricated and integrated using technology similar to the above-mentioned all-optical devices,whereas other GHz repetition sources such as mode-locked
fiber lasers and mode-locked semiconductor lasers cannot.
The pulse widths of SOA-based mode-locked ring lasers aredetermined by the magnitude of the optical nonlinearity, justas they are for other mode-locked lasers.
We recently proposed a semiconductor ring oscillator
关all-optically switched loop oscillator 共ASLOS兲兴 and predicted that it would be able to spontaneously generate shortpulses.15 This oscillator is completely different from convena兲

Electronic mail: y.ueno@ieee.org

tional mode-locked lasers in that the pulse width is passively
determined by a split delay time inside the oscillator’s ringcavity. This is because the cavity includes a SOA-based alloptical switch for which the switch-window width is passively determined by the split delay time. In this study, wewere able to observe pulses generated by the SOA-based ringoscillator.
Figure 1 shows a schematic view of the SOA-based ringoscillator. This oscillator consists of a ring cavity and an
external continuous-wave 共cw兲 laser source. The ring cavity
consists of an all-optical semiconductor switch, an energydistribution 共ED兲 Mach–Zehnder interferometer 共MZI兲, atunable delay, a variable attenuator, a polarization controller,
and an Er-doped fiber amplifier 共EDFA兲 with a 28 dB gain.
The ED MZI is an asymmetric MZI made of a customized
silica planar lightwave circuit with a fixed delay time ⌬T R ofapproximately 100 ps 共10 GHz兲 and a 50:50 split ratio.
The all-optical switch’s structure is like that of the symmetric Mach–Zehnder-type 共SMZ兲 delayed-interference
signal-wavelength convertor 共DISC兲.4,6 –9 The switch consists of a polarization-insensitive bulk-active-layer SOA, an
isolator, a MZI, and a 2.4 nm bandpass filter. The MZI in theDISC is also an asymmetric MZI, which consists of a 10.5mm-long birefringent calcite crystal, a Babinet–Soleil phaseadjustor, and two polarizers. The delay time of the MZI, thatFIG. 1. Experimental setup for the semiconductor ring oscillator.

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Appl. Phys. Lett., Vol. 79, No. 16, 15 October 2001

Ueno, Nakamura, and Tajima

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FIG. 2. Optical spectrum 共a兲 and rf spectrum 共b兲 observed at the oscillator
output after tuning the ring cavity.
FIG. 3. Typical autocorrelation trace 共a兲 and optical spectrum 共b兲 of theoutput pulses.

is, the birefringency of the calcite crystal (⌬t⫽6 ps), is used
to determine the width of the all-optical switch’s switch
window,4 and the filter blocks some of the amplified spontaneous emissions 共ASEs兲 from the SOA.
The ring cavity was built with fiber-pigtailed components 共a SOA, a filter, an ED MZI, an attenuator, and a polarization controller兲 and lens-coupled components 共DISCMZI兲. We inserted the EDFA to compensate for the large
total loss of the ring cavity 共26 dB兲 caused by the number of
optical couplings. The pulse’s round-trip time around thering was 125 ns 共8 MHz兲.
When the ring cavity is properly tuned, pulses start circulating as follows. Suppose a pulse is amplified by the SOA
inside the all-optical switch. The pulse all-optically modulates the optical phase of the copropagating cw light due tothe bandfilling effect. The phase of the cw light rises andrelaxes, and the rise time is of the order of the pulse width,
while the relaxation time is longer due to the carrier’s lifetime. At the SMZ-DISC output, only a ⌬t-long cw component survives.4 In the meantime, the input pulse polarizationis adjusted so that the amplified pulse at the SOA output isblocked by the polarizer before the calcite. As a result, a newpulse is generated at the all-optical switch’s output, whichthen circulates around the ring, returns to the SOA input, andthen modulates the phase of the cw light again. Thus, thepulse width is passively determined by the DISC-MZI’s delay time (⌬t), and can be shorter than the SOA’s carrier
lifetime, as in the case of the SMZ DISCs discussed in previous reports.4,6 –9 On the other hand, the pulse repetition
frequency is also passively determined by the ED MZI.15
This is because the ED MZI splits each pulse into two⌬T R -spaced pulses.

It should be noted that this pulse source is not a laser, butan oscillator. This is because each pulse is replaced with anew pulse after one round trip, as mentioned above. The newpulse is generated from a cw seed light through an incoherent nonlinear process, that is, a nonlinear refractive index
change due to the pulse-amplification-induced carrier-densitychange in the SOA.
We experimentally adjusted the ring oscillator shown inFig. 1 as follows. First, we observed 1558.4 nm cw lasing at
the output of the 10% output coupler by injecting a 250 mA
direct current into the SOA and a 0 dBm 1558.4 nm cw lightinto the 50:50 input coupler. We then adjusted the phase biasof the DISC MZI so that the cw component would destructively interfere with each other, and also optimized the ring’s
polarization controller in order to prevent cw lasing. Afterthe MZI bias and the polarization controller were optimized,
we observed strong ASE but no lasing at the cavity output.
We also observed a series of 8-MHz-spaced rf components
from a photodetector placed at the oscillator output.
Finally, we adjusted the round-trip time accurately withthe variable time delay. Once the delay was optimized, theoutput optical spectrum changed into a series of 10-GHzspaced discrete components 关Fig. 2共a兲兴. On the other hand,
one of the 8-MHz-spaced ring-cavity components rose up
关Fig. 2共b兲兴. These two changes occurred simultaneously anddramatically. These spectra indicated that the pulses started
circulating around the ring after one of the harmonics of thecavity frequency 共⌬8 MHz兲 precisely matched the ED MZI’sfrequency 共10.585 GHz兲. It should be noted that the two dips
observed at 1557.0 and 1559.5 nm, as shown in Fig. 2共a兲,Downloaded 17 Oct 2001 to 202.247.6.220. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

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Appl. Phys. Lett., Vol. 79, No. 16, 15 October 2001

were caused by destructive interference at the DISC-MZIoutput. The shape of the spectral envelope was thus determined by the DISC MZI, just as the pulse width was determined.
Figure 3 shows a typical autocorrelation trace 共a兲 and atypical optical spectrum 共b兲 of the 10.585 GHz output pulses
without dispersion compensation. The pulse width was estimated to be 5 ps, assuming a sech pulse shape. The limited
extinction ratio shown in Fig. 3共a兲 was attributed to the limited cavity-mode suppression ratio given in Fig. 2共b兲. We
believe that both the pulse extinction and cavity-mode suppression ratios could be improved by stabilizing or integrating the ring cavity. This is because the present cavity wasbuilt with non-polarization-maintaining fibers that had a total
length of 25 m, which are easily affected by environmental
vibration and temperature fluctuations.
As shown in Fig. 3共b兲, the width of the output spectrumenvelope was 0.42 nm. The dashed curve shows the calculated spectrum of a 6 ps sech pulse. These results indicatethat the output pulses are close to a transform limit, as theoretically discussed in detail for the DISC’s wavelengthconverted pulses given in Ref. 16.
In summary, we observed spontaneous generation of
5-ps, 10-GHz, 1558-nm pulses from a semiconductor ringoscillator 关all-optically switched loop oscillator 共ASLOS兲兴
that included a SOA-based all-optical switch. The output
pulse width of 5 ps was determined not by the magnitude ofthe SOA’s nonlinearity, but by the width 共6 ps兲 of the alloptical switch window. More specifically, the pulse width
was passively determined by an asymmetric MZI split delay
time inside an all-optical switch.

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