Development of an Optical Phase-Locked Loopfor 1-THz Optical Beat Signal Generation
by Takasaka Shigehiro*, Yasuyuki Ozeki*2, Shu Namiki*3, Misao Sakano*4 and Yu Mimura *To support larger telecommunications capacities, the speed of opticaltransmission is increasing apace, and coming transmission rates in excess of
160 Gbit/s raise hopes for all-optical signal processing technologies that make possible highspeed signal processing. Accordingly, as the clock signal source that is indispensable for signal
processing, we have developed an optical phase-locked loop (OPLL) that can generate a highspeed optical beat signal of 1 THz. The OPLL consists of an optical beat signal source and an
all-optical phase detector, and to realize a simple construction has a pair of 3-electrode
distributed feedback (DFB) laser diodes for the optical beat signal source and an all-opticalphase detector based on two-photon absorption in a silicon avalanche photodiode. As a resultof measurement of the synchronization characteristics with respect to a pulse train of 40-GHz
repetition rate, it has been confirmed that this OPLL can synchronize 160-GHz and 1-THz beatsignals with low timing jitter of 126 fs (0.016 rad 2) and 28 fs (0.03 rad 2) respectively.

ABSTRACT1.

INTRODUCTION

The technology for the extraction and recovery of theclock signal from the data signal is one of the essential
technologies for telecommunications systems. This isbecause the clock signal applies a timing reference, making possible data signal retiming at the relay point anddata confirmation at the receiving station to be carried out
accurately. In today’s optical telecommunications systems
optical signals are first converted by photodiodes into
electrical signals and clock recovery is accomplished byelectrical signal processing.
But optical transmission speeds are increasing to permit transmission of ever higher volumes of data, and at
the coming transmission speeds in excess of 160 Gbit/s,
electrical signal processing will not be feasible. To achieve
the all-optical signal processing required by higher-speed
operation will necessitate an optical clock signal, giving
great importance to techniques for optical clock signal
recovery.
Proposals for optical signal sources that will result intechnologies for generating an ultrafast optical clock signal in excess of 160 Gbit/s include a method using a
mode-locked laser diode 1) and a method for optical timedivision multiplexing a mode-locked laser at a low repeti-* FITEL Photonics Lab., R&D Div.
*2 PRESTO, Japan Science and Technology Agency (currently
with Osaka University)*3 FITEL Photonics Lab., R&D Div. (currently with National
Institute of Advanced Industrial Science and Technology)*4 Yokohama R&D Laboratory, R&D Division

tion rate 2). Each of these methods, however, presents
problems in terms of the need for sophisticated techniques for fabricating the laser diodes (LDs), the complexity of its structure and the large optical loss involved.
There is also, however, a technique whereby the opticalbeat signal is generated by interference in a continuous
combined signal of two continuous waves with different
wavelengths. This technique has the advantage of structural simplicity, plus the fact that it can generate an opticalbeat signal with a high repetition rate, not limited the
bandwidth of electrical devices. There are reports ofexperiments taking advantage of the features of this technology, that have applied optical pulse compression techniques using optical devices to generate an optical pulse
train with repetition rates as high as 1 THz 3)~5).
In order to use an optical beat signal as the clock signal
it is necessary that the beat signal be synchronized withrespect to the external optical signal. One method ofexternal synchronization of the optical beat signal uses
OPLL technology, but in currently used OPLLs the upper
limit of the frequency of the optical output beat signal isdetermined by bandwidth of its constituent elements--theelectrical phase detector and the photodiodes 6). On theother hand, as a phase detector method that is not limitedby the bandwidth of electrical devices band, there is an
all-optical phase detector, and it has been reported that a
10-GHz RF signal synchronized with an ultrafast opticalsignal can be generated using an all-optical phase detector 7). Convinced that incorporating this all-optical phasedetector into an OPLL would make possible the external
synchronization of ultrafast optical beat signals in excess
of 160 GHz, we have pursued this technological develop-

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Development of an Optical Phase-Locked Loop for 1-THz Optical Beat Signal Generation

2.

OPERATING PRINCIPLE OF OPLL

Figure 1 shows the operating principle of an OPLL using
an all-optical phase detector. Broadly speaking it has
three component elements: an optical beat signal sourcecomprising master and slave laser diodes; an all-opticalphase detector using a silicon avalanche photodiode (SiAPD); and a loop filter. Here the frequency of the opticalbeat signal source corresponds to the frequency difference between the continuous waves output by the two
LDs.
Synchronization of the optical beat signal with the external optical signal proceeds as follows: (1) The all-opticalphase detector detects the phase error that is the timing
difference between the optical beat signal and the external optical signal, and outputs a phase error signal. (2)The phase error signal is shaped by the loop filter, and,the output wavelength, that is to say the optical beat signal frequency, is controlled through feedback to the slave
LD, so that the value of the phase error signal becomeszero.
The operating principle of the all-optical phase detector
is as follows: (1) The optical beat signal and the external
optical signal, which are of corresponding polarization,
are combined and input to an Si-APD. (2) The Si-APD
generates a photocurrent in accordance with the amountof phase error, and the current is converted into a voltage
at a trans-impedance amplifier (TIA). (3) An offset voltage
unrelated to the phase error signal is subtracted from theoutput voltage of the TIA. The point to be noted here isthat since the Si-APD shows virtually no sensitivity to lightof 1.5 μm wavelength ranges, it generates a photocurrent

Optical path
Electrical pathAll-optical phase
Master LD
comparator
External
optical signal

Beat signal
sourceOptical beatsignal externallysynchronized

Optical beat signal

External optical signal

Si-APD: Silicon avalanche photodiode, TIA: Trans-impedance ampliﬁer.

Schematic of OPLL. Red solid and black dotted lines
are optical and electric paths, respectively.

t(a)Oﬀset current

τ0Timing error

Loop
ﬁlter
Slave LDSi‑APDtτ

Si‑APD
TIA

Figure 1

solely by means of the two photon absorption (TPA)
effect. In addition, since TPA is a non-resonant, non-linear
optical effect, it has a response speed of 100 THz or more
and for practical purposes places no limitation on the
speed of the input signal.
As Figure 2 shows, the fact that the photocurrent so
generated can be treated as a phase error signal is due tothe fact that the amount of photocurrent generated by
TPA is at the maximum when the optical beat signal andthe external optical signal are input simultaneously, anddecreases as the timing difference between two signals
becomes larger. This paper reports synchronization
experiments that were carried out with the midpoint
between the maximum and minimum values of photocurrent set as zero phase error. Note that the frequency ofthe photocurrent generation in the Si-APD generated by
the photocurrent is the same as the frequency of the optical beat signal, being greater than 160 GHz, but since thegenerated photocurrent is integrated by the electrical
capacitance of the Si-APD device and the TIA, the bandwidth of the output phase error signal is sufficiently narrow that it can be processed electrically.
To generate a synchronized optical beat signal of hightime accuracy requires a reduction of the timing jitter thatserves as its index. For this the OPLL loop bandwidth
must be amply large compared to LD linewidth 6). To bringthis situation about, we have used a 3-electrode DFB LD
optical beat signal source. The 3-electrode DFB LD maintains a narrow linewidth, while offering wideband FM
response characteristics. At the same time, since it is ineffect simply a 3-way split of the top electrode of an ordinary DFB LD, it can be fabricated using substantially thesame process as commercially available DFB LDs. Since
fabrication is easy, it is suitable for use as a slave LD 11).
Prototype 3-electrode DFB LDs were fabricated that had anarrow linewidth of 250 kHz and an FM response bandwidth of 100 MHz or more.

Integrated value ofphoto current

ment 8), 9).
In this paper we demonstrate that external synchronization of an ultrafast optical beat signal is possible bymeans of OPLL technology using an all-optical phasedetector. First we present the operating principle anddesign guidelines, and then report experiments conducted for external synchronization of a 160-GHz optical beatsignal 8), 9). In addition, to verify the ultrafast performance of
this OPLL, we conducted experiments for external synchronization of a 1-THz optical beat signal 10). Throughthese experimental results it has been demonstrated that
this OPLL technology makes possible the generation ofultrafast optical clock signals.

(b)

Figure 2

Operation principle of all-optical phase detector
using a Si-APD. (a): Definition of timing error τ
between optical beat signal and external optical signal, (b): An example of integrated photocurrent as afunction of timing error τ.

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Development of an Optical Phase-Locked Loop for 1-THz Optical Beat Signal Generation

As shown in Figure 3, 3-electrode DFB LD drive current
is effected by connecting a constant-current source toeach of the electrodes and controlling the drive current sothat the linewidth is reduced the minimum. Modulation of
output wavelength is accomplished by a modulation signal stacked on the center electrode. Note also that due tothe narrow linewidth of this DFB LD, it was also used as
the master LD.
If the linewidth of the LD used as the optical beat signal
source is determined, so is the loop bandwidth required
to reduce timing jitter. In practice, when an LD with a linewidth of 250 kHz is used as the optical beat signal source,a loop bandwidth in excess of 10 MHz is necessary inorder to realize a low timing jitter. This loop bandwidth is
three orders of magnitude greater that the 6-kHz loop
bandwidth of a PLL using a Si-APD in Reference 7.
Expanding the loop bandwidth requires that the bandwidth of the TIA be expanded, but at the same time the
wideband thermal noise is stacked on the phase error
signal, so the more the bandwidth of the TIA is expanded
the greater will be the degradation of the signal-to-noise
ratio. Thus in order to expand the bandwidth while maintaining the SNR needed by the synchronization operation,
we have adopted a number of approaches to increasing
the average power of the optical input signal and raising
the efficiency of TPA, such as reducing the spot size ofConstant current source

Vcc
Modulation
signalVcc

Constant current source

Vcc

Constant current
source3-electrode DFB LD

Figure 3

Wiring diagram of three-electrode DFB LD.

External
optical signal

PC

Master-LD

PC

3.

SYNCHRONIZATION EXPERIMENTS

3.1

External Synchronization of 160-GHz OpticalBeat Signal
Using this OPLL we conducted experiments for external
synchronization of a 160-GHz optical beat signal to showthat this is possible, and to confirm the amount of timing
jitter of the optical beat signal 9).
As the external optical signal we used an optical pulse
train with a center wavelength of 1540 nm, a repetition
rate of 40 GHz and a pulse width of 2 ps 13). The pulse
width of this external optical signal matches the pulse
width that was used for the 160-Gbit/s optical signal. In
order to generate a 160-GHz optical beat signal, thewavelengths of the master and slave LDs were set at1557.7 and 1559.0 nm respectively. The input power in
the Si-APD was a reference optical signal of 200 mW,
master LD: 50 mW, and slave LD: 3 mW. TIA impedance
was 5 kΩ, and the bandwidth was set at around 80 MHz.
We used a lag-lead type loop filter, and loop bandwidth
was set to ~ 20 MHz.
To confirm the synchronization operation of the OPLL,
we measured the cross-correlation of the optical beat signal and the external optical signal. Figure 6 shows theconfiguration of the measurement system. To detect thecross-correlation between the 160-GHz optical beat signal
and the external optical signal we used an Si-APD. Herethe 160-GHz optical beat signal was extracted from the
OPLL optical output using a 2-stage optical filter. The rela-

EDFA
Collimator
lens
Beam
splitterSlave-LD

oﬀset
Loop

the optical input signal in the Si-APD to 4.3 μm.
In addition, since the loop bandwidth is wide, loop delay
time is a major factor limiting loop bandwidth and phasemargin 6), 12). Here loop delay time is that time taken for thephase error signal to propagate around the loop within the
OPLL. In this OPLL the elements comprising the loop surrounded by the broken line in Figure 4 have been fabricated using free-space optics into a module measuring
145×44×110 mm. Figure 5 shows the outward appearance of the OPLL module. The delay time for this module
is approximately 1 ns, and for loop bandwidths of 100 MHz
or less, loop delay time can be disregarded.

Output
Focus
lens

TIA

ﬁlter

OPLL module

Si-APD

TIA: trans-impedance ampliﬁer,
EDFA: erbium-doped ﬁber ampliﬁer,
PC: polarization controller.
Figure 4

Configuration of OPLL.

Figure 5

Appearance of OPLL module.

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Development of an Optical Phase-Locked Loop for 1-THz Optical Beat Signal Generation

loop delay time of 5 ns 8). The timing jitter of the 160-GHz
optical beat signal output by this OPLL was 291 fs. By thismeans it was confirmed that to achieve low timing jitter inan OPLL with a wide loop bandwidth it is important toshorten the delay time.
In this way it has been demonstrated that a 160-GHz
optical beat signal can be synchronized to an external
optical signal by means of an OPLL, and that a low timing
jitter of 126 fs can be achieved.
3.2

External Synchronization of 1-THz Optical Beat
SignalSince the frequency of an optical beat signal is determined by the wavelength difference between two continuous waves, it is considered that there is no upper limit to
the frequency, and that the response speed of TPA in a
Si-APD is 100 THz or more. This holds out the hope that
this OPLL can easily synchronize optical beat signals anorder of magnitude higher in frequency than 160 GHz.
Accordingly, to confirm the ultrafast performance of thisOPLL, external synchronization experiments were conducted on 1-THz optical beat signals 10).
Since the experimental configuration is basically thesame as that shown in Figure 4, we set forth below theFree runningπPhase error（rad.）tive time of the optical beat signal and the external opticalsignal is swept by means of the optical delay line. As
Figure 7 shows, a sinewave of 6.25-ps period can beclearly perceived, demonstrating that the optical beat signal is synchronized with the external optical signal.
To measure the phase noise of the optical beat signal,
we measured the OPLL output signal with a phase error
detector using an Si-APD and TIA, and an oscilloscope
and RF spectrum analyzer. Figure 8 shows the phaseerror signal waveforms measured in the 160-GHz opticalbeat signal. The synchronization operation was also confirmed from the fact that when synchronization was not in
effect the phase error signal spread in a random manner,
whereas when synchronization was applied phase error
converged to zero. Figure 9 shows the phase noise spectrum of the 160-GHz optical beat signal. This is the power
spectrum of the phase error signal measured using an RF
spectrum analyzer, normalized to the power of the phaseerror signal under free running. Note that the power usedin normalizing was calculated based on the amplitude of
the phase error signal as measured using the oscilloscope. The synchronization operation was also confirmed
from the fact that the phase noise spectrum showed adecrease in phase noise in the frequency range below 10
MHz with respect to the spectrum under free running as
calculated from DFB LD linewidth. The amount of phasenoise can be obtained by integrating the phase noise
spectrum and multiplying by a constant. By subtracting
the value for background noise from the value under synchronized operation, we can find the amount of phasenoise that is added by the OPLL. As a result we determined that the variance and the timing jitter for the phaseerror signal were 0.016 rad 2 and 126 fs, respectively.
Meanwhile, prior to fabrication of the OPLL module, we
conducted, as a preparatory measure, a 160-GHz opticalbeat synchronization experiment using an OPLL with a

π
0

0
‑π
π

0

20

40

60

80

Under OPLL control

100

π
0

0
‑π

0

20

40

60

80

100

Time（µs）

BPF

BPF

Figure 8

160-GHz
Optical beat
Si‑APD

OPLL

Oscilloscope
EDFA

TIA

TIA: trans-impedance ampliﬁer, EDFA: erbium-doped ﬁber ampliﬁer,
3 電極 DFB-LD
BPF: band-pass ﬁlter, LPF: low-pass ﬁlter

Configuration of a cross-correlation measurement
system. Red and black lines are optical and electric
paths, respectively.

Cross-correlation（arb. units）

Figure 6

Estimated curve under free‑running‑60
Measured spectrum
under the OPLL control

‑80

‑100

3 電極 DFB-LD

Noise of the measurement system

10k

100k

1M

10M

100M

Frequency（Hz）
Delay time（6.25 ps/div.）

Figure 7

Power spectral density（dBc/Hz）

Optical pulse
source‑πFrequency（arb. units）

Waveforms of phase error signal in 160-GHz opticalbeat signal (left hand side) and its frequency graphs(right hand side). Black and gray lines are measured
signal and background noise, respectively.

LPF

40 GHz, 2 ps‑π

Waveform of cross-correlation between 160-GHz
optical beat signal and 40-GHz optical pulse train.

Figure 9

Phase noise spectrum of 160-GHz optical beat signal.
Black line: the spectrum under OPLL control, grayline: background noise, and dotted line: estimatedline under free running.

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Development of an Optical Phase-Locked Loop for 1-THz Optical Beat Signal Generation

tem shown in Figure 6, with an optical delay line fixed sothat the average value of the phase error signal becomesthe center value of the cross-correlation waveform in
Figure 11. In addition to this, the low pass filter (LPF) was
detached to achieve accurate waveform measurement.
By means of these changes it was possible to measure
the cross-correlation signal as a phase noise signal.
Figure 12 shows the result of measuring the phase error
signal with a an oscilloscope. The phase error signal,
which had random distribution when not synchronized,
converged to a phase error value of zero during synchronized operation, and its broadening was suppressed to alevel approaching background noise. From this result too,
it was possible to confirm synchronization of the 1-THz
optical beat signal. Next the phase error signal was measured using an RF spectrum analyzer, and the phasenoise spectrum obtained is shown in Figure 13. As in theexperiments described earlier, by calculating the variancefor phase error and the corresponding timing jitter, it was
found that they are 0.03 rad 2 and 28 fs respectively. Thus
it is clear that a 1-THz optical beat signal can be synchronized with a small timing jitter.
The above discussion demonstrates that this OPLL possesses the ultrafast performance to enable synchronization of even such high-speed optical signals as a 1-THz
optical beat signal.

Cross-correlation（arb. units）

points that differ from the 160-GHz optical beat signal
synchronization experiments. First of all, to generate a 1THz optical beat signal we replaced only the master LD
used in the experimental system in Section 3.1 above,and the master and slave LDs were set at 1566.3 and1558.2 nm respectively. Next, we used as the external
optical signal a 40-GHz repetition rate 500-fs optical pulse
train with a center wavelength of 1540 nm and compressed pulse width 13). The pulse width of this external
optical signal corresponds to the pulse width of the optical signal used for the 1-Tbit/s signal. The power of thevarious signals in the Si-APD--the external optical signal,
master LD and slave LD--were set at 195 mW, 70.8 mW
and 4.6 mW respectively, virtually the same values as in
the experiments in Section 3.1 above. As a result of thesettings described above, the optical spectrum in Figure
10 was obtained at the Si-APD input. Note that since thepulse width of the external optical signal had become
shorter, it became more subject to the influence of broadening pulse width due to optical fiber dispersion. Since
the amplitude of the phase error signal becomes smaller
as pulse width broadens, in these experiments we inserted an external optical fiber to compensate for the dispersion between the optical signal source and the Si-APD.
The length of the inserted optical fiber is adjusted so thatthe amplitude of the phase error signal was maximized.
To confirm synchronization of the 1-THz optical beatsignal with the external optical signal, we measured thecross-correlation waveform between the two signals.
Without synchronization there is no relationship, but withsynchronization a correlation between the two signals
appears, so we anticipated being able to measure a
waveform of 1-ps period. The configuration of the crosscorrelation measurement system was like that shown in
Figure 6, but we made re-adjustments so that the wavelength of the optical filter was optimum for these experiments. Figure 11 shows the cross-correlation waveformthat we measured. A waveform with a period of 1 ps wasobserved, confirming that the 1-THz optical beat signal
was synchronized with the external optical signal.
Next we measured the timing jitter of the 1-THz opticalbeat signal. For the purpose of timing jitter measurement,
we used the cross-correlation waveform measuring sys-001Phase error（rad）

1-THz optical beat signal
master LD

Spectrum（dBm）

External optical signal（40-GHz, 500-fs pulse train） slave LD

0

‑20

‑40

1540

1550

1560

1570

6

Free running

0

‑π
π

Under OPLL control

0

‑π

1530

5

Figure 11 Cross-correlation signal between 1-THz optical beatsignal and the external optical signal.

π

20

2
3
4
Delay time（ps）

20

40

60

80

100

Time（µs）

Wavelength（nm）

Figure 10 Optical spectrum at Si-APD input port.

Figure 12 Phase error signals of 1-THz optical beat signal(black line). Gray line is background noise.

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Development of an Optical Phase-Locked Loop for 1-THz Optical Beat Signal Generation

Estimated line under free runningPower spectral density（dBc/Hz）‑60

Measured line under OPLL control

‑70
‑80
‑90
‑100
Background noise

‑110
10k

100k

1M

10M

100M

Frequency（Hz）

Figure 13 Phase noise spectrum of 1-THz optical beat signal.

4.

CONCLUSION

We have demonstrated that an OPLL comprising an optical beat signal source using a 3-electrode DFB LD and an
all-optical phase detector using a Si-APD can synchronizea 160-GHz optical beat signal of 40-GHz repetition rateand 2-ps pulse train with a timing jitter of 126 fs, and a 1THz optical beat signal of 40-GHz repetition rate and 0.5ps pulse train with a timing jitter of 28 fs. We may therefore say that this OPLL has outstanding characteristics as
an ultrafast optical clock signal source capable of copingwith future increases in transmission speeds. In future wewill add such improvements as will render this OPLL aclock signal source that will support ultrafast all-opticalsignal processing.

ACKNOWLEDGMENT
This research is the product of a joint research effort
with the Japan Science and Technology Agency.

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