CFM1-4Threshold condition for pulse generationfrom a DISC-loop-type pulse generator
Rei Suzuki, Satoshi Kobayashi, Jun Sakaguchi, and Yoshiyasu Ueno
Univ. of Electro-Communications, Dept. of Electronic Engineering, Japansuzuki@ultrafast.ee.uec.ac.jp
The optical phase bias between the fast andslow CW components passing through the MZIwas tuned by rotating the Q and P located after
the calcite crystal. The polarization direction ofthe CW light (λ = 1554.92nm, +0.5 dBm) fromthe DFB-LD to the SOA input facet was alignedto either the TE or TM axis of the SOA. The
polarization of the pulse after one round trip was
made vertical to the polarization state of the CW
light to prevent laser oscillation. The DISC
performed polarization conversion of the pulse sothat the pulse could continue to go around.

Abstract
We improved the quality of 5-ps, 10-GHz
pulses from a DISC-loop-type pulse generator.
Separately, we observed that the pulse power
drastically increased when the pulse loop gain
reached the threshold gain.
Introduction
Ultrashort pulses with repetition rates as highas 40 GHz are needed for the ultrafast optical
time-division multiplexed (OTDM) systems
which will operate at 160 Gb/s in the near future.
To generate such pulses, various pulse sources,
such as mode-locked-lasers, have been studied.
Recently, 3.2-ps, 50-GHz and 11-ps, 10-GHz
pulse generation [1,2] using mode-locked-lasers
with a ring-cavity containing a semiconductor
optical amplifier (SOA) have been demonstrated.
In a delayed-interference-signal-wavelength
converter (DISC [3,4]) loop-type pulse generator,
the pulse width is determined by the DISC-MZI’sdelay time ∆t. So far, 5-ps, 10-GHz pulse
generation from a DISC-loop-type pulse
generator has been demonstrated [5,6]. However,asymmetric distortion was observed in the optical
spectrum of the output pulse [6]. Also, the effectof pulse loop gain on the pulse power was notexamined.
In this work, we built a 5-ps, 10-GHz
DISC-loop-type pulse generator using acommercially available SOA, and examined thepulse loop gain when the pulse was generated.

Low-distortion pulse generation andthreshold gain
Figure 2(a) shows a typical output opticalspectrum from the DISC-loop-type pulse
generator. When the optical phase bias wasslightly detuned from π, we observed alow-distortion optical spectrum. The peakwavelength was equal to the input CW
wavelength, the spectral width was 0.75 nm, andthe spectral spacing was approximately 85 pm(10 GHz). The spectral envelope was close to thecalculated spectrum of a 3.5-ps sech pulse. Figure2(b) shows a typical autocorrelation trace. The
pulse extinction ratio was 16.6 dB. The estimatedpulse width was 5.5ps, assuming a sech pulse
shape. This time-bandwidth product was 1.6
times as broad as the transform limit. Figure 2(c)shows a typical output radio frequency (RF)spectrum. The peak frequency was 10.439GHz,
and the spectral width was 40MHz. This peakfrequency
indicated
thepulse-repetition
frequency. The 4.4-MHz-spaced RF components
were the harmonics of the ring cavity frequency.
Figure 3(a) shows the dependence of the powerof the cross-phase modulation (XPM)
components on the pulse loop gain. We defined
the pulse loop gain as the sum of the DISC
transmittance, the passive device losses, and theEDFA unsaturation gains. We estimated the DISC
transmittance to be –14 dB when we input the2-ps, 12.5-GHz pulse train into the DISC unit in a
separate experiment. We defined the XPM
component power as the total of the power fromthe third, second, and first blue components andthe first, second, and third red components. The

Experimental setup
Figure 1 shows our experimental setup. Thepulse generator consisted of a ring cavity and an
external continuous-wave (CW) laser source. The
ring cavity consisted of a DISC, a tunable delay,an energy-distribution (ED) Mach-Zehnderinterferometer (MZI), Er-doped fiber amplifiers(EDFA), polarizers (P), and half- or quarter-wave
plates (H or Q). The DISC consisted of a SOA(Avanex 1901, drive current = 150 mA) and aMZI. The MZI contained a birefringent calcite
crystal (DGD, ∆t = 5.0 ps). The ED-MZI was thesame as that in Ref. 6. The ED-MZI delay time∆TMZI determined the output pulse repetition
frequency.

1522

XPM component power drastically increased by
12 dB when the pulse loop gain reachedapproximately +7 dB. This threshold gain waslarger than the 0 dB that we had expected. We
think this difference was because our estimationof the DISC transmittance was high.

pulse loop gain reached the threshold gain.
References[1] K. Vlachos, et al, IEEE J. Quantum Electronics,
10 (2003) 147.
[2] S. Gee et al, IEEE, Photonics Technol. Lett, 17(2005) 199.
[3] Y. Ueno et al, IEEE Photonics Technol Lett, 10(1998) 346.
[4] Y. Ueno et al., JOSA B19 (2002) 2573.
[5] Y. Ueno et al., Jpn. J. Appl. Phys. 39 (2000)
L803.
[6] Y. Ueno et al., Appl. Phys. Lett. 79 (2001) 2520.

Conclusion
We generated 5-ps, 10-GHz pulses with alow-distortion optical spectrum from a
DISC-loop-type pulse generator. We examined
the pulse-generation conditions and observed thatthe pulse power drastically increased when the10SOAQP

QP

90BPF
3nm

calcite∆t = 5 psQHQ

delay

10015541555

EDFA

5nm

50Autocorrelator

50QH

OSA

0.6(b)

0.40.20-500Delay (ps)50-405050BPF

EDFA Q H

1556

Wavelength (nm)ED-MZI BPF
3nm

3nm

EDFA

DFB-LD

0.5

1/∆TMZI = 10GH z

BPF

(a)

Intensity (dBm)

90 H Q

1Auto-correlation signal (V)

MZIIntensity (µW)

SMZ-DISC gate

(c)

-50
-60
-70
-80
-90

Power
Meter

10.3

10.4

10.5

Frequency (GHz)

Fig. 2. Typical output optical spectrum (a),autocorrelation trace (b), and RF spectrum (c).
OE DISC-loop-type
converter
Fig. 1. Experimentalpulse generator setup
Pulse loop gain = +16.7 d B, CW wavelength =1554.92 n m.
SOA: semiconductor optical amplifier, BPF: band pass filter The dashed curve in (a) is a 3.5-ps pulse’s spectrum.
ED-MZI: energy distribution M ach-Zehnder interferometer
The dashed line in (b) is the background level.
RFSA-100(a)-20(b)-20-40-60-80
1553

1554

1555

1556

1557

Wavelength (nm)Intensity (dBm)

0-30-40-50-60-10-40-60-80

00102030

Pulse loop gain, G loop (dB)(c)-201553

Intensity (dBm)

XPM component power, PXPM (dBm)

H: half wave plate, Q: quarter wave plate, P: polarizer
EDFA: Er-doped fiber amplifier, OSA: optical spectrum Analyzer
RFSA: radio frequency spectrum analyzer

Intensity (dBm)

OE converter

1554

1555

1556

1557

Wavelength (nm)

(d)

-20
-40
-60
-80
1553

1554

1555

1556

1557

Fig. 3. Threshold conditions for pulse generation. Wavelength (nm)(a) XPM co mponent power as the total of the power of
the third, second, and first blue components and the first, second, and third red co mponents.
The dashed line is for Gloop = P XP M as a reference.
Optical spectrum with a pulse loop gain of +16.7 dB (b), +8.63 d B (c), +4.76 dB (d)The dashed line is the CW wavelength (1554.92n m) .

1523