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Electrical characteristics: Fig. 2 shows the threshold voltages
against effective channel lengths for NMOS and PMOS devices
with 120 points in a wafer, respectively. The measurement conditions are with a drain voltage of 0.1V and a back bias voltage of
OV for NMOS devices. For PMOS devices, the reverse polarity of
drain voltages is used. The threshold voltages for NMOS devices
are sharply decreased near the effective channel length of 0.4pn,
as shown in Fig. 20.. These results come from the short channel
effect of the surface channel in NMOS devices. For PMOS
devices, however, the roll off in magnitude of the threshold voltages is suppressed and the threshold voltages are coincident within
fl.05V in all the range of the effective channel lengths (Fig. 2b).
This suppression of threshold voltages shows a good characteristic
compared with the conventional devices, and is caused by the shallow junctions of the source/drain in the buried channel PMOS
devices [3].
N E W (Leff
100
~0.6
0 IEE 1997
9 December 1996
Electronics Letters Online No: 19970262
Yong-Sun Yoon, Kyu-Ha Baek and Kee-Soo Nam (Electronics and
Telecommunications Research Institute, Yusong PO Box 106, Taejon,
305-600 Korea)
E-mail: ysyoon@cadvax.etri.re.kr
References
1
KIMURA, s., TANAKA, J., NODA, K.,TOYABE, T , and IHARA, s.: ‘Shortchannel-effect-suppressed sub-0.1-pn grooved-gate MOSFET’s
with W gate’, IEEE Electron Device Lett., 1995, 42, pp. 944100
2
TAUR, Y , WIND, S., MII, Y . J , LII, Y., MOY, D., JENKINS, K.A., CHEN, C.L.,
KLAUS, D.,
BUCCHINGNANO, J.,
ROSENFIELD, M.,
COANE, P.J.,
THOMSON, M.G.R., and POLCARI, M.: ‘High performance 0 . 1
Electron Devices Meeting, 1993, pp. 1277130
and KIMURA, s.: ‘Threshold voltage controlled
0.1-pm MOSFET utilising inversion layer as extreme shallow
source/drain’. Tech. Dig., IEEE Int. Electron Devices Meeting,
1993, pp. 123-126
4 KIMURA, s., NODA, H., HISAMOTO, D., and TAKEDA, E.: ‘A 0 . 1 ~ - g a t e
elevated source and drain MOSFET fabricated by phase-shifted
lithography’. Tech. Dig., IEEE Int. Electron Devices Meeting,
1991, pp. 950-952
3
NODA, H., MURAI, F.,
5
YAN,R.H., LEE,K.F., JEON,D.Y., KIM,Y.O., PARK,B.C., PINTO,M.R.,
RAFFERTY, C.S.,
TENNANT, D.M.,
WESTERWICK, E.H.,
CHIN, G.M.,
MORRIS, M.D.,
EARLY, K.,
MULGREW, P.,
MANSFIELD, W.M.,
WATTS, R.K., VASHCHENKOV, A.M., SWARTZ, R.G., and OURMAZD, A.:
‘High performance 0.1-pm room temperature Si MOSFET’s’.
Symp. VLSI Tech., 1992, pp. 86-87
1
NEW (Leff = 0 . 4 5 )
3
L
5
Dual-wavelength actively mode-locked
Er-doped fibre ring laser with fibre gratings
6
s u p p l y voltage, V
Fig. 3 71-stage unloaded CMOS ring oscillator characteristics
devices and conventional ones
few
Gate delay of new device: 128pdgate at power supply voltage 3.3V
e delays of conventional ring oscillator in 0.65pm channel
delays of conventional ring oscillator in 0.8pm channel length
Shenping Li, Hao Ding and K.T. Chan
Indexing terms: Fibre lasers, Gratings in fibres
A simple and novel configuration for dual wavelength operation
of actively mode-locked Er-doped fibre lasers with fibre gratings
is proposed. Simultaneously mode-locked operation with two
wavelengths at bit-rates up to 50MHz is experimentally
demonstrated.
Fig. 3 shows the CMOS ring oscillator characteristics of the
new devices compared with the conventional ones in our cases.
The gate delay of a 71-stage unloaded ring oscillator with an effective channel length of 0 . 4 5 is
~ 128pdgate at power supply voltage of 3.3V, which is represented by a solid reverse triangle in Fig.
3. With an effective channel length of 0.65pn, the gate delay of
the new devices is enhanced up to 35%, compared to that of the
conventional devices in the same effective channel length, which is
represented by a solid triangle in Fig. 3. These results are due to
the considerable reduction of the sourcei’drain capacitance using
the ASLI layer, even if the resistance of the source/drain interconnection compared with conventional ones is increased. The structure of the new device gives very shallow junction depths of <
0.1pm in both NMOS and PMOS devices. The extended diffusion
path by the ASLI layer and the exchanged thermal cycle between
the source/drain and gate steps make the shallow junctions possible. With consideration of the shallow junctions and small capacitance of the source/drain, this structure is one of the most
promising structures in the deep submicrometre regime.
Conclusions: An elegant CMOS device structure is suggested by a
self-aligned sourcei’drain technique using ASLI. The operating
speed of this device is significantly enhanced by reducing the
source/drain junction capacitance. The new device structure has
several benefits, such as (i) significant reduction of junction capacitance in sourceldrain, (ii) formation of shallow junctions in
sourceidrain, (iii) in-situ control of gate lengths by the ASLI and
oxide layer thickness, and (iv) compatibility of photomasks with
conventional CMOS devices.
390
~
CMOS devices with 1.5V power supply’. Tech. Dig., IEEE Int.
Introduction: Multiwavelength mode-locked lasers which are capable of generating high-speed short pulses with different wavelengths are of considerable interest for various applications such
as wavelength multiplexed communication systems, optical sensing
systems, and optical signal processing. Simultaneous generation of
two-wavelength optical pulses was demonstrated using an actively
mode-locked fibre laser that included a birefringent component in
its cavity [l]. Using the same laser configuration, but with two
birefringent components inserted in the laser cavity, simultaneous
four wavelength optical pulses were also obtained [2]. As the
multi-wavelength pulses were achieved by polarisation dispersion
in the birefringent component and the use of different birefringent
fibres, the selection and tuning of wavelengths are difficult in these
lasers. Dual wavelength operation in a passively mode-locked ‘figure-of-eight’ ytterbium-erbium fibre laser with two Fabry-Perot
fdters was also demonstrated [3].Because the repetition rate of the
pulse trains in passively mode-locked laser is limited by the length
of the laser cavity, the pulse trains had a repetition rate of only
2.85MHz in that laser. Furthermore, synchronised dual-wavelength pulse trains were produced in an actively mode-locked
dual-wavelength erbium-fibre laser which used a single nonlinearoptical loop mirror modulator to simultaneously modelock two
standing-wavecavities [4]. Although this laser configuration overcomes the above mentioned shortcomings, two erbium-doped fibre
amplifiers were used in such a laser. In this Letter we propose and
demonstrate a simple and novel configuration of dual-wavelength
actively mode-locked lasers. By the use of such configurations,
only one erbium-doped fibre amplifier is used and the disadvantages mentioned above can be overcome.
ELECTRONICS LETTERS
27th February 1997
Vol. 33
No. 5
Experimental setup: The configuration of our lasers is shown in
Fig. 1. The laser consisted of l m of erbium-doped fibre (EDF),
two reflection fibre gratings, one 5050 coupler (1550nm) and one
fibre polarisation controller (PC), one LiNbO, modulator, and
one polarisation dependent optical isolator (ISO). The 50:50 coupler served as both ouiput and feedback functions. The laser was
pumped through a wavelength division multiplexing (WDM) coupler (980/1550nm) by a 980nm semiconductor laser. The LiNbO,
modulator with a modulation depth of -0.94 was used to realise
active mode-locking. The two fibre Bragg gratings FGl and FG2
lengths can be separately obtained from the transmission ends of
FG2 and FG1 with very large attenuation of the other wavelength
[5]. By appropriately adjusting the distances from each of the two
gratings to the output coupler, the cavity lengths for the two
wavelengths could be made approximately equal. Thus, simultaneous mode-locking operation at those two wavelengths were
obtained when the modulator was operated around the fundamental or harmonic frequencies of the ring cavities. The optical fibre
polarisation controller were used to balance the gains (or losses) of
the two wavelengths and to optimise the pulse quality.
LiNbO3
modulator
11
'
50:50
output 1
r
Er-doped
fibre
output 2
12
I
Fig. 1 Experimental setup of dual-wavelength mode-locked Er-doped
fibre ring laser with two gratings
WDM: 980/1550nm wavelength division multiplexing coupler
PC:: polarisation controller
FGl, FG2: fibre Bragg gratings with centre reflective wavelengths at
1532.1 and 1550.0nm,respectivelv
I S 0 optical isolator
90 10 coupler is only used to observe two wavelengths simultaneously, and can be eliminated
~~
0
20
I
I
I
40
60
80
I
I
100
tirne,ns
b
j658121
Fig. 2 Output pulses of dual-wavelength mode-locked laser displayed on
fast photodetector/sampling oscilloscope when laser was actively modelocked at third harmonic of cavity fundamental frequency
Pump power: 90mW
a Mode-locked output pulses (from output 1) of light with hl =
1532.1nm
b Mode-locked output pulses (from output 2) of light with
1550.0nm
Scale of intensity axis is same for a and b
&
=
which provided -97% reflection with a bandwidth of -0.5nm at
wavelengths of hl = 1532.lnm and h, = 1550.0nm, respectively,
were used to select two fvted wavelengths. The grating FG1 (FG2)
worked as both a reflective mirror of the laser cavity for wavelength h, (&) and a filter for & (1,)
of the output light. Therefore,
by using gratings with high reflectivities, light with different wave-
ELECTRONICS LETTERS
27th February 1997
Vol. 33
I
I
I
I
I
I
I
I
I
1
Fig. 3 Optical spectrum of cavity light corresponding to pulses in Fig. 2
Spectrum was measured from output 3, and Figs. 2 and 3 were
recorded simultaneously
RL: O.OOdBm, SENS: -70dBm, 1OdBidiv
Span (wavelength) = 30nm, centre: 1540nm
RB: O.lnm. VB: 2kHz
Experimental results and discussion: By appropriate adjustment of
the polarisation controller and tuning the modulation frequency to
the harmonic or fundamental frequency of the cavities, the laser
could be simultaneously mode-locked with two wavelengths. The
output pulses were measured by a fast photodetector (20GHz) and
a sampling oscilloscope. Owing to the limited pump power, modelocking was achieved at the fundamental, second and third harmonic frequencies only. The behaviour of mode-locking at the
fundamental and two harmonic frequencies was similar. The operation of the laser was stable, and no mode-hopping was observed
when the laser was mode-locked for 1h.
Fig. 2 shows one example of the mode-locked pulses of two
wavelengths at a repetition rate of 50.4MHz corresponding to the
third harmonic of the fundamental frequency of 16.8MHz. The
light pulses at h, and h, were measured from output 1 and 2,
respectively. To simultaneously observe the cavity light spectrum,
a 90: 10 coupler was introduced inside the ring and an optical spectrum analyser with a resolution of 0.1nm was used. The spectrum
corresponding to the mode-locked pulse trains shown in Fig. 2 is
given in Fig. 3. As the reflection from the two fibre gratings is
high (-97%), most of the output light from output 1 (or output 2)
is the component of wavelength h, (or &) (-97%). As shown in
Figs. 2 and 3, because the amplitudes of the optical pulses coming
from output 1 (A,)and output 2 (&) are almost the same (see Fig.
2a and b, it should be noted that the scale of the intensity axis is
the same for the two Figures), and the light intensities of the two
wavelengths are approximately equal (see Fig. 3), we can conclude
that the two wavelengths were mode-locked simultaneously.
The repetition rate of light pulses can be increased by further
reducing the losses of the cavities or increasing the pump power in
order to let the laser operate at the higher harmonic orders. The
difference between the optimum mode-locking frequencies of the
two wavelengths was -0.1 MHz in this setup. The mode-locking
behaviour of the two wavelengths can be improved by further
reducing the difference of fibre cavity lengths for both wavelengths
so that the laser can operate at optimum mode-locking frequencies
for the two wavelengths simultaneously. In addition, it should be
mentioned that separate tunability of each wavelength can be
obtained by straining the fibre grating as a small change (-I 1p)
of the fibre grating length will produce a large shift (-lonm) in the
reflection wavelength of the grating [5] without affecting the
mode-locking frequency.
No. 5
39 1
Conclusion: We have proposed a simple and novel actively modelocked Er-doped fibre ring laser using two fibre gratings for dual
wavelength operation. Simultaneously mode-locked dual wavelength operation at up to 50.4MHz repetition rate was experimentally demonstrated. Without using additional filters, the two
wavelengths of light can come out separately from the two output
ports of the coupler when highly reflective fibre gratings are used.
Model. To calculate the eigenfunctions and eigenvalues of the electron and hole states, and band profile deformation, we self-consistently solve the multiband effective-mass equation for the
valence band, the scalar effective-mass equation for the conduction band and the Poisson equation. Fig 1 illustrates a model of
carrier capture and escape in an MQW laser structure that
Acknowledgment: Shenping Li and Hao Ding gratefully acknowledge the financial support (Postdoctoral Fellowship and Research
Associate Fellowship) of The Chinese University of Hong Kong.
Hao Ding is on leave from Shanghai Institute of Optics and Fine
Mechanics.
0 IEE 1997
10 December 1996
Electronics Letters Online No: 19970258
Shenping Li, Hao Ding and K.T. Chan (Department of Electronic
Engineering, The Chinese University of Hong Kong, Ho Sin Hang
Engiaeerirzg Building, Slzatin, New Territories, Hong Kong)
E-mail: spli@ee.cuhk.edu.hk
Fig. 1 Schematic diagram of carrier capture and escape in M Q W laser
with inclusion of intersubband transitions ( 2 + I ) and ( I + 2)
B: barrier states
References
and SARUWATARI, M.: ‘Dual
wavelength pulse generation using mode-locked erbium-doped fibre
ring laser’, Electron. Lett., 1991, 27, pp. 2072-2073
TAKARA, H., KAWANISHI, s., and SARUWATARI, M.: ‘Multiwavelength
birefringent-cavity mode-locked fibre laser’, Electron. Lett., 1992,
28, pp. 22742275
NOSKE, D U , GUY, M.J., ROTTWITT, K., KASHYAP, R., and TAYLOR, J.R.:
‘Dual-wavelength operation of a passively mode-locked ‘figure-ofeight’ ytterbium-erbium fiber soliton laser’, Opt. Commun., 1994,
108, pp. 297-301
PATTISON, D.A.,
KEAN, P.N.,
GRAY, J.W.D.,
BENNION, I.,
and
DORAN, N J.: ‘Actively modelocked dual-wavelength fibre laser with
ultra-low inter-pulse-stream timing jitter’, IEEE Photonics Technol.
Lett., 1995, 7, pp. 1415-1417
LI, S.P., DING, H., and CHAN, K.T.: ‘Novel configuration of erbiumdoped fibre lasers for dual wavelength operation’, Electron. Lett.,
1997, in printing
SCHLAGER, J B.,
KAWANISHI, S.,
Model for carrier capture and escape in
multiquantum-well lasers: Determination of
effective capture time and differential gain
includes the effects of space charge, 3D electron reflections at QW
boundaries and intersubband transitions. From small-signal analysis of the rate equations for the carrier density in the continuum,
in all subbands and for the photon density, we derive the response
function and then identify a new effective time constant and effective differential gain. In the case shown in Fig. 1, to a good
approximation we obtain
?e,‘.
= (70
+ 72+1[(1 + q2)(g1/gtot)gi1)
+ (1+41)(92 /gtot)g?’l / ( 9 2 )+7-219?’ I} / (1+7211q2/7b+l)
(1)
(aslan),,,
= (92)
+ r21g;z))/(1 + 7-1 + 7-21)
(2)
where T$) is the effective capture time, (dg/dn),ii is the effective
differential gain, g$l = (dg,idn,), g, is the optical gain for the transitions between ith conduction subband and all valence subbands, n,
is the carrier density in ith subband, g,,,= g,+g,, l/zo = l/zbil+
1 / ~ ~q,+ ~zo,/z,
r, = z6+i/ziib, and rzl s T ~ + , / T ~ +(i~= 1, 2). Here
the time constants characterising the corresponding transitions are
derived from microscopic expressions for capture and escape particle current densities by modelling these transitions as phononassisted processes. In the case of z2+]= 0, eqn. 1 corresponds to
the conventional approach [2].
VI
-.o
a ,
. , . , . , . ,,
,
,
I
A.G. Plyavenek and A.V. Lyubarskii
Indexing terms: Semiconductorjunction lasers, Semiconductor
device models, Semiconductor theory
A new model that includes the effects of band mixing, strain,
space charge, impurity doping, 3D carrier reflections at QW
boundaries and intersubband transitions is proposed to determine
effective carrier capture time and differential gain in
multiquantum-well (MQW) lasers. Results of numerical
calculations of these dynamic parameters for 1 . 3 strained
~
InGaAsPilnP MQW lasers are presented.
Introduction: The effective carrier capture time and differential
gain are important parameters of the high-speed modulation performance of multiquantum-well (MQW) lasers [1, 21. Usually a
number of factors such as space charge, 3D carrier reflections at
QW boundaries and intersubband transitions are not taken into
account in determining the dynamic parameters in MQW lasers.
Recently, we have proposed the model for carrier capture and
escape in InGaAsPiInP single-quantum-well lasers and shown that
these factors play an important role in determining the effective
capture and escape times [3].In this Letter we present the results
on the theoretical study of effective capture time and differential
gain in InGaAsP/InP MQW lasers within further development of
the model [3]. This includes the effects of band mixing, strain,
space charge, impurity doping, 3D carrier reflections at QW
boundaries and intersubband transitions.
392
.F
a,
20
60
LO
80
a
100 0
LO
borrierwidth,i
60
80
100
/543121
Fig. 2 Effective capture time and effective differential gaiii against barrier width at constant peak gain of 1000 em-’ with acceptor N, and
donor No densities in barrier layers as parameters
a Effective capture time
b Effective differential gain
-e- N, = 0, No = 0 (our model)
-IN, = IO’*cm” (our model)
-A- N D = 10’*~m-~
(our model)
- - 0 - - N A = 0, N D = 0 (conventional model)
a
b
m
Fig.3 Effective capture time and eflective 12ifferential gain against peak
gain gpeakand acceptor density N, in IOOA-thick barrier layers
a Effective capture time
b Effective differential gain
ELECTRONICS LETTERS
27th February 1997
Vol. 33
No. 5
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