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a ciurrent amplifier that takes into account parasitics, finite impedance levels, and transport delays representative of a high-speed
semiconductor process i(eqn. 2, [SI). Setting the current gain at 200
with a single pole giving anf, of SGHz produces a gain response
as shown in Fig. 1, for ii typical range of feedback resistance up to
300R. It is evident that the predicted closed-loop transimpedance
bandwidth of the amplifier approaches very closely the unity-current-gain frequency of the current amplifier itself and displays
only a very weak dependence over a decade of transimpedance
gain, as anticipated by eqn. 1.
independence and insensitivity to source capacitance in a practical
circuit, even though restrictions on g, and low Rdrconstrain the
ability of MESFET technology to generate the low input and high
output impedance values ideally required for the design of current
amplifiers. Preliminary results from work on implementation of
this new formulation with HBT technology offering device characteristics more suited to the current-mode style of operation are
encouraging [9] and will be reported shortly.
Conclusions: A transimpedance configuration formulated around a
current amplifier will display gain-bandwidth independence and
promote wideband operation through optimum matching of input
conditions, making it attractive for an optical fibre receiver frontend. Simulation results for a MMIC design based on a GaAs
MESFET process predict a -3dB bandwidth very close to the current amplifier’s f r which extends well into the microwave region
and is insensitive to capacitive source loading. Such a currentmode approach to wideband amplifier design therefore offers evident promise for future developments.
Acknowledgment: The authors wish to acknowledge the valuable
contribution of I.Z. Darwazeh to various aspects of the MMIC
design and simulation process.
0 IEE 1997
Electronics Letters Online No: 19970284
Fig. 2 Circuit diagram of MESFET current-mode transimpedance
27 January 1997
B. Wilson and J.D. Drew (Department of Electrical Engineering and
Electronics, University of Manchester Institute of Science & Technology
( U M I S T ) , PO Box 88, Manchester M60 lQD, United Kingdom)
Implementation in opto-electronic receiver: Fig. 2 illustrates the cir-
cuit of a wideband GaAs MESFET transimpedance photodiode
preamplifier based around a current amplifier. A common-gate
stage produces a low input resistance, followed by a cascode
arrangement to minimise bandwidth restrictions. Active loads are
used as the relatively high operating currents would otherwise
require unrealistically low load values or much higher supply voltages. An output buffer based on a moditied cascode structure is
used to provide true S0R matching capability with minimal penalty to overall gain level. To test the feasibility of MMIC implementation, simulation hias been carried out for a popular 20GHz
f T MESFET foundry process with full device and component characterisation data, resulting in a compact 1.5 x 2.4mm layout using
2 x 1 0 0 p MESFET devices and Nichrome resistors. Full foundry
simulation produces a composite current amplifier with an openloop unity-current-gain bandwidth of 7.6GHz before the application of feedback, resulting in a loaded closed-loop transimpedance
bandwidth for the target gain of 40dBR ofjust over 5.7GHz (Fig.
3), reducing by only 2GHz for an additional source capacitance of
2pF. An output impedance of 5 0 0 is maintained over the full useable bandwidth, with art output return loss (&) < -25dB over the
entire bandwidth. Good gain definition is also achieved, with a
difference between R, and the predicted gain of only just > ldB,
along with an input impedance of 15R.
These results clearly confirm that the new configuration can
simultaneously produa: near-optimum bandwidth, bandwidth
OGAWA, K.: ‘Considerat5ms for optical receiver design’, ZEEE J Sel.
Areas Commun., 1983, SAC-3, pp. 526532
MOI, T.v.: ‘Receiver design for high-speed optical fiber systems’, J.
Lightwave Technol., 1984, LT-3, pp. 243-267
B., and DARWAZEH, I.: ‘Transimpedance optical preamplifier with a
very low input resistance’, Electron. Lett., 1987, 23, pp. 88-89
and TERRY, M.B.: ‘The use of current amplifiers for
high-performance voltage applications’, ZEEE J. Solid State
Circuits, 1980, SC-17, pp. 155-162
WILSON, B.: ‘Current-mode circuits: analysis and CAD modelling’.
IEEE Int. Symposium on Circuits and Systems, 1990, New
Orleans, USA, Vol. 4, pp. 3242-3245
PAYNE, A., and TOUMAZOU, c.: ‘Analog amplifiers: classification and
generalisation’, IEEE Trans. Circuits Syst. -Z, 1996, 43, pp. 43-50
BRUUN, E.: ‘Bandwidth limitations in current-mode and voltagemode integrated feedback amplifiers’. IEEE Int. Symp. Circuits
Syst., 1995, Seattle, USA, Vol. 1, pp. 303-306
WILSON, B., and DREW, J.D.: ‘New current-mode transimpedance
amplifier for gain-independent bandwidth’. IEE Colloquium on
wideband circuits, modelling and techniques , Digest 1996/111,
May 1996, London, Paper 9
DREW, J.D.: ‘Wideband current-mode amplifier structures’. PhD
Thesis, University of Manchester Institute of Science &
Technology (UMIST), October 1996
Avalanche assisted upconversion in
ZBLAN glass
T.R. Gosnell
Indexing terms: Rare-earth doped fibres, Fluoride glasses,
The author reports efficient upconversion optical pumping of the
frequency, H z
’P levels of Pr3+ in ZBLAN glass codoped with Yb3+.
Measurements of Pr3+ visible emission against infrared pump
intensity demonstrate the existence of an avalanche pumping
process which comprises three components: radiationless energy
transfer from Yb3+to Pr3+,excited-state absorption in Pr3+,and
cross-relaxation between excited Pr3+ions and ground-state Yb3+
10 l o
Fig. 3 Transimpedance gain response of MESFET amplifier
= 50!2
27th February 1997
Vol. 33
No. 5
41 1
Ralationless energy-transfer phenomena involving Pr3+and Yb3+
impurity ions in solids have been known since 1991 [1-5]. In particular, optical pumping of the Yb3+2F,,2+ 2F,,2transition leads
to resonant energy transfer to the 'C, state of Pr3+,a process for
which 980nm excitation of Yb3+has been proposed as a pumping
mechanism for Pr3+-based1.31,um optical fibre amplifiers. Indeed,
our own investigation has shown that the quantum efficiency for
this process in a bulk ZBLAN sample doped with 0.3wt% Pr3+
and 2.0wt% Yb3+is 56% [6]. By shifting the pump wavelength to
near 84Onm, however, excited-state absorption on the Pr3+IC4 +
3P0,1,'I, transitions combines with Yb + Pr energy transfer to
yield a two-photon upconversion pumping mechanism that helps
populate these high-lying states of Pr3+ (Fig. la). Invoking this
mechanism, Allain et al. [l]have obtained 20mW of upconversion
laser output at 635nm in a ZBLAN fibre doped with 0.1wt% Pr3+
and 2.0wtY0Yb3+.Further, these workers were the first to observe
[2] a Pr3+/Yb3+
cross-relaxation process whereby Prj' ions in the 3P
manifold promote proximate ground-state Yb3+ ions to their 2F,,2
states while the donor ions relax to their G4states [viz. relevant
portion of Fig. 14. Remillieux et al. [7] have observed a similar
upconversion mechanism, but one in which the excited-state
absorption step is replaced by a second Yb + Pr energy transfer.
Since this process occurs only rarely, however, it does not generate
significant population in the Pr3+3P manifold.
ple with an NA = 0.95 microscope objective mounted at 90" with
respect to the pump axis. Care is taken with the collection optic to
image onto a diffraction-limited field aperture only the 12,umdiameter focal volume illuminated by the pump optic. Fluorescence signals in the visible or infrared are then detected with a
photomultiplier or InGaAs photodiode, respectively, located
immediately behind the field aperture. Bandpass interference filters
selective for either 635 or 980nm wavelengths are placed before
the field aperture depending on the detector in use.
Fig. 2 Visible emission from PP' and infrared emission from Ybs+
against pump power at two different pump wavelengths
a Visible emission from Pr3+
square law
b Infrared emission from Yb3+
0 pump at 839nm
0 pump at 800nm
linear law
Fig. I Upconversionpump mechanisms for filling of jP levels of Plj'
a Seed mechanism
b Avalanche mechanism
Solid open arrows: pump driven radiative transistions
Thin solid arrows: Non-radiative energy-transfer transitions
In [8] we proposed a combination of all but the later process as
the explanation for our obtaining significant upconversion laser
output at blue, green, orange, and red wavelengths in a ZBLAN
fibre doped with 0.3wtYo Pr3+and 2.Owt%o Yb3+.A crucial new
inference drawn in that work, however, was that the cross-relaxation step
Pr 3+ (3pO J )
Yb3+('F7p) t Pr3+(lG4) Yb3+(2F5/z)
was followed by
+ Yb3+('F512) + Pr3+(3H4)
3 2Pr3+('G4) + Yb3+('F7p)
thus yielding two Pr3+ions in the 'C, state. For the purpose of
producing upconversion emission, these Pr3+ions were then available for further excitation through excited-state absorption of the
860nm pump light used in that experiment (Fig. lb). Close inspection of eqns. 1 and 2 reveals our salient conclusion in this earlier
work, namely, that eqns. 1 and 2 participate in an avalanche
pumping mechanism capable of efficiently populating the 'Po,I
states of Pr3+when the pump wavelength lies in the range 780 880nm. The purpose of this Letter is to present direct experimental evidence for the inference drawn in this earlier work.
The sample used in the present measurements consists of a
-1 cm3 bulk cube of ZBLAN glass (ZrF,-BaF,-LaF,-AlF,-NaF)
doped with O.3wty0Pr3+and 2.OwtY0Yb3+.Upconversion pumping
of this sample is accomplished by focusing the chopped output of
a Tisapphire laser with an NA= 0.12 microscope objective
through one of the samples faces. Fluorescence emission is then
collected through an adjacent perpendicular face of the cubic sam-
41 2
Fig. 2a shows the 635nm fluorescence signal observed from the
Pr3+3P0 + :F2 transition against pump power for pump wavelengths of either 839 or 800mn. Referring to the 839nm data, the
Figure shows that a quadratic dependence of the output fluorescence signal on pump power is obtained for pump powers
<-15OmW, as shown by the solid line. This dependence is exactly
what is expected for a simple two-photon pump mechanism, more
specifically the one shown in Fig. la. Note that above 150mW,
however, a nearly 4-orders-of-magnitude increase of the visible
fluorescence signal is obtained over only a 1-order-of-magnitude
increase of the pump power. The 800nm data show an even more
dramatic dependence of the red fluorescence signal against pump
power, despite the reduced absorption cross-section of Yb3+at the
shorter wavelength. At wavelengths > -860nm, however, the visible fluorescence signal significantly decreases despite the strongly
increasing absorption by ground-state Yb3+ions. These observations combine to indicate that the proposed avalanche process
does indeed occur and dominates other proposed pump processes
such as those of Allain et al. [l] (Fig. la) and Remillienx et al. [7].
Rather, the mechanism of Fig. l a provides the seed population in
manifold required for the avalanche to begin. The
reduced signal level observed at 800 against 839nm at fixed pump
power can then be attributed to the factor-of-3 reduction in crosssection of the Pr3+excited-state-absorptiontransition between the
two wavelengths [9].
Further evidence of this avalanche mechanism is given in Fig.
2b, which shows the Yb3+980nm fluorescence signal against pump
power. In this case, the expected linear dependence is observed at
the lower pump powers, as shown by the solid line, yet a strongly
nonlinear dependence at the hgher pump powers again indicates
onset of the avalanche process. In particular, these data show that
the cross-relaxation step in eqn. 1 is directly involved.
A numerical model embodying the energy-transfer steps comprising the avalanche pumping process demonstrated in this work
is in qualitative agreement with the experimental results. That
27th February 1997
Vol. 33
No. 5
such a process appears dominant in the present measurements and
in our previous upconiversion laser experiments 181, whereas no
evidence for analogous behaviour was previously reported by
Allain et al. [l, 2, 81, may easily have resulted from the higher Pr3+
concentration used in both our bulk sample and fibres. It is possible that an even higher Pr3+concentration could produce still more
efficient avalanche upoonversion pumping in this codoped system
and thereby yield improved performance of single-wavelengthpumped Pr3+-basedupconversion lasers.
In this Letter, an optical trapping method using optical fibre [3]
is proposed. T h s method has the capabilities to solve the above
mentioned problems. This optical fibre trapping method has many
merits: (i) optical trapping systems using optical fibres are simple
and inexpensive, (ii) trapped objects can be moved freely, (iii) optical sources can be easily changed using optical connectors, (iv) the
trapping point is easily noticeable, because a fibre end points out
the focal point. These merits are verified by the following experiments.
0 IEE 1997
6 January 1997
Electronics Letters Online No: 19970220
T.R. Gosnell (Condenscd Matter and Tltermal Phvsics Groua. Los
Alumos National Labouatory, Mail Stop E543, L k Alamoi, ’ New
Mexico 87545, USA)
CCD camera
ALLAIN, J.Y., MONERIE, M., and POIGNANT, H.: ‘Red upconversion
Yb-sensitised fluoride fibre laser pumped in 0 . 8 ~region’,
Electron. Lett., 1991, 27, pp. 1156-1157
ALLAIN, J.Y., MONERIIE,M., and POIGNANT, H.: ‘Energy transfer in
Pr3+/Yb3+-dopedfluorozirconate fibres’, Electron. Lett., 1991, 27,
pp. 1012-1014
MIYAJIMA, Y., SUGAWA, T., and FUKASAKU, T.: ‘38.2dB amplification
at 1 . 3 1 ~and pot#sibility of 0 . 9 8 ~pumping in Pr3+-doped
fluoride fibre’, Electr,on. Lett., 1991, 27, pp. 1706-1707
‘Gain characteristics of Pr3+/Yb’+-codopedfluoride fibre for 1 . 3 ~
amplification’, IEEE Photonics Technol. Lett., 1991, 3, pp. 990-992
OHISHI, Y.: ‘Laser diode pumped Pr3+-dopedand Pr3+/Yb3+-codoped
Lett., 1991,
fluoride fibre amplifi’ersoperating at 1 . 3 ~ ’Electron.
22, pp. 1995-1996
XIE, P , and GOSNELL, T.R.: ‘Efficient sensitisation of praseodymium
1.31pn fluorescence by optically pumped ytterbium ions in
ZBLAN glass’, Electron. Lett., 1995, 31, pp. 191-192
energy transfer in a Yb-Pr doped ZBLAN fiber’. Compact BlueGreen Lasers and Advanced Solid-state Lasers Tech. Dig., 1993,
Vol. 2, pp. 461-463 (Optical Society of America)
XIE. P., and GOSNELL, T.R.: ‘Room temperature upconversion fibre
laser tunable in the red, orange, green and blue spectral regions’,
Opt. Lett., 1995, 20, pp. 1014-1016
QUIMBY, R.s., and ZHENG, B.: ‘New excited-state absorption
measurement technique and application to Pr3+ doped
fluorozirconate glass‘, Appl. Phys. Lett., 1992, 60, pp. 1055-1057
optical connector
Fig. 1 Experimental setup used for optical trapping
Optical trapping apparatus: Fig. 1 shows the apparatus used for
the optical trapping of a dielectric particle and a biological cell.
The laser sources are a YAG laser at 1 . 0 6 and
~ a semiconductor
laser at 1 . 4 8 The
~ output of laser light is coupled into an optical
fibre which has an optical connector at the fibre end. The trapping
fibre is attached to an xyz manipulator and laser light is introduced through the optical connector. The trapping fibre end is
polished to a tapered spherical end whose radius R, is 2, 4 and
6 p , respectively. A microscope with a liquid-immersion microscope objective is used to observe the trapped objects and the
trapping behaviour is recorded on a VTR with a CCD camera.
Optical trapping of dielectric particle and
biological cell using optical fibre
K. Taguchi, H. Ueno, T. Hiramatsu and M. Ikeda
Fig. 2 Photograph of yeast cell trapped near focal point
Indexing terms: Opticalfibres, Laser beam effects
optical trapping method using optical fibre (optical fibre
trapping method) is proposed. Optical trapping of a dielectric
particle and a biological cell is successfully demonstrated. The
experimental results show that a trapped micro object was easily
moved and freely synchronised to the trapping fibre.
Introduction: Ashkin el al. proposed the optical trapping of dielec-
tric particles by a single-beam gradient force trap for the first time
[l]. Since then, this method was developed as an optical tweezers
technology for various biological objects, such as viruses, bacteria
and yeast cells, as well as for various dielectric particles. Recently,
Sasaki et al. developed the laser scanning trapping method [2]
which utilised the sub:stantial net surrounding force. This meant
that not only transpar1:nt particles, but also metal particles could
be trapped and tramferred. However, since laser beams are
focused by objective lenses and trapped objects are transferred by
moving the focal point for these trapping systems, there are some
weak points: (i) laser beam manipulation and the scanning system
are much more complicated and expensive. (ii) the degree of freedom from restriction of motion is small.
27th February 1997
Vol. 33
Results and discussion: Polystyrene particles dispersed in ethanol
(refractive index n = 1.36) and yeast Cells dispersed in water (n =
1.33) were used as the sample microscopic objects for the trapping
experiments. The light source used for trapping a l o p diameter
polystyrene particle (n = 1.59) was a semiconductor laser. The
minimum optical power for trapping a polystyrene particle was
-1.3mW for R,= 2p11 SMF (singlemode fibre), -2.0mW for R, =
4pm SMF and -2.5mW for Rf = 6 p SMF, respectively. When
we changed the light source of a semiconductor laser to a YAG
laser, the trapping characteristics were not so different from that
of a semiconductor laser. The minimum optical power of a YAG
laser to trap a microsphere was a little less than that of the semiconductor laser. Fig. 2 is a photograph of a trapped yeast cell.
Without optical power, the yeast cells dispersed in water solution
drifted from the top to the bottom of this photograph with
Brownian motion. Under the trapping condition, we can freely
move the trapped yeast cell to forward and backward, or right and
left directions, synchronised to the trapping fibre. For this experiment, the YAG laser was used as the light source and the radius
of the spherical end was 2 p . The length of the major and minor
axis of the elliptically shaped yeast cell was -9 and 4 p , respec-
No. 5
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