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The P 2 O 5 /F combination allows a reasonably high concentration of P2O5 to be used which, besides giving a low deposition
temperature, helps to reduce considerably diffusion of OH
from the substrate tube. In the absence of fluorine, the P 2 O 5
would give an increase in refractive index of 000085 relative to
that of silica.
References
1
AINSUE, B. J., DAY, C. R., FRANCE, P. W., BEALES, K. J., a n d NEWNS, G.
R.: 'Preparation of long lengths of ultra-low-loss single-mode fibre',
Electron. Lett., 1979, 15, pp. 411-413
2
EDAHIRO, T., HORIGUCHI, M., CHIDA, K., and OHMORI, Y.: 'Spectral
loss characteristics of GeO 2 -P 2 O 5 -doped silica graded index fibres
in long wavelength band', ibid., 1979, 15, pp. 274-275
3
mode cutoff region
4
5
SENTSUI, S., FURUI, Y., YOSH1DA, K., KUROHA, T., a n d HOSAKA, T . :
'Low loss monomode fibres with P 2 O s -SiO 2 cladding in the wavelength region 1-2—1-6 fim'. Fifth European conference on optical
communication, Amsterdam, Sept. 1979
GLOGE, D.: 'Weakly guiding fibers', Appl. Opt., 1971, 10, pp.
2252-2258
ARONSON, B. s., POWERS, D. R., and SOMMER, R. G.: 'Chlorine drying of
a doped deposited silica preform simultaneous with consolidation'.
Topical meeting on optical fibre communication (Optical Society
of America), Washington, 1979, p. 54
03
6
1000
1200
1400
wavelength, nm
1600
1800
Fig. 2 Loss spectra for three lengths of single-mode fibre, measured using
tungsten-lamp/monochromator source and germanium detector
171 km
125 km
9-8 km
In Table 1 we have summarised results for three lengths of
fibre pulled from three consecutive sleeved preforms. The
quantity of material deposited was adjusted in each case to
allow for the sleeving tubes to be used, in order to achieve the
desired mode cutoff position. The 'start' ends of the preforms
were fused into the sleeving tubes and the composite structure
was collapsed and drawn into fibre in one operation. The loss
spectra for the three fibres are shown in Fig. 2, and excellent
reproducibility of loss was achieved at 1-3 /zm and at 1-6 fim.
The estimated OH content of the fibres lies in the range 30 to
100 parts in 109, inferred from the peaks at 1-25 fim in long
fibre lengths and from measurements at 1-39 /itn made on 1 km
lengths of fibre. This low OH level was achieved by using
chlorine as a drying agent during the collapse of the preform
tube. The use of chlorine or thionyl chloride as drying agents is
well known in the o.v.p.o.5 and v.a.d.6 processes. Pearson (private communication) has observed a similar effect in the
m.c.v.d. process by continuing the flow of GeCl4 during the
collapse of the substrate tube. In the m.c.v.d. process there is
always chlorine present during the deposition owing to the
decomposition of SiCl4 and other halides. At the collapse
stage, however, the halide flow is normally stopped and if no
Cl2 is introduced a high OH concentration can appear at the
centre of the fibre due to the OH present in the carrier gas or
by back-diffusion. This is particularly important in monomode
fibre where the power is concentrated at the centre of the fibre.
The intensity of the OH overtone at 1-39 /jm has been reduced
to < 3 dB/km, and further reduction should be possible.
The monochromatic pulse broadening of the 171 km length
of fibre was measured using a Nd:y.a.g. laser at 1-32 /un. No
broadening due to birefringence effects was detectable.
Conclusion: An optimised structure has been evolved to permit
long lengths of single-mode fibre to be produced with losses of
0-6 dB/km at 1-3 pm and 0-34 dB/km at 1-6 nm. The OH
impurity level has been reduced to 30-100 parts in 109 by using
chlorine as a drying agent in the m.c.v.d. process.
Acknowledgments: The authors wish to thank W. A. Merlo and
D. A. Colthorpe for technical assistance, J. V. Wright for help
in computing the single-mode power profiles, M. C. Brierley
and K. I. White for refractive-index profile measurements, B. P.
Nelson for dispersion measurements, and the Director of British Telecom Laboratories for permission to publish this work.
B. J. AINSLIE
C. R. DAY
J. RUSH
K. J. BEALES
British Telecom Laboratories
Martlesham Heath, Ipswich, Suffolk, England
10th July 1980
ELECTRONICS LETTERS 28th August 1980
SUDO, S., KAWACHI, M., EDAHIRO, T., IZAWA, T , SHIODA, T., a n d
GOTOH, H.: 'Low OH content optical fibre fabricated by vapourphase axial-deposition method', Electron. Lett., 1978, 14, pp.
534-535
0013-5194/80/180692-O2$U0/0
EFFECT OF p-DOPING ON CARRIER
LIFETIME AND THRESHOLD CURRENT
DENSITY OF 1-3 jim GalnAsP/lnP LASERS
BY LIQUID-PHASE EPITAXY
Indexing terms: Doping, Transport processes, Lasers
A correlation was found between the variation of the threshold current density and carrier lifetime with acceptor concentration in the active layer. An injected electron concentration
of 2-5-3 x 10 18 /cm 3 , independent of the acceptor concentration in the active layer, was found at the lasing threshold.
GalnAsP-InP double heterostructure (d.h.) lasers are considered as the most promising light source for optical communication between 10 and 1-55 nm. The dependence of their
threshold current density on active layer thickness has been
reported previously.1-2 Recently, Itaya et a/.3 reported that, for
a given active layer thickness i-the threshold current density
Jth of the lasers was a sensitive function of the p (zinc) doping
in the active layer. However, the reason for the dependence was
not given. In this letter, we report the variation of the spontaneous lifetime of injected carriers at the lasing threshold as a
function of the acceptor concentration in the active layer. The
increase of carrier lifetime rs with decrease in p-doping will be
shown to be the major cause of the reduction in Jth, with effects
such as free carrier absorption playing a minor role.
The d.h. lasers were grown by liquid-phase epitaxy at a temperature of ~ 65O°C using the single phase technique. They
consist of a heavily Sn-doped (> 1018/cm3) buffer layer, an
unintentionally doped GalnAsP active layer (015 to 0-3 /im), a
zinc-doped p-InP confinement layer, and a p+-quaternary cap
layer for good ohmic contact. The quaternary layers are
lattice-matched to the n+-InP substrate (Aa/a ~ 12 x 10 ~ 4 ).
To prevent cross contamination of the melts by zinc,4 all the
wells of the boat were covered by graphite caps during the
growth. The wafers were subsequently processed into oxidedefined stripe lasers with stripe widths between 10 ^m and 100
nm. Using electron-beam-induced current techniques, we
found that, for the doping range of 1017/cm3 to 2 x 1018/cm3,
the p-n junction was located at the interface of the quaternary
active layer and the n + -InP buffer layer. The active layer was
thus unintentionally doped by the diffusion of zinc from the
p-InP confining layer during the growth. This is in good
agreement with Reference 5, in which the p-doping was determined to be uniform in the active and p-InP confining layer.
Vol. 16 No. 18
693
Fig. 1 shows the variation of the threshold current density
Jth as a function of p-doping. The doping concentration was
determined from the mole fraction of zinc in the In melt.6 The
value of Jth was evaluated from the threshold current of lasers
with cavity lengths of ~ 300 /im and stripe widths of more than
30 /im. As shown, Jlh was reduced by approximately a factor of
two as the p-doping was varied from 1018/cm3 to 1017/cm3.
The lowest threshold current density achieved was ~ 1-5
kA/cm2 for devices with an active layer thickness of 015 /im
and an acceptor concentration of ~l-5 x 1017 cm" 3 .
To understand the above observation, the spontaneous carrier lifetime of the d.h. lasers at threshold was determined by
measuring their 'turn-on' delay time. The diodes were Insoldered p-side down on copper heat sinks which were then
mounted at the end of a stripline. The rise time of the package
and driving pulse were estimated to be ~ 0-5 ns. The optical
output was detected with a GaAlAsSb/GaSb avalanche
photodiode with a rise time of 100 ps or less. The turn-on delay
time td was plotted against In {//(/ — /,,,)} in Fig. 2 for three
devices, each with a different p-doping in the active layer. To
avoid complications caused by the appearance of filaments,7
only lasers with stripe widths of 10 /im were used in the above
experiment. The spontaneous carrier lifetime T, of the lasers at
threshold was estimated by the well known relation
t s = tjln {1/(1 - /,„)}•
!
3
a, 2
li
0 KX) 200 300
current ,mA
13100
13116
V3132
13148
V316A
wavelength, jjm
wavelength,
jjm
13172
rzoeTJI
Fig. 3 Emission spectrum and power/current characteristics of typical
device during c.w. operation
= 03jjm
I = HI/,,,; T = 23°C; stripe width = 22 /im; /,„ (pulsed) = 185
mA;/ r t (c.w.) = 200 mA
£3
O d= 0-15 pm
10
2
3
A
5 6 7 8 9
P-doping
10
18
fco^HI
Fig. 1 Variation of threshold cWrrent density with acceptor concentration in active layer (thickness d)
A d = 0-3 fim
•
d = 0-2 /im
O rf = 0-15/im
H122
r s = 2-6 ns
7 //V H153
/
//
^r s
= 4ns
rs r 4-1 ns
5
10
delay time t d , ns
Fig. 2 Plot of In {1/(1 — Ith)} against 'turn-on' delay time of lasers
O
O
A
694
H122; p = 1-2 x 10 18 , s = 12 /im, d = 0-3 /im, T, = 2-6 ns
H177;p = 2 x 10 17 , s= 12 /im, d = 0 1 5 /im, TS = 4 1 ns
H153; p = 3 x 10 17 , s = 13 /im, d = 0-3 /im, t , = 4 ns
For devices with p-doping of 1-2 x 1018 cm" 3 , the spontaneous carrier lifetime at threshold (Jth ~ 10 kA/cm2, s = 10 /im,
d = 0-3 /im) was estimated to be 2-6-3 ns. This is to be
compared with a value of ~ 4 ns obtained for devices with a
p-doping of ~ 3 x 1017 cm" 3 (Jth = 5-55 kA/cm2, s = 10 /im,
d = 0-3 /im). The number of electrons injected into the active
layer to achieve lasing can be easily estimated by using the
relation nlh = Jthxs/qd. We found that, independent of the original p-doping, nth is ~ 2-5-3 x 1018 cm" 3 , after taking into
account that the threshold current densities given above were
approximately twice that of broad-area devices because of current spreading. Similar conclusions were reached by Hwang
and Dyment8 for GaAs-GaAiAs lasers.
Since TS =* l/(Beffpth), where pth is the total number of holes
in the active layer at threshold, and Beff is the effective recombination constant,8 the ratio of the carrier lifetimes for devices
with different p-doping NA in the active layer can be reestimated once nth is known. Taking pth = nth + NA for charge
neutrality, a ratio of ~ 4/3 is found between lasers with
NA ~ 1018/cm3 and those with NA =* 1017/cm3. This is in good
agreement with our experimental results given above. A value
of Bejf — 8-3 x 1 0 " ' ! cm3/s is also obtained.
While the reduction in p-doping of the confining layer reduces the pulsed threshold current density at room temperature, we found that To, where J,h(T) oc exp (T/To\ is also
affected significantly by the change in doping. The effect of this
variation on their c.w. operation range will be reported in a
separate publication. A p-doping of ~ 4-5 x 1017/cm3 is
found to be optimum for c.w. operation when both the threshold current density and its temperature dependence is taken
into account. Fig. 3 shows the emission spectrum of a d.h. laser
with a stripe width of 22 /tm. A single longitudinal mode was
observed between 106 to 117 times the threshold current.
During c.w. operation at room temperature, a typical differential quantum efficiency of 20-24% per facet was obtained. C.W.
operation has been achieved for devices of this stripe width at
heat sink temperatures of ~ 63°C.
In conclusion, a good correlation was obtained between the
variation of the spontaneous carrier lifetime and threshold current density with the acceptor concentration in the active layer.
At threshold, the injected electron concentration was
~ 2-5-3 x 1018 cm" 3 . D.H. lasers with lower p-doping were
found to have longer spontaneous carrier lifetimes. They were
therefore able to generate more gain for the same injection
current. This, in turn, leads to a decrease of the threshold
current density at room temperature.
W. NG
17th July 1980
Y. Z. LIU
Rockwell International/Electronics Research Center
1049 Camino Dos Rios, Thousand Oaks, Ca. 91360, USA
ELECTRONICS LETTERS 28th August 1980
Vol. 16
No. 18
References
1
NAHORY, R. E., and POLLACK, M. A. : Threshold dependence on active
layer thickness in InGaAsP/InP d.h. lasers', Electron. Lett., 1978,
14, pp. 727-729
2
YANO, M., NISHI, H-, and TAKUSAGAWA, M.: Theoretical and exper-
imental study of threshold characteristics in InGaAsP/InP DH
lasers', IEEE J. Quantum Electron., 1979, 15, pp. 571-579
3
ITAYA, Y., SUEMATSU, Y., KATAYAMA, S., KISHINO, K., a n d ARAI, S.:
'Low threshold current density (100) GalnAsP/InP DH lasers for
wavelength 1-3 nm\ Japan. J. Appl. Phys., 1979, 18, pp. 1795-1805
4
COLEMAN, J.}., and NASH, F. R.: 'Zinc contamination and misplaced
p-n junctions in InP-GalnAsP d.h. lasers', Electron. Lett., 1978, 14,
pp. 558-559
5
ROSENTHAL, A., ITAYA, Y., and SUEMATSU, Y.: 'Measurement of Zn
doping level in InGaAsP/InP DH lasers', Japan. J. Appl. Phys.,
1979, 17, pp. 1655-1656
6
L P n mode cutoff. The cutoff wavelength determined by this
technique represents the local (2-3 mm offibrelength) physical
guide parameters and unlike the techniques using transmitted
power it is not affected by the fibre length and the associated
uncertainties.
Experiment: The experimental set-up, shown in Fig. 2, includes
a tungsten lamp/monochromator combination for variable
wavelength source, the output of which is focused to a 5-6 /im
spot on the fibre end. The normalised refracted power as a
function of wavelength is determined by comparing the refracted power under two conditions: (i) focused spot on core,
and (ii) focused spot on cladding. For (i), good results are
obtained when the focused spot is less than or equal to the core
ABRAMS, E. B., SUMSKI, S., B O N N E R , W . A., a n d COLEMAN, J. J.: ' B e
doping of liquid-phase-epitaxial InP\ J. Appl. Phys., 1979, 50, pp.
4469-4470
7
DYMENT, J. c , RIPPER, J. E, and LEE, T. p.: 'Measurement and inter-
pretation of long spontaneous lifetimes in DH lasers', ibid., 1972,
43, pp. 452-457
8
HWANG, c. J., and DYMENT, J. C : 'Dependence of threshold and
electron lifetimes on acceptor concentration in GaAs-Ga, _ , Al^ As
laser', ibid., 1973, 44, pp. 3240-3244
2
0013/5194/80/180693-03$!.50/0
Si
u
z
\
<b
Ifl
\
>
c
REFRACTED POWER TECHNIQUE FOR
CUTOFF WAVELENGTH MEASUREMENT
IN SINGLE-MODE WAVEGUIDES
Indexing terms: Optical fibres, Waveguides
A technique to measure the local (2-3 mm) values of cutoff
wavelength Xc in single-mode waveguides is described. The
technique, insensitive to the length of waveguide used, involves the spectral measurement of refracted power. The feasibility of using the cutoff wavelength determined by this
technique to predict the zero dispersion wavelength is also
studied.
Introduction: The determination of the cutoff wavelength for
the LPn mode is important for the design and verification of
the different models developed for single-mode fibres. Various
techniques1 3 have been developed to determifle this important parameter. Most of the presently available techniques (e.g.
far-field,1 near-field, and bending methods2)involve the use of
transmitted power and the results have been shown to be sensitive to the fibre length used. For fibres longer than 5 cm bending effects become significant, and for shorter lengths leaky
mode effects are important.3 In this letter, a technique is
described that is insensitive to these effects and which gives an
accurate value of the L P n mode cutoff wavelength Xc that is
representative of the local equivalent step-index guide parameters. The technique involves measurement of the normalised refracted power as a function of wavelength and from
such data the LPn mode cutoff is determined.
Principle: The radiation incident on the waveguide input face
excites different bound, leaky and radiation modes. As the
wavelength of the source is increased, some of the bound
modes become radiation modes and some transform to leaky
modes. For example, in a step-index waveguide the LPn mode
has 100% of the power propagating in the cladding at a Vparameter value of 2-405.4 By immersing the waveguide in a
fluid with a higher refractive index than the waveguide cladding, the radiation modes in the cladding can be refracted out
within a few millimetres of the waveguide input face and the
refracted power measured as a function of wavelength. As indicated in Fig. la, the fraction of power in the cladding, and
hence the normalised refracted power, alters sharply at the
ELECTRONICS LETTERS 28th August 1980
•
R
O
1
700
1
C
i
I
800
900 1000
wavelength ,nm
1100
k32/i|
Fig. 1
a Variation of ratio [Pctad/P],tne fraction of total power in cladding
as a function of K-parameter
b Normalised refracted power log [P(cladding focus ii)/F(core
focus i)] as a function of wavelength for fibre A listed in Table 1
prism
500W
monochromator
tungsten
I
lamp
I chopper
\
e
capillary
tube
U32/2J
microscope
lock-in
amplifer
Fig. 2 Schematic diagram of experimental set-up for refracted power
technique
diameter. The fibre is enclosed in an opaque capillary tube,
with only 1-2 mm of fibre exposed, to prevent the leaky mode
contribution to the measured refracted power. This arrangement is essential to obtain a sharp change in refracted power at
cutoff. This assembly is pfaced in a cell containing a fluid with
slightly higher index than the cladding. The refracted power is
measured as a function of wavelength by using a silicon detector. The refracted power measurement with focused spot on
cladding [condition (ii)] provides the reference power, which
can compensate for source coupling fluctuations and absorption in the index fluid.
Results: Fig. 16 shows the normalised refracted power as a
function of wavelength, with a sharp change in the curve at a
wavelength X? ~ 860 nm. The change at cutoff is not instantaneous as indicated in Fig. la. A major reason for this is the
instrument resolution, and the rest may be due to the depar-
Vol. 16 No. 18
695
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