Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
l~ZZ2~iZ
This invention relates to the stabilisatiorl
of the optical output characteristics of an injection
laser.
Semiconductor injection lasers (e.g. gallium
arsenide lasers) have been proposed for use in optical
communication systems to convert either digital or
analo~ue electrical signals into optical signals.
A problem in such applications is that the output
characteristics of a ~emiconductor laser change with
temperature and passage of time. The output
characteristics which can vary are-
Iq the laser threshold current,
/dP~
~ dI/ ~ ~ Iq, the laser efficiency ~ when thecurrent applied to the laser is greater than the
threshold current, and
( ~ I < Iq, the spontaneous e~ficiency ~ when
the current applied to the laser is les3 than the
threshold current
where P is the optical output power of the laser and
I 'he current applied to the laser.
In addition there is an unwanted switch-on
time delay for lasing action between the application
of the input modulating current &nd the appearance o~
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the optical output pulse when the bias current Ib
applied to the laser is below the threshold current
Iq. l'his is usually of importance 2S it is preferable
to operate the laser with its bias current Ib below Iq
for two reasons in most digital systems:
(i) to optimise the rat:io bet~een the 'on'
and 'off' pulses within the limits permitted by the
unwanted switch-on delay and the maximum permitted
peak optical output power. For low speed digital
systems e.g. 8 Mbits/s, the d.c. bias current may
be zero.
' (li) as ~ < ~ , a less sensiti~e control
of the bias current is required in order to maintain
the optical output power in the 'o~f' state at or
substantially at a predetermined level~
For intensity modulated, fre~uency modulated
and very high ,speed di~ital syst,ems it may be
preferable to apply a bias current at a value that
is greater than the thre~hold current.
'L'he switch-on delay (td) can be approximately
represente-l by the equation
td - ~ ln Im
Im + Ib Iq
where Im is the modulating current
Ib is the bias current
I~ is the threshold current
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and ~ is the carrier lifetime (typically of the order
of 1 ns to 4 ns).
Thus it can be seen that if the threshold
current varies the switch-on delay td will also vary
unless an adjustment is rnade to the d.c. bias current
Ib. A change in the switch-on delay td c~n be important~
in an optical communication system particularly if td
increases to such an extent that it becomes significant
compared to the shortest desired input pulse length.
On the assumption that the light output of
the laser at t~eshold remains substantially constant
irrespective of temperature changes or passage of
time we are proposing to monitor the d.c. light level
of the laser and t~ adjust the d.c. bias current (Ib~
applied to the laser so as to maintain the monitored
d.c. light level of the laser at or substantially at
a predetermined value which, for digital systerns, is
at or below the monitored d.c. light output at
threshold. ~or intensity and frequency rno~ulated
communication systems it may be desirable to malntain
the d.c. bias current (Ib) at a value greater than
the thresho d curre~t. In thls way it is hoped to
maintain acceptable values of optical output power
and switch-on delay td throughout the working life
of the laser given that the slope efficiency may
decrease with time and any increase in ternperature.
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According to one aspect of the present
in~ention there is provided apparatus for stab~lising
output characteristics of an injection laser comprising
first mean~ for producing an electrical signal indicative
of the d.c. optical output level of the laser, second
means for producing a reference electrical signal which
is indicative of a predetermined d.c. output light level,
third means for comparing the electrical signal with the
reference signal to produce a difference signal, and
means for feeding the difference signal to d.c. bias
control means of the laser, the d. 5 . bias control means
being responsive to said difference signal to adjust
the d.c. bia~ level such that the d.c. light output
ievel remains substantially at the predeterrnined level.
The predetermined level may be the substantially
constant threshold light output level of the laser. It
has been found that the light output of given lasers,
operating at thresho~d current, may remain approximately
con~tant, and independent of variations in temperature
or degradation resulting from the passage of time.
~ he first means may comprise a pho-todiode
and means for sensing the lowest level of the signal
produced by the photodiode. The photodiode rnay be
arranged either to sample a proportion of the light
emitted from the front face of the laser or to rnonitor
the light emitted from the back face of the la~er.
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In view of the variation of slope efficiencies
and/3 with both temperature and passage of time it
is also proposed to monitor the mean or peak or other
level of optical output powe~ o~ the laser and to
maintain this at a predetermined substantially co~stant
value.
According to another aspect of the present
invention there is provided apparatus. for stabilising
output characteristics of an injection laser compris ng
first means for producing an electrical signal
indicative of a preselected optical output level of
the laser, second means for producing a reference
electrical signal indicative of a predetermined value
of ~aid optical output level, third me~ns f~,~ cornparing
the electrical signal with the reference ~ignal to
produce a dif~erence signal, and means for feeding
the difference signal to modulating current drive
means ~or the laser, said drive means bein~ responsive
to said difference qignal to adjust the modulating
drive current applied to the laser such that the
preselected optical output le,vel remains substantially
at said pred.etermine~ level. l~r~ferably the preselected
level is the mean opti.cal output level or the peak
optical output level,
'l~e first means may comprise a photodiode and
me~ns for sensing the mean or peak level of the signal
produced by the photodiode. 'l~e photodiode may be
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arrangeA either to sample a proportion of the
light emitted from the front face of the laser
or to monitor the light emitted from the back
face of the laser.
A preferred embo~iment of the inven-tion
includes both of said aspects which are arranged
with a common photodiode.
The,invention will be de~cribed now by
way of example only with particular reference to
the accompanying drawings. In the drawings:
Figure 1 is a graph illustrating the
variation of optical power output with current
~or an injection laser;
E`igure 2 shows how the characteristics
of Figure 1 may vary with temperature;
E'igure 3 is a block schematic diagram of
one embodiment of apparatus in accor~ance with the
present in~ention;
F'i~ure 4 is a block schernatis diagram of
another embodiment of apparatus in accordance with
the present invention, and
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Figure 5 is a block schernatic diagram of
a further embodiment of apparatus in accordarlce
with the present invention.
me characteristics of an injection laser
which can vary with te~perature ~nd time are the
laser threshold current I , the lasing efficiency
(dI) I ~ Iq, and the spontaneous efficiency (dI)I < Iq.
These characteris-tics are illustrated in Figure 1
of the drawings. In addition there is a switch-on
time c;elay for lasing action between the application
of an input modulating current to the laser and the
appearance of the output of light from the laser
when the bias current applied to the laser is below
the threshold current Iq. The switch-on delay td
can be approximately represented by the following
equation
td = ~ ln Im
Im + Ib Iq
where Im is the modulating current
Ib is the bias current
Iq is the thre,shold current
and ~ is the carrier lifetime (typica]ly in the order
of 1 nS to 4 nS).
Thus the switch-on delay will vary with
~ariation in the threshold current of the laser.
This can be import~nt since, although it may be
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desirable to bias the laser below the threshcld
current (for digital systellls), the switch-on delay
may represent a limitation in 'he speed of operation
of the laser. The switch-on delay is of importance
when the laser is used in optical col~nunication systems
to transmit information in digital form. me switch-on
delay can be important in such systems when it becomes
significar.t compared to the shortest desired input
pulse duration. For intensity and frequency modulated
optical communication systems, the variation of
threshold current and slope efficiency ~ with temperature
and the passage of time is important.
We propose to control the d.c. bias by
monitoring the d.c. light output level of t~e laser
and adjusting the d.c. bias to maintain the light
output level at a substantially constant level. The
control means relies for its operation on the assumption
that the light output at threshold remains substantially
constant irrespective of temperature changes or ageing
of the laser. Th~s is illustrated in F~ure 2 for three
temperatures T1, T2 and T~.
Referring now to E'igure ~ there is shown
an apparatus for controlling the d.c. bias by monitorin~
the light output of the laser. '~he apparatus can also
control the peak output light level of the laser. The
apparatus of Figure 3 has a modulating drive current
.~ _ g _
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circui-c 15 which receives either pul~e or analogue
data and produces a drive current in response thereto
for o~erating a semiconductor ~aser 20. Li~ht from
the laser is transmitted towards an optical waveguide
(not sho~) via a semi-reflecting member 21 which
deflects a portion of the light towards a photodiode
23. rme output of the photodiode is connected to
the input of à minimum level monitoring circuit 24
and also to the input of a peak monitoring circuit 25.
me minimu~ level monitoring circuit 24 is arranged
to produce an output signal which represents the
d.c. light output level of the laser and the peak
monitorin~ circuit 25 produces an output signal
which represents t~,e peak optical power output of th~
laser 20. The output of the minimum level monitoring
circuit 24 is connected to a comp~rator 27 and the
output of the peak monitoring circuit 25 is connected
to a comparator 28. ~ach colnparator 27, 28 recelves
an input from a reference circuit 30 the output of the
reference circult 30 representing predetermined ]ight
output levels. The reference signal app~ied to the
cor~parator 27 is indicative of a predetermined d.c.
light output level and the refererloe sil~al applied to
the comparator 28 is indicative of a prede-termined peak
light output. The output of the comparator 27 is
cor~ected to a d.c. bias circuit 32 which provides the
d.c. bias signal for the laser 20, and the output of the
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il'~2Z~2
comparator 28 is connected to the modulatin~ drive
current circuit 15. An input signal level monitoring
circuit 35 is connected to the modulatin~; circuit 15
and to the bias circuit 32.
In the circuit of Figure 3 the modulating
drive current circuit 15 can comprise a bipolar
transistor which is arranged to vary the current
through the laser 20 according to the ~ifference
signal received from the comparator 25. Alternatively
the circuit 15 can be an E`ET which i~ connected in
parallel with the laser, the difference signal being
applied to the gate of the FET.
The minimum level and peak level monitoring
circuits can each comprise a conventional diode and
capacitance detector. The comparators 27, 28 each
comprise a low off,set operational amplifier. The
output of the photodiode 23 is oonnected to the
clrcuits 24, 25 by a d.c. coupled, ternperature
compensateA amplifier.
'~he d.c. bias circuit 32 ~an be a ramp
circuit which controls the d.c. bias current
according to the difference si~nal ~rom the
comparator 27.
The input signal level monitoring circuit
35 can be a diode-capacitance detection circuit lor
detecting data pulses at the inpu' to the drive cir~uit
15~ The circuit 35 is provide~ to ensure that the drive
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ll;~;~Z~2
circuit 15 hnd '~ias circuit 32 are oll]-r operational
when data pulses are fed to the laser.
In use if the d.c. light output level of
the laser varies the outpu-t of the minimum level
monitoring circuit 24 will vary and a difference
signal will be produced by the comparator 27 which
difference signal is fed to the d.c. bias circuit 32.
1`he d.c. bias circuit operates to vary -the output
level of the laser 20 so that the d.c. light output
level is returned to the value represente~ by the
reference circuit 30. In this way the d.c. light
output o~ the laser 20 is maintained at a constant
level. I'he peak monit,oring circuit 25 i~ provided
to monitor peak optical output power of the laser
and mai~tain this at a constant level in view of
the possible variation of slope elf.ciency of the
laser with ternperature and time. If the peak
output value varies this is sensed by the comparator
28 which provides an appropriate signal for the
rnodulating drive ourrent circuit. me modulating
current provided by the modulating drive current
circuit is then varied appropriately to return the
peal~ output to its predetermined va~ue.
I~lus it will be seen that the present
arrangement provides a means for controlling the
d.c. bias current in order that it may be maintained
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within certain limitations. Ln the case of digital
systems the arrangement maintains the switch-on
delay to within certain limitations. The arrangement
also monitors th~ peak output light level of the
la,ser and maintain~s this at a constant level.
The e~nbodiment described with reference to
E'igure 3 monitors the peak outp~lt iight level of a
laser. An alternative to this arral~ement is one
which monitors the mean op-ti.cal output level of the
laser.
As explained above laser switch-on delay
occurs when the d.c. bias current app].i.ed to the
laser is below the threshold current. The main
effects of the switch-on delay in the performance
of an optical co~nunications system using optical
fibres are:
(a) a reduction in the power of the pul.se.s
transmitted down the optical fibre.
(b) Mis-equalisation in the deGision
circuitry because the received pulses are of
shorter duration than an equaliser of` the circuitry
is designed to accommodate.
The relative importance of these two effect.s
depends upon the method by which the laser is driven
i.e. whether by l~RZ (non return to zero) or by RTZ
(return to zero) pulses. We have found that a
reduction in pulse power signif'icantly ~ffects the
system particularly when reduced width RTZ pulses are
used.
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il'~ZZti2
We h.ave ~evised an arrange!-1ent which monitors
the mean optical output level of a laser and maintain~
the opti.cal output -,oulse power of the laser con~tant
by varying thc amplitude of the pulse. The arrangement
is shown in E'igure 4. This arrangement has a modulati.ng
~rive current circuit 50 which receives either pulse or
analoKue data and produces a drive current in respon~.e
thereto for operating a semiconductor laser 51. Light
from the laser is transmitted towa-rds an optical
wave~uide (not shown) via a semi~reflecting member 52
which deflects a portion of the light towards a
photodiode 54, The output of the photodiode is
connected, to the input of a mean power detector 56
whi.ch produces an output signal indicative of the
mean optical power output level of the laser 51, The
OUtpl,lt o~ the detector 56 is connected to a comparator
57 which also receives a signal from a reference circuit
58, the signal from the reference circuit 58 being
indicative of a predetermined rne~n optica.l power output.
The output of the comparator 57 is connectçd to the
circuit 50~ An input signal level monitoring circuit
60 is connected between the in~ut of the dri~e current
circuit 50 an~ the output from the cornr)arator ~7.
In the arrangement o~ E`igure 4 the block~
can be constituted by similar componen,ts to those
described with reference to Fi~,ure 3 with the e~ception
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that th~ pllû~,odiGde 54 is conllecte~ to ~he co~iparator 5'7
by a large value resistor which effectively forms the
block 56. The volta~e si~nal across this resistor is
applied to the comparator 57.
In use the output signal o~ the comparator 5'7
is used to control the amplitude o~ the input current
drive pulses to rrlaintain the optical power outpu-t
constant in a manner similar to that described with
reference to Figure ~. For example, consider an
increase in switch-on delay of the laser. In the
case o~ ~TZ pu'.se operation the resu~t i,s bhat the
pulse duration is reduced and the amplitude of the
pulses increased to maintain const~nt optical power
output. In the case of i~Z pulse operation the
first pulse in a level O to level 1 transition is
o~ shûrter cluration and the ampli'ude of the
pulses is increased to maintain const~nt optical
power output.
The arran~ement shown in Figure 4 is suitable
for relatively lo~; speed systems (e.g. ~ Mbits/sec).
E'or higher speecl systems (e.g. 140 }~its/sec) the
switch-on delay when pulsin~ ~rorn zero bias ~,ay be too
large and a d.c. bias current, contro:l led relative to
threshold, may have to be employed in a-3dition to
the mean power output control. Such an arrangement
is shown in h`i~e 5. The d.c. bias current control
operates in a manner similar to that describe~l for
11222~2
E'igure 3 ~hilst the Mean power out~ju-t control is
similar to that described with reference to ~ ure 4
with the exception that it i.s desi~ned to respon~
to the mean of pulse component of the optical output
o~ the laser rather than the rnean of total opti.cal
power output.
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