AMPLIFICATEUR OPTIQUE

OPTICAL AMPLIFIER

Abrégé anglais

ABSTRACT
OPTICAL AMPLIFIER
Known optical amplifiers, which are used in a light transmis-
sion path to amplify the light signals to be transmitted, have
the disadvantage that the semiconductor lasers, used as pump
lasers therein, are very sensitive to feedback.
According to the invention, the operating current of such pump
lasers (4) is modulated at a high frequency, which causes the
pump laser to operate in a multi-mode, thereby making it in-
sensitive to feedback. For example, the operating current is
DC, on which AC is superimposed.
In the event the optical amplifier has two pump lasers, their
operating current is also modulated. Both pump lasers are
then advantageously alternately switched on and off at a suf-
ficiently high switching frequency, which also prevents recip-
rocal interference.
Figure 1.

Revendications

PATENT CLAIMS
1. An optical amplifier with a length of fiber (2) that
contains a laser-active substance for amplifying light sig-
nals, which is coupled to at least one semiconductor laser (4)
serving as a pump light source, which is activated by an oper-
ating current, characterized in that an operating current
source (5, 6) is provided, and produces a modulated operating
current for the pump laser or lasers (4, 8), where the modula-
tion frequency of the operating current is above the recipro-
cal lifetime of the energy level of the laser-active sub-
stance, which amplifies the light signals when it decays.
2. An optical amplifier according to claim 1, charac-
terized in that the operating current source contains a DC
source (5) and an AC source (6), whose output currents togeth-
er form the operating current.
3. An optical amplifier according to claim 1, with two
semiconductor lasers (4, 8), each connected as a pump light
source to one end of the length of fiber (2), characterized in
that the operating current source supplies current to the two
semiconductor lasers in such a way, that the two semiconductor
lasers are alternately switched on and off.
4. An optical amplifier according to claim 3, charac-
terized in that the operating current for each of the two
alternately switched-on semiconductor lasers (4, 8), is a DC
current that is switched on and off at the named frequency,
and is thereby modulated.
5. An optical amplifier according to claim 3, charac-
terized in that the operating current for each of the two
alternately switched-on semiconductor lasers (4, 8) is a DC
current that is switched on and off, which is modulated at the
named frequency.

Description

2~7~13
TRANSLATED FROM GERMAN
OPTICAL AMPLIFIER
Optical amplifiers serve to amplify light signals which are
transmitted over a transmission path, in particular a fiber
optical waveguide (optical fiber), and are weakened by attenu-
ation so that they must be amplified.
An optical amplifier with characteristics according to the
introductory clause of patent claim 1 is known e.g. from: M.
Yoshida et al: "Development of Compact Er3+-doped Fiber Ampli-
fiers for Practical Applications", Technical Digest on Optical
Amplifiers and their Applications 1990 (Optical So~iety of
America, Washington, DC 1990), Volume 13, pages 282 to 285.
It is a fiber amplifier with an erbium-doped length of fiber.
Erbium is a laser-active substance, which is excited by a
semiconductor laser serving as a pump laser. In this
instance, the semiconductor laser emits light at a wavelength
of A = 1480 m. This light is introduced into the doped length
of fiber by an optical coupler. Direct current is used as the
operating current to control the semiconductor laser.
Instead of a single pump laser, as in figure la, two pump
lasers as shown in figure lc can also be provided, whose pump
light is coupled into each end of the length of fiber.
A disadvantage of this optical amplifier is that the pump
light produced by the semiconductor laser i8 reflected back
into the semiconductor laser at all points along its propaga-
tion path where refractive index skipping occurs (e.g. in the
area of the opti~al coupler or in the transition between the
optical fiber and the erbium-doped length of fiber). This
degrades the efficiency of the semiconductor laser; the re-
flected light can cause amplitude fluctuations in the pumplight emitted by the semiconductor laser, because the laser-
active layer in the semiconductor laser acts as a laser reso-
nator, together with part of the transmission path (i.e. up to

~7'~81~
the area with the refractive index skip). This changes the
emission frequency of the semiconductor laser, on the one
hand, and subjects the amplitude of the pump light to oscilla-
tions, on the other. In particular, the amplifying effect of
the length of fiber is temporarily cancelled when the oscilla-
tions occur in the low frequency range, e.g. in the kilohertz
range; this can interrupt the optical transmission, under
certain circumstances.
In the event two pump lasers are present (semiconductor lasers
in each instance), the described problem exists for each of
the two pump lasers. Furthermore, in that case there is the
problem of residual pump light from each pump laser entering
the opposite pump laser, which causes optical instabilities,
which in turn lead to the above described problems of fluctua-
tions in amplification. For that reason, optical isolatorsare presently used before the pump lasers.
It is the task of the invention to produce an optical amplifi-
er with one pump laser or two pump lasers, which ensures in-
terference-free amplification of the light signals.
The task i~ fulfilled as indicated in patent claim 1.
An advantage of the invention consists in that it makes unnec-
essary the costly installation of optical isolators between
the optical coupler and the semiconductor laser, immediately
before the latter, to prevent the pump light from being re-
flected back into the semiconductor laser.
The frequency at which the operating current i5 modulated, isabove the reciprocal lifetime of the excited energy level of
the laser-active substance, e.g. erbium, which causes the
amplification of the light signals to be transmitted. In this
way, the fluctuations in the pump light amplitude are not
"perceived" by the laser-active substance, i.e. the length of

~7~
fiber acts as a low-pass filter for the varying component of
the pump light signal.
Further advantageous configurations can be found in the
subclaims.
The following describes the invention in greater detail by
means of drawings representing only one configuration, where:
Figure 1 shows an optical amplifier according to the inven-
tion with a single pump laser, and
Figure 2 shows an amplifier according to the invention with
two pump lasers.
In figure 1, an optical amplifier is inserted into a light
waveguide transmission path 1. It has a length of fiber 2,
which is doped with a laser-active substance, e.g. erbium.
Light signals are transmitted over the light waveguide trans-
mission path 1, and amplified in the length of fiber 2. Thelight signals have a wavelength of A = 1530 - 1570 nm. One
end 21 of the length of fiber 2 is connected to a semiconduc-
tor laser 4 by an optical coupler 3. The sQmiconductor laser
4 is, for example, an InGaAsP/InP-, an InGaAs/AlGaAs- or a
GaAlAs/GaAs-laser, which emits at a wavelength in the range of
= 750 - 870 nm, A = 960 - 1000 nm or A = 1460 - 1500 nm. An
electrical contact 41 connects it to a DC source 5. So far,
the description of the optical amplifier in figure 1 matches
the known types.
According to the invention~ the operating current of the laser
is not a DC current but a modulated current. For that pur-
pose, the semiconductor laser 4 in the configuration in figure
1 has an electrical contact 42, which connects it to an AC
source 6. The AC source 6 produces for example either a si-
nus-shaped or a ~quare-shaped alternating current.

207 ~
The AC source 6 produces an alternating current, which, to-
gether with the direct current from the DC source, forms the
operating current of the semi~onductor laser 4 ~pump laser).
In other words: the operating current is a direct current on
which an alternating current has been superimposed, or: the
operating current is a (periodically) modulated direct cur-
rent.
Instead of an arrangement of DC source S and AC source 6, a
modulatable DC source can also be used a~ the operating cur-
rent source. Only the operating current must be modulated insome way.
The semiconductor laser 4 emits multi-mode pump light based on
the alternating current, i.e. the modulated operating current.
For example, the light from a semiconductor laser emitting at
~ = 1480 nm, which ha~ only a few modes with direct current,
is split by the alternating current portion into 20 - 30
modes, which have a mode spacirg of 80 GHz.
The frequency of the AC, i.e. the modulation frequency of the
operating current, i8 above the reciprocal lifetime of the
excited, laser-active energy level of the erbium in fiber
length 2. Since the lifetime of the energy level is in the
millisecond range, the AC should be provided with a frequency
in the lower megahertz range. This prevents the pump light
frequency from modulating the light signals to be transmitted
and amplified.
As mentioned in the beginning, there are fiber optic amplifi-
ers with two pump lasers on each side of the amplifying length
of fiber. The problem of back reflection of pump light into
the pump laser can al80 be solved as indicated earlier, by
operating the pump laser or lasers with an operating current
which is a modulated direct current.

2 ~ 7 ~
Figure 2 shows a configuration example of such a solution.
The left portion of the figure corresponds exactly to figure
1, therefore this portion does not need to be explained again.
The second end 22 of the length of fiber is connected to a
second semiconductor laser 8 by an optical coupler 7. The
laser has two electrical contacts 81 and 82, which connect it
to the DC source 5 or the AC source 6.
Instead of the single AC source 6, each semiconductor laser 4
and 8 may have its own AC source. The same applies to DC
sour~e 5.
An operating current source of the type shown in figure 2, or
by other means, as explained above for the instance of a sin-
gle pump laser, also provides modulation of the operating
current in the case of two pump lasers.
Still, a reciprocal interference effect of the pump laserg
cannot be excluded. ~hat i8 why optical isolators are pres-
ently placed before the pump lasers. The invention permits
omitting the isolators, when the two pump lasers are alter-
nately switched on and off, so that only one is active and the
other inactive. This represents a modulation of the operating
current of each of the two pump lasers, which prevents recip-
rocal interference of the semiconductor laser6 4 and 8. The
multi-modicity of the lasers i8 automatically ensured by the
switching, if the switching frequency is sufficiently high.
If it i8 high enough for the purpose of multi-modicity of the
pump laser, it is also practically higher than the reciprocal
lifetime of the excited energy level of the laser-active sub-
stance in the length of fiber 2, which is required in all
instances, as explained earlier.
Although with two alternately switched-on pump lasers, the
average pump output in the length of fiber 2 is only as large
as that of a single, permanently operating pump laser, the
following advantages are found: bidirectional pumping simul-

2 ~ 7 /~
taneously achieves both a lower noise factor and a higheroutput. In addition, each laser is only used half of the
time, which has a positive effect on reliability.
For example, to supply both pump lasers with alternately
switched-on operating current, an ~F generator with two in-
verted outlets can be used, one of which always conducts cur-
rent while the other conducts no current.
To switch the pump lasers on and off, it is unimportant wheth-
er the operating current of one pump laser is fully switched
off during its idle phase, or is only lowered under the pump
laser's threshold current.
~he alternate on and off switching of the two pump lasers can
also be achieved with a device that supplies a square-wave AC
alternately to one and the other pump laser through a switch.
Regardless of how it is produced, the operating current of a
pump laser may be a square-shaped DC that is turned on and
off. How-ever, it can also be a square-shaped DC that is
turned on and off, on which AC is permanently superimposed, or
only during the on-phase of the pump laser, 60 that a modulat-
ed DC flows as the operating current through the pump laserduring the on-phase itself. This i8 particularly necessary
when the switching frequency of the alternate on-switching of
the two lasers i8 not high enough to make each of the pump
lasers multi-modal during its on-phase. In that instance, the
DC modulation supplie6 the multi-modicity during the on-phase.

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