Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-09-25
2004-06-08
Dang, Hung X. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S237000, C372S028000, C372S032000, C372S050121
Reexamination Certificate
active
06747776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator used for optical communication and an optical multiplexing module using a plurality of such optical modulators. More particularly, it relates to a semiconductor optical modulator of the electroabsorptive type, and to an optical multiplexing module using a plurality of electroabsorptive modulators.
2. Description of the Related Art
A block diagram of a conventional optical multiplexing module having a multiplexing factor of two is shown in FIG.
6
A. The optical multiplexing module
50
includes a combiner
1
that combines a pair of optical signals into an output optical signal LTt, a first modulator
2
that modulates a source optical signal LTs according to a first modulating voltage Vm
1
, a second modulator
3
that modulates the source optical signal LTs according to a second modulating voltage Vm
2
, a pair of attenuators
4
,
5
that attenuate the optical power of the source optical signal LTs, and a splitter
6
that splits the source optical signal LTs into two signals for output to the two attenuators
4
and
5
.
From the splitter
6
, the source optical signal follows a path of optical length L
1
through the first attenuator
4
and first modulator
2
to the combiner
1
, and another path of optical length L
2
through the second attenuator
5
and second modulator
3
to the combiner
1
. The source signal comprises, for example, a regularly spaced series of narrow pulses of light, which are modulated in the modulators
2
and
3
. Even though a source light pulse enters the two paths simultaneously, if path length L
2
is longer than path length L
1
, the two modulated light pulses will reach the combiner
1
at different timings. Time-division multiplexing can be performed by setting the path length L
2
so that the timing difference is equal to one-half the interval between successive output pulses from the first modulator
2
.
The two modulators
2
and
3
ideally have identical insertion losses. In practice, however, fabrication inaccuracies may lead to different insertion losses.
FIG. 7
shows a perspective view of the structure of one example of a conventional semiconductor optical modulator. Formed as a semiconductor chip by the use of semiconductor fabrication technology, the semiconductor optical modulator
20
includes an electroabsorptive layer
21
that absorbs light to different degrees depending on an electric field intensity, a p-type clad layer
22
disposed above the electroabsorptive layer
21
, an n-type clad layer
23
disposed below the electroabsorptive layer
21
, polyimide layers
24
, disposed on the right and left sides of the electroabsorptive layer
21
, a contact layer
26
disposed above the p-type clad layer
22
, and a pair of modulating voltage electrodes
27
,
28
disposed respectively above the contact layer
26
and below the n-type clad layer
23
.
If incident light LTin (the source optical signal LTs) enters the electroabsorptive layer
21
as indicated by the arrow in
FIG. 7
, and a modulating voltage Vm
1
is applied to the modulating voltage electrode
27
while the lower electrode
28
is grounded, the electroabsorptive layer
21
absorbs the incident light LTin to different degrees, responsive to the modulating voltage Vm
1
; consequentially, a modulated light signal is output from the electroabsorptive layer
21
. The electroabsorptive layer
21
has a limited light-absorbing capability per unit length, however, so to provide a modulation depth adequate for optical communication, the semiconductor optical modulator
20
must have at least a certain necessary length, this being the dimension of the electroabsorptive layer
21
in the direction of light propagation.
The modulators in general use are of the transmissive type, in which incident light enters at one end and is output from the other end. The necessary length dimension can be reduced by half, however, by using modulators of the reflective type, in which light enters and exits at the same end. In this case, the modulators are coupled to their respective light paths by optical circulators, not shown in the drawings.
FIG. 8A
shows a typical example of the optical power level of the output optical signal LTt in the optical multiplexing module
50
in
FIG. 6A
when the first attenuator
4
and second attenuator
5
are not controlled, that is, when the power level of the output optical signal is not adjusted. The optical power level OPS
2
of signals S
2
-
1
and S
2
-
2
from the second modulator
3
exceeds the optical power level OPS
1
of signals S
1
-
1
and S
1
-
2
from the first modulator
2
by a value D. This power difference D may arise because of an unequal splitting ratio in the splitter
6
, or because the absorption characteristics (extinction ratios) of the first modulator
2
and second modulator
3
differ, due to differing fabrication conditions.
In the conventional optical multiplexing module
50
in
FIG. 6A
, the first attenuator
4
and the second attenuator
5
adjust the power level of the source optical signal LTs so as to suppress this power difference D.
An optical multiplexing module is also known that adjusts the power levels of the output optical signals to a constant value OPSs by controlling the modulating voltages as shown in
FIG. 6B
, instead of by using attenuators. In place of the attenuators
4
and
5
in
FIG. 6A
, the optical multiplexing module
51
in
FIG. 6B
has a first modulating voltage controller
7
that biases the modulating voltage Vm
1
input to the first modulator
2
, and a second modulating voltage controller
8
that biases the modulating voltage Vm
2
input to the second modulator
3
, thereby adjusting the power levels of the optical signals output from the first modulator
2
and the second modulator
3
to a uniform level OPSs.
Due to differing fabrication conditions or an insufficiently accurate fabrication process, however, the two modulators
2
,
3
may have different extinction ratio characteristics, and thus respond differently to biasing of the modulating voltage, making a uniform output level difficult to achieve. For example, one modulator may operate in a linear region of its extinction ratio characteristic, while the other modulator operates in a nonlinear region. This is particularly apt to occur when modulators of the reflective type mentioned above are used, since their extinction ratio characteristics tend to show more non-linearity than is seen in modulators of the transmissive type.
FIG. 9
shows examples of the extinction ratio characteristics of electroabsorptive modulators of the reflective type (solid line) and transmissive type (dotted line). If a reflective modulator having the extinction ratio characteristic indicated by the solid line in
FIG. 9
is employed, and if the modulating voltage is biased at minus one volt (−1 V) with an amplitude of ±1 V, then the excellent linearity of the extinction ratio characteristic from 0 V to −2 V can be used. If the bias voltage is set to −2 V, however, then the amplitude of the modulating voltage extends into the region of poorer linearity below −2 V. Moreover, even if the bias voltage is set to −1 V, if the amplitude of the modulating voltage exceeds ±1 V, then the region of poor linearity in the extinction ratio characteristic below −2 V must be used.
If the optical multiplexing module
50
in
FIG. 6A
uses semiconductor optical modulators
20
having the imperfectly linear extinction ratio characteristics described above, then even though the optical power levels of the optical signals output from the modulators are adjusted to a constant value OPSs by the use of attenuators, fabrication variations and the non-linearity of the extinction ratio characteristics of the semiconductor optical modulators may cause the modulating fields generated by the modulating voltages Vm
1
and Vm
2
to have varying effects, so that the modulated output signals vary as shown in
FIG. 8B
, leadi
Fujii Kozo
Ozeki Yukihiro
Dang Hung X.
Oki Electric Industry Co. Ltd.
Sartori Michael A.
Tra Tuyen
Venable LLP
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