Optical communications – Transmitter – Having particular modulation
Reexamination Certificate
2000-03-01
2004-01-13
Pascal, Leslie (Department: 2733)
Optical communications
Transmitter
Having particular modulation
C359S237000
Reexamination Certificate
active
06678479
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electro-absorption (EA) modulator integrated light source (i.e., a light emission element) having a plurality of transmission properties or characteristics being different to each other, and further relates to a light emission element module for use in an optical transmission and an optical transmitter, and also an optical transmission system using therein.
Relevant prior arts will be mentioned in relation with first to third ones, below.
Relating to the first relevant art as the light emission element, by referring to attached
FIGS. 1
to
4
, the structure of a semiconductor Ea modulator integrated DFB (Distributed Feedback type) laser is explained, wherein a laser is used in a light emission portion.
The
FIG. 1
shows the semiconductor EA modulator integrated DFB laser having wavelength of 1.5 &mgr;m, for use in an optical transmission of in transmission speed of 10 Gbit/s and 20 km in distance thereof. In this figure is shown a cross-section view of a portion of the stripes of the light emission element, for explaining the structure of the light emission element. This light emission element is formed, after forming a mask of oxidization film for a selective growth method on a n-type InP semiconductor substrate
100
, with growing a lower optical separete-confinement-heterostructure
101
of n-type InGaAsP with the known selective growth method as a first growth of crystal, a strained multiple-quantum-well structure
102
composed of an undoped InGaAsP well layer and eight (8) cycles of barrier layers of undoped InGaAsP having a composition wavelength 1.15 &mgr;m, and an upper optical separete-confinement-heterostructure
103
of two (2) layers of an undoped InGaAsP layer and a p-type InGaAsP layer. With using such the method of the selective growth, in the total thickness thereof, the strained multiple-quantum-well structure in an EA modulator portion
108
is formed to be thinner than that in the laser portion
109
. Accordingly, an absorption wavelength of the strained multiple-quantum-well structure in the EA modulator portion comes to be smaller than that of the laser portion
109
. Further, the semiconductor EA modulator integrated DFB laser shown in the
FIG. 1
is manufactured, by forming a diffraction grating, a p-type InP clad layer
104
, a mesa layer and a re-growth of a Fe-InP layer
105
for concealing both sides of that mesa layer, and then electrodes
107
. The modulator length, i.e., the length for injecting current into a wave-guide portion of the EA modulator, is selected to be 157 &mgr;m, by taking a capacity of the modulator portion and an extinction ratio thereof into the consideration for determining a band of the light emission element, and on a front end surface at the side of EA modulator is treated an antireflection coating
110
, while on a terminal end surface a reflection coating.
Further, the
FIG. 2
shows a light emission element module being installed with the above-mentioned light emission element thereon. The reference numeral
201
shown in the
FIG. 2
indicates a chip carrier, on which the above-mentioned light emission element is mounted, and on which are formed strip lines for high frequency with a patterning technology or method, thereby building up a chip capacitor(s) and a terminal resistor(s), etc., within the light emission element module. Further, within the present light emission element module are installed or integrated a thermistor
202
, an isolator
203
, a lens
205
, a high frequency signal relay substrate
206
, a monitor PD install stem
209
, and a cooling stem
208
. A reference numeral
207
indicates a high frequency signal cable for electric signals.
With this optical element module, the transmission is possible on an ordinary fiber of 20 km (dispersal value: 400 ps
m). However, the transmission is impossible on the fibers other than the ordinary one, being longer than 20 km, such as the fiber of 40 km (dispersal value: 800 ps
m).
The reason of this lies in that the distance of optical transmission is restricted by chirping. Ordinarily in the transmission on the optical fiber, two factors, i.e., the chirping and an intensity of optical output can be mainly considered, of restricting the transmission distance. The restriction due to the intensity of optical output in the latter brings about no problem, since it can be amplified to a certain degree. The main problem here is the restriction due to the chirping in the former. The chirping means an expanse in the wavelength spectrum of light emitted from a semiconductor laser modulated. The reason or mechanism that the chirping restricts the transmission distance is as follows.
The chirping is caused by the following two (2) phenomena. First, during ON/OFF modulating in the light emission element, the chirping occurs in the wavelength due to changes in the refractive index and the absorption coefficient inside the light emission element. Second, it is a phenomenon that the chirping occurs since dispersion is generated when the light emitting from the light emission element propagates within the fiber. Accordingly, the larger the distance in the transmission distance of the fiber, the much more the chirping be caused by the latter. Further, when occurring the chirping too much, the wave-form of light signal is distorted to increase a pass penalty, thereby restricting the transmission distance.
A ratio, between the changes in refractive index and the absorption coefficient during the ON/OFF modulation of the light emission element, is an &agr; parameter, and that is one of the causes of bringing about the chirping, and the lower the &agr;, the less the amount of the chirping during the modulation, then it can be said that the fiber is endurable against the dispersion. Therefore, the smaller the &agr; parameter, the less the ill influence, thereby enabling to extend the transmission distance without receiving the ill influence from the dispersion.
Also, with the a parameter, there are several methods for evaluation thereof, such as, one in which a large signal is inputted into the light emission element to measure it, or other in which a small signal is inputted to measure it by a fiber-response-peak method, etc. However, in the present specification, the &agr; parameter is defined by an evaluation value in accordance with the fiber-response-peak method, wherein the small signal is inputted to the light emission element and a dispersion compensated filer is used (F. Devaux et al., “Simple Measurement of Fiber Dispersion and Chirp parameter of Intensity Modulated Light Emitter” J. Lightwave Technol., vol. 11, pp. 1937-1940, December 1993). Since the &agr; parameter is defined as a ration a change amount in refractive index to that in absorption coefficient, i.e., (change amount in refractive index)/(change amount in absorption coefficient), it varies following voltage applied to the modulator portion of the light emission element, depends upon material and MQW (multiple-Quantum-Well) structure thereof, however the light emission element comes to have almost it's own value if it is manufactured under a certain condition of a specification, though there may be brought about a fluctuation therein a little.
The dependency (hereinafter, “&agr; curve(s)”) upon voltage applied to the EA modulator having a typical &agr; parameter of the light emission element manufactured with those relevant arts, is shown in
FIG. 3
by a curve (a). In the
FIG. 3
, the a parameter depicted by a curve (a) indicates a value from 0.1 to 1.0, i.e., at applying voltage around (V
mod
−V
mod
)/2 when the voltage applied to the EA modulator is set at the modulation amplitude V
mod
of the EA modulator and the amplitude when it is set at high level V
OH
. In case of this value of the &agr;, according to the evaluation of the optical fiber with the light emission element module of those related arts, it is possible to satisfy a standard, which is desired in a pass penalty when transmitting the light signal through it at a dis
Aoki Masahiro
Fujita Minoru
Naoe Kazuhiko
Uomi Kazuhisa
Fahmy Sherif R
Opnext Japan, Inc.
Pascal Leslie
Sofer & Haroun LLP
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