Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2002-03-20
2004-08-31
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S432000, C257S433000, C257S434000, C257S435000, C257S632000, C257S791000
Reexamination Certificate
active
06784512
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photodiode for a light receiving module of optical communications, in particular, a photodiode with wavelength selectivity suitable for wavelength division multiplex (WDM) optical communications. The WDM communications signifies an optical communications system making use of more than one wavelength of light as signal light. For example, a first wavelength &lgr;
1
is allocated to transmission (upstream) light and a second wavelength &lgr;
2
is assigned to receiving (downstream) light. In this case, the photodiode (PD) of a receiving module on a subscriber should preferably not feel &lgr;
1
but sense only &lgr;
2
. Conventional photodiodes, however, have sensitivity both for &lgr;
1
and &lgr;
2
.
This application claims the priority of Japanese Patent Application No.2001-84542 filed on Mar. 23, 2001 which is incorporated herein by reference.
2. Description of Related Art
FIG. 1
shows a cross-sectional view of a typical one of conventional photodiodes (PD). This is a bottom incidence type photodiode. The material of a light receiving layer depends upon the wavelengths of signal light. For example, in the case of the optical communication systems using a wavelength band ranging from 1.3 &mgr;m to 1.6 &mgr;m, an InGaAs layer
2
is epitaxially grown directly or indirectly on an n-InP substrate
1
as a light receiving layer. InGaAs is simplified expression of a ternary mixture crystal In
1−x
Ga
x
As. Here, x is a mixture rate. The mixture rate is determined to be a definite value from the lattice matching condition between the InP substrate and the InGaAs light receiving layer. A p-type region
3
and a pn-junction
4
are produced by diffusing Zn, a typical p-type dopant, into a central part of the n-type light receiving layer
2
.
The pn-junction has ends revealing on the top surface. The revealing ends are covered with an insulating film
6
(passivation film), e.g., of silicon nitride (SiN
x
). A p-type electrode
5
is formed upon the p-type region
3
. An annular n-type electrode
7
with a central opening is made upon the bottom of the n-InP substrate
1
. An antireflection film
8
is laminated upon the opening of the InP substrate
1
. A photodiode is reversely biased by applying a lower voltage to the p-type electrode
5
(anode) and a higher voltage to the n-type electrode
7
(cathode) in use. The reverse bias induces depletion layers on both sides of the pn-junction
4
, a p-type depletion layer on the p-side and an n-type depletion layer on the n-side. The n-type depletion layer on the n-side is important. Signal light
9
goes via the antireflection film
8
into the bottom of the InP substrate
1
, and attains to the n-type depletion layer in the light receiving layer. Light having energy larger than a band gap makes pairs of electrons and holes by inducing a band gap transition of electrons from a valence band up to a conduction band. An electric field formed by the reverse bias pulls holes upward over the pn-junction into the p-type region
3
and pushes electrons downward to the n-type region, which induces photocurrent. The photocurrent is taken out from the photodiode. The band gap of the light receiving layer determines what wavelengths of light can be detected by the photodiode.
FIG. 2
shows wavelength dependence of sensitivity of the photodiode having an InGaAs light receiving layer. The abscissa is a wavelength (&mgr;m). The ordinate is sensitivity (A/W). The InGaAs photodiode has a wide range (Q) of sensitivity from 1 &mgr;m(P) to 1.6 &mgr;m(R). The InGaAs photodiodes are endowed with high utility and prevalence by the wideness of the sensitivity range.
The wide sensitivity range incurs a problem on the photodiodes in the case of the multiwavelength optical communications which includes a plurality of wavelengths of light signals. Conventional InGaAs photodiodes have sensitivity for not only the object wavelength &lgr;
2
but also the noise wavelength &lgr;
1
to which the PD should not response.
For example, in the case of a single fiber bidirectional, optical communications system making use of a 1.3 &mgr;m wavelength (&lgr;
1
) and 1.55 &mgr;m wavelength (&lgr;
2
), 1.3 &mgr;m light emitted from a laser diode (LD) is noise to a 1.55 &mgr;m-detecting PD. The 1.3 &mgr;m light emitted from the laser diode tends to go into the photodiode, which induces noise in the 1.55 &mgr;m-detecting photodiode. The above is a problem appearing in a bidirectional transmission system in the full duplex transmission mode
Otherwise, a rapidly developing dense wavelength division multiplexing (DWDM) system or coarse wavelength division multiplexing (CWDM) system contains a plurality of channels and uses a set of different wavelengths with a narrow spacing for one direction stream of signals for the channels and another set of different wavelengths with a narrow spacing for the other direction stream of signals for the channels. The DWDM and the CWDM require sophisticated laser diodes which can produce many different wavelenghs of light which are rich in monochromacity. The photodiodes having a wide range of sensitivity as shown in
FIG. 2
, are not favorable for the DWDM and the CWDM, because the PD would invite serious crosstalk among neighboring channels. On the contrary, the photodiodes having a narrow range of sensitivity with fine resolution (&Dgr;&lgr;) of a nanometer to tens of nanometers are suitable for suppressing crosstalk among different channels with small differences of wavelength. An important problem is how to give photodiodes sharp wavelength selectivity for meeting the requirement of suppressing the crosstalk among the channels.
One way of assigning wavelength selectivity is an addition of a dielectric multilayered film to a wide sensitivity range photodiode. A wavelength selective photodiode is obtained by adding a dielectric multilayered film on the market to the opening of the bottom of the wide sensitivity range PD of FIG.
1
.
{circle around (1)} Masahiro Mitsuda, Tohru Nishikawa, Tomoaki Uno, Masato Ishino, “Optical Cross-talk Reduction of LD/PD Module for ATM-PON System”, Proceedings of the 2000 Communications Society Conference of IEICE, B-10-55, p278, proposed an LD/PD module employing a double-cladding optical fiber, adding a 1.3 &mgr;m absorbing InGaAsP layer on the top of a photodiode, laying the photodiode epi-down on a glass substrate, painting an enclosure of the photodiode with a stray light absorbing resin for suppressing crosstalk. The double cladding prevents once fiber entering LD light (&lgr;
1
) from leaking out of the fiber. The InGaAsP absorption layer fitted to the top of the photodiode absorbs 1.3 &mgr;m light which is noise for the photodiode. The painted resin eliminates the 1.3 &mgr;m noise light from the photodiode.
{circle around (1)} employs three different means for eliminating noise light from the photodiode.
{circle around (2)} R. Momura, H. Yanagisawa, A. Goto, Y. Fukutomi, N. Kitamura, M. Kunitsugu, K. Kaede,” An ONU Transceiver Module using PLC for 622 Mb/s downstream ATM-PON system”, Proceedings of the 2000 Communications Society Conference of IEICE, B-10-59, p282, proposed a transceiver (LD/PD) module which protects a photodiode from invasion of LD light by a WDM (wavelength division multiplexing) filter. The WDM filter is a dielectric multilayered film made on a glass substrate having wavelength selectivity.
The dielectric multilayered film is made by piling repeatedly in turn at least two kinds of dielectric layers with different refractive indexes and different thicknesses on the glass substrate. The on-glass dielectric film has wavelength selective reflection and wavelength selective transparency. Reflection wavelength and transparent wavelength are determined by the choice of refractive indexes and thicknesses of dielectric layers. Arbitrary wavelengths can be assigned to the reflection wavelength and the transparent wavelength by selecting refractive indexes and thicknesses of the component dielectric layers. The refl
Kuhara Yoshiki
Sasaki Takashi
Yamaguchi Akira
LandOfFree
Photodiode and method of producing same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photodiode and method of producing same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photodiode and method of producing same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3352562