Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
1998-12-07
2001-04-17
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S185000, C257S190000, C257S461000, C438S093000, C438S094000
Reexamination Certificate
active
06218684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photodiode (PD) used in a system where light with a single wavelength is transmitted over a single optical fiber for bi-directional communications. More specifically, the invention relates to a photodiode used suitably for time-compression-multiplexing (TCM) transmission, or so-called “ping-pong transmission”, based on time division using 1.3-&mgr;m-region light for transmission and reception.
2. Description of the Background Art
FIG. 1
is a diagram showing a simplified constitution of bi-directional communications. A laser diode LD
1
at a central office sends an optical signal. An optical coupler
2
introduces the signal into a fiber
3
, and an optical coupler
4
guides it to a photodiode PD
2
at a subscriber, such as a house. This is downstream traffic. A signal from the subscriber is sent out by an LD
2
. The signal travels through the optical coupler
4
, the fiber
3
, and the optical coupler
2
and reaches a PD
1
at the office. This is upstream traffic.
There are two systems in bi-directional communications; one is simultaneous transmission and reception, and the other is alternate transmission and reception with time division. Some types use light having two or more wavelengths, and others use light having one wavelength. When light having two or more wavelengths are used, a wavelength-division-multiplexing coupler must be provided additionally. The present invention relates to the improvement of a photodiode used in a simple bi-directional communications system that transmits and receives signals having one wavelength on a time-division basis.
Since transmission and reception are conducted alternately, this system is called “ping-pong transmission.” It is the simplest bi-directional communications since only one kind of light is used. Still it needs optical couplers in order to transmit the upstream and the downstream light over the same optical fiber. When two optical fibers are used, optical couplers are not necessary, but the cost for installing the fibers will increase. Therefore, transmission and reception over a single fiber is desirable. Optical couplers seem essential parts when coming and going signals travel over the same single fiber, because two kinds of light must be selected so that one kind is sent to the photodiode, and the other to the fiber after receiving it from the laser diode. However, optical couplers are expensive and increase the installation cost at subscribers. Therefore, optical communication without optical couplers is desirable.
A previous invention by the present inventors enabled the realization of this seemingly unattainable object. According to the invention, a photodiode is devised to absorb a half of light having a particular wavelength, and the photodiode and a laser diode are connected in tandem so that transmitted or received light propagates linearly. The laser diode is placed behind the photodiode so that the transmitted light from the laser diode passes through the photodiode and enters an optical fiber linearly. The photodiode absorbs half the transmitted light from the laser diode, allowing the other half to pass through. Similarly, the photodiode absorbs and detects half the received light from the fiber, allowing the other half to pass through and reach the laser diode, which is inactive then because of “ping-pong transmission” and causes no bad-effects. The condition of transmitted light and received light being halved is allowed when a sufficient amount of light is transmitted. This system does not require separation of optical path and eliminates an expensive optical coupler. In the system shown in
FIG. 1
also, both the transmitted and received light are reduced to half by the optical couplers.
The previous invention described above was offered in Japanese patent application Tokuganhei 9-256107 filed on Sep. 3, 1997. This idea revolutionized the conventionally accepted knowledge that a photodiode is to absorb incident light 100 percent. Such a photodiode that absorbs a half of incident light is called a half-transmittance photodiode or simply transmittance photodiode.
FIG. 2
is a constitution diagram of a light transmission and reception module that has a combination of the above-mentioned transmittance photodiode and a laser. A fiber
62
, a lens
126
, a transmittance photodiode
64
, and a laser
70
are aligned. No optical coupler is present. The light transmission and reception module may be produced at an impressively low cost. The ping-pong transmission with time division made this possible.
A half-transmittance photodiode has a thin absorption layer. A sufficiently thick (about 4 &mgr;m) absorption layer of conventional photodiodes absorbs all the incident light. The half-transmittance photodiode is materialized by making its thickness “d” equal to (ln 2)/&agr;, where &agr; means an absorption coefficient, and ln 2 is the natural logarithm of 2. Depending on a wavelength, the thickness is very thin, 0.7 or 1.0 &mgr;m when InGaAs or InGaAsP are used for the absorption layer, for example.
The extremely thin absorption layer of a half-transmittance photodiode posed a new challenge. The present invention is directed toward further improvement of the half-transmittance photodiode. As applications of light communications spread, the use in a wider temperature range or with a lower source voltage is strongly demanded. Conventional indoor use allowed the device to operate within a range of 0 to 40° C. The demand for outdoor use is widening the temperature range to −40 to +85° C. It was difficult for the above-mentioned half-transmittance photodiode to operate stably in such a wide range of temperature. In addition, a source voltage of 5 V is being replaced by 3.3 V to preserve energy. With such a low voltage, p-n junctions sometimes do not receive sufficient voltage to operate normally.
Whereas a conventional photodiode having a thick absorption layer has a satisfactory temperature characteristic, the half-transmittance photodiode having a thin absorption layer developed by the present inventors showed a decrease in responsivity at low temperatures, a peculiar phenomenon not seen in conventional photodiodes.
FIG. 5
is a graph showing temperature characteristics of the above-mentioned half-transmittance photodiode with a parameter of source voltage. The axis of abscissa represents temperature (° C.), and the axis of ordinate responsivity (A/W). The source voltages are 4 to 10 V, 3 V, 2 V, 1 V, and 0 V. When the source voltage is 4 to 10 V, the responsivity does not decrease, remaining about 0.47 A/W. However, when the source voltage is 3 V, the responsivity reduces by half at −40° C. or below. When the voltage is 2 V, the responsivity begins to decrease at a higher temperature, 20° C., and shows about 0.16 A/W at −20° C. Such a remarkable decrease in responsivity at a temperature close to normal temperature is not seen in an ordinary photodiode, hence no such a decrease is reported.
This poor temperature characteristic deprives the invented half-transmittance photodiode of chances to be used outdoors or in cold climate. Because the photodiode is used in combination with an amplifier in an actual application, a source voltage of 3.3 V will drop to about 2 V across the photodiode. With such a low voltage, the temperature characteristic is too poor for the photodiode to be used below 25° C.
A light transmission and reception module with a half-transmittance photodiode and without an optical coupler will lose the usefulness thereof significantly if the module must be replaced by a combination of a conventional, complete-absorption-type photodiode and an optical coupler for outdoor or cold climate use.
The principal object of the present invention is to offer a half-transmittance photodiode that does not become inferior in characteristics at low temperatures. Another object of the invention is to offer a half-transmittance photodiode that operates at low source voltages.
The principle of the photodiode o
Iguchi Yasuhiro
Kuhara Yoshiki
Saito Tadashi
Terauchi Hitoshi
Mintel William
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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