Optical: systems and elements – Optical amplifier
Reexamination Certificate
2000-12-06
2003-12-09
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
C359S337000, C359S342000, C385S049000, C385S131000, C385S132000
Reexamination Certificate
active
06661567
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical amplifier, a hybrid assembly and a method of making the hybrid assembly. In particular, the invention relates to a doped glass amplifier and to a hybrid assembly including a doped glass amplifier and an optical pump, together with a method of manufacture of the same.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic diagram of a known erbium-doped fiber amplifier. In such an amplifier, a length of erbium-doped optical fiber
10
is provided to carry out the amplification. Signal light transmitted along a signal optical fiber
12
and pump light from a pump light source
14
are combined in an optical coupler
16
and sent along the erbium-doped optical fiber
10
. In the fiber, the pump light excites atomic states of the erbium atom to create an inversion i.e. a situation in which higher energy states have a greater occupancy than lower states. In this situation, the signal light can create stimulated emission of light in phase with itself so that the erbium-doped fiber acts as an optical amplifier of the signal light.
Such erbium-doped fiber amplifiers are widely used, especially in transmission backbone systems in which optical signals have to be transmitted down great lengths of optical fiber. In such transmission backbone systems, the high cost of erbium-doped fiber amplifiers is not an issue.
However, there is an increasing need for amplifiers in smaller, local systems. These may range from metro systems inter-connecting a small area, to switched backplane systems inter-connecting a business or access systems delivering high bandwidth optical fiber connectivity to end users. In such systems, the high cost of conventional erbium-doped optical fiber amplifiers is a real issue and prevents wide spread use of such amplifiers.
Moreover, another difficulty with erbium-doped optical fiber amplifiers is that the signal is input along a single optical fiber, the input optical fiber
12
of FIG.
1
. Although multiplexing techniques can be used to direct a plurality of signal down a single optical fiber for amplification, such techniques can be inconvenient. Often a separate erbium-doped fiber amplifier is required for each optical signal.
U.S. Pat. No. 5,982,973 to Yan et al describes a planar optical waveguide device, in which a specific glass composition is used. In an experimental result a net optical gain of 4.1 dB was obtained in a planar waveguide device having a length of 10 mm. However, such a device would simply replace a conventional erbium-doped optical fiber.
It is possible to integrate a number of optical components on a substrate to create a so-called optical hybrid. For example, U.S. Pat. No. 5,534,442 to Parker et al describes a process for use in manufacturing opto-electronic components in a hybrid module form. A hybrid substrate of silicon is provided with v-grooves for locating input and output optical fibres and a number of optical and electronic components are mounted on the substrate and interconnected. A number of refinements to this technique are known. For example, U.S. Pat. No. 5,574,811 to Bricheno et al, describes a method of aligning an optical fiber with a laser mounted on a silicon motherboard by using a silicon platform to which the end of the fiber is secured.
However, it is not easy to integrate Er-doped fiber amplifiers with such modules, since the fibers cannot simply be mounted directly to the substrate.
One approach has been suggested by Regener et al in U.S. Pat. No. 5,726,796. This patent describes an optical amplifier in which a waveguide is integrated on a substrate. The waveguides have a spiral configuration, and are integrated with optical couplers and a pump light source mounted on the substrate.
However, such approaches do little to reduce component count or simplify manufacturing of hybrid optical modules. Accordingly, there remains a need for an improved optical amplifier, an improved optical hybrid assembly and a corresponding method of manufacture.
SUMMARY OF THE INVENTION
According to the invention, there is provided an optical amplifier comprising a slab having opposed top and bottom surfaces and at least one edge surface extending between top and bottom surfaces, the slab defining an optical input for receiving light to be amplified on an edge surface, at least one optical waveguide extending from the optical input thorough a doped region of the slab for transmitting and amplifying the light received from the optical input, an optical output on an edge surface for delivering amplified light from the optical waveguide, and a pump light input for receiving pump light, wherein the slab is at least partially transparent and configured to distribute, through the slab, pump light incident on the pump light input over the length of the at least one optical waveguide.
The optical amplifier according to the invention can be readily incorporated into larger systems, and in particular into hybrid amplifier modules. A slab can be readily mounted on a substrate, which is not true of a fiber, and perhaps more importantly a slab optical amplifier with pump light input separate from and spaced away from the signal input/output is relatively straightforward to integrate with further components.
In prior art waveguide amplifiers, a coupler has been provided to couple both the signal light and the pump light into the input end of the waveguide. In contrast, in an optical amplifier according to the invention the pump light is pumped not into the end of the waveguide, but passes through the slab into the waveguide along the length of the waveguide. The omission of the optical coupler may simplify manufacture of systems using the optical amplifier according to the invention.
Moreover, there is no need to provide a pump light source that is capable of directing its light output down a narrow optical fiber. Instead, a broad stripe pump light source may be used; such pump light sources can provide more power at a given cost than the pump light sources conventionally used.
Also, separation of the pump light inputs and the signal light input and output means that each can have a reflection or anti-reflection coating appropriate to their own requirements. In particular, reflective material may be provided on the outer surface of the slab away from the optical input and output and the pump light input. The reflective material may multiply reflect pump light within the slab to distribute it across the interior of the slab and hence to provide pump light along the optical waveguide.
The pump light may be distributed over substantially all of the length of the optical waveguide since amplification can be most efficient if the whole waveguide is pumped. However, it is not essential that the pump light is absolutely evenly distributed.
The dopant concentration in the doped region may exceed 5×10
19
cm
−3
for providing significant amplification in a short length.
A significant advantage of the approach according to the invention is that a plurality of waveguides may be provided. The waveguides may be arranged in parallel with each other. In this way, a single optical amplifier and a single pump light source may amplifier the signals of a significant number, say ten, of input optical fibers. This approach can greatly reduce the cost of amplification because in a system in which signals on ten optical fibers need to be amplified only one amplifier is needed rather than ten.
Preferably, the slab has opposed top and bottom faces spaced apart by a distance substantially less than the smallest linear dimension of the top and bottom faces, and a pump edge face extending between the top and bottom faces at the periphery of the slab, the pump light input being on the pump edge face.
The form of a slab allows good distribution of pump light from the pump light input to the waveguide since the pump light can be retained in the slab by total internal reflection from the top and bottom faces. Light can bounce in the slab in a zigzag pattern. Furthermore, the use of the form of a slab allows good heat s
Black Thomas G.
Bookham Technology PLC
Cunningham Stephen
Renner Otto Boisselle & Sklar
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