Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2002-04-10
2004-06-15
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S047000, C385S048000, C359S634000, C359S589000, C398S085000, C398S096000
Reexamination Certificate
active
06751373
ABSTRACT:
FIELD
This patent specification relates to optical communications devices. More particularly, it relates to multiplexers/demultiplexers for use in wavelength division multiplexed (WDM) optical communications systems.
BACKGROUND
Wavelength division multiplexed (WDM) optical communication systems are based on the modulation of successive channels of information f
i
(t) onto successive optical carriers at wavelengths &lgr;
i
, which are then multiplexed onto a single fiber for transmission. In typical practical systems today, the bandwidth of each signal f
i
(t) may be about 10 GHz, the channel separation may be about 200 GHz (i.e. about 1.6 nm), and there may be about 20 channels being multiplexed onto the same fiber around a center wavelength of about 1540 nm. Thus, for example, a typical system may have 20 channels separately modulated onto carriers at 1530.0, 1531.6, 1533.2, 1536.8, . . . , 1560.4 nm, and the carriers are then optically combined into the same fiber for transmission. The above parameters are given by way of example only, the preferred embodiments described herein being applicable to any type of optical signal comprising a plurality of wavelength-division multiplexed signals at any of a variety of wavelengths.
Many useful devices in WDM communications systems are based upon WDM multiplexers/demultiplexers. Under the Principle of Reciprocity, most multiplexers are simply demultiplexers working in the opposite direction (and vice versa). Therefore, the preferred embodiments are described in terms of a demultiplexing function, it being understood that the preferred embodiments will operate as multiplexers in the opposite direction. The function of a demultiplexer is to receive a single optical beam carrying signals at &lgr;
1
&lgr;
2
&lgr;
3
&lgr;
4
. . . &lgr;
N
and generate N separate beams, each carrying a different one of the optical signals &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, . . . , or &lgr;
N
.
FIG. 1
illustrates a demultiplexer
100
according to the prior art, taken from Dutton,
Understanding Optical Communications
, Prentice-Hall (1998), which is incorporated by reference herein, in which narrow band transmissive-type dielectric thin film filters
102
are used. The thin film filters
102
are mounted on an SiO
2
substrate
106
, with GRIN lenses
104
being used to collimate the optical beam between free space and optical fibers as necessary. For the wavelength ranges of interest, each thin film filter
102
is designed to reflect all wavelengths of light except a single wavelength &lgr;
i
, with each filter being tuned to its own distinct wavelength &lgr;
i
. After a first wavelength &lgr;
1
is extracted at the first filter, the remaining wavelengths &lgr;
2
&lgr;
3
&lgr;
4
. . . &lgr;
N
are sent on to the next filter. The next filter extracts &lgr;
2
, and the remaining wavelengths &lgr;
3
&lgr;
4
. . . &lgr;
N
are sent on to the next filter, and so on. It is to be appreciated that the demultiplexer
100
also operates as a multiplexer when operated in the reverse direction, and that only the demultiplexing direction is illustrated in
FIG. 1
for clarity of presentation.
However, the use of transmissive type filters in a WDM demultiplexer results in difficulty in alignment, which is a major disadvantage. When the filters are transmissive, the “backend” wavelengths near &lgr;
N
are inevitably reflected a large number of times before being directed to their final destinations. A misalignment &Dgr;&thgr; of any reflecting surface along the way causes a 2&Dgr;&thgr; error in the trajectory of the reflected light beam from that surface onward. Even if every subsequent filter was perfectly aligned, the divergence of the beam from the intended path is equal to 2&Dgr;&thgr; (in radians) times the distance traveled to the final destination. This error can become even worse if one or more subsequent filters is also misaligned. Thus, even a small angular error in any reflecting surface can cause severe system performance degradation or even system failure. Because of this, every reflecting surface needs special care during fabrication and assembly. This drives up the cost of components and assembly.
It should be noted that Dutton, supra in
FIG. 1
, presents one method of dealing with the alignment, which is to use a precisely cut SiO
2
slab
106
as a substrate, carefully cut along the crystal axes so that the dielectric filters are precisely aligned. However, even this solution can be expensive, especially where cost savings are desired in as many aspects of a final product as possible. It would be desirable to use a less expensive material, such as plastic or standard glass, to hold the thin film filters. However, precise alignment using such low-cost materials would be very difficult, especially in view of their thermal sensitivity which can change the relative alignments in the event of uneven temperature distributions during the molding process or in field use.
Accordingly, it would be desirable to provide a WDM demultiplexer/multiplexer architecture that is more robust to small variations in the alignment of the channel filters.
SUMMARY
A WDM demultiplexer/multiplexer is provided comprising a plurality of narrow band reflective filters linearly disposed along an optical axis, each narrow band reflective filter reflecting a single channel or group of channels and transmitting the remaining channels. The narrow band reflective filters are each tilted with respect to the optical axis. In a demultiplexing mode, an optical signal initially carrying channels at &lgr;
1
&lgr;
2
. . . &lgr;
N
travels along the optical axis. Each narrow band reflective filter reflects a distinct one &lgr;
i
of the channels, directing the reflected beam away from the optical axis at twice the tilt angle toward an output. Each narrow band reflective filter is substantially transparent to the remaining channels of the optical signal, such that the remainder of the optical signal proceeds along the optical axis substantially undisturbed. Advantageously, the device is highly robust against tilt variations or other mechanical variations in the narrow band reflective filters, because such variations are not compounded as the optical signal travels through the device.
In a multiplexing mode, a plurality of optical signals &lgr;
1
, &lgr;
2
, . . . , &lgr;
N
are separately provided at the above outputs, and a multiplexed signal &lgr;
1
&lgr;
2
. . . &lgr;
N
propagates out of the first narrow band reflective filter in a direction opposite the above optical signal. Preferably, the narrow band reflective filters are tilted less than 45 degrees with respect to the optical axis, and even better performance is achieved at less than 30 degrees. In one preferred embodiment, the narrow band reflective filters are dielectric thin film filters, while in another preferred embodiment they are holographic filters. A WDM demultiplexer/multiplexer in accordance with the preferred embodiments is readily adapted for use as a channel monitor and/or an add/drop multiplexer.
According to another preferred embodiment, when many channels “N” require multiplexing/demultiplexing, the incoming beam may be split into “m” separate beams and sent to “m” separate narrowband reflective filter arrays, each comprising about N/m narrowband reflective filters. Based on the number “N” and on system parameters such as beamsplitting loss and filter transmissivity, an optimal number for “m” may be determined based on a comparison of attenuation due to beam-splitting and attenuation caused by propagation through multiple serial filters.
REFERENCES:
patent: 3953727 (1976-04-01), d'Auria et al.
patent: 4550975 (1985-11-01), Levinson et al.
patent: 4637682 (1987-01-01), Mahlein et al.
patent: 5153670 (1992-10-01), Jannson et al.
patent: 5165079 (1992-11-01), Schulz-Hennig
patent: 5258872 (1993-11-01), Johnson et al.
patent: 5521733 (1996-05-01), Akiyama et al.
patent: 5629995 (1997-05-01), Duck et al.
patent: 5652814 (1997-07-01), Pan et al.
patent: 5754718 (1998-05-01), Duck et al.
patent: 5786915 (1
Gazillion Bits Inc.
Knauss Scott A
Sanghavi Hemang
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