Optical multiplexing and demultiplexing

Details Optical multiplexing and demultiplexing Optical multiplexing and demultiplexing

1999-10-13

2002-04-02

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C385S024000, C385S032000, C385S037000, C359S199200

Reexamination Certificate

active

06366378

ABSTRACT:

FIELD OF THE INVENTION
The invention generally relates to optical transmission systems employing wavelength division multiplexing, and particularly to a so-called waveguide phased array component used therein to carry out multiplexing and demultiplexing of the optical signal.
BACKGROUND OF THE INVENTION
Wavelength Division Multiplexing (WDM) represents an efficient way to increase manyfold the capacity of an optical fiber. In wavelength division multiplexing, a number of independent transmitter-receiver pairs use the same fiber. The principle of WDM is illustrated in
FIGS. 1
a
and
1
b
by using a system comprising four parallel transmitter-receiver pairs as an example. Each of the four information sources (not shown in the figure) modulates one of the four optical transmitters each of which produces light at a different wavelength (&lgr;
1
. . . &lgr;
4
). As appears from
FIG. 1
a,
the modulation bandwidth of each source is narrower than the separation between the wavelengths, resulting in that the spectra of the modulated signals do not overlap. The signals produced by the transmitters are combined into the same optical fiber OF in a WDM multiplexer WDM
1
, which is an entirely optical (and often passive) component. At the opposite end of the fiber, a WDM demultiplexer WDM
2
, also an entirely optical (and often passive) component, separates the different spectral components of the combined signal from each other. Each of these signals is detected by a different receiver. Thus, each signal is assigned a narrow wavelength window in a specific wavelength range. A typical practical example could then be a system where the signals are in the 1550 nm wavelength range, e.g. so that the first signal is at the wavelength 1544 nm, the second signal at the wavelength 1548 nm, the third signal at the wavelength 1552 nm, and the fourth signal at the wavelength 1556 nm. Nowadays, to an ever greater extent, the de facto standard for wavelength separation is a multiple of 100 GHz (appr. 0.8 nm).
The waveguide phased array component (also known as waveguide array grating or arrayed waveguide grating) is a known component in fiber optics, most suitable for systems employing wavelength division multiplexing due to e.g. the fact that a large number of wavelengths can be transferred over it, such as a WDM signal comprising 16 or 32 wavelengths.
FIG. 2
illustrates the structure of a waveguide phased array component WGA. The component comprises, integrated on the same substrate, N optical input/output guides AWG on the first side of the component, N optical input/output guides BWG on the second side of the component, two slab waveguides SWG
1
and SWG
2
, and a grating GR constituted by optical channel waveguides WG, the grating GR connecting the slab waveguides to one another. Both sides of the component may act as the input or output side, whereby the waveguides AWG and BWG may be output or input guides. The slab waveguides, which connect input/output guides to separate channel waveguides WG in the grating, restrict propagation of light only in the plane perpendicular to the substrate but allow light propagation to the sides. The channel waveguides in the grating, instead, prevent light propagation also to the sides. The channel waveguides that connect to the slab waveguides on both sides are arranged on a circular arc so that each of them is directed towards the center waveguide of the waveguide group on the opposite side. A constant difference in length exists between two adjacent channel waveguides in the grating, the difference in length being a multiple of the center wavelength used. If light is input from the center input/output waveguide of one side at the center wavelength of the component, the light is distributed to all the waveguides of the grating. As the difference in length of the waveguides is a multiple of the center wavelength, all the waves are in the same phase upon arriving in the output slab waveguide whereupon the light is focused to the center output waveguide. In case the wavelength differs from the center wavelength, the wave front arriving in the output is slightly tilted, which means that it is not focused exactly at the center but at another waveguide of the output side. Hence, the component focuses different wavelengths to different outputs, the dimensioning of the component determining which wavelengths are focused on which output. Similarly as the wavelength of the center input waveguide determines which the output waveguide is, the location of the input waveguide determines which the output waveguide is.
The waveguide phased array component thus comprises a number of light channels whose geometry defines that they have both focusing characteristics (a lens) and dispersing characteristics (the wavelength dependency of the grating).
FIG. 3
illustrates the basic operational principle of the component in association with a case in which three different wavelengths (&lgr;
1
, &lgr;
2
, &lgr;
3
) are used to couple light alternately to each of the three input ports. As the figure shows, the output port of a specific wavelength channel depends both on the wavelength of the channel in question and which the input port of the channel in question is. The component is capable of demultiplexing N wavelength channels received from one input port so that each of the channels goes to a different output port. How the channels are distributed among the output ports depends on which the input port is. Examined from the network point of view, a situation thus exists in which a network element connected to a specific output port and receiving a signal at a specific wavelength knows, based on the output port and the wavelength, from which input port the signal originates.
In the following, the operation of the component is examined in closer detail. A symmetrical N×N phased array component has N optical ports on the A-side and N optical ports on the B-side. The component has been so designed that it multiplexes wavelengths whose separation is &Dgr;&lgr;. When optical fibers are connected to the optical ports, light is coupled between each port on the A-side and each port on the B-side on a wavelength determined from the formula: &lgr;=&lgr;
0
+&Dgr;&lgr;(i+j−2). In the formula, i stands for the port sequence number on the A-side and j for the port sequence number on the B-side, and &lgr;
0
is the wavelength coupling between the ports i=1 and j=1. The wavelength coupled between two ports is the same regardless of whether light is input to the A-side port and output from the B-side port or in the opposite direction, and the operation of the component is also in other respects symmetric as regards changes of the A- and B-sides.
The above description is also valid for a component in which the number of optical ports differs on the A-side and B-side. In such a case, N is the number of ports on the side which has the majority, and the other side may simply be seen as lacking some ports, but the coupling between the ports is nevertheless described by the above formula.
The basic function of the component as a demultiplexer is illustrated as the wavelengths coupling from one A-side port to all the B-side ports so that a dedicated wavelength is coupled to each of them. This is illustrated in
FIG. 4
a.
For example, when light is input to port i=1, the wavelengths &lgr;=&lgr;
0
+&Dgr;&lgr;(j−1) couple to the output ports. A reverse operation as a multiplexer is obtained when a wavelength is input to each A-side port, the wavelengths being selected so that all wavelengths are coupled out of the same B-side port. This is illustrated by
FIG. 4
b.
For example, when the wavelength input to each port is &lgr;=&lgr;
0
+&Dgr;&lgr;(i−1) all wavelengths are coupled out of the port j=1.
Commonly the operation of the component is periodic also with respect to wavelength, the period between the wavelengths being the Free Spectral Range (FSR). In such a case, if a coupling exists between two ports at the

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