Segmented waveguide flattening the passband of a phasar

Details Segmented waveguide flattening the passband of a phasar Segmented waveguide flattening the passband of a phasar

2000-12-13

2003-03-25

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S014000, C385S031000, C385S037000, C385S042000, C385S047000, C385S129000, C385S130000, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06539150

ABSTRACT:

FIELD OF THE INVENTION
In general, the invention relates to integrated optics formed on a chip. In particular, the invention relates to a wavelength dispersive optical structure such as an arrayed waveguide demultiplexing/switching element.
BACKGROUND ART
Arrayed waveguide (AWG) multiplexers, also referred to as phasars, have become an important component in many optical systems, particularly in wavelength division multiplexing (WDM) telecommunication systems. A WDM network uses a single optical fiber to carry a multiplicity of optical carriers at different wavelengths, each modulated with its own data signal. Electronics and opto-electronics are generally limited to data rates of 10 to 40 gigabits per second (Gbs). In WDM, with the proper optical multiplexing and demultiplexing at the ends of the fiber, the electronics can be operated in parallel on the wavelength separated carriers to achieve better utilization of the fiber bandwidth, which may be as high as 300 terahertz (THz). WDM effectively multiplies the transmission capacity of the fiber by the number of optical carriers.
An example of a WDM telecommunication system is illustrated in FIG.
1
. Multiple (N) electronic data channels enter a transmitter
10
and modulate separate optical emitters such as lasers
12
having N respective free-space output carrier wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
. The number N of WDM channels is increasing to 64 and beyond. The wavelengths correspond to respective optical frequencies f
i
=c/&lgr;
i
, where c is the speed of light, resulting in N frequencies f
1
, f
2
, . . . f
N
. Conveniently, these frequencies are arranged in a WDM frequency comb having the neighboring wavelengths f
1
, f
2
, . . . f
N
separated by a substantially constant inter-channel spacing given by
&Dgr;f
S
=f
i+1
−f
i
.  (1)
A typical frequency spacing &Dgr;f
s
is 100 GHz. Since the frequencies are narrowly spaced, it is sometimes easier to visualize the spacing in terms of wavelength spacing given approximately by
Δ
⁢
 
⁢
λ
S
⁢
 
=
 
⁢
λ
2
c
⁢
 
⁢
Δ
⁢
 
⁢
f
S
.
(
2
)
In the case of a central wavelength of 1550 nm and a channel frequency spacing of 100 GHz, the channel wavelength spacing &Dgr;&lgr;
S
is about 0.8 nm. An optical wavelength-division multiplexer
14
combines the optical signals of different wavelengths and outputs the combined signal on a single optical fiber
16
. An optical receiver
20
includes a wavelength-division demultiplexer
22
which divides its received signals according to their optical wavelength to N optical detectors
24
according to the same wavelength allocation &lgr;
1
, &lgr;
2
, . . . &lgr;
N
. In view of the reciprocity usually exhibited in passive systems, a wavelength-division demultiplexer may be substantially identical to a wavelength-division multiplexer with a reversal of their inputs and outputs.
Additionally, an optical addidrop multiplexer (ADM)
30
may be interposed on the optical path
16
between the transmitter and the receiver
20
. The optical add/drop circuit
30
removes from the optical signal on the fiber
16
one or more wavelength channels at wavelength &lgr;
AD
and inserts back onto the fiber
16
an optical data signal perhaps containing different information but at the same optical carrier wavelength &lgr;
AD
. The ADM
30
is typically implemented with technology closely resembling the WDMs
14
,
22
.
All-optical networks have been proposed in which a distributed network has many nodes each including a transmitter
10
and receiver
20
and which are linked by a functionally passive network which routes the signals between the nodes according to their wavelengths. The routing elements in such an all-optical network require switching elements similar to the ADM
30
.
In order to maximize the transmission capacity of the optical fiber
16
and to utilize the usable bandwidth of certain elements such as erbium-doped fiber amplifiers, the wavelength channels &lgr;
1
, &lgr;
2
, . . . &lgr;
N
should be placed as closely together as possible with a minimum channel spacing &Dgr;&lgr;
S
. In advanced systems, this inter-channel spacing &Dgr;&lgr;
S
is 1 nm or less for signals centered around 1300 or 1550 nm, the preferred bands for silica fiber. Such closely spaced WDM networks are referred to as dense WDM networks (DWDM).
The network design described above may be subject to a problem arising from the fact that the operation of the transmitter
10
, receiver
20
and intermediate node
30
are all referenced to the same set of WDM wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
. However each of the distributed elements must provide its own wavelength calibration. Due to environmental and aging effects, the wavelength calibration settings at one element is likely to differ from those at other elements. In view of the close spacing of the optical channels, any miscalibration between network elements is likely to produce inter-channel interference.
For an optimized optical system, the fiber
16
, the WDMs
14
,
22
, and the ADM
30
are typically designed to be single-mode at least at their ports for the optical wavelengths being used. Although each of the lasers
12
is likely emitting light across an exceedingly narrow bandwidth, the single-mode response of the frequency sensitive elements
14
,
22
,
30
usually has a wavelength (frequency) characteristic that approximates a gaussian distribution about the center wavelength &lgr;
0
of the channel F(&lgr;)=exp(−(&lgr;−&lgr;
0
)/∂&lgr;
G
). The value of the gaussian passband ∂&lgr;
G
can be fairly freely chosen for present day fabrication techniques. However, the value of the passband is subject to countervailing restraints. For dense WDM systems, the inter-channel spacing &Dgr;&lgr;
S
is made as small as possible. The gaussian passband ∂&lgr;
G
must be substantially smaller than the inter-channel spacing &Dgr;&lgr;
S
to avoid interference between channels. On the other hand, the frequency characteristics of the lasers
12
and other frequency-sensitive elements are subject to permanent or temporary variations. If the passband ∂&lgr;
G
is made too small, the peak is very narrow and small variations in wavelength away from the peak wavelength &lgr;
0
cause operation to shift to the sides of the peak, thereby degrading the signal strength. That is, for a strong signal the passband ∂&lgr;
G
should be made as large as possible to provide a broad top of the peak.
Amersfoort et al. have already recognized these problems, as disclosed in U.S. Pat. No. 5,629,992, incorporated herein by reference in its entirety. This patent describes arrayed waveguide gratings, also called phasars, of the sort described by Hunsperger et al. in U.S. Pat. No. 4,773,063, and by Dragone in U.S. Pat. Nos. 5,412,744 and 5,488,680. In particular, Amersfoort et al. describe a WDM phasar
40
exemplified in the schematic illustration of
FIG. 2. A
single-mode waveguide
42
is coupled to one end of a multi-mode waveguide
44
of length chosen to produce a doubled image of the radiation from the single-mode waveguide
42
at a port
46
on one side wall
48
of a first free-space region
50
. The width of the single-mode waveguide
42
is approximately equal to the wavelengths of the light it is carrying, taking into account the refractive index, to within near-unity constants. The multi-mode waveguide
44
has a larger width. The multi-mode waveguide
44
acts as a multi-mode interferometer (MMI). Multiple single-mode array waveguides
52
are coupled to ports on the other side of the first free-space region
50
in the form of a star coupler. The array waveguides
52
are coupled on the other end to one side of a second free-space region
54
. The array waveguides
52
have lengths with predetermined length differences between them to act as an arrayed waveguide grating (AWG), operating similarly to a planar diffraction grating. Single-mode output waveguides
56

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