Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-04-20
2004-09-28
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S047000, C385S052000
Reexamination Certificate
active
06798948
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical filter comprising an integrated wavelength dispersive element having an input for providing temperature compensation, particularly for providing passive temperature compensation in an arrayed waveguide grating.
BACKGROUND OF THE INVENTION
An arrayed waveguide grating (AWG) is a dispersive optical device used for multiplexing or demultiplexing a set of optical telecommunications channels having different wavelengths. An example of an AWG is shown in FIG.
1
. The AWG
100
is an integrated optics device formed on a substrate. The AWG has at least one input waveguide
10
for launching a multiplexed signal comprising a plurality of channels at specific wavelength bands having center wavelengths &lgr;1 to &lgr;n, into a free-space slab, or planar waveguide, such as a star coupler
12
. The star coupler
12
distributes a wavefront of the signal evenly to a plurality of waveguides that form the grating
14
. Each of the plurality of waveguides has a different optical length, the optical lengths of adjacent waveguides differing by a constant value and increasing geometrically from one side of the grating to the other. Interference caused by the relative phase differences introduced by the grating
14
occurs in a second free-space slab, or planar waveguide, such as a star coupler
16
. The dispersion of the grating
14
physically separates the different wavelengths and focuses the dispersed light on an output plane
17
of the second star coupler, where separated wavelengths are coupled into a plurality of output waveguides
18
. A center wavelength of a selected channel is located at a selected output waveguide
18
for optimized coupling. The center wavelength and the spacing of the individual wavelength bands of the channels are determined by the geometry of the AWG layout and by the effective refractive index of the waveguides of the grating. The output waveguides
18
determine the bandwidth of the individual channels by their width. Focus points T
1
O
and T
2
O
at the output plane
17
of the output planar waveguide
16
demonstrate the wavelength shift of the center wavelength that occurs as a result of a change in temperature of the device
100
with the input point fixed.
Operated in a reverse direction, multiple signals of different wavelengths are launched from the plurality of waveguides
18
and pass through the grating
14
to interfere in the star coupler
12
, and be combined as a multiplexed signal into a single waveguide
10
.
The position of the input waveguide
10
at the input plane
20
of the star coupler
12
, from which a multiplexed signal is launched, affects the location of the focused output signals. Input waveguides have been included as a part of the integrated device. However, manufacturing tolerances are not tight enough to accurately set the center wavelength in manufacture for narrow channel spacing. The index accuracy achieved with the many deposition techniques used to make AWGs is not sufficient to set the central wavelength within the required tolerances.
In U.S. Pat. No. 5,732,171, assigned to Siemens Aktiengesellschaft, Michel et al. disclose placing the input plane of the star coupler at the edge of the substrate in which the device is formed to permit coupling a waveguide at a selected location after manufacture. Tuning may be performed to align the center wavelength of the channels of the multiplexed signal with their respective output ports to optimize coupling.
Tuning by affixing a fiber pigtail is subject to alignment error over 5 degrees of freedom. With reference to
FIG. 1
, X-Y-Z coordinates are shown. The X axis indicates lateral movement along the input plane
13
of the star coupler
12
, which affects the center wavelength alignment. The Y axis indicates vertical movement with the planar slab, which is generally single mode in the vertical direction. Consequently fine alignment is necessary to reduce coupling losses. The Z axis indicates movement in and out from the input plane
20
of the star coupler
12
. Alignment in this axis affects the pitch, or separation of the focused channel outputs on the output plane
17
of the second star coupler
16
. In addition &thgr;X and &thgr;Y indicate rotational tilt about the X and Y axes, which will further affect tuning of the center wavelength and insertion loss.
A further problem in tuning the AWG is the temperature dependency of the device. Temperature change causes the refractive index of the phased array to change. This causes the wavelength bands of the channel outputs to shift position. Consequently, coupling to the output waveguides is not efficient at the center wavelength.
One solution to this problem is proposed by the present inventor in U.S. Pat. No. 5,905,824, which teaches providing an arrayed waveguide grating and a separate output waveguide chip optically coupled to the output planar waveguide of the AWG, with passive thermally responsive means for relative movement between them, or through an imaging lens passively positioned between them. Although this device provides passive temperature compensation, it does not provide means for adjusting the input waveguide for tuning the center wavelength.
Passive temperature compensation at the input of an AWG is proposed in a paper entitled, “Optical Phased Array Filter Module with Passively Compensated Temperature Dependence,” by G. Heise et al. of Siemens AG, presented at ECOC '98, 20-24, Sep. 1998 in Madrid, Spain. Heise et al. propose supporting a fiber lens pigtail adjacent the input plane of the planar waveguide using a thermal expansion rod secured to the substrate of the AWG. The thermal expansion rod provides lateral displacement of the input fiber pigtail. However, as discussed with respect to the earlier Siemens patent, alignment of the fiber pigtail is subject to alignment and coupling error over five degrees of freedom. In order to permit lateral movement of the input pigtail, a gap between the input plane and the fiber is required. Without securing the fiber to the substrate, the likelihood of misalignment is increased. In addition, the air gap between the fiber and the input slab will increase insertion losses and introduce additional problems of back reflection.
It is desired to provide an improved coupling into an arrayed waveguide grating, which will permit variable tuning to adjust the center wavelength and provide passive temperature compensation.
It is further desired to provide an arrayed waveguide having an integrated variable input waveguides to provide tuning flexibility.
SUMMARY OF THE INVENTION
The present invention has found that by providing an integrated wavelength dispersive element having a thermally responsive pivotal input structure for changing an angle of a collimated input signal launched into a focusing lens, the input point can be selected in response to changing temperature in order to compensate for thermal drift of the center wavelength. Further, the present invention has found that by providing a reflective lens assembly for focusing an input signal at a selected input point of the input planar waveguide, alignment and tuning of an input and assembly can be improved and simplified.
Accordingly, the present invention provides an input coupling for launching light into a planar waveguide of an integrated wavelength dispersive element comprising:
focusing means having optical power for focusing light at an input point on the input plane of the planar waveguide;
an input waveguide for launching a signal comprising a plurality of channels at specific wavelengths into the integrated wavelength dispersive element;
means for coupling the signal as a beam into the focusing means; and,
tilt means including a pivotal structure having a center of rotation and a thermally responsive actuator, for imparting a tilt on the beam coupled to the focusing means in response to a change in temperature.
In an alternative embodiment the present invention provides an arrayed waveguide grating comprising:
a substrate for supporting an i
Delisle Vincent
Hnatiw Alan J. P.
Bovernick Rodney
JDS Uniphase Inc.
MacLean Doug
Pak Sung
Teitelbaum Neil
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