Temperature insensitive optical waveguide device

Details Temperature insensitive optical waveguide device Temperature insensitive optical waveguide device

2002-03-18

2004-06-29

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

Planar optical waveguide

C385S014000, C385S037000, C385S142000

Reexamination Certificate

active

06757469

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates in general to optical waveguide devices, which preferably are planar, e.g., optical arrayed waveguide gratings (AWGs), multiplexers/demultiplexers, optical add/drop multiplexers and the like. More particularly, the present invention relates to such planar optical waveguide devices the channel wavelengths or central wavelengths of which do not or almost do not shift upon temperature variations of the environment. Such devices are called athermal or temperature insensitive devices.
BACKGROUND OF THE INVENTION
Today in the time of internet, multimedia and telecommunication, the amount of data which has to be transferred is heavily increasing. So called killer-applications like video-on-demand, internet-telephone, video-conferencing, virtual-reality need more bandwidth, than it was imaginable some years ago. For long distance data transfer, fiber optic cables have almost completely replaced wires made of copper and now are extending even to local areas or even to housing networks. The goal to be attained is to have an all-optical network using fiber optic cables to every housing, even to every room and almost unlimited bandwidth for everyone.
Glass fibers have the enormous bandwidth of more than 40 TBps which is more than sufficient to transfer all telephone calls in the whole world through one single glass-fiber cable. However, only a small part of this potential is used so far. Actually worldwide optical fiber long-haul transmission systems are upgraded to 40 Gbit/s data rates and Prototype-systems are runned using about one TBps. One way to use an increased part of the glass fiber bandwidth is to use signals with different wavelengths. This method is the so called Dense-Wavelength-Division-Multiplexing. (DWDM). Most dramatic changes in telecommunications have occurred during the last several years after optical DWDM has been invented. Multiple bandwidth without even changing the fiber glass cable results therefrom. The huge success of DWDM technique has encouraged the telecom equipment vendors to boost network capacities further and further by introducing systems with more and more channels.
DWDM-Components are integrated planar optical waveguide devices which can be regarded as integrated optical circuits combining miniaturized waveguides and optical devices on an planar substrate. The glass fibers are attached to the input and output channels of the waveguide device. The integrated optical waveguides perform one or more functions or processes on the transmitted light (e.g. a DWDM-System combines light of different wavelength from the input-channels into one output-channel or vice-versa). Actually common integrated optical waveguide devices are realized by using silica-based planar lightwave circuits (PLCs). Currently, the arrayed waveguide devices are mainly produced in this silica technology. Cost issues, the low wavelength tuning range and, very important, the temperature dependence of the central wavelength of silica-based AWGs are, however, major problems involved in this technology.
Drift of the channel wavelengths or the central wavelength and therefore of the filter function of an AWG with temperature is a disadvantage resulting in a bad multiplexing/demultiplexing performance if the device is used in an environment where it is exposed to thermal fluctuations. Presently, this disadvantage is tried to be overcome by using a Peltier element to keep the device at a constant temperature. However, this results in another disadvantage in that an (additional) control-circuit and a power supply is required which causes additional costs.
In order to eliminate these additional disadvantages, temperature insensitive or athermal silica-based waveguide devices have been extensively investigated. For example, a polymeric over-cladding was used to reduce the effective thermo-optical coefficient (TO coefficient) from 1*10
−5
/K to −0.24*10
−6
/K. However, this approach proved to be useful only in a relatively narrow temperature range. Another approach was to form several grooves in the arrayed-waveguide area which were filled with silicone adhesive exhibiting a negative TO coefficient. By this means, the temperature dependence was successfully suppressed with a deviation of ±0.02 nm in the temperature range from 0° to 85° C. Still another approach was reported wherein several trenches with a crescent shape in the input slab region were formed. In this way, the temperature sensitivity could be reduced to −0.0013 nm/K. However, both of the latter mentioned structures were susceptible to extra optical loss and extra phase errors. A common drawback of the said approaches is that a polyimide half-waveplate needs to be inserted in the arrayed waveguides for compensation of the polarization dependence. Respective disclosure is found in publications of international applications WO 01/51967, WO 00/46621, and WO 00/28361.
A different approach is the application of a movable input fiber in order to compensate for the temperature dependent wavelength shift which results in a complicated adjustment of the fiber.
WO 00/50937 deals with the question of affection of temperature changes to optical devices. It is contemplated that it could be beneficial to obtain an optical device wherein d&lgr;
0
/dT which expresses the temperature sensitive response could be either set to zero or controlled within a desirable range. However, this application does not teach how to obtain a value of zero or almost zero for d&lgr;
0
/dT, and consequently, the invention disclosed therein deals with optical devices which are temperature sensitive.
AWG multi/demultiplexers based on organic materials used for the waveguide have been gaining increasing attention because organic polymer devices are believed to be producible at lower cost than their conventional silica based counterparts. However, not only lower production costs cause such polymers to be interesting candidates for integrated optical circuits. In addition, the refractive index can be adjusted and tuned over a broad range and thus, a high index contrast between core and cladding can be achieved. This results in a significant reduction of the device-dimensions
Integrated optical waveguide devices (AWGs) are well known to the skilled artisan and are described in detail throughout the literature. Integrated optical waveguides made from polymeric waveguide materials are also well known to the skilled artisan and are described in detail throughout the literature. Exemplary AWGs include a silica doped waveguide circuit core or a polymeric waveguide circuit core, on a planar substrate, such as a fused silica member, a silicon wafer or the like. The use of polymeric substrates for optical waveguide devices is also known in the art, such substrates having the advantage of reducing the birefringence of the waveguide material and therefore the polarization shifts of the resulting device, see WO 01/02878.
Integrated AWG optical devices commonly contain a number of waveguide arms differing in their optical path length. Depending on the optical path length differences and the geometry, the light is assorted by its wavelength. Demultiplexing-devices comprise at least one input channel which transmits N optical signals at N different wavelengths (&lgr;
1
, &lgr;
2
, &lgr;
3
, . . . &lgr;
N
) and at least N output channels each of which transmits exactly one of the N optical signals at an exactly determined wavelength. The more channels are used, the smaller is the range of the wavelength that can pass one particular channel. Thus, the more channels (or, the smaller channel spacings, respectively) are used, the more exactly the transmission maxima of the channels of the device must fit to the related wavelengths of the signals. Vice versa, a wavelength division multiplexer includes at least N input channels and at least one output channel. Every input-channel transmits one of the N optical signals (&lgr;
1
, &lgr;
2
, &lgr;
3
, . . . &lgr;
N
) which are combined in the output-channel. Both devi

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