CA2344003A1 - Optical filter - Google Patents

Abstract

Gires-Tournois cavities are disclosed in a single tunable etalon having a lens having two input ports and two output ports in optical communication therewith; a partially reflective surface is optically coupled with an end face of the lens, and a substantially reflecting surface is spaced distance "d" from the partially reflective surface to form an optical cavity between the reflective surfaces.
A transducer for varying the distance "d" between the reflecting surfaces. The two input ports are disposed at different radial distances from an optical axis of the lens so that when separate beams of light are launched into the two input ports, the separate beams traverse optical paths between the reflecting surfaces having different optical path lengths. This filter provides a substantially flat-top narrow band output response.

Description

Doc. No: 10-337 CA Patent Optical Filter Field of the Invention This invention relates to generally to an optical filter and more particularly to an optical cavity formed about an end of a lens.
Background of the Invention Using optical signals as a means of carrying channelled information at high speeds through an optical path such as an optical waveguide i.e. optical fibres, is preferable over other schemes such as those using microwave links, coaxial cables, and twisted copper wires, since in the former, propagation loss is lower, and optical systems are immune to Electro-Magnetic Interference (EMI), and have higher channel capacities. High-speed optical systems have signalling rates of several mega-bits per second to several tens of giga-bits per second.
Optical communication systems are nearly ubiquitous in communication networks.
The expression herein "Optical communication system" relates to any system that uses optical signals at any wavelength to convey information between two points through any optical path. Optical communication systems are described for example, in Gower, Ed. Optical communication Systems, (Prentice Hall, NY) 1993, and by P.E. Green, Jr in "Fiber optic networks" (Prentice Hall New Jersey) 1993, which are incorporated herein by reference.
As communication capacity is further increased to transmit an ever-increasing amount of information on optical fibres, data transmission rates increase and available bandwidth becomes a scarce resource.
High speed data signals are plural signals that are formed by the aggregation (or multiplexing) of several data streams to share a transmission medium for transmitting data to a distant location.
Wavelength Division Multiplexing (WDM) is commonly used in optical communications systems as means to more efficiently use available resources. In WDM each high-speed data Doc. No: 10-337 CA Patent channel transmits its information at a pre-allocated wavelength on a single optical waveguide. At a receiver end, channels of different wavelengths are generally separated by narrow band filters and then detected or used for further processing. In practice, the number of channels that can be carried by a single optical waveguide in a WDM system is limited by crosstalk, narrow operating bandwidth of optical amplifiers and/or optical fiber non-linearities. A
certain minimum bandwidth is required for the WDM filters to accommodate the bandwidth of the modulated optical signal, but at the same time high isolation of the adjacent channel optical signal is required. As a result, a flat-top filter response is advantageous in obtaining both the required bandwidth and simultaneously sufficient adjacent channel isolation. In certain applications, tunable filters are necessary, to perform complex functions such as channel add and drop, or to accommodate for errors in laser wavelengths. This invention relates to a method and system for filtering or separating closely spaced channels that would otherwise not be suitably filtered by single conventional optical filters.
Currently, internationally agreed upon channel spacing for high-speed optical transmission systems, is 100 GHz, equivalent to 0.8 nm. surpassing, for example 200 GHz channel spacing equivalent to 1.6 nanometers between adjacent channels. Of course, as the separation in wavelength between adjacent channels decreases, the requirement for more precise demultiplexing circuitry capable of ultra-narrow-band filtering, absent crosstalk, increases. The use of conventional dichroic filters to separate channels spaced by 0.4 nm or less without crosstalk, is not practicable; such filters being difficult if not impossible to manufacture.
The use of fixed or tunable Fabry-Perot etalons as filters is not practicable for achieving a flat-top output response.
In a paper entitled Multifunction optical filter with a Michelson-Gires-Turnoffs interferometer for wavelength-division-multiplexed network system applications, by Benjamin B.
Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device hereafter termed the GT device provides some of the functionality provided by the instant invention. For example, the GT device as exemplified in Fig. 1 serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror 16. The

2 Doc. No: 10-337 CA Patent etalon 10 used has a 99.9% reflective back reflector 12r and a front reflector 12f having a reflectivity of about 10%; hence an output signal from only the front reflector 12f is utilized. A
beam splitting prism (BSP) 18 is disposed to receive an incident beam and to direct the incident beam to the etalon 10. The BSP 18 further receives light returning from the etalon and provides a portion of that light to the plane mirror 16 and a remaining portion to an output port. Although this known GT device appears to perform its intended function, it appears to have certain limitations: For example it requires for an optical circulator to extract one of the output signals adding to signal loss and increased cost of the device; and the requirement of a BSP which is known to have a significant polarization dependent loss.
A Michelson interferometer (MI) combined with a GT resonator as an interleaver requires an optical path length difference (8 L) between the two MI arms must be equal to (4d+~, )/8, where d is the mirror spacing of the GT resonator. which makes the filter difficult to tune, because two distances must be tuned separately and in precise proportion to one another.
In addition, to maintain this condition in spite of temperature variations, a feedback control is usually necessary. Another disadvantage of this configuration comes from the bandwidth and the filter order. Apart from the fact that these two parameters can't be independently controlled the bandwidth of such a filter remains very wide.
The present invention provides a bandpass/notch tunable filter with a substantially flat top for use in tunable add/drop applications.
It is an object of the present invention to provide a narrowband periodic filter.
It is an object of this invention to provide a method and circuit for separating an optical signal having closely spaced channels into at least two optical signals wherein channel spacing between adjacent channels is greater in each of the at least two optical signals, thereby requiring less precise filters to demultiplex channels carried by each of the at least two signals.
The present invention is believed to overcome many of the limitations of the prior art GT device and of other known multiplexing and demultiplexing devices.

3 Doc. No: 10-337 CA Patent It is an object of this invention to provide a relatively inexpensive optical circuit for interleaving or de-interleaving optical channels.
It is an object of this invention to provide an etalon based device wherein output signals from two oppositely disposed ports can be controllably interferometrically combined to yield a desired output response.
Summary of the Invention In accordance with the invention an optical filter is provided comprising:
a lens, having a first partially reflective surface having a reflectivity R, about an end face thereof;
a Gires-Tournois etalon comprising the first partially reflective surface and second reflective surface having a reflectivity R2 spaced a distance "d" from the first reflective surface;
a first input port spaced from an optical axis of the graded-index lens;
a first output port spaced from the optical axis of the graded-index lens and optically coupled with the first input port;
a second input port spaced from the optical axis of the graded-index lens;
and, a second output port spaced from the optical axis of the graded-index lens and optically coupled with the second input port.
In accordance with the invention there is provided, an optical filter comprising a graded index lens having an end face thereof coated with a first reflective coating that is at least partially transmissive, and a second reflective surface spaced a distance "d" from the first reflective coating to form an optical cavity;
a first input port offset from an optical axis of the graded index lens a distance y,;
a first output port optically coupled with the first input port and spaced a distance yi from the optical axis of the lens;
a second input port offset from an optical axis of the graded index lens a distance y2;
a second output port optically coupled with the second input port and spaced a distance y2 from the optical axis of the lens; and,

4 Doc. No: 10-337 CA Patent a coupler for coupling light received from the first and second output ports, wherein y~,-y2.
In accordance with another aspect of the invention there is provided, a double GT cavity comprising:
a GRIN lens having two input ports and two output ports;
a partially reflective surface about an end face thereof; and, a substantially reflecting surface spaced distance "d" from the partially reflective surface to form an optical cavity between the reflective surfaces; and means for varying the distance "d" between the reflecting surfaces.
Brief Description of the Drawings Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:
Fig. 1 is a circuit block diagram of a prior art Michelson-Gires-Tournois interferometer;
Fig. 2a is a side view of a Michelson GT interleaver/de-interleaver circuit having two etalons in optical communication with a 50:50 beam sputter;
Fig. 2b is a device in accordance with this invention for filtering an optical signal, wherein a waveguide chip is coupled to a graded-index lens having an end face optically coupled with a tunable GT etalon;
Fig. 2c is an alternative embodiment of the invention wherein discrete components are coupled to the end of the graded-index lens;
Fig. 2c is a side view of an interleaver/de-interleaver circuit wherein a GT
etalon is formed between an end face of a collimating GRIN lens having a partially reflective coating and a completely reflective mirror;

5 Doc. No: 10-337 CA Patent Figs. 2d and 2e are graphs illustrating the output response of the filter shown in Fig. 2c;
Fig. 3 is a perspective view of an embodiment illustrating a double pass filter wherein same elements are used as shown in Fig. 2b, and wherein a loopback to two additional ports provide means of routing once filtered signals back through the device for a second pass providing higher isolation;
Fig. 4 is a perspective view of an embodiment of the invention illustrating 2-input beam operation wherein the same device is used in two instances simultaneously to operate on two separate input signals and wherein two dual streams of de-interleaved signals are provided; and, Fig. 5 is a side view of an embodiment providing 2-input beam operation wherein respective channels of each of the two input beams do not have the same peak wavelength.
Detailed Description Fig. 2a is a Michelson interferometer combined with two GT cavities. When the optical path length difference between the two arms satisfies the following condition, 8 L=LZ-L,=(2k+1)7~ /4 and under certain conditions regarding GT resonators, the output channel 1 yields a flat top function while the output channel 2 provides a notch function. When the optical path length difference (8 L) is multiple of ~, /2, the two output channels are reversed. A
flat top bandpass/notch filter is provided when the optical path length difference is given by:
8 L=L2-L~=k7~ /4, where k is an integer.
The bandwidth and filter order of the device is affected by the choice of the two GT etalons parameters such as reflectivity and optical path lengths di and d2. The optical path length difference (8 d=d2-d,) is equal to a fraction of the operating wavelength (8 d<7~ /10). When a narrow flat-top bandwidth and a high filtering order are required, a high reflectivity r, typically 0.9 and a small b d, typically ~, /50 are chosen.

6 Doc. No: 10-337 CA Patent Turning now to Fig. 2b, an optical filter 200 in accordance with an embodiment of the invention is shown having a waveguide chip 210 at an input output end, coupled with a GRIN lens 215 having light transmissive glass spacers 218a and 218b at each end. The optical filter conveniently has all input and output ports at a same end of the device.
Providing single ended devices is generally preferable. A tunable GT etalon 220 is coupled to an end of the GRIN lens 215. Glass spacer 218b forms an end of the GT etalon 220 by having one of its faces coated with a partially reflective coating 230 having a reflectivity Ri. The GT etalon 220 has a second fully reflective surface 232 spaced a distance "d" from the coated surface 230. A
piezoelectric tuning transducer 236 is disposed between 230 and 232 for varying the distance "d" in a controlled manner in dependence upon an applied control voltage. A tunable etalon is described in United States Patent 5,283,845 in the name of Lp, assigned to JDS Fitel Inc, and issued February 1994 and is incorporated herein by reference.
The GRIN lens 215 is polished to a length less than a quarter pitch that will ensure a beam input at a point source at an end of the GRIN lens adjacent the glass spacer 218a is focussed at the surface R2. It is noted in the figure that incident beams I~ and I2 input off the optical axis of the GRIN lens, as the two upper beams are shown, have beam centres that converge to a same location on R2 as collimated beams. As is evident from Fig. 2b, and 2c beams II and IZ form angles 8 ~ and 0 2 respectively with the optical axis of the lens. The optical path length followed by beam I~ as it traverses the gap "d" between the reflective surfaces is greater than the path followed by beam I2. This yields a difference in optical path length difference within the etalon for the two beams in order to achieve a desired output response. The angles 0 i and 0 2 can be accurately controlled by selecting suitable values for y, and y2, which correspond to the amount of offset from the optical axis of the GRIN lens for the incoming beams. 'This allows the previously mentioned condition on 8 d to be satisfied. The reflected beams from the GT resonator are then combined into a 50:50 coupler.
In Fig. 2b an additional phase shift of 7~ /4 is achieved by providing a phase shifting element within the waveguide chip 210. This is to compensate for a 7~ /4 phase shift in the 4-port coupler at the output. Alternatively, the input sputter may be a 4-port coupler similar to the one at the output ports, where the input signal is introduced into one of the ports. With this arrangement,

7 Doc. No: 10-337 CA Patent the additional ~, /4 phase shift is not required, because the 4-port coupler introduces a ~, /4 phase shift. The embodiment shown in Fig. 2c has the partially transmissive coating R1 is applied to the end face of the GRIN lens 215 and the spacer element 218b is not required.
In operation, the device shown in Figs 2b and 2c functions in the following manner. An input beam of light is launched into a wavelength independent 50:50 sputter and is split into two sub-beams I1 and I2. The beams propagate along paths that are off axis through the GRIN lens and arrive with different angles 8 , and 8 ~ at the GT resonator. The reflected beams from the GT
resonator are then combined by a 50:50 coupler providing a flat t:op bandpass/notch pattern at the outputs l and 2 when the phase difference between the two beams is zero. Using optical waveguides at the input and the output allows this condition to be easily satisfied. The piezoelectric actuator allows tuning of the GT etalon cavity. By using a GT
etalon having a rear mirror having a reflectivity of nearly 100% insures a very low insertion loss.
The device has minimal polarization dependent loss due to the small angles of incidence in the GT resonator.
The functionality of the devices shown in Figs. 2b and 2c are essentially the same, however, in Fig. 2b the etalon is formed between the end face of the glass spacer 218b and the end face 232 of the etalon 220.
Figs. 2d and 2e illustrate the flat-top output response if the narrow-band filter.
Fig. 3 illustrates an alternative embodiment of the waveguide chip 210 shown in Fig. 2b. The composite waveguide chip 310 in Fig. 3 includes a loopback path in two separate stacked waveguide chips 310a and 310b wherein an output waveguide in each chip is looped back to serve as an input in a second stage of filtering. Therefore, in operation a once filtered signal is passed through the filter a second time to increase filtering and isolation.
Fig. 4 shows another embodiment wherein two stacked waveguide chips 320a and 320b are coupled in such a manner as to allow 2 input beam operation simultaneously. A
first input port IN1 is provided on the chip 320b and a second input port is provided on the chip 320a. The first input beam launched into IN1 is divided equally into two sub-beams on the chip and traverses

8 Doc. No: 10-337 CA Patent the GRIN lens and the etalon. The second input beam IN2 is launched into IN2 in waveguide chip 320b and is similarly divided into two sub-beams. Respective output ports are on opposite waveguide chips from the input beams.
Fig. 5 illustrates another embodiment of the invention wherein two beams having different peak wavelengths, or channels centre wavelengths is shown. The symmetrical coupling provided by a GRIN lens about its optical axis conveniently lends itself to this embodiment.
Of course numerous other embodiments of this invention can be envisaged without departing from the spirit and scope of the invention. For example, means for varying the gap or distance "d" between the reflective end faces of the etalon are preferably in the form of a piezoelectric transducer; however, a thermally expanding polymer spacer can be used and controlled with a heat source. Alternatively. a light transmissive layer having a high thermal coefficient of expansion can be used within the gap and can be heated or cooled to vary the gap "d".

9

Claims (21)

Claims What is claimed is:

1. An optical filter comprising:
a lens;
a first partially reflective surface having a reflectivity R1 about an end face of the lens and optically coupled therewith;
a Gires-Tournois etalon comprising the first partially reflective surface and second reflective surface having a reflectivity R2 spaced a distance "d" from the first reflective surface;
a first input port spaced from an optical axis of the lens and coupled with an end of thereof;
a first output port spaced from the optical axis of the graded-index lens and optically coupled with the first input port;
a second input port spaced from the optical axis of the lens and coupled with an end thereof; and, a second output port spaced from the optical axis of the lens and optically coupled with the second input port.

2. An optical filter as defined in claim 1 wherein the lens provides a means of collimating two beams launched therein, each having a centre at a substantially same location on the second reflective surface, and in such a manner as for the beams to be incident upon the second reflective surface at different angles of incidence.

3. An optical filter as defined in claim 1. wherein the lens is a graded-index lens.

4. An optical filter as defined in claim 3 wherein the first partially reflective surface is disposed on an end face of the graded-index lens, and wherein R1<R2.

5. An optical filter as defined in claim 3 wherein the first partially reflective surface is disposed on a light transmissive substrate directly coupled with an end face of the graded-index lens, and wherein R1<R2.

6. An optical filter as defined in claim 3, further comprising a coupler optically coupled to receive light from the first and second output ports, for providing a filtered signal.

7. An optical filter as defined in claim 6, wherein the coupler is a substantially 50:50 coupler.

8. An optical filter as defined in claim 6, wherein the optical filter is for providing two interleaved signals.

9. An optical filter as defined in claim 6, wherein the optical filter is for providing a substantially flat-top filter.

10. An optical filter as defined in claim 6, wherein the optical filter has an substantially asymmetric output response.

11. An optical filter as defined in claim 6, wherein the coupler is a four port coupler, two of four ports of the coupler being optically coupled with the first and second output ports.

12. An optical filter as defined in claim 1 wherein the first and second output ports are coupled to a means for interfering signals output thereon to provide two de-interleaved data streams.

13. An optical filter as defined in claim 6 further comprising a splitter for splitting an input signal and for providing two sub-signals to the first and second input ports.

14. An optical filter as defined in claim 13, wherein the splitter is a substantially 50:50 splitter.

15. An optical signal as defined in claim 1 further comprising means for varying the distance "d" between the two reflective surfaces.

16. An optical signal as defined in claim 11 further comprising means for varying the distance "d" between the two reflective surfaces.

17. An optical filter comprising a lens having a first reflective coating on or about an end face thereof that is at least partially transmissive, and a second reflective surface spaced a distance "d" from the first reflective coating to form an optical cavity;
a first input port offset from an optical axis of the lens a distance y1;
a first output port optically coupled with the first input port and spaced a distance y1 from the optical axis of the lens;
a second input port offset from an optical axis of the graded index lens a distance y2;
a second output port optically coupled with the second input port and spaced a distance y2 from the optical axis of the lens; and, a coupler for coupling light received from the first and second output ports, wherein y1~y2.

18. An optical filter as defined in claim 17, wherein the lens is a graded index lens and wherein the filter further comprises means for varying the distance "d"
between the first reflective coating and the second reflective surface to vary a free-spectral range of the optical cavity.

19. An optical filter as defined in claim 18, wherein the filter is a Gires-Tournois resonator and wherein second reflective surface is substantially about 100% reflective.

20. A GT cavity comprising:
a lens having two input ports and two output ports in optical communication therewith;
a partially reflective surface optically coupled with an end face thereof , and, a substantially reflecting surface spaced distance "d" from the partially reflective surface to form an optical cavity between the reflective surfaces; and means for varying the distance "d" between the reflecting surfaces, wherein the two input ports are disposed at different radial distances from an optical axis of the lens such that when separate beams of light are launched into the two input ports, the separate beams traverse optical paths between the reflecting surfaces having different optical path lengths.

21. A double GT cavity as defined in claim 20 wherein the lens is a graded-index lens.