Integrated optical device for data communication

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An integrated optical device for data communication

Lambda Crossing Ltd. C.138155 WO 01/27692 PCT/IL00/00654 -1-

An Integrated Optical Device for Data Communication

FIELD OF THE INVENTION

This invention is generally in the field of optical communication techniques, and relates to an integrated optical device and a method of its manufacture. BACKGROUND OF THE INVENTION 5 Optical communication serves as the enabling technology for the information age, and the essential backbone for long haul communication. As this technology progresses, there is a tremendous interest in providing optical routes in the short haul, metropolitan and access networks, as well as in local area networks and cable TV networks. In all these networks, the best of breed solution · for io bandwidth expansion has been the adoption of wavelength division multiplexing (WDM), which entails the aggregation of many different information carrying light streams on the same optical fiber. A device capable of accessing an individual information stream is fundamentally required in current and future networks. These devices can also add information streams to the optical fiber, as well as 15 impress information on an optical stream by optical modulation. WO 01/27692 PCT/IL00/00654 -2 -

The basic building block for optical switching is the optical modulator/switch. Various implementations of such a device have been developed, of which the most dominant is the Mach-Zender Interferometer (MZI), in which interference is produced between phase coherent light waves that have traveled 5 over different path lengths. The basic construction of the MZI is schematically illustrated in Fig. 1. Light input to a modulator 1 is via a single-mode waveguide. A beam splitter divides the light into two equal beams that travel through guides 2A and 2B, respectively. By applying voltage to electrodes 4, the effective path lengths can be varied.’ Hence, optical switching is achieved by creating a phase io difference between two arms of the device (guides 2A and 2B), and controlling the optical power at the device output.

In general, the performance criteria for the operation of the wavelength routing elements include: the following: (1) Modulation depth or contrast ratio, which signifies the ratio between 15 two (“ON” and “OFF”) or more states of a switch device; (2) Crosstalk, which defines the ability of the device to select a single optical channel while suppressing information from the other channels; (3) Electric power consumption; (4) Modulation bandwidth, which defines the speed at which the switching 20 can be achieved; and (5) Optical bandwidth within which the modulation is effective.

To achieve a good modulation performance with the MZI, the latter is typically designed with long interference arms. As a result, this device is not efficient in its implementation, and limits the scaling ability of complex optical 25 circuits. Another drawback of MZI-type devices, in their predominant implementation, is their frequency insensitivity over a desired frequency bandwidth. As a result, MZI-type devices cannot be used directly for wavelength routing.

To achieve wavelength routing, the MZI has been utilized in conjunction 30 with wavelength demultiplexers, which provide spatial separation between WO 01/27692 PCT/IL00/00654 -3- different optical frequencies. To this end, a matrix composed of at least N times (N+1) MZI is used to redirect one of the N spatially distinct wavelengths to the device output. The remaining frequencies are recombined using a wavelength' multiplexer. 5 Recently developed integrated electro-optical devices utilize resonant rings to achieve frequency selective switching. Such a device is disclosed, for example, in WO 99/17151. The main components of the device are illustrated in Fig. 2. A resonant ring 6 couples light from one fiber 8a to another fiber 8b, when the frequency of the light passing through the fiber 8a fulfils that of the resonance io condition of the ring 6. By applying an electric field or a thermal source to the ring 6, its refractive index, and consequently, its resonance condition, can be desirably adjusted. Changing the resonance condition prevents the passage of the previously coupled light and acts as a switch. Alternatively, the loss of the ring waveguide can be changed. Adding loss to the ring diminishes its operation as a resonant cavity, 15 and light cannot be coupled from fiber to fiber.

Unfortunately, the conventional resonant ring based systems require fabrication tolerances that are hard to implement by means of a conventional photolithography technique. This disadvantage becomes more essential in multiple-ring devices, wherein the distance between two locally adjacent rings is a 20 critical factor for the successful operation of the device. The use of a switching mechanism providing de-tuning of a resonant ring out of resonance condition has been proposed, being disclosed for example in WO 98/53535. This solution, however, does not meet the extinction ratio and crosstalk requirements of communication systems.

25 SUMMARY OF THE INVENTION

There is accordingly a need in the art to improve the operation of electro-optical communication devices by providing a novel electro-optical device, such as an optical frequency dependent switch, a modulator, a laser or amplifier, an Optical Add Drop Multiplexer (OADM), a spectral analyzer, or a sensor. WO 01/27692 PCT/ILO0/OO654 -4-

The optical device according to the present invention utilizes a structure formed by linear waveguides and ring waveguides (resonators). The inventors have found that the use of a ring waveguide provides for a new advantageous feature associated with the following. The conventional integrated optical devices 5 typically employ a small refractive index difference between the waveguide region and the surrounding material. Since optical waveguides can be implemented in complex manners, the universal quantity characterizing the behavior of the confined light is the effective refractive index of the waveguide. In the conventional devices, the difference between the effective refractive index of the io waveguide and the index of the surrounding medium is typically smaller then 1%. When using ring micro resonator structures, the effective refractive index of the ring waveguide has to be large, i.e., typically greater then 20%, to accommodate tight mode confinement and small losses. In these structures, however, the effective index of the ring waveguide and the linear waveguide are similar to within 15 3%. In an integrated optical device according to the present invention, the refractive index of the ring waveguide is at least 20% greater than the refractive index of the linear waveguide that receives an input signal.

The present invention takes advantage of the use of several (at least two) ring resonators. The main idea of the present invention is based on designing an 20 optical complex filter/resonator, wherein waveguide sections are specifically connected to ring resonators in a configuration which enables realization of optical switching, wavelength routing, lasing, wavelength sensitive amplification, optical filtering, etc. The device may also combine a plurality of such filters in a wavelength router module. Generally speaking, the present invention utilizes the 25 collective response of two or more closed loop resonators, which are connected to each other by two or more optical paths, for the purpose of switching or modulating a selected wavelength.

The optical resonator according to the invention is an enclosed cavity aimed at storing optical energy. As compared to tlie known devices of the kind specified, so which utilize a closed-loop type optical resonator, and several such resonators WO 01/27692 PCT/ILOO/00654 -5- cascaded in various configurations, the present invention utilizes the inclusion of a feedback path for the optical signal. In other words, in the present invention, the loop resonator serves as a frequency selective mirror within a more complex resonator. These minors together with connecting waveguides create closed loop 5 cavities with superior performance and simpler fabrication, and, mainly, are favorable for inclusion of switching capabilities and active media, as compared to the conventional devices.

The wavelength response of a structure composed of several ring resonators coupled to optical waveguides is determined by the physical and geometrical io parameters of the resonators and coupling scheme. The present invention provides novel schemes of coupling multiple resonators to achieve predetermined active filtering and modulation characteristics. These coupling schemes are relatively easy to implement, and provide desired modulation characteristics.

There is thus provided, according to one aspect of the present invention, an 15 optical resonator structure for storing optical energy comprising a combination of two spaced-apart waveguides and at least two spaced-apart resonator-cavity loops accommodated between the two waveguides and connected to each other through sections of the waveguides, said at least two spaced-apart resonator-cavity loops and said waveguide sections creating a closed loop compound resonator for storing 20 optical energy of a predetermined frequency range, the physical characteristics of the ^compound resonator being controllable to adjust the optical storage characteristics of the compound resonator.

The predetermined frequency range is determined by physical and geometrical characteristics of the compound resonator. To control the physical 25 characteristics of the waveguide and/or loop-resonators, a heating means may be used.

One of the two waveguides serves as an input and throughput waveguide, and the other serves as an output waveguide. An optical signal entering the input waveguide may include a plurality of light components having different 30 wavelengths. By actively adjusting the response of the compound resonator, using WO 01/27692 PCT/IL00/00654 -6- a heater or any other means that changes the characteristics of the waveguide sections, one of these wavelengths may be switched from the input to the output waveguide.

According to another aspect of the present invention, there is provided an 5 optical device comprising: (a) a combination of two spaced-apart waveguides and at least two spaced-apart resonator-cavity loops accommodated between the two waveguides and connected to each other through sections of the waveguides, said at least two spaced-apart resonator-cavity loops and said io waveguide sections creating a closed loop compound resonator for storing optical energy of a predetermined frequency range; and (b) a control means for controlling physical characteristics of the compound resonator to adjust its optical storage characteristics.

The device may comprise additional waveguides and additional 15 loop-resonators, forming together several such frequency selective switches, thereby providing complex optical signal switching and routing.

According to yet another aspect of the present invention, there is provided a laser device where an active material with gain is embedded in at least one of the parts comprising the above-described electro-optical device. This multiple section 20 laser can be controlled by applying the above-described control means to tune its lasing frequencyUo q-switch or to passively/actively mode lock the laser device in order to obtain pulsed operation.

According to another aspect of the present invention, there is provided a wavelength router system comprising at least one optical switch and at least one 25 optical filter, wherein the switch and the filter is constructed as the above-described electro-optical device.

According to yet another aspects of the present invention, there are provided the following: an optical spectrum analyzer, an OADM, and a sensor, each comprising the above combination of two linear waveguides and at least two 30 resonator cavity loops. WO 01/27692 PCT/IL00/0Q654 -Ί-

The real time monitoring of optical networks poses challenges for spectral analysis systems, such as the need for high resolution, short spectrum acquisition time, low cost, low loss on the optical link, and small size. In standard spectrum analyzers, where the wavelength separation element is based on gratings, high 5 resolution implies larger size and higher cost. An alternative would be to use tunable filters to scan across the optical spectrum of interest. However, existing tunable filters are limited in their ability to provide the required resolution. According to the present invention, a compound cavity, high Q optical ring resonator structure is utilized as a scanning filter, and is used for the analysis of io optical spectra.

Modem optical communications are typically based on transmitting frequency, multiplexed optical signals through an optical fiber. The OADM is capable of adding or dropping optical channels from an optical fiber, and is an essential element in modem optical communications. In the present invention, the 15 OADM is based on a combination of tunable filters, which provide the add or drop multiplexing functions. Since OADMs have to meet stringent criteria in their filtering, each ring resonator is an optical filter, and, by combining them in parallel, high order filters are obtained.

In general, the resonator-cavity loops (ring-resonators) can be replaced by 20 any other implementation of a frequency-selective element that couple between the two waveguide sections. For example, optical gratings can be used.

According to yet another aspect of the present invention, there is provided an electro-optical device comprising: - a combination of two spaced-apart waveguides and at least two 25 spaced-apart wavelength-selective elements accommodated between the two waveguides and connected to each other through sections of the waveguides, said at least two spaced-apart wavelength-selective elements and said waveguide sections creating a closed loop compound resonator for storing optical energy of a predetermined frequency range; and WO 01/27692 PCT/ILOO/00654 -8- - a control means for controlling physical characteristics of the compound resonator to adjust its optical storage characteristics.

The fabrication technology of waveguides is well developed in many classes and families of materials. The relaxation of the fabrication tolerances in the present 5 invention relates to the possibility of vertical coupling of light from the waveguides to the ring resonators. Since the vertical fabrication tolerances are much better than the horizontal tolerances, the result is a device which is simpler to manufacture. However, the details and design of the invention extend beyond such devices in which only vertical coupling exists between the waveguides and resonators, io Thus, according to yet another aspect of the present invention, there is provided a method for manufacturing the above device utilizing existing lithography techniques.

More specifically, the present invention is used with the ring-resonators and is therefore described below with respect to this application. 15

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of a non-limiting example only, with reference to the accompanying drawings, in which: 20 Fig. 1 is a schematic illustration of the conventional MZI structure;

Fig. 2 is a schematic illustration of the conventional resonant ring based :*3 electro-optical device;

Fig. 3 is a schematic illustration of an electro-optical device according to one embodiment of the invention; 25 Fig. 4 graphically illustrates some advantageous features of the device of

Fig. 2, as compared to the conventional devices;

Fig. 5 graphically illustrates simulation results of the operation of the device of Fig. 2;. φ WO 01/27692 PCT/IL00/00654 -9-

Figs. 6a to 6c schematically illustrate electro-optical devices according to three different embodiments of the invention, respectively, suitable for designing complex filter structures;

Figs. 7a to 7c illustrate three more examples, respectively, of complex filter 5 structures constructed according to the invention;

Fig. 8 graphically illustrates the operational principles of the devices of Figs. 7a-7c;

Fig. 9 schematically illustrates a block diagram of a wavelength router system utilizing the devices according to the invention; io Fig. 10 illustrates a system utilizing the optical switches and filters according to the invention, and using ASE for monitoring the status of the optical switches;

Figs. 11A and lib illustrate main constructional features and main functional features, respectively, of a single channel Optical Add Drop Multiplexer 15 (OADM) according to the invention;

Fig. 12 graphically illustrates the spectral response of one-, two- and three- ring filters for use in an OADM;

Fig. 13 schematically illustrates a four port add drop multiplexer;

Fig. 14 schematically illustrates the integration of switches and add drop 20 filters for switch-able filters;

Fig. 15 illustrates the main components of a spectral analysis filter and detector according to the invention;

Fig. 16 illustrates a tap coupler and spectral analysis system utilizing the filter of Fig. 15; 25 Fig. 17 illustrates a spectrum analyzer using several spectral filters of Fig. 15; and

Figs. 18 and 19 illustrate the main principles of a sensor device according to the invention. 30 WO 01/27692 PCT/IL00/00654 -10-

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Figs. 1 and 2 illustrate conventional MZI-type and ring-resonator type structures, respectively.

Referring to Figs. 3a and 3b, there is illustrated an electro-optical device, 5 generally designated 10, constructed and operated according to one embodiment of the invention. The device 10 includes a compound resonator, which, according to the present example, is composed of two ring-resonators 12 A and 12B (constituting resonator-cavity loops functioning as frequency-selective elements), and two waveguides 14 and 16, wherein waveguide sections 14A and 16A connects the io rings 12A and 12B to each other. As shown in Fig. 3b by way of a block-diagram, these waveguide sections 14A and 16A present a spacer S between optical cavities defined by two ring-resonators R.

Further provided in the device 10 is a heater element 18 (electrodes) placed on either one of the waveguide sections - the section 16A in the present example. 15 The operation of the heater element 18 enables to control the refractive index and, consequently, the optical phase imparted by the waveguide spacer S. Generally speaking, the change in the refractive index will induce the required phase shift to change the frequency response of the compound resonator. Such an active phase affecting may be achieved by applying any suitable thermo-optic, piezo-electric, 20 electro-optic or the like effects within the spacer or ring resonator regions.

The device 10 may be implemented as a multi-layer optical structure -;.g manufactured by a lithography technique. All the elements of the compound resonator, i.e., the ring-resonators and the waveguides, may be formed in the same optical layer. Alternatively, the compound resonator may be manufactured as a 25 multi-layer structure, namely the waveguides may be located in a locally adjacent upper or lower layer with respect to the layer containing the ring-resonators. This facilitates the manufacture to meet the requirements for small spaces between the coupling elements (i.e.. ring and waveguide).

The optical cavities (ring-resonators) are weakly coupled to the waveguides. 30 Direct coupling between the two resonators is not required by this scheme. Each of WO 01/27692 PCT/ILOO/00654 -lithe optical cavities is capable of supporting several resonance frequencies, which are determined by the geometrical and material details of the cavity. In the present example of Figs. 3a and 3b, which is the simplest case utilizing a pair of ring-resonator cavities, the two cavities 12A and 12B are identical, namely tuned 5 for the same frequency range of the resonance condition. A change in the refractive index of one of compound resonator’s elements (which is the waveguide 16 in the present example, since the heating element 18 is associated with this waveguide), will cause changes in the roundtrip phase of the entire cavity, thereby shifting the resonance condition. io As indicated above, although the ring-resonators are exemplified here, any other implementation of the frequency-selective elements (mirrors) that couple between the two waveguide sections may be employed in the compound resonator for the purposes of the present invention. Such a frequency-selective element may, for example, be an optical grating. 15 Assuming that the waveguide 14 is an input waveguide, and the waveguide 16 is an output waveguide, the response of the compound resonator 10 is essentially different from that of the conventional single-ring resonator shown in Fig. 2. This is illustrated in Fig. 4, showing two graphs Gi and G2, presenting optical power P at the output waveguide (fiber) 16 as a function of a normalized 20 wavelength ληθΓ, corresponding, respectively, to the conventional device and device 10 constructed according to the invention. It is self evident that the filtering characteristics and out of band signal suppression of the coupled resonator 10 are much better then those of the conventional single-ring resonator.

Fig. 5 illustrates simulation results of the operation of the device 10. Three 25 graphs Hi, H2 and H3 present the output power P of the device 10 as a function of normalized wavelength \or for different phase shifts: φ=0, φ=π/4 and φ=π/2, respectively.

Each of the ring-resonators (two in the present example) is comprised of a waveguide with the index or refraction (ncore) larger than its surrounding material 30 (ndadding). The waveguide is fashioned into a closed path, called “ring". The input WO 01/27692 PCT/IL00/00654 -12- waveguide passes below the ring-resonators in a manner to allow the overlap ofthe two waveguide modes and allow for transferring optical power from the input waveguide to the ring-resonator. As for the output waveguide, it is also placed so as to attain the coupling to the ring. This output waveguide may serve as the output 5 of the selected frequency. Alternatively, when the device 10 is to be used as a modulator, this waveguide can also serve as a throughput port for optical frequencies different from that ofthe modulated signal.

In general, integrated optical elements are composed with contradictory requirements. For low loss and ease of coupling to the external optical fiber, io waveguides are realized using a small difference between the refractive indices of the core and cladding. However, to achieve compact structures, tight bends are required which imply a large difference between the refractive indices of the core and cladding. An important result of the current invention relates to the ability to combine the best of both worlds by using ring resonators realized in a high index 15 core and coupled to a low index waveguide. According to the present invention, if, for example, the refractive index of a surrounding medium is 1.46, and the refractive index of an “input” linear waveguide (e.g., 14) is 1.48, the refractive index of each of the ring resonators 12A and 12B can be about 2 and would provide successful operation of the device 10. Generally, the refractive index of the 20 ring waveguide should be at least 20% greater than the refractive index of the “input” waveguide 14, to realize low loss small radius ring resonators.

The operation of the device 10 is characterized by the low loss propagation of the optical mode in the ring waveguide. This is achieved by utilizing a refractive index contrast between the waveguide and surrounding material. The ring may be 25 composed of optical glass with a refractive index of about 1.6-1.9. may be made from silicon (refractive index of 3.5) or a layered-structure made of suitable materials such as used in Vertical Emitting Cavity Lasers (VECSELs). It is known that the ring itself manifests on frequencies corresponding to its resonance condition. The resonant frequency of the ring,/#, is given by: WO 01/27692 PCT/IL00/00654 -13-

Me 2nRne/ Λ = (1) wherein R is the ring radius measured from the center of the ring to the center region of the ring waveguide; nef is the effective refractive index of the ring 5 waveguide; M is an integer value; and c is the speed of light propagation in vacuum. The effective refractive index can be determined by various known techniques.

One of the important parameters defining the characteristics of the entire device 10, is the coupling between the ring and the waveguide, since it determines io the optical bandwidth and photon lifetime, and, consequently, the modulation efficiency. The power exchange between the waveguide and the ring is denoted by Λ2, and can be calculated by computing the overlap integral of the modes ofthe ring and waveguide multiplied by the interaction length. The optical bandwidth, Af, is then determined as follows: 15 k2f 2π~Κη, (2)

As seen in Fig. 3b, the individual ring-resonator actually presents a two-port device. The throughput function describing the ring optical amplitude 20 characteristics is given by; Γ(ω) = 1 (l - fc)(l - eJM/) fxk 1-(1 (3) while the drop function is given by: Ώ(ω) = - 1-(1 25 (4). WO 01/27692 PCT/ILOO/00654 -14-

Here, ω is the radial frequency given by: ®=2f.

The matrix describing one ring is given by: (5)

=1/ T T -D D T2-D2 5

Complex structures can be obtained by multiplying the matrixes of the corresponding sections. This calculation technique is known per se, and is used in analyzing complex distributed feedback lasers.

Turning back to Fig. 3 a, it should be understood that, if at least one of the io frequency-selective elements (ring resonators) 12A or 12B or at least one of the waveguide sections 14A or 16A is filled with an active medium, the device 10 can operate as a laser.

As known, an optical communication system typically requires stringent switching and routing. A single resonant ring device usually cannot meet these 15 requirements. To this end, according to the present invention, filters are designed either as single stage coupled compound resonators, or as multiple-stage coupled compound resonators. Compound resonators of such filtering devices are illustrated in Figs. 6a-6c, being designated 20, 30 and 40, respectively. Each of these devices utilizes the compound resonator structure 10 of Fig. 3a as a frequency 20 selective switch/modulator, in which the ring-resonators R are coupled both in the forward and back directions, thereby increasing the degrees of freedom in the design of filters. It should be understood that, in all the examples, the matrix model is used in the synthesis and analysis of the filter/switch/modulator characteristics.

In the example of Fig. 6a, the entire waveguide cavity is formed by 25 waveguides 14 and 16, and three pairs of ring-resonators Ri-R2, R3-R4 and Rs-Rs enclosed therebetween. In the example of Fig. 6b, a multi-stage coupled compound resonator 30 is composed of two pairs of rings R1-R2 and R3-R4 enclosed between waveguides Wj and W2, and an additional ring-resonator R5 coupled to an additional waveguide W3. A multi-stage coupled compound resonator 40 (Fig. 5c) WO 01/27692 PCT/IL00/00654 -15- comprises two compound resonators 10, and two additional ring-resonators Rs and

Ri, the latter being is coupled to an output waveguide W4. By appropriately adjusting the index of the waveguide in the conesponding compound resonator, the desired wavelength may be switched from the input to the output waveguide. 5 Reference is now made to Figs. 7a-7c and 8, illustrating the main constructional and operational principles of three other devices 50, 60 and 70, respectively, that are capable of operating as a switch or modulator. The devices 50, 60 and 70 have somewhat different design of waveguides and rings arrangement, as compared to the previously described examples, as illustrated in 10 the figures in a self-explanatory manner using the same reference numbers for identifying those components, which are common in all examples.

Fig. 8 shows three graphs Di, D2 and D3, corresponding to simulation results of the operation of the devices 50,60 and 70, respectively. Each graph presents the optical power P at the output fiber (W3, W3 and W4, respectively) as a function of a 15 phase shift φ in the waveguide section. As clearly seen in the figure, for very small values of the phase shift Φ, more then 20dB of signal extinction is obtained. This enables the size required by the waveguide sections to be to significantly reduced, since the optical phase shift is accumulated over the length of the waveguide.

The advantages of the device according to the invention are thus 20 self-evident. The device attains attractive modulation characteristics, requires very small phase shifts, and, consequently, the interaction region, as well as the switching power, can be minimized. The extinction ratio ofthe optical signal meets optical communication standards.

Fig. 9 illustrates a block diagram of a system 100 utilizing the 25 above-described devices to form a wavelength router. The system comprises three switches SWi, SW2 and SW3, and two filter units FUi and FU2. Each of the filter units is accommodated between two locally adjacent switches, and is designed so as to, when being actuated, route a specific optical frequency. In this example, one of the filters is activated at a time, thereby enabling the routed wavelength to be 30 dynamically chosen. Obviously, a plurality of switching mechanisms can be used WO 01/27692 PCT/IL00/00654 -16- . to increase the number of drop ports. It is important to note that this technique requires a considerably lower number of switches than that of the MZI switching matrix. Indeed, for an /V-channel, M-drop system, the MZI switching matrix would require at least (N+M) by N matrix, while the system according to the present 5 invention would require N switches with an M by M matrix.

The present invention can also be used for actively monitoring the switch performance. As is known, one the crucial issues in modem communication systems is the status of the on-line switches. A non-operative switch in either the “ON” or “OFF” position can degrade the performance of the communication io network. Modem communication systems utilize an erbium-doped fiber to compensate for losses in the optical fiber, connectors and devices. The amplifier emits amplified spontaneous emission (ASE) in all optical frequencies, which are off interest.

In the present invention, it is proposed to use an ASE for monitoring the 15 status of the optical switches. This concept is illustrated in Fig. 10, showing a system 200 that utilizes the components of the above-described system 100, and two photodetectors PDj and PD2. Each photodetector is placed at the output of the corresponding switch and is coupled to a control unit (monitor) CU that monitors the optical power though this switch. Since the ASE exists at all frequencies, it can 20 be used to monitor and control the switches.

The combination of two ring waveguides accommodated between and coupled to the two linear waveguides may be advantageously utilized in various optical devices. Referring to Figs. 11A and lib, there are illustrated main constructional features and main functiohal features, respectively, of a single 25 channel Optical Add Drop Multiplexer (OADM), generally designated 300.

The OADM 300 is composed of two compound resonators 310 and 312, each constructed as described above, namely, including two ring-resonators accommodated between and coupled to two linear waveguides. Here, each ring resonator is an optical filter, and, by combining them in parallel, high order filters PCT/ILOO/00654 φ WO 01/27692 - 17- are obtained. The drop port (filter) is implemented using a double filter pass, while the add port is obtained with a single filter.

It should be noted that the number of rings per functional filter may differ to ! accommodate the specifications of a particular optical network. This concept is 5 illustrated in Fig. 12 showing graphs Gi, Gt and G3 corresponding to the optical spectral response of, respectively, one-, two- and three-ring filters.

Fig. 13 illustrates a four port add drop multiplexer. Here, multiple channel OADMs are obtained by cascading the structures of Figs. 11 A-l IB.

Fig. 14 illustrates an example of the integration of switches and add drop 10 filters for switch-able filters. Here, optical switches are added to insert and extract the ring based OADM from the optical path.

Reference is now made to Fig. 15 illustrating the main components of a system 400 formed by a spectral analysis filter 410 and detector 412. The filter 410 comprises two compound resonators 410A and 410B connected in parallel through 15 a common linear waveguide W2, and serves as a compound high Q optical ring resonator structure. The output linear waveguide W3 of the structure is connected to the detector 412. The Q of the filter is determined by the coupling factor describing the amount of light that is coupled into the filter at every round trip. The Q factor is also determined by the optical losses in the cavity and the ring radius. 20 Fig. 16 illustrates a tap coupler and spectral analysis system, generally designated 500, utilizing the above-described system 400. The filter 410 is connected to an optical network (link) 512 via a coupler 514, which taps a small amount of light, thereby minimizing the losses incurred in the optical link.

Fig. 17 illustrates a spectrum analyzer 600 utilizing several spectral analysis 25 filters - three such filters 610A, 610B and 610C in the present example, used in parallel through a common input linear waveguide Wj. Each filter has a different radius, as compared to the others, and therefore is capable of carrying out a different spectral analysis. This feature is associated with two problems that may occur when using ring resonators, namely, limited tuning range and limited free WO 01/27692 PCT/IL00/00654 -18- spectral range, resulting in that a different approach has to be adopted to scan across a wide spectrum.

Turning now to Fig. 18, there is illustrated a sensor device 700 according to the invention. The sensor device 700 comprises an environmental sensitive filter 5 710 constructed as the above-described compound resonator, which is connected to a laser 712 and a detector 714 through its input and output waveguides Wi and W2, respectively. Such a high Q optical filter structure is used as a sensor, which is suitable for various applications, such as a biological, mechanical, or temperature sensor. This is due to the fact that the filter characteristics depend on the external 10 element to be measured.

Fig. 19 shows the results of tuning the laser 712 to the edge of the filter 710. Generally speaking, the environmental element changes the resonance frequency of the filter, which results in a change of the optical power at the detector. With proper calibration, this device can be used to measure or monitor various physical, 15 mechanical or biological environmental changes.

Those skilled in the art will readily appreciate that various modifications and changes can be applied to the preferred embodiment of the present invention as herein before exemplified without departing from its scope defined in and by the appended claims. In the method claim which follow, characters which are used to 20 designate claim steps, are provided for convenience only and do not apply any . particular order of performing the steps.