Four-wave-mixing based optical wavelength converter device Number:7,324,267 from the United States Patent and Trademark Office (PTO) owispatent

Title: Four-wave-mixing based optical wavelength converter device

Abstract: A wavelength converter device is provided for generating a converted radiation at frequency .omega..sub.g through interaction between at least one signal radiation at frequency .omega..sub.s and at least one pump radiation at frequency .omega..sub.p, including an input for the at least one signal radiation at frequency .omega..sub.s, a pump light source for generating the at least one pump radiation at frequency .omega..sub.p, an output for taking out the converted radiation at frequency .omega..sub.g, a structure for transmitting the signal radiation, including two optical resonators having a non-linear material, having an optical length of at least 40*.lamda./2, .lamda. being the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, wherein by propagating through the structure, the pump and signal radiation generate the converted radiation by non-linear interaction within the optical resonators.

Patent Number: 7,324,267 Issued on 01/29/2008 to Melloni,   et al.

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Inventors:	Melloni; Andrea (Milan, IT), Morichetti; Francesco (Novara, IT), Pietralunga; Silvia Maria (Cassina de Pecchi, IT), Martinelli; Mario (San Donato Milanese, IT)
Assignee:	Pirelli & C. S.p.A. (Milan, IT)
Appl. No.:	10/518,855
Filed:	June 28, 2002
PCT Filed:	June 28, 2002
PCT No.:	PCT/EP02/07207
371(c)(1),(2),(4) Date:	September 14, 2005
PCT Pub. No.:	WO20/04/003653
PCT Pub. Date:	January 08, 2004

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Current U.S. Class:	359/330 ; 359/326
Current International Class:	G02F 1/35 (20060101); G02F 2/02 (20060101)
Field of Search:	359/325-332

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References Cited [Referenced By]

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U.S. Patent Documents

5243610	September 1993	Murata
5311605	May 1994	Stewart
5550671	August 1996	Simpson et al.
5696780	December 1997	Pieterse et al.
5808764	September 1998	Frigo et al.
5854802	December 1998	Jin et al.
6002522	December 1999	Todori et al.

Foreign Patent Documents

	0 730 191		Sep., 1996		EP
	0 981 189		Feb., 2000		EP
	WO 99/52015		Oct., 1999		WO

Other References

PP. Absil et al., "Wavelength Conversion in GaAs Micro-Ring Resonators", Optics Letters, vol. 25, No. 8, pp. 554-556, (Apr. 15, 2000). cited by other . A. Melloni et al., "Synthesis of Direct-Coupled-Resonators Bandpass Filters for WDM Systems", Journal of Lightwave Technology, vol. 20, No. 2, pp. 296-303, (Feb. 2002). cited by other . A. Yariv et al., "Coupled-Resonator Optical Waveguide: A Proposal and Analysis", Optics Letters, vol. 24, No. 11, pp. 711-713, (Jun. 1, 1999). cited by other . R. Goto et al., "Sideband Injection Locking Using Cavity-Enhanced Highly Non-Degenerate Four-Wave Mixing in DFB-LDs", Electronics Letters, vol. 34, No. 23, pp. 2249-2250, (Nov. 12, 1998). cited by other . H. van de Stadt et al., "Multimirror Fabry-Perot Interferometers", Optical Society of America, vol. 2, No. 8, pp. 1363-1370, (Aug. 8, 1985). cited by other . S.J.B. Yoo, "Wavelength Conversion Technologies for WDM Network Applications", Journal of Lightwave Technology, vol. 14, No. 6, pp. 955-966, (Jun. 6, 1996). cited by other . Legoubin et al., "Free Spectral Range Variations of Grating-Based Fabry-Perot Filters Photowritten in Optical Fibers", Journal of Optical Society of America, vol. 12, No. 8, pp. 1687-1694, (1995). cited by other . G.P. Agrawal, "Nonlinear Fibers Optics", Academic Press, 2.sup.nd Edition, p. 17, (1995). cited by other . Chou et al., "Efficient Wide-Band and Tunable Midspan Spectral Inverter Using Cascaded Nonlinearities in LiNbO.sub.3 Waveguides", IEEE Photonics Technology Letters, vol. 12, No. 1, pp. 82-84, (2000). cited by other.

Primary Examiner: Connelly-Cushwa; Michelle
Assistant Examiner: Peace; Rhonda S.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, L.L.P.

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Claims

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The invention claimed is:

1. A wavelength converter device for generating a converted radiation at frequency .omega..sub.g through interaction between at least one signal radiation at frequency .omega..sub.s and at least one pump radiation at frequency .omega..sub.p, comprising: an input for said at least one signal radiation at frequency .omega..sub.s; a pump light source for generating said at least one pump radiation at frequency .omega..sub.p; an output for taking out said converted radiation at frequency .omega..sub.g; and a structure for transmitting said signal and pump radiation, said structure including an optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g; said structure comprising a further optical resonator coupled in series to said optical resonator, said further optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, and wherein by propagating through said structure, the pump and signal radiation generate said converted radiation by non-linear interaction within each of said optical resonators.

2. The wavelength converter device according to claim 1, wherein the converted radiation is generated by four-wave-mixing.

3. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator each have an optical length lower than or equal to 7500*.lamda./2.

4. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator comprise reflectors each having a power reflectivity of at least 0.5.

5. The wavelength converter device according to claim 1, wherein the optical resonator is a Fabry-Perot like cavity bounded by two partially reflecting mirrors.

6. The wavelength converter device according to claim 5, wherein the further optical resonator is a Fabry-Perot like cavity bounded by two partially reflecting mirrors.

7. The wavelength converter device according to claim 1, wherein the optical resonator is a micro-ring-like resonator.

8. The wavelength converter device according to claim 7, wherein the further optical resonator is a micro-ring-like resonator.

9. The wavelength converter device according to claim 1, wherein the optical resonator is formed in a photonic crystal waveguide.

10. The wavelength converter device according to claim 9, wherein the further optical resonator is formed in a photonic crystal waveguide.

11. The wavelength converter device according to claim 1, further comprising an additional structure in series to the structure.

12. The wavelength converter device according to claim 11, further comprising a phase mismatch compensating element adapted to compensate for the phase mismatch accumulated by the pump and signal radiation along the structure.

13. The wavelength converter device according to claim 12, wherein the phase mismatch compensating element is placed between the structure and the additional structure.

14. The wavelength converter device according to claim 12, wherein the phase mismatch compensating element comprises a material having a non-linear refractive index n2 lower than the non-linear refractive index of the material included in the structure and the additional structure.

15. The wavelength converter device according to claim 1, wherein the pump radiation frequency .omega..sub.p and the signal radiation frequency .omega..sub.s are different.

16. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator are connected in series.

17. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator are made of the same material.

18. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator have the same optical length.

19. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator each have a free spectral range equal to or lower than about 4 THz.

20. The wavelength converter device according to claim 1, wherein the optical resonator and the further optical resonator each have a free spectral range equal to or lower than about 1000 GHz.

21. The wavelength converter device according to claim 1, wherein a ratio FSR/B between a free spectral range FSR and a bandwidth B for the optical resonator and the further optical resonator is greater than or equal to 2.

22. The wavelength converter device according to claim 1, wherein a ratio FSR/B between a free spectral range FSR and a bandwidth B for the optical resonator and the further optical resonator is less than or equal to 100.

23. The wavelength converter device according to claim 1, wherein the structure further comprises a third optical resonator cascaded to the further optical resonator, said third optical resonator comprising a non-linear material having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, wherein by propagating through said structure the pump and signal radiation generate said converted radiation by non-linear interaction within each of said optical resonator, said further optical resonator and said third optical resonator.

24. The wavelength converter device according to claim 1, wherein the structure comprises a number of cascaded optical resonators less than N.sub.max, where N.sub.max is equal to the ratio between the coherence length L.sub.coh of the structure and the physical length d of each of said cascaded optical resonators.

25. A method for generating a radiation at frequency .omega..sub.g comprising: interacting through non-linear interaction at least one pump radiation at frequency .omega..sub.p with at least one signal radiation at frequency .omega..sub.s in a structure comprising a plurality of cascaded optical resonators each comprising a non-linear material resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, and having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and wherein through said non-linear interaction the pump and signal radiation generate said converted radiation within each of said plurality of cascaded optical resonators.

26. The method according to claim 25, wherein the radiation at frequency .omega..sub.g is generated by four-wave mixing.

27. An apparatus for an optical network node, comprising: a routing element with at least one input port and a plurality of output ports for interconnecting each input port with at least one corresponding output port; at least one wavelength converter device for generating a converted radiation at frequency .omega..sub.g through interaction between at least one signal radiation at frequency .omega..sub.s and at least one pump radiation at frequency .omega..sub.p, comprising: an input for said at least one signal radiation at frequency .omega..sub.s; a pump light source for generating said at least one pump radiation at frequency .omega..sub.p; an output for taking out said converted radiation at frequency .omega..sub.g; and a structure for transmitting said signal and pump radiation, said structure including an optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, said structure comprising a further optical resonator coupled in series to said optical resonator, said further optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g; wherein by propagating through said structure the pump and signal radiation generate said converted radiation by non-linear interaction within each of said optical resonators, and said at least one wavelength converter device being optically coupled to one of the ports of said routing element.

28. The apparatus for an optical network node according to claim 27, further comprising an additional structure in series to the structure.

29. The apparatus for an optical network node according to claim 28, further comprising a phase mismatch compensating element adapted to compensate for the phase mismatch accumulated by the pump and signal radiation along the structure.

30. An optical communication line comprising an optical transmission path for transmitting at least one signal radiation at frequency .omega..sub.s, and a wavelength converter device for generating a converted radiation at frequency .omega..sub.g through interaction between said at least one signal radiation at frequency .omega..sub.s and at least one pump radiation at frequency .omega..sub.p, comprising: an input for said at least one signal radiation at frequency .omega..sub.s; a pump light source for generating said at least one pump radiation at frequency .omega..sub.p; an output for taking out said converted radiation at frequency .omega..sub.g; and a structure for transmitting said signal and pump radiation, said structure including an optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, said structure comprising a further optical resonator coupled in series to said optical resonator, said further optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g; wherein by propagating through said structure the pump and signal radiation generate said converted radiation by non-linear interaction within each of said optical resonators, said wavelength converter device being optically coupled to said optical transmission path.

31. The optical communication line according to claim 30, wherein the optical transmission path is an optical fiber length.

32. A method for altering the optical spectrum of at least one optical signal radiation at frequency .omega..sub.s comprising, interacting by non-linear interaction the optical signal radiation with an optical pump radiation at frequency .omega..sub.p in a structure comprising a plurality of cascaded optical resonators each comprising a non-linear material, resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, and having an optical length of at least 40*.lamda./2, .lamda. being the wavelength of the pump radiation, wherein the pump and signal radiation generate said converted radiation through said non-linear interaction within each of said plurality of cascaded optical resonators.

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Description

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CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based on PCT/EP2002/007207, filed Jun. 28, 2002, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength converter device comprising a structure having a plurality of cascaded optical resonators.

The present invention relates, moreover, to an use of a structure comprising a plurality of cascaded optical resonators for generating a radiation at frequency .omega..sub.g through non-linear interaction between at least one pump radiation at frequency .omega..sub.p and at least one signal radiation at frequency .omega..sub.s.

Furthermore, the present invention relates to an use of a structure, comprising a plurality of cascaded optical resonators made of a non-linear material, for altering the optical spectrum of at least one signal radiation at frequency .omega..sub.s by non-linear interaction within the material of the optical resonators.

Moreover, the present invention relates to an apparatus for an optical network node comprising a routing element and a wavelength converter device of the invention.

Additionally, the present invention relates to an optical communication line comprising an optical transmission path for transmitting at least one signal radiation at frequency .omega..sub.s and a wavelength converter device of the invention.

2. Description of the Related Art

Structures made of a plurality of cascaded optical resonators are known.

For example, A. Melloni et al. ("Synthesis of direct-coupled-resonators bandpass filters for WDM systems", Journal of Lightwave Technology, Vol. 20, No. 2, February 2002, pages 296-303) disclose a structure consisting of cascaded direct-coupled ring resonators or cascaded Fabry-Perot resonators for use as a bandpass filter.

Furthermore, U.S. Pat. No. 5,311,605 discloses an optical device comprising a length of optical waveguide having incorporated therein an extended sequence of coupled single-resonator structures for use as an optical slow wave structure. This document states that the structure may also be designed to provide a desired filter characteristic, a dispersion such as to correct for undesirable dispersion in other components of an optical system or to provide pulse expansion or compression. The Applicant notes that no mention of use of non-linear interactions is made in this document.

In a WDM (wavelength division multiplexing) optical communication system/network, wavelength management and wavelength routing control between nodes of the system is crucial for preventing wavelength blocking and facilitating cross connecting. To this end wavelength converter devices able to shift a signal radiation from an optical channel to another are required.

Non-linear wavelength converter devices using a parametric process are known.

A parametric process is a process typical of materials having a non-linearity of the .chi..sub.2 or .chi..sub.3 type according to which electromagnetic radiation at predetermined frequencies that propagate in such materials interact with each other for generating electromagnetic radiation at different frequencies from those that have generated them.

For example, a parametric process is a process according to which a pump radiation at frequency .omega..sub.p that propagates in a non-linear material, interacting with a signal radiation at frequency .omega..sub.s, generates a radiation at frequency .omega..sub.g.

Typical parametric processes are a difference frequency generation process, according to which .omega..sub.g=.omega..sub.p-.omega..sub.s, a sum frequency generation process, according to which .omega..sub.g=.omega..sub.p+.omega..sub.s, a second or third harmonic generation process, according to which .omega..sub.g=2.omega..sub.p or, respectively, .omega..sub.g=3.omega..sub.p and a degenerate four-wave mixing (FWM) process according to which .omega..sub.g=2.omega..sub.p-.omega..sub.s or .omega..sub.g=2.omega..sub.p+.omega..sub.s.

For example, one-dimensional-photonic-crystal multiresonator structures (also called photonic band-gap structures) have been proposed for wavelength conversion through a second harmonic generation parametric process.

A one-dimensional photonic crystal structure typically consists of a periodical alternation of two layers of material having different refractive indexes. The multiple reflections at the interfaces between the two layers at different refractive index generate constructive and destructive interference between the transmitted light and the reflected light, so that the propagation of electromagnetic waves in the photonic crystal structure is allowed in some intervals of frequencies (or wavelengths) and forbidden in other intervals. The layers typically have thicknesses a and b of .lamda./4n (quarter-wave layer) or .lamda./2n (half-wave layer), where .lamda. is the operating wavelength and n the refractive index of the layer, so as to form a periodic quarter-wave, half-wave or mixed quarter-half-wave structure.

WO 99/52015 describes a second harmonic generator based on a periodic photonic crystal structure. The described structure comprises a plurality of layers of a first and a second material that periodically alternate, and has a band edge at the pump radiation frequency and a second transmission resonance near the band edge of the second order band gap at the generated second harmonic frequency. The layers have thicknesses a and b of .lamda./4n or .lamda./2n.

U.S. Pat. No. 6,002,522 discloses a structure having materials with different refractive indexes and periodically arranged to form a photonic band-gap structure. Furthermore, it discloses to set the period of two different materials having different refractive indexes (that is the thickness of a pair of two different materials) at nearly half the wavelength of light used. This document teaches that the structure can be used to manufacture a wavelength converter by second harmonic generation, if a second-order non-linear optical material is used, and an optical switch if a third-order non-linear optical material is used.

Wavelength converter devices using the four-wave mixing process are known.

EP 0 981 189 discloses a non-linear wavelength converter device comprising an optical waveguide doped with a rare earth element. An input optical signal and at least one pump light cause four wave mixing (FWM) to occur in the optical waveguide and the FWM causes a converted optical signal to be produced in the optical waveguide. The optical signal and the pump light are amplified in the optical waveguide thereby the four-wave mixing converted optical power is increased.

In order to increase the four-wave mixing converted optical power, also wavelength converter devices using the four-wave mixing process in a single optical resonator have been disclosed.

P. P. Absil et al. ("Wavelength conversion in GaAs micro-ring resonators", Optics Letters, Vol. 25, No. 8, Apr. 15, 2000, pages 554-556) disclose a device comprising a single micro-ring resonator wherein a pump wave of frequency .omega..sub.p and a signal wave of frequency .omega..sub.s are launched into the ring at two different resonant frequencies. A new converted wave is generated by degenerate FWM at the frequency .omega..sub.g=2.omega..sub.p-.omega..sub.s. The Authors states that non-linear interactions are enhanced in the resonator.

U.S. Pat. No. 5,243,610 disclose a device comprising an input for an input light signal, an optical source for generating a pump light signal, a non-linear optical medium for receiving the pump and input light signals and an output. The non-linear optical medium includes a Fabry-Perot type semiconductor laser and frequency converts the input light signal to generate an output light signal using non-degenerate four-wave mixing. In this document it is stated that the FWM may be generated at a relatively lower power due to an internal electric field enlarged by confining the pump light and input signal light into a resonator.

U.S. Pat. No. 5,550,671 discloses a device comprising an input for a signal radiation, an optical source for generating a pump radiation, a laser cavity and an output. The laser cavity is composed of a rare-earth doped fiber and is defined by a pair of fiber Bragg gratings. By propagating through the laser cavity the signal and pump radiation generate by four wave mixing a new converted signal of wavelength within 10% of the signal radiation wavelength. In this document it is stated that the device can be made in compact form with a cavity length as small as 100 m and can provide inverted signals at the same intensity as the input signals.

The Applicant notes that in the above mentioned devices with a single optical resonator, the four-wave mixing converted optical power (i.e. the optical power of the radiation generated by four-wave mixing) depends on the pump power, on the resonator physical length and on the power reflectivity of the reflectors forming the resonator.

For cost, availability and reliability reasons, the pump power should be kept as low as possible. Therefore, the resonator physical length and the power reflectivity should be kept as high as possible in order to achieve high converted optical power values.

However, in this regard the Applicant notes that also the frequency difference between two consecutive resonant frequencies (free spectral range or FSR) and the bandwidth B of an optical resonator depend on the physical length and on the power reflectivity. Furthermore, for use in a WDM optical communication system, the FSR and bandwidth B of the resonator should be set according to the WDM system requirements (e.g., the bandwidth B' of the WDM optical signals and the wavelength spacing thereof which is typically selected according to ITU-T recommendations).

It follows that, the physical length and the power reflectivity of the optical resonator should be selected according to the WDM optical communication system requirements and cannot be freely set to any desired value.

Accordingly, the Applicant notes that external factors may not allow desired values of the FWM converted optical power to be achieved. Thus, the above mentioned devices, using the four-wave mixing process in a single optical resonator, are not versatile.

The Applicant has thus faced the technical problem of providing an efficient and versatile wavelength converter device, capable of achieving high converted optical power values and, at the same time, meeting WDM optical communication system requirements.

SUMMARY OF THE INVENTION

It is a first aspect of the present invention a wavelength converter device, for generating a converted radiation at frequency .omega..sub.g by interaction between at least one pump radiation at frequency .omega..sub.p and at least one signal radiation at frequency .omega..sub.s, comprising an input for said at least one signal radiation at frequency .omega..sub.s; a pump light source for generating said at least one pump radiation at frequency .omega..sub.p; an output for taking out said converted radiation at frequency .omega..sub.g; a structure for transmitting said signal and pump radiation, said structure including one optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, characterized in that said structure comprises a further optical resonator coupled in series to said optical resonator, said further optical resonator comprising a non-linear material, having an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g; wherein by propagating through said structure the pump and signal radiation generate said converted radiation by non-linear interaction within said optical resonators.

As disclosed in more detail hereinafter, in the device of the invention the converted optical power depends on the resonator optical length, the resonator power reflectivity and the number of cascaded optical resonators. Therefore, even if the resonator optical length and the resonator power reflectivity are constrained by external factors, the desired converted optical power value can still be achieved by suitably selecting the number of cascaded optical resonators.

Thus, the device of the invention can be suitably designed both to achieve high converted optical power values and to meet WDM optical communication system requirements.

In the present description and claims, the expression "resonator" is used for indicating a device with a bounded path of such dimension that a standing electromagnetic wave can be sustained by application of energy of appropriate frequency. Typical examples of an optical resonator are a guiding medium (conventionally named "resonant cavity" or "cavity") bounded by two cascaded partially reflecting mirrors or a closed-ring optical waveguide (conventionally named "microring") with a coupling portion to allow the electromagnetic radiation to enter and exit from the ring. The optical resonator has a comb of resonant frequencies that are substantially equispaced in frequency. The distance between two adjacent resonant frequencies is named free spectral range (FSR). The FSR depends on the group length L.sub.g of the resonator (FSR=c/L.sub.g). L.sub.g is defined as L.sub.g=c*.tau..sub.g, wherein c is the speed of light and .tau..sub.g is the group delay of the resonator which depends on the type of the resonator and on the material. In the case of a guiding medium bounded by two cascaded partial reflectors, non-dispersive medium and concentrated reflectors, L.sub.g is twice the optical distance between the two reflectors. In the case of distributed reflectors L.sub.g can be numerically calculated by techniques well known in the art as, for example, by means of the "coupled mode theory" (see for example S. Legoubin et al., "Free spectral range variations of grating-based Fabry-Perot filters photowritten in optical fibers", J. Optical Society of America, August 1995, Vol. 12, No. 8, pages 1687-1694). Lastly, in the case of a closed-ring optical waveguide in a non-dispersive medium L.sub.g is the optical length of the ring.

Moreover, in the present description and claims, the expression "optical length" for a radiation propagating in a propagation medium is used for indicating the product between the refractive index of the medium and the physical length thereof; "four wave mixing efficiency" is used for indicating the ratio P.sub.c/(P.sub.p.sup.2*P.sub.s) where P.sub.c is the converted radiation optical power, P.sub.p the pump radiation optical power and P.sub.s the signal radiation optical power; "bandwidth" B for an optical resonator is used for indicating the full width at half maximum (FWHM) of each resonance; "power reflectivity" or "reflectivity" is used for indicating either the ratio between the power of the radiation reflected by a partially reflecting mirror of a resonant cavity and the power of the incident radiation, or the ratio between the power of the radiation not coupled outside by the coupling portion and the power of the incident radiation in a closed-ring optical waveguide resonator; "reflector" is used for indicating an element adapted to form a resonator as, for example, a mirror of a resonant cavity, a coupling portion of a closed-ring optical waveguide resonator or a defect in a photonic crystal waveguide cavity; "partial reflector" is used for indicating a reflector having a power reflectivity lower than 100%; "multiresonator structure" is used for indicating a structure comprising a plurality of cascaded optical resonators; "multistage device" is used for indicating a device comprising a plurality of cascaded structures with phase-mismatch compensating elements interposed between one structure and the other; "non-linear material" is used for indicating a material having at least one of the .chi..sub.2, .chi..sub.3 susceptibility coefficients greater than zero.

Furthermore, in a WDM optical communication system, the signal radiation having different wavelengths are each assigned a specific band of wavelengths having predetermined width--hereinafter called "channel". Each of said channels is characterised by a central wavelength value and by a range of wavelength, centred about said central wavelength, which is defined "signal bandwidth or band" B'.

The dependent claims set out particular embodiments of the invention.

The non-linear material of the optical resonators can be of the .chi..sub.2 type. Advantageously, it is of the .chi..sub.3 type. In this latter case, the converted radiation is preferably generated by four-wave-mixing. In a preferred embodiment of the invention, the four-wave-mixing is of the degenerate type.

In a preferred embodiment of the invention, the pump radiation frequency .omega..sub.p and the signal radiation frequency .omega..sub.s are different.

Advantageously, the optical resonator and the further optical resonator are directly connected in series.

Advantageously, the optical resonator and the further optical resonator are made of the same material. This facilitates the manufacturing process of the structure.

Advantageously the optical resonator and the further optical resonator are made of a transparent material at the working wavelengths (or frequencies) of the device.

For example, the working wavelengths are selected within the interval comprised between 0.7 .mu.m and 1.9 .mu.m. According to a variant, they are selected within the interval comprised between 0.7 .mu.m and 1.8 .mu.m. Preferably, the working wavelengths are greater than 1.2 .mu.m, more preferably greater than 1.4 .mu.m. Typically, they are lower than 1.7 .mu.m.

Advantageously, the optical resonator and the further optical resonator have the same optical length.

Preferably, the optical resonator and the further optical resonator resonate at three different resonant frequencies substantially equal to the pump, signal and generated frequencies .omega..sub.p, .omega..sub.s, .omega..sub.g, respectively.

According to another embodiment of the invention, the pump, signal and generated frequencies .omega..sub.p, .omega..sub.s, .omega..sub.g fall within the same resonant frequency of the optical resonator and the further optical resonator.

Advantageously, the optical resonator and the further optical resonator each have a free spectral range equal to or lower than 4 THz. In fact, higher values of the free spectral range would imply higher values of frequency spacing .DELTA.F between the pump and signal radiation and, thus, an appreciable decrease of the FWM converted optical power due to chromatic dispersion. Preferably, the optical resonator and the further optical resonator each have a free spectral range equal to or lower than 1000 GHz. More, preferably, the optical resonator and the further optical resonator each have a free spectral range equal to or lower than 500 GHz. According to a variant, the optical resonator and the further optical resonator each have a free spectral range equal to or lower than 100 GHz. According to another variant, the optical resonator and the further optical resonator each have a free spectral range equal to or lower than 50 GHz. According to another variant, the optical resonator and the further optical resonator each have a free spectral range equal to or lower than 25 GHz.

The frequency spacing .DELTA.F between the pump and signal radiation is substantially equal to the free spectral range of the optical resonators or is an integer multiple thereof.

Advantageously, the optical resonator and the further optical resonator each have a bandwidth B of at least 100 MHz. Preferably, the bandwidth B is of at least 1 GHz. More preferably, the bandwidth B is of at least 2.5 GHz. More preferably, the bandwidth B is of at least 10 GHz. Even more preferably, the bandwidth B is of at least 20 GHz. Even more preferably, the bandwidth B is of at least 40 GHz. Even more preferably, the bandwidth B is of at least 100 GHz. The above mentioned values allow the device of the invention to be used with typical telecommunications optical signal radiation modulated at 100 Mbit/s, 1, 2.5, 10, 20 and 40 Gbit/s. Preferably, the bandwidth B of the optical resonator and of the further optical resonator is greater than the bandwidth of the optical signal radiation. More preferably, the bandwidth B of the optical resonator and of the further optical resonator is at least twice the bandwidth of the optical signal radiation.

The optical signal radiation may be continuous wave (CW) signals or may be modulated in amplitude, intensity, phase, frequency, polarization, according to any conventional technique, with any conventional format (e.g. NRZ, RZ, CRZ, soliton, duobinary).

Preferably, the optical resonator and the further optical resonator comprise reflectors having a power reflectivity of at least 0.5. Advantageously, the optical resonator and the further optical resonator comprise reflectors having a power reflectivity lower than or equal to 0.9997 (corresponding to a transmissivity of -35 dB).

Preferably, the reflectors comprised in the optical resonator have the same power reflectivity of the reflectors comprised in the further optical resonator.

Advantageously, the ratio FSR/B for the optical resonator and the further optical resonator is at least equal to 2.

Advantageously, the ratio FSR/B for the optical resonator and the further optical resonator is lower than or equal to 100.

Preferably, the optical resonator and the further optical resonator each have an optical length lower than or equal to 7500*.lamda./2. Values of optical length higher than 7500*.lamda./2 would involve too low FSR and bandwidth B values (see Eq. 2b below). On the contrary, values of optical length lower than 40*.lamda./2 would involve too high FSR values and, consequently, too high frequency spacing .DELTA.F values between the pump and signal radiation and an appreciable decrease of the FWM converted optical power, due to chromatic dispersion.

According to a first embodiment of the invention, the optical resonator is a Fabry-Perot like cavity bounded by two partially reflecting mirrors. Preferably, the further optical resonator is a Fabry-Perot like cavity bounded by two partially reflecting mirrors.

According to an embodiment, the partially reflecting mirrors are concentrated. According to another embodiment, they are distributed.

Advantageously, the Fabry-Perot like cavity is formed in a rare-earth doped bulk medium or a rare-earth doped waveguide or optical fiber.

According to a second embodiment of the invention, the optical resonator is a microring like resonator. Preferably, the further optical resonator is a microring like resonator.

According to a third embodiment of the invention, the optical resonator is formed in a photonic crystal waveguide. Preferably, the further optical resonator is formed in a photonic crystal waveguide.

Advantageously, the optical resonator and the further optical resonator comprise partial reflectors each having a uniform power reflectivity in the whole wavelength range of interest (for example, at the pump, signal and converted radiation frequencies).

Preferably, the optical power of the converted radiation is of at least 100 nW.

Advantageously, the optical power of the pump radiation is of at least 100 mW.

Advantageously, the optical power of the signal radiation is of at least 1 mW.

The non-linear material of the optical resonators is advantageously selected from the group comprising SiO.sub.2, TeO.sub.2, Al.sub.x(GaAs).sub.1-x, LiNbO.sub.3, Si, InP, polymers, such as for example, PPV [poly(phenylene-vinylene)] or MEH-PPV (poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylene-vinylene]), and combinations thereof.

The structure preferably further comprises a third optical resonator cascaded to the further optical resonator. As to the features of the third optical resonator, reference is made to what disclosed above with reference to the optical resonator and to the further optical resonator.

Preferably, the number of optical resonators cascaded in the structure is lower than N.sub.max, where N.sub.max is equal to the ratio between the coherence length L.sub.coh of the structure and the physical length d of each optical resonator (see Eq. 7 below).

Typically, the output of the device comprises an optical filter coupled at the exit of the structure, adapted to take out the radiation generated at frequency .omega..sub.g from the device and to suppress a possible residual pump radiation at frequency .omega..sub.p and a possible residual signal radiation at frequency .omega..sub.s.

According to an embodiment of the invention the wavelength converter device comprises a further structure in series to the structure. In this embodiment, the device preferably comprises also a phase mismatch compensating element adapted to compensate for the phase mismatch accumulated by the pump and signal radiation along the structure. Said phase mismatch compensating element is advantageously placed between the structure and the further structure.

As to the features of the further structure and the optical resonators therein comprised, reference is made to what disclosed above.

Preferably, the structure and the further structures have the same number of cascaded optical resonators. Furthermore, the optical resonators of the structure are preferably the same as the optical resonators of the further structure.

Advantageously, the structure and the further structures are substantially equal.

Advantageously, the phase mismatch compensating element comprises a material having a non-linear refractive index n2 lower than the non-linear refractive index of the material included in the structure and the further structure.

Preferably, the second order dispersion coefficients .beta..sub.2 and {circumflex over (.beta.)} at the pump radiation frequency of the materials included in the structures and in the phase mismatch compensating element have opposite sign.

According to another embodiment, the second order dispersion coefficients .beta..sub.2 and {circumflex over (.beta.)} at the pump radiation frequency of the materials included in the structures and in, the phase mismatch compensating element have the same sign.

The phase mismatch compensating element can comprise either an optical dielectric waveguide, a dispersive plate or an optical fiber.

According to another aspect, the present invention relates to a use of a structure comprising a plurality of cascaded optical resonators for generating a radiation at frequency .omega..sub.g through non-linear interaction of at least one pump radiation at frequency .omega..sub.p with at least one signal radiation at frequency .omega..sub.s, wherein said resonators comprise a non-linear material, resonate at the pump, signal and converted frequencies .omega..sub.p, .omega..sub.s and .omega..sub.g, and have an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation.

Preferably, the radiation at frequency .omega..sub.g is generated by four-wave mixing. More preferably, by degenerate four-wave-mixing.

As to the features of the structure and the optical resonators reference is made to what disclosed above with reference to the wavelength converter device of the invention.

According to a further aspect, the present invention relates to a use of a structure, comprising a plurality of cascaded optical resonators comprising a non-linear material, for altering the optical spectrum of at least one signal radiation at frequency .omega..sub.s propagating through it, by non-linear interaction of the optical signal radiation within the material of the optical resonators, wherein said optical resonators resonate at the signal radiation frequency .omega..sub.s and have an optical length of at least 40*.lamda./2, wherein .lamda. is the wavelength of the signal radiation.

Preferably, the optical spectrum is altered by using self-phase modulation non-linear phenomenon.

Advantageously, the optical spectrum is altered through interaction with at least one pump radiation at frequency .omega..sub.p by using cross-phase modulation non-linear phenomenon. In this case, the optical resonators preferably also resonate at the pump radiation frequency .omega..sub.p.

As to the features of the structure and the optical resonators reference is made to what disclosed above with reference to the wavelength converter device of the invention.

In a further aspect thereof, the present invention relates to an apparatus for an optical network node comprising a routing element with at least one input port and a plurality of output ports for interconnecting each input port with at least one corresponding output port; at least one wavelength converter device according to the invention optically coupled to one of the ports of said routing element.

As to the structural and functional features of the wavelength converter device, reference shall be made to what described above.

Typically, the apparatus also comprises at least one 1.times.K1 demultiplexer device. Said 1.times.K1 demultiplexer device is typically optically coupled to K1 input ports of the routing element.

Typically, the apparatus also comprises at least one K2.times.1 multiplexer device. Said K2.times.1 multiplexer device is typically optically coupled to K2 output ports of the routing element.

Advantageously, the routing element is selected from the group comprising the following elements: add-drop, cross-connect, .lamda.-router (i.e. wavelength selective router) and switch, and a combination thereof.

Advantageously, the apparatus also comprises N input optical fibers (with N.gtoreq.1). In one embodiment, the N input optical fibers are optically coupled to N respective 1.times.K1 demultiplexer devices.

Advantageously, the apparatus also comprises M output optical fibers (with M.gtoreq.1 and equal to or different from N). In one embodiment, the M output optical fibers are optically coupled to M respective K2.times.1 multiplexer devices.

In a further aspect thereof, the present invention relates to an optical communication line comprising an optical transmission path for transmitting at least one signal radiation at frequency .omega..sub.s and a wavelength converter device according to the invention, wherein said wavelength converter device is optically coupled to said optical transmission path and generates a radiation at frequency .omega..sub.g by non-linear interaction between at least one pump radiation at frequency .omega..sub.p and said at least one signal radiation at frequency .omega..sub.s.

As regards the structural and functional features of the wavelength converter device, reference shall be made to what described above.

Advantageously, the optical transmission path is an optical fibre length.

Typically, said optical communication line further comprises a transmitting station for providing said at least one signal radiation at a frequency .omega..sub.s.

Advantageously, the transmitting station is adapted to provide a plurality n of optical signals having frequencies .omega..sub.s1, .omega..sub.s2 . . . .omega..sub.sn differing from one another. Preferably, the transmitting station comprises a wavelength multiplexing device for wavelength multiplexing the plurality n of optical signals into a single WDM optical signal and for sending said WDM optical signal along the optical communication line.

Typically, said optical communication line further comprises a receiving station connected to said optical communication line.

The receiving station advantageously comprises a wavelength demultiplexer device adapted to demultiplex a WDM optical signal coming from the optical communication line. Furthermore, the receiving station is typically adapted to provide the demultiplexed signals to optional further processing stages.

According to an embodiment, the optical communication line comprises an optical node comprising an apparatus according to the invention, wherein said wavelength converter device of the line is comprised in the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will appear more clearly from the following detailed description of a preferred embodiment, made with reference to the attached drawings. In such drawings,

FIGS. 1a and 1b show a schematic view of a wavelength converter device according to first and second embodiment of the invention;

FIGS. 2a and 2b show a diagram illustrating the principle of FWM wavelength conversion according to first and second embodiment of the invention;

FIG. 3 shows a schematic view of an ideal infinite periodic Fabry-Perot like multiresonator structure;

FIG. 4 shows a plot of the ratio B/FSR versus the power reflectivity of the mirrors for the structure of FIG. 3 (FIG. 4a) and a plot of the ratio v.sub.g/v.sub.f versus the power reflectivity of the mirrors for the structure of FIG. 3 (FIG. 4b);

FIG. 5 is a plot of the conversion gain versus the power reflectivity of the mirrors for the structure of FIG. 3;

FIG. 6 shows a plot of the maximum converted optical power, which can be achieved using a single stage device of the invention, for different non-linear media;

FIG. 7 shows a schematic view of a Fabry-Perot like multiresonator structure according to a first embodiment of the invention;

FIG. 8 shows three different ways to spatially modulate the refractive index of an optical waveguide in order to form distributed partially reflecting mirrors;

FIG. 9 shows a schematic view of a microring like multiresonator structure according to a further embodiment of the invention;

FIG. 10 shows a schematic view of a photonic band gap like structure according to a further embodiment of the invention;

FIG. 11 shows a schematic view of a Fabry-Perot like structure in which an active medium is used according to a further embodiment of the invention;

FIG. 12 shows a plot of the maximum converted optical power, which can be achieved using a multistage device of the invention, for different non-linear media;

FIG. 13 shows a schematic view of a multistage device according to an embodiment of the invention;

FIG. 14 shows a schematic view of a multistage device according to a further embodiment of the invention;

FIG. 15 shows a schematic view of a multistage device according to a further embodiment of the invention;

FIG. 16 shows a schematic view of a Fabry-Perot like multiresonator structure according to a further embodiment of the invention;

FIG. 17 shows the characteristic transmission profile for the structure of FIG. 16 (FIG. 17a) and the plot of the ratio v.sub.g/v.sub.f versus f/FSR for the structure of FIG. 16 (FIG. 17b);

FIG. 18 shows a schematic view of an optical communication line according to an embodiment of the invention;

FIG. 19 shows a schematic view of an optical communication system according to an embodiment of the invention;

FIG. 20 shows an apparatus for an optical network node according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a wavelength converter device 100 comprising an input 1, an output 2, a pump light source 3 and a multiresonator structure 4 including a plurality of optical resonators (for simplicity, only 2 resonators 10, 20 are shown in FIG. 1a) directly connected in series.

Advantageously, the wavelength converter device 100 further comprises an optical isolator (not shown) for eliminating any back reflected radiation exiting the device 100 through the input 1. Typically, the optical isolator is placed between the input 1 and the first optical resonator 10.

Advantageously, the wavelength converter device 100 comprises an optical amplifier. Typically, the amplifier is placed at the output 2. For example, the optical amplifier is an erbium-doped fiber amplifier.

The input 1 is adapted to receive at least one signal radiation at frequency .omega..sub.s.

The pump light source 3 is adapted to generate at least one pump radiation at frequency .omega..sub.p.

Furthermore, the pump light source 3 is coupled to the structure 4 through a conventional optical coupler 6. Preferably, the optical coupler 6 is a wavelength selective coupler.

The structure 4 is adapted to receive said signal and pump radiation.

The optical resonators 10, 20 are made of a non-linear material, each have at least two resonant frequencies substantially equal to the pump and signal frequencies .omega..sub.p and .omega..sub.s and an optical length higher than at least 40*.lamda./2, wherein .lamda. is the wavelength of the pump radiation.

By propagating through the structure 4, the pump and signal radiation cause the generation of a converted radiation at frequency .omega..sub.g exploiting the non linear properties of the material of the optical resonators.

The output 2 is adapted to take out the converted radiation at frequency .omega..sub.g.

In a preferred embodiment of the invention, the material of the optical resonators has a non-linearity of the .chi..sub.3 type. Moreover, the signal and pump frequencies .omega..sub.s and .omega..sub.p are different and the radiation at frequency .omega..sub.g is generated using the degenerate four-wave mixing (FWM) process.

FIG. 2a schematically shows the degenerate four-wave mixing process .omega..sub.g=2.omega..sub.p-.omega..sub.s, wherein .omega..sub.s=.omega..sub.p-n.DELTA..omega., n is an integer number and .DELTA..omega. is the free spectral range of the optical resonators.

The applicant notes that, signal resonance 152 is not necessarily the first resonant frequency close to pump resonance 151, but can be shifted of an integer number n of free spectral ranges; signal resonance 152 may be at a lower frequency than pump resonance 151 or at a higher frequency as well and, according to FWM properties, the converted radiation is the phase conjugated of the signal radiation and falls into the symmetrical resonance mode 153 with respect to the pump resonance 151. Furthermore, according to the principles of FWM wavelength conversion, in case of modulated signal radiation, the same modulation is transferred to the converted radiation.

In FIG. 2a the pump radiation frequency .omega..sub.p and the signal radiation frequency .omega..sub.s are tuned to two different resonant frequencies of the optical resonators 10, 20. However, as shown in FIG. 2b, they may also fall within the same resonant frequency 154 of the optical resonators 10, 20, according to another embodiment of the invention.

The Applicant notes that an important requirement for the device of the invention is that the optical resonators 10, 20 both resonate at the pump, signal and converted frequencies. However, it is not necessary that they have the same finesse F (F=FSR/B). As shown below (see Eq. 2), this means that the optical resonators 10, 20 may have different values of power reflectivity R and optical length. However, in a preferred embodiment they have the same optical length. In a further preferred embodiment the reflectors included in the optical resonators 10, 20 have the same power reflectivity.

Some basic notions for degenerate FWM are now briefly illustrated.

For a non-resonant waveguide structure and in the hypothesis of slowly varying envelope, undepleted pump, negligible self phase modulation, lossless media, the spatial evolution of the optical power P.sub.c of the FWM converted radiation satisfies the following equation:

.function..gamma..times..times..times..times..times..times..function..DELT- A..times..times. ##EQU00001## where z is the spatial coordinate, P.sub.p is the optical pump power and P.sub.s is the optical signal power. .gamma. is a coefficient that depends on the non-linear refractive index n.sub.2 and on the waveguide effective area A.sub.eff, while .DELTA.k=2k.sub.p-k.sub.s-k.sub.c takes into account the impact on frequency conversion efficiency of the phase mismatch due to the different wave vectors of interacting fields. Since .gamma. is typically a small number (about 1.510.sup.-3 m.sup.-1W.sup.-1 in a silica fiber) high pump power (about 1 W) and extremely long device (of the order of Km) are generally required.

From this point of view the Applicant notes that the use of an optical resonator for carrying out a FWM process leads to a double advantage.

First of all, a radiation, whose frequency coincides with a resonant frequency of the optical resonator, spends much more time through the resonator of physical length L.sub.f than through a non-resonant waveguide structure of the same length. This effect derives from the fact that the group velocity v.sub.in within the optical resonator is lower than the group velocity v.sub.out within a non-resonant waveguide structure. As a consequence the interaction time between an optical pump radiation and a signal radiation is considerably increased when both optical radiation are suitably tuned near a resonant frequency. The more the propagation is slowed (v.sub.in<v.sub.out) the longer the interaction time within the resonator. Since FWM is an interaction time dependent process, the conversion efficiency in an optical resonator is higher than in an equivalent non-resonant waveguide structure of the same physical length L.sub.f.

Moreover, within the optical resonator, a radiation whose frequency coincides with a resonant frequency of the optical resonator, has a power P.sub.int strongly enhanced with respect to the power P.sub.out within a non-resonant waveguide structure. This effect too is due to the above mentioned slowed propagation group velocity and the more the propagation is slowed, the higher P.sub.int compared to P.sub.out. Since FWM is an intensity dependent process, the conversion efficiency in an optical resonator is further improved compared to that of an equivalent non-resonant waveguide structure of the same physical length L.sub.f.

The FWM process in an ideal infinite periodic structure made of cascaded optical resonators is now described.

As also described hereinafter in more detail, the cascaded optical resonators can be made from a series of directly coupled Fabry-Perot like cavities, optical microrings or a photonic band gap (PBG) waveguide with proper defects therein.

In the following description the main topics of an ideal infinite Fabry-Perot like multiresonator structure is described, even if the same teachings can be applied to optical microring and PBG structures.

FIG. 3 shows said multiresonator structure wherein partially reflecting mirrors 72 are placed at a distance d from each other into a substrate 71 of refractive index n.

This multiresonator structure has a periodic spectral response. Only a radiation whose spectrum lies inside a pass-band resonance of the structure can propagate; otherwise the radiation is backward reflected.

Important parameters of this kind of multiresonator structure have been theoretically determined by the Applicant.

The bandwidth B of such structure is given by

.times..pi..times..times..times..function. ##EQU00002## where t is the field transmission coefficient of each mirror 72 and FSR=c/2nd (Eq. 2b) is the free spectral range, i.e. the frequency difference between two consecutive resonant frequencies of the multiresonator structure. In Eq. 2b, nd is the optical length of each optical resonator.

Thus, the selective behaviour of the structure, expressed by the finesse F=FSR/B, only depends on the power