KR20070003801A - Tunable resonant grating filters - Google Patents

Abstract

The invention relates to a tunable resonant grating filter that can reflect optical radiation at a resonant wavelength, said resonant wavelength being selectively variable. The filter comprises a diffraction grating (3), a planar waveguide (4) and a light transmissive material having a selectively variable refractive index to permit tuning of the filter, said light transmissive material forming a tunable cladding layer (5) for the waveguide, preferably a liquid crystal (LC) material. The diffraction grating (3) is placed on the opposite side of the tunable layer (5) with respect to the planar waveguide (4) thereby making possible to tailor the grating structural parameters to the desired bandwidth of the filter response without significantly affecting the tunability of the filter. Within the resonant structure (1) of the present invention, the core layer, i.e., the waveguide (4), can be placed close to the tunable layer (5), either in direct contact with the tunable layer or with an interposed relatively thin intermediate layer(s) between the core and the tunable layer. Proximity of the core layer (4) to the tunable layer (5) implies that the propagation mode can significantly extend into the tunable layer (5) so that the effective refractive index of the fundamental mode in the waveguide (4) is efficiently affected by variations of the refractive index of the tunable layer (5). ® KIPO & WIPO 2007

Description

Tunable resonant grating filters

The present invention relates in particular to a tuned resonant lattice filter suitable for wavelength division multiplexed optical communication networks. More specifically, the present invention relates to a tuned resonant grating filter used as a tuned mirror in an external cavity tuned laser for wavelength division multiplexing.

Guided-mode resonance effects in planar waveguide gratings can be used to fabricate ideal or near ideal reflection filters. For an incident wavelength (or frequency) equal to each of the resonant wavelengths of the filter, the incident radiation is suppressed by the resonant reflection and no transmission through the device is achieved. For all other values of the incident wavelength, the device is substantially transparent.

The characteristics of the resonant grating filter have been studied. Applied Optics, vol. Wang S.S and R., published at 34 (1995), p. 2414. In R. Magnusson's "Multilayer waveguide-grating filters", a resonant reflective filter composed of multiple thin-film layers was seriously studied. J. Opt. Soc. Am. Donald K. J. in A (2000), p. 1241. In "Design considerations for narrow-band dielectric resonant grating reflection filters of finite length" by Donald KJ et al., A multiple-scattering interference-waveguide approach is used to predict the response of a resonant grating reflection filter. Developed.

Generally, the resonant grating filter structure has a diffraction grating having a high refractive index region and a low refractive index region in the waveguide layer, generally on a substrate supporting the layer. Waveguide layers or core layers are used as cavities through which individual modes can propagate. The grating at the top of the waveguide couples the incident illumination plane wave into individual modes within the waveguide.

IEEE Journal of Quantum Electronics, vol. 33 (1997), as described in "Resonant Grating Waveguide Structures" by Rosenblatt D. et al., Published on p. 2038, when a resonant grating waveguide structure is irradiated with an incident light beam, part of the beam Is directly transmitted and some are trapped in the waveguide layer after diffraction. Then, part of the light trapped in the waveguide layer is diffracted back to the outside and interferes with part of the transmitted light beam. At certain wavelengths and angular directions of the light beam, the structure “resonates”. In other words, complete offset interference occurs between the directly transmitted field and the diffracted contribution so that no light is transmitted. The bandwidth of the resonance is based on parameters such as depth, duty cycle (ratio of step width to grating period) and refractive index constant of the grating as well as the thickness of the waveguide layer. The bandwidth can be designed very narrowly (with a size of 0.1 nm).

Applied Physics Letters, vol. 77 (2000), p. Published in 1596. G. Levy-Yurista and A. A. A resonant lattice waveguide structure is described in AA Freisem's "Very narrow spectral filters with multilayered grating-waveguide structures", in which the lattice layer is formed by a buffer layer which is also used as an interruption layer of the etching process. It is separated from the waveguide layer. It is said that by separating the waveguide and lattice layers it can be highly optimized and controlled in thickness.

The resonant wavelength of the resonant grating filter can be controlled by varying the refractive indices of the other layers in the structure. This change in refractive index leads to a different phase match, resulting in a resonance wavelength shift. For example, a refractive index change can be induced by applying an external electric field. IEEE Journal of Quantum Electronics, vol. 37 (2001), p. Dudou Beach yen announced in 1030. In "Active Semiconductor-Based Grating Waveguide Structures" by Dudovich N. et al, an active grating waveguide structure using InGaAsP / InP materials and based on reverse voltage configuration is described.

Tunable optical filters have many applications in optical communications, in particular wavelength division multiplexing (WDM). In a WDM system, several channels having different wavelengths are transmitted through the optical fiber, and each channel carries different information. The tuned filter can be used to select and filter any one channel. To accommodate the growing optical communications traffic, high density WDM (DWMD) systems with channel spacing of 50 GHz and ultimately 25 GHz are under development. DWDM systems with 50 GHz channel spacing typically require a frequency accuracy of ± 2.5 GHz, while systems with 25 GHz typically require a frequency accuracy of ± 1.25 GHz. If the DWDM uses narrower channel spacing, the passband of the filter must also be narrowed to prevent crosstalk with other channels.

The resonant grating filter is suitable as a tunable filter, especially for applications in WDM systems, due to the relatively large inherent wavelength selectivity resulting from the resonant conditions, thereby in principle being narrow (ideally reduced to 0.1 nm or even less). One FWHM is possible.

The Proceedings of LEOS 2002, p. Hua Tan et al.'S "A tunable Subwavelength Resonant Grating Optical Filter" presented at 825 is a resonance that can be achieved by tuning the resonant wavelength by tuning a liquid crystal (LC) layer arranged as a filter cladding layer. It describes a lattice filter. While a tuning range of 17 nm was achieved, the simulation showed that the LC filter had the possibility to achieve a 55 nm tuning range and 0.1 nm bandwidth.

The use of lasers as tunable light sources can greatly enhance the reconfiguration of WDM and DWDM systems. For example, other channels may be assigned to nodes by simply tuning the wavelength.

Other approaches can be used to form tuned lasers, distributed Bragg reflector lasers, VCSEL lasers with mobile top mirrors, or external cavity diode lasers. Externally tunable lasers offer several advantages such as high output power, wide tuning range, good side mode suppression and narrow bandwidth. Various laser tuning devices, such as mechanically adjustable or electrically operated intracavity selector elements, have been developed to provide external cavity wavelength selection.

Wavelength selection and tuning of the laser cavity can be performed using an active tuned mirror. U.S. Patent No. 6,215,928 describes an active tuned mirror comprising at least a planar waveguide filling a gap in the diffraction grating and a diffraction grating formed on the cladding layer. The cladding may be formed of a liquid crystal material that can be electro-optically tuned. The resonant wavelength can be shifted by varying the voltage or current supplied to the electro-optically controlled device.

U.S. Patent No. 6,205,159 disclose an external cavity semiconductor laser tuned to each set of wavelengths by varying the voltage on a liquid crystal Fabry-Parlot (LC-FP) interferometer. Each set of wavelengths that can be tuned is formed by a static intracavity etalon. The free spectral range (FSR) of the fixed internal etalon is designed to be larger than the resolution bandwidth of the LC-FP interferometer. The FWHM line width of the fixed etalons shall be less than the external cavity longitudinal mode force.

Applicant is concerned with structural parameters such as the thickness or lattice period of the waveguide or lattice layer of the resonant grating filter, for example, in a tuned laser for a WDM system operating in an erbium C-band with channels spaced 100 GHz apart. It has been noted that it is important to achieve a filter with a relatively large, for example, 30-40 nm or greater tuning range and a relatively narrow bandwidth at FWHM, preferably 100 GHz (0.8 nm) or less, as required as the device.

Japanese Patent No. 63-244044 describes waveguide grating elements that can be used to fabricate deflection elements or focus lenses. In the device described above, light is incident laterally along the waveguide. The change in the refractive index of the LC material causes a change in the light deflection angle which can adjust the angle of the output light by the externally applied voltage.

Likewise, U.S. Patent No. 5,193,130 disclose an optical deflecting device in which a voltage signal changes the alignment direction of the LC layer near the surface of the waveguide layer. In the device described above, the waveguide light propagating along the waveguide is separated into two light beams, the first light emitted by the LC and the second light emitted towards the substrate due to the grating. The direction of light emitted out of the waveguide layer is changed by applying a voltage signal.

The present invention relates to a tuned resonant lattice filter capable of reflecting light radiation at resonant wavelengths, wherein the resonant wavelength can be selectively varied. The filter has a periodic structure having a low refractive index region and a high refractive index region, that is, a light transmissive material having a variable refractive index allowing the tuning of the diffraction grating and the planar waveguide and the filter, and forming a tuned cladding layer for the waveguide. It is provided. The light transmissive material is preferably a light transmissive material which is an electro-optical light transmissive material, more preferably a liquid crystal (LC) material.

The optical properties of the resonant grating filter are governed by structural features such as the thickness of the grating layer, the thickness of the waveguide layer or the index contrast between the low and high refractive index regions of the grating. Applicants have found that a suitable trade-off between narrow bandwidth and wide tuning occurs with an appropriate choice of the structural characteristics of the device.

Applicants have found that by placing diffraction gratings on opposite sides of the tuning layer relative to the planar waveguide, the lattice structure parameters can be tailored to the desired bandwidth of the filter response without significantly affecting the tuning of the filter. In other words, a resonant structure comprising a diffraction grating under the waveguide allows flexibility in the selection of an appropriate grating structure that achieves a desired tuning range while having a relatively small coupling efficiency required for bandwidth resolution. . Within this resonant structure, a core layer, ie, a waveguide, may be disposed near the tuning layer, and the core layer may be in direct contact with either the tuning layer or the relatively thin intervening layer interposed between the core layer and the tuning layer. have. The approach of the core layer to the tuning layer means that the propagation mode can be significantly extended to the tuning layer so that the effective refractive index of the fundamental mode in the waveguide is effectively affected by the refractive index change of the tuning layer.

Applicants have found that a grating with a relatively low coupling efficiency is desirable to obtain a resonant filter exhibiting a relatively narrow bandwidth. Preferably, in order to achieve a bandwidth of FWHM of about 0.6 nm or less (particularly suitable for tuned laser DWDM systems with 50 GHz channel spacing), the coupling efficiency should be about 0.0026 or less. More preferably, the coupling efficiency may be between about 0.001 and 0.002.

By fabricating a resonant lattice structure in which a core layer is interposed between the lattice layer and the tuning layer, manufacturing tolerances of some of the structural parameters can be relaxed as the tuning range is verified to be less affected by changes in the lattice parameters. Can be. In addition, to achieve a weak grating, that is, it is not necessary to produce a grating smaller than the coupling coefficient of 0.0026 or less, i.e. smaller than 150-200 nm.

The tuned resonant filter of the present invention is particularly suitable for application in an externally co-tuned laser for DWDM systems having 50 GHz and 25 GHz channel spacing systems. The tuned grating filter of the present invention can be tuned over the entire erbium C-band (1530-1570 nm).

In one aspect thereof, the present invention relates to an external cavity tuned laser having a tuned resonant lattice filter.

Figure 1 is a simplified example of the incident and diffraction waves associated with the layer structure of a resonant grating filter and typical incident radiation.

2A-2C show the calculated bandwidth of the FWHM of the resonant grating filter as a function of the diffraction grating thickness versus other values of the thickness of the core layer. In FIG. 2A, the specific refractive index Δn _G / n _G is 0.26, 0.07 in FIG. 2B, and 0.04 in FIG. 2C.

3 is a simplified example of the layer structure of a resonant grating filter having a "gap" layer between a core layer and a lattice layer.

4 is a resonance of the type shown in FIG. 3 as a function of the diffraction grating thickness for different values of the thickness of the gap layer from 0 nm (without the gap layer) to 300 nm (the thickness value of the core layer is the same for all curves). The calculated bandwidth in the FWHM of the lattice filter is shown.

5A and 5B show two different values for the refractive index n ₁ of the tuning layer, namely n ₁ = 1.5 (a) and n ₁ = 1.7 (b), for different layers of the resonant lattice filter structure of FIG. 1 type. The normalized optical mode variance (solid line) is shown.

6 is a simplified example of the layer structure of a resonant grating filter having a "middle" layer between a core layer and a tuning layer.

FIG. 7 shows the tuning range in which the thickness of the intermediate layer from 0 nm to 200 nm (without the intermediate layer) is calculated as a function of the core layer thickness for different values.

8 is a simplified example of the layer structure of the resonant grating filter according to the embodiment of the present invention.

9A and 9B show a comparison between the optical properties of the structure of the type of FIG. 1 and the optical properties of the structure of the type shown in FIG. 8 without any intermediate layer, where the bandwidth in the FWHM in FIG. 9A is a function of the grating thickness. While shown, the tuning range in FIG. 9B is always represented as a function of grating thickness.

10 is a simplified example of a layer structure of a resonant grating filter according to another embodiment of the present invention.

FIG. 11 shows the calculated bandwidth in the FWHM as a function of coupling efficiency, which is an example of the FHWM dependency on the “intensity” of the diffraction grating.

12A and 12B show the bandwidth (FIG. 12A) and tuning range (FIG. 12B) calculated in the FHWM of the resonant grating filter of the type shown in FIG. 10 as a function of the diffraction grating thickness for different values of the gap layer thickness. will be.

FIG. 13 shows a comparison of the resonance ranges calculated as a function of the lattice thickness of the resonant lattice filter of the type shown in FIG. 3 with different thicknesses of the gap layer from 0 nm to 300 nm (without gap layer).

14 is a schematic diagram of an external cavity tuned laser including a resonant grating filter in accordance with an embodiment of the present invention.

The applicant has studied the structure (also called resonant structure) of the resonant grating filter of the type schematically shown in FIG. The resonant structure 1 comprises a waveguide, i.e., a core layer 4 formed on the underlying cladding layer 5, and a low refractive index region 6 and a high refractive index region 7 having refractive indexes n _L and n _H , respectively. The diffraction grating layer 3 is provided. The refractive index difference between the low and high refractive index regions of the lattice is defined as

Where n _G is the average of the refractive indices of the grating and F is the duty cycle of the grating.

A cladding layer 2 is formed on the grating 3, which cladding layer is formed of a tunable material, preferably an LC material having a relatively wide selectable refractive index range. The refractive index n _C of the core layer 4 is larger than the peripheral layer to ensure confinement of the optical mode along the waveguide. The cladding layer 5 may optionally be grown on a substrate (not shown) or may be the substrate itself (functioning as a substrate). For example, the core layer 4 may be made of Si ₃ N ₄ , and the cladding layer 5 may be made of SiO ₂ grown (undoped) on a Si substrate (not shown). have. Alternatively, the cladding layer 5 may be a glass substrate on which the core layer is grown.

By defining a Cartesian coordinate system (x, y, z) with respect to the circumferential direction of the filter such that the z axis is perpendicular to the major surface of the waveguide, the light is diffracted by the grating 3 when the light beam strikes the structure 1. In the example shown in FIG. 1, the incidence is perpendicular to the structure, ie along the z-axis direction. The grating is preferably designed with a period Λ so that only zero and first diffraction orders propagate in the waveguide. All other diffraction orders disappear. The zeroth order propagates across the multilayer structure along the z direction, while the first order diffraction order depends mainly on the lattice structure and the core refractive index. If the diffraction angle exceeds the critical angle formed by the interfaces 8 and 9 between the core layer 4 and the periphery layer (in this approach the grating is treated as a small perturbation and therefore considered to be the interface), the diffraction difference The numbers propagate in the waveguide along the x axis. The critical angle of the interface between two layers with different refractive indices is defined as the minimum angle at which total reflection occurs, i.e., sinΘ _1,3 = n _1,3 / n _c with reference to FIG.

로 x축을 따라 전파하는 광은 주기적 섭동(격자)에 의해 회절되고 다시 도파관으로 나간다. 주기적 섭동은 다시 공진파장에서 z축을 따라 발생한 반사와 상기 광을 z방향으로 결합시킨다. 다른 모든 파장은 실질적으로 장치를 통해 투과된다. 공진에서, 정상파는 광이 내부 전반사에 의해 한정되는 공진공동과 같이 행동하는 도파관층에 만들어지고 결과적으로 에너지가 축적된다. Under resonant conditions, incident light is coupled to the fundamental mode propagating along the x axis to the core layer. Defined propagation constant

The light propagating along the x-axis is diffracted by periodic perturbation (lattice) and exits back to the waveguide. Periodic perturbation again couples the reflection along the z-axis and the light in the z direction at the resonant wavelength. All other wavelengths are transmitted through the device substantially. In resonance, standing waves are created in the waveguide layer where the light behaves like a resonant cavity defined by total internal reflection, resulting in energy accumulation.

값은 아래와 같이 주어지며For resonances that occur, the tangential direction at the lattice-core interface, i.e. along the x axis,

The value is given by

Where k _i ^x is a component along the x direction of the wavenumber of the incident light, k _g is the wavenumber of the grating as defined below,

Where Λ is the lattice period. As in the present case, the normal incidence is considered k _i ^x = 0. Thus, the relationship between the propagation constant and the lattice period in the basic mode is given by:

Although the vertical incidence of the light beam is shown and discussed in FIG. 1, the vertical incidence condition is not necessarily a requirement. If the incidence of the angle θ is taken into account, the resonance condition must also take into account the angular direction of the beam when occurring at a particular resonance wavelength λ ₀ and the angle θ ₀ .

The resonance wavelength λ ₀ is found to be an unobvious solution to the eigenvalue problem given below:

Where t _c is the waveguide (core) thickness, u is the modal parameter of the wave propagating in the core, and w ₂ and w ₅ are the mode parameters of the cladding layers 2 and 5 (FIG. 1). The mode parameter can be represented by the following equation:

Is the wavelength of the incident light.

Following the multipath interference approach of transversal resonance, one can derive an analytic representation of the bandwidth for a particular response of the structure at resonance as a function of the grating and waveguide parameters. The reflection efficiency of the first refraction order in the core layer can be expressed by the effective grating strength, ie, η _d , expressed in diffraction orders:

Where φ (λ) is the round trip phase expressed by the diffraction order in the waveguide region and is phase shifts coupled with the common optical path, two interface reflections and diffraction at two core interfaces Coupling due to Thus, the spectral behavior at wavelength λ around the resonant wavelength λ ₀ (and the angle around θ ₀ ) is generally Lorentzian.

The effective lattice strength η _d expresses the coupling efficiency of the grating in diffraction orders, and gratings such as grating thickness t _G , reflection refractive index Δn _G / n _G, and grating period Λ as well as wavelength, diffraction order, and grain angle. It depends on the parameter. When diffraction occurs only in the first order, the coupling efficiency corresponds to the first diffraction efficiency and can be expressed as the ratio of the light output to the incident light output of the first propagation mode. Basically, the coupling efficiency tells how effectively a grating (coupler) redirects light in a single direction of space.

The change in φ (λ) that produces half the intensity results in the following FWHM dephasing bandwidth:

The dephasing bandwidth can also be expressed as a change in tangential component as follows:

Where k ₀ is the wave number in free space at the peak wavelength λ ₀ , and dφ / dβ is defined as the modal dephasing rate and represents the rate of change of phase with respect to the tangential component. The mode dephasing ratio is defined as follows:

In general, a relatively large non-refractive difference between the core and the cladding layer results in a relatively high modal dephasing ratio and thus a relatively small bandwidth in the FWHM (see equation 10).

Equation (10) shows that the bandwidth decreases as η _d decreases and the mode dephasing ratio increases. Therefore, a structure having a lattice having a relatively small value of _d may exhibit a relatively narrow spectral response bandwidth. A grating characterized by a relatively small coupling efficiency η _d in the diffraction mode is called a "weak" grating, indicating that it has a relatively low diffraction intensity when expressed in the propagating diffraction mode.

2A and 2C are functions of the grating thickness t _G for three different values of core thickness, ie t _c = 200 nm (thick solid line), 300 nm (dashed line) and 400 nm (thin solid line with square) Figure 1 shows the bandwidth calculated in FWHM for a resonant structure of the type illustrated in FIG. The following parameters were assumed in the calculation: refractive index n _c = 1.96 of the core layer 4 while refractive index n _L = n of both the cladding layer 5 and the region 6 (low refractive index region) of the grating 3. ₃ = 1.445. The refractive index of the high refractive index region 7 of the grating takes three different values in the diagrams of FIGS. 2A-2C. FIG. 2A shows the lattice refraction constant Δn _G / n _G = 0.26, FIG. 2B shows Δn _G / n _G = 0.07, and FIG. 2C shows Δn _G / n _G = 0.04. The calculation of FWHM was derived from equation (10). The reference wavelength in the calculation is 1.55 μm.

As a general consideration, it is observed that the FWHM increases with increasing lattice thickness t _G. However, the increase is much more pronounced for relatively large refractive indices (Δn _G / n _G = 0.26) than for relatively small refractive indices (Δn _G / n _G = 0.07 and 0.04). For Δn _G / n _G = 0.26, a sharp increase with t _G is observed. Increasing the grating thickness, for example from 50 nm to 80 nm, results in a larger increase than the factor 2 in the bandwidth at the FWHM (FIG. 2A). Although not shown in FIG. 2A, the lattice thickness of about 200 nm for Δn _G / n _G = 0.26 results in a FWHM of 20 nm. Conversely, if the value of the lattice thickness is about 200 nm or more for Δn _G / n _G = 0.04, the value of FWHM may not be guaranteed even greater than 0.5 nm.

From the results shown in Figures 2A and 2C, for example, to have a relatively low bandwidth in the FWHM, i.e. less than about 0.6 nm, as required for a tuned laser for DWDM devices with 50 GHz channel spacing, the grating is It is inferred that one should choose to have t _G so that a very small thickness, i.e. a lattice non-refractive difference is relatively large, is less than 30 nm or a relatively small non-refractive difference, i.

The first choice for selecting a grating in the resonant structure of FIG. 1 with a thickness of several tens of nm (eg 30 nm), which can tolerate a relatively large refractive index Δn _G / n _G (eg 0.26), is particularly known as chemical vapor deposition or Considering that most techniques, such as etching, have manufacturing tolerances of several nanometers, lattice fabrication is particularly burdensome. On the other hand, by selecting a low specific refractive index consisting of at most 0.05, preferably 0.03 to 0.04, it is possible to produce a grating having a thickness of at least 150-200 nm.

2A and 2C show that an increase in core thickness t _c causes a decrease in FWHM for a predetermined value of grating thickness and grating non-refractive difference. More specifically, the large index of refraction of the core compared to the thickness of the relatively large core layer and / or the thickness of the cladding layer results in a relatively low bandwidth in the FWHM. However, for a large non-refractive difference (FIG. 2A), for a core thickness of 400 nm, a bandwidth in FWHM of 0.5 nm or less is obtained only for a grating thickness of less than 40 nm.

More specifically, the spectral response with low bandwidth in FWHM can be implemented in a structure with a "weak" grating, ie a grating with a relatively small coupling efficiency η _d . Weak lattice can be achieved, for example, by choosing either a thin lattice structure (small t _G ) or a lattice structure with a low specific refractive index (small Δn _G / n _G ). It is understood that the implementation of both conditions can also be guided for small values of lattice strength.

Small coupling efficiency of the grating can be achieved by inserting a layer, also called a gap layer, between the core layer and the grating layer. The presence of the gap layer reduces the overlap between the mode field and the periodic region, thereby weakening the diffraction effect of the field. FIG. 3 schematically illustrates a resonant structure 12 comprising a gap layer 10 between a core layer and a lattice layer. The structure also has a glass sheet 11 which acts as a cover plate for the tuning layer 2. The cover plate is preferred when the light transmissive material is an LC material. The same reference numerals are given to elements of the resonant structure corresponding to the resonant structure shown in FIG. 1, and detailed description thereof will be omitted.

FIG. 4 shows the resonant structure of FIG. 3 as a function of the grating thickness t _G for _gap layers t _gaps with thicknesses ranging from 0 nm (i.e. no gap layer, the structure is the same as the structure shown in FIG. 1) to 300 nm. The rental width in FWHM is shown. The calculation of FWHM was derived from equation (10), where the structural parameters were Δn _G / n _G = 0.26, t _c = 200 nm, Λ = 950 nm, n _c = 1.96 (Si ₃ N ₄ ) and n ₃ = 1.445 (Undoped SiO ₂ ). In this example, the gap layer is made of SiO ₂ . The curve for t _gap = 0 nm corresponds to the curve shown in FIG. 2A for t _c = 200 nm. For example, for the grating thickness t _G = 30 nm, the FWHM value for t _G = 30 nm is about a factor 1.7 which is smaller than the value for t _gap = 0 nm.

In addition to having a suitable bandwidth, for example a spectral response compatible with DWDM equipment, the resonant grating filter should exhibit a wide range of tunability, ie large tuning range. Preferably, the tuning range is at least 10 nm, preferably at least 30-40 nm for devices requiring a tuning range comprising the erbium C-band.

Synchronization means the shift of the resonant wavelength in the resonant grating filter. In order to shift the resonant wavelength, the solution of the eigenvalue problem (Equation 5) must be shifted. One way to change the resonance condition, i.e. shift the resonant wavelength, is to vary the refractive index of one of the layers of the resonant structure that affects the effective refractive index n _eff for the propagation mode in the core.

For the first approximation, the resonant wavelength λ ₀ can be expressed as:

Where Λ is the lattice period. Then, the tuning range Δλ ₀ of the resonant wavelength is given by:

This represents a direct proportion between Δλ ₀ of the propagation mode and the amount of change Δn _eff of the effective refractive index.

The core layer and the periodic structure (lattice) are not suitable candidates for the tuning layer, i.e., the layer whose refractive index changes to resonate the resonant wavelength, since a change in the refractive index can strongly affect the bandwidth in the FWHM. This is because the uniformity of the spectral response of the casting may be disturbed at other resonant wavelengths.

Therefore, it is desirable to achieve tuning by varying the refractive index of the cladding layer of the resonant structure. In the resonant structure of Fig. 1, the tuning layer is preferably made of a liquid crystal (LC) material having a wide selectable refractive index. The refractive index n ₁ of the LC layer is varied in response to the applied electric field, causing the resonant lattice structure to be electrically tuned.

Applicants have observed that the optical mode profile of the propagation mode must be spatially extended to significantly overlap the tuning layers in order to achieve efficient tuning. In this way, a change in the refractive index n ₁ of the tuning layer results in a significant change in the effective refractive index n _eff of the propagation mode in the waveguide. 5A and 5B show two examples of optical mode profiles for different layers of the resonant structure of the type shown in FIG. 1. 5A and 5B, the resonant structure also includes a glass plate for covering the LC material. The mode field was calculated with commercial software based on the finite difference time domain technique. The amplitude of the basic optical mode is shown at the top of FIGS. 5A and 5B. Simulations show Δn _G / n _G = 0.26, t _c = 200 nm, Λ = 950 nm, n _c = 1.96 (Si ₃ N ₄ ), n ₃ = 1.445 (undoped SiO ₂ ) and t _G = 30 nm Structural parameters were assumed. The refractive index n ₁ of the tuned cladding layer (LC material) varies from 1.5 to 1.7. In FIG. 5A, n ₁ = 1.5, while in FIG. 5B, n ₁ = 1.7. The basic propagation mode of the resonant structure is quasi-Gaussian, with a "tail" extending to the tuning layer, which represents the spatial overlap of the mode into the layer. The degree of superposition in the LC layer and the shape of the curve in the basic mode are affected by the change in refractive index n ₁ , as can be seen from the comparison between FIGS. 5A and 5B. The effective refractive index n _eff calculated for n ₁ = 1.5 was found to be 1.5995, whereas n _eff was found to be 1.6505 for n ₁ = 1.7. From equation (13), the tuning range Δλ ₀ which is 48.5 nm can be derived.

The calculation of the effective refractive index of the basic propagation mode was also performed for the same structure as that of the example of FIGS. 5A and 5B, the only difference being that the grating thickness t _G was 200 nm instead of 30 nm as in the above example. The change in effective refractive index corresponds to Δλ ₀ of 22.9 nm and was calculated to be 0.02291, which is less than 2 smaller factors than in the examples of FIGS. 5A and 5B. It is preferable that a small lattice is used to improve the synchronization as the separation between the core layer and the tuning layer increases in the structure of the type shown in FIG. 1, thereby affecting the overlapping of the basic mode into the tuning layer. That means you can. As observed above, gratings with such small thicknesses (eg, 30 nm) are often of technical importance.

Applicants have observed that the insertion of a layer called an intermediate layer between the lattice layer and the tuning layer may affect the tuning performance. FIG. 6 schematically shows a resonant grating structure 14 comprising an intermediate layer 13 between the grating layer 3 and the tuning layer 2. The intermediate layer should have a refractive index that is less than the refractive index of the core layer. The same reference numerals are given to elements of the resonant lattice structure corresponding to the resonant lattice filter shown in FIG. 1, and detailed description thereof will be omitted.

An intermediate layer between the lattice layer and the tuning layer can be considered in the design of a given resonant filter. For example, the intermediate layer may be a light transmissive conductive layer made of, for example, indium tin coral (ITO), which is used as an electrode for controlling the tuning layer. Alternatively, the intermediate layer may be intervened to enhance the attachment of the light transmitting material forming the tuning layer to the underlying layer. If the light transmissive material is an LC material, an intermediate layer made of polymide may instead be placed in contact with the LC as an alignment layer.

In FIG. 7, the tuning range of the structure of the type shown in FIG. 6 is shown as a function of waveguide (core) thickness t _c for different values of the thickness of the intermediate layer from 0 nm (ie no intermediate layer) to 200 nm. . The tuning range was calculated by Equation 13 using finite difference time zone software. The structural parameters considered in the calculation of the curve are Δn _G / n _G = 0.26, Λ = 950 nm, n _c = 1.96, n ₃ = 1.445, t _G = 30 nm and Δn ₁ = 0.2, where n ₁ is 1.5 to Reaches 1.7. The intermediate layer is taken to be made of SiO ₂ in this example. The results show that the tuning range decreases as the core thickness increases. Also, for a given core thickness, the tuning range decreases as the thickness of the intermediate layer increases.

The results shown in FIG. 7 clearly show that synchronization is significantly affected by the core thickness t _c and / or due to the presence of an intermediate layer located between the lattice layer and the tuning layer. In both cases, this results in the center of the propagation mode being further away from the tuning layer in the waveguide (typically, the fundamental mode is concentrated around the core center), thus not being able to significantly overlap the tuning layer, effectively changing the effective refractive index. It is due to the fact that it can not be.

By comparing the results illustrated in FIGS. 2A-2C with the results shown in FIG. 7, Applicants note that relatively thick core layers are preferred for relatively tight bandwidth of the spectral response, while relatively thin core layers are advantageous for broad tuning. did. As an example, by selecting a lattice with Δn _G / n _G = 0.04 and t _G = 200 nm, the core thickness t _c required to have a bandwidth at an FWHM of 0.4 nm is about 220 nm. However, this core thickness means a tuning range of 25 nm (assuming that the structural parameters are the same as the structural parameters of the calculations illustrated in the above figures). This tuning range may not be suitable for equipment of a DWDM system operating in the C-band wavelength region where, for example, a tuning range of at least 30 nm and preferably at least 40 nm is required.

Applicants have found that by placing a diffraction grating on the opposite side of the tuning layer with respect to the planar waveguide, the lattice structure parameters can be tailored to the desired bandwidth of the filter response without significantly affecting the tuning of the filter. In other words, a resonant structure comprising waveguides disposed between the lattice layer and the tuning layer has a relatively small coupling efficiency required for bandwidth resolution while allowing flexibility in the selection of an appropriate lattice structure reaching a predetermined tuning range. do.

The core layer may be disposed near the tuning layer, and the core layer may be in direct contact with either the tuning layer or one or more intervening relatively thin interlayers between the core layer and the tuning layer. The interlayer should have a thickness that does not significantly affect the tuning, and the maximum allowed thickness depends on the refractive index and the waveguide thickness and the interlayer refractive index. For example, for a 200 nm thick core layer of Si ₃ N ₄ and an intermediate layer of ITO (n = 1.9), the thickness of the ITO should be 40 nm or less. The core layer approach to the tuning layer means that the propagation mode can be partially extended to the tuning layer so that the effective refractive index is effectively affected by the change in the refractive index of the tuning layer.

8 schematically illustrates a layer structure of a resonant grating filter according to an exemplary embodiment of the present invention. The resonant structure 20 has a core layer 28 formed on the refractive lattice layer 23 including the low refractive index region 21 and the high refractive index region 22, and has a non-refractive index between the low refractive index region and the high refractive index region. The difference is Δn _G / n _G. The grating layer 23 is formed on the buffer layer 24, which is selectively formed on the substrate 25. The cladding layer 30 is formed on the core layer 28, which is formed of a tuned light transmissive material, preferably an LC material having a relatively broadly selectable refractive index.

The refractive index n _c of the core layer 28 is larger than the peripheral layer to ensure confinement of the optical mode in the waveguide. The refractive index n _c is greater than the average refractive index n _G of the grating.

The refractive index of the tuned material can be varied by changing the electric field or temperature T provided by the application of an external parameter, for example voltage V. Suitable tuning materials for the cladding layer are chosen such that the refractive index n ₁ (V) [or n ₁ (T)] remains smaller than the refractive index of the core in the entire voltage (temperature) range of interest for the function and tuning of the device. .

If the tuning layer comprises an electro-optic material, the resonant structure preferably comprises transparent electroconductive layers 26 and 28 located on opposite sides of the LC layer. Alternatively, assuming that the substrate is conductive or semiconducting (eg made of silicon), the two layers for containing the electrical structure may be layer 29 and substrate 25. Preferably, the glass plate 31 is arrange | positioned as a cover plate for LC layers.

Referring to FIG. 8, in a preferred embodiment, the low refractive index region 21 of the grating is made of the same material as the material of the buffer layer 24.

Optionally, an intermediate layer 27 is formed over the core layer. The intermediate layer can improve the adhesion and / or thickness uniformity of the conductive layer 26, particularly in manufacturing by sputtering or deposition.

Optionally, there may be an antireflective coating on the cladding layer 30 and / or an antireflective coating between the substrate 25 and the buffer layer 24 (not shown).

Although the diffraction gratings are shown as having a rectangular shape, other forms may be considered as long as the periodic structure allows the grating to allow selective radiation coupling in the waveguide region. The diffraction grating should make one-dimensional or two-dimensional periodic perturbation as in the resonance structure of FIG. The periodicity of the grating may be one or many and may or may not be dependent on the polarization of the incident light.

When the tunable cladding 20 is made of an electro-optical tunable material such as an LC material, wavelength selection is made by an electrical signal. The resonant wavelength can be shifted by varying the voltage or current applied to the electro-optically controlled material. If the electro-optic material is an LC material, the electrical signal provided for the function of the filter is an alternating voltage to prevent degradation of the LC due to dc stress. Depending on the amplitude of the voltage applied to the conductors (conductive layers 26 and 29 in FIG. 9), the tuned filter reflects radiation only at a predetermined wavelength λ ₀ . Radiation at all other wavelengths is passed through the resonant filter. Clearly the minimum of transmission occurs at λ ₀ .

Theoretically the reflectivity at λ ₀ is 100%, but the reflectivity of the resonant grating filter is generally 70% -95% as it allows the (small) part of the incident light to be transmitted, which is due to losses occurring in the substrate or buffer layer, for example.

The thickness of the LC material is about 5 μm or less, more preferably at most 2 μm, in a preferred embodiment about 1 μm.

Although preferred, the tuned cladding need not be made of an electro-optic material. For example, the use of thermo-optic materials such as polymers can also be considered. The requirement for the material to form the tunable cladding is to have a refractive index that varies relatively broadly with changes in external parameters, temperature (T) or electric field. For thermo-optic materials, the thermo-optic material must have a thermo-optic coefficient dn / dT of at least 10 ⁻⁴ / ° C. in order to achieve a few nanometers of synchronism with reasonable temperature changes. Most types of LC materials exhibit electrooptic coefficients that are large enough to achieve tuning ranges of tens of nm for reasonable changes in electric fields, typically 1-2 V / μm or more. The choice of a suitable material, either electro- or thermo-optical, is of course dependent on the application based on the tuning range required.

In the case of a tuning layer made of thermo-optic material, it should be noted that only the conductors are needed for thermal tuning. Referring to FIG. 8, only the conductive layer 29 (not the conductive layer 26) may be needed to tune the filter. In addition, the cover plate 31 may not be necessary when the layer 30 is a polymer layer.

In a preferred embodiment, the grating period Λ is chosen so that only the first order diffraction occurs in the waveguide. That is, the condition Λ> λ _max / n _c must be satisfied, where λ _max is selected to be λ _max ˜1570 nm, when the maximum wavelength of the target range, for example, the range is C-band. Preferably, the grating period should be chosen such that there is no coupling in the waveguide between the fundamental propagation mode and the first diffraction mode without the second diffraction occurring. This last condition can be expressed by the following relationship:

Here, [lambda] _min is the minimum wavelength of the target range, for example, [lambda] _min -1530 nm when the range is C-band.

For example, for t _c = 200 nm, Λ can vary from 800 to 1050 nm.

9A and 9B, a comparison between the performance for the structure of FIG. 3 and the performance for the structure of the type shown in FIG. 8 is illustrated, with both structures having no intermediate layer between the tuning layer and the core layer. With reference to FIG. 3, there is no layer 10 and with reference to FIG. 8, there is no layer 27 or 26. In FIG. 9A, the bandwidth in the FWHM is defined by the structure of FIG. 3 (thin solid line with squares labeled “grid over core”) and the type of structure illustrated in FIG. 8 (circles indicated by “grid under core”). Is shown as a function of the grating thickness for a solid solid line). In FIG. 9B, the tuning range is shown as a function of grating thickness. In the calculation, both structures are n _c = 1.96, n ₃ = 1.445, like the parameters of the low refractive index region, t _c = 200 nm, Λ = 948 nm, Δn _G / n _G = 0.04 and n _c = 1.5-1.7 Has the parameters The difference between the two structures is of course the location of the lattice layer. The data shown in FIG. 9A for a structure with a grating over the core layer corresponds to the data shown in FIG. 2C for t _c = 200 nm. The results in FIG. 9A show that, at any significant lattice thickness, the bandwidth in the FWHM of the structure with the grating disposed below the core layer is narrower than the bandwidth in the FWHM of the structure with the grating between the core layer and the tuning layer. One thing is shown clearly. In addition, FIG. 9B shows that the tuning range is significantly greater for structures having a lattice disposed below the core layer than for structures having a lattice layer over the core.

It is important to note that the tuning range for the structure with the grating under the core remains approximately constant (at about 45 nm) within a significant range of grating thickness. In contrast, the dependence of grating thickness is observed for structures with gratings on the core layer. This is in a resonant structure with a grating under the waveguide, where the tuning is less affected by fluctuations in the grating thickness, for example due to the finite accuracy associated with the manufacturing process. Thus, manufacturing tolerances of the grating can be relaxed in the case of the structure according to the invention.

10 illustrates a resonant grating filter according to still another embodiment of the present invention. In the resonant grating filter 40, a buffer layer 47 is formed on the substrate 49. A high refractive index region 50 is formed in the buffer layer to form a lattice layer having a thickness t _G , and in the lattice layer a low refractive index region 53 is adjacent to the high refractive index region so as to correspond to the region of the buffer layer. Optionally, a gap layer 51 having a thickness t _gap is formed on the grating layer, which gap layer is preferably made of the same material as the buffer layer. On the gap layer 51 (or grating layer), a core layer 46 is formed. An optional intermediate layer 45 may be formed on the core layer. An electro-optically controlled tuning layer 43 is formed on the core layer. An optional antireflective coating layer 48 is formed between the substrate 49 and the buffer layer 47. Transparent electroconductive layers 42 and 44 are preferably located on opposite sides of the tuning layer of LC material. Alternatively, the two layers comprising the electrical structure can be layer 42 and substrate 49 when the substrate is conductive or semiconductive (eg, made of silicon). In the case of an insulated substrate such as glass, an electrically conductive layer can be grown on the substrate (in FIG. 10, at a position corresponding to the selection layer 48) so that electrical contact is made between the conductive layer and the conductive layer 42 (or 45). Can be made in between. The glass cover plate 41 is located on the tuning layer 43.

The gap layer may preferably be included in the structure to reduce the overlap of the basic modes on the grating, thereby reducing the coupling efficiency of the grating without changing the properties of the grating itself.

Preferably, the resonant grating filter according to the present invention has a grating having a relatively small diffraction efficiency. FIG. 11 shows the bandwidth in FWHM as a function of coupling efficiency η _d as calculated using commercial software based on the solution of the combined wave equation for the structure of the type shown in FIG. 10. In the range of FWHMs considered, the dependence of FWHM on η _d becomes a straight line of first order approximation. If the given FHWM is 0.4 nm, the coupling efficiency should be 0.0015. Coupling efficiencies ranging from about 0.001 to 0.002 correspond to a FWHM of about 0.2 to 0.5 nm.

Applicant has noted that for all resonant structures of the examples considered so far, the bandwidth curve versus coupling efficiency in the FWHM does not change significantly. It should be noted that in the calculation the grating thickness must be varied to have the same coupling efficiency for different Δn _G / n _G values. For example, a coupling efficiency of 0.0014 is achieved by a resonant structure with a lattice on the core with Δn _G / n _G = 0.26 and t _G = 25 nm, thus grating under the core with Δn _G / n _G = 0.04 and t _G = 200 nm. By a resonant structure having a lattice under the gap layer and a core having Δn _G / n _G = 0.04, t _G = 50 nm and t _gap = 100 nm.

In the context of the present invention, the diffraction grating is considered to be "weak", ie having a relatively small coupling efficiency when η _d is less than 0.0026.

12A and 12B show the bandwidth and tuning range in the FWHM as a function of the lattice thickness for the structure of the type shown in FIG. 10 for different values of the _gap layer t _gap ranging from 0 nm to 300 nm, respectively. The solid line indicating t _gap = 0 corresponds to the structure without the gap layer. That is, the core layer is formed directly on the lattice layer. The structural parameters are n _c = 1.96, n ₃ = 1.445, t _c = 200 nm, Λ = 950 nm, Δn _G / n _G = 0.26 and n _c = 1.5-1.7 as the parameters of the low refractive index region. The curve shows only a relatively weak dependence of tuning range on the thickness of the gap layer in the considered range of at least 0-300 nm. It should be noted that the increase in gap thickness can be regarded as the same in terms of light performance against increase in grating thickness.

It should be noted that in the resonant structure of the present invention, it is not necessary to manufacture a grating having a relatively small thickness, that is, 150-220 nm, in order to obtain a weak diffraction grating. On the other hand, where a relatively thin (eg 50 nm) grating is required, the formation of the grating layer can be well controlled and regenerated, which can be achieved by a deposition process, for example plasma enhanced chemical vapor deposition (PECVD). This is because the lattice thickness can be formed by. In order to reproduce the low or high refractive index region of the grating, an etching step is generally required. Current etching techniques generally have an absolute accuracy of at least 4-5 nm. This can be a problem when the lattice layer is arranged over the core layer since the core layer itself can be affected by etching.

For comparison, the tuning range as a function of the grating thickness is shown in FIG. 13 for the type of structure shown in FIG. 3, that is, the type of structure having a grating layer disposed over the core layer and a gap layer between the core layer and the grating. Is shown. The curves show values with different thicknesses of the _gap layer t _gap ranging from 0 nm to 300 nm. The structural parameter is the same as that of the calculation shown in FIG. 4. For a given grating thickness, a significant decrease in tuning range is observed as the thickness of the gap layer is increased.

Example 1

Referring to FIG. 8, the buffer layer and the low refractive index grating region are made of SiO ₂ with a refractive index of 1.445. The high refractive index region is made of SiO _x N _y with a refractive index of 1.54. The core layer is made of SiO ₃ N ₄ with a refractive index of 1.96. The lattice thickness is 220 nm. The tuning layer is made of a nematic LC with a refractive index ranging from 1.5 to 1.7. The lattice period is 948.5 nm to have a resonant wavelength of 1526 nm (lower limit of the C-band) for the refractive index 1.5 of the LC material.

The structure can be manufactured by using standard techniques for the manufacture of semiconductor devices. As an example, a SiO ₂ layer is deposited on a Si substrate by PECVD to form a buffer layer. The surface of the buffer layer is substantially etched by dry etching, for example, to form trench regions corresponding to the high refractive index grating regions 22 formed. The trench is substantially filled with SiO _x N _y . Alternatively, a trench can be formed on the surface of which the SiO _x N _y layer is deposited on the substrate as a buffer layer and then corresponds to the low refractive index region and is substantially filled with SiO ₂ .

The resulting surface (ie grating top surface) is then flattened. In subsequent steps, a SiO ₃ N ₄ layer is deposited by PECVD to form the core layer. A thin layer of SiO _{2 with} a thickness of 40 nm or less can be selectively deposited on the core layer by PECVD. In another step, a 30 nm thick ITO layer is deposited on the core layer or the thin SiO ₂ layer. The LC cell is finally assembled on top of the structure and terminated with the ITO layer and the glass plate.

The tuning range of the resonant structure is about 40 nm.

Example 2

Referring to the structure of the type shown in FIG. 10, the buffer layer and the gap layer are made of (undoped) SiO ₂ with a refractive index of 1.445. The high refractive index region is made of SiO ₃ N ₄ with a refractive index of 1.96. The core layer is made of SiO ₃ N ₄ with a refractive index of 1.96. The lattice thickness is 50 nm, the core thickness is 200 nm, and the thickness of the gap layer is 300 nm. The tuning layer is made of nematic LC having a refractive index ranging from 1.5 to 1.7. The lattice period is 950 nm to have a resonant wavelength of 1526 nm (lower limit of the C-band) for the refractive index 1.5 of the LC material. A glass plate about 1 mm thick covers the LC cell. A transparent conductive layer made of ITO and having a thickness of 20 nm is disposed on the opposite surface of the LC cell. On the ITO layer disposed in the core layer, a polyamide layer having a thickness of 20 nm is formed to align the LC material.

The tuning range of this resonant structure is 40 nm.

The structure can be manufactured by using standard techniques for the manufacture of semiconductor devices. As an example, a SiO ₂ layer is deposited on a Si substrate by PECVD to form a buffer layer. Substantially, a SiO ₃ N ₄ layer is deposited on the buffer layer. The SiO ₃ N ₄ layer is substantially etched by dry etching, for example, to form trench regions corresponding to the low refractive index grating regions formed. A SiO ₂ layer is deposited to fill the trench and form a gap layer of SiO ₂ on the lattice.

This process has the advantage that high accuracy is not necessary in the etching step of the trench and that even slight overetching of the trench is possible. In this case, the accuracy of the lattice thickness is formed by the deposition process of the high refractive index region of SiO ₃ N ₄ .

In a preferred embodiment, the resonant grating filter according to the invention is used as a tuning element in an external cavity tuned laser. Externally tunable lasers are suitable for use in telecommunications equipment, in particular as tunable light sources for WDM and DWDM systems, to generate a central wavelength for any channel on the International Telecommunications Union (ITU) grid.

Fig. 14 schematically shows an external cavity tuned laser 60 having a tuned resonance reflecting filter in accordance with the present invention. The gain medium 61, preferably the semiconductor laser diode, has a front facet 62 and a back facet 63. The front face 62 partially reflects and serves as one of the external cavity end mirrors. The back surface 63 has low reflectivity. Generally an antireflective coating (not shown) is coated. Aim lens 64 converges the light beam emitted by the gain medium to Fabry Parrot (FP) etalon 65 having a mode fixed to the ITU channel grating. The FP etalons function as channel assignment grating elements constructed and arranged to form a plurality of equally spaced transmission peaks. After FP etalon 65, the beam impinges on the tuned resonant lattice filter 66, which forms another end mirror of the external cavity and forms the cavity physical length L ₀ with the front of the gain medium. do. The wavelength variable filter 66 is used as an end mirror of the laser cavity and is also referred to as a tuning mirror. The tuned mirror is tuned to a predetermined channel frequency by selecting one of the etalon transmission peaks.

In a preferred embodiment, the tunable mirror 66 is tuned by electrically varying the applied voltage supplied by the voltage generator 67. The applied voltage is an alternating current (AC) voltage. The laser output wavelength of the laser is selected to match the resonance wavelength λ ₀ of the tuned mirror. Tunable mirror 66 is a resonant grating filter in accordance with one of the embodiments of the present invention.

The laser system is preferably designed to produce substantially a single longitudinal mode radiation and preferably a single transverse mode radiation. Longitudinal mode means simultaneous lasing at multiple distinct frequencies within the laser cavity. The transverse mode corresponds to the spatial change in the cross section of the beam intensity in the transverse direction of the laser radiation. In general, a suitable choice of gain medium, such as a commercially available semiconductor laser diode comprising a waveguide, ensures one spatial or lateral mode operation.

In a preferred embodiment, the impact of the beam is substantially perpendicular to the waveguide face of the tuned mirror.

In the laser system of FIG. 14, the tuned mirror serves as a simple tuner that distinguishes the peaks of the grating etalon. The FWHM bandwidth of the tuned mirror is more than the FWHM bandwidth of the lattice etalons. For longitudinal single mode operation, the transmission peak of the FP etalon corresponding to a particular channel frequency must select one joint mode, i.e. transmit. Thus, FP etalons must have a fines defined as free spectrum regions (FSRs) separated by FWHMs that suppress neighboring common modes between each channel. For single mode laser emission, the longitudinal cavity mode should be specified beyond the maximum of one of the etalon transmission peaks (one selected by the tunable mirror). In this way, only the specified frequency passes through the etalon and other competing neighboring common modes are suppressed.

The laser system according to the invention is especially designed to provide fast switching over the entire C-band on an ITU 50 GHz or 25 GHz channel grid.

If the laser system is designed for 50 GHz mode spacing, the reflection band of the tuned mirror should be about 0.6 nm or less to obtain an extinction ratio of at least 5 dB between neighboring channels. Preferably the bandwidth at the FWHM of the tuned mirror should be 0.4 nm or less. The tuning of the laser system across the C-band requires a tunable mirror having a tuning range of at least 40 nm.

For application as a tunable mirror in an external cavity laser source for DWDM systems with 25 GHz channel spacing, a bandwidth of FWHM comprised between about 0.2 nm and 0.3 nm is preferred.

Bandwidths of FWHM of less than 0.2-0.3 nm are undesirable in laser systems because they make the alignment and control of the mirror in the laser more difficult.

Alternatively, the resonant grating filter according to the invention can be used in a tuned add / drop device for WDM and DWDM systems. For this application, the design preferably has an extinction ratio of side mode that is -30 dB or less and must be tailored for narrow bandwidths, such as 0.1-0.2 nm and low sidebands around the resonant wavelength, in addition to extensive tuning.

Included in the details of the present invention.

Claims (15)

In the external cavity tunable laser 60, which is configured to emit radiation at a laser emission wavelength and has an external cavity having a plurality of cavity modes, The external cavity A gain medium 61 for emitting a light beam to the external cavity; A tuned optical resonant lattice filter (66; 20; 40) for reflecting the light beam at a resonant wavelength; The filter is With a diffraction grating, Planar waveguides 28 and 46 that optically interact with the diffraction grating and form a resonant structure with the diffraction grating; And a light transmissive material having an optional variable refractive index to permit tuning of the filter and forming a tunable cladding layer (30; 43) of the planar waveguide, And the planar waveguide is disposed between the diffraction grating and the tunable cladding layer. The method of claim 1, And the emitted radiation is in a single longitudinal mode. The method of claim 1, And a channel assignment grating element arranged in said outer cavity to form a plurality of passbands substantially aligned with corresponding channels of a selected wavelength grating. The method of claim 3, wherein And an tunable resonant lattice filter arranged in said cavities for synchronously selecting one of said pass bands to select a channel for tuning said light beam. The method according to claim 3 or 4, Wherein said selected wavelength grating has a channel spacing of 50 GHz or 25 GHz. The method of claim 1, And the tuned resonant lattice filter is arranged in the outer cavity such that a light beam impinges on the filter substantially perpendicular to the major surface of the planar waveguide. In the optical resonant lattice filter (20; 40) that reflects light radiation at the resonant wavelength, A diffraction grating (23; 52) having a periodic structure including a low refractive index area (21; 53) and a high refractive index area (22; 50), and having a coupling efficiency η _d of 0.0026 or less; Planar waveguides 28 and 46 that optically interact with the diffraction grating and form a resonant structure with the diffraction grating; And a light transmissive material having an optional variable index of refraction to allow tuning of the filter and forming a tunable cladding layer 30; 43 for the planar waveguide, And the planar waveguide is disposed between the diffraction grating and the tuned cladding layer. The method of claim 7, wherein And wherein the light transmissive material is a liquid crystal material whose optional variable refractive index is controlled by an electrical signal. The method of claim 7, wherein The coupling efficiency of the diffraction grating is an optical resonance lattice filter of 0.001 to 0.002. The method of claim 7, wherein And the planar waveguide is a layer having a variable refractive index of the tuned cladding layer and a refractive index n _c that is greater than the average refractive index of the diffraction grating. The method of claim 10, And a buffer layer (24,47) disposed opposite the diffraction grating with respect to said planar waveguide, said buffer layer having a refractive index n ₃ lower than an average refractive index of the diffraction grating. The method according to any one of claims 7 to 11, And a gap layer (51) disposed between the planar waveguide and the diffraction grating, wherein the gap layer has a refractive index lower than the refractive index of the waveguide and the average refractive index of the diffraction grating. The method according to claim 11 or 12, The planar waveguide is made of silicon nitride material, the high refractive index area is made of silicon nitride or silicon oxynitride, and the low refractive index area and the buffer layer are made of silicon dioxide. Optical resonator grating filter. The method of claim 12, The gap resonant grating filter is made of silicon dioxide. The method according to any one of claims 8 to 14, And an optically transparent conductive layer (26; 42; 29; 44) arranged on opposite sides of the optically transmissive material to apply an electrical signal across the optically transmissive material.

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