Fiber bragg grating dispersion compensator

[0002] This application claims priority from U.S. Provisional Application No. 60/351,381 filed Jan. 28, 2002 entitled “THERMALLY TUNABLE FIBER BRAGG GRATING DISPERSION COMPENSATOR” and CA informal Application No. 2,369,953 filed Jan. 28, 2002 entitled “THERMALLY TUNABLE FIBER BRAGG GRATING DISPERSION COMPENSATOR”

[0003] This invention relates to fiber Bragg grating dispersion compensation, and particularly to a thermally tunable fiber Bragg grating dispersion compensator.

[0004] One of the key issues in modern high-speed optical networks is the necessity to compensate for the optical pulse broadening caused by optical fiber chromatic dispersion. With the advance of new generations of fast networks (40 Gb/sec and higher), the ability to precisely compensate for the dispersion becomes critical for the network operation thus necessitating dispersion compensation components with variable dispersion capabilities.

[0005] Efforts to compensate for chromatic dispersion have involved thus far the use of etalon-based systems, dispersion compensating fibres, dispersion compensating gratings, e.g. fiber Bragg gratings (FBG), or a combination of both. A device described in a paper “Implementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradient”, J. Lauzon et al, Optics Letters, Vol. 19, No. 23, pp. 2027-2029, December 1994, has a heat distributor and thermoelectric coolers to control the end temperatures of the distributor.

[0006] Various dispersion compensating systems are also described in patent literature, e.g. U.S. Pat. No. 5,671,307 issued to Lauzon et al., U.S. Pat. No. 6,148,127 issued Nov. 14, 2000 to Adams et al, U.S. Pat. No. 5,694,501 issued Dec. 2, 1997 to Alavie et al. (now assigned to the present assignee), and U.S. Pat. No. 6,307,988 issued Oct. 23, 2001 to Eggleton et al.

[0007] It is desirable to provide a tunable dispersion compensator (DC), preferably over a broad dispersion range. The key element of a popular type of a DC is a linearly chirped fiber Bragg grating (FBG), a diffractive grating with a linearly varying pitch (refractive index perturbation) written inside an optical fibre. Optical pulse broadening comes from the fact that the pulse's frequency components travel with different velocities, so that the longer wavelength components lag the shorter ones. In a chirped FBG, the location of resonant Bragg condition (reflection point) will be wavelength dependent. This causes a time-of-flight difference between longer and shorter wavelength equivalent to a chromatic dispersion added to the pulse.

[0008] Since silica used for fibre manufacturing has a temperature dependent refractive index and the fibre itself has certain thermal expansion coefficient, the grating local resonant wavelength becomes temperature dependent and varies as Δλ=S_TΔT , where S_T≡10 pm/K is the grating thermo-optical sensitivity.

[0009] In a linearly chirped grating, i.e. grating with the reflection position varying linearly with wavelength, group delay will be a linear function of wavelength and, after differentiation, yields uniform dispersion across the grating wavelength range (bandwidth). Any deviations of group delay from the linear profile called group delay ripple (GDR) distort the shape of an optical pulse and thus they are highly undesirable.

[0010] If one creates a uniform temperature gradient along a linearly chirped FBG, the grating chirp changes but remains linear thus giving rise to a different dispersion value. Based on this fact one can design a dispersion compensator with a thermally tuneable dispersion. Unfortunately, as is commonly known, an elongated object heated at two ends, due to thermal losses, will exhibit a non-linear temperature profile, the temperature deviation from linearity being greatest in the middle. In the case of a distributor housing a chirped Bragg grating, such thermal losses (dubbed here “temperature sagging”) amount to an undesirable group delay ripple (GDR).

[0011] It is desirable to provide a DC capable of maintaining a uniform temperature gradient on the FBG, typically a chirped FBG.

[0012] It is further desirable to provide a DC with means for varying the temperature gradient at a relatively high rate, preferably maintaining the temperature gradient linearity.

[0013] In accordance with one aspect of the invention, there is provided a dispersion compensator comprising: a length of a waveguide including a grating region having two opposite ends, a heat distributing body extending along the grating region and adjacent to the grating region, and a heating strip mounted to the body and extending along the grating region for controlled heating of the entire grating region.

[0014] The compensator may have a temperature sensor disposed intermediate the ends of the grating region, preferably at the center of the grating region, for generating a signal indicative of the temperature of the respective region of the distributor. Preferably, the grating region is a chirped Bragg grating, e.g. a linearly chirped Bragg grating to afford an efficient compensation of chromatic dispersion.

[0015] The compensator may further include a longitudinally variable heating means adjacent to and extending along the length of the distributing body and the grating region for effecting a longitudinally varying heating of the grating region, the heating means having a monotonic heating-intensity variance along the length of the grating region.

[0016] In one embodiment, the compensator has two terminal heating/cooling means adjacent the ends of the grating region for heating at least the end parts of the grating region.

[0017] As indicated above, it is desired to create a linear temperature profile along the Bragg grating. To this end, according to the invention, the grating is disposed in a close proximity of a heat distributor, preferably inside a heat distributor. Tests have shown that the provision of the distributor, made of a material of high thermal conductivity and preferably but not necessarily with a thermal expansion coefficient (CTE) identical or close to CTE of the material of the grating (glass), is beneficial in maintaining a linear variance of temperature along the grating.

[0018] Dispersion dependence on temperature difference at the ends of the grating can be expressed as $D={\left(\frac{1}{D_0}+\frac{{\mathrm{cS}}_T}{2n}\frac{\Delta T}{L}\right)}^{-1}$

[0019] where D₀is the dispersion of the grating without temperature gradient (“nominal” dispersion), L is the grating length, n is refractive index of the optical fibre in which the grating is imprinted, S_Tis as explained above and c is the speed of light. Based on the desired dispersion tuning range, one can calculate the required temperature range. In order to maintain the device's bandwidth centred at a particular wavelength, the grating center temperature should remain constant. A sensor, e.g. a thermistor placed at the centre of the distributor provides the necessary feedback for the uniform heater control loop. For a linear temperature profile, the temperature at the middle of the distributor (and thus at the middle of the grating region) should be maintained at (T1+T2)/2 where T1 and T2 are the temperatures at the respective ends of the distributor. A central control unit may be provided to respond to the signal generated by the sensor.

[0020] The invention will be explained in more detail by way of the following description in conjunction with the drawings in which

[0021] FIG. 1 is a schematic side view of the dispersion compensator o the invention,

[0022] FIG. 2 is a schematic cross-sectional view of the compensator of FIG. 1,

[0023] FIG. 3 represents a partial top view of the distributor,

[0024] FIG. 4 is a top view of another embodiment of the compensator,

[0025] FIG. 5 is a cross-sectional view of the embodiment of FIG. 4, and

[0026] FIG. 6 is a schematic top view of the distributor showing the channel and the varying wall thickness.

[0027] As shown in FIGS. 1 and 2, the dispersion compensator 10 has an elongated heat distributor 12 made e.g. from a CuW alloy or another material with high thermal conductivity, which features two end parts 14 and 16 and a middle portion 18. The distributor forms a channel 17 as seen in FIG. 2, FIG. 5 and FIG. 6. It is preferable that the cross-section of the channel 17 is constant through the length of the channel. The thickness of the side walls of the distributor 12, and/or the bottom wall thereof, may be varied, e.g. profiled as shown in FIG. 6 so that the thickness is smallest in the mid-region of the distributor. This provision is intended to make the heat flux throughout the distributor relatively constant so that the temperature gradient can be relatively constant as well. The narrowing may be in the range of a few percent, depending on the dimensions of the components, and may be determined by experimenting or modeling.

[0028] The compensator is preferably placed inside a chamber, or casing (not illustrated for clarity), made of a heat-conductive material, so that a relatively uniform ambient temperature is maintained in the vicinity of the dispersion compensator.

[0029] Two thermoelectric coolers (TEC) 20, 22 are disposed at, and attached to, the end parts 14 and 16 of the heat distributor 12. The TECs are electrically connected to a source of electric energy and to a central control unit (not shown).

[0030] A length of a waveguide 24 for reflecting an optical signal in need of dispersion compensation is placed in the channel 17 of the distributor 12 and secured to the distributor at two ends thereof. Aside from the physical (mechanical) contact between the waveguide and the distributor at the two ends of the latter, there is only thermal, non-mechanical contact between the grating region and the distributor.

[0031] The waveguide has a linearly chirped Bragg grating region 26 co-extensive with the distributor 12, the length of the distributor approximately matching the length of the grating region.

[0032] A temperature sensor, e.g. a thermistor 28 for sensing the temperature of the middle of the grating 26 is disposed approximately in the middle of the distributor 12, and is connected to the central control unit, not shown. Two thermistors 29 are mounted at the ends of the distributor. The TECs are mounted to a heat sink 31.

[0033] A strip-shaped resistive heating element 30 (FIG. 2), of uniform heat generation capacity along its length, is attached to the surface of the distributor coextensively with the grating region 26.

[0034] Because of the heat exchange between the distributor and the surroundings, temperature profile deviates from linear resulting in GDR and non-uniform dispersion. The strip-shaped heating element 30 is provided in order to compensate for the thermal losses from the distributor by delivering uniform heat. It was found that maintaining the centre of the distributor at exactly half the temperature span between the end points $T_{\mathrm{mud}}=\frac{T_1+T_2}{2}$

[0035] minimises the temperature sagging and GDR.

[0036] It is desirable to periodically dither, i.e. introduce small variations in the temperature profile (gradient) of the grating, and the resulting change in network performance (i.e. eye closure, or bit error rate) can serve as a feedback to make a decision on tuning the dispersion up or down. Typically one would require a dithering rate of ˜1 Hz over few ps/nm of dispersion change. Ideally, during the dithering cycle no additional GDR should be introduced, i.e. the shape of the grating temperature profile should remain linear. In order to introduce the capability of fast change of the grating's temperature gradient, the invention provides linearly-variable heating means. Two versions are proposed and illustrated. In the first version, illustrated in FIG. 2, two resistive strip heaters 32, 34 are attached to the distributor over the open side of the channel 17. The heaters 32 and 34 are enveloped between two layers of a supporting material 35, e.g. Kapton and connected each to a controlled voltage source. When operated, the heat from the heaters 32, 34 is transferred to the grating region 26.

[0037] As can be seen in FIGS. 2 and 3, the shape of each heater 32, 34 is tapered, with the taper directions being opposite. The shape of each taper is trimmed monotonically in such a way that the amount of resistive heat produced varies linearly along the distributor. By periodically switching the heaters on and off one after another, small linear change can be introduced in the grating temperature.

[0038] Alternatively, as illustrated in FIG. 4 and FIG. 5, the linearly variable heating means are provided by way of two uniform heating wires 36, 38 which are arranged at a variable spacing from the grating region, preferably but not necessarily inside the channel 17, the spacing defining two tapers of opposite direction as seen in FIG. 4.

[0039] While not described in detail herein, it is conceivable within the present state of the art to provide other linearly variable heating means for the above purpose.

[0040] It has been found that the provision of the uniform heating strip (30) and the temperature control (28) of the middle of the distributor, and hence of the grating region, is beneficial in maintaining a desirable linear temperature profile of the fiber grating.

[0041] Various alternatives and modifications of the above may occur to those skilled in the art, without departing from the scope and spirit of the invention as defined by the appended claims.