DE19538017A1 - Circuit arrangement for dispersion compensation in optical transmission systems with the help of chirped Bragg gratings - Google Patents

Abstract

In a dispersion compensating circuitry for optical transmission systems by means of chirped Bragg gratings, a plurality of Michelson interferometer stages each with an optical directional coupler and two chirped Bragg gratings form reflectors which completely reflect the light entering through the input optical waveguide to the output optical waveguide. The Michelson interferometer stages are interconnected in cascade so that the output optical waveguide of the interferometer of each stage passes into the input optical waveguide of the interferometer of the following stage.

Description

With optical message transmission in the Gbit / s range lying data rates over an optical fiber is the Fiber dispersion determining for the bridgeable distances length. This is especially true in the wavelength window around 1.55 µm, because here the damping by means of optical amplifiers ker can be eliminated while the dispersion of the Standard fiber with about 17 ps / nm / km quite large positive Has values. There is therefore an interest in composers ten, which have a negative dispersion and so together with the standard fiber a dispersion-free transmission measurement can form dium.

The crucial parameters of a dispersion compensation the components are the dispersion D (in ps / nm or ps / GHz), which specifies the length of the compensable route that optical bandwidth B within which compensation is possible Lich, and that caused by the dispersion compensation additional damping. It makes sense to compensate Bandwidth B at least equal to the bandwidth of the over supporting signals. However, one is desirable wide range of compensation to meet the requirements to reduce the spectral stability of the transmitter laser.

In connection with dispersion compensation, ne ben (now also commercially available) dispersion com different components Fabry-Perot interferometer, ring resonator Ren, cascaded Mach-Zehnder interferometer, cascaded birefringent crystals, open beam optics with gratings; a more recent proposal [DE-195 15 158.5] aims at an opti transverse filter.  

Another way of realizing negative disper chirped gratings (chirped gratings) are Bragg gratings with a location-dependent grating period [Francois Quellette: "Dispersion cancellation using lineary chirped Bragg grating filters in optical waveguides ", Optics Let ters, 12 (1987), 847. . . 849; K.O. Hill et al .: "Chirped In- Fiber Bragg Gratings for Compensation of Optical Fiber Dis persion, Optics Letters, 19 (1994), 1314. . . 1316]. A sol The chirped Bragg grating consists of an optical one Waveguide, whose refractive index n is in the direction of propagation tion changes periodically, the period length from the location z is dependent. Wherever there is a spectral component λ of the incident light the Bragg condition

λ = 2n _eff · Λ (z) (1)

(where n _{eff is} the mean effective refractive index of the waveguide and Λ (z) is the period length dependent on the location z), the grating acts as a mirror for these spectral components and reflects the light of this wavelength, as shown in Fig. 1 for two wavelengths λ₁ and λ₂ is indicated.

The period length λ (z) which is dependent on the location z in the chirped lattice is z over the filter length L _G. B. linearly changed:

where ΔΛ _{G is} the difference between the grating period Λ (0) at the beginning and the grating period at the end of the filter. The local change in the grating period leads to light of different wavelengths being reflected at different locations and thus having different transit times. This makes it possible to realize specific dispersions in a targeted manner.

The optical bandwidth, i. H. the frequency range B (in GHz) or the wavelength range Δλ (in nm) in which the the desired dispersion is obtained

Λ is the center wavelength of the filter.

The group delay results from the wavelength-dependent detour 2 · z _B (λ) and the speed of light c / n _eff in the medium

where z _B (λ) denotes the location in the chirped Bragg grating where the Bragg condition (1) for λ is fulfilled. With Eq. (1), (2) and (3) then apply to the dispersion D (λ)

and thus

Large bandwidth B and large dispersion D cannot therefore be achieved independently of one another; at the same time they can only be reached with a long length L _{G of} the waveguide.

The following numerical examples illustrate this connection. In order to achieve a dispersion of D = - 1000 ps / nm over a bandwidth B = 10 GHz, a filter length L _G of 8 mm is required with n _eff ≈ 1.5 and λ₀ = 1.55 µm. For larger bandwidths, which would make it unnecessary to stabilize the filter at the transmitter wavelength, there are significantly larger slopes. For D = - 1000 ps / nm and Δλ = 10 nm, a filter length L _G of 1 m is required.

The length of the components are technological limits set, namely with an integrated optical reality  dimensioning of the wafer; with a fiber optic The realization of lattice sets limits. At Using a (phase) mask, the dimensions of the Mask delimiting and in a holographic structure Stability of the receiving arrangement.

To achieve large dispersion with a wide bandwidth, meh Providing more grids in a row leads to the un certain phase position between the individual sub-grids unmanageable effects, so that these are simple in themselves Possibility practically eliminated. It is in this together menhang (from Optics Letters, (19) 1994, 1314... 1316) be knows, for dispersion compensation for each channel one WDM system to provide its own grid, the grid arranged one behind the other and each for all other Ka channels are transparent; this will make the range of one However, individual channels of the dispersion compensator are not increases.

In contrast, the invention shows another way to one Dispersion compensation in optical transmission systems.

The invention relates to a circuit arrangement for Disper sions compensation in optical transmission systems with Help of chirped Bragg grids; this circuit arrangement tion is characterized in that a Plurality of Michelson interferometer stages, each with an optical (at least approximate) 3 dB direction coupler (beam splitter) and two chirped Bragg gratings are formed as reflectors, which over the input Optical fiber incoming light (at least almost) full constantly reflect to the output optical fiber, too are connected in a cascade, in which the One stage interferometer output fiber into the input fiber of the interferometer each subsequent stage passes.  

It should be noted at this point that a Michelson-Interfe rometer, which with an optical 3 dB coupler and two ge chirped Bragg gratings are formed as reflectors, whereby a phase adjustment in the optical waveguide section between between the beam splitter and the one Bragg grating complete reflection of the light waves above the input incoming light to the output fiber enables itself (from D.C. Johnson et al .: "New design concept for a narrowband wavelength selective optical tap and combiner ", Electronics Letters, 23 (1987), 668 ... 669) is known, but without closer points of contact with the Invention are given.

The invention advantageously enables each relatively short and therefore only a small dispersion component effecting but broadband reflective Bragg- To be able to provide grids and at the same time a proportio nal to the number of interferometer series Steps to achieve increased dispersion compensation.

Further special features of the invention will become apparent from the following detailed explanation of an embodiment can be seen from the drawings. Clarified

. Figure 1 illustrates the operation of a Bragg grating;

Fig. 2 shows a schematic diagram of a Michelson Interfe rometers, as used in the invention, and

Fig. 3 shows a schematic diagram of a dispersion compensator according to the invention.

The operation of a chirped Bragg filter illustrated by FIG. 1 has already been explained above, so that further explanations are unnecessary at this point.

In Fig. 2, an integrated-optical Mi chelson interferometer is shown schematically, which is formed with an optical (at least approximate) 3 dB directional coupler (beam splitter) RK and two chirped Bragg gratings G as reflectors which reflect the light entering (almost) the input optical fiber E completely (almost) continuously to the output optical fiber A. Such a complete reflection of the light entering towards the exit occurs with the appropriate phase position of the light waves reflected by the two reflectors; Such a phase position is brought about in the arrangement sketched in FIG. 2 in the region Ph indicated schematically there for the phase adjustment. A Michelson interfe rometer, which is formed with an optical 3-dB coupler and two chirped Bragg gratings as reflectors, with a phase adjustment in the optical waveguide section between the beam splitter and the one Bragg grating a complete reflection of the over Input light waveguide allows incoming light to the output optical waveguide, is in itself (from DC Johnson et al .: "New design concept for a narrowband wavelength selective optical tap and combiner", Electronics Letters, 23 (1987), 668 669) known, so that it requires no further explanations.

In Fig. 3, an embodiment of a circuit arrangement for dispersion compensation according to the invention is shown schematically to the extent necessary for understanding the invention. In this circuit arrangement, which can be designed as an IO (integrated optics) circuit, a plurality of Michelson interferometer stages I1, I2, I3,. . , IN are provided, each of which is formed with an optical directional coupler RK and two chirped Bragg gratings G, from which the respective input optical waveguide E, E2, E3,. . . , EN incoming light completely to the respective output optical fiber A1, A2, A3,. . . , A is reflected. Such a complete reflection of the light entering towards the output occurs, as already explained in the explanation of FIG. 2, given a corresponding phase position of the two light beams reflected by the two Bragg gratings G, such a phase position by a phase adjustment in the phase adjustment range Ph is brought about between the beam splitter RK and the one Bragg grating. The interferometer stages I1, I2, I3,. . . , IN are interconnected to form a cascade, in each of which the output optical fiber of the interferometer of a stage, for example the output optical fiber A1 of interferometer I1, into the input optical fiber of the interferometer of the respective subsequent stage, in the example in the Input optical fiber E2 of the interferometer I2 passes over.

With such a waveguide arrangement, back reflections are avoided in the previous stage, so that there are no multiple reflections. It is also advantageous that this arrangement does not have to meet any special requirements for the relative positioning of the individual Bragg gratings with respect to one another. The individual, relatively short Bragg gratings (G) are expediently dimensioned such that they have a large chirp ΔΛ _G , that is to say that they each reflect broadband. They therefore have only a low (negative) dispersion and therefore only contribute to a small extent to dispersion compensation. The outlined series connection of a plurality of such interferometer stages in an optical network nevertheless enables an increased dispersion compensation in proportion to the number of interferometer stages connected in series.

With practically realized (or also numerically simulated chirped Bragg gratings can be the spectral representation development of reflection factor and dispersion undesirable wel have disabilities with an undesirable leg impairment of the suitability of the chirped gratings for dis persistence compensation. The size of the ripple depends on two design sizes of the chirped grille, namely from the course of the grating period Λ (z) and from the course  des - in the differential equations of the Coupled Wave The orie the link between the incoming and outgoing wave representative - coupling factor κ (z), the refractive index difference Δn (z) of the periodic refractive index ver with Λ (z) is dependent on the race. If you go like this in making chirped gratings by means of UV light irradiation photosensitive waveguides through a phase mask is of a cosine (or sine) refractive index course n (z)

, the coupling factor κ (z) results in

Since the grating period Λ (z) differs from its mean changes only slightly, the coupling factor is κ (z) in the we considerably proportional to the refractive index difference Δn (z).

Such ripples, which can be relatively large with a period length linear (z) linearly dependent on the location z (so-called linear chirp) and location-independent coupling factor κ (z), can be reduced in practice by the fact that the chirped Bragg grating location-dependent period length Λ (z) receives a corresponding non-linear profile across the filter length L _G and that the coupling factor κ (z) also receives a suitable location-dependent profile. In the Li teratur a gentle, often Gaussian-shaped drop in the coupling factor to the two ends of the grating is provided to reduce the ripple. By computer-aided simultaneous optimization of the period length Λ (z) and coupling factor κ (z), starting from relatively simple initial courses, the ripple can be reduced significantly further.

In order to avoid that (due to manufacturing tech boundary conditions) residual ripple with a cascading of identical Michelson- Interferometer levels are strengthened Michelson interferometer stages are provided to each other regarding the course of the period length Λ (z) and the coupling tion factor κ (z) are different, so that they Restwel other Michelson interferometer Compensate level (s).

For example, two different types of Michelson interferometer stages can be provided which (in each case in addition to their actual function) largely compensate for the residual ripple of the other type. For this purpose, the aim of the optimization program is not to specify completely flat spectral profiles of the reflection factor and dispersion, but rather a superposition of these profiles and those with which the residual ripple of the respective different type is being compensated for. In the embodiment example according to FIG. 3, interferometer stages of different types then follow one another.

It is also possible e.g. B. provide a third type of Michelson interferometer stage, which (in addition to its actual function) counteracts the ripple of Michelson interferometer stages of the first and second type. For example, in the exemplary embodiment according to FIG. 3, the Michelson interferometer stage I1 can be of the first type, the Michelson interferometer stage I2 can be of the second type and the Michelson interferometer stage I3 can be of the third type, after which this sequence expediently follows repeated to the last Michelson interferometer stage IN if necessary. The number of different types of interferometer stages that are provided in the dispersion compensator depends on the residual ripple permitted for the dispersion compensator as a whole, the number of cascaded interferometer stages and the residual ripple (possibly due to technical manufacturing constraints) individual Michelson interferometer levels.

Finally, it should be noted that with the help of optical Switches between the individual levels or by sub divide the component the number of effective compensator step up to a given smaller one in a simple manner compensating dispersion, possibly to a shorter oversize link length can be adjusted without this further explanations are required here.

Claims (7)

1. Circuit arrangement for dispersion compensation in optical transmission systems using chirped Bragg gratings, characterized in that a plurality of Michelson interferometer stages (I1, I2, I3,..., IN), each with an optical directional coupler (RK) and two chirped Bragg gratings (G) as the light entering via the input optical fiber (E, E2, E3,..., EN) to the output optical fiber (A1, A2, A3,..., A ) reflective reflectors are formed, are connected together to form a cascade, in each of which the output optical fiber (A1) of the interferometer of one stage (I1) merges into the input optical fiber (E2) of the interferometer of the subsequent stage (I2). 2. Circuit arrangement according to claim 1, characterized, that in each interferometer stage (I1, I2, I3,..., IN) in Optical fiber section between the beam splitter (RK) and the one Bragg grating (G) has a phase adjustment range (Ph) for phase adjustment for the most complete reflection possible xion of the via the respective input optical fiber (E, E2, E3,. . , EN) incoming light to the respective output Optical waveguide (A1, A2, A3,..., A) is provided. 3. Circuit arrangement according to claim 1 or 2, characterized, that different ripple spectra properties of another Michelson interfero level (s) (I1,...) compensating Michelson interfe rometer levels (..., IN) are provided. 4. Circuit arrangement according to claim 3, characterized,  that a sequence of mutually different, rest wel other Michelson interferometer Level (s) (I1,...) Compensating Michelson interfero meter steps (..., IN) are repeated one or more times. 5. Circuit arrangement according to one of claims 3 to 4, characterized by the joint optimization of period length (Λ (z)) and coupling factor (κ (z)) of the Bragg grating (G) of a Michelson interferometer stage ( 13 ) for compensation the residual ripple from (another) Michelson interferometer stage (s) (I1, I2). 6. Circuit arrangement according to one of claims 1 to 5, characterized, that to compensate for a smaller dispersion only one Part of the interferometer stages (I1, I2, I3,..., IN) effective is sam. 7. Circuit arrangement according to one of claims 1 to 6, characterized, that they are designed as an IO (integrated optics) circuit is.