DE10322110B4 - Arrangement for generating multi-wave optical signals and multi-signal source - Google Patents

Abstract

Arrangement for generating multi-wave optical signals, comprising at least
a pump source that generates pump pulses, and
a multi-core photonic crystal fiber (MF) consisting of N cores (K _i ) and capillaries (L _ij ) arranged around the cores (K _i ) of a material having a refractive index less than that of the core, each core (K _i ) with the capillaries (L _ij ) arranged around it forms a waveguide (W _j ) and all waveguides (W _j ) have small differences in the effective refractive index, wherein
Pump source and multi-core photonic crystal fiber (MF) are connected such that the pump pulses into the entire surface of the multi-core photonic crystal fiber (MF) irradiated and the passage of the pump pulses through the individual waveguide (W _j ) by means of four-wave mixing idler pulses with different discrete wavelengths are generated.

Description

The The invention relates to an arrangement for generating multi-wave optical signals and a multi-signal source.

For the generation of multi-wave signals for a big Number of channels in WDM (wavelength division multiplex) transmission systems the prior art according to previously mainly DFB laser (distributed feedback laser) with fixed wavelength used. Such lasers consist of a semiconductor laser with a grid, which in the etched active layer of the diode becomes. It is for each channel requires a separate laser with a fixed wavelength, which at the high number of up to 160 channels to considerable disadvantages and high Costs leads. Currently For example, such fixed wavelength lasers are becoming increasingly tunable Laser replaces, using different methods for the tunability of the wavelength realized be such. by temperature changes by means of thermoelectric Cooler. The tuning range can be increased by such lasers, that an array of lasers of different wavelengths into one single chip is integrated. Progress in development tunable laser lately and their comparison with the requirements of the telecommunications market are e.g. by B. Pezheski in Optics & Photonic News, Vol. 12, No. 5, pp. 34-38 (2001).

These solutions are technically very complicated and high costs.

task The invention is therefore a compact arrangement for the production specify of multi-wave optical signals, the simpler and cheaper in their production as known in the prior art solutions is.

According to the invention Task solved by that the arrangement for generating multi-wave optical signals at least one pump source that generates pump pulses and a multicore photonic crystal fiber (MCPCF) consisting of N nuclei and arranged around the nuclei Capillaries made of a material with a smaller refractive index than that of the core, with each core arranged around it Capillaries forms a waveguide and all waveguides low Have differences in effective refractive index, and pump source and MCPCF are connected such that the pump pulses into the entire Area of MCPCF radiated and the passage of the pump pulses through the single waveguide using four-wave mixing idler pulses be generated with different discrete wavelengths.

Although multicore fibers are also known in the prior art (multicore fiber), see, for example US 6,097,868 , or multi-core photonic crystal fibers, z. B. described in US 6,301,420 , However, the invention is based on a multi-core photonic crystal fiber, in which each core has a different refractive index, which is set for the purpose of generating multi-wave signals such that upon irradiation of pump pulses in the entire surface of the MCPCF the frequency of each Waveguide by means of four-wave mixing resulting Idler radiation receives a small shift. The frequencies of the resulting multi-wave signals are determined by the phase matching condition, which will be described below. The resulting multi-wave signals can be used for information transmission via WDM.

optical Multicore fibers are also described, for example, in WO 02/088802 A1 and US Pat WO 02/16985 A1. Thus, WO 02/088802 A1 is a composite Material of embedded in a matrix cylinders known whose Dispersion properties by choice of materials, diameter the cylinder and its geometric arrangement as well as by doping varied and active with other materials in a wide range is controllable. WO 02/16985 A1 discloses a multicore optical fiber structure described, whose structure is particularly favorable to other components can be adjusted. Both publications is not an indication of the inventive solution for generating multi-wave signals refer to.

In embodiments according to the invention, the different effective refractive indices in the waveguides are produced by different core materials or by a base material with different doping or by different sizes of the core diameters or by different distances and diameters of the capillaries surrounding them. The core material is, for example, quartz glass or a material with a large non-linear refractive index coefficient, for example As ₂ S ₃ glass.

The Cross sections of the capillaries arranged around the cores can be circular or be elliptical or have other shapes. Like the cross sections the capillaries of the corresponding application are adaptable also the distances adjustable to each other and to the cores.

When Pump source is in embodiments a diode laser or a diode laser with at least one amplifier or a solid-state laser intended. To increase The efficiency can be additional a second radiation source may be arranged, for example a Another diode laser, which has a spectrally broad radiation in the range the signal wavelength into the MCPCF.

In a further embodiment The invention provides for arranging a fiber amplifier at the output of the MCPCF. This - for example downstream or integrated erbium-doped fiber amplifier amplifies the multi-wave signals, so that the performance of idler pulses is the same for long distance transmission systems necessary values.

The The invention also encompasses multi-wave sources that are multiple in parallel arranged MCPFC or in the longitudinal arrangement of several of the same Pump pulses passing segments of MCPFC according to the arrangement for generating multi-wave optical signals for a number of N channels. In order to The multi-signal source generates an even larger number of channels.

Of the The essential advantage of the solution according to the invention is that for Generation of multi-wave signals in the MCPCF only one pump source is necessary while Previously the state of the art for each channel a single Laser needed becomes. An efficient conversion of pump radiation into idler radiation occurs under the condition of phase matching.

With the solution according to the invention can high demands made by currently realized WDM sources given, considerably reduce.

The inventive solution is - as already described - by a multi-core photonic crystal fiber (MCPCF).

Photonic Crystal fibers (PCF) have considerable in recent years scientific and technical interest. such Fibers consist of a core of quartz glass or another optical material surrounded by a sheath of capillaries is that filled with air (See, for example, J.C. Knight, T.A. Birks, P.S. Russel and D.M. Atkins, "All-silica single-mode optical fiber with photonic crystal cladding ", Opt. Lett. 21, 1547 (1996); T.A. Birks, W.J. Wadsworth and P.S.J. Russel, "Endlessly single-mode photonic crystal fiber "Opt. Lett. 25, 1415 (2000); WO 00/60388; WO 01/42831). The lying in the micrometer range diameter and The distance between the capillaries determines the propagation properties of the core Light. Such PCF structures to lead to new and applicatively very interesting fiber properties. Around just to name a few: the fibers can be in a very large wavelength range be monomodal, you can have very large or very small modenfelddurchmesser for the Reach and can do basic fashion novel dispersion properties are generated, e.g. abnormal Dispersion into the visible range.

The proposed in the solution according to the invention Multi-core photonic crystal fiber differs from the photonic crystal fiber described above with a core in a common coat a variety of N cores each surrounded by capillaries. Is in a photonic Crystal fiber omitted a capillary, the photonic possesses Crystal a defect. The spatial Missing capillary area forms a core for one optical waveguide, because its environment has a higher refractive index has. As a result, light in the core becomes analogous as in a conventional passed optical fiber. In an analogous manner, a photonic Crystal fiber with several cores are formed by several Capillaries are omitted. This is a light pipe in dependence the properties of the core and / or its environment in several Waveguides, consisting of core and surrounding capillaries, guaranteed. The cores can also doped with additional materials without difficulty with different doping concentrations or materials possible are. The different doping rates result in small ones Differences in the effective refractive index of the waveguide, the one for the inventive solution necessary condition.

The multi-wavelength signals suitable for use in WDM transmission systems are generated according to the invention in the described multicore photonic crystal fiber by degenerate four-wave mixing. In degenerate four-wave mixing (described for example by GP Agrawal in "Non Linear Fiber Optics ", Academic Press (1994); pp. 289-314), a pump pulse having the frequency ω _{p is} coupled into a fiber. As a result of the third-order non-linear susceptibility, the pump pulse generates an idler and a signal pulse with the frequencies ω _i and ω _s , where two photons of the pump pulse are converted into one photon each of the signal and idler waves: 2ω _p → ω _i + ω _s An efficient conversion occurs only if the condition of phase matching is met: κ = 2β (ω p ) - β (ω i ) - β (ω s ) - 2γ p P p = 0 (1) where β (ω) represents the wave number at the frequency ω _i P _p the power of the pump pulse, γ _p = n ₂ ω _p / c _eff , A _eff the effective area of the MCPCF and n ₂ the nonlinear refractive index coefficient. In order to determine the effective refractive index of a waveguide in the MCPCF, the wavenumber β (ω) is determined by numerical solution of the Helmholz equation for a waveguide structure described above. The efficiency of the conversion of pump radiation into idler radiation with the frequency ω _i can be increased by additionally coupling in the non-linear photonic crystal a signal wave with the frequency ω _s in addition to the pump wave.

The power of the idler pulse can be calculated in this case by the following formula P i (L) = P s sinh (0) 2 (gL) (2) where P _s (0) is the radiated power of the signal wave or that of the noise (if no signal wave is radiated), P _i (L) is the power of the idler radiation at the end of the PCF, g = n ₂ I _p ω _p / c the parametric gain coefficient and L are the length of the PCF. In the formula, the divergence of pulses due to different group velocities is neglected, assuming that the fiber length is smaller than the so-called "walk-off" length L _i = τ (1 / ν _g (ω _p ) -1 / ν _g (ω _i )) ^-1 , where τ is the duration of the pulses and ν _g (ω) is the group velocity of the pump pulse or the idler radiation.

In US 5,274,494 An optical amplifier is described in which signal light and pump light propagate in an optical waveguide structure. By means of four-wave mixing, the signal light is amplified in the material having non-linear properties. An application of the four-wave mixing to generate multi-wave signals for another waveguide structure is not noted.

For ordinary standard fibers, the necessary fulfillment of the phase matching condition ( 1 ), which severely restricts the use of four-wave mixing, rendering it scarcely applicable for similar purposes as set forth herein. Phase matching is possible in such fibers only in a very narrow spectral range near the zero point of the dispersion. Also by other methods of phase matching, such as by the use of birefringence, self-phase modulation or by the use of multimode fibers, such difficulties can not be substantially eliminated. On the other hand, photonic crystal fibers offer a considerably extended potential which, among other things, makes it possible to control the effective dispersion properties by means of the choice of the distances and diameters of the capillaries.

In order to generate optical signals with a certain number of discrete wavelengths for message transmission by wavelength division multiplexing (WDM), according to the invention, the individual waveguides of the multicore photonic crystal fiber have small differences in the effective refractive index that can be set during their production. As already mentioned, these refractive index differences can be generated, inter alia, by doping the cores with another material of variable concentration. In principle, different diameters of the cores or the surrounding capillaries would produce such refractive index differences, but the required tolerance limits are relatively high. By means of four-wave mixing, in each waveguide of such a multi-core photonic crystal fiber radiation with a certain discrete idler wavelength is generated, which is characterized by the condition of phase matching ( 1 ) is determined. Since the refractive index can be adjusted in its frequency dependence, for example, by small concentration differences of core dopants with another material, very accurate and targeted during production, can thereby produce a variety of optical WDM channels in the telecommunications window.

The Invention will be in the following embodiments closer by means of drawings explained.

there demonstrate:

1 the schematic structure of a multi-core PCF;

2 the schematic structure of an inventive arrangement for generating multi-wave optical signals;

3 a curve with the parameter of the dispersion of the group velocities (GVD) and the wavenumber difference κ as a function of the wavelength for a multi-core PCF made of quartz glass;

4 a curve with the parameter of the dispersion of the group velocities (GVD) and the wavenumber difference κ as a function of the wavelength for a multicore PCF made of As ₂ S ₃ glass;

5 a curve with the Idler frequency as a function of the germanium doping concentration for a multi-core PCF made of quartz glass;

6 a curve with the Idler frequency as a function of the selenium doping concentration for a multi-core PCF of As ₂ S ₃ glass;

7 Curves illustrating the conversion of pump pulses into idler pulses for a multicore PCF from As ₂ S ₃ glass
a, b: curves representing the temporal shape of the pump pulses;
c, d: curves representing the temporal shape of the idler pulses;
e; f: curves representing the spectrum of idler pulses;

8th Idler spectral lines calculated for a multicore PCF from As ₂ S _{3 (1-x)} Se _3x .

In the 1 A multicore photonic crystal fiber MF with a radius R shown in cross-section contains a number of N nuclei K _i (i = 1,..., N) of glass with a high non-linear coefficient, and of periodically arranged cylindrical capillaries L _ij with a diameter d and a distance A are surrounded to each other. For the optical guidance of waves, two layers of capillaries are generally sufficient. The cores thus have a diameter 2d - Λ. Within each of the cores is a region B _i which is doped with another optical material at the doping rate α _i , where α _i varies from core to core so that the desired idler waves are phase matched at the output of the fiber. Each core K _i forms a waveguide W _j with the capillaries L _ij surrounding it. Light is guided in each of the waveguides W _j by total reflection, similar to a standard fiber; However, the much larger difference in the effective refractive index between the core and cladding leads to considerable modifications of the dispersive properties, such as, for example, anomalous dispersion in the optical field.

In 2 an arrangement according to the invention for generating multi-wave optical signals is shown. A radiation source, for example a diode laser 1 with an amplifier 2 , Pumping pulses into the multi-core PCF MF, as in 1 is shown. By means of degenerate four-wave mixing, in each waveguide of the multi-core PCF MF, radiation is generated with a defined discrete idler wavelength, wherein due to the different effective refractive index of the individual waveguides, the desired multi-wave signals. By means of a post-amplifier, for example an erbium-doped fiber amplifier 3 If necessary, the power in each channel can be sufficiently boosted.

In the 3 are the numerical results for a multicore PCF made of quartz glass with the parameters Λ = 3.5 μm, d = 2.5 μm and in 4 for a highly non-linear multicore PCF made of As ₂ S ₃ glass with Λ = 0.53 μm, d = 0.52 μm. In the curves 2 For example, the so-called group velocity dispersion (GVD) parameter characterizing the dispersion is presented for the corresponding multicore PCF, with both material contribution and waveguide contribution included. From the figures it can be seen that the GVD parameter in these specific examples has a zero for a multicore PCF of fused silica at 1010 nm and two zeros for the As ₂ S ₃ multicore PCF at 1034 nm and 1390 nm, and the dispersion in the first case, for wavelengths greater than 1010 nm and in the second, between 1034 nm and 1390 nm is abnormal. In the curves 1 is the characteristic of the four-wave mixing wave number difference κ shown. An efficient conversion into idler radiation is only possible if the condition κ = 0 is fulfilled. From the 3 and 4 It can be seen that phase matching in the example chosen to generate idler radiation at 1504 nm in 3 (Quartz glass) or 1529 nm in 4 (As ₂ S ₃ -Glas) can be realized when the pump and the signal frequency at 980 nm and 726 nm ( 3 ) and at 1060 nm and 811 nm ( 4 ), these frequencies are in the typical range of diode lasers.

As already mentioned, for the generation of multi-wave signals in the telecommunication window by a multi-core PCF, it is necessary for each waveguide of the MCPCF to have a small difference in the effective refractive index relative to the other waveguides, thus corresponding to the phase matching condition (FIG. 1 ) the idler radiation frequency in each core gets a small shift. This results in multi-core PCF multi-wave signals that can be used for information transmission using WDM. How the necessary refractive index differences can be generated has already been mentioned several times. The refractive index of the core material can be described by the Sellmeyer formula with experimentally determined constants taking into account doping with another material according to its molar weight.

In 5 For example, the idler frequency is represented as a function of the doping concentration for a MCPCF having a core of quartz glass doped with germanium. As can be seen, by appropriate selection of the concentrations, a multiplicity of channels can be produced in the spectral range of interest for the WDM.

In 6 For example, the corresponding behavior is shown for a highly nonlinear MCPCF with a core of As ₂ S ₃ glass doped with selenium.

For applications, it is of course important to calculate the required requirements for the pump source with regard to their pulse parameters, in particular the required power and the achievable power of the multi-wave signals of the idler pulses. As a demonstration, the propagation of a pump pulse of 40 ps duration at the appropriate wavelengths was performed by the 4 characterized MCPCF from As ₂ S ₃ glass calculated numerically. The idler pulses are generated from the noise, with a noise ^10-10 times smaller than that of the pump laser. The diameter of the entire MCPCF was assumed to be 100 μm and 15 μm, respectively. The MCPCF consists of a total of 50 waveguides, each waveguide consisting of a core and capillaries arranged around this core. The other parameters are as described 4 and 6 refer to.

The results of the numerical simulation for generating idler pulses from pump pulses are in the 7 initially for a single waveguide, so for a PCF with only one core of As ₂ S ₃ glass shown. For the calculation of the individual parameters, the following equations were calculated for the amplitudes of the three pulses involved (pump pulse, signal pulse, idler pulse) taking into account their nonlinear interaction due to the nonlinear refractive index change, the different group velocities and the GVD with the GVD parameter β "= θ ² β / θω ² and linear losses (see, for example, BGP Agrawal, "Nonlinear Fiber Optics", Academic Press (1994), p.

Here, A _{j are} the amplitudes of the pulses, α the loss coefficients, η = t - z / ν _g (ω _p ) the time along with the pump pulse, z - the coordinate along the fiber and γ _j = n ₂ ω / cA _eff . The other symbols have already been introduced above.

In 7 the spectrum (e, f) and the temporal pulse shape (c, d) of the idler pulse resulting from four-wave mixing in a PCF with a core of As ₂ S ₃ glass, resulting from the conversion of the pump pulse, is shown as peak power of the pump pulse in each channel were chosen to be 3 W (a), (c), (e) and 7 W (b), (d), (f).

analog We also have bills for a PCF performed with a core of quartz glass. To the same idler performance to achieve this is the required Input power for the pump laser about 30 times higher.

For the representation of spectral lines in a MCPCF in 8th For example, calculations have been made for an MCPCF where the core material is As ₂ S _{3 (1-x)} Se _3x with x = 1% ... 4%. The other parameters have already been mentioned above. In the selected example, 50 channels are created (20 lines are shown, the remaining 30 lines are left out for better clarity). It can be seen that discrete spectral lines with the same frequency difference and almost the same amplitude arise throughout the MCPCF, which are suitable as transmission channels in WDM systems. With differently selected parameters, in particular higher pump powers, an even higher number of channels can be realized.

In the embodiments could be shown that in the inventive arrangement by means of suitable Pump laser sources a phase matching for the generation of idler radiation in an MCPCF in the spectral range of the telecommunication window can be realized. By using high-nonlinear materials for the Base material of the core with a much higher nonlinear refractive index (compared to about 100 times bigger with quartz glass), from which the multi-core photonic crystal fiber is pulled, can be significantly increase the efficiency of conversion into idler radiation Accordingly, the requirements for the performance of the pump pulses reduced become. This is from the formula (2) for the performance of the idler pulse immediately apparent.

Claims (16)

Arrangement for generating multi-wave optical signals, comprising at least one pump source which generates pump pulses, and a multi-core photonic crystal fiber (MF) comprising N cores (K _i ) and capillaries (L _ij ) arranged around the cores (K _i ) a material with a smaller refractive index than that of the core, each core (K _i ) with the capillaries (L _ij ) arranged around it forming a waveguide (W _j ) and all the waveguides (W _j ) having small differences in the effective refractive index in which the pump source and multi-core photonic crystal fiber (MF) are connected in such a way that the pump pulses are radiated into the entire surface of the multicore photonic crystal fiber (MF) and through the individual waveguides (W _j ) through four-wave mixing. Pulses are generated with different discrete wavelengths. Arrangement according to claim 1, wherein the different effective refractive indices of the waveguides (W _j ) are produced by different core materials. Arrangement according to claim 2, wherein the different Core materials by a base material with different doping are formed. Arrangement according to Claim 1, in which the different effective refractive indices of the waveguides (W _j ) are produced by different spacings or sizes of the diameters of the capillaries (L _ij ). Arrangement according to Claim 1, in which the different effective refractive indices of the waveguides (W _j ) are produced by different diameters of the cores (K _i ). Arrangement according to claim 1, wherein the core material Quartz glass is. Arrangement according to claim 1, wherein the core material a material with a big one non-linear refractive index coefficient. An assembly according to claim 7, wherein the core material is As ₂ S ₃ glass. Arrangement according to claim 1, wherein the cross section of the capillaries (L _ij ) arranged around the N cores (K _i ) are circular. Arrangement according to claim 1, wherein the cross section of the capillaries (L _ij ) arranged around the N cores (K _i ) are elliptical. Arrangement according to Claim 1, in which the pump source is a diode laser ( 1 ). Arrangement according to Claim 1, in which the pump source is a diode laser ( 1 ) with at least one amplifier ( 2 ). Arrangement according to claim 1, wherein a second radiation source is arranged, which has a spectrally broad radiation in the range of Signal wavelength irradiates. Arrangement according to Claim 1, in which a fiber amplifier ( 3 ) is arranged at the output of the multi-core photonic crystal fiber (MC). Multi-signal source, in which several multi-core photonic crystal fibers (MF) according to at least one of the preceding claims are arranged in parallel. Multi-signal source, where several of the same Pump pulse passing segments of the multi-core photonic crystal fiber (MCPCF according to at least one of the claims 1 to 14 are arranged longitudinally.