WO2001037458A1

WO2001037458A1 - Dynamic power-equalized erbium doped fiber amplifiers using transmission edge filters based on apodized linearly-chirped fiber bragg grating

Info

Publication number: WO2001037458A1
Application number: PCT/CA2000/001334
Authority: WO
Inventors: Peter W. E. Smith; David J. F. Cooper; Lawrence R. Chen
Original assignee: Smith Peter W E; Cooper David J F; Chen Lawrence R
Priority date: 1999-11-16
Filing date: 2000-11-16
Publication date: 2001-05-25
Also published as: AU1374701A

Abstract

The present invention relates to a method and devices that make use of transmission edge filters based on apodized linearly chirped (LC) fiber Bragg gratings (FBGs) to provide the variable attenuation required for power equalization among different wavelength channels in an erbium-doped fiber amplifier. This approach is especially attractive since it can easily be extended to provide dynamic power equalization by simply incorporating a feedback loop.

Description

DYNAMIC POWER-EQUALIZED ERBIUM DOPED FIBER AMPLIFIERS USING TRANSMISSION EDGE FILTERS BASED ON APODIZED LINEARLY-CHIRPED FIBER BRAGG GRATING

FIELD OF THE INVENTION

The present invention relates to a method and device for providing power equalization control in erbium doped fiber amplifiers using transmission edge filters based on apodized linearly chirped fiber Bragg gratings.

BACKGROUND OF THE INVENTION

Erbium-doped fiber amplifiers (EDFAs) are indispensable tools for providing optical amplification in wavelength-division-multiplexed (WDM) systems. However, it is difficult to transmit and amplify many WDM channels using EDFAs since the gain profile is wavelength dependent (non-uniform), while the transmission medium loss is, to first order, wavelength independent.

This creates significant differences in the signal-to-noise ratios among the different amplified WDM channels which may, depending on the system power budget or dynamic range of the receiver, cause system impairments and degrade performance. Although gain-flattened EDFAs (uniform gain profile) exist, due to possible changes in operating conditions and to network reconfiguration operations such as channel add/drop, variations can still exist among the power levels of the WDM channels amplified by an EDFA. This problem will further be compounded by the fact that many EDFAs are generally used in transmission systems. It is thus of critical importance to be able to equalize the power of the different WDM channels (after amplification) in a dynamic fashion in order to track the variations in the signal levels.

To this end, several different techniques which involve some form of variable attenuation for each of the input wavelength channels, have been considered, see [1} S. Y. Ko et al., "Gain control in erbium-doped fiber amplifiers by tuning center wavelength of a fiber Bragg grating constituting resonant cavity," Electron. Lett., vol. 34, no. 10, pp.990 - 991 (1998); S. Y. Park et al., "Dynamic gain and output power control in a gain-flattened erbium-doped fiber amplifier," IEEE Photon. Technol. Lett., vol. 10, no. 6, pp. 787 - 789 (1998); H. S. Kim et al., "Actively gain-flattened erbium-doped fiber amplifier over 35 nm by using all-fiber acoustooptic tunable filters," IEEE Photon. Technol. Lett., vol. 10, no. 6, pp. 790 - 792 (1998); and S.-K. Liaw et al., "Dynamic power-equalized EDFA module based on strain tunable fiber Bragg gratings," IEE Photon. Technol. Lett., vol. 11 , no. 7, pp. 797 - 799 (1999). In Liaw et al. the use of strain-tuned uniform fiber Bragg gratings (FBGs) operating in transmission as variable attenuators for providing power equalization among multiple channels in an EDFA module was proposed and demonstrated.

SUMMARY OF THE INVENTION

The present invention makes use of transmission edge filters based on apodized linearly chirped (LC) fiber Bragg gratings (FBGs) to provide the variable attenuation required for power equalization. This approach is especially attractive since it can easily be extended to provide dynamic power equalization by simply incorporating a feedback loop.

In one aspect of the invention there is provided a dynamic power equalized erbium-doped fiber amplifier, comprising; an erbium-doped fiber amplifier having an input optical fiber and an output optical fiber; at least one fiber Bragg grating structure in the output optical fiber including an effective refractive index modulation to give a transmission versus wavelength characteristic having a substantially linear portion over a selected wavelength region; means for tuning the at least one fiber Bragg grating structure for shifting the substantially linear portion over a selected wavelength region for varying the intensity of light at a selected wavelength λ, transmitted along the input optical fiber through the erbium-doped fiber amplifier an output through said at least one fiber Bragg grating. In another aspect of the invention there is provided a method of varying transmission of selected wavelengths of light transmitted through an erbium- doped fiber amplifier, comprising; providing an erbium-doped fiber amplifier having an input optical fiber and an output optical fiber and providing at least one fiber Bragg grating structure in the output optical fiber, the at least one fiber Bragg grating structure including an effective refractive index modulation to give a transmission versus wavelength characteristic having a substantially linear portion over a selected wavelength region; and tuning the at least one fiber Bragg grating structure to shift the substantially linear portion of the transmission versus wavelength characteristic over a selected wavelength region for selectively adjusting the intensity of light at a selected wavelength λ, transmitted through said at least one fiber Bragg grating.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a description, by way of example only, of a power stabilized erbium doped fiber amplifier constructed in accordance with the present invention, reference being had to the accompanying drawings, in which:

Figure 1 shows a calculated spectral response of a 3.5 cm linearly- ramped fiber Bragg grating in transmission;

Figure 2(a) shows the spectral response of transmission edge filters;

Figure 2(b) shows the measured variation in transmitted power as a function of applied strain; and

Figure 3 shows a dynamic power-equalized EDFA module using transmission edge filters;

Figure 4(a) illustrates the spectra of input light signals to the EDFA;

Figure 4(b) shows the spectra of output light signals from the EDFA with no power equalization; and Figure 4(c) shows the spectra of output light signals from the EDFA but with power equalized output in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A transmission edge filter is one in which the transmissivity varies linearly as a function of wavelength. They can be realized in fibers using long-period gratings (as disclosed in Fallon et al.) or tilted LCFBGs as disclosed in Y. Liu, L. Zhang, and I. Bennion, "Fabricating fiber edge filters with arbitrary spectral response based on tilted chirped grating structures," Meas. Sci. Technol., 10 pp. L1 - L3, 1999. An alternative structure is simply an apodized LCFBG. LCFBGs are routinely used in reflection for providing dispersion compensation and have been extensively characterized with the view of determining the optimal structure for dispersion compensation, as disclosed in D. Pastor, J. Capmany, B. Ortega, V. Tatay, and J. Marti, "Design of apodized linearly chirped fiber gratings for dispersion compensation," IEEE J. Lightwave. Technol., 14, pp. 2581 - 2588, 1996. It has been found that the presence of a background refractive index can significantly degrade the dispersion compensating capabilities of these gratings (by introducing non-idealities in their dispersive properties) and the preferred structures are those with no background refractive index. On the other hand, the presence of a background refractive index in an apodized LCFBG creates an asymmetric spectral response which, in transmission, approximates that of an edge filter.

A linearly ramped FBG is one in which a significant portion of the transmissivity (reflectivity) varies linearly as a function of wavelength over the grating passband (stopband), see for example Figurel . Such grating structures can be realized with the following refractive index modulation describing the grating:

n₀(z) is the background refractive index and Dn(z) and L are respectively the apodization profile and grating period. The grating period is given by

Λ(z) = Λ₀ +— z A, where Λ₀ is the nominal grating period and δΛ is the grating chirp.

Linearly ramped gratings have been fabricated in the past and their use in fiber Bragg grating sensor interrogation has been recognized, see R. W. Fallon, L. Zhang, L. A. Everall, J. A. R. Williams, and I. Bennion, "All-fiber optical sensing system: Bragg grating sensor interrogated by a long period grating," Meas. Sci. Technol., 9, pp. 1969 - 1973, 1998. The linear variation in transmissivity (reflectivity) can also be used as a means for providing variable attenuation for a signal that is transmitted (reflected) through the grating: different amounts of light are transmitted (reflected) by simply changing the wavelength of the signal. In the case of a fixed wavelength signal, the variation in the amount of power transmitted (reflected) can be obtained simply by strain- or compression-tuning the grating. In fact, the grating can simply be strain- or compression-tuned until the desired amount of transmitted (reflected) power is obtained (this can be accomplished by mounting the grating onto a piezoelectric stack and applying the necessary voltages). This is the underlying principle behind the use of linearly ramped FBGs for providing power equalization in EDFA modules.

As an example, the transmission responses of two transmission edge filters based on apodized LCFBGs gratings that we have fabricated is shown in Figure 2. For both gratings, the transmissivity varies linearly on the short wavelength side with slopes of -14 dB/nm over 0.9 nm (Grating 1 ) and -13 dB/nm over 1.1 nm (Grating 2) and has a sharp rise on the long wavelength side. The measured transmitted power as a function of strain applied to the gratings using a wavelength signal fixed on the long wavelength side of the filters, 1545.3 nm for Grating 1 and 1551.05 nm for Grating 2 is also shown in Figure 2. These results clearly demonstrate the linear relation between transmitted power and strain. Note that similar results will be obtained by setting the wavelength of the input signal to the short wavelength side of the filter and applying compression tuning.

By appropriately designing the grating apodization profile, one can obtain a 20-dB change in the transmission over a 0.4 or 0.8-nm (50 or 100 GHz) range. This can provide a sufficient variation in the power transmitted by the grating. Furthermore, this can be accomplished within a narrow enough wavelength range so that the grating passband will not affect a neighboring wavelength channel. One possible configuration of a power-equalized EDFA module employing transmission edge filters is shown in Figure 3. Note that an alternate configuration can be obtained by using similarly designed reflection edge filters and including a circulator. Figure 4 demonstrates the use of the gratings for providing power equalization among input channels to a commercial EDFA. Three wavelength signals at 1544.79 nm, 1551.19 nm, and 1553.59 nm, and having > 6 dB variation in their power levels were multiplexed together and launched into the EDFA. The output power for the signal at 1553.59 nm was then used as the reference as the reference, i.e. the grating filters were tuned to equalize the power of the signals at 1544.79 nm and 1551.19 nm to that at 1553.59 nm. The additional 3-dB loss between the spectra in Figure 4(b) and 4(c) is due to splicing loss between the EDFA and the gratings and is not insertion loss of the gratings. It will be understood that feedback is used to make the gain equalization process "automatic" and dynamic (i.e. to dynamically track variations in the power levels of the different channels). For a specific channel, once the required attenuation is determined (typically using a monitor tap), this tells us exactly how much tensile or compression strain is required to be applied on the grating.

Thus, the necessary drive voltage (electrical signal) can be applied to the grating (to effectuate the tensile or compressive strain) to get the desired amount of output power.

This feedback loop requires only a single iteration and would thus reduce the "setting" time of the device to obtain equalized output. On the other hand, one can ignore the linear characteristic of the grating and simply iterate several times (ie apply different amount of tensile and compressive strain) until the desired power is obtained.

There are several advantages unique to the method and devices disclosed herein. The principle one is that since there is a linear variation in the grating transmissivity with wavelength, a similar relationship can be easily derived between the amount of transmitted power and wavelength. This allows one to know precisely how much the wavelength of the signal would need to be changed to obtain a desired amount of transmitted power. Alternatively, for a fixed-wavelength signal, this allows one to know how much the grating needs to be strain- or compression-tuned. This last feature is particularly appealing since it can be directly incorporated into a feedback loop so that dynamic, active power equalization can be achieved.

The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.

Claims

THEREFORE WHAT IS CLAIMED IS:

1. A dynamic power equalized erbium-doped fiber amplifier, comprising; an erbium-doped fiber amplifier having an input optical fiber and an output optical fiber; at least one fiber Bragg grating structure in the output optical fiber including an effective refractive index modulation to give a transmission versus wavelength characteristic having a substantially linear portion over a selected wavelength region; and means for tuning the at least one fiber Bragg grating structure for shifting the substantially linear portion over a selected wavelength region for varying the intensity of light at a selected wavelength λ, transmitted along the input optical fiber through the erbium-doped fiber amplifier an output through said at least one fiber Bragg grating.

2. The amplifier according to claim 1 wherein said at least one fiber Bragg grating structure is N fiber Bragg gratings each having a transmission versus wavelength characteristic having a substantially linear portion over a wavelength region different from all the others, including means for tuning each of said N fiber Bragg gratings for shifting the substantially linear portion over a selected wavelength region for varying the intensity of light at selected wavelengths A.,, λ₂, λ₃, ... λ_N transmitted through said at least one fiber Bragg grating structure.

3. The amplifier according to claim 2 wherein said means for tuning each of said fiber Bragg gratings is applying tensile strain to each fiber Bragg grating.

4. The amplifier according to claim 2 wherein said means for tuning each of said fiber Bragg gratings is applying compressive strain to each fiber Bragg grating.

5. A method of varying transmission of selected wavelengths of light transmitted through an erbium-doped fiber amplifier, comprising; providing an erbium-doped fiber amplifier having an input optical fiber and an output optical fiber and providing at least one fiber Bragg grating in the output optical fiber, the at least one fiber Bragg grating including an effective refractive index modulation to give a transmission versus wavelength characteristic having a substantially linear portion over a selected wavelength region; and tuning the at least one fiber Bragg grating to shift the substantially linear portion of the transmission versus wavelength characteristic over a selected wavelength region for selectively adjusting the intensity of light at a selected wavelength λ; transmitted through said at least one fiber Bragg grating.

6. The method according to claim 5 wherein said at least one fiber Bragg grating is N fiber Bragg gratings each having a transmission versus wavelength characteristic having a substantially linear portion over a wavelength region different from all the others, including tuning each of said N fiber Bragg gratings for shifting the substantially linear portion over a selected wavelength region for varying the intensity of light at selected wavelengths λ.,, λ₂, λ₃, ... λ_N transmitted through said at least one fiber Bragg grating.

7. The method according to claim 6 wherein the fiber Bragg grating is tuned by applying strain to each fiber Bragg grating.

8. The amplifier according to claim 6 wherein the fiber Bragg grating is tuned by applying compression to each fiber Bragg grating.

9. A erbium-doped fiber amplifier with power adjustment, comprising; an erbium-doped fiber amplifier having an input optical fiber and an output optical fiber; at least one fiber Bragg grating in the output optical fiber including an effective refractive index modulation to give a transmission versus wavelength characteristic having a substantially linear portion over a selected wavelength region; wavelength control means for controlling the wavelength of light signals in said selected wavelength region transmitted through said fiber amplifier for varying the intensity of light transmitted along the input optical fiber through the erbium-doped fiber amplifier and output through said at least one fiber Bragg grating.

10. The fiber amplifier according to claim 9 including feedback control means connected to said wavelength control means