Enhanced performance on extended l-band (XL) and l-band erbium doped fiber amplifier (EDFA) by manipulating amplified spontaneous emission (ASE)

Abstract

An amplifier comprises a rare-earth doped optical fiber, a signal generator that provides a signal at a signal wavelength to the rare-earth doped optical fiber, and a pump that transmits a pump wavelength to the rare-earth doped optical fiber. A reflector that causes a reflection of C-band amplified spontaneous emission (ASE) while limiting reflections in the signal wavelength and the pump wavelength.

Description

Edvold 3-8-27-8ENHANCED PERFORMANCE ON EXTENDED L-B AND (XL) AND L-B AND ERBIUM DOPED FIBER AMPLIFIER (EDFA) BY MANIPULATING AMPLIFIED SPONTANEOUS EMISSION (ASE)BACKGROUND

[0001] Various aspects of the present disclosure relate generally to optical fibers and more specifically to extended L-band (XL) and L-band erbium doped fiber amplifiers.

[0002] The increasing demand for high data-rates in optical transmission systems has driven research to exploring spatial division multiplexing, extending the spectrum for transmission, and combining both for ultimate capacity. In commercial systems, there is a move towards combing the super C-band and the extended L-band (XL), striving to achieve six-GHz bandwidth in both bands. In the XL band, the spectrum from 1575.7 nanometers (nm) to 1627.0 nm is used. The extension of the L-band has driven the development of new erbium doped fibers (EDFs) for amplifying these wavelengths.BRIEF SUMMARY

[0003] According to aspects of the present disclosure, an amplifier comprises a rare-earth doped optical fiber, a signal generator that provides a signal at a signal wavelength to the rare-earth doped optical fiber, and a pump that transmits a pump wavelength to the rare-earth doped optical fiber. A reflector that causes a reflection of C-band amplified spontaneous emission (ASE) while limiting reflections in the signal wavelength and the pump wavelength.

[0004] In several embodiments, the amplifier includes a Bragg grating on the rare-earth doped optical fiber.

[0005] In various embodiments, the reflector includes a passive optical fiber with a Bragg grating placed before the rare-earth doped optical fiber.

[0006] In some embodiments, the reflector includes a dichroic mirror before the rare-earth doped optical fiber. In several of those embodiments, the dichroic mirror is a coating of an isolator.

[0007] In many embodiments, the reflector uses both Bragg gratings and a dichroic mirror.Edvold 3-8-27-8

[0008] In various embodiments, the reflector reflects 0.01 % - 5% of the C-band ASE, and in several of those embodiments reflects 1% of the C-band ASE.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0009] FIG. l is a block diagram illustrating a measurement system for characterizing the performance of extended L-band and L-band amplifiers (including measurement components), according to aspects of the present disclosure;

[0010] FIG. 2A illustrates gain of a rare-earth doped optical fiber with and without added reflections of backwards propagating C-band amplified spontaneous emission (ASE), according to aspects of the present disclosure;

[0011] FIG. 2B illustrates noise figure of a rare-earth doped optical fiber with and without added reflections of backwards propagating C-band amplified spontaneous emission, according to aspects of the present disclosure;

[0012] FIG. 3A illustrates simulated and measured 1627-nanometer gain for a range of reflections as a function of fiber length, according to aspects of the present disclosure;

[0013] FIG. 3B illustrates simulated and measured 1627-nanometer noise figure for a range of reflections as a function of fiber length, according to aspects of the present disclosure;

[0014] FIG. 4A illustrates simulated 1627-nanometer forward signal as a function of position within a fiber, according to aspects of the present disclosure;

[0015] FIG. 4B illustrates simulated inversion as a function of position within a fiber, according to aspects of the present disclosure;

[0016] FIG. 4C illustrates simulated total forward ASE as a function of position within a fiber, according to aspects of the present disclosure; and

[0017] FIG. 4D illustrates simulated total backward ASE as a function of position within a fiber, according to aspects of the present disclosure.DETAILED DESCRIPTION

[0018] Existing extended L-band (XL-band) and L-band amplifiers for rare-earth (e.g., erbium) doped optical fibers are inefficient due to low small signal gain at long wavelengths, so it is difficult to achieve a gain at longer wavelengths (e.g., above 1620Edvold 3-8-27-8nanometers (nm). Thus, in applications that use existing amplifiers, gain of signals passed through a rare-earth doped optical fiber is attenuated to achieve the same output power for each signal. When the signals are reduced (e.g., through a gain-flattening filter), the gain is flattened to the value of the lowest signal power. In the XL case this attenuates the gain of most signals except the longest wavelength signal at e.g. 1627 nm.

[0019] According to aspects of the present disclosure, an amplifier is disclosed that amplifies the longer wavelengths through reflections of C-band amplified spontaneous emission (ASE).

[0020] Turning now to FIG. 1, an experimental setup 100 is shown for measuring XL-band gain and noise figure (NF) of a rare-earth doped optical fiber amplifier (e.g., an erbium-doped fiber amplifier (EDFA)) in a single stage both with and without added reflection.

[0021] Signal generation comprises a wideband amplified spontaneous emission (ASE) source 102 that is carved into channels and equalized using a wavelength selective switch (WSS) 104, which creates a signal of thirty-one channels of 100-GHz bandwidth and 200-GHz spacing with center wavelengths from 1575.7 nm to 1627.0 nm.

[0022] The signal passes through a switch 106, enabling reference measurement through a reference arm 108, and amplifier measurement through a measurement arm (i.e., the EDFA 110). The measurement arm includes an isolator 112 and a wavelength-division multiplexor (WDM) 114 that combines the signal with an output from a 976-nm polarization multiplexed pump 116. After the combined signal passes through an erbium-doped optical fiber (EDF) 120 (e.g., LRXL version 2 optical fiber) the signal passes through an additional isolator 122 (to create an amplified signal). Outside the EDFA 110, a 10% tap 124 acts as attenuator and combines the signal of the reference arm 108 and amplified signal. At the output of the 10% tap 124, an optical spectrum analyzer (OSA) 128 measures the reference signal and the amplified signal.

[0023] To enable a wavelength selective reflection at the input to the EDFA, a filter WDM 130 with a reflection band from 1500 nm - 1560 nm, and a pass band from 1566 nm - 1627 nm was used, which allows the input signal to pass through, while the backwards propagating ASE is reflected into a separate fiber 134, terminated by a flat connector. The total reflection from the inlet fiber in the C-band was measured to -20 dB.Edvold 3-8-27-8

[0024] The EDFA 110 reflects some or all of the C-band (i.e., 1520-1565 nm) internally while limiting reflections in the signal wavelengths and the pump wavelengths, these reflections can be obtained using Bragg gratings in the EDF 120 directly, Bragg gratings in a passive fiber placed before the EDF 120, or both. As another example to obtain the reflections, a dichroic mirror may be added before the EDF 120. For example, the dichroic mirror may be a discrete component. As another example, an existing component may be altered (e.g., altering the anti -reflection coating of the isolator 112). In some embodiments, both the Bragg gratings and the dichroic mirror may be used.

[0025] An increased scattering loss in the EDF will yield a higher gain. However, an increased scattering loss, while providing increased gain by recapturing ASE, also attenuates the light in the signal band (i.e., XL-band, L band, or both) thus decreasing gain in the signal. A simulation shows that there is an optimum amount of background scattering loss. Simulation also shows that if losses are only caused by an absorption mechanics then increased loss will only decrease the gain.

[0026] Turning now to FIGS. 2A-B, gain and noise figure (NF) curves (respectively) for the system disclosed in FIG. 1 are shown. Both the measured results (solid lines with dots) and simulated results (dashed lines) are shown by wavelength. The darker lines show the results at 18 meters (m) and -55 dB reflection, while, the lighter lines show a length of 35 meters and -20 dB reflection. Note that the curves for the simulated and the measured results at 18 meters (m) and -55 dB reflection are so close that they overlap, so only the measured results at 18 meters (m) and -55 dB reflection are visible in FIG. 2A. The measurements show an increase in gain at 1627 nm of 3.28 dB from 8.56 dB to 11.84 dB. In this case, and in general, it is most difficult to achieve gain at the long wavelengths. In applications, a gain flattening filter is used to flatten the gain of the amplifier by attenuating the other wavelengths to the lowest level. Thus, improved gain at the wavelength with the lowest level directly translates to an improvement of total amplifier efficiency.

[0027] By increasing the reflection, NF also increases. In this case an increase of 0.7 dB is seen at 1627 nm with an average increase of 0.3 dB.

[0028] For the 18 m fiber with -55 dB reflection there is near perfect agreement between measurement and simulation, and in the 35 m / -20 dB reflection case, theEdvold 3-8-27-8agreement is reasonable. This validation enables the use of the simulations for optimization and understanding of mechanisms inside the fiber.

[0029] FIGS. 3A-B illustrate simulated 1627-nanometer gain and NF for a range of reflections as a function of fiber length. The crosses show the measured results. From the figures, to maximize the gain increase, the reflection should be maximized. However, maximizing the reflections also increases the NF and the required fiber length excessively. Thus, there needs to be a compromise between gain, NF and fiber length, which may depend on the application, but generally it is shown that a significant gain improvement can be achieved at low reflections with only a limited NF penalty. For example, a -40 dB reflection would result in a 1.58 dB gain increase while only increasing 1627 nm NF by 0.25 dB and fiber length by 7.6 m.

[0030] FIGS. 4A-D show simulations that investigate the underlying mechanisms behind the improvements from the out-of-band reflection. The forward 1627 nm signal, the inversion, the total forward propagating ASE, and backwards propagating ASE are shown as a function of fiber position for two experimentally investigated cases.

[0031] With low reflections of -55 dB, the initial inversion (FIG. 4B) is around 60%, and the peak inversion appears 2.5 m into the fiber, where the C-band ASE has built up. A low direct conversion of the 976-nm pump to the XL-band signals causes a large increase in the C-band ASE. This ASE then acts as a pump that enables the amplification of the XL-band signals. Notably, the backwards ASE (FIG. 4D) at the start of the optical fiber is large in this case. The backwards ASE is generated along the whole length of the fiber which in turn steals the inversion at the very start of the fiber. The total backwards C-band ASE power was measured to 320 mW (agreeing with the simulations), which corresponds to 40% of the pump input power.

[0032] However, when just a fraction of this ASE is reflected, it acts as a seed aiding the buildup of forwards propagating ASE that in turn improves the XL-band signal gain. This completely changes the dynamic of the amplifier. The inversion peak shifts towards the start of the fiber and causes a decrease in the backwards ASE. In the case of 1% reflection, the backwards ASE is reduced to 25 mW. This decrease in backwards ASE explains the diminishing effect of increasing the reflection. The inversion profile changes and extends further while becoming flatter, requiring a longer fiber length to achieve theEdvold 3-8-27-8maximal gain at the longest wavelength. Thus, a reflection of about 0.01% - 5% of the C-band provides desired results. For example, it has been shown that a 1% reflection of the C-band ASE a significant gain improvement of 3.28 dB is obtained.

[0033] In the L-band, typically extending to 1610 nm, the dynamic of C-band ASE pumping the signals is similar, so adding a reflection of the C-band ASE may also be beneficial in that case. Preliminary L-band simulations show that improvement in gain is possible but will most likely be smaller than in the XL-band case.

[0034] The amplifiers described herein show an improved gain of 3.28 dB with a reflection of 1% C-band ASE for a single stage XL-band EDFA, which is an improvement in efficiency of the EDFA over existing EDFAs.

[0035] Aspects of the present disclosure are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the disclosure. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general -purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0036] These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function / act specified in the flowchart and / or block diagram block or blocks.

[0037] The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on theEdvold 3-8-27-8computer or other programmable apparatus provide processes for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0038] The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and / or flowchart illustration, and combinations of blocks in the block diagrams and / or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

[0039] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0040] The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of theEdvold 3-8-27-8disclosure. Aspects of the disclosure were chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

Edvold 3-8-27-8CLAIMSWhat is claimed is:

1. An amplifier comprising:a rare-earth doped optical fiber;a signal generator that provides a signal at a signal wavelength to the rare-earth doped optical fiber;a pump that transmits a pump wavelength to the rare-earth doped optical fiber; anda reflector that causes a reflection of C-band amplified spontaneous emission (ASE) while limiting reflections in the signal wavelength and the pump wavelength.

2. The amplifier of claim 1, wherein the amplifier includes a Bragg grating on the rare-earth doped optical fiber.

3. The amplifier of claim 1, wherein the reflector includes a passive optical fiber with a Bragg grating placed before the rare-earth doped optical fiber.

4. The amplifier of claim 1, wherein the reflector includes a dichroic mirror before the rare-earth doped optical fiber.

5. The amplifier of claim 4, wherein the dichroic mirror is a coating of an isolator.

6. The amplifier of claim 4, wherein the reflector further includes a Bragg grating on the rare-earth doped optical fiber.

7. The amplifier of claim 4, wherein the reflector further includes a passive optical fiber with a Bragg grating placed before the rare-earth doped optical fiber.

8. The amplifier of claim 1, wherein the reflector reflects 0.01 % - 5% of the C-band ASEEdvold 3-8-27-8 9. The amplifier of claim 8, wherein the reflector reflects 1% of the C-band ASE.

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