Broadband hybrid optical amplifier operation in eye-safe wavelength region

A hybrid optical amplifier is proposed that includes a preamplifier element formed of single-clad Ho-doped optical fiber and a power amplifier element formed of single-clad Tm-doped (or Tm—Ho co-doped) optical fiber. The preamplifier is used to impart gain to an input signal propagating at a wavelength λS in the presence of a first pump beam operating at λP1, creating an amplified output over a defined transmission bandwidth. The power amplifier element is disposed at the output of the preamplifier element and provides an additional level of gain to the output of the preamplifier element in the presence of a second pump beam operating at λP2. A passband filter may be used between the preamplifier and the power amplifier to ensure that only wavelength components within the defined transmission bandwidth are applied as an output to the power amplifier.

TECHNICAL FIELD

The present invention relates to an optical amplifier for providing amplification in the two micron wavelength region and, more particularly, to a hybrid amplifier configuration comprising a Holmium-doped (Ho-doped) preamplifier stage and a Thulium-doped (Tm-doped) power amplifier stage, each stage based upon the use of single-clad optical fiber and amplified by a pump source based on an uncooled, multi-watt laser diode.

BACKGROUND OF THE INVENTION

There is a continuing need to develop optical systems that are capable of operating in the eye-safe wavelength range that spans generally from about 1900 nm to 2150 nm. Applications such as LIDAR, atmospheric sensing (e.g., CO2), WDM communication systems, and the like, are among those that will need to rely on high performance optical devices that operate within this eye-safe wavelength region. The ability to provide a sufficient amount of signal power for these applications necessitates the development of amplifiers and high power laser sources.

While both TDFAs and HDFAs may be able to provide acceptable output power levels at these particular wavelengths, they have to date been found to be somewhat limited in terms of operating wavelength(s) that may be utilized, and may also exhibit a limited dynamic range for input signal power. The particular design of a current HDFA approach requires a complex pump scheme using multiple pump source inputs to a double-clad Ho-doped gain fiber, which may limit its efficiency. Additionally, the noise figure of these amplifier designs has not yet been fully evaluated.

The use of double clad Tm-doped fibers requires a temperature-stabilized 793 nm pump source, which is not an optimum choice for compact high performance packaged optical amplifier modules.

SUMMARY OF THE INVENTION

The needs remaining in the art are addressed by the present invention, which relates to a hybrid HDFA/TDFA using all single-clad doped fibers. The new architecture is capable of high optical output powers, exhibits a wide operating bandwidth and low NF, and additionally operates with an uncooled pump, leading to a high performance compact packaged amplifier module for operation within the eye-safe spectral region.

In accordance with the principles of the present invention, a doped fiber amplifier is proposed that includes a single-clad HDFA used as a preamplifier and a single-clad TDFA used as a power amplifier. The HDFA preamplifier provides as an output a relatively high input signal dynamic range (for example, over a wavelength range of about 2000-2150 nm) while maintaining a low noise figure value. The TDFA power amplifier is then able to provide the desired amount of gain or power to this high dynamic range preamplifier output. High gain amplifier configurations in the eye-safe 2 μm wavelength region are particularly well-suited for applications employing pulsed input sources, whose average input power is typically −20 dBm or less.

The inventive hybrid amplifier is intended to be of particular use in a master oscillator power amplifier (MOPA) configuration, with the HDFA preamplifier functioning as the seed laser input for the (TDFA) power amplifier. The HDFA preamplifier may be used in either a CW or pulse mode as the seed laser source for the MOPA.

Both the Ho-doped preamplifier and the Tm-doped power amplifier may utilize a co-propagating pump arrangement, a counter-propagating pump arrangement, or a combination of both co- and counter-propagating pump sources. In some embodiments, the gain fiber of the power amplifier may be co-doped with both Tm and Ho.

The amplifier elements may be formed of either standard single mode optical fiber (i.e., non-polarization-maintaining), or fiber of polarization-maintaining construction. For applications that operate with a single polarization signal, polarization-maintaining fiber is preferably used in order to maintain the orientation of the propagating signal along a designated axis without the need for additional polarization controlling elements.

It is contemplated that the hybrid amplifier of the present invention is an efficient amplifier by providing both gain over a long wavelength band (e.g., 2000-2150 nm) using a single-clad Ho-doped gain fiber, and power amplification of this extended wavelength band by using a single-clad Tm-doped gain fiber. It has been found that single-clad Tm-doped fiber can exhibit a pump-to-signal efficiency on the order of 80%, a significant improvement over prior art configurations using double-clad TDFAs.

Preferred embodiments of the present invention are based on the use of an uncooled pump to generate pump beams at the wavelengths required for both the HDFA preamplifier and TDFA power amplifier. The use of an uncooled pump eliminates the need to include temperature control circuitry within the amplifier module, therefore improving the efficiency of the amplifier and simplifying the control circuitry associated with the pump sources.

An exemplary embodiment of the present invention may take the form of a hybrid optical amplifier for operation within an eye-safe wavelength region from about 2000 nm to about 2200 nm, where the hybrid optical amplifier includes a preamplifier element formed of a section of single-clad Ho-doped optical fiber and a power amplifier element formed of a section of single-clad Tm-doped (or Tm—Ho co-doped) optical fiber. The preamplifier is used to impart gain to an input signal propagating at a wavelength λSwithin the eye-safe wavelength region in the presence of a first pump beam operating at a first pump wavelength λP1, creating an amplified output over a defined transmission bandwidth within the eye-safe wavelength region. The power amplifier element is disposed at the output of the preamplifier element and provides an additional level of gain to the output of the preamplifier element in the presence of a second pump beam operating at a second pump wavelength λP2. A pump supply is included and used to provide pump beams at appropriate wavelengths to both the preamplifier element and the power amplifier element.

When the connection to an input optical signal is removed, the amplifier arrangement of the present invention may be used as a broadband ASE source, based upon the interaction of the pump beams with the gain fibers in each element.

Other and further embodiments and features of the present invention will become apparent during the course of the following discussion and by reference to the related drawings.

DETAILED DESCRIPTION

FIG.1illustrates an exemplary hybrid fiber amplifier10formed in accordance with the principles of the present invention. Here, the term “hybrid” is used to describe the use of two different rare-earth dopants (Ho and Tm), in two separate amplifier stages, to provide amplification of a propagating input signal. As mentioned above, the inventive hybrid amplifier is particularly designed to operate within an eye-safe wavelength range (e.g., from about 2000-2150 nm).FIG.1depicts an input optical signal (SIN) operating at a wavelength λSwithin this range.

Hybrid fiber amplifier10is shown as including a preamplifier stage comprising a Ho-doped fiber amplifier (HDFA)12, followed by a power amplifier stage formed as a Tm-doped fiber amplifier (TDFA)14(in some configurations, as described below, the gain fiber in the power amplifier stage may comprise a co-doped Ho—Tm fiber). Preamplifier HDFA12is focused primarily on creating gain for input signals across a relatively high dynamic range in a bandwidth from 2000-2150 nm, while maintaining a relatively low noise figure (NF). The ability to provide at least a moderate level of amplification over a wide dynamic range is important, since most conventional semiconductor laser diodes that emit in the eye-safe wavelength range are limited in the amount of power they are able to generate.

HDFA12includes a section of single-clad Ho-doped optical fiber16(referred to at times below as the “gain fiber” of HDFA12), where an inset inFIG.1depicts the “single clad” cross-section of optical fiber16as including a Ho-doped core region1and a surrounding cladding layer2. The incorporation of Ho+ions in the silica core of a single-clad optical fiber is known in the art to provide amplification of a propagating light signal in the presence of a pump beam operating at an appropriate wavelength. As shown inFIG.1, a first pump beam P1operating at a pump wavelength λP1=1940 nm is used to interact with the Ho+ions in core region1of gain fiber16in a manner that imparts gain to input signal SIN.

It is to be understood that the use of a preamplifier pump at the wavelength of 1940 nm is only one of several choices to provide amplification in the presence of Ho ions. Other pump wavelengths, corresponding to other absorption bands of Ho may be used in the HDFA preamplifier of the present invention (e.g., λP1in the range of about 1125-1150 nm, or about 1230 nm, etc.). Moreover, it has been found that increasing the pump wavelength to 2000 nm or greater shifts the spectral operating band of the preamplifier to markedly higher wavelengths, enabling access to higher output wavelengths with significant power in the region of about 2130 nm. Additionally, it is contemplated that shifting the pump wavelength to 1880 nm or lower may significantly improve the gain and output power performance of an HDFA preamplifier and, therefore, pump wavelengths in the range of 1840-1960 nm, and perhaps up to 2000 nm, should be considered as another set of available values for use in the configurations of the present invention.

A wavelength division multiplexer (WDM)18is used in this embodiment to combine the incoming optical signal SIN(operating at a wavelength λSwithin the eye-safe wavelength region) and first pump beam P1, introducing the combination into HDFA12. In particular, both SINand P1are coupled into Ho-doped core region1of single-clad Ho-doped optical fiber16. The optical energy at pump wavelength λP1interacts with the Ho+ions and transfers energy to propagating signal SIN, thus providing an amplified version of the input signal (denoted here as SA1) as the output from HDFA12. Throughout the remainder of this description, the output from HDFA12may also be referred to as the “intermediate” (i.e., inter-stage) amplified signal. Various factors influence the dynamic range achieved by HDFA12while maintaining a relatively low noise figure. For example, pump input power and pump wavelength, as well as the length and dopant concentration of gain fiber16, are but a few factors that may be adjusted to maximize the results.

TDFA14is shown inFIG.1as separated from HDFA12by an optical filter19that takes the form of a passband filter (as depicted in the diagram), with a passband coextensive with the bandwidth provided by HDFA12(e.g., 2000-2150 nm). While not required, the inclusion of passband filter19is useful in shaping the output from HDFA12so that only the transmission band of interest is applied as an input to the power amplifier TDFA14. Additionally, the presence of passband filter19prevents any amplified spontaneous emission (ASE) from TDFA14from propagating in the reverse direction into HDFA12. An optical isolator may also be disposed at the interface between HDFA12and TDFA14, with similar isolators positioned at the input and output of hybrid optical isolator10. Even though these isolators have not been particularly described or enumerated, it is understood that isolators of this type prevent the propagation of reflections (include pump beams, as discussed below) within the inventive hybrid amplifier, where the reflections are known to increase the noise level within the amplifier and diminish the available output power. Similar isolators are illustrated in the various other embodiments as will be discussed in detail below, and perform a similar well-known function.

Turning now to the particulars of TDFA14, it is seen as including a section of single-clad Tm-doped optical fiber20. An inset associated with optical fiber20shows a Tm-doped core region3surrounded by a single cladding layer4. A second pump beam P2, here operating at a wavelength λP2appropriate for interacting with Tm+ions (for example, λP2of about 1560 nm) is passed through a WDM22positioned at the output of optical fiber20. WDM22functions to inject second pump beam P2into core region3of single-clad Tm-doped optical fiber20, where it interacts with the amplified output signal SA1from HDFA12(which may be a filtered version of the preamplifier output if passband filter19is used). The use of a relatively high-power pump beam P2provides the gain necessary for a specific application, creating the amplified output signal SAMPfrom hybrid amplifier10, as shown inFIG.1. TDFA14is used to efficiently create the majority of signal power provided by hybrid amplifier10, taking full advantage of highly efficient ion-ion interactions associated with the use of Tm dopant within gain fiber20.

While HDFA12is depicted as a “co-propagating” amplifier arrangement where the input optical signal SINand pump beam P1propagate in the same direction through single-clad Ho-doped gain fiber16, TDFA14is configured in this embodiment as a counter-propagating amplifier arrangement, where second pump beam P2is coupled into the output of single-clad Tm-doped optical fiber20and therefore propagates through gain fiber20in a direction counter to that of intermediate amplified signal SA1.FIG.2illustrates an alternative embodiment of the present invention, denoted as hybrid amplifier10A, which uses a different configuration of pump sources.

In particular,FIG.2illustrates an HDFA12A that utilizes a counter-propagating pump beam P1to provide amplification within single-clad Ho-doped fiber16. A WDM18A is shown as positioned beyond the output of gain fiber16and is configured to couple pump beam P1into the output of gain fiber16so that it propagates counter to the incoming optical signal SIN. Also shown in this embodiment is the use of a co-propagating pump beam P2with TDFA14. A WDM22A is positioned at the input of single-clad Tm-doped fiber20and used to couple both the amplified output from HDFA12A and pump beam P2into gain fiber20.

For various applications, it is preferred to use polarization-maintaining (PM) fiber along the signal paths of HDFA12and TDFA14, including the use of PM fiber in the formation of single-clad Ho-doped gain fiber16and single-clad Tm-doped gain fiber20. In particular, when there is a need to provide a consistent state of polarization of a propagating optical signal, the use of PM fiber maintains the orientation of the propagating signal along a designated axis without the need for additional polarization controlling elements. While PM fiber is preferred for use along the signal path, pump source30and the pump paths between source30and each amplifier stage are generally formed of standard single mode optical fiber.

Other arrangements for providing additional output power may utilize a power amplifier that is co-doped with both Ho and Tm. Given the “hybrid” design of the present invention, the use of a co-doped gain fiber within the power amplifier allows for any residual preamplifier pump light that was not absorbed by the preamplifier's Ho-doped gain fiber to thus provide additional amplification by reacting with the Ho dopant included within the co-doped gain fiber of the power amplifier. The use of a co-doped gain fiber within the power amplifier may also eliminate the need to perhaps include a filter at the output of the preamplifier to remove residual pump, where the need to include such a filter inevitably introduces unwanted loss into the propagating signal.

As mentioned above, an aspect of the present invention is the use of an uncooled pump source, which eliminates the need to incorporate circuitry required for monitoring and controlling the operations of laser diodes that are often used to supply pump beams.

FIG.3illustrates an embodiment of the present invention, denoted as hybrid amplifier10B that utilizes a single pump source30to provide both pump beam P1(operating at λP1) to HDFA12and pump beam P2(operating at λP2) to TDFA14. In accordance with the principles of the present invention, a multi-watt semiconductor laser diode32included within pump source30is operated as an uncooled device, providing pump energy at a wavelength λLDof 940 nm. Input pump beam PINis shown as then passing through a fiber laser34, which in this case comprises a co-doped Er—Yb fiber laser. Input pump beam PINinteracts with the co-doped gain fiber within fiber laser34in a manner that creates as an output beam operating at a wavelength suitable for use as one of the pump inputs to the amplifier stages. In this particular example, fiber laser34is configured to specifically provide an output beam operating at 1560 nm, which as described above is used as second pump beam P2to generate gain within TDFA14.

Continuing with the description of pump source30, pump beam P2output from fiber laser34is next passed through a pump power splitter36, which directs a first portion of pump beam P2along a first pump path30-1toward HDFA12and directs a second portion of pump beam P2along a second pump path30-2toward TDFA14. While the power ratios used by splitter36may vary from application to application, it is often the case where an equal amount of pump power is sent along each path (i.e., splitter36is configured as a 50/50 power divider). As mentioned above, the portion of pump beam P2propagating along second pump path30-2is operating at a pump wavelength suitable for providing gain within TDFA14. Therefore, second pump path30-2is shown as coupled to the pump input port of WDM22.

Referring now to the details of providing a pump input at an appropriate wavelength λP1for HDFA12,FIG.3illustrates pump source30as further comprising an additional fiber laser38that is disposed along pump path30-1toward HDFA12. In particular, fiber laser38comprises a Tm-doped fiber laser that is configured to shift the wavelength of the propagating pump beam from λP2=1560 nm to the desired λP1value of 1940 nm. The output from fiber laser38is thus defined as first pump beam P1(operating at λP1=1940 nm), which is provided as the pump input to WDM18, as shown inFIG.3.

Also shown in the embodiment ofFIG.3is a pair of fiber Bragg gratings (FBGs)24,25may be used in conjunction with TDFA14and HDFA12, respectively. FBG24functions as a wavelength-dependent filter and is particularly formed to have a center wavelength that matches the wavelength λP2of second pump beam P2. Depending on a variety of parameters, including the length of gain fiber20and the output power of pump beam P2, there may be “residual” pump energy that exits gain fiber20and continues to propagate in the counter direction along the signal path of hybrid amplifier10B. The inclusion of FBG24functions to reflect (re-direct) pump wavelength λP2, so that any residual pump will pass a second time through gain fiber20(imparting additional gain to the propagating signal SA1). FBG25is similarly formed and provides a similar function, but in this case exhibits a center wavelength of λP1. It is to be noted that hybrid amplifier10B is illustrated as not including passband filter19, since as mentioned above the filter is useful, but not required.

FIG.4contains a plot of an exemplary spectral response from an exemplary preamplifier HDFA12over a portion of the eye-safe wavelength region, based upon an input signal SINoperating at an input signal wavelength λS=2051 nm (i.e., an input wavelength within the eye-safe band). For the purposes of collecting this data, input signal SINwas configured to maintain a constant input power of −1.6 dBm. The data is associated with the configuration as shown inFIG.3, with pump beam P1having a power of about 2.5 W at the input of HDFA12. The spectral response as shown inFIG.4includes an output power peak I of about 0.580 W for input signal SINat its propagating wavelength λSof 2051 nm. Also evident in the spectral response is a second peak (denoted II inFIG.4), which is associated with a pump feedthrough power. By virtue of using a co-propagating arrangement, the output from HDFA12thus also includes some remaining power at the pump wavelength. In this case, pump power peak II is about 23 dB below the signal peak at I, indicating efficient use of pump power within HDFA12.

FIG.5is a plot of the output spectrum of the full hybrid amplifier10; that is, the output from TDFA14for the configuration as shown inFIG.3. For this particular data, pump beam P2was configured to exhibit a power of about 5 W at the input to TDFA14. Here, the total signal output power is shown to be about 2.93 W, and the total pump feedthrough power is now about 14 dB down from the peak of the signal, the relative increase in the pump peak being an indication that TDFA14provides some additional amplification to this input pump light. Based on a −10 dB width of the ASE below the signal peak, an estimated 3 dB output power bandwidth of hybrid amplifier10is shown to be about 95 nm, spanning the eye-safe wavelength range from 2000 nm to 2095 nm.

An important aspect of the present invention is the use of an uncooled laser diode as the “seed” for generating multi-watt pump beams at the wavelengths appropriate for use with both types of gain fiber.FIG.6illustrates the signal power at the output of hybrid amplifier10(i.e., the power of amplified output signal SAMP) as a function of the input “seed” power delivered by laser diode32to pump fiber laser34, using the same input parameters as described above. The maximum output signal power for SAMPis shown to be about 2.93 W, associated with a pump power of 33.7 W from laser diode32. The variation of output power with pump power is shown as following a linear trend above 9 W with a slope of 10.5%.

FIG.7shows both experimental and simulated internal gain (G) and NF values for the hybrid amplifier10over a power range for input signal SINfrom about −35 dBm to about +5 dBm (all measured for SIN@ λS=2051). In these plots, the individual points are associated with measured experimental results, and the lines are from simulation results. The maximum small signal gain at λS=2051 nm is measured to be 49.1 dB, with the simulated gain defined as 51.5 dB, which agrees relatively well with the measured experimental data. The minimum experimental small signal NF is 6.45 dB, and the simulated value is 7.23 dB, again indicating good agreement. This high G and low NF indicate that the inventive hybrid amplifier is useful as a preamplifier for lightwave communication systems, and also as an amplifier for pulsed input signals with low duty cycles. The evolution of G and NF with Psshows a good match between experiment and simulation and follows the expected trend for a high gain fiber optical amplifier. The input signal dynamic range for G>35 dB is 37.4 dB.

It is to be noted that the lengths optical fiber chosen for the Ho- and Tm-doped fibers in the configuration used to create the data shown inFIGS.4-7was optimized for a signal wavelength λSof 2051 nm. The utilization of somewhat longer fiber lengths is expected to extend the spectral response of the inventive hybrid amplifier to wavelengths as high as 2150 nm. Additionally, the maximum output power is presumed to scale linearly with available pump power and, therefore, an output signal power of 10 W should therefore be possible using a pump source such as source30shown inFIG.3.

Another factor that influences amount of amplification that as achieved, as well as the gain bandwidth and noise figure, is the propagation direction of the pump beam through the amplifying medium. As shown and discussed above, hybrid amplifier10B ofFIG.3is based upon the use of a co-propagating pump beam P1in HDFA12and a counter-propagating pump beam P2in TDFA14.

FIG.8illustrates an alternative embodiment of the present invention, denoted as hybrid amplifier10C, which uses counter-propagating pump inputs for both amplifier stages. In particular, hybrid amplifier10C includes a counter-pumped HDFA12C, where in this case a WDM18C is disposed beyond the output of single-clad Ho-doped gain fiber16and is used to direct first pump beam P1(operating at λP1of 1940 nm) into the output of gain fiber16. Pump beam P1thus propagates in the opposite direction as input optical signal SIN. In contrast to the co-propagating arrangement for HDFA12ofFIG.3, counter-propagating pump beam P1in the arrangement ofFIG.8interacts with propagating input signal SINin a very different manner. In particular, since the power level of pump beam P1is greatest at the far-end of single-clad Ho-doped optical fiber18and thereafter diminishes as it propagates towards the input end of gain fiber12(where the power Psof the input signal SINis the greatest), the interaction provides greater slope efficiency and power conversion efficiency for a similar amount of gain (in terms of magnitude). The use of a counter-propagating pump in preamplifier HDFA12also prevents feedthrough of the pump to power amplifier TDFA14, resulting in higher signal power at the output of hybrid amplifier10C.

In some configurations of hybrid amplifier10C, an FBG26, with a center wavelength of λP1may be included at the input to HDFA12C and used in the same manner as FBGs24,25discussed above (that is, re-direct residual pump energy back into HDFA12C). The remaining components of hybrid amplifier10C, including both TDFA14and pump source30, are essentially the same (and include the same elements), as discussed above in association with hybrid amplifier10B ofFIG.3. Thus, in similar fashion, an incoming optical signal SINfirst passes through HDFA12C and is initially amplified to create intermediate amplified signal SA1. This amplified output from HDFA12C is then passed through TDFA14, where the presence of pump beam P2interacts with the Tm ions in gain fiber20to generate amplified output signal SAMPas the output of hybrid amplifier10C.

FIG.9is a plot of the simulated amplifier output SA1from HDFA12C. In comparison with the plot ofFIG.4, it is observed that the output power of intermediate amplified signal SA1is somewhat greater than that provided by the co-propagating pump configuration ofFIG.3(i.e., 0.916 W vs. 0.580 W), an increase of about 2.31 dB. Inasmuch as pump beam P1propagates in the reverse direction through gain fiber16, there is no “feedthrough” of the pump into amplified output signal SA1and thus no spike in output at the pump wavelength.FIG.10is a plot of the simulated amplified output SAMPfrom hybrid amplifier10C. In comparison to the results shown inFIG.5for hybrid amplifier10B, the total output signal power for hybrid amplifier10C is shown to be about 4.178 W, an increase of 2.31 dB over the arrangement ofFIG.3using a co-propagating pump in HDFA12. A bandwidth of at least 95 nm, and most likely somewhat greater, is expected to be achieved with this configuration.

While not explicitly illustrated, it is to be understood that other configurations of a hybrid HDFA/TDFA formed in accordance with the principles of the present invention may utilize co-propagating pump inputs for both the HDFA and TDFA, or even an arrangement where the TDFA stage is counter-pumped and the HDFA stage is co-pumped. All of these arrangements are considered to fall within the scope of the present invention.

Besides the use of WDMs (in combination with isolators) to introduce the pump beams to the amplifier stages, other embodiments of the present invention may use an optical circulator to perform this function.FIG.11illustrates an exemplary hybrid amplifier10D that embodies this approach. As with hybrid amplifier10C discussed above in association withFIG.8, hybrid amplifier10D ofFIG.11utilizes essentially the same TDFA14and pump source30.

However, instead of utilizing a WDM to introduce a counter-propagating version of first pump beam P1into single-clad Ho-doped fiber16, HDFA12D includes an optical circulator40to perform this function. Optical circulator40is arranged as a three-port circulator, with the set of ports denoted “A”, “B”, and “C”. In this case, port A is used as a “pump input”, port B is used as a “pump output”/“amplified signal input”, and port C is used as the “amplified signal” output. First pump beam P1is shown as entering port A of optical circulator40, and thereafter exiting at port B to propagate through single-clad Ho-doped gain fiber16of HDFA12D. In some applications, FBG26, with a center wavelength of λP1may be disposed at the input to HDFA12D and used in the manner described above to reflect any remaining pump beam P1back into gain fiber16.

Input signal SIN, interacting with this counter-propagating pump light in the same manner as described above, creates first amplified signal SA1, which is shown as provided as an input at port B of optical circulator40. Amplified signal SA1thereafter exits optical circulator40at port C, which is coupled to TDFA14and, more particularly, to single-clad Tm-doped gain fiber20(it is to be noted that there is no need to include an optical isolator between stages by virtue of using uni-directional optical circulators).

A slightly different configuration of a hybrid HDF/TDF amplifier is shown as amplifier10E inFIG.12. This embodiment is similar to amplifier10D as discussed above in association withFIG.11, but in this case includes a gain-shaping filter46between the output of HDFA12E and the input of TDFA14. As known in the art, and discussed in detail in our co-pending application Ser. No. 16/864,528, filed May 1, 2020 and incorporated herein by reference, a gain-shaping filter may be included at the interface between the amplifier stages to adjust the spectral response and power density of the output from HDFA12E. In some cases, the inclusion of GSF46allows for TDFA14to impart a useful amount of gain over a wider bandwidth of operation within the eye-safe wavelength region.

FIG.13illustrates another embodiment of a hybrid HDFA/TDFA amplifier formed in accordance with the present invention. Identified as hybrid amplifier10F, this arrangement is particularly configured to provide amplification of a pulsed input signal (denoted SP_IN). As with the arrangements described above, hybrid amplifier10F includes a TDFA14and pump source30that are essentially the same as those originally described in association with hybrid amplifier10ofFIG.3.

In this case, however, a four-port optical circulator42is included within an HDFA12F and used to efficiently insert a narrowband FBG centered at pulse signal wavelength λSat the output of HDFA12. As with hybrid amplifier10B discussed above, pump beam P1is introduced at port A of circulator42and thereafter directed into gain fiber16of HDFA12F. Again, pump beam P1propagates in a direction counter to the incoming pulses, and may be reflected by FBG24to pass a second time through gain fiber16.

Amplified output pulses SP_A1exiting gain fiber16are shown inFIG.13as being directed into port B of optical circulator42, where they propagate along within circulator42until reaching port C. In accordance with this embodiment of the present invention, a reflective FBG44is shown as coupled to port C of optical circulator42. Reflective FBG44is formed to have a center wavelength that matches the wavelength λSof input signal SP_IN, as well as an extremely narrowband bandwidth (on the order of 1 nm or less) to remove a significant majority of the ASE in the amplified output SP_A1from gain fiber16. While pulsed input signal SP_IN(operating at wavelength λS) is amplified within gain fiber16, broadband spontaneous emission in a region surrounding this wavelength is also present in the amplified output, as a result of the pulsed nature of the signal. Reflective FBG44is thus used in this embodiment of the present invention to remove a substantial portion of this background emission and provide a “clean” (filtered) amplified pulse train SP_A2as the input to TDFA14. The output from TDFA14, as discussed above, is the final amplified version of the input pulse train.

Besides using specific, different pump wavelengths to create gain within the Tm-doped and Ho-doped optical fibers as described in above embodiments, it is also possible to utilize “in-band” pumping at a single, common pump wavelength (denoted λPC) that is able to create gain within both types of fiber.

FIG.14illustrates an exemplary hybrid HDFA/TDFA50that is formed in accordance with the present invention to utilize in-band pumping. Similar to the arrangements described above, hybrid amplifier50includes an HDFA52as an input stage, and a TDFA54as an output stage. As shown, hybrid amplifier50includes a pump source56that is used to supply a common pump beam PCat an in-band wavelength (shown here as 1880 nm) that is able to provide amplification in both a single-clad Ho-doped optical fiber58within HDFA52and a single-clad Tm-doped optical fiber60within TDFA54.

In accordance with this embodiment of the present invention, pump source56is based upon the use of an input laser diode62(similar to the arrangements described above) to provide a pump input “seed” at a wavelength λLDof 940 nm. The emission from laser diode62is shown as applied as an input to a first fiber laser64, which comprises an Er—Yb doped fiber laser that is configured to provide an output beam operating at a wavelength of 1560 nm (as discussed above). The output from first fiber laser64is then passed through a second fiber laser66, which in accordance with this embodiment of the present invention comprises a Tm-doped fiber laser that is configured to provide a pump beam output PCat an in-band wavelength of 1880 nm that is able to impart amplification within both Ho-doped fiber58and Tm-doped fiber60. An optical splitter68is included in pump source56to provide divide pump beam PC, similar to the configurations described above, directing a first portion PC1toward HDFA52and a second portion PC2toward TDFA54.

In the arrangement ofFIG.14, a first optical circulator70is used to direct the flow of beam PC1in a counter-propagating direction through Ho-doped fiber58, and ultimately direct the amplified output signal SACout of port C and toward TDFA54. A second optical circulator72is similarly utilized as part of TDFA54to introduce pump beam PC2into Tm-doped fiber60, providing the amplified output signal SAMPout of its port C, as shown inFIG.14.

FIG.15illustrates an alternative embodiment of the present invention. Here, a hybrid amplifier80is shown as including a multi-stage preamplifier82that is used in combination with a single stage power amplifier84. In this particular configuration, multi-stage preamplifier82comprises a pair of concatenated amplifier stages86,88, with each amplifier stage taking the form of a single-clad HDFA. A single pump source90is used in this particular arrangement to supply the pump light input at λP1to both first stage86and second stage88(alternatively, it is to be understood that each stage may include its own pump source, operating at a power appropriate for that stage).

A power splitter92is used in this particular embodiment to control the ratio of pump powers within the preamplifier stages, creating two separate pump beams. A first pump beam P1Aoutput from power splitter92(operating at a first power level PP1A) is provided as the pump input to first stage86, with a second beam P1B(operating at a second power level PP1B) provided as the input source for second stage88(where the sum of PP1Aand PP1Bis ideally equal to the input power PP1of pump source90).

Referring now in particular to first preamplifier stage86, the incoming signal SINand first pump beam P1Aare provided as inputs to a first WDM94, which directs both beams along a common output fiber, which in this case is a first section of single-clad Ho-doped gain fiber96(having a length L1). The output from first preamplifier stage86, designated SA1A, is then provided as an input (amplified) signal to second preamplifier stage88. As shown, a second WDM98is disposed to receive this amplified signal SA1A, as well as the larger portion (P1B) of the pump beam. The combination of these two lightwaves is then coupled into a second section of single-clad Ho-doped gain fiber100(having a length L2), creating the preamplifier output SA1B.

Turning now to TDFA84of hybrid fiber amplifier80, the amplified output SA1Bfrom preamplifier82, and a second pump beam P2from pump source102is coupled into the Tm-doped core region of single-clad gain fiber104via a WDM106. The combination of the pump beam and signal within Tm-doped gain fiber104provides the final amplified output signal from fiber amplifier80, denoted as amplified output signal SOUTinFIG.15.

In another application, the inventive hybrid HDFA/TDFA may be used to generate an ASE optical beam. There are applications where there is a need to provide a broadband “noise” signal with a relatively high level of optical power (for example, as an input seed source for fiber optic gyroscopes).FIG.16illustrates an exemplary ASE source160formed in accordance with the present invention to provide this broadband ASE output. In contrast to the arrangements described above that are specifically directed to the amplification of an applied input signal, ASE source160provides a broadband continuum output associated with the propagation of only pump light through the sections of Ho-doped gain fiber and Tm-doped gain fiber.

ASE source160is shown as including a first stage162based upon the use of a section of single-clad, Ho-doped optical fiber164. A second stage166is based upon the use of a section of single-clad, Tm-doped optical fiber168. A pump source170, which may utilize separate, discrete devices or instead comprise an arrangement similar in form to pump source30described above, is used to provide pump light at wavelengths appropriate for generating spontaneous emission within each stage. That is, pump source170provides a first pump input PIN1at a wavelength λIN1of about 1940 nm to first stage162via a first WDM172, and a second pump input PIN2at a wavelength λIN2of about 1560 nm to second stage166via a second WDM174. In this particular configuration, both pump beams are applied in the “counter-propagating” direction with respect to the ASE output.

Also included in ASE160is a reflective termination176disposed at the input of single-clad Ho-doped gain fiber164of HDFA162. Reflective termination176may be formed as a beveled endface of gain fiber164(with a bevel angle of 8°, for example). It is clear that this reflective termination substitutes for the application of an input signal in the case of a hybrid amplifier. Therefore, when used to provide a source of ASE output, pump beam PIN1will propagate in the reverse direction along gain fiber164, then exit through termination176without reflection. Similarly, backward-propagating ASE will exit through termination176without reflection. In this manner, both the counter-propagating pump and ASE energy will have no additional effect on the forward-propagating ASE output spectrum. Alternatively, termination176may be spliced to an optical isolator (not shown), allowing for the forward ASE light of first stage162to be available as an additional broadband source as well.

In a similar manner, ASE output from first stage162is thereafter passed through single-clad Tm-doped gain fiber168of second stage166, with the additional amplification thus forming the final ASE output. Included with second stage166is an FBG178, positioned at the input to gain fiber168and configured to have a center wavelength value of λIN2. Thus, second pump beam PIN2will interact with FBG178in a manner that re-directs any remaining pump energy to pass a second time through gain fiber168and increase the amount of output power present in the output. While not rising to the amplified level of an input signal, this pump light is also sufficiently amplified, providing the relatively broadband ASE output as shown inFIG.17.

In particular, the simulation shown inFIG.17is for a configuration where first stage162is counter-pumped with a pump beam P1operating at a wavelength λIN1of 1880 nm, and having an output power of about 3 W. Second stage166was also counter-pumped for this simulation, using a 5 W pump beam P2at a wavelength λIN2of 1560 nm. Over 99% of the broadband output shown inFIG.17resides within the wavelength region of 1900-2200 nm, with a 10 dB spectral width spanning a 76 nm band from 2000-20076 nm.

FIG.18illustrates an alternative embodiment of an ASE source formed in accordance with the principles of the present invention. In particular, ASE source160A is based upon the use of an optical circulator180in combination with single-clad Ho-doped single clad gain fiber164. Similar to the arrangements using optical circulators as described above, pump light at λIN1is applied as an input to port A of optical circulator180. The pump light exits optical circulator180at port B and is introduced into the output of gain fiber164, propagating in the counter direction along the fiber. The pump light will be reflected at element176, pass again through gain fiber164(where it receives additional spectral broadening and amplification), and then be injected into port B of optical circulator100. Again, the ASE generated by gain fiber164will travel through optical circulator180and ultimately exit the device at port C. This amplified pump light then passes through second stage166in the same manner as described above to generate the ASE output from source160A. In this particular arrangement, pump source30, as discussed above, is used to provide the pump beams at λIN1and λIN2.

While certain preferred embodiments of the present invention have been illustrated and described in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the claims appended hereto. Indeed, the described embodiments are to be considered in all respects as only illustrative and not restrictive.