Broadband Tm-doped optical fiber amplifier

A broadband optical amplifier for operation in the 2 μm visible wavelength band is based upon a single-clad Tm-doped fiber amplifier (TDFA). A compact pump source uses a combination of low-power laser diode with a fiber laser to provide a multi-watt pump beam without needing to include thermal management and/or pump wavelength stability components. The broadband optical amplifier is therefore able to be relatively compact device with fiber coupled output powers of >0.5 W CW, high small signal gain, low noise figure, and large OSNR, important for use as a versatile wideband preamplifier or power booster amplifier.

TECHNICAL FIELD

The present invention relates to rare-earth doped optical fiber amplifiers and, more particularly, to a Thulium (Tm)-doped fiber amplifier particularly configured to operate over a relatively wide bandwidth within the 2 μm wavelength region.

BACKGROUND

There is a continuing need to develop optical systems that are capable of operating in the eye-safe 1.90-2.15 μm wavelength range. 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 2 μm wavelength region. In many situations, the amount of physical space that may be dedicated to these optical systems is severely limited (e.g., vehicle-based LIDAR systems) and the ability to provide sufficient amplification within a space somewhat less than the footprint of a typical smartphone is desirable.

To date, multiwatt Tm-doped fiber amplifiers (TDFAs) have been one option for providing signal gain in this 2 μm wavelength band. In most cases, the TDFAs are based upon the use of a double-clad (or triple-clad) gain fiber, where the multiple cladding layers not only result in a fiber coil of relatively large size, but also have particular coupling/connection requirements to input and output fibers.

SUMMARY OF THE INVENTION

The needs remaining in the art are addressed by the present invention, which relates to rare-earth doped fiber amplifiers and, more particularly, to a Thulium (Tm)-doped fiber amplifier particularly configured to operate over a relatively wide bandwidth within the 2 μm wavelength region.

In accordance with the principles of the present invention, a section of single-clad, Tm-doped optical fiber is used as the gain element. The use of a single-clad optical fiber allows for both the propagating optical signal and pump beam to be coupled into the core region of the fiber.

An intent of the inventive Thulium-doped fiber amplifier (TDFA) is to provide a miniaturized device with fiber coupled output powers of >0.5 W CW, high small signal gain, low noise figure, and large OSNR that can be used as a versatile wideband preamplifier or power booster amplifier.

An exemplary embodiment of the present invention takes the form of an optical amplifier for operation at an eye-safe input signal wavelength λswithin the 2 μm region, and uses a section of single-clad optical gain fiber in combination with a fiber laser-based pump source to form a relatively compact amplifier component. In particular, the section of single-clad optical gain fiber includes a Tm-doped core region, where an input signal is coupled into an input endface of the Tm-doped core region of the single-clad optical gain fiber so as to propagate therealong and exit at an output endface thereof. The pump source includes a low power (e.g., milli-watt) laser diode source, which is used with a fiber laser to create multi-watt pump light at a defined pump wavelength λP2.

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. Moreover, the propagating signal may take the form of a pulsed input signal (for transmission of digital data, for example) or a continuous wave (CW) optical input. In yet another embodiment, an amplifier of the present invention may utilize only a pump light input, generating amplified spontaneous emission (ASE) over a relatively broad range, where ASE is often used as a continuum source in systems supporting the transmission of multiple wavelengths.

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

DETAILED DESCRIPTION

FIG. 1contains a block diagram of an exemplary Tm-doped fiber amplifier (TDFA)10formed in accordance with the principles of the present invention. TDFA10is based upon the use of a section of single-clad optical fiber12that has been fabricated to include a Tm-doped core region1, surrounded by a silica-based cladding layer2(shown in the inset ofFIG. 1). In contrast to various prior art TDFAs that are based upon the use of at least a “double-clad” gain fiber, the arrangement of the present invention operates in a limited region of output power that allows for a single-clad optical fiber to be used as the gain fiber. Advantageously, the use of single-clad optical fiber simplifies the optical coupling into and out of the gain fiber, while also allowing the final product to fit within a relatively small footprint.

As mentioned above, recent developments in various types of optical-based sensing applications have created the need for optical systems that operate in the eye-safe wavelength band of 1.9-2.1 μm. One need is for an optical amplifier that is able to impart an appreciable level of gain to input signals across a relatively wide wavelength region within this eye-safe band; that is, there is an on-going need for an eye-safe broadband amplifier (the ability to function over a broad input signal range particularly useful when implemented as a preamplifier component).

FIG. 1shows an input optical signal (denoted SIN) operating within this eye-safe band being used as the input signal for TDFA10. For the sake of explanation, input signal SINis defined as being a single frequency input, operating at an input wavelength λsof 1909 nm (i.e., within this general 2 μm band associated with “eye safe” optical signals). Input optical signal SINis shown inFIG. 1as passing through an input isolator14and thereafter coupled to a signal port of a wavelength division multiplexer (WDM)16. Pump light LP2from a multi-watt pump source18operating at an appropriate pump wavelength λP2(here, shown as about 1567 nm) is coupled into a pump port of WDM16, with the output from WDM16being a combination of both the input signal SINand pump light LP2. The combination of SINand LP2is thereafter applied as an input to single-clad gain fiber12. In particular, WDM16is configured to couple these inputs into Tm-doped core region1of single-clad gain fiber12.

The arrangement as shown inFIG. 1is referred to as a “co-propagating” amplifier configuration since both the input signal SINand pump light LP2are coupled into the same end of single-clad gain fiber12. Amplification of input signal SINwithin single-clad gain fiber12is achieved via a process well-known in the art where presence of pump light LP2at an appropriate wavelength (e.g., λP2=1567 nm) functions to excite the Tm ions present in core region1of gain fiber12, resulting in amplification of input optical signal SIN. The amplified output signal SAMPfrom single-clad gain fiber12is shown inFIG. 1as passing through an output isolator19before exiting TDFA10.

In accordance with the principles of the present invention, pump source18is particularly configured to utilize the combination of a discrete, low-power semiconductor laser diode20and a fiber laser22, as shown inFIG. 1, so as to create a multi-watt power pump output LP2without needing to provide thermal management of the pump light source. That is, the utilization of a fiber laser with a discrete laser diode allows for a milli-watt power laser diode to be used as the pump light input to fiber laser22(with laser diode20provided in a simple “uncooled” subassembly), with the milli-watt laser diode output power increased to a level in the range of about 1-3 W after passing through fiber laser22. In one exemplary embodiment, laser diode20may comprise a GaAs laser diode emitting at a wavelength λP1of about 940 nm, which is used as a pump input to fiber laser22, thus generating a multi-watt pump output LP2at a useful pump wavelength λP2of about 1567 nm.

In one exemplary embodiment of pump source18, fiber laser22may comprise a section of Er—Yb co-doped fiber24, disposed between a pair of reflective surfaces261,262to form a laser cavity. To maintain a compact configuration, the reflective surfaces are preferably formed as a pair of fiber Bragg gratings (FBGs) inscribed directly in fiber24. Since the generated pump power need only be in the range of about 1-3 W, only a few meters of Er—Yb co-doped fiber may be required. The output from laser diode20is coupled into fiber laser22, where this first pump light operating at a λP1of about 940 nm resonates within the laser cavity, exciting the Er and Yb ions until reaching a sufficient energy level to be emitted as the pump light LP2, operating at the output (i.e., “second”) pump wavelength λP2of about 1567 nm. The wavelength of 1567 nm is only one wavelength that is appropriate for use in a Tm-doped gain fiber, selected here for explanatory purposes only.

In accordance with the principles of the present invention, therefore, a milli-watt, uncooled laser diode20may thus be used in conjunction with a fiber laser22to create a pump output having a power greater than 1 W. As will be discussed in detail below, it is an aspect of the present invention that the use of a pump operating in the 1-3 W regime is able to provide the desired large bandwidth and high small signal gain of the inventive amplifier, while also maintaining a relatively low noise figure (and, as a result a high optical signal-to-noise ratio (OSNR)).

Advantageously, the use of fiber laser22in this manner simplifies the design of pump source18and eliminates the need to be concerned about maintaining wavelength stability of the laser diode, providing thermal management of the laser diode (e.g., including a thermos-electric cooler in the subassembly, for example), and the like. As a result, it is possible to maintain the overall size of the inventive broadband TDFA within a relatively small footprint. Besides the ability to use an uncooled pump source, as discussed above, a miniaturized isolator/WDM combination may be used as elements14,16to further control the overall size of the amplifier. Indeed, a fully assembled and packaged TDFA formed in accordance with the present invention has been made to have the dimensions of 97×78×15 mm3, where this fully assembled structure incorporates full pump control electronics and an RS232 interface for communication purposes.FIG. 2is a depiction of an exemplary “packaged” compact inventive TDFA constructed in this manner, with a business card-sized rectangle placed on top of the package for the purposes of physical size (footprint) comparison.

FIG. 3is a graph depicting the optical slope efficiency of inventive TDFA10, the slope efficiency defined here as the change in generated output power as a function of changes in pump power (while the input signal power is held fixed). The input signal power (measured at the input of gain fiber12) is designated as Psand output signal power (measured at the output of gain fiber12) is designated as Pout. The total pump power from pump source18that is available to couple into single-clad gain fiber12is designated as PP2. For the purposes of creating the data shown inFIGS. 3-5, a linearly polarized input signal SINwas used, and all fiber sections (including single-clad gain fiber12) were formed of polarization-maintaining optical fiber. The orientation between the various fibers and components is maintained such that input signal SINpropagates through TDFA10on the slow fiber axis. It is to be understood that the use of polarization-maintaining fiber is only one exemplary embodiment of a TDFA formed in accordance with the present invention and that other configurations may be based upon standard single mode fiber (the latter suitable when the polarization of the incoming signal is not a factor to be considered in the amplifier's performance).

In particular,FIG. 3plots amplifier output power Poutas a function of changes in pump power (PP2) for an input signal SINoperating a signal wavelength λs=1909 nm, while maintaining a fixed input power (Ps=0 dBm). The measured values of Poutare shown to vary linearly with PP2as expected, with a maximum output power of 1.15 W shown as being associated with a pump power PP2of 2.58 W. An aspect of the present invention is the ability to provide an output power of at least 0.5 W over a wide region of the 2 μm band, so the 1.15 W generated by TDFA10clearly exceeds this goal, while using only single-clad Tm-doped gain fiber12, thus able to maintain the “miniaturized design” requirement.

An exemplary saturated output spectrum for this same configuration of TDFA10is shown inFIG. 4. Again, the power level of input signal SINwas maintained at Ps=0 dBm (for a single frequency input signal operating at λs=1909 nm). Pump light LP2was configured to maintain a pump power PP2of 2.58 W for the purposes of measuring the spectral response. As illustrated by the spectral data, the estimated output power bandwidth of the saturated amplifier was about 120 nm, with the peak associated with the input signal wavelength λsof 1909 nm clearly visible. The power surrounding this peak is the noise associated with amplified spontaneous emission (ASE), with the measured optical signal-to-noise ratio (OSNR) being about 58.1 dB/0.1 nm. This high OSNR value is desirable for applications such as LIDAR and DWDM transmission systems.

Recall that one goal of the present invention is to provide a fiber-based optical amplifier that is capable of operating in the eye-safe, visible 1.9-2.1 μm band, providing high small signal gain over a relatively wide bandwidth while maintaining a high OSNR (and also being relatively compact in form).FIG. 5contains plots of the gain and noise figure for TDFA10as a function of input signal power (Ps), again associated with an input signal SINoperating at a wavelength λsof 1909 nm. These gain and noise figure values are associated with pump source18operating at the wavelength of λP2of about 1567 nm, with a pump power PP2held at 1.55 W (somewhat less than saturation).

In reviewing the data ofFIG. 5, it is seen that the small signal gain G reaches a maximum value of 46 dB (for an input power of about −33 dBm), where the noise figure (NF) is approximately 4.0 dB for this level of small signal gain. As expected, G decreases with increasing Psand trends to a linear asymptote. The measured NF rises to approximately 5.5 dB for the maximum input signal power of −1.5 dBm. The high gain and low noise figure indicate that TDFA10is well-suited for use as an optical preamplifier in the visible 2 μm wavelength band.

FIG. 6illustrates an alternative embodiment of the present invention, in this case taking the form of a counter-propagating TDFA. Identified as TDFA10A, the configuration as shown inFIG. 6utilizes a similar single-clad gain fiber12, pump source18and isolators14,19as described above in association with TDFA10ofFIG. 1. Here, however, in order to create a counter-propagating fiber amplifier configuration, a WDM50is disposed at the output of single-clad gain fiber12, with pump source18coupled to a pump input port of WDM50and used to direct pump light LP2through gain fiber12in a counter-propagating direction (i.e., counter to the direction of propagation of input signal SINthrough gain fiber12). Again, pump source18is based upon the use of an uncooled laser diode in combination with a fiber laser to create a multi-watt pump beam at an appropriate pump wavelength.

TDFA10A functions in the same manner as TDFA10ofFIG. 1in terms of using pump light at a wavelength λP2of 1567 nm (for example) to excite the Tm ions present in single-clad gain fiber12and thus amplify the propagating optical signal (that is, imparting gain to the propagating signal by transferring the energy associated with the ion excitation to signal SIN), forming an output amplified signal SAMP.

In contrast to co-pumping TDFA10ofFIG. 1, counter-propagating pump light LP2in the arrangement ofFIG. 6interacts with propagating input signal SINin a very different manner, since the power level of pump light LP2is greatest at the far-end of single-clad gain fiber12and thereafter diminishes as pump light LP2propagates towards the input end of gain fiber12(where the power Psof the input signal SINis the greatest). The counter-propagating amplifier arrangement thus can create similar gain (in terms of magnitude), while also providing greater slope efficiency and power conversion efficiency than the co-propagating embodiment.

FIG. 7illustrates this difference in optical slope efficiency between co-pumped TDFA10ofFIG. 1and outer-pumped TDFA10A ofFIG. 6. Plot I inFIG. 7is associated with co-pumped TDFA10and is in fact the same data shown inFIG. 3, above. Plot II illustrates the optical slope efficiency for counter-pumped TDFA10A ofFIG. 6. It is clear that the optical slope efficiency increases for counter-pumped TDFA10A (e.g., increasing from about 48.6% to 56.5% in one example). For counter-pumped TDFA10A, plot II also shows that a maximum output power Poutof 1.265 W was achieved (again, created with a pump power PP2of about 2.58 W).

FIG. 8illustrates a comparison of the saturated optical spectrum associated with counter-pumped TDFA10A to that measured for co-pumped TDFA10(the latter discussed above in association withFIG. 4). It is noted that the spectrum associated with counter-pumped TDFA10A (plot A) exhibits a significantly different background ASE distribution than the background ASE for co-pumped TDFA10(shown in plot B ofFIG. 8; the same data as contained in the plot ofFIG. 4). This shift in ASE distribution is caused by the dramatically different distribution of pump light along the length of the counter-pumped amplifier in comparison to the distribution of pump light in the co-pumped amplifier as discussed above, and results in creating a larger operating bandwidth (on the order of about 170 nm, compared to about 120 nm for the co-pumped embodiment), but at the cost of a higher level of noise (and thus a lower OSNR). In particular, output OSNR for TDFA10A, as derived from the data in plot A, decreases to a value of about 52.4 dB/0.1 nm, compared to the 58.1 dB/0.1 nm output OSNR for TDFA10. This difference is OSNR is primarily due to the increase of the noise figure in counter-pumped TDFA10A.

The increased noise figure for counter-pumped TDFA10A is evident in the data shown inFIG. 9, which shows comparisons of both the small signal gain G and noise figure NF for both co-pumped and counter-pumped TDFA configurations. The gain curves essentially track and are the same for both pumping arrangements, but the minimum noise figure associated with counter-pumped TDFA10A has risen to about 7 dB, and is further shown as increasing rapidly with increasing input signal power.

It is to be understood that a TDFA formed in accordance with the present invention may be used in conjunction with either a “continuous wave” (CW) input optical signal or a pulsed input optical signal, where the latter is typically the case when used to amplify a digital data signal being transmitted through an optical communications network (for example).

FIG. 10illustrates this aspect of the present invention, showing an embodiment of an exemplary “pulsed input” TDFA, designated as TDFA10B. The embodiment ofFIG. 10utilizes the same components as discussed above in conjunction with TDFA10ofFIG. 1in terms of providing amplification for input signals operating within the eye-safe 2 μm wavelength region. Also particularly illustrated inFIG. 10is a driver circuit30, which is used to supply an input drive current to a laser diode32(operating within the wavelength range of about 1800-2000 nm). In this case, driver circuit30responds to an incoming electrical signal DATAIN, a digital signal that will be pulsed between “on” and “off”. As a result, driver circuit30provides a pulsed input to laser diode32, so as to create an input optical signal SINthat is not “continuous”, but is pulsed. TDFA10B thus functions to amplify the pulses, where the advantage of maintaining a high OSNR allows for clear distinction between the transmitted 1's and 0's of DATAIN.

In yet another configuration, TDFA10may be used to generate as an output an amplified spontaneous emission (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. 11illustrates an exemplary TDFA10C formed in accordance with the present invention to provide this ASE output. Here, the input signal source SINis eliminated, shown by fiber termination40inFIG. 11(which may indeed comprise a section of incoming optical fiber42cleaved at an appropriate angle, such as 8°.

Without the application of an input signal, pump light LP2is the only optical energy coupled into the TM-doped core region of gain fiber12. While not rising to the amplified level of an input signal, this pump light is also sufficiently amplified, providing the ASE output as shown inFIG. 11. Inasmuch as only pump light LP2is coupled into gain fiber12, WDM16may be eliminated, with an output fiber44from pump source18directly spliced to gain fiber12. However, it is possible that this direct coupling may introduce strong reflections (associated with the peak of the ASE output) back into pump source18. These reflections are known to degrade the quality of the pump beam and may therefore ultimately degrade the ASE output itself. Thus, the inclusion of WDM16functions as a “filter” element in this embodiment to prevent reflections from re-entering pump source18.

FIG. 12illustrates yet another embodiment of the present invention. Here, single-clad TDFA is formed as a multi-stage amplifier60, which may find particular use as a preamplifier for the reasons discussed below. In this particular configuration, multi-stage TDFA60comprises a pair of concatenated amplifier stages62,64, with each amplifier stage taking the form of a single-clad TDFA. A single pump source18is used in this particular arrangement to supply the pump light input LP2at λP2to both first stage62and second stage64(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 splitter66is used in this particular embodiment to control the ratio of pump powers within the amplifier stages, creating two separate pump beams. A first pump beam LP21output from power splitter66(operating at a first power level PP21) is provided as a pump input to first amplifier stage62, with a second beam LP22(operating at a second power level PP22) provided as the pump source for second stage64(where the sum of PP21and PP22is ideally equal to the input power PP2of pump source18).

Referring now in particular to first stage62, the incoming signal SINand first pump beam LP21are provided as inputs to a first WDM68, which directs both beams along a common output fiber, which in this case is a first section of single-clad Tm-doped gain fiber70(having a length l1). The output from first stage62, designated SA1, is then provided as an input (amplified) signal to second stage64. As shown, a second WDM72is disposed to receive this amplified signal SA1, as well as the larger portion (LP22) of the pump beam. The combination of these two lightwaves is then coupled into a second section of single-clad Tm-doped gain fiber74(having a length2).

In accordance with this multi-stage embodiment of the present invention, each amplifier stage may be separately optimized, in terms of gain fiber length and applied pump power, such that one stage provides maximum gain (for example, first stage62) and the other stage provides maximum power (here, second stage64). One particular configuration that exemplifies this optimization may use a first stage gain fiber70with1=3.0 m, pumped with 20% of the total pump power, in combination with a second stage gain fiber74of length2=2.0 m, receiving 80% of the pump power.

The use of a multi-stage TDFA as a preamplifier is particularly advantageous, since it is able to respond to relatively low power input signals and is further able to create a moderate amount of gain within such low-power inputs.

FIG. 13illustrates an alternative configuration of the multi-stage amplifier ofFIG. 12(denoted as TDFA60A inFIG. 13). The components of first stage62are the same as described above in association withFIG. 12. In this case, however, second stage64A utilizes a counter-propagating pump beam, provided via a WDM72A disposed at the output of Tm-doped gain fiber74. As with the counter-pumped single stage TDFA discussed above in association withFIG. 6, second stage64A provides improvement in slope efficiency and power conversion efficiency over the co-pumped arrangement, albeit at the cost of a decrease in overall gain.

For some applications, it may be preferable to position the fiber laser component of the pump source adjacent to the single-clad Tm-doped gain fiber.FIG. 14illustrates an exemplary TDFA120formed in this configuration. As with the various embodiments described above, TDFA120is based upon the use of a single-clad Tm-doped gain fiber122. A fiber laser124is shown as coupled to the input of gain fiber122through a splice element126. Similar to the configurations described above, fiber laser124includes a section of Er—Yb co-doped fiber128, with a pair of FBGs1301,1302defining the laser cavity itself. Co-doped fiber128is formed of double-clad optical fiber, with splice element126configured to provide efficient coupling from the core/“first cladding” combination of co-doped fiber128into the core region of single clad Tm-doped fiber122.

In accordance with this embodiment of the present invention, a discrete pump source laser diode132is utilized to create a first pump signal LP1(at a first pump wavelength λP1in the range of 910-980 nm, for example), with this beam coupled into a signal/pump combiner134(preferably formed of multimode fiber) that is spliced to an input end of Er—Yb co-doped fiber128. Input signal SINis also applied as an input to combiner134, and thus also passes through fiber laser124before reaching gain fiber122. Advantageously, input signal SIN(operating at a wavelength in the range of about 1850 nm to 2000 nm, for example) propagates through fiber laser124with minimal attenuation, thus allowing for both doped fiber sections to be disposed in this contiguous fashion, eliminating the need for a WDM to introduce the multi-watt pump into the Tm-doped fiber.

It is also to be understood that TDFA120as discussed above may also be configured in a counter-pumped arrangement. In this case, the output of co-doped fiber128(of pump source124) is not directly coupled to again fiber122, since a WDM134A is disposed in the signal path between the two gain fiber sections, as shown inFIG. 15(FIG. 15illustrating a counter-propagating embodiment denoted as TDFA120A). WDM134A is used in this case to inject the first pump light LP1(at a wavelength λP1in the range of 910-980 nm) in a counter-propagating direction through co-doped fiber128. In the through-signal direction, both incoming optical signal SINand the generated pump light LP2pass through WDM134A relatively unimpeded and are coupled into the Tm-doped core region of gain fiber122.

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. In one case, for example, a multi-watt pump laser diode may be used in place of the combination of a low power semiconductor laser diode and fiber laser. In this case, device such as a Fabry-Perot laser or distributed feedback laser may be utilized and maintained at operate in the 1-3 W range to minimize thermal management problems. Indeed, the described embodiments are to be considered in all respects as only illustrative and not restrictive.