Bi-directionally pumped polarization maintaining fiber amplifier

A bi-directionally pumped PM fiber amplifier includes an amplifier input coupled to a first WDM coupler and a second WDM coupler providing an amplifier output. A doped fiber is between the WDM couplers. A first pump light source emitting at a first wavelength along a first polarization axis is coupled to the WDM coupler through a polarization beam combiner/splitter and a polarization rotator is for downstream pumping of the doped fiber with rotated light relative to the first polarization. The fiber is upstream pumped with light having the first polarization using a second pump light source emitting at the first wavelength/first polarization, by an output of an optical power splitter with its input coupled to the first pump light source, or by a fiber-coupled rotator mirror coupled to the second WDM coupler.

FIELD

Disclosed embodiments relate to polarization maintaining (PM) fiber amplifiers.

BACKGROUND

In fiber-optics, a polarization-maintaining optical fiber (PMF or PM fiber) is a single-mode optical fiber in which linearly polarized light if properly launched into the fiber maintains a linear polarization during its propagation. The polarized light thus exits the fiber in a specific linear polarization state so there is essentially no cross-coupling of optical power between the two orthogonal polarization modes. Such fiber is used in special applications where preserving polarization is essential. PM fiber amplifiers use PM fibers and are needed in some high reliability applications in which the fiber amplifier is required to operate with as well as maintain a specific state of polarization of the received optical launch signal.

State-of-the-art PM amplifiers involve an all-PM fiber structure incorporating fiber components built with PM fibers as well as a PM doped fiber. The PM fiber includes but is not limited to PANDA fibers that have a step index profile and elliptical core fibers. Variants of these PM fibers include PM fibers that allow transmission in a specific polarization axis or non-polarizing which allow transmission on both polarization axes.

There are several known methods for pumping PM fiber amplifiers that use first and second pump lasers emitting at the same wavelength (e.g., electrically pumped laser diodes providing an emission at 980 nm for an Erbium Doped Fiber Amplifier (EDFA) which emits amplified light within the 1525-1565 nm band. Co-propagating (or forward) pumping is known for a low Noise Figure (NF), but results in low power conversion efficiency (PCE) and a low gain. Forward pumping means that the pump wave travels in the same direction as the signal wave. Counter-propagating (or backward) pumping is known for a high NF with high PCE and high gain. Backward pumping means that the pump wave travels in the opposite direction as the signal wave. Bi-directional pumping involves pumping in both the forward and backward directions simultaneously to realize the advantages of both the co-propagating and counter-propagating schemes.

Bi-directional pumping is thus often desirable since it can maintain a low NF and at the same time achieve a high PCE. However, optical cross-talk between co-propagating and counter propagating pump lasers can occur. Optical isolators such as Faraday rotators which allow light to pass only in a single direction are known to provide protection of active devices such as lasers against injection of unwanted light. However, optical isolators are generally expensive, lossy and bulky components, in particular when used for isolating light in the short wavelengths. Therefore the deployment of optical isolators is generally not practical from the system point of view in terms of cost, efficiency and size.

One other solution to mitigate the problem of pump laser crosstalk in bi-directionally pumped PM fiber amplifiers involves using pump lasers emitting at different wavelengths. These pump lasers typically employ fiber Bragg grating filters for stabilization to filter-out/reject incident signals of a wavelength outside the Bragg grating filter's pass-band.

SUMMARY

Disclosed embodiments recognize in applications including hi-reliability PM fiber amplifiers the use of bi-directionally pumping is often not possible due to the optical cross-talk between co-propagating and counter propagating pump light sources (e.g. laser diodes). In practice any residual pump light originating from the co-propagating pump light source that is not absorbed by the doped fiber will be incident onto the counter-propagating pump light source, and vice versa. The injection of light into the pump light source can affect its stability, reliability and lifetime. As a consequence the gain of the PM fiber amplifier can become unstable and the reliability of the PM fiber amplifier can be compromised. In order to improve the amplifier's performance and reliability, a solution to minimize the crosstalk between co-propagating and counter-propagating pump light sources is recognized to be needed.

Disclosed example embodiments include single wavelength bi-directionally pumped PM fiber amplifiers that provide mitigated pump light source crosstalk. A disclosed bi-directionally pumped PM fiber amplifier includes an amplifier input coupled to a first Wavelength Division Multiplexing (WDM) coupler and a second WDM coupler providing an amplifier output. A doped fiber is between the WDM couplers. A first pump light source emitting at a first wavelength along a first polarization axis is coupled to the WDM coupler through a first polarization beam combiner/splitter (PBCS), and a polarization rotator is for downstream pumping of the doped fiber with rotated light relative to the first polarization. The doped fiber is upstream pumped with light having the first polarization using (i) a second pump light source emitting at the first wavelength/first polarization coupled by a second PBCS to the second WDM coupler, (ii) an optical power splitter having an input coupled to the first pump light source having its output coupled by the second PBCS to the second WDM coupler, or (iii) a fiber-coupled rotator mirror for recirculating residual light coupled to the second WDM coupler.

DETAILED DESCRIPTION

COMPONENT IDENTIFICATION AS USED HEREIN

101,102: fiber-optic isolators103,104: WDM coupler (signal/pump combiner)105: doped fiber105a: first end of the doped fiber105105b: second end of the doped fiber105106Polarization beam combiner/splitter (PBCS)107: PBCS106a: The slow axis port of the PBCS106106b: The fast axis port of the PBCS106106c: The combined fast/slow axis port of PBCS106107a: The slow axis port of the PBCS107107b: The fast axis port of the PBCS107107c: The combined fast/slow axis port of the PBCS107108: Pump light source109: Pump light source emitting at the same wavelength as the pump light source108110: A polarization rotator112: A PBCS for re-use of received residual pump signals113: An optical power splitter114: A fiber-coupled rotator mirror

FIG. 1Ashows a depiction of an example PM doped fiber amplifier100having two pump light sources, pump light source108for providing co-propagating light and a pump light source109for providing counter propagating pump light, according to an example embodiment. Solid line arrows shown indicate the propagation direction of light from pump light source108which is on the downstream side of the propagation direction. The dashed line arrow shown shows the propagation direction of light from pump light source109which is on the upstream side of the propagation direction. When both pumps are present in a disclosed fiber amplifier system, pump light source108may be considered a first pump light source and pump light source109considered a second pump light source.

Although the pump light sources108and109are shown herein as laser diodes including symbolically as laser diodes inFIG. 1A, they can be other light sources or laser diode variants. For example, one laser diode variant is fiber pigtailed laser diodes spliced to fiber components or free-space coupled laser diodes. The free-space coupled laser diodes would be applicable especially in the case of a micro-optic assembly implementation (e.g., a laser diode chip coupled to a bulk PBCS or bulk rotator element through free-space using an optical micro-optic assembly bench).

The letters (f) and (s) shown inFIGS. 1A and 1nother FIGs. indicates the polarization state and polarization evolution of the light with (f) fast axis and (s) slow axis. All constituent components of the doped fiber amplifier100are constructed with PM fibers for maintaining the polarization state of both the signal light received and the pump light.

The PM doped fiber amplifier100includes a doped fiber105having a first end105aand a second end105b. Signal light is launched from a signal source (not shown) in the propagation direction shown left to right inFIG. 1Ain the doped fiber105from its first end105ato its second end105b. Thus, the first end105aof doped fiber105corresponds to a downstream side of the propagation direction and second end105bcorresponds to an upstream side of the propagation direction. Doped fiber105is generally doped with a rare earth (RE) element such that the doped fiber emission spectrum coincides with the signal light from the signal source to be amplified. As an example, erbium is generally the RE element of choice for signal amplification within the C-band (at about 1.55 μm). In that case the doped fiber amplifier100would be referred to as an EDFA.

Disclosed embodiments also include cladding-pumped fiber amplifiers. In this embodiment the doped fiber105can comprise a double-clad fiber including a doped core and an inner cladding, where the system further comprises optics for launching the optical signal into the doped core and optics for launching light from the pump light source108into the inner cladding. The signal light is thus launched into the doped core, while the pump light is launched into the inner cladding. As known in the art, the core can be D-shaped for more efficient pump light absorption.

Pump light source108is connected to a PBCS107. PBCS107couples light launched through its input port107ato its output port107cgiven that the light polarization is in the perpendicular axis, known as and refer to hereafter as the slow axis polarization. Light with slow axis polarization launched through PBCS107port107cwill be coupled to PBCS107port107a. Similarly PBCS107will couple light launched through its port107bto its output port107cgiven that the light polarization is in the vertical axis, known as and refer to hereafter as the fast axis polarization which is rotated 90 degrees from the slow axis. Light with fast axis polarization launched through PBCS107port107cwill be coupled to PBCS107port107b. The same operation principle applies to PBCS106which is associated with pump light source109.

According to the above operating principle, pump light source108is emitting light along the slow axis which is coupled through the polarization rotator110to port107bof PBCS107. The polarization rotator110rotates the light polarization by 90 degrees from pump light source108. Polarization rotator110can be in its simplest form implemented as a 90 degree splice joint between the PM fibers of pump light source108and the PBCS107. A 90 degree splice is generally straightforward to realize, and is not technically an optical component as it is realized during the splicing of two fibers. Basically one instructs the splicer apparatus to rotate one of the fibers by 90 degrees and then to splice the fibers. Polarization rotator110allows one to have both pump light sources108,109emitting at the vertical polarization (the slow axis) and then flip (the polarization) of light from one of the pump sources to the horizontal polarization (the fast axis), most simply using a 90 degree splice.

Polarization rotator110can also comprise a Faraday rotator. Going through polarization rotator110the polarization of light emitted by the pump light source108will be converted from the slow axis to fast axis. As such pump light from pump light source108will be coupled from port107bto port107cof PBCS107. Similarly pump light source109is emitting light in the slow axis and is coupled to PBCS106port106a. As such pump light source109will be coupled from PBCS106port106ato port106c. For the doped fiber amplifier100being an EDFA both pump light sources108and109will be generally emitting light around the 980 nm band since pump light in this band is suitable to excite erbium ions of the doped fiber105to provide optical gain in the C-band. Pumping at 1480 nm is also possible, but results on a lower absorption cross-section. Pumping at other wavelengths is also possible.

A WDM optical coupler103connects port107cof the PBCS107to first end105aof doped fiber105, and WDM optical coupler104connects port106cof PBCS106to second end105bof doped fiber105. Accordingly, light emitted by pump light sources108and109propagate in the downstream and upstream directions, respectively.

In the downstream direction a majority of the light from pump light source108generally will generally be absorbed by doped fiber105and any un-absorbed light, referred to hereafter as downstream residual pump light, which will exit the doped fiber105through its second end105b. Downstream residual pump light from pump light source108will then be coupled to port106cof PBCS106through WDM coupler104. The downstream residual pump light having a fast axis polarization will exit PBCS106through its port106bavoiding interference with the pump light source109.

In the upstream direction light from pump light source109will be absorbed by doped fiber105and any un-absorbed light, referred to hereafter as upstream residual pump light, will exit the doped fiber105through its first end105a. Upstream residual pump light from pump light source109will then be coupled to port107cof PBCS107through WDM coupler103. The upstream residual pump light having slow axis polarization will exit PBCS107through its port107aavoiding interference with the pump light source108.

With respect to signal light propagation through the doped fiber amplifier100, signal light launched is coupled to WDM coupler103which couples the signal light into first end105aof the doped fiber105and as such signal light propagates in the downstream direction, the same direction as the light signal from the pump light source108and opposite in direction relative to the light signal from pump light source109. Signal light will be amplified within the doped fiber105and will exit the doped fiber105through its second end105b. The amplified signal is coupled through the WDM coupler104. In arrangements where back-reflections can pose amplifier problems, fiber-optic isolators101and102can be added at the input and the output of the amplifier as shown inFIG. 1B(and other FIGs.) described below.

FIG. 1Bshows the PM doped fiber amplifier inFIG. 1Aas100′ along with optical isolators101and102added at the input and the output of the amplifier, according to an example embodiment. The fiber-optic isolators101and102can comprise a Faraday isolator.

FIG. 2shows the fiber amplifier100′ inFIG. 1Bnow shown as fiber amplifier200modified so that the polarization rotator110is now between the PBCS107and the WDM coupler103instead of being between the pump light source108and the PBCS107.

FIG. 3shows the fiber amplifier100′ inFIG. 1Bnow shown as fiber amplifier300modified to add another PBCS112to provide the advantage of re-use of residual pump light. Port112aof PBCS112is coupled to port107aof PBCS107and port112bof PBCS112is coupled to port106bof PBCS106, with the re-used residual pump light provided at output port112c. Light from port112ccan be used, for example, to pump a different amplifier stage within the same fiber amplifier, pump a different fiber amplifier, or used as calibration signal in a free-space laser communications (lasercom) link.

FIG. 4shows a depiction of an example PM doped fiber amplifier shown as fiber amplifier400that adds a 1×2 optical power splitter113to utilize a single pump light source shown as pump light source108to bi-directionally pump the doped fiber105of the fiber amplifier400. A polarization rotator110is positioned between the power splitter113and the PBCS107which rotates the light polarization received by 90 degrees. Downstream pumping is through the path from PBCS107through WDM coupler103and upstream pumping is through the path PBCS106through WDM coupler104. The use of a single pump light source to bi-directionally pump the fiber amplifier400provides advantages including a reduction in the fiber amplifier size and its complexity.

FIG. 5Ashows a depiction of an example PM doped fiber amplifier shown as fiber amplifier500that uses a single downstream pump light source108to bi-directionally pump the doped fiber105of the fiber amplifier500by using a fiber-coupled rotator mirror114which re-circulates residual pump light received from WDM coupler104at port114ato upstream pump the doped fiber105. As with fiber amplifier400, the use of a single pump light source to bi-directionally pump the fiber amplifier500provides the advantages of a reduction in the fiber amplifier size and its complexity.

FIG. 5Bshows a depiction of an example PM doped fiber amplifier shown as fiber amplifier550which replaces the fiber coupled rotator mirror114inFIG. 5Awith a PBCS106having port106creceiving light from WDM coupler104, according to an example embodiment. Ports106aand106bthat are coupled together through a second polarization rotator115, Advantage(s) of this arrangement is the implementation of the fiber coupled rotator mirror114with a fused device (same as the one used to couple the pump light sources into the fiber-optic amplifier path) that can be compared to a conventional solution that is based on fiber coupled micro-optics. Fused devices are generally significantly better in terms of reliability and manufacturability. In addition in the case of a high reliability system that involves qualification, this specific implementation will be more cost-effective since it does not introduce any new components as it re-uses the same component as the one used to couple the pump light source into the fiber-optic path.

As noted above, residual pump light that is injected in the at least one pump light source (108and/or109) is greatly suppressed by disclosed fiber amplifier systems without the need for using different pump light source wavelengths. This will improve the lifetime and the reliability of the pump light source(s) and hence the reliability of the fiber amplifier system. Stable fiber amplifier performance is also provided as the suppression of the cross-talk between co-propagating and counter-propagating pump light enhances the gain stability of the fiber amplifier.

Moreover, the disclosed use of a single pump light source or pump light sources both at a single emission wavelength will reduce the fiber amplifier optical assembly cost. Especially in the case of high reliability applications such as space, in which amplifiers are subjected to rigorous screening and qualification tests, the conventional selection of different pump wavelengths for a fiber amplifier means a linear increase of screening/qualification testing cost, which adds to the development cost and recurring price of the fiber amplifier as a product, which is avoided by disclosed fiber amplifiers.

Disclosed PM doped fiber amplifiers can be used in a wide variety of optical systems. For example, for applications in telecommunications such as free-space laser communications, fiber lasers, optical switching, sensing, and microwave photonics.

Those skilled in the art to which this disclosure relates will appreciate that many other embodiments and variations of embodiments are possible within the scope of the claimed invention, and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of this disclosure. For example, disclosed embodiments can also be applied to free-space implementations where the optical amplifier is constructed using free-space optical elements, with the active fiber replaced by a solid state gain medium, the WDM coupler replaced with a dichroic beam splitter, and the PBCS replaced with a waveplate.

While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.