WO1997039460A1

WO1997039460A1 - Lasers diode pump with optical fiber filter

Info

Publication number: WO1997039460A1
Application number: PCT/US1997/004826
Authority: WO
Inventors: Kevin W. Bennett; Kevin L. Sweeney
Original assignee: Atx Telecom Systems, Inc.
Priority date: 1996-04-16
Filing date: 1997-03-24
Publication date: 1997-10-23
Also published as: AU2545297A

Abstract

A laser diode pump source (26) is disclosed for pumping a gain medium (22) in a fiber optic amplifier (20). The high wavelength emission of the gain medium (22) is prevented from being transmitted back to the laser diode (26) which would otherwise cause severe degradation or catastrophic damage. In addition to the laser diode (26) for generating a pump wave, the pump source includes an optical fiber filter (28) preferably formed as a coil of single mode optical fiber (28). The coil (28) has a bend radius made sufficiently small such that the optical fiber filter (28) exhibits significant losses at the higher wavelength of the emission of the gain medium (22), but exhibits low insertion loss at the lower pump wavelength. The coil (28) allows the pump wave to pass through for pumping the gain medium (22), but substantially attenuates the back-transmitted emission generated by the gain medium (22).

Description

"LASERS DIODE PUMP WITH OPTICAL FIBER FILTER"

FIELD OF THE INVENTION This invention relates generally to pump sources for pumping laser gain media, and more particularly to laser diode pump sources for use in optical signal transmission systems.

BACKGROUND OF THE INVENTION Fiber optic amplifiers have found important applications in fiber optic communication systems, such as CATV systems. These amplifiers typically exhibit high gain, low noise, low crosstalk and intermodulation distortion, bit-rate transparency, and polarization- insensitive gain. Fiber optic amplifiers can also be fusion-spliced to standard optical fibers, thus eliminating the need for costly anti-reflection facet coatings typically required on semiconductor optical amplifiers. These properties make fiber optic amplifiers superior to semiconductor devices aε amplifiers in many optical systems. Because fiber optic amplifiers amplify optical input signals directly, they are also superior to conventional electrical amplification devices which require converting an optical signal into an electrical signal, amplifying the electrical signal, and re-converting the amplified electrical signal into an optical signal.

Among the different types of fiber optic amplifiers, those based on erbium-doped optical fibers have attracted a significant amount of attention recently. This is because the emission bandwidth of erbium ions is within the third transmission window, at a wavelength of approximately 1550 nm, where a silica optical fiber has the lowest attenuation. Different types of erbium-doped optical fibers may be selected for use in fiber optic amplifiers, depending upon various technical considerations related to the particular application. As an example, a commercially available high power 1550 nm fiber optic amplifier uses an amplifier fiber which comprises a host glass which is doped with both erbium (Er) and ytterbium (Yb) as the gain medium. The combination of erbium and ytterbium provides a broader optical absorption bandwidth. The ytterbium ions serve as a sensitizer which absorbs the pump wave at a wavelength that could not readily be absorbed without the ytterbium, and then transfers the absorbed energy to the erbium ions for emission.

The Er:Yb codoped fiber can be efficiently pumped with a high power Nd.YLF diode-pumped solid-state (DPSS) laser at 1047 nm. The output of the DPSS pump laser is coupled to one end of the codoped fiber via a wavelength division multiplexer (WDM) . The input optical signal is also coupled to the codoped fiber via the WDM, and the amplified output signal exits the fiber through another WDM at the other end of the fiber. Due to the high quality, high power output of the NdtYLF DPSS pump laser, the fiber optic amplifier is capable of generating a high output power not otherwise attainable. Such a DPSS laser-pumped Er:Yb codoped fiber optic amplifier is described in U.S. Patent No. 5,225,925, which is hereby incorporated by reference.

The output power of the ErrYb fiber optic amplifier can be further enhanced by using two high-power DPSS Nd:YLF lasers to simultaneously pump both ends of the fiber. In such an arrangement, the output of each pump laser is coupled to a WDM at each end of the fiber, and the amplified output signal exits the fiber through the WDM at the other end of the fiber. This double-pumping scheme significantly increases the output power of the fiber optic amplifier. One problem with using two Nd.YLF DPSS lasers to simultaneously pump the codoped fiber iε that the output mirrors of the two pump lasers can form a parasitic resonant cavity which includes the codoped fiber. The 1550 nm optical radiation can build up in such a cavity and become a source of noise. In order to solve this noise problem, it has been found that a coil formed of a single mode fiber could be inserted between one of the Nd:YLF pump laserε and itε aεεociated WDM to suppress the 1550 nm light buildup in such cavity.

Although the DPSS laser-pumped Er:Yb codoped fiber optic amplifiers have the advantage of relatively high output power, for certain applications it may be preferable to use optical fibers having only erbium doping. Compared to the Er:Yb codoped fiber, an optical fiber doped only with erbium can be made to have a broader gain-bandwidth curve and a gain that is less dependent on wavelength. Due to the broader gain- bandwidth and flatter output responεe, the erbium-only fiber can be more easily adapted for the so-called "WDM transmission", where several signals at different wavelengths within the gain-bandwidth are transmitted simultaneously through the fiber optic communication system. However, since the erbium-only fiber has poor absorption at the 1047 nm output of the Nd.YLF DPSS laser, it is often pumped with a laser diode having an output wavelength of approximately 980 nm, where the erbium ions have an absorption band. Erbium-doped fiber also has an absorption peak at 808 nm, which may alternatively be used for some applications.

It has been found, however, that the laser diodes themselves can be very sensitive to extraneous optical radiation of a different or higher wavelength. In fact, a small amount of incident light of approximately 1550 nm wavelength, which is the typical emission wavelength of the erbium ions, may cause degradation and sometimes destruction of the 980 nm laser diode. Although the problem has been acknowledged by various laser diode manufacturers, the exact mechanism for this breakdown phenomenon is not fully understood. However, its effects can be devastating to the manufacturability and reliability of a fiber optic telecommunication system. It is therefore desirable to find a way to prevent the harmful radiation emitted by the erbium fiber from being transmitted back to the laser diode.

This breakdown phenomenon has also been observed in optical fiber lasers, where a diode laser is directly connected to an erbium-doped optical fiber laser cavity. In this fiber laser configuration, the damage to the laser diode can be worse than that observed in a fiber optic amplifier. In fact, if the fiber laser is run in a pulsed mode of operation, the extremely high peak powers achieved in the fiber laser could shorten the . lifetime of the laser diode even further. This same breakdown phenomenon could also occur in a broadband εource based on amplified spontaneous emisεion, εuch aε found in fiber optic gyroscope applications.

One known technique for suppressing the back- transmission of the light generated by the erbium fiber involves the use of an additional WDM to couple the pump wave into the erbium-doped fiber. This WDM typically suppress the 1550 nm back-transmitted light by approximately 17 dB to 25 dB. Such a degree of suppression may be sufficient when the output power of the erbium fiber is relatively low and the amount of back-transmitted light is small. It becomes inadequate, however, when the fiber optic amplifier output power is high. As the amplifier output power increases, the amount of erbium emission transmitted back to the laser diode also increases, and the risk of damaging the laser diode is correspondingly increased. In a 25 dBm fiber optic amplifier, the risk of catastrophic damage to the laser diode becomes unacceptable if only a WDM is used. Furthermore, the addition of a WDM significantly increases the cost of the fiber optic amplifier. A fiberoptic isolator cannot readily be substituted for the WDM, since 980 nm isolators are significantly larger and much more expensive than the commonly available 1550 nm isolators used in fiber optic amplifiers.

Another known technique to further attenuate the back-transmitted 1550 nm radiation is to insert a piece of optical fiber between the laser diode and the WDM that is specially manufactured to be highly absorptive over the 1550 nm band but highly transmisεive at the 980 nm pump wavelength via specialized doping of the fiber. This filter fiber can be made εufficiently long to achieve almost any desired attenuation of the 1550 nm radiation. Alternatively, a specially manufactured optical grating could be used as a filter to suppreεε the 1550 nm emission. There are several important disadvantages associated with these techniques, however. The most apparent one is the cost, aε the specially manufactured filter fiber or grating is quite expensive. Moreover, the insertion of the filter fiber or grating also requires fusion splices at the two ends of the fiber. The fusion splicing not only increases the time and cost required to assemble the fiber optic amplifier, but also introduces additional insertion losses between the laser diode and the erbium fiber gain medium. This reduces the amount of available pump power transmitted to the erbium fiber, resulting in lower efficiency and lower output power of the fiber optic amplifier or fiber laser. A need, therefore, exists to provide an effective and inexpensive way to eliminate the risk of degradation or damage to the laser diode pump source caused by the emission from an erbium-doped fiber.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a primary object of the present invention to provide a laser diode pump source for pumping a gain medium, wherein the harmful emission generated by the gain medium is effectively prevented from damaging the laser diode in the pump source without compromising the transmisεion of the pump wave to the gain medium.

It is another object of the present invention to provide a laser diode pump εource for a fiber optic amplifier or fiber laser that has a highly effective yet inexpensive means for eliminating back-transmission of the harmful radiation emitted by the gain medium. It is a more specific object of the present invention to provide a laser diode pump source for pumping an erbium-doped fiber in a fiber optic amplifier, wherein the pump εource includes means for eliminating back-transmisεion of the harmful radiation emitted by the erbium-doped fiber gain medium to the laser diode.

It is a related object of the present invention to provide a fiber optic amplifier having an erbium-doped fiber pumped by a laser diode, wherein the back- transmitted emission from the erbium-doped fiber is effectively blocked from the laser diode.

In accordance with these and other objects of the present invention, there is provided a pump source for pumping a gain medium which absorbs optical radiation at a first wavelength and generates optical radiation at a second wavelength, the second wavelength being greater than the first wavelength. The pump source comprises: semiconductor means for generating optical radiation at the first wavelength, the semiconductor means being susceptible to damage caused by optical radiation at the second wavelength; and filter means for attenuating optical radiation at the second wavelength. The filter means includes a predetermined length of optical fiber having first and second ends, the first end directly connected to the semiconductor means, the second end coupled to the gain medium. The optical fiber is formed to have at least one bend radius sufficiently small such that the filter means has greater losses at the second wavelength than at the first wavelength.

In a preferred embodiment of the invention, a fiber optic amplifier is provided for amplifying optical radiation at a signal wavelength, the fiber optic amplifier comprising: a laser diode for generating optical radiation at a pump wavelength, the laser diode being susceptible to damage caused by optical radiation at the signal wavelength; a gain medium for absorbing optical radiation at the pump wavelength and generating optical radiation at the signal wavelength by stimulated emission; and a predetermined length of single mode optical fiber configured to have at least one bend radius sufficiently small εuch that the fiber exhibitε high losses at the signal wavelength and low losses at the pump wavelength, the fiber having a firεt end directly connected to the laser diode and a second end optically coupled to the gain medium, whereby the optical fiber attenuates optical radiation at the signal wavelength at least 10 dB without attenuating optical radiation at the pump wavelength more than 1 dB.

In the fiber optic amplifier of the preferred embodiment, the laser diode generates a pump wave at a wavelength of approximately 980 nm, and the gain medium is an erbium-doped fiber with an emission wavelength of approximately 1550 nm. The single mode optical fiber is configured in the approximate shape of a circular coil having approximately nine turns and having a diameter of approximately 19 mm. In this embodiment, the optical fiber filter coil provides approximately 100 dB attenuation to the optical radiation at 1550 nm, without providing more than 1 dB insertion loss to the optical radiation at 980 nm. By virtue of this optical fiber filter coil, the risk of degrading or damaging the laser diode by the 1550 nm light generated by the erbium fiber is significantly reduced. Moreover, the optical fiber filter is compact, inexpensive, and easily manufactured.

Other objects and advantageε of the preεent invention will become apparent with reference to the following detailed description when taken in conjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a block diagram of a fiber optic amplifier having a laser diode pump source in accordance with the practice of the present invention;

FIG. 2 is a schematic diagram illustrating the interface of a laser diode and one end facet of the an optical fiber filter according to the present invention; FIG. 3A is a cross-sectional view of a typical single mode optical fiber, and FIG. 3B is a graph illustrating the optical field distribution in the optical fiber;

FIG. 4A is a schematic view of the laser diode pump source of one embodiment of the present invention having a laser diode and a coil of single mode optical fiber, and FIG. 4B is a graph illustrating the loss for one turn of the coil of FIG. 4A as a function of wavelength and bend radius;

FIG. 5 is a graph illustrating the total loss versus number of turns of the coil of FIG. 4A at the wavelengths and bend radius of the preferred embodiment if the invention;

FIG. 6A is a perspective view of a mandrel used for forming the optical fiber filter coil, and FIG. 6B is a cross-sectional view of the mandrel taken across lines 6B-6B of FIG. 6A and showing an optical fiber filter coil formed therein;

FIG. 7A is a perspective view of a housing for enclosing the fiber optic amplifier of FIG. 1 showing a buried mandrel for forming the optical fiber filter coil, and FIG. 7B is a cross-sectional view taken across lines 7B-7B of the housing of FIG. 7A showing the buried manual.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, FIG. 1 is a block diagram of a fiber optic amplifier 20 having the laser diode pump εource with optical fiber filter according to the present invention. Generally, the fiber optic amplifier 20 has a quantity of gain medium which optically amplifies optical input signals by means of stimulated emission. The gain medium is an erbium-doped optical fiber 22. The gain medium is pumped by a laser diode pump source 24 constructed according to the teaching of the present invention. In the illustrated embodiment, the laser diode 26 in the pump source 24 is a strained-layer InGaAs single- stripe laser diode which emits light at a wavelength of approximately 980 nm. More specifically, the laser diode 26 of the preferred embodiment is a 980 nm pump module model number BFSWA0980 manufactured by SDL

Optics, having a minimum specified output power of 70 mw emitting within the range of 975 nm to 985 nm. However, any semiconductor device which may be detrimentally affected by extraneous optical radiation of a different or higher wavelength can be used. For example, the optical fiber filtering technique of the present invention may be used with an AlGaAs 880 nm laser diode if it could be used to pump a gain medium operating at a higher wavelength such as 1300 nm or 1550 nm. Although a single-stripe laser diode is used in the preferred embodiment, other semiconductor pump sources, such as laser diode arrays and broad area laser diodes, may be used if their characteristics, such as multimode operation, can be utilized in the particular application.

The device being pumped in the preferred embodiment is a gain medium comprised of an erbium-doped silica optical fiber 22. This fiber has an absorption band of approximately 980 nm which can therefore be efficiently pumped by the laser diode 26. The radiation emitted by the erbium-doped fiber 22 has a wavelength within a range of 1520 nm to 1570 nm, but is typically about 1550 nm.

As described above, the radiation emitted by the gain medium may cause degradation or even destruction to the laser diode 26 in the pump source 24 if it iε allowed to be back-tranεmitted to the output facet of the laser diode. In accordance with the teaching of the present invention, the laser diode 26 iε protected from the radiation generated by the gain medium by means of an optical fiber filter. In the preferred embodiment, the optical fiber filter is formed as a coil 28 comprised of a singular piece of single mode optical fiber 29. The coil 28 is formed in such a way that it permits the pump wave to pass through with negligible attenuation but at the same time haε high losses at the emission wavelength of the gain medium. As illustrated in FIG. 1, the coil 28 is inserted in the optical path between the laser diode and the gain medium. One end of the coil 28 is directly connected to the laser diode 26. The term "directly connected" as used herein means that there are no intervening optical components between the laser diode 26 and the coil 28 that serve to isolate the laser diode from the coil at the emission wavelength of the erbium fiber. The pump wave transmitted through the coil is then coupled into the erbium fiber 22 for pumping thereof. The back-transmitted radiation from the gain medium is attenuated in the coil to a level sufficiently low that it is unlikely to cause undesirable effects on the laser diode 26.

Using the optical fiber filter of the present invention to protect the laser diode is highly advantageous for several reasons. First, the coil 28 can readily be formed of standard single mode optical fiber, which is significantly less expensive than the specially manufactured filter fibers mentioned above. Moreover, one or more pieces of single mode optical fiber is normally required for connecting the laser diode 26 to the erbium-doped fiber 22 regardless of whether an optical fiber filter is uεed. Thus, the coil 28 can be formed manually from a section of such a connection fiber. In this way, no extra fusion splice junction is required to use the coil 28 in the fiber optic amplifier 20, thereby permitting maximum transmission of the pump wave to the erbium fiber. Furthermore, no additional fiber procesεing steps are required, and no special fibers are required to be stocked to perform the function of preventing substantially all of the back-transmitted radiation from reaching the output facet of the laser diode.

In many configurations, the output of the laser diode 26 can be directly focused into the input end 30 of the coil 28. Such an arrangement is illustrated in FIG. 2, which is a schematic diagram showing such focusing optics. The pump wave generated at the output facet 37 of the laser diode 26 is focused directly by two focusing lenses 34, 36 into the end facet 38 of the input end 30 of the optical fiber filter coil 28. The input end 30 of the optical fiber 29 is typically secured by a fixture, such as a ferrule 40, so that the fiber can be precisely aligned with respect to the laser diode for maximum transmission.

It will be appreciated to those skilled in the art that, due to practical considerations such as the lengths of optical fibers available or the ease of construction, it may sometimes be necessary or desirable to focus the laser diode output into a separate piece of optical fiber which is then connected to the input end of the coil. In such configuration, the separate piece of fiber does not perform any isolation function at the wavelengths of interest, and hence, the coil is still "directly connected" to the laser diode. Moreover, it is contemplated that the optical fiber filter of the present invention could also be made from multimode fiber in some applications. FIG. 3A showε the typical conεtruction of a single mode optical fiber 29 for forming the coil 28. The fiber 29 has a core 42 for transmitting light, and a cladding 44 surrounding the core. The cladding 44 has an index of refraction which is slightly lower than that of the core 42. In the preferred embodiment, in order to avoid multimode transmission that can cause distortion, the diameter of the core 42 of the εingle mode optical fiber 29 is made sufficiently small so that only one transverse mode will be supported in the core. The diameter of the core 42 is typically between 1 μm and 10 μm, and the diameter of the cladding is typically between 75 μm and 150 μm. The dimensions of the core and the cladding are generally selected to maximize the transmission at a given wavelength. Therefore, the exact dimensions depend on the wavelength to be transmitted and the indices of refraction of the core and cladding. In the preferred embodiment, the single mode fiber 29 is a piece of "Flexcor™ 1060" optical fiber made by Corning Inc. , which has a germania-doped silica core having a diameter of 5 μm surrounded with a pure silica cladding having a diameter of 125 μm.

A typical single mode silica fiber, without being doped to change its absorption characteristics, is highly transmissive over a very broad range of wavelengths so long as the fiber is not bent significantly. For example, the attenuation of a typical single mode optical fiber at 980 nm wavelength is less than 2.1 dB/km, and the fiber has its loweεt attenuation of approximately 0.25 dB/km at the erbium emission wavelength of 1550 nm. Thus, if a piece of straight or slightly bent single mode optical fiber is used for transmitting the 980 nm pump wave to the erbium-doped fiber 22, it would not be able to prevent the 1550 nm light generated by the erbium-doped fiber from reaching the laser diode.

When light iε transmitted through such a single mode fiber, roost of the light is confined to the core, but some of the light passes into the cladding, due to the finite difference between the indices of refraction of the core and the cladding. FIG. 3B is a graph illustrating the normalized intenεity diεtribution of optical radiation of different wavelengthε as would be seen acroεε the εingle mode fiber 29 of FIG. 3A when the fiber iε εubεtantially straight or, if it is bent, when it has a relatively large radius of curvature, such as, for example, 100 mm or greater. Points 42a and 42b of the graph correspond to the interface surfaces between the core 42 and the cladding 44. Curve 46 representε the normalized intensity distribution of light in the fiber at a first wavelength, such as 980 nm, and curve 48 represents the normalized intensity distribution of light at a second wavelength, such as 1550 nm, which is greater than the first wavelength. As shown in FIG. 3B, most of the light is confined in the area between points 42a and 42b representing the width of the core 42, but it can be seen that the field distribution has a tail representing the minimal amount of light that penetrates into the cladding 44. Generally, the amount of cladding penetration increaseε with wavelength. Thus, as illustrated, the cladding penetration at 1550 nm is greater than that at 980 nm. When the single mode fiber 29 is bent to have a sufficiently small radius of curvature, the bending will change the propagation characteristics significantly such that the intensity distribution illustrated in FIG. 3B is no longer applicable. If the bend is sufficiently sharp, a portion of the light which originally travels confined within the core as a core mode would pass into the cladding of the fiber and become cladding modes or radiation which is ultimately lost. The effects of the fiber bending on light transmisεion depend on the radius of the bend and the wavelength. As illustrated in FIG. 3B, light at the longer 1550 nm wavelength already has more field penetration into the cladding when travelling in a straight section of the fiber than the 980 nm light, and is therefore more likely to be transferred into the cladding when it enterε the bent section. Thuε, for a given bend radius, light at a longer wavelength would experience a higher loss than light at a shorter wavelength. By properly selecting the radius of the bend, a bent section of single mode fiber can be used as a filter to attenuate light at a longer wavelength without significantly affecting the transmission of light at a shorter wavelength. Therefore, the optical fiber filter of the present invention serves as a wavelength selectable filter.

In the preferred embodiment the optical fiber filter is formed from a single mode fiber shaped into a circular coil 28, as illustrated in FIG. 4A. The laser diode 26 provides the pump wave into one end of the fiber 29 of coil 28, the other end of which is coupled to the gain medium. The coil 28 has a bend radius R* as show. Once the wavelength of the pump wave and the back-transmitted radiation are known, the bend radius can be determined either empirically by measuring the attenuation at different radii, or mathematically by using the fiber manufacturer's specifications. As will be appreciated by those skilled in the art, the optical fiber filter of the present invention can be formed into various shapes other than a circular coil but still having the required bend radius or radii. For example, the optical fiber could be formed as a flattened or oblong coil, or formed having one or more S-shaped bends or figure-8 bends, or twisted to form a helix, or formed into many other configurations to accomplish the same function of attenuating the higher wavelength radiation.

FIG. 4B illustrates the loss for one turn of the coil of FIG. 4A as a function of wavelength and bend radius. Curve 50 corresponds to a coil having a bend radius R, of approximately 10 mm, and curve 52 corresponds to a coil having a bend radius R₂ of approximately 25 mm, which is greater than R,. As can be seen from the graph, the losε per turn for the coil with the larger radius R₂ is negligible (lesε than 1 dB) at the higher wavelength (1550 nm) . A coil having this bend radius therefore does not effectively attenuate the 1550 nm light. On the other hand, a coil having a bend radiuε R, haε a significant loss (greater than 10 dB) at 1550 nm, but still has a negligible loss (less than

1 dB) at 980 nm. Thus, a coil having a bend radius R, can be used to effectively attenuate the undesirable back-tranεmiεsion of the 1550 nm light while at the same time allowing the pump wave at 980 nm to be passed through to the erbium fiber without significant attenuation. In other words, the coil of radius R, functions as a filter to block the unwanted longer wavelength light but allows the shorter wavelength light to pass through. In general, at the wavelengths typically used in fiber optic amplifiers, the wavelength separation should be at least 100 nm, and the bend radius should be less than 15 mm.

In the preferred embodiment, using the particular type of single mode fiber deεcribed above, the coil 28 preferably has a bend radius within the range of 7 mm to 12 mm, and most preferably has a bend radiuε of approximately 9.5 mm. Hence, the coil diameter iε most preferably 19 mm.

As described above, the loss through a single turn of the coil of a particular optical fiber is a function of the wavelength as well as the bend radius. However, the total amount of attenuation of the optical fiber filter depends on the length of the fiber used to form the filter. If a coil shape is used to achieve the desired bend radiuε, then the total amount of attenuation can readily be controlled by increasing the number of turnε of the coil. Hence, once the bend radius R_t of the coil is determined to achieve a desired loss per turn for a given wavelength and fixed, the total losε characteriεticε of the coil can be independently controlled for any application by simply varying the number of turns of the coil.

FIG. 5 illustrates the total loss versus number of turns of the 19 mm diameter coil of FIG. 4A at the two wavelengths of interest, namely, the 1550 nm curve 54 and the 980 nm curve 56. Note that the 0-120 dB scale for the 1550 nm curve 54 is on the left vertical axis of the graph, and the 0-1.0 dB scale for the 980 nm curve 56 iε on the right. As can be seen from both FIG. 5 and FIG. 4B, the loss per turn at 1550 nm is approximately 11 dB, while the loss at 980 nm is negligible. As can further be seen from FIG. 5, the desired total attenuation at a given wavelength can be increased by increasing the number of turns in the coil 28. In the preferred embodiment, the circular coil 28 has nine turns so as to attenuate the 1550 nm light by approximately 100 dB, which appears to provide a reasonable safety margin. Even at greater than 100 db attenuation of the 1550 nm wavelength, the 980 nm pump wave has minimal attenuation, less than 0.5 dB.

It should be appreciated, however, that a different bend radius may be chosen to provide more or less attenuation per turn, and a different number of turns may be chosen to provide the more or less total loss for the desired safety margin. For example, in another embodiment, only five turns of single mode fiber formed into a 25 mm diameter coil provided approximately 40 dB of attenuation at 1550 nm, while the attenuation at 980 nm was less than 0.1 dB. Moreover, the present invention is not limited to the εpecific application of protecting a 980 nm laser diode from the 1550 nm light generated by an erbium fiber. In fact, the optical fiber filter of the present invention can be used with any semiconductor pump εource emitting at a lower wavelength than that of the undesirable back- transmisεion of light from any optical device.

Referring back to FIG. 1, the laser diode pump source 24 of the present invention is advantageouεly used in the fiber optic amplifier 20 to pump the erbium- doped fiber 22. In the preferred embodiment, the first end 30 of the coil 28 is directly connected to the laser diode 26 for receiving the pump wave. The other end 32 of the coil 28 iε connected to one input of a wavelength division multiplexer (WDM) 54 which couples the pump wave to the erbium-doped fiber 22. In the illustrated embodiment, the erbium-doped fiber gain medium is comprised of a type DF1500F amplifier fiber manufactured by Fibercore Ltd. The WDM is a model SWDM-C 980/1550 nm unit commercially available from E-Tek Dynamics, Inc. The input optical signal to the fiber optic amplifier 20 is provided via the input signal fiber 56 through an input isolator 58. The input isolator 58, which allows the 1550 nm light to be transmitted in only one direction, prevents any back reflection at 1550 nm from being transmitted to the source of the input signal. The isolator 58 is connected to the second input 33 of the WDM 54. The input signal and the pump wave are combined by the WDM 54, and its output fiber 60 is fusion spliced to the erbium-doped gain medium fiber 22. The erbium-doped fiber 22 absorbs the pump wave and amplifies the input optical signal via stimulated emission as is known in the art. The other end of the erbium fiber 22 is fusion spliced to the input fiber 61 of the output isolator 62. The output isolator 62 prevents any 1550 nm light generated or reflected by downstream optical elements from entering the fiber optic amplifier 20. The amplified optical output signal from the output isolator 62 is available at the output signal fiber 64.

The primary εource of the back-tranεmitted 1550 nm light iε the spontaneous emission in the erbium-doped fiber 22. The back-transmitted 1550 nm light is first reduced in magnitude by the WDM 54 as mentioned before, while the coil 28 provides significantly more attenuation to reduce the back-transmitted light to a level that can be tolerated by the laser diode 26. The coil 28 can be formed and retained in many different ways. For example, FIG. 6A shows a mandrel 66 which has a cylindrical body 68 having an internal bore 70 and a ledge 72 at the top covering the internal bore. The coil is formed in the bore 70 of the mandrel 66, and the ledge 72 prevents the formed coil from coming out of the mandrel once formed. Two slots 74, which are cut in to the body of the mandrel through the ledge 72, allow the fiber to enter and exit the bore 70. FIG. 6B is a cross-sectional view taken across lines 6B-6B of the mandrel 66. FIG. 6B shows the coil 28 of fiber 29 formed within the mandrel body 68. For purposes of understanding, the thickness of the single mode fiber 29 has been significantly enlarged in FIG. 6B, such that only 3.5 turns are shown. The actual diameter of the fiber is only 250 μm including the outside coating, and typically five or more turns comprise the coil. The diameter of the internal bore 70 determines the bend radius and diameter of the coil 28. The shape of the formed coil is retained by the coil tension of the single mode fiber 29 against the mandrel bore 70, and the ends of the fiber are glued into the slots 74.

FIG. 7A illustrates one embodiment of a housing 80 for enclosing the fiber optic amplifier 20 shown in FIG. 1. Instead of using a separate mandrel for forming the coil, the housing 80 provides a "buried" mandrel 82, which comprises a bore 84 formed in the base of the housing for receiving the coil 28 and retaining its shape. To keep the coil in the bore 84, a penannular retaining piece 86 is provided which is sized to fit snugly on a matching penannular seat 88 at the top end of the bore 84. Slots 81 are provided at the sides of the bore 84 to allow the single mode fiber to enter and exit the bore. A cross-sectional view of the bore 84 is shown in FIG. 7B, wherein the retaining piece 86 iε εhown in its fully-seated position. The use of the buried mandrel 82 frees up space inside the housing 80, reduces costs of a separate mandrel piece, improves manufacturability and assembly of the optical fiber filter, and allows the entire fiber optic amplifier 20 to be securely packaged in a compact housing. While only particular embodiments of the invention have been shown and described herein, various modifications and alternative constructions may be made by those skilled in the art. Accordingly, the appended claims are intended to cover all such modifications and alternative constructions that fall within the true scope and spirit of the invention as defined by the appended claims.

Claims

What is claimed is:

1. A pump source for pumping a gain medium which absorbs optical radiation at a first wavelength and generates optical radiation at a second wavelength, the second wavelength being greater than the first wavelength, the pump source comprising: semiconductor means for generating optical radiation at the first wavelength, the semiconductor means being susceptible to damage caused by optical radiation at the second wavelength; and filter means for attenuating optical radiation at the second wavelength, said filter means including a predetermined length of optical fiber having first and second ends, the first end directly connected to the semiconductor means, the second end coupled to the gain medium, the optical fiber being formed to have at least one bend radius sufficiently small such that the filter means has greater losses at the second wavelength than at the first wavelength.

2. The pump source according to claim 1, wherein said semiconductor means is a laser diode.

3. The pump source according to claim 2, wherein said optical fiber is single mode optical fiber.

4. The pump source according to claim 3, wherein said optical fiber has at least one bend radius of 15 mm or less.

5. The pump source according to claim 3, wherein said optical fiber is formed as a coil having a diameter of 25 mm or less.

6. The pump source according to claim 5, wherein said optical fiber coil has at least five turns.

7. The pump source according to claim 1, wherein said filter means attenuates optical radiation at the second wavelength at least 25 dB, and wherein said filter meanε attenuates optical radiation at the first wavelength less than 2 dB.

8. The pump source according to claim 1, wherein said gain medium includes rare-earth doped optical fiber, wherein the first wavelength is within an absorption band of the optical fiber, and wherein the second wavelength is within an emission band of the optical fiber.

9. The pump source according to claim 1, wherein said semiconductor means is a laser diode, wherein said gain medium includes a single-mode optical fiber, wherein the first wavelength is at least 100 nm less than the second wavelength.

10. The pump source according to claim 9, wherein said wherein the firεt wavelength is approximately

980 nm, and wherein the second wavelength iβ approximately 1550 nm.

11. A fiber optic amplifier for amplifying optical radiation at a signal wavelength, εaid fiber optic amplifier comprising: a laser diode for generating optical radiation at a pump wavelength, the laser diode being susceptible to damage caused by optical radiation at the signal wavelength; a gain medium for absorbing optical radiation at the pump wavelength and generating optical radiation at the signal wavelength by stimulated emission; and a predetermined length of single mode optical fiber configured to have at least one bend radius sufficiently small such that the fiber exhibits high losses at the signal wavelength and low losses at the pump wavelength, the fiber having a first end directly connected to the laser diode and a second end optically coupled to the gain medium, whereby the optical fiber attenuates optical radiation at the signal wavelength at least 10 dB without attenuating optical radiation at the pump wavelength more than 1 dB.

12. The fiber optic amplifier according to claim 11, wherein the gain medium is an erbium-doped optical fiber, wherein the pump wavelength is within an absorption band of an erbium-doped optical fiber, and wherein the signal wavelength is within an emission band of an erbium-doped optical fiber.

13. The fiber optic amplifier according to claim 12, wherein the pump wavelength is approximately 980 nm, and wherein the signal wavelength is approximately 1550 nm.

14. The fiber optic amplifier according to claim 13, wherein the optical fiber attenuates optical radiation at the signal wavelength at least 25 dB without attenuating optical radiation at the pump wavelength more than 0.5 dB.

15. The fiber optic amplifier according to claim 11, wherein the predetermined length of single mode optical fiber is configured as a coil having at leaεt five turns and having a diameter of 25 mm or less.

16. The fiber optic amplifier according to claim 15, further including a mandrel having an internal bore adapted for receiving the optical fiber and forming it into a coil and retaining the shape of the coil.

17. The fiber optic amplifier according to claim 15, further including a housing for enclosing the fiber optic amplifier, the housing including a wall having a cylindrical bore formed therein for receiving the optical fiber and forming it into a coil and retaining the shape of the coil.

18. The fiber optic amplifier according to claim 11, wherein the gain medium provides more than 10 dB gain.

19. The fiber optic amplifier according to claim 11, further including a wavelength division multiplexer connected to the second end of the predetermined length of single mode optical fiber, wherein the wavelength division multiplexer is also connected to the gain medium.

20. A laser diode pump source for providing optical radiation at a pump wavelength to a device being pumped, the laser diode pump source being susceptible to catastrophic damage due to absorption of optical radiation at a higher wavelength produced by the device being pumped, the laser diode pump source comprising: a single-stripe laser diode; a filter coil comprised of a predetermined length of single mode optical fiber configured in the approximate shape of a circular coil having at least three turns and having a bend radius of lesε than 15 mm, the optical fiber having a first end and a second end, the second end optically coupled to the device being pumped; and focusing means for optically coupling the laser diode to the first end of the fiber, whereby the filter coil provides at least 25 dB attenuation to the optical radiation at the higher wavelength produced by the device being pumped while providing less than 1 dB insertion loss to the optical radiation at the pump wavelength.