RING LASER PUMPED OPTICAL AMPLIFIER

Abstract: An optical amplifier includes an erbium doped fiber (2) having its opposite ends connected to first and second outputs (11, 12) of a ring laser (1) so that bidirectional radiation emitted from the ring laser is bidirectionally coupled into the erbium doped fiber to efficiently pump the fiber and amplify a telecommunications signal (20) passing through the fiber.

Description: Ring Laser Pumped Optical Amplifier Background of the Invention The invention relates to optical amplification of optical signals in optical fibers, preferably telecommunication and CATV signals. Optical attenuation in fibers, splices, taps, couplers, and other components limit loss margins in transmission and distribution systems thus requiring signal amplification. Amplifiers for amplifying optical signals typically include electrooptic transducers which convert optical energy into electrical energy and vice versa, and hence tend to be complicated in design and expensive especially in high data rate or speed transmission systems. Recently, a lot of attention has been given to optical amplification as a solution to this amplification problem. Several types of amplifiers including Raman and semiconductor amplifiers have been investigated. However, for various technical reasons doped fiber amplifiers appear to present the closest approach for commercial application, the dopant being either rare earth or transition metals. One of the critical issues in doped fiber amplifiers is the method of pumping of the dopant atoms to achieve population inversion and optical gain while minimizing noise. Optical pumping at several wavelengths has been investigated, such as at 514 nanometers (nm), 635 nm, 820 nm, 980 nm and 1480 nm. Of these wavelengths, 980 nm and 1480 nm are most desirable due to their availability from semiconductor laser diodes. The amount of power available from semiconductor lasers is limited by thermal effects in the devices. The doped fiber amplifiers require the highest amount of power possible that can be handled by the doped fiber. This has led amplifier designers to try several approaches to maximize the power density of pump light which is coupled into the doped fiber core. One commonly accepted technique is to use dual laser pumps with each laser pump being coupled to an opposite end of the doped fiber. This approach suffers important drawbacks, the most significant of which is cross-talk between the two counter-propagating pump lasers. Another drawback is the need to coat the pump laser facets with multilayer reflective dielectric coatings. Such coatings add cost to the laser pumps and are a cause of laser failure. Coatings also reduce output power available for pumping. Furthermore, such coatings affect a wavelength of an output of the pump lasers which in turn affects the efficiency of pumping. This makes it necessary to have tight control of manufacturing the facet coatings, further increasing the pump laser cost. Summary of the Invention It is an object of the present invention to produce an optical amplifier suitable for amplifying optical signals in an optical transmission network so as to increase the amount of pump power in a doped fiber. It is a further object of the invention to produce an optical amplifier which is reliable in design, capable of self starting reliably, and which produces high amplification with minimum noise. It is yet a further object of the invention to provide a fiber amplifier which is capable of tuning the pump wavelength to an optimum ' value, and which solves the cross-talk problem discussed above. According to a preferred embodiment of the invention, a fiber amplifier is provided which obsoletes a need for facet coatings. According to the invention, a doped fiber is pumped from each of its opposite ends by a ring laser. According to one preferred embodiment, the doped fiber forms an integral part of a closed ring path of the ring laser, and according to another preferred embodiment the doped fiber is external to a closed ring path of the ring laser. Preferably, optical outputs from both ends of a semiconductor diode chip are coupled into single mode fibers whose ends have been prepared according to conventional fiber microlensing techniques. The path to achieve lasing is provided by recirculating the optical output from one facet end back into the other via a closed ring path. Preferably, tunable filters are included in the ring path to control the lasing wavelength. For the embodiment where the doped fiber is external to the closed ring path of the ring laser, the ring laser and doped fiber are optimally interconnected by a coupler having a coupling coefficient which is designed to optimize an output power and wavelength of the ring laser. Two outputs from the coupler are then directed into the doped fiber by means of two wavelength multiplexing couplers. In the embodiment where the doped fiber forms part of the closed ring path of the ring laser, an output from one end of the semiconductor laser used for pumping is redirected to the other end via the first wavelength multiplexing coupler, the doped fiber, and a second wavelength multiplexing coupler thus completing the ring path and making the doped fiber an integral part of the ring. This embodiment has the advantage of being simpler in design and provides a single feedback loop in contrast to the other embodiment which provides two feedback paths, one to the doped fiber, the other to the ring laser. A disadvantage of the one feedback path embodiment though is that it may require higher power to initially start up since some of the power otherwise available for reaching lasing threshold is absorbed by the doped fiber, particularly at low radiation powers since the doped fiber has relatively high attenuation at such powers. However, as the power is increased, bleaching within the doped fiber drastically reduces its attenuation thus achieving lasing threshold and higher powers. These and other objects of the invention are achieved by an optical amplifier for amplifying an optical signal coupled into and out of the amplifier, comprising: a doped fiber which includes dopant ions such that optical gain is provided to the optical signal when the doped fiber is pumped by optical radiation having a wavelength different from that of the optical signal; a semiconductor ring laser which generates the optical radiation bidirectionally; means for optically interconnecting the ring laser and the doped fiber such that the bidirectional optical radiation from the ring laser is bidirectionally coupled into the doped fiber; means for coupling the optical signal into and out of the doped fiber such that the optical signal is amplified by the bidirectional optical radiation. Further objects of the invention will be apparent by reference to the following detailed description. Brief Description of the Drawings FIG 1 illustrates a first embodiment of the invention, showing a ring laser and doped fiber together forming a single closed path. FIG 2 illustrates a second embodiment of the invention with a ring laser closed path and a doped fiber closed path being interconnected by an optical coupler. Detailed Description of the Preferred Embodiment FIG 1 illustrates a first preferred embodiment of the invention whereby a semiconductor laser diode 1 is connected to a doped optical fiber 2 via an optical filter 3 (for example JDS model TB980) and first and second optical couplers, preferably passive devices 6, 7 (for example Gould model #980/1550-COW-MX-02X02). As FIG 1 illustrates, the laser diode is interconnected with the filter 3, couplers 6, 7 and doped fiber 2 (for example Photoneties fiber type EDOS-103) via an optical fiber 10 arranged in a loop such that a signal emitted through one end 11 of the laser diode is fed back into the laser diode via its opposite end 12, and vice versa, i.e. a signal emitted from the end 12 of the laser diode is routed back into the laser diode via its opposite end 11. Accordingly, in operation, the laser diode is capable of lasing using light emitted from both its ends and hence emits bidirectional radiation, and this bidirectional radiation is coupled through the doped fiber. This structure is in contrast to a distributed feedback laser which has a back end or facet which is mirrored so that light is emitted from only one end or facet of the laser. According to a preferred embodiment, the laser ends 11, 12 are angled at less than a 90° angle relative to a horizontal lasing axis of the laser 1 to prevent back reflections at the facets 11, 12 from reentering the diode cavity of the laser. This construction minimizes any adverse feedback due to undesired internal reflections at the laser ends 11, 12. Preferably, the fiber 10, which is preferably single mode fiber, has ends 13, 14 which have lenses formed thereon so as to form optimum coupling junctions with the laser ends 11, 12. According to this structure, a network having a weak signal 20 on network incoming optical fiber 21 which is required to be amplified as an amplified signal 120 on network outgoing fiber 121 is connected to the optical amplifier as described via one of two inputs of passive coupler 6. The other input to the passive coupler 6 is connected to the fiber 10 receiving an output signal 31 from the laser end face 11. Passive coupler 7 is used to couple the amplified signal 121 onto the outgoing fiber 120, with a second part of the coupler 7 interconnecting the doped fiber 2 and the laser facet 12. The doped fiber 2 is preferably a rare earth doped optical fiber, a preferred embodiment being a silicondioxide glass fiber doped with erbium. Other rare earth elements can also be used as dopants, the other rare earth elements consisting of the lanthanides which comprise Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, as well as Er. Transition metals can also be used as dopants. Preferred metals comprise aluminum, germanium, yttrium. Such a doped optical fiber has the characteristic that it is capable of efficiently transferring energy of one light wavelength to a second light wavelength, and hence can be used to "pump" the second wavelength. In the case of an erbium doped silicondioxide glass fiber, such a fiber typically is quite efficient at transferring energy from light having a wavelength of about 980 nm to a higher wavelength, one preferred higher wavelength being 1300 nm. Accordingly, if the signal 31 is optimally at a wavelength of about 980 nm and is quite large, combining it with the weak network signal 20 which is about 1550 nm using the coupler 6 results in both signals being simultaneously transmitted within the doped fiber 2 which then couples energy from the signal 31 to the signal 20 thus amplifying the signal 20 producing amplified outgoing signal 120. In addition, further amplification is achieved due to coupling between the weak signal 20 and the low wavelength signal 32, originating from the laser end face 12 and coupled into the doped fiber 2 via the coupler 7. The signal 32 preferably has a wavelength which is the same as the signal 31, as enabled by the use of the optical filter 3. According to this construction, it can readily be appreciated that a signal 31 remaining in the fiber 10 downstream from the doped fiber 2 is partially coupled by the coupler 7 towards the laser end face 12 so as to form a complete loop, and likewise a portion of the second signal 32 which originates from the laser end face 12 which is not absorbed or transferred to the signal 20 via the amplification process previously described is also coupled by the coupler 6 so as to enter the laser end face 11, thus completing essentially a bidirectional closed loop for the laser 1. Accordingly, upon activating the laser 1, a continuous feedback loop is achieved so that sufficient optical energy is routed into the laser 1 so as to insure the lasing action of the laser and high efficiency operation thereof. FIG 2 illustrates an alternate embodiment of the invention, hi this figure, elements common to those of FIG 1 are identified with similar reference numerals. In FIG 2 a third coupler 36 (for example Gould #217608 or Sifam SVR-98) has been added which is coupled at first and second ends to ends of the fiber 10, and also coupled at its first and second ends with the first and second couplers 6, 7. An advantage of this structure is that it relaxes a design criteria for a choice of the semiconductor laser 1 and the doped fiber 2. Specifically, the doped fiber 2 absorbs energy nonlinearly depending on an energy input thereinto. Specifically, the lower the energy input into the doped fiber 2, the higher the energy absorption (in dB), and this characteristic is known as bleaching. At low input power, the doped fiber 2 could have the effect of absorbing an undue percentage of the laser light such that an insufficient amount of the signal 31, 32 is looped around to an opposite end of the laser 1 so that lasing of the laser 1 is never achieved, thus resulting in very low efficiency operation of the laser 1. Though the amount of light absorption by the doped fiber 2 would go down if the laser 1 were to put out more energy, if it is not capable of lasing due to the low start up power available, the amplifier would be very inefficient. However, with the structure of FIG 2, the coupler 30 can be constructed so that a predetermined percentage of the signal 31, 32 is coupled back into the semiconductor laser 1 so that lasing powers always will be easily achieved. This results in the rapid bleaching of the fiber 2 so as to reduce its attenuation to insure that the optical amplifier is always self starting, and thus has the effect of relaxing a tolerance or design specification for a choice of the laser 1 and the doped fiber 2. Calculations have shown that the coupler 1, though beneficial as described, is not necessary in many preferred embodiments since available lasers 1 and doped fibers 2 can be found which are reliably self starting and allow the laser 1 to lase when it is turned on in spite of the attenuation imposed by the doped fiber 2 at low powers. Though the invention has been described by reference to certain preferred embodiments thereof, the invention is not to be limited thereby and only by the appended claims.

Claims: Claims 1. An optical amplifier for amplifying an optical signal coupled into and out of the amplifier, comprising: a doped fiber which includes dopant ions such that optical gain is provided to the optical signal when the doped fiber is pumped by optical radiation having a wavelength different from that of the optical signal; a semiconductor ring laser which generates the optical radiation bidirectionally; means for optically interconnecting the ring laser and the doped fiber such that the bidirectional optical radiation from the ring laser is bidirectionally coupled into the doped fiber; means for coupling the optical signal into and out of the doped fiber such that the optical signal is amplified by the bidirectional optical radiation. 2. The amplifier of claim 1, the doped fiber forming a portion of a closed ring loop path of the ring laser. 3. The amplifier of claim 2, the interconnecting means including first and second wavelength multiplexing couplers, the first coupler interconnecting a first end of the doped fiber and a first laser facet of the ring laser and the second coupler interconnecting a second end of the doped fiber and a second laser facet of the ring laser. 4. The amplifier of claim 1, the interconnecting means including an optical coupler having first, second, third and fourth ports, the first and second ports being connected to first and second portions of a ring loop path of the ring laser, the third and fourth ports interconnecting first and second ends of the doped fiber such that the doped fiber is external to the ring laser ring loop path, the coupler being capable of transferring optical radiation from the ring loop path into the doped fiber bidirectionally. 5. The amplifier of claim 4, the optical coupler having a tunable coupling coefficient and tunable coupling wavelength so as to control power output and wavelength of radiation coupled from the ring loop path to the doped fiber. 6. The amplifier of claim 1 , the ring laser having first and second facets which form an angle with a longitudinal axis of a diode cavity, the angle being sufficiently different from 90° to avoid back reflections from the facets from entering the diode cavity. 7. The amplifier of claim 1, the ring laser having first and second facets which form a peφendicular angle with a longitudinal axis of a diode cavity, and further comprising antireflection coatings disposed on the first and second facets. 8. The amplifier of claim 1 , the ring laser producing optical radiation having a wavelength of either 815 nm ± 10 nm; 980 nm ± 10 nm; or 1480 nm ± 10 nm. 9. The amplifier of claim 1 , the doped fiber being doped with one or more elements selected from the group consisting of lanthanides, aluminum, and germanium. 10. An apparatus for amplifying an optical signal in a transmission optical fiber, comprising: a semiconductor ring laser which generates bidirectional optical radiation; a doped fiber which includes dopant ions such that optical gain can be provided to a first optical signal therein when the doped fiber is pumped by optical radiation having a wavelength different from that of the first optical signal; means for optically interconnecting the ring laser and the doped fiber such that the bidirectional optical radiation is bidirectionally coupled into the doped fiber; means for coupling a signal from the transmission optical fiber into the doped optical fiber and for coupling the signal which is amplified by the doped fiber out of the doped fiber and into the transmission optical fiber.

Inventors: JABR SALIM N, JABR, SALIM, N.
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1993
Main classification: H01S3
IPC codes: H01S3, H01S5