Abstract:
The present invention is directed to providing an environmentally stable, ultra-short pulse source. Exemplary embodiments relate to passively modelocked ultra-short fiber lasers which are insensitive to temperature variations and which possess only negligible sensitivity to pressure variations. Further, exemplary embodiments can be implemented in a cost-effective manner which render them commercially practical in unlimited applications. Arbitrary fiber lengths (e.g., on the order of 1 millimeter to 1 kilometer, or greater) can be used to provide an ultra-short pulse with a cost-effective architecture which is commercially practical.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 12/625,496, filed Nov. 24, 2009, which is a divisional application of U.S. patent application Ser. No. 09/262,662, filed Mar. 4, 1999, now abandoned, each of which is hereby incorporated by reference herein in its entirety. U.S. patent application Ser. No. 09/262,662 is a divisional application of U.S. patent application Ser. No. 08/876,933, filed Jun. 16, 1997, now U.S. Pat. No. 5,923,686, which is a continuation application of U.S. patent application Ser. No. 08/169,707, filed Dec. 20, 1993, now U.S. Pat. No. 5,689,519. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to an apparatus and method for generating laser energy, and more particularly, to passively modelocked lasers for providing ultra-short pulses with enhanced stability. 
         [0004]    2. State of the Art 
         [0005]    Both actively modelocked lasers and passively modelocked lasers are well known in the laser art. For example, modelocked lasers can be formed as ultra-short pulse sources using single-mode rare-earth-doped fibers for providing compact ultra-fast technology. One particularly useful fiber pulse source is based on Kerr-type passive modelocking. Such a pulse source can be assembled using widely available standard fiber components to provide pulses at the bandwidth limit of rare-earth fiber lasers with GigaHertz repetition rates. 
         [0006]    Any practical fiber-based ultra-short pulse source must be environmentally stable in order to be commercially practical. As referenced herein, the phrase “environmentally stable” refers to a pulse source which is substantially immune to a loss of pulse generation due to environmental influences such as temperature drifts and which is, at most, only slightly sensitive to pressure variations. On the contrary, conventional fiber-based ultra-short pulse sources are susceptible to both temperature and pressure variations, and require constant monitoring and control by the user to maintain pulse generation. 
         [0007]    To address the environmental sensitivity of an ultra-short pulse source cavity, Kerr-type modelocking in a polarization maintaining erbium-doped fiber has been proposed. For example, a document entitled “Passive mode locking by using nonlinear polarization evolution in a polarization-maintaining erbium-doped fiber”, Fermann, M. E. et al., OPTICS LETTERS/Vol. 18, No. 11, Jun. 1, 1993, PP. 894-896, the disclosure of which is hereby incorporated by reference, describes Kerr-type modelocking which involves a differential excitation of two linearly polarized fundamental eigenmodes of the polarization maintaining erbium-doped fiber. The two fundamental eigenmodes accumulate a differential non-linear phase delay after a certain propagation distance. Due to interference of the eigenmodes at a polarizer, the non-linear phase delay translates into an amplitude modulation which can provide sufficient pulse-shortening per round-trip to produce passive modelocking. 
         [0008]    However, such passive modelocking suffers significant drawbacks. For example, the amplitude modulation is also sensitive to the linear phase delay between the two interfering eigenmodes. Because the linear phase delay is subject to temperature and pressure variations in the polarization maintaining erbium-doped fiber, a continuously adjustable phase control is necessary to ensure stable modelocking operation. 
         [0009]    Another attempt to address the instability problems of an ultra-short pulse source cavity uses Kerr-type modelocking in a so-called “figure of eight laser” (i.e., F8L). An F8L is described in a document entitled “Polarization maintaining figure-8 laser”, by Taverner, D. et al, presented at the Optical Soc. Am. Top. meeting on non-linear guided wave phenomena at Cambridge, 1993, paper WC No. 3. This document discloses amplitude modulation in a polarization maintaining fiber laser using Kerr-type modelocking. As described therein, use of a phase controller is avoided due to a reciprocal characteristic of the polarization maintaining F8L whereby the linear phase delay along two interfering polarization directions the F8L is always 0. 
         [0010]    However, the disclosed F8L suffers significant drawbacks. For example, passive phase modulation is translated into amplitude very inefficiently. Long lengths of polarization maintaining fiber (e.g., greater than 300 meters) are therefore required within the cavity to obtain passive modelocking, and pulses generated by the F8L are relatively long in duration (e.g., greater than 2 picoseconds). Further, the pulses are generated with a relatively small pulse energy (e.g., less than 10 picoJoules) and the F8L is limited to a relatively small stability range. Because highly-birefringent fiber is used in the cavity, accurate alignment of splices between the fiber components is required, thus increasing complexity and cost of the laser assembly. 
         [0011]    Another attempt to overcome the environmental sensitivity of the phase delay between the two interfering modes of a Kerr-type modelocked fiber laser cavity (or in this particular case the phase delay between the polarization eigenmodes of the fiber) was recently disclosed by I. N. Duling III and R. D. Esnam in a document entitled “Single-Polarisation Fibre Amplifier,” Electronics Letters, Jun. 4, 1992 Vol. 28, No. 12. In this a Faraday rotator mirror is used in conjunction with a polarizer to ensure that the output polarization state of a fiber amplifier is independent of environmental perturbations. As described, the technique is not suitable for the implementation into a Kerr-type modelocked fiber laser for several reasons. 1) The linear phase delay between the polarization eigenmodes is compensated to be exactly zero after one round-trip and cannot be adjusted. 2) The Faraday rotator mirror rotates the polarization state by 90° after one round trip, which results in a complete loss of the signal light at the polarizer, preventing the onset of laser oscillation. 3) Nonlinear polarization evolution is not controlled by this simple method, which is necessary for the operation of a modelocked fiber laser. 
         [0012]    Accordingly, it would be desirable to provide an environmentally stable ultra-short pulse source which does not require operator input and/or continuous phase control to maintain stability. Further, it would be desirable to provide such a pulse source in a cost-effective manner to provide a commercially practical, ultra-short pulse source suitable for such uses as communication environments. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention is directed to providing an environmentally stable, ultra-short pulse source. Exemplary embodiments relate to passively modelocked ultra-short pulse sources which are substantially insensitive to temperature variations and which possess only negligible sensitivity to pressure variations. Further, exemplary embodiments can be implemented in a cost-effective manner which render them commercially practical. 
         [0014]    In accordance with the present invention, operator monitoring and control, using additional phase adjusters are unnecessary. Fiber lengths on the order of 1 to 2 meters can be used to provide ultra-short pulses in a cost-effective architecture which is commercially practical. Relatively high repetition rates can be achieved without requiring the complex modulation schemes of active techniques, and without becoming susceptible to instabilities typically associated with other known passive techniques. 
         [0015]    Generally speaking, exemplary embodiments of the present invention relate to a passively modelocked laser comprising means for generating laser energy and means for pumping the laser energy generating means. The laser energy generating means includes a cavity. In accordance with exemplary embodiments, the cavity includes a gain medium for amplifying energy in the cavity, means for reflecting energy along an axis which passes through the gain medium, and means for outputting laser light generated within the cavity. The cavity further includes at least one Faraday rotator to compensate for linear phase drifts between the polarization eigenmodes of the fiber, and means for transforming linear polarization of energy in the cavity. In alternate embodiments, non-polarization maintaining fiber components are included in the cavity to simplify cavity architecture and reduce assembly cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention can be further understood with reference to the following description and the appended drawings, wherein like elements are provided with the same reference numerals. In the drawings: 
           [0017]      FIG. 1  shows an exemplary embodiment of the present invention which includes use of Kerr-type passive modelocking of an erbium-doped fiber laser; 
           [0018]      FIG. 2  shows an exemplary embodiment of a coordinate system for describing polarization evolution in a cavity; 
           [0019]      FIGS. 3A and 3B  show pulse spectra obtained at either end of an exemplary stability range of a laser; 
           [0020]      FIG. 4  shows an autocorrelation trace of a generated pulse; 
           [0021]      FIG. 5  shows an alternate embodiment of a passively modelocked laser in accordance with the present invention; 
           [0022]      FIG. 6  shows an alternate embodiment which includes an additional fiber coupler for output coupling or output coupling at a Faraday rotator mirror; and 
           [0023]      FIG. 7  shows an alternate embodiment which includes a saturable absorber or a fiber stretcher to initiate a modelocking process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    In the  FIG. 1  embodiment, an environmentally stable linear phase delay is provided between two interfering polarization directions of a Kerr-type modelocked fiber laser. In the  FIG. 1  embodiment, a passively modelocked laser is generally designated  100 , and includes a means for generating laser energy generally designated as a laser cavity  200 . The laser energy generating means can be, for example, a Fabry-Perot cavity. The passively modelocked laser further includes a means for pumping the laser energy generating means, the pumping means being generally designated  300 . 
         [0025]    In the  FIG. 1  embodiment, the laser cavity  200  includes a gain medium  202  for amplifying energy in the cavity  200 . The gain medium can be any rare-earth-doped fiber capable of providing light amplification (i.e., gain). For purposes of the following discussion, reference will be made to an optically pumped laser having an active fiber doped with erbium ions as the gain medium  202 . However, those skilled in the art will appreciate that other rare-earth-doped fibers, such as fibers doped with neodymium ions, can be used. Further, the present invention is not limited to fiber lasers, but can also be used with other types of lasers such as bulk solid-state lasers comprising again medium of bulk solid-state materials, and semiconductor lasers. Optical or electrical pumping can be used, although optical pumping is generally preferred for use with bulk solid-state lasers while electrical pumping is generally preferred for semiconductor lasers. 
         [0026]    The laser cavity  200  further includes means for reflecting energy along an axis which passes through the gain medium, the axis being generally designated by the arrow  204 . The energy reflecting means includes a first cavity mirror  206  located at a first end of the cavity  200 . The cavity mirror  206  reflects signal light within cavity  200 . The cavity mirror can be any standard laser mirror readily available and known to those skilled in the art. 
         [0027]    In an exemplary embodiment, the cavity mirror  206  also functions as a laser energy outputting means. In such an embodiment, the cavity mirror  206  serves two functions; it reflects a fraction of energy impinging onto it back into the laser cavity  200 , and it allows the remaining fraction to leak through the cavity mirror  206  to provide output energy. Alternately, the first cavity mirror  206  can be separate from an output coupler means if desired. 
         [0028]    The  FIG. 1  embodiment includes two interfering polarization directions of a Kerr-type modelocked fiber laser. These interfering polarization directions include two linear polarized eigenmodes of a highly-birefringent fiber (HBF). In the exemplary  FIG. 1  embodiment, the erbium-doped fiber used as the gain medium  202  can be a highly-birefringent fiber. 
         [0029]    The interfering polarization directions can be better understood by reference to  FIG. 2 .  FIG. 2 , illustrates a cross-section  203  of the highly-birefringent, erbium-doped fiber in gain medium  202 . In the  FIG. 2  coordinate system, axes labelled x and y can be considered the two interfering polarization directions. 
         [0030]    In accordance with the present invention, the  FIG. 1  cavity  200  also can include low-birefringent fiber (LBF). In exemplary embodiments, the length of low-birefringent fiber  220  used in the cavity  200  is relatively short in comparison to the length of the highly-birefringent fiber (e.g., on the order of eight to ten times shorter). The highly-birefringent fiber thereby dominates non-linear pulse-shaping, with such pulse-shaping being negligible in the low-birefringent fiber. 
         [0031]    By using both highly-birefringent fiber and low-birefringent fiber in the cavity, non-polarization maintaining couplers can be used for coupling light into and out of the laser cavity  200 . Such a feature simplifies laser assembly and packaging, and significantly reduces overall costs. 
         [0032]    In accordance with the present invention, the laser cavity  200  further includes means for compensating linear phase drifts of the gain medium  202 . For example, a phase drift compensating means  210  includes at least one device to control polarized light generated within the cavity and thereby compensates for linear phase drifts of the gain medium  202 . For example, Faraday rotator mirrors are known devices which, when properly chosen, reflect, in an orthogonal state, any polarization state which impinges them. The phase drift compensating means can therefore include at least one Faraday rotator mirror to compensate for linear phase drifts between the polarization eigenmodes of a gain medium such as the erbium-doped fiber  202 . 
         [0033]    In an exemplary embodiment, the phase drift compensating means eliminates linear phase drifts between the two polarization eigenmodes of the laser cavity  200  by using a pigtailed Faraday rotator mirror  210  as a second cavity mirror of the reflecting means. The Faraday rotator mirror  210  can be a 45° rotator which rotates the polarization of reflected light by 90° relative to incoming light. Reflected light therefore propagates back down the gain medium  202  in exactly an orthogonal polarization state. 
         [0034]    Due to the Faraday rotator mirror  210 , the total linear phase delay between the polarization eigenmodes of the fiber is exactly zero after one round-trip. Non-linear phase changes remain uncompensated and accumulate along the polarization eigenmodes of the highly-birefringent fiber after reflection by the Faraday rotator mirror  210 . Because the highly-birefringent fiber eliminates random mode-coupling, and because the low- birefringent fiber is relatively short in length, the non-linear phase changes are governed by the relative power in the polarization eigenmodes and are not susceptible to environmental influence. 
         [0035]    The Faraday rotator mirror  210  also eliminates spatial hole-burning in the laser cavity  200  to further improve initiation of pulse generation. The Faraday rotator mirror provides a second polarization direction which is rotated by 90°, thereby providing a relatively constant intensity along the cavity length. In addition, the Faraday rotator mirror suppresses spurious back-reflections from intra-cavity fiber ends (e.g., fiber, including the gain medium  202 , included within cavity  200 ) and consistently eliminates continuous wave lasing background. For example, scattered light which is reflected back to the Faraday rotator mirror  210  will be rotated therein and absorbed by a polarizer  216 . 
         [0036]    The Faraday rotator mirror further compensates for group-velocity walk-off of the polarization eigenmodes which can be significant in a highly-birefringent fiber. Those skilled in the art will appreciate that the differing refractive indices of polarization maintaining fiber cause light to propagate faster along one axis of the fiber (e.g., x-axis) relative to another axis (e.g., y-axis). Within each round-trip of light within the cavity, pulses of the light signal continue to spread. However, the Faraday rotator mirror  210  inhibits such group-velocity walk-off by rotating the light signal  90 ° with each reflection such that pulses of the light signal which spread during one round-trip come closer together during a subsequent round-trip. 
         [0037]    The Faraday rotator mirror can be incorporated at a point in the laser cavity  200  at which walk-off between polarization eigenmodes is maximum. Thus, non-linearity of fiber components in front of the Faraday rotator mirror can be reduced to minimize unwanted non-linearity of the low-birefringent fiber. Given these characteristics, environmentally stable operation in accordance with the present invention can be achieved using relatively long lengths of low-birefringent fiber. 
         [0038]    The phase drift compensating means can further include a second Faraday rotator mirror  212 . The second Faraday rotator  212  is also a 45° Faraday rotator which, in an exemplary embodiment, can be centrally placed in the cavity to compensate for polarization rotation of the Faraday rotator mirror  210 . 
         [0039]    Although the phase drift compensating means provides environmental stability, a polarization transformation is necessary for non-linear polarization evolution to be optimized for modelocking. This action is performed by including a means for transforming linear polarization of energy, such as the incorporation of 1 or more bulk waveplates  214  within the laser cavity  200  for introducing a linear phase delay. The cavity of  FIG. 1  also includes the polarizer  216 . Polarization eigenmodes interfere at the polarizer  216  in the cavity. The polarizer  216  can be any optical polarizing element. 
         [0040]    The polarization change induced by the waveplate  214  depends on the tilt and rotation of the waveplate or waveplates. Although use of a single waveplate will minimize scattering of light, two waveplates can be used to provide independent control of the ellipticity and rotation angle of elliptically polarized light. The polarization change induced by the waveplate, or by any arbitrary number of such waveplates, or any arbitrary polarization transformer, is considered with respect to the polarization axis of the polarizer  216 . The erbium-doped fiber of gain medium  202  can be aligned at any axis with respect to the polarizer. Polarization transformation is then uniquely defined by its overall result; i.e., the polarization transformation transforms the linearly polarized light emerging from the polarizer  216  into elliptically polarized light with an ellipticity Ψ, where the ellipse is rotated by an angle α with respect to the x-axis of the fiber, and wherein the tangent of Ψ is b/a; where b and a are minor and major axes of the polarization ellipse. 
         [0041]    The representation of this polarization transformation on the Poincaré sphere (which is well known in the field) is (0,0)→(2Ψ,2α). In the presence of the two Faraday rotators  210  and  212 , the nonlinear response R(P) as a function of intra-cavity power P in the cavity is completely defined by this polarization transformation (i.e., by the values for Ψ and α) and an effective non-linear reflectivity R(P) of the cavity mirror  206  can be defined as follows: 
         [0000]        R ( P )= f (α,Ψ,φ nl (α,Ψ, P )),   (1)
 
         [0000]    where f(α,Ψ,φ nl (α,Ψ,P)) represents a function of α,Ψ,φ nl (α,Ψ,P) and φ nl (α,Ψ,P) is the differential non-linear phase delay accumulated between the polarization eigenmodes of the highly-birefringent fiber as a function of intra-cavity power P. The range of R(P) is between 0 and 1. Passive modelocking is obtained when R(P) increases with an increase in 
         [0042]    P. 
         [0043]    The passively modelocked laser of the exemplary  FIG. 1  embodiment further includes a laser pumping means  300 . The pumping means includes an energy source (e.g., electrical or optical energy source, depending on laser type) generally represented as a pump  302 . In the  FIG. 1  embodiment, wherein an erbium fiber is used as the gain medium  202 , the pump  302  can be an optical pump. 
         [0044]    A wavelength-division multiplexing coupler  304  is provided for coupling the pumping means to the cavity  200 . The wavelength division multiplexing coupler can be any multiplexer which allows pumping of the laser cavity  200  without loss of signal light; i.e., one which allows differential coupling between the pump  302  and the signal light. In an exemplary embodiment, the pump  302  can produce energy in the  980  nanometer wavelength range, and the wavelength division multiplexer coupler can be an Aster WDM 1550/980 to accommodate a 980 nanometer pump and a 1550 nanometer signal. 
         [0045]    In accordance with an exemplary embodiment, the first Faraday rotator mirror  210  and the wavelength division multiplexer coupler  304  include low-birefringent fiber. However, those skilled in the art will appreciate that such a configuration is by way of example only. It is only significant in the exemplary embodiments described herein that the total length of the highly-birefringent fiber in the cavity  200  be relatively long in comparison with the low-birefringent fiber sections. 
         [0046]    In accordance with the exemplary  FIG. 1  embodiment, the highly-birefringent fiber section starts at the intra-cavity fiber end of the gain medium  202  (e.g., adjacent to a first lens  228 ) or as close to it as possible, to ensure that an amount of power in the polarization eigenmodes of the highly-birefringent fiber stays absolutely constant. The first Faraday rotator mirror  210 , the wavelength division multiplexer coupler  304  and the highly-birefringent fiber  218  can, in an exemplary embodiment, be interconnected using fusion splices. 
         [0047]    The exemplary embodiment of  FIG. 1  further includes means for focusing energy generated along the axis  204 . The  FIG. 1  energy focusing means includes at least the first lens  228  for focusing energy received from the gain medium  202  onto the first cavity mirror  206 , and for directing energy from the cavity mirror  206  onto the gain medium  202 . The lens can be any optical element available for focusing light from the gain medium. In exemplary embodiments, the focal point of the lens should be selected to coincide with the first cavity mirror  206  so that the power density on the cavity mirror  206  is maximized. Similarly, the focal point of the lens should be selected to coincide with maximizing power density on the gain medium  202 . 
         [0048]    In an exemplary implementation of a cavity, 2.6 meters (m) of highly-birefringent fiber were used with 0.6 m of standard communications-type low-birefringent fiber. The highly-birefringent fiber had a polarization beat length of 10 centimeters (cm) at the lasing wavelength of 1.567 microns (μm). It had an effective core area of 28 μm and the numerical aperture was 0.19. The highly-birefringent fiber was doped with ≈5×10 18  erbium ions/cm 3 . 
         [0049]    Those skilled in the art will appreciate that the laser system configuration of the  FIG. 1  embodiment is by way of example only and that alternate embodiments can be used in accordance with the present invention. For example, the entire  FIG. 1  configuration can be reversed so that the Faraday rotator mirror is to the left-hand side of the cavity and the cavity mirror  206  is to the right-hand side of the cavity. 
         [0050]    In accordance with the present invention, the exact locations of the Faraday rotator mirror  210  and the Faraday rotator  212  can readily be determined by those skilled in the art. However, in accordance with exemplary embodiments, the Faraday rotator mirror  210  and the Faraday rotator  212  define an intra-cavity portion of the cavity  200  wherein all fibers (i.e., highly-birefringent fiber and low-birefringent) are located. 
         [0051]    Further, those skilled in the art will appreciate that the non-polarization maintaining, low-birefringent fiber  220  can be located between the lens  228  and the gain medium  202  in an alternate embodiment. In such an embodiment, the erbium-doped, highly-birefringent fiber can be contained within the wavelength division multiplexer coupler  304  and/or the Faraday rotator mirror  210 . Once again, those skilled in the art will appreciate that further embodiments of the present invention can be readily implemented, with the significance being the relative lengths of low-birefringent fiber and highly-birefringent fiber within the intra-cavity portion of cavity  200 . Equally, sections of fiber with different magnitudes of group-velocity dispersion can be concatenated to maximize the energy of the oscillating pulses. 
         [0052]    In accordance with an exemplary embodiment, an 80% reflecting cavity mirror  206 , and an AR-coated prism polarizer  216  can be used. Further, an AR-coated 45° Faraday rotator  212  with a 4 mm aperture, and an AR-coated quartz zero-order waveplate  214  can be used. The waveplate  214  can be optically contacted with a thickness of 3 mm. The intra-cavity fiber end contacted with the highly-birefringent fiber  218  can be cleaved at an angle of 10° and need not be AR-coated. A movable cavity mirror  206  can be employed, in an exemplary embodiment, for translation along the axis  204  and to start the modelocking process. 
         [0053]    In accordance with an exemplary operation of the  FIG. 1  embodiment, pulses as short as 360 femptoseconds or less can be produced with an energy content of approximately 60 picoJoules. Pump power variations of, for example, plus or minus seven percent will not introduce instabilities such as the onset of a continuous wave lasing background (i.e., a laser output which is not completely modelocked) or multiple pulsing. 
         [0054]    Stable modelocking can be obtained with exemplary values of δ≈130° and α≈10°. Once the waveplate  214  and the polarizer  216  are set, additional adjustment is unnecessary and they can remain permanently fixed. In accordance with exemplary embodiments, the laser is insensitive to perturbations of the low-birefringent fiber and allowed perturbations of the highly-birefringent fiber, when the perturbation period is large compared to its beat length. Even when a strong perturbation is applied (e.g., by applying a strong twist to the fiber) and modelocking is lost, once released, the fiber springs back to its original position and modelocking characteristics. Further, such exemplary embodiments are insensitive to remaining residual intra-cavity reflections. 
         [0055]    Further, a modelocking threshold can be achieved which is, for example, fifty percent higher than the pump power level of 70 mW (measured in front of the wavelength division multiplexer coupler) at which clean continuous wave-free single pulses cap be obtained in the cavity. In an alternate embodiment, the modelocking threshold can be reduced by incorporating an AR-coated intra-cavity fiber end. 
         [0056]    Exemplary pulse spectra at the edges of an exemplary stability range are shown in  FIGS. 3A and 3B . As the pump power is increased, the pulses get shorter and their spectral width broadens, leading to an increased number of soliton periods per cavity length and a corresponding increased shedding of energy into a dispersive wave (as indicated by the increased height of the spectral resonances). 
         [0057]    A typical autocorrelation trace of exemplary pulses is shown in  FIG. 4 . The generated pulses are shown to include a typical FWHM width of 360 fsec with a time-bandwidth product of ≈0.30 for an exponentially decaying (e.g., sech 2 ) pulse shape, and are completely free of pedestals. The repetition rate of the pulses is 27 MHz and the average pulse energy measured after the output coupler is 10 picoJoules. Note that a pulse energy of 60 picoJoules or higher can be extracted when using the light rejected by the polarizer. These values translate into an exemplary average intra-cavity pulse energy of 55 picoJoules, which gives a round-trip non-linear phase delay of about 1.1π which is comparable to results obtained in standard non-polarization-maintaining Kerr-type modelocked fiber lasers. 
         [0058]    Those skilled in the art will appreciate that alternate embodiments of the present invention are possible. For example, alterations to the basic laser cavity design, in addition to those already mentioned, can be used in accordance with the present invention. For example, rather using bulk components for a polarizer, waveplate, Faraday rotator, cavity mirror and lens, integrated or pigtailed components can be used to perform these same functions. 
         [0059]    Further, alternate cavity designs can be used to output laser energy. In  FIG. 5 , an alternate cavity includes a readily available pigtailed, all-fiber polarizer or polarization beam splitter (FPBS)  502  and two readily available fiber collimators  504  (FC). Output coupling can be obtained at the all-fiber polarizer or polarization beam splitter  502  and additional fiber isolators (FI)  506  can be used to suppress unwanted back-reflections from the fiber output. 
         [0060]    In  FIG. 6 , an alternate embodiment of a cavity includes an output coupler at the Faraday rotator mirror  210  or an additional fiber output coupler via fiber isolator  602  instead of the output coupler at the polarizers. 
         [0061]    In  FIG. 7 , an alternate embodiment of a cavity includes a semiconductor saturable absorber  702  or a fiber stretcher  704  to start-up the modelocking process. Such features eliminate need to move the cavity mirror  206  to start up the modelocking process. 
         [0062]    The saturable absorber  702  can be any semiconductor saturable absorber formed on a substrate and having its band edge located in a vicinity of the laser wavelengths produced by the cavity. However, for purposes of the following discussion, reference will be made to a multiple quantum well (MQW) saturable absorber which can, for example, be based on AlInAs barriers and GaInAs wells. The saturable absorber saturation energy can be matched to the soliton energy of the fiber laser, and the total cavity length can be matched to the soliton period to ensure high-quality pulse generation without pedestals (e.g., by trial-and-error). 
         [0063]    In another alternate embodiment, the saturable absorber can be used as the principal modelocking element. In this case, there is no need for high-birefringent fiber and the cavity can be assembled completely with non-polarization-maintaining fiber. The Faraday elements thus only serve to stabilize the polarization state in the cavity. 
         [0064]    While the foregoing embodiments illustrate significant features of the present invention, those skilled in the art will readily appreciate that alternate embodiments of the invention can readily be implemented. For example, the lens illustrated in the  FIG. 1  embodiment can focus the laser energy to a point with a beam diameter of less than approximately 10 micrometers. However, the desired accuracy for a given application can be selected by the designer. Further, while lens  228  is illustrated for interconnecting various  FIG. 1  elements, those skilled in the art will appreciate that direct coupling to the fiber can be implemented such that this lens can be removed. Alternately, additional lenses can be used if desired. Further, while only a single saturable absorber is illustrated in the  FIG. 7  embodiment, more than one saturable absorber can be used if desired. 
         [0065]    In an exemplary embodiment, the power of the pump  302  can be up to 400 milliWatts or greater (e.g., typically less than 1 Watt). For example, the pump can be a 980 nanometer titanium sapphire source which produces a signal wavelength of 1.55 micrometers. Input/output leads (or ports) of the wavelength division multiplexing coupler are labelled  1 - 4 , with the lead  1  being connected to the pump  302 , the lead  3  being directed to an output coupler via gain medium  202 , the output lead  2  being terminated with all fiber ends angle-cleaved to minimize spurious reflections, and the lead  4  being connected to the Faraday rotator mirror  210 . 
         [0066]    The wavelength division multiplexing coupler can, for example, be an Aster wavelength division multiplexer coupler having two input ports and two output ports, with light being directed from the first input port (i.e., from the pump) to the output coupler via the gain medium. Light which passes from the gain medium (e.g., erbium fiber) to the Faraday rotator mirror  210  is reflected back through the wavelength division multiplexing coupler to the third input port  3  of the wavelength division multiplexing coupler  304 . Of course, alternate embodiments of the invention can include altered connections of the wavelength division multiplexer which will be readily apparent to those skilled in the art (see, for example,  FIG. 6  wherein port  4  of the wavelength division multiplexer is connected to the output coupler). 
         [0067]    The laser  100  of  FIG. 1  can be operated in a continuous mode or can be operated in a pulsed oscillation mode (POM). A typical fiber laser can produce an output power ranging from 1 to 50 milliWatts or greater (e.g., for less than 1 Watt power input). 
         [0068]    Note that the cavity  200  can also include additional bandwidth-limiting elements such as etalons or birefringent tuning plates, which can be used for wavelength-tuning the laser output. The cavity can also optionally include soliton shaping or no soliton shaping, in the presence of group-velocity walk-off between the polarization eigenmodes of the fiber or in the presence of soliton-trapping between the polarization eigenmodes of the fiber. These processes can occur simultaneously or in any combination and can stabilize the pulse formation. 
         [0069]    Thus, the foregoing has described exemplary embodiments of the present invention which relate to use of a Kerr-type modelocked fiber laser that incorporates an environmentally stable phase delay between its interfering cavity modes. The cavity is of great practical and commercial value since it operates without any continuously adjustable “knobs” to provide phase adjustment. The laser can therefore be easily assembled in a completely enclosed and sealed box. 
         [0070]    It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.