Production of optical pulses at a desired wavelength utilizing higher-order-mode (HOM) fiber

Abstract

An apparatus and method for producing optical pulses of a desired wavelength utilizes a section of higher-order-mode (HOM) fiber to receive input optical pulses at a first wavelength, and thereafter produce output optical pulses at the desired wavelength through soliton self-frequency shifting (SSFS) or Cherenkov radiation. The HOM fiber is configured to exhibit a large positive dispersion and effective area at wavelengths less than 1300 nm.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Applications 60/863,082, filed Oct. 26, 2006, and 60/896,357, filed Mar. 22, 2007, both provisional applications herein incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to the production of optical pulses at a desired wavelength using higher-order-mode fibers and, more particular, to the utilizing of HOM fiber with a positive dispersion and large effective area sufficient to generate high energy, short pulses at wavelengths below 1300 nm, considered useful for numerous applications.

BACKGROUND OF THE INVENTION

[0003] Mode-locked femtosecond fiber lasers at 1.03 .mu.m and 1.55 .mu.m have been improving significantly in the last several years, particularly with respect to the achievable output pulse energy (increasing from 1 to .about.10 nJ). Even higher pulse energy can be achieved in femtosecond fiber sources based on fiber chirped pulse amplification. However, femtosecond fiber sources, including lasers, have seen only limited applications in multiphoton imaging. The main reason is that they offer very limited wavelength tunability (tens of nanometer at best), severely restricting the applicability of these lasers, making them only suitable for some special purposes. In addition, existing femtosecond fiber sources at high pulse energy (>1 nJ) are not truly "all fiber," i.e., the output is not delivered through a single mode optical fiber. Thus, additional setup (typically involving free-space optics) must be used to deliver the pulses to imaging apparatus, partially negating the advantages of the fiber source.

[0004] Higher-order-mode (HOM) fiber has attracted significant interest recently, due to the freedom it provides to design unique dispersion characteristics in all-solid (i.e., non-"holey") silica fiber.

[0005] The `wavelength tunability` of femtosecond optical sources has been extensively studied within the phenomenon of soliton self-frequency shift (SSFS), in which Raman self-pumping continuously transfers energy from higher to lower frequencies within an optical fiber. SSFS has been exploited over the last decade in order to fabricate widely frequency-tunable, femtosecond pulse sources with fiber delivery. Since anomalous (positive) dispersion (.beta..sub.2<0 or D>0) is required for the generation and maintenance of solitons, early sources that made use of SSFS for wavelength tuning were restricted to wavelength regimes >1300 nm, where conventional silica fibers naturally exhibit positive dispersion.

[0006] In addition, Cherenkov radiation has been demonstrated in microstructured fibers pumped near their zero-dispersion wavelength. In general, an ideal soliton requires a perfect balance between dispersion and nonlinearity so that energy becomes endlessly confined to a discrete packet--both spectrally and temporally. When perturbations are introduced, this stable solution breaks down, allowing the transfer of energy between the soliton and the disturbance. Such energy transfer occurs most efficiently in fibers for solitons near the zero-dispersion wavelength. The spectral regime to which energy couples most efficiently has been dubbed "Cherenkov radiation" due to an analogous phase matching condition in particle physics. The phenomenon of Cherenkov radiation in fibers is often associated with SSFS as it allows a convenient mechanism for more efficient energy transfer between the soliton and the Cherenkov band. In particular, when the third-order dispersion is negative, SSFS will shift the center frequency of the soliton toward the zero-dispersion wavelength, resulting in efficient energy transfer into the Cherenkov radiation in the normal dispersion regime. The problem of tunability remains an issue for these arrangements capable of creating Cherenkov radiation.

[0007] The recent development of index-guided photonic crystal fibers (PCF) and air-core photonic band-gap fibers (PBGF) have relaxed this tunability requirement somewhat, with the ability to design large positive waveguide dispersion and therefore large positive net dispersion in optical fibers at nearly any desired wavelength. This development has allowed for a number of demonstrations of tunable SSFS sources supporting input wavelengths as low as 800 nm in the anomalous dispersion regime.

[0008] Unfortunately, the pulse energy required to support stable Raman-shifted solitons below 1300 nm in index-guided PCFs and air-core PBGFs is either on the very low side, a fraction of a nJ for silica-core PCFs, or on the very high side, greater than 100 nJ (requiring an input from an amplified optical system) for air-core PBGFs. The low-energy limit is due to high nonlinearity in the PCF. In order to generate large positive waveguide dispersion to overcome the negative dispersion of the material, the effective area of the fiber core must be reduced. For positive total dispersion at wavelengths less than 1300 nm, this corresponds to an effective area, A.sub.eff of 2-5 .mu.m.sup.2, approximately an order of magnitude less than conventional single mode fiber (SMF). The high-energy limit is due to low nonlinearity in the air-core PBGF where the nonlinear index, n.sub.2, of air is roughly 1000 times less than that of silica. In fact, most microstructure fibers and tapered fibers with positive dispersion are intentionally designed to demonstrate nonlinear optical effects at the lowest possible pulse energy, while air-core PBGFs are often used for applications that require linear propagation, such as pulse delivery.

[0009] For these reasons, previous work using SSFS below 1300 nm was performed at soliton energies either too low or too high (by at least an order of magnitude) for many practical applications, such as multiphoton imaging, where bulk solid state lasers are currently the mainstay for the excitation source.

[0010] The present invention is directed to overcoming these and other deficiencies in the state of the art.

SUMMARY OF THE INVENTION

[0011] The present invention relates to a higher-order-mode (HOM) fiber module operable to generate energetic, short output pulses of light at wavelengths amenable to various applications, while also providing a degree of wavelength tunability. In particular, the inventive HOM module includes a section of HOM fiber with anomalous (positive) dispersion and a large effective area, characteristics that create a soliton self-frequency shift sufficient to move an incoming stream of pulses at one wavelength to a stream of pulses at a second, desired wavelength associated with a specific application. These dispersion characteristics have also been found to allow for the creation of soliton Cherenkov radiation at wavelengths below 1300 nm, with usable energy in the range of 1-10 nJ.

[0012] Additionally, the HOM fiber module of the present invention provides the ability to compensate the dispersion of an optical pulse that is chirped at its input. Therefore, the HOM module provides a sufficient amount of dispersion to provide a transform-limited pulse at a predetermined location within the HOM fiber such that the pulse undergoes frequency shift by either of the SSFS or Cherenkov effects described above.

[0013] In accordance with the present invention, the HOM module comprises an input mode converter (for converting from the conventional LP.sub.01 mode to a higher-order mode), a section of HOM fiber coupled to the input mode converter for generating the desired self-frequency shift to a desired output wavelength, and (when necessary) an output mode converter (for converting the wavelength-shifted pulses back to the conventional LP.sub.01 mode or any other desired spatial profile).

[0014] In one embodiment, in-fiber long period gratings (LPGs) are used for the input and output mode converters, thus minimizing the amount of optical loss present at the junction between the mode converters and the HOM fiber.

[0015] The HOM fiber portion of the module is configured in one embodiment to include a wide, low index ring cladding area, separated from a high index core region by a trench. The index values and dimensions of the ring, trench and core are selected to provide the desired amount of anomalous dispersion and the size of the effective area. One set of acceptable values for use in accordance with the present invention is a dispersion on the order of +60 ps/nm-km and an effective area of approximately 44 .mu.m.sup.2. Another set of acceptable values are defined by the wavelength range within which the dispersion is anomalous (positive), this range being between 10 and 300 nm. Yet another set of acceptable values are defined by the maximum achievable dispersion in the wavelength range of interest, this value ranging from 0 to +3000 ps/nm-km. With respect to the effective area, acceptable values of A.sub.eff for the purposes of the present invention range from about 5 to 4000 .mu.m.sup.2.

[0016] The present invention also relates to a method of producing output optical pulses having a desired wavelength. The method includes generating input optical pulses and delivering the input pulses to an HOM fiber module to alter the wavelength of the input optical pulses from the first wavelength to the second, desired wavelength by soliton self-frequency shifting (SSFS) within the HOM fiber module.

[0017] In one embodiment, the method can further include converting the spatial mode of the input signal into a higher-order mode at the input of the HOM fiber module, and thereafter reconverting the output of the HOM fiber module back to the original spatial mode or to any other desired mode profile.

[0018] It is an advantage of the present invention that the HOM module is capable of achieving these characteristics at wavelengths below 1300 nm, heretofore not accomplished in an all-silica (non-holey) fiber.

[0019] Further, the HOM module of the present invention is designed such that the difference between the effective index n.sub.eff of the mode in which signal propagation is desired is separated from that of any other guided mode of the fiber by greater than 10.sup.-5, thus providing for enhanced modal stability of the signal.

[0020] In one embodiment, the input comprises a single mode fiber (SMF) spliced to the HOM fiber before mode conversion, with the properties of the splice ensuring that signal propagation in the HOM fiber occurs predominantly in the LP.sub.01 mode, further enhancing modal stability for the signal.

[0021] Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows a comparison of modal behavior between conventional LP.sub.01 (single mode fiber, top--schematic) and LP.sub.02 (bottom--simulated) modes. FIG. 1A: Near-field images. FIG. 1B: Mode profiles at various wavelengths. Conventional mode transitions from high to low index; designed HOM shows opposite evolution. Grey background denotes index profile of the fiber. FIG. 1C: Resultant total dispersion (D.sub.total, solid). Also shown are silica material dispersion (D.sub.m, dashed) and zero-dispersion line (dotted). Arrows show contribution of waveguide dispersion (D.sub.w) to total dispersion.

[0023] FIG. 2 is an index profile of the HOM fiber.

[0024] FIG. 3 shows an experimentally measured near-field image LP.sub.02 mode with an effective area A.sub.eff.about.44 .mu.m.sup.2.

[0025] FIG. 4 is a refractive index profile for an HOM fiber of the present invention, indicating the set of six different parameters than may be adjusted to provide the desired positive dispersion and large effective area;

[0026] FIG. 5 is a graph of the simulated total dispersion vs. wavelength curves for a variety of profiles, forming by adjusting one or more of the parameters shown in FIG. 4;

[0027] FIG. 6 shows index vs. radial position of the designed and fabricated fiber measured at several perform positions;

[0028] FIG. 7 illustrates an exemplary HOM module for converting an input wavelength to a desired output wavelength in accordance with the present invention; and

[0029] FIG. 8 is a graph of the transmission bandwidth of the HOM module of FIG. 7.

DETAILED DESCRIPTION

[0030] The present invention is directed to an arrangement for producing high energy, femtosecond output light pulses over a tunable wavelength range for wavelengths less than 1300 nm, using a relatively new type of fiber--higher-order-mode (HOM) fiber--that yields strong anomalous dispersion in the output wavelength range. Advantageously, the HOM fiber is an all-solid silica fiber structure (i.e., does not include air gaps or other microstructures) where the guidance mechanism is conventional index guiding. This represents a major breakthrough in fiber design, inasmuch as it was not previously considered possible to obtain anomalous dispersion at wavelengths shorter than 1300 nm in an all-silica optical fiber.

[0031] In accordance with the present invention, a higher-order-mode (HOM) fiber has been developed that is capable of achieving a strong positive (anomalous) waveguide dispersion (D.sub.w) for the LP.sub.02 mode at wavelengths less than 1300 nm. In particular, an HOM fiber has been formed that exhibits +60 ps/km-nm dispersion for the LP.sub.02 mode in the 1060-nm wavelength range. Combined with in-fiber gratings, this result has enabled the construction of an HOM anomalous dispersion element (hereinafter referred to as an "HOM module") with low loss (.about.1%), and an effective area A.sub.eff (e.g., .about.44 .mu.m.sup.2) that is ten times larger than conventional photonic crystal fibers (PCFs). Significantly, the guidance mechanism is index-guiding, as in standard fibers. Therefore, the inventive HOM fiber retains the desirable properties of such fibers, including low loss, bend resistance, and lengthwise invariance (in terms of loss, dispersion, etc.), making such a fiber attractive for a variety of applications. By utilizing the phenomenon of SSFS, for example, an input optical signal at a first, input wavelength can be shifted to a second, output wavelength after propagating through the HOM fiber of the present invention. Additionally, an HOM fiber module in accordance with the present invention can be used as a femtosecond fiber source at 1300 nm using soliton Cherenkov radiation in the HOM fiber to efficiently converter a 1030 nm femtosecond fiber source to the desired 1300 nm wavelength.

[0032] FIG. 1 provides an intuitive picture for the dispersive behavior of the guided modes by comparing the properties of the LP.sub.01 mode typically associated with convention single mode fiber, and the LP.sub.02 mode as supported within the inventive HOM fiber. In particular, FIG. 1(a) shows modal images for the fundamental LP.sub.01 mode (top) and the higher order LP.sub.02 mode (bottom). FIG. 1(b) shows the evolution of these mode profiles as a function of wavelength, in particular at 800 nm, 1040 nm and 1250 nm. The gray background in FIG. 1(b) is used to illustrate the refractive index profile of the fiber. As shown in the top set of modal images, the LP.sub.01 mode monotonically transitions from the high index central core to the surrounding lower index regions as the wavelength increases from 800 nm to 1040 nm, and finally to 1250 nm. Thus, the fraction of power traveling in lower index regions increases with increasing wavelength. Since the velocity of light increases as the refractive index of the medium drops, the LP.sub.01 mode experiences smaller group delays as wavelength increases.

[0033] Waveguide dispersion (D.sub.w), which is the derivative of group delay with respect to wavelength, is thus negative for the LP.sub.01 mode. Therefore, in wavelength ranges in which material dispersion (D.sub.m) is itself negative, the conventional LP.sub.01 mode can achieve only negative total dispersion values, where "total dispersion" D.sub.total is defined as the sum of waveguide dispersion and material dispersion. This is illustrated in FIG. 1(c) (top), which plots material dispersion D.sub.m as well as total dispersion D.sub.total of the LP.sub.01 mode in the 1060-nm wavelength range.

[0034] In contrast and in accordance with the present invention, the higher-order LP.sub.02 mode is designed to have the mode evolution shown in FIG. 1(b) (bottom). Again, the gray background is used to illustrate the refractive index profile for the fiber supporting this mode. As shown, when the wavelength increases from 800 nm to 1040 nm, and then to 1250 nm, the mode evolves in the opposite direction as the conventional fiber described above. That is, with reference to the diagrams along the bottom of FIG. 1(b), the mode transitions from the lower index regions to the higher index core as the wavelength increases from 800 nm to 1250 nm. The LP.sub.02 mode thus experiences larger group delays as the wavelength increases.

[0035] Therefore, the LP.sub.02 mode will exhibit a wavelength dispersion D.sub.w that is positive over this entire range as the mode transitions from the cladding to the core. This is illustrated in FIG. 1(c) (bottom), which shows the wavelength range where this transition occurs. Indeed, very large positive values of D.sub.w may be obtained, vastly exceeding the magnitude of the material dispersion D.sub.m (which, as mentioned above, is negative over the same range). As a result of the substantial difference in magnitude between the waveguide dispersion and the material dispersion, the LP.sub.02 mode that propagates along an HOM fiber will exhibit a total dispersion D.sub.total that is positive (anomalous dispersion).

[0036] It is to be noted that this evolution is governed by the "attractive" potential of various high index regions of the waveguide, and can thus be modified to achieve a variety of dispersion magnitudes, slopes and bandwidths. This yields a generalized recipe to obtain positive dispersion in a variety of wavelength ranges.

[0037] FIG. 2 shows the index profile of an exemplary HOM fiber formed in accordance with the present invention to provide this positive dispersion value, where a broad, low index ring 10 serves to substantially guide the LP.sub.02 mode at shorter wavelengths. As described above, the mode will then transition to a small, high index core 12 as wavelength increases (as described above in associated with FIG. 1(b), bottom). The experimentally recorded near-field image of this LP.sub.02 mode is shown in FIG. 3, where measurements have shown that this exemplary HOM fiber will exhibit an effective area A.sub.eff of approximately 44 .mu.m.sup.2 at 1080 nm.

[0038] The well-known physics of SSFS dictates that the wavelength tuning range is limited by the range within which the dispersion of the fiber mode is anomalous (positive). In other words, for a tuning range of .lamda..sub.tuning, the dispersion-zero crossings of the dispersion curves must also be separated by at least the same amount .lamda..sub.tuning. For many applications, it is desirable that this range be at least 300 nm. More broadly, a tuning range anywhere between 10 nm and 2000 nm may be considered useful. In general, the range of such tuning, and correspondingly the energy carried by the shifted soliton, scale with D*A.sub.eff for the wavelength and the mode in which the soliton signal resides.

[0039] The well-known physics of generation of Cherenkov radiation, on the other hand, requires the existence of a zero-dispersion crossing. If an optical soliton exists in its vicinity in the anomalous dispersion wavelength range, then Cherenkov radiation is generated in the spectral region on the other side of this zero-dispersion wavelength--i.e., in the region where the dispersion is normal. The exact spectral location of the generated wave is further governed by the dispersion slope of the fiber mode. Again, the energy of the converted radiation scales as D*A.sub.eff for the mode in which the optical radiation resides.

[0040] In accordance with the present invention, therefore, the fiber design problem reduces to one of configuring an HOM fiber with the required value of D*A.sub.eff at the output wavelengths of the dispersion curve. The general fiber index profile for achieving D.sub.w>0 for the LP.sub.02 mode is shown in FIG. 4. While FIG. 1 provides the physical intuition for D.sub.w>0 in an HOM fiber, achieving target dispersion D and effective area A.sub.eff values requires a numerical optimization of the six parameters shown in FIG. 4, namely, the indices and dimensions of ring 10, trench 14 and core 12. There are two ways to achieve a large dispersion (D) value; one is by increasing refractive index values .DELTA.N.sub.core and .DELTA.N.sub.ring, but this may be at the expense of the effective area A.sub.eff. The second approach is by increasing r.sub.ring as well as r.sub.trench. Increasing r.sub.ring will enhance the mode size, while increasing r.sub.trench will provide for greater effective index changes as the mode transitions, resulting in larger dispersion.

[0041] The key to achieving the desired properties is a mode that can transition (as a function of wavelength) through well-defined, sharp index steps in the fiber's index profile. Therefore, the fabrication process must be capable of producing both large index steps as well as steep index gradients, as shown in FIG. 4. The ideal means to achieve this is the Modified Chemical Vapor Deposition (MCVD) process, which affords the best layer-to-layer control of refractive index of all established fabrication technologies for fibers.

[0042] Dimensional scaling of the preform can also be used to shift the waveguide dispersion D.sub.w in order to achieve the D*A.sub.eff necessary for the desired output wavelength ranges. This is known in the art of optical waveguides as complementary scaling, which states that wavelength and dimension play a complementary role in the wave equation and, therefore, are interchangeable. However, it is to be noted that this is true only for the waveguide component of dispersion, D.sub.w. Changes in the material dispersion, D.sub.m, are not complementary and, as a result, the total dispersion D is not wavelength scalable. In other words, to move the dispersion curve that provides satisfactory operation in the 1030 nm wavelength range to the 775 nm spectral range, the dispersion D.sub.w needs to be high enough to counteract the strong negative trend for D.sub.m as wavelength decreases. Therefore, achieving similar properties at lower wavelengths needs the use of both dimensional scaling and the above-described dispersion-increasing configurations.

[0043] To achieve the higher 5- to 10-nJ output pulse energies, the design of an inventive HOM in this range requires a D*A.sub.eff value that is five to ten times greater than that associated with providing output pulses in the 1-2 nJ range. The main difficulty is to simultaneously achieve the large values of D*A.sub.eff while maintaining .lamda..sub.tuning at approximately 300 nm. FIG. 5 illustrates the simulated total D vs. wavelength curves for a variety of acceptable profiles, where the material dispersion value of silica, D.sub.m, is also shown. An important constraint applied in generating the profiles shown in FIG. 5 is that the effective index n.sub.eff of the HOM in which signal propagation is desired (i.e., the mode for which the dispersion curves are shown), is vastly separated from the n.sub.eff of any other mode that may be guided in the fiber. The large separation in n.sub.eff between modes ensures that the signal that is introduced in the HOM predominantly propagates only in that mode and does not randomly coupled to any other mode. Such random coupling may occur due to bends and other environmental perturbations, and typically the n.sub.eff difference between the modes should be greater than 10.sup.-5 to avoid this type of coupling.

[0044] FIG. 6 shows an example of the designed and fabricated index profiles for an HOM fiber formed in accordance with the present invention that yields a large positive dispersion in the 1060-nm wavelength range. The preform profiles closely match the design profile in both index values and the steep index gradients. Also shown in FIG. 6 are index profiles from different sections of the preform, showing the uniformity of the MCVD process in fabrication an HOM fiber whose properties are invariant as a function of fiber length. This robust fiber fabrication process is critical to provide a constant zero-dispersion wavelength in an HOM fiber for SSFS, and is a significant advantage of the inventive HOM fiber over the prior art bandgap fibers.

[0045] FIG. 7 illustrates an exemplary wavelength converting HOM module 20 formed in accordance with the present invention, including a section of HOM fiber 22 having the characteristics as described above in association with the above Figures to provide high energy femtosecond pulses at wavelengths less than 1300 nm. In accordance with the present invention, HOM module 20 utilizes SSFS, or a combination of SSFS with Cherenkov radiation, to shift the wavelength of an incoming signal to an output wavelength selected for a specific application (the output wavelength less than 1300 nm).

[0046] Further, in accordance with the present invention, HOM module 20 provides for dispersion compensation prior to wavelength shifting, such that chirped incoming pulses are "de-chirped" with the required amount of dispersion within HOM fiber 22. Thereafter, the de-chirped pulses undergo SSFS and/or Cherenkov radiation to generate the output pulses at the desired wavelength.

[0047] For proper operation of HOM module 20, an input mode converter 24 is needed to convert an incoming Gaussian-shaped LP.sub.01 mode signal into the desired LP.sub.02 mode. One preferred method for providing the mode conversion is with one or more in-fiber long period gratings (LPGs). This type of grating can be permanently formed in fibers by lithographically transferring a grating pattern from an amplitude mask to the fiber using a UV laser. For efficient grating formation, the fiber is typically saturated with deuterium, which acts as a catalyst for the process, resulting in UV-induced index changes in the germanosilicate glass. In another embodiment, the input mode converter may convert any arbitrary incoming spatial profile of light into the HOM that is desired to be propagated in the HOM fiber. For some applications, an output mode converter may be used to transform the higher-order-mode into another spatial mode. In the illustration of FIG. 7, an output mode converter 26 is shown as disposed at the output of HOM fiber 22 to transition the wavelength-shifted LP.sub.02 mode signal back into a conventional LP.sub.01 signal. More generally, an output mode converter can be used to convert the HOM into any desired spatial profile of light.

[0048] Alternatively, in some applications, it may be desired that no output mode converter is used, inasmuch as the wavelength-shifted radiation already exhibits the desired spatial mode profile. In these cases, therefore, the need for an output mode converted is obviated. In yet another embodiment, the HOM module may comprise a plurality of separate HOM fiber sections coupled together in series, using fiber splicing techniques or another mode converter to join together the adjacent sections. If they are joined by splices, the HOM in the first fiber is expected to adiabatically transition to the same mode order in the second fiber. If they are joined together by means of a mode converter, on the other hand, the mode order from the first fiber to the second fiber can also be changed. Such arrangements may be desired in applications where, in order to increase the .lamda..sub.tuning for SSFS, two concatenated sections will provide a much larger tuning range than that associated within only a single HOM fiber section. Alternatively, such arrangements may allow for changing the dispersion slope of the zero-dispersion crossing, as may be required for adjusting the wavelength at which Cherenkov radiation occurs. In the case where a mode converter is used to join two sections of HOM fiber, it is known from the prior art that such mode converters may be tunable, with the capability of switching light from one incoming HOM fiber to any of a set of outgoing HOM fibers (including, of course, reflecting back into the incoming HOM fiber). If a tunable mode converted is employed in this case, the resulting HOM module will additional provide a means to dynamically change the effective optical path length of the fiber and, by extension, its dispersion, dispersion-zero and/or dispersion slope (as may be desired for different SSFS and Cherenkov applications). Thus, a module with adjustable HOM fiber lengths may be designed and is considered to fall within the scope of the present invention. Indeed, in embodiments that utilize a multiple number of concatenated HOM fiber sections, tunable mode converters may be used at the interface between any two sections.

[0049] LPGs offer coupling between co-propagating modes of a fiber and have found a variety of applications as spectral shaping elements and mode-conversion devices. However, LPGs are normally narrow-band devices, and while they offer strong mode coupling (>99%), the spectral width of such coupling was typically limited to a range of 0.5 to 2 nm, too narrow for a femtosecond pulse. To overcome the spectral limitation, reports have shown that the LPG bandwidth can be extended to greater than 60 nm in some cases, if the fiber waveguide is configured to yield two modes with identical group velocities. It is to be noted that the large bandwidth of HOM module 20, as shown in FIG. 8 (i.e., approximately 51 nm), is uniquely enabled by the dispersive design of the fiber, which enables matching the group velocities of the two coupled modes. It is a significant aspect of the present invention that the utilization of the LPGs allows for the formation of an "all-fiber" tunable femtosecond pulse source.

[0050] Referring again to FIG. 7, HOM module 20 is spliced to an input single mode fiber 30 at input long period grating 24. The splice between single mode fiber 30 and input LPG 24 is configured such that the signal predominantly resides in the LP.sub.01 mode, thus ensuring mode conversion with high efficiency and also minimizing signal propagating in any mode other than the desired mode. This enables a device to be constructed in which the signal experiences high modal stability, even in the presence of bends and other environmental perturbations. Indeed, input single mode fiber 30 may be the output fiber of a laser source (not shown), avoiding any spurious mode coupling, especially in systems where the chirped output of the laser source needs to be directly coupled into the HOM fiber module.

[0051] As mentioned above, output long period grating 26 is used to convert the beam back to a Gaussian output. Dispersion-matching configurations are preferably used that yield ultra-large bandwidths, ensuring that the output pulse is always converted back to a Gaussian profile, within a tuning range of approximately 250 nm. An important consideration for output grating 26 is its length. Since the energetic output pulses are solitons for the specific combination of dispersion D and effective area A.sub.eff of the LP.sub.02 mode, nonlinear distortions may occur when the signal converts to the LP.sub.01 mode (having a smaller A.sub.eff) at the output. However, the length over which the signal travels in the LP.sub.01 mode, and hence the distortion it accumulates, can be minimized. The high index core of HOM fiber 22 enables the use of an output long period grating 26 of lengths less than 5 mm, which implies that light resides in the LP.sub.01 mode for less than 2.5 mm and therefore largely avoids nonlinear distortions. It is to be noted that the requirement for "short" LPGs actually complements the need for broad bandwidth operation, since the conversion bandwidth is typically inversely proportional to the grating length.

[0052] Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.

Claims

1. An apparatus for providing optical output pulses of a desired output wavelength from an input optical signal operating at a different wavelength, the apparatus comprising: an input mode converter for receiving the input signal and converting the spatial mode of the input signal into a higher-order-mode (HOM) signal; and a section of higher-order-mode (HOM) fiber coupled to the output of the input mode converter for receiving the input optical signal and thereafter produce as an output an optical signal at the desired output wavelength, wherein the section of HOM fiber is configured to exhibit a positive dispersion and large effective area sufficient to produce optical output pulses at the desired output wavelength, the positive dispersion also sufficient to perform dispersion compensation on the input optical signal, reducing the presence of chirp prior to producing the optical output signal.