WO1999027619A2

WO1999027619A2 - Multi-mode fiber lasers

Info

Publication number: WO1999027619A2
Application number: PCT/US1998/020493
Authority: WO
Inventors: Douglas W. Anthon
Original assignee: Scientific-Atlanta, Inc.
Priority date: 1997-10-01
Filing date: 1998-09-30
Publication date: 1999-06-03
Also published as: WO1999027619A3

Abstract

A multimode fiber laser (40) is provided for generating a laser wave having multiple modes disposed about the desired laser wavelength. The laser includes a pump source (44) for generating a laser pump wave, and an optical fiber (42) disposed in a laser cavity. The cavity itself is defined by an input mirror (11) and an output mirror (12) which may be provided as endfaces (46 and 48) of fiber optic section (42). The fiber optic section may be a ytterbium, erbium co-doped phosphate glass optical fiber.

Description

MULTI-MODE FIBER LASERS

Field of the Invention

The instant invention relates in general to lasers, and more particularly to lasers using optical fibers to generate laser waves.

Background of the Invention

It has been known for more than 30 years that glasses doped with erbium ions can operate as lasers (see, e.g., E. Snitzer & R.F. Woodcock, "Yb³ + Er³ + Glass Laser," Appl. Phys. Lett. 6, 45 (1965)). Early work on erbium glass lasers used silicate glasses and incorporated ytterbium ions as a sensitizer that would absorb pump energy and transfer it to the erbium ions. Within a few years, however, it was shown that phosphate glass was a significantly better material for the ytterbium sensitized erbium laser system (see, E. Snitzer, et al., "Phosphate Glass Er³ + Laser," IEEE J. Quantum Electronics 4, 360 (1968)). Subsequent work with other glasses and crystals demonstrated that, because of its phonon energies, phosphate glass is a uniquely efficient host material for this laser system (see, e.g., N.P. Gapontsev, et al., "Erbium Glass Lasers and Their Applications," Opt. Laser Technol., 189 (1982)).

A laser using ytterbium-sensitized erbium-doped phosphate glass as the gain medium can be pumped with different types of pump sources. Ytterbium in phosphate glass has a trivalent bonding state, and has a broad absorption peak stretching from 800 nanometers (nm) to 1100 nm, with a peak at 975 nm. Well established pump sources include InGaAs laser diodes, which generate wavelengths between 910 nm and 990 nm, and neodymium lasers generating wavelengths between 1040 nm and 1080 nm. U.S. Patent No. 3,582,820 to Snitzer discusses intracavity pumping of such a laser with neodymium lasers. End pumping with a neodymium laser has been discussed in detail by D. Hanna, et al., in Optics Commun. 63, 417 (1987). A compact intracavity pumped erbium laser has been described by D.W. Anthon, et al., in "Diode Pumped Erbium Glass Lasers," Solid State Lasers III, Proc. SPIE 1627, 8-12 (1992). Pumping with laser diodes in the 940 nm to 990 nm region has been used in a side-pumped configuration as disclosed in, for example, "Diode Array Pumped Er,Yb: Phosphate Glass Laser," Appl. Phys. Lett. 60, 1424-6 (1992) to J.A. Hutchinson, et al. Recent interest in erbium glass lasers comes from the desire to produce a suitable laser source for externally modulated CATN transmission systems. In a CATN system, analog optical signals are transmitted through optical fibers. Νd:YAG lasers operating at the wavelength of 1318 nm have been shown to be acceptable sources, and much of the experience with CATN technology has been achieved using these devices. Nevertheless, it is highly desirable to provide a suitable laser source that operates at 1550 nm wavelength. This is because a typical fused silica optical fiber has the lowest attenuation around that wavelength. Low attenuation allows an optical signal at that wavelength to be transmitted over a longer distance. Since the gain of erbium doped glass covers a range of wavelengths centered around 1550 nm, there is currently strong interest in developing suitable erbium glass lasers to transmit CATN signals.

There are, however, numerous requirements for a laser used in such an application. For example, optimal operation of a CATN transmitter requires a laser with a relatively high optical power (typically greater than 100 mW into an optical fiber), very low relative intensity noise (RIΝ) (typically less than -160 dB/Hz) between 10 MHz and 1000 MHz, and an output spectrum that is consistent with minimizing the noise and distortion associated with fiber dispersion, stimulated Brillouin scattering (SBS) and laser phase noise.

One way to minimize SBS, which is one of the main sources of distortion in high power signal transmission through an optical fiber, is to use a laser source that operates in multiple modes, where the spacing between those modes is more than twice the highest frequency in the system. With such mode spacing, the associated noise does not appear in the signal band. In a multimode laser, the output power is distributed in several spectral modes instead of being concentrated in one single mode, which is useful with respect to the suppression of SBS. The mode spacing in a typical neodymium or erbium laser is large with respect to the 100 MHz Brillouin linewidth. As a result, the individual modes act independently with respect to the onset of SBS (see, e.g., Y. Aoki, et al., "Stimulated Brillouin scattering in a long single-mode fiber excited with a multimode pump laser," J. Optical Society B 5(2), 358-363 (1988)).

The threshold for SBS is determined by the power carried in the most intense laser mode. By spreading the power over multiple spectral modes, the power in each mode can be kept below the SBS threshold, thereby suppressing noise caused by SBS. In this way, the power that can be transmitted through the fiber without significant SBS-related noise is increased. This is important for long distance analog transmission systems such as CATN, where signal-to-noise considerations dictate the use of relatively high power laser sources.

As noted above, transmission of high power optical signals in long single-mode fibers is limited by the onset of SBS. This typically occurs within a few tens of mW of power per GHz optical bandwidth. The transmitted power can be maximized by spreading the signal frequency spectrum over as large a bandwidth as possible. A maximum bandwidth is usually determined by the onset of distortions, such as fading, that are introduced by fiber dispersion.

The form of the spectral broadening is constrained by the need to avoid other forms of distortion. To avoid intermodulation distortions in an externally modulated CATN system, the optimum spectrally broadened input signal consists of independent carriers spaced by at least twice the CATN bandwidth. All carriers should be equally spaced in frequency to avoid low frequency carrier intermodulation frequencies. The maximum bandwidth is limited by fiber dispersion; signal degradation (i.e. fading or multipath interference) occurs when the individual carriers have different arrival times. Finally, it is undesirable to have carriers spaced by the exact Brillouin shift frequency; backscattered light at the appropriate frequency can reduce the power level required to produce SBS. In 1550 nm systems, for example, the combination of fiber dispersion and CATN bandwidth typically dictates carrier spacings greater than 2 GHz and a total bandwidth of less than 60 GHz.

Spectral broadening is often accomplished by phase modulating the output of a single frequency laser. This produces sidebands at the modulation frequency that act like independent carriers for subsequent intensity modulation. A system of this type, in which phase modulation is used in conjunction with an external intensity modulator is described by Chraplyvy in US patent 5420868. This approach has the advantage that it produces equal frequency spacings, but it has several disadvantages. Phase modulation with a single radio frequency (RF) tone tends to concentrate the power in only a few sidebands; this gives relatively poor SBS suppression. A more elaborate system using multiple, phase-locked harmonics is required for effective SBS suppression. In addition, single frequency lasers are generally less efficient and more troublesome than multilongitudinal mode lasers.

Conversely, multimode lasers avoid the trouble of producing a single frequency laser and an elaborate phase modulation system. Accordingly, a multimode laser with appropriate mode spacing and linewidth may be used to accomplish the same functionality as that heretofore used in connection with single-mode lasers. Multimode lasers are often used in externally modulated 1300 nm systems. Since this wavelength coincides with the dispersion minimum in silica fiber, dispersion is small and signal bandwidth of several hundred GHz can be used. Only a few of the dozens of modes under the gain curve need oscillate to achieve SBS suppression. Heretofore such lasers have typically been fabricated as solid state laser devices. An example of such a solid state device is disclosed in, for example, U.S. Patent No. 5,644,589 to Anthon, the disclosure of which is incorporated herein by reference. Accordingly there exists need to provide a multimode laser device capable of avoiding the problems inherent in the prior art. Such a device takes the form of a waveguide laser, as a fiber laser or a planar waveguide laser which should be characterized by wavelengths appropriate for desired applications, such as CATN. Moreover, multimode waveguide lasers should be amenable to fabrication with integral devices so as to provide wavelength and bandwidth selective elements.

Brief Description of the Drawings FIG. 1 is a highly schematic view of a laser cavity which has at least one normal incidence intracavity surface and has a laser structure optimized for multimode operation; and

FIG. 2 is a somewhat simplified side view of a laser arrangement in accordance with the instant invention.

Detailed Description of the Preferred Embodiment While the specification concludes with claims defining the features of the invention that are regarded as normal, it is believed that the invention will be better understood from a consideration of the following descriptions in conjunction with the drawing figures, in which like reference numerals are carried forward.

Referring now to FIG.l, there is illustrated therein a fiber optic laser structure according to the teaching of the instant invention, and optimized for multimode operation. As discussed, for example, U.S. Patent No. 5,644,589 to Anthon, the operation of a multimode laser is complicated by the presence of intracavity normal incidence surfces and by longitudinal variation in the laser gain. As disclosed previously, such surfaces and gain nonuniformities tend to mode selectivity into the resonator so that only a small fraction of the modes within the reflectivity band of the cavity reflectors actually operate. This is undesirable if the goal of the device is to encourage as many modes as possible to operate. The advantge of a waveguide laser in this application is that it can be constructed with no intracavity surfaces, with the reflective surfaces used in the prior art being replaced by photorefractive Bragg gratings.

Referring now to FIG. 2 there is illustrated therein a simplified side view of a multimode laser arrangement in accordance with the instant invention. In particular, the multimode laser arrangement 40 has been used to identify a pumped optical fiber laser arrangement in accordance with the instant invention. The arrangement 40 includes, as its main components, an optical fiber section 42 and a laser pump 44 which is of any known construction and advantageously is constituted by or includes a diode laser. A laser pump 44 is pumped directly by a laser diode having a wavelength of, for example, between 940 nm and 990 nm. A suitable laser diode is, for example, model SDL-6570-D from Spectra Diode Laboratories. The laser pump 44 issues or emits light of a wavelength lambda ( λ ) that is launched in a manner well known to those of ordinary skill in the art, into the optical fiber section 42. Light so launched is propagated longitudinally through the optical fiber. The optical fiber section 42 may include a fiber core and a cladding surrounding the core, with a stimulated light from the laser pump 44 being launched into a guide in the core. The optical fiber section 42 is shown to be provided, and the core thereof, with at least one grating 50 disposed between the input mirror and the output coupler. The at least one grating is spaced in the actual longitudinal direction of the fiber 42 from the input mirror and is shown in the embodiment of FIG. 2 as being disposed closer to endface 48. The nature and function of grating 50 will be described in greater detail herein below.

It is contemplated that the optical fiber 42 disposed in the laser cavity of the multimode laser of the instant invention is an erbium-doped glass optical fiber. In a preferred embodiment, the laser would consist of a section of YbEr codoped fiber with a resonator defined by two fiber gratings, with a third, angled, fiber grating used to define the polarization. The erbium- doped glass optical fiber may typically be fabricated in a length between 6 mm and 600 mm, and preferably in the range of about 60 mm. Assuming an index of 1.444 for the optical fiber, for example, a 2 GHz resonator length will be 51.9 mm. It is to be understood, however, that the length of the optical fiber is driven principally by the optical performance criteria dictated by the application into which the laser is incorporated. Accordingly, the length of the fiber may be between 6 and 600 mm, and preferably around 60 mm.

The pump beam would enter the resonator through a grating 50 that is highly reflective for the laser wavelength and transmisive at the pump wavelength. The other grating 52 would serve as the output coupler and possibly a reflector for the pump wavelength; its spectral width would determine the linewidth of the laser. The round trip gain in a 50 mm length of YbEr fiber at 1558 nm is approximately 1 dB. This low gain requires the use of relatively high finesse cavity reflectors, e.g. >99.5% R on the pump end and 95%R on the exit end. The laser bandwidth Dl is approximately the full wavelength range for which the grating transmission is less than twice its minimum value. The spectral width of the high reflector will need to be broader in to avoid possible parasitic oscillations involving the output mirror and the diode facet. The dispersion of the grating is also important. If the phase shift is a nonlinear function of wavelength, then the mode spacing of the resonator will not be constant across the laser bandwidth. As noted above, this could have undesirable effects in the low frequency RF spectrum.

The type of gratings used will depend on the attainable index modulation. Gratings are most commonly written into germanosilicate fiber using excimer radiation at 248 nm. Using hydrogen loading, it is possible to achieve index modulations of up to Δn = .012 in these materials. The use of the YbEr fiber complicates the problem because of its low photosensitivity. The presence of large amounts of phosphorous in the fiber inhibits the formation of the Ge defects normally used to create the gratings. Gratings have been written in phosphorous-containing core material using deuterium loading and 193 nm excitation. A refractive index change of Δn = 2χl0^-3 at 1550 nm was reported. An alternative approach is to surround the weakly photosensitive core with a highly photosensitive boron-germanium co-doped cladding ring. In this case, 248 nm excitation produces an index change of Δn = 2χl0^-3 in the cladding. Allowing for the reduced modal overlap, this gives an effective index change of approximately Δn = lxlO ⁴. Even if the maximum reported refractive index change of .012 were achieved in this geometry, it would only give an effective index change of Δn = 6χl0"⁴.

The peak reflectivity Ro of a sinusoidal grating of length L, having a period corresponding to a center wavelength lo, is given by Ro = tanh²(kL), where k = pdn/lo is the coupling constant, and dn is the effective refractive index change. The full width Dl for which T < 2 T₀, is Dl = [ In (4 / To) / 2 ]^{1 2} (lo)² / (p n L), with To = 1 - Ro and kL » 1. The desired output coupler with Dl = .48 nm (= 60 GHz) and To = .05, implies L = 1.63 mm and dn = .0013. The other reflector, with To = .005 and Dl = .48 nm, corresponds to L = 2.01 mm and dn = .0016.

The spectral widths of the gratings can also be manipulated by introducing a chirped structure. Such devices have been studied extensively for purposes of dispersion cancellation. A combination of apodization and chirp can produce nearly any desired combination of reflectivity, linewidth and phase shift. Thus, even if it is not possible to achieve the index change required for a simple grating with the desired reflectivity and spectral width, it is likely that a chirped grating exists that can be satisfy all the requirements.

The laser will be sensitive to feedback from other system elements and fiber endface reflections. The output of the laser can be launched into a fiber- coupled isolator that has low return loss. The backreflection from the fusion splice and the isolator will be small; all reflections from further along the system will be blocked by the isolator. Ideally, back reflections from the pump end will be minimized because of the cavity high reflector at this end of the laser. Wedging the diode launch end may also be important. For a single mode pump it may also be possible to use fiber bending to attenuate the 1558 nm signal without attenuating the 980 nm pump. For a multimode pump, mode scrambling may reduce the feedback level to acceptable levels. Doping the pump fiber with Co²⁺, which absorbs at 1550 nm but transmits at 980 nm, may also be useful for providing isolation. Polarization control depends on the type of fiber used. It is necessary to define a low loss and a high loss axis in the fiber in order to ensure polarized operation. For radially symmetric fiber, a polarization axis can be defined with gratings blazed at Brewster angle. For small index changes, this is approximately 45 degrees. A narrowband polarizer would consist of a relatively short (a few mm) grating with a period l times as long as that of the end reflectors. A period much longer than the wavelength would give a broadband polarizer that would function much like a Brewster plate in a bulk cavity. A variety of chirped designs are also possible. Polarizers of this type are rarely used in fiber lasers because of depolarization losses in non- PM fiber. However, because this resonator is very short and rigidly held, the round trip depolarization may be small enough that these losses will become insignificant. If this is not the case, then polarization maintaining (PM) fiber can be used. It will still be necessary to have some form of asymmetric loss aligned with the fiber axes to achieve polarized operation, but the round trip depolarization will be reduced. If single polarization (PZ) fiber is available, then the fiber will automatically have low and high loss axes, and the laser will be polarized with no additional effort. This would be the easiest solution, if such fiber could actually be made.

It is anticipated that the device would be constructed by splicing a length YbEr fiber slightly longer than the desired resonator length between a pump fiber and a polarized output fiber and then writing the gratings into the fiber from the side using an excimer laser and a phase mask. The polarized output fiber could either be PZ fiber or a PM fiber coupled to a polarizing isolator. To minimize loss, it is important that the grating polarizer be aligned to the transmissive axis of the output fiber. Once the gratings are completed, the exposed fiber section can be recoated, and the entire fiber laser can be placed in a mount to ensure that the fiber remains straight during operation. The mounting must be able to dissipate any heat generated during the laser operation. One possibility is to place the fiber in a metal channel with an ID of approximately 250 micron (μm). This could be capillary tubing or a channel formed by clamping two pieces of soft metal around a 250 μm diameter wire. Conduction through air may be sufficient to cool the device. Another choice would be to use a soft acrylic material (similar to what is used to recoat the fiber) to glue the fiber inside a tube or between two conductive plates. A sapphire plate could be placed against a metal plate with a 250 μm groove to give a combination of high thermal conductivity and sufficient transparency to allow a UN curing adhesive to be used.

The number of operating modes is maximized if the gain is longitudinally uniform. Since true uniformity is impossible with a single end pump, some compromise is required. In a laser with relatively small output coupling, the standing wave patterns are symmetrical about the center of the resonator, so effectively uniform only requires that the average of the two ends that be uniform. This sets limits on the range of pump absorption. Too weak an absorption reduces efficiency; too strong an absorption bunches all the gain at one end of the resonator and interferes with multimode operation. For a 50 mm resonator length, absorptions between 200 and 300 dB/m appear to be optimum. This can be adjusted by adjusting the Yb concentration in the fiber, or by appropriate choice of pump wavelength. For CATN applications, it is often desirable to have output powers larger than can be obtined from 150 mW single stripe laser diodes like the SDL-6570-D. This suggests the use of a double-clad design which would permit higher power broad area devices or MOPA lasers to be used. Similar techniques have been previously used for Yb-doped fiber lasers.

While the preferred embodiments of the invention have been established and described, it will be clear that the invention is not so limited.

Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art, without departing from the spirit and/or scope of the present invention as defined by the appended claims.

Claims

I claim:

1. A fiber laser for generating a coherent optical output consisting of multiple longitudinal modes about a desired laser wavelength, said laser comprising: a pump source for generating a pump wave; an input mirror and an output coupler defining a laser cavity; and a ytterbium, erbium co-doped phosphosilicate glass optical fiber disposed in said laser cavity.

2. A multimode laser as in claim 1, wherein said input mirror and output coupler are integrally formed in said optical fiber.

3. A multimode laser as in claim 1, further comprising a polarizer.

4. A multimode laser as in claim 3, wherein said polarizer is integrally formed in said optical fiber.

5. A multimode laser as in claim 3, wherein said polarizer is disposed between said input mirror and said output coupler.

6. A multimode laser as in claim 1, wherein the optical fiber has a length of between 6 mm and 600 mm.

7. A multimode laser as in claim 1, wherein the optical fiber has a length of about 60 mm.

8. A multimode laser as in claim 3, wherein said polarizer imparts a linear polarization to said laser wave.

9. A multimode laser as in claim 1, wherein at least one of said input mirrors and said output couplers further includes grating means to manipulate at least one of the spectral width or phase shift of the laser wave.

10. A multimode laser as in claim 1, wherein said pump source comprises a diode laser.

11. A fiber laser for generating a coherent optical output consisting of multiple longitudinal modes about a desired laser wavelength, said laser comprising: a pump source for generating a pump wave; an input mirror and an output coupler defining a laser cavity; and a ytterbium, erbium co-doped phosphosilicate glass optical fiber disposed in said laser cavity, wherein said input mirror and output coupler are formed as integral gratings in said optical fiber.

12. A multimode laser as in claim 11, further comprising a polarizer.

13. A multimode laser as in claim 11, wherein said polarizer is integrally formed in said optical fiber.

14. A multimode laser as in claim 13, wherein said polarizer is disposed between said input mirror and said output coupler.

15. A multimode laser as in claim 11, wherein the optical fiber has a length of between 6 mm and 600 mm.

16. A multimode laser as in claim 11, wherein the optical fiber has a length of about 60 mm.

17. A multimode laser as in claim 12, wherein said polarizer imparts a linear polarization to said laser wave.

18. A multimode laser as in claim 11, wherein at least one of said input mirrors and said output couplers further includes grating means to manipulate at least one of the spectral width or phase shift of the laser wave.

19. A multimode laser as in claim 11, wherein said pump source comprises a diode laser.