Laser generation using dual seeded nested and/or in-series Raman resonators, for telecommunications applications

Abstract

A desired N.sup.th-order Stokes output and zeroth-order Stokes pump input are seeded into a rare-earth doped amplifier where the power of the zeroth-order Stokes signal is amplified prior to both signals entering a Raman amplifier comprised of N1 Raman resonators, each uniquely tuned to one of the N1 Stokes orders, in various configurations to include one or more nested and/or in-series Raman resonators. The zeroth-order Stokes signal is converted to the N.sup.th1-order Stokes wavelength in steps and the power level of the N.sup.th-order Stokes wavelength is amplified as the two signals propagate through the Raman resonators. Each Raman resonator includes a photosensitive Raman fiber located between a pair of Bragg gratings. The linewidths of the Stokes orders can be controlled by offsetting the reflectivity bandwidths of each pair of Bragg gratings respectively located in the Raman resonators.

Claims

1. A method of generating a high-power laser signal having a narrow and controllable linewidth for telecommunications applications, comprising: a. supplying a zeroth-order Stokes wavelength pump signal having a power level and having a linewidth broad enough to prevent significant Stimulated Brillouin Scattering; b. passing the zeroth-order Stokes pump signal through a first isolator and then into a wavelength division multiplexer, c. a desired output signal having a desired output signal wavelength and a desired output signal linewidth; d. supplying a N.sup.th-order Stokes wavelength seed signal having a wavelength equal to the desired output signal wavelength, a linewidth approximating the desired output signal linewidth, and an input power level; e. passing the N.sup.th-order Stokes seed signal through a second isolator and then into the wavelength division multiplexer; f. simultaneously passing both the N.sup.th-order Stokes seed signal and the zeroth-order Stokes pump signal through the wavelength division multiplexer and then inserting them into a rare-earth-doped amplifier, whereupon the zeroth-order Stokes pump signal is amplified into an amplified zeroth-order Stokes pump signal having an amplified power lever greater than the zeroth-order Stokes pump signal power level and both the amplified zeroth-order Stokes pump signal and the N.sup.th-order Stokes seed signal are output; g. simultaneously passing both the amplified zeroth-order Stokes pump signal and the N.sup.th-order Stokes seed signal through a Raman amplifier to thereby Raman convert the amplified zeroth-order Stokes pump signal into a N.sup.th1-order Stokes pump signal which amplifies the N.sup.th-order Stokes seed signal input power level to thereby increase the input power level to an amplified output power level; and h. the Raman amplifier being comprised of one or more nested Raman resonators or one or more in-series Raman resonators or a combination of the one or more nested Raman resonators and the one or more in-series Raman resonators, with each of the foregoing resonators being connected in series, whereby i. an output laser signal comprised of the amplified N.sup.th-order Stokes signal having the desired output signal wavelength, the desired output signal linewidth and the amplified output power level is obtained.

2. The laser generating method defined in claim 1 further comprising: a. a series of long period or tilted Bragg gratings coupled with the Raman resonators tuned to sequentially increasing Stokes order wavelengths up to, at the most, the N.sup.th1-order Stokes wavelength, with an amplified Stokes pump signal and the N.sup.th-order Stokes seed signal passing first through one of the long period or tilted Bragg grating and then through the Raman resonator for each member of the series of long period or tilted Bragg grating and Raman resonator pairs; b. each of the long period or tilted Bragg gratings being tuned to one of the sequential Stokes orders hereby defined as a M.sup.th-order Stokes; and c. each of the Raman resonators being comprised of a photosensitive Raman fiber having an input end and an output end, first and second high-reflector Bragg gratings tuned to the M.sup.th-order Stokes wavelength, respectively, located on either side of the Raman fiber, with the first high-reflector Bragg grating being located between the long period or tilted Bragg grating and the input end of the Raman fiber, and the second high-reflector Bragg grating being located near the output end of the Raman fiber, and a third high-reflector Bragg grating tuned to a M.sup.th1-order Stokes wavelength located closest to the second high reflector Bragg grating between the first and second high reflector Bragg gratings, whereby d. the input amplified pump signal is sequentially converted into a higher order Stokes signal upon passing through the series of long period or tilted Bragg gratings and the Raman resonators.

3. The laser generating method defined in claim 2 wherein: an amplified output signal emanates from one of the in-series Raman resonators tuned to the N.sup.th1-order Stokes wavelength, whereby the N.sup.th-order Stokes signal passes sequentially through the series of long period or tilted Bragg gratings and Raman resonators and is thereby amplified in the Raman resonator tuned to the N.sup.th1 order Stokes wavelength.

4. The laser generating method defined in claim 1 further comprising: a. a photosensitive Raman fiber having an input and output end; b. long period or tilted Bragg gratings connected in series and respectively tuned to sequential Stokes-order wavelengths and a set of the nested Raman resonators connected in series to the long period or tilted Bragg gratings tuned to the same sequential Stokes order wavelengths; and c. the set of nested Raman resonators with an innermost Raman resonator lying between a pair of highly reflective Bragg gratings tuned to one of the sequential Stokes orders hereby defined as a M.sup.th-order Stokes, and the pair of highly reflective Bragg gratings tuned to the M.sup.th-order Stokes wavelength lying between a pair of highly reflective Bragg gratings tuned to a M.sup.th+1-order Stokes wavelength, and repeating until a pair of Bragg gratings tuned to the highest of the sequential Stokes order wavelengths comprise an outermost Raman resonator.

5. The laser generating method as defined in claim 4 further comprising: an innermost Raman resonator tuned to a 1.sup.st-order Stokes wavelength, and a highly reflective Bragg grating turned to the zeroth-order Stokes wavelength being located between the highly reflective Bragg gratings tuned to the 1.sup.st-order Stokes wavelength nearest to the output end of the Raman fiber.

6. The laser generating method as defined in claim 5 wherein the input Stokes pump signal is sequentially converted into a Stokes order equivalent to the highest of the sequential Stokes orders.

7. The laser generating method defined in claim 6 wherein: one of the nested Raman resonators is tuned to the N1.sup.st-order Stokes wavelength, whereby the N.sup.th-order Stokes seed signal is then amplified as it passes through the Raman resonator tuned to the N1.sup.st-order Stokes wavelength, and output from the second high-reflector Bragg grating of the Raman resonator tuned to the N1.sup.st-order Stokes comprises the output laser signal.

8. The laser generating method as defined in claim 4 further comprising tuning either a first or second high-reflector Bragg grating in each Raman resonator to shift a center wavelength of the respective high-reflector Bragg grating to enable narrowing of a resonating bandwidth.

9. The laser method of generating as defined in claim 4 wherein the Raman fiber in each of the Raman resonators is fabricated from one of a group of photosensitive materials consisting of germanosilicate and phosphosilicate, and is a high-dispersion Raman fiber, to break up a phase-matching condition for four-wave mixing.

10. The laser generating method defined in claim 1 further comprising: a. a photosensitive Raman fiber having an input and output end; b. long period or tilted Bragg gratings connected in series and respectively tuned to sequential Stokes-order wavelengths and a set of the nested Raman resonators connected in series to the long period or tilted Bragg gratings tuned to the same sequential Stokes order wavelengths; and c. the set of nested Raman resonators with an innermost Raman resonator lying between a pair of highly reflective Bragg gratings tuned to one of the sequential Stokes orders hereby defined as a M.sup.th-order Stokes, and the pair of highly reflective Bragg gratings tuned to the M.sup.th-order Stokes wavelength lying between a pair of highly reflective Bragg gratings tuned to a M.sup.th+1-order Stokes wavelength, and repeating until a pair of Bragg gratings tuned to the highest of the sequential Stokes order wavelengths comprise an outermost Raman resonator.

11. The laser generating method as defined in claim 10 further comprising: an innermost Raman resonator tuned to a 1.sup.st-order Stokes wavelength, and a highly reflective Bragg grating turned to the zeroth-order Stokes wavelength being located between the highly reflective Bragg gratings tuned to the 1.sup.st-order Stokes wavelength nearest to the output end of the Raman fiber.

12. The laser generating method as defined in claim 11 wherein the input Stokes pump signal is sequentially converted into a Stokes order equivalent to the highest of the sequential Stokes orders.

13. The laser generating method defined in claim 12 wherein: one of the nested Raman resonators is tuned to the N1.sup.st-order Stokes wavelength, whereby the N.sup.th-order Stokes seed signal is then amplified as it passes through the Raman resonator tuned to the N1.sup.st-order Stokes wavelength, and output from the second high-reflector Bragg grating of the Raman resonator tuned to the N1.sup.st-order Stokes comprises the output laser signal.

14. The laser generating method as defined in claim 2 further comprising tuning either the first or second high-reflector Bragg grating in each Raman resonator to shift a center wavelength of the respective high-reflector Bragg grating to enable narrowing of a resonating bandwidth.

15. A laser apparatus for generating a high-power laser signal having a narrow and controllable linewidth for telecommunications applications, comprising: a. a first signal generator for generating a zeroth-order Stokes wavelength pump signal having a power level and having a linewidth broad enough to prevent significant Stimulated Brillouin Scattering; b. a first isolator for isolating the zeroth-order Stokes pump signal then injecting the zeroth-order Stokes pump signal into a wavelength division multiplexer; c. a second signal generator for generating an N.sup.th-order Stokes wavelength seed signal having an input power level, a wavelength equal to the desired output signal wavelength, and a linewidth approximating the desired output signal linewidth; d. a second isolator for isolating the N.sup.th-order Stokes seed signal and then injecting the N.sup.th-order Stokes seed signal into a wavelength division multiplexer; e. a rare-earth-doped amplifier for receiving the zeroth-order Stokes pump signal and the N.sup.th-order Stokes seed signal from the wavelength division multiplexer and inserting them into the rare-earth-doped amplifier, whereupon the zeroth-order Stokes pump signal is amplified into an amplified zeroth-order Stokes pump signal having an amplified power lever greater than the zeroth-order Stokes pump signal power level, and for injecting the amplified zeroth-order Stokes pump signal and the N.sup.th-order Stokes seed signal into a Raman amplifier; f. the Raman amplifier containing Raman resonators sequentially tuned to a 1.sup.st through a N.sup.th1-order Stokes wavelengths, for Raman converting the amplified zeroth-order Stokes pump signal into the N.sup.th1-order Stokes pump signal and amplifying the N.sup.th-order Stokes seed signal input power level to thereby increase the input power level to an amplified output power level; and g. the Raman amplifier being comprised of one or more nested Raman resonators or one or more in-series Raman resonators or a combination of the one or more nested Raman resonators and the one or more in-series Raman resonators, with each of the foregoing resonators being connected in series, whereby h. an output laser signal comprised of the amplified N.sup.th-order Stokes signal having the desired output signal wavelength, the desired output signal linewidth and the amplified output power level is obtained.

16. The laser apparatus as defined in claim 15 wherein the in-series Raman resonator is comprised of: a. a series of long period or tilted Bragg gratings coupled with the Raman resonators tuned to sequentially increasing Stokes order wavelengths up to, at the most, the N.sup.th1-order Stokes wavelength; b. each of the long period or tilted Bragg gratings being tuned to one of the sequential Stokes orders hereby defined as a M.sup.th-order Stokes; and c. each of the Raman resonators being comprised of a photosensitive Raman fiber having an input end and an output end, first and second high-reflector Bragg gratings tuned to the M.sup.th-order Stokes wavelength, respectively, located on either side of the Raman fiber, with the first high-reflector Bragg grating being located between the long period or tilted Bragg grating and the input end of the Raman fiber, and the second high-reflector Bragg grating being located near the output end of the Raman fiber, and a third high-reflector Bragg grating tuned to a M.sup.th1.sup.st-order Stokes wavelength located closest to the second high reflector Bragg grating between the first and second high reflector Bragg gratings.

17. The laser apparatus of generating as defined in claim 16, wherein the Raman fiber in each of the Raman resonators is fabricated from one of a group of photosensitive materials consisting of germanosilicate and phosphosilicate, and is a high-dispersion Raman fiber, to break up a phase-matching condition for four-wave mixing.

18. The laser apparatus as defined in claim 15 further comprising: a. a photosensitive Raman fiber having an input and output end; b. long period or tilted Bragg gratings connected in series and respectively tuned to sequential Stokes-order wavelengths and a set of the nested Raman resonators connected in series to the long period or tilted Bragg gratings tuned to the same sequential Stokes order wavelengths as the long period or tilted Bragg gratings; and c. the set of nested Raman resonators comprising an innermost Raman resonator lying between a pair of highly reflective Bragg gratings tuned to one of the sequential Stokes orders hereby defined as a M.sup.th-order Stokes, and the pair of highly reflective Bragg gratings tuned to the M.sup.th-order Stokes wavelength lying between a pair of highly reflective Bragg gratings tuned to a M.sup.th+1-order Stokes wavelength, and repeating until a pair of Bragg gratings tuned to the highest of the sequential Stokes order wavelengths comprise an outermost Raman resonator.

19. The laser apparatus as defined in claim 18 further comprising: an innermost Raman resonator tuned to the 1.sup.st-order Stokes wavelength, and a highly reflective Bragg grating turned to the zeroth-order Stokes wavelength being located between the highly reflective Bragg gratings tuned to the 1.sup.st-order Stokes wavelength nearest to the output end of the Raman fiber.

20. The laser apparatus as defined in claim 19, wherein the Raman fiber in each of the Raman resonators is fabricated from one of a group of photosensitive materials consisting of germanosilicate and phosphosilicate, and is a high-dispersion Raman fiber, to break up a phase-matching condition for four-wave mixing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram of a generic rare-earth-doped amplifier/Raman amplifier in a fully nested configuration;

(2) FIG. 2 is a diagram of a generic rare-earth-doped amplifier/Raman amplifier in a linear (non-nested) configuration;

(3) FIG. 3 is a diagram of a sodium guidestar laser system configuration;

(4) FIG. 4 is a plot of the 1069 nm and 1178 nm cavity intensity as a function of the position in the fiber for the case of 9 meters of 20/400 fiber pumped with 59.3 W of 1069 nm and seeded with 3 W of 1178 nm for the sodium guidestar laser configuration of FIG. 3;

(5) FIG. 5 is a plot of the first Stokes line (1121 nm) cavity intensity in the forward and backward directions as a function of the position in the fiber for the case of 9 m of 20/400 fiber pumped with 59.3 W of 1069 nm and seeded with 3 W of 1178 nm;

(6) FIG. 6 is a plot of the calculated reflectivity for a pair of uniform gratings 5 cm in length with B=10 that are offset by 0.02 nm. The resulting cavity has high reflectivity only over a few GHz with these gratings;

(7) FIG. 7 is a plot of the effect of 1069 nm seed power on the 1121 nm circulating power and the relative 1178 nm output power;

(8) FIG. 8 is a plot showing the effect of 1178 nm seed power on the 1121 nm circulating power and the 1178 nm relative output power; and

(9) FIG. 9 is a plot showing the effect of the 1121 nm resonator length on the 1121 nm circulating power and the 1178 nm relative output power.

(10) FIG. 10 is a diagram of a 1240 nm rare-earth-doped/Raman amplifier in a fully nested configuration.

(11) FIG. 11 is a diagram of a 1240 nm rare-earth-doped/Raman amplifier in a linear (non-nested) configuration.

(12) FIG. 12 is a generic diagram of a 1300-1500 nm rare-earth-doped/Raman amplifier.

DETAILED DESCRIPTION

(13) The two generic embodiments representing the extremes of Raman resonator configurations for the invention described in this patent are described. One embodiment consists of a rare-earth-doped fiber amplifier spliced to a Raman resonator configuration that is fully nested. The other embodiment consists of a rare-earth-doped fiber amplifier spliced to a completely linear, unnested, Raman resonator configuration. Anyone skilled in the art will realize that other configurations lying between these two extremes are possible.

Nested Raman Resonator Embodiment

(14) A block diagram of the fully nested Raman resonator embodiment is shown in FIG. 1. The first device in the diagram is a seed source 1 tuned to the desired output wavelength, the N.sup.th order Stokes line. It may be created in a number of ways to include frequency doubling of a source at a longer wavelength. This device needs to have the linewidth desired of the output. Another device in the diagram is a seed source 2 tuned to the necessary wavelength (zeroth order Stokes line) to enable generation of an N.sup.th order Stokes signal at the desired wavelength. This device needs to have a linewidth broad enough so as to prevent SBS from being an issue. The output of the source tuned to the N.sup.th order Stokes line 1 is passed through an isolator 3 and into a WDM 5. The output of the source tuned to the zeroth order Stokes 2 is passed through an isolator 4 and into the WDM 5. The WDM 5 puts both the zeroth order and the N.sup.th order Stokes signals into the core of the fiber. Both signals then enter a rare-earth-doped amplifier 6 which amplifies the signal. The amplifier 6 may consist of one or more stages in order to reach the desired power level of the zeroth order Stokes signal. The amplified zeroth order Stokes signal and the seed for the N order Stokes line is injected into Raman resonators in a fully nested configuration as defined by pairs of Bragg gratings 8-13. Prior to entering the Raman resonators, the zeroth and N.sup.th order Stokes signals pass through several long period or tilted Bragg gratings (LP/TB) 14-16 whose role is to bring about absorption of any first through N1.sup.th order Stokes signals that leak through the Bragg gratings 8-10 back toward the rare-earth-doped amplifier 6. The Raman resonators are nested such that the innermost resonator is tuned to the first order Stokes line with Bragg gratings 10 and 11 both being high reflectors with a center wavelength at the first order Stokes line. The next innermost resonator is tuned to the second order Stokes line etc. via gratings 9 and 12 which are high reflectors centered at the second order Stokes line. The outermost Raman resonator is tuned to the N1.sup.th order Stokes line via gratings 8 and 13 which are high reflectors centered at the N1.sup.th order Stokes line. These nested Raman resonators 21 are enclosed by dashed lines in FIG. 1. The zeroth order Stokes signal passes into the nested Raman resonators and is converted to the first order Stokes signal by the resonator tuned to the first order Stokes line 10, 11. Any unconverted zeroth order Stokes signal is then reflected back into the Raman fiber 17 via a high reflector 7 tuned to the zeroth order Stokes line. The first order Stokes signal is then converted to the second order Stokes signal by the resonator tuned to the second order Stokes line 9, 12 and is eventually converted to the N1.sup.th order Stokes signal by the resonator tuned to the N1.sup.th order Stokes line 8, 13. Because there are no highly reflective Bragg gratings with a center wavelength at the N.sup.th order Stokes line, the N.sup.th order Stokes signal passes straight through the nested Raman resonators defined by the Bragg gratings 8-13 and is amplified by the N1.sup.th order Stokes signal in the Raman resonator tuned to the N1.sup.th order Stokes line. The amplification occurs in the Raman fiber 17 which is either phosphosilicate, germanosilicate or some other variety of photosensitive fiber. The output of the amplifier is the N.sup.th order Stokes signal 20. Because of potentially low seed powers at the desired output wavelength, it may be necessary to have multiple rare-earth-doped amplifier/Raman resonator configurations in series to enable amplification of the desired output wavelength to the power levels desired. This embodiment is compatible with all SBS mitigation techniques to include multiple temperature zones, strain, large mode area fibers, multiple fiber types, acoustically tailored fiber, etc. In addition, the embodiment is compatible with both polarized and unpolarized modes of operation as well as continuous wave and pulsed operation.

(15) To enable controlled linewidth operation, multiple measures can be taken. These measures include usage of a seed for the N.sup.th order Stokes signal having the desired output linewidth. In order to prevent the linewidth of the N.sup.th order Stokes seed from broadening as it is amplified, it is imperative that the linewidth of the N1.sup.th order Stokes signal be controlled. This can be accomplished by using a linewidth for the zeroth order Stokes signal that is broad enough to prevent SBS from being an issue. Although seeding the amplifier with the desired output wavelength will result in a narrower output signal than if the system were seeded by spontaneous Raman scattering, additional measures are needed within the individual Raman resonators to prevent linewidth broadening of the zeroth order Stokes signal as it is converted to higher Raman orders. Control of the linewidth of the various Stokes orders can be accomplished by shifting the center wavelength of one grating of each pair 18 slightly through either heating, bending, or stretching, e.g., through a grating controller 19. This has the effect of impeding four-wave mixing, the primary source of linewidth broadening, as well as narrowing the bandwidth that will be amplified at each Stokes order. In addition, high dispersion Raman fiber can be used to break the phase matching condition for four-wave mixing. One or more of these measures in combination should work to limit broadening of the linewidth.

(16) The factors enabling a high output power level of the N.sup.th order Stokes signal in this invention are the generation of very high levels of the N1.sup.th order Stokes signal in the Raman resonator tuned to the N1.sup.th order Stokes line in addition to seeding with the N.sup.th order Stokes signal. This occurs for two reasons, first, high power levels of the zeroth order Stokes are directly injected into the system and second, because the Raman cavities are defined by high reflector Bragg gratings, the power level in the system is able to build up to high levels in all the Raman cavities. High power levels of the N1.sup.th order Stokes signal which is created from the zeroth order Stokes signal in addition to seeding at the N.sup.th order Stokes line leads to good conversion from the zeroth order Stokes signal to the N.sup.th order Stokes signal in a short Raman fiber. In addition, the short Raman fiber enables a high SBS threshold, which enables a higher output power level of the N.sup.th order Stokes signal. Negatively, this embodiment will suffer substantial thermal stress because of high power levels in the Raman fiber and on the Bragg gratings, since all of the Raman resonators overlap.

Unnested Raman Resonator Embodiment

(17) The embodiment representing the other extreme is a series of completely unnested Raman resonators. In this configuration, multiple Raman resonators in series are used to shift the zeroth order Stoke input signal out to the N1.sup.th order Stokes line to enable amplification of the N.sup.th order Stokes input signal. The amplifier is in a linear configuration with each Raman resonator having the same or different kinds of photosensitive (or nonphotosensitive) fiber. The system is co-seeded with both the initial zeroth order Stokes signal as well as the desired N.sup.th order Stokes output signal.

(18) A block diagram of the unnested Raman resonator system is shown in FIG. 2. The first device in the diagram is a source 30 at the N.sup.th order Stokes line having the desired output linewidth. The source at the N.sup.th order Stokes line may be created in a number of ways to include frequency doubling of a source at a longer wavelength. Another device is a source 31 tuned to the wavelength of the zeroth order Stokes line. This device should have a linewidth broad enough so that SBS is not an issue. The output of the source tuned to the N.sup.th order Stokes line 30 is passed through an isolator 32 and into a WDM 34. The output of the source tuned to the zeroth-order Stokes line 31 is passed through an isolator 33 and into the WDM 34. The WDM 34 puts both the zeroth and the N.sup.th order Stokes lines into the core of the fiber. The zeroth and the N.sup.th order Stokes lines then enter an amplifier 35 which amplifies the zeroth order Stokes signal. The amplifier 35 may consist of one or more stages in order to reach the desired power level for the zeroth-order Stokes signal. The output of amplifier 35 which includes the amplified zeroth order Stokes signal and the seed for the N.sup.th order Stokes line is passed through either a long period or tilted Bragg (LP/TB) grating tuned to the first-order Stokes line 39 and is injected into the first Raman resonator in a series of Raman resonators within the dashed lines of FIG. 2. The zeroth-order Stokes signal passes through a high reflector Bragg grating 37 tuned to the first-order Stokes line and into the Raman fiber 48 which may be germanosilicate, phosphosilicate or some other kind of photosensitive (or nonphotosensitive) fiber. Any unused zeroth-order Stokes signal is reflected back into the Raman fiber 48 by a high reflector Bragg grating 36 set at the zeroth-order Stokes line. In this resonator defined by the high reflector Bragg grating 37 and the Bragg grating 38, which is an output coupler (OC), the zeroth order Stokes signal is converted to the first-order Stokes signal via the Raman process in the Raman fiber 48. The size of the Raman shift is dependent on the type of Raman fiber used. Any first-order Stokes signal that leaks out of the Bragg grating 37 back toward the rare-earth-doped amplifier 35 will be put into the cladding by the long period or tilted Bragg grating 39, where it will be absorbed. The N.sup.th and first-order Stokes signals then pass through a long period grating 43 set at the second-order Stokes line and enter the second Raman resonator in the series defined by the Bragg grating 41, which is a high reflector, and the Bragg grating 42, which is an output coupler. In this resonator the first-order Stokes signal is converted to the second-order Stokes signal in the Raman fiber 49, which may be the same or different than the Raman fiber 48. Any unused first-order Stokes signal will be reflected back into the Raman fiber 49 by the high reflector Bragg grating 40 which is centered at the first order Stokes line. Any second order Stokes signal which leaks out of the Bragg grating 41 back toward the rare-earth-doped amplifier 35 will be put into the cladding by the long period or tilted Bragg grating 43 and absorbed. Eventually, the N2.sup.th order Stokes signal which has been created from the zeroth order Stokes signal together with the N.sup.th order Stokes signal enters the last Raman resonator in the series. This resonator is tuned to the N1.sup.th order Stokes line and is defined by the Bragg gratings 45 and 46, which are both high reflectors. Within this Raman resonator the N2.sup.th order Stokes signal is converted to the N1.sup.th order Stokes signal in the Raman fiber 50, which may be the same or different than the Raman fibers 48 or 49. Any unused N2.sup.th order Stokes signal will be reflected back into the Raman fiber 50 by the Bragg grating 44, which is a high reflector tuned to the N2.sup.th order Stokes line. Also, any N1.sup.th order Stokes signal which leaks out of the Bragg grating 45 back toward the rare-earth-doped amplifier 35 will be put in the cladding by the long period or tilted Bragg grating 47 and absorbed. Upon entering the Raman resonator tuned to the N1.sup.th order Stokes line, the seed for the N.sup.th order Stokes line is amplified in a single pass through the resonator. The output of the Raman amplifier is the greatly amplified N.sup.th order Stokes line 54. Once again, because of potentially low seed powers at the desired output wavelength, it may be necessary to have multiple rare-earth-doped amplifier/Raman resonator configurations in series to enable amplification of the desired output wavelength to the power levels desired. This embodiment is compatible with all SBS mitigation techniques to include multiple temperature zones, strain, large mode area fibers, acoustically tailored fibers, different types of fiber etc. The embodiment is also compatible with both polarized and unpolarized modes of operation as well as continuous wave or pulsed operation.

(19) To enable controlled linewidth operation, multiple measures can be taken. These include usage of a seed at the N.sup.th order Stokes line having the desired output linewidth. In order to prevent the linewidth of the N.sup.th order Stokes seed from broadening, it is imperative that the linewidth of the N1.sup.th order Stokes signal amplifying it be prevented from being too broad. This can be accomplished by using a linewidth for the zeroth order Stokes signal that is broader than the linewidth required to prevent SBS from being an issue. Although seeding the amplifier with the desired output wavelength will result in a narrower output signal than if the system were seeded by spontaneous Raman scattering, additional measures are needed within the individual Raman resonators to prevent linewidth broadening of the zeroth-order Stokes signal as it is converted to higher Raman orders. Control of the linewidth of the various Stokes orders can be accomplished by shifting the center wavelength of one grating of each pair 38, 42, and 46 slightly by either heating, bending, or stretching, e.g., through the grating controllers 51-53. This will impede four-wave mixing and will help control the bandwidth being amplified at each Stokes order, thus helping to keep the linewidth of the intermediate Stokes orders from broadening. Also, usage of a high-dispersion Raman fiber to break the phase-matching condition associated with four-wave mixing is another measure that can be taken. One or more of these measures in combination should work to control the linewidth of the output of the Raman amplifier.

(20) As before, the factors enabling a high output power level of the N.sup.th order Stokes signal in this invention are the generation of high levels of the N1.sup.th order Stokes signal in the Raman resonator tuned to the N1.sup.th order Stokes line in addition to seeding with the N.sup.th order Stokes signal. Relative to the previous embodiment where the Bragg gratings defining the Raman cavities were high reflectors, power levels obtainable in the resonators in this system will be less since one Bragg grating defining each cavity will be an output coupler. In addition, the power level in each successive Raman cavity will diminish. Because of this, a decreased output power level of the N.sup.thorder Stokes signal relative to the fully nested configuration is expected. Positively, this embodiment relative to the fully nested configuration will have much less thermal stress associated with it since there will be less power in each Raman fiber and on each Bragg grating.

(21) Sodium Guidestar Laser System

(22) An exemplary application of the invention is a system that generates a narrow linewidth laser beam of about 1178 nm for second-harmonic generation to 589 nm for guide star lasers (e.g., sodium guidestar lasers). The main requirements associated with a guide star laser (which will be referred to herein as sodium guidestar laser) system are: linewidths on the order of 10 MHz along with output powers of 589 nm on D.sub.2a greater than 50 W. A linewidth of 10 MHz which equates to the natural linewidth of sodium is required in order to enable excitation of the same velocity group. The invention described in this application has sufficient output power at 1178 nm to enable the 589 nm target power levels.

(23) A block diagram of this system is shown in FIG. 3. The first device in this diagram is the second-order Stokes signal 60 at 1178 nm, the desired output signal, having a narrow linewidth (10 MHz). The system is co-seeded with a zeroth-order Stokes seed 61 at 1069 nm having a linewidth broad enough so that SBS is not an issue. The zeroth-order Stokes signal at 1069 nm is passed through an isolator 63 and into a WDM 64. The second-order Stokes signal at 1178 nm is passed through an isolator 62 and into the same WDM 64. The WDM 64 places both the zeroth and the second-order Stokes signals into the core of the fiber. Both the zeroth and second-order Stokes signals enter an ytterbium-doped rare-earth amplifier 65 where the zeroth-order Stokes signal is amplified. The amplified zeroth-order Stokes signal along with the second-order Stokes signal pass through a long-period or tilted Bragg (LP/TB) grating 69 tuned to 1121 nm and enter the Raman resonator configuration defined by high-reflector (HR) Bragg gratings 67 and 68 tuned to 1121 nm. The zeroth-order Stokes 1069 nm signal, upon entering the 1121 nm Raman resonator, is converted to the first-order Stokes signal at 1121 nm by the Raman process in the silica or germanosilicate fiber 70. FIG. 4 shows the fall-off of the zeroth order Stokes signal at 1069 nm and the growth of 1178 nm via the Raman conversion process as a function of position in the Raman fiber for the case of 9 meters of 20/400 fiber pumped with 59.3 W of 1069 nm and seeded with 3 W of 1178 nm. Any unconverted zeroth-order Stokes signal at 1069 nm will be reflected back into the Raman fiber 70 by the high-reflector Bragg grating 66 tuned to the zeroth-order Stokes line. Because high power levels of 1121 nm are obtained in the Raman resonator cavity, leakage through the Bragg grating 67 back toward the ytterbium-doped rare-earth amplifier 65 will most likely occur. To prevent this light from entering the amplifier 65, a long period or tilted Bragg grating 69 is used to divert the 1121 nm into the cladding, where it will be absorbed. FIG. 5 shows the power levels for the circulating first Stokes in the right-propagating and left-propagating directions. The second-order Stokes seed at 1178 nm will pass through the amplifier system with minimal reflections into the Raman resonator cavity tuned to 1121 nm where it will be amplified to high power and emerge from the amplifier 72. As before, because of potentially low seed powers at the desired output wavelength, it may be necessary to have multiple rare-earth-doped amplifier/Raman resonator configurations in series to enable amplification of the desired output wavelength to the power levels desired. The embodiment of the 1178 system is compatible with all techniques to suppress SBS to include large mode area fiber, multiple temperature zones, strain, acoustically tailored fiber, multiple types of fiber, etc. It is also compatible with polarized and unpolarized modes of operation as well as continuous wave or pulsed operation on the order of a microsecond. The cavity buildup time has been for this system has been calculated to be 2.5 microseconds.

(24) To prevent significant linewidth broadening of the 10 MHz 1178 nm seed, one of the 1121 nm Bragg gratings 68 needs to be offset so that four-wave mixing is impeded and the bandwidth that resonates is narrowed. For example, a cavity defined by a pair of Bragg gratings 5 cm in length with a B of 10, can achieve a grating offset of 0.02 nm with a 2 C. temperature difference between the gratings. Here B=4n n L/, where n is the mean refractive index of the fiber core, n is the modulation of the refractive index, is the fraction of light in the core, L is the grating length, and is the phase-matched wavelength. Such a cavity will have a high reflectivity over only a few GHz. The transmission of the two offset gratings is shown in FIG. 6. This will result in less broadening of the 1178 nm when it is amplified. Also, high dispersion Raman fiber can also be used to break the phase-matching condition associated with four-wave mixing. In summary, factors enabling a narrow linewidth output at 1178 nm are seeding, offset of Bragg gratings in the 1121 nm cavity, and usage of a high-dispersion Raman fiber.

(25) The required power level for the 1069 nm zeroth-order Stokes seed signal 60 depends on the seed level required within the amplifier 65 for the output power desired. Power levels associated with the 1121 nm in the resonator must be considered when designing the system due to power limitations associated with the Bragg gratings. Parameters which affect the power of 1121 nm in the resonator include: 1069 nm input power level into the Raman resonator; 1178 nm seed power level; and the length of the Raman fiber. In addition, the onset of SBS must be considered for narrow linewidth applications such as this one. As the input power level of 1069 nm into the Raman resonator increases, the 1121 nm circulating power in the resonator, the output power of 1178 nm, and the SBS increase, FIG. 7. As the input power level for the 1178 nm seed increases, the 1121 nm circulating power decreases, the 1178 nm output power increases, and the SBS increases, FIG. 8. Also, as the length of the 1121 nm resonator increases, the 1121 nm circulating power decreases, the 1178 nm output power increases, and the SBS increases, FIG. 9. Generally, SBS mitigation measures will enable an increased 1178 nm seed level or an increased resonator cavity length, a higher output power of 1178 nm, and a decreased 1121 nm circulating power in the resonator.

(26) The factors enabling a high output power level of narrow linewidth 1178 nm in this invention are the generation of very high levels of the first order Stokes line in the 1121 nm Raman resonator, in addition to the fact that the system is seeded. This high power occurs for two reasons, first, high power levels of the 1069 nm are directly injected into the system and second, because the 1121 nm cavity is defined by high reflector Bragg gratings, the power level is able to build up to high levels. High power levels of 1121 nm in addition to seeding at 1178 nm leads to good conversion from 1121 to 1178 nm in a short Raman fiber, several meters in length. In addition, the short Raman fiber enables a high SBS threshold which enables a higher output power level of 1178 nm. Because high output power levels of 1178 nm can be obtained from one amplifier, coherent combination of two or more amplifier chains is not necessary. The result is a simpler system.

(27) Remote Sensing Laser SystemNested Raman Resonator

(28) Another special case of the above embodiments are systems which generate 1240 nm for remote sensing of the water content of the earth and other planets. A block diagram for the system is shown in FIG. 10. The first device in this diagram is a third-order Stokes signal 80 tuned to 1240 nm having the desired output linewidth. The system is co-seeded with a zeroth-order Stokes signal 81 at 1066 nm. The linewidth of this should be broad enough to avoid problems with SBS, but narrower or the same as the linewidth of the desired 1240 nm output. The third-order Stokes signal at 1240 nm is passed through an isolator 82 and into a WDM 84. The zeroth-order Stokes signal at 1066 nm is passed through an isolator 83 and into a WDM 84. The WDM 84 places both the zeroth and the third-order Stokes signals into the core of the fiber. Both the zeroth and third-order Stokes signals enter a ytterbium-doped rare-earth amplifier 85 where the zeroth-order Stokes signal is amplified. The amplified zeroth-order Stokes signal at 1066 nm along with the third-order Stokes seed at 1240 nm pass through two long-period or tilted-Bragg gratings 91 and 92 tuned to 1176 and 1118 nm, respectively, and enter the Raman resonator configuration. Upon entering the 1118 nm Raman resonator defined by the highly reflective Bragg gratings 88 and 89, the 1066 nm zeroth order Stokes signal is converted to the first order Stokes line at 1118 nm by the Raman process in silica or germanosilicate Raman fiber 93. Any unconverted zeroth-order Stokes signal at 1066 nm will be reflected back into the Raman fiber 93 by the high reflector Bragg grating 86 tuned to the zeroth order Stokes line at 1066 nm. Because high power levels of 1118 nm are obtained in the Raman resonator cavity, leakage through the Bragg grating 88 back toward the ytterbium-doped rare-earth amplifier will most likely occur. To prevent this light from entering the amplifier 85, a long-period or tilted Bragg grating is used to divert the 1118 nm into the cladding where it will be absorbed. The first-order Stokes line at 1118 nm is then converted to the second-order Stokes line at 1176 nm. Any unconverted 1118 nm first-order Stokes signal is redirected back into the Raman fiber 93 by the highly reflective Bragg grating 89. Also, any 176 nm second-order Stokes signal that leaks out of the Raman resonator cavity through the Bragg grating 87 will be directed by the long-period or tilted Bragg grating 91 into the cladding where it will be absorbed. The third-order Stokes seed at 1240 nm will pass through the amplifier system into the Raman resonator cavity tuned to the second-order Stokes line at 1176 nm where it will be amplified to high power and output from the amplifier 96. SBS needs to be considered for the 1240 nm only if it is narrow linewidth. Once again, because of potentially low seed powers at the desired output wavelength, it may be necessary to have multiple, i.e., 1, 2, . . . n, rare-earth-doped amplifier/Raman resonator configurations in series to enable amplification of the desired output wavelength to the power levels desired. The embodiment shown in FIG. 10 is compatible with all techniques to suppress SBS to include large mode area fiber, strain, multiple temperature zones, acoustically tailored fiber, multiple types of fiber, etc. The embodiment is also compatible with polarized and unpolarized modes of operation as well as continuous wave or pulsed operation.

(29) To prevent significant linewidth broadening of the 1240 nm seed, one of the Bragg gratings 89 and 90 in each of the 1118 and 1176 nm resonators needs to be offset so that four-wave mixing is impeded and the bandwidth that resonates is narrowed. Offset of the Bragg gratings can be accomplished using a grating controller 95 to either stretch, heat, or bend the gratings. Finally, a high-dispersion Raman fiber can also be used to break the phase matching condition associated with four-wave mixing.

(30) Remote Sensing Laser SystemUnnested Raman Resonator

(31) Another embodiment of the system involves replacing the Raman resonators shown in the dashed box in FIG. 10 with the configuration shown in FIG. 11 where the Raman resonators are completely unnested. In this embodiment, the amplified 1066 nm (zeroth-order Stokes signal) 100 and the seed for the 1240 nm (third-order Stokes line) 101 enter the Raman resonator configuration. The amplified zeroth-order Stokes signal and third-order Stokes signal are passed through a long-period grating tuned to the first Stokes line 105 and are injected into the first Raman resonator in the series. The zeroth-order Stokes signal passes through a high reflector Bragg grating tuned to the first-order Stokes line 103 into the germanosilicate Raman fiber 110. Any unused zeroth-order Stokes signal at 1066 nm is reflected back into the Raman fiber 110 by a high-reflector Bragg grating 102 set at the zeroth-order Stokes line. In this Raman resonator, defined by the high reflector Bragg grating 103 and the output coupler Bragg grating 104, the zeroth-order Stokes signal is converted to the first-order Stokes line at 1118 nm via the Raman process. Any first-order Stokes signal that leaks out of the Bragg grating 103 back toward the rare-earth-doped amplifier will be put in the cladding by the long period or tilted Bragg grating 105, where it will be absorbed. The third-order Stokes signal at 1240 nm together with the first-Stokes signal at 1118 nm, pass through a long-period grating set at the second-order Stokes line 109 prior to entering the second Raman resonator defined by the high reflector Bragg gratings 107 and 108. In this resonator, the first-order Stokes line is converted to the second-order Stokes line in the germanosilicate Raman fiber 111. Any unused first-order Stokes signal will be reflected back into the Raman fiber 111 by the high reflector Bragg grating 106. Any second-order Stokes signal which leaks out of the Bragg grating 107 back toward the rare-earth amplifier will be put into the cladding by a long period or tilted Bragg grating 109 and absorbed. Upon entering the Raman resonator tuned to 1176 nm, the 1240 nm seed (third-order Stokes signal) is amplified to high levels and output from the system 114. SBS needs to be considered for the 1240 nm only if it is narrow linewidth. Again, because of potentially low seed powers at the desired output wavelength, it may be necessary to have multiple rare-earth-doped amplifier/Raman resonator configurations in series to enable amplification of the desired output wavelength to the power levels desired. The embodiment shown in FIG. 11 is compatible with all techniques to suppress SBS to include different types of fibers, strain, multiple temperature zones, large mode area fiber, acoustically tailored fiber, etc. In addition, the embodiment is compatible with polarized and unpolarized modes of operation as well as continuous wave or pulsed operation.

(32) To prevent significant linewidth broadening of the 1240 nm seed, one of the Bragg gratings 102 and 108 in each of the 1118 and 1176 nm resonators needs to be offset so that four wave mixing is impeded and an appropriately narrow bandwidth resonates to aid in suppression of linewidth broadening. Offset of the Bragg gratings can be accomplished using grating controllers 112 and 113 to either stretch, heat, or bend the gratings. High dispersion Raman fiber can also be used to break the phase-matching condition associated with four-wave mixing.

(33) 1300-1500 nm Laser System

(34) The final exemplary application for the invention discussed in this patent is the creation of a series of lasers in the 1300-1500 nm spectral region for telecommunications applications. A block diagram showing a general embodiment of the system is shown in FIG. 12. To enable the desired output wavelength of the N.sup.th order Stokes line, the following parameters are adjustable: the input zeroth-order Stokes wavelength, the type(s) of Raman fiber used in the system, as well as the center reflectivity wavelength for the gratings. In the diagram, the first device shown is a seed 120 for the N.sup.th order Stokes signal having a linewidth consistent with what is required of the output. Another device in the diagram is a seed 121 for the zeroth-order Stokes signal at a wavelength appropriate to enable the desired output wavelength on the N.sup.th order Stokes line. The seed for the N.sup.th order Stokes line is passed through an isolator 122 and into a WDM 124. The zeroth-order Stokes signal 121 is passed through an isolator 123 and into the same WDM 124. Both the zeroth and N.sup.th order Stokes seed signals enter a rare-earth-doped amplifier 125 where the zeroth-order Stokes seed is amplified. Both the amplified zeroth-order Stokes signal and the N.sup.th order Stokes seed enter multiple Raman resonators 128 which may be in various configurations. Within the Raman resonator configuration, the zeroth-order Stokes signal is converted to the N1.sup.th order Stokes signal in steps (zeroth order Stokes signal.fwdarw.first order Stokes signal.fwdarw.second order Stokes signal.fwdarw. . . . .fwdarw.N1.sup.th order Stokes signal) in silica, germanosilicate, and/or phosphosilicate fibers. The seed for the N.sup.th order Stokes line is then amplified to a high power level by the N1.sup.th order Stokes signal in a single pass through the system. The output of the system 129 is the N.sup.th order Stokes signal at the desired wavelength between 1300 and 1500 nm. Once again, because of potentially low seed powers at the desired output wavelength, it may be necessary to have multiple rare-earth-doped amplifier/Raman resonator configurations in series to enable amplification of the desired output wavelength to the power levels desired.

(35) For telecommunication applications, the ability to create lasers having small shifts in wavelength to enable wavelength division multiplexing in a certain bandwidth is necessary. Small shifts in the output wavelength are achievable by adjusting the wavelength of the Nth order Stokes line, the wavelength of the zeroth-order Stokes signal and/or the center wavelength of the Bragg gratings. This embodiment is compatible with all techniques to suppress SBS to include large mode area fiber, strain, multiple temperature zones, acoustically tailored fiber, etc. In addition, the embodiment is compatible with PM as well as non-PM modes of operation in addition to continuous wave and pulsed operation.