Abstract:
A multiple wavelength laser apparatus is taught to select wavelengths for simultaneous generation of short pulses at multiple wavelengths having application in wavelength-division-multiplexed communications systems, photonic microwave systems, and pump-probe applications. The multiple wavelength laser comprises an optical fiber lasing material, a laser pump source connected to said lasing material for outputting coherent light, a splitter for dividing said coherent light into parallel paths of differing wavelengths, parallel band pass filters for selecting multiple wavelengths, and parallel Faraday mirrors for operating the multiple wavelength laser in a single polarization state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 60/379,104, entitled “Multiple Wavelength Pulsed Laser Source”, filed on May 10, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention deals generally with the simultaneous generation of short pulses at multiple wavelengths, which is of particular interest to wavelength-division-multiplexed communication systems, photonic microwave systems, and pump-probe applications. A versatile and cost-effective laser source should have the following characteristics. Synchronized multiple-wavelength outputs eliminate the need for external synchronization when using more than one wavelength. Short pulses of less than 10 ps are needed for gigahertz repetition rates. The instant invention is a multiple-wavelength laser based on the sigma configuration, well known in the prior art, which has tunable wavelength, synchronized outputs, short pulses, and low noise. 
     2. Description of the Related Prior Art 
     Mode-locked pulse generation at multiple-wavelengths has been studied in various configurations of erbium-doped fiber lasers. The focus has primarily been on variations in the ring cavity design. Schlager, et al., have proposed a method that incorporates 3.4 km of birefringent fiber and a polarizing isolator in the cavity (See J. B. Schlager, S. Kawanishi, and M. Saruwatari, “Dual wavelength pulse generation using mode-locked erbium-doped fiber ring laser,” Electron. Lett., 27, 2072-2073 (1991)). Other techniques for selecting the wavelength for ring cavity lasers utilize fiber Bragg gratings, power splitters and bandpass filters, and dense-wavelength-division-multiplexers. Passively modelocked lasers are demonstrated in a figure-eight configuration by Noske, et al. and in a linear configuration by Okhotnikov, et al. (See D. U. Noske, M. J. Guy, K. Rottwitt, R. Kashyap, and J. R. Taylor, “Dual-wavelength operation of a passively mode-locked “figure-of-eight” ytterbium-erbium fiber soliton laser,” Opt. Commun., 108, 297-301 (1994) and O. G. Okhotnikov and M. Guina, “Stable Single- and Dual-Wavelength Fiber Laser Mode Locked and Spectrum Shaped by a Fabry-Perot Saturable Absorber,” Opt. Lett., 25, 1624-1626 (2000)). An actively-modelocked linear cavity laser with fiber gratings as the ends of the cavity and a nonlinear optical loop mirror in the middle has been demonstrated by Pattison et al. (See D. A. Pattison, P. N. Kean, J. W. D. Gray, I. Bennion, and N. J. Doran, “Actively Modelocked Dual-Wavelength Fiber Laser with Ultra-Low Inter-Pulse-Stream Timing Jitter,” Photon. Technol. Lett., 12, 1415-1417 (1995)) A modified sigma configuration laser, which used a circulator followed by cascaded fiber gratings in a second arm is demonstrated by Deparis, et al. (See O. Deparis, R. Kiyan, E. Salik, D. Starodubov, J. Feinberg, O. Pottiez, P. Megret, and M. Blondel, “Round-trip time and dispersion optimization in a dual-wavelength actively mode-locked Er-doped fiber laser including nonchirped fiber bragg gratings,” IEEE Photon. Technol. Lett., 11, 1238-1240 (1999)). These lasers produce either broad pulse widths (&gt;10 ps) or wide wavelength separations (&gt;15 nm). This limits the ability of the laser to produce high repetition rates and limits the total number of wavelengths that are possible within the gain bandwidth of the amplifying media. Additionally, wide wavelength tuning ranges have not previously been demonstrated. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a laser device capable of simultaneous generation of short pulses at multiple wavelengths which has application in wavelength-division-multiplexed communications systems, photonic microwave systems, and pump-probe applications. 
     Another object of the invention is to provide a laser with tunable wavelengths to allow the user to choose whichever wavelength is needed, or to scan over a range of wavelengths. 
     Another object of the invention is to provide a laser capable of synchronized multiple-wavelength outputs that eliminate the need for external synchronization when using more than one wavelength. 
     Another object of the invention is to provide a laser capable of producing short pulses of less than 10 ps which can achieve gigahertz repetition rates. 
     Another object of the invention is to provide a practical laser with amplitude fluctuations which are minimal, and timing jitter which is much less than the bit period. 
     Another object of the invention is to provide a laser with a series of fixed wavelengths in those applications which do not require tunability. 
     Another object of the invention is to provide a multiple wavelength laser based on the sigma configuration which has tunable wavelength, synchronized outputs, short pulses, and low noise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of two wavelength sigma laser. The different wavelength pulses in the cavity share the same gain medium, but each wavelength passes through a separate bandpass filter, attenuator, fiber delay line, and Faraday mirror. 
         FIG. 2  shows a schematic diagram of a multiple wavelength laser. The different wavelength pulses in the cavity share the same gain medium, but each wavelength passes through a separate bandpass filter, attenuator, fiber delay line, and Faraday mirror. 
         FIG. 3  shows optical spectra at a range of wavelength separations, tunable from 3.5 nm to 24 nm. 
         FIG. 4   a  shows RF spectrum of the laser output at 1538 nm. 
         FIG. 4   b  shows RF spectrum of the laser output at 1541.5 nm. 
         FIG. 5   a  shows a cross-correlation of pulses at 1538 nm and 1541.5 nm. 
         FIG. 5   b  shows a cross-con-elation of pulses at 1538 nm and 1560 nm. 
         FIG. 6   a  shows autocorrelations of pulses at 1538 nm and at 1541.5 nm. 
         FIG. 6   b  shows autocorrelations of pulses at a spectral width of 0.37 nm at 1541.5 nm. 
         FIG. 6   c  shows autocorrelations of pulses at a spectral width of 0.35 nm at 1538 nm. 
         FIG. 7   a  shows single-sideband amplitude noise, with zero-area spurs removed versus frequency offset from the carrier frequency. The amplitude fluctuations are &lt;0.5% (10 Hz to 1 MHz). 
         FIG. 7   b  shows single-sideband phase noise, with zero-area spurs removed, versus frequency offset from the carrier frequency. The timing jitter for each wavelength is &lt;68 fs (1 kHz to 1 MHz). 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A schematic block diagram of a dual wavelength fiber laser is shown in FIG.  1 . The laser is divided into two sections, a polarization-maintaining (PM) loop  118 , in which all components and connecting fibers maintain the circulating light in a specific linear polarization state; and a single-polarization branch  119 , in which the polarization state is not fixed. 
     Light is generated in the single-polarization branch  119 , which will be described later, and is routed into the PM loop  118  by the polarization beam splitter  103 . A single-polarization isolator  104  ensures that light circulates in the PM loop in a specific direction and in a specific linear polarization state. A portion of the circulating light exits the laser in a linearly-polarized state by means of the output coupler  105 . The remainder of the light is routed from the output coupler to the modulator  101 . 
     The modulator  101  is a Mach-Zehnder electro-optic modulator with a radio-frequency (rf) bandwidth of at least 10 gigahertz (GHz). The modulator is electrically driven by a signal originating in an rf synthesizer  101 , which produces an electrical sine-wave output with a frequency of 960 megahertz (MHz) and an output power of roughly 10 milliwatts (mW). The signal is then amplified by an rf amplifier to a level of approximately 500 mW and is converted by the step-recovery diode  108  into a periodic series of pulses with a duration of approximately 100 picoseconds (ps), an amplitude of approximately 10 volts (V), and with the same repetition rate as the synthesizer  106 . The output of the step-recovery diode  108  is sent to the electrical input port of the modulator  101 . The modulator, when thus driven, imposes an amplitude modulation on the light passing through it with the same repetition rate as the synthesizer  101 ; the duration of time that the modulator periodically allows light to pass through it is roughly the same as the pulse width generated by the step-recovery diode  108 . 
     The modulated light is then routed through a 90-degree splice  102  in the PM fiber connecting it to the polarization beam splitter  103 , to orient it into a proper polarization state to be routed by the polarization beam splitter  103  out of the PM loop  118  and into the single-polarization branch  119 . The light is amplified by a length of Erbium (Er) doped gain fiber  109 , which is excited by a laser pump source  117 ; in this instance the laser pump source  117  is a laser diode generating approximately 130 mW of optical power at a wavelength of approximately 980 nanometers (nm). The saturated output power of the Er-doped fiber amplifier is approximately 15 mW. The light then passes through approximately 270 meters (m) of SMF-28 “standard” fiber, which provides intracavity dispersion compensation; the total average dispersion of the laser cavity is approximately 11.6 ps/(nm·km). 
     The light then passes though a variable fiber delay line  111 , which, in concert with variable fiber delay lines  114  and  115 , ensures that the round-trip circulation time of a pulse through the laser is an integral submultiple of the driving frequency of the rf synthesizer  106 . The laser is thus harmonically mode-locked at the rate of the driving synthesizer  106  of 960 MHz. The light is then sent to a splitter, specifically in this embodiment the 3-dB coupler  112 , which channels the light into two parallel paths. Tunable band-pass filters  113  with bandwidths of 0.8 nm select the wavelength of the light in each path; variable attenuators  114  compensate any wavelength-dependent gain of the Er-doped fiber amplifier  109  to keep the optical intensities in each path roughly the same. The variable fiber delay lines  115  in each path ensure that the propagation time of the light through each path is approximately equal. 
     The two paths are terminated with Faraday mirrors  116 , the function of which is to reflect light into a polarization state which is orthogonal to the incident state. Light injected into the single-polarization branch by the polarization beam splitter  103  is initially linearly polarized, but the polarization state evolves in a random manner as the light propagates through the fiber and components of the branch. Since this polarization state evolution occurs in a roughly linear process, the Faraday mirrors  116  ensure that, at each point in the single-polarization branch, returning light is in a polarization state approximately orthogonal to that of the incident light. Therefore, since light injected by the polarization beam splitter is linearly polarized, the light returning to it is also approximately linearly polarized but is rotated by approximately 90 degrees. Light in this polarization state is sent by the polarization beam splitter  103  to the isolator  104 , thus completing the circuit of light through the laser. The two light paths created by the splitter  112  are recombined by the same component into one path on the return trip. The light thus makes a round trip through the single-polarization branch  119 , passing through each component twice. 
     The band-pass filters  113  in the two paths of the single-polarization branch  119  select the wavelengths of light pulses that the laser emits. Since the dispersion of the laser is maintained at a low value by the dispersion-compensating fiber  110 , the pulses exit the laser through the output coupler  105  at approximately the same time. The durations of the light pulses are determined primarily by the the duration of time that the modulator  101  periodically allows light to pass through it and by the bandwidth of the tunable bandpass filters  113 . 
     Though the components used in this embodiment have specific properties and values, the laser will operate in a similar manner with other components with other properties. For instance, though the rf synthesizer  106  in this embodiment operates at a frequency of 960 MHz, lasers of this type have been operated at rates varying from 100 MHz to more than 40 GHz; in another instance, though the duration of the pulses from this laser are in the range of approximately 10 to 20 ps, other lasers of this type have produced pulses with durations ranging from approximately 0.1 to 100 ps; and operation outside these ranges is possible with appropriate selections of component types and values. 
     An embodiment incorporating both a higher repetition rate and having a four-wavelength fiber laser is shown in the schematic block diagram of FIG.  2 . The laser is divided into two sections, a polarization-maintaining (PM) loop  218 , in which all components and connecting fibers maintain the circulating light in a specific linear polarization state; and a single-polarization branch  219 , in which the polarization state is not fixed. 
     Light is generated in the single-polarization branch  219 , which will be described later, and is routed into the PM loop  218  by the polarization beam splitter  203 . A single-polarization isolator  204  ensures that light circulates in the PM loop in a specific direction and in a specific linear polarization state. A portion of the circulating light exits the laser in a linearly-polarized state by means of the output coupler  205 . The remainder of the light is routed from the output coupler to the modulator  201 . 
     The modulator  201  is a Mach-Zehnder electro-optic modulator with a radio-frequency (rf) bandwidth of at least 10 gigahertz (GHz). The modulator is electrically driven by a signal originating in an rf synthesizer  206  which produces an electrical sine-wave output with a frequency of 10 GHz and an output power of roughly 12 milliwatts (mW). The output of the rf synthesizer  206  is sent to the electrical input port of the modulator  201 . The modulator, when thus driven, imposes an amplitude modulation on the light passing through it with the same repetition rate as the synthesizer  206 ; the duration of time that the modulator periodically allows light to pass through it is roughly the same as one period of the sine-wave output of the rf synthesizer  206 , which in this embodiment is approximately 100 ps. 
     The modulated light is then routed through a 90-degree splice  202  in the PM fiber connecting it to the polarization beam splitter  203 , to orient it into a proper polarization state being routed by the polarization beam splitter  203  out of the PM loop  218  and into the single-polarization branch  219 . The light is amplified by a length of Erbium (Er) doped gain fiber  209 , which is excited by a laser pump source  217 ; in this instance the laser pump source  217  is a laser diode generating approximately 130 mW of optical power at a wavelength of approximately 980 nanometers (tim). The saturated output power of the Er-doped fiber amplifier is approximately 20 mW. The light then passes through approximately 45 meters (in) of SMF-28 (standard) fiber, which provides intracavity dispersion compensation; the total average dispersion of the laser cavity is approximately 1.9 ps/(nm×km). 
     The light then passes through a variable delay line  211 , which, in concert with variable fiber delay lines  115 , ensures that the round-trip circulation time of a pulse through the laser is an integral submultiple of the driving frequency of the rf synthesizer  206 . The laser is this harmonically mode-locked at the rate of the driving synthesizer  206  of 10 GHz. The light is then sent to a splitter, specifically in this embodiment the broadband wavelength-division multiplexer (WDM)  212 , which channels the light into two paths, each of which includes the desired wavelengths, a narrow band WDM  220  channels the light into four paths and selects a different wavelength with a bandwidth of 2.5 nm for each path; variable attenuators  214  compensate any wavelength-dependent gain of the Er-doped fiber amplifier  209  to keep the optical intensities in each path roughly the same. The variable fiber delay lines  215  in each path ensure that the propagation time of the light through each path is approximately equal. 
     The four paths are terminated with Faraday mirrors  216 , the function of which is to reflect light into a polarization state that is orthogonal to the incident state. Light injected into the single-polarization branch by the polarization beam splitter  203  is initially linearly polarized, but the polarization state evolves in a random manner as the light propagates through the fiber and components of the branch. Since this polarization state evolution occurs in a roughly linear process, the Faraday mirrors  216  ensure that, at each point in the single-polarization branch, returning light is in a polarization state approximately orthogonal to that of the incident light. Therefore, since light injected by the polarization beam splitter is linearly polarized, the light returning to it is also approximately linearly polarized but is rotated by approximately 90 degrees. Light in this polarization state is sent by the polarization beam splitter  203  to the isolator  204 , thus completing the circuit of light through the laser. The four light paths created by the WDMs  212  and  220  are recombined by the same components into one path on the return trip. The light thus makes a round trip through the single-polarization branch  219 , passing through each component twice. 
     The narrow band WDM  220  in the single-polarization branch  219  divides the light into four paths, where the optical pass-band of each path corresponds to a select wavelength of light pulses that the laser emits. Since the dispersion of the laser is maintained at a low value by the dispersion-compensating fiber  210 , the pulses exit the laser through the output coupler  205  at approximately the same time. The durations of the light pulses are determined primarily by the duration of time that the modulator  201  periodically allows light to pass through it and by the bandwidth of the narrow band WDM  220 . 
     Though the components used in this embodiment have specific properties and values, the laser will operate with other components with other properties. For instance, though the rf synthesizer  206  in this embodiment operates at a frequency of 10 GHz, lasers of this type have been operated at rates varying from 100 MHz to more than 40 GHz; in another instance, though the duration of the pulses from this laser are in the range of approximately 10 to 20 ps, other lasers of this type have produced pulses with durations ranging from 0.1 to 100 ps; and operation outside these ranges is possible with appropriate selections of component values. Different wavelengths may be obtained by using different wavelength-division multiplexers with different optical pass-bands. 
     Variable wavelength separation operation is illustrated in  FIG. 3 , where one wavelength is fixed at 1538 nm and the other wavelength is tuned continuously from 1541.5 nm to 1562 nm. The maximum tuning range is set by the erbium gain bandwidth. Because of the dispersion in the cavity, each wavelength travels at a slightly different speed inside the cavity and thus, it is necessary to fine-tune the cavity length for each wavelength to ensure that the pulses arrive at the modulator at the same time. Therefore, the maximum tuning range of the cavity length also determines the maximum wavelength separation. The minimum wavelength separation is determined by the homogeneous linewidth of the erbium-doped fiber to be 3 to 4 nm. If the wavelength separation is less than 3 nm, gain competition between the wavelengths results in noisy operation. That is, while both wavelengths may continue to laser, pulse dropouts are observed, and eventually the lasing action of one wavelength will dominate over the other. Wavelength separation as low as 3 nm at 1550 nm and 1553 nm is possible. 
     Both wavelengths exit the laser at the same time. The laser has no pulse dropouts as evidenced by the greater than 70-dB supermode suppression shown in  FIGS. 4   a  and  4   b . We show the cross-correlation between the two wavelengths for the case of 3.5-nm separation ( FIG. 5   a ) and for 22-nm separation ( FIG. 5   b ). The cross-correlation between the 1538-nm pulse and the 1560-nm pulse appears broader because the pulse at 1560 nm is broader in than the 1541.5-nm pulse. This is because in optical fiber, the pulse width is a function of the dispersion at the pulse wavelength. Thus, while the dispersion is ideal for a transform limited pulse at 1541.5 nm, the dispersion is slightly higher at 1560 nm and the additional dispersion leads to a slightly broader pulse. 
     The autocorrelations and spectra for both wavelengths for near transform limited pulse outputs at 1538 nm and 1541.5 nm are shown in  FIGS. 6   a - 6   c . The output wavelengths are separated using 0.9-nm bandpass filters. The pulses at 1538 nm and 1541.5 nm both have full-width at half-maximum pulse widths of approximately 6.7 ps ( FIG. 6   a ), a spectral width of 0.35 nm at 1538 nm ( FIG. 6   c ) and a width of 0.37 nm at 1541.5 nm ( FIG. 6   b ), and an average power of 0.28 mW in total. The time-bandwidth products are 0.3 and 0.31 for the 1538 nm and 1541.5 nm pulses, respectively. This is consistent with transform-limited hyperbolic-secant pulse shapes. 
     The amplitude and phase noise of this laser have also been measured; both wavelengths have similar noise characteristics. The measured single-sideband amplitude noise for 1538 nm and 1541.1 nm are shown in  FIG. 7   a  (zero-area spurs have been removed). If the data is integrated over the range from 10 Hz to 1 MHz, we find less than 0.5% amplitude fluctuation. The measured single-sideband phase noise, with the spurs removed, is shown in  FIG. 7   b . Integrating over the range from 1 kHz to 1 MHz, there is less than 68 fs timing fitter. Even including the low frequencies and integrating from 10 Hz to 1 MHz, the maximum timing jitter is less than 800 fs. 
     The method to match the higher harmonic drive frequency to the fundamental cavity frequency is not limited to the delay stages used in our laser prototype. The cavity may not need any delay stages if the lengths can be specified exactly. In general, some sort of tunable delay is desirable. Examples include free-space-coupled delay stages and fiber-stretcher delay lines. 
     The number of wavelengths can also be increased by a variety of ways. Instead of a 2×2 coupler (3-dB coupler), one could use one or more M×N couplers where N is the number of paths to separate the incoming light and M is any number greater than or equal to 1. In each one of the paths, a filter, attenuator, delay stage, and Faraday mirror can be used to complete the cavity (as demonstrated in our two-wavelength version). Another method would be to use a wavelength-division-multiplexer to separate out the different wavelengths, which essentially combines the functions of our 3-dB coupler and bandpass filters. 
     The pulse spectral width can be changed by using other forms of wavelength filtering in the laser cavity. For example, a bandpass filter with a wider pass-band will allow a wider pulse spectrum to exist. An alternative to the bandpass filters is to use fiber gratings to filter the desired wavelengths. In general, any form of spectral filtering with any pass-band width can be used to set the spectral characteristics of the laser output. The cavity can be stabilized for lower noise performance, for example, similar to the method described in U.S. Statutory Invention Registration H1926. Additionally, each one of the independent paths for the different wavelengths may be stabilized to each other and/or to the external RF synthesizer. 
     Finally, the net dispersion in the laser cavity can be optimized with the addition or subtraction of any dispersive media. As examples, more optical fiber or a fiber Bragg grating can be used to change the dispersion. For longer wavelengths (such as 1560 nm), the net dispersion should be lowered to achieve transform-limited pulses. One way is to add dispersion compensation, perhaps different amounts to the independent paths, so that all the paths have the same dispersion over the range of operating wavelengths intended for that path. Alternatively, dispersion-flattening can be implemented in the main part of the cavity so that the dispersion is relatively constant over the entire gain spectra. 
     The invention demonstrates a sigma laser capable of simultaneously generating pulses at multiple wavelengths, using the same physical gain fiber. The wavelength separation is tunable from the limit of the homogeneous linewidth to the limit of the erbium gain bandwidth. Two different center wavelengths (1538 and 1541.5 nm) are shown to have nearly transform-limited 6.7-ps pulses with low noise. The amplitude fluctuation is less than 0.5% (10 Hz to 1 MHz) and the timing jitter is less than 68 fs (1 kHz to 1 MHz). In general, appropriate cavity-dispersion compensation is necessary to achieve transform-limited pulses. 
     The two-wavelength sigma laser has the same potential for high repetition-rate and low pulse duration as its single-wave-length counterpart. Extension to higher repetition rate and more wavelengths will require more gain. Additionally, with active stabilization, it can achieve lower amplitude and phase noise operation. 
     Although this invention has been described in relation to an exemplary embodiment thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in the preferred embodiment without detracting from the scope and spirit of the invention as described in the claims.