Abstract:
A laser transmitter capable of transmitting large numbers of WDM channels but requiring locking of only a single channel. Each of the channels can be individually modulated using an external modulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefits of U.S. Provisional Application Serial No. 60/286,474, filed on Apr. 25, 2001. 
    
    
     GOVERNMENT FUNDING 
     This invention was made with government support under Contract No. F1962895-C-0002 awarded by the United States Air Force. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to laser communication systems, and in particular to systems involving wavelength-division multiplexing. 
     BACKGROUND OF THE INVENTION 
     In communication systems utilizing wavelength-division multiplexing (WDM), light of multiple wavelengths (actually narrow wavelength bands) propagates through a transmission medium, typically an optical fiber. Because the wavelengths are spaced apart spectrally and do not interfere with each other, they represent separate communication channels that can be independently modulated to carry information. To select a particular channel, its wavelength is extracted—i.e., demultiplexed—from the multiple-wavelength signal. 
     The combined WDM optical signals can be amplified as a group and transported over a single fiber to increase capacity. Each carried signal can be modulated at a different rate and in a different format (SONET, ATM, data, etc.) 
     Naturally, each transmitting laser in a WDM system must be configured to operate at the wavelength corresponding to its assigned channel. Ordinarily it is necessary to exert absolute frequency control over the laser sources, particularly in the case of semiconductor lasers, which possess a nominal operating frequency that is difficult to control precisely upon fabrication and which fluctuates with injection current, junction temperature and aging. Thus, an individual frequency-locked transmitter is typically used for each channel, and consequently, as the number of WDM channels increases, the transmitter farms needed to provide them become significantly more complex. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a transmitter capable of transmitting large numbers of WDM channels but requiring locking of only a single channel. Each of the channels can be individually modulated using an external modulator. 
     The transmitter is preferably a multichannel external cavity laser similar in certain respects to those described in U.S. Pat. Nos. 6,192,062 and 6,208,679, and U.S. Ser. No. 09/708,697 now U.S. Pat. No. 6,697,192 (filed on Nov. 8, 2000), the entire disclosures of which are hereby incorporated by reference. The &#39;697 application, for example, describes external-cavity laser designs that generate coaxially overlapping outputs at multiple wavelengths. An external laser resonator may be based on a bar of light-emitting semiconductor material whose outputs emerge from a linear sequence of stripes along the length of the bar. These outputs pass through an output-coupling lens and strike a dispersive element, such as a diffraction grating. Light dispersed by the dispersive element is reflected by a mirror back along the optical path, passing through the lens and returning to the semiconductor outputs, the opposite facets of which are partially reflective. The resulting feedback produces laser amplification, and light not reflected by the partial mirror represents discrete, spatially separate outputs. 
     Thus, the partially reflective semiconductor facets and the mirror together form an ensemble of individual, external-cavity lasers, each with its own optical path. The lens and dispersive element force the individual beams into a coaxial configuration, their paths intercepting at the dispersive element. Moreover, because the beam of each of these lasers strikes the dispersive element at a different angle, each laser has a different optical path and, therefore, resonates at a different wavelength. As a result, the gain elements are forced to produce rear-face outputs at the different resonance wavelengths. 
     The spatially separated outputs are combined by a similar optical arrangement including a coupling lens and a dispersive element. Once again the lens and dispersive element force the individual beams into a coaxial configuration, causing the different wavelengths to co-propagate. The overall result is a high-power, multi-wavelength beam with high brightness due to the coaxially overlapping component beams, and which may be focused onto the end face of an optical fiber for propagation therethrough. 
     In accordance with the present invention, the individual, spatially separated outputs are modulated to encode data prior to recombination. A representative laser transmitter includes a linear array of gain elements (e.g., diodes) each having a partial reflecting surface on its outer facet; an optical device (such as a collimating lens and/or a curved mirror); a dispersive element (such as a diffraction grating or prism); and a reflective device (such as a mirror) forming an external cavity. These external-cavity elements are shared by all of the resonators of all of the array elements. The laser resonator for each array element is defined by the optical path between the partial reflector and the mirror. 
     Fast modulation of each output is facilitated by a modulator array. This design renders the invention well-suited to WDM applications, in which it is ordinarily necessary to modulate each channel independently and desirable to modulate external to the resonator in order to achieve fast modulation rates. Intracavity modulation, which has been previously proposed for these types of sources, limits the modulation bandwidth to approximately the inverse of the cavity ring-down time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 schematically illustrates a laser transmitter in accordance with the invention; and 
     FIG. 2 illustrates an alternative embodiment using an arrayed waveguide grating. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a multichannel laser transmitter  100  includes an external-cavity resonator  110  for generating the component output beams and an output stage  120  by means of which the spectrally distinct outputs are spatially combined. 
     Resonator  110  includes a set of gain elements  125  which may be implemented as a bar of light-emitting semiconductor material comprising a linear sequence of n emission elements or stripes, indicated at λ 1  . . . λ n , where n may range, for example, from two to  100 . Alternatively, the emission elements may be discrete multi-mode semiconductor amplifiers, and in still another alternative, the emission elements may be fiber amplifiers. 
     Bar  125  (or the individual emission elements) has a forward emission face  130 , which is generally antireflective, and a partially reflective rear output face  132 . That is, output face  132  is provided with a partial-mirror surface. Resonator  110  also includes an optical device (such as a collimating lens and/or a curved mirror)  140 ; a dispersive element (such as a diffraction grating or prism)  143 ; and a reflective device (such as a mirror)  146 . 
     In operation, gain-element array  125  is excited (by application of an electric current) such that elements λ 1  . . . λ n  emit radiation through face  130 . Each of the elements λ 1  . . . λ n  emits a beam of radiation having a different free-space optical path. The radiation beams from elements λ 1  . . . λ n  all pass through optical device  140  and strike dispersive element  143 . Optical device  140  causes the radiation beams to overlap as they reach dispersive element  143 . For example, device  140  may be a lens positioned, as indicated in the figure, substantially a focal-length distance away from both emission face  130  and dispersive element  143 . The light reflected from dispersive element  143  toward mirror  146  is a composite of the individual beams, which emerge from dispersive element  143  coaxially and normal to mirror  146 , which is preferably a high reflector. 
     This configuration forms a resonator. The optical paths of the beams from emission elements λ 1  . . . λ n  all pass through device  140  and are all dispersed by element  143 —that is, all beams share device  140  and dispersive element  143 —but pass through only one of the emission elements. Light reflected by mirror  146  and received through the emission face  130  is again partially reflected by output face  132 , the unreflected portion of each beam representing one of the outputs of resonator  110 . 
     Thus, the gain elements of array  125 , in combination with the other optical elements, together form an ensemble of individual external-cavity lasers. Because the beam of each of these lasers is incident on dispersive element  143  at a different angle, each lases at a different wavelength (despite the identical spontaneous emission spectra of the source emission elements). That wavelength, in turn, is determined by the beam&#39;s angle of incidence with respect to dispersive element  143  and its angle of diffraction, the optical characteristics of the gain medium, and the grating line spacing of the dispersive element  143 . Thus, by varying one or more of these parameters (most simply, the orientation and/or location of dispersive element  143  relative to emission face  130 ), the wavelengths of the lasers may be tuned. The tuning range depends on the gain bandwidth of the emission elements and the reflectivity of the output face  132 . The number of emission elements λ 1  . . . λ n  and their locations can be selected so as to generate simultaneously or sequentially any set of wavelengths within the gain width of the gain media. 
     A frequency-locking circuit  150  is desirably employed to lock the laser emissions to a WDM channel grid (e.g., separated by 50 GHz for dense WDM applications), providing wavelength stability. An important advantage of the present invention is the ability to maintain wavelength stability among all channels by monitoring only a single channel, since the channel separation of emission elements λ 1  . . . λ n  occurs by virtue of the physical arrangement of resonator cavity  110 . Frequency-locking circuit  150  typically monitors the output frequency of a single channel and generates an error signal representing deviation of the monitored frequency from a standard. In response to the error signal, the frequency of the monitored channel can be adjusted by tilting mirror  146  or grating  143 , or alternatively by translating optical device  140  and/or gainelement array  125 . Again, since the relative frequencies of all of the channels are controlled by the optical configuration, fixing the frequency of a single channel also fixes the frequency of all of the others. 
     The outputs of the emission elements through face  132  are directed to output stage  120  through a modulator array  155 , which facilitates modulation of each of the output laser beams to encode information. In one embodiment, the array comprises a linear series of electroabsorptive modulators. These are optical devices that act like very fast shutters, blocking the output of an associated emission element or letting it pass. It should be noted that the gain-element array  125  may be combined with an electroabsorptive modulator array on a common silicon platform (that is, monolithically integrated), along with driver and frequency-locking circuitry. 
     Alternatively, modulator array  155  may be a linear array of Mach-Zehnder interferometers. A Mach-Zehnder interferometer is an optical switch controlled by an external electric field. It utilizes a pair of optical waveguides, each basically a channel of dielectric material surrounded by a substrate material of lower index of refraction n; light is confined within each waveguide and confined therein by total internal reflection (that is, light originating in a material with larger n and incident on a material with lower n will be entirely reflected within the former material at angles of incidence above a critical value). The output from an emission element is split into two components, and these components travel through optical waveguides of equal length before being combined at the output. Normally, both paths have an equal index of refraction, so the beams undergo equal phase shifts as they propagate, and are combined constructively. As a result, the full power of the beam passes through the interferometer. High-voltage electrodes are placed around one of the two paths, however, and the waveguide is electrooptically responsive, so that a strong bias applied across the electrodes causes the index of refraction in that path to be changed; the two beams therefore emerge from the waveguide paths with unequal phases. The bias voltage is precisely what is needed to cause perfect destructive interference, so that no output beam appears. 
     The outputs from face  132  of gain-element array  125  can be butt-coupled to the modulator array  155  or alternatively re-imaged through the modulator array or coupled to an array of optical fibers, each fiber transmitting an individual output to one of the modulators. 
     The outputs travel from the modulator array to output stage  120 , which comprises a dispersive optical system that combines the spatially separated outputs into a single optical fiber. In the illustrated embodiment, the outputs from modulator array  155  pass through an optical device  170  (e.g., a lens) and strike another dispersive element  173  (e.g., a grating). Optical device  170  causes the beams to overlap as they reach dispersive element  173 , and a lens  175  focuses the combined outputs as a single beam onto the end face of an optical fiber  180 . 
     In another alternative, illustrated in FIG. 2, the outputs from the modulator array  155  can be coupled into a series of individual fibers, collectively indicated at  200 , and the outputs of fibers  200  then combined into a single fiber  210  using an arrayed waveguide grating (AWG)  215 . The AWG  215  essentially functions as a diffraction grating, combining the separately modulated outputs into a single, multichannel optical signal, and includes a pair of couplers  220 ,  222  and a series of waveguide arms  225  having different path lengths. 
     Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.