Abstract:
A method of wavelength conversion includes receiving a modulated optical signal of a first wavelength and using the modulated optical signal to optically pump an active medium to generate and transmit a like modulated optical signal of a second wavelength, all of which is affected entirely within an optical domain without electrical contacts. Also, an optical wavelength converter includes an active medium that is optically pumped by a modulated optical signal of a first wavelength and transmits the modulated optical signal at a second wavelength. The active medium may be contained in a detachable, external module for wavelength conversion of a fixed wavelength source.

Description:
BACKGROUND OF THE INVENTION 
     In optical networks, data from a source may be converted into an optical signal for transmission along an optical fibre. Complex optical transmission devices incorporating lasers for data transmission have been designed and packaged in single units, or modules. However, the signal wavelength outputted from these devices is predetermined by the lasing wavelength of the laser used in the device to generate the optical signal. 
     Often, it would be desirable to be able to set the output wavelength of the module during installation, and to be able to change it as required, whether for the purpose of colour choice, for compatibility with another optical device or system, or for other purposes. 
     One previous method was to manufacture a number of different fixed wavelength sourced optical transmission devices. Using the same device design, a variety of units are fabricated, each outfitted with lasers of varying wavelengths This results in expenditures associated with maintaining large inventories of multiple versions of the same device, not all of which may be used, and which may require complex processes to change the output wavelength of an installed source. 
     Another previous method was the tunable laser. For example, U.S. Pat. No. 5,949,801, issued Sep. 7, 1999 to Tayebati for Tunable Laser and Method for Operating the Same, discloses a tunable Fabry-Perot laser having a tunable Fabry-Perot filter as a wavelength-selective component. The application of a voltage to the filter changes its wavelength-selection properties. However, added complexity in the design and fabrication of tunable lasers increases the costs of such lasers, and hence, the costs associated with the device incorporating a tunable laser. Further, it is difficult to manufacture tunable lasers that are tunable, directly modulated and high speed. 
     A third method utilizes an attachable component incorporating fibre gratings, or other such component, of a period selected for a particular emitted frequency, that feed back an optical emission into a device with gain and a single mirror to form the oscillator cavity. For example, U.S. Pat. No. 5,978,400, issued Nov. 2, 1999 to Campbell et al., for Laser, discloses a laser diode coupled to an optical fibre having a grating. The laser characteristics depend on the optical phase relationship of the Bragg gratings in the external waveguide. While reducing the complexity incorporated into the optical transmission device, phase shift induced noise is increased as a result of the use of the attachable component. Also, the length of the laser cavity in such a configuration tends to be long where the spectral mode spacing is short. As a result, the likelihood of mode-hopping is increased. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention seeks to provide a method of generating a modulated optical signal and an optical wavelength converter module, which minimizes the above problems. 
     According to one aspect of the invention, there is provided a method of optical wavelength conversion including the steps of receiving a modulated optical signal of a first wavelength; stimulating an active medium using the modulated optical signal of the first wavelength for generating a like modulated optical signal of a second wavelength; and transmitting the modulated optical signal of the second wavelength; whereby wavelength conversion and modulation are affected entirely within an optical domain. 
     Also according to the invention, there is further provided an optical wavelength converter including a first part for receiving a modulated optical signal of a first wavelength; a part for stimulating an active medium using the modulated optical signal of the first wavelength for generating a like modulated optical signal of a second wavelength; and a second part for transmitting the modulated optical signal of the second wavelength; whereby wavelength conversion and modulation are affected entirely within an optical domain. 
     In one aspect of the invention, the first part, the active region, and the second part forms a resonator. The resonator may be of a vertical cavity surface emitting laser (VCSEL) type geometry, edge emitting geometry, optically pumped waveguide geometry, and the like. 
     In another aspect, the active region is contained in a module. The module is coupled to a source for generating a modulated optical signal of the first wavelength. The coupling is detachable and may be in the form of a detachable pigtail. 
     In yet another aspect of the invention, the optical signal transmitted by the second part is of a wavelength signal region suitable for optical communications, including amplified systems, wavelength division multiplexing (WDM) systems, and the like. 
     In yet a further aspect of the invention, the signal transmitted by the second part is along a same directional course as, or a different directional course than, the signal received by the first part, including at an angle to reduce back reflection of the signal received by the first part, in a direction perpendicular to the signal, or the like. 
     Advantageously, wavelength selection can be accomplished in a relatively simple and inexpensive fashion by providing the wavelength conversion of a fixed wavelength source comprising a simple resonator in an external module. By an optically pumped resonator with its own spectral and spatial filtering functions, and no electrical connections, there is no sensitivity to phase shift-induced noise resulting from use. Also, the configuration can result in oscillation in a mode best coupled to the optical fibre. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures in which: 
     FIG. 1 is a top view of a wavelength converter module coupled to an optical transmission device, in accordance with an embodiment of the invention. 
     FIG. 2 is a close-up top view of the wavelength converter module of FIG. 1, with the connector parts removed. 
     FIG. 3 is a close-up top view of the wavelength converter module with the connector parts removed, in accordance with an alternative embodiment of the invention. 
     FIG. 4 is a top view of the wavelength converter module, in accordance with another alternative embodiment of the invention. 
     FIG. 5 is a close-up top view of the wavelength converter module with the connector parts removed, in accordance with another alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, an optical transmission device  10  optically coupled to a wavelength converter module  16  is shown. 
     The optical transmission device  10  is an optical source for digital data transmission in an optical transmission system, for example, a “gigabit interface converter (GBIC)” style module, a so-called “mini dual in line (mini-DIL)” module, or a conventional 14 pin “butterfly” module. 
     The optical transmission device  10  includes a directly modulated (DM) laser  12 . The DM laser  12 , for example, may be a semiconductor laser, including distributed feedback lasers (DFB lasers), distributed Bragg reflector (DBR) lasers, or Fabry-Perot semiconductor lasers in either edge emitting or surface emitting configurations. 
     The DM laser  12  produces a pump signal  14  of a spectral width suitable to optically pump a preselected laser resonator to be described below. Any pump wavelength in a range suitable for pumping a selected resonator may be used. Examples of wavelengths used in optical communication systems are around 850 nanometers, around 980 nanometers, around 1300 nanometers or around 1480 nanometers. 
     The pump signal  14 , includes a time varying signal and conveys a data stream. A data signal is modulated into the carrier including, for example, by varying the intensity, frequency, polarisation, or phase of the carrier. 
     The pump signal  14  is coupled to and used to optically pump a laser resonator  34 , producing an output signal  39 . In the example of FIGS. 1 and 2, the resonator  34  is a semiconductor VCSEL type resonator. Preferably, the resonator  34  is a single frequency oscillator with gain, but it can be appreciated that the resonator need not have the VCSEL type geometry. 
     The VCSEL type resonator  34  of FIGS. 1 and 2 comprises an active region  38 , of preselected gain or active material. The resonator  34  may be fabricated using a variety of materials including semiconductor compounds, polymers, organics or composites, or other materials capable of providing gain when optically pumped. For example, the resonator  34  may be semiconductor edge-emitters, such as a Fabry-Perot, DFB, or DBR, rare earth or semiconductor doped glass or polymer host, organic semiconductors, or other materials and geometries. 
     As will be appreciated, the active region  38  of the resonator  34  may be fabricated using conventional methodology and materials for producing signals at a selected wavelength. The fabrication methodology could also include the addition of layers whose purpose is to improve absorption of the pump signal  14  and transfer the carriers to the active layers of the active region  38 . 
     As will be appreciated, the signal wavelength of the resonator  34  is selected possessing an acceptable spectral width and centre frequency accuracy for a particular application in an optical communications system. For example, the acceptable spectral width and accuracy of a signal in a dense wavelength division multiplexing (DWDM) network with a centre frequency of 50 Gigahertz channel separation is much less than 50 Gigahertz; typically only a few GHz. In a coarse wavelength division multiplexing (WDM) operating at 1310 nanometers with a channel separation of 10 to 20 nanometers, the acceptable width and accuracy of a signal may be a few nanometers. 
     Signal wavelength may be further selected outside of the active region  38  through the provision of physical structures such as gratings  46 , as shown as an example in FIG.  3  and further described below, or interference filters. 
     The resonator  34  lacks electrical contacts and is not provided with electrical injection current. The removal of the electrical pumping constraint allows other materials, such as dielectrics, to be used advantageously. For example, an active region  38  may be clad by a suitable material, such as silicone, to compensate for temperature variation to make a source independent of temperature. 
     The feedback mechanism consists of the two reflectors on either side of the active material, which are aligned in a manner to reflect coherent light  39  back and forth through the active medium  38 . 
     The active region is bounded on one side by a first reflector which, in the embodiment of FIG. 2, is a Distributed Bragg Reflector (DBR)  40  formed by alternating layers of semiconductor or dielectric material with differing refractive indices. The layers  40  are constructed to provide very high reflectivity (in excess of 99%) at the signal wavelength, and low reflectivity at the pump wavelength. 
     On the other side of the active region  38 , is provided a second reflector, which, in the embodiment of FIG. 2, is a mirror  42 , with lower reflectivity, though typically also in excess of 99%, to provide the feedback necessary for oscillation at the signal wavelength and typically with high reflectivity at the pump wavelength for increased efficiency. This second reflective surface may be positioned adjacent to the active region  38  or positioned on the optical fibre  18 . 
     Alternatively, the reflectivity of the mirrors  40  and  42  may be selected to establish a single pass pump. 
     An alternative embodiment positions the second mirror, now  46 , within the output optical fibre core,  22 , ensuring alignment of the oscillating mode with the optical fibre. In such a case, the resonator  34  becomes a composite, including the optical fibre  18  with Bragg gratings  46 , and the layer  42  has low reflectivity at the signal wavelength. The period of the Bragg gratings  46  is preselected to provide for a signal more accurately tuned to a particular wavelength range. 
     Alternatively, the resonator  34  may be an optically pumped waveguide. laser. For example, a semiconductor waveguide may be fabricated as a DFB laser by impressing a Bragg grating onto the waveguide by known techniques to form the second reflective surface, as exemplified in the embodiment of FIG. 3, which is then coupled to the optical fibre  18 . 
     Depending on the material and construction of the resonator  34 , stimulated emission may occur upon exposure of the active region  38  of the resonator  34  to a pump signal  14  in a narrow wavelength range, typical of atomic transitions, or in a broader range of wavelengths, typical of semiconductor materials. The pump signal  14  is absorbed in the appropriate regions of the resonator  34 . 
     The resonator  34  may be included within a connector casing  20  to facilitate coupling to the optical transmission device  10  and to optical fibre  18  for transmission. 
     Referring to FIGS. 1 and 2, the resonator  34  is coupled to the terminal end  32  of a length of optical fibre  18 . The optical fibre  18  typically comprises a core  22  and cladding  24 . A buffer  25  and a protective jacket  26  surrounds the optical fibre  18 . The optical fibre  18  is adapted to convey an optical signal propagating along its length. The signal from the resonator  34  substantially enters into the terminal end  32  and propagates along the optical fibre  18 . 
     A VCSEL type resonator  34  may be self-aligning with the terminal end  32  of the optical fibre  18 . Alternatively, resonators  34  may require manual alignment by conventional alignment methods. 
     Referring to FIG. 1, the resonator  34  is contained in a wavelength converter module  16 . The module  16  is attached to the optical transmission module  10  by a selected connector method, for example, by epoxy for a permanent attachment, or alternatively, a connector casing  20  may be provided with an attachment method adapted to mate with the optical transmission device  10 , as more fully described below. Various connector styles may be used for attachment to the optical transmission device  10 , for example, ST, FC, RJ, or LC. For example, the module  16  may be in the form of a detachable pigtail, as exemplified in FIGS. 1 and 4. 
     To assist in positioning the resonator  34  to efficiently couple the wavelength converter module  16  to the optical transmission device  10 , the connector end  30  of the connector casing  20  attaches the wavelength converter module  16  to the optical transmission device  10  at an attachment end  50  at a position to maintain the localization of the pump signal  14  from the DM laser  12  to the resonator  34 . 
     A lens  56  may be provided in the optical transmission device  10  and positioned relative to the beam of the pump signal  14  of the DM laser  12  to facilitate localization of the pump signal  14  onto the desired region of the resonator  34 , for example, by collimation or focusing. 
     A ferrule  28  protects and aligns the optical fibre  18  adjacent to the terminal end  32  within the connector casing  20 , and may also encompass the terminal end  32 . 
     Hermetic seals may be provided if required. The connector casing  20  may be mated with a corresponding Hermetic seal on the optical transmission device  10 . As an example, the lens  56  which relays the pump signal from within the package  10  can form the seal for the optical transmission device  10 , while a window (not shown) on the ferrule  28  seals the resonator  34 , the attachment end  50  being able to optically couple the pump signal  14  to the resonator  34 . 
     The resulting connection may be permanent or temporary, whereby the wavelength converter module  16  may be detached from the optical transmission device  10 . 
     In the embodiment depicted in FIG. 4, the resonator  34  is angled in relation to the direction of the pump light  14  along axis A so as to reduce reflection of the pump light  14  back into the source  12 . An equivalent function can be achieved by angling the pump signal  14  source light through angled DM laser  12  or offset lens  56 . 
     Referring to FIG. 5, the resonator  34  is in a lateral configuration with reference to the DM laser  12 . The resonator  34  is an edge-emitting resonator. Pump light  14  from the DM laser  12  is passed through a spherical, cylindrical, dup-cylindrical or elliptical lens  56 , depending on the particulars of the geometries of the DM laser  12  and the resonator  34 , to optically pump the resonator  34 , which emits a signal in a direction substantially perpendicular to the direction of the pump light  14 . 
     In operation, wavelength converter module  16  containing a VCSEL-type resonator  34  produces a signal at a particular wavelength, which is coupled to an optical fibre  18  in the connector casing  20  of a wavelength converter module  16 . The connector end  30  of the connector casing  20  is connected to the attachment end  50  of the optical transmission device  10 . The DM laser  12 , associated with the optical transmission device  10 , provides the pump signal  14  containing a modulated data stream. 
     The resonator  34  passively absorbs the pump signal  14  of a particular wavelength, which also serves to optically pump the atoms in the active region  38  of the resonator  34  from a lower to a higher energy state whereby a population inversion is created. The feedback mechanism of the resonator  34  comprising the highly reflective mirror  40  on one side of the active region and a less reflective mirror  42 , or  46 , on the other side, returns a portion of the coherent light originally produced in the active medium of the active region  38  back to the active medium for further amplification by stimulated emission. On stimulated emission, a signal modulated in a like manner to that of the pump signal is emitted through the second mirror  42  in a wavelength range for which it was configured, independent of the wavelength of the pump signal  14 , and enters into the terminal end  18  to which the resonator  34  is coupled, to propagate along the optical fibre  18 . 
     Preferably, the minimum output power for the directly modulated laser  12  is the threshold condition for the resonator  34 , and the resonator  34  should be designed to respond appropriately to the speed of the data signal,  14 . 
     Examples of signal wavelength regions in an optical communications system are 850, 1275 to 1320 or 1520 to 1620 nanometers, although other wavelengths are used as well, but the wavelengths may be used as the specifics of the gain material are changed. Within each range there will be specific wavelengths of use (for example the ITU frequency grid for DWDM communications). Wavelength converter module  16  may be detached and removed from the optical transmission device  10  so that an alternative wavelength converter with a different signal wavelength may be substituted. 
     For example, a gigabit ethernet module is an optical transmission device  10 , which could provide a pump signal  14  and data stream at about 850 nanometers, primarily modulated in terms of intensity, and may be connected to a wavelength converter module  16  including a resonator  34  configured to emit an optical signal at around 1300 nanometers for some applications, or around 1550 nm in order to produce a signal that may be used in an amplified system or a WDM system. Alternatively, the gigabit ethernet module may provide pump signal  14  and data stream at around 1300 nanometers and connected to a wavelength converter module  16 , where the resonator  34  is configured to emit a signal at around 1550 nanometers. While the output from the stock gigabit ethernet module is too low for many applications, more power can be made available. 
     The invention may be used in additional, related, forms. For example, it may at times be advantageous to have the wavelength conversion take place at a distance from the signal source. Under these conditions it is only necessary that the loss of the signal from the source to the converter and the propagation fidelity are appropriate for the task. 
     The present invention has been described with regard to preferred embodiments. However, it will be obvious to persons skilled in the art that numerous modifications, variations, and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.