Patent Publication Number: US-2006002443-A1

Title: Multimode external cavity semiconductor lasers

Description:
FIELD OF THE INVENTION  
      The present invention generally relates to laser devices such as those that may be employed in networking applications and, more particularly, to semiconductor lasers.  
     BACKGROUND OF THE INVENTION  
      Light sources such as lasers or light emitting diodes are used to produce modulated signals that carry information across optical networks. Generally, these light sources should enjoy stable operation, consistently providing signals at predetermined frequencies and with little loss. Such stable operation is increasingly more important in high-demand optical networks, such as Wavelength Division-Multiplexing (WDM) and Dense Wavelength-Division Multiplexing (DWDM) systems, where numerous data streams may be propagating simultaneously. In WDM and DWDM networks, network performance would vary channel to channel, data stream to data stream, if consistent laser operating characteristics were not maintained.  
      Laser diodes, a common type of network laser source, come in three different forms, and the specific form used is often dictated by the requirements of the network. Configurations include distributed feedback lasers (DFBs), Fabry-Perot lasers, and external cavity lasers (ECLs). Each configuration is capable of producing relatively narrow-bandwidth laser energy, via the use of different types of highly reflective laser cavities that limit the laser energy&#39;s. Low-cost Fabry-Perot lasers are often used for short-distance low data rate (&lt;2.5 Gb/s) transmissions, whereas DFB lasers are often used in high data rate transmission over longer distances. In other applications, especially where external modulators impart signal data, ECLs are used. Additional factors affecting laser configuration include whether a laser cooling system is to be used to reduce noise and output frequency fluctuations. ECLs, for example, are commonly used in cooled environments, principally because ECLs produce higher output energies, but also because network designers prefer to have more stable light sources with narrower spectral widths. In contrast, where a cooled environment is not needed, FP or DFB lasers are typically used.  
      Although there have been a number of attempts to use ECLs to replace costly DFB lasers, there are some fundamental issues that limit ECL applications for un-cooled environments. In the ECL, the resonant cavity is formed by an external element, usually a grating that provides wavelength selection. These ECL&#39;s, however, are susceptible to mode hopping, a phenomena that can occur with changes in temperature or injection/drive current, as well as with parasitic reflections. In optical networks, mode hopping can be quite problematic and induce bit error rate degradation in the system. ECLs, for example, use narrow-bandwidth reflective elements that only allow for one dominant laser mode. If a laser source is producing a laser signal operating at that mode, then the laser signal experiences minimal loss. Yet, if operating conditions in the laser change, the laser&#39;s lasing wavelength may hop to another mode. This mode could be close to the dominant lasing mode, but transition from one mode to another mode results in sudden change in optical power. As a result, even small fluctuations in operation conditions can result in a laser signal intensity dropping off dramatically, due to mode hopping.  
      Some have proposed techniques for reducing mode hopping in laser sources, but the proposals have been limited to single-mode devices that do not avoid the inherent modal dependence on output intensity. For example, thermal compensators, such as a silicone layer, could be used in an external laser cavity to counteract the effects of temperature change on the cavity length. The compensator could attempt to produce an equal and opposite temperature effect on the laser device. Yet, the technique is only able to quell mode hopping over a limited range of temperatures and, thus, not well suited for widespread commercial use. Further, while conceptually thermal compensators should reduce the affects of temperature changes, in fact, the thermal-optic coefficients of the compensating materials are non-linear, meaning that it is very difficult to achieve total thermal compensation over an entire operational temperature window of an un-cooled device. Plus, these systems merely attempt to prevent mode hopping. If mode hopping ever does occur, there will still be a dramatic drop off in signal intensity. In another example, an un-cooled ECL using a fiber Bragg grating with a moderately-widened bandwidth larger than the longitudinal mode spacing has been proposed. But the system, as with those described above, is a single-mode system that would exhibit sizable and undesirable model dependence in signal intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a perspective view of a multimode laser apparatus including a laser source coupled to a wavelength selective element.  
       FIG. 2  is a side illustration of the apparatus of  FIG. 1 .  
       FIG. 3  is a top view of an example laser apparatus having an angled facet to reduce reflection losses with a laser device.  
       FIG. 4  is a spectral plot of multiple longitudinal laser modes and grating profile with bandwidth for a laser apparatus in accordance with the example of  FIGS. 1-3 .  
       FIG. 5  illustrates another example multimode laser apparatus including a tuning element.  
       FIG. 6  illustrates an example wavelength selective element bandwidth profile for a laser apparatus at different temperature operating conditions.  
       FIG. 7  illustrates a block diagram of a transceiver including a multimode laser apparatus, in an example.  
       FIG. 8  illustrates a multi-channel, multimode laser apparatus that may be used in a wavelength division multiplexed system, in an example. 
    
    
     DETAILED DESCRIPTION  
      Although a number of devices are described with reference to illustrated examples, the disclosure is not limited to these examples. Thus, although external cavity lasers are described with an external grating element as a wavelength selective device, persons of ordinary skill in the art will recognize that other wavelength selective devices may be used, including highly reflective wavelength filters.  
       FIG. 1  illustrates an example laser apparatus  100  that has a grating profile of sufficient bandwidth (or spectral cavity profile) to support multiple longitudinal laser modes. The apparatus  100  is in an external cavity laser configuration and includes a laser source  102 , e.g., a side-emitting laser diode having a cladding region  104  surrounding laser gain region, or core,  106 . This laser source  102  is shown by way of example. The apparatus  100  may use another type of laser source, e.g., a vertical cavity surface emitting laser, fiber laser, or optical amplifier. The laser source  102  may be a III-V semiconductor laser providing laser energy at any output frequency, of which the known telecommunication frequencies band centered at 850 nm, 1310 nm, and 1550 nm are examples. In very short reach (VSR) applications, like those of an enterprise space, campus local area networks (LANs), and storage area networks (SANS), the laser source  102  may be an 850 nm GaAs/AIGaAs diode laser, for example, although any suitable light emitting material may be used. Typical operating parameters for VSRs include 10 Gb/s data transmission rates on 25-300 m fibers, including multi-mode fibers (MMFs)—although, single mode fibers (SMFs) may be used as well. The laser source  102  may alternatively provide output at the 1310 nm or 1550 nm telecommunication bands, for example, in wide area networks (e.g., longer reach, 10 km links) and metro area networks (e.g., extended reach, 10 km links). The output wavelength of the laser source  102  may be matched to reduce propagation loss and chromatic dispersion within the apparatus  100  and optical fibers coupled thereto, for example, by using a laser source producing a wavelength larger than approximately 1.1 μm, where silicon waveguides exhibit relatively low optical absorption. In some examples, the lasing material may be a non-linear material.  
      By way of example, the gain region  106  may be formed of a lasing material that has been epitaxially grown in the cladding layer  104 , or the gain region  106  may be formed via doping/implantation process to create the higher index gain region. As a laser diode chip, the laser source  102  may be batch fabricated using Silicon wafer technology and diced to produce large numbers of such sources.  
      The laser source  102  may include a first cavity reflector  107 , which may be as cleaved or coated with a dielectric, for example. In the illustrated example, the reflector  107  may reflect most of the laser energy within the laser source  102 . The laser source  102  may be disposed within a recess or cavity  108  formed in a substrate  110 , such as a semiconductor substrate or Silicon optical bench (SiOB) exposed to a pattern-and-etch lithography process. The recess  108  is positioned and sized to align the gain region  106  with a wavelength selective element, e.g., an external reflector element  112  for low-loss coupling between the two. By way of example, the laser source  102  may be bonded, glued, or fastened into the recess  108 .  
      In the illustrated example, the external reflector element  112  includes a waveguide core  113  formed within the substrate  110 , for example, via a Silicon on insulator (SOI) process, other epitaxial growth process, and/or a doping/implantation process. Alternate to these integrally formed techniques, a separately-formed waveguide may be mounted to the substrate  110 , for example by mounting a single-mode, multi-mode, or plastic optical fiber in a substrate groove, such as a V-groove or U-groove. Although not shown, coupling optics may be used between the laser source  102  and the element  112  to prevent unwanted coupling loss.  
      As illustrated in  FIG. 2 , the waveguide core  113  may extend between two cladding regions,  114  and  116 , that may be formed of the same cladding material and formed in the substrate  110 , in an example. The waveguide core  113  has an input end  118  adjacent and in communication with the laser source  102  and an exit end  120  through which laser energy from the laser source  102  is provided.  
      The element  112  forms part of a laser cavity  122  that has a longitudinal cavity mode profile that supports a plurality of longitudinal laser modes, as discussed in further detail below. To provide high reflectivity and a sufficiently broad longitudinal-laser bandwidth profile, the element  112  includes a highly-reflective, partially transmissive grating  124 . The grating  124  may by 70%-90% reflective, for example, and forms a second laser cavity reflector for laser cavity  122 .  
      The grating  124  may be formed in the waveguide core  113  using a photomasking, laser writing, etching, diamond cutting, or silicon doping, techniques. The grating  124  may be formed of silica, a polymer material, or a IIIN semiconductor structure. In an example, the grating  124  may be written by irradiating the substrate  110  with an ArF excimer laser operating at 193 nm. Such techniques may allow for grating line-width and spacing accuracy in the sub 1 nm range. Additionally, exposure saturation techniques may be used to affect the depth and index profile of the grating lines, for example, creating grating lines with uniform indexes of refraction across the entire line.  
      In another example, etching deep trenches into a SOI wafer/waveguide and filing these trenches with a poly-silicon, annealed to reduce lattice mismatch and loss, may be used to fabricate the grating. The poly-silicon may be chemically or mechanically polished to obtain a planar surface with the top of the element  112 , before the cladding layer or the remainder of the cladding layer is deposited. With this latter technique, narrow reflection bandwidths of approximately 1 nm or below may be achieved, for example, between 0.5 and 0.3 nm. The grating  124  is not limited to these fabrication techniques, however, nor is the grating  124  limited to specific reflection bandwidths, as the bandwidth may depend upon the desired conditions of the laser apparatus.  
      In the illustrated example, the grating  124  has a relatively narrow reflection bandwidth, but instead of producing a laser with single-mode operation where the side modes adjacent a principle longitudinal laser mode are suppressed, the grating bandwidth is large enough to reflect a number of longitudinal modes and, therefore, large enough to create a spectral cavity width that supports multiple laser modes. As explained in further detail below, the grating bandwidth may support  3  to  8  longitudinal modes, for example. The grating bandwidth may be adjusted by affecting the optical path length of the grating. Further, even though the grating bandwidth is large enough to support multiple longitudinal modes, the wavelength selective element  112  may also meet accepted industry standards for channel spacing and wavelength stability in systems such as WDM and DWDM systems operating over the C-band wavelength region.  
      As illustrated in  FIG. 2 , intra-cavity reflection losses within the laser source  102  may be reduced by an antireflection coating layer  126  formed on the laser source  102 . Additionally, an anti-reflection coating layer may be formed at the entrance face  118  of the element  112  to reduce reflection back into the laser cavity or back into the element  112 . Or an air gap region  128  extending between the laser source  102  and element  112  may be filled with an index of refraction matching material. With an antireflection coating or index-matching material, intra-cavity reflections may be reduced.  
      Additionally, to reduce intra-cavity reflection losses, the gain region  106  may be curved or angled from an output face surface normal to form an angled-facet output face. An example configuration is illustrated in  FIG. 3 , which shows an example laser source  200  with a gain region  202  defining an axis  204  that forms an acute angle with a surface normal on exit end  206 . Because light within the gain region  202  is incident on the exit end  206  at a non-normal angle of incidence, reflection back into the cavity is reduced.  
      In the illustrated example, the gain region  202  has a linear portion  208  and a curved portion  210  having a radius of curvature sufficiently large to prevent bending loss in the gain region  202 . As illustrated, the curved portion  210  aligns with a waveguide  212  of an external wavelength selective element  214  that also has an acute-angle entrance face  216 . Angles of less than 10° from surface normal may be used to decrease reflectively at an entrance/exit face. For example, an 8° facet angle could result in less than 10 −3  reflection at the facet, depending on the materials used. The reflection loss may be reduced even further by using an anti-reflection coating, as described above. Nevertheless, the low-loss configuration illustrated in  FIG. 3  is provided by way of example, and the illustrated enhancements are optional. Furthermore, coupling enhancements may also be used, such as forming a tapered waveguide portion of the element  212  to match the size of the gain region  202  or the light beam emanating therefrom.  
      An example plot  300  of longitudinal laser modes and grating profile of an example bandwidth is shown in  FIG. 4 . Pluralities of longitudinal laser modes ( 302 ,  304 ,  306 ,  308 , and  310 ) that may propagate within a laser apparatus are illustrated. The mode  306  may be considered a principle mode, with modes  302 ,  304 ,  308 , and  310  considered side modes, although these designations are arbitrary. Numerous other side modes are theoretically available based on the dimensions of the laser cavity extending from a first reflector to a second reflector, such as the wavelength selective device. The modes  302 ,  304 ,  306 ,  308 , and  310 , however, are the only supported modes in the illustrated example, because a grating profile with bandwidth  312  of the laser cavity envelopes these modes.  
      The grating profile  312  has a substantially flat profile (plateau)  314 , over which small reflection differences occur between the longitudinal modes  302 ,  304 ,  306 ,  308 , and  310 . The plateau  314  is smaller than a full-width half-maximum (FWHM)  316 , but large enough to support each of the modes  302 ,  304 ,  306 ,  308 , and  310 . As a result of the substantial flatness of the plateau  314  and thus the substantial flatness of the grating profile  312 , the grating profile  312  is such that no one longitudinal laser mode dominant. That is, substantially all modes  302 - 310  will see the same profile maximum value, and thus mode hopping between longitudinal laser modes or movements of a mode (for example, via thermally-induced fluctuations) will not result in a substantial reduction in intensity of the output signal. Thus, the average power of a signal may be stable, even under temperature or refractive index changes.  
      Merely by way of example, a substantially flat grating profile may be chosen with a plateau that produces approximately 10% or less grating reflectivity difference among the supported modes. The reflectivity difference between modes may reflect an intensity difference between light energy produced by the laser at the modes. As the reflectivity difference between modes is lowered, the output intensity for these modes will approach one another, becoming substantially the same for very low reflectivities. In some examples, approximately 5% or less reflectivity difference may be desired. Gratings may be chosen, such that the grating profile exhibits approximately 1% reflectivity difference between the support modes, for example.  
      The spectral width of the cavity laser is determined by the linewidth of the laser source and the profile of the wavelength selective element used therein. In the example of a grating as the wavelength selective element, the grating period, length, depth (i.e., shallowness of grating lines), and material may all be adjusted to create a grating profile with a bandwidth that results in a particular spectral width for the cavity laser. Further, the amount of saturation achieved by the source used to form the gratirig lines (e.g., excimer laser saturation) may affect the flatness of the grating profile and, thus, the flatness of the spectral widths bandwidth plateau.  
      Thus, in an example, to achieve spectral cavity widths of substantially flat profiles, the grating profile should be substantially flat as well, for example producing approximately 10% or less reflectivity difference among the supported longitudinal modes, e.g., approximately 1%-5%. These percentages are examples though, as the substantially flat grating profiles may be set to any useful tolerances.  
      By way of example, substantially flat grating profiles (or plateau widths) below 1 nm may be used. In some examples, a 0.3 or 0.4 nm grating bandwidth plateau may be used for 0.08 nm longitudinal mode spacings, with an overall cavity length of approximately 12 mm. In other examples, 8 supported modes may be formed by using a grating with profile of approximately 0.5 nm with an approximately 0.06 nm (i.e., 7.5 GHz) mode spacing. For an approximately 0.08 nm (i.e., 10 GHz) mode spacing, then six modes may be supported over 0.5 nm. Fora 0.16 nm (i.e., 20 GHz) mode spacing, three modes may be supported. These examples illustrate longitudinal mode spacings of below approximately 0.2 nm with substantially flat grating profiles across a bandwidth plateau of 1 nm and below. These values are provided by way of example, however.  
      In some examples, narrower spectral widths or grating profiles may be used for longer distance transmission systems to reduce dispersion effects. Generally, however, the grating parameters may be adjusted to create a grating profile that results in a spectral width that coincides with the distance requirement for the transmission system.  
      As the grating profile can be narrower than standard Fabry Perot lasers, the performance of laser devices in an external cavity laser configuration using a grating as the wavelength selective element may be improved. Further, temperature stability may be improved in comparison with Fabry Perot lasers, as the lasing wavelengths are defined by the grating residing in the optical waveguide. With a silicon-based waveguide for the wavelength selective element, for example, the wavelength drift may be on the order of 0.01 nm/° C. Further, the techniques may be used over much larger temperature ranges, because the techniques are substantially independent of operating temperature.  
      In the illustrated example, a substantially flat grating profile is wide enough to support five supported longitudinal laser modes, but fewer or additional numbers of modes may be supported. For example, three to eight or more modes may be simultaneously supported by the laser apparatus, where fewer supported modes means that the device will be more susceptible to mode hoping or mode shift.  
      In an implementation, a grating profile may be chosen to have a certain size, for example, by setting the number of lines in the grating to an amount corresponding to an identified bandwidth. Next, the number of modes that are to be supported for the device may be chosen based on the sensitivity or responsiveness of the laser device to fluctuations in the longitudinal mode. If the laser apparatus is to be more tolerant to mode hopping, then a few number of longitudinal modes may be selected. After the number of supported modes is selected, the cavity length of the laser apparatus may be determined and the length from the first cavity reflector to the second set. In the example of a grating as the second cavity reflector, a mid point within the grating may be referenced for setting cavity length. The cavity length will determine the spacing between the longitudinal modes.  
      Numerous examples are described above; however, it should be appreciated that these examples may be modified or changed. For example, the structures described may be replaced with other structures. The wavelength selective element, which is a waveguide grating, in some examples, may be replaced with any suitable wavelength selective element, for example, a wavelength filter, such as a thin film, etalon or Fabry Perot filter. The element should have a substantially flat bandwidth so that no dominant mode among the plurality of supported modes is created; although, this need not be the case, as one or more modes may experience higher reflectivities (or lower loss) than another mode and still be a supported mode in the device.  
      Furthermore, although the examples are described with reference to a grating of a given frequency response, a tunable wavelength selective element may be used, such as a tunable grating element or tunable waveguide.  FIG. 5  illustrates an example laser apparatus  400  including a laser diode  402  disposed in a substrate  404  and adjacent a wavelength selective element  406 . A tuning element  408 , e.g., an electrode, is positioned over a waveguide segment  410  of the wavelength selective element  406 . The waveguide segment  410  may be formed of an electrically-responsive material such as LiNbO 3  (lithium niobate) or any electro-optical material, which has an index of refraction that changes under application of an electric field. Thus, applying a voltage to the electrode  408  will change the laser cavity length of the laser apparatus  400 , thereby changing the spacing between longitudinal laser modes, and if the spacing change is large enough, changing the number of supported longitudinal laser modes. Alternatively, the bandwidth of the wavelength selective element may be tuned, for example, by positioning the electrode  408  over a grating  412  in the wavelength selective element  406 , such that the bandwidth is altered in response to application of an electric field.  
      In another alternative, instead of an electrode, a heating or thermal element may be positioned adjacent a waveguide portion, grating portion, or both of a wavelength selective element to induce an index of refraction change to change the mode spacing or bandwidth. Alternatively, the substrate of the apparatus may be mounted on a thermoelectric material that is capable of inducing temperature changes. In these examples, the wavelength selective element may be formed of a polymer having a temperature dependent index of refraction. The thermo-optic effect, for example, may be used to tune the grating by heating it, thereby tuning the frequency of the laser energy, that is, the centermost point on the grating profile and thus the centermost frequency of the spectral width of the cavity laser. Example responsiveness is approximately 12.5 nm/100° C. In another example, tuning may be achieved by mechanical techniques, such as strain/stretch. For example, mechanically stretching or compressing a fiber grating may induce a change in the propagation properties of a laser device to change the number of supported longitudinal modes.  
       FIG. 6  provides a plot  500  of the variation in bandwidth profile for a laser apparatus at different tuned temperatures for a wavelength selective element. Curves  502 ,  504 , and  506  represent different wavelength selective element bandwidths at three different temperatures that are offset from one another, i.e., the curves  500 ,  502 , and  504  have center wavelengths  508 ,  510 , and  512 , respectively. These bandwidths  500 ,  502 , and  504  will determine the spectral width of the laser and, thus, the number of supported modes. In this way, a tuning element may be used to tune the center wavelength of a bandwidth profile to coincide with a particular output frequency. Such tuning may be useful in DWDM systems that include a number of laser signal channels, each operating at a different channel frequency, such that the wavelength selective element for a particular channel is tuned to have a center frequency that corresponds to the channel frequency.  
      Other techniques for tuning either the longitudinal mode spacing or wavelength selective element bandwidth may be used. For example, either the first or second laser cavity reflector may be an external reflector mounted to a movable translation stage, thermally-, electrically-, or mechanically-controlled. That is, the laser source and the wavelength selective element may be movable relative to one another.  
      The laser devices described may be used in various applications, including transponders and transceivers, such as those used in local area networks, wide area networks, and metro area networks. An example 10 Gb/s optical transceiver  600  is illustrated in  FIG. 7 . An electrical interface  602  provides input/output data transfer with a host card or host processor environment. DC power, ground connections, various clocking channels, control signals, and monitoring channels may be coupled to the transceiver  600  via the electrical interface. The interface may take the form of a socket pluggable into a host board, for example. The width of the data bus provided by the interface  602  may vary depending on the application, as well as the bit rate through the interface  602 .  
      The transceiver  600  further includes a control system, for example a microcontroller  604 . In the illustrated example, a physical medium attachment  606  (PMA) is provided and provides the electrical functionality of the transceiver  600 . For example, the PMA  606  may provide clock multiplier/multiplexer (MUX/CMU) and/or clock data recovery/demultiplexer (CDR/DeMUX).  
      The transceiver  600  also includes an optical receiver  608 , which may represent an array of optical receivers. Examples include receivers for 10 Gb/s links based on either InP-based or GaAs-based PIN photodiodes or avalanche photodiodes. The receiver  608  converts received optical energy from an optical interface  610  into ah electrical signal provided to the microcontroller  604  and the PMA  606 . Although not shown, various circuit elements may be integrated into the optical receiver  608 , such as a transimpedance amplifier and limiting amplifier to provide high gain and high sensitivity response. The transceiver  600  also includes a transmitter  612 , or array of transmitters, that includes an external cavity laser source such as those described hereinabove.  
      The transceiver  600  is illustrated in block form. In packaged form, the transceiver  600  may include cooled or un-cooled “butterfly” packages and TO-can style packages.  
      The laser sources described herein may be combined in an array, or like fashion, to produce multi-channel laser devices. Identical laser diode chips, for example, may be batched fabricated and then diced from a processed wafer sample to be combined into a device operating at different wavelengths. WDM or DWDM transceivers are examples.  
      An example multi-channel device  700  is shown in  FIG. 8 . The device  700  includes an array of laser diodes  702 , each disposed within one of an array of recesses within a substrate  704 . As with similar laser sources described above, the laser diodes have gain regions aligned for coupling energy into a wavelength selective element  706 , for example, one of a plurality of waveguides having a grating  708  positioned therein. Each laser diode  702  and wavelength selective element  706  pair forms a different laser source associated with a different channel, and each laser source includes one tuning element  710  that may be used to tune the particular laser source to a particular center frequency, such that each laser source may operate at a different laser wavelength. Each wavelength selective element  706  is coupled to an output waveguide  712  which may be coupled to an optical multiplexer or another type of optical interface, such as an optical modulation stage.  
      Although the devices described are described in the context of laser apparatuses providing a continuous wave output, the devices may be used to produce information carrying modulated optical signals. For example, the laser apparatuses may have output waveguides coupled to optical modulators, such as electro-optical crystals formed of LiNbO 3  or III-V semiconductor compounds, including multiple quantum well structures. Waveguides may be coupled to Mach-Zehnder interferometer (MZI) modulators including two waveguide arms, each with a section for converting an applied voltage into a propagation delay between the two arms, thus modulating an incident laser signal.  
      Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalence.