Abstract:
Adverse effects to laser excitation ratio slope caused for example by ambient temperature maybe compensated for by adjusting drive current to the laser. The real time excitation ratio slope may be determined by dithering the code word by +/−1 least significant bit (LSB) of a digital-to-analog drive current source (DAC). A slight variation in laser output power caused by the dither may be detected and used to calculate in real time the laser excitation ratio slope. This may be used to select a drive current to compensate for ambient changes keeping the excitation ratio slope substantially constant.

Description:
FIELD OF THE INVENTION  
       [0001]     Embodiments of the present invention relate to lasers and, more particularly, to monitoring and controlling a laser in real time.  
       BACKGROUND INFORMATION  
       [0002]     Lasers are used in a wide variety of applications. In particular, lasers are integral components in optical communication systems where a beam modulated with vast amounts of information may be communicated great distances at the speed of light over optical fibers as well as short reach distances such as from chip-to-chip in a computing environment.  
         [0003]     Of particular interest is the so-called vertical cavity surface emitting laser (VCSEL). As the name implies, this type of laser is a semiconductor micro-laser diode that emits light in a coherent beam orthogonal or “vertical” to the surface of a fabricated wafer. VCSELs are compact, relatively inexpensive to fabricate in mass quantities, and may offer advantages over edge emitting lasers which currently comprise the majority of the lasers used in today&#39;s optical communication systems. The more traditional type edge emitting laser diodes emit coherent light parallel to the semiconductor junction layer. In contrast, VCSELs emit a coherent beam perpendicular to the boundaries between the semiconductor junction layers. Among other advantages, this tends to make it easier to couple the light beam to an optical fiber.  
         [0004]     VCSELs may be efficiently fabricated on wafers using standard microelectronic fabrication processes and, as a result, may be integrated on-board with other components. VCSELs may be manufactured using, for example, aluminum gallium arsenide (AlGaAs), gallium arsenide (GaAs), indium gallium arsenide nitride (InGaAsN), or similarly suited materials. VCSELS have been successfully manufactured in 850 nm, 1310 nm and 1550 nm ranges. This allows for a wide variety of fiber optic applications ranging from short reach applications to long haul data communications. VCSELs are promising to advance optical communication systems by providing a fast, inexpensive, energy efficient, and more reliable source of laser beam generation.  
         [0005]     Optical transceivers using VCSELs operating at line rates of 10 gigabits/second (Gb/s) have matured rapidly over the last few years and are currently available in a wide variety of form factors, each addressing a range of link parameters and protocols. These form factors are the result of Multi-Source Agreements (MSAs) that define common mechanical dimensions and electrical interfaces. The first MSA was the 300-pin MSA in 2000, followed by XENPAK, X2/XPAK, and XFP. Each of the transceivers defined by the MSAs has unique advantages that fit the needs of various systems, supporting different protocols, fiber reaches, and power dissipation levels.  
         [0006]     Temperature affects the performance of VCSELs. Nevertheless, optical transceivers are expected to operate across a wide ambient temperature range. For example, some of the MSAs may call for the transceiver to operate in conditions as cold as −25° Celsius to as hot as 85° Celsius. In optical transceiver circuits, one common problem encountered may be the change of laser ER (extinction ratio) with temperature changes. When electrons at energy level N 1  are moved to higher energy level, N 2 , energy is absorbed. When the electrons at energy level N 2  drop to level N 1 , light is emitted. The ratio of electron quantity n 2  at energy level N 2  to a total electron quantity (n1+n2) at energy levels N1 and N2 may be called the excitation ratio (ER).  
         [0007]     VCSELs have the ER characteristics as shown in  FIG. 1 . At lower temperatures, the slope efficiency is high. As the ambient temperature increases to higher temperatures, the slope efficiency drops and the turn on threshold current also increases. In order to compensate for this effect, the laser driving current may be increased as temperature rises.  
         [0008]     One way to determine the amount by which to modify drive current for a given temperature change may be to record the laser driving conditions in a memory (e.g. an EEPROM) for different temperature conditions. A driver determines the level of current to provide by referencing the look-up table in the memory to thus to compensate for the drop in slope efficiency. However, in real-world manufacturing, the manufacturer may only have one look up table to fit all different laser characteristics, which may vary due to operating conditions, age, and manufacturing variances. Hence, a laser using a “one size fits all” look-up table to determine operating conditions may tend to be inaccurate.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  graph plotting laser power vs. drive current illustrating the change in excitation ratio (ER) slope for a vertical cavity surface emitting laser (VCSEL) operating in a lower temperature and higher temperature;  
         [0010]      FIG. 2  is a plan view of a small form factor (SFF) optical transceiver package according to embodiments of the invention;  
         [0011]      FIG. 3  is a cut-away side view of a transmitter optical sub-assembly (TOSA) which may comprise the transmitter portion of the SFF shown in  FIG. 2 ;  
         [0012]      FIG. 4  is a plan view of a TO-can comprising part of the TOSA;  
         [0013]      FIG. 5  is a block diagram showing the drive control for the TOSA;  
         [0014]      FIG. 6  is a graph illustrating periodically increasing and decreasing the current code to a digital-analog current (DAC) source by +/−1 least significant bit (LSB) to determine the excitation ratio of a VCSEL in real time;  
         [0015]      FIG. 7  is a block diagram of a parallel optics module implementing one embodiment of the invention; and  
         [0016]      FIG. 8  is a block diagram of an optical router implementing the VCSEL and control scheme in one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]     Modern Small Form Factor (SFF) Optical Transceivers provide high performance integrated duplex data links for bi-directional communication over multimode optical fiber.  FIG. 2  shows one type of an SFF optical transceiver package  100 . The package may comprise a body  102  for housing electronic and optoelectronic components. Pins  103  may be provided on the body  102  for attachment to a circuit board. The front of the package  100  may include a receptacle portion  104  to receive a mating plug (not shown) to connect optical fibers or waveguides to the transceiver package  100 . In this example a transmitter receptacle  106  and a receiver receptacle  108  are shown. Slots  110  or similar features may be present to provide a locking mechanism for a mating plug.  
         [0018]     Referring to  FIG. 3 , within the transmitter receptacle  106  of  FIG. 2 , there may be a transmitter optical sub-assembly (TOSA)  200 . While the TOSA may take many configurations, the one illustrated in  FIG. 2  comprises what may be known as a transistor-outline can (TO-can) package  202 . This name refers to the shape of the TO-can  202  that resembles the shape of a discrete transistor package. The TO-can  202  hermetically houses sensitive components of the TOSA  200 . The TO-can  202  may comprise a header portion  204  having electrical leads  206 . The TO-can  202  fits within a cavity  206  with the header  204  abutting against an outer housing  208 . A spacer  210  may be used to hold the TO-can  202  against the inner walls  212  of the cavity  206 . A lens or window  214  in the top of the TO-can  202  allows light to pass to or from and optical fiber core  216 . The housing  208  is adapted to align the optical fiber  218  to the window  214  of the TO-can  202 . While the TO-can  202  is shown as a convex lens  214 , the TO-can  202  may comprise a metal can with a flat angled window. The housing  208  may form the female portion  220  of a small form factor (SFF) pluggable connector, such as an LC connector, or other standardized removable connector for optical transceivers. The fiber  218  has an extending cord section  226  and may further comprise an outer protective sheathing  224  that is held by the mating portion of the connector comprising a ferrule  225  centering the fiber  218 . The ferrule  225  may be plugged into a ferrule receptacle  222  formed in the housing  208  such that the fiber  218  is optically aligned with the window  214  of the TO-can  202 .  
         [0019]      FIG. 4  shows a more detailed view of the TO-can  202  for housing an optoelectronic assembly. The To-can  220  may include insulating base or header  204 , a metal sealing member  314 , and a metal cover  316 . Preferably, the header  204  is formed of a material with good thermal conductivity for directing dissipated heat away from the optoelectronic assembly. By using a high thermal conductivity material, the header  12  may effectively dissipate the heat of un-cooled active optical devices, e.g., diode lasers, and can incorporate integrated circuits, such as diode driver chips.  
         [0020]     The insulating header  204  includes an upper surface  318 , a lower surface  320 , and four substantially flat sidewalls  322  (two of which are shown) extending downwardly from the upper surface  318 . The thickness of the header  204  may be approximately 1 mm. Of course, it should be understood that the insulating header  204  may be thicker or thinner as desired. The header  204  may be configured as a multilayer substrate having a plurality of levels. Multiple metal layers may be provided at each of the plurality of levels, and joined together (e.g., laminated).  
         [0021]     Various devices may be housed within the TO-can  202 . For example, an active optical device  321 , such as a VCSEL  321 , and its associated integrated circuitry  323 , other optical devices  325 , such as a photodiode  325 , and various other electrical components  327  and  329  may be located within an inner region of the metal sealing member  314 .  
         [0022]     At least one electrical lead  206  may be included adapted to communicate signals from the optoelectronic and/or electrical components housed inside the package TO-can  202  to components located external to the TO-can  202  on a printed circuit board, for example. The leads  206  may be circular or rectangular in cross-section, as shown. Alternatively, the header  204  may be operatively coupled to a printed circuit board using solder connections such as, for example, ball grid array connections and/or a flex circuit.  
         [0023]     The cover  316 , may be formed of Kovar™ or other suitable metal, may be hermetically sealed to the metal sealing member  314  to contain and fully enclose the optoelectronic and electrical components mounted to the upper surface  318  of the header  204 , and to thereby seal off the TO-can  202 . Use of such a hermetically sealed cover  216  acts to keep out moisture, corrosion, and ambient air to protect the generally delicate optoelectronic and electrical components therein.  
         [0024]     The cover  316  includes a transparent portion  214  such as, for example, a flat glass window, ball lens, aspherical lens, or GRIN lens. The optoelectronic components, such as the VCSEL  325 , are positioned within the TO-can  202  in a manner such that light is able to pass to or from them through the transparent portion  214 . Typically, the transparent portion  214  is formed of glass, ceramic, or plastic. To avoid effecting the optoelectronic and electrical components housed within the TO-can  202 , the transparent portion  214  of the cover  316  may be provided with an antireflection coating to reduce optical loss and back-reflection.  
         [0025]      FIG. 5  shows a block diagram of a laser driving circuit according to one embodiment of the invention to determine the laser&#39;s excitation ratio (ER) slope in real time in order to adjust parameters to keep the slope substantially constant even as extraneous parameters, such as ambient temperature, varies. In one embodiment, a VCSEL laser  321  may be fashioned in a TOSA  220 , such as that shown in  FIG. 3 , and form part of an optical transceiver  100  such as that shown in  FIG. 2 . A photo detector (PD)  325  may also be fashioned in the TOSA  220  to detect the output of the VCSEL  321 . The photo detector (PD)  325  outputs a signal in response to the detected output of the VCSEL  321 . In one embodiment, the output of the PD  325  may be measured by monitoring changes in a voltage V PD  across a resistor  500  by a microcontroller  502 .  
         [0026]     In one embodiment, a digital-to-analog current source (DAC)  504  may be used to provide a drive current to the VCSEL  231 . DAC current sources are generally discussed for example in U.S. Pat. No. 5,001,484 to Weiss. The DAC current source  504  may typically be constructed of an array of current source transistors that produce output currents of weighted values that represent bits in a binary word or code  510 . High resolution DACs typically employ weighted current sources in which the ratio of the most significant current bit I MSB , to the least significant current bit, I LSB , ranges from 64:1, in the case of an six-bit DAC, to as high as 32,768:1, in the case of a sixteen-bit DAC. In general terms, I MSB /I LSB =2 (N−1) , where N is the number of bits.  
         [0027]     In one embodiment, as shown in  FIG. 5 , a 6-bit DAC current source  504  may be used. As shown, the VCSEL driving current  508  may be selected by inputting a 6-bit binary code  510  into the 6-bit DAC  504 . The output power of the VCSEL  321  may be monitored by the voltage V PD  from the photo detector (PD)  325  which may be located inside the TOSA package  220 .  
         [0028]     Referring to  FIG. 6 , the excitation ratio slope may be monitored in real-time by the micro-controller  502 . According to an embodiment, the microcontroller  502  may periodically dither (i.e., increase or decrease) the current code  510  by, for example, +/−1 LSB driving current without appreciable interference to the main VCSEL  321  operation. However, slight variation in VSCEL output power caused by this +/−1 LSB change may be detected by an output voltage variation V PD  of the PD  325  to reflect the difference in the laser average power. In one embodiment, the microcontroller  502  may increase and decrease the current code  510  by +/−1 LSB for example anywhere from 500-1500 times a second. Of course this number may be selected to be different according to the application. Signal V PD  feeds into the microcontroller  502  such that a representation of the excitation ratio slope efficiency  520  may be determined in real time. According to one embodiment, the slope efficiency may be determined by:  
         Slope   ⁢           ⁢   Efficiency   ⁢           ⁢     (   η   )       =           V   pd     ⁡     (       current   ⁢           ⁢   code     +     1   ⁢   LSB       )       -         V   pd     ⁡     (       current   ⁢           ⁢   code     -     1   ⁢   LSB       )       .           I   ⁡     (       current   ⁢           ⁢   code     +     1   ⁢   LSB       )       -     I   ⁡     (       current   ⁢           ⁢   code     -     1   ⁢   LSB       )               
 
         [0029]     Knowing real time excitation ratio slope efficiency then allows the microcontroller to adjust the current code to correspondingly adjust the drive current  508  driving the VCSEL  321  to maintain a substantially constant slope over various ambient temperature and conditions, thus eliminating use of EEPROM look-up tables and the drawbacks associated therewith.  
         [0030]      FIG. 7  illustrates embodiments of the invention used in a parallel optics module  700  coupled to a printed circuit board (PCB)  712 . Parallel optics module  700  may include drive controls and VCSEL TOSAs as previously described for example with relation to  FIG. 5 . Parallel optics module  700  may include an optical transmitter, an optical receiver, or an optical transceiver.  
         [0031]     Parallel optics module  700  includes an electrical connector  704  to couple module  700  to PCB  712 . Electrical connector  704  may include a ball grid array (BGA), a pluggable pin array, a surface mount connector, or the like.  
         [0032]     Parallel optics module  700  may include an optical port  706 . In one embodiment, optical port  706  may include an optical port comprising for example the SFF connector shown in  FIG. 2  or may be adapted to receive a Multi-Fiber Push On (MPO) connector  708 . MPO connector  708  may be coupled to an optical fiber ribbon  710 . In one embodiment, the optical fiber ribbon  710  includes two or more plastic optical fibers.  
         [0033]     In one embodiment, the VCSELs within the parallel optics module  700  may emit light at different wavelengths for use in Wavelength Division Multiplexing (WDM). In one embodiment, parallel optics module  700  may transmit and/or receive optical signals at approximately 850 nanometers (nm). In another embodiment, parallel optics module  700  may operate with optical signals having a transmission data rate of approximately 34 Gigabits per second (Gb/s) per channel. In yet another embodiment, optical signals transmitted and received by parallel optics module  700  may travel up to a few hundred meters. It will be understood that embodiments of the invention are not limited to the optical signal characteristics described herein.  
         [0034]      FIG. 8  illustrates an embodiment of a router  800 . Router  800  includes a parallel optics module  806  as described above. In another embodiment, router  800  may be a switch, or other similar network element. In an alternative embodiment, parallel optics module  806  may be used in a computer system, such as a server.  
         [0035]     Parallel optics module  806  may be coupled to a processor  808  and storage  810  via a bus  812 . In one embodiment, storage  810  has stored instructions executable by processor  808  to operate router  800 .  
         [0036]     Router  800  includes input ports  802  and output ports  804 . In one embodiment, router  800  receives optical signals at input ports  802 . The optical signals are converted to electrical signals by parallel optics module  806 . Parallel optics module  806  may also convert electrical signals to optical signals and then the optical signals are sent from router  800  via output ports  804 . According to embodiments of the invention, the ER slope efficiency of the lasers within the router  800  may be maintained in real time across a broad ambient temperature range.  
         [0037]     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible, as those skilled in the relevant art will recognize. These modifications can be made to embodiments of the invention in light of the above detailed description.  
         [0038]     The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the following claims are to be construed in accordance with established doctrines of claim interpretation.