Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-10-28
2003-09-23
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C372S038070, C372S038020, C372S029020, C372S029014, C372S029015
Reexamination Certificate
active
06624917
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of optical transmitters, and in particular, to optical power control circuitry for parallel optical transmitters having vertical cavity surface emitting laser (VCSEL) arrays.
2. Background Information
High speed direct coupled (DC) parallel optical data transmitters commonly use semiconductor vertical cavity surface emitting laser diode arrays (VCSEL's) as their light sources. The laser device called a VCSEL (Vertical Cavity Surface Emitting Laser) is a semiconductor laser made of many layers, e.g., 600, which emits light vertically from a lower surface and in a direction parallel to the direction of its optical cavity, as opposed to an edge-emitting type laser structure. VCSEL's have advantages over edge-emitting type structures because, for example, the edge-emitting type lasers must be precisely broken or cleaved individually to form each device during manufacturing. However, with VCSEL's, literally millions of laser devices can be made simultaneously in an etching process.
VCSEL's are currently some of the smallest lasers being produced. There is a relatively new type of VCSEL in development, the QD-VCSEL. The ‘QD’ signifies the Quantum Dots which are used in the active layer of this type of VCSEL. The QD-VCSEL promises to achieve even further size reductions.
VCSEL's have a range of uses. For example, a specially designed VCSEL has been used to create an optical latch or optical state memory, the VCSEL transitioning and latching in the ON state when an optical input is received. Arrays of such VCSEL's open up possibilities for various massively parallel optical computing applications such as pattern recognition. VCSEL's have data communications applications as well as would be clear to one skilled in the art. For more information about VCSEL's, see, for example, “LASERS, Harnessing the Atom's Light,” Harbison et al., Scientific American Library, 1998, pages 169-177.
A graph representing the optical power output P
O
in milliwatts (mW) vs. the current input I in milliamps (mA) for a typical VCSEL is shown in FIG.
1
. As is shown in the graph, the VCSEL does not begin lasing until the current through it exceeds a certain laser threshold value, shown as I
th
in the figure. The slope of the curve above I
th
is commonly referred to in the art as the differential quantum efficiency (DQE) of the VCSEL.
However, these two VCSEL diode parameters, I
th
and DQE, along with the wavelength of the output light, are dependent on operating temperature, as well as on process variations. The manufacturing process variations are, at present, not completely controllable or predictable. Therefore, a method to adjust the current through the VCSEL to compensate for these variations is required. Some methods are known, for example, from U.S. Pat. No. 4,709,370, Bednarz et al., Nov. 24, 1987 and U.S. Pat. No. 3,633,120, Battjes Jan. 4, 1972.
To prevent over-powering the laser and to meet end of life requirements, a method must exist to compensate for the effects of process and operating temperature variations on output power.
Serial optical data transmitters ordinarily use laser diodes, e.g., VCSEL's or edge-emitters, that are packaged with a photo detector that feeds back a current proportional to the optical output power of the laser diode.
FIG. 2
shows a simple circuit integrating this type of laser/photo detector package into an operational amplifier (OP AMP) negative feedback loop to control the optical output power of the laser diode. During calibration, the feedback current through the photo detector is used to adjust the average optical power out of the laser by adjusting the potentiometer labeled ‘R POT’ changing the voltage on the non-inverting input of the OP AMP which controls the laser drive transistor to provide more or less current through the laser. In operation, this feedback current serves to dynamically adjusts the laser current in response to average optical power changes caused by changes in operating temperature.
However, to use this simple method for VCSEL diode arrays would require a photo detector for each VCSEL diode in the array. VCSEL arrays used in communications, for example, commonly contain 12 or more VCSEL's, therefore 12 or more photo detectors would be required. Some problems with such an arrangement are that optical cross-talk from one VCSEL to adjacent VCSEL's, and that physical size limitations may preclude using a photo detector for each VCSEL in an array. In recently contemplated applications of VCSEL's, such as massively parallel processing, mentioned above, where perhaps millions of VCSEL's would be used at once in an array, these problems could become overwhelming.
A known solution to these problems is to dynamically adjust all the VCSEL's optical power levels using a reference VCSEL and reference photo detector in a VCSEL array, as shown in FIG.
3
. Examples of such an arrangement are described in U.S. Pat. No. 5,625,480, Swirhun et al., issued Apr. 29, 1997 and U.S. Pat. No. 5,521,736, Swirhun et al., issued May 28, 1996. These prior patents describe (Abstract) electronic circuits and methods to dynamically compensate for the effects of the substrate temperature and aging behavior of the light emitters at both the transmitter and the receiver in a parallel optical interconnect system transmitting a plurality of DC non-return to zero (NRZ) data and an independent clock signal. An arrangement of light emitters is used to reduce or avoid skew problems as well.
The reference VCSEL and reference photo detector methods, however, are based on the assumption that all VCSEL's in the array have identical I
th
and DQE. Of course, if these parameters are not identical, which is likely, some VCSEL's could be operating close to I
th
. This could cause turn on delay and increased skew between channels, which, of course, is not a desirable phenomenon, especially as data rates increase. On the other hand, some VCSEL's could be operating at higher average power than the reference VCSEL, possibly causing an over-power condition. Therefore, the reference VCSEL/photodetector methods are not a perfect solution to the problems discussed at the outset.
Another semiconductor laser diode control method is described in U.S. Pat. No. 5,019,769, Levinson, issued May 28, 1991. This patent describes (Abstract) a laser diode controller using a programmed micro-controller to accurately control the process of turning on and selecting the operating point of a laser diode. The laser diode has a front facet for transmitting light, and a back facet for monitoring the laser diode's optical output power. Once the back facet of the laser diode is calibrated, the controller can accurately monitor the laser diode's operating characteristics, and can select the best operating point current based on the current operating characteristics of the laser diode. During calibration of the laser diode, the controller can check the linearity of the laser diode's optical output power as a function of drive current, and can thereby detect defects in the laser diode. In a full duplex optical link, the controllers at the link's ends prevent the laser diodes from generating light at their full normal intensity until the integrity of the link has been established, thereby preventing light from the laser diode's from accidentally damaging user's eyes. Furthermore, the controllers can use the full duplex link to establish lower operating point drive currents that would otherwise be used to lengthen the lifetime of the laser diodes. A laser diode's operating characteristics change over time in such a way as to enable the controller to predict when the laser will fail. The controller records the operating characteristics of the laser diode in a nonvolatile memory, analyzes changes in those characteristics, and generates a failure warning message when those changes match predefined failure predict
Demsky Kevin Paul
Freitag Ladd William
Paschal Matthew James
Bussan Matthew J.
International Business Machines - Corporation
Lynt Christopher H.
Pascal Leslie
Payne Leslie J.
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