Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-03-20
2003-12-30
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C257S080000, C438S141000
Reexamination Certificate
active
06670599
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates generally to devices for monitoring the performance of optical systems. The present invention relates more particularly to devices for monitoring the performance of laser light sources used in communications and computation systems.
2. Related Art
The vertical cavity surface-emitting laser (VCSEL) is a relatively recent innovation in laser technology. It is part of a general class of devices called “surface emitting light emitting devices” (SLEDs) that have significant manufacturing and packaging advantages over conventional edge-emitting devices.
Semiconductor diode lasers have been produced for over a decade and are used extensively in both communications and in optical storage devices such as compact disks (CDs) and digital versatile disks (DVDs). The vast majority of these devices, however, rely on edge-emitting, e.g., Fabry-Perot or distributed feedback (DFB), lasers. These lasers are constructed on semiconductor wafers in such a way that when the wafer is diced, light is emitted from the cut edges. Edge emitting devices have a number of drawbacks: first, each laser takes a relatively large area on the semiconductor wafer, increasing cost; second, lasers cannot be tested until after they have been diced into individual units; third, linear arrays of lasers are more difficult to produce in high densities and two-dimensional arrays are altogether impossible to fabricate. The construction and fabrication of these lasers, however, is well known, and prices have benefited from large production volumes needed to satisfy the CD and DVD markets.
VCSEL laser cavities—rather than being patterned in the wafer plane, in a few layers of semiconductor—are patterned orthogonally to the wafer as many layers of semiconductor are deposited. The resulting lasers emit light perpendicularly to the surface of the wafer, and may be patterned in extremely high densities, either as individual devices or as one or two dimensional arrays. The result is a laser device that is inherently less expensive to produce than edge-emitting lasers. In addition, the vertical nature of these devices permits integration of additional electro-optical devices on the surface, for example adjacent each VCSEL.
Semiconductor light sources in general suffer from a number of problems associated with optical power control. Each semiconductor laser has a threshold electrical current needed before population inversion occurs i.e. there are more electrons in high energy state than in a low energy state, in its active region and coherent light is emitted. This threshold current needs to be supplied before any appreciable output is seen from a semiconductor laser. Above the threshold current, any increment in electrical current will lead to a corresponding increase in emitted optical power, up to a point. The ratio of the increase in optical power to electrical current is called the slope efficiency. Semiconductor lasers suffer from the fact that their threshold currents—and sometimes their slope efficiencies—can change significantly over operating temperature ranges and with laser age. This is a problem for a number of reasons. First, a single operating current cannot be set for the lifetime of the laser, unless it is set sufficiently high that output power will always exceed a desired minimum. This strategy has drawbacks: first, there may be eye safety issues when a laser is operated at greater than a certain power; second, operating the laser continuously at high power significantly reduces lifetime and further raises temperature, and as a result, threshold current; and finally, at high powers, high and low light output levels may be difficult to distinguish. Distinguishing between high and low light output levels is important in optical communications: a “one” and “zero” signal must be distinguishable to the receiver, and at the same time, the current levels for these signals should be as close together as possible in order to minimize switching time. It is therefore generally desirable to operate the laser at just above threshold for a “zero” level, and to use the minimum modulating current necessary to create “one” bits. The continuous current supplied is called the bias current. Thus a drift in the threshold current during operation can have highly detrimental results for users who wish to attain maximum bandwidth from such lasers while meeting eye safety and power consumption specifications. These optical power fluctuations are a problem not only for laser diodes or VCSELS; they affect other SLEDs as well, necessitating power control or monitoring for level-sensitive applications.
Various solutions have been developed for controlling optical output of diode lasers. The first category of solutions has to do with temperature monitoring and control. The idea is that one can either monitor temperature—or control it directly—of the laser device, and therefore eliminate drift in threshold current and slope efficiency tied to temperature fluctuations. The simplest solution is to place a temperature-monitoring device near the laser and to use the signal from this device to adjust the laser bias current and possibly the laser modulating current according to a pre-set formula determined from statistical sampling of the laser devices.
Another solution, which is used extensively in high-end communications modules employing edge-emitting lasers, is active control of laser temperature. The laser is placed on a substrate that has incorporated both a temperature-measuring device and a cooling device—most often a semiconductor heat pump such as a Peltier junction that, through a control loop, keep the laser base at a constant temperature where the threshold current and slope efficiency are known (and usually optimal).
Thermal control solutions require significant space and power, and although they may be suitable for long-haul communications applications, such solutions are generally unacceptable in local-area or interconnect components where space is at an extreme premium.
Thermally-based solutions do not by themselves solve the problem of laser performance degradation over its operating lifetime. They can only compensate for changes in the ambient temperature, which, although important, are far from the only factor affecting laser optical output for a given current.
The most accurate way of controlling power output from the laser is to monitor the optical output directly. A class of technologies has been developed to monitor this output for both edge- and vertically-emitting semiconductor lasers.
Direct optical power monitoring for edge-emitting lasers is relatively straightforward due to the fact that these diode lasers emit light from both front and back facets. This allows the laser to be placed in an assembly where one aperture, at the front facet, provides the useful light for the application, while the other aperture provides light to a photodiode that is aligned precisely with the back facet. The usual technology used for this alignment is referred to as a silicon workbench. A silicon wafer has a surface patterned with mechanical alignment grooves using micromachining processes to produce a silicon workbench. Generally both the laser diode and the photodiode are placed in a “vee-groove” that runs along the light emission axis.
This type of assembly is used in CD and DVD players and recorders. For VCSELs, power monitoring is more complex, because the device does not generally emit light in the rear direction, i.e., through the substrate wafer. For laser wavelengths in excess of roughly 900 nm, a GaAs wafer, the usual VCSEL substrate would be transparent to the laser light. Thus, for such devices, an optical power monitor could be built on the reverse side of the wafer. However, VCSELS used for current communications applications generally operate in the 850 nm region for multimode fiber communications, and therefore generally require a different solution. The solution currently used by most manufacturers is to place the completed VCSEL die in an enclosure fitt
Ma Eugene Y.
Payne Adam M.
Wagner Matthias
Wagner Sigurd
Aegis Semiconductor, Inc.
Hale and Dorr LLP
Lee Patrick J.
Porta David
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