A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
The present invention relates to optical focusing systems and more particularly relates to a computer-based semi-automatic focusing and assembly apparatus and method for focusing and assembling optical devices, such as laser diode assemblies, used in bar-code scanning systems.
Bar-code scanning systems are widely used in retailing, postal and parcel delivery services, manufacturing, warehousing and distribution and other industries for fast data transaction applications. The benefits of such scanning systems are countless. For example, in manufacturing, the use of bar code scanners could result in accurate raw material inventory, production routing, work-in-progress tracking, labor efficiency, inventory control, quality assurance, shipping precision, and so on.
One of the key elements of bar-code scanners is the laser diode assembly that provides the light source for the scanners. The focusing characteristics of the assembly have significant effects on the correct reading of bar codes. If the laser beam emitted from the laser diode assembly is not focused accurately, it will result in a weak signal level and therefore possible loss of information when it scans across the bar-code. The end result is that one has to scan several times before correct information can be captured. For bar-code scanners to be effective over a range of distances, the beam width of the laser beam has to be within tolerance at several specified distances (normally three distances corresponding to the near, mid and far fields are specified). To ensure that the beam width requirement is met and therefore high scanner accuracy is achieved, every laser diode assembly needs to be focused in high precision during production.
Currently such focusing adjustments are performed manually in most of the bar-code scanner production processes which limits productivity as well as focusing performance of the laser diode assemblies. The manual focusing setup typically used in manufacturing laser diode (LD) assemblies consists of the following components: the laser diode assembly consisting of a light beam emitting laser diode, a focusing lens, a lens holder with an aperture, a spring, and a diode holder; a mechanical fixture to hold the LD assembly; a beam scan head including a cylindrical rotating drum with a slit for beam width measurement, a photo detector for beam power measurement, and a level for gain adjustment; a guide rail to move the scan head for beam width measurement at different distances (the height of the scan head can also be adjusted); a beam scan block which controls the scan head and displays the beam width (the outputs of the scan block include an analog beam profile signal, a trigger signal, and a clock reference signal); a current source that supplies the current to the laser diode; and a laser power meter which is used to measure the intensity of a laser beam during the laser diode power-up stage.
The first step of the focusing process is to assemble the laser diode, the spring, and the lens together in the diode holder and use a clamp to hold the parts in place. Next, this assembly is placed in the mechanical focusing fixture. The clamp is lifted and the assembly is now ready for focusing. For each type of assembly, the operator is usually given three specified locations at which the beam width should be focused (i.e., near, mid and far fields). The operator""s next task is to connect the current source to the laser diode and slowly increase the current to a specified value. The operator then manually slides the scan head to the nearest specified location and adjusts the orientation of the LD so that the beam is centered in the aperture of the scan head. The operator then adjusts one or more focusing knobs of the focusing fixture so that the beam width displayed on the scan head is within a specified tolerance. The process is then repeated at mid and far field locations. If the operator is successful in achieving beam widths within the specified tolerances at all locations, glue is carefully applied to the lens holder to fix the lens with respect to the LD. After the curing time of the glue has elapsed, the operator removes the assembly and starts focusing the next assembly. If the operator is not able to focus the beam to acceptable beam widths at all locations, the assembly is rejected.
Following are some of the major problems associated with the manual focusing setup and procedures:
Moving Scan Head
One of the major concerns of the current setup is that the operator has to repeatedly move the scan head on the rail back and forth during focusing. For mass production, the time spent on moving the beam scan head significantly reduces productivity.
Time Consuming Focusing Check After Curing
After curing, the beam widths at specified distances have to be checked again to see if they are still within tolerances. To do this in the existing manual setup, the operator has to move the scan head to different locations again, which further slows down the process.
Human Factor
In the existing setup, the focusing criteria is quite subjective with the operator making most of the decisions based on intuition and experience. In addition, the operator is not required to optimize focusing.
Wasted Time During Curing
Currently, the operator focuses only one LD assembly at a time. During the curing period, the operator is basically idle. This period of time can be utilized more efficiently if the operator can start focusing the next assembly. With the current setup this is not possible.
No Check on Beam Centering
Another concern is that the operator cannot accurately point the laser beam to the center of the scan head aperture due to the lack of an accurate beam centering target. Because of this pointing error, the beam width displayed on the scan block may not be an accurate indication of the true beam width at the specified clip level.
Focusing Inconsistency
Currently, the operator""s task is simply to bring the beam widths within the specified tolerances. No targeting of specific beam width values within the specified ranges is required. The result of this practice is that the assembled LD assemblies lack focusing consistency from assembly to assembly, which in turn may result in performance inconsistency of bar-code scanner products.
Lack of Process Tracking
During the focusing process, some of the focusing parameters may change systematically due to errors in the focusing setup or systematic defects in the laser diodes and focusing lenses. Since no focusing data are recorded in the current focusing process, no process tracking can be done to detect such problems. Also, if a laser diode assembly results in a defective product when assembled into a bar-code scanner, no trouble shooting can be done because no data regarding to the focusing condition of the laser diode assembly at the time of assembling is available.
No Control Over the Amount of Glue Dispensed
After focusing, the amount of glue applied for curing and the way how glue is applied are crucial. The operator has to be very careful in making sure that no excessive amount of glue is applied. If this operation is not carried out with care, then the focal distance may change and the focusing may go out of tolerance. In the currently process, there is no control over the amount of glue dispensed, which could cause serious problems if the operator is not careful or not skillful.
No Access to Beam Profile Information
Another concern is that the engineers do not have easy access to the beam profile information at different distances, especially that for the assembled laser diode assemblies, to perform a comprehensive analysis of the process. The beam profiles together with beam widths can provide a more complete and in-depth knowledge about the nature of the laser beam.
Manual Power-up Procedure
The current procedure of powering up the LD is completely manual. The operator is given a target laser power level and a corresponding range of current values. The operator is supposed to manually adjust the current supply until the specified laser power level is reached. There is no specification nor control over the speed at which the operator should follow to power-up the LD. The major concern here is that rapid power-up and excessive current supply may result in permanent damage to the LD. Another concern is that there is no continuous monitoring of the laser power in the current focusing process. Therefore, if the laser power is changed, it cannot be detected and erroneous focusing may thus result.
Accordingly, it is an object of the present invention to overcome such shortcomings and to provide an improved method and apparatus for focusing and assembling visible laser diode devices.
The present invention is an apparatus and method for semi-automatic focusing and assembling of a light beam modifier, such as an optical lens, relative to a light beam source, such as a laser diode, to optimize the light source""s beam characteristics at one or more desired locations along the path of the beam. The present invention utilizes one or more light beam analyzers positioned at one or more desired locations along the path of the beam to measure a beam characteristic, such as beam width, and to generate value signals based on these measurements. The value signals are sent to a host computer for comparison with predetermined optimal beam parameter ranges. The computer generates a focusing signal, based on the comparison, which is used to axially adjust the light beam modifier relative to the light source.
Preferably, the light beam modifier and light source are held in axial alignment in a focusing fixture which is adjustable for pointing the light beam in a desired direction and for focusing the beam. The focusing station includes a stepper motor electrically connected to the host computer for receiving the focusing signal and axially adjusting the light beam modifier based on this signal. The focusing station is mounted at a fixed position on a carrier rail arrangement. The one or more light beam analyzers are slidably mounted on the carrier rail arrangement for positioning the analyzers at desired locations.
In a preferred embodiment, the present invention includes two focusing stations, three light beam analyzers and two beam splitters mounted on a perpendicular carrier rail arrangement. The two focusing stations are mounted perpendicularly and equidistant from a first beam splitter. The first beam splitter splits the light beam emitted from either focusing stations into two components. The first component is directed to a first light beam analyzer positioned at a desired location and the second component is directed to a second beam splitter. The second beam splitter further splits from the second light beam component a third component. The second and third light beam components are respectively measured by a second and a third light beam analyzer positioned on the rails at desired locations.
The spatial arrangement of the focusing stations, two beam splitters and three light beam analyzers upon carrier rails in itself allows for simultaneous measuring of beam characteristics at three locations and for alternately focusing and assembling two laser diode assemblies without the aid of a host computer. Preferably, however, the three light beam analyzers generate value signals based on their respective beam measurements and send these signals to a host computer. The host computer receives these signals and compares them to predetermined parameter ranges associated with the selected locations along the beam path. The host computer generates a focusing signal based on an optimization algorithm for the three locations and sends the focusing signal to the stepper motors for axially adjusting the light beam modifiers with respect to the light beam sources.
To provide easy pointing of the laser beam emitted from either focusing station, the present invention also preferably includes a video camera electrically connected to the host computer and facing one of the scan heads. The video camera provides a live image of the laser spot on a computer screen along with a reference target. Pointing of the laser beam is done either by manually adjusting the pointing mechanisms of the focusing stations or automatically through the use of a data acquisition system of the computer. For automatic pointing, the computer preferably includes a video frame grabber which digitizes the laser beam image captured by the video camera and detects its location on a scan head. Again, an algorithm which can be provided by computer software or other signal generating means, e.g., analog and/or digital circuitry (printed or otherwise), generates an adjustment signal based on the difference between the detected beam location and the known center of the scan head. The adjustment signal is sent to additional adjustment motors which activate the pointing mechanisms of the focusing fixture to center the beam on the scan head.
The apparatus also preferably includes a semi-automatic glue dispensing system. A valve-controlled glue dispensing system with a custom-made glue dispensing gun is used. When the operator presses a switch on the glue dispensing gun, a well-controlled amount of glue is dispensed to fix the light beam modifier relative to the light source. Once this is done, a signal is sent to the computer via the data acquisition system which initiates a curing timer to count down. The timer alerts the operator when the proper curing time has elapsed.
In another preferred embodiment of the present invention, the apparatus includes automatic laser power-up circuitry. The computer first powers up the light source to a pre-focusing current level for pointing purposes. After pointing, the circuitry then powers the light source to the nominal power level. The power level is monitored continuously and the laser power is displayed on the computer screen.
In a method for optimizing characteristics of a light beam emitted from a light source at one or more locations along the path of the light beam according to the present invention, a light beam modifier is axially aligned with a light beam source and the beam emitted from the source is continuously measured at one or more selected locations along the path of the beam. Value signals corresponding to these measurements are generated and are compared with predetermined parameter ranges. A focusing signal based on the comparison is generated and is sent to the light beam modifier for axially adjusting the modifier with respect to the light source. The focusing signal terminates when the value signals fall within the predetermined parameter ranges and the light beam modifier is fixed relative to the light source. In a preferred method, two beam splitters are positioned along the path of the light beam to split the beam into three components which, in turn, are continuously measured at three selected locations.
As a result of the present invention, a method and apparatus is provided for semi-automatic focusing and assembling of laser diode assemblies is provided. The benefits of which are the reduction of the time needed to focus a laser diode assembly, the reduction of moving parts in the system as well as the overall improvement of quality. With the present invention, beam profiles along with other important focusing data are automatically saved to data files. These data files provide optical engineers with an extensive database of information for detailed analysis of the focusing process.
For a better understanding of the present invention, reference is made to the following detailed description to be taken in conjunction with the accompanying drawings and its scope will be defined in the appended claims.