Patent Application
20080079945
An understanding of the present teachings can be gained from the following detailed description, taken in conjunction with the accompanying drawings of which like reference numerals in different drawings refer to the same or similar elements.
In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide an understanding of embodiments according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatus and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatus are clearly within the scope of the present teachings.
With specific reference to
With specific reference to
The output 211 of the first digitizing element 202 is the input to offset conditioner and prescaler 203. The offset conditioner corrects for any remaining DC offset and the prescaler adjusts the magnitude of the input signal to be within a fixed range that is most efficient for further phase processing. The output of the second digitizing element 209 is the DC offset voltage (herein “Vdc”) 210. The digital value output of the first A/D 202 is connected to offset conditioner and prescaler 203. The offset conditioner and prescaler 203 has a digital input conditioner AGC 204. The digital input conditioner AGC 204 sizes the digital output 207 to be at a predetermined value that is known to be most efficient for the function of the signal processor 105. An output of the digital input conditioner AGC 204 is a phase processor gain (herein “gp”) 206.
The digital output 207 of the offset conditioner and prescaler 203 is directed into a series of arithmetic logic units (herein “ALUs” 220), sine and cosine look up tables 221, 222, counter 223, phase latch 224, storage registers 225, FPGA 226 that calculates and updates the current frequency, phase correction and frequency update state machine 227 that produces a corrected phase 229 and updated frequency 211 values, and phase accumulator 228, all of which operate together as shown in
It is suggested that laser head reliability may be monitored and assessed based upon laser AC and DC signal intensity over time and laser frequency over time. For purposes of laser signal reliability, AC signal intensity may be calculated as:
Ka, Kb, and Kc are calibration constants for the APD 200. Calibration of the APD 200 comprises ramping the signal intensity of a calibration signal and measuring the output of the AGC 205. The resulting data is fit to a quadratic equation where the calibration constants Ka, Kb, and Kc are the quadratic, linear and constant coefficients, respectively. Kattn is then calculated as a function of Vf where:
K
attn
=K
a
V
f
2
+K
b
V
f
+K
c (2)
Mac is also a calibration constant. As part of the calibration process that ramps the signal intensity of a calibration signal, the phase processor gain 206 is also measured. The resulting phase processor gain is fit to a linear equation and Mac is the slope of the resulting fit. Therefore, AC signal intensity is calculated as a function of Vf and gp.
Also for purposes of laser signal reliability, a DC signal intensity may be calculated as:
P
dc
=K
attn(MdcVdc+Bdc) (3)
Mdc and Bdc are also calibration constants. Mdc and Bdc are calculated based upon a calibration that measures Vdc as the input calibration signal 103 ramps in intensity. The measured Vdc is fit to a linear curve and Mdc and Bdc is the slope and y-intercept of the linear fit of the collected calibration data, respectively.
With specific reference to
During operation and with specific reference to
In a specific embodiment of the measurement step 301 of the process and with specific reference to
The first laser reliability assessment uses a short time base and quantifies any instantaneous reference signal instability. Ideally, both the signal intensity and the frequency are constant over the short time base. If the reference signal is unstable, the short time base measurements of frequency and signal intensity show some amount of modulation. Instantaneous reference signal instability reflects modulation that is commonly on the order of approximately 10's of kilohertz. Accordingly, it is acceptable to measure and graph the data over a bandwidth of 100's of kilohertz. In a specific embodiment, the time base is 80 MHz and the AC and DC signal intensity and frequency data is acquired every 3.2 usec. If the laser signal is reliable, the signal intensity and frequency as functions of time are substantially constant and unmodulated. If there is instantaneous reference signal instability, the graph of signal intensity and frequency as a function of time exhibits some amount of modulation. To show modulation, the gathered data may be graphed as a function of time or can be shown in numeric form as an average and peak to peak values over a measurement cycle. If the amplitude of the frequency or signal intensity modulation exceeds a predetermined threshold, it is an indication that the reference signal is unstable or “freebling”. If the reference signal is unstable, there are a number of possible causes including, but not limited to placement of the laser source near ferrous materials, external cavity feedback and optical path feedback. The number of possible causes provides a number of possible fixes. It is ultimately up to the user to find the right fix, but the measurement and display according to the present teachings permits the user to partially diagnose a system problem by identifying that there is an issue with the reference signal.
A second laser reliability measurement uses an extended time base and indicates if the laser source is reaching an end of its useful life. Signal intensity and frequency, therefore, may be measured at a specific time every day, week, month, or other unit of periodicity. Because a date and timestamp is also collected, it is acceptable that the data not be collected at a constant periodicity, as long as the data can be correlated in time. A typical laser head product life in an interferometer system is on the order of 3-5 years. Accordingly, the periodicity of the measurement may be selected to measure a significant sample of data points over the expected life of the laser. In a specific embodiment, the signal intensity and frequency data is gathered every month and is stored or maintained in a log file in non-volatile memory with a time and date stamp. The signal intensity and frequency measurements are substantially constant during the useful life of the laser head. As the laser head reaches the end of its useful life, the signal intensity or frequency measurement begins to show a trend of degradation. Every so often, therefore, and upon request of the user, the extended time base data may be displayed 303. The extended time base measurement is most helpful as a relative measurement. The measurements may be compared to a baseline reading taken at the beginning of the laser head product life or compared to a specification. The extended time base display may be in the form of a graph of frequency and signal intensity versus time that includes the baseline reading or may be a display of a numeric showing average and peak to peak values of the baseline reading as compared to the current reading. Alternatively, the baseline reading is not displayed and a user compares the current reading to a value in the product specifications. An advantage of graphical representation of the data is that gradual degradation can be represented that provides a visual indication of a degradation trend and a qualitative rate of the degradation trend. An advantage of the numerical representation is that it can be represented without using much of the graphical user interface display space. The instantaneous and trend data may be shown upon demand or may be shown all the time in a corner of the standard display screen.
Embodiments of the teachings are described herein by way of example with reference to the accompanying drawings describing an apparatus and method for quantifying reference signal reliability in a laser system. The specific embodiment described is a heterodyne laser interferometer system. One of ordinary skill in the art with benefit of the present teachings, however, recognizes that the same principle of measuring the reference signal over time to assess reference signal reliability is also applicable to homodyne laser systems. Other variations, adaptations, and embodiments of the present teachings will occur to those of ordinary skill in the art given benefit of the present teachings.
