Method and apparatus for laser safety

A method and apparatus are provided for laser safety. A fault detector detects laser fault conditions and generates a laser fault control signal. A laser fault counter is coupled to the fault detector for counting laser fault conditions and for generating a disable laser control signal responsive to a predetermined fault count. A reset timer is coupled to the laser fault counter for identifying a predetermined reset time period responsive to the laser fault control signal and applying a reset signal to the laser fault counter. Features of laser safety method and apparatus of the invention are that the laser fault counter is reset if a second laser fault condition is not detected within the predetermined reset time period after the laser is turned on. The disable laser control signal is not generated when a safe laser optical power condition occurs between two laser fault conditions.

FIELD OF THE INVENTION 
The present invention relates to improvements in laser driver and fault 
detection arrangements for laser safety. 
DESCRIPTION OF THE PRIOR ART 
Many types of laser-based devices and systems, having a wide range of 
applications, such as in medical technology, in communications and 
computing technology, are becoming increasingly well known and 
commercially available. The lasers used in many of these devices and 
systems are often capable of producing powerful outputs that are 
potentially harmful to both people and equipment. As a result, many types 
of safety devices for use in conjunction with laser-based equipment, and 
standards designed to ensure that laser-based equipment may be safely 
operated, have been developed and continue to evolve. 
A function of a laser's DC current drive feedback loop is to maintain 
constant optical power. Known fault detection circuits monitor for 
conditions that indicate an unsafe laser optical power exists. Fault 
detection schemes exist similar to that reported in the IBM Technical 
Disclosure Bulletin Vol. 33, No. 3B., August 1990, pages 90 and 91, and 
entitled LASER FAULT DETECTION. 
FIG. 1 illustrates a prior art DC laser driver and laser fault detection 
circuits. The DC current driver is accomplished by including an 
operational amplifier (op amp) in a negative feedback loop consisting of 
op amp B1, transistors Q1 and Q2, resistor R1, the laser diode, the 
monitor diode, and potentiometer RPOT. The monitor diode current, LM, is 
fed back through potentiometer RPOT. When the laser diode current 
increases, the monitor diode current increases. This causes the voltage on 
the + terminal of op amp B1 to decrease, providing the negative feedback. 
The regulator monitor detects when the bandgap regulator voltage goes 
above 3V, which could cause a laser over power condition. The window 
detector determines when the + terminal of the DC op amp B1 goes above 
1.65V or below 1.35V. An open on the monitor diode connection would cause 
the voltage on the + terminal of the DC op amp B1 to increase above 1.65V. 
High laser optical power would cause the + terminal of the DC op amp B1 to 
decrease below 1.35V. When a fault is detected, the timing capacitor is 
charged. If the fault in still present when the timer expires, the PNP 
transistor Q2 is shut off, the + terminal of the DC op amp is forced low, 
and the output LASER FAULT is forced high. The power on reset (POR) 
initialization circuit clears all faults until the laser is powered up. 
The laser can be turned on by an external control signal LASER ON, and a 
fault can be cleared by an external control signal LASER RESET. 
SUMMARY OF THE INVENTION 
A principal object of the present invention is to provide an improved 
method and apparatus for laser safety. Other objects of the invention are 
to provide such method and apparatus substantially without negative 
effects, and that overcome disadvantages of prior art arrangements. 
In brief, a method and apparatus are provided for laser safety. A fault 
detector detects laser fault conditions and generates a laser fault 
control signal. A laser fault counter is coupled to the fault detector for 
counting laser fault conditions and for generating a disable laser control 
signal responsive to a predetermined fault count. A reset timer is coupled 
to the laser fault counter for identifying a predetermined reset time 
period responsive to the laser fault control signal and applying a reset 
signal to the laser fault counter. 
In accordance with features of the invention, the laser fault counter is 
reset if a second laser fault condition is not detected within the 
predetermined reset time period after the laser is turned on. The disable 
laser control signal is not generated when a safe optical power condition 
occurs between two laser fault conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Having reference now to the drawings, in FIG. 2, there is shown a DC laser 
driver and fault detection circuit generally designated by 200. The DC 
laser driver and fault detection circuit 200 of FIG. 2 includes the 
circuitry of FIG. 1 with an added laser fault counter 202 of the preferred 
embodiment. A laser diode and monitor diode are generally designated by 
204. 
In accordance with a feature of the invention, the laser fault counter 202 
counts the number of optical power fault conditions that occur when LASER 
RESET and LASER ON signals are used to turn the laser on. For example, 
when two consecutive laser fault conditions occur, the laser fault counter 
202 generates a control signal DISABLE LASER. 
The DC laser driver and fault detection circuit 200 includes an operational 
amplifier B1 206, a PNP transistor Q2 208, an NPN transistor Q1 210, a 
resistor R1 212, a laser diode 214, a monitor diode 216 and a 
potentiometer RPOT 218 arranged to provide drive current for constant 
optical power to the laser. An external LASER ON signal is applied to a 
PNP control 220 that operatively control the PNP transistor Q1 208. The DC 
laser driver and fault detection circuit 200 includes a reset receiver 
222, a window detector 224, a POR initialization 226, a regulator monitor 
228, a fault detector 230, a timing capacitor 232 and a band gap regulator 
234. The band gap regulator 234 provides multiple voltages, 2.0V, 1.65V, 
1.5V and 1.35V. The monitor diode current, LM, is fed back through 
potentiometer RPOT 218. When the laser diode current increases, the 
monitor diode current increases. This causes the voltage on the + terminal 
of op amp B1 206 to decrease, providing the negative feedback. The POR 
initialization 226 clears all faults until the laser is powered up. The 
regulator monitor 228 detects when the bandgap regulator voltage goes 
above 3V, which could cause a laser over power condition. The fault 
detector 230 generates a control signal LASER FAULT in response to a 
detected unsafe laser optical power fault condition. 
The external LASER ON signal is applied to the laser fault counter 202. The 
external LASER RESET signal is applied to the reset receiver 222 and the 
laser fault counter 202. A window detector 224 detects when the + terminal 
of the DC op amp B1 goes above 1.65V or below 1.35V. An open on the 
monitor diode connection would cause the voltage on the + terminal of the 
DC op amp to increase above 1.65V. High laser optical power would cause 
the + terminal of the DC op amp B1 to decrease below 1.35V. 
When a fault is detected, the timing capacitor 232 is charged. If the fault 
in still present when the timer expires, the output LASER FAULT of the 
fault detector 230 is forced high, the PNP transistor Q2 208 is shut off 
by the PNP control 220 responsive to the control signal LASER FAULT, and 
the + terminal of the DC op amp is forced low. The control signal LASER 
FAULT is applied to the PNP control 220 and to the laser fault counter 202 
of the preferred embodiment. The laser can be turned on by an external 
control signal LASER ON, and a fault can be cleared by an external control 
signal LASER RESET. 
Assume a fault is caused by high optical power out of the laser. The laser 
214 would emit this high power until the fault detection 230 shuts the 
laser down. The duration of this high power pulse is defined by a timing 
capacitor 232 in FIG. 2. The duration of this pulse, coupled with the 
frequency of the LASER RESET signal can cause an unsafe laser optical 
power. The laser fault counter 202 is used to shut down the laser if two 
consecutive laser faults are counted with no successful laser power up. 
The laser fault counter 202 is reset if a second fault is not detected 
within the predetermined time period after the LASER RESET signal goes low 
and the LASER ON signal goes high. This prevents disabling the laser 
driver when a safe optical power condition occurs between two laser fault 
conditions. 
FIG. 3 shows the laser fault counter 202 in more detail. The laser fault 
counter 202 includes a retriggerable single shot timer 302 receiving the 
control signals LASER RESET and LASER ON. A reset capacitor 304 is 
connected with the single shot timer 302 for defining a reset time period. 
The laser fault counter 202 includes an OR gate 306 applying a reset input 
to a first D flip-flop 308 of a pair of D flip-flops 308, 310 responsive 
to a clear control signal from the POR initialization 216 or responsive to 
the reset time period. The clear control signal from the POR 
initialization 216 is applied to a reset input to the second D flip-flop 
310. The control signal LASER FAULT is applied to a clock (C) input of the 
pair of D flip-flops 308, 310. The control signal LASER FAULT also is 
applied to the single shot timer 302. The data (D) input of the first D 
flip-flop 308 is set high. The output (Q) of the first flip-flop 308 is 
applied to the data (D) input of the second D flip-flop 310 and is applied 
to the single shot timer 302. A low input to the reset inputs of D 
flip-flops 308, 310 forces a low Q output independent of the clock C or 
data D inputs. When two consecutive laser faults are counted by the two D 
flip-flops 308, 310, a control signal DISABLE LASER is applied to the 
reset receiver 222 of FIG. 2 which forces the + terminal of the DC op amp 
B1 206 low, and is applied to the PNP control 220 of FIG. 2 which shuts 
off the PNP transistor Q2 208. If one laser fault is followed by a normal 
operating condition for a period of time larger than that defined by the 
reset capacitor 304, the laser fault counter 202 is reset. 
While the present invention has been described with reference to the 
details of the embodiments of the invention shown in the drawing, these 
details are not intended to limit the scope of the invention as claimed in 
the appended claims.