Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-07-18
2003-12-02
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029020, C359S199200
Reexamination Certificate
active
06658030
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of fiber optic data communications, in particular fault detection arrangements for laser safety in a duplex open loop parallel optical link.
BACKGROUND OF THE INVENTION
Laser-based devices and systems have been used widely in the fields of, for example, communications, computing technology and medical technology. The lasers utilized in these systems have output optical powers that are potentially harmful to both people and equipment. For instance, such lasers are driven at such a power so as to have damaging effects if exposed to a human eye. Among the safety methods and systems that have been developed, Method and Apparatus for Laser Safety described in U.S. Pat. No. 5,999,549 (Freitag, et al.), which is a related patent that is incorporated herein by reference, resets a laser fault counter if a second laser fault condition is not detected within a predetermined reset time period after a laser is turned on.
In the field of fiber optic data communications, fiber optic data communications links must ensure the optical power being transmitted by the laser remains below a defined level, or a “safe” level, in the event of a single failure in the link so as to avoid the aforementioned potential harm to both people and equipment. The “safe” level may include, for example, a standard established by industry and/or governmental regulations.
For serial optical links, there are at least two exemplary methods for ensuring that laser-driving, optical power does not exceed the relevant “safe” level, thus ensuring the safety of the users and any surrounding people as well as preventing any damage to the apparatus by the laser optical power. A first example method includes setting the optical power delivered by the laser to a level that is well below the “safe” level and utilize circuits on the transmitter IC to detect when the optical power level exceeds the safe level. Since the optical power in serial optical links is most often controlled by a monitor photo diode control loop, the laser average optical power is known. Therefore, fault detection circuits are able to easily determine when the average optical power exceeds a threshold limit. That is, since the current in the monitor photodiode is proportional to the optical power output by the laser, the transmitter can detect when the monitor photodiode current exceeds a threshold limit.
A second example method for ensuring that laser-driving optical power does not exceed the safe level in a serial optical link includes an Open Fiber Control (OFC) handshake protocol. This example protocol is used when the laser-driving optical power in normal data mode is set above the safe level. Thus, when a serial optical link fiber is pulled, according to the OFC protocol, the laser light is pulsed at an extremely low duty cycle (on for approximately 150 psec, off for approximately 10 sec) to ensure that the average laser optical power does not exceed the safe level. Similar to the first method, fault detection circuits on the transmitter side also ensure that a fault in the corresponding laser driving circuit does not cause the optical power to exceed the safe level.
However, such example methods of ensuring that the laser optical power remains at or below a safe level are not applicable to open loop parallel optical links. That is, in open loop parallel optical links, the average optical power is unknown, because there are no monitor photodiodes, and multiple lasers are emitting light simultaneously. Thus, the example fault detection methods described above are inappropriate since the aggregate optical power in an open loop parallel optical link is above the safe level and is much higher than that of a serial link.
For instance,
FIG. 2
shows an open loop parallel optical transmitter which includes N+1 channels. The parallel optical transmitter
200
includes global temperature coefficient adjustment DAC (TEMPCO DAC)
210
and global temperature coefficient adjustment shift register (TEMPCO SHIFT REGISTER)
220
which holds bits for the TEMPCO DAC
210
. Each of channels 0 through N include a respective laser driver
230
, a threshold current adjustment DAC
240
, a modulation current adjustment DAC
250
, and a shift register
260
to hold the bits for each DAC. EEPROM
270
stores the bits in a non-volatile memory when the parallel transmitter is powered off. This parallel transmitter, however, does not show any method for preventing the aggregate optical power out of the lasers of channels 0 through N from exceeding the “safe” level.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a method and apparatus for ensuring that the laser optical power does not exceed a “safe” level in an open loop parallel optical link. To that end, the present invention includes a duplex parallel optical link having a transmitter and receiver pair and a fiber optic ribbon that includes a designated number N of channels that cannot be split. The duplex transceiver includes a corresponding transmitter and receiver that are physically attached to each other and cannot be detached therefrom, so as to ensure safe laser optical power.
That is each duplex transceiver includes both a parallel optical transmitter and a parallel optical receiver packaged together along with a fiber ribbon cable where both directions of laser transmission are fixed together. The fiber ribbon cable includes N+1 channels which are bundled together and therefore cannot be split from each other. Channels 0 through N on both the transmitter and receiver sides include one channel which is designated as the safety channel. When the fiber ribbon is connected at both ends of the link, that is between both transceivers, the designated safety channel on both the transmitter and receiver sides function as normal data channels. However, when the fiber ribbon is pulled or is otherwise severed, the signal detectors at the receivers transmit a “loss of signal” condition to the respective transmitter. The “loss of signal” signals cause all of channels 0 through N, except for the designated safety channel, of the transmitter to shut down. That is, only the designated safety channel remains enabled.
The optical powers on the designated safety channels are set to a previously determined safe level at manufacturing. However, when the fiber is pulled or otherwise severed, a fault in the laser driver circuit could force a high current into the laser, causing the optical power to exceed the safe level. Fault detection circuits, therefore, must protect the designated safety channels from launching unsafe optical power in the event of a single failure.
The duplex transceivers each include an open loop parallel optical transmitter that includes fault detection circuits that detect a high current condition which causes the optical power to exceed the safe level when the ribbon fiber is pulled or otherwise severed.
In the event that the ribbon fiber is pulled or otherwise severed, since the aggregate power of a parallel link exceeds the safety limit, all of channels 0 through N except for the designated safety channel must be disabled by two mechanisms, which are regarded as being a redundant configuration. That is, if only one mechanism is used to disable the lasers of channels 0 through N, except for the designated safety channel, a single fault in the laser shutdown mechanism would enable the optical power to exceed the safe level. Thus, in a configuration having only one signal detector on the receiver side having its output stuck high, indicating that a signal is present, the aggregate optical power would exceed the limit if the fiber ribbon was pulled or otherwise severed.
The present invention also includes another redundant part of the safety scheme involving the laser current comparison function. A laser fault detection circuit is provided that compares the threshold current and modulation current in the laser of the designated safety channel with a redundant threshold current reference and a redundant
Baumgartner Steven John
Hedin Daniel Scott
Paschal Matthew James
Al-Nazer Leith A
Ip Paul
Rabin & Berdo PC
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