[SI-LIST] Re: question Optical Tranceivers
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 08 Oct 2004 11:02:52 -0700
Jack-
Most optical transceivers utilized for data communications applications
don't switch the emitted light completely on and off in response to
logic 1 and 0 levels. The emitted light power is switched between two
power levels P1 and P0 . A parameter called the "extinction ratio"
defines the relationship between the logic 1 and logic 0 levels ( r =
P1/P0) . Depending on the vendor or spec, this is sometimes expressed as
a fractional ratio or as dB .
Ideally, a transceiver would emit zero power in the logic 0 state and
the resulting extinction ratio would be infinite (P1/0) . However in all
the circuits that I'm familiar with the laser diode is usually biased to
just above the lasing threshold such that the device is emitting a low
level of light even during logic 0 conditions.
I would be very suprised if your transceiver has a problem responding to
the data burst. If it is emitting no light at all it sounds like either
the laser diode or the supporting driver circuitry is defective.
Ray Anderson
jan.vercammen1@xxxxxxxx wrote:
>hello SI-list,
>I am forwarding a question of a colleague:
>
>We are evaluating optical transceivers (Gbit Ethernet and Fibrechannel
>devices) for IEEE-1394 communication on GOF. The IEEE-1394 protocol issues
>a pulsed signal into the opto transmitter: every 40msec a small burst of
>some 660usec containing 20nsec pulses is generated. In between the bursts
>the signal is steady (high). The transmitter of the transceiver is always
>ENABLED, tied to GND.
>
>We find that not all optical transceivers are working with such an input
>signal: some devices do not emit light at all.
>We think that there is maybe a laser start-up time, i.e. the burst of
>660usec is too short for some transceivers to start emitting light.
>However, we cannot find any indication in the spec or datasheet.
>1. We would like to understand why certain transceivers do not transmit
>and others do?
>2. And how to spot that from a datasheet?
>
>Many thanks in advance,
>Patrick Lambrechts
>
>Kind regards,
>
>Jan vercammen
>
>
.
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