[SI-LIST] Re: question Optical Tranceivers
- From: "Tom Woodward" <tomw@xxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 8 Oct 2004 14:36:56 -0400
A couple of things about typical optical transceivers to keep in mind:
1. They are commonly AC coupled within the transceiver, so don't expect to
pass DC levels. Remember that SFP and SFF modules are primarily intended
for scrambled signals (SONET, SDH) or for 8b/10b encoded signals (Fibre
Channel, GigE). They don't expect to see long periods (like your 40 ms
interval) with no data transitions.
2. Even if your transceiver is DC coupled, some of them can become confused
by a large change in data rate. These modules normally include a laser
shutdown circuit that is intended to prevent the emission of optical power
levels above the rating of the module, thereby helping to insure eye safety
for the module. This uses a feedback signal from an optical power monitor
attached to the laser. A sudden rate change (like leaving the laser on for
a long time) can trigger this shutdown circuit, producing a "Tx-Fault"
condition. Shutdown is a latching operation, meaning that you have to at
least cycle the Tx-Enable signal to recover to normal operation.
Personally, I wouldn't try to send bursty data while Tx-Enable is turned on.
Maybe you could turn on Tx-Enable during active data bursts, and leave the
laser off during the quiescent periods.
Tom Woodward
----- Original Message -----
From: "Ray Anderson" <reanderson@xxxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Cc: <jan.vercammen1@xxxxxxxx>
Sent: Friday, October 08, 2004 2:02 PM
Subject: [SI-LIST] Re: question Optical Tranceivers
> 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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