>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 28 September 2006 13:09
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] I need a circuit for a bidirectional 
>optoisolatedTTLinterface
>
>
>Michael Rigby-Jones wrote:
>> 
>>>-----Original Message-----
>>>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]
>>>Sent: 28 September 2006 00:18
>>>To: Microcontroller discussion list - Public.
>>>Subject: Re: [EE] I need a circuit for a bidirectional 
>>>optoisolatedTTLinterface
>>>
>>>
>>>Alan B. Pearce wrote:
>>>
>>>>>So I have played around with the Linear Technology
>>>>>spice program - see attached jpeg.
>>>>>    
>>>>
>>>>You will find that the 8N137 opto is a lot faster than the 4N25, but
>>>>an even better one is the Agilent (now Avago Technologies) 
>HCPL-2300 
>>>>which has an amplifier for the PIN diode included, and has very low 
>>>>LED current requirement, and is claimed to be 8M baud. Also has UL, 
>>>>CSA and VDE approvals for working with mains voltages.
>>>>
>>>>  
>>>
>>>What I really don't understand is why these 10MB devices are so
>>>difficult to find,
>>>if you realize that optical communication through glass fiber 
>>>goes upto 
>>>10GHz and maybe faster.
>> 
>> 
>> There is a world of difference between the technologies 
>though. 10Gbit 
>> fibre optics use lasers that are often not directly modulated.  e.g. 
>> they use a mach-zehnder interferometer, or an electro-absorbtion 
>> modulator.  You can get directly modulated 10Gbit lasers, 
>but they are 
>> for short distances, and still require a relatively sophisticated 
>> driver. A lot of work also goes into the receive chain as well. 
>> Photo-transistors are far too slow, they are either PIN 
>diodes or for 
>> greater sensitivity APD's (avalanche photodiodes).  The optical 
>> components in a 10Gbit link tend to be physicaly large, uber 
>expensive 
>> and power hungry.
>> 
>> Regards
>> 
>> Mike
>
>There seem to be some "cheap" gbics around a $100.
>What would be inside those?

The short reach Gigabit ethernet modules use an interesting device called a VCSEL, (Vertical Cavity Surface Emmiting Laser).  These are cheaper to make than traditional edge emmiting lasers, but are currently limited to the shorter wavelengths (gbic is around 850nm IIRC), and have lower output.  Ideal for cheap short range stuff using multimode fibre, though in relation to the original thread would still be expensive to package up to use as an opto-isolator by the time you add a PIN diode and drive electronics.

Regards

Mike

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