Minutes of the LArg Front-end Optical Links Meeting

Present :

M-L.Andrieux - ISN
E. Auge - LAL
B.Dinkespiler - CPPM
A.Gara - Nevis
F.Henry-Couannier - CPPM
J.Lundquist - KTH     
O.Martin - CPPM
R.Mommsen - BERN
A.Patti - ISN
M.Pearce - KTH
K.Strahl - Montreal
JJ.Veillet - LAL

MINUTES
~~~~~~~

o O.Martin - CPPM
  Final Results from a complete analogue link demonstrator
  
	Olivier presented final results from a complete analogue
	link using inverted-polarity Sandia VCSEL's. Results from
	a static test demonstrated that there was a -1.8 to -2 dB
	attenuation in the VCSEL light output with a 1m pigtail
	connected to a 70m length of fibre (compared to a free
	space measurement). For the VCSEL array presented, the threshold
	current was approximately 0.7mA for all elements with a 
	light output of approximately 1.5mW for a bias current of
	5mA. A noise equivalent power (NEP) of approximately 1uW was
	demonstrated for two of the VCSEL elements for bias currents
	up to about 6mA - the NEP increased to approximately 1.5uW
	for bias currents up to 8mA. These measurements were carried out
	with an oscilloscope. Straight-cut MT connectors were used 
	through-out. There was no longer any evidence for the slope of 
        the VCSEL light output changing (onset of a new lasing mode). 
	Finally, Olivier presented results from dynamic measurements. 
	He indicated that a dynamic range of 9 bits was easily possible 
	with 10 bits `a likely conclusion'. These measurements were for a 
	signal/noise of 3. The differential non-linearity was less than 
	+-5% across the dynamic range - but more work was needed to reduce 
	the error bars, especially in the lower part of the dynamic range.

o J.Lundquist - KTH
  Gamma and Neutron Irradiation Results for VCSEL's, Fibres
  and G-Link Chips	 
  
	Johan presented results from neutron irradiation (at SARA - Sept
	1997) and gamma irradiation (at Karolinska Hospital in Stockholm - Jan
	1998) of candidate digital link components. Both these facilities
	and the test procedures were detailed by M.Pearce in the previous
	meeting. Results from the neutron irradiation of VCSEL's (from 
	Honeywell and Mitel) indicated that the threshold currents do not 
	change and that there was a light output attenuation of approximately 
	10%. Annealing was observed after approximately 12 hours
	for a bias current of 5-6 mA (Mitel) / 7-8 mA (Honeywell). 
	The dose received by the VCSEL's was approximately 
	(2.0+-0.4)e13 n(1 MeV GaAs) / cm**2. A 10m length of standard fibre was 
	irradiated to (0.7+-0.2)e13 n(1 MeV SiO2) / cm**2. For a 10m length the
	resulting attenuation was almost 20%. 

	Gamma irradiation results for VCSEL's also indicated no change
	in the threshold current. For a `10 years at LHC' dose the VCSEL
	light output decreased very slightly. A 10m length of standard
	fibre suffered an attenuation of almost 90%, however this scales
	to approximately 40% for a 2m length. As observed during the 
	neutron irradiation, the G-links chips (PLL monitored) did not
	behave abnormally during gamma irradiation. Bernard reported on a
	communication with an HP engineer who had told him that the G-link
	is manufactured in a `sea of gates' technology with 25 GHz transistors.
	The resulting small feature size may help to explain the observed
	radiation tolerance of the chip. 

        In conclusion, Johan
	stated that VCSEL's appeared resistant to both gamma and neutron
	radiation. In order to understand the radiation implications for
	fibres (especially for gammas) more information on possible routing
	scenarios is needed. Whereas gamma radiation is more damaging for
	fibres, neutrons are more damaging for VCSEL's. In the limited
	tests of the G-link PLL to date, no adverse effects have been	
	observed for either type of radiation. Finally, it was pointed out 
	that the dose rates in both the irradiations are highly unrealistic
	at 8500 (1500) times that expected in ATLAS for the gamma (neutron)
	irradiation. 

o M.Pearce - KTH
  Gamma Irradiation Studies at BNL (J.Ye and R.Stroynowski)

	Mark reported on results communicated by J.Ye and R.Stroynowski from
	SMU. A gamma source at BNL had been used to irradiate Honeywell 
	transceivers (VCSEL's and PIN's) and 50/125 GRIN fibre from Corning. 
	A total dose of 200 Gy was given (2.5 ATLAS years) with a dose rate 
	11,000 times that expected in ATLAS. The tests were totally passive
	with no measurements or biasing during the exposure. Before and
	after irradiation the transceiver exhibited clear eye-diagrams at 1.24 Gb/s.
	In both cases a bit error rate of O(10e-12) was observed. To investigate
	the effects of radiation on the fibre, an optical time domain reflectometer
	(OTDR) was used. The OTDR operated at 1310nm. 
	Before irradiation a fibre attenuation of 7.3 dB/km was
	measured (included two couplers). After irradiation, no measurement was 
	possible - the fibre appeared opaque. The attenuation was however measured
	using a calibrated laser source (1310 nm) and power meter. An attenuation 
	of 617 dB/km was measured ! This means the received signal falls below
	the transceivers sensitivity. Note that a 2*60m length of fibre was
	irradiated - which is unrealistic for ATLAS applications.

o B.Dinkespiler - CPPM 	 	
  `Poor Man's' Bit Error Rate Tester
	
	Bernard review plans for the construction of a Gb bit error rate tester.
	The device would allow on-line testing of links transferring 32 bits at
	40MHz with single bit errors detectable. The testing system would have
	in-built modularity to allow up to 16 links to be tested simultaneously.
	At present a Tektronics pattern generator is used to produce the
	32 bit input words - although this could be easily replaced by a simpler
	(home-made) dedicated device. The receiver boards will be compatible
	with the mini-ROD's to allow compatibility with test-beam electronics.
	The initial system will be based on G-link serialisers and Honeywell
	transceivers. There is also the possibility to plug in separate optics
	boards. For technical details and schematics see Bernard's transparencies. 

	As an aside Bernard reported that the G-link receiver has an `error-pin'
	which would enable simple error checking to be carried out very easily.
	An error condition is set if the control word added to the data is
	illegal, ie: due to a single event effect. This would probably form the
	basis of the first round of radiation tests in March.

o M-L.Andrieux - ISN
  Aging Tests at ISN and Kerma Factors
	
	Marie-Laure presented details of two ovens (53 l and 275 l) available at 
	ISN for aging tests of optical links. Both ovens can be computer
	controlled.

	She also reported on a revised calculation for the Kerma factor for
	SiO2. This work is being carried out in collaboration with Claude Leroy.
	The value used to date, 1.3 +- 0.3, seems to have been incorrectly 
	calculated - this is being looked into. 

o M-L.Andrieux - ISN
  Amphenol Prototype Status
	
	Marie-Laure started by reporting that CHEVAL had drilled the VCSEL
	ceramics and these had been passed by Amphenol for mechanical suitability.
	In total, 30 ceramics had now been metallised and 6 had been passed
	to Amphenol for electrical tests. The remaining 24 will be sent to 
	Elmatek for bonding with a set of VCSEL's and PIN's. Amphenol have
	made significant progress with the optical connectors as these are now
	able to mate correctly. The target of <1 dB connection loss has almost
	been met but the outer fibres in the ribbon suffer a loss of approximately
	1.5 dB. It is hoped to meet the target by improving the plug moulding and 
	polishing. The VCSEL will be aligned provisionally using a video camera 
	coupled to a microscope. It is hoped to release a complete link by 
	March 1998 in time for radiation testing.
	
o B.Dinkespiler - CPPM
  Status of Testbeam Copper Links
	
	Bernard presented the status of the copper links for the testbeam.
	The PCB's are designed and tested and can be used with FEB's
	and mini-ROD's as they become available. Six hundred cables salvaged
	from CPLEAR have been left at CERN. Plans for the final production
	of the links need to be defined with some urgency. 

		
TRANSPARENCIES
~~~~~~~~~~~~~~
	Copies of transparencies will be sent to :
		o A representative from each of the participating
		  groups (CPPM, ISN, KTH, SMU)
		o The ATLAS secretariat