PROCESOS DE FABRICACIÓN DE MATERIALES COMPUESTOS

In order to calibrate the electronics of the 0.83 pm receiver system, a propagation experim ent was simulated in the laboratory. An 830 nm laser internally m odulated by a 300 ns pulse with a p.r.f of 2 kHz, was fired at the

receiver. First of all it w as req u ired to test th e o u tp u t of the laser. The in p u t current to the laser w as varied from 0 m A to 300 m A an d the tw o vertical o u tp u ts of the PIN p hotodiode w ere com bined w ith a b ridge cable and fed to an oscilloscope. In this w ay, the experim ent tests the o u tp u t of the laser in d ep en d en tly of the receiving electronics. The am plified laser o u tp u t (actually the average of the tw o outputs) versus the cu rren t in p u t is show n in Figure 4.9. This show s the chciracteristic non-linearity of a laser driven in the low current region. At the Im perial College transm itter, the 0.83 |im laser operates at 1000 m A an d is therefore com fortably w ith in the linear region. 1200 1000 800 Û 600 < 400 200

Note that the amplifier output is the average of the two detector signals and not the "fast sum", (which is the addition of the signals).

100 200

Input to Laser (mA) or (mV).

300

Figure 4.9: Calibration of the laser output.

Detector Linearity.

Next, a test of the linearity of the detector w as required. In this experim ent, the peak voltage am plitude pulse o u tp u t to th e laser w as fixed at 1 V with the vertical o u tp u ts of the PIN p h o to d io d e connected to g eth er an d fed directly to an oscilloscope. This avoided the effects of the other electronics in the receiver system. A series of ap ertu re stops w ere then placed over the receiver a p ertu re to lim it, in a controlled w ay, the am o u n t of radiation falling onto the detector. If the response of the detector w as indeed linear, then a g rap h of the received o u tp u t voltage against the receiver ap ertu re

Chapter 4: Experimental Equipment.

area w ould also be linear. H ow ever, before this, th e transm itter ap ertu re w as scanned w ith the "mobile detector". A ppendix A.2 Figure A.2.11, to prove th at the irrad iated optical pow er was uniform over the tran sm itter aperture. Figure 4.10 show s the detector o u tp u t is a linear function of the incident radiation. The linearity of the detector has thus been confirm ed. N o te th a t th e sa tu ra tio n voltage level of the detecto r is 1.2 V p e r PIN o u tp u t channel. Such levels have never been observed in real propagation experim ents. Strong turbulence conditions, ie. clear afternoon skies d u rin g sum m er, p ro d u ce m axim um an d m inim um scintillations of the o rd er of 1000 mV an d 100 mV respectively. In practice, therefore, problem s caused by saturation of the receiver detector will not occur.

100001

The output of the laser was scanned with a 'mobile receiver* and the irradiated optical power was found to be uniform over the transmitter aperture.

r I _{y =-3 1 .8 5 4 + 9.7336X R''2 = 0.997}

I

£ I .2 2 4000-

I

< 2000 ■ 0 200 400 600 800 1000 1200 Voltage output (mV).

Figure 4.10: C alibration of the receiver.

"Fast Sum '' C ircuitry Linearity.

The "fast sum" box perform s an addition of the pulses arriving at each side of the PIN photodiode. Its operation is expected to be linear. The fast sum box is calibrated by using a pulse generator to deliver 300 ns pulses at a p.r.f of 2 kH z d irectly as in p u ts to each channel. The o u tp u t is fed to an oscilloscope an d the co rresp o n d in g voltage values noted. The v o ltag e in p u t to each channel was varied from 0 V to 1.2 V. These in p u ts w ere n o t raised above 1.2 V since this is the level at w hich the detector satu rates.

T he resu lts from this experim ent are p resen te d in F igure 4.11, an d its operation is linear. The circuit diagram of the fast su m /d ifferen ce is show n in A ppendix A.2 Figure A.2.14.

3.0 n 2 5 - 2 0- % 1.0- 0.0 0.0 0.2 0.4 0.6 0.8 1.0 12 1.4 Voltage i / p (V).

Figure 4.11: Fast Sum calibration.

Linearity of the Pulse Stretchers.

The p u lse stretcher and "'slow sum " boxes enable recording of scintillations on m ag n etic tap e sto rag e m edia. The tra n sm itte d p u lses a t th e I.C. tran sm itte r are of the o rd er of 300 ns. H ow ever, once these p u lses have passed through atm ospheric turbulence they have expanded to the o rd er of 900 ns u p o n arriv al at U.C.L. This tim e d u ra tio n is still toa sh o rt for m agnetic tape recording. The slow sum box electronically expands the tim e d u ra tio n of th e p u lses w ith a m p litu d e c o n serv atio n to th e o rd e r of m illiseconds. The in p u t pulses to the 'slow sum " circuit are atten u ated by a scaling factor so th at they fall w ith in the linearity region of the pulse stretchers in the "slow sum" box. This is 10 mV to 300 mV. A n ap p ro p riate scaling factor is selected to en su re th at the m axim um p u lse am plitude^, div id ed by the scaling factor falls w ithin the range of 10 mV to 300 mV. It is necessary to set the sam e scaling factors for both in p u ts to the "slow sum" box. The o p eratio n m u st be seen to be a linear one. The experim ental ^This is the "fast su m /2 " observed over say a few seconds on an oscilloscope.

Chapter 4: Experimental Equipment.

tech n iq u e is sim ilar to the "fast sum" calibration. Figure 4.12 show s the "slow sum " o u tp u t norm alised by the scaling factor versus the in p u t from the p u lse g en erato r to each channel. N ote th at the operation of th e "slow sum " is highly linear an d its o u tp u t is m ultiplied by 10.

1-21 _{This establishes th at th e slow sum is a}

linear operation. T he receiver am plifier saturates at 1.2 V (per channel), th u s there is no point in having in p u t voltages, (to the slow sum ), th at are greater than this. The slow su m m ultiplies by 10. s i S 1 0.8- 2 "S> 0.6-

1

0.4* 0.0 20 30 10 0 Slow su m /scaler (x. 10 V).

Figure 4.12: Slow Sum calibration.

''Extra Slow Sum"" Circuit Linearity.

The m ean po w er level is detected through the "extra slow filter" box. The filtering frequency u sed is 0.85 Hz. The circuit diagram for the filters is sh o w n in A p p en d ix A.2 Figure A.2.15. Its o u tp u t m ay be reco rd ed on m agnetic tape or displayed visually and in real time on a chart recorder. It is required to test w hether the operation of the "extra slow sum" box as well as the chart recorder is linear. The procedure w as sim ilar to th at ad o p ted previously. The "slow sum" in p u t (with ap p ro p riate scaler) w as fed into the "extra slow sum " box and displayed on bo th an oscilloscope an d the chart recorder. The speed of the chart recorder was 15/16 inches p er second an d the full-scale deflection set at 10 V. Figure 4.13 displays the voltage in p u t to each scaler of the "slow sum" and the corresponding chart recorder an d direct m eter "extra slow sum" outputs norm alised by the scaler values.

30 r30

—— X.SI0W siu n /sad er; Chart — • X.SI0W sum /scaler; Meter

20 ■20 10 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2

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«> o

I I

V o lta g e in p u t to e a c h sc a le r, (V).

Figu re 4.13: Extra S lo w S u m and chart recorder calibration.

Quotient Circuit Linearity.

The quotient box provides information about the AOA of the received beam. A visual indication of this is also provided by the ''slow difference" filter. This circuit diagram is illustrated in Appendix A.2 Figure A.2.16. Two power supplies were used to supply each of the inputs to the quotient box and the output viewed on an oscilloscope. If these are denoted and y2/ then the output of the quotient, known as "Position", is:

, , y, - Vo

' Position" \V] = —--- x 10. ^ ^ y^+yz

The output voltage range is ± 10 V and the stated rms noise for 10 V full scale is 2 mV. The voltage inputs were varied from 100 mv to 300 mV using a variety of combinations of y^ 2* Figure 4.14 shows the results of

this experiment. Inputs 2 ^re also shown. The value of "Position"

viewed on the oscilloscope is plotted against the theoretical "Position" value derived from Eq. (4.0). It is clear that the quotient box is linear within the input range of 100 mV to 300 mV to each scaler. The quotient box was then connected to a power supply such that equal voltages were passed to

Chapter 4: Experimental Equipment.

each in p u t and the o u tp u t fed to an oscilloscope. The in p u t voltage to both channels w as varied betw een 100 mV to 300 mV. This corresponds to the lin e a rity reg io n of th e "stretch er"^0 boxes. Ideally the o u tp u t of the quotient should be zero volts since both inp u ts are the same. H ow ever, as expected, a small offset from zero was found. It w as found th at by scanning the in p u t over th e full 200 mV range, the value of "Position" varied by ± 2 mV ab o u t the zero value.

i / p V I (V) i / p V2 (V) 0.100 — 0.150 0.200 0.250 0.300 0.125 0JZ50 0.100 0300 0.200

I

I

-2- ■3 ■2 •1 0 1 2 3 4

"Position" (lOV) from oscilloscope.

Figure 4.14: Q uotient box calibration.

U sing the "Position" to AOA conversion form ula, this results in an erro r of the AOA of ± 0.29 p.rads. This represents the induced error in AOA from the q u o tien t box. A nother source of erro r is from the m icrom eter itself. The best possible resolution from the m icrom eter is 0.01 mm. Relating this to the g rad ien t of the calibration curve , Figure 4.6, w e derive a position displacem ent of ± 2.89 prad. C om pounding these tw o errors w e arrive at the total error on all 0.83 pm AOA m easurem ents of ± 2.90 prad.

A fu rth e r d e sc rip tio n of the 0.83 pm system electronics w ith circuit "stretcher" boxes electronically expand the tim e d u ra tio n of the received 300 ns pulses to the o rd er of m illiseconds w hile still preserving their am plitude. This operation is linear over the 10 mV to 300 mV input range.

diagram s is reserved for A ppendix A.2.

4.3) THE 1.55|im SYSTEM.

All the 1.55 |im system electronics an d antennas w ere pro v id ed by B.T.R.L. The tran m itter an d receiver antennas are m odified astronom ical telescopes.

4.3.1) 1.55pm TRANSM ITTER.

Figure 4.15 show s a schem atic diagram of the 1.55 pm link. The transm itter consists of a Fabry-Perot 1535 nm pigtailed sem iconductor laser, an erbium fibre am plifier, an isolator an d a transm it telescope. The Fabry-Perot laser has a m ean operating w avelength of 1535 nm and can be m odulated up to 1 G b it/s directly from a BER (bit error rate) transm itter unit. The o u tp u t of the laser, typically 0 dBm m ean, is fed into an erbium po w er am plifier w hich is CO- a n d contra- p u m p ed at 1480 nm by high pow er sem iconductor lasers. The am plifier has a m axim um m ean o u tp u t pow er of +14 dBm into a sta n d a rd sin g le m o d e fibre an d is follow ed by an isolator w hich is required to p rev en t self-lasing b u t has a loss of 1 dB. The transm it telescope is of Schm idt-Cassegrain design w ith a 2 m focal length, 20 cm aperture and the cleaved fibre en d is located in its focal plane. D espite having gold coated m irrors, the telescope tran sm it pow er is 3 dB below th at in the fibre d u e to the m ism atch betw een the telescope num erical aperture, 0.05, and that of the fibre, 0.1, and the obstruction caused by the secondary m irror.

Chapter 4: Experimental Equipment.

Ill-K l'ian sn u iicf

L a s c i d io d e Is o la to r T ra n sm it te le s c o p e K c c e iv c I le a itis p littc r T e le s c o p e a m p lifie r _{4km p ath} H igh speed tc c c tv c t Pow er m eter and co m p u ter

Figure 4.15: Schematic diagram of the 1.55 |im link.

A lignm ent of the telescope is achieved by m eans of angle adjusting micrometers w ith a resolution of better than 50 prad, or 20 cm at 4 km. A boresighted sighterscope, w ith a 900 mm focal length, is attached to the transm it telescope as an aid to alignm ent. The beam produced by the telescope is diffraction lim ited. Beam w idth m easurem ents show the radiation is broadened by atmospheric turbulence to a diam eter of between 0.9 and 2 m at 4 km. Figure 4.16 is a photograph of the transm itter unit. The transm itter is located on the 12th floor (50 m high) in the plant room of the Engineering D epartm ent of Im perial College in South Kensington and the receiver is in an 11th floor laboratory of the D epartm ent of E lectrical and Electronic E ngineering, U n iv ersity C ollege London, Bloomsbury.

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Figure 4.16: Photograph of the transmitter unit.

4.3.2) 1.55pm RECEIVER.

The receiver consists of an identical telescope to that of the transmitter with a 3 dB optical beam splitter located 10 cm before the focal plane, providing two ports for experimentation. At the first port a commercial power meter is located which either logs atmospheric attenuation, via a computer system, or is used to measure the average power during high-speed data transmission experiments. Port number two contains either the high speed data receiver or a Ge quadrant photodiode which is used to measure intensity and angle of arrival fluctuations. The commercial power meter has a 2 mm diameter diode and a sensitivity of -81 dBm when the source is chopped at 270 Hz, or -61 dBm otherwise. The Ge detector has a 5 mm^ total area, a sensitivity of better than -60 dBm and a bandwidth of 1 kHz. An optical absorption long-pass filter with a 1 pm cut-off is used to reduce the ambient background light collected by the telescope. The optical loss of the telescope beam splitter and filter is 5 dB. As an additional aid to alignment, an eye piece may^slotted into the receiver telescope to view a visible light

Chapter 4: Experimental Equipment.

source from the transm itter room. Figure 4.17 shows the receiver optics for the 1.55 pm link.

Figure 4.17: Receiver optics for 1.55 pm link.

4,3.3) HIGH SPEED RECEIVER DESIGN.

A direct detection receiver is favoured over a coherent receiver since these typically only collect power from a single coherent patch of the beam and therefore utilise just a fraction of the total pow er gathered by the telescope. The receiver was designed to operate in all conditions of turbulence likely to be encountered on the link and still provide good sensitivity to im prove the pow er budget. This dem anded a large dynam ic range to encom pass intensity fluctuations and atm ospheric attenuation changes, a large area detector element to collect the moving, distorted focused spot; and a custom pre-amp design for maximum sensitivity. Figure 4.18 shows the high speed receiver sub-system. The detector element was a 1 mm diam eter InGaAs p- i-n photodiode, chosen in preference to a Ge p-i-n or Ge APD due to high leakage currents in large area Ge devices.

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