Final Presentation

ECE 4006 C

G3: Karen Cano, Scott Henderson, Di Qian

April, 23 2002

I: Project Tasks and Theory

• 1. Research on the transmitting and receiving modules.

• 2. Examine the testing board • 3. Search for the components• 4. Testing the evaluation board with

purchased components• 5. Connecting the purchased components

with parts from other groups.

Project Goal

• Duplicate the data transmitting and receiving module functionality of the Gigabit Ethernet technology with purchased components that provide optimum performance at a minimum price.

Possible Solutions• Transmitting module (laser source)

– VCSEL

• Receiving module (Photo-detector) – PIN photodiode

• Other Specs: - SC connectorized (optical)- SMA connectorized (electrical)- 850nm

- Multimode (fiber) - relatively low cost

Laser Basics• What is a Laser?

– Light Amplification by Stimulated Emission of Radiation

• How? 1) Electrons in low-energy levels bumped into high levels by injection of energy

2) When an electron drops to a lower energy level, excess energy is given off as light.

VCSELs

• Vertical Cavity Surface Emitting Lasers

• Physical makeup– Bragg mirrors– Active region

• Fabrication techniques– Molecular beam epitaxy– Vapor phase epitaxy

VCSELs

• In EELs no pre-cleaving tests can be performed, testing VCSELs is much cheaper

• Less current required for VCSELs

• Output beam easier couple into fiber and much less divergent than EELs

• Smaller and faster than EELs

VCSELs vs. EELs

• Edge Emitting Lasers - give out their light from the sides or edges, therefore no pre-cleaving tests can be performed

• Since VCSELs emit light from the top and bottom, they do not have this problem. Testing them is much cheaper

Multimode

• Multimode- light is injected into the core and can travel many paths through the cable (i.e. rattling in a tube).

• Each path is slightly different in length, so the time variance this causes, spreads pulses of data out and limits the bandwidth.

Singlemode

• Fiber has such a narrow core that light takes one path only through the glass.

• Not limited to modal-bandwidth.

• Very small amount of pulse-spreading is consequential only in Gigabit speed applications.

Photodetectors

• Necessary for light pulse detection

• Wide variety of of types – Photoconductors– Avalanche photodiodes– PIN photodiodes– MSM photodiodes

Avalanche Photodiodes

• Exemplify the “gain-bandwidth” tradeoff

• Use the p-n junction model to operate

• Take advantage of the avalanche effect– Carrier multiplication– Associated gain– Time constant associated with avalanche– Bandwidth penalty

PIN Photodiode

• PIN– Reason for name– Doped region, undoped region, doped region– Unity gain– Functions under reverse bias

• Most important parameter for operation– Transit time

Bandwidth vs. Depletion Width

• Transit time– Time for subatomic

particle to get from one electrode to the other

• Based on quickest, typically electron– e- mobility > h+ mobility

• Capacitance limited

Transit Time (continued)

• Dependence on intrinsic region length

• Minimizing this region

• High bandwidth applications

MSM Photodiode

• Metal-Semiconductor-Metal– Associated work functions– Atomic level metal-semiconductor marriage

• High speed (up to 100GHz)

• Majority carrier devices

• Not developed for Gigabit Ethernet on scale as large as PIN

II: Design OverviewKeeping track of Amps & Watts

D e s ig n S p e c if ica tio ns (L o ss le ss S ys te m )

T X D C Bias = 0 -300m AM odula t ion C u rren t = 30m A

R X D C B ias = 80 uA

T X & RXD C B ia s

D C Bias =1 .2 X Ith < T X D C Bias

P ow er Em it ted =D C Bias X S lope

V C S E LT h re sh o ld Cu rre n t (Ith )S lo p e E ff ic ie n cy (m W /m A )

C urrent into R X =P ow er E m it ted X R espons iv ity

P DR e sp o n s iv ity

W a ve len g th 8 50 nmS C C on n e cto rized

Signal Specification Overview

Honeywell VCSELs

• HFE4380-521

• Slope Efficiency 0.04 mW/mA

• I threshold - 1.5 - 6 mA

• HFE4384-522

• Slope Efficiency 0.15 mW/mA

• I theshold - 1.5 - 6mA

Link Budget

• Analysis of current and power throughout the system, starting at the TX and ending at the RX or trans-impedance amplifier.

• Losses– Not all of the light emitted by the VCSEL will reach

the PD.– Losses are incurred from the fiber and the SC

connectors.

At the PD Side

• Once the losses have been calculated into the output power range, this new range of power is to be converted back into current.

• When the power or light hits the PD, it is multiplied by the responsivity of the PD, expressed in A/W.

• This value is the current coming out of the PD and into the trans-impedance amp.

At the RX Side

• The current coming out of the PD has to be large enough to drive the trans-impedance amp, which takes at least 80 uA.

Honeywell Option #1

• Ith = 6 mA• DC bias of laser = 6 (1.2) = 7.2 mA• Slope efficiency 0.04 mW/mA• Power output at DC bias = 0.04 * 7.2 = 0.288 mW • Max TX modulation current. 300 mA• Power output at TX modulation current = 04 * 30 = 1.2 mW• Range of emission of light coming from the laser (lossless) =0.288 - 1.2 mW• Losses 3dB or 1/2 output power• Range of emission of light coming from the laser (with losses)=0.144 - 0.6 mW• Responsivity of lasermate's PD = 0.35 A/W• Min. current from PD = .000144 W * 0.35 A/W= 50.4 micro Amps• Max current from PD = .0006 W * 0.35 A/W = 210 micro Amps

• Table 1. Link budget for low slope efficiency VCSEL (HFE4380-521).

Honeywell Option #2

• Ith 6 mA• DC bias of laser = 6 (1.2) =7.2 mA• Slope efficiency 0.15mW/mA • Power output at DC bias = 7.2 * 0.15 =1.08 mW• Max TX modulation current. 300 mA• Power output at TX modulation current = .15 * 30 = 4.5 mW• Range of emission of light coming from the laser (lossless) =1.08 - 4.5 mW• Losses 3dB or 1/2 output power• Range of emission of light coming from the laser (with losses)=0.54 - 2.25 mW• Responsivity of lasermate's PD = 0.35 A/W• Min. current from PD = 0.00054 W * 0.35 A/W = 189 micro Amps• Max current from PD = 0.00225 W * 0.35 A/W= 787 micro Amps

• Table 2. Link budget for high slope efficiency VCSEL (HFE4384-522).

In Conclusion: Purchasing Choice

• Prefer VCSEL with higher slope efficiency because it can drive the trans-impedance amplifier. More importantly, it can do this with the same amount of input current that is needed for the other VCSEL.

• If the price difference is not too significant, this VCSEL is the most reliable option.

III: Construction and Testing

• Interface optical components with Maxim

• Extensive testing to meet standards

• Develop more optimal design utilizing newly ordered VCSEL and PD

• Repeat testing procedures

Circuit Layout for PD

• Optical connection on top, (SC - Light In). Electrical connections on left and right (SMA)

• R=1/(2*pi*f*CDET), where CDET = 1.5 pF (from spec. sheet). So, R = 53.1Ω.(impedance of PD).

• Capacitors were chosen based on their frequency response at 1.25 GHz. From Murata’s site: C = .01uF.

Circuit Layout for the VCSEL• Electrical connections on the left (SMA - In),

optical connections on the right (SC - Light Out).

• Constraint: Circuit’s RTOT has to be 50Ω, due to equipment requirements.

• VCSEL’s RTYPICAL=25Ω (from spec.sheet)

• So, a 25Ω resistor was placed in series with the VCSEL.

Constructing the VCSEL Board

• Radio Shack purchased “through” boards• SMA connectors, SMT components, and

VCSELs• Impracticalities of initial design• Remedy for small holes and lack of a drill• Problem: GTS-1250’s outputs are AC-

coupled• Bias-T applied and shown in new circuit

VCSEL Testing Procedure

• GTS-1250

• New VCSEL board

• Old opto-board

• Scope

• Electrical connections and optical loop

Testing Results

• IEEE 802.3z standard mask application

• Bit error rate settings in scope

• New VCSEL vs. old VCSEL

• Failure of complete Tx module thus far

VCSEL PCB Design

• Five components:

- SMT Resistor,

- LED,

- Power Connector, - 2K ohms Resistor,

- SMA Connector

VCSEL PCB Design (Con’t)

• SMT Resistor – 0.1 inch

• LED – Two Leads

• 2K Ohms Resistor – 0.5 inch

• Power Connector – hole 0.2 x 0.1 inch

The Complete PCB Design(Left: PD Circuit, Right: VCSEL Circuit)

PCB Dimensions

• SMA Leads – 0.045”X0.045”, Pad - .09”X0.09”. (given in the spec sheet)

• VCSEL and PD holes – 0.02”X0.02”

• Power Connectors – 0.125”X0.125”, Pad – 0.25”X0.25”

• SMT – 0.01”

• Resistor – Template in Library

The Completed PCBsTop Layer and Bottom Layer

Adjustment to the PCBThe SMA hole diameter was increase to 0.06-7” by using a

razor

Optimizing Circuit Design

• Minimizing inductors and component quantities in general

• Utilizing existing schematics and general EE knowledge

• Soldering followed by repeated testing

Intel/Agilent PC Interface Card

IV: Construction: Photodetector(Connectorized)

• Initial “through-component” board

• Connectorized OSI PD

• Standard SMT components

• SMA connector and DC bias via 5V connection

• PD maximum solder temperature of 500oF for total time of ten seconds

V: Testing: Photodectector(Connectorized)

• Lack of detectable eye• Simple signal analysis• 50% duty cycle,

1.25GHz square wave• Signal averaging

functionality of scope• Comparison to input• Weak output (top)

Testing (cont’d)

• Fourier analysis via oscilloscope

• Corresponding frequency concentrations, but apparent degradation

• Averaging functionality in combination with Fourier

• Input and output (bottom) still weakly associated with simple square pattern

Testing (cont’d)

• K28.5 (pseudorandom) bit test using Fourier

• Averaging still turned on

• Further degradation of signal with large amounts of stray artifacts

• Corresponding frequencies still appear to be present

Testing (cont’d)

• Drastic measures to ensure PD board integrity

• Fiber input to PD disconnected and new signal compared

• Small amount of change in Fourier, zoomed in farther

Testing (cont’d)

• Scope (BNC) connection removed

• New signal compared to previous noisy Fourier

• Signal dissipated to nearly zero, proved that scope was not causing all noise

Possible Problems at this Stage (1)

• Ascertained that the noise was coming from PD board itself

• Possibly slow or defective PD, inadequate board or construction

• Process repeated, similar results• Circuit design errors unlikely

VI: Construction: Photodetector(Unconnectorized)

• Initial “through-component” board

• Unconnectorized Hamamatsu PD

• Standard SMT components

• SMA connector and DC bias via 5V connection

• PD maximum solder temperature of 500oF for total time of ten seconds

VII: Testing: Photodetector(Unconnectorized)

• “X-Y-Z” stage setup

– Cleaved fiber (emission)

– SC connector (other end)

• Test setup identical to previous connectorized test sans the “stage”

• Simple square wave input

• Fine adjustment of fiber-vise

• No apparent output

Possible Problems at this Stage (2)

• Previous semesters board tested• Satisfactory on simple pattern, not on

K28.5• Power connector on old board fickle• Possible broken solder joint on capacitor

(VCSEL side)• Incorrect construction unlikely, but

inadequate construction possible

Concluding Remarks

• Insight into opto-electronics gained

• Work experience in team environment

• Soldering, test equipment, and test methodology learned

• Report writing and presentation experience for future job use