University of Massachusetts Lowell
The Department of
Electrical and Computer
Engineering
Subject:
Introduction to Biosensors
Wireless Optical Sensor Network
Project Report Two
Team members: Ravi G Bhatia
Wenchan Tong
Aiken Pang
Report Submit Date: 11th April, 2007
Project Overview:
Fig 1. Wireless Sensor Project
This project is about building a wireless sensor that transmits information obtained from
1550nm wavelength Fiber optic cavity wirelessly using a photodetector and a wireless
transceiver module to a computer for processing Biomolecular interactions.
Sensor Head is an optical cavity such as a Fabry Perot cavity. Biomolecular interaction
changes optical path length. Intensity change is proportional to optical path length and
as the path length changes photodetector will detect change in intensity and transmit
that information through the wireless module.
Fig2 Light Intensity before (original) and after biomolecular interaction
Progress Made:
We have purchased a photodetector for our project. It is a InGaAs pin photodetector
with responsivity from 800nm to 1700nm sitable for our project. The photodetector has
a fiber pigtail connector for directly connecting with the fiber sensor tip. Cost of the
detector is $269.
Fig3: Spectral reponse of DET01FC InGaAs pin photodetector. Reference:
“DET01CFC Operating Manual — Fiber Input InGaAs Photodetector”
Photodetector Circuitry
3.
Fig4: schematic of the photodetector circuit.
When light of intensity P is incident on the photodetector, an output voltage
appears across the load as shown in fig 2 which is proportional to the intensity of
incident light give by.
Voltage=P * Responsivity * RL
The Responsivity and RL are constants for a given circuit.
Responsivity = 0.95 A/W (from datasheet)
RL = 50 ohms (recommended by Thorlabs; Ref. datasheet DET01FC)
Therefore,
The output voltage in our circuit is equal to
Voltage = 0.95 * 50 *Intensity = 47.5 * Intensity
The voltage across the load resistor is coupled to the wireless module circuit which
converts it into digital signal and transmits the signal to a computer for detecting
whether the biomolecular interaction has taken place. But first, The wireless module
receiver has to be modified to accept the voltage corresponding to a particular light
intensity and Convert it into appropriate digital data via the built in ADC. Next the
computer software that will decode data and process whether interaction has taken
place has to be calibrated for identifying light intensity before the interaction. And the
expected intensity change after interaction. Other information such as the extent of the
intensity change which might give some information about the concentration of the
biomolecules might also have to be programmed into the system
Experimental Setup:
Initial measurements were carried out using a 1550nm Tunable Laser Source as
shown in Fig 5. Laser light of -3dbm power is coupled to optical fiber cable onto an
optical cavity show in Fig 6. Reflections form the optical cavity are detected using
the purchased photodetector. Biomolecular interaction is simulated by reducing the
length of the cavity so the the intensity of light detected by the photodetector
changes.
Fig. 5 1550nm Tunable Laser Source
Fig. 6 Optical Cavity for simulating Biomolecular Interaction
Experimental observations
In this initial experiment, light reflected form the optical cavity and detected by the
purchased detector was observed on digital oscilloscope. For a Load resistance of
20KΩ, the following output was observed as shown in Fig 7 and Fig 8. No output
was obtained for a 50Ω resistance recommended by Thorlabs due to the very weak
intensity light reflected by optical cavity.
Fig. 7 Output before simulated interaction
Fig. 8 Output after simulated interaction
Experimental Results
Results of this initil experiment are tabulated below.
Incident Power
-3dbm or 0.5mW @ λ=1550nm
Photodetector output
44mV
Before Biomolecular interaction
(simulated)
Photodetector output
101mV
After Biomolecular interaction
(simulated)
Voltage change
57mV
Table 1: Experimental Results
Observations
Output voltage is noisy and fluctuates 20-30mV. Noise is superimposed in the signal
due vibrations in the cavity or due high load resistance – 20K Ω. Recommended
resistance is 50 ohms but photodetector output is too weak to detect with 50ohms.
This setup cannot be used to detect weak intensity light and transmit information reliably
over the network. One possible solution of interest is to use a Transimpedance
Amplifier which will amplify the photodetector signal and give better noise performance.
Power amplifier
Since the range of output voltage of photodetector is 20-30mv which is not large enough
to convert by ADC, a power amplifier is needed to amplify
input output
the dynamic range of the output voltage.
min
20 mv
1v
max
30 mv
2v
The maximum input of ADC is 2.5v, so the desirable
dynamic range of the output of amplifier is 1v-2v.
Operational amplifier
Among varieties of amplifiers, operational amplifier is a solid state integrated circuit
amplifier which employs external feedback for control of its transfer function or gain.
741 chip
741 is a classic integrated circuit operational amplifier chip, made by virtually all IC
manufacturers. It is one of the most well know examples of the most versatile electronic
building blocks ever created.
Because of its good performance, stable function, easily used and cheap price, 741 is
used in this project as the power amplifier.
Circuit of amplifier
V+ = Vin
V− = K Vout
Vout = G(Vin − K Vout)
Vout/Vin = G /(1 + G K)
If G is very large, Vout/Vin comes close to 1 + (R2/R1)
Parameter: R2=2KΩ , R1=1KΩ
So that we can get the gain close to 3
Problems and solutions
1. the fiber needed was broken in the lab
2. output signal of photodetector cannot be the one need for the next process of the
project:
-The output signal of photodetector below 1V
-The signal is DC plus AC rather than just only DC needed
For the first problem, an optical fiber cable is made to use.
For the second problem, three methods have been tried.
1) Multi-meter: still cannot solve the problem of DC plus AC
2) Analog oscilloscope: has no function of sampling
3) Digital oscilloscope: can solve both problems
Wireless Sensor Network (WSN) module
ADC
In the WSN module, the ADC is a Sample and Hold 12 bits converter. Sample
and hold type ADC can provide faster sampling rate then other types (e.g. SigmaDelta). The sampling speed in our module can be 6.5MHz/17 = 382ksps.
We use the internal reference voltage for the ADC. The reference voltage can be
1.5V or 2.5V. We will use 2.5V for the prototype. The resolution is equal to (2.5v)/ (2^12)
= 0.61mV. NADC is the output digital value of the ADC.
Software platform
TinyOS was initially developed by the U.C. Berkeley EECS Department. TinyOS
is an event based operating system. Tradition OS is using time base. For example
Window is a time base. Event based mean every operation in the system is base on
event. Events in the WSN are send data, request sensor data.
This kind of design can support networked sensors with minimum hardware
implementation (fewer timers).
Programming Language
The language using in TinyOS is “NesC”. It likes C. The NesC has a custom
complier. It is used to translate Nesc to C.
Timetable
WEEK
TASK PLAN
March 21 - 27
Simulate Biosensor operation for obtaining intensity change
March 28 - 31
Finding a correct Photodetector
April
2-6
Order InGaAs APD
April
9 - 11
Testing InGaAs APD under simulation of Biosensor spec.
April
12 - 15
Circuit design, Programming wireless module
April
16 - 21
Hardware Assembly and Testing
We did fine-tune the timetable because we got the InGaAs APD on 4/6/07(not
3/25/07). We swap the time for simulate biosensor operation with Order InGaAs
APD.
Budge
Wireless sensor node: US$100 each (we need at least two)
Photosensor: US$265 each
Total cost will be $365