Physics 3330 Experiment #6 Fall 2006 Photometer and Optical Link Purpose You will design and build a photometer (optical detector) based on a silicon photodiode and a current-to-voltage amplifier whose output is proportional to the intensity of incident light. First, you will use it to measure the room light intensity. Then you will set up and investigate an optical communication link in which the transmitter is a light emitting diode (LED) and the receiver is your photodiode detector. Introduction Experiment 6 demonstrates the use of the photodiode, a special p-n junction in reverse bias used as a detector of light. The incoming radiation energy excites electrons across the silicon band gap, producing a current or a pulse of charge proportional to the incident energy deposited in the detector. In this experiment we will introduce a number of "photometric" quantities that are widely used in opto-electronics. At the end of the lab, you will use a lock-in amplifier to extract a weak signal from noise. Readings For general background on opto-electronics, see H&H Section 9.10. All of the detailed information you will need for this experiment is given below. Data Sheets for the PD204-6C silicon photodiode and the MV5752 GaAsP light emitting diode are available at the course web site. The manual for the SR510 lock-in amplifier is also posted at the web site. New Apparatus and Methods PHOTODIODE The PD204 photodiode used in this experiment is a p-intrinsic-n (PIN) silicon diode operated in reverse bias. A sketch if the photodiode structure is shown in Figure 6.1. The very thin p-type conducting layer acts as a window to admit light into the crystal. The reverse bias voltage maintains a strong electric field throughout the intrinsic region forming an extended depletion layer. The depletion layer should be thicker than the absorption length for photons in silicon in order to maximize the efficiency. Any incident photon whose energy exceeds the band-gap energy is absorbed to produce an electron-hole pair by photoelectric excitation of a valence electron into the Experiment #6 6.1 Fall 2006conduction band. The charge carriers are swept out of the crystal by the internal electric field to appear as a photocurrent at the terminals. The photocurrent is proportional to light intensity over a range of more than 6 orders of magnitude. Figure 6.1 Structure of the photodiode.+incident light+–p-type layern-type layer intrinsic region photocurrent–electron-hole pair LIGHT EMITTING DIODE The MV5752 light emitting diode acts electrically just like any diode. It emits light when forward-biased due to direct radiative recombination of electrons and holes. The forward voltage drop is about 1.7 V rather than 0.6 V because the LED is made of GaAsP instead of silicon. LOCK-IN AMPLIFIER The lock-in amplifier is a highly sensitive AC voltmeter which can detect sinusoidal signals as small as a nanovolt, even in the presence of strong interference. You must provide it with a reference sine or square wave that is coherent with the signal you want to detect (same frequency and constant phase difference). The lock-in then averages away everything in the signal except what is coherent with the reference wave. After sufficient averaging time, interfering signals will be averaged away and the desired signal will remain. The display on the lock-in reads the rms voltage of the desired signal. A very simple example of a lock-in is described in H&H Section 15.15. You can read more about the lock-ins we have in the lab (Stanford Research SR510) in the manual which is posted on our course web site. Experiment #6 6.2 Fall 2006Theory CURRENT-TO-VOLTAGE AMPLIFIER In an ordinary inverting amplifier (Exp. 4, Figure 4.3) the input voltage is applied to a resistor, and the amplifier generates an output voltage in response to the current that flows through the input resistor to the virtual ground at the negative op-amp input. A current-to-voltage amplifier (Figure 6.2) is an inverting amplifier with the input current Iin applied directly to the negative op-amp input. Since no current flows into the op-amp input, the output voltage must be Vout=–IinRF. The ideal low-frequency gain of a current-to-voltage amplifier is G =VoutIin=−RF. (1) This gain has the units of impedance, and it is often called a trans-impedance. The current-to-voltage amplifier is sometimes called a trans-impedance amplifier. 0 VFigure 6.2 Light transmitter and photodiode detector. The trim pot between pins 1 and 5 can be adjustedto compensate for any output voltage offset in the op-amp.+–356 VoutC F R F –15 V+15 V1 M237 6 4+ RTFunction GeneratorPD204R S Function GeneratorTransmitterDetectorMV57521N4002Iin In our photometer circuit the current Iin flows through the back-biased photodiode when it is illuminated (its sign is negative, so it actually flows out of the op-amp negative input node and the resulting Vout is positive). The 1 MΩ resistor is used to inject a test current, and the feedback capacitor enhances stability. Experiment #6 6.3 Fall 2006PHOTODIODE SENSITIVITY The photodiode sensitivity Sλ (in units of μA/(mW/cm2)) is defined as the photocurrent per unit light intensity incident on the photodiode. It is a function of the light wavelength λ. Thus for light intensity N(in mW/cm2) the photocurrent I (in μA) is given by I = Sλ (2) N.The sensitivity at any wavelength λ is given on the data sheet in terms of the peak sensitivity at 940 nm times a correction factor called the relative spectral sensitivity, or RSR: (3) λS).(nm 940λRSRR= The peak sensitivity may be taken from data-sheet information on the ‘reverse light current.’ LED OUTPUT To describe the output of a light source like our photodiode, it is helpful to introduce the notion of solid angle. Consider a transparent sphere of radius r, and suppose that an area A on the surface of the sphere is painted black. We then say that the blacked out region subtends a solid angle of Ω steradians (str), where Ω = A/r2. According to this definition the whole sphere subtends a solid angle of 4π str. One steradian is an area of r2, just as one radian is an arc of length r. The concept of solid angle is essential in separating the two units in which light is customarily measured. Both the lumen and the candela originated in the 18th century when the eye was the primary detector of electromagnetic