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Photodetector Technical Notes

ET-6000 Technical Notes

Photodetectors

I. Introduction

EOT Photodetectors are suitable for a variety of pulsewidth measurement and pulse profiling applications. With the exception of the ET-6000, ET series photodetectors use PIN photodiodes and a reverse bias. These photodiodes utilize the photoelectric effect to convert light energy into an electrical current. The reverse bias consists of either 3V lithium cell(s) or a wall plug-in power supply, depending on the amount of bias voltage needed and the intended application of the photodetector. All photodetectors contain their own BNC or SMA output connector. Connecting the photodetector to an oscilloscope and terminating into 50Ω at the oscilloscope is all that is required for operation. Most of EOT's photodetectors can be fitted with FC fiber optic connectors for use with fiber pigtailed light sources.

II.  Amplified Photodetectors

EOT's Amplified Photodetectors also utilize PIN photodiodes, however, these photodiodes are used in conjunction with high speed transimpedance amplifiers. These transimpedance amplifiers greatly enhance the sensitivity of the photodetectors, allowing light levels as low as 100nW to be measured. Typical gain in EOT's amplified photodetectors is 26dB. It should be noted that EOT's amplified photodetectors are AC coupled and have a low frequency cutoff of 30kHz.

III.  Applications

A.    Measuring the pulsewidth or viewing the pulse profile of Q-switched lasers.

B.     Monitoring the output of mode-locked lasers.

C.     Viewing the rapid modulation of diode lasers and externally-modulated CW laser pulses.

D.    Beamfinding/alignment of CW and pulsed lasers.

E.     Triggering applications using EOT's TTL Photodetectors, which incorporate all of the features of our biased photodetectors, plus an adjustable threshold ultrafast comparator with a TTL output accessible via a second BNC connector.

F.      Large area photodetectors can be used as power meters by using Ohm's Law to calculate power levels.

Figures 1 and 2 demonstrate some of the applications for which the ET Series photodetectors are used:

              

Figure 1                                                                         Figure 2

IV.  Definitions

Responsivity (A/W): The ratio of photocurrent to a corresponding level of incident light. Responsivity varies with wavelength.

Spectral Responsivity: Responsivity plotted as a function of wavelength.

Rise Time (Tr): The time required for the photodetector output level to change from 10% to 90% of the peak output level.

Fall Time (Tf): The time required for the photodetector output level to change from 90% to 10% of the peak output level.

Frequency Response: The electrical output response to a sinewave modulated light input. This is typically measured in dB vs. Hertz.

Cut Off Frequency: The frequency at which the detector output power decreases by 3dB from the output at 100kHz.

Bandwidth: The difference between the high and low cutoff frequencies, measured in Hertz. The bandwidth of the photodetector is approximately related to the rise time (Tr) by:

           Bandwidth (Hz) ≈ 0.35/Tr

Dark Current: The small current which flows when a reverse voltage is applied to a photodiode and no optical input is present.

Junction Capacitance: An effective capacitor is formed at the P-N junction of a photodiode. The junction capacitance is the major factor in determining the speed of a photodiode.

Reverse Breakdown Voltage: The level of reverse voltage which can cause breakdown and deterioration of the detector.

Noise Equivalent Power (NEP): The amount of incident photon energy equivalent to the intrinsic noise level of the device, providing a signal-to-noise ratio of 1.

           NEP =

Noise Current (A/√Hz)
Responsivity a λp (A/W)

           λp = wavelength of detector's peak responsivity

V.  Laser Power Calculations

Using the photodetector's responsivity at a given wavelength and Ohm's Law; V = IR, the output of EOT's photodetectors can be used to calculate the power of the laser incident on the active area of the detector:

For example, if an ET-2030 is producing an output of 20mV, the laser wavelength is 632.8nm and the detector is terminated into 50Ω, the incident power can be derived as follows:

           I = 0.02V/50Ω, or I = 0.0004A

From the responsivity curve contained in the Silicon Photodetector data sheet, the responsivity of the ET-2030 at 632.8nm is 0.4A/W. Therefore:

           0.0004A ÷ 0.4A/W = 1mW of incident power

Note: This is the amount of power incident on the detector, not necessarily the actual power of the beam as not all of the beam may be incident on the detector.

VI.  Schematics

  1. Schematic of Electrical Circuit for <2GHz Biased Silicon and InGaAs Photodetectors
     
     
  2. Schematic of Electrical Circuit for >12.5GHz Photodetectors

ET-6000 Application Notes

The ET-6000 consists of a lead selenide (PbSe) photoconductor and an amplifier with selectable gain.

A photoconductor (PC) increases in conductance with an increase in optical power.  Because the photoconductor does not generate an output current like our PIN photodiodes, it is placed in a voltage divider to generate a voltage output:  a change in PC conductance will produce a change in the divider’s output voltage.  This voltage is then amplified as shown in the figure below: 

Photoconductors use a figure of merit called Detectivity (D*):

Detectivity is used to compare the sensitivity of photoconductors for a given wavelength range and temperature in order to select the appropriate device for an application; the higher the detectivity the more sensitive the device.  The above equation normalizes active area and NEP so different size device can be compared.  For example, a larger sensor has the advantage of potentially improved photon collection, but this advantage may be offset by a larger NEP – it might not be the best device for the application.

The ET-6000 output incorporates the photoconductor and amplifier parameters into an overall responsivity in volts per watts.  The data sheet contains a graph of responsivity vs. wavelength for each gain setting.  For example, if 10μW is incident on the detector at 4.0um, the output of the ET-6000 will be 10μW x 6,400V/W = 64mV for a gain of 2 and 10μW x 320,000V/W = 3.2V for a gain of 100.  The maximum optical input power density is 10μW/mm².

In order to overcome the thermal noise inherent in mid-IR sensors an optical chopper and lock-in amplifier are recommended.  This system helps reduce flicker noise and noise proportional to the square root of the measurement bandwidth.  Flicker noise is low frequency noise proportional to the inverse of frequency (1/f): the higher the chopping frequency the lower the effect of flicker noise; EOT recommends a chopping frequency around 1,000 Hz.  The chopping frequency is synchronized with the lock-in amplifier, so the amplifier “locks in” on the chopping frequency and filters out noise not at 1,000 Hz and within the lock-in amplifier’s measurement bandwidth; as noise is proportional to the square root of the measurement bandwidth, the bandwidth should be kept as small as possible.  The lock-in amplifier will output put a value on a display, or a voltage, proportional to the optical input multiplied by the responsivity of the detector, multiplied by the gain of the lock-in amplifier.

Recommended chopper systems: 

  • Dual phase
  • 60 dB or greater dynamic range
  • Manufacturers:  Scitec, Stanford Research Systems