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1.2 to 2.6 µm Balanced Detector Prototype
What Features / Specifications Do You Need?


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Summary
The prototype Balanced Amplified Photodetector consists of two well-matched InGaAs photodiodes and an ultra-low noise, high-speed transimpedence amplifier (TIA) that generates an output voltage (RF output) proportional to the difference between the photocurrents in the two photodiodes, (i.e., the two optical input signals). Additionally, the unit has two monitor outputs to observe the optical input power levels on each photodiode separately. These outputs cannot be used to measure an RF modulation on the signal because the bandwidth is limited to 100 kHz.
Initial Prototype Specifications   Preferred Values
•Ø 1 mm Active Area (Each Photodiode)
•1.2 -2.6 μm Wavelength Range  
•2 MHz Bandwidth (Amplified Difference Signal)  
•Transimpedence Gain    
  •High Impedance (Z): 420x10³ V/A  
  •50 Ω Impedance (Z): 146x10³ V/A  
•Monitor Bandwidth: 100 kHz  
•Common Mode Rejection Ratio (CMRR): 30 dB  
•NEP (DC-2 MHz): 11 pW/√Hz  
What Package Style Do You Prefer?
The prototype was built in the housing of our existing free-space balanced detectors (see drawing above). The benefits of this package are its slim profile and the ability to position both detectors in the same plane. Since the geometrical center of the two photodiodes are 2˜ apart, the two beams paths of the incident light are well separated. An alternative design would be to make the detector housing more cube-like and have the detectors on two adjacent faces of the cube. Without further feedback, it is likely that we will finish designing the product using our existing balanced detector housing. Please use the comment field to discuss alternatives, including the possibility of switching to a fiber-coupled design.
Detectors Housing Design: Slim Cube
What Test Measurements Would you Like Made?
Responsivity of the InGaAs Photodiodes
The balanced detector was tested to determine the bandwidth of the balanced detector output. The black and red curves are the result of only one of the photodiodes being illuminated while the green curve shows the balanced detector’s output when both photodiodes are illuminated. The bandwidth of the amplified balanced output is ~2 MHz.

The Common Mode Rejection Ratio (CMRR) is a measure of the effectiveness of the subtraction of the two input signals.
CMRR = 20 · log((Vout1 - Vout2)/Vout1)

The output of the balanced detector when no light was incident on either photodetector was measured with an electrical spectrum analyzer (red curve). The black curve is the noise floor of the electrical spectrum analyzer, which was measured by removing the detector from the analyzer and replacing it with a 50 Ω terminator. Subtracting the two curves provides spectral distribution of the noise from which the NEP of the detector was calculated. The resolution bandwidth (RBW) of the electrical analyzer was set to 10 kHz during these measurements.
How Should the Advanced Design Issues Be Resolved?
1) There is a tradeoff between bandwidth of the monitor outputs, size of the photodiodes, and the gain flatness because the monitor signals are measured across the bias voltage resistor with capacitive grounding on the detector side for good RF performance (balanced output). The grounding capacitor limits the monitor bandwidth. While reducing the grounding capacitance increases monitor bandwidth, it also adversely affects the flatness of the frequency response (not equivalent to gain peaking). Alternatively, the capacitance of the photodiodes can be reduced by using a photodiode with a smaller active area, which allows for a smaller grounding capacitor without compromising the flat gain response.
2) Due to the low shunt resistance of the FIR detectors, there is a large dark current that makes the offset adjustment of both the monitor and balanced outputs challenging. In addition, the dark current is strongly temperature dependent. Two possible solutions are to cool the detector with a TEC element or to reduce the transimpedence gain, which will reduce the offset drift. Adding a TEC element with a controller that will keep the package at a constant temperature will add approximately $1,000 to the cost of the detector. One additional option is to reduce the active area of the detector. A smaller active area results in lower dark current, lower noise, and higher bandwidth.
How Will You Use the Detector?
When designing a product at Thorlabs, we try to gather as much information about the applications in which the product is being used in order to anticipate the features that the product needs to have in order to be useful in the lab. Please take a couple of minutes to provide some information about the application you had in mind for this detector and what features are required for this product in order for it to be useful. If you would prefer that we directly contact you, please fill out the email field, which will not be displayed on the forum page.
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