Features

• Optical Gain Control and Auto-Balance Function
• Avalanche Photodiodes (APDs) Excellent for Low-Input-Power Applications
• Optimized for 1300 nm
• Operating Wavelength Range: 1200 - 1700 nm
• Wide DC to 400 MHz Bandwidth
• Exceptionally Low NEP: 1.0 pW/√Hz (DC - 100 MHz)
• >35 dB Common Mode Rejection Ratio (Typical)
• <0.4 mV_RMS Output Voltage Noise
• Fiber-Coupled FC/APC Inputs
• Switchable Universal Power Supply Included

Thorlabs' PDB570C Avalanche-Photodiode-Based Balanced Detector is a balanced receiver that subtracts the two input signals from each other, resulting in the cancellation of common mode noise. The balanced detector, which uses two well-matched avalanche photodiodes (APDs) and an ultra-low-noise, ultra-low-distortion, high-speed transimpedance amplifier, is optimized for applications with low optical input powers (minimum NEP of 1.0 pW/√Hz). Each avalanche photodiode is fiber coupled to the FC/APC outputs via length-matched SMF-28e+ fibers so as not to introduce optical path differences between the input and the active area of the photodiode. Compared to our other balanced detectors that use Si or InGaAs PIN photodiodes, the PDB570C offers unique features such as exceptionally low NEP, optical gain control, and an auto-balancing function with a variable closed-loop control velocity.

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The mating sleeve can be removed to access the FC/APC ferrules for cleaning.

This balanced detector features an adjustable gain and three different detection modes: single detector, balanced (BAL), and auto-balanced (AUTO-BAL). The single detector mode allows the output of each photodiode to be measured individually. In the BAL mode, the PDB570C functions as a standard balanced detector while allowing the optical gain of both photodiodes to be simultaneously adjusted. In the AUTO-BAL mode, the balanced detector compensates for power differences between the two optical input signals that vary at a slower rate than a cutoff frequency set via the CONTROL VELOCITY knob. This power difference can be monitored via the CONTROL OUTPUT and will not contribute to the voltage generated by the balanced detector at the RF output. For details on how to operate the PDB570C, please see the Detector Operation tab.

The included post-mounting adapter plate has both 8-32 and M4 taps and can be attached to either the bottom or back of the PDB570C's shielded aluminum housing with the included M2 x 8 screws. The PDB570C's compact housing is 95 mm x 80 mm x 44.3 mm (3.74" x 3.15" x 1.74"), including the depth of the control knobs. Each detector is shipped with a ±12 V DC power supply with an input voltage of 115 V or 230 V that can be selected by a switch. A replacement power supply that supports input voltages of 100, 120, and 230 V is available below.

The exceptionally low noise of the PDB570C APD-based auto-balanced detector makes it suitable for OCT applications. The full range of our PIN photodiode-based balanced detectors that can be used for OCT applications are found here. Thorlabs also offers Fiber-Based Interferometers, which feature an integrated balanced detector.

While the PDB570C is intended for fiber-coupled applications, a free-space beam can be coupled into the FC/APC inputs via a pigtailed aspheric collimator. Care should be taken to length-match the fibers between the collimator and the inputs in an attempt to avoid introducing unwanted path length differences into the system. Thorlabs also offers balanced detectors designed for free-space applications. Additionally, some of our fiber-coupled balanced detectors with fast monitor outputs have removable FC/APC detectors for easy integration into free-space applications.

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A functional diagram of the PDB570C. The transimpedance amplifier generates an output voltage proportional to the difference between the photocurrents of the two photodiodes. The gain of the detector and cut-off frequency for the auto-balance feature can be set by the user.

Thorlabs' PDB570C Balanced Detector with Avalanche Photodiodes features an adjustable gain and three different detection modes: single detector, balanced (BAL), and auto-balanced (AUTO-BAL). The single detector mode allows the output of each photodiode to be measured individually. In this mode, sources can be connected to both INPUT+ and INPUT-, while the RF OUTPUT knob can be used to select which input you wish to monitor. The optical gain control allows the RF OUTPUT signal level to be adjusted. This feature is helpful when setting your experiment to provide equal input power levels to each APD prior to continuing with measurements in the balanced or auto-balanced detection modes.

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Front View of the PDB570C APD-Based Balanced Detector

In the BAL mode, the PDB570C functions as a standard balanced detector while allowing the optical gain of both photodiodes to be simultaneously adjusted. The output signal generated by the difference between the two input signals can then be measured via the RF OUTPUT connector.

In the AUTO-BAL mode, the optical gain control only affects INPUT-, while the gain for INPUT+ is automatically adjusted via a power-balance control loop to match the current generated by INPUT- (see the figure to the right). This circuit will compensate for power differences between the two optical input signals that vary at a slower rate than a cutoff frequency set via the CONTROL VELOCITY knob. Differences between the two input signals that vary slower than this frequency will not contribute to the voltage generated at RF OUTPUT. The CONTROL OUTPUT in auto-balance mode provides a measure of this rejected signal (i.e., the constant or slowly varying power imbalance between the two input optical signals). Therefore, the control circuit effectively functions as a lowpass filter for the CONTROL OUTPUT.

A second lowpass filter serves to remove noise from the output of the transimpedance amplifier before it is read out at RF OUTPUT. The CONTROL VELOCITY knob is not marked with graduated bandwidth values, as the control loop is affected by several factors including the optical gain, ratio of input power levels, absolute input power level, and the modulation depth of the disturbance signal.