The Thorlabs PDB2xx Series of Amplified Balanced Photodetectors are designed for optical measurement applications, offering a robust solution for detecting the difference between two optical input signals. These devices integrate two well-matched photodiodes with an ultra-low noise, high-speed transimpedance amplifier to generate an output voltage (RF OUTPUT) proportional to the difference in photocurrents. Additionally, they feature two fast monitor outputs (MONITOR+ and MONITOR-) to measure individual optical input power levels and low-frequency modulated signals on each detector separately.

Function Description:

The core function of the PDB2xx Series is to provide a balanced detection scheme, effectively suppressing common mode noise and allowing for precise measurements of differential optical signals. This is particularly useful in dual-beam setups where one optical path is for measurement and the other for an invariant reference. By balancing the optical power levels and path lengths, the devices can achieve significant common mode noise reduction, often exceeding 35 dB over their specified frequency range. The RF OUTPUT provides the primary differential signal, while the MONITOR outputs offer insights into the individual input power levels, aiding in alignment and balancing procedures. The PDB2xx Series can operate in both balanced mode (both inputs illuminated) and single detector mode, with the RF OUTPUT polarity depending on the input used in single detector mode.

Important Technical Specifications:

The PDB2xx Series includes several models with varying specifications to suit different application needs:

PDB210A(/M) and PDB220A2(/M): These models operate from DC up to 1 MHz.
- PDB210A(/M): Features Si PIN Photodiodes with a wavelength range of 320 to 1060 nm and a maximum responsivity of 0.6 A/W at 920 nm. The active detector area diameter is 5 mm. Transimpedance gain is 500 x 10³ V/A (High Z load) and 175 x 10³ V/A (50 Ω load). CW Saturation Power is 33 µW at 920 nm. Common Mode Rejection Ratio (CMRR) is typically 40 dB.
- PDB220A2(/M): Utilizes UV enhanced Si PIN Photodiodes, covering a wavelength range of 190 to 1100 nm with a maximum responsivity of 0.5 A/W at 960 nm. The active detector area diameter is 4.1 mm. Transimpedance gain is 500 x 10³ V/A (High Z load) and 175 x 10³ V/A (50 Ω load). CW Saturation Power is 36 µW at 960 nm. CMRR is typically 30 dB.
PDB210C(/M): Operates from DC up to 1 MHz.
- Features InGaAs PIN Photodiodes with a wavelength range of 800 to 1700 nm and a maximum responsivity of 1.0 A/W at 1550 nm. The active detector area diameter is 3 mm. Transimpedance gain is 500 x 10³ V/A (High Z load) and 175 x 10³ V/A (50 Ω load). CW Saturation Power is 20 µW at 1550 nm. CMRR is typically 30 dB.
PDB230A and PDB230C: These models operate from DC up to 100 MHz, offering higher bandwidth.
- PDB230A: Features Si PIN Photodiodes with a wavelength range of 320 to 1000 nm and a maximum responsivity of 0.53 A/W at 820 nm. The active detector area diameter is 0.8 mm. Transimpedance gain is 50 x 10³ V/A (High Z load) and 24.5 x 10³ V/A (50 Ω load). CW Saturation Power is 150 µW at 820 nm. CMRR is typically >30 dB (35 dB typ.).
- PDB230C: Utilizes InGaAs PIN Photodiodes, covering a wavelength range of 800 to 1700 nm with a maximum responsivity of 1.0 A/W at 1550 nm. The active detector area diameter is 0.3 mm. Transimpedance gain is 50 x 10³ V/A (High Z load) and 24.5 x 10³ V/A (50 Ω load). CW Saturation Power is 80 µW at 1550 nm. CMRR is typically >30 dB (40 dB typ.).

All models share a photo diode damage threshold of 20 mW. Electrical outputs are BNC, with 100 Ω impedance for PDB210x/PDB220A2 and 50 Ω for PDB230x. Operating temperature range is 0-40 °C. The devices are powered by an included ±12 V @ 250 mA linear power supply.

Usage Features:

Optical Input: The detectors feature two free-space optical input ports with external SM1 and internal SM05 threads, compatible with Thorlabs SM1- and SM05-threaded accessories. An SM1T1 lens tube coupler is included for mounting external SM1-threaded accessories like optics, filters, apertures, or fiber adapters. For fiber-coupled applications, Thorlabs S120-xx series fiber adapters can be easily mounted.
Mounting: The slim-line aluminum housing has three tapped holes for convenient mounting. PDB210A, PDB210C, and PDB220A2 models come in imperial (8-32) or metric (M4) versions. PDB230A and PDB230C models feature combi-threads compatible with both 8-32 and M4 posts.
Output Signals: The RF OUTPUT delivers the differential signal. MONITOR+ and MONITOR- outputs provide individual detector signals. All outputs are BNC connectors.
AC Coupling: AC-coupled versions are available upon request, which block the CW component of the optical input signal. This is beneficial for measuring weak frequency-modulated signals on a strong CW background and for use with choppers and lock-in amplifiers to further reduce noise.
Power Supply: The units are powered by an external linear power supply (LDS12B) via a PICO M8 power connector. A status LED indicates correct power operation.
Recommendations for Optimal Performance:
- Equal power levels and power densities on both photodetectors are crucial for maximum common mode noise suppression.
- Illuminating the entire active area of the detectors prevents nonlinearities.
- Equal optical path lengths are critical, especially at higher frequencies, to avoid phase differences that degrade noise reduction.
- Avoiding etalon effects and carefully shielding the detectors from unwanted stray light are essential.
- Electrostatic coupling of electrical noise from ground loops can be mitigated by using electrically isolated posts.

Maintenance Features:

General Care: The PDB2xx Series should be protected from adverse weather conditions as they are not water-resistant.
Cleaning: The housing can be cleaned with a mild detergent and a damp cloth. For cleaning the photodetector windows, gently blow off dust with compressed air and wipe gently with an optic tissue moistened with isopropanol or alcohol.
Handling: Extreme care must be taken when connecting fibers to the optical input ports to avoid damaging the protective diode glass. The damage threshold of the photodiodes is 20 mW; exceeding this value will permanently destroy the detector.
Service: The unit does not require regular user maintenance and does not contain user-serviceable components. In case of malfunction, contact Thorlabs for return instructions. Do not remove covers.
Packaging: For servicing or returns, the device must be properly packed in its complete original packaging, including plastic foam sleeves.

THORLABS PDB210A - Page 26

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