Free Space Inputs Available on Some Detectors (See Below for Details)
Switchable Power Supply Included
Switchable Transimpedance Gain Version Available
These balanced photodetectors act as a balanced receiver by subtracting the two optical input signals from each other, resulting in the cancellation of common mode noise. This allows small changes in the signal path to be extracted from the interfering noise floor.
The detectors consist of two balanced photodiodes and an ultra-low-noise, high-speed transimpedance amplifier. The two photodiodes are matched to achieve an excellent common mode rejection ratio (CMRR), leading to better noise reduction. Please see the Operation tab for more details. All of these detectors incorporate three female SMA electrical ports for reduced noise. In addition to the RF-output from the transimpedance amplifier, the Monitor+ and Monitor- ports allow the response of each photodiode to be observed individually.
To block the CW component (the unmodulated part) of the optical input signal, an AC-coupled version of each detector is offered. Note that the PDB480C-AC, PDB481C-AC, and PDB482C-AC are only available AC coupled. This improves the ability to measure a comparably weak, frequency-modulated signal over a strong CW background signal that could saturate the amplifier. The lower cut-off frequency for the AC-coupled versions, with exception to the PDB480C-AC, PDB481C-AC, and PDB482C-AC, is guaranteed to be ≤100 Hz, but will typically be below 5 Hz.
Bandwidths of DC - 350 MHz or Lower Detectors with bandwidths of DC - 350 MHz or lower are available with either Si or InGaAs photodiodes for the 320 - 1000 nm or 800 - 1700 nm range, respectively. The inputs on these detectors are not fiber coupled to the photodiodes and come with two removable FC input connectors, making them suitable for either free space or fiber applications. Although the PDB465 and PDB435 detectors also contain non-fiber-coupled sensors, the FC/PC adapters at the inputs cannot be removed.
Bandwidths of DC - 400 MHz or Higher Detectors with bandwidths of DC - 400 MHz or higher feature fiber-coupled photodiodes connected to FC/APC optical inputs with exactly length-matched SMF-28e+ or HI1060 fiber to achieve excellent CMRR values across the full detector bandwidth. All of these detectors incorporate InGaAs photodiodes and are optimized for either 1060 nm (900 - 1400 nm range) or 1300 nm (1200 - 1700 nm range). The fiber-coupled design suppresses line artifacts in the OCT image, which generally occur when detector coupling optics are used.
Our highest bandwidth balanced detectors (PDB480C-AC, PDB481C-AC, and PDB482C-AC) are offered in an AC version only. They offer bandwidths up to 2.5 GHz, leading to considerable speed improvements. The ultra-low-distortion output stage supports up to a 2 Vp-p A/D card input range, which, combined with the fiber-coupled design, improves the image quality for OCT applications considerably.
Packaging/Power Supply Housed in a shielded aluminum enclosure measuring 85 mm x 80 mm x 30 mm, these detectors are post mountable using the included adapter plate, which can be attached to the bottom or side of the housing with the included M2 screws. The unit is powered with the provided ±12 V DC power supply, for which a replacement is available below.
Thorlabs also offers Fiber-Based Interferometers, which feature an integrated balanced detector. If you are primarily interested in using our balanced detectors with OCT systems, a condensed list of detectors suitable for this application can be found here.
320 - 1000 nm
800 - 1700 nm
320 - 1000 nm
800 - 1700 nm
320 - 1000 nm
800 - 1700 nm
Max Responsivity (Typical)
Active Detector Diameter
Bandwidth (3 dB)
DC - 100 MHz AC Coupled (-AC Suffix): 100 Hz - 100 MHz
DC - 75 MHz AC Coupled (-AC Suffix): 100 Hz - 75 MHz
DC - 350 MHz AC Coupled (-AC Suffix): 100 Hz - 350 MHz
Note: For the PDB480C-AC, PDB481C-AC, and PDB482C-AC detectors the RF output signal must not exceed the RF Output Power at 1 dB Compression, which is the point at which the amplified signal at 1 GHz is compressed by 1 dB. Above this value, the amplified signal will become non-linear and begin to saturate. The RF output voltage at this saturation point can be calculated using the following formula:
where R is the load impedance (50 Ω), P0 is defined as 1 mW, and L(dBm) is the power level in dBm. Using the specified RF Output Power at 1 dB Compression of 16.5 dBm, this yields the following "maximum" output voltage:
Operation of Balanced Detectors with Bandwidths up to 400 MHz
Thorlabs' Balanced Amplified Photodetectors consist of two well-matched photodiodes and an ultra-low noise, high-speed transimpedance 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 fast monitor outputs (MONITOR+ and MONITOR-) to observe the optical input power levels on each photodiode separately. These outputs are low frequency outputs and cannot be used to measure an RF modulation on the signal. The block diagram applies to our balanced detectors with bandwidths up to 400 MHz.
Operation of the PDB480C-AC, PDB481C-AC, and PDB482C-AC Balanced Detectors
The PDB48xC-AC Balanced Amplified Photodetectors use the same basic operating principles as described above and have the following additional features. The TIA's ultra-low distortion output stage supports up to a 2 Vp-p A/D card input range. The two fast monitor outputs can be used to measure not only the individual optical input power levels, but also low frequency modulated signals up to 3 MHz (Item #s PDB480C-AC and PDB481C-AC) or 2.5 MHz (Item # PDB482C-AC). To enable the much higher frequency of these detectors, the photodiodes are connected to the inputs with length-matched fibers, only AC-coupled versions are offered, and capacitors are added to the circuit (as shown below).
Pin Diagrams for Balanced Amplified Photodetectors
Maximum Voltage is +10 V for Hi-Z & +1.5 V into 50 Ω loads.
For the PDB415, PDB425, PDB440, and PDB465 photodetectors, the maximum RF OUTPUT voltage is ±3.6 V for Hi-Z & ±1.8 V into 50 Ω loads.
The RF OUTPUT CW saturation power is 420 µW @ 1300 nm and 530 µW at 1060 nm for the PDB470C and PDB471C, respectively.
For the PDB48xC-AC photodetectors, the RF OUTPUT Power at 1 dB compression into 50 Ω loads is +16.5 dBm (Min), +18 dBm (Typical)