OCT Balanced Detectors

These balanced detectors, which are available with 5 different bandwidths, are designed specifically for OCT applications. They act as a balanced receivers 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 key feature of all of these detectors is the fast monitor ports with reduced noise.

The PDB480C-AC is the latest design for OCT Balanced Amplified Photodetectors and takes the quality of these detectors to a new level. It offers bandwidths from 30 kHz up to 1.6 GHz, leading to considerable speed improvements. The key feature of the PDB480C-AC is the ultra low distortion output stage supporting up to 2 V_p-p A/D card input range. Furthermore the fiber-coupled design supresses line artifacts in the OCT image, which generally occur when detector coupling optics are used. The latter two features improve the image quality for OCT applications considerably.

Noise Reduction
The detectors consist of two balanced photodetectors and an ultra-low-noise, high-speed transimpedance amplifier. The two balanced photodetectors are matched to achieve an excellent common mode rejection, leading to better noise reduction. Please see the Operation Tab for more details.

For the PDB480C-AC additionally the fiber input lengths are exactly matched as well to achieve an excellent CMRR value across the full detector bandwidth.

Connectors
The PDB410, PDB420, PDB440, and PDB460 detectors contain non-fiber-coupled sensors and come with two removable FC input connectors (please note that for PDB460C, the FC adapters are not removable), making them suitable for either free space or fiber-coupled applications. In contrast to that the PDB480C-AC detector is fiber-coupled, i.e. applicable for fiber based applications only. This detector comes with two FC/APC connectors (SMF28 inside) which are switchable to allow easy connector cleaning. Three SMA electrical connectors provide the balanced output signal plus a fast power monitor for each of the two input signals for all of the balanced detectors. These two monitors enable the control of the input power levels and can be used as an independent power meter as well as measuring low frequency modulated signals up to 1 MHz for each channel for the PDB410, PDB420, PDB440, and PDB460 detectors and up to 3 MHz for the PDB480C-AC detector.

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 8-32 (M4) screws. The unit is powered with the provided ±12 V DC power supply. The input voltage of 110 V or 230 V can be manually selected by a switch.

Thorlabs also offers Fiber-Based Interferometers, which feature an integrated balanced detector.

Item #	PDB410A	PDB410C	PDB420A	PDB420C
Detector Type	Si/PIN	InGaAs/PIN	Si/PIN	InGaAs/PIN
Wavelength Range	320 - 1000 nm	800 - 1700 nm	320 - 1000 nm	800 - 1700 nm
Typical Max Responsivity	0.53 A/W	1.0 A/W	0.53 A/W	1.0 A/W
Active Detector Diameter	0.8 mm	0.3 mm	0.8 mm	0.3 mm
Bandwidth (3 dB)	DC - 100 MHz		DC - 75 MHz
Common Mode Rejection Ratio	>25 dB (Typical >35 dB)		>35 dB
Transimpedance Gain *	50 x 10³ V/A		250 x 10³ V/A
Conversion Gain RF-Output	26.5 x 10³ V/W	50 x 10³ V/W	133 x 10³ V/W	250 x 10³ V/W
Conversion Gain Monitor Outputs	10 V/mW @ 820 nm	10 V/mW @ 1550 nm	10 V/mW @ 820 nm	10 V/mW @ 1300 nm
CW Saturation Power RF-Output	130 µW @ 820 nm	70 µW @ 1550 nm	27 µW @ 820 nm	15 µW @ 1300 nm
Minimum NEP (DC - 10 MHz)	7 pW/Sqrt(Hz)	3.8 pW/Sqrt(Hz)	6.5 pW/Sqrt(Hz)	3.5 pW/Sqrt(Hz)
Fiber Optical Inputs **	FC/PC or FC/APC (Removable)
Photodiode Damage Threshold	20 mW
Electrical Outputs	SMA (Qty. 3)
RF-Output Impedance	50 W
RF-Output Coupling	DC, AC-Coupling upon Request
Dimensions	85 mm x 80 mm x 30 mm (3.35" x 3.15" x 1.18")
Power Supply	±12 V @ 200 mA

*Transimpedance Gain is reduced by a factor of two for 50 Ohm loads.
**For Model PDB430C, the FC adapter is not removable

Item #	PDB440A	PDB440C	PDB460A	PDB460C
Detector Type	Si/PIN	InGaAs/PIN	Si/PIN	InGaAs/PIN
Wavelength Range	320 - 1000 nm	800 - 1700 nm	320 - 1000 nm	800 - 1700 nm
Typical Max Responsivity	0.53 A/W	1.0 A/W	0.50 A/W	1.0 A/W
Active Detector Diameter	0.8 mm	0.3 mm	0.4 mm	0.15 mm
Bandwidth (3 dB)	DC - 15 MHz		DC - 200 MHz
Common Mode Rejection Ratio	>35 dB		>25 dB (Typical >35 dB)
Transimpedance Gain *	51 x 10³ V/A		30 x 10³ V/A
Conversion Gain RF-Output	27 x 10³ V/W	51 x 10³ V/W	16 x 10³ V/W	30 x 10³ V/W
Conversion Gain Monitor Outputs	10 V/mW @ 820 nm	10 V/mW @ 1550 nm	10 V/mW @ 820 nm	10 V/mW @ 1550 nm
CW Saturation Power RF-Output	130 µW @ 820 nm	70 µW @ 1550 nm	225 µW @ 820 nm	120 µW @ 1550 nm
Minimum NEP (DC - 10 MHz)	6.4 pW/Sqrt(Hz)	3.3 pW/Sqrt(Hz)	13.2 pW/Sqrt(Hz)	6.0 pW/Sqrt(Hz)
Fiber Optical Inputs **	FC/PC or FC/APC (Removable)			FC/PC or FC/APC (Not Removable)
Photodiode Damage Threshold	20 mW
Electrical Outputs	3 x SMA
RF-Output Impedance	50 W
RF-Output Coupling	DC, AC-Coupling upon Request
Dimensions	85 mm x 80 mm x 30 mm (3.35" x 3.15" x 1.18")
Power Supply	±12 V @ 200 mA

*Transimpedance Gain is reduced by a factor of two for 50 Ohm loads.
**For Model PDB430C FC adapter is NOT removable

Item #	PDB480C-AC
Detector Type	InGaAs/PIN
Wavelength Range	800 - 1700 nm
Typical Max Responsivity	0.90 A/W
Active Detector Diameter	0.075 mm
Bandwidth (3 dB)	30 kHz - 1.6 GHz
Common Mode Rejection Ratio	>25 dB (>30 dB Typical)
Transimpedance Gain	15,000 V/A
Conversion Gain RF-Output	13,500 V/W
NEP (30 kHz - 100 MHz)	9.3 pW/Sqrt(Hz)
RF-Output
RF Output Power @ 1dB Compression	16.5 dBm (Min), 18 dBm (Typical)
Integrated Noise Power per nW RMS (RMS, 30 kHz-1.6 GHz)	600 nW
Overall Output Voltage Noise per mV RMS	9 mV
Monitor-Ports
Conversion Gain Monitor Outputs	9 V/mW
Bandwidth (@ 3 dB)	DC - 3 MHz
Overall Output Voltage Noise per mV RMS	0.65 mV
Max Output Voltage Swing (High Z)	10 V
DC-Offset	±2 mV
Output Impedance	200 Ohm
Other
Optical Inputs	FC/APC (SMF28 Fiber Inside)
Photodiode Damage Threshold	10 mW
Electrical Outputs	SMA
RF-Output Impedance	50 Ohm
RF-Output Coupling	AC-Coupling Only
Dimensions without Connectors (W x H x D)	85 mm x 80 mm x 30 mm (3.35" x 3.15" x 1.18")
Power Supply	±12 V @ 200 mA

Operation of the PDB410, PDB420, PDB440, and PDB460 Series Balanced Detectors

These 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 can be used to measure an RF modulation on the signal up to 1MHz.

Balanced receiver

Operation of the PDB480 Series Balanced Detectors

Similar to the balanced detectors described above, the PDB480C-AC Balanced Amplified Photodetector consists of two well-matched, but fiber-coupled, photodiodes with length-matched fibers and an ultra-low noise, ultra-low distortion high-speed transimpedance amplifier (TIA). Like for the other detectors above, this TIA generates an output voltage (RF OUTPUT) proportional to the difference between the photocurrent in the two photodiodes (i.e. the two optical input signals). Additionally, the unit offers two fast monitor outputs (MONITOR+ and MONITOR-) that allow the user to measure the individual optical input power levels as well as low frequency modulated signals up to 3 MHz.

Balanced receiver

RF Output

SMA Female

For the PDB410, PDB420, PDB440, and PDB460 detectors the maximum RF OUTPUT voltage is ±3.6 V for Hi-Z & ±1.8 V into 50 Ω loads.

For the PDB480 the RF OUTPUT Power at 1 dB compression into 50 Ω loads is +16.5 dBm (Min), +18 dBm (Typical)