Compact Balanced Detectors

Janis Valdmanis, Ph.D. Optics
Ultrafast Optoelectronics
General Manager

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Features

• High Quantum Efficiency Balanced Differential Photodetectors (Item # Prefix BDXQ1)
  • Low-Loss Optical Design to Maximize Efficiency
• Balanced Differential Photodetectors (Item # Prefix BDX3)
  • Low-Artifact Performance Optimized for OCT Applications
• Compact Package: 13.0 mm x 11.2 mm x 3.3 mm
• Surface Mount Component (Solders Directly to PCB)
• FC/APC, FC/PC, or LC/PC Connectors

Thorlabs' BDX family of high-speed, compact, fiber optic balanced detectors are designed for demanding OEM applications that require low-artifact performance. The surface mount package contains fiber and photodiode arrays configured on a common substrate for excellent response matching and precise timing for differential signal detection. This design achieves a high common mode rejection ratio (CMRR) and low crosstalk.

The optoelectrical configuration of these detectors is simple and compact in order to support high volume applications where space and cost are a primary concern. Additionally, the compact mounting geometry allows for easy integration with conventional circuit boards and enables high-speed connection to subsequent amplifiers or other components. Our BDX family of detectors do not have built-in amplifiers.

The pair of input fibers direct light to an array of two matched photodiodes biased with positive and negative voltages and wired as a differential pair with a single, common output. This internal optical geometry is designed to eliminate unwanted reflections from traveling either forward or backward along the intended optical path.

The BDXQ1 series of high quantum efficiency compact balanced detectors offer a typical quantum efficiency of 97% at 1550 nm with a frequency response of DC to 3 GHz. They are coupled with a pair of SMF-28e single mode input fibers with Ø900 µm loose-buffer jackets. The pair of fibers on each detector are matched in length to 1 mm and are available from stock with narrow key FC/APC or FC/PC connectors, or with LC/PC connectors upon request.

The BDX3 series of compact balanced detectors provide a frequency response bandwidth of DC to 5 GHz and are available with two single mode input fiber options. The BDX3Cx detectors have HI1060 fiber leads with Ø900 µm loose-buffer jackets for applications in the 980 - 1260 nm wavelength range, while the BDX3Bx detectors have SMF-28 Ultra fiber leads with Ø900 µm loose-buffer jackets for single mode applications in the 1260 - 1625 nm wavelength range. The two fibers in each model are matched in length to 1 mm and are available from stock with narrow key FC/APC or FC/PC connectors, or with LC/PC connectors upon request.

We also offer the BDX1EVB Evaluation Board for testing BDX balanced detectors in a lab environment with common cable connections. This board features a pre-defined footprint for soldering a BDX detector in place, and has specified points for connecting power and current monitor leads. Included with each BDX1EVB evaluation board is a micro-coaxial cable that transmits the high-speed output signal through an SMA connector.

Previous-Generation BDX1x Balanced Detectors
If your application does not require the lower-artifact performance of the BDX3 series detectors, the previous-generation BDX1x balanced detectors that share the same form factor and bandwidth are available upon request by contacting Tech Sales.

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External Pin Connections for the BDX Family of Detectors.

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Internal Electrical Configuration for the BDX Family of Detectors.

Operation of the BDX Family of Balanced Detectors

Thorlabs' BDX familly of Balanced Detectors provide differential detection of complementary signals. Each device consists of a fiber array that directs the two optical input signals to an array of well-matched photodiodes (see diagram to the near right). The output is the difference of the two input signals, which results in greatly reduced common mode noise.

The diagram to the far right indicates the external pin connections for each half-via contact of a detector. Soldering of the half-vias must be done cleanly and quickly. To avoid damaging the internal opto-electronic components, do not exceed 5 seconds of soldering time at 250 °C for each contact. When mounting to the BDX1EVB Evaluation Board or other PCB, ensure that all seven half-via contacts near the output are soldered and at least one of the ground contacts in each of the rear corners are soldered for a total of 9 contact points. All soldering must be done by hand; a reflow oven will damage the device. Please refer to the BDX1EVB Evaluation Board spec sheet for complete mounting instructions.

The components inside this module are ESD sensitive. Take all appropriate precautions to discharge personnel and equipment before making any electrical connections to the unit. This also applies to coaxial cables that easily accumulate capacitive change.

This tab describes how to solder a BDX detector to a BDX1EVB evaluation board or custom PCB. This procedure is recommended for users who are experienced with soldering. For additional details regarding the operation of the BDX1EVB board, please refer to the BDX1EVB board spec sheet.

Materials and Equipment

• Epoxy Gel, Quick-Curing at 75 °C
• Solder Appropriate for Soldering Components to PCBs
• Flux Designed to Work with the Chosen Solder
• Hot Plate Capable of Maintaining a Temperature of 75 °C
• Soldering Iron with Tip Capable of Fitting Inside BDX Detector Edge Castellations

Recommended Soldering Procedure
The steps below are the recommended procedure for soldering a BDX detector to a BDX1EVB evaluation board. Steps 5-6 are shown for two contacts in the video to the right.

Note: Soldering must be done cleanly and quickly. For each contact, do not exceed 5 seconds of soldering time at 250 °C contact temperature, to avoid damaging the internal opto-electronic components.

1. With the BDX1EVB board on the hot plate (heating off), place a small bead of epoxy gel in the center of the BDX footprint. Only a small amount of epoxy should be used to ensure that it does not leak out onto the soldering contacts. To ensure that unwanted movement does not occur, the board can be taped to the hot plate using a tape rated for 75 °C.

2. Place the BDX detector onto the footprint on the evaluation board, ensuring that the alignment pins go through the holes in the board. Ensure that the edge castellations are aligned with the pads on the board as closely as possible.

3. Turn the hot plate to 75 °C and leave for long enough to let the epoxy cure.

4. With the hot plate still at 75 °C, set the temperature of the soldering iron to 400 °C.

5. Using a small syringe, apply a small amount of flux to each pad and adjacent castellation on the detector, ensuring that both are coated, as shown in the video above.

6. Working quickly, apply a small amount of solder to the iron tip, then touch the tip to the castellation and pad simultaneously. If needed, add extra solder. A clean, shiny fillet should form between the pad and the castellation. If forming a fillet is difficult, using a lower-temperature solder may help.

7. Repeat steps 5-6 for each of the seven contacts near the output and the bias, as well as at least one of the ground contacts in each of the rear corners.

8. Clean the excess flux with a polyester cleanroom swab dipped in isopropyl alcohol.