The New Focus Models 1801 and 1811 are 125-MHz, low-noise photoreceivers designed for RF photoreception. These devices are available in both DC and AC coupled versions, offering a typical bandwidth of 125 MHz and a current gain of 40 V/mA. They are suitable for a wide range of applications including wide bandwidth low-noise detection of signals distributed over fiber-optic cables, high-resolution spectroscopy, fiber-optic sensors, and optical metrology.

Function Description:

The photoreceivers are designed to convert optical signals into electrical signals with high gain and low noise. The Model 1801 incorporates a silicon/PIN photodiode, while the Model 1811 uses an InGaAs/PIN photodiode. In both models, the photodiode is followed by a low-noise transimpedance amplifier acting as a pre-amp, with a compensating amplifier as the output stage. This design allows for the use of a large-area diode while maintaining a large bandwidth with a flat response. The compensating amplifier increases the gain of the output stage with frequency, which leads to an increase in the noise floor starting at approximately 40 MHz, with a soft peak around 110 MHz. The entire package is shielded to eliminate RF pickup.

The AC-coupled versions include additional components: two blocking capacitors, a choke, and a DC bias monitor circuit. The high-pass filter on the AC-coupled output has a corner frequency of approximately 25 kHz, and the low-pass filter on the DC bias monitor output has a corner frequency of approximately 50 kHz.

Important Technical Specifications:

General:

Bandwidth: Typical 125 MHz (DC-coupled and AC-coupled versions).
Current Gain: 40 V/mA (AC-coupled), 1 V/mA (DC-coupled for 1801), 40 V/mA (DC-coupled for 1811).
Risetime: 3 ns (typical).
Output Impedance: 50 Ω.
Power Requirements: ±15 V DC; 250 mA.
RF Output: SMA connector.
DC Bias Monitor Output (AC-coupled units only): SMB connector.

Model 1801 Specifics:

Wavelength Range: 300-1050 nm.
Minimum NEP:* 3.3 pW/√Hz (frequency dependent, see page 9 for details).
CW Saturation Power: 120 μW @ 950 nm.
Maximum Pulse Power: 5 mW.
Detector Material/Type: Silicon/PIN.
Detector Diameter: 0.8 mm.
Optical Input: FC or free space (FS).
Distance from window face to photodetector: 2.5 mm.

Model 1811 Specifics:

Wavelength Range: 900-1700 nm.
Minimum NEP:* 2.5 pW/√Hz (frequency dependent, see page 9 for details).
CW Saturation Power: 120 μW @ 950 nm.
Maximum Pulse Power: 5 mW.
Detector Material/Type: InGaAs/PIN.
Detector Diameter: 0.3 mm (FS), 0.1 mm (FC).
Optical Input: FC or free space (FS).
Distance from window face to photodetector: 0.5 mm.

Note: The equivalent input noise current at the peak (approx. 110 MHz) is greater than the low-frequency noise by a factor of 10.

Usage Features:

Power Connection: The unit requires a ±15-V power supply providing a minimum of 0.25 A of current. New Focus recommends their Model 0901 power supply. Two power cables are provided: a Model 0921 banana plug-to-microconnector cable and a Model 0922 microconnector-to-microconnector cable. The Model 0922 cable should be used with the Model 0901 power supply's 0.3-A microconnector outputs. The Model 0921 cable is for power supplies other than the 0901.
Optical Input:
- For free-space beam input, align the module so the beam is incident on the detector surface.
- For fiber-optic input, connect the fiber-optic cable from your optical source to the FC input connector on the front of the module. The detector is designed to receive an FC/PC connectorized fiber.
- To ensure operation in the linear region, keep input power levels well below the CW saturation power specification (which is wavelength dependent and inversely proportional to responsivity).
Output Connection:
- If your RF measurement instrument has a male connector, connect it directly to the SMA female output connector (labeled "AC" on AC-coupled units) on the back of the module.
- If your instrument has a female connector, use an appropriate cable.
- On AC-coupled units, the DC bias can be monitored on the output labeled "DC" using the provided SMB-to-BNC cable.

Maintenance Features:

Handling Precautions:
- Always ground yourself adequately before handling the photoreceiver or making connections to prevent electrostatic discharges, which can permanently damage the device. A ground strap is recommended.
- Ensure the optical connector is clean and undamaged before connecting it to the detector module.
Warranty: New Focus, Inc. guarantees its products to be free of defects for one year from the date of shipment. This warranty covers defects but not incidental or consequential loss.
Customer Service:
- Technical Support: Information and advice are available from applications engineers. For quickest response, provide the model and serial numbers of your product.
  - Hours: 8:00–5:00 PST, Monday through Friday (excluding holidays).
  - Toll Free: 1-866-NUFOCUS (1-866-683-6287) (from USA & Canada only).
  - Phone: (408) 284-6808.
  - Fax: (408) 980-8883.
  - Email: techsupport@newfocus.com (responses typically within one business day).
- Service: In case of malfunction or damage, contact New Focus for a return authorization number and instructions for shipping the unit back for evaluation and repair.

New Focus 1801 - Page 5

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