Features

• 7 Models Cover the 150 nm to 2.6 μm Wavelength Range
• Rise Times as Fast as 1 ns
• Thin Profile Allows Measurements in Tight Spaces
• Easy-To-Use, Compact, and Versatile
• SM05 and SM1 Threads Ideal for Fiber Coupling or Mounting ND Filters
• Internal A23 Bias Battery (Included)

Thorlabs' biased photodetectors are available in seven models that cover the wavelength range from the UV to the mid-IR (150 nm to 2.6 µm) with improved bandwidth and NEP performance over previous models. The slim housing allows the optical detector to slip into tight setups. Each model comes complete with a fast PIN photodiode and an internal bias battery packaged in a rugged aluminum housing. With a wide bandwidth DC-coupled output, these detectors are ideal for monitoring fast pulsed lasers as well as DC optical sources. Each DET has a Bias-T circuit that combines a high-frequency AC signal with a DC signal into a single output. The direct photodiode anode current is provided on a side panel BNC. This output is easily converted to a positive voltage using a terminating resistor. When looking at high-speed signals, Thorlabs recommends using a 50 Ω load resistance. For lower bandwidth applications, our variable terminator is a great time saver.

All connections and controls have been moved away from the light path, thus making the DET a more useful, easier-to-use device. The SM1, SM05, and 8-32 (M4) threadings on the DET detector housing allow it to be mounted in a cage system, lens tube system, or on a TR series post. See the Mounting Options tab for more details on how to incorporate a DET series photodetector into an optical setup.

Each DET arrives with a 12 VDC Bias battery already installed. A battery is used as the voltage source because batteries are an extremely low noise source of power. The battery can be replaced with a DET1A power adapter when the detector is being used in applications where a small increase in the signal noise due to noise in the line voltage isn't a consideration or the finite lifetime of a battery is not acceptable. The A23 battery is the replacement battery for the current DET series of photodetector, while the T505 replaces batteries used in our older models (i.e., DET1-SI and DET2-SI). Please note that due to slight positive terminal variations from manufacturer to manufacturer, Thorlabs only recommends using an Energizer battery in our DET series of photodetectors.

Battery Lifetime

When using a battery-operated photodetector, it is important to understand the battery’s lifetime and how this affects the operation of the detector. As a current output device, the output current of the photodetector is directly proportional to the light incidented on the detector. Most users will convert this current to a voltage by using a load-terminating resistor. The resistance value is approximately equal to the circuit gain. For very high speed detectors, such as the SIR5, it is very important to use a 50 Ω terminating resistor to match the impedance of standard coax cables to reduce cable reflections and improve overall signal performance and integrity. Most high-bandwidth scopes come equipped with this termination.

The battery usage lifetime directly correlates to the current used by the detector. Most battery manufacturers provide a battery lifetime in terms of mA hr. For example, if a battery is rated for 190 mA hrs, it will reliably operate for 190 hr at a current draw of 1.0 mA. This battery will be used in the following example on how to determine battery lifetime based on usage.

For this example we have a 780 nm light source with an average 1 mW power is applied to a detector. The responsivity of a biased photodetector based on the response curve at this wavelength is 0.5 A/W. The photocurrent can be calculated as:

Given the battery has a rated lifetime of 190 mA hr, the battery will last:

or 16 days of continuous use. By reducing the average incident power of the light to 10 µW, the same battery would last for about 4 years when used continuously. When using the recommended 50 Ω terminating load, the 0.5 mA photocurrent will be converted into a voltage of:

If the incident power level is reduced to 10 mW, the output voltage becomes 1 mV. For some measurement devices this signal level may be too low and a compromise between battery life and measurement accuracy will need to be made.

When using a battery-powered biased photodetector, it is desirable to use as low a light intensity as possible, keeping in mind the minimum voltage levels required. It is also important to remember that a battery will not immediately cease producing a current as it nears the end of its lifetime. Instead, the voltage of the battery will drop, and the electric potential being applied to the photodiode will decrease. This in turn will increase the response time of the detector (lowering the bandwidth of the detector). As a result, it is important to make sure that the battery is operating within its specified parameters in order to ensure the proper functioning of the biased photodetector. The battery can be tested by following the procedure described in the specifications sheet for the detector.

Another suggestion to increase the battery lifetime is to remove, or power down the light source illuminating the sensor. Without the light source, the photodetector will continue to draw current proportional to the photodetector’s dark current, but this current will be significantly smaller. This current is known as the dark current.

For applications where a DET series photodetector is continuously illuminated with a relatively high-power light source, or if having to change the battery is not acceptable, we offer the DET1A adapter and LDS2 power supply. The drawback to this option is the noise in the line voltage will add to the noise in the output signal and could cause more measurement uncertainty.