Thorlabs S120C Compact Photodiode Power Sensor, Si, 400-1100 nm

Overview

The Thorlabs S120C is a compact, general-purpose optical power head built around a large-area silicon photodiode. It covers the 400 to 1100 nm band, which spans visible alignment lasers, red and near-infrared laser diodes, HeNe sources and argon or krypton ion lasers, and it measures from 50 nW up to 50 mW. Its slim body, only 12.7 mm thick, slips into tight spaces on an optical table or inside an instrument where a bulkier thermal head would not fit, making it one of the most widely used sensors in the Thorlabs C-Series family.

Seven Star LLC supplies the Thorlabs S120C to research laboratories, universities, telecom contractors and industrial maintenance teams throughout Oman and the GCC. Customers in Muscat, Sohar, Salalah and Duqm benefit from engineering support in Muscat, and from advice on pairing the sensor with the right console and fibre adapter for each measurement task.

Working Principle

Light entering the 9.5 mm input aperture of the Thorlabs S120C first passes a reflective, diffusing neutral-density filter with an optical density of about 1. This filter reduces the incident intensity so the photodiode stays in its linear region up to 50 mW, and it spreads the beam to lessen sensitivity to spot position. The silicon detector beneath, with a 9.7 × 9.7 mm active area, generates a photocurrent proportional to the absorbed optical power; the connected meter converts that current to watts using the wavelength-dependent responsivity curve stored for this individual head. Response time is below 1 µs, so the sensor follows rapid power changes far faster than a thermal head can.

Because a photodiode’s responsivity changes strongly with wavelength, accurate readings depend on entering the correct laser wavelength at the console. The Thorlabs S120C stores its NIST- and PTB-traceable calibration data in a non-volatile memory chip inside its red Sub-D connector, so any compatible meter loads the correct curve the moment the head is plugged in. An NTC thermistor of 4.7 kΩ reports head temperature to the console for monitoring.

Engineering Design of the Thorlabs S120C

The housing is a round, black anodised disc of Ø30.5 mm by 12.7 mm, weighing about 70 g, with a 1.5 m cable ending in the 9-pin Sub-D C-Series connector. A removable annular viewing target surrounds the aperture; it fluoresces under light from 400 to 640 nm and 800 to 1700 nm, which lets a technician centre a visible or infrared beam on the active area before taking a reading. The front of the head carries an external SM1 (1.035″-40) thread in line with the aperture, so standard Thorlabs lens tubes, filters, irises and fibre adapters can be screwed directly onto the sensor.

For mounting, the Thorlabs S120C provides a combined #8-32 and M4 tapped hole, compatible with both imperial and metric posts and post holders. Optional fibre adapters are available for FC, SC, LC, SMA and ST connectors, turning the head into a fibre power checker for patch cords and pigtailed sources. There are no user-serviceable parts in the Thorlabs S120C, and the manufacturer recommends a yearly recalibration to maintain the stated uncertainty.

Key Features

Engineers specify the Thorlabs S120C for routine low-power measurement for the following reasons.

• A wide 50 nW to 50 mW range with 1 nW resolution covers both weak fibre signals and typical alignment and diode lasers.
• Measurement uncertainty is ±3% from 440 to 980 nm, widening to ±5% and ±7% at the edges of the band.
• Linearity of ±0.5% and a sub-microsecond response suit modulation checks and quick power tuning.
• The reflective OD1 diffuser raises the damage threshold to 20 W/cm² average power density and 20 µJ pulse energy.
• A slim 12.7 mm profile allows measurement inside compact setups and between closely spaced optics.
• The removable VIS/IR target makes beam centring fast and repeatable.
• SM1 threading and optional FC, SC, LC, SMA and ST adapters support both free-space and fibre-coupled measurements.
• Traceable calibration data stored in the connector enables plug-and-play use with current Thorlabs meters.

Industrial Applications

In Oman and across the Gulf, the Thorlabs S120C is a typical first sensor for any laboratory starting with optical power measurement. University physics and engineering departments in Muscat use it in teaching labs for HeNe and diode laser experiments, and research groups rely on it to check fibre coupling efficiency and the output of pigtailed laser diodes. Telecom and fibre-network contractors fit FC or LC adapters to the head to test transmitter output and patch-cord loss at 850 nm, while medical and dental equipment service teams use it to verify the power of red aiming beams and low-power therapy lasers during preventive maintenance.

In industrial settings around Sohar, Salalah and the wider GCC, maintenance engineers use the Thorlabs S120C to check the alignment and reference lasers fitted to laser-based gas analysers, optical level sensors and laser marking systems, where a gradual drop in source power can signal degraded optics or an ageing diode. Since the head is plug-and-play with consoles such as the PM100D, PM100A, PM100USB, PM200 and PM320E, one sensor can move between handheld meters and PC-based stations. Seven Star LLC supplies the Thorlabs S120C together with compatible consoles and adapters, with engineering support in Muscat.

Technical Specifications

The main Thorlabs S120C data, taken from the manufacturer’s spec sheet (rev. C), are listed below.

Related Thorlabs Equipment

To read this head you will need a compatible console; on our site see the Thorlabs PM100D handheld meter, Thorlabs PM400 touchscreen meter and Thorlabs LDC205C laser diode driver for powering diode sources under test.

How to Order

To order the Thorlabs S120C, send Seven Star LLC a formal RFQ stating quantity, the console you will use and any fibre adapters required. Our instrumentation team will confirm availability and delivery times to sites in Oman and the GCC. The full datasheet can be downloaded from this page, and our engineers in Muscat can help you decide whether a photodiode head like this one or a thermal sensor is the better fit for your laser power level.