# Mid-infrared Questions & Answers

## How do I choose among an MIR LED, a QCL, and a xenon flash lamp?

There’s a famous phrase in the spectroscopy world, “fit for purpose.” This is a perfect mantra for selecting components. Selecting the right light source starts with the desired application. What are you trying to achieve with this instrument? What is the proper wavelength and power output? What market is it serving? What is the target final cost? The answers to those questions along with the explanations below should lead to an answer.

MIR LEDs

• Wavelength: 3.3 μm (CH₄), 3.9 μm (reference light), and 4.3 μm (CO₂) are provided.
• Higher output, higher reliability, lower power consumption, faster response than lamps

QCLs

• Wavelength: 4 μm to 10 μm band
• High resolution, high output, high reliability, high-speed response

Xenon (Xe) flash lamps

• Wavelength: 0.2 μm to 5.0 μm (continuous spectrum)
• High output pulse emission in the microsecond order
• Long life

MIR LEDs
LEDs boast reliable lifetimes as well as low power consumption. They also come at a relatively low cost compared to other MIR light sources. The main tradeoff lies with power output, so these units are not intended for analytical accuracy. Portable gas monitors would be a great example application for these components.

QCLs
Quantum cascade lasers (QCLs) are the gold standard for generating light anywhere between 4-10 microns. Our DFB (distributed feedback) models provide industry-leading linewidth resolution, enabling possible ppb measurements. QCLs also have reliable lifetimes while providing high power output. All this performance comes at a much higher cost, and power requirements for lasing start at around 500mA. An instrument using a QCL will be cost intensive and require quite a bit of expertise to pull off, but nothing will touch the sensitivity it can achieve.

Xenon (Xe) flash lamps
For wide spectral output and high frequency operation, look no further than the Xe flash lamp. With output ranging from 0.2 microns to 5+ microns, these lamps make it possible to create an instrument that detects multiple gases. However, these lamps should not be considered for measuring very low concentrations due to the wide output and stability. Although Xe flash lamps with emission out to 7 microns have been developed, the relative output past 5 microns remains low. Measurements further into the fingerprint region would be very difficult to achieve.

## What is D* (D-star)?

D* is known as the “detectivity” of a detector, or the photosensitivity per unit active area in a detector. As seen in the equation below, the lower the noise equivalent power (NEP) of a detector, the higher the D* (and vice versa). NEP is the minimum power of signal needed for a detector to overcome its noise floor, or for SNR to equal 1. The lower this value, the higher the sensitivity of a detector. This relationship shows, therefore, that the higher the D*, the higher the sensitivity as well. We can also see from the equation below that the smaller the detector active area (A), the higher the D*.

${D}^{*}=\frac{\sqrt{A}}{NEP}$

D* takes into account more than just a detector’s active area, however. It is also a function of the temperature [K] or wavelength [µm] of a radiant source, the chopping frequency [Hz], and the bandwidth [Hz] of a detector—as seen in the expression of detectivity as “D* (A, B, C),” with each letter corresponding to the three characteristics mentioned.

What makes D* so useful is that it allows a comparison of different active area sizes and chemistries. While D* provides a better gauge of sensitivity, detector characteristics such as light wavelength, response time, active area shape, and number of elements, as well as the necessary electronics, should be taken into account when selecting an infrared detector.

When the applications demand more sensitivity, cooling serves the function of lowering the noise floor of a detector without reducing its quantum efficiency (QE). As a result, the lower the temperature, the higher the D* at a certain input power. It’s important to remember that cooling drives up cost and complexity, so it’s best to consider uncooled detectors first. Hamamatsu offers a wide range of uncooled detectors as well as detectors with multi-stage thermoelectric (TEC) cooling and liquid nitrogen cooling.

## Give me the short answer to “Why should I choose InAsSb?”

Photovoltaic operation typically leads to slower measurements. Hamamatsu’s InAsSb detectors mitigate that situation by boasting a rise time on the order of nanoseconds. In addition, many infrared detectors contain materials that are not RoHS compliant (mercury and lead), but InAsSb material is fully RoHS compliant. In uncooled applications, InAsSb is a strong contender for providing big cost advantages as well.

## Meet the engineers

Whether she has a camera in hand, or is working alongside her University Support Group team to solve a problem, Stephanie Butron tries to see things from a different perspective—like seeing the invisible side of mid-infrared. As an Applications Engineer at Hamamatsu Corporation, she enjoys learning more about the variety of projects and applications Hamamatsu’s customers are working on, and understanding better how Hamamatsu can help. When she isn’t helping people focus in on their research, Stephanie enjoys focusing her camera lens on the sights around her. From still lives to portraits, Stephanie tries to find new ways to look at the world.