faq - stan

The idea of calibration with highly absorptive references, usually but not always ND filters, is
- their absorption could be tested quite precisely with conventional optical loss measurements
- by design, the absorption of ND filters is homogeneous and linear up to an incident power that damages it
- what matters for the technology is the absorbed power, low enough for the signal not to overload the detection circuit
- when low absorbing samples are tested, the signal is still generated by absorbed power only, all the high power just goes through. The same amount of absorbed power will generate the same signal as in the highly absorptive reference sample.

Note that absorption of low absorbing samples is frequently nonlinear, inhomogeneous and even time-dependent. That makes it hard to use them as a reference. However, if you confirm by multiple measurements that the absorption is linear and repeatable you could switch to using a low absorption sample as a reference.

The side peaks is the price you pay for the interferometric sensitivity. With our a bit crazy one-beam interferometer you have the total probe power conserved because the probe experiences a phase distortion. When this phase distortion is converted into the amplitude contrast naturally, without a second reference beam, those effects, side-peaks, arise. The overall shape of the amplitude contrast on the probe is a central spike surrounded by a dark fringe. When this pattern crosses the central aperture of the photodetector you see those side peaks. So, they are actually those moments when the dark fringe crosses the aperture.

Standard time-scans go the following way:
- 30 s, at least, sitting at a point which could have some history of exposure
- quick shift to an unexposed point, usually by 100 um, to see the initial absorption at a fresh point and the dynamics of the time dependence.

When the sample has a strong time dependence of absorption you may see an abrupt change of absorption at the moment of the shift. When relaxation is the dominant time-dependent response - you will see a spike of absorption signal with the following relaxation. When a laser-induced absorption happens you see an instant drop of absorption to an unperturbed value and then the absorption growth.
At higher power it is hard sometimes to see the initial, unperturbed absorption when the time constant for absorption change is short. At lower power the dynamic is slower so that you have a better chance to see an unperturbed, initial absorption

L is for longitudinal, T is for transverse. L-scan is made through the sample, from outside the sample, then through the front surface, through the bulk and through the back surface. T-scan is made in a transverse direction, across the aperture. It could be made across any surface or in the bulk. Most T-scans are made across the front surface.

On L-scan, with the transmission detector and transparent sample, you see spikes from surfaces and a plateau of the bulk absorption between them. Sometimes L-scans are made only through the front surface, simply to see at which position the front sutrace is. For the reflected probe L-scans are only made to detect the position of the front surface.

T-scan helps to see inhomogeneities of coatings or the bulk absorption. While L-scan could accidentally cross the sample at an anomalous point, T-scan will give a better idea what is the regular absorption, if it is homogeneous, how many absorption spikes the sample has.

It does not matter if the pump is totally reflected. You can still test with the transmission detector if at least few % of the probe are going through the sample. If there is no probe spot seen after the sample, only then you have to switch to the reflected probe detector.

- It seems that the reflection unit is only used if the sample is very reflective to the probe wavelength. The current transmission setup should be capable of measuring most materials if the probe can pass. Correct?

Yes, you are right. The probe senses the thermal field via the thermal lensing effect. All you have to do is to catch the probe by the photodetector. Transmitted or reflected probe beams will work.

Probe is to be
- low noise since the sensitivity is defined by the signal to noise ratio. A good probe has very low RIN such that you could work near shot noise limit
- different in wavelength from any pump used in the system to avoid scattered, 100% modulated pump from reaching the photodetector. Scattered pump may add to the signal (strong spectral filters should be placed in front of the probe detector to block scattered pumps)
- "neutral" in a sense of not modifying the absorption characteristics of the object like inducing an extra absorption. This effect could be traced by reducing the probe power while measuring absorption.Also, the wavelength of the probe affects sensitivity directly like with any interferometer: the sensitivity is inversely proportional to the wavelength of the probe. Therefore you better use the shortest wavelength of probe compatible with the measurements.
Historically, we used red HeNe most of the time but also low noise IR diodes at 1310 nm, 1550 nm, 1680 nm when testing GaAs, Si and other IR materials.

The green probe is much more "active" in terms of unwanted absorption effects. Expect it to modify absorption in most coatings that use hafnia and other high-index materials. NIR probes are "neutral" in most cases, except Si and other IR materials that are not pure, contain dopants that is.

Also, the parameter that matters is RIN (Relative Intensity Noise), not anything else like the "phase stability". The probe could be a wide-spectrum superluminescence diode, for example, with the bandwidth of tens of nm if only the RIN is below -150 dB at least.

We never worked at these long pump and probe wavelengths. The longest probe was 1550 nm. What we would need is

- ZnSe optics

- 10.4 um pump laser

- MIR low noise probe

- the photodetector for that probe.

Only the bulk absorption in Ge requires a probe at the wavelength over 1950 nm. The probe should be a low RIN laser for better signal/noise ratio. Otherwise a 1550 nm probe could be used and there are plenty of low RIN lasers at this wavelength.

There is another problem with the 10.4 um pump: the divergence of the beam limits the focal spot size. The standard spot of 75 um could hardly be used. There is a solution though. We successfully used designs with a much larger focal spot, up to 1 mm. The transverse space resolution degrades but the sensitivity of the instrument is still high

F A Q

1. How are we confident that calibrating with significantly absorbing substrates can give us trustworthy and repeatable results at, say, 2 ppm?

2.What is the origin of the side peaks on a longitudinal scan?

3. What could you say about the time-scan results when the absorption is growing over time?

How are we confident that calibrating with significantly absorbing substrates can give us trustworthy and repeatable results at, say, 2 ppm?

What is the origin of the side peaks on a longitudinal scan?

What could you say about the time-scan results when the absorption is growing over time?

You named the files with L-scan and T-scan. I assume L-scan means transmission scan from fron surface through back surface, and T-scan means transmission scan in the bulk.

We would like to measure the absorption of highly reflective coatings. Should we use a reflection attachment for that?

What should be the wavelength of the probe?

For the alternate probe beam why use IR and not visible?
could we use any visible wavelength as long as the phase is very stable?

Have you any suggested temperature\humidity ranges that the PCI works best in ?

Would you be able to measure absorption at 10.4 um?

We would like to measure the absorption of our products at the wavelength of 532nm and 1064nm.

Could you advise us about the focusing lens AR-coating specification? Is it possible to prepare several pieces of them in case they have been damaged after a long time of use?

What would be calibration pieces at 213 nm and 266 nm?

E-mail: info@stan-pts.com