[Dry goods reminder] How to choose the spatial light modulator correctly?

In many applications such as imaging systems such as optical communication, microscopy and telephoto, adaptive optics, and optical enamel, optical phase modulation is involved. In this case, a new type of programmable optical instrument is needed. Light modulator.

A spatial light modulator is an optical device that uses an LCOS (Liquid Crystal On Silicon) chip to adjust the amplitude or phase of an optical wavefront. The LCOS chip is a pixel array composed of liquid crystal pixels, each of which can individually modulate light. For the same beam, the smaller the size of the pixel, the finer the modulation; the number of pixels is the resolution of the chip, and the higher the resolution, the higher the degree of freedom of modulation.

From early ferroelectric materials and twisted nematic liquid crystal structures, to the use of optoelectronic addressing. Hamamatsu's Central Research Institute and Solids Division are committed to space light modulation technology for more than 30 years. Its spatial light modulator is currently mainly in the high-end market, and is known for its high linearity, high light utilization, and high diffraction efficiency.

For the nature of the Hamsun space light modulator LCOS itself, it only changes the phase of the light without affecting the intensity and polarization state of the light (the modulation of the amplitude/light intensity needs to be achieved by the optical path). The arrangement of the liquid crystals is changed by changing the voltage, and the phase modulation changes depending on the arrangement of the liquid crystals. Through the CMOS backplane and the DVI signal output from the PC, the arrangement of the liquid crystals is single pixel controllable.

At the beginning of the article, we also mentioned that the spatial light modulator involves more applications. How do you choose one of the most effective small partners according to your research needs? Many user friends may have drawn a capitalized question mark in their minds.

In order to help your friends and relatives to say goodbye with this question, here, we start from the Hamamatsu Space Light Modulator X10468/X13267/X13138 series, and have a chat with you about the choice of spatial light modulators~

Choose the right model, do more with less.

Select resolution and cell size

LCOS is composed of pixel arrays. Currently, Hamamatsu can provide two resolutions: 792×600, 1272×1024. For 792×600 resolution products, there are two kinds of pixel sizes to choose from: 20μm, 12.5μm. .

*Different resolutions and cell sizes are indicated in series in the first half of the product model, such as X10468-08, X10468 refers to the product resolution of this model is 792 × 600, the pixel size is 20μm.

The "Effective area size" in the table refers to the area of ​​the liquid crystal surface on which the light can be modulated on the LCOS head. When the user selects the model, it is necessary to consider whether the area can accommodate the spot size of the desired modulation.

"Fill factor" refers to the percentage of the effective area of ​​a single pixel to the total area, which is critical in affecting light utilization. (Light utilization will be introduced later)

It is important to note that not all of the brand's products are labeled "Fill factor" is a concept!

Some brands' fill factor ratio does not consider the inter pixel gap, so the percentage of this item is usually very high, and the parameters appear to be “beautiful” (the calculation method of the fill factor is shown in the figure below) . But what users don't understand is that their Inter pixel gap is very large, and some even up to 1.3μm.

Hamamatsu presents the true Factor factor in a true sense based on the principle of seeking truth from facts, and maintains an ultra-small Inter pixel gap: 0.2 μm. (The calculation method of Hamamatsu fill factor ratio is shown in the figure below)

Select the appropriate band

The LCOS products of each series of Hamamatsu (X10468/X13267/X13138) are available in nine different models, indicated at the end of the model from -01 to -09. For example, -07 in X10468-07 means that the product can be modulated in the wavelength range from 620nm to 1100nm.

* The meaning of the dotted line range: For the -05 model, ultraviolet light with a wavelength of less than 400 nm will cause damage to the LCOS. Please consult us for more details. For the -08 model, the wavelength of 1350 nm to 1400 nm causes a decrease in reflectance (light utilization) of about 5% due to absorption of the glass substrate.

Attention to the light threshold

LCOS is not able to illuminate light exceeding a threshold value. If the threshold light intensity is exceeded, damage will occur, and the linearity will decrease, and the liquid crystal layer will melt.

The light threshold of Hamamatsu LCOS is a high level among similar products. The specific value will vary greatly depending on the model, wavelength, and type of light source used by the user. When selecting the model, it must be referenced to the maximum light intensity of the product in the sample. Threshold table.

In addition, as can be seen from the wavelength selection, among the nine models from -01 to -09, the three models -01, -07, and -08 have longer wavelength ranges; and -02, -03, -04, The -05, -06, and -09 models have shorter wavelength ranges because they add a dielectric mirror to the bottom layer of the liquid crystal, which greatly increases the light threshold and light utilization, but also limits the wavelength range. In a relatively small range. Users need to choose according to the needs of the application.

The parameters are read and the selection is not good.

In the parameter table of the spatial light modulator, there are several important performance parameters that require special attention from the user:

Light utilization (reflectance), first-order diffraction efficiency, linearity, stability

Light utilization rate (reflectance)

In order to improve the utilization of light, the loss of light energy is minimized during the modulation process. The LCOS of Hamamatsu is reflective. Here, the light utilization rate is defined as the reflectivity of light reflected by the LCOS liquid crystal surface, that is, the average reflected light intensity as a percentage of the incident light intensity.

Hamamatsu model light utilization reference table

* In addition, Hamamatsu can provide OEM custom products in the range of 355nm-1550nm, and optimize the required wavelengths according to the application.

On the market, some brands refer to “reflectivity” as “zero-order diffraction efficiency”, which is actually the same concept. Therefore, do not confuse “zero-order diffraction efficiency/light utilization/reflectance” with “first-order diffraction efficiency”.

First order diffraction efficiency

The "first-order diffraction efficiency" here is actually the true "diffraction efficiency" of LCOS, which is defined by the percentage of the energy of the first-order diffracted light as a percentage of the zero-order diffracted light energy when the blazed grating is loaded (using LCOS as a grating). When loading the blazed grating, the spot level is as shown below:

(a) The raster is not loaded.

(b) Load a 2-level grating with a spatial resolution of 25 line pairs/mm, in the figure, ±1 order light and 0 level light.

(c) Load a 4-level raster, equivalent to a spatial resolution of 12.5 line pairs/mm.

And to build a simple optical path, users can use their own optical power meter to achieve diffraction efficiency detection:

The higher the raster level/steps, the lower the spatial resolution, but the higher the diffraction efficiency. For the X10468 series, there is such a correspondence between the raster level/steps and the spatial resolution lp/mm:

The Hamamatsu LCOS has ultra-high diffraction efficiency close to the theoretical value due to its pure phase and high precision phase control.

X10468 Series Diffraction Efficiency Value

* For other models of data and calculation methods, as well as LCOS-SLM spatial light modulator diffraction efficiency test (first-order diffraction efficiency), please leave a message in the public number.

In addition, it is necessary to pay special attention to the fact that laboratory static electricity may cause a decrease in diffraction efficiency. Users are also reminded to pay attention to protection when using a spatial light modulator.

Linearity

The linearity of phase modulation is a critical parameter in the accuracy of light modulation. The Hamamatsu Space Light Modulator pursues high-precision modulation, so it has a recognized satisfactory performance in terms of linearity.

Hamamatsu spatial light modulator linearity

Excellent linearity in both the mean and standard deviation

For products that simply pursue phase modulation range or resolution, linearity is easily ignored, which has problems with modulation accuracy. This can be seen in the figure below:

We can see that the brand products have poor linearity in the range of 0-2Ï€, and most applications mainly use the range of 0-2Ï€. Since two phases of light having an integer multiple of the aberration of 2Ï€ are equivalent from the viewpoint of wave optics, it is equivalent.

stability

There is also an important parameter in the accuracy of light modulation, which is the stability of the phase. Because LCOS is driven by AC power, the output light will float with the drive frequency. The figure below shows the phase floating range of the -04 model. The driving frequency of X10468 and X13267 series is 240Hz (SVGA), the stability is within ±0.01π; while the driving frequency of X13138 series is 120Hz (SXGA), although it is slightly worse than the two series of SVGA drive, it can maintain Within ±0.03π.

In addition to the "stability" in the sense of parameters, expand here to talk about the stability of the "light link" of the spatial light modulator.

For products such as spatial light modulators, EU countries have such regulations. If non-EU products want to enter their markets, they need to pass strict CE certification. This requires that the wiring and connectors of the product are very strong and free from noise. Although this allows the Hamamatsu LCOS to be relatively large in size, it also guarantees that its quality and durability exceed those of other products that are not CE certified.

* Some products are EU-branded and can be sold directly in their field without CE certification.

Some users may mention the way the cable is connected, which, although it is reduced in size, is not resistant to use and has a high damage rate. In fact, when using LCOS, you only need to put the LCOS head on the optical platform, and put the control box on the shelf of the console, you can solve the problem of over-occupation.

Well, I have talked so much above, I hope to help users understand the parameters and selection of the spatial light modulator. Of course, in order to finally choose a favorite product, you may encounter many special problems. At this time, consult with Hamamatsu engineers! ~

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