---
title: Texas Instruments Phase Light Modulator (PLM)
url: https://www.emergentmind.com/topics/texas-instruments-phase-light-modulator-plm
type: topic
---

# Texas Instruments Phase Light Modulator (PLM)

The Texas Instruments Phase Light Modulator (PLM) is a MEMS-based, phase-only spatial light modulator built on Texas Instruments’ Digital Light Processing platform. In the reported implementations, it consists of a 2D mega-pixel array of electrostatically actuated piston-type micromirrors whose vertical height can be independently adjusted with 4-bit precision, enabling direct phase control at each pixel. Within recent adaptive-optics and wavefront-shaping work, the PLM is presented as a low-SWaP, chip-scale solution for simultaneous wavefront correction and beam tracking in real time, featuring over one million actuators and operation at kilohertz update rates [2409.01289; 2509.05896].

## 1. Hardware architecture and physical characteristics

The smallest commercial device described in the wavefront-correction study is the 0.67″ PLM, hosting a $1358 \times 800$ array of piston-type micromirrors, or approximately $1.09$ million actuators. Each mirror is a square of side length $10.8\,\mu\mathrm{m}$. A larger-format 0.98″ PLM increases both resolution and speed, while retaining the same underlying architecture as an electrostatically actuated micromirror array [2509.05896].

In the engineering characterization of the DLP6750 EVM, the array is likewise specified as $1358 \times 800$, with pixel pitch $10.8\,\mu\mathrm{m}$, fill factor approximately $95\%$, and mirror-coating reflectivity greater than $95\%$. Beneath each mirror are four interleaved, independently driven electrodes; by applying the appropriate binary on/off pattern across these four electrodes, the mirror is electrostatically pulled to one of 16 discrete vertical positions [2409.01289].

| Parameter | Reported value |
|---|---|
| Mirror array format | $1358 \times 800$ (1.09 MP) |
| Pixel pitch | $10.8\,\mu\mathrm{m}$ |
| Fill factor | $\approx 95\%$ |
| Phase levels | 16 (4 bit) |
| Mirror stroke | $0 \ldots 325\,\mathrm{nm}$ |
| Continuous update rate (EVM) | $1.44\,\mathrm{kHz}$ |
| Active aperture | 0.67″ diagonal ($\approx 17\,\mathrm{mm}$) |
| Package size | $\le 30 \times 30\,\mathrm{mm}^2$ |

The operating principle is electrostatic piston motion: each pixel is a rigid mirror pulled toward a backplane, producing a locally controlled phase delay. Because the actuation is capacitive, static power draw is essentially zero and only the digital driver incurs dynamic switching losses. The SWaP-oriented description reports total driver electronics on the order of a few watts, package size $\le 30 \times 30\,\mathrm{mm}^2$, and chip weight less than $10\,\mathrm{g}$; the system-level comparison for the PLM solution reports less than $30\,\mathrm{g}$, less than $5\,\mathrm{W}$, and less than $5\,\mathrm{cm}^3$ [2509.05896].

## 2. Phase modulation, quantization, and calibration

For reflective operation, a vertical displacement $h$ of a mirror increases the round-trip optical path by $2h$, so the phase shift imparted to a beam of vacuum wavelength $\lambda$ is

$$
\phi \;=\; 2k h \;=\; 2(2\pi/\lambda)h \;=\; \frac{4\pi h}{\lambda}.
$$

Each 4-bit code $i \in \{1 \ldots 16\}$ corresponds to a discrete height $d(i)$, yielding $\phi(i)=4\pi d(i)/\lambda$. In the DLP6750 EVM characterization, the total piston range is approximately $0$–$325\,\mathrm{nm}$ at maximum mirror-bias voltage, sufficient to deliver a $0$–$2\pi$ phase shift at design wavelengths between $405\,\mathrm{nm}$ and $650\,\mathrm{nm}$. In the low-SWaP wavefront-correction report, four-bit discrete levels are stated to cover at least $2\pi$ at $1.55\,\mu\mathrm{m}$, with 4-bit depth yielding a steering precision less than $30\,\mu\mathrm{rad}$ [2409.01289; 2509.05896].

The finite 16-level quantization introduces a maximum quantization error of roughly half the step size, $\Delta\phi \simeq \pi/8$ rad at mid-range. In practical hologram generation, this quantization is reported to yield diffraction efficiencies on the order of $80$–$90\%$ for typical holograms. The mirror pitch of $10.8\,\mu\mathrm{m}$ sets the highest spatial frequency and induces an aliasing limit [2409.01289; 2509.05896].

Calibration is performed by choosing an illumination wavelength, optimizing the mirror-bias voltage to stretch the full phase span to $2\pi$, and then recording interferograms while the entire chip is set sequentially to each of the 16 codes. Two calibration geometries are described: an image-plane Twyman–Green arrangement with slight reference-arm tilt to produce straight fringes, and a Fourier-plane Young’s double-pinhole arrangement in which half the chip passes through one pinhole and half through the other. The phase response $\phi(i)$ is extracted by fitting a sinusoid to fringe motion; the codes are then reordered so that $\phi(i)$ is monotonic and stored in a lookup table (LUT) [2409.01289].

## 3. Mathematical formulation for wavefront correction and tracking

In the adaptive-optics formulation, the incoming aberrated wavefront $W(x,y)$ is expanded in a finite Zernike basis $\{Z_n(x,y)\}$ over the clear aperture $D$:

$$
W(x,y) = \sum_{n=1}^{N} a_n \cdot Z_n(x,y), \qquad (x,y)\in D.
$$

The objective is to choose a mirror phase $\phi_{\mathrm{plm}}(x,y)$ that minimizes residual error. A convenient quadratic cost is the integrated squared residual phase,

$$
J[a] = \int_D |W(x,y) + \phi_{\mathrm{plm}}(x,y; a)|^2\,dx\,dy.
$$

In vector-matrix form, with $z \equiv [a_1 \ldots a_N]^T$ denoting the sensed Zernike amplitudes and $\phi_{\mathrm{plm}} \equiv H_{\mathrm{plm}} \cdot (H_{\mathrm{tr}} \cdot z)$, the same objective is written as

$$
J(z) = \|z + K_p H_{\mathrm{tr}} z\|^2_Q,
$$

where $Q$ is a weighting matrix, often the identity, and $K_p$ is a scalar gain [2509.05896].

This formulation places actuator discreteness and update-rate limits directly into the control problem. The actuator constraint is

$$
\phi_{\mathrm{plm}}(i,j) \in \{0,\Delta\phi,2\Delta\phi,\ldots,(L-1)\Delta\phi\},
$$

with $L=16$ levels. The temporal bandwidth is limited by $f_s \approx 1.44\,\mathrm{kHz}$, corresponding to a sampling period $T_s \approx 0.7\,\mathrm{ms}$. Within this framework, simultaneous adaptive optics and tracking are expressed by superposition in the same transfer-matrix chain:

$$
\phi_{\mathrm{plm}} = H_{\mathrm{plm}}\cdot(H_{\mathrm{tr,AO}}\cdot z_{\mathrm{AO}} + H_{\mathrm{tr,track}}\cdot z_{\mathrm{track}}).
$$

A plausible implication is that the PLM is not treated merely as a programmable hologram source, but as a unified control surface on which low-order tracking and higher-order aberration correction share the same basis and hardware path [2509.05896].

## 4. Control workflows and real-time algorithms

Three real-time algorithmic regimes are explicitly described. The first is closed-loop adaptive optics with a wavefront sensor (WFS). The loop measures the Zernike vector $z_{\mathrm{wfs}}$ from Shack–Hartmann spots, computes the PLM phase vector according to

$$
\phi_{\mathrm{plm}} = K_p \cdot H_{\mathrm{plm}} \cdot H_{\mathrm{tr}} \cdot z_{\mathrm{wfs}},
$$

uploads $\phi_{\mathrm{plm}}(i,j)$ to the PLM, and repeats at $f_s$. For sufficiently small $K_p$, specifically when $K_p \lambda_{\max}(H_{\mathrm{plm}}H_{\mathrm{tr}}) < 1$, the loop is reported to be strictly stable and to reduce RMS error by approximately $(1-K_p \lambda)$ [2509.05896].

The second regime is tip–tilt-only control using a camera or quad detector. Here $z_{\mathrm{wfs}}$ is replaced by a two-element vector, either $z_{\mathrm{cam}} = [\Delta x,\Delta y]^T$ or $z_{\mathrm{quad}} = [I_R-I_L, I_T-I_B]^T$. The transfer matrix $H_{\mathrm{tr}}$ is remeasured for the new sensor geometry, and the same update law is applied with $N=2$ modes. The third regime is smart search and acquisition under single-pixel feedback: the PLM is first defocused to enlarge the beam and execute a coarse angular scan, the highest-power direction is identified, the beam is refocused, and a fine spiral or conical scan is performed around the coarse direction; static higher aberrations may optionally be corrected by adding low-order Zernikes [2509.05896].

The streaming and software stack are described in detail for the DLP6750 EVM. A continuous-valued target phase profile $\theta(x,y)$ is quantized to the nearest LUT phase value, each 4-bit code is encoded in a $2\times2$ memory cell of binary pixels, and 24 such binary holograms are packed into the 8 bit-per-color channels of one RGB frame. The frame is sent over HDMI at 30 Hz or DisplayPort at 60 Hz to the PLM, which is recognized as a secondary monitor. On each video frame, the PLM unpacks the 24 holograms and displays them sequentially, each for $554\,\mu\mathrm{s}$ with a $140\,\mu\mathrm{s}$ inter-frame gap consisting of $90\,\mu\mathrm{s}$ return-to-flat and $50\,\mu\mathrm{s}$ step time, yielding an effective update rate of $24 \times 60\,\mathrm{Hz} = 1.44\,\mathrm{kHz}$. External trigger lines provide one pulse per hologram for synchronization with cameras or other hardware. The accompanying C++ library, “plmctrl,” wraps low-level graphics APIs such as DirectX 11 and provides functions including `initPLM()`, `setMirrorBias()`, `uploadRGBFrame()`, `startStreaming()`, `stopStreaming()`, and `getTriggerLine()`; bindings were tested under Windows in Python, MATLAB, and LabVIEW [2409.01289].

## 5. Reported performance and demonstrated use cases

The adaptive-optics and tracking study reports that closed-loop correction of the first five Zernikes reduced RMS wavefront error from several wavelengths to less than $1\,\lambda$ in less than $50\,\mathrm{ms}$ convergence time. In tip–tilt tracking, the residual pointing error was less than $30\,\mu\mathrm{rad}$ for $1.55\,\mu\mathrm{m}$ light at update rates up to $1\,\mathrm{kHz}$. For beam coupling into a single-mode fiber, automated reacquisition in less than $100\,\mathrm{ms}$ was obtained using conical scan plus static aberration correction, achieving greater than $80\%$ coupling efficiency [2509.05896].

The wavefront-shaping study reports several optical demonstrations. In arbitrary pattern projection, a holographic Gerchberg–Saxton design projected in the Fourier plane at $\lambda=633\,\mathrm{nm}$ with $f=300\,\mathrm{mm}$ and demagnification $\times 3$ steered approximately $70\%$ of incident power into the synthesized image, with contrast greater than $35{:}1$. Measured first-order diffraction efficiency remained above $80\%$ up to diffraction angles $\phi \lesssim 8\,\mu\mathrm{rad}$ when using a linear-ramp hologram with approximately six pixels between $2\pi$ phase wraps [2409.01289].

For aberration measurement and compensation, the PLM was divided into $25 \times 42$ super-pixels of $32 \times 32$ mirrors each. The phase of each super-pixel was measured interferometrically, requiring $1050 \times 4 = 4200$ holograms, or $3\,\mathrm{s}$ at $1.44\,\mathrm{kHz}$. Conjugating the measured aberration map and adding it to a tilt hologram yielded a diffraction-limited focus with $3.5\times$ higher peak intensity. Native chip-curvature aberration was reported to span several interference fringes, approximately $10\pi$ rad over the full aperture, while in-situ correction reduced residual wavefront error to below $\lambda/10$ RMS [2409.01289].

High-speed wavefront shaping through a multimode fiber was demonstrated with the sequence PLM $\rightarrow$ 10$\times$ objective $\rightarrow$ 1 m step-index MMF with $\mathrm{NA}=0.22$ and core radius $25\,\mu\mathrm{m}$, with the fiber output imaged on a high-speed camera. Transmission-matrix measurement employed 2680 super-pixels of $20 \times 20$ mirrors and four-step phase-shifting holography, requiring 10,720 holograms or $7.5\,\mathrm{s}$ at $1.44\,\mathrm{kHz}$. The computed input field was $u_{\mathrm{in}} = T^{-1}\cdot u_{\mathrm{target}}$, with amplitude and phase encoded using the PLM’s 4-bit profile. The achieved focus had a power ratio $p_r \approx 0.75$, and 2160 focus positions were scanned at $1.44\,\mathrm{kHz}$ by streaming 90 full 24-hologram frames. The abstract summarizes this regime as scanning over 2000 points at $1.44\,\mathrm{kHz}$ [2409.01289].

## 6. Relation to other modulators, limitations, and system implications

The PLM is positioned between several established spatial-light-modulation technologies. Relative to liquid-crystal SLMs, the reported comparison emphasizes that LC devices offer high efficiency, bit depth from 8 to 16 bit, and megapixel resolution down to $4\,\mu\mathrm{m}$ pitch, but are typically slow at 60–120 Hz, with faster models near $1.7\,\mathrm{kHz}$ via heating or overdrive, and ferroelectric devices above $3\,\mathrm{kHz}$ but with binary phase and less than $50\%$ efficiency. LC-SLMs are also polarization-sensitive, and pixel crosstalk and flicker can degrade fidelity. Relative to deformable mirrors, the comparison emphasizes multi-wavelength stroke and speeds up to a few kilohertz, but only a few hundred to a few thousand actuators and limited suitability for high-resolution holography. Relative to DMDs, the comparison emphasizes switching above $20\,\mathrm{kHz}$ but binary amplitude-only control, less than $1\%$ useful diffracted power for phase tasks, many spurious orders, and reduced degrees of freedom [2409.01289].

Within that comparison, the PLM is described as combining high efficiency above $80\%$, polarization-agnostic phase-only control, 1 MP resolution, fast sub-kHz full-frame updates, and direct 4-bit phase on every pixel; it is also described as compact, as leveraging existing DLP infrastructure, and as scaling to planned $10\,\mathrm{kHz}$ operation. At the same time, the reported trade-offs are explicit: full WFS correction with more than five modes provides the best Strehl but requires larger SWaP and more compute; camera- or quad-based tip–tilt-only control with $N=2$ minimizes SWaP and supports very high update rate but only modest alignment accuracy of about $30\,\mu\mathrm{rad}$; and smart scanning adds a few extra microseconds per step while eliminating all external beam-steering optics [2409.01289; 2509.05896].

A common misunderstanding is to equate the mirror step response with currently available continuous full-frame streaming. The device-level characterization reports single-step mirror response below $50\,\mu\mathrm{s}$ and a fundamental modulation bandwidth of approximately $20\,\mathrm{kHz}$, but current electronics limit continuous streaming to $1.44\,\mathrm{kHz}$; firmware-enabled bursts up to $5.76\,\mathrm{kHz}$ have been demonstrated, and operation up to $10\,\mathrm{kHz}$ is anticipated. Another misconception is to treat the PLM as a purely beam-steering component. The adaptive-optics study instead formulates it as a monolithic phase modulator whose single pattern can superpose tracking and higher-order AO commands, with time-multiplexing between high-order AO updates and rapid tip–tilt updates to share the 1–5 kHz bandwidth [2409.01289; 2509.05896].

Taken together, the reported results suggest a distinct role for the Texas Instruments PLM: a monolithic, chip-scale phase modulator that can unify high-resolution wavefront shaping, adaptive-optics correction, beam tracking, and acquisition workflows within a common Zernike-to-pixel transfer framework, while replacing deformable mirrors, steering mirrors, and separate acquisition subsystems in low-SWaP optical systems [2509.05896].

Source: https://www.emergentmind.com/topics/texas-instruments-phase-light-modulator-plm