Circuits that
think in light.

I am Bartu Yaman — physicist, MSc, PhD candidate and research associate at the Fraunhofer Heinrich Hertz Institute in Berlin. I build integrated photonic circuits: heterogeneous integration to allow increased functionality on photonic circuit platforms, high-frequency electro-optics, and optical neural networks — with an emphasis on mathematical modelling and design from first principles.

See the figures Get in touch
Live beam propagation, computed in your browser · Y-branch, push–pull arms, 3 dB coupler · Δn 0.02, λ 1550 nm · wavefronts drawn at a stretched wavelength

Figures

Fig. IThe EO workhorse: Mach–Zehnder modulator Fig. IIConscious parameter-space exploration & surrogates Fig. IIIOn-chip diffractive neural networks

Sections

01How I work 02What I am working on 03Where I have been

Fig. IThe EO workhorse: Mach–Zehnder modulator

This is the device that puts electronics onto light. Every high-speed link — datacom inside the datacentre, telecom between cities — needs an electrical signal turned into an optical one, and the Mach–Zehnder modulator is the workhorse that does it: broadband, linear enough to shape, and manufacturable. Its bandwidth, drive voltage and loss set the ceiling for everything downstream.

Fig. I · liveopticalelectrical
Live simulation · PAM4 symbol stream · GSG push–pull drive, ±Vπ/2 · the same signal drawn electrically, then optically

01How I work

First principles first, then whatever it takes

My core is mathematical modelling and design. I came to photonics from physics — a BSc at METU, an MSc at the Freie Universität Berlin — and I still work like a physicist: write down the physics, build the model, and let it decide what a platform or a component should be before anything reaches a mask.

A model is only as good as the chip that proves it, so I stay close to the technology. I review and design fabrication steps, design measurement setups and procedures, and measure the chips myself. A layout is a hypothesis and the wafer answers in weeks; being there for each step is how I learn which part of the model the process will honour.

Underneath it all, I solve problems. Whatever a project needs — a derivation, a new model, a process change, a test bench — I take care of it. The three figures on this page are three parts of that: one modulates, one computes, one searches.

Focus
Mathematical modellingPlatform & component designFirst-principles simulation
Hands-on
Fabrication-step review & designMeasurement setup & procedure designChip characterisation
Bartu Yaman leaning on a railing above a lake
Bartu Yaman · Berlin

02Research

What I am working on

02.1

High-speed systems on chip

Above a few tens of gigahertz the electrical and the optical design stop being separable: the microwave line and the optical mode have to travel at the same speed, over the same length, without the metal spoiling the light it is steering. So they are designed as one problem — transmission line, waveguide and drive together. That electro-optic co-design is what sets the real RF and optical performance of any electrical-to-optical or optical-to-electrical conversion.

02.2

Computing in propagation

A diffractive layer is a weight matrix you cannot see. Stack a few in a slab and the interference performs the multiply–accumulate in the time the light needs to cross the chip — no clock, no memory traffic, and no energy beyond the laser that lit it.

02.3

Surrogate modelling & advanced design

A full-wave sweep of a modulator is hours per point, and the design space has more dimensions than patience. So the sweep is replaced by a model of itself: sample a few designs, fit a surrogate to them, and let the acquisition decide where the next simulation is worth running. The search happens on the model; the solver only confirms.

02.4

Heterogeneous integration

A passive platform routes light well but cannot modulate it; an active crystal modulates beautifully but makes a poor circuit. Bond one onto the other and each does the job it is good at. The work is in the seam — mode transitions between layers, tolerance to bonding and film thickness, and how much of the crystal's figure of merit survives into the circuit. The method outlives any one material.

Fig. IIConscious parameter-space exploration & surrogates

Six design knobs and three numbers that fight over them: half-wave voltage, bandwidth, insertion loss. Only two knobs fit on a sheet of paper — the surface is the slice the other four currently cut, and it re-forms whenever they move. It is not the truth either, but a surrogate fitted to the designs sampled so far. The walker goes where the model says the cost is low and where the model is least sure.

Fig. II · livelow costhigh cost
Live radial-basis surrogate over sampled designs · expected-improvement-style acquisition, explore then exploit · the objective weighting changes each round

Fig. IIIOn-chip diffractive neural networks

The input is written as light in a slab, then crosses three phase masks — metalines of sub-wavelength scatterers. Propagation between them mixes every input into every output: the masks are the weights, the interference is the arithmetic, and the brightest detector is the answer. Move your pointer across the figure to steer the input.

Fig. III · livefield intensity
Live scalar diffraction · angular-spectrum propagation · λ 1550 nm · 256 samples across a 218 µm aperture · three phase masks solved by wavefront matching onto four detector bins

03Record

Where I have been

  1. 2025 —Research AssociateFraunhofer HHI, Berlin
  2. 2023–25Research AssistantFraunhofer HHI, Berlin
  3. 2022–25MSc PhysicsFreie Universität Berlin
  4. 2020–22Undergraduate ResearcherMETU, Ankara
  5. 2020–22Freelance ConsultantIndependent
  6. 2018–22BSc PhysicsMETU, AnkaraThesis: State Encoding With Quantum Classical Neural Networks
  7. 2021Summer of HPCPRACE, Ljubljana

Contact

Say hello

Best reached by email. I am always glad to talk about modulators, diffractive optics, or anything that turns a wafer into a system.

bartuyamann@gmail.com linkedin.com/in/brtymn