Koheron's blog
Notes on photonics, electronics and FPGA
Understanding the basics of laser diode drivers
In its most basic form, a laser driver is a current source built with a current-sense resistor and an operational amplifier. The operational amplifier measures the voltage across the sense resistor and controls its output in a feedback loop to maintain the resistor voltage as close as possible to the control voltage.
Laser I-V characteristic curve measurement
We look at I-V characteristic curves for 3 different diodes in butterfly package using the Koheron CTL200 digital laser controller (type 1, 600 mA laser current). The laser controller is connected to the computer via an USB cable. A Python script sets the laser temperature, scans the laser current and measures the laser voltage.
How current noise influences laser linewidth?
Laser diode driver current noise has strong influence on laser linewidths, but the way they are related is not obvious. To quantify a laser driver current noise, the root-mean square noise amplitude is often used. Here we show that to determine the laser final linewidth, one should rather look at the noise spectrum characteristics.
Laser frequency stabilization
Stabilizing the absolute frequency of a laser is central in many applications such as gas sensing or calibration of optical spectrum analyzers. Absolute frequency stabilization relies on the spectroscopic lines. Here we stabilize a telecom DFB laser on a HCN standard cell.
Measuring the noise of laser current sources
Laser diodes are very high efficiency devices: a photon is emitted for almost each injected electron. As a consequence, emitted light closely mimics the behavior of the driving current source. A low noise current driver is required to reach high quality emission. But what does "low noise" mean?
Common mode rejection ratio of a balanced detector
To reach the ultimate noise limit in a coherent detection setup, it is often necessary to reject the relative intensity noise (RIN) of the laser. In this post, we will present a setup to measure the common mode rejection ratio (CMRR) of the Koheron PD100B balanced photodetector.
Measure detector response using shot-noise
A photodetector combines a photodiode with a transimpedance amplifier. The amplifier conditions the photocurrent exiting the photodiode while being able to drive low impedance lines. For a wideband detector, the frequency response is determined by parasitic capacitances of many elements (photodiode, amplifier, feedback resistor, PCB tracks).
Synchronize a cluster of Red Pitayas
In its standard configuration, the Red Pitaya uses an on-board 125 MHz crystal to feed the 125 MSPS ADC and the 125 MSPS DAC. In this post, we present how to synchronize the outputs of multiple Red Pitayas on the same clock using the SATA connector (daisy-chain) available on the Red Pitaya.
Signal decimation using a compensated CIC filter
Decimation is the process of converting a time-domain signal sampled at frequency $f_s$ into a signal sampled at frequency $f_s / R$, where $R$ is the decimation rate. Downsampling alone, i.e. keeping one sample every $R$ samples is not sufficient. A low-pass filtering step must precede downsampling to prevent aliasing.
Implement a pulse-density modulator on an FPGA
Pulse-density modulation (PDM) is an attractive alternative to pulse-width modulation (PWM) in applications where the PWM technique creates unwanted spikes in the signal spectrum. We present the implementation of a pulse-density modulator on an FPGA to control the current of a laser.
Une introduction à Git
En l'espace de quelque années, Git s'est imposé comme le standard pour la gestion des versions logicielles. Le projet Git a été initié en 2005 par Linus Torvald pour gérer les sources du noyau Linux. Git repose sur une architecture distribuée : chaque copie locale d'un dépôt contient l'intégralité de l'historique du projet.
Web based interfaces for instrument control
User interface design is never an easy task: it must be cross-platform, easy to install and reactive. The first two points can be easily solved using a browser user interface. Web browsers are already installed on virtually every device (PCs, smartphones, tablets, ...). This removes the need of shipping a separate GUI application.
A simple coherent laser sensor
The Red Pitaya is a small network-attached instrument that can be thought of as a programmable oscilloscope. It features two analog inputs and two analog outputs, both with 14 bits resolution and 125 MHz sampling frequency. Although this is not the most powerful card on the market, its versatility and its price tag make it an ideal candidate to prototype custom instruments.
Concevoir une photodétection
Le photodétecteur est un composant critique dans tout système optronique, une mauvaise conception de ce dernier conduira souvent à une bien piètre qualité du système. Une erreur courante est de négliger le rapport signal-à-bruit afin d'augmenter la bande-passante. Nous allons dans ce qui suit essayer de présenter rapidement quelques principes à connaître pour la conception d'un photodétecteur performant.
New opportunities for modern electro-optical instruments
As stated by Philip C. D. Hobbs on the back cover of his book: Building a modern electro-optical instrument may be the most interdisciplinary job in all of engineering. Be it a DVD player or a laboratory one-off, it involves physics, electrical engineering, optical engineering, and computer science interacting in complex ways.
