Digital PID TEC temperature controller for DPSSL+SHG experiment

This is a very simple and low cost TEC PID controller. Here is a simple schematic of it.

TEC_controller_schematic

The controller is based on Arduino (AVR ATMega series 8bit MCU), so I haven’t spend a lot time programming it. The temperature sensor is a very cheap 10K thermistor and direct sampled by AVR’s internal ADC. The DAC part is actually a low-pass filtered PWM. The controller is connected to the computer through a USB-USART adapter.

A photo of the TEC controller is shown below. I use the power stage from my 3A CC driver in the same project as you can see from the photoTEC_controller_photo

 

I design a simple serial command line interface for system monitoring and parameter adjustment. You can simply type ‘P10’ and press enter to set parameter P to 10 for PID controller. It is same for other command such as ‘I0.5’ for setting I=0.5, ‘D1.24’ for setting D=1.24. The power stage can be turned off with command ‘OFF’ and be turned on again with ‘ON’. system states including output power, target temperature, PID parameters will be continuously updated on the serial interface.

 

3A Constant Current Driver for DPSSL+SHG experiment

This CC driver is designed for DPSSL+SHG experiment

cc_driver_schematic

This is a very simple current source, I just use it to safely drive a common high power C-mount laser diode. Do not use this driver to drive any expensive diode because there isn’t any protection circuit in this schematic. And do not expect great drift and noise performance of this circuit. If you are looking for some precision choice, you may keep a look at my design specified for precision lasers.

The left part buffers the output from TL431A (a very cheap voltage standard), to give a stable voltage about 0.041V.

The center part charge a large capacitor through 10K resistor to form a RC circuit, providing soft start function to the driver.

The right part is a simple PI controlled constant current circuit, receive voltage signal from the potentiometer which set the current we want, get the feedback from 5 parallel current sensing resistors, drive a MOSFET to control the current.

In order to reduce the heat dissipate on the MOSFET, I use a DC-DC module to provide power a little bit higher than the Vf of the diode. This  method is not practical in precision driver because the DC-DC module will bring in terrible noise to the system.

Here are photos of the driver under test and the final PCB version.

 

cc_driver_test

cc_driver_photo

 

 

Diode pumped solid state laser + SHG

This is quite a beginner level optical project.

As you may see, this project is divided into three parts, the diode laser, the pumping part and the SHG part.


The Diode Laser

A multi-mode laser diode is chosen as the core of the diode laser part. This diode laser is rated at 3W with wavelength around 808nm. In order to drive this powerful laser diode, we need to design a laser diode driver capable of providing constant current up to 3A. A picture of our 3W laser diode is shown below.
LD

Considerable amount of heat also needs to be dissipated when the diode is running at full load. However, due to characteristics of laser diode, its frequency will drift greatly when temperature changes, which will make the pumping unstable. Hence thermoelectric cooler (TEC) is introduced here together with a self-made digital controller to set the laser diode at nearly any temperature (restricted by TEC).

With some tiny but carefully designed mechanical part, I may mount the laser diode rigidly on a water cooled dock. A rendered picture of the mount part and a picture of assembled laser diode module is shown below.

LD_mount_design

The Pumping Part

The pumping part will receive 808nm laser from the diode laser and pump a 2.5*2.5*10 size Nd:YVO4 crystal (0.3% doped) to produce 1064nm laser.

First, we need to focus the laser beam into a tiny spot to create sufficient power density in the crystal. I use two 30mm focus length lens to finish the task, which works great with this fast axis compressed (FAC) laser diode. The focused laser spot is really powerful, which even burns the ceramic IR viewing card (black lines and spots).

burning_spot

The crystal need a giant heat sink too. The crystal mount is made of copper. The crystal will be installed in the slot with a small copper block pressing on it. I also place some indium foil between the crystal-copper contact surfaces to ensure good thermal contact.

crystal_mount_design

crystal_mount_photo

In this setup, one surface of the crystal is coated with 808HT+1064AR, and another is 1064HR. which means 808nm pumping light will go right into the crystal and be absorbed but the back surface will reflect any 1064nm laser to form a part of the cavity mirror. If I place a mirror that partially reflective 1064nm laser in front of the crystal  to form a whole optical cavity, we may get the 1064nm beam. The mirror in this experiment is a plain mirror, 90% transparent 10% reflective for 1064nm. I place it 30mm in front of the crystal. After some adjustment, we get the 1064nm laser spot! (The dark red spot is expand 808nm laser while the bright green spot is 1064nm laser)

1064_spot


The SHG Part

I choose KTP crystal in this experiment. KTP is a very common SHG crystal, often found in high power CW DPSSL lasers. The KTP I use is 3*3*5 in size, mounted on a copper adapter to fit into ordinary lens holders.

KTP_holder

If the KTP crystal is directly placed into the Nd:YVO4 laser cavity, a strong green laser beam can be observed. However, if we want to study the characteristic of SHG, it may be a better choice to put it outside the cavity. Adding a small prism after the KTP, I can split the 532nm beam and 1064nm beam. (Take a picture using a camera with IR filter, you can see a green 532nm spot on the left and a shining 1064nm spot on the right, indicating low SHG efficiency without cavity)

532_1064_spot


Some Measurement

Some results from Ocean Optics QE65 Pro spectrometer:

The left one shows the result when the output coupler mirror is not present. This spectrum clearly shows the fluorescence of the Nd:YVO4 crystal. Several emitting peaks and strongly absorbed 808nm peak can be found in this graph.

The right one shows the result after KTP is installed outside the cavity. Three peaks of 532/808/1064 wavelength are shown on the graph.

spectrum_1

spectrum_2


A picture of the whole setup:

whole_setup

Measurement of Electron Drift in Gas

This project aims at measuring the velocity of drifting electrons triggered by UV laser, which can be considered as a prototype of the TPC (Time Projection Chamber) laser calibration system. The introduction includes the following aspects: the design of the system, data collection, data analysis and preliminary results.

Whole System

High voltage is applied to the MPC (Multiwire Proportional Chamber) to generate a uniform electric field. Treated as a point-like particle, the laser-stimulated electrons in the field will reach a constant velocity in the working gas soon after their appearance. Since the accelerating time is short, we can assume the drifting time is approximately proportional to the drifting distance. Through a linear fit, we can get the drifting velocity of certain kind of working gas.

drift_1
The Drift Chamber

The system is designed under the principle of automatic control. The motion of MPC is dominated by a stepper motor, which is controlled by computer. The theoretical precision of motion is approximately 1 μm. The amplified signals of Laser and MPC are sampled and shown on oscilloscope. The connection between computer and oscilloscope ensures the arbitrariness of data collection. We write a LabView program to manage both of these. Abundant data are collected on each position and then exports to a file.

Apparatus Sketch
snapshot_1
Data Acquisition based on LabVIEW

In order to calculate the drifting velocity from amplitude varied data, special strategy should be applied. For each group, we find the average of maximum and minimum. Then we fit the data (either ascending or descending slope) to obtain its linear regression equation and solve for the time on that average level. The average of these time spots can be considered as the drifting time of the point. Linear fit these drifting time points to derive the drifting velocity.

Signal Analysis & Fitting

The working gas we adopted was 9.97% Methane in Argon. We sampled 20 times for each point, with total 10 sampling points in all. The drifting velocity we find is  u=(4.840±0.053)×10^4 m/s and is also supported by other researches.

DCDA
Drift Time vs Drift Distance
Results from other sources

 

Low noise power supply for everything

This is a linear power supply, which consists of several floating power regulators and LDO controllers. It will provide almost all the power you need to drive an ECDL.

Looks, strong!lnps

I even write a program on STM32 to monitor the power supply. The monitor is equipped with a touchscreen! (Everyone deserves a touchscreen nowadays, right?…)lnps_monitor