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Stabilisation of the frequency of a diode laser with an external resonator to an atomic absorption line

In this illustrative application we explore the process of stabilising the frequency of a tunable diode laser using an external resonator, specifically designed to interact with an atomic absorption line.

In this application the frequency of a tunable laser is stabilised with the help of a reference cell or a gas reference cell. Suitable lasers can include tunable diode lasers, Ti:Sa or dye lasers.

The aim is to set the laser frequency to a value at which the sample has maximum absorption (or minimum absorption).

Laselock application

This application requires the following components:

• 1x digital LaseLock with HV option
• 1x laser with tuneable frequency, here via piezo-actuator (e.g. TOPTICA DL100 diode laser)
• 1x spectroscopic absorption cell*
• 1x beam splitter
• 2x photodetectors

Principle of operation

Two different methods can be applied:

• Side-of-fringe stabilisation
• Top-of-fringe stabilisation (to maximum or minimum, ‘lock-in’-technique)

Graph showing the LaseLock side of fringe

Side-of-fringe stabilisation

This method is used when a direct discriminator signal can be derived from the measurement signal. In other words the slope of the peak signal is used to convert frequency fluctuations of the laser into amplitude fluctuations, which can be detected and subsequently stabilised.

Graph showing the LaseLock top off fringe

Top-of-fringe stabilisation

This method uses a modulation technique and phase-synchronous detection.

For this the laser frequency (or a different physical measure like the resonator length) is modulated. A detector signal is multiplied by the modulation signal and then the product signal is averaged by a low-pass filter. The resulting ‘lock-in’-signal represents the derivative of the signal with respect to the laser frequency (or the respective varied physical measure).

This signal can be used directly for physical examinations. As in most cases it contains less disturbing signal parts (noise, offsets) than the directly measured signal.

The zero-crossing of the derivative represents a maximum (or minimum) of the detected signal structure. For stabilisation of a laser or resonator towards such an extremum the ‘lock-in’ signal is processed by a regulator. This generates a suitable control signal that is fed back (either directly, or for piezo actuators via a high-voltage amplifier) to the frequency-determining element of the laser (or resonator). In this way, the control loop is closed and the laser (or resonator) is locked actively to the maximum (or minimum).

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Please get in touch with our team, we’re open Monday to Friday from 9am until 5pm, when you can call us on +44 14836 62670. Alternatively, email us at [email protected] or fill in our contact form, and a member of our team will respond within 24 hours (Mon-Fri).

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Lasers are often used within physics classrooms and labs. They allow students to conduct experiments at different wavelengths and test the different outcomes. However, switching between wavelengths can take time in order to realign the laser in a new setup with a different wavelength. 

Setting up lasers as a teacher

When using lasers for physics experiments, different wavelengths are needed for various experiments. Teachers need to manage those arrangements in advance, and this can be very time-consuming.  However, the HEXA-BEAM laser takes away this need for various experimental setups.  With a single turn of a knob, you can switch between six different wavelengths. This not only allows for a more controlled experiment setup but also means less time wasted in setting up a new laser for each experiment. 

What classes of lasers are used in physics labs? 

Only Class 2 lasers are considered safe for use in classrooms and labs. This is because Class 2 lasers are relatively weak and would not harm an eye unless a person deliberately stares into the beam. Class 2 lasers should have an output below 1 mW.

Why is a laser used in experiments and how are lasers used? 

A laser allows a student or teacher to perform an optical experiment. The laser will provide a visible and clear light source in order to obtain accurate measurements. 

What do the different laser colours mean? 

Commonly, lasers will produce 4 different colours of light- green, red and blue, and yellow. The different laser colours show the wavelength of the laser beam. The colours vary from violet, blue, and green to red and infrared. The HEXA-BEAM laser produces 6 different wavelengths of light – from violet, green, and red as standard, to additional greens and reds, and blue…  Green light is from 520 nm to 532 nm and usually appears brighter to the human eyes when at the same power, the output of each of the colours is the same.   Red light sources are between 630nm and 670nm and Violet and blue light measure from 405nm to 445nm. 

Why the HEXA-BEAM Laser is the perfect laser for physics experiments 

The HEXA-BEAM laser is able to switch between 6 different wavelengths with ease, so there is no need to re-align your equipment and restart the setup of the experiment.   The different wavelengths that the HEXA-BEAM laser produces are emitted along the same optical path and are vertically polarised.   As standard, the HEXA-BEAM contains 405nm, 520 nm, and 650 nm modules. 

What physics experiments can the HEXA-BEAM laser be used for? 

The ease of set up and flexibility of the HEXA-BEAM makes it ideal for a number of experiments within the classroom or lab including: 

• Biot’s Optical Rotation Experiment or Biot’s Sugar Experiment 
• Rotation of the Plane of Polarisation 
• The Faraday Effect  
• Poisson spot (Fresnel bright spot) experiment (showing that light behaves as a wave)
• Malus’s law
• The law of refraction, measuring refractive indices at different wavelengths of different materials, producing diffraction patterns
• Determining the emission wavelength using a simple grating
• Demonstrating Mie and Rayleigh scatterings
• Demonstrating chirality of molecules

Safety with physics lasers 

It’s important to note the health and safety precautions that must be followed when using lasers within a classroom or physics lab. Lasers should not be shone directly into a person’s eye and indirectly by reflecting the laser off a surface into the eyes. Students should be in a safe viewing zone, where the potential for the laser to be directed into the eyes is nil. 

The HEXA-BEAM laser complies with the health and safety regulations for use in undergraduate labs. 

• By having emissions powers of standard wavelengths under 1mW 
• By meeting the ‘class 2’ specifications which in turn minimises the potential health risks and hazards 

Contact Us 

For more information on how Photonics Technologies can provide your school or university with physics experiment equipment, please get in touch.   We’re open Monday to Friday from 9am until 5pm, when you can call us on +44 14836 62670. Alternatively, email us at [email protected]. Or, fill in our contact form, and a member of our team will respond within 24 hours (Mon-Fri).

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Are you running physics experiments in your undergraduate lab that require changing between different wavelengths?

Wouldn’t it be great to switch between these various wavelengths without the need to re-align the laser set-up?

Introducing the HEXA-BEAM Laser

The ideal laser for undergraduate experiments and classroom learning.

Undergraduate students conduct physics experiments as part of their learning. These experiments support students in progressing and developing their knowledge and understanding. Experiments are carried out in laboratories and will normally require lasers and other relevant physics equipment.

The use of lasers within learning environments is important to help students with their physics studies. This equipment is vital for students to learn first-hand, in a safe and suitable educational environment.

Photonics Technologies understands that conducting physics experiments can take time. So, we have developed a multi-wavelength laser, the HEXA-BEAM.

This wavelength-switching laser is versatile and designed to switch between up to 6 different wavelengths. All without changing the experiment setup, saving time and increasing control over the experiment. We’re confident that this laser will prove useful in every undergraduate University physics lab, up and down the UK.

HEXA-BEAM Laser

The HEXA-BEAM Laser has the ability to switch between different wavelengths by simply turning a knob. With one simple laser source, you can select your laser colour at the turn of a knob!

The HEXA-BEAM Laser offers the standard wavelengths of 405, 520, and 650 nm. Additional optional visible wavelengths are also available.  Please see more details about the HEXA-BEAM laser wavelengths.

Contact Us

Please get in touch with our team. We’re open Monday to Friday from 9am until 5pm, when you can call us on +44 14836 62670. Alternatively, email us at [email protected]. Or, fill in our contact form, and a member of our team will respond within 24 hours (Mon-Fri).

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Physic Experiment for Rotating the Plane of Polarisation (also known as Biot’s sugar experiment)

Biot’s optical rotation experiment demonstrates the effect of chiral media (sugar solution) on the plane of polarisation of linearly polarised light. This experiment allows for investigation into the effects of polarisation, chirality, scattering, and fluorescence.

Chirality is the name given to the property of molecules and systems whose image cannot be superimposed upon its mirror image. When linearly polarised light is passed through a chiral medium, such as a sugar solution, the plane of polarisation of the incident light is rotated due to its asymmetric structure, creating bright and dark bands that can be observed along the optical path.

Linearly polarised light is composed of left and right circularly polarised partial light waves, with both components having equal amplitudes. When these partial waves propagate through the chiral material, they encounter different refractive indices due to the asymmetry of the medium. The partial wave that oscillates through a larger refractive index will travel slower than its counterpart, producing a phase difference between them. This phase difference causes the plane of polarisation to rotate. The angle of rotation caused by the differing refractive indices is described by the following equation:

Here, is the optical angle of rotation, and are the refractive indices of the left and right circularly polarised partial waves respectively, is the length of the optical path and is the wavelength of the light beam.

The rotation of linearly polarised light causes observable bright and dark bands to appear along the cell containing the solution because the light is not entirely observable from one angle.

Equipment Required

• HEXA-BEAM Laser

The HEXA-BEAM laser makes it easy to run this experiment in different colours, allowing the student to appreciate the effect of wavelength on the experiment. With a knob at the back of the laser, students can quickly change wavelength without any realignment needed.

The HEXA-BEAM laser is aligned down the centre of the glass tube to reduce any scattering. The bands of minima/maxima are visible and their separation can be measured.

• Glass tube
• Rotatable polarising filter
• Observation screen
• Photon detector
• Rule

Analysis

The specific rotation can be determined by the distance between successive minima or maxima, as the distance between these points is equivalent to a specific rotation of .  Therefore, the specific rotation of the polarisation plane can be calculated by adjusting the values in the below equation

Where L is the distance between the consecutive extrema that are measured, and C is the concentration of the solution the laser is passing through. The values of the specific rotation can then be compared to a mathematical fit using Drude’s expression.

Sources of Error

• Inhomogeneity of solution – the presence of bubbles within the sugar solution will prevent accurate results being obtained
• Location of the maxima/minima
• Parallax error observing the maxima/minima due to the laser beam being contained within the glass tube
• Position of detector outside glass tube
• Scattering of laser beam entering the tube

Contact Us

Please get in touch with our team. We’re open Monday to Friday from 9am until 5pm, when you can call us on +44 14836 62670. Alternatively, email us at [email protected]. Or, fill in our contact form, and a member of our team will respond within 24 hours (Mon-Fri).

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Ensuring precise laser beam alignment is a daily task in physics labs, where accuracy is paramount for conducting experiments and obtaining reliable results. This critical procedure involves aligning the laser beam so that it hits the intended target with exact precision. Even the slightest misalignment can significantly impact the outcomes of experiments, making this an area of ongoing interest and debate among physicists. A particularly engaging aspect of this field is the debate between Active and Passive laser stability.

Smart electronics provide the solution, utilising specialised equipment and a continuous feedback loop to stabilise the laser accurately. We can stabilise your laser with specialised equipment which will ensure a controlled laser beam through a continuous feedback loop.

Dr. Thomas Kinder explains this groundbreaking technology in the video below.

Contact Us

Please get in touch with our team. We’re open Monday to Friday from 9am until 5pm, when you can call us on +44 14836 62670. Alternatively, email us at [email protected]. Or, fill in our contact form, and a member of our team will respond within 24 hours (Mon-Fri).

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Optical Mirror Mounts

Our Optical Mirror Mounts come in a variety of types and characteristics to deliver exceptionally high stability for the laser beam and delivery:

• Manufactured to exceptionally fine tolerances, providing a new benchmark for stability and precision with a fine- thread screws with a pitch of 170 TPI
• Constructed from specially treated materials to prevent movements from mechanical stress and changing temperatures.

You can design your own configuration – choose from the different front and back plates, adjusted screws, and colours!

Star Series SR050- Kinematic Mirror Mounts- Assorted Colours

Our Optical Mirror Mounts come in a variety of types and characteristics to deliver exceptionally high stability for laser beam and delivery:

• Manufactured to exceptionally fine tolerances, providing a new benchmark for stability and precision with a fine- thread screws with a pitch of 170 TPI
• Constructed from specially treated materials to prevent movements from mechanical stress and changing temperatures.

You can design your own configuration – choose from different front and back plates, adjusted screws, and colours!

Star Series SR050- Kinematic Mirror Mounts- Assorted Colours

Contact Us

Please get in touch with our team. We’re open Monday to Friday from 9am until 5pm, when you can call us on +44 14836 62670. Alternatively, email us at [email protected]. Or, fill in our contact form, and a member of our team will respond within 24 hours (Mon-Fri).

Explore further blogs and news articles from Photonics Technologies.