Modulating Lasers

Whether the output from your laser is Constant Wave (CW) or Q-switched, it's possible that you may need to control the laser's behavior in some way. If you need to pulse or trigger the laser, or control the output power dynamically, then you require some form of modulation. Each modulation technique has its own benefits and drawbacks, so choosing the right type of modulation for your application is a critical step in designing your system.

Types of Laser Modulation

• Embedded Electronic Modulation

• Optical Choppers

• Optical Shutters

• Acousto-optic Modulators (AOM)

• Electro-optic Modulators (EOM)

laboratory lasers feature electronic modulation, which is built into the driver circuit. This type of modulation is the easiest to use because it requires little additional equipment. All you need is a function generator to produce the modulation signal. The electronic modulation circuit controls the power delivered to the laser diode, so it is directly turning the laser on and off. This type of modulation can also control the output power of the laser by reducing the electrical power delivered to the laser diode. The maximum response frequency of this type of modulation is usually around 30 kHz, though this varies by model and some may be lower. The two types of electronic modulation are TTL and analog.

TTL:

Accepts a 0-5 VDC square-wave signal and can turn the laser on and off rapidly. Does not allow the user to adjust the power. The laser will pulse at full power.

Analog:

Accepts an arbitrary 0-5 VDC input signal. The laser's output power will track the amplitude of the input signal, within the limits of the maximum response frequency.

Advantages	Disadvantages
Already included with many lasers. Requires only a simple function generator to provide the modulation signal. Analog modulation allows for dynamic power control. Full extinction when laser is modulated to zero. Diode lasers produce clean, consistent pulses up to the specified maximum modulation frequency.	Maximum modulation frequency limited to 30 kHz. Because the power to the diode is being cycled, pulse shape may show undesirable artifacts, particularly with DPSS lasers. Pulse shape tends to degrade at higher frequencies. Rise time and fall time are limited by the laser diode's properties. Not compatible with more sensitive lasers. (Low-Noise, SLM, etc.)

Optical Choppers

An optical chopper is usually just a rotating disc punctuated with holes or slits. As the disc rotates it periodically blocks the beam and pulses the laser at a fixed frequency. These usually include an electronic controller which allows for precise control of the rotational frequency of the disc. Since they are mechanical in nature, the maximum frequency is limited to the high kHz domain, due mainly to air resistance. The diameter of the laser beam also places some constraints on the size of the slits, and therefore the maximum pulse frequency is lower for larger diameter beams.

Advantages	Disadvantages
Laser operates continuously, so pulse shape is clean and consistent. Relatively inexpensive: Basic systems start under a thousand dollars. Works well with low-noise or SLM lasers. Full extinction when laser beam diameter is less than the spacing between slits.	Maximum modulation frequency limited to high kHz range. Duty cycle of pulses cannot be adjusted. Frequency cannot be rapidly adjusted during operation. Rise time and fall time are a function of the laser beam diameter, slit diameter, and rotational speed. Can be relatively high. Can produce diffraction artifacts as the edge of the slit moves through the beam.

Optical Shutters

Optical shutters encompass a range of different devices which mechanically block or divert the laser beam. These can be in the style of guillotines, diaphragms, irises, LCD elements, or even a rotating mirror which redirects the beam. The common feature which sets them apart from choppers is that each event is individually triggered. You can open a shutter for an arbitrary amount of time and then close it again. Some of them have variable speed, so you can adjust the rise/fall time to some extent. Others can be made to open only partway, allowing for some degree of power control. This is a very broad category, so there is a lot of variability, but they are all mechanical systems and are therefore limited to some extent by their physical components.

Laserglow offers a shutter system with a maximum modulation frequency of 100 Hz, which is specifically designed for optogenetics. See our modular optics kits for full details.

Advantages

Disadvantages

Laser operates continuously, so pulse shape is clean and consistent. Relatively inexpensive: Basic systems start under a thousand dollars. Works well with low-noise or SLM lasers. Most feature full extinction. Pulses are individually controlled with fully customizable timing.

Maximum modulation frequency very limited. Maximum frequencies of 10-100 Hz are common. LCD shutters are faster (~1 kHz), but suffer from low extinction ratios. Some shutters can make a lot of noise when operating at high speed. Rise time and fall time are a function of the laser beam diameter and the shutter speed. Can be relatively high, in the millisecond domain. Can produce diffraction artifacts as the edge of the shutter moves through the beam.

Acousto-optic Modulators

An acousto-optic modulator (AOM, also sometimes called a Bragg cell), is a device which can pulse the laser or alter the transmitted power at a very high frequency. Basically, they function by sending the laser beam through a crystal which is subjected to acoustic vibrations. These vibrations alter the refractive index of the material, so the beam can be redirected from the output aperture to a beam dump in an incredibly small amount of time. The rise/fall time is limited only by the amount of time it takes for the acoustic wave to traverse the width of the crystal, so this is usually on the order of nanoseconds. An AOM can modulate a laser with essentially any arbitrary input waveform, with a response frequency in the MHz domain. They can be mounted in free-space or installed inline with fiber.

Advantages

Disadvantages

Laser operates continuously, so pulse shape is clean and consistent. Maximum modulation frequency is very high (many MHz). Rise/fall time is very low (ns). Most feature full extinction. Works well with low-noise or SLM lasers. Pulses and transmitted power are individually controlled with fully customizable timing. Can be easily mounted inline with fiber or in free-space.

Costly: The least expensive systems start in the thousands. Transmission efficiency is less than 100%. Average figures are around 80% or so, but this varies widely by type of AOM and laser being used. High-frequency acoustic noise is not detectable to humans but may affect animals and sensitive equipment. Because the laser is passing through a material, damage threshold can be an issue with very powerful lasers. Input beam diameter is limited to a few mm.

Electro-optic Modulators

An electro-optic modulator (EOM, also sometimes called a Pockels cell) is similar to an AOM except that it uses an electric field rather than an acoustic wave in order to change the optical properties of the material inside. The Pockels cell inside the EOM alters the polarization state of the beam, and a polarizer is used at the output to convert this change in polarization to a change in amplitude. Because the device uses electric fields rather than mechanical acoustic waves, it can operate faster than an AOM. Commercial models are available with a maximum frequency of up to 200 MHz.

Advantages	Disadvantages
Laser operates continuously, so pulse shape is clean and consistent. Maximum modulation frequency is extremely high (hundreds of MHz). Rise/fall time is very low (ns). Works well with low-noise or SLM lasers. Pulses and transmitted power are individually controlled with fully customizable timing. Can be easily mounted inline with fiber or in free-space.	Costly: The least expensive systems start around $5000. Does not provide full extinction. Typical values for extinction ratio are around 10 dB. Because the laser is passing through a material, damage threshold can be an issue with very powerful lasers. Input beam diameter is limited to a few mm.