Convert Continuous Wave Laser to Pulse

INTRODUCTION:

A CW laser is a continuous wave (or constant wave) laser. This acronym is simply used in order to classify "CW" lasers separately from pulsed lasers. A pulsed laser has an output which is designed to turn on / off at a specific repetition rate. The acronym "CW" stands for "continuous wave". Producing a CW laser output can be accomplished with different types of materials. These materials include gas, crystals and various semiconductor materials. This tutorial will focus on CW lasers produced by diode pumping solid state materials (CW DPSS lasers) and CW fiber lasers.

caption: block diagram of how a basic diode pumped solid state CW laser works

QUICK NAVIGATION:

• DEFINITION OF A LASER
• PRINCIPLES OF A LASER
• HOW DO LASERS WORK?
• WHAT IS A DIODE PUMPED SOLID STATE LASER?
• WHAT IS A FIBER LASER?
• WHAT IS A 532nm LASER?
• WHAT IS A CW LASER?
• DPSS CW LASER PACKAGING
• HOW MUCH DO CW LASER COSTS?
• HOW ARE CW DPSS LASERS USED?
• WHICH COMPANIES MANUFACTURE CW LASERS?

WHAT IS A LASER ?

Laser is an acronym standing for Light Amplification by Stimulated Emission of electromagnetic Radiation. It is concentrated light stimulated from changes in energy states of electrons. Similar to the light from an incandescent light bulb, laser light emits from the change in energy state of electrons through electrical energy(flow of electrons) in the filament.

WHAT ARE THE PRINCIPLES OF A LASER?

Principle number one begins with atoms. Atoms are constantly in motion, they vibrate, move and rotate, called states of excitation. If energy is applied to an atom (in the form of light, heat or electricity), termed excitation, the electrons orbiting the nucleus of the atom becomes excited (absorb the energy) to change their energy level from "ground" or their equilibrium level to a higher energy state. Without sustained energy excitation, the electrons will return to this more stable ground level. But in so doing, due to the conservation of energy principle, the excited electrons have to release the energy that they gained. They release this energy in the form of a photon. The newly released photon has a very specific vibrational frequency that depends on the energy level when the photon was released. Frequency and wavelength are related.

For those looking for the mathematical relationships of energy, frequency and wavelength, it's pretty straightforward:

Equation 1 - Energy and Frequency: E2 - E1 = h v

Where:

• E is energy measured in Joules
• h is Planck's constant, Joules-second
• v is frequency, 1/seconds

Equation 2 - Wavelength and Frequency: λ = c/v

Where:

• λ is wavelength measured in meters
• c is the speed of light in the active laser medium, meter/seconds

Equation 3 - Wavelength and Energy: E2 - E1 = h c/λ

There are four principals that form the basis of understanding how a laser output works. They are spontaneous emission, stimulated emission, population inversion and amplification. These principals are discussed below.

» Principal Number 1: The release of a photon is called spontaneous emission, i.e., the light part of the acronym "laser". Anything that produces light, such as fluorescent lights, gas lanterns or incandescent bulbs, does it through the action of electrons changing energy states and releasing photons. Even in fires where an ember is glowing red, the heated atom's electrons are changing their energy level as they cool and release photons in the visible wavelength range (red). However, a laser doesn't continuously emit concentrated light with spontaneous emission. Something more is needed.

» Principal Number 2: Stimulated emission occurs during the time that an excited electron is in a higher energy state. Another photon of an energy level equal to the difference between its present level and the lower energy level can cause or stimulate that electron into its ground or stable state. This results in the release of a second photon of equal energy and frequency (wavelength). This is the essence of a laser. All of the photons are synchronized in a way, and this is what is meant by monochromatic (same wavelength) and coherence (same phase and direction). This is the stimulated emission part of acryonym "laser". However, sustained stimulated emission is not guaranteed at any energy level. There needs to be an adequate number of electrons emitting photons.

» Principal Number 3: Population inversion refers to the number of electrons (the population), and inversion refers to the change in energy state of the electron to a higher state. Electrons are naturally at their ground level state. Sustained lasing requires a forcing function to invert that condition. In order to sustain stimulated emission as efficiently as possible, it is necessary to have a lot of electrons in higher energy states. Far more than are at the ground level. Lasing is not guaranteed just because you have excited electrons, it becomes statistically possible only when there are ample electrons in an excited state . The higher the energy level of the electrons, the higher the degree of population inversion, thus the greater the chance of amplified emission. The energy supplied for the principle of population inversion is from an external source, a light source (ie a flash lamp), another laser, or most commonly an electrical charge. This light source and lasers process based energy sources are referred to as optical pumping.

» Principal Number 4: Amplification, which is a result of both population inversion and stimulated emission. When a population inversion is present, the rate of the stimulated emission exceeds that of absorption. This is because there are more electrons at an excited energy state for which two photons are generated from each incident photon. From four photons, eight are created, from those eight, sixteen and so on. In this condition, a net optical amplification (or gain) is achieved. In a laser, this is accomplished by encapsulating the movement of electrons in what is called an optical cavity. This could be made up of a solid, a gas or even a semiconductor junction commonly called the gain medium.

HOW DO LASERS WORK?

In order for light amplification by stimulated emission (LASER) to occur, as explained in the paragraphs above, this stimulated emission and amplification in enclosed in a cavity with mirrors on each end. The cavity can be made with a lasing medium called an active medium, or a gain medium which can be a specially formulated (doped) crystal. It can also be a tube with a gas (such as helium-neon). It can also be a specific semi-conductor P-N junction typically called a Laser Diode. For a detailed technical introduction to Laser Diodes ... read more. This article by Dr. Pospiech and Sha Liu is a great in depth description of one of the most prevalent types of lasers, the semiconductor laser diode.

Inside the cavity the excited photons are reflected back and forth through the cavity interacting with other excited electrons creating the stimulated emission state. Each photon generated is a copy of the incident photon in energy, wavelength and direction. Remember from basic physics that light is also a wave. Light waves travel longitudinally as well as transversely through the gain medium. The wavelength of the laser is determined by a physical characteristic of the gain medium along with wave theory.

How does the light exit the cavity? One of the cavity mirrors is totally reflective, the second mirror typically referred to as an output coupler, is mostly reflective but allows a small percentage of light to pass out of the cavity. This is accomplished through the process of optical coating, where the optic is coated specifically to a particular or desired wavelength with a highly reflective (HR) material. The coating material is a thin layer of gold, silver, or aluminum depending on wavelength of the laser. As mentioned previously, in order to sustain the lasing process, a population inversion is needed which is generated by an energy source such as a flash lamp, a laser diode array or an electrical charge. There's much more to cavity design beyond the scope of this article. For an indepth overview on laser cavities we have found Laser Beams and Resonators paper to be a great reference. In addition the Interactive Tutorials for Laser Cavity Resonance Modes and Gain Bandwidth by Michael Davidson at Florida State University to also be very useful.

WHAT IS A CW DIODE PUMPED SOLID STATE LASER?

A CW diode pumped solid state laser (DPSS laser) is created through the optical pumping of a laser gain medium with a semiconcutor diode laser or diode laser array (a diode laser bar for example made up of a linear array of semiconductor P-N junctions). This technology is replacing previously prevalent flash lamp pumping of the solid state gain medium. The gain medium being pumped can be a solid crystal, which is synthetically produced because of the required purity of the material and specific doping recipes. Or it can be a doped optical fiber cable. Examples of solid crystals used in DPSS lasers include ND:YAG, Neodymium Doped Yttrium Aluminum Garnet, and TI:Sapphire. These two are the most commonly used crystals. The fiber based laser (called a fiber laser) utilizes a semiconductor pump laser at 910nm to 980 nm to excite ions in elements such as ytterbium or erbium doped into the fiber core. This apprach generates photons with longer wavelengths (infrared) propagating through the fiber itself.

WHAT IS A FIBER LASER?

Fiber lasers are a unique class of diode pumped solid state lasers where the fiber optic cable itself is employed as the laser gain medium. The cavity of the fiber laser is bounded by Bragg gratings at each end creating a high-power resonator. The longer wavelength light resonates between the gratings (gratings reflect light at an angle determined by the geometry of the reflecting surface; a critical angle containing the light within the fiber) while being amplified with every pass through the cavity. And like in the lasers described above, one of the gratings is partially transparent allowing the longer wavelength created laser light to exit the fiber. For a more detailed article on fiber laser basics and design fundamentals, please refer to our article "FIBER LASER BASICS AND DESIGN PRINCIPLES" »

WHAT IS A 532nm DPSS LASER?

A 532 nm laser, also known as a green laser, is a class of a DPSS laser. The wavelength of the emitted light is a second harmonic of 1064 nm. The spectral wavelength 532 nm is generated by a process known as frequency doubling using a crystal with nonlinear optical properties (polarization among others). An example of these crystals is KTP (potassium titanyl phosphate); they are synthetically produced. The process of frequency conversion is complex, well beyond the scope of this article and involves nonlinear polarization waves, propagating electromagnetic fields and phase matching. To learn more about frequency doubling you need to understand nonlinear optics. An Introduction to Nonlinear Optics by Dr. Daniel M. Mittleman at Brown University is a great resource. When 1064 nm photons interact with the nonlinear KTP crystal, nonlinear wave interactions create photons at double the frequency (half the wavelength), hence a 532 nm photon. Keeping in mind that wavelength and frequency are inversely proportional. One of the most interesting aspects of this frequency/phase interaction is that the nonlinear polarization wave interacts with the fundamental wave at 1064 nm, attenuating the 1064 nm wave, and energy is then transferred from the fundamental wave at 1064 nm to the newly dominant 2nd harmonic wave at 532 nm. Hence, the lightwaves through the crystal are composed of both the new 532 nm light and attenuated 1064 nm light. Specially coated highly reflective (HR) and anti-reflective (AR) optics are used to separate the two frequencies so that the output of the crystal is the desired 532 nm light. There are different nonlinear crystals to produce varying wavelengths of light and the conversion process itself differs whether inside or outside of a laser cavity.

These 532 nm DPSS lasers have many advantages over other laser types. One such advantage is that they are compact, have excellent beam quality and typically have a higher efficiency than other kinds of lasers. These are typically CW, quasi-CW and pulsed high-power lasers, up to the kW regime. The light can be focused into a spot several hundred microns in diameter. This creates very high-power density and makes them unique. It also makes them ideal for applications in illumination. It also makes them a very good excitation source. Other applications include holography, atom cooling and atom trapping.

WHAT IS A CW LASER?

CW simply stands for continuous wave. This in turn refers to the that fact that the light output intensity (energy) is constant over time and characterized by the amount of power it generates in Watts (W). An example of a CW laser is the laser pointer which emits a continuous beam of low power visible light. But, they can also be very high power, up into the 1000's of Watts.

A quick refresher in nomenclature may be useful at this point. Energy is best described as the heat content of the laser light output and expressed in Joules (J). The amount of energy (J) content over time is expressed as Watts (W), where 1W = 1J/s. As an example, we can consider a laser being used to heat water. A calorie is defined as the energy (J) to raise the temperature of 1 gram of water 1°C, or 4.2 J. If you want to boil a cup of water from room temperature (30°C) to boiling (100°C), this would require 66,250 J of energy (4.2 J/g x 225 g/cup). If you want to do this with a CW laser in 5 minutes, you would need a 220 W laser.

CW LASER PACKAGING

CW lasers (DPSS) are commercially available for use in many environments. They include small footprint modules for integration into another product or system (OEM) to larger chassis mount lasers used in research laboratories, medical surgery centers and manufacturing environments. They vary by application with OEM lasers being small in footprint to medium sized lasers for benchtop laboratory applications to large floor mount or rack mount systems for manufacturing. The following is a quick description of packaging of CW lasers:

» Compact CW Lasers: These lasers are self-contained smaller packaged systems for OEM applications. These are typically low power CW lasers from milli-watts up to the several Watts but can be also much higher power pulsed lasers. The small enclosure consists of the diode pumps, the laser cavity and beam shaping optics for either a free-space or fiber coupled laser output plus optional specialty devices (shutter for example). The power supplies for the diode pumps are built into the package. Connectivity is simple and includes inputs for remote control, electrical power (usually a DC power supply for OEM), laser diagnostics, remote control and safety interlocks. In very high-power pulsed CW lasers, cooling water connections are typically integrated.

» OEM Compact DPSS CW Lasers: The packaging consists of two or more components. One is the laser head containing the pump diode and laser optical system, a second is the control electronics and power supply for the laser. Connectivity is still simple with a plug and play cable between the laser head and the controller, and inputs on the controller for AC power input, remote control and safety interlocks.

» DPSS Benchtop: These larger packaged (and heavier) lasers are usually much higher power CW, quasi-CW and pulsed lasers with power output into the 10's of Watts. They often have pulsed outputs with average power into the 100's of Watts. Like the OEM packaged lasers, these can be either self-contained including laser control electronics or separated with a laser head with a separate power supply and/or controller for laser control. Depending on the power output, the separate power supplies can be smaller bench-top units or larger rack-mount units. As in the case of OEM lasers, inputs to the controllers or power supplies are for AC or DC power, remote control, safety interlocks and cooling water.

HOW MUCH DO CW LASER COSTS?

Unfortunately, there are no easy answers due to the many factors that influence price of lasers. Factors that influence price of CW lasers include:

• Wavelength
• Output Power
• Packaging
• Linewidth
• Beam Shape
• Power or Wavelength Stability Data
• Technology - DPSS, Fiber Laser, Gas, etc.

As a general rule, the more exotic the wavelength (ie deep UV), the higher the output power, or the high the quality of the beam, the higher the price. You can get an inexpensive DPSS CW laser for $15. Green laser pointers fall into this category. But higher power CW lasers with good beam quality quickly get into the tens of thousands of dollars price range.

Let's just look at a few examples of a CW laser (DPSS) and CW fiber laser. If you are looking for a CW laser diode price than we would recommend our article How Much Do Laser Diodes Cost?

The simplest DPSS laser would be a 532nm frequency doubled component which can be found for aroung $100 to $200 USD excluding the power supply. A turnkey 532nm 200mW DPSS laser can be purchased for around $1,500 but this is going to have power stability in 10% range. If you need a 1% power stability than the price can quickly go up to $3,000+.

A 10 Watt single mode 1064nm fiber laser can be found for $3,000 and the price goes up from there. For an in-depth look at 10W lasers, please visit our article on 10W laser technologies and prices. A 750 Watt fiber laser will be in the $30,000-$50,000 price range. A high beam quality, narrow linewidth fiber laser for an atomic trapping application will cost roughly the same price as the 750 Watt fiber laser.

The more flexibility you have in your requirements, whether in terms price or performance, the more likely you can find the best CW laser for your application. You can also browse our Open Product Index of CW lasers to review by specification and manufacturer.

WHERE ARE CW DPSS LASERS USED?

Like computers, lasers have revolutionized our world and are employed to some degree in almost every industry and in tens of thousands of applications. DPSS CW lasers are often the best choice for concentrated high power applications. Many of them are found in industrial applications like laser drilling, cutting, welding, soldering, ablating (removing material), marking and engraving. Consider the case of cutting or drilling a hole into any kind of metal. This has historically been a mechanical drill based application. Mechanically, it takes a hardened steel blade or drill bit of many varying sizes depending on the size of the hole. The blades and bits heat up, and over time wear out (or break) adding cost and inefficiencies to the process. There is also the process control required to adjust the speed of the cutting or drilling tool depending on factors such as material type, thickness and depth of hole. These high-power CW lasers don't have any mechanical parts to wear out, and when mounted onto a computer controlled positioning system can cut through most metals in varying patterns or drill any size hole. Maintenance is extremely low and lifetimes are much higher that their mechanical counterpart.

Next, consider welding. Welding is necessary for many industrial processes from the manufacture of automobiles and airplanes to laptops and cellphones. A laser beam can heat a smaller area of the metals being joined creating a more controlled, smaller, smoother seam and create spots on the order of millimeters rather than the centimeters yielded from conventional welding. This results in higher process efficiencies increasing productivity translating to cost savings. For example, on average, there are up to 5,000 welds spots per automobile. Multiply that number by 50 to 70 million cars manufactured per year; that's an astronomical number of welds just in this industrial sector, mostly all being done now by automated CW lasers.

CW lasers are finding applications in other industries as well. In the semiconductor industry, CW lasers are used to process the semiconductors used in all the electronics used in everything from televisions to home appliances, electronic instrumentation of every kind, office equipment, sporting equipment computers, communications systems, energy, satellites..touching almost every aspect of our modern lives.

In laboratories, DPSS CW lasers in the visible range are widely used in fluorescence applications enabling imaging of cells, organisms, sub-cellular structures and cell dynamics in live cells. To learn more about how lasers are used Fluorescence Microscopy we have found this information from University of Illinois at Urbana-Champaign useful. Visible and infrared CW lasers are also used in interferometry for precision measurements and surface diagnostics. A very unique application for DPSS CW lasers is an optical tweezer. An optical tweezer uses a highly focused laser beam to provide an attractive or repulsive force to physically hold and move microscopic dielectric objects. This is mostly used in studying biological systems.

As you might suspect from their use in processing materials, these lasers are also used extensively in the medical industry as in general surgery for cutting (laser scalpel), ablating or cauterizing. In dermatology, among other types of lasers, these lasers are selectively used in dermatology depending on the condition (removal of skin cancers for example).

WHO MAKES CW LASERS?

In the context of this overview we are referring to DPSS CW lasers and Fiber CW lasers. We have an Open-Index Product Selection Guide that lists over 40 manufacturers with data sheets of their CW Lasers. This Open-Index allows you to quickly compare key specifications including wavelength, output power, and technology to quickly find the best CW laser for your application. If you are looking for a CW laser that is made by semiconductor material, aka a laser diode we would direct you to the Open-Index for Laser Diodes.

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Source: https://www.laserlabsource.com/Solid-State-Lasers/cw-laser-basics

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