CW LASER BASICS


DPSS CW Laser

CW Laser Basics:
Learn the design fundamentals of CW Lasers, how much they cost, and their primary applications.

INTRODUCTION:

At the most basic level, 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 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.

Image with explanation of CW Laser

QUICK NAVIGATION:

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 starts 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 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.

Photon Emission from a Change in Energy State

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 4 principals to understanding how a laser output works they are Spontaneous Emission, Stimulated Emission, Population inversion and amplification. These principals are discussed below.

» Principal Number 1: This release of a photon is called spontaneous emission, i.e., the light part of LASER. Not too exciting, Anything that produces light -- fluorescent lights, gas lanterns, 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 else is needed.

» Principal Number 2: Stimulated Emission occurs during the time the 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 resulting in the release of a second photon of equal energy and frequency (wavelength). This is the essence of a laser, all the photons are in sync so to speak, this is what is meant by monochromatic (same wavelength) and coherence (same phase and direction). This is stimulated emission part of LASER. We're not there yet, sustained stimulated emission is not guaranteed at any energy level, there needs to be an adequate number of electrons emitting photons.

Diagram of Electronic Population Inversion

» Principal Number 3: Population inversion means the number of electrons and inversion meaning the change in energy state of the electron to a higher state. Electrons are comfortable in 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, more than are in the ground level. Lasing is not guaranteed just because you have excited electrons, it becomes statistically possible only when there is ample electrons in an excited state . The higher the energy level of the electron, the higher the degree of population inversion, thus the greater the chance of amplified emission. Period. The energy supplied for the principle of population inversion is from an external source, a light source (flash lamp) or another laser, or an electrical charge. This process is referred to as optical pumping. That leads us to the last principle of lasers.

Example of Stimulated Emission

» Principal Number 4: Amplification which is a result of both population inversion and stimulated emission. Simply, when a population inversion is present, the rate of stimulated emission exceeds that of absorption because there are more electrons at an excited energy state for which two photons are generated from each incident photon, then from those 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 which could be made up of a solid, gas or even a semiconductor junction commonly called the gain medium.

Figure of Laser Amplification

HOW DO LASERS WORK?

In order for light amplification by stimulated emission to occur, as mentioned in the above paragraph, we enclose this stimulated emission and amplification in a cavity with mirrors on each end. The cavity could be made with a lasing medium called an active or gain medium which can be a specially formulated (doped) crystal, it could be a tube with a gas (such as helium-neon). It could also be a specific semi-conductor P-N junction typically called a laser diode. An Introduction to Laser Diodes by Dr. Pospiech and Sha Liu is a great indepth discussion on one of the most prevalent types of lasers.

Inside the cavity the excited photons are reflected back and forth through the cavity interacting with other excited electron creating stimulated emission. 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. Lightwaves travel longitudinally as well as transversely through the gain medium. The wavelength of the laser is determined as a physical characteristic of this medium along with wave theory.

Figures of Different Types of Lasers including Solid State, Gas, and Semiconductor

But how does the light exit the cavity? One of 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 specific 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 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 DIODE PUMPED SOLID STATE LASER?

Well, now that you know the basic principles of lasers, this next section should be easy! A diode pumped solid state laser, DPSS, is simply what it states, optical pumping of a laser gain medium with a diode laser or diode laser array (a diode laser bar for example made up of a linear array of semiconductor P-N junctions). As opposed to flash lamp pumping of the same solid state laser. The gain medium being pumped can be a solid crystal, synthetically produced because of the required purity of the material and specific doping recipes, or a doped fiber cable. Examples of solid crystals used in DPSS lasers some of which you may already be familiar with or at least heard of are ND:YAG, Neodymium Doped Yttrium Aluminum Garnet (you can see why it is shortened to YAG!) and TI:Sapphire being the most common ones. The fiber-based laser (called a fiber laser) utilizes a semiconductor pump laser, 910 to 980 nm to excite ions in elements such as ytterbium or erbium doped into the fiber core generating photons with longer wavelengths (infrared) propagating through the fiber itself.

Block Diagram of DPSS Laser

WAIT, WHAT EXACTLY 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.

Schematic diagram of Fiber Laser - End Pumped and Side Pumped

WHAT IS A 532nm LASER?

A 532 nm laser also known as a green laser is a special class of a DPSS laser, the wavelength of the emitted light being a second harmonics of 1064 nm. What? The spectral wavelength 532 nm is generated by a process known as frequency doubling using a crystal with nonlinear optical properties (polarization among others for example). An example of these crystals is KTP (potassium titanyl phosphate); they are synthetically produced (not naturally occurring). 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 (see what I mean?) 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 has been a great resource for us.

In its simplest form with consideration of the basic unit of light, the photon, 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! (Remember that wavelength and frequency are inversely proportional!) One of the cool things about this frequency/phase interaction is that the nonlinear polarization wave interacts with the fundamental wave (1064nm) attenuating it, energy is transferred from the fundamental wave (1064nm) to the now dominant 2nd harmonic wave (532nm)! 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.

How a 532nm Laser Works

These 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 (10-6 meters!) in diameter creating super high-power density making them unique and ideal for applications in illumination, excitation source, and holography, atom cooling and trapping, and many other scientific applications.

WHAT IS A CW LASER?

CW simply stands for continuous wave, the light output intensity (energy) is constant over time and characterized 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.

Figure of CW and Pulsed Laser Difference

A quick refresher in nomenclature is warranted here. 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. For example a laser could be 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 wanted to do this with a CW laser in 5 minutes, you would need a 220 W laser!

You might be asking, " What do these lasers look like?"

DPSS CW LASER PACKAGING

CW diode pumped solid state lasers are commercially available for use in many environments from, integration into another product or system (OEM) to research laboratories, medical surgery centers, manufacturing environments, and outside 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 these CW lasers:

» DPSS Compact CW Laser: These lasers are self-contained smaller packaged systems for OEM applications. These are typically low power CW 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, safety interlocks and in the high-power pulsed lasers, cooling water connections.

Example Image of Compact DPSS CW Laser

» 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.

Example of OEM CW Laser Package

» DPSS Benchtop: These larger packaged (and heavier) lasers are much higher power CW, quasi-CW and pulsed lasers with power output into the 10's of Watts with 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.

Example of Benchtop CW Laser Source from Azurlight Systems

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
  • Technology - DPSS, Fiber Laser, Gas, etc...

To really do justice on this topic is outside of the scope of this overview. As a general rule, the more exotic the wavelength (UV or 2000nm) or the higher the output power or the more stringent linewidth requirements the higher the price. Want an inexpensive DPSS laser no problem you can buy a green laser pointer for $15 but that isn't going to help you in your atomic trapping application other than the presentation you will give.

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

The simplest DPSS laser would be a 532nm frequency doubled component which can be found for a $100-$200 USD excluding the power supply. A turnkey 532nm 200mW DPSS laser can be purchased for $1,500 but this is going to have power stability in 10% range want a 1% than the price can quickly go up to $4,000.

A 10 Watt single mode 1064nm fiber laser can be found for $3,000 and the price goes up from there. A 750 Watt fiber laser will be in the $30,000-$50,000 price range. Need a high-performance narrow linewidth fiber laser for your atomic trapping application you will pay about the same price as the 750 Watt fiber laser.

The more flexibility you have in your requirements be it price or performance the more likely you can find the perfect CW laser. You can also browse our Open Product Index of CW lasers to review by specification and manufacturer.

WHERE ARE CW DPSS LASERS USED?

Good question! Like computers, lasers have revolutionized our world and are employed to some degree in almost every industry in thousands of applications. There are applications where some lasers are more apt than others to perform a specific task. An example of this is the CW DPSS laser; because they are focused more around concentrated high power the bulk of them are found in industrial applications like laser drilling, cutting, welding, soldering, ablating (removing material), marking and engraving. Just to give you a little taste of how disruptive this technology is, think about cutting through or drilling a hole into any kind of metal. 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 (not to mention 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 laser don't have any mechanical parts to wear out and, when mounted on 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.

Example of CW Laser System with XY Stage

Then there's welding which 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 spots on the order of millimeters rather than centimeters from conventional welding. This results in higher process efficiencies increasing productivity translating to cost (and material) savings. For example, on average, there is up to 5000 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 all being done by automated CW lasers. That's technology at its best!!

But, these 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, CW DPSS 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 CW DPPSS lasers is an optical tweezer. We all know what a tweezer is. Well, an optical tweezer uses a highly focused laser beam to provide an attractive or repulsive force to physically hold and move microscopic dielectric objects mostly used in studying biological systems!

On a more personal level, as you might suspect from their use in processing materials, these lasers are also used 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 lasers and Fiber Lasers. We have handy Open Product Index that lists all (over 35) manufacturers with data sheets of CW Lasers. This Open Index allows you to quickly compare key specifications including wavelength, output power, and technology to quick find the best CW lasers 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.