Coherent, Inc.
Although lasers range from quantum-dot to football-field size and utilize materials from gases to solids, the underlying operating principles are always the same. This article provides the basic information about how and why lasers work.
When the laser was
first demonstrated in 1960, it sparked a wave of public interest. Soon, however,
many scientists and engineers dismissed the laser as “a solution without a
problem.” Time has proved the critics very wrong. From communications to construction,
laser technology has become a part of everyday life.
All light sources convert input energy into light.
In the case of the laser, the input, or pump, energy can take many forms, the two
most common being optical and electrical. For optical pumping, the energy source
may even be another laser.
In a conventional (incoherent) light
source, each atom excited by input energy randomly emits a single photon according
to a given statistical probability. This produces radiation in all directions with
a spread of wavelengths and no interrelationships among individual photons. This
is called spontaneous emission.
Figure 1. Spontaneous emission is a random process, whereas
stimulated emission produces photons with identical properties.
Einstein predicted that excited atoms
also could convert their stored energy into light by a process called stimulated
emission. Here, an excited atom first produces a photon by spontaneous emission.
When this photon reaches another excited atom, the interaction prompts that atom
to emit a second photon (Figure 1). This process has two important characteristics.
First, it is multiplicative — one photon becomes two. If these two photons
interact with two other excited atoms, this will yield a total of four photons,
and so forth. Most important, these two photons have identical properties: wavelength,
direction, phase and polarization. This ability to amplify light is termed optical
gain, and a wide range of solid, liquid and gas phase materials have been discovered
that exhibit gain.
The laser cavity
The laser cavity, or resonator, is at the heart
of the system. The cavity defines a unique axis with very high optical gain, which
becomes the beam direction. This axis usually is defined in two ways. First, the
laser’s shape ensures that the gain medium is longer along one axis, often
as a long thin cylinder, as is typical for gas lasers. A more extreme example of
a uniquely long gain axis is the fiber laser. This is sufficient for some high-gain
devices such as excimers. But for most, it is necessary to further enhance the gain
along this axis using cavity mirrors that produce feedback (Figure 2).
Figure 2. In the prototypical gas laser, the gain medium has a
long, thin cylindrical shape. The cavity is defined by two mirrors. One
is partially reflecting and allows the output beam to escape.
The simplest cavity is defined by two
mirrors — a total reflector and a partial reflector whose reflectance can
vary between 50 and 99 percent. Light bounces back and forth between these mirrors,
gaining intensity with each pass through the gain medium. Because some of this light
escapes the cavity, or oscillator, through the partial reflector (output coupler),
a stable equilibrium condition is reached quickly. The output beam is the light
that escapes through the output coupler.
In the ideal laser, all the photons
in the output beam are identical. This imparts several unique properties: directionality,
monochromaticity, coherence and brightness.
Monochromaticity — A photon’s
energy determines its wavelength through the relationship E = hc/λ, where c is the speed of
light, h is Planck’s constant and λ is wavelength. If our ideal laser emits all
photons with the same energy, and thus the same wavelength, it is said to be monochromatic.
Many applications are dependent on monochromaticity. For example, in telecommunications,
several lasers at different wavelengths transmit multiple streams of data down the same
fiber without crosstalk.
Coherence — Besides being the
same wavelength, the photons that make up a laser beam are all in phase (Figure
3). In the ideal case, the laser acts as one long, continuous, intense lightwave.
This enables a host of applications that rely on optical interference. For example,
the surface of precision lenses and mirrors is measured using laser interferometers.
The coherent light beam acts as an ultrafine ruler, where the wavelength of light
fulfills the dimensional role.
Figure 3. Laser light differs from conventional light in that
all the lightwaves are in phase with each other.
Directionality and brightness —
The most obvious visible difference between lasers and conventional light sources
is that laser light travels in the same direction as an intense beam. Brightness
is defined as the amount of light leaving the source per unit of surface area. Because
a laser’s photons have identical vector properties, they act as if they are
coming from the same point in space. The ideal laser thus acts as a true point source
with extremely high brightness.
This combination of directionality
and brightness has two consequences. The beam can be projected over great distances,
and it can be focused to a very small spot. In the ideal case, the divergence of
a collimated beam or the size of the focused spot are limited only by diffraction
— an inescapable property of light. This is referred to as a diffraction-limited
beam.
Continuous-wave lasers
Lasers can be divided into three main categories
— continuous wave (CW), pulsed and ultrafast.
As their name suggests, continuous
wave lasers produce a continuous, uninterrupted output. The exact wavelength(s)
at which this occurs is primarily determined by three factors: the gain bandwidth
of the lasing medium, the spectral characteristics of the cavity optics and the
longitudinal modes of the resonator.
Many laser materials in fact have several
wavelengths (or laser lines) at which emission occurs. Also, several factors (such
as the Doppler effect in the moving atoms of a gas) typically broaden the wavelength
bandwidth of the gain at each of these various lines.
The first step in determining at which
wavelength the laser will operate is to use cavity mirrors that are highly reflective
only at the desired wavelength(s). This suppresses lasing at other lines. However,
even a single laser line actually covers a band of wavelengths.
Figure 4. A resonant cavity supports only modes that meet the
resonance condition, Nλ = 2/ (cavity length). The output of a CW laser is
defined by the overlap of the gain bandwidth and these resonant cavity
modes.
The specific wavelengths of output
within this gain bandwidth are determined by the longitudinal modes of the cavity.
Figure 4 shows the basic principles of the resonant two-mirror cavity, the most
basic design. To sustain gain as light travels back and forth between the mirrors,
the waves must remain in phase, which means that the cavity round-trip distance
must be an exact multiple of the wavelength.
Nλ = 2/(cavity length)
where λ is the laser wavelength and
N is an integer called the mode number — it is usually a very large integer,
since the wavelength of light is so much smaller than a typical cavity length. In
a helium-neon laser, for example, the red output wavelength is 0.633 μm, yet
the typical cavity length is 15 to 50 cm. Wavelengths that satisfy this resonance
equation are called longitudinal cavity modes. The actual output wavelengths are
at the cavity modes that fall within the gain bandwidth, as shown in Figure 4. This
is called multi-longitudinal mode operation.
The cavity also controls the transverse
modes, or intensity cross sections. The ideal beam has a symmetric cross section:
The intensity is greater in the middle and tails off at the edges. This is called
the TEM
00 output mode. Lasers can produce many other TEM modes, a few of which are
shown in Figure 5. Typically, laser output is specified as what percentage of the
total beam intensity is in the form of the TEM
00.
Figure 5. Lasers can emit any number of transverse modes, of
which the TEM00 usually is most desirable.
A laser that produces multiple longitudinal
modes has limited coherence — different wavelengths cannot stay in phase over
extended distances. Applications such as holography, which demand excellent coherence,
often require a single longitudinal mode laser. For some laser types, single-mode
output is achieved with a very short resonant cavity; this makes the mode spacing
larger than the gain bandwidth and only one mode lases. Generally, though, a filtering
element that preferentially passes only one mode is inserted into the cavity. The
most common type of filter is called an etalon.
Various liquid and solid-state lasers
have broad bandwidths that cover tens of nanometers. Examples include dye and Ti:sapphire
lasers. Rather than being a disadvantage, this has allowed the development of tunable
and ultrafast lasers. Creating a tunable CW laser involves including an extra filtering
element in the cavity — usually a birefringent (or Lyot) filter. The birefringent
filter does two things: It narrows the bandwidth and, by rotating the filter, allows
smooth bandwidth tuning, or in the case of optically pumped semiconductor lasers
(OPSLs), it allows the final output wavelength to be exactly set to match the end
user’s specifications.
Although this sounds complicated, laser
operation is remarkably simple. High-end CW lasers include an on-board computer
or microprocessor. This automatically controls the additional cavity elements while
simultaneously maintaining optimum alignment of the mirrors.
Pulsed lasers
Some materials — ruby, rare-gas halogen
excimers, such as ArF and XeCl — sustain laser action for only a brief period
and form the basis of pulsed lasers. If the pulse duration is sufficiently long
(microseconds), the laser can be designed much like a CW laser. However, many pulsed
lasers are designed for short pulse duration; e.g., a few nanoseconds (10
–9
s). In each pulse, the light has time for very few round-trips in the cavity. The
resonant cavity designs described so far cannot control such a laser: The pulse
dies before equilibrium conditions are reached.
While two mirrors are still used in
pulsed lasers for defining the direction of highest gain, they do not act as a resonant
cavity. Instead, the usual method of controlling and tuning wavelength is a diffraction
grating (Figure 6). Some pulsed lasers, such as Nd:YAG (neodymium yttrium aluminum
garnet) can be operated with a Q-switch, an intracavity device that acts as a fast
optical gate. Light cannot pass it unless it is activated, usually by a high-voltage
pulse. Initially, the switch is closed and energy is allowed to build up in the
laser material. Then at the optimum time, the switch is opened and the stored energy
is released as a very short pulse. This can shorten the normal pulse duration by
several orders of magnitude. The peak power of a pulsed laser is proportional to
pulse energy/pulse duration. Q-switching, therefore, has an added benefit of increasing
peak power by several orders of magnitude. This effect enables neodymium (Nd)-based
solid state lasers of only modest average power to machine tougher materials such
as glasses and metals.
Figure 6. The wavelength of a pulsed laser usually is
controlled with a diffraction grating. Rotating the angle of this device
tunes the wavelength.
The wavelength purity of Q-switched
lasers can be difficult to control because of the combination of the high peak power
and the short pulse duration. However, this can be solved by stacking two or more
lasers in series: a low-power, well-controlled oscillator followed by one or more
amplifiers. For even higher performance, the oscillator itself is sometimes seeded
by another low-power laser, such as a wavelength-stabilized laser diode.
A much less common pulsing mechanism
is called cavity-dumping and is used where the laser material cannot store enough
gain for stable Q-switched operation. A cavity dumped laser has end mirrors with
nominal 100-percent efficiency in order to maximize the circulating intra-cavity
power. An intracavity switch, either an acousto-optic deflector or tillable mirror
is then flipped to allow all the trapped energy to depart the cavity in a single
round-trip time interval. Pulse energies achievable with cavity dumping are much
lower than Q-switching where the power is stored as gain in the laser medium.
Ultrafast lasers
CW lasers can produce many longitudinal modes,
and if the cavity is pulsed or modulated, it is possible to lock the phase of these
modes together. The resultant interference causes the traveling lightwaves inside
the cavity to collapse into a very short pulse. Every time this pulse reaches the
output coupler, the laser emits a part of this pulse. The pulse repetition rate
is determined by the time it takes the pulse to make one trip around the cavity.
It turns out that the more modes that
interfere, the shorter the pulse duration. In other words, the pulse duration is
inversely proportional to the bandwidth of the laser gain material. This explains
why the materials used for broadly tunable lasers produce the shortest mode-locked
pulses. The most popular ultrafast laser material is titanium-doped sapphire or
Ti:sapphire; turnkey commercial Ti:sapphire lasers now routinely deliver pulses
as short as 10 fs (20 x 10
–15 s), with typical repetition rates around 100
MHz. and peak powers approaching 1 MW. This can be amplified to the terawatt level
in custom commercial products.
With the CW power condensed into a
mode-locked pulse, the result is high peak power even for modest devices. Furthermore,
a regenerative amplifier can boost the peak power of an ultrafast laser by orders
of magnitude. These lasers can be optimized for high repetition rates (≤300
kHz) or high peak power (≥20 x 10
12 W) — the highest peak power delivered
by any class of commercial laser.
Specialized ultrafast lasers – CEP stabilization
In a mode-locked laser, interference between the
myriad longitudinal modes means that in the time domain, the output collapses from
a continuous-wave light source to a series of short pulses separated by the time
it takes for light to travel around the cavity. But in the frequency (1/wavelength)
regime, the pulse still consists of these myriad individual modes. An intensity
vs. frequency plot of these modes looks like a comb (Figure 4) and are often referred
to as a frequency comb. In recent years there has been fast growing interest, and
even a Nobel prize, for using this type of comb as a tool for high resolution spectroscopy
with unprecedented absolute precision. When using a laser in this way, the modes
act as ultrahigh precision frequency (wavelength) fiducials. This requires a specialized
laser operating approach called carrier envelope phase (CEP) stabilization.
In simple terms, each mode has a frequency given by
ν = c/λ = Nc/(2x cavity length)
Where c is the speed of light and N
is the mode number. But in real lasers, this is not exactly true. That’s because
the speed of light in any material other than a vacuum is actually defined by two
separate velocity components called group velocity and phase velocity. The group
velocity is the effective speed with which the light travels and the phase velocity
is the speed of the actual wavefronts – the waves that are electric-field
oscillations. The difference in group and phase velocity is a direct function of
the refractive index of the material(s) the light is passing through.
In a mode-locked laser this can be seen because it has the effect of adding an offset to the above formula
ν = Nc/(2x cavity length) + offset
This is called the carrier offset frequency
or CEO. In a spectroscopy measurement, the full utility of using the comb of modes
as reference frequencies means that this offset must be held constant throughout
the spectral acquisition time. But even extremely minor fluctuations and drifts
in the optical properties of the laser cavity (e.g., noise in the laser pumping
the Ti:sapphire crystal) can cause significant shifts in the CEO value. This can
be successfully addressed by using a feedback loop that measures the CEO value and
then adjusts one of the cavity optics to hold the CEO value constant. The most robust
CEO measurement tool for this purpose is the so-called 1f — 2f interferometer
whose operation has been well described elsewhere.
If the CEO is held at zero, the modes
are exact multiples of the inverse of the cavity length, enabling absolute measurement
of high resolution (hyperfine) spectrum parameters. But there is another advantage
to holding the CEO value at zero as can be seen in Figure 7. Specifically, in the
time domain, when the CEO is zero, the peak of the overall electric field oscillation
coincides with the amplitude envelope of the ultrafast pulse. Clearly this results
in the highest possible electric field peak value. This is a critical advantage
for applications involving multiple nonlinear processes, i.e., optical processes
whose efficiencies have a high order dependence on the peak electric field of the
laser. At this time, the most spectacular of these applications are in attosecond
physics using ultrashort x-ray pulses. These short pulses offer chemists and physicists
the first tool capable of probing electron motion, in contrast to slower spectroscopy
methods that can only follow the motion of the much slower nuclei.
Figure 7. In typical ultrafast laser operation, the electric field oscillation does not
have a fixed phase relationship with the pulse envelope, as shown here. The goal of CEP
stabilization is to fix the phase relationship between the overall pulse envelope
and the underlying electric field oscillation.
CEP stabilization requires a very low
noise system, including a low-noise CW laser to pump the Ti:sapphire oscillator.
In addition, the oscillator and amplifier must both incorporate feedback mechanisms
that adjust cavity dispersion in real time. Fortunately, turnkey commercial laser
systems are now available with all these attributes, enabling scientists to focus
on their experiments rather than on the nuances of CEP stabilization.
Frequency doubling
Even with the wealth of commercially available
lasers, it is not always possible to find one that exactly matches an application.
Fortunately, the required wavelength often can be generated using frequency doubling,
or shifting, or with an optical parametric oscillator. All these processes are related
and are called nonlinear phenomena since they depend nonlinearly on the laser’s
peak power.
In simple terms, when an intense and/or
tightly focused laser beam passes through a suitable condensed phase such as a liquid,
solid crystal or even dense gas jet, its oscillating electric field may interact
with the electrons of the atoms or molecules in several ways. One of these mechanisms
serves to distort the electron cloud thereby polarizing the atoms, i.e., the traveling
sine wave creates a temporary refractive index profile that is also a traveling
sine wave. When a photon interacts with this moving refractive index ripple it can
gain energy from the interaction or lose energy from the interaction. This is the
basis of frequency-doubling, where the laser frequency is doubled and the wavelength
is thus halved, as well as frequency mixing, where two laser wavelengths combine
to form photons with the sum of their original energies. It can also be used to
create long wavelength photons whose energy is the difference between two different
input laser wavelengths. Because these nonlinear interactions depend on the second
or third power of the laser intensity, they work well with pulsed lasers that have
high peak power. They can also be used with CW lasers if the beam is focused and
if the nonlinear crystal is inside the laser cavity (intracavity doubling).
Under most conditions, the light at
the new frequency (wavelength) would be destroyed by destructive interference. That’s
because it is usually created in a long (millimeters) crystal using the full length
of that crystal. But the phase velocity of the original and new wavelengths is different.
So wavelength-shifted light created at one spot in the crystal is out of phase with
that created at another position along the crystal, and so on. This difficulty is
overcome by choosing a crystal temperature and orientation that creates a so-called
phase-matching condition where the phase velocity of the fundamental and shifted
light is the same. Details of phase matching are beyond the scope of this review
article. But the most common phase-matching mechanisms depend on birefringence where
the phase velocity of light in a crystal depends on the light’s polarization
angle.
Optical parametric oscillators
The optical parametric oscillator (OPO) represents
an area of rapid development in terms of both products and applications —
thanks to its ability to produce tunable output anywhere from the mid-UV to the
mid-IR. For example, it is proving a powerful tool in the near-infrared for use
in deep-tissue microscopy.
An OPO uses a similar nonlinear mechanism
in a laserlike cavity to generate two shorter frequencies (i.e., longer wavelengths)
from one input frequency. The shorter of these new wavelengths is called the signal
wavelength, and the longer, the idler. The exact values can be smoothly varied by
tuning the cavity and rotating the OPO crystal under microprocessor control. Because
of the high power necessary for OPO operation, these devices typically have been
limited to pulsed and ultrafast systems.
Recently, the use of fan-poled nonlinear
crystals have enabled compact OPOs that can operate equally well over a wide range
of input wavelengths. By using a tunable pump laser such as a Ti:sapphire laser,
this type of OPO can thus be utilized to provide the user with two independently
tunable wavelengths. This can be very useful in several types of microscopy including
coherent anti-Stokes Raman spectroscopy (CARS) imaging.
A related device is the optical parametric
amplifier, which was developed to provide tunable pulses with much higher pulse
energy, e.g., for gas phase experiments. They are pumped by an ultrafast amplifier
– usually a regenerative amplifier in order to provide the requisite beam
quality.
Common laser types
For many years, the most common CW laser was the
helium neon laser, or HeNe. These low-power lasers (a few milliwatts) use an electric
discharge to create a low-pressure plasma in a glass tube; nearly all emit in the
red at 633 nm. In recent years, the majority of HeNe applications have switched
to visible laser diodes. Typical applications include bar-code readers, alignment
tasks in the construction and the lumber industries, and a host of sighting and
pointing applications from medical surgery to high-energy physics.
In fact, the laser diode has become
by far the most common laser type, with truly massive use throughout telecommunications
and data storage (e.g., DVDs, CDs). In a laser diode, current flow creates charge
carriers (electrons and holes) in a p-n junction. These combine and emit light through
stimulated emission. Laser diodes are available as single emitters with powers up
to tens of watts, and as monolithic linear bars, with numerous individual emitters.
These bars can be assembled into 2-D arrays with total output powers in the kilowatts
range. They are used in both CW and pulsed operation for so-called direct diode
applications. But even more importantly, laser diodes now underpin many other types
of lasers, where they are used as optical pumps that perform the initial electrical-to-optical
power conversion.
For example, higher power visible CW
applications were originally supported by argon-ion and krypton-ion lasers. Based
on a plasma discharge tube operating at high current, these gas-phase lasers are
large and very inefficient, generating a large amount of heat which must be actively
dissipated. The tube also has a finite lifetime and thus represents a costly consumable.
In most former applications the ion laser was displaced by diode-pumped solid- state
(DPSS) lasers. Here, the gain medium is a neodymium-doped crystal (usually Nd:YAG
or Nd:YVO
4) pumped by one or more laser diodes. The near-IR fundamental at 1064
nm is then converted to green 532 nm output by the use of an intracavity doubling
crystal.
The DPSS laser in turn, has now been
challenged by several newer technologies. The most successful of these is the OPSL.
Here the gain medium is a large-area semiconductor laser that is pumped by one or
more laser diodes. The OPSL offers numerous advantages, most notably wavelength
and power scalablity. Specifically, these lasers can be designed to operate at virtually
any visible wavelength, at last freeing applications from the restrictions of limited
legacy wavelength choices (i.e., 488, 514 and 532 nm). Indeed OPSLs represent a
paradigm shift in lasers because they can be designed for the needs of the application
instead of vice versa.
OPSL is now a dominant technology in
low-power bioinstrumentation applications, most notably at 488 nm. And the power
scalability and inherent low noise of OPSL technology is now seeing multiwatt green
and yellow OPSLs moving strongly into other applications including scientific research,
forensics, ophthalmology, and light shows.
At longer wavelengths, carbon dioxide
lasers, which use plasma discharge technology, emit in the mid-infrared around 10
μm. Most are CW or pseudo-CW, with commercial output powers from a few watts
to several kilowatts. Another important technology is the fiber laser, which can
be operated in CW, Q-switched and mode-locked formats. Here, laser diodes optically
pump a rare-earth doped fiber, which typically emits at about 1 μm.
Nd:YAG, CO
2, fiber and direct diode
lasers are the workhorses of industrial laser applications. Direct diode lasers
predominately service low brightness applications, such as heat treating, cladding
and some welding applications. This is because direct diode lasers offer the lowest
capital cost of any industrial laser type, as well as the lowest operating costs,
due to their high electrical efficiency.
The advent of slab-discharge technology
has allowed the size/power ratio of CO
2 lasers to be greatly scaled down, increasing
their utility in subkilowatt applications. Low-cost waveguide designs also support
a healthy market for CO
2 lasers with powers in the tens of watts, primarily in marking
and engraving applications.
Sealed CO
2 lasers and fiber lasers
have come to dominate the cutting of metals in the 2- to 4-mm thickness range. Sealed
CO
2 is usually the first choice when both metals and nonmetals must both be processed,
while fiber lasers have proved quite successful in certain markets that can benefit
from their combination of high repetition rate, low pulse energy and high brightness.
They also excel at metal cutting and welding in the 4- to 6-mm thickness range,
as well as some marking applications. Flowing gas CO
2 lasers still dominate the
market for thick metal (> 6 mm) cutting.
Nd:YAG can deliver the high peak power
for materials processing applications such as metal welding; in these heavy industrial
applications, raw power is more important than beam quality and for many years these
lasers were lamp-pumped. But the ever increasing power and lifetime characteristics
of laser diodes are causing these lasers to switch to diode pumping, i.e., DPSS
lasers.
Conversely, lower power Q-switched
DPSS lasers are often based on Nd:YVO
4. These are usually optimized for high beam
quality in order for use in micromachining and microstructuring applications with
high repetition rates (up to 250 kHz) to support high throughput processes. They
are available with powers up to tens of watts with a choice of near-infrared (1064
nm), green (532 nm) or UV (355 nm) output. The UV is popular for producing small
features in “delicate” materials because it can be focused to a very
small spot and minimizes peripheral thermal damage. Deep-UV (266 nm) versions are
starting to be used in some applications. But their relatively high cost and the
need for specialty beam delivery optics causes many potential applications to rely
instead on 355 nm lasers optimized for short pulse duration, which can produce similar
results in many materials.
Excimers represent another important
pulsed laser technology. They can produce several discrete wavelengths throughout
the UV; depending on the gas combination, emission ranges from 157 to 348 nm. The
deep-UV line at 193 nm is the most widely used source for lithography processes
in the semiconductor industry. The 308 nm wavelength is used for annealing silicon
in high performance displays. The same wavelength is also key to generating a unique
long-wear surface on the cylinder liners of high performance diesel engines. And
finally, excimers have a unique ability to produce high pulse energies – up
to 1 joule per pulse. This enables direct writing of low-cost electronic circuits
for applications such as medical disposables.
Ultrafast lasers for scientific applications
are dominated by Ti:sapphire as already described. Ultrafast is also a fast growing
technology for micromachining applications. These lasers are based on fibers, free-space
optics or some combination of the two.
And finally, there are many other types
of niche and exotic lasers that are beyond the coverage of this overview article.
Examples include Raman lasers used in telecommunications, quantum cascade lasers
used in some gas sensing applications, and chemical lasers which tend to be limited
to military programs.