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Laser pulsing technique

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Q-switching, sometimes known as giant pulse formation or Q-spoiling,[1] is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a continuous wave (constant output) mode. Compared to modelocking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. The two techniques are sometimes applied together.

Q-switching was first proposed in by Gordon Gould,[2] and independently discovered and demonstrated in or by R.W. Hellwarth and F.J. McClung at Hughes Research Laboratories using electrically switched Kerr cell shutters in a ruby laser.[3] Optical nonlinearities such as Q-switching were fully explained by Nicolaas Bloembergen, who won the Nobel prize in for this work.[4][5][6][7]

Principle of Q-switching

Q-switching is achieved by putting some type of variable attenuator inside the laser's optical resonator. When the attenuator is functioning, light which leaves the gain medium does not return, and lasing cannot begin. This attenuation inside the cavity corresponds to a decrease in the Q factor or quality factor of the optical resonator. A high Q factor corresponds to low resonator losses per roundtrip, and vice versa. The variable attenuator is commonly called a "Q-switch", when used for this purpose.

Initially the laser medium is pumped while the Q-switch is set to prevent feedback of light into the gain medium (producing an optical resonator with low Q). This produces a population inversion, but laser operation cannot yet occur since there is no feedback from the resonator. Since the rate of stimulated emission is dependent on the amount of light entering the medium, the amount of energy stored in the gain medium increases as the medium is pumped. Due to losses from spontaneous emission and other processes, after a certain time the stored energy will reach some maximum level; the medium is said to be gain saturated. At this point, the Q-switch device is quickly changed from low to high Q, allowing feedback and the process of optical amplification by stimulated emission to begin. Because of the large amount of energy already stored in the gain medium, the intensity of light in the laser resonator builds up very quickly; this also causes the energy stored in the medium to be depleted almost as quickly. The net result is a short pulse of light output from the laser, known as a giant pulse, which may have a very high peak intensity.

There are two main types of Q-switching:

Active Q-switching

Here, the Q-switch is an externally controlled variable attenuator. This may be a mechanical device such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity, or (more commonly) it may be some form of modulator such as an acousto&#;optic device, a magneto-optic effect device or an electro-optic device &#; a Pockels cell or Kerr cell. The reduction of losses (increase of Q) is triggered by an external event, typically an electrical signal. The pulse repetition rate can therefore be externally controlled. Modulators generally allow a faster transition from low to high Q, and provide better control. An additional advantage of modulators is that the rejected light may be coupled out of the cavity and can be used for something else. Alternatively, when the modulator is in its low-Q state, an externally generated beam can be coupled into the cavity through the modulator. This can be used to "seed" the cavity with a beam that has desired characteristics (such as transverse mode or wavelength). When the Q is raised, lasing builds up from the initial seed, producing a Q-switched pulse that has characteristics inherited from the seed.

Passive Q-switching

In this case, the Q-switch is a saturable absorber, a material whose transmission increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG, which is used for Q-switching of Nd:YAG lasers, a bleachable dye, or a passive semiconductor device. Initially, the loss of the absorber is high, but still low enough to permit some lasing once a large amount of energy is stored in the gain medium. As the laser power increases, it saturates the absorber, i.e., rapidly reduces the resonator loss, so that the power can increase even faster. Ideally, this brings the absorber into a state with low losses to allow efficient extraction of the stored energy by the laser pulse. After the pulse, the absorber recovers to its high-loss state before the gain recovers, so that the next pulse is delayed until the energy in the gain medium is fully replenished. The pulse repetition rate can only indirectly be controlled, e.g. by varying the laser's pump power and the amount of saturable absorber in the cavity. Direct control of the repetition rate can be achieved by using a pulsed pump source as well as passive Q-switching.

Variants

Jitter can be reduced by not reducing the Q by as much, so that a small amount of light can still circulate in the cavity. This provides a "seed" of light that can aid in the buildup of the next Q-switched pulse.

With cavity dumping, the cavity end mirrors are 100% reflective, so that no output beam is produced when the Q is high. Instead, the Q-switch is used to "dump" the beam out of the cavity after a time delay. The cavity Q goes from low to high to start the laser buildup, and then goes from high to low to "dump" the beam from the cavity all at once. This produces a shorter output pulse than regular Q-switching. Electro-optic modulators are normally used for this, since they can easily be made to function as a near-perfect beam "switch" to couple the beam out of the cavity. The modulator that dumps the beam may be the same modulator that Q-switches the cavity, or a second (possibly identical) modulator. A dumped cavity is more complicated to align than simple Q-switching, and may need a control loop to choose the best time at which to dump the beam from the cavity.

In regenerative amplification, an optical amplifier is placed inside a Q-switched cavity. Pulses of light from another laser (the "master oscillator") are injected into the cavity by lowering the Q to allow the pulse to enter and then increasing the Q to confine the pulse to the cavity where it can be amplified by repeated passes through the gain medium. The pulse is then allowed to leave the cavity via another Q switch.

Typical performance

A typical Q-switched laser (e.g. a Nd:YAG laser) with a resonator length of e.g. 10 cm can produce light pulses of several tens of nanoseconds duration. Even when the average power is well below 1 W, the peak power can be many kilowatts. Large-scale laser systems can produce Q-switched pulses with energies of many joules and peak powers in the gigawatt region. On the other hand, passively Q-switched microchip lasers (with very short resonators) have generated pulses with durations far below one nanosecond and pulse repetition rates from hundreds of hertz to several megahertz (MHz)

Applications

Q-switched lasers are often used in applications which demand high laser intensities in nanosecond pulses, such as metal cutting or pulsed holography. Nonlinear optics often takes advantage of the high peak powers of these lasers, offering applications such as 3D optical data storage and 3D microfabrication. However, Q-switched lasers can also be used for measurement purposes, such as for distance measurements (range finding) by measuring the time it takes for the pulse to get to some target and the reflected light to get back to the sender. It can be also used in chemical dynamic study, e.g. temperature jump relaxation study.[8]

Q-switched lasers are also used to remove tattoos by shattering ink pigments into particles that are cleared by the body's lymphatic system. Full removal can take between six and twenty treatments depending on the amount and colour of ink, spaced at least a month apart, using different wavelengths for different coloured inks.[9] Nd:YAG lasers are currently the most favoured lasers due to their high peak powers, high repetition rates and relatively low costs. In a picosecond laser was introduced based on clinical research which appears to show better clearance with difficult-to-remove colours such as green and light blue.[citation needed] Q-switched lasers can also be used to remove dark spots and fix other skin pigmentation issues.[citation needed]

See also

References

What&#;s the Best Tattoo Removal Machine?

Whether new or used, these laser tattoo removal machines have proven to be the best picosecond and nanosecond lasers available. We&#;ll be breaking down the machine&#;s features, pros and cons, and the price range of the machine. 

1) Candela PicoWay Laser Tattoo Removal Machine

The Candela PicoWay Laser tattoo removal machine is top of our list. This machine uses a photoacoustic effect rather than a photothermal effect which reduces the risk of the skin heating and damaging the surrounding tissue. This laser comes with the wavelengths nm and 532 nm installed. 785 nm and 730 nm are separate handpieces that can be purchased to attach to the laser machine. We recommend this laser as your ultimate tattoo removal machine due to the variety of wavelengths available and photoacoustic technology. This is also the laser that we use in our studios. See our before & after gallery for results.

Pros: This laser is FDA certified, it uses photoacoustic effects to minimize surrounding skin damage, offers 4 picosecond wavelengths to target ink color across the spectrum, and minimizes the risk of hypopigmentation for darker skin types. This laser can also be used for acne scars, wrinkles, and benign lesions. 

 Cons: The Candela PicoWay Laser is one of the most expensive lasers on the market. Retails around $175,000 for a regular consumer. It will take multiple sessions to see complete removal results and depending on your state&#;s laser regulations, it can only be operated by a certified laser professional, RN or doctor.  

Best for: All tattoos. With 4 wavelengths and adjustable handpieces, the Candela PicoWay can target ink color across the spectrum and tattoo size. 

Price range: $$$$

2) Quanta Pico Discovery Laser Tattoo Removal Machine

The Quanta Pico Discovery tattoo removal machine uses the Nd: YAG ( nm, 532 nm) wavelengths and 694 nm available. 

Pros: The Quanta Pico Discovery laser tattoo removal machine uses the  nm, 532 nm, and 694 nm wavelengths which target most ink colors across the spectrum and uses a photoacoustic effect to shatter ink while leaving the surrounding tissue unharmed. The Quanta Pico Discovery laser uses fractional treatments which take advantage of two different laser actions. The laser uses &#;LIOBs for collagen remodeling and ablation of damaged skin through picosecond &#;cold&#; ablation&#; which is good for treating wrinkles and acne scars.

Cons: Just like the Candela PicoWay laser, the Quanta Pico Discovery is more expensive than most. It will also take multiple sessions to see complete removal results and depending on your state&#;s laser regulations, it can only be operated by a certified laser professional, RN, or doctor.  Unlike the Candela PicoWay laser, the Quanta Pico Discovery laser only uses 3 wavelengths to target ink color.

Best for: Most tattoo types and sizes. 

Price range: $$$$

3) PicoSure Tattoo Removal Laser by Cynosure

The PicoSure Laser tattoo removal machine by Cynosure is the world&#;s first picosecond aesthetic laser. It uses 3 wavelengths ( nm, 755 nm, 532 nm) and photothermal effects.

Pros: The PicoSure laser has 3 wavelengths that can target most ink colors across the spectrum. It operates in picoseconds which should reduce the risk of surrounding skin damage and has an adjustable spot size of up to 10mm.

Cons: The PicoSure laser uses photothermal effects vs the PicoWay which uses photoacoustic effects. The photothermal effect uses heat to break the ink particles apart which puts the client at risk of burns and hypopigmentation. It&#;s also not safe to treat skin types 5 and 6 with the 755 nm wavelength. Hypo or hyperpigmentation can occur as a side effect, along with a higher risk of causing scar tissue. This laser will take multiple sessions to see complete removal results and retails around $100,000 for a regular consumer.

Best for: Most tattoo types and sizes. 

Price range: $$$

4) Lutronic Spectra Nano Laser Tattoo Removal Machine

 The Lutronic Spectra nano laser tattoo removal machine uses four wavelengths ( nm, 532 nm, 585 nm, 650 nm) to target all ink colors across the spectrum. It operates in short 5-10 ns pulses and has different spot sizes available.

Pros: The Lutronic Spectra laser utilizes multiple wavelengths to target all types of ink pigments and utilizes auto sensing spot sizes. The laser operates in short 5-10 ns pulses and offers .8 mm &#; 8 mm spot sizes. 

Cons: The Lutronic Spectra does not offer the largest spot size available (10 mm) which is important because smaller spot sizes target the shallow ink and larger spot sizes target the deepest ink within the dermis. It also has a disadvantage working in nanoseconds compared to picosecond lasers. The pulses are longer than the picosecond lasers which requires more energy and distributes more heat into the surrounding tissue. 

Price range: $$$

5) Cynosure RevLite SI Laser Tattoo Removal Machine

The RevLite SI laser tattoo removal machine is a q-switched multi-wavelength laser that treats tattoos, acne scars, wrinkles, and pigmented lesions.

Pros: The RevLite SI offers two wavelengths ( nm, 532 nm) to treat most ink pigments across the spectrum. It also offers a SmartInfinite (SI) handpiece, photoacoustic technology, and precision beam technology. This laser also offers multiple handpiece options to adjust spot size.

Cons: The RevLite SI only operates in two wavelengths which are best for black and red pigments but do not target blues and greens as well. Again, the laser operates in nanoseconds which is a shorter pulse duration than the picosecond lasers causing more heat to distribute to the surrounding tissue and the largest spot size is not listed as available on the website.

Price range: $$$

6) Quanta Q Plus C Laser Tattoo Removal Machine

The Q Plus laser tattoo removal machine can have two different laser sources inserted, the pure q-switched or hybrid q-switched technology. It also has 6 different configurations available for adaptability.

Pros: The Q Plus laser offers the Optibeam II handpiece which offers different flat-top spot sizes and contact skin cooler to keep the target area cool when treating. It also offers twain handpieces to treat hair removal with the Twain IPL or tattoo removal with the Twain . 

Cons: Each piece of the laser is an additional cost vs being automatically integrated. The Q Plus laser can offer Q-switched Er:YAG wavelength options at an additional cost. The laser also operates in nanoseconds vs picoseconds which distributes more heat to the surrounding tissue. The initial laser is an investment that will cost more with all of the additions. 

Price range: $$$$

7) NEATCELL Hand-Held Laser Tattoo Machine (At-Home Option)

The Neatcell Hand-Held Laser tattoo machine is a small portable laser that you can purchase from Amazon. It claims to be a picosecond pen with 9 kinds of light output frequencies and 4 kinds of light intensity. This product is mainly used to remove moles, &#;wash tattoos&#; and spots.

Pros: It can be purchased and operated by regular consumers, and it&#;s cheaper than professional removal services. It&#;s a hand-held laser pen that comes with a USB charger and protective eyewear. 

Cons: The Neatcell Hand-Held laser is not FDA certified and does not require a certified laser professional. It does not explain the 4 kinds of light intensity or 9 kinds of light output frequencies and will likely take longer to see complete removal results vs professional services. There is also the risk of hypopigmentation occurring due to the lack of knowledge and experience by the consumer. Given what you already know, the pen does not create a big enough spot size, have enough power, or pulse duration to effectively remove a tattoo.  

Best for: At-home DIYers and small tattoos.

Price range: $

For more information, please visit nd yag tattoo removal.