FAQ - Gas Lasers 2

The term 'white light laser' typically refers to one that is capable of producing a set of wavelengths which if mixed in the proper proportion can 'simulate' the effect of a white light source in full color displays and laser shows and also for some spectroscopy applications. However, they generally don't produce a broad spectrum like an incandescent light bulb.

Though under some conditions krypton lasers as such can produce wavelengths over the full visible spectrum with lines in the red, yellow, green and blue. However, The most common white light lasers are large frame ion types with a mixture argon and krypton for the gas fill. These lasers use a mix of argon and krypton. Most of them are made for a roughly 60:20:20 ratio of red, green, and blue lines for proper white balance.

It is a four level atom laser with a mixture of helium and neon. Though it lases at a number of wavelengths, its most popular output is at 633nm (red). Other available outputs are at 543nm (green), 594nm (yellow), 612nm (orange) and 1523nm (infra-red). Though neon is the lasing gas, it is the minor constituent (15% of the total mixture), helium taking the bigger share. Electrical discharge excites the helium atoms to the higher energy states, which are populated by electronic collisions.

As Argon ion lasers simultaneously run on several lines unless there is a dispersive element (prism or grating) in the cavity. With an intra-cavity prism, different lines can be selected. Most of these lasers have a hemispherical cavity, with a flat high-reflector mirror and a long-radius output coupler. The mirrors are designed for specific wavelengths. An intra-cavity prism is used for the selection of the various lines, with the prism shaped so that the beams strikes it at or near Brewster's angle on both surfaces. The prism and the high reflector are usually mounted together in a single unit.

The transitions in Helium-Cadmium laser are between energy levels of singly ionized Cadmium atoms, and there are about twelve lines. These wavelengths are in the shorter wavelength region, violet and Ultra-Violet (UV). The most prominent wavelengths are 441.6 nm and 325 nm.

Helium-Cadmium lasers can be categorized either in Metal vapour Lasers since cadmium is a metal and the lasing action in Helium Cadmium laser occurs between energy levels of cadmium ions OR a Gas Laser since the properties of Helium-Cadmium laser are similar to those of Helium Neon Laser, which is a neutral atom gas laser.

The main mechanism identified for the laser transitions is a process known as Penning ionization, in which highly excited helium atoms transfer their energy to cadmium in a way similar to the operation of He-Ne laser. However, in this case cadmium ions are produced in the process, instead of neutral atoms as in the case of neon, owing to much lower ionization potential of cadmium than of neon. Excitation energy for the helium-cadmium laser is provided by a direct-current discharge passing through the laser tube. Typical discharges are around 700 - 2000 volts, with current densities in the small-diameter bore (2 -mm) of the order of 3 - 5 amperes per square centimeter of cross section. Helium atoms in the laser gas absorb energy from the discharge and then transfer that energy to cadmium ions. The energy levels of cadmium and helium involved in the principal He-Cd lines are shown in the adjoining figure. The most prominent transitions, which can be easily, produced are 441.6-nm (blue transition) and the 325-nm (ultraviolet transition).

The most important energy transfer mechanism for the narrow-bore tubes mainly used for blue and ultraviolet lasers is Penning ionization. Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms and/or molecules. The process is named after the Dutch physicist Frans Michel Penning, who first reported it in 1927. Chemi-ionization is the formation of an ion through the reaction of a gas phase atom or molecule with an atom or molecule in an excited state and should not be confused with chemical ionization.

In Penning ionization, energy from an excited helium atom ionizes a cadmium atom:

Cadmium, a metal, is solid at room temperature and for lasing it needs to be sufficiently heated to have required partial pressure of cadmium vapors in the discharge tube. In case of cadmium, it is not practical to heat the complete chamber to approximately 260^oC because of the other components like mirrors, windows, electrodes etc. present in the chamber. Thus the challenge in making HeCd lasers operate continuous-wave (CW) is dealing with cataphoresis. This is the term given to the migration of the positively charged metal ions toward the cathode where they may condense, depleting the supply of vapor and contaminating tube components and optical surfaces. This is achieved by using cataphoresis to control the cadmium vapour distribution. In this process the cadmium metal is heated and vapourized at the anode (which is at positive potential) end of the discharge and is transported towards the cathode end of the discharge by the electric field acting upon the cadmium ions that are produced by the discharge current. The practical problem in Helium-Cadmium laser is to maintain homogeneous distribution of the metal vapor inside the electrical discharge tube. Once the vapours go out of the bore, it may deposit on cold surfaces. Thus it is important that its vapor remains at proper areas. In order to prevent coating of the windows with Cadmium, cold traps are put before the laser windows. The cadmium atoms then condense in a pocket near the cathode region.

Nitrogen laser is convenient and economical source of short, nanosecond, ultraviolet (337.1 nm) pulses.

For excitation, a fast strong electrical pulse is used where electron collisions cause the preferential population of the upper energy band first. After about 20nSec, the population of molecules at the upper laser level starts decaying to the lower laser level where it will stay because of much longer life time thereby quickly ceasing the laser action after the electrical pulse. Nitrogen lasers are hence self terminating. That is why these lasers cannot operate in CW mode. The pulse length of the low-pressure Nitrogen laser, then, is limited by the lifetime of the upper laser level i.e. 20nS. After this time, population inversion is no longer possible since half of the molecules in the upper energy state have decayed to the lower state.

Although nitrogen lasers have been operated over a range of partial pressures from a few Torr to more than 1 atmosphere, it is common to divide them into two categories: low pressure and atmospheric pressure. The first approach is a low-pressure design - it is more 'traditional' and requires a vacuum pump. The second approach is a TEA version of the nitrogen laser. TEA lasers (for Transverse Electrical-discharge at Atmospheric pressure) do not require a vacuum system at all as they operate at atmospheric or even greater pressures.

A fast high-voltage discharge populates the upper laser level, an excited electronic state with 40-ns lifetime, which emits at 337.1 nm when it drops to the lower laser level as shown in the figure. The transition is a vibronic one, in which both electronic and vibrational energy levels change, making it broadband. The lower level has a 10- microsecond decay time, much longer than the upper level, and drops to a metastable state with a lifetime of the order of seconds.

The lifetime of the upper-lasing level is dependent on the pressure in the laser tube. As pressure rises, the lifetime shortens according to:

There is 40 ns upper limit of laser lifetime at low pressures and the lifetime becomes shorter as the pressure increases. For TEA nitrogen laser where the pressure is 760 torr (one atmosphere) the lifetime is about 1 - 3 ns.

The important properties can be summarized as follows:

• Wavelength 337.1 nm
• Spectral bandwidth 0.1 nm
• Pulse width (FWHM) < 3.5 ns
• Pulse energy upto 300 μJ
• Rep rate upto 100 Hz
• Beam size 3 x 7 mm
• Small signal gain: 1-2 cm^-1
• Saturation intensity; 50 KW/cm²
• Beam divergence 4 x 6 mrad
• Long tube life - 10⁸ shots per fill
• Spark gap or DC heated thyratron triggered discharge

The name excimer refers to the electronically excited species such as monomers, dimers and other com¬plexes, which exist in the electronically excited state only. Excimers are characterized by short radiative lifetimes of the order of nanoseconds and large cross sections for stimulated emission, which enables an efficient laser operation. The term excimer stands for 'excited dimer' where a dimer refers to a molecule of two identical or similar parts. In the case of excimer lasers as we know which consists of nobel gas halides, both the molecules are different. In this case exciplex should be used for 'excited complex'. Most "excimer" lasers are of the noble gas halide type, for which the term excimer is strictly speaking a misnomer. The correct name for these lasers is exciplex laser. However, we all know these lasers as excimer lasers.

While a lot of different excimer laser transitions have been used to generate light pulses at various wavelengths between 126nm and about 660nm, the most commonly used excimer lasers are krypton fluoride (KrF, 248 nm), argon fluoride (ArF, 193 nm), xenon chloride (XeCl, 308 nm) and xenon fluoride (XeF, 351 nm).

Laser action in an excimer molecule occurs because it has a bound excited state, but a repulsive ground state. This is because inert gases such as xenon and krypton do not usually form chemical compound. However, when in an excited state, they can form temporarily-bound molecules with themselves or with halogens such as fluorine and chlorine. This bound state is the upper laser level in the case of excimer laser. The excited compound can give up its excess energy by undergoing spontonaneous or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly dissociates back into two unbound atoms. Since the excimer molecule returns to the unexcited ground state and separates into atoms, the population inversion condition is achieved the moment excited state is created, since the population of ground level is nil.

The energy level diagram of excimer laser is shown here. The general principle of the excimer laser transitions is shown in Fig. For example, in case of KrF, the upper laser level is an ionically bound charge transfer state of the rare gas positive ion and the halogen negative ion. For example for Krypton fluoride lasers, it is ²P rare gas positive ion (Kr⁺) and the ¹S halogen negative ion (F^-). The upper laser level is populated by a three-body collision involving Kr⁺, F^-, and a third collision partner (called buffer gas, for example Ne or He). While we can see that there is a minimum in the potential energy curve in the upper state it is still rather unstable. The excited laser molecules decay after several nanoseconds via emission of a photon into individual atoms like Kr and F. These form the ground state, which is covalently bonded and consists of separate Kr and F atoms for large inter-nuclear separations.

Copper vapour laser: 510.6 nm (green) and 578.2 nm (yellow)
Gold vapour laser: 627.8 nm (red)

Copper vapour laser (CVL) is three-level laser and uses copper vapours as the lasing medium. It produces green laser light at 510.6 nm and yellow laser light at 578.2 nm. It consists of a sealed zirconia tube filled with neon gas at a pressure of 25-40 torr. A solid block of pure copper metal is kept in the middle of the tube. Copper is heated above 1083⁰C (laser operating temperature is about 1650⁰C) to generate copper vapour, which is the active laser medium. High voltage is applied between the two electrodes at the end of the zirconia tube. As a result, the temperature rises inside the tube cavity to about 1400 - 1700 ⁰C, until the Copper evaporates, and the vapour pressure of the Copper is about 0.1 torr. Electrons, accelerated by the high voltage applied to the electrodes, collide with the copper vapour molecules, exciting them into one of the available high laser energy levels. The lasers are self-heated such that most of the energy provided by the discharge current provides heat to bring the plasma tube to the necessary temperature. Excitation occurs by electrons colliding with neutral copper atoms to excite them to the relevant laser-related energy levels. Inelastic collisions of electrons with copper vapour atoms causes excitation of the copper atoms, so the inversion population occur and laser oscillation due to electron transition from upper level (P_1/2, P_3/2) to the lower meta-stable level (D_5/2, D_3/2) take place at the 578.2- and 510.6nm, respectively. Two principal outputs having wavelengths of 510.6 nm (green line ²P_3/2 - ²D_5/2) and 578.2 (yellow line ²P_1/2 - ²D_3/2) are obtained. The lower laser levels (2D) are metastable leading to self-termination of the laser action. Upper and lower laser levels are shown in the energy level diagram.

The Copper Vapour Lasers (CVL) can be realized by two different ways. First one is the development of lasers using copper as such and the second one involves the vapours of copper-bearing compounds, mainly the copper halides, CuCl, CuBr, CuI. In case of copper based CVL lasers, Copper must be heated to 1400 to 1700⁰C in order to achieve a suitable vapour pressure. However, in case of CVL utilizing copper based compounds, it is possible to achieve a sufficient copper concentration for lasing in the 300 to 600⁰C range depending upon the type of compound. Typically Halide based lasers such as copper bromide (CuBr), copper chloride (CuCl) and copper iodide (CuI) lasers are necessary to be heated to 400, 500, 600 ⁰C, respectively.

Typically two energizing pulses in quick succession are required, the first to dissociate vapour molecules, and the second to cause the dissociated ions to lase. The first pulse provides copper atoms by dissociating the halide. The second pulse is delayed until an adequate copper atom concentration has built up. The second pulse is a fast discharge pulse that pumps the copper atoms to the upper laser levels by electron collisions. For better performance, the laser requires fast excitation pulses of rise time of the order of 100ns or less. The time gap between dissociation pulse and excitation pulse should be dependent on how fast the chloride and copper atoms recombine to copper halide.

Typical Parameters of Copper Vapour Lasers:

• Average Power (W): Upto 200 W
• Peak Power: 50 - 500 kW
• Pulse repetition rate: 2 - 40 kHz
• Green/Yellow ratio 1.5:1
• Pulse width (ns) 5 - 50 nS
• Efficiency (%) Greater than 1
• Small signal gain g0 = 0.05 - 0.1 cm^-1
• Saturation intensity = 9 - 12 W/cm²
• Beam Diameter: 5 - 15 mm
• Divergence: 3 - 5 mrad
• Tube diameter: 10 - 100 mm
• Tube length: 50 - 150 cm
• Warm up time: 45 - 90 min
• Lifetime: 300 - 800 hours
• Buffer gas: 25 to 50 Torr of neon

Applications of Copper Vapour Lasers

• Pumping source for tunable dye lasers and solid-state laser materials such as Ti:sapphire to obtain pico second and femto second ultrashort pulses
• High-speed flash photography and high-speed imaging with high spatial resolution and temporal resolution
• Precision material Processing
• Underwater applications
• Holography
• Particle imaging velocimetry
• Spray Pattern Measurement
• Flow Visualization
• Photodynamic therapy and detection of forensic evidence
• Laser beam can be absorbed by biological tissue components, which may be selectively destroyed. In oncology, the photodynamic therapy based on the effect of simultaneous photochemical reaction between an appropriate sensitize, laser light and oxygen, is used for a selective destructions of pathological tissues
• Dermatology
• Copper vapour lasers emitting light at 511 nm (green) ad 578 nm (yellow) have been useful for treating pigmented and vascular lesions, respectively.
• Nonlinear frequency conversion to the ultraviolet. Harmonic generation can produce 255 and 289 nm from the fundamental copper lines or tunable ultraviolet light from CVL-pumped dye lasers. So one can get pico-second and femtosecond pulses. They are particularly useful for studying Time-spatial resolved spectroscopy.
• High Resolution spectroscopy
• Frequency doubled CVL can be used for fabrication of fiber Bragg gratings (FBGs)
• Copper vapour lasers has an important application in atomic vapour isotope separation (AVLIS) as a pumping source to excite tunable dye lasers

The principle of operation and structure of the GVL is quite similar to CVL. Even the same laser tube and power supply can be used for both lasers. The only change is that instead of solid copper metal, gold wire is employed to produce gold vapour and it lases at 627.8 nm in the red region. The laser head can withstand a temperature of about 1700⁰C. The discharge circuit, like CVL, makes use of the thyratron.

As compared to Copper Vapour Lasers, which can emit more than 200 W of average power, Gold vapour can produce few tens of Watts only.

Typical efficiency of GVL (627.8 nm) is 0.2 % as compared to 1.5 %, that of Copper Vapour Laser.

Gold vapour lasers find their main applications in dermatological and experimental cancer treatment of photodynamic therapy

Energy band diagram of dye lasers is shown in the adjoining figure.

Molecule absorbs light and populates fist excited singlet state S1 with electrons

Electrons in upper vibrational levels of S1 undergo vibrational relaxation and the electrons move to the lowest vibrational level of S1. This excitation energy is then rapidly redistributed within the S1 state within a time period of few picoseconds. Thus the molecules very quickly dissipate this very high energy by internal conversion - the electron density moves to the lowest excited state, S1. Internal conversion occurs by the electron density transferring from the vibrational levels of the upper excited state to vibrational levels of a lower excited state, which are overlapping. This is a radiationless transition i.e. it does not emit a photon of energy.

The molecule decays from the S1 state to the S0 state and the energy reappears as fluorescence photons. This process may take few nanoseconds. Internal conversion may occur in S0 as well. There is a possibility of transfer of energy from S1 singlet state to T0. This happens in case the lifetime of S1 is more than of the order of 100 nanoseconds. If it happens, then the energy is lost and efficiency of dye lasers is reduced.

The simplified energy level diagram of dye lasers is also shown.

Triplet absorption in excited dye systems is a major factor that limits the proper laser action. That is, the laser pulse terminates before the pump pulse ends. In fact, the laser pulse usually terminates before the intensity of the pump pulse has fallen below the threshold excitation value. Efforts have been made to overcome this situation. For an efficient dye Laser the energy transfer from singlet state S1 to Triplet state T0 should be avoided, as it is the main loss mechanism within the dye molecule. This can be avoided if the dye molecules have low fluorescence lifetime for S1 to S0 transition as compared to time required for transfer of population from S1 singlet state to T0, the triplet state. All successful dye lasers use dyes with typical fluorescence lifetime for the S1-S0 transition of the order of few nanoseconds as compared to 100 nS for S1-T1 transition.

The other common method often used is to add a second molecule to the dye solution to act as a triplet-quenching agent. Collisions between quencher and dye molecules are responsible for this de-excitation process. Usually triplet quenchers are laser dye specific: for example, Cycloheptatriene and cyclooctatetraene (COT) are good triplet quencher for rhodamine 6G. Adamantane is also sometimes added to some dyes to prolong their life

Some of the Important Dyes and their Wavelength Tuning Range

Dye lasers can be operated in both pulsed and CW modes. In the pulsed mode, these are usually pumped either by flash lamps or by other lasers such as pulsed-nitrogen lasers or copper vapour laser or excimer lasers or frequency-doubled Nd:YAG. On the other hand, in the continuous mode, the output of a CW argon ion laser generally is preferred as the pumping source.

Although liquid dye lasers have been very successful, there has been continuous effort to find new gain media in the solid-state that would simplify the engineering of this class of lasers. The gain media used in these lasers are organic dye doped polymers. Obviously, the solid-state form has many advantages, particularly concerning handling. There are a number of materials, which have been used as solid hosts for laser dyes such as polymers, porous glasses, organically modified silicates or silicate nano-composites, polycom glass (combination of polymer and sol-gel). Further the laser damage threshold also increases significantly. Typically, Rhodamine 6G dye doped in polymer and porous glass composition can withstand pump pulses of 50 ns with energies of 7-10 J/cm².

Though solid-state lasers based on Ti: sapphire are now being used for a particular wavelength range as these can produce high enough powers without any lifetime restrictions, still dye lasers are dominating the fields of tunable lasers and ultrashort pulse generation for areas such as spectroscopy where large number of wavelengths are required. So as such the future of laser dyes particularly solid-state dye lasers continues to be good.

Important Properties of Dye Lasers

• Spectroscopy, holography, and biomedical applications.
• Treatment of port wine stain (PWS)
• Lithotripsy
• Isotope separation