SINGLE-FREQUENCY OPERATIONConsider an argon laser that has a relatively

large bandwidth, allowing a large number of longitudinal modes (with slightly different frequencies) to oscillate simultaneously in which a small section of the output is expanded to show details of each individual mode

• There are many longitudinal modes If an etalon (basically, a compact interferometer) is designed such that it is resonant only at wavelengths spaced farther apart than 5 GHz, the laser can be made to operate on a single mode (and hence a single frequency). The spacing of the resonant peaks of an etalon, called the free spectral range (FSR) of an etalon, is evident in the middle diagram in Figure 6.6.2. Only when a resonant peak of the etalon and a longitudinal mode of the laser have the same frequency will the laser have enough gain to oscillate.

• Coherence length is hence related to spectral width: The wider the spectral width, the shorter the coherence length. It is defined (in units of meters) as

• By selecting a single mode with a spectral width of about ν, the coherence length increases the distance between the successive longitudinal modes to be more distance than the range of frequencies has maximum gain curve. The reset of the modes will be out of the desired mode selected. If the maximum gain of longitudinal mode frequency q0then the other longitudinal mode will be out of the gain curve at q0-1and q0+1.

• If the distance between two successive modes given by C/2L. That is mean that the less the length L of the amplifier the more separation distant between the modes. The laser cavity must satisfy the condition:

For a laser with a larger inherent spectral width, such as an argon laser, single-frequency operation is quite desirable to allow this laser to be used as a source in making holograms (where a spectrally narrow source leads to better depth in the recorded image).

• Example:• Consider the spectral width of a regular and a

single-frequency HeNe laser. In Section 4.7 we calculated the spectral width of a HeNe laser to be 1.56 GHz and so:

Q-Switching• Fast, powerful pulses tend to ablate material quickly

without heating (and potentially altering) surrounding material or tissue.

• Fast pulses will quickly vaporize resistor material in a controlled manner, increasing the resistance of the structure by a controlled amount.

• For this reason a technique called Q-switching is used to produce very short pulses for these types of applications. The most simplistic method that can be envisioned to produce a pulsed laser is to switch the gain of the medium on and off. By switching the pump energy to the medium on and off, gain inside the laser is also switched on and off. When pump energy is sufficient to allow laser gain to exceed the threshold, an output beam appears.

• In a Q-switching technique, the laser output is switched by controlling loss within the laser cavity as outlined in Figure 7.1.1. More correctly, Q-switching is loss switching in which a loss is inserted into the cavity, thus spoiling it for laser action. In the simplest manner, a Q-switch can be thought of as an optical gate blocking the optical path to one cavity mirror and hence causing laser action to cease.

• A Q-switch consists of a mechanism that spoils the resonance of the laser cavity. There are a number of ways to accomplish this, from altering the alignment of a cavity mirror mechanically (e.g., by a rotating mirror), insertion of an optical switch within the cavity of the laser itself (e.g., an EO or AO modulator), or a saturable dye switch within the cavity. These methods are outlined in Figure 7.2.1, in which each type of Q-switch is shown within in a solid-state laser cavity.

Turning mirror at the end of the optical cavity• A rotating mirror, the simplest method, is

rarely used except in a few older military rangefinders that use ruby rods.

Electro-Optic transducerElectro optic modulators work quite literally as

optical switches, allowing intracavity light to pass only when the switch is open.

Acousto-Optic transducer • Change the transmission through the device by acoustic

signal

Saturable absorber• Saturable dye switches are simply a cell filled with organic

dye (similar to the dye used in a dye laser) placed inside the laser cavity.

• An absorber that becomes transparent when it reaches saturation. It is usually a dye solution which prevents lasing by absorption. When the radiation arrives at a certain level, this absorber comes to saturation, and since it can no longer absorb radiation, it becomes transparent. At that moment lasing can occur, and all the stored energy inside the cavity is emitted as a single pulse.

Mode LockingLocking the longitudinal optical modes inside the cavity

is achieved by locking the relative phase of all the optical modes, such that at a certain point they all have the same phase. At this point a constructive interference occur between all the laser modes, and the result is a single pulse, with very short width and very high peak power, which move between the mirrors of the cavity. These moving pulses cause the laser output to be orderly chain of pulses. The length of each pulse is from 1 [psec] (10-12 [sec]), up to 1 [nsec] (10-9 [sec]).

• This is a synchronous switching which accumulates all the energy in a single pulse moving back and forth between the cavity mirrors. Each time it reaches the output coupler, a single pulse is emitted.

Time Interval (T) between two Adjacent Pulses is the time of flight of the single pulse inside the cavity for a complete round trip:

• Example:• If the time interval between two adjacent pulses is

0.2x10-9sec, what is the length of tube?Solution:

L=TC/2=0.2x10-9x3x108=0.06m

LASER TYPES• The state of matter of the active medium:

solid, liquid, gas, or plasma. • The spectral range of the laser wavelength:

visible spectrum, Infra-Red (IR) spectrum, etc. • The excitation (pumping) method of the active

medium: Optic pumping, electric pumping, etc. • The characteristics of the radiation emitted

from the laser. • The number of energy levels which participate

in the lasing process

The active medium laserThe active medium controls the following properties

of the laser:• Laser Wavelength. • Preferred pumping method. • Order of magnitude of the laser output. • The efficiency of the laser system.• We saw that the two basic requirements for laser

action are: 1- Population Inversion between the upper and lower

laser energy levels. 2- The active medium must be transparent to the

output wavelength

Visible Gas LasersHELIUM–NEONEON LASERS• The first gas laser, the helium–neon (HeNe) laser, is still an

important source of coherent red light with uses ranging from bar-code scanning to alignment.

• HeNe lasers operate with the familiar red (632.8 nm) beam, but multiple transitions are possible, allowing the laser to operate (with suitable optics) at wavelengths in the infrared, orange, yellow, and green.

• Power output for commercially available HeNe tubes ranges from under 1 mW for a small HeNe tube to just over 100 mW for a large behemoth unit.

• These lasers typically feature excellent spectral and coherence characteristics, so are a popular choice for uses in holography, where they are still the laser of choice, preferred over semiconductor types.

Lasing Medium• The HeNe laser uses a mixture of very pure helium and

neon gases in the approximate ratio of 10 : 1.• Helium, the gas used to furnish the pump level in this

four-level system, is in greater abundance. The active medium is a noble gas Neon (Ne), and it is a 4 level laser. The energy level diagram of a Helium-Neon laser is described in the figure below. Two meta-stable energy levels

act as upper laser levels. The He-Ne laser have two lower laser levels, so quite a few wavelengths can come out of the transitions between these levels.

• The important wavelengths are: λ1= (632.8 [nm]), λ 2=1.152 [mm], λ 3=3.3913

[mm],

The role of the Helium gas in He-Ne laser is to increase the efficiency of the lasing process.

• The excitation process of the Neon atoms is a two stages process: • The high voltage causes electrons to accelerate from the cathode

toward the anode. These electrons collide with the He atoms and transfer kinetic energy to them.

• The excited Helium atoms collide with the Neon atoms, and transfer to them the energy for excitation.

• Thus Helium gas does not participate in the lasing process, but increases the excitation efficiency

• Red Wavelength out of He-Ne Laser Most of the applications of He-Ne Laser use the red wavelength, because it is the strongest line and it is in the visible region of the spectrum. As shown in figure above, this red light is emitted when the Neon atom goes from the energy level labeled E5 to the energy level labeled E2, a much bigger energy difference than for the other transitions.

• A problem with creating this red light is that a Neon atom in state E5 may also emit 3.3913 [μm] radiation. This emission decreases the population of the E5 level, without producing visible radiation.

• The solution to this problem is to use a special coating on the laser mirrors which selectively reflect only the red light. This coating causes reflection back into the optical cavity of only the desired (red) wavelength, while all other wavelengths are transmitted out, and not forced to move back and force through the active medium.

• In a similar way, other selective reflecting coating can be used on the mirrors to select other transitions. This procedure allows commercial production of He-Ne lasers at other wavelengths in the visible spectrum. For example, orange, yellow and green He-Ne lasers can be produced, but the laser efficiency is much lower than for the red.

Absorption and Amplification in He-Ne Laser • He-Ne laser is a 4 level laser, so the lifetime of the lower laser

energy level needs to be very short. In a Neon gas, which is the active lasing gas, the transition (decay) from the lower laser level is not fast enough, but it is accelerated by collisions with the tube walls. Because the number of collisions with the tube walls increase as the tube becomes narrow, the laser gain is inversely proportional to the tube radius. So, the tube diameter of a He-Ne laser must be as small as possible.

• The low gain of the active medium in a He-Ne laser limits the output power to low power. In laboratory prototypes an output power of the order of 100 [mW] was achieved, but commercial lasers are available only in the output range of 0.5-50 milliwatts [mW].

• The output coupler of He-Ne laser is a mirror with coating that transmits about 1% of the radiation to the output. This means that the power inside the optical cavity is a 100 times more than the emitted power