VARIOUS TECHNICAL ISSUES

Doped insulator solid-state lasers are most suitable for Q switching.

Since their gain media have long upper-state lifetimes and high saturation energies.

Hence the capability to store large amounts of energy.

Bulk lasers are normally preferable over fiber lasers.

Since their larger mode areas allow more energy to be stored, and their shorter resonators allow for shorter pulses.

For both active and passive Q switching, higher pulse repetition rates usually imply longer pulses.

This is because the reduced pulse energy leads to a weaker modulation of the net gain, and thus to a slower rise and decay of the optical power.

When the pulse repetition rate of an actively Q-switched laser falls below the inverse upper-state lifetime, the maximum pulse energy is achieved.

But the average power is reduced due to increased losses via spontaneous emission.

Pumping does not have to occur in a continuous-wave fashion; it is also possible to use flash lamps or quasi-cw laser diodes, fired shortly before the Q switch is opened.

This reduces the energy losses via spontaneous emission and thus allows the use of gain media with shorter upper-state lifetimes.

In most cases, the pulses in a Q-switched laser are generated by amplifying noise from spontaneous emission in many resonator round trips.

Therefore, there is usually no phase correlation between subsequent pulses, and the pattern of excited resonator modes can be random.

The nonlinear dynamics of Q switching sometimes lead to unexpected phenomena, such as the generation of double pulses and/or certain instabilities.

Numerical simulations of pulse generation can be very helpful in understanding such effects and identifying the right cure.

The Q-switched pulse train must be switched off for certain time intervals.

This often introduces the problem that the first pulse has a higher energy, if the pump source is continuously operated during the time without pulse emission.

Various methods have been developed to solve or mitigate this problem.

QUASI-CONTINUOUS-WAVE OPERATION Operation mode of lasers (e.g. diode bars)

where the pump power is switched on for short time intervals in order to limit thermal effects.

Pump source is switched on only for certain time intervals, which are short enough to reduce thermal effects significantly.

Still long enough that the laser process is close to its steady state, i.e. the laser is optically in the state of continuous-wave operation.

The duty cycle, a few percent, thus strongly reducing the heating and thermal effects, such as thermal lensing and damage through overheating.

Therefore, quasi-cw operation allows the operation with higher output peak powers at the expense of a lower average power.

Quasi-continuous-wave operation is most often used with diode bars and diode stacks.

Such devices are sometimes even designed specifically for quasi-cw operation.

Their cooling arrangement is designed for a smaller heat load, and the emitters can be more closely packed in order to obtain a higher brightness and beam quality.

Some doped-insulator solid-state lasers are also operated in quasi-cw operation. Such lasers are also called heat capacity lasers.

ULTRAFAST LASERS

The primary goal in designing and fabricating an ultrafast laser is to make its pulse as short as possible.

The fastest ultrafast lasers are capable of producing pulses of only a few femto seconds (10-15 sec) in duration. 

Frequency Spectrum, Coherence & Gain vs. Loss Mechanisms.

MODELOCKING

In the comparison of ultrafast and normal lasers, that an ultrafast laser simultaneously lases in many different modes.

But that the phases of the different modes are completely uncorrelated.  This will produce random (unpredictable) fluctuations in the intensity over time. 

Mode locking describes the set of techniques that are used to generate a known correlation between the phases and therefore make it possible to predict when the intensity maxima will occur. 

Active mode locking: a physical device is placed in the cavity which modulates the frequencies of the cavity modes.

Passive modelocking:  an intensity dependent loss-mechanism (satruable absorber) placed in the cavity causes less intense radiation to be damped out, leaving only a single, intense pulse oscillating back and forth in the cavity.

Self-modelocking: (really a special kind of passive mode-locking) the lasing medium itself has an intensity-dependent index of refraction.

LASER AMPLIFIERS

Most optical amplifiers are laser amplifiers, where the amplification is based on stimulated emission.

Here, the gain medium contains some atoms, ions or molecules in an excited state, which can be stimulated by the signal light to emit more light into the same radiation modes.

Such gain media are either insulators doped with some laser-active ions, or semiconductors, which can be electrically or optically pumped.

In addition to stimulated emission, there also exist other physical mechanisms for optical amplification, which are based on various types of optical nonlinearities.

Optical parametric amplifiers are usually based on a medium with χ(2) nonlinearity,

But there are also parametric fiber devices using the χ(3) nonlinearity of a fiber.

Other types of nonlinear amplifiers are Raman amplifiers and Brillouin amplifiers, exploiting the delayed nonlinear response of a medium.

An important difference between laser amplifiers and amplifiers based on nonlinearities is that laser amplifiers can store some amount of energy.

whereas nonlinear amplifiers provide gain only as long as the pump light is present.

MULTI PASS ARRANGEMENTS, REGENERATIVE AMPLIFIERS, AND AMPLIFIER CHAINS

A bulk-optical laser amplifier often provides only a moderate amount of gain.

Typically only few decibels. This applies particularly to ultra-short

pulse amplifiers. The effective gain may then be increased

either by arranging for multiple passes of the radiation through the same amplifier medium, or by using several amplifiers in a sequence (amplifier chains).

GAIN SATURATION

For high values of the input light intensity, the amplification factor of a gain medium saturates.

This is a natural consequence of the fact that an amplifier cannot add arbitrary levels of energy or power to an input signal.

However, as laser amplifiers store some amount of energy in the gain medium, this energy can be extracted within a very short time.

Therefore, during some short time interval the output power can exceed the pump power by many orders of magnitude.

DETRIMENTAL EFFECTS

For high gain, weak parasitic reflections can cause parasitic lasing, i.e., oscillation without an input signal, or additional output components not caused by the input signal.

This effect then limits the achievable gain. Even without any parasitic reflections,

amplified spontaneous emission may extract a significant power from an amplifier.

A related effect is that amplifiers also add some excess noise to the output.

This applies not only to laser amplifiers, where excess noise can partly be explained as the effect of spontaneous emission.

But also to nonlinear amplifiers.

ULTRAFAST AMPLIFIERS

Amplifiers of different kind may also be used for amplifying ultra short pulses.

In some cases, a high repetition rate pulse train is amplified, leading to a high average power while the pulse energy remains moderate.

In other cases, a much higher gain is applied to pulses at lower repetition rates, leading to high pulse energies and correspondingly huge peak powers.