Optical Sources

LASER By: Prof. Snehal Laddha

A laser is a device that generates light by a process called STIMULATED EMISSION.

The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation

Semiconducting lasers are multilayer semiconductor devices that generates a coherent beam of monochromatic light by laser action. A coherent beam resulted which all of the photons are in phase.

Prof. Snehal Laddha

LED is chosen for many applications using multimode fibers

The lasers tends to find more use as a single mode device in single mode fibers.

Prof. Snehal Laddha

Lasers are all around us in many places you might not realize. Besides being useful for pure science like in a physics lab, lasers are found in many real-world applications.

The grocery store The Doctor’s office Manufacturing Telecommunication The Moon Weapons

Prof. Snehal Laddha

The scanner measures the brightness of the reflected light and converts thisinformation into numbers and letters.

Prof. Snehal Laddha

Lasers are also used by dentists to fill cavities, and by doctors as a scalpel.Lasers have the advantage of being more precise and much less invasive than astandard scalpel. Lasers have become widespread in the treatment of cancer(tumor removal)

Prof. Snehal Laddha

Lasers can cut and weld complex structures out of both hard and soft materials, and at much smaller scales than traditional welding. This type of welding/cutting can be computer controlled for ultra high precision.

Prof. Snehal Laddha

Fiber-optic communication is one of the most important contributions from physics to our daily lives.

Telephone, internet, television are all transmitted using fiber-optics.

Laser light is used as a carrier for different types of information, which can then be sent huge distances with very little signal degradation and at high speeds.

Prof. Snehal Laddha

An example of application is for the light source for fiber optics communication.

Light travels down a fiber optics glass at a speed, = c/n, where n = refractive index.

Light carries with it information Different wavelength travels at different speed. This induce dispersion and at the receiving end

the light is observed to be spread. This is associated with data or information lost.

The greater the spread of information, the more loss

However, if we start with a more coherent beam then loss can be greatly reduced.

Prof. Snehal Laddha

Prof. Snehal Laddha

1. Absorption2. Spontaneous Emission3. Stimulated Emission

1. Absorption2. Spontaneous Emission3. Stimulated Emission

For atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energy

For atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energy

There is another process whereby the atom in an upper energy level can be triggered or stimulated in phase with the an incoming photon. This process is:

Stimulated emission

It is an important process for laser action

There is another process whereby the atom in an upper energy level can be triggered or stimulated in phase with the an incoming photon. This process is:

Stimulated emission

It is an important process for laser action

Therefore 3 process of light emission:

Prof. Snehal Laddha

E1

E2

When a photon of energy (E2-E1) is incident on the atom it may be excited into the higher energy state E2 through absorption of photon. This process is called as stimulated absorption.

Prof. Snehal Laddha

When the atom in higher energy state E2 makes a transition in lower energy state E1 in an entirely random manner, the process is called spontaneous emission.

Prof. Snehal Laddha

When a photon of energy equal to the energy difference between the two states (E2-E1) interacts with the atom in the upper energy state causing it to return to the lower state with the creation of a second photon, is called stimulated emission.

Prof. Snehal Laddha

In 1917 Einstein predicted that: under certain circumstances a photon

incident upon a material can generate a second photon of Exactly the same energy (frequency)PhasePolarisationDirection of propagation

In other word, a coherent beam resulted.

Prof. Snehal Laddha

Consider the ‘stimulated emission’ as shown previously.

Stimulated emission is the basis of the laser action.

The two photons that have been produced can then generate more photons, and the 4 generated can generate 16 etc… etc… which could result in a cascade of intense monochromatic radiation.

Prof. Snehal Laddha

E1

E2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

In hOut

h

E2 E2

E1 E1

Absorption, spontaneous (random photon) emission and stimulatedemission.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Prof. Snehal Laddha

The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the higher energy state E2 and lower energy state E1 by expression:E=E2-E1=hfWhere, h= 6.626x10^-34 Js is planck’s constant

Prof. Snehal Laddha

The random nature of spontaneous emission process where light is emitted by electronic transitions from a large number of atoms gives incoherent radiation.

Such process provides basic mechanism for light generation within the LED

The light generated from stimulated photon is in phase and has same polarization, i.e. coherent radiation.

Prof. Snehal Laddha

Therefore we must have a mechanism where N2 > N1 This is called POPULATION INVERSION Population inversion can be created by introducing a so call metastable

centre where electrons can piled up to achieve a situation where more N2 than N1

The process of attaining a population inversion is called pumping and the objective is to obtain a non-thermal equilibrium.

It is not possible to achieve population inversion with a 2-state system. If the radiation flux is made very large the probability of stimulated emission

and absorption can be made far exceed the rate of spontaneous emission. But in 2-state system, the best we can get is N1 = N2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E31 then atoms in state 3 relax

to state 2 non radiatively. The electrons from E2 will now jump to E1 to give out radiation.

Prof. Snehal Laddha

A thermal equilibrium Boltzmann equation shows us that: N1 > N2 > N3 Thus, the population numbers of higher energy levels are smaller than the population numbers of lower ones. This situation is called "Normal Population".

In a situation of normal population a photon impinging on the material will be absorbed, and raise an atom to a higher level.

Prof. Snehal Laddha

In population inversion, at least one of the higher energy levels has more atoms than a lower energy level. An example is described in the Figure below.

In this situation there are more atoms (N3) in an higher energy level (E3), than the number of atoms (N2) in a lower energy level (E2).

Prof. Snehal Laddha

Prof. Snehal Laddha

A schematic energy level diagram of a laser with three energy levels is the figure below. The two energy levels between which lasing occur are: the lower laser energy level (E1), and the upper laser energy level (E2).

Prof. Snehal Laddha

To achieve lasing, energy must be pumped into the system to create population inversion. So that more atoms will be in energy level E2 than in the ground level (E1). Atoms are pumped from the ground state (E1) to energy level E3. They stay there for an average time of 10-8 [sec], and decay (usually with a non-radiative transition) to the metastable energy level E2.

Prof. Snehal Laddha

Since the lifetime of the metastable energy level (E2) is relatively long (of the order of 10-3 [sec], many atoms remain in this level.

If the pumping is strong enough, then after pumping more than 50% of the atoms will be in energy level E2, a population inversion exists, and lasing can occur.

Prof. Snehal Laddha

In a four level laser compared to the equivalent diagram of a three level laser, there is an extra energy level above the ground state. This extra energy level has a very short lifetime.

Prof. Snehal Laddha

The pumping operation of a four level laser is similar to the pumping of a three level laser. This is done by a rapid population of the upper laser level (E3), through the higher energy level (E4).

To create population inversion, there is no need to pump more than 50% of the atoms to the upper laser level.

Prof. Snehal Laddha

Prof. Snehal Laddha

The lasing threshold of a four level laser is lower.

The efficiency is higher. Required pumping rate is lower. Continuous operation is possible.

Prof. Snehal Laddha

When a sizable population of electrons resides in upper levels, this condition is called a "population inversion", and it sets the stage for stimulated emission of multiple photons. This is the precondition for the light amplification which occurs in a LASER and since the emitted photons have a definite time and phase relation to each other, the light has a high degree of coherence.

Prof. Snehal Laddha

Light amplification in the laser occurs when a photon colliding with an atom in the excited energy state causes the stimulated emission of a second photon and then both these photons release two more.

Continuation of this process effectively creates avalanche multiplication, and when the electromagnetic waves associated with these photons are in phase, amplified coherent emission is obtained.

To achieve this laser action it is necessary to contain photons within the laser medium and maintain the conditions for coherence.

Prof. Snehal Laddha

This is accomplished by placing or forming mirrors (plane or curved) at either end of the amplifying medium, as illustrated in Figure

Prof. Snehal Laddha

The optical cavity formed is more analogous to an oscillator than an amplifier as it provides positive feedback of the photons by reflection at the mirrors at either end of the cavity.

Hence the optical signal is fed back many times while receiving amplification as it passes through the medium. The structure therefore acts as a Fabry– Pérot resonator.

Although the amplification of the signal from a single pass through the medium is quite small, after multiple passes the net gain can be large.

Furthermore, if one mirror is made partially transmitting, useful radiation may escape from the cavity.

Prof. Snehal Laddha

A stable output is obtained at saturation when the optical gain is exactly matched by the losses incurred in the amplifying medium.

The major losses result from factors such as absorption and scattering in the amplifying medium, absorption, scattering and diffraction at the mirrors and nonuseful transmission through the mirrors.

Prof. Snehal Laddha

Oscillations occur in the laser cavity over a small range of frequencies where the cavity gain is sufficient to overcome the above losses.

Hence the device is not a perfectly monochromatic source but emits over a narrow spectral band.

The central frequency of this spectral band is determined by the mean energy-level difference of the stimulated emission transition

Prof. Snehal Laddha

Other oscillation frequencies within the spectral band result from frequency variations due to the thermal motion of atoms within the amplifying medium (known as Doppler broadening) and by atomic collisions.

Hence the amplification within the laser medium results in a broadened laser transition or gain curve over a finite spectral width, as illustrated in Figure

Prof. Snehal Laddha

The amplifying medium the radiation builds up and becomes established as standing waves between the mirrors.

These standing waves exist only at frequencies for which the distance between the mirrors is an integral number of half wavelengths.

Thus when the optical spacing between the mirrors is L, the resonance condition along the axis of the cavity is given by

1

Prof. Snehal Laddha

where λ is the emission wavelength, n is the refractive index of the amplifying medium and q is an integer. Alternatively, discrete emission frequencies f are defined by:

Prof. Snehal Laddha

where c is the velocity of light. The different frequencies of oscillation within the laser cavity are determined by the various integer values of q and each constitutes a resonance or mode. Since Eqs (1) and (2) apply for the case when L is along the longitudinal axis of the structure, the frequencies given by Eq. (2) are known as the longitudinal or axial modes. Furthermore, from Eq. (2) it may be observed that these modes are separated by a frequency interval δf where:

Prof. Snehal Laddha

Hence substituting for δf from Eq. (3) gives:

Prof. Snehal Laddha

Prof. Snehal Laddha

The radiative properties of a junction diode may be improved by the use of heterojunctions.

A heterojunction is an interface between two adjoining singlecrystal semiconductors with different bandgap energies.

Devices which are fabricated with heterojunctions are said to have heterostructure.

Prof. Snehal Laddha

Heterojunctions are classified into either an isotype (n–n or p–p) or an anisotype (p–n).

The isotype heterojunction provides a potential barrier within the structure which is useful for the confinement of minority carriers to a small active region (carrier confinement).

It effectively reduces the carrier diffusion length and thus the volume within the structure where radiative recombination may take place.

This technique is widely used for the fabrication of injection lasers and high-radiance LEDs.

Isotype heterojunctions are also extensively used in LEDs to provide a transparent layer close to the active region which substantially reduces the absorption of light emitted from the structure

Prof. Snehal Laddha

Alternatively, anisotype heterojunctions with sufficiently large bandgap differences improve the injection efficiency of either electrons or holes.

Both types of heterojunction provide a dielectric step due to the different refractive indices at either side of the junction.

This may be used to provide radiation confinement to the active region (i.e. the walls of an optical waveguide).

The efficiency of the containment depends upon the magnitude of the step which is dictated by the difference in bandgap energies and the wavelength of the radiation.

Prof. Snehal Laddha

Prof. Snehal Laddha

A heterojunction is shown either side of the active layer for laser oscillation.

The forward bias is supplied by connecting a positive electrode of a supply to the p side of the structure and a negative electrode to the n side.

When a voltage which corresponds to the bandgap energy of the active layer is applied, a large number of electrons (or holes) are injected into the active layer and laser oscillation commences.

These carriers are confined to the active layer by the energy barriers provided by the heterojunctions which are placed within the diffusion length of the injected carriers.

Prof. Snehal Laddha

Stimulated emission by the recombination of the injected carriers is encouraged in the semiconductor injection laser (also called the injection laser diode (ILD) or simply the injection laser) by the provision of an optical cavity in the crystal structure in order to provide the feedback of photons

Prof. Snehal Laddha

High radiance due to the amplifying effect of stimulated emission. Injection lasers will generally supply milliwatts of optical output power.

Narrow linewidth on the order of 1 nm (10 Å) or less which is useful in minimizing the effects of material dispersion.

Modulation capabilities which at present extend up into the gigahertz range and will undoubtedly be improved upon

Prof. Snehal Laddha

Relative temporal coherence which is considered essential to allow heterodyne (coherent) detection in high-capacity systems, but at present is primarily of use in single-mode systems.

Good spatial coherence which allows the output to be focused by a lens into a spot which has a greater intensity than the dispersed unfocused emission.

Prof. Snehal Laddha

The basic structure of this homojunction device is shown in Figure, where the cleaved ends of the crystal act as partial mirrors in order to encourage stimulated emission in the cavity when electrons are injected into the p-type region.

However, as mentioned previously these devices had a high threshold current density (greater than 104 A cm−2 ) due to their lack of carrier containment and proved inefficient light sources.

Prof. Snehal Laddha

The DH laser structure provides optical confinement in the vertical direction through the refractive index step at the heterojunction interfaces, but lasing takes place across the whole width of the device.

Prof. Snehal Laddha

Prof. Snehal Laddha

To overcome these problems while also reducing the required threshold current, laser structures in which the active region does not extend to the edges of the device were developed.

A common technique involved the introduction of stripe geometry to the structure to provide optical containment in the horizontal plane.

Prof. Snehal Laddha

Prof. Snehal Laddha

Generally, the stripe is formed by the creation of high-resistance areas on either side by techniques such as proton bombardment or oxide isolation

The stripe therefore acts as a guiding mechanism which overcomes the major problems of the broad-area device.

However, although the active area width is reduced the light output is still not particularly well collimated due to isotropic emission from a small active region and diffraction within the structure.

The optical output and far-field emission pattern are also illustrated in Figure 2.

The output beam divergence is typically 45° perpendicular to the plane of the junction and 9° parallel to it. Nevertheless, this is a substantial improvement on the broad-area laser

Prof. Snehal Laddha

The stripe contact device also gives, with the correct balance of guiding, single transverse (in a direction parallel to the junction plane) mode operation, whereas the broad-area device tends to allow multimode operation in this horizontal plane.

Numerous stripe geometry laser structures have been investigated with stripe widths ranging from 2 to 65 μm, and the DH stripe geometry structure has been widely utilized for optical fiber communications.

Such structures have active regions which are planar and continuous.

Prof. Snehal Laddha

The Nd : YAG laser The crystalline waveguiding material which forms the

active medium for this laser is yttrium–aluminum–garnet (Y3Al5O12) doped with the rare earth metal ion neodymium (Nd3+ ) to form the Nd : YAG structure.

The energy levels for both the lasing transitions and the pumping are provided by the neodymium ions which are randomly distributed as substitutional impurities on lattice sites normally occupied by yttrium ions within the crystal structure.

However, the maximum possible doping level is around 1.5%. This laser, which is currently utilized in a variety of areas

Prof. Snehal Laddha

Single-mode operation near 1.064 and 1.32 μm, making it a suitable source for single-mode systems.

A narrow linewidth (<0.01nm) which is useful for reducing dispersion on optical links.

A potentially long lifetime, although comparatively little data is available

The possibility that the dimensions of the laser may be reduced to match those of the single-mode fiber.

Prof. Snehal Laddha

The device must be optically pumped. A long fluorescence lifetime of the order of

10^−4 seconds which only allows direct modulation of the device at very low bandwidths. Thus an external optical modulator is necessary if the laser is to be usefully utilized in optical fiber communications.

The above requirements (i.e. pumping and modulation) tend to give a cost disadvantage in comparison with semiconductor lasers.

Prof. Snehal Laddha

It comprises an Nd : YAG rod with its ends ground flat and then silvered. One mirror is made fully reflecting while the other is about 10% transmitting to give the output

Prof. Snehal Laddha

The basic structure of a glass fiber laser is shown in Figure

Prof. Snehal Laddha

An optical fiber, the core of which is doped with rare earth ions, is positioned between two mirrors adjacent to its end faces which form the laser cavity.

Light from a pumping laser source is launched through one mirror into the fiber core which is a waveguiding resonant structure forming a Fabry–Pérot cavity.

The optical output from the device is coupled through the mirror on the other fiber end face, as illustrated in Figure

Prof. Snehal Laddha

Thus the fiber laser is effectively an optical wavelength converter in which the photons at the pumping wavelength are absorbed to produce the required population inversion and stimulated emission;

This provides a lasing output at a wavelength which is characterized by the dopant in the fiber.

Prof. Snehal Laddha