Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm

2. LASER properties Nearly Monochromatic light • Example: • He-Ne Laser • λ0 = 632.5 nm • Δλ = 0.2 nm • Diode Laser • λ0 = 900 nm • Δλ = 10 nm

3. Directionality

4. Coherence Incoherent light waves Coherent light waves

5. Atomic Transitions Stimulated absorption

6. Stimulated emission

7. Condition for Amplification by Stimulated Emission • Population Inversion: • More Electrons in higher energy level • Pumping: • Process to achieve population inversion usually through external energy source • In general if N2 > N1 then MEDIA IS SAID TO BE ACTIVE

9. Basic concepts – Laser resonator • Amplification &Coherence achieved by Febry – Perot resonator • Placing mirrors at either end of the amplifying medium • Providing positive feedback • Amplification in a single go is quite small but after multiple passes the net gain can be large

10. One mirror is partially transmitting from where useful radiation may escape from the cavity. • Stable output occurs when optical gain is exactly matched with losses incurred (Absorption, scattering and diffraction)

11. Observations…. • Laser medium in a resonator produces oscillations • A spontaneous photon is duplicated over and over • Duplicated photons leak from semitransparent mirror • Photons from oscillator are identical • Coherent – identical photons • Controllable wavelength/frequency – nice colors • Controllable spatial structure – narrow beams • Controllable temporal structure – short pulses

12. ACTIVE MEDIUM Atoms: helium-neon (HeNe) laser; heliumcadmium (HeCd) laser, copper vapor lasers (CVL) •Molecules: carbon dioxide (CO2) laser, ArF and KrF excimer lasers, N2 laser •Liquids: organic dye molecules dilutely dissolved in various solvent solutions

14. Dielectric solids: neodymium atoms doped in YAG or glass to make the crystalline Nd:YAG or Nd:glass lasers •Semiconductor materials: gallium arsenide, indium phosphide crystals.

15. LASER action in Semiconductors Laser diode is similar in principle to an LED. What added geometry does a Laser diode require? An optical cavity that will facilitate feedback in order to generate stimulated emission In addition to population inversionlaser oscillation must be sustained. An optical cavity is implemented to elevate the intensity of stimulated emission. (optical resonator) Provides an output of continuous coherent radiation. A homojunction laser diode is one where the pn junction uses the same direct bandgap semiconductor material throughout the component (ex. GaAs)

17. Fundamental Laser diode: 1. Acts like an Edge emitting LED. Edge emission is suitable for adaptation to feedback waveguide. 2. Polish the sides of the structure that is radiating. 3. Introduce a reflecting mechanism in order to return radiation to the active region. Drawback: Excessive absorption of radiation in p and n layers of diode. Remedy: Add confinement layers on both sides of active region with different refractive indexes. Radiation will reflect back to active region.

19. LASER diode – Heterostructure The drawback of a homojunction structure is that the threshold current density ( I th) is too high and therefore restricted to operating at very low temperatures. Remedy: Heterostructure semiconductor laser diodes. What must be accomplished? - reduce threshold current to a usable level - improvement of the rate of stimulated emission as well as the efficiency of the optical cavity

20. To improve the performance… … Carrier confinement Confine the injected electrons and holes to a narrow region about the junction. This reduces the amount of current needed to establish the required concentration of electrons for population inversion. Photon confinement Construct a dielectric waveguide around the optical gain region to increase the photon concentration and elevate the probability of stimulated emission. This reduces the number of electrons lost traveling off the cavity axis.

22. Refer to the DH structure…… • AlGaAs has Eg of 2 eV • GaAs has Eg of 1.4 eV • P-GaAs is a thin layer (0.1 – 0.2 um) and is the Active Layer where lasing recombination occurs. • Both p regions are heavily doped • With an adequate forward bias Ec of n-AlGaAs moves above Ec of p-GaAs which develops a large injection of electrons from the CB of n-AlGaAs to the CB of p-GaAs. • These electrons are confined to the CB of the p-GaAs due to the difference in barrier potential of the two materials.

23. Note: 1.Due to the thin p-GaAs layer a minimal amount of current only is required to increase the concentration of injected carriers at a fast rate. This is how threshold current is reduced for the purpose of population inversion and optical gain. 2. A semiconductor with a wider bandgap (AlGaAs) will also have a lower refractive index than GaAs. This difference in refractive index is what establishes an optical dielectric waveguide that ultimately confines photons to the active region.

25. Advantages of stripe geometry: 1. Reduced contact reduces threshold current. 2. Reduced emission area makes light coupling to fiber easier

27. Types based on Oscillations • According to the mode of oscillations lasers are divided into 3 groups • Continuous Lasers: • Emit an continuous light beam with constant power • Require continuous steady-state pumping of the active medium • Pulsed Lasers: • Require pulsed operation of the Pumping system • Pumping achieves Population Inversion periodically for short periods • Pulsed Laser with controlled Losses: • Concentration of energy reaches a maximum so that they give rise • to giant pulses of short duration. • Peak power is in the order of 100 Watts or more • Realized by controlling the losses inside the cavity using • a device Known as the Q switch

28. Principle of Q switching The active medium is excited without feedback by blocking the reflection from one of the end mirrors of the cavity The end mirror is then suddenly allowed to reflect Suddenly applied feedback causes a rapid population inversion of the lasing levels Results in a very high peak power output pulse Duration of the light pulse is in order of 0.1 microseconds

29. Techniques for Q switching • Using a mechanically driven device • E.g., a rotating prism or mirror • Rotate one of the mirrors about an axis perpendicular to the laser • Rotating speed cannot be made very large • Hence Q switching does not take place instantaneously [semiconductor saturable absorber mirror (SESAM) ]

30. (ii) Passive device such as an electro-optical cell: • Light from optical cavity passes through a polarizer and an Electro-optic cell (controlling the phase or polarisation of the laser beam) • When appropriate voltage is applied, the material inside the cell becomes birefringent • By varying the voltage – cell blocks or transmits beam • (iii) Using a cell containing a Dye: • Passive Q switch – cell containing organic dye • Initially light output absorbed by dye, preventing reflection • After a particular intensity is reached, dye is bleached(allows light) • Now reflection from mirror is possible • Results in rapid increase in cavity gain

31. Resonator Modes Since light is a wave, when bouncing between the mirrors of the cavity the light will constructively and destructively interfere with itself. Leads to the formation of standing waves between the mirrors (a radiation pattern or a field distribution) These standing waves form a discrete set of frequencies: longitudinal modes of the cavity These modes are the only frequencies which are self-regenerating and allowed to oscillate by the resonant cavity All other frequencies of light are suppressed by destructive interference

32. Resonator Modes…… • Laser resonators have two distinct types of modes, transverse and longitudinal. • Transverse modes manifest themselves in the cross-sectional profile of the beam, that is, in its intensity pattern. • Longitudinal modes correspond to different resonances along the length of the laser cavity which occur at different frequencies or wavelengths within the gain bandwidth of the laser.

33. Modes… … Transverse modes are classified according to the number of noughts that appear across the beam cross section in two directions. The lowest-order, or fundamental mode, where intensity peaks at the centre, is known as TEM00. [TRANSVERSE ELECTROMAGNETIC WAVE] A single transverse mode laser that oscillates in a single longitudinal mode is oscillating at only a single frequency – “single mode” operation. When more than one longitudinal mode is excited, the laser is said to be in "multi-mode" operation.

34. The mode with a single nought along one axis and no nought in the perpendicular direction is TEM01 or TEM10, depending on orientation. A sampling of these modes, which is produced by stable resonators, is shown below TEM00 TEM10 TEM01

35. Mode Locking in Lasers Mode-locking is a technique in by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12s) or femtoseconds (10−15s). Need for Mode Locking When laser is oscillating with various modes and if modes are uncorrelated The output intensity i.e., The total optic electric field resulting from a multi-mode oscillation fluctuates with time To overcome this fluctuation – MODE LOCKING IS DONE i.e., the phase between the modes is to be fixed

36. Mode locking requires various Longitudinal Modes to be coupled to each other • It can be viewed as a condition in which A pulse of light is bouncing back and forth inside the cavity And every time it hits the mirror, a certain fraction is transmitted as the output pulse • The output of a mode-locked laser will be a series of pulses of extremely short duration Pulses are separated by a duration tr = 2 L / c termed the CAVITY ROUND TRIP TIME

37. multiple oscillating cavity modes laser gain profile losses wq-2 wq-1 wq wq+1 wq+2 wq+3 possible cavity modes Multiple oscillating cavity modes Sum of various modes with same relative phase – MODES LOCKED

38. Methods for producing mode-locking in a laser may be classified as either active or passive. Active methods typically involve using an external signal to induce a modulation of the intra-cavity light. Passive methods do not use an external signal, but rely on placing some element into the laser cavity which causes self-modulation of the light.

39. Active Mode Locking The most common active mode-locking technique places a standing wave acousto-optic modulator into the laser cavity. This device, when placed in a laser cavity and driven with an electrical signal, induces a small, sinusoidally varying frequency shift in the light passing through it. After some round trips, the oscillating intensity consists of a periodic train whose modes are locked and the period of the pulses is T=2L/ c Where L= length of the gain medium c = speed of light in free space

40. Passive Mode Locking • Do not require a signal external to the laser (such as the driving signal of a modulator) to produce pulses. • An intra-cavity element is introduced in the cavity. • This produces a change in the intra-cavity light. • The most common type of device which will do this is a saturable absorber • A saturable absorber is used whose absorption coefficient decreases with Increase in incident light intensity

41. SATURABLE ABSORBER • A saturable absorber is an optical device that exhibits an intensity-dependent transmission. • The device behaves differently depending on the intensity of the light passing through it. • Will selectively absorb low-intensity light, and transmit light which is of sufficiently high intensity. • When placed in a laser cavity, a saturable absorber will attenuate low-intensity constant wave light

42. Contd…. • As the light in the cavity oscillates, this process repeats, leading to the selective amplification of the high-intensity spikes, and the absorption of the low-intensity light. • After many round trips, this leads to a train of pulses and mode-locking of the laser. Mode locked pulse train appear at a frqy of c/2L

43. BEAM PROFILE MODE LOCKED CW