ECE 455 Lecture 1 What is a Introduction laser? Three Atomic ECE 455 Optical Electronics Processes Three Components of Every Laser Tom Galvin Laser Light Characteristics Gary Eden Units of Laser Spectroscopy If changes need to be made to these notes, History please contact Kavita Desai: [email protected] Laser Applications ECE Illinois Contact Information ECE 455 Lecture 1 What is a Professor: laser? Gary Eden Three Atomic Processes ECEB 2042 Three [email protected] Components of Every Laser Laser Light Characteristics Teaching Assistant: Units of Laser Spectroscopy Kavita Desai History [email protected] Laser Applications LASER ECE 455 Lecture 1 What is a laser? Light Three Atomic Processes Three Components Amplification by of Every Laser Laser Light Characteristics Stimulated Units of Laser Spectroscopy History Emission of Laser Applications Radiation Atomic Energy Levels ECE 455 Lecture 1 What is a laser? Three Atomic Individual atoms Processes have discrete energy Energy Three Components levels of Every Laser Solids will have Laser Light Characteristics energy bands Units of Laser Spectroscopy Light can cause History atoms to transition Laser between energy Applications levels Absorption ECE 455 Lecture 1 What is a laser? Three Atomic Processes 2 Three Components of Every Laser When a two-level h = − Laser Light system absorbs a Characteristics photon, it is left in Units of Laser Spectroscopy an excited state History 1 Laser Applications Spontaneous Emission ECE 455 Lecture 1 Two-level system spontaneously What is a decays from a higher laser? Three Atomic energy level to a Processes lower energy level 2 Three Components and emits a photon. of Every Laser The decay rate is Laser Light h = − Characteristics characterized by the Units of Laser spontaneous Spectroscopy emission lifetime τ History 1 Laser Emission is uniform Applications in all 4π steradians Spontaneous emission can only be explained by QED Stimulated Emission ECE 455 Lecture 1 Photon interacts with two-level What is a laser? system in an excited Three Atomic state Processes 2 Three The system goes to Components of Every Laser a lower energy level Laser Light and a photon with Characteristics the corresponding Units of Laser Spectroscopy energy is emitted History Emitted photon has 1 Laser Applications the same direction, polarization, phase and energy as the incident photon Absorption and Stimulated Emission ECE 455 Lecture 1 What is a laser? Three Atomic Processes Absorption and stimulated emission are inverse processes. Three Components The probability for a photon to be absorbed by an atom in of Every Laser its ground state is the same as the probability for the same Laser Light Characteristics photon to induce stimulated emission from an atom in its Units of Laser upper state. Spectroscopy History Laser Applications To Have Optical Gain... ECE 455 In order to have gain, there must be more atoms in the upper Lecture 1 energy state than in the lower energy state. This situation is What is a known as population inversion. laser? Three Atomic Processes N2 N2 Three Components of Every Laser Laser Light Characteristics Units of Laser Spectroscopy History N1 N1 Laser Applications Figure: Net loss will occur Figure: Net gain will occur What would happen if the populations of the upper and lower states were equal? Three Components of Every Oscillator ECE 455 Lecture 1 Power Supply What is a laser? Three Atomic Processes Three Components of Every Laser Amplifier Output Laser Light Characteristics Units of Laser Spectroscopy History Laser Applications Feedback Three Components of Every Laser ECE 455 Lecture 1 Power What is a (PUMP) laser? Three Atomic Processes M M Three Components of Every Laser Optical frequency Laser Light Characteristics amplifier (GAIN MEDIUM) Units of Laser Spectroscopy History L Laser Applications Feedback (Cavity) Characteristics of Gain Medium ECE 455 Lecture 1 What is a Provides optical gain laser? Stores energy from the pump Three Atomic Processes Energy levels in atomic media are set by nature. It is up to Three Components the laser designer to figure out how to use them. of Every Laser In semiconductor lasers, designers tweak semiconductor Laser Light Characteristics dimensions and composition to adjust the energy levels. Units of Laser Examples of gain media Spectroscopy History Titanium atoms doped into sapphire crystal (Ti:Al2O3) Laser Neon atoms in a plasma Applications Organic dye molecules in a liquid solution Quantum dots in semiconductor lasers Gain Medium ECE 455 The gain medium by itself is not very exciting. Lecture 1 What is a laser? Three Atomic Processes Three Components of Every Laser Laser Light Characteristics Units of Laser Spectroscopy History Laser Applications Ground State Atom Excited Atom Photon Characteristics of a Pump Source ECE 455 Lecture 1 What is a Source of energy for the laser laser? Three Atomic Moves the lasing species from its ground state to an Processes excited state, though not necessarily the upper level of the Three Components laser transition of Every Laser Examples of pumps Laser Light Characteristics Flashlamp Units of Laser Another laser Spectroscopy Direct electrical pumping (as in a diode laser) History Collisional pumping (electron collisions in a plasma) Laser Chemical reactions (reactions do not necessarily leave the Applications product molecule in its ground state) Gain Medium and Pump ECE 455 Lecture 1 What is a laser? Three Atomic Processes Three Components of Every Laser Laser Light Characteristics Units of Laser Spectroscopy History Laser Applications Ground State Atom Excited Atom Photon Gain Medium and Pump ECE 455 Lecture 1 Photons only pass through a fraction of the gain medium before escaping. What is a The average distance a photon travels through a gain laser? Three Atomic medium before causing stimulated emission is: Processes Three 1 Components Lopt = (1) of Every Laser σse (N2 − N1) Laser Light Characteristics where σse is a constant and N1 and N2 are the population Units of Laser number of the upper and lower states. Spectroscopy History If Lgain Lopt , there will be negligible stimulated Laser emission. Applications Excited atoms relax via spontaneous emission Light is emitted equally in all directions If a signal is introduced into one end of this device, the gain medium can be used as an optical amplifier. Characteristics of Optical Cavities ECE 455 Lecture 1 Provides feedback of the optical field What is a laser? Increases the effective length of the gain medium Three Atomic Processes Determines the spatial characteristics of the laser beam Three Resonant modes of the cavity determine the exact Components of Every Laser frequency of oscillation Laser Light Characteristics Output power strongly influenced by loss of cavity. Units of Laser Examples of cavities Spectroscopy A pair of high-reflecting mirrors may be placed around a History gain medium or form its boundaries Laser Applications Edge-emitting semiconductor lasers receive feedback from reflections at the boundaries of the semiconductor Cavities are not required for all lasers \Mirrorless Lasers") Gain Medium, Pump and Cavity ECE 455 Lecture 1 What is a laser? Pump Three Atomic Processes M M Three Components of Every Laser Laser Light Characteristics Units of Laser Spectroscopy History Gain Medium Laser Applications Ground State Atom Excited Atom Photon Gain Medium, Pump and Cavity ECE 455 Lecture 1 Mirrors force photons to pass through the gain medium several times, increasing the effective length of the gain What is a laser? medium along the axis of the cavity. Three Atomic If Lgain;eff ≥ Lopt , then on average, each photon causes Processes more than one stimulated emission before it leaves the Three Components gain medium and the number of photons grows. A laser is of Every Laser born! Laser Light Characteristics Note that the effective gain length is only increased on the Units of Laser Spectroscopy cavity's axis History Therefore strong stimulated emission is only observed Laser Applications along the axis of the cavity As atoms move to lower states via stimulated emission, spontaneous emission is suppressed (though not completely). Gain Medium, Pump and Cavity ECE 455 Lecture 1 What is a laser? Photons passing through an inverted gain medium Three Atomic experience positive feedback: Processes Each stimulated emission increases the number of photons Three Components More photons cause more stimulated emission of Every Laser In a laser, the positive feedback is balanced by two factors: Laser Light Characteristics Saturation of the gain medium { increased stimulated Units of Laser emission decreases the number of atoms in their upper Spectroscopy state History Loss of photons from the cavity, both from the laser's Laser output as well as undesirable absorption and scattering Applications Gain Medium, Pump and Cavity ECE 455 Lecture 1 The cavity provides the gain medium with a `memory' Without it, all photons emitted at time t = 0 escape by What is a laser? time Lgain Three Atomic t = (2) Processes c Three Components With a cavity, the average photon escapes the cavity in of Every Laser time Laser Light Lgain;eff Characteristics t = (3) c Units of Laser Spectroscopy where Lgain;eff is the effective length of the laser History The cavity \traps" light Laser Applications But before each photon leaves, it creates other photons with the same direction, polarization, phase and energy via stimulated emission. The net result is to stabilize the output of the laser How is Laser Light Different From Ordinary Light? ECE 455 Lecture 1 What is a Speckle Demo laser? 1 Shine laser pointer on Three Atomic Laser light is: Processes screen Three Intense 2 Components Observe pattern of spots of Every Laser Quasi-monochromatic 3 Place finger ∼30 cm in Laser Light Characteristics Temporally coherent front of face and focus eye Units of Laser Spatially coherent on it Spectroscopy 4 History Single or few mode Speckle pattern in Laser background does not go Applications out of focus What is Coherence? ECE 455 Roughly speaking..