Lab 4 Laser - Institutt for elektronikk og telekommunikasjon - NTNU

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1.2 Laser fundamentals 8 1.2.2 Laser resonators and standing waves The optical feedback and delay functions are served by an optical resonator, such as the one shown in Fig. 1.1a constructed with two mirrors that face each other. The mirrors serve to recirculate, or feed back, the light with efficiency R = R1R2, the product of the reflectivities R1 and R2 of the two mirrors. The delay time t = 2L/c is the time it takes light to complete a round trip between the mirrors. The net gain for a complete round trip is GR and must be equal to unity to sustain steady oscillation. The condition GR = 1 means that the gain compensates for the loss and the oscillator will operate with steady power. (What if GR < 1 or GR > 1?). There is an additional criterion that after each round trip the light wave crests line up with the crests from the previous round trip. This is called resonance and results in a standing wave as shown in Fig. 1.1b. This condition is similar to the resonances associated with standing sound waves in an organ pipe or on a guitar string. The condition on the mirror separation is that one round trip contains an integral number of wavelengths 2L = mλ, where m is an integer. The corresponding resonant frequencies are f m = c/λ = mf fsr , where the free spectral range f fsr = c/2L is the separation between resonance frequencies. Fig. 1.1b has harmonic number m = 4, but a typical visible wavelength laser resonator might have length L = 30 cm operating at wavelength l = 600 nm so that m = 1 million. Figure 1.2a shows resonant fre- Figure 1.1a: Mirror resonator Figure 1.1b: Standing wave in a mirror resonator Figure 1.2: Typical laser output spectrum
1.2 Laser fundamentals 9 quencies of an optical resonator near m = 1 million. For an L = 30 cm resonator, the spacing between successive resonance frequencies is f fsr = c/2L = 500 MHz while the resonance frequencies are near f m = c/l = 500 THz, one million times larger. The spectrum, or collection of different frequencies, emitted by a laser is a combination of the resonant frequencies f m of the many possible standing optical waves in the resonator (fig. 1.2a) with the range of frequencies in the gain spectrum (fig. 1.2b) produced by stimulated emission and population inversion. A representative laser output spectrum is shown in Fig. 1.2c. Modified laser resonators can suppress all but one of the resonant frequencies, permitting more precise control of the frequency, a desirable feature in scientific and engineering applications. The typical He-Ne laser illustrated in Fig. 1.3 consists of a sealed tube containing a mixture of helium and neon gasses and an optical resonator. An electrical discharge in the tube excites the helium atoms, which then collide with the neon atoms, transferring excess energy to pump the neon atoms from level 0 to level 2. The neon atoms develop a population inversion between levels 2 and 1, providing gain at one of several different visible and infrared wavelengths. The light circulating between the two mirrors is amplified by the stimulated emission from the excited neon atoms on each round trip. One mirror, called the back reflector, has high reflectivity(> 99%) while the other mirror, called the output coupler, has lower reflectivity (typically 95 − 98%) and transmits a fraction (2 − 5%) of the light to the outside this is the laser output that we observe. The power inside the laser is many (20 − 50) times higher than the power emitted. Figure 1.3: Construction of HeNe laser tube
Page 1 and 2: From pink light to red LASER Lab ex
Page 3 and 4: Chapter 1 Lab exercise: From pink l
Page 5 and 6: 1.1 Laser safety 5 Class Power Haza
Page 7: 1.2 Laser fundamentals 7 1.2 Laser
Page 11 and 12: 1.3 Lab exercises 11 1.3.5 Polariza
Page 13: Bibliography [Duc04] Stephen Duchar

Lab 4 Laser - Institutt for elektronikk og telekommunikasjon - NTNU

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