Class Note for OPTI 511L with Professor Jones at UA
Class Note for OPTI 511L with Professor Jones at UA
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Date Created: 02/06/15
R J Jones Optical Sciences OPTI 511L Fall 2008 Active Modelocking of a HeliumNeon Laser The generation of short optical pulses is important for a Wide variety of applications from time resolved measurements utilizing the short pulse durations to nonlinear optics utilizing the high peak intensity of the pulses In this experiment we explore the ideas behind active laser modelocking We Will implement a technique to modelock a long caVity Helium Neon laser by injecting frequency shifted light from each longitudinal mode of the laser into an adjacent mode Frequency shifting is accomplished With an acousto optic modulator AOM we explore the operation of the AOM in the first part of the lab Part I Operation of the AcoustoOptic Modulator Aligning for maximum de ection efficiency Properties of de ected light 55 Part II Modelocking the laser AOM alignment Electrical Spectrum Analyzer ESA Time domain measurements Optimization COP Modelocking F 2008 1 Part I Operation of the AcoustoOptic Modulator Simple analysis of Acousto Optic De ection An AOM consists of a piezo electiic transducer PZT bonded to an optical medium glass or crystalline Applying a radio frequency electrical drive to the PZT launches a high frequency traveling sound wave in the medium The pressure modulation in the sound wave is accompanied by a modulation in the index of refraction of the material this index grating moves at the speed of the sound wave and can Bragg re ect an incident laser beam For a sound wave frequency at frequency a we have ksmslvs27rls 0r 2521515 For an average index of refraction n m 24 and optical wavelength A 632nm HeNe the Bragg scattering condition is mi2 tssint9 3 t9m A n 2111 sound PZT absorber RF in gt gt sound wave i 6 input beam at Bragg angle Modelocking F 2008 2 The incident and diffracted optical waves and the sound wave must fulfill the following conditions k max I momentum conservation EPW S I i a energy conservation frequency of the sound wave Because a ltlt away we have I kiln k i e the length of the optical wavevector remains constant If we assume e A M and incident beam is aligned to fulfill the Bragg condition at frequency ms then there will be a wavevector mismatch at the new frequency a Ams Frequency tuning of the de ected optical wave can be accomplilth 37 changing the De ection occurs in the direction where the mismatch is minimized A6 Awslkv though the de ection efficiency suffers as An is increased The angle of incidence can be tuned to the new Bragg condition and the original de ection efficiency recovered Because of impedance matching conditions the PZT drive usually has limited bandwidth typically in the range 10 20 MHZ A Aligning for maximum de ection efficiency 1 Connect the AOM to the RF source function generator RF amplifier and select a drive frequency around 60 MHZ and amplitude of 8 dBm fed into the RF amplifier Direct the beam from the HeNe laser through the AOM and observe the pattern of de ected spots What happens as you change the angle of incidence 2 Align the AOM in one of the orientations that fulfills the Bragg condition for the 1St order beam Then change the RF drive frequency What happens to the power Can you recover efficient de ection by rotating the AOM Why What percentage of the total power can you obtain in the 1St diffracted order Plot the maximum diffraction efficiency of light in the 1St order as a function of RF frequency 5 8 points will do What happens to the beam position of the diffracted beams as you change the drive frequency Are any properties of the 03911 order beam affected Modelocking F 2008 3 3 Find a good way to make a careful ie precise measurement of the angle of incidence theta Assuming an index of refraction at the laser wavelength n24 and knowing the RF frequency and the wavelength of the HeNe determine the wavelength of the sound wave in this modulator What would happen to the efficiency of the AOM if you focus the beam down to less than this wavelength B Properties of de ected light 3 Align both the 1st order diffracted beam and 03911 order beam from the AOM into the 2 GHZ FPI using a lens and observe the HeNe spectrum Describe what you see Can you now use the AOM to calibrate the spectrum What is the gain bandwidth and longitudinal mode spacing of the long cavity HeNe laser Part 11 Active Modelocking with the AOM In an inhomogeneously broadened laser such as the HeNe laser oscillation in various cavity modes occurs independently When the spectrum is measured with an FPI one sees lasing in many longitudinal modes each uctuating independently in frequency and amplitude due to mirror vibrations mode pulling and other effects These independent laser oscillations can become locked or coordinated if means are found to impress some common physical restriction on all the modes simultaneously One way is to inject light from each mode into the others such that a common phase relationship between longitudinal modes is obtained In order to obtain simple analytical results the oscillation bandwidth is assumed to be spanned by N longitudinal modes each with equal amplitude A straightforward derivation then shows that i Pulses are produced with a period of 2Lc ii The peak power of the pulses is N times the average power iii Pulse widths will be bandwidth limited equal to the inverse oscillation bandwidth More realistic situations with e g unequal mode amplitudes are easily dealt with by numerical summation of the appropriate Fourier series as you will demonstrate in a pre lab assignment later For a simple estimate taking i iii into account please answer the following Q If there are 40 modes and the oscillation bandwidth is 15 GHZ what would be the shortest pulse width possible If the single wavelength power is 1 watt what would be the peak pulse power Modelocking F 2008 4 A Alignment 1 Set up the AOM modulator as shown on the following page To assure high frequency stability use the frequency synthesizer to drive the AOM In positioning mirror M3 it is important that its distance from the output mirror M2 be the same as the laser length L1 Why think of what is happening in the time domain In positioning M3 allowance should be made for the index of refraction of the AOM Estimate this correction assuming n24 for the AOM 2 Note that when the AOM is double passed it shifts the frequency of the light by 2mg With a laser caVity mode spacing cZL1 what is the appropriate AOM drive frequency Exgerimental setup for modelocking Modelocking F 2008 5 r L lt L1 2 M3 AOM a Laser tube M1 M RF drive fast photo diode detector FPI fast ESA scope Beat frequency Time domain Optical frequency pulsed output B Electric Spectrum Analyzer ESA In order to modelock effectively it is important to find and maintain a modulation frequency equal to the average longitudinal mode spacing of the laser cavity An excellent indicator for the best frequency is the collection of nearly identical beat frequencies between the laser modes see figure Unlocked Locked Modelocking F 2008 6 The radio frequency spectrum analyzer displays the power per unit bandwidth determined by the Resolution Bandwidth RBW as a function of frequency The power can be displayed in dBm log scale or Watts linear scale The range of a typical ESA is from near DC up to several GHz Therefore this instrument can be used to see the beating between optical frequencies that occurs at RF frequencies but not the optical frequencies themselves Use the ESA to observe the laser beats at c 2L1 directly from the laser without optical feedback What is the approximate bandwidth of the RF beat spectrum What accounts for this bandwidth Apply optical feedback and slowly tune the modulation frequency of the AOM When the laser locks you should see the background distribution of different beats coalesce into a single frequency see figure This indicates that the feedback from the AOM into the laser cavity has injection locked the modes such that all the laser frequencies are equally spaced by c2L and in phase with respect to each other D Optimization 11 After everything is working satisfactorily you should optimize the system to obtain the strongest modelocking The parameters which affect this most strongly are i Positioning of mirror M3 ii Alignment of the retro re ected beam into the laser ii Tuning of the AOM frequency Monitor the longitudinal modes as seen on the scanning FPI and describe the difference in their behavior when the feedback from the AOM is optimized vs blocked When the longitudinal modes of the laser are locked together you should be able to measure directly these short pulses on the oscilloscope What time duration do you measure How does this compare to their theoretical duration What determines the actual pulse width you are measuring Be sure to measurerecord at least the following 0 Over what RF bandwidth ie detuning AOM frequency can you maintain good modelocking How does this compare to the cold cavity linewidth and the Modelocking F 2008 7 estimated locking range discussed in class for the case of single frequency injection locking assume an output coupler transmission of 1 0 Over what approximate range of external cavity lengths can you still obtain good modelocking What physically accounts for this Record sketch and description the RF spectrum with vs without optical feedback Record the optical spectrum with vs without optical feedback Approximate pulse width compared measured vs theoretical Compare the average cw power with the peak power you ll need a fast DC coupled photodiode Is this what you expected If time permits try again with an external cavity length of L2 You should be able to generate a pulse train with 2 pulses in the cavity twice the repetition rate Would other external cavity lengths will work Modelocking F 2008 8
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