A Low Cost Scanning Fabry Perot Interferometer for Student Laboratory

Transcription

1 A Low Cost Scanning Faby Peot Intefeomete fo Student Laboatoy K.T.Satyajit, Suesh Doavai*, T.E.Kanakavalli **, Shaath Ananthamuthy Depatment of Physics, Bangaloe Univesity, Jnanabhaati Campus, Bangaloe *Indian Institute of Astophysics, Koamangala, Bangaloe ** Depatment of Physics, M.S.Ramaiah College, Bangaloe We have built a low-cost Scanning Faby Peot Intefeomete fo use in student laboatoy. Ou setup consists of two highly eflecting concave mios facing each othe and mounted on a home built chassis. One of the mios is attached to a piezo buzze extacted fom a motocycle hon and is diven with a tiangula wave fom a signal geneato, o though a 50 Hz AC output fom a vaiac. Light fom a lase souce to be diagnosed is made to ente the Faby Peot cavity. The beam that undegoes multiple eflections emeges fom a hole though the mio-pzt combination and is made incident on a fast photodiode. A simple amplifie cicuit optimizes the signal monitoed on an oscilloscope. The Fee Spectal Range (FSR) of the device is 5 GHz, and much lage than the FSR of the lase unde test. Using this we wee able to clealy esolve the longitudinal modes of a He-Ne 3mW lase. The FSR of the lase is measued to be 838 MHz. The Faby-Peot setup alone costs less than Rs to build. The design is simple and can be built as student pojects, and is an ideal tool fo lase diagnostics. It can be used to make accuate measuements without having to esot to commecially available intefeometes that ae expensive. 1.1 Basics of Faby Peot Cavity (Etalon) 1.INTRODUCTION The pinciple behind a Faby Peot Intefeomete (FPI) is explained in many textbooks and we will biefly summaize the elevant pats hee 1,2,3,4. FPI elies on multiple beam intefeomety. Because of multiple beam intefeence, a shap contast between dak and bight finges is seen and this esults in excellent wavelength esolution. In the etalon (cavity) multiple eflections between two closely spaced patially silveed sufaces occu. Pat of the light is tansmitted each time the light ay eaches the second suface, esulting in multiple offset beams, which can intefee with each othe. The lage numbe of intefeing ays poduces an intefeomete with extemely high esolution 3. A Faby-Peot Intefeomete, has a esolvance given by, π m = (1) Whee m = ode of intefeence 1 Which means the wavelength sepaation of two spectal lines is ( 1 ) = (2) mπ fo small angles, = eflectance of etalon sufaces. Its fee spectal ange (FSR), the change in wavelength necessay to shift the finge system by one finge, can also chaacteize the intefeomete:

2 2 = (3) Whee d is the sepaation between the mios. Standing waves will be fomed in the cavity when constuctive intefeence between the incident and eflected waves occus. Only those fequencies ae allowed, such that the cavity length is an integal numbe of half wavelengths. If the cavity length is d = n Whee n is an intege nc And ν = The fequency diffeence between adjacent longitudinal modes is C ν q+ 1 ν q = (4) Fo say, q=1, this would be just the mode sepaation in tems of fequency, and when expessed in tems of wavelength, is the same as equation (2), which is the FSR. This shows that the length d is the only detemining facto fo the FSR. Anothe measue of the pefomance of an intefeomete o lase cavity is the "finesse". This dimensionless quantity is the atio of the FSR to the esolution. (Finesse = FSR/Resolvance). Using equation (2) Finesse π = (5) With a eflectivity of 99 pecent fo both mios, the finesse will be oughly 300. If the FSR is 10 GHz, the esolution will be appoximately 33 MHz. 1.2 Scanning Faby Peot Intefeomete The FPI can be used as a scanning device in the following way: The axial position of one of the mios can be vaied vey slightly by a linea Piezo Tansduce (PZT). Diving the PZT with a tiangle wavefom and watching the esponse of the photodiode on an oscilloscope can display the longitudinal modes of the lase unde test (LUT) displayed in eal time. In essence, the comb esponse of the SFPI is used as a tunable filte to analyse the fine detail of the optical spectum of the LUT. As long as the FSR of the FPI is lage (i.e., smalle cavity length) than that of the LUT, the mode display will be unambiguous. Commecially available scanning FPIs ae pohibitively expensive and may not be affodable by most of the univesity gaduate laboatoies. Students can build this low cost scanning Faby Peot Intefeomete (SFPI) as pat of a poject. The execise can help the student gain a deepe undestanding of the subtleties behind intefeometic measuements and the wokings of lase cavities. 1.3 Uses of FPI in Classooms: 1.Illustations of axial modes of a lase: The comb esponse of the SFPI can be used as a tunable filte to analyse the lase gain pofile of the LUT. The condition fo unambiguous esolution of the longitudinal modes of lase is that the FSR of the Faby-Peot cavity be lage than the FSR of the LUT. Fom pevious discussion it evident that the FSR is lage if the length of the cavity is smalle. A cavity length of 3 cm (FSR=5 GHz) is found to easily esolve the longitudinal modes of a lase of FSR of about 750 MHz. 2. Illustations of shapes of the tansvese modes geneated in the SFPI cavity: The tansvese modes aising within the extenal cavity can be displayed by pojecting on a sceen (in place of the photodiode). The TEM modes can be clealy visualized in this manne.

3 2.Appaatus and Expeiment 2.1 The Setup The heat of a SFPI consists of two highly eflecting mios that ae held paallel, with the eflecting sufaces facing each othe. This foms a Faby Peot cavity. The lase whose output is to be analyzed is made to ente this cavity along the axial diection. Figue (1) shows a schematic of the setup. Figue 1: Schematic of the setup. The mios wee extacted fom an old and non-woking He Ne lase cavity. They have eflectivities of about 95% One of the mios (the font mio m1) is kept fixed and the othe (ea mio m2) is fitted on a piezo electic tansduce. Diving the PZT with an oscillating voltage modulates the ea mio. A photodiode is placed behind the ea mio in the axial diection fo detection. The whole setup is mounted on a home built steel chassis. Two plates ae fixed facing each othe vetically on the base. The font mio is held using a lens holde, which is theaded onto a steel disc. The PZT extacted fom a motocycle hon compises of a thin laye of the Piezo mateial, stuck on one side of a cicula bass plate. The ea mio is glued to the fee suface of the bass disc. The PZT with a space in between is held fitted to anothe steel disc. Both steel discs ae fitted onto the plates by thee adjustable scews. Figue (2) shows a magnified view of the PZT mount.

4 Figue 2: Diagam showing the PZT and Mount This PZT- mio unit is made to est on a steel disc, such that the PZT sits into a cicula step cut in the steel disc. A space is povided to electically isolate the PZT and the steel disc. The bass plate holding the PZT is clamped onto the disc at the peiphey by a steel ing, with the help of scews. Voltage is applied between two faces of the PZT (Bass disc foms one teminal and a wie soldeed on the PZT foms the othe teminal). This makes the bass disc to flex, causing the mio to move axially.a hole dilled on the PZT and the steel disc, allows light to exit though the PZT-mio combination, befoe being incident on the photodiode o the sceen. 2.2 Alignment of SFPI: A tanslucent sceen is placed behind the SFPI. Fo peliminay alignment, the font mio is emoved and light fom the lase souce is made to fall on the ea mio. The scews ae adjusted till the lase beam eflects back close to the souce. The eflected beam is steeed off axis slightly in ode to avoid undesiable finge pattens appeaing because of the lase apetue and othe eflections. This ensues nomal incidence on the ea mio. Now the font mio is epositioned. The thee adjustable scews on the both the mios ae moved synchonously to walk the beam. The meging of the two spots on the sceen indicated good alignment, and it esults in the appeaance of finges of cicula symmety. Tansvese modes fomed because of the SFPI cavity, ae seen on the sceen. By applying a DC and vaying it slowly, we can move between the vaious tansvese modes. The DC is adjusted such that the fundamental Gaussian mode is seen. This makes futhe alignment easie once the sceen is eplaced by the photodiode. The output fom the SFPI is made incident on the photodiode connected to a low noise opeational amplifie cicuit. The amplified voltage is fed into one channel of an oscilloscope, while the othe channel monitos the input voltage to the PZT. The PZT is diven with a Vaiac in seies with a DC fom a powe supply. (DC = 10 volts, AC = 20 volts.) A Hamamatsu S K photodiode was used to detect the light. We summaize below, the data including input paametes of the SFPI: Mio: fom discaded 5 mw He-Ne lase tubes (ECIL) Cavity type: Spheical Mio eflectivity (): 95 pecent.

5 Mio adius of cuvatue (RoC): 1m. Mio mounts: Thee point adjustable. Distance between mios (d): Appoximately 3 cm (FSR of 5 GHz). Tansvese mode filte (of the SFPI cavity): 1mm adius pinhole apetue. Photo detecto: Hamamatsu S K ICs used in Op-amp cicuits: OP07 and OP27. PZT: Motocycle hon (manufactue unknown). Lase Unde Test: Melles Giot (05-LHR-121) Diving souce: Vaiac (vaiable AC voltage, at 50 HZ) in seies with a constant DC (Aplabs LD3202) Display: Tektonix TDS220 oscilloscope. 3.Results and Discussion The mode stuctue is obtained by scanning and the esults ae shown in figue (3). 3.1 Longitudinal Modes Figue 3: Oscilloscope tace evealing the mode stuctue of the LUT. While the tall peaks ae that of the longitudinal modes the shot peaks ae due to the tansvese modes eithe fom the lase o the extenal cavity. The oscilloscope tace shows two scans of the lase spectum. The distance between simila peaks (about 1.730ms on the time axis) is the FSR of the SFPI. This coesponds to about 5.0 GHz (=c/2l, whee L=3cm is the distance between the SFPI mios). Theefoe the distance between the longitudinal modes of the LUT (about 0.290ms on the time axis) is 838 MHz (=0.290* 5 GHz / 1.730) and is the FSR of the LUT. This coesponds to a lase cavity length of 18cm. Futhe we obseve that intoducing a polaize between the LUT and SFPI modulates the heights of the peaks altenating, theeby confiming the mutually othogonal linea polaization of the longitudinal modes of the lase. This featue is also stated in the lase manual. We obseve that thee ae only two such modes of the given lase, and this is in ageement with the allowed numbe given the gain pofile of the lase 5. The (time scale) distance between the epeated peak patten on the scope depends on the speed of scanning. Howeve, the atio of sepaation of consecutive tall peaks to the distance between the adjacent peaks of a given ode is the same povided the esponse of the PZT is linea.

6 The PZT is ideally diven with a tiangula wave fom a function geneato, since the tiangula wavefom is linea. But the PZT thow was not sufficient in ou case (not enough to cove moe than one ode of the lase spectum), as the function geneato we used deliveed maximum amplitude of only 10 volts. The PZT we used had a single laye unlike the commecially available ones, which ae stacked, and have a lage thow. We wee able to solve this poblem by using a vaiac at 50 Hz, which can delive upto 240 volts. The non-lineaity of the sine wave output was initially an issue. Howeve at inceased input amplitudes the potion of the cuve at which the modes ae seen is appoximately linea 4. With peak-to-peak voltages upto 40 volts we could clealy esolve the longitudinal modes, by scanning two odes of the spectum. The longitudinal mode spacing on the display educed with incease in the amplitude as othe odes wee accommodated. We used a constant DC of aound 10 volts in seies with the vaaic, which impoved the thow. This is because the PZT was obseved to have bette thow in a flexed state. The mode stuctue seemed to stabilize within an hou o two afte the lase was switched on. Duing the fist hou no symmetical mode stuctue was obseved but this appeaed late. The appeaance of such symmetical mode stuctues can be a citeion fo checking the lase stabilization, in expeiments. 3.2 Tansvese mode contol: The tansvese modes that can be visualized on a sceen oiginate fom the SFPI cavity. These ae undesiable while obseving the longitudinal mode stuctue of the lase. An apetue inside the SFPI cavity helps educe the tansvese modes. We used an iis inside the SFPI cavity initially and the tansvese modes almost disappeaed when the apetue size was educed to a adius of 1mm. Howeve futhe educing the apetue esulted in a consideable attenuation of the longitudinal modes, indicating the esidual tansvese modes could be oiginating in the LUT. An apetue with a 1mm adius was late fixed inside the cavity. This is detachable when cavity alignment is needed o when the mode stuctue of the SFPI cavity modes needs to be visualized. 3.3 Visualization of tansvese modes The visualization of the tansvese modes was effective and we could clealy make out the TEM 00, TEM 10, TEM3 30, TEM 11, TEM 21, TEM 43 modes. Futhe, when the cavity was well aligned we could see ciculaly symmetic modes. Afte the initial alignment futhe fine-tuning is possible by applying DC alone and obseving the tansvese modes. Alignment is caied out until the modes attain cicula symmety. At this point the contast in the intensity distibution inceases which esults in an incease in the amplitude of the longitudinal mode peak. 4. Conclusion In conclusion, we have built a low-cost scanning Faby-Peot Intefeomete device that can be used to esolve the longitudinal mode stuctue of a He-Ne lase. The device is simple to build and can be pat of a shot-tem student poject in a couse in photonics. Using this, we wee able to esolve the two longitudinal modes of a 3mW Melles Giot He-Ne lase and detemined the FSR of the lase to be 838MHz. We have also used the SFPI to visualize the tansvese modes geneated within its cavity. Acknowledgement One of the authos (S.A) acknowledges a eseach gant fom the Boad of Reseach in Nuclea Sciences (DAE). Refeences 1. A.C.Mellissinos and J.Napolitano, Expeiments in Moden Physics, Academic Pess, A.Ghatak and K.Thyagaajan, Optical Electonics, Cambidge Univesity Pess, William L.Wolfe, Intoduction to Imaging Spectometes, SPIE, Washington, A simple Scanning Spheical Mio Faby-Peot Intefeomete, L. Pandit and D.D.Bhavalka, Indian.J.Pue Appl.Phy, 10, 487 (1972). 5. William T.Silfvast, Lase Fundamentals, Cambidge Univesity Pess, New Delhi, 1998.