Fiber Optics. Baldemar Ibarra-Escamilla. Instituto Nacional de Astrofísica, Óptica y Electrónica

Transcription

1 Fiber Optics Baldemar Ibarra-Escamilla Instituto Nacional de Astrofísica, Óptica y Electrónica 18/04/2012 Workshop on Modern Optics XII 1

2 Outline Introduction Basic principles of fiber optics New applications of fiber optics Laser definition Fiber Optics Laser Research in INAOE Conclusions 18/04/2012 Workshop on Modern Optics XII 2

3 What is Fiber Optics? Fiber Optics (FO) is the science of transmitting data, voice, and images by the passage of light through thin, transparent fibers. In telecommunications, FO technology has virtually replaced copper wire in long-distance telephone lines, and it is used to link computers within local area networks. FO is also the basis of the fiberscopes used in examining internal parts of the body (endoscopy) or inspecting the interiors of manufactured structural products. The basic medium of FO is a hair-thin fiber that is sometimes made of plastic but most often of glass. A typical glass optical fiber has a diameter of 125 μm. This is actually the diameter of the cladding. The core may have a diameter as small as 10 μm. 18/04/2012 Workshop on Modern Optics XII 3

4 The concept of light transmission has existed since the early 1840 s when French inventors Daniel Colladon and Jacques Babinet demonstrated the guiding of light over distance by refraction. 18/04/2012 Workshop on Modern Optics XII 4

5 Technological advances in communications 18/04/2012 Workshop on Modern Optics XII 5

6 Fiber Optics; Optical Amplifiers; WDM Technology; Multiple bands; C band ( nm). L band ( nm) y S band ( nm) (2001); Raman Amplifiers. Solitons. 18/04/2012 Workshop on Modern Optics XII 6

7 Technological advances in fiber optics communications 18/04/2012 Workshop on Modern Optics XII 7

8 Basic concepts of fiber optics Step-index fiber: An abrupt index change at the fiber core. Graded-index fiber: The refractive index decreases gradually inside the fiber core. 18/04/2012 Workshop on Modern Optics XII 8

9 Total internal reflection in step-index fiber Refraction at the fiber-air interface: n 0 sini n1 sin r Total internal reflection in the interface core-cladding if: c n sin 1 n n Numerical aperture (NA): Maximum angle of the incident ray n 2 max 0 sini n1 sin c n1 cos 18/04/2012 Workshop on Modern Optics XII 9 c n 2 1 n 2 2

10 Graded-index fiber Refractive index: The trajectory of a paraxial ray is obtained by: For: 2, n n n cos 0 ' pz psin pz 2 d 1 2 dz n All the rays recover their initial positions and directions at periodic intervals a 18/04/2012 Workshop on Modern Optics XII 10 n 2 ; ; a a dn d

11 It is also useful to introduce a normalized propagation constant b as: b Con The cutoff condition is defined as: n~ n2 n1 n2 0 b 1, V k 2 2 0a n1 n2 V is the normalized frequency or simply the V parameter 18/04/2012 Workshop on Modern Optics XII 11

12 Dispersion in Single-Mode Fibers Origin: Frequency dependence of the mode index ñ(w): b ~ w nw w c b b w w b w w..., where w 0 is the carrier frequency of the optical pulse. Transit time for a fiber of length L: T = L/v g = b 1 L Different frequency components travel at different speeds and arrive at different times at output end (pulse broadening). 18/04/2012 Workshop on Modern Optics XII 12

13 18/04/2012 Workshop on Modern Optics XII 13

14 Double-Clad fiber optics revestimiento The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices. 18/04/2012 Workshop on Modern Optics XII 14

15 18/04/2012 Workshop on Modern Optics XII 15

16 Not circular fibers Neodimium doped fiber with a concentration of 1300 ppm. 18/04/2012 Workshop on Modern Optics XII 16

17 Double-Clad Photonic Crystal Fiber (PCF) For high-power fiber lasers and amplifiers, PCFs can be used, where the pump cladding is surrounded by an air cladding region (air-clad fiber). Due to the very large contrast of refractive index, the pump cladding can have a very high numerical aperture (NA), which significantly lowers the requirements on the pump source with respect to beam quality and brightness. Such PCF designs can also have very large mode areas of the fiber core while guiding only a single mode for diffraction-limited output, and are thus suitable for very high output powers with excellent beam quality. Another advantage is that the pump light is kept away from any polymer coating, thus avoiding possible problems with overheating of a coating. 18/04/2012 Workshop on Modern Optics XII 17

18 - Metallic tube inserted into the artery - Diameter from 1 to 10 mm with a thickness of 100 mm Stencil fabrication for solder printing Microscopic imagen of the stent Polycrystalline silicon cut of a 250 mm thickness using a SM fiber laser of 100 W. 18/04/2012 Workshop on Modern Optics XII 18

19 In this LIDAR system the laser are directed to the see water: infrared light is reflected from the see surfaced and is detected for a receiver, blue and green light penetrate the see surface and is reflected from the see bottom. Schematic diagram of a LIDAR system. A pump laser is directed to the atmosphere and the reflected light from the molecules and the dust is recollected in a mirror system, and the n is recollected for a photomultiplier using an system of a data acquisition. 18/04/2012 Workshop on Modern Optics XII 19

20 Light Amplification by Stimulated Emission of Radiation Stimulated emission leads to a chain reaction and laser emission. If a medium has many excited molecules, one photon can become many. Excited medium This is the essence of the laser. The factor by which an input beam is amplified by a medium is called the gain and is represented by G. 18/04/2012 Workshop on Modern Optics XII 20

21 The Laser A laser is a medium that stores energy, surrounded by two mirrors. A partially reflecting output mirror lets some light out. I 0 I 1 I 3 Laser medium R = 100% with gain, G R < 100% A laser will lase if the beam increases in intensity during a round trip: that is, if I3 I0 Usually, additional losses in intensity occur, such as absorption, scattering, and reflections. In general, the laser will lase if, in a round trip: Gain > Loss This called achieving Threshold. 18/04/2012 Workshop on Modern Optics XII 21 I 2

22 Achieving inversion: Pumping the laser medium Now let I be the intensity of (flash lamp) light used to pump energy into the laser medium: I I 0 I 1 I 3 Laser medium R = 100% R < 100% I 2 Will this intensity be sufficient to achieve inversion, N 2 > N 1? It ll depend on the laser medium s energy level system. 18/04/2012 Workshop on Modern Optics XII 22

23 Achieving Laser Threshold An inversion isn t enough. The laser output and additional losses in intensity due to absorption, scattering, and reflections, occur. I 0 I 1 I 3 Laser medium Gain, G = exp(gl), and R = 100% Absorption, A = exp(-l) R < 100% I 2 The laser will lase if the beam increases in intensity during a round trip, that is, if: Gain > Loss This called achieving Threshold. It means: I 3 > I 0. Here, it means: I I exp( gl) exp( L) R exp( gl) exp( L) I ( g ) L ln(1/ R) 18/04/2012 Workshop on Modern Optics XII 23

24 Laser cavities 18/04/2012 Workshop on Modern Optics XII 24

25 Three-level system of Erbium l p = 980 nm 3 Intermediate state (Pumping state) l p = 1480 nm 2 Excited state l s 1 Ground state (a) (b) (c) (d) (a) Pumping by stimulate absorption of a 980-nm photon followed by nonradiative decay. (b) Pumping by stimulate absorption of a 1480-nm photon. (c) Stimulate emission of a signal photon. (d) Spontaneous emission of a noise photon. 18/04/2012 Workshop on Modern Optics XII 25

26 Power (dbm) Wavelength (nm) Amplified spontaneous emission (ASE) spectrum measured at the output of an optically pumped optical amplifier (ASE + ) with no signal input, showing up a characteristic peak about /04/2012 Workshop on Modern Optics XII 26

27 Axial modes of a laser, with their amplitude envelope Condition for constructive interference: for each round-trip, final phase = initial phase + N2 Modes: N N c nl N T r c 1 nl L m >> l = 1550 nm many modes within the gain bandwidth T r = Free Spectral Range (FSR) N-5 N N+5 18/04/2012 Workshop on Modern Optics XII 27

28 Modelocking 18/04/2012 Workshop on Modern Optics XII 28

29 Modelocking 18/04/2012 Workshop on Modern Optics XII 29

30 Modelocking 18/04/2012 Workshop on Modern Optics XII 30

31 Modelocking Constructive interference between phase locked cavity modes 18/04/2012 Workshop on Modern Optics XII 31

32 A pulse train is obtained when the modes have the identical phases w Nw Laser output = sum of M locked modes: Et A 0 Free-running laser: random phases The optical output intensity exhibits a periodic but irregular temporal behavior N t e j N Identical phases (locked): train of pulse whose period = round-trip time T r sin Mt / Tr I t E t A 2 sin t / T r Pulse width if M The pulse width is inversely related to the spectral bandwidth over which phases of various longitudinal modes can be synchronized. 18/04/2012 Workshop on Modern Optics XII 32

33 Spectral density Pulse shape Gaussian pulse 1.0 Amplitude Intensity Time Gaussian spectrum Amplitude Intensity Frequency For Gaussian pulses ν t = 0.44, where ν and t are measured as FWHM (Full Width at Half Maximum) for intensity 18/04/2012 Workshop on Modern Optics XII 33

34 Active mode locking Active mode-locker are able to mode-lock only with the help of some externally modulate media or device E N jwnt t Ae 1 cosw t A 2 m e A 2 w 1 N wm t jw Nt j w N wm t A e e, j m m m m m FSR 1 T rt c nl If f m = FSR (free spectral range) : For modulation, each mode transfer part of the energy to the neighbors with the same phase After many round-trips, the phases of all modes are similar Constructive interference between modes = A single pulse appear in the cavity = Active Mode Locking 18/04/2012 Workshop on Modern Optics XII 34

35 Harmonically actively mode-locked erbiumdoped fiber ring laser Erbium-doped fiber WDM coupler Pump laser diode Optical isolator Amplitude modulator DC Output coupler Optical filter Optical pulse train rf amplifier rf generator Duración: ps Cadencia: GHz 18/04/2012 Workshop on Modern Optics XII 35

36 Passive mode locking Several different physical mechanics can be used for passive mode-locking: Saturable absorbtion, SA. Self-Phase Modulation, SPM. Nonlinear Polarization Rotation, NLPR. 18/04/2012 Workshop on Modern Optics XII 36

37 Intensity Intensity Pulse shortening by a saturable absorber times through 1.0 absorber Time SEmiconductor Saturable Absorber Mirror SESAM times through the absorber Time 18/04/2012 Workshop on Modern Optics XII 37

38 A saturable absorber using nonlinear polarization rotation Fiber Polarizer Low power. No nonlinear polarization rotation. Low transmission Fiber Polarizer High power. π/2 nonlinear polarization rotation. High transmission R. J. Mears, L. Reekie, S. B. Poole, and D. N. Payne, Neodymium-doped silica single-mode fiber laser, Electron. Lett. 21, 738 (1985). 18/04/2012 Workshop on Modern Optics XII 38

39 Schematic diagram of a mode-locked figure-8 fiber laser I. N. Duling III, All-fiber ring soliton laser mode locked with a nonlinear mirror, Opt. Lett. 16, 539 (1991). 18/04/2012 Workshop on Modern Optics XII 39

40 Transmission Nonlinear Optical Loop Mirror Asymmetrical coupler: 0.5, without attenuator 1 P in 0.5 CW CCW T min 0 T max = 1 P out 1- PC 0 0 Input power P Low dynamic range No losses 18/04/2012 Workshop on Modern Optics XII 40

41 Transmission Symmetrical coupler: CW = 0.5, with attenuator T max 1 P in 0.5 PC T min = Attenuator 0 0 Input power P P out Dynamic range = Low losses It is impossible to have high dynamic range, low losses, and low critical power at the same time. 18/04/2012 Workshop on Modern Optics XII 41

42 NOLM using a symmetrical coupler and QW retarder in the loop 18/04/2012 Workshop on Modern Optics XII 42

43 Transmission Transmission Simulation (a) b / 2 = (b) y b/2 /4 0.6 / / P in l / / P in l An optimal, infinite-contrast switching characteristics (minimal transmission = 0 at low power, maximal transmission = 1 at P in = P p ) is obtained for a = b/2 + kp/2, k integer. NOLM transmission for (a) circular right and (b) linear input polarizations. O. Pottiez, E. A. Kuzin, B. Ibarra-Escamilla, and F, Méndez-Martínez Theoretical investigation of the NOLM with highly twisted fibre and a λ/4 birefringence bias, Optics Communications 254 (2005) ISSN: /04/2012 Workshop on Modern Optics XII 43

44 Experimental setup of the F8L with FBG Amplitude, a.u. Output 1 EDF PC1 C1 QWR2 Coupler 1 SMF ps WDM Pump C2 PC2 PBSQ PC3 Output 2 QWR Time, ps FBG B. Ibarra-Escamilla, O. Pottiez, E. A. Kuzin, J. W. Haus, R. Grajales-Coutiño, and P. Zaca-Moran, Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM, Optics Communications 281 (2008) B. Ibarra-Escamilla, O. Pottiez, E. A. Kuzin, R. Grajales-Coutiño, and J. W. Haus, Experimental investigation of a passively mode-locked fiber laser based on a symmetrical NOLM with highly twisted low-birefringence fiber, Laser Physics 18 (7) (2008) /04/2012 INAOE, 40 años 44

45 F8L with a Sagnac fiber filter Sagnac fiber filter transmission, nm Sagnac fiber filter wavelength shift, nm Output PC1 C1 0.8 EDF 0.6 QWR2 Coupler 1 SMF WDM Pump Coupler 2 PBSQ PC2 Output 2 QWR Wavelength, nm Sagnac filter Hi-bi fiber B. Ibarra-Escamilla et al. Wavelength-tunable picosecond pulses from a passively mode-locked figure-eight Erbium-doped fiber laser with a Sagnac fiber filter, Journal of the European Optical Society 3, (2008). M. Bello-Jiménez, et al. Optimization of the two-stage single-pump Er-doped fiber amplifier with high amplification for low frequency ns-scale pulses, Optical Engineering 46 (12) (2007) /04/2012 INAOE, 40 años Hi-bi fiber segment temperature, 0 C

46 Amplitude, a.u. Double-clad Er-Yb doped high power fiber laser DM 2 Output f 1 DM 3 f 2 f 4 Pump Laser PM-Er:Yb Fiber l/2 P DG f 3 DM Wavelength, nm B. Ibarra-Escamilla, O. Pottiez, R. Zhou, Q. Zhan, P. E. Powers, E. A. Kuzin and J. W. Haus, Wavelength tunable doubleclad Er:Yb doped high power fiber laser, Laser Physics 21 (11) (2011) /04/2012 INAOE, 40 años 46

47 Er doped fiber laser generating switchable radially and azimuthally polarized beams (a) (a) R. Zhou, B. Ibarra-Escamilla, J. W. Haus, P. E. Powers, and Q. Zhan, Fiber laser generating switchable radially and azimuthally polarized beams with 140 mw output power at 1.6 mm wavelength, Applied Physics Letter 95 (19) (2009) /04/2012 INAOE, 40 años 47

48 Conclusions We experimentally demonstrate the operation of self-starting mode-locked figure-eight fiber laser based on a symmetrical NOLM with a highly twisted low-birefringence fiber and a quarter-wave plate in the loop. The mode-locked operation is achieved by nonlinear polarization rotation in the NOLM. We demonstrated a double-clad Er:Yb wavelength-tunable fiber laser. With this configuration we can tune the wavelength from 1535 to 1567 nm using a diffraction grating. We report a fiber laser design that is capable of producing switchable radially and azimuthally polarized beams at 1.6 mm. 18/04/2012 Workshop on Modern Optics XII 48

49 Colaboradores E. Kuzin, A. González-Garcia, F. M. Maya-Ordoñez, I. Alvarez-Tamayo y M. Durán-Sánchez INAOE R. Rojas-Laguna y J. M. Estudillo-Ayala Universidad de Guanajuato O. Pottiez CIO J. Haus, and P. Powers University of Dayton M. V. Andres Universidad de Valencia 18/04/2012 Workshop on Modern Optics XII 49