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Ti : T m : L i N b O 3 Wave g u i d e A m p l i f i e r s A n d L a s e r s

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Ti : T m : L i N b O 3 Wave g u i d e A m p l i f i e r s A n d L a s e r s Thesis Submitted to the Department of Physics, Faculty of Science University of Paderborn, Germany for the degree Doktor der
Ti : T m : L i N b O 3 Wave g u i d e A m p l i f i e r s A n d L a s e r s Thesis Submitted to the Department of Physics, Faculty of Science University of Paderborn, Germany for the degree Doktor der Naturwissenschaften (Dr. rer. nat.) by mathew george reviewers: Prof. Dr. Wolfgang Sohler Prof. Dr. Donat As date of submission: date of examination: A B S T R A C T The fabrication of Ti:Tm:LiNbO 3 waveguides by diffusion doping is briefly described. The waveguides thus fabricated have been characterized by determining the propagation losses, near field intensity profile, fluorescence spectrum, absorption spectrum and Tm depth profile by secondary neutral mass spectroscopy. Emission cross sections of the gain medium Ti:Tm:LiNbO 3 have been determined by McCumber theory from the experimentally determined absorption cross sections. A sensitive experimental setup was developed to perform all spectral measurements. For the first time an in-band pumped (λ p = 165 nm)ti:tm:linbo 3 waveguide amplifier capable of broadband optical amplification in the spectral range 175 nm λ s 19 nm is demonstrated. Experimental results of small signal gain measurements were found to be in good agreement with the modeling results. Modeling results show (wavelength dependent) gain of up to 3 db in a 1 cm long single pass pumped waveguide. A double pass pumping scheme can improve the gain achievable significantly. Fabry- Pérot type lasers (λ s = 189 nm and 185 nm) have been realized by depositing specially designed mirrors at the waveguide amplifier endfaces. Laser threshold (189 nm) is at 4 mw coupled pump power; the slope efficiency is 13.3%. The slope efficiency (laser threshold) is larger (smaller) by more than an order of magnitude than those reported so far for Tm:LiNbO 3 waveguide lasers. Modeling results show that the slope efficiency can be improved to 34% by redesigning one mirror. The fabrication and characterization of Ti:Tm:LiNbO 3 waveguide for quantum memory applications is included as an appendix. Z U S A M M E N FA S S U N G Die Herstellung von Ti:Tm:LiNbO 3 Wellenleitern durch Diffusionsdotierung wird kurz beschrieben. Die so hergestellten Wellenleiter wurden sehr genau mit verschiedenen Methoden experimentell untersucht, um Ausbreitungsverluste, Nahfeldverteilungen, Fluoreszenzspektren, Absorptionsspektren und Tm-Tiefenprofile zu bestimmen. Die Emissionswirkungsquerschnitte des Verstärkungsmediums Ti:Tm:LiNbO 3 wurden mit Hilfe der McCumber Theorie aus den experimentell ermittelten Absorptionsquerschnitten berechnet. Ein experimenteller Aufbau zur Durchführung äußerst empfindlicher spektraler Messungen wurde entwickelt. Zum ersten Mal gelang es, breitbandige Verstärkung mit in-band -gepumpten (λ p = 165 nm) Ti:Tm:Li:NbO3 Wellenleitern im Wellenlängenbereich iii 175 nm λ s 19 nm zu erhalten. Die experimentell bestimmte Kleinsignalverstärkung dieser Wellenleiterverstärker ist in guter Übereinstimmung mit Modellierungsergebnissen. Weitere Rechenergebnisse sagen eine (wellenlängenabhängige) Verstärkung von bis zu 3 db in einem 1 cm langen Wellenleiter voraus. Mit einer double-pass Pumpanordnung kann die optische Verstärkung weiter deutlich verbessert werden. Durch Aufdampfen speziell entwickelter Spiegelschichten auf die Wellenleiterendflächen wurden Laser vom Fabry-Pérot Typ hergestellt mit Emissionswellenlängen von 189 nm (Resonator hoher Güte) und 185 nm (Resonator geringerer Güte). Die Laserschwelle (λ s = 189 nm) beträgt 4 mw eingekoppelter Pumpleistung; die slope efficiency ist 13.3%. Damit sind beide Werte um mehr als eine Größenordnung größer ( slope efficiency ) bzw. kleiner (Laserschwelle) als entsprechende Ergebnisse, die bisher zu Tm:LiNbO 3 Wellenleiterlasern veröffentlicht wurden. Weitere Modellierungsergebnisse zeigen, dass die slope efficiency bis zu 34% erhöht werden kann, wenn die Resonatorspiegel optimiert werden. Die Herstellung und Charakterisierung von Ti:Tm:LiNbO3 Wellenleiter für Anwendungen als Quantenspeicher werden im Anhang diskutiert. iv C O N T E N T S 1 introduction Background Motivation Organisation of Thesis 3 2 waveguide fabrication and characterization Introduction Fabrication of Ti:Tm:LiNbO 3 waveguide Tm Diffusion Doping Fabrication of Ti Waveguide Characterization of Ti:Tm:LiNbO 3 waveguide Waveguide Mode Intensity Distribution Scattering Losses Thulium Depth Profile Absorption Spectra Fluorescence Spectrum Transition Cross Sections Conclusions 14 3 Ti:Tm:LiNbO 3 waveguide amplifier Introduction In-band Pumping Scheme General Considerations Choice of Pump Wavelength Modeling Gain Spectra Power Characteristics Double Pass Pumping Further Comments Experimental Investigations Experimental Setup Experimental Results Discussion Comparison of Experimental and Modeling Results Amplifiers for Laser Applications Conclusions 37 4 Ti:Tm:LiNbO 3 waveguide laser Introduction Laser Cavity Experimental Setup Power Characteristics High-Q Operation at λ s = 189 nm Low-Q Operation at λ s = 185 nm 42 v 4.4.3 Relaxation Oscillations Spectral Properties Emission at 189 nm Radio Frequency Spectrum of Laser Output Emission at 185 nm Optimization Conclusions 49 5 conclusions and outlook Conclusions Outlook 52 a thulium doped waveguides for quantum memory applications 53 a.1 Waveguide Fabrication 53 a.2 Waveguide Characterization 55 a.3 Quantum memory 55 b optical spectrum of Ti:Er:LiNbO 3 waveguide laser 57 6 acknowledgements 59 bibliography 61 vi I N T R O D U C T I O N background Lasers are sources of coherent, diffraction limited electromagnetic beams. The acronym laser (light amplification by stimulated emission of radiation) is based on the fact that lasers rely on the phenomenon called stimulated emission of radiation for amplification of light. When reported for the first time by Maiman [1] in 196, its critics described lasers as a solution looking for a problem. Now, more than five decades after that remarkable invention, lasers find applications in day to day life in devices like CD players, bar code scanners etc. and in a variety of fields, like communications, metrology [2], medicine [3], etc. to name a few. Integrated optics refers to the integration of various optical devices like lasers, modulators, beam splitters etc. into one substrate [4]. Its essential feature is the fabrication of multifunctional chips, like in the case of integrated circuits in the domain of electronics. Four decades after the first proposal [5] of integrated optics, a lot of progress has been made in fabricating various devices as well as in the integration of several devices in one chip [6]. Integrated lasers have improved properties in comparison to their bulk counter-parts [7] due to (i) the reduction of cavity mode volume due to optical confinement, (ii) higher optical gain, (iii) lower thresholds and (iv) the possibility for integrating several devices (including the laser) in one chip. Lithium niobate (LiNbO 3 ) is a versatile substrate for integrated optics because of its excellent electro-optic, acousto-optic, nonlinear optic properties combined with the possibility to fabricate low loss optical waveguides [8]. Moreover, it can be easily doped with rare-earth ions (to get a laser active material and) to take advantage of the favorable properties of both, the host material and the rare earth ions simultaneously. A prominent example is the demonstration of a whole family of erbium doped waveguide lasers namely Fabry-Pèrot type lasers, Distributed Bragg Reflector- (DBR-) lasers, acousto-optically tunable lasers, electro-optically Q-switched lasers and harmonically mode-locked lasers [9] and of neodymium doped waveguide lasers [1, 11]. Also attractive properties of rare earth-doped waveguide amplifiers in LiNbO 3 have been shown [11, 12]. Another more recent example is the demonstration of a thulium-doped waveguide quantum memory in LiNbO 3 [13]. 1 1.2 motivation The research presented in this thesis started with the discovery of self-pulsations from a Fabry-Pérot type Ti:Er:LiNbO 3 waveguide laser (λ p = 148 nm, λ s = 1611 nm) [14]. The laser was found to emit pulses, as in the case of a passively mode locked laser, without the aid of any intracavity elements to induce such pulsations. This observation immediately raised several open questions. Would it make any difference if the laser active ions are different? In order to find the answer, Ti:Tm:LiNbO 3 waveguide amplifier and subsequently lasers were developed. This is described in detail in this thesis. Although the answer to the question posed above seems to be no as hinted by the laser results, it is yet to be proved beyond doubt. Apart from the academic interests which motivated this research there are other equally interesting aspects from the point of view of applications of the integrated lasers mentioned above. Lasers operating at wavelengths longer than 138 nm are termed eye safe. This is because of the strong absorption of these wavelengths by the front part of the human eye (cornea and vitreous humor) resulting in some sort of protection 1 to the retina [15]. There are several applications where such eye safe lasers are needed because the radiation cannot be guided. Laser radar, remote sensing, communications etc. are examples of such applications. In particular Tm-doped lasers, depending on the host material, emit in the wavelength range 165 nm to beyond 2 nm. Two example applications of Tm lasers are mentioned in the following. Several gas molecules have absorption bands falling in this spectral region. Tm-doped lasers can be used for the spectroscopy and detection of such gases. Tm lasers emitting at the appropriate wavelength can be used for performing laser surgery. This is because of the strong water absorption lines falling in the emission spectrum of Tm. There is a growing interest in Tm-doped LiNbO 3 waveguide lasers utilizing the 3 F 4 3 H 6 transition to get emission in the wavelength range 16 nm λ 19 nm [16, 17] and in Tm-doped waveguide lasers in other hosts [18, 19]. Up to now, Tm:LiNbO 3 lasers were pumped at 795 nm wavelength exploiting the strong absorption by the 3 H 6 3 H 4 transition. This thesis discusses the development of in-band pumped (λ p = 165 nm) Ti:Tm:LiNbO 3 waveguide amplifiers and Fabry-Pérot type Ti:Tm:LiNbO 3 waveguide lasers (λ s = 189 nm and 185 nm). 1 This applies only to stray radiations of low powers. With high power radiation adequate saftey measures should be taken. 2 1.3 organisation of thesis The core component of the waveguide amplifiers and laser cavities mentioned in this thesis is a Ti:Tm:LiNbO 3 waveguide. The fabrication of Ti:Tm:LiNbO 3 waveguides and their characterization are mentioned in chapter 2. Chapter 3 discusses how a Ti:Tm:LiNbO 3 waveguide can be used as an amplifier. An in-band pumped Ti:Tm:LiNbO 3 waveguide amplifier capable of achieving broadband optical gain in the wavelength range 175 nm λ 19 nm is demonstrated. This chapter includes a discussion of in-band pumping, extensive modeling of amplifiers, the specially developed experimental setup for small signal gain measurements, results of small signal gain measurements which are in good agreement with modeling results and a discussion on amplifiers for laser applications. The waveguide lasers operating near 189 nm and 185 nm developed using a Ti:Tm:LiNbO 3 waveguide amplifier is presented in chapter 4. This chapter includes a discussion of the formation of laser cavity, results of laser experiments done with both lasers and optimization of the 189 nm laser. The highlight is the first demonstration of an in-band pumped Ti:Tm:LiNbO 3 waveguide laser (i) emitting at the longest emission wavelength, (ii) with the smallest laser threshold and the highest output power reported from a Tm:LiNbO 3 waveguide laser, so far. A Ti:Tm:LiNbO 3 waveguide can be used as a solid state quantum memory [13]. Appendix A briefly outlines the fabrication and characterization of Ti:Tm:LiNbO 3 waveguides for quantum memory applications. 3 WAV E G U I D E FA B R I C AT I O N A N D C H A R A C T E R I Z AT I O N introduction By doping lithium niobate (LN) with rare earth (RE) ions, it is possible to fabricate integrated optical devices which make use of the properties of the host material as well as the rare earth ion. RE doped integrated lasers [2, 1] in LN are examples of such devices where the possibility to fabricate low loss waveguides in LN is combined with the laser properties of the rare earth ion. The laser(s) described in this thesis were fabricated by three processing steps: (i) indiffusing the RE ion (Er or Tm) into congruent lithium niobate (CLN), (ii) formation of Ti indiffused waveguides fabricated on the RE doped surface and (ii) subsequently the laser cavity was formed by coating suitable mirrors on waveguide endfaces. Laser active ions can be incorporated into LN lattice by various techniques. For example Er can be doped into LN lattice by indiffusion, ion implantation, pulsed laser deposition or by growing the crystal from a doped melt [21]. Our group has performed Pr doping [22] and Er doping [2, 9] in the past as well as Tm doping [23, 24] of LN by indiffusion recently. Particularly, detailed investigations on diffusion doping of Er were done and the results are described in [21]. Irrespective of the dopant used, we start by coating a planar layer of the RE atoms on the surface of a CLN wafer. Due to the lithium deficiency (Li/Nb=.94) in CLN, the dopant atoms occupy regular lattice sites and a dopant concentration of several mole % can be realized. Afterwards, this layer is indiffused at a temperature lower than the Curie temperature of LN (114 C), so that the ferro electric phase of the lattice is not disturbed during the diffusion process. In this chapter a brief overview on RE doping of LN by indiffusion (with Tm-doping as an example), subsequent Ti waveguide fabrication and the characterization of the fabricated waveguides thus formed are given. 2.2 fabrication of ti:tm:linbo 3 waveguide The RE doped waveguides (and laser cavities) used for the research mentioned in this thesis were fabricated by Raimund Ricken and Viktor Quiring from the technology wing of our lab. 5 2.2.1 Tm Diffusion Doping Fig. 2.1 depicts the different steps of waveguide fabrication which were discussed above. Commercially available.5 mm thick Z-cut wafer of undoped optical grade CLN were Tm-doped near the +Zsurface before waveguide fabrication. A vacuum deposited Tm layer of 32 nm thickness was in-diffused at 113 C during 15 hours in an argon atmosphere followed by a post treatment in oxygen (1 hour) Fabrication of Ti Waveguide On the Tm-doped surface 14 nm thick Ti layer was deposited first. Subsequently stripes with widths ranging from 4.5 µm to 8.5 µm in steps of.5 µm were defined by a photolithography step and indiffused at 16 C for 9.6 hours to form 6 mm long optical strip waveguides. Subfigure 6 of Fig. 2.1 shows the co-ordinate system of the crystal (capitals) with respect to the laboratory frame (small letters). Tm Tm:LiNbO 3 LiNbO 3 1 LiNbO 3 2 Ti Photoresist 3 4 LiNbO 3 LiNbO 3 Ti stripe Ti:Tm:LiNbO 3 Y,z 5 -Z,y X,x 6 LiNbO 3 LiNbO 3 Figure 2.1: Ti:Tm:LiNbO 3 waveguide fabrication steps. 1 & 2 - Tm deposition and indiffusion, 3 - Ti deposition, 4 & 5 Ti stripe defenition and 6 - Ti indiffusion to realize Ti:Tm:LiNbO 3 waveguide. The thickness of the RE layer, the diffusion temperature and the indiffusion time are adjusted in such a way that the (i) resultant RE depth profile has a good overlap with the fundamental waveguide mode of both pump and signal emissions as well as (ii) the reservoir (RE layer) is completely depleted resulting in a smooth surface which is essential for the realization of low loss optical waveguides. The 6 thickness of the Ti layer, the diffusion temperature and the indiffusion time are adjusted in such a way that the waveguides are mono mode for wavelengths larger than 15 nm. Indiffusion of RE ions is done prior to the fabrication of Ti waveguides. This is due to the fact that the Ti indiffusion occurs at a much faster rate in comparison to the RE indiffusion. Afterwards, the sample was cut to 5 mm, 1 mm, 15 mm and 3 mm long pieces (breadth = 12 mm, in all cases) and the end faces of each piece were polished perpendicular to the waveguides. The 15 mm long sample was used for investigations to determine waveguide properties, except for the fluorescence measurement which was done with the 5 mm long sample. Subsequently, the 15 mm long sample was used for amplifier experiments and the 3 mm long sample was used for laser (fabrication and) experiments. The width of the waveguide which was used for both amplifier and laser experiments is 6.5 µm. 2.3 characterization of Ti:Tm:LiNbO 3 waveguide The various properties of the waveguide namely, mode intensity distribution, scattering loss, Tm doping profile, absorption spectrum, fuorescence spectrum and transition cross sections were characterized as mentioned below Waveguide Mode Intensity Distribution Whether the waveguide is monomode as designed can be tested by measuring the near field intensity distributions. The near field intensity distribution of the waveguide mode was measured by imaging the magnified (1x) waveguide mode onto an infrared camera. Amplified spontaneous emission from a 165 nm diode laser operated below threshold was used as the light source for this measurement so as to minimze the potential errors that could arise from interference effects. From the captured image (see Fig. 2.2) the dimensions of the intensity profile is calculated using a specially developed software 1. The measured full width at half maximum of the fundamental TE mode at 165 nm is 7.8 µm 6.2 µm. On the other hand the calculated FWHM of the TE mode at 165 nm, with fabrication parameters of Pb34z input to Focus [25] are 7. µm 5.2 µm. This is slightly smaller in comparison with the measured values. The software takes the bulk density of Ti for performing the calculation whereas the actual density is slightly lower. This is the reason for the discrepancy. 1 by Dr.Ansgar Hellwig. 7 Depth (µ m) Width (µ m) Figure 2.2: Measured (left) and calculated (right) mode intensity profiles of a 6.5 µm waveguide. The blue line (in measurement data) corresponds to 1 µm. Isolines are drawn at intensity levels corresponding to 9%,7%,5% etc Scattering Losses The propagation losses of Ti:Tm:LN waveguides were measured by the Fabry-Pérot method which was developed in our group [26]. A waveguide with polished endfaces perpendicular to the waveguide itself form a low finesse Fabry-Pérot resonator because of the residual endface reflectivities arising from the air:ln interface. During the experiment the transmitted optical power behind such a waveguide resonator is measured as a function of wavelength (or as a function of the resonator length) using the setup shown in Fig.2.3. A narrow band laser emission with a coherence length longer than twice the optical path length of the waveguide is used as the light source in the experiment so that the Fabry-Pérot resonances of the low finesse cavity can be measured. From the measured curve (see actual measurement data in Fig.2.4), we can calculate the waveguide propagation losses with the following relation, α = 4.34 (ln[r] + ln[2] ln[k]) (2.1) L where the contrast, K = I max I min I max +I min, R is the waveguide endface reflectivity and L is the length of the waveguide in cm. With the above expression α is obtained in units of db/cm. Narrowband laser Polarization controller InGaAs Ti:RE:LN waveguide Figure 2.3: Experimental setup used to measure waveguide propagation loss. 8 Narrow band emission centered at 155 nm (from a tunable extended cavity diode laser) was used for the measurement because of (i) the weak Tm absorption at this wavelength and (ii) the fact that only the fundamental mode will be supported by the waveguide. Apart from the 6.5 µm waveguide, the adjacent 7
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