External cavity multiline semiconductor laser for WDM applications

External cavity multiline semiconductor laser for WDM applications Igor S. Moskalev a, Sergey B. Mirov * a, Tasoltan T.Basiev b, Vladimir V.Fedorov a, Gary J. Grimes a, I. Edward Berman c a Univ of Alabama at Birmingham, Birmingham, AL 35294 b General Physics Institute, Moscow 117942, Russia c Atlantic Vision, Inc., Westborough, MA 01581 ABSTRACT A novel technical approach to build a multiwavelength laser source for DWDM applications is described. The basic idea of this system is to maintain simultaneous lasing operation in a gain medium at different wavelengths without mode competition. The system uses a novel dispersive cavity. By designing this cavity structure appropriately, the system creates its own microcavities-channels each lasing independently at different wavelengths across the complete gain spectrum of the laser active material. Multifrequency lasing on the basis of a single diode laser chip was analyzed theoretically and demonstrated experimentally. A simultaneous generation across ~80% of the FWHM of the luminescence bandwidth of the GaAlAs active medium (657-667 nm) in multifrequency regime with a special pre-assigned spectral composition (8 independent lines were obtained and up to tens of lines possible) was realized. The minimum spectral separation between lines was 6.2 GHz. The linewidth of each oscillation line was less than 2 GHz and the total output power of the laser was about 50mW. It was demonstrated that the output radiation could be collimated to laser beam with a divergence less than 5 mrad. For DWDM applications a version of this superbroadband semiconductor laser operating in one of the telecommunication wavelength bands around 1.3 or 1.5 µm is proposed. Keywords: Semiconductor laser, Multiwavelength laser, Mode competition, Wavelength Division Multiplexing. 1. INTRODUCTION Dense wavelength division multiplexing (DWDM) promises to greatly expand the capacity of optical fibers, both those yet to be installed, and perhaps more importantly, those already deployed in terrestrial and undersea systems. For viable DWDM systems, low cost laser sources maintaining accurate adherence to the particular channel wavelength spacing are required. As the channel spacing of DWDM network decreases, short-cavity lasers, such as distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, and vertical cavity surface emitting lasers (VCSELs), may have to be externally wavelength stabilized to control the temperature dependent output wavelength drift. To improve the wavelength stability one can use a longer external cavity and produce a diffractively stabilized lasing wavelength separation. Examples of these lasers include multichannel grating cavity lasers with bulk-optic [1-4] and integrated design [5-7]. However, some of these systems lack of flexibility in the selection of wavelengths, total amount of available channels, and wavelength spacing. In addition to this, a compensation for optical cross talk can be of significant issue. This paper describes the theoretical design and practical realization of a new type of source, a multifrequency, superbroadband diode laser, for use in DWDM systems. It is based on a novel external cavity construction where optical components of the cavity maintain distinct gain channels in a single stripe diode chip, reduce cross talk, suppress mode competition, and force each channel to lase at specific stabilized wavelength. By designing this cavity structure appropriately, the system creates its own microcavities each lasing at different wavelengths across the complete gain spectrum of the active material. Recently we demonstrated a true continuous and multiline lasing of color center laser [8-13] with a spectral width practically coinciding with the luminescence spectrum of the gain medium. Now we demonstrate an application of this novel cavity design to build a multiline/superbroadband (ML/SB) laser based on a conventional commercial diode chip. The construction of the cavity is flexible and also allows utilization of multistripe diode chips, diode arrays, as well as individual diode lasers made from different materials. The laser could be readily integrated with an external modulator. With a multistripe diode chip or diode array cavity a current modulation of individual channels can be realized, being limited only by * Correspondence: E-mail: mirov@uab.edu; WWW: http://lorentz.phy.uab.edu/~mirov/; Telephone (205) 934-8088; Fax (205) 934-8042 128 In-Plane Semiconductor Lasers V, Luke J. Mawst, Ramon U. Martinelli, Editors, Proceedings of SPIE Vol. 4287 (2001) 2001 SPIE 0277-786X/01/$15.00

a cavity roundtrip and carrier recombination times. 2. PRINCIPLES OF OPERATION The basic optical scheme of the laser cavity is shown in Fig.1. The laser operates as follows. Emission from the active medium passes through the spatial mask and focusing lens L 1 into the off-axis mode suppression element, aperture A, which together with the spatial mask divides active zone of p-n junction into a number of channels and separates from the amplified emission of individual channel only part of it that is spread parallel to the resonator axis. This separated radiation is diffracted on the diffraction grating G. The Littrow mount grating works as a retroreflector in the auto-collimating regime in the first order of diffraction and returns part of radiation back to the aperture. The off-axis mode suppression element, aperture, in turn extracts from the diffracted radiation only the radiation of the main laser modes. Secondary laser modes, which diverge from the optical axes, are expelled from the process of generation. Hence, the aperture should simultaneously select the fundamental transverse modes for all existing channels in the cavity. Positioning the aperture in the place where all the channels intersect can do it. This place corresponds to the focal plane of the lens L 1. The width of the aperture is estimated as the mode size in the focal plane of the lens. w m w c w L Aperture A λ 1 λ 2 λ 3 Mirror M 1 x 1 x 2 α β Active medium Spatial mask Focusing lens L 1 w G Grating G Fig.1 General design of the SB/ML laser. The radiation of the main laser modes, each with a distinct wavelength, is collimated by the focusing lens and directed back to the active medium. Each mode with its different wavelength has its own trajectory. This radiation along with the stimulated radiation provoked by it are reflected directly back by the mirror M 1. This process gives rise to the superbroadband multi-frequency oscillation an independent oscillation of the different parts of active medium with different wavelengths, covering practically the whole spectral region of the active medium amplification band. This results in each part of the crystal parallel to the laser axis working as an independent laser with its own wavelength lying within the broad AM luminescence spectrum. If we assume that each beam passing through active element is located at a coordinate x (Fig.1), in the transversal direction, with respect to the position of the optical axis of the laser, α is the angle of intersection of the current beam x and the optical axis and f is the focal length of the focusing lens L 1 then we can write: x α (2.1) f Proc. SPIE Vol. 4287 129

If the normal to diffraction grating forms an angle β to the optical axis then one can say that each beam x arrives at the diffraction grating at the certain angle determined as β + α. The diffraction grating works in the autocollimation scheme causing the first order of diffraction to be retroreflected into the cavity. Since each beam strikes the diffraction grating at a different angle, the cavity for each beam selects a different wavelength determined from well-known Littrow mount condition: k λ = 2t sin( β + α), (2.2) where k is the order of diffraction (in our particular case k=1), and t is the grating spacing period. The zero order of diffraction is used as an output of the laser. The angle β is found from auto-collimating condition for the central wavelength λ 0 : λ 0 β = arcsin (2.3) 2t So, each fraction of the crystal located at a distance x i from the optical axis (Fig.1) generates its own wavelength λ i determined as: λ xi λ + i 2t sin arcsin 0 (2.4) 2t f Since each main laser mode λ i (that is parallel to the optical axis in the crystal) doesn t overlap with others (in the crystal) we obtain independent oscillation of the different parts of active medium at different wavelengths covering practically the whole spectral region of the active medium amplification band. The spatial mask selects a number of active cavities (active wavelengths). Choosing parameters of the mask (size of its holes and their separation) one can set a certain spectral structure of the output radiation. Furthermore, one can tune the laser wavelengths by simply moving the mask in transverse direction. The spatial mask in conjunction with other optical components in our cavity has the same functional role as multiple stripes in diode structures. However, it provides an additional flexibility in choosing output spectral composition from the amplification band of the gain medium. 3. THEORETICAL ANALYSIS OF THE SUPERBROADBAND DIODE LASER IN A MULTILINE OUTPUT MODE In this paper we analyze a diode ML/SB laser satisfying laser transmitter requirements for DWDM system, in particular, maximum amount of emitted lines. Cavity analysis is based on the Gaussian beam approximation [14,15]. First, we define optimal beam radii in the crystal and dispersive element, and focal length of the intracavity lens providing maximum amount of lines and minimal cross-talk between channels. Then exact equations relating the intracavity beam parameters with parameters and relative positions of the cavity optical elements are derived and numerically solved. In the analysis a stable external cavity AlGaAs diode laser with a physical size of the laser chip 500x200x1 µm, featuring an amplification band of about 20nm wide centered around 662nm is considered. The diffraction grating has a spacing period t=(1/1800)mm. The spectral linewidth, line separation, and maximum amount of lines emitted without a cross talk from a single chip diode laser along with parameters of the external resonator are estimated below. 3.1. Maximum amount of independent channels in a single chip diode laser Maximum amount of lines, oscillating in a single diode chip, can be realized when the active medium is split to as many independent channels as possible by means of decreasing the mode diameters in the crystal. Due to diffraction divergence any two adjacent modes could overlap with each other causing a mode competition. To minimize the latter effect the overlap volume needs to be minimized. For our particular optical scheme (see Fig.1) the mirror M 1 is positioned closely to the rear 130 Proc. SPIE Vol. 4287

facet of the crystal, therefore, the beam waist of each mode is located on that facet. The beam diameter increases along the crystal. The amount of non-overlapping channels is maximum when wc = 2w m, (3.1) where w m and w c are the beam radii at the mirror and output facet of the crystal, respectively. If the physical length of the crystal is d 0 and its refraction index is n the beam radius on the output facet of the crystal is calculated as: w + 2 2 0 c = wm 1 2 2 4 n π wm λ d 1 2 (3.2) Solving (3.1) and (3.2) with respect to w m we obtain a numerical estimation for the beam waist on the mirror M 1 for a particular crystal described in the beginning of this section: w m ~5µm (3.3) Dividing the transverse size of the crystal x~200µm by the mode size 2w m =10µm we obtain maximum possible number of independent cavities in the crystal to be n max ~20. This gives an estimation of the upper limit for a number of independent spectral lines that we can expect from our laser working in a multiline operation regime. 3.2. Spectral properties of the ML/SB diode laser According to equation (2.4) for given crystal, diffraction grating, and oscillation wavelength, the focal length f of the intracavity lens L 1 is connected to the output laser bandwidth λ. Taking derivative dλ(x i )/dx i one obtains focal length f as: f = 2 t x λ cos arcsin 0, (3.4) λ 2t where the central wavelength λ 0 =0.662µm. For a characteristic semiconductor laser amplification band of λ=20nm, width of p-n junction of x=200µm, and diffraction grating with a period t=(1/1800) mm, we obtain f~8.9 mm. From the fact that the maximum number of spectral channels in our laser is about n max =20 and the overall bandwidth of the resonator radiation is λ=20 nm it follows that the maximum spectral separation between two adjacent channels is about δλ~1nm. To achieve this spectral separation the spectral resolution of the diffraction grating should be less or equal to δλ. It is well known that the maximum theoretical resolving power of a grating, R=λ/δλ, equals to the total number of illuminated grooves N times the order of diffraction k (in our case k=1). If for an oscillation channel with a wavelength λ its beam size at the grating is 2w g then spectral resolution of the grating is determined as follows: λ λ δλ = = (3.5) mn m ( 2w / t) Substituting λ~662nm, δλ~1nm and t=1/800mm in the equation (3.5) one finds for minimum beam radius at the grating: g λ t w g 200µ m (3.6) m δλ The upper limit of the bandwidth in our laser system obviously coincides with channel spacing. An additional factor that can increase the bandwidth is instability of the cavity caused by temperature fluctuations and mechanical vibrations. This factor can be significantly decreased by proper construction of the resonator and applying appropriate means for thermal stabilization. Proc. SPIE Vol. 4287 131

3.3. Matrix analysis of the multiline cavity The requirements on the beam radii in the crystal, the focal length of the lens L 1, and the radii of the channels on the diffraction grating were established in the previous section. In our further analysis of the laser resonator we use the wellknown matrix method for Gaussian beams [14,15] to find the equations relating the above parameters with the positions of the lens and the diffraction grating in the cavity. In the framework of this method we approximate cavities corresponding to each channel as those consisting of two plain mirrors, intracavity lens, and dielectric medium inside the resonators. Modeling the diffraction grating by a plane mirror comes from the fact that the grating works in autocollimating regime. The matrix of equivalent resonator will be calculated as follows: where: 1 M = 0 l 1 1 1 f 0 1 1 0 d g1 eff = g1g 2 1 1 L L, (3.7) g 2 d = d1 (3.8) n 0 d eff z + + - effective distance between mirror M 1 and the lens L 1 ; z is a separation between the mirror and the rear facet of the crystal (in our case z=0); d o is length of the crystal, d 1 is distance from the output facet of the crystal to the lens; ldeff L = deff + l (3.9) f - effective length of the resonator, l is separation between lens L 1 and grating G, f-focal length of the lens; l g1 =1 (3.10) f -effective g 1 parameter [14,15]; -effective g 2 parameter [14,15]; deff g2 =1 (3.11) f The parameters g 1 and g 2 are the stability parameters of the resonator. For stable resonators the condition g 1 g 2 <1 must be satisfied. Then we obtain beam radii for individual channels in the characteristic planes of the cavity planes of the crystal facets, grating, lens, and focal plane of the lens, where the aperture is placed. By this means, w m on the mirror M 1 [14,15]: w m = Lλ g 2 π g1( 1 g1g2 ) (3.12) On the grating G [14,15]: 132 Proc. SPIE Vol. 4287

w G = Lλ π g g1 1 g g ) 2 ( 1 2 (3.13) On the output facet of the crystal: 2 d 0 z + w = 1+ n c wm (3.14) z 0 with confocal parameter of the beam at the mirror M 1 z o =πw m 2 /λ; On the lens L 1 : On the aperture A (at the focal plane of the lens): 2 deff w f = wm 1+ (3.15) z0 w D λf = (3.16) πw m From equations (3.7)-(3.16) one can analytically find the appropriate cavity configuration satisfying requirements (3.1), (3.3) and (3.6). The focal length of the lens L 1 is known from equation (3.4), the size of the crystal, d o, is fixed, values of the beam waists on the mirror and grating are known from (3.3) and (3.6), parameter z is zero since HR coating on the rear facet of the crystal serves as the mirror M 1. Hence, after substituting these data in (3.7)-(3.16) we end up with two equations (3.12) and (3.13) with two unknowns, namely separations d 1 and l from crystal to the lens, and from the lens to the grating, respectively. By obtaining them one gets a complete description of the resonator. Alternatively we performed numerical analysis to find the resonator parameters. As the variables of the analysis we used the two distances and the focal length of the lens. Varying the two distances and the focal length of the lens our computer program checked stability condition ( g 1 g 2 <1) first. After calculating beam waists on the mirror, grating and output facet of the crystal it checked conditions (3.1), (3.3), (3.4), and (3.6). As the successful solutions the only solutions satisfying the last four conditions were chosen. The results of calculation show that for crystal of length d 0 =0.5mm, focal length of the lens L 1 f=9.0mm, distance from the crystal to the lens d 1 =8.9mm, distance from the lens to the grating (with t=1/1800mm), l=40mm, and aperture width of 200 µm we can expect laser radiation in multiline operation with a number of lines 20, line separation of 1nm and overall bandwidth of about 20nm. The result of analysis demonstrate that a single diode chip multiline laser emitting around 1550 nm can be realized with output parameters completely satisfying telecommunication requirements for 100 GHz channel spacing system. 4. EXPERIMENTAL RESULTS The experimental setup is similar to shown in Fig.1. Instead of a single-lens focusing element it employs a proprietary three lens focusing system providing a manifold enhancement of individual channel diameters on the grating with respect to ones in the active medium. In our experiments we used a AlGaAs diode with a size of p-n junction of 500x200x1 µm. Rear facet of the crystal was (HR) coated with reflection of about 5% and served as mirror M 1 of the laser resonator, the output facet was (AR) coated with reflection of about 0.1%. As an output coupler we used a diffraction grating with a period 1/1800mm and diffraction efficiency of about 80%. Laser driver SDL-840 served as a power supply for CW laser operation. Proc. SPIE Vol. 4287 133

Spectrograph SpectroPro150 and a CCD camera were used for wide oscillation spectra measurements with a spectral resolution of about 0.5nm. For a high resolution measurements we used two Fabry-Perot Etalons with finesse of 14 and free spectral ranges of 141 and 30 GHz, respectively. The luminescence spectrum of the diode is shown in Fig.2a. As it can be seen from the figure, the spectrum is centered at 660nm and has bandwidth of about 20nm at 1/e 2 level. The oscillation spectrum of the diode laser biased at current of 2300 ma in the non-selective cavity (when diffraction grating and all lenses are removed) is shown in Fig.2b. In this case the diode operates as a conventional diode laser with the chip facets serving as laser mirrors. As one can see lasing occurs at the maximum of the luminescence band. The measured oscillation linewidth determined by registration system was less than 0.5 nm. Fig. 2c-d demonstrates the results of the multiline operation regime. As one can see from Fig. 2c three-lines oscillation with a spectral lines separation 2.5 nm was achieved on a single diode chip biased at current of 1500 ma. Changing the modes diameters in the crystal, by means of repositioning of the intracavity lens, allows changing the number of oscillating channels. In Fig. 2d a simultaneous oscillation of five spectral lines is shown. We obtained up to 8 simultaneously oscillating channels. The overall width of the oscillation spectrum is about 8 nm, which constitutes ~ 80% of the FWHM of the luminescence bandwidth. The total output power of the laser was about 50mW. (a) Intensity, arbitrary units (b) (c) (d) 640 650 660 670 680 Wavelength, nm Fig.2 Comparison of fluorescence spectrum (a), and outputs of the diode laser in the nonselective Fabry- Perot cavity (b), and Superbroadband external cavity with three (c) and five line (d) outputs. 656 658 660 662 664 666 Wavelength, nm Fig.3 Continuous tuning of the laser output in a dual-wavelength regime of operation. Fig.3 shows a tunable dual-wavelength laser operation. By rotating the diffraction grating in the dispersion plane we can tune the dual-wavelength laser oscillation. A dual-wavelength laser operation with 7 nm tuning range and with 1 nm spectral distance between lines was achieved with 1500 ma injection current. We also may change the spectral distance between oscillation lines by changing the focusing system arrangement. The minimum spectral separation between lines in our experiments, measured with a Fabry-Perot etalon, was 6.2 GHz (see Fig.4). The oscillation linewidths of each line were less than 2 GHz. The accuracy of the measurement was limited by the interferometer resolution. The free spectral range of the interferometer was about 30 GHz, its finesse was about 14. So, the resolution of the etalon was about 2.1 GHz, which is very close to the longitudinal mode spectral separation of our laser (~2.1 GHz, for laser length of 7 cm). From the results we conclude that each channel operates in almost single-longitudinal mode regime. 134 Proc. SPIE Vol. 4287

Etalon FSR ν=30 GHz Line separation ν=6 GHz Intensity Output laser oscillation exhibits different divergence in the horizontal (diffraction grating) plane and vertical (fast) plane. Experiments show that a collimated output radiation with divergence less than 5 mrad for both planes can be obtained when optimal collimation lens is used. Such radiation can be focused into a single-mode fiber for further data input modulation. 5. DISCUSSION As it was shown above, the experimental results are in a good agreement with the developed theory of operation of SB/ML laser. Further optimization of the optical scheme will increase reliability and performance of the ML/SB diode laser. Using appropriate single focusing lens in the resonator in place of the complex three-lens focusing system will simplify the laser resonator, its adjustment and reliability. The spatial mask used to form a multiline structure of the laser spectrum is much more flexible compared to a conventional multistripe technology. Moving the mask in transverse direction we can tune frequencies of the laser and set desirable spectral structure. Another advantage of such a scheme is that it is much simpler to produce a spatial filter than to make a special multistripe structure of a diode. It is notable that in the case when the spectral linewidth of the single channel is smaller than the spectral separation between channels the multiline oscillation could be achieved without placing intracavity spatial mask. The considered example of SB diode laser working in a visible spectrum range has been designed to check our basic ideas and principle possibility of realization such a laser source based on a single diode chip. The next step we are going to perform is designing analogous device in the near infrared spectrum, namely laser operating around 1550nm for telecom applications. Calculation performed for this case shows that one can expect about 20 independent lines from laser based on a crystal with a transverse size of about 400µm. Further improvement of the laser scheme is in usage of a diode array instead of a single broad stripe edge emitting diode. Calculations show that we can significantly improve performance of the ML/SB laser. Furthermore, in such an arrangement we will be able to perform a direct modulation of individual channels. At the same time all the lasers in the array will be driven by one common external cavity providing stable channel separation. 6. CONCLUSION In this paper we have analyzed theoretically and demonstrated experimentally the possibility of creation of multiwavelength/super-broadband diode laser based on a novel laser resonator scheme. Multi-frequency oscillation (up to 8 spectral lines simultaneously) with a linewidth of 2 GHz and a spectral line separation of 6.2 GHz has been achieved in a single diode operating at 660nm. Total bandwidth of the laser radiation was about 80% of the FWHM luminescence band of the diode. In the multi-frequency regime the total output power was about 50mW. After collimation of the output beam its Pixel Fig.4 High resolution measurement of linewidth of each channel in twowavelength operation with a Fabry-Perot interferometer. This result demonstrates a single longitudinal mode operation of the laser. Proc. SPIE Vol. 4287 135

divergence was 5 mrad both in horizontal and vertical directions. Possibility of tuning of the laser wavelength within the luminescence band has been demonstrated. One of the major advantages of our laser scheme is that we can use any type of the active medium: we can use a single diode chip as well as a diode array or a multi-striped diode. Although we believe that the best choice is a diode array since, it s the most flexible scheme in terms of arranging the desirable spectral structure of the output radiation. Theoretical analysis showed that we can build an analogous device operating in the near infrared spectral region, namely at 1.3-1.55µm. This range corresponds to the existing DWDM standards. In this paper we have demonstrated a very promising and successful attempt to create a multiline laser source. The superbroadband/multiline laser is believed to be a leading candidate for optical DWDM systems for telecommunication applications. The excellent reliability of the ML laser combined with its remarkable ability to tailor a spectral response of arbitrary nature and the temperature stability of its wavelength controlling external cavity make it an excellent candidate for telecommunication applications. The ML/SB laser is particularly applicable to secure telecom applications. It can be much more compact than individual lasers and can be made very rugged. By replacing a totally passive spatial filter with an active reconfigurable spatial filter such as a liquid crystal or smart pixel array the SBL can be instantly reprogrammed to emit an entirely different sets of wavelengths. This feature could be used in combination with electronically alterable fiber Bragg grating receivers to make a complex multiwavelength frequency hopping regime. This would be the most secure system imaginable and would be essentially impossible to defeat even with physical access to the system. In summary, we believe that the ML/SBL will be capable of providing physical security, cryptography using high data rates, small form and rugged, wavelength stability, and the flexibility to custom tailor the lines in the field. These advantages are critical in military and commercial applications, particularly in metropolitan area optical networks. REFERENCES 1. I.H.White, K.O.Nyairo, Demonstration of a 1x2 multichannel grating cavity laser for wavelength division multiplexing (WDM) applications, Electron. Lett., vol. 26, pp.832-833, 1990. 2. I.H.White, A multichannel grating cavity laser for wavelength division multiplexing applications J. Lightwave Technology, vol. 9, 893-899, 1991. 3. G.C.Papen, G.M.Murphy, and D.J.Brady, A.T.Howe, J.M.Dallesasse, R.Y.Dejule, D.J.Holmgren, Multiplewavelength operation of a laser-diode array coupled to an external cavity, Optics letters, vol. 18, pp.1441-1443, 1993. 4. C-L.Pan, C-L.Wang, A novel tunable dual-wavelength external-cavity diode array and its applications, Optical and Quantum Electronics, vol. 28, pp.1239-1257, 1996. 5. J.B.D. Soole, K. Poguntke, A.Schere, H.P.LeBlanc, C.Chang-Hasnain, C.Caneau, R.Bhat, and M.A.Koza, Multistripe array grating integrated cavity (MAGIC) laser: a new semiconductor laser for WDM applications, Electron. Lett., vol. 28, pp.1805-1807, 1992. 6. M.Zirngibl, C.H.Joyner, L.W.Stulz, U. Koren, M.D.Chen, M.G.young, B.I.Miller, Digitally tunable laser based on the integration of waveguide grating multiplexer and an optical amplifier, IEEE Photon. Technol. Lett., vol. 6, pp. 516-518, 1994. 7. C.R.Doerr, C.H.Joyner, L.W.Stulz, J.C.Centanni, Wavelength selectable laser with inherent wavelength and singlemode stability, IEEE Photon. Technol. Lett., vol. 9, pp. 1430-1432, 1997. 8. T.T.Basiev, S.B. Mirov, Room Temperature Tunable Color Center Lasers, Laser Science and Technology books series vol. 16 pp. 1-160. Gordon and Breach Science Publishers/Harwood Academic Publishers, 1994. 9. T.T.Basiev, S.B.Mirov, P.G.Zverev, I.V.Kuznetsov, R.Sh. Tedeev, (October 1995) Solid State Laser with Superbroadband or Control Generation Spectrum. U.S. Patent No5, 461,635. 10. T.T. Basiev, P.G. Zverev, S.B. Mirov, V.V. Fedorov, "Superbroad-Band Laser on LiF Color Center Crystal for Near - Infrared and Visible Spectral Regions", Abstr. Rep. International Conf. "LASER-93", Munich, Germany, 1993. 11. T.T. Basiev, P.G. Zverev, S.B. Mirov, V.V. Fedorov, Solid State Laser with Superbroadband or Control Generation Spectrum SPIE, vol. 2379, 54-61, 1995. 136 Proc. SPIE Vol. 4287

12. T.T.Basiev, P.G.Zverev, V.F.Fedorov, S.B. Mirov, Multiline, Superbroadband And Sun-Color Oscillation Of LiF:F 2 - Color Center Laser, Applied Optics, 36, No 12, 1997. 13 N. W. Jenkins, S. B. Mirov, Solid-State White-Light Laser Using LiF:F 2 + ** Color Center Crystals OSA Trends in Optics and Photonics Vol.34, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds. (Optical Society of America, Washington, DC 2000), pp. 364-372. 14. Optical Resonators. Fundamentals, Advanced Concepts and Applications N.Hodgson and H.Weber, Springer- Verlag, London 1977. 15. Laser Handbook, A.M.Prokhorov, Ed., Moscow, 1976. Proc. SPIE Vol. 4287 137