cw fiber laser of NdLa pentaphosphateH. P. Weber, P. F. Liao, B. C. Tofield, and P. M. Bridenbaugh Citation: Applied Physics Letters 26, 692 (1975); doi: 10.1063/1.88038 View online: http://dx.doi.org/10.1063/1.88038 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/26/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Miniature Ndpentaphosphate laser with bonded mirrors side pumped with lowcurrentdensity LED’s Appl. Phys. Lett. 33, 309 (1978); 10.1063/1.90350 Magnetic ordering in CeLa and NdLa alloys J. Appl. Phys. 49, 2115 (1978); 10.1063/1.324756 Intracavity secondharmonic generation in a Nd pentaphosphate laser Appl. Phys. Lett. 29, 176 (1976); 10.1063/1.89014 Laser performance of large Ndpentaphosphate crystals Appl. Phys. Lett. 28, 189 (1976); 10.1063/1.88718 NdLa pentaphosphate laser performance Appl. Phys. Lett. 23, 519 (1973); 10.1063/1.1654982

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cw fiber laser of NdLa pentaphosphate H. P. Weber, P. F. Liao, B. C. Tofield,* and P. M. Bridenbaugh

Bell Telephone Laboratories, Holmdel, New Jersey 07733 (Received \3 February 1975; in final form 7 April 1975)

cw operation at 1.051 p.m of a waveguide laser consisting of a crystalline fiber of Ndo,Lao.,P ,014, 12 X 12 p.m in cross section and 0.68 mm in length, is reported. When longitudinally pumped with a Kr ion laser at 752.5 nm the room-temperature laser threshold was less than lO-mW absorbed power.

An important step toward miniaturization of an opti­cally pumped solid -state laser is the demonstration of a wave gUiding structure such as a fiber formed by the amplifying medium itself. Such a structure eliminates critical alignment of the resonator mirrors and con­fines radiation in the active pumped area. This geome­try can result in a lower pump threshold, and a better control of the high -order transverse modes. In addition, such a scheme is directly compatible with the geometry of a fiber optical transmission medium.

We report here the first successful operation of a laser using the high-Nd-concentration materiall

Ndo.5Lao,5P5014 in a guided mode structure. The high concentration of Nd allows the laser to be very short and hence reduces (i) the number of longitudinal modes and, (ii) the total loss due to scattering at the interface be­tween the crystalline core and cladding. In addition, materials with a high concentration of active ions are well suited for transverse pumping schemes in contrast to dilute systems. Transverse pumping schemes open the possibility of using spatially incoherent pump sources such as LED's.

Our lasing medium consists of Single-crystal needles of Ndo,5Lao.5P50l4' This material is preferable to pure NdP50 l4 since it results in a % reduction in the heat generated per unit volume2 and has a fluorescence life­time which is nearly twice as long (200 Ilsec)3,4 as that in NdP5014' Both factors lead to a lower threshold for the dilute material. Our crystals have a cross-section­al area a"" 12x12 Ilm and length I"" O. 68 mm. Sohd­state lasers with guided wave geometries with 20-100 times larger active volume have been reported recently using Nd-doped glass fibers 5 and YAG:Nd fibers 6 as the gain media.

The crystals were grown from phosphoric acid as described elsewhere3 with minor modifications of the apparatus. 7 These crystalline needles [Fig. l(a)] are of excellent optical quality as grown and their side faces are smooth. The end faces are also frequently perfect as grown, Spectroscopic and x-ray investigations reveal that the needles are crystallographically identical to their bulk counterparts. The axis of the needles corre­sponds to the crystallographic a axis.

Figure l(b) illustrates the construction of the crystal holder. A glass tube of 5-mm o,d. and O. 5-mm Ld. and about 20 mm length was fiUed with hollow quartz fibers, each with apprOximately 150-llm o,d. and typically 25 Ilm L d. They extended about 10 mm beyond either end of the glass tube. The hollow fibers were imbedded in the tube and cemented with ()' -cyanoacry-

692 Applied Physics Letters, Vol. 26, No. 12, 15June 1975

o

late, 8 a monomer that polymerizes quickly as soon as it is brought into small cavities, After curing, slices of 2-mm thickness and surfaces orthogonal to the rod axis were cut and the surfaces polished. The 25-llm holes were cleaned in an ultrasonic bath and subsequent-1y with a jet stream of hexane. The crystalline NdQ.5Lao,5P5014 needles were then inserted into the cleaned holes under a microscope with micromanipula­tors. The crystals were used as grown and only cleaned by rinsing in water. These needles were cemented in the hollow quartz fibers with ()' -cyanoacrylate. The surfaces of the disk were further polished until the crystalline fiber ends appeared exposed at the disk surfaces. This rather complicated procedure assured that throughout the machining and polishing processes, the fiber axis remained normal to the polished surface. Such a prepared disk was placed between two plane mirrors with reflectivities 99.8% at 1. 05 Ilm. To re-

0.2 0.4 0.6 0.8 L 0

I I I I mm

(a)

,

500 Mm

(b)

FIG. 1. Crystalline fibers of Ndo.5LaO.5P5014 and their mounting. (a) Photograph of a typical fiber with apprOXimately 10 x10-Mm cross section. (b) Polished mounting block for fi­bers. Hollow quartz fibers with inside diameter of approxi­mately 25 I-lm are cemented parallel to each other in a glass tube oriented normal to the polished surface of the block. Two crystals which are laid onto the block in the illustration are subsequently inserted and cemented into the holes. Then the polished ends are further polished until the fiber ends are exposed.

Copyright © 1975 American Institute of Physics 692

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FIG. 2. Laser arrangement. The fiber in its mount is placed between two plane mirrors with reflectivity R = 99. 8% at 1.05 11m. Benzyl alcohol is used at the interfaces to reduce reflection and scattering losses.

duce the loss at the interface between crystal and mir­ror, the interface was immersed in benzyl alcohol (n = 1. 54). In order to guarlJ.ntee good contact between the mirrors and the disk, we arranged the laser axis in vertical orientation (90 0 rotated from that shown in Fig. 2). The top mirror was merely floating on a liquid film. The weight of the top mirror ensured that the thicknesses of the liquid films were kept to a minimum. No mirror adjustments were made for such a laser array 0 For SimpliCity only one of the hollow quartz fibers is shown in Fig. 2.

One penta phosphate laser at a time in the array was longitudinally pumped at room temperature with the

o

I

(a)

o

I

l---(b)

FIG. 3. Time behavior of output intenSity lof fiber laser. (a) Pump beam is chopped. Large, irregular relaxation spikes appear. Time scale 5 msec/cm. (b) cw pump is used. Time scale 0.1 sec/cm. The upper trace is the baseline, slightly modulated by 60-Hz electronic pickup.

693 Appl. Phys. Lett., Vol. 26, No. 12, 15 June 1975

752. 5-nm beam of a Kr+ ion laser which was focused through a 10-mm -focal-length lens. For this laser, which represented our first successful approach to the construction of a fiber laser, we measured a threshold of less than 10 mW of absorbed power for cw operation. This value does not far exceed the best reported for a bulk laser of a pentaphosphate, 9 and further optimization seems possible in our case.

The gain per pass supplied by the Nd3+ ions is given approximately by

G = exp(Nal),

where N is the number of pumped Nd3 + ions per unit volume, a is the effective emission cross section, 10

and 1 is the active length. N at threshold is given by

N= WTTxp/hcal,

(1)

(2)

where W T is the absorbed -power at threshold, T is the lifetime of the 4F3/Z state, Xp is the pump wavelength, and a is the cross-sectional area of the fiber. For our measured absorbed power Eqs. (1) and (2) yield a gain of 1.9 per pass at threshold and hence imply nearly 50% loss per pass 0 This loss probably results predominantly from scatter due to an imperfect interface between the fiber and the Q! -cyanoacrylate cement. It is not unrea­sonable to antiCipate a reduction of this loss to 1% per pass which would result in a threshold of 160 /lW of ab­sorbed power for the present geometry.

The confocal parameter for the crystal diameter of 12 /lm is 0.23 mm, which is only one-third the distance between the mirrors (Fresnel number 0.05). This short confocal parameter would result in a diffraction loss of about 90% per pass in the absence of waveguiding and would far exceed the calculated gain at threshold. Thus the waveguiding effect of the crystalline fiber is an essential feature of operation of this laser.

Figure 3(a) shows the output intensity ot the laser when the pump beam is chopped (time scale 5 msec/

en I-iE ::::>

4

a:i 3 co:: c

>­I-

~2 .... I-~ co:: .... en c .....

o

-If-­RES

1.054 1.052 1.05 1.048 WAVELENGTH IJLml

FIG. 4. Emission spectrum of fiber laser. Spectra for chopped and cw pump beam are identical.

Weber et al. 693

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cm), Relaxation -type oscillations are clearly evidenL Figure 3(b) shows the output for cw pumping with the same detector time resolution but on a time scale of 0,1 sec/cm. The intensity fluctuations of approximately 10% in output are due to mechanical instabilities in the ex­perimental apparatus.

From the mirror separation d == O. 68 mm one calcu­lates a longitudinal mode spacing of .Q.X == 5.0 A. Figure 4 shows the output spectrum as measured with a grating spectrometer, which confirms this value, The refrac­tive indices of the pentaphosphate waveguide core and the polymer cladding are 1. 62 and 1,48, respectively, and hence the approximately 12-l1m-diam core can support up to the 14th-order transverse mode. Most likely the laser oscillates in the lower-order transverse modes because they have lower loss.

In conclusion, we have constructed a cw Ndo.5Lao.5P5014 waveguide laser with crystals 12x12 11m in cross section and 0.68 mm in length with external mirrors. In this first attempt, threshold was less than 10 mW of absorbed power; however, with optimized parameters the threshold should be drastically reduced.

It is a pleasure to acknowledge the skillful help of

F. A. Dunn who expertly polished, handled, and mounted the samples, and of G. D. Aumiller, who designed the mounting apparatus for the crystals. We also would like to thank J, Stone for helpful discussions concerning problems inherent to fiber lasers.

*Present address: Materials Physics Division, AERE, Harwell, Didcot, Oxfordshire, OXll ORA, England.

IFor a review and list of references, see, e.g., B.C. Tofield, H.P, Weber, T.C. Damen, and P.F. Liao, J. Solid State Chern. 12, 207 (1975).

2H. p. Weber, B. C. Tofield, IEEE J. Quantum Electron. (to be published).

3B.C. Tofield, H.P. Weber, T.C. Damen, andG.A. Pasteur, Mater. Res. Bull. 9, 435 (1974).

4S. Singh, D.C. Miller, J.R. Potopowics, and L.K. Shick, J. Appl. Phys. 46, 1191 (1975).

5J. Stone and C.A. Burrus, Appl. Opt. 13, 1256 (1974). BC.A. Burrus and J. Stone, Appl. Phys. Lett. 26, 318

(1975). 7p. M. Bridenbaugh and B. C. Tofield (unpublished). BAron Alpha® #202, Vigor Comp., Div, of B. Jadow and Sons, Inc., New York, N. Y. 10010.

9G. Huber, J.P. Jeser, W.W. Kriihler, and H.G. Daniel­meyer, IEEE J. Quantum Electron. QE-10, 766 (1974).

tOJf.P. Weber, P.F. Liao, and B.C. Tofield, IEEE J. Quan­tum Electron. QE-10, 563 (1974).

Far-infrared reflectivity studies of dielectric function of a Pb­Sn-Te epitaxial film*

W. E. Tennant and J. A. Cape

Science Center, Rockwell International, Thousand Oaks, California 91360 (Received 11 December 1974; in final form 10 March 1975)

Far-infrared reflectivity measurements from "'50 to "'250 cm- l on a Pbo.s2SIloIsTe epitaxial film (P'" 1016

cm- 3) on a high-carrier-density substrate (P'" lOIS cm- 3

) of similar composition showed interference oscillations. These detailed reflectivity data were fitted using the standard two-oscillator model with a spatially variable plasma frequency. The fit determines the phonon frequencies, electron and phonon linewidths, high­frequency dielectric constant, and, using existing effective-mass data, the spatial profile of the free-carrier density.

PACS numbers: 78.65., 71.85.F, 63.20.D

Optimal utilization of layered semiconductor struc­tures (e. g., epitaxial films) for optoelectronic applica­tions depends on a knowledge of the spatial dependence of the optical and electronic properties. We report here the use of far-infrared reflectivity measurements to determine both the frequency dependence and the spatial profile of the dielectric function of a thin epitaxial film ofp-type PbO.S2SnO.1STe on ap· Pbo.sSno.2Te substrate. The profile obtained extends from the front surface

computer. Our detector was a liquid-helium-cooled doped silicon bolometer,

of the film through most of the interface region between film and substrate. The method used to obtain the di­electric function appears promising for the nondestruc­tive study of a variety of layered semiconductor materials.

At frequencies higher than the highest longitudinal

>­I-

> 1.0 I-U W ~ 0.5 w a:::

100 150 200 250 w (cm- I)

Room -temperature reflectivity spectra in the region 50-250 cm-1 at near-normal incidence were obtained using a Beckman far-infrared Fourier-transform spectrophotometer connected on line to a PDP-9 digital

FIG. 1. Reflectivity data (points) and theoretical fit (solid curve) using the dielectric function of Eq. (1) and a spatially dependent plasma frequency given by Eq. (3). The oscillations arise from interference between the surface and the partially reflecting interface region.

694 Applied Physics Letters, Vol. 26, No. 12, 15 June 1975 Copyright © 1975 American Institute of Physics 694

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