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Attenuation characteristics of a planar dielectric coated metallic waveguide for 10.6 μm radiation
Volume
5 8, number
5
OPTICS COMMUNICATIONS
ATTENUATION CHARACTERISTICS OF A PLANAR METALLIC WAVEGUIDE FOR 10.6 pm RADIATION
1 July 1986
DIELECTRIC
COATED
J. GOMBERT
and M. GAZARD
THOMSON-CSF,
Laboratoire Central de Recherches, Domaine de Corbeville, B.P. No. IO, 91401 Orsay Cedex, France
Received
7 February
1986; revised manuscript
received
7 April 1986
Flexible planar aluminum waveguides coated with various thicknesses of dielectric layer (CaF,) have been realized for a 10.6 pm application. The coupling efficiency between a gaussian CO, laser beam and the guides has been discussed: multimode or single mode excitation can be obtained,depending on the value of the ratio q between the laser waist and guide width. Transmissions and attenuations of these guides have been measured confirming the theoretical tendency of a dielectric layer varying from 0 to an optical thickness of X/4 to reduce TM mode losses while increasing TE mode losses.
1. Introduction
The use of flexible metallic hollow waveguides to transmit IR radiation has been described earlier [ 1,2]. Realization of such waveguides has been completed by Garmire [3] who demonstrated with a rectangular (0.5 X 10 mm) gold plated guide that it was possible to transmit 960 W power with a loss of 0.18 dB/m [4]. Recently Miyagi [5,6] reported the fabrication of a cylindrical dielectric-coated metallic hollow waveguide. The dielectric layer (germanium) improved the attenuation of the metallic guide by a factor of 5 (nickel waveguide: 1.5 dB/m; germanium-nickel waveguide 0.34 dB/m). The purpose of this letter is to present our first experimental results describing the evolution of the attenuations of TE and TM modes in 2-D planar metallic waveguides coated with a dielectric when the optical thickness of the dielectric varies from 0 to h/4.
(1
xK%allX sin[26(af
(n2 t K2.q
- 1)“2,0koT-
I+$&
CP =2
4(af - l)n2
l-
1 -1 I
GTE]
,
I22
112
+ql
(
xaizaIi 1+
1 l--
44
Cz; - l(n2 t ,2)2
, - 1)1/2nokOT-
+,,I
2. Theory
where J/TE and $TM are defined by
Using the calculations of Miyagi for a two dimensional slab waveguide, we computed the attenuation of a planar aluminum waveguide coated with a variable thickness of calcium fluoride. According to ref. [7 ] the attenuation constant aTE and aTM of the TE and TM modes can be written as
J/TE = tan-l
Science Publishers Division)
$TM
B.V.
I -1
X sin[26($
0 030-4018/86/$03.50 0 Elsevier (North-Holland Physics Publishing
1’2
=
1
tan-l
307
Volume 58, number 5
1 July 1986
OPTICS COMMUNICATIONS
3. Experimental
dB/m
3.1. Fabrication of the guide
10-4,
Two strips of a plastic material (polycarbonate) were metallized with 200 nm evaporated aluminum under a vacuum of 2 X low6 Torr. Films of calcium fluoride were evaporated on the metallized strips with a thickness uniformity of f 5%. Material homogeneity was controlled by X-rays, imaging a sample with an electron microprobe. The two metallized strips were than placed face to face and separated with a polycarbonate wedge as shown on fig. 2.
I
0
1
0.5
3.2. Measurement
P Fig. 1. Attenuations of several TE and TM modes in a CaF2coated aluminum slab waveguide as a function of CaF2 thickness -[6T=p(h/4)(n: - 1)-1/2].
TE ‘metal QTM I metal
= nOkO
Hi
(n,k,T)3
n/(n2
t K2)
’ I IZ
n - jK : complex refractive index of the metal, al : refractive index of the dielectric (al = 1.28 at 10.6 pm for CaF,), 2T: the hollow core width, n0 N 1: refractive index of the air, k, = 27r/h, u. = i(p + 1)~ (p = 0, 1,2.. .), 6 T: dielectric thickness. The value of the complex refractive index of aluminum has been extrapolated from the data given in ref. [8] : F = 27.5 - j 93.7. Fig. 1 shows the theoretical attenuations of TE and TM modes versus the optical thickness 6 T = t = p(X/4) (a: - 1)-1/2 (p varying from 0 to 1). As expected, for t = 0 (purely metallic guide) the low order TE modes have low attenuations whereas respective TM modes have high attenuations. When the optical thickness of the dielectric is increased from t = 0 to t = (h/4)@: 1)-112 (p = 0 top = l), the attenuations of TE modes increase to a maximum for p = 1, while in the same time the attenuations of TM modes decrease to reach a minimum for p = 1. Nevertheless the attenuations of TM modes for p = 1 remain higher than these of respective TE modes for p = 0.
set-up
The set-up for the guides optical properties measure. ments is shown in fig. 3. By changing the distance between L, and M2 the beam width was allowed to vary. For the transmission measurements the launching guide G.A. was removed. 3.3. Coupling The theoretical coupling efficiency of a gaussian laser beam into a slab waveguide was calculated as a function of the ratio of the beam width w. divided by the waveguide width T: q = we/T. The results depend on the nature of the waveguide walls. In a waveguide with metallic walls the coupling efficiency into TE, mode is maximum for 77= 0.7 and reaches 98%, as it is shown in fig. 4a. The optimum
.
20 mm
c
I
2 T=lmm
ro~ycar~onare
Fig. 2. Guide structure.
308
wedge
OPTICS COMMUNICATIONS
Volume 5 8, number 5
“‘Y LC
S /
M2
/
m
P
F
Fig. 3. Guide measurement set-up. Mr : mirror, Mz: cylindrical mirror, S: beam splitter, h/2: h/2 wave plate, F: slit, G.A.: launching guide, G: guide, P: polarizor, Lc: cylindrical lens, D: detector.
loo
%Coupling 000
. -A\.
/
.d=, mln
,/
1 July 1986
input coupling into TM0 mode is lower. For a given value of 1) the coupling efficiency into TE, or TM, mode decreases when the mode order p increases. In a guide with dielectric coated metallic walls, for a given value of n, the coupling efficiencies into TE, and TM, modes are the same (fig. 4b) and decrease when p increases. An optimum coupling efficiency of 98% into TE, or TM, mode is calculated for 77= 0.7. The coupling efficiency has been experimentally determined as a function of 71in the case of a metal waveguide, by studying the beam at the output of a short aluminum guide (8 cm long), in which the losses are mainly due, for low order modes, to the coupling losses. A far field pattern of the modes was displayed on a scope using a scanning mirror; a lock in amplifier and a cooled Hg CdTe detector allowed to visualize the multimode or monomode propagation. According to our calculations, for Q < 0.7 the laser beam of a few hundred milliwatts coupled into the first order modes and the measured output power indicated that all power entered the guide. The measured power efficiencies are represented by points on fig. 4a. For n = 0.7 a single mode coupling into TE, was obtained. 3.4. Transmission and attenuation of the guides
(a)
lorpr
50\ 0
rl
1
1.5
(b)
Fig. 4. Theoretical coupling efficiency of a guassian beam into (a) a metallic waveguide. The points are experimental data obtained for several guide widths, (b) a dielectric coated metallic waveguide.
The transmissions of the fundamental modes TEn and TM, in aluminum guides 1.2 mm width, coated with variable thicknesses of calcium fluoride, have been measured assuming that the coupling loss was the same in all cases -which is consistent with the results given before. A far field pattern of the guide output was recorded and fitted to a cosine square curve. When the fit was obtained a good alignment and a monomode propagation was assumed. 17was equal to 0.7 in all the following experiments. The transmitted powers have been normalized and plotted versus the CaF, thickness (fig. 5). The results are consistent with the theoretical predictions. TEn and TM, modes have the same attenuation for a thickness of CaF, of 2.5 pm (2.35 pm are expected from fig. 1). The value of the dielectric thickness for p = 1 is also slightly higher when determined experimentally. This small shift may be due to the large uncertainty on the value of the CaF, refractive index in the 10.6 pm region. The attenuations of Teu and TM, modes of two of former guides have been measured by placing the guide 309
OPTICS COMMUNICATIONS
Volume 58, number 5
I
I
1
2
thickness
3
~
1 July 1986
which an aluminum layer was evaporated. A guide with a section of 1 X 20 mm allows only a TE propagation (o,E = 0.8 dB/m). The deposition of a dielectric layer (CaF2) onto the metal reduces the loss of TM modes while increasing TE modes attenuation. The first measured attenuations (2 dB/m for TE, and 3 dB/m for TM,) even higher than predicted show clearly the tendency of a dielectric layer to reduce TM modes loss. Further experiments are underway to optimize the guide structure and the deposition in order to decrease the attenuations of TE and TM modes.
4
pm
Acknowledgements
Fig. 5. Transmitted power of the guides versus CaFz thickness.
after a 30 cm long launching guide. The far field determination was used to align the guides. Coupling between the two guides was assumed to be 100%. The power Is, after the two guides and Is1 after the launching guide were measured and the attenuation calculated. Attenuations of 0.8 dB/m and 9 dB/m were measured respectively for TE, and TM,, in an aluminum waveguide without dielectric. Coating the metallic walls with 1.5 pm of CaF, increased the loss of TE, mode to 2 dB/m but reduced the attenuation of TM,, to 3 dB/m. In both cases the differences between the measured values and the theoretical values of losses may be explained by the guide structure: the two strips of polycarbonate glued on the wedges were not parallel and as a result microbending occurred, leading to a “mild regime” bend propagation [lo] that increased the loss.
4. Conclusion Flexible planar metallic waveguides have been designed and realized using a polymeric substrate on
310
This work was supported by the Direction des Recherches, Etudes et Techniques. The authors wish to thank P. Terlinden for her useful collaboration, P. Kayoun and M. Sella (CNRS Meudon) for helpful1 discussions.
References [II E.A.J. Marcatili and T.A. Schmeltzer, Bell. Syst. Tech. .I. 43 (1964) 1783. PI C.C. Eaglesfield, IEEE paper no. 3748 E (1962) p. 26. [31 E. Garmire, T. McMahon and M. Bass, Appl. Phys. Lett. 31 (1977) 92. [41 E. Garmire, Proc. SPIE, Vol. 484, May, Arlington, ed. P. Klocek p. 112-116. iSI M. Miyagi, A. Hongo, Y. Aizawa and S. Kawakami, Appl. Phys. Lett. 43 (1983) 430. [61 M. Miyagi, K. Harada, Y. Aizawa and S. Kawakami, Proc. SPIE, Vol. 484, May, Arlington, ed. P. Klocek, p. 117123. [71 M. Miyagi, A. Hongo and S. Kawakami, IEEE J. Quant. Electron QE 19 (1983) 136. [81 M.A. Ordal, L.L. Long, R.J. Bell, S.E. Bell, R.R. Bell, R.W. Alexander, Jr and C.A. Ward, Appl. Optics 22 (1983) 1099; values of E. Shiles et al., Phys. Rev. B 22 (1980) 1612. [91 R. Terlinden, These de 38me Cycle, Universite de ParisSud, F&rier 1985. 1101 H. Krammer, Appl. Optics 16 (1977) 2163.
5 8, number
5
OPTICS COMMUNICATIONS
ATTENUATION CHARACTERISTICS OF A PLANAR METALLIC WAVEGUIDE FOR 10.6 pm RADIATION
1 July 1986
DIELECTRIC
COATED
J. GOMBERT
and M. GAZARD
THOMSON-CSF,
Laboratoire Central de Recherches, Domaine de Corbeville, B.P. No. IO, 91401 Orsay Cedex, France
Received
7 February
1986; revised manuscript
received
7 April 1986
Flexible planar aluminum waveguides coated with various thicknesses of dielectric layer (CaF,) have been realized for a 10.6 pm application. The coupling efficiency between a gaussian CO, laser beam and the guides has been discussed: multimode or single mode excitation can be obtained,depending on the value of the ratio q between the laser waist and guide width. Transmissions and attenuations of these guides have been measured confirming the theoretical tendency of a dielectric layer varying from 0 to an optical thickness of X/4 to reduce TM mode losses while increasing TE mode losses.
1. Introduction
The use of flexible metallic hollow waveguides to transmit IR radiation has been described earlier [ 1,2]. Realization of such waveguides has been completed by Garmire [3] who demonstrated with a rectangular (0.5 X 10 mm) gold plated guide that it was possible to transmit 960 W power with a loss of 0.18 dB/m [4]. Recently Miyagi [5,6] reported the fabrication of a cylindrical dielectric-coated metallic hollow waveguide. The dielectric layer (germanium) improved the attenuation of the metallic guide by a factor of 5 (nickel waveguide: 1.5 dB/m; germanium-nickel waveguide 0.34 dB/m). The purpose of this letter is to present our first experimental results describing the evolution of the attenuations of TE and TM modes in 2-D planar metallic waveguides coated with a dielectric when the optical thickness of the dielectric varies from 0 to h/4.
(1
xK%allX sin[26(af
(n2 t K2.q
- 1)“2,0koT-
I+$&
CP =2
4(af - l)n2
l-
1 -1 I
GTE]
,
I22
112
+ql
(
xaizaIi 1+
1 l--
44
Cz; - l(n2 t ,2)2
, - 1)1/2nokOT-
+,,I
2. Theory
where J/TE and $TM are defined by
Using the calculations of Miyagi for a two dimensional slab waveguide, we computed the attenuation of a planar aluminum waveguide coated with a variable thickness of calcium fluoride. According to ref. [7 ] the attenuation constant aTE and aTM of the TE and TM modes can be written as
J/TE = tan-l
Science Publishers Division)
$TM
B.V.
I -1
X sin[26($
0 030-4018/86/$03.50 0 Elsevier (North-Holland Physics Publishing
1’2
=
1
tan-l
307
Volume 58, number 5
1 July 1986
OPTICS COMMUNICATIONS
3. Experimental
dB/m
3.1. Fabrication of the guide
10-4,
Two strips of a plastic material (polycarbonate) were metallized with 200 nm evaporated aluminum under a vacuum of 2 X low6 Torr. Films of calcium fluoride were evaporated on the metallized strips with a thickness uniformity of f 5%. Material homogeneity was controlled by X-rays, imaging a sample with an electron microprobe. The two metallized strips were than placed face to face and separated with a polycarbonate wedge as shown on fig. 2.
I
0
1
0.5
3.2. Measurement
P Fig. 1. Attenuations of several TE and TM modes in a CaF2coated aluminum slab waveguide as a function of CaF2 thickness -[6T=p(h/4)(n: - 1)-1/2].
TE ‘metal QTM I metal
= nOkO
Hi
(n,k,T)3
n/(n2
t K2)
’ I IZ
n - jK : complex refractive index of the metal, al : refractive index of the dielectric (al = 1.28 at 10.6 pm for CaF,), 2T: the hollow core width, n0 N 1: refractive index of the air, k, = 27r/h, u. = i(p + 1)~ (p = 0, 1,2.. .), 6 T: dielectric thickness. The value of the complex refractive index of aluminum has been extrapolated from the data given in ref. [8] : F = 27.5 - j 93.7. Fig. 1 shows the theoretical attenuations of TE and TM modes versus the optical thickness 6 T = t = p(X/4) (a: - 1)-1/2 (p varying from 0 to 1). As expected, for t = 0 (purely metallic guide) the low order TE modes have low attenuations whereas respective TM modes have high attenuations. When the optical thickness of the dielectric is increased from t = 0 to t = (h/4)@: 1)-112 (p = 0 top = l), the attenuations of TE modes increase to a maximum for p = 1, while in the same time the attenuations of TM modes decrease to reach a minimum for p = 1. Nevertheless the attenuations of TM modes for p = 1 remain higher than these of respective TE modes for p = 0.
set-up
The set-up for the guides optical properties measure. ments is shown in fig. 3. By changing the distance between L, and M2 the beam width was allowed to vary. For the transmission measurements the launching guide G.A. was removed. 3.3. Coupling The theoretical coupling efficiency of a gaussian laser beam into a slab waveguide was calculated as a function of the ratio of the beam width w. divided by the waveguide width T: q = we/T. The results depend on the nature of the waveguide walls. In a waveguide with metallic walls the coupling efficiency into TE, mode is maximum for 77= 0.7 and reaches 98%, as it is shown in fig. 4a. The optimum
.
20 mm
c
I
2 T=lmm
ro~ycar~onare
Fig. 2. Guide structure.
308
wedge
OPTICS COMMUNICATIONS
Volume 5 8, number 5
“‘Y LC
S /
M2
/
m
P
F
Fig. 3. Guide measurement set-up. Mr : mirror, Mz: cylindrical mirror, S: beam splitter, h/2: h/2 wave plate, F: slit, G.A.: launching guide, G: guide, P: polarizor, Lc: cylindrical lens, D: detector.
loo
%Coupling 000
. -A\.
/
.d=, mln
,/
1 July 1986
input coupling into TM0 mode is lower. For a given value of 1) the coupling efficiency into TE, or TM, mode decreases when the mode order p increases. In a guide with dielectric coated metallic walls, for a given value of n, the coupling efficiencies into TE, and TM, modes are the same (fig. 4b) and decrease when p increases. An optimum coupling efficiency of 98% into TE, or TM, mode is calculated for 77= 0.7. The coupling efficiency has been experimentally determined as a function of 71in the case of a metal waveguide, by studying the beam at the output of a short aluminum guide (8 cm long), in which the losses are mainly due, for low order modes, to the coupling losses. A far field pattern of the modes was displayed on a scope using a scanning mirror; a lock in amplifier and a cooled Hg CdTe detector allowed to visualize the multimode or monomode propagation. According to our calculations, for Q < 0.7 the laser beam of a few hundred milliwatts coupled into the first order modes and the measured output power indicated that all power entered the guide. The measured power efficiencies are represented by points on fig. 4a. For n = 0.7 a single mode coupling into TE, was obtained. 3.4. Transmission and attenuation of the guides
(a)
lorpr
50\ 0
rl
1
1.5
(b)
Fig. 4. Theoretical coupling efficiency of a guassian beam into (a) a metallic waveguide. The points are experimental data obtained for several guide widths, (b) a dielectric coated metallic waveguide.
The transmissions of the fundamental modes TEn and TM, in aluminum guides 1.2 mm width, coated with variable thicknesses of calcium fluoride, have been measured assuming that the coupling loss was the same in all cases -which is consistent with the results given before. A far field pattern of the guide output was recorded and fitted to a cosine square curve. When the fit was obtained a good alignment and a monomode propagation was assumed. 17was equal to 0.7 in all the following experiments. The transmitted powers have been normalized and plotted versus the CaF, thickness (fig. 5). The results are consistent with the theoretical predictions. TEn and TM, modes have the same attenuation for a thickness of CaF, of 2.5 pm (2.35 pm are expected from fig. 1). The value of the dielectric thickness for p = 1 is also slightly higher when determined experimentally. This small shift may be due to the large uncertainty on the value of the CaF, refractive index in the 10.6 pm region. The attenuations of Teu and TM, modes of two of former guides have been measured by placing the guide 309
OPTICS COMMUNICATIONS
Volume 58, number 5
I
I
1
2
thickness
3
~
1 July 1986
which an aluminum layer was evaporated. A guide with a section of 1 X 20 mm allows only a TE propagation (o,E = 0.8 dB/m). The deposition of a dielectric layer (CaF2) onto the metal reduces the loss of TM modes while increasing TE modes attenuation. The first measured attenuations (2 dB/m for TE, and 3 dB/m for TM,) even higher than predicted show clearly the tendency of a dielectric layer to reduce TM modes loss. Further experiments are underway to optimize the guide structure and the deposition in order to decrease the attenuations of TE and TM modes.
4
pm
Acknowledgements
Fig. 5. Transmitted power of the guides versus CaFz thickness.
after a 30 cm long launching guide. The far field determination was used to align the guides. Coupling between the two guides was assumed to be 100%. The power Is, after the two guides and Is1 after the launching guide were measured and the attenuation calculated. Attenuations of 0.8 dB/m and 9 dB/m were measured respectively for TE, and TM,, in an aluminum waveguide without dielectric. Coating the metallic walls with 1.5 pm of CaF, increased the loss of TE, mode to 2 dB/m but reduced the attenuation of TM,, to 3 dB/m. In both cases the differences between the measured values and the theoretical values of losses may be explained by the guide structure: the two strips of polycarbonate glued on the wedges were not parallel and as a result microbending occurred, leading to a “mild regime” bend propagation [lo] that increased the loss.
4. Conclusion Flexible planar metallic waveguides have been designed and realized using a polymeric substrate on
310
This work was supported by the Direction des Recherches, Etudes et Techniques. The authors wish to thank P. Terlinden for her useful collaboration, P. Kayoun and M. Sella (CNRS Meudon) for helpful1 discussions.
References [II E.A.J. Marcatili and T.A. Schmeltzer, Bell. Syst. Tech. .I. 43 (1964) 1783. PI C.C. Eaglesfield, IEEE paper no. 3748 E (1962) p. 26. [31 E. Garmire, T. McMahon and M. Bass, Appl. Phys. Lett. 31 (1977) 92. [41 E. Garmire, Proc. SPIE, Vol. 484, May, Arlington, ed. P. Klocek p. 112-116. iSI M. Miyagi, A. Hongo, Y. Aizawa and S. Kawakami, Appl. Phys. Lett. 43 (1983) 430. [61 M. Miyagi, K. Harada, Y. Aizawa and S. Kawakami, Proc. SPIE, Vol. 484, May, Arlington, ed. P. Klocek, p. 117123. [71 M. Miyagi, A. Hongo and S. Kawakami, IEEE J. Quant. Electron QE 19 (1983) 136. [81 M.A. Ordal, L.L. Long, R.J. Bell, S.E. Bell, R.R. Bell, R.W. Alexander, Jr and C.A. Ward, Appl. Optics 22 (1983) 1099; values of E. Shiles et al., Phys. Rev. B 22 (1980) 1612. [91 R. Terlinden, These de 38me Cycle, Universite de ParisSud, F&rier 1985. 1101 H. Krammer, Appl. Optics 16 (1977) 2163.