- Email: [email protected]
Resonant array input couplers for optically pumped FIR lasers
I~@aredPhys. Technot.Vol. 35. No. 7, pp. 859%%2,1994
Pergamon
1350-4495(94)ooo31-x
RESONANT
Copyright c 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved I350-4495/94 $7.00 + 0.00
ARRAY INPUT COUPLERS FOR OPTICALLY PUMPED FIR LASERS
P. J. KING,’ B. W. JAMES,’ W. WRIGHT*~~ and L. B. WHITBOUR~~ ‘School of Physics, University of Sydney, NSW 2006, *CSIRO Division of Applied Physics and ‘CSIRO Division of Exploration Geoscience, Lindfield, NSW 2070, Australia (Received 19 May 1994) Abstract-An input coupler consisting of an array of conducting rings on a ZnSe substrate has been used in an optically pumped formic acid laser operating at 433 pm. By using a uniform pump beam ofoptimized diameter a higher conversion efficiency than obtainable with hole input coupling has been achieved.
I.
INTRODUCTION
In the most common design of an optically pumped far infrared (FIR) laser, the pump radiation from a CO, laser is coupled into a FIR waveguide resonator by focusing it through a small hole in one of the resonator mirrors. The diameter of the hole is usually sufficiently small that FIR losses due to the hole are negligible. The laser output is extracted at the other end of the resonator by means of a partially reflecting FIR mirror, which preferably should have a high reflectance at the CO, laser wavelength so that any unabsorbed pump radiation is reflected back along the resonator for further absorption. Hole input coupling has the disadvantage that the diverging pump beam leads to incomplete and very non-uniform pumping of the FIR mode volume as well as loss of pump radiation if the beam intercepts the walls of the waveguide resonator. Occasionally other pumping geometries have been tried. One scheme admits the CO, pump beam through a Brewster angle window which also serves as a FIR mirror, bending the FIR axis away from the pumping axis. This method has been used for both pulsed oscillators”) and superradiant lasers’*) where increased FIR losses due to poor reflection from the window can be tolerated. Another superradiant design U) has used a Brewster window coated on the inside with a reflecting coating to reflect the CO, pump beam, which enters via a side window, along the axis of the FIR waveguide to a fully reflecting mirror; at the same time the Brewster window provides high transmission for the FIR superradiant output. A similar design has also been used successfully for a cw FIR 1aser.‘4’In this case the angle of incidence of the CO, laser radiation on the intracavity Brewster window was chosen to exploit reststrahlen reflection from the quartz. However, due to FIR losses in the Brewster window, this laser produced lower output power in comparison with the standard hole input coupling technique. Yet another desig#’ uses a grating as a dichroic mirror: the FIR radiation reflects specularly from the grating while the CO2 pump beam enters the resonator by diffraction into the first order. In this paper we describe a new design of input coupler which allows pumping of the FIR resonator with a pump beam of constant cross-section. The coupler transmits a CO, laser beam of arbitrary diameter, allowing the overlap of the pump and FIR beams in the FIR resonator to be optimized. At the same time, due to its high reflectance at the laser wavelength, the coupler functions as a mirror for the FIR resonator.
tPresent address: School of Electrical Engineering, Cornell University, Ithaca. NY 14853, U.S.A. 859
II.
THE
INPllT
COUPLER
The input coupler consists of a periodic array of circular or square conducting rings supported on a ZnSe substrate which is backed by an antireflection coating for the pump radiation. Examples of array patterns are shown in Fig. I. The ZnSe substrate is transparent at CO, laser wavelengths and the scale of the conducting rings is such that they exhibit resonant reflectance at the FIR laser line, while their very small fractional cross-section causes little attenuation of the pump radiation. Such input couplers are characterized reflectance at the FIR laser wavelength.
by high transparency at the pump wavelength This structure is the complement of the resonant as a FIR laser output
coupler.“’
and high slot array
structure
which has been used very successfully
combines allowing
high reflectance at CO, laser wavelengths with resonant transmission at FIR wavelengths. the desired FIR transmittance to be obtained by appropriate choice of dimensions.
The latter structure
Although a rigorous theory for annular slot arrays. based on modal analysis, has been published.‘q’ a comparable analysis for arrays of conducting rings is not available. Babinet’s Principle. which relates the electromagnetic interaction with a screen to that of its complementary structure cannot be used in this case as the presence of the supporting dielectric substrate destroys the symmetry needed for the application of Babinet’s Principle. In the limit of wavelengths much smaller than the size of the rings the transmittance of the array is determined by the fraction of the area which is uncoated. For wavelengths close to the
0000
Resonant
array
input couplers
loo 80
100
1
-
g ;60
-
5 3 40
7
s
-
20
861
0))
JL 0
i
I
0
300
600
900
Frequency
(GHz)
1200
1500
0
300
600 Frequency
!m
1200
1500
(GHz)
Fig. 2. (a) Reflectance and (b) transmittance of one of the input couplers as a function of frequency, measured with a polarizing Michelson interferometer. The coupler consisted of square rings of dimensions (see Fig. 1): H’ = 2.0 pm, d = 73.2pm, I = 50.3 pm. The prominent spikes are due to incomplete cancellation of water vapour lines. The formic acid line of interest is at 693 GHz.
circumferential length of the rings there is resonantly enhanced reflection and hence a minimum in transmission. For longer wavelengths the transmission again increases to high values. Parker et al.@)have shown that annular rings of outer dia 3.0 mm and width 0.35 mm exhibit such bandstop transmission behaviour at frequencies in the vicinity of 20 GHz (iv = 15 mm). With appropriate scaling to produce high reflectance at the relevant FIR laser wavelength, such structures might be expected to be useful as input couplers for optically pumped FIR lasers. A prototype input coupler, consisting of rings of average radius 70.5 pm in an array of period 160 pm, was fabricated using conventional photolithographic techniques on a ZnSe substrate with a broadband antireflection coating for 8-12 pm on its other side. The reflectance and transmittance were measured using a polarising Michelson interferometer”’ over a range of wavelengths which included the region of resonant behaviour. The resonance was found to be at 3 10 GHz. Using these results to scale the mask dimensions, several input couplers were fabricated with predicted reflection maxima close to the formic acid laser line of 433 pm. An example of the interferometric measurements is shown in Fig. 2. To select the coupler with the highest reflectance at 433 pm for testing in a laser, their reflectances were measured at 433 pm using a formic acid laser as the radiation source. The parameters of the selected coupler are given in Table 1.
III.
RESULTS
The input coupler was tested by using it to replace the hole input coupler in a formic acid laser. A CO, laser pump beam of relatively constant diameter along the length of the FIR waveguide was used. By passing the beam through a telescope its diameter was optimized for maximum FIR output power in the EH,, mode of the 433 pm line. For a pump power of 32 W, the FIR output power in the EH,, mode was 15 mW. This was comparable to, but slightly lower than, the 18 mW obtained using the same pump power with hole input coupling. FIR powers were measured with a Scientech 362 calorimeter and the readings corrected for incomplete absorption by the calorimeter element.““’ As the reflectance of the input coupler at 433 /lrn was 89%, the FIR loss at this end Table I. Parameters for the coupler tested with the HCOOH at 433 brn Average radius Width Array period Reflectance at 433 iirn
input laser
31.0/11 2.0 I’m I IO~~m x9 & 5”/u
862
P. J. KIHG CI ul
was similar to the transmission
loss of 13% at the output
coupler.
When this is taken into account
it is clear that there is an increased conversion efficiency with the new input coupler the same laser using hole input coupling (0.09% compared with 0.06%).
IV.
compared
with
DISCUSSION
These measurements show that an input coupler which allows a pump beam of constant diameter leads to a significant increase in the conversion efficiency of the lasing medium. In contrast to a hole input coupler, resonator. However
it allows a much better match to the mode volume of the FIR waveguide the resonant array input coupler used to achieve a constant diameter pump
beam in the present
work introduced
additional
losses which resulted
in slightly lower laser output
power than for the same laser with hole input coupling. Nevertheless the use of this coupler has the compensating advantage of eliminating the need for maintaining precise alignment of the pump beam. This may be of considerable importance in applications where the laser cannot accessed. Even in the absence of expansion of the pump beam to its optimum diameter produced output of the order of IO mW for a pump power of 32 W.
be easily the laser
V. CONCLUSION By using an input coupler consisting of arrays of conducting rings on a ZnSe substrate, it has been possible to pump a formic acid FIR laser with a CO, beam of constant cross-section along the length of the FIR waveguide. For optimum diameter of the pump beam the conversion efficiency from pump radiation to FIR radiation within the FIR resonator was approx. 50% greater than for the same laser with hole input coupling. However, due to offsetting losses introduced by the input coupler, the output power was slightly reduced. It can be concluded that a pumping configuration which allows a uniform pump beam diameter offers the potential of considerable improvements in the efficiency of optically pumped FIR lasers. .3ckno~r,led~pmenl.s---This work was supported by a grant from the Australian Research Council. grateful for support from the Science Foundation for Physics within the University of Sydney.
The authors
are also
REFERENCES I. A. Semet and N. C. Luhmann Jr, Appl. P~J’.F. Lerr. 28, 659 (1976). 2. T. K. Plant. L. A. Newman. E. J. Danielewicz. T. A. deTemple and P. D. Coleman. lEEE Trans. Microww Theor?, T~I. MTT-22, 988 (1974). 3. T. K. Plant and T. A. deTemple. J. Appl. Phys. 47, 3042 (1976). 4. P. J. Kmg, B. W. James, I. S. Falconer and L. B. Whitbourn, Irrfiured Phys. 30, 359 (1990). 5. Z. Solajic and J. Heppner, Appi. Phys. B 33, 23 (1984). 6. M. D. Bowden, B. W. James, 1. S. Falconer, D. H. Dawes, P. A. Krug, J. C. Macfarlane. W. Wright and L. B. Whitbourn. Opf. Cormnun. 89, 419 (1992). 7. A. Roberts and R. C. McPhedran, IEEE Trans. Anrennus Propug. AP-36, 607 (1988). 8. E. A. Parker, S. M. A. Hamdy and R. J. Langley. Elecrron. Lett. 17, 880 (1981). 9. J. A. How and L. B. Whitbourn, In{. J. Infrared Millimerer Wut’es 4, 335 (1983). IO. F. B. Foote, D. T. Hodges and H. B. Dyson, Ifr/. J. hfrared Millimefer Waws 2, 773 (1981).
Pergamon
1350-4495(94)ooo31-x
RESONANT
Copyright c 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved I350-4495/94 $7.00 + 0.00
ARRAY INPUT COUPLERS FOR OPTICALLY PUMPED FIR LASERS
P. J. KING,’ B. W. JAMES,’ W. WRIGHT*~~ and L. B. WHITBOUR~~ ‘School of Physics, University of Sydney, NSW 2006, *CSIRO Division of Applied Physics and ‘CSIRO Division of Exploration Geoscience, Lindfield, NSW 2070, Australia (Received 19 May 1994) Abstract-An input coupler consisting of an array of conducting rings on a ZnSe substrate has been used in an optically pumped formic acid laser operating at 433 pm. By using a uniform pump beam ofoptimized diameter a higher conversion efficiency than obtainable with hole input coupling has been achieved.
I.
INTRODUCTION
In the most common design of an optically pumped far infrared (FIR) laser, the pump radiation from a CO, laser is coupled into a FIR waveguide resonator by focusing it through a small hole in one of the resonator mirrors. The diameter of the hole is usually sufficiently small that FIR losses due to the hole are negligible. The laser output is extracted at the other end of the resonator by means of a partially reflecting FIR mirror, which preferably should have a high reflectance at the CO, laser wavelength so that any unabsorbed pump radiation is reflected back along the resonator for further absorption. Hole input coupling has the disadvantage that the diverging pump beam leads to incomplete and very non-uniform pumping of the FIR mode volume as well as loss of pump radiation if the beam intercepts the walls of the waveguide resonator. Occasionally other pumping geometries have been tried. One scheme admits the CO, pump beam through a Brewster angle window which also serves as a FIR mirror, bending the FIR axis away from the pumping axis. This method has been used for both pulsed oscillators”) and superradiant lasers’*) where increased FIR losses due to poor reflection from the window can be tolerated. Another superradiant design U) has used a Brewster window coated on the inside with a reflecting coating to reflect the CO, pump beam, which enters via a side window, along the axis of the FIR waveguide to a fully reflecting mirror; at the same time the Brewster window provides high transmission for the FIR superradiant output. A similar design has also been used successfully for a cw FIR 1aser.‘4’In this case the angle of incidence of the CO, laser radiation on the intracavity Brewster window was chosen to exploit reststrahlen reflection from the quartz. However, due to FIR losses in the Brewster window, this laser produced lower output power in comparison with the standard hole input coupling technique. Yet another desig#’ uses a grating as a dichroic mirror: the FIR radiation reflects specularly from the grating while the CO2 pump beam enters the resonator by diffraction into the first order. In this paper we describe a new design of input coupler which allows pumping of the FIR resonator with a pump beam of constant cross-section. The coupler transmits a CO, laser beam of arbitrary diameter, allowing the overlap of the pump and FIR beams in the FIR resonator to be optimized. At the same time, due to its high reflectance at the laser wavelength, the coupler functions as a mirror for the FIR resonator.
tPresent address: School of Electrical Engineering, Cornell University, Ithaca. NY 14853, U.S.A. 859
II.
THE
INPllT
COUPLER
The input coupler consists of a periodic array of circular or square conducting rings supported on a ZnSe substrate which is backed by an antireflection coating for the pump radiation. Examples of array patterns are shown in Fig. I. The ZnSe substrate is transparent at CO, laser wavelengths and the scale of the conducting rings is such that they exhibit resonant reflectance at the FIR laser line, while their very small fractional cross-section causes little attenuation of the pump radiation. Such input couplers are characterized reflectance at the FIR laser wavelength.
by high transparency at the pump wavelength This structure is the complement of the resonant as a FIR laser output
coupler.“’
and high slot array
structure
which has been used very successfully
combines allowing
high reflectance at CO, laser wavelengths with resonant transmission at FIR wavelengths. the desired FIR transmittance to be obtained by appropriate choice of dimensions.
The latter structure
Although a rigorous theory for annular slot arrays. based on modal analysis, has been published.‘q’ a comparable analysis for arrays of conducting rings is not available. Babinet’s Principle. which relates the electromagnetic interaction with a screen to that of its complementary structure cannot be used in this case as the presence of the supporting dielectric substrate destroys the symmetry needed for the application of Babinet’s Principle. In the limit of wavelengths much smaller than the size of the rings the transmittance of the array is determined by the fraction of the area which is uncoated. For wavelengths close to the
0000
Resonant
array
input couplers
loo 80
100
1
-
g ;60
-
5 3 40
7
s
-
20
861
0))
JL 0
i
I
0
300
600
900
Frequency
(GHz)
1200
1500
0
300
600 Frequency
!m
1200
1500
(GHz)
Fig. 2. (a) Reflectance and (b) transmittance of one of the input couplers as a function of frequency, measured with a polarizing Michelson interferometer. The coupler consisted of square rings of dimensions (see Fig. 1): H’ = 2.0 pm, d = 73.2pm, I = 50.3 pm. The prominent spikes are due to incomplete cancellation of water vapour lines. The formic acid line of interest is at 693 GHz.
circumferential length of the rings there is resonantly enhanced reflection and hence a minimum in transmission. For longer wavelengths the transmission again increases to high values. Parker et al.@)have shown that annular rings of outer dia 3.0 mm and width 0.35 mm exhibit such bandstop transmission behaviour at frequencies in the vicinity of 20 GHz (iv = 15 mm). With appropriate scaling to produce high reflectance at the relevant FIR laser wavelength, such structures might be expected to be useful as input couplers for optically pumped FIR lasers. A prototype input coupler, consisting of rings of average radius 70.5 pm in an array of period 160 pm, was fabricated using conventional photolithographic techniques on a ZnSe substrate with a broadband antireflection coating for 8-12 pm on its other side. The reflectance and transmittance were measured using a polarising Michelson interferometer”’ over a range of wavelengths which included the region of resonant behaviour. The resonance was found to be at 3 10 GHz. Using these results to scale the mask dimensions, several input couplers were fabricated with predicted reflection maxima close to the formic acid laser line of 433 pm. An example of the interferometric measurements is shown in Fig. 2. To select the coupler with the highest reflectance at 433 pm for testing in a laser, their reflectances were measured at 433 pm using a formic acid laser as the radiation source. The parameters of the selected coupler are given in Table 1.
III.
RESULTS
The input coupler was tested by using it to replace the hole input coupler in a formic acid laser. A CO, laser pump beam of relatively constant diameter along the length of the FIR waveguide was used. By passing the beam through a telescope its diameter was optimized for maximum FIR output power in the EH,, mode of the 433 pm line. For a pump power of 32 W, the FIR output power in the EH,, mode was 15 mW. This was comparable to, but slightly lower than, the 18 mW obtained using the same pump power with hole input coupling. FIR powers were measured with a Scientech 362 calorimeter and the readings corrected for incomplete absorption by the calorimeter element.““’ As the reflectance of the input coupler at 433 /lrn was 89%, the FIR loss at this end Table I. Parameters for the coupler tested with the HCOOH at 433 brn Average radius Width Array period Reflectance at 433 iirn
input laser
31.0/11 2.0 I’m I IO~~m x9 & 5”/u
862
P. J. KIHG CI ul
was similar to the transmission
loss of 13% at the output
coupler.
When this is taken into account
it is clear that there is an increased conversion efficiency with the new input coupler the same laser using hole input coupling (0.09% compared with 0.06%).
IV.
compared
with
DISCUSSION
These measurements show that an input coupler which allows a pump beam of constant diameter leads to a significant increase in the conversion efficiency of the lasing medium. In contrast to a hole input coupler, resonator. However
it allows a much better match to the mode volume of the FIR waveguide the resonant array input coupler used to achieve a constant diameter pump
beam in the present
work introduced
additional
losses which resulted
in slightly lower laser output
power than for the same laser with hole input coupling. Nevertheless the use of this coupler has the compensating advantage of eliminating the need for maintaining precise alignment of the pump beam. This may be of considerable importance in applications where the laser cannot accessed. Even in the absence of expansion of the pump beam to its optimum diameter produced output of the order of IO mW for a pump power of 32 W.
be easily the laser
V. CONCLUSION By using an input coupler consisting of arrays of conducting rings on a ZnSe substrate, it has been possible to pump a formic acid FIR laser with a CO, beam of constant cross-section along the length of the FIR waveguide. For optimum diameter of the pump beam the conversion efficiency from pump radiation to FIR radiation within the FIR resonator was approx. 50% greater than for the same laser with hole input coupling. However, due to offsetting losses introduced by the input coupler, the output power was slightly reduced. It can be concluded that a pumping configuration which allows a uniform pump beam diameter offers the potential of considerable improvements in the efficiency of optically pumped FIR lasers. .3ckno~r,led~pmenl.s---This work was supported by a grant from the Australian Research Council. grateful for support from the Science Foundation for Physics within the University of Sydney.
The authors
are also
REFERENCES I. A. Semet and N. C. Luhmann Jr, Appl. P~J’.F. Lerr. 28, 659 (1976). 2. T. K. Plant. L. A. Newman. E. J. Danielewicz. T. A. deTemple and P. D. Coleman. lEEE Trans. Microww Theor?, T~I. MTT-22, 988 (1974). 3. T. K. Plant and T. A. deTemple. J. Appl. Phys. 47, 3042 (1976). 4. P. J. Kmg, B. W. James, I. S. Falconer and L. B. Whitbourn, Irrfiured Phys. 30, 359 (1990). 5. Z. Solajic and J. Heppner, Appi. Phys. B 33, 23 (1984). 6. M. D. Bowden, B. W. James, 1. S. Falconer, D. H. Dawes, P. A. Krug, J. C. Macfarlane. W. Wright and L. B. Whitbourn. Opf. Cormnun. 89, 419 (1992). 7. A. Roberts and R. C. McPhedran, IEEE Trans. Anrennus Propug. AP-36, 607 (1988). 8. E. A. Parker, S. M. A. Hamdy and R. J. Langley. Elecrron. Lett. 17, 880 (1981). 9. J. A. How and L. B. Whitbourn, In{. J. Infrared Millimerer Wut’es 4, 335 (1983). IO. F. B. Foote, D. T. Hodges and H. B. Dyson, Ifr/. J. hfrared Millimefer Waws 2, 773 (1981).