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Hybrid output mirror for optically pumped far infrared lasers
Volume 13, number 4
HYBRID OUTPUT MIRROR FOR OPTICALLY
E.J. DANIELEWICZ,
April 1975
OPTICS COMMUNICATIONS
PUMPED FAR INFRARED
LASERS*
T.K. PLANT, and T.A. DeTEMPLE
Electra-Physics Laboratory,
Department
of Electrical Engineering,
University of Illinois, Urbana, Illinois 61801,
USA
Received 24 January 1975
A hybrid metallic mesh multilayer dielectric coating mirror has been developed for use as an output mirror for optically pumped far infrared lasers. The metallic mesh provides a high reflectance in the far infrared while the multilayer dielectric coating is chosen to provide a maximum reflectivity at the pump wavelength (10 pm). This hybrid mirror has increased the output power of a CHsF waveguide laser at 496 pm by a factor of 350 over that obtained with a hole coupling mirror. In addition, this mirror results in a far infrared output beam which has a minimum angular divergence limited only by the particular oscillating transverse waveguide mode (the EHi r mode for this experiment).
Recently developed optically pumped far infrared (FIR) lasers have produced hundreds of new laser lines in the spectral region from 40-l 800 pm. Most FIR lasers have utilized simple hole output mirrors which have the advantage of low loss in the FIR and operate over a broad spectral range. On the other hand, the output from a hole coupled cavity is highly diverging and lacks a well-defined transverse intensity profile. Furthermore, it is not easy to optimize the laser output by changing the diameter of the coupling hole because the laser mode will distort or switch to a higher order transverse mode which minimizes the loss through the hole [l] . In contrast, metallic mesh mirrors which have been used for HCN lasers at 337 nm have a uniform reflectance and transmittance over the entire mirror [2]. This couples the total mode out of the laser, and by varying the mesh geometry, allows one to change the output coupling and hence optimize the laser output. A disadvantage is that the mesh reflectivity in the infrared is low. Unfortunately, for efficient operation the optically pumped FIR lasers require mirrors which are highly reflecting both in the FIR and at the pump wavelength (typically 10 pm). To illustrate this, one can define a * Research supported by AFOSR Grant 71-1981, NSF Grant GK 4331 and the University of Illinois Industrial Affiliates program.
366
gas absorption efficiency (which should be related to the FIR output) assuming small absorption as cu!/(al t y) where o(, I and y are the gas absorption coefficient, cavity length and infrared exponential cavity loss coefficient per pass, respectively. If the cavity is very lossy in the infrared then y s crl so the absorption efficiency becomes cyl/y < 1. For a small cavity loss the absorption efficiency becomes -1 which indicates that the dominant loss of infrared radiation is the gas absorption which is what is desired. Hence a low-loss in the infrared requires high reflecting mirrors. In this paper we present experimental results on a hybrid output mirror for optically pumped FIR lasers. The mirror is composed of a metallic mesh FIR reflector and multilayer dielectric films for high reflectance in the infrared. This combination utilizes the desirable properties of the metallic mesh mirror while maintaining a high infrared reflectance which is desirable from the FIR laser standpoint. The geometry of the metal mesh-dielectric (MMD) mirror is illustrated in fig. 1. The metal mesh is in the form of an inductive grid deposited on a wedged Si substrate using standard photolithographic techniques. The four layer infrared dielectric mirror is then deposited on top of the mesh-substrate combination resulting in a reflectivity > 98% throughout the CO, laser bands. The FIR reflectivity may be varied by changing the grid parameters or the substrate index of refraction [24] .
Volume 13, number 4
OPTICS COMMUNICATIONS
April 1975
MODE
I
0)
Mi, MODE
bl Si
II
Fig. 1. a) Geometry of inductive grid metal mesh; t - 0.6 I.rm of Ni, 2~ - 10 pm; for gvalues see text. b) Hybrid metallic mesh multilayer dielectric fllm mirror. The MMD mirror was used as the output mirror of a FIR waveguide resonator [5]. The cavity was composed of the MMD mirror located 120 cm from a flat Au coated mirror which had a 1.5 mm diameter hole in it to allow the pump radiation to enter. The waveguide was laboratory grade Pyrex tubing 22 mm ID, 119.5 cm long. The pump radiation from a cw CO2 laser was focused into the FIR cavity with a 30 cm focal length lens. In fig. 2 is shown the FIR output from the waveguide laser as the cavity length was scanned over one free spectral range with CH3F as the lasing gas at 496 pm [6] . For these data a 19.7 squares/mm mesh (- 85% reflectivity) was used for a), a 13 squares/mm mesh (- 73% reflectivity) was used for b), and a flat Au mirror with a 1.5 mm diameter hole (- 98% reflectivity) was used for c). No laser action was observed when a 13 squares/mm mesh (- 6 1% reflectivity) was used. From these observations and an estimate of the waveguide loss, the small signal FIR gain coefficient is bounded between 0.23 m-l < CYFIR < 0.3 m-l. It is evident from these data that a number of transverse waveguide modes can oscillate depending on the specific cavity condition. The pronounced fringes in
cl
*
Fig. 2. FIR output power versus cavity length for different output mirrors; a) g = 5.30 nm, b) g = 76.2 pm, c) Au flat with a 1.5 mm diameter hole. Frequency increases to the left.
a) and b) are spaced by one-half the CO2 pump wavelength and thus represent the cavity resonances in the infrared, These fringes are greatly diminished in c) because the FIR laser is operating further above threshold and is thus less sensitive to small changes in the circulating infrared power. The system did not lase when a 19.7 squares/mm mesh (- 85% reflectivity) mirror without the dielectric coating was used thus confirming our premise that a low infrared cavity loss 367
April 1975
OPTICS COMMUNICATIONS
Volume 13, number 4
waveguide
Fig. 3. Transverse intensity profile of modes I and II of fig. 2a.
is required. At the optimum pressure of 34 mtorr with 7.6 W of CO, pump power, a FIR output power of 3 mW was obtained with the 85% reflecting mesh. This FIR power was a factor of 350 times larger than that obtained with the 1.5 mm diameter hole coupler! In fig. 3 is shown the transverse mode patterns of modes I and II of fig. 2. These data were obtained by scanning the FIR beam with a rotating mirror across a detector-aperture combination located 77 cm from the waveguide. Both modes were found to be linearly polarized perpendicular to the CO, polarization. The scan direction was perpendicular to the FIR direction
of polarization. Mode I is identified as the EHl I mode which has the lowest loss in a dielectric guide [7] . The central dip is thought to be due to the combined effects of the perturbation caused by the coupling hole in the back mirror of the FIR cavity and the near-field diffraction dip discussed by Degnan [8] . The measured divergence half-angle of 24 mr is very close to the theoretical value of 26 mr. Mode II was found to be non-circularly symmetric and to htve the same intensity profile as the LP,, mode [8] . Table 1 summarizes the output characteristics of the waveguide laser with the MMD mirror for two different gases, CH3 F and HCOOH [9] . These data are indicative of the broadband nature of the cavity and the MMD mirror. In conclusion, we note that improvements in the MMD mirror may be made by interchanging the position of the metal mesh and multilayer dielectric films which should reduce the effect of FIR film loss on the FIR laser performance. In addition, an equivalent antireflection coating may be obtained by grinding the Si substrate to an integral multiple of hF1R/2n. Using existing technology, MMD mirrors can be extended to wavelengths perhaps as short as 50 pm where high quality dielectric mirrors are already available. Finally, the MMD mirror has withstood an incident power density of -1 MW/cm2 without damage thus making possible the development of efficient cw and pulsed optically pumped lasers throughout the far infrared. * The LPi 1 mode is oniy allowed when n ide z ncOIe however radial variations in the gain causing $ ens effects may cause this mode to oscillate.
Table 1 Summary of experimental results Mesh mirror
Laser gas
@+/mm)
FIR
Relative
wavelength (rmi
output power”’
line (crml
CO2
pump
19.7 13.0 19.7
CHaF CHaF HCOOH
496.1 496.1 432.5
1.0 0.6 0.49
P(20) 9.6 P(20) 9.6 R(20) 9.6
19.7 19.7
HCOOH HCOOH
419.5 393.6
0.06 0.31
R(22) 9.6 R(18) 9.6
a) Output powers are normalized to the maximum output power of CHsF at h = 496.1 Wm.
368
“,
Volume 13, number 4
OPTICS COMMUNICATIONS
The authors wish to acknowledge the help of Carl Meyer, Delco Electronics Corp., for providing the Si substrates and technical assistance.
References [ 1] G.T. McNice and V.E. Derr, IEEE J. Quantum Electron. QE-5 (1969) 569. [2] R. Ulrich, T.J. Bridges and M.A. Pollack, Appl. Opt. 9 (1970) 2511.
April 1975
[3] R. Uhich, K.F. Renk and L. Genzel, IEEE Trans. Microwave Theory Tech. (1963) 363. [4] V.Y. Balakhanov, Sov. Phys. Doklady 10 (1966) 788. [S] D.T. Hodges and T.S. Hartwick, Appl. Phys. Lett. 23 (1973) 252. [6] T.Y. Chang and T.J. Bridges, Opt. Commun. 1 (1970) 423. [7] E.A.J. Marcatili and R.A. Schmeltzer, BeB System Tech. Jour. 43 (1964) 1783. [8] J.J. Degnan, Appl. Optics 12 (1973) 1026. [9] R.J. Wagner, A.J. Zelano and L.H. Ngai, Opt. Commun. 8 (1973) 46.
369
HYBRID OUTPUT MIRROR FOR OPTICALLY
E.J. DANIELEWICZ,
April 1975
OPTICS COMMUNICATIONS
PUMPED FAR INFRARED
LASERS*
T.K. PLANT, and T.A. DeTEMPLE
Electra-Physics Laboratory,
Department
of Electrical Engineering,
University of Illinois, Urbana, Illinois 61801,
USA
Received 24 January 1975
A hybrid metallic mesh multilayer dielectric coating mirror has been developed for use as an output mirror for optically pumped far infrared lasers. The metallic mesh provides a high reflectance in the far infrared while the multilayer dielectric coating is chosen to provide a maximum reflectivity at the pump wavelength (10 pm). This hybrid mirror has increased the output power of a CHsF waveguide laser at 496 pm by a factor of 350 over that obtained with a hole coupling mirror. In addition, this mirror results in a far infrared output beam which has a minimum angular divergence limited only by the particular oscillating transverse waveguide mode (the EHi r mode for this experiment).
Recently developed optically pumped far infrared (FIR) lasers have produced hundreds of new laser lines in the spectral region from 40-l 800 pm. Most FIR lasers have utilized simple hole output mirrors which have the advantage of low loss in the FIR and operate over a broad spectral range. On the other hand, the output from a hole coupled cavity is highly diverging and lacks a well-defined transverse intensity profile. Furthermore, it is not easy to optimize the laser output by changing the diameter of the coupling hole because the laser mode will distort or switch to a higher order transverse mode which minimizes the loss through the hole [l] . In contrast, metallic mesh mirrors which have been used for HCN lasers at 337 nm have a uniform reflectance and transmittance over the entire mirror [2]. This couples the total mode out of the laser, and by varying the mesh geometry, allows one to change the output coupling and hence optimize the laser output. A disadvantage is that the mesh reflectivity in the infrared is low. Unfortunately, for efficient operation the optically pumped FIR lasers require mirrors which are highly reflecting both in the FIR and at the pump wavelength (typically 10 pm). To illustrate this, one can define a * Research supported by AFOSR Grant 71-1981, NSF Grant GK 4331 and the University of Illinois Industrial Affiliates program.
366
gas absorption efficiency (which should be related to the FIR output) assuming small absorption as cu!/(al t y) where o(, I and y are the gas absorption coefficient, cavity length and infrared exponential cavity loss coefficient per pass, respectively. If the cavity is very lossy in the infrared then y s crl so the absorption efficiency becomes cyl/y < 1. For a small cavity loss the absorption efficiency becomes -1 which indicates that the dominant loss of infrared radiation is the gas absorption which is what is desired. Hence a low-loss in the infrared requires high reflecting mirrors. In this paper we present experimental results on a hybrid output mirror for optically pumped FIR lasers. The mirror is composed of a metallic mesh FIR reflector and multilayer dielectric films for high reflectance in the infrared. This combination utilizes the desirable properties of the metallic mesh mirror while maintaining a high infrared reflectance which is desirable from the FIR laser standpoint. The geometry of the metal mesh-dielectric (MMD) mirror is illustrated in fig. 1. The metal mesh is in the form of an inductive grid deposited on a wedged Si substrate using standard photolithographic techniques. The four layer infrared dielectric mirror is then deposited on top of the mesh-substrate combination resulting in a reflectivity > 98% throughout the CO, laser bands. The FIR reflectivity may be varied by changing the grid parameters or the substrate index of refraction [24] .
Volume 13, number 4
OPTICS COMMUNICATIONS
April 1975
MODE
I
0)
Mi, MODE
bl Si
II
Fig. 1. a) Geometry of inductive grid metal mesh; t - 0.6 I.rm of Ni, 2~ - 10 pm; for gvalues see text. b) Hybrid metallic mesh multilayer dielectric fllm mirror. The MMD mirror was used as the output mirror of a FIR waveguide resonator [5]. The cavity was composed of the MMD mirror located 120 cm from a flat Au coated mirror which had a 1.5 mm diameter hole in it to allow the pump radiation to enter. The waveguide was laboratory grade Pyrex tubing 22 mm ID, 119.5 cm long. The pump radiation from a cw CO2 laser was focused into the FIR cavity with a 30 cm focal length lens. In fig. 2 is shown the FIR output from the waveguide laser as the cavity length was scanned over one free spectral range with CH3F as the lasing gas at 496 pm [6] . For these data a 19.7 squares/mm mesh (- 85% reflectivity) was used for a), a 13 squares/mm mesh (- 73% reflectivity) was used for b), and a flat Au mirror with a 1.5 mm diameter hole (- 98% reflectivity) was used for c). No laser action was observed when a 13 squares/mm mesh (- 6 1% reflectivity) was used. From these observations and an estimate of the waveguide loss, the small signal FIR gain coefficient is bounded between 0.23 m-l < CYFIR < 0.3 m-l. It is evident from these data that a number of transverse waveguide modes can oscillate depending on the specific cavity condition. The pronounced fringes in
cl
*
Fig. 2. FIR output power versus cavity length for different output mirrors; a) g = 5.30 nm, b) g = 76.2 pm, c) Au flat with a 1.5 mm diameter hole. Frequency increases to the left.
a) and b) are spaced by one-half the CO2 pump wavelength and thus represent the cavity resonances in the infrared, These fringes are greatly diminished in c) because the FIR laser is operating further above threshold and is thus less sensitive to small changes in the circulating infrared power. The system did not lase when a 19.7 squares/mm mesh (- 85% reflectivity) mirror without the dielectric coating was used thus confirming our premise that a low infrared cavity loss 367
April 1975
OPTICS COMMUNICATIONS
Volume 13, number 4
waveguide
Fig. 3. Transverse intensity profile of modes I and II of fig. 2a.
is required. At the optimum pressure of 34 mtorr with 7.6 W of CO, pump power, a FIR output power of 3 mW was obtained with the 85% reflecting mesh. This FIR power was a factor of 350 times larger than that obtained with the 1.5 mm diameter hole coupler! In fig. 3 is shown the transverse mode patterns of modes I and II of fig. 2. These data were obtained by scanning the FIR beam with a rotating mirror across a detector-aperture combination located 77 cm from the waveguide. Both modes were found to be linearly polarized perpendicular to the CO, polarization. The scan direction was perpendicular to the FIR direction
of polarization. Mode I is identified as the EHl I mode which has the lowest loss in a dielectric guide [7] . The central dip is thought to be due to the combined effects of the perturbation caused by the coupling hole in the back mirror of the FIR cavity and the near-field diffraction dip discussed by Degnan [8] . The measured divergence half-angle of 24 mr is very close to the theoretical value of 26 mr. Mode II was found to be non-circularly symmetric and to htve the same intensity profile as the LP,, mode [8] . Table 1 summarizes the output characteristics of the waveguide laser with the MMD mirror for two different gases, CH3 F and HCOOH [9] . These data are indicative of the broadband nature of the cavity and the MMD mirror. In conclusion, we note that improvements in the MMD mirror may be made by interchanging the position of the metal mesh and multilayer dielectric films which should reduce the effect of FIR film loss on the FIR laser performance. In addition, an equivalent antireflection coating may be obtained by grinding the Si substrate to an integral multiple of hF1R/2n. Using existing technology, MMD mirrors can be extended to wavelengths perhaps as short as 50 pm where high quality dielectric mirrors are already available. Finally, the MMD mirror has withstood an incident power density of -1 MW/cm2 without damage thus making possible the development of efficient cw and pulsed optically pumped lasers throughout the far infrared. * The LPi 1 mode is oniy allowed when n ide z ncOIe however radial variations in the gain causing $ ens effects may cause this mode to oscillate.
Table 1 Summary of experimental results Mesh mirror
Laser gas
@+/mm)
FIR
Relative
wavelength (rmi
output power”’
line (crml
CO2
pump
19.7 13.0 19.7
CHaF CHaF HCOOH
496.1 496.1 432.5
1.0 0.6 0.49
P(20) 9.6 P(20) 9.6 R(20) 9.6
19.7 19.7
HCOOH HCOOH
419.5 393.6
0.06 0.31
R(22) 9.6 R(18) 9.6
a) Output powers are normalized to the maximum output power of CHsF at h = 496.1 Wm.
368
“,
Volume 13, number 4
OPTICS COMMUNICATIONS
The authors wish to acknowledge the help of Carl Meyer, Delco Electronics Corp., for providing the Si substrates and technical assistance.
References [ 1] G.T. McNice and V.E. Derr, IEEE J. Quantum Electron. QE-5 (1969) 569. [2] R. Ulrich, T.J. Bridges and M.A. Pollack, Appl. Opt. 9 (1970) 2511.
April 1975
[3] R. Uhich, K.F. Renk and L. Genzel, IEEE Trans. Microwave Theory Tech. (1963) 363. [4] V.Y. Balakhanov, Sov. Phys. Doklady 10 (1966) 788. [S] D.T. Hodges and T.S. Hartwick, Appl. Phys. Lett. 23 (1973) 252. [6] T.Y. Chang and T.J. Bridges, Opt. Commun. 1 (1970) 423. [7] E.A.J. Marcatili and R.A. Schmeltzer, BeB System Tech. Jour. 43 (1964) 1783. [8] J.J. Degnan, Appl. Optics 12 (1973) 1026. [9] R.J. Wagner, A.J. Zelano and L.H. Ngai, Opt. Commun. 8 (1973) 46.
369