Tunable linearly polarized TEM 00 operation of CO 2 laser with a concave diffraction grating Enrique Bernal G. and Rolf McClellan E. Bernal G. is with Honeywell Corporate Research Cen- ter, Bloomington, Minnesota 55420. When this work was done R. McClellan was with PTR Optics Corporation; his address is now 8 Beverly Road, Acton, Massachusetts 01720. Received 2 April 1975; revised manuscript received 8 July 1976. With the standard cavity configuration (10-m radius of curvature concave total reflector and flat output reflector) the Coherent Radiation model 41 CO 2 laser emits an output beam with a mode structure close to TEM 00 , i.e., the intensity profile of the beam is approximately Gaussian. However, when the 2956 APPLIED OPTICS / Vol. 15, No. 12 / December 1976
Transcript
Tunable linearly polarized TEM00 operation of CO2 laser with a concave diffraction grating
Enrique Bernal G. and Rolf McClellan E. Bernal G. is with Honeywell Corporate Research Center, Bloomington, Minnesota 55420. When this work was done R. McClellan was with PTR Optics Corporation; his address is now 8 Beverly Road, Acton, Massachusetts 01720. Received 2 April 1975; revised manuscript received 8 July 1976.
With the standard cavity configuration (10-m radius of curvature concave total reflector and flat output reflector) the Coherent Radiation model 41 CO2 laser emits an output beam with a mode structure close to TEM00, i.e., the intensity profile of the beam is approximately Gaussian. However, when the
Fig. 1. Transverse mode structure obtained when using flat intra-cavity grating for wavelength selection. The two photos represent
different attempts to optimize power distribution.
silicon substrate with 1800 lines/in. and a blaze wavelength of 9.6 μm. The mode structure obtained from the concave grating-flat output mirror configuration is shown in Fig. 2. Figure 2(a) shows a nearly TEM0 0 mode structure at 10.6 μm, and Fig. 2(b) shows a similar beam quality at 9.6 μm. It was possible to obtain TEM 0 0 operation for all the wavelengths of interest in both the P and R branches of carbon dioxide transitions, but it was necessary to adjust the gas mixture and discharge current for each emission wavelength. A measure of the degree of control over the mode structure possible with this configuration is shown in Fig. 3, in which we have adjusted the mirrors and gas mixture to produce a TEM 0 1 mode. It is possible by adjustment of the operating conditions of the laser to obtain various modes, and it is not difficult to obtain the fundamental mode.
As has been indicated above, the experimental observations would be fully explained by the negative lensing in the excited medium of a high power, axial flow, electric discharge CO2 laser that has been previously reported2 and analyzed.2-3 To account for our observations, however, the negative lensing would have to be distributed nonuniformly along the length of the cavity. Under this condition, interchanging end re-
laser is converted to wavelength-tunable operation by installing (1) the CR accessory grating mount with a plane grating replacing the curved total reflector and (2) a 10-m radius partial mirror replacing the flat output reflector, the output exhibits an uncontrollable higher order mode structure under all operating conditions of discharge current and gas mixtures; i.e., it is impossible to achieve TEM 0 0 operation. Typical optimized far-field beam profiles, recorded as burn patterns on reference cards or tongue depressors 4 m (400 times the beam waist) from the output reflector, are shown in Figs. 1(a) and (b).
In this Letter we report the finding that by replacing the plane diffraction grating with a concave grating and using a flat output reflector, beams with nearly TEM 0 0 modes are easily obtained, with the laser exhibiting the tunability and polarization stability1 typical of grating/mirror cavities.
The poor spatial mode structure obtained with the plane grating was at first suspected to be due to irregularities in the diffracted wavefront of gratings available prior to 1973. However, the use of ML series gratings from PTR Optics, which have greatly improved ruling uniformity, failed to improve the beam structure. To verify that the higher order mode structure was not related to the presence of the grating in the cavity, the laser was operated with a flat total reflector and a 10-m output mirror. A multimode output beam was again observed, strongly suggesting asymmetrical negative lensing in the excited medium.2 A series of attempts to compensate for this negative lensing by the use of output reflectors with radii of curvature less than 10 m proved unsuccessful.
Because of the difficulties experienced in coupling output from spherical mirrors, it was decided to simulate the standard cavity configuration of the laser (flat mirror in the output location) by using a 10-m radius of curvature concave grating as a total reflector. The grating was ruled on an aluminized
Fig. 2. TEM00 mode structure produced with concave intracavity grating for wavelength selection: (a) 10.6 μm; (b) 9.6 μm,
Fig. 3. Transverse mode structure produced with concave intracavity grating. Cavity adjusted for TEM01 to show that different modes can
flectors will not, in general, yield laser cavities that are equivalent in the sense of Gordon and Kogelnik,4 with the result that one version of the cavity can yield a TEM0 0 output, though the version obtained by merely interchanging end reflectors yields a multimode output beam.
The Littrow (Eagle) mounted concave grating does, of course, exhibit spherical aberrations,5 including astigmatism whereby the grating has, for a given wavelength, two different radii of curvature about the axes parallel and perpendicular to the grooves. These radii are given, respectively, by
where R is the radius of curvature of the grating substrate and i is the littrow angle from the grating equation. Therefore the grating used in this experiment had radii of 9.3 m and 10.8 m at 10.6 μm. In designing a stable TEM 0 0 laser cavity with a concave grating, one simply ensures that the stability and mode quality criteria3 are met in both axes, i.e., for both radii. Previous experience by Gustafson6 has indicated that this is not troublesome.
The power output of the laser with the concave grating approaches that obtainable with the standard mirror configuration (~250 W). However, care must be exercised to cool the grating properly to prevent damaging it at the flux densities incident upon it when operating at high power. The standard grating mount supplied by the manufacturer1 holds the grating almost completely isolated from the cooled mirror mounts to isolate it electrically since it must be held at high voltage inside the gas discharge. For this reason, it is not advisable to exceed an output power of approximately 10 W while operating with this grating mount. At this power level it was found that after 30 min of operation the temperature of the grating had increased by 12°C. This is in good agreement with the temperature rise to be expected for the thermal mass of the aluminum holder, assuming that there is no heat transferred to the laser structure or the environment. Operation of the laser at power outputs of 100 W, which corresponds to an intracavity power of 300 W, has been experimentally observed to lead to destruction of the grating by actually burning the front surface. A grating mount designed with provisions for cooling would be highly desirable since the grating shows no evidence of intrinsic damage at the flux densities required for full power operation of this laser.
In conclusion, we have found a concave grating useful in circumventing the effects of negative lensing in the medium of a commercial, axial flow CO2 laser. We have also shown that, despite its inherent astigmatism, the concave grating can be used to construct stable, near TEM 0 0 laser cavities. Other possible laser applications of concave gratings include use as wavelength-selective elements for tunable, unstable resonators and as a compensation mechanism for astigmatism introduced into the laser cavity by the medium or other optical components.
We are indebted to A. O. Forseth for his able assistance and to H. Gustafson for sharing his experience on the successful use of concave gratings in CO2 lasers.
References 1. The manufacturer's literature, Coherent Radiation model 41
brochure, specifies output to be linearly polarized. However, without an intracavity grating the laser output is elliptically polarized.
2. H. K. V. Lotsch and W. C. Davis, Appl. Opt. 9, 2725 (1970). 3. H. Kogelnik, Bell Syst. Tech. J. 44, 455 (1965). 4. J. P. Gordon and H. Kogelnik, Bell Syst. Tech. J. 43, 28?3
