High-precision reflectivity measurement technique for low-loss laser mirrors

Virgil Sanders Litton Industries, Guidance & Control Systems Division, Woodland Hills, California 91364. Received 12 June 1976. As in many other fields of laser research, one of the major

quests in ring laser gyro research is for low-loss, high optical quality, thin film dielectric coatings for mirrors. The quality of these mirror coatings is a function of the energy lost on re­flection from absorption and scatter phenomena. Back-scattered energy from one direction into the other causes the locking phenomena in ring lasers, and absorption reduces the Q of the laser cavity. Both effects are detrimental to ring laser gyro performance.

Most of the established vendors of multilayer dielectric mirror coatings have some means of measuring transmittance to ±0.0001. None have the means, with the equivalent high degree of accuracy, to measure reflectance. Thus, when specifying reflection parameters to a laser mirror vendor one specifies a particular transmittance and can only hope for minimum loss to absorption and scatter phenomena. We report here a simple apparatus and technique which we use to measure reflectance of ring laser mirrors (typical reflectance >0.9970) with an accuracy better than ±0.0001. The method is general and may be used to determine the total loss associ­ated with any optical component in a laser cavity.

Figure 1 is a top view schematic representation of the ap­paratus. The measurement indicated here is total reflection loss of p polarized light at some oblique angle of incidence by a sample mirror. The figure indicates an implied 45° angle of incidence on the sample mirror. However, the apparatus may easily be constructed to accommodate angles of incidence as small as 1°. The technique involves two intralaser cavity measurements, one with the sample mirror in the cavity and one without. Figure 1 indicates two laser geometry configu­rations: first without the sample mirror; second with the sample mirror. Mirror # 2 remains fixed for both configu­rations, and mirror # 1 is moved to position # 2 for the second measurement and configuration. There are two necessary restrictions associated with the geometry of the laser cavities and the sample mirror which ensure equivalent gain and loss to the two lasers—the over-all laser cavity length of the two configurations must be the same, and the sample mirror must be piano. This ensures the same mode geometry, thus, the same gain and diffraction losses associated with the plasma tube.

The orientation of the windows indicated in the laser cavity is dependent on the orientation of the plane of polarization of interest. The orientation shown in Fig. 1 indicates p po­larization with respect to the sample mirror. The windows at the ends of the plasma tube are oriented at Brewster's angle relative to the laser beam. For s polarization, the windows would be rotated 90° about the laser beam axis. The rotatable window is approximately 2 mm thick and has less than 10 sec of arc of wedge. It is mounted on a turntable with an angular reading accuracy of 1 min of arc and rotates about an axis through the center of the window and parallel to the window surface.

The rotatable window serves as a measurable loss to the laser cavity. The window is first oriented nominally at Brewster's angle relative to the laser beam axis and then ro­tated in one direction until laser action is quenched as a result of the induced reflection loss. Next, the window is rotated back through Brewster's angle in the opposite direction until laser action is quenched again. As the window is rotated the

Fig. 1. Reflectance measurement apparatus (two laser cavity configurations).

Fig. 3. Percent loss vs included angle φ (drawn from Fig. 2).

January 1977 / Vol. 16, No. 1 / APPLIED OPTICS 19

beam axis is displaced laterally. The plasma gain tube must be displaced by an equal amount to minimize diffraction losses from the tube. The mirrors should be adjusted for optimum alignment after each movement of the rotatable window. Using Fresnel's laws of reflection1 one may calculate from this measured included angle φ the amount of loss required to quench the laser action (see Figs. 2 and 3). The graphs in Figs. 2 and 3 are associated with the rotatable window mentioned earlier for the 6328-Å wavelength of the He-Ne laser. Figure 3 is drawn from Fig. 2. This measurement is made twice: first without the sample mirror (configuration # 1) using only mirrors # 1 and #2 , then with the sample mirror. The dif­ference in the measured losses required to quench laser action is the amount of loss introduced by the sample laser mirror.

The accuracy and repeatability of the measurement is a function of the sharpness and stability of the lasing threshold as an observable end point. An effort must be made to hold the gain and losses constant from one measurement to the next. The reproducibility is limited by the quality of the optics, stability of the current flowing through the plasma discharge, precision angle reading associated with the rota­table window, dust that might fall unnoticed on a piece of the optics, and to some extent mechanical and thermal stability of the setup. The termination point of an included angle measurement is exceedingly sharp and easy to determine with the eye for visible laser light: lasing or not lasing. It is ad­visable to set the gain level so one can obtain an included angle (see Fig. 3) greater than 20°. This portion of the curve in Fig. 3 is somewhat linear, allowing us to use the slope of the curve for our calculation. Typically we set the gain for an included angle of 22-25° (slope ≅ 0.17% per degree) and find each measurement repeatable (four out of five times) to within ±3 min, indicating a repeatability of ±0.0001. The scatter and absorption characteristics of a mirror might not be considered constant; i.e., the amount of the laser beam light scattered by a particulate on or in a mirror coating is a function of the transverse field distribution and spot size dimension of the laser beam, the location relative to the laser beam axis, and the size of the particulate relative to the laser beam dimen­sions and wavelength of the light. High intensity laser light might change both the scatter and absorption characteristics of an inclusion on or in a mirror coating. However, to the extent that these characteristics can be considered constant, if one can ensure to some particular degree of accuracy that the only gain-loss difference in the two measurement setups is the loss contributed from absorption, scatter, and trans-mittance by the sample mirror, one can claim to have mea­sured the absolute specular reflectance of the sample mirror to that particular degree of accuracy. In our setups the ±3 min repeatability in the included angle measurement indicates ±0.0001 accuracy. With considerable effort and expense, this degree of accuracy could no doubt be bettered. However, with commercially available optics and power supply and moderate care with respect to cleanliness, one may breadboard construct the setup indicated in Fig. 1 in the open air and expect to at­tain this ±0.0001 accuracy and stability.

The apparatus and technique have proved useful in eval­uating various vendors' multilayer dielectric coatings. This apparatus served a study as one of the tools used to evaluate multilayer dielectric coated mirrors from various vendors. It is not uncommon to observe a change in total loss of 0.0005 by moving 2-3 mm off the center of a coating (diameter of coating = 12 mm). Microabrasions from a light wipe with lens tissue may change the scatter loss by as much as 0.0002. Absorption plus scatter losses may differ by as much as 0.002 in two sep­arate coating runs using identical equipment, materials, and coating parameters. The manufacturers of these coatings were not aware of these properties associated with their

20 APPLIED OPTICS / Vol. 16, No. 1 / January 1977

product. We concluded from this study that there is a great need for such a tool to make this reflectance measurement.

Reference 1. F. A. Jenkins and H. E. White, Fundamentals of Optics

(McGraw-Hill, New York, 1957).