Dual Wavelength Infrared Laser

1. Field of the Invention

This invention relates to laser range finders and in particular to a multiple wave length laser source for use therein. Laser rangefinders exist that operate in both the 3-5 .mu.m atmospheric window (DF at 3.8 .mu.m) and the 8-13 .mu.m window (CO.sub.2 at 10.6 .mu.m). At sea level, variations in the attenuation of radiation in the 8-13 .mu.m band are due to variations in the amount of atmospheric water vapor, because of a water vapor continuum absorption located in this spectral region. Variations in the attenuation of the 3-5 .mu.m band are primarily due to changes in the aerosol concentration in the atmosphere i.e. visibility changes; the longer wavelengths of the 8-13 .mu.m band are less affected by aerosol scattering losses.

A major design consideration for a laser radar is the selection of the operating wavelength. Both the CO.sub.2 and DF laser wavelengths lie in the "eye-safe" region, so that differences in their atmospheric propagation properties under envisioned operating conditions of the laser rangefinder should decide the optimum operating wavelength. For maximum flexibility, both laser wavelengths should be incorporated into the transmitter. If two independent wavelength laser devices are used, however, problems in optical alignment of components which can be opaque to visible radiation are encountered. Furthermore, complex beam-mixing optics would not be as rugged as a single cavity system under field conditions. It is evident that a considerable reduction in system complexity is possible if the two lasers can be operated in the same optical cavity.

2. Description of the Prior Art

In U.S. Pat. No. 3,548,212 which issued Dec. 15, 1970, Whetter describes a multi beam laser tracking system in which laser beams of different wavelengths and from different optical cavities are employed. The lasers appear to share a common back mirror, although they use separate portions of that mirror, while the output mirrors are the partially reflecting ends of the crystals. It is noted that the laser beams are not combined into a common beam, but instead are directed forward to a common target at a known distance from the source.

In U.S. Pat. Nos. 3,437,820 which issued Apr. 8, 1969 to Thompson et al and 3,549,256 which issued Dec. 22, 1970 to Brienza et al, optical rangefinders are described in which a single output beam is produced by combining the two beams from different optical cavities with a dichroic mirror. Such devices require careful alignment of steering mirrors to ensure that the direction of propagation of the two different wavelength laser beams is the same.

Applicant has now discovered that if two lasers operating on different wavelengths that lie in spectral regions with low molecular absorption by atmospheric gases and disposed in tandem in the same optical cavity may be operated in that cavity, resulting in a co-linear laser output which is a mixture of the two laser beams.

Thus, according to the invention, a dual wavelength laser source is contemplated, comprising optical cavity means including totally reflecting mirror means and partially transmitting mirror means; two pulsed laser means having different wave length outputs disposed in tandem in said optical cavity; timing circuit means simultaneously creating laser pulses from said laser means; so constructed and arranged such that laser pulses from said two laser means are received by said totally reflecting mirror means and are directed to said partially transmitting mirror means to transmit a co-linear laser output which is a mixture of said two laser beams and which has two different wavelengths.

FIG. 1 is a schematic diagram of an experimental set-up used to confirm the operation of the dual wavelength laser source according to the invention; and

FIG. 2 is a theoretical illustration relating to target ranging.

With reference to FIG. 1, it will be seen that a pulsed CO.sub.2 laser, conveniently a Tachisto TAC II discharge module was used for the CO.sub.2 laser discharge, and a pulsed DF laser, conveniently a Lumonics TE 211 module was used for the DF laser discharge. These modules (laser cavities) were placed in tandem and aligned along the longitudinal axis of reflection of an optical resonant cavity formed by a totally reflecting gold-coated mirror M1 with a radius of curvature of 10 m (M1), and an output mirror M2 which is partially transmitting at CO.sub.2 and DF wavelengths, also with a 10 m radius of curvature. The selection of radii of curvature of the cavity mirrors is governed by laws for "stable" cavities as defined in "Laser Beams and Resonators" H. Kogelnick and T. Li., Applied Optics Vol. 5, No. 10, pp. 1550-1567 (1966), or "unstable" cavities as defined in "Unstable Optical Resonators" A. D. Siegman, Applied Optics Vol. 13, No. 2, pp. 353-367 (1974).

The reflectivity of the output mirror (M2) should be sufficiently high near 3.8 .mu.m and at 9.6 .mu.m to permit lasing at these wavelengths. The distance between the mirrors was 2.5 m. The CO.sub.2 laser was free running at a repetition rate of 0.5 Hz. Trigger pulses from the CO.sub.2 module were used to trigger a timing circuit, conveniently oscilloscope 1. It will be appreciated that any conventional timing circuit which generates a suitable fixed delay may be employed. The delayed oscilloscope trigger of oscilloscope 1 was used as a trigger pulse of variable delay to trigger the DF module so that the laser pulses occurred simultaneously. The direct oscilloscope trigger of oscilloscope 1 was used to trigger oscilloscope 2 that sampled the laser output via the detector.

The detector represents three detectors, used one at a time, to sample the output from mirror M2, depending upon the measurement being performed.

(a) For the measurement of laser pulse energies, a pyroelectric joulemeter is used e.g. a Gen Tec Model ED-200

(b) For the determination of the DF laser pulse shape; an indium antimodide (InSb) detector cooled to 77.degree. K. with liquid nitrogen was employed.

(c) For the determination of the CO.sub.2 laser pulse shape, a photon drag germanium detector operating at room temperature was employed.

More specifically, in operation, the CO.sub.2 laser (TAC2) and the DF laser (TE211) are pulsed lasers. The CO.sub.2 laser was used in the internal trigger mode, with a repetition rate selected by the operator. If both laser discharges were produced simultaneously, the CO.sub.2 laser pulse would appear later than the DF laser pulse, due to the nature of the molecules involved. Hence, a delay between the initiation of the CO.sub.2 laser discharge and the DF laser discharge is introduced. This delay was produced by connecting the synch pulse from the CO.sub.2 laser to oscilloscope 1, and then triggering the DF laser discharge with the delayed trigger output of oscilloscope 1. The operator has control of the delay of this pulse after the initiation of the oscilloscope trace. In a complete cycle, both the CO.sub.2 and DF lasers are fired once each, the CO.sub.2 laser before the DF laser, with the delay set to permit the CO.sub.2 and DF laser pulses to be emitted simultaneously. In addition oscilloscope 1 is connected to a oscilloscope 2 which responds to the signal developed by the detector.

As discussed briefly above, the two laser beams must be colinear in order that the same target will be intercepted by the two beams or that the two lasers sample the same volume in space. Even a small angular misalignment is detrimental to the obtaining of optimum results, as ranges of several kilometers are frequently encountered. FIG. 2 illustrates an instance in which a small target 3 will give unequal response for the two wavelengths, due to misalignment; laser 1 fills the target entirely, while laser 2 only partially fills it. For example, for a target range of 5 kms, and with lasers whose beams have a divergence of 0.5 mR (a reasonable value), the two beams could not intercept a target 5 m in diameter if the beams were misaligned by as little as 1 mR; this misalignment is very slight, and difficult to detect or correct for, when one has to align steering mirrors and a dichroic mirror in accordance with prior art devices. Furthermore, this alignment tolerance must be maintained under field conditions.

By placing the active media in the same optical resonant cavity, only one set of resonator mirrors is required, and no alignment involving a dichroic mirror is necessary. Instead, the optical resonant cavity forces the two beams to be colinear, so that the alignment problems are solved automatically. If there are any beam steering effects, both beams are affected equally, and the laser beams remain colinear. Thus, this arrangement is much more stable for field use.

The following experiments were performed to confirm the operational characteristics of the dual wavelength laser source according to the invention.

Most experiments were carried out in a cavity whose output mirror had a reflectivity of 90% at 9.6 and 10.6 .mu.m, and 75 to 80% near 3.8 .mu.m. In the present case, a CO.sub.2 laser and a DF laser were placed in the same optical resonant cavity. It is known that the SF.sub.6, necessary for the DF laser, inhibits the CO.sub.2 laser from operating on the normal transition at 10.6 .mu.m, but does not affect the CO.sub.2 laser performance as the output wavelength is shifted to 9.6 .mu.m. This wavelength has nearly the same atmospheric absorption characteristics as the conventional 10.6 .mu.m wavelength radiation. By itself, the CO.sub.2 module produced 250 mJ of laser energy at 10.6 .mu.m with a gas mix of CO.sub.2 :N.sub.2 :He in a ratio of about 2:1:5 at a total pressure of 107 kPa. When 9 kPa of SF.sub.6 and D.sub.2 in a ratio of 10:1 were introduced into the DF module, the CO.sub.2 laser output shifted to the P(22) transition of the 00.degree.1-02.degree.0 manifold at a wavelength of 9.6 .mu.m; the CO.sub.2 laser energy was unchanged with this wavelength shift. The DF laser module produced 15 mJ of laser radiation near 3.8 .mu.m; subsequent experiments demonstrated that the DF laser output was severly limited by aperturing by the CO.sub.2 module and that there was no absorption of DF radiation by the CO.sub.2 molecules. The DF module is capable of producing 150 mJ of laser energy when the CO.sub.2 laser module was removed from the optical cavity. When the output mirror M2 was a mirror reflecting 50% at 3.8 .mu.m and only 70% at 9.6 .mu.m, the CO.sub.2 laser did not operate while the DF laser output increased to 25 mJ with this mirror, indicating that the output mirror reflectivity must be chosen to optimize both lasers.

Mention has been made of pulse energies and operating pressures, but only to ensure that the presence of one laser medium does not influence the performance of the other laser. The only effect noted was to cause the CO.sub.2 laser to operate on the 00.degree.1 to 02.degree.0 transition instead of the normal 00.degree.1 to 10.degree.0 transition, resulting in a wavelength shift from 10.6 .mu.m to 9.6 .mu.m.

Table I lists the DF lasing transitions that were observed when a 10/1 mixture of SF.sub.6 /D.sub.2 was excited at a pressure of 9 kPa; the weak and strong transitions are identified in the table. The observed transitions were noted both with and without the CO.sub.2 laser in operation.

The transitions listed in table I correspond to DF molecular transitions. These lines were observed both when the CO.sub.2 laser contained no CO.sub.2 and when the CO.sub.2 laser was operational. The significance of this result is that the CO.sub.2 laser does not affect the performance of the DF laser.

The shape of the 9.6 .mu.m pulses was observed on oscilloscope 2 in response to a signal received from the Photon drag detector. It was similar to the 10.6 .mu.m pulses produced when SF.sub.6 was not present in the cavity. This is not surprising as the two transitions have the 00.degree.1 upper level in common. The shape of the 3.8 .mu.m DF laser pulse was also observed. The 9.6 .mu.m CO.sub.2 laser appears to be partially mode locked, while the shape of the DF pulse is determined by the cascading that characterizes multilevel systems; both pulses appear to be approximately 500 ns in duration. When the two laser pulses were produced simultaneously, there was no detectable change in the pulse shapes. In this experiment, the DF laser discharge was delayed with respect to the CO.sub.2 laser discharge, because of the longer cavity build-up time for the CO.sub.2 laser pulse. It was also noted that the cavity build-up time of the 9.56 .mu.m transition was 500 ns longer than was the case for the 10.6 .mu.m pulse; this longer cavity build-up time for the 9.6 .mu.m laser pulse is due to the reduced gain on this transition relative to that of the 10.6 .mu.m transition when no SF.sub.6 is present.

An estimate of the divergence of the CO.sub.2 laser was made by measuring the burn size made by the beam as it is focussed onto a piece of exposed Polaroid.RTM. film. The size of the burn pattern was not altered when the DF laser was operated with the 9.6 .mu.m CO.sub.2 laser. The measurement indicated a beam divergence of approximately 0.6 mrad.

From the above experiments it is apparent that the output mirror reflectivity is ideally about 50% at 3.8 .mu.m and 90% at 9.6 .mu.m to optimize the transmission of both wavelengths.

According to the latest in formation available e.g. McClatchey, R. A. and Selby, J. E., "Atmospheric Attenuation of Laser Radiation from 0.76 to 31.25 microns", AFCRL-TR-74-0003, it is apparent that a CO.sub.2 laser transmits through the atmosphere just as well at wavelength of 9.6 .mu.m and 10.6 .mu.m for all practical purposes. Accordingly, the shift in operating wavelength of the CO.sub.2 laser from the 10.6 .mu.m to 9.6 .mu.m in no way reduces the potential effectiveness of the CO.sub.2 laser for use as a range finder.

The invention has been described in relation to a CO.sub.2 laser which operates in the 8 to 13 micron window and a DF laser which operates in the 3 to 5 micron window. Although only the DF/CO.sub.2 combination is described, these two lasers are the most important gas lasers operating in the above mentioned spectral windows. Accordingly, no further examples of combinations of lasers for rangefinding applications are needed.