Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2002-09-30
2003-10-21
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S034000, C372S032000, C372S033000
Reexamination Certificate
active
06636536
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to passive temperature compensation of rotatable gratings and other wavelength selecting devices used to tune lasers.
2. Prior Art
Laser radar (LIDAR) systems, utilizing tunable lasers, can be used to transmit different wavelengths of light into airborne suspensions (such as smog or poison gasses) which have differing reflectance's or absorption to different wavelengths. The reflected light intensity is then measured for remote spectrographic analysis of suspension samples. It is advantageous to maximize the stability and repeatability of the output at each different wavelength. It is also advantageous to minimize intervals between transmitting wavelengths in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting different wavelengths with constant power at the laser's maximum cyclic rate.
Tunable lasers typically include an intra-cavity diffraction grating. The wavelength of such lasers is tuned by adjusting the angle of incidence of the laser cavity beam against the diffraction grating. Such intra-cavity tuning requires very high accuracy and stability. Tuned CO
2
lasers, for instance, require a grating angular range of typically 0.2 radians and an accuracy of 10 or 20 &mgr;radians. Output laser power is a sensitive function of the tuning angle near a particular wavelength. This accuracy can only be achieved through careful design of housings, optics, sensors, feedback loops, actuators and system configuration.
In
FIG. 1
of the prior art, a representative directly driven grating implementation is shown. Intra-cavity beam
22
is tuned by rotary grating
24
responsively changing the Littrow reflection incidence angle relative to cavity
22
, thereby selecting lasing wavelength. Actuator
20
in response to control by position control
28
rotates grating
24
to produce different wavelengths. Grating position is sensed by position sensor
32
and ambient temperature is sensed by sensor
31
. Position control
28
includes position data for sequencing between desired positions and therefore wavelengths and provides output drive to actuator
20
, responsive to said position data, actual position information from position sensor
32
and ambient temperature sensor
31
.
Temperature induced variations, especially in the grating, position sensor, and sensor coupling pose a serious threat to the functionality of the tuner. These variations are the result of absorbed cavity as well as ambient energy. Military applications may require high cavity power and oppressively wide operating temperature ranges. Changes in temperature of the grating blank or base material, for instance, modify the apparent ruling of the grating and therefore its tuned frequency. The position sensor and its coupling have a direct effect on the tuned frequency of the cavity. Cavity detuning results in output power fluctuations.
Several techniques have been used to nullify the effects of these errors. Grating
24
can use a blank material of low coefficient of thermal expansion (CTE). Invar as a grating blank material, for instance, would have a CTE of typically 1.3 ppm/K. Special invar can reduce this somewhat further. Even this small CTE would generate an excessive error of 65 &mgr;radians for a 40° C. ambient rise and 10° C. cavity induced temperature rise.
Another technique for ambient correction is to use the temperature sensor
31
to crudely modify the desired position address data in position control
28
in a calibrated way to compensate for the thermal induced errors of the grating and sensor. Thermal errors generated by laser cavity energy are not compensated unless complex rotating or additional non-contact thermal sensors are used.
These techniques are inordinately complex and do not function well even for the relatively small errors generated by low CTE gratings. Most serious of the deficiencies are the nonlinear and unpredictable temperature variations of most sensors and the thermal time constant mismatch between the elements. Additionally, invar is a poor material to work with. Invar has high density, low thermal predictability and low thermal conductivity. It is difficult to machine and has dissimilar metals incompatibility with many optical surface materials. Its density and thermal conductivity discourage use at high power or rapid tuning rates. The use of lighter, more compatible blank materials with unavoidably higher CTEs function even more poorly.
FIG. 2
of the prior art is based on an adaptive resonant positioner disclosed in U.S. Pat. No. 5,450,202. As disclosed, high accuracy and speed are combined as a result of the adjustment of adjacent pairs of drives on.a pattern delayed basis. Intra-cavity beam
22
is tuned by rotating agile mirror
42
which reflects beam
22
as beam
22
a
onto fixed grating
44
responsively determining the incidence angle between cavity
22
a
and grating
44
thereby selecting lasing wavelength. A second position of mirror
42
results in cavity beam
22
b
for another incidence angle and lasing wavelength. As shown, the cavity beams
22
a
and
22
b
intersect the grating at
26
a
and
26
b
for two of the tuned wavelengths in a tuning band. Mirror
42
is rotated by actuator
40
in response to control by agile position control
46
. Mirror position is sensed by position sensor
48
and ambient temperature is sensed for calibrated correction by sensor
31
. Agile position control
46
provides output drive to actuator
40
responsive to the internally defined desired position data, actual position information from position sensor
48
and ambient(temperature sensor
31
.
This system operates better in a high tuning rate, thermally hostile environment for a number of reasons. Since the grating is fixed mounted, a massive and therefore thermally improved version including active thermal control
45
is possible. Alternately, even without thermal control
45
, better thermal coupling reduces the thermal differences between ambient and grating temperature. More importantly, the mirror is no longer a thermally sensitive element and can be constructed of light material for high speed and compatibility with optical coatings. The position sensor is typically a linear Si sensor and is thus more predictable and stable than other types of sensors.
Although this approach enjoys speed and thermal advantages over the
FIG. 1
prior art, it is still too complex, requires a larger grating for the same beam size, is susceptible to mismatched thermal time constants and falls short in performance for stringent military requirements. Wide operating temperature ranges still require the use of invar for grating blanks. Additionally, for high power tuners, the mirror damage threshold drops by one half since a reflection occurs twice as often for each cavity round trip.
SUMMARY OF INVENTION
Preferred embodiments of a passive thermal compensation for wavelength agile laser tuners according to the present invention include wavelength selection means, thermal shim means, sensor coupler means and tuner controller means. Thermal energy from lasing or environmental injection coupled into the wavelength selection means is tightly coupled to the thermal shim means. Temperature variations of the thermal shim means produce angular corrections to the operation of the wavelength selection means to produce corrected wavelength tuning at a typically narrow band of wavelengths and at a rate which closely tracks the error producing thermal expansion of the wavelength selector. Sensor coupler means receives thermal energy from environmental injection and modifies sensor tuning sensitivity to produce corrected tuning at a wider band of wavelengths.
LIDAR and other types of remote chemical sensors utilize the acute absorption of energy at specific wavelengths as compared with relatively low absorption at others. Unfortunately, laser return energy already includes unavoidabl
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