Solid-etalon for the CALIPSO lidar receiver

Transcription

1 Solid-etalon for the CALIPSO lidar receiver Neal H. Zaun*, Carl Weimera, Yakov Sidorin', David Lunt" aball Aerospace & Technologies Corp., P0 Box 1062, Boulder, CO USA ; bcoronado Technology Group LLC, 1674 South Research Loop Suite 436, Tucson, AZ USA ABSTRACT In 2005 a lidar instrument will be launched aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite for measuring the three dimensional distribution ofatmospheric clouds and aerosols. A key part ofthe lidar instrument is a 532 nm tunable etalon, which allows daytime operation. The design rationale and measured optical performance ofthe etalon and its mounting system during assembly and integration are presented. Keywords: Etalon, lidar, spectral filters 1.INTRODUCTION The lidar instrument on the Cloud-Aerosol Lidar and infrared Pathfinder Satellite Observations (CALIPSO) will provide a better understanding ofhow clouds and atmospheric aerosols influence the Earth's climate.1 This instrument contains a diode-pumped, Q-switched Nd:YAG laser with 20 nanosecond polarized output pulses ( Hz) of I 1 0 mj (1064 nm) and I 10 mj (532 nm). The laser beam passes through a beam expander that produces a 70 m eye-safe spot on the ground from the 705 km altitude. Co-aligned to the transmitter is a one-meter all beryllium-receiving telescope with a 125 micro-radian field-of-view. The receiver beam from the telescope is collimated to about one inch diameter, and then a dichroic mirror is used to split the O64 nm and 532 nm channels. The 1064 nm channel uses a 0.40 nm bandwidth interference filter and an avalanche photodiode for detection. While, the 532 nm channel uses an etaloninterference filter combination and two photomultiplier (PMT) detectors for detection ofthe two crossed polarizations. Since the signal-to-noise ratio (SNR) of the 532 nm channel is background limited during the daytime, a key for obtaining a high SNR is the etalon-interference filter combination. The development and performance of the etaloninterference filter combination is discussed in this paper. 2. REQUIRENMENTS AND TRADES Since the signal-to-noise ratio (SNR) of the 532 nm channel is background limited during daytime operation the basic requirement ofthe ofthe spectral filtering system are to: match the laser wavelength, have a bandpass which is close to the laser bandwidth of 33 pm and block all other radiation over the PMT response region. For space use the etaloninterference filter combination and its housing must survive the launch vibration environment, space survival temperatures and the space radiation environment. The requirements for the etalon-interference filter combination are given in Table I. Etalon wavelength tuning is required because first, the laser wavelength at 532 nm varies with the temperature of the Nd:YAG crystal by 2.6 pm/ C2. The laser is cooled using a radiative cooler that will cause the laser to have a temperature, which will vary due to orbital changes, and the difference from ground and in-orbit operation. Second, the etalon and laser developments were done in parallel, so the exact final operating temperature of the laser crystal, and therefore the wavelength was not known until the laser system had been completed. The etalon types examined included: bare solid etalon, solid etalon sandwiched between two thick fused silica windows ("sandwich etalon"), air-gap etalon, tunable piezoelectric controlled etalon, and pressure tunable etalon. The tunable piezoelectric controlled etalon and pressure tunable etalon both require more weight and cost to implement as a space qualified unit than other approaches, as well as introduce higher risk because of complexity (including a feedback * nzaunball.com; phone Earth Observing Systems IX, edited by William L. Barnes, James J. Butler, Proc. of SPIE Vol (SPIE, Bellingham, WA, 2004) X/04/$15 doi: /

2 control system). The simple solid design vastly simplified the electronics and optical requirements making space qualification much easier. In addition, their capability ofgreater tuning range than the other approaches is not required. Table I. Etalon-interference filter combination requirements and measured performance. Etalon PARAMETER REQUIREMENT PERFORMANCE Clear aperture (etalon and filter) <29 mm 30 mm Free Spectral Range pm 710 pm Bandwidth (FWHM) pm 39 pm Peak transmission (collimated beam) > Contrast ratio > Angular tilt <22 mrad I 5.2 mrad at 39.5 C Temperature wavelength tuning range >16 pm 54 pm Polarization effects <1/1000 transfer oflinearly polarized light Satisfied Interference Filter (two parts) Bandwidth (FWHM) pm 770 pm Peak transmission > Number ofcavities 3 3 Out-of-band blocking <10 ( nm) <106 ( nm) Space qualified (both when mounted) Survival temperature -30 C to +60 C Radiation Launch vibration Qualified Since the exact laser wavelength ofthe laser was not known when the etalon was being developed and there is a slight manufacturing error in the etalon thickness it was decided that tilt tuning would be used to match the laser wavelength on the ground. In space, wavelength-matching adjustment would be accomplished by temperature tuning the etalon. Shown in Table 2 is a general comparison ofthe solid etalon and air-gap etalon. The bare solid etalon was found hard to mount for launch vibration survivability and still maintain the required optical performance. When the required forces were exerted on the bare solid etalon, distortions in the etalon caused an unacceptable increase in the bandwidth. As shown below the air-gap etalon is more susceptible to performance degradation with tilt tuning than the solid etalon. Therefore the "sandwich etalon" was selected and proved to be very mechanically stable and ideal for space use were only a small amount ofwavelength tuning is required. Table 2. General comparison of solid and air-gap etalons. PARAMETER AIR-SPACED ETALONS SOLID-SPACED ETALONS Temperature Stability Good Average Temperature Tuning No Yes Angular Tuning Yes Yes Dispersion No Yes Fabrication Tolerances Good Good Size Variable (typically larger) Variable (typically smaller) Angular Acceptance (field ofview) Fixed and Limited Cost $$ $ Proportional to effective index ofthe cavity 142 Proc. of SPIE Vol. 5542

3 The output beam diameter of the one-meter telescope after a collimator was selected to be approximately 25 mm. This beam diameter minimized the etalon and interference filter fabrication requirements and provided for reasonable sized optics throughout the aft-optical system. The 25 mm beam size, meant that the +125 micro-radian field ofview caused the beam going through the etalon to have a 2.5 mrad. Tilt tuning ofthe etalon can only be used over a limited range since tilting the etalon increases the bandwidth and decreases the peak transmission. This occurs because the etalon is operating in a divergent beam rather than a collimated beam. The new bandwidth (A2'iiew) 5 given in Equation l, for a bandwidth with no tilt (AX), a tilt angle of x, wavelength of20, and a gap with an index Ofng. Equation 2 gives the relative peak transmission as a function of tilt angle. A2new A22 +((2xxxex2o)/ng2). (I) z. = A2 xarctan (2xXx x20)/ng2 2 (2 A (2xxx x2o)/ng These equations are plotted for both an air gap and solid etalon made of fused silica in Figures 1 and 2, which shows the solid etalon is more tolerant to tilt than the air-gap etalon. SNR considerations allow the etalon's bandwidth to increase to 45 pm and the peak transmission to decrease to 85% from its initial values. This allows the etalon to be tilted up to 22 rnrad that corresponds to a wavelength shift of6o pm, that can be used for matching the laser wavelength during integration on the ground Air-gap etalon Mm Value I :_ :.. a. ir-gap etalon 0. ''''''' 0.0. Max Value f 25 \\ O.2 U Si ca e a on Fused silica etalon TiltAngle (mrad) Tilt Angle (mrad) Figure 1 Bandwidth versus tilt angle for air-gap and Figure 2 Relative peak transmission versus tilt angle fused silica etalons with a 35 pm bandwidth at zero tilt. for air-gap and fused silica etalons. 3. DESIGN AND MANUFACTURE Fused silica was chosen as a material for the spacer layer primarily because of its thermo-optical characteristics: refractive index n =.461 and thermo-optic coefficient 3 dnidt 10x106 (IC') at 532 nm, coefficient of thermal expansion a 0.5x106 (IC'), thermal conductivity of about 1.3 (Wni'K'), and low radiation damage. In practice the fused silica solid-spaced etalon is a simple, robust, modest-cost, alignment-free low-rate temperature-adjustable filter system. Its mechanical characteristics allow for excellent manufacturability using continuous ring polishing technique, which results in low residual micro-roughness of the etalon surfaces (several Angstrom rms) and flatness figures of about X/l00 across the clear aperture. In practice, the approximate 130 jim thick fused silica etalon plate is sandwiched Proc. of SPIE Vol

4 between and directly optically wet-contacted to two fused silica glass AR coated etalon plates that also serve as mounting substrates, see Figure 3. To achieve the required effective finesse of about FE 2 1, high reflective etalon coatings (R>90%) were required across the clear aperture ofabout 30 mm. In general, the affects of change of ambient temperature on the etalon's peak wavelength are defined by the material and optical characteristics ofthe spacer layer. Specifically, for fused silica the thermal dispersion ofrefractive index of the spacer layer defines most of the spectral tuning of the peak wavelength via the temperature dependent change of optical length ofthe cavity. Throughout the flight laser and etalon development a Coherent WaveMaster Laser Wavelength Meter was used to measure the wavelength of the CALIPSO flight laser manufactured by Fibertek, Inc. In addition, a laboratory narrow band (<1 pm) laser was used to measure the etalon performance at Coronado and Ball. The Wavemaster acted as a wavelength "transfer" standard. Testing was done using a temperature controlled engineering oven-housing. A special etalon flight oven-housing was developed for both temperature tuning the etalon and provides a highperformance mount to survive the launch environment, shown in Figure 4. The etalon is housed in a temperature controlled oven-housing that provided thermal adjustment of the peak wavelength and was nominally operated at 40 C. The housing has a fused silica window at one end and the interference filter, discussed below, at the other end to help maintain a uniform etalon temperature. The heaters were designed to tune the etalon temperature from approximately C and a survival heater was used to maintain the etalon above 30 C when the regular heaters are off. Launch survival was guaranteed with a special etalon mount and a housing mount, which used flexures. Numerous mounting techniques were designed and experimented with to obtain the final etalon mounting technique to guarantee launch survival. In addition, the mounting technique had to maintain a stress level on the etalon such that the polarization properties ofthe etalon were such that less than 1/1000 ofthe linearly polarized light parallel to the laser output beam is induced into the cross-polarization direction. This required that the mounting technique minimize the amount ofbirefringence induced into the fused silica, stress birefringence. Heaters Window Flexures Figure 3 "Sandwich etalon" thin solid etalon sandwiched between two thick fused silica windows Figure 4. Etalon-interference filter combination housing and oven. The out-of-band blocking of the etalon was accomplished with an interference filter, which blocked radiation over the response range of the photomultiplier tube (EMR-54 1 E-OI-13) from nm. The filter was manufactured by Barr Associates and is made of two parts. The first part is a fused silica substrate that contains the interference coatings, 144 Proc. of SPIE Vol. 5542

5 which defined the bandpass and provided for some blocking. The filter had a bandpass of 770 pm which closely matched the etalon's free spectral range of7lo pm. The interference filter was mounted in the etalon oven housing and is also heated to 40 C, so its shift with temperature of2i pml C, had to be accounted for. The second part ofthe filter is a shortwave blocking filter element made of Schott GG 495 color glass. With the window, etalon and interference filter all made of fused silica the only element susceptible to space radiation is the blocking filter. The expected transmission loss of the blocking filter over the three year mission life is expected to be less than one percent by mounting the fused silica interference filter on the outside. 4. RESULTS The flight etalon was tilted to an angle of 15.2 mrad and operated at 39.5 C to pass the flight laser wavelength of nm. The performance results exceeded the requirements as shown in Table I. The etalon transmission was measured by wavelength scanning a narrow band laser, <I pm, through the receiver optical system. With the etalon at a fixed temperature of39.5 C the peak transmission is at nm and the bandwidth is 39 pm as shown in Figure 5. The transmission curve closely matches the Airy function. Figure 6 shows the etalon transmission as a function of the etalon temperature and the laser at a fixed wavelength. These measurements give a thermal tuning coefficient of 3.5 pml C, which compares favorably with 3.9 pm/ C using the physical properties of fused silica. The etalon housing has the capability of changing the etalon temperature from C which provides an on-orbit tuning range of 54 pm. C H H Data Airy fit Wavelength (nm) Data Airy fit Temperature (C) Figure 5. Etalon transmission versus wavelength using a tunable narrow band laser. Figure 6. Etalon transmission versus temperature for laser at nm. ACKNOWLEDGM ENTS This work was supported by NASA Langley Research Center. The authors would like to acknowledge the contribution of Dewi Feaver for designing and testing the etalon housing and Jason Ensher, Justin Spelman, and Paula Wamsley for testing ofthe ofthe completed receiver system. REFERENCES I. C.S. Weimer, R. Schwiesow, and M. LaPole, "CALIPSO: Lidar and wide-field camera performance", Proc. SPIE, V S. Z. Xing and J. C. Bergquist, "Thermal Shifts of the Spectral Lines in the 4F312 to I11/2 Manifold of an Nd:YAG Laser" IEEE J. Quantum Electron., vol. 24, No. 9, pp , Sept H. A. Macleod, Thin-Film Optical Filters, McGraw-Hill, Proc. of SPIE Vol