Laser Doppler optical air-data system: feasibility demonstration and systems specifications

Hans W. Mocker; T. J. Wagener

doi:10.1364/AO.33.006457

Applied Optics
Vol. 33,
Issue 27,
pp. 6457-6471
(1994)
•https://doi.org/10.1364/AO.33.006457

Laser Doppler optical air-data system: feasibility demonstration and systems specifications

Hans W. Mocker and T. J. Wagener

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Hans W. Mocker and T. J. Wagener, "Laser Doppler optical air-data system: feasibility demonstration and systems specifications," Appl. Opt. 33, 6457-6471 (1994)

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Abstract

A compact laser Doppler velocimeter with diode-pumped solid-state laser technology at 1.06 μ and a novel solid-block beam-splitting interferometer was demonstrated. This system allows for accurate Doppler velocity measurements even in a vibration environment. A comparison of the backscatter measurements with a backscatter sonde from the University of Wyoming showed excellent agreement. Based on demonstrated performance and technology projection, the system specifications of an optical air-data system at 2.015 μm are determined, and a detailed design concept is presented. In addition, the potential for a multifunctional sensor that can determine air data and detect wind shear and wake vortices is addressed.

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Atmospheric Layer	Typical Values	After Volcanic Eruption
Lower troposphere (0–6 km)	β_10.6μm ~ 10⁻⁶ to 10⁻⁹ (1/msr)	Minimal long-term effect
	β_1.06μm ~ 10⁻⁵ to 10⁻⁸ (1/msr)
	β_10.6μm/β_1.06μm ~ 2.5 × 10⁻²
Upper troposphere (6–10 km)	β_10.6μm ~ 10⁻⁸ to 10⁻¹⁰ (1/msr)	Minimal long-term effect
	β_1.06μm ~ 10⁻⁷ to 10⁻⁹ (1/msr)
Stratosphere (10–20 km)	β_10.6μm ~ 10⁻⁸ to 10⁻¹¹ (1/msr)	β_10.6μm ~ 10⁻⁸ (1/msr)
	β_1.06μm ~ 10⁻⁷ to 10⁻⁹ (1/msr)	β_10.6μm/β_1.06μm ~ 2.9 × 10⁻²
	β_10.6μm/β_1.06μm ~ 0.7 × 10⁻²

Wavelength (μm)	Desired Aperture Size (cm²)	Required Energy (mJ)	Energy Density at Aperture (mJ/cm²)	Permissible Energy Density (mJ/cm²) at 50 Hz	Percent of Permissible Exposure
1.06	0.6	3	5	5 × 10⁻⁴	10⁶
1.54	0.9	3	3	2	150
2.015	1.1	3	2.7	2	135
10.6	5.9	3	0.5	2	25

	Wavelength (μm)

Parameter	1.06	2.015	10.6
SNR (dB)	5	5	5
η	0.4	0.4	0.4
β (altitude 20 km)	1 × 10⁻⁹ (msr)⁻¹	3 × 10⁻¹⁰ (msr)⁻¹	1 × 10⁻¹¹ (msr)⁻¹
B for cw laser system (kHz)	800	400	80
Laser power for cw system (kW)	2	1	0.2
Pulse length (μs)	0.25	0.5	2.5
B for pulsed-laser system (MHz)	4	2	0.4
Laser peak power for pulsed system (kW)	12	6	1.2
Laser pulse energy (mJ)	3	3	3

Parameter	Range	Accuracy	Resolution	Update Rate (Hz)
True airspeed (TAS)	20–1200 kts	±2.5 kts	0.1 kts	50
Angle of attack (AOA)
− 15°–30°	TAS > 60 kts − 15° ≤ AOS ≤ 15°	±0.5°	0.1°	50
±90°	20 < TAS < 60 kts −60° ≤ AOS ≤ 60°	±3°
	TAS > 60 kts −80° ≤ AOS ≤ 80°	±3°
Angle of sideslip (AOS)
±15°	TAS > 60kts −15° ≤ AOS ≤ 30°	±1°	0.1°	50
±90°	TAS > 20kts −90 ≤ AOA ≤ 90°	±3°

Parameter	Nd:YAG	Er³⁺:Glass	Tm³⁺:YAG
Wavelength (μm)	1.064	1.535	2.015
Fluorescence polarization	Isotopic	Isotopic	Isotopic
Index of refraction n	1.82	1.531	1.8315
Temperature coefficient dn/dT (1/°C)	7.3 × 10⁻⁶	−6.3 × 10⁻⁶	7.3 × 10⁻⁶
Gain cross section σ (cm²)	6.5 × 10⁻¹⁹	8 × 10⁻²¹	7 × 10⁻²¹
Fluorescent lifetime τ	230 μs	8.8 ms	10 ms; under intense pumping (i.e., 10 kW/cm²) 1.8 ms
Peak absorption at laser diode emission (cm⁻¹)	4–8	5–11	6
Absorption bandwidth (nm)	~ 3	~ 60	4
Pumping laser diode wavelength at absorption peak (nm)	808.5	930–1000	781 and 785
Thermal conductivity (W/m °C)	10 or 13	0.82	14
Thermal expansion a (1/°C)	7–8 × 10⁻⁶	11.4 × 10⁻⁶	6.9 × 10⁻⁶
Optimum doping (%)	0.6–1.4	0.15	6

Parameter	Value
Gain medium	Tm³⁺:YAG
Optical wavelength	2.015 μm
Pulse length	0.5 μs
Pulse energy	3 mJ
Repetition rate	> 200 Hz
Average power	~ 1 W
Linewidth	~ 200 kHz
Efficiency	> 10%
Temperature	−60–70 °C
Pumping laser diodes (λ)	AlGaAs (785 nm)
Reliability	> 5000 h

Parameter	Value
Optical wavelength	2.015 μm
Optical throughput	99%
Optical polarization	Elliptical
Scan angle	45 ± 0.5° w/r to rotation axis
Angular wobble	< 1 arcmin
Clear aperture diameter	2.5 cm
Motor drive	3000 rpm ± 0.1%
Position pickoff	Every 90° with 1 arcmin accuracy
Temperature	−60–70 °C
Pressure	Sea level to 20 km
Reliability	> 20,000 h

Parameter	Value
Material type	InGaAs-p-i-n diode
Optical wavelength	2.015 μm
Diameter	75 μm
Quantum efficiency	1.1
Dark current	30 nA
Noise current	$~ 7.0 pA / \sqrt{Hz}$
Capacitance	3 pF
Frequency response	250 MHz
Temperature	−60–70 °C

Parameter	Value
Processor type	FFT with frequency band selection
Algorithm	Poly-pulse pair
Doppler frequency range	1–250 MHz
Burst length	0.5 μs
Doppler frequency update	50 Hz on each axis
Accuracy of frequency measurement	200 kHz or better
Temperature	−60–70 °C

	Demonstration Unit		Production Unit

Component	Weight (kg)	Power (W)	Weight (kg)	Power (W)	Reliabilitya
Injection laser	2.0	30	0.7	25	> 10
Ring laser	4.5	200	2.3	160	> 10
Optics and detection	1.1	1	1.1	1	> 100
Scanner	1.8	15	1.8	15	> 20
Processor	2.7	72	1.8	50	> 100
Power supplies	1.4	62	0.7	49	> 100
Package	9.1	0	6.8	0	> 100
Total	22.6	380	15.2	300	> 4

Altitude	Flight Pattern	Preferred Sensing Mode
High (> 500 m)	Cruise	OADS and CAT
Medium (100–500 m)	Airport approach	OADS and WSS
Low (< 100 m)	Landing and takeoff	OADS and VVD

	Function

Parameter	OADS	WSS		WVD
Approach	Doppler-shifted backscatter from air Pulsed, coherent transmit–receive system that can perform all three functions
Measurement medium	Air containing aerosols, precipitation, and clouds	Air containing aerosols, precipitation, and clouds	Air containing aerosols, precipitation, clouds, and jet exhaust
Altitude (m)	0–20000	100–500	< 100
Range (m)	20	2000–4000	< 500
Scan pattern	Three-axis measurement—collimated or focused (need not be orthogonal)	Volumetric scan (θ and φ with scanner and distance with range gating)	Volumetric scan (θ and φ, with scanner and distance with range gating)
β_min for 2-μm system (1/msr)	3 × 10⁻¹⁰	l0⁻⁸	l0⁻⁸
Velocity accuracy (m/s)	0.20	<1	0.3
Minimum SNR for laser pulse	~ 5 dB at altitude of 20 km	~ 10 dB at range of 4 km in dry microburst	~ 15 dB at range of 500 m

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Applied Optics