The signal-to-noise ratios have been derived for IPDA lidar using a direct detection receiver for both pulsed and sinewave laser modulation techniques, and the results and laboratory measurements are presente

Abshire, James B.

Sun, Xiaoli

NASA Technical Reports Server

Signal to Noise Ratios of Pulsed and Sinewave Modulated 
Direct Detection Lidar for IPDA Measurements 
 
Xiaoli Sun and James B. Abshire 
NASA Goddard Space Flight Center, Code 694, Greenbelt, Maryland 20771, USA 
xiaoli.sun@nasa.gov  
 
Abstract: The signal-to-noise ratios have been derived for IPDA lidar using a direct detection 
receiver for both pulsed and sinewave laser modulation techniques, and the results and laboratory 
measurements are presented. 
 
1.  Introduction 
Integrated path differential absorption (IPDA) lidar are used to remotely measure the column density of 
gases in the path to a scattering target [1]. Usually a direct detection receiver is used to measure the ratio of the laser 
echo signal with the laser wavelength tuned onto the gas absorption line (on-line) to that tuned off the line (off-line). 
When using transmitters based on tunable diode lasers followed by modulators and amplifiers, the modulation 
pattern is flexible, and the laser can operate CW or with its intensity modulated with pulses or different patterns. For 
space measurements there is interest in examining laser modulation techniques in order to minimize the transmitted 
laser power to achieve a high received signal-to-noise ratio (SNR).  
Here we consider the performance of two measurement techniques for direct detection IPDA lidar. One is 
the pulsed modulation, where the laser wavelength is switched between on-line and off-line wavelengths on 
alternate pulses [2]. The other technique uses two CW lasers modulated by sinewaves of different frequencies and 
using lock-in type processing at each frequency in the receiver [3]. In this case, both the on-line and off-line lasers 
transmit simultaneously but are intensity modulated at different frequencies. Figure 1 shows a schematic of the laser 
signals for these two modulation techniques. Figure 2 shows the corresponding receiver block diagrams. 
Generally the pulse modulation and detection is a time resolved measurement and requires a higher peak 
power laser operating at a relatively low duty cycle. Sinewave modulation and lock-in type detection approach uses 
a laser operating at lower peak power and higher duty cycle, and a very narrow electrical bandwidth post detection 
receiver to reject noise. The on-line and off-line lasers can be transmitted at the same time without interference. This 
paper compares the received SNR as a function of the average received laser power for both approaches.
 
Figure 1. Pulsed and sine wave modulation of the laser power. 
 
 
Figure 2. Receiver block diagrams for the receivers using pulsed 
detection and sine wave modulation lock-in detection. Two similar 
post detector circuits are used for the lock-in detection, one for 
each modulation frequency.
2. Receiver SNR – Theory 
 
We have derived the expressions for the mean signal and noise variance from the direct detection receiver for both 
cases, assuming a ideal modulation of the laser power, an ideal photon counting detector and processing electronics. 
The SNR for the pulsed modulation can be written as 
https://ntrs.nasa.gov/search.jsp?R=20110008281 2019-08-30T15:04:27+00:00Z
€ 
SNR ≡ meanstdev =
1
2
⋅
ηdet
hf Psig
ηdet
hf Psig +Pbgτ pw f pulse( ) +
I darkτ pw f pulse
q
Tint      (1) 
where ηdet is the detector quantum efficiency, hf is the photon energy, <Psig> is the average received signal power, 
Pbg is the CW background power, τpw is the pulse width,  fpulses is the pulse rate, Idark is the detector dark current, q is 
the electron charge, and Tint is the receiver integration time.  For sine wave modulation and lock-in detection, the 
receiver shot noise may be derived from frequency domain [4] and SNR can be expressed as 
      
€ 
SNR = meanstdev =
1
2⋅
ηdet
hf ⋅
Psig
2
2 ηdethf ⋅ Psig +Pbg( ) +
I dark
q
⎡ 
⎣ 
⎢ 
⎤ 
⎦ 
⎥ BW ⋅ 2
=
1
4 2
⋅
ηdet
hf ⋅ Psig
ηdet
hf ⋅ Psig +Pbg( ) +
I dark
q
⋅
1
2BWn
                       (2)  
where BWn is the receiver noise bandwidth. If an ideal integrators is used as the lowpass filters, noise bandwidth and 
the integration times follows the relationship 
€ 
BWn = 1/2Tint .  The results are plotted in Figure 1, which shows the 
SNR for pulsed detection is at least 4 times that of sinewave detections. Since the pulse duty cycle 
€ 
τ pw f pulse <<1, the 
background noise and detector dark noise have much less impact on pulse detection than on sinewave modulation.  
 
3. Receiver SNR – Experiment 
We conducted laboratory experiments to measure the SNRs for both pulse detection and lock-in detection under 
similar conditions. The power from the 1060 nm laser diode was modulated by a waveform generator. A near 
infrared photomultiplier was used as photon counting detector. For pulsed detection a multichannel scaler was used 
as a time resolved histogrammer. For lock-in detection, a bandpass filter was used after the detector followed by an 
oscilloscope to record the analog waveforms into a PC for the signal processing. Figure 3 shows the measurement 
results along with the calculations from Eqs. (1) and (2) and the parameter values used in the experiments. 
 
 
Experiment Parameters: 
Laser: 1060 nm laser diode, intensity modulated by an  
           arbitrary waveform modulator 
Detector: Hamamatsu H10330-75 PMT used in photon  
                counting configuration 
 
Pulsed Modulation: 
Pulse width: 1 µsec, rectangular shape 
Pulse rate: 10 kHz, alternating on line and off-line  
Receiver integration time: Tint= 0.2 sec 
 
Sinewave Modulation Lock-in Detection: 
Sinewave frequency, on-line: 50 kHz  
Sinewave frequency, off-line: 51 kHz 
Anti alaising filter before oscilloscope: 10 kHz bandpass 
Lowpass filter type: 9th order Bessel 
Lowpass filter bandwidth: 5 Hz  
Figure 3. Measurement of the receiver SNR for both pulse and lock-in detection (symbols) along with the theoretical 
calculations (lines) using Eqs. (1) and (2). The  parameter values are also listed. 
 
Figure 3 shows that there is good agreement between the theory and the measurements. At high signal conditions, 
where the performance is limited by signal shot noise, the pulsed SNR is 4 times higher that of sinewave 
modulation. It also shows that the pulsed modulation requires about 1/16 the average laser power to achieve the 
same SNR compared to sinewave modulation. The differences become larger at lower signal levels. More details 
about the derivation and experiment will be described in the presentation. 
 
References 
[1] R.M. Measures, Laser Remote Sensing, Fundamentals and Applications, Krieger, Malabar Florida, 1992.  
[2] J. B. Abshire et al., ‘Pulsed airborne lidar measurements of atmospheric CO2 column absorption,’ Tellus Series B, 2010. 
[3] J. Pruitt et al., ‘High-speed CW lidar retrieval using spectral lock=in algorithm,’ SPIE 5154, 2003. 
[4] R. M. Gagliardi and S. Karp, Optical Communications, Ch. 4, 2nd edition, John Wiley and Sons, New York, 1995. 


Signal to Noise Ratios of Pulsed and Sinewave Modulated Direct Detection Lidar for IPDA Measurements

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