Difference Frequency Technique Research Articles

Research on the frequency stabilization of pulsed differential absorbing lidar for CO2 detection based on matching algorithm

The differential absorption lidar (DIAL) can help us to obtain the vertical distribution of the atmospheric CO2 concentration, which is important to the study of carbon sources and carbon sinks. We design a seeder injected pulsed laser system, working as the laser source of the CO2 DIAL. Unlike the other CO2 DIALs, our laser source is the result of difference frequency of two lasers at the wavelengths of 1064 nm and 634 nm, respectively. It should be pointed out that the high frequency (wavelength) accuracy and stability of the emission laser, especially the on-line one, are greatly required in the CO2 DIAL system. However, the mechanical properties of the dye laser (634 nm) and the application of laser difference frequency technique make the wavelength drift constantly; besides, the extremely unstable energy of the pulsed laser increases the difficulty in identifying and stabilizing the on-line wavelength. Hence, a fast and efficient frequency (wavelength) stabilization method is needed to achieve a stable emission wavelength. Aiming at the research gap of the high precision requirements of on-line laser for this kind of pulsed DIAL, we propose a frequency stabilization method based on matching algorithm. The basic idea is to utilize the saturable absorption of CO2 molecule, by measuring the differential residual-intensity after the laser passing through dual absorption cells to calculate the optical depth (OD) and obtain the so-called pseudo CO2 absorption spectrum, which can be used to identify the on-line laser accurately. Finally, based on the matching algorithm of one-dimensional image, treating the OD as the gray value in the image, we implement the OD matching as a most important part in the process of frequency stabilization, and determine the exact position of the real-time output laser in the measured pseudo absorption spectrum. Thus, when some errors happen to the monitored ODs, by continuously adjusting the wavelength of the laser, the proposed method can fulfill the wavelength adjustment and accomplish the continuous frequency stabilization for on-line laser. Experimental results show that the frequency stabilization algorithm based on OD matching can satisfy the requirements for pulsed on-line laser frequency stabilization, and the sum of squares of deviation method is the optimal one in this application, with a stabilization accuracy of 0.3 pm. Besides, the proposed method can also be introduced in other laser frequency stabilization.

Measurement of carrier-envelope phase of few cycles Ti:sapphire laser by difference frequency technique

The self-reference technique is widely used to measure the carrier-envelope phase offset (CEO) of femtosecond laser pulses in the normal femtosecond laser frequency comb system. However, the instability of the frequency comb system is increased because the photonic crystal fiber is introduced into the system to broaden the spectrum of femtosecond laser pulses outside the cavity. In this paper, we presented another method to measure specifically the CEO for the broad spectrum Ti:sapphire oscillator with the pulse duration of 7fs, namely the difference frequency technique, with which we have obtained the beat frequency signal with a S/N ratio greater than 31dB. This provided a good precondition for the next generation of the femtosecond laser frequency comb system without the photonic crystal fiber.

Multi-order predistortion of power amplifiers using a second harmonic based technique

A novel multi-order predistortion linearizer is proposed to achieve independent control of the third- and fifth-order intermodulation products in high power amplifier. The approach is based on a combination of the second harmonic technique and the difference frequency technique. The second harmonic and difference frequency terms are generated using an envelope detector and two frequency multipliers. The RF predistorter has the advantage of low insertion loss and the requirement of a short delay line. Experimental results demonstrate an adjacent channel power ratio (ACPR) reduction of 11 dB for W-CDMA, at 2140 MHz.

Difference frequency technique to achieve frequency-temperature compensation in whispering-gallery sapphire resonator-oscillator

A new difference frequency technique to obtain frequency-temperature compensation using high quality anisotropic dielectric resonators is presented (patent pending). Compensation was demonstrated near room temperature by frequency locking two microwave oscillators to orthogonally polarised whispering-gallery modes in the same sapphire resonator.

Improvement of third-order intermodulation product of RF and microwave amplifiers by injection

This paper discusses the improvement in the third-order intermodulation product (IM3) performance obtainable from RF and microwave amplifiers by two alternative injection techniques. The first is the addition to the amplifier input of the second harmonics of the input spectrum and the second is the addition to the amplifier input of the difference frequencies between the spectral components of the input signal. Both techniques are considered in theory, by simulation and in practice. Both techniques give useful improvements in two-tone IM3 performance. The second harmonic technique reduced the IM3 level by 43 dB in an amplifier at 835 MHz. The difference-frequency technique gave a reduction of 48 dB in an amplifier at 880 MHz. The difference-frequency technique also gives a greater improvement for complex spectra signals.

A difference frequency technique for improving IM performance of RF amplifiers

A difference frequency technique for reducing the IM product levels in a nonlinear RF amplifier is presented. The technique is based on the standard two-tone test. The two sinusoid signals together with their difference frequency signal are fed into the amplifier. CAD simulation shows that the third-order IM level at the output of a nonlinear amplifier can be reduced using this technique. The results of a related experiment are reported, which gave a measured of 50 dB reduction in the level of the third-order intermodulation without affecting the fundamental power levels. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 208–210, 2000.

Heterodyne frequency measurements and analysis of CO 2 laser hot band transitions

New frequency measurements of molecular lines in the P branch of the 01 11-[11 10, 03 10] I band of 12C 16O 2, observed in laser emission, and lines in the R branch, observed in absorption with a tunable-diode laser, have been made by heterodyne difference frequency techniques, with stabilized CO 2 lasers generating the reference frequencies. The data obtained, plus additional results of measurements made by two other groups of researchers, were combined in a leastsquares fit to obtain improved rovibrational constants for this band. The new constants predict more reliably the frequencies of both the P- and R-branch transitions. A table of transition frequencies and their estimated uncertainties is given for this band.

Infrared spectrometry

A brief history of the early work on Fourier spectroscopy at the Johns Hopkins University is given as well as a general description of the techniques of Fourier and Hadamard transform spectroscopy. The techniques of tunable diode laser spectroscopy and the difference frequency technique for IR spectral studies are also briefly described including some of their characteristics.

Doppler-limited molecular spectroscopy by difference-frequency mixing*

A cw difference-frequency spectrometer has been developed for the 2.2 to 4.2 μm infrared region by mixing argon and tunable dye lasers in LiNbO3. A resolution of 5 × 10−4 cm−1 with continuous scans up to 1 cm−1 and a scan precision better than 2 × 10−3 cm−1 have been achieved. Tunable ir powers of ~1 μW have been obtained, enabling sensitive absorption measurements and real-time spectroscopy. The range, resolution, speed, and sensitivity of this difference-frequency technique are demonstrated by absorption traces of water vapor, ammonia, methane, and nitrous oxide.