Dual-frequency Laser Research Articles

Coherent continuous-wave dual-frequency high-Q external-cavity semiconductor laser for GHz-THz applications.

We report a continuous-wave highly-coherent and tunable dual-frequency laser emitting at two frequencies separated by 30 GHz to 3 THz, based on compact III-V diode-pumped quantum-well surface-emitting semiconductor laser technology. The concept is based on the stable simultaneous operation of two Laguerre-Gauss transverse modes in a single-axis short cavity, using an integrated sub-wavelength-thick metallic mask. Simultaneous operation is demonstrated theoretically and experimentally by recording intensity noises and beat frequency, and time-resolved optical spectra. We demonstrated a >80 mW output power, diffraction-limited beam, narrow linewidth of <300 kHz, linear polarization state (>45 dB), and low intensity noise class-A dynamics of <0.3% rms, thus opening the path to a compact low-cost coherent GHz to THz source development.

Coherent Interferometric Dual-Frequency Laser Radar for Precise Range/Doppler Measurement

A novel coherent interferometric dual frequency laser radar that merges the concept of laser and radio detection and ranging (ladar and radar, respectively), for both range and velocity measurement, is presented and experimentally demonstrated. The innovative architecture combines the broadband tunability of dual-wavelength optical sources, enabling a dynamic trade-off between precision and robustness in Doppler estimation, with the high stability of low frequency RF sources for the interferometric measure of the target range with extreme accuracy. The possibility to easily reconfigure the employed frequencies, allows to change the Doppler resolution, as well as the range ambiguity and precision, to dynamically adapt the system to reliably operate in different environments. Moreover, the coherent detection allows to enhance the signal to noise ratio reaching excellent performances also with low level of received power. The laboratory characterization provides an estimation of the system performances, in terms of resolution and sensitivity, as well as the indoor demonstration with targets of opportunity proves the effectiveness of the proposed architecture to operate in real scenarios.

Doppler-free spectroscopy on the Cs D1 line with a dual-frequency laser.

We report on Doppler-free laser spectroscopy in a Cs vapor cell using a dual-frequency laser system tuned on the Cs D1 line. Using counter-propagating beams with crossed linear polarizations, an original sign reversal of the usual saturated absorption dip and large increase in Doppler-free atomic absorption is observed. This phenomenon is explained by coherent population trapping (CPT) effects. The impact of laser intensity and light polarization on absorption profiles is reported in both single-frequency and dual-frequency regimes. In the latter, frequency stabilization of two diode lasers was performed, yielding a beat note fractional frequency stability at the level of 3×10-12 at 1 s averaging time. These performances are about an order of magnitude better than those obtained using a conventional single-frequency saturated absorption scheme.

Dual-frequency injection-locked continuous-wave near-infrared laser.

We report a dual-frequency injection-locked continuous-wave near-infrared laser. The entire system consists of a Ti:sapphire ring laser as a power oscillator, two independent diode lasers employed as seed lasers, and a master cavity providing a frequency reference. Stable dual-frequency injection-locked oscillation is achieved with a maximum output power of 2.8 W. We show its single longitudinal/transverse mode characteristics and practical power stability, as fundamental performance features of this laser system. We also demonstrate arbitrary selectivity of the two frequencies and flexible control of their relative powers by simply manipulating the seed lasers, as advanced features.

Frequency-locked chaotic opto-RF oscillator.

A driven opto-RF oscillator, consisting of a dual-frequency laser (DFL) submitted to frequency-shifted feedback, is experimentally and numerically studied in a chaotic regime. Precise control of the reinjection strength and detuning permits isolation of a parameter region of bounded-phase chaos, where the opto-RF oscillator is frequency-locked to the master oscillator, in spite of chaotic phase and intensity oscillations. Robust experimental evidence of this synchronization regime is found, and phase noise spectra allow us to compare phase-locking and bounded-phase chaos regimes. In particular, it is found that the long-term phase stability of the master oscillator is well transferred to the opto-RF oscillator, even in the chaotic regime.

Birefringent dual-frequency laser Doppler velocimeter using a low-frequency lock-in amplifier technique for high-resolution measurements.

A birefringent dual-frequency laser with a half-intracavity has been used to develop a laser Doppler velocimeter (LDV). The developed LDV utilizes a new signal-processing method based on a lock-in amplifier to achieve high-resolution velocity measurements and the discrimination of positive and negative velocities. Theoretical analysis and simulation results are presented. The velocity measurement experiments by using a high-precision linear stage are performed to verify the performance of the LDV. Compared with the previous dual-frequency LDVs, the average velocity resolution of the developed LDV is improved from 0.31 mm/s to 0.028 mm/s for a target without the rotational velocity. The measurement results show that our new technique can offer a powerful instrument for metrology sciences.

T-shaped cavity dual-frequency Nd:YAG laser with electro-optical modulation

A T-shaped cavity dual-frequency Nd:YAG laser with electro-optical modulation is proposed, which consists of both p- and s-cavities sharing the same gain medium of Nd:YAG. Each cavity was not only able to select longitudinal mode but also tune frequency using an electro-optic birefringent filter polarization beam splitter + lithium niobate. The frequency difference of dual frequency was tuned through the whole gain bandwidth of Nd:YAG, which is far above the usually accepted free spectral range value in the case of a single-axis laser. As a result, the simultaneous operation of orthogonally and linearly polarized dual-frequency laser was obtained, which coincides with the theoretical analysis based on Jones matrices. The obtained frequency difference ranges from 0 to 132 GHz. This offers a simple and widely tunable source with potential for portable frequency reference applications in terahertz-wave generation and absolute-distance interferometry measurement areas.

Frequency difference stabilization in dual-frequency laser by stress-induced birefringence closed-loop control.

The frequency difference of dual-frequency lasers is increasingly becoming an area of focus in research. The stabilization of beat frequency is of significance in fields such as synthetic wavelength and shows great potential in precise measurement. In this paper, a novel device based on stress-induced birefringence closed-loop control is proposed. Experiments are carried out on a dual-frequency He-Ne Zeeman-birefringence laser with the output mirror sealed in the opposite direction. The results show that the device is capable of controlling the frequency difference variation in 1.3%, in a convenient and highly cost-effective way, and it can increase the quantity of frequency difference, which is crucial to the application of precise measurement through dual-frequency lasers.

In-phase and antiphase self-intensity regulated dual-frequency laser using two-photon absorption.

A 25 dB reduction of resonant intensity noise spectra is experimentally demonstrated for both the antiphase and in-phase relaxation oscillations of a dual-frequency solid-state laser operating at telecommunication wavelengths. Experimental results demonstrate that incorporation of an intracavity two-photon absorber that acts as a buffer reservoir reduces efficiently the in-phase noise contribution, while it is somewhat ineffective in lowering the antiphase noise contributions. A slight spatial separation of the two modes in the nonlinear two-photon absorber reduces the antiphase resonant intensity noise component. These experimental results provide a new approach in the design of ultra-low noise dual-frequency solid-state lasers.

50 W low noise dual-frequency laser fiber power amplifier.

A three-stage dual-frequency laser signal amplification system is presented. An output from a narrow-linewidth Nd:YAG nonplanar ring-oscillator (NPRO) is split into two parts, one of them is frequency shifted by an acoustooptic modulator (AOM) then coupled into a single mode optical fiber. The other part is coupled into another single mode fiber then combined with the frequency-shifted beam with a 2 to 1 single mode fiber coupler. The combined beam has a power of 20 mW containing two frequency components with frequency separation of 150 ± 25 MHz. The dual-frequency signal is amplified via a three-stage Yb3+-doped diode pumped fiber power amplifier. The maximum amplified power is 50.3 W corresponding to a slope efficiency of 73.72% of the last stage. The modulation depth and signal to noise ratio (SNR) of the beat signal are well maintained in the amplifying process. The dual-frequency laser fiber power amplifier provides robust optical carried RF signal with high power and low noise.

Effect of pump polarization direction on power characteristics in monolithic microchip Nd:YAG dual-frequency laser.

The pump polarization direction can greatly influence the characteristics of the laser diode end-pumped monolithic microchip Nd:YAG dual-frequency laser. We experimentally observe the lasing thresholds and the optical powers of two splitting modes versus the pump polarization direction. The effect of the pump-induced gain anisotropy on the mode oscillation sequence is analyzed. And the effect on the intensities of these modes is also proved with a rate equation model. This study contributes to the improvement of the stability and the reliability of the Nd:YAG dual-frequency laser.

System for simultaneously measuring 6DOF geometric motion errors using a polarization maintaining fiber-coupled dual-frequency laser.

A novel method for simultaneously measuring six degree-of-freedom (6DOF) geometric motion errors is proposed in this paper, and the corresponding measurement instrument is developed. Simultaneous measurement of 6DOF geometric motion errors using a polarization maintaining fiber-coupled dual-frequency laser is accomplished for the first time to the best of the authors' knowledge. Dual-frequency laser beams that are orthogonally linear polarized were adopted as the measuring datum. Positioning error measurement was achieved by heterodyne interferometry, and other 5DOF geometric motion errors were obtained by fiber collimation measurement. A series of experiments was performed to verify the effectiveness of the developed instrument. The experimental results showed that the stability and accuracy of the positioning error measurement are 31.1 nm and 0.5 μm, respectively. For the straightness error measurements, the stability and resolution are 60 and 40 nm, respectively, and the maximum deviation of repeatability is ± 0.15 μm in the x direction and ± 0.1 μm in the y direction. For pitch and yaw measurements, the stabilities are 0.03″ and 0.04″, the maximum deviations of repeatability are ± 0.18″ and ± 0.24″, and the accuracies are 0.4″ and 0.35″, respectively. The stability and resolution of roll measurement are 0.29″ and 0.2″, respectively, and the accuracy is 0.6″.

3D robust digital image correlation for vibration measurement.

Discrepancies of speckle images under dynamic measurement due to the different viewing angles will deteriorate the correspondence in 3D digital image correlation (3D-DIC) for vibration measurement. Facing this kind of bottleneck, this paper presents two types of robust 3D-DIC methods for vibration measurement, SSD-robust and SWD-robust, which use a sum of square difference (SSD) estimator plus a Geman-McClure regulating term and a Welch estimator plus a Geman-McClure regulating term, respectively. Because the regulating term with an adaptive rejecting bound can lessen the influence of the abnormal pixel data in the dynamical measuring process, the robustness of the algorithm is enhanced. The robustness and precision evaluation experiments using a dual-frequency laser interferometer are implemented. The experimental results indicate that the two presented robust estimators can suppress the effects of the abnormality in the speckle images and, meanwhile, keep higher precision in vibration measurement in contrast with the traditional SSD method; thus, the SWD-robust and SSD-robust methods are suitable for weak image noise and strong image noise, respectively.

Phase noise reduction by using dual-frequency laser in coherent detection

Dual-frequency laser radar (DFLR) uses laser with two coherent frequency components as transmitting wave. The method is based on the use of an optically-carried radio frequency (RF) signal, which is the frequency difference between the two components. Due to the two optical waves are generally subject to the same first order phase noise, the synthesis wave is predicted to have a stronger resistance to atmospheric turbulence than the single-frequency wave. To the best of our knowledge, the model proposed in this paper is the first model that detailedly illustrates that the dual-frequency laser has an advantage over the single frequency laser in atmospheric turbulence resistance. Experiments are carried out to compare the performances of single and dual frequency lidars under atmospheric turbulence. The experimental results show that, with the increase of atmospheric turbulence intensity, the signal to noise ratio (SNR) of beat signal decreases and its central frequency stability (CFS) becomes worse in conventional single frequency coherent laser radar (SFCLR). While for the DFLR, the SNR and CFS are almost unaffected by atmospheric turbulence, which are in good agreement with the theoretical model.

Laparoscopic Transcystic Treatment Biliary Calculi by Laser Lithotripsy.

Background and Objectives:Laparoscopic transcystic common bile duct exploration (LTCBDE) is a complex procedure requiring expertise in laparoscopic and choledochoscopic skills. The purpose of this study was to investigate the safety and feasibility of treating biliary calculi through laparoscopic transcystic exploration of the CBD via an ultrathin choledochoscope combined with dual-frequency laser lithotripsy.Methods:From August 2011 through September 2014, 89 patients at our hospital were treated for cholecystolithiasis with biliary calculi. Patients underwent laparoscopic cholecystectomy and exploration of the CBD via the cystic duct and the choledochoscope instrument channel. A dual-band, dual-pulse laser lithotripsy system was used to destroy the calculi. Two intermittent laser emissions (intensity, 0.12 J; pulse width 1.2 μs; and pulse frequency, 10 Hz) were applied during each contact with the calculi. The stones were washed out by water injection or removed by a stone-retrieval basket.Results:Biliary calculi were removed in 1 treatment in all 89 patients. No biliary tract injury or bile leakage was observed. Follow-up examination with type-B ultrasonography or magnetic resonance cholangiopancreatography 3 months after surgery revealed no instances of retained-calculi–related biliary tract stenosis.Conclusion:The combined use of laparoscopic transcystic CBD exploration by ultrathin choledochoscopy and dual-frequency laser lithotripsy offers an accurate, convenient, safe, effective method of treating biliary calculi.

激光强度起伏及相位噪声对光学双频探测的影响

激光强度起伏及相位噪声对光学双频探测的影响

Dual-polarization fiber grating laser-based laser Doppler velocimeter

A dual-frequency laser Doppler velocimeter implemented by a dual-polarization fiber grating laser is proposed, with the two laser frequencies produced by the two orthogonally polarized laser outputs of the fiber grating laser. The reflected laser outputs from a moving target experience the Doppler frequency shift, which is shown to be linearly related to the velocity and the beat note frequency difference between the laser outputs and the reflected light. With a digital frequency demodulation scheme, a high sensitivity of 0.64 MHz/(m/s) and a velocity resolution of less than 0.5% of the velocity for velocity measurement are demonstrated, which shows that the proposed laser Doppler velocimeter is capable of measurement of wide range of velocity.

时频扰动对双频激光相干探测系统的影响

时频扰动对双频激光相干探测系统的影响

抑制Y 型腔正交偏振双频激光器频差闭锁及失谐磁场结构设计

抑制Y 型腔正交偏振双频激光器频差闭锁及失谐磁场结构设计

Measuring Method of Radius of Curvature Based on Dual-Frequency Laser Interferometer

In order to achieve high-accuracy measurement of radius of curvature of optical sphere, ultra-high accuracy radius of curvature testing device is developed by dual-frequency laser interferometer and Fizeau interferometer based on cat’s eye and confocal method. Through analyzing the error source models of radius of curvature testing, optical configuration of the testing device has been optimized. Precise environment control and real-time monitoring system is also established to reduce the errors caused by environment. Through the above processes, the radius of curvature measurement relative accuracy is better than 2 ppm. One optical sphere, R88.5 mm, test aperture 59 mm, has been tested. Testing result is 88499.465 ± 0.176 μm, meeting the design requirement. The method has high accuracy and practical advantages.